γ-Al2O3 catalysts for the oxidative dehydrogenation of propane.

Applied Catalysis A: General 207 (2001) 421–431

Mo/␥-Al2 O3 catalysts for the oxidative dehydrogenation of propane. Effect of Mo loading M.C. Abello a,∗ , M.F. Gomez a , O. Ferretti b a

INTEQUI, Instituto de Investigaciones en Tecnolog´ıa Qu´ımica (UNSL-CONICET), Chacabuco y Pedernera, 5700 San Luis, Argentina b CINDECA, Centro de Investigación y Desarrollo en Procesos Cataliticos (UNLP-CONICET) and Depto. de Ing. Qca. (UNLP), 47 No. 257, 1900 La Plata, Argentina Received 5 February 2000; received in revised form 21 June 2000; accepted 22 June 2000

Abstract Oxidative dehydrogenation of propane over MoO3 /␥-Al2 O3 catalysts with different Mo loading was studied. The catalysts were characterized by XRD, DRS, TPR, DTP and by isopropanol decomposition reaction. The results indicated that with Mo loading increasing from 3.6 to 12.7 wt.%, propane conversion increased parallel to the acidity and to the reducibility. At a conversion higher than 20% the selectivity to propene leveled off around 25% irrespective of the molybdenum content. The catalytic properties of Mo/␥-Al2 O3 appear to be related to the coordination of Mo species of the active sites and to its Brøsnted acidity. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Propane; Oxidative dehydrogenation of alkanes; Molybdenum-supported catalyst

1. Introduction In the last decade, great interest has been observed in the development of highly selective catalysts for the transformation of light alkanes into more valuable organic compounds. The oxidative dehydrogenation is a valid alternative for the production of these chemicals. This provides several advantages over the nonoxidative process based on engineering and economic considerations [1]. The oxidative dehydrogenation of propane to propene is particularly attractive if it is taken into account that forecasts for basic olefins indicate that the annual growth rate of propene will be 4.8% over the next 15 years [2] and the existing capacity (principally from steam-cracking of naphtha and FCC in oil refining) may become insufficient to meet ∗ Corresponding author. Fax: +54-652-26711. E-mail address: [email protected] (M.C. Abello).

this demand [3]. Vanadium and molybdenum oxides are important components of catalysts used for selective oxidation of light alkanes [4–21]. Molybdenum supported on a variety of different oxides has recently been reported as promising propane oxydehydrogenation catalysts. Grzybowska et al. [14] have reported studies over Mo/TiO2 consisting of one or five monolayers of Mo with addition of alkaline cations in an attempt to improve the propene selectivity. Meunier et al. [22] have investigated catalysts consisting of molybdenum supported on alumina, zirconia, silica, titania and magnesia. Among these, titania anatase was found to be the most effective support. They also studied the effect of molybdenum loading. A molybdenum coverage of more than one monolayer was found to be the most selective on titania. Redox and surface acid–base properties of catalysts have been proved to play an important role in partial oxidation of light paraffins. Many authors agree

