Influence of framework silicon to aluminium ratio on aluminium coordination and distribution in zeolite Beta investigated by ~27Al MAS and ~27A1 MO MAS NMR

摘要

~27A1 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 ~27A1 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 A1-O-A1 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.