Our journey of developing multifunctional metal-organic frameworks

Abstract

Metal–organic frameworks (MOFs) are organic-inorganic hybrid solids constructed from the coordination interaction of metal ions/clusters with organic linkers, which currently represent one of the most rapidly expanding platforms for new functional materials. Based on well-established approaches, involving tuning the pore sizes, incorporation of functional sites and post-synthetic modification, the pore structures of MOFs can be readily controlled for multifunctional applications. In this brief review, we summarize and highlight our research progresses during our journey on developing functional MOFs for various applications including gas storage, gas separations, luminescent sensing, proton conduction, and molecular recognitions.

1. Introduction

Metal–organic frameworks (MOFs, also known as PCPs for porous coordination polymers) have long been the preeminent platform for the worldwide scientists and engineers to explore novel multifunctional materials [1,2]. MOFs are crystalline porous media straightforwardly self-assembled through the coordination of suitable organic linkers to metal ions/clusters, which features exceptional porosity, high modularity and diverse functionality. The initial efforts to construct porous structures of coordination polymers can be dated back to late 1980s [3,4]. The establishment of permanent porosity was realized until late 1990s for the very first MOFs [5–8], showing superior porosity to traditional porous materials at the time, which represented a turning-point in the field, since it initiated intensive research interests. To boost the porosity and surface areas of MOFs, the approach of secondary building units (SBUs) and isoreticular principle were subsequently proposed within the concept of reticular chemistry to design novel MOFs with extraordinarily high porosity [9–12]. In principle, incorporating SBUs featuring rigidity and directionality can translate their geometry into infinite nets while the adjustments of organic struts can control the pore size and porosity of MOFs. Hence, it is feasible to design this unique class of porous materials with tunable pore sizes as well as predictable topology and structure [13]. Moreover, the control over the interpenetration of MOFs affords another approach to control and stabilize the pore architecture [14,15]. The implementation of simple solvothermal synthesis of MOFs in 2001 [15], as exemplified by the assembly of MOF-14, represents an important contribution in this field since it outlined a general approach to synthesize MOFs. To date, the research on MOFs represents one of the most rapidly expanding fields of science.

Besides the control over pore sizes, another important aspect for MOFs as multifunctional materials is the introduction of functional sites, referring to various types of Lewis acidic/basic sites, for selective binding or recognition of guest molecules. One type of important functional sites is represented by open metal sites (OMSs) [16–18]. The first example of coordinatively unsaturated metal centers in activated MOFs was demonstrated by a copper tetracarboxylate MOF (MOF-11), which exhibits open Cu sites after full dehydration [16]. It is well recognized that MOFs with OMSs are very promising for molecular recognition, selective gas sorption and catalysis [17–20]. To immobilize OMSs, a preconstructed metalloligand approach was proposed for the construction of mixed-metal-organic frameworks (M′MOFs), which feature dual open metal sites at one single metal centers [21]. In contrast, other types of functional sites (e.g. amino, hydroxyl, pyridyl groups) can be straightforwardly incorporated into MOFs by directly using corresponding organic linkers [22,23]. Furthermore, the implementation of other functionalized approaches, namely post-synthetic modification [24,25] and multivariate MOFs [26–28], have greatly widens the scope and dimension of functional MOFs.

By virtue of control over pore size and pore chemistry, MOFs have been realized with ultrahigh porosity (500–10,000 m² g⁻¹), tunable pore size (3–100 Å), high thermal stability (up to 500 °C), and exceptional chemical stability [2]. The above rational design principles dramatically facilitates the exploration of novel functional MOFs for a wide variety of applications, including gas storage [29–32] and separation [33–36], optical materials [37,38], electric and magnetic materials [39,40], chemical sensing [41–43], catalysis [44–47], and biomedicine [48,49].

Our ongoing research effort has long been dedicated to the development of functional MOFs for various applications (Fig. 1), especially for gas storage and separation. Our seminal contributions on the applications of MOF materials include the implementation of MOFs with superior performance for methane storage, important industrial gas separations and corresponding evaluation standards (fixed-bed adsorption and/or breakthrough experiments). By taking the advantages of pore and function engineering, we have been able to target some unique porous MOFs with multifunctional properties and applications [50,51]. Here, we highlight our important research progresses during the journey on exploring multifunctional MOFs, involving representative examples for applications such as methane storage, gas chromatographic separation of hexane isomers, carbon dioxide capture, kinetic separation of hydrogen isotope, acetylene/ethylene separation, acetylene/carbon dioxide separation, propylene/propyne separation, ethylene/ethane separation, luminescent sensing, enantioselective separation of racemic mixtures, and so on. In specific, three followed approaches have been involved: (a) precise control of pore size and pore chemistry within MOFs for methane storage and highly selective separation of gas molecules; (b) developing MOFs with dual-functionality, featuring optimal pore size with strong binding sites for selective recognition of guest molecules; (c) construction of porous mixed-metal-organic frameworks (M′MOFs) to immobilize open metal sites for small molecule separation.

Fig. 1.

Multifunctional MOFs are being developed for many applications ranging from energy storage, gas separation, and luminescent sensing to proton conduction.

2. Gas storage

The storage and delivery of fuel gases in a convenient, cheap and safe way is a very challenging but important, especially for low-density gases like H₂ and CH₄. Particularly, methane (main component of natural gas) is a highly attractive clean fuel as considering its low carbon dioxide emission and natural abundance. The high density CH₄ storage can be achieved by using porous media as adsorbents. A good example about CH₄ storage is the naturally occurring methane clathrate (or natural gas hydrate, a potentially important future fuel source), which is an icy solid containing a large amount of CH₄ trapped by water cavity, giving a volumetric total uptake capacity of 180 v(STP)/v after dissociation and corresponding to about 15 wt% (by mass). Compared to such onetime energy source, reversible and rechargeable storage under mild conditions is more desirable. Given that MOFs show large surface areas and low densities, it is very promising to use them for fuel gas storage.

MOFs have been demonstrated the ability to concentrate methane within their pore systems at ambient temperature long time ago. In 1997, Kitagawa et al. reported [Co₂(4,4′-bpy)₃(NO₃)₄] (4,4′-bpy = 4,4′-bipyridine) as the first example of MOFs for CH₄ storage, [5] showing a total uptake capacity of 2.3 mmol g⁻¹ at 298 K and 30 bar, which initiated the application of MOFs for gas storage. In 2000, a higher methane uptake of 6.5 mmol g⁻¹ was achieved by using [Cu (4,4′-bpy)₂(SiF₆)] at 298 K and 36 bar, [52] which is superior than that of zeolite 5A (3.7 mmol g⁻¹ under the same conditions). In 2002, the uptake capacity of MOFs for methane was further advanced to 0.17 g g⁻¹ (155 cm³ (STP) cm⁻³) at 298 K and 36 bar, by using a series of MOFs with pore sizes ranging from 3.8 to 28.8 Å that are constructed via isoreticular approach. [10] Following these pioneering works, many representative MOFs including MIL-100, MIL-101, HKUST-1, [Zn₂(dabco)(bdc)₂] (dabco = 1,4-diazabicyclo[2.2.2]octane, bdc²⁻ = benzene-1,4-dicarboxylate), PCN-14 and MOF-74 have been evaluated for methane storage, [53–56] giving a record at that time, that is 220 cm³ (STP) cm⁻³ for PCN-14 [55]. Then from 2011, intense research endeavours have been performed on this application. Accordingly, MOF materials with improved properties have been developed. In particular, HKUST-1 shows a very high volumetric storage capacity of 267 cm³ (STP) cm⁻³ at 298 K and 65 bar [57], while Co(bdp) (bdp²⁻ = 1,4-benzenedipyrazolate) shows a record working capacity of 197 cm³ (STP) cm⁻³ from unique stepwise sorption behavior at 298 K and operating pressure of 65–5.8 bar [58].

