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Bulk mixed Mo–V–Te–O catalysts for propane oxidation to acrylic acid
Applied Catalysis A: General 274 (2004) 123–132
Bulk mixed Mo–V–Te–O catalysts for propane oxidation to acrylic acid Vadim V. Guliants a,∗ , Rishabh Bhandari a , Jamal N. Al-Saeedi a,1 , Vijay K. Vasudevan a , Rajiv S. Soman b , Olga Guerrero-Pérez c , Miguel A. Bañares c a
Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA b College of Applied Science, University of Cincinnati, Cincinnati, OH 45206-2839, USA c Instituto de Catalisis y Petroleoquimica, CSIC, E-28049 Madrid, Spain Received in revised form 24 May 2004; accepted 29 May 2004 Available online 8 July 2004
Abstract The model Mo–V–Te–O catalysts containing the orthorhombic (M1) and hexagonal (M2) phases with well-defined crystal morphologies and promising for the selective oxidation of propane to oxygenates were obtained hydrothermally over a wide range of synthesis compositions (Mo0.70–0.30 V0.60–0.20 Te0.15–0.05 ). The bulk (ICP), local (EDS) elemental and structural (XRD) analyses of this compositionally simple model system indicated that the M1 phase with the bulk Mo/V ratio of ∼2 was dominant in the Mo–V–Te–O catalysts. This phase has been proposed as active and selective for propane oxidation to oxygenates and ammoxidation to acrylonitrile. The model catalysts displayed high selectivity to acrylic acid in the presence of water vapor in the feed, which is believed to enhance the apparent rates of formation of oxygenates as well as maintain the catalytic surface in a partially reduced oxidation state. The selectivity to acrylic acid over these model catalysts correlated with the extent of exposure of the surface ab planes of the M1 phase proposed to contain the active and selective surface sites. Therefore, the Mo–V–Te–O catalysts represent a well-defined and highly promising model system for elucidating the surface molecular structure–activity/selectivity relationships in propane oxidation to acrylic acid over multicomponent Mo–V–Te–Nb–O catalysts. © 2004 Elsevier B.V. All rights reserved. Keywords: Mixed Mo–V–Te oxides; Hydrothermal synthesis; Phase diagram; Propane oxidation
1. Introduction The past several years have seen a significant growth in the number of industrial patents and publications on the one-step propane oxidation to oxygenates, i.e. acrolein and acrylic acid, and ammoxidation to acrylonitrile over several promising bulk mixed metal oxides [1–16]. The recently discovered Mo–V–Te–Nb–O catalytic system [6,9,11,12,16] appears to be the most active and selective for these transformations. These catalysts are prepared in aqueous medium by reacting metal oxide sources, e.g. ammonium heptamolybdate, vanadyl sulfate, tellurium dioxide and ammonium niobium oxalate at 353–433 K and calcined in an inert N2 environ∗
Corresponding author. Tel.: +1 513 556 0203; fax: +1 513 556 3473. E-mail address: [email protected] (V.V. Guliants). 1 Current address: Department of Chemical and Petroleum Technology, College of Technological Studies, Kuwait. 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.049
ment to prevent the phase separation of metal oxide components. This synthesis results in layered V–Mo–Te–Nb–O phases with a 4 Å d-spacing that are commonly observed in multicomponent solid solutions of V, Nb and other metal ions in Mo5+ /Mo6+ suboxides [17]. The multicomponent vanadium molybdate catalysts were reported to contain two crystalline phases, so-called “M1” and “M2”, proposed to be responsible for the propane activation to propylene and its subsequent oxidation to oxygenates or ammoxidation to acrylonitrile [16]. Recently, the M1 and M2 phases were shown to have, respectively, orthorhombic and hexagonal structures with the (Te2 O)M20 O56 and (TeO)M2 O9 (M = Mo, V, Nb) compositions [18–22]. The studies of propane (amm)oxidation indicated that the orthorhombic phase is the most active and selective for both of these transformations, although a synergism due to a cooperation between these phases was observed at high propane conversion [18,23]. The M1 and M2 phases possessing well-defined rod-like
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crystal morphology were also obtained hydrothermally in the Nb-free system (Mo0.6 V0.3 Te0.1 ) that selectively transformed ethane and propane to ethylene and acrylic acid, respectively [7,21,24,25]. Therefore, the model Mo–V–Te–O system appears to be particularly promising for studies of the surface molecular structure–activity/selectivity relationships in selective transformation of propane to oxygenates and acrylonitrile due to its compositional simplicity and well-defined crystal morphology. In this study, we investigate the synthesis phase diagram of this model catalytic system as well as the role of pH in the nucleation and growth of mixed Mo–V–Te–O phases with distinct crystal morphologies. We further examine the local composition of the M1 and M2 phases, their vibrational spectroscopic characteristics (IR, Raman) and performance in selective oxidation of propane to acrylic acid.
2. Experimental 2.1. Catalyst preparation Mixed Mo–V–Te–O catalysts were prepared hydrothermally using ammonium paramolybdate (81–83% as MoO3 , Alfa Aesar), vanadyl sulfate (99.9%, Alfa Aesar) and TeO2 (99.99%, Alfa Aesar) as the metal oxide sources. The Mo–V–Te–O phase diagram was explored for 28 synthesis compositions in the Mo0–0.9 V0–0.9 Te0.05–0.5 range. In a typical synthesis, exemplified here for the Mo0.6 V0.3 Te0.1 composition, ammonium paramolybdate (6.80 g, 38.7 mmol of Mo6+ ) was dissolved in 100 ml of deionized water while stirring at room temperature. TeO2 (0.96 g, 6.0 mmol of Te4+ ) was added to the resulting solution leading to white suspension and the mixture was stirred for 0.5 h. Vanadyl sulfate (4.68 g, 19.2 mmol of V4+ ) was dissolved in 60 ml of deionized water at room temperature and added dropwise to the Mo–Te suspension. The mixture became viscous brown upon V4+ addition, then a dark pink homogeneous gel. The gel was then divided into portions in which pH was adjusted using NH4 OH (0.5 M) and HNO3 (0.5 M) to values in the pH = 1.0–4.0 range (the synthesis gel has the pH = 2.2 ± 0.1). The resultant dark pink gel was placed in a Teflon-lined stainless steel autoclave and heated for 72 h at 175 ◦ C. The black solid was filtered, washed with deionized water and dried for 12 h at 80 ◦ C. The mixed metal oxide product was calcined at 600 ◦ C for 2 h under flowing N2 (40–60 cm3 /min) in a tubular furnace. The temperature was ramped to 600 ◦ C at 5 ◦ C/min. The mixed Mo–V–Te–Nb–O catalyst with a Mo1.00 V0.31 Te0.17 Nb0.19 synthesis composition was synthesized in a control experiment under hydrothermal conditions [16]. 2.2. Physicochemical characterization The elemental composition of the Mo–V–Te oxides was characterized by a Leeman Labs’ ICP/Echelle Spectrome-
ter, Model PS1000 ICP, equipped with a Hildebrand Grid nebulizer and using Mo, V and Te elemental standards (Alfa Aesar). The BET surface areas were determined from the N2 adsorption isotherms (0.05 < P/P0 < 0.3) at −196 ◦ C using a Micromeritics TriStar 3000 porosimeter. The model Mo–V–Te–O catalysts were characterized by powder X-ray diffraction employing Siemens D500 diffractometer (Cu K␣ radiation). The SEM images were collected using Hitachi S-900 scanning electron microscope. Transmission electron microscopy (TEM) was performed at the Argonne National Laboratory using a Philips CM30 transmission electron microscope operated at 200 kV with a useful limit of information of 0.14 nm. This instrument was equipped with an EDAX-NX2 (spatial resolution ∼1 nm) for the EDS analysis of the local elemental composition. MoO3 , V2 O5 and TeO2 were used as the EDS elemental standards to determine the respective Cliff-Lorimer sensitivity k-factors. The IR spectra were collected in the transmission mode on a BioRad FTS60. The Raman spectra were collected using a single monochromator Renishaw System 1000 equipped with a cooled CCD detector (−73 ◦ C) and holographic super-notch filter. The samples were excited with the 514 nm Ar+ line; the spectral resolution was ca. 3 cm−1 and spectrum acquisition consisted of 20 accumulations of 30 s each. The Raman spectra under dehydrated conditions were collected using a hot stage (Linkam TS-1500). The kinetic studies of propane oxidation were conducted at 350–450 ◦ C using a feed of 6.3 vol.% propane, 9.4 vol.% oxygen, 0–66.4 vol.% water vapor and balance He and 1.0 g of catalyst packed in a 3/8 in. quartz tubular microreactor in a programmable oven. An HP 5890II gas chromatograph equipped with an FID and TCD was employed for the reaction product analysis. The carbon balances agreed within 5 mol%.
