Al2O3 catalyst

Applied Catalysis A: General 264 (2004) 73–80

Mechanistic features of the ethane oxidative dehydrogenation by in situ FTIR spectroscopy over a MoO3/Al2 O3 catalyst Eleni Heracleous a , Angeliki A. Lemonidou a,∗ , Johannes A. Lercher b a b

Department of Chemical Engineering, Aristotle University of Thessaloniki and Chemical Process Engineering Research Institute (CERTH/CPERI), P.O. Box 1517, University Campus, GR-54006 Thessaloniki, Greece Institüt für Technische Chemie, Technische Universität München, Lichtenbergstr. 4, Garching D85748, Germany Received in revised form 19 December 2003; accepted 19 December 2003

Abstract Aspects of the surface pathway of the ethane oxidative dehydrogenation (ODH) were investigated by in situ infrared spectroscopy. Adsorption and surface reactions were studied on MoO3 /Al2 O3 and Al2 O3 , in order to investigate contributions of the support as well as selective and unselective routes of the ODH reaction. The interaction of ethane with the oxides was investigated at isothermal conditions (50 ◦ C) in the absence and presence of oxygen by time resolved IR spectroscopy between 50 and 500 ◦ C. Catalytic testing of these materials in ethane oxidative dehydrogenation showed that the molybdena catalyst is highly selective (initial C2 H4 selectivity was 96%), while mainly COx was formed with alumina. The strong acid sites on alumina, detected by NH3 -TPD, are speculated to be responsible for the unselective conversion of ethane to COx , while the moderate acid strength introduced by molybdena allows selective activation of ethane and inhibits the further oxidation of ethylene produced. The spectroscopic data indicate that the activation of the ethane C–H bond proceeds through the formation of alkoxides, which decompose to ethylene and a surface OH group or are oxidized to surface bound oxygenates. MoO3 /Al2 O3 favors the first route, while pure alumina favors formation of oxygenates and full oxidation. © 2004 Elsevier B.V. All rights reserved. Keywords: In situ Infrared Spectroscopy; Ethane; Oxidative dehydrogenation; Molybdena-alumina catalysts; ␥-Alumina; Surface mechanism

1. Introduction The selective conversion of alkanes to alkenes is a significant challenge. Currently, light olefins are mainly produced by steam cracking and catalytic dehydrogenation, two processes that operate under severe thermal conditions and are highly energy consuming. Catalytic oxidative dehydrogenation (ODH) of alkanes overcomes the thermodynamic limitations of these approaches and the exothermic character may allow developing an energetically well-balanced process. However, industrial realization of ODH is impeded by low yields at higher conversions due to formation of COx . Thus, active and selective catalysts appear to be the key for the realization of the process. Oxidative dehydrogenation of alkanes has been extensively studied over catalysts based on early transition metal ∗ Corresponding author. Tel.: +30-2310-996273; fax: +30-2310-996184. E-mail address: [email protected] (A.A. Lemonidou).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.030

oxides (mainly Mo and V oxides), which activate alkanes at relatively low temperatures (<600 ◦ C) [1]. Recent studies on the selective oxidation of ethane to ethene over molybdena catalysts showed that alumina-supported molybdena catalysts are active and highly selective for the production of ethene. The Mo concentration of the catalyst was an important parameter affecting the physicochemical properties and the catalytic performance. Low concentrations of Mo on the catalysts led to low olefin selectivity, attributed to accessible alumina surface patches, whereas complete coverage of the support by Mo oxide led to optimum activity and selectivity [2]. Over reducible oxides a parallel and consecutive reaction scheme to COx , i.e., direct oxidation of the reacting alkane and consecutive over-oxidation of the produced olefin are observed. The activation of the first C–H bond in the reacting alkane is usually the rate-determining step of the overall reaction. This proceeds on a Men + –O2− center, often considered as an acid–base pair. Reaction is suggested to proceed via a redox (Mars and van Krevelen) type

