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A novel selective oxidation catalyst: ultrafine complex molybdenum based oxide particles
3rd World Congress on Oxidation Catalysis R.K. Grasselli, S.T. Oyama, A.M. Gaffney and J.E. Lyons (Editors) 9 1997 Elsevier Science B.V. All fights reserved.
903
A novel selective oxidation catalyst: ultrafine complex molybdenum based oxide particles Y. Fan a, W. Kuang a , W. Zhang a'b and Yi Chen a aDepartment of Chemistry, Nanjing University, Nanjing 210093, China bNational Normal College of Inner Mongolia, Tongliao 028043, China
1. INTRODUCTION C-H activation leading to selective oxidation is one of the most challenging problems in terms of surface science and catalysis. In the past decades, many kinds of metal oxides especially Mo-based and V-based oxides have been widely used as selective oxidation catalysts, and the studies on structure and catalytic properties of these oxides have demonstrated that the nature of oxygen species in the oxides is one of the most important parameters influencing catalytic selectivity. For the oxidation of olefins and aromatics the nucleophilic lattice oxygen ions (0 2) are responsible for the selective oxidation, while electrophilic oxygen species (O2,O') may attack C=C and benzene ring because of higher electron density in these regions, which leads to C-C bond cleavage and complete oxidation[ 1-2]. In order to increase the reactivity of lattice oxygen species and to improve the catalytic selectivity, great efforts have been made to modify the state of lattice oxygen species by adding some promoters to these oxides[3]. It has been found that Bi, Fe, Sn, W and rare earth oxides are the effective promoters of the molybdenum based and vanadium based oxide catalysts for selective oxidation of toluene to benzaldehyde[4-9]. Very recently, ultrafine metal oxides have attracted much research interests in terms of materials science and heterogeneous catalysis[10-12]. These new catalytic materials are expected to have unique catalytic properties because of their nano-scale particle sizes. In this work, a novel catalyst for selective oxidation of toluene to benzaldehyde, i.e. ultrafine complex molybdenum based oxide particles, has been developed. It has been found that the reactivity of lattice oxygen ions can be improved by decreasing the oxide particle size to nano-scale and that the ultrafine oxide particles exhibit unique catalytic properties for selective oxidation. Our results have revealed that the ultrafine complex oxide particles are potentially new catalytic materials for selective oxidation reactions.
* Research project supported by the National Natural Science Foundation of China and SINOPEC
904 2. EXPERIMENTAL
2.1 Catalyst preparation The Mo-Ce(sg) oxide particle samples were prepared by a sol-gel method in which the mixed aqueous solutions containing Ce(NO3)y6H20, (NH4)6Mo70246H20 and citric acid with (Ce+Mo)/citric acid =3(tool/tool) and different atomic ratios of cerium to molybdenum were first kept in a water bath at 80~ until gelation was completed, and then the as-prepared gels were dried at 120 ~ for 4h and calcined in air at 400 ~ for 4h. The mono-component MoO3 and Ce02 oxides were prepared by same procedures but without adding cerium nitrate or ammonium molybdate to the mixed solution. For comparison, the conventional coprecipitation method was also used to prepare Mo-Ce(cp) oxide, in which cerium nitrate aqueous solution was mixed with ammonium molybdate aqueous solution, and the precipitates formed were dried and calcined. 2.2 Catalytic oxidation of toluene The toluene oxidation reaction was used as a probe to study the catalytic properties of the Mo-Ce complex oxides. The as-prepared oxides were introduced into a U-type quartz fixed bed microreactor and their catalytic properties for selective oxidation of toluene to benzaldehyde were evaluated under the reaction conditions of 0.1MPa, 400 ~ air/toluene = 9 (vol/vol), F/W =1900 ml/h.g.cat. The reaction products were analyzed by an on-line gas chromatography. Under the above reaction conditions, the main products were CO, CO2, H20 and benzaldehyde. 2.3 Characterization The morphology, particle size and structure of the oxides were determined by using transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). BET surface area of the samples were measured by using Micromeritics ASAP-2000 instrument. The interaction of Ce with Mo and its effect on the nature of active oxygen species in the complex oxides were studied by using temperature-programmed reduction(TPR) and laser Raman spectroscopy (LRS).