Considering the non-polar and low polarizability nature of methane molecule, it is a very challenging task to advance the volumetric storage and deliverable working capacities of MOF adsorbents for methane. After years of continuous efforts, the comprehensive studies in this field can afford some empirical principles [31]. In general, at high pressure (over 60 bar), the total gravimetric CH₄ storage capacities of MOFs are highly relied on their pore volumes and Brunauer-Emmett-Teller (BET) surface areas [57–60]. In contrast, under moderate condition, besides pore volume and surface area, other structural features of targeted adsorbent should also be taken into account, namely pore size, open metal sites, ligand functionalization, and so on [61]. Consequently, it is quite straightforward to achieve high methane storage capacity by using MOFs with ultrahigh surface area. It is generally recognized that the optimal pore structure of rechargeable methane adsorbents for high volumetric storage capacity features pore size of 11 Å, volumetric BET surface area of 1400–2400 m² cm⁻³ and open metal sites, with optimal adsorption enthalpy of 17 kJ mol⁻¹ [62]. But there is still a long way to go before achieving the latest ambitious targets of 350 cm³ (STP) cm⁻³ and 0.5 g g⁻¹ at room temperature, which is set by US Department of Energy (DOE).

The continuous efforts of our group on MOFs for methane storage previously focus on exploring dicopper paddle-wheel compounds with open metal sites [63], which have been demonstrated as efficient methane adsorbents owing to their appropriate pore structures. To construct MOFs with open metal sites of high-density for high CH₄ storage capacity, in 2011 we realized a unique MOF [Cu₃(BHB)] (termed as UTSA-20, H₆BHB = 3,3′,3″,5,5′,5″-benzene-1,3,5-triyl-hexabenzoic acid) containing paddle-wheel Cu₂(COO)₄ SBUs with intensive open Cu sites, which shows a novel zyg topology of trinodal (3,3,4) net (Fig. 2a) [64]. UTSA-20 contains two kinds of pore structures with aperture size of about 3.4 × 4.8 Ų for rectangular ones and 8.5 Å in diameter for cylindrical ones as observed along the crystallographic $c$ axis, giving a porosity of 63%. The BET surface area of UTSA-20 was measured as 1156 m² g⁻¹. It is generally recognized that the open copper sites exposed on pore surface are accessible for methane molecules to preferentially binding. Therefore, with moderate porosity and high density of open Cu sites, UTSA-20 is very promising for methane storage. At 300 K and 35 bar, the absolute volumetric storage capacity of UTSA-20 for methane is 195 cm³ (STP) cm⁻³, corresponding to a methane storage density in micropores of UTSA-20 that is 0.22 g cm⁻³, which is comparable to that of the compressed methane at 300 K and 340 bar, and is higher than those of many high-performance MOFs under same condition. A derivative of UTSA-20, UTSA-61, showing ntt-type framework, was also been developed by our group in 2017 for enhanced methane storage performance [65]. UTSA-61 is constructed from a $C_2$-symmetry ligand with one ethynyl moiety on one branch of BHB, showing higher BET surface area of 2171 m² g⁻¹ as compared to UTSA-20. At 298 K and 65 bar, UTSA-61a exhibits a high methane storage capacity of 244 cm³ (STP) cm⁻³, giving a working capacity of 176 cm³ (STP) cm⁻³ (between 5 and 65 bar).

Fig. 2.

(a) Packing structure of UTSA-20 viewed along the crystallographic [0 0 1] axis, and corresponding high-pressure CH₄ sorption isotherms at various temperatures. Reprinted with permission [64]. Copyright 2014, Royal Society of Chemistry. (b) Crystal structure of MOF-505/NOTT-100, and corresponding high-pressure CH₄ sorption isotherms for its analogues. Reprinted with permission [59]. Copyright 2013, Royal Society of Chemistry.

In 2013, our group systematically investigated the methane storage capacities of a series of copper diisophthalate MOFs (MOF-505 or NOTT-100, NOTT-101, 102, 103, and 109) [59], derived from the prototypal MOF-505 (or NOTT-100) [17], which shows nbo topology. These variants show high methane storage capacity ranging from 181 to 196 cm³ (STP) cm⁻³ at 300 K and 35 bar (Fig. 2b), giving working capacity of 136–140 cm³ (STP) cm⁻³ under the operating pressure of 35–5 bar. Importantly, a derived empirical equation was obtained from the results of these isoreticular MOFs, which can be used to reasonably predict the methane storage performance, thus affording a tool for preliminary screening of potential porous MOFs for methane storage. This study also demonstrated that the rational control over pore structure of MOFs through the variation of ligand length can optimize their methane storage capacity.

In 2014, we realized to advance volumetric methane storage capacity of MOFs by incorporating functional groups/sites onto their pore surfaces [66,67]. In specific, by substituting the benzene spacer to pyridine, pyridazine, and pyrimidine groups, isomorphous MOFs with Lewis basic nitrogen sites were obtained, namely ZJU-5, UTSA-75 and UTSA-76 respectively, which are ideal for exclusive investigation of functionalizating effect on methane storage (Fig. 3). At 298 K and 65 bar, these MOFs show high total volumetric methane uptakes ranging from 249 to 257 cm³ (STP) cm⁻³, giving corresponding working capacities of 188–197 cm³ (STP) cm⁻³ at operating pressure of 65–5 bar, which are obviously higher than those for the prototypal NOTT-101 (237 and 181 cm³ (STP) cm⁻³ for total and working capacity, respectively). Particularly, among these MOFs, the pyrimidine-functionalized UTSA-76 shows a record-high volumetric working capacity of 197 cm³ (STP) cm⁻³ at room temperature [67]. In addition, the CH₄ storage capacities of derived multivariate MOFs that integrating different ligands are proportional to the ratios of nitrogen-containing linkers. Systematical studies revealed that the incorporation of N-containing moieties significantly enhances the dynamic feature of organic linkers, facilitating adsorption rearrangement under high pressure for high methane packing efficiency. Later, other functional groups such as CF₃ and F were also immobilized into the organic linker, giving UTSA-88 and NOTT-108 [68,69], which show total volumetric methane uptakes of 248 and 247 cm³ (STP) cm⁻³ at 298 K and 65 bar with working capacities of 185 and 186 cm³ (STP) cm⁻³, respectively.

Fig. 3.

Total volumetric methane storage capacities of NOTT-101a, ZJU-5a, UTSA-75a, and UTSA-76a (from bottom to top, triangle) at 65 bar and room temperature. The insets are their corresponding crystal structures. Reproduced with permission from Ref. [66]. Copyright 2015, Royal Society of Chemistry.

Very recently, inspired by the role of pyrimidine moiety for methane storage performance of UTSA-76, we developed a new MOF material (UTSA-110a) based on an extended tetracarboxylic acid ligand that immobilizes two pyrimidine groups to advance methane storage (Fig. 4) [70]. Owing to the longer linkers, UTSA-110a shows a larger BET surface area of 3241 m² g⁻¹ as compared to the prototypal UTSA-76 (2820 m² g⁻¹). At 298 K and 65 bar, UTSA-110a exhibits high gravimetric total methane uptake capacity of 402 cm³ (STP) g⁻¹, corresponding to 241 cm³ (STP) cm⁻³ in volumetric, which gives very high working capacities of 317 cm³ (STP) g⁻¹ and 190 cm³ (STP) cm⁻³ (at operating pressure of 65–5.8 bar) in gravimetric and volumetric, respectively. These results are higher than those of top-performing MOFs like UTSA-76a, HKUST-1 and NOTT-102, showing a remarkable increase in gravimetric working capacity while maintain its high volumetric capacity. The optimized methane storage performance of UTSA-110a implies high methane packing efficiency can be realized through rational design of porous media.

Fig. 4.

(a) Comparison of crystal structures of NOTT-101, UTSA-76, and UTSA-110. (b) A detailed comparison of the methane storage capacities of UTSA-110a with two best-performing MOFs (UTSA-76a and HKUST-1). (c) The CH₄ gravimetric/volumetric working capacities (between 5.8 and 65 bar) for UTSA-110a in comparison to the best robust MOFs reported. Reprinted with permission from Ref. [70]. Copyright 2018, Wiley-VCH.

Overall, our continuous efforts on the application of MOFs for methane storage have brought in gradually enhanced methane storage performance [71]. It should be noted that the reported performance of methane storage in MOFs is still lower than the target set by US DOE, the implementation of which requires further endeavours in the field. On the other hand, in terms of high pressure methane storage, MOF materials are too modest under current storage conditions, considering their exceptional saturated capacities at cryogenic temperatures. Certainly, methane storage in MOFs under relatively moderate conditions might be also applicable for the implementation of such important application, which should be also paid attention to.

