Fractionation of Lignin with Decreased Heterogeneity: Based on a Detailed Characteristics Study of Sequentially Extracted Softwood Kraft Lignin

Abstract

Industrial lignin fractionation is attracting increasing interest due to its enormous potential in the development of high value-added materials. However, the widely reported fractionation approaches are primarily focused on the separation of fractions with a low polydispersity index (PDI). In this study, based on the detailed characteristic examination of carefully sequential-extracted softwood Kraft lignin fractions, a novel method to isolate lignin fraction with decreased heterogeneity (LGF-dh), was established in consideration of impurities, elemental composition, molar mass distribution, carbohydrate content, functional hydroxyl content, and the content of ligninrelevant aromatic units. To characterize the mentioned properties, an elemental analyzer, SEC-MALS, GC−MS, GC-FID, Py/GC− MS, 31 P-NMR, and HSQC-NMR were used to compare the differences of the sequential lignin fractions that were obtained by methyl tert-butyl ether (MTBE), ethyl acetate (EtOAc), ethanol (EtOH), methanol (MeOH), acetone, and dioxane. Moreover, a practical and feasible three-step extraction process was proposed to separate the low heterogeneity lignin fraction from industrial lignin according to the different solubilities of each fraction in the green cosolvent system of EtOH/water, MeOH/water, and acetone/water. Overall, this work presented a comprehensive study on the properties of softwood lignin as well as proposed a feasible and convenient method to reduce the heterogeneity of lignin, which would promote its valorization.

Phenolic Group Distribution as a Function of Molecular Weight in Lignins and Their Fractions

“…It is well recognized that solvent fractionation strategies applied to kraft lignins are able to reduce the intrinsic heterogeneity in terms of molecular weight distribution 23 and also refine some structural chemical features 24 either functional groups (such as alcohols, 25 phenols, carboxylic acids) or structural intermonomeric motifs. 26 The recently developed selective derivatization with dansyl chloride of lignin phenolic functionalities coupled with GPC-FL 19 was used in this work to focus the investigation on this important chemical group in lignin, and verify that solvent fractionation permitted not only to obtain fractions more or less rich in phenols but those fractions were also more homogeneous in terms of phenol distribution and molecular weight distribution.…”

Influence of Carbohydrates Covalently Bonded with Lignin on Solvent Fractionation, Thermal Properties, and Nanoparticle Formation of Lignin

“…DMK has been confirmed to be an excellent solvent to selectively extract abundant amounts of fractions with a trace amount of carbohydrates both in previous reports as well as in this study. ,, Since DMK could extract 67% of the lignin fraction with a trace amount of carbohydrates by single solvent extraction, we therefore chose DMK to separate the prepurified lignin into two parts: acetone-soluble with a trace amount carbohydrates (LG-TC0) and acetone-insoluble with large amounts of carbohydrates (LG-LC0), as shown in Figure . In a previous study, we demonstrated that lignin fractions with increasing Mw could be obtained by the solvent sequence of EtOAc, EtOH, MeOH, acetone, and dioxane. Moreover, the carbohydrate signals were detected from the fraction extracted by EtOH, MeOH, and dioxane.…”

“…In addition, sequential solvent extraction, which uses solvents with different solvent parameters to extract lignin by the precipitation of redissolved lignin in a gradient manner, has been proven an effective strategy to obtain more homogeneous lignin samples. Up to date, solvent sequences, for instance, DCM → MeOH, DCM → isopropanol → MeOH → mixture of MeOH/DCM, ethyl acetate → isopropanol → EtOH → MeOH → actone, , and isopropanol → EtOH → MeOH, have been used to decrease the heterogeneity of lignin. And, the existence of carbohydrates in fractions extracted by isopropanol, EtOH, and MeOH has been confirmed.…”

“…In a previous study, we carried out the sequential extraction by the solvent order of methyl tert -butyl ether (MTBE), ethyl acetate (EtOAc), ethanol (EtOH), methanol (MeOH), acetone (dimethyl ketone, DMK), and dioxane to study the component characteristics of industrial softwood kraft lignin. One interesting finding was that the obvious carbohydrate signals were only detected from the fractions extracted by EtOH, MeOH, and dioxane, which are solvents with a higher hydrogen bonding ability.…”

Recent Advances in the Valorization of Lignin: A Key Focus on Pretreatment, Characterization, and Catalytic Depolymerization Strategies for Future Biorefineries

“…Compared to 13 C NMR, 31 P NMR takes relatively shorter time for the analysis. [113,114] Phosphitylation of lignin using 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP) is commonly being done to quantify the hydroxyl groups, [113,114] as shown below in Figure 3c. The phosphitylated hydroxyl products are quantified using cyclohexanol as an internal standard.…”