Tuning the distribution of Al atoms in the framework of beta zeolites and its impact on the alkylation of phenol

Abstract

The distribution of Al atoms in the framework of beta zeolites is tailored by tuning the anions in the template of tetraethylammonium (TEA⁺). The strong polarizability induced by OH⁻ on TEA⁺ results in the highest density of total acid sites with Brønsted acid sites primarily located in the channels along the a and b axes of beta zeolites. Notably, Al pairs at the α and γ sites exhibit strong steric hindrance due to spatial constraints, while Al pairs at the β site are close to the large 12 MR channels with low diffusion limitation. Moreover, the ²⁷Al MAS NMR confirms that unpaired Al atoms are located in T7 sites, while Al pairs are located in 6 MRs with T6-6 sites that directly face the same 12 MR channels. Therefore, the resultant beta-C sample exhibits high activity, robust stability and the desired selectivity for 4-TBP in a continuous alkylation reaction.