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MgAl2O4 catalysts
Catalysis Communications 56 (2014) 119–122
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effect of calcination temperature on isobutane dehydrogenation over Mo/MgAl2O4 catalysts Guowei Wang, Wenlong Wu, Xiaolin Zhu, Yanan Sun, Chunyi Li ⁎, Honghong Shan State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 28 June 2014 Accepted 12 July 2014 Available online 17 July 2014 Keywords: Calcination temperature Isobutane Isobutene Dehydrogenation Mo/MgAl2O4
a b s t r a c t The catalytic activity of a novel catalyst Mo/MgAl2O4 was studied in isobutane dehydrogenation, and the effect of calcination temperature has been comprehensively investigated. For catalysts with MoO3 loading below monolayer coverage (5Mo/MgAl2O4), isobutane conversion increases with calcination temperature, while the selectivity to isobutene decreases. An opposite trend is observed for catalysts with MoO3 loading above monolayer coverage (30Mo/MgAl2O4). In combination with characterization results, it can be concluded that calcination temperature affects catalyst structure and properties in many aspects including surface area, acidity and reducibility, as well as the interaction between Mo species and support, and thus influences the catalytic behaviors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Light alkenes are an important and versatile class of chemistry intermediate, and dehydrogenation of light alkanes are receiving an increasing attention in recent years. Compared with industrialized Pt and CrOx-based catalysts, Mo-based catalysts, relatively inexpensive and environment-friendly, which have been extensively studied in oxidative dehydrogenation (ODH) of propane [1–3] and butanes [4,5], may be a good alternative. However, in the presence of oxidants, deep oxidative reactions are inevitable and the generated alkenes are more susceptible to be overoxidized to COx [6,7]. Fortunately, our recent study demonstrated that Mo catalysts supported on MgAl2O4 are active and stable for isobutane dehydrogenation in the absence of oxidants [8]. Although the active species for isobutane dehydrogenation were speculated in our previous study, it is still not enough to prepare an ideal dehydrogenation catalyst. Calcination temperature is an important factor to influence the catalyst structure and properties, such as the dispersion of the active sites [9–11], the interaction between active component and support [12], as well as the acid–base properties [13,14]. Besides, as directly related to the dispersion and structure of the surface Mo species [15–17] and significantly affecting the acid–base properties and redox behaviors of the catalysts [18,19], Mo loading also imposes great impact on the catalytic performance. In this respect, 5 wt.% and 30 wt.%, representing Mo loadings below and above monolayer coverage
⁎ Corresponding author. Tel.: +86 532 86981862; fax: +86 532 86981718. E-mail address: [email protected] (C. Li).
http://dx.doi.org/10.1016/j.catcom.2014.07.018 1566-7367/© 2014 Elsevier B.V. All rights reserved.
respectively, were selected as typical loading amounts for the purpose of separate investigation of calcination temperature effect. In general, this paper is a follow-up research of our previous work [8], aiming at systematically investigating the influence of calcination temperature (550–800 °C) on catalyst structure, properties, Mo-support interactions and dehydrogenation performance of Mo/MgAl2O4 catalysts with different Mo loadings, which has not been reported before. 2. Experimental 2.1. Catalysts preparation MgAl2O4 was obtained by sol–gel method. MoO3/MgAl2O4 catalysts were prepared through wetness impregnation using the corresponding amounts of ammonium heptamolybdate solution. The impregnated samples were dried at 140 °C for 12 h, and then were respectively calcined at 550, 600, 700 and 800 °C for 2 h. The investigated MoO3 contents of the catalysts were 5 wt.% and 30 wt.%, and corresponding samples are denoted as xMo/MgAl2O4 further in the paper, where x represents the MoO3 contents by weight in the prepared catalysts. 2.2. Characterization methods Nitrogen adsorption–desorption measurements were performed on a Quadrasorb SI instrument and porosimetry analyzer at liquid nitrogen temperature. The specific surface areas and pore volumes of the samples were determined with the BET and BJH methods. X-ray diffraction (XRD) analyses of the samples were carried out on X'Pert PRO MPD diffractometer system using Cu Kα radiation at 40 kV and 40 mA, running
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Table 1 Properties and reaction results of xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C). Catalysts
SBETa (m2/g)
Vpb (cm3/g)
NH3 desorbedc (mmol/g cat)
Tpd (°C)
Xisobutanee (wt.%)
Rf (mmol h−1 g−1)
TOFg (h−1)
5Mo/MgAl2O4-550 5Mo/MgAl2O4-600 5Mo/MgAl2O4-700 5Mo/MgAl2O4-800 30Mo/MgAl2O4-550 30Mo/MgAl2O4-600 30Mo/MgAl2O4-700 30Mo/MgAl2O4-800
102 100 102 76 74 65 38 36
0.24 0.23 0.24 0.13 0.12 0.12 0.12 0.11
0.163 0.169 0.179 0.206 0.179 0.149 0.123 0.122
177 183 192 209 201 190 186 181
17.4 27.1 44.9 62.1 36.9 32.6 28.8 17.8
0.23 0.36 0.60 0.83 0.49 0.44 0.39 0.24
0.67 1.05 1.73 2.39 1.42 1.26 1.11 0.69
a b c d e f g
Specific surface area. Pore volume. Amount of NH3 desorbed within 110-510 °C calculated from the TPD profiles. Peak temperature of the NH3-TPD profiles. Isobutane conversion. Reaction rate of isobutane per mass of catalyst. Turn over frequency calculated as: the amount of isobutane transformed per hour divided by the amount of Mo on the catalyst.
