Gas-Phase Catalytic Oxidation of Benzene to Phenol over Cu

Liquid phase hydroxylation of benzene to phenol over Cu/ZnO catalysts. İsmail Boz , Tuba Gürkaynak Altınçekiç. Reaction Kinetics, Mechanisms and ...
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Ind. Eng. Chem. Res. 2005, 44, 8765-8772

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Gas-Phase Catalytic Oxidation of Benzene to Phenol over Cu-Impregnated HZSM-5 Catalysts Yusuke Shibata,† Rei Hamada,† Takahiro Ueda,† Yuichi Ichihashi,‡ Satoru Nishiyama,§ and Shigeru Tsuruya*,‡ Division of Chemical Science and Engineering, Graduate School of Science and Technology, Kobe University, Nada, Kobe 657-8501, Japan, Department of Chemical Science and Engineering, Faculty of Engineering, Kobe University, Nada, Kobe 657-8501, Japan, and Center for Environmental Management, Kobe University, Nada, Kobe 657-8501, Japan

The utilization of copper-impregnated HZSM-5 (Cu/HZSM-5) zeolites as catalysts for the gasphase catalytic oxidation of benzene was attempted. The major products were phenol, carbon dioxide, and carbon monoxide. The Si/Al ratio in the HZSM-5 support of the Cu/HZSM-5 catalyst influenced both the yield of and the selectivity for phenol, both of which increased with a decrease in the Si/Al atomic ratio of the HZSM-5 support with constant Cu loading. The amount of carbon monoxide uptake of the prereduced Cu/HZSM-5 catalysts decreased with a decrease in the Si/ Al atomic ratio of the HZSM-5 support of the Cu/HZSM-5 catalyst. The yield of phenol over the Cu/HZSM-5 catalyst increased, along with both the yield of CO2 and CO, by adding water vapor in the reaction system. The oxygen species adsorbed on the Cu/HZSM-5 catalyst were suggested to be responsible for the formation of phenol, based on the results of the transition response experiment. The amount of the isolated Cu2+ species with square-pyramidal configuration on the Cu/HZSM-5 catalyst estimated from the ESR observation was found to have a correlation with the yield of phenol. 1. Introduction More than 90% of phenol, a versatile chemical, is currently produced by a cumene method which consists of three steps and yields acetone as a byproduct. The one-step synthesis of phenol through the direct oxidation of benzene is desirable and attractive from an economical point of view but has been known to be one of the most difficult oxygenation reactions. Studies on phenol synthesis through the liquid-phase and/or the gas-phase catalytic oxidation of benzene have been reported over a catalytic system using some oxidants. Fenton-type systems using H2O2 as an oxidant and producing OH radicals have been employed1-4 for the transformation of benzene to phenol through hydroxylation. The oxidation of benzene to phenol using oxygen gas, instead of H2O2, in the presence of CuCl occurred under ambient conditions.5-9 More recently, we also reported the liquidphase oxidation of benzene to phenol over supported Cu10-14 and vanadium15,16 catalysts using gaseous oxygen as an oxidant and ascorbic acid as a reducing reagent, respectively. Vanadium-substituted heteropolyacid, in the presence of both oxygen and ascorbic acid, has been reported17 to be active for the liquid-phase oxidation of benzene to phenol. The gas-phase catalytic oxidation of benzene to phenol over a V2O5/SiO2 catalyst was carried out using N2O as an oxidant.18 Fe-supported ZSM-5 zeolites were reported to be very effective for the gas-phase oxidation of benzene to phenol using N2O as an oxidant,19-21 and the surface oxygen species produced * To whom correspondence should be addressed. Tel. and fax: +81-78-803-6171. E-mail: [email protected]. † Division of Chemical Science and Engineering, Graduate School of Science and Technology. ‡ Department of Chemical Science and Engineering, Faculty of Engineering. § Environmental Management Center.

