Silica membranes with extremely low defect concentrations have been prepared using sol–gel synthesis starting from tetraethyl orthosilicate (TEOS). An asymmetric structure is obtained by applying two silica layers on top of a γ-Al2O3 layer, supported by an α-Al2O3 support. The selective silica top layers have a total thickness of 30 nm and micropores with a pore diameter ∼5 Å, determined by physical adsorption on unsupported silica membrane material. The morphology of the homogenous silica layer is analysed by FE-SEM and TEM. The transport properties of the membranes are measured in the temperature range of 50–300°C and at pressure differences ranging from 0.5 to 3 bar. The membranes have reproducible high permeances (2×10−6 mol/m2·s·Pa) for H2 and much lower permeances for CO2, N2, and O2 (∼10× lower), CH4 (∼500× lower). The silica membranes show a slight increase of permeance with increasing temperature for H2, CH4, N2, O2 and a slight decrease for CO2. For the time being the pore size of the microporous supported silica membranes can be estimated best from the relation between the permeance and kinetic diameter (dk) of the gases used. The pore size can be derived from the value of the dk of the smallest gas where permeance is no more observed. The silica membranes can be applied for different types of separations: CO2/CH4 with a permselectivity (Fα) of 75, O2/N2 with Fα=4, H2/CH4 with Fα>500 and H2/CO2 with Fα=70, all at 200°C. Separation factors obtained from gas separation experiments with 50/50 (vol.%) gas mixtures are very similar to permselectivities calculated from single gas permeance experiments.
- Microporous membranes
- Gas separations