from 5° to 75° with scanning speed at 10°/min. The acidic properties of the samples were determined by temperature programmed desorption (TPD) of NH3. The redox behaviors of the samples were obtained by temperature programmed reduction (TPR) of H2.
2.3. Catalytic activity measurements Catalytic tests were performed in a fixed bed micro-reactor at atmospheric pressure and 560 °C. For each test, 4 g of catalysts with 80– 180 μm were loaded into the reactor. During the test, a flow of isobutane diluted by nitrogen with a fixed molar ratio of 1:6 was fed into the microreactor, at a total flow rate of 14 mL/min. The feed and the reaction products were both analyzed with a Bruker 450 Gas Chromatograph.
3. Results and discussion 3.1. Structural characterization and acid properties The BET surface area and pore volume of Mo/MgAl2O4 catalysts calcined at different temperatures (Tc) are listed in Table 1. With Mo content increasing from 5 wt.% to 30 wt.%, an apparent loss of surface area is observed, probably resulting from the plugging effect of Mo species on MgAl2O4 support. As for 5Mo/MgAl2O4 catalysts, the specific surface area remains almost unchanged with Tc increasing from 550 to 700 °C, while a sharp decrease is observed when Tc reaches 800 °C. Moreover, the diffraction peaks of all 5Mo/MgAl2O4 catalysts investigated (Fig. 1a) are assigned to MgAl2O4, and the intensity of MgAl2O4 increases gradually with Tc, indicating the well dispersed nature of Mo species on MgAl2O4 surface.
In the case of 30Mo/MgAl2O4 catalysts, the specific surface area decreases with increasing Tc, suggesting that small pore walls collapse gradually, consistent with the increasing trend of pore size (not given). Compared with the well dispersed Mo species over 5Mo/MgAl2O4 catalysts, aggregated Mo phases on the support are observed for 30Mo/MgAl2O4 catalysts (see Fig. 1b). For 30Mo/MgAl2O4-550 sample, both MgMoO4 and Al2(MoO4)3 are detected. The intensity of MgMoO4 increases with Tc, whereas that of Al2(MoO4)3 decreases. As Tc is reaching 800 °C, the characteristic diffraction peaks corresponding to Al2(MoO4)3 completely disappear. All these facts suggest that calcination temperature has a great impact on the interaction between Mo species and the support, i.e., high calcination temperature facilitates the formation of MgMoO4 rather than Al2(MoO4)3. NH3-TPD was further used to determine the acid properties of the catalysts, and the results are integrated in Table 1. As for 5Mo/MgAl2O4 catalysts, both the peak temperature and the amount of NH3 desorbed increase with increasing Tc, indicating the increase of strength and amount of acid sites, respectively. While the acidity of 30Mo/MgAl2O4 catalysts decreases monotonously with increasing Tc, possibly related to variations of interaction between Mo species and the support.
