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MgO catalysts
Applied Catalysis A: General 309 (2006) 10–16 www.elsevier.com/locate/apcata
Effect of additives on properties of V2O5/SiO2 and V2O5/MgO catalysts I. Oxidative dehydrogenation of propane and ethane A. Klisin´ska, K. Samson, I. Gressel, B. Grzybowska * Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Krako´w, Poland Received 18 July 2005; received in revised form 23 March 2006; accepted 6 April 2006 Available online 5 June 2006
Abstract Oxidative dehydrogenation, ODH of propane and ethane was studied on VOx/SiO2 (VSi) and VOx/MgO (VMg) catalysts of the same vanadium loading (1.5 theoretical monolayer of vanadia) and doped with additives of main group elements (K and P) and of transition metal ions (Ni, Cr, Nb, and Mo). The additives modify both the specific activity and selectivity to olefins, the modifying effect (its sign and extent) depending on the type of the catalyst and on the alkane nature. The main difference between the two series of the catalysts consist in: (a) increase in the specific activity in the ODH reactions on introduction of the additives (except K) for VSi and decrease for VMg catalysts, (b) considerable increase in the selectivity to propene in ODH of propane for K-doped VSi catalyst and decrease for K-doped VMg catalyst (Ni, Cr, and Mo increasing and P decreasing the selectivity for the both series), and (c) higher selectivities to ethene as compared with those to propene for VSi and lower to ethene than to propene for VMg catalysts. In contrast to the propane ODH the additives exert only very small effect on the selectivities to ethene in the ODH of ethane. # 2006 Elsevier B.V. All rights reserved. Keywords: Vanadia catalysts; Additives; Oxidative dehydrogenation of propane and ethane
1. Introduction Vanadia-based catalysts active and selective in oxidation reactions of various hydrocarbons mainly to oxygenated products, in particular to organic anhydrides [1], have been found also promising for oxidative dehydrogenation, ODH of lower alkanes, in particular of propane [2,3]. The ODH reactions of lower alkanes have attracted in the last decade much interest as an alternative to classical dehydrogenation, DH for production of olefins [4–6]. They permit to avoid such inconveniences of DH as low equilibrium constants, deactivation by the coke formation and hence necessity of the catalyst regeneration, and the endothermicity of the reaction. On the other hand ODH reactions are accompanied by formation of considerable amounts of undesirable carbon oxides, produced mainly by consecutive oxidation of olefins, and – to some extent – by parallel to ODH total oxidation of alkanes. This problem – general for all selective oxidation reactions – is even more acute for ODH, since olefins (except ethene) – intermediate products in the chain of hydrocarbon oxidation from hydrocarbons to the most favoured
* Corresponding author. Tel.: +48 12 639 5100; fax: +48 12 425 1923. E-mail address: [email protected] (B. Grzybowska). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.04.028
thermodynamically carbon oxides – are more active than the substrate alkanes. Catalytic properties of oxide catalysts can be improved by introduction of additives. The patent literature provides ample examples of additives of various chemical nature introduced to industrial catalysts, the fundamental studies on mechanism of their action are, however, scarce. The brief review of academic research on the problem [7] demonstrates the variety of effects of the additives on structure, physicochemical and catalytic properties of oxide catalysts in selective oxidation reaction, and show a need for systematic studies on the problem. Most of the to date studies have been concerned with alkali metal additives. For the ODH reactions of lower alkanes alkali metal additives have been found to improve the selectivity to propene in the ODH of propane on V2O5/TiO2, MoO3/TiO2 [8,9], V2O5/ Al2O3 [3] and MoO3/MgO–g-Al2O3 [10] catalysts, the effect being explained by modification of acid–base properties of the catalysts by alkalis. On the other hand the K additive was found to decrease the selectivity to olefins in ODH of propane and butane on catalysts based on VMgO system [11–13]: no explanation has been given for this fact. Moreover, for the same catalysts the effect of alkalis may depend on the length of a hydrocarbon chain. Lo´pez Nieto et al. reported that in ODH of ethane and propane on VOx/Al2O3 catalysts the K additive
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decreased the selectivity to ethane and increased that to propene [3,14]. Little is known about the effect of other additives, in particular of transition metal ions, which may modify other – beside acidobasic – properties, such as M–O bond strength on the surface or in the bulk of oxide catalysts, and the type of the surface oxygen species. Among non-alkali additives, Fe, Ca, Al in vanadia– titania system were found to increase the selectivity to propene in the propane ODH, whereas W decreased it [15]. The effect was ascribed to the change of electronic properties (work function), which may control the type of oxygen species. Additives of Bi, Mo, K, and P to vanadia–alumina system modified redox properties of the catalysts; the activity in ODH of propane increased with the decrease in the reducibility [16]. Addition of Mo [17] and of Mo, W, B, Al, Ga, Sb [18] to VMgO system was found to increase the selectivity to olefins and dienes in ODH of n-butane (with the exception of W). Mo added to VMgO system increased the selectivity to styrene in ODH of ethyl benzene, whereas Cr and Co doping increased the activity [19]: no clear explanation of these effects was, however, provided. We have undertaken studies on the effect of additives of various nature on catalytic and physicochemical properties of vanadiabased catalysts. Two series of the catalysts have been selected, containing vanadia on supports of different nature: (a) SiO2, weakly interacting with the dispersed vanadia phase, for which crystalline vanadia of relatively high acidity is observed even at low V loading [2], and (b) MgO, which can react with vanadia [2], and for which lower acidity of vanadium centres could be expected. The additives represent both main group elements (K and P) and transition metals of different valency (Ni, Cr, Nb, and Mo). In selection of the second group of additives (introduced to the catalysts in the ionic form), we adopted a simple hypothesis, assuming, that the cationic additives of lower electronegativity (lower valency) than vanadium would shift the electron density towards the V–O active centre, rendering it less acidic and more basic (in the Lewis sense), whereas the additives of the higher than vanadium electronegativity (higher valences) should have a reverse effect. Besides acido-basic properties modification of the electron density around the active V–O centre may affect also redox properties. It is generally recognized that both acidobasic and redox properties (often interrelated), influence catalytic performance in selective oxidation reaction [20–22]. In the present work (part I) these two series have been examined in ODH of propane and ethane. In part II [23], we will describe the effect of the additives on the structure and physicochemical properties of the two series of the catalysts and will attempt to correlate them with the catalytic performance. The molecular aspect of the effect of the additives to vanadia-based catalysts has been approached by quantum chemical calculations [24]. 2. Experimental 2.1. Catalysts The V2O5/SiO2 (VSi) catalysts were prepared by impregnation with aqueous solution of ammonium metavanadate of a
