Effective Production of Levulinic Acid from Biomass through

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Effective Production of Levulinic Acid from Biomass through Pretreatment Using Phosphoric Acid, Hydrochloric Acid, or Ionic Liquid Yosuke Muranaka, Tatsuya Suzuki, Hiroyuki Sawanishi, Isao Hasegawa, and Kazuhiro Mae* Department of Chemical Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT: Levulinic acid is one of the top value-added chemicals. It can be obtained through the hydrolysis of some types of saccharides such as glucose, the constituent unit of cellulose. Cellulose is the most abundant natural resource on this planet; however, its rigid structure prevents the easy utilization of cellulose. In this study, an efficient method for the conversion of cellulose to levulinic acid was sought by reforming the structure of cellulose through several types of pretreatment. Pretreatment with highly concentrated acid was found to enhance the conversion efficiency of the subsequent acidic hydrothermal treatment. Through pretreatment with highly concentrated acid at 50 °C and acidic hydrothermal treatment in the range of 200−210 °C, cellulose was converted into levulinic acid by 40.1 C-% (48.2 mol %) and 49.2 C-% (59.1 mol %) when phosphoric acid and hydrochloric acid, respectively, were used as the reagent. When ionic liquid was used as the reagent for pretreatment, 60.7 C-% (72.9 mol %) of cellulose was successfully converted into levulinic acid. In addition, the key factors for the conversion were clarified to be the decrease of crystallinity; the solubilization of cellulose; and in the case of treatment using ionic liquid, an appropriate interaction between the ionic liquid and cellulose.

1. INTRODUCTION The annual energy consumption in the world is increasing each year. Energy production mostly relies on fossil resources, and the mass consumption of these resources, leading to their exhaustion, is considered to be an important issue. CO2 discharged through the use of fossil resources is also considered as a problem for promoting global warming, with gradually increasing effects. To reduce societal dependence on fossil resources, biomass is considered as a potential alternative material. The amount of CO2 discharged through the use of biomass is comparable to the amount of CO2 absorbed by the plants that make up the biomass. This unique nature of biomass, which is known as “carbon neutrality”, provides no increase of CO2 in the air if biomass is continuously used as the resource.1,2 With this nature, biomass has the potential to be a substitute for fossil resources with the advantage of restraining CO2 emissions, so the investigation of biomass utilization is considered to be important. Furthermore, in contrast to fossil resources, which are one-way disposable resources, biomass is sustainably usable for its short span of growth, which makes biomass an important element in a recycling-based society.3,4 Biomass can possibly be utilized as an alternative energy resource or an alternative raw material to fossil resources. However, as the source of energy, biomass has huge barriers such as a relatively low energy content, seasonality, discrete geographic availability, high moisture content, and high collection costs.5 In addition, the amount of biomass available for energy use is highly limited. As an alternative raw material, on the other hand, biomass would be a beneficial resource because chemical production requires far lower volumes of biomass to satisfy demand.6 To utilize biomass as an alternative raw material, we should aim at not just a particular component but various types of components. This is because biomass consists of three main components, namely, cellulose, hemi© XXXX American Chemical Society

cellulose, and lignin, and they have different characteristics from each other, so that aiming just a particular component would lead to the waste of the other potential components. In 2004, the National Renewable Energy Laboratory of the U.S. Department of Energy identified 12 chemicals that should be made from biomass.7 These chemicals, which could be used as building-block chemicals, potentially enable biomass to be a main resource for making chemicals that are produced from petroleum today. Levulinic acid is one of these 12 chemicals, and it can be obtained through the hydrolysis of some types of saccharides such as glucose or fructose.8,9 The uses of levulinic acid are quite various. For example, it can be used to make materials such as nylon-like polymer, artificial rubber, and plastics.10 It can also be an intermediate for medical supplies or materials for making some other valuable chemicals.11 For example, 2-methyltetrahydrofuran, which is an alternative highboiling-temperature solvent to tetrahydrofuran, can be made through the dehydration and hydrogenation of levulinic acid.12 2-Methyltetrahydrofuran is also available as the electrolyte for lithium rechargeable batteries or as an alternative fuel. Other examples of valuable chemicals that can be made from levulinic acid are γ-valerolactone and ethyl levulinate.13,14 A molecule of levulinic acid consists of five carbons, and theoretically, 1 mol of levulinic acid can be obtained from 1 mol of hexose. To obtain levulinic acid from biomass, the degradation of cellulose, one of the main components of biomass, is required. Scheme 1 shows the pathway for the degradation of cellulose to levulinic acid. For the beginning step, hydrolysis of cellulose produces glucose, and then glucose Received: May 2, 2014 Revised: June 24, 2014 Accepted: June 27, 2014

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dx.doi.org/10.1021/ie501811x | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Scheme 1. Conversion Pathways of Cellulose into Levulinic Acid

with an internal volume of 60 cm3 and a glass bottle with an internal volume of 50 cm3. 2.1. Samples. Filter-paper cellulose and pulverized cedar (Cryptomeria japonica) were used as the raw materials. Amorphous cellulose was also used for comparison. Both types of cellulose were obtained from Kao Corporation (Tokyo, Japan). The ultimate analyses of the samples used are listed in Table 1. Phosphoric acid (85 wt %), hydrochloric

can isomerize into fructose. Fructose can be dehydrated into 5hydroxymethylfurfural (HMF). Levulinic acid is produced through the hydrolysis of HMF under acidic conditions. Concerning the degradation of cellulose to glucose, enzymatic saccharification is a well-known method.15−18 The huge advantage of enzymatic saccharification is high selectivity. However, the bioprocess requires high costs because it requires high technology of sewage treatment and complicated reactor control. In addition, sludge produced by the process decreases the efficiency and leads to long reaction times. Considering these disadvantages of biochemical technology, it is necessary to develop a thermochemical technology, but thermochemical technology has the disadvantage of low selectivity under the present circumstances. Many studies of the conversion of biomass or saccharides to levulinic acid by thermochemical technology have been reported. In particular, as can be assumed from Scheme 1, there are many studies on methods of converting fructose or glucose in high yields of about 80 mol % (the definition of conversions are provided in section 2.5. Definition of Conversions).19,20 In one of these reports on the conversion of cellulose, Girisuta et al. obtained levulinic acid in a yield of 60 mol % at 150 °C using 1 M sulfuric acid;9 however, the yield was lower than that obtained using monosaccharides. This is because the rigid crystalline structure of cellulose containing strong hydrogen bonds. For the production of levulinic acid from inedible plant biomass, a method for reforming this rigid structure of cellulose and converting the materials into easily decomposable ones is an important key. Although many cellulose reforming technologies have been invented so far, most of them are for the production of saccharides from cellulose, whereas few of them are for the production of levulinic acid. In this study, considering pretreatment followed by acidic hydrothermal treatment of the target chemical, levulinic acid, we first conducted pretreatments using highly concentrated acid. In addition, according to the many recent reports about the high efficiency of ionic liquids in the production of saccharides from cellulose, we also examined the efficiency of pretreatments using ionic liquid for the conversion.

