Ionic-Liquid-Phase Hydrolysis of Pine Wood - ACS Publications

Ind. Eng. Chem. Res. 2009, 48, 1277–1286

1277

APPLIED CHEMISTRY Ionic-Liquid-Phase Hydrolysis of Pine Wood Carsten Sievers, Mariefel B. Valenzuela-Olarte, Teresita Marzialetti, Ildar Musin, Pradeep K. Agrawal,* and Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst DriVe NW, Atlanta, Georgia 30332-0100

Depolymerization of cellulose and hemicellulose from loblolly pine wood is studied in the ionic liquid 1-butyl3-methylimidazolium chloride, which is capable of dissolving carbohydrates and lignin. In the presence of an acid catalyst, the carbohydrate fraction is converted into water-soluble products under milder conditions than reported for similar reactions in the aqueous phase. The water-soluble products included monosaccharides, oligosaccharides, furfural, and 5-hydroxymethylfurfural (HMF). The lignin fraction is recovered as a solid residue. It is found by 13C CP MAS NMR spectroscopy that chemical modifications of lignin occurred only to a very moderate extent. The influence of the reaction temperature, water content, and acid concentration is investigated. In particular, the presence of water is found to reduce the solubility of carbohydrate but also to be required for its conversion. Under harsh conditions (high temperature, high acid concentration), solid degradation products, so-called humins, form. The main building blocks of humins are sugars that are linked by additional components forming linkages that are more resistant to hydrolysis than glycosidic bonds in carbohydrates. 1. Introduction To supplement oil resources, the development of technologies for the processing and utilization of nonpetroleum feedstocks (e.g., gas, coal, and biomass) is of enormous importance for fulfilling the needs of modern society for liquid transportation fuels. Among these feedstocks, biomass is particularly attractive because it is both sustainable and CO2-neutral. As a result, the development of synthesis routes for the production of biofuels has received much attention over the past several years.1-3 It is estimated that the U.S. could produce 1.3 × 109 metric tons of dry biomass per year.4 The energy content of this biomass would be equivalent to 52% of current oil consumption. Currently, the production of bioethanol from corn is performed on an industrial scale in the U.S.2 However, the increased demand for corn could lead to competition with food and feed producers. Therefore, the utilization of nonedible biorenewable feedstocks, such as lignocellulosic biomass, is highly desirable. The major constituents of lignocellulosic biomass are polymeric carbohydrates (cellulose and hemicellulose) and substituted aromatics (lignin). The production of liquid fuels from the carbohydrate fraction of lignocellulosic biomass requires its depolymerization into monosaccharides or other low-molecular-weight products. This can be achieved by acid- or enzyme-catalyzed hydrolysis in the aqueous phase.2,5 A high selectivity to monosaccharides is obtained in enzyme-catalyzed processes, but such processes typically require several days to achieve the desired conversion. Moreover, the feedstock needs to be pretreated to increase its digestibility.5,6 Faster conversion of biomass can be reached via acid-catalyzed hydrolysis, but controlling the selectivity of this reaction is difficult under typical process conditions. Therefore, it is desirable to develop a process for the hydrolysis of biomass that eliminates the need * To whom correspondence should be addressed. E-mail: [email protected] (P.K.A.), [email protected] (C.W.J.).

for pretreatment and uses mild reaction conditions for the hydrolysis step. In addition to monosaccharides, 5-hydroxymethylfurfural (HMF) and furfural are potential intermediates for the production of fuels and chemicals from biorenewables. In particular, several HMF-derived compounds (2,5-furan dicarboxylic acids, levulinic acid) were proposed as top value-added chemicals from biomass in a recent U.S. Department of Energy (DOE) report.8 HMF and furfural are formed by acid-catalyzed dehydration of hexoses and pentoses, respectively.9-11 The dehydration reaction is much faster for ketoses than for aldoses.10 Therefore, it is possible that the formation of HMF from glucose and other aldoses occurs via fructose as an intermediate (Figure 1). A variety of different acid catalysts have been used for the production of HMF from fructose and sucrose.12-14 Once formed, HMF can be consumed by a number of different subsequent reactions. In particular, its conversion into levulinic acid and formic acid has been reported for reactions in water.7 Ionic liquids are a group of solvents with unique properties,15 such as very low vapor pressures, unusual modes of coordination, and the tendency to form solvent cages.16 Rogers and coworkers demonstrated that the ionic liquid 1-butyl-3-methylimidazolium chloride (BMImCl) is capable of dissolving up to 25 wt % of cellulose, forming a highly viscous solution.17 It was also shown that a major fraction of wood can be dissolved in BMImCl.18,19 An NMR study demonstrated that the unusual solubility of cellulose in BMImCl is due to a disruption of the hydrogen bonds in cellulose by the chloride anions of the ionic liquid.20 Several studies have investigated ionic-liquid-phase reactions that are relevant as potential routes for the production of biofuels. It was shown that acid hydrolysis of cellulose and wood in ionic liquids is possible.21,22 Moreover, the conversion of glucose to HMF over a CrCl2 catalyst was demonstrated.23 However, only very limited information on the products formed in these processes has been reported.

10.1021/ie801174x CCC: $40.75  2009 American Chemical Society Published on Web 01/05/2009

1278 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009

Figure 1. Formation and conversion of 5-hydroxymethylfurfural in acidic medium.7

In this report, we describe acid-catalyzed hydrolysis of loblolly pine wood in ionic liquids. In this medium, the carbohydrate fraction of the wood is converted into soluble products under very mild conditions without any chemical pretreatment. The complex reaction network consisting of hydrolysis, dehydration, and other reactions is discussed. 2. Experimental Section 2.1. Materials. Loblolly pine wood was harvested near Oglethorpe, GA. The logs were debarked, chipped, and milled. For the hydrolysis experiments, the -25 + 60 mesh fraction was used. The feedstock was stored in a freezer prior to use. Cellulose (fibrous medium) was purchased from Sigma-Aldrich. Before the hydrolysis experiments, wood and cellulose feedstocks were dried at 105 °C overnight. 1-Butyl-3-methylimidazolium chloride (BMImCl) was purchased from Solvent Innovations. Prior to use, BMImCl was molten and dried under vacuum. Trifluoroacetic acid (TFA, 99.9%) from EDM was used as received. 2.2. Characterization of Loblolly Pine Wood. The amounts of extractives, ash, lignin, and carbohydrates in loblolly pine wood were determined following TAPPI methods.24-27 Before analysis, the sample was dried overnight at 105 °C. The ash content was measured as the residual mass of the sample after it had been heated in air to 525 °C and held at this temperature for 2 h.24 The extractives content was determined by Soxhlet extraction in dichloromethane for 24 cycles.25 The lignin content was measured as the residue from hydrolysis in a 72% H2SO4 solution after filtration and drying.26 The acid-soluble fraction of lignin was then determined by UV spectrophotometric analysis of the liquid phase. The carbohydrates in loblolly pine wood (glucose, arabinose, galactose, xylose, and mannose) were measured after the acid hydrolysis of the raw material as described for the determination of the lignin content.27 The liquid product obtained after filtration was analyzed using a highperformance anion-exchange chromatograph with a pulsed amperometric detector (HPAEC-PAD, Dionex) and a CarboPac PA10 column. 2.3. Ionic-Liquid-Phase Hydrolysis. Ionic-liquid-phase hydrolysis reactions were conducted in sealed pressure tubes. The batches consisted of 5 g of vacuum-dried 1-butyl-3-methylimidazolium chloride (BMImCl), 0.25 g of oven-dried loblolly pine wood or cellulose, and 10 µL (0.2 wt %) of trifluoroacetic acid (TFA). The reactions were performed for 2 h (based on the addition of acid to the preheated sample to establish time zero) at 120 °C, except where otherwise noted. The pressure tubes were sealed immediately after addition of the acid to prevent TFA vapor from leaving the tube. The mixture was stirred at 300 rpm. No increase of the soluble products and monosaccharide yield from loblolly pine was found upon increasing the stirring rate to 600 rpm. After the reactions, the ionic liquid

