Transition-Metal-Substituted Phosphomolybdates ... - ACS Publications

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Transition-Metal-Substituted Phosphomolybdates: Catalytic and Kinetic Study for Liquid-Phase Oxidation of Styrene Soyeb Pathan and Anjali Patel* Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of Science, The M. S. University of Baroda, Vadodara, 390 002 Gujarat, India S Supporting Information *

ABSTRACT: Mono-transition-metal-substituted phosphomolybdates, PMo11M (M = Co, Mn, Ni), were synthesized and characterized by various physicochemical techniques. They were efficiently used for liquid-phase nonsolvent oxidation of styrene under mild reaction conditions. It was found that all of the catalysts are efficient in achieving higher conversions with high turnover numbers and are selective toward benzaldehyde. A detailed kinetic study was also carried out. Although all of the catalysts are homogeneous under the present oxidation conditions, they were regenerated and reused for up to two cycles.

1. INTRODUCTION Benzaldehyde is a very valuable chemical product that has widespread applications in perfumery, dyestuff, and agro chemical industries.1 It is the second most important aromatic molecule (after vanillin) used in the cosmetics and flavor industries. However, the traditional procedures suffer from a lack of selectivity, the use of organic solvents, toxicity of the reagents, and waste production. It is therefore a challenge for chemists to develop new environmentally benign and nonpolluting oxidation procedures. Catalytic oxidations of organic substrates with environmentally benign oxidants, such as H2O2, have been of growing interest because of recent environmental concerns.2−4 After molecular oxygen, H2O2 is among the best and most atom-efficient oxidant. The major advantages of using H2O2 are that it has a high oxygen content (47%), exists in high purity, and can be safely stored and used.5,6 Apart from this, it is environmentally friendly because the only byproduct of the reaction is water, eliminating the need for costly disposal treatments.4,7−9 The search for active, green, and recyclable catalysts to activate H2O2 for the oxidation of organic compounds is the focus of many researchers in both academia and industry. Keggin-type polyoxometalates (POMs) meet both activity and stability criteria. Also, the use of POMs as catalysts is important in the so-called clean technologies because objections to environmental pollution and corrosion of the traditional technologies are avoided.10−13 Recently, a subclass of POMs, transition-metal-substituted POMs (TMSPOMs), is getting more popular because substitution of a transition metal into the POM has been explored as a route to increasing the range of applications of these compounds.14−17 It has been reported that the addition of transition metals is expected to influence the redox properties considerably, particularly when they are incorporated in the primary structure of the Keggin ion.18,19 Among all TMSPOMs, transition-metal-substituted phosphomolybdates are the most important because of their unique redox properties. Reports on the oxidation of alkenes, e.g., vapor-phase oxidation of propene,20 oxidation of norbornene with aqueous H2O2 as an oxidant in different solvents,21 and © 2013 American Chemical Society

epoxidation of alkenes with TBHP/H2O2 in CH3CN and CH2Cl2,22 over vanadium-substituted phosphomolybdates are also available. Vapor-phase oxidation of cyclohaxene over ironsubstituted phosphomolybdate23 has also been reported. Neumann et al. reported the synthesis and characterization of ruthenium-substituted phosphomolybdates and explored their use as catalysts for the epoxidation of alkenes by molecular oxygen in acetonitrile.24 Almost all of the reports on the oxidation of alkenes catalyzed by transition-metal-substituted phosphomolybdates describe reactions either in the presence of solvent or in a vapor phase with molecular oxygen. As per our knowledge, no reports describing the study of solvent-free liquid-phase oxidation of styrene catalyzed by transition-metal-substituted (cobalt, manganese, and nickel) phosphomolybdates are available in art. Recently, liquid-phase nonsolvent oxidation of styrene over cobalt- and manganesesubstituted phosphotungstates was reported by our group.25 The obtained results encourage us to carry out detailed studies on transition-metal-substituted phosphomolybdates. As an extension of our work, it was thought to develop a catalytic system for styrene oxidation, comprising an environmentally benign oxidant as well as a highly efficient catalyst such as transition-metal-substituted (cobalt, manganese, and nickel) phosphomolybdates because they are of considerable interest because of the redox properties and variable oxidation states of these metal ions. Further, no reports on the kinetic study of oxidation of styrene using transition-metal-substituted (cobalt, manganese, and nickel) phosphomolybdates as catalysts are available. Recently, we have reported the synthesis and detailed characterization of a Keggin-type cesium salt of transitionmetal-substituted phosphomolybdates, Cs 5 [PCo(H 2 O)Mo11O39]·6H2O and Cs5[PMn(H2O)Mo11O39]·6H2O.26 As an extension of our work, in the present paper we report the Received: Revised: Accepted: Published: 11913

