Liquid-Phase Oxidation of Benzene to Phenol over V-Substituted

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Ind. Eng. Chem. Res. 2005, 44, 1-7

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Liquid-Phase Oxidation of Benzene to Phenol over V-Substituted Heteropolyacid Catalysts Sho-ta Yamaguchi,† Shingo Sumimoto,† Yuichi Ichihashi,‡ Satoru Nishiyama,§ and Shigeru Tsuruya*,‡ Division of Molecular Science, Graduate School of Science and Technology, Department of Chemical Science and Engineering, Faculty of Engineering, and Environment Management Center, Kobe University, Nada, Kobe 657-8501, Japan

The liquid-phase oxidation of benzene was carried out in aqueous acetic acid solvent over V-substituted heteropolyacids (V-HPAs) using molecular O2 and ascorbic acid as the oxidant and reducing reagent, respectively. Phenol was exclusively obtained as the oxygenation product. The elimination of the V species from the V-HPA (Keggin structure) catalyst into the reaction solvent during the benzene oxidation reaction was largely inhibited by ion-exchanging the proton of the V-HPA catalyst with Cs+. The main active species were assumed to be the V species anchored in the HPA. The influences of the reaction temperature, the concentration of acetic acid in the aqueous solvent, and the reaction pressure on the yield of phenol were investigated to obtain the optimal reactions condition for phenol formation. The reuse of the V-HPA catalyst caused gradual deactivation for phenol formation, despite the retention of the structure of the V-HPA catalyst. The deactivation was suggested to be due to the reduction of the V species in the V-HPA catalyst on the basis of the diffuse reflection spectra of the used catalysts. 1. Introduction Phenol is a versatile chemical compound that is mainly manufactured at present using the cumene process, which comprises three steps. Although each of these three steps gives a high yield, acetone, in an amount equivalent to the formed phenol, is concomitantly produced as the byproduct. Direct oxygenation of benzene to phenol by molecular oxygen is an attractive method for phenol production, even though it is one of the most difficult oxidation reactions. A heteropolyacid (HPA) as a solid catalyst has two attractive features: its redox ability can be controlled by varying the component elements, and it exhibits pseudo-liquid properties. Heteropolyacids with W as polyatoms and various types of structures were attempted as a catalyst for benzene oxidation to phenol using H2O2 as the oxidant.1 The benzene oxidation was conducted in the presence of both palladium and VPI5, which is an aluminophosphate microporous material consisting of 18 tetrahedral atoms, or MCM-41, in which a pore heteropolyacid was introduced, using O2 as the oxidant.2,3 We have reported the liquid-phase oxidation of benzene over supported vanadium (V/SiO2, V/Al2O3) catalysts using O2 and ascorbic acid as the oxidant and reducing reagent, respectively.4,5 One drawback of the * To whom correspondence should be addressed. Tel. and Fax: +81-78-803-6171. E-mail: [email protected]. † Division of Molecular Science, Graduate School of Science and Technology. ‡ Department of Chemical Science and Engineering, Faculty of Engineering. § Environment Management Center.

supported V catalysts was the considerable elution of the supported V species into the reaction solution during the benzene oxidation.4,5 In this study, we applied several V-substituted heteropolyacids as catalysts for the liquid-phase oxidation of benzene using O2 and ascorbic acid as the oxidant and reducing reagent, respectively. We focused on the activity of phenol formation and also the extent of leaching of the V species during benzene oxidation. The V-substituted HPA catalysts were also found to be active for phenol formation using zinc powder, in place of ascorbic acid, as the reducing reagent. 2. Experimental Section 2.1. Catalysts. A vanadium catalyst (V/Al2O3) impregnated on Al2O3 support was prepared4 by a conventional impregnation method using VO(C5H7O2)2 (Nacalai Tesque, guaranteed reagent) dissolved in ethanol and Al2O3 (JRC-ALO-6). A small amount of oxalic acid was added to the ethanol in order to dissolve the V salt homogeneously. After evaporation to dryness, the resulting V catalyst was dried at 393 K overnight and calcined at 623 K for 3 h in flowing air. Vanadium substituted heteropolyacids (V-HPA) with Mo as the polyatoms were prepared at the P/Mo/V ratio of 1:(12 - n):n (n ) 0-3) using MoO3, V2O5, and aqueous 85% H3PO4.6 A 9.3-11.3-g sample of MoO3 (Nacalai Tesque, guaranteed reagent) and 0.65-1.95 g of V2O5 (Nacalai Tesque, guaranteed reagent) were added to 180 cm3 of ion-exchanged water. A fraction of one-third of 0.5 cm3 of an aqueous 85% H3PO4 (Nacalai Tesque, guaranteed reagent) was added three times to the

