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Catalytic performance for propane selective oxidation and surface properties of 12-molybdophosphoric acid treated with pyridine
Applied Catalysis A: General 182 (1999) 357±363
Catalytic performance for propane selective oxidation and surface properties of 12-molybdophosphoric acid treated with pyridine W. Lia, K. Oshiharab, W. Uedab,* a
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama, 226-0087, Japan b Department of Materials Science and Engineering, Science University of Tokyo in Yamaguchi, 1-1-1 Daigaku-dori, Onoda, Yamaguchi, 756-0884, Japan Received 9 December 1998; received in revised form 19 January 1999; accepted 25 January 1999
Abstract When molybdophosphoric acid, H3PMo12O40, was treated with pyridine and heated in N2 ¯ow at 4208C as an optimized temperature, a black solid catalyst was formed with a structure of orthorhombic phase and in a reduced state. This reduced H3PMo12O40(Py) catalyst showed a high potentiality in the propane and isobutane oxidation with molecular oxygen to acrylic acid and methacrylic acid above 3008C. It was proved that the higher the reduction degree of the catalyst is, the higher the oxidation activity and selectivity to partial oxidation products are. The FT-IR study revealed that, in the lattice of the heattreated H3PMo12O40(Py) catalyst, pyridinium ion remained to assume the highly resistant orthorhombic secondary structure against reoxidation, and on the surface, Lewis acid sites were generated with the formation of the primary oxygen-de®cient Keggin structure. A possible reaction mechanism was proposed for alkane oxidation, where protons and electrons in the reduced H3PMo12O40(Py) catalyst cooperate to activate molecular oxygen. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Heteropolymolybdophosphate catalyst; Alkane selective oxidation; Reduced state; Surface acid property; Oxidation mechanism
1. Introduction Alkenes and aromatics obtained from petroleum have been the main raw materials. However, the petrochemical industries will probably move to the direct use of alkanes, which are even more economical and readily available raw materials of low environmental cost [1]. In the last decade much progress has been made, particularly in the selective partial *Corresponding author. Tel.: +81-836-88-4559; fax: +81-83688-4559; e-mail: [email protected]
oxidation of light alkanes with molecular oxygen in gas phase [2,3]. For economic reasons, molecular oxygen is used as the primary oxidant predominantly [4]. To promote both the conversion of reactants and the selectivities of partial oxidation products, many kinds of metal compounds are used to create catalytically active sites in different oxidation reaction processes [5]. The most well-known oxidation of lower alkanes is the selective oxidation of n-butane to maleic anhydride using crystalline V±P±O complex oxide catalysts [6]. The selective conversions of methane to
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00030-7
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W. Li et al. / Applied Catalysis A: General 182 (1999) 357±363
2.2. Catalytic test
methanol, formaldehyde, and higher hydrocarbons (by oxidative coupling of methane (OCM)) are also widely investigated [7±9]. The oxidation of ethane has also received attention [10,11]. Heteropolycompounds, having unique structures and the resulting acidic and redox properties, exhibit high oxidation abilities in the selective oxidations of alkanes [2,3,12,13]. Especially, the interesting fact should be noted is that the heteropolycompounds containing molybdenum showed high catalytic activities in the partial oxidation of hydrocarbons to corresponding acids [11,14±25]. We recently found that a reduced 12-molybdophosphoric acid which was prepared by heat-treatment of its pyridinium salt can catalyze the propane and isobutane oxidation to acrylic acid and methacrylic acid, respectively [26±28]. After activation in N2 ¯ow at 4208C for 2 h the catalyst shows a reduced state of molybdenum and a new stable structure. This paper focus on the catalytic performance for propane oxidation and structural and surface properties of 12-molybdophosphoric acid catalysts prepared by the pyridine pretreatment.
Propane oxidation was carried out at atmospheric pressure in a conventional ¯ow system with a ®xed bed Pyrex tubular reactor (f12 mm). The dried samples were used for the catalytic oxidation after the thermal treatment in N2 ¯ow (50 ml minÿ1) at desired temperatures for 2 h. The catalyst (3 g) was diluted by 2 g quartz sand to prevent the catalyst from overheating during the reaction. The feed compositions were controlled with mass ¯ow controller (KOFLOC 3510). The feedstock and products were analyzed with an on-line gas chromatograph operating with two sequential columns, molecular sieve 13X 1 m at room temperature for separation of O2, N2 and CO, and Porapak Q 4 m at 60±1408C for hydrocarbons and CO2. There was a ice cooling trap to collect acid products at the outlet of the reactor, and the collected products were analyzed quantitatively by FID of Shimadzu GC-14A gas chromatograph with TCWAX 60 m capillary. Other minor oxygenated products were analyzed from the tail gas abstracted from the outlet of the reactor.
