A simple strategy is reported for catalytic conversion of glucose to 5-hydroxymethylfurfural (HMF) over AlI3 in N,N-dimethylacetamide (DMAC). When the reaction was conducted in DMAC at 120°C for 15 min over AlI3 catalyst, HMF was obtained with a yield of 52%. The reaction course was monitored by 13C NMR spectroscopy and HPLC analysis. The results suggest that AlI3 catalyzes the three consecutive reactions consisting of mutarotation of α-glucopyranose to β-glucopyranose, isomerization of glucose to fructose, and dehydration of fructose to HMF.
Conversion of Glucose to 5-Hydroxymethylfurfural Catalyzed by Metal Halide in N,N-Dimethylacetamide
Qiuhe Ren,a,b Yizheng Huang,a Hong Ma,a Feng Wang,a Jin Gao,a and Jie Xu a,*
A simple strategy is reported for catalytic conversion of glucose to 5-hydroxymethylfurfural (HMF) over AlI3 in N,N-dimethylacetamide (DMAC). When the reaction was conducted in DMAC at 120oC for 15 min over AlI3 catalyst, HMF was obtained with a yield of 52%. The reaction course was monitored by 13C NMR spectroscopy and HPLC analysis. The results suggest that AlI3 catalyzes the three consecutive reactions consisting of mutarotation of α-glucopyranose to β-glucopyranose, isomerization of glucose to fructose, and dehydration of fructose to HMF.
Keywords: Glucose; Dehydration; Metal halide; 5-Hydroxymethylfurfural
Contact information: a: State Key Laboratory of Catalysis; Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, CAS, Dalian 116023, PR China; b: University of Chinese Academy of Sciences, Beijing 100039, PR China; *Corresponding author: firstname.lastname@example.org
In view of the costs and challenges inherent in safely and continuously providing fossil energy resources, increasing attention has been paid to the conversion of biomass to biofuels and chemicals (Gallezot 2012; Zhou et al. 2011; Bozell 2010; Alonso et al. 2010). 5-Hydroxymethylfurfural (HMF), an important biomass-derived platform chemical, can be converted to biofuels and a broad range of chemicals (Rosatella et al. 2011; Román-Leshkov et al. 2007; Zhao et al. 2007; Huber et al. 2005), which are currently produced from petroleum. Recently, our group has developed methods for converting HMF to maleic anhydride (Du et al. 2011), which is now mainly produced via selective oxidation of benzene, o-xylene, or n-butane in industry. In addition, we have developed methods for converting HMF to furan-based polyester and copolyester materials, fluorescent material, porous organic frameworks, and liquid fuel products, respectively (Ma et al. 2011; Ma et al. 2012a,b,c; Che et al. 2012). However, a simple and reliable way of acquiring HMF remains as a bottleneck for extending biomass chains.
HMF could be formed through acid-catalyzed dehydration of fructose (Román-Leshkov et al. 2006; Tong et al. 2010; James et al. 2010). However, glucose, the isomer of fructose, is a better feedstock for HMF production because it is cheaper. It is the most abundant monosaccharide in nature, and it can be obtained from starch and cellulose. However, in comparison with fructose, it is more difficult to convert glucose to HMF. Zhang and co-workers reported the conversion of glucose to HMF using a CrCl2/ionic liquid system (Zhao et al. 2007). Some other Cr-containing catalytic systems have also been developed (Yong et al. 2008; Binder and Raines 2009; Yu et al. 2009; Li et al. 2009; Qi et al. 2010; Zhao et al. 2011; Zhang and Zhao 2011; Yuan et al. 2011; Hu et al. 2012). More efforts have been taken to develop low-toxic catalytic systems. In ionic liquids, Yb(OTf)3, H3BO3, H-ZSM-5, GeCl4, ZrO2, and SnCl4 have been used to catalyze this process. HMF was obtained with yields of 24 to 61% (Stahlberg et al. 2010; Stahlberg et al. 2011; Jadhav et al. 2012; Zhang et al. 2011; Qi et al. 2012; Hu et al.2009). Under microwave irradiation, the reactions over TiO2 and AlCl3 catalysts gave 37 to 61% yields of HMF (Dutta et al. 2011; De et al. 2011; Yang et al. 2012). In addition, ScCl3, ZnCl2 combined with HCl, SO42-/ZrO2-Al2O3, hydrotalcite combined with Amberlyst-15, Sn-Mont, Sn-Beta zeolite combined with HCl, and AlCl3 combined with HCl have also been used to catalyze this process; HMF was obtained with yields of 30 to 62% (Beckerle and Okuda 2012; Deng et al. 2012; Yan et al. 2009; Ohara et al. 2010; Wang et al. 2012; Nikolla et al.2011; Pagán-Torres et al. 2012). Unlike previous studies, we herein demonstrate a simple method for the catalytic conversion of glucose to HMF over metal halide. It was found that aluminum halides in N,N-dimethylacetamide (DMAC) were efficient for this important reaction, and the catalytic performance of aluminum halides decreases in the order of AlI3 > AlBr3> AlCl3. A tentative reaction route, including mutarotation, isomerization, and dehydration, is proposed based on NMR and HPLC analysis.