10.1021/ie040220b CCC: $30.25 © 2005 American Chemical Society Published on Web 12/09/2004

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aqueous solution at intervals of 10 min followed by the addition of 80 cm3 of ion-exchanged water and refluxing at 343-353 K for 24 h. After the solution had been evaporated to dryness and left at room temperature overnight, the resulting H3+nPMo12-nVnO40 (HPMoVn) solid was further dried at 393 K for 3 h. Cs3+nPMo12-nVnO40 (CsPMoVn) was prepared by adding the specified amount of HPMoVn in 10 cm3 of an aqueous CsNO3 (0.01 mol/L, Nacalai Tesque, guaranteed reagent) solution to obtain the Cs/P ratio of 10:1. The resulting precipitate of Cs ion-exchanged PMoVn (CsPMoVn) was dried at room temperature overnight and then at 393 K for 3 h. Rb5PMo10V2O40 was prepared in a similar way using an aqueous CH3COORb (0.01 mol/L, Nacalai Tesque, guaranteed reagent) solution. A vanadium-substituted heteropolyacid (V-HPA) with W as the polyatoms was prepared using NaVO3, Na3PO4, and Na2WO4.7 To NaVO3 (0.61-1.83 g, Nacalai Tesque, guranteed reagent) dissolved in 80 mL of deionized water at 80 °C were added 10 mL of aqueous Na3PO4 (1.9 g, Nacalai Tesque, guranteed reagent) solution and 0.5 mL of HCl (35-37%) followed by 20 mL of Na2WO4‚12H2O (14.8-19.8 g, Nacalai Tesque, guranteed reagent) and 8.5 mL of HCl (35-37%) under stirring [P/W/V ) 1:(12 - n):n (n ) 0-3)]. A 10-mL portion of an aqueous CsNO3 solution (0.05 mol/L) was added to the resulting solution to produce Cs3+nPW12-nVnO40 (CsPWVn) precipitate. The precipitate (CsPWVn) was filtered off and dried at room temperature overnight and at 393 K for 3 h. 2.2. Liquid-Phase Oxidation of Benzene. Benzene (Nacalai Tesque, guaranteed reagent), ascorbic acid (Nacalai Tesque, guaranteed reagent), and acetic acid (Nacalai Tesque, guaranteed reagent) were used as received without further purification. A 0.5-cm3 sample of benzene (5.6 mmol), V-substituted HPA catalyst (V, 0.039 mmol), ascorbic acid (0-5 mmol), and 5 cm3 of an aqueous acetic acid solution were added into a stainless steal pressured reactor (Taiatsu Glass Co.; inner diameter, 1.7 cm; height, 11.2 cm). The oxidation was conducted for 24 h under stirring with a magnetic stirrer at 303-353 K under an O2 atmosphere of 0.4 MPa. After the oxidation, 5 cm3 of 2-propanol, which was confirmed as unchanged during the aftertreatment, was added to the reaction mixture as an internal standard. After centrifugal separation of the solid catalyst, the reaction products were analyzed at 363 K using a gas chromatograph (Shimazu, GC-8A) equipped with a 3-m stainless steel column packed with Silicon OV-17. No other product except phenol was detected, and the carbon balance was usually more than 90%. The percentage of the V leaching from the catalyst to the reaction solution during the oxidation was obtained by measuring the V amount eluted in the reaction solution using a Shimazu AA-6200 atomic absorption instrument. When zinc powder was used in place of ascorbic acid as the reducing reagent, the procedures and the conditions were the same except for the difference in the reducing reagent. 3.3. Measurement of the IR Spectra of the Catalysts. The IR spectra of the fresh and used catalysts were measured using a KBr disk mounted in an infrared spectrophotometer (Nihon Bunko VALOR-III). The sample (inner diameter, ca. 20 mm) was prepared by grinding a catalyst (ca. 1 mg) and KBr (ca. 100 mg) in an agate mortar and pressing the KBr sample powder at about 200 kg/cm2. The used catalysts were washed three times with acetone and dried at room temperature

