The role of CO2 in the dehydrogenation of propane over WOx-VOx/SiO2

A series of WOx-VOx catalysts supported on porous silica, with W/V molar ratios between 0 and 0.6, are examined for propane dehydrogenation in the presence and absence of CO2 from 500 degrees C to 600 degrees C and at atmospheric pressure. Catalysts characterization using temperature programmed reduction (H-2-TPR), temperature programmed oxidation (CO2-TPO), Raman spectroscopy, X-ray diffraction and transmission electron microscopy shows that the combination of the two metal components allows retention of VOx dispersion during the reaction. CO2 has the ability to oxidize V2O3 to V2O4 and participates in the oxidative dehydrogenation of propane to propylene. When the reaction is carried out with D-2 present in the feed together with C3H8 and CO2 (D-2:C3H8:CO2 = 1:1:1), only 45% of the resulting water contains D2O. This confirms that the reaction follows the oxidative dehydrogenation route of propane but is also accompanied by the reverse water gas shift reaction in combination with the non-oxidative dehydrogenation route. Moreover, one of the major roles of CO2 is the suppression of the formation of surface carbon. A partially reduced vanadia dimer was used to represent the active site and density functional theory (DFT) calculations were performed. This allowed to confirm that propane dehydrogenation in the presence of CO2 can proceed simultaneously via direct oxidative dehydrogenation and non-oxidative dehydrogenation followed by the reverse water gas shift reaction. According to the DFT-calculated Gibbs free energy profile at 600 degrees C, the activation of the secondary C-H bond of propane (E-DFT,E-act = 158 kJ/mol) is rate-limiting, while re-oxidation of the catalyst with CO2 is potentially much faster. The catalyst with a W/V = 0.1 M ratio has the highest C3H6 average turnover frequency but higher selectivities were obtained with W/V = 0.6. In agreement with the value predicted from DFT, the experimental apparent activation energy for all investigated W/V ratios is similar and varies from 127 +/- 11 kJ/mol to 147 +/- 12 kJ/mol with W/V molar ratios between 0 and 0.6. (C) 2015 Elsevier Inc. All rights reserved.