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Partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde
Applied Catalysis A: General 242 (2003) 329–334
Partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde Min Chen, Ren-Xian Zhou, Xiao-Ming Zheng∗ Institute of Catalysis, Zhejiang University, Xixi Campus, Hangzhou, PR China Received 8 January 2002; received in revised form 8 July 2002; accepted 21 September 2002
Abstract In this paper, studies of V-Cs-Cu and V-Cs-Cu-Tl catalysts used in the partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde are reported. The two series of catalysts are prepared by an impregnation method. The effects of temperature, space velocity, concentration, ratio of O2 /p-tert butyl toluene and the stability of catalysts are examined. The results indicate that both the catalysts show good catalytic activity in partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde; but the series of V-Cs-Cu-Tl catalysts exhibit more excellent catalytic activity. Under the reaction conditions of space velocity at 10,000 h−1 and temperature at 440 ◦ C, the conversion of p-tert butyl toluene can reach 12 mol% and the selectivity of p-tert butyl benzaldehyde is 85 mol%. In addition, the good stability in the series of V-Cs-Cu-Tl catalysts is another important character: the catalytic activity still remained significant after stability test for 365 h. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Partial oxidation reaction; p-Tert butyl benzaldehyde; Catalyst
1. Introduction p-Tert butyl benzaldehyde is mainly used in foodstuff, and medicine and also enjoys wide demand in the chemical industry [1,2]. Few studies of the vapor phase partial oxidation of p-tert butyl toluene to produce p-tert butyl benzaldehyde have been reported, because it is quite difficult to get a good correspondence between the conversion and selectivity [3–6]. There are three reaction paths in the vapor phase partial oxidation of p-tert butyl toluene. They are (1) side chain oxidation, (2) oxidative couping reaction, (3) direct oxidation to CO2 and H2 O. However, it still has been an open question has remained as to how to find an excellent catalyst system and how to im∗ Corresponding author. E-mail address: minchen [email protected] (X.-M. Zheng).
prove catalytic performance for the reaction. There are also many reports about the catalysts used in selective toluene oxidation, for example, the properties for oxidation of toluene to benzadehyde are improved by silver additions [7,8] and small addition of potassium to the V/Ti-oxide catalyst can increase the activity and selectivity [9]. Our aim is to identify a good catalyst for selective oxidation from p-tert butyl toluene to p-tert butyl benzaldehyde. Hence, the activities of V-Cs-Cu and V-CsCu-Tl series of catalysts are assigned to the reaction. From our study, it is evident that these two system catalysts are optimal to the reaction. However, selective oxidation of p-tert butyl toluene is seldom reported. This is a novel catalyst system and an effective application in selective oxidation of p-tert butyl toluene. The main aim of our work is to find a new catalyst system that has good catalytic properties in both
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M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
Scheme 1.
conversion and selectivity. In this paper, a new series of V-Cs-Cu-Tl/TiO2 ·SiC catalysts have been studied. V2 O5 is used as chief catalyst and the promoters Cs, Cu, Tl are used as active components. TiO2 and silicon carbide (commercial) are used as the support. Our study has been to focus on using an active route for vapor phase catalytic oxidation method to produce p-tert butyl benzaldehyde, the obtained results are optimum values and have good advantages in both conversion and selectivity. The reaction mechanism for the formation of p-tert butyl benzaldehyde from p-tert butyl toluene is shown in Scheme 1.
2. Experimental 2.1. Preparation of the catalyst The catalysts are prepared by an impregnation method. In this study, cupric nitrate Cu(NO3 )2 ·3H2 O, thallous nitrate (TlNO3 ) and cesium nitrate (CsNO3 ) are used for active compounds. Vanadylic citric acid solution is also added. The mixture was stirred enough, admixed with TiO2 ·SiC used as the supports, then dried by evaporation. After being dried at 250 ◦ C for 2 h, the samples were calcined at 600 ◦ C for 5 h and then pulverized into particles of 20–40 meshes. The composition of the catalysts in molar ratio is as follows: V/Cs is 0.2, V/Cu is 0.15 and V/Tl is n (n = 0.01–0.03). The catalysts are denoted as V-Cs-Cu and V-Cs-Cu-Tl. 2.2. Activity measurement The catalytic activity test for partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde is performed in a flow reactor containing a fixed-bed of 3 ml of catalyst. The temperature is varied from room temperature to 500 ◦ C. The total feed is 40 l/min (STP).
