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4PMo12 − xVxOn mixed oxides from heteropolycompound precursors for selective oxidation of isobutane
Catalysis Communications 18 (2012) 81–84
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of Te(12 − x)/4PMo12 − xVxOn mixed oxides from heteropolycompound precursors for selective oxidation of isobutane Wenli Ding, Lulu Liu, Fanpeng Shang, Jing Hu, Qiubin Kan ⁎, Jingqi Guan ⁎ College of Chemistry, Jilin University, Changchun, 130023, PR China
a r t i c l e
i n f o
Article history: Received 14 August 2011 Received in revised form 12 November 2011 Accepted 16 November 2011 Available online 28 November 2011 Keywords: Isobutane Selective oxidation Tellurium Heteropolycompound
a b s t r a c t A series of Te(12 − x)/4PMo12 − xVxOn (x=0–3) catalysts have been prepared from the corresponding heteropolycompound precursors and tested for the partial oxidation of isobutane. It has been found that Te(12 − x)/4PMo12 − xVxOm heteropolycompounds are more active than the corresponding mixed oxides, while similar activity can be obtained if higher reaction temperatures are used for mixed oxides (e.g. 390 °C). In addition, the selectivity to methacrolein increases remarkably over Te(12 − x)/4PMo12 − xVxOn (x=0–3) mixed oxides compared with the corresponding heteropolycompound precursors. It is suggested that Te 4 + in the mixed oxide system should play a key role for improvement of the catalytic behavior. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The direct oxidation of isobutane to methacrylic acid (MAA) looks promising due to the relatively low cost of the raw material, the simplicity of the one-step process, the absence of inorganic coproducts, and the low environmental impact. The first reported catalysts for this reaction should be heteropolycompounds with Keggin structure [1]. Actually, up to now, the most effective catalysts for direct conversion of isobutane to MAA should be heteropolycompounds [2–12]. Therefore, there are many reports about the modification of molybdophosphoric salts with Keggin structure for this reaction, e.g. substitution of protons by potassium or caesium, transition metal ion (i.e. Cu2 +, Fe3 +, Ni2 +, Te4 +, etc.) substitution in counter cations, polyatom substitution (typically V-substitution for Mo) [13–16]. Improvement of catalytic behavior can be observed by effective modification process. However, although heteropolycompounds are active and selective for the direct oxidation of isobutane to methacrolein (MAL) and MAA, the industrial application of heteropolycompounds as catalysts for this reaction has been so far limited due to their structural instability at the reaction conditions. It is well known that the Keggin anion is gradually decomposed in the reaction conditions resulting in decrease of the catalytic activity and reducing the selectivity. Therefore, other different catalytic systems, i.e., V/MCM-41, Te-Mo-based mixed oxides etc., have been tentatively applied into this reaction [17–25]. Zhang et al. reported that MAL was formed with selectivity of ca. 20% at isobutane conversion of ca.7% over vanadium-containing MCM-41 catalysts [17].
⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Kan), [email protected] (J. Guan). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.11.031
Paul et al. reported selectivity to MAL of 33.7% at 2.8% of isobutane conversion can be obtained over Mo10V10Sb80Ox catalyst [18]. Liu et al. reported that the selectivity to MAL was 45% at 7% isobutane conversion over SbRe2O6 catalyst [19]. However, the yield to MAL reported by these literatures is still too low. In our previous studies, high selectivity to MAL (as high as 48%) has been achieved at 15% isobutane conversion over MoVTeSb catalyst at 390 °C [20]. It is found that tellurium is a very important element to achieve high selectivity to MAL and V-Te4 +containing phase is active and selective in the selective oxidation of isobutane to MAA [20–23]. Therefore, it is very interesting to know the catalytic performance of Te-containing mixed oxides decomposed from their corresponding heteropolycompound precursors for partial oxidation of isobutane. In the present study, Keggin-type phosphomolybdic acids with molybdenum partially substituted by vanadium and protons substituted by tellurium were designedly decomposed to obtain Mo-V-Te-P-O mixed oxides. The structure of these oxides and catalytic performance for partial oxidation of isobutane were investigated. 2. Experimental 2.1. Catalyst preparation A series of H3 + xPMo12 − xVxOm (x = 0–3) heteropolycompound catalysts were prepared using published methods [24]. In a typical synthesis, 7.1 g of Na2HPO4 was dissolved in 100 mL water and mixed with NaVO3 that had been dissolved in 100 ml of hot water. The mixture was cooled and acidified to a red color with 5 ml of concentrated sulfuric acid. To this mixture was added a solution of Na2MoO4. Finally, 85 ml of concentrated sulfuric acid was added
W. Ding et al. / Catalysis Communications 18 (2012) 81–84
