Synthesis and Characterization of ZSM-5 Zeolite Membranes

Citation

Lai, Re (2001) Synthesis and Characterization of ZSM-5 Zeolite Membranes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/m5xs-mq20. https://resolver.caltech.edu/CaltechETD:etd-11102005-103556

Abstract

Synthesis of ZSM-5 zeolite membranes supported on porous alumina substrates was conducted in autoclave reactors. The film morphologies and structures were characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Gas adsorption was used to estimate the crystalline fraction of the products. The gas permeation properties of the membranes were evaluated for H₂, CH₄, O₂, N₂, and n-butane and other gases singly or in mixtures.

ZSM-5 film formation on alumina and other surfaces in certain clear, tetrapropylammonium(TPA⁺)-containing synthesis solutions was found to be preceded by a gel layer. The presence of alumina and other substrates of a high Hamaker constant facilitated formation of the surface gel layer and accelerated zeolite crystallization. Aluminum added to the solution or leached from alumina substrates had dual effects, to induce surface gelation and to retard crystallization of that layer as well as crystallization in the bulk solution.

Growth of ZSM-5 membranes was facilitated by using alumina tubes pre-coated with silicalite seeds. Accompanied with gradual growth of an external polycrystalline layer, siliceous deposits accumulated as deep as 100 µm inside the pores of the support, converting gradually from amorphous to crystalline. Pure gas permeation results are presented for membranes prepared in a solution of composition SiO₂: 0.15TPAB4: 0.7NaOH: 98H₂O using 0.4 and 2 µm seeds.

Organic-free hydrogel reaction mixtures were further used to grow ZSM-5 membranes on seeded porous alumina substrates, eliminating the crack-prone calcination step for removing organic TPA⁺ templates from as-synthesized ZSM-5. The optimum composition SiO₂: 0.0125Al₂O₃: 0.2675Na₂O: 46H₂O was identified to produce membranes with permeation selectivities for H₂ over n-butane above 10⁴ and for O₂ over N₂ 9-10. The permeation was strongly activated with the activation energies increasing sharply with molecular size.

Parallel synthesis of zeolite films was developed to expedite composition screening. The films prepared in an array reactor of multiple wells each containing 35 micro-liter synthesis solution displayed morphology similar to that produced by the conventional synthesis. The method was applied to explore the composition space of clear, organic-free synthesis solutions for ZSM-5 films growth: SiO₂: (700-300)Al₂O₃: (0.5-0.7)NaOH: 80H₂O.

Thesis Files