To enhance lignin valorization is absolutely critical to assure the isolation of as much as possible uniform lignin fragments with defined reactivity/functionality and of course to assure the process repeatability. The development of the specific fractionation protocol employing the anti-solvent methodology in combination with the process conditions enables isolation of lignin with the specific properties. In this study, organosolv process has been applied to isolate lignin from various lignocellulosic biomass sources (hardwood, softwood and straws), which contain different lignin structures (different ratios of monolignol units and thus functional groups). A gradual change of the solubility parameter attained with the addition of the anti-solvent induced the formation of the insoluble lignin fractions. Additionally, the effect of different process temperature to the structure of isolated lignin has been investigated.<br/>Fractional lignin precipitation enabled lignin separation according to the molecular weight. Therefore, isolated lignin fractions were characterized using size-exclusion chromatography, to obtain molecular weight distributions of the acetylated fractions, while the NMR analysis provided information about the functional group distribution and the main structural features. Specifically, <sup>31</sup>PNMR was used to quantify the different OH groups, while 2D NMR showed the distribution of the main lignin motifs within the fractions. Such a fundamental approach disclosed the limits of the possible tailoring of lignin characteristics employing solely water and the process temperature. Lignin of the reduced complexity has significantly higher potential to be further used for different applications such as polymer synthesis, depolymerisation, (de)functionalization.

Lignin fractionation: Effects of lignocellulose type and process temperature on structure, functionality and molecular weight

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