Figure 1. FT-IR spectra of HPMoVn: (a) HPMo, (b) HPMOV1, (c) HPMoV2, (d) HPMoV3.

overnight and then at 393 K for 3 h before the IR measurement. 3.4. Measurement of X-ray Diffraction (XRD) patterns of the Catalysts. The powder XRD patterns of the fresh and used catalysts were observed at room temperature using a Rigaku RINT 2000 XRD instrument with a Cu KR source. The used catalysts were washed three times with acetone and dried at room temperature overnight and then at 393 K for 3 h before the XRD measurement. 3.5. Measurement of BET Surface Areas of the Catalysts. The BET surface areas of the catalysts were obtained from the observed amount of N2 adsorption at liquid N2 temperature using a static gas adsorption vacuum line. The sample (100 mg) was degassed at 473 K for 1 h. 3.6. Measurement of Diffuse Reflectance (DR) Spectra of the Catalysts. The DR spectra of the catalysts were measured in an in situ sample cell using an electronic absorption spectrophotometer (Hitachi U-3210D) equipped with an integral sphere (Hitachi 150-0902). The sample was degassed at room temperature for 1 h in the pretreatment chamber and transferred to the measured part of the in situ sample cell. The obtained reflectance data were converted to the Kubelka-Munk (K-M) function using an application program (U-3210/U-3410). 3. Results and Discussion The IR spectra of the prepared V-HPA, H3+nPMo12-nVnO40 (HPMoVn, n ) 0-3) are illustrated in Figure 1. All of the IR spectra were confirmed to have four IR peaks (PsO, 1070 cm-1; MosOsMo, 965 cm-1; ModO, 870, 790 cm-1) characteristic for a Keggin-type heteropolyacid.8 The increase in the amount of V introduced caused a shift of the IR peak at around 1080 cm-1 to a lower wavenumber, in agreement with the results reported previously.6,9 The prepared HPMoVn (n ) 0-3) catalysts were yellowish to orange in color, very soluble in water, and took on a blue color upon treatment with a mild reducing reagent; these results also provide qualitative support for the conclusion that the prepared HPMoVn (n ) 0-3) to have HPA structures. The Cs ion-exchanged counterparts (CsPMo, CsPMoV1,

Ind. Eng. Chem. Res., Vol. 44, No. 1, 2005 3 Table 1. Oxidation of Benzene at 303 K over V-Supported and V-Substituted HPA Catalystsa catalyst

phenol yield (%)

amount of V leached (%)

V/Al2O3 H5PMo10V2 Cs5PMo10V2

6.0 1.1 1.1

40.7 14.5 2.1

a V amount of catalyst, 0.039 mmol; benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 80 vol % acetic acid; ascorbic acid, 1 mmol; oxidant, 0.4 MPa O2; reaction temperature, 303 K; reaction time, 24 h.

Figure 2. FT-IR spectra of CsPMoVn: (a) CsPMo, (b) CsPMOV1, (c) CsPMoV2, (d) CsPMoV3.

