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Conversion of fructose and glucose into 5-hydroxymethylfurfural catalyzed by a solid heteropolyacid salt
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 6 5 9 e2 6 6 5
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Conversion of fructose and glucose into 5-hydroxymethylfurfural catalyzed by a solid heteropolyacid salt Chunyan Fan, Hongyu Guan, Hang Zhang, Jianghua Wang, Shengtian Wang, Xiaohong Wang* Key Laboratory of Polyoxometalate, Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, 5268 Renmin street, Changchun, 130024, Jilin Province, PR China
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abstract
Article history:
A solid heteropolyacid salt Ag3PW12O40 has been used as a heterogeneous catalyst for the
Received 20 November 2010
production of 5-hydroxymethylfurfural (HMF) from fructose and glucose. The fructose was
Received in revised form
selectively dehydrated into HMF with the HMF yield as high as 77.7% and selectivity of
1 March 2011
93.8% within 60 min at 120 C. In addition, Ag3PW12O40 also exhibited catalytic activity for
Accepted 2 March 2011
conversion of glucose into HMF. Moreover, the catalyst is tolerant to high concentration
Available online 30 March 2011
feedstock and can be recycled. The results illustrate that the Ag3PW12O40 is an excellent acid and environmentally benign solid catalyst for the production of HMF from fructose
Keywords:
and glucose. ª 2011 Elsevier Ltd. All rights reserved.
Fructose Glucose Dehydration 5-hydroxymethylfurfural Heterogeneous acid catalysis
1.
Introduction
Biofuels, which have fewer environmental concerns, are a promising renewable and sustainable alternative for limited fossil fuels [1,2]. Among current biofuel sources, 5hydroymethylfurfural (HMF) converted from biomass is a versatile and key intermediate and it is attracting much attention in biofuels chemistry and the petroleum industry [3]. Lewkowski [4] indicated that nearly a hundred inorganic and organic compounds positively qualify as catalysts for the production of HMF. Compared with homogeneous catalysts, heterogeneous ones are easier to be separated and recycled,
and have shown a superior behavior in terms of HMF’s selectivity [5]. However, solid acid catalysts tend to give low conversion and low yield due to the formation of various byproducts, under higher temperature and long reaction times greater than 2 h. Since high feedstock concentration leads to condensation products such as polymers and humins [6] as elevating temperature and prolonging reaction time, use of low feedstock concentration limits the real applications. Some heterogeneous catalysts had been developed to improve the conversion of sugar. The fructose conversion of 89% and 60% selectivity were achieved with acidic ionexchange resin at fructose concentration (30 wt%) in
* Corresponding author. Tel.: þ86 431 88930042; fax: þ86 431 85099759. E-mail address: [email protected] (X. Wang). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.03.004
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a biphasic system at 90 C for 8e16 h [7]. Watanabe [8] reported a cation exchange resin in acetone-water mixtures to obtain 99.3% of conversion and 44.3% selectivity of 20 wt% fructose solution by microwave heating at 150 C for 20 min. Sulfated zirconia [9] had been used as a solid acid catalyst for the dehydration of 2 wt% fructose solution with 93.6% conversion, 72.8% HMF yield and 77.7% selectivity by microwave heating at 180 C and 20 min in acetone-DMSO(dimethyl sulfoxide) mixture. Recently, Shimizu et al. evaluated the activity of solid acid catalysts: heteropolyacid, zeolite, and acidic resin by simple water removal methods [10]. In that report, FePW12O40 and Cs2.5H0.5PW12O40 could catalyze fructose dehydration with 100% and 91e97% HMF yields under evacuation at 0.97 105 Pa at 120 C for 2 h, which is the best result to date (3 wt% fructose in DMSO). For high concentration fructose (50 wt%), the yields reached about 100% and 50%, respectively, corresponding to Amerlyst-15 and FePW12O40. However, it is hard to separate 5-HMF product from high-boiling solvent DMSO. In addition, 1butanol and 2-butanol were also investigated as extracting agents, using the salting out effect to improve the selectivity of HMF [2]. Some researchers used ionic liquid to improve the conversion of sugar. Indeed, ionic liquid is beneficial for the conversion of sugars into HMF [11], but the solubility of sugar is relatively low compared to its solubility in water, which limits the extent of feedstock concentration that can be converted into HMF. Therefore, seeking an efficient catalyst is essential for converting high concentration sugar into HMF with high conversion and good selectivity under mild conditions. Among various heterogeneous catalysts, heteropolyacids (HPAs) have been attracted much attention due to their unique properties such as well-defined structure, Brønsted acidity, possibility to modify their acid-base by changing their chemical composition, ability to accept and release electrons, high proton mobility [12,13]. Among heteropolyacids, dodecatungstophosphoric acid H3PW12O40 is characterized by its strong acidity being classified into a superacid [14], but it is soluble in water and polar solvents that limits its application. The substituted protons of H3PW12O40 by large monovalent cations such as Csþ, NH4þ, Kþ and Agþ could form a range of insoluble, microporous solid acidic catalysts effective in various catalytic reactions. Up to now, so many attentions have been paid for Csþ, NH4þ, Kþ salts of H3PW12O40. Markedly less attention has been directed to catalytic properties of insoluble silver salts of H3PW12O40 [15]. The reasons of selecting Ag3PW12O40 are (1) the insolube and microporous heterogeneous catalyst. (2) the mild Lewis acidity of Ag and favorable redox potential leading to Ag-based catalysts to be likely the first choice of reagent for many organic reactions [16e18]. These two factors might be suitable for overcoming the drawbacks in HMF production process. In the present study, we reported the catalytic conversion of fructose and glucose into 5-HMF in a two-phase reactor system with high feed concentration by Ag3PW12O40. The experimental results demonstrated that Ag3PW12O40 exhibited higher selectivity (93.8%) and excellent efficiency (relatively high yield 77.7%) for fructose conversion with high concentration (30 wt%). In addition, Ag3PW12O40 was active for the conversion of glucose into HMF. Therefore, this solid acid Ag3PW12O40 was shown to
be an efficient catalyst for the conversion of monosaccharides into HMF with high selectivity. Moreover, this catalyst is tolerant to high concentration feedstock and the catalyst can be recycled.
2.
Experimental
2.1.
