Multi-scale utilization of lignin: A catalytic hydrogenation strategy based on catechyl lignin nanoparticles

With an abundance of oxygen-containing groups on the hydrophilic shell of lignin nanoparticles (LNPs), this biomaterial can serve as a catalytic support to adsorb and load metals. Herein, catechyl lignin oligomers from castor seed coats ( Cas ) were extracted via solvothermal pretreatment and prepared as LNPs using the solvent shifting method. Pd 2+ ions were then stabilized and reduced in situ on the surface of LNPs, which were further utilized for the catalyzed hydrogenation of lignin-derived aldehydes. Due to the homogeneous building units and linear linkages, the lignin oligomers from Cas exhibited superior self-assembly capacity with a lower π-π interaction energy than those from conventional lignocelluloses, resulting in a high yield of LNPs with a smooth surface. The high content of phenolic hydroxyl groups in catechyl-dominant LNPs resulted in efficient Pd 2+ ions adsorption via chemisorption of monomolecular layer and a partial reduction of the Pd 2+ ions into metallic states. In addition, the Cas- derived Pd/LNPs exhibited exceptional catalytic activity for the hydrogenation of lignin-derived aldehydes with high selectivity to alcohols under mild conditions. This could be attributed to the interaction preference of the reaction substrate according to the construction of LNPs: strong hydrogen bond between the aldehyde group of the substrate and the hydrophilic shell of LNPs, as well as the hindered π-π interactions between the benzene ring of the substrate and the hidden hydrophobic core of LNPs. This study, therefore, successfully employed lignin with a broad range of molecular weights from oligomers to monomers, paving the way for a comprehensive lignin biorefining industry.