Dialkyl Phosphate-Related Ionic Liquids as Selective Solvents for Xylan

Article pubs.acs.org/Biomac

Dialkyl Phosphate-Related Ionic Liquids as Selective Solvents for Xylan Carmen Froschauer,*,†,‡ Michael Hummel,§ Gerhard Laus,† Herwig Schottenberger,† Herbert Sixta,‡,§ Hedda K. Weber,‡ and Gerhard Zuckerstaẗ ter‡ †

Faculty of Chemistry and Pharmacy, University of Innsbruck, 6020 Innsbruck, Austria Competence Center of Wood Composites and Wood Chemistry K-Plus, 4021 Linz, Austria § Department of Forest Products Technology, Aalto University, 00076 Aalto, Finland ‡

S Supporting Information *

ABSTRACT: Herein we describe a possibility of selective dissolution of xylan, the most important type of hemicellulose, from Eucalyptus globulus kraft pulp using ionic liquids (ILs). On the basis of the IL 1-butyl-3-methylimidazolium dimethyl phosphate, which is well-known to dissolve pulp, the phosphate anion was modified by substituting one oxygen atom for sulfur and selenium, respectively. This alteration reduces the hydrogen bond basicity of the IL and therefore prevents dissolution of cellulose fibers, whereas the less ordered xylan is still dissolved. 1H NMR spectra of model solutions and Kamlet−Taft parameters were used to quantify the solvent polarity and hydrogen bond acceptor properties of the ILs. These parameters have been correlated to their ability to dissolve xylan and cellulose, which was monitored by 13C NMR spectroscopy. It was found that the selectivity for xylan dissolution increases to a certain extent with decreasing hydrogen-bond-accepting ability of anions of the ILs.

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INTRODUCTION The biorefinery of wood and the processing of cellulose, the earth’s most abundant biomacromolecule, using ionic liquids (ILs) acquired a lot of interest in the past.1,2 Graenacher first described the dissolution of cellulose in molten salts back in the 1930s.3 However, his invention was not fully appreciated at that time due to the high melting temperature of the used Nethylpyridinium chloride (melting temperature: 120 °C). It required almost seven decades, and the task-specific utilization of low melting salts, now termed ILs, until their potential for dissolving lignocellulose4−6 and even wood sawdust7 was discovered. Moreover, the need for improved and environmentally benign separation techniques for biomass attracts more and more attention.8 So far, ILs have been predominantly used for the dissolution of microcrystalline cellulose (MCC),9 pulp, or the entire wood, whereas in biorefinery processes an important additional objective constitutes selective fractionation of the following three main wood components:10,11 the semicrystalline polysaccharide cellulose (40−50%), the amorphous phenolic macromolecule lignin (18−35%), and the amorphous polysaccharide hemicellulose (20−40%).2 Via selective and easy separation of the basic biomass biopolymers, the single fractions could independently serve as feedstock for polymeric composite materials, base chemicals, or even as fuels and therefore represent an alternative to the present economy, which is dominated by nonrenewable fossil resources.1 © 2012 American Chemical Society

Numerous studies have focused on methods for delignification of lignocellulosic material using ILs.12−15 Lignin is quite stable toward dissolution or external forces due to its crosslinked structure, strong chemical bonds, and complex compositions.16 The separation of cellulose from lignocellulose using ILs has so far met limited success, and only fractions of moderate yields, free of lignin and hemicellulose, could be demonstrated.5,17 Despite the numerous existing approaches for the isolation of cellulose and lignin, only a few practicable methods for the extraction of hemicellulose using ILs have been reported. Known procedures for xylan removal, such as steam hydrolysis at elevated pressure, alkaline extraction,18 oxidative, and reductive treatments are accompanied by unwanted cellulose degradation.19 However, for the production of biocomposite materials, it is important to dissolve and process cellulose without degradation.20 Under harsh conditions the reaction rate for the hydrolysis of hemicellulose is similar to that observed for the hydrolysis of cellulose.21 Even the enhanced method of hemicellulose extraction using Ni(tren), to produce dissolving pulp from paper-grade pulps,18,22,23 is handicapped by the necessity of an effective removal of the residual nickel and the fact that the softwood hemicellulose glucomannan is almost insoluble under these extraction Received: April 16, 2012 Revised: May 14, 2012 Published: May 16, 2012 1973

