Synthesis, Characterization, and Catalytic Properties of Vanadium

A series of V2O5/AlPO4 catalysts with vanadia loadings ranging from 2.5 to 20 wt % was prepared and characterized by X-ray diffraction (XRD), Fourier ...
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Langmuir 2003, 19, 4548-4554

Articles Synthesis, Characterization, and Catalytic Properties of Vanadium Oxide Catalysts Supported on AlPO4† Komandur V. R. Chary,* Gurram Kishan, Kanaparthi Ramesh, Chinthala Praveen Kumar, and Guggilla Vidyasagar Catalysis Division, Indian Institute of Chemical Technology, Hyderabad 500 007, India Received April 26, 2002. In Final Form: January 15, 2003 A series of V2O5/AlPO4 catalysts with vanadia loadings ranging from 2.5 to 20 wt % was prepared and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy, electron spin resonance spectroscopy, temperature-programmed reduction (TPR), temperature-programmed desorption (TPD) of NH3, BET surface area, and oxygen chemisorption methods. The catalytic properties were evaluated for the vapor phase ammoxidation of 3-picoline to nicotinonitrile. TPR results indicated that the appearance of a single reduction peak corresponds to V5+ to V3+. The XRD results suggest that, at low vanadia loadings AlPO4 is found to be amorphous. However, at V2O5 loading of 7.38 wt % and above, AlPO4 exist as R-cristobalite form in addition to V2O5. TPD results show that the acidity increases with vanadia loading and decreases at 7.38% and above loadings. The ESR spectra obtained under ambient conditions for the samples reduced at 640 K show the presence of V4+ in axial symmetry. Dispersion of vanadia was determined by oxygen chemisorption at 640 K and by the static method on the samples prereduced at the same temperature. The oxygen uptake measured at 640 K increases with increase in vanadia loading, whereas dispersion of vanadia was decreased. The decrease in dispersion of vanadia was discussed in terms of formation of crystalline forms of V2O5 and AlPO4. The ammoxidation activity and selectivity of nicotinonitrile formation increase with vanadia loading up to 9.5 wt % and did not change appreciably at higher loadings. The catalytic properties during ammoxidation are related to the oxygen chemisorption sites.

Introduction Supported vanadia catalysts consist a group of technologically important catalysts and have been used extensively for the selective oxidation of o-xylene to phthalic anhydride,1,2 ammoxidation of alkyl aromatics,3-8 and selective catalytic reduction of NOx with NH3.9,10 In addition to these oxidation reactions, supported vanadia catalysts have also been investigated for the oxidative dehydrogenation of alkanes to olefins,11-13 oxidation of butane to maleic anhydride,14,15 and selective oxidation of methanol to formaldehyde16 or methyl formate.17 The most * Corresponding author. Fax: +91-40-27160921. Tel: +91-4027193162. E-mail: [email protected]. † IICT Communication number: 020405. (1) Bond, G. C.; Tahir, S. F. Appl. Catal. 1991, 71, 1. (2) Nikolov, V.; Klissurski, D.; Anastasov, A. Catal. Rev. Sci. Eng. 1991, 33, 1. (3) Cavalli, F.; Cavani, F.; Manenti, I.; Trifiro, F. Catal. Today 1987, 1, 245. (4) Sanati, M.; Anderson, A. J. Mol. Catal. 1990, 59, 233. (5) Andersson, A.; Lundin, S. T. J. Catal. 1980, 65, 9. (6) Chary, K. V. R.; Kishan, G.; Srilaksmi, K.; Ramesh, K. Langmuir 2000, 16, 7692. (7) Andersson, A.; Bovin, J. O.; Walter, P. J. Catal. 1986, 98, 204. (8) Andersson, A. J. Catal. 1982, 76, 144. (9) Wachs, I. E.; Deo, G.; Weckhuysen, B. M.; Andreini, A.; Vuurman, M. A.; Michiel de Boer.; Amiridis, M. D. J. Catal. 1996, 161, 211. (10) Sorrentino, A.; Roga, S.; Sannino, D.; Magliano, A.; Ciambelli, P.; Santacesaria. E. Appl. Catal., A 2001, 209, 45. (11) Khodakov, Andrei.; Olthof, Bryan.; Bell, Alexis T.; Eglesia Enrique. J. Catal. 1999, 181, 20. (12) Lindblad, T.; Rebenstorf, B.; Yan, Z. G.; Andersson, S. L. T. Appl. Catal. 1994, 112, 187. (13) Andersson, S. L. T. Appl. Catal. 1994, 112, 209. (14) Hardling, H.; Birkeland, K. E.; Kung, H. H. Catal. Lett. 1994, 28, 1. (15) Ramsetter, A.; Baerns, M. J. Catal. 1988, 109, 303.

