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Effect of pH in the preparation of γ-Bi2MoO6 for oxidative dehydrogenation of n-butene to 1,3-butadiene: Correlation between catalytic performance and oxygen mobility of γ-Bi2MoO6
Catalysis Communications 8 (2007) 625–628 www.elsevier.com/locate/catcom
Effect of pH in the preparation of c-Bi2MoO6 for oxidative dehydrogenation of n-butene to 1,3-butadiene: Correlation between catalytic performance and oxygen mobility of c-Bi2MoO6 Ji Chul Jung a, Heesoo Kim a, Ahn Seop Choi 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 23 May 2006; received in revised form 11 August 2006; accepted 11 August 2006 Available online 22 August 2006
Abstract c-Bi2MoO6 catalysts were prepared by a co-precipitation method with a variation of pH value (pH 1–7), and they were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene in a continuous flow fixed-bed reactor. Temperature-programmed reoxidation (TPRO) measurements were carried out to determine the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 consumed in the reaction. It was observed that the catalytic performance of c-Bi2MoO6 was closely related to the oxygen mobility of c-Bi2MoO6. The yield for 1,3-butadiene was monotonically increased with decreasing TPRO peak temperature and with increasing TPRO peak area. Among the catalysts tested, the c-Bi2MoO6 catalyst prepared at pH 3 showed the best catalytic performance due to its facile oxygen mobility. Ó 2006 Elsevier B.V. All rights reserved. Keywords: c-Bi2MoO6; Effect of pH; n-Butene; 1,3-Butadiene; Oxidative dehydrogenation; Oxygen mobility; Temperature-programmed reoxidation (TPRO)
1. Introduction Selective oxidation of olefins has been considered to be an important subject, because this process can be used in petrochemical industries for the large-scale synthesis of various chemical intermediates [1–3]. In particular, oxidative dehydrogenation of n-butene has attracted much attention as a promising process that can produce 1,3-butadiene in a single unit [4]. A number of catalysts have been investigated for the oxidative dehydrogenation of n-butene, including ferrite-type catalyst [5], vanadium-containing catalyst [6], manganese oxide molecular sieve [7], Cu–Mo catalyst [8], and bismuth molybdate catalysts [9–12]. Among these catalysts, bismuth molybdates have been widely *
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 888 7295. E-mail address: [email protected] (I.K. Song).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.08.019
investigated as efficient catalysts for this reaction [9–15]. Typically, three types of bismuth molybdate catalysts (aBi2Mo3O12, b-Bi2Mo2O9, and c-Bi2MoO6) have been considered for the oxidative dehydrogenation of n-butene [9,13–15]. It was reported that the oxidative dehydrogenation of nbutene to 1,3-butadiene follows Mars–van Krevelen mechanism [16]. That is, lattice oxygen in the bismuth molybdate catalyst directly reacts with n-butene, and oxygen in the gas phase makes up oxygen vacancy in the catalyst [9,16]. This means that oxygen mobility of bismuth molybdate catalyst plays an important role on the catalytic performance in the oxidative dehydrogenation of n-butene [12,17–19]. In other words, a bismuth molybdate catalyst with facile oxygen mobility may show an excellent catalytic performance in this reaction [9,18–21]. This was well supported by previous works [12,17,18] reporting that c-Bi2MoO6 exhibited the
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best catalytic performance in the oxidative dehydrogenation of n-butene due to the highest oxygen mobility among three types of bismuth molybdate catalysts. Co-precipitation method has been generally used in the preparation of c-Bi2MoO6 catalyst [9,12,22]. It was reported that pH value should be maintained constant during the co-precipitation step in the preparation of cBi2MoO6 catalyst, although the reported pH values were somewhat different case by case [9,23,24]. This implies that precise control of pH value during the co-precipitation step is very important for the successful preparation of cBi2MoO6 catalyst. However, any systematic investigations to see the effect of pH value in the preparation of cBi2MoO6 on the catalytic performance and oxygen mobility have not been attempted yet. In this work, a series of c-Bi2MoO6 catalysts were prepared by a co-precipitation method with a variation of pH value (pH 1–7), and they were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Temperature-programmed reoxidation (TPRO) measurements were carried out to determine the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 consumed in the reaction. A correlation between catalytic performance and oxygen mobility of c-Bi2MoO6 catalysts was established. 