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Conversion of glucose into furans in the presence of AlCl3 in an ethanol–water solvent system
Bioresource Technology 116 (2012) 190–194
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Conversion of glucose into furans in the presence of AlCl3 in an ethanol–water solvent system Yu Yang a,b, Changwei Hu a,⇑, Mahdi M. Abu-Omar b,⇑ a b
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China Department of Chemistry and the Center for direct Catalytic Conversion of Biomass to Biofuels (C3Bio), Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
a r t i c l e
i n f o
Article history: Received 12 December 2011 Received in revised form 30 March 2012 Accepted 31 March 2012 Available online 13 April 2012 Keywords: Glucose AlCl3 Ethanol 5-Hydroxymethylfurfural 5-Ethoxymethylfurfural
a b s t r a c t Glucose was converted into furans (5-hydroxymethylfurfural and 5-ethoxymethylfurfural) in the presence of AlCl3 in an ethanol–water solvent system. The system showed high activity for the conversion of glucose into furans but low activity for the subsequent formation of LAs (levulinic acid and ethyl levulinate). High furans yield of 57% with low LAs yield of 11% can be obtained at 160 °C within 15 min. Glucose-based disaccharides (sucrose, maltose and cellobiose) and polysaccharides (starch but not cellulose) can also be converted to furans effectively under the same condition. AlCl3 can be used to prepare furans from biomass-derived compounds in ethanol–water, a green solvent system. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Glucose, the monomer of cellulose and starch, is the most abundant and cheapest monosaccharide and extensive research is being conducted on the conversion of glucose into chemicals and biofuels. Different solvents have been used in the process of glucose conversion, such as water (Aida et al., 2007; Yang et al., 2010), dimethylsulfoxide (Yan et al. 2009), ionic liquids (Zhao et al., 2007; Zhang and Zhao, 2010), methanol (Tominaga et al., 2011) and ethanol (Peng et al., 2011; Hu et al., 2011). The use of alcohols as solvents for carbohydrate conversion has received increased attention because alcohols suppress humin formation (Hu et al., 2011) and afford biodiesel-like products, (the ether of 5-hydroxymethylfurfural (HMF) and/or the ester of levulinic acid (LA)), in one-pot reactions (Peng et al., 2011; Tominaga et al., 2011; Hu et al., 2011; Zhu et al., 2011; Saravanamurugan et al., 2011). Peng et al. (2011) used SO42 as a catalyst for glucose conversion in ethanol to give ethyl levulinate (LAE) with a 30% yield. The use of Amberlyst-15 can give 80% LAE yield from glucose (Hu et al., 2011). Tominaga et al. (2011) used In (OTf)3 in methanol for the conversion of glucose to afford methyl levulinate with a 58% yield after 5 h at 160 °C. The focus of these papers has been on the
conversion of glucose to the esters of LA. Furans (HMF and its ethers) appear to be the intermediates in these reactions. Compared to LAs (LA and its esters), furans are more valuable. HMF can serve as a platform chemical for liquid fuels and renewable polyesters (Corma et al., 2007). Furthermore, the ethers of HMF are excellent additive for diesel. For example, 5-ethoxymethylfurfural (EMF) has a high energy density of 8.7 kWh/L, similar to that of regular gasoline (8.8 kWh/L), nearly as good as that of diesel (9.7 kWh/L), and significantly higher than that of ethanol (6.1 kWh/ L) (Gruter and Dautzenberg, 2007). A catalytic system that can convert glucose to furans selectively in alcohol solvents is highly desirable. In the present paper, the conversion of glucose into furans in the presence of AlCl3 in an ethanol–water solvent system is reported. AlCl3 is a cheap, nontoxic, and abundant chemical, and AlCl3 in a water-tetrahydrofuran solvent system showed high activity for conversion of glucose into HMF (Yang et al. 2012). Ethanol was chosen because it is regarded as a green solvent and can be prepared from biomass (Saxena et al., 2009). The effect of the reaction conditions and possible reaction pathway are discussed. Some complex carbohydrates were also used as raw materials. 2. Methods 2.1. Materials
⇑ Corresponding authors. Tel.: +86 1 765 494 5302; fax: +86 1 765 494 0239 (M. M. Abu-Omar). E-mail addresses: [email protected] (C. Hu), [email protected] (M.M. Abu-Omar). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.03.126
Ethyl glucoside (EGL) was purchased from Carbosynth (USA). All other chemicals were purchased from Sigma–Aldrich (USA). All reagents were of analytical grade and used as received.
