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Study on the surface adsorbates of an enantioselective, heterogeneously catalysed, hydrogenation reaction

Taylor, Robert Jon 2010. Study on the surface adsorbates of an enantioselective, heterogeneously catalysed, hydrogenation reaction. PhD Thesis, Cardiff University.

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Surface Enhanced Raman Spectroscopy (SERS), Cyclic Voltammetry (CV) and Density Functional Theory (DFT) have been used to investigate the Orito reaction - a heterogeneously catalysed enantioselective hydrogenation reaction. Using a spectro-electrochemical flowcell, designed and built in-house, the boundaries for in situ measurements have been pushed closer to the real Orito reaction conditions than ever before. By increasing the surface coverage of adsorbed hydrogen, it was found that the reactants methyl pyruvate (MP), ethyl pyruvate (EP) and ethyl benzoylformate (EBF) all formed a half-hydrogenated state (HHS) on a platinum surface under both gas phase and electrochemical conditions. Through controlled solution concentration experiments on MP and EP under electrochemical conditions, the surface coverage of hydrogen was found to decrease as a function of higher pyruvate concentrations. By co-adsorbing MP or EP with cinchonidine (CD) - a cinchona alkaloid frequently used to chirally modify the catalyst surface in the Orito reaction, the coverage of adsorbed hydrogen was found to remain high (relative to unmodified surfaces) even at high pyruvate concentrations and the strength of the M-H bond was found to be slightly weaker (as deduced from a red shift in the frequency of the M-H stretching vibration). It was found that the modifier competes with the HHS for adsorption sites and creates a chiral pocket into which hydrogen can also adsorb and interact with the modifier causing the M-H bond to weaken via hydrogen bonding with the tertiary nitrogen atom of the quinuclidine substituent of CD. It is proposed here that the rate enhancement commonly observed for modified catalysts, is due to an increase in the coverage of surface hydrogen and an activation of surface hydrogen towards reaction with the pyruvate reactant. Adsorption studies of EP on well defined platinum surfaces using CV revealed a trend for adsorption and decarbonylation decreasing in the order {100}>{110}>{111}. The same trend for the equivalent palladium surfaces was also found, but with no evidence of decarbonylation.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
ISBN: 9781303190995
Date of First Compliant Deposit: 30 March 2016
Last Modified: 09 Oct 2014 13:13

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