Permeation and separation studies on microporous sol-gel modified ceramic membranes

Permeation and separation experiments with H2, CO2, O2, N2, CH4 and isobutane with microporous sol-gel modified supported ceramic membranes were performed to determine the gas transport characteristics and the hydrogen separation performance of these membranes. It is found that the permeation is activated, and for defectfree membranes the apparent activation energies are in the ranges 13¿15 and 5¿6 kJ mol¿1 for H2 and CO2, respectively. Correction for the pressure drop over the support results in apparent activation energies for the silica top-layer on the order of 17¿22 and 10¿15 kJ mol¿1 for H2 and CO2 respectively. Due to the very thin top-layer, the permeation is relatively high, with representative values of 6·10¿7 and 20·10¿7 mol m¿2s¿1 Pa¿1 for H2 at 25 and 200°C, respectively. The H2 permeation is almost pressure-independent up to pressures of at least 5 bar. Typical separation factors for H2---CH4 and H2---isobutane are approximately ¿40 and ¿200, respectively, at 200°C for high-quality membranes. For moderate-quality membranes the H2---CH4 separation factor is around 10, while the H2---isobutane separation factor remains at a high value of around 100 at 200°C and 120 at 300°C

Microstructural properties of non-supported microporous ceramic membrane top-layers obtained by the sol-gel process

Dried and calcined non-supported membrane top-layers of SiO2, SiO2/TiO2, SiO2/ZrO2 (10, 20 and 30 mol% TiO2 and ZrO2, respectively) and SiO2/Al2O3 (10 mol% AlO1.5) were prepared using acid catalyzed hydrolysis and condensation of alkoxides in ethanol. The microstructure was determined using nitrogen physisorption. The modified Horváth-Kawazoe model for nitrogen adsorption in cylindrical pores was used for pore size assessment. SiO2 non-supported membrane top layers were 100% microporous with an average porosity of 30¿37%, depending on drying conditions. The bimodal pore size distribution shows a maximum at an effective pore diameter of 0.5 nm, and a broader tail with a weaker maximum around 0.75 nm. Microporous non-supported binary systems can be prepared with porosities between 15 and 40%. The high reactivity of the Ti, Zr, Al-alkoxides requires carefully chosen conditions. Too much water results in dense materials. The pore size distributions (PSDs) of the binary systems resemble the PSDs for silica

Polymeric-silica-based sols for membrane modification applications:sol-gel synthesis and characterization with SAXS

Polymeric SiO2 and binary SiO2/TiO2, SiO2/ZrO2 and SiO2/Al2O3 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

In This Issue

The formation is described of supported microporous membranes (by IUPAC definition rpore < 1 nm), prepared by modification of mesoporous γ-alumina membranes with polymeric sols. The mesoporous γ-alumina membranes, with a top-layer thickness in the order of 7–10 μm, and with pore radii of 2–2.5 nm, have a very high surface finish (mean roughness 40 nm). The amorphous microporous top-layer thickness is in the order of 60–100 nm. Gas transport properties are effectively improved as is shown by activated permeation and molecular sieve-like separation factors in the order of 50–200 for H2/CH4. These microporous top-layers can be prepared from a relatively wide range of sol structures; from inorganic oligomers which are too small to result in significant scattering with SAXS, to polymeric structures with fractal dimensions in the range: 1 <df <2.04, and radii of gyration between 0.8 and 4 nm