Extending the scope of reductive lignin depolymerization toward new feedstocks and innovative non-metal approaches

Lignin is the most abundant source of renewable aromatics on Earth, but its tendency to undergo irreversible condensation reactions during lignocellulose pretreatment hampers its valorization in current biorefinery schemes. A strategy for circumventing this issue is the so-called reductive catalytic fractionation (RCF), which relies on the use of redox catalysts and hydrogen gas to promote the reductive cleavage of lignin and to prevent lignin condensation, ultimately yielding low-molecular weight phenolics, along with a delignified (hemi)cellulose fraction. Some limitations must be overcome to improve the potential of RCF. The treatment of biomass feedstocks alternative to the widely explored virgin wood is important for extending the range of applications of this method. In addition, the recurrent adoption of high pressures of hydrogen gas poses safety and equipment constraints. This dissertation reports on research undertaken to investigate the RCF of wheat straw, an inexpensive and abundantly available agricultural residue, with a focus on the study of hydrogen-free process configurations. Moreover, an innovative dithionite-assisted organosolv fractionation (DAOF) method for the reductive depolymerization of lignin within biomass pretreatment is developed, based on the use of sodium dithionite as a reducing agent, instead of hydrogen gas and redox catalysts. Overall, this dissertation shows that RCF and DAOF represent promising approaches for the sustainable production of light aromatics via the reductive depolymerization of lignin.