Figure 4. Influence of reaction temperature on the yield of phenol and the percentage of V leached. Catalyst, 47 mg of Cs5PMo10V2O40 (amount of V, 0.039 mmol); benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 80 vol % acetic acid; oxidant, 0.4 MPa O2; ascorbic acid, 1 mmol; reaction time, 24 h. Table 2. Oxidation of Benzene at 353 K over V-Supported and V-Substituted HPA Catalystsa

Figure 3. XRD patterns of CsPWVn: (a) CsPW, (b) CsPWV1, (c) CsPWV2, (d) CsPWV3.

CsPMoV2, and CsPMoV3,) of HPMoVn (n ) 0-3) were also confirmed to have a Keggin structure from their IR spectra (Figure 2). The IR spectra of the CsPWVn (n ) 0-3) also had four IR peaks in the vicinity of 807 and 887 cm-1 (WsOsW), 985 cm-1 (WdO), and 1080 cm-1 (PsO) (figure not shown). The introduction of a V atom to the CsPWVn (n ) 0) also caused a slight shift of the IR peaks in the neighborhood of 985 and 1080 cm-1 to a lower wavenumber, as reported previously.6 The XRD patterns of the CsPWVn (n ) 0-3) catalysts were substantially similar to those reported recently10 (Figure 3), although the CsPWV3 included some XRD patterns in addition to the ones based on the heteropolyanion, because of the instability of the heteropolyanion structure of CsPWV3.7 Rb ion-exchanged V-substituted HPA (RbPMoV2) was also identified as having a Keggin-type structure from the IR spectra. 3.1. Influence of Reaction Conditions on the Liquid-Phase Oxidation of Benzene over V-Supported and V-Substituted HPA Catalysts. The liquid-phase oxidation of benzene was carried out at 303 K in an aqueous solvent containing 80 vol % acetic acid using V/Al2O3, HPMoV2, and CsPMoV2 catalysts (Table 1). The yield of phenol over the V/Al2O3 catalyst was considerably higher than those over both the HPMoV2 and CsPMoV2 catalysts. However, nearly half of the supported V species leached out to the reaction solution during the benzene oxidation. The percentage of V leaching decreased significantly when the CsPMoV2 catalyst was used, even though the catalytic activity for

catalyst

phenol yield (%)

amount of V leached (%)

V/Al2O3 Cs5PMo10V2

7.6 7.2

37.6 5.2

a V amount of catalyst, 0.039 mmol; benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 80 vol % acetic acid; ascorbic acid, 1 mmol; oxidant, 0.4 MPa O2; reaction temperature, 353 K; reaction time, 24 h.

phenol formation was comparatively low. The Keggin structure, which firmly retains the V atoms of the CsPMoV2 catalyst, is thought to prevent V leaching. Thereafter, we attempted to explore the reaction conditions required to promote the activity for phenol formation using alkali metal ion-exchanged V-substituted HPA catalysts. The influence of the reaction temperature on the yield of phenol was investigated using the CsPMoV2 catalyst (Figure 4). The percentage of V leached was less than 5% even at the higher reaction temperatures. The yield of phenol and the percentage of V leached over V/Al2O3 and CsPMoV2 catalysts were compared at a reaction temperature of 353 K (Table 2). Both the proton and alkali metal HPAS have been reported to maintain their structures up to about 723 K and their melting points, respectively.10 The V-substituted HPA was reported to begin to decompose at more than 473 K.7,11 Thus, the CsPMoV2 catalyst is thought to maintain its original structure during benzene oxidation in the temperature range of 303-363 K. The yield of phenol over the CsPMoV2 catalyst steadily increased with increasing reaction temperature as shown in Figure 4. The yield of phenol over V-impregnated SiO2 (V/SiO2) catalyst was previously reported4 to have a maximum value at a reaction temperature of around 333 K and inversely

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Figure 6. Influence of O2 pressure on the yield of phenol and the percentage of V leached. Catalyst, 47 mg of Cs5PMo10V2O40 (amount of V, 0.039 mmol); benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 60 vol % acetic acid; oxidant, O2; ascorbic acid, 1 mmol; reaction temperature, 353 K; reaction time, 24 h.