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conditions.24 Yields of more than 80% of the wood xylan can be obtained by cold alkaline postextraction.25 In most hemicellulose extraction experiments xylan is used as a standard because it is commercially available and shows typical hemicellulosic characteristics (degree of polymerization from 70 to 220). In general, the dissolution of hemicelluloses in ILs and the hydrolysis of hemicelluloses by catalysts are easier than for cellulose, due to their amorphous structures and lower molecular weights.26 The absence of the C6 primary alcohol reduces the ability to form inter- and intramolecular H-bonds and therefore facilitates dissolution.27 Hemicelluloses interact strongly with the lignin network in the cell wall matrix and are therefore liberated with difficulty.28 A long-term goal should be the development of biocomposites from natural fibers, processed via ILs using improved and green separation techniques for biomass. The properties of these biocomposites should be adjustable by varying the content of cellulose, lignin, hemicelluloses, chitosan, keratin fibers, and the like,29 but the single components would have to be isolated successfully in a reasonably high quality first. Hemicelluloses on their own can be used as sweetening agents (monomers), prebiotics (oligomers), or as emulsifiers (polymers).30 Homogenous esterification of xylan-rich hemicelluloses with maleic anhydride in ILs offers an efficient method to prepare novel and important functional biopolymers for biomaterials.31 Furthermore, the removal of hemicelluloses is an important part of lignocellulose pretreatment to enhance accessibility of the cellulose fraction to enzymes, resulting in higher glucose yields from cellulose.32,33 The interaction of solvents with solutes can be described, inter alia,34 by the three well-established polarity parameters conceived by Kamlet and Taft: the general dipolarity/ polarizability (π*), the hydrogen bond acidity (α), and the hydrogen bond basicity (β). The procedure has been successfully applied to characterize molecular solvents35 as well as ILs.36 ILs showing a strong ability to accept hydrogen bonds, thus with a strong hydrogen basicity, show high β values and are good cellulose dissolving agents.37 It was found that for ILs the anion has the major influence on the ability to dissolve cellulose and, consequently, for example on the pretreatment efficiency of wood chips.38 Because xylan should be easier dissolved than cellulose with respect to the hydrogen-bonding mechanism,39 one can develop an IL with a lower “H-bond basicity” than cellulose-dissolving ILs. To get more systematic data correlating with the influence of structural aspects of the IL on the dissolution of xylan and cellulose, we investigated the interactions of selected ILs and ethanol as a simple model solute for cellulose by means of 1H NMR spectroscopy. Because 1,3-dialkylimidazolium O,O-dimethyl phosphates already resulted in cellulose solutions in commercially attractive concentrations40−42 owing to their adequate hydrogen bond basicity,43 we modified the phosphate anion by substituting the O atom of one methoxy group with a S or a Se atom. These ILs containing O,S-dimethyl phosphorothioate (dmpt) and O,Sedimethyl phosphoroselenoate (dmpSe)44 anions have already been proven as solvation media for cellulosic materials.45 In the present study, the dissolution selectivity of O,Se-dimethyl phosphoroselenoate-based ILs toward xylan, versus cellulose, is disclosed in detail.