from the N2O oxidant were suggested to be responsible for the phenol formation. HZSM-5 calcined at 1120 K was reported to be an active catalyst for the gas-phase catalytic oxidation of benzene to phenol using N2O as an oxidant.22 The gas-phase catalytic oxidation of benzene to phenol over a composite Cu-Pd catalyst was attempted in the presence of both O2 and H2.23,24 A vanadium-molybdenum composite oxide catalyst supported on SiO2 was used as a catalyst for benzene oxidation to phenol in the presence of water vapor.25 We have reported the gas-phase catalytic oxidation of benzene to phenol over Cu-ion-exchanged HZAM-5 catalysts in the presence of O2.26 The high temperature calcination of the Cu-supported HZAM-5 catalyst prepared by a solid ion-exchanged, an ion-exchanged, or an impregnated method caused an increase in the catalytic activity for the phenol formation.27 In the extension of the study on the gas-phase catalytic oxidation of benzene with O2 over Cu-supported HZSM-5 catalysts, we have investigated benzene oxidation over the Cu catalysts (Cu/HZSM-5) impregnated on HZSM-5 zeolites with different Si/Al atomic ratios and the influence of the reaction conditions on both the yield of and selectivity for phenol. To investigate the role of oxygen species in the benzene oxidation, the behaviors of both the phenol and carbon oxide (CO2 + CO) formations were followed by a transient response experiment. The coordination and valence states of the Cu species on the Cu/HZSM-5 catalysts were studied using both ESR spectra and CO adsorption data, the data from which were used to elucidate what kind of Cu species is responsible for the phenol formation. 2. Experimental Section 2.1. Catalysts. The ZSM-5 zeolites were synthesized by a conventional hydrothermal synthesis according to

10.1021/ie050821i CCC: $30.25 © 2005 American Chemical Society Published on Web 10/07/2005

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a patent.28 To obtain NaZSM-5 zeolites ion-exchanged with almost 100% Na, the as-synthesized ZSM-5 zeolites were ion-exchanged with a 1 N NaNO3 aqueous solution at 353 K for 1 h, washed with deionized water, dried at 383 K overnight, and calcined at 773 K for 5 h in flowing air. HZSM-5 zeolites were obtained by ion-exchanging the NaZSM-5 zeolties in a 1 N NH4NO3 aqueous solution at 353 K for 6 h, washed with deionized water, and calcined at 773 K for 5 h in flowing air. Copperimpregnated HZSM-5 (Cu/HZSM-5) zeolites were prepared by impregnating with Cu(CH3COO)2‚H2O (Nacalai Tesque, guaranteed reagent), drying at 394 K, and calcining at 1123 K for 5 h in a N2 flow. All the Cu weight percent contents were calculated based on the corresponding Cu amount fed in the impregnation preparation. Alkali-metal-added Cu/HZSM-5 zeolites were obtained by impregnating with alkali metal acetates, drying at 393 K, and calcining at 773 K for 5 h. The Si/Al atomic ratios of the synthesized ZSM-5 zeolites were determined using atomic absorption spectroscopy (Shimazu type AA-630-01) after dissolving the zeolite samples homogeneously in a few drops of aqueous hydrogen fluoride (Sterachemi, guaranteed reagent, 47 vol %). 2.2. X-ray Diffraction (XRD) Measurements of the Catalysts. The XRD patterns of the Cu-supported HZSM-5 catalysts, in addition to the corresponding HZSM-5 zeolites, were observed at room temperature using XRD equipment (Rigaku Ultima+) with a Cu KR source. 2.3. BET Surface Area Measurements of the Catalysts. The BET surface areas of the Cu-impregnated catalysts, which were degassed at 773 K for 1 h as a pretreatment, were measured at 77 K using a microadsorption apparatus (residual pressure below 10-3 Pa). 2.4. Gas-Phase Catalytic Oxidation of Benzene. Benzene (Nacalai Tesque, guaranteed reagent) was employed as-received after confirming that no impurity was detected by gas-liquid chromatography (GLC) analysis. The reaction was performed using a conventional continuous fixed-bed flow Pyrex reactor (i.d., 18 mm) vertically placed in an electric furnace. Before starting the reaction, the catalyst in the reactor was calcined at 773 K for 2 h in flowing air, followed by a decrease in the temperature to 673 K in flowing N2. The oxidation was started by feeding benzene with a microfeeder in both flowing O2 and N2 under atmospheric pressure. The oxidation of benzene with H2O addition was performed by feeding H2O from another microfeeder. The typical reaction conditions were as follows: W/F, 236 g-cat min/mol (W, catalyst weight, 0.5 g; F, amount of total gas supply, 2.12 × 10-3 mol/min); N2/ O2/benzene, 4:1:0.18 (mole ratio) (in the case of H2O vapor addition, N2/O2/benzene/H2O ) 3.14-3.79:1:0.18: 0.21-0.86 (N2, as balance gas)); reaction temperature, 673 K. The gaseous products and the liquid product (and unreacted benzene) were separated by trapping the latter using a refrigerant consisting of dry ice and diethyl malonate (223 K). The trapped liquid portion was diluted with a mixed solvent of toluene and 2-propanol (1 cm3 each) and transferred for GLC analysis (Shimazu GC-14B) using a 1-m stainless steel column filled with silicon OV-17 in the temperature range of 453-493 K (8 K/min) in flowing N2. A 2-cm3 gas sample removed with a microsyringe was analyzed by GLC (Shimazu GC-14B) in a flowing H2 carrier with an

