Oxidative dehydrogenation of ethane on γ-Al2O3 supported vanadyl and iron vanadyl phosphates

Applied Catalysis A: General 226 (2002) 41–48

Oxidative dehydrogenation of ethane on ␥-Al2 O3 supported vanadyl and iron vanadyl phosphates Physico-chemical characterisation and catalytic activity M.P. Casaletto a , L. Lisi b,∗ , G. Mattogno a , P. Patrono c , G. Ruoppolo d , G. Russo b a

d

Institute of Materials Chemistry, CNR, P.O. Box 10, I-00016 Monterotondo Scalo (RM), Italy b Institute of Research on Combustion, CNR, P. le Tecchio 80, 80125 Naples, Italy c Institute of Advanced Inorganic Methodologies, CNR, I-00016 Monterotondo Scalo (RM), Italy Department of Chemical Engineering, University Federico II, P. le Tecchio 80, 80125 Naples, Italy

Received 5 June 2001; received in revised form 21 September 2001; accepted 26 September 2001

Abstract

␥-Al2 O3 supported vanadyl and iron vanadyl phosphates (VOPO4 and Fe0.23 (VO)0.77 PO4 ) calcined at 550 or 650 ◦ C have been investigated as catalysts for the oxidative dehydrogenation (ODH) of ethane to ethylene in the temperature range 450–650 ◦ C in a fixed bed reactor operating under atmospheric pressure. Catalysts have been characterised by X-ray diffraction (XRD), BET surface area measurements, X-ray photoelectron spectroscopy (XPS), NH3 temperature programmed desorption (TPD) and temperature programmed reduction (TPR). A good dispersion of the active phase has been obtained. The presence of various vanadium species (vanadyl phosphate, V4+ and V3+ ions, V5+ and V4+ oxides) have been detected on the catalysts surface with a significantly different distribution between supported vanadyl and iron vanadyl phosphate, V4+ ion being the prevailing species on VOPO4 /Al2 O3 (VOP/Al). The different surface species show different acidity and reducibility. Supported vanadyl phosphate catalysts are more active than iron modified samples and catalysts calcined at 650 ◦ C give better catalytic performances than those calcined at 550 ◦ C. Catalytic tests promote the formation of V3+ , whose fraction increases with the reaction temperature. The better catalytic properties of supported vanadyl phosphate have been attributed to the presence of surface V4+ ions. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Vanadium phosphate; Oxidative dehydrogenation; Fe doping; X-ray photoelectron spectroscopy

1. Introduction Bulk and supported vanadium phosphorous catalysts have been widely investigated for the partial oxidation of C2 –C5 hydrocarbons. Depending on the reaction conditions, olefins or oxygenated compounds are produced [1–8]. Bulk vanadyl phosphate shows a very high selectivity but gives low conversion in ∗ Corresponding author. Fax: +39-81-5936-936. E-mail address: [email protected] (L. Lisi).

the oxidative dehydrogenation (ODH) of ethane [5]. Good selectivities and better conversion with respect to bulk VOPO4 have been obtained by dispersing vanadyl phosphate on TiO2 [6]. It has been shown that catalytic properties are dramatically modified by the interaction between vanadyl phosphate and the support which produces a change of the local structure of the deposed phase whose extent depends on the nature of the support itself [6–9]. Specifically, oxide-type supports strongly modify features of bulk phase due to deep interaction with vanadyl phosphate [6–8],

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 1 ) 0 0 8 8 1 - X

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M.P. Casaletto et al. / Applied Catalysis A: General 226 (2002) 41–48

whereas, very acid oxide supports as SiO2 [8] or supports not containing oxygen as SiC [9] tend to preserve the crystalline structure of vanadyl phosphate. Bordes [10] reported that VOPO4 (␥ or ␣) like structures are the prevailing phases for supported vanadyl phosphate catalysts. The presence of VO(H2 O)2+ – P–O species has also been detected using ESR for TiO2 supported vanadyl phosphate and related to the sites promoting the formation of ethylene in the ethane ODH [6]. On the other hand, it is reported [11,12] that Co and Fe doping affects the VOPO4 /(VO)2 P2 O7 ratio for bulk vanadyl phosphate. In a recent paper, Casaletto et al. [13] showed that dispersion on ␥-Al2 O3 of Fe substituted vanadyl phosphate results in the formation of vanadium oxides in addition to VOPO4 . The promotional effect of different elements on the catalytic activity of vanadyl phosphate has been widely studied [14], Fe ion giving high yields of maleic anhydride from n-butane. In this paper, a study on the correlation between physico-chemical properties and catalytic activity in the ethane ODH of vanadyl phosphate and iron vanadyl phosphate supported on ␥-Al2 O3 is reported. Different techniques as X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and NH3 temperature programmed desorption (TPD) have been employed in order to gain information on different vanadium sites on the catalyst surface. The effect of the iron substitution and of the thermal treatment under oxidising or reaction conditions on the surface structure of the active phase and on the catalytic activity is also discussed.