commonly employed supports are alumina, silica, and titania. In recent years aluminum phosphate is receiving considerable attention as a support for a variety of catalytic reactions.18-23 Amorphous aluminum phosphate is built of tetrahedral units of AlO4 and PO4 and is structurally similar to silica. AlPO4 is also used as a support for metal catalysts, for example, as support for nickel used for hydrogenation reactions24,25 and for chromium used for polymerization of ethylene.26,27 For oxidation reactions, it was recently used to support vanadium-phosphorus oxide catalysts.28 Some reasons for using amorphous AlPO4 as support are high surface area and large average pore size, which may be obtained by proper synthetic procedures.22,23 Also specific interaction with the supported species can be achieved, and this may affect their catalytic properties. The fact that a specific (16) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (17) Forzatti, P.; Tronconi, E.; Busca, G.; Tittarelli, P. Catal. Today 1987, 1, 209. (18) Moffat. J. B. Catal. Rev. Sci. Eng. 1978, 18, 199. (19) Rebenstorf, B.; Lindblad, T.; Andersson, S. L. T. J. Catal. 1991, 128, 293. (20) Campelo, J. M.; Garcia, A.; Herencia, J. F.; Luna, D.; Marinas J. M. Romero, A. A. J. Catal. 1995, 151, 307. (21) Jhansi Lakshmi, L.; Srinivas, S. T.; Kanta Rao, P.; Nosov, A. V.; Lapina, O. B.; Mastikin. V. M. Solid State Nucl. Magn. Reson. 1995, 4, 59. (22) Cheung, T. T. P.; Wilcox, K. W.; McDaniel, M. P.; Johnson, M. M.; Bronniman, C.; Frye, J. J. Catal. 1986, 102, 10. (23) Campelo, J. M.; Marinas, J. M.; Mendioroz, S.; Pajares, J. A. J. Catal. 1986, 101, 484. (24) Marcelin, G.; Vogel, R. G.; Swift, H. E. J. Catal. 1983, 83, 42. (25) Campele, J. M.; Garcia, A.; Luna, D.; Marinas, J. M. Bull. Soc. Chim. Belg. 1983, 92, 851. (26) Mc. Daniel, M. P.; Johnson, M. M. J. Catal. 1986, 101, 446. (27) Mc. Daniel, M. P. Ind. Eng. Chem. Res. 1988, 27, 1559. (28) Kuo, P. S.; Yang, B. L. J. Catal. 1989, 117, 301.

10.1021/la0203943 CCC: $25.00 © 2003 American Chemical Society Published on Web 04/29/2003