2. Experimental
2.3. Characterization
2.1. Catalyst preparation
Phases of the prepared catalysts were identi®ed by XRD (Rigaku diffractometer RAD-1VB, Cu K radiation) measurements. FT-IR spectra for the catalysts after the thermal treatment and the catalytic reaction were recorded at room temperature by KBr method with a Perkin-Elmer paragon 1000 Fourier-transform infrared spectrometer with 2 cmÿ1 resolution. In situ FT-IR spectra were obtained with the same spectrometer by employing an in situ IR cell which was connected to a vacuum system. The IR samples were held on a KBr plate by the deposition of the powder suspended in acetone. Pyridine adsorption was carried out by exposing the samples for 20 min to 25 torr of pyridine at room temperature in the in situ IR cell. Then the sample was evacuated at desired temperatures and the spectra were recorded at room temperature. A Shimadzu ESCA-750 X-ray photoelectron spectrometer with an aluminum anode (1486.6 eV) was used to obtain XPS spectra. All binding energies were referenced to gold (Au 4f7/2 line; 83.8 eV) which was deposited on the samples in vacuum. The samples
The 12-molybdophosphoric acid (H3PMo12O40 H2O) was obtained from Nippon Inorganic Color & Chemical. H3PMo12O40H2O was dissolved in distilled water, then the solution was ®ltered and evaporated at 40±508C with stirring until there were small crystals on the surface of the solution. The mixture was kept at 58C overnight to recrystallize, and the crystal was ®ltered and dried at 408C for 8 h. In this way, H3PMo12O4010H2O (denoted by HPMo) was obtained; the water content was determined by Elementary Analysis. Pyridine-treated HPMo, denoted by HPMo(Py), was prepared by a precipitation method. The recrystallized HPMo was dissolved in distilled water with stirring at 40±508C, and an aqueous solution containing a desired amount (7 mol eq) of pyridine was added very slowly, then the solution containing precipitates was evaporated to dryness at 408C. The obtained yellow solid was further dried in N2 ¯ow at 1208C for 8 h.
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were outgassed in the preparation chamber of the spectrometer under 10ÿ8 torr for 15 min. 3. Results and discussion 3.1. Effect of heat-treatment The effect of pretreatment temperature of the HPMo(Py) catalyst on the conversion of propane and the selectivities is shown in Fig. 1. The change of the conversion of propane exhibited a mountain shape with the increase of the pretreatment temperature from 3408C to 5308C. The conversion of propane reached the top at 4208C. The selectivity of acrylic acid also increased with the increase of the pretreatment temperature (T<4708C), and then decreased greatly at the higher pretreatment temperature than 4708C. In place of that the selectivity to propene increased, as can be seen in Fig. 1. The selectivity of deeply oxidized products (CO and CO2) decreased with the increase of the pretreatment temperature. The selectivity to acetic acid was unchanged. Obviously, the catalyst pretreatment temperature strongly in¯uenced the catalytic activity of HPMo(Py). It appears that a catalyst state created through the thermal pre-
Fig. 2. XRD patterns of the HPMo(Py) catalysts heat-treated at various temperatures. (a) Before and (b) after use for the propane oxidation at 3408C for 8 h. The reaction conditions for the propane oxidation are the same as in Fig. 1. Orthorhombic phase (*); cubic phase (oxidized) (~); cubic phase (reduced) (*); MoO3 (X).
Fig. 1. Changes of the propane conversion (*), selectivities ((&) acrylic acid, (&) propene), and XRD peak intensity (*) of the orthorhombic phase as the function of pretreatment temperature of the HPMo(Py) catalyst under N2 for 2 h. The propane oxidation was conducted under the following reaction conditions: reaction temperature, 3408C, catalyst weight, 3 g; feed composition, C3H6/ O2/H2O/N220/10/20/50; total flow rate, 50 ml minÿ1.
treatment between 4008C and 4708C is responsible for the activity and the selective formation of acrylic acid in the propane oxidation. The XRD patterns of the HPMo(Py) catalysts pretreated at various temperatures before and after the reaction are shown in Fig. 2 and the relevant FT-IR results are shown in Fig. 3. When the catalyst was heat-treated at 4208C, where the maximum oxidation activity was achieved, the observed XRD pattern was monophasic and seems to be that of a orthorhombic system. Although the structure giving this XRD pattern based on the primary Keggin structure has not yet been determined, it is obvious that this structural phase is responsible for the activity and the selectivity to
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system and the orthorhombic phase became major after the catalytic reaction. However, the catalyst showed a very low activity for the propane oxidation. The reason is that some parts of the catalyst after the reaction are in the oxidized state, which is proved by the observation that the intensity of P=O band at 1062 cmÿ1 was relatively strong, and that oxidized phases like MoO3 and cubic HPMo were recognized in the XRD patterns. A similar situation also happened more clearly in the case of the catalyst treated at the highest temperature at 5308C. After this heat-treatment the fresh catalyst became cubic, as shown in Fig. 2, but was very unstable under the reaction conditions, changing easily into the oxidized phases, MoO3 and cubic HPMo. At the same time it showed a relatively strong band of P=O in the FT-IR spectra, although the fresh catalyst was in a highly reduced state as revealed by the very weak P=O band intensity (Fig. 3). This catalyst also showed a low activity as expected. As a consequence, the higher the reduction degree of the catalyst is, the higher the oxidation activity and selectivity to partial oxidation products are. Fig. 3. FT-IR spectra of the HPMo(Py) catalysts heat-treated at various temperatures. (a) Before and (b) after use for the propane oxidation at 3408C for 8 h. The reaction conditions for the propane oxidation are the same as in Fig. 1.