3. Gas separation

Among many potential applications of MOF materials, gas separation/purification is the most promising one, which is widely involved during many important industrial processes, including the manufacture of hydrocarbon commodities, natural gas upgrading, the removal of harmful gas, carbon dioxide capture and so on. It is generally recognized that adsorptive separation based on physical porous media is more energy efficient as compared with conventional energy intensive approaches like distillation and chemical adsorbents. MOFs have been proven to be great promising for gas separation and purification owing to their unique pore structures and pore surfaces. Tremendous research efforts have been dedicated to using MOFs for gas separations, and significant progresses have been witnessed in the last decade. Our ongoing research effort has long been dedicated to the exploration of porous MOFs for gas separation and purification (Fig. 5) [72]. We have made seminal contributions on MOFs for gas chromatographic separation of hexane isomers (at 2005), kinetic D₂/H₂ separation (at 2008), C₂H₂/C₂H₄ separation (at 2011) and C₃H₄/C₃H₆ separation (at 2017). We also made very important contributions to other challenging gas separation, including CO₂ capture, C₂H₄/C₂H₆ and C₂H₂/CO₂ separation. The initial efforts of MOF community on gas separation generally feature predicting separation potential based on single-component gas sorption isotherms. At the early stage, actual separation of gas mixtures directly using MOFs was scarcely realized. Our group is one of the first groups who have envisioned the practical promise of MOFs for these important and challenging gas separations, and examined their separation performance through the fixed-bed adsorption and/or breakthrough experiments. Our ongoing research efforts on MOFs for gas separation have eventually targeted some highly sieving MOF materials for highly challenging C₂H₂/CO₂, C₂H₂/C₂H₄, C₂H₄/C₂H₆ and C₃H₄/C₃H₆ separation. In the following of this section, we high-lighted our major contribution on MOFs for gas separations.

Fig. 5.

Chronology of key progresses made by our group (highlighted in gray) in the field of MOFs for gas separation.

In 2005, our group reported the first example of MOF materials for alkane isomers separation by using experimental setup of gaschromatographic (GC) column [73]. A double interpenetrated MOF [Zn₂(bdc)₂(bpy)] (MOF-508, H₂bdc = 1,4-benzenedicarboxylic acid; bpy = 4,4′-bipyridine) was used to separate mixtures of linear and branched isomers of pentane and hexane (Fig. 6a). This MOF is 3D framework with pcu topology that composing of 6-connected paddle-wheel zinc clusters as metal nodes, bdc²⁻ and bpy as bridged linkers. The 1D channels in as-synthesized MOF-508a are about 4.0 Å in diameter, which are accessible for the diffusion of linear alkanes while excluding their branched isomers, as considering the pore size is only slightly larger than size of methane molecules (kinetic diameter: 3.8 Å). After flowed through the GC column of MOF-508, the alkanes mixtures can be well separated into single components, with a retention time order of linear > monobranched > dibranched alkanes. For mixture of hexane isomers, pure 2,2-dimethylbutane (22DMB) eluted firstly from the column, followed by 2-methylpentane (2MP), while their linear isomer n-hexane (nHEX) shows the longest retention time. Like-wise, branched 2-methylbutane can also be separated apart from n-pentane. The selective separation of alkane isomers was attributed to the different van der Waals interactions between these molecules and pore surface of MOF-508. The retention time of an alkane isomer mainly depends on the length of its linear part, facilitating efficient separation of alkane isomers. In 2007, experimental fixed-bed breakthrough study was firstly performed on the separation of hexane isomers in a 3D MOF [Zn₂(bdc)₂(dabco)] with pore sizes of about 7.5 Å × 7.5 Å and 3.8 Å × 4.7 Å (Fig. 6b) [74]. Breakthrough separation experiments of binary and ternary isomers mixtures of hexane demonstrated that this MOF can well separate the monobranched 3-methylpentane (3MP) and dibranched 22DMB from linear nHEX alkane. This fixed-bed breakthrough approach enables real-time dynamic monitoring of component concentrations.

Fig. 6.

(a) Space-filling packing structures diagram of MOF-508a, which contains 1D channels of 4.0 Å × 4.0 Å, and chromatograms of alkane mixtures separated on a MOF-508 column. S = thermal conductivity detector response. Reprinted with permission from Ref. [73]. Copyright 2006 Wiley-VCH. (b) Perspective illustration of 3D intersecting channels in the X-ray crystal structure of [Zn₂(bdc)₂(dabco)] with two types of pore apertures, and corresponding binary breakthrough curves for an equimolar mixture of 22DMB/nHEX at T = 313 K. Reprinted with permission from Ref. [74]. Copyright 2007 American Chemical Society.

In 2008, fixed-bed breakthrough was applied to the separation of gaseous molecules, representing a turning point in the field. The breakthrough experiments were performed on a double interpenetrated [Zn₂(bdc)₂(bpy)] (MOF-508) for the separation of CO₂/N₂ and CO₂/CH₄ mixtures (Fig. 7a) [75]. The pore size of MOF-508 (4.0 Å) is similar to the molecular sizes of CO₂ (3.30 Å), CH₄ (3.76 Å) and N₂ (3.64 Å), that is suitable for the capture and separation of CO₂ from related binary and ternary mixtures. Single-component gas adsorption isotherms revealed that the uptake capacities of MOF-508b for CO₂, CH₄ and N₂ at 4.5 bar and 303 K are 26.0, 5.5 and 3.2 wt%, respectively. The adsorption enthalpy at zero coverage for CO₂ was determined to be 14.9 kJ mol⁻¹, which is much higher than those for CH₄ and N₂. Firstly, single-component breakthrough experiments were applied to evaluate the separation potential of MOF-508b, showing a breakthrough time hierarchy of CO₂ > CH₄ > N₂, which confirms that MOF-508b can preferentially adsorb CO₂ over CH₄ and N₂. Further fixed-bed breakthrough experiments indicated that CO₂ can be efficiently removed from corresponding CO₂/CH₄, CO₂/N₂ and CH₄/N₂/CO₂ mixtures, giving a CO₂/CH₄ selectivity of 3 and a CO₂/N₂ selectivity of 6 as estimated from breakthrough experiments. This study represents the first example in MOF community of using fixed-bed breakthrough to mimic practical gas separation process; here specific involved the separation of CO₂ over CH₄ and N₂ from their binary and ternary mixtures. Since then, this very important technology has gradually become a metrics for the evaluation of MOFs in gas separation.

Fig. 7.

(a) One-dimensional micropores of about 4.0 Å × 4.0 Å in MOF-508. (b) Single-component breakthrough curves for N₂ (square), CH₄ (circle), and CO₂ (triangle) at 323 K and 1 bar. Reprinted with permission from Ref. [75]. Copyright 2008 American Chemical Society. (c) The cavity structure of UTSA-16 (yellow ball of about 4.5 Å in diameter). (d) Comparison of the adsorption isotherms of CO2 at 296 K. From top to bottom: MgMOF-74, UTSA-16, ZnMOF-74, bio-MOF-11, CuBTC, Cu-TDPAT, UTSA-20a, ZIF-78, Zn₅(BTA)₆(TDA)₂, Zn(bdc)(dabco), MIL-101, Yb(BPT) and MOF-177. Reproduced with permission from Ref. [76]. Copyright 2012 Nature Publishing Group.

In terms of CO₂ capture and separation [77], we realized a benchmark MOF in 2012, namely [K(H₂O)₂Co₃(cit)(Hcit)] (UTSA-16), from a very cheap chemical citric acid (Hcit) to exhibit remarkable performance for CO₂ capture (Fig. 7b) [76]. UTSA-16 is a 3D framework with dia topology containing 3D channels of about 3.3 × 5.4 Ų and small cavities of 4.5 Å, which is composed of Co₄O₄ clusters and K⁺-polyhedra linkers via face-sharing and further infinite 3D heteronuclear M-O-M connections. The BET surface area of UTSA-16 is 628 m² g⁻¹. Under ambient conditions, the activated UTSA-16 exhibited high CO₂ uptake of 160 cm³ (STP) cm⁻³, high CO₂/CH₄ selectivity (29.8) for 50/50 CO₂/CH₄ mixture, and high CO₂/N₂ selectivity (314.7) for 15/85 CO₂/N₂ mixture. Simulated breakthrough experiments demonstrated the high CO₂ adsorption capacity and selectivity of UTSA-16 for binary CO₂/CH₄ and CO₂/N₂ gas mixtures. Powder neutron diffraction studies revealed that the remarkable performance of UTSA-16 for CO₂ capture can be attributed to its optimal pore size and suitable binging sites for CO₂ molecules (on the terminal coordinated water molecules). The optimized separation performance, low cost and good stability of UTSA-16 renders it as a potential candidate for CO₂ capture and removal.

