Lignin as a Multifunctional Photocatalyst for Solar-Powered Biocatalytic Oxyfunctionalization of C-H Bonds

Lignin is a key structural material in all terrestrial plants that is responsible for cell wall formation, water transportation, seed protection, and stress adaptation. Each year, pulp and paper industry produces approximately 50 million metric tons of lignin as waste, 95% of which is combusted or abandoned. Here, we report a new multifunctionality of lignin as a photocatalyst (e.g., synergistic formation of H2O2 formation through O2 reduction and H2O oxidation, use of H2O as an electron donor, and OH• -scavenging activity). Our spectroscopic and photoelectrochemical analyses reveal the photophysical characteristics (e.g., light absorption, charge separation/transfer) of lignin models [e.g., lignosulfonate (LS) and kraft lignin (KL)] and their electronic properties [HOMO-LUMO gap: 2.67 eV (LS), 2.95 eV (KL), LUMO: -0.VRHE (LS) and -0.26 VRHE (KL), HOMO: 2.44 VRHE (LS) and 2.69 VRHE (KL)]. We demonstrate lignin-sensitized redox chemistry, such as (i) H2O2 formation through O2 reduction using H2O as an electron donor and (ii) O2 evolution through H2O oxidation, under visible light. Furthermore, the integration of lignin and H2O2-dependent unspecific peroxygenases (UPOs) enables enantiospecific oxyfunctionalization reactions (e.g., benzylic hydroxylation, alkane hydroxylation, styrene epoxidation). Lignin photocatalysts solve existing issues (e.g., requirement of artificial electron donors, H2O2- or OH• -driven inactivation of UPO) related to the sustainable activation of UPO. The lignin/UPO hybrid achieves a total turnover number of enzyme of 81070, the highest value ever recorded for solar-powered biocatalytic oxyfunctionalization in photochemical platforms. This work demonstrates the propriety of lignin in robust photocatalyst/biocatalyst hybrids for artificial photosynthesis.