slowly with vigorous stirring of the solution. The heteropoly acid was then extracted with 400 ml of ethyl ether after the water solution was cooled. After separation, a stream of air was passed through the heteropoly etherate layer to free ether. The remained solid was dissolved in 50 ml of water, and then allowed to crystallize. The formed crystals were filtered, washed with water, and air dried. Te(12 − x)/4PMo12 − xVxOm heteropolycompound catalysts were prepared as follows: aqueous solution of telluric acid was added dropwise to an aqueous solution of H3 + xPMo12 − xVxOm. The resulting solution was evaporated to dryness at 80 °C. The powder samples were collected, dried at 110 °C for 12 h to obtain Te(12 − x)/4PMo12 − xVxOm heteropolycompounds. The heteropolycompounds were then heat treated in a flow of nitrogen at 600 °C for 2 h to obtain the final metal oxides Te(12 − x)/4PMo12 − xVxOn (TG curves showed that the Te(12 − x)/4PMo12 − xVxOm heteropolycompounds are completely decomposed at 600 °C). BET surface areas were 1–3 m2∙g− 1 for all the samples examined. For comparison, PMo12On and PMo9V3On mixed oxide catalysts have been prepared by decomposing heteropolycompound H3PMo12O40 and H6PMo9V3O40 respectively at 600 °C. 2.2. Catalyst characterization Powder X-ray diffraction patterns (XRD) were collected using a Shimadzu XRD-6000 scanning (4°/min) with Cu Kα radiation (40 kV, 30 mA). The infrared spectra (IR) of various samples were recorded at room temperature using a NICOLET Impact 410 spectrometer. Specific surface areas of the catalysts were measured based on the adsorption isotherms of N2 at −196 °C using the BET method (Micromeritics ASAP2010). TG and DTA measurements were carried out on a Shimadzu DTA-60 working in an air or N2 stream. The temperature was raised up to 700 °C at a heating rate of 20 °C min − 1. X-ray photoelectron spectra (XPS) were recorded on a VG ESCA LAB MK-II X-ray electron spectrometer using AlKa radiation (1486.6 eV, 10.1 kV). The spectra were referenced with respect to the C 1 s line at 284.7 eV. The measurement error of the spectra was ±0.2 eV. 2.3. Catalytic tests The reaction was performed in a stainless steel tubular fixed bed reactor (16 mm i.d., 400 mm long) under atmospheric pressure. Reaction feed was controlled by a mass flow controller, and water was fed by a mini-pump. The catalytic reaction condition was as follows: molar ratio of the feed gas i-C4H10: O2: N2: H2O = 1:1:2:1, gas hourly space velocity (GHSV) = 1000 mL·h − 1gcat − 1. The experiments were carried out at 350–410 °C, and the time allotted at each temperature
Intensity (a.u.)
Te2.25PMo9V3On
Te2.5PMo10V2On
Te2.75PMo11VOn
Te2.25PMo9V3On Te2.5PMo10V2On
Transmitance (a. u.)
82
1200
30
40
50
60
2 Theta (o) Fig. 1. XRD patterns of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts.
1000
800
600
400
Fig. 2. The FT-IR spectra of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts.
during catalytic runs was about 3.5 h. The products were then fed via heated lines to an on-line gas chromatography for analysis. Methacrolein (MAL), methacrylic acid (MAA), COx (CO, CO2), and propene (C3=) were the main products. Carbon mass balances of ≥97% were typically observed.
3. Results and discussion 3.1. Characterization XRD patterns of the Te(12 − x)/4PMo12 − xVxOn mixed oxides are shown in Fig. 1. Only peaks corresponding to MoO3 (2θ = 12.8 , 23.5 , 25.7 , 27.3 , 33.9 , 39.0 , 39.7 , 45.8 , 46.3 , and 49.3 ) [JCPDS 35–0609] could be found. Fig. 2 displays the IR spectra of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts. The bands at 990, 876, 820, 590, and 490 cm − 1 are characteristic of MoO3. The infrared spectra along with the XRD patterns indicate that in both cases, MoO3 phase mainly exists in the mixed oxide catalysts. To gain deeper insight into the surface constitution and properties of these catalysts, their Mo 3d5/2, V 2p3/2, Te 3d5/2 and P 2p binding energies were investigated with the results listed in Table 1. The Mo 3d5/2 peak of the catalysts could be fitted into two components at 231.7 and 232.8 eV, which can be assigned to Mo 5 + and Mo 6 + species, respectively [21,25]. Thus, Mo 6 + cations are mainly present on the surface of the Te(1.5 + 0.5x)PMo12 − xVxOn catalysts, and a small amount of Mo 5 + ions coexist on the catalyst surfaces. The V 2p3/2 peak of catalysts could be fitted into two components at 516.2 and 517.3 eV, which can be related to V 4 + and V 5 + species, respectively [21,25]. It can be seen that V 4 + and V 5 + coexist on the surface of the Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts. The Te 3d5/2 peak of catalysts could be fitted into two components at 576.2 and 577.3 eV, which can be related to Te4 + and Te6 + species,
Table 1 XPS data of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts. Sample
20
Te3PMo12On
Wavenumber (cm-1)
Te3PMo12On
10
Te2.75PMo11VOn
Te3PMo12On Te2.75PMo11VOn Te2.5PMo10V2On Te2.25PMo9V3On
Binding energy (eV)
Surface composition (at%)
Mo 3d5/2
V 2p3/2
Te 3d5/2
P 2p
O1s
Mo
V
Te
P
O
233.2 233.1 233.1 233.2
– 516.7 516.8 516.8
577.0 577.1 577.1 577.2
133.8 133.7 133.8 133.8
530.8 530.9 530.8 530.9
15.1 14.2 13.6 13.2
– 1.2 2.6 4.4
3.7 3.5 3.4 3.3
1.2 1.3 1.4 1.5
80.0 79.8 79.0 77.6
i-C4H10 MAL MAA
40
30
20
10
Conversion and selectivity (%)
Conversion and selectivity (%)
W. Ding et al. / Catalysis Communications 18 (2012) 81–84
70
i-C4H10
60
MAL MAA
83
50 40 30 20 10 0
370
380
390
400
0
410
Reaction temperature (oC)
1
2
3
x in Te(12-x)/4PMo12-xVxOn
Fig. 3. Conversion/selectivity versus the reaction temperature obtained during the oxidation of isobutane over Te2.25PMo9V3On mixed oxide catalyst.
