Fractionation of Lignin for Valorization

Abstract

Biorefining produces technical lignins that are not readily usable as a precursor for the production of value-added materials and chemicals, thus yielding a technological gap in complete utilization of lignocellulosic biomass. Various processes have been developed and demonstrated for fractionation of technical lignins, with the purpose of increasing the suitability of technical lignins as a precursor for commercial production of fuels and chemicals. Fractionation of lignins reduces the amount of impurities and generates lignin streams with smaller internal variations in chemical and structural properties. Examples of such processes, including membrane filtration, solvent extraction, and acid gradient precipitation, are reviewed in this chapter.

1. Introduction

Lignin is a loosely defined term that is used to describe different types of plant-based materials depending on the context. Lignin in different plants can be different in monolignol composition and molecular structure. On top of this variability is the susceptibility of lignins to various lignin extraction processes, as well as the various chemical and structural changes of extracted lignins during processing. In this chapter, the stream of lignins derived from a biorefining process (e.g., kraft pulping, cellulosic ethanol production) are referred to as “technical lignins”, which are compositionally and structurally different from the native lignins present in plant cell walls.

Depending on the isolation process, technical lignins contain components other than phenolic polymers. Some of these components (e.g., sodium salt, solvents, carbohydrates, extractives) may complicate the downstream processing of value-added products. The most common types of technical lignin are kraft lignin and sulfite lignin, of which 5 × 10⁷ tons are produced annually by the pulp and paper industry [1]. These lignins contain partially depolymerized plant lignins of various molecular weight, along with sodium salts as well as sulfur-containing organic and inorganic components. Lignins derived from cellulosic biorefining (e.g., biochemical conversion of lignocellulose feedstocks via saccharification and fermentation) contain ash components, undigested plant polysaccharides as well as oligomeric and monomeric sugars [2, 3]. These non-lignin components have various influences on lignin valorization. For instance, ash components introduce defects and negatively affects the crystallinity of carbon fiber produced from lignins. Sugars and carbohydrates participate in condensation reactions with lignins under thermal treatment, which complicate the processing of lignins into thermoplastic materials.

Fractionation of technical lignins is a necessary step in the upgrading of lignins to high-value products, which has higher requirements in product purity and/or performance compared to other lower-value uses of technical lignins (e.g., boiler fuel, dust control agent). In the conversion process of technical lignins to lignin fiber and carbon fiber, soften spinning lignin is complicated by the high polydispersity of and branching of technical lignins [4]. As for catalytic hydrogenolysis of technical lignins, the noble metal catalysts lose activity in presence of the sulfur impurities in technical lignins [5]. With fractionation processes, these adverse effects of structural heterogeneity and chemical impurity can be addressed as suitable precursors for valorization is derived from technical lignins.

2. pH-based precipitation

Lignins obtained via kraft pulping processes have been commercially separated from black liquor via pH-based precipitation. As pH decreases, lignin molecules become protonated and have less intermolecular electrostatic repulsion, thus favoring the formation of lignin precipitates. Sequential pH adjustment has been studied as a process for producing kraft lignin fractions with different molecular weights, different chemical structures, and different ash content. Magalhães et al. studied a sequential acidification process and obtained low-ash, fully protonated lignins at pH 3 from kraft lignins derived from both hardwood and softwood feedstocks [6]. Helander and Theliander reported the positive correlation between precipitation pH and molecular weight in the recovered fractions, and the retention of most sulfur content in the low molecular weight lignin fraction [7].

Lignin precipitation is also affected by anionic strength, and the effects of different anions on lignin prepitation follow the same order as the Hofmeister series [8]. Sewring and Theliander compared the difference between chloride and sulfate anions regarding their effect on lignin precipitation, and discovered that sulfates not being an effective anion for salting out kraft lignins [9]. When carbon dioxide is used in lignin precipitation, lignin precipitates can be formed with low ash content (<1 wt%) [10]. This process has been commercialized (e.g., LignoBoost) and shows a potential as a sustainable technology given the availability of renewable CO₂.

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 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.

4. Flocculation and membrane separation

The use of flocculants increases the Zeta potential of black liquor and promotes lignin flocculation. Piazza and Garcia studied the application of bovine blood and poly(diallyldimethylammonium chloride) in the recovery of soda lignin derived from wheat straw, and both flocculants are effective (87–92%) in removing lignin from the supernatant [30].

Ultrafiltration is the most commonly used commercial process that recovers lignosulfonates during the sulfite pulping process, and is also used in the recovery of lignins from kraft pulping black liquor. Via ultrafiltration and nanofiltration with membranes of different cut-off, various lignin fractions can be obtained from black liquors. The applications of membrane filtration in lignin recovery has been reviewed by Humpert and Czermak [1]. After partial depolymerization (via pulping and/or other chemical processes), technical lignins can be fractionated via membrane separation and thus the low- and high-molecular-weight fractions can be utilized for different applications. For instance, Hulteburg et al. demonstrated the use of polymeric membrane to recover low molecular weight phenolics from softwood kraft lignins that have been partially depolymerized with aqueous NaOH [31]. The recovered phenolic compounds have a molecular weight of 250–450 g/mol and can be metabolized by some Pseudomonas strains [32, 33]. Mendes et al. reported the use of ultrafiltration to recover 83% of the lignins in the lignin-lean aqueous phase derived from the acidification of kraft black liquor [34]. Labidi et al. compared the performance of several ceramic ultrafiltration membranes with different cut-offs (5–15 kDa) in the fractions of soda pulping black liquor from Miscanthus, and discovered that other than the highest molecular weight fraction (15 kDa and higher; likely contaminated by lignin-carbohydrate complexes), the other lignin fractions from different membranes have similar molecular structures (i.e., types of linkages among monolignols) despite their differences in molar mass [35]. Keyoumu et al. studied the ultrafiltration of hardwood (birch) and softwood (spruce-pine mixture) black liquor using ceramic membranes, and reported a negative correlation between membrane cut-offs and the phenolic group content in the obtained fractions [36].

5. Summary

Lignin fractionation has the potential of transforming low quality, heterogenous lignins to streams of lignin fractions with less impurities and narrower internal variation in chemical and structural properties, thus may enable the development of novel value-added products from lignin. To achieve this goal, cost-effective technologies that deliver lignin fractions with specific properties (e.g., low polydispersity, low ash, high/low phenolics content) are required. Methods for characterization of lignin fractions, as well as technologies for controlling lignin fraction characteristics, are instrumental in lignin fractionation for valorization. Therefore, new delignification, fractionation, characterization, and upgrading methods should be developed in tandem so that a complete pipeline of lignin valorization can be accomplished.