Figure 5. Influence of concentration of acetic acid on the yield of phenol and the percentage of V leached. Benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing acetic acid; oxidant, 0.4 MPa O2; ascorbic acid, 1 mmol; reaction temperature, 353 K; reaction time, 24 h; catalyst, (A) 47 mg of Cs5PMo10V2O40 (CsPMoV2) (amount of V, 0.039 mmol), (B) 64 mg of Cs5PW10V2O40 (CsPWV2) (amount of V, 0.039 mmol).

decreased with the further increase in the reaction temperature, thus differing from the CsPMoV2 catalyst. The decrease in the solubility of gaseous O2 with increasing reaction temperature is one of the reasons for having a maximum value over the V/SiO2 catalyst at a specified reaction temperature.4 The difference in the catalytic activity vs the reaction temperature between the CsPMoV2 and V/SiO2 catalysts might suggest that the oxygen species in the CsPMoV2 catalyst, rather than gaseous O2, directly participate in phenol formation.8,12 We observed variation in the color of the CsPMoV2 catalyst during redox treatment. The fresh CsPMoV2 catalyst, weak orange in color, changed to heteropoly-blue color in an aqueous solution including ascorbic acid, followed by another change to yellow-toorange color upon exposure to air. However, the reduced CsPMoV2 catalyst with heteropoly-blue color remained intact under a N2 atmosphere. This visual observation indicates that the reduced CsPMoV2 catalyst is oxidized by taking up gaseous O2 into the HPA structure. The influences of the acetic acid concentration on both the yield of phenol and the percentage of V leached were investigated at 353 K using both the CsPMoV2 and CsPWV2 catalysts (Figure 5A,B). The amount of V leached was below 10%, irrespective of the volumetric concentration of acetic acid. The yields of phenol over both catalysts increased with increasing acetic acid concentration and had maximum values at an acetic acid concentration of around 60 vol %. The solvent containing around 100 vol % acetic acid caused a decline in the phenol yields over both catalysts. Hydrogen peroxide

formed in the present reaction system during benzene oxidation is thought4,5 to play a pivotal role in phenol formation. H2O2, which directly participates in phenol formation, can be stably present in an acidic medium; on the other hand, the reduction of ascorbic acid, a step necessary for H2O2 formation, becomes difficult in a strong acid medium. These two contradicting effects of the acid medium will contribute to make a volcanic shape in the relationship between the acetic acid concentration and the yield of phenol. We investigated hereafter some factors affecting the yield of phenol using the aqueous solvent containing 60 vol % acetic acid. The influence of the O2 pressure on the yield of phenol over CsPMoV2 catalyst at 353 K is illustrated in Figure 6. The leaching of V species was inhibited in the range of O2 pressures applied. The yield of phenol increased with increasing O2 pressure, although the phenol yield tended to level off beyond an O2 pressure of around 0.5 MPa. The higher solubility of O2 in the solvent under the higher O2 pressure is thought to contribute to the promotion of phenol formation. Thus, the oxygendeficient sites within the catalyst, from which oxygen species were consumed during the benzene oxidation reaction as described above, are thought to be more easily filled with the dissolved oxygen if the O2 pressure is higher, so that the solubility of oxygen is higher. The effects of the amount of ascorbic acid on both the yield of phenol and the percentage of V leached was investigated at 353 K using both the CsPMoV2 and CsPWV2 catalysts (Figure 7A,B). No phenol was produced when no ascorbic acid was present as the reducing reagent. The yield of phenol increased with increasing amount of ascorbic acid up to 1.0-1.5 mmol, but a further increase in the amount of ascorbic acid inversely caused a decrease in the yield of phenol. The extra amount of ascorbic acid interacts with the H2O2 produced, which plays a pivotal role in phenol formation, to retard the phenol production. The percentage of V leached from the CsPMoV2 catalyst abruptly surged for amounts of ascorbic acid of around more than 2.0-3.0 mmol, although the V species in the CsPWV2 catalyst was comparatively resistant to the higher amounts of ascorbic acid. Both the IR spectra and the XRD patterns of the catalysts used under more than 3.0 mmol of ascorbic acid indicated the retention of the HPA structure in these used catalysts (figures not shown). The V species are thought to be easily leached from the HPA structure in the presence of an excess amount of ascorbic acid, although we do not know why. The leaching of the

