Bioresource Technology 151 (2014) 361–366
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Efficient conversion of cellulose into biofuel precursor 5-hydroxymethylfurfural in dimethyl sulfoxide–ionic liquid mixtures Shaohua Xiao, Bing Liu ⇑, Yimei Wang, Zhongfeng Fang, Zehui Zhang ⇑ College of Chemistry and Material Science, Key Laboratory of Catalysis and Material Sciences of the State Ethnic Affairs Commission & Ministry of Education, South-Central University for Nationalities, Wuhan 430074, China
h i g h l i g h t s Conversion of cellulose into HMF was developed in DMSO catalyzed by AlCl3. HMF yield of 20.5% was obtained from cellulose in DMSO. Ionic liquid [BMIM]Cl largely promoted the conversion of cellulose into HMF in DMSO. High HMF yield of 54.9% was obtained in a mixed solvent DMSO-[BMIM]Cl (10 wt.%).
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Article history: Received 15 August 2013 Received in revised form 25 October 2013 Accepted 28 October 2013 Available online 6 November 2013 Keywords: Cellulose 5-Hydroxymethylfurfural Dehydration AlCl3 Ionic liquids
a b s t r a c t In recent years, cellulose has received increasing attention as a potential material for the production of biofuels and bio-based chemicals. In this study, a new process for the efficient conversion of cellulose into 5-hydroxymethylfurfural (HMF) was developed by the use of AlCl3 as the catalyst in DMSO–ionic liquid ([BMIM]Cl) mixtures. Various reaction parameters such as reaction time, reaction temperature, solvent and catalyst dosage were investigated in detail. A high HMF yield of 54.9% was obtained from cellulose at 150 °C after 9 h in a mixed solvent of DMSO–[BMIM]Cl (10 wt.%). More importantly, the catalytic system could be reused for several times despite of the slight loss of its catalytic activity. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction The rapid development of human society requires much more energy and chemicals, and it is estimated to grow by more than 50% by 2025 (Ragauskas et al., 2006). However, the fossil fuels becomes shrinking gradually, which is the world’s main source of energy and chemicals at present. This issue has prompted a direction to replace fossil fuels resources with renewable and sustainable resources (Espinosa et al., 2013; Greenhalf et al., 2013). Biomass is the only widely available carbon source apart from fossil resources. Thus, the abundant biomass is the unique renewable resources that can be converted into liquid fuels, and value-added chemicals through effective biorefinery routs (Grootscholten et al., 2013; Liu and Zhang, 2013). The majority (60–90 wt.%) of plant biomass is the biopolymer carbohydrates stored in the form of cellulose and hemicellulose. However, the structure of cellulose is ⇑ Corresponding authors. Tel./fax: +86 27 67842572. E-mail addresses:
[email protected] (B. Liu),
[email protected] (Z. Zhang). 0960-8524/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.biortech.2013.10.095
highly crystalline, due to the presence of the extensive intra- and inter-molecular hydrogen bonds and Van der Waals interactions Nishiyama et al., 2003. The inert structure of cellulose makes it difficult to be accessed by other reagents. Therefore, it is still challenging to effectively convert cellulose into liquid fuels and valuable fine chemicals, due to its poor solubility in many conventional solvents such as organic solvents and water (Hidayat et al., 2012; Hundt et al., 2013). Nowadays, one of the most attractive and promising approaches is to convert C6-based carbohydrates into 5-hydroxymethylfurfural (HMF) which have been recognized as important versatile platform compounds for the synthesis of a broad range of new products as well as for the replacement of fossil resources-derived fuels and chemicals (Scheme 1) (van Putten et al., 2013). HMF, which is formed from the dehydration of hexose, was first reported at the end of the 19th century. As shown in Scheme 1, HMF is simultaneously an aromatic aldehyde, an aromatic alcohol and a furan ring system. Thus, HMF has been called a ‘‘sleeping giant’’. As shown in Scheme 1, HMF can be used for the production of various chemicals and liquid fuels such as
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suitable candidates for the economically large-scale synthesis of HMF. Therefore, it is strongly desired to completely or partially replace expensive ionic liquids with water or highly polar organic solvents for the economical and sustainable conversion of cellulose into HMF. Recently, AlCl3 with low toxicity was found to show high catalytic activity in the conversion of carbohydrates into HMF (PaganTorres et al., 2012; Yang et al., 2012). However, there was no report on the synthesis of HMF from cellulose using AlCl3 as the catalyst in the cheap solvent systems. Herein, in the present work, we firstly report our results on the synthesis of HMF from cellulose catalyzed by AlCl3 in a mixed solvent of DMSO and [BMIM]Cl (Scheme 2). 2. Experimental section 2.1. Methods Scheme 1. Catalytic conversion of HMF into various chemicals and liquid fuels.
