Oxidative dehydrogenation of propane over calcined vanadate-exchanged Mg,Al-layered double hydroxides

Applied Catalysis A: General 185 (1999) 65±73

Oxidative dehydrogenation of propane over calcined vanadateexchanged Mg,Al-layered double hydroxides K. Bahranowskia, G. Buenob, V. CorteÂs CorberaÂnb, F. Koolid, E.M. Serwickac,*, R.X. Valenzuelab, K. Wcisøoc a

Faculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy, 30-059 KrakoÂw, al. Mickiewicza 30, Poland b Instituto de Catalisis y Petroleoquimica, C.S.I.C., Campus U.A.M. Cantoblanco, 28049 Madrid, Spain c Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, 30-239 KrakoÂw, ul. Niezapominajek 1, Poland d National Institute for Research in Inorganic Materials, Namiki 1-1, Tsukuba, Ibaraki 305, Japan Received 9 November 1998; accepted 14 April 1999

Abstract Mg,Al-layered double hydroxides (LDHs) exchanged with decavanadate and pyrovanadate anions have been prepared and characterized by elemental analysis, FTIR, PXRD, XPS and BET. The mixed oxide materials obtained by calcination at 823 K differ in catalytic performance depending on the type of intercalated vanadate. The activity of the calcined decavanadateexchanged LDH is, depending on temperature, comparable or slightly better than that of the reference V±Mg±O sample, while its areal activity is much higher, pointing to the formation of centres of higher intrinsic activity. The activity of catalyst obtained from pyrovanadate-exchanged LDH is low. Selectivity-conversion pro®les indicate that propene formed in the reaction is subsequently oxidized mainly to CO, the process being favoured over catalyst derived from the decavanadateexchanged LDH. The selectivity pattern is related to the catalysts acid±base properties. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydrotalcites; Layered double hydroxides; Oxidative dehydrogenation; V±Mg±O mixed oxides

1. Introduction The oxidative dehydrogenation of alkanes to alkenes offers an attractive route to convert the low cost saturated hydrocarbons into highly demanded chemicals. The task is by no means straightforward because of the known chemical inertness of alkanes. Activation of alkane C±H bond requires high tem*Corresponding author.

perature, conditions which favour the complete oxidation. The role of the catalyst is to increase the rate of formation of the desired products. In the case of the oxidative dehydrogenation of propane to propene, the V±Mg±O mixed oxide system has proved as one of the most active and selective [1±4]. The catalysts are usually prepared by impregnation of a magnesiumcontaining support, (e.g. MgO or Mg(OH)2) with vanadium precursor solution, followed by calcination at elevated temperature, or by solid state reaction between magnesia and vanadia. Their activities and

0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 1 1 3 - 1

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selectivities depend on the V/Mg ratio and are usually related to the oxide phase composition. It has been shown that the latter can be controlled to a large degree by the preparation method [5]. This prompted us to employ synthetic procedures used for the preparation of layered double hydroxides to obtain the precursors of the potentially active mixed oxide system. Layered double hydroxides (LDHs), known also as hydrotalcites or anionic clays, may be represented by the general formula [M(II)1ÿxM(III)x(OH)2] Anÿ x=n
mH2 O. They consist of brucite layers which, as a result of partial substitution of divalent by trivalent cations, acquire excess positive charge. This excess charge is compensated by the incorporation of anions into the interlayer space. An important feature of these compounds is the relative facility of anion exchange. In the present study we use the exchange properties of a parent magnesium±aluminium layered double hydroxide to load the LDH matrix with vanadium-containing anions. The catalytic properties of calcined vanadium-containing LDHs are tested in the reaction of oxidative dehydrogenation of propane. Their performance is compared with the behaviour of a conventionally prepared V±Mg±O mixed oxide catalyst of optimum activity/selectivity characteristics. The catalytic studies are supplemented by characterization of the samples with chemical analysis, XRD, FTIR, XPS, BET, and decomposition of isopropanol ± a probe reaction of the acido-basic properties. 2. Experimental 2.1. Materials and methods The parent LDH was prepared according to the standard coprecipitation method. 0.12 M of Mg(NO3)2
6H2O and 0.06 M of Al(NO3)3
9H2O were dissolved in 50 ml distilled water and added dropwise to 100 ml of distilled water at pH 10, controlled by dropwise addition of 2 M NaOH. Care was taken to avoid CO2 contamination during synthesis. The suspension was magnetically stirred overnight at 328 K, the precipitate centrifuged, washed several times with hot distilled water and left in the form of suspension used subsequently in the exchange experiments. The product obtained is referred to as MgAlNO3-LDH.

