Dehydrogenation of Ethylbenzene with Carbon Dioxide Using

In addition to the produced styrene, equivalent amounts of carbon monoxide and water were formed. These results suggest that the dehydrogenation of ...
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Ind. Eng. Chem. Res. 2000, 39, 1228-1234

Dehydrogenation of Ethylbenzene with Carbon Dioxide Using Activated Carbon-Supported Catalysts Na-oki Ikenaga,* Tadatoshi Tsuruda, Kazuhiro Senma, Takanori Yamaguchi, Yoshihiro Sakurai, and Toshimitsu Suzuki Department of Chemical Engineering, Faculty of Engineering, Kansai University, Suita, Osaka 564-8680, Japan

Dehydrogenation of ethylbenzene to styrene under excess carbon dioxide flow was carried out over activated carbon-supported metal oxide catalysts (Cr, Mn, Co, Ni, Mo, Ru, La, and Ce) at 823 K and W/F ) 35 g of catalyst‚h/mol. The highest yield of styrene (about 40%) with selectivity of above 80% was obtained using activated carbon-supported chromium oxide (Cr/AC) and cerium oxide (Ce/AC) catalysts. The initial activities of the Cr/AC and Ce/AC catalysts were comparable to that of an iron-loaded activated carbon catalyst reported previously. Only chromium(III) oxide and cerium(IV) oxide were detected by X-ray diffraction before and after reactions at higher loading levels, and these species might have been active forms. However, a reduced chromium oxide species was detected by X-ray photoelectron spectroscopy after reaction under argon. In addition to the produced styrene, equivalent amounts of carbon monoxide and water were formed. These results suggest that the dehydrogenation of ethylbenzene to styrene proceeds via two reaction paths. One is the simple dehydrogenation and an oxidation reaction of hydrogen formed with carbon dioxide. The other is the oxidative dehydrogenation of ethylbenzene through the redox cycle of chromium(III) oxide. Introduction Industrially styrene is manufactured by a catalytic dehydrogenation of ethylbenzene (eq 1) with potassium-

C6H5CH2CH3 f C6H5CHdCH2 + H2 - 125 kJ/mol (1) promoted iron(III) oxide. Because the dehydrogenation reaction is endothermic (125 kJ/mol), the dehydrogenation of ethylbenzene is carried out at 550-650 °C with an excess amount of steam as a diluent and a heat carrier (steam/ethylbenzene ) 10-15 mol/mol). An iron oxide-chromium oxide-potassium carbonate based catalyst is used as an industrial catalyst, and improvement is carried out with addition of various metal oxides such as vanadium, cerium, molybdenum, and also manganese compounds.1,2 Coulter et al. demonstrated that K-promoted iron oxide catalysts prepared under ultrahigh-vacuum conditions could increase the turnover frequency and lower the activation energy as compared to unpromoted iron oxide catalyst.2 Wu et al. indicated that TiO2-Fe2O3, ZrO2-Fe2O3, and TiO2-Fe2O3-ZrO2 showed higher activities than the conventional K-promoted iron catalysts.3 Jebarathinam et al. tried to use spinel oxides containing Ni, Cr, Zn, Cu, Fe, and Al for the dehydrogenation of ethylbenzene and reported that basic sites and acidic sites on the catalyst surface influence the ethylbenzene conversion.4 On the other hand, the catalytic activities of various metal oxides and phosphates were investigated for the oxidative dehydrogenation of ethylbenzene to styrene. Oganowski et al. reported that the activity of the V-MgO mixed catalysts could be promoted by Cr, Co, * Corresponding author. Phone: +81-6-6368-0792. Fax: +81-6-6388-8869. E-mail: [email protected].

