Enhanced Levulinic Acid Production from Cellulose by Combined

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Kinetics, Catalysis, and Reaction Engineering

Enhanced Levulinic Acid Production from Cellulose by Combined Brønsted Hydrothermal Carbon and Lewis Acid Catalysts Tat Boonyakarn, Piyaporn Wataniyakul, Panatpong Boonnoun, Armando T Quitain, Tetsuya Kida, Mitsuru Sasaki, Navadol Laosiripojana, Bunjerd Jongsomjit, and Artiwan Shotipruk Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05332 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 31, 2019

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Industrial & Engineering Chemistry Research

Enhanced Levulinic Acid Production from Cellulose by Combined Brønsted Hydrothermal Carbon and Lewis Acid Catalysts

Tat Boonyakarn,† Piyaporn Wataniyakul,† Panatpong Boonnoun,‡ Armando T. Quitain,§ Tetsuya Kida,§ Mitsuru Sasaki,§ Navadol Laosiripojana,∥ Bunjerd Jongsomjit† and Artiwan Shotipruk*,†

†

Chemical Engineering Research Unit for Value Adding of Bioresources, Department of

Chemical Engineering, Faculty of Engineering, Chulalongkorn University, Phayathai Road, Bangkok 10330, Thailand ‡

Department of Industrial Engineering, Faculty of Engineering, Naresuan University,

Phitsanulok 65000, Thailand §

Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto

University, Kumamoto 860-8555, Japan ∥

The Joint Graduate School of Energy and Environment, King Mongkut’s University of

Technology Thonburi, Prachauthit Road, Bangmod, Bangkok 10140, Thailand

* Corresponding author. Tel: +66-2-218-6868; Fax: +66-2-218-6877 Email: [email protected] (A. Shotipruk)

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract This study presents a synergistic catalytic system using the combination of Brønsted hydrothermal carbon-based acid (HTCG-SO3H) and Lewis acid catalysts for one-pot conversion of cellulose to levulinic acid (LA). Chromium chloride (CrCl3), among a number of other Lewis acidic metal chlorides, was found to give the highest LA yields, and was therefore used in combination with the HTCG-SO3H, for cellulose conversion to LA. With the appropriate amount of HTCG-SO3H, the formation of side products could be reduced, resulting in improved selectivity of LA. Compared with that obtained by CrCl3 alone, at 5 wt.% HTCG-SO3H, 0.015 M CrCl3, 200 °C and 5 min reaction time, the LA yield was considerably enhanced from 30 wt.% to 40 wt.%. Since HTCG-SO3H is a heterogeneous catalyst that can be easily prepared from biomass at moderate temperature, its use in such combined catalyst system offers economic and environmental benefits, thus making largescale implementation of such process potentially feasible.

Keywords: Cellulose; Levulinic acid; hydrothermal carbon-based acid catalyst; Lewis acid; Brønsted acid


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1. Introduction Lignocellulosic biomass consisting mainly of cellulose, hemicelluloses and lignin, is a promising renewable material for production of high valuable chemicals. A recent review on catalytic conversion of lignocellulosic biomass to important platform chemicals ranging from C2 basic chemical such as ethanol to C5 basic chemicals such as furfural and levulinic acid (LA) and C6 chemicals such as hydroxymethylfurfural (HMF), can be found in Mika et al., 2018.1 Among these platform chemicals, LA has gained increasing interest as an alternative to petroleum-based chemicals, for uses as, plasticizer, coating, fuel additive and antifreeze, and in processing of resin, textile, and animal feed.2 Global market demand of LA is estimated to be 3820 tons by 2020, which is a significant increase from the market demand of 2606.2 tons in 2013.3 For LA production, cellulose is generally decomposed through hydrolysis to glucose, a major sugar unit in lignocellulosic biomass. This sugar is then converted to HMF via dehydration, and subsequently to LA via rehydration. In general, homogeneous Brønsted acid catalysts, such as sulfuric acid (H2SO4), hydrochloric acid (HCl), and solid Brønsted acid catalysts such as Amberlyst 704, carbon-based acid,5 graphene oxide (GO),6,7 are used for hydrolysis of cellulose, dehydration of sugars and rehydration of HMF. However, due to the difficulty of dehydration to HMF directly from glucose, it is common that glucose is isomerized first to fructose, which is more easily dehydrated. Isomerization of glucose to fructose, on the other hand, is not effectively catalyzed by a Brønsted acid catalyst, but requires use of a Lewis acid8-11 or a Brønsted base.12-14 Recently, production of biomass derived HMF and LA has been achieved with use of a so called bi-functional catalysts, which refer to catalysts that consist of two



