Quantum Chemical Studies of the Wacker Process
The Wacker process is one of the first successes of industrial catalytic science. However, despite 45 years of substantial experimental and theoretical studies, the intrinsic mechanisms have never fully been understood. Especially of interest is the identity of the rate determining step and the detailed intermediate steps as well as the mechanisms of reprocessing cycles of palladium and copper. With the motivation of using the Wacker process as a test case, a thorough and continuous mechanism is determined with density functional theory (DFT) methods and solvation models simulating structures optimized in solution.
Part I: Olefin oxidation by Pd(II) Chloride
To the best of our knowledge this is the first consistent theoretical study that shows the complete olefin oxidation mechanism with palladium chloride consistent with results from experimental kinetic studies under standard Wacker process conditions of low [Cl-]. Our results indicate that the rate-determining step is related to chloride dissociation that leads to the vacant site necessary for vinyl-alcohol formation. Though the trans- attack by the nucleophile is a lower energy pathway thermodynamically (+12.3 kcal/mol), calculations indicate a cis- attack is realistic (~16 kcal/mol), and actually is better suited to elucidate kinetics observations. Product formation is a result of water-catalyzed reductive elimination. As a testament to performance of this model, the energetics of the rate determining step is characterized to the experimentally observed barrier within the error of the calculation method.
Personnel: John Keith