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Figure 7. Influence of the amount of ascorbic acid on the yield of phenol and the percentage of V leached. Catalyst, 47 mg of Cs5PMo10V2O40 (amount of V, 0.039 mmol); benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 60 vol % acetic acid; oxidant, 0.4 MPa O2; reaction temperature, 353 K; reaction time, 24 h; catalyst, (A) 47 mg of Cs5PMo10V2O40 (CsPMoV2) (amount of V, 0.039 mmol), (B) 64 mg of Cs5PW10V2O40 (CsPWV2) (amount of V, 0.039 mmol)

V species is thus thought to be one of the causes of the decline in the yield of phenol in the presence of an excess amount of ascorbic acid. The dependence of both the yield of phenol and the percentage of V leached on the reaction time was examined using the CsPMoV2 and CsPWV2 catalysts as illustrated in Figure 8A,B. The yields of phenol over both catalysts sharply increased in the initial reaction stages; during the initial stages, the leaching of the V species was low. This result implies that the V species within the HPA framework, rather than the V species dissolved in the solvent, directly participate in phenol formation. 3.2. Influence of the V Atoms Present in the V-Substituted HPAs. Both the catalytic activities for phenol formation and the percentage of leaching of the V species were investigated at 353 K using the V-substituted HPA catalysts [CsPMoVn (n ) 0-3)] varying the number of incorporated V atoms (Table 3). The CsPMo catalyst having no V atom was found to have some catalytic activity for phenol formation, although the yield of phenol was comparatively low. The CsPMoVn (n ) 1-3) catalysts had almost the same activities for phenol formation. As the V species present in all of the catalysts were set to the same content (V, 0.039 mmol), the amount of V present in the reaction system, irrespective of the CsPMoVn (n ) 1-3) catalysts, is suggested to govern the catalytic activity. The percentage of V leached was comparatively low, irrespective of the catalysts. The catalytic activity for phenol formation, together with the surface area obtained by the BET method, was compared using CsPMoV2, RbPMoV2, and CsPWV2 catalysts (Table 4). The result that the ratios of the yield

Figure 8. Dependence of the yield of phenol and the percentage of V leached on the reaction time. Catalyst, 47 mg of Cs5PMo10V2O40 (amount of V, 0.039 mmol); benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 60 vol % acetic acid; oxidant, 0.4 MPa O2; ascorbic acid, 1 mmol; reaction temperature, 353 K; catalyst, (A) 47 mg of Cs5PMo10V2O40 (CsPMoV2) (amount of V, 0.039 mmol), (B) 64 mg of Cs5PW10V2O40 (CsPWV2) (amount of V, 0.039 mmol). Table 3. Oxidation of Benzene over CsPMoVn (n ) 0-3) Catalystsa catalyst

phenol yield (%)

amount of V leached (%)

CsPMo CsPMoV CsPMoV2 CsPMoV3

0.8 6.5 7.2 7.2

4.6 5.2 4.5

a V amount of catalyst, 0.039 mmol; benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 80 vol % acetic acid; ascorbic acid, 1 mmol; oxidant, 0.4 MPa O2; reaction temperature, 353 K; reaction time, 24 h.