Article

EXPERIMENTAL SECTION

Materials. All ILs used in this study contain 1,3-dialkylimidazolium-based cations. Because 1-ethyl-3-methylimidazolium acetate (EMIM OAc, 143314-17-4, 1) is able to dissolve cellulose very well,46 this substance was used as a reference in the dissolution experiments, although it is known to cause dramatic changes in cellulosic structures.37 1-Butyl-3-methylimidazolium dimethyl phosphate (BMIM dmp, 891772-94-4, 2) is also a generally stable IL with an appropriate hydrogen bond basicity38,40,43 and was reported to be an efficient solvent for cellulose and wood as well.41−43,47 BMIM dmp was prepared by a published procedure, namely, by reaction of 1-butyl3-methyimidazolium chloride (BMIM Cl) and trimethylphosphate.48 1-EMIM OAc (97%) and BMIM Cl (99%) were purchased from Sigma Aldrich and IoLiTec (Germany), respectively, and used as received. By analogy to BMIM dmp and the highly recognized BMIM Cl,4,49 we decided to investigate 1-butyl-3-methylimidazolium dimethyl phosphorothioate (BMIM dmpt, 1331734-68-9, 3), 1-allyl-3-methylimidazolium dimethyl phosphorothioate (AMIM dmpt, 5), 1-butyl-3methylimidazolium dimethyl phosphoroselenoate (BMIM dmpSe, 6), and 1-allyl-3-butylimidazolium O,Se-dimethyl phosphoroselenoate (ABIM dmpSe, 7). These four ILs were prepared by alkylation of the respective 1-alkylimidazole with alkyl halides, followed by a metathesis step, as recently described.45 The resulting metathesis products contain a residual chloride fraction of about 1 to 2%, which represents a usual value for an IL synthesized by metathesis.49,50 Because of unsatisfying NMR spectroscopy results, ABIM dmpSe was not further studied. To investigate the influence of the residual chloride on the dissolution process, an already described45 alternative reaction (ar) using trialkylphosphate and a starting dialkylimidazolium halide was selected to prepare chloride-free IL, in particular, 1-butyl-3methylimidazolium dimethyl phosphorothioate (BMIM dmpt ar, 4).45 Therefore, we were able to investigate the influence of exchanging one oxygen atom of the anion for S or Se and also to analyze the influence of chloride in the anion composition upon the solution power of the IL. Any impact of the corrosiveness of the chloride fraction in the ILs on different materials50 was disregarded for this study because it is beyond the scope of this investigation. The chemical structures of all imidazolium-based ILs used for the present study are shown in Chart 1.

Chart 1. ILs Used in This Study

Synthetic and structural details about the prepared substances are available in the Supporting Information. All ILs used in this study were vacuum-dried and virtually water-free; a water content of ∼0.5% was determined by Karl Fischer titration. The imidazolium O,S-dimethyl phosphorothioate-based ILs are in the same range of thermal stability as EMIM OAc (Td = 210 °C) and decompose at around 200 to 230 °C. The related O,Se-dimethyl 1974

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N,N-diethyl-4-nitroaniline (DENA)) in the selected ILs. Because all three parameters are temperature-dependent,35,53 the measurements were conducted at 25 °C. Stock solutions of Kamlet−Taft dyes were prepared in acetone (60 mg of RD, 2.4 mg of NA, and 2.4 mg of DENA in 8 mL of acetone). We transferred 100 μL of each dyesolution by pipet into three separate vials, where the solvent was evaporated using a stream of dry nitrogen. 0.8 g of the vacuum-dried IL (water content 0.5%, determined by Karl Fischer Titration) was added to the dried dye, carefully mixed, and transferred by syringe into a cuvette of 1 mm thickness; the quartz cell was capped and sealed. The absorbance spectra were recorded between 200 and 800 nm using a PC-controlled Shimadzu UV 2500 with thermostatted sample holder at 25.0 °C (thermostat precision 0.1 °C). The wavelength at the maximum absorption (λmax) was determined and used to calculate the Kamlet−Taft parameters according to reported equations.54 OH-Shift Measurements. About 1 g of the virtually water-free IL was placed in a vial, and an equimolar amount of ethanol was added with a syringe. The vials were vortexed until a homogeneous solution was obtained, and the mixture was transferred into a standard NMR tube. No further locking agent was needed because only the difference between the OH and the CH3 shifts of ethanol is required. The NMR tube was sealed to avoid absorption of ambient moisture. 1H and 13C NMR spectra were recorded at room temperature (20 to 23 °C). The 1 H NMR spectra obtained were interpreted as described in literature: the difference between the chemical shifts of the ethanol−CH3 and the ethanol−OH group gives the so-called OH-shift, whereas the difference between the chemical shifts of the acidic C2-proton in 1methylimidazolium-based cations and the CH3-group of the methylside chain of the cation is indicated as C2−H-shift. C2−H-shifts were determined from 1H NMR spectra of the neat IL without any other solvents.

