Tungsten and Molybdenum Heteropolyanions with Different Central Ions—Correlation between Theory and Experiment

Abstract

Density functional theory calculations were carried out to investigate the electronic structures of Keggin-typed [XMo12O40]n− and [XW12O40]n− anions with different heteroatoms (X = Zn2+, B3+, Al3+, Ga3+, Si4+, Ge4+, P5+, As5+, and S6+). The influence of solvent on redox properties of heteropolyanions was discussed. For [XW12O40]n− systems two linear correlation: first, between the experimental redox potential and energies of LUMO orbital; and second, between the experimental redox potential and total energy interaction (calculated between internal tetrahedron (XO4n−), and rest of Kegging anion skeleton, (W12O36)) were designated. Taking into account the similarity of XW12O40n− and XMo12O40n− systems (in geometry and electronic structure), the estimated redox potential of molybdenum heteropolyanions (with X being p block elements) in different solvent were proposed.

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 structure of HPA shows a high degree of T d symmetry. As shown in Figure , the model of HPA with the α-Keggin structure is specific and clear, which is composed of a central atom X (commonly seen were B, Al, Ga, Si, Ge, P, As, S, and so on), , coordination atoms M (mainly including Mo, W, V, and Nb), , and oxygen atoms, and the molecular formula is XM 12 O 40 m – ( m = 2∼5). There are four types of oxygen: terminal oxygen O T , coordinating to only one skeleton metal atom M; bridge oxygen O C1 and O C4 , connecting two M 3 O 13 trimers; bridge oxygen O C2 and O C3 , connecting two coordinated oxygen atoms in the same M 3 O 13 trimer, and central tetrahedral oxygen O I .…”

“…where k B is Boltzmann's constant, T is absolute temperature, h is Planck's constant, and ΔG ≠ is the difference in Gibbs free energy between the initial state and transition state of the reaction. of a central atom X (commonly seen were B, Al, Ga, Si, Ge, P, As, S, and so on), 45,46 coordination atoms M (mainly including Mo, W, V, and Nb), 46,47 Molybdenum-based HPA compounds had been successfully applied in the selective oxidation of MAL to MAA due to their suitable acid and redox properties. 1,2 To study the influence of transition metals with similar properties in the fifth and sixth secondary groups on the redox properties of phosphorus molybdenum HPAs, the models were established by replacing one molybdenum atom with tungsten, vanadium, niobium, and tantalum atoms, namely, PMo 11 Figure 2 shows diagrams of substituted HPAs after structure optimization.…”