March 12, 2013 July 17, 2013 July 30, 2013 July 30, 2013 dx.doi.org/10.1021/ie400797u | Ind. Eng. Chem. Res. 2013, 52, 11913−11919

Industrial & Engineering Chemistry Research

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product was identified by a comparison with the authentic samples and finally by gas chromatography−mass spectroscopy.

use of Cs5[PM(H2O)Mo11O39]·6H2O (M = Co, Mn, Ni) as catalysts for solvent-free oxidation of styrene with H2O2 under mild reaction conditions. The catalytic activity was evaluated for nonsolvent liquid-phase oxidation of styrene with aqueous 30% H2O2. The conditions for maximum conversion as well as selectivity for the desired products were optimized by varying different parameters. A comparative study on the kinetic behavior of the catalysts was also done. The catalytic properties for a recycled catalyst were also evaluated under optimized conditions. Even though they behave as homogeneous catalysts under the present oxidation conditions, we could suggest a method for regeneration and the catalysts were reused for up to two cycles.

3. RESULTS AND DISCUSSION 3.1. Analytical Techniques. Elemental analysis shows that found values are in good agreement with analytical values. The results are given as follows. Anal. Calcd for PMo11Co: Cs, 25.88; Mo, 41.20; P, 1.20; Co, 2.30; O, 28.75. Found: Cs, 25.90; Mo, 41.29; P, 1.16; Co, 2.33; O, 28.69. Anal. Calcd for PMo11Mn: Cs, 26.00; Mo, 41.29; P, 1.21; Mn, 2.15; O, 28.79. Found: Cs, 26.34; Mo, 41.40 ; P, 1.18; Mn, 2.04; O, 28.61. Anal. Calcd for PMo11Ni: Cs, 26.03; Mo, 41.34; P, 1.21; Ni, 2.30; O, 28.84. Found: Cs, 26.12; Mo, 41.39; P, 1.24; Ni, 2.32; O, 28.89. It is observed from Table 1 that all of the catalysts show a weight loss of 4.2−4.83% at 150 °C, corresponding to seven

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used were of analytical reagant grade. 12-Molybdophosphoric acid (H3PMo12O40), sodium hydroxide, styrene, 30% aqueous H2O2, dichloromethane, CsCl, Co(CH 3 COO) 2 ·4H 2 O, Mn(CH 3 COO) 2 ·4H 2 O, and Ni(CH3COO)2·4H2O were obtained from Merck and used as received. 2.2. Synthesis of Transition-Metal-Substituted Phosphomolybdates.26 A total of 1 mmol of H3PMo12O40 (1.825 g; PMo12) was dissolved in water (10 mL), and the pH of the solution was adjusted to 4.3 using a sodium hydroxide solution. A total of 1 mmol of M(CH3COO)2·4H2O (M = Co, Mn, Ni) dissolved in a minimum amount of water was mixed with the above hot solution. The pH of the solution was adjusted to 4.3. The solution was heated at 80 °C with stirring for 1 h and filtered hot. To the hot-filtrate-saturated solution was added dropwise CsCl, and the resulting solution was allowed to stand. Because the obtained crystals were very poorly soluble in any solvent, no recrystallization was carried out. The obtained crystals were filtered, air-dried, and designated as PMo11Co, PMo11Mn, and PMo11Ni. 2.3. Characterization. A detailed study on the characterization of synthesized materials can be found in our earlier publications.26 However, in the present paper, Fourier transform infrared (FT-IR), thermal analysis, X-ray diffraction (XRD) and diffuse-reflectance spectroscopy (DRS) are given for convenience. The synthesized materials were characterized by elemental analysis, thermogravimetric−differential thermal analysis (TG-DTA), FT-IR, XRD, and DRS. Elemental analysis was carried out using a JSM 5610 LV EDX-SEM analyzer. TGDTA was carried out on a Mettler-Toledo Star SW 7.01 up to 600 °C in air with a heating rate of 5 °C/min. FT-IR spectra of the samples were recorded as KBr pellets on a Perkin-Elmer instrument. The powder XRD pattern was obtained by using a Philips diffractometer (model PW-1830). The conditions used were Cu Kα radiation (1.5417 Å). The DR-UV−visible spectrum was recorded at room temperature on a PerkinElmer LAMBDA instrument using a 1 cm quartz cell. 2.4. Catalytic Reaction. The oxidation reaction was carried out in a borosilicate glass reactor provided with a double-walled condenser. The desired catalyst, styrene, and H2O2 mixtures were intensively stirred in the reactor at a set constant temperature for the entire duration of the reactions. The same reaction was carried out by varying different parameters such as the molar ratio of styrene to H2O2, amount of catalyst, and reaction time. After completion of the reaction, the catalyst was removed and the product was extracted with dichloromethane. The product was dried with magnesium sulfate and analyzed on a gas chromatograph using a BP-1 capillary column. The

