Crystal Mechanism of Cage-Based Small-Pore Molecular Sieves: A Case Study of CHA and LEV Structures

Abstract

The elucidation of the crystallization mechanism of zeolites is one of the most important topics in zeolite science. Here we have investigated the formation pathway for CHA and LEV zeolite crystals, the aluminosilicate and silicoaluminophosphate versions, respectively, in the presence of the organic structure-directing agents (SDAs), TMAdA+, TEA+, AN, and HMI. The crystallization process was monitored using a combination of ex-situ 1H-13C CP MAS NMR and IR spectra of a series of solid products recovered as a function of time during the crystallization process. We demonstrated that in the overall crystallization mechanism of CHA and LEV zeolite, the construction of large cha- and lev-cages, with complete incorporation of organic SDAs, precedes that of the small d6r-cages in the aluminoslilicate and silicoaluminophosphate forms. Crystallization can be viewed as an initial construction of randomly distributed cha- and lev-cages at the nucleation stage. The next step is that d6r-cages are constructed by linking the tops and bottoms of the cha- and lev- cages, and finally, this linking promotes the formation and growth of CHA and LEV zeolite crystals. We also showed that the SDAs play an important role in the initial condensation and nucleation, as well as participating in the crystal growth.