intermediate cell method29 using 1-m and 4-m stainless steel columns filled with activated charcoal at column temperatures of 413 K and room temperature, respectively, and a 1-m stainless steel column filled with molecular sieves 5A at a column temperature of room temperature. The carbon balance, the yield, and the selectivity were defined as follows: carbon balance (%) ) (sum of the mole numbers of all the products corresponding to benzene fed and unreacted benzene)/(mole number of benzene fed) × 100; yield (%) ) (mole number of a reaction product corresponding to benzene fed)/ (mole number of benzene fed) × 100; selectivity ) (mole number of a product corresponding to benzene fed)/(sum of the mole numbers of all products corresponding benzene fed) × 100. The carbon balances in this study were usually 90% to around 100%. The yield of a product was evaluated using the average values of times on stream of 3 and 4 h. 2.5. Measurement of Electron Spin Resonance (ESR) Spectra of the Cu Catalysts. The ESR spectra of the Cu/HZSM-5 catalysts were measured at room temperature using a JES-TE300 ESR instrument. The catalyst samples (ca. 0.2 g) were treated under various conditions in a sidearm and then were transferred in situ to quartz sample tubes. 2.6. Measurement of CO Adsorption on the Prereduced Cu Catalysts. The CO adsorption on the Cu/ HZSM-5 catalysts (0.05 g) was conducted using a semimicro constant-volume gas-adsorption apparatus equipped with a capillary sample tube. The Cu sample was degassed at 773 K for 0.5 h, followed by calcination at 773 K for 1 h under 20 kPa of O2 and degassing at 773 K for 2 h. After cooling to room temperature, the dead volume was measured by introducing 1.3 kPa of He, followed by degassing for 0.5 h. The total amount of CO adsorption was measured after introducing 2.6 kPa of CO and confirming the attainment of adsorption equilibrium. The amount of reversible adsorption was obtained by repeating the measurement of the CO adsorption after degassing at room temperature for 0.5 h. The amount of irreversible adsorption was estimated by subtracting the amount of reversible adsorption from the amount of total adsorption. 2.7. Measurement of O2 Uptake on the Prereduced Cu Catalysts. The amount of O2 uptake of the prereduced Cu/HZSM-5 catalysts was measured using a semimicro constant-volume gas-adsorption apparatus equipped with a capillary sample tube. A 0.05 g portion of Cu/HZSM-5 catalyst degassed at 773 K for 0.5 h was calcined at 773K for 1 h, followed by degassing at 773 K for 1 h and reducing under 20 kPa of CO at 773 K for 1 h. After degassing at 773 K for 1 h, cooling to 673 K, and introducing 1.3 kPa of He, the dead volume was measured, followed by degassing at 673 K for 0.5 h. The total amount of O2 uptake was measured, after adsorption equilibrium was attained, by introducing 2.6 kPa of O2. After degassing at 673 K for 1 h, the amount of reversible O2 uptake was obtained by repeating the measurement of O2 uptake. The amount of irreversible O2 uptake was estimated by subtracting the amount of reversible O2 uptake from the total amount of O2 uptake. 3. Results and Discussion The products detected in the gas-phase catalytic reaction of benzene over Cu-supported HZSM-5 catalysts were phenol, CO2, and CO. Only a trace of biphenyl

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Figure 1. Dependence of the yields of phenol, CO2, and CO and the selectivity for phenol on the reaction temperature: catalyst, 0.5 g of Cu(0.7)/HZSM-5(96); N2/O2/benzene, 4:1:0.18; b, yield of phenol; 2, yield of CO2; 9, yield of CO; O, selectivity for phenol.

Figure 2. Dependence of the yields of phenol, CO2, and CO and the selectivity for phenol on the partial pressure of O2: catalyst, 0.5 g of Cu(0.7)/HZSM-5(96); N2/O2/benzene, 4:1:0.18; reaction temperature, 673 K; b, yield of phenol; 2, yield of CO2; 9, yield of CO; O, selectivity for phenol.

