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α-Al2O3 catalysts
Applied Catalysis A: General 210 (2001) 355–361
Partial oxidation of methane over VOx /␣-Al2 O3 catalysts Mar´ıa A. Volpe Planta Piloto de Ingenier´ıa Qu´ımica, UNS-CONICET, Complejo CRIBABB, Edificio 2, Camino Carrindanga km. 7, Casilla de Correos 717, 8000 Bah´ıa Blanca, Argentina Received 19 May 2000; received in revised form 5 October 2000; accepted 6 October 2000
Abstract The catalytic behavior of a series of VOx /␣-Al2 O3 catalysts for the partial oxidation of methane has been evaluated. Samples with different vanadia loading were prepared from NH4 VO3 and V(AcAc)3 . Characterization performed by TPR and oxygen uptake measurements indicates that different VOx species are present on the samples. The catalytic patterns indicate that each V-surface species possesses different activity and selectivity. Isolated vanadates are the most active and selective towards HCHO, while V2 O5 crystallites are detrimental to the catalytic performance. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Partial oxidation of methane; Vanadia-supported catalysts; VOx
1. Introduction The worldwide surplus of methane is an incentive for the investigation of the partial oxidation of methane (POM) with oxygen to C1-oxygenates over a large number of catalysts [1]. Nonselective, complete oxidation to CO2 is the most frequent result. This fact is related to the thermodynamic aspect of the oxidation of methane, since the free energy change of the complete oxidation reaction is larger than that corresponding to the oxidation to C1-oxygenate. In addition, formaldehyde is thermodynamically unstable with respect to its decomposition. The POM is, therefore, a kinetic problem and deep oxidation must be suppressed. Vanadia supported on silica is a selective catalyst towards partial oxidation products, as methanol and formaldehyde. On the other hand, vanadia supported on ␥-alumina or on titania are active systems, but they only produce CO2 and CO [2,3]. For the case E-mail address: [email protected] (M.A. Volpe).
of VOx /␥-Al2 O3 , vanadia sites are likely to catalyze the formation of partial oxidation products, but they would undergo further oxidation on active acid sites on the surface of the support [2]. Based on this supposition, it seems interesting to test the catalytic performance of vanadia catalysts supported on low acidic ␣-Al2 O3 . The present paper reports the catalytic performance of VOx /␣-Al2 O3 catalysts for the POM. A series of VOx /␣-Al2 O3 catalysts, with vanadium content varying in the 0.4–2.1 wt.% range have been prepared. These samples contain different types of vanadia species, since, depending on loading, monomeric vanadate, polymeric surface species and crystalline V2 O5 may be present on the surface of the support [4,5]. Here, we report our attempt to determine the role of different vanadia species on the activity and selectivity for the POM. It is generally accepted that the partial oxidation of hydrocarbons on oxide catalysts involves a redox mechanism. For this reason, in the present paper, the reducibility of VOx catalysts is studied. In order
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to do so, the catalysts are characterized by temperature programmed reduction and by oxygen uptake measurements.
2. Experimental 2.1. Catalysts preparation The support was ␣-alumina form Rhône Poulenc, with 13 m2 /g. VOx /␣-Al2 O3 samples will be referred to as xV, where x indicates the V weight percent of the solid, as determined by atomic absorption spectroscopy. 2.2. Samples prepared from NH4 VO3 Two VOx /␣-Al2 O3 samples were obtained by mixing 10 ml of an aqueous solution of ammonium metavanadate at pH 4 with approximately 5 g of the support. When necessary, pH was adjusted by adding aliquots of a diluted solution of HNO3 . For the 0.8V sample, the solid was filtered after 24 h of contact. For the 2.1V sample, after 24 h of contact, a solution of nitric acid was added in order to lower pH from 4 to 2. After 10 min, the solid was filtered. In both cases, the solids were calcined at 500◦ C for 4 h. 2.3. Samples prepared from V(AcAc)3 The 0.4V catalyst was prepared by reaction of the support with a solution of V(AcAc)3 in toluene (5 × 10−2 g/ml). The solid was filtered after 24 h of reaction, washed in fresh toluene and calcined in air flow at 500◦ C for 4 h. In order to obtain a more loaded sample (0.9V catalyst), a certain amount of 0.4V sample was put in contact with the V(AcAc)3 solution. The same preparation procedure as for sample 0.4V was followed. 2.4. NH4 OH treated samples The four VOx /␣-Al2 O3 samples were treated with an ammoniacal solution. Approximately 300 mg of the solids were put in contact with 10 ml of 0.3N NH4 OH solution at room temperature for 5 min.
2.5. Catalysts characterization 2.5.1. Temperature programmed reduction (TPR) The catalysts were pretreated by calcination at 530◦ C in dry air for 1 h, and subsequently purged in Ar at the same temperature and cooled to 100◦ C. Then, they were reduced from 100 to 530◦ C with a heating rate of 10◦ C/min in a H2 /Ar gas mixture (5 vol.% of H2 ) flowing at 20 cm3 /min. The H2 consumption was monitored by a thermoconductivity detector. 2.5.2. Oxygen uptake measurements Oxygen uptake measurements were performed in a volumetric apparatus operating in the pulse mode. Before measurements, the samples were reduced in a H2 flow for 4 h at 530◦ C, and then they were evacuated at the same temperature for the same period of time. Then, the samples were cooled to 300◦ C. Doses of chromatographic oxygen were admitted into the sample cell. The oxygen uptake was measured for each pulse. The specific amount of oxygen consumed, as millimoles of gas consumed per gram of vanadium, was calculated by extrapolating the total oxygen uptake to zero pressure. 2.5.3. Catalytic test All VOx /␣-Al2 O3 catalysts were tested for the POM. The sample weight was in the 50–300 mg range, and it was selected for each sample so as to have approximately the same weight of vanadium for all the samples. Test were run in a flow pyrex reactor operating at atmospheric pressure in the temperature range 400–530◦ C, using a reaction mixture He/CH4 /O2 in the molar ratio 97:1:2 flowing at a rate of 50 STP cm3 /min. Analysis of the reactants and products was performed by gas chromatography equipped with a thermal conductivity detector and two columns in parallel. Helium was the carrier gas. The line from the outlet of the reactor to the gas chromatograph was heated to approximately 60◦ C in order to avoid condensation of the products. Blank runs were performed on the same reactor packed with glass spheres without any detectable conversion. The conversion, expressed as the number of moles of methane converted, was measured at 400, 450, 500 and 530◦ C. The specific activity, TOF, was calculated as the conversion per gram of V per second. The selectivity (%) to HCOH, CO and CO2 was calculated
M.A. Volpe / Applied Catalysis A: General 210 (2001) 355–361
as 100 × (number of moles of methane converted to product/total number of moles of methane converted).
