Mechanistic effects resulting from the cesium-doping of a NiMoO 4 catalyst in n-butane oxidative dehydrogenation

Transcription

1 Applied Catalysis A: General 281 (2005) Mechanistic effects resulting from the cesium-doping of a NiMoO 4 catalyst in n-butane oxidative dehydrogenation Luis M. Madeira a, *, Manuel F. Portela b a LEPAE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, Porto, Portugal b GRECAT (UQUIMAF, ICEMS, Lisboa), Departamento de Engenharia Química, Instituto Superior Técnico, Universidade Técnica de Lisboa, Av. Rovisco Pais, Lisboa, Portugal Received 27 July 2004; received in revised form 17 November 2004; accepted 17 November 2004 Available online 25 December 2004 Abstract In this paper, the catalytic n-butane oxydehydrogenation was studied over two catalysts: undoped and Cs-doped a-nimoo 4. The effects of both reaction temperature and contact time on the catalytic performances were evaluated, providing a further insight about the reaction network. It is shown that 1-butene and 2-butenes isomers are primary products, being butadiene formed through a consecutive dehydrogenation step via any n-butene. Carbon oxides are also secondary products, resulting from the deep oxidation of any hydrocarbon species, but direct oxidation of butane must also be considered, particularly to CO 2. The effects of cesium-doping are an increase in the butenes production with a simultaneous inhibition of the deep oxidation to CO x species, practically not affecting the butenes to butadiene dehydrogenation step. # 2004 Elsevier B.V. All rights reserved. Keywords: Butane; Oxydehydrogenation; Mechanism; Nickel molybdate; Cesium promoter 1. Introduction It is well known that the petrochemical industry s trend points to the direct use of alkanes as raw materials instead of alkenes, due to their greater abundance and lower price [1]. To convert the cheaper paraffins into the industrially important olefins, two main routes have been proposed: direct dehydrogenation and oxidative dehydrogenation (ODH). Recent studies suggest however that the ODH process is more promising [1,2]. The thermodynamics of direct dehydrogenation requires operation at high temperatures, and this has several disadvantages, like the difficulty of controlling undesirable reactions (e.g. cracking of hydrocarbons) and coke formation. Moreover, the conversion in classic dehydrogenation is limited by the thermodynamic equilibrium. In spite of this, ODH of light alkanes is not yet industrially implemented, mainly because no * Corresponding author. Tel.: ; fax: address: mmadeira@fe.up.pt (L.M. Madeira). adequate catalyst was already found and the deep oxidation of both paraffins and specially olefins is thermodynamically favoured, thus limiting the achieved selectivity levels. In the particular case of n-butane oxydehydrogenation, yielding butenes and butadiene as the desired products, several catalytic systems have already been successfully tested [2], being particularly noteworthy the results achieved with the Ni Mo O one [2,3], and especially with the stoichiometric NiMoO 4 catalyst [3,4]. Nevertheless, it was previously found that the performance of this catalyst, namely the selectivity towards dehydrogenation, can be significantly enhanced by doping it with alkali metals [5]. Cesium has shown to be particularly interesting [5], and when changing the promoter loading, a maximum in selectivity was recorded for a surface content of 3% (atomic ratio Cs/Mo = 0.03) [6]. Several kinetic studies, models and theories have been developed concerning the ODH of alkanes, e.g. [7 10], and some have specially focused on butane conversion [11 14]. It is usually accepted that this type of reactions involve X/$ see front matter # 2004 Elsevier B.V. All rights reserved. doi: /j.apcata

2 180 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) complex mechanisms, with series parallel reactions, but numerous aspects of the reaction mechanism still need to be elucidated and consequently the basic factors that determine selectivity are far from being clarified. To better understand some fundamental aspects of the n-butane oxydehydrogenation mechanism, a study was carried out in which the effects of some important parameters (namely contact time and temperature) over catalytic performance were evaluated, using two promising catalysts: undoped and 3% Cs-doped a-nimoo 4. The promoted catalyst was also selected so that the mechanistic effects of alkali metal doping might be clarified. 