Experiments were combined with atomically detailed simulations and density functional theory (DFT) calculations to understand the effect of incorporation of an ionic liquid (IL), 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF]), into a metal organic framework (MOF with a zeolitic imidazolate framework), ZIF-8, on the CO separation performance. The interactions between [BMIM][PF] and ZIF-8 were examined in deep detail, and their consequences on CO/CH, CO/N, and CH/N separation have been elucidated by using experimental measurements complemented by DFT calculations and atomically detailed simulations. Results suggest that IL-MOF interactions strongly affect the gas affinity of materials at low pressure, whereas available pore volume plays a key role for gas adsorption at high pressures. Direct interactions between IL and MOF lead to at least a doubling of CO/CH and CO/N selectivities of ZIF-8. These results provide opportunities for rational design and development of IL-incorporated MOFs with exceptional selectivity for target gas separation applications.

Metal-organic frameworks (MOFs) have been widely studied for different applications owing to their fascinating properties such as large surface areas, high porosities, tunable pore sizes, and acceptable thermal and chemical stabilities. Ionic liquids (ILs) have been recently incorporated into the pores of MOFs as cavity occupants to change the physicochemical properties and gas affinities of MOFs. Several recent studies have shown that IL/MOF composites show superior performances compared with pristine MOFs in various fields, such as gas storage, adsorption and membrane-based gas separation, catalysis, and ionic conductivity. In this review, we address the recent advances in syntheses of IL/MOF composites and provide a comprehensive overview of their applications. Opportunities and challenges of using IL/MOF composites in many applications are reviewed and the requirements for the utilization of these composite materials in real industrial processes are discussed to define the future directions in this field.

Zeolites are aluminosilicate materials that contain regular three-dimensional arrays of molecular-scale pores, and they can act as hosts for catalytically active metal clusters. The catalytic properties of such zeolites depend on the sizes and shapes of the clusters, and also on the location of the clusters within the pores. Transmission electron microscopy has been used to image single atoms and nanoclusters on surfaces, but the damage caused by the electron beam has made it difficult to image zeolites. Here, we show that aberration-corrected scanning transmission electron microscopy can be used to determine the locations of individual metal atoms and nanoclusters within the pores of a zeolite. We imaged the active sites of iridium catalysts anchored in dealuminated HY zeolite crystals, determined their locations and approximate distance from the crystal surface, and deduced a possible cluster formation mechanism.

Highly dealuminated Y zeolite-supported mononuclear iridium complexes with reactive ethylene ligands were synthesized by chemisorption of Ir(C2H4)2(C5H7O2). The resultant structure and its treatment in He, CO, ethylene, and H2 were investigated with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR spectra show that Ir(C2H4)2(C5H7O2) reacted readily with surface OH groups of the zeolite, leading to the removal of C5H7O2 ligands and the formation of supported mononuclear iridium complexes, confirmed by the lack of Ir−Ir contributions in the EXAFS spectra. The EXAFS data show that each Ir atom was bonded to four carbon atoms at an average distance of 2.10 Å, consistent with the presence of two ethylene ligands per Ir atom and in agreement with the IR spectra indicating π-bonded ethylene ligands. The EXAFS data also indicate that each Ir atom was bonded to two oxygen atoms of the zeolite at a distance of 2.15 Å. The supported iridium−ethylene complex reacted with H2 to give ethane, and it also catalyzed ethylene hydrogenation at atmospheric pressure and 294 K. Treatment of the sample in CO led to the formation of Ir(CO)2 complexes bonded to the zeolite. The sharpness of the υCO bands indicates a high degree of uniformity of these complexes on the support. The iridium−ethylene complex on the crystalline zeolite support is inferred to be one of the most nearly uniform supported metal complex catalysts. The results indicate that it is isostructural with a previously reported rhodium complex on the same zeolite; thus, the results are a start to a family of analogous, structurally well-defined supported metal complex catalysts.

