Advancing from MOFs and COFs to Functional Macroscopic Porous Constructs

Abstract

Metal–organic frameworks (MOFs) and covalent‐organic frameworks (COFs) are the highly porous rising stars of reticular chemistry. However, most face challenges such as poor macroscopic structuring capability, inadequate mechanical robustness, and inaccessible porosities for target reactants, which hinder their practical applications. This review explores various strategies to assemble MOFs and COFs into macroscopic 3D‐structured multi‐scale porous structures, such as aerogels, foams, and sponges. The methods discussed include direct mixing, self‐shaping, in situ growth, template‐assisted approaches, and 3D printing. These strategies enable macroscopic MOF or COF porous structures to achieve excellent mechanical strength and tunable porosity from the molecular level and micro‐scale up to the macroscopic level. This structural tunability allows the MOF or COF porous structures to outperform their neat powders by making their micro‐ and meso‐porosities more accessible to target reactants. Such improvements pave the way for the functionality of MOF or COF species at larger scales, addressing urgent societal needs, including environmental remediation, CO₂ capturing, value‐added catalytic reactions, water harvesting, electromagnetic (EM) shielding, and beyond.

This review study investigates the recent progress and methodologies for manufacturing metal–organic framework (MOF) or covalent–organic framework (COF)‐based 3D structured macroscopic porous constructs with high structural integrity, providing the possibility to control their porosity across dimensions. This improves the capability of MOFs/COFs far beyond their powdery form, enabling a wealth of opportunities in various disciplines toward crucial societal demands.

1. Introduction

Metal–organic frameworks (MOFs) and covalent–organic frameworks (COFs) are advanced and topologically diverse porous structures that hold significant importance for both science and industry. MOFs and COFs are valued for their remarkable intrinsic properties, including large specific surface area, tunable porosity, and chemical versatility. Such engineerable features and controllable functionality make MOFs and COFs promising for a wide set of crucial societal demands.^{[ 1 ]}

MOFs are known as porous crystalline products of coordination chemistry formed by bonding metal clusters to organic ligands. MOFs were first discovered by Yaghi and Li in 1995,^{[ 2 ]} and since then, have spurred much interest owing to their remarkable characteristics.^{[ 3 ]} These specifications include an ultra‐large specific surface area (up to $\sim$ 10 000 m² g⁻¹) and abundant porosities, along with customizable porosity, composition, and structural features.^{[ 4 ]} Notably, by adjusting the length of the organic linker, MOF pore size can be simply tuned from a few angstroms to a few nanometers (up to about $\sim$9.8 nm).^{[ 3b ]} Such features make MOF promising for a wide range of applications, such as energy storage/conversion,^{[ 5 ]} gas capture/storage,^{[ 6 ]} gas separation,^{[ 7 ]} sensors,^{[ 8 ]} catalysis,^{[ 9 ]} environmental remediation,^{[ 10 ]} drug delivery,^{[ 11 ]} anti‐bacterial applications,^{[ 12 ]} water harvesting,^{[ 13 ]} and beyond.^{[ 14 ]}

Similarly, COFs are another class of crystalline organic compounds that were first discovered by Yaghi's research group.^{[ 15 ]} in 2005. COFs are defined as 2D or 3D porous crystalline materials where their molecular building blocks are linked together through robust covalent bonds in an ordered manner.^{[ 16 ]} This passes through precise control of organic monomers that enable a highly porous, lightweight organic compound with well‐defined porosities, mainly composed of light elements such as C, B, N, and O.^{[ 17 ]} Similar to MOFs, COFs also possess engineerable porosities with pore sizes ranging from 0.7 to 5 nm,^{[ 18 ]} making them suitable for a multitude of applications.^{[ 14a ]}

Despite the numerous intriguing potentials of MOFs and COFs, attributed to their abundant porosities and high specific surface area, they face challenges that hinder their practicality. Both MOFs and COFs possess abundant micro‐ and meso‐porosities, but they suffer from intrinsic fragility and poor processability, rendering them easy to crumble and limiting their potential for 3D macro‐structuring.^{[ 19 ]} Additionally, most of these MOF and COF porosities are not accessible for target reactants, hence diminishing their potential to the optimum level.^{[ 17} , ^{20 ]} For instance, in the case of COFs, strong van der Waals forces and robust π–π stacking interactions cause COF agglomeration due to their lamellar structure. This agglomeration reduces the number of accessible active binding sites, thereby deteriorating their adsorption capability.^{[ 21 ]}

These challenges necessitate the integration of MOFs and COFs into free‐standing, multi‐scale porous 3D‐structured constructs. This approach improves their structural integrity, performance, and handling while enhancing the diffusibility of target reactants into their micro‐ or meso‐porosities. Interestingly, when MOFs or COFs are embedded into a macroscopic porous structure (e.g., aerogels or foams), they retain their intrinsic properties while exhibiting significantly higher adsorption kinetics and capacity compared to their neat powdered forms.^{[ 22 ]} The macroscopic arrangement also benefits from the proper mechanical integrity of supporting scaffolds while providing a broad range of porosity, from the micro‐ and meso‐porosity of MOF or COF species to the macroscopic arrangement originating from the supporting building block, like aerogel or foam. This tunability allows the engineering of aerogel porosity from the molecular to the macroscopic scale, bridging the gap between MOF or COF materials and their practicality in a multitude of applications.

This review offers a critical and comprehensive insight into the design of MOF or COF porous structures with hierarchical multi‐scale porosities, such as aerogels, foams, and sponges, utilizing a broad array of methodologies. These methodologies are categorized into four main subgroups: direct mixing, self‐shaping, in situ growth, and template‐assisted synthesis. Following this, we delve into the processes for integrating these approaches into 3D printing to control the spatial arrangement of the resulting structures, achieving macroscopic porous constructs with on‐demand morphological features. Next, the practicality of hierarchically porous macroscopic MOF or COF structures is investigated for current challenges in environmental remediation, CO₂ capturing and transformation into value‐added products via catalytic processes, water harvesting, and electromagnetic (EM) interference shielding. Finally, we discuss in detail the challenges and future prospects associated with MOF or COF porous structures.

2. Manufacturing Methodologies of Macroscopic Porous Structures

There are several main methodologies to introduce MOFs or COFs into the hierarchical architectures of aerogels or foams, including i) direct mixing or post‐crosslinking, ii) self‐shaping method, iii) in situ growth, and iv) template‐assisted method. Each method consists of its own technical details, but most aerogel production methods involve gelation, aging, and freeze‐drying. These manufacturing processes result in hierarchically porous 3D‐structured aerogels with porosities ranging from the molecular level to micro‐ and macro‐scales, thanks to the incorporation of MOFs or COFs into the system (Figure  1 ). Such porosity tuning across scales enables the integration of various pore sizes in the hierarchical structures, ranging from micro‐porosity (<2 nm) and meso‐porosity (2–50 nm) to macro‐porosity (>50 nm), which is challenging to achieve with traditional aerogel production approaches.^{[ 17} , ^{19 ]} The hybridization of the porous structures’ manufacturing methods with 3D printing also enables on‐demand 3D spatial arrangement and complex macroscopic geometries.^{[ 23 ]}

Figure 1.

Schematic illustration demonstrating the engineering of porosity and the 3D macroscopic arrangements of MOF or COF porous structures; such porosity engineering enables controlling the porosity level from nano‐ and micro‐scales up to the macroscopic level. The purple beads are selected to represent ZIF‐67, a widely used MOF for aerogel production, due to its ease of synthesis.

Regarding the aerogel production approaches, direct mixing or post‐crosslinking (Figure  2A) involves adding the pre‐synthesized MOF or COF powders to the initial compounds with the assistance of other external binders or gel precursors, viz., supporting scaffolds. The formed gel can act as a template for aerogel production through further aging, freezing, and drying.^{[ 17} , ^{19 ]} The self‐shaping approach is mostly used to manufacture COF porous structures and involves the simultaneous growth of COF along with self‐crosslinking without using any external binder. The self‐shaping approach allows the production of hierarchically porous structures from the pure COF (Figure 2B).^{[ 17 ]} The in situ growth approach involves the in situ growth of functional MOF or COF on a 3D structured porous substrate or a supporting scaffold such as aerogels, foams, or sponges. The in situ growth approach allows MOF or COF to be decorated on the outer surface of porous structures, making them more accessible to target reactants (Figure 2C).^{[ 24 ]} The template‐assisted method involves incorporating post‐synthesis removable template molecules into MOF‐ or COF‐based hierarchical constructs, creating engineerable porosities or vacancies within the final structure (Figure 2D). This approach holds great promise for inducing customizable, shape‐memory, and stimuli‐responsive vacancies in multi‐scale porous structures when necessary.^{[ 25 ]}

Figure 2.

Schematic illustration of MOF or COF porous structures manufacturing approaches, including A) direct mixing or post‐crosslinking, B) self‐shaping method, C) in situ growth, and D) template‐assisted synthesis method.

The liquid within the generated MOF or COF gels can be extracted through several drying methods, including ambient pressure drying, supercritical drying, and freeze drying.^{[ 26 ]} The ambient pressure drying can take place when the cell walls are robust enough to withstand the capillary pressure, precluding the irreversible shrinkage of porous structures. Two main strategies have been considered for drying at ambient pressure: i) employing a solvent exchange step using low surface tension solvents, such as ethanol, acetone, and hexane, to minimize capillary pressure, and ii) enhancing the gel's robustness to withstand the applied capillary stress.

Supercritical drying is another drying approach that allows the preservation of the porous framework with the least shrinkage. This approach enables capillary force minimization and solvent removal within the pores of the gel under supercritical conditions. Supercritical‐based drying approaches usually take place through two main strategies: i) the low‐temperature supercritical drying that involves the replacement of the synthesis solvent with a lower‐temperature solvent, e.g., CO₂ at supercritical state (90 atm, 40 °C), and ii) high‐temperature supercritical drying, employing the alcohol at supercritical states (120 atm, 265 °C) to dry the gel and obtain an aerogel. In the low‐temperature state, this method has some limitations in water removal owing to the poor CO₂ solubility in water at supercritical conditions, necessitating the replacement with an organic solvent.^{[ 26 ]}

Freeze‐drying is another widely used gel‐drying approach, enabling aerogel production with the least shrinkage and excellent pore preservation. In this method, the drying process takes place at low temperatures and pressures to sublimate ice crystals instead of gas‐liquid transformation.^{[ 26 ]} Both supercritical and freeze‐drying are capable of preventing structure shrinkage as a result of capillary pressures caused by the outer surface menisci. This prevents the collapse of the cell walls and maintains the integrity of the 3D structured porous frameworks even at ultra‐low densities.^{[ 17} , ^{27 ]} The formed porous constructs are named xerogel, cryogel, or aerogel according to the drying method used. The term “xerogel” typically refers to materials dried at ambient pressure and temperature, resulting in a structure with higher density compared to cryogels and aerogels. In contrast, porous structures created through freeze‐drying (lyophilization), which involves freezing the gel and sublimating the ice crystals, are known as “cryogels”. When supercritical drying is used, the resulting porous structure is called an “aerogel”.^{[ 28 ]} Notably, the literature on MOF and COF porous structures commonly uses the term “aerogels” for the majority of the resulting structures. Therefore, we adopt the term “aerogels” to align with the terminology used in published research. However, it is important to recognize that the distinct terms xerogel, cryogel, and aerogel more accurately describe the different forms.

The porous constructs can be formed from pure MOF or COF or their hybrid with other compounds as integrity boosters or supporting scaffolds, e.g., graphene oxide (GO),^{[ 29 ]} cellulose,^{[ 30 ]} chitosan,^{[ 31 ]} melamine,^{[ 32 ]} and so on. Figure  3 showcases a summary of the most common supporting scaffolds in preparing porous MOF or COF structures in the form of aerogel or foam. The integration of MOFs or COFs with nanomaterials or robust supporting scaffolds addresses their inherent weaknesses, leading to more homogeneous distribution within macroscopic structures, minimized aggregation, and enhanced mechanical, chemical, and physical properties.^{[ 33 ]} Furthermore, hybridizing MOFs/COFs with supporting nanomaterials facilitates their processing into porous macroscopic constructs, functional composites, or membranes, thereby expanding their potential for advanced applications with tunable functionality.^{[ 19} , ³³ , ^{34 ]} Notably, the combination of MOFs with conductive nanomaterials such as MXene or graphene significantly improves the electrical conductivity of the hybrid systems, opening new opportunities in energy storage and electromagnetic shielding applications.^{[ 35 ]} Another promising strategy involves the hybridization of MOFs and COFs with each other to form core‐shell MOF/COF structures, which demonstrate enhanced catalytic performance.^{[ 36 ]} Although each MOF or COF species possesses unique physicochemical performance, their integration produces a new class of porous crystalline materials with superior properties compared to their individual counterparts, effectively addressing their limitations.^{[ 37 ]}

Figure 3.

The most commonly used supporting scaffolds to yield free‐standing MOF or COF porous structures include wood aerogel, carbon aerogel, chitosan‐based aerogel, aramid nanofiber‐based fibrous aerogels, melamine foams, polyacrylonitrile (PAN) fibrous aerogels, and cellulose aerogels.

Considering the four primary MOF or COF porous structures' manufacturing approaches from a technical standpoint, each method presents distinct advantages and challenges that merit careful consideration before making a choice. The direct mixing method stands out as a scalable and straightforward approach for producing MOF or COF porous structures, allowing for the creation of cryogels or aerogels by simply blending ingredients, followed by gelation and freeze‐drying. Importantly, pre‐synthesized and purified MOFs or COFs can be easily integrated into the gel precursors, saving considerable time and expense by eliminating extensive post‐production purification steps that might damage the macroscopic structure. However, achieving a homogeneous distribution of MOFs or COFs within the host substrate is quite a challenge, potentially leading to agglomeration and hindering the accessibility of reactants to the MOF or COF species in the scaffold. Moreover, incorporating high loadings of MOFs or COFs into the gel precursors prior to freeze‐drying could also deteriorate the mechanical properties of the resulting porous structure.

In contrast, the self‐shaping method enables the fabrication of all‐COF macroscopic porous constructs with robust mechanical performance. However, this method is primarily applicable to COFs and is less commonly employed for MOFs. The scalability of the self‐shaping approach, along with cost minimization, post‐manufacturing activation/purification, and resistance to degradation, are other important factors that require further attention. The in situ growth strategy allows for high loading of MOFs or COFs on supporting substrates while preserving structural integrity and mechanical performance. It enhances the accessibility of reactants by decorating MOFs or COFs on the outer surface of the scaffold. Yet, precise control over MOF or COF loading on building blocks remains a challenge, and multiple purification steps limit its suitability for large‐scale production, potentially damaging the supporting scaffold.

Template‐assisted synthesis facilitates the creation of artificial porosities in macroscopic porous structures or utilizes removable templates to achieve maximum loading of MOFs or COFs. However, the method necessitates multiple washing cycles to remove templates or their residues, posing significant challenges. The emerging self‐templating technique offers promise by eliminating the need for soft or hard templates, allowing for the creation of macroscopic porous constructs from MOFs or COFs. Nevertheless, it is more applicable to smaller structures, presenting difficulties in scaling up for larger MOF‐ or COF‐based porous constructs. In subsequent sections, recent advancements in developing MOF and COF porous structures will be comprehensively analyzed based on these manufacturing strategies.

2.1. Direct Mixing Method

The direct mixing approach for manufacturing porous structures involves mixing and gelation of pre‐synthesized and purified MOF or COF powder with an integrity booster. This process forms multi‐scale porous hierarchical building blocks after subsequent aging and freeze‐drying steps.^{[ 17 ]} These integrity boosters involve using biopolymers such as chitosan^{[ 31a ]} and cellulose‐based^{[ 38 ]} nanocompounds or the immobilization of MOF or COF on 2D GO sheets toward free‐standing ultra‐lightweight porous constructs with induced microporosity.^{[ 29a ]} These integrity boosters enhance the structural stability of the resulting porous structures. Moreover, employing directional freezing and emulsification prior to the drying steps allows for the engineering of porosities’ alignment and distribution throughout the construct, enabling advanced functions and designs.^{[ 30a ]} To date, many practices have been implemented to incorporate MOF or COF species into hierarchical porous building blocks using various supporting scaffolds. These practices are discussed in the following sections.

2.1.1. Cellulose‐Based Porous Structures

In a study by Zhu et al.,^{[ 38 ]} a longstanding processing challenge associated with embedding high MOF loadings in porous structures was addressed by entrapping MOFs into hierarchical cellulose nanocrystal (CNC) aerogels. They reported a straightforward approach to integrating structural CNCs with functional MOFs to generate aerogels without using chemical modifiers. In this case, the MOF aerogel with up to 50 wt% loadings was prepared by directly mixing MOFs, such as ZIF‐8, UiO‐66, or MIL‐100(Fe), with crosslinked cellulose nanocrystals (CNCs), forming a stable colloidal suspension in an aqueous medium. This was followed by the addition of carboxymethyl cellulose (CMC) as the crosslinker. The crosslinked CNCs were aldehyde modified (CNC‐CHO), while the CMC was hydrazide modified (CMC‐NHNH₂). The resulting mixture contained a crosslinked cluster with a high MOF loading trapped within the crosslinked network of CNC and CMC, which was found to be colloidally stable. The MOF interaction with the host CNC–CMC was based on the physical entanglement and van der Waals attraction forces. Freezing and lyophilization of this composition led to free‐standing aerogels with a high MOF loading (Figure S1A–C, Supporting Information). The outcome of the process also demonstrated the high structural integrity of the aerogel, even when immersed and compressed under liquid. Likewise, the results also showcased the preservation of MOF crystallinity and porosity during aerogel formation, providing MOF‐active sites across the hierarchical porous cellulose aerogel for water purification. These outcomes clearly demonstrated the potential of nanocellulose‐based compounds as promising templates or supporting substrates for making free‐standing aerogels with high MOF loadings. However, increasing the MOF loading to more than 50 wt% in this system significantly reduced the structural integrity of the resulting porous framework, causing it to fall apart due to looser connections.

In another attempt, hybrid MOF/MXene (Ti₃C₂T _x )/cellulose aerogels were developed in which cellulose and MXene acted as the supporting scaffold and conductivity booster, respectively.^{[ 39 ]} The aerogel production process started by preparing a uniform suspension of cellulose in sodium hydroxide (NaOH)/urea (NaOH:urea:deionized (DI) water weight ratio of 7:12:81), followed by thorough direct mixing with MXene and zeolitic imidazolate framework‐67 (ZIF‐67). Next, N,N’‐methylene bisacrylamide (MBA) was added to the suspension as the crosslinker, and the formed complex was molded and aged, viz., allowing the gel to form upon reaction between precursors, at room temperature (RT) to obtain a hydrogel. The hydrogel was then soaked in a DI water bath to remove the remaining NaOH, urea, and MBA. The resulting hydrogel was then frozen and lyophilized for 72 h to produce free‐standing porous and conductive MOF‐based hierarchical aerogels. A further pyrolysis process turned the resulting aerogels into potent hybrid carbon‐based aerogels with desired electrical conductivity and magnetization (Figure S1D, Supporting Information). Further morphological analyses via SEM showcased the formation of a hybrid porous structure with aligned porosities in which the ZIF‐67 MOFs were well‐positioned on the outer wall of the aerogel. Notably, the highly aligned and openly porous architecture of the aerogels was preserved even after the carbonization process. This approach enabled the creation of conductive porous pathways through hybrid compositions containing rhombic dodecahedral Co/C nanoparticles derived from thermally treated ZIF‐67 (Figure S1E–J, Supporting Information). This method created numerous magnetic/electric hetero‐interfaces, which are beneficial for trapping and dissipating EM waves. Moreover, the resulting aerogels were shown to be ultra‐lightweight and robust, capable of withstanding considerably higher loads relative to their low density (85.6 mg cm⁻³), with a total compressive strength of 282.6 kPa at a strain of about 80% (Figure S1K,L, Supporting Information).

Cellulose‐based scaffolds also demonstrated promising potential in preserving the integrity of COF‐based aerogels. In a study by Zhang et al.,^{[ 40 ]} COF‐LZU1 was synthesized solvothermally (Figure S2A, Supporting Information), and cellulose‐based scaffolds were used to generate free‐standing high COF‐loading aerogels. The process was inspired by former MOF‐based aerogels,^{[ 38 ]} and a crosslinked aldehyde‐modified CNC and hydrazine‐modified CMC through the sol–gel process was used to yield COF aerogels. Accordingly, the desired amount of COF powder, up to 50 wt%, was first suspended with 1 wt% CNC–CHO, followed by uniform mixing with a 1 wt% CMC–NHNH₂ aqueous suspension. Freezing (at −20 °C) and lyophilization of the obtained composition led to free‐standing COF‐cellulose aerogels with well‐ordered micropores, afforded by the introduction of COF to the system (Figure S2B, Supporting Information). Another attempt involved manufacturing smart moisture‐sensitive hierarchically structured CNF/COF aerogels through coupling proton‐conductive mono‐sulfonated and bi‐sulfonated COFs (COF‐SO₃H and COF‐2SO₃H) with carboxylated cellulose nanofibers (CNF‐C) as the supporting scaffold and suspending agent (Figure S3A, Supporting Information).^{[ 41 ]} Accordingly, COF‐SO₃H and COF‐2SO₃H were synthesized through Schiff‐base condensation between the aldehyde and amine functional groups of monomers and acid catalysts. Both COFs were then added to the 1 wt% CNF‐C mixture, resulting in a homogeneous dispersion due to hydrogen bond‐driven self‐assembly between the components. This hydrogen bonding facilitates the formation of a stable CNF/COF dispersion, while the sole COF was sedimented after 1 day (Figure S3B, Supporting Information). Freezing and lyophilization of the stable CNF/COF dispersion led to free‐standing ultra‐lightweight (density of 30 mg cm⁻³) COF‐cellulose aerogel. Employing the inherent characteristics of COFs and CNF‐C in synergy significantly improved the water uptake and ion conductivity of the resulting dispersion. In this case, through asymmetric moisturization, a self‐maintained moisture gradient was generated, creating a concentration variation for mobile H⁺ and Na⁺ ions. This led to effective charge separation and diffusion.

2.1.2. Chitosan‐Based Porous Structures

Apart from cellulose‐based scaffolds, chitosan has also shown promising potential as a robust supporting scaffold, enabling the creation of free‐standing COF‐based aerogels with customized morphological features and multi‐scale porosities. In this regard, Li et al.^{[ 31a ]} employed chitosan as the supporting scaffold to develop COF‐based porous aerogels through a facile direct mixing aerogel manufacturing approach. The TpPa‐1, TpPa‐NO₂, NUS‐2, and TpTe‐1 COFs were synthesized through the reaction of 1,3,5‐triformylphloroglucinol with 1,4‐phenylenediamine, 2‐nitro‐1,4‐ phenylenediamine, hydrazine, and terephthalohydrazide, respectively, in the presence of acetic acid as the catalyst via solvothermal reaction (120 °C for 72 h) (Figure  4A). The stabilization and homogeneous dispersion of the as‐synthesized COFs with chitosan, in the presence of 1,4‐butanediol diglycidyl ether as the crosslinker, led to hydrogel formation after aging at RT for several hours (Figure 4B). Accordingly, the chitosan's primary amine interacts with the epoxy moieties via an amine‐epoxy reaction, leading to a highly crosslinked network. Freezing and lyophilization of the obtained gel produced free‐standing, robust aerogels capable of being reinforced with up to 50 wt% COF powder relative to the chitosan scaffold. The obtained aerogels from various COF species showed ultra‐lightness (density 56–69 mg cm⁻³) with remarkable mechanical robustness (Figure 4C,D). Importantly, the entrapped COFs within the aerogel porous network retained their porosity, crystallinity, and accessibility, making them formidable catalytic microreactors for pollutant removal. Notably, by simple molding, the aerogel can be turned into desired shapes or useful forms for advanced functions and designs.

Figure 4.

A) Schematic illustration of TpPa‐1, TpPa‐NO₂, NUS‐2, and TpTe‐1 COF structures, along with their corresponding powder photograph and SEM images. B) Manufacturing process of chitosan/COF hybrid aerogels. C) Free‐standing chitosan/COF (50 wt%) aerogel based on TpPa‐1, TpPa‐NO₂, NUS‐2, and TpTe‐1 COFs. D) Digital photograph showcasing the mechanical robustness of aerogels capable of withstanding 250 g weight; the left image is chitosan/TpPa‐1, and the right images show chitosan/TpPa‐NO₂ and chitosan/TpPa‐1 aerogel. E) Chitosan/TpPa‐1 on a spider plant's grassy leaves. F) Different types of chitosan/COF aerogels are shaped according to the features of the used mold. Reprinted with permission.^{[ 31a ]} Copyright 2023, Royal Society of Chemistry.

Another attempt involves developing a double crosslinking strategy toward preparing mechanically robust and compressible porous chitosan‐COF aerogels through direct mixing.^{[ 31b ]} By harnessing the physical interaction, e.g., electrostatic interaction and hydrogen bonding, combined with chemical crosslinking, different types of sulfonated ionic COFs (TpPa‐SO₃H) were synthesized through a solvent‐free approach and turned into free‐standing porous structures. To this end, the as‐synthesized COFs were crosslinked with the chitosan scaffold, generating a 3D structured hierarchical porous network. Chitosan‐COF aerogel was prepared using a hydrogel template, followed by aging and freeze‐drying of COF‐reinforced chitosan. For this aim, the chitosan was first dispersed in 0.16 mol L⁻¹ acetic acid solution to obtain a homogeneous transparent aqueous mixture, followed by COF addition. Next, the 1,3,5‐triformylphloroglucinol (Tp) crosslinker was introduced to the system, and the resulting mixture was aged for 12 h at RT to facilitate hydrogel formation. Lyophilization of the formed hydrogels led to chitosan‐TpPa‐SO₃H aerogels (Figure  5A). Herein, acetic acid plays a crucial role, not only assisting in the uniform dispersion of chitosan but also acting as a potent catalyst toward reaction acceleration between the Tp's aldehyde groups and amine groups of chitosan. In this system, the crosslinked Tp is the key to manufacturing robust chitosan‐COF aerogels. Moreover, the reaction of the Tp crosslinker with abundant amino functional groups of chitosan led to imine bond generation via Schiff‐base reaction, leading to a highly crosslinked chitosan network formation. The residual Tp's aldehyde groups react with the edge amino functional groups of COFs, generating a covalent linkage between chitosan and COFs. Additionally, the ‐NH‐, C═O, and ‐SO₃H functional groups of TpPa‐SO₃H COF interact with ‐NH₂ (amine) and hydroxyl (‐OH) functional groups of chitosan through hydrogen bonding or electrostatic interaction, highly enhancing the mechanical robustness of aerogels.

Figure 5.

