Synthesis of gas and vapor molecular sieving silica membranes and analysisof pore size and connectivity

Pervaporation and gas permeation properties of microporous silica membranes made by a sol-gel method are discussed. Defect free molecular sieving memb ranes are prepared by a dip coating process. The molecular sieving performa nce was measured and controlled based on gas permeation behavior of the mem branes. The apparent activation energy for helium permeation and He/N-2 per m-selectivity values were used as the parameters for optimization of the me mbrane performance. The membranes with very high activation energy for He d iffusion were used for pervaporation studies with a methanol/MTBE mixture a t 323 K. Separation factor values as high as 260 were measured at a total l iquid flux of 0.3 kg/m(2) hr. Sorption studies performed on corresponding s ilica gels revealed a separation mechanism based on diffusion of vapor mole cules. Permeation of the vapor molecules through the micropores followed an activated diffusion mechanism. The gas permeation data could provide an un derstanding of the pore size distribution of the membrane, and the vapor so rption and diffusion data on the size and connectivity of the membrane pore s.