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Reduction potential, UV–visible absorption edge energy, and oxidation catalysis of niobium-containing H3+xPW12−xNbxO40 Keggin and H6+xP2W18−xNbxO62 Wells-Dawson heteropolyacid catalysts
Applied Catalysis A: General 394 (2011) 201–208
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Reduction potential, UV–visible absorption edge energy, and oxidation catalysis of niobium-containing H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson heteropolyacid catalysts Dong Ryul Park, Jung Ho Choi, Sunyoung Park, In Kyu Song ∗ School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
a r t i c l e
i n f o
Article history: Received 8 November 2010 Received in revised form 23 December 2010 Accepted 31 December 2010 Available online 8 January 2011 Keywords: Heteropolyacid Niobium Reduction potential Absorption edge energy Oxidation catalysis
a b s t r a c t Niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson heteropolyacids (HPAs) were prepared in this work to explore their redox properties and oxidation catalysis. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy, respectively. The trend of reduction potential is well consistent with the trend of absorption edge energy with respect to niobium substitution in both series of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts. Absorption edge energy of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts shifted to lower value with increasing reduction potential of the HPA catalysts, regardless of the identity of HPA catalysts; an HPA catalyst with higher reduction potential exhibited lower absorption edge energy. In order to probe oxidation catalysis of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts, vapor-phase benzyl alcohol oxidation was carried out as a model reaction. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heteropolyacids (HPAs) are early transition metal-oxygen anion clusters that exhibit a wide range of molecular sizes, compositions, and architectures [1–3]. One of the great advantages of HPA catalysts is that their catalytic properties can be tuned in a systematic way by changing the identity of counter-cation, central heteroatom, and framework polyatom [4–6]. Among various HPA structural classes, Keggin HPAs [7] have been widely employed as acid-base and oxidation catalysts in commercial processes [8–10]. Some of these reactions include hydration of propylene [11], nbutene [12], and isobutene [13], polymerization of tetrahydrofuran [14], oxidation of methacrolein [15], and Wacker and related olefin oxidations [16]. Recently, however, Wells-Dawson HPAs [17] have also attracted considerable attention as promising catalysts due to their excellent catalytic performance for several reactions [18–20]. Nonetheless, much progress has not been made on the catalytic performance of Wells-Dawson HPAs, and only limited information on the catalytic properties of Wells-Dawson HPAs is currently available.
∗ Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.12.042
The catalytic redox property of HPAs has attracted much attention for several oxidation reactions [21–23]. HPAs have been investigated as oxidation catalysts to make useful chemicals directly from hydrocarbon raw materials, including oxidation of propane to acrylic acid [24–26], and oxidation of isobutane to methacroelin and methacrylic acid [27,28]. Therefore, fundamental understanding of reduction potential (oxidizing power) of HPA catalysts is of great importance in designing HPA catalysts for oxidation reactions. Several theoretical and instrumental methods have been employed to determine the reduction potential of HPAs. For example, quantum chemical studies have attempted to elucidate the reduction potentials of selected HPAs [29,30]. Another method is to determine the reduction potential of HPA catalysts from negative differential resistance (NDR) peak voltages in tunneling spectra measured with a scanning tunneling microscopy (STM) [31]. However, the most conventional technique to determine the reduction potential of HPAs is an electrochemical measurement in solution [32]. In the electrochemical measurement, the reduction potentials of HPA catalysts depend on the experimental conditions such as pH, composition of electrolyte solution, and identity of electrode [33,34]. Therefore, the measurement of reduction potential by a consistent method under consistent conditions is required, because direct comparison of reduction potentials of HPA catalysts from literature data is not a simple task [35].
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Another promising approach to probe the reduction potentials of HPA catalysts is to measure the absorption edge energies from UV–visible spectra of the HPAs [33]. Absorption edge energy in the UV–visible spectrum of HPA catalyst measures the energy required for electron transfer from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) [36]. The HOMO energy is not greatly affected by changes in the HPA framework, while the LUMO energy is greatly affected by framework polyatom substitution [37]. The band gap energy between the HOMO and the LUMO of an HPA catalyst corresponds to the reduction potential of the HPA catalyst. Thus, the absorption edge energy of an HPA catalyst measured by UV–visible spectroscopy reflects the reduction potential of the HPA catalyst, as reported in the previous works [38,39]. On the other hand, it has been reported that niobium-containing HPA showed the excellent catalytic oxidation performance due to its high reduction potential [39]. Therefore, a systematic investigation on the redox property and oxidation catalysis of niobium-containing H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs would be worthwhile. In this work, niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) WellsDawson HPAs were prepared. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy in solution, respectively. Vapor-phase benzyl alcohol oxidation was carried out as a model reaction to probe the oxidation catalysis of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts. Correlations between redox properties (reduction potential and absorption edge energy) and catalytic oxidation activity of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts were then established.
2. Experimental 2.1. Preparation of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs Niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were prepared using Na2 WO4 ·2H2 O (Junsei Chem.), phosphoric acid (Sigma–Aldrich), acetic acid (Samchun Chem.), NbCl5 (Sigma–Aldrich), oxalic acid (Sigma–Aldrich), KCl (Junsei Chem.), diethyl ether (Samchun Chem.), and HCl (Sigma–Aldrich). For example, typical procedures for the preparation of H9 P2 W15 Nb3 O62 Wells-Dawson HPA are as follows. K6 P2 W18 O62 Wells-Dawson HPA was prepared according to the method in the literature [40]. 20 g of K6 P2 W18 O62 HPA was then dissolved in 50 ml of distilled water. Upon slowly adding 15 g of NaClO4 into the solution containing K6 P2 W18 O62 , a white precipitate of KClO4 was immediately formed. After removing the precipitate by filtration, 1 M Na2 CO3 solution was added dropwise into the resulting solution. A white precipitate of Na12 P2 W15 O56 lacunary HPA was obtained by filtering the precipitate at pH 9. In order to incorporate niobium into Wells-Dawson HPA structure, 7.0 g of NbCl5 was dissolved in 650 ml of 5% H2 O2 solution and pH of the solution was adjusted to 1.4 using 1 M NaOH solution. 36 g of Na12 P2 W15 O56 lacunary HPA was then added into the solution containing Nb precursor with vigorous stirring at 80 ◦ C. The mixed solution was further maintained at 80 ◦ C with stirring for 2 h. After the solution was cooled to room temperature, 100 g of KCl was added into the solution. The precipitate was filtered and successively washed with distilled water, ether, and ethanol to yield K9 P2 W15 Nb3 O62 . In order to transform potassium-containing HPA into proton-containing HPA, K9 P2 W15 Nb3 O62 was dissolved in
