Ind. Eng. Chem. Res. 1996, 35, 2137-2143
2137
Oxidative Dehydrogenation of Propane over Molybdenum Supported on MgO-γ-Al2O3 M. C. Abello, M. F. Gomez, and L. E. Cadu ´ s* INTEQUI† (Instituto de Investigaciones en Tecnologı´a Quı´mica (UNSL-CONICET)), Casilla de Correo 290, 5700 San Luis, Argentina
Catalysts of Mo supported on MgO-γ Al2O3 were studied in the oxidative dehydrogenation of propane to propene. The catalysts were active and very stable, but the dehydrogenation selectivity was reduced by the formation of carbon oxides. Characterizations by XRD, XPS, Raman spectroscopy, electron paramagnetic resonance, and BET surface measurements were performed. The catalyst preparation method led to large and stable magnesium molybdate particles on the surface. EPR and XPS measurements gave clues about the fact that the active centers for the reaction include Mo5+ ions. A scheme for the surface architecture is proposed. Introduction The transformation of light alkanes into more valuable organic compounds is being extensively studied. Many papers on the oxidative dehydrogenation (ODH) of ethane, propane, and butane can be found in the most recent literature (Blasco et al., 1995; Corma et al., 1992, 1993a,b; Chaar et al., 1987, 1988; Gao et al., 1994a,b; Grabowski et al., 1995; Guerrero-Ruiz et al., 1992; Juarez-Lopez et al., 1995; Kaddouri et al., 1992; Mamedov and Cortes-Corberan, 1995; Mazzocchia et al., 1991; Michalakos et al., 1993; Siew-Hew-Sam et al., 1990). A wide variety of catalytic systems have been proposed. Different vanadium oxides including unsupported and supported V2O5, vanadates, solid solution, and mixed phases, have particularly attracted attention since they have shown good performance. The ODH of light alkanes usually produces a considerable amount of carbon oxides because of the low selectivities of the catalysts employed. Research in developing catalysts capable of decreasing the formation of undesired byproducts has grown in the last decade. In an early work about the ODH of propane, Cadus et al. (1996) found that the MgMoO system provided high selectivity to propene (around 95%). The conversion of propane was below 6%, and a deactivation caused by a solid-state reaction and a sintering process was observed. It has also been pointed out that a slight excess of MoO3 was necessary to make the catalyst an active one. In those catalysts, at least two phases were simultaneously identified. An exhaustive characterization has allowed us to explain their different catalytic behaviors in the ODH of propane. Sites containing unsaturated Mo5+ species on the surface would be the active sites. The specific surface areas of MgMoO catalysts were extremely low, probably because of the way in which the precipitation and calcination processes have been carried out. A well-known alternative to increase the specific surface area and the stability of the catalyst consists of dispersing the active component on a support. The superficial coverage and the chemical properties of the support are the most important parameters in defining the monolayer structure. Eon et al. (1994) and Gao et al. (1994a) have obtained active catalysts for the ODH of propane when vanadium oxides and magnesium vanadates were supported on γ-Al2O3 and TiO2. †
FAX: 54-652-26711. E-mail:
[email protected].
S0888-5885(95)00677-4 CCC: $12.00
A new class of materials referred to as composite oxides have been recently obtained by modifying the surface of bulk oxides with attached clusters of other oxides (Contescu et al., 1991; You-Chang Xie and YouQi Tang, 1990). The most important difference with the binary oxides is that the second component is very well dispersed on the surface of the main oxide. Even though the second phase is present in a considerable quantity, it did not show up as peaks in the X-ray diffraction patterns. Taking into account the results of the MgMoO system (Cadus et al., 1996) and the new strategies for preparing catalysts with specific properties, a study of catalytic features of molybdenum species on a MgO-γ-Al2O3 composite oxide in the oxidative dehydrogenation of propane has been carried out in this work. The differences in catalytic activity and selectivity of the different phases present have been discussed. Emphasis was placed on the physicochemical characterization of catalysts with the aims of (i) studying the nature of the active sites and (ii) investigating the architectural aspect of the surface. Experimental Section Catalyst Preparation. Support. The composite oxide used as the support consisted of MgO dispersed on commercial γ-Al2O3 in powder form. The alumina was pretreated at 873 K for 2 h before being used. The