The Influence of Melting on Catalysis in Propane Oxidation

A model catalyst composed of crystalline potassium pentavanadate (K3V5O14) supported on silica was studied to elucidate the effect of phase transitions on the performance of potassium‐promoted vanadia catalysts in propane oxidation. Operando calorimetry shows a clear correlation between a drop in activity and an increase in selectivity to propylene upon melting of the crystalline K3V5O14 phase under reaction conditions. The pentavanadate phase itself is not active in propane oxidation, neither in the solid nor in the molten state. The activity of the catalyst mainly originates from highly‐active, i. e., unselective VxOy surface species anchored to silica and formed during synthesis in addition to the supported pentavanadate phase. Melting of K3V5O14 leads to the coverage of these VxOy species preventing the overoxidation of propylene and leading to an increase in propylene selectivity. The change in catalyst properties is therefore due to a physical effect and not to a change in the chemical properties of the predominant crystalline phase. Liquefaction cools down the catalyst. Using operando differential scanning calorimetry, a clear correlation between the melting of a surface phase and the drastic change in the catalytic properties of an alkali‐modified catalyst in propane oxidation could be demonstrated. By covering highly active vanadium centers, the inactive melt can increase selectivity in oxidation catalysis.