Kinetics of the propane oxidative dehydrogenation on vanadia/titania catalysts from steady-state and transient experiments

Oxidative dehydrogenation of propane over V2O5/TiO2 (anatase) catalysts has been studied by steady-state and transient kinetic methods. Steady-state experiments were carried out by measuring conversions and selectivities for various feed compositions including C3H8, O-2 and He, and for various contact times, at 225, 250 and 280 degreesC. Transient tests consisted in submitting a fixed bed of catalyst to a constant flow rate of feed with alternating concentration steps of O-2 in He and C3H8 in He, separated with steps of pure He. The influence of reaction temperature and partial pressure of C3H8 on the concentrations of the reaction products (C3H6, CO, CO2, H2O) has been studied in this way in the temperature range 166-446degreesC. The results obtained by both methods could be interpreted on the basis of a mechanism in which the C3H6 formation proceeds through a Eley-Rideal sequence of steps, i.e. without participation of the adsorbed propane species. On the basis of the above results, two reaction models have been compared, considering the lattice oxygen as the only oxygen species which participate in the reaction. In the first model, different reaction orders with respect to concentration of the surface oxide species have been considered, for the selective ODH reaction (order = 1) and for total combustion reactions (order = 2). In the second model, two different types of lattice oxygen were assumed, one of them giving rise to selective oxidation, the other to total combustion products. Both models provided equally good representation of the experimental results, but the first one seems more plausible on the basis of additional arguments. Kinetic constants for both models (activation energies, pre-exponential factors) obtained on the basis of steady-state results are given. (C) 2002 Elsevier Science B.V. All rights reserved.