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

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