All reagents were of analytical grade and were used as purchased without further purification unless otherwise stated. AlI3 and methyl benzoate were purchased from Alfa Aesar. HMF, GaCl3, and InCl3 were purchased from Sigma-Aldrich. D-glucose and D-fructose were purchased from Tianjin Kermel. AlCl3, DMAC, and other reagents were purchased from Shanghai Chemical Reagent Company. DMAC was distilled under reduced pressure before being used.
Typical Procedure for Glucose Conversion
All the reaction experiments were conducted in a 50 mL two-necked flask equipped with a condenser and a magnetic stirrer. Typically, 0.5 mmol glucose and 0.1 mmol AlI3 were mixed in 2 mL DMAC under N2. The mixture was stirred in a preheated oil bath at the desired temperature for a certain period of time. After reaction, the mixture was immediately cooled in an ice bath to terminate the reaction, followed by filtering off solid particles. The filtrate was added to a certain amount of methyl benzoate as internal standard and was diluted with ethyl acetate to 10 mL. The sample was filtrated with a 0.2 μm micropore membrane before its analysis by GC and HPLC.
HMF Quantification Procedure
HMF was analyzed with an Agilent 4890D GC device equipped with a flame ionization detector and FFAP capillary column (30 m × 0.32 mm × 0.4 μm). HMF was confirmed by 1H NMR measured with a Bruker DRX-400 spectrometer, as well as an Agilent 6890N GC device equipped with an Agilent 5973 mass selective detector and HP-5 capillary column (30 m × 0.25 mm × 0.3 μm). HMF yield was determined by the internal standard curve method with methyl benzoate as internal standard.
Glucose and Fructose Quantification Procedure
Glucose and fructose are analyzed with a Waters 2695 HPLC equipped with 2414 refractive index detector at 30oC and high performance carbohydrate column (4.6 mm × 250 mm) at 30oC controlled with the column oven. Acetonitrile/water solution (75:25) with a flow rate of 1.4 mL/min was used as the mobile phase. The injection amount was 10 μL. Both glucose conversion and fructose yield were calculated by using an external standard curve method.
RESULTS AND DISCUSSION
In the initial experiment, AlCl3 (10 mol% based on glucose) was used as the catalyst to convert glucose in DMAC at 100oC for 15 min. Very low yield (6%) of HMF was obtained. The yield of HMF was increased to 36% when the reaction temperature was 130oC. Prolonging the reaction to 240 min had little effect on the yield of HMF (Table 1, entries 1 to 3). When GaCl3 and InCl3 were used, lower yields of HMF were obtained (Table 1, entries 4 and 5). Several other metal chlorides such as FeCl3, LaCl3, CuCl2, and NiCl2 were tested. Much lower yields of HMF were obtained (Table 1, entries 6-9). However, when AlBr3 and AlI3 were used, high yields of HMF were achieved (Table 1, entries 10 and 11). In particular, when 20 mol% AlI3 was used, HMF was obtained with a yield of 50% (Table 1, entry 12). Therefore, AlI3 was selected as the catalyst for the following study. Furthermore, a blank experiment was conducted without adding any metal chloride and only little HMF was obtained (Table 1, entry 13).
Table 1. Conversion of Glucose to HMF Catalyzed by Metal Halides a
The effect of reaction temperature on the conversion of glucose to HMF was optimized. As shown in Fig. 1, from 90oC to 120oC, higher reaction temperatures gave higher conversion and HMF yields. When the reaction was performed at 120oC, glucose conversion reached over 99%, and HMF was obtained with a yield of 52%. An increase in the temperature to 130-150oC gave slightly decreased yields of HMF. Moreover, the increase of temperature generated more dark-brown insoluble solid polymers, commonly known as humin (Binder and Raines 2009), originating from the side-reaction of the decomposition of glucose. Thus, to avoid the formation of humin, the optimized temperature was set at 120oC.
Fig. 1. Effect of temperature on the conversion of glucose to HMF catalyzed by AlI3. Reaction conditions: 0.5 mmol glucose, 0.1 mmol AlI3, 2 mL DMAC, 15 min
Fig. 2. Effect of solvent on the conversion of glucose to HMF catalyzed by AlI3; Reaction conditions: 0.5 mmol glucose, 0.1 mmol AlI3, 2 mL Solvent, 120oC, 15 min
Conversions of glucose to HMF over AlI3 in different solvents were conducted. The polar aprotic solvents that have good solubility for glucose, such as DMAC, N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), and dimethyl sulfoxide (DMSO), were investigated. As shown in Fig. 2, solvents remarkably influenced the catalytic efficiency. The effect of solvents on glucose conversion and HMF yield were in the order DMAC > DMF > NMP > DMSO. Neither levulinic acid nor formic acid (the rehydration products of HMF) was detected in the reaction mixtures, which indicated that the rehydration side reaction of HMF was prevented.
