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Sorption experiments with H2, CO2, CH4 and iso-C4H10 were performed on microporous SiO2 and SiO2/TiO2 (30 mol% TiO2) non-supported membrane top-layers using volumetric and gravimetric techniques. For silica, the sorption capacity decreases in the order CO2>iso-C4H10>CH4>H2 at temperatures

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

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.

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.

Microporous SiO2 and SiO2/MO2 (M=Ti, Zr, Al; 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, SiO2/ZrO2 and SiO2/Al2O3 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: 1

Alumina membranes with a binary top layer consisting of either SiO2-TiO2 or SiO2-Al2O3 were prepared. Adsorption/desorption experiments show that both binary systems are microporous (type I isotherms). The porosity for SiO2-TiO2 is 15% and for SiO2-Al2O3 1-2%. Permeability experiments have been performed on these membranes with H2, CO2 and CH4. For membranes with a SiO2-TiO2 top layer relatively high permeabilities (2.2*10-7 mol/m2.s.Pa at 28°C and 6.7*10-7 mol/m2.s.Pa at 200°C, respectively) were obtained for H2. For CO2 the permeabilities are a factor of 4 lower at 28°C and a factor of 12 lower at 200°C. The permeability of CH4 is a factor of 4 lower at 28°C compared with H2. For SiO2-Al2O3, the permeabilities are a factor of three to four lower than those of SiO2-TiO2 membranes. All gases show an increase in permeability with increasing temperature and thus an activated transport mechanism prevails.

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.

Mesoporous alumina membranes were modified with microporous silica. The polymeric silica sols consist of very weakly branched polymeric molecules with a fractal dimension of 1.3 and Guinier radius of ≈ 2.0 nm. The resulting top layer material is microporous with a porosity of 40% and a layer thickness of 60 nm. Gas transport is activated with an activation energy of 11 kJ/mol for hydrogen and molecular sieve like separation factors have been obtained. From the mechanism of gas transport the conclusion can be drawn that the pores are of molecular dimensions (0.7 – 1 nm diameter).