The temperature is measured both before and after the catalyst bed. The flow of p-tert butyl toluene is fed using a mass flow controller. Liquid p-tert butyl toluene was pumped into heated evaporators through which the steam of air is passing before reaching the preheater. The analysis of the reactor effluent is performed with an on-line SP-2304 model vapor chromatograph with a FID. 2.3. Structure and surface characterization The active compounds’ analysis of catalyst before and after reaction was performed on the inductively coupled plasma atomic emission spectrometry (ICP-AES) instrument type PLASMA-SEEC, made in the US by Leeman Company. The X-ray powder diffraction (XRD) patterns are collected on a Rigaku D/max-IIIB instrument using monochromatized Cu K␣ radiation (40 kV and 30 mA). The X-ray photoelectron spectroscopy (XPS) analysis is performed on a PHI-550 model ESCA/ SAM spectrometer equipped with an Al anode (hν = 1486.6 eV). The binding energies are referenced to the binding energy level of O 1s at 531.8 eV. This value is an average of the O 1s binding energy determined by referring to the C 1s spectra (284.6).
3. Results and discussion 3.1. Effect of space velocity on catalyst activity The results of changes in the space velocity over V-Cs-Cu and V-Cs-Cu-Tl catalysts are shown in Fig. 1. At the reaction temperature of 460 ◦ C, it can be seen that the selectivity of p-tert butyl benzaldehyde increases with the increasing of space velocity from 5000 to 10,000 h−1 , but the conversion of p-tert butyl toluene varies oppositely. At space velocity of 10,000 h−1 , the conversion of p-tert butyl toluene and the selectivity of p-tert butyl benzaldehyde both increase to the maximum valves in these two catalysts. However, by comparison of V-Cs-Cu and V-Cs-Cu-Tl catalyst systems, the latter obtains better selectivity than the former at the condition of the space velocity at 10,000−1 , but the selectivity decreases with the further increasing of space velocity. Such a result can be explained by the fact that reaction heat can be
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
Fig. 1. Effect of space velocity on the V-Cs-Cu and V-Cs-Cu-Tl catalyst.
taken away more quickly from the catalyst surface with the increasing of space velocity and then the deep oxidation is prevented.
331
Fig. 2. Effect of p-tert butyl toluene concentration on the V-Cs-Cu and V-Cs-Cu-Tl catalyst.
lysts show good catalytic activities, but in comparison with the V-Cs-Cu catalyst, the V-Cs-Cu-Tl catalyst is more active.
3.2. Effect of temperature on catalyst activity
3.3. Effect of p-tert butyl toluene concentration on catalyst activity
Here, we present the influence of temperature for the series of V-Cs-Cu and V-Cs-Cu-Tl catalysts. The results are given in Table 1. At the conditions of space velocity at 10,000 h−1 and the p-tert butyl toluene concentration at 0.9–1.0 mol/m3 , for V-Cs-Cu catalyst, the conversion of p-tert butyl toluene reaches 16.2 mol% and the selectivity of p-tert butyl benzaldehyde reaches 83.3 mol% at the temperature of 460 ◦ C. And for V-Cs-Cu-Tl catalyst, the conversion of p-tert butyl toluene and the selectivity of p-tert butyl benzaldehyde reach >12.0 and 85.0 mol%, respectively, at the lower temperature of 440 ◦ C. Both series of cata-
The effect of p-tert butyl toluene concentration on the reaction properties was determined. At the conditions of reaction temperature at 460 ◦ C and the velocity at 10,000 h−1 , the results are given in Fig. 2. With the increasing of p-tert butyl toluene concentration, the conversion of p-tert butyl toluene increases in both catalysts, while the selectivity of p-tert butyl benzaldehyde of V-Cs-Cu catalyst attains a maximum at the concentration of 1.0 mol/m3 . However, there is little change in the selectivity of p-tert butyl benzaldehyde of V-Cs-Cu-Tl catalyst: this indicates that V-Cs-Cu-Tl catalyst shows more applicability than V-Cs-Cu
Table 1 Effect of reaction temperature on the V-Cs-Cu-Tl and V-Cs-Cu catalyst Catalyst
Temperature (◦ C)
Conversion (mol%)
Selectivity (mol%)
VCs0.2 Cu0.15 /TiO2 ·SiC
440 460 480
13.6 16.2 22.1
40.8 83.3 74.8
VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC
440 460
12.0 18.4
85.0 78.9
332
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
