ZrO2 catalysts for CO2 dehydrogenation of ethane to ethylene

Studies in Surface Science and Catalysis, volume 147 X. Bao and Y. Xu (Editors) {)2004 Elsevier B.V. All rights reserved.

691

Cr203/ZrO2 catalysts for CO2 dehydrogenation of ethane to ethylene S h a o b i n W a n g a and K a z u h i s a M u r a t a b

aDepartment of Chemical Engineering, Curtin University of Technology, GPO Box U 1987, Perth, WA 6845, Australia. Email: [email protected] bResearch Institute for Green Technology, AIST, Tsukuba, Ibaraki 305-8565, Japan ABSTRACT Several Cr203 catalysts supported on zirconia and its modified forms were prepared, characterized and tested for oxidative dehydrogenation of ethane into ethylene by CO2. The effects of sulfation and tungstation of zirconia on the catalytic activity were investigated. CrzO3/ZrO2 exhibited high activity in this reaction with 60% ethylene selectivity at 57% ethane conversion at 650 ~ Addition of WO3 reduced the surface area, redox potential and acidity, resulting in decreases in activity and selectivity, whereas SO42- additive could enhance the ethylene selectivity and yield due to the change of property of acid sites. 1. INTRODUCTION Ethylene is an important raw material for petrochemical industry and plastics production. Oxidative dehydrogenation of ethane by oxygen into ethylene has been proposed as a good alternative to the process of thermal cracking of ethane because it provides several advantages, such as lower operation temperature and less coking [1, 2]. However, over-oxidation of ethylene to carbon oxides is a serious concern. In the past several years, some researchers have attempted to use weak oxidants like CO2 [3-8] or N20 [9-11] for the oxidative dehydrogenation of hydrocarbons. We have found that zirconia-supported C r 2 0 3 exhibits good activity for dehydrogenation of ethane and CO2 can promote the dehydrogenation [12]. Sulfation and tungstation of zirconia will change the surface properties of the catalysts thus promoting the activity in oxidative dehydrogenation of ethane by oxygen [13, 14]. Here, we report our further investigation on the effect of modification of zirconia by sulfation and tungstation on their catalytic behaviors in the dehydrogenation of ethane by carbon dioxide.

692 2. EXPERIMENTAL ZrO2 samples were prepared by heat-treatment of the amorphous ZrO2 obtained by precipitation of ZrO(NO3)2. The sulfated and tungstated zirconia samples were then prepared by wetness impregnation of ammonium sulfate and ammonium tungstate salts on the amorphous ZrO2 with 6 wt% loading of sulfate or WO3, calcination at 700 ~ for 3 h. All chromium-based catalysts were prepared by impregnation of Cr(NO3)3 at a Cr203 loading of 5 wt% on the supports, calcination at 700 ~ for 3 h. BET surface area of the catalysts was obtained by nitrogen adsorption a t 196 ~ XRD measurements were conducted on a Philips PW 1800 X-ray diffractometer at 40 kV and 40 mA. XPS measurements were carried out on a PHI 5500 ESCA system (Perkin-Elmer) with Mg Ka as radiation source. Temperature-programmed reduction (TPR) experiments were conducted in a fixed-bed reactor loaded with 0.5 g samples. The samples were reduced in 10% Hz/Ar flow at a rate of 30 ml/min from ambient temperature to 700 ~ at a heating rate of 3 ~ The TPD profiles were obtained on a special NH3-TPD apparatus under vacuum conditions with the temperature varying from 100-800 ~ at a heating rate of 10 ~ The oxidative dehydrogenation of ethane by CO2 was performed at atmospheric pressure on a fixed-bed reactor packed with 1 g catalysts and 2 g quartz sands. The reactant stream C2H6: C 0 2 : N 2 = 10%: 50%: 40% was introduced into the reactor at a flow rate of 60 ml/min while no back-mixing effect was found. The reaction temperature ranged between 500-650~ The products were analyzed by two gas chromatographs (Shimadzu, GC-8A).