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types of functional groups, one being the Brønsted acid, and another being enzyme, Lewis acid, or Brønsted base.15-24 Interestingly, Lewis acidic metal salts such as Cr(III), Cr(II), Al(III), Zn(II), Sn(IV), Fe(III), Cu(II) salts, for example, act both as Lewis acid and Brønsted acid, and have been reported to sufficiently drive hydrolysis, dehydration, and rehydration reactions, without requiring external addition of Brønsted acid.25 However, these intrinsic bifunctional acidic metal salts give poor selectivity to HMF and LA, since the Lewis sites of these salts generally promote the formation of humin as by-product, particularly from sugars and HMF.26 To increase the selectivity of HMF and LA, use of external Brønsted acid together with a Lewis acidic metal salt has been proposed.27 The synergistic effect of combined Lewis acid and external Brønsted acids have been reported for biomass conversion in a number of previous research studies, in which various combined catalytic systems have been employed, including the systems of chromium chloride (CrCl3) and HCl,25 CrCl3 and ammonium halides,27 CrCl3 and carbon dioxide,28 aluminum chloride (AlCl3) and HCl,8 AlCl3 and phosphoric acid (H3PO4).29 Despite the improvement in HMF and LA yields by these combined Brønsted acid and Lewis acid systems, the homogeneous external Brønsted acids employed in the previous studies are highly corrosive and cannot be easily recycled. Furthermore, side reactions easily occur to form unwanted by-products. To avoid these problems, heterogeneous Brønsted acid catalysts may be used instead of the homogeneous mineral acids. Other catalytic systems involving use of solid niobium based solid acid catalysts, containing both Lewis acid and Brønsted acid sites have also been employed on cellulose conversion and a high LA yield of 53% was achieved with Al-NbOPO4 at 180 °C after 24 h in water.30 However, a problem with most solid catalysts when applied to the conversion of lignocellulosic feedstocks is that the separation of the catalyst from the


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solid biomass residue and the reaction by-products remains to be challenging. On this account, use of solid carbon-based catalysts would be advantageous as the catalyst can be recovered together with the remaining biomass residue, and can be reused after regeneration by acid functionalization. A relatively new type of heterogeneous carbon-based Brønsted acid catalysts has been prepared via a two-step synthesis method: hydrothermal carbonization of a low cost a carbonaceous material (i.e. glucose), followed by functionalization of the resulted carbonized material by acid (normally sulfuric acid).31 The synthesis of this type of catalyst consumes less energy compared with the conventionally synthesized carbon-based catalyst via incomplete combustion since the carbon material used as a catalyst support is prepared in hot water at relatively mild temperatures under autogenerated pressure.32-34 In this work, the combined Lewis acidic metal salt and hydrothermal carbonbased acid catalysts (HTCG-SO3H) were investigated on cellulose conversion to LA. Firstly, the most suitable metal salt was selected based on cellulose conversion. Then, in the system of combined metal salt and HTCG-SO3H catalysts, the effects of variables such as reaction temperature, reaction time, HTCG-SO3H dosage and the concentration of the metal salt on cellulose conversion were determined.

2. Materials and Methods 2.1. Materials and chemicals Glucose, hydroxymethylfurfural (HMF), levulinic acid (LA), concentrated sulfuric acid (98%), acetone, ethanol, methanol and chromium (III) chloride (hexahydrate) (CrCl3), manganese (II) chloride (MnCl2), cadmium chloride (CdCl2), cobalt (II) chloride (CoCl2), iron (III) chloride (hydrate) (FeCl3) and iron (II) chloride



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(tetrahydrate) (FeCl2) were purchased from Wako Pure Chemical Company (Osaka, Japan). Cellulose (powdered α-cellulose from cotton linters) was purchased from HiMedia Laboratories Pvt. Ltd. (Mumbai, India).

