Article pubs.acs.org/IECR
Efficient One-Pot Synthesis of 5‑(Ethoxymethyl)furfural from Fructose Catalyzed by a Novel Solid Catalyst Liu Bing, Zehui Zhang,* and Kejian Deng* Key Laboratory of Catalysis and Materials Sciences of the State Ethnic Affairs Commission & Ministry of Education, College of Chemistry and Material Science, South-Central University for Nationalities, Wuhan, 430074, P. R. China ABSTRACT: A novel method has been developed for the direct conversion of fructose into 5-(ethoxymethyl)furfural (EMF) catalyzed by an organic−inorganic hybrid solid catalyst [MIMBS]3PW12O40. First, etherification of 5-(hydroxymethyl)furfural (HMF) by ethanol was studied over a series of catalysts, and [MIMBS]3PW12O40 showed the best catalytic activity with an EMF yield of 90.7% at 70 °C. Then the one-pot dehydrative etherification of fructose into EMF was studied. It indicated that a low temperature (70 °C) was not favored for the dehydrative etherification process, and a high temperature (110 °C) was favored to promote side reactions. A high EMF yield of 90.5% was obtained from fructose at 90 °C with 5 mol % catalyst for 24 h. The recycling test demonstrated that [MIMBS]3PW12O40 could be reused several times without losing catalytic activity, with an average EMF yield of approximate 90%. This work provides a meaningful method for the conversion of carbohydrates into fine chemicals and biofuel additives.
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In addition, EMF has been widely established as a flavor and aroma component additive in wines and beers, due to its low toxicity.13 Therefore, there is a great interest in the synthesis of EMF from renewable resources. Previous work in this area has occurred by first replacing the OH group of HMF with a Cl atom, thereby forming 5(chloromethyl)furfural (CMF).14,15 The nucleophilic substitution of CMF with ethanol forms EMF and HCl. Although a high EMF yield was obtained in this method, there were some concerns about the recycle of HCl and the waste disposal problems. In addition, the introduction of unreacted halides into the final product was problematic. Recently, there were some reports on the production of EMF through etherification of HMF with ethanol using solid acid catalysts such as mesoporous silica catalysts.16 While relatively high EMF yields were obtained, the production of EMF directly from fructose by a one-pot process would result in energy savings. In 2012, Balakrishnan et al.17 reported the direct conversion of fructose in ethanol at 110 °C for 30 h to EMF with a yield of 70% using silica sulfuric acid as catalyst. Although fructose has been the preferable feedstock for EMF production, it is clear that the large scale, sustainable use of EMF will require cellulosic biomass as feedstocks. As glucose is the monomer unit of cellulosic biomass, in this sense, isomerization of glucose into fructose is a particularly relevant reaction. High fructose corn syrups (HFCS) have now largely been produced on industrial scales using a solid-supported biocatalytic process. However, the enzyme is very expensive, and enzymatic reactions generally proceed slowly. New catalytic methods are now required, and in a recent work, Moliner et al.18 found that tin-containing zeolites were highly active catalysts for the isomerization of
INTRODUCTION The awareness of climate change and diminishing fossil fuel reserves necessitates the replacement of current hazardous and nonrenewable processes with sustainable, green, and environmentally benign practices.1 In the next two decades, petroleum production is unlikely to keep pace with the growing demand for fuels and chemicals. Thus, new synthetic routes and related technologies for generating fuels and chemicals from renewable feedstock are needed.2,3 Biomass is the most abundant renewable resource with an estimated global production of around 1.0 × 1011 tons per year, and it has received considerable attention as an alternative source for both fuels and chemicals.4,5 Recently, 5-(hydroxymethyl)furfural (HMF) has been recognized as a key molecule in biorefinery processes.6 HMF is considered to be an important intermediate which can be converted into energy products (2,5-dimethylfuran, an octane booster), monomers for high-value polymers (furan-2,5dicarboxylic acid and 2,5-bis(hydroxymethyl)furan), and valuable intermediates for fine chemistry. In the past few years, the preparation of HMF through the dehydration of biomass-based sugars has received much attention. HMF has been successfully prepared from hexoses such as glucose and fructose and even from cellulose and lignocelluloses.7,8 In recent years, processing HMF into drop-in biofuel candidates has attracted much interest. For example, 5(ethoxymethyl)furfural (EMF) and 2,5-dimethylfuran (DMF) are considered to be the two most potential biofuel candidates, which can be obtained from the etherification and hydrogen reduction of HMF, respectively.9,10 In terms of EMF, it is considered to be an excellent additive for diesel. It has a high energy density of 8.7 kWh/L, which is similar to regular gasoline (8.8 kWh/L), nearly as good as diesel (9.7 kWh/L), and significantly higher than ethanol (6.1 kWh/L).11 EMF has been used as a blend in commercial diesel in engine tests, and EMF generated positive results with a significant reduction of soot (fine particulates) and a reduction of the SOx emissions.12 © 2012 American Chemical Society
Received: Revised: Accepted: Published: 15331
July 31, 2012 August 25, 2012 September 4, 2012 September 4, 2012 dx.doi.org/10.1021/ie3020445 | Ind. Eng. Chem. Res. 2012, 51, 15331−15336
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Figure 1. Synthesis of EMF from fructose in ethanol.