and Mo doping5 and that the Mg3(VO4)2-MgO system exhibited a higher activity in the dehydrogenation of ethylbenzene.6 In addition, the activities of Zr-Sn mixed phosphates,7 Pd-NaBr/SiO2,8 and a carbon molecular sieve9 have been discussed. Carbon dioxide is conceivable as one of the major sources of the greenhouse effect gas, and its effective utilization is of current importance. Several researchers tried to use carbon dioxide as an oxidizing agent in the dehydrogenation of ethylbenzene. The catalytic activity of iron oxide based catalysts decreased in the dehydrogenation of ethylbenzene in a carbon dioxide atmosphere.10-12 Several research groups containing us have reported the positive effect of carbon dioxide on the catalytic activities for the dehydrogenation. We have reported the dehydrogenation of ethylbenzene under carbon dioxide using an activated carbon-supported iron oxide catalyst and that the iron oxide catalyst exhibited a higher activity via the redox cycle between iron oxide and iron metal. The deactivation of iron catalyst could be suppressed with a small amount of lithium salt to the iron catalyst.13 The catalytic behaviors of zeolite-supported iron oxide,14 alkali metal-promoted iron/activated carbon,15 and Fe2O3/Al2O316 have been investigated in the dehydrogenation of ethylbenzene. In this work, we will deal with the dehydrogenation of ethylbenzene to styrene catalyzed by various metal oxides (vanadium, chromium, manganese, cobalt, nickel, molybdenum, ruthenium, lanthanum, and cerium) supported on an activated carbon in the presence of excess carbon dioxide. Experimental Section Materials. All chemicals were purchased from commercial sources and used without further purification. A granular activated carbon (Wako Pure Chemical

10.1021/ie990426q CCC: $19.00 © 2000 American Chemical Society Published on Web 03/28/2000

Ind. Eng. Chem. Res., Vol. 39, No. 5, 2000 1229 Table 1. Dehydrogenation of Ethylbenzene with Activated Carbon-Loaded Transition-Metal Catalystsa run catalystb surface area (m2/g) EB feed (mmol) EB convn (mmol) ST yield (mmol) EB convn (%) ST yield (%) ST select. (%) 1 2 3 4 5 6 7 8 9 10 11 12

none AC V/AC Cr/AC Mn/AC Fe/AC Co/AC Ni/AC Mo/AC Ru/AC La/AC Ce/AC

950 760 820 810 830 780 790 785 815 790 820

2.60 2.45 2.62 2.59 2.92 3.02 2.65 2.41 2.96 2.58 2.97 2.57

0.08 0.24 0.90 1.24 0.87 1.33 0.76 0.44 0.98 0.52 1.24 1.23

0.02 0.18 0.71 1.05 0.72 1.02 0.50 0.25 0.71 0.40 1.12 1.01

3.2 9.9 34.2 47.9 29.8 44.1 28.4 18.4 33.2 20.0 41.7 47.9

0.8 7.5 27.1 40.5 24.8 33.6 18.7 10.5 24.1 15.3 37.7 39.3

23.8 75.9 79.4 84.6 83.2 76.3 65.6 57.3 72.5 76.6 90.5 82.1

a 823 K, 2 h, W/F ) 35 g of catalyst‚h/mol, catalyst ) 50 mg, CO (Ar) ) 30 mL/min. Pretreatment: Ar, 973 K, 10 min; CO , 973 K, 10 2 2 min. b Loading level of metal ) 0.5 mmol/g of carbon. Abbreviations: EB, ethylbenzene; ST, styrene.

