Corn Stover Pretreatment by Ionic Liquid and Glycerol Mixtures with

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Corn stover pretreatment by ionic liquid and glycerol mixtures with their density, viscosity, and thermogravimetric properties Joan G Lynam, Genica I Chow, Phillip L Hyland, and Charles J Coronella ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00480 • Publication Date (Web): 31 May 2016 Downloaded from http://pubs.acs.org on June 4, 2016

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ACS Sustainable Chemistry & Engineering

Corn stover pretreatment by ionic liquid and glycerol mixtures with their density, viscosity, and thermogravimetric properties Joan G. Lynam*, Genica I. Chow, Phillip L. Hyland, and Charles J. Coronella Department of Chemical & Materials Engineering, University of Nevada, Reno, 1664 N. Virginia St., MS0170, Reno, NV 89557, USA *

Corresponding author. E-mail address: [email protected] (J. Lynam)

Abstract As a green, low-pressure pretreatment for biomass, ionic liquids (ILs) have great potential, but their high cost has limited commercial application to date. Glycerol, a safe by-product of the growing biodiesel industry, was investigated as an IL cosolvent. In this study, the ILs 1-ethyl-3methylimidazolium formate ([C2mim][O2CH]) and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) were diluted with up to 75% glycerol and used to pretreat corn stover (CS) at 145 °C and ambient pressure. The pretreated CS residues were enzymatically hydrolyzed, yielding over 90% of the cellulose in the raw CS as glucose and 80% of the xylan as xylose. These mixture pretreatments gave yields 40% higher than the pure IL pretreatments. Fourier transform infrared spectroscopy indicated that lignin had been removed by the glycerol-IL pretreatments. Compared to the ILs used, glycerol’s viscosity was an order of magnitude higher at ambient temperature, but at the processing temperature used the viscosities of the ILs and their mixtures with glycerol were quite similar. An explanation for enhanced pretreatment may relate to the positive excess molar volume of the mixtures. Keywords Biomass, Lignin, FTIR, TGA, DSC, Corn stover, Density, Pretreatment

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Introduction Biomass has great potential to reduce the world’s dependence on fossil fuels. In recognition of the need to reduce carbon pollution, the U.S. Environmental Protection Agency (EPA) has set cellulosic biofuel requirements under the Renewable Fuel Standard (RFS) program in 2016 of 230 million gallons (870.6 million L), seven times more than was produced in the U.S. in 2014. To achieve such cellulosic biofuel production, many types of lignocellulosic (non-food) biomass will need to be converted to biofuel. In addition to biofuels, lignocellulosic biomass will be required as a renewable and sustainable resource for chemical and polymer production. While biomass for conversion can be grown as primary crops, such as poplar or switchgrass, using biomass byproducts from food production is intuitively a better use of resources. Corn stover (CS) shows great promise as a feedstock for cellulosic ethanol production or chemical production in the United States because of its low cost, concentrated distribution, and availability of 31 billion kg per year without soil depletion.1 Prior to its conversion, however, CS requires pretreatment due to the recalcitrance of its lignocellulosic structure. Traditional pretreatment methods include acids, bases, ammonia, or volatile organic compounds. 2-4 A relatively new method to deconstruct lignocellulosic biomass utilizes ionic liquids (ILs), salts that are molten below 100 °C.4-6 Their large cations cause poor coordination between cation and anion, thus lowering their melting points. As salts, they have extremely low vapor pressure, a property that allows for recycling and makes them “green” for biomass pretreatment. Some ILs used in pretreatment have been found to be environmentally friendly, such as 1-ethyl-3methylimidazolium acetate ([C2mim][OAc]), which has a low toxicity (LD50 > 2000 mg kg−1).7 Pretreatment in [C2mim][OAc] has been reported to increase glucose yield after enzymatic hydrolysis.8 If halogens and sulfur are avoided in ILs, then their decomposition products are