3. Results 3.1. Samples prepared from NH4 VO3 The way to prepare the 0.8V sample is generally followed to obtain highly dispersed supported vanadia [2]. For this catalyst, the number of vanadium atoms fixed per square nanometer was 7. The value of the specific concentration was similar to those corresponding to VOx monolayer deposited on a wide variety of supports (␥-Al2 O3 , TiO2 , ZrO2 , Nb2 O5 ) [6]. Thus, it was considered that the 0.8V sample contained molecularly dispersed vanadia, and that no V2 O5 crystallites were present in this case. The amount of vanadium fixed by the 2.1V sample was higher than for the 0.8V catalyst. The difference in the vanadium concentration would be related to the fact that after acidification of the methavanadate solution, a red solid precipitated onto the solid. It is interesting to note that the vanadium species in acidic solution strongly depend on pH. The aggregation and coordination of vanadium atoms increase with diminishing pH, as methavandate, decavanates, and V2 O5 crystallites [7]. Thus, the red precipitate would be crystalline V2 O5 . Therefore, it could be postulated that the 2.1V sample is composed of two different species: highly dispersed vanadia and V2 O5 crystallites. 3.2. Samples prepared from V(AcAc)3 For the 0.4V catalyst, only half of the vanadium formerly present in the organic solution was retained by the support. The saturation of ␣-Al2 O3 was probably due to the fact that the anchored vanadium complex occupies a large area [8]. Subsequent calcination of the solid eliminated the remaining acetyl-acetonate ligands, and more hydroxyl groups were available for further reaction with the complex. This was the reason why the vanadium loading of the 0.9V sample was higher than the 0.4V sample. The similarity between the vanadium concentration of the 0.8V and 0.9V samples prepared from different precursors is worth noting. It could indicate that for
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Table 1 Vanadium concentration and oxygen uptake measurements for VOx /␣-Al2 O3 Sample
Preparation method
V (wt.%)
O2 consumed (mmol/g V)
0.4V 0.9V 0.8V 2.1V 2.1tV
V(AcAc)3 V(AcAc)3 NH4 VO3 (pH 2) NH4 VO3 (pH 4) NH4 OH treatment
0.4 0.9 0.8 2.1 0.6
8.3 4.9 4.3 6.8 4.8
both preparation methods, the maximum amount of highly dispersed VOx that ␣-Al2 O3 can accommodate was achieved. 3.3. NH4 OH treated samples The vanadium contents of some of the samples treated with the ammoniacal solution are reported in Table 1. The treatment removed considerable amount of vanadium for the 2.1V sample; from now on, this sample will be named as 2.1tV. On the other hand, a negligible concentration was removed from the 0.4V and the 0.8V samples. As established by Yoshida et al. [9], only vanadia with weak interaction with the support is soluble in a diluted ammoniacal solution. In line with this, for the 2.1V catalysts approximately half of vanadium, probably bulk like vanadia, was loosely bound. On the other samples, the major part of the vanadia would be strongly anchored to the support. 3.4. Temperature programmed reduction The TPR profile of 0.4V, 0.8V, 0.9V and 2.1V catalysts are shown in Fig. 1. The reduction pattern of the 0.4V sample was characterized by two peaks centered at 380◦ C (the major one) and at 450◦ C. The TPR spectra corresponding to the 0.9V and 0.8V samples were quite similar. Both of them outlined a main peak centered at approximately 460◦ C. For the 2.1V catalyst, the main hydrogen consumption appeared at approximately 520◦ C, with a shoulder at the lower temperature side. It is generally accepted that monomeric vanadates and polyvanadates chains are formed at low vanadia loading. When the concentration increases, polyvanadates chains condense to form surface vanadia monolayer. Finally, crystalline V2 O5 appears for concentrations above the monolayer
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Fig. 1. TPR profiles for VOx /␣-Al2 O3 catalysts: (a) 0.4V; (b) 0.9V; (c) 0.8V; (d) 2.1V samples.
Fig. 2. TPR profiles for 2.1V and 2.1tV samples.
capacity [4,6,10,13]. In line with this, for the 0.4V sample, the peak at 380◦ C would be related to isolated vanadates, while the consumption at 450◦ C should be assigned to polyvanadates or to a vanadia monolayer. The last peak also appeared in the profile corresponding to the 0.9V and 0.8V catalysts, that could be mainly constituted of a surface monolayer. In the case of the 0.8V sample, a shoulder at the low temperature side could also be detected. One could attribute this shoulder to monovanadate species. For 2.1V, the main hydrogen consumption would be related to the partial reduction of crystalline V2 O5 . The reduction of pure V2 O5 occurs at approximately 600◦ C [2]. However, it is generally accepted that the supported oxide is more readily reduced than unsupported V2 O5 . The shoulder at the side of lower temperatures should be assigned to the reduction of polyvanadates chains. Fig. 2 comparatively shows the TPR profiles of the 2.1V and the 2.1tV samples. The high temperature consumption peak clearly diminished for the treated sample, due to elimination of crystalline V2 O5 . It could be observed that a slight decrease of the shoulder also occurred. This would indicate that a small amount of polyvanadates chains was also removed by contacting 2.1V with the ammoniacal solution.