Measurements
All solvents and chemicals used were obtained from commercial suppliers. H3PW12O40 was prepared following the ref [19]. Elemental analyses were carried out using a Leeman Plasma Spec (I) ICP-ES. IR spectra (4000 - 500 cm-1) were recorded in KBr discs on a Nicolet Magna 560 IR spectrometer. Morphology of sample and the energy dispersive X-ray analysis (EDS) of W and Ag elements in asprepared and reused samples were studied by means of Field Emission Scanning Electron Microscope XL30 ESEM FEG at 25 k. The specific surface areas of samples were calculated from the nitrogen adsorptionedesorption isotherms at 77 K in Micromeritics ASAP 2010 instrument. Prior to the measurements, the samples were preheated and degassed, under vacuum at 473 K for 2 h. XRD patterns of the sample were collected on Japan Rigaku Dmax 2000 X-ray diffractometer with Cu Ka radiation (l ¼ 0.154178 nm). XPS were recorded on an Escalab-MK II photoelectronic spectrometer with Al Ka (1200 eV). Centrifugation was performed on T4C Centrifuge (4000 rpm, 5 min, Beijing Yiyao). The consumption of saccharide was measured by measuring its concentration in the aqueous phase by Highperformance liquid chromatography (HPLC, Shimadzu LC-10A) was conducted on a system equipped with a refractive index detector. The saccharide was quantified by HPLC with Aminex HPX-87H column, using MilliQ water (pH ¼ 2) as the mobile phase at a flow rate of 0.6 ml min1and column temperature of 303 K. And also the concentration of HMF in the aqueous phase was determined by HPLC with ION-300H column using a 2: 8 v/v methanol: water (pH ¼ 2) gradient at a flow rate of 0.7 ml/min and a column temperature of 303 K using a UV detector. The concentration of HMF in the organic phase was determined by gas chromatography (Agilent 6890) equipped with Agilent 19091J-416 capillary column and flame ionization detector. Qualitative analysis was performed by Gas chromatography-mass spectrometry (GC-MS, Agilent 5970) with scan parameters: low mass 20.0, high mass 700.0, and threshold 150. The analytical error in HMF yield and substrates conversion was evaluated to be in the 1% range.
2.2.
Preparation of catalyst
1 mmol of H3PW12O40$6H2O was dissolved in 10 mL distilled water. The above solution was added dropwisely into 10 mL, 1M of AgNO3 aqueous solution with vigorous stirring at room temperature. The mixture was continuously stirred for about 30 min, the precipitate was aged for 12 h at room temperature. The white precipitate was filtrated and washed with water several times until no Agþ being detected. The sample was dried for 12 h at 200 C. The resulting Ag3PW12O40 was obtained with yield 50%. Anal. Calc. for Ag3PW12O40: W, 69.20; P, 0.97; Ag, 10.05%. Found: W, 69.18; P, 0.96; Ag, 10.05%.
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 6 5 9 e2 6 6 5
2.3.
Dehydration reaction
In a typical reaction protocol, 2400 mg of sugar was mixed with 8 g of water and 18 mL MIBK in a Parr reactor (size, 50 mL) and 0.080 g of Ag3PW12O40 was added as a catalyst. The experiments were carried out at 120 C for 60 min with electrically-heated oil bath (It needs 15 min from room temperature to 120 C). The reaction was stopped by rapidly cooling the reactor in an ice bath at 0 C for 30 min. All experiments were repeated six times. After the reaction, the mixture was stood still to form two layers: the upper layer-organic phase consisted of HMF and MIBK; and the lower layer-water phase contained HMF, unreacted sugar and catalyst. These two layers were separated by sedimentation. The catalyst Ag3PW12O40 was centrifuged and then was rinsed with water three times to wash out unreacted sugar. The catalyst was calcinated at 80 C for 3 h to remove water. The total amount of Ag3PW12O40 leaching through six runs of the reaction was detected using a Leeman Plasma Spec (I) ICP-ES.
3.
Results and discussion
3.1.
Characterization of catalyst
From the result of elemental analysis, the contents of the material are W, 69.20; P, 0.97; Ag, 10.05%, respectively, and it can be seen that the ratio of W: P: Ag is 12:1:3 corresponding to the formula of Ag3PW12O40. The FT-IR spectrum of Ag3PW12O40 was investigated and result was given in Supplementary Information (S.I.) section (see Fig. S1a). The peaks in the IR spectral range 600e1100 cm1 corresponding to [PW12O40]3- structural vibrations could be distinguished easily at 1080, 983, 888 and 815 cm1, which were attributed to the asymmetry vibrations P-Oa (internal oxygen connecting P and W), W-Od (terminal oxygen bonding to W atom), W-Ob (edge-sharing oxygen connecting W) and W-Oc (corner- sharing oxygen connecting W3O13 units), respectively. The surface acidity of Ag3PW12O40 was determined by pyridine adsorption infrared spectroscopy (see S.I. section, Fig. S1b). It can be seen that the IR spectrum exhibited strong bands at 1446 cm1 and 1602 cm1, which were attributed to the Lewis acidity. Meanwhile, the peaks at 1638 and 1542 cm1 were attributed to Brønsted acidity. The peaks at 1490 cm1 was characteristic of pyridine adsorbed on both Lewis and BrØnsted acid sites. The result showed that Ag3PW12O40 exhibited both Brønsted acidity and Lewis acidity. To identify the structure of Ag3PW12O40, XRD spectroscopy was performed and result was given in S.I. section (see Fig. S2). In comparison with tetragonal H3PW12O40 phase (Fig. S2a) (JCPDS no.75-2125), there are no reflexes originating from crystalline H3PW12O40, whereas the new ones assigned to crytalline phase of Ag3PW12O40 salt (see Fig. S2b) can be seen [17]. Thus, there is only one crystalline phase e Ag3PW12O40 in the silver salt. This observation is in accordance with previously reported data [17]. The morphology of Ag3PW12O40 was observed by Scanning electron micrograph (SEM). The SEM image in Fig. S3 showed
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that the sample displayed well shaped crystalline particles with average diameters 8 mm. The porous property of Ag3PW12O40 was analyzed by nitrogen sorption analysis. As seen in S.I. section (see Fig. S4), the adsorption isotherm exhibited Type II isotherm: a steep gas uptake at low relative pressure (P/P0 < 0.2) and a flat course in the intermediate section, thus reflecting the microporous nature of the polymer networks. The hysteresis was not observed upon desorption which is usually found for microporous networks [20]. Pore size was about 0.86789 nm and the surface area of Ag3PW12O40 was 5.08 m2/g.
3.2.
Catalytic activity
3.2.1.
Effect of solvent
In the present work, the dehydration of sugar into HMF by heterogeneous HPAs catalyst was explored in a water-methylisobutylketone (MIBK) biphasic system. According to the literature [21], MIBK is a good solvent that could suppress unwanted side reactions for fructose dehydration in water using acid catalysts, and could extract more HMF into organic phase with good partitioning of HMF compared to other solvents. So MIBK and water biphasic system was used to evaluate the catalytic activity of Ag3PW12O40. The influence of amount of MIBK on conversion and selectivity was given in Fig. 1. It can be seen that the HMF selectivity was low in pure water due to the side products. The selectivity increased significantly using MIBK and water biphasic system. As the amount of MIBK in biphasic system increased, the conversion and HMF selectivity increased. The fructose conversion and HMF selectivity reached maximum values of 82.8% and 93.8%, respectively, at volume ratio of 1:2.25 of water to MIBK. Increasing volume ratio to 1:3 did not increase the conversion and selectivity significantly. So the volume ratio of water to MIBK is 1:2.25 in the whole experiment whatever fructose or glucose was used as feedstock.
3.2.2.
