Effect of Synthesis pH and H 2 O Molar Ratio on the Structure and Morphology of Aluminum Phosphate (AlPO-5) Molecular Sieves

AlPO-5 molecular sieves were prepared by the hydrothermal reaction of a gel mixture with the following compositions: Al 2 O 3 : P 2 O 5 : Et 3 N : H 2 O = 1:1:1.5: x , where x is between 100 and 750 H 2 O 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 H 2 O = 100 to a hexagonal pillar shape at H 2 O = 450. With pH control in the range of about 2.5-3.5, the hexagonal pillar crystals began to form at H 2 O = 100 and an island of hexagonal pillars with radiation form appeared at H 2 O = 300-450 due to the formation of a tridymite type of dense AlPO 4 phase. It appears that the formation rate of hexagonal pillar crystals to form a dense AlPO 4 phase is favorable under acidic conditions, and an amorphous AlPO-5 structure forms under basic conditions. Thus, the H 2 O concentration and pH value have a dramatic effect on the AlPO-5 structure.