catalyst. In addition, if the p-tert butyl toluene concentration were quite high that would cause the excess dehydrogenation, which would result in a decreasing in selectivity. 3.4. Effect of O2 /p-tert butyl toluene ratio on catalyst activity At the reaction condition of temperature at 460 ◦ C and the space velocity at 10,000 h−1 , the effects of changing O2 /p-tert butyl toluene ratio on V-Cs-Cu and V-Cs-Cu-Tl catalysts are shown in Fig. 3. With the increasing of O2 /p-tert butyl toluene ratio up to 5, there are apparently a maximum in conversion of p-tert butyl toluene and selectivity of p-tert butyl benzaldehyde. Here is the proper reaction: Fig. 3. Effect of O2 /p-tert butyl toluene on the V-Cs-Cu and V-Cs-Cu-Tl catalyst.
If there were 1.0 mol p-tert butyl toluene converted into corresponding p-tert butyl benzaldehyde, the O2 /p-tert butyl toluene ratio should be to 1 theoretically. If the complete oxidation is as follows:
catalyst, it is worthwhile to test its stability. From Fig. 4, it can be seen clearly that, after 365 h reaction, the catalytic activity of V-Cs-Cu-Tl catalyst is stable. No significant changes in either the conversion or the selectivity could be observed during the stability experiment. 3.6. Catalytic activity of toluene
The O2 /p-tert butyl toluene ratio should rise to 15 theoretically. But in the vapor phase particle oxidation reaction, both mentioned reactions appear impossible. The aim of our study is to obtain the good results both in the suitable conversion and the highest selectivity. So the ratio of O2 /p-tert butyl toluene should be adjusted to a ratio between 1 and 15. From the results indicated in Fig. 3, on can conclude that an O2 /p-tert butyl toluene ratio of about 5 is appropriate for the purpose of obtaining good catalytic activity in both conversion and selectivity.
In order to investigate these two series of catalysts in other substances, the catalytic activity for toluene was also tested; the result is listed in Table 2. At the condition of velocity at 10,000 h−1 , reaction temperature at 460 ◦ C and toluene concentration at 1.2 mol/m3 ,
3.5. Test of catalyst stability When one studies the influence of space velocity, temperature, p-tert butyl toluene concentration and O2 /p-tert butyl toluene on the reaction properties of
Fig. 4. Effect of the reaction time on VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC catalyst.
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
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Table 2 The result of partial oxidation toluene to benzaldehyde Catalyst
Temperature (◦ C)
Conversion (mol%)
Selectivity (mol%)
VCs0.2 Cu0.15 /TiO2 ·SiC
440 460
1.2 2.2
20.3 40.7
VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC
440 460
2.0 3.1
13.6 17.2
the result of 40.7% selectivity of benzaldehyde is obtained, but the conversion is very poor: 2.2 mol% in V-Cs-Cu-Tl catalyst. But in V-Cs-Cu catalyst, the catalyst has hardly any effect on the catalytic activity from toluene to benzaldehyde. However, the result indicates that these two series of catalysts show relatively lower activity and selectivity in the vapor phase oxidation of toluene to benzaldehyde. Associated with the novel results of these two series catalysts obtained in the selective oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde, the above result can be explained by the different reaction mechanisms in different reaction substances such as p-tert butyl toluene and toluene on V-Cs-Cu and V-Cs-Cu-Tl catalyst. 3.7. Structure and surface characterization of catalyst Table 3 shows the results for the content of active compounds of V-Cs-Cu-Tl catalyst before and used after 365 h reaction. From Table 3, it can be seen that there is little difference in the content of the catalyst active compounds. Combined with stability testing results, these results show clearly that a catalyst of high stability is related with the fact that the contents of the catalyst active compounds remain almost unchanged after reaction. The surface oxygen species of the catalysts is another important factor. Fig. 5 shows the XPS of O 1s analysis of the samples before and after 365 h reaction. A survey of XPS spectra indicates the presence of O 1s
Fig. 5. O 1s spectra of VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC catalyst: (a) fresh catalyst; (b) spent catalyst.
