Similarities in Recalcitrant Structures of Industrial Non‐Kraft and Kraft Lignin

Abstract

This work compares the structure of industrially isolated lignin samples from kraft pulping and three alternative processes: butanol organosolv, supercritical water hydrolysis, and sulfur dioxide/ethanol/water fractionation. Kraft processes are known to produce highly condensed lignin, with reduced potential for catalytic depolymerization, whereas the alternative processes have been hypothesized to impact the lignin less. The structural properties most relevant to catalytic depolymerization are characterized by elemental analysis, quantitative 13C and 2 D HQSC NMR spectroscopy, gel permeation chromatography, and thermogravimetric analysis. Quantification of the β‐O‐4 ether bond content shows partial depolymerization, with all samples having less than 12 bonds per 100 aromatic units. This results in theoretical monomer yields of less than 5 %, strongly suggesting the alternative fractionation processes generate highly condensed lignin structures that are no more suitable for catalytic depolymerization than kraft lignin. However, the different thermal degradation profiles suggest there are physicochemical differences that could be leveraged in other valorization strategies.

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

“…The DP of K2 and Soda lignins is lower and of about 6 aromatic units. The content in β-O-4 bonds is lower than 10 linkages per 100 aromatics units (table 2) for all the lignins, which is in agreement with previous studies 7,26,48 . The K2 lignin presents the highest content in β-O-4 and β-5 linkages.…”

“…The fraction of cleavable bonds (FCB) and the theoretical yields in monomers (TMY) are presented in table 2. They are based on the approach proposed in previous work 26,43 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 [42][43][44][45] .…”

“…This point may be explained by the combination of a lower molecular weight and higher β-O-4 content of this K2 lignin. These 2 structural behaviours are integrated in the calculation of the theoretical monomer yield (TMY) (see table 2) 26 . K2 presents the highest TMY.…”

Lignin‐Based Solid Polymer Electrolytes: Lignin‐Graft‐Poly(ethylene glycol)

“…In particular, lignin is the largest renewable aromatic resource, with 50 million tons produced each year globally. [ 1 ] Among various kinds of lignins, more than 85% of lignin product is kraft lignin that is generated through the paper pulping process. [ 2 ] Generally, the isolated lignin has high mechanical strength and high thermal stability.…”

“…However, intensive studies are solving the solvent issue through testing and optimizing various solvents with multiple industrial lignins. [ 1,26,27 ] In addition, in‐depth discussion of scalability can be found from literatures. [ 8,28–30 ] These works demonstrate potentials of industrial‐scale production of lignin‐based polymers as a low‐cost renewable resource with high economic value.…”