Two vanadate solutions were prepared by dissolving 0.05 M of NaVO3 in 150 ml distilled water, one at pH 4.5 where V10 O6ÿ 28 decavanadate species are supposed to dominate, the other at pH 9.5 which favours in the formation of pyrovanadate anions V2 O4ÿ 7 [6]. Ion exchange was carried out by a dropwise addition of an appropriate vanadate solution to the MgAlNO3-LDH suspension, at 328 K. The decavanadate was added at pH 4.5 controlled by a dropwise addition of 0.5 M HCl, while in the case of pyrovanadate addition the pH 9.5 was adjusted with a dropwise addition of 2 M NaOH. The exchange products are referred to as MgAlV10O28-LDH and MgAlV2O7-LDH, respectively. Prior to catalytic experiments the samples were converted to mixed oxide (MO) phases by 24 h calcination in air at 823 K. The calcined materials are referred to as MgAlV10O28-MO and MgAlV2O7-MO. The V±Mg±O reference catalyst was obtained by impregnation procedure described originally by Chaar et al. [1]. The MgO support was prepared by thermal decomposition in air at 973 K of a MgCO3 precipitated by reaction of magnesium nitrate solution with ammonium carbonate. Subsequently, an appropriate amount of MgO was impregnated with an ammonium metavanadate solution and calcined at 823 K to form a mixed V±Mg oxide catalyst. From a series of V±Mg± O samples prepared in such a way the catalyst containing 24 wt% V2O5, showing best catalytic performance in the oxidative dehydrogenation of propane, has been chosen as a reference. This sample is further referred to as 24VMgO. Samples were characterized with PXRD, FTIR, XPS, BET and chemical analysis. The PXRD patterns were obtained with a DRON-3 diffractometer using Ni-®ltered Cu K radiation. FTIR spectra were recorded using the KBr pellet technique in a Nicolet 800 FTIR instrument. The XPS spectra were obtained with a VG-ESCA 3 photoelectron spectrometer using non-monochromatized Al Ka1;2 radiation and calibrated against the F 1s line position assumed to be 685.5 eV. The BET speci®c surface area of the samples was determined from argon adsorption at 77 K, after outgassing at 473 K for 2 h. Chemical analysis was carried out on an ICP±AES Plasma 40 Perkin-Elmer spectrometer, after dissolution of the samples in nitric acid. Oxidative dehydrogenation of propane was studied in a tubular, down-¯ow quartz reactor operating at

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Table 1 Composition of LDH samples from chemical analysis Sample

Mga

Ala

Va

Xb

V/Mgc

MgAlNO3 MgAlV2O7-LDH MgAlV10O28-LDH

2.02 2.04 1.50

1.00 1.00 1.00

± 0.69 1.60

0.33 0.33 0.40

± 0.34 (0.25) 1.07 (1.12)

a

Atomic ratios with respect to Al. Assuming the LDH formula [Mg1ÿxAlx(OH)2]Anÿ x=n . c Theoretical values in parentheses. b

atmospheric pressure. Catalyst particles (0.25±0.42 mm) were diluted with SiC chips up to a total bed volume of 1 cm3. A C3H8/O2/He mixture of 4:8:88 molar ratio was fed into the reactor at a total ¯ow of 0.23 mol/h and residence time 12.3 g
h/mol C3. SiC chips were placed above the catalytic bed to suppress empty volume. To suppress the post-catalytic empty volume the reactor tube was narrowed just below the catalytic bed. Under these conditions the homogeneous conversion of propane was negligible, being equal to 0.2 mol% at 773 K. Reactants and products were analyzed on-line with a Varian 3400 CX GC equipped with a TCD detector. Decomposition of isopropanol to propene and acetone was studied at 473 K with the pulse method, using dry helium as a carrier gas. The amount of the catalyst was 0.1 g (grain size 0.2±0.25 mm). Two ml of isopropanol were injected in a stream of dried helium at the ¯ow rate 30 ml/min. The isopropanol pulses were injected successively till constant values of conversion and yield of products were attained (usually after three pulses). The rates of products formation reported further in the text are the mean values obtained in three successive pulses after the stationary state of the activity was attained. The products were analyzed with gas chromatography with FID detection. 3. Results and discussion 3.1. Characterization of the catalysts The vanadate ions used for exchange experiment show complex equilibria in solution, depending on both the pH and the concentration of vanadium [6]. At pH 4.5 and the concentration used in the present experiment, the initial metavanadate species are sup-

