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, 02, N2, CH4 and iso-CaHlo 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-~ 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-t.Pa-~ and 20 × 10-7 mol.m-2.s-~.Pa-~ for H2 at 25°C and 200°C, respectively. Separation factors for H2/CH4 and H2/iso-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-t for H2 and CO2, respectively. Comparison with zeolite diffusion data shows that these activation energies are higher than for zeolite 4A (dpo~ = 4 /~), indicating that the average pore size of the sol-gel derived membranes is probably smaller.

No abstract

Polymeric SiO 2 and binary SiO2/TiO a, SiO2/ZrO 2 and SiO2/A120 3 sols, for ceramic membrane modification applications, have been prepared by acid-catalyzed hydrolysis and condensation of alkoxides in alcohol. The sols were characterized with small angle X-ray scattering, using synchrotron radiation. Directly after synthesis, the sols were found to consist of weakly branched polymeric structures with typical fractal dimensions of around 1.5 and radii of gyration of = 2 nm. The aggregation for silica sols obeys the tip-to-tip cluster-cluster aggregation model in the initial stages. Prehydrolysis of TEOS was found to be the best method to synthesize polymeric binary systems. Based on an analysis of film formation from sols consisting of weakly branched polymers, it is expected that consolidation of these polymers will result in microporous materials.

No abstract

MATERIALS RESEARCH SOCIETY UPDATE YOUR STAFFPrepare for the future. Keep your staff up-to-date on the latest advances in the materials sciences and engineering. CUSTOM DESIGN MRS courses can be adapted and tailored to meet the specific needs of your organization. EXPERTISE MRS can develop a course that meets the needs of your staff. COST EFFECTIVE • CONVENIENTThis program provides cost-effective continuing education for your organization. There are no travel costs for your staff and minimal time away from their work. QUALITY MRS instructors are acknowledged leaders in their individual fields of expertise.

No abstract

Microporous SiO2 and SiOJMO2 (M = Ti, Zr, A1; 10 mol% MOx) materials for gas separation membrane applications have been prepared from polymeric sols. Characterization of these sols with SAXS showed that the mean fractal dimension of the SiO2 sols is 1.3-1.4 with a radius of gyration of approximately 2.5 nm. The dried and calcined films are microporous and the pore size distribution was bimodal with maxima at diameters of 0.5 nm and 0.75 nm. For the SiO2/TiO2, SiOJZrO2 and SiOJA12Oa systems, much milder reaction conditions proved to be necessary to obtain sols with comparable fractal dimensions due to the high reactivity of the Ti/Zr/Al-alkoxides. Microporous supported membranes with molecular sieve-like gas transport properties can be prepared from a relatively wide range of sol structures: from polymers too small to characterize with SAXS to structures with fractal dimensions: I < dy < 2.04.

Aging experiments on microporous sol-gel-derived nonsupported Si02 membranes were performed. Microstructure characterization was performed using nitrogen physisorption. It is found that both chemical aging and thermal aging result in densification of the microstructure, without pore growth. The influence of aging on supported SiO2-modified membranes was investigated using gas permeation and separation experiments. As for the nonsupported materials, some densification takes place. This leads to lower permeation rates, but a strong positive effect was observed on the separation properties. This might be attributed to a decrease of the pore size. Separation factors ranging from 50 to 125 have been measured for H2/CH4 at temperatures in the order of 250 "C.