Oxidative Dehydrogenation — Vanadia

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 

---

For example, the oxidative dehydrogenation of ethane and propane was examined via UV-visible and Raman spectra. The study investigated the catalytic properties vanadia formulations that possessed a range of VO surface species density (1.4—34.2 V/nm ) on an AI2O3 support. The observations showed increased surface densities, greater than 2.3 V/nm , favored two-dimensional polyvanadates. At lower surface densities, ca. 2.3 V/nm , predominately isolated monovanadate species were observed. Further increasing surface densities to more than 7.0 V/nm yielded V2O5 crystallites. ... 

After this paper was accepted for publication in November, 1992, a number of reports have appeared that deal with the subject of oxidative dehydrogenation of light alkanes. The effect of the structure of vanadia on a support has been investigated for the oxidation of butane [87J and propane [88-90], The evidence supports the concepts that the bridging oxygen in V — O — V plays an important role in the oxidation reaction [87, 90], The data also show that vanadia species of different structures on these supports have different catalytic properties, and that isolated V04 units are the most selective [91]. 

Smits, Seshan and Ross have studied the selective oxidative dehydrogenation of propane to propylene over Nb20s and Nb2C>5 supported on alumina.32 Vanadia supported on MgO has typically been used in these reactions, although reduced surface vanadyl ions can give rise to decreasing selectivity to propylene. Niobia on the other hand, is much more difficult to reduce than vanadyl but there have been few studies with this oxide in such reactions. 

Grime P, Wolfram T, Pelzer K, Schlogl R, Trunschke A. Role of dispersion of vanadia on SBA-15 in the oxidative dehydrogenation of propane. Catalysis Today. 2010 157(1—4) 137-142. 

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. 

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]. 

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]. 

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 energetics predicted may depend on the cluster size used to model this system or on the actual particle size. For oxidative dehydrogenation of alkanes catalyzed by vanadia, it has been shown that the activity per vanadium atom increases with increasing size of the vanadium particle. This is due to reduced electron transfer of oxygen to vanadium on the smaller particle and, hence, to a the lower reducibility of the smaller nano-sized particles . The lower coordination number of vanadium implies that an increase in charge is less easily accommodated on the smaller particle. 

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. 

Rozanska, X., Kondratenko, E. and Sauer, J. (2008). Oxidative Dehydrogenation of Propane Differences between N2O and O2 in the Reoxidation of Reduced Vanadia Sites and Consequences for Selectivity, J. Catal, 256, pp. 84-94. 

Owens, L. and Kung, H. (1992). Effects of Loading and Cesium Modifier on Silica-Supported Vanadia in Oxidative Dehydrogenation of Butane, Preprints-American Chemical Society, Division of Petroleum Chemistry, 37(4), pp. 1194-1200. 

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... 

R. Rulkens, T.D. Tilley - A Molecular Precursor Route to Active and Selective Vanadia-Silica-Zirconia Heterogeneous Catalysts for the Oxidative Dehydrogenation of Propane, J. Am. Chem. Soc. 120, 9959,1998. 

Heracleous, E., Machli, M., Lemonidou, A. A., and Vasalos, I. A. Oxidative dehydrogenation of ethane and propane over vanadia and molybdena supported catalysts. J. Mol. Catal. A Chem. 232,29-39 (2005). 

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). 

As a specific example of the correlations we have observed, our experimental results demonstrate that the reactions of vanadium and niobium oxide cluster cations with methanol lead to dehydrogenation of methanol under single collision conditions producing neutral formaldehyde [16]. Formaldehyde is, indeed, a major product formed from methanol over condensed-phase vanadia surfaces [17]. Another study conducted in our laboratory showed that,, Clj products were formed during the reaction between certain clusters and CCl, leaving phosgene (COClj) as the most likely neutral product [15, 18]. The decomposition of... 

---