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Latin American applied research

Print version ISSN 0327-0793

Lat. Am. appl. res. vol.34 no.2 Bahía Blanca Apr./June 2004

 

Oxidative dehydrogenation of propane and n-butane over aluminia supported vanadium catalysts

V. Murgia1,2, E. Sham1,2, J. C. Gottifredi1,2 and E. M. Farfan Torres1,3

1INIQUI - 2Facultad de Ingeniería – 3Facultad de Ciencias Exactas, Universidad Nacional de Salta, Buenos Aires 177, 4400-Salta, Argentina.
sham@unsa.edu.ar

Abstract — Structural properties of vanadium dispersed species on g-A1203 are investigated with the scope to detect changes related with V loading in the oxidative dehydrogenation (ODH) of propane and nbutane. XPS, FTIR, and FTIR of absorbed pyridine were used to study the nature of vanadium supported species.
Tetrahedral V5+ and probably V4+ species were detected. For vanadium loadings higher than 4.3 % wt octahedral species were also observed.
In the n-butane ODH reaction, the selectivity to ODH products decreases when vanadium content increases. However, for propane ODH, the selectivity seems to be independent of vanadium loadings. Low oxygen/alkane feeding ratios favor selectivity to olefins. It is also shown that low V loading catalysts reach selectivities as good as best reported V-Mg-O catalyst.

Keywords — Oxidative Dehydrogenation; Vanadium Catalysts; propane; n-butane.

I. INTRODUCTION

Vanadium oxide dispersed on metal oxide supports exhibits interesting catalytic properties for partial oxidation of alkanes, aromatics and olefins (Bielanski and Haber, 1979; Dadyburjor at al., 1979; Mamedov and Cortés Corberán, 1995; Blasco et al., 1995). The activity and selectivity of vanadia supported catalysts are deeply modified by the structure and physicochemical properties of the dispersed species supported on the surface (Dadyburjor et al., 1979; Mamedov and Cortés Corberán, 1995; Yoshida et al., 1988; Eon et al., 1992; Koranne et al., 1994).
   MgO supported vanadium was reported as a very selective catalyst in the oxidative dehydrogenation of propane and n-butane (Kung and Chaar, 1988), while g-A12O3 supported vanadium catalyst was found to present a good selectivity to olefin products for ethane ODH but a poor selectivity in the ODH of n-butane (Concepcion et al., 1994).
   The acid-base character of the support explained this different behavior. On MgO, a support with basic properties, the interaction between vanadium species and the supports leads to the formation of vanadate compounds. In the case of more acid supports, such as SiO2 or A12O3 a weak interaction is expected leading to less dispersed vanadium species on the surface which, in turns favors the formation of V2O5 crystallites (Blasco at al., 1995; Galli at al., 1995).
   Alumina-supported vanadia has been the object of several structural investigations (Haber et al., 1986; Nag et al., 1988; Eckert and Wachs, 1989; Andersen and Kung, 1992; Michalakos et al., 1993). These studies show, in general, that at low loading vanadium form highly dispersed amorphous phases, whose structure changes from isolated tetrahedral vanadium, to polyvanadates species at medium loading. Crystalline V2O5 also appears at high loading in addition to amorphous vanadia phases.
The binding strength of oxygen lattice has been postulated as the parameter that governs activities and selectivities of these catalysts (Yoshida et al., 1988). In this way VO4 tetrahedra formed at low vanadium contents are related with high selectivities to alkenes because all the oxygen ions in these species are bridged between V and the metal ions of the support oxide (Michalakos et al., 1993; Corma et al., 1993a).
Other authors considered that -V2O7- units favor alkane ODH and suggested that the bridging oxygen ion between two vanadium ions (V-O-V) plays an important role in the propane ODH. The removal of' this oxygen results in a local structural change from the -V2O7- unit to two edge-sharing square-based -VO3-- units where vanadium is present as V4+ cations (Eon et al., 1994). These stabilized V4+ ions were postulated as responsible for the dehydrogenation selectivity of pyrovanadate structure.
As V4+ ions are also present in selective ODH catalysts like Mg pyrovanadate and Mg orthovanadate (Kung and Kung, 1992; Gao et al., 1994), Mamedov and Cortés Corberán (1995) suggest that a certain extent of reduction is needed for the selective dehydrogenation of alkanes. But they also emphasize that a good selectivity can be achieved by controlling reaction conditions (temperature and alkane to oxygen ratio). The aim of this paper is to further study the nature of active sites and the influence of the acid-base character of the vanadium supported catalysts on the selectivity for the ODH of propane and n-butane. For this purpose V/g-A12O3 catalysts have been prepared with different degrees of coverage using incipient wet impregnation
techniques.