2. Experimental 2.1. Catalysts preparation and characterization The experiments were performed using two catalysts: (i) pure a-nimoo 4 (alpha denotes the nickel molybdate polymorphic form which is stable from room temperature up to 650 8C, with molybdenum in a distorted octahedral environment), which was prepared by co-precipitation as described by Mazzocchia et al. [15], and (ii) cesium-doped a-nimoo 4. For the preparation of this catalyst, the undoped nickel molybdate sample was used as starting material, which was then submitted to wet impregnation, using cesium nitrate. The sample was finally filtered, dried and calcined in dry air flow in the same conditions as pure NiMoO 4 (2 h at 550 8C). Further details can be found elsewhere [5]. The composition of the catalysts was analysed by inductively coupled plasma spectroscopy (ICP) on a Perkin- Elmer Plasma 400 equipment and by atomic absorption (AA) on a Perkin-Elmer 4100 ZL unit. The specific areas of both catalysts were measured by the BET method using a Micromeretics ASAP 2000 unit, while the X-ray diffractograms were obtained in a Rigaku apparatus using Cu Ka radiation (Ni filter) from 108 to 508, at28/min. The surface composition of the catalysts was analysed by X-ray photoelectron spectroscopy (XPS) using a XSAM 800 (Kratos) equipment operated in the FAT mode with the nonmonochromatic Mg X-radiation (hn = ev). The base pressure in the chamber was in the range of 10 7 Pa. The XPS binding energy (BE) values have been corrected assuming the adventitious C 1s peak to have a BE of ev. The basicity of the catalysts was analysed by temperature-programmed desorption of CO 2 (TPD-CO 2 ). In these runs, and after heating the catalyst (ca. 0.3 g) in He flow until 550 8C to clean the surface, the sample was cooled and then saturated with CO 2 at 30 8C. Purging was carried out during 20 min in a He stream, and finally CO 2 was desorbed by heating at 10 8C/min in a He flow of 1 cm 3 /s. The response was measured on-line using the thermal conductivity detector of a Shimadzu GC-8A gas chromatograph. Finally, the temperature-programmed reduction (TPR) runs involved a pre-treatment at 150 8C during 60 min in argon flow (50 ml/min) followed by reduction of the catalyst (50 mg) from room temperature up to 800 8C with a mixture of 5% hydrogen in argon (60 ml/min). The heating rate was 10 8C/min and on-line gas analysis was also performed with a TC detector. More details and results regarding the characterization of the samples can be found in previous papers [5,6,16,17] Catalytic activity tests For the catalytic runs, a continuous-flow fixed-bed tubular reactor was used (i.d. 20 mm; length 650 mm), which was putted inside a vertical TermoLab electrical furnace. The temperature of this furnace was controlled through a Shimaden SR24 apparatus, the unit being provided with a forced air circulation system. The reactor was made of quartz and was equipped with a coaxially centred thermocouple (from Thermocoax, KI type), connected to a Newport Electronics digital display unit. The catalyst particles (d p < 0.25 mm, for which internal diffusion resistances are absent) were diluted with inert quartz from Aldrich (50 70 mesh) in a catalyst to quartz volumetric ratio of 1:2, while the rest of the reactor was filled with inert Carborundum, from Carlo Erba. In this way, potential gasphase reactions at higher temperatures are minimised. Preliminary experiments changing the gas velocity evidenced that external mass transfer resistances can also be neglected. As reactor feed, a mixture of the hydrocarbon, oxygen and nitrogen was used and the flow rates were adjusted in each run with a mass-flow controller system from Brooks, model 5878, in order to change the contact time. The ratio W/F (where W is the catalyst weight and F the butane flow rate), which is proportional to the contact time, was used. Experiments were performed until steady-state conditions were reached, and the reactor was operated at 1.10 bar (total pressure). The catalytic effects of the contact time and temperature were investigated. For the runs with the unpromoted catalyst, a catalyst charge of g (or g to reach higher contact times) was used, and the experiments were carried out at 460, 500 and 540 8C. For the experiments with the Cs-doped NiMoO 4, a charge of g was used and the reaction was studied at 500, 520, 540 and 560 8C. In both cases the reactants partial pressures employed were: P O2 ¼ 0:10 bar and P butane = 0.05 bar. Some experiments were also performed with 1-butene or cis-2-butene in the reactor feed. The experimental conditions used in both cases were T = 520 8C; 2% of the hydrocarbon and 10% of O 2, for a total flow rate of 11.8 l/h. The remaining conditions were identical to those previously described. In all the experimental conditions employed, for both catalysts and for any hydrocarbon used in the reactor feed (butane or butenes), the only products detected were C 4 s (1- butene, trans-2-butene, cis-2-butene and butadiene), carbon