Effect of the catalyst
The performance of acid catalysts was examined through the degradation of sugar (Table 1 and Table 2) under the reaction
Fig. 1 e Effect of MIBK on the dehydration of fructose Reaction conditions: 2400 mg of fructose in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL, 80 mg of Ag3PW12O40, 120 C, 60 min.
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Table 1 e The influence of the catalyst usage on conversion of fructose and HMF selectivity. a Time (min) Catalyst (g)
Rb
60 120 60 120 30 60 90 120 60
7.5
0.32 0.16
15
0.08
30
no
Conversion (%) Yield (%) HMForg (mg/mL) HMFaq (mg/mL) Levulinic acid (%) TOF c (103) 94.7 100 84.3 100 73.4 82.8 92.6 94.1 46.9
0.4 0.5 0.6 0.6 0.5 0.3 0.5 0.2 0.4
75.4 70.2 74.6 68.4 59.7 77.7 77.8 74.9 33.2
0.3 0.4 0.3 0.2 0.5 0.4 0.6 0.2 0.3
57.0 52.8 58.8 53.4 47.9 63.4 61.6 59.7
0.2 0.4 0.4 0.3 0.6 0.2 0.2 0.7
30.1 28.6 22.1 23.4 17.5 20.4 24.5 23.0
0.4 0.4 0.2 0.2 0.3 0.1 0.8 0.2
11.3 13.5 7.7 8.4 1.2 1.6 3.9 7.9
0.2 0.3 0.2 0.4 0.1 0.2 0.3 0.2
31.19 14.60 61.27 28.49 198.81 127.22 86.27 62.46
0.3 0.2 0.3 0.1 0.4 0.1 0.2 0.6
a Reaction conditions, T ¼ 120 C, water: MIBK ¼ 1:2.25. b R is the ratio of the fructose usage (g) to the weight of catalyst (g). c Turnover frequency for HMF on the catalyst in units of mol g1h1.
conditions, such as water to MIBK volume ratio 1:2.25, 300 mg/ mL of sugar, and a certain amount of catalyst at 120 C for certain time. Without any catalyst, fructose could be dehydrated with 46.9% conversion effect and 33.2% yield, respectively, and the dehydration of fructose could be promoted by using solid catalyst and be enhanced with increasing the usages of Ag3PW12O40. For instance, the fructose conversion and HMF yield could reach 92.6 and 77.8% using 0.01 g catalyst per 1 mL unit volume. As the usage of catalyst increased, the selectivity decreased due to the decomposition being accelerated at high usage of catalyst (0.320 g). This decrease was attributed to the availability of acidic sites that favored the rehydration of HMF to levulinic acid (11.3%). The best value of TOF was 198.81 at 0.080 g of catalyst with 30 min reaction time and R value of 30. Therefore, the usage of 0.080 g was chosen as an appropriate catalyst concentration in subsequent experiments. The influence of catalyst usage on glucose’s dehydration was given in Table 2. It can be seen the same influence of usage on glucose dehydration. The usage of 0.32 g was chosen as an appropriate catalyst concentration in subsequent experiments. It can be seen that it needs higher temperature, higher amount of catalyst and longer time to convert glucose into HMF than the dehydration of fructose. The comparison of the different catalysts was given in Fig. 2. By pure Brønsted acid HCl (0.075 mmol, corresponding to the same effective acid concentration as 0.025 mmol of
H3PW12O40) and H3PW12O40 catalysts, higher conversion of hexose and lower selectivity could be achieved under the same reaction conditions for 60 min. This result was attributed to their strong acidity, which may favor the subsequent rehydration of HMF to levulinic acid and formic acid, which was also observed on other literature [22]. By Lewis acid AgNO3, low conversion and low yield were obtained, showing that this kind of Lewis acid catalyst can not give the high conversion of sugar. Cs3PW12O40 owned no Brønsted and Lewis acidity leading to low catalytic activity. By Ag3PW12O40, high conversion and high selectivity were obtained. Exchanging of Hþ with Agþ can modify the acidity strength of H3PW12O40 and introduce some Lewis acidity into the molecules. The acidity capacity of Ag3PW12O40 was also measured by titration by n-butylamine with the indicator anthraquinone (pKa ¼ 8.2), and the high value of 3.01 mmol/g acid sites was detected. And from the result of pyridine adsorption infrared spectroscopy, Ag3PW12O40 exhibited both Brønsted and Lewis acidity, which may be suitable for the dehydration of fructose into HMF and lead to high selectivity for HMF. Therefore, the higher catalytic activity of Ag3PW12O40 was attributed to the synergistic effect of Lewis acid sites and BrØnsted acid sites. It is reported that the selective conversion of glucose into HMF is still a challenge [23], and some Lewis acid catalyzed transformations in ionic liquids were reported [24,25]. The effect of Lewis acid is thought to involve the coordination of mononuclear metal with glucose leading to the isomerization
Table 2 e The influence of the catalyst dosage on conversion of glucose and HMF selectivity. a Time (h) 1 2 3 4 1 2 3 4
Catalyst (g) 0.16
0.32
Rb 15
7.5
Conversion (%)
yield (%)
HMForg (mg/ml)
16.8 0.6 27.9 0.4 34.3 0.2 45.7 0.4 54.9 0.3 68.2 0.5 74.9 0.2 89.5 0.2
8.4 0.3 13.8 0.5 26.5 0.4 36.4 0.3 42.2 0.7 54.4 0.4 60.3 0.6 76.3 0.5
5.28 0.4 10.2 0.6 19.6 0.5 23.7 0.4 28.9 0.5 40.3 0.3 44.8 0.9 58.3 0.7
a Reaction conditions, T ¼ 130 C, water: MIBK ¼ 1:2.25. b R is the ratio of the glucose usage (g) to the weight of catalyst (g). c Turnover frequency for HMF on the catalyst in units of mol g1h1.
HMFaq (mg/ml) 5.85 5.93 11.6 23.1 23.6 23.4 25.8 28.9
0.3 0.5 0.2 0.1 0.6 0.3 0.4 0.3
Levulinic acid (%) 0.5 0.1 1.3 0.1 2.5 0.2 3.9 0.1 0.8 0.2 1.9 0.3 4.6 0.5 6.1 0.4
TOF c (103) 6.98 5.71 10.08 7.62 17.54 11.27 8.41 7.94
0.3 0.2 0.4 0.1 0.5 0.2 0.5 0.6
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Fig. 2 e Dehydration of sugar by different catalysts Reaction conditions: 2400 mg of sugar in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL 0.025 mmol of catalyst, 120 C, 60 min for fructose and 0.1 mmol of catalyst, 130 C, 4 h for glucose, respectively.
of glucose to fructose [26]. Here, we selected Ag3PW12O40 as Lewis acid catalyst to achieve the dehydration of glucose to HMF. From Fig. 2, it can be seen that Ag3PW12O40 exhibited high catalytic activity compared to Brønsted acids HCl (0.3 mmol, corresponding to the same effective acid concentration as 0.1 mmol of H3PW12O40) and H3PW12O40. AgNO3, a pure moderated Lewis acid, was not efficient for the dehydration of glucose into HMF. Therefore, the higher catalytic activity of Ag3PW12O40 was attributed to the synergistic effect of Lewis acid sites and BrØnsted acid sites. The highest activity of Ag3PW12O40 is attributed to accumulation of substrate on the catalyst. The IR spectra of Ag3PW12O40 and its adsorbed fructose and glucose confirm the absorption of substrate on the part of the catalyst (Fig. S5). Compared to the spectrum of Ag3PW12O40, the vibration bands shift to a higher frequency, indicating that some interaction occurs between the O atom from substrate and the terminal oxygen atom from the HPA molecule.