2,5-dihydroxymethylfuran (DHMF), 2,5-dimethylfuran (DMF), 2,5-dimethyltetrahydrofuran (DMTHF), 5-ethoxymethylfurfural, 2,5-diformylfuran (DFF), 2,5-furandicarboxylic acid (FDCA), levulinic acid (LA) and linear alkanes (Hu et al., 2012). In the past decades, the dehydration of fructose and inulin has been reported using acid catalysts in the presence of water, organic solvents, ionic liquids and two-phase systems. Moderate to high HMF yields were obtained from fructose based carbohydrates (Zhu et al., 2011). However, it should be pointed out that fructose is not abundant and very expensive, which limits the large-scale and sustainable production of HMF from fructose. On the contrary, glucose has been considered as the preferred feedstock for the production of HMF, as it is the most abundant and the cheapest monosaccharide. However, synthesis of HMF from glucose due to the stable pyranoside ring structure of glucose. Therefore, there is a strong incentive to develop an efficient process for the production of HMF using glucose as starting material. A major breakthrough was developed by Zhao et al. (2007). They reported the conversion of glucose in ionic liquid [EMIM]Cl using CrCl2 as catalyst, which gave a high HMF yield of 70%. Later, other metal salts such as GeCl4, SnCl4, and ZrCl4 in ionic liquids also showed catalytic activity for the conversion of glucose into HMF (Hu et al., 2009; Stahlberg et al., 2010; Zhang et al., 2011). Cellulose, which is made up of glucose unit via b-1,4-glycosidic bonds, is the most abundant carbohydrate in nature. Synthesis of HMF directly from cellulose is desirable for large-scale production of HMF because it can reduce the step of the production of glucose and the corresponding costs. However, the tight H-bond network and van der Waals interactions in cellulose makes it notoriously resistant to hydrolysis. In 2002, Seri et al. (2002) reported that catalytic conversion of cellulose by LaCl3 in water at 250 °C could produce HMF with a low yield of 19%. Until recently, a significant attention has been paid to the transformation of cellulose into HMF in ionic liquids due to the excellent dissolvability of cellulose in ionic liquids (Swatloski et al., 2002). For instance, Su et al. (2009) reported the formation of HMF with 55.4% yield from cellulose using a combination of CrCl2 and CuCl2 in 1-ethyl-3-methylimidazolium chloride [EMIM]Cl. Later, other methods have also been developed for the conversion of cellulose into HMF catalyzed by CrCl3 in ionic liquid (Tan et al., 2011; Zhang et al., 2010). However, in view of the toxicity and polluting characteristics of CrCl2 or CrCl3, some other Lewis acids with low toxicity such as ZrCl4 (Liu et al., 2013) and InCl3 (Li et al., 2013) have also been applied to catalyze the conversion of cellulose into HMF in ionic liquids. However, the high cost of ionic liquids makes them probably not
1-Butyl-3-methylimidazole chloride ([BMIM]Cl) was prepared according to our previous work (Zhang et al., 2009). Glucose was purchased from ABCR GmbH & Co. (Karlsruhe, Germany). Avicel PH-101 cellulose was purchased from Sigma (St Louis, USA). Acetonitrile (99.99%) was purchased from Tedia Co. (Fairfield, USA). HMF was purchased from Beijing Chemicals Co., Ltd. (Beijing, China). Dimethyl sulfoxide (DMSO) and AlCl3 (99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other chemicals were supplied by local suppliers and used without further purification. 2.2. Typical procedure for the catalytic conversion of cellulose into HMF In a typical run, cellulose (100 mg) and AlCl3 (8.2 mg) were added into a mixed solvent of DMSO (4.5 g) and [BMIM]Cl (0.5 g) in a 10 mL round-bottom flask equipped with a condenser. The reactor was then immersed into the preheated oil bath, and the reaction mixture was stirred at the speed of 600 rpm for a given reaction time. Time zero was recorded when the reactor was immersed into the preheated oil bath. After reaction, the reaction mixture was diluted with deionized water, and the solution was centrifuged at 10,000 rpm for 8 min. The clear liquid was further employed for product analysis. 