phosphoroselenoates are slightly less stable as they decompose at 190 to 200 °C.45 In contrast with the low toxicity of dimethyl phosphorothioates,51 dimethyl phosphoroselenoates liberated small amounts of hazardous selanes, as discernible by their typical odor, when exposed to adventive moisture. Because selenium compounds are generally known to exhibit a small toxicological tolerance, the dimethyl phosphoroselenoates introduced herein can be considered for restricted use in analytical research laboratories. Eucalyptus globulus kraft pulp from Ence, Spain (817 mL/g; R10 = 87.93; R18= 91.54), CCE xylan (81.0% xylan), and cotton linters cellulose were donated from the Competence Centre of Wood Composites and Wood Chemistry K-Plus, Austria, and were used airdried with a water content of 4 to 5%. Ethanol absolute (≥99.8%) for OH-shift measurements was purchased from Sigma Aldrich and used as received (certified water content ≤0.2%). All declarations in percent are perceived as mass fractions. NMR Sample Preparation. All NMR samples were prepared by weighing 100 mg of air-dried ENCE pulp (18.5% xylan content), 4.0 g of predried IL, and 0.4 mL of DMSO-d6 into glass screw-top flasks. After 24 h of stirring at 800 rpm and 80 °C under air-atmosphere in closed flasks, the viscous solutions were poured into NMR tubes (10 mm o.d.). By visual inspection of the NMR tubes one can already anticipate which ILs are able to dissolve xylan only and which ones can

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RESULTS AND DISCUSSION Selective Dissolution Experiments Observed by 13C NMR. In preliminary studies, it seemed as if 0.5% of Eucalyptus globulus kraft pulp dissolved in O,S-dimethyl phosphorothioate ILs at 100 °C (as confirmed by optical microscopy). Preparing the mixtures in O,Se-dimethyl phosphoroselenoate ILs resulted in turbid solutions, which showed unaffected cellulose fibers under the microscope (Figure 2), whereas the dissolution of up

Figure 1. EMIM OAc and BMIM dmp are able to dissolve the pulp as a whole and give clear solutions; ILs with S- and Se-containing anions result in turbid solutions in NMR tubes. dissolve the whole pulp (Figure 1). ILs like BMIM dmp and EMIM OAc, which are known to dissolve cellulose very well, resulted in clear solutions, whereas ILs containing sulfur- or selenium-modified phosphate anion resulted in turbid suspensions. Furthermore, solutions of cotton linters cellulose in BMIM Cl and of pure CCE xylan in BMIM dmpt have been prepared the same way to record reference spectra for the single components xylan and cellulose. It was previously demonstrated, that the addition of DMSO-d6 to the solution facilitates the sample preparation, reduces the line width in the NMR spectra, and, additionally, has no impact on the chemical shifts observed for any of the wood components.52 NMR Spectroscopy. All NMR spectra were acquired on a Bruker Avance DPX300 NMR spectrometer, operating at 7.05 T magnetic field strength. The spectrometer was equipped with a 10 mm 1H/BB observe probe without gradients. 13C NMR measurements were performed at T = 80 °C and involved the following acquisition parameters: excitation pulse angle 30°, spectral width 245 ppm, acquisition time 0.5 s, repetition interval 0.5 s, 100.000−200.000 accumulations, inverse-gated WALTZ-16 1H decoupling. Chemical shifts were referenced to δTMS = 0.0 ppm (TMS = tetramethylsilane). All acquired FIDs were multiplied with an exponential window function using a line broadening of 5 Hz prior to Fourier transform. Zero- and first-order phase correction and linear baseline correction yielded the final spectra. Measurement of Kamlet−Taft Parameters. The Kamlet−Taft parameters (π*, α, β) were determined by preparing solutions of Kamlet−Taft dyes (Reichardt’s dye (RD), 4-nitroaniline (NA), and

Figure 2. 1-Allyl-3-butylimidazolium O,Se-dimethyl phosphoroselenoate with (A) 3% CCE xylan (2 h, 100 °C, residual fibers are designated to inhomogeneities of the starting material) and (B) 0.5% Eucalyptus globulus kraft pulp (4 h, 100 °C).

