Probing reaction pathways on model catalyst surfaces: Vinyl acetate synthesis and olefin metathesis

The reaction pathway for the palladium-catalyzed synthesis of vinyl acetate from acetic acid, ethylene and oxygen is investigated using reflection-absorption infrared spectroscopy by monitoring the rate of acetate titration by gas-phase ethylene. This reveals that acetate species are removed by reaction with gas-phase ethylene resulting in vinyl acetate formation. Reaction with C2D4 reveals a large (similar to 6) isotope effect indicating that hydrogen is involved in the rate-limiting step. This also results in the appearance of an infrared feature that is assigned to an acetoxyethyl-palladium. intermediate. Acetate reaction rates are different for the isotopomers, CH2CD2 and CHDCHD. These observations are consistent with a reaction pathway first proposed by Samanos in which ethylene reacts with an acetate species to form an acetoxyethyl-palladium intermediate, which then reacts to form vinyl acetate by a P-hydride elimination reaction. The Herisson-Chauvin pathway for olefin metathesis, which proposes that reaction occurs between a surface carbene and alkene to form a C-3 metallacycle, which decompose by the reverse of this pathway to form metathesis products, is tested on a MoAl alloy formed by reactions with MO(CO)(6) with an alumina thin film grown on a refractory metal substrate. This is the accepted pathway in homogeneous phase, and is demonstrated to occur on the alloy surface by grafting intermediates using iodine-containing precursors. (C) 2007 Elsevier B.V. All rights reserved.