Table 1. TG-DTA of PMo11Co, PMo11Mn, and PMo11Ni DTA catalyst

TGA (% wt loss at 150 °C)

endothermic

exothermic

PMo11Co PMo11Mn PMo11Ni

4.83 4.21 4.33

130 125 137

415 430 425

H2O molecules. Similarly, DTA of all catalysts showed an endothermic peak due to water of crystallization. An exothermic peak in the region 415−430 °C indicates crystallization of the MoO3 phase after decomposition of the Keggin unit. The frequencies of the FT-IR bands for PMo12, PMo11Co, PMo11Mn, and PMo11Ni are shown in Figure 1. The FT-IR spectrum of PMo12 showed bands at 1070, 965, and 870 and 790 cm−1, corresponding to the symmetric stretching of P−O, Mo−O, and Mo−O−Mo bonds, respectively. The FT-IR spectrum showed P−O bond frequencies at 1050, 1043, and 1046 cm−1 for PMo11Co, PMo11Mn, and PMo11Ni, respectively. The shift in the band position compared to PMo12 indicates that the transition metal was introduced into the octahedral lacuna. There is also a shift in the stretching vibration of MoO and Mo−O−Mo for all three compounds, indicating complexation of the transition metals. An additional band at 480, 422, and 442 cm−1 is attributed to the Co−O, Mn−O, and Ni−O vibrations, respectively. Thus, the FT-IR spectra clearly show the incorporation of Co/Mn/Ni into the Keggin framework. The powder XRD patterns of PMo12, PMo11Co, PMo11Mn, and PMo11Ni indicate that the synthesized complexes are crystalline (Figure 2). The peaks between 2θ = 15 and 30° indicate the presence of a Keggin ion. The shifts in the 2θ values for PMo11Co, PMo11Mn, and PMo11Ni compared to PMo12 may be due to substitution of the transition metal into the lacuna. Figure 3 shows DRS spectra for PMo11, PMo11Co, PMo11Mn, and PMo11Ni. In the case of PMo11, the intense absorption band at ∼285 nm is due to O → Mo charge transfer. The DRS spectra of PMo11Co, PMo11Mn, and PMo11Ni show two peaks. The peak at around 270 nm corresponds to O → Mo charge transfer, indicating formation of the PMo11O39 lacuna in the synthesized compounds. The observed shifting compared to that of PMo11 may be due to substitution of the transition metal. The DRS spectra of PMo11Co, PMo11Mn, and PMo11Ni show broad bands in the regions 560−580, 390−430, 11914

dx.doi.org/10.1021/ie400797u | Ind. Eng. Chem. Res. 2013, 52, 11913−11919


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Figure 3. DRS spectra of (a) PMo11 (b) PMo11Co, (c) PMo11Mn, and (d) PMo11Ni.

3.2.1. Effect of the Mole Ratio of Styrene to H2O2. The reaction was carried out by varying the mole ratio of styrene to H2O2 with 20 mg of catalyst for 10 h at 80 °C (Figure 4). It is

Figure 1. FT-IR spectra of (a) PMo12, (b) PMo11Co, (c) PMo11Mn, and (d) PMo11Ni.