2.2. Synthesis of hydrothermal carbon-based acid catalyst from glucose To prepare HTCG-SO3H, a two-step process consisting of hydrothermal carbonization and acid functionalization was employed following the procedure described in Wataniyakul et al. (2018).31 For hydrothermal carbonization, 30 g of glucose and 300 ml of deionized water were mixed in a 520 ml SUS-316 stainlesssteel closed batch reactor and the reactor was then heated at 220 °C for 6 hours. After the reaction was completed, the resulting black solid sample or hydrothermal carbon (HTCG) was washed with water, ethanol, and acetone, each for 1 hour under sonication. The resulting HTCG was filtered by Whatman filter paper (No.1) and dried at 110 °C overnight in a hot air oven. For the acid functionalization, 50 ml of concentrated sulfuric acid was added to 5 g of dried HTCG. The mixture, in a 3-neck rounded bottom flask was then heated at 150 °C for 15 hours. After the heating process was complete, the resulting solid was washed with boiling distilled water until no pH change in the wash water was observed. The solid catalyst (HTCG-SO3H) was then washed with 500 ml of ethanol, then with 300 ml of acetone, dried overnight at 110 °C, and was then ground to powder. The detailed physicochemical characterization results of the HTCG-SO3H prepared by the aforementioned procedures were reported in our previous work and as supplementary data.

2.3. Cellulose conversion tests


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In this study, two sets of cellulose conversion tests were carried out. The first one was carried out to determine the most suitable metal salt. Based on Peng et al., (2010)35, six commonly used transition metal salts: FeCl2, FeCl3, CoCl2, MnCl2, CdCl2 and CrCl3 shown to have high reactivity on cellulose conversion in an aqueous system were selected for our comparative analysis. The second set of cellulose conversion test was carried out to evaluate the combined Brønsted HTCG-SO3H and the selected Lewis acid catalysts. The results were compared with those obtained from the reactions catalyzed by a single catalyst (either HTCG-SO3H or the Lewis acid catalyst). In addition, the effects of various process variables were determined in the reaction system of combined catalysts. In all cases, the cellulose conversion reactions were carried out in an 8.8 ml SUS-316 stainless-steel closed batch reactor (AKICO Co., Japan). In the first set of experiment to determine the most suitable Lewis acid catalyst, 0.1 g of cellulose and 5 ml of a 0.005 M solution of metal salt (in DI water) were charged into the reactor. The reactor was shaken and heated to 200 °C by an electric heater, connected to a temperature controller. It is noted that heating was performed in two steps. In the first step, the reactor was first preheated in a hot-air oven (at 450 °C) for approximately 10-15 min. The reactor was then heated in the second step using another oven whose temperature was controlled precisely at the reaction temperature, and in which the reaction time was set to start. The reaction was allowed to take place for 5 min, after which the reactor was quenched in a water bath. With an additional amount of 5 ml of de-ionized water, the liquid reaction product and the remaining solid were completely removed from the reactor, and were separated from each other by a Whatman filter paper (No. 1). The amounts of glucose, HMF and LA in the liquid products were determined by a high performance liquid chromatography (HPLC).



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The most suitable Lewis acid, yielding the highest LA yield, was then combined with HTCG-SO3H and the combined catalysts were evaluated for cellulose conversion. As a preliminary evaluation of the combined catalyst, 0.1 g of cellulose and 5 ml of the selected metal salt solution (in DI water) were used and the reaction was carried out at 200 °C for 5 min. Furthermore, detailed cellulose conversion experiments were also carried out to determine the effect of reaction temperature (160220 °C), reaction time (0-60 min), HTCG-SO3H dosage (0-40 wt. %) and metal salt concentration (0-0.02 M) on the glucose, HMF and LA yields. The quantification of glucose, HMF, and LA in the liquid reaction product was conducted using a HPLC (JASCO AS-2055 plus, Japan) consisting of a Jasco RI-2031 plus detector, Jasco UV-970 detector, Jasco PU980 pump system, sugai U-620 column heater and a Jasco AS-2055 plus automated sampler injector equipped with a Shodex SUGAR SH1011 (8.0mmID*300 mm) column at 60 °C. The concentrations of HMF and LA were analyzed based on UV absorbance at 220 nm and the concentrations of glucose were analyzed based on RI. Perchloric acid (HClO4) was used as an eluent at a flow rate of 0.5 ml/min. The sample injection volume was 10 μl. The retention time for glucose, HMF and LA were 16.0, 39.4, and 23.3 min, respectively. The yields of the reaction products: glucose, HMF and LA were calculated as mass percentages of the products to the mass of the starting cellulose.

