Preparation of zeolite MFI membranes on alumina hollow fibers with high flux for pervaporation

Polycrystalline zeolite MFI (silicalite-1) membranes are successfully prepared by the secondary growth method on the α-Al2O3 hollow fibers with a high surface/volume ratio. Influences of the seed suspension concentration, the crystallization time and the content of the structure-directing agent (SDA, e.g., TPAOH) in the synthesis solution are investigated. As-synthesized zeolite seeds and membranes are characterized by X-ray diffraction (XRD), dynamic light scattering (DLS) and scanning electron microscopy (SEM) with energy dispersive X-ray (EDX). The membrane formation and integrity are found to be sensitively depending on the SDA contents in the synthesis solution and the seed amount of the seeded layer. High TPAOH content may cause aluminum leaching from the alumina support seriously and low TPAOH content may lead to non-continuous zeolite layer, leading to poor separation performance. A continuous and dense zeolite seed layer is necessary to obtain the silicalite-1 membrane with good quality. However, too thick seed layer may cause peeling and cracking of the membrane and thus decrease the separation performance. Appropriate TPAOH and seed amount may prevent the α-Al2O3 support leaching effectively. The silicalite-1 membranes prepared in this paper show the highest flux and relatively high separation factor of ethanol/water mixtures compared with the literature data.