A) Schematic demonstration of the manufacturing process of chitosan/TpPa‐SO₃H aerogel through direct mixing. Reprinted with permission.^{[ 31b ]} Copyright 2023, Elsevier. B) Fabrication process of chitosan/COF‐IL aerogel along with the crystalline structure of the COF‐IL. Digital photographs from C) COF‐IL crystalline powder, and D) resulting chitosan/COF‐IL (80 wt%) aerogel. Reprinted with permission.^{[ 42 ]} Copyright 2019, Royal Society of Chemistry.

Another research employed the chitosan scaffold followed by further crosslinking with allylimidazole ionic liquid (IL) decorated COF (COF‐IL) toward hybrid chitosan‐COF aerogels.^{[ 42 ]} The chemical crosslinking occurred through a photoinduced thiol‐ene reaction between COF‐IL and thiol (‐SH) decorated chitosan. The eco‐friendly thiol‐functionalized chitosan was found to be promising for hydrogel generation owing to its in situ gelation and mucoadhesiveness. Direct mixing of the thiol‐functionalized chitosan and COF‐IL in an acidic medium at RT led to a robust hydrogel. Upon exposing the hydrogel to UV irradiation (300 W, 365 nm) in the presence of 2‐hydroxy‐2‐methylpropiophenone (HMPP) as a crosslinker and subsequently placing the developed covalently crosslinked aerogel in a cooler, a highly crosslinked network was formed. Freeze‐drying the crosslinked chitosan‐COF led to a 3D structured macroporous aerogel (Figure 5B). The resulting aerogel can be easily molded into various shapes while containing high COF loadings (up to 80 wt%) and maintaining its structural integrity (Figure 5C,D). These ultra‐lightweight aerogels (density range of 25–32 mg cm⁻³) exhibited remarkable mechanical robustness, with an ultimate strength of 13.8 MPa, capable of withstanding a 250 g weight without any signs of plastic deformation.

2.1.3. Graphene‐Based Porous Structures

Immobilizing MOFs on GO sheets through direct mixing, followed by subsequent gelation and freeze‐drying, was found to be an effective approach toward generating robust free‐standing multi‐scale porous aerogels. Accordingly, Huang et al.^{[ 29a ]} reported a facile direct mixing approach for manufacturing hybrid reduced graphene oxide‐MOF (rGO‐MOF) aerogels based on the GO gelation and reduction, in which the gelation/reduction process was initiated with the aid of MOF crystals. The MIL‐88A nanorods were selected as the MOF in which the free metal ions on their surfaces acted as potent linkers to bind with GO nanoflakes, forming a 3D‐structured hierarchical porous network. The aerogel production process started from the dispersion of MIL‐88A and GO within the water, followed by heating at 95 °C for 5 h in an oven to generate interconnected hydrogels. Freezing (24 h) and lyophilization of the as‐developed hydrogel yielded graphene‐MIL‐88A aerogel (Fe₃O₄@C/rGO) (Figure  6A). The crosslinking between the MIL‐88A nanorods and GO nanoflakes occurred due to the metal–oxygen covalent and/or electrostatic interactions between the free Fe³⁺ ions in the MOF crystals and the oxygen‐based functionalities, such as ‐COOH and ‐OH, throughout the GO structure. This process also prevents the sedimentation of MIL‐88A, allowing its homogeneous and stable dispersion. Importantly, the GO can be effectively reduced with the aid of MIL‐88A without any other chemicals, generating a conductive MOF‐based porous framework. Additionally, the crystallinity and morphology of rGO‐MIL‐88A aerogel can be engineered by altering the gelation time (Figure 6B). In this case, the shape of the nanorods changed from a hexagonal cone to a dome‐shaped one after prolonging the gelation process. The resulting Fe₃O₄@C/rGO aerogel showcased an ultralightweight (density of 6.2 mg cm⁻³) porous network with magnetic responses (Figure 6C,D). This originated from the interlinking of rGO nanoflakes in synergy with MIL‐88A, where the MIL‐88A was uniformly dispersed throughout the porous network. The resulting porous construct was used as a potent MOF‐based EM shielding system.

Figure 6.

A) Schematic illustration of the manufacturing process of rGO/MOF aerogel, achieved by immobilizing pre‐synthesized MOF on GO sheets, wherein the MOF triggers the gelation of GO. B) Digital images of constructs from gelation to aerogel formation after freeze‐drying. Digital images showcasing the aerogel's C) lightness and D) magnetic response. Reprinted with permission.^{[ 29a ]} Copyright 2022, Springer Nature under Creative Commons Attribution 4.0 International License.

2.1.4. Porosity Engineering in Direct Mixing Method

Employing unidirectional freeze casting^{[ 30} , ^{43 ]} or emulsification^{[ 44 ]} techniques allows for the engineering of the arrangement and alignment of porosities in MOF‐based aerogels toward advanced applications. Accordingly, a study developed MOF aerogels through unidirectional freeze casting, in which the pre‐synthesized GO, cellulose, and metal‐embedded ZIF‐8 were directly mixed, followed by unidirectional freezing and lyophilization to yield free‐standing flexible aerogels.^{[ 43 ]} The generated aerogels underwent subsequent pyrolysis and leaching, resulting in hierarchically porous aerogels with aligned porosities (Figure  7 ). In this procedure, the presence of cellulose along with GO nanosheets facilitates their crosslinking, while the metal‐embedded ZIF‐8 acts as a source of nitrogen and metal. The metallic atoms, such as Cu, Au, Ru, and Ni, could be trapped in the pores or embedded in the framework as mixed metal nodes. In this context, ZIF‐8 acts as a protector to prevent the agglomeration of metal atoms. During the carbonization process, the Zn nodes of ZIF‐8 evaporate, leaving a skeleton with numerous nitrogen‐rich defective sites. These sites enable coordination with metal cations, resulting in the formation of a hybrid structure. The process also prevents the agglomeration of single metal atoms and enhances the electrical conductivity of the resulting MOF‐based aerogels. Another study involved developing a unidirectional freeze‐casted CNF/ZIF‐67 aerogel by directly mixing pre‐synthesized CNF and ZIF‐67. This was followed by unidirectional freezing, achieved by placing the homogeneous CNF/ZIF‐67 dispersion on a copper cylinder immersed in liquid nitrogen.^{[ 30a ]} This led to the vertically aligned porosities in the direction of ice crystal formation. Lyophilization of the unidirectionally frozen assemblies led to aerogel formation, which turned into ultralightweight (density of 1.74 mg cm⁻³) conductive aerogel upon carbonization with an aligned porosity homogeneously decorated with ZIF‐67 (Figure S4, Supporting Information).

Figure 7.

A) Schematic demonstration of hybrid MOF‐based aerogel through unidirectional freeze casting; B–D) SEM images from the bottom and cross‐section of the aerogel showcasing the alignment of the porosities in the freezing direction. Reprinted with Permission.^{[ 43 ]} Copyright 2022, Wiley‐VCH.

Another attempt to engineer the porosities of MOF‐based aerogels through direct mixing involves employing an emulsification technique.^{[ 44 ]} This technique includes the design of a high‐internal‐phase emulsion (HIPE) stabilized by the assembly of MOF at the interface of water and oil. HIPEs are recognized as stabilized emulsion systems (either oil‐in‐water or water‐in‐oil) formed using surfactants or particles with densely packed droplets, enabling the fabrication of highly porous macroscopic constructs.^{[ 45 ]} Three types of MOF structures with different morphologies were developed and used to create porous ultralightweight aerogels (density of 10 mg cm⁻³) with varying porous structures (Figure S5, Supporting Information).^{[ 44 ]} These structures include Cu₃(BTC)₂ nanoparticles, Mn₃(BTC)₂ nanowires, and Ni(BDC) nanosheets. Accordingly, by direct mixing of the MOF species, water, and oil at RT, the HIPE is formed upon MOF assembly at the oil/water interface. Then, the inner oil and outer water phases were extracted through supercritical CO₂ drying followed by lyophilization, leading to free‐standing emulsion‐templated MOF aerogels. This technique allows for tuning the structure and porosities of aerogels by altering the emulsion templates during emulsification, holding great promise toward porosity‐specific applications. The direct mixing technique, as a scalable and versatile approach for manufacturing MOF or COF aerogels, has been utilized in numerous studies to produce free‐standing, multi‐scale porous structures for various applications. A list of recent practices is provided in Table 1 .

Table 1.

Recently developed MOF and COF porous structures through the direct mixing approach and their application.

MOF/COF type	Supporting scaffold	Drying method	Remarks	Refs.
ZIF‐67 MOF	CNF	FD	Carbonized CNF‐ZIF‐67 with aligned porosities via unidirectional freeze casting for EMI shielding.	[30a]
ZIF‐8 MOF	GO & cellulose	FD	Development of ZIF‐8 embedded GO‐cellulose aerogel toward dichlorination processes.	[43]
ZIF‐8 MOF UiO‐66 MOF MIL (100) Fe MOF	CNC‐CHO & CMC‐NHNH2	FD	Developing high MOF loading (up to 50 wt%) aerogels for ion removal application.	[38]
ZIF‐67 MOF	MXene and cellulose	FD	Carbonization of hybrid MXene‐cellulose‐ZIF‐67 aerogel toward EMI shielding.	[39]
MIL‐88A MOF Ni‐doped MIL‐88A MOF	GO	FD	Development of hybrid magnetic rGO aerogel via chemical‐induced gelation of GO using the MIL‐88A MOF itself.	[29a]
ZIF‐8 MOF	Agarose	FD	Carbonized agarose‐ZIF‐8 aerogels toward oil adsorption.	[46]
COF‐LZU1	Chitosan	FD	Preparation of chitosan‐COF‐LZU1 aerogel by using glutaraldehyde as a crosslinker toward radioactive iodine adsorption.	[47]
COF‐LZU1	CNC–CHO & CMC–NHNH2	FD	High‐loading COF‐LZU1 (50 wt%) CNC‐CMC aerogel toward radioactive iodine adsorption.	[40]
TpPa‐SO3H COF	Chitosan	FD	Directly mixed chitosan‐COF aerogel toward the adsorption of sulfamerazine.	[31b]
COF‐IL	Chitosan	FD	Fabrication of high COF loading (up to 80 wt%) chitosan‐COF‐IL hydrogel via photoinitiated thiol‐ene reaction and subsequent aerogel manufacturing through freezing and lyophilization; the aerogel was used for direct CO2 adsorption and CO2 cycloaddition.	[42]
TpPa‐1 COF TpPa‐NO2 COF NUS‐2 COF TpTe‐1 COF	Chitosan	FD	Fabrication of Pd‐loaded COF‐reinforced chitosan aerogels (up to 50 wt%) toward chlorobenzene dichlorination from continuous water streams at room temperature.	[31a]
TAPB‐DMTP COF	SA	FD	Development of SA‐COF aerogel as a solid‐phase extractor toward the determination of quinolone antibiotics.	[48]
COF‐2SO3H COF‐SO3H	CNF	FD	Hybrid CNF‐COF aerogels toward water harvesting and powder generation from human respiration.	[41]

Abbreviations: Cellulose nanofiber (CNF), electromagnetic interference (EMI), freeze‐drying (FD), cellulose nanocrystal (CNC), carboxymethyl cellulose (CMC), graphene oxide (GO), reduced graphene oxide (rGO), decorated COF with allylimidazolium ionic liquid (COF‐IL), and sodium alginate (SA).

2.2. Self‐Shaping Method

The self‐shaping approach is a promising method for manufacturing aerogels from pure MOF or COF species, involving their simultaneous in situ synthesis followed by self‐crosslinking. This process creates a self‐shaped template for aerogel manufacturing, followed by further aging and drying steps. The self‐shaping method is more commonly used for COF‐based aerogels, with very limited practices for MOF‐based constructs. This is due to the more robust and self‐supported porous structures formed through the in situ and interconnected crosslinking of COF species. In the case of self‐shaped MOF porous structures, Yang et al.^{[ 49 ]} developed a 3D structured MOF‐5‐derived carbon aerogel using the self‐shaping aerogel production method, followed by compression and annealing (Figure S6, Supporting Information). Employing MOFs in carbon aerogel generation allows for control over porosity across various scales, from micro‐ and meso‐porosities inherent to MOFs to macro‐porosities induced by a hierarchical structure. This, combined with the induced proper conductive domains, high BET surface area, and acceptable mechanical robustness, makes MOF‐derived carbon aerogels promising candidates for a wide range of applications. In this regard, synthesizing MOF‐5 and compressing it into a cylinder (height: 5 mm, diameter: 13 mm) at 3 and/or 10 tons of compressive force for 5 min generates a compressed MOF‐5 cylinder. Thermal annealing of these compressed cylinders at 900 °C under a mixed atmosphere of 15% H₂/85% N₂ at a flow rate of 0.05 L min⁻¹ led to the formation of free‐standing, flexible carbon aerogels. The resulting carbonized MOF‐5 aerogel without compression showcased a high surface area of 1916 m² g⁻¹. However, the BET surface area and crystallinity of the MOF‐5 porous structure deteriorated upon further compression, a decline associated with the structural collapse of the MOF due to pressure‐induced phase transitions. Under mechanical pressure, the cubic MOF‐5 framework undergoes significant distortions and compression, shrinking the pore size due to changes in the 1,4‐benzenedicarboxylate ligand's distance and deformation of Zn₄O bond angles. This method holds great promise for developing carbon aerogels, where controlling micro‐porosity or post‐synthesis processability is crucial.

On the other hand, COFs have shown promising potential for creating mechanically robust, free‐standing, multi‐scale porous structures through the self‐shaping approach. This method maximizes COF loading for advanced applications and functions. Consequently, several attempts have been made to develop all‐COF porous structures using the self‐shaping method. Jia and coworkers^{[ 50 ]} developed a versatile, scalable, and straightforward self‐shaping approach for generating all‐COF porous structures through a group‐protection COF synthesis strategy. As proof of concept and to demonstrate the universality of the approach, ten different types of highly crystalline COF organo‐hydrogels with high solvent/freezing resistance coupled with proper mechanical properties were developed. The as‐developed organo‐hydrogels can be easily transformed into organogels, hydrogels, and/or hierarchical porous aerogels through subsequent solvent exchange and freeze‐drying steps, bridging the gap between various types of COF porous constructs. The so‐called group‐protection method slows down the formation rate of COFs and tunes their morphology, resulting in COF gels formed through a hydrogen‐bonding crosslinked network of COF nanosheets and nanofibers.

Moreover, the formed hydrogen bonding networks between the COF's functional groups and water/organic binary solvents play a critical role in generating organo‐hydrogels and maintaining their solvent or freezing resistance. Figure  8 showcases the details of the proposed universal methodology for all‐COF porous structures. In this case, the Boc (tertbutoxycarbonyl) chemical group was employed to protect the amine monomers, which control the polymerization rate. This allows the formation of a hydrogen bonding network between the solvents and COF nanosheets/nanofibers, facilitating the generation of COF organo‐hydrogels. Additionally, chemical functional groups such as amine (‐NH₂) and hydroxyl (‐OH) are introduced into the COF monomers to promote hydrogen bonding. Some specific types of organic solvents, such as n‐butanol, dioxane, and o‐dichlorobenzene, with hydrogen‐bonding acceptor functionalities (N‐ and O‐based groups) were also incorporated into the COF porous structure's fabrication approach. These solvents form clusters upon hydrogen bonding with water, facilitating interactions with functionalities on the COF skeleton. This engineerability enabled the generation of a wide range of COF porous structures with excellent photothermal conversion, solar energy absorption, and water transmission capabilities.

Figure 8.

A) The Production procedure of COF gels, B) the employed COF gel monomers, and C) a schematic illustration of 10 different types of COF along with their chemical structure, including TpPa, TAPB‐Tp, TpAzo, DHTA‐Pa, NKCOF‐1, NKCOF‐60, TpBD, NKCOF‐58, NKCOF‐61, and NKCOF‐59. Reprinted with permission.^{[ 50 ]} Copyright 2023, American Chemical Society.

Zhu et al.^{[ 51 ]} developed a versatile self‐shaping all‐COF aerogel production approach through in situ gelation and then supercritical CO₂ (ScCO₂) drying (see Section 2 for further information on supercritical drying) using six different types of COF structures without using any binder or additive (Figure S7, Supporting Information). Accordingly, a stochiometric ratio of amino monomers, i.e., tris(4‐aminophenyl)benzene (TAPB), tris(4‐aminophenyl)amine (TAPA), and (1,1′‐biphenyl])‐4,4′‐diamine (BPDA), and aldehyde monomers, i.e., terephthaldehyde (PDA), 2,5‐dibromoterephthalaldehyde (BrPDA), 2,5‐dimethoxyterephthalaldehyde (OMePDA), and tris(4‐formylphenyl)amine (TFPA), were mixed in 1 mL DMSO, as the reaction solvent, followed by sonication till obtaining a homogeneous clear solution. Then, 6 m acetic acid was added to the mixture as a catalyst, leading to in situ gelation after about 1 min. The obtained wet gel was aged for 12 h at 80 °C till reaction completion, resulting in a hazy gel. Next, the gel was immersed in THF, acetone, and ethanol to remove the impurities. The purified gel was then dried through ScCO₂ drying. Two types of processes were introduced to generate highly crystalline all‐COF aerogel through the self‐shaping method (Figure  9 ). The type I method involved TAPB‐OMePDA and TAPB‐TFPA chemistries that pass through in situ COF formation and gelation, followed by drying. The type II process involved an additional reactivation and ScCO₂ drying process to turn amorphous TAPB‐PDA, TAPA‐TFPA, BPDA‐BTCA, and TAPB‐BrPDA COFs into crystalline structures. The reactivation was conducted using a mixture of mesitylene and dioxane, followed by solvent exchange and ScCO₂ drying steps. Notably, during the reactivation process, the COF aerogels retained their shape and macroscopic features. The outcome of this process led to free‐standing ultralightweight (density 20–40 mg cm⁻³) all‐COF aerogels with customized compositions toward environmental remediation and rapid pollutant removal applications.

Figure 9.

Manufacturing process of self‐shaped COF aerogel through either Type I gelation followed by supercritical CO₂ drying or Type II gelation, reactivation, and supercritical CO₂ drying. Reprinted with Permission.^{[ 51 ]} Copyright 2021, American Chemical Society.

Another attempt involved the fabrication of crystalline all‐COF self‐shaped porous foams via a facile one‐step thermo‐molding approach to manufacture hierarchically porous olefine‐linked COFs through melt polymerization.^{[ 52 ]} The obtained crystalline porous structures demonstrated excellent processability and moldability, featuring nanoscale (0.58 nm) eclipsed stacking porosities within the 3D building blocks of the resulting foams. The versatility of this approach enabled the synthesis of all‐COF foams based on different COF types, such as TMT‐TPT, TMT‐DMTP, TMT‐TPA, TMT‐BPA, and NKCOF‐12, using the so‐called thermomolding method. The abbreviations correspond to trimethyltriazine (TMT), 2,4,6‐tris(4‐formylphenyl)‐1,3,5‐triazine (TPT), 2,5‐dimethoxyterephthalaldehyde (DMTP), terephthalaldehyde (TPA), 4,4′‐biphenyldicarboxaldehyde (BPA), and Nankai Covalent Organic Framework 12 (NKCOF‐12). The synthesis process involved charging a Pyrex tube with TMT and benzoic anhydride, followed by the addition of TPT, DMTP, TPA, BPA, and/or 1,3,5‐triformylbenzene to the tube. The Pyrex tube was evacuated to a pressure of 50 mTorr and then well‐sealed, followed by heating for 12 h at 180 °C. The formed porous solid construct was collected and washed with DMF and methanol. The purified foam was then dried under vacuum at 120 °C to obtain the expected crystalline 3D structured all‐COF foam. The outcome of the process, based on the stoichiometric synthesis of each COF structure, resulted in custom moldable, robust, and free‐standing foams.

A series of attempts have been made to develop all‐COF aerogels through a three‐step protocol involving gelation, solvent exchange, and supercritical drying. In the initial effort, Martin‐Illian et al.^{[ 53 ]} reported the first aerogel production attempt based on the above‐mentioned three‐step procedure. The first step, i.e., gelation, involved the synthesis of COF gels by mixing aldehyde‐ and amine‐based monomers in glacial acetic acid (AcOH) with 10 vol% water within a dialysis membrane at RT. In this reaction, the aldehyde monomers involve 1,3,5‐benzenetricarbaldehyde (BTCA) or terephthalaldehyde (PDA), while the amine monomers include 1,3,5‐tris(4‐aminophenyl)benzene (TAPB) or 1,4‐diaminobenzene (PPDA). The resulting monomer mixture is left stationary in contact with AcOH in an oven at 25 °C for 5 days to ensure the proper crystallinity of the COF gels. With shorter incubation times, the crystallinity of the COFs deteriorates. AcOH plays a critical role in this process by facilitating optimal solvent exchange prior to supercritical drying. Additionally, the gelation of the monomer mixture does not require additional heating, but to ensure homogeneity, it requires the addition of 10 vol% water. The second step involves solvent exchange with THF and ethanol to remove unreacted monomers and prevent gel collapse due to solvent–structure interactions. In the third step, the self‐shaped COF gel undergoes ScCO₂ drying (venting at a rate of ∼8–10 bar per hour to atmospheric pressure) to obtain a 3D‐structured, free‐standing, lightweight (density of 20 mg cm⁻³) imine‐based aerogel (Figure  10A,B). Interestingly, the generated aerogels, which inherit the micro‐ or meso‐porosity of the COFs, exhibited elastic behavior up to moderate strains of 25%–30%.

Figure 10.

A series of COF aerogels produced through the self‐shaping method; A) Schematic represents the manufacturing process of hierarchical COF‐based aerogel generated via sol–gel process according to the following steps: i) mixing the building blocks in the AcOH, ii) formation of COF gel, iii) solvent exchange followed by supercritical CO₂ drying; the bottom right of the image showcases the COF aerogel prepared via TAPB‐BTCA‐AGCOF; scale bar is equal to 0.5 cm. B) A view of TAPB‐BTCA‐AGCOF aerogel standing on top of a superlight dandelion. Reprinted with Permission.^{[ 53 ]} Copyright 2021, Wiley‐VCH. C) Fabrication of COF‐based aerogel through a sol–gel process that includes the following steps: I) preparing a mixture of molecular components within the AcOH, II) gel formation, III) solvent exchange and supercritical drying, IV) breaking process, and V) compressing the COF aerogel parts. The final represented construct after compression is TAPB‐BTCA‐MCOF‐membrane; the scale bar is equal to 0.3 cm. Reprinted with Permission.^{[ 54 ]} Copyright 2022, Wiley‐VCH under Creative Commons CC BY license. D) Generation of COF‐electrode out of COF aerogel, including the following steps: i) making a mixture of COF monomer within the AcOH to make COF gel, ii) solvent exchange step, iii) supercritical CO₂ drying (scCO₂), iv) breaking down the COF aerogel into smaller pieces, v) adding carbon super P, vi) making a uniform solid mixture of COF aerogel and carbon super P, and vii) carbon@COF aerogel composite's compression. Reprinted with Permission.^{[ 55 ]} Copyright 2022, Wiley‐VCH.

In a following attempt, the same group utilized this three‐step procedure to generate COF membranes made of crushed and compressed self‐shaped aerogel pieces.^{[ 54 ]} Fabricating the COF membrane started with the manufacturing of the imine‐based COF aerogel through the previous method,^{[ 53 ]} followed by breaking the aerogel into smaller pieces (about 1–3 mm size) in the presence of AcOH; accordingly, 5 µL AcOH was utilized for each 10 mg of COF. A fixed amount of the mixture (37 mg cm⁻²) was then compressed for 5 min at 120 MPa to generate free‐standing COF membranes (Figure 10C), which were used for gas separation. In further work, the same group developed COF porous structures using the previously introduced three‐step method, resulting in flexible COF electrodes that retained the inherent micro‐ or meso‐porosity of the COFs used.^{[ 55 ]} Accordingly, the broken aerogel pieces were well mixed with Carbon‐C60 (30 wt%) using mortar to enhance the electrical conductivity of the mixture and gain a homogeneous solid composition. The solid mixture was then compressed for 5 min at 120 MPa, generating flexible and free‐standing hybrid carbon/COF aerogel film toward double‐layer electrochemical capacitors (Figure 10D).

In another work by Li et al.,^{[ 56 ]} a fast gelation strategy was developed through a self‐shaping approach, followed by solvent exchange and freeze‐drying to produce free‐standing all‐COF aerogels. Using a versatile and moderate synthetic method, they created COF aerogels catalyzed by scandium (III) trifluoromethanesulfonate (Sc(OTf)₃). Due to the significant catalytic activity of Sc(OTf)₃ on aldimine condensation, the distribution of densified COF crystal nuclei can be induced under mild conditions, leading to the rapid formation of a crystalline COF gel in just 5 min. This process is superior to acetic acid‐catalyzed reactions toward COF formation, as they involve slow accumulation of large 2D lamellae. Three types of COF aerogels were developed, namely COFA‐1, COFA‐2, and COFA‐3. For this matter, the appropriate amounts of aldehyde and amine monomers were mixed and added to DMF, followed by the addition of the Sc(OTf)₃ catalyst into the reaction vessel, allowing the reaction to proceed at RT. The aldehyde and amine monomers for each COF species were as follows: 1,4‐benzenedicarboxaldehyde and 1,3,5‐tris(4‐aminophenyl) benzene for COFA‐1, 1,3,5‐triformylbenzene and 1,3,5‐tris(4‐aminophenyl) benzene for COFA‐2, and triformylbenzene and 1,4‐diaminobenzene for COFA‐3. The gel formed under mild conditions after about 5 min, and it was subsequently transformed into free‐standing all‐COF aerogels through solvent exchange and freeze‐drying (Figure  11 ). This led to the formation of highly crystalline COF aerogels with high surface areas using an interconnected hierarchical porous architecture with a wide range of COF chemistries for pollutant removal and environmental remediation. Notably, under the catalytic activity of Sc(OTf)_3, the gelation is not limited to protic solvents, and it could occur under different types of solvents or their combinations.

Figure 11.

A) Manufacturing process of COF gel and powder, B) imine formation mechanism catalyzed via Sc(OTf)₃, and C) structural view of investigated COFs, including COFA‐1, COFA‐2, and COFA‐3. Reprinted with Permission.^{[ 56 ]} Copyright 2022, American Chemical Society.