3.2.3.
with high concentration into HMF, while a high selectivity of 87.4% was obtained at 50 wt% initial concentration under the same reaction conditions.
3.2.4.
Effect of temperature
Dehydration of fructose is normally performed above the boiling point of the water. We studied the dehydration of sugar at 100 Ce130 C in order to determine the effect of the reaction temperature on the formation of HMF (Table 3). It can be seen that the reaction temperature had a large effect on the dehydration of fructose into HMF. The 54.3% conversion was obtained at a temperature of 100 C. Meanwhile, the fructose conversion reached about 100% at 130 C within the same time. Therefore, the temperature played a positive role on the
Effect of feedstock’ concentration
Under the reaction conditions as 8 mL of water, 16 mL of MIBK, 0.080 g of Ag3PW12O40, at 120 C for 60 min, we investigated the influence of initial concentrations including 10 wt%, 30 wt% and 50 wt% (Fig. 3) on dehydration of fructose. It can be seen that the conversion and the HMF selectivity were affected by different initial concentrations of fructose. The conversion of fructose increased but the HMF selectivity decreased with the increase of feedstock’s concentrations. The highest HMF selectivity reached 93.8% at initial concentration 30 wt%. The loss of selectivity for 10 wt% of fructose was attributed to the more catalytic sites compared to the high concentration’s fructose, resulting in more by-products with 14.52% yield of levulinic acid. Compared to 30 wt% feedstock, the 6% reduction of selectivity for 50 wt% fructose might be attributed to the higher fructose concentration leading to higher rates of condensation reactions. This result was in agreement with literature reports [7]. Nevertheless, the Ag3PW12O40 heterogeneous catalyst was efficient for the transformation of fructose
Fig. 3 e The influence of the initial concentration of fructose on the conversion and selectivity Reaction conditions: certain amount of fructose in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL, 80 mg of Ag3PW12O40, 120 C, 60 min.
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Table 3 e The influence of temperature on the yield and selectivity of HMF. Temperature ( C)
Yield (%) (from fructose a)
Selectivity (%) (from fructose)
54.3 0.6 71.2 0.4 73.3 0.7 77.7 0.4 87.6 0.5
100 110 115 120 130
76.8 93.9 94.6 93.8 87.9
Yield (%) (from glucose b)
0.2 0.8 0.5 0.6 0.3
11.5 30.7 45.8 52.5 76.3
0.5 0.2 0.5 0.5 0.5
Selectivity (%) (from glucose) 53.8 55.4 57.3 60.3 85.3
0.8 0.6 0.4 0.3 0.3
a Reaction condition: 2400 mg of fructose, 0.08 g of catalyst, water: MIBK ¼ 1:2.25 for 60 min. b Reaction condition: 2400 mg of glucose, 0.32 g of catalyst, water: MIBK ¼ 1:2.25 for 4 h.
conversion of fructose. As for HMF selectivity, it improved as the temperature increased from 100 C to 115 C, and could reach highest 94.6% at 115 C. However, higher temperatures (130 C) gave rise to by-products and unknown insoluble byproducts leading to lower HMF selectivity. From the point of economic view and selectivity of HMF, 120 C was selected as the reaction temperature. The influence of temperature on glucose’s conversion showed the same variety. In addition, longer reaction time and higher temperature were needed for the dehydration of glucose.
3.2.5.
Reusability of the catalyst
The Ag3PW12O40 catalyst was easily separated from the production mixture because, at the end of the reaction, the catalyst decanted to the bottom of the reactor. It could be used one more time after washing with water. The recycled reaction procedure retained high activity throughout the conversion of fructose into HMF. To test the leaching of the Ag3PW12O40 catalyst [27], the catalyst was filtered after reacting for 30 min (ca. 73.4% fructose conversion) and the filtrate was allowed to react over further 30 min under the same reaction conditions. 74.1% conversion of fructose was obtained, which showed a little leaching of Ag3PW12O40 into the mixture. The total amount of Ag3PW12O40 leaching through six runs of the reaction reached 5.1% of the starting amount of Ag3PW12O40 (Fig. 4). The IR peaks of recycled
Ag3PW12O40 at 1078, 981, 886 and 805 cm1 basically correspond with the IR spectrum of its parent PW12. Therefore, the original frameworks of HPA are not destroyed after reaction. Therefore, this solid acid catalyst was stable and can be reused up to at least six reaction cycles.
4.
Conclusion
A heteropolyacid salt Ag3PW12O40 has been developed for the selective conversion of fructose and glucose into HMF. The yield HMF of 77.7% and selectivity of 93.8% were obtained within 60 min at 120 C in the dehydration of fructose. It is also important that Ag3PW12O40 is suitable for the dehydration of glucose into HMF with high yield of 76.3% under mild conditions. Moreover, this catalyst is tolerant to high concentration feedstock and can be recycled.
Acknowledgments This work was supported by the National Natural Science Foundation of China (20871026). And it was supported by analysis and testing foundation of Northeast Normal University and the major projects of Jilin Provincial Science and Technology Department.
Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biombioe.2011.03.004.
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Fig. 4 e The catalyst activity in six reaction cycles Reaction conditions: 2400 mg of fructose in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL, 80 mg of Ag3PW12O40, 120 C, 60 min.
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[17] Haber J, Pamin K, Matachowski L, Napruszewska B, Po1towicz J. Potassium and silver salts of Tungstophosphoric acid as catalysts in dehydration of ethanol and hydration of ethylene. J Catal 2002;207:296e306. [18] Parent MA, Moffat JB. Cation/proton interactions and acid strengths in salts of 12-tungstophosphoric acid prepared from 1A, 1B and 3B monovalent cations. Catal Lett 1997;48: 135e43. [19] Rocchiccioli-Deltcheff C, Fournier M, Franck R, Thouvenot R. Vibrational investigations of polyoxometalates. 2. Evidence for anion-anion interactions in molybdenum(VI) and tungsten(VI) compounds related to the Keggin structure. Inorg Chem 1983;22:207e16. [20] Naodovic M, Yamamoto H. Asymmetric silver-catalyzed reactions. Chem Rev 2008;108:3132e48. [21] Merville GF, Toulouse AG, Toulouse J-P. G, Toulouse, ET, Lille, P. S. Process for manufacturing 5-hydroxymethylfurfural. US patent 181744.1982. [22] By van Dam HE, Kieboom APG, van Bekkum H. The conversion of fructose and glucose in acidic media: formation of hydroxymethylfurfural. Starch/Starke 1986;38: 95e101. Delft. [23] Pidko EA, Degirmenci V, vanSanten RA, M.Hensen EJ. Glucose activation by transient Cr2þ dimers. Angew Chem Int Ed 2010;49:2530e4. [24] Hu SQ, Zhang ZF, Song JL, Zhou YX, Han BX. Efficient conversion of glucose into 5-hydroxymethylfurfural catalyzed by a common Lewis acid SnCl4 in an ionic liquid. Green Chem 2009;11:1746e9. [25] Li CZ, Zhang ZH, Zhao ZK. Direct conversion of glucose and cellulose to 5-hydroxymethylfurfural in ionic liquid under microwave irradiation. Tetrahedron Lett 2009;50:5403e5. [26] Binder Joseph B, Raines RT. Simple chemical transformation of lignocellulosic biomass into furans for fuels and chemicals. J Am Chem Soc 2009;131:1979e85. [27] Misono M. Heterogeneous catalysis by heteropoly compounds of molybdenum and tungsten. Catal Rev 1987;29: 269e321.