HCl solution and it was extracted with diethyl ether. The etherate was maintained at 50 ◦ C to obtain a crude solid product. The whitecolored solid was washed with distilled water, and then it was dried in a convection oven at 60 ◦ C overnight. These washing and drying processes were repeated several times to obtain H9 P2 W15 Nb3 O62 Wells-Dawson HPA. H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs were also prepared according to the similar method as H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. For example, typical procedures for the preparation of H6 PW9 Nb3 O40 Keggin HPA catalyst are as follows. 120 g of Na2 WO4 ·2H2 O was dissolved in 150 ml of distilled water. 3 ml of phosphoric acid and 20 ml of acetic acid were successively added to the solution containing tungsten precursor. The precipitate was filtered to obtain white-colored Na8 HPW9 O34 . The methods for incorporation of niobium into Keggin HPA structure were similar to those for H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs. 6.5 g of NbCl5 was dissolved in 650 ml of 5% H2 O2 solution and pH of the solution was adjusted to 1.4 using 1 M NaOH solution. 24 g of Na8 HPW9 O34 lacunary HPA was then added into the solution containing Nb precursor with vigorous stirring at 50 ◦ C. The mixed solution was further maintained at 50 ◦ C with stirring for 2 h. After the solution was cooled to room temperature, 50 g of KCl was added into the solution. The precipitate was filtered and successively washed with distilled water, ether, and ethanol to yield K6 PW9 Nb3 O40 . K6 PW9 Nb3 O40 was dissolved in HCl solution and it was extracted with diethyl ether. The etherate was maintained at 50 ◦ C to obtain a crude solid product. The white-colored solid was washed with distilled water, and then it was dried in a convection oven at 60 ◦ C overnight. These washing and drying processes were repeated several times to obtain H6 PW9 Nb3 O40 Keggin HPA. All H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were thermally treated at 300 ◦ C for 2 h in a stream of nitrogen (30 ml/min) prior to characterization and catalytic reaction. In this work, H3+x PW12−x Nbx O40 Keggin HPAs with x = 0, 1, 2, and 3 were denoted as K-Nb0 , K-Nb1 , K-Nb2 , and K-Nb3 , respectively. H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs with x = 0, 1, 2, and 3 were denoted as WD-Nb0 , WD-Nb1 , WD-Nb2 , and WD-Nb3 , respectively. For example, WD-Nb1 represents the Wells-Dawson HPA catalyst with mono-niobium substitution (H7 P2 W17 Nb1 O62 ). 2.2. Characterization H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were characterized by FT-IR (Nicolet, Nicolet 6700), 31 P NMR (Bruker, AVANCE 600), and ICP-AES (Shimadzu, ICPS-1000IV) analyses to confirm the successful formation of HPAs. 31 P NMR measurement was performed using D2 O and H3 PO4 as a solvent and as an external reference, respectively. Crystalline phases of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were investigated by XRD measurements (Rigaku, D-MAX2500-PC). Surface areas of the HPA catalysts were measured using a BET apparatus (Micromeritics, ASAP 2010). 2.3. Reduction potential and UV–visible spectroscopy Reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts were measured by an electrochemical method in solution. The electrochemical measurements were made using a Potentiostat/Galvanostat (Eco Chemie, Autolab 302N) instrument with a computer-controlled cyclovoltametry system. The working electrode was Pt disk with an electrode diameter of 0.2 mm. A Pt rod and Ag/AgCl (NaCl saturated) were used as a counter electrode and a reference electrode, respectively. To obtain cyclovoltagrams,
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Fig. 1. FT-IR spectra of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs.
1 mM of each HPA catalyst dissolved in 0.5 M Na2 SO4 aqueous electrolyte solution (10 ml) was prepared. The sample solutions were purged with nitrogen (50 ml/min) for 3 min and then maintained for 1 min for stabilization prior to the cyclovoltametry measurements. Cyclovoltagrams were obtained at a scan rate of 10 mV/s. UV–visible spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) HPA catalysts in solution were measured at room temperature with a Lambda-35 spectrometer (PerkinElmer). 1 mM of each HPA catalyst dissolved in distilled water was used for UV–visible spectroscopy measurement. The Kubelka-Munk function (F(R∞ )) [41] was used to convert reflectance measurements into equivalent absorption spectra using the reflectance of BaSO4 as a reference, and to obtain absorption edge energies directly from the [F(R∞ )·h]1/2 curves. 2.4. Vapor-phase benzyl alcohol oxidation Vapor-phase benzyl alcohol oxidation over niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) HPA catalysts was carried out in a continuous flow fixed-bed reactor at atmospheric pressure. Each catalyst (0.3 g) was charged into a tubular quartz reactor, and was pretreated with a mixed stream of nitrogen (10 ml/min) and oxygen (10 ml/min) at 320 ◦ C for 1 h. Benzyl alcohol (2.90 × 10−3 mol/h) was sufficiently vaporized by passing through a pre-heating zone and continuously fed into the reactor together with oxygen and nitrogen carrier. Feed composition (molar ratio) was fixed at benzyl alcohol (0.24):oxygen (1.0):nitrogen (1.0). Catalytic reaction was carried out at 300 ◦ C for 5 h. Reaction products were periodically sampled and analyzed with a gas chromatograph (HP 5890II, FID) equipped with a Supelco VO COLTM column (60 m (L) × 0.25 mm (D)). Conversion of benzyl alcohol and selectivity for product were calculated on the basis of carbon balance. Yield for benzaldehyde was calculated by multiplying conversion of benzyl alcohol and selectivity for benzaldehyde. 3. Results and discussion 3.1. Formation of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs Successful formation of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) WellsDawson HPAs was confirmed by FT-IR analyses. Fig. 1 shows the FT-IR spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson
HPAs. The formation of heteropolyanions could be identified by four characteristic IR bands in the range of 700–1200 cm−1 (P–O, polyatom = O, inter-octahedral polyatom-O-polyatom, and intraoctahedral polyatom-O-polyatom bands) [42]. These four characteristic IR bands were clearly observed in all H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs, indicating successful formation of heteropolyanions in both families of Keggin and Wells-Dawson HPAs. Successful formation of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) WellsDawson HPAs was further confirmed by 31 P NMR measurements. Fig. 2 shows the 31 P NMR spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs dissolved in D2 O. In the 31 P NMR spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs, all HPAs exhibited a single main peak. Although small extra peaks were observed in the H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs due to the impurity of HPA sample, a main single peak appeared at −14.9 ppm, −13.5 ppm, −11.9 ppm, and −10.6 ppm in the KNb0 , K-Nb1 , K-Nb2 , and K-Nb3 , respectively (Fig. 2(a)). This result was in good agreement with the previous results [33,43,44]. It is interesting to note that these peaks shifted to higher chemical shift (ca. +1.4 ppm) with increasing niobium substitution. This was also well consistent with the result reported in the previous work [43]. For H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs (Fig. 2(b)), a single peak appeared at −12.6 ppm in the WDNb0 HPA, in good agreement with the previous report [44]. This indicates that the two phosphorous atoms in the Wells-Dawson heteropolyanion of WD-Nb0 (H6 P2 W18 O62 ) were equivalent. On the other hand, two main peaks were observed in the 31 P NMR spectra of niobium-containing WD-Nb1 , WD-Nb2 , and WD-Nb3 HPAs. This was due to two unequivalent phosphorous atoms in the Wells-Dawson heteropolyanions, PW9 and PW9−x Nbx with x = 1, 2, and 3 [44]. The higher value of peaks (−13.1 ppm, −13.6 ppm, and −14.0 ppm for WD-Nb1 , WD-Nb2 , and WD-Nb3 , respectively) corresponding to PW9 in the Wells-Dawson heteropolyanion shifted to lower chemical shift (ca. −0.5 ppm) with increasing niobium substitution. The lower value of peaks (−10.8 ppm, −9.2 ppm, and −7.6 ppm for WD-Nb1 , WD-Nb2 , and WD-Nb3 , respectively) was assigned to the phosphorus atom affected by the substituted framework niobium atom (PW9−x Nbx with x = 1, 2, and 3) [44]. These peaks shifted to higher chemical shift (ca. +1.6 ppm) with increasing niobium substitution. This was well consistent with the trend observed for niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs. The above results strongly indicate success-
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(a)
-10.6 ppm
-7.6 ppm
(b)
-14.0 ppm
K-Nb3
WD-Nb3 -9.2 ppm
-13.6 ppm
-11.9 ppm K-Nb2
WD-Nb2
-13.5 ppm
-10.8 ppm
-13.1 ppm WD-Nb1
K-Nb1 -12.6 ppm
-14.9 ppm
WD-Nb0
K-Nb0
-10
-11
-12
-13
-14
-15
-16
-7
-8
C h e m i c al s h i f t ( p p m)
-9
-10
-11
-12
-13
-14
-15
-16
Chemical shift (ppm)
Fig. 2. 31 P NMR spectra of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs dissolved in D2 O. The chemical shift was recorded using H3 PO4 as an external reference. Table 1 Chemical compositions of phosphorous, tungsten, and niobium in the H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs determined by ICP-AES analyses. Ratio of P:W:Nb Theoretical value
Measured value
K-Nb3 K-Nb2 K-Nb1 K-Nb0
1.0:9.0:3.0 1.0:10.0:2.0 1.0:11.0:1.0 1.0:12.0:0.0
1.2:9.0:2.8 1.1:10.0:2.1 0.9:11.0:0.9 0.9:12.0:0.0
WD-Nb3 WD-Nb2 WD-Nb1 WD-Nb0
2.0:15.0:3.0 2.0:16.0:2.0 2.0:17.0:1.0 2.0:18.0:0.0
2.0:15.0:3.0 2.2:16.0:2.1 2.1:17.0:1.1 2.0:18.0:0.0
Intensity (A. U.)
