V-containing SBA-15 and MCF prepared by direct synthesis as catalysts of oxidation reactions

In the oxidative dehydrogenation (ODH) of propane, the most studied catalysts are vanadium oxides (VOx) supported on different oxides [1], usually prepared by impregnation. Recently, the flame pyrolysis method (FP) has been proposed for the preparation of VOx-SiO2 and VOx-Al2O3 systems (nominal content of V2O5: 5-50 % by weight) [2,3]. Catalytic tests, showed that VOx-Al2O3 systems were more active [3], whereas VOx-SiO2 systems were more selective [2]. The most selective catalysts resulted to be that with a 10% nominal content of V2O5 (V10Si). The catalytic performance of such systems may be correlated to their physico-chemical properties, as evaluated by FT-IR and Micro-Raman spectroscopies. IR spectra of VOx-SiO2 samples showed a band at 930 cm-1, due to the vibration of SiO44- groups strongly polarized by interaction with vicinal vanadium atoms: such [SiOδ- Vδ+] species were not observed with a sample prepared by impregnation (V10Si-i), indicating that V incorporation in the silica framework takes place during FP. No evidence of V incorporation in the framework of alumina was instead detected in VOx-Al2O3 systems. Micro-Raman analysis showed the presence of catalytically active isolated V=O groups (signals 1027 and 512 cm-1) only with sample V10Si, whereas with all other VOx-SiO2 and VOx-Al2O3 samples only the typical bands of crystalline V2O5 were observed [2,3]. Adsorption of CO at nominal 77 K on V10Si sample showed the presence of different OH species, with the following scale of relative acidity: isolated SiOH < H-bonded SiOH < V-OH [2]. The Brønsted acidity was studied by means of NH3 adsorption on samples outgassed at 500 °C. The blue-shift (ΔνNH4+) of the band due to the bending vibration of NH4+ species with respect to that of free NH4+ (1410 cm-1) can be used as a measure of the acidic strength of Brønsted sites: the smaller the shift the higher the Brønsted acidity. An interesting correlation has been observed between the acidic strength of Brønsted sites and the selectivity to propylene for both VOx-SiO2 and VOx-Al2O3 systems: the Figure reports selectivity to propylene, measured at propane isoconversion, as a function of the Brønsted acidity, measured as ΔνNH4+. For both systems, the selectivity increases with the decreasing of the Brønsted acidic strength, which favors side-reactions catalyzed by acids leading to a higher amount of COx [4]. In particular, Brønsted acidity seems to play an important role in the formation of CO, although also the reaction C + CO2  2CO shall also contribute to its formation [3]. CO is mainly a product of the secondary oxidation of propylene, whereas CO2 forms from primary combustion of propane [4-6]. EPR spectra revealed a weaker V=O bond in VOx- Al2O3 systems [3]: being V=O bond strength an index of oxygen availability, this explains their higher activity. In conclusion, for FP catalysts: (i) a better dispersion of the active phase can be obtained with silica rather than with alumina; (ii) selectivity to propylene depends on Brønsted acidity; (iii) stronger Brønsted acidity leads to higher CO production. Therefore, the best catalyst (V10Si) was characterized by isolated vanadium species, less acidic Brønsted sites and stronger V=O bonds. References. 1. F. Cavani, N. Ballarini, A. Cericola, Catal. Tod. 127 (2007) 113-131. 2. 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. 3. 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., in press, ISSN:0920-5861 DOI:10.1016/j.cattod.2008.05.020. 4. K. Chen, A. Khodakov, J. Yang, A. T. Bell, E. Iglesia, J. Catal. 186 (1999) 325-333. 5. O. R. Evans, A. T. Bell, T. Don Tilley, J. Catal. 226 (2004) 292-300. 6. M. D. Argyle, K. Chen, A. T. Bell, E. Iglesia, J. Catal. 208 (2002) 139-142.