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interaction may be obtained has been shown by FT-IR studies of adsorbed carbon monoxide at low temperatures.29-31 Chromocene in toluene solution primarily reacts with the more acidic surface P-OH groups.31 Impregnation with chromium salts, in ethanol or water, leads to interaction mainly with P-OH groups for CrII species, whereas CrIII interacts with both Al-OH and P-OH groups.29 Other divalent cations such as MnII, FeII, and CoII appear also to interact primarily with P-OH groups.30 Not much is known about the interaction of vanadia with AlPO4 surfaces. For selective oxidation of aromatic side chains V-P-O compounds are not beneficial although selectivity may vary with vanadia loading. Lindblad12 and Andersson et al.13 reported amorphous AlPO4 supported vanadia catalysts for the selective oxidative dehydrogenation of propane and butane. They found that tetrahedral vanadium sites were found to be active in this reaction. Several authors32,33 also reported that a tetrahedral vanadium species would form over this support. Determination of dispersion of active phase in supported metal oxide systems is an interesting topic in recent years to understand their role in determining catalytic activity/ selectivity during oxidation reactions. To this end, a method such as oxygen chemisorption was studied extensively in recent years to find active phase dispersion in supported vanadium systems.34-40 We have reported in our earlier studies the characterization and reactivity of vanadium oxide supported on alumina35 and titania.39 The ammoxidation of 3-picoline to nicotinonitrile is a reaction of interest since the nicotinonitrile can be further transformed either to nicotinamide, a component of the vitamin B group, or to the provitamin nicotinic acid. Both compounds are important for the metabolism of humans and animals and are used as food additives. Vanadium oxide based catalysts are generally used for this ammoxidation process.41-45 It is generally believed that lattice oxygen is consumed in the selective oxidation reaction and that the reduced catalyst surface is reoxidized by molecular oxygen. In the present investigation, we report a systematic study on the characterization of vanadia catalysts supported on amorphous aluminum phosphate by X-ray diffraction (XRD), electron spin resonance (ESR), Fourier transform infrared (FT-IR), temperature-programmed reduction (TPR), and temperature-programmed desorption (TPD) of NH3 and oxygen chemisorption measurements. The catalytic properties were evaluated for am(29) Robenstorf, P.; Lindblad, T. J. Catal. 1991, 128, 303. (30) Lindblad, T.; Robenstorf, P. Acta Chem. Scand. 1991, 45, 342. (31) Robenstorf, B.; Lindblad, T. Acta Chem. Scand. 1990, 44, 789. (32) Hong, S. B.; Hwang, B. W.; Yeom, Y.; Kim, S. J.; Uh, Y. S. Stud. Surf. Sci. Catal. 1991, 60, 179. (33) Song, M. K.; Yeom, Y. H.; Kim, S. J.; Uh, Y. S. Appl. Catal. 1993, 102, 93. (34) Oyama, S. T.; Went, G. T.; Lewis, K. B.; Bell, A. T.; Somarjai, G. A. J. Phys. Chem. 1989, 93, 6786. (35) Chary, K. V. R.; Kishan, G. J. Phys. Chem. 1995, 99, 14424. (36) Chary, K. V. R.; Rao, B. R.; Subrahmanyam, V. S. Appl. Catal. 1991, 74, 1. (37) Nag, N. K.; Chary, K. V. R.; Rao, B. R.; Subrahmanyam, V. S. Appl. Catal. 1987, 31, 87. (38) Fierro, J. L. G.; Gambaro, L. A.; Cooper, T. A.; Kremen, G. Appl. Catal. 1983, 6, 363. (39) Chary, K. V. R.; Kishan, G.; Bhaskar, T.; Sivaraj, C. J. Phys. Chem. 1998, 102, 6792. (40) Chary, K. V. R.; Kishan, G.; Narayana, K. V.; Bhaskar, T. J. Chem. Res., Synop. 1998, 314. (41) Sanati, M.; Andersson, A. J. Mol. Catal. 1990, 59, 233. (42) Busca, G.; Cavani, F.; Trifiro, F. J. Catal. 1987, 106, 471. (43) Kumar, S.; Zahed Hussain, S. Chem. Eng. Sci. 1980, 35, 1425. (44) Andersson, A. J. Catal. 1989, 69, 465. (45) Baiker, A.; Zollinger, A. Appl. Catal. 1984, 10, 231.

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moxidation of 3-picoline to nicotinonitrile. The purpose of this work is to estimate the dispersion of vanadia supported on AlPO4 and also to identify the changes in the structure of the vanadium oxide species as a function of an active component loading. Experimental Section Catalyst Preparation. Amorphous aluminum phosphate with a P/Al ratio of 0.9 was prepared from Al(NO3)3‚6H2O and (NH4)2HPO4 following the procedure described elsewhere.12 The starting materials were dissolved in water and acidified with nitric acid. A hydrogel was then formed by adding concentrated ammonia to the aqueous solution until a pH of 8 was achieved. After 1 h the solvent was filtered off and the hydrogel was washed with twice its volume of distilled water. The hydrogel was dried at 383 K for 16 h and calcined at 773 K in air for 30 min. The resulting aluminum phosphate support had a N2 BET surface area of 180 m2/g. V2O5/AlPO4 catalysts with vanadia contents ranging from 2.5 to 20 wt % were prepared by the impregnation with an aqueous solution containing ammonium metavanadate. The catalysts were subsequently dried at 383 K for 16 h and calcined at 723 K for 5 h in air at a heating rate of 10 K/min. Atomic Absorption Spectroscopy (AAS). A 0.1 g portion of catalyst sample was finely ground in an agate mortar. A solution containing 10% HCl (AR grade) was added to it and digested for 3-4 h until a clear solution was obtained. The solution was filtered into a 100 mL volumetric flask, and the volume was made up to the mark. A desired amount of this solution was diluted to get a concentration of 10-15 µg. A perkin-Elemer Analyst 300 double beam spectrometer was used to analyze the samples. Before the test solutions were analyzed, three to four standard solutions were used for calibrating the instrument. An acetylene-nitrous oxide flame and the resonance line of wavelength 318.5 nm of vanadium cathode lamp were used. Test samples were analyzed by keeping the concentration of vanadium less than the amount present in the standard solution. X-ray Diffraction and Electron Spin Resonance. X-ray diffraction studies were carried out on a Philips diffractometer using Cu KR radiation. ESR measurements were recorded at room temperature on Bruker ER 200D-SRC X-band spectrometer with 100 kHz modulation. The reduced catalysts for the ESR study were prepared in quartz tubes (25 cm long, 4 mm diameter). The samples were prereduced at 640 K for 2 h in a continuous flow (40 mL/min) of purified hydrogen. The setup was subsequently evacuated for 1 h at 10-6 Torr. The catalyst thus prepared was transferred into the ESR tube and sealed off under vacuum. Oxygen Chemisorption. Oxygen chemisorption was measured by the static method using an all Pyrex glass system capable of attaining 10-6 Torr. The details of the experimental setup are given elsewhere.37 Prior to the adsorption measurements the samples were prereduced in a flow of hydrogen (40 mL/min) at 640 K for 2 h and evacuated at the same temperature for an hour. Oxygen chemisorption uptakes were determined as the difference of two successive adsorption isotherms measured at 640 K. The surface areas of the catalysts were determined by the BET method using nitrogen physisorption at 77 K and taking 0.162 nm2 as its cross-sectional area. FT-IR Spectroscopy. Fourier transform infrared spectra of the samples were recorded on Nicolet 740 FT-IR spectrometer at ambient conditions. Self-supporting disks containing the catalyst samples were prepared with KBr by applying pressure. These disks were used for recording FT-IR spectra. Temperature-Programmed Reduction. Temperatureprogrammed reduction (TPR) studies were carried out on an AutoChem 2910 (Micromeritics, USA) instrument. In a typical TPR experiment, about 250 mg of oven-dried V2O5/AlPO4 sample was taken in a U-shaped quartz sample tube. The catalyst was packed on a quartz wool plug in one arm of the sample tube. Prior to TPR, the catalyst sample was pretreated at 673 K for 2 h in flowing hydrocarbon-free dry air in order to eliminate the moisture and to ensure complete oxidation. After pretreatment the sample was cooled to room temperature and the carrier gas consists of 5% hydrogen balance argon (50 mL/min), which is purified by passing through an oxy-trap, and molecular sieves