3 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) oxides (CO and CO 2 ) and water. The formation of oxygencontaining products like aldehydes, ketones or carboxylic acids was always negligible, and the error in the carbon mass balance was smaller than 2%. Blank runs evidenced an insignificant conversion (X < 0.4%), even at the highest temperatures employed. Reactants and products were analyzed with an on-line Shimadzu GC-8A gas chromatograph with two columns, as described elsewhere [5]. 3. Results 3.1. Catalysts characterization The chemical analyses performed showed that the prepared Ni Mo O catalyst has the expected atomic ratio (Mo/Ni = 1.0), for which the recorded X-ray diffractogram (Fig. 1) evidences the typical pattern of the a-nimoo 4 (e.g. characteristic peak of the a-phase located at 2u = JCPDS powder diffraction file card no ). Moreover, the XPS analyses also confirmed the stoichiometric surface atomic ratio for Mo/Ni, in agreement with the chemical analysis. It is noteworthy that after the preparation procedure used to dope the nickel molybdate catalyst with cesium, only traces of alkali metal were detected in the sample (chemical analysis was performed using ICP), and the concentration of Cs on the external surface of the catalyst, determined by XPS, was Cs/Mo = 0.03 (preparation atomic ratio). In addition, the X-ray diffraction pattern recorded for NiMoO 4 remains unchanged with Cs addition. In Table 1 are summarised some of the most important characterization data for both catalysts. It seems that addition of the alkali metal does not change the structure of the NiMoO 4 catalyst, as evidenced by XRD, but remains on its surface, blocking pores and thus decreasing the BET surface area. Moreover, although no major changes in the binding energies (BE) of nickel and oxygen due to the addition of cesium have been noticed, a slight decrease of the BE values for the Mo 3d bands was recorded (Table 1), suggesting an increase of the average electron density of the molybdenum atoms, as was observed in other mixed oxide catalysts with respect to the nucleophilicity of their oxygens. The effect of cesium-doping is particularly remarkable on the nickel molybdate surface basicity (evaluated through integration of the CO 2 -TPD curves), which increases by a factor above 2. The promoter also increases the NiMoO 4 Fig. 1. X-ray diffraction pattern of undoped and doped NiMoO 4 catalyst at room temperature. resistance to reduction, as evidenced by the increase in the temperature of onset of reduction during TPR runs (Table 1) Catalytic experiments with the pure NiMoO 4 catalyst Effect of the contact time The effect of the contact time (W/F) on butane conversion level (X) and on yield (Y) and selectivity (S) to dehydrogenation products (C 4 s) is illustrated in Fig. 2. Although at high W/F values both X and Y C4 s show some curvature, especially the C 4 s yield, they seem to increase linearly at low contact times (thus constant reaction rates). A similar behaviour was also described by Mamedov and Corberán [7], according to whom the alkane conversion into alkenes and carbon oxides should occur on the catalyst surface. This idea was supported by the linear relationship found between the alkane conversion level and the contact time for the MgV and NiVSb systems, suggesting the absence of contributions from homogeneous (gas phase) reactions. Extrapolation of our X versus W/F fitting curves for zero contact time yields a null conversion value, even at the highest temperature, in agreement with the results obtained in the blank runs, mentioned above. In what concerns the selectivity towards dehydrogenation products, Fig. 2C shows that it decreases when increasing the contact time, due to overoxidation of the olefins to carbon oxides at high conversion levels. A similar trend was observed by Stern and Grasselli, who used NiMoO 4 in propane oxydehydrogenation [18]. They found that propylene selectivity decreases with the propane conversion due to consecutive reactions, with overoxidation of the propylene to carbon oxides. As regards the product distribution, Fig. 3 shows quite different trends. While the carbon oxides formation is favoured by an increase of contact time, for 1-butene and 2- butenes an opposite trend is noticed. Finally, the effect of W/ Table 1 Main characterization data for undoped and Cs-doped nickel molybdate Sample S BET (m 2 g 1 ) XPS binding energies (ev) CO 2 -TPD area (a.u.) T a onset (8C) Mo 3d 5/2 Mo 3d 3/2 NiMoO %Cs-NiMoO a Temperature of onset of reduction.