acrylic acid. In fact, the activity change as the function of the pretreatment temperature very nicely coincides with that of the relative XRD peak intensity of the orthorhombic phase, as shown in Fig. 1. The effect of the heat pretreatment on the primary Keggin structure was studied by FT-IR measurement on the heat-treated samples (Fig. 3). The most prominent point is that a very weak P=O band at 1062 cmÿ1 was observed in the heat-treated sample at 4208C, implying that the obtained orthorhombic phase is a highly reduced one. This reduced state structure was quite stable even after the catalytic oxidation, as ascertained by both Figs. 2 and 3, although a slight recovery of the P=O bond intensity was observed after the reaction. In the case of the HPMo(Py) catalyst treated at 3408C in N2 ¯ow for 2 h, it can be seen that a part of the catalyst has begun to change to the orthorhombic
3.2. Surface properties Based on quantitative analyses of elements of the catalysts, the chemical formulas of the dried and heat-treated (4208C) catalysts were determined to be C5 H5 NH3 C5 H5 NPMoVI 12 O40 2H2 O and V H C5 H5 NHPMoVI 9 Mo3 O38 , respectively. The latter formula agrees well with the FT-IR data which showed the existence of pyridinium ions in the solid and revealed the reduced state as described above. The most important point from this formula is that the Keggin unit is oxygen-de®cient. In order to study this point, we measured in situ FT-IR spectra of adsorbed pyridine and XPS spectra. The in situ IR spectra of pyridine adsorbed on the activated HPMo(Py) catalyst are shown in Fig. 4. The IR absorption bands at 1538 and 1443 cmÿ1 are known to relate to pyridine adsorbed on Brfnsted acid sites and Lewis acid sites, respectively. The IR spectrum of the freshly heat-treated sample clearly showed a band at 1538 cmÿ1, indicating the existence of pyridine adsorbed on Brfnsted acid sites, but in the present case pyridinium ions in the bulk. When this sample was exposed to pyridine atmosphere at 308C, a
W. Li et al. / Applied Catalysis A: General 182 (1999) 357±363
Fig. 4. In situ FT-IR spectra of pyridine adsorbed on the HPMo(Py) catalysts heat-treated at 4208C for 2 h. (a) Before adsorption, (b) after adsorption under 25 torr pyridine at 308C for 1 h and evacuation at 308C for 1 h, and (c) after further evacuation at 3008C for 10 min.
clear band at 1443 cmÿ1 was observed; this was stable even after the heat-treatment for 3008C for 30 min. Most of the pyridine adsorbed on Lewis acid sites at 1443 cmÿ1 remained on the activated HPMo(Py) after the evacuation at 3008C, suggesting that the Lewis acid sites are strong. The results reveal that Lewis sites are present on the heat-activated HPMo(Py) catalyst, and support the oxygen-de®cient Keggin structure. The Lewis acid sites around the oxygen-de®cient Keggin surface structure seem to be created by lattice oxygen removal by the reaction with pyridine during the thermal pretreatment.
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The XPS results are summarized in Table 1. Normal HPMo shows the binding energies of Mo in 6 valence at 234.6 and 231.5 eV. After it was treated with pyridine and dried in N2 ¯ow at 1208C for 8 h, the oxidation state of surface Mo was not varied, but the binding energies of doublet peaks of Mo slightly decreased to 234.2 and 231.1 eV. The result indicates the in¯uence of the basic lattice pyridine on the surface state of the catalyst. The similar result was also obtained in the XPS spectra of O1s. The bonding energy of O1s decreased from 528.6 to 528.0 eV after the treatment with pyridine. It is obvious that the electronic density of Keggin units in 12-molybdophosphoric acid was increased by the lattice pyridine, so that the strength of Mo±O and P=O bonds becomes weaker. After the HPMo(Py) catalyst was treated in N2 ¯ow at 4208C for 2 h, two peaks of Mo3d3/2 were observed at the binding energies of 233.2 and 234.3 eV. The former peak represents MoV. The superior peak intensity of 233.2±234.3 eV in the Mo3d3/2 XPS spectra revealed that most of Mo on the surface is in the reduced state of MoV. On the other hand, the O1s spectrum exhibits three states of 528.3, 529.6 and 531.2 eV, respectively, only in the case of the activated HPMo(Py) catalyst. The binding energy at 528.3 eV is obviously responsible for lattice oxygen ions. Because the XPS spectra were not taken by in situ method, there is a possibility for oxygen to adsorb on the surface of the activated catalyst. However, it seems to be dif®cult to attribute those species having higher binding energies to adsorbed oxygen species because hydroxyl groups may exist on the surfaces of catalysts [29]. Nevertheless there are some papers [30±32] considering them to be active oxygen ions. Certainly, in the present experiments, they cannot be attributed simply to adsorbed oxygen ions with lower charge. However,
Table 1 Binding energies for H3PMo12O40 catalysts by XPS Samples
Mo3d3/2
Mo3d5/2
O1s
H3PMo12O40 H3PMo12O40(Py) (after dryness) H3PMo12O40(Py) (after activation at 4208C in N2)
234.6 234.2 234.3 233.2
231.5 231.1 231.2 230.3
528.6 528.0 531.5 529.6 528.3
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Table 2 Selective oxidation of propane over heteropolymolybdophosphatesa Catalyst
Conversion (%)
Selectivityb (%)
State
Precursorc
Acidityd
C3H8
O2
C3H6
AA
AcOH
COx
Oxidized
H3PMo12O40 H3PMo12O40 H3PW12O40 (NH4)3PMo12O40
Medium Weak Strong Strong
0.9 0 0 4.5
2.7 0 0 28.8
73.0 ± ± 15.9
1.3 ± ± 5.6
10.6 ± ± 26.2
11.8 ± ± 49.2
Reduced
(PyH)3PMo12O40 (PyH)3PMo12O40e
Medium Medium
7.5 Tr
43.7 Tr
Tr ±
28.5 ±
15.3 ±
55.0 ±
a
Reaction conditions: reaction temperature, 3408C, catalyst wt, 3 g, feed, C3/O2/H2O/N220/10/20/50, flow rate, 50 ml minÿ1. AA: acrylic acid, AcOH: acetic acid. c The precursor were pretreated at desired temperatures before the reaction. d Rough estimation. e Pyridine was added in the feed. C3/O2/H2O/N2/C5H5N20/10/20/50/0.5. b
it might be oxygen ions species, because these peaks were observed only on the reduced catalysts having the oxygen-de®cient surfaces. 3.3. Propane oxidation over reduced Mo surface The catalytic activities of H3PMo12O40(HPMo), (NH4)3PMo12O40 (denoted by NHPMo), H3PMo12O40(Py)(HPMo(Py)), and related heteropolycompounds in the propane oxidation are summarized in Table 2. The non-reduced, acidic form of molybdophosphate catalyst, H3PMo12O40, revealed an activity and yielded propene mainly. In addition, the H3PW12O40 catalyst having very poor redox property showed no activity. The reduced state HPMo(Py) catalysts showed distinctly enhanced activities for the propane oxidation, and the catalyst gave a signi®cantly different product distribution, where acrylic acid and acetic acid were the main organic oxygenated products. Such drastic change of the product distribution was observed even at a low conversion of the propane under a short contact time. Table 2 also shows the catalytic performance of the activated NHPMo catalyst having a similar surface area of the HPMo(Py) catalyst. Non-activated NHPMo was completely inactive but became active after the heat-treatment under N2 ¯ow at 4508C. The catalyst, however, gave the broad product distribution because this catalyst was in a reduced state when fresh but easily reoxidized during the catalytic oxidation.
