High temperature hydrogen (H2) separation membranes may play an important role in future cost-effective production of H2 from catalytic reforming of coal, natural gas, and biomass for use in the emerging fuel cell-based automobiles and clean power systems. The microporous inorganic membranes, particularly molecular sieve zeolite membranes, are potentially useful for high temperature H2 separation from catalytic reforming streams. Highly siliceous MFI zeolite (i.e. silicalite and ZSM-5) membranes with zeolitic pore size of ~0.56nm offer high permeance of H2 and extraordinary hydrothermal and chemical stabilities. However, the H2 selectivity over small gases like CO2 was found to be low at high temperatures because transport of molecules with sizes significantly smaller than the pore diameter may be primarily governed by Knudsen diffusion. Furthermore, nanometer scale intercrystalline pores and grain boundaries exist in the polycrystalline membranes which can severely reduce the membrane selectivity especially when the separation relies on molecular sieving or size and shape selectivity. To enhance the selectivity for H2 separation from small molecule gases, the MFI membranes must be modified to minimize the nonselective intercrystalline pores as well as reduce the intracrystalline zeolitic pore size.

In this study, alpha-alumina-supported MFI zeolite membranes were modified by catalytic cracking of methyldiethoxysilane (MDES) molecules inside the zeolitic channels on-stream during separation of H2/CO2 gas mixture at 450oC. The MDES vapor was carried by the H2/CO2 feed stream and the effect of modification was monitored continuously through analysis of the permeate stream. A dramatic increase in H2 selectivity over CO2 was achieved with a moderate loss in H2 permeance on the modified membrane. The modified membrane exhibited good stability in separation of H2/CO2 gas mixture containing water vapor at 450oC and atmospheric pressure.