2. Experimental 2.1. Preparation of materials VOPO4 ·2H2 O, thereafter indicated as VOP, was prepared by refluxing V2 O5 in 85 wt.% H3 PO4 aqueous solution for 16 h according to Ladwig’s method [15]. The iron substituted compound (FeVOP) was prepared by adding Fe(NO)3 ·9H2 O to this solution. Pure ␥-Al2 O3 (AKZO CK-300) with 190 m2 g−1 surface area was used as support. The supported VOP catalyst was prepared by impregnation, dissolving an amount of the active phase (VOP) corresponding to the theoretical mono-layer coverage (14 wt.%) in wa-

ter in the presence of the support. Supported FeVOP catalyst was prepared by mechanical mixing of the support and FeVOP (14 wt.%) in bi-distilled water, which was slowly evaporated in a rotative evaporator. This procedure was needed in this case because of the insolubility in water of the bulk iron containing compound. All samples were dried at 110 ◦ C and then calcined under flowing air at 550 or 650 ◦ C for 3 h. The supported samples will be referred to with the code VOP/Al-x and FeVOP/Al-x, respectively, with x = 5 (550 ◦ C) or x = 6 (650 ◦ C) depending on the calcination temperature. 2.2. Physico-chemical characterisation A Philips PW 1100 diffractometer was employed for obtaining X-ray diffraction (XRD) patterns of the materials at room temperature. Ni-filtered Cu K␣ radiation was used and the 2ϑ measurements were accurate to 0.05◦ . BET surface areas were measured by N2 adsorption at 77 K with a Carlo Erba 1900 Sorptomatic instrument. The surface chemical composition of the samples was studied by XPS (Surface Analysis Group of the ICMAT-CNR) in an ultrahigh vacuum (UHV) VG ESCALAB MkII spectrometer, equipped with a standard Al K␣ excitation source (hν = 1486.6 eV) and a five-channeltron detection system. Samples were positioned at the electron take-off angle normal to the surface with respect to the hemispherical analyser, set at 20 eV constant pass energy. The binding energy (BE) scale was corrected for charging effects by assigning a value of BE = 285.0 eV to the C 1s peaks from the surface contamination. The accuracy of the measured BE is ±0.1 eV. Photoemission data were collected and processed by using a Digital Micro-PDP computer system and VG S5250 software. After Shirley background subtraction, a non-linear least-squares curve-fitting routine with a Gaussian/Lorentzian product function was used for the analysis of XPS spectra, separating elemental species in different oxidation states. The software was also used to correct the spectra for the Al K␣3,4 satellite peaks of O 1s, giving a spurious contribution to the vanadium peaks. Relative concentrations of chemical elements were calculated by a standard quantification routine, including Wagner’s energy

M.P. Casaletto et al. / Applied Catalysis A: General 226 (2002) 41–48

dependence of attenuation length [16] and standard set of VG Escalab sensitivity factors. TPR with hydrogen and TPD of NH3 were carried out using a Micromeritics TPD/TPR 2900 analyser equipped with a TC detector and coupled with a Hiden HPR 20 mass spectrometer. In the TPR experiments, the sample was reduced with a 2% H2 /Ar mixture (25 cm3 min−1 ) by heating at 10 ◦ C min−1 to 650 ◦ C. In the TPD experiments, the sample was saturated with pure ammonia at room temperature for 1 h and, after purging with pure He for 2 h, it was heated (10 ◦ C min−1 ) to 650 ◦ C in flowing He (25 cm3 min−1 ). The samples were treated in flowing air at 550 ◦ C for 2 h before each test. 2.3. Catalytic tests Catalytic activity tests were carried out with the experimental apparatus described in [17], equipped with a fixed bed quartz micro-reactor operating under atmospheric pressure. The reaction products were analysed with a Hewlett Packard series II 5890 gas-chromatograph, equipped with a thermal conductivity detector for the analysis of O2 , CO and CO2 and a flame ionisation detector for the analysis of hydrocarbons. The concentrations of O2 , CO and CO2 were also measured online with a Hartmann & Braun URAS 10 E continuous analyser. Water produced during the reactions was kept by a silica gel trap, in order to avoid condensation in the cold part of the experimental apparatus. The catalyst was diluted 1:10 with inert ␣-Al2 O3 particles having the same dimension. The section upside the catalytic bed was filled with larger ␣-Al2 O3 particles in order to limit homogeneous reactions among radicals possibly formed in the catalytic bed. The feed composition was 4% C2 H6 and 2% O2 in a balance of He. The

