High Potential of Interzeolite Conversion Method for Zeolite Synthesis

Zeolites are a class of microporous crystalline aluminosilicates with a special type of pore system that are widely used in adsorption, catalysis, and ion-exchange applications. This diversity of applications is possible because of their unusual properties such as unique framework, high internal surface area, solid acidity, and molecular sieve and ion-exchange properties. In general, medium-to-large pore size and high-silica zeolites are synthesized by hydrothermal treatment of amorphous aluminosilicate hydrogel as a starting material in the presence of various organic structure-directing agents (OSDAs). According to the International Zeolite Association (IZA), 206 framework types of zeolites have been registered until now1). However, the OSDAs occlude the cages and pores in the final zeolite, and must be removed, generally by calcination at high temperatures, which generates harmful gaseous pollutants. Furthermore, the use of expensive OSDAs also increases the cost of the zeolite. Therefore, recent development toward OSDA-free zeolite synthesis using seed crystals has attracted considerable attention, and several research groups have succeeded in OSDA-free synthesis2)~11). In most zeolite syntheses, an amorphous phase is directly converted to a given type of zeolite, but the transformation is usually gradual and follows the sequence: amorphous phase→ less stable zeolite→most stable zeolite. On this basis, several researchers have pointed out the high potential of another method for zeolite formation, based on the hydrothermal conversion of one zeolite type into another type (interzeolite conversion)12)~29). This study describes our recent investigations of the synthesis of several types of zeolites by interzeolite conversion of FAU (the three characters indicate the framework type code), beta (hereinafter, *BEA), and LEV type zeolites in the presence and absence of OSDAs30)~44), and reviews the potential of interzeolite conversion as an alternative method for zeolite synthesis.