Fluorine-free crystallization of triclinic AlPO4-34

Abstract

Triclinic AlPO₄-34 arises from HF with fluoride ions located in the double Al–F–Al bridges. Previously, it has only been obtained in the presence of fluorine. In the present paper, a new synthetic method has been explored for the preparation of a microporous crystalline framework of triclinic AlPO₄-34 avoiding the use of toxic HF acid. Fluoride-free triclinic AlPO₄-34 (named as AlPO₄-34FF) has been prepared by using a mixture of water and ethylene glycol as the mixing solvent. The effects of morpholine and ethylene glycol on the crystallization of AlPO₄-34FF have been investigated by combining it with the model for constructing aluminum phosphate frameworks. It has been found that the stability of double Al–OH–Al bridges during crystallization is key for the formation of AlPO₄-34FF. High morpholine concentration and a suitable ratio of ethylene glycol/water will regulate the extent of hydrolysis and condensation reaction to keep double Al–OH–Al bridges stable and play important roles in the crystallization. Meanwhile, the narrow crystallizing area for crystallizing AlPO₄-34FF in mixing solvent has been understood by successfully using the quantitative relationship of 3 ethylene glycol molecules bonding to 4 water molecules in the mixing solvent. On this basis, fluoride-free crystallization of triclinic AlPO₄-34 has been achieved in an aqueous system by limiting the water content in the gel to control the degree of hydrolysis and condensation of the reacting active species. Powder XRD patterns and single crystal refinement have verified the formation of this fluoride-free triclinic AlPO₄-34 phase. A vibration band from diffuse reflectance FT-IR spectroscopy at 3640 cm⁻¹ provides the information of a double Al–OH–Al bridge in triclinic AlPO₄-34. Thermal analysis has shown that a double Al–OH–Al bridge is weaker than a double Al–F–Al bridge and led to the fluorine-free triclinic AlPO₄-34 (space group P) to transform into AlPO₄-34 (space group R) at lower temperatures.