Membranes and films of zeolite and zeolite-like materials

An ideal structure of zeolite membrane should be a slice of a perfect zeoli te crystal attached on a porous metal or ceramic support. To maximize the t hroughput, the zeolite layer must be very thin, limited only by the cell di mension of zeolite. Separation of a mixture may then be achieved based on t he molecular sieving ability of zeolite, which allows only molecules smalle r than a critical size to pass through. A variety of methods have been repo rted for the preparation of zeolite membranes, but so far a perfect epitaxi al zeolite layer is still out of reach and only a polycrystalline zeolite m embrane can be obtained. The first part of this review focuses on the perme ation of gases and vapors through a polycrystalline zeolite membrane as a s eparation means. The effect of microstructure on permeance will also be dis cussed, as well as the preparation methods leading to different microstruct ures. In addition to the usage as a shape-selective membrane, thin films of zeolite and zeolite-like molecular sieves can also serve as hosts for the encapsulation and orientation of guest atoms and molecules and their cluste rs. In the second part of this review, the production of layers of aligned microporous molecular sieve crystals on supports and the fabrication of sup ported thin zeolite-like nanoporous silica films as well as their potential applications on the preparation of advanced materials are discussed. (C) 2 001 Elsevier Science Ltd. All rights reserved.