Vinyl Acetate Formation by the Reaction of Ethylene with Acetate Species on Oxygen-Covered Pd(111)

Abstract

The reaction pathway of vinyl acetate synthesis is scrutinized by reacting gas-phase ethylene (at an effective pressure of 1 x 10-4 Torr) with eta2-acetate species (with a coverage of 0.31 +/- 0.02 monolayer) on a Pd(111)-O(2x2) model catalyst surface in ultrahigh vacuum. It is found that the 1414 cm-1 infrared feature due to the symmetric OCO stretching mode of the acetate species decreases in intensity due to reaction with gas-phase ethylene, while temperature-programmed desorption experiments demonstrate that vinyl acetate is formed. The formation of ethylidyne species is detected when almost all of the acetate species have been removed. The experimental removal kinetics are reproduced by a model in which adsorbed acetates react with an ethylene-derived (possibly ethylene or vinyl) species, where ethylene adsorption is blocked by the acetate present on the surface.

Elucidation of the Reaction Mechanism for the Palladium‐Catalyzed Synthesis of Vinyl Acetate

“…The structures of the adsorbed h 2 -acetate [7] and ethylene [8] species were determined separately on a Pd(111) surface by using low-energy electron diffraction (LEED) and reflectionabsorption IR spectroscopy (RAIRS), [6][7][8][9] which showed that both species adsorb on the bridge site. Herein, we discuss the investigation into the nature of the reaction pathway by using isotopically labeled reactants, which allowed the mechanism for the formation of the VAM to be unequivocally identified.…”

“…[10] Kinetic measurements were typically carried out by the collection of spectra over 100 scans, which is a relatively short procedure that took 25 seconds because of the large intensity of the asymmetric vibrational mode of the acetate moiety (O-C-O) at 1414 cm À1 . [6] Some experiments were carried out with a larger number of scans to yield spectra with better signal-tonoise ratios and, therefore, allow additional surface species to be identified. A flux of ethylene impinged onto the sample from a collimated dosing source to obtain an enhanced flux at the Pd(111) single-crystal surface while the background pressure was minimized.…”

“…A flux of ethylene impinged onto the sample from a collimated dosing source to obtain an enhanced flux at the Pd(111) single-crystal surface while the background pressure was minimized. [6] were transferred into glass bottles, which were attached to the gas-handling line for introduction into the vacuum chamber.…”

“…[6] The experimental titration curves obtained with acetate species can be explained by using a procedure described previously, [6] in which it was assumed that adsorbed ethylene underwent reaction with the acetate species and that ethylene adsorption was blocked by the acetate species. Acetate species adsorbed on clean Pd(111) surfaces (namely, in the absence of the (22) oxygen overlayer) are also removed by reaction with ethylene and also can be analyzed by using the same procedure used to reproduce the kinetic data for the oxygen-covered Pd(111) surface; furthermore, these acetate species yield similar rate constants to those found for reactions on the oxygen-covered surface, and therefore the presence of coadsorbed oxygen does not substantially affect the reaction kinetics.…”

Promotional Effects of Au in Pd-Au Catalysts for Vinyl Acetate Synthesis

“…Pd is an active catalyst for the acetoxylation of ethylene to vinyl acetate (VA), see equation (1) [2,3,[24][25][26][27][28][29][30][31][32][33][34][35][36]. However Pd is also an active catalyst for the combustion of ethylene and acetic acid and thus exhibits a low selectivity for VA synthesis (equations (2) and (3) 3) The addition of small amounts of Au to a supported Pd catalysts can significantly enhance its selectivity and activity for this reaction, as shown in Fig.…”