Postsynthesis of high silica beta by cannibalistic dealumination of OSDA-free beta and its catalytic applications

Abstract

Partially removing the framework Al atoms from low-silica aluminosilicates is an old but still challenging problem, because a high framework Al content generally induces lower acid strength and fast deactivation. An inappropriate dealumination method may cause structural collapse. The organic structure-directing agent (OSDA)-free Beta zeolite is expected to serve as a green solid acid catalyst, but decreasing the Al content before being used as a catalyst is needed to enhance the stability and acid strength. Al₂(SO₄)₃ was used to extract effectively the framework Al species from Al-rich OSDA-free Beta (Si/Al = 4.3) under mild conditions, forming a natroalunite phase. The natroalunite phase was then selectively washed away with diluted NaOH solution, resulting in dealuminated zeolite (DeAl Beta) with a pure *BEA phase and higher Si/Al ratios of 7.6–12. The dealuminated Beta zeolites were revealed to possess abundant mesopores while maintaining high crystallinity. The decrease of the total acid amount in DeAl Beta zeolites was accompanied by the enhancement of acid strength, resulting in higher selectivity of deeper cracking products and lower deactivation rate in the 1,3,5-triisopropylbenzene cracking reaction. The DeAl Beta also showed a high catalytic activity comparable to commercial nanosized Beta in the liquid-phase Friedel–Crafts acylation, which was much higher than the parent Al-rich Beta. This study would provide a promising method for developing useful solid acid catalysts by the combination of OSDA-free synthesis and the eco-efficient dealumination process.