Industries, Ltd.; 10-30 mesh, 950 m2/g) was used as a support. Cr(NO3)3‚9H2O, Ce(NO3)3‚9H2O (Nacalai tesque, Inc.), VOCl3, Mn(NO3)2‚9H2O, Fe(NO3)3‚9H2O, Co(NO3)2‚ 6H2O, Ni(NO3)2‚6H2O, (NH4)6Mo7O24(NO3)2‚4H2O, La(NO3)3‚6H2O, Ce(NO3)3‚6H2O (Wako Pure Chemical Industries Ltd.), and RuCl3‚3H2O (Mitsuwa Pure Chemicals, Ltd.) were used as a metal oxide precursor. Carbon-supported metal oxide catalysts were prepared by the usual impregnation method: 2 g of the activated carbon was suspended in 10 g of aqueous solution containing 0.2-6.0 mmol of meal salts. After impregnating metal salts to the support by immersing it in aqueous solutions for 24 h, water was evaporated in vacuo, and then the activated carbon-supported catalysts were dried at 343 K under reduced pressure. Apparatus and Procedure. The reaction was carried out in a conventional flow type reactor made of a stainless steel tube (4.3 mm i.d., 8 mm o.d., 400 mm length) operated at atmospheric pressure. The catalyst (50-200 mg) was placed in the center of the reactor by using a quartz glass wool plug. The reaction was carried out at 723-953 K for 2-5 h. Before ethylbenzene was introduced into the reactor, the catalyst was pretreated under an argon stream from room temperature to 973 K at a heating rate of 35 K/min and was maintained at this temperature for 10 min. The catalyst was further treated with carbon dioxide at 973 K for 10 min, and then the reactor was cooled to the desired reaction temperature under a stream of carbon dioxide. Ethylbenzene was fed (ca. 1.3 mmol/h) to the reactor by passing carbon dioxide (30 mL/min) through the ethylbenzene saturator thermostated at 316 K. The effluent from the reactor was condensed in two traps containing heptane, externally cooled in an ice water bath. The gaseous products were collected into a gas bag. The liquid products (ethylbenzene, styrene, benzene, and toluene) were quantified with a flame ionization detector gas chromatograph (Shimadzu GC-l4APF equipped with a SE-30 column: 3.0 mm × 3 m) by using ethylcyclohexane as an internal standard. Analyses of gaseous products (CO, CO2, and CH4) were carried out with a gas chromatograph (Shimadzu GC-8APT) equipped with a thermal conductivity detector using a 3 mm × 2.5 m stainless steel column packed with an activated carbon. The amount of water formed during the reaction was determined by the Karl Fischer method using an Aquacounter (Hiranuma AQV-5S). Analyses of Catalysts. Metal species before and after the dehydrogenation were analyzed with powder

X-ray diffraction (XRD) method using an X-ray diffractometer (JEOL JDX-35305) with monochromated Cu KR radiation and X-ray photoelectron spectroscopy (XPS; JEOL JPS-9000). The surface area of the catalyst was measured with a Brunauer-Emmett-Teller method using nitrogen as the adsorbate at 77 K and a relative pressure of p/p0 ) 0.05-0.3 with a Micromeritics Gemini-2375. Results and Discussion Activities of Various Metal Oxide Supported Catalysts. The results of the dehydrogenation of ethylbenzene are shown in Table 1, with various metal oxide based catalysts under a carbon dioxide atmosphere at 823 K for 2 h. Very low conversion of ethylbenzene (3.2%) and styrene yield (0.8%) were observed (run 1) in the dehydrogenation carried out under a carbon dioxide flow at 823 K in the absence of a catalyst. In the oxidative dehydrogenation of ethylbenzene with air, activated carbon17 and graphite18 were reported to be active catalysts. Active carbon support itself was used as a catalyst (run 2), and low ethylbenzene conversion (9.9%) and styrene yield (7.5%) were obtained. This shows that the activated carbon acted as a catalyst in the dehydrogenation of ethylbenzene in carbon dioxide. Higher ethylbenzene conversion and styrene yield were observed with various metal-loaded activated carbon catalysts (0.5 mmol/g of carbon, runs 3-11) than that with the activated carbon. Cr/AC and Ce/AC afforded the highest ethylbenzene conversion (48%) and styrene yield (about 40%) with more than 82% selectivity, and these values were higher than those of Fe/AC catalyst reported in the previous paper13 and commercially used Cr2O3/Al2O3 catalyst (conversion, 16.0%; yield, 12.8%; selectivity, 79.8%) for the dehydrogenation of saturated hydrocarbons. The order of an activity of supported metal oxide was as follows: Cr ) Fe ) Ce > Mn ) Mo ) V > Co > Ru > Ni. These findings agree well with the results that iron oxide and chromium oxide supported on γ-Al2O3 showed high activities in the oxidative dehydrogenation of ethylbenzene with air.19 Effects of Metal Loading Level. Figure 1 illustrates the effects of the amount of chromium and cerium loaded on the activated carbon on the ethylbenzene conversion, the styrene yield, and the styrene selectivity. In the reactions with Cr/AC catalyst (Figure 1a), the ethylbenzene conversion and the styrene yield increased with an increase in the amount of chromium, reached maxima (conversion, 47.9%; yield, 40.5%) at the