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2.2. General reaction procedure
2.3. Analytic methods Quantitative analyses of glucose, EGL, fructose, HMF, EMF and LA were performed by HPLC using a Waters 1525 pump, an aminex column HPX-87 column (Agilent) and Waters 2412 Refractive Index detector. 0.005 M H2SO4 solution was used as the mobile phase at a flow rate of 0.6 mL/min, and the column temperature was maintained at 338 K. The amounts of glucose, EGL, fructose, HMF, EMF and LA were calculated based on external standard curves constructed with authentic standards. LAE was characterized by gas chromatography (Agilent 6890 with a DB-5 column and flame ionization detector (FID). The temperature of the injection was 270 °C. The temperature of the column was maintained at 120 °C for 3.3 min and then raised to 200 °C with a ramp rate of 80 °C/ min. LAE yield was determined by standard curves using furfuryl alcohol as an internal standard. The analytical error was evaluated to be in the range of ±1%. The pH value of the ethanol–water solution (10 wt.% water content) of AlCl36H2O was measured on an Accumet AB15/15+ pH meter (±0.01 pH units) calibrated with standard buffer solutions. The pH value of the solution at 25 °C was 2.56. 3. Results and discussion 3.1. Production of furans from glucose 3.1.1. Effect of water Fig. 1 shows the effect of water content in the ethanol–water solvent on glucose conversion and the distribution of products at 160 °C. When pure ethanol was the solvent (the water content was 0 wt.%), glucose was essentially converted after 15 min, and a furans yield of 44% was observed. EMF was the major product with EMF/HMF molar ratio of 4.8. Some LAs was also obtained, but its yield was low (5%). This result is in sharp contrast to those of previous reports (Peng et al., 2011; Hu et al., 2011; Tominaga et al., 2011). Therefore, using AlCl3 in ethanol, the reaction rate
4
60
40 2
Molar ratio of EMF/HMF
Glucose conversion EGL yield furans yield LAs yield molar ratio of EMF/HMF
80
Percent (%)
Reactions were carried out in a Discover TM microwave batch reactor (CEM Corporation). In a typical experiment for the conversion of glucose, a 10-mL reaction tube was charged with 1 mmol glucose, 0.4 mmol AlCl36H2O, 0.4 g deionized water and 3.6 g ethanol and heated to 160 °C for 15 min. Experiments were conducted at 130, 140, 150, 160 and 170 °C to know the effect of reaction temperature. In a typical experiment for the conversion of intermediates or products, a 10-mL reaction tube was charged with 1 mmol of starting material, 0.4 mmol AlCl36H2O, 0.4 g deionzed water and 3.6 g ethanol and heated to 160 °C for 15 min. In a typical experiment for the conversion of other carbohydrates, a 10-mL reaction tube was charged with 1 mmol of carbohydrates (based on monosaccharide units), 0.4 mmol AlCl36H2O, 0.4 g deionzed water and 3.6 g ethanol and heated to 160 °C for 15 min. All solutions were mixed at the maximum stirring rate of the microwave reactor using a magnetic stir bar. Temperatures in the reactor were measured by a fiber optic sensor. The reaction vessel was pressurized due to the vapor pressure of the solution at the employed reaction temperatures. Zero time was taken as soon as the set temperature was reached. The reaction was stopped by nitrogen flow cooling. The reaction time means the period during which the highest constant temperature was maintained. Samples were filtered with a 0.2-mm syringe filter prior to analysis. All optimized experiments were run in triplicates and the results reported are an average. The standard deviation was ca. 2% between different reaction runs.
6 100
20
0
0 0
20
40
60
80
100
Water content (wt%) Fig. 1. The effect of water content in the ethanol–water solvent on the products distribution for glucose conversion. Reaction condition: Solvent, 4 g; glucose, 1 mmol; AlCl36H2O, 0.4 mmol; reaction temperature, 160 °C; reaction time, 15 min.
for furans formation from glucose must be faster than the rate for furans conversion to LAs. Thus, high yield of furans with low yield of LAs can be obtained. In addition to furans and LAs, a 19% yield of EGL was also obtained. In order to inhibit the formation of EGL, water was introduced into the system. With increasing water content, the EGL yield decreased gradually. As a result of less EGL being formed, furans yield increased to 57% at 10 wt.% of water content. In fact a furans yield of 57% is comparable to using fructose as the starting material (Zhu et al., 2011). However, the introduction of water also inhibited the conversion of HMF to EMF; thus, the molar ratio of EMF/HMF decreased to less than 2 at a 10 wt.% water content. LAs yield also increased with increasing water content as water favors the rehydration reaction rate (Peng et al., 2011). Thus the use of AlCl3 in pure water produced high LAs but low furans yield (Peng et al., 2010). Humin formation also increased with higher water content. It has been well documented that large amount of humin was formed in aqueous medium (Hu et al., 2011). As the water content in the solvent exceeded 10 wt.%, furans yield declined. The optimal water content for this ethanol–water solvent system was 10 wt.% (Fig. 1). 3.1.2. Effect of the reaction temperature Fig. 2 shows the effect of temperature on the distribution of products. At 130 °C, high glucose conversion (93%) was observed. However, the yield of furans was low (28%). Moreover, the molar ratio of EMF/HMF was 0.63. A high yield of EGL was obtained. The formation of EGL is preferred at low reaction temperature (Hu et al., 2011). An increase in the reaction temperature decreased EGL yield and increased furans yield. At a reaction temperature of 170 °C, EGL yield decreased to 3% and the furans yield approached 59%. The increase in reaction temperature also increased the reaction rate and/or equilibrium of HMF etherification to EMF. The molar ratio of EMF/HMF reached 1.4 at 170 °C. Higher reaction temperatures were not examined because the system pressure at 180 °C exceeded the pressure limitation of the microwave batch reactor of 300 psi. However, considering that LA yields and insoluble humin gradually increased with increasing reaction temperature, further increase of the reaction temperature would lead to a decrease in furans selectivity and yield. 3.1.3. Effect of AlCl36H2O amount The dependence of glucose conversion and product distributions on the amount of AlCl36H2O employed is shown in Fig. 3.