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were allowed to pass over the sample and the temperature was increased from ambient to 1273 K at a heating rate of 10 K/min and the data were recorded simultaneously. The hydrogen concentration in the effluent stream was monitored with the thermal conductivity detector, and the areas under the peaks were integrated using the GRAMS/32 software. The high-purity Ag2O has been used as a standard sample to calibrate the thermal conductivity detector. Once the TCD is calibrated, pulse calibration with 5% H2 in Ar is done to quantify the hydrogen consumption values using GRAMS/32 software. Temperature-Programmed Desorption (TPD) of Ammonia. TPD experiments were also conducted on Auto Chem 2910 instrument. In a typical experiment for TPD studies, the sample was pretreated by passage of high-purity (99.995%) helium (50 mL/min) at 573 K for 1 h. After pretreatment, the sample was saturated with high-purity anhydrous ammonia (75 mL/min) with a mixture of 10% NH3-He at 353 K for 1 h and subsequently flushed with He flow (50 mL/min) 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 is calculated using GRAMS/ 32 software. Ammoxidation of 3-Picoline. A down flow fixed bed reactor operating at atmospheric pressure and made of Pyrex glass was used for the catalyst testing during the ammoxidation of 3-picoline to nicotinonitrile. About 2 g of the catalyst with 18-25 mesh particle size diluted with an equal amount of quartz grains was charged into the reactor and was supported on a glass wool bed. Prior to introducing the reactant 3-picoline with a syringe pump, the catalyst was prereduced at 673 K for 2 h in purified hydrogen flow (40 mL/min). After the prereduction, the reactor was fed with 3-picoline, ammonia, and air keeping the mole ratio of 3-picoline/H2O/NH3/air at 1:13:22:44 and contact time 0.6 s. The reaction was carried out at various temperatures ranging from 598 to 753 K. The liquid products, mainly nicotinonitrile, were analyzed by the gas chromatograph HP-6890 equipped with a flame ionization detector (FID) using an OV-17 column. Traces of carbon oxides were also formed during the reaction.

