Use of diffusion barriers in the preparation of supported zeolite ZSM-5 membranes

Abstract

Zeolite ZSM-5 membranes were prepared by in situ crystallization on porous α-Al_2O_3 disks that contained a diffusion barrier to limit excessive penetration of siliceous species into the alumina pores. The barrier was introduced into the alumina pores by impregnating the porous disk with a 1:1 molar mixture of furfuryl alcohol (FA) and tetraethylorthosilicate (TEOS), polymerizing the mixture retained in the disk, and carbonizing the resulting polymer at 600°C in N_2. Following carbonization, a partial carbon burn-off was carried out by catalyzed oxidation in 2% O_2N_2 at 600°C to generate a carbon-free region near the surface of the support. After zeolite crystallization, the remaining carbon and the organic structure directing agent were removed by calcination in air, at 500°C. It was found that pure carbon does not support zeolite growth while the solid obtained from a mixture of FA and TEOS does, due to the presence of dispersed silica. Membranes synthesized using barriers have n-butane flux and n-butane/isobutane selectivity 2.7 × 10^(−3) mol m^(−2) s^(−1) and 45, respectively, at 185°C, which are, respectively, ca. 1.6 and 4 times as large as those of membranes prepared without the use of barriers. Electron probe microanalysis (EPMA) and X-ray diffraction (XRD) revealed that the internal layer of the barrier-pretreated membrane has smaller thickness and higher crystallinity accounting for the increased flux and selectivity.