CHEMSUSCHEM FULL PAPERS DOI: 10.1002/cssc.201300773
Switchable Ionic Liquids as Delignification Solvents for Lignocellulosic Materials Ikenna Anugwom,[a] Valerie Eta,[a] Pasi Virtanen,[a] Pivi Mki-Arvela,[a] Mattias Hedenstrçm,[b] Michael Hummel,[c] Herbert Sixta,[c] and Jyri-Pekka Mikkola*[a, d] The transformation of lignocellulosic materials into potentially valuable resources is compromised by their complicated structure. Consequently, new economical and feasible conversion/ fractionation techniques that render value-added products are intensely investigated. Herein an unorthodox and feasible fractionation method of birch chips (B. pendula) using a switchable ionic liquid (SIL) derived from an alkanol amine (monoethanol amine, MEA) and an organic super base (1,8-diazabicyclo[5.4.0]-undec-7-ene, DBU) with two different trigger acid gases (CO2 and SO2) is studied. After SIL treatment, the dissolved fractions were selectively separated by a step-wise method using an antisolvent to induce precipitation. The SIL was recy-
cled after concentration and evaporation of anti-solvent. The composition of undissolved wood after MEA-SO2-SIL treatment resulted in 80 wt % cellulose, 10 wt % hemicelluloses, and 3 wt % lignin, whereas MEA-CO2-SIL treatment resulted in 66 wt % cellulose, 12 wt % hemicelluloses and 11 wt % lignin. Thus, the MEA-SO2-SIL proved more efficient than the MEACO2-SIL, and a better solvent for lignin removal. All fractions were analyzed by gas chromatography (GC), Fourier transform infrared spectroscopy (FT-IR), 13C nuclear magnetic resonance spectroscopy (NMR) and Gel permeation chromatography (GPC).
Introduction Lignocellulosic materials represent a vast resource for the production of biomolecules and commodity chemicals. However, the utilization of these renewable resources has been compromised by several factors, such as the close associations and interconnections that exists between the three main plant cell wall components: cellulose, lignin, and hemicelluloses. Consequently, the low efficiencies by which lignocellulosic substrates can be fractionated via existing methods is a problem, and methods such as enzymatic hydrolysis and fermentation have their limitations.[1]
[a] I. Anugwom, Dr. V. Eta, Dr. P. Virtanen, Dr. P. Mki-Arvela, Prof. J.-P. Mikkola Laboratory of Industrial Chemistry and Reaction Engineering Process Chemistry Centre, bo Akademi University bo-Turku, 20500 (Finland) E-mail: [email protected] [b] M. Hedenstrçm Computation Life Science Cluster (CLIC) Department of Chemistry, Chemical–Biological Center Ume University 901 87, Ume (Sweden) [c] Dr. M. Hummel, Prof. H. Sixta Department of Forest Products Technology School of Chemical Technology Aalto University P.O. Box 16300, 00076 Aalto (Finland) [d] Prof. J.-P. Mikkola Technical Chemistry, Department of Chemistry Chemical-Biological Center Ume University 901 87, Ume (Sweden) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300773.
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It has been established that each of the three main components of lignocellulosic material are themselves valuable resources, if selectively separated. Lignin is considered one of the most abundant natural polymers and is expected to play a vital role as a raw material for the production of bioproducts. During delignification, the pulp and paper industry produces large amounts of lignin as byproduct.[2] The degraded (and sulfonated) lignin is mainly used in low-tech applications and energy production. Extracting lignin in its native form is rather challenging because of its structure, which is that of an irregular, highly condensed cross-linked polymer network that provides the lignocellulosic material with both mechanical strength and the rigidity to resist external forces.[3] Additionally, lignin is known to bind physically and chemically both to cellulose and hemicelluloses by covalent bonding, for example by benzyl ether, benzyl ester, and phenyl glycoside bonds, forming lignin–carbohydrate complexes (LCCs) in plant cell walls.[4, 5] In principle, however, three general approaches can be employed to overcome the limitations of selectively separating or fractionating lignocellulosic material. The first strategy involves extracting the lignin at high temperatures and pressures, in the range of 160–260 8C and 0.69–4.82 MPa, respectively. The process only takes a few seconds and leaves cellulose-rich pulp.[6, 7] The second approach takes advantage of even more severe conditions to depolymerize the lignocellulosic material; added mineral acids have been applied at elevated temperatures to liberate sugars for fermentation, also.[8] The third approach is dissolution of lignocellulose in ionic liquids (ILs) followed by precipitation.[9]
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Scheme 1. Flow chart illustrating the fractionation process for birch treatment using SILs.
Ionic liquids (ILs) have been widely studied as fractionation solvents for lignocellulosic materials. Their dissolution properties are unique and can, by an appropriate choice of cations and/or anions, even offer an environmentally friendly option. ILs are salts composed mainly of organic cations and organic or inorganic anions. Their melting points that are usually below 100 8C, and they also exhibit high thermal and chemical stability and a wide liquidus range.[10] Many ILs are able to dissolve lignocellulosic material or one of its major components: cellulose, hemicelluloses, and/or lignin.[9, 11–14] As a new alternative, switchable ionic liquids (SILs) have been studied as fractionation solvents for lignocellulosic materials.[15, 16] SILs are solvents capable of ionic/non-ionic switching by addition or removal of one compound, a so-called trigger.[17, 18] In addition, they can be readily prepared from abundant chemicals such as monoethanol amine (MEA), an amidine such as 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU), while a typical component of acidic industrial flue gases, such as CO2 or SO2, can serve as trigger. The use of SILs as solvents for lignocellulosic material can facilitate selective extraction or selective enrichment of the material by the choice of the trigger (CO2 or SO2), alkanol amine, or amidine constituting the SIL. The SILs are hereafter referred to by the acronyms MEA-CO2-SIL and MEA-SO2-SIL (Scheme 1).
Typically, lignin extraction from lignocellulosic material can be facilitated by means of a range of industrial processes, commonly applied in pulping of wood chips and other fibers. An example of such an industrial process is the Kraft process, operating at ca. 170 8C.[19] Lignin undergoes two main structural changes during chemical pulping; firstly, the lignin macromolecules are fragmented via cleavage of ether groups (which are widely present in lignin).[3, 11, 20] In the next step, nucleophilic groups are introduced. Consequently, the lignin fragments are dissolved and removed. However, almost all existing processes used in the fractionation and separation of lignin from lignocellulosic material are capital-intensive because of the use of high pressures and elevated treatment temperatures, or because of the need for processing equipment made from special materials due to the use of acids and bases at high temperature.[11, 21] The aim of this study is to demonstrate fractionation of birch wood chips using SILs based on monoethanol amine (MEA), an amidine, (DBU) and triggers such as CO2 and SO2. The aim is to dissolve one or two of the main components of the wood. Selective extraction of lignin from the lignocellulosic material using SILs would be the most desirable option: because these SILs can be produced in situ they do not require dry conditions (a notorious feature of ‘classical’ ILs) and are easy to handle.