ful formation of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. Successful formation of niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs was also confirmed by ICP-AES analyses. Chemical compositions of phosphorous, tungsten, and niobium in the H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 WellsDawson HPAs determined by ICP-AES analyses are summarized in Table 1. The measured P:W:Nb ratios in the H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPAs were in good agreement with the theoretical values, indicating successful formation of niobium-
3.2. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts Reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson
(a)
(b)
K-Nb3
WD-Nb3
K-Nb2
K-Nb1
Intensity (A. U.)
Catalyst
containing H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs. It was also confirmed by ICP-AES analyses that the amount of potassium in all HPA samples was negligible. This means that substitution of potassium with proton was successful. In order to examine the physical properties of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 WellsDawson HPAs, crystalline phases and BET surface areas of the catalysts were investigated. Fig. 3 shows the XRD patterns of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. It was found that H3 PW12 O40 and H6 P2 W18 O62 catalysts showed the characteristic XRD peaks. For niobium-containing H3+x PW12−x Nbx O40 (x = 1, 2 and 3) and H6+x P2 W18−x Nbx O62 (x = 1, 2 and 3) HPAs, on the other hand, their XRD patterns were different from those of H3 PW12 O40 and H6 P2 W18 O62 catalysts. The difference in the XRD patterns was due to the flexible secondary structure of the HPAs. BET surface areas of the niobium-containing HPAs are listed in Table 2. It was observed that all niobiumcontaining H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs exhibited very low surface area less than 10 m2 /g, in good agreement with the previous result [4].
WD-Nb2
WD-Nb1
K-Nb0
10
20
30
2 Theta (Degree)
40
50
WD-Nb0
10
20
30
40
50
2 Theta (Degree)
Fig. 3. XRD patterns of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs.
D.R. Park et al. / Applied Catalysis A: General 394 (2011) 201–208
K-Nb3 K-Nb1
-0.539 V
-0.559 V
(b)
Current (A.U.)
Current (A.U.) -0.3
WD-Nb3 WD-Nb1
(a) -0.544 V -0.566 V
205
-0.4
-0.5
-0.6
Potential (volts)
-0.3
-0.4
-0.5
-0.6
Potential (volts)
Fig. 4. Typical cyclovoltagrams of (a) K-Nb1 and K-Nb3 HPA catalysts and (b) WD-Nb1 and WD-Nb3 HPA catalysts dissolved in 0.5 M Na2 SO4 aqueous electrolyte solution.
Table 2 BET surface areas of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. Catalyst
BET surface area (m2 /g)
Catalyst
BET surface area (m2 /g)
K-Nb3 K-Nb2 K-Nb1 K-Nb0
1.5 1.6 1.2 1.4
WD-Nb3 WD-Nb2 WD-Nb1 WD-Nb0
1.4 1.3 0.9 1.6
HPA catalysts were measured by an electrochemical method in solution. Fig. 4 shows the typical cyclovoltagrams of K-Nb1 and KNb3 HPA catalysts along with WD-Nb1 and WD-Nb3 HPA catalysts dissolved in 0.5 M Na2 SO4 aqueous electrolyte solution. Reduction waves of the HPA catalysts were observed with one peak potential at −0.544 V and −0.566 V for K-Nb1 and K-Nb3 HPA catalysts, respectively, and at −0.539 V and −0.559 V for WD-Nb1 and WDNb3 HPA catalysts, respectively. According to the previous work [42], HPAs exhibited mono-oxo-type reduction-oxidation ability. An electrochemical investigation [45] of the reduction potentials of HPAs dissolved in H2 SO4 or Na2 SO4 electrolyte solution revealed that the half-wave one-electron reduction potentials (first electron reduction potentials) of HPA catalysts were unaltered and pH-independent. Therefore, the first electron reduction potential of HPA catalyst was taken as the representative reduction potential of the HPA catalyst in this work. The first electron reduction poten-
tials of K-Nb1 and K-Nb3 HPA catalysts were found to be −0.544 V and −0.566 V, respectively, while those of WD-Nb1 and WD-Nb3 HPA catalysts were −0.539 V and −0.559 V, respectively. UV–visible spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts in solution were measured for simple diagnostic of their redox property. It has been reported that UV–visible spectra of HPAs in solid are sensitive to the number of coordinated water molecules in the HPAs [46]. This can be explained by the fact that HPAs in solid form a flexible secondary structure depending on the number of crystalline water molecules. In an aqueous solution, on the other hand, HPAs are dissociated and heteropolyanions fully solvated by water molecules. Therefore, UV–visible spectra of HPAs in solution at constant concentration may give directly comparable results. It has been reported that UV–visible spectra of HPA catalysts measured in solution state showed the same trends as those of HPA catalysts measured in solid state under consistent conditions, although the absorption edge energy of an HPA catalyst measured in solid state was smaller than that measured in solution state. Furthermore, UV–visible spectra of HPA catalysts in solution are easier to obtain and give more consistent results than those of HPA catalysts in solid. Fig. 5 shows the [F(R∞ )·h]1/2 curves of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs in solution. Absorption edge energy was determined by the intercept of a linear fit to the absorption edge. As
Fig. 5. [F(R∞ )·h]1/2 curves of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs in solution.
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It was observed that the HPA catalysts with niobium substitution showed the excellent redox properties (reduction potential and absorption edge energy) compared to the HPA catalysts without niobium substitution in both families of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts. The effect of polyatom substitution on the redox property of HPA catalysts was elucidated by a molecular orbital study on the Hn PM12−x Vx O40 (M = Mo, W; x = 0–3) HPAs [30]. The molecular orbital study [30] revealed that the energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) was well correlated with the reduction potential of the HPA. It has been reported that the HOMO for all HPAs consists primarily of nonbonding p-orbitals on the oxygens of the HPAs, while the LUMO consists of an antibonding combination of d-orbitals on the metal (polyatom) centers and p-orbitals on the neighboring bridging oxygens [29,47]. Thus, substitution of polyatoms into HPA framework does not affect the energies of the HOMOs since they are almost entirely centered on the oxygens. The substitution of vanadium, however, stabilizes the LUMOs because these orbitals derive substantially from vanadium d-orbitals which have been assumed to be more stable than those of molybdenum and tungsten [30]. In this manner, it can be inferred that niobiumcontaining HPA catalyst showed high redox property due to the stabilization of LUMOs derived from substitution of niobium into the HPA structure.
