Sol-gel synthesis of molecular sieving silica membranes

Polymeric silica sol was synthesized by the acid catalyzed hydrolysis and condensation of tetra-ethyl-ortho-silicate. Calcined unsupported membranes made from this sol showed microporous nature. Supported membranes on alumina were prepared by dipping and calcining. Helium showed activated diffusion with an apparent activation energy of 17 kJ mol(-1). H-2 permeation was comparable to that of helium under identical conditions. N-2, Ar, O-2, C3H6, C3H8, n-C4H10 and i-C4H10 permeation values were extremely small and therefore difficult to fit appropriate diffusion models. At 303 K hydrocarbon permeation was about 2 times higher than that of N-2, Ar or O-2. He/N-2 permselectivity around 1000 and helium permeation in the order of 10(-7)-10(-8) mol m(-2) s(-1) Pa-1 were measured in the temperature range of 303-460 K. Comparison of E-act, selectivity and He and N-2 permeation of different samples evidenced the dependence of nitrogen flux on processing defects. Obviously permeation rate of nitrogen molecule was insignificant through majority pores of the membrane.