Table 1. Ultimate Analysis of Samples Used crystalline cellulose amorphous cellulose cedar

C (wt %)

H (wt %)

O (wt %)

H/C

42.95 43.37 46.87

6.20 6.12 4.84

50.85 50.52 48.28

0.14 0.14 0.10

acid (20 wt %), and the ionic liquids 1-ethyl-3-methylimidazolium bromide ([EMIM]Br) and 1-ethyl-3-methylimidazolium methylphosphonate ([EMIM]P) were used for pretreatments, and sulfuric acid (5 wt %) was used as the hydrolysis reagent. 2.2. Treatment with Acid. Filter-paper cellulose was pretreated with phosphoric acid. The reason for choosing phosphoric acid as the reagent is that phosphoric acid is known to change the type of cellulose by breaking its hydrogen bonds.21 Cellulose, as a highly crystalline material, is known to dissolve in highly concentrated acids. Unlike solubilization by hydrolysis using sulfuric or hydrochloric acid, for instance, cellulose solubilized using phosphoric acid is regenerated as a deposit by adding water. Regenerated cellulose no longer has the original structure of the cellulosic particles or fibers. Zhang et al. reported the details of the changes in the structure of cellulose upon phosphoric acid treatment.22 One gram of filterpaper cellulose was pretreated at room temperature using 3 mL of phosphoric acid in a sealed batch reactor. The pretreatment time was set to 0−168 h (1 week). After the pretreatment, 50 mL of pure water was added to the sample, and the batch reactor was sealed again. For the second treatment, the reactor vessel was put in an oil bath heated to 160 °C for 120 min. The reactor was cooled in a water bath, and the products were filtered by suction after the second treatment. Filtrates were diluted with 250 mL of pure water and analyzed by total organic carbon (TOC) analysis and ion chromatography to identify the carbon conversions into solution and levulinic acid. On the other hand, the effect of the type of pretreatment reagent was also examined. One gram of filter-paper cellulose was pretreated in a sealed batch reactor at 50 or 100 °C for 24 h using 2 mL of phosphoric or hydrochloric acid. After the

2. EXPERIMENTAL SECTION Levulinic acid is one of the 12 chemicals identified by the U.S. Department of Energy as being capable of being obtained through the degradation of cellulose. To achieve the thermochemical degradation effectively, we examined the effect of pretreatment using some acids. The experiments were performed using a Swagelok (316 stainless steel) batch reactor B



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cellulose.31 Water-soluble cellulose is available for many processes such as liquid-phase reactions. We investigated the effect of pretreatment with [EMIM]P on the products of acidic hydrothermal degradation. For the pretreatment, 0.5 g of cellulose or cedar and 5.0 g of [EMIM]P were mixed in a glass bottle and put in an oil bath heated to 150 °C for 1 h under stirring. Immediately after heating, the products were cooled, and 5 mL of pure water was added. For the cellulose sample, because the products are converted into water-soluble components by [EMIM]P, 200 mL of ethanol was added for the recovery after the addition of 5 mL of water, and then the deposit was separated by centrifugation. The deposit and the solubilized cellulose without the addition of ethanol are denoted as as P-cellulose-deposit and P-cellulose, respectively, in this article. For the cedar sample, because it was mostly insoluble, the products were filtered by suction and washed with pure water. The residue is denoted as P-cedar hereafter in this article. The solid samples, P-cedar and P-cellulose-deposit, were dried in vacuo and then analyzed by CHNS elemental analysis, TG analysis, and XRD. The flowchart for the pretreatment of cellulose with [EMIM]P is shown in Scheme 2. For the acidic hydrothermal treatment, because the states

pretreatment, 50 mL of pure water was added to the sample, and the batch reactor was sealed again. For the second treatment, the reactor vessel was put in an oil bath heated to 160−220 °C for 120 min. The reactor was cooled in a water bath, and the products were filtered by suction after the second treatment. The filtrates were diluted with 250 mL of pure water and analyzed by TOC analysis and ion chromatography to identify the carbon conversions into solution and levulinic acid. For comparison, we conducted the experiments without the pretreatment to examine the effect of the pretreatment. 2.3. Treatment with Ionic Liquid. Ionic liquid is a salt that stays as liquid at relatively low temperature such as room temperature. Although the melting points of salts are generally high, it is possible to decrease the melting point by increasing the radii of the cation and anion and weakening the electrostatic force between them. Despite the weakened electrostatic force between ions, the force is still strong enough to have features such as nonvolatility, incombustibility, and ionic conductivity. Because of their unique character, ionic liquids are used as lubricants, solvents, and electrolytes, and there have been many studies on ionic liquid recently.23−26 Among the many types of ionic liquids, it is known that the types containing the imidazole group are effective in the solubilization of cellulose.27,28 Solubilization of cellulose with an ionic liquid enables some reagents such as enzymes or acids to attack the active sites of cellulose more effectively.29 In addition, solubililzed cellulose is easily recovered as a deposit by the addition of nonsolubilizing reagent, and this deposit is supposed to be changed to an amorphous structure that has a higher reactivity than crystalline cellulose. In this study, the effect of pretreatment on crystallinity was examined first, and then acidic hydrothermal treatment was conducted on pretreated cellulose. [EMIM]Br and [EMIM]P were chosen as the ionic liquids. 2.3.1. Treatment with [EMIM]Br. [EMIM]Br breaks the rigid structure of cellulose and solubilizes biomass. Cellulose solubilized with this ionic liquid is known to be recovered by the addition of another liquid that has no ability to dissolve cellulose.30 We focused on this unique character, which enables decrystallization and easy separation of cellulose. For the pretreatment, 0.3 g of cellulose or cedar and 9.7 g of [EMIM]Br were mixed in a glass bottle and put in a water bath heated to 80−120 °C for 1−6 h under stirring. Immediately after heating, the products were cooled, 30 mL of pure water was added for the extraction of the solute, and the mixture was filtered by suction. The deposit was washed with pure water; dried in vacuo; and then analyzed by CHNS elemental analysis, thermogravimetric (TG) analysis, and X-ray diffraction (XRD). The deposit is denoted as Br-cellulose or Br-cedar hereafter in this article. For the acidic hydrothermal treatment, 0.1 g of Br-cellulose or Br-cedar and 9.9 g of 5 wt % H2SO4 were mixed in a sealed batch reactor and put in an oil bath heated to 220 °C, where they were held for 2 h. The reactor was cooled in a water bath, and the products were filtered by suction after the reactions. Filtrates were diluted with 100 mL of pure water and analyzed using high-performance liquid chromatography (HPLC). For comparison, the acidic hydrothermal treatment was conducted using the raw materials (cedar, filter-paper cellulose, or amorphous cellulose) as samples according to the same method. 2.3.2. Treatment with [EMIM]P. [EMIM]P has a very high ability to break the rigid structure of cellulose, which not only solubilizes cellulose but transforms it into water-soluble