II. EXPERIMENTAL

A. Catalyst Preparation
Alumina support was Aldrich, surface area 132 m2g-1. Catalysts were prepared by impregnation of the support with an aqueous solution of ammonium metavanadate (from Fluka p.a.). The impregnated samples were dried at 110 ºC over night, and then calcined at 550 ºC in air 16 hours. Catalysts having loadings of 1.4, 2.7, 4.3 and 5.1 wt % of V were prepared with alumina by successive impregnation cycles. They will be denoted as Vx/A12O3, being x the vanadium wt % content.

B. Catalyst Characterization
Infrared spectra were recorded between 1200 and 800 cm-1 using a Bruker IFS 88B on samples dispersed in KBr.
   The catalysts acidity was also studied using pyridine as probe molecule. FTIR spectra of adsorbed pyridine were obtained from wafers of self supported samples mounted in a pyrex vacuum cell with KRS-5 windows.
   The samples were degassed out for 1,5 h at 450ºC and then cooled at room temperature (RT) to obtain the reference spectra. Then, pyridine was adsorbed at room temperature and desorbed at different temperatures from RT to 250ºC for 30 minutes in each case. The spectra were taken at room temperature and the reference spectra substracted in all cases.
    The XPS spectra were taken in a Physical Electronics 5700 ESCA spectrometer. The exciting radiation was Mg Ka (1253.6 eV). The binding energies (BE) were calculated with respect to Cls peak set at 284.5 eV.

C. Catalytic Test
The catalytic studies were performed in a fixed bed tubular reactor. The samples were diluted with quartz. The catalytic volume was 1.2 cm3. Reactants and reaction products were analyzed by online chromatography using a Varian 3700 and a Shimadzu GC-3BT chromatographs for heavy and light products respectively.
    For n-butane test, the reactor was fed with a mixture 20/15/65 of n-butane, oxygen, and nitrogen respectively at a constant flow rate of 150 mL min-1.
The weight of catalytic samples (155-255 mm) was varied to achieve different contact times. Experiments were carried out at constant temperature (703 and 773K). No noticeable deactivation was observed over a period of 24 hours of reaction at the maximum temperature. Blank tests on the reactor showed no activity in the range of temperatures studied.
    For propane reaction, two different mixtures of propane, oxygen and nitrogen 20/15/65 and 4/8/88 were fed. The flow rate of the reactants was varied (150-250 mL/min) to achieve different contact times.

III. RESULTS

A. Catalyst Characterization
Surface area, vanadium weight content and the resultant theoretical coverage for the catalysts are given in Table 1.