43

reaction temperatures ranged from 450 to 650 ◦ C. The contact time ranged from 0.01 to 0.75 g s Ncm−3 . Carbon balance was closed within 3% error in all experiments.

3. Results and discussion XRD patterns of all supported samples show only signals of ␥-Al2 O3 . No signals attributable to any VOPO4 phase nor to alumina phases different from ␥ were detected, suggesting that a quite homogeneous dispersion of the active phase can be obtained up to the content corresponding to the theoretical mono-layer coverage, even by using the chemical mixing technique. Moreover, the presence of the active phase does not promote any transition of the support to phases ␦, ␤ or ␣. Nevertheless, the values of the surface area of the catalysts (see Table 1) show that a small reduction of the specific surface area occurs upon calcination at higher temperature. As a consequence, elimination of alumina micropores likely starts at 650 ◦ C, determining lower values of surface area. Otherwise, the lower values of surface area of the sample prepared by chemical mixing can be an indication of the formation of very small FeVOP crystallite not detectable by XRD analysis. The binding energy values of the main XPS peaks (V 2p, P 2p, O 1s, Al 2p) and the relative chemical composition of the investigated samples are reported in Table 2. The elemental concentrations are normalised to the vanadium species. In all the samples the binding energy values (BE) are characteristic of elements in their normal oxidation states: V5+ at BE = 518.5 eV; P5+ at BE = 134.1 eV and Al3+ at BE = 74.3 eV, which are in good agreement with data reported in the literature [13,18–21].

Table 1 Surface area, H2 uptake and peak temperature evaluated from TPR curves and H2 /V ratio estimated from TPR and XPS data Catalyst

Surface area (m2 g−1 )

H2 uptake (× 104 mol g−1 )

Tmax (◦ C)

H2 /V from TPR

H2 /V from XPS

VOP/Al-5 VOP/Al-6 FeVOP/Al-5 FeVOP/Al-6 VOP/Al-6 after ODH at 550 ◦ C FeVOP/Al-6 after ODH at 550 ◦ C

162 144 134 126

5.2 5.4 4.4 4.9 4.5 3.8

543 529 566 515 501 519

0.60 0.63 0.65 0.72 0.52 0.57

0.57 0.65 0.92 0.83 0.58 0.60

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Table 2 BE values of the XPS peaks and relative chemical composition of the samples as a function of the temperature of catalysisa Catalyst

Dried VOP/Al VOP/Al-5 VOP/Al-5 after ODH at 450 ◦ C VOP/Al-5 after ODH at 550 ◦ C VOP/Al-6 VOP/Al-6 after ODH at 550 ◦ C a b

V 2p3/2

P 2p

O 1s

Al 2p

C 1s

518.5b

134.1b

531.5b

74.3b

285.0b

1.0 1.0 1.0 1.0 1.0 1.0

0.9 1.1 1.1 0.9 1.1 1.1

35.9 33.1 36.3 36.7 35.1 35.9

16.9 19.2 20.3 21.1 19.4 18.6

0.8 4.4 8.0 7.0 10.0 5.9

Surface atomic ratio (P/V)

0.9 1.1 1.1 0.9 1.1 1.1

Elemental concentrations are normalised to V. BE values (eV).

The surface atomic ratio (P/V) corresponds to the stoichiometric value, being always around 1. The V 2p spectrum is dominated by a major peak at BE = 518.5 eV with a spin-orbit splitting of ∼7.5 eV, as expected for the V 2p3/2 and V 2p1/2 components of the doublet [19]. The fairly large V 2p3/2 peak (FWHM ∼ 4 eV) is consistent with the presence of different single components. The analysis of the XPS spectrum has been performed by a curve-fitting procedure in order to distinguish vanadium species in different oxidation states. Vanadyl phosphates as VOPO4 samples, have been analysed and used as reference samples. The XPS spectrum of the VOPO4 sample consists of a single peak at BE = 518.4 eV, which can be assigned to V5+ species in the vanadyl phosphate compound [13,18,21,22].