posed to polymerize to produce predominantly decavanadate anions V10 O6ÿ 28 . In basic solution, at pH 9.5, pyrovanadate species V2 O4ÿ 7 are expected to form. Both anions may appear as partly protonated species. The composition of the samples, expressed as atomic ratios of metal elements, is given in Table 1. One can see that in the parent LDH material the Mg/Al ratio is close to the theoretical value. It is preserved in the MgAlV2O7-LDH sample exchanged in the basic conditions. Upon exchange in the acidic medium, used in the synthesis of MgAlV10O28-LDH, the LDH matrix becomes depleted of magnesium relative to the Al content. The V/Mg ratio depends, as expected, on the preparation conditions. However, while in the MgAlV10O28-LDH sample it is close to the value expected for the theoretical LDH formula with xˆ0.40 and V10 O6ÿ 28 as compensating anions, in the case of the MgAlV2O7-LDH sample there is more vanadium incorporated into the solid than expected for xˆ0.33 and V2 O4ÿ 7 species. As mentioned above, the equilibria between different vanadate ions include the formation of partly protonated anions. In the case of pyrovanadate species a HV2 O3ÿ 7 ion is a likely possibility. For this species as a compensating interlayer anion the expected V/Mg ratio is 0.33, i.e. very close to the experimentally observed value. However, also partial incorporation of metavanadate ions, VOÿ 3 , for which the expected V/Mg ratio is 0.50, could lead to the observed enhanced vanadium content. Thus, it cannot be excluded that a spectrum of vanadate ions exist in the interlayer of the MgAlV2O7-LDH sample. FTIR spectra of the parent MgAlNO3-LDH sample and the vanadate-exchanged LDHs are presented in Fig. 1. The major changes occurring upon incorporation of the vanadate anions consist in disappearance of bands characteristic of nitrate anions and in appearance of new bands in the 500±1000 cmÿ1 range.

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K. Bahranowski et al. / Applied Catalysis A: General 185 (1999) 65±73

Fig. 1. FTIR spectra of LDH samples: (a) MgAlNO3-LDH, (b) MgAlV2O7-LDH, (c) MgAlV10O28-LDH.

However, although the nitrate band at 1381 cmÿ1 is gone in both vanadate-exchanged samples, in the case of MgAlV2O7-LDH a small amount of carbonate anion impurity, absorbing in a similar range, appears at 1360 cmÿ1. Obviously, despite precautions, CO2 from the atmosphere must have contaminated the sample prepared at basic pH. The new bands in the 500±1000 cmÿ1 region indicate the presence of vanadate species. The band at 910 cmÿ1, visible in the spectrum of MgAlV2O7-LDH, is close to the (VO2)(asy) vibration typical of both the HV2 O3ÿ 7 ÿ1 and the …VO3 †nÿ n anions, while the band at 630 cm may be due to the (VOV)(asy) in …VO3 †nÿ n chains [7]. Thus, on the basis of the IR spectra, it is not possible to

Fig. 2. XRD patterns of LDH samples: (a) MgAlNO3-LDH, (b) MgAlV2O7-LDH, (c) MgAlV10O28-LDH.

decide unequivocally which of the two vanadium anionic forms have been incorporated. The spectrum of MgAlV10O28-LDH coincides with that reported by Lopez Salinas and Ono [8] and is characteristic of decavanadate species, the band at 964 cmÿ1 being due to the V=O terminal stretching mode. The powder XRD patterns of the parent LDH and the vanadate-exchanged forms are presented in Fig. 2. Indexation of the diffraction lines for a hydrotalcitelike material can be done assuming a hexagonal cell with three brucite layers per cell (polytype 3R) [9,10]. The MgAlNO3-LDH gives strong peaks at 8.77 and Ê , characteristic of (0 0 3) and (0 0 6) re¯ections 4.40 A of nitrate-containing LDH structures [11] (Fig. 2(a)).