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relatively benign.9 Reducing the cost of IL pretreatment by dilution with low-cost and nonhazardous co-solvents would make biomass conversion more feasible, since pretreatment is a large part of the overall conversion cost.10 As a byproduct of the biodiesel industry, glycerol production has increased substantially in the USA in recent years, from 100 million kg/year in 2010 to 600 million kg/year in 2013.11 Crude glycerol has declined in price to near US $0.11/kg.12 This less expensive form of glycerol has been used to pretreat wheat straw before subsequent enzymatic hydrolysis, resulting in improved sugar yields.13 Lynam and Coronella have added glycerol to [C2mim][OAc] and [C2mim][O2CH] to pretreat loblolly pine and rice hulls.9, 14 In those studies, pretreatments that diluted the ILs with 50 % glycerol gave greatly increased glucose and also xylose yields, compared to untreated biomass.9, 14 Refined glycerol can be used in food and so is a safe cosolvent. Its boiling point is 290 °C, meaning that it exhibits low vapor pressure at usual IL pretreatment temperatures of 100 °C – 160 °C. Thus, glycerol could be an appropriate and lower-cost cosolvent for ILs used for corn stover pretreatment. If dilution of an IL with glycerol does not have a negative effect on pretreatment effectiveness of CS, then the reduced use of IL would lead to a more feasible process. This work investigated the effect of IL-glycerol solvents on CS deconstruction. In addition, the properties of these mixtures were studied to determine the underlying basis for their behavior.

Experimental Materials. CS was acquired from Idaho National Lab (Idaho Falls, ID, USA). As described in the Supporting Information (Table S1), the CS contains 30.7% cellulose, 9.8% lignin, 27.2% hemicellulose, and 5.1% ash, with the balance comprised of extractives.15 CS was meshed to obtain the fraction of 0.297-0.595 mm in diameter, then rinsed with 11x deionized (DI) water at 3 ACS Paragon Plus Environment


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approximately 55 °C for 1 h, and separated by vacuum filtration. Rinsed particles were dried at 105 °C for 24 h. Weighing of CS and pretreated CS was performed only after drying at 105 °C so that all mass yields are on a dry basis. The ILs 1-ethyl-3-methylimidazolium acetate >95% ([C2mim][OAc]) and 1-ethyl-3methylimidazolium formate > 97% ([C2mim][O2CH]) were purchased from IoLiTec, Inc. (http://www.iolitec-usa.com). Cellulase (powder) from Trichoderma reesei ATCC 26921, hemicellulase (powder) from Aspergillus niger, cellobiase (liquid) from Aspergillus niger, and glycerol (Reagentplus, >= 99.0%), were purchased from Sigma-Aldrich Corp. (St.Louis, MO, USA). Sodium citrate dehydrate, ACS, 99.0% min, and sodium azide, 99% min, were purchased from Alfa Aesar (Ward Hill, MA, USA). Nylon 66 membranes of pore size 20 µm were purchased from Sigma Aldrich. Methods. IL pretreatment. Each IL-glycerol mixture was dried for 24 h at 105 °C prior to use. A mass of 0.75 g of dried, raw CS was added to 10 g of the IL-glycerol mixture, which was then heated in a muffle furnace at 140-145 °C for three hours. Vacuum filtering at 100 °C with a 0.3 mm nylon filter separated solid biomass residue from the IL-glycerol mixture. After each pretreatment experiment, approximately 50% of the IL-glycerol and pure glycerol solvents were recovered using this method. Only small amounts (≤10%) of the pure IL solvents could be recovered using this method. The gel-like structure of the CS pretreated with pure IL tends to trap IL so that filtration without rinsing removes little IL.16-17 The pretreated CS was put into a vial which was placed in an orbital shaker (SC20XR, Torrey Pines Scientific, Inc., Carlsbad, CA, USA) with 15 ml deionized (DI) water and shaken at 200 RPM at 50 °C for 24 h. The water was decanted and the water-washing process was repeated for another 24 h at 50 °C to ensure that the IL was removed. Vacuum filtering with a 20 µm nylon filter separated the rinsed, pretreated CS from the 4 ACS Paragon Plus Environment

