(770a) Microporous Membranes with Gated Pore Structure

Inorganic membranes for molecular separations include non-porous and microporous membranes. For non-porous, dense membranes the perm-selectivity is determined by the interaction of the membrane material with specific gases and the permeance is controlled by both the thickness of the dense membrane layer and surface reaction kientics. Non-porous membranes are only perm-selective to oxygen, hydrogen or carbon-dioxide. Microporous membranes are versatile and can be designed for the separation of many gas or liquid mixtures. For microporous membranes the perm-selectivity depends on the pore size and solubility (adsorption equilibrium) and the permeance is determined by the pore size, porosity and thickness of the microporous layer. Pore size reduction has been studied for the improvement of perm-selectivity of microporous membranes but such effort often leads to a reduction in porosity and hence the permeability for the permeating molecule. The more effective way to improve the perm-selectivity with minimum reduction in permeability is to reduce the pore size without much change in porosity. Experimental results of chemical vapor deposition modification of mesoporous alumina membranes will be presented to demonstrate these two structures. Modification of microporous membranes to improve perm-selectivity with a minimum reduction in permeance for the perm-selective molecule can be achieved with a design of gated-pore-structure, i.e., inserting a gate in the micropores to reduce the pore size in a narrow region of the membrane. This strategy will be demonstrated using two examples. One is the catalytic cracking deposition of silica on microporous MFI zeolite membranes to form a gate that effectively narrows the MFI zeolitic pores. The modified MFI zeolite membranes show a 5 to 30 fold increase in hydrogen/carbon-dioxide selectivity with only about 30% reduction in hydrogen permeance. The other example is the membrane surface ligand exchange modification of ZIF-8 membranes (exchanging partially the original ligand linkers in ZIF-8 with a bulkier one). The ligand exchange is designed to narrow the aperture of the crystallites on the surface of the ZIF-8 membrane. Such post-synthesis modification can improve propylene/propane selectivity of ZIF-8 membranes from about 100 to 200, with virtually no reduction in propylene permeance. However, challenges remain including controlled mofidication and characterization of the structure of modified membranes. All these results show promise of post-synthesis modification based on the design of the gated-pore structure to obtain membranes with high perm-selectivity and permeance (or permeability).