Fig. 4. Selective oxidation of isobutane over Te(12 − x)/4PMo12 − xVxOm heteropoly compounds (left) at 350 °C, and Te(12 − x)/4PMo12 − xVxOn mixed oxides (right) at 390 °C.
respectively [21,25]. It is found that Te4 + is mainly formed in the catalysts. However, Te0 (binding energy of 573.0 eV) is not observed. The P 2p binding energies of surface phosphorus are very similar (ca. 133.8 eV) for all of the samples, which indicates that P atoms are mainly present as P 5 + in Te(12 − x)/4PMo12 − xVxOn catalysts [25].
selectivity to MAA is considerably improved by one V substitution for Mo atom in the molybdophosphoric system. In addition, for Te(12 − x)/4PMo12 − xVxOn mixed oxide system, the conversion of isobutane slightly increases, while the selectivity to MAL and MAA is gradually improved by the increase of V-content. In addition, it can be seen that higher selectivity to MAL and MAA can be obtained over Te2.25PMo9V3On mixed oxide than over Te2.25PMo9V3Om heteropolycompound catalyst. Moreover, the selectivities to MAL over Te(12 − x)/4PMo12 − xVxOn mixed oxides are generally higher than those over heteropolycompound catalysts at similar isobutane conversion. It has been generally accepted that a V5 + surface site is associated with alkane activation, a Te 4 + site is due to α-H abstraction once the alkene has formed, and a (Mo6 +)2 site is related to the O insertion [26]. Our previous experiment results have proved that Te-Mo-based mixed oxides are very active and selective in the partial oxidation of isobutane and the main product should be MAL, not MAA for the Te-Mo-based mixed oxides [20–23]. According to the XRD and IR results, Te-containing phases have not been detected in the Te(12 − x)/ 4PMo12 − xVxOn mixed oxide system. Moreover, it is generally accepted that MoO3 phase is relatively low activity for partial oxidation of alkanes. Therefore, the oxidation process may be carried out through the cooperation of Mo, V, Te, and/or P [21,27]. It can be found that there are Mo 6 +, V 5 +, Te 4 +, and P 5 + on the Te(12 − x)/ 4PMo12 − xVxOn mixed oxide surfaces according to XPS studies. From Table 2, it can be found that V-containing catalysts exhibited relatively good selectivity to MAA compared with V-free catalysts, while Te-containing catalysts exhibited better selectivity to MAL compared with Te-free ones. Therefore, the role of vanadium may be related to alkane activation and the formation of the redox cycle V 4 + + Mo 6 + ↔ V 5 + + Mo 5 + to improve the redox capability of Mo species, while the role of tellurium may be associated with α-H abstraction for improvement of selectivity to MAL.
