Comparative Study on the Catalytic Performance of Single-Phase Mo

through N2 adsorption at 77 K. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was carried out with VISTA-PRO apparatus (Varian)...
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Ind. Eng. Chem. Res. 2006, 45, 607-614

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Comparative Study on the Catalytic Performance of Single-Phase Mo-V-O-Based Metal Oxide Catalysts in Propane Ammoxidation to Acrylonitrile Nobufumi Watanabe and Wataru Ueda* Catalysis Research Center, Hokkaido UniVersity, N-21, W-10, Kita-ku, Sapporo 001-0021, Japan

Five single-phase Mo-V-O-based mixed metal oxides, Mo-V-O, Mo-V-Te-O, Mo-V-Sb-O, MoV-Te-Nb-O, and Mo-V-Sb-Nb-O, all of which assume the same orthorhombic structure, were prepared by hydrothermal method, and propane ammoxidation to acrylonitrile (AN) using these mixed oxides as catalysts was performed in order to clarify roles of the constituent elements. All of the catalysts were found to be active for propane ammoxidation. The AN selectivity was increased by approximately a factor of 2 by the introduction of Te or Sb to the Mo-V-O oxide catalyst and further increased by the introduction of Nb to the Mo-V-Te-O and Mo-V-Sb-O catalyst. At the same time, the oxidative decomposition of ammonia to nitrogen was retarded by the introduction of Te, Sb, and Nb. Reaction network analyses for each catalyst revealed that Mo and V in a framework structure are responsible for oxidative activation of propane to propene, which is the rate-determining step, and that Te or Sb clearly promotes the conversion of the formed propene to AN, whereas catalysts without Te or Sb clearly promote the destructive conversion of propene to COx. The data also revealed that Nb suppresses the further reaction of AN to undesired products. Active sites for the selective ammoxidation of propane to AN are discussed on the basis of the catalyst crystal structure. 1. Introduction Direct synthesis of acrylonitrile from propane is a very attractive approach, and much research on catalytic propane ammoxidation to acrylonitrile has been done recently.1-15 The most promising catalyst among those investigated in such studies is Mo-V-Te(Sb)-Nb-O mixed metal oxides, which were developed by Mitsubishi Chemical Corporation.1,16 These catalysts have high activities for the production of acrylonitrile and achieve 50-60% acrylonitrile yields at relatively low reaction temperatures.16 Mo-V-Te-Nb-O and Mo-V-SbNb-O catalysts that are active and selective for propane ammoxidation normally contain an orthorhombic structure and a pseudohexagonal structure, both of which are formed during calcination under nitrogen atmosphere. The former phase with the orthorhombic structure is reported a three-dimensionally complex structure with five-, six- and seven-membered rings units composed of an arrangement of MO6 octahedra and MO7 pentagonal bipyramids (M ) Mo, V, Nb) that is isotypic with Cu-Nb-O-X (X ) Cl, Br, I) and Cs-Nb-W-O systems (Figure 1).17,18 Te and Sb are located in the hexagonal channels as ascertained by EXAFS measurements and 125Te Mo¨ssbauer spectroscopy reported by Millet et al.19,20 Recently, we have succeeded in the preparation of singlephase Mo-V-O, Mo-V-Te-O, and Mo-V-Te-Nb-O oxides with an orthorhombic structure using hydrothermal synthesis and reported that they had high activities for the selective oxidation of propane to acrylic acid,21,22 which clearly demonstrates that, in the Te-containing catalyst system, it is the orthorhombic phase that is catalytically active and selective for propane oxidation. In the Sb-containing catalyst system, on the other hand, the catalysts prepared by the drying method often used for this catalyst system always contained both the orthorhombic and the pseudohexagonal structure, so that it has been difficult to evaluate the intrinsic activity and selectivity * To whom correspondence should be addressed. Tel.: +81 11 706 9164. Fax: +81 11 706 9163. E-mail: [email protected].

Figure 1. Structural model (a-b plane) of the orthorhombic phase of MoV-O-based oxide catalyst. Mo, V, and Nb are located in octahedral sites and pentagonal channels. Te or Sb is accommodated in hexagonal channels.

because of the lack of data about relative quantities and interactions between the phases. There are some methods for preparing single-phase materials in the Sb-containing system, in which, for instance, a high vanadium concentration, approximately twice the Mo concentration, has to be applied to the preparation or a treatment with H2O2 solution is needed to remove the pseudohexagonal phase from the mixture.20,23 These are obviously unfavorable techniques because the effects of the vanadium content and H2O2 solution treatment on the catalyst are unclear. Preparation of single-phase oxide catalysts using a similar method is highly important from the perspective of understanding intrinsic catalytic properties. In this study, we present a new method that enables the production of single-phase Mo-V-Sb-O and Mo-V-SbNb-O catalysts and tested the propane ammoxidation to acrylonitrile over the single-phase Sb-containing catalyst system along with the Mo-V-O catalyst and the Te-containing system. On the basis of a comparison of catalytic performance, the roles of constituent elements in the single-phase Mo-V-O-based catalysts are discussed. 2. Experimental Section 2.1. Catalyst Preparation. All hydrothermal reactions were performed using a stainless steel autoclave with a Teflon inner

10.1021/ie0509286 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/01/2005

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Table 1. Elemental Compositions and Surface Areas of Mo-V-O-Based Oxide Catalysts composition (Mo/V/Te or Sb/Nb) catalysts

preparativea

bulkb

surfacec

calcinationd

surface area (m2/g)

Mo-V-O Mo-V-Te-O Mo-V-Te-Nb-O Mo-V-Sb-O Mo-V-Sb-Nb-O

1.00/0.25/-/1.00/0.50/0.17/1.00/0.30/0.17/0.12 1.00/0.33/0.17/1.00/0.42/0.17/0.12