3.2. Redox behaviors TPR profiles of Mo/MgAl2O4 catalysts calcined at different temperatures are displayed in Fig. 2. It has been reported that the reduction pattern of supported MoO 3 follows the stepwise path Mo6 + → Mo4 + → Mo0 [20,21], i.e. low temperature peak is ascribed to the reduction of Mo 6 +, while high temperature peak to that of Mo4 +. Since the reduced Mo4 + species are the active sites for dehydrogenation [8], only the low temperature peak is analyzed here. As
Fig. 1. XRD patterns of xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C): a. 5Mo/MgAl2O4; b. 30Mo/MgAl2O4.
G. Wang et al. / Catalysis Communications 56 (2014) 119–122
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Fig. 2. H2-TPR profiles of xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C): a. 5Mo/MgAl2O4; b. 30Mo/MgAl2O4.
for 5Mo/MgAl2 O 4 catalysts, the H 2 consumption area increases slightly with increasing T c and the peak temperature decreases simultaneously, demonstrating that 5Mo/MgAl2O4 catalysts calcined at higher temperatures are much easier to be reduced. An opposite variation trend is observed for 30Mo/MgAl2O4 catalysts, i.e., the H2 consumption area decreases and the peak temperature increases obviously with increasing Tc, which may be attributed to the different molybdates formed on the catalyst surface. 3.3. Catalytic activity for isobutane dehydrogenation The influence of Tc on dehydrogenation activity of Mo/MgAl2O4 catalysts was examined, and the experimental results obtained at 560 °C and 1.5 h (for the development of the active Mo4 + species) are shown in Fig. 3. With regard to 5Mo/MgAl2O4 catalysts, isobutane conversion increases linearly from 17.4 wt.% at 550 °C to 62.1 wt.% at 800 °C. Meanwhile, the selectivity to isobutene declines steadily, more obviously at higher Tc, corresponding to accelerated cracking reactions mainly to methane, propene and propane. Taking both isobutane conversion and the selectivity to isobutene into account, optimal Tc for 5Mo/MgAl2O4 catalysts is 800 °C, with the achievement of maximum yield of isobutene (34.8 wt.%). While for 30Mo/MgAl2O4 catalysts, isobutane conversion decreases linearly with Tc, and the selectivity to isobutene increases at the same time, suggesting the formation of cracking by-products are aggravated at low Tc. Finally, a maximum yield of isobutene (23.8 wt.%) is obtained at 700 °C. 3.4. Discussion As discussed above, obvious difference is observed for the influence of Tc on the catalytic performance of Mo/MgAl2O4 catalysts with different Mo loadings. With regard to 5Mo/MgAl2O4 catalysts with well dispersed
Mo species on the catalyst surface, high Tc facilitates the activation of isobutane. While for 30Mo/MgAl2O4 catalysts with aggregated Mo species, although increased Tc inhibits the generation of side products, the activity decreases significantly. According to Xie and Tang [22], a MoO3 molecule occupies 20 Å2. This fact and previous nitrogen adsorption–desorption measurements jointly indicate that samples with 5 wt.% Mo species possess sufficient surface area for monolayer coverage, even with Tc up to 800 °C. In contrast, insufficient surface area for monolayer coverage is observed for 30Mo/MgAl2O4 catalysts and aggregated molybdates appear on the catalyst surface. Therefore, 5 wt.% and 30 wt.% can be regarded as typical Mo loadings representing below and above monolayer coverage, respectively, as indicated by the XRD results. Abello et al. [15] reported that the presence of acid sites is beneficial for the abstraction of the first H from C\H bond of relatively inert alkane molecule, which is regarded as the rate-determining step in dehydrogenation. Therefore, as the increase of acid amount and strength with Tc for 5Mo/MgAl2O4 catalysts, both the reaction rate of isobutane and TOF increase steadily (Table 1), in accordance with the results obtained by other researchers [5,18]. Although easier reduced Mo species on the sample with higher Tc facilitates the conversion of isobutane, high Tc is not beneficial for the selectivity to isobutene. With increasing Tc, electron transfer probably occurs from the support to Mo species and leads to the electron-deficient state of the support, which increases the strength and amount of Lewis acid sites, as evidenced by data listed in Table 1. Consequently, the increase of acidity probably promotes isobutene adsorption, increases the likelihood of secondary reactions [23,24], and further results in the decrease of selectivity to isobutene (see Fig. 3b). For 30Mo/MgAl2O4 catalysts, in addition to the acid property, the interaction of Mo species with the support is very important. Al2(MoO4)3 is the main form of Mo species on the catalyst calcined at 550 °C, while
Fig. 3. Catalytic activity test results for xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C).
