Permeation properties of silica-zirconia composite membranes supported on porous alumina substrates

A hydrogen-selective silica-zirconia composite membrane was prepared on a macroporous alumina support by chemical vapor deposition of tetraethylorthosilicate (TEOS) and zirconium (IV) tert-butoxide (ZTB) at 923 K. The resulting membrane had a high H₂permeance of 3.8×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹with selectivities over CO₂, N₂and CH₄of 1100, 1400 and 3700, respectively. Studies of the temperature dependence of the permeance of He, H₂, and Ne demonstrated that the permeation mechanism was similar to that of dense silica membranes, involving solid-state diffusion with jumps of the permeating species between solubility sites. Parameters such as the site density, jump distance, and jump frequency were calculated and were physically plausible, and varied in reasonable manner with the mass and size of He, H₂, and Ne. An alternative mechanism involving an activated gas translational mechanism was shown to fit the data but to give physically unrealistic parameters. The silica-zirconia membrane showed hydrothermal stability over a limited testing period of 48 h. After exposure to 16 mol% water vapor at 923 K for 48 h, a pure silica membrane showed a 68% decline with a H₂permeance of 4.5×10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹and a H₂over N₂selectivity of 800, both of which continued to deteriorate. In comparison, a 10% ZrO₂-SiO₂membrane showed a decline of 56% but to a level of 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹with a H₂over N₂selectivity of 5700. Importantly, the deterioration largely stabilized at that permeance level.