For the rest of the samples (not shown in Fig. 2 for the sake of simplicity), no important differences were observed when comparing the TPR after and before the NH4 OH treatment, since VOx strongly bound to ␣-Al2 O3 was stable to the ammoniacal solution treatment. According to the TPR results, the reducibility of VOx /␣-Al2 O3 showed the following order: 0.4V > 0.9V ≈ 0.8V ≈ 2.1tV > 2.1V. If there exits a relationship between reducibility of the VOx species and their activity and selectivity for oxidation reactions, VOx /␣-Al2 O3 samples are expected to have different catalytic behavior for the partial oxidation of methane. 3.5. Oxygen uptake measurements It is worthwhile to note that the reduction pretreatment at 520◦ C led to the reduction of the supported VOx species of all the samples (see TPR profiles). This reduction process was associated with a change of the color of solids from yellow (calcined samples) to greenish (reduced samples). Subsequent to reduction, oxygen uptake measurement was performed at a temperature high enough (300◦ C) to assure the complete reoxidation of vanadia. The amount of oxygen
M.A. Volpe / Applied Catalysis A: General 210 (2001) 355–361
necessary for the reoxidation is related to the nature of different vanadia species [9,10]. Oxygen uptake measurement results are shown in Table 1, as atoms of oxygen consumed per gram of vanadium. The specific consumption corresponding to the 0.4V sample was the highest among all the catalysts; this uptake would be mainly related to the reoxidation of isolated vanadate. It can be observed that the oxygen uptake for the 0.8V, 0.9V and 2.1tV samples were quite similar. This suggests that these catalysts consisted of the same VOx species (polymeric VOx ). The consumption for 2.1V was different from the other samples. For this sample, it has been inferred that V2 O5 crystallites coexist with a VOx surface monolayer. Thus, the specific oxygen uptake value, 6.8 mmol/g, would be ascribed to the reoxidation of both species. Since, the concentrations of bulk-like vanadia and molecularly dispersed VOx cannot be exactly measured, it is not possible to determine the specific uptake corresponding to each species. Summing up, the specific amount of oxygen consumed during the reoxidation of catalysts varied in the VOx /␣-Al2 O3 series due to the different nature of the V surface species. 3.6. Catalytic test All catalysts and the bare ␣-Al2 O3 were tested. It was observed that the support did not exhibit activity in the 400–530◦ C range. Table 2 shows the methane conversion and TOF number at 450 and 530◦ C for the
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Fig. 3. Dependence of the conversion of methane with temperature for (䉬) 0.4V, (䉫) 0.8V,(䊉) 0.9V, (䊏) 2.1V and ( ) 2.1tV.
catalysts. TOF numbers were calculated assuming that all vanadium atoms were active sites in the reaction. They should be considered as “lower limit” values, since they are not expressed as per exposed vanadium atoms. This fact is particularly true for the 2.1V sample, which contains bulky vanadium oxide. The conversion values for methane oxidation for all VOx /␣-Al2 O3 catalysts as a function of the reaction temperature are shown in Fig. 3. It can be observed that conversion increased with temperature for all
Table 2 Conversion of CH4 , TOF and selectivity for POM on VOx /␣-Al2 O3 Sample
Temperature (◦ C)
CH4 conversion (%)
HCHO selectivity (%)
CO2 selectivity (%)
CO selectivity (%)
TOF (mol g/V s)
0.4V
450 530
9 21
60 8
22 21
18 71
1.4 × 10−3 3.3 × 10−3
0.8V
450 530
5 13
42 15
30 35
28 55
7.6 × 10−4 2.0 × 10−3
0.9V
450 530
1 16
46 15
31 30
23 55
1.5 × 10−4 2.4 × 10−3
2.1V
450 530
2 8
– –
23 21
77 79
2.5 × 10−4 9.4 × 10−4
2.1tV
450 530
3 17
40 23
27 24
33 53
4.7 × 10−4 2.6 × 10−3
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samples. This trend is well pronounced for the 0.4V, 0.8V and 2.1tV samples, while it is less evident for 2.1V. At any temperatures, the 0.4V sample was the most active catalyst for the POM reaction, with conversion values in the 10–20% range. On the other hand, the sample 2.1V was the least active one, with conversion values in the 2–8% range. TOF and conversion values for the 0.8V, 0.9V and 2.1tV catalysts were quite similar for all the temperatures. The specific activity corresponding to these samples were intermediate between 0.4V and 2.1V. The activity of VOx /␣-Al2 O3 catalysts seems to be associated with their reducibility, as determined by the TPR experiences; the following order of activity–reducibility can be established: 0.4V > 0.9V ≈ 0.8V ≈ 2.1tV > 2.1V. In order to relate activity and reducibility, it is necessary to note that it is generally accepted that the mechanism of the POM follows a Mars-van Krevelen-type mechanism in which the active site is partially reduced by the hydrocarbon. Besides, another reaction mechanism has been proposed, which indicates that the reactivity of the catalyst depends on its capability to activate and stabilize gas phase oxygen on reduced sites [13]. For both mechanisms, reducibility would be related to the number of sites that can activate methane or oxygen and, therefore, it would be connected with activity. Oxidation of CH4 yielded HCHO, CO and CO2 . The presence of C2 Hn products was not observed. The selectivities to products at 450 and 530◦ C are reported in Table 2. At any temperature, the 0.4V sample was the most selective to HCHO (60% for a conversion value of 9%). Isolated surface V-species would be dominant in this sample, and they appear to be involved in the production of formaldehyde. A low selectivity to formaldehyde was observed for the 0.9V, 0.8V and 2.1tV catalysts at the lowest temperature (approximately 40%). These results show that V–O–V bonds are also selective, though in a lower extent, since these samples are mainly conformed by a vanadia monolayer or at least by polyvanadates chains for which the V–O–V bonds are more abundant than the V–O–support bonds. Another interpretation consistent with the present activity–selectivity data is that the monovanadate species is the only selective site responsible for the high selectivity of the 0.4V sample. The low selectivities of the 0.8V, 0.9V and 2.1tV catalysts is due to
low concentrations of the active site in these samples. However, from the analysis of TPR profiles of Figs. 1 and 2, one can observe that the concentration of the monovanadate species is undetectable for 2.1tV sample, and it is much less than a half of the total VOx species for the 0.8V sample. In spite of this, the selectivities for both samples were not much lower than the 0.4V sample (approximately 40% for the former catalysts and 60% for the latter sample at 450◦ C). For the 2.1V catalyst, which contains bulk-like vanadia and a surface monolayer, the yield of formaldehyde was zero. Probably, the polymeric vanadates are able to produce HCHO. However, it is subsequently oxidized to CO on V2 O5 crystallites. Bulk-like vanadia is known to suppress the formation of partial oxidation products without any beneficial influence on the activity [14]. The dependence of the selectivity to HCHO on activity is presented in Fig. 4 for 0.4V and 0.8V. An increase in the formation of deep oxidation products, CO2 and mainly CO, were detected when increasing temperature and, therefore, activity. The loss in the selectivity would be due to the decomposition of formaldehyde to CO [12,15]. When trying to combine oxygen uptake measurements data with the catalytic pattern of VOx /␣-Al2 O3 , a simple correlation cannot be obtained, neither selec-
Fig. 4. Dependence of selectivity of HCHO on the conversion of methane for (䉬) 0.4V and (䉫) 0.8V samples.
