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Copper phosphate nanostructures catalyze dehydration of fructose to 5-hydroxymethylfufural
Catalysis Communications 29 (2012) 96–100
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Copper phosphate nanostructures catalyze dehydration of fructose to 5-hydroxymethylfufural Pongtanawat Khemthong a, Pornlada Daorattanachai a, b, Navadol Laosiripojana b, Kajornsak Faungnawakij a,⁎ a b
Nanomaterials for Energy and Catalysis Laboratory, National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Pathumthani 12120, Thailand The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 15 August 2012 Received in revised form 18 September 2012 Accepted 21 September 2012 Available online 28 September 2012 Keywords: Fructose dehydration 5-Hydroxymethylfurfural Copper phosphate Hot compressed water X-ray absorption fine structure
a b s t r a c t The nanostructured catalysts of copper hydrogen phosphate monohydrate and copper pyrophosphate were developed for catalyzing dehydration of fructose to 5-hydroxymethylfurfural under hot compressed water at 200 °C. As evidenced by X-ray absorption fine structure, X-ray diffraction, SEM, and TEM analyses, the CuHPO4·H2O (as-synthesized) exhibits needle-like nanocrystals, while the samples calcined at 600 °C and 900 °C display α-Cu2P2O7 phases in rod-like nanostructure and irregularly shaped microcrystal, respectively. Each copper is distributed throughout the phosphate network. Among all samples, the Cu2P2O7 catalysts with weak acid strength (+3.3 ≤ H0 ≤ +4.8) was highly active and selective for 5-hydroxymethylfurfural production with a yield of 36% and maximum turnover number, while no metal leaching was observed after the reaction. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The growing attention for chemical production from renewable resources has spread across the globe because of the diminishing reserves and demanding cost of fossil fuels. Fructose can serve as a renewable chemical feedstock because this abundant and inexpensive sugar can be obtained from biomass. Fructose can be used in biorefinery industry, i.e. production of biofuels, biopolymer, organic carbon, and so on. Particularly, biofuel has potential to prevent global warming by decreasing greenhouse gas emission which is generally generated from utilization of fossil fuel/coal [1]. The transformation of fructose into synthetic bio-fuels has been investigated and the process involves the integrated reactions of dehydration, hydrolysis, isomerization, reforming, aldol condensation, hydrogenation and oxidation [2]. More specifically, the furonic products are the chemical platforms, being a key player bridging between carbohydrate chemistry and mineral oil-based industrial chemistry because it can be supplied as a starting material instead of fossil-derived chemicals. Among various furonic products, 5-hydroxymethylfurfural (5-HMF) has been considered a promising platform for producing fine chemical, pharmaceuticals and furan-based polymers. Moreover, the dehydration of fructose into 5-HMF by acid catalysts has been considered a key step for consecutive formation of other chemical platforms and biofuels [3–7]. The formation of fructose can be implemented in acid-catalyzed conditions with FeCl3, CrCl3, HCl, H3PO4 and H2SO4 as homogeneous ⁎ Corresponding author. Tel.: +66 2 564 7100x6638; fax: +66 2 564 6981. E-mail address: [email protected] (K. Faungnawakij). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.09.025
catalysts [2,3,6,8]. Since the homogenous catalysts are generally corrosive, harmful, and non-reusable, various heterogeneous catalysts, including H-zeolites [8], Nb2O5 [9,10], metal sulphate [11], and metal (IV) phosphate [12,13] have been proposed to overcome the drawbacks. The solid catalysts can be applicable in the presence of aqueous, non-aqueous and biphasic systems [2,14]. The dehydration of fructose to 5-HMF in non-aqueous and biphasic solutions provides a high product yield, and suppresses the undesired side reactions [15]. Moreover, these reaction systems are currently obstructed by high cost of manufacturing and environmental issues. The aqueous process is very promising in ecological point of view as a green synthesis of 5-HMF, while the efficiency of this system is not sufficient and need to be improved. The problem with the efficient preparation of pure 5-HMF in one step is still unresolved, while it is difficult to find an effective, economic and easy-to-use mode of the production process. In order to obtain a high yield of 5-HMF without promoting a sequential side reaction or continuous removal of 5-HMF, one can apply a suitable catalyst and design a better reaction condition. It has been reported that metal based-phosphates are solid acid catalyst which can promote the dehydration of fructose. Therefore, in this context, we have reported the use of heterogeneous metal phosphate catalysts, and show their catalytic potential when water was employed under hot compressed water. Also, it is interesting to investigate due to their acid properties which consequently making them encouraging catalysts for acid-catalyzed reactions [16]. There are, however, a few studies on fructose catalytic reaction using copper phosphate. Copper phosphate was synthesized in wet processes by precipitation from solution, followed by calcinations in air at 600
P. Khemthong et al. / Catalysis Communications 29 (2012) 96–100