Results and Discussion MEA-CO2-SIL and MEA-SO2-SIL treatments of milled birch Selective fractionation of lignocellulosic material (i.e., milled birch wood) was performed in an SIL based on an alkanol amine, amidine, and two types of acid gases (CO2 or SO2) at 120 8C (Scheme 2. The resulting fluffy material from the treatment can be compared to that obtained using a typical pulping method before bleaching (Figure 1). When comparing scanning electron microscopy (SEM) images of the untreated
Scheme 2. Structure of the SILs MEA-DBU-CO2 and MEA-DBU-SO2.
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by using a sulfur-containing ionic liquid.[11, 13] Concerning the removal of pectins and uronic acids, the results were similar for both SILs. Also, similar results were achieved for the removal of the other wood sugars using both SILs (Figure 3 C). Alternatively, the glucose yield (cellulose content originating from cellulose hydrolysis, in line with the analysis procedure) was much higher for the wood treated with MEA-SO2-SIL (680 mg g 1), resulting in higher glucose yields compared to Figure 1. A) Milled birch (~ 1 mm), B) MEA-CO2-SIL-treated birch, and C) MEASO2-SIL-treated birch. both native wood and the MEA-CO2-SIL-treated wood, the yields of which were 420 mg g 1 and 660 mg g 1, respectively (Figure 3 B). The recovered material from the spent SIL was precipitated using methanol (Figure 4). It was found that the recovered materials mainly composed of hemicelluloses, in addition to some of lignin (Table 1). The recovered materials were lean in cellulose, meaning that the SIL did not dissolve cellulose, but mainly Figure 2. Scanning electron microscopy (SEM) images of A) milled birch (~ 1 mm), B) MEA-CO2-SIL-treated birch, and C) MEA-SO2-SIL-treated birch. Magnification: 1000 . lignin and hemicelluloses.
Infrared spectroscopy analysis of undissolved, treated wood wood and wood treated with either SIL, structural changes appeared minor (Figure 2). The result revealed that weight reductions of 44 % and 40 % were obtained after treatment with the MEA-SO2-SIL and MEACO2-SIL, respectively.
The spectra in Figure 5 show native birch wood and birch treated using MEA-SO2 SIL and MEA-CO2 SIL. The strong absorption band at 3400 cm 1 is due to O H stretching and C H stretching was observed at 2910 cm 1. Furthermore, several peaks in the finger-print region, between 1800 and 600 cm 1,
Chemical analysis The wood material undissolved after SIL treatment was subjected to chemical analysis to determine its lignin, cellulose, and hemicellulose contents (Figure 3 and Table 1). The lignin content of the wood was reduced by 81 wt % after MEA-SO2-SIL treatment, but only by 50 wt % using MEA-CO2-SIL. The SO2-containing SIL is evidently more selective towards lignin dissolution compared to the CO2-triggered SIL. The presence of SO2 could be the reason: analogous affinity toward lignin has been achieved
Figure 3. Hemicelluloses contents (sugar yields) for native birch and SIL-treated undissolved birch using either MEA-CO2-SIL or MEA-SO2-SIL.
Table 1. Mass balance of native and treated as well as the recovered solid from spent MEACO2/MEASO2- SIL after milled birch treatment.[a, b] Sample MEA-SO2-SIL (undissolved fraction) Recovered fraction from spent SO2-SIL MEA-CO2-SIL (undissolved fraction) Recovered fraction from spent CO2-SIL
Total components quantity [%] mass [g]
Cellulose quantity [%]
mass [g]
Hemicelluloses quantity [%] mass [g]
Lignin quantity [%]
mass [g]
total [%]
56 1.3 37.5 1.3 60 1.8 10 1.9
80.5 1.9 0.4 1.0 66.5 1.0 0.5 1.6
68 0.1 59.9 0.8
10.8 1.5 60.8 1.9 12.0 1.1 60 1.9
3.9 1.1 30.9 1.1 11.9 1.1 27 1.9
3.3 7.7 11 4
95 92 90 92
85 25 90 15
9.2 15 11 9
[a] The “mass” columns correspond to the amounts of undissolved wood material, or the methanol-precipitated material from the spent SIL after the SIL treatment. [b] The inability to close the initial mass balance is because ashes and the wood extractives are not considered in the calculation, furthermore there can be some residual SIL left on the material.
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Figure 4. Solids recovered from A) MEA-SO2-SIL-treated birch, and B) MEACO2-SIL-treated birch.
Figure 5. FT-IR spectra of A) milled birch (~ 1 mm), B) MEA-SO2-SIL treated birch C) MEA-CO2-SIL treated birch.
show well-defined changes.[29, 30] The strong carbonyl band observed at 1740 cm 1 (Figure 5) can be attributed to a high content of xylan in the hardwood.[31] However a clear, well-marked reduction of this peak after treatment with SIL is visible, also. The peaks at 1650 and 1590 cm 1 are assigned to absorbed O H and conjugated C O, respectively. The reduction in intensity of the peak at 1504 cm 1, attributed to vibrations of aromatic skeletons,[32] after SIL treatment further suggests lignin removal.
Nuclear magnetic resonance analysis of undissolved, treated wood 13
C NMR spectra of undissolved material after SIL treatment, together with a reference spectrum of native birch, are shown in Figure 6. A signal assigned to cellulose is in the region between 50 and 105 ppm, while the signal at 89 ppm relates to C-4 of the highly ordered cellulose of the crystallite interiors, and the broader upfield signal at 84 ppm is assigned to the C4 of disordered cellulose.[33, 34] The signals resonating at 21 and 173 ppm, respectively, denote the methyl and carboxylic carbon of acetyl groups attached to hemicelluloses and their intensities were reduced after both SO2 and CO2 based SIL treatment, indicating the removal of hemicelluloses from the wood. An obvious reduction of the peak in the region between 125 and 160 ppm was observed assigned mainly to the aromatic carbons of lignin, upon comparison of the spectra from untreated birch and treated birch wood. Although the NMR results from should be considered semiquantitative, they can be interpreted as to reflect the nearly complete removal of
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Figure 6. NMR spectra for A) milled birch (~ 1 mm), B) MEA-CO2-SIL-treated birch and C) MEA-SO2-SIL-treated birch.