Table 4. Benzene Oxidation over CsPMoV2, RbPMoV2, and CsPWV2 Catalystsa

catalyst

phenol yield (%)

surface area (m2/g)

yield of phenol per unit surface area (%‚g/m2)

CsPMoV2 RbPMoV2 CsPWV3

7.2 5.5 6.0

125 82 159

0.058 0.067 0.038

a V amount of catalyst, 0.039 mmol; benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 60 vol % acetic acid; ascorbic acid, 1 mmol; oxidant, 0.4 MPa O2; reaction temperature, 353 K; reaction time, 24 h.

of phenol to the surface areas of these catalysts were different from catalyst to catalyst suggests that the phenol yield is independent of the BET surface area of the catalyst. Taking into account the result that the V species held in the HPA framework mainly participate in phenol formation, the V species within the bulk of the HPA structure, in addition to those present on the

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Table 5. Oxidation of Benzene over the Used CsPMoV2 Catalystsa trial

phenol yield (%)

amount of V leached (%)

1 2 3

7.2 5.1 1.3

5.2 3.4 3.6

a V amount of catalyst (CsPMoV ), 0.039 mmol; benzene, 5.6 2 mmol; solvent, 5 cm3 of an aqueous solution containing 80 vol % acetic acid; ascorbic acid, 1 mmol; oxidant, 0.4 MPa O2; reaction temperature, 353 K; reaction time, 24 h.

Figure 9. Diffuse reflectance (DR) spectra of the fresh and used CsPMoV2 catalysts: (a) fresh CsPMoV2 catalyst, (b) used CsPMoV2 catalyst (first), (c) used CsPMoV2 catalyst (second).

surface, are also thought to contribute to phenol formation, although further experimental evidence is necessary. Thus the V-substituted HPA catalysts are assumed to be present in what we call a pseudo-liquid state, in which state the bulk V species in the HPA structure are accessible to a reactant, as in the present reaction system. It has been reported11 that HPA behaves similarly to a pseudo-liquid phase. 3.3. Behavior and State of Used CsPMoV2 Catalysts. The catalytic behavior in the reuse of the CsPMoV2 catalyst is reported in Table 5. The used catalyst was washed with acetone three times, dried at room temperature overnight and at 393 K for 3 h each time before being used as the catalyst in benzene oxidation. The yield of phenol decreased over the used CsPMoV2 catalyst with each iteration, despite the low percentages of V leached. The decrease in the phenol yield thus seems not to be due to the leaching of V species from the CsPMoV2 framework. The IR spectra of these used catalysts (figure not shown) confirmed that the HPA structures of all the used CsPMoV2 catalysts are maintained even after the benzene oxidation. Visual observations of the fresh and used CsPMoV2 catalysts indicated that the color of the catalysts gradually changed from yellow to heteropoly blue, suggesting that the used CsPMoV2 catalysts are in the reduced state. The diffuse reflectance (DR) spectra of the fresh and the used CsPMoV2 catalysts were measured as illustrated in Figure 9. The DR peaks at around 380 nm of the used CsPMoV2 catalysts shifted to shorter wavelengths, suggesting that the used catalysts are in more reduced states than the fresh catalyst. Judging from the reaction results (Table 5) and the reducing character of the used catalysts, the decline in the phenol yield over the used catalyst is due to the reduced state of the used catalyst. To attempt to regenerate the used CsPMoV2 catalyst, a twice-used sample was reoxidized by two methods: (i) treatment at 473 K for 5 h in oxygen flow and (ii)

Figure 10. Benzene oxidation over CsPMoV2 catalyst using zinc powder as the reducing reagent. Benzene, 5.6 mmol; solvent, 5 cm3 of an aqueous solution containing 60 vol % acetic acid; oxidant, 0.4 MPa O2; reaction temperature, 353 K; reaction time, 24 h; catalyst, (A) 47 mg of Cs5PMo10V2O40 (CsPMoV2) (amount of V, 0.039 mmol), (B) 64 mg of Cs5PW10V2O40 (CsPWV2) (amount of V, 0.039 mmol).