Another study focused on developing COF aerogels based on the effective catalytic reaction of Sc(OTf)₃ at RT, which enabled the rapid formation of crystalline COF gels. These gels were subsequently transformed into aerogels through freeze‐drying.^{[ 57 ]} The COF porous structures were developed with robust semi‐rigid or rigid building blocks based on TFPT‐HZ‐COF or TPT‐HZ‐COF, with densities ranging from 29 to 40 mg cm⁻³ (Figure S8, Supporting Information). The COF's building blocks were synthesized using D3h symmetric aldehydes, specifically 2,4,6‐tris(4‐formylphenoxy)‐1,3,5‐triazine (TPT) and 2,4,6‐tris(4‐formylphenyl)‐1,3,5‐triazine (TFPT), as the nuclei. Symmetrical D2h hydrazine hydrate was used as the main building block to generate a hexagonal frame structure. To produce TFPT‐HZ‐COF aerogel, TFPT was added to a vial containing ethanol and o‐dichlorobenzene, and then ultrasonicated to obtain a homogeneous mixture. Next, the hydrazine hydrate was added to the resulting mixture and further sonicated, followed by the dropwise addition of Sc(OTf)₃ catalyst (mixed in ethanol) to the substrate. The reaction vial was sealed to allow gelation to occur over 12 h at RT. The gel was then subjected to Soxhlet extraction using THF for 24 h to remove impurities and activate the porous construct. Next, the solvent exchange step was carried out via tert‐butanol at RT, and the aerogel was generated through freeze‐drying of the obtained gel. The TPT‐HZ‐COF aerogel was prepared using a similar approach, replacing TPT with TFPT while maintaining the stoichiometric balance.

Using supporting scaffolds has also demonstrated promising potential for developing hybrid self‐shaped COF aerogels. Accordingly, a work investigated the in situ growth of TpDq‐COF on the surface of GO to form a self‐shaped gel template for aerogel production.^{[ 58 ]} The hybrid rGO‐COF gel was obtained via the in situ reaction of diaminoanthraquinone (Dq) and the 1,3,5‐triformylphloroglucinol (Tp), as the organic linker, in the presence of GO through a hydrothermal reaction. This process led to the simultaneous reduction of GO and uniform formation of the COF throughout the rGO nanoflake (Figure  12A). The formed hydrogel served as a template for aerogel manufacturing, turning into an ultralightweight, conductive, and mechanically robust hybrid rGO‐COF aerogel after freeze‐drying. Hence, in addition to the inherent micro‐ or meso‐porosities of the COF species, the system could benefit from supporting scaffolds to produce flexible hybrid COF aerogels. A similar approach involved decorating sulfonate anionic COFSO₃Na on the surface of GO through a hydrothermal process. This served as a self‐shaped template for aerogel production, simultaneously reducing the GO and uniformly synthesizing the COF across the GO surface.^{[ 59 ]} The hybrid rGO‐COF gel was prepared by mixing the monomer (sodium 2,5‐ diaminobenzenesulfonate (DB‐SO₃Na)), crosslinker (1,3,5‐triformylphloroglucinol (Tp)), and catalyst (p‐toluenesulfonic acid (PTSA)) in the presence of GO in water as the solvent or reaction substrate. The homogeneous mixture was then treated in a hydrothermal autoclave at 120 °C for 24 h, resulting in the reduction of GO and its simultaneous uniform decoration with 2 nm thick sulfonate ion‐containing COFs (Figure 12B). The formed hydrogel transformed into a lightweight (density of 7.1 mg cm⁻³) hierarchical porous construct after washing and freeze‐drying. The system offers a broad range of tunable porosities, from micro‐ and meso‐pores, achieved through COF modification, to macro‐pores, enabled by tailoring the graphene framework. The self‐shaping method offers a versatile approach to producing macroscopic 3D‐structured COF/aerogels for diverse applications. Table  2 summarizes recent practices in developing 3D‐structured porous structures using the self‐shaping approach.

Figure 12.

A) Schematic illustration of rGO/COF preparation protocol. Reprinted with permission.^{[ 58 ]} Copyright 2020, Nature Publishing Group under Creative Commons license. B) Production procedure of graphene/COF (COF‐SO₃Na) aerogel through a self‐shaping approach. Reprinted with permission.^{[ 59 ]} Copyright 2022, Wiley‐VCH.

Table 2.

Recently developed COF porous constructs using the self‐shaping method and their potential applications.

COF type	Formation catalyst	Supporting scaffold	Drying method	Remarks	Refs.
COF‐C3H7NH2 COF‐C4H9NH	Acetic acid	Self‐supported	VD	Flexible self‐supported COF aerogels toward the adsorption of bisphenol‐A.	[60]
TAPA‐TFPA COF TAPB‐PDA COF TAPB‐ DMTP COF TAPB‐BrPDA COF TAPB‐TFPA COF BPDA‐BTCA COF	Acetic acid	Self‐supported	ScCO2	Generating self‐supported COF aerogels based on six different types of COFs toward oil absorption, organic/inorganic pollutant removal, and iodine capture.	[51]
TMT‐TPT‐COF TMT‐DMTP‐COF TMT‐TPA‐COF TMT‐BPA‐COF NKCOF‐12	‐	Self‐supported	VD	Developing a facile thermo‐molding manufacturing of olefin‐linked COF foams using melt polymerization. The COF foams were used for oil/water separation and C2H2 purification.	[52]
TpDq‐COF	‐	GO	FD	Development of flexible rGO‐COF aerogel toward oil/water separation and supercapacitor applications.	[58]
UiO‐67 on MCA COF	Acetic acid	Chitosan	FD	Synthesis of MOF on COF, using chitosan as the supporting scaffold, toward hybrid chitosan‐based aerogels for the removal of pollutants from wastewater.	[61]
BT‐Dg COF	‐	CNF	FD	CNF‐BT‐Dg COF aerogel toward the removal of dyes and ions from wastewater.	[62]
TFPT‐HZ COF TPT‐HZ COF	Sc(SO₃CF₃)₃	Self‐supported	FD	Developing flexible and hydrophobic self‐supported COF aerogels toward thermal insulation application.	[57]
TAPB‐BTCA COF PPDA‐BTCA COF TAPB‐PDA COF	Acetic acid	Self‐supported	ScCO2	Cutting the aerogel into smaller pieces and making free‐standing membranes upon compression of aerogel pieces toward CO2 adsorption. The aerogels showed far better performance than the generated membranes by compressing the pieces of the aerogel.	[54]
COF‐SO3Na	PTSA	GO	FD	Free‐standing ultralightweight GO‐COF aerogel toward the removal of organic pollutants.	[59]
TAPB‐BTCA COF PPDA‐BTCA COF TAPB‐PDA COF	Acetic acid	Self‐supported	ScCO2	Development of imine‐based COF aerogels toward oil/water separation.	[53]
TAPB‐BTCA COF TZ‐BTCA COF	Acetic acid	Self‐supported	ScCO2	Compression of generated carbon‐COF flexible electrode toward supercapacitor applications.	[55]
Co‐coupled Bpy–COF	Acetic acid	GO	FD	Hybrid graphene‐Co‐COF aerogel toward water splitting.	[63]
TpPa COF DHTA‐Pa COF TpBD COF TpAzo COF TAPB‐Tp COF NKCOF‐58 COF NKCOF‐59 COF NKCOF‐1 COF NKCOF‐60 COF NKCOF‐61 COF	TFA	Self‐supported	FD	Developing a set of versatile self‐shaped COF aerogels using a group‐protection synthesis approach. The generated aerogels showcased promising potential for photothermal conversion, solar energy harvesting, and water transmission.	[50]
NNS‐VCOF	TfOH	Self‐supported	FD	Developing thiazole‐derived COF with vinyl linkage toward free‐standing ultralight COF aerogels for thermal insulation, flame retardancy, and hydrogen evolution reaction.	[64]
COFA‐1 to 15	Sc(SO₃CF₃)₃	Self‐supported	FD	Development of a universal method based on the Sc(SO₃CF₃)₃ catalyst toward manufacturing 15 different types of COF aerogels for adsorption of iodine and organic pollutants.	[56]
THB‐TAPB COF	Acetic acid	Self‐supported	FD	Removing lipids from human plasma samples using THB‐TAPB COF aerogel.	[65]

Abbreviations: Vacuum dried (VD), supercritical CO₂ drying (ScCO₂), graphene oxide (GO), freeze‐drying (FD), cellulose nanofiber (CNF), scandium(III) trifluoromethanesulfonate (Sc(SO₃CF₃)₃), p‐toluenesulfonic acid (PTSA), trifluoroacetic acid (TFA), and trifluoromethanesulfonic acid (TfOH).

2.3. In Situ Growth Approach

The in situ growth aerogel manufacturing procedure involves embedding MOF or COF growth sites into a hierarchical porous host, such as aerogels or foams. The synthesis is then initiated to achieve a homogeneously MOF‐ or COF‐decorated porous construct. Such aerogel surface decoration with MOF or COF species possesses micro‐ and meso‐porosities, making these species more accessible for interactions with target chemicals or molecules. Moreover, this approach allows for the engineering of aerogel porosity across multiple scales, starting from MOF‐ or COF‐induced micro‐ and meso‐porosities up to the macro‐porosity formed by the hierarchical porous host scaffold. This is highly challenging to achieve with traditional aerogel manufacturing processes. The advantage of in situ methods is that they enable the creation of hierarchical porous constructs with full‐porosity scales, offering remarkable opportunities for the development of functional porous structures.^{[ 24} , ^{66 ]} Various types of supporting scaffolds have been used for MOF or COF growth, which will be discussed in the following sections.

2.3.1. Graphene‐Based Supporting Scaffolds

Jiang and coworkers^{[ 24a ]} employed the graphene aerogel as a hosting scaffold for in situ and vertically aligned growth of two‐dimensional COFs. The vertically aligned TAPB‐BTCA COF (1,3,5‐tris(4‐aminophenyl)benzene (TAPB) and 1,3,5‐benzenetricarbaldehyde (BTCA)) was chosen for graphene aerogel decoration, providing numerous sites with micro‐ and meso‐porosities. This aerogel was established via an imine‐linked scaffold and selected because of the excellent mechanical robustness of its pores, combined with a 2D topology.

The freeze‐dried graphene aerogel scaffold, with uniformly distributed 5 µm pore sizes, was impregnated with a COF monomer solution consisting of TAPB, BTCA, benzaldehyde, aniline monofunctional compounds, and the highly active Sc(OTf)₃ catalyst (Figure  13A). In this reaction, the monofunctional compounds effectively maintained the reaction rate and prevented COF self‐coagulation. This enabled controllable and uniform COF growth with excellent crystallinity and a well‐defined 2D morphology throughout the graphene aerogel scaffold. After 48 h of reaction, the resulting in situ COF‐decorated multi‐scale porous construct was washed, dried, and subjected to high‐temperature annealing (800 °C for 2 h) under an argon atmosphere. This process enhanced the conductivity of the aerogel and induced porosities across multiple scales. These porosities include macro‐porosities (1–50 µm) derived from the host graphene aerogel scaffold, meso‐porosities induced by COFs (20 nm thickness and an average size of 200 nm), and molecular‐scale micro‐porosities from the 1–2 nm nanopores within the highly crystalline topological skeleton of COF (Figure 13B–F). The great advantage of this approach stems from two main factors: i) obtaining structural integrity and mechanical stability from the host scaffold and ii) engineering micro‐ or meso‐porosities within a hierarchically porous macroscopic framework by tuning the COF's structural design.

Figure 13.

Engineering the features of in situ prepared COF aerogels across scales. A) Manufacturing the vertically aligned COF‐graphene aerogel (v‐COF‐GA) through the in situ vertical COF growth across the macroporous graphene building block through a reversible polycondensation termination approach. B) Engineering the features of in situ prepared COF aerogels from macro‐ to nano‐scale. C) The 3D X‐ray tomography of v‐COF‐GA demonstrates the aerogel's structural features at the millimeter scale. The top and side scans of the aerogels can be seen at the bottom of the image. D) High‐resolution nano‐CT showcasing the homogeneous distribution of COF across the microporous GA as the supporting scaffold. The orange and gray parts stand for high‐density iodine‐labeled COFs and low‐density graphene scaffold, respectively. E) SEM images from the surface and cross‐section (top and bottom images, respectively) of the hybrid aerogel, showcasing the accommodation of vertically aligned COFs throughout the graphene scaffold. F) PXRD from the v‐COF, GA, and hybrid v‐COF‐GA. Reprinted with permission.^{[ 24a ]} Copyright 2022, Wiley‐VCH.

The in situ growth of COF species on a GO scaffold, followed by subsequent aerogel formation, has been found to be an effective approach for generating potent and mechanically robust porous structures with engineerable porosities across scales. In a study by Li et al.,^{[ 29b ]} hybrid graphene‐COF hydrogels with simultaneous hydrophilic and hydrophobic regions were developed through a facile in situ COF growth strategy. These hydrogels can be transformed into porous aerogels upon subsequent freeze‐drying (Figure  14A).

Figure 14.

A) Schematic illustration of the graphene‐COF aerogels with dual‐region hydrogels capable of accelerating solar‐driven water evaporation. B) Variation of the water contact angle of the lyophilized samples along with the schematic demonstrations showing dual regions in the formed aerogels, where blue and orange areas correspond to the pure reduced graphene oxide (rGO) and COF‐modified rGO, respectively. Reprinted with permission.^{[ 29b ]} Copyright 2022, American Chemical Society. In situ formation of rGO‐COF aerogels through the hydrothermal method, followed by lyophilization to form free‐standing flexible aerogel; the images show the C) lightness of the formed rGO‐COF aerogels, D) rGO and hybrid rGO‐COF hydrogels, and E) their corresponding rGO‐COF aerogels after lyophilization that can support higher weights with respect to the density of aerogel. Reprinted with permission.^{[ 67 ]} Copyright 2022, American Chemical Society.

In this case, the graphene provides hydrophobic regions, while hydrophilic regions were generated by in situ COF species coverage. The precise engineering of these numerous hydrophilic/hydrophobic sites effectively altered the light‐harvesting, wettability (Figure 14B), and water evaporation capabilities of the generated porous constructs. This is achieved through the controlled in situ decoration of rGO with COF‐SO₃H, viz., a sulfonic acid functionalized COF, via the hydrothermal method. To synthesize rGO‐COF aerogel, 2,5‐diaminobenzenesulfonic acid (DASA) and p‐toluenesulfonic acid (PTSA) as monomer and catalyst, respectively, were dispersed in water, followed by the addition of GO to the mixture in two sequential steps. The resulting complex was then hydrothermally treated in an autoclave for 2 days at 120 °C. The obtained hydrogel was washed with water, hot acetone, and water again, and it served as a template for aerogel production. This process showcased controllable surface wettability and heterogeneous hydrophilic COF sites combined with hydrophobic graphene sites, which are suitable for a multitude of applications.

Another attempt employed GO as a scaffold for the in situ growth of COF using the hydrothermal approach, followed by freeze‐drying.^{[ 67 ]} In this method, p‐toluenesulfonic acid monohydrate (PTSA), 2,6‐diaminoanthraquinone (Dq), and ascorbic acid were mixed in deionized water. The yellowish mixture was then added to the GO suspension and sonicated, followed by the addition of the 2,4,6‐triformylphloroglucinol (Tp) crosslinker. The mixture was treated in a hydrothermal autoclave at 120 °C for 24 h, and the formed hydrogel was transformed into a free‐standing, lightweight, and robust aerogel through freeze‐drying (Figure 14C–E). Employing the GO scaffold has also been found to be a promising approach for the in situ growth of MOF species toward mechanically robust aerogels with a hierarchical porous framework, in which the MOF precursors are added to the suspension of GO, followed by the drying of the resulting hydrogel through lyophilization.^{[ 68 ]}

2.3.2. Melamine‐Based Supporting Scaffolds

Melamine foams are another promising supporting scaffold for in situ growth of MOFs or COFs toward a multitude of applications. The in situ growth of MOF or COF species on melamine foam leverages the flexible and mechanically robust nature of the macroscopic supporting scaffold. This process induces micro‐ or meso‐porosities throughout the outer surface of the melamine foam via the in situ growth of MOF or COF species.

Melamine foam also possesses excellent processability, combined with lightness and a hierarchical macro‐porous framework, making it readily engineerable for decoration with a wide range of micro‐ or meso‐porous materials.^{[ 32} , ^{69 ]} Several research studies have practiced the use of melamine as a supporting scaffold for COF growth. In an attempt, a facile, scalable approach was introduced to generate a COF‐decorated superhydrophobic (contact angle 154.3°) melamine sponge with a high specific surface area of 153 m² g⁻¹ combined with excellent chemical and mechanical stability of the sponge scaffold (Figure S9A, Supporting Information).^{[ 32b ]} The COF decoration on the melamine sponge was performed through a one‐pot synthetic approach at RT. The prepared melamine sponge was immersed in the acetonitrile solution composed of 1,3,5‐tris(4‐aminophenyl)benzene (TAPB) and 2,5‐divinylterephthalaldehyde (DVA). This was followed by the addition of 12 m ethyl acetate to the reaction mixture, which was then kept for 12 h. The sponge was then squeezed several times to remove excess COF solution. The resulting in situ COF decorated sponge was thoroughly washed with acetone and dried under vacuum at RT.

Another research employed the porous melamine sponge scaffold toward in situ one‐pot decoration of TpTt COF throughout the sponge's structure via the “reactive seeding” approach with robust photocatalytic activity (Figure S9B, Supporting Information).^{[ 70 ]} In this regard, a mixture of 1,3,5‐triformylphloroglucinol (Tp) in combination with melamine and 1,3,5‐triazine‐2,4,6‐triamine (Tt) was added to DMSO, followed by immersing the melamine sponge into the mixture and further sonication. The obtained mixture was then transferred into an autoclave and treated at 150 °C for 24 h. Next, the COF‐decorated melamine sponge was washed with DMF and ethanol in an ultrasonic bath. The resulting sponge was dried for 12 h at 60 °C, resulting in a robust COF‐coated sponge with a unique multi‐scale porous morphology (Figure S9C–F, Supporting Information).

Another practice investigated the uniform in situ decoration of melamine foam with imine‐linked TPDABpy COF via a one‐pot solvothermal synthesis approach.^{[ 69 ]} The abundant amine functional groups on the surface of melamine foam provide active sites for the uniform growth of the COF throughout the macroporosities of the sponge. The process includes adding N,N,N’,N’‐tetrakis(4‐aminophenyl)‐1,4‐phenylenediamine (TPDA) and 2,2′ – bipyridine‐5,5′ ‐dicarboxaldehyde (Bpy) into 1,4‐dioxane, followed by ultrasonication to obtain a homogeneous mixture. The melamine foams were then added to the homogenized mixture, and after ultrasonication and vortex mixing, a proper amount of 6 M acetic acid (catalyst) was added. The resulting composition was transferred into an autoclave and treated for 72 h at 120 °C. The COF‐decorated melamine foam was obtained after proper purification and drying steps (Figure  15A).

Figure 15.

A) In situ fabrication process of COF‐decorated (COF‐TPDABpy) melamine foam. Reprinted with permission.^{[ 69 ]} Copyright 2023, Elsevier. B) Schematic demonstration of in situ growth of DAB‐COF on carbonized melamine‐graphene foam scaffold. Reprinted with permission.^{[ 71 ]} Copyright 2022, Elsevier.

The in situ growth of COF on a flexible graphene‐decorated carbonized melamine foam (GCF) scaffold was also performed to embed the micro‐ or meso‐porosities of COF into a conductive carbonaceous domain, aiming to create potent supercapacitors.^{[ 71 ]} The COF crystals were in situ grown on the surface of GCF via Schiff‐base reaction, generating an intimate mixture of both carbonaceous and COF phases. In this case, the carbonaceous framework ensures proper mechanical robustness with fatigue resistance and compressibility, combined with the rapid electronic transfer because of the carbonized domain. To decorate GCF with COF, a Pyrex tube was first charged with 1,3,5‐tris(4‐aminophenyl)benzene (TAB) and 2,5‐dihydroxy‐1,4‐benzenedicarboxaldehyde (DHA), followed by the addition of n‐butyl alcohol and 1,2‐dichlorobenzene. The mixture was homogenized, and then the GCF was immersed in it and left stationary for 24 h to ensure optimal soaking with the COF monomers. Acetic acid was added as the catalyst, and the system was sonicated. The tube was then flash‐frozen in a liquid nitrogen bath at 77 K. After degassing through three freeze‐thaw cycles, the sealed tube was heated for 3 days at 200 °C. Upon completion of the reaction, impurities were removed by washing with multiple solvents, and the material was finally dried to obtain COF‐decorated GCF (Figure 15B).

The potential of melamine foam for stable and homogeneous immobilization of MOF was also investigated. Accordingly, mesoporous PCN‐22 MOF was decorated throughout the pores of micro‐porous melamine foam using a one‐pot synthesis process.^{[ 32a ]} The immobilized MOFs preserved their porosity and crystallinity, while their decoration on the foam surface made them more accessible for target reactions, i.e., cholesteryl esters epoxidation. The PCN‐224(Fe) MOF was selected for decoration on melamine foam through the assembly of zirconium chloride and tetrakis(4‐carboxyphenyl)‐porphyrin (TCPP(Fe)) ligand within the acid‐penetrated foam. The foam was immersed in a DMF solution containing the reaction precursors, followed by the addition of benzoic acid and subsequent treatment of the sealed container at 120 °C for 72 h. The color change of the foam from white to dark brown indicated the formation of PCN‐224(Fe) crystals throughout its structure. The resulting foam showcased thermal stability up to 300 °C, with excellent compressibility and handleability for practical applications (Figure S10, Supporting Information).

2.3.3. Cellulose‐Based Supporting Scaffolds

3D structured porous cellulose constructs, as biodegradable and green scaffolds, are promising supporting building blocks for generating macroscopic porous structures. The engineerable chemical functionalities of cellulose derivatives, such as CNF, combined with their excellent mechanical stability and flexibility in aerogel form, make them promising candidates for in situ growth of MOF through coordination chemistry.^{[ 24} , ⁶⁶ , ^{72 ]} For this purpose, Zhou et al.^{[ 66 ]} decorated the aluminum‐based Al‐MIL‐53 MOF on the CNF aerogels through in situ interfacial synthesis. This process involves the crosslinking and continuous nucleation of MOF monolayers on the individual CNFs (Figure  16A). The manufacturing of such CNF‐MOF aerogel was conducted in three sequential steps, starting from i) interfacial synthesis of hybrid CNF‐MOF nanofibers, ii) freeze‐drying of the formed assemblies, and iii) their further crosslinking to produce superelastic free‐standing aerogel. For the synthesis of hybrid CNF‐MOF nanofibers, Cladophora cellulose was extracted and carboxylated through 2,2,6,6‐Tetramethylpiperidine‐1‐oxyl (TEMPO) assisted oxidation. The as‐prepared carboxylated CNF was subjected to ion exchange with Al(NO₃)₃∙9H₂O to generate a complex between CNF‐COO⁻ and Al³⁺. This acts as a nucleation site for the growth of MOF. Subjecting the ion‐complexed CNF‐COO⁻–Al³⁺ to disodium terephthalate (Na₂BDC) as the ligand, along with polyvinylpyrrolidone (PVP) as the crystallization agent and surfactant, led to the formation of hybrid CNF‐MOF nanofibers (Figure 16B–D). Freeze‐drying of the formed CNF‐MOF suspension led to a free‐standing aerogel with a hierarchical porous framework with combined micro‐porosity from MOF species and macro‐porosity from CNF scaffold (Figure 16E–G). The formed aerogel was again immersed in ligand/Al(NO₃)₃∙9H₂O to facilitate the further growth of Al‐MIL‐53 MOF. Further freeze‐drying of this assembly led to the final ultralightweight (3 mg cm⁻³) MOF‐loaded CNF aerogel, which can withstand 80% compression and recover its shape immediately upon force release. The aerogel also showcased specific stress and specific compression modulus values of approximately 100 and 200 MPa cm³ g⁻¹, respectively, indicating robust mechanical properties even with the incorporation of MOF into the porous cellulosic structure.

Figure 16.

A) Schematic demonstration of CNF@Al‐MIL‐53 aerogel preparation via interfacial synthesis, followed by freeze‐drying and subsequent crosslinking. Accordingly, the algae‐extracted CNFs were decorated with Al‐MIL‐53 MOF through in situ crosslinking. The outcome of this process led to the formation of ultra‐lightweight CNF‐MOF aerogels with low density. B) TEM images of the CNF‐MOF nanofibers in which CNF was decorated with Al‐MIL‐53. SEM images of C) neat CNF and D) Al‐MIL‐53 coated nanofibers. E‐G) SEM images of the CNF@Al‐MIL‐53 aerogel at different zooms. The showcased circle areas represent the joints between MOFs and crosslinked nanofibers. Reprinted with permission.^{[ 66 ]} Copyright 2019, Springer Nature under a Creative Commons Attribution 4.0 International License.

Another attempt leveraged robust hierarchical macroscopic cellulose scaffolds for MOF immobilization, effectively addressing the processing challenges of MOF porous structures manufacturing. This included controlling MOF loading, spatial distribution, composition, and confinement using various bio‐based scaffolds.^{[ 24b ]}

This, combined with the accessibility of MOF species to target reactions or chemicals and the robustness and formability of the cellulose support, makes them ideal for practical applications. The process involved immobilizing the MOF on microspheres with crosslinked shells, followed by centrifugation and freeze‐drying to yield high internal phase emulsion (HIPE) foam with a spiderweb‐like porous framework. Using CNF with bovine serum albumin (BSA) as a crosslinker enables the generation of engineerable foams with multi‐scale porosities, i.e., micro‐porosity and macro‐porosity, due to the MOFs and hierarchical arrangement, respectively. The unique arrangement of MOF allows for their uniform distribution across the network at high loadings (up to 86 wt%), improving the accessibility and performance of MOF within the porous framework.

One advantage of the proposed approach is the separate fabrication of microspheres and the HIPE template, preventing the adverse effects of MOF loading on HIPE formation, even at higher loadings, such as poor stability in HIPE and non‐uniform MOF distribution or aggregation. The porous structure's manufacturing was carried out through several stages: i) generation of shell‐crosslinked microspheres via acoustic cavitation, ii) in situ growth of target MOF on the microsphere template, iii) concentrating the MOF‐loaded microspheres to fabricate stable HIPE, and iv) freezing and lyophilization of the formed assemblies to yield free‐standing aerogels.

To generate the porous structures, TEMPO‐oxidized CNF was initially prepared and treated with 3‐aminopropyltriethoxysilane (APTES) to introduce amine functional groups along with pre‐existing carboxylic groups on CNF, transforming it into a nanoparticle surfactant with dual functionalities. This improves the crosslinking and interaction of CNF with BSA, followed by ultrasonication of the CNF/BSA aqueous complex with hexane as the oil phase. This led to covalent shell crosslinking of the CNF/BSA complex at the oil/water interface, generating microspheres with trapped oil droplets, known as “emulsion interfacial polymerization”. The prepared microspheres serve as a template for in situ MOF synthesis and immobilization. ZIF‐8 was then synthesized in situ on microspheres upon the dropwise addition of zinc nitrate and ligand (2‐methylimidazole) to the microsphere suspension. The Zn²⁺ ions attach to microspheres through their negative charges, followed by their coordination with ligands and initiation of ZIF‐8 nanoparticle nucleation growth. The resulting complex is then concentrated by centrifugation, excluding the excess aqueous phase and forming an emulsion gel with a solid‐like behavior. Freeze‐drying the formed assemblies resulted in free‐standing, ultra‐lightweight foams with high MOF loading and a spiderweb‐like hierarchical arrangement (Figure  17 ).