lignin from wood. Furthermore, considerable amounts of the hemicelluloses were removed. Gel permeation chromatography analysis of the undissolved treated and untreated wood The molecular weight distribution (MWD) of the carbohydrates for native birch wood and the SIL-extracted residues were determined after removal of lignin by the sodium chlorite method (see Experimental Section). Birch wood depicts a bimodal distribution which originates from the hemicelluloses (lowmolecular-weight region) and cellulose (high molar mass), as shown in Figure 7 (black curve). Assuming a Gaussian distribution of the single carbohydrate types, the MWD curve has been deconvoluted into the single compounds (see Supporting Information). Integrating the Gaussian curves provides the cellulose and hemicellulose contents. The results are in excellent agreement with the carbohydrate analysis performed by gas chromatography (GC; see Supporting Information Table S1). The MWD curve of the MEA-CO2-SIL-treated residue also displays a bimodal distribution (Figure 7, blue solid line). However, the content of cellulose and hemicellulose were in disagreement with the carbohydrate analysis when deconvoluted. This indicates that the cellulose and hemicellulose cannot be described by simple Gaussian functions, which invalidates this deconvolution approach. Most likely, some carbohydrate degradation has occurred, resulting in lower average molecular weights and causing an overlap with the hemicellulose. The cellulose depolymerization is even more pronounced and visible for the MEA-SO2-SIL-treated residue (Figure 7, red solid ChemSusChem 2014, 7, 1170 – 1176
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www.chemsuschem.org Experimental Section Materials
Figure 7. Molecular weight distribution of birch wood (black line), the undissolved residue (solid line), and the precipitated material (dashed line). MEASO2-SIL: red, MEACO2-SIL: blue. The curves have been weighted according the mass balances given in Table 1.
SILs were prepared from DBU, MEA, and either CO2 or SO2 by methods earlier described in detail.[15] An equimolar mixture of DBU and MEA was added into a three-neck roundbottom flask. Acid gas was then bubbled through the mixture under rigorous stirring. The exothermic reaction was occurring until the reaction to form the SIL was finished. The air-dried, industrial size birch chips and milled birch (ca.1 mm) (Betula pendula) were provided by the Finnish Forest Research Institute, (Metla). 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) (99 %) and monoethanol amine (99 %) were used as-received and provided by Sigma–Aldrich. Supercritical-grade CO2 (99.999 %, H2O < 0.5 ppm) and SO2 (99.998 %, H2O < 3 ppm) were provided by AGA Oy. Methanol (Merck, 99 %) was used as antisolvent and for washing of the fractions obtained, and was used as-received.
Treatment of milled birch wood in SILs line). It seems that the high-molecular-weight fractions were entirely degraded, resulting in the disappearance of the initial bimodal shape.
Conclusions The use of SIL as a solvent for the fractionation of birch (B. pendula) is successfully demonstrated, and the SILs were synthesized using an alkanol amine (monoethanol amine), an organic superbase (1,8-diazabicyclo-[5.4.0]-undec-7-ene), and either CO2 or SO2 as trigger. The undissolved material recovered after treating the milled birch wood with MEA-CO2-SIL contained 60 wt % cellulose, 12 wt % hemicelluloses, and 11 wt % lignin, while the MEA-SO2-SIL-treated undissolved material contained 80 wt % cellulose 10 wt % hemicelluloses, and 4 wt % lignin. These results indicate that over 50 wt % and 80 wt % lignin extraction was achieved using MEA-CO2-SIL or MEA-SO2-SIL, respectively. Furthermore, 90 wt % of the wood pectin and uronic acids from the birch wood were dissolved. The treatment using either MEA-CO2-SIL or MEA-SO2-SIL resulted in 40 wt % and 44 wt % weight reduction, respectively, after 24 h treatment at 120 8C, under vigorous agitation. The lignin extraction was also confirmed by NMR spectroscopy since the signal between 160 ppm and 120 ppm assigned to lignin decreased after SIL treatment. In accordance with GPC results, less degradation of the cellulose was obtained by the treatment using MEA-CO2-SIL compared to MEA-SO2-SIL. Based on these results, one can selectively extract components from birch wood by the choice of the acid gas upon SIL preparation, thus having almost pure fractions of the birch material. Simple solvent extraction can be applied to separate dissolved fractions from the SIL. Further studies will focus on an improved recovery of the dissolved lignin in other to achieve a complete fractionation process, where all fractions are properly accounted for and remain in their native form as much as possible. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Upon fractionation of wood into cellulose, hemicellulose, and lignin with the SILs, the strategy described in Scheme 2 was followed. Milled wood (approx. 150 g) was immersed in the SILs (approx. 750 g) at 120 8C under normal atmospheric pressure for 24 h under vigorous agitation. While the mixture was still hot, the undissolved wood fraction was separated using vacuum filtration. The undissolved wood material was washed several times with warm water (about 60 8C) until no visual evidence of residual SILs was seen on the undissolved fraction. The treated and dried wood samples were weighed and the weight changes were calculated. The undissolved material was then submitted to acid hydrolysis for determination of the cellulose content, whereas the amount of hemicelluloses was determined by acid methanolysis. The lignin content was determined using the Klason lignin method with slight modifications (see next section). The washing water and recovered spent SIL were collected and concentrated using a rotary evaporator and vacuum treatment to remove excess water. The spent SIL was then treated in two ways. At first, the methanol was added to the recovered SIL, which induced the precipitation of the first fraction of the dissolved material. After centrifuging and filtration a precipitate was obtained and washed several times with methanol, followed by drying and scaling.
Carbohydrate analysis The carbohydrate content of the samples was analyzed by gas chromatography (GC) after acid methanolysis followed by silylation for the determination of hemicelluloses and acid hydrolysis followed by silylation for the determination of the cellulose content.[22, 23] About 2 mL of the silylated sample was injected through a split injector (260 8C, split ratio 1:5) into the capillary column coated with dimethyl polysiloxane (HP-1, Hewlett Packard). The column length, internal diameter, and film thickness were 30 m, 320 mm, and 0.17 mm, respectively. The following temperature program was applied: 100 8C–4 8C min 1–175 8C followed by 175 8C– 12 8C min 1–290 8C. The detector (FID) temperature was 290 8C. Hydrogen was used as a carrier gas. The different peaks were identified using GC–MS. The following analytical-grade sugars or their acids were used as standards for GC calibration: arabinose, rhamnose, xylose, galactose, glucose, mannose, glucuronic acid, and galacturonic acid. The calibration factors were determined for each series of analyses by performing the methanolysis or hydrolysis, siChemSusChem 2014, 7, 1170 – 1176
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CHEMSUSCHEM FULL PAPERS lylation, and GC analysis on two parallel samples containing equal amounts (0.1 mg) of the above-mentioned sugars and their derivatives. The calibration factors were determined by calculating the ratio of the total area of the different sugar unit peaks to the area of the sorbitol peak. The calibration factor for 4-O-methylglucoronic acid was assumed to be equal to the calibration factor of glucuronic acid.[23]
Lignin content The lignin content was determined using the Klason lignin method with slight modification facilitating complete hydrolysis of the polysaccharides with an autoclave treatment at 125 8C, at 1.4 bar for 90 min.[24, 25]
Scanning electron microscopy Scanning electron microscopy (SEM) images were taken using a Leo Gemini 1530 scanning electron microscope equipped with a ThermoNORAN Vantage X-ray detector for EDXA analysis. Conditions: SEBElectron detector at 15 kV, and the ILSE detector at 2.70 kV.