Results and Discussion X-ray diffraction patterns of various V2O5/AlPO4 catalysts with vanadia loadings ranging 2.27 to 19.4 wt % are shown in Figure 1. For samples with vanadium oxide loading less than 7.38%, no XRD peaks are observed, indicating that vanadium oxide is present in a welldispersed amorphous state. However the presence of vanadia crystallites having size less than 4 nm cannot be ruled out, which is beyond the detection capacity of the XRD technique. XRD results suggest that AlPO4 synthesized is found to be X-ray amorphous. The peak at 2θ corresponding with 21 (4.226 Å) is due to amorphous AlPO4. This finding is in good agreement with XRD results of Lindblad et al.12 With increasing vanadium loading from 7.38 to 19.4%, XRD peaks become more intense and developed into specific peaks for the R-crystobalite phase of AlPO446 (peaks are shown with open circles in Figure 1). The XRD also reveals that the peaks for crystalline V2O5 became more intense at vanadia loading 7.38% (V2O5 peaks are shown with down triangles in Figure 1) and above where the formation of crystalline AlPO4 of the R-crystobalite structure occurs. This is remarkable since amorphous AlPO4 is stable even at 1073 K.22,23,46 Enhancing of R-crystobalite crystallization caused by vanadium oxide loading is likely that the pH of solution changed when vanadium loading increased, because ammonium metavanadate (NH4VO3) was used as a vanadium precursor. This pH variation allows the dissolution of small crystals and the growth of the R-crystobalite. The presence of crystalline V2O5 in the sample facilitates and accelerates the crystallization of AlPO4 and is reflected in the (46) Florke, O. W. Z. Kristallogr. 1967, 125, 1934.

Figure 1. X-ray diffractograms of V2O5/AlPO4 catalysts: (3) reflections due to V2O5; (O) reflections due to R-crystobalite phase of AlPO4. Table 1. Results of Oxygen Uptake, Dispersion, Oxygen Atom Site Density, and Surface Area of Various V2O5/ AlPO4 Catalysts surface V2O5 surface area loading area,b oxygen atom O2 a site density dispersionc on AlPO4, calicned, uptake, m2/g % (w/w) m2/g µmol/g reduced (1018), m-2 O/V 2.27 4.86 7.38 9.50 15.1 19.4

145 125 114 97 78 72

73.1 131.3 165.8 195.3 226.2 300.3

143 128 117 102 81 75

0.62 1.23 1.70 2.30 3.36 4.82

0.59 0.49 0.41 0.37 0.27 0.28

a T (reduction) ) T (adsorption) ) 640 K. b BET surface area determined after oxygen chemisorption. c Dispersion ) fraction of vanadium atoms at the surface assuming Oads/Vsurf ) 1.

decreased surface area of 7.38% V2O5/AlPO4 (Table 1). The amount of vanadia needed for total coverage of support as two-dimensional monolayer was estimated by Bond et al.1 as 0.145 wt %/m2 of the support surface. Therefore on AlPO4 support of having a specific surface area of 180 m2/g requires 16.2 wt % of V2O5. However, the V2O5 crystallites appear starting from 7.38 wt % indicating that a large portion of the support surface remains uncovered. This observation is also in agreement with oxygen chemisorption results reported later in this section. The present XRD results also indicate that there is no compound formed between AlPO4 and V2O5. However, the possibility for the formation of VOPO4 with crystallite size less than 4 nm cannot be ruled out, which is

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Figure 2. Oxygen uptake plotted as a function of V2O5 loading on AlPO4 (Tadsorption ) Treduction ) 640 K). The dashed line corresponds to one oxygen atom per one vanadium atom.

undetectable by the XRD technique. Cheung et al.22 observed that when the P/Al ratio in solution is equal to or greater than 1, AlPO4 is so obtained as crystallite material, if P/Al < 1, then the resulting material remains amorphous. In the present study, the pure AlPO4 contains a P/Al ratio of 0.9 (confirmed by AAS); hence we observed only the amorphous AlPO4 by X-ray diffraction. The surface area of the AlPO4 prepared was 180 m2 g-1, and is decreased as a function of vanadia content shown in Table 1. This decrease of surface area with increasing vanadia loading might be due to blocking pores of the support by crystalline vanadium oxide. Figure 2 shows that the oxygen uptake of various V2O5/ AlPO4 catalysts measured at 640 K as a function of vanadia content on AlPO4. The other information, such as oxygen atom site density, dispersion, etc., derived from it, is also given in Table 1. Oxygen atom site density can be defined as the number of oxygen atoms chemisorbed per unit area of the reduced metal oxide surface. The dispersion of vanadia was found to decrease with vanadia content, and it was found to be 0.59 for 2.27% V2O5/AlPO4 and 0.28 for 19.4% V2O5/AlPO4 catalysts (Table 1). The method of oxygen chemisorption employed here was the procedure described by Oyama et al.34 According to Oyama et al.34 dispersion of vanadia is defined as the fraction of total oxygen atoms (determining from oxygen chemisorption) to total vanadium atoms in the sample. These findings are also supported by the XRD results of the samples (Figure 1) wherein, crystalline vanadia phase appears above 7.38% V2O5 and increases with further increase of vanadia content. The dashed line in Figure 2 corresponds to one oxygen atom per one vanadium atom. For the same composition of V2O5 the dispersion of V2O5/AlPO4 was found to be less than the vanadia supported on Al2O3,35 TiO2,39 and ZrO2.40 These results are in agreement with the findings of Lindblad et al.12 that at 2% V the highly dispersed species may cover about 15% of the support surface and decreases further by increase of vanadia content. According to Lindblad et al.,12 low dispersion of vanadia is due to a large portion of the support surface that remains uncovered as indicated by the Raman spectra, which showed contributions from the support samples. However, it was shown earlier that a good coverage could result in strongly decreased support-bond intensity.47 The oxygen atom site density values (Table1) derived from oxygen chemisorption capacities were (47) Huuhtanen, J.; Sanati, M.; Andersson, A.; Andersson, S. L. T. Appl. Catal., A 1993, 97, 197.