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and 900 °C. The structure of these materials was further studied with various techniques. Then their catalytic potential for transformation of fructose to furan compounds was investigated under hot compressed water conditions. 2. Experimental 2.1. Catalyst synthesis In a typical synthesis, the synthesis method was modified from the literature described elsewhere [17]. CuCO3 powder (1.2 g) was dissolved in 5 ml of 70% H3PO4, and then the resultant solution was added by 30 ml of acetone under vigorously stirred condition. The precipitated product was filtered and washed with acetone several times. The samples were dried in an oven overnight, and then calcined at 600 and 900 °C. 2.2. Catalyst characterization Crystalline phase compositions were confirmed by X-ray diffractometer (XRD: D8 ADVANCE, Bruker) with Cu Kα radiation at 40 kV and 40 mA. The morphology of the particles was obtained by scanning electron microscope (SEM: JSM-6301F, JEOL) and transmission electron microscope (TEM: JEM-2100, JEOL). The local geometry of Cu was studied by X-ray absorption fine structure (EXAFS) at beamline 8 of the Synchrotron Light Research Institute (Public Organization), Thailand. The absorption spectra were collected in ion chambers, filled with helium gas. The data fitting to model structures was performed using Athena and EXAFSPAK. The Fourier transform was analyzed on k3-weighted EXAFS oscillations in the range of 2.5–10.2 Å −1 (Supplementary Material). Acid strengths of the samples were examined via several indicators including neutral red (pKa = +6.8), methyl red (pKa = +4.8), dimethyl yellow (pKa = +3.3), crystal violet (pKa = +0.8), 4-(phenylazo)diphenylamine (pKa = +0.42), and dicinnamalactone (pKa = −0.3). 2.3. Fructose dehydration to 5-HMF The catalytic testing was performed in 10 ml of stainless steel micro reactor. The reaction was carried out in an aqueous solution consisting of 0.1 g of fructose and 0.01 g of catalysts dissolved in 1 ml of DI water. Next, the reactor was sealed with stainless steel screw cap and purged with nitrogen gas to remove air inside the reactor at pressure of 25 bars. The mixture was allowed to react at 200 °C for 5 min. The 5-HMF and furfural products were analyzed through highperformance liquid chromatographs (HPLC) equipped with a Shodex RSpak KC811 column and a UV detector. The sugar content was determined by gel permeation chromatographs (GPC) equipped with a Shodex Sugar SP810 column and a refractive index (RI) detector. Other plausible by-products were qualitatively analyzed by a gas chromatography-mass spectrometry (GC-MS). The amount of metal leaching in the reaction media after the reaction tests was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). 3. Results and discussion In Fig. 1, the XRD results of as-synthesized and calcined samples shows the presence of two crystalline phases of copper hydrogen phosphate monohydrate (CuHPO4·H2O indexed according to PDF-00-035-0087) and copper pyrophosphate (α-Cu2P2O7 indexed according to PDF-00-021-0880), respectively. Generally, CuHPO4·H2O has a monoclinic structure containing one hydrogen bond as OH group [18] which pointed toward its solid surface. Therefore, Cu2P2O7 has two polymorphic phases including α-Cu2P2O7 (low-temperature) and β-Cu2P2O7 (high-temperature) manifest mono-clinic crystal structures [19]. This material always appears in α-Cu2P2O7 form at room
Fig. 1. XRD patterns of copper phosphates; (a) CuHPO4·H2O, (b) α-Cu2P2O7-600 and (c) α-Cu2P2O7-900.
temperature. Although calcination was above their respective α-to-β phase transition temperature, the crystalline phase after left at ambient temperature is reversed from β-to-α form as indicated by XRD. These results are corresponding to the work reported by Pogorzelec-Glaser and co-workers [20]. It has been shown that nucleation and crystal growth of the phosphate were observed over the sample calcined at 600 °C, suggested by the broadening of the diffraction lines. From 600 to 900 °C the diffraction lines were sharpened with the same peak shapes. We can suggest that the products were aggregation without any phase transformation. This observation is supported by SEM images. In order to obtain further information about their respective average local environment, the structural investigation was further studied by X-ray absorption technique, especially in EXAFS region. This technique was therefore the good method of choice for obtaining information on short range structural of a specific atom type in a material. The initial EXAFS model was taken from parameter obtained from the literatures [6]. The first and second shells were modelled corresponding to Cu–O. A third shell was then added to the model to explain the Cu-P interaction; addition of other shells did not improve the quality of fit. The k-space EXAFS data and Fourier transform of the samples are shown in Fig. 2 and the results from fitting are listed in Table 1. In all EXAFS data analyses, the data show that the first shell surrounding Cu 2+ contains only oxygen atoms at a mean bond distance of 1.94–1.96 Å. The Cu–O distance decreases with increasing calcinations temperature. The observed Cu(–O) coordination numbers are in the range of 3.23–3.69. The second and third correlation shells were found to be associated with oxygen and phosphorus around the central Cu 2+ atom, respectively. These compounds would present a layered structure of Cu2O8 dimers cross-linked by the phosphate tetrahedral which is related to the dittmarite family [21]. Based on the structural analysis of the interionic distances between Cu(II) and O 2− in CuHPO4·H2O and α-Cu2P2O7, we suggest that the CuHPO4·H2O is smaller than α-Cu2P2O7. The EXAFS results also indicate that the copper is distributed throughout the phosphate network and not phase segregated into domains of CuO. We supposed that the existence of phosphate sheet in the structure may induce the Cu(II) center more reactive susceptible to the reaction with fructose regarding on the distortion of centrosymmetric dimmers. The SEM micrograph of as-synthesized CuHPO4·H2O (Fig. 3a) revealed a specified shape as an assembly of needle-like crystals, whose size ranged up to several hundreds of nanometer. The SEM image of the sample calcined at 600 °C (Fig. 3c) also showed the rod-like structure but with higher size of each rod due to its partial
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Fig. 2. FT module Cu K-edge of copper phosphates; (a) CuHPO4·H2O, (b) α-Cu2P2O7-600 and (c) α-Cu2P2O7-900.