Table 3 The content of active compounds in fresh and used catalyst Catalyst
Tl (ω/10−2 )
V (ω/10−2 )
Cs (ω/10−2 )
Cu (ω/10−2 )
VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC (before reaction) VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC (after reaction)
0.37 0.34
1.82 1.77
0.67 0.64
0.52 0.50
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M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
with two peaks in sample (a) and (b). One O 1s peak is near 532.00 eV and the other is near 529.00 eV. The ␣ peak is represented by sorption oxygen, while the  peak is represented by lattice oxygen [10]. They play different roles in the catalytic reaction. Obviously, the intensity of  peak decreases appreciably in sample (b) after 365 h reaction, while the ␣ peak is almost unchanged. Meanwhile, it can be seen that the binding energies of both the ␣ peak and the  peak decrease. This can be explained by assuming that the V-Cs-Cu-Tl catalyst has the ability to transform any adsorbed oxygen species rapidly into lattice oxygen, which is required for selective oxidation. Therefore, we propose that the lowering of the peak area of  peak should be attributed to the lattice oxygen. Fig. 5 indicates that the lattice oxygen has participated in this selective oxidation reaction. The result that the active oxygen numbers decrease also shows a good correlation with the high selectivity on V-Cs-Cu-Tl catalyst. On the basis of the XPS results, it can be concluded that the lattice oxygen is the main contributor in particle oxidation from p-tert butyl toluene to p-tert butyl benzaldehyde. However, the V-Cs-Cu-Tl catalyst which has been used at 440 ◦ C for 365 h is examined by X-ray diffraction analysis. No new phases are created in the catalyst. Thus, it is in accordance with the results of the content of active compounds test.
4. Conclusion From the results mentioned above, one can see that the V-Cs-Cu-Tl catalyst system has a significant effect on the reaction of partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde. After 365 h reaction, the series of catalysts still keep the high catalytic activity. On the basis of ICP and XPS results, we conclude that the high catalytic activity of V-Cs-Cu-Tl catalyst is related to the content of the catalyst active compounds remaining unchanged after a long reaction and to the function of the lattice oxygen. References [1] S.T. Noboru, N.H. Isao, U.T. Michio, T.K. Kazuhiro, N.S. Isao, United States Patent 5,136,104 (1992). [2] M. Ai, J. Catal. 40 (1975) 327. [3] V.D. Sokolovskii, Catal. Rev.-Sci. Eng. 32 (1990) 1. [4] Y. Moro-Oka, W. Ueda, Adv. Catal. 40 (1991) 23. [5] H.K. Matralis, C. Papadopoulou, C. Kordulis, et al., Appl. Catal. A 126 (1995) 365. [6] J. Miki, Y. Osada, T. Konoshi, Y. Taachibana, et al., Appl. Catal. A 137 (1996) 93. [7] Y.L. Andreikov, A.A. Lyapkin, V.L. Volkov, Petrol. Chem. U.S.S.R. 17 (1978) 155. [8] Y. Wu, Z. Wu, Chihua Xuebao (China) 9 (1983) 210. [9] D.A. Bulushev, L. Kiwi-Minsker, V.I. Zaikovskii, et al., Appl. Catal. A 202 (2000) 243. [10] J.L.G. Flerro, G. Tejuca, Appl. Surf. Sci. 27 (1987) 453.
Partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde Min Chen, Ren-Xian Zhou, Xiao-Ming Zheng∗ Institute of Catalysis, Zhejiang University, Xixi Campus, Hangzhou, PR China Received 8 January 2002; received in revised form 8 July 2002; accepted 21 September 2002
Abstract In this paper, studies of V-Cs-Cu and V-Cs-Cu-Tl catalysts used in the partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde are reported. The two series of catalysts are prepared by an impregnation method. The effects of temperature, space velocity, concentration, ratio of O2 /p-tert butyl toluene and the stability of catalysts are examined. The results indicate that both the catalysts show good catalytic activity in partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde; but the series of V-Cs-Cu-Tl catalysts exhibit more excellent catalytic activity. Under the reaction conditions of space velocity at 10,000 h−1 and temperature at 440 ◦ C, the conversion of p-tert butyl toluene can reach 12 mol% and the selectivity of p-tert butyl benzaldehyde is 85 mol%. In addition, the good stability in the series of V-Cs-Cu-Tl catalysts is another important character: the catalytic activity still remained significant after stability test for 365 h. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Partial oxidation reaction; p-Tert butyl benzaldehyde; Catalyst
1. Introduction p-Tert butyl benzaldehyde is mainly used in foodstuff, and medicine and also enjoys wide demand in the chemical industry [1,2]. Few studies of the vapor phase partial oxidation of p-tert butyl toluene to produce p-tert butyl benzaldehyde have been reported, because it is quite difficult to get a good correspondence between the conversion and selectivity [3–6]. There are three reaction paths in the vapor phase partial oxidation of p-tert butyl toluene. They are (1) side chain oxidation, (2) oxidative couping reaction, (3) direct oxidation to CO2 and H2 O. However, it still has been an open question has remained as to how to find an excellent catalyst system and how to im∗ Corresponding author. E-mail address: minchen [email protected] (X.-M. Zheng).
prove catalytic performance for the reaction. There are also many reports about the catalysts used in selective toluene oxidation, for example, the properties for oxidation of toluene to benzadehyde are improved by silver additions [7,8] and small addition of potassium to the V/Ti-oxide catalyst can increase the activity and selectivity [9]. Our aim is to identify a good catalyst for selective oxidation from p-tert butyl toluene to p-tert butyl benzaldehyde. Hence, the activities of V-Cs-Cu and V-CsCu-Tl series of catalysts are assigned to the reaction. From our study, it is evident that these two system catalysts are optimal to the reaction. However, selective oxidation of p-tert butyl toluene is seldom reported. This is a novel catalyst system and an effective application in selective oxidation of p-tert butyl toluene. The main aim of our work is to find a new catalyst system that has good catalytic properties in both
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 5 3 0 - 6
330
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
Scheme 1.
conversion and selectivity. In this paper, a new series of V-Cs-Cu-Tl/TiO2 ·SiC catalysts have been studied. V2 O5 is used as chief catalyst and the promoters Cs, Cu, Tl are used as active components. TiO2 and silicon carbide (commercial) are used as the support. Our study has been to focus on using an active route for vapor phase catalytic oxidation method to produce p-tert butyl benzaldehyde, the obtained results are optimum values and have good advantages in both conversion and selectivity. The reaction mechanism for the formation of p-tert butyl benzaldehyde from p-tert butyl toluene is shown in Scheme 1.
2. Experimental 2.1. Preparation of the catalyst The catalysts are prepared by an impregnation method. In this study, cupric nitrate Cu(NO3 )2 ·3H2 O, thallous nitrate (TlNO3 ) and cesium nitrate (CsNO3 ) are used for active compounds. Vanadylic citric acid solution is also added. The mixture was stirred enough, admixed with TiO2 ·SiC used as the supports, then dried by evaporation. After being dried at 250 ◦ C for 2 h, the samples were calcined at 600 ◦ C for 5 h and then pulverized into particles of 20–40 meshes. The composition of the catalysts in molar ratio is as follows: V/Cs is 0.2, V/Cu is 0.15 and V/Tl is n (n = 0.01–0.03). The catalysts are denoted as V-Cs-Cu and V-Cs-Cu-Tl. 2.2. Activity measurement The catalytic activity test for partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde is performed in a flow reactor containing a fixed-bed of 3 ml of catalyst. The temperature is varied from room temperature to 500 ◦ C. The total feed is 40 l/min (STP).