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increased isobutane conversion and decreased selectivity to isobutene. Nevertheless, for the catalyst with MoO3 loading above monolayer coverage (30Mo/MgAl2O4), increasing calcination temperature decreases the acidity and reducibility, and also exerts a remarkable effect on the interaction between the support and Mo species, which is reflected by different molybdates formed on the catalyst surface. Exactly such effects lead to decreased isobutane conversion and increased selectivity to isobutene with calcination temperature. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. U1362201) and the National 973 Program of China (No. 2012CB215006). Appendix A. Supplementary data
Fig. 4. Conversion of isobutane and selectivity to isobutene for MgMoO4 and Al2(MoO4)3.
as the increase of Tc, MgMoO4 becomes the dominant species. Different molybdates present on the catalyst resulting from changing interactions between Mo species and the support under various Tc may be the real reason for the variation of catalytic performance. To further confirm that, activity test for isobutane dehydrogenation was carried out over Al2(MoO4)3 and MgMoO4 at 560 °C (see Fig. 4), respectively. Although isobutane conversion is much lower over MgMoO4, the selectivity to isobutene reaches 82.3 wt.%, indicating that it promotes selective dehydrogenation of isobutane. Compared with MgMoO4, the conversion increases to 20.7 wt.% for Al2(MoO4)3, but the selectivity decreases significantly. Such results are consistent with the catalytic performance over 30Mo/MgAl2O4 catalysts calcined at different Tc, demonstrating that the interaction of Mo species with the support becomes an important factor influencing dehydrogenation performance. And another reason is the different reducibility of Mo species over 30Mo/MgAl2O4 catalysts with various Tc, i.e., Mo species are much easier to be reduced for the catalysts calcined at lower temperature, showing higher reactivity of the catalysts. 4. Conclusions For Mo/MgAl2O4 catalysts, calcination temperature significantly affects the microstructure, acidity property, redox behavior and the interaction between the support and Mo species, and further influences the catalytic performance of isobutane dehydrogenation. However, obvious difference is observed for its influence on the catalysts with different Mo loadings. For the catalysts with MoO3 loading below monolayer coverage (5Mo/MgAl2O4), higher calcination temperature leads to lower surface area and higher acidity and reducibility, further resulting in
Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2014.07.018. These data include MOL files and InChiKeys of the most important compounds described in this article. References [1] L. Yuan, S. Bhatt, G. Beaucage, V.V. Guliants, S. Mamedov, R.S. Soman, J. Phys. Chem. B 109 (2005) 23250–23254. [2] M. Abello, M. Gomez, L. Cadus, Ind. Eng. Chem. Res. 35 (1996) 2137–2143. [3] L.E. Cadus, M.C. Abello, M.F. Gomez, J.B. Rivarola, Ind. Eng. Chem. Res. 35 (1996) 14–18. [4] L.M. Madeira, J.M. Herrmann, F.G. Freire, M.F. Portela, F.J. Maldonado, Appl. Catal. A Gen. 158 (1997) 243–256. [5] Y. Zhang, I. Rodriguez-Ramos, A. Guerrero-Ruiz, Catal. Today 61 (2000) 377–382. [6] X. Lin, C.A. Hoel, W.M.H. Sachtler, K.R. Poeppelmeier, E. Weitz, J. Catal. 265 (2009) 54–62. [7] M. Cherian, M.S. Rao, A.M. Hirt, I.E. Wachs, G. Deo, J. Catal. 211 (2002) 482–495. [8] G. Wang, C. Li, H. Shan, W. Wu, Ind. Eng. Chem. Res. 52 (2013) 13297–13304. [9] D.Y. Yoon, J.H. Park, H.C. Kang, P.S. Kim, I.S. Nam, G.K. Yeo, J.K. Kil, M.S. Cha, Appl. Catal. B Environ. 101 (2011) 275–282. [10] G.Y. Wang, W.X. Zhang, H.L. Lian, D.Z. Jiang, T.H. Wu, Appl. Catal. A Gen. 239 (2003) 1–10. [11] H. Li, W.L. Dai, J.F. Deng, Appl. Catal. A Gen. 207 (2001) 151–157. [12] D. Duvenhage, N. Coville, Appl. Catal. A Gen. 233 (2002) 63–75. [13] L. Ma, J. Li, H. Arandiyan, W. Shi, C. Liu, L. Fu, Catal. Today 184 (2012) 145–152. [14] Z. Boukha, L. Fitian, M. López-Haro, M. Mora, J.R. Ruiz, C. Jiménez-Sanchidrián, G. Blanco, J.J. Calvino, G.A. Cifredo, S. Trasobares, J. Catal. 