Table 1. Characteristic of supported vanadium catalysts

XPS spectra show that V2p3/2 signals are broadened in the case of supported samples compared to the corresponding signal of bulk vanadium pentoxide. This broadening is more noticeable for low vanadium loadings samples (Vl.4/A12O3, V2.7/A12O3) and can be correlated with the formation of highly dispersed vanadium species over the carrier.
    The assignment of vanadium oxidation state was made by assuming that lines with high BE values (516.9-517.2 eV) could be related to the presence of V5+ and those with low BE (515.6-5l6.3 eV) to the presence of some reduced species, probably V4+ (Gao et al. (1994) and Matralis et al., 1995).
As can be seen in Table 1 samples with very low vanadium coverage (V1.4/A12O3, V2.7/A12O3 and V4.3/A12O3) present V2p3/2 lines more related to values of V4+ oxidation state. However, V2p3/2 bands are asymmetric suggesting that both V4+ and V5+ species are present. When the vanadium coverage increases (V5.1/A12O3) the measured BE increases to 516.6 eV indicating that V5+ species are predominant.
   The FTIR spectra of the Vx/A12O3 catalysts along with that of unsupported V2O5 are presented in Fig.1. Bulk V2O5 presents two peaks, one at 1018 cm-l which has been assigned to the stretching of the short double bond (V=O)3+ (Matralis et al., 1995), and a second absorption at 835 cm-1 corresponding to V-O vibrational modes (Gao et al., 1994). The IR spectrum of V1.4/A12O3 catalyst shows essentially a broad absorption band at 991 cm-l which has been assigned to two-dimensional vanadium oxide amorphous phases (Matralis et al., 1995; Miyata et al., 1997). This band has also been correlated with the vibration of isolated V=O bonds of the type VO2+ (Corma et al., 1993b). When vanadium loading increases broad bands with maxima centered at 1010-1016 cm-1 are observed showing the beginning of V2O5 phase formation. The shifting to lower frequencies indicates a weakening of the (V=O)3+ bonding coincident with an amorphous oxide phase or the presence of neighboring V4+ ions. In Figure 2 the infrared spectra of pyridine adsorbed on V2.7/A12O3 (A), V4.3/A12O3 (B) and V5.1/A12O3 (C) catalysts after evacuation at temperatures of 150ºC (a)
and 250ºC (b) are presented.
   Bands related to physisorbed and coordinately bonded pyridine are observed for all catalysts. After evacuation at 250ºC, bands at 1452, 1493, 1577 and 1624 cm-1, characteristic of pyridine retained on Lewis acid sites, are only observed.
   The peak at 1539 cm-1 associated to pyridine adsorbed on Brönsted acid sites is observed only for V5.1/A12O3. This band disappeared after evacuation at 250 ºC.


Fig. 1- FTIR spectra of: (A) V2O5, (B) V1.4/Al2O3, (C) V2.7/Al2O3, (D) V4.3/Al2O3 and (E) V5.1/Al2O3.


Fig. 2- FTIR spectra of pyridine adsorbed on: (A) V2.7/Al2O3, (B) V4.3/Al2O3 and (C) V5.1/Al2O3 samples obtained at evacuation temperatures of 150ºC (a) and 250ºC (b).

B. Catalytic Study
The catalytic testing results for the oxidative dehydrogenation of n-butane are presented in Table 2. In general C4-olefins (l-butene, cis-2-butene, trans-2- butene and butadiene), CO and CO2 are the main reaction products, while cracking products are minor or absent. The carbon balance is satisfactory in all the cases with differences lower than 1%.

Table 2. Oxidative dehydrogenation of n-butane on supported vanadium catalysts

    It can be seen that, in all cases, selectivity decreases when conversion increases. However, significantly, the higher selectivities are observed with low vanadium content catalysts. The Rv and Rv x Selectivity ODH at 500ºC is higher for lower vanadia loadings.
Also selectivity slightly increases when the temperature increases from 430ºC to 500ºC. The effect of O2/C4 ratio was investigated for V1.4/Al2O3 at different temperatures. As O2/C4 ratio varies from 0.8 to 1.5, at a given temperature, overall n-butane conversion is increased, while ODH selectivity decreases. However, as shown in Table 3, C4 olefins yield is almost unaffected.

