(574e) Mechanistic Details of Vinyl Acetate Synthesis on Pd/SiO2

Vinyl acetate (VA) is an important chemical intermediate for the production of polymers. It is primarily produced by oxidative acetoxylation of ethylene on Pd-based heterogeneous catalysts. However, the mechanistic understanding of VA formation under steady-state catalysis remains incomplete despite extensive past surface science and density functional theory (DFT) studies that mainly probed titrations of surface acetates with ethylene. In this work, we use experiment and theory to develop a more complete mechanistic framework that captures effects of all reactants and inhibiting products and of coverage on rates and selectivity over broad reactant pressure ranges.

Measured VA rates are strongly inhibited by CO and increase monotonically with CH₃COOH pressure but increase and then decrease with O₂/C₂H₄ pressure (Fig.1a). These complex trends are described well by rate expression derived from catalytic cycles involving coupled acetate-formation, acetate-ethylene coupling and acetate oxidation steps proposed based on DFT analysis (Fig.1b). DFT-derived activation energy barrier for acetate formation is much lower than that of acetate consumption via C-O coupling and C-H activation at low coverage. However, the energy barrier for the formation step increases much more rapidly with acetate coverage, demonstrating that steady-state operation is achievable only over a narrow coverage range where the acetate formation and consumption barriers are similar. This also explains why the kinectic relevance of each elementay step can be altered at different steady state reaction conditions, as detected by measured kinetic isotope effects. The acetate coverage at different steady-state reaction conditions is probed via in situ infrared spectroscopy, and the ability of proposed steps to describe VA selectivity is also demonstrated. The insights developed from these studies provide a comprehensive framework for understanding VA synthesis reaction over Pd catalyst and form a basis for understanding the effects of composition on rates and selectivity.