1.00/0.34/-/1.00/0.36/0.06/1.00/0.25/0.13/0.11 1.00/0.33/0.06/1.00/0.33/0.11/0.11

1.00/0.23/-/1.00/0.23/0.08/1.00/0.11/0.25/0.16 1.00/0.16/0.08/1.00/0.15/0.19/0.20

N2, 773 K air, 553 K + N2, 873 K N2, 873 K air, 593 K + N2, 873 K N2, 873 K

6.5 12.2 19.8 13.8 11.2

a Preparative composition of elements in the slurry. b Determined by ICP-AES. c Determined by XPS. d Calcined samples were ground in an agate mortar for 10 min.

tube at 448 K. All chemicals were used as purchased without any further purification. Some of the preparative conditions are listed in Table 1. 2.1.1. Preparation of Mo-V-O Mixed Oxide Catalyst.22 (NH4)6Mo7O24 (8.82 g) was dissolved in 120 mL of distilled water. Separately, an aqueous solution of VOSO4 was prepared by dissolving 3.28 g of hydrated VOSO4 in 120 mL of distilled water. These two solutions were mixed at 293 K and stirred for 10 min before being introduced into the autoclave (300-mL Teflon inner tube). The concentration of Mo in the mixed solution was 0.2 mol‚L-1. After 10 min of nitrogen bubbling to replace the residual air, the hydrothermal reaction was carried out for 20 h. The obtained gray solid was washed with distilled water, dried at 353 K overnight, and then calcined under nitrogen flow (50 mL‚min-1) at 773 K for 2 h. 2.1.2. Preparation of Mo-V-Te-O Mixed Oxide Catalyst.24-27 To an aqueous solution of (NH4)6Mo7O24 (5.35 g in 20 mL of distilled water) was successively added 0.80 g of TeO2 powder. The obtained slurry was stirred for 30 min. Separately, an aqueous solution of VOSO4 was prepared by dissolving 3.94 g of hydrated VOSO4 in 20 mL of distilled water. The solution was added dropwise to the slurry at 293 K and stirred for 1 h before being introduced into the autoclave (50-mL Teflon inner tube). After 5 min of nitrogen bubbling, the hydrothermal reaction was carried out for 72 h. The obtained black solid was washed with distilled water and dried at 353 K overnight. The dried solid was first calcined in air at 553 K for 2 h and then under nitrogen flow (50 mL‚min-1) at 873 K for 2 h. 2.1.3. Preparation of Mo-V-Te-Nb-O Mixed Oxide Catalyst.25 (NH4)6Mo7O24 (5.35 g) was dissolved into 20 mL of distilled water heated at 353 K, and a 1.16-g portion of H6TeO6 was successively added (first solution). An aqueous solution of VOSO4 (second solution) was prepared by dissolving 2.37 g of hydrated VOSO4 in 10 mL of distilled water. A third solution was simultaneously prepared by dissolving a 2.33-g portion of hydrated Nb2(C2O4)5 in 10 mL of distilled water heated at 353 K. The second solution was then added to the first solution, and the resulting solution was stirred for 5 min. The third solution was finally added to the mixed solution, and the resulting slurry was stirred for 10 min at 353 K before being introduced into the autoclave. After 5 min of nitrogen bubbling, the hydrothermal reaction was carried out for 48 h. The obtained dark blue powder was washed with distilled water, dried at 353 K overnight, and then calcined under nitrogen flow (50 mL‚min-1) at 873 K for 2 h. 2.1.4. Preparation of Mo-V-Sb-O Mixed Oxide Catalyst. A 21.20-g portion of (NH4)6Mo7O24 was dissolved in the 250 mL of distilled water heated at 353 K, and a 5.32-g portion of Sb2(SO4)3 powder was successively added. This mixed solution were then stirred and heated at 353 K for 13 h. Separately, an aqueous solution of VOSO4 was prepared by dissolving a 10.52-g portion of hydrated VOSO4 in 40 mL of distilled water. After 13 h, the solution containing VOSO4 was added to the mixed solution containing Sb and Mo, and the

resulting slurry was stirred at 353 K for 15 min. After 15 min of stirring, the obtained dark slurry was filtered using a glass filter with a pore size range of 4-5.5 µm connected to a filtration flask. The filtrate separated from this slurry was introduced into the autoclave (300-mL Teflon inner tube). After 5 min of nitrogen bubbling, the hydrothermal reaction was carried out for 24 h. The obtained black solid was washed with distilled water and dried at 353 K overnight. It was first calcined in air at 593 K for 2 h and then under nitrogen flow (50 mL‚min-1) at 873 K for 2 h. 2.1.5. Preparation of Mo-V-Sb-Nb-O Mixed Oxide Catalyst. A 5.30-g portion of (NH4)6Mo7O24 was dissolved in 20 mL of distilled water heated at 353 K, and a 1.33-g portion of Sb2(SO4)3 powder was successively added. This mixed solution was stirred for 15 min. Separately, an aqueous solution of vanadium was prepared by dissolving a 3.32-g portion of hydrated VOSO4 in 10 mL of distilled water. After 15 min of stirring, this solution was then added to the Mo-Sb mixed solution, and the resulting slurry was stirred for 15 min. The obtained dark slurry was filtered using a glass filter with a pore size range of 4-5.5 µm connected to a filtration flask. The filtrate was finally mixed with a solution prepared by dissolving a 1.08-g portion of (NH4)3[Nb(O2)2(C2O4)2] in 20 mL of distilled water.28 After 5 min of stirring, the resulting mixed solution was introduced into the autoclave. After 5 min of nitrogen bubbling, the hydrothermal reaction was carried out for 48 h. The obtained dark purple powder was washed with distilled water and dried at 353 K overnight. It was calcined at 873 K for 2 h under nitrogen flow (50 mL‚min-1). 2.2. Characterization. All of the catalysts were characterized by the following analytical techniques: X-ray diffraction (XRD) patterns were collected with a RINT Ultima+ apparatus (Rigaku) using Cu KR irradiation. X-ray photoelectron spectroscopy (XPS) measurements were carried out with a JPS-9010MC spectrometer (JEOL) using Mg KR irradiation. BrunauerEmmett-Teller ( BET) surface areas were measured using a BELSORP18 instrument (BEL) through N2 adsorption at 77 K. Inductively coupled plasma atomic emission spectrometry (ICPAES) was carried out with VISTA-PRO apparatus (Varian). Scanning electron microscopy (SEM) was performed on a JSM6360LA instrument (JEOL). 2.3. Catalytic Test for Propane Ammoxidation. Propane ammoxidation tests were performed at atmospheric pressure using a fixed-bed flow reactor. The inner diameter of the reactor was 13 mm. Powder samples obtained after calcination were ground in an agate mortar for 10 min. After grinding, the samples were pressed to thin disks. These disks were crushed, and the obtained particles were adjusted into 250-500-µm sizes using sieves. These particles were diluted to 31 wt % with carborundum (250-500-µm sizes). The amount of the particles was varied (63-1000 mg) to investigate the influence of the contact time (W/F ) 0.0012-0.019 gcat‚min‚mL-1). The diluted catalysts were set into the reactor and heated at the desired temperatures under helium flow. The catalytic reaction was

