Elucidating structure-performance correlations in hierarchically porous aluminophosphates through catalytic transformations.

Abstract

The ability to redesign traditional microporous catalysts with an additional and adaptable mesoporous network is an exciting area of catalyst synthesis. The size of the interconnected pores in microporous zeotype catalysts can significantly restrict the diffusion and mass transport of molecules to the active sites. This can lead to slower reaction rates, shorter catalyst lifetimes, and restricts the scope of substrates as they are limited by the size of the micropores. However, by incorporating mesopores into microporous materials to create hierarchically porous catalysts the limitations of microporous frameworks can be overcome. The additional mesopores can aid molecular diffusion to and from the active site, preventing the accumulation of unwanted by-products and improving the catalytic lifetime. However, less effort has been made to understand the inherent structure and chemistry of these hierarchical materials. This thesis aims to expand the knowledge of these materials and use catalytic behaviour and spectroscopic insight to elucidate structure-property correlations in a range of novel hierarchical doped aluminophosphate systems.
One of the most versatile methods for synthesising hierarchical zeotype frameworks is a one-step soft templating approach using an amphiphilic surfactant molecule. Using an organosilane surfactant as a mesoporogen has been shown to not only template mesopores within zeotypes but provide additional acidic functionalities through surface silanol groups. Coupled with the formation of discrete solid-acid framework sites through isomorphous substitution by heteroatom dopants, a range of tailored Brønsted acid sites can be introduced into the aluminophosphate structure (HP MAlPO). Two acid-catalysed model reactions, n-butane isomerisation and ethanol dehydration have been utilised in conjunction with multi-technique characterisation to probe the physicochemical properties of the HP MAlPO catalysts and rationalise their behaviour. The effect of dopant choice has been explored within a HP AlPO-5 framework showing that modulation of Brønsted acid strength can be achieved through choice of isomorphous dopant. When compared to analogous microporous counterparts, the benefits of a hierarchical system with additional silanol functionalities were shown to depend on the catalytic reaction with HP MAlPO-5 catalysts; demonstrating superior activity for ethanol dehydration but not n-butane isomerisation. The nature of the substituted metal active site was also explored, and Ni incorporation in HP AlPO-5 was shown to also form extra-framework active sites which exhibit appreciable activity for n-butane hydrogenolysis. Cobalt doped HP AlPOs with contrasting microporous frameworks (AFI vs AEI) were also compared to understand how the interplay between micro- and mesopores and framework acidity changes with different topologies. It was found that different framework types modulated the strength of the dopant acid sites, and micropore topology was a key descriptor in reaction pathway. The role of mesoporosity appeared to vary depending on the framework type and catalytic application suggesting the interconnectivity and accessibility between micro- and mesopores changes with the underlying micropore topology. Small angle neutron scattering (SANS), positron annihilation lifetime spectroscopy (PALS), nuclear magnetic resonance (NMR), and ammonia temperature programmed desorption (NH3-TPD) were used to expand understanding of the morphology and chemistry of mesopores within silicon doped AlPO-5 and further rationalise the behaviour of these materials in acid-catalysed transformations.

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Identifiers

ORCID for Alice Elizabeth Oakley: orcid.org/0000-0002-5489-9424

ORCID for Robert Raja: orcid.org/0000-0002-4161-7053

ORCID for Mark Light: orcid.org/0000-0002-0585-0843

ORCID for Marina Carravetta: orcid.org/0000-0002-6296-2104

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