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mechanism in two steps, i.e., reduction of the catalyst by the alkane with extraction of the lattice oxygen, followed by re-oxidation of the reduced catalyst with molecular dioxygen. Several studies have addressed the details of the primary and secondary steps of ODH. According to Kung [3], the first step in the oxidative dehydrogenation of ethane is the breaking of the C–H bond to form an ethyl species on a surface metal ion of the catalyst. Dehydrogenation occurs, if the ethyl species undergoes ␤-hydrogen elimination. Alternatively, the alkyl species could also form a C–O bond, producing ethoxide species that lead to oxygen-containing products (organic and total oxidation products). The relative rate between these two competitive pathways determines the selectivity to the desired product. On the other hand, IR studies on the activation of alkane C–H bonds on oxidation catalysts by Busca et al. [4] suggest that alkoxides are the first abundant intermediates upon contact of the alkane with the catalyst. The spectroscopic data indicate that hydrocarbons are activated towards both selective and total oxidation by cleavage of the C–H bond. The interaction results in the formation of a hydroxyl group by link of the hydrogen with an oxygen anion, while the alkyl group links to another oxygen anion giving rise to an alkoxy group. This alkoxide can either rapidly decompose to the corresponding olefin, or, if strongly bound to the surface, convert to acetate and formate species that are eventually oxidized to COx [5]. The choice between selective and total oxidation mainly depends on the relative rates of alkoxy group decomposition and oxidation [6]. However, despite the efforts described the surface chemistry of alkane activation is still under discussion. Here, oxidative dehydrogenation of ethane to ethylene is investigated by in situ IR spectroscopy over a MoO3 /Al2 O3 catalyst and pure alumina in order to identify intermediate species formed on the surface and obtain information on the selective and unselective pathways of the reaction.

2. Experimental part 2.1. Catalyst preparation The catalyst used is a 20 wt.% MoO3 /Al2 O3 . The catalyst was prepared by wet impregnation of the ␥-Al2 O3 support (Engelhard) with hot aqueous solution of ammonium heptamolybdate, (NH4 )6 Mo7 O24 ·4H2 O (Fisher), to ensure full dissolution of the precursor. Prior to impregnation, the support was crushed and sieved to a particle size of 106–180 ␮m. After impregnation, the solvent was removed by evaporation under reduced pressure and the resulting solid was dried over night at 120 ◦ C and calcined in synthetic air at 650 ◦ C for 6 h. The catalyst is referred to as 20MoAl. ␥-Al2 O3 used for the experiments had a particle size of 106–180 ␮m and was calcined in synthetic air at 600 ◦ C for 6 h.

2.2. Catalyst characterization The concentration of molybdenum in the catalyst was measured by ICP using a Plasma 400 Perkin-Elmer spectrometer. Surface areas of the samples were determined by N2 adsorption at 77 K, using the multipoint BET analysis method, with an Autosorb-1 Quantachrome flow apparatus. Prior to the measurements, the samples were dehydrated in vacuum at 250 ◦ C overnight. The crystalline structure of the materials was studied by X-ray diffraction (XRD) analysis with a Siemens D500 diffractometer, using Cu K␣ radiation. NH3 -temperature-programmed desorption (TPD) was used to determine the acid properties. In a typical experiment, 0.1 g of the sample were loaded in a U-shaped quartz reactor, pretreated at 500 ◦ C for 0.5 h and then cooled to 100 ◦ C under He flow. The pretreated samples were saturated with 5% NH3 /He for 1 h at 100 ◦ C, with subsequent flushing at 100 ◦ C for 1 h to remove the physisorbed ammonia. TPD analysis was carried out from 100 to 700 ◦ C, at a heating rate of 10 K/min. The composition of the exit gas was monitored on-line by a quadrupole mass analyzer (Omnistar, Balzers). The (m/z) fragments registered were as follows: NH3 = 15; H2 O = 18; N2 = 28; NO = 30; N2 O = 44 and NO2 = 46. 2.3. Flow reactor studies Catalytic tests were carried out in a fixed-bed quartz reactor, equipped with a coaxial thermocouple. The oxidative dehydrogenation of ethane was investigated between 450 and 600 ◦ C. For the determination of the activity of the catalysts as a function of temperature, the weight of the sample was 0.3 g and the total flow was 55 cm3 /min (composition of reaction mixture C2 H6 /O2 /He = 5/5/45). In order to obtain different ethane conversion levels at constant reaction temperature (550 ◦ C), the W/F ratio was varied from 0.04 to 0.36 g s cm−3 . Prior to the experiments, the samples were activated in flowing oxygen at 500 ◦ C for 30 min. The reaction products were analyzed on line by a Varian 3700 gas chromatograph equipped with a thermal conductivity detector (TCD). Two columns in a series-bypass configuration were used in the analysis: a Porapak Q and a MS 5A. Negligible amounts of oxygenates were observed at the reactor exit. The ethane conversion and the selectivity to the reaction products were calculated on a carbon basis. Closure of the carbon mass-balance was better than ±1%. Conversion of ethane at experiments using an empty-volume reactor was lower than 2%, confirming that gas-phase reactions are negligible at the experimental conditions used for the activity tests. 2.4. IR measurements IR measurements were performed in situ with a Bruker IFS-88 spectrometer equipped with a flow-cell to investigate the intermediate species in ethane oxidative dehydrogenation over molybdena-alumina catalysts. The spectrometer