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Conversion of fructose and glucose into 5-hydroxymethylfurfural catalyzed by a solid heteropolyacid salt Chunyan Fan, Hongyu Guan, Hang Zhang, Jianghua Wang, Shengtian Wang, Xiaohong Wang* Key Laboratory of Polyoxometalate, Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, 5268 Renmin street, Changchun, 130024, Jilin Province, PR China
article info
abstract
Article history:
A solid heteropolyacid salt Ag3PW12O40 has been used as a heterogeneous catalyst for the
Received 20 November 2010
production of 5-hydroxymethylfurfural (HMF) from fructose and glucose. The fructose was
Received in revised form
selectively dehydrated into HMF with the HMF yield as high as 77.7% and selectivity of
1 March 2011
93.8% within 60 min at 120 C. In addition, Ag3PW12O40 also exhibited catalytic activity for
Accepted 2 March 2011
conversion of glucose into HMF. Moreover, the catalyst is tolerant to high concentration
Available online 30 March 2011
feedstock and can be recycled. The results illustrate that the Ag3PW12O40 is an excellent acid and environmentally benign solid catalyst for the production of HMF from fructose
Keywords:
and glucose. ª 2011 Elsevier Ltd. All rights reserved.
Fructose Glucose Dehydration 5-hydroxymethylfurfural Heterogeneous acid catalysis
1.
Introduction
Biofuels, which have fewer environmental concerns, are a promising renewable and sustainable alternative for limited fossil fuels [1,2]. Among current biofuel sources, 5hydroymethylfurfural (HMF) converted from biomass is a versatile and key intermediate and it is attracting much attention in biofuels chemistry and the petroleum industry [3]. Lewkowski [4] indicated that nearly a hundred inorganic and organic compounds positively qualify as catalysts for the production of HMF. Compared with homogeneous catalysts, heterogeneous ones are easier to be separated and recycled,
and have shown a superior behavior in terms of HMF’s selectivity [5]. However, solid acid catalysts tend to give low conversion and low yield due to the formation of various byproducts, under higher temperature and long reaction times greater than 2 h. Since high feedstock concentration leads to condensation products such as polymers and humins [6] as elevating temperature and prolonging reaction time, use of low feedstock concentration limits the real applications. Some heterogeneous catalysts had been developed to improve the conversion of sugar. The fructose conversion of 89% and 60% selectivity were achieved with acidic ionexchange resin at fructose concentration (30 wt%) in
* Corresponding author. Tel.: þ86 431 88930042; fax: þ86 431 85099759. E-mail address: [email protected] (X. Wang). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.03.004
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a biphasic system at 90 C for 8e16 h [7]. Watanabe [8] reported a cation exchange resin in acetone-water mixtures to obtain 99.3% of conversion and 44.3% selectivity of 20 wt% fructose solution by microwave heating at 150 C for 20 min. Sulfated zirconia [9] had been used as a solid acid catalyst for the dehydration of 2 wt% fructose solution with 93.6% conversion, 72.8% HMF yield and 77.7% selectivity by microwave heating at 180 C and 20 min in acetone-DMSO(dimethyl sulfoxide) mixture. Recently, Shimizu et al. evaluated the activity of solid acid catalysts: heteropolyacid, zeolite, and acidic resin by simple water removal methods [10]. In that report, FePW12O40 and Cs2.5H0.5PW12O40 could catalyze fructose dehydration with 100% and 91e97% HMF yields under evacuation at 0.97 105 Pa at 120 C for 2 h, which is the best result to date (3 wt% fructose in DMSO). For high concentration fructose (50 wt%), the yields reached about 100% and 50%, respectively, corresponding to Amerlyst-15 and FePW12O40. However, it is hard to separate 5-HMF product from high-boiling solvent DMSO. In addition, 1butanol and 2-butanol were also investigated as extracting agents, using the salting out effect to improve the selectivity of HMF [2]. Some researchers used ionic liquid to improve the conversion of sugar. Indeed, ionic liquid is beneficial for the conversion of sugars into HMF [11], but the solubility of sugar is relatively low compared to its solubility in water, which limits the extent of feedstock concentration that can be converted into HMF. Therefore, seeking an efficient catalyst is essential for converting high concentration sugar into HMF with high conversion and good selectivity under mild conditions. Among various heterogeneous catalysts, heteropolyacids (HPAs) have been attracted much attention due to their unique properties such as well-defined structure, Brønsted acidity, possibility to modify their acid-base by changing their chemical composition, ability to accept and release electrons, high proton mobility [12,13]. Among heteropolyacids, dodecatungstophosphoric acid H3PW12O40 is characterized by its strong acidity being classified into a superacid [14], but it is soluble in water and polar solvents that limits its application. The substituted protons of H3PW12O40 by large monovalent cations such as Csþ, NH4þ, Kþ and Agþ could form a range of insoluble, microporous solid acidic catalysts effective in various catalytic reactions. Up to now, so many attentions have been paid for Csþ, NH4þ, Kþ salts of H3PW12O40. Markedly less attention has been directed to catalytic properties of insoluble silver salts of H3PW12O40 [15]. The reasons of selecting Ag3PW12O40 are (1) the insolube and microporous heterogeneous catalyst. (2) the mild Lewis acidity of Ag and favorable redox potential leading to Ag-based catalysts to be likely the first choice of reagent for many organic reactions [16e18]. These two factors might be suitable for overcoming the drawbacks in HMF production process. In the present study, we reported the catalytic conversion of fructose and glucose into 5-HMF in a two-phase reactor system with high feed concentration by Ag3PW12O40. The experimental results demonstrated that Ag3PW12O40 exhibited higher selectivity (93.8%) and excellent efficiency (relatively high yield 77.7%) for fructose conversion with high concentration (30 wt%). In addition, Ag3PW12O40 was active for the conversion of glucose into HMF. Therefore, this solid acid Ag3PW12O40 was shown to
be an efficient catalyst for the conversion of monosaccharides into HMF with high selectivity. Moreover, this catalyst is tolerant to high concentration feedstock and the catalyst can be recycled.
