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Reactivity of n-butene isomers over a multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst in the oxidative dehydrogenation of n-butene
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Catalysis Communications 9 (2008) 1676–1680 www.elsevier.com/locate/catcom
Reactivity of n-butene isomers over a multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst in the oxidative dehydrogenation of n-butene Ji Chul Jung a, Howon Lee a, Heesoo Kim a, Young-Min Chung b, Tae Jin Kim b, Seong Jun Lee b, Seung-Hoon Oh b, Yong Seung Kim b, In Kyu Song a,* a
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, Republic of Korea b SK Energy Corporation, Yuseong-ku, Daejeon 305-712, Republic of Korea Received 8 November 2007; received in revised form 20 January 2008; accepted 21 January 2008 Available online 1 February 2008
Abstract Oxidative dehydrogenation of n-butene isomers to 1,3-butadiene was carried out over a multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst. The yield for 1,3-butadiene over the catalyst decreased in the order of 1-butene > mixture of 1-butene and 2butene > 2-butene. The oxidative dehydrogenation of n-butene to 1,3-butadiene favorably occurred with increasing 1-butene content, while the total oxidation of n-butene to CO2 was promoted with increasing 2-butene content. It is believed that the Co9Fe3Bi1Mo12O51 catalyst retained more selective oxygen species for the reaction with 1-butene than for the reaction with 2-butene, leading to the facile oxidative dehydrogenation of 1-butene to 1,3-butadiene. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Multicomponent bismuth molybdate; Reactivity of n-butene isomers; Oxidative dehydrogenation; 1,3-Butadiene; Selective oxygen species
1. Introduction Selective oxidation of hydrocarbons has been considered to be an important subject in the petrochemical industries due to its significance in producing large-scale chemical intermediates [1–5]. In particular, oxidative dehydrogenation of n-butene over multicomponent bismuth molybdate catalysts has attracted much attention as a promising process for producing 1,3-butadiene [6–9]. Three n-butene isomers (1-butene, cis-2-butene, and trans-2-butene) or their mixtures have been generally used as a n-butene source in the oxidative dehydrogenation of n-butene [10–15]. It has been reported that the reactivity of n-butene isomers strongly depends on the catalyst system [16], indicating that the favorable feed compositions of n-butene are different *
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song).
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.01.026
depending on the catalyst system. This implies that the feed composition of n-butene is one of the important factors determining the catalytic performance of multicomponent bismuth molybdate catalysts in the oxidative dehydrogenation of n-butene. A number of multicomponent bismuth molybdate catalysts can be formed depending on the constituent metal components and their compositions [6,17–22]. In our previous work [23], it was revealed that MII9MIII3Bi1Mo12O51 (MII: divalent metal, MIII: trivalent metal) catalysts were efficient for the oxidative dehydrogenation of n-butene. It has been generally accepted that Ni or Co is suitable for MII, while Fe is suitable for MIII in the multicomponent bismuth molybdate catalyst system [6,23]. In this work, therefore, Co9Fe3Bi1Mo12O51 was chosen as a model catalyst to investigate the reactivity of n-butene isomers over the multicomponent bismuth molybdate catalyst in the oxidative dehydrogenation of n-butene.
J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
A multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst was prepared by a co-precipitation method, and was applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. 1-Butene, 2-butene, or a mixture of 1-butene and 2-butene was used as a n-butene source to investigate the reactivity of n-butene isomers over the Co9Fe3Bi1Mo12O51 catalyst. 1-Butene-TPD and 2-butene-TPD experiments were carried out to identify selective oxygen species, and thus, to elucidate different reactivity of n-butene isomers over the Co9Fe3Bi1Mo12O51 catalyst. 2. Experimental 2.1. Catalyst preparation The Co9Fe3Bi1Mo12O51 catalyst was prepared by a coprecipitation method. 1.5 g of bismuth nitrate (Bi(NO3)3 5H2O, Sigma–Aldrich) was dissolved in 10 ml of distilled water that had been acidified with 3 ml of concentrated nitric acid. The solution was then added to 100 ml of an aqueous solution containing 7.9 g of cobalt nitrate (Co(NO3)2 6H2O, Sigma–Aldrich) and 3.7 g of ferric nitrate (Fe(NO3)3 9H2O, Sigma–Aldrich) to obtain a mixed nitrate solution. The mixed nitrate solution was added dropwise into 50 ml of an aqueous solution containing 6.4 g of ammonium molybdate ((NH4)6Mo7O24 4H2O, Sigma–Aldrich) under vigorous stirring. After stirring the mixed solution vigorously at room temperature for 1 h, a solid product was obtained by evaporation. The solid product was dried overnight at 175 °C, and it was then calcined at 475 °C for 5 h in an air stream to yield the Co9Fe3Bi1Mo12O51 catalyst. The formation of Co9Fe3Bi1Mo12O51 catalyst was confirmed by XRD (MAC Science, M18XHFSRA) measurements. The atomic ratio of constituent metal elements was determined by ICP-AES (Shimadz, ICP1000IV) analyses. 2.2. Temperature-programmed desorption (TPD) experiments To identify selective oxygen species and to elucidate different reactivity of n-butene isomers over the Co9Fe3Bi1Mo12O51 catalyst, 1-butene and 2-butene were used as a probe molecule in the TPD experiments. The Co9Fe3Bi1Mo12O51 catalyst (0.2 g) was charged into a tubular quartz reactor of the conventional TPD apparatus. The catalyst was pretreated at 200 °C for 1 h under a flow of helium (20 ml/min) to remove any physisorbed organic molecules. 20 ml of n-butene (1-butene or 2-butene) was then pulsed into the reactor every minute at room temperature under a flow of helium (5 ml/min), until the adsorption sites of the catalyst were saturated with n-butene. The physisorbed n-butene was removed by evacuating the catalyst sample at 50 °C for 1 h. The furnace temperature was increased from room temperature to 500 °C at a heating rate of 5 °C/min under a flow of helium (10 ml/min). The desorbed mole-
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Table 1 Compositions of n-butene employed in the oxidative dehydrogenation reaction n-Butene
1-Butene Mixture-(1-butene+ 2-butene) 2-Butene
Components (wt%) 1Butene
trans-2Butene
cis-2Butene
nButane
Others
99.1 61.4
0.1 23.9
0.1 12.4
0.1 0.4
0.6 1.9
0
26.5
72.5
0.2
0.8
cules were detected using a GC-MSD (Agilent, MSD6890N GC). 2.3. Oxidative dehydrogenation of n-butene isomers The oxidative dehydrogenation of n-butene isomers to 1,3-butadiene was carried out in a continuous flow fixedbed reactor in the presence of air and steam. The Co9Fe3Bi1Mo12O51 catalyst (0.5 g) was pretreated at 470 °C for 1 h with an air stream (16 ml/min). Water was sufficiently vaporized by passing through a pre-heating zone and was continuously fed into the reactor together with n-butene and air. Feed composition was fixed at n-butene:O2: steam = 1:0.75:15. 1-Butene, 2-butene, or a mixture of 1-butene and 2-butene was used as a n-butene source. The compositions of n-butene employed in the catalytic reaction are summarized in Table 1. Air was used as an oxygen source (nitrogen in air served as a carrier gas). The catalytic reaction was carried out at 420 °C. GHSV (gas hourly space velocity) was fixed at 475 h1 on the basis of n-butene. Reaction products were periodically sampled and analyzed with gas chromatographs. Conversion of n-butene and selectivity for 1,3-butadiene were calculated on the basis of carbon balance as follows. Yield for 1,3butadiene was calculated by multiplying conversion and selectivity moles of n-butene reacted moles of n-butene supplied Selectivity for 1; 3-butadiene moles of 1; 3-butadiene formed ¼ moles of n-butene reacted
Conversion of n-butene ¼
3. Results and discussion 3.1. Formation of Co9Fe3Bi1Mo12O51 catalyst The successful formation of Co9Fe3Bi1Mo12O51 catalyst was confirmed by XRD and ICP-AES measurements. Fig. 1 shows the XRD pattern of Co9Fe3Bi1Mo12O51 catalyst. Each phase was identified by its characteristic diffraction peaks using JCPDS. It was found that the Co9Fe3Bi1Mo12O51 catalyst was composed of four major mixed phases of b-CoMoO4, a-CoMoO4, Fe2(MoO4)3, and c-Bi2MoO6. This result was well consistent with a