phase was diluted with a 24-fold amount of water. This dilution was required for high-performance liquid chromatography (HPLC) analysis of the liquid-phase products. Moreover, it dramatically decreases the solubility of unprocessed cellulose.17,28 Solid and liquid products were separated using nylon filters (0.2 µm pore size). The amount of solid residue was determined by weighing the filters before and after filtration. For this purpose, the filters were dried at 105 °C. The yield of soluble products (Y) was defined as the percentage of the dry starting material that was not recovered by the procedure described above: Y)

mfeedstock,dry - mresidue × 100 mfeedstock,dry

(1)

Likewise, all other yields were defined relative to the dry feedstock. The experimental error for the soluble products yield was estimated as 2%. In the reactions with cellulose as the feedstock, the residue could not be separated from the filter after drying because of its small volume. Therefore, an additional experiment was required at a larger scale to provide enough solid residue for characterization. This reaction was performed in a three-neck flask with a batch that was scaled up by a factor of 4, and the reaction was performed overnight (14 h). After dilution of the mixture with distilled water, the solid residue was separated from the liquid phase using a centrifuge. The solid was washed four times with 100 mL of distilled water to remove residual ionic liquid. The washed residue was dried in a glass dish at 105 °C overnight. 2.4. Analysis of the Liquid Phase. The concentration of monosaccharides was determined using a HPAEC instrument (Dionex) equipped with a CarboPac PA10 column and a pulsed amperometric detector (PAD). For quantification of the chromatograms, an external calibration was used. For this purpose, a standard solution of glucose, arabinose, galactose, xylose, and mannose in a mixture of water and BMImCl (1:24) was prepared. Calibration factors were determined from the peak areas in the chromatogram of this standard. Standard chromatograms were obtained before and after each series of analyses. In addition, the retention times of rhamnose and fructose were determined, so that these compounds could be detected when potentially present in the sample. The HMF and furfural yields were measured using a Shimadzu HPLC system. In order to obtain a high signal-to-noise ratio, a UV detector was used at a wavelength of 210 nm. A Biorad Aminex HPX-87H column was used for separation. The column oven temperature was 50 °C. Modifications of the column by the ionic liquid were minimized by utilizing a guard column. The eluent was 0.005 N H2SO4. Several standards with different concentrations of HMF and furfural were analyzed to determine the extinction coefficients of HMF and furfural at 210 nm. A linear correlation

Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 1279

Figure 3. Hydrolysis of loblolly pine wood at different temperatures. Figure 2. Soluble product yield for hydrolysis of cellulose and loblolly pine wood in the ionic liquid phase at 120 °C, as well as carbohydrate fraction of wood (assuming that the soluble products from loblolly pine wood are formed from this fraction). Table 1. Composition of Loblolly Pine Wood fraction

concentration (wt %)

carbohydrates glucose mannose galactose xylose arabinose Klason lignin acid-soluble lignin extractives ashes

63.4 41.3 10.7 2.8 7.3 1.3 31.6 0.3 1.7 0.6

between the signal of the UV detector and the concentration of these compounds was established. For secondary hydrolysis, 174 µL of 72% H2SO4 were added to 5 mL of the water/BMImCl mixture. The samples were heated to 121 °C in an autoclave for 1 h and analyzed by HPAEC (vide supra). 2.5. Analysis of the Solid Residue. For the solid-state 13C CP MAS NMR measurements, the solid samples were pressed into 7-mm zirconia rotors and spun at 5 kHz (using magic angle spinning). The measurements were conducted on a Bruker Avance 300 spectrometer with a resonance frequency of 75.48 MHz for 13C. The signal-to-noise ratio was enhanced by applying cross polarization.29 The contact time was 1 ms. Spectra were recorded as the sum of 2400 scans and were calibrated using the methine carbon atoms of adamantane, which was used as an external standard (δ ) 29.47 ppm). The Klason lignin reference sample was prepared by the same method as described for the analysis of the lignin content in loblolly pine wood (vide supra).26 3. Results 3.1. Characterization of Loblolly Pine Wood. The composition of the loblolly pine was determined by proximate analysis (Table 1). The carbohydrate fraction was 63.4 wt % of the dry loblolly pine wood and consisted of 86 wt % hexoses (glucose, galactose, and mannose) and 14 wt % pentoses (xylose and arabinose). As expected for pine wood, only small amounts of extractives and ash were found.30 3.2. Comparison of Cellulose and Pine Wood. Loblolly pine wood and cellulose were hydrolyzed in the ionic liquid phase at 120 °C using dilute TFA as the acid catalyst (Figure 2). TFA was chosen as the soluble acid catalyst in order to be able to compare the results with our ongoing research on aqueous-phase hydrolysis.31 No major difference was observed

when the same molar concentration of H2SO4 was used as the catalyst (not shown). It should be mentioned that, in the ionic liquid phase, a continuous exchange occurs between the cations and anions present.32 Therefore, the Brønsted acid primarily serves as a proton source. Because of the large excess of chloride anions in the present case, the active species could be described as HCl independent of the actual proton source. The anions from the proton source might affect the catalytic reaction by forming complexes with the reactant or intermediates, but this effect is expected to be minor. Within 2 h, 97% of the cellulose feedstock was converted into water-soluble products. When the reaction time was extended beyond this point, a slight decrease in the yield of soluble products was observed. When loblolly pine wood was used as the feedstock, the soluble products yield was 62% after 2 h. This value is equivalent to 97% of the carbohydrate fraction in the wood sample being dissolved. It is interesting to note that, in the initial stage of the reaction, a higher yield of soluble products was found for the carbohydrate fraction in loblolly pine wood (79% after 30 min) than for cellulose (60% after 30 min, Figure 2). As the presence of lignin, extractives, and ash did not appear to hinder the conversion of the carbohydrate fraction into soluble products, we decided to conduct more detailed investigations of the influence of the reaction conditions on the conversion of loblolly pine wood. 3.3. Influence of Experimental Conditions. Ionic-liquidphase hydrolysis reactions were performed at 100, 120, and 150 °C (Figure 3). As expected, the initial rate of formation of soluble products increased with increasing temperature. In addition to an increase of the intrinsic rate, the decreasing viscosity of the mixture is likely to facilitate the reaction. At a reaction temperature of 150 °C, the entire carbohydrate fraction of the wood was converted into water-soluble products within 0.5 h. However, the soluble products yield decreased progressively after this point. The monosaccharide yields at different temperatures are shown in Figure 4a. In the reactions at 120 and 150 °C, the monosaccharide yield reached a maximum after approximately 2.0 and 0.5 h, respectively (Figure 4a). After this point, the rate of consumption of monosaccharides by reactions such as dehydration was faster than the rate of formation of these species. Such a maximum was not reached in the reaction at 100 °C. However, the formation of monosaccharides was significantly lower in the reaction at this temperature. At first sight, the initial rate of formation of monosaccharides appears to be similar for the reactions at 120 and 150 °C. However, it has to be kept in mind that the monosaccharide yield reflects the net rate of monosaccharide formation. An


