Tetraalkylammonium hydroxide-assisted ionothermal synthesis and characterization of LTA-type aluminophosphate zeotypes with high structural stability after detemplation and hydration

Abstract

LTA-type aluminophosphate zeotype (AlPO-LTA) was ionothermally prepared with the assistance of several tetraalkylammonium hydroxides (TAAOH) containing alkyl chains of different lengths. The shape of the TAA⁺ cations had no determinant effect on the product structure selectivity, but had some influence on the crystallization kinetics, resulting in the formation of cubic and rhombohedral AlPO-LTA with different crystallinities. Combining the results of ¹H → ¹³C CP/MAS NMR and TG–DTA characterizations, it was demonstrated that tetramethylammonium (TMA⁺) was occluded in the LTA structure acting as co-template, whereas the other three TAA⁺ with longer alkyl chains were not occluded. The structure of AlPO-LTA after detemplation and hydration in moisture was carefully studied using XRD and ²⁷Al MAS NMR characterizations, manifesting its high structural stability with reversible water-adsorption capability, irrespective of TAAOH used in the synthesis.