Environmental Separation and Reactions: Zeolite Membranes
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Zeolites are three-dimensional, microporous, crystalline solids with well-defined structures that contain aluminum, silicon, and oxygen in their regular framework. When zeolite crystals are intergrown to form continuous layers, the resulting membranes can separate gas mixtures with high selectivities because their pore sizes are comparable to the molecular dimensions. Separation is possible by molecular sieving, preferential adsorption, or differences in diffusion rates. Zeolitic membranes can be classified as either symmetric membranes (self-supported) or asymmetric membranes (supported). Different from the mesoporous inorganic membranes used in gas diffusion process for uranium isotope separation, zeolite membranes consist of many crystallites packed together without crystallite boundary gap in ideal cases. Different types of membranes have varying crystal structure, pore size, and surface properties, thus exhibiting different separation and/or catalytic properties.
Up to date, some zeolite membranes such as the MFI type have been well developed in the last decade since some early works in this area. Preparation techniques have been developed and fine-tuned in recent years, which makes it possible to fabricate a large and pinhole-free zeolite membrane. Zeolite-membrane-based separations and reactions have generated much interest in recent years. They are attractive for a variety of reasons, including steady state operation, tailored selectivity, low energy consumption, compatibility to high-temperature and high-pressure conditions, and potential for effective separation and combined reaction-separation systems. Such merits offer the opportunity to solve problems associated with high production costs. Zeolite membranes have been the highlight in the separation technology and novel reactor engineering fields, and is widely expected to find commercial applications in the near future. Some excellent reviews on the preparation and application of zeolite membranes have appeared in the last several years. However, the rapid development of this field necessitates a timely update on this topic, particularly on preparation of zeolite membrane and its application to environmental separation and reactions.