Catalytic hydrothermal liquefaction of alkali lignin at low temperature: Effect of acid and base catalysts on phenolic monomers production

Developing efficient process and technologies for the conversion of lignin into valuable functional chemicals is an important aspect of the biorefinery concept. In this study, the effect of low-cost base catalyst (NaOH, KOH, K₂CO₃ and Na₂CO₃) and acid catalyst formic acid (FA) was examined for the effective depolymerization of lignin into monomers of phenolic compounds at low temperatures (140, 160 and 180 °C). Also, different reaction residence time (30, 60 and 120 min.), catalyst amounts have been varied to optimize the liquefaction reaction conditions. In the case of non-catalytic experiments, maximum bio-oil yield (14.2 wt.%) was obtained at 160 °C for 30 min. reaction time. With K₂CO₃ catalysts, maximum bio-oil yield of 26.0 wt.% was observed, whereas in case of formic acid (FA) catalyst lignin depolymerization is very effective as it produced maximum bio-oil yield of 78.0 wt.%. Bio-oils were characterized using GC–MS, FT-IR, NMR analytical methods. Bio-oil analysis showed that it contained higher amount of phenolic monomers after lignin depolymerization. Higher amount of vanillin (41.9%) and isovanillin (44.9%) was found without catalyst and with FA liquefaction reaction. However with base catalyst, dibutyl phthalate (28.8%) was observed as the major compound in lignin depolymerization derived bio-oil. Solid residue analysis shows that base and acid catalyst depolymerized lignin in different ways by breaking of C-O and C–C bonds.