The structure-directing role of heterologous seeds in the synthesis of zeolite

Zeolites have been widely used as catalysts, ion-exchangers, and adsorbents in chemical industries, detergent industry, steel industry, glass industry, ceramic industry, medical and health field, and environmental field, and recently applied in energy storage. Seed-assisted synthesis is a very effective approach in promoting the crystallization of zeolites. In some cases, the target zeolite cannot be formed in the absence of seed zeolite. In homologous seed-assisted synthesis, the structure of the seed zeolite is the same to that of the target zeolite, while the structure of the seed zeolite is different to that of the target zeolite in the heterologous seed-assisted synthesis. In this review, we briefly summarized the heterologous seed-assisted syntheses of zeolites and analyzed the structure-directing effect of heterologous seeds and surveyed the "common composite building units (CBUs) hypothesis" and the "common secondary building units (SBUs) hypothesis". However, both hypotheses cannot explain all observations on the heterologous seed-assisted syntheses. Finally, we proposed that the formation of the target zeolite does need nuclei with the structure of target zeolite and the formation of the nuclei of the target zeolite can be promoted by either the undissolved seed crystals with the same CBUs or SBUs to the target zeolite or by the facilitated appropriate distribution of the specific building units due to the presence of the heterologous seed that does not have any common CBUs and SBUs with the target zeolite. (c) 2023 Institute of Process Engineering, Chinese Academy of Sciences. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co., Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).