Siliceous ZSM-5 membranes by secondary growth of b-oriented seed layers

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-NN-(tripropylammoniumhexamethylene) di-NN-propylammoniurn 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&h0h/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.