A facile top-down protocol for postsynthesis modification of hierarchical aluminum-rich MFI zeolites

Abstract

High aluminum content constitutes a major hurdle for the postsynthesis modification of hierarchical zeolites. A facile protocol comprising fluorination and sequential alkaline treatment is presented for the postsynthesis modification of hierarchical Al-rich MFI zeolites. By virtue of this protocol, uniform intracrystalline mesoporosity is introduced in an Al-rich MFI zeolite (Si/Al = 14.3). The obtained hierarchical zeolites exhibit a significant mesopore size distribution, centered around 6 nm, and show improved conversions in catalytic cracking of bulky aromatic molecules. The fundamental implications of the fluorination-alkaline treatment protocol are related to the formation of F-bearing tetrahedral aluminum species in the antecedent fluorination step, which alleviates the resistance of Al sites to the alkaline medium and causes Al-F complexation for regulated hydrolysis of the Al species during the alkaline treatment process. This top-down protocol and the derived mechanistic understandings are expected to be applied in the synthesis of hierarchical Al-rich zeolites with other framework topologies.

Keywords: aluminum; fluorides; porosity; postsynthesis modification; zeolites.