Figure 4. Monosaccharide yield in hydrolysis of loblolly pine wood at different temperatures: (a) total monosaccharide yield, (b) fraction of hexoses.

increase in temperature simultaneously influences the rates of reactions both forming and converting monosaccharides. The fraction of hexoses among the monosaccharides is shown in Figure 4b. In the beginning of the reactions at 100 and 120 °C, the concentration of hexoses within the carbohydrate fraction was less than the corresponding value of 86 wt % in the starting material, indicating that hemicellulose, which contains all of the pentoses in biomass, is more susceptible to hydrolysis than cellulose. The hexose fraction increased with increasing reaction time, indicating the preferential consumption of pentoses in the ionic liquid phase. Moreover, the fraction of hexoses present at a given time increased with increasing temperature. The formation of 5-hydroxymethylfurfural (HMF) in the beginning of the reaction increased with increasing temperature (Figure 5a). However, in the reaction at 150 °C, a maximum in the HMF yield was observed after approximately 1 h. This shows that HMF is consumed in consecutive reactions, including, perhaps, the formation of humins (vide infra). In the reaction at 100 °C, the initial rate of formation of HMF was 0, which demonstrates that HMF is a secondary product. It is assumed that this observation would also be made at higher temperatures if samples were taken at shorter times. The furfural yield showed trends comparable to those of the HMF yield (Figure 5b). However, in the reaction at 120 °C, the furfural yield reached a maximum after 2 h. In all cases, the furfural yield was significantly lower than the HMF yield. In each reaction, the sum of the yields of monosaccharides, HMF, and furfural was significantly lower than the total soluble products yield. Possible explanations for this observation are incomplete hydrolysis of cellulose or hemicellulose to watersoluble oligosaccharides or degradation reactions of monosaccharides, HMF, and furfural. To probe for the presence of oligosaccharides, a secondary hydrolysis was performed for the products of ionic-liquid-phase hydrolysis of loblolly pine wood at 120 °C (Figure 6). Secondary hydrolysis consistently

Figure 5. Yield of dehydration product in reaction at different temperatures: (a) HMF and (b) furfural.

Figure 6. Monosaccharide yield after ionic-liquid-phase hydrolysis at 120 °C and after additional secondary hydrolysis in the aqueous phase.

increased the monosaccharide yield by almost an order of magnitude, as oligosaccharides were converted to monosaccharides during this treatment. Note that the values after secondary hydrolysis include the monosaccharides originally present in the product mixture. Moreover, it is important to keep in mind that the yields determined by this method represent only lower limits. In fact, it is likely that some monosaccharides that are formed during the secondary treatment are consumed in subsequent reactions, such as dehydration of hexoses to HMF. The addition of small amounts of water led to an increase in the soluble products yield (see Figure 7a). In the presence of 1-4 wt % water (relative to BMImCl), the soluble products yield was equal to the carbohydrate content of the feedstock. However, when the concentration of water was increased to more than 4 wt %, the soluble products yield decreased sharply. Note that the soluble products yield of 30 wt % in the presence of 9 wt % water (Figure 7a) is comparable to but marginally higher than that of TFA-catalyzed aqueous-phase hydrolysis under similar conditions.31 The monosaccharide yield increased with increasing water concentration up to 2 wt %, indicating that, at lower water content, the hydrolysis reaction might be limited by the amount of available water (Figure 7b). When the water concentration


Figure 7. Hydrolysis of loblolly pine wood at 120 °C for 2 h with different water concentrations: (a) soluble products yield; (b) monosaccharide, HMF, and furfural yields; c) fraction of hexoses in the carbohydrate fraction.

was increased further, the monosaccharide yield decreased again. It is worth mentioning that, up to a water content of 4 wt %, the monosaccharides were almost exclusively hexoses, whereas the fraction of hexoses was lower than in the starting material when the concentration of water was increased to 9 wt % (Figure 7c). The HMF and furfural yields initially increased upon addition of water (Figure 7b). When the water content increased to 9 wt %, only small amounts of HMF were found. In contrast, the furfural yield increased steadily with increasing water content (Figure 7b). The influence of the acid concentration on the conversion and product distribution is shown in Figure 8a. The highest soluble products yield was found in the presence of 0.4 wt % TFA. A marked decrease in the monosaccharide yield with increasing acid content was observed, showing that an increasing acid concentration favors conversion of monosaccharides over their formation (Figure 8b). A continuous increase of the fraction of hexoses indicates that this trend is even more pronounced for pentoses (Figure 8c). In contrast to the continuous decrease of the monosaccharide yield, the HMF and furfural yields went through a maximum when 0.4 wt % acid was used. 3.4. Analysis of Solid Residues. The 13C CP MAS NMR spectrum of chemically untreated loblolly pine wood is shown in Figure 9a. In agreement with previous reports, the main resonances are assigned to CH3 in acetyl groups of hemicellulose (21 ppm); methoxy groups in lignin (56 ppm); C6 carbon atoms in carbohydrates (66 ppm); C2, C3, and C5 carbon atoms in carbohydrates (73 and 74 ppm); sugars in hemicellulose (82 ppm); C4 carbon atoms in carbohydrates (89 ppm); C1 carbon atoms in carbohydrates (106 ppm); unsubstituted olefinic or aromatic carbon atoms in lignin (115 ppm); quaternary olefinic

Figure 8. Hydrolysis of loblolly pine wood at different acid concentrations (after 2 h at 120 °C): (a) soluble products yield; (b) monosaccharide, HMF, and furfural yields; (c) fraction of hexoses in the carbohydrate fraction.