3. Results and Discussion 3.1. Selection of metal salt as Lewis acid catalyst As shown in Figure 1, different metal salts gave different yields of HMF and LA under identical conditions, and in the decreasing order: CrCl3> FeCl2= FeCl3> CoCl2 > MnCl2> CdCl2. When CoCl2, CdCl2, and MnCl2 were used as catalysts, low


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glucose, HMF and LA yields were obtained. The reason for different cellulose conversion has been described in a previous study,31 to be attributed to the acidity of metal chloride as well as the type of metal. Low acidity metal chlorides such as CoCl2, CdCl2 and MnCl2 (pH 7) lack catalytic activity for cellulose conversion to glucose, and as a result, HMF and LA yields were low. It is also interesting to note that when Fe type catalysts were used, the yields of HMF and LA were relatively low, while that of glucose was relatively high. Having high acidity, FeCl3 (pH 2.5) was expected to have high catalytic activity for cellulose hydrolysis, however, the Fe type metal chlorides are not favourable for glucose isomerization compared with Cr type. CrCl3 (pH 3.8) which is relatively favorable in term of acidity, and is highly favorable in term of metal type, was found to give the highest overall yields of cellulose conversion products (glucose, HMF and LA). It was therefore selected for use in combination with HTCG-SO3H in the subsequent study. 25 LA

HMF

glucose

20

Yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 10 5 0 FeCl₂

FeCl₃

CoCl₂

MnCl₂

Type of Lewis acid

CdCl₂

CrCl₃

Figure 1. Yields of glucose, HMF and LA from cellulose conversion at 200oC for 5 min, catalyzed with various acidic metal salts (0.005 M).



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3.2. Combination of Lewis acid catalyst with hydrothermal carbon-based acid catalyst Figure 2 shows the comparison of glucose, HMF and LA yields from cellulose conversion carried out at 200 °C for 5 min, catalyzed by three systems of catalysts: HTCG-SO3H, CrCl3, and the two catalysts combined. As HTCG-SO3H is a carbonbased Brønsted acid catalyst, it is expected to catalyze cellulose hydrolysis, but is inefficient for further conversion of glucose to HMF,5 the LA yield of the reaction catalyzed by HTCG-SO3H alone was therefore expected to be low (Figure 2). On the contrary, significantly higher LA yield was observed using CrCl3 alone. This is due to the presence of ion complex, [Cr(H2O)5OH]2+, formed in presence of water, that catalyzes isomerization of glucose to fructose, and the presence of the intrinsic Brønsted acidity, that drives hydrolysis, fructose dehydration to HMF, and further HMF rehydration to LA. Nevertheless, it has been reported that Lewis acid sites are non-selective, as they catalyze not only the aldose-to-ketose isomerization, but also the side reactions such as the generation of humin, an undesirable product, from sugars as well as from HMF.26 By working synergistically with the CrCl3 catalyst, the addition of HTCG-SO3H as a Brønsted acid catalyst led to approximately two folds enhancement of LA yield (from 11.2 wt. % to 22.9 wt. %). In agreement with previous studies, the result here demonstrated that further improvements in LA yield could be achieved by addition of a Brønsted acid catalyst, to facilitate the hydrolysis of cellulose, dehydration of fructose, and rehydration of HMF to LA before the side reactions took place by the action of the Lewis acid.26 Given these results, further studies were conducted to help gain the insight on how the combined effect of CrCl3 and HTCG-SO3H catalysts behave at various conditions, and to find the most suitable reaction conditions.


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40 35

Yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


LA

HMF

glucose

30 25 20 15 10 5 0 HTCG-SO₃H

CrCl₃

CrCl₃/HTCG-SO₃H

Type of catalyst Figure 2. Yields of Glucose, HMF and LA from cellulose conversion at 200oC for 5 min, catalyzed with CrCl3 (0.005 M), HTCG-SO3H (5 wt.%), and both catalysts combined.

3.3. Effects of temperature and time The glucose, HMF and LA yields obtained from cellulose conversion carried out under different reaction temperatures and times are shown in Figure 3. As seen from Figure 3, at the reaction temperatures of 160 °C and 180 °C, the yields of all products were relatively low at all reaction times. As the reaction temperatures increased to 200 °C and 220 °C, the increase in the overall yields of all products was observed due to the increased rates of reaction at higher temperatures. In addition, at high temperatures, the number of [Cr(H2O)5OH]2+ complexes and the H+ ions increased; with the former, promoting glucose isomerization, while the latter, promoting hydrolysis, dehydration, and rehydration.25 The results in Figure 3 also clearly demonstrated the influence of reaction time on cellulose conversion. At the reaction temperatures between 160 °C and 200 °C, glucose and HMF yields increased initially with time, and started to decrease after a certain point. While LA, on the other hand, continued to increase with time, suggesting that glucose was readily converted to HMF, and HMF to LA as the reaction 11 ACS Paragon Plus Environment


