Radical and (photo)electron transfer induced mechanisms for lignin photo- and electro-catalytic depolymerization

As one of the three major components of woody biomass, lignin is a kind of natural organic polymer and the only abundant natural renewable resource with aromatic nucleus. Chemical catalysis induced depolymerization is an important and effective approach for lignin utilization. In particular, photocatalysis and electrocatalysis show great potential in accurately activating C–O/C–C bonds, which is a critical point of selective cleavage of lignin. In this contribution, we focus on radical and (photo)electron transfer induced reaction mechanisms of the photo(electro)catalytic depolymerization of lignin. Primarily, the general situation of Carbon-centered radicals and active oxygen species mediated lignin conversion has been discussed. Then the mechanisms for (photo)electron transfer mediated lignin depolymerization have been summarized. At the end of this review, the challenges and opportunities of photo(electro)catalysis in the applications of lignin valorization have been forecasted.