Lignin Depolymerization into Aromatic Monomers Using Supported Metal Catalysts in Supercritical Water

Lignin is a potentially useful source of aromatic monomers. Depolymerization of organosolv-lignin into aromatic monomers was investigated over charcoal-supported palladium, platinum, rhodium, or ruthenium catalyst (Pd/C, Pt/C, Rh/C, and Ru/C) in supercritical water (658-688 K) for reaction times of 0.5-3 h. Ru/C showed no catalytic activity for lignin depolymerization into aromatic monomers, but did catalyze conversion of lignin into gaseous products such as methane and carbon dioxide. In contrast, Pd/C, Pt/C, and Rh/C showed catalytic activity for lignin depolymerization into aromatic monomers. Yield of aromatic monomers decreased in the order Pd/C > Rh/C > Pt/C at 673 K and 1 h. Pd/C showed higher selectivity for phenolic aromatic monomers than Rh/C and Pt/C. Investigation of the activity of Pd/C at 673 K over time (0.5-3 h) found the highest yield of aromatic monomers (12.7 %) was obtained at 2 h.