In 2008, our group realized the first experimental example of MOFs for the challenging kinetic sieving H₂/D₂ separation [78]. We developed a mixed-metal–organic framework (M′MOF) [Zn₃(bdc)₃(CuPyen)] (M′MOF-1, PyenH₂ = 5-methyl-4-oxo-1,4-dihydro-pyridine-3-carbaldehyde) with two open sites per metal center, showing enhanced affinity for hydrogen molecules, which realized kinetic quantum molecular sieving of D₂ from H₂ (Fig. 8). M′MOF-1 is constructed from trinuclear Zn₃(RCOO)₆ clusters that bridged by bdc²⁻ and preconstructed metalloligand Cu(Pyen), giving 3D framework with two types of micropores (both ≤5.6 Å). After activation, the immobilized Cu centers with two open sites are accessible for gas molecules on the pore surface. Gas sorption study of M′MOF-1 for H₂ and D₂ gave comparable D₂/H₂ molar ratios $(n{\text{D}}_2/n{\text{H}}_2)$ of 1.09–1.11, which can be attributed to quantum effect. The adsorption isosteric enthalpies at zero coverage $(Q_{\text{st,n}=0})$ were calculated to be 12.29(53) and 12.44(50) kJ mol⁻¹ for H₂ and D₂, respectively. Adsorption kinetics analyses for the diffusion of H₂ and D₂ during adsorption in M′MOF-1 revealed that the heavier D₂ were faster than H₂, showing average ratios of $k{\text{D}}_2/k{\text{H}}_2$ rate constants of 1.62 and 1.38 for two components ($k_1$ and $k_2$), respectively, which was attributed to different diffusion along two types of pores. Such adsorption difference was attributed to quantum effects referring to the higher effective collision cross section of H₂ related to its higher zero point energy that results in a higher barrier for diffusion along the pores as compared to D₂. The D₂ kinetic barriers related to its zero-point energy (activation energy) were determined to be 12.52 and 8.04 kJ mol⁻¹ for both components, which are slightly lower than those of H₂ kinetics (13.35 and 8.56 kJ mol⁻¹). This work represents the first experimental results of MOF materials for kinetic isotope quantum molecular sieving of hydrogen [79]. Later research efforts from chemistry and material communities have been dedicated to this important application [80–82].

Fig. 8.

(a) X-ray crystal structure of M′MOF-1 [Zn₃(bdc)₃Cu(Pyen)] showing (i) one trinuclear Zn₃(COO)₆ secondary building unit, (ii) one 36 tessellated Zn₃(bdc)₃ 2-D sheet that is pillared by the Cu(Pyen) to form a 3-D microporous M′MOF-1 having (iii) curved pores of about 5.6 Å × 12.0 Å and (iv) irregular ultramicropores along different directions. Color scheme: Zn (magenta), Cu (green), O (red), N (blue), C (gray), H (white). (b) Isotherms for H₂ and D₂ adsorption on M′MOF-1 at 77.3 and 87.3 K. (c) Comparison of H₂ and D₂ kinetic profiles and the corresponding fitting for DE kinetic model for adsorption on M′MOF-1 at 77.3 K (0.2–0.4 kPa). Reprinted with permission from Ref. [78]. Copyright 2008 American Chemical Society.

In 2011, we reported the first example of microporous MOFs for the important but challenging separation of acetylene/ethylene during petrochemical industrial processes [83]. Based on metalloligand Cu(SalPycy), two isostructural mixed-metalorganic frameworks M′MOFs Zn₃(bdc)₃[Cu(SalPycy)] (M′MOF-2) and Zn₃(cdc)₃[Cu(SalPycy)] (M′MOF-3, H₂cdc = 1,4-cyclohexanedicarboxylate acid) have been developed for optimized C₂H₂/C₂H₄ selectivities (Fig. 9). Both MOFs contain chiral pore cavities with immobilization of open Cu centers. M′MOF-2 and M′MOF-3 are 3D isomorphic pillar-layered frameworks, in which 2D Zn₃(bdc)₃ or Zn₃(cdc)₃ layers are composed of trinuclear Zn₃(RCOO)₆ clusters that connected by bdc²⁻ or cdc²⁻ ligands while the chiral metalloligands Cu(SalPyCy) serving as pillars. M′MOF-2 and M′MOF-3 contain chiral pore cavities of about 6.4 Å with pore accessible volume of 52% and 48%, respectively, showing BET surface areas of 388 and 110 m² g⁻¹. At 195 K, M′MOF-3a showed a high C₂H₂ uptake capacity of 147 cm³ g⁻¹ and small C₂H₄ uptake capacity of 30 cm³ g⁻¹, giving significantly high C₂H₂/C₂H₄ selectivity of 25.5, while M′MOF-2a showed a low C₂H₂/C₂H₄ selectivity of 1.6. The higher selectivity of M′MOF-3a was attributed to enhanced sieving effects of smaller pores in M′MOF-3a. Compared with the molecular size of C₂H₄ (kinetic diameters: 4.2 Å), the smaller C₂H₂ molecules (3.3 Å) are able to efficiently fill the micropores of M′MOF-3a. At 295 K, the C₂H₂/C₂H₄ selectivity of M′MOF-3a was calculated to be 5.2, rendering this material a practically promising adsorbent for this important separation. Later, more isostructural M′MOFs have been developed for the removal of C₂H₂ from 1/99 C₂H₂/C₂H₄ mixture [84], which are competent to reduce the C₂H₂ concentration to <40 ppm that meets the polymerization requirement, but suffer from low gas uptake capacity owing to limited pore space. Our research endeavors were also extended to another series of MOFs with open metal sites, M-MOF-74, for C₂H₂/C₂H₄ separation [85], which show high uptake capacity and strong binding affinity for C₂H₂ and C₂H₄, being lack of high selectivity.

Fig. 9.

(a) The three-dimensional pillared framework with chiral pore cavities for M′MOF-3. (b-c) Adsorption isotherms of C₂H₂ (blue square), CO₂ (red dot) and C₂H₄ (green triangle) on (b) M′MOF-2a and (c) M′MOF-3a at 295 K. Reproduced with permission from Ref. [83]. Copyright 2011 Nature Publishing Group.

Ideal MOF materials for C₂H₂/C₂H₄ separation should exhibit high C₂H₂/C₂H₄ selectivity and optimal C₂H₂ adsorption capacity at ambient conditions. Accordingly, MOFs with dual-functionality featuring suitable pore/aperture size and accessible strong binding sites were then proposed as very promising candidates. Tremendous research efforts have been dedicated to using MOFs for C₂H₂/C₂H₄ separation [89], and significant progresses have been realized since the initiation of such application. In 2015, we developed a dual-functionalized MOF [Cu(atbdc)] (UTSA-100, H₂atbdc = 5-(5-amino-1H-tetrazol-1-yl)-1,3-benzenedicarboxylic acid) with apo topology for efficient C₂H₂ removal from 1/99 C₂H₂/C₂H₄ mixture (Fig. 10a-b) [86]. UTSA-100 features 1D channels of ~4.3 Å with small cavities of 4.0 Å (aperture size: 3.3 Å), exhibiting moderate BET surface area of 970 m² g⁻¹, and –NH₂ groups as binding sites. Consequently, UTSA-100a shows high C₂H₂ uptake capacity of 95.6 cm³ (STP) g⁻¹ while adsorbs fewer C₂H₄ of 37.2 cm³ (STP) g⁻¹ at ambient conditions, giving a high C₂H₂/C₂H₄ selectivity of 10.7 for 1/99 C₂H₂/C₂H₄ mixture. Compared to M′MOFs, M-MOF-74 series and NOTT-300, UTSA-100a shows superior separation performance in trace acetylene removal, owing to its suitable pore size and binding sites.

Fig. 10.

