Dehydrogenation of ethylbenzene and propane over Ga2O3–ZrO2 catalysts in the presence of CO2

Catalysis Communications 8 (2007) 1317–1322 www.elsevier.com/locate/catcom

Dehydrogenation of ethylbenzene and propane over Ga2O3–ZrO2 catalysts in the presence of CO2 Huiyun Li a

a,c

, Yinghong Yue a, Changxi Miao b, Zaiku Xie b, Weiming Hua

a,*

, Zi Gao

a

Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China b Shanghai Research Institute of Petrochemical Technology, Shanghai 201208, PR China c Department of Chemistry, Anyang Normal University, Anyang 455002, Henan, PR China Received 30 August 2006; received in revised form 22 November 2006; accepted 25 November 2006 Available online 1 December 2006

Abstract Ga2O3–ZrO2 catalysts were prepared by a coprecipitation method. The catalysts were characterized by XRD, Raman, DRS, XPS, SEM, EDX, IR, NH3-TPD, CO2-TPD and N2 adsorption methods. Dehydrogenation of ethylbenzene and propane in the presence of CO2 over these catalysts has been investigated and compared. Ga2O3–ZrO2 catalysts exhibit different catalytic behavior for the two dehydrogenation reactions.  2006 Elsevier B.V. All rights reserved. Keywords: Dehydrogenation; Ethylbenzene; Propane; Styrene; Propene; CO2

1. Introduction Styrene is an important monomer for synthetic polymers. It is commercially produced mainly by the vapor phase catalytic dehydrogenation of ethylbenzene (EB) on potassium-promoted iron oxide catalysts in the presence of large excess of superheated steam at 823–923 K [1]. However, the present industrial process is thermodynamically limited and, moreover, it is very energy consuming due to the required excess of superheated steam and higher reaction temperatures. Recently, the dehydrogenation of EB in the presence of CO2 has attracted increasing interest [2]. This new process is energy-saving and environmentally friendly. It was estimated that the energies required for producing 1 t of styrene by a present commercial process using steam and by a new process using CO2 were 1.5 · 109 and 1.9 · 108 cal, respectively [3]. The development of catalysts with improved dehydrogenation activity for the new process is required, since the commercial K*

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promoted iron oxide catalysts do not work effectively for EB dehydrogenation in the presence of CO2. A variety of metal oxide catalysts, such as iron oxide [4,5], vanadia [6,7], chromia [8,9], ceria [8] and zirconia [10] have been studied. Propene is an important raw material for the production of polypropene, polyacrylonitrile, acrolein and acrylic acid. The catalytic dehydrogenation of propane to propene is of increasing significance due to the growing demand for propene. Dehydrogenation of propane is an endothermic reaction that requires relatively high temperature to obtain a high yield of propene. However, the high reaction temperature favours thermal cracking reactions to coke and light alkanes, resulting in a decline in product yield and an increase in catalyst deactivation. Although the oxidative dehydrogenation of propane by oxygen has been proposed as an alternative to the process, the propene selectivity of the catalytic reaction remains an unsolved problem due to the overoxidation of propane to carbon dioxide. Recently, the oxidative dehydrogenation of propane by carbon dioxide instead of oxygen has been reported to give rather high propene selectivity [2,11]. Since CO2 is one of

