Valorization of Lignin to Simple Phenolic Compounds over Tungsten Carbide: Impact of Lignin Structure

Lignins isolated from representative hardwood, softwood, and grass materials were effectively hydrocracked to aromatics catalyzed by tungsten carbide over activated carbon (W2C/AC). The effects of botanical species and fractionation methods on lignin structure and the activity of W2C/AC were studied in detail. Gas permeation chromatography (GPC), FTIR, elemental analysis, and 2D HSQC NMR showed that all the extracted samples shared the basic skeleton of lignin, whereas the fractionation method significantly affected the structure. The organosolv process provided lignin with a structure more similar to the native lignin, which was labile to be depolymerized by W2C/AC. Softwood lignins (i.e., spruce and pine) possessed higher molecular weights than hardwood lignins (i.e., poplar and basswood); whereas corn stalk lignin that has noncanonical subunits and exhibited the lowest molecular weight owing to its shorter growth period. beta-O-4 bonds were the major linkages in all lignin samples, whereas softwood lignins contained more resistant linkages of beta-5 and less beta-beta than corn stalk and hardwood lignins; as a result, lowest hydrocracking efficiency was obtained in softwood lignins, followed by corn stalk and hardwood lignins. 2D HSQC NMR spectra of lignin and the liquid oil as well as the solid residue showed that W2C/AC exhibited high activity not only in beta-O-4 cleavage, but also in deconstruction of other ether linkages between aromatic units, so that high yield of liquid oil was obtained from lignin.