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Figure 2. XRD patterns of calcined (a) Mo-V-O, (b) Mo-V-Sb-O, (c) Mo-V-Te-O, (d) Mo-V-Sb-Nb-O, and (e) Mo-V-Te-Nb-O catalysts. The calcination conditions are summarized in Table 1.

started by introducing the reactant gas with composition of C3H8/ NH3/O2/He ) 5.7:8.6:17.1:68.6 (the total flow rate was 52.5 mL‚min-1). The reaction temperature range was 633-713 K. The reactants and products were analyzed with three on-line gas chromatographs with columns, Molecular Sieve MS13X, Gaskuropack 54, and Porapak QS. The carbon balance was always more than 95%, but the selectivities were calculated on the basis of the product sum. The initial reaction rates of propane were evaluated using the results obtained with various contact times at a reaction temperature of 683 K. The rate constants for the main products (propene, acrylonitrile, and COx) were estimated using a fitting calculation between the experimental results and a model simulated from the reaction pathways. The activation energies of C3H8 conversion for propane ammoxidation on the catalysts between 653 and 683 K were calculated under low-conversion conditions (around 10% propane and ammonia conversion). 3. Results and Discussion 3.1. Formation of Single Orthorhombic Phase in MoV-O-Based Oxide Catalysts. Figure 2 shows XRD patterns of the calcined Mo-V-O mixed-oxide catalysts. All of the catalysts showed almost the same diffraction pattern with characteristic peaks at 2θ ) 6.7°, 7.8°, 9.0°, 22°, 45°, and ∼27°, which were derived from the orthorhombic structure. No peaks related to any impurities were observed. The formation of the pseudohexagonal structure, which is often formed when a conventional drying method is used for preparation, was negligible in every catalysts prepared hydrothermally in this study. Particularly, the preparation procedure reported here for the first time was successfully applied for the preparation of Sb-containing catalysts. We have reported that it was possible to synthesize single orthorhombic phase Mo-V-O, Mo-V-Te-O, and Mo-VTe-Nb-O catalysts through the hydrothermal synthesis. For the Mo-V-Sb-O and Mo-V-Sb-Nb-O catalysts, preparation in a single-phase state has been difficult. Millet et al. reported a procedure for preparing a single-phase Mo-VSb-O catalyst using an aqueous slurry of (NH4)6Mo7O24‚4H2O, Sb2(SO4)3, and VOSO4. In this case, they needed the preparative condition of a high vanadium concentration, Mo/V/Sb ) 1:1.7: 0.17.23 For the Mo-V-Sb-Nb-O catalyst, two-phase formation has been inevitable when the drying method has been used. Therefore, a treatment of calcined samples, which contain both phases, in a 15% hydrogen peroxide at room temperature had to be applied to remove the phase with the pseudohexagonal structure.20 In the procedure reported here, single-phase Mo-V-Sb-O and Mo-V-Sb-Nb-O catalysts can be obtained with the simple operation described in the experimental section. The most