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the major greenhouse gases, the use of CO2 is attractive not only economically but also ecologically. Propane dehydrogenation in the presence of CO2 has been studied over various catalysts, such as Cr2O3 [12,13], rare earth vanadates [14] and Ga2O3 [15–17]. It is interesting to compare dehydrogenation of EB and propane in the presence of CO2. In this work, dehydrogenation of ethylbenzene and propane over Ga2O3–ZrO2 catalysts has been investigated and compared. The catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), Fourier transform infrared spectroscopy (FTIR), temperature programmed desorption of NH3 and CO2 (NH3-TPD and CO2-TPD) and N2 adsorption methods. The catalytic activities are correlated with the results of surface acidity and basicity measurements. 2. Experimental 2.1. Catalyst preparation Ga2O3–ZrO2 mixed oxide catalysts were prepared by a coprecipitation method. An aqueous solution of ammonia was added dropwise under vigorous stirring to a mixed solution of Ga(NO3)3 and ZrOCl2 until the final pH = 8.5 was attained. The precipitate was then filtered, washed with distilled water and dried at 383 K overnight. After calcination in static air at 923 K for 4 h, the catalysts were obtained. They are denoted as nGZ, where n represents the weight percentage of Ga2O3 in the catalysts. Pure ZrO2 catalyst is designated as Z. 2.2. Characterization of catalysts The BET surface areas of the catalysts were measured by N2 adsorption at 77 K on a Micromeritics ASAP 2000 instrument. XRD patterns were recorded on a Rigaku D/ MAX-IIA diffractometer with Ni-filtered Cu Ka radiation operated at 30 kV and 20 mA. Laser Raman spectra in the 100–800 cm 1 range were taken on a Dilor LabRam-1B spectrometer using a laser at 632.8 nm line as the excitation source. DRS spectra in the 190–600 cm 1 range were recorded on a JASCO V-550 spectrometer. The data were acquired and analyzed by a computer at the rate of 100 nm min 1. XPS spectra were obtained with Al Ka radiation (1486.6 eV) on a Perkin–Elmer PHI 5000C ESAC system at a base pressure of 1 · 10 9 Torr. All binding energy values were referenced to the C(1s) peak at 284.6 eV. SEM and EDX analysis were performed on a Philips XL30 microscope. In situ FTIR spectra of pyridine adsorption were recorded on a Nicolet Nexus 470 FT-IR spectrometer which was furnished with an in situ sample cell. A self-supporting disk of the sample (13.6 mg) was pretreated at 573 K for 4 h under a vacuum of 10 2 Pa and then cooled to room temperature. After the pyridine

adsorption at room temperature for 30 min and evacuation at 423 K for 1 h, IR spectra were recorded. Surface acidity was measured by NH3-TPD in a flow-type fixed-bed reactor at ambient pressure. The sample (100 mg) was preheated at 773 K for 3 h, and then cooled to 393 K in flowing He. At this temperature, sufficient pulses of NH3 were injected until adsorption saturation, followed by purging with He for 2 h. The temperature was then raised from 393 to 773 K at a rate of 10 K min 1, and the NH3 was collected in a liquid N2 trap and detected by gas chromatography. Surface basicity was measured by CO2-TPD with the same apparatus, and the method is similar to that of NH3-TPD except that the adsorption temperature of CO2 was 353 K. 2.3. Catalytic tests The dehydrogenation of EB in the presence of CO2 was carried out in a flow-type fixed-bed microreactor at ambient pressure. To supply the reactant, a gas mixture of N2 and CO2 (19:1 molar ratio) at a flow rate of 60 ml min 1 was passed through a glass evaporator filled with liquid EB maintained at 273 K. The molar ratio of N2 to CO2 to EB was 361:19:1. The catalyst load was 100 mg and the reaction temperature was 873 K. Prior to the reaction, the catalyst was pretreated at 873 K in N2 for 2 h. The hydrocarbon products were analyzed using an on-line gas chromatograph equipped with a 2-m packed column of DNP and a flame ionization detector (FID). The gas components, such as N2, CO and CO2, were analyzed by another gas chromatograph equipped with a 2-m packed column of carbon molecular sieve 601 and a thermal conductivity detector (TCD). Propane dehydrogenation in the presence of CO2 was performed at 873 K in a flow-type fixed-bed microreactor at ambient pressure. The catalyst load was 200 mg, and it was activated at 873 K in N2 for 2 h prior to the reaction. The gas reactant contained 2.5 vol% propane, 5 vol% carbon dioxide and balancing nitrogen. The total flow rate of gas reactant was 20 ml min 1. The hydrocarbon reaction products were analyzed using an on-line gas chromatograph equipped with a 6-m packed column of Porapak Q and a FID. The gas components, such as N2, CO and CO2, were analyzed by another gas chromatograph equipped with a 2-m packed column of carbon molecular sieve 601 and a TCD. 3. Results and discussion 3.1. XRD and textural characterization XRD patterns of GZ catalysts calcined at 923 K are shown in Fig. 1. It can be seen that both tetragonal and monoclinic ZrO2 are present in pure ZrO2. Moreover, the fraction of monoclinic zirconia, estimated by the formula proposed by Toraya et al. [18], is obviously higher than that of tetragonal zirconia. For GZ catalysts, the transformation