to 7% of cold caustic extracted (CCE) xylan was possible.55 To conduct reliable NMR experiments it was necessary to increase the pulp concentration to 2.5%, thermostatted at a lower measurement temperature of 80 °C. By changing these parameters, O,S-phosphoroselenoate ILs also gave turbid solutions as described above. It seems evident that xylan gets dissolved very easily, whereas cellulose fibers remain undissolved. Unfortunately, the filtration of the pulp-solution in lab-scale was not possible due to the very high viscosity56 of the resulting cellulose/xylan-solutions.57 Therefore, some of the newly synthesized ILs were chosen to prove our assumption of a selective solvation ability of xylan by 1975

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Figure 3. 13C NMR spectra of pure cellulose (in BMIM Cl), pure xylan (in BMIM dmpt), and hemirich (ENCE) pulp solutions in different ILs at 80 °C.

means of 13C NMR spectroscopy. It is possible to identify whether xylan, cellulose, or both components get dissolved by analyzing the different NMR-signals of the pulp mixtures. NMR studies on cellulose solutions in ILs have already been conducted to investigate the interaction between ILs and cellulose by 1H, 13C, and 35/37Cl NMR relaxation measurements.58−60 The 35/37Cl relaxation rates for the anion and subsequent computer modeling studies61,62 indicated the major impact of the anion on the solubility of cellulose. From previous investigations we know that, in our case, the dissolution power of the ILs increases with the chloride content in the anion composition of the ILs. However, neither the newly synthesized ILs containing residual chloride nor the ones without any halide anions were able to dissolve 0.5% of Eucalyptus globulus kraft pulp completely. ILs synthesized by salt metathesis showed a higher dissolution power for xylan; for example, BMIM dmpt was able to dissolve 3% of xylan, whereas only 1.2% xylan was soluble in BMIM dmpt ar (chloride-free). In summary, the absence of chloride notably reduces the dissolution power for isolated xylan. This is in accordance with previous reports stating that ILs containing phosphate-based anions do not affect lignocellulose and its components as severely as the analogous chlorides.38 The 13C NMR measurements have been used to investigate the dissolved pulp in different ILs. Comparison with the spectrum of pure cellulose and pure xylan recorded under identical conditions reveals that both components are dissolved in EMIM OAc and BMIM dmp, whereas BMIM dmpt, BMIM dmpt ar, and AMIM dmpt preferably dissolve xylan, and BMIM dmpSe selectively dissolves xylan (Figure 3). Cellulose gives signals at δ = 102.7, 79.7, 75.6, 75.2, 73.9, and 60.5 ppm that correspond to the resonances of the C1, C4, C5, C3, C2, and C6 carbons of the anhydroglucose unit.59,63 The characteristic signals for xylan are located at 102.4, 76.1, 74.8, 73.8, and 63.8 ppm, respectively.58 The results support recent considerations that the structure of both the anions and the cations is

significantly affecting the solubility of cellulose and xylan, with the anion playing a key role in the disruption of the inter- and intramolecular hydrogen bonds.64,65 Comparing the solutions of BMIM dmpt and BMIM dmpt ar (chloride-free), it can be seen that a residual chloride content of up to 1 to 2% does not affect the solvation behavior significantly. As an additional goal, we evaluated the applicability of 77Se-NMR, 31P NMR, as well as 2D homonuclear and heteronuclear Overhauser effect NMR spectroscopy (1H−1H-NOESY, 1H−77Se HOESY, 1H−31PHOESY) on a O,Se-dimethyl phosphoroselenoate IL sample to probe xylan−solvent interactions. However, our attempts were unsuccessful due to the insensitivity of HOESY NMR spectroscopy and a strong signal overlap in all of the other specified experiments. Kamlet−Taft Parameters. Kamlet−Taft parameters were determined and correlated to the OH-shift of the IL to explain the selective solubility of xylan in some of the investigated ILs. To dissolve polysaccharides, it is necessary to design ILs with a strong hydrogen bond basicity to be effective in weakening the hydrogen-bonding network of the polymer chains.43,66 The abilities of the ILs to interact with solutes were measured spectrophotometrically using a series of three solvatochromic dyes (RD, NA, and DENA). The polarizability expressed by π* is relatively high for all ILs and reflects the degree of delocalization of charge on the cation. The α value, which expresses the ability to donate hydrogen bonds, depends mainly on the cation of the IL. The β value has been established as a measure of the hydrogen-bond-accepting ability in ILs and is generally affected by the nature of the anion species.36 A high β value indicates a pronounced hydrogen-bond-accepting ability and thus renders cellulose soluble in the respective IL.9 In our study, the β parameters of the O,S-dimethyl phosphorothioate and O,Se-dimethyl phosphoroselenoatebased ILs range between 1.107 for EMIM OAc and 0.824 for BMIM dmpSe. 1976