Gas separation performance of zeolitic imidazolate framework (ZIF-8) was improved by incorporating an ionic liquid (IL), 1-n-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF 4 ]). Detailed characterization based on X-ray diffraction (XRD) and scanning electron microscopy (SEM) confirmed that the morphology of ZIF-8 remains intact upon IL incorporation up to 28 wt%. Thermogravimetric analysis indicated the presence of direct interactions between the IL and metal organic framework (MOF). FTIR spectroscopy illustrated that the anion of the IL was shared between the imidazolate framework and[BMIM] + cation. Adsorption isotherms of CO 2 , CH 4 , and N 2 measured for pristine ZIF-8 and IL-loaded ZIF-8 samples, complemented by Grand Canonical Monte Carlo (GCMC) simulations, showed that these interactions influence the gas uptake performance of ZIF-8. CH 4 and N 2 uptakes decreased in the whole pressure range, while CO 2 uptake first increased by approximately 9% at 0.1 bar in 20 wt% IL-loaded sample, and then, decreased as in the case of other gases. As a result of these changes in gas uptakes occurring at different extend, the corresponding CO 2 /CH 4 , CO 2 /N 2 , and CH 4 /N 2 selectivities enhanced especially at low pressure regime upon IL incorporation. Results showed that CO 2 /CH 4 selectivity increased from 2.2 to 4, while CO 2 /N 2 selectivity more than doubled from 6.5 to 13.3, and CH 4 /N 2 selectivity improved from 3 to 3.4 at 0.1 bar at an IL loading of 28 wt%. The heat of adsorption values (Q st ) measured and simulated for each gas on each sample indicated that interactions between the IL and ZIF-8 strongly influence the gas adsorption behaviors. The change in Q st of CO 2 upon IL-incorporation was more significant than that of other gases, leading to an almost doubling of CO 2 selectivity over CH 4 and N 2 , specifically at low pressures. On the other hand, the selectivity improvement was lost at high pressures because of a strong decrease in the available pore space due to the presence of IL in ZIF-8. These results suggest that such IL/MOF combinations with tunable structures have huge potential towards high performance gas separation applications.

The efficient separation of gases has industrial, economic, and environmental importance. Here, the gas separation performance of a metal organic framework (MOF) is enhanced by ionic liquid (IL) incorporation. One of the most commonly used ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), was incorporated into a commercially available MOF, CuBTC. Detailed characterization by combining spectroscopy with diffraction, electron microscopy, and thermal analysis confirmed that the structures were intact after incorporation. Adsorption isotherms of CH4, H2, N2, and CO2 in IL-incorporated CuBTC were experimentally measured and compared with those of pristine CuBTC. Consequently, ideal selectivities for CO2/CH4, CO2/N2, CO2/H2, CH4/N2, CH4/H2, and N2/H2 separations were calculated. The results showed that the CH4 selectivity of CuBTC over CO2, H2, and N2 gases becomes at least 1.5 times higher than that of pristine CuBTC upon the incorporation of IL. For example, the CH4/H2 selectivity of CuBTC increased from 26 to 56 at 0.2 bar when the IL loading was 30 wt %. These results show that the incorporation of ILs into MOFs can lead to unprecedented improvements in the gas separation performance of MOFs. The tunable physicochemical properties of ILs combined with a large number of possible MOF structures open up opportunities for the rational design of novel materials for meeting future energy challenges.

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The structure of a catalyst often changes as a result of changes in the reactive environment during operation. Examples include changes in bulk phases, extended surface structures, and nanoparticle morphologies; now we report real-time characterization of changes in the structure of a working supported catalyst at the molecular level. Time-resolved extended X-ray absorption fine structure (EXAFS) data demonstrate the reversible interconversion of mononuclear iridium complexes and tetrairidium clusters inside zeolite Y cages, with the structure controlled by the C(2)H(4)/H(2) ratio during ethene hydrogenation at 353 K. The data demonstrate break-up of tetrairidium clusters into mononuclear complexes indicated by a decrease in the Ir-Ir coordination number in ethene-rich feed. When the feed composition was switched to first equimolar and then to a H(2)-rich (C(2)H(4)/H(2) = 0.3) feed, the EXAFS spectra show the reformation of tetrairidium clusters as the Ir-Ir coordination number increased again. When the feed composition was cycled from ethene-rich to H(2)-rich, the predominant species in the catalyst cycled accordingly. Evidence confirming the structural change is provided by IR spectra of iridium carbonyls formed by probing of the catalyst with CO. The data are the first showing how to tune the structure of a solid catalyst at the molecular scale by choice of the reactant composition.