Effect of Synthesis pH and H₂O Molar Ratio on the Structure and Morphology of Aluminum Phosphate (AlPO-5) Molecular Sieves

Abstract

AlPO-5 molecular sieves were prepared by the hydrothermal reaction of a gel mixture with the following compositions: Al₂O₃ : P₂O₅ : Et₃N : H₂O = 1:1:1.5:x, where x is between 100 and 750 H₂O molar ratio. The structure and morphology of the AlPO-5 molecular sieves depend on the gel mixture's composition, hydrothermal temperature, hydrothermal reaction time, and pH. Without pH control, the AlPO-5 structure changed from a spherical shape at H₂O = 100 to a hexagonal pillar shape at H₂O = 450. With pH control in the range of about 2.5-3.5, the hexagonal pillar crystals began to form at H₂O = 100 and an island of hexagonal pillars with radiation form appeared at H₂O = 300-450 due to the formation of a tridymite type of dense AlPO₄ phase. It appears that the formation rate of hexagonal pillar crystals to form a dense AlPO₄ phase is favorable under acidic conditions, and an amorphous AlPO-5 structure forms under basic conditions. Thus, the H₂O concentration and pH value have a dramatic effect on the AlPO-5 structure.