3.2. Catalytic properties The catalytic property obtained by changing reaction temperature in a range of 330–370 °C over Te2.25PMo9V3On mixed oxide catalyst is plotted in Fig. 3. As expected, the conversion of isobutane increases with temperature. In the meanwhile, the selectivity to MAL and MAA gradually decreases with the increase of temperature. The total yield to MAL and MAA gradually increases with enhancing the reaction temperature. The catalytic results obtained for the oxidation of isobutane over PMo12On, PMo9V3On, and Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts at 390 °C have been given in Table 2. It can be found that the selectivity to MAL is very low over the Te-free PMo12On and PMo9V3On mixed oxide catalysts. Contrastively, the Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts are active and selective for the partial oxidation of isobutane. The selectivity to MAL and MAA is considerably improved by augmentation of V-content. In addition, very little isobutene is formed over these catalysts (b0.5%). It can be found that Te2.25PMo9V3On catalyst achieved the best MAL and MAA selectivity (56.5%) and yield (6.6%) at 390 °C. Fig. 4 shows the results of the oxidation of isobutane catalyzed by Te(12 − x)/4PMo12 − xVxOn mixed oxides at 390 °C and their corresponding heteropolycompound precursors Te(12 − x)/4PMo12 − xVxOm at 350 °C. For Te(12 − x)/4PMo12 − xVxOm heteropolycompound catalysts (XRD patterns shown in Fig. 1s), the conversion of isobutane and
Table 2 Catalytic data of PMo12On, PMo9V3On, and Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts for isobutane oxidation at 390 °C. Catalysts
PMo12On PMo9V3On Te3PMo12On Te2.75PMo11VOn Te2.5PMo10V2On Te2.25PMo9V3On
Conversion
Selectivity (%)
(%)
MAL
MAA
CO
CO2
C3=
others
5.1 9.6 9.3 9.8 10.5 11.8
9.6 10.5 27.8 28.3 29.2 34.4
12.3 14.4 9.3 13.8 15.3 22.1
38.2 33.8 32.8 31.3 30.3 22.3
27.8 22.6 18.2 17 16.1 13.4
5.8 5.7 6.6 6.5 6.3 5.3
6.2 12.9 5.2 3.0 2.7 2.4
4. Conclusions A series of Te(12 − x)/4PMo12 − xVxOn (x= 0–3) mixed oxide catalysts have been synthesized by pyrolysis of the corresponding heteropolycompound precursors for the selective oxidation of isobutane. It has been found that MoO3 is the main crystalline phase in the Te(12 − x)/ 4PMo12 − xVxOn (x= 0–3) mixed oxide catalysts. The addition of tellurium in the mixed oxide system should modify the physicochemical properties (e.g. surface composition, and surroundings of Mo), which affect their catalytic performance for the selective oxidation of isobutane to MAL and MAA. Under our reaction conditions, the Te2.25PMo9V3On
84
W. Ding et al. / Catalysis Communications 18 (2012) 81–84
oxide catalyst reached the best MAL+MAA selectivity (56.5%) and yield (6.6%) at 390 °C. Acknowledgments This work was supported by Jilin province (20090591 and 201105006), Jilin University (450060445017) and Specialized Research Fund for the Doctoral Program of Higher Education (20100061120083). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.catcom.2011.11.031. References [1] K. Harold, K.L. Samuel, Rhom and Haas Co., European Patent No. 0010902, 1979. [2] Q. Huynh, Y. Schuurman, P. Delichere, S. Loridant, J.M.M. Millet, Journal of Catalysis 261 (2009) 166–176. [3] N. Mizuno, M. Tateishi, M. Iwamoto, Journal of Catalysis 163 (1996) 87–94. [4] E. Etienne, F. Cavani, R. Mezzogori, F. Trifirò, G. Calestani, L. Gengembre, M. Guelton, Applied Catalysis A: General 256 (2003) 275–290. [5] S. Paul, V. Le Courtois, D. Vanhove, Industrial and Engineering Chemistry Research 36 (1997) 3391–3399. [6] L. Jalowiecki-Duhamel, A. Monnier, Y. Barbaux, G. Hecquet, Catalysis Today 32 (1996) 237–241. [7] W. Li, K. Oshihara, W. Ueda, Applied Catalysis A: General 182 (1999) 357–363. [8] F. Cavani, A. Tanguy, F. Trifirò, M. Koutyrev, Journal of Catalysis 174 (1998) 231–241.