3. RESULTS AND DISCUSSION The morphology of Mo-Ce oxide samples (Ce/Mo atomic ratio = 2/3) prepared by different methods are presented in Figures 1 and 2. It can been seen that the particle size of Mo-Ce(sg) was in the range of 20-80 nm, while that of Mo-Ce(cp) was higher than 500nm. These results have shown that the Mo-Ce oxides prepared by the sol-gel method are actually the ultrafine oxide particles(<100nm). The BET surface areas for these two samples were 19m2/g MoCe(sg) and 3m2/g Mo-Ce(cp), respectively. XRD patterns of the above two samples revealed that molybdenum oxide and cerium oxide in the Mo-Ce complex oxide samples formed a solid solution with sheelite-type structure of Ce2(MoO4)3. On increasing Ce/Mo atomic ratio to higher than 2/3, however, Ce2(MoO4)3and CeO2 were found to coexist in the complex oxide samples. Oxidation of toluene was employed to evaluate the catalytic activity and selectivity of the Mo-Ce complex oxides. The particle size, surface area and structure of ultrafine complex
905
Figure 1. Morphology of Mo-Ce(sg) oxide prepared by sol-gel method
Figure 2. Morphology of Mo-Ce(cp) oxide prepared by coprecipitation method
Table 1
The catalytic properties of complex Mo-Ce oxide catalysts (Ce/Mo atomic ratio = 1) for oxidation of toluene MoO3 Particle size(m) >200 Surface area(m2/g) 5.1 Conversion of toluene (mol%) 24.0 Benzaldehyde selectivity (%) 6.0 Benzaldehyde yield (10-6mol/m 2 s) 0.4
Ce02 10-20 41.0 54.8 0.0 0.0
Mo-Ce(cp) >200 12.1 35.5 16.0 0.6
Mo-Ce(sg) 20-40 19.0 34.0 37.0 0.9
Mo-Ce oxide particles were not changed. This suggested that the ultrafine complex oxide particles were quite stable during the catalytic oxidation. After reaction for 7h, the catalytic oxidation reached a steady state. The catalytic properties of CeO2, MoO3 and two Mo-Ce oxides catalysts (Ce/Mo atomic ratio =1) are given in Table 1. It can be seen from these results that under the same reaction conditions, the toluene conversion on the above oxides catalysts showed the order as CeO2 >Mo-Ce(cp)-~Mo-Ce(sg) >MOO3. The oxidation products on mono-component CeO2 catalyst were CO,CO2 and H20 but without benzaldehyde or other selective oxidation products, indicating that CeO2 is an active component for complete oxidation of toluene. By adding ceria to MOO3, however, the selectivity to benzaldehyde was remarkably improved, so that the complex oxides showed higher catalytic selectivity to benzaldehyde than the mono-component MoO3 catalyst. Interesting, the conversions of toluene on both complex Mo-Ce oxides were very similar, but the benzaldehyde
906
Cl
b .I
300
I
500
1
700 Temperature, ~
I
900 ~.
t
1100
Figure 3. TPR profiles of the mono-component MoO3 and the complex Mo-Ce oxide. (a) MoO3, (b) complex Mo-Ce oxide.