M.A. Volpe / Applied Catalysis A: General 210 (2001) 355–361
tivity nor activity correlated with the specific amount of oxygen uptake by the catalysts. For example, the oxygen uptake for the 2.1V catalyst was an intermediate value between those of the other samples, however, the activity and selectivity of this sample was the lowest. This lack of correlation seems to indicate that the oxygen involved in the oxygen uptake measurements, the lattice oxygen, is not associated with the formation of the reaction products, at least in a simple way. Thus, it could be speculated that the reactive oxygen species arises in the reaction conditions, and it should be evaluated on the “working catalyst”. Summing up, from the characterization and the catalytic results, it can be inferred that each species presents a different catalytic pattern. Highly dispersed monomeric species are more active and selective than polymeric VOx , while bulk-like vanadia is quite inactive, and it catalyzes deep oxidation reactions. In effect, for VOx /␣-Al2 O3 catalysts, the yields of HCHO were low, since the highest selectivities were achieved at low conversion values, and selectivity dropped drastically with increasing conversion. The selection of appropriate experimental conditions (reactor design, CH4 /O2 stoichiometry, etc.) as well as the deliberate addition of impurities could improve the limited yield, however, these modifications are beyond the scope of the present study. An important feature of the POM reaction performed on VOx /␣-Al2 O3 catalyst was that considerable conversions were observed at relatively low reaction temperatures. In the majority of the studies devoted to the POM reaction over vanadia-supported catalysts, measurable conversions are observed for reaction temperatures at 600–700◦ C [2,3,11,12]. This result could be related to the fact that VOx species supported on ␣-Al2 O3 are more easily reduced than the analogous species supported on ␥-Al2 O3 or SiO2 . Another interesting point emerges when comparing VOx supported on ␣-Al2 O3 with vanadia supported on ␥-Al2 O3 ; the oxidation of methane using ␥-Al2 O3 as a support only produces CO and CO2 , no HCHO or CH3 OH were detected. Two reasons have been suggested in order to explain this result: (I) partial oxidation products are formed, but they are further
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oxidized on acid sites of the support and exposed sites of alumina are present even for V concentration corresponding to a vanadia monolayer [16]; (II) the nature of V-species on this support renders the catalyst nonselective. According to the present results, both monomeric and polymeric vanadates are active and selective towards partial oxidation products. Thus, the formation of deep oxidation products would be related to the acidic character of ␥-Al2 O3 . 4. Conclusion The nature of VOx surface species supported on ␣-Al2 O3 controls the catalytic pattern of VOx /␣-Al2 O3 for the partial oxidation of methane. Isolated and polymeric vanadates are active and selective to HCHO at low conversion. The presence of bulk-like vanadia decreases both activity and selectivity of the title reaction. References [1] O. Krylov, Catal. Today 18 (1993) N3. [2] M. Koranne, J. Goodwin, G. Marcelin, J. Catal. 148 (1994) 388. [3] A. Parmaliana, F. Frusteri, D. Miceli, A. Mezzapica, M. Scurrell, Appl. Catal. 78 (1991) L7. [4] F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijin, J. Medema, V.H. de Beer, P.J. Gellings, J. Phys. Chem. 84 (1980) 2783. [5] M.A. Vuurman, I.E. Wachs, Phys. Chem. 96 (1992) 5008. [6] I.E. Wachs, B. Weckhuysen, Appl. Catal. A: Gen. 157 (1997) 67. [7] J. Eon, R. Olier, J.C. Volta, J. Catal. 145 (1994) 318. [8] C. Neyertz, M. Volpe, Surface Colloids (1998). [9] J. Haber, A. Kozlowska, R.J. Kozlowski, J. Catal. 102 (1986) 52. [10] M. Ferreira, M. Volpe, J. Mol. Catal. A 149 (1999) 33. [11] I.E. Wasch, R. Saleh, S. Chan, C. Chersich, Appl. Catal. 15 (1995) 339. [12] M. Banares, L. Alemany, M. López Granados, M. Faraldos, J. Fierro, Catal. Today 33 (1997) 73. [13] A. Parmaliana, V. Sokolovskii, D. Miceli, F. Arena, N. Giordano, J. Catal. 148 (1994) 514. [14] L. Owens, H. Kung, J. Catal. 144 (1993) 202. [15] A. Parmaliana, F. Arena, J. Catal. 167 (1997) 57. [16] G. Deo, I.E. Wachs, J. Catal. 146 (1994) 323.