was detected when the yield of phenol was comparatively high. 3.1. Effect of the Reaction Temperature on Benzene Oxidation. The yield of phenol over the Cu(0.7)/HZSM-5(96) (Cu, 0.7 wt %; Si/Al atomic ratio of HZSM-5, 96) catalyst increased with the reaction temperature until around 693 K, at which point a further increase in the temperature caused a decrease in the yield of phenol (Figure 1). Both the yields of CO2 and CO monotonically increased with an increase in the reaction temperature. The effect of the reaction temperature on the oxidation products was similar to that27 obtained over the Cu/HZSZM-5 catalyst prepared by a solid-state reaction method, though the yields of phenol over the present catalyst were higher. 3.2. Effect of the Partial Pressure of O2 on Benzene Oxidation. The yield of phenol over the Cu(0.7)/HZSM-5(32) catalyst (Cu, 0.7 wt %; Si/Al atomic ratio, 32) increased with increased O2 partial pressure up to an O2 pressure of around 20 kPa: a further increase in the partial pressure of O2 inversely caused a decline in the yield of phenol (Figure 2). Thus, an optimum value of the partial pressure of O2 existed for the formation of phenol. The yields of both CO2 and CO increased monotonically with an increase in the partial pressure of O2. The formed phenol is thought to be consecutively oxidized to CO2 and/or CO under a higher partial pressure of O2. From the log-log plot of the space-time yield (STY) of phenol (mol min-1 g-cat-1) and the partial pressure of O2, the reaction orders in the O2

Figure 3. Influence of the amount of loaded Cu on the benzene oxidation: N2/O2/benzene, 4:1:0.18; reaction temperature, 673 K; b, yield of phenol; 2, yield of CO2; 9, yield of CO; O, selectivity for phenol; A, 0.5 g of Cu/HZSM-5(32); B, 0.5 g of Cu/HZSM-5(75); C, 0.5 g of Cu/HZSM-5(160).

partial pressure were calculated to be 0.20 at the lower partial pressures of O2 (e20 kPa) and -0.31 at the higher O2 partial pressures (g20 kPa) (figure not depicted). 3.3. Influence of the Amount of Loaded Cu on Benzene Oxidation. The influence of the amount of loaded Cu on the benzene oxidation was investigated using a Cu/HZSM-5(32) (Si/Al atomic ratio, 32) catalyst (Figure 3A). The yield of phenol increased sharply with an increase in the amount of loaded Cu and had a maximum value at around 0.7 wt % loaded Cu, and a further increase in the amount of loaded Cu caused a decline in the phenol yield. The yields of both CO2 and CO increased with the amount of loaded Cu, though the formation of both CO2 and CO tended to level off at

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Table 1. Influence of Alkali Metal Added to the Cu/ HZSM-5 Catalyst on Benzene Oxidationa phenol catalyst

yield (%)

selectivity (%)

CO2 yield (%)

CO yield (%)

Cu/HZSM-5 Li/Cu/HZSM-5 Na/Cu/HZSM-5 K/Cu/HZSM-5 Rb/Cu/HZSM-5 CsCu/HZSM-5

3.3 2.5 3.3 4.4 3.6 3.6

30.1 14.9 15.2 23.5 17.1 22.7

4.4 9.4 11.8 9.0 11.1 7.7

3.4 5.1 6.4 5.4 6.3 5.3

a Catalyst, 0.5 g of Cu(0.6)/HZSM-5(80); alkali metal/Cu atomic ratio, 0.2.

