Highly hydrogen selective ceramic membranes: application to the transformation of greenhouse gases

This paper reports the preparation of modified Vycor glass membranes by several methods: polymerization, dip coating, sol–gel impregnation, and chemical vapor deposition (CVD). It was found that a membrane (here called Nanosil) formed by the high-temperature CVD of tetraethylorthosilicate (TEOS) in inert gas exhibited hydrogen selectivities of 100% with respect to CH4, CO, CO2 and H2O. The transport of hydrogen was not through Knudsen diffusion, but was activated, and probably involved atomic hydrogen species. The membrane was also stable to hydrothermal stresses (10% H2O at 873 K and 1 bar) for over 150 h of operation. The membrane was employed in a catalytic reactor for the dry reforming of methane, CH4+CO2⇌2CO+2H2, using a 1% Rh/Al2O3 catalyst. As a result of simultaneous reaction and separation, conversions were higher than those obtained in a packed-bed reactor operated at the same conditions, and exceeded equilibrium levels.