First electron reduction potential (SCE, volts)
-0.51 -0.52 -0.53 -0.54
K-Nb1 WD-Nb1
-0.55
WD-Nb3
-0.56
-0.58 -0.59
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K-Nb3
-0.57
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WD-Nb0 K-Nb0
-0.60 -0.61 -0.62 3.80
3.78
3.76
3.74
3.72
3.70
3.68
3.66
Absorption edge energy (eV) Fig. 6. Correlation between absorption edge energies and reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts.
shown in Fig. 5, a systematic variation of absorption edge energies was observed in both families of HPA catalysts. The absorption edge energies of H3+x PW12−x Nbx O40 Keggin HPA catalysts decreased in the order of K-Nb0 > K-Nb3 > K-Nb2 > K-Nb1 . The absorption edge energies of H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts showed the same trend as those of H3+x PW12−x Nbx O40 Keggin HPA catalysts with respect to niobium substitution.
3.4. Vapor-phase benzyl alcohol oxidation over H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 catalysts
3.3. Correlation between reduction potential and absorption edge energy
In order to probe oxidation catalysis and redox properties (reduction potential and absorption edge energy) of niobiumcontaining H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts, vapor-phase benzyl alcohol oxidation was carried out as a model reaction. It is known that benzaldehyde is formed by the oxidation catalysis of HPA, while toluene is produced by the acid catalysis of HPA [48,49]. In our catalytic reaction, benzaldehyde and toluene were produced as main products. Selectivity for benzaldehyde was more than 90%, while selectivity for toluene was less than 7%. This means that the HPA catalysts dominantly affected the oxidation catalysis rather than the acid catalysis. This result indicates that the benzyl alcohol oxidation was suitable as a model reaction to probe the oxidation catalysis and redox property of HPA catalysts. In our catalytic reaction, negligible amounts of CO, CO2 , and benzene were produced as by-products. Fig. 7 shows the typical catalytic performance of WD-Nb3 HPA catalyst in the vapor-phase benzyl alcohol oxidation with time on stream at 300 ◦ C. WD-Nb3 HPA catalyst showed a stable catalytic performance during the 5 h-reaction (Fig. 7(a)).
Fig. 6 shows the correlation between absorption edge energies and reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts. The absorption edge energies of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts were directly correlated with the reduction potentials of the HPA catalysts. Reduction potentials of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts increased with decreasing absorption edge energies of the HPA catalysts, regardless of the identity of HPA catalysts. The smaller absorption edge energy corresponded to the higher reduction potential of the HPA catalyst without HPA structural sensitivity. This implies that the absorption edge energies measured by UV–visible spectroscopy can serve as a correlating parameter or as an alternative parameter for the reduction potentials of niobium-containing Keggin and Wells-Dawson HPA catalysts.
100
100
(a)
Selectivity for benzaldehyde (%)
Conversion of benzyl alcohol (%)
80 70 60 50 40 30 20 10
90
90
(b)
80
80
70
70
60
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50
50
40
40
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0
60
120
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Selectivity for toluene (%)
206
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Fig. 7. Typical catalytic performance of WD-Nb3 HPA catalyst in the vapor-phase benzyl alcohol oxidation with time on stream at 300 ◦ C.
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on the basis of absorption edge energy measured by UV–visible spectroscopy. 4. Conclusions
Fig. 8. Correlations between yield for benzaldehyde and reduction potential of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts, and between yield for benzaldehyde and absorption edge energy of the HPA catalysts.
Selectivities for benzaldehyde and toluene were also almost constant during the reaction, as shown in Fig. 7(b). The other catalysts also showed a stable catalytic behavior with time on stream. In the vapor-phase benzyl alcohol oxidation performed at 300 ◦ C for 5 h, yield for benzaldehyde increased in the order of K-Nb0 (51.3%) < K-Nb3 (55.9%) < K-Nb2 (56.8%) < K-Nb1 (60.8%) for H3+x PW12−x Nbx O40 Keggin HPA catalysts, and WD-Nb0 (54.7%) < WD-Nb3 (59.0%) < WD-Nb2 (60.2%) < WD-Nb1 (63.8%) for H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts. Among the catalysts tested, WD-Nb1 HPA catalyst with the highest reduction potential and the lowest absorption edge energy showed the highest yield for benzaldehyde. Fig. 8 shows the correlations between yield for benzaldehyde and reduction potential of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts, and between yield for benzaldehyde and absorption edge energy of the HPA catalysts. It is interesting to note that yield for benzaldehyde monotonically increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts, regardless of the identity of HPA catalysts (without HPA structural sensitivity). As mentioned earlier, the most conventional technique to determine the reduction potentials of HPA catalysts is an electrochemical measurement. On the other hand, absorption edge energy determined from UV–visible spectrum of HPA catalyst measures the energy required for electron transfer from the HOMO to the LUMO. The band gap energy between the HOMO and the LUMO of an HPA catalyst corresponds to the reduction potential of the HPA catalyst. Thus, the absorption edge energy of an HPA catalyst measured by UV–visible spectroscopy reflects the reduction potential of the HPA catalyst, as reported in the previous works [38,39]. As shown in Fig. 6, absorption edge energies of niobium-containing HPA catalysts were well correlated with the reduction potentials of the HPA catalysts. This indicates that the absorption edge energy measured by UV–visible spectroscopy can serve as a correlating parameter or as an alternative parameter for the reduction potential of the HPA catalyst. For benzyl alcohol oxidation over the niobium-containing HPA catalysts, catalytic oxidation activity was well correlated with the reduction potentials and absorption edge energy of the HPA catalysts. The catalytic oxidation activity over the HPA catalysts monotonically increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts, regardless of the identity of HPA catalysts. These demonstrate that the absorption edge energies measured by UV–visible spectroscopy can be utilized as a probe of oxidation catalysis of the HPA catalysts. This implies that an HPA catalyst can be rationally designed to have a suitable oxidizing power to meet the need for selective oxidation reaction,
In this work, niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were prepared to elucidate their redox properties and oxidation catalysis. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy, respectively. Absorption edge energy of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts shifted to lower value with increasing reduction potential of the HPA catalysts, regardless of the identity of HPA catalysts; an HPA catalyst with higher reduction potential exhibited lower absorption edge energy. The correlations between redox properties (reduction potential and absorption edge energy) of both niobium-containing Keggin and Wells-Dawson HPA catalysts and catalytic oxidation activity (yield for benzaldehyde) of the HPA catalysts in the benzyl alcohol oxidation revealed that the catalytic oxidation activity monotonically increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts. Thus, it is concluded that the absorption edge energy of niobiumcontaining H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs determined from UV–visible spectra can be utilized as a correlating parameter for reduction potential of the HPA catalysts, and in turn, may track the oxidation catalysis of the HPA catalysts in the benzyl alcohol oxidation. Acknowledgements This work was supported by Mid-career Researcher Program of National Research Foundation (NRF) grant funded by the Korea government (MEST) (No. 2010-0000301). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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Applied Catalysis A: General journal homepage: www.elsevier.com/locate/apcata
Reduction potential, UV–visible absorption edge energy, and oxidation catalysis of niobium-containing H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson heteropolyacid catalysts Dong Ryul Park, Jung Ho Choi, Sunyoung Park, In Kyu Song ∗ School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea
a r t i c l e
i n f o
Article history: Received 8 November 2010 Received in revised form 23 December 2010 Accepted 31 December 2010 Available online 8 January 2011 Keywords: Heteropolyacid Niobium Reduction potential Absorption edge energy Oxidation catalysis
a b s t r a c t Niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson heteropolyacids (HPAs) were prepared in this work to explore their redox properties and oxidation catalysis. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy, respectively. The trend of reduction potential is well consistent with the trend of absorption edge energy with respect to niobium substitution in both series of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts. Absorption edge energy of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts shifted to lower value with increasing reduction potential of the HPA catalysts, regardless of the identity of HPA catalysts; an HPA catalyst with higher reduction potential exhibited lower absorption edge energy. In order to probe oxidation catalysis of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts, vapor-phase benzyl alcohol oxidation was carried out as a model reaction. Yield for benzaldehyde increased with increasing reduction potential and with decreasing absorption edge energy of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Heteropolyacids (HPAs) are early transition metal-oxygen anion clusters that exhibit a wide range of molecular sizes, compositions, and architectures [1–3]. One of the great advantages of HPA catalysts is that their catalytic properties can be tuned in a systematic way by changing the identity of counter-cation, central heteroatom, and framework polyatom [4–6]. Among various HPA structural classes, Keggin HPAs [7] have been widely employed as acid-base and oxidation catalysts in commercial processes [8–10]. Some of these reactions include hydration of propylene [11], nbutene [12], and isobutene [13], polymerization of tetrahydrofuran [14], oxidation of methacrolein [15], and Wacker and related olefin oxidations [16]. Recently, however, Wells-Dawson HPAs [17] have also attracted considerable attention as promising catalysts due to their excellent catalytic performance for several reactions [18–20]. Nonetheless, much progress has not been made on the catalytic performance of Wells-Dawson HPAs, and only limited information on the catalytic properties of Wells-Dawson HPAs is currently available.