2. Experimental A series of c-Bi2MoO6 were prepared by a co-precipitation method with a variation of pH value during the co-precipitation step. Known amount of bismuth nitrate (Bi(NO3)3 Æ 5H2O from Aldrich) was dissolved in distilled water acidified with nitric acid. The solution was then added dropwise into the aqueous solution containing known amount of ammonium molybdate ((NH4)6Mo7O24 Æ 4H2O from Sigma) under vigorous stirring. During the co-precipitation step, pH value of the mixed solution was precisely controlled using ammonia solution. The pH value was varied from 1 to 7 with an interval of 1 in order to prepare seven c-Bi2MoO6 catalysts. 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 °C, and then it was calcined at 475 °C for 5 h to yield c-Bi2MoO6 catalyst. The prepared c-Bi2MoO6 catalysts were characterized by XRD (MAC Science, M18XHF-SRA), Raman spectroscopy (Horiaba Jobin Yvon, T64000), BET (Micromeritics, ASAP 2010), and ICP-AES (Shimadz, ICP-1000IV) analyses. In order to measure the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 catalysts, temperature-programmed reoxidation (TPRO) experiments were carried out. Prior to the TPRO measurement, each catalyst was partially reduced by carrying out the oxidative dehydrogenation of n-butene at 420 °C for 3 h in the absence of oxygen feed. 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. Furnace temperature was then raised from room
temperature to 500 °C at a heating rate of 5 °C/min. The amount of oxygen consumed was detected using a thermal conductivity detector. Oxidative dehydrogenation of n-butene to 1,3-butadiene was carried out in a continuous flow fixed-bed reactor in the presence of air and steam. Water was sufficiently vaporized by passing a pre-heating zone and fed into the reactor continuously together with n-butene and air. Feed composition was fixed at n-butene:O2:steam = 1:0.75:15. C4 raffinate-3 containing 72.5 wt% n-butene (1-butene(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). Each catalyst was pretreated at 470 °C for 1 h with an air stream, and then catalytic reaction was carried out at 420 °C. GHSV (gas hourly space velocity) was fixed at 300 h1 on the basis of n-butene. Reaction products were periodically sampled and analyzed with gas chromatography. Conversion of n-butene and selectivity for 1,3-butadiene were calculated on the basis of carbon balance as followings. Yield for 1,3-butadiene 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 Formation of c-Bi2MoO6 catalysts was well confirmed by XRD, Raman spectroscopy, and ICP-AES measurements. Fig. 1 shows the XRD patterns and Raman spectra of c-Bi2MoO6 catalysts prepared at different pH. XRD patterns and Raman spectra were well consistent with those reported in previous works [9,12,25,26], indicating successful formation of c-Bi2MoO6 catalysts. Bi/Mo atomic ratios of c-Bi2MoO6 catalysts determined by ICP-AES measurements were in the range of 1.94–1.97, in good agreement with theoretical value of 2.0. The above results demonstrate that c-Bi2MoO6 catalysts were successfully prepared within the wide range of pH values. BET surface areas of cBi2MoO6 catalysts were found to be very low, and they were in the range of 2.1–4.5 m2/g with no great difference. Fig. 2 shows the catalytic performance of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene at 420 °C after 12-h catalytic reaction, plotted as a function of pH value. In this reaction, CO and CO2 were mainly produced as by-products. Selectivity for 1,3-butadiene over cBi2MoO6 catalysts was almost constant (ca. 90%). What is noticeable is that conversion of n-butene and yield for 1,3butadiene showed volcano curves with respect to pH value. As shown in Fig. 2, the c-Bi2MoO6 catalyst prepared at pH 3 showed the best catalytic performance in this reaction. In order to correlate catalytic performance with oxygen mobility to make up lattice oxygen of c-Bi2MoO6 catalysts,
J.C. Jung et al. / Catalysis Communications 8 (2007) 625–628
a
pH = 7
627
b
pH = 7
pH = 5 pH = 4 pH = 3 pH = 2
pH = 6
Absorbance (A. U.)