2.3. Determination of the products Quantitative analysis of HMF was performed by high-performance liquid chromatography (HPLC) method, using a reversedphase C18 column (200 4.6 mm) and an ultraviolet detector at 280 nm. Acetic acid aqueous solution (1 wt.%) and acetonitrile with the volume ratio of 15:85 were used as the mobile phase at a rate of 1.0 mL/min, and the column oven temperature was maintained at 30 °C. The concentration of HMF was in samples was canculated based on the standard curve obtained with the standard substances. 3. Results and discussion 3.1. Catalytic conversion of glucose into HMF in DMSO by various Lewis acids As glucose was the important intermediate for the one-pot conversion of cellulose, the catalytic conversion of glucose into HMF was initially carried out in DMSO in the presence of some common Lewis acids at 130 °C for 4 h. The blank experiment was carried out in DMSO without catalyst, and a negligible HMF yield was
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Scheme 2. Schematic illustration for the synthesis of HMF from cellulose.
observed (Table 1, entry 1). As shown in Table 1, Lewis acid catalysts showed a remarkable effect on the dehydration of glucose into HMF. AlCl3 was found to show the best catalytic performance on the dehydration of glucose with HMF yield of 53.2% (Table 1, entry 2). Our results were consistent with the previous results in which they also found that AlCl3 showed high catalytic activity in the dehydration of glucose into HMF (Pagan-Torres et al., 2012; Yang et al., 2012). The main reason was that AlCl3 could effectively promote the isomerization of glucose into fructose, followed by the dehydration of fructose into HMF (Pagan-Torres et al., 2012). Although the dehydration of glucose catalyzed by CrCl3 produced high HMF yield in ionic liquids (Zhao et al., 2007), a moderate HMF yield of 23.9% was obtained from glucose in the presence of CrCl3 in DMSO (Table 1, entry 3). HMF yields were obtained less than 5% when the dehydration of glucose was catalyzed by other Lewis acids such as FeCl3, CuCl2, MnCl2 and ZnCl2 (Table 1, entries 4–7). These results indicated that those Lewis acids could not effectively catalyze the isomerization of glucose into fructose. The degradation products such as levulinic acid and retro-aldol product of glyceraldehydes were always found as the byproducts during the dehydration of glucose (Pilath et al., 2010). In addition, insoluble humins were observed, these species being produced by the cross-polymerization of glucose (Kuster, 1990). Based on the above results, the catalytic activity on the dehydration of glucose decreased in an order: AlCl3 > CrCl3 > FeCl3 > ZnCl2 > MnCl2 > CuCl2. It was noted that trivalent metal chlorides showed higher catalytic activity than divalent metal chlorides on the dehydration of glucose into HMF. Liu et al. (2009) also found that trivalent metal chlorides showed higher catalytic acidity than divalent metal chlorides on the hydrolysis of hemicelluloses. 3.2. Catalytic conversion of cellulose into HMF under various reaction conditions As AlCl3 showed the highest catalytic activity on the dehydration of glucose into HMF, catalytic conversion of cellulose into HMF was carried out in DMSO using AlCl3 as the catalyst in the following experiments. Firstly, the conversion of cellulose was carried out in DMSO in the presence of 10 mol.% AlCl3 at 150 °C. To our delight, HMF yield of 20.5% was obtained from cellulose after 9 h at 150 °C in a one-pot reaction, albeit it was lower than that from
Table 1 The results of glucose dehydration into HMF catalyzed by various Lewis acids.a Entry
Catalyst
Catalyst amount (mg)
HMF yield (%)
1 2 3 4 5 6 7
– AlCl3 CrCl3 CuCl2 ZnCl2 FeCl3 MnCl2
– 8.2 9.8 8.2 8.3 10.0 7.7
0.1 53.2 23.9 0.3 3.6 4.7 1.3
a Reaction conditions: Glucose (180 mg, 1 mmol) and catalyst (10 mol.%) were added into DMSO (3 mL), then the reaction was carried out at 130 °C for 4 h.
