Abstract
Zeolites are a unique type of heterogenous catalysts that have soared in importance since their initial use within industry in the 1950’s, largely due to their ability to be shape selective. Octahedral manganese oxides are another type of heterogenous catalyst that have emerged in significance and have been identified recently to be highly active and selective in the hydrogenation of α, β unsaturated aldehydes. In this thesis, using density functional theory (DFT) and Molecular Dynamics (MD), we conducted numerous studies aiming to understand heterogenous catalytic reactions over zeolites and two different octahedral manganese oxides.To further understand shape-selectivity in zeolites, a complete free energy profile for different chain-sized alkynes in Cu-SSZ-13 was studied, using DFT and biased MD calculations. The results unequivocally show that the alkyne with size most commensurable to the hole to be thermodynamically favoured to adsorb. In addition, the elongation of the chain by just one methylene group from the optimum size has a major negative impact on the free energy within the zeolite. Finally, the effect of van der Waals (vdW) forces was quantified for the optimally sized alkyne. It was found that without the inclusion of vdW interactions, the molecule will not enter the hole from a thermodynamic perspective.
To identify the reaction mechanism for selective hydrogenations of α, β-unsaturated aldehydes on manganese oxides, a thorough investigation of the possible mechanisms was conducted on birnessite (OL) and cryptomelane (OMS-2). Using acetaldehyde as the substrate molecule, the mechanism on Birnessite, is proposed to proceed via a solvent assisted Eley-Rideal mechanism with molecular hydrogen in the liquid phase. Birnessite has no available adsorption site without the introduction of an oxygen vacancy and a traditional Langmuir-Hinshelwood style mechanism cannot occur because substrate adsorption within the vacancy is too large, which effectively poisons the catalyst. Furthermore, the solvent molecule is more thermodynamically favoured to fill the vacancy and also more likely from a concentration perspective.
The selectivity of the reaction is also found to be influenced by the choice the solvent used when using cinnamaldehyde as the chosen α, β-unsaturated aldehyde. The formation of the unsaturated alcohol, cinnamyl alcohol, is favoured in pure methanol, whilst formation of the saturated aldehyde, hydrocinnamaldehyde, is favoured in water / methanol (10:1). The change in selectivity is reported to be governed by the hydrogen distance from the surface and the ability of the hydrogen molecule to diffuse within solution.
On cryptomelane, the catalytic surface has an open adsorption spot for the acetaldehyde substrate. The adsorption on this site is not too strong and thus the hydrogenation reaction can occur via a traditional Langmuir-Hinshelwood mechanism.
It is proposed that the reaction is most likely to occur via a Horiuti-Polanyi Mechanism where the hydrogen molecule dissociates on the surface. Finally, for cinnamaldehyde, a simple investigation found that the adsorption geometry of the molecule can have a profound impact on the reaction barriers for CC or CO hydrogenation. The reaction favours hydrogenation at the CC bond when the molecule is adsorbed horizontally across the surface, whilst the reaction barrier is smaller for the CO bond when adsorbed vertically and perpendicular to the surface.
| Date of Award | Jul 2020 |
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| Original language | English |
| Awarding Institution |
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| Sponsors | Engineering & Physical Sciences Research Council |
| Supervisor | Peijun Hu (Supervisor) & Meilan Huang (Supervisor) |