Mechanistic Study of Vinyl Acetate Synthesis On Pd and Au-Pd Model Catalysts

CALAZA, FLORENCIA C.; STACCHIOLA, DARIO J.; LI, ZHENJUN; TYSOE, WILFRED T.

Lincolnshire, IL

Encuentro; 38TH GREAT LAKES REGIONAL MEETING ACS; 2009

AMERICAN CHEMICAL SOCIETY

The industrial synthesis of vinyl acetate monomer (VAM) from acetic acid, ethylene and oxygen using palladium as a catalyst is an important reaction which has been used for about thirty years, even though some controversy existed about the mechanism by which this reaction occurs. The addition of gold to the catalyst improves this process by increasing its activity and selectivity. The understanding of the mechanism of this reaction and the role of Au on the surface of the catalyst, can lead to further improvement. By using isotopically labeled ethylene to react with acetate species on Pd(111) and on O-Pd(111), the mechanism for VAM synthesis is proposed to be the Samanos pathway in which ethylene molecules insert into the Pd-OAc bond forming an acetoxyethyl palladium intermediate (VAMH) which then ƒÀ-dehydrogenates to form VAM. The rate limiting step for this reaction is the ƒÀ-hydride elimination. Kinetics studies reveal that this step has an activation energy of 59 E} 3 kJ/mol. Au- Pd(111) alloys model catalysts were prepared in the Lab and tested for reactivity during VAM synthesis. Overall, these alloys showed to be much less active in terms of decomposition of reactants and product. It was found that for the gold-rich alloys, the reaction follows the mechanism proposed by Moiseev, by first activating ethylene molecules on the surface forming vinyl species which in turn insert into the Pd-OAc bond forming VAM. For gold coverages on the surface of ~20%, now there are large patches of palladium atoms in which the reaction follows the Samanos pathway as proposed for the pure palladium catalyst.