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commercial silica support (Aerosil 200), evaporation of the solute, followed by drying for 12 h at 120 8C and calcination under a flow of air for 5 h at 500 8C. Before impregnation the support was pre-treated in water at room temperature and dried at 90 8C for 12 h: its specific surface area after such treatment was 175 m2/g. The additives (A) were introduced by adding appropriate amounts of their soluble salts or acids [KNO3, Ni(NO3)2, Cr(NO3)3, H3PO4], ammonium paramolybdate, and niobia dissolved in oxalic acid to the metavanadate solution. All the reactants used were of p.p.a. grade. The atomic ratio A/V in the calcined catalysts was 0.2 with the exception of K, for which K/V was 0.1. The K/V ratio of 0.1 was shown in previous studies as optimal for VOx/SiO2 [25] and VOx/TiO2 catalysts [26], the selectivity to propene decreasing slightly at higher K content. The choice of the A/V ratio for other additives was arbitrary: we adopted A/V of 0.2 basing on the literature data, which show that evident modification of properties (e.g. shift of Raman frequencies of V O bonds [27,28] or of reducibility [16]) are observed at A/V ratios higher than 0.1. On the other hand we wanted to avoid formation of mixed A–V–O bulk compounds e.g. V–Nb–Mo, which may lead the alkane reactions to oxygenated products. The V2O5/MgO (VMg) catalysts were prepared in the same way, using commercial MgO (Ubichem, 72 m2/g). The content of vanadia in the samples corresponded to 1.5 theoretical monolayers of V2O5 per the support surface, and was calculated with the assumption that one monolayer contains 10 V atoms per 1 nm2 of the support. The symbols of the samples, adopted further in the text, are VSiA, or VMgA for respectively vanadia–silica and vanadia–magnesia catalysts, where A is the additive. Additionally samples without vanadium, containing only the additives (the same amounts as in VSiA and VMgA) on SiO2 and MgO supports, were prepared and tested in the same conditions. Table 1 gives a list of the catalysts, their specific surface area obtained with the BET method and the Table 1 List of samples of V2O5/SiO2 and V2O5/MgO catalysts List of catalysts
SSA (m2/g)
V2O5 (wt.%)
Additive (wt.%)
xia of additives
VSi VSiK SiNi VSiCr VSiNb VSiP VSiMo
116.6 92.2 110.8 109.1 104.2 70.2 76.8
28.4 28.0 27.4 27.5 26.8 27.9 26.8
– 1.2 3.5 3.2 5.5 1.9 5.7
17.9b 2.5 9.6 11.6 17.6 24.1 19.4
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
79.7 135.6 80.3 63.4 164.4 175.1 155.7
14.1 14.0 13.8 13.9 13.7 13.8 13.7
– 0.6 1.8 1.6 2.8 1.0 2.9
17.9b 2.5 9.6 11.6 17.6 24.1 19.4
a Approximate value of xi electronegativity of introduced ions calculated from xzþ Me ¼ ð1 þ 2zÞxMe , where xMe is an electronegativity of an atom and z its valency. b V5+.
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approximate values of the electronegativities of the additive ions. For the VSi series the specific surface area of the catalysts, SSA was lower than that of the pure SiO2 support (175 m2/g). The presence of the additives led to the further decrease of SSA, the effect being the biggest for the P, Mo and K additives. Thus, both vanadia and the additives lead to the sintering of the support. For the VMg series SSA was, with the exception of VMgCr, higher than that of the MgO support (72 m2/g), the presence of additives (in particular of P, Mo, Nb and K) leading to a considerable increase in SSA. It can be mentioned, that similar increase of the specific surface area was observed in some cases for vanadia [29– 31] and chromia [32] deposited on MgO. It was found [32], that the treatment of magnesia with water, followed by evaporation and drying in the same conditions as those in the preparation of the catalysts, led to the formation of microcrystalline Mg(OH)2, as verified by XRD: on calcination the magnesium hydroxide decomposed into MgO which had a higher specific surface area than the original support.
studied reactions (conversions at 500 8C < 2%). For Cr and Ni samples without vanadium, the activity was higher, but the main product of the reactions was CO2. At 450 8C, the temperature and which the vanadia catalysts were compared further in the text, conversions of propane were about 15 and 10% for CrSi and NiSi samples, with selectivities to propene of 15 and 13%, respectively, and 13 and 6% for CrMg and NiMg, with the propene selectivities of about 4% for both Cr and Ni samples on MgO. 3. Results and discussion 3.1. ODH of propane
2.2. Catalytic activity measurements
Fig. 1 presents the variations of the propane conversion with the reaction temperature, for some of the catalysts of VSiA and VMgA series. For both series of the catalysts the increase of the conversion with the temperature is usually accompanied by the decrease of the selectivity to propene and the increase of the selectivity to carbon oxides. Fig. 2 illustrates the changes of the
The activity of the catalysts in oxidative dehydrogenation of propane and ethane was measured in a fixed bed flow apparatus in the temperature range 360–520 8C for propane and at 500 8C for ethane. A stainless steel reactor (120 mm long, internal diameter 13 mm) was coupled directly to a series of gas chromatographs. Propene or ethene and carbon oxides (CO and CO2) were found to be the main reaction products. The amounts of the degradation, C2 products and of oxygenates were below 1% of the total amount of products. The reaction mixture contained 7.1 vol.% of propane and 17 vol.% of ethane in air. 0.5 ml (0.25–0.30 g for VSiA, and 0.30–0.40 g for VMgA series) for propane reaction and 1 ml for ethane of a catalyst sample of grain size 0.63–1 mm, diluted with quartz beads, was used. The composition of the reaction mixture was controlled with the mass flowmeters. Since the selectivity to olefins decreased with the increasing conversion (a typical behaviour for the ODH reactions on oxide catalysts [2–5,33]) the selectivities at iso-conversions were compared. To obtain the same conversions for a series of catalysts of different activities, the total flow of the reaction mixture at a given reaction temperature varied between 20 and 120 ml/min, which corresponded to the contact time t = 0.2–1.5 s. On the other hand the flow rate (contact time) had to be adjusted for some catalysts to keep the conversions below 20%: above these values the conversions were limited by the high consumption of oxygen. The carbon balance for conversions higher than about 10% was better than 97 2%. At lower conversions the balance was poorer and hence the selectivities to different P products were calculated from the formula: Si ¼ ci = ci , where ci are concentrations of products i. The total conversion Xp was calculated as: Xp ¼ c0p cp =c0p , where c0p and cp are the concentrations of propane at the inlet and outlet of the reactor, respectively. The pure supports were not active in the studied conditions, the total propane conversion not exceeding 1% and that of ethane 0.5% at 500 8C. The samples without vanadium containing K, P, Mo, and Nb were very little active in the
Fig. 1. Changes of the propane conversion (at t = 0.5 s) with the reaction temperature for VSiA (a) and VMgA (b) catalysts.