samples were obtained by impregnating alumina in a solution of Mg(NO3)2 at 320 K. Then they were dried and calcined. Calcination proceeded from room temperature to 723 K at a heating rate of 3 K/min, being kept at this temperature for 2 h. Different loadings of MgO were realized by successive impregnation and calcination. MgO contents (wt %) were 8, 12, and 16, leading to samples named 8S, 12S, and 16S, respectively. Hydroxylation. Because MgO is partially dissolved when the support is impregnated with an aqueous solution of ammonium heptamolybdate, the procedure given by Llorente et al. (1992) was attained. Supports S were suspended in an aqueous ammonia solution at pH 11.9 and magnetically stirred for 2 h, the solvent was withdrawn in a rotary vacuum desiccator at 323 K, and the sample was finally calcined as was mentioned above. Impregnation. The hydroxylated supports S with different MgO loadings were impregnated with ammonium heptamolybdate by using solutions whose pH’s © 1996 American Chemical Society
2138
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
were previously adjusted to 11.9 (with ammonia) and calcined. Calcination was performed in two steps: from room temperature to 723 K at 3 K/min and maintaining this temperature for 2 h and then at 873 K for 6 h. The amount of ammonia heptamolybdate was chosen to yield a final content of MgMoO4 according to the MgO content after calcination. MoO3 contents (wt %) were 24, 32.7 and 39.4, leading to samples named Mo8S, Mo12S, and Mo16S, respectively. Catalyst Characterization. Specific surface areas (SA, m2/g) of all samples were determined from nitrogen adsorption isotherms at 77 K by the BET method. A Micromeritics Accusorb 2100E was used. X-ray diffraction (XRD) patterns were obtained by using a Rigaku diffractometer operated at 35 kV and 30 mA by employing Ni-filtered Cu KR radiation (λ ) 0.154 18 nm). The EPR measurements were obtained with a Bruker spectrometer at room temperature and a Klystron frequency of 9.7 GHz and 100-kHz magnetic field modulation. The determinations of surface composition were performed by X-ray photoelectron spectroscopy (XPS) on a Shidmadzu equipment employing a Mg KR X-ray excitation source (hν ) 1253 eV). The line C 1s was used as reference. Ratios of atomic concentrations in the outer layers of the samples were estimated from the corresponding XPS area ratios by using the effective ionization cross section of ejected electrons tabulated by Scofield and the formulas given by Seah and Dench (1979). Raman spectra were collected with a JASCO TRS600SZP multichannel monochromatic spectrometer. The samples were pressed into self-supporting wafers. The spectra were recorded at room temperature and with 514.5-nm line excitation radiation of an Ar ion laser. The laser power used was 200 mW. Catalytic Test. The catalysts (700 mg, 0.5-0.85-mm particle diameter) were tested in a fixed bed, quartz tubular reactor operated at atmospheric pressure and between 723 and 823 K. The temperature was measured with a coaxial thermocouple. The feed was a mixture of 4 vol % propane, 4 vol % oxygen, and the balance helium. The flow rate was 100 mL/min at room temperature. The reactants and reaction products were alternately analyzed on-line by a Shimadzu GC9A gas chromatograph equipped with a thermal conductivity detector. A Porapaq Q (80-100-mesh) column for separating hydrocarbons and CO2 and a 2-m activated carbon (30-50-mesh) column for carbon monoxide, methane, and oxygen were used. The homogeneous contribution was tested with the empty reactor. These runs showed no activity below 853 K. The results were very similar with and without the use of quartz particles. The conversion and selectivity for products were evaluated for the exit stream (Cadus et al., 1996). Results Catalyst Characterization. Surface Texture. Table 1 includes the change in the specific surface areas of the supports and the supported molybdenum samples. Increasing the Mg loading leads to a decrease in the SA of the composite oxides (85, 80, and 74% of the value of the bare support). The SA increases after hydroxylation, which is in agreement with Llorente et al. (1992). After impregnation with molybdenum, the decrease in the SA is larger due to pore blocking. The SA does not change after reaction.
Table 1. Relation of MgO Content and Surface Properties of Catalysts MgO content of catalysts, wt % 0
8
12
16
184.6
156.5 165.4 97.7 30
146.5 157.3 83.6 29
136.0 153.0 55.8 29
m2/g
SA, support S support SOHa Mo-supported catalyst r mean, Å a
29
SOH supports after hydroxylation.
Figure 1. X-ray diffraction patterns of molybdenum-supported samples. (2) Peaks due to MgMoO4, (0) γ-Al2O3.