The temperature is measured both before and after the catalyst bed. The flow of p-tert butyl toluene is fed using a mass flow controller. Liquid p-tert butyl toluene was pumped into heated evaporators through which the steam of air is passing before reaching the preheater. The analysis of the reactor effluent is performed with an on-line SP-2304 model vapor chromatograph with a FID. 2.3. Structure and surface characterization The active compounds’ analysis of catalyst before and after reaction was performed on the inductively coupled plasma atomic emission spectrometry (ICP-AES) instrument type PLASMA-SEEC, made in the US by Leeman Company. The X-ray powder diffraction (XRD) patterns are collected on a Rigaku D/max-IIIB instrument using monochromatized Cu K␣ radiation (40 kV and 30 mA). The X-ray photoelectron spectroscopy (XPS) analysis is performed on a PHI-550 model ESCA/ SAM spectrometer equipped with an Al anode (hν = 1486.6 eV). The binding energies are referenced to the binding energy level of O 1s at 531.8 eV. This value is an average of the O 1s binding energy determined by referring to the C 1s spectra (284.6).
3. Results and discussion 3.1. Effect of space velocity on catalyst activity The results of changes in the space velocity over V-Cs-Cu and V-Cs-Cu-Tl catalysts are shown in Fig. 1. At the reaction temperature of 460 ◦ C, it can be seen that the selectivity of p-tert butyl benzaldehyde increases with the increasing of space velocity from 5000 to 10,000 h−1 , but the conversion of p-tert butyl toluene varies oppositely. At space velocity of 10,000 h−1 , the conversion of p-tert butyl toluene and the selectivity of p-tert butyl benzaldehyde both increase to the maximum valves in these two catalysts. However, by comparison of V-Cs-Cu and V-Cs-Cu-Tl catalyst systems, the latter obtains better selectivity than the former at the condition of the space velocity at 10,000−1 , but the selectivity decreases with the further increasing of space velocity. Such a result can be explained by the fact that reaction heat can be
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
Fig. 1. Effect of space velocity on the V-Cs-Cu and V-Cs-Cu-Tl catalyst.
taken away more quickly from the catalyst surface with the increasing of space velocity and then the deep oxidation is prevented.
331
Fig. 2. Effect of p-tert butyl toluene concentration on the V-Cs-Cu and V-Cs-Cu-Tl catalyst.
lysts show good catalytic activities, but in comparison with the V-Cs-Cu catalyst, the V-Cs-Cu-Tl catalyst is more active.
3.2. Effect of temperature on catalyst activity
3.3. Effect of p-tert butyl toluene concentration on catalyst activity
Here, we present the influence of temperature for the series of V-Cs-Cu and V-Cs-Cu-Tl catalysts. The results are given in Table 1. At the conditions of space velocity at 10,000 h−1 and the p-tert butyl toluene concentration at 0.9–1.0 mol/m3 , for V-Cs-Cu catalyst, the conversion of p-tert butyl toluene reaches 16.2 mol% and the selectivity of p-tert butyl benzaldehyde reaches 83.3 mol% at the temperature of 460 ◦ C. And for V-Cs-Cu-Tl catalyst, the conversion of p-tert butyl toluene and the selectivity of p-tert butyl benzaldehyde reach >12.0 and 85.0 mol%, respectively, at the lower temperature of 440 ◦ C. Both series of cata-
The effect of p-tert butyl toluene concentration on the reaction properties was determined. At the conditions of reaction temperature at 460 ◦ C and the velocity at 10,000 h−1 , the results are given in Fig. 2. With the increasing of p-tert butyl toluene concentration, the conversion of p-tert butyl toluene increases in both catalysts, while the selectivity of p-tert butyl benzaldehyde of V-Cs-Cu catalyst attains a maximum at the concentration of 1.0 mol/m3 . However, there is little change in the selectivity of p-tert butyl benzaldehyde of V-Cs-Cu-Tl catalyst: this indicates that V-Cs-Cu-Tl catalyst shows more applicability than V-Cs-Cu
Table 1 Effect of reaction temperature on the V-Cs-Cu-Tl and V-Cs-Cu catalyst Catalyst
Temperature (◦ C)
Conversion (mol%)
Selectivity (mol%)
VCs0.2 Cu0.15 /TiO2 ·SiC
440 460 480
13.6 16.2 22.1
40.8 83.3 74.8
VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC
440 460
12.0 18.4
85.0 78.9
332