treatment at 353 K for 24 h under 0.8 MPa of O2 in an aqueous solution containing 60 vol % acetic acid. The yield of phenol over the used CsPMoV2 catalyst treated by method i was 1.6%, a value almost similar to that (1.3%) of the untreated one. The used catalyst treated by method i exhibited a green to blue color. Thus, the used CsPMoV2 catalyst is thought to be insufficiently reoxidized through treatment method i. However, calcination at more than 473 K causes the elimination of V species from the HPA framework. We attempted to reoxidize the used CsPMoV2 catalyst in the reaction solution as indicated in method ii. The used catalyst treated by method ii yielded 4.1% phenol; the color of the used catalyst after the treatment of method ii was yellow to green. These results indicate that the used CsPMoV2 catalyst was to some extent, but not perfectly, reoxidized through method ii. 3.4. Benzene Oxidation over CsPMoV2 Catalyst Using Zn Powder as the Reducing Reagent. To find an effective reducing reagent that is less expensive than ascorbic acid, zinc powder was used as the reducing reagent in benzene oxidation over the CsPMoV2 and CsPWV2 catalysts (Figure 10A,B). The yields of phenol over both of the V-substituted heteropolyacid catalysts increased almost linearly with the increase in the amount of Zn. No phenol was produced over whether the CsPMoV2 or the CsPWV2 catalyst using ZnO or Zn(NO3)2‚6H2O in place of Zn powder. Zn0 species are thus thought to be effective as a reducing reagent in the liquid-phase oxidation of benzene to phenol. The percentage of V leached increased with increasing amount of Zn up to around 1-2 mmol of Zn, but a further increase of the amount of Zn caused a leveling off of the leaching of V. The Zn ions produced from the oxidation of Zn powder during benzene oxidation might substitute for the V species in the HPA framework. A

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study of the efficiency of zinc powder as the reducing reagent over supported V catalysts including the V-substituted HPA catalysts is now under way and will be reported soon. 4. Conclusions Phenol was exclusively produced in the liquid-phase oxidation of benzene carried out in the aqueous acetic acid solvent over V-substituted heteropolyacid (V-HPA, i.e., heteropolymolybdate, heteropolytungstate) catalysts using molecular O2 and ascorbic acid as the oxidant and reducing reagent, respectively. The elimination of the V species from the V-HPA (Keggin structure) catalyst into the reaction solvent during the benzene oxidation reaction was largely inhibited by ion-exchanging the proton of the V-HPA catalyst with Cs+. The yield of phenol had an optimal value when both the concentration of acetic acid in the aqueous solvent and the amount of ascorbic acid were varied. The oxidizing V species anchored in the HPA were assumed to be the main active species for phenol formation. Zinc powder (Zn0) was found to be an effective reducing reagent for phenol formation over the V-HPA catalysts, similarly to ascorbic acid. Acknowledgment The authors thank Mr. Kenji Nomura of Kobe University for his technical assistance during this study. Literature Cited (1) Nomiya, K.; Yanagibayashi, H.; Nozaki, C.; Kondoh, K.; Hiramatsu, E.; Shimizu, Y. Hydroxylation of Benzene Catalyzed by Selectively Site-Substituted Vanadium(V) Heteropolytungstates in the Presence of Hydrogen Peroxide. J. Mol. Catal. A: Chem. 1996, 114, 25. (2) Passoni, L. C.; Cruz, A. T.; Buffon, R.; Schuchardt, U. Direct Selective Oxidation to Phenol Using Molecular Oxygen in the Presence of Palladium and Heteropolyacids. J. Mol. Catal. A: Chem. 1997, 120, 117.

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Received for review August 13, 2004 Revised manuscript received October 21, 2004 Accepted October 26, 2004 IE040220B