Highly efficient organosolv fractionation of cornstalk into cellulose and lignin in organic acids
Introduction
Corn has become one of the largest agricultural products in Canada. In 2015, around 2035000 acres of grain corn were harvested in Ontario, Canada (Kulasekera, 2015). As a crop residue with limited applications as an organic fertilizer, silage for livestock and energy source for heat and electricity, cornstalk has been paid less attention. Cornstalk is a typical lignocellulosic biomass, consisting of lignin (16–20%), cellulose (30–35%), hemicellulose (25–30%), ash, extractives and protein. Lignin is a complex amorphous polymer consisting of phenylpropanes, and cellulose, the dominant structural polysaccharide in cornstalk cell walls, is a linear polymer of β(1 → 4)-linked d-glucopyranoside monomer units. Generally in plant cell walls, cellulose has degrees of polymerization with 5000–7000 glucose monomer units. However, hemicellulose only has a polymerization degree of 200, and it is a heterogeneous polymer of pentoses (xylose, arabinose), hexoses (mannose, glucose, galactose), and sugar acids (Saha, 2003). Lignin, due to its phenolic nature can be a source of bio-based phenols or polyols for the production of various bio-based materials such as bio-based phenol formaldehyde resins and polyurethane resins (Wang et al., 2009, Mahmood et al., 2016). Cellulose can be hydrolyzed into sugar for the production of ethanol or butanol bio-fuels (Tian et al., 2016) or bio-based materials for thermal/electrical energy storage applications (Chen et al., 2013, Kim et al., 2012). Thus, fractionation of lignocellulosic biomass for the production of its chemical components (mainly lignin and cellulose) has attracted increasing interest.
By far, various lignocellulosic biomass fractionation processes have been investigated, including physical treatment (ball-mill, hydrothermolysis), chemical treatment (using acids, alkali, organic solvents) and physico-chemical treatment (e.g., steam explosion) (Yu et al., 2009). Organosolv fractionation or called organosolv pulping, fraction of lignocellulosic biomass using various organic solvents such as dioxane, methanol, ethanol, methanol, ethylene glycol, glycerol, acetone, acetic acid or formic acid, has been more intensively studied in recent years due to its relatively mild conditions, high yield and purity of cellulose and lignin products (Koo et al., 2011, Xu et al., 2006, Liu et al., 2010, Huijgen et al., 2008, Zhang et al., 2010a, Zhang et al., 2010b). Organosolv fractionation involves mixing a lignocellulosic feedstock such as wood chips with an aqueous organic solvent (commonly ethanol-water or acetone-water mixtures with the concentration of the organic solvent in water ranging from 40 to 80%) at temperatures ranging from 140 to 220 °C. This causes lignin to break down by hydrolytic cleavage of alpha aryl-ether links into fragments that are soluble in the solvent system. Organosolv fractions has also demonstrated to be an effective approach for lignocellulosic biomass pretreatment for hydrolysable and fermentable substrate for bio-ethanol production due to its advantages such as being environment-friendly, low capital investment and high solvent recovery (Villaverde et al., 2015, Viell et al., 2013, Zhang et al., 2016, Tian et al., 2016). Higher boiling solvents can also be used as they have the advantage of lower process pressure, but recovery of the solvent by distillation would be a challenge (Sarkanen, 1980). Ethanol has been suggested as the preferred solvent due to its easy recovery. For instance, Pan et al. (2007) recovered 79% of the lignin in a woody biomass at 170 °C, with 1.1% w/w H2SO4 as catalyst and ethanol-water solvent (65% v/v) for 60 min.
Organic acids especially acetic acid and formic acid have been applied in lignocellulosic biomass fractionation because of their close Hildebrand’s solubility to lignin, which favors the delignification of lignocellulosic biomass (Zhao et al., 2009). It has been reported that fractionation of rice straw in 90% formic acid-water solution at 100 °C for 60 min resulted in 85% lignin removal (Lam et al., 2001). Xu et al. (2006) fractionated wheat straw in aqueous alcohol or aqueous organic acid solvents with a liquor/solid ratio of 20:1 (mL/g) at 85 °C for 4 h, and the aqueous organic acids were found to be more effective than the aqueous organic alcohols for delignification of wheat straw. According to the study of Villaverde et al. (2009), lignin obtained from lignocellulosic biomass fractionation in acetic acid-water solvent contained more acetyl groups at C-a and C-c positions than the milled wood lignin, implying the cleavage of ester linkages and the acetylation of lignin by the acid. Similarly, formylation of lignin could occur during the fractionation process with formic acid. In lignin obtained from organosolv fractionation of a hardwood biomass, guaiacyl unit was the main structure, and the content of syringyl unit was higher than that of the hydroxybenzyl unit, with both β-O-4 and β-5 bonds (Zhang et al., 2010a, Zhang et al., 2010b). Combination of formic acid and acetic acid was found to favor lignocellulosic biomass fractionation by promoting the cleavages of the linkages between lignin and carbohydrates (Vanderghem et al., 2011, Jahan et al., 2014).
This study aimed to investigate organosolv fractionation of cornstalk in formic acid/acetic acid/water mixed solvents at a high solid/solvent ratio, i.e., 1:5 (g/mL), under mild operating conditions. Currently, the solid/solvent ratios tested in literature were mostly in the range of 1:5–1:10 (w/w), due to the relatively lower density of the cornstalk. For the economics consideration, we chose the biomass-to-solvent ratio to be at 1:5 (g/mL) in this work. Effects of fractionation solvents, catalysts, temperatures and residence time on the fractionation efficiency in terms of product yield, purity and chemical composition were investigated.
Section snippets
Materials
Cornstalk was supplied by Ontario Federation of Agriculture (OFA). Before use, the cornstalk was air-dried at room temperature, and then ground into particles to pass 20 mesh sieve. Elemental composition and chemical components in cornstalk are listed in Table 1. The chemicals used in this study, such as formic acid, acetic acid, ethyl acetate, hydrochloric acid and sulfuric acid were purchased from Caledon laboratory chemicals, and were used as received.
Fractionation procedure
Fractionation of cornstalk was carried
FTIR analysis of cornstalk and crude cellulose
Cornstalk and typical crude celluloses obtained from cornstalk fractionation at 90 °C for 180 min were characterized by FTIR, and the resulted FTIR spectra are displayed in Supplemental Fig. S1. As clearly shown in the figure, spectra of various crude cellulose samples display very similar transmittance characteristics with that of the cornstalk, all showing characteristic cellulose peaks at 3400 cm−1 (OH), 2900 cm−1 (CH), 1382 cm−1 (CH), 1317 cm−1 (COC), 1120 cm−1 (COC), 1030 cm−1 (COH), and lignin
Conclusions
Cornstalk was efficiently fractionated into crude cellulose and crude lignin in mixed solvents of acetic acid, formic acid and water. Effects of fractionation agents, catalysts, temperatures and residence time on the product yields, purity and composition of the final products were investigated. The optimal conditions for organosolv fractionation of cornstalk were as follows: mixed solvent of acetic acid/formic acid/water (3:6:1, v/v/v), HCl as the catalyst, 90 °C and 180 min residence time,
Acknowledgements
This work was supported by Ontario Ministry of Agriculture, Foods and Rural Affairs (OMAFRA) and Natural Sciences and Engineering Research Council of Canada (NSERC).
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Note: The co-author Shanghuan Feng has made the similar contributions to this work as the first author Tao Shui.