sintering. The sample calcined at 900 °C (Fig. 3e) illustrated a re-texturing and coalescence in aggregates of irregularly shaped particles of different size in a micron-size region. TEM images of typical CuHPO4·H2O (Fig. 3b) and α-Cu2P2O7-600 (Fig. 3d) represented that they are compound-crystalline, being condensed to large particles. All samples show unspecified shape. In addition, all phosphate catalysts have acid strength in the same range of +3.3 ≤ H0 ≤ +4.8, indicating their weak acid strength. The catalytic transformation activity for the dehydration of 10% aqueous solution of fructose was consequently studied under hot compressed water and the reaction result is given in Table 2. To the best of our knowledge, this is the first time that these catalysts have been applied for this reaction. All the heterogeneous copper-based catalysts have an ability to dehydrate fructose to 5-HMF in aqueous solution with conversion in a range of 82–94% at reaction time of 5 min. It has been reported that 83.6% conversion is obtained from fructose using anatase TiO2 assisted with microwave irradiation at 200 °C after 5 min [22]. We can suggest that the employment of the copper hydrogen phosphate and pyrophosphate can promote the conversion of fructose via dehydration reaction and exhibit high catalytic activity. Among the investigated metal phosphate samples, the α-Cu2P2O7-900 offered the best performances, in terms of both activity and selectivity, and displaying the highest yields to 5-HMF of 35.8%. Furfural was partially observed as a side product with a yield below 3%. Meanwhile, CuHPO4·H2O and α-Cu2P2O7-600 also exhibited promising performance of 14.3% and 29.8% yield to 5-HMF, respectively, under the same condition. The turnover number (TN) is also used for evaluation of catalyst activity on the reaction. It was found that the α-Cu2P2O7-900 expressed for the best TN at 34.0 mmol 5-HMF/ h·g of catalyst when compared with other solid
Table 1 Structural parameters obtained from the fitting of the EXAFS data. Catalysts
Shell
CNa
R (Å)b
σ2 (Å2)c
E0 (eV)d
CuHPO4·H2O
Cu–O Cu–O Cu–P Cu–O Cu–O Cu–P Cu–O Cu–O Cu–P
3.21 3.66 1.01 3.69 3.29 2.28 3.29 2.71 0.99
1.97 2.94 3.59 1.95 3.03 3.63 1.94 3.02 3.61
0.003 0.005 0.004 0.003 0.005 0.025 0.003 0.001 0.004
10.40
α-Cu2P2O7-600
α-Cu2P2O7-900
The weighted F-factor of all analysis was less than 28. a The coordination number, b The nearest-neighbor distance, c The Debye–Waller factor. d The energy shift.
8.62
1.82
phosphates and homogenous H3PO4 catalyst. It should be noted that, however, the as-synthesized CuHPO4·H2O manifested a lower value of the %yield to 5-HMF and TN than phosphoric acid. At this reaction temperature, most part of the surface Bronsted sites of CuHPO4·H2O can induce the continuous reaction. It is possible that such a reaction can form levulinic and formic acids, yielding a low 5-HMF product. The induced side reactions may stem from the modification of the surface acidity. Therefore, the incorporation of copper into phosphate, rather than merely using phosphate, can further improve catalytic performance, resulting in an increase in the number of effective acid sites. In addition, there is at least a study indicating that copper phosphate can promote the oxidation of fructose [16]. On the other hand, under this condition, the condensed materials α-Cu2P2O7 can effectively promote the dehydration, thereby bringing about the desired product. Thus, two different unique sites of copper phosphates are involved in the two catalytic activities. One belongs to the hydroxyl group or Brønsted site, while the other is potentially a metal center or Lewis acid type. Based on the obtained results from different surface catalyst types, it can be concluded that the surface hydroxyl group was not solely accountable for the catalytic activities. Therefore, the Lewis acid is considered to play an important role in the catalytic reactions. It can be suggested that the Lewis acid sites on α-Cu2P2O7 might be responsible for the substantial improvements of both the catalytic activity and selectivity of the fructose dehydration process. The catalyst provided the appropriate active sites for selective formation of 5-HMF. It is also noteworthy to consider the fact that in the presence of homogeneous Brønsted acid catalyst of H3PO4, the dehydration could result in a 29% 5-HMF yield. The reaction pathway for fructose dehydration with cupric phosphate is proposed (as shown in Scheme 1). According to the literature [6], the beginning of the cyclodehydration reaction involves the protonation of hydroxyl group of fructose into H2O +, resulting in the release of three water molecules. During 5-HMF production, furfural can be produced as a side product. The HMF production occurs in tandem with various side-reactions as dehydration and polymeric products as indicated by the adhered chemical on the surface of catalysts and the dark soluble products for each catalyst. GC-MS analysis confirmed the presence of acids such as levulinic and formic acids, as well as aldehydes in the product solutions. In addition, the copper leaching was negligible (b 1%) as determined by ICP-OES. This result confirmed the stability of the catalyst which solely acted as a heterogonous catalyst. 4. Conclusions We have successfully synthesized CuHPO4·H2O and α-Cu2P2O7 nanostructured catalysts by a facile precipitation. The obtained products
P. Khemthong et al. / Catalysis Communications 29 (2012) 96–100
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Fig. 3. SEM images (left) and TEM images with electron diffraction patterns (right) of CuHPO4·H2O (a, b); α-Cu2P2O7-600 (c, d); α-Cu2P2O7-900 (e).
