Siliceous ZSM‐5 Membranes by Secondary Growth of b‐Oriented Seed Layers

Abstract

A modified seeded growth procedure, which allowed us to prepare the first functional b‐oriented siliceous ZSM‐5 membrane (Z. Lai et al., Science, 2003, 300, 456), is described and discussed in detail. The procedure involves growing a b‐oriented ZSM‐5 seed monolayer into a dense b‐oriented membrane. Preservation of the orientation is achieved by enhancing the growth rate of siliceous ZSM‐5 crystals along the b‐axis using trimer‐TPAOH (bis‐N,N‐(tripropylammoniumhexamethylene) di‐N,N‐propylammonium trihydroxide) as the structure‐directing agent (SDA). Under similar synthesis conditions but using monomer‐TPAOH (tetrapropylammonium hydroxide) as the SDA for the secondary growth of the b‐oriented seed layer, two other films with mixed grain orientations, namely b&a‐ or b&a&h 0 h/c‐oriented ZSM‐5 membranes were obtained depending on temperature of the secondary growth. Membrane microstructures were characterized by X‐ray diffraction, pole figure analysis, scanning electron microscopy, fluorescent confocal optical microscopy, electron probe microanalysis, and X‐ray photoelectron spectroscopy. The separation performance of the three types of membranes was tested by using a mixture of xylene isomers. Significant improvement in both permeance and separation factor is achieved by the b‐oriented siliceous ZSM‐5 membrane compared to the other two types of membranes described here or other reported ZSM‐5 membranes in the literature. The permeation results indicate that the membrane performance depends strongly on membrane microstructure.