E. Heracleous et al. / Applied Catalysis A: General 264 (2004) 73–80 40350

30350 NH3 MS signal

was used in the transmission–absorption mode with a resolution of 4 cm−1 . The IR cell consisted of a stainless steel chamber equipped with CaF2 windows and a resistance-heated furnace inside which a golden sample holder was placed. A spectrum of the empty cell was used as reference (I0 ) to convert the single beam spectra (I) into absorbance spectra (log I0 /I). The samples were pressed into thin, self-supporting wafers, which were placed in the gold sample holder in the furnace of the cell. Prior to the measurements, the samples were activated in situ under He flow at 500 ◦ C (heating rate 10 ◦ C/min) for 60 min. After activation, the system was cooled to 50 ◦ C.

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(A)

3. Results and discussion

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6200 NH3 MS signal

The MoO3 loading of the 20MoAl catalyst (20.25 wt.% MoO3 , measured by ICP) corresponds to a surface coverage of 1.25, using 22 Å2 as the mean surface area occupied by one Mo6+ oxide unit on ␥-Al2 O3 [7]. The impregnation of the alumina support with molybdena caused a decrease in the surface area, from 184 m2 /g for the bare support to 124 m2 /g for the 20MoAl catalyst. This decrease could be attributed to partial coverage of the narrower pores of the support by the molybdenum phase. Crystalline phases in the samples were characterized by X-ray diffraction. Both samples exhibited diffraction lines characteristic of ␥-alumina. The diffractogram of the 20MoAl catalyst exhibited additional lines corresponding to aluminum molybdate, Al2 (MoO4 )3 [2]. However, the low intensity of these peaks indicates that for the most part Mo is highly dispersed on the alumina surface as MoOx species and only a very small part reacts with Al2 O3 to form Al2 (MoO4 )3 crystallites. The acidic properties of 20MoAl and Al2 O3 investigated by NH3 -TPD are compiled in Fig. 1A and B, respectively. The main desorption product was in both cases NH3 , while traces of N2 , NO and N2 O were also detected. The 20MoAl catalyst is characterized by a narrow desorption peak, in the range 100–400 ◦ C, with maximum temperature of desorption at 206 ◦ C. In contrast, the NH3 -TPD with Al2 O3 showed two peaks, one asymmetric with a maximum at 186 ◦ C and a shoulder extended up to 450 ◦ C and a second peak at 600 ◦ C. The incorporation of molybdenum on alumina causes a significant increase in the total acidity (906 ␮mol NH3 /g for the 20MoAl catalyst compared to 312 ␮mol/g for Al2 O3 ) and generation of new acid sites of weak and moderate strength. The strong acidity decreases (as evidenced by the narrower desorption profile of the catalyst), while the very strong acid sites of alumina (Tdes > 500 ◦ C), are totally eliminated, probably covered by the molybdenum phase at such high loadings. Although we are not able to discriminate between Lewis and Brønsted acidity, literature data suggest that the new acid sites generated from the introduction of Mo are of

75

4800 3400 2000 600

(B)

100

200

Fig. 1. NH3 -TPD profiles: (A) 20 wt.% MoO3 /Al2 O3 ; (B) Al2 O3 .

