Abstract
Lignins are valuable renewable resources for the potential production of a large array of biofuels, aromatic chemicals and biopolymers. Yet native and industrial lignins are complex, highly branched and heterogenous macromolecules, properties that have to date often undermined their use as starting materials in lignin valorisation strategies. Reliable knowledge of weight average molar mass, conformation and polydispersity of lignin starting materials can be proven to be crucial to and improve the prospects for the success of such strategies. Here we evaluated the use of commonly-used size exclusion chromatography (SEC)—calibrated with polystyrene sulphonate standards—and under-used analytical ultracentrifugation—which does not require calibration—to characterise a series of lignin fractions sequentially extracted from soda and Kraft alkaline lignins using ethyl acetate, methyl ethyl ketone (MEK), methanol and acetone:water (fractions F01–F04, respectively). Absolute values of weight average molar mass (Mw) determined using sedimentation equilibrium in the analytical ultracentrifuge of (3.0 ± 0.1) kDa and (4.2 ± 0.2) kDa for soda and Kraft lignins respectively, agreed closely with previous SEC-determined Mws and reasonably with the size exclusion chromatography measurements employed here, confirming the appropriateness of the standards (with the possible exceptions of fraction F05 for soda P1000 and F03 for Indulin). Both methods revealed the presence of low (~ 1 kDa) Mw material in F01 and F02 fractions followed by progressively higher Mw in subsequent fractions. Compositional analysis confirmed > 90% (by weight) total lignins successively extracted from both lignins using MEK, methanol and acetone:water (F02 to F04). Considerable heterogeneity of both unfractionated and fractionated lignins was revealed through determinations of both sedimentation coefficient distributions and polydispersity indices. The study also demonstrates the advantages of using analytical ultracentrifugation, both alongside SEC as well as in its own right, for determining absolute Mw, heterogeneity and conformation information for characterising industrial lignins.
Highlights
Lignins are valuable renewable resources for the potential production of a large array of biofuels, aromatic chemicals and biopolymers
Yields obtained after each solvent extraction step differed between the two lignin types, especially after ethyl acetate extraction (F01) and the resultant residue (F05), though similar yields resulted using methyl ethyl ketone (MEK) and methanol extractions (F02 and F03)
Residual carbohydrate compositions of both untreated lignins (2.8% and 1.5% for soda and Kraft, respectively) (Table 1) were higher than those reported previously for organosolv lignins (e.g. 0.3–0.8%65; 0.2–1.1%43), consistent with previous reports that residual carbohydrate is generally higher in alkaline lignins compared with these other lignins, possibly originating from polysaccharides remaining covalently-bound to the lignin as a Lignin-Carbohydrate Complex (LCC) or from trapped, non-covalently-bound carbohydrates that arise during the lignin precipitation step[43,66]
Summary
Lignins are valuable renewable resources for the potential production of a large array of biofuels, aromatic chemicals and biopolymers. Absolute values of weight average molar mass (Mw) determined using sedimentation equilibrium in the analytical ultracentrifuge of (3.0 ± 0.1) kDa and (4.2 ± 0.2) kDa for soda and Kraft lignins respectively, agreed closely with previous SEC-determined Mws and reasonably with the size exclusion chromatography measurements employed here, confirming the appropriateness of the standards (with the possible exceptions of fraction F05 for soda P1000 and F03 for Indulin) Both methods revealed the presence of low (~ 1 kDa) Mw material in F01 and F02 fractions followed by progressively higher Mw in subsequent fractions. These lignins arise as side-products of industrial bioprocessing of native lignocellulose biomass to extract the carbohydrates and the remaining lignin content is relatively high They are mainly produced by alkaline and organosolv processes and amongst the commonly-employed alkaline-based processes, Kraft and soda pulping involve use of either sodium sulphide under aqueous alkaline conditions (Kraft)[17] or sodium hydroxide alone (soda). Sequential solvent extraction has become a widely-adopted strategy to isolate lignin fractions with controlled properties[39,40,41], with their own characteristics, depending on the lignin s ource[42]
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