An assessment of zeolite beta reactivity based on 27Al MAS NMR study of T-site distribution

Abstract

A series of model catalysts has been synthesized by hydrothermal crystallization in a fluoride medium by varying H₂O/SiO₂ from 3.6 to 9.6 in the reaction gel. The zeolites obtained show similar structure, morphology and textural characteristics, but they are characterized by different distribution of Al atoms over T-sites and acidic properties: the higher the H₂O/SiO₂ ratio, the more is the number of strong acid sites according to NH₃-TPD data. In the ²⁷Al MAS NMR spectra of the synthesized samples, 4 signals of nonequivalent Al atoms with δ_iso = 54.3, 55.7, 58.6 and 59.8 ppm have been distinguished, which are assigned to T5 + T8 + T9, T1 + T2, T6 + T7 and T3 + T4 sites, respectively. In the series of model zeolites, intensity of the signals at 58.6 and 59.8 ppm increases with the H₂O/SiO₂ ratio in the gel. Beta-7.6 and Beta-9.6 samples show higher catalytic activity in the toluene disproportionation reaction compared to Beta-3.6 and Beta-5.6, which coincides with the NH₃-TPD data. A correlation between the initial rate of xylenes’ formation and the number of Al atoms corresponding to the signals at 58.6 ppm (T6 + T7 sites) and 59.8 ppm (T3 + T4 sites) in the ²⁷Al MAS NMR spectra has been established. In the course of crystallization T6 and T7 positions in zeolite framework tend to be the mostly occupied with an increase of the induction period by changing Si/Al or H₂O/SiO₂ ratio in the reaction gel.