selectivity of the ultrafine particle catalyst was much higher than that of the larger particles. These results have revealed that ultrafine oxide particle catalysts have unique catalytic properties for the selective oxidation of toluene to benzaldehyde. In particular, the specific activity (benzaldehyde yield) of the ultrafine complex oxide particles was higher than those of the larger complex oxide particles catalyst and the mono-component MoO3 catalyst, which
907 can not be explained by the effect of mere particle size. Apparently, the differences in the nature of active species in the oxide catalysts should be taken into consideration. As pointed out by Haber[1-2], the lattice oxygen ions in molybdenum oxides are the main active species for selective oxidation of lower olefins to aldehydes. The state of lattice oxygen species in the mono-component MoO3 oxide and Mo-Ce complex oxides were studied by using TPR and LRS. As can be seen in Figure 3, which shows the TPR profiles of MoO3 and Mo-Ce complex oxide, the hydrogen consumption peaks for MoO3 appeared at 670, 755 and 990~ By adding ceria to MOO3, the hydrogen consumption peaks shift to lower temperatures: 510, 715 and 860 ~ These results suggest that due to interaction of Ce with Mo in the complex oxides, the molybdenum oxides are easier to reduce to lower valance. By using IR spectroscopy, Jonson et al.[13] showed that the vibrational frequency of Mo=O in MoO3 was 995cmq. Figure 4 shows the Raman bands of Mo=O species in the complex MoCe oxides. For the complex Mo-Ce(cp) oxide, the vibrational frequency of Mo=O was 953cmq, which is lower than that of Mo=O in MOO3. The red shift of vibrational frequency indicates a weaker bonding between Mo=O in the complex Mo-Ce oxide. This is consistent
953
1100
1000
900
850
Rarnan shift, cmq
Figure 4. Laser raman spectra of Mo=O species in the complex Mo-Ce oxide catalysts prepared by different methods. (a) Mo-Ce(cp), (b) Mo-Ce (sg).
908 with the TPR result. As the lattice oxygen species in the complex Mo-Ce oxides have higher mobility, their reactivity for selective oxidation of toluene are increased. This can account for the higher specific activity of complex Mo-Ce oxide catalysts for oxidation of toluene to benzaldehyde. Moreover, it can be seen from Figure 4 that the vibrational frequency of Mo=O species in Mo-Ce(sg) was much lower than in Mo-Ce(cp), indicating that Mo=O chemical bonding in the ultrafine oxide particles was even weaker and the lattice oxygen ions in ultrafine complex oxide particles have a higher mobility. The higher reactivity of lattice oxygen in the matrix of ultra:fine Mo-Ce complex oxides can account for the reason that ultrafine complex oxide particles exhibit unique catalytic properties for selective oxidation of toluene to benzaldehyde. Our results have clearly confirmed the reactivity of lattice oxygen ions can be improved by decreasing the oxide particle size to nano-scale. These results suggest that the ultrafine complex oxide particles are potentially new catalytic materials for selective oxidation reactions. Reference
1. J. Haber, in Proceedings of the 8th Intemational Congress on Catalysis, July, 1984, Berlin (Wes0, Vol. 1, Verlag Chemie, 1984, P. 85. 2. J. Haber, in Heterogeneous Hydrocarbon Oxidation, ACS Symposium Series 638, American Chemistry Society, 1996, P. 20. 3. Y. Moro-oka and W. Ueda, Adv. Catal., 40 (1994) 233. 4. K. Van der Wide and P. J. Van den Berg, J. Catal., 39 (1975) 473. 5. J. Buiten, J. Catal., 21 (1968) 188. 6. S. Tan, Y. Moro-oka and A. Ozaki, J. Catal., 17 (1970) 125. 7. M. Ai and T. Ikama, J. Catal., 40 (1975) 203. 8. K.A. Reddy and L. K. Doraiswamy, Chem. Eng. Sci., 24 (1969) 1415. 9. Z. Yan and S. Lars T. Andersson, J. Catal., 131 (1991) 350. 10. C. Simon, R. Bredesen, H. Gronadal, A. G. hustoit and E. Tangstad, J. Mater. Sci., 30(1995) 5554. 11. K. Maede, F. Mizukami, M. Watanabe, N. Arai, S. Niwa, M. Toba and K. Shimizu, J. Mater. Sci. Lett., 9 (1990) 522. 12. J. Y. Guo, F. Gitzhofer and M. I. Boulos, J. Mater. Sci., 30 (1995) 5589. 13. B. Jonson, R. Larsson and B. Rebenstoff, J. Catal., 102 (1986) 29.