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Table 2 Total activity in ODH of propane and ethane on of V2O5/SiO2 and V2O5/MgO catalysts Catalyst symbol
Propanea
Ethaneb
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo
0.70 0.09 0.71 0.74 0.92 3.57 1.61
0.85 0.17 0.99 1.15 1.08 5.61 1.88
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
0.87 0.24 0.66 0.67 0.40 0.45 0.40
1.85 1.11 1.81 2.14 1.86 1.17 1.38
a b
Fig. 2. Changes of the selectivity to C3H6, CO and CO2 in propane–oxygen reactions with the total propane conversion at 450 8C: (a) VSi catalyst and (b) VMg catalyst.
selectivities to different products with the conversion at 450 8C for undoped VSi and VMg catalysts. Similar curves were obtained for the doped catalysts. For VSi catalyst the course of the changes in selectivities with conversion is typical of this reported for other vanadia-based catalysts [2–5,33]: the decrease of the selectivity to propene and increase that to CO with the increasing conversion, indicates the consecutive path of the reaction, propene formed in the first step of the reaction being over-oxidized to CO. The selectivity to CO2 is practically independent of the conversion, suggesting that this latter compound is formed by the path parallel to ODH. The selectivities to CO, at lower conversions are considerably higher than those to CO2. In contrast, for VMgA catalysts the selectivities vary only slightly with the conversion, and CO2 formation (increasing slightly with the conversion) predominates over CO. The excess of CO2 over CO in total combustion products of the propane ODH over vanadia–magnesia catalysts was reported also in [29,34,35]. The difference between vanadia–magnesia and vanadia on other supports in distribution of CO and CO2, implies different
Area-specific activity (mmol m2 min1)
450 8C. 500 8C.
type or different population of the active oxygen species on catalysts of the both types. The selectivity to propene for the VMg catalyst without the additives, are lower than those reported in the literature (at comparable conversions) for catalysts of similar loading of vanadia phase and are close to those obtained for lower loadings of vanadia [34–41]. For VSi catalyst the selectivities are close to those reported by Lo´pez Nieto et al. [42] for similar loading of vanadia, and lower than in the data of Parmaliana et al. [43]. The total specific activity (Table 2) at 450 8C for VSiA series follows the sequence: VSiK VSi ¼ VSiNi ¼ VSiCr < VSiNb < VSiMo VSiP: Thus all the additives except potassium do not affect, or increase the total activity of the VSiA catalysts. The activity of the samples with the Nb, P and Mo additives is higher than that of the undoped sample, whereas the effect of Cr and Ni additives is practically not observed. Potassium decreases considerably the activity. It can be observed that the previous studies on vanadia-based catalysts showed that the addition of the alkali metals generally lowers the total activity of the catalysts [3,9,32,40]. The effect was ascribed in the first place to blocking of the active VOx centres by alkalis. The rough dependence of the activity (an increase) with the increasing electronegativity of the additive ion can be observed for this series (Fig. 3a). For VMgA series the sequence of the activities (Table 2) is: VMgK VMgMo ¼ VMgNb ¼ VMgP < VMgNi ¼ VMgCr < VMg and no correlation with the x values has been observed. Thus all the additives decrease the total activity of the VMgA catalysts in the reaction of propane.
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Fig. 3. Changes of: (a) the activity in ODH reactions of ethane (500 8C) and propane (450 8C) and (b) the selectivity to C2H4 and C3H6 at 10% conversion, with the electronegativity of the additives’ ions for VSiA catalysts.
Fig. 4. Selectivity to various products in ODH of propane at 10% (2) propane conversion for VSiA (a) and VMgA (b) catalysts. Reaction temperature: 450 8C, *470 8C.
Fig. 4 compare selectivities to various products in the propane ODH at isoconversion (10%) of propane for the reaction temperature of 450 8C for the two series of the catalysts. The predominance of CO2 over CO (CO2/CO = 2.5– 2) on VMgA catalysts, and of CO over CO2 for VSiA series (CO2/CO = 0.4–0.9) are again clearly seen. The selectivities to different products change on introduction of the additives as compared with undoped catalysts. For both VMgA and VSiA series the Ni, Cr, Nb and Mo additives increase the selectivity to propene, whereas P slightly decreases it. The striking difference between the two series can be observed in the effect of K, this element increasing considerably the selectivity to propene for VSiK catalyst and decreasing it for the VMgK sample. The different for VSiA and VMgA catalysts effect of K on the selectivity to propene suggests that this additive affects different properties of the catalysts in the both cases.
500 8C. Lower conversions of ethane as compared with those of propane can be due to the higher C–H bond energy in the ethane molecule as compared with propane, the breaking of this bond being the rate determining step in the oxidation reactions of hydrocarbons. The specific activity was referred to the same alkane partial pressure as in the ODH of propane (7 kPa), assuming a first-order dependence of the rate of the ethane ODH on the ethane partial pressure. This assumption seems to be justified as the conversions of ethane taken in the calculations were below 10%. The total specific activities for both series follow the sequence:
3.2. ODH of ethane
The sequence of the activities for the VSiA catalysts is the same as that for the ODH of propane, the activity increasing with the electronegativity of the additives (Fig. 3a), whereas it is different for the VMgA series. For VMgA catalysts the Cr and Nb
In view of the lower activity of the catalysts in the ethane ODH, the catalysts were compared at higher temperature of
VSiK < VSi < VSiNi ¼ VSiNb ¼ VSiCr < VSiMo VSiP; VMgK ¼ VMgP < VMgMo < VMgNi ¼ VMg ¼ VMgNb < VMgCr
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alkanes is practically no marked effect of K on the selectivity to ethene on VSiA catalysts, and a considerable increase in the selectivity to propene for K-doped VSi. For VMgA catalysts the K additive increases very slightly the selectivity to ethene. A plot of the selectivities to propene and ethene at 10% conversion at for VSiA catalysts as a function of the electronegativity of the additive ion (Fig. 3b) shows, that the selectivity to ethene does not depend on the electronegativity, whereas a decrease of the selectivity with this parameter is observed for propene. For VMgA catalysts such dependence is not observed. The differences in the effect of additives on the selectivity to olefins in ODH of ethane and propane have been observed previously for K-doped VOx/Al2O3 catalysts [3,14] and for VPO catalysts with Bi, La, Mo and Zr additives [45]. The literature and the present data show then, that ethane and propane ODH have different requirements towards the catalyst active centre, putting some doubt on the possibility of finding a common catalyst for the ODH reactions of these two alkanes. Lo´pez Nieto et al. [3,14] point out that the differences in the propene and ethane selectivity may be due to the different basicity of the two olefins. It cannot be forgotten either, that, out of all the olefins, the ethene molecule has C–H bonds stronger (less easily activated) than the parent alkane. It can be thus further oxidized by a different mechanism than other olefins, not involving necessarily the initial activation of a C–H bond, but, for instance, the addition of oxygen across the double bond. 4. Conclusions
Fig. 5. Selectivity to various products in ODH of ethane at 10% (2) ethane conversion for VSiA (a) and VMgA (b) catalysts. Reaction temperature: 500 8C.