or aromatic carbon atoms in lignin (123 and 134 ppm); olefinic or aromatic carbon atoms with OH or OR substituents in lignin (149 and 151 ppm); and acetyl groups in hemicellulose (173 ppm).33-35 Several low-intensity resonances are observed between 200 and 225 ppm. It is assumed that these signals are spinning side bands from resonances in the region of unsaturated carbon atoms. Additionally, aldehyde and ketone groups would resonate in this region. Spinning sidebands of equal intensity would also be expected between 47 and 80 ppm. However, this region is dominated by other resonances, so the spinning sidebands are not observed. The spectrum of the residue from ionic-liquid-phase hydrolysis of loblolly pine (2 h at 120 °C, 0.2 wt % TFA) showed marked differences from that of untreated loblolly pine (Figure 9b). Resonances that are characteristic for carbohydrates were almost completely absent. Only the weak, broad peak at 73 ppm might indicate the presence of a small amount of sugars. New peaks were observed in the aliphatic region (0-50 ppm). Moreover, peaks corresponding to aliphatic alcohols or ethers (73 and 85 ppm) were observed, and esters or carboxylic acids (170 ppm) might be present in the sample. Comparison of the spectrum of the ionic-liquid-phase hydrolysis residue with that of Klason lignin from loblolly pine (Figure 9c) shows that lignin is the main constituent of the residue. However, a number of differences were observed between the spectra, and these are assigned to modifications by the respective treatments. The most important difference is the lower intensity of resonances corresponding to aliphatic and unsaturated carbon atoms with alcohol or ether substituents in the spectrum of Klason lignin. The other significant difference between the spectra is that the concentration of aliphatic carbon atoms is much higher in the spectrum of Klason lignin.


Figure 9. 13C CP MAS NMR spectra: (a) loblolly pine wood, (b) solid residues from the TFA-catalyzed conversion of loblolly pine in BMImCl, (c) Klason lignin derived from loblolly pine. Spinning sidebands are indicated by asterisks.

In the previous sections, efficient conversion of the carbohydrate fraction into soluble products was demonstrated. However, it was also shown that, in some cases, these soluble products can be converted into insoluble degradation products when more severe reaction conditions (high temperature, long time, high acid concentration) are applied. These products are commonly referred to as humins (vide infra). To gain insight into the nature of these residues, an additional experiment was conducted in which cellulose was subjected to the experimental conditions for 14 h. The 13C CP MAS NMR spectrum of the humins formed during conversion of cellulose (Figure 10a) is dominated by the resonances that are also present in the spectrum of untreated cellulose (Figure 10b). The resonances in the cellulose spectrum include the typical resonances of carbohydrate (vide supra).35 The similarity of the two spectra indicates that the humins are predominantly composed of saccharides. In contrast to the spectrum of cellulose, several resonances were observed in the region between 0 and 50 ppm in the humins spectrum, which are assigned to aliphatic carbon atoms.36 Moreover, resonances corresponding to unsaturated carbon species (olefins, aromatics) were observed in the region between 120 and 170 ppm, and a signal at 203 ppm indicated the presence of ketones or aldehydes. 4. Discussion 4.1. Dissolution and Hydrolysis of Wood in Ionic Liquids. The potential of ionic liquids as efficient solvents for cellulose was first suggested by Rogers and co-workers.17,18,20,37,38 It was demonstrated by 35Cl and 13C NMR spectroscopy that the chloride anions of the ionic liquid interact with the hydroxyl groups in cellulose and, thus, disrupt intermolecular hydrogen bonds.20 In the absence of these interactions, the individual cellulose chains become soluble and form a viscous solution.17 The solubility is reduced dramatically in the presence of water. Swatloski et al. reported that cellulose precipitates from the ionic-liquid-phase solution when more than 1 wt % water is

Figure 10. 13C CP MAS NMR spectra: (a) solid residues from the TFAcatalyzed conversion of cellulose in BMImCl, (b) cellulose.

added.17 The crystallinity of cellulose decreases significantly as a result of the treatment. It has been shown that this effect can be used to pretreat lignocellulosic biomass in order to increase the rate of enzymatic hydrolysis of the carbohydrate fraction.19,39,40 In a recent study, it was demonstrated that enzyme-catalyzed ionic-liquid-phase hydrolysis is possible with certain combinations of enzymes and ionic liquids.41 In addition to their ability to dissolve cellulose, imidazoliumbased ionic liquids are capable of dissolving a significant fraction of lignin.18,19,42 As for cellulose, it was suggested that the solubility is predominantly influenced by the anion, with methylsulfate anions being more efficient than chloride ions.42 However, the exact nature of the interactions that provide the solubility of lignin is not understood as well as in the case of cellulose. In additional studies, it was shown that 58-70 wt % of several wood species (oak, eucalyptus, poplar, and pine) can be dissolved in a mixture of BMImCl and dimethyl sulfoxide (DMSO) within 24 h.18 In ionic liquid solution, carbohydrates can be readily converted in a variety of chemical reactions because the accessibility of the saccharide functional groups increases significantly. For example, complete acetylation of cellulose was reported using 1-allyl-3-methylimidazolium chloride as the reaction medium.19,43 A particularly interesting route for the production of biofuels and commodity chemicals is depolymerization of the carbohydrate fraction by hydrolysis. Hydrolysis of cellulose21 and lignocellulosic biomass22 (corn stalk, rice straw, pine wood, and bagasse) was described in recent publications. The authors reported total reducing sugar yields of 66-81% (based on the carbohydrate content) for different types of lignocellulosic biomass. However, the determination of the sugar yields in this study depended solely on the absorbance of the sample at 540 nm. This method results in a considerable ambiguity because other reaction products could well absorb light at this wavelength. In particular, it is possible that a small amount of a chromophore that is formed as a side product could provide a major contribution to the yield determined in this way. For this reason, a more careful analytical approach was chosen in the present study, in which the monosaccharide yield was deter-


mined by HPAEC using a pulsed amperometric detector (PAD) that is well-suited for monosaccharide analysis. The results of this study are in agreement with previous publications in the conclusion that ionic-liquid-phase hydrolysis has significant potential for depolymerization of the carbohydrate fraction in lignocellulosic biomass under mild conditions.21,22 In the present case, 97% of the cellulose or 62 wt % of loblolly pine wood (equivalent to 97% of the carbohydrate fraction) was converted into water-soluble products within 2 h at 120 °C. Note that the data do not allow conclusions regarding the extent to which these water-soluble products are depolymerized, as only limited monosaccharides and some monosaccharide degradation products could be quantified (vide infra). However, the fact that the products are water-soluble (unlike cellulose and most constituents of wood) shows that the original wood must have been converted to some degree. The presence of a considerable amount of soluble oligosaccharides was demonstrated by secondary hydrolysis in an aqueous phase, which increased the monosaccharide concentration in the solution considerably. Although lignin is likely to be partially dissolved during the reaction, it is not hydrolyzed to a major extent under acidic conditions and remains as a solid residue, as demonstrated by NMR spectroscopy (vide infra). It is important to note that the reaction temperature applied in this study is much lower than the conditions required to hydrolyze crystalline cellulose in an aqueous phase (140-220 °C).31,44,45 In addition to the decreased crystallinity of cellulose, BMImCl also provides a reaction medium in which the presence of lignin does not seem to hinder the hydrolysis reactions. In contrast, it has been reported for aqueous-phase reactions that the presence of lignin limits the accessibility of the glycosidic bonds in the polysaccharide fraction.46 Consequently, the enzymatic digestibility of lignocellulosic biomass decreases with increasing lignin content.47-49 The present results show that ionic-liquid-phase hydrolysis has the potential to eliminate this hindrance. It is assumed that the solubility of lignin in ionic liquids such as BMImCl greatly facilitates the separation of different fractions of lignocellulosic biomass.18,19,42 A particularly interesting observation was that, in the initial stage of the reaction, the formation of soluble products was faster for the carbohydrate fraction of loblolly pine wood than for cellulose (Figure 2). A possible explanation is that the amorphous parts of the carbohydrate fraction (e.g., hemicellulose) in loblolly pine wood are converted faster than crystalline cellulose. Alternatively, the differences in the initial rates could be caused by mass-transport limitations. Fibrous cellulose is dissolved in BMImCl in a relatively short time, forming a highly viscous solution.17 It is assumed that the high viscosity leads to diffusion limitations on the reactions under the given conditions. In contrast to the dissolution of cellulose, the carbohydrates from the wood particles are gradually released into the ionic liquid phase.18 It has been reported that the nondestructive dissolution of carbohydrates and lignin from wood in ionic liquids takes several hours.18,19 In the presence of an acid catalyst, the dissolved carbohydrates are depolymerized by hydrolysis. It is suggested that this transformation occurs sufficiently fast to limit the concentration of dissolved polymeric carbohydrate to a level where transport limitations would be much less severe than in the case of the conversion of cellulose. If this was the case, a gradual release of carbohydrates into the solution would allow for faster conversion. 4.2. Consecutive Reactions of Monosaccharides in Ionic Liquids. Although the literature on acid-catalyzed conversion of monosaccharides in ionic liquids is limited,23,50,51 a multitude