0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 6 8 0 - 3

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that the oxidation reactions proceed via Mars and van Krevelen mechanism [23] and metallic cations as V, Mo, etc. are needed in order to change their oxidation state in the course of the reaction. During the oxidation reaction lattice oxygen oxidizes the organic molecules leaving a reduced center that is reoxidized by O2 . Besides, it is also accepted that the adsorption and activation of the hydrocarbon molecule are related to acid–base interactions and the adsorption and desorption rates of products are related to acid–base properties [24]. Then, an active and selective catalyst should have a peculiar combination of acid and redox properties. The present work reports a systematic study about molybdenum loading on alumina and its influence in the oxidative dehydrogenation of propane. The reactivity of Mo/␥-Al2 O3 catalysts with Mo loading from 3.6 to 12.7 wt.% of MoO3 and the role of their acid–base and redox properties are investigated. The nature of the active sites is also discussed. 2. Experimental part 2.1. Catalyst preparation The materials used were ␥-Al2 O3 (surface area, 184.6 m2 g−1 , precalcined at 873 K for 3 h), and ammonium heptamolybdate from Baker. Supported Mo oxide was prepared by a single impregnation of the support with an aqueous solution containing ammonium heptamolybdate, AHM, 5×10−3 M at pH 5.6 by using the standard technique. The volume of AHM was chosen in order to add the desired amount of Mo. The weight loading of MoO3 varied between 3 and 13%. After evaporation of the solvent under reduced pressure the samples were dried at 373 K overnight and further calcined in air at atmospheric pressure according the following procedure: temperature was raised linearly for 2.5 h up to 723 K, kept constant at 723 K for 3 h, raised linearly up to 873 K, and then maintained for 5 h at 873 K. Supported molybdenum catalysts were referred to xMo/␥-Al2 O3 where x indicates the wt.% of MoO3 related to weight of catalyst. 2.2. Catalyst characterization All samples were characterized using the following physicochemical methods.

2.2.1. BET surface area BET surface areas were measured by using a Micromeritics Accusorb 2100E instrument by adsorption of nitrogen at 77 K on 200 mg of sample previously degassed at 473 K under high vacuum atmosphere for 2 h. 2.2.2. Chemical composition The molybdenum content was determined by atomic absorption spectroscopy. The samples were brought into solution by alkali fusion with KSO4 H and subsequent dissolution with diluted HCl solution. The measurements were carried out by standard addition solution method by using a Varian AA275 equipment. 2.2.3. X-ray diffraction (XRD) XRD diffraction patterns were obtained with a RIGAKU diffractometer operated at 30 kV and 20 mA by using Ni-filtered Cu K␣ radiation (λ=0.15418 nm). The samples in powder form were analyzed without previous treatment after deposition on a quartz sample holder. The identification of crystalline phases was made by using references from the JCPDS files. 2.2.4. Diffuse reflectance spectroscopy (DRS) Spectra in the UV–VIS region were collected by a Varian Super Scan 3 spectrophotometer equipped with a reflectance attachment. AHM and sodium molybdate were used as reference compounds. Spectra were recorded in the 200–600 nm range. 2.2.5. Temperature programmed reduction (TPR) Studies were performed in a conventional TPR unit. This apparatus consists of a gas handling system with mass flow controllers (Matheson), a tubular reactor, a linear temperature programmer (Omega, model CN 2010), a PC for data retrieval, a furnace and various cold traps. In each experiment, the sample size was ca. 100 ␮mol of Mo to assure a good resolution in the experimental conditions used. Before each run, the samples were oxidized in a 30 ml min−1 flow of 20 vol.% O2 in He at 723 K for 30 min and then, cooled at 323 K. After that, helium was admitted to remove oxygen. The samples were subsequently contacted with a 30 ml min−1 flow of 10 vol.% H2 in Ar and heated, at a rate of 10 K min−1 , from 323 K to a final temperature of 963 K and held at 963 K for 2 h. Hydrogen