The results again suggest that the reduced state is responsible for the activity and for the formation of acrylic acid in the propane oxidation. Obviously, the pyridine treatment has an effect to make the catalysts able to be active for alkane oxidation and to keep highly reduced states during the oxidation. However, no catalytic activity was observed even on the above activated catalyst when a small amount of pyridine was added in the reactant feed, as shown in Table 2. This is due to the facts that pyridine molecule adsorbs on both the Brfnsted acid sites and Lewis acid sites to block the adsorption and activation of propane and oxygen. Similarly, the K3PMo12O40 catalyst showed no activity. As a consequence, both the acidic properties and the reduced state of molybdenum in the HPMo(Py) catalyst played substantial roles in the course of the alkanes oxidation with molecular oxygen. The particular oxygen-de®cient structure which provides Lewis acid sites is also very important. Normally the formation of active oxygen on the surface and the activation of alkanes with the active oxygen thus formed are the key steps in the catalytic alkane oxidation with molecular oxygen. On the basis of the above results, we propose a possible reaction mechanism for alkane oxidation, where protons and electrons in the reduced HPMo(Py) catalyst cooperate to activate molecular oxygen. The reaction scheme is shown as follows, along with the subsequent activation of propane and the consecutive
W. Li et al. / Applied Catalysis A: General 182 (1999) 357±363
oxidation: 2H 2eÿ reduced Mo O2 ! Mo ÿ O H2 O Mo ÿ O C3 H8 ! MoC3 H7 or C3 H7 O MoC3 H7 ! C3 H6 ! Acrolein; Acryli acid MoC3 H7 O ! Propional; Acrolein ! Propionic acid; Acrylic acid Molecular oxygen ®rst reacts with two protons and reduced Mo to form one molecule of water. The remaining oxygen atom is coordinated on Mo site and becomes the active oxygen species for propane activation. Then propane changes into either adsorbed propene-like species, followed by the rapid consecutive allylic oxidation to acrylic acid, or adsorbed alcoholic species, followed by oxidative dehydrogenation. It should be noted that the highly reduced state is suitable for both consecutive oxidations because the reduced state provides not only coordinatively unsaturated Mo for the adsorption or stabilization of intermediate species but also basic lattice oxygen which may only be formed near the reduced Mo sites. A part of formed acrylic acid can be oxidized deeply to acetic acid, CO and CO2 by the activated oxygen species on the surface. Acetic acid and the other C±C bond degradation products are also formed by various proton-catalyzed reactions. We, therefore, need to control the protonic property of the catalyst in order to achieve much higher selectivities to unsaturated oxygenate products. References [1] S. Albonetti, F. Cavani, F. Trifiro, Catal. Rev.-Sci. Eng. 38 (1996) 413. [2] Y. Moro-oka, W. Ueda, Catalysis 11 (1994) 223. [3] F. Cavani, F. Trifiro, Catalysis 11 (1994) 247.