[9] M. Langpape, J.M.M. Millet, U.S. Ozkan, M. Boudeulle, Journal of Catalysis 181 (1999) 80–90. [10] F. Cavani, E. Etienne, M. Favaro, A. Galli, F. Trifirò, G. Hecquet, Catalysis Letters 32 (1995) 215–226. [11] H. Imai, M. Nakatsuka, A. Aoshima, Japanese Patent 62,132,832 (1987), attributed to Asahi Chem. Ind. Co. K. [12] Kawakami, S. Yamamatsu, T. Yamaguchi, Japanese Patent 03,176,438 (1991), attributed to Asahi Chem. Ind. Co. [13] N. Mizuno, M. Tateishi, M. Iwamoto, Applied Catalysis A: General 118 (1994) L1–L4. [14] M. Langpape, J.M.M. Millet, Applied Catalysis A: General 200 (2000) 89–101. [15] N. Dimitratos, J.C. Védrine, Catalysis Today 81 (2003) 561–571. [16] Q. Huynh, A. Selmi, G. Corbel, P. Lacorre, J.M.M. Millet, Journal of Catalysis 266 (2009) 64–70. [17] Q. Zhang, Y. Wang, Y. Ohishi, T. Shishido, K. Takehira, Journal of Catalysis 202 (2001) 308–318. [18] J.S. Paul, P.A. Jacobs, P.-A.W. Weiss, W.F. Maier, Applied Catalysis A: General 265 (2004) 185–193. [19] H. Liu, E.M. Gaigneaux, H. Imoto, T. Shido, Y. Iwasawa, Applied Catalysis A: General 202 (2000) 251–264. [20] J. Guan, H. Wang, K. Song, C. Xu, Z. Wang, Q. Kan, Catalysis Communications 10 (2009) 1437–1440. [21] J. Guan, S. Wu, H. Wang, S. Jing, G. Wang, K. Zhen, Q. Kan, Journal of Catalysis 251 (2007) 354–362. [22] J. Guan, C. Xu, Z. Wang, Y. Yang, B. Liu, F. Shang, Y. Shao, Q. Kan, Catalysis Letters 124 (2008) 428–433. [23] J. Guan, K. Song, H. Xu, Z. Wang, Y. Ma, F. Shang, Q. Kan, Catalysis Communications 10 (2009) 528–532. [24] B. Tsigdinos, C. Hallada, Inorganic Chemistry 7 (1968) 437–441. [25] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of XPS, Perkin-Elmer, USA, 1992. [26] R.K. Grasselli, J.D. Burrington, D.J. Buttrey, P. DeSanto Jr., C.G. Lugmair, A.F. Volpe Jr., T. Weingand, Topics in Catalysis 23 (2003) 5–22. [27] R.K. Grasselli, Topics in Catalysis 15 (2001) 93–101.
Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of Te(12 − x)/4PMo12 − xVxOn mixed oxides from heteropolycompound precursors for selective oxidation of isobutane Wenli Ding, Lulu Liu, Fanpeng Shang, Jing Hu, Qiubin Kan ⁎, Jingqi Guan ⁎ College of Chemistry, Jilin University, Changchun, 130023, PR China
a r t i c l e
i n f o
Article history: Received 14 August 2011 Received in revised form 12 November 2011 Accepted 16 November 2011 Available online 28 November 2011 Keywords: Isobutane Selective oxidation Tellurium Heteropolycompound
a b s t r a c t A series of Te(12 − x)/4PMo12 − xVxOn (x=0–3) catalysts have been prepared from the corresponding heteropolycompound precursors and tested for the partial oxidation of isobutane. It has been found that Te(12 − x)/4PMo12 − xVxOm heteropolycompounds are more active than the corresponding mixed oxides, while similar activity can be obtained if higher reaction temperatures are used for mixed oxides (e.g. 390 °C). In addition, the selectivity to methacrolein increases remarkably over Te(12 − x)/4PMo12 − xVxOn (x=0–3) mixed oxides compared with the corresponding heteropolycompound precursors. It is suggested that Te 4 + in the mixed oxide system should play a key role for improvement of the catalytic behavior. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The direct oxidation of isobutane to methacrylic acid (MAA) looks promising due to the relatively low cost of the raw material, the simplicity of the one-step process, the absence of inorganic coproducts, and the low environmental impact. The first reported catalysts for this reaction should be heteropolycompounds with Keggin structure [1]. Actually, up to now, the most effective catalysts for direct conversion of isobutane to MAA should be heteropolycompounds [2–12]. Therefore, there are many reports about the modification of molybdophosphoric salts with Keggin structure for this reaction, e.g. substitution of protons by potassium or caesium, transition metal ion (i.e. Cu2 +, Fe3 +, Ni2 +, Te4 +, etc.) substitution in counter cations, polyatom substitution (typically V-substitution for Mo) [13–16]. Improvement of catalytic behavior can be observed by effective modification process. However, although heteropolycompounds are active and selective for the direct oxidation of isobutane to methacrolein (MAL) and MAA, the industrial application of heteropolycompounds as catalysts for this reaction has been so far limited due to their structural instability at the reaction conditions. It is well known that the Keggin anion is gradually decomposed in the reaction conditions resulting in decrease of the catalytic activity and reducing the selectivity. Therefore, other different catalytic systems, i.e., V/MCM-41, Te-Mo-based mixed oxides etc., have been tentatively applied into this reaction [17–25]. Zhang et al. reported that MAL was formed with selectivity of ca. 20% at isobutane conversion of ca.7% over vanadium-containing MCM-41 catalysts [17].
