Publisher: University of Glasgow

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Publication date(s): 2021 (print) 2021/06/24 (online)

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Journal: Doctoral Thesis

Link: https://theses.gla.ac.uk/82282/

URL: https://theses.gla.ac.uk/82282/

The Methanol-to-Hydrocarbons reaction is a group of industrially relevant reactions which focus on the use of methanol as a feedstock to turn it into light olefins (Methanol-to-Olefins) and/or gasoline-range aromatics (Methanol-toGasoline) over an acidic zeolite catalyst. Understanding the shift between two product regimes is important in order to be able to shift between the two. There is a lack of understanding when it comes to the mechanism of the Methanol-toHydrocarbons reaction, with the most widely accepted mechanism being the ‘Hydrocarbon Pool’ mechanism. An autocatalytic cycle in which the product distribution depends on the composition of the formed hydrocarbons within the zeolite pores. In literature, a combination of IR, Raman, ssNMR and other spectroscopies have been used in order to try and identify the identity of the hydrocarbons at different parts of the reaction coordinate. Inelastic Neutron Scattering has only been recently used in order to study this, with this thesis focusing on expanding on those studies and identifying the hydrocarbons retained within the ZSM-5 catalyst during MTH conditions at different temperatures and time-on-stream. INS spectra showed that at a mature steady state, the retained hydrocarbons gave the same spectra irrespective of temperature. This, as well as comparing the INS spectra given from using both dimethyl ether and methyl acetate as reactants, has suggested that the spectra are the ‘vibrational fingerprint of the hydrocarbon pool’. At high temperatures of 400oC, and at more than 20 wt% coke, the INS spectra were dominated by coke, with the coke being assigned to ‘glassy-like’ carbon suggesting that the coke is formed from growing polycyclic aromatics. Studying the diffusion of methane through the coked samples has highlighted the challenges of using coked species to complete such experiments. The interaction between methanol and ZSM-5 at room temperature has been investigated as well. INS studies have shown that the OH functionality of the methanol has disappeared when the methanol was dosed in the zeolite at room temperature. This contradicted results obtained with other spectroscopic techniques. A variety of scenarios were explored in trying to rationalise the contradictory results. A comparison study using ssNMR, DRIFTS and INS has been completed, with it showing that methanol is present within the zeolite intact with the possibility of methoxy being present at an occupancy of less than 1% chance.

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