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Glycerol oxidation: An investigation into the control of product distribution and mechanistic pathways using novel supports

Evans, Christopher Dean 2017. Glycerol oxidation: An investigation into the control of product distribution and mechanistic pathways using novel supports. PhD Thesis, Cardiff University.
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Abstract

There is a major limitation preventing the use of bio-fuels as they are currently not economically viable when compared to fossil fuels. To make bio-fuels more economically attractive, new chemical processes for converting waste by-products into a valuable commodity are necessary. One of the main by-products from the production of bio-fuels is glycerol. This thesis investigates the oxidation of glycerol with the aim of selectively producing lactic acid. Glycerol oxidation is a relatively complex reaction with two potential pathways leading to a variety of products. The oxidation pathway leads to the C3 products glyceric acid and tartronic acid, with further oxidation leading to C2 and C1 products. The dehydration pathway leads to lactic acid, through a currently unknown mechanism. The reaction scheme sets out two challenges to solve; what conditions are optimal for a high yield of lactic acid and what is the mechanism. When observing the two challenges, one begets the other. By characterising what conditions are optimal for lactic acid production, it would then be plausible that an experiment could be designed to confirm the mechanism of lactic acid formation. Using a standard 1 wt% AuPt/TiO2 catalyst, an investigation into the reaction conditions was conducted and it was found that the conditions had a significant effect on the product distribution. A progressive elimination was designed so as to progressively find the optimal conditions for the formation of lactic acid. The conditions mapped were; temperature, O2 pressure, base substrate ratio, metal substrate ratio and stabilising agent used to produce the catalyst. Temperature was found to increase selectivity to lactic acid as it was increased. Increasing the O2 pressure showed an increase in lactic acid selectivity up to 3 bar, at which point there was an increase in C-C scission products. By increasing the base ratio, lactic acid selectivity increased dramatically, providing an insight that would later be used to help confirm the mechanism. Higher metal substrate ratio was found to increase the C-C scission products formed. Four types of stabilising agent were used to observe the effect of stability and activity, with PVA providing a balance between stability and selectivity. The parameter map led to a series of conditions that produced a yield of 80 % lactic acid. Using the conditions found from the parameter mapping study, an experiment was designed to attempt to confirm the mechanism of formation for lactic acid. Two current theories that lactic acid formation proceeds either via a benzylic rearrangement or a Cannizzaro reaction, provided an insight as to how this would be achieved, as the position of the carbon environments in lactic acid would change dependent on the mechanism followed. Through a 13C labelling study, NMR spectroscopy was used to show that there was no net movement of the carbon environments during the mechanism, discounting the benzylic rearrangement. Through a logical discussion, the Cannizzaro reaction was also discounted as technically this is an intermolecular hydride shift, with the mechanism being confirmed to be an intramolecular hydride shift. Supercritically prepared perovskite supports were used to see if the lactic acid yield could be increased further through the change of the B site cation. Contrary to initial belief, LaMnO3 heavily favoured the oxidation pathway at the same conditions the standard AuPt/TiO2 produced a high yield to lactic acid. This was found to correlate with the oxygen adsorption to a clean lattice structure, as perovskites with high oxygen adsorption favoured the oxidation pathway whilst low oxygen adsorption favoured the dehydration pathway. LaCrO3 was found to produce a yield of up to 86 % lactic acid under the optimal conditions outlined in the parameter mapping study. The selective control of reaction products could ultimately lead to more economically viable bio-fuel production.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 8 March 2018
Last Modified: 19 Mar 2019 02:30
URI: http://orca-mwe.cf.ac.uk/id/eprint/109739

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