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consumption was monitored by a thermal conductivity detector after removing the water formed. The areas under the peaks were integrated to determine the H2 consumed previous calibration. 2.2.6. Acid–base properties Acidity measurements were determined by ammonia temperature-programmed desorption, TPD, by using a conventional flow system with a thermal conductivity detector. In each experiment, ca. 0.2 g were used. All samples were pretreated in the same way: they were heated under helium flow from room temperature to 873 K at 10 K min−1 . After cooling down to room temperature, the adsorption step was started. This step was carried out in pure ammonia flow for 30 min. Then, the samples were swept with helium for 30 min and finally, the desorption step was performed from room temperature to 873 K at a heating rate of 10 and 30 ml min−1 of helium total flow. Continuous voltages from the detector cell and reactor thermocouple were converted to digital signals, amplified with a data acquisition workstation and stored in a PC. An arbitrary scale for the different acid strengths was selected. Weak, medium and strong acidity were related to the number of NH3 molecules desorbed up to 473 K, between 473 and 673, and above 673 K, respectively. The acidity distribution of catalysts has been expressed as the integrated area of NH3 band developed in a given range of temperature. The total integrated area of the NH3 band was considered as a measure of total acidity. Acidity was calculated by dividing the total area by the BET surface area of the sample. Decomposition of isopropanol, IPA, was also used for determining the acid–base properties of the samples [25]. Since IPA needs an acid–base pair to be converted to acetone, the acetone-to-propene ratio reflects the basicity, whereas the propene formation rate is due to the acidity. The reaction was carried out between 433 and 473 K in a fixed-bed continuous flow reactor under atmospheric pressure and in the absence of oxygen. The feed consisted of 4.5% IPA and the balance helium. The weight of catalyst was ca. 0.5 g and the flow rate was 40 ml STP min−1 . The data for rate calculations were taken after the stationary state was reached and the conversion of isopropanol was <15%. Product analysis was performed by gas chromatography using a Carbowax 20 M on Chromosorb W column and a thermal conductivity detector.

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2.3. Catalytic test The catalysts (0.5–0.85 mm particle diameter) were tested in a fixed bed, quartz tubular reactor operated at atmospheric pressure 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−1 at room temperature. The reactants and reaction products were alternately analyzed on-line by a Shimadzu GC9A gas chromatography 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. For the base case the weight of catalytic samples was 0.7 g. In order to obtain different propane conversion levels, by variation of W/F, the weight for 3Mo/␥-Al2 O3 and 13Mo/␥-Al2 O3 catalysts was varied between 0.2 and 0.9 g. The conversion and selectivity to products were evaluated for the exit stream. The moles of O2 consumed per mole of propane feed were calculated as X XSi νO2 R O2 = where X is conversion, Si the selectivity for product i and νO2 the oxygen stoichiometry to form product i [26]. 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 filling the void space. A set of experimental data, obtained by varying the mass velocity of the feed, while keeping W/F constant, indicated the mass-transfer effects were negligible at the space velocities used.

3. Results Results of the Mo analysis of impregnated catalysts are included in Table 1, together with the specific surface area values, SBET and the resultant coverage, θ , defined as a fraction of a theoretical monolayer. The coverage was calculated using 22 Å2 as the mean surface area occupied by one Mo6+ oxide unit on ␥-Al2 O3 [27] and from the initial BET area of the

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Table 1 Characteristics of Mo/␥-Al2 O3 catalysts Catalyst (m2 g−1

SBET catalyst) SBET (m2 g−1 support) θ MoO3 (AAS) (% w/w) MoO3 theor. (% w/w)

3Mo/␥-Al2 O3

7Mo/␥-Al2 O3

10Mo/␥-Al2 O3

13Mo/␥-Al2 O3

182.3 189 0.18 3.6 3.5

181.0 196 0.36 7.7 6.7

174.4 193 0.53 10.0 9.7

166.0 190 0.72 12.7 12.6

support. The theoretical monolayer value is 16.7 wt.% MoO3 . In all cases, Mo loading was well below those required for monolayer coverage. Chemical analysis data indicated that calcination at 873 K had no significant effect on the molybdenum content. Then, the loss of Mo by sublimation could be considered negligible. The specific surface areas expressed per gram of catalyst decrease with increasing Mo loading. This behavior agrees with literature [28]. If the comparison is done per gram of support no changes were found. Thus, it can be considered that MoO3 contributes only to the mass of the catalysts covering partially the support area. The X-ray diffraction patterns of impregnated Mo catalysts only showed that the presence of characteristic peaks of ␥-Al2 O3 (JCPDS 10-425), MoO3 and Al2 (MoO4 )3 were not detected in any sample. Al2 (MoO4 )3 is a thermodynamically stable phase and it has been reported to form in highly loaded catalysts at temperature up to 993 K [29]. Using Raman spectroscopy, Cheng and Schrader [30] have shown that aluminium molybdate is not formed when Mo oxide on ␥-Al2 O3 is calcined at temperature of 823 K even though the MoO3 content was 20 wt.%. Taking into account the XRD results, the calcination temperature (873 K) and the experimental conditions during preparation (a very dilute impregnation solution and high specific surface area of support) it can be considered that the formation of Al2 (MoO4 )3 is avoided and all molybdena is deposited in the form of adsorbed species and not in the form of precipitated bulk amorphous phase. Then a high dispersion degree of molybdenum should be present on ␥-Al2 O3 . After being used in propane oxidation, no significant changes were observed in the XRD patterns. Acidic properties were investigated using the adsorption of probe molecules by TPD. NH3 TPD profiles of samples shown in Fig. 1 are characterized