In dried VOP/Al sample, the V 2p3/2 peak can be deconvolved into two components at BE = 518.5 and 517.4 eV, which can be assigned to V5+ species in VOPO4 [18,22] and V4+ ions [13,19–21], respectively. Besides the V5+ species as in VOPO4 , a major amount of V4+ species is detected, as calculated by the peak areas of single components and reported in Table 3. The presence of V4+ ions, as VO(H2 O)2+ –O–P, was detected by EPR also for VOP/TiO2 catalysts [6]. Quantitative information on the relative surface amount of vanadium species can be inferred from Table 3, together with their evolution as a function of the temperature of catalysis. After calcination at 550 and 650 ◦ C, the V 2p3/2 peak of the VOP/Al sample shows the same components attributable to V5+ and to V4+ species, but it can be fitted by inserting a new component at BE =

Table 3 Relative surface composition of vanadium species as a function of the temperature of catalysis, as resulting from the curve-fitting of the V 2p3/2 photoelectron peaka Catalyst BE (eV) Dried VOP/Al VOP/Al-5 VOP/Al-5 after ODH at 450 ◦ C VOP/Al-5 after ODH at 550 ◦ C VOP/Al-6 VOP/Al-6 after ODH at 550 ◦ C a

V5+ (VOPO4 ) 518.5

V4+ (ion) 517.4

V3+ (V2 O3 ) 516.1

40.5 31.1 34.3

59.5 52.4 44.2

– 16.5 21.5

29.6

37.5

32.9

40.9 27.6

49.0 60.9

10.1 11.5

Catalyst BE (eV) Dried FeVOP/Al FeVOP/Al-5 FeVOP/Al-5 after ODH at 450 ◦ C FeVOP/Al-5 after ODH at 550 ◦ C FeVOP/Al-6 FeVOP/Al-6 after ODH at 550 ◦ C

V5+ (VOPO4 ) 518.4

V5+ (V2 O5 ) 517.2

V4+ (V2 O4 ) 516.5

V3+ (V2 O3 ) 516.0

73 65 30

15 20 25

11 15 25

– – 20

27

32

18

23

47 27

31 28

10 16

12 29

Concentrations (conc. %) are expressed as percentage of the total area of V 2p3/2 peak (taken as 100%).

M.P. Casaletto et al. / Applied Catalysis A: General 226 (2002) 41–48

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Fig. 1. Curve-fitting of the V 2p3/2 peak for the VOP/Al sample after calcination at 550 ◦ C. The scattered points refer to the raw data, while the solid line to the fitting results. The single components are plotted as dotted curves: (1) V5+ ; (2) V4+ and (3) V3+ ions.

516.1 eV, which can be assigned to V3+ ionic species [13,19,21]. The V 2p3/2 peak fitting of the VOP/Al-5 sample is reported in Fig. 1, where the scattered points refer to the experimental data, while the solid line to the fitting results. The single components are plotted as dotted curves and indicated as: (1) V5+ ; (2) V4+ and (3) V3+ ions. The amount of V3+ in the VOP/Al-5 and VOP/Al-6 catalysts is balanced by a decrease of both V4+ and V5+ species. The VOP/Al-5 sample has been tested for the ODH of ethane to ethylene at reaction temperatures, T r = 450 and 550 ◦ C. After performing the catalytic reaction at different temperatures, the V 2p3/2 spectra of the samples can be fitted with the same three components at BE = 518.5, 517.4 and 516.1 eV, assigned to V5+ , V4+ and V3+ species, respectively. The relative quantitative distribution of vanadium species as a function of the temperature of catalysis is shown in Table 3. Quantitative XPS analysis shows that no significant changes occur on the sample surface after performing catalysis at lower temperature, as evidenced by comparing the VOP/Al-5 sample before and after catalysis at 450 ◦ C (see Table 3). Nevertheless, the amount of V3+ species is continuously growing as a function of the temperature of catalysis. A higher concentration of V3+ species is detected after performing catalysis

at higher temperature (i.e. at 550 ◦ C), as shown in Table 3. A progressive reduction of vanadium species is evidenced in the samples as effect of the temperature of catalysis. The same trend has been observed after catalytic tests performed on the VOP/Al-6 sample at 550 ◦ C, i.e. an increase of the V3+ content in the catalyst after reaction. It is worth noting that no changes in the core-level spectra have been found after a prolonged X-ray irradiation, thus excluding any possible damage of the sample during a long exposure in UHV conditions. An analogous behaviour has been found in the FeVOP/Al samples, which have been tested for the same catalytic reaction [13]. The results reported in [13] are summarised in Table 3 in order to compare these data with those resulting from VOP/Al samples. TPR experiments carried out on the samples calcined at 550 and 650 ◦ C did not reveal significant differences either for VOP/Al and FeVOP/Al catalysts, except for the peak temperature which is slightly lower for the samples calcined at 650 ◦ C (see Table 1). All samples give rise to a single, narrow and quite symmetric peak due to the strong reducing power of hydrogen which leads to a rapid formation of V3+ , whatever the initial oxidation state of vanadium. A fairly good agreement between TPR and XPS results