3. Results and discussions 3.1. Hydrothermal synthesis of Mo–V–Te–O system The ternary synthesis diagram of the mixed Mo–V–Te–O system was investigated at pH = 2.2 with a particular focus on the formation of the M1 and M2 phases. The M1 phase was identified by the major XRD reflections at 6.6◦ , 7.8◦ , 9.0◦ , 13.0◦ , 22.1◦ , 26.2◦ , 26.8◦ , 27.6◦ and 45.2◦ , while the M2 phase was characterized by the major reflections at 22.1◦ , 28.3◦ , 36.2◦ and 44.6◦ [15,16,18–25]. Although a significant overlap of the XRD reflections is observed for these two phases, the M1 phase may be detected by the presence of reflections at 2Θ < 10◦ , while the presence of the M2 phase is manifested in the characteristic reflection at 2Θ = 28.3◦ . The four-component Mo1.00 V0.31 Te0.17 Nb0.19 oxide catalyst prepared as a reference under hydrothermal conditions contained predominantly M1 and some M2 phase (Fig. 1). The M1 and M2 phases were observed exclusively for the Mo0.70–0.30 V0.60–0.20 Te0.15–0.05 synthesis compo-
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Fig. 1. XRD patterns of mixed Mo0.6 V0.3 Te0.1 and Mo1.00 V0.31 Te0.17 Nb0.19 oxides. The peaks at 38.5◦ and 44.7◦ 2Θ correspond to the aluminum material of the sample holder.
sitions (Figs. 1 and 2), while the impurity phases, such as MoO3 (JCPDS 76-1003), (Mo0.93 V0.07 )5 O14 (JCPDS 31-1437) and Mo5 TeO16 (JCPDS 31-0874), were detected for more Mo- and Te-rich synthesis compositions (not shown here). The XRD patterns of three freshly calcined Mo–V–Te–O catalysts with the synthesis compositions Mo0.6 V0.3 Te0.1 , Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 are shown in Fig. 2. The XRD patterns of these three compositions indicated the presence of essentially pure M1 phase with the exception of the Mo0.3 V0.6 Te0.1 catalyst, which also contained some M2 phase. The M1 crystallite size and aspect ratios in the Mo–V–Te–O catalysts were estimated by the Scherrer formula [26] for the (1 2 0), (2 1 0) and (0 0 1) reflections of the M1 phase at 7.8◦ , 9.0◦ and 22.1◦ 2Θ, respectively,
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and shown in Table 1. This analysis revealed that the Mo–V–Te–O phases possessed nanoscale dimensions in agreement with previous observations [15]. The M1 crystal size in the c-axis direction was approximately the same in these catalysts, while the crystal dimensions in the ab plane varied from sample to sample. The ab planes have been recently proposed to contain the active and selective surface sites for propane oxidation to acrylic acid and ammoxidation to acrylonitrile [7,23,24,25]. In order to verify this hypothesis, we estimated the relative exposure of the ab planes of the M1 phase in these catalysts assuming the crystals to have a platelet shape as recently observed by high-resolution TEM [15]. The estimates obtained (Table 1) suggested that the ab planes are not exposed to a great degree for the platelet morphology of these catalysts and that other crystallographic planes are predominant. However, among the three catalysts, the Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 catalysts exposed the ab planes to a greater degree (∼40% higher) than the Mo0.6 V0.3 Te0.1 catalyst. Theoretical surface area estimates for these M1 crystals, S(theory) in Table 1, are much higher than the experimental BET surface areas for the model Mo–V–Te–O catalysts, suggesting that these catalysts contain dense aggregates of multiple M1 crystals. Grinding these catalysts has a beneficial effect on their selectivity to acrylic acid [7], because it breaks up the M1 aggregates and, to some degree, individual crystals reducing their dimensions along the c-axis, and, therefore, enhances the exposure of the ab planes. The morphology and elemental composition of the three selected catalysts (Mo0.6 V0.3 Te0.1 , Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 ) were studied by SEM (Fig. 3a–c), ICP and EDS (Table 2). The N2 adsorption measurements indicated that the BET surface areas of all catalysts were quite low (Table 1). The synthesis composition had a dramatic im-
Fig. 2. XRD pattern of three selected compositions of mixed Mo–V–Te–O system.
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Table 1 Crystal sizes and surface areas of the M1 phase present in model Mo–V–Te–O catalysts Synthesis composition
a-Axis (nm)
b-Axis (nm)
c-Axis (nm)
ab planes (%)
S(theory)a (m2 /g)
SBET (m2 /g)
Mo0.6 V0.3 Te0.1 Mo0.5 V0.4 Te0.1 Mo0.3 V0.6 Te0.1 Mo0.6 V0.3 Te0.1 (pH = 2.8)
25 50 65 35
130 70 65 50
105 95 100 75
17 23 24 22
30 24 21 32
7.6 6.4 1.9 5.5
a
For the density estimate of 3.8 g/cm3 for the M1 phase [22] with the Mo/V = 2 in the Mo–V–Te–O system.
Table 2 The bulk (ICP) and local (TEM/EDS) elemental composition of the M1 and M2 phases present in model Mo–V–Te–O catalysts Synthesis composition
Bulk composition
M1 composition
(Mo/V)M1
M2 composition
(Mo/V)M2
Mo0.6 V0.3 Te0.1 Mo0.5 V0.4 Te0.1 Mo0.3 V0.6 Te0.1
Mo0.60 V0.30 Te0.09 Mo0.50 V0.29 Te0.11 Mo0.30 V0.27 Te0.08
Mo0.60 V0.32 Te0.07 Mo0.50 V0.30 Te0.22 Mo0.30 V0.15 Te0.05
1.90 1.65 2.00
Mo0.60 V0.76 Te0.08 Mo0.50 V0.74 Te0.16 Mo0.30 V0.31 Te0.02
0.80 0.68 0.96
pact on the morphology of mixed Mo–V–Te–O system. The Mo0.6 V0.3 Te0.1 and Mo0.6 V0.3 Te0.1 (pH = 2.8) catalysts showed the presence of uniform 10–30 m rods with 1–2 m cross-sections (Fig. 3a and d, respectively). The Mo0.5 V0.4 Te0.1 catalyst contained crystals with pinwheel shape (Fig. 3b) and the Mo0.3 V0.6 Te0.1 catalyst showed the presence of a more ordered and uniform pinwheel
crystals (Fig. 3c). The elemental ICP composition of the Mo0.6 V0.3 Te0.1 and Mo0.5 V0.4 Te0.1 catalysts was close to both their synthesis composition and the local EDS composition of the M1 phase in these catalysts, i.e. the Mo/V ratio ∼ 2 (Table 2). The agreement between the bulk (ICP) and local (EDS) elemental compositions for the M1 phase in the Mo0.6 V0.3 Te0.1 and Mo0.5 V0.4 Te0.1 catalysts confirmed the previous XRD observation that M1 was the dominant phase in these two catalysts. On the other hand, the Mo0.3 V0.6 Te0.1 catalyst had a much higher Mo/V ratio by ICP as compared to its synthesis composition. Furthermore, its bulk composition was closer to that of the M2 phase, i.e. the Mo/V ratio ∼ 1. According to XRD, this catalyst contained some M2 phase manifested in the presence of the characteristic reflection at 2Θ = 28.3◦ and the absence of the XRD reflections of the M1 phase at 2Θ < 10◦ . The local Mo/V stoichiometries of the M1 and M2 phases for the Mo–V–Te–O system agreed well with those previously reported for the four-component Mo–V–Te–Nb–O system [18–21]. Therefore, the bulk elemental composition of the two catalysts, namely Mo0.6 V0.3 Te0.1 and Mo0.5 V0.4 Te0.1 , agreed well with that of the M1 phase, while the lower ICP Mo/V ratio of the Mo0.3 V0.6 Te0.1 catalyst indicated the presence of the M2 phase. The Mo–V–Te–O catalyst with the Mo0.6 V0.3 Te0.1 composition reported previously by Ueda and Oshihara [7] was further selected to investigate the effects of the synthesis pH and water vapor present in the feed on propane oxidation to acrylic acid over the model Mo–V–Te–O catalysts. 3.2. Effect of synthesis pH
Fig. 3. Morphology of model Mo–V–Te–O catalysts after thermal treatment in N2 at 600 ◦ C for 2 h: (a) Mo0.6 V0.3 Te0.1 , (b) Mo0.5 V0.4 Te0.1 , (c) Mo0.3 V0.6 Te0.1 and (d) Mo0.6 V0.3 Te0.1 (pH = 2.8).