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100
1.6
1.6
100 1.4
0.8 Glucose conversion EGL yield fruans yield LAs yield Molar ratio of EMF/HMF
40
20
0.4
1.2 60
40
20
0
0.0 130
140
150
160
100 Glucose conversion EGL yield furans yield LAs yield molar ratio of EMF/HMF
Percent (%)
2
40 1 20
Molar ratio of EMF/HMF
3
0 0 0.1
0.2
0.3
0.4
5
10
15
20
25
30
Time (min)
Fig. 2. The effect of reaction temperature on the products distribution. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; glucose, 1 mmol; AlCl36H2O, 0.4 mmol; reaction time, 15 min.
0.0
0.6
0.4 0
Temperature ( C)
60
0.8
0
170
o
80
1.0
Glucose conversion Fructose yield Total sugars furans yield LAs yield Molar ratio of EMF/HMF
Molar ratio of EMF/HMF
60
80
Percent (%)
Percent (%)
1.2
Molar ratio of EMF/HMF
80
0.5
0.6
AlCl3 6H2O amount (mmol) Fig. 3. The effect of AlCl36H2O amount on the products distribution. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; glucose, 1 mmol; reaction temperature, 160 °C; reaction time, 15 min.
The amount of AlCl36H2O was varied from 0.05 mmol to 0.6 mmol. The amount of AlCl36H2O in the examined range had little to no effect on glucose conversion. Even at the low amount of 0.05 mmol (5 mol% relative to glucose), the reaction reached 93% conversion after 15 min. However, the product distributions varied significantly with AlCl36H2O amount. At the low AlCl36H2O amount of 0.05 mmol, the main product was EGL; and its yield reached 62% and the furans yield was only 18%. An increase in AlCl36H2O amount caused a significant dip in EGL yield and an increase in furans yield. At 0.4 mmol of AlCl36H2O, the EGL yield was only 9% and the furans yield was 57%. A further increase in the AlCl36H2O amount to 0.6 mmol did not produce additional increase in furans yield; instead more LAs and humin were formed. The molar ratio of EMF/HMF continued to rise with increasing AlCl36H2O amount. 3.2. Reaction pathway for the conversion of glucose using AlCl3 based catalyst in ethanol–water solvent system The conversion of glucose in the presence of AlCl3 in an ethanol–water system is a complex multistep process. The time profile for glucose conversion in the ethanol–water system is depicted in Fig. 4. High glucose conversion of 94% was achieved within 30 s; meanwhile respectable furans and EGL yields of 35% each were
Fig. 4. Time profile for glucose conversion in ethanol–water system. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; glucose, 1 mmol; AlCl36H2O, 0.4 mmol; reaction temperature, 160 °C.
observed over the same 30 s. As the reaction progressed, EGL was consumed and the furans yield increased to a plateau of 57%. This observation is consistent with EGL being converted to furans at longer reaction times. Peng et al. (2011) proposed EGL as an intermediate en route from glucose to EMF. However, when EGL was used under the present conditions instead of glucose as the starting material (Table 1, entry 2 vs. entry 1), more glucose and EGL remained unconverted, and the yields of EMF and LAE were lower than those observed for glucose. A possible interpretation is that the formation of EGL is a competing pathway to that leading to EMF and LAE. Fortunately, EGL formation is reversible in the presence of water. Thus introduction of water reduces EGL formation and improves the yield of furans (as seen in section 3.1.1 above). Furthermore, the amount of AlCl36H2O becomes crucial in accelerating the hydrolysis of EGL back to glucose and hence affecting the product distributions. At the start of the reaction, fructose was observed. It was reported that AlCl3 in water can catalyze the isomerization of glucose-to-fructose (Peng et al., 2010; Rasrendra et al., 2010; Yang et al. 2012). Therefore, using AlCl36H2O in the ethanol–water solvent system, glucose-to-fructose isomerization was observed. In the presence of water, AlCl36H2O hydrolyzes to release HCl. The pH of the solution (0.4 mmol AlCl36H2O; ethanol, 3.6 g; H2O, 0.4 g) was 2.56. In a control experiment, directly using hydrochloric acid for the conversion of glucose at pH 2.56 under the same reaction conditions, fructose was not observed. Although glucose conversion reached 75% after 15 min, the major product was EGL (71% yield) and only trace amounts of furans were obtained. These results indicated that the catalytic contribution of HCl, which
Table 1 Probing the reaction pathway by studying the catalytic action of AlCl3 based catalyst on reactants, intermediates, and products.a Entry
1 2 3 4 5 6 7
Starting material
Glucose EGL Fructose HMF EMF LA LAE
Products yield (%) Glucose
EGL
HMF
EMF
LA
LAE
1 1 trace – – – –
9 14 trace – – – –
24 25 12 15 7 – –
33 27 46 70 82 – –
3 2 3 3 3 10 7
8 3 11 12 8 90 93
a Reaction condition: Ethanol: 3.6 g, H2O: 0.4 g, Staring material: 1.0 mmol, AlCl36H2O: 0.4 mmol, temperature: 160 °C, reaction time: 15 min.
