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. 2021 Aug 6;7(8):1434–1440. doi: 10.1021/acscentsci.1c00743

Large-Scale Production of Hierarchically Porous Metal–Organic Frameworks by a Reflux-Assisted Post-Synthetic Ligand Substitution Strategy

Guorui Cai 1, Xing Ma 1, Meruyert Kassymova 1, Kang Sun 1, Meili Ding 1, Hai-Long Jiang 1,*
PMCID: PMC8393232  PMID: 34471687

Abstract

graphic file with name oc1c00743_0006.jpg

The mass production of hierarchically porous metal–organic frameworks (HP-MOFs) with adjustable morphology and size as well as retained crystallinity is highly desirable yet challenging. Herein, we have developed a versatile post-synthetic ligand substitution (PSLS) strategy to convert typical microporous MOFs and even their composites to HP-MOFs and their composites at a 10 g level and beyond in a simple reflux system. The resulting HP-MOFs feature intrinsic micropores and abundant defective mesopores, which greatly facilitate the transport and activation of large substrates for stable and efficient heterogeneous catalysis. Furthermore, the presence of defective mesopores in the HP-MOF composites improves activity and selectivity for large molecule-involved one-pot tandem catalysis. This strategy opens a new door to fast, facile, general, and scale-up production of HP-MOFs and related composites for expanding applications of conventional microporous MOF-based materials.

Short abstract

A post-synthetic ligand substitution strategy has been developed for the mass production of hierarchically porous MOFs and their composites for enhanced large molecule-involved one-pot tandem catalysis.

Introduction

Metal–organic frameworks (MOFs) as a relatively new class of crystalline porous solids have been widely accepted as a promising materials platform in many fields, benefiting from their high porosity and well-defined and tailored porous structures.1−12 Their intrinsic micropores endow the particular capability of size-sieving and small molecule access, which are of importance in diverse applications. Nevertheless, in many cases, the small pores limit their applications involving large molecules.13−17 A straightforward solution to the above issues is the extension of the bridging ligand length to afford mesoporous MOFs.18−22 Unfortunately, the targeted products with high porosity are mostly thermodynamically instable and framework interpenetration is hardly avoidable.18−22 Moreover, the long and complex organic linkers are usually insoluble and commercially unavailable, posing synthetic challenges. Alternatively, it is possible to introduce larger pores into parent microporous MOFs, producing hierarchically porous MOFs (HP-MOFs).23 The intrinsic micropores conduce to high surface area and abundant active sites, while the introduced mesopores or macropores across the microporous matrix would facilitate the diffusion and transport of substrates and/or products, which significantly extend their application potentials and enhance their performance. To achieve this target, some in situ synthetic methods have been developed to fabricate HP-MOFs, on the basis of soft/hard template, nontemplate, ligand fragment, modulator-assisted strategies, etc.24−38 The key point of these methods is to explore special templating reagents (hard template, soft surfactant, defective ligand, or special modulator), which not only possess special interaction with the MOF skeleton to create the large pores but also can be removed subsequently without disrupting the overall MOF structure. Accordingly, the increased process complexity and/or high cost are usually unavoidable, making it difficult for large-scale synthesis toward real-world applications.39

In this context, the introduction of meso/macropores into intrinsically microporous MOFs by a post-treatment process would be a promising strategy that meets the practical demands. Although several related studies, such as heat treatment, acid/alkali etching, etc., have been reported;40−47 unfortunately, the harsh chemical/thermal treatments often damage their pristine MOF structures and even introduce some impurities, such as metal oxides.40,41 Moreover, the etching/destruction of the parent MOF structures is hard to control, which places high demand on particular etching reagents and MOF stability. Therefore, a simple, fast, general, and scalable synthetic approach to the conversion of microporous MOFs into hierarchically porous structures remains imperative.