Brønsted type, while only Lewis acid sites were identified on the alumina support [8,9]. 3.2. Flow reactor studies The performance of the 20MoAl catalyst and the bare support (calcined ␥-Al2 O3 ) in the oxidative dehydrogenation of ethane was examined between 450 and 600 ◦ C. The activity of the samples as a function of reaction temperature is shown in Fig. 2. Ethane conversion up to 48% at 600 ◦ C was observed with 20MoAl. Alumina was also active, with conversion reaching 28% at 600 ◦ C. The main products of the oxidative dehydrogenation reaction were C2 H4 , CO2 , CO and H2 O, while in the case of Al2 O3 H2 was also detected. Because selectivity is strongly related to conversion, conversions were varied at 550 ◦ C by varying W/F from 0.04 to 0.36 g s/cm3 . Figs. 3 and 4 show the selectivity to the reaction products as a function of ethane conversion for 20MoAl and the Al2 O3 support, respectively. At low ethane conversions, the main reaction product with 20MoAl was ethylene with selectivity reaching 90%. Extrapolated to zero conversion, the initial C2 H4 selectivity was 96%, indicating that ethane is initially exclusively converted to ethylene. As the conversion increased the olefin selectivity decreased, while the CO selectivity increased. The selectivity to CO2 hardly

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60 20MoAl Al2O3

C2H6 conversion, %

50 40 30 20 10 0

450

500

550

600

Temperature, ˚C Fig. 2. Ethane conversion as a function of temperature over 20 wt.% MoO3 / Al2 O3 catalyst and Al2 O3 (reaction conditions: W/F = 0.33 g s cm−3 , C2 H6 /O2 = 1/1).

3.3. In situ IR spectroscopy

100

Selectivity, %

80

C2H4 CO2

60

CO

40 20 0

varied. Thus, we conclude that CO is formed exclusively from ethene via secondary oxidation. Alumina was highly unselective, with olefin selectivity not exceeding 12% even at very low conversion levels. The increase of the conversion with increasing W/F did not cause substantial variations in the product distribution. The ethylene selectivity decreased slightly over the conversion range studied, suggesting only minor secondary reactions. Due to the significant amounts of H2 in the products, we speculate that ethylene is mostly produced from pure dehydrogenation of ethane on the alumina surface in agreement with earlier reports [10]. CO2 and CO were the main reaction products, with the CO2 selectivity slightly increasing at higher conversions due to oxidation of CO. Based on these results, we conclude that the strong acid sites of alumina catalyze the unselective conversion of ethane to COx , while the moderate acidity introduced by molybdena allows the selective activation of ethane to ethylene.

0

4

8 12 16 C2H6 conversion, %

20

24

Fig. 3. Product distribution as a function of ethane conversion over 20 wt.% MoO3 /Al2 O3 (reaction conditions: T = 550 ◦ C, C2 H6 /O2 = 1/1).

3.3.1. Activated adsorbents Typical IR spectra of Al2 O3 and of 20MoAl at 50 ◦ C in He flow, after the in situ pretreatment and prior to exposure to ethane, are presented in Figs. 5 and 6, respectively. These spectra were subtracted in subsequent figures from spectra obtained in the presence of ethane. Al2 O3 exhibits bands at 3764, 3721 and 3677 cm−1 , typical of the νOH bands of alumina hydroxyls. The band at 3764 cm−1 is assigned to basic hydroxyl groups, bound to a single tetrahedrally coordinated aluminum atom, while the band at 3721 cm−1 is due to bridged OH groups shared by an octahedrally and a tetrahedrally coordinated aluminum cation. The band at 3677 cm−1 is assigned to hydroxyl groups bound to three octahedrally coordinated aluminums [11]. The decrease in frequency has been associated with increasing acid strength. The absence of these bands in the spectrum of the 20MoAl catalyst (Fig. 6) suggests that the alumina surface is covered

100 80

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Selectivity, %

0.35

C2H4 CO2 CO

0.05 0

4

8 12 C2H6 conversion, %

16

20

Fig. 4. Product distribution as a function of ethane conversion over Al2 O3 (reaction conditions: T = 550 ◦ C, C2 H6 /O2 = 1/1).