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HO HO HO
O O OH
EtOH H2O
HO
O OH HO OH OH
Fructose OH OH
LAE
EtOH H2 O
EtOH H2 O
O
OH
HMF
Glucose
Furans
saccharides
O
EMF
O
O
O
O
O
EGL
HO HO HO
O
O
O OH
LA
O
LAs
Scheme 1. Proposed reaction pathway for AlCl3 based catalytic conversion of glucose in ethanol–water system.
formed during the hydrolysis of AlCl36H2O, to glucose-to-fructose isomerization was rather low. The active species for the isomerization of glucose-to-fructose is therefore likely an Al-containing species. In the system applied in the current study, similar furans yield can be obtained from glucose and fructose (Table 1, entry 1 vs 3). In a system where the isomerization of glucose-to-fructose could not be catalyzed, the conversion of glucose into furans is significantly harder than that of fructose (Zhao et al. 2007; Yang et al., 2010), making furans yield from glucose lower than that from fructose. Using Amberlyst-15 as catalyst in methanol, the furans yield reached 44% from fructose (Zhu et al., 2011) while the maximum yield of furans from glucose was less than 1% (Hu et al., 2011). Therefore, the conversion of glucose into furans in the current system was much more effective than that of previously reported systems where the catalyst did not catalyze the isomerization of glucose-to-fructose (Tominaga et al., 2011; Hu et al., 2011). When fructose was used as starting material (Table 1, entry 3), trace amounts of glucose and EGL were detected in the reaction mixture. These can be attributed to the reversible isomerization of glucoseto-fructose (Dehkordi et al., 2009). The use of HMF, EMF, LA, and LAE as starting materials demonstrated that LA/LAE was formed from HMF/EMF, and under the present reaction conditions, the formation of EMF and LAE was favored. Based on these results, a plausible reaction pathway for catalytic conversion of glucose in ethanol–water system over AlCl3 is shown in Scheme 1.
HMF EMF
60
Fruans Yield (%)
50
3.3. Production of furans from complex carbohydrates Disaccharides (sucrose, maltose and cellobiose) and polysaccharides (starch and cellulose) were examined as substrates. The results are summarized in Fig. 5. The furans yield from sucrose was the highest, which is consistent with the hypothesized reaction pathway in Scheme 1 as sucrose contains one fructose molecule per unit. The furans yield from maltose was comparable to that obtained from glucose. Conversion of cellobiose and starch to furans was slightly lower in efficiency than that of maltose and sucrose. The use of AlCl3 in the ethanol–water solvent system was effective in hydrolyzing the glycosidic linkage of disaccharides and starch. However, cellulose conversion was poor, affording only a 9% furan yield. The majority of the cellulose remained unconverted as a solid suspension. This poor reactivity with cellulose is likely due to ineffective depolymerization and decrystallization in this system at 160 °C. Higher temperature is often required for cellulose conversion (Cheng et al., 2011; Yang et al. 2012).
4. Conclusion AlCl3 in an ethanol–water solvent system is effective in converting glucose to furans (HMF and EMF) under reasonably mild conditions, 160 °C. The formation of EGL is a competing route to the formation of furans. The reversion of EGL to glucose can be promoted by increasing of water content, the reaction temperature, and AlCl36H2O amount. A maximum furans yield was obtained in water content of 10 wt.%. Higher water content leads to further transformation of furans to LAs. Complex carbohydrates but not cellulose can also be converted to furans effectively over AlCl3 in ethanol–water solvent system at 160 °C.
References
40 30 20 10 0
Sucrose
Maltose
Cellobiose
Starch
Cellulose
Fig. 5. Furans yield from different carbohydrates. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; carbohydrate, 1 mmol based on monosaccharide units; AlCl36H2O, 0.4 mmol; reaction temperature, 160 °C; reaction time, 15 min.
Aida, T.M., Sato, Y., Watanabe, M., Tajima, K., Nonaka, T., Hattori, H., Arai, K., 2007. Dehydration of D-glucose in high temperature water at pressures up to 80 MPa. J. Supercrit. Fluids 40, 381–388. Cheng, M., Shi, T., Wang, S., Guan, H., Fan, C., Wang, X., 2011. Fabrication of micellar heteropolyacid catalysts for clean production of monosaccharides from polysaccharides. Catal. Commun. 12, 1483–1487. Corma, A., Iborra, S., Velty, A., 2007. Chemical routes for the transformation of biomass into chemicals. Chem. Rev. 107, 2411–2502. Dehkordi, A.M., Tehrany, M.S., Safari, I., 2009. Kinetics of glucose isomerization to fructose by immobilized glucose isomerase (Sweetzyme IT). Ind. Eng. Chem. Res. 48, 3271–3278. Gruter, G. J. M., Dautzenberg, F., Eur. Pat. Appl., 2007, 1834950A1. Hu, X., Lievens, C., Larcher, A., Li, C.Z., 2011. Reaction pathways of glucose during esterification: effects of reaction parameters on the formation of humin type polymers. Bioresour. Technol. 102, 10104–10113. Peng, L., Lin, L., Zhang, J., Zhuang, J., Zhang, B., Gong, Y., 2010. Catalytic conversion of cellulose to levulinic acid by metal chlorides. Molecules 15, 5258–5272.