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The β values decrease almost linearly with a decreasing dissolution ability versus cellulose because the acetate anion is a considerably stronger hydrogen bond acceptor (HBA) than O,S-dimethyl phosphorothioate or O,Se-dimethyl phosphoroselenoate. Table 1 summarizes the Kamlet−Taft parameters for this series of ILs.

differences in OH shifts than alteration of the anion. These findings are consistent with previously published literature.45,73 Surprisingly, the amount of 1 to 2% chloride in the anion composition of the ILs had almost no influence on these parameters. The differences between the BMIM and AMIM cations are very small (Table 2). The determined OH shift of EMIM OAc is consistent with the OH shift stated in literature.73

Table 1. ILs Used in This Study and Their Corresponding Kamlet−Taft Parametersa

a

entry

ionic liquid

β

π*

1 2 3 4 5 6

EMIM OAc BMIM dmp BMIM dmpt BMIM dmpt ar AMIM dmpt BMIM dmpSe

1.107 1.052 0.964 0.964 0.922 0.824

1.039 0.946 1.020 1.020 1.094 0.983

Table 2. OH-Shift (Ethanol−OH-Proton with Reference to the Methyl Protons of Ethanol) and C2−H-Shift (ImidazolC2−H Referred to the Methyl Protons of the CH3 Side Chain, Neat)

Measurements were conducted at 25 °C.

It was not possible to determine α parameters because RD is affected by the ILs containing sulfur and selenium. This phenomenon is indicated by a change of color of RD from dark green to colorless. The IL solutions appeared light yellow and not purple as it would be expectable for these ILs. This behavior has been noticed previously for other ILs and was ascribed to protonation of RD,67,68 which, however, could not be an explanation in this case. OH-Shift. The mechanism of cellulose dissolution in ILs is still ambiguous. The role of the cation has been underestimated and neglected for a long time.58,69 Recent studies, however, demonstrated a pronounced influence of the cation. Computational studies have highlighted the cation’s abilities to interact with cellulose via, for example, dispersive forces.64,65,70 The importance of the anions’ interactions with the OH groups of the anhydroglucose unit71 as the main driving force for cellulose dissolution is broadly accepted.72 Therefore, it was reasonable to determine the interaction of different anions of ILs with a model solvent containing a hydroxyl functionality (in this case ethanol). As previously reported, the anion-dependent HBA properties of the ILs can be correlated with the difference between the chemical shifts of the ethanol−CH3 group and the ethanol−OH group in 1H NMR spectra.73 The stability of the CH3-signal and thus its suitability as internal reference have been previously described.74 This OH shift is used to distinguish between cellulose-dissolving and nondissolving ILs and should provide insight into the HBA properties of the anion and the hydrogen bond donor (HBD) properties of the OH group.73 The effect of the anion on the ethanol OH group increases in the same way as the impact on the HBD site of the cation.75 The 1H NMR chemical shift of the OH-group in the probe molecule ethanol can be used as a measure of the capacity of the anion to form hydrogen bonds,73 which is crucial for the breaking of inter- and intramolecular hydrogen bonds of the cellulose chains40,76 and thus for the dissolution of cellulose. Ethanol, acting as an HBD, was equimolarly mixed with the virtually water-free IL, followed by conducting 1H and 13 C NMR spectroscopy measurements. The chemical shift of the OH group (using the CH3 group as an internal reference) was obtained as described in the Experimental Section and reflects the mean strength of the interaction between all HBAs and ethanol. A downfield shift of the OH signal identifies severe interactions and therefore a strong hydrogen bond basicity of the IL. Variations of the cation result in much smaller

entry

ionic liquid

OH-shift [ppm]

