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Structure and Reactivity of Molybdenum Oxide Catalysts Supported on La2O3-Stabilized Tetragonal ZrO2 Kondakindi Rajender Reddy, Thallada Bhaskar, and Komandur V. R. Chary* Catalysis Division, Indian Institute of Chemical Technology, Hyderabad-500 007, India Received October 11, 2002. In Final Form: July 28, 2003 A series of molybdenum oxide catalysts with Mo loadings in the range 2.5-12.5 wt % Mo supported on La2O3-stabilized ZrO2 was prepared by impregnation. X-ray diffraction results of the catalysts showed the diffraction patterns of crystalline MoO3 from 10 wt % Mo. Dispersion of molybdena was determined by the oxygen chemisorption method, and it was found to decrease with an increase in Mo loading due to formation of MoO3 crystallites at higher Mo loadings. Temperature-programmed reduction results suggest that the reducibility is found to decrease with Mo loading as a result of the decrease in dispersion. The acidity of the catalysts was found to increase with Mo loading, and the activity of the catalysts also found to increase with the increase in Mo loading up to 7.5 wt % Mo loading. Laser Raman spectra of the catalysts show the Raman bands due to MoO3 from 7.5 wt % Mo at 819 and 997 cm-1. At lower Mo loadings, highly dispersed species were observed. The catalytic properties were evaluated during the ammoxidation of toluene to benzonitrile and were related to the oxygen chemisorption sites on the support surface.
Introduction Catalysts containing molybdenum oxide/sulfide as an active component have been extensively employed in the recent past for the partial oxidation of hydrocarbons and alcohols and also extensively used in hydroprocessing reactions for the petroleum industry.1-11 The catalytic properties of the active molybdenum oxide phase can be greatly influenced by the nature of supported oxide and the dispersion of the active component. Zirconia-based materials have attracted considerable interest in recent years for their potential use as catalyst supports.12-15 In the catalytic sense, they appear to have some advantages in the areas of practical application over traditional oxides, such as SiO2 and Al2O3.16-19 Zirconia is often included in heterogeneous systems, either as a promoter or more generally as a support. In CO hydrogenation for instance, zirconia addition to metal-supported catalysts induces a long-term stability.20,21 It also enhances both * Corresponding author. Tel.: +91-40-27193162. Fax: +91-4027160921. E-mail:
[email protected]. (1) Massoth, F. E. Adv. Catal. 1978, 27, 265. (2) Grange, P. Catal. Rev.sSci. Eng. 1980, 21, 135. (3) Reddy, B. M.; Chary, K. V. R.; Subrahmanyam, V. S.; Nag, N. K. J. Chem. Soc., Faraday Trans. I 1985, 81, 1655. (4) Chung, J. S.; Miranda, R.; Bennett, C. O. J. Catal. 1988, 144, 898. (5) Louis, C.; Tatibouet, J. M.; Che, M. J. Catal. 1988, 109, 354. (6) Masuoka, Y.; Niwa, M.; Murakami, Y. J. Phys. Chem. 1990, 94, 1477. (7) Zhang, W.; Desikan, A. N.; Oyama, S. T. J. Phys. Chem. 1995, 99, 14468. (8) Desikan, A. N.; Zhang, W.; Oyama, S. T. J. Catal. 1995, 157, 740. (9) Miyata, H.; Mukai, T.; Ono, T.; Kubokawa, Y. J. Chem. Soc., Faraday Trans. I 1988, 84, 4137. (10) Matsuura, I.; Oda, H.; Hoshida, K. Catal. Today 1993, 16, 547. (11) Quincy, R. B.; Houalla, M.; Procter, A.; Hercules, D. M. J. Phys. Chem. 1990, 94, 1520. (12) Chen, K.; Xie, S.; Iglesia, E.; Bell, A. T. J. Catal. 2000, 189, 421. (13) Calafat, A.; Avilan, L.; Aldana, J. Appl., Catal. A 2000, 201, 215. (14) Indovina, V. Catal. Today 1998, 41, 95. (15) Maity, S. K.; Rana, M. S.; Srinivas, B. N.; Bej, S. K.; Muralidhar, G.; Rao, T. S. R. P. J. Mol. Catal. A: Chem. 2000, 153, 121. (16) Silver, R. G.; Hou, C. J.; Ekerdt, J. G. J. Catal. 1989, 118, 400. (17) Jackson, N. B.; Ekerdt, J. G. J. Catal. 1990, 126, 31. (18) Pajonk, G. M.; Tanany, A. E. React. Kinet. Catal. Lett. 1992, 47, 167. (19) Benedetti, A. J. Catal. 1990, 122, 330. (20) Andersen, K. J.; Candia, R.; Rostrup-Neilsen, J. Ger. Offen. 1974, 760, 122.