Partial oxidation of methane over VOx /␣-Al2 O3 catalysts Mar´ıa A. Volpe Planta Piloto de Ingenier´ıa Qu´ımica, UNS-CONICET, Complejo CRIBABB, Edificio 2, Camino Carrindanga km. 7, Casilla de Correos 717, 8000 Bah´ıa Blanca, Argentina Received 19 May 2000; received in revised form 5 October 2000; accepted 6 October 2000
Abstract The catalytic behavior of a series of VOx /␣-Al2 O3 catalysts for the partial oxidation of methane has been evaluated. Samples with different vanadia loading were prepared from NH4 VO3 and V(AcAc)3 . Characterization performed by TPR and oxygen uptake measurements indicates that different VOx species are present on the samples. The catalytic patterns indicate that each V-surface species possesses different activity and selectivity. Isolated vanadates are the most active and selective towards HCHO, while V2 O5 crystallites are detrimental to the catalytic performance. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Partial oxidation of methane; Vanadia-supported catalysts; VOx
1. Introduction The worldwide surplus of methane is an incentive for the investigation of the partial oxidation of methane (POM) with oxygen to C1-oxygenates over a large number of catalysts [1]. Nonselective, complete oxidation to CO2 is the most frequent result. This fact is related to the thermodynamic aspect of the oxidation of methane, since the free energy change of the complete oxidation reaction is larger than that corresponding to the oxidation to C1-oxygenate. In addition, formaldehyde is thermodynamically unstable with respect to its decomposition. The POM is, therefore, a kinetic problem and deep oxidation must be suppressed. Vanadia supported on silica is a selective catalyst towards partial oxidation products, as methanol and formaldehyde. On the other hand, vanadia supported on ␥-alumina or on titania are active systems, but they only produce CO2 and CO [2,3]. For the case E-mail address: [email protected] (M.A. Volpe).
of VOx /␥-Al2 O3 , vanadia sites are likely to catalyze the formation of partial oxidation products, but they would undergo further oxidation on active acid sites on the surface of the support [2]. Based on this supposition, it seems interesting to test the catalytic performance of vanadia catalysts supported on low acidic ␣-Al2 O3 . The present paper reports the catalytic performance of VOx /␣-Al2 O3 catalysts for the POM. A series of VOx /␣-Al2 O3 catalysts, with vanadium content varying in the 0.4–2.1 wt.% range have been prepared. These samples contain different types of vanadia species, since, depending on loading, monomeric vanadate, polymeric surface species and crystalline V2 O5 may be present on the surface of the support [4,5]. Here, we report our attempt to determine the role of different vanadia species on the activity and selectivity for the POM. It is generally accepted that the partial oxidation of hydrocarbons on oxide catalysts involves a redox mechanism. For this reason, in the present paper, the reducibility of VOx catalysts is studied. In order
0926-860X/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 0 ) 0 0 8 1 0 - 3
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M.A. Volpe / Applied Catalysis A: General 210 (2001) 355–361
to do so, the catalysts are characterized by temperature programmed reduction and by oxygen uptake measurements.
2. Experimental 2.1. Catalysts preparation The support was ␣-alumina form Rhône Poulenc, with 13 m2 /g. VOx /␣-Al2 O3 samples will be referred to as xV, where x indicates the V weight percent of the solid, as determined by atomic absorption spectroscopy. 2.2. Samples prepared from NH4 VO3 Two VOx /␣-Al2 O3 samples were obtained by mixing 10 ml of an aqueous solution of ammonium metavanadate at pH 4 with approximately 5 g of the support. When necessary, pH was adjusted by adding aliquots of a diluted solution of HNO3 . For the 0.8V sample, the solid was filtered after 24 h of contact. For the 2.1V sample, after 24 h of contact, a solution of nitric acid was added in order to lower pH from 4 to 2. After 10 min, the solid was filtered. In both cases, the solids were calcined at 500◦ C for 4 h. 2.3. Samples prepared from V(AcAc)3 The 0.4V catalyst was prepared by reaction of the support with a solution of V(AcAc)3 in toluene (5 × 10−2 g/ml). The solid was filtered after 24 h of reaction, washed in fresh toluene and calcined in air flow at 500◦ C for 4 h. In order to obtain a more loaded sample (0.9V catalyst), a certain amount of 0.4V sample was put in contact with the V(AcAc)3 solution. The same preparation procedure as for sample 0.4V was followed. 2.4. NH4 OH treated samples The four VOx /␣-Al2 O3 samples were treated with an ammoniacal solution. Approximately 300 mg of the solids were put in contact with 10 ml of 0.3N NH4 OH solution at room temperature for 5 min.
2.5. Catalysts characterization 2.5.1. Temperature programmed reduction (TPR) The catalysts were pretreated by calcination at 530◦ C in dry air for 1 h, and subsequently purged in Ar at the same temperature and cooled to 100◦ C. Then, they were reduced from 100 to 530◦ C with a heating rate of 10◦ C/min in a H2 /Ar gas mixture (5 vol.% of H2 ) flowing at 20 cm3 /min. The H2 consumption was monitored by a thermoconductivity detector. 2.5.2. Oxygen uptake measurements Oxygen uptake measurements were performed in a volumetric apparatus operating in the pulse mode. Before measurements, the samples were reduced in a H2 flow for 4 h at 530◦ C, and then they were evacuated at the same temperature for the same period of time. Then, the samples were cooled to 300◦ C. Doses of chromatographic oxygen were admitted into the sample cell. The oxygen uptake was measured for each pulse. The specific amount of oxygen consumed, as millimoles of gas consumed per gram of vanadium, was calculated by extrapolating the total oxygen uptake to zero pressure. 2.5.3. Catalytic test All VOx /␣-Al2 O3 catalysts were tested for the POM. The sample weight was in the 50–300 mg range, and it was selected for each sample so as to have approximately the same weight of vanadium for all the samples. Test were run in a flow pyrex reactor operating at atmospheric pressure in the temperature range 400–530◦ C, using a reaction mixture He/CH4 /O2 in the molar ratio 97:1:2 flowing at a rate of 50 STP cm3 /min. Analysis of the reactants and products was performed by gas chromatography equipped with a thermal conductivity detector and two columns in parallel. Helium was the carrier gas. The line from the outlet of the reactor to the gas chromatograph was heated to approximately 60◦ C in order to avoid condensation of the products. Blank runs were performed on the same reactor packed with glass spheres without any detectable conversion. The conversion, expressed as the number of moles of methane converted, was measured at 400, 450, 500 and 530◦ C. The specific activity, TOF, was calculated as the conversion per gram of V per second. The selectivity (%) to HCOH, CO and CO2 was calculated
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as 100 × (number of moles of methane converted to product/total number of moles of methane converted).
