Probing the structure and reactivity of zeolites in their interactions with water

Abstract

The work presented in this thesis has been performed to increase the understanding of the interactions of zeolites with water, under a range of reaction conditions. The systems studied have been selected on the basis of their known hydrolytic instability or as model systems for which new avenues for hydrolytic instability can more easily be followed. The focus of research detailed in this thesis is split into three main sections. The mechanism of the ADOR (Assembly, Disassembly, Organisation, Reassembly) process for the synthesis of new zeolites from germanosilicate UTL is probed to deepen the understanding of the intermediate species formed and how the local structure and long-range order of the products are affected by reaction variables. Subsequently, the CHA framework has been selected as a model system for the study of a newly-identified bond lability exhibited by some non-‘ADORable’ aluminosilicate zeolite materials. To further aid characterisation and understanding of this lability, a series of inter-zeolite transformations from the FAU to CHA framework have been adapted to allow ¹⁷O enrichment and ¹⁷O NMR spectroscopic investigation, to characterise and identify key intermediate and framework species. The complexity and high structural disorder of the materials studied in this work mean that a multivariate approach to characterisation and analysis is required. Primarily, solid-state NMR spectroscopy and powder X-ray diffraction have been used to characterise the materials, elucidating differences between their local structure and long-range order. In addition, energy-dispersive X-ray spectroscopy, scanning electron microscopy, thermogravimetric analysis and density functional theory calculations have also been used to further aid characterisation and understanding of these complex systems. Results presented demonstrate that both zeolites and water interact strongly with one another when combined. Zeolites and water are found to be reactive even under mild conditions, e.g. room temperatures, where ¹⁷O NMR spectroscopy has shown the rapid exchange of H₂¹⁷O oxygen into the frameworks of UTL, CHA and FAU zeolites, by resolving signals attributed to framework zeolite linkages. Whilst favourable under ambient conditions, these bond lability processes are also found to be selective, with observed enrichment found to differ between frameworks with different topologies, com- positions and extraframework cations. In some cases, observed exchange of framework oxygen species has been attributed in part to the defective nature of zeolite frameworks. A further mechanistic explanation for the enrichment of zeolites for which the structure is not thought to be defective has also been predicted by collaborators using molecular dynamics. Whilst this mechanism may explain the facile bond lability seen for CHA frameworks, its widespread applicability to other zeolite frameworks that show rapid exchange of framework oxygen with that in water is not known. Water is also found to interact strongly with zeolite frameworks undergoing structural transformations, such as during the ADOR process or during post-synthetic transformations. Here, under the elevated temperatures of reaction conditions, ¹⁷O supplied from H₂¹⁷O reagent has been shown to be incorporated into the structure of zeolite frameworks, especially in regions considered non-reactive within the reaction. These results demonstrate that both zeolites and water interact and react when combined and provide further evidence to the contrary that zeolites are perceived as static, inert scaffolds. The findings have potential implications for the commercial uses of zeolites, where a balance between framework stability and flexibility is important.