Broadening the Applicable Scope of Seed-Directed, Organic Structure-Directing Agent-Free Synthesis of Zeolite to Zincosilicate Components: A Case of VET-Type Zincosilicate Zeolites

摘要

Seed-directed synthesis of zeolite without using organic structure-directing agents (OSDAs) has been considered to be a low-cost, environmentally friendly technique, potentially for application in the commercial manufacture of zeolites. The synthesis of zincosilicate zeolites by this method is thought to be more difficult than the aluminosilicate and borosilicate analogues, mainly because of the highly dissolution of zincosilicates in alkali solutions and the lack of comprehensive, structural information of intermediate zincosilicate species in zeolite synthesis systems. Here, a successful synthesis of zincosilicate zeolites possessing the VET-type structure by the seed-directed, OSDA-free method is reported for the first time. The obtained products show comparable pore characteristics and superior acidities to those of the seed crystals synthesized with OSDAs. In addition, VET-type zincoaluminosilicate zeolites are synthesized, which show better acidities than those of zincosilicate counterparts. In contrast, a layered silicate magadiite is formed under the identical synthesis conditions but without seeding; interestingly, a 5—3 unit, a structural unit with 8 tetrahedral atoms consisting of a 5-membered ring and liner 3 tetrahedral atoms, is found to be a common structural unit between magadiite and VET-type zeolite. Analyses of the intermediate matters formed during the synthesis reveal that the intermediates have similar chemical compositions and medium-range orders (silicate ring-structures) to the resultant zeolites, suggesting that selective incorporations of Zn and Li ions into solids and minimal structural changes during synthesis are critical to achieve VET-type zincosilicate zeolite. It is anticipated that these findings can help and stimulate us to broaden the applicable scope of the seed-directed, OSDA-free method, thus far mostly limited to aluminosilicate and borosilicate zeolites, to zincosilicate and other metallosilicate compositions.