glucose (Table 2, entry 1). To the best of our knowledge, this is the first report on the synthesis of HMF from cellulose in DMSO. As it known to us, cellulose is insoluble in DMSO, thus the transformation of cellulose into HMF most probably occurred on the surface of cellulose. Tan et al. (2011) reported that the conversion of cellulose into HMF catalyzed by CrCl2 could be improved by the addition of zeolite, in which zeolite promoted the hydrolysis of cellulose. Inspired by the results reported by Tan et al. (2011), the combination of AlCl3 and H2SO4 was used for the conversion of cellulose into HMF in DMSO, and HMF yield was elevated from 20.5% to 29.6% by the use of H2SO4 as the co-catalyst (Table 2, entries 1 vs 2). These results indicated that H2SO4 played a role in the hydrolysis of cellulose into glucose in DMSO, followed by the dehydration of glucose into HMF catalyzed by AlCl3. It was reported that the reaction medium also played a key role in the chemical reactions (Zhang et al., 2012). As the common ionic liquid [BMIM]Cl is considered to be an excellent solvent in the process of cellulose, the conversion of cellulose was catalyzed by AlCl3 was further carried out in a mixed solvent of DMSO and [BMIM]Cl. Although only 10 wt.% of [BMIM]Cl was used, HMF yield increased largely from 20.5% to 54.9% (Table 2, entries 1 vs 3). Binder et al. (2009) also found that HMF yield from cellulose catalyzed by CrCl3 in N, N-dimethylacetamide (DMA) and lithium chloride (LiCl) systems could also be improved by the addition of [BMIM]Cl. The following two reasons would be helpful to understand the positive effect of [BMIM]Cl on the transformation of cellulose into HMF. On the one hand, [BMIM]Cl had high hydrogen bond acceptor ability to break the extensive intra- and inter-molecular hydrogen bonds in cellulose, thus the catalyst accessed the cellulose chain much more easily (Swatloski et al., 2002). On the second hand, it was reported that imidazolium-based ionic liquids may take a role of activating the reactants and stabilized the intermediates of each step through hydrogen-bonding interactions (Lai and Zhang, 2010). Interestingly, it was found that HMF yield decreased from 54.9% to 41.7% when the conversion of cellulose in a mixed solvent of DMSO and [BMIM]Cl were catalyzed by the combination of AlCl3 and H2SO4 (Table 2, entries 3 vs 4). These results indicated that AlCl3 could both effectively catalyze the hydrolysis of cellulose into glucose and the dehydration of glucose into HMF in DMSO–[BMIM]Cl mixtures, and the use of H2SO4 as the co-catalyst inversely promoted the degradation of glucose and HMF into other byproducts (Table 2, entries 3 vs 4) (Patil and Lund, 2011). The phenomenon was much more obvious when much more H2SO4 was used (Table 2, entries 4 vs 5).
