Accepted Manuscript Facile Pulping of Lignocellulosic Biomass Using Choline Acetate Fangchao Cheng, Hui Wang, Gregory Chatel, Gabriela Gurau, Robin D. Rogers PII: DOI: Reference:

S0960-8524(14)00675-0 http://dx.doi.org/10.1016/j.biortech.2014.05.016 BITE 13424

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

8 March 2014 4 May 2014 5 May 2014

Please cite this article as: Cheng, F., Wang, H., Chatel, G., Gurau, G., Rogers, R.D., Facile Pulping of Lignocellulosic Biomass Using Choline Acetate, Bioresource Technology (2014), doi: http://dx.doi.org/10.1016/j.biortech. 2014.05.016

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Facile Pulping of Lignocellulosic Biomass Using Choline Acetate

Fangchao Cheng,a,b Hui Wang,a Gregory Chatel,a,† Gabriela Gurau,a and Robin D. Rogersa,*

a

Center for Green Manufacturing and Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487, USA

b

Key Laboratory of Bio-based Material Science and Technology of Ministry of Education of China, College of Material Science and

Engineering, Northeast Forestry University, Harbin, 150040, China

Abstract Treating ground bagasse or Southern yellow pine in the biodegradable ionic liquid (IL), choline acetate ([Cho][OAc]), at 100 °C for 24 h led to dissolution of hemicellulose and lignin, while leaving the cellulose pulp undissolved, with a 54.3% (bagasse) or 34.3% (pine) reduction in lignin content. The IL solution of the dissolved biopolymers can be separated from the undissolved particles either by addition of water (20 wt% of IL) followed by filtration or by centrifugation. Hemicellulose (19.0 wt% of original bagasse, 10.2 wt% of original pine, containing 14-18 wt% lignin) and lignin (5.0 wt% of original bagasse, 6.0 wt% of original pine) could be subsequently precipitated. The pulp obtained from [Cho][OAc] treatment can be rapidly dissolved in 1-ethyl-3-methylimidazolium acetate (e.g., 17 h for raw bagasse vs. 7 h for pulp), and precipitated as cellulose-rich material (CRM) with a lower lignin content (e.g., 23.6% for raw bagasse vs. 10.6% for CRM). Keywords: biomass; pulping; biopolymer recovery; ionic liquids *

Corresponding author, Tel.: +1 205-348-4323; Email: [email protected] † Current Address: Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), CNRS/Université de Poitiers ENSIP,B1, 1 rue Marcel Doré, 86073 Poitiers Cedex 9, France

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1. Introduction Pulping processes are still some of the most prevalent methods to treat abundant and renewable lignocellulosic resources to obtain cellulose for the production of paper, fiber, membranes, and other commodity materials and chemicals (Pilate et al., 2002). Among the different pulping processes currently employed in the paper industry, kraft pulping is the most dominant chemical process (FAO, 2013). During this process, lignin is removed from cellulose and hemicellulose by chemical degradation with sodium hydroxide and sodium sulfide solutions at high temperatures and pressures. Despite its popularity and efficiency, kraft pulping involves the use of a variety of toxic and hazardous chemicals (e.g., sodium sulfide) and generates large amounts of air and water pollutants, including sulfur dioxide, hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide (Bajpai, 2011). In addition, significant degradation of the biopolymers during the extraction process (e.g., lignin and hemicellulose can degrade into molecules with low molecular weights) is a major obstacle for the development of a viable technology to efficiently separate them (Agbor et al., 2011). Among the alternative pulping processes which have been studied that degrade lignin and hemicellulose but leave the cellulose pulp undissolved (e.g., organosolv (Johansson et al., 1987), sulfite (Wang et al., 2009), soda (Zhao et al., 2002)), the discovery of efficient dissolution of cellulose (Swatloski et al., 2002) by some hydrophilic ionic liquids (ILs, generally defined as salts that are liquid below 100 °C (Rogers & Seddon, 2003)) has attracted the attention of scientists in using ILs as a key element of the Biorefinery (Sun et al., 2011). Many reports have focused on the feasibility of using ILs, such as 1-butyl-3-

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methylimidazolium chloride ([C4 mim]Cl) and 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]), to dissolve, separate, and recover cellulose, hemicellulose, and lignin from lignocellulosic biomass (Fort et al., 2007; Li et al., 2011; Sun et al., 2009; Wang et al., 2012). A key feature of the IL process compared with traditional pulping methods is that the ILs dissolve all of the lignocellulosic biopolymers, while pulping processes, such as kraft pulping, rely on destruction and dissolution of lignin and hemicellulose while leaving as much cellulose intact and undissolved as possible (Brandt et al., 2013). Current limitations of IL-based processing of lignocelluloses include high cost of the ILs, the energy needed to recycle the IL, low biodegradability of the ILs, potential derivatization of biopolymers, and incomplete separation of cellulose and lignin, among others (Sun et al., 2011; Tadesse & Luque, 2011; Ebner et al., 2008). In this context, attention was turned to development of biomass pretreatment processes using lower-cost, environmentally more benign, and renewable material-based ILs under milder conditions. Choline, an essential nutrient, is non-toxic and biodegradable (Blusztajn, 1998), and choline-based ILs typically exhibit low toxicity, excellent biodegradability (Petkovic et al., 2010), as well as lower-cost synthesis (Plaquevent et al., 2008). Cholinebased ILs have also shown potential in the delignification of bamboo and as solvents for the production of 5-hydroxymethylfurfural from polysaccharides and biomass feedstocks (Muhammad et al., 2011; Tong et al., 2010). Choline acetate ([Cho][OAc]) is easy to synthesize, with low viscosity at higher temperatures. It is also much more biodegradable than dialkylimidazolium-based ILs. Petkovic et al. (2010) reported aqueous [Cho][OAc] solutions can be degraded up to 52% in P. corylophilum cultures after 14 days, in comparison to negligible biodegradation (0–