Table 3. V1.4/Al2O3 changes of the selectivity and yield to C4 olefins with O2/C4 ratios

   To compare the catalytic behavior of V1.4 and V5.1/Al2O3 solid olefin selectivity, results at 500ºC and 430ºC are presented in Fig. 3 and 4, respectively. For both catalysts it was observed that the selectivity to lbutene and 2-butenes decreases with the conversion, while butadiene selectivity slightly increases. For low vanadium content, the predominant isomer was 1- butene, and the ratio cis-2/trans-2-butene was near 1. For V5.1/Al2O3 catalysts the molar ratio 1-butene/cis-2- butene/trans-2-butene was closer to the equilibrium predictions (1/1/1.1).


Fig. 3- Selectivity to olefins versus conversion of n-butane at 500ºC for V1.4/Al2O3: n 1-butene; s 2-tbutene; m 2-c-butene; 5 butadiene.


Fig. 4- Selectivity to olefins versus conversion of n-butane at 430ºC for V5.1/Al2O3: n 1-butene; s 2-tbutene; m 2-c-butene; 5 butadiene.

   From Table 2 it can be seen that combustion products (CO and CO2) selectivity increases up about 18% conversion level. For V2.7, V4.3 and V5.1/Al2O3 catalysts cracking products, less than 1%, are observed for conversions higher than 20%. Also for V5.1/Al2O3 and conversion higher than 18 %, the selectivity to ODH increases with conversion. For this solid the CO and CO2 selectivities initially increase and then decrease when conversions are higher than 18%.
Figures 5 and 6 show the changes in selectivity to olefins with C3H6 conversion for V2.7 and V5.1/Al2O3 catalysts at 430 and 470ºC, with two different O2/C3H8 feed ratios. It was observed that the selectivity decreases when the conversion increases. However, for propane ODH the selectivity seems to be independent of both, temperature and vanadium loadings, in the studied range which is in agreement with previously reported results (Mamedov and Cortés Corberán, 1995; Eon et al., 1992).


Fig. 5- Selectivities to olefins versus propane conversion for 2/1 O 2 /C3 ratio: V2.7/Al2O3 and V5.1/Al2O3 at 430°C; DV2.7/Al2O3 and V5.1/Al2O3 at 470°C.


Fig. 6. Selectivities to olefins versus propane conversion for 0.8/1 O2/C3 ratio: V2.7/Al2O3 and V5.1/Al2O3 at 430°C; DV2.7/Al2O3 and V5.1/Al2O3 at 470°C.

   In Table 4, it is shown that the specifc rate per vanadium mol,Rv, and Rv x Selectivity ODH at 500ºC reach a maximum for V2.7/Al2O3 catalysts. In this table, the effect of the O2/C3H8 ratio at constant temperature is also observed. In general, it can be seen, that increasing this ratio from 0.75 to 2, the propane conversion increases and the selectivity decreases. This decrease could be only related with the conversion and not with the feed ratio. As can be seen in Table 5 when the conversion level is constant, the selectivity and yield show to be independent of this ratio, except for V1.4/Al2O3. For this catalyst the higher selectivity is achieved at the higher conversions levels when O2/C3H8 ratio is low. On the other side, for this catalyst at low conversion levels the selectivity to ODH products is favored for high O2/C3H8 ratios.

Table 4. Oxidative dehydrogenation of propane on supported vanadium catalysts

Table 5. Variation of the selectivity and yield to C3 olefins from propane with O2/C3 ratio

IV. DlSCUSSION

A. Structure of V/Al2O3 catalysts
Our XPS and IR studies confirm the presence of V5+ and probably V4+ supported species on the surface of catalysts.
   An interesting fact is that catalysts with low vanadium coverage (V1.4/Al2O3 and V2.7/Al2O3) present a more pronounced shift in the BE for the V2p3/2 line confirming the presence of both V4+ and V5+ surface species. It is known from literature (mostly from studies using XPS, ESR, and chemical titration) that V4+ species are stabilized, when vanadium is in strong interaction with the carrier for V2O5/TiO2 and V2O5 / Al2O3 systems (Matralis et al., 1995 and references there in).
From IR and XPS results we can conclude that formation of this partially reduced vanadium species, in low loading catalysts, could be correlated with the formation of isolated VOx species where the vanadium is in strong interaction with the support surface. Under these conditions there are very few possibilities of side interactions that could favor the formation of polyvanadates or V2O5 crystallites. In the opposite side the formation of polyvanadates like species is favored when vanadium loading increases leading to predominance of V5+ species in tetrahedral or octahedral environment.
   IR results of adsorbed pyridine indicate that only Lewis acid sites are present in the low vanadium content catalysts, being mainly related to g-Al2O3. At higher vanadium loadings, Lewis and Brönsted acid sites are both present. Brönsted sites are related with octahedral V5+ species, for vanadium contents higher than 4 wt %. Thus, a lower selectivity for butane ODH could be correlated with a higher acid character of the vanadium surface species present in the catalyst.