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Peng, L., Lin, L., Zhang, J., Shi, J., Liu, S., 2011. Solid acid catalyzed glucose conversion to ethyl levulinate. Appl. Catal., A 397, 259–265. Rasrendra, C., Makertihartha, I.G.B.N., Adisasmito, S., Heeres, H., 2010. Green chemicals from D-glucose: systematic studies on catalytic effects of inorganic salts on the chemo-selectivity and yield in aqueous solutions. Top. Catal. 53, 1241–1247. Saravanamurugan, S., Nguyen Van Buu, O., Riisager, A., 2011. Conversion of monoand disaccharides to ethyl levulinate and ethyl pyranoside with sulfonic acidfunctionalized ionic liquids. Chem. Sus. Chem. 4, 723–726. Saxena, R., Adhikari, D., Goyal, H., 2009. Biomass-based energy fuel through biochemical routes: a review. Renewable Sustainable Energy Rev. 13, 167–178. Tominaga, K., Mori, A., Fukushima, Y., Shimada, S., Sato, K., 2011. Mixed-acid systems for the catalytic synthesis of methyl levulinate from cellulose. Green Chem. 13, 810–812. Yan, H., Yang, Y., Tong, D., Xiang, X., Hu, C., 2009. Catalytic conversion of glucose to 5-hydroxymethylfurfural over SO42 /ZrO2 and SO42 /ZrO2-Al2O3 solid acid catalysts. Catal. Commun. 10, 1558–1563.
Yang, F., Liu, Q., Bai, X., Du, Y., 2010. Conversion of biomass into 5hydroxymethylfurfural using solid acid catalyst. Bioresour. Technol. 102, 3424–3429. Yang, Y., Hu, C., Abu-Omar, M.M., 2012. Conversion of carbohydrates and lignocellulosic biomass into 5-hydroxymethylfurfural using AlCl3 6H2O catalyst in a biphasic solvent system. Green Chem. 14, 509–513. Zhang, Z., Zhao, Z.K., 2010. Production of 5-hydroxymethylfurfural from glucose catalyzed by hydroxyapatite supported chromium chloride. Bioresour. Technol. 102, 3970–3972. Zhao, H., Holladay, J., Brown, H., Zhang, Z., 2007. Metal chlorides in ionic liquid solvents convert sugars to 5-hydroxymethylfurfural. Science 316, 1597–1600. Zhu, H., Cao, Q., Li, C., Mu, X., 2011. Acidic resin-catalysed conversion of fructose into furan derivatives in low boiling point solvents. Carbohydr. Res. 346, 2016– 2018.
Contents lists available at SciVerse ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Conversion of glucose into furans in the presence of AlCl3 in an ethanol–water solvent system Yu Yang a,b, Changwei Hu a,⇑, Mahdi M. Abu-Omar b,⇑ a b
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China Department of Chemistry and the Center for direct Catalytic Conversion of Biomass to Biofuels (C3Bio), Purdue University, 560 Oval Drive, West Lafayette, IN 47907, USA
a r t i c l e
i n f o
Article history: Received 12 December 2011 Received in revised form 30 March 2012 Accepted 31 March 2012 Available online 13 April 2012 Keywords: Glucose AlCl3 Ethanol 5-Hydroxymethylfurfural 5-Ethoxymethylfurfural
a b s t r a c t Glucose was converted into furans (5-hydroxymethylfurfural and 5-ethoxymethylfurfural) in the presence of AlCl3 in an ethanol–water solvent system. The system showed high activity for the conversion of glucose into furans but low activity for the subsequent formation of LAs (levulinic acid and ethyl levulinate). High furans yield of 57% with low LAs yield of 11% can be obtained at 160 °C within 15 min. Glucose-based disaccharides (sucrose, maltose and cellobiose) and polysaccharides (starch but not cellulose) can also be converted to furans effectively under the same condition. AlCl3 can be used to prepare furans from biomass-derived compounds in ethanol–water, a green solvent system. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Glucose, the monomer of cellulose and starch, is the most abundant and cheapest monosaccharide and extensive research is being conducted on the conversion of glucose into chemicals and biofuels. Different solvents have been used in the process of glucose conversion, such as water (Aida et al., 2007; Yang et al., 2010), dimethylsulfoxide (Yan et al. 2009), ionic liquids (Zhao et al., 2007; Zhang and Zhao, 2010), methanol (Tominaga et al., 2011) and ethanol (Peng et al., 2011; Hu et al., 2011). The use of alcohols as solvents for carbohydrate conversion has received increased attention because alcohols suppress humin formation (Hu et al., 2011) and afford biodiesel-like products, (the ether of 5-hydroxymethylfurfural (HMF) and/or the ester of levulinic acid (LA)), in one-pot reactions (Peng et al., 2011; Tominaga et al., 2011; Hu et al., 2011; Zhu et al., 2011; Saravanamurugan et al., 2011). Peng et al. (2011) used SO42 as a catalyst for glucose conversion in ethanol to give ethyl levulinate (LAE) with a 30% yield. The use of Amberlyst-15 can give 80% LAE yield from glucose (Hu et al., 2011). Tominaga et al. (2011) used In (OTf)3 in methanol for the conversion of glucose to afford methyl levulinate with a 58% yield after 5 h at 160 °C. The focus of these papers has been on the
conversion of glucose to the esters of LA. Furans (HMF and its ethers) appear to be the intermediates in these reactions. Compared to LAs (LA and its esters), furans are more valuable. HMF can serve as a platform chemical for liquid fuels and renewable polyesters (Corma et al., 2007). Furthermore, the ethers of HMF are excellent additive for diesel. For example, 5-ethoxymethylfurfural (EMF) has a high energy density of 8.7 kWh/L, similar to that of regular gasoline (8.8 kWh/L), nearly as good as that of diesel (9.7 kWh/L), and significantly higher than that of ethanol (6.1 kWh/ L) (Gruter and Dautzenberg, 2007). A catalytic system that can convert glucose to furans selectively in alcohol solvents is highly desirable. In the present paper, the conversion of glucose into furans in the presence of AlCl3 in an ethanol–water solvent system is reported. AlCl3 is a cheap, nontoxic, and abundant chemical, and AlCl3 in a water-tetrahydrofuran solvent system showed high activity for conversion of glucose into HMF (Yang et al. 2012). Ethanol was chosen because it is regarded as a green solvent and can be prepared from biomass (Saxena et al., 2009). The effect of the reaction conditions and possible reaction pathway are discussed. Some complex carbohydrates were also used as raw materials. 2. Methods 2.1. Materials
⇑ Corresponding authors. Tel.: +86 1 765 494 5302; fax: +86 1 765 494 0239 (M. M. Abu-Omar). E-mail addresses: [email protected] (C. Hu), [email protected] (M.M. Abu-Omar). 0960-8524/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2012.03.126
Ethyl glucoside (EGL) was purchased from Carbosynth (USA). All other chemicals were purchased from Sigma–Aldrich (USA). All reagents were of analytical grade and used as received.
