DFT Insights into the Effect of Metal Substitution on the Catalytic Activity of Molybdenum-Based Heteropoly Acid in the Oxidation of Methacrolein to Methacrylic Acid

The selective oxidation of methacrolein (MAL) to methacrylic acid (MAA) over heteropolyacid has received great attention, while the "structure–activity" descriptor for catalyst development and improvement needs intensive exploration. Herein, the selective oxidation of MAL to MAA on phosphorus molybdenum heteropolyanion PMo12O403– (HPA) with different substitutions of coordinated metal atoms (M = W, V, Nb, and Ta) was calculated using density functional theory (DFT). The effect of substituted metal atoms on the electronic properties of HPA, adsorption and oxidation of MAL to MAA on HPA and PMo11MO40n– (M-HPA; n = 3 or 4), and activation of oxygen by vacancy were revealed as well as the catalytically active sites. The differences in the energy barriers of the rate-determining step were compared and explained. The accessible electronegativity was used as the descriptor associated with the energy barrier of the rate-determining step, and an inverted volcano relationship was unraveled. To further understand the break and the formation of crucial bond during the reaction, a formula defined as matched-degree was proposed. These studies would be beneficial for the efficient screening and design of heteropoly acid catalysts.