Effect of metal formates on hydrothermolysis of organosolv lignin for the production of bio-oil

•Metal formates catalyzed hydrothermolysis of sulphur free organosolv lignin.•Special emphasis on sodium formate as an in-situ base producer for lignin depolymerization.•Influence of metal formates on the composition of monoaromatic compounds present in bio-oils. This research work investigated the use of various metal formates with emphasis on sodium formate for catalyzing depolymerization of organosolv lignin to bio-oils rich in monoaromatic phenolic compounds. The addition of sodium or calcium formate to the reaction mixture greatly influenced the bio-oil composition, whereas magnesium formate showed minor influence compared to non-catalytic reactions. The optimum yield of bio-oil was 13.9% of feed lignin achieved at 310 °C after 60 min of reaction time using sodium formate and contained 92 mg monoaromatics per gram of feed lignin. All the bio-oil samples demonstrated narrow molar mass distribution having an average molar mass of 207–277 g mol−1 containing syringol, guaiacol, catechol and methoxy catechol as major compounds.