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.