Time Evolution of an Aluminogermanate Zeolite Synthesis: Segregation of Two Closely Similar Phases with the Same Structure Type

Abstract

A synthesis of zeolite-like aluminogermanates using N,N,N-trimethyl-terc-butylammonium (TMTBA) and fluoride evolved from an initially crystallized zeolite with the AST Zeolite Framework Type and very little Al in it into an Al-free GeO2 phase with the same structure but a slightly smaller unit cell. In spite of both zeolites having the same framework type, same symmetry, a much similar unit cell size and almost the same framework composition, phase segregation occurred at intermediate crystallization times as the initial AST phase was gradually replaced by the new one. For reaction mixtures with a high Al content complete replacement of the large cell by the smaller cell phase was accomplished. The solids have been characterized by powder X-ray diffraction (XRD), Rietveld refinement from synchrotron diffraction data, multinuclear Nuclear Magnetic Resonance (NMR) of both intact and disolved solids and infrarred spectroscopy. The study reveals TMTBA can degrade under the crystallization conditions to initially yield triethylamine (tMA) while transmethylation between TMTBA and tMA produces tetramethylammonium (TMA), as probed by liquid NMR. The TMA cation can also structure-direct to AST zeolites but with a smaller unit cell volume and no Al in the framework. Phase segregation occurs despite the lack of structural mismatch because both crystallization events are decoupled, occurring at different moments and from largely different solutions. When both TMTBA and TMA cations are intentionally added at the start, no phase segregation but crystallization of a solid solution occurs instead.