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Abstract:
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 an extinction of activity and increase in selectivity to propylene upon melting of the crystalline K3V5O14 phase, which itself is not active in the solid state. Importantly, the melt itself, i.e., the K3V5O14 phase in the liquid state, also has no oxidative effect. The measured activity 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.