the activity22-25 and the selectivity toward alcohol.26,27 Zirconia powders can be prepared from various methods; however, they sinter easily.28 This sintering occurs even during the thermal pretreatments to remove the chlorine of the starting material. The structures of the tetragonal and cubic phases of zirconia can, however, be kinetically stabilized at room temperature by the incorporation of various metal oxides, namely, MgO, CaO, La2O3, CeO2, and Y2O3. The amount of dopant required for the stabilization of either structure depends on the dopant and the method of preparation. Three processes are known to change pore structure and surface area of the material: (i) phase transformation, (ii) crystallite growth, and (iii) intercrystallite sintering. It is known that the process of crystallite growth is strongly inhibited by stabilizing the structure of the zirconia into the tetragonal form by alloying it with oxides such as Y2O3, La2O3, and CeO2.16,29-32 La2O3 and Y2O3 are known as structural stabilizers for zirconia.33 Silver et al.16 studied the CO hydrogenation over zirconium oxide and yttriastabilized zirconium oxide. More significant is the fact that additives can bring about a strong modification of the surface structure of zirconia,24,34-36 in which case substitution of Zr4+ with dopant cations results in a rise (21) Andersen, K. J.; Candia, R.; Rostrup-Neilsen, J. U.S. Patent 3,988,262, 1976. (22) Lisitsyn, A. S.; Kuznetsov, V. L.; Yermakov, Y. I. React. Kinet. Catal. Lett. 1980, 14, 445. (23) Bruce, L. A.; Matthews, J. F. Appl. Catal. 1982, 4, 353. (24) Bruce, L. A.; Hope, G. J.; Matthews, J. F. Appl. Catal. 1983, 8, 349. (25) Iizuka, T.; Tanaka, Y.; Tanabe, K. J. Mol. Catal. 1982, 17, 381. (26) Chang, C. D.; Perkins, P. D. U.S. Patent 4,440,668, 1984. (27) Ichikawa, M. Bull. Chem. Soc. Jpn. 1978, 51, 2268. (28) Szymansky, R. Ph.D. Thesis, Universite´ Catholique de Lyon, Lyon, France, 1985. (29) Singh, P.; Sainkar, S. R.; Kuber, M. V.; Gunjikar, V. G.; Shinde, R. F.; Date, S. K. Mater. Lett. 1990, 9, 65. (30) Bastide, B.; Odier, P.; Coutures, J. P. J. Am. Ceram. Soc. 1988, 71, 449. (31) Theunissen, G. S. A. M. Ph.D. Thesis, Twente University of Technology, Twente, The Netherlands, 1991. (32) Lange, F. F. J. Am. Ceram. Soc. 1986, 69, 240. (33) Mercera, P. D. L. Ph.D. Thesis, Twente Institute of Technology, Twente, The Netherlands, 1991. (34) Tanabe, K. Mater. Chem. Phys. 1987, 13, 347. (35) Prokhorenko, E. V. Kinet. Catal. 1988, 29, 702. (36) Gavalas, G. R. J. Catal. 1984, 88, 54.
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in anion vacancy concentrations and conductivity. Indeed, this is the basis of its redox properties and the catalytic use of stabilized zirconia. Doped zirconia has been shown to be active at a relatively low temperature as acid/base and a redox catalyst for isomerization, (de)hydration, aldol condensation, hydrogenation, oxidative coupling, and so forth.25,37,38 The ammoxidation of toluene is a very important reaction to produce benzonitrile. Benzonitrile is used as a precursor for resins and coatings. It is also used as an additive in fuels and fibers. Sanati et al.39 studied toluene oxidation and ammoxidation over various V2O5/ZrO2 catalysts. The selectivity toward the formation of benzonitrile and the conversion of toluene was found to increase over V2O5 supported on zirconia. Unsupported V2O5 and ZrO2 leads to the combustion of the reactants producing carbon oxides, which leads to rapid catalyst deactivation. In the present investigation, a systematic study was undertaken on the characterization of MoO3/La2O3-ZrO2 catalysts by various techniques such as BrunauerEmmett-Teller (BET) specific surface area, X-ray diffraction (XRD), laser Raman spectroscopy, oxygen chemisorption, temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, and pore size distribution measurements. The catalytic properties were evaluated during ammoxidation of toluene, and the results were discussed in relation with the structural properties. The aim of the present work is to investigate the properties of the La2O3-ZrO2 material to be used as a catalyst support for ammoxidation of toluene to benzonitrile and to relate the catalytic activity to oxygen chemisorption