Ceramic zeolite composite membranes, comprising a ceramic substrate and a dense pure zeolite thin layer on the surface of the substrate, have been developed by in-situ synthesis. The zeolite thin layer has been shown by X-ray diffraction, SEM and optical microscopy to have a pure zeolite phase, in which individual zeolite crystallites have intergrown in three dimensions into a polycrystal zeolite thin disc. The nitrogen permeability of these composite membranes, after calcination, reaches 1-4 m(3)/m(2)-hr-bar at room temperature. The ideal selectivities are 2.81 for He over N-2, and 47.7 for N-2 over n-butane, which are far beyond the range of Knudsen diffusion. After improvement of the preparation procedure the membrane even shows a selectivity of 6.2 for n-butane over i-butane. Combined effects of ''shape-selectivity'' of the zeolite and capillary condensation of the non-zeolite pores are considered to be responsible for the high selectivities.