∗ Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song). 0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2010.12.042
The catalytic redox property of HPAs has attracted much attention for several oxidation reactions [21–23]. HPAs have been investigated as oxidation catalysts to make useful chemicals directly from hydrocarbon raw materials, including oxidation of propane to acrylic acid [24–26], and oxidation of isobutane to methacroelin and methacrylic acid [27,28]. Therefore, fundamental understanding of reduction potential (oxidizing power) of HPA catalysts is of great importance in designing HPA catalysts for oxidation reactions. Several theoretical and instrumental methods have been employed to determine the reduction potential of HPAs. For example, quantum chemical studies have attempted to elucidate the reduction potentials of selected HPAs [29,30]. Another method is to determine the reduction potential of HPA catalysts from negative differential resistance (NDR) peak voltages in tunneling spectra measured with a scanning tunneling microscopy (STM) [31]. However, the most conventional technique to determine the reduction potential of HPAs is an electrochemical measurement in solution [32]. In the electrochemical measurement, the reduction potentials of HPA catalysts depend on the experimental conditions such as pH, composition of electrolyte solution, and identity of electrode [33,34]. Therefore, the measurement of reduction potential by a consistent method under consistent conditions is required, because direct comparison of reduction potentials of HPA catalysts from literature data is not a simple task [35].
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Another promising approach to probe the reduction potentials of HPA catalysts is to measure the absorption edge energies from UV–visible spectra of the HPAs [33]. Absorption edge energy in the UV–visible spectrum of HPA catalyst measures the energy required for electron transfer from the Highest Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied Molecular Orbital (LUMO) [36]. The HOMO energy is not greatly affected by changes in the HPA framework, while the LUMO energy is greatly affected by framework polyatom substitution [37]. The band gap energy between the HOMO and the LUMO of an HPA catalyst corresponds to the reduction potential of the HPA catalyst. Thus, the absorption edge energy of an HPA catalyst measured by UV–visible spectroscopy reflects the reduction potential of the HPA catalyst, as reported in the previous works [38,39]. On the other hand, it has been reported that niobium-containing HPA showed the excellent catalytic oxidation performance due to its high reduction potential [39]. Therefore, a systematic investigation on the redox property and oxidation catalysis of niobium-containing H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs would be worthwhile. In this work, niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) WellsDawson HPAs were prepared. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy in solution, respectively. Vapor-phase benzyl alcohol oxidation was carried out as a model reaction to probe the oxidation catalysis of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts. Correlations between redox properties (reduction potential and absorption edge energy) and catalytic oxidation activity of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts were then established.
2. Experimental 2.1. Preparation of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs Niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were prepared using Na2 WO4 ·2H2 O (Junsei Chem.), phosphoric acid (Sigma–Aldrich), acetic acid (Samchun Chem.), NbCl5 (Sigma–Aldrich), oxalic acid (Sigma–Aldrich), KCl (Junsei Chem.), diethyl ether (Samchun Chem.), and HCl (Sigma–Aldrich). For example, typical procedures for the preparation of H9 P2 W15 Nb3 O62 Wells-Dawson HPA are as follows. K6 P2 W18 O62 Wells-Dawson HPA was prepared according to the method in the literature [40]. 20 g of K6 P2 W18 O62 HPA was then dissolved in 50 ml of distilled water. Upon slowly adding 15 g of NaClO4 into the solution containing K6 P2 W18 O62 , a white precipitate of KClO4 was immediately formed. After removing the precipitate by filtration, 1 M Na2 CO3 solution was added dropwise into the resulting solution. A white precipitate of Na12 P2 W15 O56 lacunary HPA was obtained by filtering the precipitate at pH 9. In order to incorporate niobium into Wells-Dawson HPA structure, 7.0 g of NbCl5 was dissolved in 650 ml of 5% H2 O2 solution and pH of the solution was adjusted to 1.4 using 1 M NaOH solution. 36 g of Na12 P2 W15 O56 lacunary HPA was then added into the solution containing Nb precursor with vigorous stirring at 80 ◦ C. The mixed solution was further maintained at 80 ◦ C with stirring for 2 h. After the solution was cooled to room temperature, 100 g of KCl was added into the solution. The precipitate was filtered and successively washed with distilled water, ether, and ethanol to yield K9 P2 W15 Nb3 O62 . In order to transform potassium-containing HPA into proton-containing HPA, K9 P2 W15 Nb3 O62 was dissolved in
HCl solution and it was extracted with diethyl ether. The etherate was maintained at 50 ◦ C to obtain a crude solid product. The whitecolored solid was washed with distilled water, and then it was dried in a convection oven at 60 ◦ C overnight. These washing and drying processes were repeated several times to obtain H9 P2 W15 Nb3 O62 Wells-Dawson HPA. H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs were also prepared according to the similar method as H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. For example, typical procedures for the preparation of H6 PW9 Nb3 O40 Keggin HPA catalyst are as follows. 120 g of Na2 WO4 ·2H2 O was dissolved in 150 ml of distilled water. 3 ml of phosphoric acid and 20 ml of acetic acid were successively added to the solution containing tungsten precursor. The precipitate was filtered to obtain white-colored Na8 HPW9 O34 . The methods for incorporation of niobium into Keggin HPA structure were similar to those for H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs. 6.5 g of NbCl5 was dissolved in 650 ml of 5% H2 O2 solution and pH of the solution was adjusted to 1.4 using 1 M NaOH solution. 24 g of Na8 HPW9 O34 lacunary HPA was then added into the solution containing Nb precursor with vigorous stirring at 50 ◦ C. The mixed solution was further maintained at 50 ◦ C with stirring for 2 h. After the solution was cooled to room temperature, 50 g of KCl was added into the solution. The precipitate was filtered and successively washed with distilled water, ether, and ethanol to yield K6 PW9 Nb3 O40 . K6 PW9 Nb3 O40 was dissolved in HCl solution and it was extracted with diethyl ether. The etherate was maintained at 50 ◦ C to obtain a crude solid product. The white-colored solid was washed with distilled water, and then it was dried in a convection oven at 60 ◦ C overnight. These washing and drying processes were repeated several times to obtain H6 PW9 Nb3 O40 Keggin HPA. All H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were thermally treated at 300 ◦ C for 2 h in a stream of nitrogen (30 ml/min) prior to characterization and catalytic reaction. In this work, H3+x PW12−x Nbx O40 Keggin HPAs with x = 0, 1, 2, and 3 were denoted as K-Nb0 , K-Nb1 , K-Nb2 , and K-Nb3 , respectively. H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs with x = 0, 1, 2, and 3 were denoted as WD-Nb0 , WD-Nb1 , WD-Nb2 , and WD-Nb3 , respectively. For example, WD-Nb1 represents the Wells-Dawson HPA catalyst with mono-niobium substitution (H7 P2 W17 Nb1 O62 ). 