日本ゼオライト学会　刊行物 Publication of Japan Zeolite Association

Organic structure-directing agents (OSDAs) have enabled us to synthesize zeolites with novel framework types; however, the use of OSDAs is undesirable from a practical view point because of the high environmental burdens as well as their high cost by the complex total process. Therefore, the synthesis of zeolites without the use of OSDAs has been one of the most important topics for the commercialization of zeolites. Recently, the OSDA-free syntheses of useful zeolites have been achieved by utilizing the zeolite seed crystals. In this review, the overview of recent advance in seed-assisted, OSDA-free synthesis of zeolites is shown, and a new working hypothesis named “Composite Building Unit (CBU) Hypothesis” proposed by us for broadening zeolite types by seed-assisted synthesis is explained. This hypothesis focuses on the structural relationship between seed (target) zeolite and the zeolite obtained from the reactant gel without seeds. The background of the proposal of this hypothesis is explained, and the validity of the hypothesis is introduced. It is confirmed that several zeolites such as ZSM-11, ZSM-12 and ECR-18 can be synthesized by applying this hypothesis. In addition, some requirements for the successful synthesis of zeolites by this method and the unique features of zeolites obtained are explained.