(a) Pore structure of UTSA-100 showing the zigzag channels along the $c$ axis and the cage with the diameter of about 4.0 Å in the pore wall. (b) Acetylene (red) and ethylene (blue) sorption at 296 K. Reproduced with permission from Ref. [86]. Copyright 2015 Nature Publishing Group. (c) Neutron crystal structures of $\text{SIFSIX-1-Cu}\supset {\text{C}}_2{\text{D}}_2$ at 200 K from Rietveld analysis. (d) Sorption isotherms of C₂H₂ (filled circles) and C₂H₄ (triangles) in SIFSIX-1-Cu (red), SIFSIX-2-Cu (green), SIFSIX-2-Cu-i (blue), SIFSIX-3-Zn (light blue), and SIFSIX-3-Ni (orange) at 298 K, 1.0 bar. Open circles are desorption isotherms of C₂H₂. Reproduced with permission from Ref. [87]. Copyright 2016 American Association for the Advancement of Science. (e) Crystal structure of UTSA-200a with the packing diagram viewed along crystallographic [0 0 1] axis. (f) Sorption isotherms of C₂H₂ (circles) and C₂H₄ (triangles) in UTSA-200a and SIFSIX-2-Cu-i at 298 K, 1.0 bar. Reproduced with permission [88]. Copyright 2017 Wiley-VCH.

To overcome the trade-off of adsorption capacity versus selectivity for C₂H₂/C₂H₄ separation, precise control of pore chemistry and size over series of SIFSIX materials was performed (Fig. 10c-d) [87]. In 2016, systematic investigations on these pillar-layered SIFSIX materials revealed that SIFSIX-2-Cu-i shows exceptionally high C₂H₂ uptake capacity of 2.1 mmol g⁻¹ (47 cm³ g⁻¹) at 0.025 bar and 298 K, giving record C₂H₂/C₂H₄ selectivities of 39.7–44.8 at that time. The separation mechanism of the superior SIFSIX-2-Cu-i was rationalized to be that C₂H₂ molecules are interacts strongly with two different ${\text{SiF}}_6^{2-}$ groups through cooperative C–H · · · F hydrogen bonds, while extra van der Waals interactions forms within suitable pore with aperture size of 4.4 Å, resulting in preferentially binding of C₂H₂ molecules from C₂H₂/C₂H₄ mixture. Other SIFSIX materials including SIFSIX-1-Cu (with pore size of 8 Å), SIFSIX-2-Cu (with pore size of 10.5 Å) and SIFSIX-3-M (with pore size of 4.2 Å) were also studied systematically for comparison. Breakthrough experiments and simulations for C₂H₂/C₂H₄ mixtures (1/99 and 50/50) demonstrated these SIFSIX materials as highly efficient adsorbents for the production of high-purity ethylene, as they can reduce the acetylene impurity to be less than 2 ppm under ambient conditions. This work represents one of milestones for MOFs in the field of hydrocarbon separations.

In 2017, we realized a unique SIFSIX material [Cu(azpy)₂(SiF₆)] (UTSA-200, also SIFSIX-14-Cu-i, azpy = 4,4′-azopyridine) with optimal pore structure (aperture size of ~3.4 Å) and high density of strong binding sites, showing molecular sieving of C₂H₂ over C₂H₄ under ambient conditions (Fig. 10e-f) [88]. By controlling the pore structure at more precise level, the interpenetrated UTSA-200 shows comparable porosity as that of its analogue SIFSIX-2-Cu-i, but smaller pore aperture size (3.4 vs 4.4 Å). Consequently, activated UTSA-200a shows a large acetylene uptake capacity of 3.65 mmol g⁻¹ (~82 cm³ g⁻¹) at 1 bar and 298 K that comparable to SIFSIX-2-Cu-i, while takes up much less ethylene of <0.25 mmol g⁻¹ (~5.6 cm³ g⁻¹) at low pressures as compared to that of SIFSIX-2-Cu-i (2.28 mmol g⁻¹, ~51 cm³ g⁻¹). UTSA-200a thus exhibits a record high selectivity up to 6000 for 1/99 C₂H₂/C₂H₄ mixture at 1 bar and 298 K. During a single breakthrough cycle, the C₂H₄ production from the outlet effluent is up to 85.7 mmol g⁻¹, while 1.18 mmol g⁻¹ C₂H₂ is captured from the mixture. Crystal structures, molecular modeling, selectivity calculation, and experimental breakthrough experiments comprehensively demonstrated this unique material for sieving C₂H₂ from C₂H₄, which represents the best separation performance of MOFs for C₂H₂/C₂H₄ mixture. The molecular sieving of C₂H₂/C₂H₄ has also been realized by virtue of another SIFSIX material UTSA−300a [90], which features dynamic pore structure that can exclusively opened by C₂H₂ molecules. Benefiting from our control of pore engineering in MOFs, MOF adsorbents are well competent for the very challenging C₂H₂/C₂H₄ separation.

Among various C₂H₂−containing mixtures, C₂H₂/CO₂ is one of the most challenging mixtures for adsorptive separation by porous media, as considering their identical sizes, shapes and physical properties. Structurally, C₂H₂ molecules can interact with Lewis acidic sites like open metal sites through its pi system of carbon−carbon triple bond, while interact with Lewis basic sites using its H atoms as H−bonding donors. In contrast, CO₂ molecules bind to Lewis acidic sites (coordinates to metal site) through its electronegative O atoms, while interact with Lewis basic sites using the electropositive C atoms. Taking these into account, several approaches have been proposed to boost the C₂H₂−selective adsorption [92–96]. In 2016, we reported an unique MOF−74 isomer [Zn₂(dobdc)(H₂O)] 0.5H₂O (UTSA−74, H₄dobdc = 2,5−dihy droxy−1,4−benzenedicar−boxylic acid) with two accessible binding sites per open metal center for enhanced C₂H₂/CO₂ separation (Fig. 11a–c) [91]. UTSA−74 exhibits a framework topology of four connected fgl with 1D open channels of ~8.0 Å along the crystallo−graphic [0 0 1] direction. Particularly, the potential open Zn centers in UTSA−74 exhibit two accessible gas binding sites after the removal of corresponding axial water molecules. In contrast, in the well−known MOF−74 series, there is only one accessible site per open metal center for the binding of gas molecules. Given that both MOFs show equal average density of open metal sites, UTSA−74a exhibits a comparable acetylene uptake capacity (145 cm³ cm⁻³) as Zn−MOF−74 do. However, for CO₂ adsorption under ambient condition, UTSA−74a shows a much smaller uptake capacity of 90 cm³ cm⁻³ than that of Zn−MOF−74 (146 cm³ cm⁻³), giving a C₂H₂/CO₂ uptake ratio of 1.5. It is because that each open Zn site in UTSA−74 binds two C₂H₂ molecules with terminal−binding mode, while binds only one CO₂ molecule as each CO₂ molecule can bridge then be shared by two open metal sites, as confirmed by X−ray crystal structures and molecular modeling studies. UTSA−74a shows a fairly superior C₂H₂/CO₂ selectivity of 9 for 50/50 C₂H₂/CO₂ mixture at ambient condition, comparing to typical porous media. Breakthrough experiment over a column of UTSA−74a reveals well separation of C₂H₂ from the 50/50 C₂H₂/CO₂ mixture. This work represents the first example that clearly establishes C₂H₂/CO₂ separation performance of MOFs by experimental breakthrough.

Fig. 11.

(a) X−ray single crystal structure of $\text{UTSA}-74\supset \text{C}{\text{O}}_2$, and (b) corresponding local coordination environment. The Zn ions with two accessible binding sites are highlighted in gold, and the tetrahedral ions are shown in green tetrahedron. (c) Comparison of sorption isotherms of C₂H₂ and CO₂ for UTSA−74 and Zn−MOF−74 at 298 K. Reproduced with permission from Ref. [91]. Copyright 2016 American Chemical Society. (d) Perspective views of cage unit in UTSA−300 showing aperture size of 3.3 Å (Zn, Si, F, S, N, and C are represented by purple, orange, green, bright yellow, light blue, and gray, respectively, and solvent molecules are omitted for clarity). (e) Packing diagram of $\text{UTSA-}300\supset {\text{C}}_2{\text{D}}_2$ from neutron powder diffraction data, C₂D₂ molecules are shown in a CPK model. (f) C₂H₂, CO₂, and C₂H₄ single−component sorption isotherms for UTSA−300a at 273 K. Reproduced with permission from Ref. [90]. Copyright 2017 American Chemical Society.