Intensity (A. U.)
pH = 6
pH = 5 pH = 4 pH = 3 pH = 2
pH = 1
20
25
30
35
40
45
50
pH = 1
200
300
400
500
600
700
800
900
1000 1100
Raman shift (cm-1)
2 Theta (degree)
Fig. 1. (a) XRD patterns and (b) Raman spectra of c-Bi2MoO6 catalysts prepared at different pH.
100 90 Conversion of n-butene
70
Selectivity for 1,3-butadiene
60
Yield for 1,3-butadiene
pH = 1
O2 consumption (A. U.)
50 40 30
320 C pH = 3
280 C pH = 7
20
344 C
10
0
100
200
0
300
400
500
Temperature ( C ) 2
3
4
5
6
7
pH Fig. 2. Catalytic performance of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene at 420 °C after 12 h-catalytic reaction, plotted as a function of pH value.
TPRO experiments were carried out. Prior to the TPRO experiments, oxidative dehydrogenation of n-butene was carried out at 420 °C for 3 h in the absence of oxygen feed in order for the catalyst to consume lattice oxygen. Fig. 3 shows the TPRO profiles of selected partially reduced cBi2MoO6 catalysts. The amount of oxygen consumption measured by TPRO experiment is equivalent to the amount of lattice oxygen vacancy in the partially reduced catalyst, that is, to the amount of lattice oxygen consumed during the catalytic reaction in the absence of oxygen feed. It is inferred that TPRO peak temperature and TPRO peak area reflect the oxygen mobility and the capability for oxygen make-up, respectively. Fig. 3 clearly shows that cBi2MoO6 catalysts exhibited different oxygen mobility and different capability for oxygen make-up depending on pH value employed during the co-precipitation step. Fig. 4 shows a comprehensive correlation between catalytic performance and oxygen mobility to make up lattice oxygen of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene. Relative TPRO peak area was calculated as the ratio of TPRO peak area of c-Bi2MoO6
Fig. 3. TPRO profiles of selected partially reduced c-Bi2MoO6 catalysts.
280
TPRO peak temperature ( C )
1
pH=4
1.2
pH=3 300
1.0
pH=5 pH=1 pH=2
320
0.8
pH=6
0.6
pH=7
340
0.4 360
Relative TPRO peak area
Percentage
80
0.2 10
15
20
25
30
35
40
45
50
Yield for 1,3-butadiene Fig. 4. A comprehensive correlation between catalytic performance and oxygen mobility to make up lattice oxygen of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene.
prepared at a given pH with respect to that of c-Bi2MoO6 prepared at pH 3. Catalytic performance data were taken from Fig. 2. As shown in Fig. 4, yield for 1,3-butadiene was well correlated with TRPO peak temperature and relative TPRO peak area. The yield for 1,3-butadiene was
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monotonically increased with decreasing TRPO peak temperature and with increasing relative TPRO peak area. It is clear that the catalytic performance of c-Bi2MoO6 was closely related to the oxygen mobility to make up lattice oxygen consumed in the reaction. The c-Bi2MoO6 catalyst prepared at pH 3, which showed the lowest TPRO peak temperature and the largest TPRO peak area, exhibited the best catalytic performance among the catalysts tested in this work. High catalytic performance of c-Bi2MoO6 prepared at pH 3 was attributed to its facile oxygen mobility to make up lattice oxygen consumed in the reaction. 4. Conclusions A series of c-Bi2MoO6 catalysts were prepared by a coprecipitation method with a variation of pH value (pH 1– 7), and they were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Successful formation of cBi2MoO6 catalysts was well confirmed by XRD, Raman spectroscopy, and ICP-AES analyses. Temperature-programmed reoxidation (TPRO) measurements were carried out to determine the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 consumed in the reaction. It was observed that the catalytic performance of c-Bi2MoO6 was well correlated with the oxygen mobility to make up lattice oxygen. Among the catalysts tested, the c-Bi2MoO6 catalyst prepared at pH 3 showed the best catalytic performance due to its facile oxygen mobility to make up lattice oxygen consumed in the reaction. Acknowledgement The authors acknowledge the support from Korea Energy Management Corporation (2005-01-0090-3-010).