sites. Experimental Section Catalyst Preparation. A series of MoO3/La2O3-ZrO2 catalysts with Mo loadings in the range of 2.5-12.5 wt % Mo was prepared by impregnation with requisite ammonium heptamolybdate. After impregnation with ammonium heptamolybdate, the catalysts were dried at 383 K for 24 h and calcined in air at 773 K for 6 h. The La2O3-ZrO2 support (MEL Chemicals, U.K., 630/01, 6 mol % La2O3, BET specific surface area 72 m2/g) was calcined at 773 K for 6 h before use. Catalyst Characterization. XRD patterns were recorded on a Siemens D-5000 diffractometer using graphite-filtered Cu KR radiation. Oxygen chemisorption was measured by a static method using an all Pyrex glass system capable of attaining a vacuum of 10-6 Torr. Oxygen chemisorption measurements were performed on the catalysts prereduced in a flow of hydrogen (40 mL/min) at 623 K for 2 h and evacuated at the same temperature for 1 h. The oxygen chemisorption uptakes were determined as the difference of two successive adsorption isotherms measured at 623 K. The specific surface areas of the catalysts were determined by the BET method using nitrogen physisorption at 77 K taking 0.162 nm2 as the cross-sectional area of the N2 molecule. Pore size distribution measurements were carried out on AutoPore III (Micromeritics, U.S.A.) instrument by the mercury penetration method. TPR experiments were carried out on an AutoChem 2910 (Micromeritics, U.S.A.) instrument. Prior to TPR, the catalyst sample was pretreated by passing ultrahigh-purity (99.999%) helium (50 mL/min) at 673 K for 2 h. After pretreatment, the sample was cooled to room temperature and the carrier gas consisting of 5% hydrogen and balance argon (50 mL/min) was allowed to pass over the sample. The temperature was increased (37) Baiker, A.; Kilo, M.; Maciejewski, M.; Menzi, S.; Wokaun, A. Proc. 10th Int. Congr. Catal. Budapest 1992, 1257. (38) Sun, Y.; Sermon, P. A. J. Chem. Soc., Chem. Commun. 1993, 1242. (39) Sanati, M.; Andersson, A.; Wallenburg, L. R.; Rebenstorf, B. Appl. Catal. 1993, 106, 51.
Reddy et al. from ambient to 1273 K at a heating rate of 5 K/min, and the data were recorded simultaneously. The hydrogen consumption values and Tmax positions were calculated using GRAMS/32 software. More details concerning the TPR experiment were reported elsewhere.40 The TPD of NH3 experiments were conducted on the same AutoChem 2910 instrument that was used for TPR. Prior to TPD, the sample was pretreated by passage of high purity (99.999%) helium (50 mL/min) at 573 K for 1 h. After pretreatment, the sample was saturated with a 10% NH3 and balance He mixture (75 mL/min) at 353 K for 1 h and subsequently flushed at 378 K for 2 h to remove physisorbed ammonia. TPD analysis was carried out from ambient temperature to 1023 K at a heating rate of 10 K/min. The amount of NH3 desorbed was calculated using GRAMS/32 software. More details concerning TPD were reported elsewhere.40 The Raman spectra were recorded with a LabRam spectrometer (DILOR) equipped with a confocal microscope (Olympus) and a He-Ne laser. The slit width was set to 200 µm, resulting in a spectral resolution of 2 cm-1 with 1800 grating. The laser power of the He-Ne laser attached to the LabRam spectrometer was set at 0.14 mW by neutral density filters. Ammoxidation of Toluene. The ammoxidation of toluene to benzonitrile reaction was carried out in a down-flow, fixedbed, cylindrical Pyrex reactor with a 20-mm internal diameter. About 0.5 g of the catalyst with a 18-25 mesh size (0.5 mm) diluted with an equal amount of quartz grains of the same dimensions was charged into the reactor and supported on a glass wool bed. Prior to introducing the reactant toluene with a syringe pump (B-Braun perfusor, Germany) the catalyst was treated in air at 673 K for 2 h in air flow (40 mL/min), and then the reactor was fed with toluene, ammonia, and air in the mole ratio of 1:14:30. The preheated zone is filled with quartz glass particles heated to 423 K for adequate vaporization of the liquid feed. The reaction products were analyzed by a HP 6890 gas chromatograph. The only byproducts formed during the reaction are carbon oxides and were determined by HP-5973 gas chromatography-mass spectrometry using a porapak Q column.