3. Results 3.1. Samples prepared from NH4 VO3 The way to prepare the 0.8V sample is generally followed to obtain highly dispersed supported vanadia [2]. For this catalyst, the number of vanadium atoms fixed per square nanometer was 7. The value of the specific concentration was similar to those corresponding to VOx monolayer deposited on a wide variety of supports (␥-Al2 O3 , TiO2 , ZrO2 , Nb2 O5 ) [6]. Thus, it was considered that the 0.8V sample contained molecularly dispersed vanadia, and that no V2 O5 crystallites were present in this case. The amount of vanadium fixed by the 2.1V sample was higher than for the 0.8V catalyst. The difference in the vanadium concentration would be related to the fact that after acidification of the methavanadate solution, a red solid precipitated onto the solid. It is interesting to note that the vanadium species in acidic solution strongly depend on pH. The aggregation and coordination of vanadium atoms increase with diminishing pH, as methavandate, decavanates, and V2 O5 crystallites [7]. Thus, the red precipitate would be crystalline V2 O5 . Therefore, it could be postulated that the 2.1V sample is composed of two different species: highly dispersed vanadia and V2 O5 crystallites. 3.2. Samples prepared from V(AcAc)3 For the 0.4V catalyst, only half of the vanadium formerly present in the organic solution was retained by the support. The saturation of ␣-Al2 O3 was probably due to the fact that the anchored vanadium complex occupies a large area [8]. Subsequent calcination of the solid eliminated the remaining acetyl-acetonate ligands, and more hydroxyl groups were available for further reaction with the complex. This was the reason why the vanadium loading of the 0.9V sample was higher than the 0.4V sample. The similarity between the vanadium concentration of the 0.8V and 0.9V samples prepared from different precursors is worth noting. It could indicate that for
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Table 1 Vanadium concentration and oxygen uptake measurements for VOx /␣-Al2 O3 Sample
Preparation method
V (wt.%)
O2 consumed (mmol/g V)
0.4V 0.9V 0.8V 2.1V 2.1tV
V(AcAc)3 V(AcAc)3 NH4 VO3 (pH 2) NH4 VO3 (pH 4) NH4 OH treatment
0.4 0.9 0.8 2.1 0.6
8.3 4.9 4.3 6.8 4.8
both preparation methods, the maximum amount of highly dispersed VOx that ␣-Al2 O3 can accommodate was achieved. 3.3. NH4 OH treated samples The vanadium contents of some of the samples treated with the ammoniacal solution are reported in Table 1. The treatment removed considerable amount of vanadium for the 2.1V sample; from now on, this sample will be named as 2.1tV. On the other hand, a negligible concentration was removed from the 0.4V and the 0.8V samples. As established by Yoshida et al. [9], only vanadia with weak interaction with the support is soluble in a diluted ammoniacal solution. In line with this, for the 2.1V catalysts approximately half of vanadium, probably bulk like vanadia, was loosely bound. On the other samples, the major part of the vanadia would be strongly anchored to the support. 3.4. Temperature programmed reduction The TPR profile of 0.4V, 0.8V, 0.9V and 2.1V catalysts are shown in Fig. 1. The reduction pattern of the 0.4V sample was characterized by two peaks centered at 380◦ C (the major one) and at 450◦ C. The TPR spectra corresponding to the 0.9V and 0.8V samples were quite similar. Both of them outlined a main peak centered at approximately 460◦ C. For the 2.1V catalyst, the main hydrogen consumption appeared at approximately 520◦ C, with a shoulder at the lower temperature side. It is generally accepted that monomeric vanadates and polyvanadates chains are formed at low vanadia loading. When the concentration increases, polyvanadates chains condense to form surface vanadia monolayer. Finally, crystalline V2 O5 appears for concentrations above the monolayer
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Fig. 1. TPR profiles for VOx /␣-Al2 O3 catalysts: (a) 0.4V; (b) 0.9V; (c) 0.8V; (d) 2.1V samples.
Fig. 2. TPR profiles for 2.1V and 2.1tV samples.
capacity [4,6,10,13]. In line with this, for the 0.4V sample, the peak at 380◦ C would be related to isolated vanadates, while the consumption at 450◦ C should be assigned to polyvanadates or to a vanadia monolayer. The last peak also appeared in the profile corresponding to the 0.9V and 0.8V catalysts, that could be mainly constituted of a surface monolayer. In the case of the 0.8V sample, a shoulder at the low temperature side could also be detected. One could attribute this shoulder to monovanadate species. For 2.1V, the main hydrogen consumption would be related to the partial reduction of crystalline V2 O5 . The reduction of pure V2 O5 occurs at approximately 600◦ C [2]. However, it is generally accepted that the supported oxide is more readily reduced than unsupported V2 O5 . The shoulder at the side of lower temperatures should be assigned to the reduction of polyvanadates chains. Fig. 2 comparatively shows the TPR profiles of the 2.1V and the 2.1tV samples. The high temperature consumption peak clearly diminished for the treated sample, due to elimination of crystalline V2 O5 . It could be observed that a slight decrease of the shoulder also occurred. This would indicate that a small amount of polyvanadates chains was also removed by contacting 2.1V with the ammoniacal solution.