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] R.K. Grasselli, Topics Catal. 21 (2002) 79. [5] J.A. Toledo-Antonio, N. Nava, M. Matı´nez, X. Bokhimi, Appl. Catal. A 234 (2002) 137. [6] J.M. Lo´pez Nieto, P. Concepcio´n, A. Dejoz, H. Kno¨zinger, F. Melo, M.I. Va´zquez, J. Catal. 189 (2000) 147. [7] V.V. Krishnan, S.L. Suib, J. Catal. 184 (1999) 305. [8] P.N. Tiwari, T.G. Alkhazov, K.U. Adzamov, A.K. Khanmamedova, J. Catal. 120 (1989) 278. [9] A.P.V. Soares, L.D. Dimitrov, M.C.R.A. de Oliveira, L. Hilaire, M.F. Portela, R.K. Grasselli, Appl. Catal. A 253 (2003) 191. [10] M.F. Portela, M.M. Oliveira, M.J. Pires, Polyhedron 5 (1986) 119. [11] D.A.G. van Oeffelen, J.H.C. van Hooff, G.C.A. Schuit, J. Catal. 95 (1985) 84. [12] Ph.A. Batist, J.F.H. Bouwens, G.C.A. Schuit, J. Catal. 25 (1972) 1. [13] M. Egashira, K. Matsuo, S. Kagawa, T. Seiyama, J. Catal. 58 (1979) 409. [14] Ph.A. Batist, B.C. Lippens, G.C.A. Schuit, J. Catal. 5 (1966) 55. [15] A.C.A.M. Bleijenberg, B.C. Lippens, G.C.A. Schuit, J. Catal. 4 (1965) 581. [16] R.K. Grasselli, in: G. Ertl, H. Kno¨zinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 5, Wiley, New York, 1997, p. 2302. [17] G.C.A. Schuit, J. Less Comm. Metals 36 (1974) 329. [18] Ph.A. Batist, A.H.W.M. Der Kinderen, Y. Leeuwnburgh, F.A.M.G. Metz, G.C.A. Schuit, J. Catal. 12 (1968) 45. [19] E. Ruckenstein, R. Krishnan, K.N. Rai, J. Catal. 45 (1976) 270. [20] M.F. Porteal, Topics Catal. 15 (2001) 241. [21] Y.M. Oka, W. Ueda, Adv. Catal. 40 (1994) 233. [22] P. Boutry, R. Montarnal, J. Wrzyszcz, J. Catal. 13 (1969) 75. [23] G.W. Keulks, J.L. Hall, C. Daniel, K. Suzuki, J. Catal. 34 (1974) 79. [24] B. Grzybowska, J. Haber, J. Komorek, J. Catal. 25 (1972) 25. [25] E.V. Hoefs, J.R. Monnier, G.W. Keulks, J. Catal. 57 (1979) 331. [26] T. Uda, T.T. Lin, G.W. Keulks, J. Catal. 62 (1980) 26.