FT–IR spectroscopy Fourier Transform infrared (FT–IR) spectra of the samples were recorded using a Bruker infrared fiber sensor (IFS; Bruker optical GmbH, Germany), 66/S Fourier Transform Infrared (FT–IR) spectrometer equipped with an attenuated total reflectance (ATR) cell and with a deuterated tri glycine sulfate (DTGS) detector. The KBr disc method for solids was applied (300 mg KBr containing 1 % finely ground sample). Spectra were recorded at the range between 4000 and 500 cm 1, 64 scans were collected at a resolution of 4 cm 1.
NMR spectroscopy 13
C nuclear magnetic resonance spectra of solid samples were recorded using a Bruker 500 MHz AMX spectrometer. Acquisition time: approx. 5 h (8192 scans). A Gaussian window function was used in the processing. All samples were milled in a Retsch ZM 200 mill with a 0.5 mm sieve to obtain particles of uniform size. 50 % water was added before packing the sample into a 4 mm rotor.13C NMR results gives only absolute intensities since no internal standard was used.
Gel permeation chromatography Acid chlorite delignification conditions were as follows: in 40 mL water containing, a sample of 1 g was delignified with 0.8 g NaClO2 and 0.16 g AcOH at 75 8C for 2 h; the chemicals were added in two portions. The suspension was cooled in an ice bath and centrifuged to separate the holocellulose, which was extensively washed (10 x water and 2 x acetone). The conditions upon treatment of the birch meal were more severe: 1.5 g NaClO2 and 0.3 mL AcOH were added in three equal portions over 3 h. EDA pretreatment of birch holocellulose was done with the following method.[26] A sample of 0.5 g was incubated in 5 mL EDA for 1 day at room temperature and then washed with 5 mL ethylene diamine (EDA) and 10 mL DMAc. EDA-soluble material was precipitated with 1-propanol. The combined holocellulose samples were solvent-exchanged in 5 mL DMAc for 1 day at room temperature. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Molar mass distribution of pulps was determined by means of gel permeation chromatography (GPC) as reported elsewhere.[27] All samples were first activated in a water–acetone–N,N-dimethylacetamide (DMAc) sequence. The activated samples were dissolved in 90 g L 1 lithium chloride (LiCl) containing DMAc, at room temperature and under gentle stirring. The samples were then diluted to 9 g L 1 LiCl/DMAc, filtered with 0.2 mm syringe filters, and analyzed in a Dionex Ultimate 3000 system with a guard column, four analytical columns (PLgel Mixed-A, 7.5 300 mm), and RI-detection (Shodex RI-101). The eluent flow rate and temperature were 0.75 mL min 1 and 25 8C, respectively. Narrow pullulan standards (343 Da-2500 kDa, PSS GmbH) were used to calibrate the system. The molar masses (M) of the pullulan standards were modified to correspond to those of cellulose (Mcellulose = q Mppullulan), as proposed by Berggren et al.[28] The coefficients q = 12.19 and p = 0.78 were found by a least-squares method, using data published in their report.[28]
Acknowledgements The Finish Bio economic Cluster (FIBIC) and the Finnish Funding Agency for Technology and Innovation (TEKES) are gratefully acknowledged for financial support. In Sweden, the Bio4Energy program and Kempe Foundations are acknowledged. The authors are gratefully to Steliana Aldea and Sebastian von Schoultz for assisting with the spectroscopy. Keywords: alkanol amines · biomass · delignification · ionic liquids · organic superbases [1] L. P. Ramos, Quim. Nova 2003, 26, 863 – 871. [2] R. El Hage, N. Brosse, L. Chrusciel, C. Sanchez, P. Sannigrahi, A. Ragauskas, Polym. Degrad. Stab. 2009, 94, 1632 – 1638. [3] H. Nimz, Angew. Chem. 1974, 86, 336 – 344; Angew. Chem. Int. Ed. Engl. 1974, 13, 313 – 321. [4] M. Lawoko, G. Henriksson, G. Gellerstedt, Biomacromolecules 2005, 6, 3467 – 3473. [5] F. Yaku, R. Tanaka, T. Koshijima, Holzforschung 1981, 35, 177 – 181. [6] Y. Sun, J. Cheng, Bioresour. Technol. 2002, 83, 1 – 11. [7] N. S. Mosier, C. Wyman, B. Dale, R. Elander, Y. Y. Lee, M. Holtzapple, M. R. Ladisch, Bioresour. Technol. 2005, 96, 673 – 686. [8] P. Kumar, D. Barrett, M. J. Delwiche, P. Stroeve, Ind. Eng. Chem. Res. 2009, 48, 3713 – 3729. [9] P. Mki-Arvela, I. Anugwom, P. Virtanen, R. Sjçholm, J.-P. Mikkola, Ind. Crops Prod. 2010, 32, 175 – 201. [10] P. Wasserscheid, T. Welton, Ionic liquids in synthesis, Wiley-VCH, Weinheim, 2006. [11] S. S. Y. Tan, D. R. MacFarlane, J. Upfar, L. A. Edye, W. O. S. Doherty, A. F. Patti, J. M. Pringle, J. L. Scott, Green Chem. 2009, 11, 339 – 345. [12] N. Sun, M. Rahman, Y. Qin, M. L. Maxim, H. Rodriguez, R. D. Rogers, Green Chem. 2009, 11, 646 – 655. [13] Y. Pu, N. Jiang, A. Ragauskas, J. Wood Chem. Technol. 2007, 27, 23 – 33. [14] D. A. Fort, R. C. Remsing, R. P. Swatloski, P. Moyna, G. Moyna, R. D. Rogers, Green Chem. 2007, 9, 63 – 69. [15] I. Anugwom, P. Mki-Arvela, P. Virtanen, S. Willfçr, R. Sjçholm, J.-P. Mikkola, Carbohydr. Polym. 2012, 87, 2005 – 2011. [16] I. Anugwom, P. Mki-Arvela, P. Virtanen, S. Willfçr, P. Damlin, M. Hedenstrçm, J.-P. Mikkola, Holzforschung 2012, 66, 809 – 815. [17] P. G. Jessop, D. J. Heldebrant, L. Xiaowang, C. A. Eckert, C. L. Liotta, Nature 2005, 436, 1102. [18] I. Anugwom, P. Mki-Arvela, P. Virtanen, P. Damlin, R. Sjçholm, J.-P. Mikkola, RSC Adv. 2011, 1, 452 – 457. [19] E. Sjçstrçm, Wood Chemistry. Fundamentals and Applications, Academic Press, New York, 1993. [20] A. Vishtal, A. Kraslawski, Bioresources 2011, 6, 3547 – 3568.