Figure 17.

Proposed strategy to fabricate ZIF‐8 porous structures with a spiderweb‐like arrangement. A) Extraction of CNF from grape seed and its further TEMPO‐assisted oxidation, followed by modification with APTES. B) Generating microspheres via emulsification with crosslinked shells employing CNF and bovine serum albumin as the shell components. Next, the obtained solution containing ZIF‐8 ingredients, e.g., zinc nitrate and 2‐methylimidazole, was added to the emulsion system to initiate the in situ growth of the ZIF‐8 on the surface of microspheres. C) Centrifugation of the ZIF‐8 decorated microspheres at 2000 g for 5 min to generate a high internal phase emulsion (HIPE) with a gel‐like behavior. D) Freeze‐drying of the resulting gel to make porous hierarchical ZIF‐8 foam with a spiderweb‐like porous construct. E) SEM images of the ZIF‐8 foams with different contents of ZIF‐8 (from 0 to 86 wt%). Reprinted with permission.^{[ 24b ]} Copyright 2020, Wiley‐VCH.

Zeolitic imidazolate frameworks embedded in porous cellulose structures have demonstrated significant potential for generating conductive MOF‐cellulose aerogels through subsequent carbonization. To achieve this, a study investigated the in situ growth of ZIF‐67 on bacterial cellulose, resulting in a free‐standing aerogel after freeze‐drying (Figure  18A).^{[ 72 ]} To manufacture the aerogel, the bacterial cellulose gel was initially immersed in a solution containing 1 wt% KOH for 5 h at 70°C, thoroughly washed with deionized water, and then treated with CO(NO₃)₂.6H₂O by mixing for 6 h at RT. Next, the aqueous solution containing the ligand (2‐methylimidazole) was added dropwise to the system and stirred for 2 h, followed by aging at RT for 18 h. The resulting gel was washed with deionized water and freeze‐dried to generate aerogel. In the final step, the aerogel was calcined for 3 h at 900 °C with a gradual temperature increase of 2 °C per minute. The morphological analyses clearly demonstrated the uniform nucleation growth of ZIF‐67 throughout the macro‐porous bacterial cellulose network (Figure 18B–H). This led to the formation of a hybrid conductive/magnetic MOF‐derived aerogel.

Figure 18.

A) Schematic presentation of in situ synthesis of ZIF‐67 on bacterial cellulose (BC) aerogel, followed by carbonization to generate conductive multi‐scale porous aerogel. B) Digital photograph of the carbonized BC‐ZIF‐67 aerogel on a petal. SEM images of C) CNF, D & G) non‐reduced BC‐ZIF‐67 aerogel, and E, F, H) carbonized BC‐ZIF‐67 aerogel. Reprinted with permission.^{[ 72 ]} Copyright 2020, Elsevier.

Another study employed the hybrid composition of bacterial cellulose/chitosan as the supporting scaffold for the in situ nucleation growth of ZIF‐67.^{[ 73 ]} The aerogel manufacturing process started with the preparation of a neutral suspension of bacterial cellulose and chitosan suspension, stabilized with 1 wt% acetic acid. The formed complex was subjected to freezing and lyophilization to produce a bacterial cellulose/chitosan aerogel. This aerogel was then soaked in a solution of Co(NO₃)₂·6H₂O, followed by the addition of 2‐methylimidazole. Upon the addition of monomer, the ZIF‐67 nucleation growth was triggered from Co²⁺ entrapped sites, leading to the homogeneous decoration of the hybrid aerogel with ZIF‐67. The resulting aerogel was thoroughly washed with a methanol/deionized water solution, transforming into a free‐standing and highly porous aerogel with a BET surface area of 268.7 m² g⁻¹ and 46.1% ZIF‐67 loading through an in situ manufacturing process (Figure S11A, Supporting Information).

In a practical and scalable approach, Bai et al.^{[ 74 ]} developed a versatile in situ MOF synthesis method. This method involved substrate seeding followed by secondary in situ nucleation growth, resulting in a hybrid aerogel with uniformly coated MOF layers throughout the aerogel's porous framework. The process starts by pre‐seeding the organic ligand onto the aerogel, followed by spraying a controlled amount of ion solution on the as‐prepared aerogel. The sprayed ion solution diffuses into the aerogel, leading to uniform nucleation growth of MOF along the macro‐porous aerogel walls. The proposed strategy benefits from its simplicity and scalability, resulting in hierarchical MOF aerogels with numerous accessible active sites with proper mass transfer for catalytic activities. The detailed manufacturing process of aerogels starts from the fabrication of a cellulose–ligand aerogel scaffold by mixing an appropriate amount of terephthalic acid (organic ligand) with NaOH in deionized water, followed by the stepwise addition of carboxymethylcellulose (CMC). The formed gel was then subjected to freezing and lyophilization to yield pre‐seeded cellulose aerogel. Afterward, the copper acetate dissolved in DMF was sprayed onto the aerogel for 200 s with a flow rate of 1 µL s⁻¹. The resulting aerogel was naturally dried, washed with DMF to remove impurities, and vacuum‐dried at 120 °C to yield CMC–CuBDC aerogel (Figure S11B, Supporting Information). The proposed approach is versatile, allowing for the synthesis of various types of MOF species by changing the type of ligand and the sprayed ion solution. This enables nucleation and growth throughout the porous structure's framework.

2.3.4. Polyacrylonitrile‐Based Supporting Scaffolds

The electrospun fibers have also been found to be potential scaffolds for the immobilization of MOF or COF species through an in situ growth strategy. For instance, a study investigated the scalable production of ultralightweight fibrous COF aerogels employing the epitaxial growth synergistic assembly (EGSA) method.^{[ 75 ]} Accordingly, the COF fibers were grown on the electrospun polyacrylonitrile (PAN) fibers (average diameter of 1.7 µm) with urea linkage. This was followed by the removal of the PAN through solvent extraction to achieve hollow COF fibers (Figure  19 ). The resulting fibrous COF aerogels exhibited ultra‐lightness, with densities ranging from 14.1 to 15.5 mg cm⁻³. They featured a hierarchical porous framework, encompassing micro‐pores, meso‐pores, and macro‐pores. The derived COF aerogels also demonstrated proper mechanical properties, capable of being compressed up to 50% with instant shape recovery upon force release. They showed only about 5% stress degradation after 20 compression cycles.

Figure 19.

A) Schematic representation of COF‐based fiber aerogels through epitaxial growth synergic assembly (EGSA) approach. B–E) and F‐I) showcase the structural features plus X‐ray diffractograms of TpBD‐Me₂ and TpBD COF aerogel fibers, respectively. Each part shows B,F) the packing models, C,G) powder X‐ray diffractogram graphs, D,H) SEM images, and E,I) TEM images. Reprinted with permission.^{[ 75 ]} Copyright 2024, Wiley‐VCH.

2.3.5. Aramid Nanofiber‐Based Supporting Scaffolds

Another study utilized aramid nanofibers as the supporting scaffold for the seeding and nucleation growth of MOFs, resulting in free‐standing MOF aerogels.^{[ 22c ]} The aramid nanofiber is a kind of p‐phenylene terephthalamide (PPTA) fibrous construct at the nanoscale with excellent thermal and chemical stabilities. The preparation of aramid nanofiber aerogel was conducted through a deprotonation and subsequent protonation strategy (Figure  20A). To achieve this, the micron‐sized aramid yarns made of PPTA were suspended in a mixture of KOH/DMSO and properly dispersed till obtaining a yellowish‐dark brown solution. In this process, deprotonation occurs, wherein KOH weakens the intermolecular hydrogen bonds and encapsulates the PPTA molecular chain through interaction with amide groups. The protonation step was then conducted using water as the proton donor, which facilitates proton transfer in combination with KOH. This improves the stiffness of the aramid nanofiber chains and facilitates their self‐assembly in an aerogel framework. The HKUST‐1 was selected as the MOF for the in situ growth on the aramid nanofibers with high loading (Figure 20B). In this case, the aramid nanofiber gel was immersed in an ethanolic metal ion (Cu(NO₃)₂) solution and reacted with the organic ligand (1,3,5‐benzenetricarboxylate (H₃BTC)). In this process, a large portion of copper ions (Cu²⁺) are adsorbed on the surface of aramid nanofiber through electrostatic interaction, viz., mainly through van der Waals interaction, with the negatively charged oxygen‐based functionalities serving as pre‐seeded nucleation sites for in situ MOF growth. After interacting with the organic ligand's solution, the resulting mixture was transferred to an autoclave and treated for 12 h at 80 °C. After washing with ethanol and performing a solvent exchange with tert‐butyl alcohol, the sample was freeze‐dried. This resulted in ultra‐lightweight (density of 5.86 mg cm⁻³) and highly porous (99.33% hierarchical porosity with a specific surface area of 636.62 m² g⁻¹) aerogel featuring meso‐ and macro‐porosities. The fibrous construct made of MOF‐decorated aramid nanofibers showcased the uniform decoration of HKUST‐1 on nucleation sites with an average diameter of 86–128 nm compared with the neat aramid nanofiber diameter of 40 ± 20 nm (Figure 20C–E). The mass fraction of the MOF can be adjusted from 32% to 72% by altering the processing conditions and composition of reactants. This versatility makes these materials promising for environmental applications, such as CO₂ removal and capture, where high MOF loading is required.

Figure 20.

A) Fabrication process of aramid nanofiber (ANF) aerogel. B) Manufacturing process of HKUST‐1 aerogel through in situ decoration of ANF. C) A view of the cleaved cluster of ANFs and HKUST‐1. D‐E) SEM images of HKUST‐1 aerogels at various magnifications. Reprinted with permission.^{[ 22c ]} Copyright 2023, Elsevier.

2.3.6. Wood‐Based Supporting Scaffolds

Wu et al.^{[ 76 ]} employed the porous balsa wood scaffold for in situ MOF growth, enabling the nucleation site plantation and synthesis of a wide range of MOF via coordination chemistry. For this purpose, a Janus‐structured wood‐based porous structure was developed, featuring one side made of wood and the other side covered with MOF species. The Janus morphology significantly enhanced the dehydration capacity and catalytic performance of the MOF‐loaded construct, surpassing the results achieved with neat MOF powders. The fabrication of MOF‐loaded wood aerogel was performed in two stages: i) chemical treatment along with oxidation of wood structure and ii) in situ growth of MOF through nucleation site plantation. In this regard, the balsa wood as the supporting scaffold was initially treated with NaClO₂ and then NaOH to remove the lignin and hemicellulose from the wood structure. The obtained porous wood structure with proper mechanical properties and honeycomb arrangement was subjected to TEMPO oxidation to transform the hydroxyl groups into carboxylic functional groups. These groups act as trapping sites for metal ions, facilitating the uniform growth of the MOF species throughout the construct. For in situ MOF synthesis, the wood aerogel was first immersed in a metal ion solution for 2 h. It was then placed in a second solution containing organic ligands and metal salts to promote the nucleation and growth of MOFs on the wooden scaffold. The wooden scaffold was first washed with methanol, then subjected to solvent exchange with tert‐butyl alcohol, and finally freeze‐dried to form wood‐MOF aerogel (Figure  21A). To manufacture MOF‐loaded Janus wood aerogel, the wood aerogel was immersed in a 2 m metal salt solution for 24 h to achieve metal‐ion chelation. Next, the obtained structure was washed with deionized water, underwent solvent exchange, and then freeze‐dried. Afterward, a certain volume of wood aerogel was blocked with hexane and transferred into a metal ion solution for 2 h. The type of solvent and metal ion varied depending on the MOF type. The corresponding solution of the organic ligand was added to the metal ion solution to facilitate the MOF growth on nucleation sites. The aerogel was freeze‐dried after washing and solvent exchange to generate a free‐standing MOF‐loaded Janus wood aerogel (Figure 21B). Various kinds of MOFs, including ZIF‐8, ZIF‐67, Fe‐BTC, HKUST‐1, and UiO‐66, were in situ synthesized on the wooden structure, showcasing the versatility of the proposed approach toward selective catalytic applications. Table  3 presents a list of recently developed multiscale porous MOF or COF structures created using in situ growth strategies.

Figure 21.

Schematic demonstration of A) MOF/wood aerogel and B) Janus MOF/wood aerogel manufacturing processes. Reprinted with permission.^{[ 76 ]} Copyright 2021, American Chemical Society.

Table 3.

Recently developed MOF/COF aerogels through in situ growth strategy and their potential applications.

MOF/COF type	Supporting scaffold	Drying method	Remarks	Refs.
ZIF‐67 MOF	MXene	FD	Hybrid MXene‐MOF aerogel as an engineerable supporting scaffold toward alkali‐ion batteries.	[77]
ZIF‐67 MOF	BC	FD	Carbonized bacterial cellulose‐ZIF‐67 aerogel toward EMI shielding.	[72]
ZIF‐L MOF UiO‐66 MOF	Balsa wood	FD	In situ synthesis of MOFs on the balsa wood as a porous supporting scaffold toward a potent recyclable air filter.	[78]
MOF‐808	TCNF	FD	In situ assembly of MOF‐808 on TCNF toward free‐standing aerogels for ion removal.	[79]
UiO‐66 UiO‐66‐NH2	Cellulose aerogel	FD	In situ synthesis of UiO‐66 or UiO‐66‐NH2 on the outer surface of cellulose aerogel toward heavy metal removal.	[80]
ZIF‐8 MOF UiO‐66‐NH2	MWCNT	FD	In situ MOF synthesis on MWCNT toward flexible free‐standing aerogels for pesticide removal from water.	[81]
HKUST‐1 MOF	ANF	FD	In situ HKUST‐1 MOF growth on ANF toward robust multiscale porous aerogels for selective CO2 adsorption.	[22c]
ZIF‐9 MOF ZIF‐12 MOF	Cellulose aerogel	FD	In situ ZIF‐9 or ZIF‐12 growth on cellulose aerogel as a supporting scaffold toward organic pollutants degradation.	[82]
ZIF‐67	BC‐chitosan	FD	In situ growth of ZIF‐67 on hybrid bacterial cellulose‐chitosan scaffold toward removing organic dyes and heavy metal ions.	[73]
Al‐MIL‐53 MOF	TCNF	FD	In situ loading of Al‐MIL‐53 MOF on the TCNF toward flexible free‐standing aerogels for thermal insulation and flame retardancy.	[66]
MIL‐101 (Fe)	GO	FD	In situ growth of MIL‐101 (Fe) MOF on GO as the supporting substrate, followed by aerogel production and its carbonization toward EMI shielding.	[68a]
HKUST‐1 MOF	Ru‐graphene aerogel	FD	In situ nucleation growth of HKUST‐1 MOF on the Ru‐graphene aerogel supporting scaffold toward the catalytic oxidation of CO.	[83]
ZIF‐8 MOF HKUST‐1 MOF UiO‐66 MOF	Wood aerogel	FD	TEMPO‐oxidized balsa wood aerogel was used to generate a Janus MOF‐aerogel. In this structure, one side of the wood was blocked with hexane while the other side was in situ decorated with MOF species. The Janus MOF‐wood aerogel was employed for catalytic reactions.	[76]
UiO‐66‐NH2 MOF	CNF	FD	In situ development of hybrid CNF‐BPQD‐UiO‐66 aerogel toward promoted photo‐induced uranium extraction.	[84]
CuBDC MOF CoNiBDC MOF	CMC	FD	Spray‐assisted in situ synthesis of MOF on the outer accessible surface of CMC aerogel toward CO2 cycloaddition reaction and electrochemical OER.	[74]
ZIF‐8 MOF	CNF	FD	In situ growth of MOFs on shell‐crosslinked microspheres, followed by the creation of a HIPE template and subsequent freeze‐drying, generated a spiderweb‐like aerogel with high MOF loading (up to 86 wt%). These aerogels were utilized for pollutant adsorption and oil/water separation.	[24b]
PCN‐224 MOF	MF	VD	In situ decoration of MF with PCN‐224 MOF toward catalytic reactions.	[32a]
ZIF‐67 MOF	EPVA nanofiber	FD	In situ growth of ZIF‐67 on EPVA nanofiber toward free‐standing robust MOF aerogels for organic pollutants degradation.	[85]
ZIF‐8 MOF	AF	FD	In situ growth of ZIF‐8 on AF to generate hybrid aerogel toward environmental remediation. The aerogel showed promising potential toward ion removal, organic dye degradation, and oil/water separation.	[86]
MOFN‐40	Te NWs	FD	Generating aerogel by the in situ decoration of MOFN‐40 on Te NWs toward energy conversion and storage applications.	[87]
TAPB‐DVA COF	MF	VD	In situ synthesis of COF on MF toward superhydrophobic, mechanically robust, and multi‐scale porous foams for oil and organic pollutant absorption.	[32b]
COF‐366‐Fe	Graphene aerogel	FD	In situ decoration of COF‐366‐Fe on graphene aerogel as the supporting scaffold for detecting released NO from living cells.	[88]
TpTt COF	MF	VD	In situ decoration of MF with TpTt COF for tetracycline degradation under visible light.	[70]
2,3‐Dha‐Tph COF 2,5‐Dha‐Tph COF	Dopamine decorated‐PS	AD	In situ synthesis of COFs on dopamine decorated PS toward removal and subsequent extraction of cadmium from wastewater.	[89]
Dha‐Tab COF COF DTF	MF	VD	In situ COF decoration on MF for oil/water separation.	[90]
TPDA‐Bpy COF	MF	VD	In situ TPDA‐Bpy COF decoration on MF for oil/water separation and food safety applications.	[69]
TPDA‐2,3Dha COF	MF	AD	In situ TPDA‐2,3Dha COF decoration on MF for ultra‐fast and effective removal of cadmium ions from wastewater.	[91]
DAB‐COF	MF‐GO	FT	In situ decoration of DAB‐COF on hybrid flexible carbonized MF‐GO scaffold followed by subsequent heat treatment for supercapacitor application.	[71]
TAPB‐BTCA COF	Graphene aerogel	VD	In situ decoration of TAPB‐BTCA COF on the graphene aerogel followed by thermal treatment toward high‐performance electrode for energy storage devices.	[24a]
COF‐SO3H	GO	FD	Dual hydrophilic‐hydrophobic region COF‐graphene scaffold for water harvesting application.	[29b]
TpBD‐Me2 COF TpBD COF	PAN	FD	In situ COF decoration on PAN scaffold toward free‐standing flexible lightweight aerogel for absorption of organic pollutants.	[75]

Abbreviations: Freeze‐drying (FD), bacterial cellulose (BC), electromagnetic interference (EMI), cellulose nanofiber (CNF), TEMPO‐oxidized CNF (TCNF), multi‐walled carbon nanotube (MWCNT), aramid nanofiber (ANF), 2,2,6,6,‐tetramethyl‐1‐piperidinyloxy radical (TEMPO), black phosphorus quantum dots (BPQD), carboxymethylcellulose (CMC), oxygen evolution reaction (OER), high internal phase emulsion (HIPE), melamine foam (MF), vacuum dried (VD), electrospun poly(vinyl alcohol) (EPVA), amyloid fibrils (AF), nanowire (NW), polyurethane sponge (PS), air dried (AD), freeze‐thawing (FT), graphene oxide (GO), reduced graphene oxide (rGO), and polyacrylonitrile (PAN).

2.4. Template‐Assisted Method

The template‐assisted approach is a highly engineerable manufacturing process that involves embedding template molecules or nanoparticles in MOF‐ or COF‐based hierarchical structure and their subsequent extraction by non‐destructive approaches. Several techniques have been implemented to produce MOF‐ or COF‐based constructs, including i) soft‐templating, ii) hard‐templating, and iii) self‐templating. In the following sections, the above approaches are discussed in detail.

2.4.1. Soft‐Templating

The soft‐templating technique is a versatile methodology for engineering the porosity of hierarchical COF or MOF porous constructs using soft templates and their subsequent removal/extraction. The typical substances for the soft templating method involve ionic liquid,^{[ 92 ]} emulsions,^{[ 93 ]} surfactants,^{[ 94 ]} and so on. For this purpose, Tang et al.^{[ 93a ]} developed hollow COF microspheres with interfacially driven defects through emulsion interfacial polymerization using bifunctional dodecyltrimethylammonium bromide (DTAB) surfactant. Accordingly, 2,5‐dimethoxyterephthaldehyde (DMTP) and 1,3,5‐tris(4‐aminophenyl)benzene (TAPB) were initially dissolved in n‐butanol as the oil phase. Subsequently, an aqueous phase was prepared by dissolving DTAB in DI water. The surfactant‐enriched aqueous phase was then added to the oil phase to produce an emulsion, followed by adding Sc(OTf)₃ catalyst and then shaking to allow complexation. The emulsion was then heated for 30 min at 70 °C to trigger COF growth and simultaneous template removal. The hollow COF microspheres were finally obtained through centrifugation, followed by washing with methanol and drying (Figure S12A, Supporting Information). Of note, the organic linker and catalyst were positioned on both sides of the emulsion's interface, facilitating the COF growth and crystallization along the emulsion droplets’ interface. The microemulsion system also served as a potent temporary soft template, which could be removed through the evaporation of emulsion droplets. This process yielded hollow COF spheres with controllable shell thickness and size. The introduced method proved to be a potent approach for defect introduction into the system owing to its slow crystal growth.

Another attempt involved the soft‐templating manufacture of hierarchically porous TpAzo COF constructs using ionic liquid under mild conditions.^{[ 95 ]} The technique employed allowed for the induction of tunable meso‐porosities by adjusting the ionic liquids’ alkyl chain length. To achieve this, 1,3,5‐triformyl phloroglucinol (Tp) and 4,4'‐azobenzenediamine (Azo) were introduced into the ionic liquid and reacted at 50 °C for 12 h. This process generated a soft‐templated COF porous structure with a 92% yield after isolation, filtration, and methanol‐assisted Soxhlet extraction (Figure S12B, Supporting Information). Interestingly, the X‐ray diffractogram showcased nearly similar crystallinity for the as‐prepared hierarchically porous TpAzo COF with a far shorter synthesis time than the solvothermally synthesized TpAzo COF (Figure S12C, Supporting Information). The nitrogen sorption isotherm showcased a broader pore size distribution for hierarchically porous TpAzo COF than the conventional type of TpAzo COF (Figure S12D,E, Supporting Information). The employed technique enabled the introduction of additional meso‐porosities to the system while maintaining its original micro‐porosities. The ionic liquid employed in this technique also served as both a soft template and reaction catalyst. The technique allowed for the manufacturing of different types of hydrazone‐ and imine‐based hierarchically porous COFs, which is challenging to achieve through conventional approaches.

2.4.2. Hard‐Templating

Hard templating is another versatile approach for generating shape‐ and size‐controllable porosities. This method involves embedding hard templates made of rigid solids with different shapes at the nano‐ or micro‐scale into MOF or COF porous constructs. This is followed by their non‐destructive extraction, which keeps the structure intact and enables the introduction of engineerable porosities to the target construct. This broadens their porosity levels across various scales.^{[ 25 ]} The commonly used hard templates for generating MOF or COF porous structures include polymeric nanospheres (e.g., polystyrene (PS)),^{[ 96 ]} polyacrylonitrile (PAN) electrospun nanofibers,^{[ 97 ]} sodium chloride (NaCl),^{[ 98 ]} silica nanoparticles (SiO₂),^{[ 99 ]} and carbonaceous materials.^{[ 100 ]} Among these hard templates, polystyrene (PS) is widely used due to its controllable size and dimensions through synthetic approaches. The uniform morphology of PS allows for the formation of densely packed nanoparticles, capable of filling the gaps within the COF precursors in the synthesis solution. PS could also be easily removed using organic solvents, preserving the structure with artificially induced porosities across desired scales. Moreover, PS microspheres act as potent and robust supporting scaffolds, maintaining the integrity of the structure during COF synthesis.^{[ 96} , ^{101 ]}

Many practices employed PS as a potential hard template to generate hierarchical MOF or COF porous structures. In this regard, Shen et al.^{[ 101b ]} manufactured ZIF‐8 MOF single crystals with highly ordered and aligned macro‐pores. The methodology relies on the hard‐template shaping effect of PS nanospheres coupled with a double solvent heterogeneous MOF nucleation approach. The introduced method enables the in situ growth of ZIF‐8 within the well‐ordered voids, rendering a hierarchically porous MOF with ordered micro‐ and macro‐porous constructs (Figure  22A). To manufacture the hard‐templated porous ZIF structure, the mono‐dispersed PS nanospheres were initially assembled within a highly aligned 3D structure, followed by filling the construct with ZIF precursors, viz., Zn(NO₃)₂ and 2‐methylimidazole. The resulting structure was then soaked into a mixed solution consisting of CH₃OH and NH₃.H₂O. The CH₃OH stabilized the precursors and maintained the balance between the nucleation and growth of ZIF‐8 MOF. The NH₃.H₂O also triggered the rapid crystallization of MOF precursors.

Figure 22.

A) Hard‐template‐assisted fabrication of porous SOM ZIF‐8 porous structure with hierarchical porosity induced by embedding and extracting the PS spheres; SOM corresponds to a single‐crystal ordered macropore. Reprinted with permission.^{[ 101b ]} Copyright 2018, Science. B) Manufacturing process of 3D‐structured COF‐300 porous structure by employing the well‐packed PS microspheres as hard templates. SEM images of COF‐300 multi‐scale porous structures at C) low and D–F) high magnifications. Reprinted with permission.^{[ 96 ]} Copyright 2023, American Chemical Society. G) Manufacturing process of TpBpy COF porous structure's fabrication with micro‐ and macro‐porosities using PS spheres as hard templates. Reprinted with permission.^{[ 101c ]} Copyright 2019, American Chemical Society.

Another attempt was made to develop a 3D‐structured hard‐templated single‐crystal hierarchical COF (COF‐300 and COF‐303) porous structure with macro‐ and micro‐porosities using monodisperse PS microspheres.^{[ 96 ]} Accordingly, COF species were crystallized within the colloidal PS hard template using an aniline‐modulated strategy, facilitating the transformation from amorphous to crystalline. This process generated interconnected 3D‐structured macro‐pores. In a typical proof‐of‐concept synthesis (Figure 22B), the closely packed PS microspheres were used as a hard template, followed by the addition of COF‐300 precursors to the template. The precursors, consisting of a DMSO solution enriched with terephthaldehyde (BDA), tetrakis(4‐aminophenyl)methane (TAM), and aniline, were infiltrated into the empty spaces between well‐packed PS microspheres. Next, the glacial acetic acid was added to the previous composition and reacted for 3 days at 50 °C. The template was then removed by soaking and washing with THF, leading to the formation of a hierarchically porous COF structure (Figure 22C–F).