272 (2010) 121–130. [15] M. Abello, M. Gomez, O. Ferretti, Appl. Catal. A Gen. 207 (2001) 421–431. [16] M. Harlin, L. Backman, A. Krause, O. Jylhä, J. Catal. 183 (1999) 300–313. [17] K. Chen, S. Xie, A.T. Bell, E. Iglesia, J. Catal. 198 (2001) 232–242. [18] M. Abello, M. Gomez, M. Casella, O. Ferretti, M. Banares, J. Fierro, Appl. Catal. A Gen. 251 (2003) 435–447. [19] S. Rajagopal, J. Marzari, R. Miranda, J. Catal. 151 (1995) 192–203. [20] F. Arena, A. Parmaliana, J. Phys. Chem. 100 (1996) 19994–20005. [21] J. Medema, C. Van Stam, V. De Beer, A. Konings, D. Koningsberger, J. Catal. 53 (1978) 386–400. [22] Y.C. Xie, Y. Tang, Adv. Catal. 37 (1990) 1–43. [23] K. Chen, S. Xie, E. Iglesia, A.T. Bell, J. Catal. 189 (2000) 421–430. [24] L. Madeira, M. Portela, C. Mazzocchia, A. Kaddouri, R. Anouchinsky, Catal. Today 40 (1998) 229–243.
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Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Effect of calcination temperature on isobutane dehydrogenation over Mo/MgAl2O4 catalysts Guowei Wang, Wenlong Wu, Xiaolin Zhu, Yanan Sun, Chunyi Li ⁎, Honghong Shan State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580, China
a r t i c l e
i n f o
Article history: Received 23 May 2014 Received in revised form 28 June 2014 Accepted 12 July 2014 Available online 17 July 2014 Keywords: Calcination temperature Isobutane Isobutene Dehydrogenation Mo/MgAl2O4
a b s t r a c t The catalytic activity of a novel catalyst Mo/MgAl2O4 was studied in isobutane dehydrogenation, and the effect of calcination temperature has been comprehensively investigated. For catalysts with MoO3 loading below monolayer coverage (5Mo/MgAl2O4), isobutane conversion increases with calcination temperature, while the selectivity to isobutene decreases. An opposite trend is observed for catalysts with MoO3 loading above monolayer coverage (30Mo/MgAl2O4). In combination with characterization results, it can be concluded that calcination temperature affects catalyst structure and properties in many aspects including surface area, acidity and reducibility, as well as the interaction between Mo species and support, and thus influences the catalytic behaviors. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Light alkenes are an important and versatile class of chemistry intermediate, and dehydrogenation of light alkanes are receiving an increasing attention in recent years. Compared with industrialized Pt and CrOx-based catalysts, Mo-based catalysts, relatively inexpensive and environment-friendly, which have been extensively studied in oxidative dehydrogenation (ODH) of propane [1–3] and butanes [4,5], may be a good alternative. However, in the presence of oxidants, deep oxidative reactions are inevitable and the generated alkenes are more susceptible to be overoxidized to COx [6,7]. Fortunately, our recent study demonstrated that Mo catalysts supported on MgAl2O4 are active and stable for isobutane dehydrogenation in the absence of oxidants [8]. Although the active species for isobutane dehydrogenation were speculated in our previous study, it is still not enough to prepare an ideal dehydrogenation catalyst. Calcination temperature is an important factor to influence the catalyst structure and properties, such as the dispersion of the active sites [9–11], the interaction between active component and support [12], as well as the acid–base properties [13,14]. Besides, as directly related to the dispersion and structure of the surface Mo species [15–17] and significantly affecting the acid–base properties and redox behaviors of the catalysts [18,19], Mo loading also imposes great impact on the catalytic performance. In this respect, 5 wt.% and 30 wt.%, representing Mo loadings below and above monolayer coverage
⁎ Corresponding author. Tel.: +86 532 86981862; fax: +86 532 86981718. E-mail address: [email protected] (C. Li).