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Received: July 30, 2013 Published online on March 11, 2014
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Switchable Ionic Liquids as Delignification Solvents for Lignocellulosic Materials Ikenna Anugwom,[a] Valerie Eta,[a] Pasi Virtanen,[a] Pivi Mki-Arvela,[a] Mattias Hedenstrçm,[b] Michael Hummel,[c] Herbert Sixta,[c] and Jyri-Pekka Mikkola*[a, d] The transformation of lignocellulosic materials into potentially valuable resources is compromised by their complicated structure. Consequently, new economical and feasible conversion/ fractionation techniques that render value-added products are intensely investigated. Herein an unorthodox and feasible fractionation method of birch chips (B. pendula) using a switchable ionic liquid (SIL) derived from an alkanol amine (monoethanol amine, MEA) and an organic super base (1,8-diazabicyclo[5.4.0]-undec-7-ene, DBU) with two different trigger acid gases (CO2 and SO2) is studied. After SIL treatment, the dissolved fractions were selectively separated by a step-wise method using an antisolvent to induce precipitation. The SIL was recy-
cled after concentration and evaporation of anti-solvent. The composition of undissolved wood after MEA-SO2-SIL treatment resulted in 80 wt % cellulose, 10 wt % hemicelluloses, and 3 wt % lignin, whereas MEA-CO2-SIL treatment resulted in 66 wt % cellulose, 12 wt % hemicelluloses and 11 wt % lignin. Thus, the MEA-SO2-SIL proved more efficient than the MEACO2-SIL, and a better solvent for lignin removal. All fractions were analyzed by gas chromatography (GC), Fourier transform infrared spectroscopy (FT-IR), 13C nuclear magnetic resonance spectroscopy (NMR) and Gel permeation chromatography (GPC).
Introduction Lignocellulosic materials represent a vast resource for the production of biomolecules and commodity chemicals. However, the utilization of these renewable resources has been compromised by several factors, such as the close associations and interconnections that exists between the three main plant cell wall components: cellulose, lignin, and hemicelluloses. Consequently, the low efficiencies by which lignocellulosic substrates can be fractionated via existing methods is a problem, and methods such as enzymatic hydrolysis and fermentation have their limitations.[1]
[a] I. Anugwom, Dr. V. Eta, Dr. P. Virtanen, Dr. P. Mki-Arvela, Prof. J.-P. Mikkola Laboratory of Industrial Chemistry and Reaction Engineering Process Chemistry Centre, bo Akademi University bo-Turku, 20500 (Finland) E-mail: [email protected] [b] M. Hedenstrçm Computation Life Science Cluster (CLIC) Department of Chemistry, Chemical–Biological Center Ume University 901 87, Ume (Sweden) [c] Dr. M. Hummel, Prof. H. Sixta Department of Forest Products Technology School of Chemical Technology Aalto University P.O. Box 16300, 00076 Aalto (Finland) [d] Prof. J.-P. Mikkola Technical Chemistry, Department of Chemistry Chemical-Biological Center Ume University 901 87, Ume (Sweden) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cssc.201300773.
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It has been established that each of the three main components of lignocellulosic material are themselves valuable resources, if selectively separated. Lignin is considered one of the most abundant natural polymers and is expected to play a vital role as a raw material for the production of bioproducts. During delignification, the pulp and paper industry produces large amounts of lignin as byproduct.[2] The degraded (and sulfonated) lignin is mainly used in low-tech applications and energy production. Extracting lignin in its native form is rather challenging because of its structure, which is that of an irregular, highly condensed cross-linked polymer network that provides the lignocellulosic material with both mechanical strength and the rigidity to resist external forces.[3] Additionally, lignin is known to bind physically and chemically both to cellulose and hemicelluloses by covalent bonding, for example by benzyl ether, benzyl ester, and phenyl glycoside bonds, forming lignin–carbohydrate complexes (LCCs) in plant cell walls.[4, 5] In principle, however, three general approaches can be employed to overcome the limitations of selectively separating or fractionating lignocellulosic material. The first strategy involves extracting the lignin at high temperatures and pressures, in the range of 160–260 8C and 0.69–4.82 MPa, respectively. The process only takes a few seconds and leaves cellulose-rich pulp.[6, 7] The second approach takes advantage of even more severe conditions to depolymerize the lignocellulosic material; added mineral acids have been applied at elevated temperatures to liberate sugars for fermentation, also.[8] The third approach is dissolution of lignocellulose in ionic liquids (ILs) followed by precipitation.[9]
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Scheme 1. Flow chart illustrating the fractionation process for birch treatment using SILs.
Ionic liquids (ILs) have been widely studied as fractionation solvents for lignocellulosic materials. Their dissolution properties are unique and can, by an appropriate choice of cations and/or anions, even offer an environmentally friendly option. ILs are salts composed mainly of organic cations and organic or inorganic anions. Their melting points that are usually below 100 8C, and they also exhibit high thermal and chemical stability and a wide liquidus range.[10] Many ILs are able to dissolve lignocellulosic material or one of its major components: cellulose, hemicelluloses, and/or lignin.[9, 11–14] As a new alternative, switchable ionic liquids (SILs) have been studied as fractionation solvents for lignocellulosic materials.[15, 16] SILs are solvents capable of ionic/non-ionic switching by addition or removal of one compound, a so-called trigger.[17, 18] In addition, they can be readily prepared from abundant chemicals such as monoethanol amine (MEA), an amidine such as 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU), while a typical component of acidic industrial flue gases, such as CO2 or SO2, can serve as trigger. The use of SILs as solvents for lignocellulosic material can facilitate selective extraction or selective enrichment of the material by the choice of the trigger (CO2 or SO2), alkanol amine, or amidine constituting the SIL. The SILs are hereafter referred to by the acronyms MEA-CO2-SIL and MEA-SO2-SIL (Scheme 1).
Typically, lignin extraction from lignocellulosic material can be facilitated by means of a range of industrial processes, commonly applied in pulping of wood chips and other fibers. An example of such an industrial process is the Kraft process, operating at ca. 170 8C.[19] Lignin undergoes two main structural changes during chemical pulping; firstly, the lignin macromolecules are fragmented via cleavage of ether groups (which are widely present in lignin).[3, 11, 20] In the next step, nucleophilic groups are introduced. Consequently, the lignin fragments are dissolved and removed. However, almost all existing processes used in the fractionation and separation of lignin from lignocellulosic material are capital-intensive because of the use of high pressures and elevated treatment temperatures, or because of the need for processing equipment made from special materials due to the use of acids and bases at high temperature.[11, 21] The aim of this study is to demonstrate fractionation of birch wood chips using SILs based on monoethanol amine (MEA), an amidine, (DBU) and triggers such as CO2 and SO2. The aim is to dissolve one or two of the main components of the wood. Selective extraction of lignin from the lignocellulosic material using SILs would be the most desirable option: because these SILs can be produced in situ they do not require dry conditions (a notorious feature of ‘classical’ ILs) and are easy to handle.