0 3800

3300

2800 -1 Wavenumber, cm

2300

Fig. 5. IR spectra of Al2 O3 at 50 ◦ C in He flow.

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E. Heracleous et al. / Applied Catalysis A: General 264 (2004) 73–80

0.1 0.08

3540 1645

0.06 20 min 10 min 5 min 1 min

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77

3300

2800 -1 Wavenumber, cm

2300

1800

Fig. 6. IR spectra of the 20 wt.% MoO3 /Al2 O3 catalyst at 50 ◦ C in He flow.

by molybdenum oxide units and full coverage has been achieved. The metal oxide species seem to anchor to the support by reacting with the surface hydroxyl groups and are consequently located on the surface of the oxide. The broad band observed at 3450 cm−1 is attributed to strongly perturbed OH bands indicating a substantial hydroxylation of the surface. The IR spectrum also shows a strong band at 2000 cm−1 assigned to the first overtone of the Mo=O terminal bonds of the surface molybdenum oxide species. The presence of one vibrational band suggests that only one terminal Mo=O bond exists on each surface molybdena species present on the alumina surface (mono-oxo structure), since a di-oxo structure would be expected to give rise to several combination bands [12]. If coupled O=Mo=O bonds were present, two bands separated by 30–50 cm−1 and with different relative intensities should be detected in the IR spectrum [13]. The single terminal Mo=O bond could belong to both isolated and polymerized species, with bridging bonds either to the support cation or another Mo, respectively. The IR bands of the dispersed Mo-oxo species on alumina below 1000 cm−1 cannot be detected because of the IR absorption of the alumina support, thus making it impossible to obtain more information on the bridging bonds, which typically vibrate in the 400–900 cm−1 region. 3.3.2. Ethane adsorption and reaction Fig. 7 shows the time-dependent IR spectra of surface species formed upon contact of Al2 O3 with C2 H6 . The (minute) bands at 1492, 1470, 1460, 1405 and 1375 cm−1 are due to deformation bands of gaseous ethane, while the bands in the 2800–3100 cm−1 region correspond to C–H stretching bands of ethane. Interaction of ethane with alumina caused the formation of bands at 1645 and 3540 cm−1 , the intensity of which increased with time, and negative bands in the 3800–3600 cm−1 region. The band at 1660–1640 cm−1 is attributed to a νC=C vibration of an olefin, strongly bound to a strong Lewis acid site [14]. Accordingly, the results suggest the first detectable adsorbed

-0.02 3800

3300

2800

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-1

Wavenumber, cm

Fig. 7. IR spectra during exposure of Al2 O3 to 5% C2 H6 /He at 50 ◦ C.

species to be adsorbed ethylene. The negative bands at 3764, 3721 and 3677 cm−1 , assigned to the alumina –OH groups, indicate an interaction between the surface hydrocarbon species and the hydroxyl groups associated with the support. A more intense decrease with time is observed for the 3764 cm−1 band, assigned to the most basic –OH group of alumina, indicating that the terminal hydroxyls are more reactive than the bridging hydroxyls, in agreement with studies investigating the interaction of propylene with the ZrO2 surface [15]. Knözinger et al. [11] have also identified this type of alumina hydroxyl groups as possible chemisorption sites for olefins. The broad band at 3540 cm−1 increased in intensity with the concentration of adsorbed hydrocarbon species. It is attributed to hydrogen-bonded hydroxyl groups formed in the surface reaction, i.e., from the abstraction of hydrogen from ethane [11]. The analysis of the corresponding IR spectra for the 20MoAl catalyst did not show evidence of adsorbed species. The only changes observed in the IR spectra noted were the increase of the band at 1615 cm−1 , due to the deformation vibrations of H2 O [16] and the generation of a broad band at 3460 cm−1 . A negative band at 2000 cm−1 , which corresponds to the terminal Mo=O bonds of the surface molybdenum oxide species (associated with Mo6+ ), is attributed to the reduction of the Mo species by the hydrocarbon to lower oxidation states [12]. The intensity of this band has been used in literature to estimate the reduction degree of metal components [17]. The variation of its intensity with time (see Fig. 8) suggests a nearly monotonous decrease with contact time. After the ethane flow was stopped, the intensity of the 2000 cm−1 band did not regain its initial value suggesting an irreversible alteration of the molybdena phase. The decrease and reduction of the Mo=O bonds by the hydrocarbon could imply the direct involvement of these bonds in the ethane or ethene surface reaction. However, the loss of the Mo=O band may be a consequence of the reduction of Mo by abstraction of a bridging oxygen. The replenishment of this oxygen by the oxygen of the terminal bonds could account for the decrease observed. As alkanes

