Dehydrogenation vanadia catalysts

Vanadia catalysts exhibit high activity and selectivity for numerous oxidation reactions. The reactions are partial oxidation of methane and methanol to formaldehyde, and oxidative dehydrogenation of propane to propene and ethane to ethcnc.62 62 The catalytic activity and selectivity of... 

The physicochemical properties of potassium-, bismuth-, phosphorous- and molybdenum-doped (MeA7 atomic ratios of 0 to 1) V2O5/Y-AI2O3 catalysts and their catalytic behavior in the oxidative dehydrogenation of propane have been compared. The incorporation of metal oxides modifies the catalytic behavior of alumina-supported vanadia catalysts by changing both their redox and their acid-base properties. In this way, the addition of potassium leads to the best increase in the selectivity to propylene. This performance can be related to the modification of the acid character of the surface of the catalysts. The possible role of both redox and acid-base properties of catalysts on the selectivity to propylene during the oxidation of propane is also discussed. 

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]. 

Banares, M. and Khatib, S. (2004). Structure-activity Relationships in Alumina-supported Molybdena-vanadia Catalysts for Propane Oxidative Dehydrogenation, Catal. Today, 96, pp. 251-257. 

Banares, M. A., and Khatib, S. J. Structure-activity relationships in alumina-supported molybdena-vanadia catalysts for propane oxidative dehydrogenation. Catal. Today 96,251-257(2004). 

Quantificahon greatly aids the understanding of the catalytic contributions of different vanadia species during catalytic reactions. For example, our preliminary activity test over these supported catalysts showed that the I.2V/6-AI2O3 sample exhibits better stability than higher loading catalysts for butane dehydrogenation in dilute feed [57]. The explanation is that the monovanadate species (ca 50% on the surface) dilute the polyvanadate species so that the two-dimensional coke species responsible for catalyst deactivation are less likely to form [40, 57]. 

Argyle, M.D., Chen, K.D., Bell, A.T. and Iglesia, E. (2002) Effect of catalyst structure on oxidative dehydrogenation of ethane and propane on alumina-supported vanadia. Journal of Catalysis, 208 (1), 139-49. 

Molecular strucmre and reactivity of vanadia-based catalysts for propane oxidative dehydrogenation smdied by in sim Raman spectroscopy and catalytic activity measurments. Journal of Catalysis, 111 (2), 293-306. 

Figure 15.8 Desorption of butane at room temperature in a flow of 2% 02/Ar from vanadia/alumina catalysts after butane dehydrogenation at 873 K and 1 bar.

A large variety of oxide catalysts have been claimed as beeing effective in the oxidative dehydrogenation (ODH) of propane [5-18]. Mainly vanadium based catalysts such as VPO, VMgO solids have been developed and some of them have been extensively studied in order to identify the active vanadate phases [7, 9-14], Recently, a vanadia on precipitated silica catalyst has been found to exhibit high yield in the ODH of propane [17], However, utilization of rare earth catalysts in oxidation reactions seems also to be attractive [18], because solids composed of oxides of Ce, Sm, Nd or Y and CeFs are able to preserve high selectivity at high conversion [16]. 

In recent years, much attention has been devoted to the oxidative dehydrogenation (ODH) of light paraffins [1] and alcohols to aldehydes [2]. Among all the explored catalysts, supported vanadia-based catalysts have been seen promising both in terms of activity and selectivity. A large number of factors may determine their catalytic performances such as (i) the nature,... 

The large-scale production of sulfuric acid, overall one of the most important products in the chemical industry, exploits the redox activity of vanadium(V) oxide. Vanadia-, molybdena-, and tin oxide-based catalysts are used further for a host of selective hydrocarbon oxidation processes including dehydrogenation, oxidative coupling, and oxygenation (3,41. Much recent activity in this area has been aimed at the development of more active and more selective catalysts by dispersing oxides in the form of monolayers on bulk oxide supports. This approach has proven extremely successful for tailoring catalyst properties to... 

Neither substance catalyzes hydrogen disproportionation reaction of cycloolefins. An essential difference was found in the action of these catalysts on alcohols. In the presence of vanadia, alcohols are hydro-genolyzed to the corresponding paraffins. At comparable conditions in the presence of chromia, alcohols undergo a dehydrogenation-condensation reaction with production of ketones. 

Since vanadium oxide had been used as an effective catalyst for the dehydrogenation of hydrocarbons, it was expected from purely thermodynamic considerations that conditions could be found for the reverse reaction of hydrogenation to take place. Experiments carried out in our laboratory with coprecipitated vanadia-alumina catalyst showed this to be true. 

Processes Occurring during Deactivation/Regeneration of a Vanadia/AIumina Catalyst under Propane Dehydrogenation Conditions... 

Shiju, N., Anilkumar, M., Mirajkar, S., ef al. (2005). Oxidative dehydrogenation of ethylbenzene over vanadia-alumina catalysts in the presence of nitrous oxide structure-activity relationship, J. Catal., 230, pp. 484-492. 

Notably, catalysts with redox properties, such as molybdenum-, chromium-, and vanadia-based catalysts, show high activity in various oxidative dehydrogenation reactions of hydrocarbons [45 8]. Factors influencing the reaction also include acid-base bifunctionality, which plays an important role in CO2-mediated dehydrogenation reactions [49]. Both basic sites and Lewis-acid vacant sites are important for hydrocarbons activation [50]. In fact, an enhanced basicity results in an improved performance because of the rapid desorption of the electron-rich alkenes, whereas Lewis acid sites enhance the dehydrogenation process [51]. In addition, in the presence of CO2 as feed, surface basicity favors the adsorption and reactivity of the acid CO2 molecules [52] (see also previous chapters). 

Evans OR, Bell AT, Tilley TD (2004) Oxidative dehydrogenation of propane over vanadia-based catalysts supported on high-surface-area mesoporous MgAl204. J Catal 226 292-300... 

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