Lewis Acid Dominant Windmill-Shaped V8 Clusters: A Bifunctional Heterogeneous Catalyst for CO2 Cycloaddition and Oxidation of Sulfides

Abstract

Carbon dioxide (CO₂) and sulfides in gasoline are the main causes of air pollution. Considerable attention has been devoted to solving the problems, and the catalytic reaction seems to be a good choice. Owing to the high density of Lewis acid (LA) active sites and large numbers of open methoxide groups, polyoxovanadates (POVs) are an undisputed option as a heterogeneous catalyst for the CO₂ cycloaddition reaction and catalytic oxidation of sulfides. On the basis of the above, a series of V₈ clusters, [(C₂N₂H₈)₄(CH₃O)₈V^IV₈O₁₂]·CH₃OH (V₈-1a), [(C₂N₂H₈)₄(CH₃O)₄V^IV₄V^V₄O₁₆]·4CH₃OH (V₈-1), [(C₃N₂H₁₀)₄(CH₃O)₄V^IV₄V^V₄O₁₆]·5H₂O (V₈-2), [(C₆N₂H₁₄)₄(CH₃O)₄V^IV₄V^V₄O₁₆]·5CH₃OH·2H₂O (V₈-3), have been legitimately designed and triumphantly isolated. In the synthesis process, three different kinds of Lewis bases (LBs), ethanediamine, 1,2-diaminopropane, and 1,2-cyclohexanediamine, were used to modify LA {V₈} clusters to form four diverting windmill-shaped configuration. Among them, the vanadium atoms in V₈-1a are +4 valence of V^IV, while the vanadium atoms in V₈-1-3 are mixed valence states of V^IV and V^V. Magnetic property investigation indicates that the antiferromagnetic coupling interactions between V^IV ions all exist in the four compounds. The compound V₈-1 also demonstrated high catalytic activity in the cycloaddition of CO₂ to several epoxides under relatively mild conditions (70 °C, 0.5 MPa). More importantly, the reaction pressure 0.5 MPa is the lowest among the high nuclear polyoxometallates (POMs). Furthermore, V₈-1 also has an excellent catalytic conversion for the oxidation of sulfides. The catalytic tests manifested that V₈-1 was a very efficient difunctional heterogeneous catalyst for CO₂ cycloaddition reaction and catalytic oxidation of sulfides.