by a broad asymmetric desorption pattern, spanning the range 298–873 K. Acid strength distribution calculated as explained in Section 2, is illustrated in Fig. 1. The total acidity increases with Mo loading and the new acid sites are of weak or moderate strength probably of Brønsted type as proposed by literature [31–33]. At loadings higher than 3% Mo, the strong acid sites of the alumina have been mostly eliminated. The abundance of strong sites decreases from 18.7% on 3Mo/␥-Al2 O3 to 6.5% on 13Mo/␥-Al2 O3 . According to the references, these strongly acidic sites are probably Lewis sites. In addition, the different catalysts were compared using the test reaction of isopropanol. In Fig. 2, the influence of Mo loading on propene and acetone formation rates at 433 K is shown. The dehydration to propene was the main reaction in all cases indicating an increase in the acidity of the samples with Mo loading. These results agree with those obtained by TPD. The acetone formation rate also increased but this effect is less marked than for the dehydration reaction. Di-isopropylether was also detected; however, propene remains by far the main product. In order to compare, the bare alumina was also tested. It was poorly active at 433 K and the reaction products on support were propene and di-isopropylether. The isopropanol decomposition proceeds by two parallel routes: dehydration to propene on acidic (rather weak or moderate) sites and dehydrogenation to acetone on redox (basic) sites [34]. The isopropanol decomposition cannot distinguish between the Brønsted and Lewis sites. Nevertheless, Aramend´ıa et al. [35] have reported that the dehydrating activity in the isopropanol decomposition could be correlated with Brønsted acidity. Then, it can be inferred that a higher content of Mo the Brønsted acidity increases. Rajagopal et al. [31] have suggested that new Brønsted acid sites are generated with MoO3 loading and they are associated with Mo

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Fig. 1. NH3 -TPD profiles of Mo supported catalysts:

: 3Mo/␥-Al2 O3 ;

species strongly bound in a monodentate manner to Al or to Mo in a polymolybdate layer. The redox properties were examined by thermal programmed reduction. The spectra of fresh samples are shown in Fig. 3. With the only exception of 3Mo/␥-Al2 O3 , the samples presented two well-resolved reduction peaks. For all samples, the maximum in the first reduction peak appeared in the same temperature range where the oxydehydro-

: 7Mo/␥-Al2 O3 ;

: 10Mo/␥-Al2 O3 ;

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: 13 Mo/␥-Al2 O3 .

genation reaction was carried out. The second peak evolved when the temperature ramp had already finished (963 K). Besides, no significant effects of H2 reduction were observed bellow 573 K. The first Tmax shifted to lower temperature with increasing Mo loading, indicating that the interaction of Mo species with the support decreased. This effect is important when MoO3 content was varied from 3.6 to 12.7%. In this case the temperature change was 58 K (from 783 to

Fig. 2. Propene and acetone formation rates in isopropanol decomposition on Mo/␥-Al2 O3 catalysts.

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Fig. 3. H2 -TPR profiles of Mo supported catalysts.

