Liquefaction of lignocellulosic biomass into phenolic monomers and dimers over multifunctional Pd/NbOPO₄ catalyst

For the first time, a tandem catalytic material, 5 wt % Pd/NbOPO ₄, was utilized in the depolymerization of wood in supercritical ethanol under hydrogen atmosphere. The experiments were conducted under various conditions, with fresh, and acetone extracted birch. A comprehensive analysis was performed to elucidate the dissolution efficiency and achieved product distribution. The results indicated that with fresh birch, 34 wt % of lignin monomer yield with 84 wt % delignification efficiency were obtained while with extracted wood, 35 wt % of lignin monomer yield with 78 wt % delignification efficiency were achieved. The total lignin monomer content extracted from the fresh birch is composed of 76.9 wt % of dimethoxyphenols and 16.5 wt % with the guaiacol structure. Major lignin monomer product was homosyringaldehyde (61.9 wt %). With extracted wood, 93.2 wt % of dimethoxyphenols (63.6 wt % homosyringaldehyde) and 6.8 wt % of guaiacol-monomers were achieved. It was concluded that the depolymerization occurred via breaking of the ether bonds in lignin, including ether hydrolysis by Lewis acid sites over the solid acid catalyst and with subsequent deoxygenation of monophenols over Pd. In addition, an extraction process was proposed to extract the aromatic fraction from the obtained biocrude.