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&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.

Effective manipulation of the microstructure of zeolite film by hydrothermal pretreatment

“…Synthesis of MFI microcrystals MFI (silicalite-1) microcrystals were synthesized with slight modification to the well-documented procedure [5]. For a typical synthesis, 7.6 g TPAOH (tetrapropylammonium hydroxide, 20 wt%, Aldrich) and 7.5 g TEOS (tetraethyl orthosilicate, 98%, Kermal) were mixed with 60.0 g DDI (distilled de-ionized) water.…”

“…Taking Mordenite Framework Inverted (MFI) zeolite [1] (symmetry: orthorhombic; space group: P nma ; unit cell constants: a = 20.1 Ǻ , b = 19.9 Ǻ , and c = 13.4 Ǻ ) as an example, the pore system consists of straight channels (pore size: 0.53 9 0.56 nm) interconnected by zigzag channels (0.51 9 0.55 nm) [2]. In addition to their traditional uses as catalysts, adsorbents and ionic exchangers [3], zeolites can also be fabricated into thin films and used as separation membranes [4][5][6][7][8], selective sensors [9,10], low-k materials [11,12], corrosion-resistant coatings [13], and optical materials [14,15]. In the synthesis of high-quality zeolite films, the accurate manipulation and optimization of their microstructures, such as crystallographic preferred orientation (CPO), thickness, intergrowth and grain boundary structures, is critical.…”

“…In subsequent electrochemical testing, these films showed excellent molecular sieving properties as selective sensors [10]. Tsapatsis et al fabricated twin-free and highly b-oriented MFI membranes by secondary growth of b-oriented MFI seed monolayers [4,5]. Compared to randomly oriented membranes, the b-oriented membrane demonstrated dramatic improvements in both permeance and selectivity in the separation of p-/o-xylene vapors.…”

Zeolite Membranes in Catalysis—From Separate Units to Particle Coatings

“…A secondary growth method with vacuum seeding for the preparation of A-type zeolite membranes [74] and diverse techniques for crystal orientation in zeolite membranes [75][76][77][78][79][80][81][82] were proposed. A vapor-phase transport (VPT) method was first reported by Xu et al as an alternative approach to the hydrothermal synthesis for the preparation of ZSM-5 zeolite [83].…”

Double aluminum recovery and its reuse in wastewater treatment

“…The mixture was compacted in a 22-mmdiameter stainless steel holder applying 6 tons/cm 2 of pressure. The disk was kept at room temperature until it was used in the electrochemical reactor (Guerrero et al 2010;Lai et al 2004). …”