For the rest of the samples (not shown in Fig. 2 for the sake of simplicity), no important differences were observed when comparing the TPR after and before the NH4 OH treatment, since VOx strongly bound to ␣-Al2 O3 was stable to the ammoniacal solution treatment. According to the TPR results, the reducibility of VOx /␣-Al2 O3 showed the following order: 0.4V > 0.9V ≈ 0.8V ≈ 2.1tV > 2.1V. If there exits a relationship between reducibility of the VOx species and their activity and selectivity for oxidation reactions, VOx /␣-Al2 O3 samples are expected to have different catalytic behavior for the partial oxidation of methane. 3.5. Oxygen uptake measurements It is worthwhile to note that the reduction pretreatment at 520◦ C led to the reduction of the supported VOx species of all the samples (see TPR profiles). This reduction process was associated with a change of the color of solids from yellow (calcined samples) to greenish (reduced samples). Subsequent to reduction, oxygen uptake measurement was performed at a temperature high enough (300◦ C) to assure the complete reoxidation of vanadia. The amount of oxygen
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necessary for the reoxidation is related to the nature of different vanadia species [9,10]. Oxygen uptake measurement results are shown in Table 1, as atoms of oxygen consumed per gram of vanadium. The specific consumption corresponding to the 0.4V sample was the highest among all the catalysts; this uptake would be mainly related to the reoxidation of isolated vanadate. It can be observed that the oxygen uptake for the 0.8V, 0.9V and 2.1tV samples were quite similar. This suggests that these catalysts consisted of the same VOx species (polymeric VOx ). The consumption for 2.1V was different from the other samples. For this sample, it has been inferred that V2 O5 crystallites coexist with a VOx surface monolayer. Thus, the specific oxygen uptake value, 6.8 mmol/g, would be ascribed to the reoxidation of both species. Since, the concentrations of bulk-like vanadia and molecularly dispersed VOx cannot be exactly measured, it is not possible to determine the specific uptake corresponding to each species. Summing up, the specific amount of oxygen consumed during the reoxidation of catalysts varied in the VOx /␣-Al2 O3 series due to the different nature of the V surface species. 3.6. Catalytic test All catalysts and the bare ␣-Al2 O3 were tested. It was observed that the support did not exhibit activity in the 400–530◦ C range. Table 2 shows the methane conversion and TOF number at 450 and 530◦ C for the
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Fig. 3. Dependence of the conversion of methane with temperature for (䉬) 0.4V, (䉫) 0.8V,(䊉) 0.9V, (䊏) 2.1V and ( ) 2.1tV.
catalysts. TOF numbers were calculated assuming that all vanadium atoms were active sites in the reaction. They should be considered as “lower limit” values, since they are not expressed as per exposed vanadium atoms. This fact is particularly true for the 2.1V sample, which contains bulky vanadium oxide. The conversion values for methane oxidation for all VOx /␣-Al2 O3 catalysts as a function of the reaction temperature are shown in Fig. 3. It can be observed that conversion increased with temperature for all
Table 2 Conversion of CH4 , TOF and selectivity for POM on VOx /␣-Al2 O3 Sample
Temperature (◦ C)
CH4 conversion (%)
HCHO selectivity (%)
CO2 selectivity (%)
CO selectivity (%)
TOF (mol g/V s)
0.4V
450 530
9 21
60 8
22 21
18 71
1.4 × 10−3 3.3 × 10−3
0.8V
450 530
5 13
42 15
30 35
28 55
7.6 × 10−4 2.0 × 10−3
0.9V
450 530
1 16
46 15
31 30
23 55
1.5 × 10−4 2.4 × 10−3
2.1V
450 530
2 8
– –
23 21
77 79
2.5 × 10−4 9.4 × 10−4
2.1tV
450 530
3 17
40 23
27 24
33 53
4.7 × 10−4 2.6 × 10−3
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samples. This trend is well pronounced for the 0.4V, 0.8V and 2.1tV samples, while it is less evident for 2.1V. At any temperatures, the 0.4V sample was the most active catalyst for the POM reaction, with conversion values in the 10–20% range. On the other hand, the sample 2.1V was the least active one, with conversion values in the 2–8% range. TOF and conversion values for the 0.8V, 0.9V and 2.1tV catalysts were quite similar for all the temperatures. The specific activity corresponding to these samples were intermediate between 0.4V and 2.1V. The activity of VOx /␣-Al2 O3 catalysts seems to be associated with their reducibility, as determined by the TPR experiences; the following order of activity–reducibility can be established: 0.4V > 0.9V ≈ 0.8V ≈ 2.1tV > 2.1V. In order to relate activity and reducibility, it is necessary to note that it is generally accepted that the mechanism of the POM follows a Mars-van Krevelen-type mechanism in which the active site is partially reduced by the hydrocarbon. Besides, another reaction mechanism has been proposed, which indicates that the reactivity of the catalyst depends on its capability to activate and stabilize gas phase oxygen on reduced sites [13]. For both mechanisms, reducibility would be related to the number of sites that can activate methane or oxygen and, therefore, it would be connected with activity. Oxidation of CH4 yielded HCHO, CO and CO2 . The presence of C2 Hn products was not observed. The selectivities to products at 450 and 530◦ C are reported in Table 2. At any temperature, the 0.4V sample was the most selective to HCHO (60% for a conversion value of 9%). Isolated surface V-species would be dominant in this sample, and they appear to be involved in the production of formaldehyde. A low selectivity to formaldehyde was observed for the 0.9V, 0.8V and 2.1tV catalysts at the lowest temperature (approximately 40%). These results show that V–O–V bonds are also selective, though in a lower extent, since these samples are mainly conformed by a vanadia monolayer or at least by polyvanadates chains for which the V–O–V bonds are more abundant than the V–O–support bonds. Another interpretation consistent with the present activity–selectivity data is that the monovanadate species is the only selective site responsible for the high selectivity of the 0.4V sample. The low selectivities of the 0.8V, 0.9V and 2.1tV catalysts is due to
low concentrations of the active site in these samples. However, from the analysis of TPR profiles of Figs. 1 and 2, one can observe that the concentration of the monovanadate species is undetectable for 2.1tV sample, and it is much less than a half of the total VOx species for the 0.8V sample. In spite of this, the selectivities for both samples were not much lower than the 0.4V sample (approximately 40% for the former catalysts and 60% for the latter sample at 450◦ C). For the 2.1V catalyst, which contains bulk-like vanadia and a surface monolayer, the yield of formaldehyde was zero. Probably, the polymeric vanadates are able to produce HCHO. However, it is subsequently oxidized to CO on V2 O5 crystallites. Bulk-like vanadia is known to suppress the formation of partial oxidation products without any beneficial influence on the activity [14]. The dependence of the selectivity to HCHO on activity is presented in Fig. 4 for 0.4V and 0.8V. An increase in the formation of deep oxidation products, CO2 and mainly CO, were detected when increasing temperature and, therefore, activity. The loss in the selectivity would be due to the decomposition of formaldehyde to CO [12,15]. When trying to combine oxygen uptake measurements data with the catalytic pattern of VOx /␣-Al2 O3 , a simple correlation cannot be obtained, neither selec-
Fig. 4. Dependence of selectivity of HCHO on the conversion of methane for (䉬) 0.4V and (䉫) 0.8V samples.