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Figure 1. Effects of loading level of Cr (a) and Ce (b) on ethylbenzene conversion, styrene yield, and selectivity: 823 K; 2 h; W/F ) 35 g of catalyst‚h/mol; catalyst, 50 mg; CO2, 30 mL/min; (O) ethylbenzene conversion, (4) styrene yield, (2) styrene selectivity.

Figure 2. Effect of reaction temperature on dehydrogenation of ethylbenzene using Cr/AC (a) and Ce/AC (b) catalysts: 2 h; W/F ) 35 g of catalyst‚h/mol; catalyst, 50 mg; CO2, 30 mL/min; (O) ethylbenzene conversion, (4) styrene yield, (2) styrene selectivity.

loading level of 0.5 mmol/g of carbon (3.7 wt % Cr2O3), and then decreased slightly further with an increase in the amount of chromium oxide. The styrene selectivity reached 85% at 0.3 mmol of chromium/g of carbon (2.2 wt % Cr2O3) and remained almost constant with an increase in the loading level of chromium oxide. In the dehydrogenation of ethylbenzene with Ce/AC catalyst (Figure 1b), the ethylbenzene conversion and the styrene yield increased to 47.9% and 39.3%, respectively, with an increase in the amount of cerium to 0.5 mmol/g of carbon (7.9 wt % CeO2), and then they remained constant. The styrene selectivity reached a maximum (82.1%) at the loading level of 0.5 mmol/g of activated carbon, and above this level it leveled off. These trends observed in the dehydrogenation with chromium and cerium oxide loaded activated carbon catalysts were similar to that observed in the reactions with iron-loaded activated carbon catalyst.13 Effects of Reaction Temperature with Chromium- and Cerium-Loaded Activated Carbon Catalysts. Effects of reaction temperature on the dehydrogenation of ethylbenzene with Cr(0.3)/AC and Ce(0.5)/ AC catalysts were examined. The ethylbenzene conversion, the styrene yield, and the styrene selectivity as a function of reaction temperature are shown in Figure 2. In the reaction with the Cr(0.3)/AC catalyst (Figure 2a), the ethylbenzene conversion increased markedly

from 12.9 to 77.9% with an increase in the reaction temperature from 723 to 953 K. However, the ethylbenzene conversion hardly increased in the temperature range of 823-923 K. In the reaction under an argon atmosphere (not shown in the figure), the ethylbenzene conversion linearly increased with an increase in the reaction temperature. These results suggest that strong interaction between carbon dioxide and the catalyst surface did occur in this temperature range, and as a result the dehydrogenation of ethylbenzene would not increase with an increase in the reaction temperature. The styrene yield reached a maximum (49.3%) at 823 K and decreased slightly with further increases in the reaction temperature. The styrene selectivity remarkably decreased from 82.9% at 773 K to 49.0% at 953 K, because ethylbenzene and/or styrene decomposed to form methane, benzene, toluene, and deposited carbon. In the reaction with the Ce(0.5)/AC catalyst (Figure 2b), behavior similar to that in the Cr/AC catalyst was observed. The styrene yield increased to 40.0% with an increase in the reaction temperature to 823 K and remained constant with further increases in the reaction temperature to 923 K. However, the styrene selectivity remained constant (about 83%) in the range between 723 and 823 K, and then it decreased noticeably to 56.0% at 923 K. At temperatures above 823 K, the amounts of cracked hydrocarbons such as methane, benzene, and toluene increased.