Scheme 2. Preparation of Pretreated Cellulose with [EMIM]P

differed between samples, we changed the ratio of sample and 5 wt % H2SO4 as samples. For the liquid sample, P-cellulose, 2.1 g of sample (1.0 g of [EMIM]P, 1.0 g of pure water, and 0.1 g of cellulose) and 7.9 g of 5 wt % H2SO4 were mixed in a sealed batch reactor. For the solid samples, P-cellulose-deposit and Pcedar, 0.1 g of sample and 9.9 g of 5 wt % H2SO4 were mixed in a sealed batch reactor. The reactors were put in an oil bath heated to 220 °C, where they were held for 2 h. The reactors were cooled in a water bath, and the products were filtered by suction after the reactions. The filtrates were diluted with 100 mL of pure water and analyzed by HPLC. We conducted another acidic hydrothermal treatment following the same methods using 0.1 g of P-cedar or P-cellulose-deposit and 9.9 g of 5 wt % H2SO4 as samples. 2.4. Analyses of Products. Ultimate analysis of the samples was performed using a CHNS elemental analyzer (BEL Japan, Inc., ECS4010). The crystallinity of the samples was measured by XRD (Rigaku Corporation), and TG curves during pyrolysis were measured using a TG analyzer (Shimadzu, TGA-50). For the organic acid analysis, an aqueous solution containing 951 mg/L p-toluene sulfonic acid, 4185 mg/L Bis-Tris, and 29 mg/L ethylenediaminetetraacetic acid was used as the eluent, and it was fed at 0.8 mL/min to the HPLC instrument equipped with a sulfonated polystyrene gel column (Shim-pack SCR-102H) and an electrical conductivity detector (Shimadzu, CDD-6A). The yield of each organic acid component estimated from the above measurements was represented on the basis of dry and ash-free samples. The C



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ment were levulinic acid, formic acid, glucose, and cellobiose; a small amount of HMF was also produced. However, the yields of formic acid, glucose, cellobiose, and HMF were very low, so we discuss only the yield of levulinic acid. Figure 1 shows the

TOC in the aqueous solution was estimated using a TOC analyzer (Shimadzu, TOC-VCSH). 2.5. Definition of Conversions. The results are expressed in terms of carbon conversion (C-%) in this article. The definition of carbon conversion is C‐% = [carbon content of the objective substance (g)] /[carbon content of the material (g)] × 100

For example, the carbon conversion of cellulose into solution is calculated from the carbon content in solution (calculated as TOC) divided by the carbon content of cellulose. When cedar was used as the sample, the carbon conversion was calculated on the basis of cellulose content, which means carbon content of the material (g) = [weight of cedar (g)] × [cellulose content of cedar (wt %)] × (carbon content of cellulose)

For the final yield of levulinic acid, the conversion is also expressed as mole percentage. This was calculated by the following equation yield (mol %) = [obtained amount of levulinic acid (mol)] /[initial amount of cellulose used (mol)] × 100

where initial amount of cellulose used (mol) = [initial amount of cellulose used (g)] /[molecular weight of the constituent unit of cellulose (=162) (g/mol)] × 100

The theoretical maximum yields for levulinic acid in the two expressions are 83.3 C-% and 100 mol %.

3. RESULTS AND DISCUSSION 3.1. Effect of Pretreatment Time. To investigate the effect of pretreatment time, we pretreated cellulose with phosphoric acid for 0, 1, 3, 5, 12, 24, 48, and 168 h at room temperature. The pretreatment was followed by reaction at 160 °C to obtain levulinic acid. Only 10 C-% of the cellulose was solubilized through the treatment of the raw material. On the other hand, 25−40 C-% of the pretreated cellulose converted into the soluble component, and its ratio increased with increasing pretreatment time. This means that the pretreatment worked effectively on the solubilization of cellulose. At that time, the yield of levulinic acid was the lowest when cellulose was degraded without pretreatment. However, the yields for all treatments were very low, in the range of 1−5 C-%. Considering that the pretreatment somehow had an effect on the yield of levulinic acid and that the pretreatment time would not matter very much, we decided to pretreat cellulose for 24 h in the following experiments. 3.2. Effect of Pretreatment Reagents. For the further investigation of the effect of pretreatment on the yield of levulinic acid, we conducted acidic hydrothermal degradation in the range of 160−220 °C for 120 min using raw filter-paper cellulose and samples pretreated with phosphoric acid. The pretreatment temperature and time were set to 50 °C and 24 h, respectively. The main products obtained through the treat-

Figure 1. Results of acidic hydrothermal treatment with phosphoric acid (2 h, cellulose/acid/H2O = 1 g/2 mL/50 mL): (a) conversion into solution, (b) conversion into levulinic acid, and (c) selectivity. White, virgin; black, pretreated.

results of the acidic hydrothermal treatment of cellulose with phosphoric acid. Pretreated cellulose increased the carbon conversion into the soluble component at all hydrothermal temperatures, especially lower temperatures, 160−180 °C, as shown in Figure 1a. Figure 1b shows the change in the levulinic acid yield as a function of hydrothermal temperature. The yield of levulinic acid also increased through pretreatment by at most 10 C-% for 200 °C acidic hydrothermal degradation, reaching 40.1 C-% (48.2 mol %). Figure 1c shows the selectivity of levulinic acid, which is defined as selectivity of levulinic acid = [yield of levulinic acid (C‐%)] /[soluble fraction (C‐%)]

The dotted line located in the graph represents the theoretical maximum selectivity 0.833. This value is equal to 5/6 because D



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one glucose unit in cellulose (C6H10O5) produces at most one molecule of levulinic acid (C5H8O3) and formic acid (CH2O2). The selectivity also reached the highest value of 55.2% at 200 °C for the pretreated sample. The highest scores for each factor, namely, carbon conversion into solution, yield of levulinic acid, and selectivity of levulinic acid, are summarized in Table 2. The values in parentheses are the reaction Table 2. Highest Scores for Each Factor in Carbon Conversion acid

cellulose

phosphoric phosphoric hydrochloric hydrochloric

virgin pretreated virgin pretreated

solutiona 68.9 72.7 82.9 83.6

(200) (200) (200) (200)

levulinic acida 34.0 40.1 39.9 49.2

(210) (200) (210) (210)

selectivitya 50.5 55.2 53.5 62.1

(210) (200) (220) (210)

a

Values in parentheses are the temperatures at which the scores were obtained.