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1% after 28 days) of imidazolium-based ILs (Gathergood et al. 2006). Zhang et al. (2012) reported [Cho][OAc] itself does not dissolve cellulose, but when combined with certain quaternary ammonium-chlorides the mixtures can dissolve and decrystallize microcrystalline cellulose (MCC) with good rates (2–6 wt% of MCC within 5–10 min at 100 °C). For raw biomass, [Cho][OAc] can extract suberin (a complex cross-linked polymer composed of aromatic and aliphatic domains) from cork with a 39.7% extraction yield after 4 h processing at 100 °C (Ferreira et al., 2012). [Cho][OAc] was also used to pretreat bamboo powder to enhance its subsequent enzymatic saccharification under ultrasound irradiation (Ninomiya et al., 2013). The cellulose saccharification ratio of bamboo powder was 55% under conventional heating (100 °C, 1 h), and increased to 92% under ultrasound irradiation (25 °C, 1 h). Here, the crystallinity of the cellulose was lower after ultrasound assisted pretreatment and no degradation of the cellulose was reported. The present work demonstrates the ability of [Cho][OAc] to pulp lignocellulosic biomass by dissolving hemicellulose and lignin, which can be subsequently separated and recovered from the IL solution by adding anti-solvents. This work also reports the influence of different methods for separation of the biopolymers on the pulp yield, lignin content in the pulp, biopolymer yields, composition, total loss, and characterization of the recovered biopolymers by 13C NMR, FT-IR, and powder X-ray diffraction. In addition, the recovered pulp was further treated with [C2mim][OAc], an IL widely used to completely dissolve biomass, to determine if pretreatment with [Cho][OAc] would improve the dissolution, separation, and recovery of the cellulose.

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2. Materials and Methods 2.1. Materials Microcrystalline cellulose (MCC) with degree of polymerization 270 and xylan (from beechwood) was purchased from Sigma-Aldrich (St. Louis, MO). Indulin AT (lignin from the kraft pulping process) was provided by MeadWestvaco Corporation (Glen Allen, VA). Deuterated DMSO (DMSO-d6) was purchased from Cambridge Isotope Laboratories, Inc. (Andover, MA). Deionized (DI) water was obtained from a commercial deionizer (Culligan, Northbrook, IL) with specific resistivity of 17.38 MΩ cm at 25 °C. [C2mim][OAc] (95%) was purchased from Iolitec USA (Ionic Liquids Technologies Inc., Tuscaloosa, AL). Choline acetate with a melting point of 63.4 °C was synthesized following the reported (Zhang et al., 2012) procedure. All other solvents and reagents were obtained from SigmaAldrich (St. Louis, MO) and used as received. Bagasse was donated by the Sugar Cane Growers Cooperative of Florida (Belle Glade, FL). Southern yellow pine was received from Seaman Timber Co. (Montevallo, AL) as shavings. The biomass samples were ground into powder using a lab mill (Janke & Kunkel Ika Labortechnik, Wilmington, NC), separated using brass sieves (Ika Labortechnik, Wilmington, NC) to particles having diameters of less than 0.25 mm (bagasse) and 0.125 mm (pine), and then dried overnight in an oven (Precision Econotherm Laboratory Oven, Natick, MA) at 90 °C. The pine sample was Soxhlet-extracted with a toluene/ethanol mixture (2:1, v/v) for 8 h as described in literature (Milne et al., 1992).

2.2. Determination of the solubilities of biopolymers

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The solubilities of three model compounds, microcrystalline cellulose (MCC), xylan (model compound for hemicellulose), and kraft lignin (Indulin AT), in [Cho][OAc] and in a 5:1 w/w [Cho][OAc]/H2O mixture were determined. Typically, 0.01 g model compound (0.005 g for MCC) was added to 1.0 g IL or IL/H2O at 100 °C (for [Cho][OAc]) or 90 °C (for [Cho][OAc]/H2O) with stirring, and visually checked for any undissolved particles. If the solution was homogeneous, 0.01 g more sample (0.005 g for MCC) was added. If the biopolymer/IL solution remained heterogeneous after 24 h, the solubility was reported as ‘> x wt%’ (x wt% was calculated based on the amount of added biopolymer just before the solution became heterogeneous). 2.3. Biomass dissolution and heating methods For each trial, 0.5 g biomass powder was mixed with 10.0 g [Cho][OAc] and heated for specific time periods with different heating methods (H-1, H-2, and H-3) as shown in Table 1. Microwave irradiation was conducted in a domestic microwave oven (SHARP Carousel R-209KK, Mahwah, NJ) at full power (800 W). (Caution: Care must be taken when using microwave heating because ILs are efficient microwave absorbers and heating occurs rapidly which can easily lead to degradation of the ILs and biopolymers, or even explosions of the system.) 2.4. Precipitation and separation of the biopolymers Treatment of the biomass with choline acetate at 100 °C for 24 h afforded a suspension (Mixture A) that contained undissolved pulp and dissolved hemicellulose and lignin. Two different methods were used for separation of the pulp from the dissolved biopolymers as presented in Table 2. Hemicellulose was recovered from the IL solution by adding ethanol

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(95%, v/v) followed by filtration, and then water was added to the filtrate to precipitate lignin. In the recovery of hemicellulose and lignin, ultrasound irradiation was used to facilitate the separation of lignin and hemicellulose. Separation details are described in Table 2. The abbreviations used in the mass balance analyses of biomass are summarized in Table 3. Pulp yield (PY), hemicellulose yield (HY), and lignin yield (LY) were calculated as the mass percentage of the recovered material to the mass of original biomass (%B). Biopolymers yield (BY) was calculated by the sum of the hemicellulose yield and lignin yield. LR represents the reduction of the lignin in the pulp compared to the lignin in the original biomass as calculated in eq. 1:

LR (%) =

m B × LB − m P × LCP × 100% mB × LB

(1)

where mB and mP stand for the mass of original biomass and pulp, and LB and LCP are the lignin contents in the original biomass and in the pulp, respectively. All of the biopolymers hydrolyzed in the process and all the lost biopolymer mass in the washing and separation steps were combined and designated by the term “biopolymer mass lost” (BML), which was calculated according to eq. 2.