of pathways have been reported in literature for reactions in aqueous phase.9-14,52-55 These routes include isomerization,52-54 dehydration,9-14 and C-C bond cleavage (e.g., retro-aldol reactions).52,54,55 One of the most frequently discussed reactions is the acidcatalyzed formation of HMF and furfural from hexoses and pentoses, respectively.9-14 As the dehydration is significantly faster for ketoses than for aldoses,10 it has been suggested that the formation of HMF occurs via fructose as an intermediate.9 In fact, high selectivities and yields were reported for HMF production from fructose,12,23,56 whereas the selectivity is usually much lower when glucose is the starting material.57 Exceptionally selective conditions were reported by Zhao et al. in the ionic liquid phase (1-ethyl-3-methylimidazolium chloride) using chromium chlorides (CrCl2 and CrCl3) to catalyze the conversion of glucose to HMF.23 The authors suggested that the key step of the reaction, the isomerization of glucose to fructose, is greatly facilitated by anionic chromium chloride species that catalyze proton transfer. Much lower HMF yields were obtained with other metal chlorides and sulfuric acid. It is interesting to note that, in imidazolium-based ionic liquids, HMF can be produced from fructose even without an additional catalyst.50 HMF and furfural can be converted in various consecutive reactions including hydrolysis to form levulinic acid and formic acid7 or by polymerization.58,59 The observations in our study are in line with the commonly proposed pathway for HMF formation involving a sequence of hydrolysis of polysaccharides to monosaccharides followed by dehydration of the monosaccharides. Although fructose might be formed as an intermediate,9 it was never detected as a liquidphase product. This shows that either there is a direct route from glucose to HMF or fructose is a relatively short-lived intermediate. In principal, the reaction pathways for the formation and consumption of pentoses and hexoses are comparable. However, distinct differences in the reactivity were observed. A fraction of hexoses below the corresponding value in the feedstock (86 wt %) in the initial stage of the reactions at 100 and 120 °C indicates preferential hydrolysis of the pentose-rich hemicellulose in the beginning of the reaction. As the reaction proceeds, the fraction of hexoses increases progressively, showing that pentoses are more readily converted to secondary products, such as furfural, which is formed by dehydration. However, rather low furfural yields were observed in this study, which is only partially explained by the fact that pentoses are much less abundant in loblolly pine wood than hexoses (Table 1). It is suggested that the conversion of furfural into other degradation products appears to be quite rapid. The present data suggest pronounced differences in the relative reactivities of pentoses and hexoses that deserve more detailed studies in the future. 4.3. Influence of Water and Acid Concentrations. The concentrations of water and TFA in the reaction mixture had a considerable influence on the conversion and product distribution. In particular, the concentration of water needs to be optimized. On one hand, water is required for the hydrolysis reaction, in which one molecule of water is consumed for every glycosidic bond that is broken. On the other hand, polymeric carbohydrates are no longer soluble in ionic liquids when the water content exceeds a certain level.17 The influence of these constraints was clearly observed in the present study. Small amounts of water (up to 2 wt %) facilitate hydrolysis and, therefore, increase the soluble products yield and, in particular, the monosaccharide yield. It is likely that soluble oligosaccharides are among the main products when the water concentration is below 2 wt %. When no water is


added, dehydration of monosaccharides to HMF or furfural is likely to provide the main water source for additional hydrolysis. We note that the concentration of residual water in the dried wood and ionic liquid was high enough to start the reaction. As the concentration of water increases, it increasingly competes with the hydroxyl groups in carbohydrates for chloride ions as a hydrogen-bonding partner.17 However, the formation of hydrogen bonds between the chloride anions of BMImCl and hydroxyl groups is essential for the solubility of cellulose in this ionic liquid.37 If the hydrogen-bonding interactions are prevented, cellulose or the cellulose fraction in wood remains in its crystalline form, which is less accessible for hydrolysis by soluble acids. It is interesting to note that Swatloski et al. reported a water concentration of 1 wt % as the upper limit for the solubility of cellulose,17 whereas in our study, the decrease of the soluble products yields was observed only when more than 4 wt % water was present. This indicates that it might not be necessary to form a true solution of carbohydrates in the ionic liquid but that a sufficiently strong interaction with the ionic liquid is enough to render the glycosidic bonds susceptible to cleavage. When a certain critical concentration of water is exceeded, the reactivity is similar to that of acid-catalyzed reactions in an aqueous phase, including significantly lower yields of soluble products and monosaccharides. Studies of depolymerization of lignocellulosic biomass with dilute mineral acids in an aqueous phase have shown that, under mild conditions (as applied here), only the noncrystalline hemicellulose fraction is susceptible to hydrolysis.44 However, it has been proposed that the crystalline cellulose fraction is modified under these ionic liquid conditions, resulting in improved accessibility.44 Similarly to the monosaccharide yield, the HMF and furfural yields initially increased with increasing water content. This trend might appear counterintuitive because these products are formed in a dehydration reaction. However, the observation can be rationalized by considering the increased concentration of monosaccharides in solution that can be converted into HMF and furfural. The decrease of the HMF yield at high water content might be explained by consumption of HMF in an acidcatalyzed hydrolysis reaction, forming levulinic acid and formic acid (Figure 1). The concentration of acid has a pronounced influence on the composition of the product mixture. The data in Figure 8 suggest that the highest soluble products yield is obtained in the presence of 0.4 wt % TFA. It is assumed that a lower acid concentration is not sufficient for complete conversion of the carbohydrate fraction within the reaction time of 4 h whereas a higher acid concentration favors the formation of insoluble degradation products (humins). The decrease of the HMF and furfural yields at higher acid concentrations indicates that these compounds are lost in consecutive reactions, which might include the formation of humins. In agreement with Li et al., we found that the type of acid does not play a major role as long as sufficiently strong acids are used.22 4.4. Modification of Solid Residues. Acid-catalyzed hydrolysis in aqueous media is commonly performed to convert the carbohydrate fraction of biomass into monosaccharides.31,60,61 The solid residue from such reactions contains most of the lignin that was present in the starting material. Although typically only a small part of the lignin is dissolved, lignin can by modified by various chemical reactions.62,63 13 C CP MAS NMR spectroscopy is a powerful tool for the characterization of solid carbon-based materials, such as biomass residues. It has to be mentioned that cross polarization (CP)