higher amounts of loaded Cu. The influence of the amount of loaded Cu on the benzene oxidation is illustrated in Figure 3B and C using both the Cu/HZSM5(75) and the Cu/HZSM-5(160) catalysts, respectively. The Cu/HZSM-5 catalysts with different Si/Al atomic ratios had an optimum amount of loaded Cu for phenol formation; the optimum loaded Cu amount varied with the Si/Al atomic ratio of the HZSM-5 support. The maximum yield of phenol over the Cu/HZSM-5 catalysts with different Si/Al ratios increased with a decrease in the Si/Al atomic ratio (Si/Al ratio ) 160, 75, and 32; yield of phenol ) 1.7%, 3.3%, and 4.3%, respectively). We attempted to utilize the HZSM-5 (a kind of silicalite) zeolite with a higher Si/Al atomic ratio (Si/Al ratio ) 894) as a support for Cu species. The yields of phenol, CO2, and CO over the Cu(0.7)/HZSM-5(894) (Cu(0.7)/ silicalite) catalyst at 673 K (N2/O2/benzene ) 4:1:0.18) were 0.01%, 50.5%, and 0.7%, respectively. The HZSM-5 (silicalite) support with a high Si/Al ratio was thus confirmed not to be effective for phenol formation but to be appropriate for complete oxidation. As discussed later (section 3.7), the Cu species impregnated on the silicalite support were suggested to aggregate to form Cu oxides based on the ESR spectra of the Cu/silicalite. These Cu oxides are thought to be responsible for the complete oxidation of benzene, because the ion-exchanged sites for Cu ions are scarce. One of the roles of the H+ acidity will be, thus, to offer the sites of Cu-ionexchange to form isolated Cu species. 3.4. Influence of Alkali Metal Added to the Cu/ HZSM-5 Catalyst on Benzene Oxidation. The influence of alkali metal (alkali metal/Cu atomic ratio, 0.2) added to the Cu(0.6)/HZSM-5(80) catalyst on benzene oxidation is shown in Table 1. The yield of phenol increased with the addition of K, Rb, or Cs to the Cu/ HZSM-5 catalyst. However, because the addition of these alkali metals also caused an increase in the yields of both CO2 and CO, the selectivity for phenol declined on adding alkali metals to the Cu/HZSM-5 catalyst. The promotion effect of the alkali metal added to the Cu/ HZSM-5 catalyst for the formation of phenol may be due to activation of the gaseous O2 to form dissolved oxygen species on the Cu species, although we do not have a definite scheme for the participation of the oxygen species in the phenol formation. The simultaneous increase in the yield of carbon oxides (CO2 and CO) may be attributed to the participation of the added alkali metal, just like a NaZSM-5 support,26,27 dominating the complete (deep) oxidation of benzene. 3.5. Influence of the Introduction of Water Vapor on Benzene Oxidation. The influence of the introduction of water vapor (12 kPa) to the reaction system on benzene oxidation was investigated using Cu/ HZSM-5(96) catalysts with various amounts of loaded

Figure 4. Influence of H2O vapor on the yield of and the selectivity for phenol: catalyst, 0.5 g of Cu/HZSM-5(96); reaction temperature, 673 K; partial pressure of H2O, 12 kPa; O2/benzene, 1:0.18 (N2 was used as a balance gas); b, yield of phenol without H2O vapor; O, yield of phenol with H2O vapor; 9, selectivity for phenol without H2O vapor; 0, selectivity for phenol with H2O vapor.

Figure 5. Behavior of the used Cu/HZSM-5 catalyst with and without H2O vapor: catalyst, 0.5 g of Cu/HZSM-5(96); reaction temperature, 673 K; partial pressure of H2O, 12 kPa; O2/benzene, 1:0.18 (N2 was used as a balance gas); b, yield of phenol without H2O vapor; O, yield of phenol with H2O vapor.

Cu (Figure 4). The yield of phenol hardly increased with the lower amounts of loaded Cu on introducing water vapor. However, the introduction of water vapor to the reaction system brought a considerable increase in the yield of phenol with an amount of loaded Cu of around 0.5 wt %, though the selectivity for phenol did not increase. Thus, the yields of both CO2 and CO also increased with the introduction of water vapor. The dependence of the yields of phenol both with and without water vapor on the time on stream after the used Cu(0.6)/HZSM-5(96) catalyst was recalcined at 773 K for 5 h in flowing air is illustrated in Figure 5. The yields of phenol both with and without water vapor over the used Cu(0.6)/HZSM-5(96) catalyst which was recalcined were almost the same as those over fresh catalysts. The catalytic activity of the used Cu/HZSM-5 catalyst was thus recovered by calcining at 773 K in flowing air. The pentasil structure of the used Cu/ HZSM-5 catalyst is thought to be almost intact during benzene oxidation even in the presence of water vapor. This assumption was confirmed by comparing the XRD pattern of the fresh Cu/HZSM-5 catalyst with that of a used one in the presence of water vapor (figure not depicted). 3.6. Benzene Oxidation over the Cu/HZSM-5 Catalyst in the Absence of Gaseous O2. To investigate the role of O2 during benzene oxidation over the Cu/HZSM-5 catalyst, the yields of phenol, CO2, and CO were followed with time on stream after halting the

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Figure 6. Benzene oxidation over the Cu/HZSM-5 catalyst in the absence of gaseous O2: catalyst, 0.5 g of Cu(0.6)/HZSM-5(80); N2/ O2/benzene, 4:1:0.18; reaction temperature, 673 K; b, yield of phenol; 4, yield of CO2; 0, yield of CO.