Figure 4. Effect of the mole ratio: % conversion is based on styrene; time = 10 h; amount of PMo11Co = 20 mg; temperature = 80 °C.

seen from Figure 4 that, with an increase in the mole ratio of H2O2, i.e., upon variation of the styrene to H2O2 mole ratio from 1:1 to 1:4, there is a drastic change in the conversion of styrene to benzaldehyde. The increase in conversion may be due to an increase in the concentration of H2O2. On the other hand, conversion decreases with an increase in the concentration of styrene. The conversion of styrene is 98.9%, and the product selectivity for benzaldehyde is >97% when the mole ratio of styrene to H2O2 is 1:3. 3.2.2. Effect of the Amount of Catalyst. The reaction was carried out with different amounts of catalyst with a 1:3 mole ratio of styrene to H2O2 for 10 h at 80 °C. The conversion and selectivity are reported in Figure 5. It is seen from Figure 5 that the conversion increases with an increase in the amount of PMo11Co from 5 to 30 mg. With 20 mg of catalyst, the

Figure 2. XRD patterns of (a) PMo12, (b) PMo11Co, (c) PMo11Mn, and (d) PMo11Ni.

and 550−600 nm corresponding to the presence of CoII, MnII, and NiII in the compounds, respectively. 3.2. Oxidation of Styrene Using Hydrogen Peroxide. A detailed study was carried out on the oxidation of styrene over PMo11Co to optimize the conditions. In the present study, styrene upon oxidation gives benzaldehyde, styrene oxide, acetophenone, and benzoic acid. However, benzaldehyde was characterized as the major oxidation product. To ensure the catalytic activity, all reactions were carried out without catalyst. It was found that no oxidation takes place. 11915



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3.2.4. Effect of the Reaction Time. The oxidation of styrene with H2O2 was carried out by varying the reaction time at a mole ratio of 1:3 using 20 mg of catalyst. It is seen from Figure 7 that, with an increase in the reaction time, the conversion also

Figure 5. Effect of the amount of catalyst: % conversion is based on styrene; time = 10 h; temperature = 80 °C; molar ratio of styrene to H2O2 = 1:3.

conversion was 98.9%. Hence, further optimization of the conditions was carried out with 20 mg of catalyst. 3.2.3. Effect of the Temperature. The effect of the temperature on the oxidation of styrene was investigated at three different temperatures, viz., 60, 80, and 100 °C, keeping the other parameters fixed, namely, the amounts of styrene (10 mmol), 30% H2O2 (30 mmol), and PMo11Co (20 mg) and reaction time (10 h). The results are shown in Figure 6, which

Figure 7. Effect of the time: % conversion is based on styrene; amount of PMo11Co = 20 mg; temperature = 80 °C; molar ratio of styrene to H2O2 = 1:3.

increases. This is due to the fact that more time is required for the formation of a reactive intermediate (substrate + catalyst), which is finally converted into the products. The optimum conditions for maximum conversion of styrene to benzaldehyde are as follows: mole ratio of styrene to H2O2 = 1:3; amount of catalyst = 20 mg; reaction time = 10 h; temperature = 80 °C. 3.2.5. Oxidation of Styrene Catalyzed by PMo11M (M = Co, Mn, Ni). To optimize the conditions for other PMo11M (M = Mn, Ni) catalysts, detailed studies were carried out on the oxidation of styrene, and the results are summarized in Table S1 in the Supporting Information. At the same time, to compare the catalytic activity of synthesized catalysts, the oxidation of styrene was also carried out over parents PMo12, PMo11Mn, and PMo11Ni under the optimized conditions of PM11Co, and the results are presented in Table 2. It is seen Table 2. Comparative Data for the Oxidation of Styrenea

Figure 6. Effect of the temperature: % conversion is based on styrene; time = 10 h; amount of PMo11Co = 20 mg; molar ratio of styrene to H2O2 = 1:3.

catalyst

% conversion

% selectivity for benzaldehyde

TON

PMo12 PMo11Co PMo11Mn PMo11Ni

68.4 98.9 78.1 92.7

96.5 97.1 98.0 87.1

759 1264 998 1174

a % conversion is based on styrene; styrene to H2O2 = 1:3; amount of catalyst = 20 mg; temperature = 80 °C; time = 10 h.