H. Li et al. / Catalysis Communications 8 (2007) 1317–1322 ∗ ∗

Intensity / a.u.

20GZ

Intensity / a.u.

15GZ 10GZ 5GZ ∗

10

20

∗ •∗

30

•

∗

40

•

∗ •

50

60

70

∗ ∗∗

∗∗

Z

• •

•

•

400

5GZ 10GZ 15GZ 20GZ

600

800

Wavenumber / cm

Fig. 2 shows the Raman spectra of GZ catalysts calcined at 923 K. The bands at 178, 189, 221, 306, 332, 345, 380, 475, 500, 536, 556, 615 and 635 cm 1 are the characteristic of the monoclinic ZrO2, while the bands at 149, 269, 317, 462 and 645 cm 1 are indicative of the tetragonal ZrO2 [19–21]. For pure ZrO2, both tetragonal and monoclinic ZrO2 were observed, and the monoclinic phase is dominant. For 10GZ, 15GZ and 20GZ catalysts, only the tetragonal ZrO2 phase is detected. These observations are in good agreement with the XRD results. However, the Raman spectrum of 5GZ catalyst reveals the co-existence of tetragonal and monoclinic ZrO2, which is different from the XRD observation. This discrepancy could be due to the fact that Raman spectroscopy is more surface-sensitive than XRD. The above result also indicates that the surface phase is different from the bulk phase for the 5GZ catalyst. Table 1 Surface area and NH3-TPD data of the GZ catalysts NH3 desorbed (mmol g 1) 393–623 K

623–773 K

Total

0.18 0.08 0.18 0.21 0.19

0.17 0.08 0.20 0.22 0.19

0.35 0.16 0.38 0.43 0.38

-1

Fig. 2. Raman spectra of GZ catalysts calcined at 923 K. (*) Monoclinic ZrO2; (d) tetragonal ZrO2.

The sharpness of all Raman peaks and, in particular, the resolution of two bands at 269 and 317 nm indicates that ZrO2 and Ga2O3–ZrO2 mixed oxides are highly crystalline. As the Ga2O3 content is increased, both the intensity of Raman peaks and the resolution of the above two bands become lower, suggesting a decrease in degree of crystalline ZrO2 order. This is in line with the XRD observation. 3.3. UV–Vis DRS study Fig. 3 shows UV–Visible diffuse reflectance spectra of GZ catalysts calcined at 923 K. For the ZrO2 catalyst, two intense adsorption maxima are observed at 210 and 228 nm. A broad shoulder peak with a maximum at ca. 305 nm, extending from 260 up to 400 nm, is also observed. For GZ catalysts, the band at 228 nm disappears. All bands may be attributed to charge transfer transitions. The bands at 210 and 228 nm are associated with the normal energy gap transition in crystalline ZrO2, and the shoulder peak at ca. 305 nm is related to transitions involving localised levels near the valence and conduction bands, due to defects and non-stoichiometry [22–24]. The occurrence of two bands at 210 and 228 nm is probably related to two different crystalline ZrO2 phases. The former band is related to the tetragonal ZrO2 and the latter band to

210

Absorbance / a.u.