903
A novel selective oxidation catalyst: ultrafine complex molybdenum based oxide particles Y. Fan a, W. Kuang a , W. Zhang a'b and Yi Chen a aDepartment of Chemistry, Nanjing University, Nanjing 210093, China bNational Normal College of Inner Mongolia, Tongliao 028043, China
1. INTRODUCTION C-H activation leading to selective oxidation is one of the most challenging problems in terms of surface science and catalysis. In the past decades, many kinds of metal oxides especially Mo-based and V-based oxides have been widely used as selective oxidation catalysts, and the studies on structure and catalytic properties of these oxides have demonstrated that the nature of oxygen species in the oxides is one of the most important parameters influencing catalytic selectivity. For the oxidation of olefins and aromatics the nucleophilic lattice oxygen ions (0 2) are responsible for the selective oxidation, while electrophilic oxygen species (O2,O') may attack C=C and benzene ring because of higher electron density in these regions, which leads to C-C bond cleavage and complete oxidation[ 1-2]. In order to increase the reactivity of lattice oxygen species and to improve the catalytic selectivity, great efforts have been made to modify the state of lattice oxygen species by adding some promoters to these oxides[3]. It has been found that Bi, Fe, Sn, W and rare earth oxides are the effective promoters of the molybdenum based and vanadium based oxide catalysts for selective oxidation of toluene to benzaldehyde[4-9]. Very recently, ultrafine metal oxides have attracted much research interests in terms of materials science and heterogeneous catalysis[10-12]. These new catalytic materials are expected to have unique catalytic properties because of their nano-scale particle sizes. In this work, a novel catalyst for selective oxidation of toluene to benzaldehyde, i.e. ultrafine complex molybdenum based oxide particles, has been developed. It has been found that the reactivity of lattice oxygen ions can be improved by decreasing the oxide particle size to nano-scale and that the ultrafine oxide particles exhibit unique catalytic properties for selective oxidation. Our results have revealed that the ultrafine complex oxide particles are potentially new catalytic materials for selective oxidation reactions.
* Research project supported by the National Natural Science Foundation of China and SINOPEC
904 2. EXPERIMENTAL
2.1 Catalyst preparation The Mo-Ce(sg) oxide particle samples were prepared by a sol-gel method in which the mixed aqueous solutions containing Ce(NO3)y6H20, (NH4)6Mo70246H20 and citric acid with (Ce+Mo)/citric acid =3(tool/tool) and different atomic ratios of cerium to molybdenum were first kept in a water bath at 80~ until gelation was completed, and then the as-prepared gels were dried at 120 ~ for 4h and calcined in air at 400 ~ for 4h. The mono-component MoO3 and Ce02 oxides were prepared by same procedures but without adding cerium nitrate or ammonium molybdate to the mixed solution. For comparison, the conventional coprecipitation method was also used to prepare Mo-Ce(cp) oxide, in which cerium nitrate aqueous solution was mixed with ammonium molybdate aqueous solution, and the precipitates formed were dried and calcined. 2.2 Catalytic oxidation of toluene The toluene oxidation reaction was used as a probe to study the catalytic properties of the Mo-Ce complex oxides. The as-prepared oxides were introduced into a U-type quartz fixed bed microreactor and their catalytic properties for selective oxidation of toluene to benzaldehyde were evaluated under the reaction conditions of 0.1MPa, 400 ~ air/toluene = 9 (vol/vol), F/W =1900 ml/h.g.cat. The reaction products were analyzed by an on-line gas chromatography. Under the above reaction conditions, the main products were CO, CO2, H20 and benzaldehyde. 2.3 Characterization The morphology, particle size and structure of the oxides were determined by using transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). BET surface area of the samples were measured by using Micromeritics ASAP-2000 instrument. The interaction of Ce with Mo and its effect on the nature of active oxygen species in the complex oxides were studied by using temperature-programmed reduction(TPR) and laser Raman spectroscopy (LRS).