http://dx.doi.org/10.1016/j.catcom.2014.07.018 1566-7367/© 2014 Elsevier B.V. All rights reserved.
respectively, were selected as typical loading amounts for the purpose of separate investigation of calcination temperature effect. In general, this paper is a follow-up research of our previous work [8], aiming at systematically investigating the influence of calcination temperature (550–800 °C) on catalyst structure, properties, Mo-support interactions and dehydrogenation performance of Mo/MgAl2O4 catalysts with different Mo loadings, which has not been reported before. 2. Experimental 2.1. Catalysts preparation MgAl2O4 was obtained by sol–gel method. MoO3/MgAl2O4 catalysts were prepared through wetness impregnation using the corresponding amounts of ammonium heptamolybdate solution. The impregnated samples were dried at 140 °C for 12 h, and then were respectively calcined at 550, 600, 700 and 800 °C for 2 h. The investigated MoO3 contents of the catalysts were 5 wt.% and 30 wt.%, and corresponding samples are denoted as xMo/MgAl2O4 further in the paper, where x represents the MoO3 contents by weight in the prepared catalysts. 2.2. Characterization methods Nitrogen adsorption–desorption measurements were performed on a Quadrasorb SI instrument and porosimetry analyzer at liquid nitrogen temperature. The specific surface areas and pore volumes of the samples were determined with the BET and BJH methods. X-ray diffraction (XRD) analyses of the samples were carried out on X'Pert PRO MPD diffractometer system using Cu Kα radiation at 40 kV and 40 mA, running
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Table 1 Properties and reaction results of xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C). Catalysts
SBETa (m2/g)
Vpb (cm3/g)
NH3 desorbedc (mmol/g cat)
Tpd (°C)
Xisobutanee (wt.%)
Rf (mmol h−1 g−1)
TOFg (h−1)
5Mo/MgAl2O4-550 5Mo/MgAl2O4-600 5Mo/MgAl2O4-700 5Mo/MgAl2O4-800 30Mo/MgAl2O4-550 30Mo/MgAl2O4-600 30Mo/MgAl2O4-700 30Mo/MgAl2O4-800
102 100 102 76 74 65 38 36
0.24 0.23 0.24 0.13 0.12 0.12 0.12 0.11
0.163 0.169 0.179 0.206 0.179 0.149 0.123 0.122
177 183 192 209 201 190 186 181
17.4 27.1 44.9 62.1 36.9 32.6 28.8 17.8
0.23 0.36 0.60 0.83 0.49 0.44 0.39 0.24
0.67 1.05 1.73 2.39 1.42 1.26 1.11 0.69
a b c d e f g
Specific surface area. Pore volume. Amount of NH3 desorbed within 110-510 °C calculated from the TPD profiles. Peak temperature of the NH3-TPD profiles. Isobutane conversion. Reaction rate of isobutane per mass of catalyst. Turn over frequency calculated as: the amount of isobutane transformed per hour divided by the amount of Mo on the catalyst.
from 5° to 75° with scanning speed at 10°/min. The acidic properties of the samples were determined by temperature programmed desorption (TPD) of NH3. The redox behaviors of the samples were obtained by temperature programmed reduction (TPR) of H2.
2.3. Catalytic activity measurements Catalytic tests were performed in a fixed bed micro-reactor at atmospheric pressure and 560 °C. For each test, 4 g of catalysts with 80– 180 μm were loaded into the reactor. During the test, a flow of isobutane diluted by nitrogen with a fixed molar ratio of 1:6 was fed into the microreactor, at a total flow rate of 14 mL/min. The feed and the reaction products were both analyzed with a Bruker 450 Gas Chromatograph.
3. Results and discussion 3.1. Structural characterization and acid properties The BET surface area and pore volume of Mo/MgAl2O4 catalysts calcined at different temperatures (Tc) are listed in Table 1. With Mo content increasing from 5 wt.% to 30 wt.%, an apparent loss of surface area is observed, probably resulting from the plugging effect of Mo species on MgAl2O4 support. As for 5Mo/MgAl2O4 catalysts, the specific surface area remains almost unchanged with Tc increasing from 550 to 700 °C, while a sharp decrease is observed when Tc reaches 800 °C. Moreover, the diffraction peaks of all 5Mo/MgAl2O4 catalysts investigated (Fig. 1a) are assigned to MgAl2O4, and the intensity of MgAl2O4 increases gradually with Tc, indicating the well dispersed nature of Mo species on MgAl2O4 surface.