J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
10
20
30
40
50
2 Theta (Degree) Fig. 1. XRD pattern of Co9Fe3Bi1Mo12O51 catalyst.
previous report [6]. The atomic ratio of constituent metal components determined by ICP-AES analyses was found to be Co:Fe:Bi:Mo = 9.0:3.2:1.0:11.4, in good agreement with the theoretical value. The above results indicate that the Co9Fe3Bi1Mo12O51 catalyst was successfully prepared in the present work. 3.2. Effect of feed composition on the catalytic performance of Co9Fe3Bi1Mo12O51 Fig. 2 shows the catalytic performance of Co9Fe3Bi1Mo12O51 with a variation of n-butene source in the oxidative dehydrogenation of n-butene at 420 °C after a 6 hreaction. In the catalytic reaction, CO2 was mainly produced as a by-product. The experimental results clearly showed that the catalytic performance of Co9Fe3Bi1Mo12O51 strongly depended on the composition of nbutene. It was revealed that the selectivity for 1,3-butadiene obtained with a mixture of 1-butene and 2-butene (91.7%) was slightly higher than that obtained with 1-butene (86.7%) or 2-butene (87.3%) feed. In our catalytic reaction system, isomerization between 1-butene and 2-butene also
Conversion of n-butene Selectivity for 1,3-butadiene Selectivity for CO2 Yield for 1,3-butadiene
100
Percentage
80
60
40
20
occurred to a small extent. However, the degree of isomerization between 1-butene and 2-butene could not be calculated in the case of mixed feed (1-butene and 2-butene), due to the simultaneous presence of two components. Therefore, it can be inferred that the slightly low selectivity for 1,3-butadiene observed for 1-butene and 2-butene feeds was due to the occurrence of a small degree of isomerization of pure n-butene. Importantly, it is noteworthy that both the conversion of n-butene and the yield for 1,3-butadiene decreased in the order of 1-butene > mixture of 1butene and 2-butene > 2-butene. This means that 1-butene is more favorable than 2-butene for the production of 1,3butadiene in the oxidative dehydrogenation of n-butene over the Co9Fe3Bi1Mo12O51 catalyst. In order to ensure the different reactivity of n-butene isomers, yields for 1,3-butadiene and CO2 were plotted as a function of 1-butene content in the n-butene feed. As shown in Fig. 3, the correlations clearly show that the yield for 1,3-butadiene increased with increasing 1-butene content in the feed, while the yield for CO2 decreased with increasing 1-butene content in the feed. Once again, this result supports that 1-butene was more favorable than 2butene in the oxidative dehydrogenation of n-butene to 1,3-butadiene over the multicomponent bismuth molybdate catalyst. The total oxidation of n-butene to CO2 was promoted with increasing 2-butene content in the feed. 3.3. Selective oxygen species for the formation of 1,3butadiene Fig. 4 shows the 1-butene-TPD and 2-butene-TPD profiles over Co9Fe3Bi1Mo12O51 catalyst. In the TPD experiments, n-butene (1-butene and 2-butene) was not desorbed in its pure form. Instead, the adsorbed n-butene (1-butene and 2-butene) was desorbed in the form of 1,3butadiene and CO2 by the reaction with oxygen species in the catalyst. It was found that only a small amount of
85
8.0
80
7.5
75
7.0
70
6.5
65 0 1-Butene
Mixture (1-Butene + 2-Butene)
6.0 0
2-Butene
Fig. 2. Catalytic performance of Co9Fe3Bi1Mo12O51 with a variation of nbutene source in the oxidative dehydrogenation of n-butene at 420 °C after a 6 h-reaction.
Yield for CO2 (%)
α-CoMoO4 γ-Bi2MoO6
Intensity (A. U.)
β-CoMoO4 Fe2(MoO4)3
Yield for 1,3-butadiene (%)
1678
20
40
60
80
100
1-Butene content in the n-butene feed (wt%) Fig. 3. Yields for 1,3-butadiene and CO2 over Co9Fe3Bi1Mo12O51 catalyst in the oxidative dehydrogenation of n-butene, plotted as a function of 1butene content in the n-butene feed.
J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
Mass signal intensity (A. U)
5x10
4x10
b
3
Mass signal intensity (A. U)
a
3
CO2 (x 0.1) 1,3-Butadiene (x 1)
3x10
2x10
1x10
3
3
3
1679
3
5x10
3
4x10
1,3-Butadiene (x 1)
CO2 (x 0.1)
3
3x10
3
2x10
3
1x10
0
0 0
100
200
300
400
500
0
Temperature (ºC)
100
200
300
400
500
Temperature (ºC)
Fig. 4. (a) 1-Butene-TPD and (b) 2-butene-TPD profiles over Co9Fe3Bi1Mo12O51 catalyst.