The post-synthetic ligand exchange (PSE, named by Kim et al.48) has been widely used to introduce functionalized ligands into MOFs under mild conditions, where a new bridging ligand is generally used to replace the pre-existing ligand of MOFs while preserving the bulk crystallinity.11,48−52 Inspired by this mild conversion process, we describe a facile post-synthetic ligand substitution (PSLS) strategy by substituting the bridging ligand in microporous MOFs with a terminal ligand to create additional defective spaces, which is a mild process without an obvious disruption of the morphology and size of the parent MOF particles. The mesopores and even macropores can be generated and tailored to afford HP-MOFs by simply controlling the ratio/amount of terminal ligand (Figure 1). The HP-MOFs with different structural topologies, including UiO-66 with different sizes and morphologies,53 UiO-67,53 MOF-808,54 and MIL-53,55 can be produced (Figure S1), demonstrating the good generality of this strategy. This reflux-assisted process is very easy to operate and scale up to at least 10 g in one pot.56 The resultant HP-MOFs not only feature intrinsic micropores enriching small-size substrates and meso-/macro-pores entrapping large functional species but also possess abundant defective sites, which are able to interact and activate substrates. The CO2 cycloaddition of epoxides and one-pot cascade hydrogenation/condensation reactions have been conducted, as a proof-of-concept, highlighting the collaborative strengths of ordered micropores and defective mesopores toward efficient and selective catalysis.

Figure 1.

Figure 1

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Schematic showing the conversion of microporous MOFs into corresponding hierarchically porous structures on the basis of the PSLS strategy.

Results and Discussion

Given its high chemical and thermal stability, the well-known microporous MOF, UiO-66, was first selected as a representative model for the PSLS study. Typically, the UiO-66 powder was introduced into an aqueous solution of sodium acetate, followed by a reflux treatment at 100 °C to promote the ligand substitution process (Figure 1, Supporting Information, Section S2.2). Upon activation in N,N-dimethylformamide (DMF) solution with hydrochloric acid to remove the residual ligands in MOF pores, hierarchically porous UiO-66 (denoted HP-UiO-66) was obtained. It is worth noting that, in contrast to the conventional solvothermal synthesis in an autoclave, this PSLS process by refluxing treatment in a flask is very simple and promising for conversion in a large scale. Mass production can be achieved by simply using a large-volume flask. As a laboratory demonstration, we have successfully completed the conversion of 10 g of MOFs (HP-MOF yield: 8.44 g) in a 1 L flask (Figure 2a–c).

Figure 2.

Figure 2

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(a–c) Experimental photos of reflux setup (1 L) showing the scale-up conversion of UiO-66 into HP-UiO-66 in one pot. (d) N2 sorption isotherms at 77 K and (e) powder XRD patterns of UiO-66 and HP-UiO-66. (f) SEM and (g) TEM images of HP-UiO-66.

Nitrogen sorption measurements indicate that the pristine UiO-66 possesses a high microporosity with a pore volume of 0.52 cm3/g. After the post-synthesis conversion, the resulting HP-MOF exhibits an increased total pore volume (0.68 cm3/g) with the introduction of mesopores, while only a slightly decreased specific BET surface area (SBET) compared with that of their pristine MOFs (SBET: 802 vs 995 m2g–1). The pore size distribution data show that the intrinsic micropores (<2 nm) in MOFs are basically maintained, while mesoporous features (2–20 nm) clearly appear after the PSLS process (Figure 2d and Figure S2). Powder X-ray diffraction (XRD) patterns show that no noticeable decrease in the crystallinity occurred after this mild PSLS process (Figure 2e). Scanning electron microscopy (SEM) images show that the morphology and size of UiO-66 particles are mostly maintained (Figure 2f and Figure S3). From both SEM and transmission electron microscopy (TEM) images, the additional mesopores are observable in HP-UiO-66 (Figure 2g). The above results unambiguously demonstrate that this strategy is able to create additional mesopores while keeping UiO-66 structure, microporosity, particle size, and morphology, which is highly desired for various practical applications.

A series of control experiments have been attempted to figure out the real role of sodium acetate in the mesopore generation of HP-UiO-66. In view of the fact that the hydrolysis of sodium acetate would produce hydroxide ion (OH–), whether the mesopore formation related to the OH– etching should be checked. However, this concern is ruled out by employing a neutral strong electrolyte, ammonium acetate, by which a similar hierarchically porous structure can be obtained (Figure S4). This point is further evidenced by using NaOH solution to etch UiO-66; no additional pore can be generated even under the same pH as the aqueous solution of sodium acetate (Figure S5). Similarly, when post-treating with the same molar amount of aqueous acetic acid, no additional pore can be produced (Figure S6). This result is probably attributed to the weak electrolyte feature of acetic acid (higher pKa than terephthalic acid), which, dissociation in only a slight degree, is not able to provide sufficient acetate ions to drive the PSLS process. As expected, when the weak carboxylic acid is replaced by corresponding carboxylates, such as sodium formate, sodium propionate, sodium benzoate, etc., the conversion to HP-UiO-66 can be successfully achieved after this PSLS process (Figures S7–S9). To unveil the mechanism, some carboxylates with a long chain are deliberately used; the steric resistance stemmed from the MOF micropores suppresses the entrance of these bulky carboxylates. As a result, only the marginal layer of the MOF particles can be etched, while no noticeable mesopores and structural change can be found in the MOF interior (Figures S10 and S11). However, if sodium acetate was directly added in the synthetic system of conventional UiO-66, no crystalline UiO-66 can be obtained (Figure S12), ruling out the possibility of in situ defective mesopore formation accompanied by the MOF assembly.