Effect of pH in the preparation of c-Bi2MoO6 for oxidative dehydrogenation of n-butene to 1,3-butadiene: Correlation between catalytic performance and oxygen mobility of c-Bi2MoO6 Ji Chul Jung a, Heesoo Kim a, Ahn Seop Choi 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 23 May 2006; received in revised form 11 August 2006; accepted 11 August 2006 Available online 22 August 2006
Abstract c-Bi2MoO6 catalysts were prepared by a co-precipitation method with a variation of pH value (pH 1–7), and they were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene in a continuous flow fixed-bed reactor. Temperature-programmed reoxidation (TPRO) measurements were carried out to determine the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 consumed in the reaction. It was observed that the catalytic performance of c-Bi2MoO6 was closely related to the oxygen mobility of c-Bi2MoO6. The yield for 1,3-butadiene was monotonically increased with decreasing TPRO peak temperature and with increasing TPRO peak area. Among the catalysts tested, the c-Bi2MoO6 catalyst prepared at pH 3 showed the best catalytic performance due to its facile oxygen mobility. Ó 2006 Elsevier B.V. All rights reserved. Keywords: c-Bi2MoO6; Effect of pH; n-Butene; 1,3-Butadiene; Oxidative dehydrogenation; Oxygen mobility; Temperature-programmed reoxidation (TPRO)
1. Introduction Selective oxidation of olefins has been considered to be an important subject, because this process can be used in petrochemical industries for the large-scale synthesis of various chemical intermediates [1–3]. In particular, oxidative dehydrogenation of n-butene has attracted much attention as a promising process that can produce 1,3-butadiene in a single unit [4]. A number of catalysts have been investigated for the oxidative dehydrogenation of n-butene, including ferrite-type catalyst [5], vanadium-containing catalyst [6], manganese oxide molecular sieve [7], Cu–Mo catalyst [8], and bismuth molybdate catalysts [9–12]. Among these catalysts, bismuth molybdates have been widely *
Corresponding author. Tel.: +82 2 880 9227; fax: +82 2 888 7295. E-mail address: [email protected] (I.K. Song).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.08.019
investigated as efficient catalysts for this reaction [9–15]. Typically, three types of bismuth molybdate catalysts (aBi2Mo3O12, b-Bi2Mo2O9, and c-Bi2MoO6) have been considered for the oxidative dehydrogenation of n-butene [9,13–15]. It was reported that the oxidative dehydrogenation of nbutene to 1,3-butadiene follows Mars–van Krevelen mechanism [16]. That is, lattice oxygen in the bismuth molybdate catalyst directly reacts with n-butene, and oxygen in the gas phase makes up oxygen vacancy in the catalyst [9,16]. This means that oxygen mobility of bismuth molybdate catalyst plays an important role on the catalytic performance in the oxidative dehydrogenation of n-butene [12,17–19]. In other words, a bismuth molybdate catalyst with facile oxygen mobility may show an excellent catalytic performance in this reaction [9,18–21]. This was well supported by previous works [12,17,18] reporting that c-Bi2MoO6 exhibited the
626
J.C. Jung et al. / Catalysis Communications 8 (2007) 625–628
best catalytic performance in the oxidative dehydrogenation of n-butene due to the highest oxygen mobility among three types of bismuth molybdate catalysts. Co-precipitation method has been generally used in the preparation of c-Bi2MoO6 catalyst [9,12,22]. It was reported that pH value should be maintained constant during the co-precipitation step in the preparation of cBi2MoO6 catalyst, although the reported pH values were somewhat different case by case [9,23,24]. This implies that precise control of pH value during the co-precipitation step is very important for the successful preparation of cBi2MoO6 catalyst. However, any systematic investigations to see the effect of pH value in the preparation of cBi2MoO6 on the catalytic performance and oxygen mobility have not been attempted yet. In this work, a series of c-Bi2MoO6 catalysts were prepared by a co-precipitation method with a variation of pH value (pH 1–7), and they were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Temperature-programmed reoxidation (TPRO) measurements were carried out to determine the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 consumed in the reaction. A correlation between catalytic performance and oxygen mobility of c-Bi2MoO6 catalysts was established. 