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catalyst. In addition, if the p-tert butyl toluene concentration were quite high that would cause the excess dehydrogenation, which would result in a decreasing in selectivity. 3.4. Effect of O2 /p-tert butyl toluene ratio on catalyst activity At the reaction condition of temperature at 460 ◦ C and the space velocity at 10,000 h−1 , the effects of changing O2 /p-tert butyl toluene ratio on V-Cs-Cu and V-Cs-Cu-Tl catalysts are shown in Fig. 3. With the increasing of O2 /p-tert butyl toluene ratio up to 5, there are apparently a maximum in conversion of p-tert butyl toluene and selectivity of p-tert butyl benzaldehyde. Here is the proper reaction: Fig. 3. Effect of O2 /p-tert butyl toluene on the V-Cs-Cu and V-Cs-Cu-Tl catalyst.
If there were 1.0 mol p-tert butyl toluene converted into corresponding p-tert butyl benzaldehyde, the O2 /p-tert butyl toluene ratio should be to 1 theoretically. If the complete oxidation is as follows:
catalyst, it is worthwhile to test its stability. From Fig. 4, it can be seen clearly that, after 365 h reaction, the catalytic activity of V-Cs-Cu-Tl catalyst is stable. No significant changes in either the conversion or the selectivity could be observed during the stability experiment. 3.6. Catalytic activity of toluene
The O2 /p-tert butyl toluene ratio should rise to 15 theoretically. But in the vapor phase particle oxidation reaction, both mentioned reactions appear impossible. The aim of our study is to obtain the good results both in the suitable conversion and the highest selectivity. So the ratio of O2 /p-tert butyl toluene should be adjusted to a ratio between 1 and 15. From the results indicated in Fig. 3, on can conclude that an O2 /p-tert butyl toluene ratio of about 5 is appropriate for the purpose of obtaining good catalytic activity in both conversion and selectivity.
In order to investigate these two series of catalysts in other substances, the catalytic activity for toluene was also tested; the result is listed in Table 2. At the condition of velocity at 10,000 h−1 , reaction temperature at 460 ◦ C and toluene concentration at 1.2 mol/m3 ,
3.5. Test of catalyst stability When one studies the influence of space velocity, temperature, p-tert butyl toluene concentration and O2 /p-tert butyl toluene on the reaction properties of
Fig. 4. Effect of the reaction time on VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC catalyst.
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
333
Table 2 The result of partial oxidation toluene to benzaldehyde Catalyst
Temperature (◦ C)
Conversion (mol%)
Selectivity (mol%)
VCs0.2 Cu0.15 /TiO2 ·SiC
440 460
1.2 2.2
20.3 40.7
VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC
440 460
2.0 3.1
13.6 17.2
the result of 40.7% selectivity of benzaldehyde is obtained, but the conversion is very poor: 2.2 mol% in V-Cs-Cu-Tl catalyst. But in V-Cs-Cu catalyst, the catalyst has hardly any effect on the catalytic activity from toluene to benzaldehyde. However, the result indicates that these two series of catalysts show relatively lower activity and selectivity in the vapor phase oxidation of toluene to benzaldehyde. Associated with the novel results of these two series catalysts obtained in the selective oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde, the above result can be explained by the different reaction mechanisms in different reaction substances such as p-tert butyl toluene and toluene on V-Cs-Cu and V-Cs-Cu-Tl catalyst. 3.7. Structure and surface characterization of catalyst Table 3 shows the results for the content of active compounds of V-Cs-Cu-Tl catalyst before and used after 365 h reaction. From Table 3, it can be seen that there is little difference in the content of the catalyst active compounds. Combined with stability testing results, these results show clearly that a catalyst of high stability is related with the fact that the contents of the catalyst active compounds remain almost unchanged after reaction. The surface oxygen species of the catalysts is another important factor. Fig. 5 shows the XPS of O 1s analysis of the samples before and after 365 h reaction. A survey of XPS spectra indicates the presence of O 1s
Fig. 5. O 1s spectra of VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC catalyst: (a) fresh catalyst; (b) spent catalyst.