Another study employed PS hard templates to induce macro‐porosity in inherently micro‐porous TpBpy COFs.^{[ 101c ]} To fabricate the multi‐scale porous COF structures, Tp linker and 2,2ˊ‐bipyridine‐5,5′‐diamine (Bpy) were employed in the presence of PS templates using the Schiff‐base reaction. For this matter, the colloidal monodispersion of PS with a concentration of 10 wt% was mixed with Bpy in the presence of p‐toluenesulfonic acid (PTSA) catalyst, forming an organic salt. The obtained salt was then thoroughly shaken with Tp and poured into a petri dish overnight at RT for excess water evaporation. Further drying of the complex at 80 °C for 24 h resulted in an orange PS‐embedded TpBpy composite. This composite was subsequently transformed into a micro‐ and macro‐porous COF construct by extracting the PS spheres and excess monomer using THF‐assisted Soxhlet purification (Figure 22G). Interestingly, upon introducing bipyridine functionalities into the COF building block, metallic ions like Co²⁺ can coordinate with the as‐developed hierarchical porous structure and yield TpBpy‐Co structures with dual porosities.

Employing electrospun nanofibers of polyacrylonitrile (PAN) is another type of potent hard‐templating‐based approach for the preparation of fibrous COF membranes. Ding et al.^{[ 97 ]} developed a practical process based on a sacrificial electrospun membrane to embed the COF powder (TpPa COF) into a flexible, manageable, and mechanically robust COF membrane (Figure  23A). To produce the COF membrane, a hybrid p‐phenylenediamine (Pa) embedded PAN electrospun membrane was prepared at different Pa loadings (100% (PAN/Pa‐100) and 200% (PAN/Pa‐200)) and vacuum dried at RT for 12 h. The PAN/Pa membranes were then added to dichloromethane containing Tp linker and acetic acid as the catalyst. The reaction was followed for 1 day at 120 °C, yielding fibrous PAN/COF membranes. The membranes were washed three times using dichloromethane and acetone, followed by vacuum drying. Finally, the as‐prepared PAN/COF fibrous membranes were washed with DMF using a Soxhlet extractor at 160 °C for 1 day until the PAN template was fully removed from the membrane, resulting in a COF membrane after vacuum drying. This led to highly practical, flexible, and mechanically robust fibrous COF membranes with a high degree of crystallinity in conjunction with a high specific surface area of 1153 m² g⁻¹. The outcome of this hard templating process paves the way for developing scalable COF membrane production for catalytic, separation, and energy applications.

Figure 23.

A) The three‐step process for the fabrication of porous fibrous COF membranes. Morphological assessment through SEM analysis from B) neat PAN, C) PAN/Pa‐100, D) PAN/Pa‐200, E) neat TpPa COF powder, F1–2) PAN/COF‐100, G1–2) PAN/COF‐200, H1–2) COF‐100, and I1–2) COF‐200. Reprinted with permission.^{[ 97 ]} Copyright 2021, Wiley‐VCH under Creative Commons CC BY license.

Sodium chloride (NaCl) and SiO₂ are other types of potent and removable economic hard templates for the production of multi‐scale porous COF structures.^{[ 98} , ^{99 ]} Correspondingly, a study employed NaCl as a removable and recyclable hard template to produce a multi‐scale hierarchically porous TpBD COF structure.^{[ 98 ]} The fabrication of the NaCl‐templated COF structure was initiated by mixing p‐toluene sulfonic acid monohydrate (PTSA), used as a recyclable catalyst, with benzidine through mortar mixing followed by thorough grinding. Next, a desired amount of NaCl was added to the mixture as a removable hard template. The Tp crosslinker was then added to the mixture, followed by adding 100 µL water and further polymerization in a mold for 5 min at 170 °C. The as‐prepared mixture was cooled down to RT and then turned into free‐standing foams by sequential washing (water, dimethylacetamide, and acetone) and subsequent drying (Figure S13A, Supporting Information). The resulting COF foam showcased an excellent surface area of ∼700 m² g⁻¹ and demonstrated fantastic effectiveness in removing pollutants from aqueous systems. NaCl, as a crystalline pore‐forming hard template, served as a supporting substance to provide proper integrity for COF growth and the formation of a macroscopic porous structure with minimized shrinkage. These templates can be easily removed with water and recycled for further use. Notably, the foam's volume can be adjusted by changing the mass fraction of the hard template during synthesis, promoting macro‐porosity and preventing the stacking of COF crystals.

SiO₂ is another potent sacrificial template for manufacturing hard‐templated porous COF structures. In this case, a study developed hierarchical porous COFs by adding a specific amount of SiO₂ hard template to a COF synthesis medium, resulting in a multi‐scale porous structure.^{[ 99 ]} The fabrication of the SiO₂‐templated structure started by adding a certain amount of SiO₂ to the mortar‐mixed PTSA/TPDA powder composition, resulting in a uniform amorphous COF‐SiO₂ gray precursor mixture. Adding water to the resulting mixture and treating it in an autoclave at 120 °C for 3 days generated a dark COF‐SiO₂ powder. The residual PTSA monomer was removed through Soxhlet extraction using water and 1,4‐dioxane. The dried powder was then subjected to multiple steps to remove the SiO₂ template. The SiO₂ nanoparticles were extracted from the COF structure by dispersing it in 1m NaOH and subjecting it to ultrasonication for five repeated cycles. The final multi‐scale porous COF structure was then collected by washing with water and drying (Figure S13B, Supporting Information).

2.4.3. Self‐Templating Approach

The self‐templating approach involves using a portion of the material as a self‐guided template to regulate the formation of hierarchical porous COF structures with engineerable porosities, generated through the material's inherent self‐assembly capability. This approach allows for the fabrication of hierarchically porous COF constructs with a broader range of porosities. Additionally, by adjusting the self‐guided template, it is possible to engineer the porosities of these constructs over a wider range. This method also addresses the challenge of template removal, which typically requires a series of steps to extract the soft or hard templates, potentially leading to pore contamination or shell collapse.^{[ 25 ]} However, the self‐templated method requires a specific monomer and reaction conditions design, which hinders its universality in the synthesis of porous COF structures.

One of the most common approaches to yielding self‐templated COF structures is through Ostwald ripening. Ostwald ripening is a phenomenon in liquid or solid solutions that involves the time‐dependent variation of an inhomogeneous construct. This includes the dissolution of small crystals and their redeposition on larger crystals.^{[ 102 ]} By employing the combined effect of nanoparticle assembly, inside‐out Ostwald ripening, and epitaxial growth, Wang et al.^{[ 103 ]} managed to develop dioxane‐based microflowers (5–7 µm) of COF‐316 with interconnected hollow petals. Due to the interconnected hollow morphology and the intrinsic porosity of the resulting structure, a uniform COF‐316 and polypyrrole (PPy) composite can be created using both interior and exterior functionalization. This enhances charge transfer and structural stability during charge‐discharge cycles. Figure 24A represents the manufacturing process of hollow COF‐316 microflowers. These microflowers were prepared via nucleophilic aromatic substitution reactions between the linear tetrafluorophthalonitrile (TFPN) and triangular 2,3,6,7,10,11‐hexahydroxytriphenylene (HHTP). To this end, HHTP, TFPN, trimethylamine, and 1,4‐dioxane were sequentially added into a fully dried Pyrex tube, followed by sonication and freezing in liquid nitrogen (N₂ bath, 77 K). The frozen composition then underwent two freeze‐thaw cycles and was subsequently heated for 72 h at 120 °C to yield microflowers. The brownish product was thoroughly washed with DMF (five times), immersed in 50 °C DMF for 1 day, and treated with a Soxhlet extractor for 24 h using THF to remove unreacted precursors. The product was then placed in acetone for 24 h and dried in a vacuum oven, generating hollow COF‐316 microflowers.

Figure 24.

A) Schematic illustration of the manufacturing process of hollow COF‐316 microflowers as a result of the combined action of nanoparticles assembly, inside‐out Ostward ripening, and epitaxial growth. Reprinted with permission.^{[ 103 ]} Copyright 2021, Wiley‐VCH. B) Hollow COF spheres formation mechanism. Reprinted with permission.^{[ 104 ]} Copyright 2015, Nature Publishing Group under Creative Commons Attribution 4.0 International licence.

Another study developed hollow DhaTab COF spheres with mesoporous walls generated through a single‐step, cost‐effective self‐templating approach that does not require any sacrificial template.^{[ 104 ]} The formation of these spheres occurred via inside‐out Ostwald ripening, resulting in crystalline, highly porous (specific surface area of 1500 m² g⁻¹), and chemically stable COF constructs. These hollow DhaTab COF porous structures feature 0.5–2 µm macro‐porous inner cavities enclosed by a 20‐40 nm meso‐porous COF shell. Notably, the excellent chemical stability of these COF‐based structures stems from their robust intermolecular hydrogen bonding. The manufacturing of hollow COF structures was carried out through a Schiff‐base reaction between the 2,5‐dihydroxyterephthalaldehyde (Dha) and 1,3,5‐tris (4‐aminophenyl)benzene (Tab) within the mesitylene/dioxane hybrid solvent in the presence of acetic acid as the reaction catalyst. In this process, the Tab can also be replaced with N1,N1‐bis(4‐aminophenyl) benzene‐1,4‐diamine (Bad).

A yellowish or red‐colored DhaTab or DhaBad COF was obtained, followed by filtration and sequential washing (dimethylacetamide, DI water, and ethanol). The self‐templated hollow COF structures were primarily produced due to the Kirkendall effect and Ostwald ripening, with Ostwald ripening being the main factor in generating hollow spheres of metal oxides and sulfide‐based compounds. Notably, the Kirkendall effect corresponds to the variation in intrinsic diffusivities of chemical precursors in solid solutions.^{[ 105 ]} As a proof of concept, the highly crystalline DhaTab COFs with rod‐shaped morphology were formed 12 h after random self‐assembly of crystallites into a denser or coiled sphere. In this case, the crystallites positioned in the inner part of the spheres possess higher surface energy compared with those located at the outer surface. Consequently, they trigger inside‐out Ostwald ripening after 24 h, generating hollow DhaTab COF spheres (Figure 24B). The formed assemblies are highly crystalline/porous and can preserve their spherical architecture within phosphate buffers or water, thanks to their excellent chemical stability. This remarkable chemical stability stems from their robust intramolecular hydrogen bonding, which locks in the phenyl ring in one plane and protects the imine nitrogen from possible nucleophilic invasions.

In another study, Wang et al.^{[ 106 ]} employed the Kirkendall conversion of COFs through asymmetric monomer exchange to precisely engineer porous hollow structures. Accordingly, altering the monomer feeding strategies by adding one or more monomers either simultaneously or sequentially led to the generation of hollow constructs made of single or multiple COFs with an engineerable spatial distribution. The method can be employed to manufacture centimeter‐sized self‐templated COF structures with compressive strength and Young modulus of 45 and 263 MPa, respectively, as well as 20% elastic deformation. Interestingly, the COF structures exhibited excellent anti‐fragility, capable of being remolded after five cycles of crushing and grinding with negligible variations. The proposed method provides a practical approach to producing COF structures on a macroscopic scale. Table 4 presents a list of recently developed MOF or COF porous structures generated through template‐assisted manufacturing approaches.

Table 4.

Recently developed template‐assisted MOF or COF porous structures and their potential applications.

MOF/COF type	Method	Template	Supporting scaffold	Drying	Remarks	Refs.
TAPB‐DMTP COF	S‐T	ET	ET	VD	Fabrication of emulsion‐templated hollow COF microspheres toward laccase immobilization to degrade tetracycline.	[93a]
TpAzo COF	S‐T	IL	Self‐supported	AD	Ionic liquid templated TpAzo COF porous structure for C‐C coupling catalytic reactions.	[95]
ZIF‐8 MOF	H‐T	PS	Self‐supported	–	PS hard‐templated ZIF‐8 MOF porous structure toward catalytic reactions.	[101b]
ZIF‐8 MOF	H‐T	PS	Self‐supported	–	Generation of hollow ZIF‐8 MOF beads.	[107]
TpBPy COF	H‐T	PS	Self‐supported	VD	Hard‐templated COF porous structure toward the adsorption or removal of perfluorinated compounds.	[101a]
H‐COF	H‐T	SiO2	Self‐supported	VD	Hard‐templated porous COF structure for the food chemical analysis.	[99]
H‐COF	H‐T	Cu‐MOF	Self‐supported	VD	MOF templated COF porous structure via Schiff base reaction at RT toward food safety analysis.	[108]
COF‐300	H‐T	PS	Self‐supported	AD	PS hard‐templated COF‐300 porous structure toward iodine adsorption.	[96]
TpBPy COF	H‐T	PS	Self‐supported	AD	PS hard‐templated TpBPy COF porous structure for the OER and effective electrocatalysis reactions.	[101c]
TpBPy COF	H‐T	SiO2	Self‐supported	AD	Silica templated TpBPy COF porous structure toward effective electrocatalysis reactions.	[109]
TpBD COF	H‐T	NaCl	Self‐supported	VD	NaCl templated TpBD COF porous structure for the effective removal of sulfamerazine.	[98]
TpPa COF	H‐T	E‐PAN	Self‐supported	VD	E‐PAN templated TpPa COF toward generating porous COF fibrous constructs, effective for a multitude of applications such as energy, separation, and catalysis.	[97]
ZIF‐8 MOF HKUST‐1 MOF	Self‐T	N/A	CNF	FD	Self‐templated CNF‐MOF aerogels for the rapid and effective adsorption of organic dyes.	[110]
COF‐316	Self‐T	N/A	Self‐supported	VD	Flower‐shaped self‐templated COF‐316 porous structures for supercapacitor and energy storage applications.	[103]
DhaTab COF	Self‐T	N/A	Self‐supported	FT/VD	Developing self‐templated hollow spherical DhaTab COF porous structures toward trypsin uptake and immobilization.	[104]

Abbreviations: Emulsion‐template (ET), ionic liquid (IL), polystyrene (PS), soft‐template (S‐T), hard‐template (H‐T), self‐templated (Self‐T), vacuum drying (VD), room temperature (RT), hierarchical COF (H‐COF), air‐dried (AD), oxygen evolution reaction (OER), electrospun polyacrylonitrile (E‐PAN), not applicable (N/A), freeze‐drying (FD), and freeze‐thawing (FT).

3. Additive Manufacturing of MOF or COF Porous Structures

3D printing is a versatile fabrication method within additive manufacturing processes, enabling the creation of complex 3D structures with customized macroscopic arrangements through precise layer‐by‐layer deposition.^{[ 111 ]} So far, various 3D printing approaches have been employed to manufacture 3D‐structured MOF or COF constructs, including direct ink writing (DIW),^{[ 23 ]} stereolithography (SLA),^{[ 112 ]} fused deposition modeling (FDM),^{[ 113 ]} and selective laser sintering (SLS).^{[ 114 ]} Among these techniques, DIW is one of the most favorable 3D printing methods. It relies on the viscoelastic characteristics of the ink, which directly affect its printability and shape retention. This method requires proper shear‐thinning behavior, where the ink exhibits lower viscosity at higher shear rates, allowing it to flow through the nozzle. Conversely, at lower shear rates, the ink behaves like a solid, preventing spreading after printing. This characteristic is crucial for maintaining the ink's shape retention and ensuring printing precision.^{[ 111} , ^{115 ]}

3D printing of the MOF or COF offers a set of advantages over their neat powders. The most noteworthy benefit of this technique is the ability to create multi‐scale hierarchically porous complex 3D structures. This is achieved by combining the macroscopic arrangement obtained through 3D printing with the inherent micro‐ and meso‐porosities of the MOF or COF species.^{[ 22} , ^{116 ]} The formed assembly also exhibits significantly better mechanical robustness, enhancing handleability and improving the stability with minimal material loss compared to neat powder.^{[ 117 ]} Interestingly, the formed interconnected macrostructure within the 3D‐printed structures accelerates mass transport and enhances the pollutant removal capability of the construct beyond the potential of neat MOF or COF powder.^{[ 22a ]} Such features enhance the practicality of MOF or COF structures in advancing functional applications. Generally, the manufacturing of MOF and COF porous structures through 3D printing has been performed either by directly mixing MOF or COF in the ink or by their in situ growth on a pre‐printed supporting scaffold. Direct mixing is found to be less challenging for the 3D printing of the MOF or COF structures, as the pre‐synthesized MOF or COF is mixed with the 3D printing ink, followed by 3D printing and possible post‐treatments. In contrast, in situ growth requires multiple time‐consuming sequential steps. Notably, a study employed a self‐shaping strategy to develop a 3D printed COF‐enriched construct with multi‐scale porosities. This enabled the formation of 3D structures upon the reaction of COF precursors after printing.^{[ 22a ]}

However, the 3D printing of MOF or COF porous structures faces some noteworthy challenges. These challenges include: i) poor rheological characteristics of MOF‐ or COF‐based inks, ii) crack propagation and ultimate failure in 3D‐printed MOF or COF porous structures due to the weak interconnected macroscopic structure, and iii) smaller specific surface area of MOF or COF porous structures compared to their neat powders.^{[ 118 ]} Notably, the decrease in the specific surface area of such porous structures can be attributed to several factors, including: i) pore surface coverage and pore blockage of MOFs or COFs by additives in the ink, leading to dead mass accumulation, ii) deformation or decomposition of MOF or COF crystals due to pressure or heat during the 3D printing process, iii) weak adhesion between the supporting building block and MOFs or COFs, resulting in crack propagation, structural collapse, and material loss, and iv) corrosion of the MOF or COF structure within the ink due to the nature of the solvent used.^{[ 118} , ^{119 ]}

To address these challenges, researchers have implemented several practices to produce 3D‐structured MOF or COF porous structures with high structural integrity. For instance, a study employed a SiO₂‐based 3D‐printed hierarchically porous ceramic building block as a supporting scaffold for the in situ growth of MIL‐100(Fe) and HKUST‐1 via hydrothermal processes, aimed at catalytic degradation.^{[ 116 ]} In this case, the SiO₂‐based thixotropic ink was initially prepared, and the fumed inks were employed to prepare a 3D‐structured hierarchical porous scaffold through DIW. The printing was carried out using a nozzle with an internal diameter (I.D.) of 500 µm, with a printing speed of 4 mm s⁻¹. The printing was performed on a Teflon substrate with a constant extrusion pressure of 0.2–0.3 MPa to easily detach the 3D‐printed scaffold from the substrate after drying for 12 h at RT. The scaffold was then dried in an oven at 110 °C to prevent crack formation, and the temperature was subsequently raised to 350 °C to eliminate the organic compounds. The final hierarchical porous ceramic scaffold was obtained through annealing at 900 °C. Next, the surface of the scaffold was modified through functionalization with dopamine to provide active sites for the in situ growth of MOF species. Finally, the dopamine‐decorated scaffold was subjected to MIL‐100(Fe) and/or HKUST‐1 MOF precursors, and the MOF was synthesized through the hydrothermal process (Figure  25A). The resulting MOF‐decorated 3D‐structured ceramic scaffold showcased a promising potential for the catalytic degradation of organic dyes thanks to their high surface area combined with numerous active sites across the multi‐scale porous structure.

Figure 25.

A) Schematic illustration of in situ growth of MOF species on the surface of a 3D printed ceramic scaffold, enabling controlling the porosity across scales. Reprinted with Permission.^{[ 116 ]} Copyright 2020, Elsevier. B) The schematic process represents the formation of a 3D‐printed UV‐cured scaffold for in situ growth of HKUST‐1 that starts from I) adding precursors together, II) initiating the UV‐curing, and III) in situ formation of the MOF through immersing the scaffold in the ion solution. IV) Schematic representation of the process from 3D printing and UV curing to in situ HKUST‐1 MOF growth. Reprinted with Permission.^{[ 120 ]} Copyright 2020, American Chemical Society.

Another study employed a UV‐cured 3D‐structured supporting scaffold for the in situ growth of HKUST‐1 MOF, addressing the brittleness of MOF powders and creating a robust MOF‐embedded construct.^{[ 120 ]} In this regard, a 3D printable ink was prepared by mixing several components together: i) The first part consisted of acrylamide monomer (AAm) with the crosslinker N,N'‐methylenebisacrylamide (MBA) and photoinitiator (Irgacure 2959) to form a highly crosslinked network upon UV irradiation, ii) the second part involved adding sodium alginate to form a double polymeric network, iii) the third part included HKUST‐1 MOF's deprotonation agent (triethylamine, TEA) and organic ligand (trimesic acid, H3BTC), and iv) the final part involved adding a shear‐thinning agent (hydroxyethylcellulose, HEC) to enable 3D printing of the UV‐curable ink via DIW (Figure 25BI–III). The as‐prepared crosslinked mixture was 3D printed into desired shapes. This was followed by the activation of the photoinitiator through UV irradiation at a wavelength of 365 nm, initiating the free‐radical polymerization of the monomer. Upon polymerization, covalent crosslinking was established between the amine group of polyacrylamide and the carboxylic acid functional group of sodium alginate. The cured scaffold was then immersed in an ion solution bath (Cu(NO₃)₂) to initiate the in situ growth of HKUST‐1 MOF on the 3D‐printed polymeric supporting scaffold (Figure 25BIV). This process resulted in a stretchable (453%) 3D‐printed MOF‐loaded scaffold with excellent dye adsorption capability.

Aramid nanofibers have also demonstrated promising potential for creating mechanically robust 3D‐printed supporting scaffolds for in situ MOF growth. To this end, a study employed aramid nanofibers as a supporting scaffold for creating high in situ ZIF‐67 MOF‐loaded (63.4%) textiles with proper mechanical robustness and a high specific surface area (756.6 m² g⁻¹).^{[ 121 ]} The process started by preparing a 3D‐printable aramid nanofiber suspension by adding Kevlar pulp into the KOH/DMSO mixture. The optimized 3D‐printable ink was then printed on a cold substrate (−10 °C) to yield a 3D‐printed scaffold. Next, the frozen gel was subjected to solvent exchange by immersing in ethanol to remove the DMSO/KOH, followed by immersing in cobalt nitrate‐methanol solution (0.2 mol L⁻¹). The resulting ion‐enriched gel was immersed in the organic ligand bath (0.8 mol L⁻¹ 2‐methylimidazole) to facilitate the nucleation growth of MOF on ion sites. The aerogel was obtained after two cycles of solvent exchange (1:1 tert‐butanol:H₂O) and subsequent freeze‐drying (Figure  26 ). A similar procedure was followed to develop ZIF‐8 and/or MIL‐88 MOF‐decorated hybrid 3D printed aramid nanofiber aerogel. The resulting structure demonstrated a recyclable MOF‐loaded scaffold for pollutant removal.

Figure 26.

A) The in situ growth of ZIF‐67 on the 3D‐printed aramid nanofiber supporting scaffold. B) 3D printing of aramid nanofiber‐based scaffold with desired shapes, resulting in in situ MOF‐coated aerogels. C) Hybrid aramid nanofiber‐ZIF‐67 aerogel textile. D) Foldable ZIF‐67 decorated aerogel textile. E) SEM image from the surface of the hybrid aerogel and F–I) cross‐sectional SEM images from the aerogel's filaments. Reprinted with permission.^{[ 121 ]} Copyright 2024, American Chemical Society.

The 3D printing of COF porous structures has also shown promising potential for engineering macroscopic geometries for advanced applications. A research study investigated the 3D printing of GO and COF precursors, followed by reaction initiation to yield a 3D‐structured, free‐standing, multi‐scale porous structure through a COF self‐shaping strategy.^{[ 22a ]} The formed hierarchically porous 3D‐printed GO‐COF (TpBD COF) foam possessed multi‐scale porosities in the following manner: i) well‐ordered micro‐porosities between the range of 2–2.2 nm, ii) disordered meso‐/macro‐porosities in the range of 50 nm–200 µm, and iii) ordered macro‐porosities in the range of 1.5 mm–2 cm. Such a wide porosity range makes these structures promising for environmental remediation. The COF synthesis was carried out in situ via solid‐state mixing of amine and aldehyde linkers in the presence of p‐toluenesulfonic acid (PTSA) as the reaction catalyst for the imine condensation reaction. To this end, the amine linkers, i.e., benzidine (BD), 2,6‐diaminoanthraquinone (Dq), Azo, and catalyst (PTSA), were well mixed in water, followed by the sequential addition and thorough dispersion of the Tp crosslinker and GO until a brownish hydrogel was obtained. The optimized ink was then 3D‐printed into a grid‐shaped structure and subjected to thermal treatment in a sealed environment to synthesize the COF. The resulting complex was freeze‐dried, immersed in water for catalyst removal, and freeze‐dried again to produce a free‐standing, ultra‐lightweight 3D‐structured GO‐COF multi‐scale porous structure (Figure  27 ).

Figure 27.

A) Schematic demonstration of hybrid GO‐COF foams 3D printing via self‐shaping approach, where the COF precursors were added to the ink and the reaction took place after 3D printing the desired geometry. B) Chemical structure of the formed COF species. C) i‐ii: SEM images from the GO‐COF grid‐shaped foam with pore size and a print resolution of about 1.5 mm and 0.7 mm, respectively; iii‐iv: digital images of the 3D printed GO‐COF foam grids. D) The precursors used to synthesize the COF. E) TpBD COF's space‐filled model. F) i: 3D view of the 3D printed TpBD COF's foam with macro‐porosities derived from the generated hierarchical structure; ii: 3D printed TpBD COF's foam SEM image; iii: graphical view of the macroscopic GO‐COF foam. G) i: A view of the free‐standing GO‐COF foam and ii: schematic illustration of the grid‐shaped GO‐COF foam generated via self‐shaping after the 3D printing of the scaffold. Reprinted with permission.^{[ 22a ]} Copyright 2020, American Chemical Society.

In an interesting attempt by Zhang et al.,^{[ 23 ]} a versatile approach was developed to integrate COFs with imine and/or β‐ketoenamine linkages into hierarchically porous 3D‐printed constructs containing single or multiple COF components. The method employs enhanced DIW with hierarchical co‐assembly and dual‐staged network reorganization after 3D printing. For this reason, Pluronic F127 was introduced as a 3D‐printing template for the co‐assembly with imine polymeric compounds within an aqueous medium. By limiting the imine polycondensation degree during COF generation, Pluronic F127 and the amorphous imine polymer co‐assembly enabled the formation of a 3D‐printed hydrogel scaffold with self‐healing and shear‐thinning properties. After removing Pluronic F127 from the construct and initiating the amorphous‐to‐crystalline transformation, various types of imine‐ and/or β‐ketoenamine‐based COF species were developed (Figure  28A–C).

Figure 28.

