The development of a chemo-enzymatic one-pot cascade towards cyclohexane oxidation and biomass degradation via in-situ generated H2O2

Brehm, Joseph 2023. The development of a chemo-enzymatic one-pot cascade towards cyclohexane oxidation and biomass degradation via in-situ generated H2O2. PhD Thesis, Cardiff University. Item availability restricted.

	PDF - Accepted Post-Print Version Restricted to Repository staff only until 1 May 2025 due to copyright restrictions. Download (8MB)
	PDF (Cardiff University Electronic Theses and Dissertations Form - Memorandum of Understanding) - Supplemental Material Restricted to Repository staff only Download (125kB)

Abstract

The work presented in this thesis is focused on the development of tandem chemocatalytic/enzymatic cascades for chemical valorisation and biomass degradation, utilising insitu generated H2O2. Primarily, this has focused on (1) the utilisation of the unspecific peroxygenase (UPO) PaDa-1 for the selective oxidation of C-H bonds, focusing on the oxidative upgrading of cyclohexane to KA oil (cyclohexanol and cyclohexanone), and (2) the degradation of biomass, primarily β-chitin, through combination of chemo-catalysts with the lytic polysaccharide monooxygenase enzyme (LPMO), CBP-21. The direct synthesis of H2O2 from molecular H2 and O2 over chemocatalysts is an environmentally friendly, non-toxic alternative to the current industrial route to production dominated by the anthraquinone oxidation process. As such, the in-situ synthesis of H2O2 using a supported metal catalyst can find application in enzymatic reactions that use H2O2 as a substrate, replacing alternative approaches to H2O2 supply, including multi-enzymatic cascades or drip-feeding the commercially produced oxidant. In all cases, the supply of H2O2 in an enzymatic system must be carefully controlled to prevent enzyme deactivation by oxidative damage, limiting overall process efficacy. The initial part of this work focuses on the synthesis and characterisation of a series of stable gold-palladium supported catalysts that can produce H2O2 at controlled rates for future use in chemo-enzymatic experiments. The effect of Au: Pd ratio and metal loading on the direct synthesis of H2O2 and its subsequent degradation from molecular H2 and O2 was investigated under reaction conditions optimised for H2O2 synthesis (i.e. under sub-ambient temperatures, elevated pressures and using a water-methanol solvent system). Notably, catalytic performance was found to be related directly to the metal loading of the catalyst, identifying the potential for control over H2O2 synthesis rates and optimisation when used in conjunction with enzymatic components of the cascade system. However, this work also identified the concerns associated with catalyst stability; while no metal leaching was detected, H2O2 synthesis rates decreased considerably upon reuse (up to 60 % loss), with this associated with the loss of surface chloride species upon initial use. These observations highlight the need to consider the presence of residual halide species, which are associated with metal precursors, and may direct future attention to alternative routes of catalyst synthesis. ii Building on these initial studies, the second part of this work focuses on utilising the supported gold-palladium catalysts in the valorisation of cyclohexane through chemocatalytic/enzymatic cascades in conjunction with the UPO PaDa-1. It was observed that the chemo-catalyst/PaDa-1 cascade is limited by the H2O2 synthesis rate, and it is possible to modulate process efficiency through control of the chemo-catalyst formulation. Additionally, further catalyst design studies identified the efficacy of a range of Pd-based bimetallic catalysts, particularly focusing on catalyst formulations that have been previously identified to promote catalytic performance towards H2O2 synthesis under conditions considered unsuitable for the enzymatic component. Notably, Pd-based alloyed catalysts consisting of Pt, In, Fe, Zn, Co, Ni, and Cu were studied and benchmarked against well-studied AuPd catalyst formulation. Interestingly, a range of chemo-catalyst formulations (PdIn, PdCo and PdZn) were identified, which offered improved process efficiency when combined with PaDa-1. Finally, the scope of chemo-catalytic/enzymatic compatibility was broadened, focusing on the use of a AuPd chemo-catalyst with the lytic polysaccharide monooxygenase known as CBP-21 for the degradation of β-chitin, in this case exploring the potential to replace molecular oxygen as the enzymatic co-substrate with in-situ generated H2O2. It was found that the rate of β-chitin degradation can be significantly increased by the in-situ approach. Indeed, the degree of β-chitin degradation via a purely aerobic approach required a minimum of 30 h to achieve that observed over the in-situ H2O2 route over a timeframe of 4 h. However, the activity of the in-situ H2O2 approach was found to not be able to be sustained, with a rapid plateau in performance observed at approximately 5 % total saccharification. Further study identified three factors which may contribute to the activity plateau: (1) the availability of the reducing agent (ascorbic acid), with this undergoing H2O2-mediated oxidation over the course of the reaction, leading to the deactivation of the active oxidation state of the enzymatic metal centre; (2) the potential of product inhibition effects and (3) direct oxidative damage to CBP-21 caused by the build-up of H2O2.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Date of First Compliant Deposit:	1 May 2024
Last Modified:	01 May 2024 14:47
URI:	https://orca.cardiff.ac.uk/id/eprint/168558

Actions (repository staff only)

Edit Item