ions increase the activity, while for the reaction of propane they decrease it. For both alkanes and for both series of the catalysts, however, potassium decreases the activity, though to the smaller extent for the ethane reaction. Fig. 5 give selectivities to different products at 10% of ethane conversion at 500 8C for VSiA and VMgA series, respectively. It can be observed, that, on the whole, the VMgA catalysts are less selective in ethene formation than the VSiA series. Moreover, similarly to the propane ODH, the VMgA catalysts yield more CO2 than CO, whereas CO is the main product of overoxidation on VSiA series. For the VMgA system the selectivity to ethene is considerably lower than that to propene in the propane ODH, the latter result being in agreement with the data reported earlier by Kung et al. [44]. In contrast, the VSiA catalysts are slightly more selective for propene than for ethene formation. On the whole, the effect of the additives on the selectivity to ethene is less evident than in the case of the propane ODH, the same additive having in some cases opposite effect. The most striking difference in the selectivities to olefins for the two
The effect of additives, including main group elements (K and P) and transition metal ions (Ni, Cr, Nb, and Mo), introduced to vanadia-based catalysts VOx/SiO2 and VOx/MgO, on their catalytic performance in oxidative dehydrogenation of propane and ethane depends on the type of a catalyst and on the oxidized alkane: (1) For VSiA catalysts (a) all the additives with the exception of K increase the total area-specific activity in both ODH of propane and ethane, the presence of the K additive leads to the decrease in the activity. The activity increases with the increasing electrogenativity of the additive; (b) K, Ni, Cr, Mo, and Nb additives increase the selectivity to propene (the highest effect being exerted by potassium), P decreases it slightly. The selectivity to propene decreases with the increasing electronegativity of the additives; (c) the additives have only slight effect on the selectivity to ethane; (d) CO is the main product of total combustion and is formed in a consecutive oxidation of propene, CO2 is a minor product and is mainly formed in a parallel reaction. (2) For VMg catalysts (a) all the additives decrease the area-specific activity in the propane ODH. For the ODH of ethane Cr and Nb lead to
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the increase in the activity, whereas Ni, Mo, P and K decrease it; (b) the K additive leads to the decrease in the selectivity to propene, whereas other additives (Ni, Cr, Nb, and Mo) increase it or have no effect (P). In ethane ODH all the additives increase slightly the selectivity to ethane; (c) the activity and selectivity to olefins do not depend on the electronegativity of the additives; (d) CO2 is the main product of total combustion, CO is formed only in minor quantities. Acknowledgments The work has been supported by the Polish State Committee for Scientific Research, KBN grant nos. PBZ/KBN/018/T09/ 99/4b and 4T09B 108 25. The authors want to thank Ms. Z. Czuła for determination of the BET specific surface area. References [1] B. Grzybowska-S´wierkosz, F. Trifiro`, J.C. Vedrine (Eds.), Vanadia Catalysts for Selective Oxidation of Hydrocarbons and Their Derivatives, Appl. Catal. A 157 (1/2) (1997) (special issue). [2] E.A. Mamedov, V. Corte´s Corbera´n, Appl. Catal. A 127 (1995) 1, and references therein. [3] T. Blasco, J.M. Lo´pez Nieto, Appl. Catal. A 157 (1997) 117. [4] H.H. Kung, Adv. Catal. 40 (1994) 1. [5] S. Albonetti, F. Cavani, F. Trifiro`, Catal. Rev. Sci. Eng. 38 (1996) 413. [6] F. Cavani, F. Trifiro`, Catal. Today 36 (1997) 431. [7] B. Grzybowska-S´wierkosz, Top. Catal. 21 (2002) 35. [8] B. Grzybowska, P. Meksˇs, R. Grabowski, K. Wcisło, Y. Barbaux, L. Gengembre, Stud. Surf. Sci. Catal. 82 (1994) 151. [9] R. Grabowski, B. Grzybowska, K. Samson, J. Słoczyn´ski, J. Stoch, K. Wcisło, Appl. Catal. A 125 (1995) 129. [10] M.C. Abello, M.F. Gomez, L.E. Cadus, Catal. Lett. 53 (1998) 185. [11] M.C. Kung, H.H. Kung, J. Catal. 134 (1992) 668. [12] H.H. Kung, M.C. Kung, Appl. Catal. A 157 (1997) 105. [13] W.D. Harding, H.H. Kung, V.L. Kozhevnikov, K.R. Poeppelmeier, J. Catal. 144 (1993) 597. [14] A. Galli, J.M. Lo´pez Nieto, A. Dejoz, M.I. Vazquez, Catal. Lett. 34 (1995) 51. [15] B. Grzybowska, J. Słoczyn´ski, R. Grabowski, K. Samson, I. Gressel, K. Wcisło, L. Gengembre, Y. Barbaux, Appl. Catal. A 230 (2002) 1.
[16] J.M. Lo´pez Nieto, R. Coenraads, A. Dejoz, M.I. Vazquez, Stud. Surf. Sci. Catal. 110 (1997) 443. [17] A. Dejoz, J.M. Lo´pez Nieto, F. Ma´rquez, M.I. Va´zquez, Appl. Catal. A 180 (1999) 83. [18] D.L. Stern, J.N. Michaels, L. Decaul, R.K. Grasselli, Appl. Catal. A 153 (1997) 21. [19] W. Oganowski, J. Hanuza, H. Drulis, W. Mis´ta, L. Macalik, Appl. Catal. A 136 (1996) 143. [20] A. Bielan´ski, J. Haber, Oxygen in Catalysis, Marcel Dekker, New York, 1991. [21] J.C. Vedrine, J.M.M. Millet, J.-C. Volta, Catal. Today 32 (1996) 115. [22] B. Grzybowska-S´wierkosz, Top. Catal. 11/12 (2000) 23. [23] A. Klisin´ska, S. Loridant, B. Grzybowska, J. Stoch, I. Gressel Part II, this issue. [24] A. Klisin´ska, A. Haras, K. Samson, M. Witko, B. Grzybowska, J. Mol. Catal. A: Chem. 210 (2004) 87. [25] A. Klisin´ska, Ph.D. Thesis, Krako´w, 2005. [26] B. Grzybowska, I. Gressel, K. Samson, K. Wcisło, J. Stoch, M. Mikołajczyk, F. Dautzenberg, Polish J. Chem. 75 (2001) 1513. [27] G. Deo, I.E. Wachs, J. Haber, Crit. Rev. Surf. Chem. 4 (1994) 141, and references therein. [28] G. Deo, I.E. Wachs, J. Catal. 146 (1994) 335. [29] M.A. Chaar, D. Patel, H.H. Kung, J. Catal. 109 (1988) 463. [30] R. Wang, M. Xie, P. Li, C. Ng, Catal. Lett. 24 (1994) 67. [31] A. Corma, J.M. Lo´pez Nieto, N. Paredes, J. Catal. 144 (1993) 425. [32] R. Grabowski, B. Grzybowska, J. Słoczyn´ski, K. Wcisło, Appl. Catal. A 144 (1996) 335. [33] J. Słoczyn´ski, R. Grabowski, K. Wcisło, B. Grzybowska-S´wierkosz, Polish J. Chem. 71 (1997) 1585. [34] A. Pantazidis, C. Mirodatos, Stud. Surf. Sci. Catal. 101B (1996) 1029. [35] R. Burch, E.M. Crabb, Appl. Catal. A 100 (1993) 111. [36] D. Siew Hew Sam, V. Soenen, J.C. Volta, J. Catal. 123 (1990) 417. [37] A. Corma, J.M. Lo´pez Nieto, N. Paredes, A. Dejoz, I. Vazquez, Stud. Surf. Sci. Catal. 82 (1994) 113. [38] R.X. Valenzuela, E.A. Mamedov, I.A. Vasalos, React. Kinet. Catal. Lett. 55 (1995) 213. [39] F. Arena, F. Frusteri, A. Parmaliana, Catal. Lett. 60 (1999) 59. [40] A.A. Lemonidou, N. Nalbandian, I.A. Vasalos, Catal. Today 61 (2000) 333. [41] C. Pak, A.T. Bell, T.D. Tilley, J. Catal. 206 (2002) 49. [42] A. Corma, J.M. Lo´pez Nieto, N. Paredes, M. Perez, Y. Shen, H. Cao, S.L. Suib, Stud. Surf. Sci. Catal. 72 (1992) 213. [43] A. Parmaliana, V. Sokolovskii, D. Mieli, N. Giordano, Appl. Catal. A 135 (1996) L1. [44] P.M. Michalakos, M.C. Kung, I. Jahan, H.H. Kung, J. Catal. 140 (1993) 226. [45] M. Ga˛sior, I. Gressel, V.A. Zazhigalov, B. Grzybowska, Polish J. Chem. 77 (2003).