2.2. Characterization H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were characterized by FT-IR (Nicolet, Nicolet 6700), 31 P NMR (Bruker, AVANCE 600), and ICP-AES (Shimadzu, ICPS-1000IV) analyses to confirm the successful formation of HPAs. 31 P NMR measurement was performed using D2 O and H3 PO4 as a solvent and as an external reference, respectively. Crystalline phases of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were investigated by XRD measurements (Rigaku, D-MAX2500-PC). Surface areas of the HPA catalysts were measured using a BET apparatus (Micromeritics, ASAP 2010). 2.3. Reduction potential and UV–visible spectroscopy Reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts were measured by an electrochemical method in solution. The electrochemical measurements were made using a Potentiostat/Galvanostat (Eco Chemie, Autolab 302N) instrument with a computer-controlled cyclovoltametry system. The working electrode was Pt disk with an electrode diameter of 0.2 mm. A Pt rod and Ag/AgCl (NaCl saturated) were used as a counter electrode and a reference electrode, respectively. To obtain cyclovoltagrams,
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Fig. 1. FT-IR spectra of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs.
1 mM of each HPA catalyst dissolved in 0.5 M Na2 SO4 aqueous electrolyte solution (10 ml) was prepared. The sample solutions were purged with nitrogen (50 ml/min) for 3 min and then maintained for 1 min for stabilization prior to the cyclovoltametry measurements. Cyclovoltagrams were obtained at a scan rate of 10 mV/s. UV–visible spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) HPA catalysts in solution were measured at room temperature with a Lambda-35 spectrometer (PerkinElmer). 1 mM of each HPA catalyst dissolved in distilled water was used for UV–visible spectroscopy measurement. The Kubelka-Munk function (F(R∞ )) [41] was used to convert reflectance measurements into equivalent absorption spectra using the reflectance of BaSO4 as a reference, and to obtain absorption edge energies directly from the [F(R∞ )·h]1/2 curves. 2.4. Vapor-phase benzyl alcohol oxidation Vapor-phase benzyl alcohol oxidation over niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) HPA catalysts was carried out in a continuous flow fixed-bed reactor at atmospheric pressure. Each catalyst (0.3 g) was charged into a tubular quartz reactor, and was pretreated with a mixed stream of nitrogen (10 ml/min) and oxygen (10 ml/min) at 320 ◦ C for 1 h. Benzyl alcohol (2.90 × 10−3 mol/h) was sufficiently vaporized by passing through a pre-heating zone and continuously fed into the reactor together with oxygen and nitrogen carrier. Feed composition (molar ratio) was fixed at benzyl alcohol (0.24):oxygen (1.0):nitrogen (1.0). Catalytic reaction was carried out at 300 ◦ C for 5 h. Reaction products were periodically sampled and analyzed with a gas chromatograph (HP 5890II, FID) equipped with a Supelco VO COLTM column (60 m (L) × 0.25 mm (D)). Conversion of benzyl alcohol and selectivity for product were calculated on the basis of carbon balance. Yield for benzaldehyde was calculated by multiplying conversion of benzyl alcohol and selectivity for benzaldehyde. 3. Results and discussion 3.1. Formation of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs Successful formation of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) WellsDawson HPAs was confirmed by FT-IR analyses. Fig. 1 shows the FT-IR spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson
HPAs. The formation of heteropolyanions could be identified by four characteristic IR bands in the range of 700–1200 cm−1 (P–O, polyatom = O, inter-octahedral polyatom-O-polyatom, and intraoctahedral polyatom-O-polyatom bands) [42]. These four characteristic IR bands were clearly observed in all H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs, indicating successful formation of heteropolyanions in both families of Keggin and Wells-Dawson HPAs. Successful formation of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) WellsDawson HPAs was further confirmed by 31 P NMR measurements. Fig. 2 shows the 31 P NMR spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs dissolved in D2 O. In the 31 P NMR spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs, all HPAs exhibited a single main peak. Although small extra peaks were observed in the H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs due to the impurity of HPA sample, a main single peak appeared at −14.9 ppm, −13.5 ppm, −11.9 ppm, and −10.6 ppm in the KNb0 , K-Nb1 , K-Nb2 , and K-Nb3 , respectively (Fig. 2(a)). This result was in good agreement with the previous results [33,43,44]. It is interesting to note that these peaks shifted to higher chemical shift (ca. +1.4 ppm) with increasing niobium substitution. This was also well consistent with the result reported in the previous work [43]. For H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs (Fig. 2(b)), a single peak appeared at −12.6 ppm in the WDNb0 HPA, in good agreement with the previous report [44]. This indicates that the two phosphorous atoms in the Wells-Dawson heteropolyanion of WD-Nb0 (H6 P2 W18 O62 ) were equivalent. On the other hand, two main peaks were observed in the 31 P NMR spectra of niobium-containing WD-Nb1 , WD-Nb2 , and WD-Nb3 HPAs. This was due to two unequivalent phosphorous atoms in the Wells-Dawson heteropolyanions, PW9 and PW9−x Nbx with x = 1, 2, and 3 [44]. The higher value of peaks (−13.1 ppm, −13.6 ppm, and −14.0 ppm for WD-Nb1 , WD-Nb2 , and WD-Nb3 , respectively) corresponding to PW9 in the Wells-Dawson heteropolyanion shifted to lower chemical shift (ca. −0.5 ppm) with increasing niobium substitution. The lower value of peaks (−10.8 ppm, −9.2 ppm, and −7.6 ppm for WD-Nb1 , WD-Nb2 , and WD-Nb3 , respectively) was assigned to the phosphorus atom affected by the substituted framework niobium atom (PW9−x Nbx with x = 1, 2, and 3) [44]. These peaks shifted to higher chemical shift (ca. +1.6 ppm) with increasing niobium substitution. This was well consistent with the trend observed for niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin HPAs. The above results strongly indicate success-
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(a)
-10.6 ppm
-7.6 ppm
(b)
-14.0 ppm
K-Nb3
WD-Nb3 -9.2 ppm
-13.6 ppm
-11.9 ppm K-Nb2
WD-Nb2
-13.5 ppm
-10.8 ppm
-13.1 ppm WD-Nb1
K-Nb1 -12.6 ppm
-14.9 ppm
WD-Nb0
K-Nb0
-10
-11
-12
-13
-14
-15
-16
-7
-8
C h e m i c al s h i f t ( p p m)
-9
-10
-11
-12
-13
-14
-15
-16
Chemical shift (ppm)
Fig. 2. 31 P NMR spectra of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs dissolved in D2 O. The chemical shift was recorded using H3 PO4 as an external reference. Table 1 Chemical compositions of phosphorous, tungsten, and niobium in the H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs determined by ICP-AES analyses. Ratio of P:W:Nb Theoretical value
Measured value
K-Nb3 K-Nb2 K-Nb1 K-Nb0
1.0:9.0:3.0 1.0:10.0:2.0 1.0:11.0:1.0 1.0:12.0:0.0
1.2:9.0:2.8 1.1:10.0:2.1 0.9:11.0:0.9 0.9:12.0:0.0
WD-Nb3 WD-Nb2 WD-Nb1 WD-Nb0
2.0:15.0:3.0 2.0:16.0:2.0 2.0:17.0:1.0 2.0:18.0:0.0
2.0:15.0:3.0 2.2:16.0:2.1 2.1:17.0:1.1 2.0:18.0:0.0
Intensity (A. U.)