B. Catalytic Behavior
n-Butane ODH
Since ODH reactions involve redox mechanisms, the catalytic behavior could be associated with the redox behavior of the vanadium species dispersed on the catalyst surface. According with Mamedov and Cortés Corberán's conclusions (1995), a certain extent of V reduced ions species is needed to achieve good activities and selectivities to olefin products. These authors also stated that using low oxygen-to-alkane ratios an optimum extent of reduction could be achieved. For this reason, in the case of n-butane ODH reaction, the reactor was fed with a reaction mixture of this characteristic (15% oxygen, 20% n-butane and 65% nitrogen) instead of those classically used in literature. In Table 2, if we compare ODH selectivity for similar conversion values, it can be observed that selectivity to olefin products increases when the V content decreases. Characterization results show that low vanadium content catalyst (V1.4/Al2O3) presents isolated VOx species with an important fraction of V4+. While the higher vanadium content catalyst (V5.1 /Al2O3) presents polyvanadates with a predominant V5+ oxidation state. For this catalyst, when conversion levels are near 15-18 % and oxygen conversion is higher than 99% the selectivity to ODH is increased. This improvement could be correlated with changes in both the surface structure of vanadate species and in their V4+ /V5+ ratio (Eon et al., 1992 and Mamedov and Cortés Corberán, 1995) due to the reducing character of the reaction atmosphere. In this way V5.1 /Al2O3 catalyst improves its ODH selectivity.

C. Low Vanadium Content Catalysts
If we consider the results of low content catalysts (V1.4/Al2O3 and V2.7/Al2O3) it is observed that the selectivity to 1-butene and 2-butenes decreases with the conversion while the selectivity to butadiene increases. This behavior suggest that l-butene is a primary product as was early reported. The same conclusion can be derived for 2-butenes. At low conversion the formation of these primary products is predominant.
   On the other hand, butadiene can be considered a secondary product formed from butenes since selectivity to butadiene slightly increases with conversion.
Selectivity to CO and CO2 increases with the conversion for these catalysts showing the same trend of' butadiene. This result could indicate that they are also secondary products. As CO production increases with the conversion and begins to be detected at the same time that olefins, its formation could be related with the combustion of olefins strongly adsorbed on the catalyst surface.
   The high selectivity to 1-butene for low vanadium content could also be related to the high rate of the second hydrogen abstraction, producing a 1-butene/cis- 2-butene/trans-2-butene ratio of 1.4/1/1, which responds to a probabilistic distribution.

D. High Vanadium Contents Catalyst
For high vanadium loadings the selectivity to 1-butene and 2-butenes decreases with the conversion up to near 15-18%. For higher conversion levels, the oxygen conversion is higher than 99%, the selectivity to butenes (1 and 2-butenes) increases and the formation of CO and CO2 of decreases. In this case a kinetic effect could be related with the observed results. In fact as conversion increases oxygen availability becomes the limiting factor. Under these conditions the olefins formation is favored.
   Experimentally it was confirmed that no reaction takes place in complete absence of oxygen, showing that both reduced and oxidized species are necessary to sustain ODH reaction.
   Assuming that the rate of the second hydrogen abstraction is the selectivity determining step, the result suggests that this is slower for acid catalyst, that is to say those with higher vanadium content. In this case the product distribution follows the equilibrium ratio (1butene / cis-2-butene / trans-2-butene, 1 /1 /1.1). Another possible selectivity determining step is the olefinic intermediates desorption, determined by the interaction of the alkenes with the catalyst surface. This interaction will depend on the acid-base character of both, the products and the catalyst. As olefins interact more strongly with acid catalysts than with basic ones, the desorption rate will decreased, favoring high selectivities to 2-butene and C 4 olefins distribution must be close to the equilibrium ratio.