from Table 2 that synthesized catalysts are more efficient than the cesium salt of parent PMo12. It is also observed from the results of Tables 2 and S1 in the Supporting Information (turnover frequency) that the order of activity of catalysts for the oxidation of styrene is PMo11Co > PMo11Ni > PMo11Mn. The difference in the reactivity of these catalysts can be explained on the basis of the reduction potential of the transition-metal ion substituted into the lacuna of the phosphomolybdate moiety. The values for the reduction potentials for cobalt, nickel, and manganese are −0.28, −0.27,

reveals that 73.9, 98.9, and 100% conversion was found to correspond to 60, 80, and 100 °C, respectively. At elevated temperature (100 °C), 100% styrene conversion was found with 72.1% selectivity of benzaldehyde and 27.9% selectivity for benzoic acid; i.e., a decrease in the selectivity for benzaldehyde was observed. This is due to overoxidation of benzaldehyde to benzoic acid at elevated temperature. Hence, further optimization of the conditions was carried out with 80 °C temperature. 11916



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and −1.18 V, respectively. Among all of the catalysts, PMo11Mn shows the lowest conversion compared to other PMo11M (M = Co, Ni). It is well-known that the more negative the reduction potential, the poorer its oxidizing ability. From the reduction potential value, it is clear that a catalyst consisting of manganese is expected to be less active compared to those of cobalt and nickel. So, the results are expected. However, a significant difference in the selectivity of benzaldehyde was observed (Table 2). Excellent conversion and selectivity was observed for PMo11Co compared to those of the other catalysts. 3.3. Recycling of the Catalyst. All of the catalysts are insoluble in aqueous as well as organic solvents. Upon the addition of H2O2, the insoluble catalysts become soluble in the reaction mixture because of the formation of a peroxo species. This peroxo species gave oxygen to the substrates and at the end of the reaction changed back to the insoluble state, which can be easily separated by simple filtration. Decomposition or leaching of the metal content from PMo11M was confirmed by carrying out an analysis of the used catalyst (EDX) as well as the product mixtures (atomic absorption spectroscopy). For all catalysts, analysis of the used catalyst did not show appreciable loss in the metal content compared to the fresh catalyst. Analysis of the product mixtures showed that if any metal was present, it was below the detection limit, which corresponded to less than 1 ppm. The separated catalysts were washed with dichloromethane and dried at 100 °C. The recycled catalyst PMo11Co obtained was characterized by FT-IR. The FT-IR spectra for the fresh as well as regenerated catalyst (two cycles) are represented in Figure 8. No appreciable shift in the FT-IR

Table 3. Oxidation of Styrene with Fresh and Recycled Catalystsa catalyst

% conversion

% selectivity of benzaldehyde

TON

PMo11Co R1-PMo11Co R2- PMo11Co PMo11Mn R1-PMo11Mn R2- PMo11Mn PMo11Ni R1-PMo11Ni R2-PMo11Ni

98. 9 95.1 94.2 78.1 78.0 77.8 92.7 92.0 92.0

>97

1264 1216 1204 998 996 994 1174 1165 1165

>98

>87

a

% conversion is based on styrene; amount of catalyst = 20 mg; temperature = 80 °C; molar ratio of styrene to H2O2 = 1:3, time = 10 h.