temperatures at which the corresponding scores were obtained. These results indicate that the phosphoric acid pretreatment was effective for the production of levulinic acid. When the solid component obtained through pretreatment with phosphoric acid and filtration was analyzed by Fourier transfer infrared spectroscopy, the sample showed a peak for the type of cellulose with weak hydrogen bonds in the structure. This indicates that the reforming of cellulose by phosphoric acid, which is known to be an effective process upon enzymatic saccharification, was achieved and that it was effective for the production of levulinic acid as well. As mentioned in the Introduction, the degradation path of cellulose to levulinic acid as shown in Scheme 1 was considered in this work. This degradation path is most probably correct considering the agreement with the products such as glucose, HMF, formic acid, and levulinic acid. In that case, because it is known that the degradation of cellulose to glucose depends highly on the acidity of the reagent, the use of a stronger acid as a reagent might help increase the yield of levulinic acid. Therefore, we conducted the same experiments using hydrochloric acid as the reagent instead of phosphoric acid. Hydrochloric acid is a volatile acid, so product recovery after treatment using hydrochloric acid would possibly be easier.32 Judging from the results of the treatments using phosphoric acid, probably the highest yield would be obtained at a temperature of around 200 °C, so we conducted the acidic hydrothermal degradation using hydrochloric acid in the range of 180−220 °C. On the other hand, the pretreatment was conducted at both 50 and 100 °C, to examine the effect of pretreatment temperature. Figure 2 shows the results of the acidic hydrothermal treatment of cellulose with hydrochloric acid. As shown in Figure 2a, cellulose pretreated at 50 °C was converted into soluble components more than untreated cellulose, but the increase was quite slight compared to the case with phosphoric acid. However, according to Figure 2b, the increase in the yield of levulinic acid was remarkable, especially at 200 and 210 °C, where the values were 17 and 10 C-%, respectively. The yield reached 49.2 C-% (59.1 mol %) for acidic hydrothermal degradation at 210 °C with a selectivity of 62.1%. When the hydrothermal temperature was higher than 210 °C, the samples overreacted, and undesirable cohesion of the samples occurred, producing a residue and decreasing the conversions into solution and levulinic acid. For the samples

Figure 2. Results of acidic hydrothermal treatment with hydrochloric acid (2 h, cellulose/acid/H2O = 1 g/2 mL/50 mL): (a) conversion into solution, (b) conversion into levulinic acid, and (c) selectivity. White, virgin; black, pretreated at 50 °C; gray, pretreated at 100 °C.

pretreated at 100 °C, there was little difference in the carbon conversions into solution and levulinic acid no matter how high the acidic hydrothermal temperature was. In addition, based on the degradation pathways shown in Scheme 1, it is known that humins, which are deactivating substances, are usually produced as byproducts from the acidic hydrothermal treatment of saccharides. Therefore, the reason for this slight difference in carbon conversion was probably because some amount of cellulose overreacted and deactivated, producing humins, through the pretreatment at 100 °C with highly concentrated hydrochloric acid. As the support for this hypothesis, huge amounts of residue were produced after the pretreatment at 100 °C. Table 2 shows the highest scores for each factor, namely, carbon conversion into solution, carbon conversion into levulinic acid, and selectivity of levulinic acid through the pretreatment at 50 °C. Because pretreatment worked more effectively at 50 °C than at 100 °C, only the scores for pretreatment at 50 °C are included in the table. The values in parentheses represent the reaction temperatures at which the corresponding scores were obtained. Comparing the results for phosphoric acid and hydrochloric acid, one can see that all of E



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the scores were higher when hydrochloric acid was used as a reagent. In summary of this section, we confirmed two things: First, the assumption that the yield of levulinic acid increases with the use of stronger acid was correct. Second, pretreatment with acid is very effective in the production of levulinic acid from cellulose. As a result, a two-step treatment with hydrochloric acid was found to enhance the yield of levulinic acid, which reached 49.2 C-% (59.1 mol %) with a selectivity of 62.1% through acidic hydrothermal degradation at 210 °C. However, as with many reports, it should be noted that the proposed method for both acids has the issue of a very low sample concentration at the hydrothermal step, which is less than 2 wt %. 3.3. Treatment with Ionic Liquid. Cellulose is a waterinsoluble material. However, obtaining levulinic acid from cellulose requires the conversion of the latter into water-soluble components once through the process such as acidic saccharification. Consequently, we conducted acidic hydrothermal degradation after the pretreatment of solubilizing cellulose using ionic liquid. Because it was found to be better to use a stronger acid for acidic hydrothermal degradation in the previous section, we used 5 wt % sulfuric acid as the reagent. For the recovery of sulfuric acid, the addition of ammonia would be helpful because ammonium sulfate was recovered as a deposit. This deposit does not need to be wasted because of its potential for use as fertilizer. 3.3.1. Treatment of Cellulose with [EMIM]Br. After the pretreatment using [EMIM]Br at 80 or 120 °C for 1, 3, or 6 h, no solid cellulose was recognized in the samples before the addition of pure water. All of the samples recovered by the addition of water showed yields of over 98 wt %. Little change in the elemental compositions of the samples was detected, according to the CHNS analyses. Regarding the change in crystallinity, which is one of the main purposes of pretreatment, it decreased with pretreatment time. Cellulose samples pretreated at 80 and 120 °C showed quite similar results, so we decided to conduct the pretreatment with [EMIM]Br at 80 °C. Upon pretreatment at 80 °C, the decreases in intensity from crystalline cellulose calculated by XRD analyses were about 0% for 1 h, 22% for 3 h, and 28% for 6 h. Judging from the decreases between 1 and 3 h and between 3 and 6 h, no decrease was expected for treatments longer than 6 h, so we decided to set the pretreatment conditions as 80 °C for 6 h. After treatment under the determined conditions, 99.0 wt % of the cellulose was recovered as the deposit (Br-cellulose). Figure 3 shows the changes in the XRD profiles upon pretreatment. The crystallinity of cellulose decreased by about 30% through the pretreatment. However, this decrease is not large considering that cellulose was completely solubilized once, so cellulose seemed to be recrystallized during the process of recovery. Table 3 lists the elemental compositions, and Figure 4 shows the TG profiles of various samples. Unlike the crystallinity, the elemental composition and TG profile exhibited little change upon the pretreatment with [EMIM]Br. This indicates that only the crystallinity of the cellulose changed upon pretreatment. Then, Br-cellulose was degraded under the acidic hydrothermal conditions at 220 °C for 2 h using 5 wt % sulfuric acid. For comparison, the acidic hydrothermal treatment was conducted using filter-paper cellulose and amorphous cellulose as samples according to the same method. Figure 5 shows the carbon conversion obtained through the treatment.