were CuHPO4·H2O for the as-synthesized sample and α-Cu2P2O7 for the calcined one. The size and shape of products strongly depend on the heat treatment process. Therefore, the morphology of CuHPO4·H2O exhibits
Table 2 Fructose dehydration to 5-HMF carried out in water medium in the presence of copper phosphate-based catalysts (batch experiments). Catalysts
%Conversion
%Yield of 5-HMF
TNa
%Leaching
CuHPO4·H2O α-Cu2P2O7-600 α-Cu2P2O7-900 H3PO4
92.3 94.4 82.2 93.5
14.3 29.8 35.8 29.1
13.6 28.3 34.0 27.7
0.0 0.0 1.9 –
a
TN is Turnover number which is expressed as mmol of 5-HMF/g of catalyst h.
needle-like nanocrystals, while α-Cu2P2O7 samples calcined at 600 °C and 900 °C display rod-like nanostructure and irregularly shaped microcrystal, respectively. Each copper species is fully distributed throughout the phosphate network. All phosphates possess weak acid strength in a range of +3.3 ≤ H0≤ +4.8. Employed as heterogeneous catalysts in the synthesis of 5-HMF from fructose dehydration, the phosphate acid catalysts show a significant improvement with respect to mineral acid H3PO4. The α-Cu2P2O7-900 exhibited superior catalytic activity and %yield to 5-HMF (36%) to CuHPO4·H2O. Their catalytic activities are related to surface properties in the terms of Brønsted sites and Lewis acid sites. Finally, we count on the optimization of reaction condition and on the modification of catalyst structures to further improve the 5-HMF production efficiency.
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O-
O P HO
OH
OH
Cu2+ O-
O
-3H2O O
O HO
+ OH O
OH
HO
-
O
D-Fructose
O O
P
O Cu2+ O-
-3H2O
Furfural
5-HMF
-
O P O
Cu2+ O-
OH O
O
O
OH
+ Polymer Products
Levulinic acid
Formic acid
Scheme 1. Reaction network for fructose dehydration to 5-HMF catalyzed by cupric phosphate under hot compressed water.
Acknowledgements We acknowledged the Synchrotron Light Research Institute (Public Organization), Thailand for XAS measurement, and the Thailand Toray Science Foundation for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2012.09.025. References [1] [2] [3] [4] [5]
G.W. Huber, J.A. Dumesic, Catalysis Today 111 (2006) 119–132. X.T. Yang, Y. Ma, Y. Li, Applied Catalysis A: General 385 (2010) 1–13. T.V. Stein, P.M. Grande, W. Leitner, P.D. de Maria, ChemSusChem 4 (2011) 1592–1594. N. Nikbin, S. Caratzoulas, D.G. Vlachos, ChemCatChem 4 (2012) 504–511. F. Benvenuti, C. Carlini, P. Patrono, A.M.R. Galletti, G. Sbrana, M.A. Massucci, P. Galli, Applied Catalysis A: General 193 (2000) 147–153. [6] F.S. Asghari, H. Yoshida, Carbohydrate Research 34 (2006) 2379–2387.
