**3. Solvent extraction**

Solvent extraction, e.g., partitioning and extraction with organic solvent, has been investigated both as a method of isolating lignins from biomass (for production of Organosolv lignins and Acetosolv lignins) and as a process for fractionating technical lignins. The structure and composition of extracted lignins is affected by the type of solvent used and extraction process parameters. Saddler et al. reported the lignins extracted via an ethanol organosolv process and obtained lignins with Mw ≈ 2100 and a polydispersity (PDI) of 1.8–2.0 [11], which are similar to those of organosolv lignins obtained from Alcell (Mw = 2100–8000, PDI = 3.5–13) and methanol organosolv (Mw = 1200–3800, PDI = 1.6–2.4) processes [12]. With increasing severity of the extractions, the average molecular weight of the extracted lignins decreases and

### *Fractionation of Lignin for Valorization DOI: http://dx.doi.org/10.5772/intechopen.107338*

the number of functional groups in the extracted lignins decreases. The increase in functional groups changes the lignins' reactivity during derivatization reactions, thus improving their suitability in upgrading processes via chemical modification.

Extracted fractions of technical lignins are also used in the synthesis of lignin derivatives and copolymers. Generally speaking, organic solvent extracted lignins have lower average molecular weight and lower glass transition temperature compared to the lignins prior to the extraction [13, 14]. Organosolv lignin (e.g., Alcell), which is extracted from hardwood using a mixture of organic solvents, has been demonstrated as a viable precursor for the synthesis of polyols [15] and lignin esters [16, 17]. Pan and Saddler compared the performance of organosolv and Kraft lignins as polyol substitutes in the systhesis of rigid polyurethane foam, and the results showed organosolv lignin being a better option with greater miscibility with commercially-available synthetic polyols [18]. The amount of polyol groups in organosolv lignin can be affected by the type of organic solvent used in the extraction process, thus enabling some degree of tuning in the reactivity of extracted lignins [14, 19].

Solvent fractionation of technical lignins removes inorganic salts and retains the low molecular weight fractions of lignins, thus improving the suitability of lignins as a precursor of melt spinning in carbon fiber production. Baker et al. reported the use of a solvent extraction process in obtaining a lignin fraction that has undetectable level of ash (compared to 2.7 wt% of ash content in the hardwood lignin prior to the extraction), low crosslinking reactivity (thus forming a more stable melt), and easier extrusion [20]. Rials et al. explored the use of organosolv lignins obtained from extraction of switchgrass and yellow poplar using an acidified solvent mixture (i.e. a mixture of methyl isobutyl ketone, ethanol, and water), and discovered that the blend of lignins from the two biomass feedstocks demonstrated better mechanical properties compared to organosolv lignins solely derived from switchgrass [21], due to the plasticizing effect of hardwood lignins. It is worth noting that for fractionated lignins to be a suitable precursor for carbon fiber production, it has to have high purity (<1000 ppm of ash), low volatile content (<5 wt % at 250°C), and low content of non-volatile particulates (<500 ppm) [22]. These requirements can be met with organic solvent extraction processes which aims to dissolve polymeric lignins while partitioning lignins from impurities.

Extraction with low toxicity and renewable solvent mixtures have been identified as a promising approach to fractionating and purifying lignins. Thies et al. reported the use of the mixture of acetic acid and water as a solvent for removing metal salts from kraft lignin under elevated temperature (95°C). Upon mixing the hot acetic acid–water mixture with kraft lignin, two phases are formed: one solvent-rich phase with extracted metal salts and one lignin-rich phase with <100 ppm metals content [23]. Hodge and Thies described a modified version of the aforementioned extraction process, where pressurized carbon dioxide is introduced to the lignin-acid-water mixture to produce a lignin solution in a CO2-expanded solvent [24]. By controlling the pressure of carbon dioxide, lignin fractions with various average molecular weights can be obtained from kraft lignin. In addition to concentrated acetic acid aqueous solution, deep eutectic organic solvents with low vapor pressures are also reported as candidates for solvent fractionation of lignins, as summarized in a review by Ragauskas and Wan [25]. Hou et al. studied the use of solvents composed of choline chloride and urea (or oxalic acid) in the dissolution of kraft lignins, and obtained lignin solutions with 10–16% w/w concentrations [26]. These eutectic solvents are also used in the extraction of lignins from bamboo [27], corncob [28], and poplar [29] biomass. Due to the high costs with the production and recovery of deep eutectic solvents, this approach to lignin fractionation has not yet been commercialized.