Later in 2017, we realized an unique microporous material [Zn (dps)₂(SiF₆)] (UTSA−300, dps = 4,4′−dipyridylsulfide) showing exclusively binding to C₂H₂ molecules over CO₂ under ambient conditions (Fig. 11d–f) [90]. UTSA−300 contains high density of hexafluorosilicate F sites and undulating 2D pore channels with aperture size of ~3.3 Å. Apparently, the Lewis basic sites in this structure can afford specific binding for gas molecules, while the pore aperture size of ~ 3.3 Å enables possible molecular sieving. This structure exhibits a structural transformation of pore opening to pore closing during activation/desolvation, giving a closed−pore framework UTSA−300a with dispersed 0D cavities. Although the pore size of UTSA−300 matches well with molecular sizes of both CO₂ and C₂H₂, UTSA−300a only adsorbs C₂H₂ molecules under ambient conditions with uptake capacity of 76.4 cm³ g⁻¹ while exclude CO₂ and C₂H₄ molecules, giving unprecedented C₂H₂/CO₂ uptake ratio (19) and selectivity that is superior to any other MOFs. This porous material shows remarkable binding affinity for C₂H₂ with adsorption heat of 57.6 kJ mol⁻¹. As demonstrated by crystal structures and modeling studies, the dynamic nature of UTSA−300 revealed by the structural transformation of gate−opening involving the formation of H−bonding interactions dominates to the selective recognition of CO₂ over C₂H₂. This work represents significant progress for complete molecular exclusion of CO₂ from C₂H₂.

Similarly, using MOFs for propyne removal related to the production of propylene also came into our view for its significance. In 2017, our group reported the first example using MOFs for this challenging C₃H₄/C₃H₆ separation (Fig. 12a–c) [97]. ELM−12 featuring rigid square−grid copper bipyridine scaffold with dynamic dangling OTf⁻ groups was targeted owing to its suitable pore size (cavity size: 6.1 Å × 4.3 Å × 4.3 Å and 6.8 Å × 4.0 Å × 4.2 Å). After activation, the C₃H₄ uptake capacity of ELM−12 at low pressure is up to 2.55 mmol/g (298 K), while that for C₃H₆ is only 0.67 mmol/g, giving a IAST selectivity of 84 for 1/99 C₃H₄/C₃H₆ mixture. The propyne molecules were found to exhibiting strong interactions with oxygen atoms of trifluoromethanesulfonate (OTf⁻) groups through cooperative C–H·· ·O interactions as examined by neutron powder diffraction of C₃D₄−loaded sample, giving high adsorption heat of 60.6 kJ mol⁻¹ for propyne that are much higher than that for propylene (32.3 kJ mol⁻¹). Therefore, ELM−12 exhibits remarkable separation performance as confirmed by breakthrough experiments. Trace C₃H₄ can be removed from 1/99 C₃H₄/C₃H₆ mixture through the fixed bed affording a polymer-grade C₃H₆ (>99.9998%). This work represents the first example of MOFs for challenging C₃H₄/C₃H₆ separation, which initiates intensive research efforts on this important hydrocarbon separation [99]. Also, this work has broken the stereotype of flexible MOFs being inferior for gas separation.

Fig. 12.

(a) Schematic diagrams of cavity in ELM−12 (Cu, green; C, gray; O, red; S, yellow; F, light green). (b) Neutron diffraction crystal structure of $\text{ELM-}12\supset {\text{C}}_3{\text{D}}_4$ showing the preferential binding sites for C₃D₄ molecules and their close contacts with the framework. (c) C₃H₄ and C₃H₆ adsorption isotherms of ELM−12 at ambient conditions. Reproduced with permission from Ref. [97]. Copyright 2017 American Chemical Society. (d) Crystal structure of UTSA−200a with the packing diagram viewed along crystallographic [0 0 1] axis. (e) DFT−D calculated structure and binding site of $\text{UTSA-200a}\supset {\text{C}}_3{\text{H}}_4$. (f) Sorption isotherms of C₃H₄ and C₂H₄ in UTSA−200a at 298 K. Reproduced with permission [98]. Copyright 2018 Wiley−VCH.

Very recently, we realized enhanced molecular sieving of C₃H₄ from C₃H₆ by virtue of UTSA−200 (Fig. 12d−e) [98], [Cu(azpy)₂(SiF₆)], from comprehensive screening of 20 MOFs featuring various framework structures, pore sizes and functional sites. Among various types of porous MOFs, UTSA−200a features suitable pore aperture size and strong gas binding affinity, showing a record high C₃H₄ uptake capacity of 95 cm³ cm⁻³ at very low pressure with negligible C₃H₆ adsorption, which gives superior C₃H₄/C₃H₆ uptakes ratio of ~150 that is one order of magnitude larger than previous benchmark MOF materials. The C₃H₄/C₃H₆ selectivity of UTSA−200a was calculated to be >20,000 based on IAST approach. Despite of the dynamic nature of dipyridine linkers in UTSA−200, followed by a certain adsorption uptake of C₃H₆ at high pressure region, this MOF still shows unprecedented C₃H₄/C₃H₆ separation performance, giving a record C₃H₆ productivity of 62.9 mmol g⁻¹ for 1/99 C₃H₄/C₃H₆ mixture under ambient condition, as confirmed by simulated and experimental breakthrough studies. This work represents the best C₃H₄/C₃H₆ separation performance reported so far.

In terms of hydrocarbon separation, the separation of ethylene from ethane takes the most important part as they are essential bulk chemical commodities and their productions are energy−intensive processes. Various strategies including immobilization of open metal sites have been proposed to develop efficient adsorbent for adsorptive separation of C₂H₄/C₂H₆, resulting significant progresses. Very recently, our group reported two breakthroughs on this very challenging, important C₂H₄/C₂H₆ separation [100,101]. The first breakthrough was achieved after our screening of MOFs with pore size of 3.4–4.4 Å, using molecular sieving approach over an ultramicroporous material [Ca(C₄O₄)(H₂O)] (termed as UTSA−280) (Fig. 13a−b) [100]. This MOF was synthesized from calcium nitrate and squaric acid, that possesses rigid 1D channels with aperture sizes of 3.2 × 4.5 and 3.8 × 3.8 Ų. These apertures are of a similar size to the minimum cross−sectional area of ethylene molecules (3.28 × 4.18 Ų), but larger than those of ethane molecules (3.81 × 4.08 Ų). Particularly, this structure exhibits high pore rigidity owing to the well constraints of organic moieties by rigid coordination geometry, which was well confirmed by comprehensive crystal structural studies on UTSA−280. Consequently, UTSA−280 exclusively adsorbs C₂H₄ molecules with uptake capacity of 88 cm³ cm⁻³ (2.5 mmol g⁻¹) at 298 K and 1 bar, while shows negligible adsorption uptake for ethane. Such molecular sieving behavior for C₂H₄/C₂H₆ separation realized by UTSA−280 is the first example of MOFs for this separation. The efficiency of this molecular sieve for the separation of C₂H₄/C₂H₆ mixtures is validated by breakthrough experiments with high C₂H₄ productivity of 1.86 mol kg⁻¹ under ambient condition. Further− more, this material is water−stable and can be easily synthesized at the kilogram scale based on an environmentally friendly method, which is important for potential industrial implementation. The strategy of using highly rigid MOFs with well defined and rigid pores could also be extended to other porous materials for chemical separation processes.

Fig. 13.

(a) Single crystal structure of $\text{UTSA-280}\supset {\text{C}}_2{\text{H}}_4$. (b) Single−component sorption isotherms of UTSA−280 for C₂H₄ and C₂H₆ at 298 K. Reproduced with permission from Ref. [100]. Copyright 2018 Nature Publishing Group. (c) Neutron diffraction crystal structures of $\text{Cu}{(\text{Qc})}_2\supset {\text{C}}_2{\text{D}}_4$. (d) Single−component sorption isotherms of Cu(ina)₂ and Cu (Qc)₂ for C₂H₄ and C₂H₆ at 298 K. Reproduced with permission from Ref. [102]. Copyright 2018 American Chemical Society. (e) Neutron diffraction crystal structures of ${\text{Fe}}_2\left({\text{O}}_2\right)\text{(dobdc)}\supset {\text{C}}_2{\text{D}}_6$. (f) Single−component sorption isotherms of Fe₂(O₂)(dobdc) for C₂H₄ and C₂H₆ at 298 K. Reproduced with permission from Ref. [101]. Copyright 2018 American Association for the Advancement of Science.