Figure 3. XRD curves of cellulose samples.

Table 3. Elemental Compositions of the Samples

Br-cellulose P-cellulose-deposit Br-cedar P-cedar

C (wt %)

H (wt %)

O (wt %)

N (wt %)

P (wt %)

44.10 38.16 49.82 40.80

6.10 7.13 4.34 6.02

49.80 42.83 45.79 42.25

− 5.64 0.05 5.19

− 6.24 − 5.74

Figure 4. TG curves of cellulose samples.

Figure 5. Carbon conversions of samples pretreated with [EMIM]Br into levulinic acid.

All of the data are plotted on the basis of the initial amounts of sample used. The yield of levulinic acid from Br-cellulose was significantly higher than that from filter-paper cellulose. However, amorphous cellulose was converted into levulinic acid better than Br-cellulose, which indicates that the crystallinity of cellulose is one of the important factors for producing levulinic acid. F



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3.3.2. Treatment of Cedar with [EMIM]Br. After the pretreatment of cedar with [EMIM]Br at 80 °C for 6 h and the addition of pure water, 93.4 wt % of the cedar was recovered as the deposit (Br-cedar). The solubilized components (6.6 wt %) were not recovered even upon the addition of a huge amount of pure water. This means that the solubilized components derived not from cellulose but from lignin or hemicellulose and also that the separation of [EMIM]Br and some solubilized components of cedar is not an easy process. The XRD and TG curves of Br-cedar are shown in Figures 6 and 7, respectively. Although there were

ment was conducted using the raw material, namely, pulverized cedar, as the sample according to the same method. Figure 5 shows the carbon conversion after the acidic hydrothermal treatment on the basis of the initial sample amount. There was a small decrease in the conversion into residue for Br-cedar, which was probably somewhat due to the loss of cedar through pretreatment. Because the difference in the yield of levulinic acid was so small, the structure of cellulose in cedar does not seem to be broken through the pretreatment. In summary of this section, pretreatment with [EMIM]Br on cedar caused only the extraction of lignin or hemicellulose but not the destruction of the cellulose structure. Furthermore, it was considered that the separation of lignin or hemicellulose and [EMIM]Br was difficult and the yield of levulinic acid was not increased well through the pretreatment. From these results, we determined that pretreatment with [EMIM]Br is not useful for the conversion of cedar into levulinic acid. 3.3.3. Treatment of Cellulose with [EMIM]P. The structure of cellulose reformed by [EMIM]P treatment has been studied by many researchers. Liu et al. reported that imidazolium ionic liquids containing phosphorous convert cellulose into watersoluble components effectively at around 150 °C.33 Vo et al. also reported that the interaction between cellulose and [EMIM]P changes dramatically with the reaction temperature in the range of 140−160 °C.31 Thus, we decided to conduct the pretreatment at 150 °C for 1 h. Unlike for the pretreatment with [EMIM]Br, cellulose pretreated at 150 °C for 1 h using [EMIM]P was solubilized completely and then never recovered upon the addition of pure water. To examine the change in crystallinity and thermodegradability, a great deal of ethanol was added to the sample, and cellulose was recovered as Pcellulose-deposit. The yield of the deposit was 130.0 wt %, which means that some amount of [EMIM]P was recovered together with the cellulose. Through the CHNS and TG analyses, it was confirmed that 72.4 wt % of the cellulose was recovered as P-cellulose-deposit and the rest was derived from the ionic liquid. The XRD and TG curves of P-cellulose-deposit are shown in Figures 3 and 4, respectively. Cellulose was decrystallized very well, and the thermal degradability was increased by the pretreatment. Then, P-cellulose-deposit was degraded under the acidic hydrothermal conditions at 220 °C for 2 h using 5 wt % sulfuric acid. Because the recovery of cellulose requires plenty of ethanol, we conducted acidic hydrothermal degradation on the water-added cellulose solution, P-cellulose, as well. Although ionic liquid is an expensive reagent and its recovery is indispensable, there is no significant difference between these two samples in this respect

Figure 6. XRD curves of cedar samples.

Figure 7. TG curves of cedar samples.

changes in these curves upon pretreatment, they were quite slight. To examine the effect of pretreatment on the conversion of biomass into levulinic acid, Br-cedar was degraded under the acidic hydrothermal conditions at 220 °C for 2 h using 5 wt % sulfuric acid. For comparison, the acidic hydrothermal treat-

Figure 8. Carbon conversions of samples pretreated with [EMIM]P into levulinic acid (cellulose or cedar/5 wt % sulfuric acid = 0.1 g/9.9 g). Changing factors: (a) sample (cellulose), (b) pretreatment temperature, (c) ratio of sample to [EMIM]P, (d) acid, (e) sample (cedar). P-cellulose was used as a sample for b−d. G



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units of glucose, which is a water-soluble material. The yield of glucose was only 22.0 C-% through the pretreatment of cellulose; however, the yields of levulinic acid from the two materials, pretreated cellulose and glucose, were quite the same. This was probably because the mass-transfer limitations of acid inside the cellulose particles were reduced under the homogeneous conditions through the solubilization and this improved the effectiveness of cellulose conversion into levulinic acid to equal that of glucose conversion. This means that cellulose is not required to be degraded further to glucose but rather that only solubilization is the important factor for the production of levulinic acid. This result suggests that a saccharification process with expensive enzyme is not required for the production of levulinic acid from cellulose. However, the yield was not yet as high as the 60.7 C-% obtained from Pcellulose, which indicates that [EMIM]P has another effect than only solubilization. Consequently, we conducted [EMIM]P pretreatment on glucose to examine whether [EMIM]P breaks not only the rigid structure of cellulose but the structure of glucose as well. In this case, 0.1 g of glucose and 1.0 g of [EMIM]P were heated at 150 °C under stirring. The product was analyzed using ion chromatography, and the amount of glucose remaining was 4.3%. However, the amount of glucose remaining as calculated by the 3,5-dinitrosalicylic acid (DNS) method was 15.2%. When [EMIM]P was analyzed by the DNS method, it was revealed that [EMIM]P has no reduction ability. These facts indicate that the column used for HPLC was not able to separate [EMIM]P and glucose in pretreated glucose, which was not the case for the mixed sample of unpretreated glucose and [EMIM]P. In other words, glucose was somehow captured in [EMIM]P. The same thing seems to occur as for cellulose, and that makes H2SO4 able to access the active sites of cellulose effectively in the subsequent acidic hydrothermal reaction. In addition, fructose and HMF, the chemicals closer to levulinic acid than glucose in the conversion pathways, were not detected from pretreated glucose. This fact confirms that the yield of levulinic acid was increased not because cellulose was degraded further than glucose through the pretreatment but because cellulose was captured in [EMIM]P. In summary, the keys to producing levulinic acid from cellulose are the solubilization and the capture of cellulose by [EMIM]P through the pretreatment, which promotes the acidic hydrothermal degradation with H2SO4, reaching a yield of 60.7 C-% (72.9 mol %). In addition, some of the keys for the effective conversion of cellulose to levulinic acid are quite possibly related to the each conversion pathway shown in Scheme 1, such as the isomerization of glucose to fructose or the dehydration of fructose to HMF. Regarding these pathways, for example, the production of HMF or the isomerization of glucose, many researchers have reported that the presence of phosphorous helps give a high conversion.34 Considering the previous reports and the result in the previous section that phosphoric acid broke the hydrogen bonds of cellulose, phosphorous in [EMIM]P probably contributed to the significant increase in yield that was not observed when [EMIM]Br was used as a reagent. Subsequently, we sought more proper conditions for obtaining levulinic acid. We conducted experiments changing some conditions such as the pretreatment temperature, sample concentration for pretreatment, and type of acid for hydrothermal degradation. For these experiments, cellulose was not recovered by the addition of ethanol after the pretreatments