⁎ Corresponding authors. E-mail addresses: [email protected] (Q. Kan), [email protected] (J. Guan). 1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2011.11.031
Paul et al. reported selectivity to MAL of 33.7% at 2.8% of isobutane conversion can be obtained over Mo10V10Sb80Ox catalyst [18]. Liu et al. reported that the selectivity to MAL was 45% at 7% isobutane conversion over SbRe2O6 catalyst [19]. However, the yield to MAL reported by these literatures is still too low. In our previous studies, high selectivity to MAL (as high as 48%) has been achieved at 15% isobutane conversion over MoVTeSb catalyst at 390 °C [20]. It is found that tellurium is a very important element to achieve high selectivity to MAL and V-Te4 +containing phase is active and selective in the selective oxidation of isobutane to MAA [20–23]. Therefore, it is very interesting to know the catalytic performance of Te-containing mixed oxides decomposed from their corresponding heteropolycompound precursors for partial oxidation of isobutane. In the present study, Keggin-type phosphomolybdic acids with molybdenum partially substituted by vanadium and protons substituted by tellurium were designedly decomposed to obtain Mo-V-Te-P-O mixed oxides. The structure of these oxides and catalytic performance for partial oxidation of isobutane were investigated. 2. Experimental 2.1. Catalyst preparation A series of H3 + xPMo12 − xVxOm (x = 0–3) heteropolycompound catalysts were prepared using published methods [24]. In a typical synthesis, 7.1 g of Na2HPO4 was dissolved in 100 mL water and mixed with NaVO3 that had been dissolved in 100 ml of hot water. The mixture was cooled and acidified to a red color with 5 ml of concentrated sulfuric acid. To this mixture was added a solution of Na2MoO4. Finally, 85 ml of concentrated sulfuric acid was added
W. Ding et al. / Catalysis Communications 18 (2012) 81–84
slowly with vigorous stirring of the solution. The heteropoly acid was then extracted with 400 ml of ethyl ether after the water solution was cooled. After separation, a stream of air was passed through the heteropoly etherate layer to free ether. The remained solid was dissolved in 50 ml of water, and then allowed to crystallize. The formed crystals were filtered, washed with water, and air dried. Te(12 − x)/4PMo12 − xVxOm heteropolycompound catalysts were prepared as follows: aqueous solution of telluric acid was added dropwise to an aqueous solution of H3 + xPMo12 − xVxOm. The resulting solution was evaporated to dryness at 80 °C. The powder samples were collected, dried at 110 °C for 12 h to obtain Te(12 − x)/4PMo12 − xVxOm heteropolycompounds. The heteropolycompounds were then heat treated in a flow of nitrogen at 600 °C for 2 h to obtain the final metal oxides Te(12 − x)/4PMo12 − xVxOn (TG curves showed that the Te(12 − x)/4PMo12 − xVxOm heteropolycompounds are completely decomposed at 600 °C). BET surface areas were 1–3 m2∙g− 1 for all the samples examined. For comparison, PMo12On and PMo9V3On mixed oxide catalysts have been prepared by decomposing heteropolycompound H3PMo12O40 and H6PMo9V3O40 respectively at 600 °C. 2.2. Catalyst characterization Powder X-ray diffraction patterns (XRD) were collected using a Shimadzu XRD-6000 scanning (4°/min) with Cu Kα radiation (40 kV, 30 mA). The infrared spectra (IR) of various samples were recorded at room temperature using a NICOLET Impact 410 spectrometer. Specific surface areas of the catalysts were measured based on the adsorption isotherms of N2 at −196 °C using the BET method (Micromeritics ASAP2010). TG and DTA measurements were carried out on a Shimadzu DTA-60 working in an air or N2 stream. The temperature was raised up to 700 °C at a heating rate of 20 °C min − 1. X-ray photoelectron spectra (XPS) were recorded on a VG ESCA LAB MK-II X-ray electron spectrometer using AlKa radiation (1486.6 eV, 10.1 kV). The spectra were referenced with respect to the C 1 s line at 284.7 eV. The measurement error of the spectra was ±0.2 eV. 2.3. Catalytic tests The reaction was performed in a stainless steel tubular fixed bed reactor (16 mm i.d., 400 mm long) under atmospheric pressure. Reaction feed was controlled by a mass flow controller, and water was fed by a mini-pump. The catalytic reaction condition was as follows: molar ratio of the feed gas i-C4H10: O2: N2: H2O = 1:1:2:1, gas hourly space velocity (GHSV) = 1000 mL·h − 1gcat − 1. The experiments were carried out at 350–410 °C, and the time allotted at each temperature
Intensity (a.u.)
Te2.25PMo9V3On
Te2.5PMo10V2On
Te2.75PMo11VOn
Te2.25PMo9V3On Te2.5PMo10V2On
Transmitance (a. u.)
82
1200
30
40
50
60
2 Theta (o) Fig. 1. XRD patterns of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts.
1000
800
600
400
Fig. 2. The FT-IR spectra of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts.
during catalytic runs was about 3.5 h. The products were then fed via heated lines to an on-line gas chromatography for analysis. Methacrolein (MAL), methacrylic acid (MAA), COx (CO, CO2), and propene (C3=) were the main products. Carbon mass balances of ≥97% were typically observed.