The effect of synthesis pH on the formation of the M1 and M2 phases was investigated for the Mo0.6 V0.3 Te0.1 composition. The synthesis pH was adjusted in the 1–4 range by 0.5 M NH4 OH or 0.5 M HNO3 relative to the original synthesis pH ∼ 2.2. The synthesis pH was critical for con-
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Fig. 4. XRD patterns of model Mo0.6 V0.3 Te0.1 oxides obtained in hydrothermal synthesis for 72 h at (a) pH = 1.0, (b) pH = 2.2 (natural), (c) pH = 2.8, (d) pH = 3.0 and (e) pH = 3.8.
trolling the phase composition and morphology of the resultant phases. For the synthesis pH = 1.0, the hexagonal Mo0.87 V0.13 O2.935 phase (JCPDS 48-0766) with the characteristic X-ray reflections at 2Θ = 9.6◦ , 16.7◦ , 19.3◦ , 25.6◦ , 29.4◦ and 45.3◦ was observed after hydrothermal synthesis at 175 ◦ C for 72 h (Fig. 4a). The M1 and M2 phases were obtained at synthesis pH = 2.0–2.8 (Fig. 4b and c), while increasingly disordered and poorly defined layered phases were observed at pH = 3.0–3.8 (Fig. 4d and e). Finally, extremely low yields of solid products were observed at pH > 4.0. At the synthesis pH > 2, the metal oxide precursors were present in aqueous solution as polyanionic hydroxo species, e.g. [H3 Mo7 O24 ]3− , [H2 Mo7 O24 ]4− and [HMo7 O24 ]5− in the case of Mo, and VO3 (OH)3− , V2 O7 6− and VO4 4− in the case of V [27,28]. The presence of Te species probably facilitated the formation of the mixed oxide M1 and M2 phases under hydrothermal conditions. Also, the low yield of solid products at pH > 4.0 may be explained by the increased negative charge of the polyanionic Mo and V species in solution. Therefore, it appears that well-defined M1 and M2 phases can be obtained only at synthesis pH near the isoelectric points of constituent metal oxide species.
475 and 428 cm−1 are likely due to the vibrations of Mo–O, V–O, Te–O, and bridging Mo–O–M (M = Mo, V, and Te) bonds, which are expected in this range [30]. The Raman spectra of the Mo0.6 V0.3 Te0.1 oxide after thermal activation in N2 displayed bands in the 945–830 cm−1 range assigned to the Mo–O vibrations (Fig. 6), whereas the band at ∼660 cm−1 were likely due to Mo–O, V–O or Te–O vibrations [31]. The spectra of all catalysts studied were dominated by the Raman bands at 991, 870, 830, 335, 282, 241, 214, 196 and 152 cm−1 that corresponded to the
3.3. Vibrational spectroscopy of Mo–V–Te–O system The IR spectrum of the Mo0.60 V0.30 Te0.10 catalyst after thermal activation in N2 is shown in Fig. 5. The broad IR bands were observed below 1000 cm−1 , most of which are attributable to the M=O and M–O–M bond vibrations (M = Mo or V). The IR peaks at 991, 848 and 580 cm−1 may be assigned to the Mo–O vibrations [29]. The peaks at 542,
Fig. 5. IR spectrum of model Mo0.6 V0.3 Te0.1 oxide after a thermal treatment in N2 at 600 ◦ C for 2 h.
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Fig. 6. Raman spectra of model Mo0.6 V0.3 Te0.1 oxide after thermal treatment in N2 at 600 ◦ C for 2 h: (a) under flowing air at 25 ◦ C and (b) under flowing propane at 300 ◦ C.
Mo-oxide phases (Fig. 6) [31]. When the Raman spectra were collected in the presence of pure propane at 300 ◦ C, the band at 870 cm−1 became more intense. The bands in the 860–880 cm−1 range corresponded to the bridging M–O–M modes of various metal oxide species in these mixed metal oxide phases. In the presence of propane, weak band at 1060 cm−1 was observed, which may be associated with the vibrations of adsorbed propoxy species on the Mo and V surface sites [32,33] indicating the activation of propane and formation of surface propoxy species at temperatures close to the reaction temperature of propane oxidation to acrylic acid.
to propylene, the feed contained 11.6 vol.% propane, 18.8 vol.% oxygen and the balance of He. For the selective oxidation of propane to acrylic acid, the kinetic data were collected at 380 ◦ C using a feed containing 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% water vapor and balance of He. The mixed Mo0.6 V0.3 Te0.1 oxide was a rather poor ODH catalyst as compared to the reported catalysts [1–5]. The presence of water vapor in the feed dramatically
3.4. Kinetic studies of mixed Mo–V–Te system 3.4.1. Effect of synthesis composition The Mo–V–Te–O catalysts (Mo0.6 V0.3 Te0.1 , Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 ) were studied in the selective oxidation of propane to acrylic acid. One of these compositions (Mo0.6 V0.3 Te0.1 ) has been studied previously in this reaction [7]. The performance of the representative Mo–V–Te–O catalysts in propane oxidation is shown in Tables 3–6 and Figs. 7 and 8. The results of propane oxidation to acrylic acid over the Mo0.6 V0.3 Te0.1 catalyst agreed well with that reported previously by Ueda and Oshihara [7] and shown in Table 3 for comparison. The acrylic acid and propylene yields observed were almost identical in both studies. In the case of oxidative dehydrogenation (ODH) of propane
Fig. 7. Propane oxidation to acrylic acid (AA) at 400 ◦ C over model Mo–V–Te oxide catalysts. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
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Table 3 Oxidative dehydrogenation (ODH) and selective oxidation of propane over mixed Mo0.6 V0.3 Te0.1 oxide catalyst at 380 ◦ C Conversion (C), selectivity (S) and yield (Y) (mol%)
ODHa
Selective oxidationb (this work)
Selective oxidationc (Ueda and Oshihara [7])
C(propane) S(Y) to propylene S(Y) to acrylic acid (AA) S(Y) to COx
64 26(15) 1(1) 73(44)
28 21(6) 49(14) 30(7)
26 14(4) 48(12) 26(7)
a b c
Feed composition: 11.6 vol.% propane, 18.8 vol.% oxygen and the balance He. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He. Feed composition: 3.3 vol.% propane, 10 vol.% oxygen, 46.7 vol.% H2 O and the balance N2 ; reaction temperature: 390 ◦ C.
Table 4 Selective oxidation of propane over mixed Mo0.6 V0.3 Te0.1 oxide catalysta T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
23 28 34 42 54
20 21 15 14 9
47 49 42 29 24
24 18 11 10 14
8 12 31 47 53
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
Table 5 Selective oxidation of propane over mixed Mo0.5 V0.4 Te0.1 oxide catalysta T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
23 34 40 54 64
26 7 5 5 4
44 67 68 42 31
22 16 10 10 8
7 11 17 44 57
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
Table 6 Selective oxidation of propane over mixed Mo0.3 V0.6 Te0.1 oxide catalysta T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
17 20 24 32 38
18 13 11 9 9
55 64 63 56 46
11 6 6 5 6
16 17 20 30 39
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
Fig. 8. Propane oxidation to acrylic acid (AA) at 380 ◦ C over the Mo0.5 V0.4 Te0.1 oxide catalyst. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
enhanced the selectivity of propane oxidation to acrylic acid. The effect of water vapor on the selectivity of propane oxidation to acrylic acid is discussed in more detail below. Our findings indicated that the selectivity to acrylic acid reached a maximum (49–68 mol%) at ∼380 ◦ C for all three catalysts. Moreover, the selectivity to propylene decreased with propane conversion, while the production of acrylic acid increased confirming previous observations that propylene is a reaction intermediate during propane oxidation to acrylic acid [1–6]. The Mo–V–Te–O catalysts can be ranked in terms of acrylic acid yield in the following order: Mo0.5 V0.4 Te0.1 > Mo0.3 V0.6 Te0.1 Mo0.6 V0.3 Te0.1 (Fig. 7). The Mo0.5 V0.4 Te0.1 oxide displayed the highest yield to acrylic acid of ∼27 mol% at 380–400 ◦ C which significantly exceeded previously reported performance of the Mo0.6 V0.3 Te0.1 catalyst [7]. A detailed selectivity versus
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Fig. 9. Selectivity to acrylic acid (AA) over the model Mo–V–Te–O catalysts at 380 ◦ C and 20–34% propane conversion as a function of relative exposure of the ab planes of the M1 phase. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He. Fig. 10. Arrhenius plots of the rate constants of propane oxidation over the model Mo–V–Te–O catalysts.