Compared with C₂H₄−selective adsorbents, C₂H₆−selective separation can directly generate C₂H₄ of desired purity in one single separation cycle, saving tremendous energy consumption. Recently, we realized to boost C₂H₆/C₂H₄ selectivity by controlling over pore structures of two isoreticular MOFs termed as Cu(ina)₂ and Cu(Qc)₂ (or Qc−5−Cu−sql [103]) with aperture size of 4.1 and 3.3 Å (Fig. 13c−d) [102], which feature weakly polar pore surface. Both MOFs show C₂H₆−selective adsorption behavior, with C₂H₆/C₂H₄ uptake capacity of 67.4/64.3 and 60.0/25.3 cm³ cm⁻³, respectively, giving a record C₂H₆/C₂H₄ uptake ratio of 237% for Cu(Qc)₂. At 298 K and 1 bar, the C₂H₆/C₂H₄ selectivity of Cu(Qc)₂ for 50/50 C₂H₆/C₂H₄ mixture is 3.4, significantly higher than that of Cu (ina)₂ (1.3). The enhanced C₂H₆/C₂H₄ separation performance of Cu(Qc)₂ is attributed to more close van der Waals contacts with C₂H₆ molecules in its self−adaptive pore structure. Breakthrough experiments comprehensively demonstrate this Cu(Qc)₂ as an efficient C₂H₆−selective adsorbent for C₂H₄ purification. This work presents a feasible approach to advance C₂H₆/C₂H₄ separation.

Our another breakthrough on ethylene−ethane separation was realized after metal−peroxo sites was incorporated into MOFs, which usually serve as catalytic active center in alkane C–H activation and here was proposed to selective bind C₂H₆ molecules (Fig. 13e−f) [101]. The post−synthesized MOF Fe₂(O₂)(dobdc) exhibits C₂H₆−selective adsorption with capacity of 74.3 cm³ g⁻¹ at 1 bar and 298 K, corresponding to ~1.1 C₂H₆ molecules per Fe₂(O₂)(dobdc) formula. This material shows a record adsorption selectivity of 4.4 for 50/50 C₂H₆/C₂H₄ separation, which represents a new benchmark for C₂H₆−selective adsorbents. Fe−peroxo sites do pay a key role for the selective recognition of C₂H₆ molecules as demonstrated by neutron powder diffraction studies and theoretical calculations. The separation performance of Fe₂(O₂)(dobdc) for the C₂H₆/C₂H₄ mixtures has been validated by simulated and experimental breakthrough curves. During the first adsorption cycle, polymer−grade C₂H₄ with purity of 99.99% can be straightforwardly produced after C₂H₆/C₂H₄ mixtures through a fixed−bed column packed with Fe₂(O₂)(dobdc), demonstrating the potential of Fe₂(O₂)(dobdc) in saving energy consumption for this important industrial separation under ambient conditions.

As presented above, our continuous efforts on advancing MOF materials for various gas separations have realized some breakthroughs, rendering MOFs as an exceptional platform for exploring novel porous media. These progresses highlight the potential of MOF materials in addressing some challenging separation for important chemicals. It should be noted that there are still challenges to overcome before MOFs come into real application in industrial separation processes, including stability, impurity tolerance, processability, persistence, material cost, recyclability. Besides fixed−bed separation approach, other advancing approaches including film/membranes also show great potential for gas separation, affording numerous opportunities for the communities of materials and chemistry.

4. Luminescent sensing

By integrating the merits of organic and inorganic chromophores, as well as host−guest chemistry, MOFs can exhibit diversified luminescence. Since such type of luminescence show responses to external stimulus, taking the intrinsic porosity of MOFs into account, MOFs are unparalleled luminescent materials for detecting chemical species. In particular, luminescence sensing is a fast, highly sensitive and straightforward approach for in−field and real−time detection. More importantly, by controlling the pore structures and pore chemistry, specific binding sites can be easily incorporated onto pore surface for the selective recognition of various small molecules, metal ions and anions [38]. These collaborative features for luminescent MOFs have enabled us to realize some porous luminescent materials as chemical sensors.

In 2007, our group realized a lanthanide MOF [Eu(BTC)(H₂O)] 1.5H₂O (BTC³⁻ = benzenetricarboxylate) for the selective sensing of organic solvent molecules by incorporating potential open Eu³⁺ sites (Fig. 14a−b) [104]. This MOF contains 1D pore channels with pore size of 6.6 × 6.6 Ų. This MOF shows different luminescent responses to various solvent molecules including DMF, CH₃−CN, CHCl₃, 2−propanol, 1−propanol, methanol, THF, ethanol, and acetone, among which significant fluorescent enhancement was observed for DMF while almost complete fluorescence quenching for acetone. These different fluorescent responses were attributed to different binding interactions between potential open Eu³⁺ sites and various solvent molecules.

Fig. 14.

(a) Crystal structure of [Eu(BTC)(H₂O)]∙solvents, featuring Lewis basic pyridyl sites oriented toward pore centers. (b) The PL spectra of Eu(BTC) introduced into various pure solvent emulsions. Reprinted with permission [104]. Copyright 2007 Wiley−VCH. (c) Crystal structure of activated Tb(BTC) in methanol containing NaF with the model of fluoride (green) at the center of the channel involving its hydrogen bonding interaction with terminal methanol molecules. (d) Excitation (dotted) and PL spectra (solid) of activated Tb(BTC) solid in different concentrations of NaF methanol solution (excited and monitored at 353 and 548 nm, respectively). Reprinted with permission from Ref. [105]. Copyright 2008 American Chemical Society. (e) Crystal structure of [Eu(PDC)_1.5(DMF)]∙solvents, featuring Lewis basic pyridyl sites oriented toward pore centers. Color codes: Eu green polyhedral, C gray, N purple, O red. (f) The excitation (dotted) and PL spectra (solid) of activated Tb(BTC) solid in different concentrations of Cu (NO₃)₂ DMF solution (excited and monitored at 321 and 618 nm, respectively). Reprinted with permission [106]. Copyright 2009 Wiley−VCH.

In 2008, our group reported another example of luminescent MOFs for sensing and recognition of anions by using [Tb(BTC)]∙G (MOF−76b), showing highly selective luminescent response toward F⁻ (Fig. 14c−d) [105]. This MOF shows luminescent enhancement toward anions such as F⁻, Cl⁻, Br⁻, ${\text{CO}}_3^{2-}$, and ${\text{SO}}_4^{2-}$ owing to the hydrogen bonding interactions. Among different anions, F⁻ shows much stronger interactions with the terminal coordinated O–H group and thus reduce its vibration frequency and corresponding quenching effect upon Tb centers, showing significant luminescent turn−on response, which demonstrates this type of luminescent MOFs are promising for the sensing of fluoride anion.

In 2009, we realized a lanthanide MOF [Eu(PDC)_1.5(DMF)]·0.5D MF 0.5H₂O (H₂PDC = pyridine−3,5−dicarboxylate acid) with incorporating of Lewis basic sites for selective binding and recognition of metal ions (Fig. 14e−f) [106]. This MOF contains 1D channels of about 6.3 × 8.5 Ų along crystallographic [1 0 0] axis with uncoordinated pyridyl N atoms that point to the center of channels, which are accessible for potential selective luminescent sensing. As expected, this MOF exhibits selective luminescent quenching to metal ions, particularly to Cu²⁺. It was speculated that the binding of the uncoordinated pyridyl group to Cu²⁺ reduces the antenna efficiency of the PDC organic linkers to magnify the f−f transitions of Eu centers, resulting in luminescent quenching. The quenching effect of Cu²⁺ is more significant than alkali and alkaline−earth metal ions, which was attributed to the preferential binding of free pyridyl N atoms for Cu²⁺ over other metal ions.

After these pioneered works, we have also developed luminescent MOFs for sensing of various other chemical species including nitroaromatic explosives [107], oxygen gas [108]. In principle, rational design porous luminescent materials based on MOF chemistry can afford novel sensing materials. There are great opportunities lie in overcoming the challenges of the fabrication of sensing devices with good stability, impurity tolerance, persistence and so on.