proceeded. This was also true for the reaction at 220 °C, with the glucose and HMF yields decreased abruptly from 0 to 5 min. At 220 °C, the maximum LA yield was observed at the reaction time of 20 min to be 37 wt.%. It is, nevertheless, interesting to note that, at this temperature, LA yield dropped considerably from 37 wt.% to 19.5 wt.% upon increasing the reaction time from 20 min to 40 min. This result suggested that the LA was decomposed at high temperature and long time, which is in agreement with the study of Yan et al. (2008)36 which reported that LA was easily dehydrated to unsaturated lactone above approximately 200 °C. Because of the observed instability of LA at 220 °C, the reaction temperature and time were fixed at 200 °C and 5 min in the subsequent study, in which the effects of HTCG-SO3H catalyst dosage and CrCl3 concentration on the production of LA were further determined. At 200 °C and 5 min, not only does LA remain relatively stable, the yield of HMF, which is the LA precursor, were found to be at maximum. The high HMF yield would therefore lead potentially to high LA production.

10

Glucose Yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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160°C

180°C

200°C

220°C

(a)

8 6 4 2 0 0

5

20

40

60

Time (min)


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HMF Yield (wt.%)

12 160°C

10

180°C

200°C

(b)

220°C

8 6 4 2 0 0

5

20

40

60

Time (min)

50 160°C

LA Yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


180°C

200°C

220°C

(C)

40 30 20 10 0 0

5

20

40

60

Time (min) Figure 3. Effects of temperature and time on yields of (a) glucose, (b) HMF, and (c) LA from cellulose conversion with combination of CrCl3 (0.010 M) and HTCG-SO3H (10 wt.%)

3.4. Effect of hydrothermal carbon-based acid catalyst dosage For cellulose conversion carried out at 200 °C for 5 min, the results in Figure 4 clearly demonstrated the considerable influence of the catalyst dosage on cellulose conversion. Specifically, the increase in HTCG-SO3H dosage from 0 to 5 wt.% resulted in significant increase in HMF and LA yields. The highest LA yield of 22.9 wt.% was observed at 5 wt.% HTCG-SO3H dosage. At higher HTCG-SO3H dosage from 10 wt.% to 40 wt.%, overall LA yield decreased with increasing HTCG-SO3H 13 ACS Paragon Plus Environment


dosage, whereas the overall glucose yield, on the other hand, increased. This could be due to the fact that the external Brønsted acid added to the system drove the CrCl3 hydrolysis backward, causing the shift of active form of chromium chloride (for glucose isomerization), [Cr(H2O)5]OH2+ to the less active form, Cr(H2O)63+.25 This result, therefore, suggested that the excess amount of HTCG-SO3H inhibited glucose isomerization, and was likely to have a negative impact on LA production.

30 glucose

HMF

LA

25

Yield (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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20 15 10 5 0 0

5

10

20

30

40

HTCG-SO3H (wt.%)

Figure 4 . Effects of HTCG-SO3H dosage on yields of glucose, HMF and LA from cellulose conversion at 200oC for 5 min, catalyzed with combined CrCl3 (0.005 M) and HTCG-SO3H catalysts.

3.5. Effect of CrCl3 concentration Unlike increasing HTCG-SO3H, increasing CrCl3 concentration favored cellulose conversion, especially in glucose isomerization. As seen in Figure 5, at high concentration of CrCl3 (from 0.005 M to 0.02 M), both glucose and HMF yields decreased, since glucose and HMF can be more readily converted to LA in presence of [Cr(H2O)5]OH2+ ion complexes and H+ ions, respectively. As seen in Figure 5, the LA 14 ACS Paragon Plus Environment

