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.