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Redox behavior and oxidation catalysis of HnXW12O40 (X = Co2 +, B3 +, Si4 +, and P5 +) Keggin heteropolyacid catalysts
Catalysis Communications 43 (2014) 155–158
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Redox behavior and oxidation catalysis of HnXW12O40 (X=Co2 +, B3 +, Si4 +, and P5 +) Keggin heteropolyacid catalysts Jung Ho Choi, Tae Hun Kang, Ji Hwan Song, Yongju Bang, In Kyu Song ⁎ School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-Gu, Seoul 151-744, South Korea
a r t i c l e
i n f o
Article history: Received 22 July 2013 Received in revised form 12 September 2013 Accepted 16 September 2013 Available online 8 October 2013 Keywords: Heteropolyacid Redox behavior Reduction potential Oxidation of benzaldehyde
a b s t r a c t Redox behavior and oxidation catalysis of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) Keggin heteropolyacid catalysts were investigated. Successful formation of HnXW12O40 catalysts was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Reduction potentials of HnXW12O40 catalysts were determined by electrochemical measurements. First electron reduction potential of HnXW12O40 catalysts decreased with increasing overall negative charge of heteropolyanion. HnXW12O40 catalysts were then applied to the liquidphase oxidation of benzaldehyde to benzoic acid. Yield for benzoic acid increased with increasing first electron reduction potential. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Heteropolyacids (HPAs) are early transition metal–oxygen anion clusters that exhibit a wide range of molecular structures and compositions. Because of their structural and chemical versatility, HPAs have found successful applications in electrochemistry [1], medicine [2], material science [3], and catalysis [4]. Among various HPA structural classes, Keggin HPAs have been widely employed as catalysts because of their outstanding structural stability, acid–base property, and unique redox property. Comprehensive understanding about redox behavior of HPAs is very important in designing HPAs as an oxidation catalyst [5,6]. One of the most conventional methods to investigate the redox behavior of HPA catalysts is to determine the reduction potential (oxidizing ability) of HPAs by electrochemical measurement in solution. However, electrochemical measurement is strongly influenced by a number of experimental conditions. Electrochemical measurement under consistent experimental conditions only gives comparable results, and therefore, direct comparison of results in literatures is not a simple task. It is obvious that there is a need for systematic investigations about redox properties of HPAs using electrochemical measurements to provide a basis for designing HPAs as an oxidation catalyst. Nevertheless, only a few electrochemical investigations have been made and limited information is currently available. In this work, redox behavior and oxidation catalysis of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts were investigated. Successful formation of HnXW12O40 HPA catalysts was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Reduction ⁎ Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.09.015
potentials of HnXW12O40 catalysts were determined by electrochemical measurements. Reduction potential was determined under consistent environments to provide a systematic basis to estimate the redox behavior of heteroatom-substituted HPA catalysts. A correlation between first electron reduction potential and overall negative charge of heteropolyanion was established. HnXW12O40 catalysts were then applied to the homogeneous liquid-phase oxidation of benzaldehyde to benzoic acid. The catalytic activity was also correlated with the redox behavior (first electron reduction potential) of HnXW12O40 catalysts. Through this work, we have successfully demonstrated that first electron reduction potential determined by electrochemical measurements was well correlated with catalytic performance in the oxidation of benzaldehyde. 2. Experimental 2.1. Catalyst preparation HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts were prepared according to the methods in the literatures [7–10]. Assynthesized alkali-metal salts were converted into acid forms by etherate method using sulfuric acid and ethyl ether. Successful formation of XW12On− 40 heteropolyanion frameworks was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy using a Nicolet 6700 (Nicolet) spectrometer. Each HPA catalyst was thermally treated at 200 °C for 3 h prior to characterization and catalytic reaction. 2.2. Electrochemical measurements Cyclovoltammograms of HnXW12O40 (X=Co2+, B3+, Si4+, and P5+) HPA catalysts were measured by a conventional three-electrode system.