A) Fabrication of 3D printed TpPa‐1 COF using Pluronic F127 template‐assisted co‐assembly, followed by framework reorganization after printing; the inset shows the robust COF porous structure capable of withstanding higher loadings (100 g) considering its lightness (32 mg). Manufacturing of 3D printed porous structure of B) 3D‐TpBD‐Me₂ and C) 3D‐TPE‐COF. D) Dual ink 3D‐printing toward generating heterogeneous COF constructs composed of 3D‐TpPa‐1/TpBD‐Me₂ and/or 3D‐TpPa‐1/TPE‐COF; the inset shows the proper mechanical integrity of the 3D‐printed porous structure upon withstanding considerably higher loadings than its weight. E) SEM image from the interface between the 3D‐TpPa‐1 and 3D‐TpBD‐Me₂ within the developed heterogeneous 3D‐printed structure. Reprinted with Permission.^{[ 23 ]} Copyright 2019, American Chemical Society.

The resulting porous constructs possessed an exceptional specific surface area, a hierarchically porous macroscopic structure, high structural integrity, and mechanical robustness. Interestingly, this approach enabled the printing of different COF precursor inks, resulting in dual‐component heterogeneous 3D‐structured COF constructs with high printing resolution (Figure 28D,E). The manufactured 3D‐structured COFs retained the same degree of crystallinity as neat COF powders while exhibiting improved handleability and macroscopic mechanical integrity when assembled into a hierarchical 3D‐structured framework. These developed porous structures also potentially enhance the mass transportation rate for applications in separation, catalysis, and molecular storage.

4. Functional MOF or COF Macrostructures

Embedding MOF and COF species into macroscopic, mechanically robust, multi‐scale porous structures holds significant potential for transitioning these materials from lab scale to active use, addressing a wide range of critical demands. This advantage stems from the large specific surface area of MOFs and COFs, which, in synergy with macroscopic scaffolds, creates a hierarchical porous structure with numerous accessible active binding sites. These features enhance mass transfer and diffusion of reactants, resulting in high adsorption capacities and efficient pollutant removal. Given these potentials, we explore the applications and functionality of MOF and COF porous structures in various societal demands, including pollutant removal, environmental remediation, carbon capture, atmospheric water harvesting, and mitigating the hazardous effects of electromagnetic (EM) waves through lossy MOF‐based aerogels or porous structures.

4.1. MOF or COF Porous Structures toward Environmental Remediation

The growing industrial activities and rapid expansion of human civilization led to massive disposal of pollutants into the ecosystem and water streams, causing substantial challenges and serious public health issues.^{[ 122 ]} Among these challenges, access to clean and safe drinking water, plus the removal of hazardous contaminants, has become an essential need for public health and environmental sustainability.^{[ 123 ]} To this end, porous structured adsorbents like metal oxides,^{[ 124 ]} carbon‐based materials,^{[ 125 ]} zeolites,^{[ 126 ]} natural minerals,^{[ 127 ]} MOFs,^{[ 128 ]} and COFs^{[ 129 ]} have shown significant potential in water treatment applications. MOFs and COFs stand out due to their unique structural properties, including a high specific surface area, abundant binding sites, controllable pore sizes, and recyclability.^{[ 14a ]} These properties allow them to adsorb a wide range of contaminants effectively, including heavy metals and organic pollutants.

Interestingly, integrating MOFs or COFs into macroscopic hierarchical porous structures promotes their mechanical properties and performance, allowing them to act far beyond what is expected from their constituent MOF or COF species.^{[ 17} , ^{110 ]} It also enables controlling the morphological features of the porous structure in the form of a membrane and/or aerogel to meet the requirements of various environmental and industrial applications.^{[ 54} , ^{130 ]} This offers effective solutions for pollution control and resource recovery, highly improving the practicality and handleability of MOFs and COFs while embedded in macroscopic structures.^{[ 17 ]} Generally speaking, adsorption involves the attachment of contaminants onto the surface of an adsorbent through various host‐guest interactions, including electrostatic forces, π–π interactions, coordination interactions, hydrogen bonding, and van der Waals forces. The type of interaction between the pollutant and the adsorbent is highly dependent on the surface charge of the adsorbent, the pH of the solution, and the charge state of the pollutant in that medium.^{[ 131 ]} Hence, besides MOF or COF porous structures’ manufacturing, the optimization of governing adsorption parameters and the engineering of surface functionalities are crucial to maximizing pollutant removal.

MOF and COF species embedded in supporting aerogel scaffolds have been extensively used for environmental remediation. In this regard, MOFs integrated with cellulose scaffolds (CelloMOFs) in the form of foam and aerogel have been introduced to remove organic pollutants from aqueous media. These composite materials exhibit exceptional mechanochemical strength and high adsorption efficacy, coupled with rapid response times. CelloMOFs are frequently utilized as adsorbents for a diverse range of compounds, such as iodine, volatile organic compounds (VOCs), phenolic compounds, pharmaceuticals, antibiotics, and other industrial pollutants, effectively purifying water.^{[ 132 ]}

Bimetal Hofmann‐type MOFs (Co–Fe)^II(pz)[Ni^II(CN)₄] and cellulose aerogels were fabricated through in situ growth (CoFe@CA‐IS) and doping method (CoFe@CA‐D) to develop efficient materials for iodine capture.^{[ 133 ]} The in situ method involved immersing cellulose aerogels in a water‐methanol mixture containing Co(NO₃)₂⋅6H₂O, Fe(ClO₄)₂⋅xH₂O, and pyrazine for 4 h, followed by immersion in K₂[Ni(CN)₄]⋅xH₂O solution for another 4 h. The aerogels were then washed with water and methanol several times and dried at RT, resulting in the hybrid cellulose‐MOF aerogel. The doping method involved converting a neutral gel into a uniform gel fluid, mixing in (Co–Fe)II(pz)[NiII(CN)₄], stirring the mixture thoroughly, and finally dropping the mixture into an orifice plate. After freezing at −20 °C for 2 h, the mixture was freeze‐dried to obtain the hybrid cellulose‐MOF‐based aerogel. The doping method notably increased the loading of (Co–Fe)^II(pz)[Ni^II(CN)₄] from 13.56% (for the in situ method) to 45.80%, significantly enhancing the adsorption performance. The porosity of aerogels prepared with in situ and doping methods reached 86.7% and 87.8%, respectively, compared to 45.7% for pure cellulose aerogels. This increased porosity contributed to the superior adsorption capacities observed in these materials when coupled with MOFs.

In terms of iodine removal, the maximum adsorption capacity for CoFe@CA‐D was 457.99 mg g⁻¹, substantially higher than the 194.34 mg g⁻¹ achieved by CoFe@CA‐IS. This enhanced capacity in CoFe@CA‐D is attributed to the higher MOF loading and better dispersion within the aerogel matrix provided by the doping method. The adsorption process for the pure cellulose aerogel followed pseudo‐first‐order kinetics, whereas the hybrid aerogels exhibited pseudo‐second‐order kinetics, indicating chemisorption as the primary adsorption mechanism. The equilibrium adsorption values for CoFe@CA‐IS and CoFe@CA‐D closely matched the values predicted by the pseudo‐second‐order model, showing the reliability and efficiency of these hybrid aerogels. Additionally, both CoFe@CA‐IS and CoFe@CA‐D displayed excellent reusability. After five adsorption–desorption cycles, CoFe@CA‐IS retained 81% of its initial adsorption capacity, while CoFe@CA‐D retained 91%. This high level of reusability, combined with the high adsorption capacities, makes these hybrid aerogels promising candidates for real‐world applications in managing nuclear‐related iodine pollution.^{[ 133 ]}

Apart from MOF‐based aerogels, COF‐based aerogels, and porous structures also demonstrate efficient potential in iodine removal applications, offering high surface area, tunable pore sizes, and chemical stability. Wang et al.^{[ 47 ]} synthesized chitosan (CS)‐COF aerogels and evaluated their performance for iodine adsorption (Figure S14A,B, Supporting Information). These aerogels were prepared using the freeze‐drying method, resulting in COF nanoparticles tightly attached to the CS network, thereby creating a stable 3D porous structure. Figure S14A (Supporting Information) shows the digital photograph of the CS‐COF aerogel, which maintains the color of pure COF nanoparticles, indicating uniform distribution. The aerogel's density of 12.4 mg cm⁻³ allows it to stand steadily on green bristlegrass, demonstrating its remarkable lightness. Morphological studies revealed that the pure CS aerogel had a smooth surface and an interlaced sheet‐like morphology. In contrast, the CS–COF (30 wt%) aerogel exhibited a hierarchically porous structure with uniformly distributed COF nanoparticles, enhancing the aerogel's stability and adsorption capacity.

Figure S14C (Supporting Information) illustrates the possible adsorption mechanisms of iodine on the CS–COF aerogels. As demonstrated, the imine groups and benzene rings in the COF structure interact strongly with iodine molecules, while the amino groups in the chitosan contribute to the adsorption process. The combination of these interactions and the aerogel's porous structure allows for effective and efficient iodine capture. Accordingly, the as‐prepared aerogels showcased exceptional iodine vapor adsorption capacity, achieved 5.62 g g⁻¹ within 84 h (Figure S14D,E, Supporting Information). The dynamic filtration tests demonstrate the aerogel's capability to remove iodine from solution rapidly, with a removal efficiency of 97.7%. This excellent performance is attributed to the synergistic effect of COF and CS. The COF provides a high surface area and active sites, while the CS network offers structural support and additional active sites for iodine adsorption.

COFs can also be used in cellulose‐based scaffolds for environmental applications. Zhang et al. proposed encapsulating COFs in cellulose‐based aerogels to capture and remove iodine from the environment.^{[ 40 ]} The synthesis of these hybrid aerogels involves introducing COF‐LZU1 particles into the sol–gel process of functionalized cellulose nanocrystals (CNCs) and carboxymethyl cellulose (CMC). This method enabled the creation of structurally stable and ultralight aerogels with high porosity, which is crucial for effective iodine adsorption. The aerogels demonstrated an exceptional iodine capture ability, with an uptake capacity of 399 mg g⁻¹ for dissolved iodine and 6.8 g g⁻¹ for volatilized iodine. Furthermore, the adsorption kinetics follow a pseudo‐second‐order model, indicating chemisorption as the dominant process. The equilibrium adsorption capacity was positively correlated with the initial iodine concentration, reaching an equilibrium uptake and removal efficiency of 399 mg g⁻¹ and 92.7%, respectively, at an initial iodine concentration of 1400 mg L⁻¹. Dynamic iodine uptake tests also highlighted the practical applications of these hybrid aerogels. When shaped into columns, the aerogels could effectively remove iodine from both liquid and vapor phases (Figure S14F–I, Supporting Information). For instance, a column with 50 mg of hybrid aerogel removed 90% of iodine from a 100 mg L n‐hexane solution at a flow rate of 0.5 mL min⁻¹. Similarly, the aerogel columns showed a removal efficiency of over 80% after filtering 30 mL of solution. For iodine vapor, the aerogels maintained high efficiency even after 9 h of continuous operation, reducing the iodine concentration to 7.8 µg mL⁻¹.

Tan et al.^{[ 24b ]} developed a novel spiderweb‐like MOF‐embedded multifunctional foam. This innovative approach effectively addressed the prevalent issue of MOF aggregation and allowed for precise control over the MOF's loading, composition, spatial distribution, and confinement within bio‐originated macroscopic supports. The method utilized ensured the dispersion of individual MOF nanoparticles within a spiderweb‐like network in each macro‐void, maintaining high loadings up to 86 wt%. This configuration provided highly accessible foam pores, enhancing the material's adsorption and catalytic capacities. The resultant MOF foams demonstrated superior adsorption capacity and rapid adsorption kinetics due to their hierarchical porosity and large specific surface area. For instance, the ZIF‐8 foams showed a maximum equilibrium adsorption capacity (q_e) of 83.3 mg g⁻¹ and an adsorption rate constant (k₂) of 0.036 g mg⁻¹ h⁻¹ for Rhodamine B dye, which was significantly higher than those of bulk ZIF‐8 powders. The foam's spiderweb‐like structure facilitated rapid dye removal from aqueous solutions, effectively decolorizing the water. This high adsorption performance is attributed to the well‐dispersed ZIF‐8 nanoparticles and the interconnected porous network within the foam, providing ample active sites for dye molecules to attach.

Another study developed ultralight and robust COF fiber aerogels (FAGs) for oil absorption, utilizing an epitaxial growth synergistic assembly (EGSA) strategy to enhance their structural properties and functional performance.^{[ 75 ]} The synthesis process involved dissolving urea‐based linkers and PAN in DMF, followed by electrospinning to form fibrous membranes, which were then subjected to various treatments to form COF FAGs. Figure  29A–C illustrates the resultant ultralight TpPa‐1 FAG, showcasing its intricate structure of tangled hollow microfibers and dense nanofibers. These structural features contribute to its exceptional porosity and low density. The novelty of this work lies in the integration of COF materials into fiber aerogels, achieving a unique hierarchical hydrophobic structure that significantly enhances oil absorption capabilities. Interestingly, the integration of COF species into the fibrous aerogel form significantly improved the adsorption capacities when compared with neat COFs (Figure 29D,E). The hierarchical porosity of the COF FAGs also allowed solvents to be absorbed not only by large interstitial pores but also by the hollow fibers and COF frameworks themselves, as depicted in Figure 29F. This comprehensive design facilitates efficient capillary action and solvent retention, making these COF FAGs highly effective in oil absorption applications.

Figure 29.

A) Digital photograph of TpPa‐1 filamentous aerogel on light dandelion hairs, and B,C) FESEM images from its porous filamentous framework. D) Oil absorption capacity of filamentous COF aerogels, E) comparing the absorption capability of filamentous COF aerogels with their constituent COF species. F) Absorption of organic solvent into the porosities of COF aerogels. Reprinted with Permission.^{[ 75 ]} Copyright 2024, Wiley‐VCH.

Graphene‐based MOF or COF aerogels and porous constructs represent another class of structures used in environmental remediation. A study by Li et al.^{[ 58 ]} presents the development and application of rGO‐COF aerogels for the efficient absorption of oils and organic pollutants from water. The aerogels were synthesized through a hydrothermal approach, which allows the COFs to grow in situ along the surface of 2D graphene sheets, forming a self‐shaped 3D hierarchical porous structure after freeze‐drying. This unique structure provides the aerogels with ultralow density, excellent mechanical strength, and high surface area, making them highly effective for environmental remediation. The aerogels demonstrated selective absorption of different types of solvents and oils, such as dyed silicone oil and chloroform from water, with absorption capacities ranging from 98 to 240 times their own weight, depending on the solvent. The recyclability of the rGO‐COF aerogel was also tested, maintaining above 87% absorption capacity after 20 cycles of ethanol absorption and drying. These results revealed the potential of rGO‐COF aerogels for practical and efficient oil spill clean‐ups and for removing organic pollutants from water resources.

Another similar investigation focused on the development of thin rGO‐COF aerogels designed for enhanced organic pollutant removal.^{[ 59 ]} The synthesis process involved a green, hydrothermal method where COFs were grown on a graphene template at 120 °C, resulting in an ultrathin COF layer approximately 2 nm thick. This hybrid aerogel combines the large active surface area and porous structure of COFs with the 2D graphene flakes, leading to improved adsorption performance. The adsorption kinetics and isotherms, particularly for dye pollutants, indicate the remarkable efficiency of the rGO‐COF aerogel. The rGO‐COF hybrid demonstrated rapid adsorption kinetics, achieving over 90% removal efficiency for methylene blue (MB), crystal violet (CV), and rhodamine B (RhB) within just 1 min. The maximum adsorption capacities recorded were 334 mg g⁻¹ for MB, 328 mg g⁻¹ for CV, and 368 mg g⁻¹ for RhB, significantly higher than those of COF powder and rGO alone. Additionally, the rGO‐COF aerogel maintained its high removal efficiency across a range of pH conditions and displayed excellent reusability, retaining its performance over at least five adsorption–desorption cycles.

Another study involved the preparation of COF‐based aerogels through a simple three‐step method involving sol–gel transition, solvent exchange, and supercritical CO₂ drying.^{[ 53 ]} This process resulted in ultralight, highly porous aerogels with a sponge‐like architecture, constructed from interconnected fiber‐like structures of COF nanosheets. The COF aerogels maintained the micro‐ and meso‐porosity of the COF constituents, exhibiting extremely low densities (only about three times the density of air) and high structural integrity. Moreover, the COF aerogels exhibited high removal efficiencies (around 99%) and excellent reusability, retaining their adsorption capacity over 10 consecutive adsorption–desorption cycles. From the mechanical integrity point of view, the COF aerogels showed acceptable elasticity and robustness. They maintained their structural integrity under compressive stress and exhibited elastic behavior up to 25%–35% strain, transitioning to plastic deformation without failure at higher strains.

Similar to COFs, MOFs integrated with carbonaceous materials can be used for organic pollutant absorption. In this regard, octylamine‐appended/reduced graphene oxide (rGO‐OctA) composite aerogels were synthesized for oil absorption applications.^{[ 134 ]} The unique combination of the MOF's hydrophobic properties and the mechanical stability of rGO makes these aerogels particularly suitable for environmental remediation applications, such as oil spill cleanup. The absorption performance of the rGO‐OctA aerogels revealed absorption capacities ranging from 4700 to 16 122 wt% for different organic liquids, significantly outperforming the individual components (OctA and rGO/Mg²⁺). This high absorption capacity is attributed to the synergistic effects of the hydrophobic MOF particles and the porous structure of the rGO matrix, which together provide numerous active sites for oil absorption.

Shapeable fibrous aerogels of MOFs templated with nanocellulose have been developed for rapid and large‐capacity adsorption applications.^{[ 110 ]} These MOF aerogels were manufactured by synthesizing MOF crystals on the template of TEMPO‐oxidized CNFs. The synthesis involved a process of ionic gelation, template synthesis of MOF crystals, and freeze‐drying (Figure  30A). CNFs interact ionically with metal ions, which serve as cross‐linkers to form a homogeneous fibrous hydrogel. Subsequently, MOF crystals such as ZIF‐8, HKUST‐1, and ZIF‐67 were grown around the CNF networks, resulting in fibrous MOF aerogels that maintain high porosity and low density (Figure 30B). The resultant MOF aerogels demonstrated superior adsorption capacity and rapid adsorption kinetics due to their hierarchical porosity and high specific surface area. For example, the ZIF‐8 aerogels demonstrated a significantly higher maximum iodine adsorption capacity compared to conventional MOF powders. Additionally, the resulting ZIF‐8 aerogels demonstrated effective dye adsorption efficiency, with their fibrous structure facilitating rapid dye removal from aqueous solutions. The adsorption rate constant (k₂) for ZIF‐8 aerogels with 33 wt% content was 0.036 g mg⁻¹ h⁻¹, and the equilibrium adsorption capacity (q_e) reached 83.3 mg g⁻¹.

Figure 30.

A) Schematic representation of steps in preparing fibrous CelloMOF constructs using CNF as the supporting scaffold. B) Optical and SEM images of freeze‐dried CNF‐MOF‐based aerogels. Reprinted with permission.^{[ 110 ]} Copyright 2023, American Chemical Society. C) Digital photograph of CNF‐MOF‐808‐EDTA aerogel on light cilia of a Setaria. D,E) SEM images of the CNF‐MOF‐808‐EDTA aerogels with hierarchical porosities. F) Cu (II) adsorption mechanism of CNF‐MOF‐808‐EDTA aerogel. Reprinted with Permission.^{[ 79 ]} Copyright 2021, Elsevier.

In another similar investigation on CelloMOF aerogels, a novel approach was developed to synthesize ultralight and shapeable aerogels with a hierarchical cellular architecture.^{[ 79 ]} The synthesis process involved the use of TEMPO‐oxidized CNF (TCNF) and MOF‐808‐EDTA. MOF‐808 was functionalized with ethylene diamine tetraacetic acid (EDTA) via a solvent‐assisted linker exchange method, followed by the freeze‐drying technique to create the hybrid aerogel. This method achieved high loading of chelating groups and enhanced adhesion between MOF‐808‐EDTA and TCNF, resulting in a sustainable platform for high‐performance adsorption materials. The SEM and optical images revealed that the MOF‐808‐EDTA@TCNF aerogels possess a well‐defined and interconnected hierarchical cellular architecture (Figure 30C–E). This architecture facilitates the efficient transport and capture of heavy metals such as copper ions. The ion adsorption mechanism of these MOF‐based aerogels stems from their hierarchical pores, which allow for adequate ion transportation channels and promote the rapid diffusion of ions to the interior binding sites (Figure 30F). The negatively charged surface yields electrostatic interaction with free ions, enhancing the adsorption capacity.

Melamine‐based foams integrated with COFs have shown remarkable potential in heavy metal adsorption applications. A study focused on melamine foam functionalized with COFs, specifically designed for the removal of cadmium (Cd²⁺) from water.^{[ 69 ]} The melamine foam‐COF composites demonstrated an exceptional adsorption capacity of 278.45 mg g⁻¹. The stable macro‐porous structure of the melamine foam provided extensive active sites and rapid ion diffusion channels, enabling the aerogel to reach adsorption equilibrium within just 1–3 s. This rapid and efficient removal is crucial for real‐time water purification applications.

Furthermore, polyimide‐based aerogels offer substantial benefits for environmental applications, particularly in gas absorption and filtration. The inherent thermal stability and mechanical robustness of polyimide materials make them ideal candidates for harsh environmental conditions. When integrated with COFs, polyimide aerogels demonstrate enhanced sorption capacities for various pollutants. A study by Hu et al.^{[ 130 ]} focused on the use of polyimide‐COF aerogels for the adsorption of volatile organic compounds (VOCs) in the gas state. The hybrid structure of polyimide and COFs provides a synergistic effect, combining the structural integrity of polyimides with the high surface area and porosity of COFs, resulting in a material that is both durable and highly effective in pollutant removal.

In addition to adsorption applications, MOF‐ and COF‐based porous structures have been explored for catalytic applications. Palladium‐loaded COF (Pd@COF‐QA) aerogels served as an efficient phase transfer catalyst for the Suzuki–Miyaura coupling reaction in water.^{[ 135 ]} The COF‐based catalyst demonstrated exceptional catalytic activity, achieving a 99% yield for the coupling reaction under mild conditions (50 °C). The Pd@COF‐QA catalyst maintained its performance over multiple catalytic cycles with minimal loss in activity. This proved another potential of COF‐based porous constructs as catalysts in green chemistry applications. Water splitting, a critical process for hydrogen production, has also benefited from the development of COF‐based aerogels. A study on graphene‐COF aerogels demonstrated their effectiveness as electrocatalysts for hydrogen evolution reactions (HER) and oxygen evolution reactions (OER).^{[ 63 ]} The hybrid aerogels exhibited a low overpotential and high current density for HER, indicating efficient catalytic activity. The integration of COFs with graphene enhanced the electronic conductivity and provided abundant active sites for the catalytic reactions, resulting in improved performance. These findings suggest that graphene‐COF aerogels could play a significant role in developing sustainable and efficient hydrogen production technologies, contributing to the advancement of clean energy solutions.

COF and MOF aerogels have also shown promise in photocatalytic applications.^{[ 64 ]} In particular, due to their photocatalytic performance, these materials can be engineered to absorb a wide spectrum of light wavelengths, from UV to visible, by adjusting their organic ligands and metal centers. This results in changes in the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels in their structures.^{[ 136 ]} Furthermore, MOFs and COFs promote efficient separation and migration of photogenerated electron‐hole pairs, facilitated by their high surface areas and active sites, which minimizes the recombination rate. The catalytic performance is often driven by redox‐active sites, which can be fine‐tuned by selecting appropriate metal centers in MOFs or functional groups in COFs.^{[ 137 ]}

When exposed to visible or UV light, COFs and MOFs absorb photons, creating electron‐hole pairs. These charges move to active sites where redox reactions occur, enabling processes such as pollutant degradation, organic transformations, and energy conversion. The creation of hybrid composites by combining COFs with MOFs or other materials results in heterostructures that further improve photocatalytic efficiency.^{[ 138 ]} These composites benefit from enhanced light absorption and charge separation, reducing electron‐hole recombination. By extending the absorption capacity through the integration of MOFs or COFs into porous structures and improving electron transfer during the photocatalytic reaction, these combinations amplify the photocatalytic capabilities of MOFs and COFs, making them highly effective under visible and UV illumination.^{[ 139 ]}

A study investigated TiO₂@Cd‐MOF nanocomposite aerogels to evaluate their performance on the photocatalytic degradation of methyl orange as an organic dye.^{[ 140 ]} The aerogels demonstrated high efficiency in degrading methyl orange dye, achieving over 94.1% degradation within 90 min of exposure to light. This high photocatalytic activity is attributed to the synergistic effect of the MOF's high surface area and the efficient relocation of photo‐induced electrons, which reduces recombination rates of electron‐hole pairs and enhances the removal performance during the photocatalytic process.

Although the advantages of MOF/COF‐based porous structures toward environmental remediation are clearly articulated, it is also essential to contextualize their performance in comparison with more conventional materials commonly used in water purification applications, such as activated carbon, zeolites, and metal oxides. These traditional materials are valued for their wide availability, low cost, and relatively straightforward processing.^{[ 141 ]} Activated carbon, for example, has long been considered the benchmark in adsorption‐based water treatment due to its relatively high surface area and ability to remove a wide variety of contaminants.^{[ 142 ]} For instance, granular activated carbon (GAC) achieved only 43% removal of standard polypropylene microplastics after 7 h with a 0.5 g L⁻¹ dose and up to 90% removal when the dosage was increased to 1.5 g L⁻¹.^{[ 143 ]} The adsorption followed a pseudo‐first‐order kinetic model and was primarily driven by weak physical interactions, with FTIR analysis showing no significant chemical bonding to the GAC surface, which reflects a relatively low interaction specificity.

Zeolites such as clinoptilolite, mordenite, and synthetic zeolite 4A have been widely used for their ion‐exchange properties, particularly for removing ammonium from water.^{[ 144 ]} In synthetic solutions, these materials showed effective ammonia nitrogen removal, with efficiency reaching over ∼80% in ideal conditions. However, their performance declined significantly in real water systems, such as the Yamuna River water and treated sewage effluents, due to competition with other ions. For example, clinoptilolite's ammonia removal dropped to ∼37.5% in municipal effluent water, demonstrating a strong dependency on water matrix composition and limiting its practical utility in complex real‐world conditions.

Similarly, metal oxides such as TiO₂, ZnO, and NiO are known photocatalysts and have been extensively studied for degrading dyes and toxic compounds.^{[ 145 ]} In their unmodified forms, they primarily absorb in the UV region, which limits efficiency under natural light. Efforts to improve their photocatalytic performance through the integration of silver nanoparticles have shown only moderate enhancements.^{[ 146 ]} In one comparative study, Ag‐doped TiO₂ showed the highest activity, while NiO/Ag composites exhibited weaker photocatalytic degradation for dye pollutants like methyl orange, achieving degradation efficiencies of around 86% under visible light after 3 h of irradiation.^{[ 147 ]} The performance was attributed to surface plasmon resonance effects and better charge separation, yet it fell short compared to newer materials like MOF/COF composites, particularly in terms of degradation rate and selectivity.