The effect of reaction time on the conversion of glucose to HMF at 120oC was investigated, and the results are shown in Fig. 3. Glucose conversion increased in the course of the reaction and reached over 99% in 30 min. The maximal HMF yield of 52% was obtained in 15 min. Fructose was detected. Initially, fructose yield reached 8% in 1 min, and then decreased slowly until no fructose could be detected in 30 min. The variation of glucose conversion and HMF yield with reaction time was also investigated at 110oC and 130oC, and the results were similar to those obtained at 120oC.
Fig. 3. Variation of glucose conversion, HMF yield, and fructose yield with reaction temperature and time; Reaction conditions: 0.5 mmol glucose, 0.1 mmol AlI3, 2 mL DMAC
In the reaction of converting glucose to HMF, water was produced. AlI3 was probably hydrolyzed to aluminum hydroxide (Al(OH)3) and hydroiodic acid (HI), which might also catalyze the conversion of glucose to HMF. In order to clarify whether Al(OH)3 or HI catalyzed this reaction, some comparison experiments were conducted. The results are listed in Table 2. Both Al(OH)3 and HI were found to be inactive, no matter whether they were used alone or together, suggesting that AlI3 itself, not its hydrolyzed products, catalyze the conversion of glucose to HMF.
Table 2. Effects of AlI3 and its Hydrolyzed Products on Converting Glucose to HMF a
To gain more insight into the process for glucose conversion to HMF catalyzed by AlI3, we collected a series of 13C NMR spectra of glucose in DMAC under different conditions (Fig. 4). It can be seen from Fig. 4a that in DMAC at 120oC for 1 min, glucose was in the α-glucopyranose form. The six peaks appearing in Fig. 4a belong to the six carbons of α-glucopyranose.When AlI3 was added, a mixture of α-glucopyranose and β-glucopyranose was obtained. The twelve peaks in Fig. 4d belong to six carbons of α-glucopyranose and six carbons of β-glucopyranose (Duquesnoy et al. 2008; Roslund et al. 2008). This indicated that AlI3 promoted the mutarotation of α-glucopyranose to β-glucopyranose. Similarly, it can be seen from Figs. 4b and 4c that AlBr3 and AlCl3 also promoted the mutarotation of α-glucopyranose to β-glucopyranose.
Fig. 4. 13C NMR spectra of 0.5 mmol glucose in 2 mL DMAC (with d6-DMSO as external standard) at 120oC for 1 min under different conditions: a) without AlX3; b) in the presence of 0.1 mmol AlCl3; c) in the presence of 0.1 mmol AlBr3; d) in the presence of 0.1 mmol AlI3
Scheme 1. Speculated reaction route of conversion of glucose to HMF catalyzed by AlI3
It should be noted that fructose was obtained during the reaction process. However, no fructose was obtained in the absence of AlX3 under the same conditions, which indicated that AlI3could promote the isomerization of glucose to fructose. To clarify whether AlI3 can catalyze the dehydration of fructose to HMF, an experiment with fructose as feedstock was conducted, and HMF was obtained with a yield of 54%, while only little HMF was obtained in the absence of AlI3. These results clearly indicated that AlI3 could catalyze the dehydration of fructose to HMF. On the basis of the NMR study and the experiments described above, a speculated reaction route is proposed in Scheme 1. Potentially catalytic applications in conversion of various carbohydrates and reaction mechanism investigation are currently under study.
- Aluminum halides in DMAC can provide a simple and efficient system for converting glucose to HMF. The catalytic performance of aluminum halides decreases in the order of AlI3> AlBr3 > AlCl3. When AlI3 in DMAC was used, HMF was obtained with a yield of 52% at 120oC in 15 min.
- 13C NMR spectra indicated that aluminum halides could promote the mutarotation of α-glucopyranose to β-glucopyranose.
- HPLC and comparison experiments indicated that AlI3 could promote the isomerization of glucose to fructose and catalyze the dehydration of fructose to HMF.
The authors are grateful for support from Main Direction Program of Knowledge Innovation of Chinese Academy of Science, Grant. No. KSCX2-EW-G-5; and the National Natural Science Foundation of China, Grant. No. 21233008, 1273231 and 21073184.
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Article submitted: October 12, 2012; Peer review completed: December 21, 2012; Revised version received: January 30, 2013; February 1, 2013; Published: February 4, 2013.