Effect of additives on properties of V2O5/SiO2 and V2O5/MgO catalysts I. Oxidative dehydrogenation of propane and ethane A. Klisin´ska, K. Samson, I. Gressel, B. Grzybowska * Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Krako´w, Poland Received 18 July 2005; received in revised form 23 March 2006; accepted 6 April 2006 Available online 5 June 2006
Abstract Oxidative dehydrogenation, ODH of propane and ethane was studied on VOx/SiO2 (VSi) and VOx/MgO (VMg) catalysts of the same vanadium loading (1.5 theoretical monolayer of vanadia) and doped with additives of main group elements (K and P) and of transition metal ions (Ni, Cr, Nb, and Mo). The additives modify both the specific activity and selectivity to olefins, the modifying effect (its sign and extent) depending on the type of the catalyst and on the alkane nature. The main difference between the two series of the catalysts consist in: (a) increase in the specific activity in the ODH reactions on introduction of the additives (except K) for VSi and decrease for VMg catalysts, (b) considerable increase in the selectivity to propene in ODH of propane for K-doped VSi catalyst and decrease for K-doped VMg catalyst (Ni, Cr, and Mo increasing and P decreasing the selectivity for the both series), and (c) higher selectivities to ethene as compared with those to propene for VSi and lower to ethene than to propene for VMg catalysts. In contrast to the propane ODH the additives exert only very small effect on the selectivities to ethene in the ODH of ethane. # 2006 Elsevier B.V. All rights reserved. Keywords: Vanadia catalysts; Additives; Oxidative dehydrogenation of propane and ethane
1. Introduction Vanadia-based catalysts active and selective in oxidation reactions of various hydrocarbons mainly to oxygenated products, in particular to organic anhydrides [1], have been found also promising for oxidative dehydrogenation, ODH of lower alkanes, in particular of propane [2,3]. The ODH reactions of lower alkanes have attracted in the last decade much interest as an alternative to classical dehydrogenation, DH for production of olefins [4–6]. They permit to avoid such inconveniences of DH as low equilibrium constants, deactivation by the coke formation and hence necessity of the catalyst regeneration, and the endothermicity of the reaction. On the other hand ODH reactions are accompanied by formation of considerable amounts of undesirable carbon oxides, produced mainly by consecutive oxidation of olefins, and – to some extent – by parallel to ODH total oxidation of alkanes. This problem – general for all selective oxidation reactions – is even more acute for ODH, since olefins (except ethene) – intermediate products in the chain of hydrocarbon oxidation from hydrocarbons to the most favoured
* Corresponding author. Tel.: +48 12 639 5100; fax: +48 12 425 1923. E-mail address: [email protected] (B. Grzybowska). 0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2006.04.028
thermodynamically carbon oxides – are more active than the substrate alkanes. Catalytic properties of oxide catalysts can be improved by introduction of additives. The patent literature provides ample examples of additives of various chemical nature introduced to industrial catalysts, the fundamental studies on mechanism of their action are, however, scarce. The brief review of academic research on the problem [7] demonstrates the variety of effects of the additives on structure, physicochemical and catalytic properties of oxide catalysts in selective oxidation reaction, and show a need for systematic studies on the problem. Most of the to date studies have been concerned with alkali metal additives. For the ODH reactions of lower alkanes alkali metal additives have been found to improve the selectivity to propene in the ODH of propane on V2O5/TiO2, MoO3/TiO2 [8,9], V2O5/ Al2O3 [3] and MoO3/MgO–g-Al2O3 [10] catalysts, the effect being explained by modification of acid–base properties of the catalysts by alkalis. On the other hand the K additive was found to decrease the selectivity to olefins in ODH of propane and butane on catalysts based on VMgO system [11–13]: no explanation has been given for this fact. Moreover, for the same catalysts the effect of alkalis may depend on the length of a hydrocarbon chain. Lo´pez Nieto et al. reported that in ODH of ethane and propane on VOx/Al2O3 catalysts the K additive
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decreased the selectivity to ethane and increased that to propene [3,14]. Little is known about the effect of other additives, in particular of transition metal ions, which may modify other – beside acidobasic – properties, such as M–O bond strength on the surface or in the bulk of oxide catalysts, and the type of the surface oxygen species. Among non-alkali additives, Fe, Ca, Al in vanadia– titania system were found to increase the selectivity to propene in the propane ODH, whereas W decreased it [15]. The effect was ascribed to the change of electronic properties (work function), which may control the type of oxygen species. Additives of Bi, Mo, K, and P to vanadia–alumina system modified redox properties of the catalysts; the activity in ODH of propane increased with the decrease in the reducibility [16]. Addition of Mo [17] and of Mo, W, B, Al, Ga, Sb [18] to VMgO system was found to increase the selectivity to olefins and dienes in ODH of n-butane (with the exception of W). Mo added to VMgO system increased the selectivity to styrene in ODH of ethyl benzene, whereas Cr and Co doping increased the activity [19]: no clear explanation of these effects was, however, provided. We have undertaken studies on the effect of additives of various nature on catalytic and physicochemical properties of vanadiabased catalysts. Two series