USE OF DIFFUSION-BARRIERS IN THE PREPARATION OF SUPPORTED ZEOLITE ZSM-5 MEMBRANES

Zeolite ZSM-5 membranes were prepared by in situ crystallization on po rous alpha-Al2O3 disks that contained a diffusion barrier to limit exc essive penetration of siliceous species into the alumina pores. The ba rrier was introduced into the alumina pores by impregnating the porous disk with a 1 : 1 molar mixture of furfuryl alcohol (FA) and tetraeth ylorthosilicate (TEOS), polymerizing the mixture retained in the disk, and carbonizing the resulting polymer at 600 degrees C in N-2. Follow ing carbonization, a partial carbon bum-off was carried out by catalyz ed oxidation in 2% O-2-N-2 at 600 degrees 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 r emoved by calcination in air, at 500 degrees 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 x 10(-3) mol m(-2) s(-1) and 45, respective ly, at 185 degrees C, which are, respectively, ea. 1.6 and 4 times as large as those of membranes prepared without the use of barriers. Elec tron probe microanalysis (EPMA) and X-ray diffraction (XRD) revealed t hat the internal layer of the barrier-pretreated membrane has smaller thickness and higher crystallinity accounting for the increased flux a nd selectivity.