Solid-state NMR spectroscopy of structure and chemical reactivity in zeolites

Abstract

Zeolites are microporous aluminosilicates comprised of corner-sharing TO₄ tetrahedra, which are assembled into rings and cages to create three- dimensional frameworks. They have found widespread industrial applications, including in heterogeneous catalysis, molecular sieving, ion- exchange and biomedicine. The incorporation of trivalent aluminium into tetrahedral sites results in an anionic framework, which is typically charge balanced by the presence of alkali metal cations, small organic molecules or by the protonation of apical oxygen sites to give a bridging hydroxyl. These bridging hydroxyl species have catalytically-active Brønsted acidity and zeolites have potential applications as solid-acid catalysts in the petrochemical and fine chemical industries. Mordenite and ferrierite have found application within the petrochemical industry as a cost-effective, greener alternatives to the Monsanto and BP Cativia™ processes for the carbonylation of dimethyl ether and methanol. In this thesis, the structure and chemical reactivity of these two industrially-important zeolites is explored by combining solid-state NMR spectroscopy, first-principles calculations and heterogenous catalysis.

Aluminosilicate zeolites are hydrophilic, and the presence of water in their pores has a significant impact on the structure of the zeolite. In this thesis, it has been shown that dehydration of H- and Na-zeolites leads to a large distortion of the symmetry of the coordination environment of the tetrahedral Al species, which arises from the interaction between charge- balancing H⁺/Na⁺ cations and the zeolite framework. This distortion is removed upon hydration as the cations are solvated. Furthermore, in H- zeolites, the coordination number of some Al species can increase from four (AlIV) to six (AlVI) upon hydration. Although this is a well-known phenomenon, the detailed structure of the AlVI species is unknown but often thought to be an extra-framework species which sits in the pores of the zeolite. It is demonstrated here that their formation occurs with little change to the Si/Al ratio, and can be fully reversed by dehydration, ion-exchange or ammonia adsorption, indicating that the Al remains connected (at least, in part) to the zeolite framework. The presence and quantity of the framework-AlVI species is been shown to be important in the catalytic turnover and selectivity for the isomerisation of n-butane and important for determining the rate of deactivation and coking for the induction period of the DME carbonylation reaction.

Characterisation of the oxygen sites in zeolites is also crucial as hydroxyl species (i.e., Brønsted acid protons and defect silanols) are often responsible for the catalytic activity in zeolites. However, ¹⁷O NMR spectroscopy is rarely used in the characterisation of zeolites, owing to its extremely low natural abundance (0.037 %), moderate gyromagnetic ratio and quadrupolar spin (I = 5/2). Traditionally, ¹⁷O enrichment of zeolites is carried out using high-temperature post-synthetic exchange with ¹⁷O₂(g), which could lead to degradation of the zeolite framework. In this thesis, a novel method for room temperature enrichment of framework oxygen species via lower-temperature hydration with H₂¹⁷O(l) is explored. The facile ¹⁷O enrichment on rapid timescales reveals an unexpected lability of oxygen linkages in zeolite frameworks. It has been shown that this phenomenon occurs for multiple topologies and in different forms of the zeolite, without degradation to the framework. This challenges the conception that the bulk framework acts as a rigid scaffold and suggests that zeolite frameworks are dynamic even at low temperature under aqueous conditions, which in turn raises wider questions about the role of the framework during aqueous-based processes.