of the catalysts have been selected, containing vanadia on supports of different nature: (a) SiO2, weakly interacting with the dispersed vanadia phase, for which crystalline vanadia of relatively high acidity is observed even at low V loading [2], and (b) MgO, which can react with vanadia [2], and for which lower acidity of vanadium centres could be expected. The additives represent both main group elements (K and P) and transition metals of different valency (Ni, Cr, Nb, and Mo). In selection of the second group of additives (introduced to the catalysts in the ionic form), we adopted a simple hypothesis, assuming, that the cationic additives of lower electronegativity (lower valency) than vanadium would shift the electron density towards the V–O active centre, rendering it less acidic and more basic (in the Lewis sense), whereas the additives of the higher than vanadium electronegativity (higher valences) should have a reverse effect. Besides acido-basic properties modification of the electron density around the active V–O centre may affect also redox properties. It is generally recognized that both acidobasic and redox properties (often interrelated), influence catalytic performance in selective oxidation reaction [20–22]. In the present work (part I) these two series have been examined in ODH of propane and ethane. In part II [23], we will describe the effect of the additives on the structure and physicochemical properties of the two series of the catalysts and will attempt to correlate them with the catalytic performance. The molecular aspect of the effect of the additives to vanadia-based catalysts has been approached by quantum chemical calculations [24]. 2. Experimental 2.1. Catalysts The V2O5/SiO2 (VSi) catalysts were prepared by impregnation with aqueous solution of ammonium metavanadate of a
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commercial silica support (Aerosil 200), evaporation of the solute, followed by drying for 12 h at 120 8C and calcination under a flow of air for 5 h at 500 8C. Before impregnation the support was pre-treated in water at room temperature and dried at 90 8C for 12 h: its specific surface area after such treatment was 175 m2/g. The additives (A) were introduced by adding appropriate amounts of their soluble salts or acids [KNO3, Ni(NO3)2, Cr(NO3)3, H3PO4], ammonium paramolybdate, and niobia dissolved in oxalic acid to the metavanadate solution. All the reactants used were of p.p.a. grade. The atomic ratio A/V in the calcined catalysts was 0.2 with the exception of K, for which K/V was 0.1. The K/V ratio of 0.1 was shown in previous studies as optimal for VOx/SiO2 [25] and VOx/TiO2 catalysts [26], the selectivity to propene decreasing slightly at higher K content. The choice of the A/V ratio for other additives was arbitrary: we adopted A/V of 0.2 basing on the literature data, which show that evident modification of properties (e.g. shift of Raman frequencies of V O bonds [27,28] or of reducibility [16]) are observed at A/V ratios higher than 0.1. On the other hand we wanted to avoid formation of mixed A–V–O bulk compounds e.g. V–Nb–Mo, which may lead the alkane reactions to oxygenated products. The V2O5/MgO (VMg) catalysts were prepared in the same way, using commercial MgO (Ubichem, 72 m2/g). The content of vanadia in the samples corresponded to 1.5 theoretical monolayers of V2O5 per the support surface, and was calculated with the assumption that one monolayer contains 10 V atoms per 1 nm2 of the support. The symbols of the samples, adopted further in the text, are VSiA, or VMgA for respectively vanadia–silica and vanadia–magnesia catalysts, where A is the additive. Additionally samples without vanadium, containing only the additives (the same amounts as in VSiA and VMgA) on SiO2 and MgO supports, were prepared and tested in the same conditions. Table 1 gives a list of the catalysts, their specific surface area obtained with the BET method and the Table 1 List of samples of V2O5/SiO2 and V2O5/MgO catalysts List of catalysts
SSA (m2/g)
V2O5 (wt.%)
Additive (wt.%)
xia of additives
VSi VSiK SiNi VSiCr VSiNb VSiP VSiMo
116.6 92.2 110.8 109.1 104.2 70.2 76.8
28.4 28.0 27.4 27.5 26.8 27.9 26.8
– 1.2 3.5 3.2 5.5 1.9 5.7
17.9b 2.5 9.6 11.6 17.6 24.1 19.4
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
79.7 135.6 80.3 63.4 164.4 175.1 155.7
14.1 14.0 13.8 13.9 13.7 13.8 13.7
– 0.6 1.8 1.6 2.8 1.0 2.9
17.9b 2.5 9.6 11.6 17.6 24.1 19.4
a Approximate value of xi electronegativity of introduced ions calculated from xzþ Me ¼ ð1 þ 2zÞxMe , where xMe is an electronegativity of an atom and z its valency. b V5+.
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approximate values of the electronegativities of the additive ions. For the VSi series the specific surface area of the catalysts, SSA was lower than that of the pure SiO2 support (175 m2/g). The presence of the additives led to the further decrease of SSA, the effect being the biggest for the P, Mo and K additives. Thus, both vanadia and the additives lead to the sintering of the support. For the VMg series SSA was, with the exception of VMgCr, higher than that of the MgO support (72 m2/g), the presence of additives (in particular of P, Mo, Nb and K) leading to a considerable increase in SSA. It can be mentioned, that similar increase of the specific surface area was observed in some cases for vanadia [29– 31] and chromia [32] deposited on MgO. It was found [32], that the treatment of magnesia with water, followed by evaporation and drying in the same conditions as those in the preparation of the catalysts, led to the formation of microcrystalline Mg(OH)2, as verified by XRD: on calcination the magnesium hydroxide decomposed into MgO which had a higher specific surface area than the original support.