BML(%B)= (1- PY - BY)×100%

(2)

The carbohydrate (cellulose and hemicellulose) mass lost, noted CML (from carbohydrate in the original biomass, %C) and lignin mass lost, LML (from lignin in the original biomass, %L) were calculated according to eqs. 3 and 4:

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CML (%C) =

m B × (1 − LB) − m P × (1 − LCP) − m H × (1 - LCH ) × 100% m B × (1 − LB)

LML (%L) =

m B × LB − m P × LCP − m H × LCH − m L × 100% m B × LB

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(3)

(4)

where mB, mP, mH, and mL represent the mass of the original biomass, pulp, extracted hemicellulose, and extracted lignin, respectively. 2.5. Recycling of [Cho][OAc]

The filtrate was collected after the lignin was separated, and the IL was recycled from the filtrate by evaporating the solvents under vacuum (rotary evaporator, Büchi Rotary R210, Flawil, Switzerland). The resulting mixture was still in the liquid state due to the presence of some remaining water, and was filtered and then dried under a Schlenk line for 10 h at 85 °C to obtain solid [Cho][OAc] which was dark brown. The recycled IL was characterized by 1H NMR by dissolving in DMSO-d6. The recycled IL was also used to process bagasse and pine with H-2 and S-1 methods as shown in Tables 1 and 2. 2.6. Further processing of the [Cho][OAc] treated pulp

The recovered pulp was further treated with the IL [C2mim][OAc]. The time required for complete dissolution of 0.25 g original biomass (bagasse or pine) or the corresponding [Cho][OAc] treated pulp in 5 g [C2mim][OAc] were determined to compare the tractability of the biomass and pulp following the report of Sun et al. (2009). Briefly, a drop of the mixture was taken out at the end of the specified dissolution time and placed under a microscope (Reichert Stereo Star Zoom 580, Depew, NY) at 25 °C to detect if there were any undissolved particles.

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Precipitation and separation of biomass and cellulose-rich material (CRM) were also carried out according to the previous report (Sun et al., 2009). Instead of using the biomass/[C2mim][OAc] solutions mentioned above (obtained with different dissolution time), here, a series of biomass/[C2mim][OAc] solutions were prepared under the same reaction conditions: 0.25 g original biomass (bagasse or pine) or the corresponding [Cho][OAc] processed pulp and 5 g [C2mim][OAc] were heated at 110 °C for 16 h. CRM was precipitated by adding 100 mL of acetone/water (1:1, v/v), and separated by filtration. The obtained CRM was washed three times with acetone/water to ensure that all carbohydrate-free lignin was washed out. Lignin was recovered from the IL solution by evaporating the acetone, and separated by filtration, leaving the IL in the remaining solution. Both the lignin and CRM were dried overnight in the oven at 75 °C. 2.7. Characterization

The original biomass, pulp, xylan, Indulin AT, and recovered hemicellulose and lignin were characterized by Fourier transform infrared spectroscopy (FT-IR) using a Bruker Alpha FT-IR instrument, Bruker Optics Inc. (Billerica, MA) with 10 scans at 2 cm-1 resolution. Xylan, Indulin AT, and recovered hemicellulose and lignin were also analyzed by 13C NMR using a Bruker AVANCE 500 MHz NMR spectrometer (Karlsruhe, Germany). About 5 wt% biopolymer was heated in DMSO-d6 at 90 °C for 15 min to prepare the NMR samples. A total of 10,000 scans were collected for 13C NMR at 125.76 MHz to get better resolved spectra. 1H NMR of fresh and recovered [Cho][OAc] were collected at 500 MHz in DMSO-d6 to compare the IL before and after the dissolution process.

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The lignin content of the original or pretreated biomass, pulp, and recovered hemicellulose were determined by TAPPI methods (TAPPI, 1991 and 1998) with a scaled down process as described in the previous report (Sun et al., 2009). Briefly, 0.1 g of the material was first hydrolyzed using 2 mL 72 wt% H2SO4, and then the acid was diluted to 3% and refluxed for 4 h. The acid insoluble lignin was measured gravimetrically (Sartorius AC 210 P balance, Bohemia, NY) after drying in the oven. The acid soluble lignin was determined by UV spectroscopy using a Lambda XLS absorption spectrophotometer (Perkin Elmer Ltd, Beaconsfield, UK) at 205 nm. The melting point of [Cho][OAc] was determined under nitrogen by differential scanning calorimetry (DSC) using a DSC2920 Modulated DSC, TA Instruments, Inc. (New Castle, DE) cooled with a liquid nitrogen cryostat. The calorimeter was calibrated for temperature and cell constants using indium (Tm = 156.61 °C; C = 28.71 J g−1). 8.4 mg [Cho][OAc] in an aluminum pan was initially cooled a rate of 5 °C min−1 from 25 °C to 75 °C followed by a 10 min isotherm at −75 °C, and heated at a rate of 5 °C min−1 to 100 °C followed by a 10 min isotherm. The cycle was repeated twice, and the third heating run was used for data collection.

