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Catalytic performance of bismuth molybdate catalysts in the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene
Applied Catalysis A: General 317 (2007) 244–249 www.elsevier.com/locate/apcata
Catalytic performance of bismuth molybdate catalysts in the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene Ji Chul Jung a, Heesoo Kim a, Yong Seung Kim b, Young-Min Chung b, Tae Jin Kim b, Seong Jun Lee b, Seung-Hoon Oh 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, South Korea b SK Corporation, Yuseong-ku, Daejeon 305-712, South Korea Received 20 July 2006; received in revised form 9 October 2006; accepted 11 October 2006 Available online 20 November 2006
Abstract a-Bi2Mo3O12 and g-Bi2MoO6 were prepared by a co-precipitation method, and were applied to the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene. Both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts were thermally and structurally stable during the catalytic reaction. They exhibited a stable catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 without catalyst deactivation. However, the catalytic performance of g-Bi2MoO6 was superior to a-Bi2Mo3O12 due to the facile oxygen mobility of g-Bi2MoO6. The reactivity of n-butene isomers in the C4 raffinate-3 decreased in the order of 1-butene > trans-2-butene > cis-2-butene over both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts. Steam played an essential role in suppressing CO2 formation, and furthermore, served as a heat sink for preventing hot spots or reactor run-away. In the catalytic reaction with respect to reaction temperature, the maximum conversion of n-butene (ca. 66%) and the maximum yield for 1,3-butadiene (ca. 60%) were achieved at 440 8C over the g-Bi2MoO6 catalyst (n-butene:oxygen:steam = 1:0.75:15). # 2006 Elsevier B.V. All rights reserved. Keywords: Bismuth molybdate; C4 raffinate-3; 1,3-Butadiene; Oxidative dehydrogenation; Catalyst stability; Effect of steam; Effect of reaction temperature
1. Introduction Bismuth molybdates have been widely investigated as efficient catalysts for the selective oxidation of olefins such as the oxidation of propylene to acrolein, the ammoxidation of propylene to acrylonitrile, and the oxidative dehydrogenation of n-butene to 1,3-butadiene [1–4]. In particular, the oxidative dehydrogenation of n-butene over bismuth molybdate catalysts has attracted much attention as a promising process for manufacturing 1,3-butadiene [3–7], an important raw material for manufacturing a large number of chemical products in the petrochemical industries [8,9]. The oxidative dehydrogenation of n-butene has many advantages because this process can be operated as a single unit and is independent of the naphtha cracking unit in producing 1,3-butadine in the sense that no additional major naphtha cracking products such as ethylene
* Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.10.021
and propylene are produced for the production of 1,3-butadiene [3–5,10–14]. C4 raffinate-3 is a residue obtained after separating 1,3butadiene, isobutene, and 1-butene from the C4 raffinate stream in a naphtha-cracking unit. The C4 raffinate-3, therefore, is mainly composed of 2-butene (trans-2-butene and cis-2butene), n-butane, and 1-butene. It is expected that the commercial value of C4 raffinate-3 can be much enhanced if 1,3-butadine is directly produced using C4 raffinate-3 as a nbutene source. Therefore, developing an efficient catalyst for the oxidative dehydrogenation of C4 raffinate-3 to 1,3butadiene would be worthwhile. Among various catalysts, three types of bismuth molybdates (a-Bi2Mo3O12, b-Bi2Mo2O9, and g-Bi2MoO6) have been widely studied as efficient catalysts for the oxidative dehydrogenation of n-butene [2,15–17]. In previous studies [3,18–20], however, it was reported that b-Bi2Mo2O9 was thermally unstable and decomposed into a-Bi2Mo3O12 and gBi2MoO6 in the temperature range of 400–550 8C. This indicates that b-Bi2Mo2O9 can not serve as an efficient catalyst
J.C. Jung et al. / Applied Catalysis A: General 317 (2007) 244–249
in the oxidative dehydrogenation of n-butene at reaction temperature of 400–550 8C. Therefore, only a-Bi2Mo3O12 and g-Bi2MoO6 catalysts were considered for use in the oxidative dehydrogenation of n-butene to 1,3-butadiene at 400–550 8C. In catalytic dehydrogenation reactions, steam is generally used as a diluent, oxidant, coke remover, and heat sink [21,22]. However, a large amount of steam is not only thermodynamically unfavorable in the oxidative dehydrogenation reaction, but also undesirable in the viewpoint of energy management. Therefore, it is necessary to optimize the amount of steam used in the oxidative dehydrogenation reaction. Nonetheless, not much progress has been made on studying the effect of steam on the catalytic performance of bismuth molybdate catalysts in the oxidative dehydrogenation of n-butene. In this work, a-Bi2Mo3O12 and g-Bi2MoO6 were prepared by a co-precipitation method, and were applied to the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene in a continuous flow fixed-bed reactor. Catalyst stability was confirmed by conducting the oxidative dehydrogenation reaction with time on stream and by characterizing the catalyst before and after the reaction. The role and effect of steam on the catalytic performance of g-Bi2MoO6 catalyst were examined with an aim of maximizing 1,3-butadine production and minimizing CO2 formation. The effect of reaction temperature on the catalytic performance of g-Bi2MoO6 catalyst was also investigated. 2. Experimental 2.1. Preparation of bismuth molybdate catalysts a-Bi2Mo3O12 and g-Bi2MoO6 catalysts were prepared by a co-precipitation method. A known amount of bismuth nitrate (Bi(NO3)35H2O from Sigma–Aldrich) was dissolved in distilled water that had been acidified with concentrated nitric acid. The solution was then added dropwise into an aqueous solution containing a known amount of ammonium molybdate ((NH4)6Mo7O244H2O from Sigma–Aldrich) under vigorous stirring. During the co-precipitation step, the pH of the mixed solution was precisely controlled using known amounts of ammonia solution. The pH values were kept at 1.5 and 3.0 in the preparation of a-Bi2Mo3O12 and g-Bi2MoO6, respectively [18,23,24]. After the resulting solution was stirred vigorously at room temperature for 1 h, the precipitate was filtered to obtain a solid product. The solid product was dried overnight at 110 8C, and it was then calcined at 475 8C for 5 h in an air stream to yield the bismuth molybdate catalyst.