Results and Discussion XRD patterns of various MoO3/La2O3-ZrO2 samples are shown in Figure 1. From the figure, it is very clear that in all the samples the zirconia exists in the tetragonal phase with the d spacings at 2.96, 2.54, 2.60, 1.83, 1.81, 1.57, and 1.54 Å (the corresponding 2θ values are 30.16, 35.3, 34.5, 49.8, 50.4, 58.7, and 60°, respectively). The reflections corresponding to crystalline MoO3 were seen in the 10 wt % Mo sample and above (d ) 3.26 and 3.46 Å), and these reflections were shown with closed circles in Figure 1. In all other samples, molybdenum oxide is present in a “highly dispersed state and may be present in a two-dimensional state”. However, at lower Mo loadings ( MoO2, while the reverse sequence is observed for the formation of carbon oxides. They also reported that the rates of formation of benzonitrile are more than a factor of 2 higher in the absence of gaseous oxygen. Table 5 shows the activity of various supported molybdenum oxides in comparison to the present results. The results presented in Table 5 are from our laboratory, and more details about preparation and reactivity are found elsewhere.42,67 The conversion of toluene is found to be more in the case of the MoO3/La2O3-ZrO2 catalyst compared to those of the TiO2- and Nb2O5-supported catalysts, and the selectivity is more in the case of the MoO3/TiO2 catalyst. To find the relation between the rate of toluene conversion and Mo content, a plot of the turnover frequency (66) Andersson, A.; Hansen, S. Catal. Lett. 1988, 1, 377. (67) Chary, K. V. R.; Reddy, K. R.; Bhaskar, T.; Sagar, G. V. Green Chem. 2002, 4, 206.
Figure 7. Ammoxidation of toluene over various Mo/La2O3ZrO2 catalysts. Catalyst weight, 0.5 g; reaction temperature, 673 K; feed rate, 1 mL/h. Toluene, ammonia, and air are in the mole ratio of 1:14:30.
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already existing monolayer, and this post-monolayer phase does not appreciably chemisorb oxygen (Figure 2). In the perspective of the previously described background, the correlation shown here indicates that the catalytic functionality of the dispersed molybdena phase supported on La2O3-ZrO2, which is responsible for the ammoxidation of toluene to benzonitrile, is located on a patchy-monolayer phase, and this functionality can be titrated by the oxygen chemisorption method reported in this work. Conclusions
Figure 8. Relation between TOF and surface Mo content. Table 5. Catalytic Activity of Various Supported Molybdenum Oxide Catalystsa catalyst
% conversion
% selectivity
7.5% MoO3/La2O3-ZrO2 7.5% MoO3/Nb2O567 7.5% MoO3/TiO242
65 57.5 58
68 69 76
a
Similar conditions were employed as in Figure 7.
(TOF) versus the surface Mo content is shown in Figure 8, where the TOF is defined as the number of toluene molecules converted per second per site. The TOF was found to be almost constant (≈6.8 × 10-3 s-1) up to 7.5 wt % Mo and decreased at higher Mo loadings. Up to 7.5 wt % Mo loading, persite activity (constant TOF) is constant with the increase in surface Mo sites. The decrease in the TOF beyond 7.5 wt % Mo loading indicates the presence of different molybdena species and is due to MoO3 particles, but not enough so to attain the bulk nature. The absence of direct correlation between TOF and Mo loading may be due to the fact that, in addition to oxygen adsorption, ammonia adsorption may also be responsible for this reaction. Oxygen is chemisorbed selectively on coordinatively unsaturated sites generated upon reduction, having a particular coordination environment. These sites are determined by oxygen chemisorption and are located on a highly dispersed molybdenum phase, which is formed only at low molybdenum loadings and remains as a “patchy monolayer” on the support surface. At higher molybdena loadings, a second phase is formed, in addition to the
La2O3-stabilized zirconia is found to be a good support material for supporting MoO3 for vapor phase ammoxidation of toluene to benzonitrile. XRD results of the catalysts showed the presence of reflections due to MoO3 at higher Mo loadings (from 10 wt % Mo). Dispersion of molybdena was found to decrease with the Mo loading, and this is due to formation of MoO3 crystallites at higher Mo loadings. At lower Mo loadings, the dispersion approached the 100% dispersion line and gradually decreased with further increase in Mo loading. Laser Raman spectroscopy reveals that MoO3 bands appeared from 7.5 wt % Mo. At lower Mo loadings, highly dispersed species are observed and the band positions are found to shift to the high-frequency side as a result of the formation of octahedral polymolybdate species and crystalline MoO3 at higher Mo loadings. TPR results suggested that the reduction occurs in two stages and the Tmax positions of both the peaks were found to shift to the highertemperature side, which clearly indicates that the dispersion is decreasing with the increase of Mo loading. The acidity of the catalysts was also found to increase up to 10 wt % Mo, and the acidity of the 10 and 12.5 wt % samples was found to be same, where the activity of these two catalysts was also found to be same. The activity of the catalysts was found to increase up to 7.5 wt % and remained constant at higher loadings in lines similar to oxygen chemisorption results and acidity measurements. Acknowledgment. We thank Dr. Gerhard Mestl, Fritz-Haber Institute, Berlin for providing laser Raman spectra of our samples. We also thank MEL Chemicals for providing the La2O3-ZrO2 gift sample. K.R.R. thanks Council of Scientific and Industrial Research (CSIR) for Senior Research Fellowship (SRF). LA020843Z
