Hydrophobic silica membranes for gas separation

The synthesis and properties of hydrophobic silica membranes are described. These membranes show very high gas permeance for small molecules, such as H2, CO2, N2, O2, and CH4, and permselectivities of 20–50 for these gases with respect to SF6 and larger alkanes like C3H8 and i-C4H10. The membranes are prepared by repeated dip coating of supported γ-alumina membranes in a silica sol solution, followed by drying and calcining. The hydrophobic nature of the membranes is obtained by adding methyl-tri-ethoxy-silane (MTES) to the sol prepared by acid-catalysed hydrolysis and condensation of tetra-ethyl-ortho-silicate (TEOS). The double silica membrane layer has a total thickness of 60 nm and a pore Ø of ca. 0.7 nm. The membranes are 10× more hydrophobic than the state-of-the-art silica membranes which makes them more suitable for application in humid process streams. Besides that, the very high permeance obtained for N2 and O2 of 4 and 7 × 10−7 mol/m2 s Pa, respectively, offer perspectives on dedicated air purification in which larger impurity molecules are blocked by molecular sieving effects.