because P-cellulose-deposit also contains some ionic liquid and it should be recovered later anyway. Figure 8a shows the carbon conversion obtained through the treatment. All of the data are plotted on the basis of the initial amounts of sample used. Both pretreated cellulose samples provided markedly increased yields of levulinic acid, especially P-cellulose, which reached 60.7 C-% (72.9 mol %). In the previous section, the crystallinity was revealed to be an important factor for the production of levulinic acid. However, this remarkable increase could not be explained only by the decrease of crystallinity, considering that the yield of 60.7 C-% was higher than that from amorphous cellulose. Most probably, [EMIM]P had a huge effect on the degradation and on the increase of the yield of levulinic acid. Therefore, we conducted some more experiments to examine the effect of [EMIM]P on the production. First, we examined whether [EMIM]P worked as a catalyst. In this case, 0.1 g of cellulose, 1.0 g of [EMIM]P, 1.0 g of pure water, and 7.9 g of 5 wt % H2SO4 were mixed in a sealed batch reactor and put in an oil bath heated to 220 °C without any pretreatment. The yield of levulinic acid reached only 15.4 C-%, which was actually much smaller than the yield without the use of [EMIM]P. This fact means that the addition of [EMIM]P itself does not work effectively, or that [EMIM]P even works as an inhibitor, and that the important steps are to pretreat cellulose and to convert it into soluble components once. Next, to examine the importance of the solubilization of cellulose, we conducted the acidic hydrothermal degradation through the pretreatment using relatively highly concentrated H2SO4 instead of [EMIM]P. The pretreatment temperature was set to room temperature. As the concentration of H2SO4 was increased from 50 wt % in intervals of 10 wt %, we found that cellulose was solubilized well when 70 wt % H2SO4 was used, so we pretreated 0.2 g of cellulose with 1.43 g of 70 wt % H2SO4 for 24 h at room temperature. Through the pretreatment, 97.6 C-% of the cellulose was converted into soluble components, and the yield of glucose was 22.0 C-%. After the pretreatment, 18.37 g of pure water was added to the sample to dilute the concentration of H2SO4 to 5 wt %. Then, the sample was treated at 180 or 200 °C for 5−60 min. As shown in Figure 9, the highest yield was 47.4 C-% when the acidic hydrothermal treatment was conducted at 180 °C for 30 min. When 0.2 g of glucose and 19.8 g of 5 wt % H2SO4 were mixed and heated to 180 or 200 °C for 5−60 min, the highest yield was 50.5 C-% for the sample treated at 200 °C for 15 min. Cellulose consists of

Figure 9. Carbon conversions of glucose and pretreated cellulose (with sulfuric acid) into levulinic acid (sample/5 wt % sulfuric acid = 0.1 g/9.9 g): cellulose (○, 180 °C; ●, 200 °C), glucose (□, 180 °C; ■, 200 °C). H



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pretreatment conditions must be very precisely controlled. As mentioned before, the effect of pretreatment depends highly on the temperature in the range of 130−170 °C. Therefore, the reproducibility of levulinic acid was examined, and the yield was distributed, as it probably depends on the profile of stirring, slight differences in temperature, and so on. The yield reported in this article (60.7 C-%) is the average yield obtained from several repeated experiments, whereas the highest yield was 66.0 C-% (79.2 mol %). When the acidic hydrothermal treatment was conducted using the sample from the same pretreated sample, the yields showed little error in the range of ±1 C-%. This confirms that the error derives from the conditions of pretreatment. 3.3.4. Treatment of Cedar with [EMIM]P. Cedar treated with [EMIM]P at 150 °C for 1 h turned into slurry-like components. Part of the [EMIM]P existing as a liquid could be recovered by filtration. After filtration, no solid component was recovered upon the addition of pure water or ethanol to the filtrate. This means that cellulose was not extracted from the biomass by [EMIM]P or that cellulose was solubilized with its rigid structure being well broken. Through the analysis of the filtrate, it was revealed that there was no cellulose or hemicellulose derivative in the liquid. Regarding the residue, 145.8 wt % was recovered on the basis of the amount cedar used. Through CHNS and TG analyses, it was clarified that 83.3 wt % of the 145.8 wt % derived from the cedar and the remaining 62.5 wt % was from [EMIM]P. Considering that cedar contains 34.1 wt % lignin, 49.0 wt % of the lignin (16.7/ 34.1) was extracted by [EMIM]P. Because the [EMIM]P captured in the residue could not be recovered even after many rinses and because crystallinity of cellulose decreased well (Figure 6), it seems that the [EMIM]P was captured strongly by cellulose. The pretreatment with [EMIM]P was apparently effective in decreasing the crystallinity of cellulose in cedar by [EMIM]P being captured. Subsequently, to examine the effect of pretreatment on the production of levulinic acid, we conducted acidic hydrothermal treatment on P-cedar. The yield of levulinic acid is shown in Figure 8e along with that from a raw cedar sample plotted on the basis of the cellulose content in the initial amount of sample used. Because lignin was somewhat lost through the pretreatment, the amount of residue was smaller for the pretreated sample after the acidic hydrothermal treatment. According to Figure 8e, the yield of levulinic acid was markedly increased through the pretreatment, from 21.9 C-% (26.3 mol %) to 53.8 C-% (64.5 mol %). As is the case for cellulose, the error derived from the pretreatment conditions was large for the case of cedar as well. Thus, a yield of 53.8 C-% was obtained as the average yield from several experiments, whereas the highest was 57.3 C% (68.8 mol %). Pretreatment with [EMIM]P was clarified to be remarkably effective in the production of levulinic acid from cedar, with [EMIM]P being captured by the sample. However, [EMIM]P extracted lignin from cedar through pretreatment, which was hard to separate and would be a significant hurdle for the recovery of [EMIM]P. For this reason, even though the pretreatment method was effective directly on cedar, it would be better as a process to separate cellulose from biomass before pretreatment with [EMIM]P.