C2−H-shift [ppm]

1 2 3 4 5 6

EMIM OAc BMIM dmp BMIM dmpt BMIM dmpt ar AMIM dmpt BMIM dmpSe

5.94 5.39 4.73 4.99 4.71 4.48

9.05 6.21 6.05 5.73 5.87 5.70

If the OH shift indicates primarily the interaction of the anion with the solute, then the C2−H-shift can be used to represent the interaction of the anion with the corresponding cation. Similar to the solute, the cation acts as the HBD and the C2 proton of the imidazolium ring influences the hydrogen bond acidity.77 Depending on the hydrogen bond basicity of the anion, the C2−H-position of the cation is shifted more or less downfield in 1H NMR measurements using the CH3 side chain as an internal reference. For the determination of the C2−H-shifts, NMR measurements were conducted using the neat IL to focus on the anion−cation interaction. For interest, the achieved values were compared with the C2−H-shifts from the ethanolic solutions as previously reported.73 Intuitively, the C2−H-shift of the ethanolic solution should be noticeably smaller, as the anion interacts simultaneously with the solute ethanol and with the cation. However, this effect is only observable for EMIM OAc (C2−H-shift neat: 9.05 ppm; C2− H-shift in EtOH: 6.29 ppm) and slightly for BMIM dmp (C2− H-shift neat: 6.21 ppm; C2−H-shift in EtOH: 5.99 ppm). Regarding all other ILs, no significant differences of the shifts are recordable. This might be attributed to dilution effects.78 A correlation of the OH shift (interaction anion-solute) and the chemical shift of the acidic C2-proton of the cation (interaction anion−cation) is presented in Figure 4. Both chemical shifts represent the apparent overall hydrogen bond interactions of the anion. The chemical shift of the OH-proton increases almost linearly with the β value from the solvatochromic study (Figure 5) and hence with the HBA ability and the solubility of cellulose. According to our results, ILs with an OH shift smaller than 4.7 are not able to dissolve cellulose any more. Comparing the solubility of cellulose and xylan in these ILs by looking at these parameters, both qualify as good indicators to evaluate whether the ILs selectively dissolve xylan or both cellulose and xylan. Certainly, it must be stated that there are other conditions that also have to be taken into consideration in evaluating the dissolution process. The good solubility of pulp in EMIM OAc, for example, results not only from its hydrogen basicity but also from the increased Brønsted basicity of the acetate anion and the low viscosity, which facilitate the 1977

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Article

ASSOCIATED CONTENT

S Supporting Information *

Detailed preparation and analytical data of the synthesized ILs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +43-512-50757080. Fax: +43-512-507-57099. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support was provided by the Austrian government, the provinces of Lower Austria, Upper Austria, and Carinthia as well as by the Lenzing AG. We also express our gratitude to the Johannes Kepler University, Linz, the University of Natural Resources and Applied Life Sciences, Vienna, and the Lenzing AG for their kind contributions. We would like to thank Lauri K.J. Hauru for his skillful introduction into measurement of Kamlet−Taft parameters.

Figure 4. Correlation between OH-shift (ethanol-IL mixtures) and C2−H-shift (pure IL).

