Mixed alumina–chromia pillared layered α-zirconium phosphate

Abstract

Mixed aluminium(III)–chromium(III) oligomers have been intercalated into colloidal α-zirconium phosphate by in situ, polymerisation from buffered mixed Al–Cr solutions at the reflux temperature. Single mixed species are formed in a wide range of added Al : Cr ratios, between 10 : 90 and 70 : 30. All materials have high basal spacings between 25.7 and 42.1 Å at room temperature. ²⁷Al MAS NMR studies suggest that the mixed Al–Cr oligomeric species intercalated at high Al : Cr ratios are higher polymers than the tridecamer [AlO₄Al₁₂(OH)₂₄(H₂O)₁₂]⁷⁺. On calcination at 400 °C most intercalation compounds maintained free heights between 13.5 and 24.8 Å, suggesting that true pillared materials are formed that are thermally stable up to 700–800 °C, after which they began to decompose into Cr₂O₃ and collapsed structures of aluminium zirconium phosphate. The surface of zirconium phosphate shows strong affinity for aluminium at high Al : Cr ratios as indicated by XPS analysis. The mixed alumina–chromia pillared α-zirconium phosphate materials presented high BET surface areas (158–634 m² g^–1) and high micropore volumes (0.072–0.322 cm^–3 g^–1) up to an Al : Cr ratio of 70 : 30. Above this ratio, the materials are barely porous with low surface areas.