4 182 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) Fig. 2. Effect of the contact time on the butane conversion level (A) and on the yield (B) and selectivity (C) to dehydrogenation products with the NiMoO 4 catalyst. (&) 460 8C; (*) 500 8C; (~) 540 8C. Experimental conditions: W = or g; P C4 H 10 ¼ 0:05 bar; P O2 ¼ 0:10 bar. F on the selectivity towards butadiene depends on the temperature: at 540 8C the trend is similar to that found in the formation of any C 4 product, i.e., selectivity decreases with the contact time; at lower temperatures a maximum in selectivity is observed, defining a range of W/F values where transition of behaviour occurs Effect of the temperature As expected, the butane conversion level increases with the reaction temperature (Fig. 2A). The yield to dehydrogenation products shows a similar behaviour (Fig. 2B), although the increase is not so pronounced because the reaction temperature has a negative effect on the C 4 s selectivity, at equal W/F values (Fig. 2C). The temperature increase is particularly beneficial for the formation of carbon oxides (especially CO), or for butadiene production, at low contact times (Fig. 3). In all the experimental conditions tested the selectivities towards 1- butene, trans-2-butene or cis-2-butene decrease with the reaction temperature (Fig. 3), this effect being a consequence of the higher conversion, as discussed below. The results herein presented evidence for 1-butene and 2- butenes a character of primary products, while carbon oxides are essentially secondary products, resulting from the complete oxidation of any hydrocarbon present in the system. The role of butadiene in the reaction network will be discussed later on Catalytic experiments with the Cs-doped NiMoO 4 catalyst Effect of the contact time Fig. 4 shows the butane conversion levels and the yields and selectivities to dehydrogenation products as a function of the contact time, for the 3% Cs-NiMoO 4 sample. Once this catalyst is much less active than pure nickel molybdate (BET surface area is considerably smaller, as shown in Table 1), higher contact times (and temperatures) were used with the alkali-doped sample (W/F values in the range g h/mol butane, Fig. 4) in comparison with those employed for the undoped one (W/F values between 3.1 and 15.9 g h/mol butane, Fig. 2). Even so, the butane conversion levels achieved and reported in Fig. 4A were much below those obtained with a-nimoo 4 (see Fig. 2). Once again, both conversion and yield to C 4 s increase with W/F, while selectivity to dehydrogenation shows the opposite trend (Fig. 4). It is remarkable that at the highest contact time and highest temperature tested the Cs-doped catalyst exhibits a selectivity to C 4 s of almost 80%, and this performance was only reached with pure NiMoO 4 at the

5 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) Fig. 3. Effect of the contact time on the selectivity for the reaction products with the NiMoO 4 catalyst. (&) 460 8C; (*) 500 8C; (~) 540 8C. Experimental conditions identical to those of Fig. 2. Fig. 4. Effect of the contact time on the butane conversion level (A) and on the yield (B) and selectivity (C) to dehydrogenation products with the 3%Cs- NiMoO 4 catalyst. (*) 500 8C; (&) 520 8C; (~) 540 8C; () 560 8C. Experimental conditions: W = g; P C4 H 10 ¼ 0:05 bar; P O2 ¼ 0:10 bar.