3. Results and Discussion 3.1.

Solubility of biopolymer models in [Cho][OAc] and [Cho][OAc]/H2O

The solubility study of MCC, xylan, and kraft lignin Indulin AT, in [Cho][OAc] showed that [Cho][OAc] can dissolve xylan and Indulin AT to more than 13 wt% and 20 wt% at 100 °C, respectively. In the 20 parts water to 100 parts [Cho][OAc] (w/w) mixture, the

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solubility of xylan was essentially the same (>13 wt%), although the temperature used to dissolve the xylan in the [Cho][OAc]/H2O mixture was lower. The solubility of Indulin AT, however, decreased sharply from > 20 wt% to > 9 wt%. In either [Cho][OAc] or [Cho][OAc]/H2O, the solubility of cellulose was quite limited (< 0.5 wt%), which is in accordance with the literature (Zhang et al., 2012). These results suggested that [Cho][OAc] could be used to selectively dissolve and separate hemicellulose and lignin from lignocellulosic biomass to produce a cellulose-rich pulp. 3.2. Pulping of bagasse and Southern yellow pine using [Cho][OAc]

Bagasse with an initial lignin content of 23.6 wt% was selected to investigate first because of its more porous structure compared to the tight, compact structure of wood, which makes bagasse readily processable in ILs (Li et al., 2011), than Southern yellow pine, a softwood, which is usually considered to be the most difficult to process (Brandt et al., 2013). To ensure the thermal stability of the IL during the process, a heating temperature of 100 °C (far below the 169 °C degradation temperature of [Cho][OAc]) using an oil bath was chosen (Petkovic et al., 2010). Additionally, microwave irradiation has been shown to be able to facilitate the dissolution and delignification of woody biomass in ILs (Wang et al., 2013), and microwave irradiation was also selected as a comparative heating method. Fig. 1 (see also Table 1) represents the three methods for heating, including adding

solid [Cho][OAc] and bagasse followed by heating in an oil bath at 100 °C for 24 h (H-1); melting the [Cho][OAc] then adding the bagasse followed by heating in an oil bath at 100 °C for 24 h (H-2); and melting the [Cho][OAc], adding the bagasse, and heating using a domestic microwave with 60 x 2 s pulses at full power (H-3). In each case the resulting

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dark, viscous mixture (designated as Mixture A) in Fig. 1 contained a biopolymer/IL solution and undissolved pulp. Dissolution of lignocellulosic biomass in ILs designed to dissolve all of the biopolymers (e.g., [C2mim][OAc]) requires a multistep precipitation and separation process (Sun et al., 2009). The carbohydrate-rich materials (CRM) are precipitated with the addition of antisolvent (e.g., acetone/H2O, 1:1, v/v) to the IL biomass solution. After the separation of the CRM, lignin dissolved in the solution is recovered by evaporating part of the antisolvent. Because it is almost impossible to separate the dissolved cellulose and hemicellulose with an antisolvent, the dissolution of cellulose in the ILs makes it difficult to separate hemicellulose from biomass in this process. Since the solubility studies and the literature indicated cellulose would not be dissolved by [Cho][OAc], it should be possible here, to separate and precipitate the hemicellulose. Since the mixture of [Cho][OAc] and water (5:1, w/w) has the ability to dissolve hemicellulose (> 13 wt%) and lignin (> 9 wt%), water (20 wt% of the IL) was added to Mixture A to facilitate the separation of the undissolved pulp, followed by filtration (Table 2, Fig. 2, S-1). The undissolved pulp was also tried to be separated from the solution by

repeatedly heating and centrifuging Mixture A directly (S-2) since the addition of water decreases the solubility of lignin in [Cho][OAc]. Hemicellulose was precipitated from the IL solution by the addition of ethanol, which is an effective antisolvent for hemicellulose and which is totally miscible with [Cho][OAc]. The hemicellulose particles were recovered by filtration and the lignin was then recovered by evaporating the ethanol and adding water to the IL solution, followed by filtration. The recovered pulp, hemicellulose, and lignin were dried overnight in an oven. Fig. 2 provides a flowchart for processing bagasse using

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the different separations methods explored in this work. The pulp in the Mixtures A using different heating methods (H-1, H-2, and H-3) were separated using Separation Method S-1, which was shown to be the most efficient method. The amounts of biopolymers dissolved in the IL using H-1 and H-2 were almost the same (37.4% of the original bagasse for H-1 vs. 36.2% for H-2). However, when the H-3 method was used, only 12.8% of the original bagasse could be dissolved in the IL. The yields of the recovered biopolymers from the IL solution when using the Heating Method H-1 (Trial 1 in Table 4) were similar to those using the Heating Method H-2 (Trial 2 in Table 4). However,

the lignin content in the recovered pulp was lower using the H-2 method with premelting of the IL compared to that with no premelting (H-1; 16.9 wt% vs. 18.6 wt%, Table 4 and Fig. 2, Trials 1 and 2), and the process with premelting also led to a lower loss rate (12.2 wt% vs.