conversion study was conducted for this catalyst in order to find conditions that maximized the acrylic acid yield. The study was carried out at 380 ◦ C with the same feed composition while varying the feed flow rate (4.8–58.0 cm3 /min or GHSV of 288–3480 h−1 ). The highest yield of acrylic acid of ∼31 mol% was obtained for the Mo0.5 V0.4 Te0.1 oxide catalyst at 380 ◦ C and propane conversion of ∼65 mol% (Fig. 8), which approached the acrylic acid yields reported for the optimized Mo–V–Te–Nb–O system [6,9–12]. The performance of the model Mo–V–Te–O catalysts in propane oxidation to acrylic acid was further correlated with the relative exposure of the ab planes of the M1 phase proposed to contain the active and selective surface sites for this reaction. This correlation was done for the propane oxidation data at 380 ◦ C corresponding to the range of propane conversions of 20–34 mol% (Tables 4–6) and shown in Fig. 9. Although the selectivity to acrylic acid shows some dependence on propane conversion in this range, the data shown in Fig. 9 indicated that the selectivity of these catalysts in propane oxidation to acrylic acid exhibit a trend with the extent of exposure of the ab planes. This observation suggests the importance of the surface ab planes for selective oxidation of propane to acrylic acid. We further examined the apparent activation energy of propane oxidation over the model Mo–V–Te–O catalysts. The apparent activation energy for this reaction determined from the propane consumption rate data was E1 = 37 ± 5 kJ/mol for all three catalysts (Fig. 10). This value of activation energy for propane oxidation is significantly below
the values reported in the literature for the propane ODH to propylene over well-defined supported vanadia (99 kJ/mol) and molybdena (117 kJ/mol) catalysts [34]. Chen et al. [34] noted that the propane ODH rates reflect an apparent activation energy given by the sum of the enthalpy of propane adsorption and the C–H bond activation energy, E1 = H1ads + HIIr . Since molecular adsorption of propane is very weak, the apparent activation energy values reflect predominantly HIIr values. The low values of E1 and HIIr observed for propane activation over the Mo–V–Te–O catalysts are due to concerted reaction of C–H bond activation by lattice oxygen atoms, which leads to HIIr values much lower than C–H bond dissociation energies. Evidently, the unique structure of the surface ab planes of the M1 phase which provides reducible V(V) and moderately strong Lewis acid sites [18,23] is responsible for the ability of this phase to activate propane selectively and control adsorption of the surface intermediates, such as propylene and acrolein, during their oxidation to acrylic acid. 3.4.2. Effect of synthesis pH The effect of the synthesis pH on the catalytic performance of the Mo–V–Te–O system was studied for the representative Mo0.6 V0.3 Te0.1 composition obtained at both the natural pH ∼ 2.2 and pH = 2.8 (Tables 4 and 7). This pH = 2.8 was selected because it was the highest synthesis pH at which only M1 and M2 phases could be obtained (Fig. 4d).
Table 7 Selective oxidation of propane over Mo0.6 V0.3 Te0.1 oxide catalyst synthesized at pH = 2.8a T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
28 43 51 54 63
16 17 16 13 11
49 50 43 32 24
21 16 17 19 21
13 16 25 36 45
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
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The bulk elemental analysis indicated a higher Mo/V ratio in this catalyst (ICP composition: Mo0.60 V0.21 Te0.12 ) as compared to the Mo0.6 V0.3 Te0.1 catalyst obtained at the natural pH (Table 2). The Mo0.6 V0.3 Te0.1 catalyst (pH = 2.8) was characterized by higher activity in propane oxidation and selectivity to acrylic acid (Table 7) as compared to the Mo0.6 V0.3 Te0.1 catalyst synthesized at natural pH (Table 4). This catalyst showed higher selectivity to acrylic acid at higher propane conversion during propane oxidation in the presence of water vapor in the feed at 380–400 ◦ C than the Mo0.6 V0.3 Te0.1 catalyst synthesized at natural pH ∼ 2.2. The examination of the M1 phase present in this catalyst (Table 1) suggested that the ab planes were exposed to a higher degree than in the original Mo0.6 V0.3 Te0.1 catalyst obtained at pH = 2.2, i.e. 22% versus 17% (Table 1), which may account for its higher selectivity to acrylic acid during propane oxidation. 3.4.3. Effect of water vapor The effect of water vapor was studied at 400 ◦ C employing the model Mo0.6 V0.3 Te0.1 oxide catalyst and the feed containing propane:O2 :He = 11.6:18.8:69.6 (mol% on dry basis) and 0–66 mol% water vapor as a function of time on stream (0–14 days). The presence of water vapor in the feed had a profound effect on the selectivity of propane oxidation to acrylic acid. The results shown in Fig. 11 indicate that the selectivity to acrylic acid continuously increased as a function of water content in the feed at constant propane conversion ∼43 mol%. For the entire range of water vapor concentrations studied, the propylene and CO2 yields decreased as the water content increased, while the formation of CO was enhanced with the water content in the feed. Formation of acetic acid was unaffected by the changes in the
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water vapor concentration in the feed, suggesting that different active surface sites were involved in propane oxidation to acetic acid. These observations indicated that water has two distinct effects in propane oxidation over bulk multicomponent vanadium molybdate catalysts. Firstly, water enhances the rate of propylene oxidation to acrylic acid, probably through its hydration to isopropanol, manifested in decreasing yields of propylene with the water content in the feed. The presence of water provides an abundant source of OH− species on the catalyst surface, which according to Volta and coworkers exhibit electrophilic properties required for selective oxidation of hydrocarbons [33]. Recent findings for propane oxidation to acrylic acid over structurally distinct Mo–V–Sb–Nb–O phases further suggested that water participates directly in oxygen insertion steps, enhances the apparent rates of formation of oxygenates as well as maintains the catalytic surface in a partially reduced oxidation state [35–37]. Secondly, water plays a role in controlling desorption of acrylic acid manifested in the opposite trends observed for the rates of acrylic acid and CO versus CO2 formation in the presence of water.
4. Conclusions The Mo–V–Te–O catalysts containing the M1 and M2 phases with well-defined crystal morphologies were obtained hydrothermally over a wide range of synthesis compositions (Mo0.70–0.30 V0.60–0.20 Te0.15–0.05 ). The elemental and structural analyses of this compositionally simple model system indicated that the orthorhombic M1 phase with the bulk Mo/V ∼ 2 ratio is dominant in the Mo–V–Te–O catalysts. This phase has been proposed as active and selective for propane oxidation to oxygenates and ammoxidation to acrylonitrile [18–23]. These model catalysts displayed high yields of acrylic acid in the presence of water vapor in the feed, which is believed to enhance the apparent rates of formation of oxygenates as well as maintain the catalytic surface in a partially reduced oxidation state. The selectivity to acrylic acid over these model catalysts correlated with the extent of exposure of the surface ab planes of the M1 phase proposed to contain the active and selective surface sites [7,21,23]. Therefore, the Mo–V–Te–O catalysts represent a well defined and highly promising model system for elucidating the surface molecular structure–activity/selectivity relationships in propane oxidation to acrylic acid over multicomponent Mo–V–Te–Nb–O catalysts.
Acknowledgements Fig. 11. Effect of water on propane oxidation over Mo0.6 V0.3 Te0.1 oxide catalyst at 380 ◦ C and 45 mol% propane conversion. Feed composition (on dry basis): 11.6 vol.% propane, 18.8 vol.% oxygen and 69.6 vol.% He (AA: acrylic acid).