2.
Experimental
2.1.
Measurements
All solvents and chemicals used were obtained from commercial suppliers. H3PW12O40 was prepared following the ref [19]. Elemental analyses were carried out using a Leeman Plasma Spec (I) ICP-ES. IR spectra (4000 - 500 cm-1) were recorded in KBr discs on a Nicolet Magna 560 IR spectrometer. Morphology of sample and the energy dispersive X-ray analysis (EDS) of W and Ag elements in asprepared and reused samples were studied by means of Field Emission Scanning Electron Microscope XL30 ESEM FEG at 25 k. The specific surface areas of samples were calculated from the nitrogen adsorptionedesorption isotherms at 77 K in Micromeritics ASAP 2010 instrument. Prior to the measurements, the samples were preheated and degassed, under vacuum at 473 K for 2 h. XRD patterns of the sample were collected on Japan Rigaku Dmax 2000 X-ray diffractometer with Cu Ka radiation (l ¼ 0.154178 nm). XPS were recorded on an Escalab-MK II photoelectronic spectrometer with Al Ka (1200 eV). Centrifugation was performed on T4C Centrifuge (4000 rpm, 5 min, Beijing Yiyao). The consumption of saccharide was measured by measuring its concentration in the aqueous phase by Highperformance liquid chromatography (HPLC, Shimadzu LC-10A) was conducted on a system equipped with a refractive index detector. The saccharide was quantified by HPLC with Aminex HPX-87H column, using MilliQ water (pH ¼ 2) as the mobile phase at a flow rate of 0.6 ml min1and column temperature of 303 K. And also the concentration of HMF in the aqueous phase was determined by HPLC with ION-300H column using a 2: 8 v/v methanol: water (pH ¼ 2) gradient at a flow rate of 0.7 ml/min and a column temperature of 303 K using a UV detector. The concentration of HMF in the organic phase was determined by gas chromatography (Agilent 6890) equipped with Agilent 19091J-416 capillary column and flame ionization detector. Qualitative analysis was performed by Gas chromatography-mass spectrometry (GC-MS, Agilent 5970) with scan parameters: low mass 20.0, high mass 700.0, and threshold 150. The analytical error in HMF yield and substrates conversion was evaluated to be in the 1% range.
2.2.
Preparation of catalyst
1 mmol of H3PW12O40$6H2O was dissolved in 10 mL distilled water. The above solution was added dropwisely into 10 mL, 1M of AgNO3 aqueous solution with vigorous stirring at room temperature. The mixture was continuously stirred for about 30 min, the precipitate was aged for 12 h at room temperature. The white precipitate was filtrated and washed with water several times until no Agþ being detected. The sample was dried for 12 h at 200 C. The resulting Ag3PW12O40 was obtained with yield 50%. Anal. Calc. for Ag3PW12O40: W, 69.20; P, 0.97; Ag, 10.05%. Found: W, 69.18; P, 0.96; Ag, 10.05%.
b i o m a s s a n d b i o e n e r g y 3 5 ( 2 0 1 1 ) 2 6 5 9 e2 6 6 5
2.3.
Dehydration reaction
In a typical reaction protocol, 2400 mg of sugar was mixed with 8 g of water and 18 mL MIBK in a Parr reactor (size, 50 mL) and 0.080 g of Ag3PW12O40 was added as a catalyst. The experiments were carried out at 120 C for 60 min with electrically-heated oil bath (It needs 15 min from room temperature to 120 C). The reaction was stopped by rapidly cooling the reactor in an ice bath at 0 C for 30 min. All experiments were repeated six times. After the reaction, the mixture was stood still to form two layers: the upper layer-organic phase consisted of HMF and MIBK; and the lower layer-water phase contained HMF, unreacted sugar and catalyst. These two layers were separated by sedimentation. The catalyst Ag3PW12O40 was centrifuged and then was rinsed with water three times to wash out unreacted sugar. The catalyst was calcinated at 80 C for 3 h to remove water. The total amount of Ag3PW12O40 leaching through six runs of the reaction was detected using a Leeman Plasma Spec (I) ICP-ES.
3.
Results and discussion
3.1.
Characterization of catalyst
From the result of elemental analysis, the contents of the material are W, 69.20; P, 0.97; Ag, 10.05%, respectively, and it can be seen that the ratio of W: P: Ag is 12:1:3 corresponding to the formula of Ag3PW12O40. The FT-IR spectrum of Ag3PW12O40 was investigated and result was given in Supplementary Information (S.I.) section (see Fig. S1a). The peaks in the IR spectral range 600e1100 cm1 corresponding to [PW12O40]3- structural vibrations could be distinguished easily at 1080, 983, 888 and 815 cm1, which were attributed to the asymmetry vibrations P-Oa (internal oxygen connecting P and W), W-Od (terminal oxygen bonding to W atom), W-Ob (edge-sharing oxygen connecting W) and W-Oc (corner- sharing oxygen connecting W3O13 units), respectively. The surface acidity of Ag3PW12O40 was determined by pyridine adsorption infrared spectroscopy (see S.I. section, Fig. S1b). It can be seen that the IR spectrum exhibited strong bands at 1446 cm1 and 1602 cm1, which were attributed to the Lewis acidity. Meanwhile, the peaks at 1638 and 1542 cm1 were attributed to Brønsted acidity. The peaks at 1490 cm1 was characteristic of pyridine adsorbed on both Lewis and BrØnsted acid sites. The result showed that Ag3PW12O40 exhibited both Brønsted acidity and Lewis acidity. To identify the structure of Ag3PW12O40, XRD spectroscopy was performed and result was given in S.I. section (see Fig. S2). In comparison with tetragonal H3PW12O40 phase (Fig. S2a) (JCPDS no.75-2125), there are no reflexes originating from crystalline H3PW12O40, whereas the new ones assigned to crytalline phase of Ag3PW12O40 salt (see Fig. S2b) can be seen [17]. Thus, there is only one crystalline phase e Ag3PW12O40 in the silver salt. This observation is in accordance with previously reported data [17]. The morphology of Ag3PW12O40 was observed by Scanning electron micrograph (SEM). The SEM image in Fig. S3 showed
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that the sample displayed well shaped crystalline particles with average diameters 8 mm. The porous property of Ag3PW12O40 was analyzed by nitrogen sorption analysis. As seen in S.I. section (see Fig. S4), the adsorption isotherm exhibited Type II isotherm: a steep gas uptake at low relative pressure (P/P0 < 0.2) and a flat course in the intermediate section, thus reflecting the microporous nature of the polymer networks. The hysteresis was not observed upon desorption which is usually found for microporous networks [20]. Pore size was about 0.86789 nm and the surface area of Ag3PW12O40 was 5.08 m2/g.
3.2.
Catalytic activity
3.2.1.
