Elementary Reaction Steps of Vinyl Acetate in the Liquid Phase

2 Elementary Reaction Steps of Vinyl Acetate in the Liquid Phase 

The aqueous solution layer that forms at the metal interface can ultimately provide a medium for the dissolution of Pd ions or oxidized Pd clusters into the supported liquid layer where they can then act as homogeneous catalysts. As was discussed earlier, the acetoxylation of ethylene can be carried out over various Pda,OAcj, clusters where alkali metal acetates are typically used as promoters. DFT calculations were carried out on both the Pd2(OAc)2 and Pd3(OAc)e clusters in order to examine the paths that control the solution-phase chemistry. The Pd3(OAc)e cluster is the most stable structure but is known experimentally to react to form the Pd2(OAc)2 dimer and monomer complexes in the presence of alkali metal acetates. The reaction proceeds by the dissociative adsorption of acetic acid to form acetate ligands. Elthylene subsequently inserts into a Pd-acetate bond. The cation is then reduced by the reaction to form the neutral Pd°. The reaction is analogous to the Wacker reaction in which ethylene is oxidized over Pd + to form acetaldehyde. Pd° is subsequently reoxidized by oxygen to form pd2+[35,36,44] 

The reductive elimination of vinyl acetate can then proceed through either an inner-or an outer-sphere reaction channel. The inner-sphere mechanism involves the reaction between two ligands that are both already coordinated to the metal center. The outer-sphere mechanism involves the reaction between ethylene which is coordinated to the metal center and an acetate anion which resides in solution. A sketch of both the inner-and outer-sphere reactions is given in Fig. 6.19. 

Kragten et al.I carried out DFT calculations to determine the reaction energies and the activation barriers for a sequence of elementary steps that make up both the inner- and outer-sphere mechanisms. The effects of solution are included via the explicit introduction of one or two acetic acid molecules along with an overall reaction field. The solute is modeled by a cluster in which the charge is balanced by the coordination of protons to 

The overall potential energy profiles were calculated for tx)th the inner- and outer-sphere mechanisms for the homogeneous acetoxylation of ethylene to vinyl acetate. The results, which include the effect of solvation, are shown in Fig. 6.20. 

---