V.V. Guliants acknowledges the National Science Foundation for the NSF Career Award (CTS-0238962). J.N. Al-Saeedi thanks the Public Authority for Applied Education, Kuwait, for the Ph.D. fellowship. Rajiv Soman thanks
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Bulk mixed Mo–V–Te–O catalysts for propane oxidation to acrylic acid Vadim V. Guliants a,∗ , Rishabh Bhandari a , Jamal N. Al-Saeedi a,1 , Vijay K. Vasudevan a , Rajiv S. Soman b , Olga Guerrero-Pérez c , Miguel A. Bañares c a
Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, USA b College of Applied Science, University of Cincinnati, Cincinnati, OH 45206-2839, USA c Instituto de Catalisis y Petroleoquimica, CSIC, E-28049 Madrid, Spain Received in revised form 24 May 2004; accepted 29 May 2004 Available online 8 July 2004
Abstract The model Mo–V–Te–O catalysts containing the orthorhombic (M1) and hexagonal (M2) phases with well-defined crystal morphologies and promising for the selective oxidation of propane to oxygenates were obtained hydrothermally over a wide range of synthesis compositions (Mo0.70–0.30 V0.60–0.20 Te0.15–0.05 ). The bulk (ICP), local (EDS) elemental and structural (XRD) analyses of this compositionally simple model system indicated that the M1 phase with the bulk Mo/V ratio of ∼2 was dominant in the Mo–V–Te–O catalysts. This phase has been proposed as active and selective for propane oxidation to oxygenates and ammoxidation to acrylonitrile. The model catalysts displayed high selectivity to acrylic acid in the presence of water vapor in the feed, which is believed to enhance the apparent rates of formation of oxygenates as well as maintain the catalytic surface in a partially reduced oxidation state. The selectivity to acrylic acid over these model catalysts correlated with the extent of exposure of the surface ab planes of the M1 phase proposed to contain the active and selective surface sites. Therefore, the Mo–V–Te–O catalysts represent a well-defined and highly promising model system for elucidating the surface molecular structure–activity/selectivity relationships in propane oxidation to acrylic acid over multicomponent Mo–V–Te–Nb–O catalysts. © 2004 Elsevier B.V. All rights reserved. Keywords: Mixed Mo–V–Te oxides; Hydrothermal synthesis; Phase diagram; Propane oxidation
1. Introduction The past several years have seen a significant growth in the number of industrial patents and publications on the one-step propane oxidation to oxygenates, i.e. acrolein and acrylic acid, and ammoxidation to acrylonitrile over several promising bulk mixed metal oxides [1–16]. The recently discovered Mo–V–Te–Nb–O catalytic system [6,9,11,12,16] appears to be the most active and selective for these transformations. These catalysts are prepared in aqueous medium by reacting metal oxide sources, e.g. ammonium heptamolybdate, vanadyl sulfate, tellurium dioxide and ammonium niobium oxalate at 353–433 K and calcined in an inert N2 environ∗
Corresponding author. Tel.: +1 513 556 0203; fax: +1 513 556 3473. E-mail address: [email protected] (V.V. Guliants). 1 Current address: Department of Chemical and Petroleum Technology, College of Technological Studies, Kuwait. 0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2004.05.049
ment to prevent the phase separation of metal oxide components. This synthesis results in layered V–Mo–Te–Nb–O phases with a 4 Å d-spacing that are commonly observed in multicomponent solid solutions of V, Nb and other metal ions in Mo5+ /Mo6+ suboxides [17]. The multicomponent vanadium molybdate catalysts were reported to contain two crystalline phases, so-called “M1” and “M2”, proposed to be responsible for the propane activation to propylene and its subsequent oxidation to oxygenates or ammoxidation to acrylonitrile [16]. Recently, the M1 and M2 phases were shown to have, respectively, orthorhombic and hexagonal structures with the (Te2 O)M20 O56 and (TeO)M2 O9 (M = Mo, V, Nb) compositions [18–22]. The studies of propane (amm)oxidation indicated that the orthorhombic phase is the most active and selective for both of these transformations, although a synergism due to a cooperation between these phases was observed at high propane conversion [18,23]. The M1 and M2 phases possessing well-defined rod-like
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crystal morphology were also obtained hydrothermally in the Nb-free system (Mo0.6 V0.3 Te0.1 ) that selectively transformed ethane and propane to ethylene and acrylic acid, respectively [7,21,24,25]. Therefore, the model Mo–V–Te–O system appears to be particularly promising for studies of the surface molecular structure–activity/selectivity relationships in selective transformation of propane to oxygenates and acrylonitrile due to its compositional simplicity and well-defined crystal morphology. In this study, we investigate the synthesis phase diagram of this model catalytic system as well as the role of pH in the nucleation and growth of mixed Mo–V–Te–O phases with distinct crystal morphologies. We further examine the local composition of the M1 and M2 phases, their vibrational spectroscopic characteristics (IR, Raman) and performance in selective oxidation of propane to acrylic acid.
2. Experimental 2.1. Catalyst preparation Mixed Mo–V–Te–O catalysts were prepared hydrothermally using ammonium paramolybdate (81–83% as MoO3 , Alfa Aesar), vanadyl sulfate (99.9%, Alfa Aesar) and TeO2 (99.99%, Alfa Aesar) as the metal oxide sources. The Mo–V–Te–O phase diagram was explored for 28 synthesis compositions in the Mo0–0.9 V0–0.9 Te0.05–0.5 range. In a typical synthesis, exemplified here for the Mo0.6 V0.3 Te0.1 composition, ammonium paramolybdate (6.80 g, 38.7 mmol of Mo6+ ) was dissolved in 100 ml of deionized water while stirring at room temperature. TeO2 (0.96 g, 6.0 mmol of Te4+ ) was added to the resulting solution leading to white suspension and the mixture was stirred for 0.5 h. Vanadyl sulfate (4.68 g, 19.2 mmol of V4+ ) was dissolved in 60 ml of deionized water at room temperature and added dropwise to the Mo–Te suspension. The mixture became viscous brown upon V4+ addition, then a dark pink homogeneous gel. The gel was then divided into portions in which pH was adjusted using NH4 OH (0.5 M) and HNO3 (0.5 M) to values in the pH = 1.0–4.0 range (the synthesis gel has the pH = 2.2 ± 0.1). The resultant dark pink gel was placed in a Teflon-lined stainless steel autoclave and heated for 72 h at 175 ◦ C. The black solid was filtered, washed with deionized water and dried for 12 h at 80 ◦ C. The mixed metal oxide product was calcined at 600 ◦ C for 2 h under flowing N2 (40–60 cm3 /min) in a tubular furnace. The temperature was ramped to 600 ◦ C at 5 ◦ C/min. The mixed Mo–V–Te–Nb–O catalyst with a Mo1.00 V0.31 Te0.17 Nb0.19 synthesis composition was synthesized in a control experiment under hydrothermal conditions [16]. 2.2. Physicochemical characterization The elemental composition of the Mo–V–Te oxides was characterized by a Leeman Labs’ ICP/Echelle Spectrome-
ter, Model PS1000 ICP, equipped with a Hildebrand Grid nebulizer and using Mo, V and Te elemental standards (Alfa Aesar). The BET surface areas were determined from the N2 adsorption isotherms (0.05 < P/P0 < 0.3) at −196 ◦ C using a Micromeritics TriStar 3000 porosimeter. The model Mo–V–Te–O catalysts were characterized by powder X-ray diffraction employing Siemens D500 diffractometer (Cu K␣ radiation). The SEM images were collected using Hitachi S-900 scanning electron microscope. Transmission electron microscopy (TEM) was performed at the Argonne National Laboratory using a Philips CM30 transmission electron microscope operated at 200 kV with a useful limit of information of 0.14 nm. This instrument was equipped with an EDAX-NX2 (spatial resolution ∼1 nm) for the EDS analysis of the local elemental composition. MoO3 , V2 O5 and TeO2 were used as the EDS elemental standards to determine the respective Cliff-Lorimer sensitivity k-factors. The IR spectra were collected in the transmission mode on a BioRad FTS60. The Raman spectra were collected using a single monochromator Renishaw System 1000 equipped with a cooled CCD detector (−73 ◦ C) and holographic super-notch filter. The samples were excited with the 514 nm Ar+ line; the spectral resolution was ca. 3 cm−1 and spectrum acquisition consisted of 20 accumulations of 30 s each. The Raman spectra under dehydrated conditions were collected using a hot stage (Linkam TS-1500). The kinetic studies of propane oxidation were conducted at 350–450 ◦ C using a feed of 6.3 vol.% propane, 9.4 vol.% oxygen, 0–66.4 vol.% water vapor and balance He and 1.0 g of catalyst packed in a 3/8 in. quartz tubular microreactor in a programmable oven. An HP 5890II gas chromatograph equipped with an FID and TCD was employed for the reaction product analysis. The carbon balances agreed within 5 mol%.
