Supported vanadium oxide catalysts, ethane oxidation

Gao, X. Banares, M.A. Wachs, EE. Ethane and w-butane oxidation over supported vanadium oxide catalysts An in situ UV-visible diffuse reflectance spectroscopic investigation. J. Catal. 1999,188, 325-331. 

In this paper selectivity in partial oxidation reactions is related to the manner in which hydrocarbon intermediates (R) are bound to surface metal centers on oxides. When the bonding is through oxygen atoms (M-O-R) selective oxidation products are favored, and when the bonding is directly between metal and hydrocarbon (M-R), total oxidation is preferred. Results are presented for two redox systems ethane oxidation on supported vanadium oxide and propylene oxidation on supported molybdenum oxide. The catalysts and adsorbates are stuped by laser Raman spectroscopy, reaction kinetics, and temperature-programmed reaction. Thermochemical calculations confirm that the M-R intermediates are more stable than the M-O-R intermediates. The longer surface residence time of the M-R complexes, coupled to their lack of ready decomposition pathways, is responsible for their total oxidation. 

Ethane Oxidation on Supported Vanadium Oxide. Figure 1 shows the rates of production of the major products of ethane oxidation over a series of silica-supported vanadium oxide catalysts. As was described earlier, the structure of the catalyst changed considerably with the active-phase loading (77). The low loading samples (0.3 -1.4%) were shown to consist primarily of 0=V03 monomeric units, while the high loading catalysts (3.5 - 9.8%) were composed of V2O5 crystallites. 

Methane and ethane have been photooxidised to the corresponding aldehydes using a solid-supported vanadium oxide catalyst, V205/S102-IW (incipient wetness) at elevated temperatures.Both processes are highly sensitive to reaction temperature and to the method by which the catalyst is prepared. 

Supported vanadium oxides have been proposed as selective catalysts in partial oxidation reactions [1] and more specifically in the oxidative dehydrogenation (ODH) of short chain alkanes [2, 3]. However, it has been observed that the catalytic behavior of these catalysts during the oxidation of alkanes depends on the vanadium loading and the acid-base character of metal oxide support. In this way, alumina-supported vanadia catalysts with low V-loading are highly active and selective during the ODH of ethane [4-7] and propane [8] but they show a low selectivity in the ODH of n-butane [4, 5, 9, 10]. 

Wada, K Yamada, H Watanabe, Y and Mitsudo, T. (1998) Selective photo-assisted catalytic oxidation of methane and ethane to oxygenates using supported vanadium oxide catalysts. /. Chem. Soc. Faraday Trans., 94> 1771 1778. 

Ethylene represents the major feedstock for the petrochemical industry and is generally produced by ethane and propane steam cracking, which also produces a wide range of hydrocarbon products. Currently, there is much interest in developing a selective catalytic route for the oxidative conversion of ethane to ethylene. Supported vanadium oxide catalysts are particularly good for this process because... 

Martinez-Huerta, M., Gao, X., Tian, H., et al. (2006). Oxidative Dehydrogenation of Ethane to Ethylene over Alumina-supported Vanadium Oxide Catalysts Relationship between Molecular Structures and Chemical Reactivity, Catal. Today, 118, pp. 279-287. 

Martinez-Huerta, M., Deo, G., Pietro, J., et al. (2007). Changes in Ceria-supported Vanadium Oxide Catalysts during the Oxidative Dehydrogenation of Ethane and Temperature-programmed Treatments, 7. Phys. Chem. C, 111, pp. 18708-18714. 

Gao, X., Banares, M. and Wachs, 1. (1999). Ethane and n-Butane Oxidation over Supported Vanadium Oxide Catalysts An In Situ UV-Visible Diffuse Reflectance Spectroscopic Investigation, J. Catal, 188, pp. 325-331. 

In the case of the ODH of ethane, the catalytic behavior of supported V-catalysts is completely different, and a positive role of acid sites in catalysts on the selectivity to ethylene is observed as a consequence of i) the presence of acid sites which do not have a negative effect on the olefin stability, and ii) the positive effect of the support surface acid sites on the catalytic activity of V-atoms." " In this way, the catalytic activity of supported vanadium oxide catalysts, when using metal oxide supports with an acid character (such as AI2O3), is higher than when supported on metal oxides with a basic character (such as MgO). ... 

Le Bars, J., Auroux, A., Forissier, M., and Vedrine, J. C. Active sites of YfiJg-AiPj catalysts in the oxidative dehydrogenation of ethane. J. Catal. 162,250-259 (19%). Khodakov, A., Olthof, B., Bell, A. T., and Iglesia, E. Structure and catalytic properties of supported vanadium oxides Support effects on oxidative dehydrogenation reactions. J. Catal. 181,205-216 (1999). 