studied reactions (conversions at 500 8C < 2%). For Cr and Ni samples without vanadium, the activity was higher, but the main product of the reactions was CO2. At 450 8C, the temperature and which the vanadia catalysts were compared further in the text, conversions of propane were about 15 and 10% for CrSi and NiSi samples, with selectivities to propene of 15 and 13%, respectively, and 13 and 6% for CrMg and NiMg, with the propene selectivities of about 4% for both Cr and Ni samples on MgO. 3. Results and discussion 3.1. ODH of propane
2.2. Catalytic activity measurements
Fig. 1 presents the variations of the propane conversion with the reaction temperature, for some of the catalysts of VSiA and VMgA series. For both series of the catalysts the increase of the conversion with the temperature is usually accompanied by the decrease of the selectivity to propene and the increase of the selectivity to carbon oxides. Fig. 2 illustrates the changes of the
The activity of the catalysts in oxidative dehydrogenation of propane and ethane was measured in a fixed bed flow apparatus in the temperature range 360–520 8C for propane and at 500 8C for ethane. A stainless steel reactor (120 mm long, internal diameter 13 mm) was coupled directly to a series of gas chromatographs. Propene or ethene and carbon oxides (CO and CO2) were found to be the main reaction products. The amounts of the degradation, C2 products and of oxygenates were below 1% of the total amount of products. The reaction mixture contained 7.1 vol.% of propane and 17 vol.% of ethane in air. 0.5 ml (0.25–0.30 g for VSiA, and 0.30–0.40 g for VMgA series) for propane reaction and 1 ml for ethane of a catalyst sample of grain size 0.63–1 mm, diluted with quartz beads, was used. The composition of the reaction mixture was controlled with the mass flowmeters. Since the selectivity to olefins decreased with the increasing conversion (a typical behaviour for the ODH reactions on oxide catalysts [2–5,33]) the selectivities at iso-conversions were compared. To obtain the same conversions for a series of catalysts of different activities, the total flow of the reaction mixture at a given reaction temperature varied between 20 and 120 ml/min, which corresponded to the contact time t = 0.2–1.5 s. On the other hand the flow rate (contact time) had to be adjusted for some catalysts to keep the conversions below 20%: above these values the conversions were limited by the high consumption of oxygen. The carbon balance for conversions higher than about 10% was better than 97 2%. At lower conversions the balance was poorer and hence the selectivities to different P products were calculated from the formula: Si ¼ ci = ci , where ci are concentrations of products i. The total conversion Xp was calculated as: Xp ¼ c0p cp =c0p , where c0p and cp are the concentrations of propane at the inlet and outlet of the reactor, respectively. The pure supports were not active in the studied conditions, the total propane conversion not exceeding 1% and that of ethane 0.5% at 500 8C. The samples without vanadium containing K, P, Mo, and Nb were very little active in the
Fig. 1. Changes of the propane conversion (at t = 0.5 s) with the reaction temperature for VSiA (a) and VMgA (b) catalysts.
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Table 2 Total activity in ODH of propane and ethane on of V2O5/SiO2 and V2O5/MgO catalysts Catalyst symbol
Propanea
Ethaneb
VSi VSiK VSiNi VSiCr VSiNb VSiP VSiMo
0.70 0.09 0.71 0.74 0.92 3.57 1.61
0.85 0.17 0.99 1.15 1.08 5.61 1.88
VMg VMgK VMgNi VMgCr VMgNb VMgP VMgMo
0.87 0.24 0.66 0.67 0.40 0.45 0.40
1.85 1.11 1.81 2.14 1.86 1.17 1.38
a b
Fig. 2. Changes of the selectivity to C3H6, CO and CO2 in propane–oxygen reactions with the total propane conversion at 450 8C: (a) VSi catalyst and (b) VMg catalyst.
selectivities to different products with the conversion at 450 8C for undoped VSi and VMg catalysts. Similar curves were obtained for the doped catalysts. For VSi catalyst the course of the changes in selectivities with conversion is typical of this reported for other vanadia-based catalysts [2–5,33]: the decrease of the selectivity to propene and increase that to CO with the increasing conversion, indicates the consecutive path of the reaction, propene formed in the first step of the reaction being over-oxidized to CO. The selectivity to CO2 is practically independent of the conversion, suggesting that this latter compound is formed by the path parallel to ODH. The selectivities to CO, at lower conversions are considerably higher than those to CO2. In contrast, for VMgA catalysts the selectivities vary only slightly with the conversion, and CO2 formation (increasing slightly with the conversion) predominates over CO. The excess of CO2 over CO in total combustion products of the propane ODH over vanadia–magnesia catalysts was reported also in [29,34,35]. The difference between vanadia–magnesia and vanadia on other supports in distribution of CO and CO2, implies different
Area-specific activity (mmol m2 min1)
450 8C. 500 8C.
type or different population of the active oxygen species on catalysts of the both types. The selectivity to propene for the VMg catalyst without the additives, are lower than those reported in the literature (at comparable conversions) for catalysts of similar loading of vanadia phase and are close to those obtained for lower loadings of vanadia [34–41]. For VSi catalyst the selectivities are close to those reported by Lo´pez Nieto et al. [42] for similar loading of vanadia, and lower than in the data of Parmaliana et al. [43]. The total specific activity (Table 2) at 450 8C for VSiA series follows the sequence: VSiK VSi ¼ VSiNi ¼ VSiCr < VSiNb < VSiMo VSiP: Thus all the additives except potassium do not affect, or increase the total activity of the VSiA catalysts. The activity of the samples with the Nb, P and Mo additives is higher than that of the undoped sample, whereas the effect of Cr and Ni additives is practically not observed. Potassium decreases considerably the activity. It can be observed that the previous studies on vanadia-based catalysts showed that the addition of the alkali metals generally lowers the total activity of the catalysts [3,9,32,40]. The effect was ascribed in the first place to blocking of the active VOx centres by alkalis. The rough dependence of the activity (an increase) with the increasing electronegativity of the additive ion can be observed for this series (Fig. 3a). For VMgA series the sequence of the activities (Table 2) is: VMgK VMgMo ¼ VMgNb ¼ VMgP < VMgNi ¼ VMgCr < VMg and no correlation with the x values has been observed. Thus all the additives decrease the total activity of the VMgA catalysts in the reaction of propane.
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Fig. 3. Changes of: (a) the activity in ODH reactions of ethane (500 8C) and propane (450 8C) and (b) the selectivity to C2H4 and C3H6 at 10% conversion, with the electronegativity of the additives’ ions for VSiA catalysts.
Fig. 4. Selectivity to various products in ODH of propane at 10% (2) propane conversion for VSiA (a) and VMgA (b) catalysts. Reaction temperature: 450 8C, *470 8C.
Fig. 4 compare selectivities to various products in the propane ODH at isoconversion (10%) of propane for the reaction temperature of 450 8C for the two series of the catalysts. The predominance of CO2 over CO (CO2/CO = 2.5– 2) on VMgA catalysts, and of CO over CO2 for VSiA series (CO2/CO = 0.4–0.9) are again clearly seen. The selectivities to different products change on introduction of the additives as compared with undoped catalysts. For both VMgA and VSiA series the Ni, Cr, Nb and Mo additives increase the selectivity to propene, whereas P slightly decreases it. The striking difference between the two series can be observed in the effect of K, this element increasing considerably the selectivity to propene for VSiK catalyst and decreasing it for the VMgK sample. The different for VSiA and VMgA catalysts effect of K on the selectivity to propene suggests that this additive affects different properties of the catalysts in the both cases.
