Elucidation of the Reaction Mechanism for the Catalytic Synthesis of Vinyl Acetate by Palladium

CALAZA, FLORENCIA C.; STACCHIOLA, DARIO J.; BURKHOLDER, LUKE; TYSOE, WILFRED T.

Milwaukee, WI

Encuentro; The 37th Great Lakes Regional Meeting ACS; 2006

AMERICAN CHEMICAL SOCIETY

The palladium-catalized synthesis of vinyl acetate monomer (VAM) from acetic acid, ethylene and oxygen was first discovered some thirty years ago and currently approximately 3000 metric tonnes are produced annually by this reaction. This is the first observation of the reaction between gas-phase ethylene and acetate species adsorbed on oxygen-covered Pd(111). The nature of the surface is scrutinized using reflection-absortion infrared spectroscopy (RAIRS) to monitor the time-dependent evolution of the surface species. Gas phase species are monitored by a mass spectrometer while the sample temperature is ramped in a TPR experiment. In order to study the effect of pre-adsorbed oxygen the reaction was also performed on the clean surface. Two mechanisms have been proposed. The first involves the coupling of ethylene directly with chemisorbed acetate. The resulting acetoxyethyl-palladium intermediate then undergoes beta-hydride elimination to form vinyl acetate. Alternatively, ethylene could first dehydrogenate to form a vinyl-palladium intermediate, which then couples with a surface acetate species to form VAM directly. We have proven the first mechanism to be the correct one.