Catalytic hydrothermal liquefaction of lignin over carbon nanotube supported metal catalysts for production of monomeric phenols

Abstract

The catalytic hydrothermal liquefaction (HTL) of lignin was studied at different temperature (260, 280 and 300 °C) with different metal supported (Co, Ni and Fe) on carbon nanotube (CNT) in the presence of different solvents including water, ethanol and methanol for 30 min Reaction time. In comparison with water solvent liquefaction, the bio-oil yield significantly increased under alcoholic solvent (ethanol and methanol). As the reaction temperature increased from 260 to 300 °C, there was an increase in the conversion (70.9–86.9 wt%) of lignin. The highest bio-oil yield (66.2 wt%) was found with 10.wt.% Co/CNT catalyst using an ethanol solvent system at 280 °C. Moreover lower temperature showed lower yield of bio-oil. Catalyst acidity and surface area were seen to affects the alkali lignin depolymerization. Co/CNT and Ni/CNT catalysts are mainly responsible for the depolymerization of lignin into higher monomer phenolic compounds. A higher area percentage of vanillin obtained during catalytic liquefaction reaction as the presence of a catalyst, and alcoholic solvent accelerated the β-O-4 bond cleavages. Also elemental and GPC analysis showed that during the liquefaction reaction promotes the hydrogenation/hydrodeoxygenation and produced lower molecular weight bio-oils.