PREPARATION AND SEPARATION PROPERTIES OF SILICALITE COMPOSITE MEMBRANES

Silicalite-alumina composite membranes were prepared by an in situ zeo lite synthesis method using an alumina membrane tube with a 5-nm pore diameter, gamma-alumina layer as a substrate. Single gas permeances of H-2, Ar, n-C4H10, i-C4H10, and SF6 were measured and mixtures of H-2/ i-C4H10 and H-2/SF6 were separated to characterize the silicalite memb rane. These measurements were made from 300 to 737 K, and are compared to an alumina membrane without a silicalite layer. Permeances were lo wer in the silicalite membrane (a factor of 8 for Ar at 298 K). Permea nces for the alumina membrane decreased as the temperature increased, and separation selectivities were lower than values expected for Knuds en diffusion. Transport through the alumina membrane was by Knudsen fl ow and surface diffusion. The silicalite membrane showed dramatically different behavior, and transport appeared to be controlled by molecul ar size and adsorption properties. Permeances of all components studie d were activated in the silicalite membrane, and activation energies r anged from 8.5 to 16.2 kJ/mol. The ratio of single gas permeances was as high as 136 for H-2 to SF6 and 1100 for H-2 to i-C4H10 at 298 K. Se paration selectivities at elevated temperatures were significantly abo ve Knudsen diffusion selectivity for the silicalite membrane and were larger than ratios of pure gas permeances at the same temperature. The largest permeance ratio for the separation of mixtures was 12.8 for H -2/SF6 at 583 K. Separation selectivities for both membranes were high er when a pressure drop was maintained across the membrane than when a n inert sweep gas was used because of counter diffusion of the sweep g as.