O'Malley, AJ; (2016) Hydrocarbon Dynamics in Microporous Catalysts. Doctoral thesis , UCL (University College London).

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Abstract

The dynamics of hydrocarbons inside microporous zeolite catalysts are studied using neutron scattering methods and complementary molecular simulations, to investigate quantitatively a crucial component of industrial zeolite catalysis. The diffusion of longer n-alkanes in the siliceous analogue of ZSM-5, silicalite is modelled using state-of-the-art molecular dynamics (MD) simulations. The measured diffusivities show far improved agreement with quasielastic neutron scattering (QENS) experiments. Isobutane diffusion in silicalite is also modelled, giving good agreement with diffusion coefficients and jump diffusion parameters obtained by neutron spin-echo experiments. The simulations give interesting insights into preferred siting locations, contradicting previous studies of isobutane dynamics in the MFI structure due to the use of a more accurate framework model. Tandem QENS and MD studies of octane isomer diffusion in zeolite HY show a counterintuitive increase in diffusion with branching, due to alkane clustering in the faujasite supercage. The difference in intermolecular forces (dictated by molecular shape) slow the diffusion of n-octane significantly more than 2,5-dimethyhexane in the faujasite structure. The behaviour contrasts with that in the MFI structure where branching is known to hinder alkane diffusion. Methanol diffusion in commercial HY and H-ZSM-5 samples was studied using QENS, showing free methanol diffusion in HY, but not in H-ZSM-5 due to room temperature methoxylation as confirmed by inelastic neutron scattering (INS) spectroscopy and quantum mechanical calculations of vibrational spectra. QM/MM embedded cluster calculations were also performed to compare the acidity and methanol adsorption energy of HY, and at three locations in the H-ZSM-5 structure. The diffusion component of the recently patented SAPO-37 catalysed Beckmann rearrangement is also studied using QENS to measure cyclohexanone oxime mobility in zeolites HY and SAPO-37, highlighting diffusion differences correlatable to catalytic activity despite sharing the same faujasite structure. This thesis illustrates the power of complementary neutron scattering and computational studies of sorbate dynamics in zeolites, future work aims to incorporate these studies into the design of new microporous catalytic processes.

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