3. RESULTS AND DISCUSSION 3.1. Catalyst characterization Table 1 presents the surface areas of the Cr-based catalysts prepared. It is seen that ZrO: has the lowest surface area and the surface area of the sulfate modified Cr-based catalyst is about the same as that of the unpromoted Cr/ZrO2 catalyst. However, tungstated Cr/ZrO: exhibits much lower surface area. These results indicate that sulfation has little effect on the surface area while tungstation induces a remarkable reduction in the surface area. The XRD patterns of ZrO: and Cr-based catalysts are shown in Fig.1. The calcined support, ZrO:, presents crystalline phase of monoclinic ZrO2. For Cr/ZrO:, tetragonal ZrO2 is the major phase with minor monoclinic ZrO2 while two ZrO: phases, monoclinic and tetragonal ZrO:, coexist in those modified Cr/ZrO: catalysts. However, the two phases show a different ratio. Monoclinic ZrO: is the major phase for Cr/S-ZrO: whereas the tetragonal ZrO: dominates

693 Table 1 Physico-chemical properties of chromium oxide based catalysts. Catalyst SBET NH3 Cr Concentration Cr/Zr Cr 3+ BE (ev) (m2/g) (gmol/g) (%) ZrO2 22 . . . . . Cr/ZrO2 62 20.7 4.5 0.197 576.7 Cr/W-ZrO2 36 8.4 3.2 0.187 577.0 Cr/S-ZrO2 60 14.0 2.8 0.134 575.8

Cr 6+ BE

(ev) 579.1 579.0 577.4

the ZrO2 phase in Cr/W-ZrO2. These results suggest that the additives on ZrO2 can influence the transformation of ZrO2 phases, favoring the stability of tetragonal phase. For all zirconia-supported catalysts, no significant Cr203 diffraction peaks can be observed, probably due to low amount and welldispersed Cr203 particles on ZrO2 surface. The surface Cr species on the various Cr/ZrO2 catalysts were determined by XPS and their concentrations and Crzp spectra are shown in Table 1 and Fig. 2, respectively. Cr/ZrO2 has the largest Cr surface concentration while Cr/S-ZrO2 shows the lowest surface Cr value. The ratio of Cr/Zr also indicates the greatest dispersion of Cr on Cr/ZrO2. The Crzp spectra demonstrate that three catalysts display somewhat different XPS spectral patterns. Two Cr species can be found by curve fitting and they are assigned to Cr 3+ and Cr 6+, whose binding energies (BE) are given in Table 1. The peaks in the Cr/S-ZrO2 spectrum show a shift to lower binding energy while others shows a similar position in BE values. The spectra also show that the concentrations of Cr 3+ and Cr 6+are different for three Cr-based catalysts. The ammonia adsorption and TPD profiles over the Cr-based catalysts are presented in Table 1 and Fig. 3, respectively. As seen, sulfation and tungstation i

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Fig. 1. XRD patterns of ZrO2 and Cr-based catalysts.

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Fig. 2. XPS Cr2p profiles of Cr-based catalysts.

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Fig. 3. TPD profiles of NH3 on Cr-based catalysts.

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Fig. 4. TPR profiles of ZrO2 and Cr-based catalysts.

also reduce the ammonia adsorption and the addition of WO3 results in the greatest reduction in acidity. From NH3-TPD profiles, one can see that Cr/ZrO2 exhibits only one strong desorption peak occurring at 100 - 300 ~ Two broad peaks can be observed in the TPD profile of Cr/S-ZrO2. The stronger one appears at 100 - 300 ~ the same position as that on Cr/ZrO2, and the weak one occurs at 360 - 500 ~ For Cr/W-ZrO2, three peaks are present in the TPD profile. The strongest one appears at 100 - 300 ~ while the other two weak peaks appear at 380 - 450 ~ and 480 - 520 ~ respectively. The TPR profiles of the calcined ZrO2 and Cr-based catalysts are illustrated in Fig. 4. The TPR profiles of ZrO2 and the Cr-based catalysts display significantly different behavior. ZrO2 shows a strong reduction peak only at 550 - 650 ~ Cr/ZrO2 exhibits strong and broad reduction peaks after 200 ~ Two distinct reduction peaks occur on the TPR profile of Cr/S-ZrO2, centred at 290 ~ and 460 ~ while no significant reduction peak appears on the TPR profile of Cr/W-ZrO2. Those results suggest that impregnation of Cr203 on ZrO2 enhances the redox potential while modification of Cr/ZrO2 significantly decreases the redox potential of the catalysts. 3.2. Catalytic performance Table 2 gives the results of catalytic activity and selectivity of all catalysts at the temperatures 500-650 ~ One can see that ethane and CO2 conversions increase as the temperature increases while ethylene selectivity decreases with the increasing ethane conversion. The consumption of CO2 suggests the involvement of CO2 in the reaction. The calcined ZrO2 exhibits little activity at low temperatures but can induce reaction at high temperature of 650 ~ giving 93 % ethylene selectivity at 10 % ethane conversion and also producing some

