Fully-occupied Keggin type polyoxometalate as solid base for catalyzing CO₂ cycloaddition and Knoevenagel condensation

The highly negatively charged alkaline Keggin type polyoxometalate (POM) Na₁₆[SiNb₁₂O₄₀]·xH₂O (NaSiNb₁₂) was synthesized and used as an efficient solid base catalyst for CO₂ cycloaddition and Knoevenagel condensation reactions. This is the first report of the basicity of a fully-occupied POM with the Keggin type structure. Theoretical calculations by the density functional theory (DFT) method revealed the high negative NBO (natural bond orbital) charges (-0.919 - 1.029) of the oxygen atoms in [SiNb₁₂O₄₀]^16-, suggesting the existence of basicity, which was confirmed by CO₂-TPD and the in situ FT-IR spectra of methanol absorbed on the POM solid. Catalysis tests indicated that the POM solid base was very efficient in catalyzing the heterogeneous Knoevenagel condensation, giving high conversion and selectivity for several substrates. The catalyst NaSiNb₁₂ also presented high activity in the cycloaddition of CO₂ to epichlorohydrin under relatively mild conditions (120 °C, 1 MPa) in the absence of any solvent or co-catalyst. This solid base catalyst could be reused five times without an obvious decrease in the conversion and selectivity.