Enhanced production of phenolic monomers from lignocellulose via a two-step depolymerization strategy and solvent system optimization

The catalytic valorization of lignin has been widely studied using diverse feedstocks and catalysts, yet achieving high yields of aromatic monomers remains a challenge. In this study, a two-step strategy was employed to produce lignin-derived oil via flow-through solvolysis and subsequently generate high-yield aromatic monomers through catalytic depolymerization. Nearly complete lignin removal from birch wood was accomplished using a methanol/water mixture (8:2, v/v) at 225 °C and 8 MPa for 3 h. The solvolysis liquor was then depolymerized over a Pd/activated carbon catalyst under 3 MPa initial H₂ pressure at 290 °C for 3 h, affording a 69.3 C% yield of aromatic monomers via complete β–O–4 bond cleavage and partial C–C bond cleavage. Glucan was almost completely recovered and is suitable for downstream applications such as cellulosic ethanol. Solvent composition and depolymerization parameters were optimized, and the product distribution was analyzed to elucidate solvolysis and depolymerization mechanisms. When other lignocellulosic feedstocks, including oak, pine, rice husk, and kenaf, were tested, delignification efficiency varied. Oak and pine retained residual lignin, while rice husk and kenaf showed nearly complete delignification. However, aromatic monomer yields from these feedstocks were lower (29.4–39.0 C%) than from birch due to their lower sinapyl alcohol-to-coniferyl alcohol ratios. This two-step approach offers a promising route for lignin valorization, combining high delignification, selective monomer production, and efficient catalyst recovery, and may be broadly applicable to lignocellulosic biorefineries targeting high-value aromatic chemicals.