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[4] R.A. Scheldon, in: D.H.R. Barton et al. (Ed.), The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum Press, New York, 1933, p. 9. [5] H.H. Kung, Ind. Eng. Chem. Prod. Res. Dev. 25 (1986) 171. [6] B.K. Hodnett, Catal. Rev.-Sci. Eng. 27 (1985) 373. [7] G.J. Hutchings, Chem. Soc. Rev. 18 (1989) 251. [8] Y. Amenomiya, Catal. Rev.-Sci. Eng. 32 (1990) 163. [9] G.N. Kastanes, G.A. Tsigdinos, J. Schwank, Proc. AIChE Mgt. 172 (1983) C4. [10] E.M. Thorsteinson, T.P. Wilson, F.G. Young, P.H. Kasai, J. Catal. 52 (1978) 116. [11] M. Roy, M. Gubelmann-Bonneau, H. Ponceblanc, J.C. Volta, Catal. Lett. 42 (1996) 93. [12] M.T. Pope, Heteropoly and Isopoly Oxometalates, Springer, New York, 1983. [13] R.J.J. Jansen, Recl. Trav. Chim. Pays-Bas 113 (1994) 115. [14] K. Harold, European Patent 0010 902, 1979. [15] J.B. Black, J.D. Scott, E.M. Serwick, J.B. Goodenough, J. Catal. 106 (1987) 16. [16] E.M. Serwick, J.B. Black, J.B. Goodenough, J. Catal. 106 (1987) 23. [17] G.B. McGarvey, J.B. Moffat, J. Catal. 132 (1991) 100. [18] J.P. Battek, US Patent 5 198 580, 1993. [19] A. Aboukais, D. Ghoussoub, E. Blouet-Crusson, M. Rigole, M. Guelton, Appl. Catal. A 111 (1994) 109. [20] N. Mizuno, M. Tasaki, M. Iwamoto, Appl. Catal. A 118 (1994) L1. [21] S. Albonetti, F. Cavani, F. Tifiro, M. Gazzano, M. Koutyrev, F.C. Aissi, A. Aboukais, J. Catal. 146 (1994) 491. [22] N. Mizuno, M. Tasaki, M. Iwamoto, Appl. Catal. A 128 (1995) L165. [23] F. Cavani, E. Etienne, M. Favaro, A. Galli, F. Trifiro, Catal. Lett. 32 (1995) 215. [24] N. Mizuno, M. Tasaki, M. Iwamoto, J. Catal. 163 (1996) 87. [25] S. Yamamatsu, T. Yamaguchi, Japan Patent 02-042 032 1990. [26] W. Ueda, Y. Suzuki, Chem. Lett. (1995) 541. [27] W. Ueda, Y. Suzuki, W. Li, S. Imaoka, Stud. Surf. Sci. Catal. 101 (1996) 1047. [28] W. Li, W. Ueda, Stud. Surf. Sci. Catal. 110 (1997) 433. [29] M. Che, Adv. Catal. 32 (1983) 1. [30] B. Grzybowska, J. Haber, W. Marczewski, L. Ungier, J. Catal. 42 (1976) 327. [31] C.T. Au, M.W. Roberts, A.R. Zhu, Surf. Sci. 115 (1982) L117. [32] C.R. Brundle, L.P. Metcalf, J. Chem. Soc., Faraday Trans. II 78 (1982) 2101.
Catalytic performance for propane selective oxidation and surface properties of 12-molybdophosphoric acid treated with pyridine W. Lia, K. Oshiharab, W. Uedab,* a
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama, 226-0087, Japan b Department of Materials Science and Engineering, Science University of Tokyo in Yamaguchi, 1-1-1 Daigaku-dori, Onoda, Yamaguchi, 756-0884, Japan Received 9 December 1998; received in revised form 19 January 1999; accepted 25 January 1999
Abstract When molybdophosphoric acid, H3PMo12O40, was treated with pyridine and heated in N2 ¯ow at 4208C as an optimized temperature, a black solid catalyst was formed with a structure of orthorhombic phase and in a reduced state. This reduced H3PMo12O40(Py) catalyst showed a high potentiality in the propane and isobutane oxidation with molecular oxygen to acrylic acid and methacrylic acid above 3008C. It was proved that the higher the reduction degree of the catalyst is, the higher the oxidation activity and selectivity to partial oxidation products are. The FT-IR study revealed that, in the lattice of the heattreated H3PMo12O40(Py) catalyst, pyridinium ion remained to assume the highly resistant orthorhombic secondary structure against reoxidation, and on the surface, Lewis acid sites were generated with the formation of the primary oxygen-de®cient Keggin structure. A possible reaction mechanism was proposed for alkane oxidation, where protons and electrons in the reduced H3PMo12O40(Py) catalyst cooperate to activate molecular oxygen. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Heteropolymolybdophosphate catalyst; Alkane selective oxidation; Reduced state; Surface acid property; Oxidation mechanism
1. Introduction Alkenes and aromatics obtained from petroleum have been the main raw materials. However, the petrochemical industries will probably move to the direct use of alkanes, which are even more economical and readily available raw materials of low environmental cost [1]. In the last decade much progress has been made, particularly in the selective partial *Corresponding author. Tel.: +81-836-88-4559; fax: +81-83688-4559; e-mail: [email protected]
oxidation of light alkanes with molecular oxygen in gas phase [2,3]. For economic reasons, molecular oxygen is used as the primary oxidant predominantly [4]. To promote both the conversion of reactants and the selectivities of partial oxidation products, many kinds of metal compounds are used to create catalytically active sites in different oxidation reaction processes [5]. The most well-known oxidation of lower alkanes is the selective oxidation of n-butane to maleic anhydride using crystalline V±P±O complex oxide catalysts [6]. The selective conversions of methane to
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(99)00030-7
358
W. Li et al. / Applied Catalysis A: General 182 (1999) 357±363
2.2. Catalytic test
methanol, formaldehyde, and higher hydrocarbons (by oxidative coupling of methane (OCM)) are also widely investigated [7±9]. The oxidation of ethane has also received attention [10,11]. Heteropolycompounds, having unique structures and the resulting acidic and redox properties, exhibit high oxidation abilities in the selective oxidations of alkanes [2,3,12,13]. Especially, the interesting fact should be noted is that the heteropolycompounds containing molybdenum showed high catalytic activities in the partial oxidation of hydrocarbons to corresponding acids [11,14±25]. We recently found that a reduced 12-molybdophosphoric acid which was prepared by heat-treatment of its pyridinium salt can catalyze the propane and isobutane oxidation to acrylic acid and methacrylic acid, respectively [26±28]. After activation in N2 ¯ow at 4208C for 2 h the catalyst shows a reduced state of molybdenum and a new stable structure. This paper focus on the catalytic performance for propane oxidation and structural and surface properties of 12-molybdophosphoric acid catalysts prepared by the pyridine pretreatment.