3.2. Raman spectroscopy

19.0 34.8 42.0 53.9 59.1

∗ ∗∗

200

from the metastable tetragonal phase to the monoclinic phase is retarded. The tetragonal phase persists alone in these catalysts at 923 K. As the Ga2O3 content is increased, the intensity of diffractive peaks for GZ catalysts is weakened, suggesting that the degree of crystalline ZrO2 order is decreased. The characteristic peaks of Ga2O3 are not observed in all the catalysts, implying that Ga2O3 is rather homogeneously mixed with zirconia. After calcination at 923 K, the specific surface area of ZrO2 is 19.0 m2 g 1. The incorporation of Ga2O3 into ZrO2 leads to significant increment in surface area, as shown in Table 1. Furthermore, the surface area of GZ catalysts increases with an increase in Ga2O3 content.

Z 5GZ 10GZ 15GZ 20GZ

∗ •

∗

80

Fig. 1. XRD patterns of GZ catalysts calcined at 923 K. (*) Monoclinic ZrO2; (d) tetragonal ZrO2.

SBET (m2 g 1)

•

∗

Z

2 Theta / degree

Catalyst

1319

228

305 Z 5GZ 10GZ 15GZ 20GZ 200

300

400

500

600

Wavelength / nm Fig. 3. UV–Visible diffuse reflectance spectra of GZ catalysts calcined at 923 K.

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H. Li et al. / Catalysis Communications 8 (2007) 1317–1322

the monoclinic ZrO2 [25]. Thus, it can be concluded from the DRS results that pure ZrO2 exists in two crystalline phases after calcination at 923 K: a metastable tetragonal phase and a thermodynamically favoured monoclinic phase. For GZ catalysts, only the tetragonal phase appears. The above observation is in consistence with the XRD results. 3.4. XPS study The XPS spectra for 15GZ were recorded, and the deconvoluted spectra in the Ga(3d) region are shown in Fig. 4. Only two peaks appear in the spectrum of 15GZ at 24.2 and 21.3 eV, respectively, which can be assigned to O(2s) and Ga3+(3d) band according to the literature [26]. The Ga 3d banding energy of pure b-Ga2O3 is 20.8 eV [16]. This shift in binding energy (0.5 eV) can be attributed to a strong interaction between the Ga2O3 species and ZrO2. The strong interaction was also observed in our recent work when gallium oxide was supported on ZrO2 or Al2O3 [17]. 3.5. SEM and EDX analysis The SEM image of 15GZ is given in Fig. 5. It is clear that the catalyst is made up of particles with identical morphology, specifically spheres. Aggregates of such spheres were also observed. The EDX analysis at different particles has clearly shown homogeneous distribution of Ga in the particles. 3.6. Acid–base properties The surface acidity of GZ catalysts was measured by the NH3-TPD method, and the results are listed in Table 1. The number of medium to strong acid sites (expressed as the amount of NH3 desorbed at 623–773 K) present on GZ catalysts increases with the Ga2O3 content up to 15%, and then decreases as the Ga2O3 content is further increased. The same variation trend for the number of weak and total acid sites with the Ga2O3 content was

Fig. 5. SEM of the 15GZ catalyst.

observed. The acidity of ZrO2 is enhanced after the incorporation of appropriate amount of Ga2O3 into ZrO2. The nature of acid sites present on the 15GZ catalyst as a representative was measured in situ by FTIR spectra of pyridine adsorption (not shown). It can be seen that Lewis (characteristic at 1445, 1490 and 1606 cm 1) acid sites are present on the catalyst. In general, the band at 1490 cm 1 can be assigned to chemisorbed pyridine adsorbate complexes bonded to both Brønsted and Lewis acid sites. When Lewis acid sites are exclusively present in the sample, the 1490 cm 1 band intensity is one-third that of the 1445 cm 1 band. This is the case for our sample. Moreover, no band occurs at about 1540 cm 1 (pyridinium ion) showing that no Brønsted acidity is detectable. The above results indicate that only Lewis acid sites are present on the 15GZ catalyst. The surface basicity of GZ catalysts was measured by the CO2-TPD method, and the results are summarized in Table 2. As the Ga2O3 content is increased, the number of total base sites present on GZ catalysts first increases and then decreases. A maximum value appears at a Ga2O3 content of 15%. The number of weak base sites (expressed as the amount of CO2 desorbed at 353–623 K) follows the same variation trend. The basicity of ZrO2 is enhanced after the addition of appropriate amount of Ga2O3 into ZrO2. There is a much smaller number of base sites than acid sites on GZ catalysts, suggesting that Ga2O3–ZrO2 mixed oxides are primarily acid catalysts. 3.7. Dehydrogenation of ethylbenzene and propane The dehydrogenation of EB was carried out in the presence of CO2 at 873 K. Styrene is the major product of the reaction, whereas benzene and toluene are minor