K. Bahranowski et al. / Applied Catalysis A: General 185 (1999) 65±73

The XRD pattern of the MgAlV2O7-LDH sample Ê , which contains broad peaks at 9.17 and 4.60 A may be indexed as the (0 0 3) and (0 0 6) re¯ections of the LDH structure. The estimated gallery height is Ê . This is slightly less than the transverse then 4.4 A dimension of the pyrovanadate anion, estimated by Ê . The basal spacings Twu and Dutta [12] to be ca. 5 A reported for other pyrovanadate-containing LDH Ê [12,13], but can structures are around 10.5±10.8 A be lower if a grafting of pyrovanadate anions onto the brucite sheets occurs [14]. On the other hand, Delmas and coworkers [15,16] reported the interlayer distance Ê for an LDH material intercalated with of 9.15 A polymeric metavanadate species. In view of this, the broad maxima present in the XRD pattern of the MgAlV2O7-LDH sample may be interpreted as containing contributions from both the pyrovanadate- and metavanadate-intercalated areas of LDH structure. This result is compatible with the chemical analysis data showing that the composition of this sample cannot be interpreted strictly in terms of pyrovanadate ions, and with the IR spectra consistent with the presence of both types of vanadate anions. The XRD pattern of MgAlV10O28-LDH consists of a broad Ê , followed by sharp re¯ections re¯ection at ca. 10.0 A Ê at 5.83 and 3.91 A as well as several other less intense peaks. This type of pattern is frequently encountered in LDHs intercalated with polyoxoanions in acidic medium [17±19]. The origin of the broad re¯ection has been frequently discussed over the years. It has been assigned either to a polyoxometalate salt impurity formed by reaction of the extracted magnesium (and some aluminium) with the polyoxometalate anions used in the intercalation experiment [17] or to a defect LDH structure resulting from acid damage [18]. This peak obscures the (0 0 3) re¯ection of the decavanadate-exchanged magnesium±aluminium LDHs, so that the basal spacing has to be evaluated from the sharp (0 0 6) and (0 0 9) higher order re¯ecÊ tions. In the present work the d003 is equal 11.70 A Ê , gives the which, assuming the layer thickness of 4.8 A Ê , in agreement with the pregallery height of 6.9 A viously published data [8,17±20]. Calcination of vanadate-exchanged LDHs at 823 K produces mixed oxide materials whose XRD patterns are presented in Fig. 3, together with the XRD pattern of 24VMgO reference sample. The LDH-derived sample of lower V/Mg ratio, i.e. MgAlV2O7-MO

69

Fig. 3. XRD patterns of (a) 24VMgO, (b) MgAlV2O7-MO, (c) MgAlV10O28-MO.

(V/Mgˆ0.34), gives similar pattern as the 24VMgO catalyst (V/Mgˆ0.15) showing only MgO diffraction lines (Fig. 3(a) and (b)). The most intense peaks at Ê observed for 24VMgO are due, 2.11 and 1.49 A respectively, to the (2 0 0) and (2 2 0) re¯ections of MgO lattice. In the mixed oxide material obtained by decomposition of LDH precursor the lines are slightly shifted towards higher angles and are observed at 2.08 Ê , indicating that in the presence of alumiand 1.46 A nium a defect magnesia structure, usually described as magnesia±alumina solid solution [19], is formed. The lack of de®nite diffraction lines originating from crystalline vanadates shows that in both samples the vanadium-containing phase must be quasi-amor-

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Table 2 Surface composition (atomic ratios) of investigated catalysts from XPS Sample

V/Mga

V/O

MgAlV2O7-MO MgAlV10O28-MO 24VMgO

0.179 (0.34) 0.855 (1.07) 0.111 (1.15)

0.090 0.177 0.092

a

Bulk values in parentheses.