glucose in water. Lew et al.10 have reported the one-pot synthesis of EMF from glucose with a yield of 31%, using SnBEA and Amberlyst 131 acidic catalyst. Sn-BEA and Amberlyst were used to catalyze isomerization of glucose into fructose and for dehydrative etherification of fructose into EMF, respectively. In recent years, organic−inorganic hybrid materials have attracted much attention due to the flexible tailorability of both organic and inorganic groups.19 Recently, the design and application of heteropolyacid-based ionic liquid (HPA-based IL) hybrid materials have been proposed as a catalyst upgrade for organic syntheses, and some thermal/solvent-responsive HPA-based IL hybrid catalysts have been reported.20 The advantages of heteropolyacid-based solid catalysts are not only low corrosiveness to equipment but also low energy consumption during the separation process. In this paper, as an expansion of our previous work in the production of HMF from carbohydrates,21−23 the direct conversion of fructose into EMF was studied (Figure 1). First, production of EMF from the etherification of HMF was carried out using a stable and easily prepared HPA-based IL hybrid catalyst, methylimidazolebutylsulfate phosphotungstate ([MIMBS]3PW12O40). Then the catalytic activity of [MIMBS]3PW12O40 was further investigated in the direct one-pot conversion of fructose in ethanol into EMF.
Figure 2. Catalyst preparation.
three times with ethyl acetate, and then dried in vacuum. Then an aqueous solution of H3PW12O40 was added, and the mixture was stirred at room temperature for 24 h. Water was removed in vacuum, and the final product was obtained as a white solid, which was named as [MIMBS]3PW12O40. Typical Procedure for EMF Production. Etherification of HMF in Alcohols. First, HMF (0.126 g, 1 mmol), ethanol (5 mL), and catalyst (5 mol %) were added to a 25 mL roundbottom flask with a condenser. Then the reaction mixture was carried out at a specified temperature for the desired time under atmospheric pressure with a magnetic stirrer. Samples were withdrawn, diluted with water, centrifuged at 10 000 rpm for 5 min, and analyzed by an HPLC system.10 To obtain the detailed characterization of EMF, the reaction mixture was filtered to remove the solid catalyst after reaction, and then the solvent was removed by rotary evaporation under reduced pressure to give a crude product, which was purified on silica gel, eluting with EtOAc and petroleum ether (1:6, v/v). 1 H NMR (400 MHz, CDCl3) δ: 9.60 (s, 1H, CHO), 7.20−7.21 (d, 1H, J = 4.0 Hz, furan ring), 6.51−6.52 (d, 1H, J = 4.0 Hz, furan ring), 4.52 (s, 2H, OCH2-furan ring), 3.56−3.60 (q, 2H, J = 8.0 Hz, CH2O), 1.21−1.24 (t, 3H, J = 6.0 Hz, CH3). 13C NMR (100 MHz, CDCl3) δ: 178.5, 158.9, 150.2, 122. 7, 109.9, 70.6, 65.8, 15.7. HRMS (EI): m/z [M]+ calcd for C8H10O3: 154.16; found: 154.06. One-Pot Dehydrative Etherification of D-Fructose into EMF. First, D-fructose (0.180 g, 1 mmol) was suspended in ethanol (5 mL) and stirred at a set temperature until a clear solution was formed. Then catalyst (5 mol %) was added to the reaction sytem. Otherwise, steps were worked up in a manner similar to that discussed above. Determination of HMF and EMF. HPLC analysis of HMF and EMF was performed on a Varian ProStar 210 HPLC system coupled with a UV detector. The samples were separated using a reversed-phase C18 column (200 × 4.6 mm). The column temperature was maintained at 30 °C. The optimized mobile phase consisted of acetonitrile and 0.1 wt % acetic acid aqueous solution with a volume ratio at 15:85. The flow rate was set at 1.0 mL/min. The detection wavelength was 280 nm. The content of HMF and EMF in samples was obtained directly by interpolation from calibration curves, with a coefficient of 0.999. Recycling of Catalyst. At the end of the reaction, [MIMPS]3PW12O40 was recovered by filtration. The residues were washed three times with 10 mL of ethanol and three times