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water. The pretreated CS was next placed in a drying oven set at 105 °C for 24 h prior to weighing. All pretreatments were performed in duplicate, except for glycerol pretreatment, which was performed 4 times, and averages are reported. Enzymatic hydrolysis. After 24 h of drying at 105 °C, 0.1 g samples of raw and pretreated CS were placed in a 22 mL vial with 5 ml of pH 5.05 sodium citrate buffer where 100 ul of a 2% sodium azide solution was added and deionized water was added to give a volume of 10 mL total, as described in NREL’s Enzymatic Saccharification of Lignocellulosic Biomass LAP 009 protocol.18 Cellulase was added at a concentration of 5 units per 0.1 g sample, hemicellulase was added at 50 units per 0.1 g sample, and cellobiase at a concentration of 14 units per 0.1 g sample. The cellulase with cellobiase showed a value of 5 FPU/ml. Samples were shaken in an SC20XR orbital shaker at 200 RPM at 50 °C. Aliquots were taken for analysis at 8 h, 18 h, 24 h, 48 h, and 72 h and filtered through a 0.45 µm syringe filter, and then stored at 4 °C prior to analysis. Glucose yield is defined as the fraction of cellulose available in the biomass that is recovered as glucose, while xylose yield is defined as the fraction of xylan. Anhydrous factors of 0.9 and 0.88 were used for glucose and xylose, respectively. The formulas used for glucose and xylan yield are: %

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0.9 ∗ 10 ℎ ∗ ( ℎ ) ∗ ! "# ∗

% $

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0.88 ∗ 10 ℎ ∗ $ ( ℎ ) ∗ ! $ "# ∗

HPLC analysis. The HPLC system (SHIMADZU, CA, USA) consisted of a system controller (SCL-10A), an auto-injector (SIL-10AD),a liquid pump (LC-10AD), a column oven (CTO-10A) 5 ACS Paragon Plus Environment


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and a refractive index detector (RID-6A). Aliquots were diluted 12:1 with nanopure DI water and 15 µL injected into an Aminex 87-H column from Bio-Rad, with 5 mM H2SO4 as the mobile phase, at 0.7 mL/min flow, at a column temperature of 55 °C. The column was calibrated for glucose and xylose. The UV-VIS detector was utilized at 208 nm and 290 nm for quantitative analysis,. Standard errors for glucose yields of raw CS and glycerol-pretreated CS were less than 10% for all aliquots. For xylose yields, standard errors for raw CS and glycerol-pretreated CS were less than 5% for all aliquots. Fourier transform infrared spectroscopy (FTIR). A Nicolet 6700 FTIR-ATR with a SmartiTR diamond ATR (Themo Scientific, Waltham, MA, USA) using 32 scans for each sample at 2 cm-1 was used from 4000 to 600 cm-1. FTIR analysis was performed on both raw and pretreated samples. Spectra were recorded in triplicate and averaged. Peak intensities were measured from the plateaus at lower wavenumbers. Bomb Calorimetry. Heats of combustion (HHV) were recorded for raw and pretreated solid in a Parr 1241 adiabatic oxygen bomb calorimeter fitted with continuous temperature recording. Samples (~0.1 g) were dried at 105 °C for 1 h prior to analysis. The standard error was ± 0.5 MJ/kg.. Density and excess molar volume measurements (VmE ). Corning Pyrex borosilicate glass Gay-Lussac specific gravity bottles of 2 cm3 capacity were used to measure density. The bottles were calibrated using nanopure DI water to determine actual volume within a standard uncertainty of 0.3%. Bottles with ILs, glycerol, or their mixtures were placed in an oven at 140 °C for twenty minutes to equilibrate. Temperature control was ± 3 °C The filled bottles with the fluid exactly to the top were weighed on top of insulating Styrofoam to prevent scale inaccuracies due to heat conduction to the scale mechanism. Densities were measured at least


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three times and standard errors in density were found to be

Corn Stover Pretreatment by Ionic Liquid and Glycerol Mixtures with

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