present catalyst, PMo11Co, is better in terms of conversion of styrene as well as selectivity of benzaldehyde. Moreover, reported reactions were carried out in acetonitrile as the solvent, while the present catalytic system offered solvent-free oxidation reactions. Comparative catalytic data of transition-metal-substituted (cobalt and managanese) phosphomolybdates, i.e., PMo11M with their phosphotungstate analogues,25 are also summarized in Table 4. PW11Co showed >99% conversion of styrene in 14 h, whereas in the case of PMo11Co, almost the same conversion was observed only in 10 h under the same reaction conditions. Similarly, 20 mg of PW11Mn showed 83% conversion of styrene in 14 h under similar conditions, and 25 mg of PMo11Mn showed complete conversion of styrene. From these results, it is clear that PMo11M are better catalysts compared to their tungstate analogues for the present catalytic systems. 3.4. Kinetics. A study of the kinetic behavior was carried out for all three catalysts. In all of the experiments, reaction mixtures were analyzed at fixed intervals of time using gas chromatography. 3.4.1. Determination of the Order as Well as Rate of Reaction. The plot of ln C0/C versus time (Figure 9) shows a linear relationship of the styrene consumption with respect to time. With an increase in the reaction time, there is a gradual decrease in the styrene concentration over both catalysts. A slight deviation from linearity was observed for all of the catalysts. These observations indicate that the oxidation of styrene is expected to be pseudo-first-order. This was further supported by the study of the effect of the catalyst concentration on the rate of oxidation of styrene. The catalyst amount was varied from 5 to 25 mg at a fixed substrate concentration of 10 mmol and at a temperature of 80 °C. The plot of the reaction rate versus catalyst amount (Figure 10) also shows a linear relationship for PMo11Mn and a pseudolinear relationship for PMo11Co and PMo11Ni. As the concentration of the active species increased from 5 to 20 mg, the rate of reaction also increased for both catalysts. The above study confirms that the initial rates of oxidation of styrene follow pseudo-first-order kinetics with respect to the substrate as well as the catalyst for all of the catalysts. On the basis of the above results, the rate law was also deduced for both catalysts and was found to obey eq 1. Furthermore, the rate constants were determined using eq 1, and the values for the same are reported in Table 5.

Figure 8. FT-IR spectra of (a) PMo11Co and (b) R-PMo11Co.

band position of the regenerated catalyst compared to fresh PMo11Co indicates retention of the Keggin-type structure of PMo11Co; i.e., PMo11Co is stable under the present reaction conditions. The regenerated catalyst showed no significant decrease in the conversion as well as selectivity (Table 3). Thus, the above study indicates that the catalysts are stable and can be regenerated for up to two cycles. Table 4 represents a comparison of the catalytic activity of the present catalyst with those of reported catalysts. Hu et al. reported the use of XW11Co (X = P, Si) as catalysts for the oxidation of styrene with H2O2 using acetonitrile as the solvent.27 54.7% and 56.1% conversion with 67.0% and 64.3% selectivity of benzaldehyde was found in 12 h for PW11Co and SiW11Co, respectively. Upon comparison, it was found that the 11917



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Table 4. Comparison of the Catalytic Activity with Those of Reported Catalysts

a

entry

catalyst

reaction conditionsa

T (°C)

% conversion

% selectivity of benzaldehyde

ref

1 2 3 4 5 6

PMo11Co PMo11Mn PW11Co PW11Mn PW11Co SiW11Co

10:30:20:10:0 10:30:25:14:0 10:30:20:14:0 10:30:20:14:0 2:2:100:12:5 2:2:100:12:5

80 80 80 80 60 60

98.9 98.2 >99 83.0 54.7 56.1

97.1 94.4 99.0 99.0 67.0 64.3

present work present work 25 25 27 27

% conversion is based on styrene:styrene to H2O2:amount of catalyst (mg):time (h):solvent (mL).

This may be due to activation of the catalytic species with temperature. The graph of ln k versus 1/T was plotted (Figure 11), and the value of the activation energy (Ea) was determined from the plot.

Figure 9. Styrene consumption as a function of the reaction time (catalyst amount = 20 mg; T = 353 K): (a) PMo11Co; (b) PMo11Ni; (c)PMo11Mn. Figure 11. Arrhenius plot: (a) PMo11Co; (b) PMo11Ni; (c) PMo11Mn.

The catalytic as well as kinetic data indicate that all of the catalysts are highly reactive toward solvent-free liquid-phase oxidation of styrene under mild reaction conditions. From the catalytic study as well as Ea value (Table 5), it is clear that the order of activity of the catalysts is PMo11Co > PMo11Ni > PMo11Mn for the present reaction.

4. CONCLUSION The present contribution reports clean and environmentally benign solvent-free oxidation of styrene over PMo11M (M = Co, Mn, Ni). The superiority of the present work lies in obtaining higher conversion and selectivity for the desired product with high TON. The catalysts can be reused for up to two cycles without any significant loss in conversion as well as selectivity. Catalytic and kinetic studies revealed that PMo11Co is the best catalyst among all and the order of activity of catalysts for the oxidation of styrene is PMo11Co > PMo11Ni > PMo11Mn. Encouraging results of the oxidation of styrene show that the present catalysts can be used for solvent-free selective oxidation of other organic substrates with H2O2.