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stability of a-Bi2Mo3O12 and g-Bi2MoO6 was examined by conducting XRD (MAC Science, M18XHF-SRA) analyses before and after the catalytic reactions, and before and after the TPRO (temperature-programmed reoxidation) measurements. 2.3. Oxidative dehydrogenation of C4 raffinate-3 The oxidative dehydrogenation of C4 raffinate-3 to 1,3butadiene was carried out in a continuous flow fixed-bed reactor. C4 raffinate-3 containing 72.5 wt.% n-butene (1butene(14.2 wt.%) + trans-2-butene(38.3 wt.%) + cis-2-butene(20.0 wt.%)) was used as a n-butene source, and air was used as an oxygen source (nitrogen in air served as a carrier gas). C4 raffinate-3 was composed of 72.5 wt.% n-butene, 26.9 wt.% nbutane, 0.4 wt.% cyclobutane, 0.1 wt.% methyl cyclopropane, and 0.1 wt.% residue. Water was sufficiently vaporized by passing through a pre-heating zone and fed into the reactor continuously together with the C4 raffinate-3 and air. The feed composition was fixed at n-butene:oxygen:steam = 1:0.75:15 in the normal catalytic reactions. Prior to the catalytic reaction, the catalyst was pretreated with air at 470 8C for 1 h. The catalytic performance test was conducted at 420 8C in normal cases. The GHSV (gas hourly space velocity) was fixed at 300 h1 on the basis of n-butene. In order to examine the effect of steam on the catalytic performance of g-Bi2MoO6, we carried out catalytic performance tests at 420 8C with a variation of n-butene/steam ratio (n-butene:oxygen:steam = 1:0.75:0–30). The effect of reaction temperature on the catalytic performance of g-Bi2MoO6 was investigated by carrying out the catalytic reaction at temperatures ranging from 380 to 460 8C (n-butene:oxygen:steam = 1:0.75:15). Reaction products were periodically sampled and analyzed with a gas chromatograph. Conversion of n-butene and selectivity for 1,3-butadiene were calculated on the basis of carbon balance as follows. The yield for 1,3butadiene was calculated by multiplying conversion and selectivity. Conversion of n-butene ¼
moles of n-butene reacted moles of n-butene supplied
Selectivity for 1; 3-butadiene ¼
moles of 1; 3-butadiene formed moles of n-butene reacted
3. Results and discussion 3.1. Formation of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts
2.2. Characterization The formation of pure a-Bi2Mo3O12 and g-Bi2MoO6 catalysts was confirmed by XRD (MAC Science, M18XHFSRA) and Raman spectroscopy (Horiaba Jobin Yvon, T64000) measurements. The Bi/Mo atomic ratios of the prepared catalysts were determined by ICP-AES (Shimadz, ICP1000IV) analyses. Surface areas of the catalysts were measured using an ASAP 2010 instrument (Micromeritics). The catalyst
The formation of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts was verified by XRD, Raman spectroscopy, and ICP-AES measurements. Fig. 1 shows the XRD patterns and Raman spectra of prepared a-Bi2Mo3O12 and g-Bi2MoO6 catalysts. These XRD patterns and Raman spectra are in good agreement with those reported in previous works [16,19,24–26], indicating the successful formation of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts. Bi/Mo atomic ratios of a-Bi2Mo3O12 and g-Bi2MoO6
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Fig. 1. (a) XRD patterns and (b) Raman spectra of prepared a-Bi2Mo3O12 and g-Bi2MoO6 catalysts.