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REFERENCES

(1) Sun, N.; Rodriguez, H.; Rahman, M.; Rogers, R. D. Chem. Commun. 2011, 47, 1405−1421. (2) Mora-Pale, M.; Meli, L.; Doherty, T. V.; Linhardt, R. J.; Dordick, J. S. Biotechnol. Bioeng. 2011, 108, 1229−1245. (3) Graenacher, C. Cellulose Solution. U.S. Patent 1,943,176, 1934. (4) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. J. Am. Chem. Soc. 2002, 124, 4974−4975. (5) Fort, D. A.; Remsing, R. C.; Swatloski, R. P.; Moyna, P.; Moyna, G.; Rogers, R. D. Green Chem. 2007, 9, 63−69. (6) Swatloski, R. P.; Rogers, R. D.; Holbrey, J. D. Dissolution and Processing of Cellulose Using Ionic Liquids. WO Patent 03/029329, 2003. (7) Kilpeläinen, I.; Xie, H.; King, A.; Granstrom, M.; Heikkinen, S.; Argyropoulos, D. S. J. Agric. Food. Chem. 2007, 55, 9142−9148. (8) Rahman, M.; Qin, Y.; Maxim, M. L.; Rogers, R. D. Ionic Liquid Systems for the Processing of Biomass, Their Components and/or Derivatives, and Mixtures Thereof. WO2010056790A1 Patent, 2010. (9) Xu, A.; Wang, J.; Wang, H. Green Chem. 2010, 12, 268−275. (10) Zhu, S.; Wu, Y.; Chen, Q.; Yu, Z.; Wang, C.; Jin, S.; Ding, Y.; Wu, G. Green Chem. 2006, 8, 325−327. (11) Winterton, N. J. Mater. Chem. 2006, 16, 4281−4293. (12) Sun, N.; Rahman, M.; Qin, Y.; Maxim, M. L.; Rodriguez, H.; Rogers, R. D. Green Chem. 2009, 11, 646−655. (13) Pu, Y.; Jiang, N.; Ragauskas, A. J. J. Wood Chem. Technol. 2007, 27, 23−33. (14) Myllymaeki, V.; Aksela, R. Dissolution and Delignification of Lignocellulosic Materials with Ionic Liquid Solvent under Microwave Irradiation. WO2005017001 Patent, 2005. (15) Lee, S. H.; Doherty, T. V.; Linhardt, R. J.; Dordick, J. S. Biotechnol. Bioeng. 2009, 102, 1368−1376. (16) Wang, J.; Zheng, Y.; Zhang, S. The Application of Ionic Liquids in Dissolution and Separation of Lignocellulose. In Clean Energy Systems and Experiences; Eguchi, K., Ed.; Sciyo: Croatia, 2010; pp 71− 84. (17) Sun, N.; Jiang, X.; Maxim, M. L.; Metlen, A.; Rogers, R. D. ChemSusChem 2011, 4, 65−73. (18) Puls, J.; Schroeder, N.; Stein, A.; Janzon, R.; Saake, B. Macromol. Symp. 2006, 232, 85−92. (19) Kettenbach, G.; Stein, A. Verfahren zum Abtrennen von Hemicellulosen aus hemicellulosehaltiger Biomasse. EP1366231B1 Patent, 2003.

Figure 5. Linear correlation of β value and the OH shift.

disruption of the inter- and intramolecular hydrogen bonds in biopolymers and the processing of the solution.1

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CONCLUSIONS In summary, it appears that the solubility of cellulose and xylan can be well-described using different parameters for the hydrogen-bond basicity. The obtained OH shift correlates with the anion-dependent β value; the higher the β value the higher the OH shift and the higher the dissolution capacity for cellulose. Cellulose dissolution occurs if the β value is >0.85 or if the OH shift is >4.7 ppm.73 Therefore, the new O,S-dimethyl phosphorothioates and O,Se-dimethyl phosphoroselenoates can be considered to be at the borderline for cellulose dissolution in ILs. Among the investigated ILs, BMIM dmpSe was identified as the most effective IL for fractionation of xylan and cellulose in agreement with a low β value and a low OH shift. By recording 13C NMR spectra it was possible to show that this IL can extract xylan from hemicellulose-rich pulp. The insoluble portion is assumed to comprise unaffected cellulose fibers. Unfortunately, the high viscosity of the IL/pulp-solution and toxicological aspects limit the applicability of these new ILs for up-scaled applications. The use of IL/cosolvent mixtures may be a solution to obviate this drawback79 and is currently investigated.80 Furthermore, the respective fine-tuning by auxiliary antisolvents81 holds promise to establish IL-based techniques for fractionation of wood with industrial applications.82,83 1978

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