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MATERIALS AND METHODS Materials. Keggin-type phosphotungstic acid was purchased from Tianjin Damao Chemical Reagent Factory (Tianjin, China). N-Methylimidazole (99%) and ethanol (99.5%) were obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai China) and were freshly distilled before use. 1,4Butane sultone (99%) was supplied by Wuhan Fengfan Chemical Co., Ltd. (Wuhan, China). 5-(Hydroxymethyl)furfural (98%) was supplied by Beijing Chemicals Co. Ltd. (Beijing, China). Fructose was from Sanland-Chem International Inc. (Xiamen, China). HY (Si/Al = 5) zeolite was supplied from Nankai University Catalyst Co., Ltd. (Tianjin, China). Sulfonated ion-exchange resin NKC-9 was purchased from the Chemical Plant of NanKai University (Tianjin, China). Acetonitrile (HPLC grade) was purchased from Merck & Co. (Darmstadt, Germany). NMR spectra were measured with a Bruker DRX-400 spectrometer at 298 K. An Agilent 6890 network GC/5973 MS equipped with a HP-5 silica capillary column, size 30 m × 0.25 mm, was used to determine molecular weights of the final products. Fourier transform infrared (FT-IR) measurements were recorded on a Nicolet NEXUS-6700 FTIR spectrometer with a spectral resolution of 4 cm−1 in a wavenumber range of 500−4000 cm−1. Preparation of the Catalyst. The catalyst was prepared according to the known procedure without any modifications (Figure 2), as described in our previous work.23 Briefly, Nmethylimidazole and 1,4-butane sultone were reacted at 50 °C for 24 h. After reaction, the slurry mixture was filtered, washed 15332
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with 10 mL of diethyl ether before drying at 80 °C for 8 h in a vacuum oven.
butylsulfate group in the organic cation provides the active acid site responsible for the high performance. On the other hand, the heteropolyanions with high valence and large volume are able to endow the novel “ionic liquids” with a high melting point, leading to insolublity in the reaction system. Effect of Catalyst Loading on the Etherification of HMF into EMF. The effect of the catalyst loading on the etherification of HMF was studied, and the results are shown in Figure 3. An increase in the catalyst loading from 2.5 mol % to
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RESULTS AND DISCUSSION Etherification of HMF into EMF in Ethanol with Various Catalysts. Initially, the etherification of HMF by ethanol was carried out using various catalysts to evaluate the catalytic performance of our prepared catalyst, and the results are summarized in Table 1. First, homogeneous acids H2SO4 Table 1. Results of the Etherification of HMF in Ethanol with Various Catalystsa entry
catalyst
time (h)
HMF conversion (%)
EMF yield (%)
1 2 3b 4 5 6 7
H2SO4 p-TSA HY-zeolite NKG-9 [BmimSO3H]3PW12O40 H3PW12O40 methylimidazolebutylsulfate
18 18 24 24 24 24 24
100 100 10.0 100 98.1 100 1.2
79.8 80.6 8.5 82.8 90.7 85.3 0
a
Reaction conditions: a mixture of HMF (126 mg, 1 mmol) and 5 mol % catalyst in 5 mL of ethanol was heated at 70 °C. bFor HY-zeolite as catalyst, the catalyst amount was 30 mg. Figure 3. The effect of catalyst loading on etherification of HMF by ethanol. Reaction conditions: HMF (126 mg, 1 mmol) was added to 5 mL of ethanol, and then a set amount of [MIMPS]3PW12O40 was added at 70 °C.