ful formation of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. Successful formation of niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs was also confirmed by ICP-AES analyses. Chemical compositions of phosphorous, tungsten, and niobium in the H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 WellsDawson HPAs determined by ICP-AES analyses are summarized in Table 1. The measured P:W:Nb ratios in the H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPAs were in good agreement with the theoretical values, indicating successful formation of niobium-
3.2. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts Reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson
(a)
(b)
K-Nb3
WD-Nb3
K-Nb2
K-Nb1
Intensity (A. U.)
Catalyst
containing H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs. It was also confirmed by ICP-AES analyses that the amount of potassium in all HPA samples was negligible. This means that substitution of potassium with proton was successful. In order to examine the physical properties of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 WellsDawson HPAs, crystalline phases and BET surface areas of the catalysts were investigated. Fig. 3 shows the XRD patterns of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. It was found that H3 PW12 O40 and H6 P2 W18 O62 catalysts showed the characteristic XRD peaks. For niobium-containing H3+x PW12−x Nbx O40 (x = 1, 2 and 3) and H6+x P2 W18−x Nbx O62 (x = 1, 2 and 3) HPAs, on the other hand, their XRD patterns were different from those of H3 PW12 O40 and H6 P2 W18 O62 catalysts. The difference in the XRD patterns was due to the flexible secondary structure of the HPAs. BET surface areas of the niobium-containing HPAs are listed in Table 2. It was observed that all niobiumcontaining H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs exhibited very low surface area less than 10 m2 /g, in good agreement with the previous result [4].
WD-Nb2
WD-Nb1
K-Nb0
10
20
30
2 Theta (Degree)
40
50
WD-Nb0
10
20
30
40
50
2 Theta (Degree)
Fig. 3. XRD patterns of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs.
D.R. Park et al. / Applied Catalysis A: General 394 (2011) 201–208
K-Nb3 K-Nb1
-0.539 V
-0.559 V
(b)
Current (A.U.)
Current (A.U.) -0.3
WD-Nb3 WD-Nb1
(a) -0.544 V -0.566 V
205
-0.4
-0.5
-0.6
Potential (volts)
-0.3
-0.4
-0.5
-0.6
Potential (volts)
Fig. 4. Typical cyclovoltagrams of (a) K-Nb1 and K-Nb3 HPA catalysts and (b) WD-Nb1 and WD-Nb3 HPA catalysts dissolved in 0.5 M Na2 SO4 aqueous electrolyte solution.
Table 2 BET surface areas of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs. Catalyst
BET surface area (m2 /g)
Catalyst
BET surface area (m2 /g)
K-Nb3 K-Nb2 K-Nb1 K-Nb0
1.5 1.6 1.2 1.4
WD-Nb3 WD-Nb2 WD-Nb1 WD-Nb0
1.4 1.3 0.9 1.6
HPA catalysts were measured by an electrochemical method in solution. Fig. 4 shows the typical cyclovoltagrams of K-Nb1 and KNb3 HPA catalysts along with WD-Nb1 and WD-Nb3 HPA catalysts dissolved in 0.5 M Na2 SO4 aqueous electrolyte solution. Reduction waves of the HPA catalysts were observed with one peak potential at −0.544 V and −0.566 V for K-Nb1 and K-Nb3 HPA catalysts, respectively, and at −0.539 V and −0.559 V for WD-Nb1 and WDNb3 HPA catalysts, respectively. According to the previous work [42], HPAs exhibited mono-oxo-type reduction-oxidation ability. An electrochemical investigation [45] of the reduction potentials of HPAs dissolved in H2 SO4 or Na2 SO4 electrolyte solution revealed that the half-wave one-electron reduction potentials (first electron reduction potentials) of HPA catalysts were unaltered and pH-independent. Therefore, the first electron reduction potential of HPA catalyst was taken as the representative reduction potential of the HPA catalyst in this work. The first electron reduction poten-
tials of K-Nb1 and K-Nb3 HPA catalysts were found to be −0.544 V and −0.566 V, respectively, while those of WD-Nb1 and WD-Nb3 HPA catalysts were −0.539 V and −0.559 V, respectively. UV–visible spectra of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts in solution were measured for simple diagnostic of their redox property. It has been reported that UV–visible spectra of HPAs in solid are sensitive to the number of coordinated water molecules in the HPAs [46]. This can be explained by the fact that HPAs in solid form a flexible secondary structure depending on the number of crystalline water molecules. In an aqueous solution, on the other hand, HPAs are dissociated and heteropolyanions fully solvated by water molecules. Therefore, UV–visible spectra of HPAs in solution at constant concentration may give directly comparable results. It has been reported that UV–visible spectra of HPA catalysts measured in solution state showed the same trends as those of HPA catalysts measured in solid state under consistent conditions, although the absorption edge energy of an HPA catalyst measured in solid state was smaller than that measured in solution state. Furthermore, UV–visible spectra of HPA catalysts in solution are easier to obtain and give more consistent results than those of HPA catalysts in solid. Fig. 5 shows the [F(R∞ )·h]1/2 curves of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs in solution. Absorption edge energy was determined by the intercept of a linear fit to the absorption edge. As
Fig. 5. [F(R∞ )·h]1/2 curves of (a) H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and (b) H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs in solution.
D.R. Park et al. / Applied Catalysis A: General 394 (2011) 201–208
It was observed that the HPA catalysts with niobium substitution showed the excellent redox properties (reduction potential and absorption edge energy) compared to the HPA catalysts without niobium substitution in both families of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts. The effect of polyatom substitution on the redox property of HPA catalysts was elucidated by a molecular orbital study on the Hn PM12−x Vx O40 (M = Mo, W; x = 0–3) HPAs [30]. The molecular orbital study [30] revealed that the energy gap between the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) was well correlated with the reduction potential of the HPA. It has been reported that the HOMO for all HPAs consists primarily of nonbonding p-orbitals on the oxygens of the HPAs, while the LUMO consists of an antibonding combination of d-orbitals on the metal (polyatom) centers and p-orbitals on the neighboring bridging oxygens [29,47]. Thus, substitution of polyatoms into HPA framework does not affect the energies of the HOMOs since they are almost entirely centered on the oxygens. The substitution of vanadium, however, stabilizes the LUMOs because these orbitals derive substantially from vanadium d-orbitals which have been assumed to be more stable than those of molybdenum and tungsten [30]. In this manner, it can be inferred that niobiumcontaining HPA catalyst showed high redox property due to the stabilization of LUMOs derived from substitution of niobium into the HPA structure.