Figure 3. FT-IR spectra of pure AlPO4, pure V2O5, and various V2O5/AlPO4 catalysts.

found to be less than the average number density of VdO groups on the low index planes of V2O5 (5 × 1018 m-2).34 This suggests that the oxygen chemisorption method described here samples the surface but not the bulk. The FT-IR spectra of various V2O5 /AlPO4 catalysts of various V2O5 loadings obtained at ambient conditions are shown in Figure 3. The spectra of pure AlPO4 exhibit bands of the PO43- tetrahedran due to the P-O stretching vibration around 1100 cm-1.48,49 This band becomes less broadened with higher loading of vanadium. The spectra of the catalysts with V2O5 content of 9.5 wt % and above show two minute bands at 1020 and 820 cm-1, respectively. These two bands are characteristic of V2O5 corresponding to VdO bond stretching and V-O-V deformation, respectively.50-53 Thus, vanadia content of up to 9.5% is (48) Farmer, V. C. In The Infrared spectra of Minerals; Farmer, V. C., Ed.; Mineralogical Society: London, 1974. (49) Baiker, A.; Dollenmeir, P.; Glinski, M.; Reller, A. Appl. Catal. 1987, 35, 351. (50) Nag, N. K.; Chary, K. V. R.; Reddy, B. M.; Rao, B. R.; Subrahmanyam. Appl. Catal. 1984, 9, 225. (51) Busca, G.; Lavelley. J. C. Spectrochim. Acta 1986, 424, 443. (52) Busca, G. Mater. Chem. Phys. 1988, 19, 157. (53) Nakagawa, Y.; Ono, O.; Miyata, H.; Hubokawa, Y. J. Chem. Soc., Faraday Trans. 1983, 79, 2929.

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Chary et al. Table 2. Spin Hamiltonian Parameters of V4+ in V2O5/ AlPO4 Catalysts (Samples Reduced at 640 K for 2 h) wt % of V2O5 on AlPO4

g|

A|

g⊥

A⊥

R2

K (cm-1)

2.27 4.86 7.38 9.5 19.4

1.9206 1.9259 1.9286 1.9286 1.9286

206 204 201 201 197

1.9851 1.9851 1.9834 1.9805 1.9766

81 81 82 81 77

0.8771 0.8656 0.8363 0.8376 0.8478

0.01273 0.01277 0.01274 0.01263 0.01211

catalysts, and this can be attributed to spin-spin coupling. The hyperfine structure also becomes diffuse gradually with the increase of vanadia loading. The well-resolved ESR spectra at lower loadings might be due to the presence of the highly dispersed vanadia phase as determined by the oxygen chemisorption and XRD results of the present study. The temperature-programmed reduction profiles of various V2O5/AlPO4 samples and bulk V2O5 are shown in Figure 5. It was found that bulk V2O5 exhibited multiple major reduction peaks when treated in 5% H2-in-Ar up to 1273 K. Koranne et al.54 and Bosch et al.55 have reported a similar observation, and they have attributed this phenomenon to the following reduction sequence:

Figure 4. ESR spectra of hydrogen reduced V2O5/AlPO4 catalysts.

stabilized by interaction with the support surface and is present in a form not detectable as bulk V2O5. However, different species of vanadium oxide could be traced in the monolayer region. For example, Nakagawa et al.53 reported a shift of VdO bond stretching frequency from 1020 cm-1 (bulk V2O5) to 980 cm-1 in the IR spectra of the vanadium oxide monolayer species supported on titania. According to these authors, the vanadium oxide on the support surface is present as an amorphous V2O5 at lower vanadia loadings and amorphous and crystalline V2O5 at higher contents. Several studies using IR techniques have also supported these observations.1 These findings are good agreement with XRD and oxygen chemisorption results. Electron spin resonance spectroscopy has been widely used for characterizing supported vanadia catalysts to determine the possible coordination environment, i.e., symmetry of vanadium and the effect of support on vanadyl bond strength. The V4+ species in reduced catalysts were found to exhibit clear hyperfine splitting due to interaction between electron spin (S ) +1/2) and nuclear spin (I ) +7/2). The ESR spectra of hydrogen reduced (640 K) V2O5/ AlPO4 catalysts recorded at ambient temperature are shown in Figure 4, and the Hamiltonian parameters such as gII, AII, g⊥, A⊥, R2, and k values are given in Table 2. A well-resolved spectrum with hyperfine splitting (hfs) can be seen in Figure 4 at lower vanadia loadings. An examination of ESR spectra of various samples (Figure 4) suggests that the spectra are well resolved at lower vanadia compositions and the intensity of the ESR signals decreases gradually as vanadia loading increases in the