Intensity / a.u.

A B

Table 2 CO2-TPD data of the GZ catalysts

28

26

24

22

20

18

16

14

Binding energy / eV Fig. 4. XPS spectra of the 15GZ catalyst. (A) Ga3+(3d) signal peak; (B) O(2s) signal peak.

Catalyst

CO2 desorbed (mmol g 1) 353–623 K

623–773 K

Total

Z 5GZ 10GZ 15GZ 20GZ

0.014 0.008 0.013 0.017 0.005

0.002 0.002 0.004 0.002 0.002

0.016 0.010 0.017 0.019 0.007

H. Li et al. / Catalysis Communications 8 (2007) 1317–1322

by-products. As the reaction goes on, a decline in EB conversion is observed and meanwhile the selectivity to styrene increases slightly. The yield of styrene decreases progressively with time on stream. As an example, the typical time course of EB dehydrogenation over the 15GZ catalyst is depicted in Fig. 6. The reaction data after 30 min on stream are compared, and the results are listed in Table 3. The activity of the ZrO2 catalyst is enhanced markedly after the incorporation of gallium oxide, whereas the selectivity to styrene declines to some extent. The activity of GZ catalysts is ca. 2 times as high as that of ZrO2. As the Ga2O3 content is increased, the conversion of EB on GZ catalysts first increases and then decreases. A maximum of 36.9 conversion appears at a Ga2O3 content of 15%. The selectivity to styrene at the maximum conversion is 93.0%. The same variation trend was observed for the yield of styrene, since the selectivity to styrene is similar. It has been suggested by various authors [27–30] that the oxidative dehydrogenation of EB with O2 or CO2 proceeds through the following steps: the adsorption of EB on an acid site, the abstraction of a-hydrogen from EB by a basic OH group adjacent to the acid site, the formation of an anion vacancy via desorption of water, the filling of the vacancy site by O species formed from adsorbed O2 or CO2 and the abstraction of b-hydrogen from EB by the O species to give styrene and basic OH group. According to this reaction mechanism, both acid and base sites are necessary for the reaction. The medium to strong acid sites would enhance the adsorption and activation of EB molecules, and the availability of adjacent basic OH groups on the catalyst would promote the abstraction of a-hydrogen from EB molecules to form water. This means that one may correlate the catalytic activity of GZ catalysts for EB dehydrogenation in the presence of CO2 with acid–base properties. The same variation trend of GZ catalysts for EB dehydrogenation activity, the number of medium to strong acid sites and total base sites with the Ga2O3 content is observed. Wu [31] also observed that the catalytic activities of EB dehydrogenation correlate well with the acidity and basicity of ZrO2–Fe2O3 catalysts.

80

80

60

60

40

40

20

20

0 0

1

2

3

4

Selectivity / %

100

Conversion / %

100

0

Time on stream / h Fig. 6. EB dehydrogenation and propane dehydrogenation over the 15GZ catalyst as a function of reaction time. (j) EB conversion; (d) styrene selectivity; (m) propane conversion; (.) propene selectivity.

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Table 3 Catalytic properties of GZ catalysts for the dehydrogenation of ethylbenzene (EB) in the presence of CO2 Catalyst

Z 5GZ 10GZ 15GZ 20GZ a b

EB conv. (%)

16.8 31.4 35.5 36.9 29.4

Selectivity (%)a

Yieldb (%)

STY

B

T

100 95.0 93.8 93.0 94.1

0 4.1 5.2 5.8 4.4

0 0.9 1.0 1.2 1.5

16.8 29.8 33.3 34.3 27.7

STY = styrene, B = benzene, T = toluene. Styrene yield.