First electron reduction potential (SCE, volts)
-0.51 -0.52 -0.53 -0.54
K-Nb1 WD-Nb1
-0.55
WD-Nb3
-0.56
-0.58 -0.59
K-Nb2
K-Nb3
-0.57
WD-Nb2
WD-Nb0 K-Nb0
-0.60 -0.61 -0.62 3.80
3.78
3.76
3.74
3.72
3.70
3.68
3.66
Absorption edge energy (eV) Fig. 6. Correlation between absorption edge energies and reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts.
shown in Fig. 5, a systematic variation of absorption edge energies was observed in both families of HPA catalysts. The absorption edge energies of H3+x PW12−x Nbx O40 Keggin HPA catalysts decreased in the order of K-Nb0 > K-Nb3 > K-Nb2 > K-Nb1 . The absorption edge energies of H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts showed the same trend as those of H3+x PW12−x Nbx O40 Keggin HPA catalysts with respect to niobium substitution.
3.4. Vapor-phase benzyl alcohol oxidation over H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 catalysts
3.3. Correlation between reduction potential and absorption edge energy
In order to probe oxidation catalysis and redox properties (reduction potential and absorption edge energy) of niobiumcontaining H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts, vapor-phase benzyl alcohol oxidation was carried out as a model reaction. It is known that benzaldehyde is formed by the oxidation catalysis of HPA, while toluene is produced by the acid catalysis of HPA [48,49]. In our catalytic reaction, benzaldehyde and toluene were produced as main products. Selectivity for benzaldehyde was more than 90%, while selectivity for toluene was less than 7%. This means that the HPA catalysts dominantly affected the oxidation catalysis rather than the acid catalysis. This result indicates that the benzyl alcohol oxidation was suitable as a model reaction to probe the oxidation catalysis and redox property of HPA catalysts. In our catalytic reaction, negligible amounts of CO, CO2 , and benzene were produced as by-products. Fig. 7 shows the typical catalytic performance of WD-Nb3 HPA catalyst in the vapor-phase benzyl alcohol oxidation with time on stream at 300 ◦ C. WD-Nb3 HPA catalyst showed a stable catalytic performance during the 5 h-reaction (Fig. 7(a)).
Fig. 6 shows the correlation between absorption edge energies and reduction potentials of H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPA catalysts. The absorption edge energies of H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts were directly correlated with the reduction potentials of the HPA catalysts. Reduction potentials of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts increased with decreasing absorption edge energies of the HPA catalysts, regardless of the identity of HPA catalysts. The smaller absorption edge energy corresponded to the higher reduction potential of the HPA catalyst without HPA structural sensitivity. This implies that the absorption edge energies measured by UV–visible spectroscopy can serve as a correlating parameter or as an alternative parameter for the reduction potentials of niobium-containing Keggin and Wells-Dawson HPA catalysts.
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Fig. 7. Typical catalytic performance of WD-Nb3 HPA catalyst in the vapor-phase benzyl alcohol oxidation with time on stream at 300 ◦ C.
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on the basis of absorption edge energy measured by UV–visible spectroscopy. 4. Conclusions
Fig. 8. Correlations between yield for benzaldehyde and reduction potential of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts, and between yield for benzaldehyde and absorption edge energy of the HPA catalysts.
Selectivities for benzaldehyde and toluene were also almost constant during the reaction, as shown in Fig. 7(b). The other catalysts also showed a stable catalytic behavior with time on stream. In the vapor-phase benzyl alcohol oxidation performed at 300 ◦ C for 5 h, yield for benzaldehyde increased in the order of K-Nb0 (51.3%) < K-Nb3 (55.9%) < K-Nb2 (56.8%) < K-Nb1 (60.8%) for H3+x PW12−x Nbx O40 Keggin HPA catalysts, and WD-Nb0 (54.7%) < WD-Nb3 (59.0%) < WD-Nb2 (60.2%) < WD-Nb1 (63.8%) for H6+x P2 W18−x Nbx O62 Wells-Dawson HPA catalysts. Among the catalysts tested, WD-Nb1 HPA catalyst with the highest reduction potential and the lowest absorption edge energy showed the highest yield for benzaldehyde. Fig. 8 shows the correlations between yield for benzaldehyde and reduction potential of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts, and between yield for benzaldehyde and absorption edge energy of the HPA catalysts. It is interesting to note that yield for benzaldehyde monotonically increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts, regardless of the identity of HPA catalysts (without HPA structural sensitivity). As mentioned earlier, the most conventional technique to determine the reduction potentials of HPA catalysts is an electrochemical measurement. On the other hand, absorption edge energy determined from UV–visible spectrum of HPA catalyst measures the energy required for electron transfer from the HOMO to the LUMO. The band gap energy between the HOMO and the LUMO of an HPA catalyst corresponds to the reduction potential of the HPA catalyst. Thus, the absorption edge energy of an HPA catalyst measured by UV–visible spectroscopy reflects the reduction potential of the HPA catalyst, as reported in the previous works [38,39]. As shown in Fig. 6, absorption edge energies of niobium-containing HPA catalysts were well correlated with the reduction potentials of the HPA catalysts. This indicates that the absorption edge energy measured by UV–visible spectroscopy can serve as a correlating parameter or as an alternative parameter for the reduction potential of the HPA catalyst. For benzyl alcohol oxidation over the niobium-containing HPA catalysts, catalytic oxidation activity was well correlated with the reduction potentials and absorption edge energy of the HPA catalysts. The catalytic oxidation activity over the HPA catalysts monotonically increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts, regardless of the identity of HPA catalysts. These demonstrate that the absorption edge energies measured by UV–visible spectroscopy can be utilized as a probe of oxidation catalysis of the HPA catalysts. This implies that an HPA catalyst can be rationally designed to have a suitable oxidizing power to meet the need for selective oxidation reaction,
In this work, niobium-containing H3+x PW12−x Nbx O40 (x = 0, 1, 2 and 3) Keggin and H6+x P2 W18−x Nbx O62 (x = 0, 1, 2 and 3) Wells-Dawson HPAs were prepared to elucidate their redox properties and oxidation catalysis. Reduction potentials and absorption edge energies of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts were measured by an electrochemical method and UV–visible spectroscopy, respectively. Absorption edge energy of H3+x PW12−x Nbx O40 and H6+x P2 W18−x Nbx O62 HPA catalysts shifted to lower value with increasing reduction potential of the HPA catalysts, regardless of the identity of HPA catalysts; an HPA catalyst with higher reduction potential exhibited lower absorption edge energy. The correlations between redox properties (reduction potential and absorption edge energy) of both niobium-containing Keggin and Wells-Dawson HPA catalysts and catalytic oxidation activity (yield for benzaldehyde) of the HPA catalysts in the benzyl alcohol oxidation revealed that the catalytic oxidation activity monotonically increased with increasing reduction potential and with decreasing absorption edge energy of the HPA catalysts. Thus, it is concluded that the absorption edge energy of niobiumcontaining H3+x PW12−x Nbx O40 Keggin and H6+x P2 W18−x Nbx O62 Wells-Dawson HPAs determined from UV–visible spectra can be utilized as a correlating parameter for reduction potential of the HPA catalysts, and in turn, may track the oxidation catalysis of the HPA catalysts in the benzyl alcohol oxidation. Acknowledgements This work was supported by Mid-career Researcher Program of National Research Foundation (NRF) grant funded by the Korea government (MEST) (No. 2010-0000301). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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