In the case of 30Mo/MgAl2O4 catalysts, the specific surface area decreases with increasing Tc, suggesting that small pore walls collapse gradually, consistent with the increasing trend of pore size (not given). Compared with the well dispersed Mo species over 5Mo/MgAl2O4 catalysts, aggregated Mo phases on the support are observed for 30Mo/MgAl2O4 catalysts (see Fig. 1b). For 30Mo/MgAl2O4-550 sample, both MgMoO4 and Al2(MoO4)3 are detected. The intensity of MgMoO4 increases with Tc, whereas that of Al2(MoO4)3 decreases. As Tc is reaching 800 °C, the characteristic diffraction peaks corresponding to Al2(MoO4)3 completely disappear. All these facts suggest that calcination temperature has a great impact on the interaction between Mo species and the support, i.e., high calcination temperature facilitates the formation of MgMoO4 rather than Al2(MoO4)3. NH3-TPD was further used to determine the acid properties of the catalysts, and the results are integrated in Table 1. As for 5Mo/MgAl2O4 catalysts, both the peak temperature and the amount of NH3 desorbed increase with increasing Tc, indicating the increase of strength and amount of acid sites, respectively. While the acidity of 30Mo/MgAl2O4 catalysts decreases monotonously with increasing Tc, possibly related to variations of interaction between Mo species and the support.
3.2. Redox behaviors TPR profiles of Mo/MgAl2O4 catalysts calcined at different temperatures are displayed in Fig. 2. It has been reported that the reduction pattern of supported MoO 3 follows the stepwise path Mo6 + → Mo4 + → Mo0 [20,21], i.e. low temperature peak is ascribed to the reduction of Mo 6 +, while high temperature peak to that of Mo4 +. Since the reduced Mo4 + species are the active sites for dehydrogenation [8], only the low temperature peak is analyzed here. As
Fig. 1. XRD patterns of xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C): a. 5Mo/MgAl2O4; b. 30Mo/MgAl2O4.
G. Wang et al. / Catalysis Communications 56 (2014) 119–122
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Fig. 2. H2-TPR profiles of xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C): a. 5Mo/MgAl2O4; b. 30Mo/MgAl2O4.
for 5Mo/MgAl2 O 4 catalysts, the H 2 consumption area increases slightly with increasing T c and the peak temperature decreases simultaneously, demonstrating that 5Mo/MgAl2O4 catalysts calcined at higher temperatures are much easier to be reduced. An opposite variation trend is observed for 30Mo/MgAl2O4 catalysts, i.e., the H2 consumption area decreases and the peak temperature increases obviously with increasing Tc, which may be attributed to the different molybdates formed on the catalyst surface. 3.3. Catalytic activity for isobutane dehydrogenation The influence of Tc on dehydrogenation activity of Mo/MgAl2O4 catalysts was examined, and the experimental results obtained at 560 °C and 1.5 h (for the development of the active Mo4 + species) are shown in Fig. 3. With regard to 5Mo/MgAl2O4 catalysts, isobutane conversion increases linearly from 17.4 wt.% at 550 °C to 62.1 wt.% at 800 °C. Meanwhile, the selectivity to isobutene declines steadily, more obviously at higher Tc, corresponding to accelerated cracking reactions mainly to methane, propene and propane. Taking both isobutane conversion and the selectivity to isobutene into account, optimal Tc for 5Mo/MgAl2O4 catalysts is 800 °C, with the achievement of maximum yield of isobutene (34.8 wt.%). While for 30Mo/MgAl2O4 catalysts, isobutane conversion decreases linearly with Tc, and the selectivity to isobutene increases at the same time, suggesting the formation of cracking by-products are aggravated at low Tc. Finally, a maximum yield of isobutene (23.8 wt.%) is obtained at 700 °C. 3.4. Discussion As discussed above, obvious difference is observed for the influence of Tc on the catalytic performance of Mo/MgAl2O4 catalysts with different Mo loadings. With regard to 5Mo/MgAl2O4 catalysts with well dispersed