phous, possibly responsible for the broad envelope centred around 2ˆ358. The XRD pattern of MgAlV10O28-MO sample (V/Mgˆ1.07) is presented Ê , assignable in Fig. 3(c). The peaks at 2.08 and 1.46 A to defect MgO, are more intense and narrower than in the case of MgAlV2O7-MO sample, pointing to better crystallinity of this material. Beside the lines characteristic of Al-doped magnesia, new peaks emerge in the area where the most intense re¯ections of aMg2V2O7 phase are expected. Indeed, when this sample is calcined at temperature by 100 K higher, the XRD pattern (not shown) exhibits evident diffraction lines characteristic of magnesium pyrovanadate. Table 2 shows the results of the XPS analysis. Surface enrichment in magnesium is observed in all samples. The values of V/O show that the density of surface V centres measured with respect to the surface oxygens is ca. two times higher in MgAlV10O28-MO than in the MgAlV2O7-MO and 24VMgO samples. The BET speci®c surface areas of the investigated samples are 9, 46 and 41 m2/g for MgAlV10O28-MO, MgAlV2O7-MO and 24VMgO, respectively. The lower surface area found for the mixed oxide sample derived from decavanadate-exchanged LDH is consistent with the XRD analysis pointing to a better crystallinity of this material. 4. Catalysis The changes of the total conversion of propane with the reaction temperature for the MgAlV10O28-MO, MgAlV2O7-MO and 24VMgO samples are presented in Fig. 4(a). The plot for MgAlV10O28-MO is very close to that of the reference 24VMgO catalyst, the activity of LDH-derived sample being slightly better in the temperature range 773±823 K. MgAlV2O7-MO is less active than the two other samples. However,

Fig. 4. Catalytic activity of investigated samples: (a) conversion vs. temperature, (b) areal activity vs. temperature.

when the surface area of the catalysts is taken into account, the calculations show that the activity of the MgAlV10O28-MO sample, expressed as the areal rate (per surface unit), is much higher than in the reference 24VMgO catalyst and the other LDH-derived material (Fig. 4(b)). When comparing the areal activities, two contributing factors should be considered, i.e. the density of active sites and the intrinsic reactivity of active sites. The XPS determined density of surface V centres in the MgAlV10O28-MO catalyst is ca. twice that of the 24VMgO sample. The observed difference in the areal activities of the two materials is, however, much higher, the rate of reaction being ca. ®ve times greater on the LDH-derived catalyst (Fig. 4(b)). This means that the higher surface density of V centres in MgAlV10O28-MO may account only in part for the enhanced areal rate. The total increase of the activity per surface unit can be only explained in terms of higher intrinsic reactivity of V centres created at the surface of this catalyst. On the other hand, the MgAlV2O7-MO sample, whose surface concentration

K. Bahranowski et al. / Applied Catalysis A: General 185 (1999) 65±73

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of V centres is similar as in the reference catalyst, has lower areal activity. This suggests that here vanadium sites of lower intrinsic reactivity have been created. The major reactions products are propene, CO2 and CO. The overall process may be described with a simpli®ed reaction scheme, shown below, typical of selective oxidation reactions of hydrocarbons on oxide catalysts. It involves a parallel-consecutive mechanism in which the products of complete oxidation are obtained either directly from propane or in a consecutive reaction of propene combustion, and was observed previously on other V-containing catalysts [2,21±24]:

Fig. 5(a)±(c) presents the selectivity/conversion pro®les for propene, CO2 and CO. It should be noted that the changes in conversion were induced by changes in the reaction temperature (see Fig. 4(a)). A similarity of activation energies for the propene formation and its consecutive oxidation to carbon oxides has been reported for several V-based systems, including V±Mg±O [25], V±Mg±Sb±O [26] and V± Ti±O [27] oxide systems which justi®es comparison of the data gathered at different temperatures. For MgAlV10O28-MO and 24VMgO, which qualitatively show similar behaviour, the selectivity to propene decreases with increasing conversion (Fig. 5(a)). The selectivity to CO2 varies only little (Fig. 5(b)). In the case of the MgAlV10O28-MO catalyst the initial minor decrease is followed, at higher conversions, by a slight increase. On the reference sample the selectivity to CO2 shows a mildly increasing trend. The selectivity to CO (Fig. 5(c)) increases with increasing conversion for both samples. The observed tendencies in selectivity-conversion patterns imply that, as observed previously on other V-containing systems, the total oxidation processes gain on importance at higher conversions (higher temperatures). They also indicate that consecutive oxidation of propene results mainly in the formation of CO [24,26]. The selectivities to CO are particularly high for the MgAlV10O28-MO catalyst which suggests that on this catalyst the consecutive oxidation of propene becomes more signi®cant. A possible explanation for

Fig. 5. Selectivity-conversion profiles: (a) selectivity to propene, (b) selectivity to CO2, (c) selectivity to CO.