3. Results and discussions 3.1. Hydrothermal synthesis of Mo–V–Te–O system The ternary synthesis diagram of the mixed Mo–V–Te–O system was investigated at pH = 2.2 with a particular focus on the formation of the M1 and M2 phases. The M1 phase was identified by the major XRD reflections at 6.6◦ , 7.8◦ , 9.0◦ , 13.0◦ , 22.1◦ , 26.2◦ , 26.8◦ , 27.6◦ and 45.2◦ , while the M2 phase was characterized by the major reflections at 22.1◦ , 28.3◦ , 36.2◦ and 44.6◦ [15,16,18–25]. Although a significant overlap of the XRD reflections is observed for these two phases, the M1 phase may be detected by the presence of reflections at 2Θ < 10◦ , while the presence of the M2 phase is manifested in the characteristic reflection at 2Θ = 28.3◦ . The four-component Mo1.00 V0.31 Te0.17 Nb0.19 oxide catalyst prepared as a reference under hydrothermal conditions contained predominantly M1 and some M2 phase (Fig. 1). The M1 and M2 phases were observed exclusively for the Mo0.70–0.30 V0.60–0.20 Te0.15–0.05 synthesis compo-
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Fig. 1. XRD patterns of mixed Mo0.6 V0.3 Te0.1 and Mo1.00 V0.31 Te0.17 Nb0.19 oxides. The peaks at 38.5◦ and 44.7◦ 2Θ correspond to the aluminum material of the sample holder.
sitions (Figs. 1 and 2), while the impurity phases, such as MoO3 (JCPDS 76-1003), (Mo0.93 V0.07 )5 O14 (JCPDS 31-1437) and Mo5 TeO16 (JCPDS 31-0874), were detected for more Mo- and Te-rich synthesis compositions (not shown here). The XRD patterns of three freshly calcined Mo–V–Te–O catalysts with the synthesis compositions Mo0.6 V0.3 Te0.1 , Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 are shown in Fig. 2. The XRD patterns of these three compositions indicated the presence of essentially pure M1 phase with the exception of the Mo0.3 V0.6 Te0.1 catalyst, which also contained some M2 phase. The M1 crystallite size and aspect ratios in the Mo–V–Te–O catalysts were estimated by the Scherrer formula [26] for the (1 2 0), (2 1 0) and (0 0 1) reflections of the M1 phase at 7.8◦ , 9.0◦ and 22.1◦ 2Θ, respectively,
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and shown in Table 1. This analysis revealed that the Mo–V–Te–O phases possessed nanoscale dimensions in agreement with previous observations [15]. The M1 crystal size in the c-axis direction was approximately the same in these catalysts, while the crystal dimensions in the ab plane varied from sample to sample. The ab planes have been recently proposed to contain the active and selective surface sites for propane oxidation to acrylic acid and ammoxidation to acrylonitrile [7,23,24,25]. In order to verify this hypothesis, we estimated the relative exposure of the ab planes of the M1 phase in these catalysts assuming the crystals to have a platelet shape as recently observed by high-resolution TEM [15]. The estimates obtained (Table 1) suggested that the ab planes are not exposed to a great degree for the platelet morphology of these catalysts and that other crystallographic planes are predominant. However, among the three catalysts, the Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 catalysts exposed the ab planes to a greater degree (∼40% higher) than the Mo0.6 V0.3 Te0.1 catalyst. Theoretical surface area estimates for these M1 crystals, S(theory) in Table 1, are much higher than the experimental BET surface areas for the model Mo–V–Te–O catalysts, suggesting that these catalysts contain dense aggregates of multiple M1 crystals. Grinding these catalysts has a beneficial effect on their selectivity to acrylic acid [7], because it breaks up the M1 aggregates and, to some degree, individual crystals reducing their dimensions along the c-axis, and, therefore, enhances the exposure of the ab planes. The morphology and elemental composition of the three selected catalysts (Mo0.6 V0.3 Te0.1 , Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 ) were studied by SEM (Fig. 3a–c), ICP and EDS (Table 2). The N2 adsorption measurements indicated that the BET surface areas of all catalysts were quite low (Table 1). The synthesis composition had a dramatic im-
Fig. 2. XRD pattern of three selected compositions of mixed Mo–V–Te–O system.
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Table 1 Crystal sizes and surface areas of the M1 phase present in model Mo–V–Te–O catalysts Synthesis composition
a-Axis (nm)
b-Axis (nm)
c-Axis (nm)
ab planes (%)
S(theory)a (m2 /g)
SBET (m2 /g)
Mo0.6 V0.3 Te0.1 Mo0.5 V0.4 Te0.1 Mo0.3 V0.6 Te0.1 Mo0.6 V0.3 Te0.1 (pH = 2.8)
25 50 65 35
130 70 65 50
105 95 100 75
17 23 24 22
30 24 21 32
7.6 6.4 1.9 5.5
a
For the density estimate of 3.8 g/cm3 for the M1 phase [22] with the Mo/V = 2 in the Mo–V–Te–O system.
Table 2 The bulk (ICP) and local (TEM/EDS) elemental composition of the M1 and M2 phases present in model Mo–V–Te–O catalysts Synthesis composition
Bulk composition
M1 composition
(Mo/V)M1
M2 composition
(Mo/V)M2
Mo0.6 V0.3 Te0.1 Mo0.5 V0.4 Te0.1 Mo0.3 V0.6 Te0.1
Mo0.60 V0.30 Te0.09 Mo0.50 V0.29 Te0.11 Mo0.30 V0.27 Te0.08
Mo0.60 V0.32 Te0.07 Mo0.50 V0.30 Te0.22 Mo0.30 V0.15 Te0.05
1.90 1.65 2.00
Mo0.60 V0.76 Te0.08 Mo0.50 V0.74 Te0.16 Mo0.30 V0.31 Te0.02
0.80 0.68 0.96
pact on the morphology of mixed Mo–V–Te–O system. The Mo0.6 V0.3 Te0.1 and Mo0.6 V0.3 Te0.1 (pH = 2.8) catalysts showed the presence of uniform 10–30 m rods with 1–2 m cross-sections (Fig. 3a and d, respectively). The Mo0.5 V0.4 Te0.1 catalyst contained crystals with pinwheel shape (Fig. 3b) and the Mo0.3 V0.6 Te0.1 catalyst showed the presence of a more ordered and uniform pinwheel
crystals (Fig. 3c). The elemental ICP composition of the Mo0.6 V0.3 Te0.1 and Mo0.5 V0.4 Te0.1 catalysts was close to both their synthesis composition and the local EDS composition of the M1 phase in these catalysts, i.e. the Mo/V ratio ∼ 2 (Table 2). The agreement between the bulk (ICP) and local (EDS) elemental compositions for the M1 phase in the Mo0.6 V0.3 Te0.1 and Mo0.5 V0.4 Te0.1 catalysts confirmed the previous XRD observation that M1 was the dominant phase in these two catalysts. On the other hand, the Mo0.3 V0.6 Te0.1 catalyst had a much higher Mo/V ratio by ICP as compared to its synthesis composition. Furthermore, its bulk composition was closer to that of the M2 phase, i.e. the Mo/V ratio ∼ 1. According to XRD, this catalyst contained some M2 phase manifested in the presence of the characteristic reflection at 2Θ = 28.3◦ and the absence of the XRD reflections of the M1 phase at 2Θ < 10◦ . The local Mo/V stoichiometries of the M1 and M2 phases for the Mo–V–Te–O system agreed well with those previously reported for the four-component Mo–V–Te–Nb–O system [18–21]. Therefore, the bulk elemental composition of the two catalysts, namely Mo0.6 V0.3 Te0.1 and Mo0.5 V0.4 Te0.1 , agreed well with that of the M1 phase, while the lower ICP Mo/V ratio of the Mo0.3 V0.6 Te0.1 catalyst indicated the presence of the M2 phase. The Mo–V–Te–O catalyst with the Mo0.6 V0.3 Te0.1 composition reported previously by Ueda and Oshihara [7] was further selected to investigate the effects of the synthesis pH and water vapor present in the feed on propane oxidation to acrylic acid over the model Mo–V–Te–O catalysts. 3.2. Effect of synthesis pH
Fig. 3. Morphology of model Mo–V–Te–O catalysts after thermal treatment in N2 at 600 ◦ C for 2 h: (a) Mo0.6 V0.3 Te0.1 , (b) Mo0.5 V0.4 Te0.1 , (c) Mo0.3 V0.6 Te0.1 and (d) Mo0.6 V0.3 Te0.1 (pH = 2.8).
