Exploration on the mechanism of crystal morphology transformation in mordenite†

Abstract

Mordenite is extensively utilized in the domains of adsorption and catalysis owing to its excellent selectivity, stability, and renewability. In this work, the effects of crystal seeds, the ratio of SiO₂/Al₂O₃, crystallization temperature, silicon source, and structure directing agents on the morphologies of mordenite are investigated by utilizing hexamethyleneimine (HMI) as the OSDA. Results indicate that the transformation of rod-like and flake-like mordenites is mainly related to the increasing ratio of SiO₂/Al₂O₃, and the formation of flake-like morphology is related to various conditions. On a macro level, the rod-like and flake-like zeolites are formed by transforming amorphous precursors into small crystals and growing them epitaxially. On a micro level, HMIH⁺ progressively displaces Na⁺ to balance the framework charge with the increasing ratio of SiO₂/Al₂O₃. DFT calculation and nuclear magnetic resonance results confirm the specific guidance of HMI on the T₂ and T₄ sites of the rod-like and flake-like mordenites. The conclusion can serve as a reference for regulating the morphology and active sites of mordenite.