Charge density mismatch synthesis of zeolite beta in the presence of tetraethylammonium, tetramethylammonium, and sodium ions: Influence of tetraethylammonium decomposition

Abstract

The effects of a temperature change on the phase selectivity of crystallization in a tetraethylammonium (TEA+)-tetramethylammonium (TMA+)-Na+ mixed structure-directing agent (SDA) system, which had been previously used in the synthesis of UZM-9 and UZM-5 at 100 and 150 oC, respectively, have been further investigated in order to shed more light on the course of charge density mismatch (CDM) syntheses of aluminosilicate zeolites. Nanocrystalline zeolite beta with a Si/Al ratio of 8.1, essentially the same ratio as that (8.0) of its initial synthesis mixture, was the phase crystallized at 155 oC, whereas heating at higher temperatures ranging from 160 to 200 oC for up to 21 days gave no crystalline products. The course of chemistry at these temperatures is highly influenced by the increasing instability of the TEA+ SDA. The overall results of this study demonstrate that the TMA+, as well as the TEA+ and Na+ ions, plays an important role in the nucleation step of zeolite beta prepared here, providing another example of SDA cooperation in a CDM zeolite synthesis. An idealized charge density disproportionation model is employed to track the evolving chemistry of the aluminosilicate solids and solutions. The discrepancy between the observed solution chemistry and that expected by the model suggests the presence of undesirable reactions in the system, namely TEA+ decomposition.