Zeolite Seeds: Third Type of Structure Directing Agents in the Synthesis of Zeolites

Zeolites have been widely applied in the industrial processes and their syntheses normally require the presence of templates, structure-directing agents (SDA). The first type of SDA is inorganic cations such as sodium, and the second type is organic cations such as organic ammonium, which direct the crystallization of zeolites. In this review, we briefly summarize recent works on the organotemplate-free syntheses of zeolites in the presence of zeolite seeds, where the zeolite seeds direct the crystallization of zeolite crystals. Therefore, the zeolite seeds in the seed-directed synthesis (SDS) routes are reasonably considered to be a third type of SDA. We focus here on the SDS syntheses of zeolites with abundant Al sites, high-silica zeolites, and pure silica zeolites, as well as the use of exotic zeolite seeds. Compared with organic SDAs, the use of zeolite seeds in organotemplate-free synthesis not only significantly reduces zeolite cost but also completely avoids calcination of organic SDAs in zeolites for opening micropores. More importantly, this SDS approach is universal and simple for synthesizing a series of aluminosilicate zeolites.