Selective monophenol and alkylphenol production in Organosolv lignin depolymerization: The role of acid sites on Pd-based catalysts

This work studies the influence of acid sites on the catalytic depolymerization of Organosolv lignins using Pd supported on TiO 2 and Nb 2 O 5 . The reactions were conducted in ethanol at 250 °C under 40 bar of H 2 . Catalyst-free depolymerization of Organosolv lignin produces the compounds derived from the primary monolignols (coniferyl, sinapyl alcohol) constituents of the lignin structure. The yield of monophenols depends on the type of lignocellulosic biomass, with beech wood yielding the highest amount. The degree of depolymerization of wheat straw and beech lignins significantly increases in the presence of a Pd/TiO 2 catalyst. The yield to monophenols follows the order: eucalyptus (7.6%) < wheat straw (30.9%) < beech (37.7%). Pd/TiO 2 catalyst promoted the hydrogenation and hydrogenolysis of monomers formed in the solvolysis of lignin with ethanol. Pd/Nb 2 O 5 catalyst exhibits different product distribution, with the formation of monophenols as well as alkylphenols. The change in product distribution is likely due to the higher acidity and the presence of Brønsted acid sites of the Pd/Nb 2 O 5 catalyst. A reaction mechanism is proposed for the depolymerization of Organosolv lignins with both catalysts.