1,3-butadiene was formed and released at low temperature. On the other hand, CO2 was dominantly formed by the total oxidation of n-butene (1-butene and 2-butene) and released at high temperature in the TPD measurements. This result indicates that the oxygen species in the Co9Fe3Bi1Mo12O51 catalyst directly reacted with n-butene to form 1,3-butadiene and CO2. This reaction mechanism is in good agreement with the Mars-van Krevelen mechanism, which is a generally accepted mechanism for the oxidative dehydrogenation of n-butene [24–28]. According to this mechanism, lattice oxygen in the catalyst directly reacts with n-butene, and in turn, oxygen in the gas phase makes up the oxygen vacancy in the catalyst [29–33]. Some kinetic studies on this reaction also support that the oxidative dehydrogenation of n-butene follows the Mars-van Krevelen mechanism [34,35]. It is known that the reaction is zeroorder with respect to oxygen within a wide range of temperature, indicating that oxygen in the gas phase does not directly take part in the oxidative dehydrogenation of n-butene. Therefore, it is believed that the desorbed 1,3butadiene was attributed to the selective oxygen species in the catalyst involved in the oxidative dehydrogenation of n-butene, while the desorbed CO2 was due to the nonselective oxygen species in the catalyst causing the total oxidation of n-butene. This implies that the peak areas of 1,3-butadiene and CO2 reflect the amount of selective and nonselective oxygen species in the Co9Fe3Bi1Mo12O51 catalyst, respectively. The main peak positions (peak temperatures) of 1,3butadiene and CO2 in the 1-butene-TPD and 2-buteneTPD profiles were almost identical to one another. It is interesting to note that the Co9Fe3Bi1Mo12O51 catalyst retained two types of selective oxygen species for the reaction with 1-butene to form 1,3-butadiene (Fig. 4a). On the other hand, the Co9Fe3Bi1Mo12O51 catalyst provided only a single selective oxygen species for the reaction with 2-butene to form 1,3-butadiene (Fig. 4b). Furthermore, the total peak area of 1,3-butadiene profile obtained in
the 1-butene-TPD measurement (Fig. 4a) was much larger than that obtained in the 2-butene-TPD measurement (Fig. 4b). This indicates that the Co9Fe3Bi1Mo12O51 catalyst retained more selective oxygen species for the reaction with 1-butene than for the reaction with 2-butene in the formation of 1,3-butadiene. This result strongly supports the conclusion that 1-butene was more favorable than 2-butene for the formation of 1,3-butadiene in the oxidative dehydrogenation of n-butene over the Co9Fe3Bi1Mo12O51 catalyst. In the case of CO2 formation, two types of nonselective oxygen species were observed in the 2-butene-TPD profile (Fig. 4b), while only a single oxygen species was detected in the 1-butene-TPD profile (Fig. 4a). Furthermore, the peak area of CO2 profile observed in the 2-butene-TPD profile (Fig. 4b) was much larger than that observed in the 1-butene-TPD profile (Fig. 4a). This means that the Co9Fe3Bi1Mo12O51 catalyst retained more nonselective oxygen species for the reaction with 2-butene than for the reaction with 1-butene in the formation of CO2. This leads to a promoted total oxidation of 2-butene to CO2. 4. Conclusions A multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst was prepared by a co-precipitation method, and was applied to the oxidative dehydrogenation of n-butene isomers to 1,3-butadiene in a continuous flow fixed-bed reactor. 1-Butene, 2-butene, or a mixture of 1butene and 2-butene was used as a n-butene source. The yield for 1,3-butadiene over the Co9Fe3Bi1Mo12O51 catalyst decreased in the order of 1-butene > mixture of 1butene and 2-butene > 2-butene, indicating that 1-butene was much more efficient than 2-butene for the formation of 1,3-butadiene in the oxidative dehydrogenation of nbutene over the Co9Fe3Bi1Mo12O51 catalyst. This was attributed to the abundant selective oxygen species in the Co9Fe3Bi1Mo12O51 catalyst for the reaction with 1-butene in the formation of 1,3-butadiene. On the other hand, the
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J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
yield for CO2 over the Co9Fe3Bi1Mo12O51 catalyst increased in the order of 1-butene < mixture of 1-butene and 2-butene < 2-butene. This was due to the fact that the Co9Fe3Bi1Mo12O51 catalyst retained abundant nonselective oxygen species for the reaction with 2-butene in the formation of CO2, leading to a promoted total oxidation of 2-butene to CO2 over the Co9Fe3Bi1Mo12O51 catalyst. Acknowledgement The authors wish to acknowledge support from the Korea Energy Management Corporation (2005-01-0090-3010). References [1] S.C. Oh, H.P. Lee, H.T. Kim, K.O. Yoo, Korean J. Chem. Eng. 16 (1999) 543. [2] L.M. Madeira, M.F. Portela, Catal. Rev. 44 (2002) 247. [3] Y.H. Kim, H.S. Yang, Korean J. Chem. Eng. 17 (2000) 357. [4] Ph.A. Batist, J.F.H. Bouwens, G.C.A. Schuit, J. Catal. 25 (1972) 1. [5] R.K. Grasselli, Topics Catal. 21 (2002) 79. [6] Y. Moro-oka, W. Ueda, Adv. Catal. 40 (1994) 233. [7] W. Ueda, K. Asakawa, C.-L. Chen, Y. Moro-oka, T. Ikawa, J. Catal. 101 (1986) 360. [8] M.W.J. Wolfs, Ph.A. Batist, J. Catal. 32 (1974) 25. [9] D.-H. He, W. Ueda, Y. Moro-oka, Catal. Lett. 12 (1992) 35. [10] D.A.G. van Oeffelen, J.H.C. van Hooff, G.C.A. Schuit, J. Catal. 95 (1985) 84. [11] J.C. Jung, H. Kim, A.S. Choi, Y.-M. Chung, T.J. Kim, S.J. Lee, S.-H. Oh, I.K. Song, J. Mol. Catal. A 259 (2006) 166. [12] M.F. Porteal, Topics Catal. 15 (2001) 241.
