Preparation for ZSM-5 membranes by a two-stage varying-temperature synthesis

Thin (7 mum) polycrystalline zeolite membrane with high permeances was repr oducibly prepared on an alpha -Al2O3 support, whose pore size is about 2 mu m, by a two-stage varying-temperature synthesis. Compared to the synthesis at constant temperature, equiaxed particles in the membrane were achieved; thin and dense membrane was obtained. Since nucleation occurred during an i nduction period and large amounts of nuclei could form at low temperature, for ZSM-5 membrane preparation, if the temperature change were made after c ompletion of the nucleation period, nuclei on the support would grow quickl y to form thin and continuous membrane. The membrane was characterized by X RD, SEM and permeation measurements using single gases and mixtures of n-bu tane and iso-butane. At room temperature, the highest H, permeance is 2.4 x 10(-6) mol/m(2) s Pa, the highest ideal selectivities are 129 for H-2/n-bu tane, 1548 for H-2/iso-butane. The separation selectivity for n-butane/iso- butane has maximum (e.g. 78 at 373 K) as a function of temperature and is h igher than the ideal selectivity, It is probably that n-butane effectively blocks the permeation of iso-butane in a mixture, which decreases the perme ance of iso-butane. (C) 2001 Elsevier Science B.V. All rights reserved.