Density functional study of the palladium acetate catalyzed Wacker reaction in acetic acid

In the Wacker process, palladium acetate complexes catalyze the homogeneous reaction of ethylene and water to form acetaldehyde. We have studied the m echanism of this reaction in detail, using density functional theory comput ational methods. The putative most active catalyst is a dimer complex, whic h has been modeled by clusters of two palladium ions coordinated by acetate ligands. The active site is formed by one of the palladium ions. In the Wa cker process as catalyzed by palladium acetate, ethylene coordinates to pal ladium. Next, coupling with hydroxyl species from the solution takes place in an outer-sphere mechanism. A series of hydrogen transfers, in which term inal acetate participates, converts the hydroxyethyl ligand into acetaldehy de. Finally, the product desorbs. The overall reaction enthalpy is exotherm ic. One of the hydrogen transfers, the step that results in acetaldehyde fo rmation, is the rate-determining step. This step costs 61 kJ/mol. All react ions presumably take place within the coordination sphere, and thus hydroge n from the solvent is not incorporated into the product. Solvent effects ar e explicitly taken into account in all steps.