because the yield was much higher when P-cellulose was used than when P-cellulose-deposit was. All of the results are summarized in Figure 8. First, Figure 8b compares the effects of the pretreatment temperature on the levulinic acid yield. The yield was highest for the sample pretreated at 150 °C. The color of the cellulose solution after pretreatment became darker as the temperature rose. Because the yield decreased when cellulose was pretreated at 170 °C, we tried to deposit the solubilized cellulose at 170 °C by the same method as we used to obtain P-cellulose-deposit at 150 °C pretreatment and analyze it. However, the solubilized cellulose at 170 °C was never recovered by the addition of ethanol, no matter how much ethanol was added. The viscosity of the pretreated sample became lower as the pretreatment temperature rose, and the sample pretreated at 130 °C was kind of a gelatinous material. Therefore, the pretreatment temperature had a crucial effect on cellulose, and the yield of levulinic acid reached its highest value upon 150 °C pretreatment by forming the appropriate interaction between cellulose and [EMIM]P. Figure 8c shows the effects of the amount of sample. When the cellulose/[EMIM]P ratio was increased by a factor of 2, from 0.5/5 to 1/5, the yield of levulinic acid was less than half, which means that sufficient amounts of [EMIM]P are required for effective conversion. Judging from these results, the pretreatment conditions seem to be better when the temperature is 150 °C and the cellulose/ionic liquid ratio is 0.5/5. Next, to examine the effects of the type of acid used for the acidic hydrothermal degradation, 5 wt % hydrochloric acid was used instead of sulfuric acid (Figure 8d). However, the yield was 15 C-% less than that obtained with the treatment using sulfuric acid. In summary, 60.7 C-% (72.9 mol %) of cellulose pretreated with [EMIM]P converted to levulinic acid through the acidic hydrothermal degradation using sulfuric acid. After this conversion, the carbon balance was considered. 9.5 C-% of the cellulose was obtained as a residue. Because the cellulose was solubilized completely once, it is not reasonable to consider that the residue still consisted of cellulose. Thus, the residue was assumed to consist mostly of humic substances. Specifically, 7.3 C-% was identified as organic acids (excluding levulinic acid), which consisted of formic acid (4.7 C-%), acetic acid (1.9 C-%), and glycolic acid (0.7 C-%). According to the stoichiometry, formic acid should be produced from cellulose in a ratio of 1:5 against levulinic acid in carbon conversion. However, the yield of formic acid was less than 12.1 C-% (= 60.7/5). Formic acid decomposes into carbon dioxide under severe conditions.35 Therefore, 7.4 C-% was probably lost as volatile components. So far, 84.9 C-% was identified (9.5 C-% as residue, 60.7 C-% as levulinic acid, 7.3 C-% as the other organic acids, 7.4 C-% as volatile components). The remaining 15.1 C-% was assumed to be other volatile components or water-soluble humins. It was not able to identify these components because TOC in the solution was undetectable due to the presence of carbon derived from [EMIM]P. Saccharides such as glucose, fructose, and cellobiose were not detected at all, so we concluded that the material was fully converted through the reaction. Although the yield of levulinic acid was high, there also are some issues with this conversion method. One of them is that, as in the case of acidic treatment, the concentration of the sample is very low at 1 wt %. This is a common problem for the production of levulinic acid, because a high sample concentration generally results in a high conversion to humins. The other is that, when [EMIM]P is used as a reagent, the

4. CONCLUSIONS It was clarified that pretreatment using highly concentrated acid or ionic liquid works effectively in the conversion of cellulose or cedar into levulinic acid. The decrease in crystallinity and I



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(9) Girisuta, B.; Janssen, L. P. B. M.; Heeres, H. J. Kinetic Study on the Acid-Catalyzed Hydrolysis of Cellulose to Levulinic Acid. Ind. Eng. Chem. Res. 2007, 46, 1696. (10) Cha, J. Y.; Hanna, M. A. Levulinic acid production based on extrusion and pressurized batch reaction. Ind. Crops Prod. 2002, 16, 109. (11) Andersson-Engels, S.; Berg, R.; Svanberg, K.; Svanberg, S. Multicolour fluorescence imaging in connection with photodynamic therapy of δ-amino levulinic acid (ALA) sensitised skin malignancies. Bioimaging 1995, 3, 134. (12) Bozell, J. J.; Moens, L.; Elliott, D. C.; Wang, Y.; Neuenscwander, G. G.; Fitzpatrick, S. W.; Bilski, R. J.; Jarnefeld, J. L. Production of levulinic acid and use as a platform chemical for derived products. Resour., Conserv. Recycl. 2000, 28, 227. (13) Yan, Z.; Lin, L.; Liu, S. Synthesis of γ-Valerolactone by Hydrogenation of Biomass-Derived Levulinic Acid over Ru/C Catalyst. Energy Fuels 2009, 23, 3853. (14) Pasquale, G.; Vázquez, P.; Romanelli, G.; Baronetti, G. Catalytic upgrading of levulinic acid to ethyl levulinate using reusable silicaincluded Wells−Dawson heteropolyacid as catalyst. Catal. Commun. 2012, 18, 115. (15) Kamm, B.; Kamm, M. Principles of biorefineries. Appl. Microbiol. Biotechnol. 2004, 64, 137. (16) Kadam, K. L.; Chin, C. Y.; Brown, L. W. Flexible biorefinery for producing fermentation sugars, lignin and pulp from corn stover. J. Ind. Microbiol. Biotechnol. 2008, 35, 331. (17) Ojeda, K.; Kafarov, V. Exergy analysis of enzymatic hydrolysis reactors for transformation of lignocellulosic biomass to bioethanol. Chem. Eng. J. 2009, 154, 390. (18) Harun, R.; Danquah, M. K. Enzymatic hydrolysis of microalgal biomass for bioethanol production. Chem. Eng. J. 2011, 168, 1079. (19) Kuster, B. F. M.; van der Baan, H. S. The influence of the initial and catalyst concentrations on the dehydration of D-fructose. Carbohydr. Res. 1977, 54, 165. (20) Chang, C.; Ma, X.; Cen, P. Kinetics of Levulinic Acid Formation from Glucose Decomposition at High Temperature. Chin. J. Chem. Eng. 2006, 14, 708. (21) Haan, R. D.; Rose, S. H.; Lynd, L. R.; van Zyl, W. H. Hydrolysis and fermentation of amorphous cellulose by recombinant Saccharomyces cerevisiae. Metab. Eng. 2007, 9, 87. (22) Zhang, Y. H. P.; Cui, J.; Lynd, L. R.; Kuang, L. R. A Transition from Cellulose Swelling to Cellulose Dissolution by o-Phosphoric Acid: Evidence from Enzymatic Hydrolysis and Supramolecular Structure. Biomacromolecules 2006, 7, 644. (23) Forsyth, S. A.; Pringle, J. M.; MacFarlane, D. R. Ionic Liquids An Overview. Aust. J. Chem. 2004, 57, 113. (24) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. Thermophysical Properties of Imidazolium-Based Ionic Liquids. J. Chem. Eng. Data 2004, 49, 954. (25) Iglesias, P.; Bermúdez, M. D.; Carrión, F. J.; Martínez-Nicolás, G. Friction and wear of aluminium−steel contacts lubricated with ordered fluidsNeutral and ionic liquid crystals as oil additives. Wear 2004, 256, 386. (26) Phillips, B. S.; Zabinski, J. S. Ionic liquid lubrication effects on ceramics in a water environment. Tribol. Lett. 2004, 17, 533. (27) Swatloski, R. P.; Spear, S. K.; Holbrey, J. D.; Rogers, R. D. Dissolution of Cellulose with Ionic Liquids. J. Am. Chem. Soc. 2002, 124, 4974. (28) Sievers, C.; Valenzuela-Olarte, M. B.; Marzialetti, T.; Musin, I.; Agrawal, P. K.; Jones, C. W. Ionic-Liquid-Phase Hydrolysis of Pine Wood. Ind. Eng. Chem. Res. 2009, 48, 1277. (29) Uju; Shoda, Y.; Nakamoto, A.; Goto, M.; Tokuhara, W.; Noritake, Y.; Katahira, S.; Ishida, N.; Nakashima, K.; Ogino, C.; Kamiya, N. Short time ionic liquid pretreatment on lignocellulosic biomass to enhance enzymatic saccharification. Bioresour. Technol. 2012, 103, 446. (30) Barthel, S.; Heinze, T. Acylation and carbanilation of cellulose in ionic liquids. Green Chem. 2006, 8, 301.