supply of only gaseous O2, following the attainment of a steady state in the formation of phenol, as illustrated in Figure 6 (a transition response method). Phenol continued to be produced even after halting the O2 supply, though the yield of phenol became considerably lower. On the other hand, the yields of both CO2 and CO were lower in the initial stage of cessation of the O2 supply and became zero at 30 min after the stoppage of the O2 supply. These results imply that the formation of phenol needs the oxygen species sorbed on the Cu/ HZSM-5 catalysts, rather than gaseous O2, and that CO2 and CO are mainly formed in the presence of gaseous O2. 3.7. ESR Spectra of the Cu/HZS-5 Catalysts. The ESR spectra of the Cu/HZSM-5(32) catalysts and the parallel components were observed with variation in the amounts of loaded Cu and are illustrated in Figure 7A and B, respectively. Three kinds of ESR signals were confirmed from the parallel components (Figure 7B) of the ESR spectra. (First: g| ) 2.33, A| ) 157 G. Second: g| ) 2.31, A| ) 159 G. Third: g| ) 2.28, A| ) 171 G.) These three signals are identified as the squarepyramidal Cu2+ species, distorted square-pyramidal Cu2+ species, and square-planar Cu2+ species, respectively, according to Kucherov et al.30,31 Schoonheydt and co-workers32 reported Cu2+ coordination structures in Cu/ZSM-5 based on a density functional theory (DFT)/ ab initio assignment of the ESR spectra. The axially symmetric signal with g| ) 2.30-2.33 was assigned to a five-fold or distorted three-fold Cu2+ coordination in a six-ring with a bridging T-site, containing two lattice Al’s. The axially symmetric species with g| ) 2.26-2.28 was assigned to a square-planar Cu2+ coordination in six-rings and a square-pyramidal Cu2+ coordination in five-rings, with both rings containing only one Al and no extralattice oxygen. The ESR spectra of both the Cu/ HZSM-5(75) and the Cu/HZSM-5(160) (figures not shown) were similar to those of the Cu/HZSM-5(32). Torre-Abreu et al.33 proposed that the Cu species on the Cu-ion-exchanged HZSM-5 are present as isolated Cu2+ ions with a lower rate of Cu-ion-exchange and are present as CuO and coexisting Cu2+ and Cu1+ ions with the higher Cu-ion-exchanged Cu/HZSM-5 catalyst, based on the results obtained from the H2-TPR (temperature programmed reduction) experiment. The concentrations of the square-pyramidal Cu species including the distorted Cu species, together with those of the squareplanar Cu species, detected by the ESR spectra were plotted against the weight percentage of loaded Cu (Figure 8). The concentration of the square-pyramidal Cu2+ species (Figure 8, b) increased with an increase

Figure 7. ESR spectra of the Cu/HZSM-5 catalysts with different amounts of loaded Cu: Si/Al atomic ratio of the HZSM-5 support, 75; A, entire ESR spectra; B, ESR spectra of the g| region.

Figure 8. Variation in the relative concentration of Cu2+ with Cu loading: catalyst, Cu/HZSM-5 (Si/Al atomic ratio ) 160); b, square-pyramidal Cu2+ ions; O, square-planar Cu2+ ions.

in the weight percentage of loaded Cu up to around 0.75 wt % and decreased with a further increase in loaded Cu. The square-planar Cu2+ species (Figure 8, O) had a maximum value at a loaded Cu amount of around 1 wt %, though the relative concentration was considerably low. The maximum values of the square-pyramidal Cu2+ species of both the Cu/HZSM-5(75) and the Cu/HZSM5(160) catalysts were obtained at weights of loaded Cu of around 0.7 wt % and 0.5 wt %, respectively (no figure

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Figure 10. CO adsorption on prereduced Cu/HZSM-5 catalysts with different Si/Al atomic ratios: catalyst, 0.05 g with Cu, 0.7 wt %. The catalysts were oxidized with O2 at 773 K for 1 h, followed by degassing at 773 k for 2 h, and adsorbed at room temperature. Figure 9. ESR spectra of Cu/HZSM-5 catalyst treated with oxidizing and reducing conditions: catalyst, Cu(0.6)/HZSM-5(96); (a) treated with O2 (20 kPa) at 773 K for 1 h followed by degassing at room temperature; (b) sample a treated with carbon monoxide (CO, 20 kPa) at 773 K for 1 h followed by degassing at 773 K for 1 h; (c) sample b treated with H2O (2.0 kPa) at room temperature for 2 h followed by degassing at 773 K for 30 min.