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Figure 3. Effects of W/F on dehydrogenation of ethylbenzene using Cr/AC (a) and Ce/AC (b) catalysts: 823 K; 2 h; catalyst, 50 mg; CO2, 30 mL/min; (O) ethylbenzene conversion, (4) styrene yield, (2) styrene selectivity.

Figure 4. Effect of time on stream on dehydrogenation of ethylbenzene using Cr/AC (a) and Ce/AC (b) catalysts: 823 K; W/F ) 35 g of catalyst‚h/mol; catalyst, 50 mg, CO2, 30 mL/min; (O) ethylbenzene conversion, (4) styrene yield, (2) styrene selectivity.

Effects of Contact Time and Reaction Time. Figure 3 illustrates the effects of contact time (W/F) on the ethylbenzene conversion, the styrene yield, and the styrene selectivity with Cr(0.3)/AC (Figure 3a) and Ce(0.5)/AC (Figure 3b) catalysts. In these experiments, W/F was changed by varying the amount of catalysts charged under a constant ethylbenzene feed rate (1.4 mmol/h). In the reaction with both Cr(0.3)/AC and Ce(0.5)/AC catalysts, the ethylbenzene conversion and the styrene yield increased with an increase in W/F from 35 to 150 g of catalyst‚h/mol. However, the styrene yield only slightly increased with increases in W/F, and as a result, the selectivity decreased. Increases in the contact time increased the conversion of ethylbenzene. However, side reactions proceeded simultaneously to give cracked products and carbon deposition. Similar behavior was observed in the dehydrogenation of isobutane.20,21 The ethylbenzene conversion, the styrene yield, and the styrene selectivity are shown in Figure 4 as a function of time on stream at the reaction temperature of 823 K with Cr(0.3)/AC (Figure 4a) and Ce(0.5)/AC catalysts (Figure 4b). In both catalysts, the ethylbenzene conversion and the styrene yield decreased markedly in the initial 2 h of the reaction period and then decreased gradually with further progress of the reaction. However, the decreases in the ethylbenzene conversion and the styrene yield with the Cr/AC catalyst are more significant as compared to those with the Ce/

AC catalyst. The styrene selectivity with the Cr/AC catalyst decreased slightly from 86.6% to 70.6% with the progress in the reaction from 1 to 5 h. The surface areas of the Cr/AC and Ce/AC catalysts decreased from 820 to 380 m2/g after reaction for 5 h at 823 K. Such decreases in the surface area could be attributed to the carbon deposition on the catalyst, as reported in the previous paper.13 Therefore, the remarkable decreases in the ethylbenzene conversion, the styrene yield, and the styrene selectivity seemed to be due to the carbon deposition on the catalyst surface. Role of Carbon Dioxide and XRD Analyses of Catalysts. To understand the role of carbon dioxide in the dehydrogenation of ethylbenzene, the reactions under an argon flow were done with the Cr/AC and Ce/ AC catalysts. The results are shown in Table 2. The ethylbenzene conversion under a carbon dioxide flow was the same as that under an argon flow using both Cr/AC and Ce/AC catalysts. On the other hand, the styrene yield and selectivity in a carbon dioxide atmosphere were higher than those in an argon atmosphere in the reaction with the Cr/AC catalyst (runs 4, 13, 15, and 16). However, the effect of carbon dioxide was negative in the reaction with Ce/AC (runs 12 and 17) on the styrene yield and selectivity. Under a carbon dioxide flow with Cr(0.5)/AC (run 13), the amounts of carbon monoxide (0.92 mmol) and water (0.97 mmol) formed

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Table 2. Effect of CO2 in the Dehydrogenation of Ethylbenzene with Cr/AC Catalystsa run

catalystb

feed gas

EB feed (mmol)

EB convn (mmol)

ST yield (mmol)

TOL yield (mmol)

BZ yield (mmol)

EB convn (%)

ST yield (%)

ST select. (%)