To elucidate the influence of the terminal ligand amount on the substitution process, 0, 2, 4, 6, 8, or 10 mmol of sodium acetate was used in the synthesis of HP-UiO-66. As indicated in N2 sorption curves (Figure S13), a small amount of terminal ligand does not cause the change in the pore feature of UiO-66. The mesoporous characteristics, i.e., hysteresis loop in N2 sorption curves, start to appear by increasing the sodium acetate amount to 4 mmol. A further increased concentration of sodium acetate results in the gradual increase of mesoporosity. Accordingly, the mesoporous behavior can be tuned by changing the amount of the terminal ligand. However, most of the UiO-66 skeleton is etched away when the concentration of sodium acetate is too high, resulting in structural collapse. All these results jointly suggest that both concentration and size of carboxylate play key roles in this PSLS process. In addition, it is noteworthy that the introduction of polyvinylpyrrolidone (PVP) in the sodium acetate aqueous solution is necessary to promote the formation of mesopores while maintaining a high pristine microporosity (Figure S14a). Moreover, the difference between the TEM images (Figure 2g and Figure 14b) and the similar high yields (88.5%–92.9%) of HP-UiO-66 obtained with different amounts of PVP indicate that the PVP might mainly influence the distribution of the acetate to etch the MOF,51 not the etching rate/reaction equilibrium. Therefore, we propose that the PVP might behave as the protective coating to prevent the destruction of the MOF particle surface during the diffusion of etchant (acetate) into the MOF interior, thereby promoting the production of homogeneous mesopores in the entire MOF particles (Figure 2g).

The UiO-66 before and after PSLS process have been characterized by 1H NMR. Results show that the treated MOFs present some characteristic peaks assignable to the acetate, and these peaks disappear after activation treatment (Figure 3a). The ratio of acetate/1,4-benzenedicarboxylate (BDC) calculated by integral area of 1H NMR peaks in HP-UiO-66 before HCl activation is obviously higher than those in conventional UiO-66, indicating a higher degree of defect in the former (0.33 vs 0.10; Figure S15). Furthermore, the FT-IR peak at 2975 cm–1 assignable to the saturated alkane appears after the PSLS treatment and disappears after activation, indicating the successful substitution of acetate into the MOF skeleton and subsequent removal in the activation process (Figure S16). To gain visual evidence on the mesopore generation, the resulting HP-UiO-66 was soaked into dye solutions with different charges. While the anionic dyes (methyl orange, MO) can be absorbed, the cationic dyes (methylene blue, MB) are hardly captured (Figure 3b,c), indicating the cationic framework due to the missing of linkers. When soaking HP-UiO-66 in a DMF solution of Coomassie Brilliant Blue R250 (R250), the solution gradually fades and the white HP-UiO-66 accordingly turns to brilliant blue (Figure 3d). In stark contrast, due to the larger size of R250 than the micropore opening, the pristine UiO-66 is not able to uptake R250 and thus its color remains unchanged even after longer soaking time. This clear comparison again manifests the presence of defective mesopores in HP-UiO-66. Altogether, the above information unambiguously supports that the acetate coordinates to the Zr-oxo clusters, creating the structural defects, and subsequently, it can be removed to release the mesoporous space after activation.

Figure 3.

Figure 3

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(a) 1H NMR spectra for CH3COONa and HP-UiO-66 before and after activation. UV/vis absorption spectra recording the adsorption of (b) small-sized anionic dye (MO), (c) small-sized cationic dye (MB), and (d) large-sized anionic dye (R250) by UiO-66 and HP-UiO-66. Insets in b and c: photos of dye solution (left) before and (right) after adding HP-UiO-66. Insets in d: photos of dye solution after adding (left) UiO-66 and (right) HP-UiO-66.