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J.H. Choi et al. / Catalysis Communications 43 (2014) 155–158
Electrochemical measurements were carried out using a Potentiostat/ Galvanostat (Eco Chemie, Autolab 302N) instrument. Glassy carbon electrode with diameter of 3.0 mm and platinum rod were used as a working electrode and a counter electrode, respectively. Saturated calomel electrode (KCl saturated) was used as a reference. Each HPA catalyst (1 mM) was dissolved in 0.5 M Na2SO4 aqueous solution and purged for 20 min under nitrogen flow (20 ml/min) prior to electrochemical measurement. Cyclovoltammograms were acquired at scan rates of 25, 50, 75, 100, 125, and 150 mV/s under the atmospheric conditions. 2.3. Homogeneous liquid-phase oxidation of benzaldehyde Homogeneous liquid-phase oxidation of benzaldehyde was carried out over HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts in a glass reactor equipped with a reflux condenser. Benzaldehyde (10 ml, 0.098 mol) was charged into a reactor. Acetonitrile (100 ml) was successively charged into the reactor as a solvent. Reactor was heated to 80 °C and then each HPA catalyst (0.3 g) was added into the reactor. Liquid-phase oxidation of benzaldehyde was carried out for 1 h under oxygen bubbling (30 ml/min). Reaction products were sampled and analyzed after 1 h-reaction using a gas chromatograph (YL6100 GC, Younglin) equipped with a flame ionization detector. DB-5 (Agilent, 60 m × 0.32 mm) capillary column was used for product separation. Conversion of benzaldehyde and selectivity for benzoic acid (a main product) were calculated on the basis of carbon balance. Yield for benzoic acid was calculated by multiplying conversion of benzaldehyde and selectivity for benzoic acid. 3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the DRIFT spectra of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts. The spectrum of H6CoW12O40 catalyst exhibited the characteristic bands at 960, 889, and 761 cm−1, which were assigned to W = Od, W–Ob–W, and W–Oc–W stretching vibrations, respectively. The characteristic band for Co–Oa stretching vibration (445 cm−1) was not observed in the scan range. The spectrum of H5BW12O40 catalyst exhibited the characteristic bands at 955, 904, 814, and 753 cm−1, which were assigned to W = Od, W–Oa, W–Ob–W,
H6CoW12O40 *ν(Co-Oa): 445 cm-1
Transmittance (A. U.)
H5BW12O40 W=Od
W-Ob-W
*ν(B-Oa): 1410, 504 cm-1
W-Oc-W
B-Oa H4SiW12O40
W=Od
W-Oa
W-Ob-W W-Oc-W
Si-Oa H3PW12O40
W-Ob-W
W=Od Si-Oa
P-Oa W=Od
1200
1100
1000
W-Ob-W
900
W-Oc-W
W-Oc-W
800
700
Wavenumber (cm-1) Fig. 1. DRIFT spectra of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts. Oa: oxygen connected to the central heteroatom, Ob: corner-shared oxygen, Oc: edge-shared oxygen and Od: terminal oxygen.
and W–Oc–W stretching vibrations, respectively. There were three characteristic bands for B–Oa stretching vibrations at 1410, 1004, and 504 cm−1 [11]. The band at 1004 cm−1 corresponding to B–Oa stretching vibration appeared in the scan range. The spectrum of H4SiW12O40 catalyst exhibited the characteristic bands at 979, 885, and 787 cm−1, which were assigned to W = Od, W–Ob–W, and W–Oc–W stretching vibrations, respectively. The bands for Si–Oa were observed at 1020 and 924 cm−1 [12]. The spectrum of H3PW12O40 catalyst exhibited the characteristic bands at 984, 888, and 810 cm−1, which were assigned to W = Od, W– Ob–W, and W–Oc–W stretching vibrations, respectively. The band for P– Oa was observed at 1080 cm−1. Unlike other polyatom-substituted heteropolyanions or other structural classes of heteropolyanions, X–O bands of HnXW12O40 HPA catalysts did not show any band splitting, indicating the existence of Td local symmetry of a central XO4 unit that is a characteristic feature of α-Keggin structure. All positions of characteristic IR bands of HnXW12O40 HPA catalysts were in good agreement with the results of previous works [11–13], indicating successful formation of heteropolyanion frameworks. 