this lies in the acid±base properties of the catalysts. To check on this we have tested the behaviour of the samples in the decomposition of isopropanol, a probe reaction for the acid±base properties of oxide catalysts [28±30]. The alcohol may decompose through two parallel routes: dehydration to propene on acid type centres and dehydrogenation to acetone on basic redox centres. The rate of the propene formation may be regarded as a measure of the catalyst acidity, whereas the acetone/propene ratio as a measure of the catalyst basicity. Table 3 shows the results of the isopropanol

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K. Bahranowski et al. / Applied Catalysis A: General 185 (1999) 65±73

Table 3 Results of the isopropanol decomposition at 473 K Sample

Propene (10ÿ6 Acetone (10ÿ6 Acetone/ mole/m2/s) mole/m2/s) propene

The more detailed discussion of various vanadium centres present in the vanadium-containing LDHderived mixed oxide catalysts is the subject of a forthcoming study.

MgAlV2O7-MO MgAlV10O28-MO 24VMgO

0.02 0.20 0.04

5. Conclusions

0.17 0.56 0.36

8.5 2.8 9.0

decomposition over the investigated catalysts. The rate of propene formation is distinctly higher on the MgAlV10O28-MO sample than on any of the other two catalysts. At the same time the value of the acetone/ propene ratio observed for this material is the lowest. Thus, this sample is the most acidic and the least basic of all investigated samples. Propene molecule exhibits basic properties and should be more strongly held on the more acidic and less basic surface. The longer retention time provides conditions for subsequent oxidation of propene to carbon oxides which is manifested as enhanced selectivity to CO over MgAlV10O28-MO. The increase in the selectivity to COx with the increase in the catalyst acidity and decrease in basicity has been previously observed in ODH of propane on other vanadium-based catalysts [21,27]. This observation suggests that the preparation of decavanadate-exchanged LDHs with parent Mg,AlLDH of higher Mg/Al ratio than that used in this study might lead to further improvement of the catalytic performance of LDH-derived material. In the case of MgAlV2O7-MO sample the initial selectivity/conversion patterns for propene and carbon dioxide show different trends than both the reference catalyst and the other LDH-derived sample. Here the selectivity to propene unexpectedly increases with increasing conversion (temperature), while the selectivity to CO2 strongly decreases. The selectivity to CO shows a mildly increasing trend. However, after repeating the catalytic cycles several times, the performance of this sample, in terms of selectivity, approaches that of other catalysts (Fig. 5(a)±(c), full symbols). This shows that, in contrast to the other catalysts, the surface of MgAlV2O7-MO requires considerable time to reach the steady state condition. It is worthwhile to note that it is only the selectivity pattern that changes upon conditioning in the reaction mixture, the activity of the MgAlV2O7-MO catalyst remaining at the same, rather low level.

The use of vanadate-exchanged magnesium±aluminium layered double hydroxides as precursors of mixed-oxide catalysts provides materials differing in catalytic performance depending on the nature of intercalated vanadate. The most active in the oxidative dehydrogenation of propene is the sample derived from the decavanadate-exchanged LDH. The overall activity of the MgAlV10O28-MO catalyst is, depending on temperature, comparable or slightly better than the reference 24VMgO sample prepared in a conventional way. The much higher intrinsic activity of this catalyst with respect to the 24VMgO sample is assigned both to the higher density of the surface vanadium sites and to their higher speci®c activity. The MgAlV2O7-MO catalyst is the least active of all samples studied. Analysis of the selectivity-conversion pro®les indicates that propene formed in the oxidative dehydrogenation reaction is subsequently oxidized mainly to CO, the process being favoured over MgAlV10O28-MO catalyst. The latter effect is related to the highest acidity and the lowest basicity of this catalyst, making the desorption of the propene molecules more dif®cult. The performance of the MgAlV2O7-MO sample changes upon conditioning in the reaction mixture to approach those of other samples, indicating that surface transformation is required for this sample to reach a steady state performance. Acknowledgements This work was partially ®nanced by the State Committee for Scienti®c Research, KBN, Warsaw, within the research project 6 P04D 040 14. The authors are very grateful to Prof. B. GrzybowskaSÂwierkosz for the critical reading of the manuscript and stimulating discussions. Thanks are also due to Dr. J. Stoch for providing the XPS analysis and to Mrs. I. Gressel for carrying out the isopropanol tests.

K. Bahranowski et al. / Applied Catalysis A: General 185 (1999) 65±73

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