Progress in seed-assisted synthesis of zeolites without using organic structure-directing agents

• Seed-assisted, OSDA-free synthesis can be applied to some of zeolites.
• CBU hypothesis can assist in determining synthesis conditions.
• Seed-assisted synthesis of zeolites can lead to the production of zeolites with unique features.

Recent progress in the development of methods that do not require the use of organic structure-directing agents (OSDAs) has enabled the synthesis of useful zeolites under the use of zeolite seed crystals. This novel approach has attracted considerable attention as a low-cost, environmentally friendly technique for the production of zeolites. In this review, the conventional seed-assisted synthesis of zeolites in addition to the seed-assisted preparation of zeolite membranes are introduced, and an overview of recent developments in the seed-assisted, OSDA-free synthesis of various zeolites, such as beta, ZSM-12, TTZ-1, levyne, and so on, is presented. Although some mechanistic studies have been reported, the details of seed-assisted synthesis remain under investigation; recent OSDA-free syntheses have been estimated to be kinetically controlled systems. A new working hypothesis, designated as the “composite building unit (CBU) hypothesis” was proposed by us in order to broaden the range of zeolite types that can be prepared via seed-assisted synthesis. This hypothesis focuses on the structural relationship between the seed (target) zeolite and the zeolite obtained from a reactant gel without seeds. It was confirmed that several zeolites, such as ZSM-11, ECR-18, and so on, can be synthesized without OSDAs for the first time, by applying this hypothesis. The requirements for the successful synthesis of zeolites are introduced in addition to the unique features of the zeolites that are obtained by the seed-assisted synthesis method.

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