Lignin as a multifunctional photocatalyst for solar-powered biocatalytic oxyfunctionalization of C–H bonds

Each year, the pulp and paper industry produces approximately 50 million metric tons of lignin as waste, 95% of which is combusted or abandoned. Here, we report the use of lignin as a photocatalyst that forms H2O2 by O2 reduction and H2O oxidation under visible light. We investigated the photophysical and electronic properties of two lignin models, lignosulfonate and kraft lignin, by spectroscopic and photoelectrochemical analyses, and demonstrated the photoredox chemistry of lignin using these and other lignin models (for example, native-like cellulolytic enzyme lignin, artificial lignin dehydrogenation polymer and phenolic β-aryl ether-type lignin dimer). Furthermore, the integration of lignin and H2O2-dependent unspecific peroxygenases (UPOs) enabled the highly enantioselective oxyfunctionalization of various C–H bonds. The use of lignin photocatalysts solves a number of the challenges relating to the sustainable activation of UPOs, notably, eliminating the need for artificial electron donors and suppressing the HO·-mediated inactivation of UPOs. Thus, the lignin–UPO hybrid catalyst achieved a total turnover number of UPO of 81,000 for solar-powered biocatalytic oxyfunctionalization in photochemical platforms. The pulp and paper industry produces approximately 50 million metric tons of lignin per year as a waste product. Here, lignin is shown to act as a photocatalyst for the solar-driven synthesis of hydrogen peroxide from H2O and O2 under visible light. Coupling this photocatalytic process with unspecific peroxygenases enables the enantioselective oxyfunctionalization of C–H bonds.