was used here to enhance the signal-to-noise ratio. This pulse sequence is not necessarily quantitative because the sensitivity of a given nucleus depends on its chemical environment.29,64 Therefore, it is not possible to determine the exact concentrations of various carbon species without additional information. However, it is possible to compare relative intensities of different resonances and to observe qualitative trends in the concentrations of corresponding species. For the discussion of the differences between the residues produced in ionic-liquid-phase hydrolysis of loblolly pine and from Klason lignin produced from the same starting material, it is important to remember that both treatments are invasive and likely lead to modifications of lignin. In particular, the treatment with 72% sulfuric acid in the Klason method exposes the biomass sample to strongly acidic conditions. This might lead to carbenium-ion-based depolymerization and repolymerization reactions similar to those under steam explosion conditions where aryl ethers are converted to phenols and ketones.62 Comparison with the 13C CP MAS NMR spectrum of Klason lignin (Figure 9c) demonstrates that lignin is the main constituent in the spectrum of the residue from ionic-liquid-phase hydrolysis (Figure 9b). However, it is also clear that different structural units are present in the samples, indicating that loblolly pine lignin is modified by at least one of the treatments. The most pronounced difference is a reduction of the peak corresponding to oxygen-substituted olefinic or aromatic carbon atoms in the spectrum of Klason lignin, indicating that the strongly acidic conditions applied in this method alter the molecular structure of lignin significantly. Another indication of modifications of the lignin fraction is the observation of considerable resonances in the aliphatic region, in particular, in the spectrum of Klason lignin. These species must be formed during the treatment, because only small amounts of aliphatics were reported to be present in untreated loblolly pine wood.63 The same observation was made during hydrolysis of loblolly pine wood with dilute aqueous acid solution.63 The present data provide evidence for a significant transformation of lignin in aqueous media, whereas these modifications appear to be much less dramatic in ionic liquid phase. In addition to modified or unmodified constituents of the starting material, solid residues can contain compounds that are formed in liquid-phase reactions. Various scenarios have been suggested for the formation of these products, which are commonly referred to as humins. One such possibility is polymerization of HMF and furfural, which was suggested to occur without involving other reactants.58,59 The 13C CP MAS NMR spectrum of humins from ionicliquid-phase hydrolysis of loblolly pine wood (Figure 10a) shows that sugars are the dominant building block. However, the formation of insoluble polysaccharides by simple condensation of monosaccharides or soluble oligosaccharides can be excluded because it was shown that these materials are readily converted into soluble species under the conditions used in this study. Therefore, the inverse reaction, spontaneous repolymerization of cellulose under unchanged conditions, would be rather unlikely. It is suggested that the connections between these saccharide units are more resistant to hydrolysis than glycosidic bonds, which are readily cleaved under the given experimental conditions. We speculate that HMF, furfural, or anhydrous sugars might be among the constituents of the humins. Additionally, aliphatic species that are also observed in the spectrum might be incorporated as bridging units. These compounds might be


formed by repeated dehydration and hydration reactions starting from monosaccharides. 4.5. Potential of Ionic-Liquid-Phase Conversion of Biomass for Industrial Processes. The development of ionic-liquid-phase processes for the conversion of biomass will require an efficient method of separating the products from the reaction media. This is particularly challenging when polar monosaccharides need to be separated from very polar reaction media, such as ionic liquids. The similar polarities of the products and the reaction medium will render separation by extraction rather difficult. Likewise, isolation of monosaccharides by distillation will be almost impossible because of the negligible vapor pressure of ionic liquids and monosaccharides. In contrast to monosaccharides, HMF and furfural have physicochemical properties that are sufficiently different from those of the ionic liquid. Therefore, it is possible that these products can be isolated from ionic liquid phase. Extraction of HMF from aqueous phase with a relatively nonpolar solvent has been demonstrated successfully.65 However, extractions are always limited by the partition coefficients under given conditions. An alternative approach might be distillation, in which the products, TFA, and water are removed from the reaction mixture while the ionic liquid remains in the reaction vessel. However, the boiling points of furfural and HMF are 162 °C (at ambient pressure) and 115 °C (at 10 mTorr), respectively. As the boiling points of water and TFA are below this range, separation by distillation would have to include recycling of these compounds. It is likely that the selectivity toward HMF and furfural can be increased if extraction or distillation is applied continuously. In addition to a possible improvement of the selectivity, continuous operation would also facilitate the integration of new processes into an existing refining infrastructure. 5. Conclusions Acid-catalyzed conversion of loblolly pine wood is facilitated in 1-butyl-3-methylimidazolium chloride because the ionic liquid increases the accessibility of glycosidic bonds in the carbohydrate fraction. It was shown that almost the entire carbohydrate fraction of the starting material can be converted into watersoluble products. Unlike in aqueous-phase reactions, the presence of the lignin matrix does not hinder the hydrolysis process. At the same time, most of the lignin fraction remains as a solid residue, so that almost complete sugar-lignin fractionation is achieved. Nearly complete conversion of the carbohydrate fraction into water-soluble products is readily observed at 120 °C, which is much lower than the temperatures typically applied for aqueous-phase hydrolysis. The focus of future work should be on improving the selectivity of the reaction and developing efficient strategies for separation of the products, as these appear to be key limiting issues of this approach. Moreover, a controlled, continuous addition of water could help to improve the selectivity to monomeric compounds (e.g., monosaccharides, HMF). Acknowledgment This work was supported by Chevron through the Georgia Tech Strategic Energy Institute. We thank Prof. Art Ragauskas for obtaining the loblolly pine feedstock and Kathleen Poll for its proximate analysis. Literature Cited (1) Huber, G. W.; Dumesic, J. A. An overview of aqueous-phase catalytic processes for production of hydrogen and alkanes in a biorefinery. Catal. Today 2006, 111, 119.