important procedure is the filtration just before hydrothermal synthesis. When Sb2(SO4)3 powders were added to the aqueous solution of (NH4)6Mo7O24 and the mixture was allowed to be stirred at 353 K for 13 h, a redox reaction between Sb and Mo took place, giving a black solution containing Sb5+ and Mo5+. Then, when the aqueous solution of VOSO4 was added dropwise to the aqueous Mo and Sb solution, we could observed a rapid color change of the mixed solution with the formation of the slurry, indicating that a reduction-oxidation reaction between Mo and V also took place. This redox reaction might also be a key point for the structure formation. After filtration, black solids and filtrates were separated from the slurry on the glass filter and in the filtration flask, respectively. When this filtrate was allowed to react under the hydrothermal conditions, a singlephase Mo-V-Sb-O catalyst was obtained. When the black solids on the glass filter were treated under the hydrothermal conditions, mainly the pseudohexagonal structure was obtained. Furthermore, when hydrothermal synthesis was conducted on the slurry without filtration, a mixture containing both orthorhombic and pseudohexagonal phase was obtained. These results clearly indicate that the black solids on the glass filter are engaged in the formation of the pseudohexagonal structure and that the filtrate gives the solid with the orthorhombic structure during the hydrothermal treatment. This is the reason we successfully obtained single-phase materials by employing simple filtration. We speculate that the black solid as a precursor of the pseudohexagonal phase forms by a reaction between Sb3+ and Mo5+ (and/or V) and that, on the other hand, the orthorhombic phase requires Sb5+. During the redox reaction between Mo6+ and Sb3+, Mo5+ and Sb5+ were formed, the former of which inevitably induces the formation of the precursor of the pseudohexagonal phase and the latter of which seems to be an essential species for the formation of the orthorhombic phase. Te4+, which is the oxidation state of the starting Te source, might have the same structural role in the orthorhombic phase as Sb5+, so that we could easily obtain a single phase in the Te-containing catalyst preparation. The slurry filtration procedure was also effective in the preparation of the Mo-V-Sb-Nb-O catalyst. Similarly, the obtained slurry was filtered before the Nb chemical source was added, and the hydrothermal reaction was carried out after the addition of the (NH4)3[Nb(O2)2(C2O4)2] solution to the filtrate. It should be noted that the Nb source for the Sb system is different from that for Te system, that is, (NH4)3[Nb(O2)2(C2O4)2] and Nb2(C2O4)5, respectively. As mentioned above, Sb5+ is necessary for the formation of the orthorhombic phase, so that, to keep the high oxidation state of Sb, (NH4)3[Nb(O2)2(C2O4)2] is preferable to Nb2(C2O4)5, which is rather a strong reducing agent. The Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O catalysts after the hydrothermal synthesis showed amorphous XRD patterns with very broad peaks around 2θ ) 22° and 45°, and the characteristic peaks of the orthorhombic structure appeared after calcination under a nitrogen atmosphere. However, the Mo-V-O, Mo-V-Te-O, and Mo-V-Sb-O catalysts already exhibited the characteristic peaks after the hydrothermal synthesis. These results indicate that Nb is not necessary to construct the orthorhombic structure. Nb is, conversely, considered to prevent crystallization during the hydrothermal synthesis. We have already reported that the introduction of Nb affects the morphology of the Mo-V-Te-Nb-O catalyst and confirmed that the addition of niobium oxalate prevents the formation of large particles, according to SEM images.25 A similar change of morphology was observed in the case of the

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Mo-V-Sb-Nb-O catalyst. The particle size of the Mo-VSb-Nb-O catalyst (length, 1-2 µm; width, 0.3 µm) was smaller than that of the Mo-V-Sb-O catalyst (length, 5-10 µm; width, 1-2µm). These results also support the conclusion that crystallization is suppressed through the introduction of Nb. 3.2. Elemental Composition of Mo-V-O-Based Oxide Catalysts. The bulk compositions of the Mo-V-O-based oxides determined by ICP-AES and the surface compositions calculated from the XPS peaks are summarized in Table 1. First, these data confirm that all of the constituent elements were introduced into the orthorhombic structure. The bulk V/Mo ratios of the Mo-V-Sb-O and Mo-V-Te-Nb-O catalysts are not very different from those of the preparative compositions, whereas these ratios for the Mo-V-O, Mo-V-Te-O, and Mo-V-Sb-Nb-O catalysts are different from those of the preparative compositions. The bulk Te/Mo and Sb/Mo ratios of the Mo-V-Te-O and Mo-V-Sb-O catalysts are about one-third those of the preparative composition, but those of the Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O catalysts were almost the same as those of the preparative compositions. A comparison of the Te/Mo and Sb/Mo ratios between the Nbcontaining catalysts and Nb-free ones shows that those of the Nb-containing catalysts were higher than those of the Nb-free ones, for example, 0.06 for Mo-V-Te-O and 0.13 for MoV-Te-Nb-O. The bulk Nb/Mo ratios of the Mo-V-TeNb-O and Mo-V-Sb-Nb-O catalysts were not very different from those of the preparative compositions. The XRD patterns (Figure 2) and the bulk composition (Table 1) of the Mo-V-O catalyst clearly indicate that the fundamental orthorhombic structure, which is composed of MO6 octahedra, MO7 pentagonal bipyramids (M ) Mo, V, Nb), and six- and seven-membered ring units consisting of MO6 octahedra (Figure 1), can be formed with Mo and V only. We then would like to focus on the location of Te, Sb, and Nb in the structure, although Te or Sb as the third constituent element has been reported to be exclusively located in hexagonal channel.19,23 The bulk Mo/(Mo + V) and V/(Mo + V) ratios of the Mo-V-O, Mo-V-Te-O, and Mo-V-Sb-O catalysts and the bulk Mo/ (Mo + V + Nb) and V/(Mo + V + Nb) ratios of the MoV-Te-Nb-O and Mo-V-Sb-Nb-O catalysts were calculated. As compared to the bulk Mo/(Mo + V) ratios of the MoV-O, Mo-V-Te-O, and Mo-V-Sb-O catalysts, these ratios were not very different among the catalysts (0.75, 0.74, and 0.75, respectively). The bulk V/(Mo + V) ratios of these three catalysts were also essentially the same (0.25, 0.26, and 0.25, respectively). These results support the conclusion that Te and Sb locate in the hexagonal channels. The bulk Mo/(Mo + V + Nb) ratios of the Mo-V-Te-Nb-O and Mo-VSb-Nb-O catalysts were 0.74 and 0.69, respectively, and the bulk V/(Mo + V + Nb) ratios were 0.18 and 0.23, respectively. The lower bulk Mo/(Mo + V + Nb) ratio (0.69) of the MoV-Sb-Nb-O catalyst compared to the bulk Mo/(Mo + V) ratio (0.75) of the Mo-V-Sb-O catalyst implies that Nb substituted for some of the Mo. On the other hand, Nb in the Mo-V-Te-Nb-O catalyst substituted for some of the V, because the bulk V/(Mo + V + Nb) ratio (0.18) of the MoV-Te-Nb-O catalyst is lower than the V/(Mo + V) ratio (0.26) of the Mo-V-Te-O catalyst. For the surface compositions determined by XPS, the surface V/Mo ratios of all catalysts were lower than the bulk V/Mo ratios. In the cases of the Mo-V-Sb-O, Mo-V-Te-NbO, and Mo-V-Sb-Nb-O catalysts, the surface V/Mo ratios of these catalysts were about a factor of 2 smaller than the bulk V/Mo ratios. The surface Te/Mo and Sb/Mo ratios of the Mo-

