Improvement of direct liquefaction of technical alkaline lignin pretreated by alkaline hydrogen peroxide

•Low concentration of alkaline hydrogen peroxide was applied on technical alkaline lignin.•The hydrogen peroxide of 0.5% concentration could produce 93.5% methanol soluble lignin.•The pretreatment could decrease weight average molecular weight from 1692 to 1354 g/mol.•Mild pretreatment could preserve most of functional groups in the alkaline lignin.

Technical alkaline lignin was first pretreated with alkaline hydrogen peroxide and then liquefied using methanol-water mixture or methanol solvents. When the technical lignin was pretreated by 0.5% H2O2 at 80 °C, the liquid yield of the liquefaction in methanol-water mixture and methanol increased from 84.2% to 93.5% and from 79.2% to 90.5%, respectively. The alkaline lignin (AL), pretreated lignin (PL) and resulting liquid (RL) were characterized by using GPC, FT-IR, and 1H NMR methods. The GPC results illustrated that the pretreatment by 0.5% H2O2 could decrease the weight average (Mw) and number average (Mn) molecular weight from 1692 to 1354 g/mol and from 1376 to 1207 g/mol, respectively. The FT-IR and 1H NMR results confirmed that some phenols were oxidized to quinone or aliphatic chain structures, and parts of side chains were separated from lignin.