3.2. Electrochemical measurement Fig. 2 shows the cyclovoltammograms of HnXW12O40 (X=Co2+, B3+, Si , and P5+) HPA catalysts at a scan rate of 100 mV/s. Electrochemical measurements were performed under consistent experimental conditions to provide comparable results. All HnXW12O40 HPA catalysts exhibited well-defined reversible and stepwise one- or two-electron redox transitions during the electrochemical measurements. Five couples of one-electron redox waves were observed for H3PW12O40 catalyst. Three couples of redox waves were observed for H4SiW12O40 catalyst with electron ratio of 1:1:2. Two couples of redox waves with electron ratio of 2:1 were observed for H5BW12O40 catalyst. Three couples of one-electron redox waves were observed for H6CoW12O40 catalyst. Overall shapes of cyclovoltammograms were maintained at a scan rate up to 150 mV/s, and only peak current increased monotonically. Each inset figure in Fig. 2 shows the dependence of peak current on square root of scan rate (plotted for peak current of first electron reduction). Peak current was proportional to square root of scan rate, indicating that this process was a diffusion-controlled process which was expressed by the Randles– Sevcik equation [11]. It has been suggested that reducibility (oxidizing ability) of HPAs can be determined by the highest potential in the cyclovoltammograms [14]. Therefore, first highest reduction potential was taken as representative reduction potential to estimate reducibility of HPAs in this work. First electron reduction potential increased in the order of H6CoW12O40 (−0.554V)bH5BW12O40 (−0.511V)bH4SiW12O40 (−0.227 V) b H3PW12O40 (−0.020 V). Fig. 3 shows the correlation between first electron reduction potential and overall negative charge of heteropolyanion in a series of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts. The correlation shows that first electron reduction potential decreased with increasing overall negative charge of heteropolyanion. Among HPA catalysts tested, H3PW12O40 catalyst with the smallest overall negative charge showed the highest reduction potential. It is well known that added electrons to α-Keggin heteropolytungstates were delocalized over tungsten sites. This indicates that each redox transition during the electrochemical measurements corresponded to the electron transfer in tungsten sites, and nature of central atom would not directly but indirectly affect the reduction potential [15]. Variation of reduction potentials can be rationalized by taking into account of its enthalpy, entropy, and electronic properties. However, enthalpy and entropy terms show small variation because overall size of heteropolyanion is nearly identical regardless of the nature of central atom in a series of α-Keggin heteropolytungstates, indicating that variation in reduction potentials is mainly due to electronic properties [16]. Previous density functional theory (DFT) calculations [17,18] have reported the effect of heteroatom charge on electronic structure using a clathrate model, in which tetrahedral XOn− units are encapsulated 4 4+
J.H. Choi et al. / Catalysis Communications 43 (2014) 155–158
b 3.0 2.5 2.0
Current (A. U.)
1.5 1.0 0.5 4
6
8
10
12
Square root of scan rate
Peak current (10-6 A)
3.5
Peak current (10-6 A)
Current (A. U.)
a
157
12 11 10 9 8 7 6 5 4
6
8
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12
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-0.227 V
-0.020 V
H3PW12O40 0.6
0.4
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0
-0.2
-0.4
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H4SiW12O40
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9
-0.511 V
7 6 5 4
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d Peak current (10-6 A)
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19 18 17 16 15 14 13 12 11 10 9
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H5BW12O40 0.6
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0
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0
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Potential (volts vs. SCE)
Fig. 2. Cyclovoltammograms of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts at a scan rate of 100 mV/s. Each inset figure shows the dependence of peak current on square root of scan rate (plotted for peak current of first electron reduction).
within neutral W12O40 cage (XOn− 4 @W12O40). It has been demonstrated that Keggin heteropolyanion with larger negative charge resulted in poor reducibility of heteropolyanion due to smaller capacity to accept electrons, in good agreement with the results of this work, indicating that first electron reduction potential determined by electrochemical measurements could be utilized as a promising parameter to
Overall negative charge of heteropolyanion
-3 H3PW12O40 H4SiW12O40
-4
H5BW12O40
-5
H6CoW12O40
-6
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
0.1
First electron reduction potential (volts vs. SCE) Fig. 3. Correlation between first electron reduction potential and overall negative charge of heteropolyanion in a series of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts.
estimate reducibility of HPAs in a series of HnXW12O40 (X = Co2 +, B3 +, Si4 +, and P5 +) HPA catalysts.