6 184 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) Fig. 5. Effect of the contact time on the selectivity for the reaction products with the 3%Cs-NiMoO 4 catalyst. (*) 500 8C; (&) 520 8C; (~) 540 8C; () 560 8C. Experimental conditions identical to those of Fig. 4. lowest contact time and temperature studied (Fig. 2C). Such performance is attributed to the higher basicity of the Cs- NiMoO 4 catalyst surface, evidenced by the CO 2 -TPD experiments (Table 1). The direct relationship found between surface basicity and selectivity can be accounted for by the electron-donating character of olefinic species and the consequent easier desorption from a more basic surface, thus preventing further overoxidation into carbon oxides. When the selectivity to each product is plotted versus W/F, different trends are noticed, once again (Fig. 5). While the selectivities to 1-butene, trans-2-butene and cis-2-butene decrease with the contact time, for butadiene, CO and CO 2 the opposite behaviour is found Effect of the temperature One can see, in Fig. 4, that an increase in the reaction temperature leads to an increase of both the butane conversion level and yield to C 4 products, showing a negative effect in selectivity for dehydrogenation. Besides, it must be pointed out that the selectivities towards 1-butene and 2-butenes tend to decrease with the temperature (for the same contact time), while for the other compounds (butadiene, CO and CO 2 ) selectivity increases with the reaction temperature (Fig. 5). It is also noteworthy that at 500 8C selectivities to CO and butadiene become null at low contact times, while for CO 2 selectivities are always above 3%. This suggests that CO and butadiene are essentially secondary products, i.e., they are formed from 1-butene and/or 2-butenes, while CO 2 may also result directly from butane. This fact is also supported by previous results obtained without oxygen in the reactor feed stream [19]. In such experiments a significant decrease in the butane conversion was noticed, probably due to deep catalyst reduction and/or coke formation. However, a small production, perfectly measurable, of butenes and CO 2 was still recorded. After introducing O 2 in the gas phase, conversion increased again and both CO and butadiene were now formed with high yields Experiments with 1-butene and cis-2-butene To try to clarify the pathway that leads to butadiene, which can derive from 1-butene and/or 2-butenes previously formed, as well as the possible existence of isomerization reactions, some experiments were performed feeding the reactor with 1-butene or cis-2-butene. However, due to the different surface conditions and the different reactivity of n- butane, 1-butene and cis-2-butene, the distributions of products obtained in these runs can be also quite different. The results obtained are shown in Table 2. A significant conversion level was obtained with both hydrocarbons, because they are more reactive than butane. For 1-butene a higher conversion was achieved, but such greater reactivity only leads to a larger production of carbon oxides. Similar butadiene yields were recorded with 1-butene and cis-2- butene and, therefore, it seems that a preferential pathway for its formation does not exist. Important information was

7 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) Table 2 Conversion and yields recorded with the 3% Cs-NiMoO 4 catalyst when feeding the reactor with 1-butene or cis-2-butene a Hydrocarbon reagent 1-Butene cis-2-butene Conversion (%) Yield (%) CO 14 7 CO Butene 6 trans-2-butene 5 5 cis-2-butene 6 Butadiene a Experimental conditions: T = 520 8C; 2% of hydrocarbon and 10% of O 2 in the feed; W = 0.3 g; W/F = 31.1 g h/mol hydroc. also obtained regarding the isomerization reactions. Indeed, in both cases a low isomerization degree was found between 1-butene and 2-butenes. In addition, when cis-2-butene was fed to the reactor, a small trans-2-butene production was obtained. A low isomerization degree between butenes was noticed. 