Table 2. Product Distribution in the Ammoxidation of Propane over Mo-V-O-Based Catalysts catalyst Mo-V-O Mo-V-Te-O Mo-V-Te-Nb-O Mo-V-Sb-O Mo-V-Sb-Nb-O

conversion (%)a reaction temp (K) C3H8 NH3 O2 682 683 682 683 682

24.2 47.7 52.7 42.5 33.8

25.2 23.0 33.2 28.7 19.0

51.9 59.9 57.7 58.4 39.6

selectivity (%)b AN

PEN AcN COx

25.2 38.8 61.0 48.9 46.6

28.4 22.1 15.8 18.4 26.3

5.3 3.5 4.1 4.0 4.1

40.0 35.4 18.2 28.3 22.4

a Reaction conditions: amount of all catalysts, 0.5 g; total flow rate, 52.5 mL‚min-1; gas composition, C3H8/NH3/O2/He ) 5.7:8.6:17.1:68.6 (%). b AN, acrylonitrile; PEN, propene; AcN, acetonitrila. The selectivities to hydrogen cyanide, acetic acid, and acrylic acid were less than 1%.

V-Te-O and Mo-V-Sb-O catalysts were identical to the bulk Te/Mo and Sb/Mo ratios of these catalysts. It is interesting to note that the surface Nb/Mo ratios of both Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O catalysts were greater than the bulk ratios and that, at the same time, the Te/ Mo and Sb/Mo ratios of the Mo-V-Te-Nb-O and Mo-VSb-Nb-O catalysts were greater than those of the Mo-VTe-O and Mo-V-Sb-O catalysts. The increase of the Nb/ Mo ratio could reflect the suppression of crystallization during the hydrothermal synthesis. Nb5+ ions are very unstable in aqueous solution and easily hydrolyzed in water.29 If the hydrolyzed Nb remains on the crystal surface, preventing crystal growth, it is reasonable that the surface Nb/Mo ratio would be higher than the bulk ratio. The Te/Mo and Sb/Mo ratios of the Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O catalysts were clearly increased as a result of the introduction of Nb. This probably relates to charge compensation, but detailed factors in the increase of the ratio are still ambiguous. 3.3. Catalytic Performance of Mo-V-O-Based Oxides in Propane Ammoxidation. Five catalysts, namely, Mo-V-O, Mo-V-Sb-O, Mo-V-Te-O, Mo-V-Sb-Nb-O, and MoV-Te-Nb-O, were tested for propane ammoxidation to acrylonitrile. From the XRD patterns of the catalysts after the catalytic reaction, it was observed that the Mo-V-O catalyst had decomposed into an unknown oxide similar to MoO2 (JCPDS 32-0671) at reaction temperatures above 693 K. The reaction temperature applied to the Mo-V-O catalyst was therefore limited to the range of 633-693 K. Propane conversions and product distributions are listed in Table 2. Analyzed products are acrylonitrile (AN), propene (PEN), acetonitrile (AcN), hydrogen cyanide, acetic acid, acrylic acid, and COx. The selectivities to hydrogen cyanide, acetic acid, and acrylic acid were far less than 1% and, therefore, are omitted from the table. All of the catalysts were found to be active for propane ammoxidation. The propane conversions of the Tecontaining catalyst (47.7% for the Mo-V-Te-O catalyst and 52.7% for the Mo-V-Te-Nb-O catalyst) were slightly higher than those of the Sb-containing catalysts (42.5% for the MoV-Sb-O catalyst and 33.8% for the Mo-V-Sb-Nb-O catalyst). All of the catalysts exhibited similar activities for ammonia conversion of around 20-30%. Figure 3 shows the evolution of the propane oxidation rate per unit surface area as a function of the reaction temperature in the range of 633-713 K. Some difference in the propane oxidation rates among the five catalysts tested can be seen, but the difference is not large, despite the fundamental differences in the catalyst compositions. It is, therefore, rational to assume that Mo and V as common constituent elements largely contribute to the creation of active sites for the activation of propane in the presence of ammonia. On the other hand, product selectivity is strongly dependent on the catalyst compositions, as can be seen in Table 2. Over the Mo-V-Te-O, Mo-V-Sb-O, Mo-V-Te-Nb-O, and

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Figure 3. Rate propane oxidation per unit surface area over (2) Mo-VO, (]) Mo-V-Te-O, ([) Mo-V-Te-Nb-O, (O) Mo-V-Sb-O, and (b) Mo-V-Sb-Nb-O catalysts. Reaction conditions: reaction temperature, 633-713 K; total flow rate, 52.5 mL‚min-1; gas composition, C3H8/ NH3/O2/He ) 5.7:8.6:17.1:68.6 (%); amount of all catalysts, except the Mo-V-O catalyst (1.0 g), was 0.5 g; diluted with carborundum.

Figure 4. Selectivity changes of (a,b) AN, (c,d) PEN, (e,f) COx vs C3H8 conversion achieved at reaction temperatures in the range of 633-713 K over (2) Mo-V-O, (]) Mo-V-Te-O, ([) Mo-V-Te-Nb-O, (O) Mo-V-Sb-O, and (b) Mo-V-Sb-Nb-O catalysts. Reaction conditions: total flow rate, 52.5 mL‚min-1; gas composition, C3H8/NH3/O2/He ) 5.7:8.6:17.1:68.6 (%); amount of all catalysts, except the Mo-V-O catalyst (1.0 g), was 0.5 g; diluted with carborundum.

