The catalytic hydrothermal liquefaction of lignin to produce aromatics over nickel metal hydrotalcite catalysts

Abstract

In this study, a series of Ni(5,10,15 and 20) wt%/MgAl2O4 catalysts were prepared and used for the catalytic depolymerization of lignin under three varying solvents namely, methanol, ethanol and water, couple with varying reaction environment (N2, and H2). Highest bio-oil yield (81.8 wt%) was found with 15 wt%Ni/MgAl2O4 catalyst in ethanol solvent under hydrogen environment at 280 °C for 30 min reaction time. Under this condition higher heating value 36.2 MJ/kg of bio-oil was obtained, which is significantly increases from 24.5 MJ/kg of lignin. Under nitrogen environments bio-oil analysis showed that, it was consisted of mainly of alkyl phenols (Ca. 68%) and methoxy phenols (24.0%) compounds. Intriguingly, a deoxygenating and hydrogenation reaction was promoted by hydrogen, which further produced 38.2% of higher cyclohexane compounds and 19% of aromatic hydrocarbon. GPC analysis showed that hydrogen enhanced the lower molecular weight compounds. Catalysts stability test showed excellent recyclability.