Results and Discussion MEA-CO2-SIL and MEA-SO2-SIL treatments of milled birch Selective fractionation of lignocellulosic material (i.e., milled birch wood) was performed in an SIL based on an alkanol amine, amidine, and two types of acid gases (CO2 or SO2) at 120 8C (Scheme 2. The resulting fluffy material from the treatment can be compared to that obtained using a typical pulping method before bleaching (Figure 1). When comparing scanning electron microscopy (SEM) images of the untreated
Scheme 2. Structure of the SILs MEA-DBU-CO2 and MEA-DBU-SO2.
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by using a sulfur-containing ionic liquid.[11, 13] Concerning the removal of pectins and uronic acids, the results were similar for both SILs. Also, similar results were achieved for the removal of the other wood sugars using both SILs (Figure 3 C). Alternatively, the glucose yield (cellulose content originating from cellulose hydrolysis, in line with the analysis procedure) was much higher for the wood treated with MEA-SO2-SIL (680 mg g 1), resulting in higher glucose yields compared to Figure 1. A) Milled birch (~ 1 mm), B) MEA-CO2-SIL-treated birch, and C) MEASO2-SIL-treated birch. both native wood and the MEA-CO2-SIL-treated wood, the yields of which were 420 mg g 1 and 660 mg g 1, respectively (Figure 3 B). The recovered material from the spent SIL was precipitated using methanol (Figure 4). It was found that the recovered materials mainly composed of hemicelluloses, in addition to some of lignin (Table 1). The recovered materials were lean in cellulose, meaning that the SIL did not dissolve cellulose, but mainly Figure 2. Scanning electron microscopy (SEM) images of A) milled birch (~ 1 mm), B) MEA-CO2-SIL-treated birch, and C) MEA-SO2-SIL-treated birch. Magnification: 1000 . lignin and hemicelluloses.
Infrared spectroscopy analysis of undissolved, treated wood wood and wood treated with either SIL, structural changes appeared minor (Figure 2). The result revealed that weight reductions of 44 % and 40 % were obtained after treatment with the MEA-SO2-SIL and MEACO2-SIL, respectively.
The spectra in Figure 5 show native birch wood and birch treated using MEA-SO2 SIL and MEA-CO2 SIL. The strong absorption band at 3400 cm 1 is due to O H stretching and C H stretching was observed at 2910 cm 1. Furthermore, several peaks in the finger-print region, between 1800 and 600 cm 1,
Chemical analysis The wood material undissolved after SIL treatment was subjected to chemical analysis to determine its lignin, cellulose, and hemicellulose contents (Figure 3 and Table 1). The lignin content of the wood was reduced by 81 wt % after MEA-SO2-SIL treatment, but only by 50 wt % using MEA-CO2-SIL. The SO2-containing SIL is evidently more selective towards lignin dissolution compared to the CO2-triggered SIL. The presence of SO2 could be the reason: analogous affinity toward lignin has been achieved
Figure 3. Hemicelluloses contents (sugar yields) for native birch and SIL-treated undissolved birch using either MEA-CO2-SIL or MEA-SO2-SIL.
Table 1. Mass balance of native and treated as well as the recovered solid from spent MEACO2/MEASO2- SIL after milled birch treatment.[a, b] Sample MEA-SO2-SIL (undissolved fraction) Recovered fraction from spent SO2-SIL MEA-CO2-SIL (undissolved fraction) Recovered fraction from spent CO2-SIL
Total components quantity [%] mass [g]
Cellulose quantity [%]
mass [g]
Hemicelluloses quantity [%] mass [g]
Lignin quantity [%]
mass [g]
total [%]
56 1.3 37.5 1.3 60 1.8 10 1.9
80.5 1.9 0.4 1.0 66.5 1.0 0.5 1.6
68 0.1 59.9 0.8
10.8 1.5 60.8 1.9 12.0 1.1 60 1.9
3.9 1.1 30.9 1.1 11.9 1.1 27 1.9
3.3 7.7 11 4
95 92 90 92
85 25 90 15
9.2 15 11 9
[a] The “mass” columns correspond to the amounts of undissolved wood material, or the methanol-precipitated material from the spent SIL after the SIL treatment. [b] The inability to close the initial mass balance is because ashes and the wood extractives are not considered in the calculation, furthermore there can be some residual SIL left on the material.
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Figure 4. Solids recovered from A) MEA-SO2-SIL-treated birch, and B) MEACO2-SIL-treated birch.
Figure 5. FT-IR spectra of A) milled birch (~ 1 mm), B) MEA-SO2-SIL treated birch C) MEA-CO2-SIL treated birch.
show well-defined changes.[29, 30] The strong carbonyl band observed at 1740 cm 1 (Figure 5) can be attributed to a high content of xylan in the hardwood.[31] However a clear, well-marked reduction of this peak after treatment with SIL is visible, also. The peaks at 1650 and 1590 cm 1 are assigned to absorbed O H and conjugated C O, respectively. The reduction in intensity of the peak at 1504 cm 1, attributed to vibrations of aromatic skeletons,[32] after SIL treatment further suggests lignin removal.
Nuclear magnetic resonance analysis of undissolved, treated wood 13
C NMR spectra of undissolved material after SIL treatment, together with a reference spectrum of native birch, are shown in Figure 6. A signal assigned to cellulose is in the region between 50 and 105 ppm, while the signal at 89 ppm relates to C-4 of the highly ordered cellulose of the crystallite interiors, and the broader upfield signal at 84 ppm is assigned to the C4 of disordered cellulose.[33, 34] The signals resonating at 21 and 173 ppm, respectively, denote the methyl and carboxylic carbon of acetyl groups attached to hemicelluloses and their intensities were reduced after both SO2 and CO2 based SIL treatment, indicating the removal of hemicelluloses from the wood. An obvious reduction of the peak in the region between 125 and 160 ppm was observed assigned mainly to the aromatic carbons of lignin, upon comparison of the spectra from untreated birch and treated birch wood. Although the NMR results from should be considered semiquantitative, they can be interpreted as to reflect the nearly complete removal of
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Figure 6. NMR spectra for A) milled birch (~ 1 mm), B) MEA-CO2-SIL-treated birch and C) MEA-SO2-SIL-treated birch.
