The heterogeneity of technical lignins, due to the intrinsic lack of a primary structure coupled with the harsh conditions adopted during the extraction processes, limits the exploitation of the great potential of these materials. To overcome this constraint and achieving more homogeneous physicochemical properties, lignins have been in the last years subjected to various fractionation strategies.[1] In this contribution, an overview of our recent work on the topic will be provided. Specific focus will be given to the developed fractionation protocols for lignosulfonates (LS), either by sequential solvent extraction or precipitation. Both these strategies, seldom explored in the past in favor of membrane-based approaches, mainly preferred because of the high water solubility of LS, evidenced their suitability towards the obtainment of cuts with well-defined properties. As a matter of fact, standardized chromatographic (GPC), spectroscopic (31P and HSQC NMR) and elemental analysis techniques evidenced a clear relationship between the used solvent and the physicochemical characteristics of the fraction, independently of the starting LS (hardwood or softwood). Furthermore, a recently disclosed segmented continuous flow fractionation (SCFF) technique will be presented. The SCFF allowed for an enhanced control over the process parameters (i.e. temperature and pressure) together with the possibility of adjusting the solvent gradients to generate different fractions with tailored features, either very similar to conventional batch fractions or with novel/altered properties when applying overheated solvents in pressurized systems. Last, but not least, scale-up can be easily realized in linear and parallel mode and a high degree of automation can be readily implemented to better address the needs of industrial production regimes. Overall, the research outcomes here illustrated open up new possibilities for the achievement of specific lignin cuts to be successively transformed into high added-value specialties.

Tailoring lignin physicochemical properties by solvent-based fractionation strategies

ACS Spring 2022

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