Preparation of large and well-shaped LTA-type AlPO4 crystals by using crown ether Kryptofix 222 as structure directing agent

In a detailed study, the influence of the most relevant synthesis parameters on the crystallization of LTA-type AlPO4 using Kryptofix 222 (K222) as structure directing agent (SDA) was investigated with the aim to develop a reliable and reproducible strategy for the preparation of large and well-shaped crystals. Only with sufficient mixing time for the aluminophosphate gel before adding the SDAs, well-shaped cubic and large LTA-type AlPO4 crystals could be obtained. With increasing water content in the starting gel, the crystal size became larger and the cubic crystals displayed less truncated. Hydrofluoric acid was found to be a more efficient mineralizer to form large and well-shaped cubic crystals than ammonium fluoride. When Al(OH)3 was used instead of our standard aluminium source Al2O3, the crystal size was not influenced. When Al[OCH(CH3)2]3 was the aluminium source, the size of the LTA-type AlPO4 crystals decreased much, probably since a larger number of nuclei was formed upon hydrolysis of Al[OCH(CH3)2]3. When Al2(SO4)3 was used as aluminium source, no LTA-type AlPO4 crystals could be obtained. With increasing substitution of aluminium by gallium, the crystals size increased and the crystal surface became rough. All diffraction patterns of mixed (Al + Ga)PO4 crystals were characteristic of the LTA framework. The same Ga/Al and (Al + Ga)/P ratio of the synthesis gel was also found in the (Al + Ga)PO4 crystals. Low temperature in combination with long synthesis time was found to be beneficial for the preparation of large and well-shaped AlPO4 crystals.