were determined to be 0.6 and 1.96, respectively. These values are in good agreement with the theoretical values (Bi/Mo = 2/3 for a-Bi2Mo3O12 and Bi/Mo = 2 for g-Bi2MoO6). This result also supports the conclusion that bismuth molybdate catalysts were successfully prepared in this work. a-Bi2Mo3O12 and gBi2MoO6 catalysts showed very low BET surface areas of 1.9 and 3.5 m2/g, respectively, as reported in previous works [2,26,27]. 3.2. Catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 Fig. 2 shows the catalytic performance of a-Bi2Mo3O12 and g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 performed at 420 8C for 48 h (n-butene:oxygen:steam = 1:0.75:15). As shown in Fig. 2, g-Bi2MoO6 exhibited a better catalytic performance than a-Bi2Mo3O12 in terms of the conversion of n-butene, selectivity for 1,3-butadine, and yield for 1,3-butadine. Yields for 1,3-butadine over a-Bi2Mo3O12 and g-Bi2MoO6 were ca. 10 and 46%, respectively. The enhanced catalytic performance of g-Bi2MoO6 can be attributed to the facile oxygen mobility of g-Bi2MoO6, as reported in previous works [18,26,28–31]. This result was strongly supported by the TPRO (temperature-programmed reoxidation) measurements for a-Bi2Mo3O12 and g-Bi2MoO6
catalysts. For the TPRO experiment, each catalyst was partially reduced by carrying out the oxidative dehydrogenation of C4 raffinate-3 in the absence of the oxygen feed at 420 8C for 3 h in order for the catalyst to consume lattice oxygen for the reaction. After the reduced catalyst was placed in a conventional TPRO apparatus, a mixed stream of oxygen (10%) and helium (90%) was introduced to the catalyst sample and the furnace temperature was increased from room temperature to 500 8C (heating rate = 5 8C/min). The amount of oxygen consumed was detected using a thermal conductivity detector (TCD). The amount of oxygen consumption measured by TPRO experiment is equivalent to the amount of oxygen vacancies in the partially reduced catalyst. Experimental results showed that the TPRO peak in the g-Bi2MoO6 appeared at 285 8C while that in the aBi2Mo3O12 appeared at 390 8C. Furthermore, the TPRO peak area for the g-Bi2MoO6 was larger than that for the aBi2Mo3O12. These results strongly suggest that the g-Bi2MoO6 catalyst retains a higher oxygen mobility than a-Bi2Mo3O12 catalyst. The reactivity values of n-butene isomers (1-butene, trans-2butene, and cis-2-butene) in the C4 raffinate-3 in the oxidative dehydrogenation reaction over a-Bi2Mo3O12 and g-Bi2MoO6 catalysts are summarized in Table 1. The reactivity of n-butene isomers was calculated according to the following equation (it has a meaning of conversion). The reactivity of n-butene
Fig. 2. Catalytic performance of (a) a-Bi2Mo3O12 and (b) g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 performed at 420 8C for 48 h (nbutene:oxygen:steam = 1:0.75:15): (&) conversion of n-butene; (*) selectivity for 1,3-butadeiene; (~) yield for 1,3-butadiene.
J.C. Jung et al. / Applied Catalysis A: General 317 (2007) 244–249 Table 1 Reactivity of n-butene isomers in the C4 raffinate-3 over a-Bi2Mo3O12 and gBi2MoO6 catalysts at 420 8C (n-butene:oxygen:steam = 1:0.75:15) Catalyst
1-Butene
Trans-2-butene
Cis-2-butene
a-Bi2Mo3O12 g-Bi2MoO6
17.0 58.5
14.5 50.3
12.8 44.1
isomers over both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts decreased in the order of 1-butene > trans-2-butene > cis-2butene, which is consistent with the trend reported in the literature [32]. This result indicates that 1-butene is the most favorable feed in the oxidative dehydrogenation of n-butene over bismuth molybdate catalysts. However, a preliminary experimental result revealed that the reactivity of n-butene isomers over Zn-ferrite catalyst decreased in the order of cis-2butene > trans-2-butene > 1-butene. This means that the reactivity of n-butene isomers strongly depends on the catalyst system. It is noteworthy that the oxidative dehydrogenation of n-butane to n-butene did not occur over the bismuth molybdate catalysts in our reaction system. This was well confirmed by analyzing the amount of n-butane in the reactor inlet and outlet streams. Furthermore, the isomerization between 1-butene and 2-butene was not observed within a detectable range. Reactivity of individual n-butene moles of individual n-butene reacted ¼ moles of individual n-butene supplied 3.3. Catalyst stability It is noteworthy that both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts exhibited a stable catalytic performance with time on stream without catalyst deactivation, as shown in Fig. 2. In order to ensure the catalyst stability, XRD measurements were conducted before and after the catalytic reaction. Fig. 3 shows the XRD patterns of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts obtained before and after the catalytic reaction (48-h reaction at 420 8C). It was observed that the a-Bi2Mo3O12 and g-Bi2MoO6 catalysts showed no difference in XRD patterns before and after the reaction, indicating that the a-Bi2Mo3O12 and g-Bi2MoO6
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catalysts are thermally stable during the catalytic reaction performed at 420 8C. Catalyst stability of a-Bi2Mo3O12 and g-Bi2MoO6 was further confirmed by conducting XRD measurements before and after the TPRO experiments. For this purpose, TPRO experiments were carried out as described in the previous section. It has been demonstrated that lattice oxygen in the bismuth molybdate catalyst plays a key role in the oxidative dehydrogenation of n-butene, because the reaction follows the Mars–van Krevelen mechanism [2,32]. Fig. 4 shows the XRD patterns of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts obtained before and after the TPRO measurement. No difference in XRD patterns before and after the TPRO measurement was observed for either a-Bi2Mo3O12 or g-Bi2MoO6 catalysts. This indicates that the reduced catalysts were completely reoxidized by molecular oxygen. The above result also indicates that both the a-Bi2Mo3O12 and the g-Bi2MoO6 catalysts are structurally stable during the catalytic reaction performed at 420 8C. It can be concluded that catalyst stability leads to a stable catalytic performance of a-Bi2Mo3O12 and g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 without catalyst deactivation (Fig. 2). 3.4. Effect of steam The role and effect of steam on the catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 were investigated. For this purpose, g-Bi2MoO6, which shows a better catalytic performance than a-Bi2Mo3O12, was chosen as a model catalyst. The catalytic reactions were carried out at 420 8C with a variation of n-butene/steam ratio (n-butene:oxygen:steam = 1:0.75:0–30). Fig. 5 shows the catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 at 420 8C as a function of steam/n-butene ratio. It is known that CO and CO2 are mainly produced as by-products in the oxidative dehydrogenation reaction [2–4]. In our catalyst system, however, the formation of CO was negligible and CO2 was the major by-product. As shown in Fig. 5, the conversion of n-butene and the yield for 1,3-butadiene were decreased with increasing steam/nbutene ratio. However, maximum selectivity for 1,3-butadiene and minimum selectivity for CO2 were observed at a steam/
Fig. 3. XRD patterns of (a) a-Bi2Mo3O12 and (b) g-Bi2MoO6 catalysts obtained before and after the catalytic reaction (48-h reaction at 420 8C).
