DFT Studies on the Influence of the Substituted Central Atom on Molybdenum-Based Heteropolyanions Used for the Catalytic Oxidation of Methacrolein to Methacrylic Acid

Heteropolyacids have been used as efficient and sustainable catalysts for the selective oxidation of methacrolein (MAL) to methacrylic acid (MAA) in recent years due to their adjustable structure. Herein, the influence of the substituted central atom (X = Al, Ga, Si, Ge, As, and S) on the structural and electronic properties of primary molybdenum-based heteropolyanions was studied. The density functional theory results indicated that the bridge oxygen of the heteropolyanion was more prone to breakage than the terminal oxygen. The interaction between MAL and catalytic sites of the molybdenum-based heteropolyanion having different central atoms and the corresponding adsorption energies were calculated. The reaction mechanism of MAL oxidation catalyzed by the heteropolyanion was systematically studied, which included the activation of the carbonyl group, hydrogen transfer, and generation of ester-based intermediates, formation of a carboxyl structure, and oxygen activation. The first step of carbonyl group activation is confirmed as the rate-determining step of this reaction. As a result, a linear relationship was discovered between the activation barrier of the rate-determining step and the lowest unoccupied molecular orbital (LUMO) of the heteropolyanion as the LUMO energy reflected the redox potential, which indicated that an increase in the LUMO energy would lead to poor catalytic performance. In addition, the activation energy of the rate-determining step would decrease in positive proportion to the ratio of the negative charge of the heteropolyanion to the electronegativity of the central atom. This correlation result indicated that the reactivity of the heteropolyanion could be predicted by calculating the negative charge and electronegativity. These findings would provide theoretical insights into the heteropolyanion at the molecular level and assist in the modification of the heteropolyacid structure to improve the catalytic performance.