Simultaneous Control of Aluminum Atoms and Defects in the MOR Zeolite Framework by Postsynthetic Treatments

The framework Al sites and defects in zeolite structures realize their functionalities as catalysts, adsorbents, and ion exchangers. Therefore, controlling the number and state of Al atoms and silanol species in zeolites is crucial for achieving better performance. Among the numerous synthetic approaches for this purpose, postsynthetic approaches show promise for achieving the modification of Al atoms and defects for various zeolites. In this study, the effects of steam, acid, and fluoride treatment on MOR zeolites were analyzed. Steam treatment eliminated framework Al atoms to form extra-framework Al species, and framework vacancies formed by dealumination were healed by silicate migration. A 27Al triple-quantum magic-angle spinning nuclear magnetic resonance (3QMAS NMR) spectroscopy study combined with gauge-including projector augmented wave (GIPAW) calculations for comprehensive defect models revealed the increased distortion of tetrahedral coordination around Al atoms and the change in Al siting during steam treatment. Acid treatment removed extra-framework Al species. Fluoride treatment caused realumination without changing the bulk composition. Additionally, fluoride treatment decreased the number of defects, whereas fluoride treatment of steamed zeolite increased the number of defective silanol sites in the case of MOR zeolites. The different roles of the postsynthesis treatments in this study are expected to aid in facilitating the postsynthetic modification of the atomic distribution in various zeolites.