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Fig. 4. XRD patterns of (a) a-Bi2Mo3O12 and (b) g-Bi2MoO6 catalysts obtained before and after the TPRO measurement.
Fig. 5. Catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 at 420 8C as a function of steam/n-butene ratio (n-butene:oxygen:steam = 1:0.75:0–30).
n-butene ratio of 15–20. Judging from the conversion of nbutene and yield for 1,3-butadiene, one may conclude that no steam is required for the maximum production of 1,3-buatdiene from C4 raffinate-3. However, one should consider the amount of CO2 (CO2 selectivity) formed in the reaction for the practical application of the oxidative dehydrogenation of C4 raffinate-3. Table 2 summarizes the heats of reaction for the formation of 1,3-butadiene, CO2, and CO from 1-butene, cis-2-butene, and trans-2-butene at 420 8C, which were calculated according to the methods in the literature [33,34]. The heat of reaction for the formation of CO2 and CO is ca. 20 and 10 times higher, respectively, than that for the formation of 1,3-butadine. This means that the formation of excess amounts of CO2 in our catalytic reaction system may cause hot spots or reactor runaway in a large-scale catalytic process. Therefore, minimizing CO2 formation is very important for the safe operation of oxidative dehydrogenation process, although maximizing 1,3butadiene production is also important. As shown in Fig. 5, steam plays an essential role in suppressing CO2 formation by blocking the active sites for CO2 formation, and furthermore, serves as a heat sink for preventing hot spots or reactor run-away. In addition to this, steam plays roles in lowering the partial pressure, decreasing contact time, and regenerating catalyst active sites in the oxidative dehydrogenation reaction.
3.5. Effect of reaction temperature The effect of reaction temperature on the catalytic performance of g-Bi2MoO6 catalyst was examined in the oxidative dehydrogenation of C4 raffinate-3. For this purpose, catalytic reactions were carried out at temperatures ranging from 380 to 460 8C (n-butene:oxygen:steam = 1:0.75:15). Fig. 6 shows the catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 as a function of reaction temperature. As shown in Fig. 6, the selectivity for 1,3butadiene over g-Bi2MoO6 was almost constant (ca. 90%). Furthermore, the selectivity for CO2 was less than 5% at reaction temperatures of 380–460 8C. However, the conversion of n-butene and the yield for 1,3-butadiene showed volcanoshaped curves with respect to reaction temperature. The Table 2 Heat of reaction for the formation of 1,3-butadiene, CO2, and CO from 1butene, cis-2-butene, and trans-2-butene at 420 8C (kJ mol1) Reactant
1-Butene Cis-2-butene Trans-2-butene
Product C4H6 + H2O
4CO2 + 4H2O
4CO + 4H2O
128.1 118.9 116.8
2552.6 2543.4 2541.3
1414.9 1406.1 1403.6
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Fig. 6. Catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 as a function of reaction temperature (n-butene:oxygen:steam = 1:0.75:15).
maximum conversion of n-butene (ca. 66%) and the maximum yield for 1,3-butadiene (ca. 60%) were observed at 440 8C. 4. Conclusions a-Bi2Mo3O12 and g-Bi2MoO6 were prepared by a coprecipitation method for use in the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene. It was found that both aBi2Mo3O12 and g-Bi2MoO6 catalysts were thermally and structurally stable during the catalytic reaction. They exhibited a stable catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 without catalyst deactivation. However, gBi2MoO6 showed a better catalytic activity than a-Bi2Mo3O12 due to the facile oxygen mobility of g-Bi2MoO6. The reactivity of n-butene isomers in the C4 raffinate-3 decreased in the order of 1-butene > trans-2-butene > cis-2-butene over both aBi2Mo3O12 and g-Bi2MoO6 catalysts. It was also revealed that steam played an important role in suppressing CO2 formation, and furthermore, served as a heat sink for preventing hot spots or reactor run-away. In addition to this, steam played roles in lowering the partial pressure, decreasing contact time, and regenerating catalyst active sites. The reaction temperature also strongly affected the catalytic performance. The maximum conversion of n-butene (ca. 66%) and the maximum yield for 1,3-butadiene (ca. 60%) were observed at 440 8C over the gBi2MoO6 catalyst (n-butene:oxygen:steam = 1:0.75:15). Acknowledgement The authors acknowledge the support from the Korea Energy Management Corporation (2005-01-0090-3-010). References [1] J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal. 63 (1980) 235–254. [2] A.P.V. Soares, L.D. Dimitrov, M.C.A. Oliveira, L. Hilaire, M.F. Portela, R.K. Grasselli, Appl. Catal. A 253 (2003) 191–200. [3] Ph.A. Batist, J.F.H. Bouwens, G.C.A. Schuit, J. Catal. 25 (1972) 1–11.