From adsorption-desorption isotherms of nitrogen at 77 K on all samples, the pore size distribution was determined. The mean pore radius around 29 Å remained unchanged after impregnation. X-ray Diffraction. From the X-ray diffraction patterns of γ-Al2O3 and MgO-γ-Al2O3 supports, no differences were found when the MgO content was 0, 8, 12, and 16 wt %, respectively. The diffraction peak of MgO crystals (d ) 2.11) was not detected. This indicates that the MgO is well dispersed on the surface of the support. These results agree with some literature (Caixia et al., 1995; You-Chang Xie and You-Qi Tang, 1990). The X-ray diffraction patterns of supported molybdenum samples are shown in Figure 1. MgMoO4 is characterized by the presence of the principal line at 2θ ) 26.4, d ) 3.37. The MgMoO4 pattern is similar to the precipitated material extensively characterized in a previous work (Cadus et al., 1996). From these spectra, it is inferred that the molybdenum reacts with magnesium, leading to fairly large magnesium molybdate crystallites, thus accounting for their XRD peaks. The broadening of the peaks observed suggests also that the magnesium molybdate may be distorted. From XRD data, there is no identification of the presence of another phase or structure. There is no evidence of the existence of a solid solution, although such determinations are difficult. After being used in propane oxidation, no remarkable changes were observed in the XRD patterns. EPR Spectroscopy. EPR measurements were applied to examine the presence of Mo5+. Spectra of pure samples of MoO3 and MgMoO4 recorded at room temperature revealed no significant presence of Mo5+ ions. However, spectra of all Mo-supported catalysts showed a similar signal characteristic of Mo5+. On fresh samples, the spectra were well resolved and exhibited an asymmetric EPR signal assigned to Mo5+ in the environment of nonaxial symmetry (Cadus et al., 1996; Oganowski et al., 1975). The average g values and line
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2139 Table 2. EPR Parameters of the Signals for Mo-Supported Catalysts av ga
a
catalyst
br
ar
∆H, G
Mo8S Mo12S Mo16S
1.928 1.928 1.928
1.928 1.925 1.929
64 63 61
ar, after reaction; br, before reaction.
Figure 3. Raman spectra of Mo8S (a), Mo12S (b), and Mo16S (c) before reaction.
Figure 2. EPR spectra (a) for Mo12S before and after the reaction recorded at room temperature and (b) for Mo8S, Mo12S, and Mo16S after reaction. Table 3. XPS Results binding energy Al 2p 8S 12S 16S Mo8S Mo12S Mo16S
Mo 3d5/2
Mg 2p
74.95 50.85 74.60 50.25 74.35 50.00 75.30 233.4 50.80 74.80 233.2 50.50 74.75 233.2 50.50
atomic ratio coverage, Mg/Al Mo/Al Mo/Mg % 0.23 0.29 0.45 0.52 0.55 0.68
0.20 0.24 0.32
0.37 0.44 0.47
18.7 22.5 31.0 41.9 44.1 50.0
width at peak-to-peak maximum (∆H) are presented in Table 2. These values are very close to those values reported by other workers (Oganowski et al., 1975; Reddy et al., 1992). Figure 2a shows the EPR spectra given by sample Mo12S before and after oxidative dehydrogenation of propane. The intensities of the signal were different, and a comparison on the basis of a similar ∆H value can be made. The intensities of the lines for used samples were similar and remarkably more intense than for fresh samples. This behavior was observed in all Mo-supported catalysts. EPR spectra of all used samples are shown in Figure 2b. The EPR signal in the samples unequivocally indicated the presence of Mo5+ species, but EPR is not a surface technique. XPS Measurements. Table 3 summarizes the binding energies of the most intense peaks of metal cations together with their surface atomic ratios for fresh samples. Using the metal-to-aluminum ratios listed in Table 3, the metal coverage was calculated. The Mo/ Mg atomic ratio for Mo16S is rather close to the value observed on (1/1)MgMoO bulk catalyst (0.52) in which a slight excess of MoO3 was detected simultaneously to MgMoO4 (Cadus et al., 1996). However, the Mo/Mg atomic ratio for Mo8S of 0.37 should indicate a partial coverage of Mg. The deconvoluted XPS spectra were used to determine the relative surface concentration of Mo6+ and Mo5+. A 3d5/2-to-3d3/2 peak area ratio of 3:2 was used in the deconvolution of the spectra, and the separation of the
Figure 4. Raman spectra of Mo8S (a), Mo12S (b), and Mo16S (c) after reaction.
