Improved performance of silica membranes for gas separation

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 (d_k) of the gases used. The pore size can be derived from the value of the d_k 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 (F_α) of 75, O₂/N₂ with F_α=4, H₂/CH₄ with F_α>500 and H₂/CO₂ with F_α=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.