Alumina-supported vanadia

A similar comparison of results of TPR/TPO-Raman spectroscopy with those of quantitative TPR has also been made for alumina-supported vanadia (Kanervo et al., 2003). However, the Raman signal of V205 crystals is at least ten times more intense than that of surface VOx species for excitation in the wavelength range of 514—532 nm, because of resonance enhancement (Xie et al., 2000). Thus, only a minor fraction of the surface VO species on alumina aggregated to form microcrystals during reduction and oxidation cycles. 

This trend in the TOF values was found not to correspond with the variations in the strength of the terminal V=0 bond as measured by the respective Raman shifts (Banares, 1999 Wachs et al., 1996). Potassiumdoping of alumina-supported vanadia catalysts resulted in lower V = O frequencies, which indicated a weakened terminal V = O bond (Cortez et al., 2003). However, the propane conversion and the catalyst reducibility decreased. Therefore, it was not considered to be likely that the terminal V=0 bond is the active site for alkane ODH on supported vanadia. The same effect was observed for titania-supported vanadia. DFT calculations described a close interaction of potassium ions with both the supported vanadia and the titania support (Si-Ahmed et al., 2007 Lewandowska et al., 2008). Such an interaction leads to an elongated V=0 bond with a... 

These results led to questioning of the presumed similar performance of polymeric and isolated surface species. Raman spectroscopy and UV-vis DRS investigations of zirconia- and alumina-supported vanadia showed that the oxidation state of the surface vanadium oxide species was determined by the propane-to-C>2 ratio in the feed (Garcra-Cortez and Banares, 2002). The combination of two spectroscopic techniques provided more detail about the structural state of the supported species during moderate reduction under reaction conditions (Gao et al., 2002). 

These observations are consistent with those of other UV-vis experiments (Puurunen and Weckhuysen, 2002 Puurunen et al., 2001). Raman spectroscopy of a working alumina-supported vanadia catalyst, showed that the surface population ratio of polymeric-to-isolated vanadia species decreased during reduction (as indicated by a relative loss in intensity of the 1009-cm 1 band relative to that of the 1017-cm 1 band), whereas the total activity and selectivity in propane ODF1 essentially remained unaffected (Garcia-Cortez and Banares, 2002). This result suggested that the active sites for ODH of propane and of ethane on alumina-supported vanadia should be isolated surface vanadia sites, whereas other arrangements of vanadium sites such as polymeric species did not seem to be crucial. 

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. 

Koranne, M.M., Goodwin, J.G.Jr and Marcelin, G. (1994) Gharacterization of silica- and alumina-supported vanadia catalysts using temperature programmed reduction. Journal of Catalysis, 148 (1), 369-77. 

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]. 

Alumina-supported vanadia catalyst (V/AL) was prepared by "wet" impregnation method of a Girdler T126 y-Al203 support (Sbet= 188 m g" ), with an ammonium metavanadate solution at a pH of 7 [5]. The concentration of the vanadia solution was selected in order to achieve a final vanadium loading of 3.5 wt % of vanadium atoms. The sample was calcined at 600°C for 6 h. 

Mo-, P-, Bi- and K-doped catalysts were prepared by impregnation of the alumina-supported vanadia catalyst with an aqueous solution of a salt of the corresponding metal. The amount of metal oxide was varied in order to obtain MeA/ atomic ratios between 0 and 1. All the catalysts tested were previously calcined at 600°C during 6 h. The catalysts will be named with the metal promoter and, in parenthesis, the MeA/ atomic ratio. Table 1 shows the physico-chemical characteristics of some of the studied catalysts. 

The intensity of the band at 1450 crn l observed in the spectrum of the undoped alumina-supported vanadia catalyst decreases upon the addition of bismuth, potassium, phosphorous or molybdenum. These results indicate a reduction of the number of Lewis acid sites after the incorporation of metal oxides. In the case of K-doped catalysts the low intensity of the band at 1450 cm 1 clearly demonstrates that the majority of the surface acid sites disappears with the incorporation of potassium. Thus, Lewis acid sites have completely disappeared in the K(0.7) catalyst. 

It has been suggested that the Incorporation of alkali metals on Ti02-vanadia catalysts decreases both the V=0 stretching frequencies and their polarizing power, while the incorporation of acid anions produces an opposite trend [20]. In addition, the presence of alkali ions decreases the heat of the propylene adsorption [17,18, 21]. Thus the different catalytic behavior of doped alumina supported vanadia catalysts, could be explained on the bases of the influence of the acid-base character of catalysts on the adsorption/desorption of propane and propene. In any case, the redox properties must be also considered. In this way, it will be interesting to study if, realy, a lower reducibility of the active sites could favor a lower rate of the consecutive reactions, as it has been observed in the case of K-doped catalysts. 

In conclusion, this paper shows the effect of the addition of different metal oxides (K, Bi, P and Mo) on the catalytic behavior of an alumina-supported vanadia catalysts in the ODH of propane. In all cases, the addition of small amounts of metal oxide (MeA/ atomic ratio of 0.1) increases the selectivity to propylene, probably as a consequence of the elimination of non selective sites (Lewis acid sites) on the surface of the support. However, only in the case of K-doped catalysts the selectivity and the yield of propylene increases with the metal content. The varition of the acid-base character of catalysts and its influence on the adsorption/desorption of reactants and products could be responsible of the different performances obsen/ed. In this way. 

Figure 17.6. Raman spectra of an alumina-supported vanadia catalyst during propane ODH vs O2/C3H8 molar ratio at 400°C in the operando Raman-GC cell during the spectroscopic study of propane ODH (A) and conversion of propane and selectivity to CO, CO2 and propylene analyzed online (B). Reprinted with permission from Cortez, G. and Banares, M. (2002), J. Catal.y299, p. 197, copyright 2002 from Elsevier.

Ermini, V., Finocchio, E., Sechi, S., et al. (2000). Propane Oxydehydrogenation over Alumina-supported Vanadia Doped with Manganese and Potassium, A/)/)/. Catal A, 198, pp. 67-79. 

An IR study has been performed on the adsorption, from the vapour phase, and the coadsorption of tert-butyl hydroperoxide (TBHP) and benzothiophene (BT) over the active catalyst, alumina-supported vanadia. ... 

TEXTURE AND SURFACE PROPERTffiS OF SUPPORTED METALLIC OXIDE CATALYSTS Na-DOPED, TITANIA AND ALUMINA-SUPPORTED VANADIA... 

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