3.3. Effect of the concentration of [BMIM]Cl on the conversion of cellulose into HMF Based on the above results, [BMIM]Cl played an important role in the one-pot conversion of cellulose into HMF in DMSO. Therefore, the effect of the amount of [BMIM]Cl on the transformation of cellulose into HMF was studied. As shown in Fig. 1, HMF yield increased with the increase of the amount of [BMIM]Cl in the range from 2.5 wt.% to 10 wt.%. HMF yield of 29.5% was obtained after 9 h
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Table 2 Catalytic conversion of cellulose into HMF by AlCl3 under various conditions.a Entry
Solvent
Catalyst
HMF yield (%)
1 2
DMSO (5.0 g) DMSO (5.0 g)
20.5 31.2
3
DMSO (4.5 g) & [BMIM]Cl (0.5 g) DMSO (5.0 g) & [BMIM]Cl (0.5 g) DMSO (5.0 g) & [BMIM]Cl (0.5 g)
AlCl3 (8.2 mg) AlCl3 (8.2 mg) & H2SO4 (23 mg) AlCl3 (8.2 mg) AlCl3 (8.2 mg) & H2SO4 (23 mg) AlCl3 (8.2 mg) & H2SO4 (46 mg)
41.7
4 5 a
54.9
29.8
Reaction conditions: Cellulose (100 mg, 1 mmol), AlCl3 (10 mol.%), 150 °C for
9 h.
Fig. 2. Effect of the reaction temperature on the conversion of cellulose into HMF. Reaction conditions: cellulose (100 mg) and AlCl3 (10 mol.%) were added into a mixed solvent DMSO (4.5 g) and [BMIM]Cl (0.5 g), then the reaction was carried out at different reaction temperatures.
Fig. 1. Conversion of cellulose into HMF with various concentration of [BMIM]Cl. Reaction conditions: Cellulose (100 mg, 1 mmol) and AlCl3 (10 wt.%) were added into a mixed solvent of DMSO and [BMIM]Cl, then the reaction was carried out at 150 °C for 9 h.
with 2.5 wt.% of [BMIM]Cl, and HMF yields increased to 43.2% and 54.9% for 5.0 wt.% of [BMIM]Cl and 10.0 wt.% of [BMIM]Cl, respectively. However, HMF yields did not increased when the amount of [BMIM]Cl was beyond 10 wt.%. These results indicated that [BMIM]Cl indeed showed a positive effect on the conversion of cellulose into HMF, and the possible reasons were stated as above. Due to the high cost of [BMIM]Cl, 10 wt.% of [BMIM]Cl was an appropriate dosage in DMSO from an economic standpoint. Interestingly, it was found that HMF yield in [BMIM]Cl was a little lower than that in the mixed solvents with 10 wt.% of [BMIM]Cl. The reason might be that the [BMIM]Cl ionic liquid has very high viscosity, which was not facile for the mass transfer between the catalyst and cellulose (Govinda et al., 2011). 3.4. Effect of the reaction temperature on the conversion of cellulose into HMF To show the relationship between reaction temperature and the yield of HMF, the effect of reaction temperature on HMF yield is presented in Fig. 2. Experiments were carried out at 110, 130, 150, and 180 °C, respectively. As shown in Fig. 2, the reaction temperature showed a remarkable effect on the conversion of cellulose into HMF. Higher reaction temperature clearly produced higher HMF yield at an early reaction stage. The conversion of cellulose at 110 °C produced no HMF after 12 h (data not given in Fig. 2). After reaction, cellulose samples precipitated out when the
reaction mixture was diluted with water. The results indicated that the cleavage of cellulose chain did not occur at 110 °C. Thus the addition of water competed with hydroxyl groups of cellulose to form hydrogen bonds, resulting in the precipitation of cellulose (Swatloski et al., 2002). The conversion of cellulose could proceed slowly at 130 °C with a low HMF yield of 7.4% after 12 h. However, the dehydration of glucose at 130 °C could produce HMF yield of 54.9%. Thus the main reason for the low HMF yield from cellulose at 130 °C was that the hydrolysis of cellulose into glucose at 130 °C was not effective. In order to improve the HMF yield from cellulose, the reaction temperature was further elevated to 150 °C. As seeing in Fig. 2, the reaction proceeded smoothly at 150 °C. HMF yield increased gradually before 9 h and then slowly decreased. The maximum HMF yield of 54.9% was obtained after 9 h, and then it slightly decreased to 51.6% after 12 h. Further increasing the reaction temperature to 180 °C, it only required 3 h to obtain the maximum HMF yield of 52.3%. However, HMF yield decreased sharply from 3 h to 12 h, indicating that HMF was not stable at high reaction temperature. Our results were consistent with the previously reported results on the effect of reaction temperature on synthesis of HMF (Ding et al., 2012). These results showed that 150 °C was an appropriate reaction temperature for the conversion of cellulose into HMF in our catalytic system.