The effect of synthesis pH on the formation of the M1 and M2 phases was investigated for the Mo0.6 V0.3 Te0.1 composition. The synthesis pH was adjusted in the 1–4 range by 0.5 M NH4 OH or 0.5 M HNO3 relative to the original synthesis pH ∼ 2.2. The synthesis pH was critical for con-
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Fig. 4. XRD patterns of model Mo0.6 V0.3 Te0.1 oxides obtained in hydrothermal synthesis for 72 h at (a) pH = 1.0, (b) pH = 2.2 (natural), (c) pH = 2.8, (d) pH = 3.0 and (e) pH = 3.8.
trolling the phase composition and morphology of the resultant phases. For the synthesis pH = 1.0, the hexagonal Mo0.87 V0.13 O2.935 phase (JCPDS 48-0766) with the characteristic X-ray reflections at 2Θ = 9.6◦ , 16.7◦ , 19.3◦ , 25.6◦ , 29.4◦ and 45.3◦ was observed after hydrothermal synthesis at 175 ◦ C for 72 h (Fig. 4a). The M1 and M2 phases were obtained at synthesis pH = 2.0–2.8 (Fig. 4b and c), while increasingly disordered and poorly defined layered phases were observed at pH = 3.0–3.8 (Fig. 4d and e). Finally, extremely low yields of solid products were observed at pH > 4.0. At the synthesis pH > 2, the metal oxide precursors were present in aqueous solution as polyanionic hydroxo species, e.g. [H3 Mo7 O24 ]3− , [H2 Mo7 O24 ]4− and [HMo7 O24 ]5− in the case of Mo, and VO3 (OH)3− , V2 O7 6− and VO4 4− in the case of V [27,28]. The presence of Te species probably facilitated the formation of the mixed oxide M1 and M2 phases under hydrothermal conditions. Also, the low yield of solid products at pH > 4.0 may be explained by the increased negative charge of the polyanionic Mo and V species in solution. Therefore, it appears that well-defined M1 and M2 phases can be obtained only at synthesis pH near the isoelectric points of constituent metal oxide species.
475 and 428 cm−1 are likely due to the vibrations of Mo–O, V–O, Te–O, and bridging Mo–O–M (M = Mo, V, and Te) bonds, which are expected in this range [30]. The Raman spectra of the Mo0.6 V0.3 Te0.1 oxide after thermal activation in N2 displayed bands in the 945–830 cm−1 range assigned to the Mo–O vibrations (Fig. 6), whereas the band at ∼660 cm−1 were likely due to Mo–O, V–O or Te–O vibrations [31]. The spectra of all catalysts studied were dominated by the Raman bands at 991, 870, 830, 335, 282, 241, 214, 196 and 152 cm−1 that corresponded to the
3.3. Vibrational spectroscopy of Mo–V–Te–O system The IR spectrum of the Mo0.60 V0.30 Te0.10 catalyst after thermal activation in N2 is shown in Fig. 5. The broad IR bands were observed below 1000 cm−1 , most of which are attributable to the M=O and M–O–M bond vibrations (M = Mo or V). The IR peaks at 991, 848 and 580 cm−1 may be assigned to the Mo–O vibrations [29]. The peaks at 542,
Fig. 5. IR spectrum of model Mo0.6 V0.3 Te0.1 oxide after a thermal treatment in N2 at 600 ◦ C for 2 h.
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Fig. 6. Raman spectra of model Mo0.6 V0.3 Te0.1 oxide after thermal treatment in N2 at 600 ◦ C for 2 h: (a) under flowing air at 25 ◦ C and (b) under flowing propane at 300 ◦ C.
Mo-oxide phases (Fig. 6) [31]. When the Raman spectra were collected in the presence of pure propane at 300 ◦ C, the band at 870 cm−1 became more intense. The bands in the 860–880 cm−1 range corresponded to the bridging M–O–M modes of various metal oxide species in these mixed metal oxide phases. In the presence of propane, weak band at 1060 cm−1 was observed, which may be associated with the vibrations of adsorbed propoxy species on the Mo and V surface sites [32,33] indicating the activation of propane and formation of surface propoxy species at temperatures close to the reaction temperature of propane oxidation to acrylic acid.
to propylene, the feed contained 11.6 vol.% propane, 18.8 vol.% oxygen and the balance of He. For the selective oxidation of propane to acrylic acid, the kinetic data were collected at 380 ◦ C using a feed containing 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% water vapor and balance of He. The mixed Mo0.6 V0.3 Te0.1 oxide was a rather poor ODH catalyst as compared to the reported catalysts [1–5]. The presence of water vapor in the feed dramatically
3.4. Kinetic studies of mixed Mo–V–Te system 3.4.1. Effect of synthesis composition The Mo–V–Te–O catalysts (Mo0.6 V0.3 Te0.1 , Mo0.5 V0.4 Te0.1 and Mo0.3 V0.6 Te0.1 ) were studied in the selective oxidation of propane to acrylic acid. One of these compositions (Mo0.6 V0.3 Te0.1 ) has been studied previously in this reaction [7]. The performance of the representative Mo–V–Te–O catalysts in propane oxidation is shown in Tables 3–6 and Figs. 7 and 8. The results of propane oxidation to acrylic acid over the Mo0.6 V0.3 Te0.1 catalyst agreed well with that reported previously by Ueda and Oshihara [7] and shown in Table 3 for comparison. The acrylic acid and propylene yields observed were almost identical in both studies. In the case of oxidative dehydrogenation (ODH) of propane
Fig. 7. Propane oxidation to acrylic acid (AA) at 400 ◦ C over model Mo–V–Te oxide catalysts. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
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Table 3 Oxidative dehydrogenation (ODH) and selective oxidation of propane over mixed Mo0.6 V0.3 Te0.1 oxide catalyst at 380 ◦ C Conversion (C), selectivity (S) and yield (Y) (mol%)
ODHa
Selective oxidationb (this work)
Selective oxidationc (Ueda and Oshihara [7])
C(propane) S(Y) to propylene S(Y) to acrylic acid (AA) S(Y) to COx
64 26(15) 1(1) 73(44)
28 21(6) 49(14) 30(7)
26 14(4) 48(12) 26(7)
a b c
Feed composition: 11.6 vol.% propane, 18.8 vol.% oxygen and the balance He. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He. Feed composition: 3.3 vol.% propane, 10 vol.% oxygen, 46.7 vol.% H2 O and the balance N2 ; reaction temperature: 390 ◦ C.
Table 4 Selective oxidation of propane over mixed Mo0.6 V0.3 Te0.1 oxide catalysta T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
23 28 34 42 54
20 21 15 14 9
47 49 42 29 24
24 18 11 10 14
8 12 31 47 53
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
Table 5 Selective oxidation of propane over mixed Mo0.5 V0.4 Te0.1 oxide catalysta T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
23 34 40 54 64
26 7 5 5 4
44 67 68 42 31
22 16 10 10 8
7 11 17 44 57
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
Table 6 Selective oxidation of propane over mixed Mo0.3 V0.6 Te0.1 oxide catalysta T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
17 20 24 32 38
18 13 11 9 9
55 64 63 56 46
11 6 6 5 6
16 17 20 30 39
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
Fig. 8. Propane oxidation to acrylic acid (AA) at 380 ◦ C over the Mo0.5 V0.4 Te0.1 oxide catalyst. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
enhanced the selectivity of propane oxidation to acrylic acid. The effect of water vapor on the selectivity of propane oxidation to acrylic acid is discussed in more detail below. Our findings indicated that the selectivity to acrylic acid reached a maximum (49–68 mol%) at ∼380 ◦ C for all three catalysts. Moreover, the selectivity to propylene decreased with propane conversion, while the production of acrylic acid increased confirming previous observations that propylene is a reaction intermediate during propane oxidation to acrylic acid [1–6]. The Mo–V–Te–O catalysts can be ranked in terms of acrylic acid yield in the following order: Mo0.5 V0.4 Te0.1 > Mo0.3 V0.6 Te0.1 Mo0.6 V0.3 Te0.1 (Fig. 7). The Mo0.5 V0.4 Te0.1 oxide displayed the highest yield to acrylic acid of ∼27 mol% at 380–400 ◦ C which significantly exceeded previously reported performance of the Mo0.6 V0.3 Te0.1 catalyst [7]. A detailed selectivity versus
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Fig. 9. Selectivity to acrylic acid (AA) over the model Mo–V–Te–O catalysts at 380 ◦ C and 20–34% propane conversion as a function of relative exposure of the ab planes of the M1 phase. Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He. Fig. 10. Arrhenius plots of the rate constants of propane oxidation over the model Mo–V–Te–O catalysts.
