The oxidative dehydrogenation of propane over V-containing mesoporous silicas: the effect of vanadium dispersion, surface acidity and support properties on the catalytic activity

Among the many materials considered for propane ODH, V-containing mesoporous silicas characterized by high surface area, large pores and well-dispersed VOx species prove to be most effective [1,2]. V-based catalysts are usually obtained by impregnation: it has been observed, though, that V-containing mesoporous systems (i.e. V-SBA-15 and V-MCF) prepared by direct synthesis exhibit better V-dispersion and superior catalytic performances in both selective and total oxidation reactions, reactions both based on V reduction and subsequent reoxidation (MvK mechanism) [3,4]. As it is known, isolated V-species are beneficial in terms of propene selectivity, whereas higher activity is usually associated to polyvanadates (up to the formation of VOx monolayer) [1-5].In this work, a V-SBA-15 and a V-MCF sample [3,4] (V cont. 2.5 wt.%) prepared by direct synthesis were tested in propane ODH. Their physico-chemical and catalytic properties were compared with those of materials with the same V-content but prepared by different ways: i) two impregnated samples (V-SBA-15-i and V-MCF-i) and ii) a non-porous sample obtained by flame pyrolysis (V-SiO2), a high temperature synthesis technique allowing high V-dispersion to be achieved [2]. As a whole, both V-SBA-15 and V-MCF exhibited better textural properties (i.e. higher SSA) and higher V dispersion (isolated V-species), as compared to impregnated ones, in which polymeric VOx and micro-crystalline V2O5 were detected by TEM, H 2-TPR micro-Raman and DR UV-Vis [4,5]. On the other hand, both impregnated samples showed higher Brønsted acidity (related to V-OH groups), as shown by IR-spectroscopy of adsorbed CO and NH3 Better catalytic performances in terms of selectivity to propene were achieved with both samples directly synthesized, as consequence of higher V dispersion and lower surface acidity. Brønsted acidity seems indeed to be negatively correlated with the selectivity to propene, since stronger Brønsted acidic sites favor the formation of CO.. In contrast, samples directly synthesized show supported V species that can be converted into Lewis acidic sites by dehydroxylation much more easily than impregnated samples (Figure 1). x and other by-products. A similar correlation was previously observed with a set of non-porous VOx-SiO2 samples prepared by flame pyrolysis (results to be published). Moreover, higher selectivity to propene was obtained over V-MCF as compared to V-SBA-15 (a similar trend was observed with impregnated ones). This behavior was related to the different porous network of the two catalysts: the structure of MCF, featuring a 3-D network with ultra-large cells (20-40 nm), favors molecular diffusion along each direction, unlike cylindrical channels of SBA-15 (3-6 nm) occurring in one dimension mainly (axial diffusion). Molecules can be much more retained inside channels of SBA-15 (deeper oxidation) as compared to MCF. On the other hand, the non-porous sample, with V-species well dispersed and incorporated into the silica framework, exhibited worse catalytic results than mesoporous samples, confirming the important role of mesoporosity in oxidation processes. References [1] F. Cavani, N. Ballarini, E. Cericola, Catal. Today 127 (2007) 113. [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. [3] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, L. Gaberova, M. Armandi, E. Garrone, Catal. Today, 176 (2011) 458. [4] M. Piumetti, B. Bonelli, P. Massiani, S. Dzwigaj, I. Rossetti, S. Casale, M. Armandi, C. Thomas, E. Garrone, Catal. Today doi:10.1016/j.cattod.2011.06.028 [5] H. Dai, A.T. Bell, E. Iglesia, J. Catal 221 (2004) 49.