Influence of Framework Silicon to Aluminum Ratio on Aluminum Distribution and Lewis Acidity in Zeolite Beta Investigated by 27Al MAS and MQ MAS NMR.

Abstract

Al-27 magic-angle spinning (MAS) and triple quantum (3Q) MAS NMR spectroscopic techniques were used to characterise zeolite Beta samples with framework Si/Al ratios between 9 and 215, obtained by synthesis in fluoride medium. A carefully controlled stepwise calcination procedure was adopted to obtain H-Beta. A partial resolution of the T-sites was observed in the Al-27 MAS NMR spectra, the resolution increasing with increasing the Si/Al ratios. The relative intensity of these peaks varied gradually, with Si/Al ratio showing that the relative occupancy of the crystallographic T-sites changes with Si/Al ratio. The tetraethylammonium cation, used as an organic structure-directing agent in Beta synthesis, affects the average chemical shift of aluminium atoms in different T-sites. In H-Beta, octahedrally coordinated framework-associated aluminium atoms that could be quantitatively reverted into tetrahedral coordination were observed. The amount of this octahedral aluminium species decreases with increasing Si/Al ratio and it was absent for the two high-silica H-Beta samples with Si/Al = 110 and 215. Specific framework tetrahedral T-sites tend to convert to framework-associated octahedral sites during calcination It is suggested that two aluminium T-sites, which are adjacent or close to each other, obeying the Loewenstein's rule (i.e., no Al-O-Al linkage), are required for the hydrolysis of a Si-O-Al bond for the formation of octahedrally coordinated aluminium. The distribution of aluminium in zeolite Beta is a function of the Si/Al ratio and is non-uniformly distributed over the crystallographic T-sites.