Hydrogen peroxide assisted oxidative liquefaction of alkaline lignin over cobalt-nickel loaded zirconia catalysts

Abstract

Research toward the oxidative fractionation of lignin is gaining attention due to the selective breaking of a sub-aromatic unit of lignin's complex structure into monomeric phenolic. This study aims to investigate the liquefaction tendency of alkaline lignin by screening the synergistic effect of Co and Ni loaded on synthesized (syn) ZrO2 catalyst under the applied mild oxidative reaction conditions of molecular oxygen and hydrogen peroxide. Compared to the non-catalytic reaction (38.1 wt%), a significant improvement in the bio-oil yield (53.7 wt%) was obtained with the 5Co–5Ni/ZrO2-syn catalyst under the oxidative condition of molecular oxygen. However, only 44.3 wt% bio-oil was observed with commercial (com) 5Co–5Ni/ZrO2-com catalyst. The synergistic study of Co and Ni metal exhibited a diverse impact on the fractionation of lignin into respective product yields (bio-oil, gas, and char). Notably, adding a catalytic amount of hydrogen peroxide showed a remarkable improvement in liquefaction. The maximum bio-oil yield (72.1 wt%), and a maximum conversion (77.9%) were noticed with 0.5 mL H2O2 at 120 °C/30 min under the atmospheric molecular oxygen. The GC-MS analysis of bio-oil (5Co–5Ni/ZrO2-syn-HP) attributed to a high quantity of guaiacyl phenolic with a maximum proportion of benzaldehyde-3-hydroxy-4-methoxy (65.3%). The 5Co–5Ni/ZrO2-syn catalyst was also examined for the three catalytic cycles, which did not show any significant loss in product yield.