Metal oxide catalysts for the valorisation of bio-renewable feedstocks

Rabiu, Hamza 2023. Metal oxide catalysts for the valorisation of bio-renewable feedstocks. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The fact that biofuels are currently not economically competitive with conventional fossil fuels is a fundamental constraint on their use. The improvement of novel chemical processes to convert biproducts from these reactions into high value-added chemicals could be one technique to lessen the financial shortfall between these two industries. First and second generations biofuels produce glycerol and 5-hydroxymethyl furfural as byproducts. This thesis investigates the possibility of producing high-value chemicals through oxidation of these bio-derived compounds with supported gold-platinum catalysts. The first part of this thesis involves the oxidation of 5-hydromethyl furfural (HMF) with the aim of selectively producing 2,5-furandicarboxylic acid (FDCA). Initial experiments were conducted with different lanthanum based perovskite supports under basic conditions, and it was found that AuPt/LaMnO3 gave the highest yield of FDCA. The AuPt/LaMnO3 catalyst was then used for an investigation into the reaction conditions and it was found that the conditions had a significant effect on the product distribution. A progressive elimination study was designed to determine the optimal conditions for the formation of FDCA. The conditions mapped were temperature, O2 pressure and base to substrate ratio. Temperature was found to increase selectivity to FDCA as it was increased. Increasing the O2 pressure showed an increase in FDCA selectivity up to 3 bar, at which point 100% FDCA selectivity was achieved. By increasing the base to substrate ratio, FDCA selectivity increased, however, the carbon balance decreased due to the formation of byproducts (humins). The best compromise was found to be a base to substrate ratio of 1:1 which gave a reasonable carbon balance of around 94% with 100% selectivity to FDCA. This provided an insight that would later be used to help confirm the mechanism. The parameter mapping led to a series of conditions that produced a yield of 100 % FDCA. These conditions enabled the design of experiments under base free conditions to extend the catalyst modification study. The second part of the thesis described research into the oxidation of glycerol which 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 reaction mechanism. When observing the two challenges, one begets the other. By identifying the conditions that 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. The gold platinum catalysts were also investigated for the production of lactic acid through the exchange of the B site cation on the perovskite supports. It was found that the LaMnO3 support heavily favoured the oxidation pathway under similar conditions to that for the initial catalysts screening in HMF oxidation to FDCA. This was found to correlate with the oxygen adsorption capacity on the clean perovskite lattice, 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 same conditions. The selective control of reaction products could ultimately lead to more economically viable biofuel production.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Funders:	Petroleum Technology Development Funds (PTDF)
Date of First Compliant Deposit:	23 October 2023
Last Modified:	24 Oct 2023 09:26
URI:	https://orca.cardiff.ac.uk/id/eprint/163413

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