Mo-V-Sb-Nb-O catalysts, AN was the major product, whereas overoxidation products such as COx were mainly produced over the Mo-V-O catalyst. The catalytic performances of all of the catalysts are further compared in Figure 4, where the product selectivity is plotted against the propane conversion achieved at various reaction temperatures. Irrespective of the catalyst, the selectivity to PEN was higher at conditions achieving lower propane conversions, whereas the selectivity to AN increased as the conversion increased. This implies that PEN is the primary product of the ammoxidation of propane. Notable is that the selectivity to PEN is not dependent on the catalyst, whereas the other selectivities are strongly dependent as follows: The AN selectivity of the MoV-O catalyst (Figure 4a) increased slightly to about 20% as

Figure 5. Rate per unit surface area of (a) NH3 consumption and (b) N2 formation over (2) Mo-V-O, (]) Mo-V-Te-O, ([) Mo-V-Te-NbO, (O) Mo-V-Sb-O, and (b) Mo-V-Sb-Nb-O catalysts. Reaction conditions: reaction temperature, 633-713 K; total flow rate, 52.5 mL‚min-1; gas composition, C3H8/NH3/O2/He ) 5.7:8.6:17.1:68.6 (%); amount of all catalysts, except the Mo-V-O catalyst (1.0 g), was 0.5 g; diluted with carborundum.

the propane conversion increased. The PEN selectivity (Figure 4c) decreased as the propane conversion increased, whereas the COx selectivity (Figure 4e) increased dramatically. The AN selectivities of the Mo-V-Te-O and Mo-V-Sb-O catalysts were higher by about a factor of 2 (Figure 4a,b) than those of the Mo-V-O catalyst. The COx selectivities of the Mo-VTe-O and Mo-V-Sb-O catalysts were lower than that of the Mo-V-O catalyst and almost constant even at high propane conversion levels. In the case of the Mo-V-Te-Nb-O catalyst, the AN selectivity was further progressed and higher than that of the Mo-V-Te-O catalyst for the entire range of propane conversions. The AN selectivity of the Mo-V-SbNb-O catalyst was not very different from that of the MoV-Sb-O catalyst. All of these results indicate that the introduction of Te, Sb, and Nb is responsible for the AN selectivity and also that the effects of the introduction of Nb are different for the Mo-V-Te-Nb-O and Mo-V-SbNb-O catalysts. Figure 5 shows the evolution of the ammonia consumption rate per unit surface area and the formation rate of nitrogen molecule as a function of reaction temperature in the range of 633-713 K. The rate of ammonia consumption increased with increasing reaction temperature, and interestingly, this trend is almost independent of the catalyst. On the other hand, the rate of nitrogen molecule formation decreased markedly in the following order: Mo-V-O > Mo-V-Te-O ) Mo-VSb-O ) Mo-V-Sb-Nb-O > Mo-V-Te-Nb-O. These results indicate that the utilization efficiency of ammonia in the reaction over the Mo-V-O catalyst was the worst among the five catalysts but that this utilization efficiency could be improved by the introduction of Te, Sb, or Nb. The rate of nitrogen formation is considered to decrease naturally as a result of the higher selectivity to AN, which involves the insertion of nitrogen, under the condition that both the rates of propane conversion (Figure 3) and ammonia consumption (Figure 5a) are almost independent of the catalyst. It appears that the oxidative activations of propane and ammonia take place simultaneously at the same active sites that have a surface acidic property, as ammonia is a basic compound. 3.4. Kinetic Comparison of Mo-V-O-Based Catalysts in Propane Ammoxidation. In many studies, reaction networks of propane selective oxidation to acrylic acid3,10,30-36 and propane ammoxidation2,37-39 have been proposed. These reaction networks involve the common route that propane is first oxidized to propene. Propene is further converted to acrylic acid or acrylonitrile. A proposed reaction network for propane ammoxidation is shown in Scheme 1. The direct route from propane to acrylonitrile or COx is also regarded. Here, we

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Figure 6. Selectivity changes of (a) AN, (b) PEN, and (c) COx vs C3H8 conversion at various contact times over (2) Mo-V-O, (]) Mo-V-TeO, ([) Mo-V-Te-Nb-O, (O) Mo-V-Sb-O, and (b) Mo-V-SbNb-O catalysts. Reaction conditions: reaction temperature 683 K; gas composition C3H8/NH3/O2/He ) 5.7:8.6:17.1:68.6 (%); contact time W/F, 0.0012-0.0190 gcat‚min‚mL-1.

Scheme 1. Proposed Propane Ammoxidation Pathways Used for Calculation of Rate Constants

compare our five catalysts in terms of kinetics on the basis of the ammoxidation steps shown in Scheme 1. Figure 6 shows product distribution vs the propane conversion for various contact times at the reaction temperature of 683 K. Propene was a major product at low propane conversion levels, i.e., at short contact times. The PEN selectivities of all the