2. Experimental A series of c-Bi2MoO6 were prepared by a co-precipitation method with a variation of pH value during the co-precipitation step. Known amount of bismuth nitrate (Bi(NO3)3 Æ 5H2O from Aldrich) was dissolved in distilled water acidified with nitric acid. The solution was then added dropwise into the aqueous solution containing known amount of ammonium molybdate ((NH4)6Mo7O24 Æ 4H2O from Sigma) under vigorous stirring. During the co-precipitation step, pH value of the mixed solution was precisely controlled using ammonia solution. The pH value was varied from 1 to 7 with an interval of 1 in order to prepare seven c-Bi2MoO6 catalysts. 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 °C, and then it was calcined at 475 °C for 5 h to yield c-Bi2MoO6 catalyst. The prepared c-Bi2MoO6 catalysts were characterized by XRD (MAC Science, M18XHF-SRA), Raman spectroscopy (Horiaba Jobin Yvon, T64000), BET (Micromeritics, ASAP 2010), and ICP-AES (Shimadz, ICP-1000IV) analyses. In order to measure the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 catalysts, temperature-programmed reoxidation (TPRO) experiments were carried out. Prior to the TPRO measurement, each catalyst was partially reduced by carrying out the oxidative dehydrogenation of n-butene at 420 °C for 3 h in the absence of oxygen feed. 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. Furnace temperature was then raised from room
temperature to 500 °C at a heating rate of 5 °C/min. The amount of oxygen consumed was detected using a thermal conductivity detector. Oxidative dehydrogenation of n-butene to 1,3-butadiene was carried out in a continuous flow fixed-bed reactor in the presence of air and steam. Water was sufficiently vaporized by passing a pre-heating zone and fed into the reactor continuously together with n-butene and air. Feed composition was fixed at n-butene:O2:steam = 1:0.75:15. C4 raffinate-3 containing 72.5 wt% n-butene (1-butene(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). Each catalyst was pretreated at 470 °C for 1 h with an air stream, and then catalytic reaction was carried out at 420 °C. GHSV (gas hourly space velocity) was fixed at 300 h1 on the basis of n-butene. Reaction products were periodically sampled and analyzed with gas chromatography. Conversion of n-butene and selectivity for 1,3-butadiene were calculated on the basis of carbon balance as followings. Yield for 1,3-butadiene 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 Formation of c-Bi2MoO6 catalysts was well confirmed by XRD, Raman spectroscopy, and ICP-AES measurements. Fig. 1 shows the XRD patterns and Raman spectra of c-Bi2MoO6 catalysts prepared at different pH. XRD patterns and Raman spectra were well consistent with those reported in previous works [9,12,25,26], indicating successful formation of c-Bi2MoO6 catalysts. Bi/Mo atomic ratios of c-Bi2MoO6 catalysts determined by ICP-AES measurements were in the range of 1.94–1.97, in good agreement with theoretical value of 2.0. The above results demonstrate that c-Bi2MoO6 catalysts were successfully prepared within the wide range of pH values. BET surface areas of cBi2MoO6 catalysts were found to be very low, and they were in the range of 2.1–4.5 m2/g with no great difference. Fig. 2 shows the catalytic performance of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene at 420 °C after 12-h catalytic reaction, plotted as a function of pH value. In this reaction, CO and CO2 were mainly produced as by-products. Selectivity for 1,3-butadiene over cBi2MoO6 catalysts was almost constant (ca. 90%). What is noticeable is that conversion of n-butene and yield for 1,3butadiene showed volcano curves with respect to pH value. As shown in Fig. 2, the c-Bi2MoO6 catalyst prepared at pH 3 showed the best catalytic performance in this reaction. In order to correlate catalytic performance with oxygen mobility to make up lattice oxygen of c-Bi2MoO6 catalysts,
J.C. Jung et al. / Catalysis Communications 8 (2007) 625–628
a
pH = 7
627
b
pH = 7
pH = 5 pH = 4 pH = 3 pH = 2
pH = 6
Absorbance (A. U.)
Intensity (A. U.)
pH = 6
pH = 5 pH = 4 pH = 3 pH = 2
pH = 1
20
25
30
35
40
45
50
pH = 1
200
300
400
500
600
700
800
900
1000 1100
Raman shift (cm-1)
2 Theta (degree)
Fig. 1. (a) XRD patterns and (b) Raman spectra of c-Bi2MoO6 catalysts prepared at different pH.
