The Overlooked Roles of Long-Chain Alkyl Quaternary Ammonium Cations in the Mesopore Engineering of ZSM-5 Zeolite

Abstract

The introduction of a specific concentration of long-chain alkyl quaternary ammonium cations inhibits the construction of mesoporosities inside MFI zeolites through alkaline treatment. The spontaneously formed micelles bear an electrical double-layer structure, which imposes repulsion on the caustic hydroxide ions (OH^–) and therefore restrains the dissolution of the zeolite in the alkaline media. The aforementioned inhibitive effect of long-chain alkyl quaternary ammonium cations can be disabled by the addition of an inorganic salt, such as NaBr, which induces a transformation in micelle morphology and the micellar electrical double-layer structure. In the case of alkyl quaternary ammonium cations with short alkyl chains (e.g., trimethyloctylammonium: C₈TA⁺), such an inhibitive effect cannot be eliminated by the addition of an inorganic salt like NaBr. These findings update the understanding of the roles of alkyl quaternary ammonium cations and their micelle forms in tailoring the porosities inside zeolites.