Article pubs.acs.org/IECR
Efficient Pretreatment of Wheat Straw Using Novel Renewable Cholinium Ionic Liquids To Improve Enzymatic Saccharification Huan Ren, Min-Hua Zong, Hong Wu, and Ning Li* State Key Laboratory of Pulp and Paper Engineering, School of Food Science and Technology, South China University of Technology, Guangzhou 510640, China S Supporting Information *
ABSTRACT: Nine cholinium ionic liquids (ILs) were synthesized. A high solubility of lignin (up to 483 mg g−1) and xylan (up to 721 mg g−1) was observed in four ILs containing organic anions, while cellulose, chitosan, and keratin were scarcely soluble in all ILs. These ILs were used for wheat straw pretreatment. Among the nine ILs tested, cholinium taurate ([Ch][Tau]) was the best. The effects of biomass particle sizes, water contents, and biomass loadings on the IL pretreatment were studied. Readily digestible residues were obtained after wheat straw up to 2 mm size was pretreated. Additionally, this IL pretreatment process was highly tolerant toward moisture. A good reducing sugar yield (79.7%) was achieved in the enzymatic hydrolysis of wheat straw pretreated by [Ch][Tau] at a biomass loading of 10% under 80 °C for 6 h. Therefore, renewable sulfonate-based cholinium ILs may be promising solvents for pretreatment and fractionation of lignocelluloses.
1. INTRODUCTION Production of biobased fuels and chemicals from lignocellulosic biomass, which mainly consists of cellulose, hemicellulose, and lignin, has attracted considerable interest in the past decade.1,2 For example, polysaccharides present in lignocelluloses could be hydrolyzed into glucose and xylose by enzymes, which could be transformed into biofuels such as ethanol, H2, and biodiesel by microorganisms, and into valuable biobased platform compounds such as 5-hydroxymethylfurfural and furfural via chemical dehydration.3 Lignin, the richest renewable resource of aromatics in nature, has been considered as a promising feedstock for production of value-added products, especially aromatic compounds.4,5 However, valorization of lignocellulosic biomass into value-added products remains challenging, since it is highly recalcitrant to chemical and biological degradation due to its heterogeneous and complicated structures as well as extensive chemical cross-linking between the components. Therefore, pretreatment of lignocellulosic biomass is generally necessary to deconstruct and fractionate this biomass prior to its subsequent conversion. Ionic liquids (ILs), types of salts with melting points of 99%), and N,N-dimethylglycine (98%) were from Shanghai Macklin Biochemical Co., Ltd. (China). Levulinic acid (99%), and 2-furoic acid (98%) were purchased from Aladdin Industrial Inc. (Shanghai, China), and other chemicals were of the highest purity commercially available. 2.2. Synthesis of Cholinium ILs. A choline hydroxide solution was added dropwise to an aqueous acid solution of equal mole under N2 at 4 °C. The mixture was stirred at room temperature for 48 h. Water was then removed under reduced pressure at 55 °C. The ILs were dried in vacuo for 48 h at 70 °C. The water contents of the ILs were determined to be less than 2 wt % with a Karl Fischer moisture titrator (Metrohm 787 KF Titrino, Switzerland). The NMR data and spectra of the ILs are available as Supporting Information. 2.3. Assessment of Biopolymer Solubility in ILs. To determine the solubility of lignin, xylan, cellulose, and chitosan, a 1 mg sample was added to a glass vial containing 0.5 g of IL at a designated temperature (60 or 90 °C) under N2 with stirring, and visually checked whether it was dissolved. If the solution was clear, 2.5 or 5 mg of sample was added. The solubility was calculated when the solution remained heterogeneous over 24 h. 2.4. Pretreatment of Wheat Straw with ILs. The IL pretreatment of wheat straw was conducted as described previously,14 with slight modifications. Typically, 150 mg straw samples (250−420 μm) were incubated under N2 in 3 g of ILs while being stirred at 80 °C for 6 h. Biomass size, and loading, pretreatment temperature, and time were stated for each experiment. Then, the suspension was diluted with equal volume of deionized water and centrifuged (18 000g, 15 min). The residues were washed with deionized water until the supernatant was colorless. Then, the residues were lyophilized and stored in a sealed bag at −20 °C prior to use. Lignin extractability and polysaccharide loss were calculated, respectively, according to the following equations:
polysaccharide digestibility (%) = (amount of the polysaccharide hydrolyzed /amount of the polysaccharide in the biomass used for
lignin extractability (%) ⎛ lignin amount in the recovered residues ⎞ = ⎜1 − ⎟ × 100 lignin amount in the untreated biomass ⎠ ⎝
enzymatic hydrolysis) × 100 reducing sugar yield (%) = (released reducing sugar amount/theoretic reducing
polysaccharide loss (%)
sugar amount in native biomass) × 100
= (1 − (residue recovery × polysaccharide content in the recovered residues/polysaccharide content in
The initial saccharification rate was calculated from the amount of reducing sugar released in the initial 30 min. 2.9. Error Analysis. The standard uncertainty (u) was determined for all of the obtained results. Each weighing was made considering an uncertainty of u(m) = 0.1 mg. The
native biomass)) × 100
2.5. Compositional Analysis of Wheat Straw. Polysaccharide and lignin contents of wheat straw samples were B
DOI: 10.1021/acs.iecr.5b03729 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Solubility of Lignin, Xylan, Cellulose, and Chitosan in Cholinium ILs lignin (mg/g)
a
xylan (mg/g)
cellulose (mg/g)
entry
IL
60 °C
90 °C
60 °C
90 °C
1 2 3 4 5 6 7 8
[Ch][Sar] [Ch][DiMeGly] [Ch][Tau] [Ch][Lev] [Ch][Fur] [Ch][HSO4] [Ch][H2PO4] [Ch]2[HPO4]
282 ± 5 354 ± 5 236 ± 5 245 ± 5 95 ± 5 25 ± 5 12 ± 3 37 ± 5
483 ± 5 430 ± 5 355 ± 5 358 ± 5 321 ± 5 25 ± 3 25 ± 3 69 ± 5
166 ± 5 256 ± 5 629 ± 5 262 ± 5 55 ± 5 66 ± 5 20 ± 3 20 ± 3
213 ± 5 283 ± 5 721 ± 5 321 ± 5 60 ± 5 88 ± 5 30 ± 3 27 ± 3
chitosan (mg/g)
90 °C < < < < < < <