725 K). The appearance of the second peak suggests that a fraction of Mo species reduce at higher temperatures likely due to its high dispersion state on the support. The extent of reduction has been characterized by the change in average oxidation number of Mo, 1ON, which depends on the amount and the reducibility of Mo species and on the experimental conditions of TPR [28]. For example, 1ON=2 indicates an average reduction of Mo6+ to Mo4+ . Table 2 shows the amount of H2 uptake and the corresponding 1ON calculated from the whole TPR spectra. It can be observed that the extent of reduction increased with Mo loading. For 3Mo/␥-Al2 O3 the reduction produced a 1ON<1, then, Mo species has been mainly reduced from Mo6+ to Mo5+ . For 7Mo/␥-Al2 O3 the Mo average oxidation state is higher than four. 1ON for 10Mo/␥-Al2 O3 and 13Mo/␥-Al2 O3 indicate that molybdenum was reduced to Mo4+ . These results are consistent with conclusions of De Canio et al. [36]. Characterization of Mo species was achieved by diffuse reflectance spectroscopy. Spectra are shown

in Fig. 4, together with the spectra of molybdenum compounds in tetrahedral coordination (sodium molybdate) and in octahedral coordination (ammonium heptamolybdate). From the spectrum of sodium molybdate, the molybdenum in tetrahedral coordination exhibits two absorption bands, at 220 and 260 nm, whereas the molybdenum in octahedral coordination also presents a band around 220 nm, and in addition, another bands at higher wavelengths (270–290 and 310–350 nm). From literature, the Mo=O bond of tetrahedral molybdate and Mo–O–Mo bridge bond of the octahedral species possess electronic absorptions at 220–250 and 320 nm, respectively [37]. Spectra of Mo/␥-Al2 O3 samples are similar and they exhibit bands at 220 and 260 nm and a broad band above 280 nm which becomes more intensive at increasing Mo loading. From these spectra it can be suggested that a mixture of tetrahedral and octahedral Mo species are present on the support surface. The fraction of tetrahedral species seems to be bigger at low Mo loading whereas the concentration of octahedral

Table 2 Change in average oxidation number during TPR up to 963 K of Mo/␥-Al2 O3 catalysts Catalyst

3Mo/␥-Al2 O3

7Mo/␥-Al2 O3

10Mo/␥-Al2 O3

13Mo/␥-Al2 O3

H2 uptake (␮moles) 1ON

41.32 0.80

60.32 1.20

108.38 2.16

111.44 2.23

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427

Fig. 4. DR spectra of Mo supported catalysts: (a) Na2 MoO4 ; (b) AHM.

species predominates at high loading. Recently, these catalysts were investigated by XAFS techniques and this study determined that the fraction of tetrahedral Mo species decreased from 40% to near 10% when the Mo loading changed from 3.6 to 12.7 wt.% MoO3 [38]. These results also agree with literature [39,40].

The catalytic results obtained in the oxidative dehydrogenation of propane are shown in Table 3. Propene and carbon oxides were the main products. Oxygenated products other than carbon oxides were not observed. Conversion (X%) increased with Mo loading suggesting that the active sites are related to molybdenum species. It can be also seen that ␥-Al2 O3

Table 3 Catalytic results for propane oxidation over Mo supported catalystsa Catalyst

T (K)

RO2 (%)

X (%)

YC3 (%)

Selectivity (%) CO

CO2

C2

C3 H6

3Mo/␥-Al2 O3

723 773 823

6.9 39.1 79.4

3.5 12.9 25.5

23.1 43.4 39.6

17.4 27.6 32.0

– 1.9 3.1

59.4 27.1 25.3

2.1 3.5 6.5

7Mo/␥-Al2 O3

723 773 823

12.0 49.3 96.4

4.8 15.9 29.3

24.5 39.7 38.4

28.2 31.4 36.6

– 1.1 1.5

47.3 27.8 23.4

2.3 4.4 6.9

10Mo/␥-Al2 O3

723 773 823

24.4 66.7 100

8.6 21.6 32.4

37.5 38.9 39.4

26.9 31.7 31.6

– 1.1 1.7

35.6 28.2 27.3

3.1 6.1 8.8

13Mo/␥-Al2 O3

723 773 823

39.9 88.1 100

13.6 28.5 36.2

40.6 44.2 47.2

27.0 28.2 25.7

– 1.0 2.0

32.4 26.6 25.1

4.4 7.6 9.1

␥-Al2 O3

723 773 823

7.5 34.5 61.4

1.6 10.4 19.1

17.1 46.3 43.2

82.4 32.1 32.1

0.5 3.0 4.7

– 18.6 20.0

– 1.9 3.8

a

W/FC3 =78.6 gcat h (mol C3 H8 )−1 ; X: propane conversion; YC3 : yield to propene; RO2 : oxygen consumption.