13.0 wt%, Table 4, Trials 1 and 2). The results (Table 4, Fig. 2) indicated that microwave heating (H-3) led to higher pulp yield, but also higher lignin content (20.6 wt%) compared with conventional heating (18.6 wt% for the H-1 method and 16.9 wt% for the H-2 method) under the tested conditions. Based on these results, the H-2 method was chosen for all subsequent trials (Table 4). Comparison of the two separation methods (Table 4, Fig. 3) indicated that the recovered biopolymer yields obtained using these separation methods were almost the same (hemicellulose: 24.0 wt% for S-1, Trial 2 vs. 22.2 wt% for S-2, Trial 5; lignin: 5.0 wt% for S-1, Trial 2 vs. 4.2 wt% for S-2, Trial 5). However, compared to the centrifugation method

(S-2), method S-1 with water addition led to lower lignin loss (18.6 wt% vs. 32.2 wt%). Slightly higher lignin contents in the pulp (16.9 wt% vs. 14.0 wt%) and in the hemicellulose (18.3 wt% vs. 14.0 wt%) were obtained with the S-1 method compared to the

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S-2 method because of the precipitation of lignin with the addition of such small amounts

of water. The separation of the gel residue from the [Cho][OAc] layer by centrifugation (Fig. 2) in method S-2 led to a loss of lignin and hemicellulose in the processing. The IL solution (with the dissolved biopolymers) was unavoidably left on the residue particles, and only ca. 40 wt% of the original IL solution could be recovered. Moreover, filtration with the addition of water in S-1 is technically easier than the repeated melting and centrifugation in S-2. Separation Method S-1 is therefore more practical and efficient for the separation of

hemicellulose and lignin biopolymers from bagasse/IL solution compared with S-2. Low frequency ultrasound irradiation has been reported to help break the linkages between hemicellulose and lignin without changes of lignin composition and structure as a result of the cavitation (Sun & Tomkinson, 2002). To understand if sonication could facilitate the separation of the lignin and hemicellulose in the present study, sonication was employed after adding ethanol to the IL/biopolymers solution. The EtOH/IL/biopolymers solution (Fig. 3, S-2, C2) was sonicated for 15 min, and the recovered hemicellulose had lower lignin content (13.2 wt% vs. 18.0 wt%; Table 4, Trials 5 and 6), and the lignin yield was correspondingly increased (7.8 wt% vs. 4.2 wt%). Ultrasonic treatment should thus be considered to improve this separation and the recovery of more pure biopolymer fractions. It should be noted that the ultrasonic treatment was not applied in the [Cho][OAc] processing of pine, because the amount of recovered hemicellulose was not sufficient to measure the lignin content. The optimized heating (H-2) and separation (S-1) methods were also used to treat Southern yellow pine. Table 4 and Fig. 3 summarize the results obtained for bagasse and

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pine. The less efficient dissolution of pine in [Cho][OAc] led to higher pulp yield than bagasse, but with lower delignification (34.3 wt% for Trial 4 vs. 54.3 wt% for Trial 2, Fig. 4), which is attributed to the loose structure of bagasse compared to pine and the improved

accessibility of bagasse and increased penetration and diffusion of [Cho][OAc] into its interior (Lan et al., 2011). The separation process for pine led to lower recovered biopolymers yield compared to bagasse (16.2 wt% for Trial 4 vs. 24.0 wt% for Trial 2) and the isolated lignin yield was only 6.0 wt%. 3.3. Characterization of the recovered biopolymers

The biopolymers obtained in the present studies were characterized by 13C NMR and FTIR. These spectra indicated that the extracted hemicellulose mainly contained xylan with minor amounts of lignin, consistent with the results of the lignin content determination. The results also indicated that the structure of the extracted lignin is very similar to that of milled wood lignin, indicating that the original structure of lignin was preserved in the [Cho][OAc] processing. Powder X-ray diffractograms of the original bagasse and pulp are also similar, indicating that the processing of bagasse with [Cho][OAc] has limited influence on the crystal structure of cellulose. 3.4.

Recycling of [Cho][OAc]

After filtration of the biopolymers from the IL solution, [Cho][OAc] was recycled by evaporating the solvents and drying under high vacuum with recovery yields of ca. 95 wt%. The recycled IL was darker in color compared to the fresh IL. Analysis of the original and recycled IL by 1H NMR indicated no apparent differences in the spectra. The recycled [Cho][OAc] is still efficient in the pulping of bagasse and pine (Trials 7-9, Table 4, Fig. 4).

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3.5.

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Further processing of the pulp using [C2mim][OAc]

The pulp obtained by [Cho][OAc] processing with H-2 and S-1 methods was dissolved in [C2mim][OAc], an IL widely used in biomass processing to dissolve cellulose (Wang et al., 2012), at 110 °C to determine its tractability and compared to the dissolution of unprocessed biomass under the same conditions. The results indicated that the time to completely dissolve the pulp in [C2mim][OAc] was remarkably reduced (by 40-60%) after [Cho][OAc] processing (bagasse: 17 h for original vs. 7 h for pulp; pine: 45 h for original vs. 27 h for pulp). Precipitation and separation of the biomass and [Cho][OAc] processed pulp were also performed after their dissolution in [C2mim][OAc]. The results are summarized in Table 5. Much higher CRM yields can be achieved with the [Cho][OAc] processed pulp (81.289.5%, Trials 11 and 13) than with the original biomass (65.9-66.7%, Trials 10 and 12), consistent with the reduced lignin and hemicellulose contents of the processed pulp. In addition, the final lignin content in the CRM was lower when using the [Cho][OAc] processed pulp (10.6-24.8% for Trials 11 and 13 vs. 14.5-27.5% for Trials 10 and 12) with lower lost mass (6.8-16.5% for Trials 11 and 13 vs. 27.4-29.3% for Trials 10 and 12). As expected, the delignification and lignin yield for the [Cho][OAc] processed pulp (24.244.1% lignin removal, 2.3-3.7% lignin yield, Trials 11 and 13, Fig. 4) were lower than those for the original biomass (40.6-59.0% lignin removal, 4.0-6.7% lignin yield, Trials 10 and 12). Thus, [Cho][OAc] processed pulp can be more easily processed with [C2mim][OAc] than the raw biomass. In this process, lignin can be further removed from the biomass, leaving most of the carbohydrate in the obtained CRM.