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Catalysis Communications 9 (2008) 1676–1680 www.elsevier.com/locate/catcom
Reactivity of n-butene isomers over a multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst in the oxidative dehydrogenation of n-butene Ji Chul Jung a, Howon Lee a, Heesoo Kim a, Young-Min Chung b, Tae Jin Kim b, Seong Jun Lee b, Seung-Hoon Oh b, Yong Seung Kim b, In Kyu Song a,* a
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, Republic of Korea b SK Energy Corporation, Yuseong-ku, Daejeon 305-712, Republic of Korea Received 8 November 2007; received in revised form 20 January 2008; accepted 21 January 2008 Available online 1 February 2008
Abstract Oxidative dehydrogenation of n-butene isomers to 1,3-butadiene was carried out over a multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst. The yield for 1,3-butadiene over the catalyst decreased in the order of 1-butene > mixture of 1-butene and 2butene > 2-butene. The oxidative dehydrogenation of n-butene to 1,3-butadiene favorably occurred with increasing 1-butene content, while the total oxidation of n-butene to CO2 was promoted with increasing 2-butene content. It is believed that the Co9Fe3Bi1Mo12O51 catalyst retained more selective oxygen species for the reaction with 1-butene than for the reaction with 2-butene, leading to the facile oxidative dehydrogenation of 1-butene to 1,3-butadiene. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Multicomponent bismuth molybdate; Reactivity of n-butene isomers; Oxidative dehydrogenation; 1,3-Butadiene; Selective oxygen species
1. Introduction Selective oxidation of hydrocarbons has been considered to be an important subject in the petrochemical industries due to its significance in producing large-scale chemical intermediates [1–5]. In particular, oxidative dehydrogenation of n-butene over multicomponent bismuth molybdate catalysts has attracted much attention as a promising process for producing 1,3-butadiene [6–9]. Three n-butene isomers (1-butene, cis-2-butene, and trans-2-butene) or their mixtures have been generally used as a n-butene source in the oxidative dehydrogenation of n-butene [10–15]. It has been reported that the reactivity of n-butene isomers strongly depends on the catalyst system [16], indicating that the favorable feed compositions of n-butene are different *
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song).
1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.01.026
depending on the catalyst system. This implies that the feed composition of n-butene is one of the important factors determining the catalytic performance of multicomponent bismuth molybdate catalysts in the oxidative dehydrogenation of n-butene. A number of multicomponent bismuth molybdate catalysts can be formed depending on the constituent metal components and their compositions [6,17–22]. In our previous work [23], it was revealed that MII9MIII3Bi1Mo12O51 (MII: divalent metal, MIII: trivalent metal) catalysts were efficient for the oxidative dehydrogenation of n-butene. It has been generally accepted that Ni or Co is suitable for MII, while Fe is suitable for MIII in the multicomponent bismuth molybdate catalyst system [6,23]. In this work, therefore, Co9Fe3Bi1Mo12O51 was chosen as a model catalyst to investigate the reactivity of n-butene isomers over the multicomponent bismuth molybdate catalyst in the oxidative dehydrogenation of n-butene.
J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
A multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst was prepared by a co-precipitation method, and was applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. 1-Butene, 2-butene, or a mixture of 1-butene and 2-butene was used as a n-butene source to investigate the reactivity of n-butene isomers over the Co9Fe3Bi1Mo12O51 catalyst. 1-Butene-TPD and 2-butene-TPD experiments were carried out to identify selective oxygen species, and thus, to elucidate different reactivity of n-butene isomers over the Co9Fe3Bi1Mo12O51 catalyst. 2. Experimental 2.1. Catalyst preparation The Co9Fe3Bi1Mo12O51 catalyst was prepared by a coprecipitation method. 