Propane oxidation was carried out at atmospheric pressure in a conventional ¯ow system with a ®xed bed Pyrex tubular reactor (f12 mm). The dried samples were used for the catalytic oxidation after the thermal treatment in N2 ¯ow (50 ml minÿ1) at desired temperatures for 2 h. The catalyst (3 g) was diluted by 2 g quartz sand to prevent the catalyst from overheating during the reaction. The feed compositions were controlled with mass ¯ow controller (KOFLOC 3510). The feedstock and products were analyzed with an on-line gas chromatograph operating with two sequential columns, molecular sieve 13X 1 m at room temperature for separation of O2, N2 and CO, and Porapak Q 4 m at 60±1408C for hydrocarbons and CO2. There was a ice cooling trap to collect acid products at the outlet of the reactor, and the collected products were analyzed quantitatively by FID of Shimadzu GC-14A gas chromatograph with TCWAX 60 m capillary. Other minor oxygenated products were analyzed from the tail gas abstracted from the outlet of the reactor.
2. Experimental
2.3. Characterization
2.1. Catalyst preparation
Phases of the prepared catalysts were identi®ed by XRD (Rigaku diffractometer RAD-1VB, Cu K radiation) measurements. FT-IR spectra for the catalysts after the thermal treatment and the catalytic reaction were recorded at room temperature by KBr method with a Perkin-Elmer paragon 1000 Fourier-transform infrared spectrometer with 2 cmÿ1 resolution. In situ FT-IR spectra were obtained with the same spectrometer by employing an in situ IR cell which was connected to a vacuum system. The IR samples were held on a KBr plate by the deposition of the powder suspended in acetone. Pyridine adsorption was carried out by exposing the samples for 20 min to 25 torr of pyridine at room temperature in the in situ IR cell. Then the sample was evacuated at desired temperatures and the spectra were recorded at room temperature. A Shimadzu ESCA-750 X-ray photoelectron spectrometer with an aluminum anode (1486.6 eV) was used to obtain XPS spectra. All binding energies were referenced to gold (Au 4f7/2 line; 83.8 eV) which was deposited on the samples in vacuum. The samples
The 12-molybdophosphoric acid (H3PMo12O40 H2O) was obtained from Nippon Inorganic Color & Chemical. H3PMo12O40H2O was dissolved in distilled water, then the solution was ®ltered and evaporated at 40±508C with stirring until there were small crystals on the surface of the solution. The mixture was kept at 58C overnight to recrystallize, and the crystal was ®ltered and dried at 408C for 8 h. In this way, H3PMo12O4010H2O (denoted by HPMo) was obtained; the water content was determined by Elementary Analysis. Pyridine-treated HPMo, denoted by HPMo(Py), was prepared by a precipitation method. The recrystallized HPMo was dissolved in distilled water with stirring at 40±508C, and an aqueous solution containing a desired amount (7 mol eq) of pyridine was added very slowly, then the solution containing precipitates was evaporated to dryness at 408C. The obtained yellow solid was further dried in N2 ¯ow at 1208C for 8 h.
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were outgassed in the preparation chamber of the spectrometer under 10ÿ8 torr for 15 min. 3. Results and discussion 3.1. Effect of heat-treatment The effect of pretreatment temperature of the HPMo(Py) catalyst on the conversion of propane and the selectivities is shown in Fig. 1. The change of the conversion of propane exhibited a mountain shape with the increase of the pretreatment temperature from 3408C to 5308C. The conversion of propane reached the top at 4208C. The selectivity of acrylic acid also increased with the increase of the pretreatment temperature (T<4708C), and then decreased greatly at the higher pretreatment temperature than 4708C. In place of that the selectivity to propene increased, as can be seen in Fig. 1. The selectivity of deeply oxidized products (CO and CO2) decreased with the increase of the pretreatment temperature. The selectivity to acetic acid was unchanged. Obviously, the catalyst pretreatment temperature strongly in¯uenced the catalytic activity of HPMo(Py). It appears that a catalyst state created through the thermal pre-
Fig. 2. XRD patterns of the HPMo(Py) catalysts heat-treated at various temperatures. (a) Before and (b) after use for the propane oxidation at 3408C for 8 h. The reaction conditions for the propane oxidation are the same as in Fig. 1. Orthorhombic phase (*); cubic phase (oxidized) (~); cubic phase (reduced) (*); MoO3 (X).
Fig. 1. Changes of the propane conversion (*), selectivities ((&) acrylic acid, (&) propene), and XRD peak intensity (*) of the orthorhombic phase as the function of pretreatment temperature of the HPMo(Py) catalyst under N2 for 2 h. The propane oxidation was conducted under the following reaction conditions: reaction temperature, 3408C, catalyst weight, 3 g; feed composition, C3H6/ O2/H2O/N220/10/20/50; total flow rate, 50 ml minÿ1.
treatment between 4008C and 4708C is responsible for the activity and the selective formation of acrylic acid in the propane oxidation. The XRD patterns of the HPMo(Py) catalysts pretreated at various temperatures before and after the reaction are shown in Fig. 2 and the relevant FT-IR results are shown in Fig. 3. When the catalyst was heat-treated at 4208C, where the maximum oxidation activity was achieved, the observed XRD pattern was monophasic and seems to be that of a orthorhombic system. Although the structure giving this XRD pattern based on the primary Keggin structure has not yet been determined, it is obvious that this structural phase is responsible for the activity and the selectivity to
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system and the orthorhombic phase became major after the catalytic reaction. However, the catalyst showed a very low activity for the propane oxidation. The reason is that some parts of the catalyst after the reaction are in the oxidized state, which is proved by the observation that the intensity of P=O band at 1062 cmÿ1 was relatively strong, and that oxidized phases like MoO3 and cubic HPMo were recognized in the XRD patterns. A similar situation also happened more clearly in the case of the catalyst treated at the highest temperature at 5308C. After this heat-treatment the fresh catalyst became cubic, as shown in Fig. 2, but was very unstable under the reaction conditions, changing easily into the oxidized phases, MoO3 and cubic HPMo. At the same time it showed a relatively strong band of P=O in the FT-IR spectra, although the fresh catalyst was in a highly reduced state as revealed by the very weak P=O band intensity (Fig. 3). This catalyst also showed a low activity as expected. As a consequence, the higher the reduction degree of the catalyst is, the higher the oxidation activity and selectivity to partial oxidation products are. Fig. 3. FT-IR spectra of the HPMo(Py) catalysts heat-treated at various temperatures. (a) Before and (b) after use for the propane oxidation at 3408C for 8 h. The reaction conditions for the propane oxidation are the same as in Fig. 1.