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4. Conclusions [Cho][OAc], a nontoxic and biodegradable IL, can partially dissolve bagasse and Southern yellow pine, leaving a cellulose-rich pulp undissolved. The IL solution of dissolved biopolymers can be separated from the undissolved pulp either by addition of water (20 wt% of IL) followed by filtration or by direct centrifugation. The pulp obtained from [Cho][OAc] treatment can be rapidly dissolved in [C2mim][OAc] and precipitated as CRM with a much lower lignin content. Thus, treatment with [Cho][OAc] might be considered as either a pretreatment method before enzyme hydrolysis of biomass or an effective way of biopolymer extraction from biomass.

5. Note Added in Proof While this manuscript was under review, a relevant publication appeared in Green Chemistry (Sun et al., 2014), where [Cho][OAc] was among four ILs investigated for pretreatment of switchgrass leading to enhance enzymatic digestion of sugars. Sun et al. treated switchgrass with [Cho][OAc] in 20 g scale at 90 °C for 5 h or at 140 °C for 1 h, followed by a precipitation via addition of 100 mL DI water. Although the biomass and the process steps used are not directly comparable, for example these authors did not separate dissolved lignin and hemicellulose prior to precipitation, they did report that [Cho][OAc] could partially delignify switchgrass (17% and 50% lignin removal, for the two respective conditions used).

Acknowledgements

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The authors would like to thank 525 Solutions Inc. (U.S. Department of Energy SBIR Award, Grant No. DE-SC0004198) and the China Scholarship Council (No. 201206600006) for financial support.

Appendix A. Supplementary data Supplementary data associated with this article, including synthesis of choline acetate, optimization of microwave heating conditions, 1H NMR of the recycled IL, and characterization of the recovered biopolymers and pulps, can be found in the online version at http://dx.doi.org/10.1016/j.biortech.XXXX.XX.XXX.

References 1. Agbor, V.B., Cicek, N., Sparling, R., Berlin, A., Levin, D.B., 2011. Biomass pretreatment: Fundamentals toward application. Biotechnol. Adv., 29, 675-685. 2. Bajpai, P. 2011. Environmentally Friendly Production of Pulp and Paper. John Wiley & Sons, Hoboken. 3. Blusztajn, J.K., 1998. Choline, a Vital Amine. Science, 281, 794-795. 4. Brandt, A., Grasvik, J., Hallett, J.P., Welton, T., 2013. Deconstruction of lignocellulosic biomass with ionic liquids. Green Chem., 15, 550-583. 5. Ferreira, R., Garcia, H., Sousa, A.F., Petkovic, M., Lamosa, P., Freire, C.S.R., Silvestre, A.J.D., Rebelo, L.P.N., Pereira, C.S., 2012. Suberin isolation from cork using ionic liquids: characterisation of ensuing products. New J. Chem., 36, 2014-2024. 6. Food and Agriculture Organization (FAO) of the United Nations, 2013. FAO Yearbook of Forest Products 2011, Roma.

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7. Fort, D.A., Remsing, R.C., Swatloski, R.P., Moyna, P., Moyna, G., Rogers, R.D., 2007. Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3-methylimidazolium chloride. Green Chem., 9, 63-69. 8. Gathergood, N., Scammells, P.J., Garcia, M.T., 2006. Biodegradable ionic liquids Part III. The first readily biodegradable ionic liquids. Green Chem., 8, 156-160. 9. Johansson, A., Aaltonen, O., Ylinen, P., 1987. Organosolv pulping — methods and pulp properties. Biomass, 13, 45-65. 10. Lan, W., Liu, C.F., Sun, R.C., 2011. Fractionation of bagasse into cellulose, hemicelluloses, and lignin with ionic liquid treatment followed by alkaline extraction. J. Agric. Food. Chem., 59, 8691-8701. 11. Li, W.Y., Sun, N., Stoner, B., Jiang, X.Y., Lu, X.M., Rogers, R.D., 2011. Rapid dissolution of lignocellulosic biomass in ionic liquids using temperatures above the glass transition of lignin. Green Chem., 13, 2038-2047. 12. Milne, T.A., Chum, H.L., Agblevor, F., Johnson, D.K., 1992. Standardized analytical methods. Biomass Bioenerg., 2, 341-366. 13. Muhammad, N., Man, Z., Bustam, M., Mutalib, M.I.A., Wilfred, C., Rafiq, S., 2011. Dissolution and delignification of bamboo biomass ssing amino acid-based ionic liquid. Appl. Biochem. Biotechnol., 165, 998-1009. 14. Ninomiya, K., Ohta, A., Omote, S., Ogino, C., Takahashi, K., Shimizu, N., 2013. Combined use of completely bio-derived cholinium ionic liquids and ultrasound irradiation for the pretreatment of lignocellulosic material to enhance enzymatic saccharification. Chem. Eng. J., 215–216, 811-818.