E. Propane ODH
Selectivity results to propene at different temperatures show that all points fit the same selectivity-conversion curve. That could mean that at constant conversion, the selectivity does not depend on the temperature and on vanadium concentration, in the range of coverages studied. This behavior could be related with the fact that propane is more acidic than n-butane and in consequence the interaction between the active sites on the catalysts and the reactant molecule is lower. Also in this case there are no possibilities of equilibrium reactions, because the only two possible ODH products are the olefin or carbon oxides.
   An interesting fact is that although the same selectivity is obtained at a constant conversion level with samples having different vanadia loadings, the specific activity per vanadium mol and Rv x Selectivity ODH values reach a maximum for V2.7/Al2O3.

V. CONCLUSIONS

Our studies allow concluding that both reduced and oxides species are necessary to obtain good selectivities to olefins in the case of the n-butane ODH. In this way low loading vanadium catalysts present the best selectivities. In this way when ODH reactions is conducted under reductive conditions reduced species are favored and then alkene selectivities must be enhanced.
   For propane ODH the vanadium catalyst content do not affect the selectivity.

ACKNOWLEDGEMENTS

Financial support by CONICET from Argentina is acknowledged. We are also grateful to the Inorganic Chemistry Department of the University of Málaga, Spain, for the facilities in XPS experiments.