100 90 Conversion of n-butene
70
Selectivity for 1,3-butadiene
60
Yield for 1,3-butadiene
pH = 1
O2 consumption (A. U.)
50 40 30
320 C pH = 3
280 C pH = 7
20
344 C
10
0
100
200
0
300
400
500
Temperature ( C ) 2
3
4
5
6
7
pH Fig. 2. Catalytic performance of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene at 420 °C after 12 h-catalytic reaction, plotted as a function of pH value.
TPRO experiments were carried out. Prior to the TPRO experiments, oxidative dehydrogenation of n-butene was carried out at 420 °C for 3 h in the absence of oxygen feed in order for the catalyst to consume lattice oxygen. Fig. 3 shows the TPRO profiles of selected partially reduced cBi2MoO6 catalysts. The amount of oxygen consumption measured by TPRO experiment is equivalent to the amount of lattice oxygen vacancy in the partially reduced catalyst, that is, to the amount of lattice oxygen consumed during the catalytic reaction in the absence of oxygen feed. It is inferred that TPRO peak temperature and TPRO peak area reflect the oxygen mobility and the capability for oxygen make-up, respectively. Fig. 3 clearly shows that cBi2MoO6 catalysts exhibited different oxygen mobility and different capability for oxygen make-up depending on pH value employed during the co-precipitation step. Fig. 4 shows a comprehensive correlation between catalytic performance and oxygen mobility to make up lattice oxygen of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene. Relative TPRO peak area was calculated as the ratio of TPRO peak area of c-Bi2MoO6
Fig. 3. TPRO profiles of selected partially reduced c-Bi2MoO6 catalysts.
280
TPRO peak temperature ( C )
1
pH=4
1.2
pH=3 300
1.0
pH=5 pH=1 pH=2
320
0.8
pH=6
0.6
pH=7
340
0.4 360
Relative TPRO peak area
Percentage
80
0.2 10
15
20
25
30
35
40
45
50
Yield for 1,3-butadiene Fig. 4. A comprehensive correlation between catalytic performance and oxygen mobility to make up lattice oxygen of c-Bi2MoO6 in the oxidative dehydrogenation of n-butene to 1,3-butadiene.
prepared at a given pH with respect to that of c-Bi2MoO6 prepared at pH 3. Catalytic performance data were taken from Fig. 2. As shown in Fig. 4, yield for 1,3-butadiene was well correlated with TRPO peak temperature and relative TPRO peak area. The yield for 1,3-butadiene was
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monotonically increased with decreasing TRPO peak temperature and with increasing relative TPRO peak area. It is clear that the catalytic performance of c-Bi2MoO6 was closely related to the oxygen mobility to make up lattice oxygen consumed in the reaction. The c-Bi2MoO6 catalyst prepared at pH 3, which showed the lowest TPRO peak temperature and the largest TPRO peak area, exhibited the best catalytic performance among the catalysts tested in this work. High catalytic performance of c-Bi2MoO6 prepared at pH 3 was attributed to its facile oxygen mobility to make up lattice oxygen consumed in the reaction. 4. Conclusions A series of c-Bi2MoO6 catalysts were prepared by a coprecipitation method with a variation of pH value (pH 1– 7), and they were applied to the oxidative dehydrogenation of n-butene to 1,3-butadiene. Successful formation of cBi2MoO6 catalysts was well confirmed by XRD, Raman spectroscopy, and ICP-AES analyses. Temperature-programmed reoxidation (TPRO) measurements were carried out to determine the oxygen mobility to make up lattice oxygen of c-Bi2MoO6 consumed in the reaction. It was observed that the catalytic performance of c-Bi2MoO6 was well correlated with the oxygen mobility to make up lattice oxygen. Among the catalysts tested, the c-Bi2MoO6 catalyst prepared at pH 3 showed the best catalytic performance due to its facile oxygen mobility to make up lattice oxygen consumed in the reaction. Acknowledgement The authors acknowledge the support from Korea Energy Management Corporation (2005-01-0090-3-010).
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