In contrast, MOF and COF aerogels present a unique set of advantages. They not only possess exceptionally high surface areas and accessible hierarchical porosity but their frameworks can also be readily modified to introduce task‐specific functional groups that enable selective adsorption and degradation of a wide variety of contaminants.^{[ 148 ]} For instance, while activated carbon typically relies on non‐specific interactions for adsorption, MOFs and COFs can be designed to include binding sites that interact specifically with target molecules, including pharmaceuticals, heavy metals, and organic micropollutants, often at trace concentrations.^{[ 149 ]} Furthermore, the integration of these frameworks into aerogel form significantly improves mass transfer and enhances both adsorption kinetics and capacity, overcoming one of the key limitations seen in conventional porous powders.^{[ 150 ]} From a scalability standpoint, recent advances in low‐temperature synthesis, green chemistry approaches, and gelation‐based structuring methods have made MOF/COF aerogels increasingly viable for industrial applications.^{[ 150} , ^{151 ]} Although their production costs remain somewhat higher than those of activated carbon or zeolites, the superior performance and tunability of MOF/COF aerogels offer long‐term value, particularly in scenarios where high selectivity and multifunctionality are needed. As such, their development marks a significant advancement in the design of next‐generation materials for sustainable water treatment technologies.

All in all, the development and application of MOF‐ and COF‐based aerogels have opened new avenues in environmental remediation. These materials offer unique properties such as high porosity, high surface area, and tunable functionality, making them highly effective for applications like adsorption, photocatalysis, and catalysis.^{[ 152 ]} MOF and COF aerogels can be engineered to possess specific characteristics that enhance their performance in these applications, as previously mentioned. Despite these promising developments, there are still challenges that need to be addressed to fully realize the potential of MOF and COF aerogels in environmental applications. One of the primary challenges is the scalability of the synthesis processes, which often require precise control over reaction conditions and the use of expensive reagents. Additionally, the stability and mechanical integrity of these materials, especially in harsh environmental conditions, require further improvement to ensure their long‐term performance. Future research should focus on developing more cost‐effective and scalable synthesis methods, as well as enhancing the structural stability of MOF and COF aerogels. By addressing these challenges, MOF and COF aerogels can become more viable alternatives to traditional materials, offering sustainable and efficient solutions for environmental remediation and beyond.

4.2. MOF or COF Porous Structures toward CO₂ Capturing

Since the Industrial Revolution, the increasing emission of carbon dioxide (CO₂) into the atmosphere due to human activity has been adversely affecting the Earth's temperature through the greenhouse effect. MOFs and COFs, known for their high porosity, have demonstrated excellent CO₂ adsorption capacity and affinity, attributed to their organic linkers and overall structure. However, for effective CO₂ capture in industrial and practical applications, it is necessary to embed MOFs or COFs into durable and recyclable porous macroscopic structures with adequate mechanical stability. CO₂ capture and reduction using MOF or COF porous structures typically occur through two main approaches: i) direct CO₂ adsorption by capturing it on active sites within the MOFs and COFs, and ii) CO₂ capture followed by its transformation into value‐added products.^{[ 22} , ⁴² , ⁵⁴ , ^{74 ]}

In this regard, several attempts have been made to investigate the CO₂ uptake capacity of MOF or COF hierarchically porous aerogels.^{[ 22} , ⁴² , ^{54 ]} By employing the self‐shaping aerogel manufacturing approach, a study developed porous, free‐standing aerogels and subsequently transformed these aerogels into membranes by compressing crushed aerogel pieces. Various types of imine‐based COFs were used for CO₂ capture.^{[ 54 ]} The developed aerogel or membranes were based on TAPB‐BTCA, PPDA‐BTCA, and TAPB‐PDA COF species, in which their aerogel and membrane forms showcased proper structural integrity and mechanical performance. This promotes the practicality and handleability of porous structures for real‐world applications. Interestingly, the COF aerogel showcased remarkable CO₂ adsorption, where the highest value of 22.6 mmol g⁻¹ was recorded for the TAPB‐PDA COF aerogel (Figure  31A). However, as the membranes were produced from the compressed aerogel pieces, they showcased far lower CO₂ adsorption capacity (5.40 mmol g⁻¹ for TAPB‐PDA COF membrane, Figure 31B). This decline in practicality was due to the reduced porosity of the membrane compared to the corresponding aerogel. For instance, in the case of TAPB‐PDA COF aerogel and membrane, the BET‐specific surface area decreased from 2150 to 170 m² g⁻¹ when the aerogel was transformed into the membrane. This reduction was primarily due to the gate‐closing effect, highlighting the adverse impact of compression on the active specific surface area of COF aerogels.

Figure 31.

CO₂ adsorption performance of A) aerogel (black‐colored) and B) membrane (red‐colored) of various types of imine‐based COFs, including TAPB‐BTCA, PPDA‐BTCA, and TAPB‐PDA COFs, showcased by line with sphere, square, and triangle symbols, respectively; all measurements were taken at 200 k at 1 bar. Reprinted with Permission.^{[ 54 ]} Copyright 2022, Wiley‐VCH under Creative Commons CC BY license. Adsorption of CO₂, N₂, and CH₄ gases via the chitosan‐COF‐IL aerogel at temperatures of C) 273 K and D) 298 K. E) Using the reactor as the mold for the preparation of chitosan‐COF‐IL aerogel, enabling the scaling up of the CO₂ cycloaddition process. Reprinted with Permission.^{[ 42 ]} Copyright 2019, Royal Society of Chemistry. F) CO₂ adsorption capacity of neat aramid nanofiber, HKUST‐1 MOF, and MOF aerogels. G) i‐ii: Center of mass (COM) probability distribution illustrations, showing the density of carbon dioxide in the MOF aerogel at 10 kPa from various directions; colors show: grey (carbon), blue (nitrogen), white (hydrogen), red (oxygen), and orange (copper). Reprinted with Permission.^{[ 22c ]} Copyright 2023, Elsevier.

Another attempt involved employing the mechanically robust chitosan‐COF‐IL aerogel prepared via a direct mixing approach, in which the chitosan was used as a robust supporting scaffold.^{[ 42 ]} The chitosan‐COF‐IL aerogel was formed through the chemical crosslinking of thiol chitosan with COF‐IL via a thiol‐ene photoinitiated reaction. This process enabled high COF loadings of up to 80 wt% within the macroscopic aerogel framework. This provides numerous sites for strong and selective CO₂‐capturing, owing to the excellent affinity of COF‐IL to CO₂. In this case, both the neat crystalline powder and high‐COF loading aerogels demonstrated strong CO₂ adsorption, showing a greater affinity for CO₂ compared to interfering gases such as CH₄ and N₂. The neat COF‐IL showed CO₂ adsorption of 106.04 and 59.37 cm³ g⁻¹ at temperatures of 273 and 298 K (pressure of 1 atm), respectively. In comparison, the chitosan‐COF‐IL (80 wt%) aerogel exhibited CO₂ adsorption values of 38.77 and 25.83 cm³ g⁻¹ at the same respective temperatures (Figure 31C,D). The as‐prepared aerogel also revealed high‐yield CO₂ cycloaddition capacity through a scalable and recyclable approach. The CO₂ cycloaddition on styrene oxide, as an aromatic compound, was carried out under mild conditions (1 atm pressure) without the presence of a co‐catalyst. The outcomes indicated a meaningful correlation between the catalytic performance of COF‐IL and reaction temperature, reaction time, and amount of catalyst. Increasing the reaction temperature from 25 to 80 °C enhanced the reaction yield from 4% to 97% after 120 h (1.5 mol% COF‐IL) in solvent‐free situations. Moreover, a high catalytic loading of 3 mol% slightly increased the yield. Under optimized reaction conditions, a reaction time of 48 h was found to be effective in achieving a yield of approximately 98%. The nature of the aerogel manufacturing approach enabled the fabrication of the chitosan‐COF‐IL aerogel in a reactor, facilitating the easy scaling up of the CO₂ cycloaddition process. (Figure 31E).

The in situ decorated HKUST‐1 MOF on the aramid nanofiber, as the supporting scaffold, was also employed as an effective hierarchically porous MOF aerogel for excellent CO₂ adsorption.^{[ 22c ]} The manufactured ultralightweight aramid nanofiber‐HKUST‐1 MOF aerogel possessed a high specific surface area and numerous active sites with a high affinity toward CO₂ capturing. Accordingly, the meso‐ and macro‐porosities of the HKUST‐1 act as CO₂ transport channels, allowing the CO₂ to be captured on numerous micro‐porous active sites upon entry into the interior of the MOF. The CO₂ capturing assessment at a pressure of 0–1 bar and temperature of 0 °C for neat aramid nanofiber aerogel and crystalline powder of HKUST‐1 MOF showed a CO₂ adsorption capacity of 0.96 and 3.99 mmol g⁻¹, respectively. Surprisingly, upon in situ HKUST‐1 MOF growth on aramid nanofiber aerogel, reaching a loading of 72%, the CO₂ adsorption capacity increased to 8.17 mmol g⁻¹, significantly surpassing that of the neat powder (Figure 31F). It also showed far better affinity toward CO₂ than other gases, with an adsorption selectivity of 42 and 39 for CO₂/O₂ and CO₂/N₂, respectively. This significant enhancement is rooted in the synergistic effect of the MOF with the macroscopic structure, which increases the accessibility of the MOF micro‐pores for CO₂ capture. The simulation of the center of mass (COM) also highlighted the role of micro‐porosities of HKUST‐1 in effective CO₂ capturing (Figure 31G).

Using MOF‐embedded porous structures is a promising strategy to adsorb and transform CO₂ into value‐added products. Toward this aim, a study investigated fabricating MOF‐embedded aerogels for CO₂ cycloaddition in the liquid phase along with electrochemical oxygen evolution reaction (OER).^{[ 74 ]} The aerogel manufacturing process involved pre‐seeding a CMC aerogel by adding the organic ligand, followed by the controlled spraying of an ion solution to facilitate the in situ nucleation growth of MOFs, such as CuBDC or CoNiBDC, throughout the outer wall of the aerogel scaffold. The in situ growth of MOFs on aerogel scaffolds not only made the MOF porosities more accessible to target reactants but also promoted the mass transfer and catalytic activity of the porous microreactor. The hierarchical porous aerogel based on CMC‐CuBDC showed promising potential as a microreactor for CO₂ cycloaddition to various types of epoxides, yielding cyclic carbonates. (Figure  32A). As demonstrated in Figure 32B, in situ growth of CuBDC on the CMC aerogel scaffold can promote the phenyl cyclic carbonate yield to about 91.5% with CuBDC loading of about 9.1 wt%. This method is 1.6 times more effective than the neat CuBDC nanosheets under the same reaction parameters. Notably, the recovery and reusability assay (Figure 32C) through simple washing and drying steps showed that the CuBDC‐embedded aerogels could retain 96% of their performance toward the CO₂ cycloaddition reaction. In contrast, the catalytic performance of CuBDC nanosheets dropped to 77% under the same conditions. This outcome suggests the superior performance of MOF aerogels compared to their neat powder form, which could be attributed to the reduction in the number of Cu active sites caused by the agglomeration of the MOF powder during the catalytic reaction. Embedding the MOF on the accessible outer wall of the aerogel prevents such adverse effects, maintaining its catalytic performance at an optimal level after several reuse cycles. Additionally, the system showed promising potential for electrochemical oxygen evolution reactions (OER) when CoNiBDC MOF was synthesized in situ on the CMC aerogel scaffold. Upon carbonization of the MOF aerogel, the electrical conductivity of the scaffold improved, making it a potential candidate for CO₂ electrochemical reactions by combining conductivity with efficient mass transfer within the multi‐scale porous aerogel. These results clearly highlight the potential of macroscopic hierarchically porous scaffolds in enhancing the catalytic potential of MOFs beyond what can be achieved with their neat powders.

Figure 32.

A) Schematic demonstration of the catalytic performance of the CMC‐CuBDC microreactor toward cycloaddition of CO₂ with epoxide to generate cyclic carbonate compounds. B) The product yield of the microreactor compared with CuBDC nanosheets toward cyclic carbonates. C) Successive generation of phenyl cyclic carbonate after multiple washing‐drying cycles using the microreactor compared with CuBDC nanosheets. Reprinted with Permission.^{[ 74 ]} Copyright 2021, Wiley‐VCH.

4.3. MOF or COF Porous Structures toward Atmospheric Water Harvesting

Combining water harvesting and solar evaporation within MOF‐ or COF‐based porous structures presents a cutting‐edge solution to address water scarcity challenges, particularly in arid and semi‐arid regions.^{[ 41} , ^{153 ]} The synergistic integration of these technologies combines the high water‐sorption capacity of porous materials with the efficient thermal management of solar‐driven processes, offering a sustainable and scalable solution for clean water production. MOFs and COFs, with their high surface area, tunable porosity, and hydrophilic functionality, can capture atmospheric moisture even under low relative humidity.^{[ 154 ]} MOF‐ or COF‐based porous structures, due to their robust structural integrity and ease of handling compared to powder forms, provide a practical medium for real‐world applications. Compared to powder forms, monolithic MOF or COF structures exhibit improved structural integrity and handling, making them more suitable for practical applications. These constructs can be further optimized by engineering pore size distribution and surface chemistry to enhance water uptake performance.^{[ 155 ]}

The process typically involves two main stages: i) nighttime water vapor adsorption (or absorption) onto the porous structure under conditions of higher humidity and lower temperature, followed by ii) daytime desorption and water release, often in liquid form.^{[ 156 ]} Desorption requires thermal energy to overcome the interactions, physisorption or chemisorption, between the water molecules and the porous network. Solar evaporation offers a renewable and effective means to supply this energy. By coupling solar collectors with MOF or COF‐based structures, solar irradiation can be locally converted into heat,^{[ 157 ]} triggering the release of adsorbed water. The integration can be further enhanced by embedding photothermal materials into the porous matrix to boost solar‐to‐thermal conversion efficiency. Common additives include plasmonic nanoparticles, carbon‐based materials, and metal oxides, which efficiently absorb sunlight and convert it into heat.^{[ 156} , ^{158 ]}

The overall efficiency of this combined water harvesting‐solar evaporation system depends on the interplay between sorption properties and heat management.^{[ 159 ]} Critical parameters include the thermal conductivity of the porous structure, which governs heat transfer and directly affects the desorption rate. Moreover, the design of the solar collection system must ensure uniform heating across the porous material to avoid thermal gradients that could compromise efficiency. Advanced systems may incorporate concentrator photovoltaics or solar evaporators to intensify solar exposure and enhance water recovery performance.^{[ 160 ]}

For instance, Li et al.^{[ 29b ]} developed a graphene‐COF dual‐region porous structure to address the limitations of high energy demand and low evaporation efficiency. This hybrid construct consists of hydrophilic COF‐loaded reduced graphene oxide (rGO‐COF) and amphiphilic reduced graphene oxide (rGO) regions, formed by the controlled deposition of sulfonic acid‐functionalized COF (COF‐SO₃H) via hydrothermal synthesis. As illustrated in Figure  33A, this dual‐region hydrogel structure enhances light absorption, water retention, and energy conversion, achieving a steam generation rate of 3.69 kg m⁻² h⁻¹ and a solar‐to‐vapor efficiency of 92% under one sun irradiation. The material enables efficient solar‐driven desalination and purification of seawater and contaminated water sources. Its design, balancing hydrophilic and hydrophobic domains, regulates channel size and wettability, significantly reducing the energy required for steam generation. This system also holds promise for broader applications, including membranes and separation devices. Nevertheless, challenges remain, particularly in thermal management, as excessive heating may degrade MOF stability or reduce operational lifespan. Addressing these limitations will require novel materials and hybrid composites with improved thermal resilience.

Figure 33.

A) Schematic demonstration of dual hydrophilic‐hydrophobic region graphene‐COF porous structure toward solar water evaporation. Reprinted with Permission.^{[ 29b ]} Copyright 2022, American Chemical Society. B) Lightweight CNF‐COF aerogel capable of water harvesting and humidity‐driven electricity generation. C) Moisture adsorption of the carboxylated CNF and deviated CNF‐COF aerogels. D) Water uptake of the CNF‐COF aerogel from the human respiration behind a mask (temperature of 25 °C and RH of 80%); the inset shows the infrared image of the human breathing behind the mask. E) I: Functional groups distribution within the CNF‐COF aerogel, II: moisture uptake by the aerogel enables aerogel's ionization and release of the charged ions, i.e., Na⁺ and H⁺, III: migration of charged ions to each side and subsequent charge separation. Reprinted with Permission.^{[ 41 ]} Copyright 2024, American Chemical Society.

In addition to performance, these systems offer substantial environmental benefits. By utilizing solar energy and ambient humidity, they reduce reliance on conventional freshwater sources and lower the carbon footprint associated with water production. For instance, a recent innovation involved embedding proton‐conductive COF‐2SO₃H into a carboxylated cellulose nanofiber (CNF‐C) network to create an intelligent, moisture‐sensitive hybrid aerogel (Figure 33B).^{[ 41 ]} CNF‐C acts as a dispersing and stabilizing agent, forming hierarchical porous structures with enhanced ion conductivity and water sorption capacity. This hybrid aerogel demonstrated approximately 1.6 times greater water uptake compared to CNF‐C alone and generated a stable output voltage of ∼0.55 V for at least 5 h under ambient conditions (Figure 33C–E). The aerogel also functioned effectively as a solar‐driven energy harvester and sensor. When integrated into a wearable mask, it exhibited heightened sensitivity to human respiration, producing a peak voltage of ∼1.0 V and self‐charging within 3 min.

Ongoing research continues to enhance the water uptake capacity, adsorption/desorption kinetics, and energy efficiency of such hybrid systems. Future developments may focus on designing advanced MOF/COF composites to improve multifunctional performance. By harnessing the unique physicochemical properties of porous materials and the renewable energy of sunlight, this technology provides a viable and eco‐friendly path toward water harvesting in harsh, remote, or low‐humidity environments.

4.4. MOF‐based Porous Structures toward Electromagnetic Shielding

Electromagnetic interference (EMI) shielding is indispensable in contemporary technology due to the widespread deployment of electronic devices, which are susceptible to disruptions, malfunctions, or failures caused by EMI. Robust EMI shielding is crucial for preserving the integrity and functionality of electronic systems, particularly in sectors such as telecommunications, healthcare, military, and aerospace, where precision and reliability are critical.^{[ 161 ]} The primary EMI shielding mechanisms include reflection, absorption, multiple reflection, and internal scattering. Reflection occurs at the surface of conductive materials due to impedance mismatch between the shielding material and free space. Although reflection effectively prevents EMI from penetrating the shield's body, it can cause secondary pollution, as the reflected waves can interfere with nearby equipment.^{[ 162 ]}

On the contrary, absorption involves the conversion and dissipation of electromagnetic wave energy into heat within the shielding material. This mechanism primarily depends on the material's electrical conductivity, dielectric permittivity, magnetic permeability, and thickness. Multiple reflections refer to electromagnetic waves reflecting repeatedly between the front and back surfaces of a shield, thereby minimizing the EM wave's transmission and contributing to its dissipation. Internal scattering, on the other hand, involves multiple back‐and‐forth reflections of EM waves within the porosities and/or vacancies of porous shielding systems. Internal scattering further enhances absorption by prolonging the path of electromagnetic waves within the shield. This mechanism is especially prominent in materials with numerous internal interfaces, such as porous structures. The extended path increases the possibility of interactions with the material's free charges and electric/magnetic dipoles, gradually dissipating the EM wave's energy and minimizing transmission.^{[ 163 ]} For optimal EMI shielding, it is essential to minimize reflection and maximize absorption. Accordingly, the balance between these mechanisms is achieved by carefully engineering the material's structure and properties, such as electrical conductivity, thickness, and internal architecture, to enhance absorption and reduce reflection. This approach leads to absorption‐dominant shielding materials that provide superior EMI protection by converting electromagnetic waves’ energy into heat efficiently.^{[ 164 ]}

MOFs are considered one of the novel materials whose properties can be meticulously engineered based on their structure. The highly tunable nature of MOFs allows for precise manipulation of their chemical composition and porosity, enabling the design of materials tailored for specific applications, including EMI shielding.^{[ 35b ]} Embedding MOFs into porous supporting scaffolds enhances the internal scattering mechanism by creating multiple pathways for electromagnetic waves to traverse. This increased scattering facilitates more interactions between the waves and the material, leading to higher energy dissipation. As a result, MOFs can provide high dielectric and magnetic losses in the EMI shielding mechanism, promoting the dissipation of EM waves.^{[ 35} , ^{165 ]}

In this regard, hierarchical MOF‐derived/graphene hybrid aerogels were designed to evaluate their efficacy as microwave‐absorbing materials.^{[ 68a ]} These aerogels were prepared using MOF‐derived magnetic γ‐Fe₂O₃@C and GO via a scalable freeze‐drying process followed by a thermal annealing process. The interaction between MOFs and GO ensures uniform dispersion of magnetic nanoparticles within the GO‐based cell walls, enhancing Ohmic and magnetic losses after thermal annealing of the resulting aerogel. The core‐shell structure of MOF derivatives and hierarchical porosity facilitates multiple internal scatterings, significantly boosting the dissipation of EM waves. The aerogels achieved an impressive minimum reflection loss (RL) of −60.5 dB at 9.51 GHz and a broad effective absorption bandwidth (EAB) of 7.76 GHz at a mere 5 wt.% filler loading. MOF‐derived γ‐Fe₂O₃@C particles provide both dielectric and magnetic losses, essential for efficient EM wave absorption. The magnetic nanoparticles embedded in the carbonaceous medium improve impedance matching and absorption capacity. The porous structure and abundant heterogeneous interfaces in the aerogel contribute to the enhancement of dielectric losses, leading to further EM waves’ dissipation. Hence, the combination of these factors promotes the dissipation of EM waves by converting their energy into heat through multiple internal scattering and polarization mechanisms.

Another study investigated a novel approach using double‐layered nanocomposites made from ZIF‐67‐derived porous carbon (C‐ZIF67) and graphene nanoplate (GNP) films reinforced with cellulose nanofibers.^{[ 166 ]} These constructs, with a thickness of 0.1 mm, demonstrate outstanding mechanical strength (46.33 MPa) and an EMI shielding effectiveness (EMI SE) of 50.5 dB in the X‐band frequency range with an absorption coefficient of 0.87, showcasing the absorption‐dominant nature of the resulting aerogel. In this investigation, the incorporated MOFs play a pivotal role in the microwave absorption process. The C‐ZIF67 layer significantly improves impedance matching and provides extensive pathways for electromagnetic waves, enhancing internal scattering and extending interaction times. This results in an efficient “absorption–reflection–reabsorption” mechanism. The porous structure of C‐ZIF67 facilitates internal scattering and strong dielectric and magnetic losses, which are essential for converting EM waves’ energy into heat and enhancing absorption efficiency. On the other hand, the combination of the C‐ZIF67 and GNP layers minimizes reflection and maximizes absorption, rendering these composite films highly effective for EMI shielding in advanced electronic applications.

CNF‐ZIF‐67‐based aerogels were prepared through a combination of ZIF‐67 and cellulose nanofibers (CNF), followed by carbonization at various pyrolysis temperatures (Figure  34A).^{[ 30a ]} This innovative fabrication process led to a 3D interconnected network with Co/C nanoparticles embedded within the carbon framework. These aerogels exhibited high surface area, low density, and exceptional microwave absorption properties, with optimal performance observed at a pyrolysis temperature of 900 °C. At this temperature, the aerogel demonstrated an EMI SE of 35.1 dB and a specific SE of 20172.4 dB cm³ g⁻¹. The MOF‐derived Co/C significantly enhanced both dielectric and magnetic losses, which are crucial for obtaining an absorption‐dominant shielding system. As can be seen in Figure 34B, the aerogel's porous structure promoted internal scattering, leading to enhanced energy dissipation in the form of heat. To this end, the high pyrolysis temperature increased the graphitization degree and the purity of the Co nanoparticles, further improving the aerogel's dielectric and magnetic properties. This resulted in a dominant absorption mechanism, effectively minimizing the surface reflection and maximizing the dissipation of EM waves.

Figure 34.

A) Schematic illustration of Co/C@CNF aerogel preparation process and B) corresponding EMI shielding mechanisms. Reprinted with Permission.^{[ 30a ]} Copyright 2020, Elsevier. C) Optical and SEM images of MOF‐derived magnetic γ‐Fe₂O₃@C and graphene aerogels, and D) schematic demonstration of related microwave absorption mechanisms at macroscopic and microscopic scales. Reprinted with Permission.^{[ 29a ]} Copyright 2022, Springer Nature.

Huang et al. synthesized MOF‐derived magnetic γ‐Fe₂O₃@C and graphene lightweight aerogels through a scalable freeze‐drying process followed by thermal annealing (Figure 34C).^{[ 29a ]} The EM wave absorption mechanism of γ‐Fe₂O₃@C/rGO aerogels, as illustrated in Figure 34D, is attributed to the synergistic effects of impedance matching and EM wave attenuation parameters. At the macroscopic level, the 3D porous structure of the aerogels facilitates the entry of EM waves, reducing surface reflection. This structure promotes multiple random reflections and scatterings of EM waves within the microcellular free spaces, thereby lengthening the pathway of EM waves and facilitating their dissipation. At the microscopic level, the synergistic dielectric and magnetic losses are crucial for the absorption‐dominant attenuation mechanism. The incident EM waves were captured and attenuated through interaction with numerous interfaces. Various polarization mechanisms in the aerogels, including dipolar polarization due to defects and functional groups on the rGO skeleton and heterogeneous interfacial polarizations due to numerous conductive and magnetic interfaces, e.g., ferromagnetic nanoparticles, Fe₃O₄@C and Ni‐Fe₃O₄@C nanocapsules, and graphene flakes, significantly contributed to EM waves’ absorption. The interconnected and conductive structure of the aerogels facilitated Ohmic loss, while spatially dispersed ferromagnetic nanoparticles within the highly porous 3D frameworks enhanced the magnetic losses. Such synergistic effects of multiple parameters resulted in high attenuation capability and good impedance matching, leading to superior EM wave absorption performance.

