A high-performance gas-separation membrane containing submicrometer-sized metal-organic framework crystals.

Metal–organic frameworks (MOFs) are an emerging class of nanoporous materials comprising metal centers connected by various organic linkers to create one-, two-, and threedimensional porous structures with tunable pore volumes, surface areas, and chemical properties. Several thousand MOF materials have been synthesized and their numbers continue to grow rapidly. MOFs are predicted to be highly attractive for application in gas-separation membranes and also have a range of other potential applications, for example in selective gas adsorption, hydrogen storage, catalysis, and sensing. Recently, thin continuous MOFmembranes for gas separation have been reported by several authors using MOFs such as MOF-5, HKUST-1 (Cu3(BTC)2), [8] Cu(hfipbb)(H2hfipbb)0.5, [9] and ZIF-8. However, the gas-permeation properties (permeability and selectivity) have so far not been found to be technologically attractive. This may have several reasons, such as membrane defects and related processing issues, use of MOFs with low selectivity, and unfavorable orientation of crystals in the membrane. An alternative route to high-performance MOF membranes is to incorporate them into polymers to obtain nanocomposite (mixed-matrix) membranes. The incorporation of nanoporous molecular sieves such as zeolites into polymeric membranes has attracted much attention, since one can in principle combine the size/shape selectivity of nanoporous materials with the processibility and mechanical stability of polymers. However, zeolite/polymer composite membranes often have defective morphologies characterized by void spaces between the zeolite particles and the polymeric matrix, leading to poor gas-separation performance since the gas molecules bypass the zeolite particles. Recent approaches to address the issue of interface compatibilization are emerging. On the other hand, the use of MOFs in mixed-matrix membranes provides several potential advantages over zeolites. The control of MOF/polymer interface morphology is easier than that of the zeolite/polymer interface, since the organic linkers in MOFs have better affinity with polymer chains than the inorganic zeolites do, and the surface properties of MOFs can be easily tuned by functionalization with various organic molecules if necessary. In general, MOFs also have higher pore volumes and lower density than zeolites, and hence their effect on the membrane properties can be greater for a given mass loading. Recently, several MOFmixed-matrix membranes such as Cu-BPY-HFS (Cu-4,40-bipyridine hexafluorosilicate) in Matrimid, HKUST-1 in poly(sulfone), MOF-5 in Matrimid, and Cu-TPA (terephthalic acid) in poly(vinyl acetate) have been reported. Although a high degree of MOF/polymer adhesion (as characterized by scanning electron microscopy) was found, the gas-separation performance of these membranes was not high. In addition to the control of interface morphology, the selection of appropriate MOF/polymer pairs is indispensable for high-performance mixed-matrix membranes, a fact emphasized in theoretical predictions. ZIF-90 (zeolitic imidazolate framework-90) is an attractive MOF for application in CO2-selective mixed-matrix membranes. ZIF-90 has a sodalite cagelike structure with 0.35 nm pore windows, through which size exclusion of CH4 from CO2/CH4 mixtures is possible. [20] Furthermore, the imidazole linker in ZIF-90 contains a carbonyl group, which has a favorable chemical noncovalent interaction with CO2. [21] Submicrometer-sized crystals of a related MOF material, ZIF-8, have recently been reported. So far, ZIF-90 crystals have been synthesized by the conventional solvothermal method. However, their size (ca. 100 mm) is too large for use in thin mixed-matrix membranes (which require submicrometer-sized crystals). Herein, we describe the synthesis of submicrometer-sized ZIF-90 crystals by a novel method, namely nonsolvent-induced crystallization. The ZIF-90 crystals were thoroughly characterized, and we compare them with solvothermally synthesized ZIF-90. Mixed-matrix membranes were then fabricated using three poly(imide)s as polymer matrices, and their CO2/CH4 separation properties were investigated. In particular, we demonstrate the first MOF-containing gas-separation membranes with technologically attractive properties. The morphology of our ZIF-90 crystals is shown in Figure 1. In general, the synthesis of smaller crystals requires reaction conditions that favor nucleation over crystal growth. Particle-size control proved difficult in conventional solvothermal synthesis. We crystallized small ZIF-90 particles at room temperature by the rapid addition of a nonsolvent to the reagent solution (see the Supporting Information), leading to supersaturation of the solution. The nucleation rate can be [*] Dr. T. -H. Bae, J. S. Lee, Dr. W. Qiu, Prof. Dr. W. J. Koros, Prof. Dr. C. W. Jones, Prof. Dr. S. Nair School of Chemical & Biomolecular Engineering Georgia Institute of Technology 311 Ferst Drive NW, Atlanta, GA 30332-0100 (USA) Fax: (+1)404-894-4200 E-mail: christopher.jones@chbe.gatech. edu sankar.nair@chbe.gatech. edu