doublets was assumed to be the same for both oxidation states. The calculated 3d5/2 and 3d3/2 binding energies were 233.3 and 236.3 eV for Mo6+ and 232.6 and 235.6 eV for Mo5+. These results are consistent with previous data obtained by Ward et al. (1977) and Cadus et al. (1993). The Mo5+ concentrations in samples Mo12S and Mo16S were similar, 40.5 and 40.1%, respectively, and higher than in Mo8S, 29.1%. Raman Spectroscopy. The Raman spectra of Mosupported catalysts are shown in Figures 3 and 4. The presence of magnesium molybdate on the support was confirmed. The Raman bands at 966, 955, 908, and 852 cm-1 were ascribed to magnesium molybdate. A weak band at 815 cm-1 ascribed to the Mo-O-Mo of MoO3 was also observed on fresh Mo8S sample. MoO3 is likely to be present in all samples because a slight excess of heptamolybdate was used during the preparation of the catalysts. The comparison of the spectra obtained over fresh and used Mo8S catalyst shows a degree of reduction as evidenced by the intensity loss of the MoO3 band at 815 cm-1. Ozkan et al. (1992) reported a relative intensity loss of MoO3 bands at 815 and 992 cm-1, greater than the relative intensity loss of the MnMoO4 bands at 926 and 860 cm-1 for a reduced MnMoO4MoO3 sample at 450 °C. Reductions of the Mo-supported catalysts were also shown by a color change in the sample from white to pale gray. Over the deformation frequency range, δ(OMoO) bands at 383, 369, 347, 333, and 272 cm-1 were also detected. The sharpening of the Raman band with increasing coverage observed in the samples has already been observed for other
2140
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
Table 4. Catalytic Results for the Oxidative Dehydrogenation of Propane on Mo-Supported Catalystsa Al2O3 at T (K) X, % X/m2 % SC32% SCO % SCO2 % SC2 % CO/CO2
Mo8S at T (K)
Mo12S at T (K)
Mo16S at T (K)
723
773
823
723
773
823
723
773
823
723
773
823
1.62 0.01
7.67 0.11 49.2 35.9 14.8
19.8 0.29 34.4 47.8 17.8
16.3
14.2
10.8 0.28 54.2 33.0 12.7
2.42
2.68
0.00
2.50
27.8 0.48 39.0 41.8 16.9 2.30 2.48
2.64 0.07 85.8
1.54
32.1 0.47 26.7 48.7 22.5 2.29 2.16
12.1 0.21 55.9 31.5 12.6
0.21
18.5 0.14 22.1 44.2 28.2 5.45 1.57
3.58 0.06 83.6
17.1 82.9
10.2 0.08 22.6 46.9 30.5
0.00
2.60
23.4 0.60 38.4 42.0 16.9 2.63 2.48
a
Reaction conditions: W/F ) 185 g‚h/mol of C3; molar ratio C3/O2/He ) 4/4/92; X (%), conversion of propane; X/m2, conversion per specific surface area; % Si, selectivity to product i.
Figure 5. Influence of the molybdenum content of the catalysts on the selectivity to propene at two different propane conversions per specific surface area. (b) X/m2 ) 0.1; (2) X/m2 ) 0.3. Figure 7. Influence of the propane conversion on the CO/CO2 ratio obtained on Mo-supported catalysts. For comparative purposes, the results obtained on γ-Al2O3 and composite oxides are also included.
Figure 6. Propene selectivity as a function of the propane conversion: (a) Mo8S, (b) Mo12S, and (c) Mo16S.
supported oxides and has been attributed to an increase of lateral interactions among surface species (Eon et al., 1994) Catalytic Results. The catalytic results obtained during the oxidative dehydrogenation of propane on molybdenum-supported catalysts are shown in Table 4. For comparative purposes, the results on γ-Al2O3 are also included. Propene, CO2, and CO were the main products. Ethene and methane (the latter only as traces) were also detected at high-temperature and high-conversion levels of propane. Oxygenated products other than carbon oxides were not observed. Since there exists differences in the SA in the Mosupported catalysts, the influence of the Mo content on the propene selectivity at conversion per unit of specific surface area is shown in Figure 5. It can be observed that the selectivity to propene increases when increasing the Mo content. The selectivity to propene as a function of propane conversion is shown in Figure 6. The conversion was varied by changing the reaction temperature. The higher the conversion of propane, the
lower the selectivity to propene obtained. The catalytic behaviors of Mo12S and Mo16S were rather close. On Mo12S and Mo16S, the higher selectivity for propene around 85% was obtained at 723 K where carbon monoxide formation was not detected. In Figure 7, the influence of propane conversion in the CO/CO2 ratio obtained on different Mo-supported catalysts is shown. These results are compared to those on γ-Al2O3 and composite supports. Carbon monoxide is not detected at conversion levels below 3%, which agrees with other results found on sepiolite-supported vanadium catalysts (Corma et al., 1993a). On Mosupported catalysts, the CO/CO2 ratio reaches an almost constant value (around 2.5), and at higher conversion, a slight decrease was observed. From these results, it seems that on γ-Al2O3, carbon monoxide is formed directly from propane, whereas on molybdenum-supported catalysts and composite oxides, CO is a secondary product formed from degradation of propene. Comparing the catalytic results of γ-Al2O3 and the composite oxides, the activities at different temperatures were similar but the product distributions were different. Table 5 shows the results for supports S. On the bare support γ-Al2O3, propene formation was observed (selectivity of 22%) at 773 K. Simultaneously, a carbonaceous deposit was formed. However, the composite oxide supports were not selective. Propene was detected at 823 K (selectivity of 9%), and no coke formation was observed. This behavior could be indicating the presence of an active coke formed on γ-Al2O3 under the reaction conditions. Discussion Magnesium molybdate with a slight excess of MoO3 was found to have good performance in the ODH of
