A low-temperature CVI method for pore modification of sol–gel silica membranes

A low-temperature (300 °C) CVI technique was employed to modify commercial sol–gel silica membranes, in order to close meso- or micropores and repair defects often formed in their top sol–gel layer. Membranes were characterized before and after modification in terms of permeance and permselectivity with the use of He, H2, N2, CO2 and SF6 gas probe molecules. The sol–gel silica membranes had a low selectivity indicative of a Knudsen diffusion mechanism which reveals the presence of defects on the silica layer. The pore size of the CVI modified silica membranes decreased to less than 0.55 nm with a concomitant decrease of permeance and a significant improvement of permselectivities (H2/N2: 91, H2/CO2: 50, He/N2: 244 at 300 °C). Activation energies of permeation for various gases reveal a significant improvement of permselectivity with temperature for industrially important gas pairs, such as H2 and He over CO2 and N2. This hybrid CVI technique on sol–gel silica membranes provides an effective method for closure of pores and for the repair of top-layer defects, at temperatures (300 °C) where available sealing technologies can be used.