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

Abstract

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. Pd2+ 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 Pd2+ ions adsorption via chemisorption of monomolecular layer and a partial reduction of the Pd2+ 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.