depicted). The maximum values of the yields of phenol over the Cu/HZSM-5 catalysts with Si/Al atomic ratios of 32, 75, and 160 against the Cu weight percentage (Figure 3) were approximately similar to those of the concentration of the square-pyramidal Cu2+ species (Si/ Al ratio ) 32, 75, and 160; Cu wt % at phenol yieldmax (at square-pyramidal ESRmax) ) 0.75 wt % (0.8 wt %), 0.7 wt % (0.7 wt %), and 0.5 wt % (0.5 wt %)). The isolated Cu2+ species with a square-pyramidal configuration are thus thought to be responsible for the formation of phenol. No relationship between the yield of phenol and the concentration of the Cu2+ species with the square-planar configuration was observed as a function of the weight percentage of loaded Cu of the Cu/HZSM-5 zeolites. The redox behavior of the Cu(0.6)/HZSM-5(96) catalysts without and with H2O vapor was investigated through ESR observation (Figure 9). Anisotropic ESR peaks based on the typical isolated Cu2+ ions were observed in the oxidized form of the Cu(0.6)/HZSM-5(96) catalyst (Figure 9 line a). The oxidized catalyst was reduced in the absence of H2O vapor by treating with CO at 773 K for 2 h and degassing at 773 K for 1 h, and its ESR intensity decreased, as illustrated in Figure 9 line b. The reduced form of the Cu(0.6)/HZSM-5(96) catalyst was treated with H2O vapor at a pressure of 2.0 kPa at room temperature for 2 h; the intensity of the ESR spectrum (Figure 9 line c) was almost recovered to that of the original oxidized ESR spectrum (Figure 9 line a). This result indicates that the H2O vapor contributed to the reoxidation of the reduced Cu1+ to Cu2+ ions. Kuroda et al.34 reported that the Cu1+ species on Cu/NaZSM-5 form both Cu2+ and Cu0 in the presence of H2O. The presence of H2O vapor caused a promotion of phenol formation, in addition to the formation of CO2 and CO, over the Cu(0.6)/HZSM-5(96) catalyst (Figure 4). The ease of reoxidation of the Cu species on the catalyst with exposure to H2O vapor will be one of the reasons for the promoted oxidation of benzene. The ESR spectrum of the Cu impregnated on the silicalite (Cu(0.7)/silicalite ≡ Cu(0.7)/HZSM-5(894), HZSM-5 with a Si/Al atomic ratio of 894) was shown to

have a very low intensity of the isolated Cu2+ signal (figure not depicted), compared with that of the corresponding Cu(0.7)/HZSM-5(96), the ESR spectrum of which showed typical anisotropic peaks based on the isolated Cu2+ ions, as shown in Figure 7A. One of the reasons for this is that the Cu species impregnated on the silicalite are present in an aggregated form, such as copper oxide (CuO), rather than as isolated Cu species. These aggregated Cu species are thought to promote the deep oxidation of benzene, rather than the formation of benzene, in connection with the results of benzene oxidation over the Cu(0.7)/silicalite catalyst described in section 3.3. 3.8. CO Adsorption and O2 Uptake of the Prereduced Cu/HZSM-5 Zeolites with Different Si/Al Atomic Ratios. The adsorption of carbon monoxide (CO) over the Cu(0.7)/HZSM-5 catalysts with Si/Al ratios of 32-160 prereduced by degassing at 773 K for 2 h was measured in order to evaluate the degree of reduction of the Cu2+ to Cu1+ ions (Figure 10). The amount of adsorbed CO is a measure of the amount of Cu1+ ions reduced.35-37 The amount of adsorbed CO increased with an increase in the Si/Al atomic ratio of the HZSM-5 support. The H2-TPR results reported in ref 32 showed that the Cu2+ ions on a low Cu-ionexchanged HZSM-5 are difficult to reduce to Cu1+ ions, which is in agreement with the results shown in Figure 10. The Cu/HZSM-5 catalyst with a lower Si/Al atomic ratio had a lower amount of adsorbed CO, and thus a lower amount of reduced Cu1+, than that with a higher Si/Al ratio at a constant amount of loaded Cu. As mentioned in section 3.3, the catalytic activity for phenol formation became higher as the Si/Al atomic ratio of the Cu-impregnated HZSM-5 support became lower (Figure 3). The amounts of O2 uptake of the Cu(0.7)/HZSM-5 catalysts prereduced with carbon monoxide (see the Experimental Section) were measured using catalysts with different Si/Al atomic ratios. The amount of O2 uptake decreased to a large extent using the prereduced catalysts with low Si/Al atomic ratios (Si/Al ) 32, 123; O2 uptake ) 0.08 µmol g-cat-1, 1.5 µmol g-cat-1, respectively); these results are in agreement with those for CO adsorption. From the results of CO adsorption, O2 uptake, and benzene oxidation, the formation of phenol is thought to require the isolated squarepyramidal Cu species which are comparatively easily reoxidized, though we cannot depict a clear scheme including the active isolated Cu species for phenol formation at this stage.

