Understanding lignin depolymerization to phenols via microwave-assisted solvolysis process

Abstract

In this study, microwave assisted degradation of lignin, a paper industry and biorefinery waste product, is investigated in presence of different organic solvents such as ethylene glycol (EG), dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) at different temperatures (100–140°C) and treatment periods (20–80min). Experiments performed in batch microwave reactor revealed that, in all the solvents, a major reduction in weight average molecular weight (Mw) of lignin was observed in the time period of 20–40min. In a polar protic solvent with high loss tangent (EG), maximum reduction in Mw was observed at 120°C, while in polar aprotic solvents (DMSO and DMF), Mw decreased with increase in temperature. Significant yield of phenolics (20wt.%) containing acetosyringone, guaiacol, syringaldehyde, anisole and lignin dimers (m/z 306, 322Da) was obtained at 100°C in DMF and DMSO, while at 140°C, 11wt.% phenolics were obtained in EG. Plausible reactions for the formation of phenolic monomers and dimers include etheric CβO and CαCβ bond cleavages induced by solvent interactions. The shift of molecular weight distribution to higher molecular weights suggested the involvement of recombination reactions at long treatment periods. This study shows that solvent-assisted microwave processing is a promising technique to convert lignin to phenols.