A spectroscopic study of the homogeneous catalytic conversion of ethylene to vinyl acetate by palladium acetate

We have studied the various aspects of the homogeneous acetoxylation of ethylene to vinyl acetate by palladium acetate using ultraviolet-visible (UV/vis), Raman, and infrared spectroscopic methods. Vibrational assignments for the Raman and the infrared spectra of crystalline palladium acetate have been made. Density functional theoretical calculations were used to help with our assignments. Raman spectroscopy demonstrated that Pd acetate solvated in glacial acetic acid maintains a trimeric structure, similar to that of the crystal. Upon addition of potassium acetate in concentrations between 0.10 and 0.20 M, the palladium acetate trimers decompose to dimers. It remains, however, an open question whether monomers, or perhaps dimers that are fully coordinated by acetate ligands, form with increasing potassium acetate concentration. In the presence of ethylene, dimeric species appear to form by decomposition of the trimers, and it is the dimers which are the catalytically active and selective species in the formation of vinyl acetate. During reaction with ethylene, palladium(II) in palladium acetate is reduced to palladium black, and the hydrocarbon products vinyl acetate and acetaldehyde are formed. Monomeric palladium acetate species are apparently not formed by this reaction.