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2.2. General reaction procedure
2.3. Analytic methods Quantitative analyses of glucose, EGL, fructose, HMF, EMF and LA were performed by HPLC using a Waters 1525 pump, an aminex column HPX-87 column (Agilent) and Waters 2412 Refractive Index detector. 0.005 M H2SO4 solution was used as the mobile phase at a flow rate of 0.6 mL/min, and the column temperature was maintained at 338 K. The amounts of glucose, EGL, fructose, HMF, EMF and LA were calculated based on external standard curves constructed with authentic standards. LAE was characterized by gas chromatography (Agilent 6890 with a DB-5 column and flame ionization detector (FID). The temperature of the injection was 270 °C. The temperature of the column was maintained at 120 °C for 3.3 min and then raised to 200 °C with a ramp rate of 80 °C/ min. LAE yield was determined by standard curves using furfuryl alcohol as an internal standard. The analytical error was evaluated to be in the range of ±1%. The pH value of the ethanol–water solution (10 wt.% water content) of AlCl36H2O was measured on an Accumet AB15/15+ pH meter (±0.01 pH units) calibrated with standard buffer solutions. The pH value of the solution at 25 °C was 2.56. 3. Results and discussion 3.1. Production of furans from glucose 3.1.1. Effect of water Fig. 1 shows the effect of water content in the ethanol–water solvent on glucose conversion and the distribution of products at 160 °C. When pure ethanol was the solvent (the water content was 0 wt.%), glucose was essentially converted after 15 min, and a furans yield of 44% was observed. EMF was the major product with EMF/HMF molar ratio of 4.8. Some LAs was also obtained, but its yield was low (5%). This result is in sharp contrast to those of previous reports (Peng et al., 2011; Hu et al., 2011; Tominaga et al., 2011). Therefore, using AlCl3 in ethanol, the reaction rate
4
60
40 2
Molar ratio of EMF/HMF
Glucose conversion EGL yield furans yield LAs yield molar ratio of EMF/HMF
80
Percent (%)
Reactions were carried out in a Discover TM microwave batch reactor (CEM Corporation). In a typical experiment for the conversion of glucose, a 10-mL reaction tube was charged with 1 mmol glucose, 0.4 mmol AlCl36H2O, 0.4 g deionized water and 3.6 g ethanol and heated to 160 °C for 15 min. Experiments were conducted at 130, 140, 150, 160 and 170 °C to know the effect of reaction temperature. In a typical experiment for the conversion of intermediates or products, a 10-mL reaction tube was charged with 1 mmol of starting material, 0.4 mmol AlCl36H2O, 0.4 g deionzed water and 3.6 g ethanol and heated to 160 °C for 15 min. In a typical experiment for the conversion of other carbohydrates, a 10-mL reaction tube was charged with 1 mmol of carbohydrates (based on monosaccharide units), 0.4 mmol AlCl36H2O, 0.4 g deionzed water and 3.6 g ethanol and heated to 160 °C for 15 min. All solutions were mixed at the maximum stirring rate of the microwave reactor using a magnetic stir bar. Temperatures in the reactor were measured by a fiber optic sensor. The reaction vessel was pressurized due to the vapor pressure of the solution at the employed reaction temperatures. Zero time was taken as soon as the set temperature was reached. The reaction was stopped by nitrogen flow cooling. The reaction time means the period during which the highest constant temperature was maintained. Samples were filtered with a 0.2-mm syringe filter prior to analysis. All optimized experiments were run in triplicates and the results reported are an average. The standard deviation was ca. 2% between different reaction runs.