1.5 g of bismuth nitrate (Bi(NO3)3 5H2O, Sigma–Aldrich) was dissolved in 10 ml of distilled water that had been acidified with 3 ml of concentrated nitric acid. The solution was then added to 100 ml of an aqueous solution containing 7.9 g of cobalt nitrate (Co(NO3)2 6H2O, Sigma–Aldrich) and 3.7 g of ferric nitrate (Fe(NO3)3 9H2O, Sigma–Aldrich) to obtain a mixed nitrate solution. The mixed nitrate solution was added dropwise into 50 ml of an aqueous solution containing 6.4 g of ammonium molybdate ((NH4)6Mo7O24 4H2O, Sigma–Aldrich) under vigorous stirring. After stirring the mixed solution vigorously at room temperature for 1 h, a solid product was obtained by evaporation. The solid product was dried overnight at 175 °C, and it was then calcined at 475 °C for 5 h in an air stream to yield the Co9Fe3Bi1Mo12O51 catalyst. The formation of Co9Fe3Bi1Mo12O51 catalyst was confirmed by XRD (MAC Science, M18XHFSRA) measurements. The atomic ratio of constituent metal elements was determined by ICP-AES (Shimadz, ICP1000IV) analyses. 2.2. Temperature-programmed desorption (TPD) experiments To identify selective oxygen species and to elucidate different reactivity of n-butene isomers over the Co9Fe3Bi1Mo12O51 catalyst, 1-butene and 2-butene were used as a probe molecule in the TPD experiments. The Co9Fe3Bi1Mo12O51 catalyst (0.2 g) was charged into a tubular quartz reactor of the conventional TPD apparatus. The catalyst was pretreated at 200 °C for 1 h under a flow of helium (20 ml/min) to remove any physisorbed organic molecules. 20 ml of n-butene (1-butene or 2-butene) was then pulsed into the reactor every minute at room temperature under a flow of helium (5 ml/min), until the adsorption sites of the catalyst were saturated with n-butene. The physisorbed n-butene was removed by evacuating the catalyst sample at 50 °C for 1 h. The furnace temperature was increased from room temperature to 500 °C at a heating rate of 5 °C/min under a flow of helium (10 ml/min). The desorbed mole-
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Table 1 Compositions of n-butene employed in the oxidative dehydrogenation reaction n-Butene
1-Butene Mixture-(1-butene+ 2-butene) 2-Butene
Components (wt%) 1Butene
trans-2Butene
cis-2Butene
nButane
Others
99.1 61.4
0.1 23.9
0.1 12.4
0.1 0.4
0.6 1.9
0
26.5
72.5
0.2
0.8
cules were detected using a GC-MSD (Agilent, MSD6890N GC). 2.3. Oxidative dehydrogenation of n-butene isomers The oxidative dehydrogenation of n-butene isomers to 1,3-butadiene was carried out in a continuous flow fixedbed reactor in the presence of air and steam. The Co9Fe3Bi1Mo12O51 catalyst (0.5 g) was pretreated at 470 °C for 1 h with an air stream (16 ml/min). Water was sufficiently vaporized by passing through a pre-heating zone and was continuously fed into the reactor together with n-butene and air. Feed composition was fixed at n-butene:O2: steam = 1:0.75:15. 1-Butene, 2-butene, or a mixture of 1-butene and 2-butene was used as a n-butene source. The compositions of n-butene employed in the catalytic reaction are summarized in Table 1. Air was used as an oxygen source (nitrogen in air served as a carrier gas). The catalytic reaction was carried out at 420 °C. GHSV (gas hourly space velocity) was fixed at 475 h1 on the basis of n-butene. Reaction products were periodically sampled and analyzed with gas chromatographs. Conversion of n-butene and selectivity for 1,3-butadiene were calculated on the basis of carbon balance as follows. Yield for 1,3butadiene was calculated by multiplying conversion and selectivity moles of n-butene reacted moles of n-butene supplied Selectivity for 1; 3-butadiene moles of 1; 3-butadiene formed ¼ moles of n-butene reacted
Conversion of n-butene ¼
3. Results and discussion 3.1. Formation of Co9Fe3Bi1Mo12O51 catalyst The successful formation of Co9Fe3Bi1Mo12O51 catalyst was confirmed by XRD and ICP-AES measurements. Fig. 1 shows the XRD pattern of Co9Fe3Bi1Mo12O51 catalyst. Each phase was identified by its characteristic diffraction peaks using JCPDS. It was found that the Co9Fe3Bi1Mo12O51 catalyst was composed of four major mixed phases of b-CoMoO4, a-CoMoO4, Fe2(MoO4)3, and c-Bi2MoO6. This result was well consistent with a
