XAS enlightening of the local structure of VOx sites in catalysts for the ODH of propane

The need for light olefins is progressively growing, with a higher rate for propene, leading to the necessity of alternative production routes. The oxidative dehydrogenation (ODH) technology can be hardly considered mature and it has not yet found commercial application due to selectivity issues, especially when applied to propane. V-based catalysts proved the most interesting for propane ODH, but different possible reaction pathways have been proposed [1,2], depending on the polymerisation degree of surface V species. A detailed assessment of V local structure may help in elucidating activity and selectivity issues, in particular when coupled with an investigation on acidic properties. Therefore, the present investigation aims at defining the active site structure of VOx/SiO2 and VOx/Al2O3 catalysts characterised by variable metal loading and prepared by different techniques. X-ray absorption analysis has been used to assess the local environment of the active metal and the results, compared with Raman, FT-IR, EPR data, helped in interpreting the catalytic performance. An innovative flame pyrolysis (FP) method has been used for the preparation of AlVO4 and various silica and alumina supported samples with V loading ranging from 10 to 50 wt%. Comparative samples have been prepared by impregnation of FP-prepared SiO2 and Al2O3 from a NH4VO3 solution (V10Si-i and V10Al-i) and of a commercial silica (VAG1984). A different preparation route has also been used, namely co-precipitation from TEOS and ammonium vanadate [3] achieving a V2O5 content of 6.7wt% (VAG1) and 15wt% (VAG3). X-ray absorption spectra were collected at the GILDA beamline in the ESRF facility (Grenoble, France) on ca. 50 mg of each sample, ground, mixed with cellulose powder and pressed to obtain a thin wafer. Samples of V2O5 and AlVO4 were used as reference. The XANES spectra of all the samples are reported in Fig. 1. By comparing such patterns with the selected reference materials one may immediately notice that most samples can be defined V2O5-like, with the exception of ALVO4, V10Si and, to some extent V10Al, which may be defined AlVO4-like. A possible structure of V sites embedded in the SiO2 matrix has been also proposed on the basis of the EXAFS spectra (Fig. 2,3). Fig. 1: XAS spectra. Fig. 2: Schetch of V site structure in AlVO4-like samples, such as V10Si (Yellow: V5+; green: Si4+). Fig. 3: Schetch of V site structure in V2O5-like samples. No vicinal V sites have been detected for sample V10Si (high V dispersion) and the XAFS spectra have been fitted with an oxygen ion at ca. 1.5Å (vanadyl oxygen) and 4 identical oxygen ions located at ca. 1.8Å. A further oxygen ion is present at a distance of ca. 3 Å, ascribed to the silica matrix (Fig. 2). A similar picture may be drawn for AlVO4 and V10Al. By contrast, many different oxygen species (and vicinal V ions) have been detected for the other samples. A much higher V dispersion of the FP-prepared samples with respect to those prepared by impregnation, at the same or even higher V-loading, can be concluded by XAS analysis. This reflected on catalyst performance [4-6], showing a higher selectivity at isoconversion for sample V10Si with respect to the homologue prepared by impregnation or coprecipitation, especially when operating under anaerobic conditions. References: [1] X. Rozanska, R. Fortrie, J. Sauer, J. Phys. Chem. C, 111 (2007) 6041. [2] K.D. Chen, A.T. Bell, E. Iglesia, J. Catal., 209 (2002) 35. [3] N. Ballarini, F. Cavani, M. Ferrari, R. Catani, U. Cornaro, J. Catal. 213 (2003) 95. [4] I. Rossetti, L. Fabbrini, N. Ballarini, F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E. A. Blekkan, L. Forni, J. Catal. 256 (2008) 45-61. [5] I. Rossetti, L. Fabbrini, N. Ballarini, C. Oliva , F. Cavani, A. Cericola, B. Bonelli, M. Piumetti, E. Garrone, H. Dyrbeck, E.A. Blekkan, L. Forni, Catal. Tod., 141 (2009) 271-281. [6] C. Oliva, S. Cappelli, I. Rossetti, N. Ballarini, F. Cavani, L. Forni, Chem. Eng. J., 154 (2009) 131.