lignin from wood. Furthermore, considerable amounts of the hemicelluloses were removed. Gel permeation chromatography analysis of the undissolved treated and untreated wood The molecular weight distribution (MWD) of the carbohydrates for native birch wood and the SIL-extracted residues were determined after removal of lignin by the sodium chlorite method (see Experimental Section). Birch wood depicts a bimodal distribution which originates from the hemicelluloses (lowmolecular-weight region) and cellulose (high molar mass), as shown in Figure 7 (black curve). Assuming a Gaussian distribution of the single carbohydrate types, the MWD curve has been deconvoluted into the single compounds (see Supporting Information). Integrating the Gaussian curves provides the cellulose and hemicellulose contents. The results are in excellent agreement with the carbohydrate analysis performed by gas chromatography (GC; see Supporting Information Table S1). The MWD curve of the MEA-CO2-SIL-treated residue also displays a bimodal distribution (Figure 7, blue solid line). However, the content of cellulose and hemicellulose were in disagreement with the carbohydrate analysis when deconvoluted. This indicates that the cellulose and hemicellulose cannot be described by simple Gaussian functions, which invalidates this deconvolution approach. Most likely, some carbohydrate degradation has occurred, resulting in lower average molecular weights and causing an overlap with the hemicellulose. The cellulose depolymerization is even more pronounced and visible for the MEA-SO2-SIL-treated residue (Figure 7, red solid ChemSusChem 2014, 7, 1170 – 1176
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www.chemsuschem.org Experimental Section Materials
Figure 7. Molecular weight distribution of birch wood (black line), the undissolved residue (solid line), and the precipitated material (dashed line). MEASO2-SIL: red, MEACO2-SIL: blue. The curves have been weighted according the mass balances given in Table 1.
SILs were prepared from DBU, MEA, and either CO2 or SO2 by methods earlier described in detail.[15] An equimolar mixture of DBU and MEA was added into a three-neck roundbottom flask. Acid gas was then bubbled through the mixture under rigorous stirring. The exothermic reaction was occurring until the reaction to form the SIL was finished. The air-dried, industrial size birch chips and milled birch (ca.1 mm) (Betula pendula) were provided by the Finnish Forest Research Institute, (Metla). 1,8-diazabicyclo-[5.4.0]-undec-7-ene (DBU) (99 %) and monoethanol amine (99 %) were used as-received and provided by Sigma–Aldrich. Supercritical-grade CO2 (99.999 %, H2O < 0.5 ppm) and SO2 (99.998 %, H2O < 3 ppm) were provided by AGA Oy. Methanol (Merck, 99 %) was used as antisolvent and for washing of the fractions obtained, and was used as-received.
Treatment of milled birch wood in SILs line). It seems that the high-molecular-weight fractions were entirely degraded, resulting in the disappearance of the initial bimodal shape.
Conclusions The use of SIL as a solvent for the fractionation of birch (B. pendula) is successfully demonstrated, and the SILs were synthesized using an alkanol amine (monoethanol amine), an organic superbase (1,8-diazabicyclo-[5.4.0]-undec-7-ene), and either CO2 or SO2 as trigger. The undissolved material recovered after treating the milled birch wood with MEA-CO2-SIL contained 60 wt % cellulose, 12 wt % hemicelluloses, and 11 wt % lignin, while the MEA-SO2-SIL-treated undissolved material contained 80 wt % cellulose 10 wt % hemicelluloses, and 4 wt % lignin. These results indicate that over 50 wt % and 80 wt % lignin extraction was achieved using MEA-CO2-SIL or MEA-SO2-SIL, respectively. Furthermore, 90 wt % of the wood pectin and uronic acids from the birch wood were dissolved. The treatment using either MEA-CO2-SIL or MEA-SO2-SIL resulted in 40 wt % and 44 wt % weight reduction, respectively, after 24 h treatment at 120 8C, under vigorous agitation. The lignin extraction was also confirmed by NMR spectroscopy since the signal between 160 ppm and 120 ppm assigned to lignin decreased after SIL treatment. In accordance with GPC results, less degradation of the cellulose was obtained by the treatment using MEA-CO2-SIL compared to MEA-SO2-SIL. Based on these results, one can selectively extract components from birch wood by the choice of the acid gas upon SIL preparation, thus having almost pure fractions of the birch material. Simple solvent extraction can be applied to separate dissolved fractions from the SIL. Further studies will focus on an improved recovery of the dissolved lignin in other to achieve a complete fractionation process, where all fractions are properly accounted for and remain in their native form as much as possible. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Upon fractionation of wood into cellulose, hemicellulose, and lignin with the SILs, the strategy described in Scheme 2 was followed. Milled wood (approx. 150 g) was immersed in the SILs (approx. 750 g) at 120 8C under normal atmospheric pressure for 24 h under vigorous agitation. While the mixture was still hot, the undissolved wood fraction was separated using vacuum filtration. The undissolved wood material was washed several times with warm water (about 60 8C) until no visual evidence of residual SILs was seen on the undissolved fraction. The treated and dried wood samples were weighed and the weight changes were calculated. The undissolved material was then submitted to acid hydrolysis for determination of the cellulose content, whereas the amount of hemicelluloses was determined by acid methanolysis. The lignin content was determined using the Klason lignin method with slight modifications (see next section). The washing water and recovered spent SIL were collected and concentrated using a rotary evaporator and vacuum treatment to remove excess water. The spent SIL was then treated in two ways. At first, the methanol was added to the recovered SIL, which induced the precipitation of the first fraction of the dissolved material. After centrifuging and filtration a precipitate was obtained and washed several times with methanol, followed by drying and scaling.
Carbohydrate analysis The carbohydrate content of the samples was analyzed by gas chromatography (GC) after acid methanolysis followed by silylation for the determination of hemicelluloses and acid hydrolysis followed by silylation for the determination of the cellulose content.[22, 23] About 2 mL of the silylated sample was injected through a split injector (260 8C, split ratio 1:5) into the capillary column coated with dimethyl polysiloxane (HP-1, Hewlett Packard). The column length, internal diameter, and film thickness were 30 m, 320 mm, and 0.17 mm, respectively. The following temperature program was applied: 100 8C–4 8C min 1–175 8C followed by 175 8C– 12 8C min 1–290 8C. The detector (FID) temperature was 290 8C. Hydrogen was used as a carrier gas. The different peaks were identified using GC–MS. The following analytical-grade sugars or their acids were used as standards for GC calibration: arabinose, rhamnose, xylose, galactose, glucose, mannose, glucuronic acid, and galacturonic acid. The calibration factors were determined for each series of analyses by performing the methanolysis or hydrolysis, siChemSusChem 2014, 7, 1170 – 1176
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CHEMSUSCHEM FULL PAPERS lylation, and GC analysis on two parallel samples containing equal amounts (0.1 mg) of the above-mentioned sugars and their derivatives. The calibration factors were determined by calculating the ratio of the total area of the different sugar unit peaks to the area of the sorbitol peak. The calibration factor for 4-O-methylglucoronic acid was assumed to be equal to the calibration factor of glucuronic acid.[23]
Lignin content The lignin content was determined using the Klason lignin method with slight modification facilitating complete hydrolysis of the polysaccharides with an autoclave treatment at 125 8C, at 1.4 bar for 90 min.[24, 25]
Scanning electron microscopy Scanning electron microscopy (SEM) images were taken using a Leo Gemini 1530 scanning electron microscope equipped with a ThermoNORAN Vantage X-ray detector for EDXA analysis. Conditions: SEBElectron detector at 15 kV, and the ILSE detector at 2.70 kV.