Systematic characterizations have been conducted to figure out the function of the (DMF + aqueous HCl) treatment in the activation process. To evaluate if there are Cl– ions in the activated HP-UiO-66, the content of Cl– ions in the suspension of HP-UiO-66 dissolved by NaOH aqueous solution has been detected by ion chromatography, which indicates a Cl– ion content of 17 μg/mgHP-UiO-66. Moreover, the obvious Cl 2p signal peak at 198.2 eV in the X-ray photoelectron spectroscopy (XPS) spectrum can be identified in the activated HP-UiO-66, suggesting the charge balance role of Cl– ion in the defective MOF structure (Figure S17).57 In addition, the TGA curves of HP-UiO-66 before and after the (DMF + aqueous HCl) treatment pose an obvious deviation on the mass drop below 150 °C (Figure S18). The increased mass drop in the activated HP-UiO-66 can be attributed to physiosorbed/coordinated H2O proven by the OH stretch peaks (∼3400 cm–1) in the FTIR spectrum (Figure S16). Therefore, we propose that the coordinated acetate was removed by the (DMF + aqueous HCl) activation, which causes a chlorine containing MOF (Figure S19).58−60

In addition, the Zr content in the above two solutions was detected by inductively coupled plasma atomic emission spectrometer (ICP-AES), which indicates that the Zr concentrations are 1.44 and 0.01 mg/mL, respectively, in the sodium acetate reflux solution and the (DMF + aqueous HCl) activation solution. The high concentration of Zr in the former indicates that UiO-66 is etched by acetate and thus the formation of mesopores. To evaluate if the (DMF + aqueous HCl) treatment would further etch the MOF structure and introduce additional mesopores, the same treatment was performed on the pristine UiO-66 powder. All characterizations demonstrate that no mesoporous feature can be identified (Figure S20). In addition, both UiO-66 and HP-UiO-66 are stable even in concentrated HCl (Figure S21). The above results indicate that the low concentration of Zr in the (DMF + aqueous HCl) solution is not caused from the further etching of the HP-UiO-66 skeleton by HCl. It might come from the Zr residual in the MOF pores formed in the acetate reflux process.

With the above results, we hypothesize that the PSLS might be a reversible process in the generation of HP-UiO-66, on the basis of the equilibrium of dissociation and reassembly (Figure 4a).48−52 Upon soaking MOF crystals into the terminal ligand solution with high concentration, the equilibrium state of the pristine MOF would be broken. As a result, some linkers in the MOFs will be substituted by the terminal ligands after reaching a kinetic equilibrium state and thus producing small defective pores (Figure 4b,c). If the terminal ligand concentration in the solution is further increased, the above equilibrium state would be broken and pushed to another new kinetic equilibrium state. Along with an increased amount of terminal ligand, the bridging ligands around the metal cluster are gradually substituted, and even the metal cluster will be simultaneously released from the MOF matrix, resulting in a larger pore space (Figure 4c,d). On the basis of this mechanism, the etching extent of MOF skeleton can be controlled to generate different ranges of pores by using carboxylate as the terminal ligand. The terminal ligands coordinated to the metal-oxo clusters can be removed to afford the coordinatively unsaturated metal centers, behaving Lewis acid sites as discussed below, in the HP-MOFs after the activation process.

Figure 4.

Figure 4

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(a) Typical growth mechanism of common MOF crystals. (b–d) Proposed mechanism for the typical conversion of a microporous MOF into hierarchically porous structure with different levels of structural defect on the basis of the PSLS strategy.

On the basis of the above mechanism, the generality of this conversion strategy has been investigated. Taking UiO-66 as a representative, HP-UiO-66 with different sizes (typical examples, 800 nm, 2 μm) and morphologies (in octahedral, dodecahedral, or cubic shape) can be obtained, depending on the original geometry of their parent MOFs (Figures S22–S29). Other types of MOFs, such as UiO-67, MIL-53, and MOF-808, featuring even completely different structures and compositions, have also been attempted. Encouragingly, they all can be converted into their corresponding hierarchically porous structures (denoted HP-UiO-67, HP-MIL-53, and HP-MOF-808, respectively) (Figures S30–S32). In view of the operation simplicity of this refluxing process followed by activation process, both of which are realized by stirring the sample in a flask, it is expected to achieve a large volume of HP-MOFs in one pot. To demonstrate this scale-up potential in the laboratory, when a 10-g-scale conversion (100 times of magnification vs typical experiment) was attempted in a 1 L flask, 8.44 g of HP-MOFs was harvested (Figure 2a–c). The obtained HP-UiO-66 is uniform in its size and microstructure (Figures S33 and S34), similar to those in common-scale synthesis (Figure S3). In addition, the same PSLS process has been applied for preparing MOF-based composites. Taking UiO-66 encapsulating platinum nanoparticles (denoted Pt@UiO-66) as an example, to our delight, which can be successfully converted into the corresponding composite featuring hierarchical pores (denoted Pt@HP-UiO-66, Figures S35–S38). It is believed that this is a highly general approach, and predictably, a variety of MOF-based composites featuring hierarchical pores can be achieved in mass production on the basis of this conversion strategy, which is definitely promising for broad and practical applications.