[7] G.R. Akien, L. Qi, I.T. Horváth, Chemical Communications 48 (2012) 5850–5852. [8] R. Weingarten, G.A. Tompsett, W.C. Conner Jr., G.W. Huber, Journal of Catalysis 279 (2011) 174–182. [9] C. Carlini, M. Giuttari, A.M.R. Galletti, G. Sbrana, T. Armaroli, G. Busca, Applied Catalysis A: General 183 (1999) 295–302. [10] P. Carniti, A. Gervasini, M. Marzo, Catalysis Communications 12 (2011) 1122–1126. [11] X. Qi, H. Guo, L. Li, Industrial and Engineering Chemistry Research 50 (2011) 985–989. [12] G.A. Halliday, R.J. Young Jr., V.V. Grushin, Organic Letters 5 (2003) 2003–2005. [13] X. Qi, M. Watanabe, T.M. Aida, R.L. Smith Jr., Catalysis Communications 10 (2009) 1771–1775. [14] J. Wang, W. Xu, J. Ren, X. Liu, G. Lu, Y. Wang, Green Chemistry 13 (2011) 2678–2681. [15] L. Cottier, G. Descotes, Trends in Heterocyclic Chemistry 2 (1991) 233–248. [16] B. Boonchom, N. Vittayakorn, Materials Letters 64 (2010) 275–277. [17] B. Boonchom, N. Phuvongpha, Materials Letters 63 (2009) 1709–1711. [18] C. Günther, H. Görls, D. Stachel, Acta crystallographica E65 (2009) i85. [19] B.E. Robertson, A. Calvo, Canadian Journal of Chemistry 46 (1968) 605–612. [20] K. Pogorzelec-Glaser, A. Pietraszko, B. Hilczer, M. Połomska, Phase Transitions 79 (2006) 535–544. [21] A. Pujana, J.L. Pizarro, L. Lezama, A. Goñi, M.I. Arriortua, T. Rojo, Journal of Materials Chemistry 4 (1998) 1055–1060. [22] X. Tong, Y. Ma, Y. Li, Applied Catalysis A: General 385 (2010) 1–13.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Copper phosphate nanostructures catalyze dehydration of fructose to 5-hydroxymethylfufural Pongtanawat Khemthong a, Pornlada Daorattanachai a, b, Navadol Laosiripojana b, Kajornsak Faungnawakij a,⁎ a b
Nanomaterials for Energy and Catalysis Laboratory, National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency, Pathumthani 12120, Thailand The Joint Graduate School of Energy and Environment, King Mongkut's University of Technology Thonburi, Bangkok 10140, Thailand
a r t i c l e
i n f o
Article history: Received 15 August 2012 Received in revised form 18 September 2012 Accepted 21 September 2012 Available online 28 September 2012 Keywords: Fructose dehydration 5-Hydroxymethylfurfural Copper phosphate Hot compressed water X-ray absorption fine structure
a b s t r a c t The nanostructured catalysts of copper hydrogen phosphate monohydrate and copper pyrophosphate were developed for catalyzing dehydration of fructose to 5-hydroxymethylfurfural under hot compressed water at 200 °C. As evidenced by X-ray absorption fine structure, X-ray diffraction, SEM, and TEM analyses, the CuHPO4·H2O (as-synthesized) exhibits needle-like nanocrystals, while the samples calcined at 600 °C and 900 °C display α-Cu2P2O7 phases in rod-like nanostructure and irregularly shaped microcrystal, respectively. Each copper is distributed throughout the phosphate network. Among all samples, the Cu2P2O7 catalysts with weak acid strength (+3.3 ≤ H0 ≤ +4.8) was highly active and selective for 5-hydroxymethylfurfural production with a yield of 36% and maximum turnover number, while no metal leaching was observed after the reaction. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The growing attention for chemical production from renewable resources has spread across the globe because of the diminishing reserves and demanding cost of fossil fuels. Fructose can serve as a renewable chemical feedstock because this abundant and inexpensive sugar can be obtained from biomass. Fructose can be used in biorefinery industry, i.e. production of biofuels, biopolymer, organic carbon, and so on. Particularly, biofuel has potential to prevent global warming by decreasing greenhouse gas emission which is generally generated from utilization of fossil fuel/coal [1]. The transformation of fructose into synthetic bio-fuels has been investigated and the process involves the integrated reactions of dehydration, hydrolysis, isomerization, reforming, aldol condensation, hydrogenation and oxidation [2]. More specifically, the furonic products are the chemical platforms, being a key player bridging between carbohydrate chemistry and mineral oil-based industrial chemistry because it can be supplied as a starting material instead of fossil-derived chemicals. Among various furonic products, 5-hydroxymethylfurfural (5-HMF) has been considered a promising platform for producing fine chemical, pharmaceuticals and furan-based polymers. Moreover, the dehydration of fructose into 5-HMF by acid catalysts has been considered a key step for consecutive formation of other chemical platforms and biofuels [3–7]. The formation of fructose can be implemented in acid-catalyzed conditions with FeCl3, CrCl3, HCl, H3PO4 and H2SO4 as homogeneous ⁎ Corresponding author. Tel.: +66 2 564 7100x6638; fax: +66 2 564 6981. E-mail address: [email protected] (K. Faungnawakij). 1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2012.09.025
