The role of steam in selective oxidation of methacrolein over H₃PMo₁₂O₄₀

Role of steam in selective oxidation of methacrolein with molecular oxygen over H₃PMo₁₂O₄₀ catalyst was investigated. Addition of steam to feed gas significantly enhanced both catalytic activity and selectivity to methacrylic acid, which were fivefold and twice increases, respectively, under the optimal steam pressure (P_H2O = 0.13 atm). Kinetic analysis demonstrated that the addition of steam caused 200-fold increase in the pre-exponential factor for the formation of methacrylic acid, leading to the significant increase in the activity. The steam in the feed gas varied hydrous state of H₃PMo₁₂O₄₀ under the reaction conditions, while did not alter redox property, molecular and crystalline structures, and surface area of the catalyst. In the presence of steam at 573 K, three H₂O per one H₃PMo₁₂O₄₀ were absorbed and hydrated protons like [H₃O]⁺ were formed in the bulk of H₃PMo₁₂O₄₀. Methacrolein was adsorbed on the surface of the hydrous catalyst, but not on anhydrous one at all. Based on the results, it was concluded that activation of methacrolein readily occurred on the catalyst in the presence of steam, leading to the significant increase in the pre–exponential factor. Quantum chemical calculation supported the smooth activation of methacrolein by the reaction with [H₃O]⁺ without any transition state.