REFERENCES

1. Andersen, P.J. and H.H Kung,, "Differential Heat of Reoxidation of Reduced V2O5/ g Al2O 3". J. Phis.Chem., 96, 3114 (l992).
2. Bielanski, A. and J. Haber. "Oxygen in Catalysis on Transition Metal Oxides". Catal. Rev. Sci. Eng, 19,1 (1979).
3. Blasco, T.; J. M. López Nieto, A. Dejoz and M. I. Vázque. "Influence of the Acid-Base Character of Supported Vanadium Catalysts on Their Catalytic Properties for the Oxidative Dehydrogenation of n-Butane". J. Catal., 157, 271 (l995).
4. Concepción, P.; A. Dejoz; J. M. López Nieto and M. I. Vázquez. Proc. l4th Iberoamerican Simp. on Catalysis, Concepcion ( Chilean Chemical Society,), p. 769 (1994).
5. Corma, A.; J.M López Nieto,. and N. Paredes. "Influence of the Preparation Methods of V-Mg-O Catalysts on their Catalytic Properties for the Oxidative Dehydrogenation of Propane". J. Catal., 144, 425 ( 1993a).
6. Corma, A.; López Nieto, J.M. and Paredes, N. "Preparation of V-Mg-O Catalysts: Nature of Active Species Precursors". App. Cata/. A: General, 104, 161 (1993b).
7. Dadyburjor, D.B.; .S. S. Jewr, and E. Ruckenstein. "Selective Oxidation of Hydrocarbons on Composite Oxides". Catal. Rev. Sci. Eng., 19. 293 (1979).
8. Eckert, H. and I. E. Wachs. "Solid-State 51V NMR Structural Studies on Supported Vanadium (V) Oxide Catalysts Vanadium Oxide Layers on Alumina and Titania Supports". J. Phys. Chem., 93, 6796 (1989).
9. Eon, J. G.; R. Olier, and J. C. Volta. "Oxidative Dehydrogenation of Propane on g- Al2O3 Supported Vanadium Oxides". J. Catal., 134, 668 (1992).
10. Eon, J.G.; P. G. Pries de Oliveira,  F. Lefebvre and J. C. Volta. "Comparision between gamma-aluminum niobate supported vanadium oxides in propane oxidative dehydrogenation", in New Developments in Selective Oxidation II, V, Cortés Corberán and S. Vic Bellon Eds., p. 83, Elsevier Science B.V. (l994).
11. Galli, A.; J. M. López Nieto, A. Dejoz and M. I. Vázquez. "The Effect of Potasium on the Selective Oxidation of n-Butane and Ethane over Al2O3 Supported Vanadia Catalysts". Catal. Lett., 34, 51 (1995).
12. Gao, X.; P. Ruiz; Q. Xin, X. Guo and B. Delmon. "Effect of Coexistence of Magnesium Vanadate Phases in the Selective Oxidation of Propane to Propene". J. Catal., 148, 56 (1994).
13. Haber. J.; A. Kozlowskca, and R. Kozlowski. "Nature of The Support-.Active Phase Enhacement in Vanadium Oxide Monolayer Catalysts". J. Catal, 102, 52 (1986).
14. Koranne, M. M.; J. G. Godwin Jr.and G. Marcelin. "Characterization of Silica- and Alumina Supported Vanadia Catalysts Using Temperature Programmed Reduction". J.Catal., 148, 369 (1994).
15. Kung, H. H. and M. A. Chaar. U.S. Patent 4772319, (1988).
16. Kung, M.C. and H. H. Kung. "The Potassium in the Preparation of Magnesium Orthovanadate and Pyrovanadate on the Oxidative Dehydrogenation of Propane and Butane".J. Catal., 134, 668 (1992).
17. Mamedov. E. A. and V. Cortés Corberán. "Oxidative dehydrogenation of lower vanadium oxide-based catalysts. The present state of the art and outlooks". Appl.Catal.A: General, 127, 1 (1995).
18. Matralis, H.; M. Ciardelli, M. Ruwet, and P. Grange. "Vanadia Catalysts Supported on Mixed TiO2-Al2O3 Supports: Effect of Composition on the Structure and Acidity".J. Catal, 157, 368 (1995).
19. Michalakos, P. M.; M. C. Kung; I. Jaban and H. H. Kung. "Selectivity Patterns in Alkane Oxidation over Mg3(VO4)2-MgO, Mg 2 V2O7 and (VO)2P2O7". J.Catal., 140, 226 (1993).
20. Miyata, H.; K. Fujii, I. Ono, Y. Kubokawa, T. Ohno, and F. Hatayama. "Carrier Effect on the Nature of V4+ and Active Oxygen Species in Vapor-phase. Oxidation of Butadiene over Supported Divanadium Pentoxide Catalysts". J. Chem. Soc.Faraday Trans.. 1, 83, 675 (1987).
21. Nag, N. K.; K. V. R. Chary, B. M.  Reddy, B. R. Rao, and V. S. Subrahmanyan. "Characterization of Supported Vanadium Oxide Catalysts by Low Temperature Oxygen Chemisorption Technique. II: V2O5/SiO2 system". Appl. Catal., 40, 191 (l988).
22. Roozeboom, F.; M.C. Mittelmeijer-Hazeleger; J. A. Moulijn; J. Medema; V. H. J. Beer and P. J. Gellings. "Vanadium Oxide Monolayer Catalysts. 3. A Raman Spectroscopic and Temperature Programmed Reduction Study of Monolayer and Crystal-Type Vanadia on Various Supports". J. Phys. Chem., 84, 2783 (1980).
23. Yoshida, S.; T. Tanaka, Y. Nishima, H. Mizutani, and T. Funabilki. "The Local Structures of Vanadium Oxide on Silica and g;-Alumina Studied by X-Ray Absorption (XANES-EXAFS) Spectroscopy-The Effect of Hydration". Proceed. 9th. Intern.. Congr. Catal., 3, 1473 (l988).
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Received: January 15, 2003.
Accepted for publication: May 13, 2003.
Recommended by Subject Editor Ricardo Gómez

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