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yields obtained from the system of combined catalysts were higher than those of CrCl3 alone, for the entire CrCl3 concentration range studied. In both cases, the increased LA yields with increasing CrCl3 concentrations were evident. These results confirmed the synergistic effect of the combination of CrCl3 and HTCG-SO3H. The presence of HTCG-SO3H promoted not only hydrolysis, dehydration and rehydration, but also reduced side reactions caused by CrCl3. Comparable LA yields (ca. 40 wt. %) were found at CrCl3 concentrations of 0.02 and 0.015 M. Compared with complex systems involving reaction in relatively expensive solvents such as ionic liquids37-39 or gammavalerolactone40, employed in previous studies, the LA yield achieved in aqueous system in this study may not be as high, since these solvents offer additional advantage in solubilizing the cellulose substrate. Nevertheless, the LA yield from this study is superior to those of the catalytic systems of similar nature carried out in water. To the best of our knowledge, there were only two studies that previously reported the production of LA from the conversion of cellulose,28,41 using combined Brønsted acids (HCl, H2SO4, or CO2) and CrCl3 as Lewis acid catalysts (Table 1), the slightly higher LA yield was obtained in much shorter reaction time with the combined CrCl3 and HTCG-SO3H system at suitable reaction temperature, concentration of CrCl3, and amount of HTCG-SO3H catalyst. Since HTCG-SO3H is a biomass-derived catalyst, it can be easily recovered together with the remaining reaction residues without separation, and can be recycled after regeneration with sulfuric acid42. As for CrCl3, the recovery of CrCl3 from an aqueous solution has been suggested previously by Peng et al. (2010)35. Based on their measurements with atomic absorption spectroscopy of the liquid product, it was suggested that, as the reaction progressed at high temperature, Cr3+ that was originally in the solution was transformed to the oxide form. This chromium oxide was found to be deposited on



humin, a solid side product formed from the side-reactions of the acid-catalyzed decompositions of cellulose, glucose and HMF. The Chromium oxides can then be separated from humin in the form of Cr2O3 upon calcining at 400 °C.

Glucose Yield (wt. %)

12

CrCl₃

(a)

CrCl₃/HTCG-SO₃H

10 8 6 4 2 0 0

0.0025

8

0.005 0.01 [CrCl₃] molar

CrCl₃

7 HMF yield (wt. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.015

0.02

0.015

0.02

(b)

CrCl₃/HTCG-SO₃H

6 5 4 3 2 1 0 0

0.0025

0.005 0.01 [CrCl₃] molar


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50 LA Yield (wt. %)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

45

CrCl₃

40

CrCl₃/HTCG-SO₃H

35 30 25 20 15 10 5 0 0

0.0025

0.005 0.01 [CrCl₃] molar

0.015

0.02

Figure 5. Effects of CrCl3 concentration on yields of (a) glucose, (b) HMF, and (c) LA from cellulose conversion at 200oC for 5 min, catalyzed with combined CrCl3 and HTCG-SO3H (5 wt.%) catalysts.

Table 1. Comparison of reaction conditions and LA yields for cellulose conversion catalyzed by various systems of combined Brønsted acid and Lewis acid catalysts.

Entry 125 225 325 425 533 633 7* *This

work

Brønsted acid (conc.) HCl (0.25 M) H2SO4 (0.25 M) CO2 (4 MPa) CO2 (4 MPa) CO2 (1 MPa) CO2 (1.2 MPa) HTCG-SO3H (5 wt.%)

Lewis acid (conc.) CrCl3 (0.017 M) CrCl3 (0.017 M) CrCl3 (0.017 M) CrCl3 (0.017 M) CrCl3 (0.016 M) CrCl3 (0.024 M) CrCl3 (0.015 M)

LA (wt.%)

t (min)

T (°C)

32

90

160

23

90

160

19

90

160

22

90

180

29

360

190

28

360

195

40

5

200

4. Conclusions 17 ACS Paragon Plus Environment


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The synergistic effect of the combination of HTCG-SO3H as Brønsted acid and CrCl3 as Lewis acid on cellulose conversion to LA was demonstrated. Under a suitable condition of 5 wt.% of HTCG-SO3H, 0.015 M of CrCl3, 200 °C and 5 min, approximately 40 wt.% LA yield could be produced in a one-pot cellulose conversion. The combined catalyst system described herein thus offers great economical and environmental potentials for future development of large-scale process for biomass conversion into useful chemicals.

Acknowledgements Financial support from e-ASIA Joint Research Program (e-ASIA JRP) and Thailand Research Fund (RTA598006, RSA5880047 and IRG5780014) are greatly appreciated. The first author would also like to thank the financial support from the Japan Student Services Organization (JASSO) Scholarship.

Supporting Information XRD pattern of HTCG-SO3H (Figure S1), FTIR spectrum of HTCG-SO3H (Figure S2), TGA pattern of HTCG-SO3H (Figure S3), Surface area, pore volume, and pore size, total acidity and sulfur content of HTCG-SO3H (Table S1), Conversion, selectivities and yields of reaction products at 200oC for 5 min, catalyzed with combination CrCl3 (0-0.02M) and HTCG-SO3H (5 wt.%) catalysts (Table S2).

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Table of Contents/Abstract Graphics


Enhanced Levulinic Acid Production from Cellulose by Combined

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