solubilization of cellulose were clarified to be the important factors for the conversion. Phosphoric acid broke the hydrogen bonds in cellulose and converted 40.1 C-% (48.2 mol %) of the cellulose into levulinic acid. Hydrochloric acid as the stronger acid helped the conversion, enabling 49.2 C-% (59.1 mol %) of cellulose to be converted into levulinic acid. Ionic liquid, a recent notable reagent, was found to work effectively as well. In particular, [EMIM]P markedly enhanced the yield of levulinic acid, enabling cellulose to be converted into water-soluble components and capturing cellulose in its structure. The effective conversion method was developed through experiments under several conditions, and 60.7 C-% (72.9 mol %) of cellulose was converted into levulinic acid through a two-step treatment with [EMIM]P.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +81 75 383 2678. Fax: +81 75 383 2638. E-mail: kaz@ cheme.kyoto-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan through a Grant-in-Aid for Scientific Research (A) (Grant 25249109).

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ABBREVIATIONS DNS = 3,5-dinitrosalicylic acid [EMIM]Br = 1-ethyl-3-methylimidazolium bromide [EMIM]P = 1-ethyl-3-methylimidazolium methylphosphonate HMF = 5-hydroxymethylfurfural HPLC = high-performance liquid chromatography TG = thermogravimetric TOC = total organic carbon XRD = X-ray diffraction

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REFERENCES

(1) Liu, S.; Abrahamson, L. P.; Scott, G. M. Biorefinery: Ensuring biomass as a sustainable renewable source of chemicals, materials, and energy. Biomass Bioenergy 2012, 39, 1. (2) Tilman, D.; Hill, J.; Lehman, C. Carbon-Negative Biofuels from Low-Input High-Diversity Grassland Biomass. Science 2006, 314, 1598. (3) Clark, J. H. Green chemistry for the second generation biorefinerySustainable chemical manufacturing based on biomass. J. Chem. Technol. Biotechnol. 2007, 82, 603. (4) Mohanty, A. K.; Misra, M.; Drzal, L. T. Sustainable BioComposites from Renewable Resources: Opportunities and Challenges in the Green Materials World. J. Polym. Environ. 2002, 10, 19. (5) Lipinsky, E. S. Chemicals from Biomass: Petrochemical Substitution Options. Science 1981, 26, 1465. (6) FitzPatrick, M.; Champagne, P.; Cunningham, M. F.; Whitney, R. A. A biorefinery processing perspective: Treatment of lignocellulosic materials for the production of value-added products. Bioresour. Technol. 2010, 101, 8915. (7) Fernando, S.; Adhikari, S.; Chandrapal, C.; Murali, N. Biorefineries: Current Status, Challenges, and Future Direction. Energy Fuels 2006, 20, 1727. (8) Jow, J.; Rorrer, G. L.; Hawley, M. C.; Lamport, D. T. A. Dehydration of D-Fructose to Levulinic Acid over LZY Zeolite Catalyst. Biomass 1987, 14, 185. J



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(31) Vo, H. T.; Kim, Y. J.; Jeon, E. H.; Kim, C. S.; Kim, H. S.; Lee, H. Ionic-Liquid-Derived, Water-Soluble Ionic Cellulose. Chem.Eur. J. 2012, 18, 9019. (32) Rackemann, D. W.; Doherty, W. O. S. The conversion of lignocellulosics to levulinic acid. Biofuels, Bioprod. Biorefin. 2011, 5, 198. (33) Liu, W.; Hou, Y.; Wu, W.; Ren, S.; Wang, W. Complete conversion of cellulose to water soluble substances by pretreatment with ionic liquids. Korean J. Chem. Eng. 2012, 29, 1403. (34) Nakajima, K.; Noma, R.; Kitano, M.; Hara, M. Selective glucose transformation by titania as a heterogeneous Lewis acid catalyst. J. Mol. Catal. A: Chem. 2014, 388, 100. (35) McMurray, T. A.; Byrne, J. A.; Dunlop, P. S. M.; Winkelman, J. G. M.; Eggins, B. R.; McAdams, E. T. Intrinsic kinetics of photocatalytic oxidation of formic and oxalic acid on immobilized TiO2 films. Appl. Catal. A 2004, 262, 105.

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Effective Production of Levulinic Acid from Biomass through

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