Figure 10. Effect of the catalyst amount on the rate of reaction: (a) PMo11Co; (b) PMo11Ni; (c)PMo11Mn.

−d[BA] = k[sty][cat] dt

(1)

Table 5. Kinetic Parameters for the Oxidation of Styrene over PMo11Co, PMo11Mn, and PMo11Ni catalyst

rate consant k × 10−3 (m−1)

activation energy Ea (kJ/mol)

preexponential factor A

PMo11Co PMo11Mn PMo11Ni

3.87 2.03 3.03

62.9 76.1 67.3

5.9 × 106 2.6 × 108 2.3 × 107

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ASSOCIATED CONTENT

S Supporting Information *

Optimized reaction conditions for oxidation of styrene catalyzed by PMo11M (M = Co, Mn, Ni). This material is available free of charge via the Internet at http://pubs.acs.org.

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3.4.2. Effect of the Temperature. Because most of the oxidation reactions are temperature-sensitive, the effect of the temperature on the oxidation of styrene was also studied by varying the temperature between 333 and 373 K and keeping the styrene to H2O2 ratio at 1:3 and the catalyst amount at 20 mg. As the temperature increases from 333 to 373 K, the conversion of styrene also increases drastically for all catalysts.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91 265 2795552. Fax: +91 265 2795392. Notes

The authors declare no competing financial interest. 11918



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Substituted Heteropolyacid Catalysts. Ind. Eng. Chem. Res. 2005, 44, 1−7. (20) Benadji, S.; Eloy, P.; Leonard, A.; Su, B.-L.; Rabia, C.; Gaigneaux, E. M. Characterization of H3+xPMo12−xVxO40 heteropolyacids supported on HMS mesoporous molecular sieve and their catalytic performance in propene oxidation. Microporous Mesoporous Mater. 2012, 154, 153−163. (21) Kala Raj, N. K.; Ramaswamy, A. V.; Manikandan, P. Oxidation of norbornene over vanadium-substituted phosphomolybdic acid catalysts and spectroscopic investigations. J. Mol. Catal. A: Chem. 2005, 227, 37−45. (22) Benlounes, O.; Cheknoun, S.; Mansouri, S.; Rabia, C.; Hocine, S. Catalytic activation of C−H bonds of hydrocarbons by heteropolycompounds. J. Taiwan Inst. Chem. Eng. 2011, 42, 132−137. (23) Kala Raj, N. K.; Deshpande, S. S.; Ingle, R. H.; Raja, T.; Manikandan, P. Immobilized molybdovanadophosphoric acids on SBA-15 for selective oxidation of alkenes. Stud. Surf. Sci. Catal. 2005, 156, 769−778. (24) Neumann, R.; Dahan, M. Ruthenium substituted Keggin type polyoxomolybdates: synthesis, characterization and use as bifunctional catalysts for the epoxidation of alkenes by molecular oxygen. Polyhedron 1998, 17, 3557−3564. (25) Shringarpure, P.; Patel, K.; Patel, A. First Series Transition Metal Substituted Phosphotungstates as Catalysts for Selective NonSolvent Liquid Phase Oxidation of Styrene to Benzaldehyde: A Comparative Study. J. Cluster Sci. 2011, 22, 587−601. (26) Patel, A.; Pathan, S. Keggin-type cesium salt of first series transition metal-substituted phosphomolybdates: one-pot easy synthesis, structural, and spectral analysis. J. Coord. Chem. 2012, 65, 3122−3132. (27) Hu, J.; Li, K.; Li, W.; Ma, F.; Guo, Y. Selective oxidation of styrene to benzaldehyde catalyzed by Schiff base-modified ordered mesoporous silica materials impregnated with the transition metalmonosubstituted Keggin-type polyoxometalates. Appl. Catal., A 2009, 364, 211−220.

ACKNOWLEDGMENTS S.P. is thankful to UGC-MANF, New Delhi, India, for financial support.

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