3. Results and discussion 3.1. Characterization XRD patterns of the Te(12 − x)/4PMo12 − xVxOn mixed oxides are shown in Fig. 1. Only peaks corresponding to MoO3 (2θ = 12.8 , 23.5 , 25.7 , 27.3 , 33.9 , 39.0 , 39.7 , 45.8 , 46.3 , and 49.3 ) [JCPDS 35–0609] could be found. Fig. 2 displays the IR spectra of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts. The bands at 990, 876, 820, 590, and 490 cm − 1 are characteristic of MoO3. The infrared spectra along with the XRD patterns indicate that in both cases, MoO3 phase mainly exists in the mixed oxide catalysts. To gain deeper insight into the surface constitution and properties of these catalysts, their Mo 3d5/2, V 2p3/2, Te 3d5/2 and P 2p binding energies were investigated with the results listed in Table 1. The Mo 3d5/2 peak of the catalysts could be fitted into two components at 231.7 and 232.8 eV, which can be assigned to Mo 5 + and Mo 6 + species, respectively [21,25]. Thus, Mo 6 + cations are mainly present on the surface of the Te(1.5 + 0.5x)PMo12 − xVxOn catalysts, and a small amount of Mo 5 + ions coexist on the catalyst surfaces. The V 2p3/2 peak of catalysts could be fitted into two components at 516.2 and 517.3 eV, which can be related to V 4 + and V 5 + species, respectively [21,25]. It can be seen that V 4 + and V 5 + coexist on the surface of the Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts. The Te 3d5/2 peak of catalysts could be fitted into two components at 576.2 and 577.3 eV, which can be related to Te4 + and Te6 + species,
Table 1 XPS data of Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts. Sample
20
Te3PMo12On
Wavenumber (cm-1)
Te3PMo12On
10
Te2.75PMo11VOn
Te3PMo12On Te2.75PMo11VOn Te2.5PMo10V2On Te2.25PMo9V3On
Binding energy (eV)
Surface composition (at%)
Mo 3d5/2
V 2p3/2
Te 3d5/2
P 2p
O1s
Mo
V
Te
P
O
233.2 233.1 233.1 233.2
– 516.7 516.8 516.8
577.0 577.1 577.1 577.2
133.8 133.7 133.8 133.8
530.8 530.9 530.8 530.9
15.1 14.2 13.6 13.2
– 1.2 2.6 4.4
3.7 3.5 3.4 3.3
1.2 1.3 1.4 1.5
80.0 79.8 79.0 77.6
i-C4H10 MAL MAA
40
30
20
10
Conversion and selectivity (%)
Conversion and selectivity (%)
W. Ding et al. / Catalysis Communications 18 (2012) 81–84
70
i-C4H10
60
MAL MAA
83
50 40 30 20 10 0
370
380
390
400
0
410
Reaction temperature (oC)
1
2
3
x in Te(12-x)/4PMo12-xVxOn
Fig. 3. Conversion/selectivity versus the reaction temperature obtained during the oxidation of isobutane over Te2.25PMo9V3On mixed oxide catalyst.
Fig. 4. Selective oxidation of isobutane over Te(12 − x)/4PMo12 − xVxOm heteropoly compounds (left) at 350 °C, and Te(12 − x)/4PMo12 − xVxOn mixed oxides (right) at 390 °C.
respectively [21,25]. It is found that Te4 + is mainly formed in the catalysts. However, Te0 (binding energy of 573.0 eV) is not observed. The P 2p binding energies of surface phosphorus are very similar (ca. 133.8 eV) for all of the samples, which indicates that P atoms are mainly present as P 5 + in Te(12 − x)/4PMo12 − xVxOn catalysts [25].
selectivity to MAA is considerably improved by one V substitution for Mo atom in the molybdophosphoric system. In addition, for Te(12 − x)/4PMo12 − xVxOn mixed oxide system, the conversion of isobutane slightly increases, while the selectivity to MAL and MAA is gradually improved by the increase of V-content. In addition, it can be seen that higher selectivity to MAL and MAA can be obtained over Te2.25PMo9V3On mixed oxide than over Te2.25PMo9V3Om heteropolycompound catalyst. Moreover, the selectivities to MAL over Te(12 − x)/4PMo12 − xVxOn mixed oxides are generally higher than those over heteropolycompound catalysts at similar isobutane conversion. It has been generally accepted that a V5 + surface site is associated with alkane activation, a Te 4 + site is due to α-H abstraction once the alkene has formed, and a (Mo6 +)2 site is related to the O insertion [26]. Our previous experiment results have proved that Te-Mo-based mixed oxides are very active and selective in the partial oxidation of isobutane and the main product should be MAL, not MAA for the Te-Mo-based mixed oxides [20–23]. According to the XRD and IR results, Te-containing phases have not been detected in the Te(12 − x)/ 4PMo12 − xVxOn mixed oxide system. Moreover, it is generally accepted that MoO3 phase is relatively low activity for partial oxidation of alkanes. Therefore, the oxidation process may be carried out through the cooperation of Mo, V, Te, and/or P [21,27]. It can be found that there are Mo 6 +, V 5 +, Te 4 +, and P 5 + on the Te(12 − x)/ 4PMo12 − xVxOn mixed oxide surfaces according to XPS studies. From Table 2, it can be found that V-containing catalysts exhibited relatively good selectivity to MAA compared with V-free catalysts, while Te-containing catalysts exhibited better selectivity to MAL compared with Te-free ones. Therefore, the role of vanadium may be related to alkane activation and the formation of the redox cycle V 4 + + Mo 6 + ↔ V 5 + + Mo 5 + to improve the redox capability of Mo species, while the role of tellurium may be associated with α-H abstraction for improvement of selectivity to MAL.
