One-step alcoholysis of lignin into small-molecular aromatics: Influence of temperature, solvent, and catalyst

Lignin valorization is a challenge because of its complex structure and high thermal stability. Supercritical alcoholysis of lignin without external hydrogen in a self-made high-pressure reactor is investigated under different temperatures (450-500 °C) and solvents as well as catalysts by using a reactant suspension mode. Small-molecular arenes and mono-phenols (C₇-C₁₂) are generated under short residence time of 30 min. High temperature (500 °C) favors efficient deoxy-liquefaction of lignin (70%) and formation of small-molecular arenes (C₆-C₉). Solvents methanol and ethanol demonstrate much more synergistic effect on efficient deoxy-liquefaction of lignin than propanol. The catalyst Cu-C has the optimal activity and selectivity in methanol (70% of conversion, 83.93% of arenes), whereas Fe-SiC possesses the optimal catalytic deoxygenation in ethanol, resulting in the formation of arenes other than phenols. Further analysis indicates that lignin is converted into arenes by efficient cleavages of C-O ether bonds and C-C bonds under high temperature and pressure.