conversion study was conducted for this catalyst in order to find conditions that maximized the acrylic acid yield. The study was carried out at 380 ◦ C with the same feed composition while varying the feed flow rate (4.8–58.0 cm3 /min or GHSV of 288–3480 h−1 ). The highest yield of acrylic acid of ∼31 mol% was obtained for the Mo0.5 V0.4 Te0.1 oxide catalyst at 380 ◦ C and propane conversion of ∼65 mol% (Fig. 8), which approached the acrylic acid yields reported for the optimized Mo–V–Te–Nb–O system [6,9–12]. The performance of the model Mo–V–Te–O catalysts in propane oxidation to acrylic acid was further correlated with the relative exposure of the ab planes of the M1 phase proposed to contain the active and selective surface sites for this reaction. This correlation was done for the propane oxidation data at 380 ◦ C corresponding to the range of propane conversions of 20–34 mol% (Tables 4–6) and shown in Fig. 9. Although the selectivity to acrylic acid shows some dependence on propane conversion in this range, the data shown in Fig. 9 indicated that the selectivity of these catalysts in propane oxidation to acrylic acid exhibit a trend with the extent of exposure of the ab planes. This observation suggests the importance of the surface ab planes for selective oxidation of propane to acrylic acid. We further examined the apparent activation energy of propane oxidation over the model Mo–V–Te–O catalysts. The apparent activation energy for this reaction determined from the propane consumption rate data was E1 = 37 ± 5 kJ/mol for all three catalysts (Fig. 10). This value of activation energy for propane oxidation is significantly below
the values reported in the literature for the propane ODH to propylene over well-defined supported vanadia (99 kJ/mol) and molybdena (117 kJ/mol) catalysts [34]. Chen et al. [34] noted that the propane ODH rates reflect an apparent activation energy given by the sum of the enthalpy of propane adsorption and the C–H bond activation energy, E1 = H1ads + HIIr . Since molecular adsorption of propane is very weak, the apparent activation energy values reflect predominantly HIIr values. The low values of E1 and HIIr observed for propane activation over the Mo–V–Te–O catalysts are due to concerted reaction of C–H bond activation by lattice oxygen atoms, which leads to HIIr values much lower than C–H bond dissociation energies. Evidently, the unique structure of the surface ab planes of the M1 phase which provides reducible V(V) and moderately strong Lewis acid sites [18,23] is responsible for the ability of this phase to activate propane selectively and control adsorption of the surface intermediates, such as propylene and acrolein, during their oxidation to acrylic acid. 3.4.2. Effect of synthesis pH The effect of the synthesis pH on the catalytic performance of the Mo–V–Te–O system was studied for the representative Mo0.6 V0.3 Te0.1 composition obtained at both the natural pH ∼ 2.2 and pH = 2.8 (Tables 4 and 7). This pH = 2.8 was selected because it was the highest synthesis pH at which only M1 and M2 phases could be obtained (Fig. 4d).
Table 7 Selective oxidation of propane over Mo0.6 V0.3 Te0.1 oxide catalyst synthesized at pH = 2.8a T (◦ C)
C(propane) (mol%)
S(C3 H6 ) (mol%)
S(AA) (mol%)
S(CO) (mol%)
S(CO2 ) (mol%)
360 380 400 420 450
28 43 51 54 63
16 17 16 13 11
49 50 43 32 24
21 16 17 19 21
13 16 25 36 45
a
Feed composition: 6.3 vol.% propane, 9.4 vol.% oxygen, 47.3 vol.% H2 O and the balance He.
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The bulk elemental analysis indicated a higher Mo/V ratio in this catalyst (ICP composition: Mo0.60 V0.21 Te0.12 ) as compared to the Mo0.6 V0.3 Te0.1 catalyst obtained at the natural pH (Table 2). The Mo0.6 V0.3 Te0.1 catalyst (pH = 2.8) was characterized by higher activity in propane oxidation and selectivity to acrylic acid (Table 7) as compared to the Mo0.6 V0.3 Te0.1 catalyst synthesized at natural pH (Table 4). This catalyst showed higher selectivity to acrylic acid at higher propane conversion during propane oxidation in the presence of water vapor in the feed at 380–400 ◦ C than the Mo0.6 V0.3 Te0.1 catalyst synthesized at natural pH ∼ 2.2. The examination of the M1 phase present in this catalyst (Table 1) suggested that the ab planes were exposed to a higher degree than in the original Mo0.6 V0.3 Te0.1 catalyst obtained at pH = 2.2, i.e. 22% versus 17% (Table 1), which may account for its higher selectivity to acrylic acid during propane oxidation. 3.4.3. Effect of water vapor The effect of water vapor was studied at 400 ◦ C employing the model Mo0.6 V0.3 Te0.1 oxide catalyst and the feed containing propane:O2 :He = 11.6:18.8:69.6 (mol% on dry basis) and 0–66 mol% water vapor as a function of time on stream (0–14 days). The presence of water vapor in the feed had a profound effect on the selectivity of propane oxidation to acrylic acid. The results shown in Fig. 11 indicate that the selectivity to acrylic acid continuously increased as a function of water content in the feed at constant propane conversion ∼43 mol%. For the entire range of water vapor concentrations studied, the propylene and CO2 yields decreased as the water content increased, while the formation of CO was enhanced with the water content in the feed. Formation of acetic acid was unaffected by the changes in the
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water vapor concentration in the feed, suggesting that different active surface sites were involved in propane oxidation to acetic acid. These observations indicated that water has two distinct effects in propane oxidation over bulk multicomponent vanadium molybdate catalysts. Firstly, water enhances the rate of propylene oxidation to acrylic acid, probably through its hydration to isopropanol, manifested in decreasing yields of propylene with the water content in the feed. The presence of water provides an abundant source of OH− species on the catalyst surface, which according to Volta and coworkers exhibit electrophilic properties required for selective oxidation of hydrocarbons [33]. Recent findings for propane oxidation to acrylic acid over structurally distinct Mo–V–Sb–Nb–O phases further suggested that water participates directly in oxygen insertion steps, enhances the apparent rates of formation of oxygenates as well as maintains the catalytic surface in a partially reduced oxidation state [35–37]. Secondly, water plays a role in controlling desorption of acrylic acid manifested in the opposite trends observed for the rates of acrylic acid and CO versus CO2 formation in the presence of water.
4. Conclusions The Mo–V–Te–O catalysts containing the M1 and M2 phases with well-defined crystal morphologies were obtained hydrothermally over a wide range of synthesis compositions (Mo0.70–0.30 V0.60–0.20 Te0.15–0.05 ). The elemental and structural analyses of this compositionally simple model system indicated that the orthorhombic M1 phase with the bulk Mo/V ∼ 2 ratio is dominant in the Mo–V–Te–O catalysts. This phase has been proposed as active and selective for propane oxidation to oxygenates and ammoxidation to acrylonitrile [18–23]. These model catalysts displayed high yields of acrylic acid in the presence of water vapor in the feed, which is believed to enhance the apparent rates of formation of oxygenates as well as maintain the catalytic surface in a partially reduced oxidation state. The selectivity to acrylic acid over these model catalysts correlated with the extent of exposure of the surface ab planes of the M1 phase proposed to contain the active and selective surface sites [7,21,23]. Therefore, the Mo–V–Te–O catalysts represent a well defined and highly promising model system for elucidating the surface molecular structure–activity/selectivity relationships in propane oxidation to acrylic acid over multicomponent Mo–V–Te–Nb–O catalysts.
Acknowledgements Fig. 11. Effect of water on propane oxidation over Mo0.6 V0.3 Te0.1 oxide catalyst at 380 ◦ C and 45 mol% propane conversion. Feed composition (on dry basis): 11.6 vol.% propane, 18.8 vol.% oxygen and 69.6 vol.% He (AA: acrylic acid).
V.V. Guliants acknowledges the National Science Foundation for the NSF Career Award (CTS-0238962). J.N. Al-Saeedi thanks the Public Authority for Applied Education, Kuwait, for the Ph.D. fellowship. Rajiv Soman thanks
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