Effect of solvent
In the present work, the dehydration of sugar into HMF by heterogeneous HPAs catalyst was explored in a water-methylisobutylketone (MIBK) biphasic system. According to the literature [21], MIBK is a good solvent that could suppress unwanted side reactions for fructose dehydration in water using acid catalysts, and could extract more HMF into organic phase with good partitioning of HMF compared to other solvents. So MIBK and water biphasic system was used to evaluate the catalytic activity of Ag3PW12O40. The influence of amount of MIBK on conversion and selectivity was given in Fig. 1. It can be seen that the HMF selectivity was low in pure water due to the side products. The selectivity increased significantly using MIBK and water biphasic system. As the amount of MIBK in biphasic system increased, the conversion and HMF selectivity increased. The fructose conversion and HMF selectivity reached maximum values of 82.8% and 93.8%, respectively, at volume ratio of 1:2.25 of water to MIBK. Increasing volume ratio to 1:3 did not increase the conversion and selectivity significantly. So the volume ratio of water to MIBK is 1:2.25 in the whole experiment whatever fructose or glucose was used as feedstock.
3.2.2.
Effect of the catalyst
The performance of acid catalysts was examined through the degradation of sugar (Table 1 and Table 2) under the reaction
Fig. 1 e Effect of MIBK on the dehydration of fructose Reaction conditions: 2400 mg of fructose in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL, 80 mg of Ag3PW12O40, 120 C, 60 min.
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Table 1 e The influence of the catalyst usage on conversion of fructose and HMF selectivity. a Time (min) Catalyst (g)
Rb
60 120 60 120 30 60 90 120 60
7.5
0.32 0.16
15
0.08
30
no
Conversion (%) Yield (%) HMForg (mg/mL) HMFaq (mg/mL) Levulinic acid (%) TOF c (103) 94.7 100 84.3 100 73.4 82.8 92.6 94.1 46.9
0.4 0.5 0.6 0.6 0.5 0.3 0.5 0.2 0.4
75.4 70.2 74.6 68.4 59.7 77.7 77.8 74.9 33.2
0.3 0.4 0.3 0.2 0.5 0.4 0.6 0.2 0.3
57.0 52.8 58.8 53.4 47.9 63.4 61.6 59.7
0.2 0.4 0.4 0.3 0.6 0.2 0.2 0.7
30.1 28.6 22.1 23.4 17.5 20.4 24.5 23.0
0.4 0.4 0.2 0.2 0.3 0.1 0.8 0.2
11.3 13.5 7.7 8.4 1.2 1.6 3.9 7.9
0.2 0.3 0.2 0.4 0.1 0.2 0.3 0.2
31.19 14.60 61.27 28.49 198.81 127.22 86.27 62.46
0.3 0.2 0.3 0.1 0.4 0.1 0.2 0.6
a Reaction conditions, T ¼ 120 C, water: MIBK ¼ 1:2.25. b R is the ratio of the fructose usage (g) to the weight of catalyst (g). c Turnover frequency for HMF on the catalyst in units of mol g1h1.
conditions, such as water to MIBK volume ratio 1:2.25, 300 mg/ mL of sugar, and a certain amount of catalyst at 120 C for certain time. Without any catalyst, fructose could be dehydrated with 46.9% conversion effect and 33.2% yield, respectively, and the dehydration of fructose could be promoted by using solid catalyst and be enhanced with increasing the usages of Ag3PW12O40. For instance, the fructose conversion and HMF yield could reach 92.6 and 77.8% using 0.01 g catalyst per 1 mL unit volume. As the usage of catalyst increased, the selectivity decreased due to the decomposition being accelerated at high usage of catalyst (0.320 g). This decrease was attributed to the availability of acidic sites that favored the rehydration of HMF to levulinic acid (11.3%). The best value of TOF was 198.81 at 0.080 g of catalyst with 30 min reaction time and R value of 30. Therefore, the usage of 0.080 g was chosen as an appropriate catalyst concentration in subsequent experiments. The influence of catalyst usage on glucose’s dehydration was given in Table 2. It can be seen the same influence of usage on glucose dehydration. The usage of 0.32 g was chosen as an appropriate catalyst concentration in subsequent experiments. It can be seen that it needs higher temperature, higher amount of catalyst and longer time to convert glucose into HMF than the dehydration of fructose. The comparison of the different catalysts was given in Fig. 2. By pure Brønsted acid HCl (0.075 mmol, corresponding to the same effective acid concentration as 0.025 mmol of
H3PW12O40) and H3PW12O40 catalysts, higher conversion of hexose and lower selectivity could be achieved under the same reaction conditions for 60 min. This result was attributed to their strong acidity, which may favor the subsequent rehydration of HMF to levulinic acid and formic acid, which was also observed on other literature [22]. By Lewis acid AgNO3, low conversion and low yield were obtained, showing that this kind of Lewis acid catalyst can not give the high conversion of sugar. Cs3PW12O40 owned no Brønsted and Lewis acidity leading to low catalytic activity. By Ag3PW12O40, high conversion and high selectivity were obtained. Exchanging of Hþ with Agþ can modify the acidity strength of H3PW12O40 and introduce some Lewis acidity into the molecules. The acidity capacity of Ag3PW12O40 was also measured by titration by n-butylamine with the indicator anthraquinone (pKa ¼ 8.2), and the high value of 3.01 mmol/g acid sites was detected. And from the result of pyridine adsorption infrared spectroscopy, Ag3PW12O40 exhibited both Brønsted and Lewis acidity, which may be suitable for the dehydration of fructose into HMF and lead to high selectivity for HMF. Therefore, the higher catalytic activity of Ag3PW12O40 was attributed to the synergistic effect of Lewis acid sites and BrØnsted acid sites. It is reported that the selective conversion of glucose into HMF is still a challenge [23], and some Lewis acid catalyzed transformations in ionic liquids were reported [24,25]. The effect of Lewis acid is thought to involve the coordination of mononuclear metal with glucose leading to the isomerization
Table 2 e The influence of the catalyst dosage on conversion of glucose and HMF selectivity. a Time (h) 1 2 3 4 1 2 3 4
Catalyst (g) 0.16
0.32
Rb 15
7.5
Conversion (%)
yield (%)
HMForg (mg/ml)
16.8 0.6 27.9 0.4 34.3 0.2 45.7 0.4 54.9 0.3 68.2 0.5 74.9 0.2 89.5 0.2
8.4 0.3 13.8 0.5 26.5 0.4 36.4 0.3 42.2 0.7 54.4 0.4 60.3 0.6 76.3 0.5
5.28 0.4 10.2 0.6 19.6 0.5 23.7 0.4 28.9 0.5 40.3 0.3 44.8 0.9 58.3 0.7
a Reaction conditions, T ¼ 130 C, water: MIBK ¼ 1:2.25. b R is the ratio of the glucose usage (g) to the weight of catalyst (g). c Turnover frequency for HMF on the catalyst in units of mol g1h1.