Mo species on the catalyst surface, high Tc facilitates the activation of isobutane. While for 30Mo/MgAl2O4 catalysts with aggregated Mo species, although increased Tc inhibits the generation of side products, the activity decreases significantly. According to Xie and Tang [22], a MoO3 molecule occupies 20 Å2. This fact and previous nitrogen adsorption–desorption measurements jointly indicate that samples with 5 wt.% Mo species possess sufficient surface area for monolayer coverage, even with Tc up to 800 °C. In contrast, insufficient surface area for monolayer coverage is observed for 30Mo/MgAl2O4 catalysts and aggregated molybdates appear on the catalyst surface. Therefore, 5 wt.% and 30 wt.% can be regarded as typical Mo loadings representing below and above monolayer coverage, respectively, as indicated by the XRD results. Abello et al. [15] reported that the presence of acid sites is beneficial for the abstraction of the first H from C\H bond of relatively inert alkane molecule, which is regarded as the rate-determining step in dehydrogenation. Therefore, as the increase of acid amount and strength with Tc for 5Mo/MgAl2O4 catalysts, both the reaction rate of isobutane and TOF increase steadily (Table 1), in accordance with the results obtained by other researchers [5,18]. Although easier reduced Mo species on the sample with higher Tc facilitates the conversion of isobutane, high Tc is not beneficial for the selectivity to isobutene. With increasing Tc, electron transfer probably occurs from the support to Mo species and leads to the electron-deficient state of the support, which increases the strength and amount of Lewis acid sites, as evidenced by data listed in Table 1. Consequently, the increase of acidity probably promotes isobutene adsorption, increases the likelihood of secondary reactions [23,24], and further results in the decrease of selectivity to isobutene (see Fig. 3b). For 30Mo/MgAl2O4 catalysts, in addition to the acid property, the interaction of Mo species with the support is very important. Al2(MoO4)3 is the main form of Mo species on the catalyst calcined at 550 °C, while
Fig. 3. Catalytic activity test results for xMo/MgAl2O4-Tc catalysts (x = 5/30, Tc = 550–800 °C).
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increased isobutane conversion and decreased selectivity to isobutene. Nevertheless, for the catalyst with MoO3 loading above monolayer coverage (30Mo/MgAl2O4), increasing calcination temperature decreases the acidity and reducibility, and also exerts a remarkable effect on the interaction between the support and Mo species, which is reflected by different molybdates formed on the catalyst surface. Exactly such effects lead to decreased isobutane conversion and increased selectivity to isobutene with calcination temperature. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. U1362201) and the National 973 Program of China (No. 2012CB215006). Appendix A. Supplementary data
Fig. 4. Conversion of isobutane and selectivity to isobutene for MgMoO4 and Al2(MoO4)3.
as the increase of Tc, MgMoO4 becomes the dominant species. Different molybdates present on the catalyst resulting from changing interactions between Mo species and the support under various Tc may be the real reason for the variation of catalytic performance. To further confirm that, activity test for isobutane dehydrogenation was carried out over Al2(MoO4)3 and MgMoO4 at 560 °C (see Fig. 4), respectively. Although isobutane conversion is much lower over MgMoO4, the selectivity to isobutene reaches 82.3 wt.%, indicating that it promotes selective dehydrogenation of isobutane. Compared with MgMoO4, the conversion increases to 20.7 wt.% for Al2(MoO4)3, but the selectivity decreases significantly. Such results are consistent with the catalytic performance over 30Mo/MgAl2O4 catalysts calcined at different Tc, demonstrating that the interaction of Mo species with the support becomes an important factor influencing dehydrogenation performance. And another reason is the different reducibility of Mo species over 30Mo/MgAl2O4 catalysts with various Tc, i.e., Mo species are much easier to be reduced for the catalysts calcined at lower temperature, showing higher reactivity of the catalysts. 4. Conclusions For Mo/MgAl2O4 catalysts, calcination temperature significantly affects the microstructure, acidity property, redox behavior and the interaction between the support and Mo species, and further influences the catalytic performance of isobutane dehydrogenation. However, obvious difference is observed for its influence on the catalysts with different Mo loadings. For the catalysts with MoO3 loading below monolayer coverage (5Mo/MgAl2O4), higher calcination temperature leads to lower surface area and higher acidity and reducibility, further resulting in
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