Synthesis of Nano-crystalline Zeolites and Applications to Zeolite Membranes

Zeolites are crystalline aluminosilicate materials and possess intracrystalline pores and nanospaces of similar sizes to the molecules of the lighter hydrocarbons. Moreover, zeolites have strong acid sites on the nanopore surfaces within and on the external surfaces of the crystalline structure. These properties enable zeolites to be used as in reaction and separation processes, and these potential uses have led to the development of zeolite-based structured materials, such as zeolite films and membranes1)~3). Zeolite membranes combine the properties of zeolites with those of inorganic membranes, so are attractive materials for various applications, such as selective reaction membranes4),5) and separation membranes6)~9). ZSM-5 zeolite membranes were first prepared by Sano et al.10),11), and a great deal of subsequent research has focused on the use of zeolite membranes for reaction and separation. To prepare a zeolite membrane by hydrothermal synthesis, zeolite seed crystals are deposited on a porous support, then secondary growth of the zeolite occurs to form the zeolite membrane. The uniformity of the membrane affects the reaction/separation properties, so the seeding of the zeolite crystals and the secondary growth process must occur uniformly12)~16). Therefore, nano-sized zeolite crystals are expected to have good properties as seed crystals, because the smaller size allows for greater control of the seeding and growth processes. Mono-dispersed zeolite nanocrystals are expected to form uniform zeolite membranes. This review describes a method for preparing nanocrystalline zeolites in a solution consisting of a surfactant, an organic solvent, and water (called the emulsion method17)~21)), and a method based on the catalytic cracking of silane22) (called the CCS method) for the regioselective deactivation of acid sites using silane compounds with various organic substituents. An MFI zeolite (ZSM-5) membrane was applied to the reaction of methanol to olefins23),24), to investigate the effect of the regioselective deactivation of acid sites by the CCS method on the olefin yields. A hydrophilic silicalite-1 membrane25) was prepared using silicalite-1 nanocrystals, to examine the effect of the crystal size on the separation properties26),27).