Reducibility and Dispersion Influence the Activity in Silica-Supported Vanadium-Based Catalysts for the Oxidative Dehydrogenation of Propane: The Case of Sodium Decavanadate

Calcined silica-supported sodium decavanadate (Na6V10O28/SiO2) is more active for the oxidative dehydrogenation of propane (ODP) than the thermodynamically stable alpha-polymorph of sodium metavanadate (alpha-NaVO3/SiO2) and the silica-bound, site-isolated terminal vanadium oxo [VO4]/SiO2 benchmark catalyst. Calcination of Na6V10O28/SiO2 in air at 600 degrees C leads to a mixture of Na1+xV3O8, interacting with the silica support, and the metastable polymorph of sodium metavanadate, beta-NaVO3. The formation of beta-NaVO3 at this temperature is unexpected as beta-NaVO, supported on silica and calcined at the same conditions transforms into alpha-NaVO3. At 450 degrees C (temperature of the ODP reaction) in an inert atmosphere, Na6V2O28/SiO2 transforms predominantly to the reduced phase alpha'-NaV2O5 that displays poor activity in ODP. However, the deactivated material recovers the high activity of calcined Na6V10O28/SiO2 after ca. 3 h time on stream (TOS) or after 1 h in air (450 degrees C). This observation is consistent with the proposed link between the high catalytic activity in ODP and the reducibility of a V phase as neither the catalytic performance nor characteristic Raman bands of alpha-NaVO3/SiO2 and [VO4]/SiO2 change significantly in an inert atmosphere at 450 degrees C. Vanadium K-edge operando X-ray absorption near-edge structure (XANES) and in situ Raman mapping show that the oxidation of alpha'-NaV2O5 to a mixture of Na1+xV3O8 and beta-NaVO3 occurs under ODP conditions within several minutes. In contrast, the initial activity recovers within hours (depending on the conditions), and it is explained mostly by slow redispersion of the Na1+xV3O8 phase on SiO2.