J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
10
20
30
40
50
2 Theta (Degree) Fig. 1. XRD pattern of Co9Fe3Bi1Mo12O51 catalyst.
previous report [6]. The atomic ratio of constituent metal components determined by ICP-AES analyses was found to be Co:Fe:Bi:Mo = 9.0:3.2:1.0:11.4, in good agreement with the theoretical value. The above results indicate that the Co9Fe3Bi1Mo12O51 catalyst was successfully prepared in the present work. 3.2. Effect of feed composition on the catalytic performance of Co9Fe3Bi1Mo12O51 Fig. 2 shows the catalytic performance of Co9Fe3Bi1Mo12O51 with a variation of n-butene source in the oxidative dehydrogenation of n-butene at 420 °C after a 6 hreaction. In the catalytic reaction, CO2 was mainly produced as a by-product. The experimental results clearly showed that the catalytic performance of Co9Fe3Bi1Mo12O51 strongly depended on the composition of nbutene. It was revealed that the selectivity for 1,3-butadiene obtained with a mixture of 1-butene and 2-butene (91.7%) was slightly higher than that obtained with 1-butene (86.7%) or 2-butene (87.3%) feed. In our catalytic reaction system, isomerization between 1-butene and 2-butene also
Conversion of n-butene Selectivity for 1,3-butadiene Selectivity for CO2 Yield for 1,3-butadiene
100
Percentage
80
60
40
20
occurred to a small extent. However, the degree of isomerization between 1-butene and 2-butene could not be calculated in the case of mixed feed (1-butene and 2-butene), due to the simultaneous presence of two components. Therefore, it can be inferred that the slightly low selectivity for 1,3-butadiene observed for 1-butene and 2-butene feeds was due to the occurrence of a small degree of isomerization of pure n-butene. Importantly, it is noteworthy that both the conversion of n-butene and the yield for 1,3-butadiene decreased in the order of 1-butene > mixture of 1butene and 2-butene > 2-butene. This means that 1-butene is more favorable than 2-butene for the production of 1,3butadiene in the oxidative dehydrogenation of n-butene over the Co9Fe3Bi1Mo12O51 catalyst. In order to ensure the different reactivity of n-butene isomers, yields for 1,3-butadiene and CO2 were plotted as a function of 1-butene content in the n-butene feed. As shown in Fig. 3, the correlations clearly show that the yield for 1,3-butadiene increased with increasing 1-butene content in the feed, while the yield for CO2 decreased with increasing 1-butene content in the feed. Once again, this result supports that 1-butene was more favorable than 2butene in the oxidative dehydrogenation of n-butene to 1,3-butadiene over the multicomponent bismuth molybdate catalyst. The total oxidation of n-butene to CO2 was promoted with increasing 2-butene content in the feed. 3.3. Selective oxygen species for the formation of 1,3butadiene Fig. 4 shows the 1-butene-TPD and 2-butene-TPD profiles over Co9Fe3Bi1Mo12O51 catalyst. In the TPD experiments, n-butene (1-butene and 2-butene) was not desorbed in its pure form. Instead, the adsorbed n-butene (1-butene and 2-butene) was desorbed in the form of 1,3butadiene and CO2 by the reaction with oxygen species in the catalyst. It was found that only a small amount of
85
8.0
80
7.5
75
7.0
70
6.5
65 0 1-Butene
Mixture (1-Butene + 2-Butene)
6.0 0
2-Butene
Fig. 2. Catalytic performance of Co9Fe3Bi1Mo12O51 with a variation of nbutene source in the oxidative dehydrogenation of n-butene at 420 °C after a 6 h-reaction.
Yield for CO2 (%)
α-CoMoO4 γ-Bi2MoO6
Intensity (A. U.)
β-CoMoO4 Fe2(MoO4)3
Yield for 1,3-butadiene (%)
1678
20
40
60
80
100
1-Butene content in the n-butene feed (wt%) Fig. 3. Yields for 1,3-butadiene and CO2 over Co9Fe3Bi1Mo12O51 catalyst in the oxidative dehydrogenation of n-butene, plotted as a function of 1butene content in the n-butene feed.
J.C. Jung et al. / Catalysis Communications 9 (2008) 1676–1680
Mass signal intensity (A. U)
5x10
4x10
b
3
Mass signal intensity (A. U)
a
3
CO2 (x 0.1) 1,3-Butadiene (x 1)
3x10
2x10
1x10
3
3
3
1679
3
5x10
3
4x10
1,3-Butadiene (x 1)
CO2 (x 0.1)
3
3x10
3
2x10
3
1x10
0
0 0
100
200
300
400
500
0
Temperature (ºC)
100
200
300
400
500
Temperature (ºC)
Fig. 4. (a) 1-Butene-TPD and (b) 2-butene-TPD profiles over Co9Fe3Bi1Mo12O51 catalyst.
