Oxidative depolymerization of lignin to high value aromatics via metal-free catalyst under ambient condition

The depolymerization of lignin into value added chemicals or liquid fuels provide additional revenue for current polysaccharides based biorefinery processes. Lignin is usually depolymerized into low molecular chemicals via either oxidation or reduction approaches and the conversion processes usually require the use of expensive metal catalyst. In this study, a metal-free catalytic system (quinone with sodium persulfate) was used to depolymerize lignin and its model compounds. Comparing to our previous Fenton reaction, Quinone activated sulfate free radical is proposed to act as a stronger oxidant and deconstruct lignin structure at relatively low temperature. Herein, we explore capabilities of this new catalyst system for facilitating oxidation, cleavage and depolymerization of two typical lignin model compounds with β-O-4 linkage at nearly ambient condition. The effects of solvents, the catalyst loading as well as temperature (25 -120 degree C) and reaction time were studied to achieve the optimized conditions to depolymerize lignin. The GC/MS results indicate the successful breakdown of beta-O-4 linkage with the successful detection of two degraded aromatic monomers.  This catalyst was further used to successfully depolymerize real lignin to low molecular weight hydrocarbon fuel precursors under mild reaction conditions.