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Catalytic performance of bismuth molybdate catalysts in the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene Ji Chul Jung a, Heesoo Kim a, Yong Seung Kim b, Young-Min Chung b, Tae Jin Kim b, Seong Jun Lee b, Seung-Hoon Oh 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, South Korea b SK Corporation, Yuseong-ku, Daejeon 305-712, South Korea Received 20 July 2006; received in revised form 9 October 2006; accepted 11 October 2006 Available online 20 November 2006
Abstract a-Bi2Mo3O12 and g-Bi2MoO6 were prepared by a co-precipitation method, and were applied to the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene. Both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts were thermally and structurally stable during the catalytic reaction. They exhibited a stable catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 without catalyst deactivation. However, the catalytic performance of g-Bi2MoO6 was superior to a-Bi2Mo3O12 due to the facile oxygen mobility of g-Bi2MoO6. The reactivity of n-butene isomers in the C4 raffinate-3 decreased in the order of 1-butene > trans-2-butene > cis-2-butene over both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts. Steam played an essential role in suppressing CO2 formation, and furthermore, served as a heat sink for preventing hot spots or reactor run-away. In the catalytic reaction with respect to reaction temperature, the maximum conversion of n-butene (ca. 66%) and the maximum yield for 1,3-butadiene (ca. 60%) were achieved at 440 8C over the g-Bi2MoO6 catalyst (n-butene:oxygen:steam = 1:0.75:15). # 2006 Elsevier B.V. All rights reserved. Keywords: Bismuth molybdate; C4 raffinate-3; 1,3-Butadiene; Oxidative dehydrogenation; Catalyst stability; Effect of steam; Effect of reaction temperature
1. Introduction Bismuth molybdates have been widely investigated as efficient catalysts for the selective oxidation of olefins such as the oxidation of propylene to acrolein, the ammoxidation of propylene to acrylonitrile, and the oxidative dehydrogenation of n-butene to 1,3-butadiene [1–4]. In particular, the oxidative dehydrogenation of n-butene over bismuth molybdate catalysts has attracted much attention as a promising process for manufacturing 1,3-butadiene [3–7], an important raw material for manufacturing a large number of chemical products in the petrochemical industries [8,9]. The oxidative dehydrogenation of n-butene has many advantages because this process can be operated as a single unit and is independent of the naphtha cracking unit in producing 1,3-butadine in the sense that no additional major naphtha cracking products such as ethylene
* Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 889 7415. E-mail address: [email protected] (I.K. Song). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.10.021
and propylene are produced for the production of 1,3-butadiene [3–5,10–14]. C4 raffinate-3 is a residue obtained after separating 1,3butadiene, isobutene, and 1-butene from the C4 raffinate stream in a naphtha-cracking unit. The C4 raffinate-3, therefore, is mainly composed of 2-butene (trans-2-butene and cis-2butene), n-butane, and 1-butene. It is expected that the commercial value of C4 raffinate-3 can be much enhanced if 1,3-butadine is directly produced using C4 raffinate-3 as a nbutene source. Therefore, developing an efficient catalyst for the oxidative dehydrogenation of C4 raffinate-3 to 1,3butadiene would be worthwhile. Among various catalysts, three types of bismuth molybdates (a-Bi2Mo3O12, b-Bi2Mo2O9, and g-Bi2MoO6) have been widely studied as efficient catalysts for the oxidative dehydrogenation of n-butene [2,15–17]. In previous studies [3,18–20], however, it was reported that b-Bi2Mo2O9 was thermally unstable and decomposed into a-Bi2Mo3O12 and gBi2MoO6 in the temperature range of 400–550 8C. This indicates that b-Bi2Mo2O9 can not serve as an efficient catalyst
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in the oxidative dehydrogenation of n-butene at reaction temperature of 400–550 8C. Therefore, only a-Bi2Mo3O12 and g-Bi2MoO6 catalysts were considered for use in the oxidative dehydrogenation of n-butene to 1,3-butadiene at 400–550 8C. In catalytic dehydrogenation reactions, steam is generally used as a diluent, oxidant, coke remover, and heat sink [21,22]. However, a large amount of steam is not only thermodynamically unfavorable in the oxidative dehydrogenation reaction, but also undesirable in the viewpoint of energy management. Therefore, it is necessary to optimize the amount of steam used in the oxidative dehydrogenation reaction. Nonetheless, not much progress has been made on studying the effect of steam on the catalytic performance of bismuth molybdate catalysts in the oxidative dehydrogenation of n-butene. In this work, a-Bi2Mo3O12 and g-Bi2MoO6 were prepared by a co-precipitation method, and were applied to the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene in a continuous flow fixed-bed reactor. Catalyst stability was confirmed by conducting the oxidative dehydrogenation reaction with time on stream and by characterizing the catalyst before and after the reaction. The role and effect of steam on the catalytic performance of g-Bi2MoO6 catalyst were examined with an aim of maximizing 1,3-butadine production and minimizing CO2 formation. The effect of reaction temperature on the catalytic performance of g-Bi2MoO6 catalyst was also investigated. 2. Experimental 2.1. Preparation of bismuth molybdate catalysts a-Bi2Mo3O12 and g-Bi2MoO6 catalysts were prepared by a co-precipitation method. A known amount of bismuth nitrate (Bi(NO3)35H2O from Sigma–Aldrich) was dissolved in distilled water that had been acidified with concentrated nitric acid. The solution was then added dropwise into an aqueous solution containing a known amount of ammonium molybdate ((NH4)6Mo7O244H2O from Sigma–Aldrich) under vigorous stirring. During the co-precipitation step, the pH of the mixed solution was precisely controlled using known amounts of ammonia solution. The pH values were kept at 1.5 and 3.0 in the preparation of a-Bi2Mo3O12 and g-Bi2MoO6, respectively [18,23,24]. After the resulting solution was stirred vigorously at room temperature for 1 h, the precipitate was filtered to obtain a solid product. The solid product was dried overnight at 110 8C, and it was then calcined at 475 8C for 5 h in an air stream to yield the bismuth molybdate catalyst.