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15. Petkovic, M., Ferguson, J.L., Gunaratne, H.Q.N., Ferreira, R., Leitao, M.C., Seddon, K.R., Rebelo, L.P.N., Pereira, C.S., 2010. Novel biocompatible cholinium-based ionic liquids-toxicity and biodegradability. Green Chem., 12, 643-649. 16. Pilate, G., Guiney, E., Holt, K., Petit-Conil, M., Lapierre, C., Leple, J.C., Pollet, B., Mila, I., Webster, E.A., Marstorp, H.G., Hopkins, D.W., Jouanin, L., Boerjan, W., Schuch, W., Cornu, D., Halpin, C., 2002. Field and pulping performances of transgenic trees with altered lignification. Nat. Biotech., 20, 607-612. 17. Plaquevent, J.C., Levillain, J., Guillen, F., Malhiac, C., Gaumont, A.C., 2008. Ionic liquids: New targets and media for α-amino acid and peptide chemistry. Chem. Rev., 108(12), 5035-5060. 18. Rogers, R.D., Seddon, K.R., 2003. Ionic liquids--Solvents of the future? Science, 302, 792-793. 19. Sun, N., Rahman, M., Qin, Y., Maxim, M.L., Rodriguez, H., Rogers, R.D., 2009. Complete dissolution and partial delignification of wood in the ionic liquid 1-ethyl-3methylimidazolium acetate. Green Chem., 11, 646-655. 20. Sun, N., Rodriguez, H., Rahman, M., Rogers, R.D., 2011. Where are ionic liquid strategies most suited in the pursuit of chemicals and energy from lignocellulosic biomass? Chem. Commun., 47, 1405-1421. 21. Sun, N., Parthasarathi, R., Socha, A.M., Shi, J., Zhang, S., Stavila, V., Sale, K.L., Simmons, B.A., Singh, S., 2014. Understanding pretreatment efficacy of four cholinium and imidazolium ionic liquids by chemistry and computation. Green Chem., 16, 25462557.

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22. Sun, R., Tomkinson, J., 2002. Comparative study of lignins isolated by alkali and ultrasound-assisted alkali extractions from wheat straw. Ultrason. Sonochem., 9, 85-93. 23. Swatloski, R.P., Spear, S.K., Holbrey, J.D., Rogers, R.D., 2002. Dissolution of cellose with ionic liquids. J. Am. Chem. Soc., 124, 4974-4975. 24. Tadesse, H., Luque, R., 2011. Advances on biomass pretreatment using ionic liquids: An overview. Energ. Environ. Sci., 4, 3913-3929. 25. TAPPI T222om-98, 1998, Acid-insoluble lignin in wood and pulp. 26. TAPPI Useful Methods, UM 250, 1991, Acid-soluble lignin in wood and pulp. 27. Tong, X., Ma, Y., Li, Y., 2010. Biomass into chemicals: Conversion of sugars to furan derivatives by catalytic processes. Appl. Catal. A: Gen., 385, 1-13. 28. Wang, G.S., Pan, X.J., Zhu, J.Y., Gleisner, R., Rockwood, D., 2009. Sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) for robust enzymatic saccharification of hardwoods. Biotechnol. Progr., 25, 1086-1093. 29. Wang, H., Gurau, G., Rogers, R.D., 2012. Ionic liquid processing of cellulose. Chem. Soc. Rev., 41, 1519-1537. 30. Wang, H., Maxim, M.L., Gurau, G., Rogers, R.D., 2013. Microwave-assisted dissolution and delignification of wood in 1-ethyl-3-methylimidazolium acetate. Bioresour. Technol., 136, 739-742. 31. Zhang, Q., Benoit, M., Vigier, K.D.O., Barrault, J., Jérôme, F., 2012. Green and inexpensive choline-derived solvents for cellulose decrystallization. Chem. Eur. J., 18, 1043-1046. 32. Zhao, J., Li, X., Qu, Y., Gao, P., 2002. Xylanase pretreatment leads to enhanced soda pulping of wheat straw. Enzyme Microb. Technol., 30, 734-740.

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Figure captions Fig. 1. Bagasse heating methods using [Cho][OAc] (see also Table 1). The numbers on the

right indicate the percentage of the bagasse dissolved.

Fig. 2. Flowchart for the processing of biomass using [Cho][OAc] and separation of the

biopolymers.

Fig. 3. Mass balance for trials with different heating and separation methods. The numbers

stand for the trial numbers in Tables 4-5.

Fig. 4. Comparison of the pulp yield and the lignin content in the pulp with kraft pulping.

The numbers represent the trial numbers in Tables 4-5.

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Fig. 1.

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Fig. 2.

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Fig. 3.

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Fig. 4.

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Table 1. Heating methods to dissolve the lignocellulosic biomass. Method

Preparationa

H-1

Biomass powder (0.5 g) added to Oil bath and stirring (720 rpm) solid [Cho][OAc]b (10 g) at 23 °C followed by heatingb

24 h, 100 °C

H-2

Melting of [Cho][OAc] (10 g) followed by addition of biomass powder (0.5 g)c

Oil bath and stirring (720 rpm)

24 h, 100 °C

H-3

Melting of [Cho][OAc] (5 g) followed by addition of biomass powder (0.25 g)d

Microwave irradiation (Pmax = 800 W) and vigorous stirring between pulses (manually)d

60 × 2 s pulsese

Heating

Conditions

a

The dark mixture obtained (Mixture A) was composed of an IL phase (with biopolymers dissolved) and undissolved pulp. b Melting point of [Cho][OAc]: 63.4 °C. c Performed in a 20 mL vial. d Performed in a 50 mL beaker. e The mixture was manually stirred for 10–15 s between each two pulses. The temperature was not controlled between pulses.