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4. Conclusions A copper-ion-impregnated HZSM-5 (Cu/HZSM-5) catalyst was found to be active for phenol formation, in addition to the formation of both CO2 and CO, in the gas-phase catalytic oxidation of benzene using gaseous O2 as an oxidant. An optimum value for the amount of impregnated Cu was observed for the formation of phenol. The optimum yield of phenol tended to increase using a Cu/HZSM-5 catalyst with a low Si/Al atomic ratio of the HZSM-5 support. Phenol was hardly yielded over the Cu catalyst (Cu/silicalite) impregnated on silicalite (ZSM-5 type zeolite with a high Si/Al atomic ratio (894)), and CO2 was almost exclusively obtained over the Cu/silicalite catalyst. The introduction of H2O vapor during the benzene oxidation promoted phenol formation, along with the formation of CO2 and CO. H2O vapor was suggested to contribute to the reoxidation of the reduced Cu1+ ions to Cu2+ ions on the Cu/HZSM-5 catalyst during benzene oxidation, based on the results of ESR observation. The transition response experiment indicated that the O2 species adsorbed on the Cu/ HZSM-5 catalyst are mainly responsible for the phenol formation, rather than gaseous O2, which contributes to the deep oxidation of benzene. The amount of the isolated Cu2+ species with a square-pyramidal configuration on the Cu/HZSM-5 catalyst was found to correlate with the yield of phenol, based on the ESR spectra of the catalysts. Acknowledgment We would like to thank Mr. Kenji Nomura of Kobe University for his technical assistance during this work. Literature Cited (1) Walling, C. Fenton’s Reagent Revisited. Acc. Chem. Res. 1975, 8, 125. (2) Dixon, W. T.; Norman, R. O. C. An Intermediate in Homolytic Aromatic Substitution. Proc. Chem. Soc., London 1963, 97. (3) Dixon, W. T.; Norman, R. O. C. Electron Spin Resonance Studies of Oxidation. Part IV. Some Benzenoid Compounds. J. Chem. Soc. 1964, 4857. (4) Dorfman, L. M.; Taub, I. A.; Buhler, R. E. Pulse Radiolysis Studies. I. Transient Spectra and Reaction-Rate Constants in Irradiated Aqueous Solutions of Benzene. J. Chem. Phys. 1962, 36, 3051. (5) Kunai, A.; Hata, S.; Ito, S.; Sasaki, K. Mechanistic Study of Air Oxidation of Benzene in Sulfuric Acid Catalyzed by Cuprous Ions. J. Org. Chem. 1986, 51, 3471. (6) Ito, S.; Yamasaki, T.; Okada, H.; Okino, S.; Sasaki, K. Oxidation of Benzene to Phenols with Molecular Oxygen Promoted by Copper(I) Chloride J. Chem. Soc., Perkin Trans. 2 1988, 285. (7) Ito, S.; Kunai, A.; Okada, H.; Sasaki, K. Direct Conversion of Benzene to Hydroquinone. Cooperative Action of Copper(I) Ion and Dioxygen. J. Org. Chem. 1988, 53, 296. (8) Sasaki, K.; Ito, S.; Saheki, Y.; Kinoshita, T.; Yamasaki, T.; Harada, J. One-Step Oxidation of Benzene to Phenol under Ambient Conditions. Chem. Lett. 1983, 37. (9) Kitano, T.; Kuroda, Y.; Itoh, A.; Li-Fen, J.; Kunai, A.; Sasaki, K. Liquid-Phase Oxidation of Benzene under Ambient Conditions. J. Chem. Soc., Perkin Trans. 2 1990, 1991. (10) Ohtani, T.; Nishiyama, S.; Tsuruya, S.; Masai. M. In Liquid-Phase Oxidation of Benzene with Molecular Oxygen Catalyzed by Cu-Zeolites; Proceedings of 10th International Congress on Catalysis, Budapest, Hungary; Guczi, L., Solymosi, F., Tetenyi, P., Eds.; Elsevier: Amsterdam, 1993; p 1999. (11) Ohtani, T.; Nishiyama, S.; Tsuruya, S.; Masai, S. LiquidPhase Benzene Oxidation to Phenol with Molecular Oxygen Catalyzed by Cu-Zeolites. J. Catal. 1995, 155, 158. (12) Okamura, J.; Nishiyama, S.; Tsuruya, S.; Masai, S. Formation of Cu-Supported Mesoporous Silicates and Alumino-

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Received for review July 12, 2005 Revised manuscript received September 1, 2005 Accepted September 2, 2005 IE050821I