ANALYSIS AND THEORY OF GAS-TRANSPORT IN MICROPOROUS SOL-GEL DERIVED CERAMIC MEMBRANES

Sol-gel modification of mesoporous alumina membranes is a very success ful technique to improve gas separation performance. Due to the formed microporous top layer, the membranes show activated transport and mol ecular sieve-like separation factors. This paper concentrates on the m echanism of activated transport (also often referred to as micropore d iffusion or molecular sieving). Based on a theoretical analysis, resul ts from permeation and separation experiments with H-2, CO2, O-2, N-2, CH4 and iso-C4H10 on microporous sol-gel modified supported ceramic m embranes are integrated with sorption data. Gas permeation through the se membranes is activated, and for defect-free membranes the activatio n energies are in the order of 13-15 kJ.mol(-1) and 5-6 kJ.mol(-1) for H-2 and CO2 respectively. Representative permeation values are in the order of 6 x 10(-7) mol.m(-2).s(-1).Pa-1 and 20 X 10(-7) mol.m(-2).s( -1).Pa-1 for H-2 at 25 degrees C and 200 degrees C, respectively. Sepa ration factors for H-2/CH4 and H-2/iso-butane are in the order of 30 a nd 200 at 200 degrees C, respectively, for high quality membranes. Pro cesses which strongly determine gas transport through microporous mate rials are sorption and micropore diffusion. Consequently, the activati on energy for permeation is an apparent one, consisting of a contribut ion from the isosteric heat of adsorption and the activation energy fo r micropore diffusion. An extensive model is given to analyse these co ntributions. For the experimental conditions studied, the analysis of the gas transport mechanism shows that interface processes are not rat e determining. The calculated activation energies for micropore diffus ion are 21 kJ.mol(-1) and 32 kJ.mol(-1) for H-2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation en ergies are higher than for zeolite 4A (d(pore) = 4 A), indicating that the average pore size of the sol-gel derived membranes is probably sm aller.