Investigating the use of isotopic enrichment to study inorganic solids using solid-state NMR spectroscopy

Abstract

This thesis focuses on the use of solid-state nuclear magnetic resonance (NMR) spectroscopy and, specifically, the application of ¹⁷O and ²⁹Si isotopic enrichment, for the characterisation of inorganic solids. The materials of interest (metal-organic frameworks, phosphate-based frameworks, and zeolites) are all microporous, and their characterisation is complicated by the high level and different types of disorder present. Solid-state NMR spectroscopy is an ideal technique for studying disordered solids owing to its sensitivity to the atomic-scale environment and is particularly powerful when combined with density functional theory calculations, X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy. The work presented explored a range of isotopic enrichment methods (due to the low natural abundances of ¹⁷O and ²⁹Si), to facilitate experiments and enable measurements that aren’t possible at natural abundance, providing new information on local structure and insight into chemical reactivity.

Firstly, a series of mixed-metal (Al,Sc)-MIL-53 materials were synthesised and characterised. The use of post-synthetic ¹⁷O enrichment provides information on the framework cation distribution, although significant practical challenges (e.g., with the formation of impurities) were encountered. Secondly, a range of post-synthetic ¹⁷O-enrichment methods were investigated for phosphate-based frameworks. The methods tested were more successful for SAPO-34, than AlPO-34. The work conducted suggested the reactivity is more complex than what has been previously thought. Finally, the feasibility of in-situ experiments to understand the formation of novel zeolites was investigated using ²⁹Si-enriched starting materials and reagents. This allowed reactions to be followed in real time, and the effect of varying reaction conditions to be understood. The enrichment methods developed could be extended to other frameworks to aid the study of structure, disorder, and reactivity. As the desire to design new microporous materials continues, solid-state NMR spectroscopy and isotopic enrichment have the potential to be key techniques for understanding the complex structure-property relationships present.