4. Discussion The catalytic results obtained, with variation of the contact time and reaction temperature, seem to suggest that, in the experimental conditions used, 1-butene and 2-butenes are the main primary products of this reaction. The differences recorded with the two catalysts, undoped or cesium-doped NiMoO 4, concern the selectivity to butadiene. Indeed, the results obtained with the Cs-doped catalyst evidence a secondary character for butadiene because its selectivity increases with both W/F and temperature (Fig. 5). With pure a-nimoo 4 (Fig. 3) such behaviour is only found at low contact times and low temperature (460 8C). At this temperature and for W/F values above 6.2 g h/mol butane butadiene selectivity decreases, and this trend is observed at any contact time for the higher temperatures (particularly at 540 8C). This apparent different behaviour between the two catalysts can be due to their different activity. In fact, the overall analysis of Figs. 2 5 shows that at low conversions the increase of the contact time and temperature leads to an increase in the butadiene selectivity. At high conversions selectivity decreases when increasing the contact time, certainly due to the overoxidation to CO or CO 2. It should be noted, however, that other authors, working with vanadium and magnesium oxides, suggest that butadiene formation as a secondary product from desorbed and readsorbed butenes is not the only way it is formed [11,12]. In fact, the formation of butadiene as a primary product must also be taken into account. The secondary products are then butadiene (which results from the subsequent dehydrogenation of the previously formed butenes) and carbon oxides (which result from the complete oxidation of the hydrocarbon species present in the reaction). The selectivity versus conversion plots, at constant temperature, allow the investigation of the cesium-doping Fig. 6. Effect of the butane conversion on the selectivity for the reaction products with the NiMoO 4 ((*) 500 8C; (~) 540 8C) and 3%Cs-NiMoO 4 ((*) 500 8C; (D) 540 8C) catalysts. Experimental conditions: P C4 H 10 ¼ 0:05 bar; P O2 ¼ 0:10 bar.

8 186 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) effects in the reaction mechanism. Data shown in Fig. 6 evidence that, as expected, the selectivity towards carbon oxides increases in both catalysts with the butane conversion level (by increasing the contact time), and this trend is directly related with the evolution of the 1-butene, trans-2- butene and cis-2-butene selectivities, which exhibit the opposite behaviour. Moreover, the positive effect of cesiumdoping on the selectivity towards butenes is accompanied by an inhibition of CO x formation. Regarding butadiene, one can see that at low conversion levels selectivity increases, decreasing later on for conversions above ca. 5%. It is also worthy of note that with the Cs-doped catalyst, extrapolation to zero conversion results in a CO 2 selectivity different from zero, evidencing that it might also result directly from butane, what corroborates the findings mentioned above (see Section 3.3.2). In addition, the CO to CO 2 ratio increases in both catalysts with the n-butane conversion, suggesting that the parallel reaction of butane oxidation leads to both carbon oxides, while the consecutive oxidation of butenes and butadiene evolves mainly to CO. Fig. 7A shows that the selectivity towards dehydrogenation decreases with the butane conversion level, which is typical in ODH reactions. In this figure it is also visible that at the same temperature, and at equal conversion level, the doped catalyst is much more selective to C 4 s than pure NiMoO 4. For instance, while for a butane conversion of 6% and at 540 8C the selectivity towards dehydrogenation is around 61% for NiMoO 4, with the 3% Cs-NiMoO 4 sample it reaches a value of 85%. It is also noteworthy that under isoconversion conditions, selectivity to ODH products increases with temperature, particularly for the undoped catalyst. A similar behaviour was also commonly found with other catalytic systems, for instance with V-based catalysts [20]. According to Nieto et al., operating conditions that facilitate the redox activity of the catalyst (the selective process) should result in a higher selectivity to the desired products [20]. Therefore, a higher reaction temperature results in a higher selectivity to dehydrogenation products, as it strongly promotes the redox processes on the catalyst. For the Ni Mo O system, the above-mentioned tests performed in the absence of oxygen in the reactor feed, which showed conversion of butane into C 4 s with high selectivities, allowed to put in evidence that lattice oxygen plays a crucial role in selectivity [19]. The involvement of lattice oxygen in the oxidation reaction, i.e. the existence of a redox mechanism, was also recently evidenced using Fig. 7. Effect of the butane conversion on the selectivity to dehydrogenation products (A), on the 2-butenes/(1-butene + butadiene) ratio (B) and on the 1-butene/cis-2-butene ratio (C) with the NiMoO 4 ((*) 500 8C; (~) 540 8C) and 3%Cs-NiMoO 4 ((O) 500 8C; (D) 540 8C) catalysts. Experimental conditions identical to those of Fig. 6.