acrylic acid. In fact, the activity change as the function of the pretreatment temperature very nicely coincides with that of the relative XRD peak intensity of the orthorhombic phase, as shown in Fig. 1. The effect of the heat pretreatment on the primary Keggin structure was studied by FT-IR measurement on the heat-treated samples (Fig. 3). The most prominent point is that a very weak P=O band at 1062 cmÿ1 was observed in the heat-treated sample at 4208C, implying that the obtained orthorhombic phase is a highly reduced one. This reduced state structure was quite stable even after the catalytic oxidation, as ascertained by both Figs. 2 and 3, although a slight recovery of the P=O bond intensity was observed after the reaction. In the case of the HPMo(Py) catalyst treated at 3408C in N2 ¯ow for 2 h, it can be seen that a part of the catalyst has begun to change to the orthorhombic
3.2. Surface properties Based on quantitative analyses of elements of the catalysts, the chemical formulas of the dried and heat-treated (4208C) catalysts were determined to be C5 H5 NH3 C5 H5 NPMoVI 12 O40 2H2 O and V H C5 H5 NHPMoVI 9 Mo3 O38 , respectively. The latter formula agrees well with the FT-IR data which showed the existence of pyridinium ions in the solid and revealed the reduced state as described above. The most important point from this formula is that the Keggin unit is oxygen-de®cient. In order to study this point, we measured in situ FT-IR spectra of adsorbed pyridine and XPS spectra. The in situ IR spectra of pyridine adsorbed on the activated HPMo(Py) catalyst are shown in Fig. 4. The IR absorption bands at 1538 and 1443 cmÿ1 are known to relate to pyridine adsorbed on Brfnsted acid sites and Lewis acid sites, respectively. The IR spectrum of the freshly heat-treated sample clearly showed a band at 1538 cmÿ1, indicating the existence of pyridine adsorbed on Brfnsted acid sites, but in the present case pyridinium ions in the bulk. When this sample was exposed to pyridine atmosphere at 308C, a
W. Li et al. / Applied Catalysis A: General 182 (1999) 357±363
Fig. 4. In situ FT-IR spectra of pyridine adsorbed on the HPMo(Py) catalysts heat-treated at 4208C for 2 h. (a) Before adsorption, (b) after adsorption under 25 torr pyridine at 308C for 1 h and evacuation at 308C for 1 h, and (c) after further evacuation at 3008C for 10 min.
clear band at 1443 cmÿ1 was observed; this was stable even after the heat-treatment for 3008C for 30 min. Most of the pyridine adsorbed on Lewis acid sites at 1443 cmÿ1 remained on the activated HPMo(Py) after the evacuation at 3008C, suggesting that the Lewis acid sites are strong. The results reveal that Lewis sites are present on the heat-activated HPMo(Py) catalyst, and support the oxygen-de®cient Keggin structure. The Lewis acid sites around the oxygen-de®cient Keggin surface structure seem to be created by lattice oxygen removal by the reaction with pyridine during the thermal pretreatment.
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The XPS results are summarized in Table 1. Normal HPMo shows the binding energies of Mo in 6 valence at 234.6 and 231.5 eV. After it was treated with pyridine and dried in N2 ¯ow at 1208C for 8 h, the oxidation state of surface Mo was not varied, but the binding energies of doublet peaks of Mo slightly decreased to 234.2 and 231.1 eV. The result indicates the in¯uence of the basic lattice pyridine on the surface state of the catalyst. The similar result was also obtained in the XPS spectra of O1s. The bonding energy of O1s decreased from 528.6 to 528.0 eV after the treatment with pyridine. It is obvious that the electronic density of Keggin units in 12-molybdophosphoric acid was increased by the lattice pyridine, so that the strength of Mo±O and P=O bonds becomes weaker. After the HPMo(Py) catalyst was treated in N2 ¯ow at 4208C for 2 h, two peaks of Mo3d3/2 were observed at the binding energies of 233.2 and 234.3 eV. The former peak represents MoV. The superior peak intensity of 233.2±234.3 eV in the Mo3d3/2 XPS spectra revealed that most of Mo on the surface is in the reduced state of MoV. On the other hand, the O1s spectrum exhibits three states of 528.3, 529.6 and 531.2 eV, respectively, only in the case of the activated HPMo(Py) catalyst. The binding energy at 528.3 eV is obviously responsible for lattice oxygen ions. Because the XPS spectra were not taken by in situ method, there is a possibility for oxygen to adsorb on the surface of the activated catalyst. However, it seems to be dif®cult to attribute those species having higher binding energies to adsorbed oxygen species because hydroxyl groups may exist on the surfaces of catalysts [29]. Nevertheless there are some papers [30±32] considering them to be active oxygen ions. Certainly, in the present experiments, they cannot be attributed simply to adsorbed oxygen ions with lower charge. However,
Table 1 Binding energies for H3PMo12O40 catalysts by XPS Samples
Mo3d3/2
Mo3d5/2
O1s
H3PMo12O40 H3PMo12O40(Py) (after dryness) H3PMo12O40(Py) (after activation at 4208C in N2)
234.6 234.2 234.3 233.2
231.5 231.1 231.2 230.3
528.6 528.0 531.5 529.6 528.3