Table 3 The content of active compounds in fresh and used catalyst Catalyst
Tl (ω/10−2 )
V (ω/10−2 )
Cs (ω/10−2 )
Cu (ω/10−2 )
VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC (before reaction) VCs0.2 Cu0.15 Tl0.03 /TiO2 ·SiC (after reaction)
0.37 0.34
1.82 1.77
0.67 0.64
0.52 0.50
334
M. Chen et al. / Applied Catalysis A: General 242 (2003) 329–334
with two peaks in sample (a) and (b). One O 1s peak is near 532.00 eV and the other is near 529.00 eV. The ␣ peak is represented by sorption oxygen, while the  peak is represented by lattice oxygen [10]. They play different roles in the catalytic reaction. Obviously, the intensity of  peak decreases appreciably in sample (b) after 365 h reaction, while the ␣ peak is almost unchanged. Meanwhile, it can be seen that the binding energies of both the ␣ peak and the  peak decrease. This can be explained by assuming that the V-Cs-Cu-Tl catalyst has the ability to transform any adsorbed oxygen species rapidly into lattice oxygen, which is required for selective oxidation. Therefore, we propose that the lowering of the peak area of  peak should be attributed to the lattice oxygen. Fig. 5 indicates that the lattice oxygen has participated in this selective oxidation reaction. The result that the active oxygen numbers decrease also shows a good correlation with the high selectivity on V-Cs-Cu-Tl catalyst. On the basis of the XPS results, it can be concluded that the lattice oxygen is the main contributor in particle oxidation from p-tert butyl toluene to p-tert butyl benzaldehyde. However, the V-Cs-Cu-Tl catalyst which has been used at 440 ◦ C for 365 h is examined by X-ray diffraction analysis. No new phases are created in the catalyst. Thus, it is in accordance with the results of the content of active compounds test.
4. Conclusion From the results mentioned above, one can see that the V-Cs-Cu-Tl catalyst system has a significant effect on the reaction of partial oxidation of p-tert butyl toluene to p-tert butyl benzaldehyde. After 365 h reaction, the series of catalysts still keep the high catalytic activity. On the basis of ICP and XPS results, we conclude that the high catalytic activity of V-Cs-Cu-Tl catalyst is related to the content of the catalyst active compounds remaining unchanged after a long reaction and to the function of the lattice oxygen. References [1] S.T. Noboru, N.H. Isao, U.T. Michio, T.K. Kazuhiro, N.S. Isao, United States Patent 5,136,104 (1992). [2] M. Ai, J. Catal. 40 (1975) 327. [3] V.D. Sokolovskii, Catal. Rev.-Sci. Eng. 32 (1990) 1. [4] Y. Moro-Oka, W. Ueda, Adv. Catal. 40 (1991) 23. [5] H.K. Matralis, C. Papadopoulou, C. Kordulis, et al., Appl. Catal. A 126 (1995) 365. [6] J. Miki, Y. Osada, T. Konoshi, Y. Taachibana, et al., Appl. Catal. A 137 (1996) 93. [7] Y.L. Andreikov, A.A. Lyapkin, V.L. Volkov, Petrol. Chem. U.S.S.R. 17 (1978) 155. [8] Y. Wu, Z. Wu, Chihua Xuebao (China) 9 (1983) 210. [9] D.A. Bulushev, L. Kiwi-Minsker, V.I. Zaikovskii, et al., Appl. Catal. A 202 (2000) 243. [10] J.L.G. Flerro, G. Tejuca, Appl. Surf. Sci. 27 (1987) 453.