Valorization of wood biomass-lignin via selective bond scission: A minireview

Non-food biomass are examined for the production of fuel and chemicals, since petroleum is becoming less accessible. Among wood biomasses, the most underutilized component-lignin with highly crosslinked and randomly polymerized aromatic ether units becomes robust and recalcitrant to current chemical processing. Increased attentions to develop catalytic methods for lignin depolymerization are observed recently, as lignin can be the largest natural source of aromatics. The existing technologies of drastic reaction conditions do nonselective random bond cleavage yielding large variety of small molecules with less yields. Recently, selective bond scission (C-C or C-O) via catalytic thermal and photochemical routes have been discovered which include hydrogenolysis by transition metal complexes, metals on different supports, bimetallic catalysts with synergistic effect; bond activation with photoredox catalysts, etc. This mini review discusses recent progress in selective bond scission techniques with homogeneous and heterogeneous catalytic systems for the final goal of lignin valorization to value-added products.