Dd. Kragten et al., A density functional study of the acetoxylation of ethylene to vinyl acetate catalyzed by palladium acetate, J PHYS CH A, 103(15), 1999, pp. 2756-2765
Vinylacetate can be formed in the homogeneous reaction of ethylene with pal
ladium acetate in glacial acetic acid. We propose a Wacker-like mechanism,
which has been studied using density functional theory computational method
s. The palladium acetate dimer, which is presumably the active catalyst, ha
s been modeled by clusters of two palladium ions coordinated by acetate lig
ands. The active site is formed by a single palladium ion which is part of
the dimer. In this mechanism, ethylene coordinates to palladium by substitu
tion of a terminal acetate. Next, the ligand couples with an acetate ion, a
nd consecutive beta-hydrogen transfer forms the product vinyl acetate. The
coupling probably takes place via an outer sphere attack by acetate. Theory
suggests that the rate-determining step is the beta-hydrogen transfer, and
the: activation energy is predicted to be 67 kJ/mol. Molecules from the so
lvent act as a catalyst in this step. However, at high acetate concentratio
n, formation of a vacancy at a terminal acetate site is inhibited, which re
sults in a negative reaction order with respect to acetate. Solvent effects
are explicitly taken;into account in all steps as a correction to the ener
gies obtained in a vacuum.