MgO-supported vanadium catalysts have been proposed as active and selective catalysts for the ODH of propane and n-butane [1,2], while AljOg-supported vanadium catalysts present a high selectivity to ethene during selective oxidation of ethane [3],... 

In this work, the activity and selectivity of catalysts based on niobium and vanadium oxides supported on high surface area anatase Ti02 in ethane ODH have been investigated. Specifically, the influence of the cooperation of vanadium and niobium oxides supported phases as components inducing respectively redox and acid properties, together with the effect of the preparation conditions on the catal3dic performances have been studied. 

Supported metal oxides are currently being used in a large number of industrial applications. The oxidation of alkanes is a very interesting field, however, only until recently very little attention has been paid to the oxidation of ethane, the second most abundant paraffin (1). The production of ethylene or acetaldehyde from this feed stock is a challenging option. Vanadium oxide is an important element in the formulation of catalysts for selective cataljdic reactions (e. g. oxidation of o-xylene, 1-3, butadiene, methanol, CO, ammoxidation of hydrocarbons, selective catalytic reduction of NO and the partial oxidation of methane) (2-4). Many of the reactions involving vanadium oxide focus on the selective oxidation of hydrocarbons, and some studies have also examined the oxidation of ethane over vanadium oxide based catalysts (5-7) or reviewed the activity of vanadium oxide for the oxidation of lower alkanes (1). Our work focuses on determining the relevance of the specific oxide support and of the surface vanadia coverage on the nature and activity of the supported vanadia species for the oxidation of ethane. 

The work was strongly inspired by Union Carbide s Ethoxene process, a route for manufacturing ethylene from ethane and oxygen by oxidative dehydrogenation. The first catalysts consisted of molybdenum, vanadium, and niobium oxides. The selectivity for ethylene was very high but, unfortunately, the conversion of ethane was low ( 10%). Therefore, scientists at the time focused on the co-production of ethylene and acetic acid. A catalyst consisting of molybdenum, vanadium, niobium, calcium, and antimony supported on a molecular sieve was developed (63% selectivity to acetic acid, 14% selectivity to ethylene, and 3% conversion of ethane). In addition, Rhone-Poulenc (catalyst vanadium oxide or vanadyl pyrophosphate) and BP (catalyst combination of rhenium and tungsten) patented processes for the production of acetic acid from ethane. Very efficient catalysts were also disclosed by Hoechst (molybdenum vanadate, promoted with Nb, Sb, Ca, and Pd, 250-280 °C, 15 bar, 86% selectivity to acetic add at 11% conversion of ethane per pass) and Sabic (phosphorus-modified molybdenum-niobium vanadate, 260 °C, 14 bar, 50% selectivity to acetic acid at 53% conversion of ethane). 

The reactivity of vanadium oxide in ethane ODH is highly dependent on the type of support [18, 37, 42, 47-51]. The performance of the catalysts depends on the characteristics of the supports and the nature of the active species formed on these supports. At a given surface density, the vanadia-support interactions determine the type of VO structure formed. Banares and co-workers [52-53] found that in ethane ODH, the turnover frequency values vary with the specific oxide support for both the isolated and polymeric surface VO species. Thus, the support has a significant influence on the reaction parameters. The support cation directly affects the reactivity of the bridging V-O-support bond. 

Solsona, B., Blasco, T, L6pez Nieto, J. M., Pena, M. L., Rey, R, and Vidal-Moya, A. Vanadium oxide supported on mesoporous MCM-41 as selective catalysts in the oxidative dehydrogenation of alkanes. J. Catal. 203,443 52 (2001). Martinez-Huerta, M. V, Fierro, J. L. G., and Banares, M. A. Monitoring the states of vanadium oxide during the transformation of TiO anatase-to-rutile under reactive environments reduction and oxidative dehydrogenation of ethane. Catal. Commun. 

Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 g. i.s.ml 1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (A1203, HMS, MFI). 

Oxidative dehydrogenation of ethane over vanadium and niobium oxides supported catalysts... 

There seems to be no literature about the direct oxidation of ethane to acetic acid over heteropolycompounds catalysts. Nevertheless, there is a limited amount of literature[10,26-28] about direct oxidation of ethane to acetic acid over oxide catalysts at low temperature (200-350 C). It seems that vanadium and molybdenum are necessary to those catalysts, and the addition of water is useful to increase the production of acetic acid. Roy et al. [10] has proved that vanadium and molybdenum phosphates supported on Ti02-anatase were effective in the direct oxidation of ethane to acetic acid. Considering previous research results, it is suggested that other promoters, such as trcmsition-metal oxides, are necessary to enhance the catalytic activity of the activated H3PMol2O40(Py) in the direct oxidation of ethane to acetic acid. 

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