3.3. Oxidation catalysis of HnXW12O40 HPA catalysts Homogeneous liquid-phase oxidation of benzaldehyde with molecular oxygen was carried out over HnXW12O40 HPA catalysts in order to track their oxidation catalysis. In the catalytic reaction, benzoic acid was formed as a main oxidation product. Yield for benzoic acid obtained after 1 h-reaction increased in the order of H6CoW12O40 (13.2%) b H5BW12O40 (15.6%) b H4SiW12O40 (16.2%) b H3PW12O40 (19.7%). Fig. 4 shows the first electron reduction potential plotted as a function of yield for benzoic acid in the homogeneous liquidphase oxidation of benzaldehyde after 1 h-reaction at 80 °C. As shown in Fig. 4, yield for benzoic acid increased with increasing first electron reduction potential of HnXW12O40 HPA catalysts. This indicates that reduction potential can be used as a correlating parameter to track the oxidation catalysis of HnXW12O40 HPA catalysts in the liquid-phase oxidation of benzaldehyde. Previous works [13,19] on the TPR measurements of HPAs demonstrated that both 12-molybdophosphoric acid and 12-tungstophosphoric acid were reduced via two reduction steps; H2 → 2H+ + 2e− → H2O. It was also revealed that reducibility of both HPAs determined by TPR measurements was well correlated with catalytic activity for oxidation of methacrolein, which was in good agreement with the results of this work. Although heteropolytungstates are less active than heteropolymolybdates in the oxidation reactions, it is quite meaningful to study heteropolytungstate for fundamental understanding
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First electron reduction potential (volts vs. SCE)
0.1 H3PW12O40
0 -0.1 H4SiW12O40
-0.2 -0.3 -0.4 -0.5
Acknowledgments H5BW12O40
H6CoW12O40
This work was supported by Mid-career Researcher Program of National Research Foundation (NRF) grant funded by the Korea government (MEST) (No. 2012-R1A2A4A01001146).
-0.6 13
14
15
16
17
18
19
20
Yield for benzoic acid (%) Fig. 4. First electron reduction potential plotted as a function of yield for benzoic acid in the homogeneous liquid-phase oxidation of benzaldehyde after 1 h-reaction at 80 °C.
to elucidate the effect of heteroatom-substitution on redox behaviors of HPAs due to structural diversity of heteropolytungstates. 4. Conclusions Redox behavior and oxidation catalysis of HnXW12O40 (X=Co2+, B3+, Si , and P5+) Keggin HPA catalysts were investigated by electrochemical measurements. Successful formation of HnXW12O40 HPA catalysts was confirmed by DRIFT spectroscopy. Reduction potentials of HnXW12O40 HPA catalysts were determined by electrochemical measurements. First electron reduction potential of HnXW12O40 HPA catalysts decreased with increasing overall negative charge of heteropolyanion. HPA catalyst with larger negative charge resulted in poor reducibility of heteropolyanion due to smaller capacity to accept electrons. The result suggests that first electron reduction potential can be utilized as a 4+
promising parameter to estimate the reducibility (oxidizing power) of HPA catalyst in a series of HnXW12O40 HPA catalysts. HnXW12O40 HPA catalysts were also applied to the homogeneous liquid-phase oxidation of benzaldehyde. It was revealed that yield for benzoic acid increased with increasing first electron reduction potential, indicating that first electron reduction potential can also be used as a correlating parameter to track the oxidation catalysis of HnXW12O40 HPA catalysts in the liquid-phase oxidation of benzaldehyde.