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stability of a-Bi2Mo3O12 and g-Bi2MoO6 was examined by conducting XRD (MAC Science, M18XHF-SRA) analyses before and after the catalytic reactions, and before and after the TPRO (temperature-programmed reoxidation) measurements. 2.3. Oxidative dehydrogenation of C4 raffinate-3 The oxidative dehydrogenation of C4 raffinate-3 to 1,3butadiene was carried out in a continuous flow fixed-bed reactor. C4 raffinate-3 containing 72.5 wt.% n-butene (1butene(14.2 wt.%) + trans-2-butene(38.3 wt.%) + cis-2-butene(20.0 wt.%)) was used as a n-butene source, and air was used as an oxygen source (nitrogen in air served as a carrier gas). C4 raffinate-3 was composed of 72.5 wt.% n-butene, 26.9 wt.% nbutane, 0.4 wt.% cyclobutane, 0.1 wt.% methyl cyclopropane, and 0.1 wt.% residue. Water was sufficiently vaporized by passing through a pre-heating zone and fed into the reactor continuously together with the C4 raffinate-3 and air. The feed composition was fixed at n-butene:oxygen:steam = 1:0.75:15 in the normal catalytic reactions. Prior to the catalytic reaction, the catalyst was pretreated with air at 470 8C for 1 h. The catalytic performance test was conducted at 420 8C in normal cases. The GHSV (gas hourly space velocity) was fixed at 300 h1 on the basis of n-butene. In order to examine the effect of steam on the catalytic performance of g-Bi2MoO6, we carried out catalytic performance tests at 420 8C with a variation of n-butene/steam ratio (n-butene:oxygen:steam = 1:0.75:0–30). The effect of reaction temperature on the catalytic performance of g-Bi2MoO6 was investigated by carrying out the catalytic reaction at temperatures ranging from 380 to 460 8C (n-butene:oxygen:steam = 1:0.75:15). Reaction products were periodically sampled and analyzed with a gas chromatograph. Conversion of n-butene and selectivity for 1,3-butadiene were calculated on the basis of carbon balance as follows. The yield for 1,3butadiene was calculated by multiplying conversion and selectivity. Conversion of n-butene ¼
moles of n-butene reacted moles of n-butene supplied
Selectivity for 1; 3-butadiene ¼
moles of 1; 3-butadiene formed moles of n-butene reacted
3. Results and discussion 3.1. Formation of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts
2.2. Characterization The formation of pure a-Bi2Mo3O12 and g-Bi2MoO6 catalysts was confirmed by XRD (MAC Science, M18XHFSRA) and Raman spectroscopy (Horiaba Jobin Yvon, T64000) measurements. The Bi/Mo atomic ratios of the prepared catalysts were determined by ICP-AES (Shimadz, ICP1000IV) analyses. Surface areas of the catalysts were measured using an ASAP 2010 instrument (Micromeritics). The catalyst
The formation of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts was verified by XRD, Raman spectroscopy, and ICP-AES measurements. Fig. 1 shows the XRD patterns and Raman spectra of prepared a-Bi2Mo3O12 and g-Bi2MoO6 catalysts. These XRD patterns and Raman spectra are in good agreement with those reported in previous works [16,19,24–26], indicating the successful formation of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts. Bi/Mo atomic ratios of a-Bi2Mo3O12 and g-Bi2MoO6
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Fig. 1. (a) XRD patterns and (b) Raman spectra of prepared a-Bi2Mo3O12 and g-Bi2MoO6 catalysts.