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Fig. 5. Variation of propene selectivity with the propane conversion obtained in the ODH of propane. 3Mo/␥-Al2 O3 (open symbols); 13Mo/␥-Al2 O3 (solid symbols): (䊊 䊉) 723 K; (䊐 䊏) 773 K; (4 䉱) 823 K.

is able to activate the reaction but its selectivity to propene is low. Fig. 5 shows the values of propene selectivity against conversion. The higher conversion of propane the lower selectivity to propene. The selectivity is less dependent of reaction temperature than conversion. While the selectivity to propene decreases as result of the marked increase of the selectivity to CO, the formation of CO2 is less affected. At higher conversion levels, the propene selectivity tends to a

stationary value ca. 25% irrespective of the molybdenum content. In spite of selectivity of propene shows a very slight increase with Mo loading, it is observed that approximately all points fit the same curve and it can, thus, postulate that the same type of active site is present on the surface which should differ only in the superficial site concentration; this result is consistent with DRS experiments. The selectivity conversion trend suggests that propane is initially oxidized to

Fig. 6. Relationship between oxygen consumption and propane conversion in the oxydehydrogenation reaction for Mo/␥-Al2 O3 .

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429

Fig. 7. Consumption rate of propane in ODH at 723 K against formation rate of propene in IPA decomposition.

propene and that propene is further oxidized to COX . CO2 could be a primary product as well, since its selectivity at zero conversion limit does no appear to be zero. High levels of oxygen conversion were observed at 823 K and this limited the propane conversion on 10Mo/␥-Al2 O3 and on 13Mo/␥-Al2 O3 catalysts. A change of color of the catalyst particles was observed along the catalytic bed, from white of the fresh material to gray or black. This color change could be accounted for either by reduction of the molybdenum species since the reaction atmosphere became reductive as the oxygen conversion was high or by coke formation. The moles of oxygen consumed per mole of propane feed as a function of conversion are plotted in Fig. 6. The slope of the line is 3.1, which represents the weighted average oxygen stoichiometry [26]. C2 were also detected at higher temperatures and high conversion levels of propane. The selectivity to C2 was higher at low Mo loading and likely related to the uncovered fraction of ␥-Al2 O3 surface. It is known that hydrocarbon cracking is favored on strong acid sites such as present on ␥-Al2 O3 . From TPD studies the strong acidity decreases at increasing Mo loading. Then the formation of C2 is expected to decrease as well. The conversion at the lowest temperature is low enough (<10%) for the differential reactor assumption

to be valid, then the consumption rate of propane in the oxydehydrogenation reaction, ODH, can be calculated. These rates against the formation rate of propene in the IPA decomposition reaction were plotted in Fig. 7. The oxydehydrogenation activity linearly increases with increasing acidic properties of the catalysts. From these results it can be suggested that acidic sites have a very important role in the activation of alkane. These acidic sites are involved in the first hydrogen abstraction from propane, which is considered as the rate-determining step.

