Separation of Lignin in the Cellulosic Ethanol Process
Traditionally, lignin is fractionated from lignocellulosic biomass in the paper industry by three methods: sulfite, Kraft, and soda processes. The sulfite process is carried out with either sulfites (SO32-) or bisulfites (HSO3-) depending on the pH. The counter ion can be sodium, potassium, ammonium, calcium, or magnesium. The Kraft process uses sodium hydroxide and sodium sulphide under strong
Figure 10.2 Polymerization of coniferyl alcohol to lignin. The reaction has two alternative routes catalyzed by peroxidases or oxidases.
alkaline conditions to cleave the ether bonds in lignin. The lignin may be recovered from the alkaline liquid remaining after pulp extraction, which is called "black liquor" by lowering the pH to 5-7.5 with sulfuric acid. In the soda process, alkaline lignin is recovered through extraction with sodium hydroxide. Separated lignin is normally referred to as "soda lignin," which is normally difficult to recover from non-wood sources by filtration or centrifugation because of its high carboxylic acid content arising from oxidation of aliphatic hydroxy groups.
With the large-scale production of cellulosic ethanol via cellu- lolysis route additional sources of lignin will be available through
various pretreatment technologies. These pretreatment technologies are discussed in detail in Chapter 5. In organic solvent-based pretreatment processes lignin can be separated from cellulose and hemicellulose under relatively mild conditions. Lignin separated by this process is called organosolv lignin, the benefits of organo — solv lignin over sulfonated and Kraft lignins include; no contamination with sulfur, greater ability to be derivatized, lower ash content, higher purity (due to lower carbohydrate content), generally lower molecular weight and more hydrophobic character [19]. Lignins can be efficiently recovered in ionic liquid-based pretreatment processes as well. Sun and coworkers have studied the effect of ionic liquid/organic solvent pretreatment on the structural properties of isolated lignins [20]. In this study corncob was submitted to pretreatments with 1-ethyl-3-methylimadazolium acetate and water/ organic solvents (DMSO, DMF, and DMAc) followed by alkaline extraction to isolate lignin. The lignin fractions obtained were analyzed and the results showed that a maximum yield of 85.04% (based on the original lignin) can be achieved for DMSO-lignin prepared with the EMIMAc/DMSO pretreatment. The fractions prepared with EMIMAc/organic solvents contained lower amounts of carbohydrates (0.48-1.40%) than milled wood lignin (MWL, 8.73%) and had similar molecular weights (2050-2430 g/mol) to MWL. On the other hand, the fraction H2O-lignin prepared with EMIMAc/ H2O contained relatively large amounts of carbohydrates (11.19%) and had a higher molecular weight (4310 g/mol) than milled wood lignin (MWL) [20].
There are few recent reports on the isolation and analysis of lignin from the biomass residue from actual cellulosic ethanol pilot plants [21-25]. In one account published in 2012, Guo and coworkers reported a study on cellulosic ethanol production residue obtained from a large-scale pilot plant where steam-exploded cornstalk was used in the simultaneous saccharification and fermentation (SSF) method [21]. Its lignin and ash contents were determined to be 62.18% and 9.91%, respectively, following standard procedures, and the remaining component was carbohydrates. In this work they developed a solvent extraction method to separate lignin from bioethanol production residue. Benzyl alcohol, dioxane and ethanol were used as extraction solvents, and the results were compared to the conventional alkali-solution and acid-isolation methods. They found that benzyl alcohol and dioxane extraction could reach higher lignin yields of 71.55% and 74.14%, respectively. FTIR
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Table 10.2 Weight average (Mw), number average (Mn) of molecular weights and polydispersities (Mw/Mn) of lignin samples extracted by different methods [21].
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and XRD analysis results indicate that sodium hydroxide solution dissolved most of the lignin in the raw material. However, the low lignin yield by this method may be attributed to the products loss during the complex separation process. GPC and 1H NMR results revealed that the dioxane-lignin had closer molecular weight with alkali-lignin, lower S/G ratio (0.22) and higher OHPh/OHAl ratio (0.45) with respect to benzyl alcohol-lignin. The results divulge that the lignin products separated from bioethanol production residue by dioxane extraction has better chemical activity and good potential as a feedstock in chemical industry [21]. Additionally, they compared the molecular weights of lignin extracted from cellulosic ethanol pilot plant residues using benzyl alcohol and dioxane as solvents with alkali-lignin and these results are shown in Table 10.2.