1,3-butadiene was formed and released at low temperature. On the other hand, CO2 was dominantly formed by the total oxidation of n-butene (1-butene and 2-butene) and released at high temperature in the TPD measurements. This result indicates that the oxygen species in the Co9Fe3Bi1Mo12O51 catalyst directly reacted with n-butene to form 1,3-butadiene and CO2. This reaction mechanism is in good agreement with the Mars-van Krevelen mechanism, which is a generally accepted mechanism for the oxidative dehydrogenation of n-butene [24–28]. According to this mechanism, lattice oxygen in the catalyst directly reacts with n-butene, and in turn, oxygen in the gas phase makes up the oxygen vacancy in the catalyst [29–33]. Some kinetic studies on this reaction also support that the oxidative dehydrogenation of n-butene follows the Mars-van Krevelen mechanism [34,35]. It is known that the reaction is zeroorder with respect to oxygen within a wide range of temperature, indicating that oxygen in the gas phase does not directly take part in the oxidative dehydrogenation of n-butene. Therefore, it is believed that the desorbed 1,3butadiene was attributed to the selective oxygen species in the catalyst involved in the oxidative dehydrogenation of n-butene, while the desorbed CO2 was due to the nonselective oxygen species in the catalyst causing the total oxidation of n-butene. This implies that the peak areas of 1,3-butadiene and CO2 reflect the amount of selective and nonselective oxygen species in the Co9Fe3Bi1Mo12O51 catalyst, respectively. The main peak positions (peak temperatures) of 1,3butadiene and CO2 in the 1-butene-TPD and 2-buteneTPD profiles were almost identical to one another. It is interesting to note that the Co9Fe3Bi1Mo12O51 catalyst retained two types of selective oxygen species for the reaction with 1-butene to form 1,3-butadiene (Fig. 4a). On the other hand, the Co9Fe3Bi1Mo12O51 catalyst provided only a single selective oxygen species for the reaction with 2-butene to form 1,3-butadiene (Fig. 4b). Furthermore, the total peak area of 1,3-butadiene profile obtained in
the 1-butene-TPD measurement (Fig. 4a) was much larger than that obtained in the 2-butene-TPD measurement (Fig. 4b). This indicates that the Co9Fe3Bi1Mo12O51 catalyst retained more selective oxygen species for the reaction with 1-butene than for the reaction with 2-butene in the formation of 1,3-butadiene. This result strongly supports the conclusion that 1-butene was more favorable than 2-butene for the formation of 1,3-butadiene in the oxidative dehydrogenation of n-butene over the Co9Fe3Bi1Mo12O51 catalyst. In the case of CO2 formation, two types of nonselective oxygen species were observed in the 2-butene-TPD profile (Fig. 4b), while only a single oxygen species was detected in the 1-butene-TPD profile (Fig. 4a). Furthermore, the peak area of CO2 profile observed in the 2-butene-TPD profile (Fig. 4b) was much larger than that observed in the 1-butene-TPD profile (Fig. 4a). This means that the Co9Fe3Bi1Mo12O51 catalyst retained more nonselective oxygen species for the reaction with 2-butene than for the reaction with 1-butene in the formation of CO2. This leads to a promoted total oxidation of 2-butene to CO2. 4. Conclusions A multicomponent bismuth molybdate (Co9Fe3Bi1Mo12O51) catalyst was prepared by a co-precipitation method, and was applied to the oxidative dehydrogenation of n-butene isomers to 1,3-butadiene in a continuous flow fixed-bed reactor. 1-Butene, 2-butene, or a mixture of 1butene and 2-butene was used as a n-butene source. The yield for 1,3-butadiene over the Co9Fe3Bi1Mo12O51 catalyst decreased in the order of 1-butene > mixture of 1butene and 2-butene > 2-butene, indicating that 1-butene was much more efficient than 2-butene for the formation of 1,3-butadiene in the oxidative dehydrogenation of nbutene over the Co9Fe3Bi1Mo12O51 catalyst. This was attributed to the abundant selective oxygen species in the Co9Fe3Bi1Mo12O51 catalyst for the reaction with 1-butene in the formation of 1,3-butadiene. On the other hand, the
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yield for CO2 over the Co9Fe3Bi1Mo12O51 catalyst increased in the order of 1-butene < mixture of 1-butene and 2-butene < 2-butene. This was due to the fact that the Co9Fe3Bi1Mo12O51 catalyst retained abundant nonselective oxygen species for the reaction with 2-butene in the formation of CO2, leading to a promoted total oxidation of 2-butene to CO2 over the Co9Fe3Bi1Mo12O51 catalyst. Acknowledgement The authors wish to acknowledge support from the Korea Energy Management Corporation (2005-01-0090-3010). References [1] S.C. Oh, H.P. Lee, H.T. Kim, K.O. Yoo, Korean J. Chem. Eng. 16 (1999) 543. [2] L.M. Madeira, M.F. Portela, Catal. Rev. 44 (2002) 247. [3] Y.H. Kim, H.S. Yang, Korean J. Chem. Eng. 17 (2000) 357. [4] Ph.A. Batist, J.F.H. Bouwens, G.C.A. Schuit, J. Catal. 25 (1972) 1. [5] R.K. Grasselli, Topics Catal. 21 (2002) 79. [6] Y. Moro-oka, W. Ueda, Adv. Catal. 40 (1994) 233. [7] W. Ueda, K. Asakawa, C.-L. Chen, Y. Moro-oka, T. Ikawa, J. Catal. 101 (1986) 360. [8] M.W.J. Wolfs, Ph.A. Batist, J. Catal. 32 (1974) 25. [9] D.-H. He, W. Ueda, Y. Moro-oka, Catal. Lett. 12 (1992) 35. [10] D.A.G. van Oeffelen, J.H.C. van Hooff, G.C.A. Schuit, J. Catal. 95 (1985) 84. [11] J.C. Jung, H. Kim, A.S. Choi, Y.-M. Chung, T.J. Kim, S.J. Lee, S.-H. Oh, I.K. Song, J. Mol. Catal. A 259 (2006) 166. [12] M.F. Porteal, Topics Catal. 15 (2001) 241.
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