6 100
20
0
0 0
20
40
60
80
100
Water content (wt%) Fig. 1. The effect of water content in the ethanol–water solvent on the products distribution for glucose conversion. Reaction condition: Solvent, 4 g; glucose, 1 mmol; AlCl36H2O, 0.4 mmol; reaction temperature, 160 °C; reaction time, 15 min.
for furans formation from glucose must be faster than the rate for furans conversion to LAs. Thus, high yield of furans with low yield of LAs can be obtained. In addition to furans and LAs, a 19% yield of EGL was also obtained. In order to inhibit the formation of EGL, water was introduced into the system. With increasing water content, the EGL yield decreased gradually. As a result of less EGL being formed, furans yield increased to 57% at 10 wt.% of water content. In fact a furans yield of 57% is comparable to using fructose as the starting material (Zhu et al., 2011). However, the introduction of water also inhibited the conversion of HMF to EMF; thus, the molar ratio of EMF/HMF decreased to less than 2 at a 10 wt.% water content. LAs yield also increased with increasing water content as water favors the rehydration reaction rate (Peng et al., 2011). Thus the use of AlCl3 in pure water produced high LAs but low furans yield (Peng et al., 2010). Humin formation also increased with higher water content. It has been well documented that large amount of humin was formed in aqueous medium (Hu et al., 2011). As the water content in the solvent exceeded 10 wt.%, furans yield declined. The optimal water content for this ethanol–water solvent system was 10 wt.% (Fig. 1). 3.1.2. Effect of the reaction temperature Fig. 2 shows the effect of temperature on the distribution of products. At 130 °C, high glucose conversion (93%) was observed. However, the yield of furans was low (28%). Moreover, the molar ratio of EMF/HMF was 0.63. A high yield of EGL was obtained. The formation of EGL is preferred at low reaction temperature (Hu et al., 2011). An increase in the reaction temperature decreased EGL yield and increased furans yield. At a reaction temperature of 170 °C, EGL yield decreased to 3% and the furans yield approached 59%. The increase in reaction temperature also increased the reaction rate and/or equilibrium of HMF etherification to EMF. The molar ratio of EMF/HMF reached 1.4 at 170 °C. Higher reaction temperatures were not examined because the system pressure at 180 °C exceeded the pressure limitation of the microwave batch reactor of 300 psi. However, considering that LA yields and insoluble humin gradually increased with increasing reaction temperature, further increase of the reaction temperature would lead to a decrease in furans selectivity and yield. 3.1.3. Effect of AlCl36H2O amount The dependence of glucose conversion and product distributions on the amount of AlCl36H2O employed is shown in Fig. 3.
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100
1.6
1.6
100 1.4
0.8 Glucose conversion EGL yield fruans yield LAs yield Molar ratio of EMF/HMF
40
20
0.4
1.2 60
40
20
0
0.0 130
140
150
160
100 Glucose conversion EGL yield furans yield LAs yield molar ratio of EMF/HMF
Percent (%)
2
40 1 20
Molar ratio of EMF/HMF
3
0 0 0.1
0.2
0.3
0.4
5
10
15
20
25
30
Time (min)
Fig. 2. The effect of reaction temperature on the products distribution. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; glucose, 1 mmol; AlCl36H2O, 0.4 mmol; reaction time, 15 min.
0.0
0.6
0.4 0
Temperature ( C)
60
0.8
0
170
o
80
1.0
Glucose conversion Fructose yield Total sugars furans yield LAs yield Molar ratio of EMF/HMF
Molar ratio of EMF/HMF
60
80
Percent (%)
Percent (%)
1.2
Molar ratio of EMF/HMF
80
0.5
0.6
AlCl3 6H2O amount (mmol) Fig. 3. The effect of AlCl36H2O amount on the products distribution. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; glucose, 1 mmol; reaction temperature, 160 °C; reaction time, 15 min.
The amount of AlCl36H2O was varied from 0.05 mmol to 0.6 mmol. The amount of AlCl36H2O in the examined range had little to no effect on glucose conversion. Even at the low amount of 0.05 mmol (5 mol% relative to glucose), the reaction reached 93% conversion after 15 min. However, the product distributions varied significantly with AlCl36H2O amount. At the low AlCl36H2O amount of 0.05 mmol, the main product was EGL; and its yield reached 62% and the furans yield was only 18%. An increase in AlCl36H2O amount caused a significant dip in EGL yield and an increase in furans yield. At 0.4 mmol of AlCl36H2O, the EGL yield was only 9% and the furans yield was 57%. A further increase in the AlCl36H2O amount to 0.6 mmol did not produce additional increase in furans yield; instead more LAs and humin were formed. The molar ratio of EMF/HMF continued to rise with increasing AlCl36H2O amount. 3.2. Reaction pathway for the conversion of glucose using AlCl3 based catalyst in ethanol–water solvent system The conversion of glucose in the presence of AlCl3 in an ethanol–water system is a complex multistep process. The time profile for glucose conversion in the ethanol–water system is depicted in Fig. 4. High glucose conversion of 94% was achieved within 30 s; meanwhile respectable furans and EGL yields of 35% each were
Fig. 4. Time profile for glucose conversion in ethanol–water system. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; glucose, 1 mmol; AlCl36H2O, 0.4 mmol; reaction temperature, 160 °C.