catalysts [2,3,6,8]. Since the homogenous catalysts are generally corrosive, harmful, and non-reusable, various heterogeneous catalysts, including H-zeolites [8], Nb2O5 [9,10], metal sulphate [11], and metal (IV) phosphate [12,13] have been proposed to overcome the drawbacks. The solid catalysts can be applicable in the presence of aqueous, non-aqueous and biphasic systems [2,14]. The dehydration of fructose to 5-HMF in non-aqueous and biphasic solutions provides a high product yield, and suppresses the undesired side reactions [15]. Moreover, these reaction systems are currently obstructed by high cost of manufacturing and environmental issues. The aqueous process is very promising in ecological point of view as a green synthesis of 5-HMF, while the efficiency of this system is not sufficient and need to be improved. The problem with the efficient preparation of pure 5-HMF in one step is still unresolved, while it is difficult to find an effective, economic and easy-to-use mode of the production process. In order to obtain a high yield of 5-HMF without promoting a sequential side reaction or continuous removal of 5-HMF, one can apply a suitable catalyst and design a better reaction condition. It has been reported that metal based-phosphates are solid acid catalyst which can promote the dehydration of fructose. Therefore, in this context, we have reported the use of heterogeneous metal phosphate catalysts, and show their catalytic potential when water was employed under hot compressed water. Also, it is interesting to investigate due to their acid properties which consequently making them encouraging catalysts for acid-catalyzed reactions [16]. There are, however, a few studies on fructose catalytic reaction using copper phosphate. Copper phosphate was synthesized in wet processes by precipitation from solution, followed by calcinations in air at 600
P. Khemthong et al. / Catalysis Communications 29 (2012) 96–100
97
and 900 °C. The structure of these materials was further studied with various techniques. Then their catalytic potential for transformation of fructose to furan compounds was investigated under hot compressed water conditions. 2. Experimental 2.1. Catalyst synthesis In a typical synthesis, the synthesis method was modified from the literature described elsewhere [17]. CuCO3 powder (1.2 g) was dissolved in 5 ml of 70% H3PO4, and then the resultant solution was added by 30 ml of acetone under vigorously stirred condition. The precipitated product was filtered and washed with acetone several times. The samples were dried in an oven overnight, and then calcined at 600 and 900 °C. 2.2. Catalyst characterization Crystalline phase compositions were confirmed by X-ray diffractometer (XRD: D8 ADVANCE, Bruker) with Cu Kα radiation at 40 kV and 40 mA. The morphology of the particles was obtained by scanning electron microscope (SEM: JSM-6301F, JEOL) and transmission electron microscope (TEM: JEM-2100, JEOL). The local geometry of Cu was studied by X-ray absorption fine structure (EXAFS) at beamline 8 of the Synchrotron Light Research Institute (Public Organization), Thailand. The absorption spectra were collected in ion chambers, filled with helium gas. The data fitting to model structures was performed using Athena and EXAFSPAK. The Fourier transform was analyzed on k3-weighted EXAFS oscillations in the range of 2.5–10.2 Å −1 (Supplementary Material). Acid strengths of the samples were examined via several indicators including neutral red (pKa = +6.8), methyl red (pKa = +4.8), dimethyl yellow (pKa = +3.3), crystal violet (pKa = +0.8), 4-(phenylazo)diphenylamine (pKa = +0.42), and dicinnamalactone (pKa = −0.3). 2.3. Fructose dehydration to 5-HMF The catalytic testing was performed in 10 ml of stainless steel micro reactor. The reaction was carried out in an aqueous solution consisting of 0.1 g of fructose and 0.01 g of catalysts dissolved in 1 ml of DI water. Next, the reactor was sealed with stainless steel screw cap and purged with nitrogen gas to remove air inside the reactor at pressure of 25 bars. The mixture was allowed to react at 200 °C for 5 min. The 5-HMF and furfural products were analyzed through highperformance liquid chromatographs (HPLC) equipped with a Shodex RSpak KC811 column and a UV detector. The sugar content was determined by gel permeation chromatographs (GPC) equipped with a Shodex Sugar SP810 column and a refractive index (RI) detector. Other plausible by-products were qualitatively analyzed by a gas chromatography-mass spectrometry (GC-MS). The amount of metal leaching in the reaction media after the reaction tests was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES). 3. Results and discussion In Fig. 1, the XRD results of as-synthesized and calcined samples shows the presence of two crystalline phases of copper hydrogen phosphate monohydrate (CuHPO4·H2O indexed according to PDF-00-035-0087) and copper pyrophosphate (α-Cu2P2O7 indexed according to PDF-00-021-0880), respectively. Generally, CuHPO4·H2O has a monoclinic structure containing one hydrogen bond as OH group [18] which pointed toward its solid surface. Therefore, Cu2P2O7 has two polymorphic phases including α-Cu2P2O7 (low-temperature) and β-Cu2P2O7 (high-temperature) manifest mono-clinic crystal structures [19]. This material always appears in α-Cu2P2O7 form at room
Fig. 1. XRD patterns of copper phosphates; (a) CuHPO4·H2O, (b) α-Cu2P2O7-600 and (c) α-Cu2P2O7-900.