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tivity nor activity correlated with the specific amount of oxygen uptake by the catalysts. For example, the oxygen uptake for the 2.1V catalyst was an intermediate value between those of the other samples, however, the activity and selectivity of this sample was the lowest. This lack of correlation seems to indicate that the oxygen involved in the oxygen uptake measurements, the lattice oxygen, is not associated with the formation of the reaction products, at least in a simple way. Thus, it could be speculated that the reactive oxygen species arises in the reaction conditions, and it should be evaluated on the “working catalyst”. Summing up, from the characterization and the catalytic results, it can be inferred that each species presents a different catalytic pattern. Highly dispersed monomeric species are more active and selective than polymeric VOx , while bulk-like vanadia is quite inactive, and it catalyzes deep oxidation reactions. In effect, for VOx /␣-Al2 O3 catalysts, the yields of HCHO were low, since the highest selectivities were achieved at low conversion values, and selectivity dropped drastically with increasing conversion. The selection of appropriate experimental conditions (reactor design, CH4 /O2 stoichiometry, etc.) as well as the deliberate addition of impurities could improve the limited yield, however, these modifications are beyond the scope of the present study. An important feature of the POM reaction performed on VOx /␣-Al2 O3 catalyst was that considerable conversions were observed at relatively low reaction temperatures. In the majority of the studies devoted to the POM reaction over vanadia-supported catalysts, measurable conversions are observed for reaction temperatures at 600–700◦ C [2,3,11,12]. This result could be related to the fact that VOx species supported on ␣-Al2 O3 are more easily reduced than the analogous species supported on ␥-Al2 O3 or SiO2 . Another interesting point emerges when comparing VOx supported on ␣-Al2 O3 with vanadia supported on ␥-Al2 O3 ; the oxidation of methane using ␥-Al2 O3 as a support only produces CO and CO2 , no HCHO or CH3 OH were detected. Two reasons have been suggested in order to explain this result: (I) partial oxidation products are formed, but they are further
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oxidized on acid sites of the support and exposed sites of alumina are present even for V concentration corresponding to a vanadia monolayer [16]; (II) the nature of V-species on this support renders the catalyst nonselective. According to the present results, both monomeric and polymeric vanadates are active and selective towards partial oxidation products. Thus, the formation of deep oxidation products would be related to the acidic character of ␥-Al2 O3 . 4. Conclusion The nature of VOx surface species supported on ␣-Al2 O3 controls the catalytic pattern of VOx /␣-Al2 O3 for the partial oxidation of methane. Isolated and polymeric vanadates are active and selective to HCHO at low conversion. The presence of bulk-like vanadia decreases both activity and selectivity of the title reaction. References [1] O. Krylov, Catal. Today 18 (1993) N3. [2] M. Koranne, J. Goodwin, G. Marcelin, J. Catal. 148 (1994) 388. [3] A. Parmaliana, F. Frusteri, D. Miceli, A. Mezzapica, M. Scurrell, Appl. Catal. 78 (1991) L7. [4] F. Roozeboom, M.C. Mittelmeijer-Hazeleger, J.A. Moulijin, J. Medema, V.H. de Beer, P.J. Gellings, J. Phys. Chem. 84 (1980) 2783. [5] M.A. Vuurman, I.E. Wachs, Phys. Chem. 96 (1992) 5008. [6] I.E. Wachs, B. Weckhuysen, Appl. Catal. A: Gen. 157 (1997) 67. [7] J. Eon, R. Olier, J.C. Volta, J. Catal. 145 (1994) 318. [8] C. Neyertz, M. Volpe, Surface Colloids (1998). [9] J. Haber, A. Kozlowska, R.J. Kozlowski, J. Catal. 102 (1986) 52. [10] M. Ferreira, M. Volpe, J. Mol. Catal. A 149 (1999) 33. [11] I.E. Wasch, R. Saleh, S. Chan, C. Chersich, Appl. Catal. 15 (1995) 339. [12] M. Banares, L. Alemany, M. López Granados, M. Faraldos, J. Fierro, Catal. Today 33 (1997) 73. [13] A. Parmaliana, V. Sokolovskii, D. Miceli, F. Arena, N. Giordano, J. Catal. 148 (1994) 514. [14] L. Owens, H. Kung, J. Catal. 144 (1993) 202. [15] A. Parmaliana, F. Arena, J. Catal. 167 (1997) 57. [16] G. Deo, I.E. Wachs, J. Catal. 146 (1994) 323.