V2O5 f V6O13

(1)

V6O13 f V2O4

(2)

V2O4 f V2O3

(3)

The sharp peak at 965 K corresponds to the reduction of V2O5 to V6O13 (first peak), the peak at 1003 K is associated with the reduction of V6O13 to V2O4 (second step), and the peak at 1067 K corresponds to V2O3 formed by the reduction of V2O4. The reduction conditions applied were similar to those of the supported V2O5/AlPO4 catalysts. The results of TPR by various V2O5/AlPO4 catalysts show a schematic change in the reduction of vanadia with increase of vanadia loading. The TPR profiles for all samples have shown only one prominent maximum (Tmax), the dependence of Tmax on V2O5 content and the amount of hydrogen consumed during TPR are given in Table 3. Baiker et al.49 reported that titania-supported vanadium oxide catalysts exhibited only a single reduction peak during TPR, if less than four layers of vanadium oxide were deposited. Our previous studies39 also confirm these findings. From Figure 5 and Table 3 it has been observed that the Tmax values during TPR increases from 867 to 960 K with vanadia loading until a loading corresponding to 9.5% of V2O5 is reached. This is due to increase in the vanadia crystal size. It is clear that when the crystal size increases, its dispersion decreases, which would be in good agreement with the common knowledge that smaller crystals are reduced faster than the bigger ones. The Tmax shift indicates that the reducibility of vanadia decreases with vanadia loading on AlPO4 and it also suggests that the interactions between vanadia and aluminum phosphate increases with increase in vanadia content. TPR of unsupported V2O5 reveals that it reduces at higher temperature than V2O5/AlPO4 catalysts due to increased diffusional limitations in bulk V2O5, which is in agreement with the work of Koranne et al.54 and Bosch et al.55 They have shown that supported vanadium oxide catalysts reduce at much lower temperature than bulk vanadia (54) Koranne, Manoj M.; Goodwin, James, G., Jr.; Marcelin, G. J. Catal. 1994, 148, 369. (55) Bosch, H.; Kip, B. J.; Van Ommen, J. G.; Gellings, P. J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 2479.

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Figure 6. Ammoxidation activity of 3-picoline over V2O5/AlPO4 catalysts. (Reaction temperature 640 K.)

Figure 5. Temperature-programmed reduction profile (TPR) of pure V2O5 and various V2O5/AlPO4 catalysts. Table 3. Temperature-Programmed Reduction (TPR) Results of Various V2O5/AlPO4 Catalysts wt % of V2O5 on AlPO4

Tmax (K)

H2 consumption, µmol/g

2.27 4.86 7.38 10.0 15.1 19.4

867.2 908.5 948.4 961.1 969.0 967.0

312.3 589.0 959.2 1445.5 1740.0 2052.3

Table 4. Temperature-Programmed Desorption Results of Various V2O5/AlPO4 Catalysts wt % of V2O5 on AlPO4

Tmax (K)

NH3 uptake (µmol/g)

0 2.27 4.86 7.38 9.5 15.1 19.4

564 565 565 544 496 483 549

1960 2110 2200 1470 1600 1710 1780

and the reducibility of vanadia is strongly influenced by the kind of support used. Bond et al.56 reported that the VOx monolayer species is reduced in a single step from V5+ to V3+. The acidity measurements have been carried out by the ammonia TPD method. The ammonia uptake by various catalysts is given in Table 4. The acidic sites are in the temperature region of 480-550 K due to moderate acidic sites. It is found that the acidity increases with (56) Bond, G. C.; Zurita, J. P.; Flamerz, S.; Gellings, P. J.; Bosch, H.; Van Ommen, J. G.; Kip, B. J. Appl. Catal. 1986, 22, 361.

