Aluminum Phosphates Derived from Alumina and Alumina‐Sol–Gel Systems

Aluminum phosphate products formed by the reactions of alumina and alumina‐gel systems with acidic phosphates were analyzed. Drying of alumina‐gel to form microcrystalline boehmite and conversion to γ‐alumina by thermal treatment was indicated by the appearance of octahedral, pentacoordinate, or tetrahedral sites, which were established using 27 Al magic‐angle‐spinning solid‐state nuclear magnetic resonance spectroscopy. Crystalline aluminum phosphate products and amorphous material were identified using this technique. α‐alumina and heat‐treated alumina‐gel that were reacted with phosphate in an Al:P ratio of 1:1 yielded dramatically different aluminum orthophosphate:aluminum metaphosphate product ratios of 8.2:1 and 1:1.1, respectively. When alumina‐gel was heat‐treated with phosphate, an abundance of aluminum orthophosphate, aluminum metaphosphate, and hydrated aluminum phosphate products were affected by varying conditions of temperature and time of heat treatment and by the amount of phosphate present. An α‐alumina/alumina‐gel composite sol–gel phase that was reacted with phosphoric acid (H 3 PO 4 ) in a Al:P ratio of 1:1 exhibited an increased quantity of aluminum metaphosphate products compared with an α‐alumina:H 3 PO 4 ratio of 1:1 and a higher percentage of reaction (79%) compared with the reactions of an α‐alumina:H 3 PO 4 ratio of 1:1 or an alumina‐gel:H 3 PO 4 ratio of 1:1. The morphologies of aluminum triphosphate hydrate and aluminum metaphosphate product phases were observed using scanning electron microscopy.