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Fig. 5 Lignin extraction processes and their products. 3
sulfonation at the a -carbon, which leads to cleavage of aryl- ether bonds and subsequent crosslinking. 68 Both Kra and sul te black liquor typically contain signi cant amounts of carbohydrate and inorganic impurities. 64 The soda- anthraquinone process is mostly applied in the paper industry on non-woody materials like sugarcane bagasse or straw. 64 The material is treated with an NaOH solution (13 – 16 wt%) at high pressures and temperatures of 140 – 170 °C, where anthraqui- none is added to stabilize hydrocelluloses. 64,69 The resulting soda lignin is sulfur-free and contains little hemicellulose or oxidized moieties. Organosolv lignin is produced in an extrac- tion process using organic solvents and results in separation of dissolved and depolymerized hemicellulose, cellulose as residual solids, and lignin that can be precipitated from the cooking liquor. 65 Various solvent combinations are possible, such as ethanol/water (Alcell process) or methanol followed by methanol and NaOH and antraquinone (Organocell process), which will a ff ect structure of the resultant materials. 51 Common for all organosolv lignins is that their structures are closer to that of natural lignins, in particular compared to Kra ligninor lignosulfonates. They are additionally sulfur-free and tend to contain less than 1% carbohydrates. 65 Several other methods of biomass processing have been developed that are targeted at lignin extraction, rather than producing cellulose bers, where lignin is as a byproduct. 64 Milled wood lignin (MWL) can be produced to closely emulate native lignin, but at the expense of process yields. 70 This method is considered gentle but time consuming, o en requiring weeks of processing, making it viable only in a laboratory setting. 58 Other techniques that aim to produce native lignin analogues include cellulolytic enzymatic lignin (CEL) and enzymatic mild acidolysis lignin (EMAL). The CEL procedure was developed as an improvement of the MWL process, where higher yields were obtained without increasing milling duration. 46 By adding an additional acidolysis step, Guerra et al. were able to again improve on the yield, while still producing lignin that closely resembled the native structure. 70 The physicochemical pretreatments aim to reduce lignin particle size through mechanical force, extrusion, or other. These techniques include
is found in pine and eucalyptus wood, while switchgrass contains only 17 – 18% lignin, 60 and less than 15% is typically found in annual plants. 61 The rst step in lignin valorization is biomass fractionation, where the cellulose, hemicellulose, and lignin are separated from each other. Several techniques have been developed, which can be grouped into sulfur and sulfur- free pulping from the paper and pulp industry, and bio- re nery processes that aim to produce of materials, chemicals, and energy from biomass. 59 The latter may speci cally be designed to isolate lignin of high purity and reactivity, whereas pulping originally produced lignin as a by- or waste-product. An overview is given in Fig. 5. The three industrial extraction methods for lignin are kra , sul te and soda pulping. In addition, organosolv pulping has been developed to extract lignin and separate the pulp bers. Commercialization of this process has not yet been done, but interest has risen recently in this technology, as organosolv pulping produces a technical lignin of high purity and reac- tivity. Several other methods also exist, but these are mainly used in lab-scale and are referred to as biore nery concepts, or “ pretreatments ” . 62 In the Kra pulping process, the lignocellu- losic biomass is mixed with a highly alkaline cooking liquid containing sodium hydroxide (NaOH) and sodium sul te (Na 2 S), at elevated temperatures of 150 – 180 °C. From the resulting black liquor, kra lignin can be precipitated out by lowering the pH to around 5 – 7.5. 46 In the LignoBoost process, this precipitation is done by adding rst CO 2 and then sulfuric acid. Kra lignin has a sulfur content of 1 – 3%, is highly condensed, contains low amounts of b -O-4 linkages, and is frequently burned for energy and chemical recovery at the mills. 63 – 65 In the Kra process, lignin is fragmented through a - aryl ether or b -aryl ether bonds, which results in increased phenolic OH content in the resultant lignin. 66 The sul te process is another specialized pulping technique, which utilizes a cooking liquor containing sodium, calcium, magnesium or ammonium sul te and bisul te salts. 65 Treatments are typically conducted at 120 – 180 °C under high pressures, which gives lignosulfonates that contain 2.1 – 9.4% sulfur, mostly in the benzylic position. 67 Lignosulfonates are cleaved mainly through
RSCAdv. , 2023, 13 , 12529 – 12553 | 12533
© 2023 The Author(s). Published by the Royal Society of Chemistry
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