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

Abstract

Lignins isolated from representative hardwood, softwood, and grass materials were effectively hydrocracked to aromatics catalyzed by tungsten carbide over activated carbon (W C/AC). The effects of botanical species and fractionation methods on lignin structure and the activity of W C/AC were studied in detail. Gas permeation chromatography (GPC), FTIR, elemental analysis, and 2 D 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 W C/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. β-O-4 bonds were the major linkages in all lignin samples, whereas softwood lignins contained more resistant linkages of β-5 and less β-β than corn stalk and hardwood lignins; as a result, lowest hydrocracking efficiency was obtained in softwood lignins, followed by corn stalk and hardwood lignins. 2 D HSQC NMR spectra of lignin and the liquid oil as well as the solid residue showed that W C/AC exhibited high activity not only in β-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.

Rapid Nondestructive Fractionation of Biomass (≤15 min) by using Flow‐Through Recyclable Formic Acid toward Whole Valorization of Carbohydrate and Lignin

“…Because these chemical linkages require more energy to be cleaved, condensed lignin tends to be less susceptible to depolymerization by chemical catalysis. Therefore, fractionation methods that preserve the dominant ether linkages between monolignols are preferable, if the goal is to valorize lignin to hydrocarbon biofuels and renewable aromatic chemicals . Because WIL from RFF preserves ether linkages, it may be more easily utilized for subsequent chemical upgrading.…”

A Comparison of Phenolic Monomers Produced from Different Types of Lignin by Phosphotungstic Acid Catalysts

“…As compared to poplar and pine, straw lignin has less content of G (32.7 wt %) structure units and less content of H (20.8 wt %) structure units (2D‐HSQC NMR results in Figure ), these units are randomly distributed in the straw lignin structure. This may result in its lower molecular weight . In parallel, straw lignin samples contain less β‐O‐4 aryl ether linkages, resulting in lower molecular weight .…”

Natural Phenol Polymers: Recent Advances in Food and Health Applications

“…Valorization of lignins to get fine chemicals [32] or bio-oil [33] relies on fine tuning of each of these steps, which may range from bioengineering to get lignins featuring a restricted subset of linkages or precursor units in order to facilitate chemical (or enzymatic) deconstruction [34], to a catalytic treatment to get a specific chemical transformation or cleavage of a specific linkage [35], to the delignification or removal of lignocellulosic matrix via lignin depolymerization that is commonly performed via Na 2 S/NaOH treatment yielding the kraft lignin [36,37] or to organic solvent extraction [38]. …”