Primary Processes of Depolymerization of Lignin Dispersed into Gas Phase

The primary depolymerization processes of hydrolytic lignin (HL) are examined, focusing on the formation of intermediate oligomers and bulky environmentally persistent free radicals (EPFRs). Fragmentation of HL was conducted in a continuous atomization (CA) fast flow reactor, where HL, dissolved in a 9:1 acetone-to-water solution, was dispersed. Results indicated that HL fragmentation occurs significantly faster in the gas phase in comparison to the literature rate of formation of major biofuel-phenolic compounds. In other words, the formation of phenolic compounds occurs at much lower rate constants being the limiting stage for lignin depolymerization. The critical role of surface associated reactions for formation of biofuel compounds developed in our previous work was highlighted. Using spin trapping with electron paramagnetic resonance (EPR) spectroscopy, it was shown that intermediate EPFRs, as hydroxyl radical generators, may act as biologically active intermediates in aqueous environments relevant to anthropogenic activities, wildfires, tobacco smoke, and other combustion processes. The addition of a highly hydroxylated 5% CuO/SiO₂ catalyst at concentrations of 1-3% (relative to an initial lignin concentration of 1 g/L in a 9:1 acetone-to-water mixture) did not significantly alter EPFR yields. However, an increasing trend in EPFR yield was observed with catalyst concentrations at 5%. A mechanistic scheme for the formation of CuO-surface-associated EPFRs is discussed.