References [1] R. Wloadarczyk, M. Chojak, K. Miecznikowski, A. Kolary, P.J. Kulesza, R. Marassi, J. Power Sources 159 (2006) 802–809. [2] M. Fukuma, Y. Seto, T. Yamase, Antiviral Res. 16 (1991) 327–339. [3] W. Bu, S. Uchida, N. Mizuno, Angew. Chem. Int. Ed. 48 (2009) 8281–8284. [4] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171–198. [5] I.K. Song, H.S. Kim, M.S. Chun, Korean J. Chem. Eng. 20 (2003) 844–849. [6] D.R. Park, J.H. Choi, S. Park, I.K. Song, Appl. Catal. A Gen. 394 (2011) 201–208. [7] L.C.W. Baker, T.P. McCutcheon, J. Am. Chem. Soc. 78 (1956) 4503–4510. [8] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot, Inorg. Chem. 22 (1983) 207–216. [9] E.O. North, J.C. Bailar, F.G. Jonelis, Inorg. Synth. 1 (1939) 129–132. [10] J.C. Bailar, H.S. Booth, M. Grennert, Inorg. Synth. 1 (1939) 132–133. [11] L. Adamczyk, K. Miecznikowski, J. Solid State Electrochem. 17 (2013) 1167–1173. [12] T. Rajkumar, G.R. Rao, Mater. Chem. Phys. 112 (2008) 853–857. [13] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113–252. [14] P. Villabrille, G. Romanelli, L. Gassa, P. Vázquez, C. Cáceres, Appl. Catal. A Gen. 324 (2007) 69–76. [15] B. Keita, Y. Jean, B. Levy, L. Nadjo, R. Contant, New J. Chem. 26 (2002) 1314–1319. [16] M.T. Pope, G.M. Varga Jr., Inorg. Chem. 5 (1966) 1249–1254. [17] X. López, J.A. Fernández, J.M. Poblet, Dalton Trans. (2006) 1162–1167. [18] I.-M. Mbomekallé, X. López, J.M. Poblet, F. Sécheresse, B. Keita, L. Nadjo, Inorg. Chem. 49 (2010) 7001–7006. [19] K. Katamura, T. Nakamura, K. Sakata, M. Misono, Y. Yoneda, Chem. Lett. (1981) 89-82.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Redox behavior and oxidation catalysis of HnXW12O40 (X=Co2 +, B3 +, Si4 +, and P5 +) Keggin heteropolyacid catalysts Jung Ho Choi, Tae Hun Kang, Ji Hwan Song, Yongju Bang, In Kyu Song ⁎ School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, 1 Gwanak-ro, Gwanak-Gu, Seoul 151-744, South Korea
a r t i c l e
i n f o
Article history: Received 22 July 2013 Received in revised form 12 September 2013 Accepted 16 September 2013 Available online 8 October 2013 Keywords: Heteropolyacid Redox behavior Reduction potential Oxidation of benzaldehyde
a b s t r a c t Redox behavior and oxidation catalysis of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) Keggin heteropolyacid catalysts were investigated. Successful formation of HnXW12O40 catalysts was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Reduction potentials of HnXW12O40 catalysts were determined by electrochemical measurements. First electron reduction potential of HnXW12O40 catalysts decreased with increasing overall negative charge of heteropolyanion. HnXW12O40 catalysts were then applied to the liquidphase oxidation of benzaldehyde to benzoic acid. Yield for benzoic acid increased with increasing first electron reduction potential. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Heteropolyacids (HPAs) are early transition metal–oxygen anion clusters that exhibit a wide range of molecular structures and compositions. Because of their structural and chemical versatility, HPAs have found successful applications in electrochemistry [1], medicine [2], material science [3], and catalysis [4]. Among various HPA structural classes, Keggin HPAs have been widely employed as catalysts because of their outstanding structural stability, acid–base property, and unique redox property. Comprehensive understanding about redox behavior of HPAs is very important in designing HPAs as an oxidation catalyst [5,6]. One of the most conventional methods to investigate the redox behavior of HPA catalysts is to determine the reduction potential (oxidizing ability) of HPAs by electrochemical measurement in solution. However, electrochemical measurement is strongly influenced by a number of experimental conditions. Electrochemical measurement under consistent experimental conditions only gives comparable results, and therefore, direct comparison of results in literatures is not a simple task. It is obvious that there is a need for systematic investigations about redox properties of HPAs using electrochemical measurements to provide a basis for designing HPAs as an oxidation catalyst. Nevertheless, only a few electrochemical investigations have been made and limited information is currently available. In this work, redox behavior and oxidation catalysis of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts were investigated. Successful formation of HnXW12O40 HPA catalysts was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy. Reduction ⁎ Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song). 1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.catcom.2013.09.015
