ADVANCED MOLECULAR SIEVE MEMBRANES Qilei Song 1,4 , Shuai Cao 2 , Shan Jiang 3 , Andrew I. Cooper 3 , Anthony K. Cheetham 2 , and Easan Sivaniah 1,5 1 Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK 2 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, UK 3 Department of Chemistry, University of Liverpool, Liverpool UK 4 Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK 5 Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Kyoto 606-8501, Japan Email: q.song@imperial.ac.uk Introduction Microporous materials with well-defined and uniform molecule- sized pores, such as conventional zeolites and novel metal-organic frameworks (MOFs), 1 present high internal surface areas and size and shape-selective sieving properties on a molecular level. Owing to their tailored pore size, chemical functionality and high surface area, these ordered molecular sieves are promising for applications in gas sorption, storage, catalysis, and separations. One particular promising application of these microporous materials is in membrane separation processes that are important for applications in global energy and environment processes, for example, natural gas purification, carbon capture, hydrogen production, and water desalination and purification. Ideally, the selective adsorption and rapid diffusion of molecules in ordered framework materials can give both intrinsically high molecular permeability and remarkable selectivity. However, it remains difficult to fabricate crystalline molecular sieves to separation membranes on a large scale, particularly for gas separations. The technical challenges involve many aspects, such as defects and pinholes in crystals, chemical and steam stability, stress, sealing, and cost of production. In contrast, industrial molecular- selective membranes are dominantly made of polymers which can be easily fabricated by solution processing or interfacial polymerization. The molecular transport in conventional polymers generally follows a solution-diffusion mechanism, and presents a trade-off between permeability and selectivity, known as an upper bound. Microporous molecular sieving materials are desirable for the next-generation high-performance membranes to achieve both high permeability and high selectivity. MOFs PIMs POCs Figure 1. Three types of microporous materials, metal-organic frameworks (MOFs), polymers of intrinsic microporosity (PIMs), and amorphous porous organic cage (POCs) molecules. In this paper, we summarize our recent work on design and fabrication of molecular sieve membranes from several novel classes of microporous materials, including MOFs, polymers of intrinsic microporosity (PIMs), 2-3 porous organic cages (POCs), 4 as shown in Figure 1. By adopting versatile processing methods, we are able to tune the physical properties and particularly the distribution, size, and architecture of channels and free volume elements and consequently molecular sieving properties. Versatile processing methods and post- synthetic transformation approaches were developed, such as incorporation of MOF nanocrystals into polymer matrix forming nanocomposites, transformation of molecular packing by photo- oxidation and thermal oxidative crosslinking.. The intermolecular interactions, including weak van der Waals forces, hydrogen bonding and strong covalent crosslinking, play important role in tuning the packing of molecules and architecture of free volume elements. Molecular sieve membranes derived from PIMs polymer show ground-breaking separation performance in terms of gas permeability and selectivity, for separation of CO 2 from CH 4 , air separation (O 2 /N 2 ), and separation of H 2 from large gas molecules, and great potential for many other molecular-level separations. Experimental Synthesis of Materials. Zeolitic imidazolate framework ZIF-8, one type of MOFs, was synthesized from zinc salts and 2- methylimidazole. The PIM-1 polymer was synthesized following the method invented by Budd et al., 2 from polycondensation reaction of 3,3,3′,3′-tetramethyl-1,1′- spirobisindane-5,5′,6,6′-tetrol and 2,3,5,6- tetrafluoroterephthalonitrile in the presence of K 2 CO 3 in anhydrous dimethylformamide. Porous organic cages were synthesized from reactions between 1,3,5-triformylbenzene and various diamines in dichloromethane. 4 Formation of Films and Membranes. Thick dense membranes were prepared by solution casting process, with solvent slowly evaporated resulting in transparent and free-standing films. Thin films were prepared by spin-coating of dilution solution on different substrates. Characterization Methods. The materials including solid powders, films, and membranes were characterized with various physical and chemical techniques, including X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), gel permeation chromatograph (GPC), Fourier Transform Infrared Spectra (FTIR), thermal gravimetric analysis and differential thermal analysis (TGA-DTA), and mechanical property measurements. Gas Sorption and Permeation Methods. Low-pressure gas sorption was performed using a Micromeritics ASAP 2020 instrument at pressures up to 1 bar. High-pressure pure-gas sorption isotherms were measured using a home-made dual-volume pressure decay apparatus at pressures up to 35 bar. Pure gas permeation tests were carried out on a home-made constant-volume pressure-increase apparatus. The mixed gas permeation properties were measured in another membrane cell using the constant-pressure variable-volume method. The gas compositions were analysed by a gas chromatograph. Results and Discussion We fabricated polymer nanocomposite gas-separation membranes by incorporating crystalline microporous zeolitic imidazolate frameworks (ZIFs) nanocrystals into a conventional polyimide polymer (Matrimid) via solution mixing. 5 The resulting nanocomposite membranes showed excellent dispersion of nanoparticles, good adhesion at the interface, and enhanced gas permeability while the selectivity remained at high level. Advanced characterization techniques indicate the increase of the free volume elements which correlates with the gas transport properties. However, the gas transport properties in these nanocomposites are still limited by the dense molecular packing of flexible polymer chains. We then developed a new membrane material system based on polymers of intrinsic microporosity (PIMs). PIMs polymers have rigid and contorted macromolecular structure; consequently, the polymer chains cannot pack efficiently in the solid state, generating