temperature. Although calcination was above their respective α-to-β phase transition temperature, the crystalline phase after left at ambient temperature is reversed from β-to-α form as indicated by XRD. These results are corresponding to the work reported by Pogorzelec-Glaser and co-workers [20]. It has been shown that nucleation and crystal growth of the phosphate were observed over the sample calcined at 600 °C, suggested by the broadening of the diffraction lines. From 600 to 900 °C the diffraction lines were sharpened with the same peak shapes. We can suggest that the products were aggregation without any phase transformation. This observation is supported by SEM images. In order to obtain further information about their respective average local environment, the structural investigation was further studied by X-ray absorption technique, especially in EXAFS region. This technique was therefore the good method of choice for obtaining information on short range structural of a specific atom type in a material. The initial EXAFS model was taken from parameter obtained from the literatures [6]. The first and second shells were modelled corresponding to Cu–O. A third shell was then added to the model to explain the Cu-P interaction; addition of other shells did not improve the quality of fit. The k-space EXAFS data and Fourier transform of the samples are shown in Fig. 2 and the results from fitting are listed in Table 1. In all EXAFS data analyses, the data show that the first shell surrounding Cu 2+ contains only oxygen atoms at a mean bond distance of 1.94–1.96 Å. The Cu–O distance decreases with increasing calcinations temperature. The observed Cu(–O) coordination numbers are in the range of 3.23–3.69. The second and third correlation shells were found to be associated with oxygen and phosphorus around the central Cu 2+ atom, respectively. These compounds would present a layered structure of Cu2O8 dimers cross-linked by the phosphate tetrahedral which is related to the dittmarite family [21]. Based on the structural analysis of the interionic distances between Cu(II) and O 2− in CuHPO4·H2O and α-Cu2P2O7, we suggest that the CuHPO4·H2O is smaller than α-Cu2P2O7. The EXAFS results also indicate that the copper is distributed throughout the phosphate network and not phase segregated into domains of CuO. We supposed that the existence of phosphate sheet in the structure may induce the Cu(II) center more reactive susceptible to the reaction with fructose regarding on the distortion of centrosymmetric dimmers. The SEM micrograph of as-synthesized CuHPO4·H2O (Fig. 3a) revealed a specified shape as an assembly of needle-like crystals, whose size ranged up to several hundreds of nanometer. The SEM image of the sample calcined at 600 °C (Fig. 3c) also showed the rod-like structure but with higher size of each rod due to its partial
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Fig. 2. FT module Cu K-edge of copper phosphates; (a) CuHPO4·H2O, (b) α-Cu2P2O7-600 and (c) α-Cu2P2O7-900.
sintering. The sample calcined at 900 °C (Fig. 3e) illustrated a re-texturing and coalescence in aggregates of irregularly shaped particles of different size in a micron-size region. TEM images of typical CuHPO4·H2O (Fig. 3b) and α-Cu2P2O7-600 (Fig. 3d) represented that they are compound-crystalline, being condensed to large particles. All samples show unspecified shape. In addition, all phosphate catalysts have acid strength in the same range of +3.3 ≤ H0 ≤ +4.8, indicating their weak acid strength. The catalytic transformation activity for the dehydration of 10% aqueous solution of fructose was consequently studied under hot compressed water and the reaction result is given in Table 2. To the best of our knowledge, this is the first time that these catalysts have been applied for this reaction. All the heterogeneous copper-based catalysts have an ability to dehydrate fructose to 5-HMF in aqueous solution with conversion in a range of 82–94% at reaction time of 5 min. It has been reported that 83.6% conversion is obtained from fructose using anatase TiO2 assisted with microwave irradiation at 200 °C after 5 min [22]. We can suggest that the employment of the copper hydrogen phosphate and pyrophosphate can promote the conversion of fructose via dehydration reaction and exhibit high catalytic activity. Among the investigated metal phosphate samples, the α-Cu2P2O7-900 offered the best performances, in terms of both activity and selectivity, and displaying the highest yields to 5-HMF of 35.8%. Furfural was partially observed as a side product with a yield below 3%. Meanwhile, CuHPO4·H2O and α-Cu2P2O7-600 also exhibited promising performance of 14.3% and 29.8% yield to 5-HMF, respectively, under the same condition. The turnover number (TN) is also used for evaluation of catalyst activity on the reaction. It was found that the α-Cu2P2O7-900 expressed for the best TN at 34.0 mmol 5-HMF/ h·g of catalyst when compared with other solid
Table 1 Structural parameters obtained from the fitting of the EXAFS data. Catalysts
Shell
CNa
R (Å)b
σ2 (Å2)c
E0 (eV)d
CuHPO4·H2O
Cu–O Cu–O Cu–P Cu–O Cu–O Cu–P Cu–O Cu–O Cu–P
3.21 3.66 1.01 3.69 3.29 2.28 3.29 2.71 0.99
1.97 2.94 3.59 1.95 3.03 3.63 1.94 3.02 3.61
0.003 0.005 0.004 0.003 0.005 0.025 0.003 0.001 0.004
10.40
α-Cu2P2O7-600
α-Cu2P2O7-900
The weighted F-factor of all analysis was less than 28. a The coordination number, b The nearest-neighbor distance, c The Debye–Waller factor. d The energy shift.