potentials of HnXW12O40 catalysts were determined by electrochemical measurements. Reduction potential was determined under consistent environments to provide a systematic basis to estimate the redox behavior of heteroatom-substituted HPA catalysts. A correlation between first electron reduction potential and overall negative charge of heteropolyanion was established. HnXW12O40 catalysts were then applied to the homogeneous liquid-phase oxidation of benzaldehyde to benzoic acid. The catalytic activity was also correlated with the redox behavior (first electron reduction potential) of HnXW12O40 catalysts. Through this work, we have successfully demonstrated that first electron reduction potential determined by electrochemical measurements was well correlated with catalytic performance in the oxidation of benzaldehyde. 2. Experimental 2.1. Catalyst preparation HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts were prepared according to the methods in the literatures [7–10]. Assynthesized alkali-metal salts were converted into acid forms by etherate method using sulfuric acid and ethyl ether. Successful formation of XW12On− 40 heteropolyanion frameworks was confirmed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy using a Nicolet 6700 (Nicolet) spectrometer. Each HPA catalyst was thermally treated at 200 °C for 3 h prior to characterization and catalytic reaction. 2.2. Electrochemical measurements Cyclovoltammograms of HnXW12O40 (X=Co2+, B3+, Si4+, and P5+) HPA catalysts were measured by a conventional three-electrode system.
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Electrochemical measurements were carried out using a Potentiostat/ Galvanostat (Eco Chemie, Autolab 302N) instrument. Glassy carbon electrode with diameter of 3.0 mm and platinum rod were used as a working electrode and a counter electrode, respectively. Saturated calomel electrode (KCl saturated) was used as a reference. Each HPA catalyst (1 mM) was dissolved in 0.5 M Na2SO4 aqueous solution and purged for 20 min under nitrogen flow (20 ml/min) prior to electrochemical measurement. Cyclovoltammograms were acquired at scan rates of 25, 50, 75, 100, 125, and 150 mV/s under the atmospheric conditions. 2.3. Homogeneous liquid-phase oxidation of benzaldehyde Homogeneous liquid-phase oxidation of benzaldehyde was carried out over HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts in a glass reactor equipped with a reflux condenser. Benzaldehyde (10 ml, 0.098 mol) was charged into a reactor. Acetonitrile (100 ml) was successively charged into the reactor as a solvent. Reactor was heated to 80 °C and then each HPA catalyst (0.3 g) was added into the reactor. Liquid-phase oxidation of benzaldehyde was carried out for 1 h under oxygen bubbling (30 ml/min). Reaction products were sampled and analyzed after 1 h-reaction using a gas chromatograph (YL6100 GC, Younglin) equipped with a flame ionization detector. DB-5 (Agilent, 60 m × 0.32 mm) capillary column was used for product separation. Conversion of benzaldehyde and selectivity for benzoic acid (a main product) were calculated on the basis of carbon balance. Yield for benzoic acid was calculated by multiplying conversion of benzaldehyde and selectivity for benzoic acid. 3. Results and discussion 3.1. Catalyst characterization Fig. 1 shows the DRIFT spectra of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts. The spectrum of H6CoW12O40 catalyst exhibited the characteristic bands at 960, 889, and 761 cm−1, which were assigned to W = Od, W–Ob–W, and W–Oc–W stretching vibrations, respectively. The characteristic band for Co–Oa stretching vibration (445 cm−1) was not observed in the scan range. The spectrum of H5BW12O40 catalyst exhibited the characteristic bands at 955, 904, 814, and 753 cm−1, which were assigned to W = Od, W–Oa, W–Ob–W,
H6CoW12O40 *ν(Co-Oa): 445 cm-1
Transmittance (A. U.)