4. Discussion Catalytic results show a clear dependence of the activity data on the molybdenum content. Mo/␥-Al2 O3 are active but poorly selective catalysts in the oxidative dehydrogenation of propane at least for propane conversion level of real interest. At the steady state, propane conversions >20% are obtained at 823 K with a selectivity to propene around 25%. It is well known that several Mo species can be present on the support and the nature and their relative ratio depend on preparation method, calcination temperature, Mo concentration, etc. The characterization of these catalysts by XRD, TPR, NH3 -TPD, IPA decomposition and DRS, reveals that molybdenum loading determines the na-

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ture of the supported species as well as their acid–base and redox properties. Results obtained in this work show that molybdenum forms well dispersed phases, in which the contribution of octahedral (MoO ) species increases with molybdenum loading. From Fig. 4 an important fraction of Mo species octahedral coordinated is present on xMo/␥-Al2 O3 even on 3Mo/␥-Al2 O3 . Kim et al. [41] identified by Raman spectroscopy the presence of Mo7 O24 6− and MoO4 2− species (tetrahedral coordination) on Mo/␥-Al2 O3 containing 4.2 wt.% of MoO3 whereas Mo8 O26 4− species were predominant on catalyst with 13.9 wt.%. Okamoto and Imanaka have also reported that tetrahedral coordination is maintained in samples with 3.5 wt.% MoO3 on Al2 O3 [42]. These changes in structure are responsible of the increase in acidity and in reducibility. The number of sites interacting with NH3 increases after the addition of Mo (Fig. 1) and the strength of the acid sites changes. The strong acid sites show a very important diminution and new weak or moderate sites are created. These favor dehydration over dehydrogenation in the isopropyl alcohol decomposition. The dehydration activity in IPA decomposition was appreciably correlated to Brønsted acidity [35]. Then, it can be inferred that the addition of molybdenum to alumina surface produces Brønsted sites of moderate or weak strength. Concerning the redox properties, Fig. 3 and Table 2 reflect the well known tendency of increasing reducibility of alumina-supported MoO3 with increasing Mo loading. The first Tmax shifts to lower temperature, clearly indicating that the interaction between Mo species and support decreases. At least two Mo(VI) species are present on these samples: MoT and MoO . These structures exhibit different reduction behavior. The first peak could be assigned to the reduction of a large fraction of octahedral coordinated species from Mo6+ to Mo5+ which are easily reducible. The second one to the reduction of all Mo species to lower oxidation state including highly dispersed tetrahedral species which are more difficult to reduce. Grunert et al. [43] have reported that the reduction of Mo/␥-Al2 O3 in H2 produces surfaces on which Mo(VI), Mo(V), Mo(IV), Mo(II) and at reduction temperatures above 900 K, Mo(0) coexist. From Figs. 1–3 and Table 3 a parallelism between activity, reducibility and acidity can be observed. The

Scheme 1.

linear relationship between the catalytic activity to form propene from propane and the catalyst acidity determined by the dehydration of isopropyl alcohol (Fig. 7) suggests that the methylene C–H bond cleavage of propane is assisted by surface acidic sites. A detailed mechanism of propane oxidation cannot be clearly elucidated at this stage but the acidic nature of Mo surface species has some effect on the reaction rate. Hydrogen abstraction from alkanes which is generally accepted as the rate-determining step, may be the result of the attack of moderate Brønsted site. The Brønsted acidity should be associated with octahedral coordinated species in fresh samples and it can be also created from the water produced during reaction. The ability of Brønsted acid sites to interact with saturated alkanes has been recognized by other authors [44]. A model for the intermediate showing how the Brønsted acid sites assist in the first step of the C–H activation of saturated alkane and the role of bridging Mo–O–Mo, is tentatively presented in Scheme 1. As mentioned above Mo/␥-Al2 O3 catalysts increase their Brønsted acidity with Mo loading favoring oxydehydrogenation to propene, but at the same time Brønsted acids also favor combustion of olefins which are very reactive towards electrophilic sites. Then, it is expected that selectivity to propene should be penalysed because of COx formation. Thus, our results show that propene selectivity is only slightly increased with the molybdenum content, fitting almost the same selectivity-conversion curve (Fig. 6) for different Mo/␥-Al2 O3 catalysts. The slight increase of propene selectivity could be explained by the decrease of the C2 0 s formation rates due to a diminution of strong acidic sites responsible of cracking reactions. The oxidation state of molybdenum also could play an important role in the catalytic properties, if it takes

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