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2141 Table 5. Catalytic Results for the Oxidative Dehydrogenation of Propane on Composite Oxides MgO-γ-Al2O3a 8S at T (K) 723 X, % X/m2 % SC32% SCO % SCO2 % SC2 % CO/CO2
773
823
4.9 10.3 20.8 0.04 0.09 0.19 9.1 52.1 50.7 40.8 47.9 49.3 10.6 9.5 1.1 1.0 1.0
12S at T (K) 723
773
823
16S at T (K) 723
2.2 9.0 17.5 2.6 0.02 0.09 0.17 0.03 9.7 7.4 50.9 43.8 17.1 92.6 45.6 36.8 82.9 3.5 9.7 0.9 1.1 1.2 0.2
773
823
10.2 19.2 0.11 0.2 9.3 53.1 44.3 43.9 37.9 3.1 8.5 1.2 1.2
a Reaction conditions: W/F ) 185 g‚h/mol of C ; molar ratio 3 C3/O2/He ) 4/4/92.
propane (Cadus et al., 1996). These catalysts were highly selective, but their specific surface areas were low and severely reduced after 7 h in reaction. In the present work, those drawbacks are overcome when molybdate ions are impregnated on a composite support of magnesia-alumina. The preparation method of the support allows a suitable composite oxide to be obtained in order to prepare more stable catalysts. The absence of the characteristic XRD peak of MgO even when magnesia loading was considerable indicates that this component is highly dispersed on the alumina matrix. The surface atomic ratio given by XPS (Table 3) suggests that aluminum is also present on the surface. Analysis of composite oxides by SEM-EDAX reveals that Mg and Al are always present on the surface, but the results do not provide evidence to verify that a new stoichiometric component has been formed. The calcination temperature used during the support preparation is high enough to form MgO but sufficiently low so that other phases such as magnesium aluminate spinel are avoided. In a recent work, Vriedland et al. (1996) reported the formation of magnesium aluminate spinel on alumina when the samples were calcined at 873 K. Molybdate ions from an ammonia solution (pH 11.9) are impregnated on magnesia-alumina support, and the practical importance of this impregnation stems from the fact that Mo goes preferably to the magnesia component of the support and forms very stable crystallites of MgMoO4 even at the first stage of the calcination procedure. The impregnation sequence avoids the formation of Al2(MoO4)3, which is not the catalyst of our present interest. If the impregnation of γ-Al2O3 had been made with a MgMoO4 precursor compound in solution, Al2(MoO4)3 would have been formed on the support. It was observed that alumina impregnated with a heptamolybdate solution and calcined at 723 K forms Al2(MoO4)3, which was clearly detected by XRD. When this sample was impregnated with Mg(NO3)2, crystallites of both MgMoO4 and Al2(MoO4)3 coexisted on the catalyst surface. You-Chang Xie and You-Qi Tang (1990) have also reported that MoO3 dispersed onto the surface of γ-Al2O3 forms Al2(MoO4)3 when the sample was heated at a temperature over 773 K. The crystallites of MgMoO4 formed on composite oxides are rather large, thus accounting for their XRD peaks (Figure 1). They cover different percentages of surface (Table 3), causing a blockage of pores and a decrease in the SA. A sharpening and intensification of XRD peaks as well as the largest decrease in SA are observed for Mo16S. In Mo8S, the Raman spectrum reveals the MoO3 band at 815 cm-1 which decreases in intensity after reaction. This band
corresponds to unreacted MoO3 located far away from the composite surface which could be highly interdispersed among the MgMoO4 particles or over its surface. It is quite likely that MoO3 can be present on the other samples. Concerning the catalytic behavior of composite oxides, the activity of propane (Table 5) is rather close to the bare alumina, but no propene formation and coke are detected in the 723-773 K temperature range. However, propene was formed on alumina at 773 K which could be directly related to the formation of a carbonaceous material. The catalytic activity of carbonaceous material on alumina has been described in the literature (Cadus et al., 1990; Schraut et al., 1987). This active coke has been considered to be the true catalytically active substance in the oxydehydrogenation of ethylbenzene to styrene. The amount of Mo incorporated into composite oxides influences the catalytic behavior of the catalysts (Table 4). Although the conversion tends to decrease when increasing the Mo content, the selectivity to propene is clearly related to these species. In Figure 6, the selectivity to propene increases with Mo content at the same conversion per unit of specific surface area, thus confirming the previous conclusion that molybdenum species act as active sites for propane oxidation (Cadus et al., 1996). The catalytic behaviors of Mo12S and Mo16S are rather close and consistent with the similar atomic coverage given by XPS and with the superficial concentration of Mo5+. The Mo/Mg atomic ratio for Mo16S is near the value of pure MgMoO4 ([Mo/Mg]XPS ) 0.476). It could be stated that a single active site is present in these samples. There is a good correlation between the Mo5+ surface concentration (XPS) and catalytic selectivity. However, the catalytic behavior of Mo8S should indicate that other kinds of sites on the surface contribute to modify its catalytic properties. The Mo5+ concentration given by the quantitative curve fittings of the XPS Mo 3d is lower. Two possibilities could be considered for its lower selectivity to propene. One is that unreacted Mg, which is nonselective, would be present on the surface of γ-Al2O3. Its lower XPS atomic ratio compared to the value of pure MgMoO4 would indicate the presence of small amounts of magnesia remaining on the surface. Another possibility is the higher surface concentration of aluminum since the support itself is nonselective. The catalytic results obtained for Mo-supported catalysts show a high CO/CO2 ratio (Figure 7). This ratio cannot be attributed to a deficit of oxygen which would increase the carbon monoxide formation but to the increase of both carbon oxides at the expense of the selectivity to propene. The selectivity to carbon oxides is higher in the composite oxides than in the alumina, which is consistent with the fact that MgO changes the acidity of the support and prevents coke formation. In all examined catalysts, the formation of ethene, C2, was observed at the highest temperature studied. It is worth commenting on the evolution of the cracked product. C2 is already formed in supports S at 773 K. The selectivity to C2 increases with the propane conversion, and its yield is high at 823 K. In contrast, the selectivity to C2 did not exceed 3% on Mo-supported catalysts, and it was almost the same in all of them. This means that the formation of C2 should be related to the superficial presence of magnesia. Taking into account the differences in specific surface areas, the selectivity to C2 on γ-Al2O3 is also less than the observed
2142
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996
value on composite oxides. The high dispersion of Mg can explain the fact that coke is not formed. Nature of Active Sites. The EPR spectra of Mosupported catalysts clearly show the signal corresponding to Mo5+ ions which is not found in the reference phases (MgMoO4 and MoO3). The presence of Mo5+ ions in fresh samples could be related to the calcination process (Oganowski et al., 1975), whereas in used catalysts, Mo5+ ions are generated under the propane atmosphere during the reaction. The intensities of the EPR signals for used samples were higher than for the fresh ones. This finding leads to the conclusion that the active sites become a dynamic process on the surface. There was no exact correlation with Mo5+concentration given by EPR. The higher the Mo5+ surface concentration from the quantitative curve fittings of the XPS Mo 3d, the higher the propene selectivity. Then, it is reasonable to propose that the active sites are a form of Mo5+, which is present on the surface as a small fraction of the total Mo5+. More studies are necessary to understand how these surface modifications take place in the reaction environment. Active sites in selective oxidation catalysts are usually related to the formation of a redox-type complex. In Mg-Mo-O mixed catalysts, Cadus et al. (1996) have found that the active sites are related to the formation of Mo5+ ion in an environment of nonaxial symmetry. In mechanical mixtures of MgMoO4 and MoO3, we have also arrived at the same conclusion. Khan and Somorjai (1985) proposed that the active sites on the molybdenum-based catalysts for methane conversion are pairs of coupled Mo5+ and Mo6+ species which transfer active oxygen and interact with methyl radicals, undergoing a redox mechanism. A catalytic reaction mechanism through a redox cycle between V5+ and V4+ has also been proposed on vanadium oxide based catalysts (Siew-HewSam et al., 1990; Chaar et al., 1987). Conclusions From the results described above, it can be concluded that the strategy of preparation from a composite support allows stable catalysts to be obtained with interesting activities in the ODH of propane. Nevertheless, the oxidation of propene to carbon oxides reduces the dehydrogenation selectivity. The incorporation of Mo over the magnesia-alumina support leads to the formation of large and very stable MgMoO4 particles. The characterization techniques suggest that the composite support is not fully covered for a molybdate layer and there is a small amount of MoO3 on the MgMoO4 particles. Other phases such as Al2(MoO4)3 are not present in significant amounts. A fair description of the composite oxide surface showing the magnesia well dispersed on the alumina is shown in Figure 8a, and a scheme of Mo-supported catalysts is depicted in Figure 8b. Mo5+ ions are proposed as the active site for the oxidative dehydrogenation of propane. This reduced active form is present on the surface as a small fraction of the total Mo5+, and it is generated by propane reduction of the surface. Acknowledgment We are grateful to the Japan International Cooperation Agency for the grant of the laser raman spectrom-
Figure 8. Scheme depicting the formation of composite oxides (a) and Mo-supported catalysts (b).
