Hydrocarboxylation of Olefins Catalyzed by Polyoxometalate-Anchored Palladium Single-Atom Catalysts

Herein, the Keggin-type polyoxometalate (POM)-anchored palladium single-atom catalysts [(n-C₄H₉)₄N]₆[SiW₁₁O₃₉Pd] and [(n-C₄H₉)₄N]₅[PW₁₁O₃₉Pd] were constructed and employed in the olefin hydrocarboxylation reaction under two-phase conditions. Both the conversion of styrene and the yield of the carboxylic acids can reach as high as 95% at 110 °C. The characterization by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), electrospray ionization mass spectrometry (ESI-MS), and in situ Fourier-transform infrared (FT-IR) spectra of CO adsorption indicated the incorporation of the isolated Pd atom into the POM structure. The POM-anchored Pd catalysts can be directly stored under an ambient atmosphere and easily recovered in consecutive catalytic recycling. Tetrabutylammonium bromide (TBAB) played a crucial role in stabilizing/dispersing the Pd catalyst. Moreover, the catalytic systems showed a unique “pseudo-liquid-phase” behavior and the reactants were absorbed into the flexible POM anions. The catalysts exhibited a wide substrate suitability and have been applied under phosphine ligand-free conditions. The catalytic mechanism studies involving D₂O showed that the hydrocarboxylation reaction proceeds through the “Pd–H” pathway.