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Table 2. Methods for separating pulp from the dissolved biopolymers/IL solution.a Method

Step 1: Pulp recovery

Step 2: Hemicellulose recovery

Step 3: Lignin recovery

S-1

Cool the Mixture A to ca. 90 °C, add 20 parts water to 100 parts Mixture A,b and recover the precipitated pulp (B1) by filtrationc

Concentrate the resulting filtrate (C1),d add 100 mL ethanol (95%, v/v), and recover the precipitated hemicellulose (D1) by filtratione

Concentrate the resulting filtrate (E1),d add 100 mL water, and recover the precipitated lignin (F1) by filtrationf

S-2

Repeatedly centrifuge the hot Mixture A three times,g separate and recover the gel residue which was washed and dried to obtain the pulp (B2)c

Combine the wash filtrate and IL layer (C2), concentrate,d add 100 mL ethanol (95%, v/v), and recover the precipitated hemicellulose (D2) by filtratione

Concentrate the filtrate (E2),d add 100 mL water, and recover the precipitated lignin (F2) by filtrationf

S-2US

Centrifuge the hot Mixture A,g separate and recover the gel residue which was washed and dried to obtain the pulp (B2)c

Combine the washing filtrate and IL layer (C2), concentrate,c add 100 mL ethanol (95%, v/v), ultrasonic irradiation,h and recover precipitated hemicellulose (D2US) by filtratione

Concentrate filtrate (E2US) by evaporation of ethanol,d add 100 mL water, and recover precipitated lignin (F2US) by filtrationf

a All the residues and filtrates were designated by a letter (A to F) associated to the number of the method, and were included in Figs. 1 and 2. b 2 g water (only 1 g for H-3 sample) added under stirring at 90 °C for 10 min. cB1/B2 (pulp) were separated by vacuum filtration in a ceramic funnel with 10 µm nylon filter paper, washed with water (3 × 100 mL), and dried in an oven at 90 °C overnight. d Concentrated to about 20 mL under vacuum at 50 °C using a rotary evaporator. eD1/D2 (hemicellulose) were recovered by vacuum filtration in a ceramic funnel with 10 µm nylon filter paper, washed with ethanol (3 × 100 mL), and dried in an oven at 75 °C overnight. f F1/F2 (lignin) were recovered by vacuum filtration with 0.8 µm nylon filter paper, washed with water (3 × 100 mL), and dried in an oven at 75 °C overnight. g The hot Mixture A was directly centrifuged at 100 × g for 15 min, and heated to liquid state (90 °C). After repeating these steps twice, the mixture was centrifuged at 100 × g for 1 min. h The ethanol/IL/biopolymers solution (C2 in ethanol) was sonicated (42 kHz; 135 W) at 25 °C for 15 min.

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Table 3. Abbreviations used in the mass balance analysis Abbreviation CRM PY LCP LR BY HY LCH LY BML CML LML

Description Cellulose-rich material from dissolved cellulose using [C2mim][OAc] Pulp yield (material undissolved when treated with [Cho][OAc]) Lignin content in pulp Lignin removed in the process Biopolymers (hemicellulose and lignin) yield Hemicellulose yield Lignin content in hemicellulose Lignin yield Biopolymer mass lost (all of the biopolymers hydrolyzed in the process and all the lost biopolymers in the washing and separation steps) Carbohydrate (cellulose and hemicellulose) mass lost Lignin mass lost

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Table 4. Mass balance and compositional analysis of the biomass after processing. Pulp Methods and Materials Trial (Dissolution/Separation, PY(%B)a LCP(%)b LR(%)c Biomass Resource)

Recovered Biopolymers

Lost Components

BY(%B)d HY(%B)e LCH(%)f LY(%B)g

BML (%B)

CML h

(%C)

LML i

(%L)j

1

H-1/S-1, bagasse k

62.6

18.6

50.7

24.4

19.4

18.1

5.0

13.0

12.6

14.4

2

H-2/S-1, bagasse k

63.8

16.9

54.3

24.0

19.0

18.3

5.0

12.2

10.2

18.6

n

l

3

H-3/S-1, bagasse

87.2

20.6

23.9

9.6

4.4

-

5.2

3.2

3.6

10.9

4

H-2/S-1, pinem

74.2

27.0

34.3

16.2

10.2

-n

6.0

9.6

7.4

14.7

5

H-2/S-2, bagasse

66.2

14.0

60.7

22.2

18.0

14.0

4.2

11.6

5.2

32.2

6

H-2/S-2US, bagasse

67.8

13.5

61.2

21.0

13.2

11.4

7.8

11.2

8.1

21.2

7

H-2/S-1, bagasse, first recycle H-2/S-1, bagasse, second recycle H-2/S-1, pine, recycle

53.2

14.3

67.8

29.1

21.1

20.6

8.0

17.7

18.4

33.8

56.5

13.7

67.2

28.7

22.5

20.8

6.2

14.8

12.9

40.9

76.3

29.3

26.7

10.3

8.0

-

2.3

13.4

10.9

8 9 a

b

19.2 c

Pulp yield (PY) is the mass percentage of the isolated pulp to the mass of original biomass (%B). Lignin content in pulp (LCP). Lignin removed (LR) is the reduction of the lignin mass in the pulp compared to the lignin mass in the original biomass. dBiopolymers yield (BY) is the mass percentage of the recovered biopolymers (hemicellulose and lignin) to the mass of original biomass (%B). eHemicellulose yield (HY) is the mass percentage of the isolated hemicellulose to the mass of original biomass (%B). f Lignin content in hemicellulose (LCH,%). g Lignin yield (LY, %B) is the mass percentage of the isolated lignin to the mass of original biomass (%B). hBML is the mass percentage of lost components in the process to the mass of original biomass (%B). iCML (%C) is the carbohydrates (cellulose and hemicellulose) mass lost with respect to the carbohydrate in the original biomass. jLML is lignin mass lost with respect to the lignin in the original biomass. kUse of 0.5 g bagasse powder (

Facile pulping of lignocellulosic biomass using choline acetate.

Treating ground bagasse or Southern yellow pine in the biodegradable ionic liquid (IL), choline acetate ([Cho][OAc]), at 100°C for 24h led to dissolut...
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