catalysts indicated an exponentially decresing trend with increasing propane conversion. The AN selectivities of all the catalysts, except for the Mo-V-O catalyst, increased with decreasing PEN selectivity. The AN selectivities of the MoV-Te-O and Mo-V-Te-Nb-O catalysts remained essentially unchanged even though the propane conversion increased, and the highest AN selectivity was attained over the Mo-V-Te-Nb-O catalyst. The AN selectivities of the MoV-Sb-O and Mo-V-Sb-Nb-O catalysts decreased slightly as the propane conversion increased. The COx selectivities of all of the catalysts increased with increasing propane conversion. The COx selectivity of the Mo-V-O catalyst was the highest of all of the catalysts. All of these trends were not strongly affected by the reaction temperature. Using the above results, we calculated the initial reaction rate of propane at 683 K and the activation energy for propane oxidation. The results are presented in Table 3. As already mentioned, the initial reaction rates are not exactly the same for all of the catalysts, but by taking facet dependency of activity40 into account, it can be concluded that the first step of the ammoxidation of propane, which is the formation of propene and the rate-determining step, is predominantly controlled by Mo and V, which construct the octahedra network in the orthorhombic structure (Figure 1). In accordance with this fact, the activation energy values were found to be similar. The results now allow us to calculate relative reaction constants using the data shown in Figure 6 for comparing five catalysts on the basis of kinetics. Then, we can discuss the role of the constituent elements in the Mo-V-O-based oxide catalysts. 3.5. Roles of Constituent Elements in Propane Ammoxidation. The relative rate constants for the main products (propene, acrylonitrile, and COx) were estimated by a fitting calculation between the experimental results (Figure 6) and a model simulated from the reaction pathways. The values for each relative rate constant listed in Table 3 are normalized relative to k1 + k5 + k6, which corresponds to the total propane conversion. The propane conversion and propene oxidation were assumed to be a pseudo-first-order and a first-order reaction, respectively.41 Good agreement was obtained in each case, and the calculated rate constants at the reaction temperature of 683 K are summarized for the five catalysts in Table 3. There are several observations that should be noted: (1) k1 was the same for all of the catalysts, indicating that neither Te, Sb, nor Nb is involved in the activation of propane. (2) Direct conversion of propane to COx was negligible for every catalyst, so the first oxidative step takes place selectively. (3) Although the calculation gave some values for k6, these are just possibilities at the present stage. (4) k4, which is for the step involving the destructive conversion of propene, was extraordinarily high for the Mo-V-O catalyst, whereas for the other catalysts, the values were about one-third. On the other hand, k2, which is for the step involving the allylic ammoxidation of the formed propene, was the lowest for the Mo-V-O catalyst, but there

Table 3. Initial Reaction Rates, Activation Energies for C3H8 Conversion, and Simulated Rate Constants of Mo-V-O-Based Oxide Catalysts (683 K)

b

catalyst

initial rate (µmol‚m-2‚min-1)a

activation energy (kJ‚mol-1)b

k1

k2

Mo-V-O Mo-V-Te-O Mo-V-Te-Nb-O Mo-V-Sb-O Mo-V-Sb-Nb-O

9.6 13.8 12.6 12.6 9.6

131 105 131 122 107

0.84 0.82 0.81 0.84 0.86

5.77 8.89 10.37 9.82 7.20

relative rate constanta k3 k4 2.46 1.68 0.46 1.23 1.99

13.17 3.79 4.20 5.06 2.34

k5

k6

0.02 0.06 0.02 0.02 0.03

0.14 0.12 0.17 0.14 0.11

a Reaction conditions: reaction temperature, 683 K; W/F, 0.0012-0.019 g ‚min‚mL-1; gas composition, C H /NH /O /He ) 5.7:8.6:17.1:68.6 (%). cat 3 8 3 2 Reaction conditions: reaction temperature, 653-683 K; W/F, 0.0012 gcat‚min‚mL-1; gas composition, C3H8/NH3/O2/He ) 5.7:8.6:17.1:68.6 (%).

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is not a significant difference in k2 between the Nb-containing catalysts (Mo-V-Te-Nb-O and Mo-V-Sb-Nb-O) and the non-Nb catalysts (Mo-V-Te-O and Mo-V-Sb-O). These data clearly indicate that Te and Sb promote the allylic oxidation selectively without affecting the intrinsic oxidation activity derived from Mo and V. This is probably due to the fact that Te and Sb locate in the central position of the hexagonal channels. (5) k3, which is for the step involving the destructive conversion of the formed acrylonitrile to COx, was the lowest for the Mo-V-Te-Nb-O catalyst, for which the highest acrylonitrile selectivity was attained. This decrease of the destructive conversion was, however, not observed in the Sbcontaining systems. As indicated in the section on elemental composition, Nb can be substituted for V in the case of the Te-containing system but only for Mo in the case of the Sb system. This means that structural dilution of V in the octahedra network can be achieved as a result of the introduction of Nb in the Te-containing system, presumably preventing the further oxidation. As a consequence, all of the elements, which are located in the network separately, are involved in each reaction step cooperatively. We discuss here the effect of ammonia. As is well-accepted, propane selective oxidation and ammoxidation involve the common route that propane is oxidized to propene, which is further converted to acrylonitrile (Scheme 1) or acrylic acid by selective oxidation in the absence of NH3. It is clear that all of the catalysts tested are active for the activation of propane in ammoxidation as well as selective oxidation,25,40 although the ammoxidation requires a higher reaction temperature than the selective oxidation because of ammonia. As for the selectivity, the acrylic acid (AA) selectivity of the Mo-V-O catalyst in the propane selective oxidation reaction was quite low, and overoxidation to COx primarily occurred (24% for propane conversion, 5.7% for AA selectivity, and 68% for COx selectivity).21 However, in the propane ammoxidation reaction, the MoV-O catalyst showed a relatively higher selectivity to AN (24% for propane conversion, 25% for AN selectivity, and 40% for COx selectivity) than to AA. It is considered that ammonia in reactant gases strongly interacts with acidic sites on the catalysts and suppresses the adsorption of propene, thus lowering the overoxidation of propene to COx. Hence, a relatively high AN selectivity was attained by the Mo-V-O catalyst. Even so, the Mo-V-O catalyst is still unfavorable in terms of the efficient utilization of ammonia, because the oxidative decomposition of ammonia to nitrogen proceeds rapidly. The other catalysts, on the other hand, apparently showed high selectivities to AN and high utilization efficiencies of ammonia because of the presence of Te or Sb, which can promote allylic oxidation. By taking all of the data into account, the following explanation for the action of Te and Sb can be deduced. Because the activities of propane conversion (Figure 3) and ammonia conversion (Figure 5a) are not strongly dependent on the introduction of Te or Sb, it appears that the Te or Sb element makes no contribution toward constructing acidic active sites for propane and ammonia activation. Te and Sb evidently promote propene selective ammoxidation to acrylonitrile, suggesting that propene preferentially adsorbs on Te or Sb sites but not on acidic sites that promote overoxidation. Then, an allylic intermediate from propene forms on the sites and readily reacts with ammonia activated near acid sites, so that ammonia decomposition to nitrogen can be retarded. 4. Conclusion We successfully demonstrate the roles of the constituent elements of Mo-V-O-based oxide catalysts in the ammoxi-