Guo et al. investigated the manufacturing and EM wave absorption performance of MOF‐based aerogels, specifically focusing on a hierarchical WS₂/CoS₂@carbonized cotton fiber (CCF) structure derived from ZIF‐67 MOFs.^{[ 167 ]} The synthesis process involved anchoring ZIF‐67 nanosheets onto cotton fibers, followed by tungsten etching, sulfurization, and carbonization. This process resulted in a 3D interconnected network with WS₂ and CoS₂ nanoparticles uniformly distributed within the carbon matrix, enhancing the material's electromagnetic properties. The aerogels exhibited proper microwave dissipation capabilities, achieving a minimum reflection loss (RL) of −51.26 dB at 17.36 GHz with a thickness of 2 mm and a broad effective absorption bandwidth (EAB) of 6.72 GHz. The role of the MOF‐derived WS₂ and CoS₂ nanoparticles is crucial in enhancing the dielectric and magnetic losses. These nanoparticles improve impedance matching and facilitate multiple interfacial polarizations, dipole polarizations, and conductive losses, which are essential for effective EM waves’ attenuation.

From a mechanistic perspective, while most MOFs are not conductive and their shielding performance as individual components is inferior to the best practices in the field, they are considered promising EM wave dissipators when combined with conductive and magnetic materials. Embedding MOFs within porous shielding building blocks facilitates the dissipation of EM waves as heat through various mechanisms, including the formation of conductive microcurrent channels, dielectric, magnetic, and Ohmic losses, as well as multiple internal scatterings primarily originating from the porous host.^{[ 35} , ¹⁶⁴ , ^{165 ]} Achieving effective EMI shielding with MOF‐based materials requires enhancing electrical conductivity, either through carbonization or by incorporating conductive additives. This creates a conductive network with multiple interfaces and defects, which contribute to EM wave dissipation via the Ohmic loss mechanism.^{[ 35} , ^{165 ]} Additionally, MOFs' hybridization with heterogeneous compounds generates numerous interacting interfaces. This induces a micro‐ or nano‐capacitance effect, which facilitates the dissipation of EM waves’ energy through dielectric loss, thereby enhancing overall absorbance. Also, embedding magnetic MOFs into the shields can contribute to a stronger EM wave dissipation via magnetic losses, mainly stemming from magnetic eddy current loss and magnetic resonance. The magnetic effects also help increase magnetic permeability, contributing to improved impedance matching by enhancing the total impedance of the shielding system and reducing its mismatch with free space (377 Ω).^{[ 164} , ^{165 ]} These factors position MOFs as promising materials for enhancing EM wave dissipation within shielding structures, effectively contributing to the overall absorbance of the system.

5. Challenges and Future Prospects

This review delved into strategies to convert MOFs and COFs into handleable porous hierarchical structures using a wide array of methodologies. These methodologies include direct mixing, self‐shaping, in situ growth, template‐assisted approaches, and 3D printing for generating aerogels with controllable spatial arrangements. The developed hierarchical porous structures showcased promising potential in a wide range of crucial applications, including environmental remediation, CO₂ capture/reduction, water harvesting, EMI shielding, and beyond. Transforming MOFs or COFs into macroscopic hierarchical porous structures holds great potential for advanced applications, allowing the essence of reticular chemistry to transition into active functional uses.

The primary advantage of developing such porous structures is addressing the poor structuring capability and intrinsic rigidity of MOFs or COFs by embedding them into flexible or stiff macroscopic constructs with high structural integrity. The macroscopic construct itself enables the development of structures with engineerable porosities across scales, i.e., micro‐ (<2 nm), meso‐ (2–50 nm), and macro‐porosities (>50 nm), allowing porosity tuning from the molecular level and micro‐scale up to the macroscopic level. Such control over porosity was not possible with traditional approaches, leading to enhanced diffusion of reactants or pollutants into multi‐scale porous MOF‐ or COF‐based constructs. Additionally, immobilizing the MOFs or COFs on macroscopic supporting scaffolds increases the volume of active sites and makes their effective micro‐ and meso‐porosities more accessible to target reactants. These features allow MOF or COF porous structures to perform far beyond the capability of their neat powders.

These benefits, combined with the customization of macroscopic morphology via additive manufacturing, their excellent chemical stability, and improved MOF or COF adaptability and handleability, make 3D‐structured porous constructs of MOFs or COFs potential tools to address current societal needs. However, the industrialization of the MOF/COF‐based constructs faces a set of limitations as discussed in the following section:

5.1. Industrialization Challenges of MOF/COF‐Based Constructs

MOF/COF‐based constructs possess remarkable potential toward crucial societal challenges in environmental remediation, combating greenhouse gas emissions (CO₂ capture and conversion), atmospheric water harvesting in arid and semi‐arid areas, EMI shielding, and beyond. However, leveraging their full potential and transitioning from laboratory to industrial scale requires overcoming several critical challenges, including scalability limitations, stability issues, cost barriers, and environmental concerns.

5.1.1. Scalability Issue

In terms of scalability, most MOF species can be effectively synthesized at the laboratory scale, typically yielding from a few milligrams to grams of material. Laboratory‐scale synthesis allows easier provision and control of the essential conditions, such as humidity levels, high temperatures, extended reaction times, and the handling of hazardous chemicals. However, scaling up to kilogram‐scale production at an industrial level presents significant challenges.^{[ 168 ]} Key issues include the availability and cost of raw materials, the economic feasibility of synthesis methods, difficulties in maintaining consistent reaction conditions, and environmental concerns, all of which hinder large‐scale MOF production. Consequently, despite the existence of over 100 000 different MOFs, only a limited number have successfully reached kilogram‐scale production.^{[ 33a ]}

Solvothermal approaches are predominantly used for synthesizing MOFs, typically involving long reaction times within pressure‐sealed vessels placed in heating ovens.^{[ 169 ]} This presents a significant barrier to industrial‐scale manufacturing due to challenges in achieving uniform mixing and maintaining stable reaction conditions, such as humidity, pressure, temperature, and gas flow. Additionally, poor control over reaction conditions can lead to decreased yields and inconsistencies in product quality.^{[ 168} , ^{170 ]}

Various methods have been developed to facilitate kilogram‐scale production of MOFs, including mechanochemical synthesis, spray drying, flow chemistry, and electrochemical methods.^{[ 33a ]} Although each method offers distinct advantages, they also have notable limitations. For instance, flow chemistry enables continuous production with effective heat and mass transfer control, along with reduced environmental and safety risks.^{[ 168} , ^{171 ]} However, clogging issues in the flow path during solid formation present significant operational challenges.^{[ 168} , ^{172 ]} Mechanochemical synthesis, such as ball milling, offers a greener and more sustainable approach by reducing solvent usage and reaction time while enhancing conversion efficiency.^{[ 173 ]} Nevertheless, mechanochemical processes often result in structural defects, dislocations, and loss of crystallinity, leading to amorphous products.^{[ 33} , ^{174 ]} Moreover, critical parameters such as ligand availability, choice of organic solvent, control over particle size, anion accumulation, morphology, and activation techniques must be carefully managed during large‐scale MOF production.

Similarly, large‐scale production of COFs faces challenges related to high reactant costs and synthesis complexity. The solvothermal method, widely employed for COF synthesis, poses significant scalability issues, primarily due to its operational complexity, requirement for high temperatures and pressures, use of organic solvents, generation of hazardous byproducts, and typically limited yields at the milligram scale, far below commercial objectives.^{[ 34a ]} Therefore, shifting toward kilogram‐scale yields is crucial for meeting industrial demands. Key considerations for scaling COF synthesis include precise cost estimation, reaction yield and time, safety factors, sustainability, and reusability.^{[ 175 ]} Various strategies have been explored to achieve high‐yield COF production, including flux synthesis, continuous flow synthesis, mechanochemical approaches, microwave‐assisted synthesis, room‐temperature synthesis, and sonochemical methods. Among these, flux synthesis has demonstrated significant potential for industrial‐scale, high‐quality COF production, capable of achieving kilogram or even ton‐scale quantities.^{[ 34a ]}

5.1.2. Stability Issue

Another major barrier to the industrialization of MOFs/COFs is their reduced stability under continuous operational conditions, which significantly limits their practical applicability in industrial settings. Although MOFs generally demonstrate excellent performance in laboratory environments, maintaining stability becomes considerably more challenging during the transition to industrial‐scale applications. Many MOF species experience performance degradation due to environmental factors such as humidity, light, temperature, gas atmosphere, and pressure. Moreover, chemical stability in acidic or basic environments is another critical limiting factor for industrial applicability, as numerous MOFs degrade when exposed to acids or bases. The stability of MOFs largely depends on the nature and strength of the metal‐ligand bonds and the overall structural topology of the framework. For instance, MOFs containing metal‐carboxylate linkages, such as MOF‐5 and ZN₄O(BDC)₃ (BDC:1,4‐benzodicarboxylate), exhibit reduced performance in humid conditions and readily hydrolyze in aqueous solutions.^{[ 176 ]} Consequently, enhancing the stability of MOFs in aqueous environments is vital, especially for applications ranging from environmental remediation and biomedical uses to catalysis.^{[ 33} , ^{177 ]}

One effective strategy to address MOF instability in water involves introducing a trifluoromethyl group into their frameworks, resulting in improved water stability through increased steric hindrance.^{[ 176b ]} Additionally, synthesizing MOFs with more robust metal‐ligand linkages can significantly enhance their stability. For example, UiO‐66 demonstrates high stability across a broad pH range (4–8) due to its robust Zr₆ clusters surrounded by twelve strong organic linkers.^{[ 178 ]} To overcome stability challenges in acidic and basic industrial environments, chemically stable MOFs such as MIL‐101 can be employed effectively even under conditions involving SO₂ and NO₂ gases.^{[ 179 ]}

In applications involving CO₂ capture from humid flue gases, the strong affinity of many hydrophilic MOFs for water molecules substantially diminishes their CO₂ uptake capacity. For example, HKUST‐1 shows approximately a 77% reduction in CO₂ uptake capacity under 50% relative humidity compared to dry CO₂ conditions, primarily due to its high affinity for water. Similarly, HKUST‐1 loses approximately 90% of its ammonia uptake capability after 7 days at room temperature under 90% relative humidity.^{[ 180 ]} Additionally, MOF‐5 experiences structural degradation when water content reaches approximately 8 wt%, severely limiting its industrial applications, particularly in gas separation and storage. Hence, enhancing MOF stability under harsh operational conditions is crucial for their practical industrial use.^{[ 33} , ¹⁷⁶ , ^{181 ]}

Similarly, COFs face stability challenges due to chemically induced reversibility, adversely affecting their chemical durability and suitability for advanced functions. Specifically, COFs with imine, boroxane, or borate linkages generally exhibit poor chemical stability despite their high crystallinity. Effective utilization of COFs in liquid‐phase applications, such as wastewater treatment, requires enhanced stability in water, acids, bases, and radiation exposure. Preparing COFs with minimal or non‐reversible linkages (e.g., amine‐linked, olefin‐linked, triazine‐linked, and β‐ketone‐linked COFs) can significantly improve stability, but often results in reduced crystallinity. Therefore, achieving an optimal balance between chemical stability and crystallinity is crucial for advancing COFs toward viable industrial applications.^{[ 34a ]}

5.1.3. Cost Barriers

The high production costs associated with MOF/COF constructs, primarily stemming from expensive precursors, solvents, and complex synthesis methods, significantly limit their widespread industrial application and their ability to replace conventional materials.^{[ 174} , ^{182 ]} Thus, developing simpler, greener, and more cost‐effective synthetic strategies, where solvents used in both synthesis and purification can be efficiently recycled, is essential. Recycling solvents would substantially reduce overall production costs, given that solvents constitute a significant portion of MOF production expenses.^{[ 168a ]} Transitioning from toxic organic solvents to water can greatly minimize toxic chemical consumption, alleviate environmental concerns, and further lower overall production costs.

Another effective approach to cost reduction involves using cheaper, widely available metal clusters and organic ligands. For instance, the production cost of Ni‐MOF‐74 is approximately 886.7 USD per kilogram, with about 78% attributed to the ligand. Replacing the conventional ligand H₄dobdc (2,5‐dihydroxyterephthalic acid) in Ni‐MOF‐74 with a more affordable alternative, such as 4,6‐dihydroxy‐1,3‐benzenedicarboxylic acid, can significantly reduce the total cost while maintaining similar material properties.^{[ 33a ]}

Solvent use is another major contributor to MOF production expenses. For example, synthesizing 1 kg of MOF‐5 requires approximately 81.30 liters of DMF, resulting in a cost of around 527.3 USD per kilogram, with 78.6% directly related to solvent costs. Similarly, solvent costs represent a substantial portion of expenses in the production of UiO‐66. Therefore, implementing solvent recycling processes during synthesis and purification and employing cost‐effective or aqueous solvents could significantly decrease expenses, thus enhancing the economic feasibility of large‐scale MOF production.^{[ 33a ]}

5.1.4. Environmental Concerns

Another barrier to the industrialization of MOFs/COFs is the use of a massive amount of solvents and hazardous precursors. This poses safety issues and environmental concerns regarding the disposal of MOF/COF synthesis wastes into nature.^{[ 34} , ^{182 ]} For instance, some synthesis protocols require using DMF as a toxic solvent, which raises environmental concerns. Additionally, the disposal of heavy metals used as metal clusters in MOFs also requires careful consideration. Hence, using greener approaches with systematic solvent recycling or the use of aqueous synthesis systems with safer metal clusters could significantly reduce the environmental concerns and health risks associated with the mass‐scale production of MOFs.^{[ 33a ]}

5.2. MOF/COF Development Challenges and Future Prospects

Based on the current progress and gaps in the field, as demonstrated in Figure  35 , we propose the following possible prospects for the future development of MOF or COF hierarchical porous structures.

Figure 35.

Challenges and future prospects of porous MOF‐ or COF‐based macroscopic structures.

5.2.1. Minimizing the Production Cost and Scalable Production

Cost and scalability are two crucial factors for the widespread use of a technology. Hence, it is necessary to integrate the MOF or COF into scalable porous structures to promote their accessible active sites and reduce powder consumption. This allows for achieving favorable performance beyond what can be expected from neat MOF or COF powder at considerably lower concentrations. Moreover, developing a one‐pot synthesis approach along with using recoverable solvents for activation and purification of MOFs or COFs is another approach to minimize the production cost. Also, the drying method plays an important role in aerogels’ cost minimization. To this end, developing an aerogel based on air‐drying instead of supercritical or freeze‐drying assists in scalability and cost minimization for industrial applications. The recyclability or reusability of the MOF or COF porous constructs is another important issue in cost minimization. This enables the reactivation and reusing of the resulting constructs toward target reactants for multiple cycles with negligible performance reduction. All in all, the considered porous structure's manufacturing approach must meet the necessities of economic feasibility for scaling up through a simplified approach with minimized energy consumption toward real‐world applications.

5.2.2. Environmental Aspects

Another important factor is the interrelation of environmental protection and manufacturing strategies. This necessitates developing more environmentally friendly manufacturing approaches by using sustainable raw materials and solvents. This also includes designing synthesis approaches that minimize hazardous wastes with the potential to recycle manufactured materials, minimizing pollutant disposal into nature.

5.2.3. Selecting Robust Supporting Scaffolds

Employing supporting scaffolds allows for addressing one of the great disadvantages of MOFs or COFs, which is their poor structuring capability and intrinsically brittle nature. So far, these scaffolds have included materials such as graphene oxide, carbonaceous materials, chitosan, cellulose, aramid nanofibers, polyacrylonitrile, and beyond. Among these materials, bio‐based materials such as cellulose derivatives, chitin, and chitosan have great potential to be used as mechanically robust and biodegradable aerogel building blocks to host MOFs or COFs owing to their highly tunable chemistry. Wood templates are also another type of potent scaffold for the immobilization of MOFs or COFs, improving their accessibility and effectiveness toward target reactants through a scalable in situ manufacturing approach.

5.2.4. Engineering the Mechanical Performance of Porous Structures

The tunability of aerogels and foams by altering their main supporting component allows control over their mechanical character, ranging from flexible to stiffer structures. Such engineerability allows for obtaining a hierarchically porous structure with the required structural integrity/robustness combined with abundant porosities derived from the intrinsic porous nature of MOFs or COFs. Additionally, more attention is needed to the cyclic mechanical stability of the resulting hybrid MOF‐ or COF‐based porous structures to minimize the loss in structural integrity upon numerous loading‐unloading cycles. This enhancement ensures that the resulting MOF‐ or COF‐based structures remain effective for long‐term service in practical applications.

5.2.5. Porosity Engineering

Adding MOFs or COFs to hierarchically porous structures unlocks a great feature for aerogels and foams: the ability to engineer porosity from the molecular level up to micro‐ and macro‐scales. This level of porosity engineering was not possible with traditional aerogel or foam production methods. A systematic roadmap is required to apply this feature for practical applications such as carbon capture, gas adsorption, and beyond. Additionally, this approach provides a roadmap for pore connectivity, facilitating the efficient transport of reactants to the active sites of the MOFs or COFs embedded within the porous macroscopic structure.

5.2.6. Tuning the Porosity Level Across Scales

Maintaining the micro‐ or macro‐scale porosity of hierarchical porous MOF‐ or COF‐based constructs enhances the accessibility of MOFs or COFs to target reactants. This can be achieved through directional freezing or altering the freezing temperature before the lyophilization of the aerogels, thereby significantly promoting their performance and mechanical robustness. Additionally, employing 3D printing allows for precise control over the 3D spatial macroscopic arrangements of aerogels, enabling the creation of complex geometries that are difficult to achieve with traditional methods.

5.2.7. In Situ Synthesis of MOFs or COFs on Supporting Nanomaterials and Their Homogeneity in the Hosting Medium

Direct mixing of MOF or COF powder with aerogel binders at high loadings can significantly degrade the mechanical properties of the final porous structure and lead to potential agglomerations. Similarly, in situ growth also faces challenges in controlling the exact weight ratio of MOF or COF through nucleation growth. Hence, it is suggested to first synthesize the MOFs or COFs on aerogel's 1D/2D building blocks, such as cellulose nanofibers or graphene oxide, and then use these to create aerogels through simple molding, freezing, and lyophilization. This approach allows for the maximum weight ratio of MOFs or COFs while maintaining the mechanical integrity of the porous structure and ensuring the homogeneity of MOF or COF species throughout the framework. Additionally, further studies are required to develop a manufacturing roadmap that maximizes the loading of MOFs or COFs in porous structures while preserving structural integrity and mechanical robustness. This should be accompanied by strategies to promote the uniform dispersion of MOFs or COFs within the supporting matrix, thereby maximizing the accessibility of MOFs or COFs' active sites for chemical reactions or adsorption processes.

5.2.8. Employing Emulsification Techniques

One area that requires further investigation in MOF or COF aerogels is the generation of aerogels using emulsification techniques. This approach allows for the creation of MOF‐embedded water‐in‐oil (W/O) or oil‐in‐water (O/W) emulsions with controllable compositions for a multitude of applications. Employing emulsification also enables meaningful control of the macro‐/micro‐scale porosity of the resulting aerogels by altering the soft template prior to freezing and lyophilization.

5.2.9. Reticular Chemistry and Interfacial Science

Interfacial complexation and the assembly or synthesis of MOFs or COFs at the liquid‐liquid interface are other interesting topics that have not been properly investigated. This approach enables MOFs or COFs to yield structural liquids with customizable features, which can be used as templates for aerogel production. The nature of the interfacial assemblies allows for meaningful engineering of hybrid templates with customizable features for specific, aim‐oriented tasks.

5.2.10. Intrinsically Conductive MOF Aerogels

Porous MOFs suffer from inadequate electrical conductivity, which forces scientists to undergo a carbonization process or employ electrically conductive additives to generate a conductive pathway for applications requiring a conductive structure, such as EMI shielding and energy. Therefore, it is highly recommended that manufacturing strategies for generating aerogels and foams made of intrinsically conductive MOFs be explored. This would enable revolutionary energy and shielding platforms by combining molecular‐level porosity with conductive channels through coordination chemistry.

5.2.11. Resistance to Degradation

The manufactured macroscopic porous MOF‐ or COF‐based structures should be able to retain their chemical stability in the presence of various types of solvents, working temperatures, and pH conditions to ensure their long‐term function with optimum performance.

5.2.12. Employing Artificial Intelligence and Simulation for Optimizing MOF‐ or COF‐Based Porous Structures

Employing artificial intelligence (AI) and simulation tools allows for optimizing the arrangement and composition of the MOF or COF porous structures toward target applications. This approach avoids unnecessary costs due to trial and error and provides a scientific and theoretical roadmap for yielding potent hierarchically porous MOF or COF structures based on required features. Accordingly, high‐throughput screening (HTS) approaches provide a wide range of opportunities to discover new types of functional MOFs for advanced or industrial applications.^{[ 183 ]} To achieve this, large libraries of MOF constructs can be considered for task‐oriented functions and applications through computational methodologies. AI and machine learning improve the HTS via data‐driven predictions, improving and accelerating MOF discovery processes.^{[ 184 ]} Furthermore, studying the reaction mechanisms and MOF's active sites enables the discovery of new approaches to improve their performance toward chemical reactions, advanced applications, and engineering chemistry. Moreover, employing density functional theory (DFT) based on theoretical studies and molecular dynamics simulations provides deep insight into the catalytic activity, adsorption energy, and electronic energies of MOFs.^{[ 185 ]} These efforts contribute to the effective enhancement of MOF reactive sites and the optimization of their detailed design for task‐specific processes such as sensing, gas separation, and catalysis. A similar approach can be applied to the advancement of COFs through AI‐driven methods and in‐depth simulations.

5.2.13. Avoid blocking the MOF or COF Porosities

Some additive manufacturing methods employ high‐force compression and supporting resins to create MOF or COF membranes and 3D‐structured monoliths. This approach has a significant disadvantage as it considerably reduces the porosity level of MOFs or COFs and damages their structure. Therefore, it is recommended to use in situ growth strategies by synthesizing MOFs or COFs on constituent nanoparticles or pre‐made supporting scaffolds, such as electrospun nanofibers, aerogels, and foams. This approach helps maintain the porosity level and BET‐specific surface area.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Biographies

Seyyed Alireza Hashemi obtained the Ph.D. degree in Mechanical Engineering from the University of British Columbia (UBC) and is currently serving at the same institute as a Postdoctoral Research Fellow. Dr. Hashemi is an active nanotechnologist focused on the design and synthesis of advanced nanomaterials with customized features toward interfacial complexation, liquid–liquid phase separation, structured liquids, additive manufacturing, soft materials, and engineerable 3D‐structured multi‐scale porous building blocks, e.g., aerogels, for applications in electromagnetic interference (EMI) shielding, sensors, and beyond.

Ahmadreza Ghaffarkhah attained the Ph.D. degree in Mechanical Engineering from the University of British Columbia (UBC). He is currently a Postdoctoral Researcher at the Bioproducts Institute (BPI) and the Nanomaterials and Polymer Nanocomposites Laboratory (NPNL) at UBC. Dr. Ghaffarkhah's research focuses on the synthesis of nanomaterials, soft materials, polymer processing and forming, and additive manufacturing. He is also actively engaged in liquid streaming, liquid‐in‐liquid printing, and the development of all‐liquid reconfigurable systems based on nanoscale materials.

Ali Akbar Isari is currently pursuing the Ph.D. degree at the University of British Columbia (UBC) under the supervision of Dr. Mohammad Arjmand. His research focuses on the development of 2D material‐based aerogels and hydrogels for applications in electromagnetic interference (EMI) shielding and environmental technologies.

Mahyar Panahi‐Sarmad is a Ph.D. student at the University of British Columbia (UBC), specializing in biofabrication and biomacromolecules. With a background in polymer engineering, his early research focused on aerogels and nanocomposites. As an Elite Scholar, he worked on shape memory soft matter for biomedical applications and later contributed to additive manufacturing and functional textiles as a remote research associate at the Key Laboratory of Eco‐Textiles. At UBC, his current work focuses on sustainable biofabrication techniques—particularly all‐aqueous (miscible) liquid‐in‐liquid printing—emphasizing interfacial interactions and hierarchical structure, and includes projects on colloidal soft materials, living matter, and energy‐harvesting systems, all rooted in bio‐based material design.

Feng Jiang is an Associate Professor with the Department of Wood Science at the University of British Columbia, and Tier II Canada Research Chair in Sustainable Functional Biomaterials. He was trained in wood science and macromolecular science and engineering, and his current research focuses on developing advanced functional materials from lignocellulosic biomass for packaging, energy, textile, electronic, and sensor applications.

Orlando J. Rojas is the Canada Excellence Research Chair in Forest Bioproducts at the University of British Columbia and Director of the Bioproducts Institute. A global leader in sustainable materials and soft matter, he has authored nearly 600 publications, accumulating over 50 000 citations and an h‐index of 105. He is the recipient of the Anselme Payen Award and a Fellow of the American Chemical Society, the Finnish Academy of Science and Letters, and TAPPI. Prof. Rojas holds honorary and guest professorships in the United States, Europe, and Asia, and serves on scientific advisory boards for the Max Planck Institute of Colloids and Interfaces, the Institute of Materials (IMATUS) at the University of Santiago de Compostela, Spain, and several global corporations.

Stefan Wuttke created the research group “WuttkeGroup for Science”, initially hosted at the Institute of Physical Chemistry at the University of Munich (LMU, Germany). Currently, he is an Institute Professor and Director of the Department of Functional Materials and Nanomagnetism at the Academic Centre for Materials and Nanotechnology of the AGH University of Krakow (Poland), and also a visiting Professor of Functional Materials at Lincoln University (UK). His principal focus is the design, synthesis, and functionalization of MOFs and their nanometric counterparts to target diverse applications. At the same time, he aims to establish a basic understanding of the chemical and physical elementary processes involved in the synthesis, functionalization, and application of these hybrid materials.

Mircea Dincă is the Alexander Stewart 1886 Professor of Chemistry at Princeton University. He grew up in Romania and moved to the United States to pursue a Bachelor's degree in Chemistry at Princeton University. Graduate studies in Inorganic Chemistry at UC Berkeley were followed by a Postdoctoral appointment at MIT. He started his independent career in 2010 at MIT and moved his research group to Princeton in 2025. His group focuses on the synthesis of new multifunctional materials for applications in electrical and electronic devices, heterogeneous catalysis, and various uses in clean and renewable energy.

Mohammad Arjmand is recognized as a dedicated researcher in the fields of nanotechnology and polymer engineering. His impact can be seen through his roles at the University of British Columbia, Okanagan Campus, as a faculty member, a Canada Research Chair in Advanced Materials and Polymer Engineering, an Inductee of the Royal Society of Canada, and the Lead of the Plastic Recycling Research Cluster. Dr. Arjmand directs the Nanomaterials and Polymer Nanocomposites Laboratory (NPNL), which focuses on the synthesis of multifunctional nanomaterials and the development of their polymer nanocomposites and assemblies.

Hashemi S. A., Ghaffarkhah A., Isari A. A., et al. “Advancing from MOFs and COFs to Functional Macroscopic Porous Constructs.” Adv. Mater. 37, no. 52 (2025): 2411617. 10.1002/adma.202411617