Figure 7. Dependence of conversion or selectivity on the reaction temperature (K) on catalyst 7.38% V2O5/AlPO4.

vanadia loading until 4.86 wt % V2O5 and decreases at loadings of 7.38 wt % and above. The decrease in acidity at higher loadings may be due to the formation of R-crystobalite phase. The activity results during ammoxidation of 3-picoline also increase up to 7.38 wt % and constant with further increase of vanadia loading. This clearly indicates that the acid sites are responsible for picoline ammoxidation. The TPD results also suggest that the strength of acid sites plays a crucial role in determining the catalytic activity during ammoxidation of 3-picoline, and the acidity of the catalysts is mainly due to the vanadia phase since ammonia uptake increases with increasing vanadia loading. Busca57 reported a systematic study on the surface acidity of solid oxides by IR spectroscopic methods. Figure 6 shows the dependence of activity/ selectivity of the V2O5/AlPO4 catalysts during the ammoxidation of 3-picoline to nicotinonitrile at 640 K. It can be clearly seen from the Figure 6 that the conversion of 3-picoline as well as selectivity of nicotinonitrile was found to increase up to 9.5 wt % V2O5 and remains constant above this loading. Pure AlPO4 was found to be inactive for the nicotinonitrile formation under the same experi(57) Busca, G. Phys. Chem. Chem. Phys. 1999, 1, 723.

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Figure 8. Relationship between V2O5 loading and the rate of 3-picoline conversion.

mental conditions of the reaction. Lindblad et al.12 also shown the activity is almost constant above 9.5 wt % V2O5 loading in V2O5/AlPO4 catalysts during the oxidation of toluene. Andersson et al.6,10,58 extensively studied 3-picoline ammoxidation and observed that it is favored by the presence of active V2O5/V6O13 boundaries, which are probably a region of nonstoichiometric V2O5 or V6O13 phase with VdO species adjacent to V4+ ions. Andersson et al.59 also observed that the high activity of catalyst should have a high concentration of both acidic and basic sites. Thus, the ammoxidation of 3-picoline activity of vanadia supported on aluminum phosphate might be due to the presence of the acidic and basic nature of AlPO4. Figure 7 shows the dependence of activity/selectivity on reaction temperature during the 3-picoline ammoxidation on 7.38 wt % V2O5/AlPO4 catalyst. The conversion of 3-picoline and the selectivity of nicotinonitrile were found to increase up to a reaction temperature of 698 K. Above 673 K the conversion and selectivity decrease due to the formation of carbon oxides, which were also confirmed by quadrupole gas chromatography-mass spectrometry. To find the relation between ammoxidation activity of 3-picoline and the dispersion of vanadia, a plot 1/TOF vs (58) Andersson, A. J. Catal. 1986, 100, 414. (59) Andersson, A.; Bovin, J. O.; Walter, P. J. Catal. 1986, 98, 204.

Chary et al.

V2O5 loading is shown in Figure 8, where TOF is equal to the rate of picoline molecules converted per second per surface V2O5. Each surface V2O5 molecule present on AlPO4 is shown in Figure 8. This relationship clearly demonstrates that 3-picoline conversion is directly related to oxygen chemisorption measured at 640 K. As reported elsewhere in connection with vanadium oxide catalysts supported on alumina50 and on silica,37 oxygen is chemisorbed at low temperatures selectively on coordinatively unsaturated sites, generated upon reduction, having a particular coordination environment. These sites are located on a highly dispersed vanadia phase, which is formed only at low vanadia loadings and remains as a “patchy monolayer” on the support surface. At higher vanadia loadings, a second phase is formed, in addition to the already existing monolayer, and this post-monolayer phase does not appreciably chemisorb oxygen (see Figure 2). In the perspective of the above background, the correlation shown here indicates that the catalytic functionality of the dispersed vanadia phase supported on AlPO4 responsible for the ammoxidation of 3-picoline to nicotinonitrile is located on a patchy monolayer phase and this functionality can be titrated by the oxygen chemisorption method described in this work. Conclusions The results of oxygen chemisorption in combination with XRD reveal that at lower vanadia loadings AlPO4 is amorphous, whereas at higher V loadings amorphous AlPO4 turns into R-cristobalite crystalline form along with crystalline V2O5. TPR results demonstrated that the reducibility of V2O5 is decreased with vanadia loading in V2O5/AlPO4 catalysts. Oxygen uptakes measured at 640 K with a prereduction of the sample at the same temperature reveals that vanadium oxide is found to be poorly dispersed on the AlPO4 support and the dispersion decreases with vanadia loadings. ESR, TPD of NH3, and FT-IR results further support the above findings. The catalytic activity during 3-picoline ammoxidation is directly related to dispersion of vanadia. Acknowledgment. G.K., K.R., and C.P.K. thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for the award of Senior Research Fellowships. G.V. thanks the Council of Scientific and Industrial Research (CSIR) for the award of a Junior Research Fellowship. LA0203943