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Chemical physics of zeolites

State of the art simulation and quantum mechanical techniques were used to model:
the structural properties of zeolite crystals;
the sorption and diffusion of molecules within the microporous structures;
the mechanisms of key reactions.

Methodologies involved in simulating the structures of microporous solids have been refined and applied to a variety of complex systems. Work concentrated on:
modelling of Chabazite (the framework structure of this system has been successfully described by current interatomic potentials with minimization techniques but more difficulty was encountered in modelling the distribution of extraframework cations);
complex aluminophosphates systems (simulations have been used to assist the determination of previously unknown crystal structures);
complex heteroatom containing zeolites including catalytic titanium silicalite for which detailed predictions have been made regarding the location of the titanium in framework sites.

Molecular dynamic techniques were applied to the study of hydrocarbon diffusion in zeolites. Intracrystalline diffusion of linear hydrocarbon molecules in the zeolite silicalite and in the zeolite Chabazite were studied. Through modelling the trend of the diffusion with increasing loading was clearly established, and activation energies in good agreement with experimental values were obtained. Also, it was possible to extract a number of conclusions on the role played by the framework dynamics in the diffusion process.

An investigation of the computational and methodological aspects of calculating proton affinities and aluminium substitution energies has been performed for 1 site in silicalite. A cluster embedded in a point charge array yielded similar energetics to conventional hydrogen terminated models, provided the hydrogen positions are chosen correctly, while basis set quality was found to be more critical.
Adsorption of ammonia, water, methanol, dimethyl ether and the allyl cation at a Broensted acid site have been simulation. The nature of the binding geometries can be rationalized in terms of the conformation of the neighbouring T sites around the aluminium ion.

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