Synthesis of AlPO₄-34 and its large single crystal by partially substituting framework fluorine with oxygen species

Abstract

AlPO₄-34 is a fluorine containing aluminophosphate with two fluorine atoms located in the bridge of two octahedrally coordinated Al atoms of a 4-ring. It has a unit cell composition (C₄H₁₀NO⁺)₂ (AlPO₄)₆F₂ ⁻ with the stoichiometric value of Al:P:F being 3:3:1 (F/Al = 0.33) and to be named as AlPO₄-34F. The presence of fluorine in the reacting gels favors the formation of AlPO₄-34 structure, but hinders the growth of its large crystals. In present work, a seesaw effect between organic template and hydrofluoric acid on the crystallization of AlPO₄-34 has been found and used to synthesize AlPO₄-34 with F/Al in the framework lower than 0.33. Large single crystal of AlPO₄-34 has been prepared from a gel having the molar ratio of HF/Al₂O₃ = 0.125. Single crystals refinement has shown that the unit cell formula of the AlPO₄-34 is (C₄H₁₀NO⁺)₂(AlPO₄)₆F ⁻_1.18 (OH) ⁻_0.82 (H₂O)_0.5. The molar ratio of Al:P:F:O_b is 3:3:0.59:0.41 with 40% fluoride ions being substituted by oxygen species (O_b: oxygen atoms in the bridges of Al–O–Al). Ion chromatography analysis has confirmed the partial substitution of fluorine by oxygen. It was named as AlPO₄-34P. Thermal analyses have shown that oxygen species in the bridge position of two octahedrally coordinated Al atoms are more instable than fluoride ions. Under heating, the AlPO₄-34P will transform to chabazite structure at a temperature lower than AlPO₄-34F.