Chemo-enzymatic cascades via in-situ generated h2o2 over supported aupd nanoparticles

Stenner, Alexander 2024. Chemo-enzymatic cascades via in-situ generated h2o2 over supported aupd nanoparticles. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The work presented in this thesis investigates and enables exceptional catalytic performance upon combining heterogeneous catalysts and enzymes under one-pot conditions. Unspecific peroxygenase (UPO) from Agrocybe aegerita and chloroperoxidase (CPO) from Caldariomyces fumago are naturally evolved heme enzymes which utilize H2O2 to perform biocatalytic oxidation reactions. UPO and CPO are ideal biocatalysts that rival conventional synthetic approaches, owing to their high H2O2 utilization efficiency, relatively high stability, and capacity to perform industrially relevant chemical transformations under mild operating conditions. However, the upscale of peroxy-enzymes is severely limited by their susceptibility towards oxidative inactivation, even at modest H2O2 concentrations. Therefore, a continuous low-concentration stream of H2O2 is required to maintain enzyme activity, with in-situ H2O2 generation a highly attractive approach to achieve this. Continuous addition of commercial H2O2 is a well-established strategy, however, this results in rapid dilution of valuable reaction products, contamination of the reaction media, large volumes of aqueous waste, and the formation of H2O2 ‘hot-spots’ in the reaction media. As such, the development of highly efficient, industrially scalable, and enzymatically compatible in-situ H2O2 generation represents a significant challenge in the context of upscaling peroxy-enzymatic processes. This study employs supported AuPd-based nanoparticles to generate in-situ H2O2 from H2 and O2 for peroxy-enzymatic utilization. This represents a highly atom efficient and scalable approach, while concomitantly offering high compatibility with current industrial reactors. However, H2O2 direct synthesis is a highly challenging reaction, owing to prevalence of competing side-reactions and the thermodynamic favourability of unselective H2O formation. As such, H2O2 direct synthesis requires high reaction pressure, sub-ambient temperature, and acidic reaction media to promote catalytic activity and selectivity. Therefore, a reaction conditions gap exists between the requirements to maintain chemo-catalytic activity and enzymatic stability. Indeed, reaction conditions which maintain peroxy-enzymatic stability (near neutral pH, ambient temperature, low-pressure) severely hamper in-situ H2O2 synthesis rates, which renders chemo-catalytic activity as rate-limiting, relative to peroxy-enzymatic utilization. Herein, this study has focused on enhancing in-situ H2O2 synthesis rates under conditions which maintain enzymatic stability, with the aim to design highly efficient and kinetically balanced chemo-enzymatic cascade systems. The first part of this study builds on previous works by investigating chemo-enzymatic cyclohexane oxidation with an UPO enzyme, with the produced cyclohexanol and cyclohexanone critical feedstocks for Nylon production. The aim of this study was to promote in-situ H2O2 synthesis rates, and hence chemo-enzymatic cascade efficacy, through rational catalyst design. Trimetallic Au-Pd-Pt catalysts were compared to the mono- and bi-metallic analogues, where dopant concentrations of Pt (0.02 wt.%) were found to offer enhanced H2O2 synthesis rates. This was observed under the optimized high-pressure conditions for H2O2 synthesis, as well as the low-pressure conditions required to maintain UPO activity. Upon utilization in the chemo-enzymatic cascade, the 0.49%Au-0.49%Pd-0.02%Pt/TiO2 formulation offered 1.3- and 1.5-fold enhanced cyclohexane oxidation rates, relative the mono- and bi-metallic counterparts, respectively. Despite this, the presence of Pt as both heterogeneous and homogeneous species, was found to contribute towards extensive UPO deactivation. As such, it was concluded that chemo-enzymatic cascade efficacy could be promoted via chemo-catalyst design, however, optimising a highly compatible reaction system is paramount. iii To expand the established system, a CPO enzyme was utilized for indole oxidation to 2-oxindole, which is a highly relevant reaction for pharmaceutical feedstock production. Endeavours to enhance chemo-catalytic H2O2 synthesis rates focused on the modification of reaction conditions, rather than chemo-catalyst design, with a series of 1 wt.% AuPd/TiO2 catalysts employed. The reaction modification process required careful consideration of the requirements to maintain CPO indole oxidation activity, which favours an acidic reaction environment and the incorporation of a radical scavenging organic cosolvent. As such, a new set of tailored reaction conditions were established for chemo-enzymatic CPO indole oxidation, which subsequently offered a 63 % enhancement in H2O2 synthesis rates and 5- fold reduction in metal leaching for the 0.5%Au-0.5%Pd/TiO2 formulation. The modified reaction conditions were subsequently employed for chemo-enzymatic CPO indole oxidation. The cascade efficacy was probed as a function of catalyst design and chemo-catalyst to enzyme ratio, allowing the kinetic balance between in-situ H2O2 synthesis and peroxy-enzymatic utilization to be tuned. A highly efficacious indole oxidation system was reported, reaching > 99 % conversion in 1 hour reaction time with 93 % selectivity towards 2-oxindole. The system offered highly competitive total turnover numbers (TTNs) when compared to alternative CPO cascades employing in-situ generated H2O2, with TTNs reaching 33,000 following substrate re-charging experiments. Despite this, significant chemo-catalytic deactivation was observed over multiple uses. HAADF ACSTEM X-EDS imaging revealed restructuring of nanoparticle morphology following exposure to reaction conditions, with extensive AuPd segregation observed in the used formulation. Further catalytic instability was observed for CPO, wherein extensive deactivation was induced upon exposure to reaction conditions. In particular, the combination of pressure (2 bar) and the organic cosolvent (50 vol.% tert-butanol) resulted in a coaction effect towards CPO deactivation. As such, addressing chemo-catalytic and enzymatic stability under reaction conditions remains a critical challenge to enhance cascade performance further. Overall, this study established highly efficacious chemo-enzymatic cascade systems for key pharmaceutical and industrial reactions, which builds towards the large-scale utilization of enzymatic oxidation reactions.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Schools > Chemistry
Date of First Compliant Deposit:	24 April 2025
Last Modified:	24 Apr 2025 10:25
URI:	https://orca.cardiff.ac.uk/id/eprint/177873

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