Photocatalytic biohybrids for sustainable chemical synthesis

Abstract

Biocatalyst artificial photosynthesis converts solar energy into high value-added compounds by integrating photo(electro)catalytic reactions with redox enzyme reactions. Photo(electro)catalyst absorbs solar energy to generate photoexcited charges that are transferred to redox enzymes. Subsequently, activated oxidoreductases synthesize industrially useful fine chemicals (e.g., pharmaceutical intermediates, chiral compounds) and chemical fuels (e.g., hydrogen, methanol) with excellent reaction specificities and efficiencies. In this dissertation, I demonstrate that lignin and microplastic wastes can be used in sustainable semi-artificial photosynthesis. Chapter 1 demonstrates that lignin—as a major component of plants and a waste from the pulp, paper and biofuel industry—possesses the photocatalytic ability to produce H2O2 under solar light. Furthermore, the integration of lignin-driven photocatalysis and peroxygenase-mediated oxyfunctionalization reactions enables the highly enantioselective oxyfunctionalization of various C-H bonds. The use of lignin solves challeneges relating to the sustainable activation of peroxygenases, such as (i) no requirement of artificial electron donor, and (ii) suppression of peroxygenase’s inactivation mediated by radicals. This chapter establishes lignin as an energy conversion material for producing fuels and chemicals, presenting an example of waste-to-wealth conversion. Chapter 2 commences with a conceptual discussion of biocatalytic photoelectrochemical (PEC) platforms and highlights recent advances in PEC-based biosynthesis through cofactor regeneration or direct transfer of charge carriers to (or from) oxidoreductases on enzyme-conjugated electrodes. In addition, future perspectives and potential next steps are addressed in the vibrant field of biocatalytic photosynthesis. Chapter 3 reveals the function of nonrecyclable real-world microplastics as electron feedstocks to accelerate enzymatic reactions in biocatalytic PEC systems. A hematite-based photoanode extracts electrons from hydrolyzed polyethylene terephthalate (PET) solutions obtained from post-consumer PET wastes (e.g., drinking bottles) and transfers the electrons to the bioelectrocatalytic site. Carbon-based cathodes receive the electrons to redox enzymes for organic synthetic reactions, such as stereoselective oxyfunctionalization, reductive amination, and asymmetric hydrogenation. This chapter presents a photoelectrocatalytic approach for integrating environmental remediation and biocatalytic photosynthesis towards sustainable solar-to-chemical synthesis.