9 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) Table 3 Rates of butane conversion and of products formation (equivalent of converted butane) over the two tested catalysts and respective ratios Catalyst T (8C) W/F (g h/mol butane ) r i 10 5 (mol/(h m 2 )) r butane r co r CO2 r 1-butene r trans-2-butene r cis-2-butene r butadiene NiMoO %Cs-NiMoO NiMoO 4 3%Cs NiMoO n.d n.d n.d. not defined. several techniques, particularly in situ electrical conductivity [21]. Moreover, a clear increase of the nickel molybdate resistance to reduction after Cs addition was noticed (Table 1), and this is well correlated with the smaller activity of the Cs-doped nickel molybdate catalyst, thus also supporting a redox-type mechanism. Besides this effect on activity, the different properties of both solids might also have an important impact in selectivity. It was suggested that undoped a-nimoo 4 (an n-type semiconductor) has possibly a higher concentration of weakly adsorbed oxygen species [21], which may account for the lower selectivity in oxydehydrogenation because the presence of those species is usually associated with total oxidation whereas lattice oxygen is crucial for selective oxidation. Obviously, selectivity will be also governed by the catalytic surface s basic nature. Fig. 7B presents the effect of butane conversion on the 2-butenes/(1-butene + butadiene) ratio. Such parameter decreases with X because the effect of the butane conversion on the selectivity towards 2-butenes (negative effect) and butadiene (positive or negative effect, depending on X) exceeds that in 1-butene. Curiously the 2-butenes/(1- butene + butadiene) ratio seems to be independent of the catalyst used. Other authors found that such ratio decreases with the basic character of the catalyst, because isomerization of olefins becomes more important on acid sites [22]. Nevertheless, with our catalysts, the ratio does not differ significantly from one catalyst to another. The ratio 1-butene/cis-2-butene also seems to be independent of the catalyst used and decreases with the butane conversion (Fig. 7C), pointing for a higher extent of 1-butene oxidation compared with cis-2-butene (or trans-2- butene, once the ratio between trans- and cis-isomers is practically unaffected by the conversion level). The decrease of the ratio shown in Fig. 7C results from the higher reactivity of 1-butene, as shown in Table 2. Some recorded data at low conversions allowed us to compare also the catalytic performances achieved by both catalysts in terms of reaction rates per unit surface area. Data obtained are shown in Table 3. At a first glance it is evident that Cs-doping leads to much smaller butane conversion rates (r butane = X/(W/FS BET )), under equal reaction conditions. As butane conversion in these catalysts seems to proceed by a redox or Mars van Krevelen mechanism [21], such effect can be ascribed to the change in the solid reducibility by Cs-doping (see TPR results Table 1). Nevertheless, the effect of the alkali metal is much more significant in the reduction of the carbon oxides formation rates, which in some cases decreases by almost two orders of magnitude as compared with pure NiMoO 4 (Table 3). The reaction rates for 2-butenes decrease only slightly after Cs-doping or, in some cases, even increase (ratio of reaction rates NiMoO 4 /3%Cs NiMoO 4 < 1). For butadiene, the decrease in the reaction rate is of the same order of magnitude as that found for butane. These results suggest that the largest selectivity to C 4 s achieved with the doped catalyst is due to the decrease of production of carbon oxides, with a simultaneous increase in dehydrogenation rates. It is noteworthy that this effect is more pronounced in the case of 1-butene and 2-butenes, being almost imperceptible for butadiene. In this regard we have also noticed, in a kinetic study previously reported [17], that for 1-butene and 2-butenes the partial order with respect to butane increases about twice after doping the molybdate with Cs. In the case of butadiene such effect is much less significant, while for carbon oxides a significant decrease of that parameter was observed [17]. Therefore, in the series parallel network proposed and represented in Scheme 1, it seems that the effect of doping the nickel molybdate catalyst with cesium is the promotion of the processes that lead to butenes (1) while simultaneously reducing/inhibiting those that lead to CO x formation (2), practically not affecting the dehydrogenation pathways to butadiene (3). The inhibition of the carbon oxides formation can be ascribed to (i) the easier desorption of the formed olefins from a more basic surface and/or (ii) a decrease of the amount and electro-