5. Enantioselective separation

Enantioselective separation of racemic substrates is of great industrial and pharmaceutical importance, and has long been a great challenging for chemist. Using porous media with chiral binding sites that mimic the chiral environment in biological systems was proposed for binding small molecules of specific chirality to produce fine chemicals of enantiopure. By integrating the merits of permeable porosity and highly ordered structure with functionalization at atomic−level, MOFs show great potential in enantioselective separation. Incorporating chiral ligands, synthesis with chiral templates or post−synthesis are usually applied for the construction of homochiral MOFs [109].

Chiral secondary alcohols are valuable and important intermediates for the synthesis of various pharmaceutical, agricultural, and fine chemicals. By using chiral metalloligands preconstructed from chiral diamine and carbaldehyde, we have realized series of mixed−metal–organic frameworks (M′MOFs) with chiral pore cavities, showing great potential for enantioselective separation of different secondary alcohols. In 2011, we developed two isostructural homochiral MOFs, Zn₃(bdc)₃[Cu(SalPycy)] (M′MOF−2) and Zn₃(cdc)₃[Cu(SalPycy)] (M′MOF−3), featuring chiral pore cavities with accessible open Cu centers [83]. Both MOFs contain 2D metal dicarboxylate layers from trinuclear M₃(RCOO)₆ clusters connected by different dicarboxylate linkers, which are further pillared by the chiral metalloligands. The chiral cavities can be tuned by incorporating different dicarboxylate linkers, showing pore size of ~6.4 Å. In particular, M′MOF−3 can exclusively take up S−1−phenylethyl alcohol (S−PEA) from racemic PEA to form $\text{M}\prime \text{MOF}-3\supset \text{S}-\text{PEA}$ (Fig. 15), which can be eluted by methanol immersion. Both MOFs show enantioselective separation of S−PEA from racemic PEA, with ee values of 21.1% for M′MOF−2 and 64% for M′MOF−3 with smaller pore. The chiral pore environments in these M′MOFs enable them for enantioselective separation of different secondary alcohols. In 2012, we developed four homochiral MOFs, Cd₃(bdc)₃[Cu(SalPyMeCam)] (M′MOF−4), Zn₃(cdc)₃[Cu (SalPyMeCam)] (M′MOF−5), Cd₃(bdc)₃[Cu(SalPytBuCy)] (M′MOF−6) and Zn₃(cdc)₃[Cu(SalPytBuCy)] (M′MOF−7) for chiral separation of secondary alcohols [84]. Particularly, M′MOF−7 shows high enantioselectivity for separation of 1−phenylethanol, 2−butanol and 2−pentanol, with the ee values of 82.4%, 77.1% and 65.9%, respectively, which can be attributed to size/shape matching of chiral pore structures with substrates.

Fig. 15.

Schematic diagram of encapsulating S−PEA molecules within the chiral pore cavities of M′MOF−3 (highlighted as spheres). (Zn, pink; Cu, cyan; O, red; C, gray; N, blue; H, white). Reproduced with permission [83]. Copyright 2011 Nature Publishing Group.

In short, MOFs are excellent platform for exploring novel chiral porous materials, which facilitate the implementation of the enantioselective separation of important chiral molecules. The adsorptive approach based on porous materials to separate racemic mixtures is very promising to advance the production of enantiopure chemicals or medicines, which would reap great benefits for related fields.

6. Other applications

Besides the aforementioned efforts on pore and function engineering for multifunctional MOF materials, our ongoing active research endeavours also involve the applications of MOFs for proton conducting, white−light emitting, luminescent thermometers, two−photon luminescence, nonlinear optical materials, catalysis, and so on [110].

For example, in 2017, we reported an imidazole−loaded hybrid MOF material based on [Cu₁₂(BTC)₈(H₂O)₁₂][HPW₁₂O₄₀]∙Guest (NENU−3) showing high proton conductivity [111]. The targeted material Im@(NENU−3) was synthesized in a straightforward one− step approach by directly loading of imidazole molecules into the pristine sample of NENU−3, which shows a very high proton conductivity of up to 1.82 × 10⁻² S cm⁻¹ at 90% RH and 70 °C. Interestingly, loading imidazole molecules into the activated sample NENU−3a through a two−step approach produced a different sample Im−Cu@(NENU−3a), which shows much lower proton conductivity of 3.16 × 10⁻⁴ S cm⁻¹ under same conditions. It was speculated that the proton transportation pathway in Im−Cu@ (NENU−3a) has been blocked, resulting in low conductivity, while one−step straightforward strategy allows high concentration of free imidazole molecules within Im@(NENU−3), which facilitates the formation of successive proton−hopping pathways through hydrogen bonded networks. Crystallographic study of Im−Cu@(NENU−3a) reveals that imidazole molecules mainly terminal bound onto the open Cu sites, isolating the lattice water molecules, which well supports the proposed mechanism.

Our continuous efforts on pore and function engineering have enabled us to develop functional MOF materials for their broad applications.

7. Conclusion and outlook

In this review, we have summarized the ongoing research endeavors during our exploration and discovery of functional MOFs for various applications ranging from gas storage and separation to luminescent sensors. Some important progresses we achieved are also highlighted, including methane storage, gas chromatographic separation of hexane isomers, carbon dioxide capture, kinetic separation of hydrogen isotope, acetylene/ethylene separation, acetylene/carbon dioxide separation, propylene/propyne separation, ethylene/ethane separation, luminescent sensing, and enantioselective separation of racemic mixtures. These functionalities and applications result from the thorough understanding of MOF chemistry. Also, several rational strategies to construct multi− functional MOFs for unprecedented discoveries have also been outlined.

Definitely, MOFs are unparalleled for their high porosity, diversified/designable structures and tunable pore sizes. The emergence of MOFs has revolutionized the field of molecular chemistry, affording an ideal platform for exploring novel functional materials. In terms of variety and multiplicity, functional MOFs are more extensive than any other class of porous media. This type of hybrid materials integrates the merits of organic materials and inorganic materials, showing collaborative functionalities. After years of intense research efforts from science and engineering communities, substantial progresses on multifunctional MOFs have been achieved since their original inception. The power of MOF chemistry in material design and synthesis has been demonstrated to be far beyond our imaginations. New dimensions and regions of research on MOF materials are continuous emerging. For example, the incorporation of MOF materials with inorganic/polymer materials or biomaterials can afford novel supermolecular systems showing synergistic functionalities. Furthermore, there are numerous opportunities in the fabrication of MOF−based devices such as thin films and membranes to advance their direct applications in our daily life. On the other hand, intensive worldwide efforts from certain commercial companies have been dedicated to promote functional MOFs towards industrial applications. Overall, MOFs are taking their steps from academia to industrial application and from laboratories to factories. It can be believed that the close collaborations among scientists and engineers as well as industrial partners will bring a bright future of MOFs.

Abbreviations:

RON

Research Octane Number

nHEX

n-hexane

2MP

2-methylpentane

3MP

3-methylpentane

22DMB

2,2-dimethylbutane

23DMB

2,3-dimethylbutane

H₂bdc

1,4-benzenedicarboxylic acid

44′-bpy

4,4′-bipyridine

dabco

1,4-diazabicyclo[2,2,2]octane

bdp²⁻

1,4-benzenedipyrazolate

H₆BHB

3,3′,3″,5,5′,5″-benzene-1,3,5-triyl-hexabenzoic acid

PyenH₂

5-methyl-4-o xo-1,4-dihydro-pyridine-3-carbaldehyde

H₂cdc

1,4-cyclohexanedicarboxylate acid

BTC³⁻

benzenetricarboxylate

H₂atbdc

5-(5-amino-1H-tetrazol-1-yl)-1,3-be nzenedicarboxylic acid

azpy

4,4′-azopyridine

BET

Brunauer-Emmett-Teller

DFT-D

dispersion-corrected density-functional theory

Hcit

citric acid

H₄dobdc

2,5-dihydroxy-1,4-benzenedicarboxylic acid

dps

4,4′-dipyridylsulfide

OTf⁻

trifluoromethanesulfonate

H₂PDC

pyridine-3,5-dicarboxylate acid

S-PEA

S-1-phenylethyl alcohol