Analysis and theory of gas transport in microporous sol-gel derived ceramic membranes

Sol-gel modification of mesoporous alumina membranes is a very successful technique to improve gas separation performance. Due to the formed microporous top layer, the membranes show activated transport and molecular sieve-like separation factors. This paper concentrates on the mechanism of activated transport (also often referred to as micropore diffusion or molecular sieving). Based on a theoretical analysis, results from permeation and separation experiments with H2, CO2, O2, N2, CH4 and iso-C4H10 on microporous sol-gel modified supported ceramic membranes are integrated with sorption data. Gas permeation through these membranes is activated, and for defect-free membranes the activation energies are in the order of 13–15 kJ.mol−1 and 5–6 kJ.mol−1 for H2 and CO2 respectively. Representative permeation values are in the order of 6×10−7 mol.m−2.s−1.Pa−1 and 20×10−7 mol.m−2.s−1.Pa−1 for H2 at 25°C and 200°C, respectively. Separation factors for H2CH4 and H2iso-butane are in the order of 30 and 200 at 200°C, respectively, for high quality membranes. Processes which strongly determine gas transport through microporous materials are sorption and micropore diffusion. Consequently, the activation energy for permeation is an apparent one, consisting of a contribution from the isosteric heat of adsorption and the activation energy for micropore diffusion. An extensive model is given to analyse these contributions. For the experimental conditions studied, the analysis of the gas transport mechanism shows that interface processes are not rate determining. The calculated activation energies for micropore diffusion are 21 kJ.mol−1 and 32 kJ.mol−1 for H2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation energies are higher than for zeolite 4A (dpore=4Å), indicating that the average pore size of the sol-gel derived membranes is probably smaller.