High-Resolution Profiling of the Functional Heterogeneity of Technical Lignins

Abstract

In technical lignins, functionality is strongly related to molar mass. Hence, any technical lignin exhibits concurrent functionality-type distribution (FTD) along its molar mass distribution (MMD). This study combined preparative size-exclusion chromatography with offline characterizations to acquire highly resolved profiles of the functional heterogeneity of technical lignins, which represent crucial information for their material use. The shape of these profiles showed considerable dissimilarity between different technical lignins and followed sigmoid trends. Determining the dispersity in functionality ( Đ F ) of lignins via their FTD revealed a rather homogeneous distribution of their functionalities ( Đ F of 1.00–1.21). The high resolution of the acquired profiles of functional heterogeneity facilitated the development of a robust calculation method for the estimation of functional group contents of lignin fractions based simply on their MMD, an invaluable tool to simulate the effects of intended purification processes. Moreover, a more thorough evaluation of separations based on functionality becomes accessible.

Fractionation and Absolute Molecular Weight Determination of Organosolv Lignin and Its Fractions: Analysis by a Novel Acetone-Based SEC─MALS Method

“…This obstacle is further complexed by lignin species obtained from different raw materials with various phenylpropanoid monomer unit ratios, resulting essentially in a cluster of copolymers . Additionally, interactions of lignin molecules with the stationary phase are almost inevitable due to its three-dimensional and intrinsic branched structure, which can prolong the elution time and consequently lead to misleading molar masses. , In this context, the aggregation of lignin molecules by intermolecular forces, especially initiated by hydrogen bonding of hydroxyl groups, has been discussed earlier . Furthermore, it has been reported that clustering of lignin aggregates occurs due to the π–π interaction between the aromatics .…”

“…Classifying the size and chemistry by (solvent) fractionation leads to classes of more homogeneous lignin molecules, which will most likely influence the d n /d c value. Additionally, technical lignins can exhibit absorption (and fluorescence) behavior. , Once the sample absorbs incident light, this can cause a change in the scattered intensity and lead to misleading calculation in the MW. In a recent work, Zinovyev et al discussed the dependency of the d n /d c value on the MW and the absorption phenomena for Kraft lignin and fractions.…”

“…Technical lignins are still not sufficiently utilized even though they hold considerable potential for production of renewable chemicals and materials. , The direct material usage is still hindered by a number of barriers, prominently by the lignin heterogeneity caused by a high molecular weight (MW) dispersity, various interunit linkages, and impurities. − Fractionation of technical lignins by preparative size exclusion chromatography (SEC), solvent, or membrane fractionation has been revealed to provide narrower fractions of MW classes, which further showed enhanced performance in specific applications such as bio-based polyurethane foams or carbon fibers. ,, In particular, solvent fractionation, with multiple solvents in a certain order, classifies technical lignins in size . However, the solvent fractionation method, used for this work, in only acetone–water mixtures is much more process- and cost-efficient in regard to scale-up.…”

“…SEC ideally separates polymers solely in terms of their size in solution rather than by their absolute M W and requires, therefore, the use of a previous calibration . Technical lignin molecules are challenging solutes in SEC since their solubility is hampered in common solvents, and usually, derivatization is necessary for full solubilization. , As no narrow lignin standards are available, the conventional molar mass analysis is based on narrow synthetic polymer standards. However, applying this calibration method is not sufficiently accurate for lignin, and obtained molecular weights remain questionable .…”

Lignin-Based Polyurethanes from the Blocked Isocyanate Approach: Synthesis and Characterization

“…Its aromatic structure is composed of three basic phenylpropane units, guaiacyl (G-unit), syringyl (S-unit), and p -hydroxyphenyl (H-unit), which are derived from coniferyl, sinapyl, and p -coumaryl alcohol, respectively . These units are linked by carbon–carbon (β–5, 5–5, β–1, and β–β) and carbon–oxygen linkages (β–O–4, α–O–4, and 4–O–5), forming an amorphous and complex three-dimensional structure with a variety of functional groups, such as carbonyl, methoxyl, and aliphatic and phenolic hydroxyl groups. , Among the functional groups, the hydroxyls are the most abundant and most reactive and therefore the preferred target for reactions to develop lignin-based materials, such as PUs. Furthermore, the abundant aromatic moieties impart high stiffness to lignin .…”