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M.P. Casaletto et al. / Applied Catalysis A: General 226 (2002) 41–48

Fig. 2. NH3 TPD curves of ␥-Al2 O3 , VOP/Al-5, FeVOP/Al-5 and FeVOP/Al-6 after catalytic test at 550 ◦ C.

is obtained, as shown in Table 1, by the comparison between the H2 /V ratio evaluated from the integration of TPR profiles and that calculated on the base of vanadium average oxidation state estimated from XPS results hypothesising a complete reduction to V3+ . After catalytic tests at 550 ◦ C either vanadyl and iron vanadyl phosphate are in a more reduced state. Similar results were found by Ciambelli et al. [6] for TiO2 supported VOPO4 catalysts. Results of ammonia TPD experiments carried out on VOP/Al-5 and FeVOP/Al-5 samples are reported in Fig. 2. Both samples show a signal peaked at about 120 ◦ C, which can be associated to the contribution of the support by comparison with the TPD profile of ␥-Al2 O3 , also reported in Fig. 2. In addition to this tailed peak, which represents the unique contribution for VOP/Al-5, the iron substituted catalyst gives rise to two extra signals at higher temperature, suggesting that the different nature of surface vanadium species results also in a different acidity. The complexity of the TPD curve of FeVOP/Al-6 sample after catalytic test at 550 ◦ C gives a further confirmation to this hypothesis. From the analysis of XPS data it can be inferred that neither V5+ phosphate nor V4+ ion, the two prevailing species for VOP/Al-5 sample, give a signifi-

cant contribution to the surface acidity, suggesting that vanadium oxides present on FeVOP/Al sample, even in larger amount after reaction at 550 ◦ C, contain acid centres adsorbing ammonia at higher temperature. Catalytic activity tests were carried out at 450 and 550 ◦ C on all catalysts and also at 650 ◦ C on samples calcined at this temperature. Tests performed in absence of catalyst showed that ethane conversion due to homogeneous reactions is still negligible at 650 ◦ C. Catalytic activity of pure ␥-Al2 O3 has been also evaluated in order to estimate the possible contribution to the activity of supported VOP catalysts. Under the same reaction conditions used for catalytic tests, alumina oxidises ethane producing mainly CO and CO2 with a rate more than one order of magnitude lower with respect to the catalysts investigated. As a consequence, its contribution has been neglected. In Table 4, the rate of ethane consumption evaluated under differential conditions (ethane conversion <10%) at different reaction temperatures is reported for all catalysts. In Fig. 3, ethylene selectivity is reported as a function of ethane conversion for both VOP and FeVOP based catalysts at 550 ◦ C, whilst CO and CO2 selectivity have been reported in Table 5 for the same reaction temperature.

M.P. Casaletto et al. / Applied Catalysis A: General 226 (2002) 41–48 Table 4 Ethane consumption rate for VOP/Al and FeVOP/Al calcined at 550 and 650 ◦ C Catalyst

VOP/Al-5 VOP/Al-6 FeVOP/Al-5 FeVOP/Al-6

Ethane consumption rate (× 106 mol s−1 g−1 ) Tr = 450 ◦ C

Tr = 550 ◦ C

Tr = 650 ◦ C

0.37 0.44 0.31 0.45

7.5 10.2 3.1 6.5

– 54 – 37

All catalysts produce only C2 H4 , CO and CO2 . However, selectivity to CO2 , always <10% for VOP/Al catalysts and almost negligible for FeVOP/Al samples, suggests the dominance of a hetero-homogeneous mechanism for both systems, CO being considered the product of the further oxidation of ethylene in the gas phase, whilst CO2 the product of its surface oxidation [23]. Comparison with TiO2 supported vanadium oxide catalysts [24] shows that the supported vanadyl phosphate reported in this paper gives a better ethylene selectivity under the same experimental conditions. The alumina supported VOP samples, however, show worse catalytic performances com-

Fig. 3. Ethylene selectivity as function of ethane conversion for VOP/Al-5, VOP/Al-6, FeVOP/Al-5 and FeVOP/Al-6 catalysts (reaction temperature = 550 ◦ C).