HMFaq (mg/ml) 5.85 5.93 11.6 23.1 23.6 23.4 25.8 28.9
0.3 0.5 0.2 0.1 0.6 0.3 0.4 0.3
Levulinic acid (%) 0.5 0.1 1.3 0.1 2.5 0.2 3.9 0.1 0.8 0.2 1.9 0.3 4.6 0.5 6.1 0.4
TOF c (103) 6.98 5.71 10.08 7.62 17.54 11.27 8.41 7.94
0.3 0.2 0.4 0.1 0.5 0.2 0.5 0.6
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Fig. 2 e Dehydration of sugar by different catalysts Reaction conditions: 2400 mg of sugar in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL 0.025 mmol of catalyst, 120 C, 60 min for fructose and 0.1 mmol of catalyst, 130 C, 4 h for glucose, respectively.
of glucose to fructose [26]. Here, we selected Ag3PW12O40 as Lewis acid catalyst to achieve the dehydration of glucose to HMF. From Fig. 2, it can be seen that Ag3PW12O40 exhibited high catalytic activity compared to Brønsted acids HCl (0.3 mmol, corresponding to the same effective acid concentration as 0.1 mmol of H3PW12O40) and H3PW12O40. AgNO3, a pure moderated Lewis acid, was not efficient for the dehydration of glucose into HMF. Therefore, the higher catalytic activity of Ag3PW12O40 was attributed to the synergistic effect of Lewis acid sites and BrØnsted acid sites. The highest activity of Ag3PW12O40 is attributed to accumulation of substrate on the catalyst. The IR spectra of Ag3PW12O40 and its adsorbed fructose and glucose confirm the absorption of substrate on the part of the catalyst (Fig. S5). Compared to the spectrum of Ag3PW12O40, the vibration bands shift to a higher frequency, indicating that some interaction occurs between the O atom from substrate and the terminal oxygen atom from the HPA molecule.
3.2.3.
with high concentration into HMF, while a high selectivity of 87.4% was obtained at 50 wt% initial concentration under the same reaction conditions.
3.2.4.
Effect of temperature
Dehydration of fructose is normally performed above the boiling point of the water. We studied the dehydration of sugar at 100 Ce130 C in order to determine the effect of the reaction temperature on the formation of HMF (Table 3). It can be seen that the reaction temperature had a large effect on the dehydration of fructose into HMF. The 54.3% conversion was obtained at a temperature of 100 C. Meanwhile, the fructose conversion reached about 100% at 130 C within the same time. Therefore, the temperature played a positive role on the
Effect of feedstock’ concentration
Under the reaction conditions as 8 mL of water, 16 mL of MIBK, 0.080 g of Ag3PW12O40, at 120 C for 60 min, we investigated the influence of initial concentrations including 10 wt%, 30 wt% and 50 wt% (Fig. 3) on dehydration of fructose. It can be seen that the conversion and the HMF selectivity were affected by different initial concentrations of fructose. The conversion of fructose increased but the HMF selectivity decreased with the increase of feedstock’s concentrations. The highest HMF selectivity reached 93.8% at initial concentration 30 wt%. The loss of selectivity for 10 wt% of fructose was attributed to the more catalytic sites compared to the high concentration’s fructose, resulting in more by-products with 14.52% yield of levulinic acid. Compared to 30 wt% feedstock, the 6% reduction of selectivity for 50 wt% fructose might be attributed to the higher fructose concentration leading to higher rates of condensation reactions. This result was in agreement with literature reports [7]. Nevertheless, the Ag3PW12O40 heterogeneous catalyst was efficient for the transformation of fructose
Fig. 3 e The influence of the initial concentration of fructose on the conversion and selectivity Reaction conditions: certain amount of fructose in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL, 80 mg of Ag3PW12O40, 120 C, 60 min.
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Table 3 e The influence of temperature on the yield and selectivity of HMF. Temperature ( C)
Yield (%) (from fructose a)
Selectivity (%) (from fructose)
54.3 0.6 71.2 0.4 73.3 0.7 77.7 0.4 87.6 0.5
100 110 115 120 130
76.8 93.9 94.6 93.8 87.9
Yield (%) (from glucose b)
0.2 0.8 0.5 0.6 0.3
11.5 30.7 45.8 52.5 76.3
0.5 0.2 0.5 0.5 0.5
Selectivity (%) (from glucose) 53.8 55.4 57.3 60.3 85.3
0.8 0.6 0.4 0.3 0.3
a Reaction condition: 2400 mg of fructose, 0.08 g of catalyst, water: MIBK ¼ 1:2.25 for 60 min. b Reaction condition: 2400 mg of glucose, 0.32 g of catalyst, water: MIBK ¼ 1:2.25 for 4 h.
conversion of fructose. As for HMF selectivity, it improved as the temperature increased from 100 C to 115 C, and could reach highest 94.6% at 115 C. However, higher temperatures (130 C) gave rise to by-products and unknown insoluble byproducts leading to lower HMF selectivity. From the point of economic view and selectivity of HMF, 120 C was selected as the reaction temperature. The influence of temperature on glucose’s conversion showed the same variety. In addition, longer reaction time and higher temperature were needed for the dehydration of glucose.
3.2.5.
Reusability of the catalyst
The Ag3PW12O40 catalyst was easily separated from the production mixture because, at the end of the reaction, the catalyst decanted to the bottom of the reactor. It could be used one more time after washing with water. The recycled reaction procedure retained high activity throughout the conversion of fructose into HMF. To test the leaching of the Ag3PW12O40 catalyst [27], the catalyst was filtered after reacting for 30 min (ca. 73.4% fructose conversion) and the filtrate was allowed to react over further 30 min under the same reaction conditions. 74.1% conversion of fructose was obtained, which showed a little leaching of Ag3PW12O40 into the mixture. The total amount of Ag3PW12O40 leaching through six runs of the reaction reached 5.1% of the starting amount of Ag3PW12O40 (Fig. 4). The IR peaks of recycled
Ag3PW12O40 at 1078, 981, 886 and 805 cm1 basically correspond with the IR spectrum of its parent PW12. Therefore, the original frameworks of HPA are not destroyed after reaction. Therefore, this solid acid catalyst was stable and can be reused up to at least six reaction cycles.
4.
Conclusion
A heteropolyacid salt Ag3PW12O40 has been developed for the selective conversion of fructose and glucose into HMF. The yield HMF of 77.7% and selectivity of 93.8% were obtained within 60 min at 120 C in the dehydration of fructose. It is also important that Ag3PW12O40 is suitable for the dehydration of glucose into HMF with high yield of 76.3% under mild conditions. Moreover, this catalyst is tolerant to high concentration feedstock and can be recycled.
Acknowledgments This work was supported by the National Natural Science Foundation of China (20871026). And it was supported by analysis and testing foundation of Northeast Normal University and the major projects of Jilin Provincial Science and Technology Department.
Appendix. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.biombioe.2011.03.004.
references
Fig. 4 e The catalyst activity in six reaction cycles Reaction conditions: 2400 mg of fructose in a Parr reactor (size, 50 mL), the total volume of biphasic system 26 mL, 80 mg of Ag3PW12O40, 120 C, 60 min.
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