Effect of phosphate ions on the surface chemistry and microstructure of amorphous alumina

Abstract

High-surface-area amorphous Al₂O₃–AlPO₄ samples (P/Al ratio: 0–0.17) have been prepared by reductive decomposition of aluminium nitrate in the presence of phosphoric acid. The materials obtained are amorphous up to 1100–1200 K and present “zeolitic-type” monomodal microporosity (r= 10–15 Å). They are weakly acidic. The Lewis acidity changes from being weak (associated with octahedrally coordinated Al³⁺ ions) to being strong by increasing P content due to the formation of tetrahedral Al³⁺ ions induced by phosphate species. Brønsted sites are also present; these are associated with phosphate species. The catalytic activity in methanol dehydration increases with P content up to P/Al = 10/90. P inhibits both the crystallization of the γ-alumina phase and the γ- to α-alumina phase transition.