3. RESULTS AND DISCUSSION The morphology of Mo-Ce oxide samples (Ce/Mo atomic ratio = 2/3) prepared by different methods are presented in Figures 1 and 2. It can been seen that the particle size of Mo-Ce(sg) was in the range of 20-80 nm, while that of Mo-Ce(cp) was higher than 500nm. These results have shown that the Mo-Ce oxides prepared by the sol-gel method are actually the ultrafine oxide particles(<100nm). The BET surface areas for these two samples were 19m2/g MoCe(sg) and 3m2/g Mo-Ce(cp), respectively. XRD patterns of the above two samples revealed that molybdenum oxide and cerium oxide in the Mo-Ce complex oxide samples formed a solid solution with sheelite-type structure of Ce2(MoO4)3. On increasing Ce/Mo atomic ratio to higher than 2/3, however, Ce2(MoO4)3and CeO2 were found to coexist in the complex oxide samples. Oxidation of toluene was employed to evaluate the catalytic activity and selectivity of the Mo-Ce complex oxides. The particle size, surface area and structure of ultrafine complex
905
Figure 1. Morphology of Mo-Ce(sg) oxide prepared by sol-gel method
Figure 2. Morphology of Mo-Ce(cp) oxide prepared by coprecipitation method
Table 1
The catalytic properties of complex Mo-Ce oxide catalysts (Ce/Mo atomic ratio = 1) for oxidation of toluene MoO3 Particle size(m) >200 Surface area(m2/g) 5.1 Conversion of toluene (mol%) 24.0 Benzaldehyde selectivity (%) 6.0 Benzaldehyde yield (10-6mol/m 2 s) 0.4
Ce02 10-20 41.0 54.8 0.0 0.0
Mo-Ce(cp) >200 12.1 35.5 16.0 0.6
Mo-Ce(sg) 20-40 19.0 34.0 37.0 0.9
Mo-Ce oxide particles were not changed. This suggested that the ultrafine complex oxide particles were quite stable during the catalytic oxidation. After reaction for 7h, the catalytic oxidation reached a steady state. The catalytic properties of CeO2, MoO3 and two Mo-Ce oxides catalysts (Ce/Mo atomic ratio =1) are given in Table 1. It can be seen from these results that under the same reaction conditions, the toluene conversion on the above oxides catalysts showed the order as CeO2 >Mo-Ce(cp)-~Mo-Ce(sg) >MOO3. The oxidation products on mono-component CeO2 catalyst were CO,CO2 and H20 but without benzaldehyde or other selective oxidation products, indicating that CeO2 is an active component for complete oxidation of toluene. By adding ceria to MOO3, however, the selectivity to benzaldehyde was remarkably improved, so that the complex oxides showed higher catalytic selectivity to benzaldehyde than the mono-component MoO3 catalyst. Interesting, the conversions of toluene on both complex Mo-Ce oxides were very similar, but the benzaldehyde
906
Cl
b .I
300
I
500
1
700 Temperature, ~
I
900 ~.
t
1100
Figure 3. TPR profiles of the mono-component MoO3 and the complex Mo-Ce oxide. (a) MoO3, (b) complex Mo-Ce oxide.