2 4 13 12 14 15 16 17

AC Cr(0.5)b/AC Cr(1.5)/AC Ce(0.5)/AC AC Cr(0.5)/AC Cr(1.5)/AC Ce(0.5)/AC

CO2 CO2 CO2 CO2 Ar Ar Ar Ar

2.45 2.59 2.90 2.57 2.56 2.56 2.27 2.89

0.24 1.24 1.35 1.23 0.39 1.15 1.13 1.17

0.18 1.05 1.14 1.01 0.20 0.81 0.77 1.04

0.01 0.03 0.03 0.03 0.01 0.03 0.03 0.03

0.02 0.04 0.03 0.2 0.03 0.03 0.03 0.01

9.9 47.9 46.5 47.9 15.2 45.0 49.9 40.6

7.5 40.5 39.1 39.3 7.8 31.6 34.0 35.9

75.9 84.6 84.2 82.1 51.0 70.1 68.2 88.6

a 823 K, 2 h, W/F ) 35 g of catalyst‚h/mol, catalyst ) 50 mg, CO (Ar) ) 30 mL/min. Pretreatment: Ar, 973 K, 10 min; CO , 973 K, 10 2 2 min. b Numerals in parentheses indicate mmol of metal/g of carbon. Abbreviations: EB, ethylbenzene; ST, styrene; TOL, toluene; BZ, benzene.

during the reaction are in fair agreement with the amount of styrene produced (1.05 mmol). Under an argon flow (run 15), however, the amount of hydrogen formed (1.92 mmol) was about 2 times as large as that of the styrene yield (0.81 mmol) and carbon monoxide and water were scarcely produced. As reported in the previous paper on the dehydrogenation of isobutane,21 two reaction paths for the formation of water in a carbon dioxide atmosphere were proposed. The first one is a direct process that carbon dioxide oxidizes the reduced chromium oxide species formed during the course of the dehydrogenation of ethylbenzene to give carbon monoxide. The second one is the reverse water-gas shift reaction (eq 2), where carbon

CO2 + H2 f CO + H2O

(2)

dioxide reacts with hydrogen from the dehydrogenation of ethylbenzene. In fact, the slight formation of carbon monoxide and water were confirmed in the reaction of hydrogen with carbon dioxide at 823 K in the absence of ethylbenzene. To confirm the possibility of oxidation by carbon dioxide, the XRD analyses of the Cr(3)/AC and Ce(5)/ AC catalysts before and after the reaction were compared in Figures 5 and 6, respectively. No diffraction peaks in the XRD profiles of Cr(0.5)/AC and Ce(0.5)/ AC, which afforded the highest ethylbenzene conversion, were observed. Parts a and b of Figure 5 indicate the XRD patterns of the Cr/AC catalyst at a higher loading level (3 mmol/g of carbon) after pretreatment with either argon or carbon dioxide at 973 K for 20 min. Clear diffraction peaks of Cr2O3 were seen under both argon and carbon dioxide flows. The differences between argon and carbon dioxide pretreatment of the catalyst were not observed by XRD analyses. The XRD profiles of the catalysts after reaction with ethylbenzene under argon and carbon dioxide flows at 823 K for 2 h were illustrated in parts c and d of Figure 5, respectively. Sharp diffraction peaks assigned as Cr2O3 appeared even after the dehydrogenation of ethylbenzene with argon and carbon dioxide. No changes in the diffraction peaks were observed as compared with the diffraction peaks obtained after pretreatment in both argon and carbon dioxide. This result indicated that Cr2O3 was an active species for the dehydrogenation of ethylbenzene, but a redox cycle of Cr2O3 could not be observed during reaction with the Cr/AC catalyst. Parts a-d of Figure 6 show the XRD profiles of the Ce(5)/AC catalysts after pretreatment with argon (a) and carbon dioxide (b) at 973 K for 20 min and after reaction under an argon flow (c) and a carbon dioxide flow (d) at 823 K for 2 h, respectively. The diffraction

