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Cr/SiO2 catalysts (Cr loading in the 0.25-2.0 wt% range) have been prepared by direct hydrothermal synthesis in the presence of templating agents, in order to attain porous systems with high specific surface area (in the 600-1000 m 2 ⋅g -1 range), and then characterized and tested in the oxidative dehydrogenation of propane in the presence of CO2 or CO2+O2 as oxidant. The extent and regularity of mesopores decreased significantly by increasing the Cr content (X-ray diffraction, N2 adsorption, transmission electron microscopy), but this did not limit the catalytic performances of the catalysts with higher Cr loadings. In all cases the only chromium species found were surface chromates (diffuse reflectance electronic spectroscopy and X-ray absorption near edge spectroscopy), accompanied by Brønsted acid centres (infra-red spectra of adsorbed NH3). All catalysts appeared stable towards irreversible deactivation, even after ca. 900 min of testing, and yields in propene as high as 40% were attained. The combination of spectroscopic and catalytic results allowed to rationalize, at least in part, the role of different oxidants in defining the chromium oxidation state and a tentative correlation of the oxidation state of Cr species during the reaction (Cr 2+ /Cr 3+ ) with selectivity in propene is suggested. -the sum of all selectivities was never equal to 100%, as C atoms were also involved in the formation of coke. At the beginning of the process ∑SC3+C2+C1 for 2.0-Cr/DHS was ca. 80% and only after ca.10 h it approached to 100%, due to the stabilization of the coke formation, accompanied with a significant the increase of propene selectivity and some decrease in the activity of the cracking processes.

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