E. Heracleous et al. / Applied Catalysis A: General 264 (2004) 73–80

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3721

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0.07

0

5

10 Adsorption time, min

15

20

-0.03 3800 (A)

Fig. 8. Variation of IR band intensity of Mo=O bonds of the MoOx species on 20 wt.% MoO3 /Al2 O3 catalyst during exposure to 5% C2 H6 /He at 50 ◦ C.

3.3.3. Temperature-programmed reaction of C2 H6 /O2 mixtures The IR spectra of surface species arising from the interaction of a 5% C2 H6 –5% O2 /He gas mixture with Al2 O3 are shown in Fig. 9A. Temperature was raised from 50 to 500 ◦ C with a heating rate of 5 ◦ C/min and spectra were recorded at various temperatures. At low temperature (50–100 ◦ C), bands at 1640 and 1230 cm−1 arising from olefinic C=C and CHx stretching vibrations [14,19] indicate the formation and strong adsorption of ethylene on alumina. The appearance of these bands was accompanied by the decrease of the terminal –OH groups of the support, as indicated by the negative band at 3761 cm−1 , and formation of hydrogen-bonded hydroxyls (broad band at 3540 cm−1 ) (see Fig. 9B). As the temperature increased, the bands assigned to hydrocarbon species progressively disappeared and strong bands appeared around 1590, 1460 and 1350 cm−1 , attributed to contributions of νCOO asymmetric and symmetric stretchings and δCH bending vibrations of carboxylates, i.e., surface acetate (1580, 1460 and 1355 cm−1 ) and formate species (1590, 1355 and 1370 cm−1 ) [20,21]. At temperatures above 350 ◦ C, the formation of surface carbonates is evidenced by a shoulder at 1650 cm−1 , assigned to bidentate carbonates [22] and bands at 1550, 1465 and 1360 cm−1 . The bands at 1550 and 1360 cm−1 are assigned to the symmetric and asymmetric O–C–O stretching of unidentate carbonates, while the band at 1465 cm−1 is due to the symmetric O–C–O stretching of bicarbonates [23]. The 3800–3400 cm−1 region of the IR spectra at temperatures higher than 200 ◦ C (Fig. 9B) exhibited additional negative bands at 3721 and 3677 cm−1 , with more pronounced decrease of the bands at 400 and 500 ◦ C. The formation of bicarbonates involves the surface

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1600 1400 -1 Wavenumber, cm

1200

Fig. 9. IR spectra during exposure of Al2 O3 to 5% C2 H6 –5% O2 /He from 50 to 500 ◦ C: (A) in the carbon–oxygen frequency; (B) in the hydroxyl region.

hydroxyls, as demonstrated by studies on the adsorption of CO2 on alumina [24], and thus, is associated with the hydroxyl consumption at high temperatures. Note that the band at 3540 cm−1 disappeared above 300 ◦ C and a new band at 3650 cm−1 was formed, the intensity of which increased 1510 1650 1705 1590

2000 0.12

Absorbance

interact only weakly with most metal oxide catalysts [18], the absence of bands affiliated with adsorbed species may be either due to their low concentration, or their overlap with bands of gaseous C2 H6 .