selectivity of the ultrafine particle catalyst was much higher than that of the larger particles. These results have revealed that ultrafine oxide particle catalysts have unique catalytic properties for the selective oxidation of toluene to benzaldehyde. In particular, the specific activity (benzaldehyde yield) of the ultrafine complex oxide particles was higher than those of the larger complex oxide particles catalyst and the mono-component MoO3 catalyst, which
907 can not be explained by the effect of mere particle size. Apparently, the differences in the nature of active species in the oxide catalysts should be taken into consideration. As pointed out by Haber[1-2], the lattice oxygen ions in molybdenum oxides are the main active species for selective oxidation of lower olefins to aldehydes. The state of lattice oxygen species in the mono-component MoO3 oxide and Mo-Ce complex oxides were studied by using TPR and LRS. As can be seen in Figure 3, which shows the TPR profiles of MoO3 and Mo-Ce complex oxide, the hydrogen consumption peaks for MoO3 appeared at 670, 755 and 990~ By adding ceria to MOO3, the hydrogen consumption peaks shift to lower temperatures: 510, 715 and 860 ~ These results suggest that due to interaction of Ce with Mo in the complex oxides, the molybdenum oxides are easier to reduce to lower valance. By using IR spectroscopy, Jonson et al.[13] showed that the vibrational frequency of Mo=O in MoO3 was 995cmq. Figure 4 shows the Raman bands of Mo=O species in the complex MoCe oxides. For the complex Mo-Ce(cp) oxide, the vibrational frequency of Mo=O was 953cmq, which is lower than that of Mo=O in MOO3. The red shift of vibrational frequency indicates a weaker bonding between Mo=O in the complex Mo-Ce oxide. This is consistent
953
1100
1000
900
850
Rarnan shift, cmq
Figure 4. Laser raman spectra of Mo=O species in the complex Mo-Ce oxide catalysts prepared by different methods. (a) Mo-Ce(cp), (b) Mo-Ce (sg).
908 with the TPR result. As the lattice oxygen species in the complex Mo-Ce oxides have higher mobility, their reactivity for selective oxidation of toluene are increased. This can account for the higher specific activity of complex Mo-Ce oxide catalysts for oxidation of toluene to benzaldehyde. Moreover, it can be seen from Figure 4 that the vibrational frequency of Mo=O species in Mo-Ce(sg) was much lower than in Mo-Ce(cp), indicating that Mo=O chemical bonding in the ultrafine oxide particles was even weaker and the lattice oxygen ions in ultrafine complex oxide particles have a higher mobility. The higher reactivity of lattice oxygen in the matrix of ultra:fine Mo-Ce complex oxides can account for the reason that ultrafine complex oxide particles exhibit unique catalytic properties for selective oxidation of toluene to benzaldehyde. Our results have clearly confirmed the reactivity of lattice oxygen ions can be improved by decreasing the oxide particle size to nano-scale. These results suggest that the ultrafine complex oxide particles are potentially new catalytic materials for selective oxidation reactions. Reference
1. J. Haber, in Proceedings of the 8th Intemational Congress on Catalysis, July, 1984, Berlin (Wes0, Vol. 1, Verlag Chemie, 1984, P. 85. 2. J. Haber, in Heterogeneous Hydrocarbon Oxidation, ACS Symposium Series 638, American Chemistry Society, 1996, P. 20. 3. Y. Moro-oka and W. Ueda, Adv. Catal., 40 (1994) 233. 4. K. Van der Wide and P. J. Van den Berg, J. Catal., 39 (1975) 473. 5. J. Buiten, J. Catal., 21 (1968) 188. 6. S. Tan, Y. Moro-oka and A. Ozaki, J. Catal., 17 (1970) 125. 7. M. Ai and T. Ikama, J. Catal., 40 (1975) 203. 8. K.A. Reddy and L. K. Doraiswamy, Chem. Eng. Sci., 24 (1969) 1415. 9. Z. Yan and S. Lars T. Andersson, J. Catal., 131 (1991) 350. 10. C. Simon, R. Bredesen, H. Gronadal, A. G. hustoit and E. Tangstad, J. Mater. Sci., 30(1995) 5554. 11. K. Maede, F. Mizukami, M. Watanabe, N. Arai, S. Niwa, M. Toba and K. Shimizu, J. Mater. Sci. Lett., 9 (1990) 522. 12. J. Y. Guo, F. Gitzhofer and M. I. Boulos, J. Mater. Sci., 30 (1995) 5589. 13. B. Jonson, R. Larsson and B. Rebenstoff, J. Catal., 102 (1986) 29.