Figure 5. XRD patterns of Cr(3)/AC catalyst before and after reaction: (a) before reaction activated with argon at 973 K for 20 min; (b) before reaction activated with carbon dioxide at 973 K for 20 min; (c) after reaction with ethylbenzene under an argon flow; (d) after reaction with ethylbenzene under a carbon dioxide flow.

peaks assigned as CeO2 appeared in all of the XRD profiles, and they were broad as compared to the XRD patterns of the Cr(3)/AC catalyst. Because noncrystalline species cannot be identified by XRD, the Cr/AC catalysts before and after the dehydrogenation were subjected to XPS analyses. Only the electron ascribed to Cr2O3 species (binding energy of 576.3 eV, Cr2p3/2) was observed after pretreatment with argon and carbon dioxide at 973 K and after the dehydrogenation of ethylbenzene under a carbon dioxide flow at 823 K. However, two peaks were observed at 576.3 and 575.1 eV when the dehydrogenation was carried out under an argon flow at 823 K. Although the

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(about 40%) with over 80% selectivity at 823 K, carbon dioxide/ethylbenzene ) 50 mol/mol and W/F ) 35 g of catalyst‚h/mol. An addition of 0.3-0.5 mmol of chromium and 0.5 mmol of cerium supported on 1 g of activated carbon resulted in significant increases in the catalytic activity. The initial activities of the Cr/AC and Ce/AC catalysts were comparable to that of an iron-loaded activated carbon catalyst which was reported in a previous paper. Chromium(III) oxide and cerium(IV) oxide were detected by XRD before and after reactions, and they may act as active species. However, the formation of reduced chromium oxide species on the catalyst surface after the reaction under argon was recognized by XPS. In the dehydrogenation of ethylbenzene in a carbon dioxide atmosphere, carbon monoxide and water besides styrene were formed as products, and the amounts of carbon monoxide and water formed during the reaction agreed well with the amount of styrene. These indicated that the dehydrogenation of ethylbenzene to styrene proceeds via two reaction paths. One is the simple dehydrogenation and an oxidation reaction of hydrogen formed with carbon dioxide. The other is the oxidative dehydrogenation of ethylbenzene through the redox cycle of chromium(III) oxide. The deactivation of catalysts during the dehydrogenation under both carbon dioxide and argon flow was attributed to carbon deposition on the catalyst surface. Acknowledgment This work is partially supported by a cooperative research fund from Kansai University. Figure 6. XRD patterns of Ce(5)/AC catalyst before and after reaction: (a) before reaction activated with argon at 973 K for 20 min; (b) before reaction activated with carbon dioxide at 973 K for 20 min; (c) after reaction with ethylbenzene under an argon flow; (d) after reaction with ethylbenzene under a carbon dioxide flow.

peak at 575.1 eV cannot be assigned to definite Cr species, formation of reduced chromium oxide species on the catalyst surface was evident. This fact clearly indicates that the role of carbon dioxide in the dehydrogenation of ethylbenzene over the Cr2O3/AC catalyst is to keep the chromium(III) oxide phase during the reaction. Therefore, we can safely propose two reaction paths for the dehydrogenation of ethylbenzene to styrene with the Cr/AC and Ce/AC catalysts under carbon dioxide. The first one is the simple dehydrogenation of ethylbenzene followed by the reverse water-gas shift reaction (eqs 1 and 2). This made it possible to reduce the hydrogen partial pressure and to release the thermodynamic limitation of equilibrium. The second one is the direct process that ethylbenzene reduces chromium(III) oxide to reduce chromium oxide species on the catalyst surface and carbon dioxide reoxidates it to chromium(III) oxide (eqs 3 and 4).

C6H5CH2CH3 + Cr2O3 f C6H5CHdCH3 + 2CrO + H2O (3) 2CrO + CO2 f Cr2O3 + CO

(4)

Conclusion Activated carbon-supported chromium (Cr/AC) and cerium (Ce/AC) catalysts exhibited a high styrene yield

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Received for review June 15, 1999 Revised manuscript received December 1, 1999 Accepted February 9, 2000 IE990426Q