Given the unique hierarchical structures, these micropores and mesopores would synergistically promote catalytic reactions, where micropores expose active sites and enrich substrates while the defective mesopores benefit the rapid transport of substrates and provide a large number of open Zr sites as the Lewis acid sites (Figures S39–S42). As a proof of example, UiO-66 and HP-UiO-66 have been employed for the catalytic CO2 cycloaddition of epoxides, upgrading CO2 to high-value products (Figure 5a,b).61−63 The better mass transfer capability together with a higher concentration of defect sites in HP-UiO-66 are responsible for its significantly higher activity than the parent UiO-66. The conversion of various epoxides with different sizes illustrates the advantage and high activity of HP-UiO-66, particularly in large-size molecule involved mass transfer, far superior to microporous UiO-66 (Figure 5a).

Figure 5.

Figure 5

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(a) Catalytic cycloaddition reactions between CO2 and epoxides with different sizes on the basis of UiO-66 and HP-UiO-66 catalysts. (b) Recycling test of the CO2 cycloaddition with epibromohydrin over HP-UiO-66. (c) One-pot cascade synthesis of secondary arylamines through the hydrogenation of nitrobenzene and the reductive amination of benzaldehyde reactions over Pt@UiO-66 and Pt@HP-UiO-66.

Not limited to the enhanced activity of straightforward reactions, HP-MOFs are even able to promote complex reactions with both improved activity and selectivity. Taking Pt@HP-UiO-66 as an example, it exhibits particular strengths in one-pot multistep cascade reactions (Figure S43). The hierarchically porous composites can convert nitrobenzene (A) into N-(naphthalen-1-ylmethyl)aniline (D) in one-pot cascade reaction, with the selectivities of <1%, ∼ 3%, and ∼97% to aniline (B), N-phenylnaphthalene-1-(methanimine) (C), and D, respectively. In sharp contrast, their microporous composite mainly converted A into intermediate C with a selectivity ∼71%, and only a 20% selectivity to the D in the same reaction time length, due to the restrict effect of micropores on intermediate products (Figure 5c and Figure S44). All above clearly highlight the synergistic strengths of the ordered micropores and generated defective mesopores toward catalytic applications involving large-size substrates and products.

Conclusions

In summary, we have developed a simple yet highly effective and general strategy to the conversion of common microporous MOFs and their composites into their corresponding hierarchically porous structures. This conversion can be easily scaled up to 10 g and even higher amounts, during which the pristine MOF structure (and related micropores) can be maintained, while some structural domains are disassembled to create additional defective mesopores in resulting HP-MOFs. The incorporation of guest species inside MOF particles does not disturb this conversion and the synthetic strategy can be applicable to diverse MOF-based composites. With the cooperation of coexisted micropores and defective mesopores, the HP-MOFs and related composites greatly boost catalytic reactions and even multistep cascade reactions in one pot, exhibiting improved activity and selectivity in reference to the parent microporous MOF-based materials. This work opens a novel and general avenue to MOFs with hierarchical pores into mass production and pushes one-step forward for MOF-based materials toward broad and practical applications.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (21725101, 22161142001, and 21521001), the DNL Cooperation Fund, Chinese Academy of Sciences (DNL201911), and Fundamental Research Funds for the Central Universities (WK3450000007).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscentsci.1c00743.

  • Discussions of materials and instrumentation used, experimental details, uptake of guest species, and catalytic activity evaluation and figures of crystal structures, pore size distributions, SEM and TEM images, N2 sorption isotherms, powder XRD patterns, NMR spectra, FT-IR patterns, XPS spectra, thermogravimetric curves, proposed defective structures, CO2 sorption isotherms, adsorption curve, proposed mechanisms, and recycling tests (PDF)

Author Contributions

† G.C. and X.M. contributed equally to this work.

The authors declare no competing financial interest.

Supplementary Material

oc1c00743_si_001.pdf (2.7MB, pdf)

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