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Table 2 Selective oxidation of propane over heteropolymolybdophosphatesa Catalyst
Conversion (%)
Selectivityb (%)
State
Precursorc
Acidityd
C3H8
O2
C3H6
AA
AcOH
COx
Oxidized
H3PMo12O40 H3PMo12O40 H3PW12O40 (NH4)3PMo12O40
Medium Weak Strong Strong
0.9 0 0 4.5
2.7 0 0 28.8
73.0 ± ± 15.9
1.3 ± ± 5.6
10.6 ± ± 26.2
11.8 ± ± 49.2
Reduced
(PyH)3PMo12O40 (PyH)3PMo12O40e
Medium Medium
7.5 Tr
43.7 Tr
Tr ±
28.5 ±
15.3 ±
55.0 ±
a
Reaction conditions: reaction temperature, 3408C, catalyst wt, 3 g, feed, C3/O2/H2O/N220/10/20/50, flow rate, 50 ml minÿ1. AA: acrylic acid, AcOH: acetic acid. c The precursor were pretreated at desired temperatures before the reaction. d Rough estimation. e Pyridine was added in the feed. C3/O2/H2O/N2/C5H5N20/10/20/50/0.5. b
it might be oxygen ions species, because these peaks were observed only on the reduced catalysts having the oxygen-de®cient surfaces. 3.3. Propane oxidation over reduced Mo surface The catalytic activities of H3PMo12O40(HPMo), (NH4)3PMo12O40 (denoted by NHPMo), H3PMo12O40(Py)(HPMo(Py)), and related heteropolycompounds in the propane oxidation are summarized in Table 2. The non-reduced, acidic form of molybdophosphate catalyst, H3PMo12O40, revealed an activity and yielded propene mainly. In addition, the H3PW12O40 catalyst having very poor redox property showed no activity. The reduced state HPMo(Py) catalysts showed distinctly enhanced activities for the propane oxidation, and the catalyst gave a signi®cantly different product distribution, where acrylic acid and acetic acid were the main organic oxygenated products. Such drastic change of the product distribution was observed even at a low conversion of the propane under a short contact time. Table 2 also shows the catalytic performance of the activated NHPMo catalyst having a similar surface area of the HPMo(Py) catalyst. Non-activated NHPMo was completely inactive but became active after the heat-treatment under N2 ¯ow at 4508C. The catalyst, however, gave the broad product distribution because this catalyst was in a reduced state when fresh but easily reoxidized during the catalytic oxidation.
The results again suggest that the reduced state is responsible for the activity and for the formation of acrylic acid in the propane oxidation. Obviously, the pyridine treatment has an effect to make the catalysts able to be active for alkane oxidation and to keep highly reduced states during the oxidation. However, no catalytic activity was observed even on the above activated catalyst when a small amount of pyridine was added in the reactant feed, as shown in Table 2. This is due to the facts that pyridine molecule adsorbs on both the Brfnsted acid sites and Lewis acid sites to block the adsorption and activation of propane and oxygen. Similarly, the K3PMo12O40 catalyst showed no activity. As a consequence, both the acidic properties and the reduced state of molybdenum in the HPMo(Py) catalyst played substantial roles in the course of the alkanes oxidation with molecular oxygen. The particular oxygen-de®cient structure which provides Lewis acid sites is also very important. Normally the formation of active oxygen on the surface and the activation of alkanes with the active oxygen thus formed are the key steps in the catalytic alkane oxidation with molecular oxygen. On the basis of the above results, we propose a possible reaction mechanism for alkane oxidation, where protons and electrons in the reduced HPMo(Py) catalyst cooperate to activate molecular oxygen. The reaction scheme is shown as follows, along with the subsequent activation of propane and the consecutive
W. Li et al. / Applied Catalysis A: General 182 (1999) 357±363
oxidation: 2H 2eÿ reduced Mo O2 ! Mo ÿ O H2 O Mo ÿ O C3 H8 ! MoC3 H7 or C3 H7 O MoC3 H7 ! C3 H6 ! Acrolein; Acryli acid MoC3 H7 O ! Propional; Acrolein ! Propionic acid; Acrylic acid Molecular oxygen ®rst reacts with two protons and reduced Mo to form one molecule of water. The remaining oxygen atom is coordinated on Mo site and becomes the active oxygen species for propane activation. Then propane changes into either adsorbed propene-like species, followed by the rapid consecutive allylic oxidation to acrylic acid, or adsorbed alcoholic species, followed by oxidative dehydrogenation. It should be noted that the highly reduced state is suitable for both consecutive oxidations because the reduced state provides not only coordinatively unsaturated Mo for the adsorption or stabilization of intermediate species but also basic lattice oxygen which may only be formed near the reduced Mo sites. A part of formed acrylic acid can be oxidized deeply to acetic acid, CO and CO2 by the activated oxygen species on the surface. Acetic acid and the other C±C bond degradation products are also formed by various proton-catalyzed reactions. We, therefore, need to control the protonic property of the catalyst in order to achieve much higher selectivities to unsaturated oxygenate products. References [1] S. Albonetti, F. Cavani, F. Trifiro, Catal. Rev.-Sci. Eng. 38 (1996) 413. [2] Y. Moro-oka, W. Ueda, Catalysis 11 (1994) 223. [3] F. Cavani, F. Trifiro, Catalysis 11 (1994) 247.
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