8.62
1.82
phosphates and homogenous H3PO4 catalyst. It should be noted that, however, the as-synthesized CuHPO4·H2O manifested a lower value of the %yield to 5-HMF and TN than phosphoric acid. At this reaction temperature, most part of the surface Bronsted sites of CuHPO4·H2O can induce the continuous reaction. It is possible that such a reaction can form levulinic and formic acids, yielding a low 5-HMF product. The induced side reactions may stem from the modification of the surface acidity. Therefore, the incorporation of copper into phosphate, rather than merely using phosphate, can further improve catalytic performance, resulting in an increase in the number of effective acid sites. In addition, there is at least a study indicating that copper phosphate can promote the oxidation of fructose [16]. On the other hand, under this condition, the condensed materials α-Cu2P2O7 can effectively promote the dehydration, thereby bringing about the desired product. Thus, two different unique sites of copper phosphates are involved in the two catalytic activities. One belongs to the hydroxyl group or Brønsted site, while the other is potentially a metal center or Lewis acid type. Based on the obtained results from different surface catalyst types, it can be concluded that the surface hydroxyl group was not solely accountable for the catalytic activities. Therefore, the Lewis acid is considered to play an important role in the catalytic reactions. It can be suggested that the Lewis acid sites on α-Cu2P2O7 might be responsible for the substantial improvements of both the catalytic activity and selectivity of the fructose dehydration process. The catalyst provided the appropriate active sites for selective formation of 5-HMF. It is also noteworthy to consider the fact that in the presence of homogeneous Brønsted acid catalyst of H3PO4, the dehydration could result in a 29% 5-HMF yield. The reaction pathway for fructose dehydration with cupric phosphate is proposed (as shown in Scheme 1). According to the literature [6], the beginning of the cyclodehydration reaction involves the protonation of hydroxyl group of fructose into H2O +, resulting in the release of three water molecules. During 5-HMF production, furfural can be produced as a side product. The HMF production occurs in tandem with various side-reactions as dehydration and polymeric products as indicated by the adhered chemical on the surface of catalysts and the dark soluble products for each catalyst. GC-MS analysis confirmed the presence of acids such as levulinic and formic acids, as well as aldehydes in the product solutions. In addition, the copper leaching was negligible (b 1%) as determined by ICP-OES. This result confirmed the stability of the catalyst which solely acted as a heterogonous catalyst. 4. Conclusions We have successfully synthesized CuHPO4·H2O and α-Cu2P2O7 nanostructured catalysts by a facile precipitation. The obtained products
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Fig. 3. SEM images (left) and TEM images with electron diffraction patterns (right) of CuHPO4·H2O (a, b); α-Cu2P2O7-600 (c, d); α-Cu2P2O7-900 (e).
were CuHPO4·H2O for the as-synthesized sample and α-Cu2P2O7 for the calcined one. The size and shape of products strongly depend on the heat treatment process. Therefore, the morphology of CuHPO4·H2O exhibits
Table 2 Fructose dehydration to 5-HMF carried out in water medium in the presence of copper phosphate-based catalysts (batch experiments). Catalysts
%Conversion
%Yield of 5-HMF
TNa
%Leaching
CuHPO4·H2O α-Cu2P2O7-600 α-Cu2P2O7-900 H3PO4
92.3 94.4 82.2 93.5
14.3 29.8 35.8 29.1
13.6 28.3 34.0 27.7
0.0 0.0 1.9 –
a
TN is Turnover number which is expressed as mmol of 5-HMF/g of catalyst h.
needle-like nanocrystals, while α-Cu2P2O7 samples calcined at 600 °C and 900 °C display rod-like nanostructure and irregularly shaped microcrystal, respectively. Each copper species is fully distributed throughout the phosphate network. All phosphates possess weak acid strength in a range of +3.3 ≤ H0≤ +4.8. Employed as heterogeneous catalysts in the synthesis of 5-HMF from fructose dehydration, the phosphate acid catalysts show a significant improvement with respect to mineral acid H3PO4. The α-Cu2P2O7-900 exhibited superior catalytic activity and %yield to 5-HMF (36%) to CuHPO4·H2O. Their catalytic activities are related to surface properties in the terms of Brønsted sites and Lewis acid sites. Finally, we count on the optimization of reaction condition and on the modification of catalyst structures to further improve the 5-HMF production efficiency.
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O-
O P HO
OH
OH
Cu2+ O-
O
-3H2O O
O HO
+ OH O
OH
HO
-
O
D-Fructose
O O
P
O Cu2+ O-
-3H2O
Furfural
5-HMF
-
O P O
Cu2+ O-
OH O
O
O
OH
+ Polymer Products
Levulinic acid
Formic acid
Scheme 1. Reaction network for fructose dehydration to 5-HMF catalyzed by cupric phosphate under hot compressed water.
Acknowledgements We acknowledged the Synchrotron Light Research Institute (Public Organization), Thailand for XAS measurement, and the Thailand Toray Science Foundation for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2012.09.025. References [1] [2] [3] [4] [5]
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