(2) Huber, G. W.; Iborra, S.; Corma, A. Synthesis of Transportation Fuels from Biomass: Chemistry, Catalysts, and Engineering. Chem. ReV. 2006, 106, 4044. (3) Valenzuela, M. B.; Jones, C. W.; Agrawal, P. K. Batch aqueousphase reforming of woody biomass. Energy Fuels 2006, 20, 1744. (4) Perlack, R. D.; Wright, L. L.; Turhollow, A.; Graham, R. L.; Stokes, B.; Erbach, D. C. Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply; Oak Ridge National Laboratory: Oak Ridge, TN, 2005. (5) Sun, Y.; Cheng, J. Y. Hydrolysis of lignocellulosic materials for ethanol production: A review. Bioresour. Technol. 2002, 83, 1. (6) Wyman, C. E.; Dale, B. E.; Elander, R. T.; Holtzapple, M.; Ladisch, M. R.; Lee, Y. Y. Coordinated development of leading biomass pretreatment technologies. Bioresour. Technol. 2005, 96, 1959. (7) Horvat, J.; Klaic, B.; Metelko, B.; Sunjic, V. Mechanism of Levulinic Acid Formation. Tetrahedron Lett. 1985, 26, 2111. (8) Werpy, T.; Petersen, G. : Vol. 1 Top Value Added Chemicals from Biomass: Volume 1sResults of Screening for Potential Candidates from Sugars and Synthesis Gas; Technical Report NREL/TP-510-35523; National Renewable Energy Laboratory: Golden, CO, 2004. (9) Antal, M. J.; Mok, W. S. L.; Richards, G. N. Kinetic studies of reactions of ketoses and aldoses in water at high temperature. 1. Mechanism of formation of 5-(hydroxymethyl)-2-furaldehyde from D-fructose and sucrose. Carbohydr. Res. 1990, 199, 91. (10) Robyt, J. F. Essentials of Carbohydrate Chemistry; Springer: New York, 1998. (11) Moreau, C.; Durand, R.; Razigade, S.; Duhamet, J.; Faugeras, P.; Rivalier, P.; Ros, P.; Avignon, G. Dehydration of fructose to 5-hydroxymethylfurfural over H-mordenites. Appl. Catal. A-Gen. 1996, 145, 211. (12) Roman-Leshkov, Y.; Chheda, J. N.; Dumesic, J. A. Phase modifiers promote efficient production of hydroxymethylfurfural from fructose. Science 2006, 312, 1933. (13) Moreau, C.; Durand, R.; Alies, F. R.; Cotillon, M.; Frutz, T.; Theoleyre, M. A. Hydrolysis of sucrose in the presence of H-form zeolites. Ind. Crop Prod. 2000, 11, 237. (14) Kuster, B. F. M. 5-Hydroxymethylfurfural (HMF)sA review focussing on its manufacture. Starch-Stärke 1990, 42, 314. (15) Paˆrvulescu, V. I.; Hardacre, C. Catalysis in Ionic Liquids. Chem. ReV. 2007, 107, 2615. (16) Sievers, C.; Jimenez, O.; Mu¨ller, T. E.; Steuernagel, S.; Lercher, J. A. Formation of solvent cages around organometallic complexes in thin films of supported ionic liquid. J. Am. Chem. Soc. 2006, 128, 13990. (17) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. Dissolution of cellose with ionic liquids. J. Am. Chem. Soc. 2002, 124, 4974. (18) Fort, D. A.; Remsing, R. C.; Swatloski, R. P.; Moyna, P.; Moyna, G.; Rogers, R. D. Can ionic liquids dissolve wood? Processing and analysis of lignocellulosic materials with 1-n-butyl-3-methylimidazolium chloride. Green Chem. 2007, 9, 63. (19) Kilpela¨inen, I.; Xie, H.; King, A.; Granstrom, M.; Heikkinen, S.; Argyropoulos, D. S. Dissolution of wood in ionic liquids. J. Agric. Food Chem. 2007, 55, 9142. (20) Remsing, R. C.; Swatloski, R. P.; Rogers, R. D.; Moyna, G. Mechanism of cellulose dissolution in the ionic liquid 1-n-butyl-3methylimidazolium chloride: a C-13 and Cl-35/37 NMR relaxation study on model systems. Chem. Commun. 2006, 1271. (21) Li, C.; Zhao, Z. K. Efficient Acid-Catalyzed Hydrolysis of Cellulose in Ionic Liquid. AdV. Synth. Catal. 2007, 349, 1847. (22) Li, C.; Wanga, Q.; Zhao, Z. K. Acid in ionic liquid: An efficient system for hydrolysis of lignocellulose. Green Chem. 2008, 10, 177. (23) Zhao, H. B.; Holladay, J. E.; Brown, H.; Zhang, Z. C. Metal chlorides in ionic liquid solvents convert sugars to 5-hydroxymethylfurfural. Science 2007, 316, 1597. (24) TAPPI Test Method T 211 om-93; TAPPI Press: Atlanta, GA, 1993. (25) TAPPI Test Method T 264 cm-97; TAPPI Press: Atlanta, GA, 1997. (26) TAPPI Test Method T 222 om-98; TAPPI Press: Atlanta, GA, 1998. (27) TAPPI Test Method T 249 cm-00; TAPPI Press: Atlanta, GA, 2000. (28) Gutowski, K. E.; Broker, G. A.; Willauer, H. D.; Huddleston, J. G.; Swatloski, R. P.; Holbrey, J. D.; Rogers, R. D. Controlling the aqueous miscibility of ionic liquids: Aqueous biphasic systems of water-miscible ionic liquids and water-structuring salts for recycle, metathesis, and separations. J. Am. Chem. Soc. 2003, 125, 6632. (29) Laws, D. D.; Bitter, H.-M. L.; Jerschow, A. Solid-State NMR Spectroscopic Methods in Chemistry. Angew. Chem., Int. Ed. 2002, 41, 3096. (30) Sjo¨stro¨m, E. Wood Chemistry; 2nd ed.; Academic Press: New York, 1999.