3.5. Effect of the catalyst dosage on the conversion of cellulose into HMF Fig. 3 shows how varying the amount of AlCl3 affects the conversion of cellulose into HMF. As shown in Fig. 3, the catalyst dosage also showed a significant effect on the HMF yield. HMF yield increased with the increase of the amount of AlCl3 from 2.5 mol.% to 10 mol.% (based on glucose unit). A HMF yield of 32.1% was obtained after 9 h at 150 °C with 2.5 mol.% of AlCl3, and the HMF yield increased to 45.4% and 54.9% after 9 h when using 5 mol.% and 10 mol.% of AlCl3, respectively. The increase of the HMF yield with the increase of the catalyst dosage at the same reaction time point can be attributed to an increase in the availability and number of catalytically active sites. However, HMF yield decreased with the increase of the amount of AlCl3 from 10 mol.% to 20 mol.%. The possible reason might be that high catalyst loading not only accelerated the conversion of cellulose into HMF, but also promoted other side reactions such as rehydration
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completely remove water and residual ethyl acetate. The remaining [BMIM][Cl] with AlCl3 was used directly in the next run by adding fresh cellulose sample and DMSO under the same reaction conditions. There processes were repeated for 5 times, and the results are shown in Fig. 4. It was noted that there was a slight decrease in the yield of HMF. HMF yield was 54.9% for the first run, while it slightly decreased to 46.4% for the fifth run. The loss of the catalytic activity was possible due to the mass loss during the recycling procedure. 4. Conclusion
Fig. 3. The effect of the catalyst dosage on the conversion of cellulose into HMF. Reaction conditions: Cellulose (100 mg, 1 mmol) and a set amount of AlCl3 were added into a mixed solvent of DMSO (4.5 g) and [BMIM]Cl (0.5 mg), then the reaction was carried out at 150 °C for 9 h.
We demonstrated a one-pot process for the conversion of cellulose into valuable HMF in DMSO catalyzed by AlCl3. It was found that a low amount of [BMIM]Cl in DMSO was crucial to obtain a high HMF yield. Under mild conditions, AlCl3 could effectively promote cellulose to produce HMF with a yield of 54.9%. The one-pot method for the synthesis of HMF from cellulose shows promising potential in large-scale production of HMF because it can reduce the step of the production of glucose and the corresponding costs. In addition, the catalytic system was effectively recycled five times. The method described herein should be valuable and economical to facilitate cost-effective conversion of lignocellulosic biomass into biofuels and bio-based products. Acknowledgements The Project was supported by National Natural Science Foundation of China (Nos. 21203252 & 21206200). References
Fig. 4. Recycling of [BMIM]Cl and AlCl3 for the synthesis of HMF from cellulose. Reaction conditions: Cellulose (100 mg, 1 mmol), [BMIM]Cl (0.5 mg) and AlCl3 (10 mol.%) were added into DMSO (4.5 g), then the reaction was carried out at 150 °C for 9 h.
of HMF into levulinic acid and cross-polymerization reactions (Kuster, 1990), resulting in a low HMF yield. 3.6. Recycling experiments Catalyst recycling is an important goal in terms of green and sustainable chemistry. Thus, the recycling experiments of [BMIM][Cl] and AlCl3 were carried out. The conversion of cellulose into HMF was used as the model reaction. Experiments were carried out were carried out at 150 °C for 9 h in a mixed solvent of [BMIM][Cl] and DMSO with 10 mol.% of AlCl3 as the catalyst. After reaction, most of DMSO and HMF were removed from the reaction mixture by distillation under reduced pressure. Then, 1 mL of deionized water was added into the remaining mixture, and the mixture was stirred vigorously. Then the resulting mixture was centrifuged at 10,000 rpm for 5 min to remove the insoluble substances such as humins. Then the remaining HMF in the aqueous solution was extracted with ethyl acetate until no HMF was detected in ethyl acetate. The resulting aqueous phase which contained AlCl3 and [BMIM][Cl] was evaporated in vacuum to
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