Unlocking Structure–Reactivity Relationships for Catalytic Hydrogenolysis of Lignin into Phenolic Monomers

Abstract

Lignin depolymerization into aromatic monomers with high yields and selectivity is essential for the economic feasibility of biorefinery. However, the relationship between lignin structure and its reactivity for upgradeability is still poorly understood, in large part owing to the difficulty in quantitative characterization of lignin structural properties. To overcome these shortcomings, advanced NMR technologies [2D HSQC (heteronuclear single quantum coherence) and 31P] were used to accurately quantify lignin functionalities. Diverse lignin samples prepared from Eucalyptus grandis with varying β‐O‐4 linkages were subjected to Pd/C‐catalyzed hydrogenolysis for efficient C−O bond cleavage to achieve theoretical monomer yields. Strong correlations were observed between the yield of monomeric aromatic compounds and the structural features of lignin, including the contents of β‐O‐4 linkages and phenolic hydroxyl groups. Notably, a combined yield of up to 44.1 wt % was obtained from β‐aryl ether rich in native lignin, whereas much lower yields were obtained from technical lignins low in β‐aryl ether content. This work quantitatively demonstrates that the lignin reactivity for acquiring aromatic monomer yields varies depending on the lignin fractionation processes.

Depolymerization of lignin for biological conversion through sulfonation and a chelator-mediated Fenton reaction

“…For lignins with 50% β-O-4 content, in theory only about 50% of the lignin can be released as monomers and dimers through cleaving ether bonds and in practice the yield is typically in the range of 20% or less. 14,17,18 Higher monomer yields have been achieved for lignins containing higher β-O-4 content (50–70%) through lignin-first processing of intact biomass thereby avoiding isolation processes that result in condensation 13,19–22 or by altering the natural lignin synthetic pathways. 11 Catalytic approaches performed at high temperature and high pressure may cleave C–C bonds along with C–O bonds resulting in increased monomer yields.…”

Depolymerization of Technical Lignins in Supercritical Ethanol: Effects of Lignin Structure and Catalyst

“…They are based on the approach proposed in previous work , and on our present analyses. It has been previously shown in several studies that the monomer yields under solvent depolymerization conditions can be related to the content in β- O -4 bonds. − Therefore, it is of interest to calculate a theoretical yield in monomers by assuming a complete conversion of the β- O -4 bonds , in order to compare the theoretical potential of our lignins. TMY increases for lower-DP lignins and higher β- O -4 contents.…”

“…Amiri et al 43 have completed the approach of Phongpreecha et al 42 by using a simple ether cleavage model to predict the final depolymerization yields in monomers. Recently, Xiao et al 44 have studied the effect of diverse lignins prepared from Eucalyptus with varying β-O-4 contents (on a large range) and phenolic hydroxyl groups. They clearly show a relation between monomers yields and β-O-4 contents.…”

Lignin‐First Depolymerization of Lignocellulose into Monophenols over Carbon Nanotube‐Supported Ruthenium: Impact of Lignin Sources

“…Gas chromatography‐mass spectrometry (GC‐MS) and GC experiments were performed to qualitative and quantitative analysis the aromatic monomers, respectively, as described previously [10j,15b] . Briefly, GC‐MC analyses of the lignin oily product were carried out on a Shimadzu GCMS‐QP2010SE equipped with a HP‐5 MS (30 m×250 μm ×0.25 μm, Agilent) capillary column and an MS detector.…”