10 188 L.M. Madeira, M.F. Portela / Applied Catalysis A: General 281 (2005) Scheme 1. Reaction network proposed (black arrows main reaction pathways). negative character of the oxygen associated with deep oxidation sites. While possibility (i) is corroborated by the CO 2 -TPD experiments (Table 1), hypothesis (ii) has been suggested based on the results obtained using in situ electrical conductivity measurements with both solids [21], although no shifts in the oxygen binding energies were detected by XPS after doping. It is also reasonable to consider that two types of active sites are involved in the overall process, selective and non-selective, as suggested by other authors [13,23]. Considering this dual-site approach, the addition of Cs seems to affect both types (once it increases butenes production and decreases deep oxidation rates), with an overall positive effect in selectivity. It is worth mentioning that Blasco et al. [24] proposed two different networks for butane ODH, depending on the acid base character of the supported vanadium catalyst used. They found that both distribution of C 4 -olefins and selectivity to dehydrogenation products depend strongly on the acid base character of the catalyst surface. Catalysts with a basic nature favour initial mono-olefins formation and its subsequent conversion to butadiene (the main product of consecutive reactions), and a network identical to that shown in Scheme 1 was proposed. On the other hand, on catalysts with an acid character both mono- and di-olefins are initially formed (with a high content of 2-butenes), being carbon oxides the only secondary products, formed mainly by consecutive reactions [24]. 5. Conclusions The main results of a systematic study were presented, in which some parameters (namely the contact time and temperature) were studied over a wide range of experimental conditions. The study was focused on the catalytic oxidative dehydrogenation of n-butane using two catalysts: undoped and cesium-doped a-nimoo 4. It is widely accepted that the parallel reactions that occur on oxydehydrogenation processes have a common surface intermediate formed by alkane activation, possibly the alkyl species, which is formed by dissociation of a C H bond on a secondary carbon atom [1,22]. A second H-abstraction (b elimination) leads to formation of butenes, which may desorb or undergo further oxidation to butadiene and/or CO x. Although some authors suggest the existence of subsequent gas phase reactions, e.g. [25], our results evidence that alkane conversion to either alkenes or carbon oxides occurs on the catalyst surface. Moreover, the catalytic runs were performed at relatively low temperatures (in the range C), at which gas phase reactions are not significant. It was shown that both 1-butene and 2-butenes are primary products, formed possibly via the butyl species, while butadiene and carbon oxides are essentially secondary products. We could not obtain conclusive information regarding the preferential provenience of butadiene when feeding the reactor with 1-butene or cis-2-butene. Therefore, in the reaction mechanism proposed it is assumed that butadiene can be formed through further dehydrogenation of any butene, though its direct formation from butane cannot be ruled out. Formation of carbon dioxide directly from butane must also to be considered. The mechanistic effects of doping the NiMoO 4 catalyst with an alkali metal (cesium) are the promotion of the first reaction step, butane! butenes, while simultaneously inhibiting the deep oxidation to carbon oxides. 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