and p-toluenesulfonic acid (p-TSA) with strong acidity were used to catalyze the etherification of HMF (Table 1, entries 1 and 2), and yields of EMF were obtained in 79.8% and 80.6% for H2SO4 and p-TSA with HMF conversion of 100%, respectively. These results were similar to those reported by Balakrishnan et al.17 At the end of the reaction, no HMF was detected, and the byproducts were mainly caused by the ringopening alcoholysis of HMF leading to ethyl levulinate. Incidently, H2SO4 and p-TSA were dissolved in the reaction system, resulting in a difficult catalyst separation and recycle. Recently, the use of hetergeneous catalysts has attracted much interest in terms of green chemistry. EMF was only achieved in a low yield of 8.5% with a low conversion of 10.0% when the commercial solid acid catalyst HY-zeolite was used (Table 1, entry 3). This is ascribed to the weak acidity of the catalytic sites. A sulfated functionalized ion-exchange resin NKG-9 as a strong solid acid catalyst was also used to catalyze the reaction. As expected, a high EMF yield of 82.8% was reached in 24 h (Table 1, entry 4). However, the structure of NKG-9 was destroyed. The globose catalyst was pulverized from the reaction and the particles became smaller after the reaction; therefore, the separation and recycle of NKG-9 catalyst became difficult. A high EMF yield of 90.7% was obtained with HMF conversion of 98.1% when using the prepared catalyst [MIMPS]3PW12O40 (Table 1, entry 5). These results indicated that [MIMPS]3PW12O40 was an efficient catalyst for the etherification of HMF. As a control experiment, H3PW12O40 and methylimidazolebutylsulfate (MIMBS) as the precursors of this catalyst were also used to catalyzed the etherification of HMF with ethanol to produce EMF (Table 1, entries 6 and 7). A relatively high yield of EMF of 85.3% with H3PW12O40 was obtained. However, it was soluble in the reaction mixture. When methylimidazolebutylsulfate (MIMBS) was used, almost no HMF was converted into EMF because methylimidazolebutylsulfate is a neutral salt and could not catalyze this reaction. All the results indicated that [MIMBS]3PW12O40 exhibited the best catalytic performance of all the screened catalysts. The
10 mol % resulted in an increase in EMF yield especially at the initial reaction stage. For example, EMF yield was only 10.7% in 2 h with 2.5 mol % catalyst whearas those yields reached 23.5% and 35.3% with the catalyst loading being 5 mol % and 10 mol % in 2 h, respectively. A higher etherification rate of HMF with an increase in catalyst loading can be attributed to an increase in the availability and number of catalytically active sites. Maximal EMF yields depended on the catalyst loadings. Maximal yields were 70.8% for 2.5 mol % catalyst, 90.7% for 5 mol % catalyst, and 85.7% for 10 mol % catalyst at 24 h, respectively, and the conversion of fructose reached 78.5%, 98.1%, and 100%, respectively. These results indicated that a low catalyst loading led to a low HMF conversion and EMF yield but that a high catalyst loading led to a high HMF conversion of 100% with a relative low selectivity. Therefore, 5 mol % is an appropriate catalyst loading for the etherification of HMF by ethanol with high HMF conversion and high EMF selectivity. One-Pot Dehydrative Etherification of Fructose into EMF. HMF is the dehydration product of fructose, and it is known that the dehydration of fructose is readily promoted by acid catalysts similar to those used to promote the etherification of HMF. Therefore, one-pot dehydrative etherification of fructose into EMF was investigated in the following experiments. First, one-pot dehydrative etherification of fructose in ethanol was carried out at 70 °C under conditions similar to those used to promote the etherification of HMF (Figure 4). The yield of EMF very slowly increased from 2.3% at 2 h to 15.4% at 24 h, and HMF yields were kept at a low level around 10%. The results revealed that dehydration of fructose required higher temperatures, which was unlike the conditions for etherification of HMF. Therefore, the reaction temperature was increased to 90 °C. As expected, the yield of EMF was largely 15333
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temperature, the higher the dehydrative etherification rate. Furthermore, when the one-pot conversion of fructose into EMF was further carried out at 110 °C, EMF yields were slightly higher than those at 90 °C in the initial 4 h. However, when the reaction time was further prolonged beyond 4 h, EMF yield was lower than that at 90 °C, and the maximum EMF yield was 73.8% at 24 h. The HMF yield of 26.0% at 110 °C for 2 h was much higher than that of 15.0% at 90 °C for 2 h (Figure 4). These results also indicated that a higher temperature was benefical for the dehydration of fructose. The lower EMF yield at 110 °C can be attributed to two reasons. On the one hand, it was reported that alkyl levulinate side products were readily promoted at high temperature from the alcoholysis of HMF.17 They reported that ethyl levulinate was achieved in 62% with 5 mol % H2SO4 at 120 °C for 30 h. On the other hand, HMF itself was not stable at high temperature. Some humins were reported to be formed from the polymerization and cross-polymerization of HMF.24 Therefore, one-pot dehydrative etherification of fructose was preferably carried out at 90 °C with 5 mol % catalyst in achieving a high EMF yield of 90.5%. Finally, glucose was also used as substrate for the production of EMF. Experiments starting with glucose were carried out at 90 °C under the conditions as described for fructose. Even though the reaction time was prolonged to 24 h, no HMF and EMF were detected. As described in the mechanism for the EMF formation from glucose and fructose (Figure 5), fructose was the important intermediate for the synthsies of EMF from glucose. No EMF was obtained from glucose, indicating that the acidic catalyst [MIMBS]3PW12O40 could not promote the iosmerization of glucose into fructose wherase it might catalyze the reaction of glucose with ethanol, resulting in ethyl
Figure 4. Effect of reaction tempeature on one-pot dehydrative etherification of fructose in ethanol. Reaction conditions: fructose (180 mg, 1 mmol) and 5 mol % [MIMPS]3PW12O40 were added into 5 mL of ethanol, and then the reaction was carried out at a set temperature.