The dehydrogenation of propane was performed in the presence of CO2 at 873 K. The major product formed in the reaction is propene, and the major by-products are methane, ethane and ethylene. The effect of reaction time on the catalytic performance of 15GZ as a representative catalyst is given in Fig. 6. The catalysts deactivate rather fast in the first 1 h, followed by a relatively slow decease in activity. Meanwhile, the selectivity to propene increases progressively with time on stream. Comparison between two dehydrogenation reactions shows that the catalysts deactivate more seriously in propane dehydrogenation than in EB dehydrogenation. Moreover, the selectivity to propene is lower than that to styrene, indicating that the side reaction of cracking is more serious during the dehydrogenation of propane. The reaction data for propane dehydrogenation after 10 min on stream are compared, and the results are listed in Table 4. The activity of the ZrO2 catalyst is very low. After the incorporation of gallium oxide into ZrO2, the catalyst becomes much more active, suggesting that Ga2O3 is the major active component for propane dehydrogenation in the presence of CO2. This may be the reason why the propane conversion on GZ catalysts increases with increasing the Ga2O3 content. The propene yield rises slightly due to the decline in propene selectivity. The different catalytic behavior of GZ catalysts in EB dehydrogenation and propane dehydrogenation with varying the Ga2O3 content could be related to the fact that the former reaction is easier to proceed than the latter one. In order to understand the role of CO2 in the reaction, the amount of CO in the reaction product on 15GZ as a representative catalyst was measured. For EB dehydrogenation, Table 4 Catalytic properties of GZ catalysts for the dehydrogenation of propane in the presence of CO2 Catalyst

Conv.a (%)

Z 5GZ 10GZ 15GZ 20GZ

5.6 27.9 32.8 38.6 41.8

a b

Propane conversion. Propene yield.

Yieldb (%)

Selectivity (%) C3H6

CH4

C2H4

C2H6

77.8 54.3 48.0 45.4 43.1

10.2 34.5 41.3 41.6 43.4

10.2 9.9 9.0 9.2 9.3

1.8 1.3 1.7 3.8 4.2

4.4 15.2 15.7 17.5 18.0

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the amount of CO formed is 1.5 as much as that of hydrocarbon products (styrene, benzene and toluene) after 30 min on stream. For propane dehydrogenation, the amount of CO formed is 2.8 as much as that of hydrocarbon products (propene, methane, ethane and ethene) after 10 min on stream. In the absence of CO2, EB conversion and propane conversion over the 15GZ catalyst are 32.3% and 26.1%, respectively, lower than those in the presence of CO2. Therefore, one role of CO2 is to promote the dehydrogenation reaction through the reverse water–gas shift reaction. The other role of CO2 could be the elimination of coke by the Boudouard reaction. 4. Conclusions In the present work we have shown that the transformation from the metastable tetragonal phase to the monoclinic phase is retarded after the incorporation of Ga2O3 into ZrO2. A strong interaction between the Ga2O3 species and ZrO2 was observed. Ga2O3–ZrO2 catalysts display different catalytic behavior for the dehydrogenation of ethylbenzene and propane in the presence of CO2, which is probably caused by the fact that the former reaction is easier to proceed than the latter one. Correlation between catalytic activity for EB dehydrogenation in the presence of CO2 and surface acidity and basicity of GZ catalysts has been found. Acknowledgments This work was financially supported by the State Basic Research Project of China (2006CB806103), Shanghai Research Institute of Petrochemical Technology, the National Natural Science Foundation of China (20303004) and the Shanghai Natural Science Foundation (06ZR14008). References [1] W.D. Mross, Catal. Rev. Sci. Eng. 25 (1983) 591. [2] S.B. Wang, Z.H. Zhu, Energy Fuels 18 (2004) 1126.

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