500 8C. Lower conversions of ethane as compared with those of propane can be due to the higher C–H bond energy in the ethane molecule as compared with propane, the breaking of this bond being the rate determining step in the oxidation reactions of hydrocarbons. The specific activity was referred to the same alkane partial pressure as in the ODH of propane (7 kPa), assuming a first-order dependence of the rate of the ethane ODH on the ethane partial pressure. This assumption seems to be justified as the conversions of ethane taken in the calculations were below 10%. The total specific activities for both series follow the sequence:
3.2. ODH of ethane
The sequence of the activities for the VSiA catalysts is the same as that for the ODH of propane, the activity increasing with the electronegativity of the additives (Fig. 3a), whereas it is different for the VMgA series. For VMgA catalysts the Cr and Nb
In view of the lower activity of the catalysts in the ethane ODH, the catalysts were compared at higher temperature of
VSiK < VSi < VSiNi ¼ VSiNb ¼ VSiCr < VSiMo VSiP; VMgK ¼ VMgP < VMgMo < VMgNi ¼ VMg ¼ VMgNb < VMgCr
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alkanes is practically no marked effect of K on the selectivity to ethene on VSiA catalysts, and a considerable increase in the selectivity to propene for K-doped VSi. For VMgA catalysts the K additive increases very slightly the selectivity to ethene. A plot of the selectivities to propene and ethene at 10% conversion at for VSiA catalysts as a function of the electronegativity of the additive ion (Fig. 3b) shows, that the selectivity to ethene does not depend on the electronegativity, whereas a decrease of the selectivity with this parameter is observed for propene. For VMgA catalysts such dependence is not observed. The differences in the effect of additives on the selectivity to olefins in ODH of ethane and propane have been observed previously for K-doped VOx/Al2O3 catalysts [3,14] and for VPO catalysts with Bi, La, Mo and Zr additives [45]. The literature and the present data show then, that ethane and propane ODH have different requirements towards the catalyst active centre, putting some doubt on the possibility of finding a common catalyst for the ODH reactions of these two alkanes. Lo´pez Nieto et al. [3,14] point out that the differences in the propene and ethane selectivity may be due to the different basicity of the two olefins. It cannot be forgotten either, that, out of all the olefins, the ethene molecule has C–H bonds stronger (less easily activated) than the parent alkane. It can be thus further oxidized by a different mechanism than other olefins, not involving necessarily the initial activation of a C–H bond, but, for instance, the addition of oxygen across the double bond. 4. Conclusions
Fig. 5. Selectivity to various products in ODH of ethane at 10% (2) ethane conversion for VSiA (a) and VMgA (b) catalysts. Reaction temperature: 500 8C.
ions increase the activity, while for the reaction of propane they decrease it. For both alkanes and for both series of the catalysts, however, potassium decreases the activity, though to the smaller extent for the ethane reaction. Fig. 5 give selectivities to different products at 10% of ethane conversion at 500 8C for VSiA and VMgA series, respectively. It can be observed, that, on the whole, the VMgA catalysts are less selective in ethene formation than the VSiA series. Moreover, similarly to the propane ODH, the VMgA catalysts yield more CO2 than CO, whereas CO is the main product of overoxidation on VSiA series. For the VMgA system the selectivity to ethene is considerably lower than that to propene in the propane ODH, the latter result being in agreement with the data reported earlier by Kung et al. [44]. In contrast, the VSiA catalysts are slightly more selective for propene than for ethene formation. On the whole, the effect of the additives on the selectivity to ethene is less evident than in the case of the propane ODH, the same additive having in some cases opposite effect. The most striking difference in the selectivities to olefins for the two
The effect of additives, including main group elements (K and P) and transition metal ions (Ni, Cr, Nb, and Mo), introduced to vanadia-based catalysts VOx/SiO2 and VOx/MgO, on their catalytic performance in oxidative dehydrogenation of propane and ethane depends on the type of a catalyst and on the oxidized alkane: (1) For VSiA catalysts (a) all the additives with the exception of K increase the total area-specific activity in both ODH of propane and ethane, the presence of the K additive leads to the decrease in the activity. The activity increases with the increasing electrogenativity of the additive; (b) K, Ni, Cr, Mo, and Nb additives increase the selectivity to propene (the highest effect being exerted by potassium), P decreases it slightly. The selectivity to propene decreases with the increasing electronegativity of the additives; (c) the additives have only slight effect on the selectivity to ethane; (d) CO is the main product of total combustion and is formed in a consecutive oxidation of propene, CO2 is a minor product and is mainly formed in a parallel reaction. (2) For VMg catalysts (a) all the additives decrease the area-specific activity in the propane ODH. For the ODH of ethane Cr and Nb lead to
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the increase in the activity, whereas Ni, Mo, P and K decrease it; (b) the K additive leads to the decrease in the selectivity to propene, whereas other additives (Ni, Cr, Nb, and Mo) increase it or have no effect (P). In ethane ODH all the additives increase slightly the selectivity to ethane; (c) the activity and selectivity to olefins do not depend on the electronegativity of the additives; (d) CO2 is the main product of total combustion, CO is formed only in minor quantities. Acknowledgments The work has been supported by the Polish State Committee for Scientific Research, KBN grant nos. PBZ/KBN/018/T09/ 99/4b and 4T09B 108 25. The authors want to thank Ms. Z. Czuła for determination of the BET specific surface area. References [1] B. Grzybowska-S´wierkosz, F. Trifiro`, J.C. Vedrine (Eds.), Vanadia Catalysts for Selective Oxidation of Hydrocarbons and Their Derivatives, Appl. Catal. A 157 (1/2) (1997) (special issue). [2] E.A. Mamedov, V. Corte´s Corbera´n, Appl. Catal. A 127 (1995) 1, and references therein. [3] T. Blasco, J.M. Lo´pez Nieto, Appl. Catal. A 157 (1997) 117. [4] H.H. Kung, Adv. Catal. 40 (1994) 1. [5] S. Albonetti, F. Cavani, F. Trifiro`, Catal. Rev. Sci. Eng. 38 (1996) 413. [6] F. Cavani, F. Trifiro`, Catal. Today 36 (1997) 431. [7] B. Grzybowska-S´wierkosz, Top. Catal. 21 (2002) 35. [8] B. Grzybowska, P. Meksˇs, R. Grabowski, K. Wcisło, Y. Barbaux, L. Gengembre, Stud. Surf. Sci. Catal. 82 (1994) 151. [9] R. Grabowski, B. Grzybowska, K. Samson, J. Słoczyn´ski, J. Stoch, K. Wcisło, Appl. Catal. A 125 (1995) 129. [10] M.C. Abello, M.F. Gomez, L.E. Cadus, Catal. Lett. 53 (1998) 185. [11] M.C. Kung, H.H. Kung, J. Catal. 134 (1992) 668. [12] H.H. Kung, M.C. Kung, Appl. Catal. A 157 (1997) 105. [13] W.D. Harding, H.H. Kung, V.L. Kozhevnikov, K.R. Poeppelmeier, J. Catal. 144 (1993) 597. [14] A. Galli, J.M. Lo´pez Nieto, A. Dejoz, M.I. Vazquez, Catal. Lett. 34 (1995) 51. [15] B. Grzybowska, J. Słoczyn´ski, R. Grabowski, K. Samson, I. Gressel, K. Wcisło, L. Gengembre, Y. Barbaux, Appl. Catal. A 230 (2002) 1.
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