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oxygenates. Cr/ZrO2 shows a high activity, but low selectivity to ethylene, especially at higher temperatures. At 650 ~ ethane conversion is 57 % while ethylene selectivity is only 60%, giving ethylene yield of 35%. Addition of sulfate or tungstate oxide on CffZrO2 shows a different effect on catalytic behavior. Cr/W-ZrO2 exhibits lower ethane conversion and ethylene selectivity. Although ethane conversion is reduced on Cr/S-ZrO2 while ethylene selectivity is improved. Due to the higher selectivity to ethylene, Cr/S-ZrO2 produces even higher ethylene yield at high temperatures (600-650 ~ Therefore, the catalytic activity of ZrO2 supported catalysts presents an order as Cr/ZrO2 > Cr/S-ZrO2 > Cr/W-ZrO2 > ZrO2 in terms of ethane conversion. It has been proposed that the dehydrogenation and oxidative dehydrogenation of ethane due to the introduction of CO2 and its decomposition to produce surface oxygen species will be the parallel reaction paths [12]. It is well known that selective oxidation reactions on oxides proceed through a redox mechanism. The acid-base and redox properties of the oxide surface play an important role in catalytic activity. Therefore, the different activity of Cr-based catalysts can be attributed to the varying chromium structure and redox properties of the catalysts prepared. From Table 1, it is seen that tungstation of Cr/ZrO2 significantly reduces the surface area while sulfation shows little change on the surface area of Cr/ZrO2. The order of surface area is the same as the ethane conversion of the catalysts. The amounts of ammonium adsorption on three CrzO3-based catalysts show that Cr/W-ZrO2 and Cr/S-ZrO2 have lower Table 2 Catalytic activity and product selectivity over chromium oxide based catalysts at 500-650 ~ Catalyst Temp Conv (%) Sel (%) Yield (%) ZrO2

Cr/ZrO2

Cr/W-Zr02

Cr/S-ZrO2

(~

C2H6

CO2

C2H4

CH4

C3H8

EtOH

C2H4

500 550 600 650 500 550 600 650 500 550 600 650 500 550 600 650

0.1 0.9 1.8 10.6 10.3 22.9 37.9 57.3 7.4 18.5 33.6 49.0 7.0 17.5 35.0 50.6

1.0 1.9 2.4 4.0 4.6 10.2 19.2 29.1 4.3 7.2 15.0 23.0 2.2 8.9 17.0 21.9

81.8 97.9 84.0 93.4 95.3 87.2 74.6 60.4 94.0 84.9 67.0 54.6 94.3 91.3 84.2 76.0

18.2 2.1 16.0 6.1 4.6 12.7 25.3 39.4 5.7 14.0 31.6 44.7 5.7 8.7 15.7 23.9

0 0 0 0.05 0.03 0.05 0.09 0.10 0.31 0.72 0.85 0.47 0 0.02 0.06 0.10

0 0 0 0.5 0 0 0 0.08 0 0.29 0.51 0.27 0 0 0 0

0.1 0.9 1.5 9.9 9.8 20.0 28.3 34.6 6.9 16.0 22.6 26.7 6.6 16.0 29.4 38.4

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acidities than that of Cr/ZrO2, similar to the order of ethane conversion. While the profile of TPD on Cr/S-ZrO2 shows a different pattern from that of the other two catalysts, which can contribute to the difference in ethylene selectivity. The temperature-programmed reduction by hydrogen also shows that ZrO2 and its supported Cr203 catalysts demonstrate varying redox potentials. ZrO2 only exhibits one reduction peak centered at 650 ~ while others show two broad peaks both at lower temperatures before 500 ~ which indicates a higher redox potential. The XPS results also show a different pattern of Cr6+/Cr3+ on three Cr-based catalysts with Cr/ZrO2 showing the highest value, which favors the redox cycle of Cr6+/Cr3+. 4. C O N C L U S I O N Cr203/ZrO2 based catalysts are effective for oxidative dehydrogenation of ethane with CO2. Sulfate shows a better effect on catalytic activity by enhancing the ethylene selectivity while tungstate oxide suppresses the activity and ethylene selectivity. Addition of sulfate changes the property of acid sites responsible for the ethylene selectivity while addition of tungstate oxide reduces the surface area, acidity, and redox potential resulting in decreases in activity and selectivity to ethylene. REFERENCES

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