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Table 5 Ethane conversion and CO and CO2 selectivity for VOP/Al-5, VOP/Al-6, FeVOP/Al-5 and FeVOP/Al-6 catalysts (reaction temperature, T r = 550 ◦ C) Catalyst

X(C2 H6 ) (%)

S(CO) (%)

S(CO2 ) (%)

VOP/Al-5

9.2 13.7 15.8 20.2 23.2

20.8 25.7 26.5 27.5 30.0

5.9 8.0 8.3 10.0 9.0

VOP/Al-6

12.5 14.8 21.2

27.7 27.6 30.1

5.6 7.7 10.0

FeVOP/Al-5

4.5 7.6 14.6

17.0 19.8 29.4

– 3.3 4.6

FeVOP/Al-6

3.2 11.9 13.6

20.5 27.5 29.2

– 3.9 5.1

pared to TiO2 supported VOP samples under the same conditions as reported in [25] suggesting a marked effect of the interaction between VOP and support. VOP/Al catalysts are more active than FeVOP/Al catalysts, even if the lower amount of vanadium, which is supposed to be the active ion for the reaction, is considered for FeVOP supported samples. The catalytic activity at a given temperature generally enhances by increasing the calcination temperature either for VOP/Al and FeVOP/Al catalysts, in agreement with the higher reducibility of catalysts calcined at 650 ◦ C, whereas, ethylene selectivity is not significantly affected by the calcination temperature basically depending on ethane conversion (see Fig. 3). Nevertheless, the specific activity evaluated at 450 ◦ C does not show a significant effect of the catalysts nature, likely due to the very low values calculated at this temperature. The different slope of conversion/selectivity curves, shown by vanadyl and iron vanadyl phosphate catalysts in Fig. 3, confirms the different nature of the surface active phase which, for FeVOP/Al catalysts, more easily promotes the further C2 H4 oxidation to CO2 at high contact times. As already mentioned before, catalytic tests give rise to the formation of a larger amount of V3+ either for VOP/Al and FeVOP/Al catalysts. This derives mainly from the reduction of V4+ ions for VOP/Al

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samples, since the fraction of vanadium present as vanadyl phosphate does not change significantly. On the other hand, the formation of V3+ species on FeVOP/Al after catalytic tests is balanced by the reduction of vanadyl phosphate and, in lesser extent, of V5+ and V4+ oxides. This could suggest that for VOP/Al catalysts V4+ ionic species, likely as VO(H2 O)2+ –O–P, are involved in the ODH reaction and that their activity prevails on that of other vanadium species. The formation and the increase of V3+ as ion in these samples after calcination and catalytic tests reasonably indicates that V4+ ions should play an important role in the reaction mechanism, probably acting through a redox cycle V4+ –V3+ –V4+ . The higher activity of VOP/Al-6 catalyst, containing a larger amount of V4+ ions with respect to VOP/Al-5 catalyst after ODH reaction, confirms this hypothesis. Tessier et al. [4], who found a partial segregation between vanadium and phosphorous for TiO2 supported vanadyl phosphate catalysts, also proposed that isolation of vanadium sites by phosphate groups can be responsible for the selective formation of acetic acid from ethane. As concerns FeVOP/Al samples, both vanadium phosphate and vanadium oxides could contribute to the global activity evaluated for these catalysts. Furthermore, the involvement of Fe ions in the ethane activation cannot be excluded.

4. Conclusions ␥-Al2 O3 supported vanadyl and iron vanadyl phosphate (VOP/Al and FeVOP/Al) show good catalytic performances in the oxidative dehydrogenation of ethane to ethylene. Dispersion of VOPO4 or Fe0.23 (VO)0.77 PO4 on alumina results in the formation of a not homogeneous phase containing, in addition to VOPO4 , V4+ and V3+ ions (probably bound to phosphorus through oxygen bond) for VOP/Al and V5+ , V4+ and V3+ oxides for FeVOP/Al catalysts. These species have different redox and acid properties, which are related to different catalytic performances. The better catalytic properties of VOP/Al catalysts with respect to FeVOP/Al catalysts have been attributed to the presence of V4+ ions, which could be involved in the ODH reaction through a redox cycle V4+ –V3+ –V4+ .

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