Encapsulating subnanometric metal clusters in zeolites for catalysis and their challenges

Subnanometric metal clusters encapsulated within zeolites are of great interests for the industrial catalysis. Herein we review for the first time the encapsulation of subnanometric metal clusters in zeolites. The concepts of both subnanometric metal clusters and zeolites are briefly introduced. The recent advancements of synthesis methods, such as impregnation, ion-exchange followed by post-treatment, template-guidance approach, in situ hydrothermal synthesis and interzeolite transformation are summarized. Further, the encapsulation effects including metal loading, ligand property, zeolite type and calcination condition for subdividing subnanometric metal clusters and nanoparticles in zeolites using quite similar synthesis procedures during in situ hydrothermal synthesis are firstly reviewed to explore the underlying mechanism. The important catalytic applications mainly contained propane dehydrogenation, formic acid decomposition, ammonia borane hydrolysis, cyclohexane oxidation, water-gas shift reaction and hydrogenation reaction are demonstrated. This review concludes with the challenges and status of both the stability issue under high temperature and advanced characterization techniques as well as the industrial perspectives.