Synthesis and Gas Separation of Chabazite Zeolite Membranes

Chabazite (CHA) zeolite membranes exhibit superior performances in light gas separations owing to the eight-membered ring channel structure with small pore size (0.38 nm), adjustable surface characteristics, and high material stability and preparation reproducibility, and have gradually become one of the hot spots of zeolite membrane research in recent years. This review article first introduces the basic characteristics, the two typical CHA zeolite membranes (SAPO-34 and SSZ-13 membranes), then compares the synthesis and preparation methods of CHA zeolite membranes (in-situ synthesis, secondary growth synthesis, microwave heating methods) and analyzes their advantages and disadvantages in application status. The influences of their key synthesis conditions of the secondary growth as the mainstream synthesis method on the qualities of SSZ-13 and SAPO-34 membranes have been elaborated in detail, mainly including 1) the seeding conditions, such as carrier type, seeding crystal and seeding approach; 2) the hydrothermal synthesis conditions, such as crystallization time and temperature, water content, silica-to-alumina ratio, structure directing agent, and cation type; 3) the calcination approaches, such as conventional calcination, staged calcination, and rapid heating treatments. After comparative analysis, the preferred synthesis conditions of the above two typical CHA zeolite membranes are proposed. Furthermore, the modulation of membrane surface chemistry is discussed for the enhancement in gas separation, such as silica-to-alumina ratio adjustment, ion exchange, heteroatom substitution, amino-group functionalization, and surface modification. The detailed characteristics of gas separation in various gas mixture systems and the permeation properties of different single gases on CHA zeolite membranes are analyzed and summarized as well. Finally, the future development of CHA zeolite membranes is prospected.