were determined to be 0.6 and 1.96, respectively. These values are in good agreement with the theoretical values (Bi/Mo = 2/3 for a-Bi2Mo3O12 and Bi/Mo = 2 for g-Bi2MoO6). This result also supports the conclusion that bismuth molybdate catalysts were successfully prepared in this work. a-Bi2Mo3O12 and gBi2MoO6 catalysts showed very low BET surface areas of 1.9 and 3.5 m2/g, respectively, as reported in previous works [2,26,27]. 3.2. Catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 Fig. 2 shows the catalytic performance of a-Bi2Mo3O12 and g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 performed at 420 8C for 48 h (n-butene:oxygen:steam = 1:0.75:15). As shown in Fig. 2, g-Bi2MoO6 exhibited a better catalytic performance than a-Bi2Mo3O12 in terms of the conversion of n-butene, selectivity for 1,3-butadine, and yield for 1,3-butadine. Yields for 1,3-butadine over a-Bi2Mo3O12 and g-Bi2MoO6 were ca. 10 and 46%, respectively. The enhanced catalytic performance of g-Bi2MoO6 can be attributed to the facile oxygen mobility of g-Bi2MoO6, as reported in previous works [18,26,28–31]. This result was strongly supported by the TPRO (temperature-programmed reoxidation) measurements for a-Bi2Mo3O12 and g-Bi2MoO6
catalysts. For the TPRO experiment, each catalyst was partially reduced by carrying out the oxidative dehydrogenation of C4 raffinate-3 in the absence of the oxygen feed at 420 8C for 3 h in order for the catalyst to consume lattice oxygen for the reaction. After the reduced catalyst was placed in a conventional TPRO apparatus, a mixed stream of oxygen (10%) and helium (90%) was introduced to the catalyst sample and the furnace temperature was increased from room temperature to 500 8C (heating rate = 5 8C/min). The amount of oxygen consumed was detected using a thermal conductivity detector (TCD). The amount of oxygen consumption measured by TPRO experiment is equivalent to the amount of oxygen vacancies in the partially reduced catalyst. Experimental results showed that the TPRO peak in the g-Bi2MoO6 appeared at 285 8C while that in the aBi2Mo3O12 appeared at 390 8C. Furthermore, the TPRO peak area for the g-Bi2MoO6 was larger than that for the aBi2Mo3O12. These results strongly suggest that the g-Bi2MoO6 catalyst retains a higher oxygen mobility than a-Bi2Mo3O12 catalyst. The reactivity values of n-butene isomers (1-butene, trans-2butene, and cis-2-butene) in the C4 raffinate-3 in the oxidative dehydrogenation reaction over a-Bi2Mo3O12 and g-Bi2MoO6 catalysts are summarized in Table 1. The reactivity of n-butene isomers was calculated according to the following equation (it has a meaning of conversion). The reactivity of n-butene
Fig. 2. Catalytic performance of (a) a-Bi2Mo3O12 and (b) g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 performed at 420 8C for 48 h (nbutene:oxygen:steam = 1:0.75:15): (&) conversion of n-butene; (*) selectivity for 1,3-butadeiene; (~) yield for 1,3-butadiene.
J.C. Jung et al. / Applied Catalysis A: General 317 (2007) 244–249 Table 1 Reactivity of n-butene isomers in the C4 raffinate-3 over a-Bi2Mo3O12 and gBi2MoO6 catalysts at 420 8C (n-butene:oxygen:steam = 1:0.75:15) Catalyst
1-Butene
Trans-2-butene
Cis-2-butene
a-Bi2Mo3O12 g-Bi2MoO6
17.0 58.5
14.5 50.3
12.8 44.1
isomers over both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts decreased in the order of 1-butene > trans-2-butene > cis-2butene, which is consistent with the trend reported in the literature [32]. This result indicates that 1-butene is the most favorable feed in the oxidative dehydrogenation of n-butene over bismuth molybdate catalysts. However, a preliminary experimental result revealed that the reactivity of n-butene isomers over Zn-ferrite catalyst decreased in the order of cis-2butene > trans-2-butene > 1-butene. This means that the reactivity of n-butene isomers strongly depends on the catalyst system. It is noteworthy that the oxidative dehydrogenation of n-butane to n-butene did not occur over the bismuth molybdate catalysts in our reaction system. This was well confirmed by analyzing the amount of n-butane in the reactor inlet and outlet streams. Furthermore, the isomerization between 1-butene and 2-butene was not observed within a detectable range. Reactivity of individual n-butene moles of individual n-butene reacted ¼ moles of individual n-butene supplied 3.3. Catalyst stability It is noteworthy that both a-Bi2Mo3O12 and g-Bi2MoO6 catalysts exhibited a stable catalytic performance with time on stream without catalyst deactivation, as shown in Fig. 2. In order to ensure the catalyst stability, XRD measurements were conducted before and after the catalytic reaction. Fig. 3 shows the XRD patterns of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts obtained before and after the catalytic reaction (48-h reaction at 420 8C). It was observed that the a-Bi2Mo3O12 and g-Bi2MoO6 catalysts showed no difference in XRD patterns before and after the reaction, indicating that the a-Bi2Mo3O12 and g-Bi2MoO6
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catalysts are thermally stable during the catalytic reaction performed at 420 8C. Catalyst stability of a-Bi2Mo3O12 and g-Bi2MoO6 was further confirmed by conducting XRD measurements before and after the TPRO experiments. For this purpose, TPRO experiments were carried out as described in the previous section. It has been demonstrated that lattice oxygen in the bismuth molybdate catalyst plays a key role in the oxidative dehydrogenation of n-butene, because the reaction follows the Mars–van Krevelen mechanism [2,32]. Fig. 4 shows the XRD patterns of a-Bi2Mo3O12 and g-Bi2MoO6 catalysts obtained before and after the TPRO measurement. No difference in XRD patterns before and after the TPRO measurement was observed for either a-Bi2Mo3O12 or g-Bi2MoO6 catalysts. This indicates that the reduced catalysts were completely reoxidized by molecular oxygen. The above result also indicates that both the a-Bi2Mo3O12 and the g-Bi2MoO6 catalysts are structurally stable during the catalytic reaction performed at 420 8C. It can be concluded that catalyst stability leads to a stable catalytic performance of a-Bi2Mo3O12 and g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 without catalyst deactivation (Fig. 2). 3.4. Effect of steam The role and effect of steam on the catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 were investigated. For this purpose, g-Bi2MoO6, which shows a better catalytic performance than a-Bi2Mo3O12, was chosen as a model catalyst. The catalytic reactions were carried out at 420 8C with a variation of n-butene/steam ratio (n-butene:oxygen:steam = 1:0.75:0–30). Fig. 5 shows the catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 at 420 8C as a function of steam/n-butene ratio. It is known that CO and CO2 are mainly produced as by-products in the oxidative dehydrogenation reaction [2–4]. In our catalyst system, however, the formation of CO was negligible and CO2 was the major by-product. As shown in Fig. 5, the conversion of n-butene and the yield for 1,3-butadiene were decreased with increasing steam/nbutene ratio. However, maximum selectivity for 1,3-butadiene and minimum selectivity for CO2 were observed at a steam/
Fig. 3. XRD patterns of (a) a-Bi2Mo3O12 and (b) g-Bi2MoO6 catalysts obtained before and after the catalytic reaction (48-h reaction at 420 8C).