FT–IR spectroscopy Fourier Transform infrared (FT–IR) spectra of the samples were recorded using a Bruker infrared fiber sensor (IFS; Bruker optical GmbH, Germany), 66/S Fourier Transform Infrared (FT–IR) spectrometer equipped with an attenuated total reflectance (ATR) cell and with a deuterated tri glycine sulfate (DTGS) detector. The KBr disc method for solids was applied (300 mg KBr containing 1 % finely ground sample). Spectra were recorded at the range between 4000 and 500 cm 1, 64 scans were collected at a resolution of 4 cm 1.
NMR spectroscopy 13
C nuclear magnetic resonance spectra of solid samples were recorded using a Bruker 500 MHz AMX spectrometer. Acquisition time: approx. 5 h (8192 scans). A Gaussian window function was used in the processing. All samples were milled in a Retsch ZM 200 mill with a 0.5 mm sieve to obtain particles of uniform size. 50 % water was added before packing the sample into a 4 mm rotor.13C NMR results gives only absolute intensities since no internal standard was used.
Gel permeation chromatography Acid chlorite delignification conditions were as follows: in 40 mL water containing, a sample of 1 g was delignified with 0.8 g NaClO2 and 0.16 g AcOH at 75 8C for 2 h; the chemicals were added in two portions. The suspension was cooled in an ice bath and centrifuged to separate the holocellulose, which was extensively washed (10 x water and 2 x acetone). The conditions upon treatment of the birch meal were more severe: 1.5 g NaClO2 and 0.3 mL AcOH were added in three equal portions over 3 h. EDA pretreatment of birch holocellulose was done with the following method.[26] A sample of 0.5 g was incubated in 5 mL EDA for 1 day at room temperature and then washed with 5 mL ethylene diamine (EDA) and 10 mL DMAc. EDA-soluble material was precipitated with 1-propanol. The combined holocellulose samples were solvent-exchanged in 5 mL DMAc for 1 day at room temperature. 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org Molar mass distribution of pulps was determined by means of gel permeation chromatography (GPC) as reported elsewhere.[27] All samples were first activated in a water–acetone–N,N-dimethylacetamide (DMAc) sequence. The activated samples were dissolved in 90 g L 1 lithium chloride (LiCl) containing DMAc, at room temperature and under gentle stirring. The samples were then diluted to 9 g L 1 LiCl/DMAc, filtered with 0.2 mm syringe filters, and analyzed in a Dionex Ultimate 3000 system with a guard column, four analytical columns (PLgel Mixed-A, 7.5 300 mm), and RI-detection (Shodex RI-101). The eluent flow rate and temperature were 0.75 mL min 1 and 25 8C, respectively. Narrow pullulan standards (343 Da-2500 kDa, PSS GmbH) were used to calibrate the system. The molar masses (M) of the pullulan standards were modified to correspond to those of cellulose (Mcellulose = q Mppullulan), as proposed by Berggren et al.[28] The coefficients q = 12.19 and p = 0.78 were found by a least-squares method, using data published in their report.[28]
Acknowledgements The Finish Bio economic Cluster (FIBIC) and the Finnish Funding Agency for Technology and Innovation (TEKES) are gratefully acknowledged for financial support. In Sweden, the Bio4Energy program and Kempe Foundations are acknowledged. The authors are gratefully to Steliana Aldea and Sebastian von Schoultz for assisting with the spectroscopy. Keywords: alkanol amines · biomass · delignification · ionic liquids · organic superbases [1] L. P. Ramos, Quim. Nova 2003, 26, 863 – 871. [2] R. El Hage, N. Brosse, L. Chrusciel, C. Sanchez, P. Sannigrahi, A. Ragauskas, Polym. Degrad. Stab. 2009, 94, 1632 – 1638. [3] H. Nimz, Angew. Chem. 1974, 86, 336 – 344; Angew. Chem. Int. Ed. Engl. 1974, 13, 313 – 321. [4] M. Lawoko, G. Henriksson, G. Gellerstedt, Biomacromolecules 2005, 6, 3467 – 3473. [5] F. Yaku, R. Tanaka, T. Koshijima, Holzforschung 1981, 35, 177 – 181. [6] Y. Sun, J. Cheng, Bioresour. Technol. 2002, 83, 1 – 11. [7] N. S. Mosier, C. Wyman, B. Dale, R. Elander, Y. Y. Lee, M. Holtzapple, M. R. Ladisch, Bioresour. Technol. 2005, 96, 673 – 686. [8] P. Kumar, D. Barrett, M. J. Delwiche, P. Stroeve, Ind. Eng. Chem. Res. 2009, 48, 3713 – 3729. [9] P. Mki-Arvela, I. Anugwom, P. Virtanen, R. Sjçholm, J.-P. Mikkola, Ind. Crops Prod. 2010, 32, 175 – 201. [10] P. Wasserscheid, T. Welton, Ionic liquids in synthesis, Wiley-VCH, Weinheim, 2006. [11] S. S. Y. Tan, D. R. MacFarlane, J. Upfar, L. A. Edye, W. O. S. Doherty, A. F. Patti, J. M. Pringle, J. L. Scott, Green Chem. 2009, 11, 339 – 345. [12] N. Sun, M. Rahman, Y. Qin, M. L. Maxim, H. Rodriguez, R. D. Rogers, Green Chem. 2009, 11, 646 – 655. [13] Y. Pu, N. Jiang, A. Ragauskas, J. Wood Chem. Technol. 2007, 27, 23 – 33. [14] D. A. Fort, R. C. Remsing, R. P. Swatloski, P. Moyna, G. Moyna, R. D. Rogers, Green Chem. 2007, 9, 63 – 69. [15] I. Anugwom, P. Mki-Arvela, P. Virtanen, S. Willfçr, R. Sjçholm, J.-P. Mikkola, Carbohydr. Polym. 2012, 87, 2005 – 2011. [16] I. Anugwom, P. Mki-Arvela, P. Virtanen, S. Willfçr, P. Damlin, M. Hedenstrçm, J.-P. Mikkola, Holzforschung 2012, 66, 809 – 815. [17] P. G. Jessop, D. J. Heldebrant, L. Xiaowang, C. A. Eckert, C. L. Liotta, Nature 2005, 436, 1102. [18] I. Anugwom, P. Mki-Arvela, P. Virtanen, P. Damlin, R. Sjçholm, J.-P. Mikkola, RSC Adv. 2011, 1, 452 – 457. [19] E. Sjçstrçm, Wood Chemistry. Fundamentals and Applications, Academic Press, New York, 1993. [20] A. Vishtal, A. Kraslawski, Bioresources 2011, 6, 3547 – 3568.
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Received: July 30, 2013 Published online on March 11, 2014
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