Chemical state of non-framework aluminum in USY zeolite and its influence on zeolite acidity

The type and the chemical state of non-framework aluminum and the related influence on zeolite acidity were investigated by means of ²⁷Al MAS NMR, N₂ adsorption-desorption, XRD Rietveld refinement. The results showed that non-framework aluminum corresponding to the chemical shift of 0 in ²⁷Al MAS NMR existed in an isolate state in the supercage of USY zeolite and the non-framework aluminum corresponding to the chemical shift of 30-50 may be bound to Y zeolite framework. The crystallographically identifiable non-framework Al³⁺ located at SI' and SII' sites of the sodalite cage, and the non-framework Al³⁺ at SI' site coordinates with three framework O3 atoms and one O atom at 111 ternary axis to form unstable four fold coordination structure, and the non-framework Al³⁺ at SII' site could not coordinate with framework O atom. There was not any crystallographically identifiable non-framework Al³⁺ in the supercage. Polymeric non-framework aluminum blocked up pore channels of Y zeolite, influencing microporous adsorption ability and covering Bronsted acid sites. By removing polymeric non-framework aluminum, pore channels of Y zeolite became free from blockage and Bronsted acid sites were recovered.