dation of propane on the basis of the common crystal structures with different elemental compositions. It was found that all of the catalysts, namely, Mo-V-O, Mo-V-Te-O, Mo-V-TeNb-O, Mo-V-Sb-O, and Mo-V-Sb-Nb-O, showed high activities for propane ammoxidation but different product selectivities depending on the constituents. From a comparison of catalytic performance and a kinetics analysis, Mo and V in the octahedra network in the orthorhombic structure were found to be responsible for the activation of propane. The introduction of Te or Sb as a third construction elements plays an important role in the allylic oxidation of the formed propene, yielding a dramatic improvement in the AN selectivity. Moreover, the oxidative decomposition of ammonia is also suppressed by Te or Sb. Nb, which seems to have a dilution effect on V located in the network, brings about a further improvement of the AN selectivity, resulting in the Mo-V-Te-Nb-O catalyst exhibiting the highest AN yield. Acknowledgment Analytical support (ICP-AES) from Nippon Kayaku Corp. is gratefully acknowledged. Literature Cited (1) Ushikubo, T.; Oshima, K.; Kayou, A.; Vaarkamp, M.; Hatano, M. Ammoxidation of Propane over Catalysts Comprising Mixed Oxides of Mo and V. J. Catal. 1997, 169, 394-396. (2) Oliver, J. M.; Lo´pez Nieto, J. M.; Botella, P. Selective Oxidation and Ammoxidation of Propane on a Mo-V-Te-Nb-O Mixed Metal Oxide Catalyst: A Comparative Study. Catal. Today 2004, 96, 241-249. (3) Grasselli, R. K.; Burrington, J. D.; Buttrey, D. J.; DeSanto, P., Jr.; Lugmair, C. G.; Volpe, A. F., Jr.; Weingand, T. Multifunctionality of Active Centers in (Amm)Oxidation Catalysts: From Bi-Mo-Ox to Mo-V-Nb(Te, Sb)-Ox. Top. Catal. 2003, 23, 5-22. (4) DeSanto, P., Jr.; Buttrey, D. J.; Grasselli, R. K.; Lugmair, C. G.; Volpe, A. F.; Toby, B. H.; Vogt, T. Structural Characterization of the Orthorhombic Phase M1 in MoVNbTeO Propane Ammoxidation Catalyst. Top. Catal. 2003, 23, 23-38. (5) Baca, M.; Pigamo, A.; Dubois, J. L.; Millet, J. M. M. Propane Oxidation on MoVTeNbO Mixed Oxide Catalysts: Study of the Phase Composition of Active and Selective Catalysts. Top. Catal. 2003, 23, 3946. (6) Ballarini, N.; Cavani, F.; Giunchi, C.; Masetti, S.; Trifiro`, F.; Ghisletti, D.; Cornaro, U.; Catani, R. Rutile-Type Cr/Sb Mixed Oxides as Heterogeneous Catalysts for the Ammoxidation of Propane to Acrylonitrile. Top. Catal. 2001, 15, 111-119. (7) Guerrero-Pe´rez, M. O.; Al-Saeedi, J. N.; Guliants, V. V.; Ban˜ares, M. A. Catalytic Properties of Mixed Mo-V-Sb-Nb-O Oxides Catalysts for the Ammoxidation of Propane to Acrylonitrile. Appl. Catal. A: Gen. 2004, 260, 93-99. (8) Olea, M.; Florea, M.; Sack, I.; Prada Silvy, R.; Gaigneaux, E. M.; Marin, G. B.; Grange, P. Evidence for the Participation of Lattice Nitrogen from Vanadium Aluminum Oxynitrides in Propane Ammoxidation. J. Catal. 2005, 232, 152-160. (9) Derouane-Abd Hamid, S. B.; Centi, G.; Pal, P.; Derouane, E. G. Site Isolation and Cooperation Effects in the Ammoxidation of Propane with VSbO and Ga/H-ZSM-5 Catalysts. Top. Catal. 2001, 15, 161-168. (10) Shishido, T.; Konishi, T.; Matsuura, I.; Wang, Y.; Takaki, K.; Takehira, K. Oxidation and Ammoxidation of Propane over Mo-V-Sb Mixed Oxide Catalysts. Catal. Today 2001, 71, 77-82. (11) Bergh, S.; Cong, P.; Ehnebuske, B.; Guan, S.; Hagemeyer, A.; Lin, H.; Liu, Y.; Lugmair, C. G.; Turner, H. W.; Volpe, A. F., Jr.; Weinberg, W. H.; Woo, L.; Zysk, J. Combinatorial Heterogeneous Catalysis: Oxidative Dehydrogenation of Ethane to Ethylene, Selective Oxidation of Ethane to Acetic Acid, and Selective Ammoxidation of Propane to Acrylonitrile. Top. Catal. 2003, 23, 65-79. (12) Zenkovets, G. A.; Kryukova, G. N.; Tsybulya, S. V.; Anufrienko, V. F.; Larina, T. V.; Burgina, E. B. The Structure of Oxide Ga-Sb-NiP-W-O/SiO2 Catalyst and Its Catalytic Properties in Propane Ammoxidation. Kinet. Catal. 2002, 43, 384-390. (13) Masetti, S.; Trifiro`, F.; Blanchard, G. Fluid-Bed Tin-Based Catalyst for Propane Ammoxidation. Appl. Catal. A: Gen. 2001, 217, 119-129.

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ReceiVed for reView August 11, 2005 ReVised manuscript receiVed October 18, 2005 Accepted October 18, 2005 IE0509286