H5BW12O40 W=Od
W-Ob-W
*ν(B-Oa): 1410, 504 cm-1
W-Oc-W
B-Oa H4SiW12O40
W=Od
W-Oa
W-Ob-W W-Oc-W
Si-Oa H3PW12O40
W-Ob-W
W=Od Si-Oa
P-Oa W=Od
1200
1100
1000
W-Ob-W
900
W-Oc-W
W-Oc-W
800
700
Wavenumber (cm-1) Fig. 1. DRIFT spectra of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts. Oa: oxygen connected to the central heteroatom, Ob: corner-shared oxygen, Oc: edge-shared oxygen and Od: terminal oxygen.
and W–Oc–W stretching vibrations, respectively. There were three characteristic bands for B–Oa stretching vibrations at 1410, 1004, and 504 cm−1 [11]. The band at 1004 cm−1 corresponding to B–Oa stretching vibration appeared in the scan range. The spectrum of H4SiW12O40 catalyst exhibited the characteristic bands at 979, 885, and 787 cm−1, which were assigned to W = Od, W–Ob–W, and W–Oc–W stretching vibrations, respectively. The bands for Si–Oa were observed at 1020 and 924 cm−1 [12]. The spectrum of H3PW12O40 catalyst exhibited the characteristic bands at 984, 888, and 810 cm−1, which were assigned to W = Od, W– Ob–W, and W–Oc–W stretching vibrations, respectively. The band for P– Oa was observed at 1080 cm−1. Unlike other polyatom-substituted heteropolyanions or other structural classes of heteropolyanions, X–O bands of HnXW12O40 HPA catalysts did not show any band splitting, indicating the existence of Td local symmetry of a central XO4 unit that is a characteristic feature of α-Keggin structure. All positions of characteristic IR bands of HnXW12O40 HPA catalysts were in good agreement with the results of previous works [11–13], indicating successful formation of heteropolyanion frameworks. 3.2. Electrochemical measurement Fig. 2 shows the cyclovoltammograms of HnXW12O40 (X=Co2+, B3+, Si , and P5+) HPA catalysts at a scan rate of 100 mV/s. Electrochemical measurements were performed under consistent experimental conditions to provide comparable results. All HnXW12O40 HPA catalysts exhibited well-defined reversible and stepwise one- or two-electron redox transitions during the electrochemical measurements. Five couples of one-electron redox waves were observed for H3PW12O40 catalyst. Three couples of redox waves were observed for H4SiW12O40 catalyst with electron ratio of 1:1:2. Two couples of redox waves with electron ratio of 2:1 were observed for H5BW12O40 catalyst. Three couples of one-electron redox waves were observed for H6CoW12O40 catalyst. Overall shapes of cyclovoltammograms were maintained at a scan rate up to 150 mV/s, and only peak current increased monotonically. Each inset figure in Fig. 2 shows the dependence of peak current on square root of scan rate (plotted for peak current of first electron reduction). Peak current was proportional to square root of scan rate, indicating that this process was a diffusion-controlled process which was expressed by the Randles– Sevcik equation [11]. It has been suggested that reducibility (oxidizing ability) of HPAs can be determined by the highest potential in the cyclovoltammograms [14]. Therefore, first highest reduction potential was taken as representative reduction potential to estimate reducibility of HPAs in this work. First electron reduction potential increased in the order of H6CoW12O40 (−0.554V)bH5BW12O40 (−0.511V)bH4SiW12O40 (−0.227 V) b H3PW12O40 (−0.020 V). Fig. 3 shows the correlation between first electron reduction potential and overall negative charge of heteropolyanion in a series of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts. The correlation shows that first electron reduction potential decreased with increasing overall negative charge of heteropolyanion. Among HPA catalysts tested, H3PW12O40 catalyst with the smallest overall negative charge showed the highest reduction potential. It is well known that added electrons to α-Keggin heteropolytungstates were delocalized over tungsten sites. This indicates that each redox transition during the electrochemical measurements corresponded to the electron transfer in tungsten sites, and nature of central atom would not directly but indirectly affect the reduction potential [15]. Variation of reduction potentials can be rationalized by taking into account of its enthalpy, entropy, and electronic properties. However, enthalpy and entropy terms show small variation because overall size of heteropolyanion is nearly identical regardless of the nature of central atom in a series of α-Keggin heteropolytungstates, indicating that variation in reduction potentials is mainly due to electronic properties [16]. Previous density functional theory (DFT) calculations [17,18] have reported the effect of heteroatom charge on electronic structure using a clathrate model, in which tetrahedral XOn− units are encapsulated 4 4+
J.H. Choi et al. / Catalysis Communications 43 (2014) 155–158
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Fig. 2. Cyclovoltammograms of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts at a scan rate of 100 mV/s. Each inset figure shows the dependence of peak current on square root of scan rate (plotted for peak current of first electron reduction).