observed over the same 30 s. As the reaction progressed, EGL was consumed and the furans yield increased to a plateau of 57%. This observation is consistent with EGL being converted to furans at longer reaction times. Peng et al. (2011) proposed EGL as an intermediate en route from glucose to EMF. However, when EGL was used under the present conditions instead of glucose as the starting material (Table 1, entry 2 vs. entry 1), more glucose and EGL remained unconverted, and the yields of EMF and LAE were lower than those observed for glucose. A possible interpretation is that the formation of EGL is a competing pathway to that leading to EMF and LAE. Fortunately, EGL formation is reversible in the presence of water. Thus introduction of water reduces EGL formation and improves the yield of furans (as seen in section 3.1.1 above). Furthermore, the amount of AlCl36H2O becomes crucial in accelerating the hydrolysis of EGL back to glucose and hence affecting the product distributions. At the start of the reaction, fructose was observed. It was reported that AlCl3 in water can catalyze the isomerization of glucose-to-fructose (Peng et al., 2010; Rasrendra et al., 2010; Yang et al. 2012). Therefore, using AlCl36H2O in the ethanol–water solvent system, glucose-to-fructose isomerization was observed. In the presence of water, AlCl36H2O hydrolyzes to release HCl. The pH of the solution (0.4 mmol AlCl36H2O; ethanol, 3.6 g; H2O, 0.4 g) was 2.56. In a control experiment, directly using hydrochloric acid for the conversion of glucose at pH 2.56 under the same reaction conditions, fructose was not observed. Although glucose conversion reached 75% after 15 min, the major product was EGL (71% yield) and only trace amounts of furans were obtained. These results indicated that the catalytic contribution of HCl, which
Table 1 Probing the reaction pathway by studying the catalytic action of AlCl3 based catalyst on reactants, intermediates, and products.a Entry
1 2 3 4 5 6 7
Starting material
Glucose EGL Fructose HMF EMF LA LAE
Products yield (%) Glucose
EGL
HMF
EMF
LA
LAE
1 1 trace – – – –
9 14 trace – – – –
24 25 12 15 7 – –
33 27 46 70 82 – –
3 2 3 3 3 10 7
8 3 11 12 8 90 93
a Reaction condition: Ethanol: 3.6 g, H2O: 0.4 g, Staring material: 1.0 mmol, AlCl36H2O: 0.4 mmol, temperature: 160 °C, reaction time: 15 min.
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HO HO HO
O O OH
EtOH H2O
HO
O OH HO OH OH
Fructose OH OH
LAE
EtOH H2 O
EtOH H2 O
O
OH
HMF
Glucose
Furans
saccharides
O
EMF
O
O
O
O
O
EGL
HO HO HO
O
O
O OH
LA
O
LAs
Scheme 1. Proposed reaction pathway for AlCl3 based catalytic conversion of glucose in ethanol–water system.
formed during the hydrolysis of AlCl36H2O, to glucose-to-fructose isomerization was rather low. The active species for the isomerization of glucose-to-fructose is therefore likely an Al-containing species. In the system applied in the current study, similar furans yield can be obtained from glucose and fructose (Table 1, entry 1 vs 3). In a system where the isomerization of glucose-to-fructose could not be catalyzed, the conversion of glucose into furans is significantly harder than that of fructose (Zhao et al. 2007; Yang et al., 2010), making furans yield from glucose lower than that from fructose. Using Amberlyst-15 as catalyst in methanol, the furans yield reached 44% from fructose (Zhu et al., 2011) while the maximum yield of furans from glucose was less than 1% (Hu et al., 2011). Therefore, the conversion of glucose into furans in the current system was much more effective than that of previously reported systems where the catalyst did not catalyze the isomerization of glucose-to-fructose (Tominaga et al., 2011; Hu et al., 2011). When fructose was used as starting material (Table 1, entry 3), trace amounts of glucose and EGL were detected in the reaction mixture. These can be attributed to the reversible isomerization of glucoseto-fructose (Dehkordi et al., 2009). The use of HMF, EMF, LA, and LAE as starting materials demonstrated that LA/LAE was formed from HMF/EMF, and under the present reaction conditions, the formation of EMF and LAE was favored. Based on these results, a plausible reaction pathway for catalytic conversion of glucose in ethanol–water system over AlCl3 is shown in Scheme 1.
HMF EMF
60
Fruans Yield (%)
50
3.3. Production of furans from complex carbohydrates Disaccharides (sucrose, maltose and cellobiose) and polysaccharides (starch and cellulose) were examined as substrates. The results are summarized in Fig. 5. The furans yield from sucrose was the highest, which is consistent with the hypothesized reaction pathway in Scheme 1 as sucrose contains one fructose molecule per unit. The furans yield from maltose was comparable to that obtained from glucose. Conversion of cellobiose and starch to furans was slightly lower in efficiency than that of maltose and sucrose. The use of AlCl3 in the ethanol–water solvent system was effective in hydrolyzing the glycosidic linkage of disaccharides and starch. However, cellulose conversion was poor, affording only a 9% furan yield. The majority of the cellulose remained unconverted as a solid suspension. This poor reactivity with cellulose is likely due to ineffective depolymerization and decrystallization in this system at 160 °C. Higher temperature is often required for cellulose conversion (Cheng et al., 2011; Yang et al. 2012).
4. Conclusion AlCl3 in an ethanol–water solvent system is effective in converting glucose to furans (HMF and EMF) under reasonably mild conditions, 160 °C. The formation of EGL is a competing route to the formation of furans. The reversion of EGL to glucose can be promoted by increasing of water content, the reaction temperature, and AlCl36H2O amount. A maximum furans yield was obtained in water content of 10 wt.%. Higher water content leads to further transformation of furans to LAs. Complex carbohydrates but not cellulose can also be converted to furans effectively over AlCl3 in ethanol–water solvent system at 160 °C.
References
40 30 20 10 0
Sucrose
Maltose
Cellobiose
Starch
Cellulose
Fig. 5. Furans yield from different carbohydrates. Reaction condition: Ethanol, 3.6 g; H2O, 0.4 g; carbohydrate, 1 mmol based on monosaccharide units; AlCl36H2O, 0.4 mmol; reaction temperature, 160 °C; reaction time, 15 min.
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