1286 Ind. Eng. Chem. Res., Vol. 48, No. 3, 2009 (31) Marzialetti, T.; Valenzuela-Olarte, M. B.; Sievers, C.; Hoskins, T. J. C.; Agrawal, P. K.; Jones, C. W. Dilute Acid Hydrolysis of Loblolly Pine: A Comprehensive Approach. Ind. Eng. Chem. Res. 2008, 47, 7131. (32) Dietz, M. L.; Dzielawa, J. A. Ion-exchange as a mode of cation transfer into room-temperature ionic liquids containing crown ethers: Implications for the ‘greenness’ of ionic liquids as diluents in liquid-liquid extraction. Chem. Commun. 2001, 2124. (33) Capanema, E. A.; Balakshin, M. Y.; Kadla, J. F. A comprehensive approach for quantitative lignin characterization by NMR spectroscopy. J. Agric. Food Chem. 2004, 52, 1850. (34) Nimz, H. H.; Robert, D.; Faix, O.; Nemr, M. C-13 NMR spectra of lignins 8. Structural differences between lignins of hardwoods, softwoods, grasses and compression wood. Holzforschung 1981, 35, 16. (35) Dudley, R. L.; Fyfe, C. A.; Stephenson, P. J.; Deslandes, Y.; Hamer, G. K.; Marchessault, R. H. High-Resolution Carbon-13 CP/MAS NMR Spectra of Solid Cellulose Oligomers and the Structure of Cellulose-II. J. Am. Chem. Soc. 1983, 105, 2469. (36) Hesse, M.; Meier, H.; Zeeh, B. Spektroskopische Methoden in der organischen Chemie, 5th ed.; Thieme: Stuttgart, Germany, 1995. (37) Moulthrop, J. S.; Swatloski, R. P.; Moyna, G.; Rogers, R. D. Highresolution C-13 NMR studies of cellulose and cellulose oligomers in ionic liquid solutions. Chem. Commun. 2005, 1557. (38) Fort, D. A.; Swatloski, R. P.; Moyna, P.; Rogers, R. D.; Moyna, G. Use of ionic liquids in the study of fruit ripening by high-resolution C-13 NMR spectroscopy: ‘Green’ solvents meet green bananas. Chem. Commun. 2006, 714. (39) Dadi, A. P.; Varanasi, S.; Schall, C. A. Enhancement of cellulose saccharification kinetics using an ionic liquid pretreatment step. Biotechnol. Bioeng. 2006, 95, 904. (40) Dadi, A. P.; Schall, C. A.; Varanasi, S. Mitigation of cellulose recalcitrance to enzymatic hydrolysis by ionic liquid pretreatment. Appl. Biochem. Biotechnol. 2007, 137, 407. (41) Zhao, H.; Baker, G. A.; Song, Z.; Olubajo, O.; Crittle, T.; Peters, D. Designing enzyme-compatible ionic liquids that can dissolve carbohydrates. Green Chem. 2008, 10, 696. (42) Pu, Y.; Jiang, N.; Ragauskas, A. J. Ionic Liquid as a Green Solvent for Lignin. J. Wood Chem. Technol. 2007, 27, 23. (43) Wu, J.; Zhang, J.; Zhang, H.; He, J. S.; Ren, Q.; Guo, M. Homogeneous acetylation of cellulose in a new ionic liquid. Biomacromolecules 2004, 5, 266. (44) Torget, R.; Walter, P.; Himmel, M.; Grohmann, K. Dilute-acid pretreatment of corn residues and short-rotation woody crops. Appl. Biochem. Biotechnol. 1991, 28-9, 75. (45) Mosier, N.; Wyman, C.; Dale, B.; Elander, R.; Lee, Y. Y.; Holtzapple, M.; Ladisch, M. Features of promising technologies for pretreatment of lignocellulosic biomass. Bioresour. Technol. 2005, 96, 673. (46) Garrote, G.; Dominguez, H.; Parajo, J. C. Hydrothermal processing of lignocellulosic materials. Holz Roh Werkst. 1999, 57, 191. (47) Yang, B.; Boussaid, A.; Mansfield, S. D.; Gregg, D. J.; Saddler, J. N. Fast and efficient alkaline peroxide treatment to enhance the enzymatic digestibility of steam-exploded softwood substrates. Biotechnol. Bioeng. 2002, 77, 678. (48) Gharpuray, M. M.; Lee, Y. H.; Fan, L. T. Structural modification of lignocellulosics by pretreatments to enhance enzymatic hydrolysis. Biotechnol. Bioeng. 1983, 25, 157. (49) Grabber, J. H.; Ralph, J.; Lapierre, C.; Barriere, Y. Genetic and molecular basis of grass cell-wall degradability. I. Lignin-cell wall matrix interactions. C. R. Biol. 2004, 327, 455.

(50) Zhao, H.; Holladay, J. E.; Zhang, Z. C. Methods for conversion of carbohydrates in ionic liquids to value-added chemicals. U.S. Patent 20080033187, 2008. (51) Moreau, C.; Finiels, A.; Vanoye, L. Dehydration of fructose and sucrose into 5-hydroxymethylfurfural in the presence of 1-H-3-methyl imidazolium chloride acting both as solvent and catalyst. J. Mol. Catal. A: Chem. 2006, 253, 165. (52) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Glucose and fructose decomposition in subcritical and supercritical water: Detailed reaction pathway, mechanisms, and kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888. (53) Kooyman, C.; Vellenga, K.; Dewilt, H. G. J. Isomerization of D-glucose into D-fructose in aqueous alkaline solutions. Carbohydr. Res. 1977, 54, 33. (54) Sasaki, M.; Goto, K.; Tajima, K.; Adschiri, T.; Arai, K. Rapid and selective retro-aldol condensation of glucose to glycolaldehyde in supercritical water. Green Chem. 2002, 4, 285. (55) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Lowtemperature catalytic gasification of lignin and cellulose with a ruthenium catalyst in supercritical water. Energy Fuels 2004, 18, 327. (56) Bao, Q. X.; Qiao, K.; Tomida, D.; Yokoyama, C. Preparation of 5-hydroymethylfurfural by dehydration of fructose in the presence of acidic ionic liquid. Catal. Commun. 2008, 9, 1383. (57) Aida, T. M.; Sato, Y.; Watanabe, M.; Tajima, K.; Nonaka, T.; Hattori, H.; Arai, K. Dehydration of D-glucose in high temperature water at pressures up to 80 MPa. J. Supercrit. Fluids 2007, 40, 381. (58) Asghari, F. S.; Yoshida, H. Kinetics of the decomposition of fructose catalyzed by hydrochloric acid in subcritical water: Formation of 5-hydroxymethylfurfural, levulinic, and formic acids. Ind. Eng. Chem. Res. 2007, 46, 7703. (59) Hashaikeh, R.; Butler, I. S.; Kozinski, J. A. Selective promotion of catalytic reactions during biomass gasification to hydrogen. Energy Fuels 2006, 20, 2743. (60) Nguyen, Q. A.; Tucker, M. P.; Keller, F. A.; Beaty, D. A.; Connors, K. M.; Eddy, F. P. Dilute acid hydrolysis of softwoods. Appl. Biochem. Biotechnol. 1999, 77-9, 133. (61) Nagle, N.; Ibsen, K.; Jennings, E. A process economic approach to develop a dilute-acid cellulose hydrolysis process to produce ethanol from biomass. Appl. Biochem. Biotechnol. 1999, 77-9, 595. (62) Li, J. B.; Henriksson, G.; Gellerstedt, G. Lignin depolymerization/ repolymerization and its critical role for delignification of aspen wood by steam explosion. Bioresour. Technol. 2007, 98, 3061. (63) Sievers, C.; Marzialetti, T.; Hoskins, T. J. C.; Valenzuela-Olarte, M. B.; Agrawal, P. K.; Jones, C. W., Quantitative characterization of residues from aqueous phase acid hydrolysis by solid state NMR spectroscopy. Bioresour. Technol., in press. (64) Conte, P.; Spaccini, R.; Piccolo, A. State of the art of CPMAS C-13-NMR spectroscopy applied to natural organic matter. Prog. Nucl. Magn. Reson. Spectrosc. 2004, 44, 215. (65) Roman-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; A., D. J. Production of dimethylfuran for liquid fuels from biomass-derived carbohydrates. Nature 2007, 447, 982.

ReceiVed for reView April 3, 2008 ReVised manuscript receiVed November 10, 2008 Accepted November 12, 2008 IE801174X

Ionic-Liquid-Phase Hydrolysis of Pine Wood - ACS Publications

Recommend Documents