promoted at 90 °C, which increased from 10.5% at 2 h to 90.5% at 24 h (Figure 4). The yield of the intermediate HMF decreased from 15.0% at 2 h to 1.2% at 24 h. The results also indicated that dehydration of fructose into HMF was much more difficult than the etherification of HMF. Upon comparison of the results at 70 °C with those at 90 °C, we saw that the reaction temperature had a remarkable effect on the in situ dehydration and etherification process for the conversion of fructose into EMF. The higher the reaction
Figure 5. Possible mechanism of EMF formation from glucose and fructose in ethanol. 15334
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glucopyranoside, similar to results previously reported for Brønsted acid catalysts in methanol and butanol.25,26 Under acidic conditions, D-furan fructose initially lost one water molecular to form fructofuranosyl oxocarbenium ion. Then fructofuranosyl oxocarbenium ion released one H+ to form the enol furan intermediate, which subsequently dehydrated twice to form HMF. The etherification of HMF with ethanol was promoted under acidic conditions to form EMF. Reusability of [MIMBS]3PW12O40 for One-Pot Conversion of Fructose into EMF. Finally, the reusability and stability of [MIMBS]3PW12O40 was evaluated. The reusability of the catalyst was investigated in the one-pot dehydrative etherification of fructose in ethanol at 90 °C. After the reaction was finished at 24 h with the use of fresh catalyst, the used catalyst was collected by filtration, washed with ethanol and diethyl ether, and dried at 80 °C for 8 h in a vacuum oven. Then the recovered catalyst was added into a fresh solution of reaction mixture, and the second cycle was carried out under the same conditions. These steps were repeated five times, and the reusability results are shown in Figure 6. The yields of EMF
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +86-27-67842572. Fax: +86-27-67842572. E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Project was supported by National Natural Science Foundation of China (Nos. 21203252, 21206200) and the Special Fund for Basic Scientific Research of Central Colleges, South-Central University for Nationalities (CYZ2012005).
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REFERENCES
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Figure 6. Recycle experiments for the catalyst [MIMBS]3PW12O40.
were maintained around 90%. Therefore, this novel hybrid solid acid catalyst [MIMBS]3PW12O40 can be reused without losing catalytic activity, being favorable for the potential application in large scale synthesis from an industrial point of view.
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CONCLUSION Production of EMF through one-pot dehydrative etherification of fructose in ethanol was developed. The novel organic− inorganic hybrid solid catalyst [MIMBS]3PW12O40 showed high catalytic activity and selectively for the etherification of HMF in ethanol with a high EMF yield of 90.7% and HMF conversion of 98.1% at 70 °C for 24 h. More importantly, the one-pot dehydrative etherification of fructose in ethanol was also realized by this solid catalyst. The reaction temperature had a notable effect on EMF yield. Unlike for the etherification of HMF in ethanol, one-pot production of EMF from fructose was much more difficult at 70 °C. A high EMF yield of 90.5% was achieved from fructose at 90 °C whereas further elevation of temperature to 110 °C decreased the maximum yield of EMF to 78.3% due to side reactions. Finally, [MIMBS]3PW12O40 can be reused several times without losing catalytic activity. Due to the simple preparative procedures and the commercial availability of materials for the synthesis of [MIMBS]3PW12O40, the large-scale production of EMF from renewable carbohydrates using this novel catalyst shows promise for the future. 15335
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