3700

1570 1465

0.08

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1700 1500 -1 Wavenumber, cm

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Fig. 10. IR spectra during exposure of 20 wt.% MoO3 /Al2 O3 catalyst to 5% C2 H6 –5% O2 /He from 50 to 500 ◦ C.

E. Heracleous et al. / Applied Catalysis A: General 264 (2004) 73–80 H2C CH2

H3C CH3

H O

79

H2C CH2

- ßH CH3 CH2 O

COx H O

-α H

HC O

CH3 H O

O

CH3

H

C

C

O O

O

Scheme 1. Tentative surface mechanism for the ethane oxidative dehydrogenation reaction.

with temperature up to 500 ◦ C. It is attributed to the valence vibration of an OH group of bicarbonates [25]. The spectra obtained during the same procedure with 20MoAl catalyst are presented in Fig. 10. Note that strong bands associated with surface species were not observed at all temperatures studied. Thus, conclusions are based on very weak bands. At 50 and 100 ◦ C, a band at 1705 cm−1 —assigned to aldehydic C=O vibrations—was observed. If the formation of oxygenates from alkanes on metal oxides proceeds through the formation of alkoxides [4], the observation of weakly bound aldehyde implies that the ethoxides can react to aldehyde and a hydroxy group. With increasing temperature the aldehydes are concluded to react to carboxylates (formate/acetate species at 1590, 1570 and 1465 cm−1 ), which transform to surface carbonates at 400 and 500 ◦ C with broad bands centered on 1650 and 1510 cm−1 . The hydroxyl region of the spectra exhibits a broad band at 3460 cm−1 indicating the generation of water on the catalytic surface, which disappears at temperatures over 100 ◦ C. The downward shift of the vibrational band of the terminal Mo=O bonds with increasing temperature is related to the anharmonicity of the vibration and the constant integral absorbance indicates hardly interaction with surface species. We speculate that no significant reduction of the Mo species takes place in the presence of oxygen in the reaction mixture, in contrast to the decreased absorbance observed in ethane reducing atmosphere.

or strongly bound to the catalytic surface. In the case of alumina (unselective route), ethene formed is strongly bound to the surface and is oxidized to acetates/formates, the precursors for the formation of COx . With the molybdena catalyst, ethene is able to desorb, because the lower acid strength (compared to alumina) permits desorption of the olefin. The unselective route to COx occurs also over the molybdena catalyst via the further oxidation of ethoxide to acetaldehyde, which transforms to acetates/formates at the relatively high temperatures of the ODH reaction. The aldehydic intermediates observed on the catalyst could also stem from secondary reactions of ethene, i.e., that the ethoxide–ethene transformation is reversible. The extent of the side reactions to COx is very small, as evidenced by the very weak bands corresponding to acetate/formate species on the 20MoAl catalyst (Fig. 10) compared with the strong bands observed over alumina (Fig. 9A). The data presented indicate that the activation of ethane proceeds via labile (not directly observed) surface ethoxides. On the alumina support the subsequent reactions lead to strong bonding of the surface species and finally to carboxylates and carbonates, which decompose to CO and CO2 . The weaker interaction with the molybdena surface allows desorption of ethene and leads, therefore, to high ethene selectivity.

Acknowledgements 4. Conclusions The surface species described allows to formulate a tentative surface mechanism for the oxidative dehydrogenation of ethane over molybdena catalysts (Scheme 1). The activation of the C–H bond of ethane on the catalytic surface seems to proceed through the formation of alkoxides. The species is not detected directly, as the ethoxy group is highly unstable. Given that oxygenates are formed via alkoxides, the detection of the aldehyde species on the molybdena catalyst at low temperature suggests indirectly the formation of ethoxides. The unstable ethoxide rapidly dehydrates to ethene or is oxidized to acetaldehyde. The formed olefin resulting from a ␤-hydrogen abstraction from the alkoxide can be weakly

The authors acknowledge the Greek–German bilateral program and the GSRT (PENED2001) for the financial support.

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