Preparation of Aluminium Orthophosphates in Organic Solvents and Their Properties

Various kinds of crystalline aluminium orthophosphates have been prepared in the reaction of alumini um hydroxide with phosphoric acid under the various conditions of heating temperatures, reaction times, water contents, and sort of organic solvent. (1) Aluminium orthophosphate, AlPO₄ (berlinite, cristobalite-form, tridymite-form, and A-form), is formed by the dehydration of aluminium orthophosphate dihydrate, AlPO₄ 2 H₂O (metavariscite). The formation of these crystalline aluminium orthophosphates strongly depended on the sort of organic solvent, the water content and the amount of phosphoric acid. (2) Mixing of water and organic solvent shortened the r eaction time comparing with the conventional method to get metavariscite. Insoluble organic solvent having a boiling point around 100° C improved effectively the chemical reaction. The suitable condition for the preparation of metavariscite was as follows; H₃PO₄, Al(OH)₃, H₂O, and organic solvent were mixed with the molar ratio of 0.33: 0.30: 4.0: 1.5, and then they were heated with stirring under reflux for 15 h. (3) The optimum condition to prepare each species of AlPO₄ was as follows; For berlinite and -cristobalite-form the starting material with the same molar ratio as mentioned in (2) was treated in 1butanol and isobutyl methyl ketone, respectively. Berlinite and cristobalite-form were formed by the dehydration for 12 and 6 h after the refluxing for 12 and 20 h, respectively. Tridymite-form and A-form of AlPO₄ were obtained by the dehydration of metavariscite. The former was prepared by dehydrating with P₂O₅ (O'-form ) in 1-butanol for 48 h, the latter in 1-butanol for 6h. (4) Physicochemical properties (acidity and acid strength of the solid, s pecific surface area, average particle size, and adsorption of ammonia gas) showed that especially cristobalite-form and tridymite -form were superior on the surface activity to those formed by the conventional heat-treatment method.