Equilibrium and kinetic characteristics of five single gases in a methyltriethoxysilane-templating silica/α-alumina composite membrane

The equilibrium and kinetic characteristics of a methyltriethoxysilane (MTES)-templating silica/α-alumina composite membrane were studied using unsteady- and steady-state permeation of pure CO2, N2, CH4, H2 and He gases. The saturated amount of adsorption, Langmuir parameter, and diffusivity could be estimated by the generalized Maxwell–Stefan model, the incorporating dust gas model, and the Langmuir model using transient and steady-state measurements. Due to the surface diffusion effect, CO2, which has a strong adsorption affinity, was found to be more permeable than either N2 or CH4. On the contrary, H2 and He both exhibited greater permeation fluxes relative to the other gases because of their molecular size, structure, and non-adsorbable properties. CH4 and N2 both displayed very low permeability and a slow rate of permeation. The result can primarily be ascribed to the molecular size and structure of CH4 and N2, which likely contributed to steric hindrance or molecular sieving within the membrane pore. Using the calculated equilibrium and kinetic parameters, the permeation fluxes for the various gases through the membrane could be successfully calculated at unsteady-state pressurization and depressurization steps. Since the permeation flux in the MTES membrane was affected by molecular sieving effects as well as surface diffusion properties according to molecular properties, the kinetic and equilibrium separation should be considered simultaneously in the membrane.