Investigating the structural stability and reactivity of zeolites utilising solid-state NMR

Abstract

The primary focus of this thesis, discussed in chapter 5, explores the ¹⁷O enrichment of zeolites utilising H₂¹⁷O (l). With a focus on exploring what factors affect the lability of zeolites under these aqueous conditions. To contrast this method with the better established ¹⁷O₂ gas enrichment method, explored in section 6. Furthermore, due to the complexity of interpreting the NMR results of zeolites, computational methods have been employed to better understand the results obtained, with these results discussed in section 7.

The results shown in this thesis contrast heavily with what had been previously believed about the stable frameworks of zeolites, with it being shown that zeolites are extremely labile in the presence of miniscule amounts of water at room temperature. This lability is witnessed in both CHA and MOR zeolite topologies with the framework showing to have a notable effect on the enrichment seen and the rate at which this exchange occurs.

Investigating further into high temperature ¹⁷O₂ gas enrichment yields notable changes to the framework, with evidence of dealumination being found in both the ²⁹Si and ²⁷Al spectra obtained. This dealumination is seen more readily in the proton form of zeolites in contrast to other charge-balancing cations. From this it’s clear that the proton form of a zeolite demonstrates this room temperature lability with water in exchange for its thermal stability, by contrast other ion-exchanged forms show high thermal stability but reduced lability in an aqueous environment.

These results have changed how we view zeolitic frameworks and their stability, as it becomes evident that despite their widespread use in industry, we still do not fully understand zeolites. These findings could have a significant impact on the future use of zeolites in industrial processes, opening up new potential applications.