3.2. Catalytic properties The catalytic property obtained by changing reaction temperature in a range of 330–370 °C over Te2.25PMo9V3On mixed oxide catalyst is plotted in Fig. 3. As expected, the conversion of isobutane increases with temperature. In the meanwhile, the selectivity to MAL and MAA gradually decreases with the increase of temperature. The total yield to MAL and MAA gradually increases with enhancing the reaction temperature. The catalytic results obtained for the oxidation of isobutane over PMo12On, PMo9V3On, and Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts at 390 °C have been given in Table 2. It can be found that the selectivity to MAL is very low over the Te-free PMo12On and PMo9V3On mixed oxide catalysts. Contrastively, the Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts are active and selective for the partial oxidation of isobutane. The selectivity to MAL and MAA is considerably improved by augmentation of V-content. In addition, very little isobutene is formed over these catalysts (b0.5%). It can be found that Te2.25PMo9V3On catalyst achieved the best MAL and MAA selectivity (56.5%) and yield (6.6%) at 390 °C. Fig. 4 shows the results of the oxidation of isobutane catalyzed by Te(12 − x)/4PMo12 − xVxOn mixed oxides at 390 °C and their corresponding heteropolycompound precursors Te(12 − x)/4PMo12 − xVxOm at 350 °C. For Te(12 − x)/4PMo12 − xVxOm heteropolycompound catalysts (XRD patterns shown in Fig. 1s), the conversion of isobutane and
Table 2 Catalytic data of PMo12On, PMo9V3On, and Te(12 − x)/4PMo12 − xVxOn mixed oxide catalysts for isobutane oxidation at 390 °C. Catalysts
PMo12On PMo9V3On Te3PMo12On Te2.75PMo11VOn Te2.5PMo10V2On Te2.25PMo9V3On
Conversion
Selectivity (%)
(%)
MAL
MAA
CO
CO2
C3=
others
5.1 9.6 9.3 9.8 10.5 11.8
9.6 10.5 27.8 28.3 29.2 34.4
12.3 14.4 9.3 13.8 15.3 22.1
38.2 33.8 32.8 31.3 30.3 22.3
27.8 22.6 18.2 17 16.1 13.4
5.8 5.7 6.6 6.5 6.3 5.3
6.2 12.9 5.2 3.0 2.7 2.4
4. Conclusions A series of Te(12 − x)/4PMo12 − xVxOn (x= 0–3) mixed oxide catalysts have been synthesized by pyrolysis of the corresponding heteropolycompound precursors for the selective oxidation of isobutane. It has been found that MoO3 is the main crystalline phase in the Te(12 − x)/ 4PMo12 − xVxOn (x= 0–3) mixed oxide catalysts. The addition of tellurium in the mixed oxide system should modify the physicochemical properties (e.g. surface composition, and surroundings of Mo), which affect their catalytic performance for the selective oxidation of isobutane to MAL and MAA. Under our reaction conditions, the Te2.25PMo9V3On
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oxide catalyst reached the best MAL+MAA selectivity (56.5%) and yield (6.6%) at 390 °C. Acknowledgments This work was supported by Jilin province (20090591 and 201105006), Jilin University (450060445017) and Specialized Research Fund for the Doctoral Program of Higher Education (20100061120083). Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.catcom.2011.11.031. References [1] K. Harold, K.L. Samuel, Rhom and Haas Co., European Patent No. 0010902, 1979. [2] Q. Huynh, Y. Schuurman, P. Delichere, S. Loridant, J.M.M. Millet, Journal of Catalysis 261 (2009) 166–176. [3] N. Mizuno, M. Tateishi, M. Iwamoto, Journal of Catalysis 163 (1996) 87–94. [4] E. Etienne, F. Cavani, R. Mezzogori, F. Trifirò, G. Calestani, L. Gengembre, M. Guelton, Applied Catalysis A: General 256 (2003) 275–290. [5] S. Paul, V. Le Courtois, D. Vanhove, Industrial and Engineering Chemistry Research 36 (1997) 3391–3399. [6] L. Jalowiecki-Duhamel, A. Monnier, Y. Barbaux, G. Hecquet, Catalysis Today 32 (1996) 237–241. [7] W. Li, K. Oshihara, W. Ueda, Applied Catalysis A: General 182 (1999) 357–363. [8] F. Cavani, A. Tanguy, F. Trifirò, M. Koutyrev, Journal of Catalysis 174 (1998) 231–241.
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