eter to CENACA. Financial support is also acknowledged to CONICET and to Universidad Nacional de San Luis. Literature Cited Blasco, T.; Concepcio´n, P.; Lopez Nieto, J. M.; Perez Pariente, J. Preparation, Characterization and Catalytic properties of VAPO-5 for the oxydehydrogenation of propane. J. Catal. 1995, 152, 1. Cadus, L. E.; Gorriz, O. F.; Rivarola, J. B. Nature of active coke in the oxydehydrogenation of Ethylbenzene to Styrene. Ind. Eng. Chem. Res. 1990, 29, 1143. Cadus, L. E.; Gotor, F. J.; Acosta, D.; Naud, J.; Ruiz, P; Delmon, B. Solid state reaction in the iron-molybdenum-antimony oxide system. Solid State Ionics 1993, 63, 743. Cadus, L. E.; Abello, M. C.; Gomez, M. F.; Rivarola, J. B. Oxidative dehydrogenation of propane over MgMoO catalysts. Ind. Eng. Chem. Res. 1996, 35, 14. Caixia, Qi; Tingfang, Bai; Lidum, An. Influence of MgO Loading on the Structure and Catalytic performance of Pd/γ-Al2O3 I. Effect of interaction between MgO and γ-Al2O3. React. Kinet. Catal. Lett. 1995, 54 (1), 131. Chaar, M. A.; Patel, D.; Kung, H. H. Selective Oxidative Dehydrogenation of Propane over V-Mg-O Catalysts. J. Catal. 1988, 109, 463. Chaar, M. A.; Patel, D.; Kung, M. C.; Kung, H. H. Selective Oxidative Dehydrogenation of Butane over V-Mg-O Catalysts. J. Catal. 1987, 105, 483. Contescu, Cr.; Sirajav, Ch.; Schuwarz, J. A. Selective ion exchange of palladium on alumina-silica composite oxides. Appl. Catal. 1991, 74, 95. Corma, A.; Lopez Nieto, J. M.; Paredes, N.; Perez, M.; Shen, Y.; Cao, H.; Suib, S. L. Oxidative dehydrogenation of propane over supported-vanadium oxide catalysts. New Development in Selective Oxidative by Heterogeneous Catalysis (Stud. Surf. Sci. Catal., Vol. 72); Elsevier: Amsterdam, The Netherlands, 1992; p 213. Corma, A.; Lopez Nieto, J. M.; Paredes, N. Oxidative dehydrogenation of propane on vanadium supported on magnesium silicates. Appl. Catal. A: Gen. 1993a, 97, 159. Corma, A.; Lopez Nieto, J. M.; Paredes, N. Preparation of V-Mg-O catalysts: Nature of active species precursors. Appl. Catal. A: Gen. 1993b, 104, 161. Eon, J. G.; Olier, R.; Volta, J. C. Oxidative dehydrogenation of propane on γ-Al2O3 supported vanadium oxides. J. Catal. 1994, 145, 318. Gao, X.; Xin, Q.; Guo, X. Support effects on magnesium-vanadium mixed oxides in the oxidative dehydrogenation of propane. Appl. Catal. A: Gen. 1994a, 114, 197.
Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2143 Gao, X.; Ruiz, P.; Xin, Q.; Guo, X.; Delmon, B. Effect of Coexistence of Magnesium Vanadate Phases in the selective oxidation of propane to propene. J. Catal. 1994b, 148, 56. Grabowski, R.; Grzybowska, B.; Samson, K.; Sloczynski, J.; Stoch, J.; Wcislo, K. Effect of alkaline promoters on catalytic activity of V2O5/TiO2 and MoO3/TiO2 catalysts in oxidative dehydrogenation of propane and in isopropanol decomposition. Appl. Catal. A: Gen. 1995, 125, 129. Guerrero-Ruiz, A.; Rodriguez Ramos, I.; Fierro, J. L. G.; Sonnen, V.; Herrmann, J. M.; Volta, J. C. Nature of surface sites in the selective oxide hydrogenation of propane over V-Mg-O Catalysts. New Developments in Selective oxidative by Heterogeneous Catalysis (Stud. Surf. Sci. Catal., Vol. 72); Elsevier: Amsterdam, The Netherlands, 1992; p 203. Juarez-Lopez, R.; Godjayeva, N. S.; Cortes Corberan, V.; Fierro, J. L. G.; Mamedov, E. A. Oxidative dehydrogenation of ethane on supported vanadium-containing oxides. Appl. Catal. A: Gen. 1995, 124, 281. Kaddouri, A.; Anouchinsky, R.; Carballo, L.; Mazzocchia, C. Oxidative dehydrogenation of ethane on the R and β phases of NiMoO4. New Developments in Selective Oxidation by Heterogeneous Catalysts (Stud. Surf. Sci. Catal., Vol. 72); Elsevier: Amterdam, The Netherlands, 1992; p 34. Khan, M. M.; Somorjai, G. A.; A kinetic study of partial oxidation of methane with nitrous oxide on a molybdena-silica catalyst. J. Catal. 1985, 91, 263. Llorente, J. M. M.; Rives, V.; Malet, P.; Gil-Llambias, F. J. MoO3/ MgO Systems: Effect of preparation Method on their physicochemical Properties. J. Catal. 1992, 135, 1. Mamedov, E. A.; Cortes-Corberan, V. Oxidative dehydrogenation of lower alkanes on vanadium oxide-based catalysts. Appl. Catal. A: Gen. 1995, 127, 1. Mazzocchia, C.; Aboumard, C.; Diagne, C.; Tempesti, E.; Herrmann, J. M.; Thomas, G. On the NiMoO4 oxidative dehydrogenation of propane to propene: some physical correlations with the catalytic activity. Catal. Lett. 1991, 10, 181. Michalakos, P. M.; Kung, M. C.; Jahan, Y.; Kung, H. H. Selective Patterns in Alkane Oxidation over Mg3(VO4)2-MgO, Mg2V2O7, and (VO)2P2O7. J. Catal. 1993, 140, 226.
Oganowski, W.; Hanuza, J.; Jezowska-Trzebiatowska, B.; Wrzyszcz, J. Physicochemical Properties and Structure of MgMoO4-MoO3 Catalysts. J. Catal. 1975, 39, 161. Ozkan, V. S.; Smith, M. R.; Driscoll, S. A. Investigation of synergy effects in selective oxidation catalysts through in situ Laser Raman Spectroscopy/Isotopic Labeling Technique. J. Catal. 1992, 134, 24. Reddy, B. M.; Reddy, E. P.; Srinivas, S. T. Dispersion and activity of molybdena-alumina catalyst prepared by impregnation and solid/solid wetting methods. J. Catal. 1992, 136, 50. Schraut, A.; Emig, G.; Sockel, H. G. Composition and Structure of active coke in the oxydehydrogenation of ethylbenzene. Appl. Catal. 1987, 29, 311. Seah, M. P.; Dench, W. A. Quantitative Electron Selective Spectroscopy of Surfaces. Surf. Int. Anal. 1979, 1 (1), 2. Siew-Hew-Sam, D.; Soenen, V.; Volta, J. C. Oxidative Dehydrogenation of Propane over V-Mg-O Catalysts. J. Catal. 1990, 123, 417. Vrieland, G. E.; Khazai, B.; Murchison, C. B. Anaerobic oxidation of butane to butadiene over magnesium molybdate catalysts. Appl. Catal. A: Gen. 1996, 134, 123. Ward, M. B.; Lin, M. J.; Lunsford, J. H. The oxidative dehydrogenation of ethane by nitrous oxide over molybdenum oxide supported on silica gel. J. Catal. 1977, 50, 306. You-Chang Xie; You-Qi Tang. Spontaneous monolayer dispersion of oxides and salts onto surfaces of supports: Applications to heterogeneous catalysis. Adv. Catal. 1990, 37, 1.
Received for review November 7, 1995 Accepted April 16, 1996X IE9506779
X Abstract published in Advance ACS Abstracts, June 1, 1996.