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Fig. 4. XRD patterns of (a) a-Bi2Mo3O12 and (b) g-Bi2MoO6 catalysts obtained before and after the TPRO measurement.
Fig. 5. Catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 at 420 8C as a function of steam/n-butene ratio (n-butene:oxygen:steam = 1:0.75:0–30).
n-butene ratio of 15–20. Judging from the conversion of nbutene and yield for 1,3-butadiene, one may conclude that no steam is required for the maximum production of 1,3-buatdiene from C4 raffinate-3. However, one should consider the amount of CO2 (CO2 selectivity) formed in the reaction for the practical application of the oxidative dehydrogenation of C4 raffinate-3. Table 2 summarizes the heats of reaction for the formation of 1,3-butadiene, CO2, and CO from 1-butene, cis-2-butene, and trans-2-butene at 420 8C, which were calculated according to the methods in the literature [33,34]. The heat of reaction for the formation of CO2 and CO is ca. 20 and 10 times higher, respectively, than that for the formation of 1,3-butadine. This means that the formation of excess amounts of CO2 in our catalytic reaction system may cause hot spots or reactor runaway in a large-scale catalytic process. Therefore, minimizing CO2 formation is very important for the safe operation of oxidative dehydrogenation process, although maximizing 1,3butadiene production is also important. As shown in Fig. 5, steam plays an essential role in suppressing CO2 formation by blocking the active sites for CO2 formation, and furthermore, serves as a heat sink for preventing hot spots or reactor run-away. In addition to this, steam plays roles in lowering the partial pressure, decreasing contact time, and regenerating catalyst active sites in the oxidative dehydrogenation reaction.
3.5. Effect of reaction temperature The effect of reaction temperature on the catalytic performance of g-Bi2MoO6 catalyst was examined in the oxidative dehydrogenation of C4 raffinate-3. For this purpose, catalytic reactions were carried out at temperatures ranging from 380 to 460 8C (n-butene:oxygen:steam = 1:0.75:15). Fig. 6 shows the catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 as a function of reaction temperature. As shown in Fig. 6, the selectivity for 1,3butadiene over g-Bi2MoO6 was almost constant (ca. 90%). Furthermore, the selectivity for CO2 was less than 5% at reaction temperatures of 380–460 8C. However, the conversion of n-butene and the yield for 1,3-butadiene showed volcanoshaped curves with respect to reaction temperature. The Table 2 Heat of reaction for the formation of 1,3-butadiene, CO2, and CO from 1butene, cis-2-butene, and trans-2-butene at 420 8C (kJ mol1) Reactant
1-Butene Cis-2-butene Trans-2-butene
Product C4H6 + H2O
4CO2 + 4H2O
4CO + 4H2O
128.1 118.9 116.8
2552.6 2543.4 2541.3
1414.9 1406.1 1403.6
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Fig. 6. Catalytic performance of g-Bi2MoO6 in the oxidative dehydrogenation of C4 raffinate-3 as a function of reaction temperature (n-butene:oxygen:steam = 1:0.75:15).
maximum conversion of n-butene (ca. 66%) and the maximum yield for 1,3-butadiene (ca. 60%) were observed at 440 8C. 4. Conclusions a-Bi2Mo3O12 and g-Bi2MoO6 were prepared by a coprecipitation method for use in the oxidative dehydrogenation of C4 raffinate-3 to 1,3-butadiene. It was found that both aBi2Mo3O12 and g-Bi2MoO6 catalysts were thermally and structurally stable during the catalytic reaction. They exhibited a stable catalytic performance in the oxidative dehydrogenation of C4 raffinate-3 without catalyst deactivation. However, gBi2MoO6 showed a better catalytic activity than a-Bi2Mo3O12 due to the facile oxygen mobility of g-Bi2MoO6. The reactivity of n-butene isomers in the C4 raffinate-3 decreased in the order of 1-butene > trans-2-butene > cis-2-butene over both aBi2Mo3O12 and g-Bi2MoO6 catalysts. It was also revealed that steam played an important role in suppressing CO2 formation, and furthermore, served as a heat sink for preventing hot spots or reactor run-away. In addition to this, steam played roles in lowering the partial pressure, decreasing contact time, and regenerating catalyst active sites. The reaction temperature also strongly affected the catalytic performance. The maximum conversion of n-butene (ca. 66%) and the maximum yield for 1,3-butadiene (ca. 60%) were observed at 440 8C over the gBi2MoO6 catalyst (n-butene:oxygen:steam = 1:0.75:15). Acknowledgement The authors acknowledge the support from the Korea Energy Management Corporation (2005-01-0090-3-010). References [1] J.D. Burrington, C.T. Kartisek, R.K. Grasselli, J. Catal. 63 (1980) 235–254. [2] A.P.V. Soares, L.D. Dimitrov, M.C.A. Oliveira, L. Hilaire, M.F. Portela, R.K. Grasselli, Appl. Catal. A 253 (2003) 191–200. [3] Ph.A. Batist, J.F.H. Bouwens, G.C.A. Schuit, J. Catal. 25 (1972) 1–11.
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