within neutral W12O40 cage (XOn− 4 @W12O40). It has been demonstrated that Keggin heteropolyanion with larger negative charge resulted in poor reducibility of heteropolyanion due to smaller capacity to accept electrons, in good agreement with the results of this work, indicating that first electron reduction potential determined by electrochemical measurements could be utilized as a promising parameter to
Overall negative charge of heteropolyanion
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First electron reduction potential (volts vs. SCE) Fig. 3. Correlation between first electron reduction potential and overall negative charge of heteropolyanion in a series of HnXW12O40 (X = Co2+, B3+, Si4+, and P5+) HPA catalysts.
estimate reducibility of HPAs in a series of HnXW12O40 (X = Co2 +, B3 +, Si4 +, and P5 +) HPA catalysts.
3.3. Oxidation catalysis of HnXW12O40 HPA catalysts Homogeneous liquid-phase oxidation of benzaldehyde with molecular oxygen was carried out over HnXW12O40 HPA catalysts in order to track their oxidation catalysis. In the catalytic reaction, benzoic acid was formed as a main oxidation product. Yield for benzoic acid obtained after 1 h-reaction increased in the order of H6CoW12O40 (13.2%) b H5BW12O40 (15.6%) b H4SiW12O40 (16.2%) b H3PW12O40 (19.7%). Fig. 4 shows the first electron reduction potential plotted as a function of yield for benzoic acid in the homogeneous liquidphase oxidation of benzaldehyde after 1 h-reaction at 80 °C. As shown in Fig. 4, yield for benzoic acid increased with increasing first electron reduction potential of HnXW12O40 HPA catalysts. This indicates that reduction potential can be used as a correlating parameter to track the oxidation catalysis of HnXW12O40 HPA catalysts in the liquid-phase oxidation of benzaldehyde. Previous works [13,19] on the TPR measurements of HPAs demonstrated that both 12-molybdophosphoric acid and 12-tungstophosphoric acid were reduced via two reduction steps; H2 → 2H+ + 2e− → H2O. It was also revealed that reducibility of both HPAs determined by TPR measurements was well correlated with catalytic activity for oxidation of methacrolein, which was in good agreement with the results of this work. Although heteropolytungstates are less active than heteropolymolybdates in the oxidation reactions, it is quite meaningful to study heteropolytungstate for fundamental understanding
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Acknowledgments H5BW12O40
H6CoW12O40
This work was supported by Mid-career Researcher Program of National Research Foundation (NRF) grant funded by the Korea government (MEST) (No. 2012-R1A2A4A01001146).
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Yield for benzoic acid (%) Fig. 4. First electron reduction potential plotted as a function of yield for benzoic acid in the homogeneous liquid-phase oxidation of benzaldehyde after 1 h-reaction at 80 °C.
to elucidate the effect of heteroatom-substitution on redox behaviors of HPAs due to structural diversity of heteropolytungstates. 4. Conclusions Redox behavior and oxidation catalysis of HnXW12O40 (X=Co2+, B3+, Si , and P5+) Keggin HPA catalysts were investigated by electrochemical measurements. Successful formation of HnXW12O40 HPA catalysts was confirmed by DRIFT spectroscopy. Reduction potentials of HnXW12O40 HPA catalysts were determined by electrochemical measurements. First electron reduction potential of HnXW12O40 HPA catalysts decreased with increasing overall negative charge of heteropolyanion. HPA catalyst with larger negative charge resulted in poor reducibility of heteropolyanion due to smaller capacity to accept electrons. The result suggests that first electron reduction potential can be utilized as a 4+
promising parameter to estimate the reducibility (oxidizing power) of HPA catalyst in a series of HnXW12O40 HPA catalysts. HnXW12O40 HPA catalysts were also applied to the homogeneous liquid-phase oxidation of benzaldehyde. It was revealed that yield for benzoic acid increased with increasing first electron reduction potential, indicating that first electron reduction potential can also be used as a correlating parameter to track the oxidation catalysis of HnXW12O40 HPA catalysts in the liquid-phase oxidation of benzaldehyde.
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