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

Abstract

Finding the right pathway: Reaction of isotopomers of ethylene with acetate species (see scheme) adsorbed on a Pd(111) surface in ultrahigh vacuum has shown that vinyl acetate is produced by insertion of ethylene into the acetate species to form an acetoxyethyl intermediate, which decomposes by β‐hydride elimination to yield vinyl acetate.

Final Technical Report for DOE Grant, number DE-FG02-05ER15701; Probing Surface Chemistry Under Catalytic Conditions: Olefin Hydrogenation,Cyclization and Functionalization.

“…The calculated barriers indicate that there is a change in the reaction mechanism from that found over Pd(111) [19,36]. Despite the high gold coverage, molecular oxygen can still dissociate on these surfaces as the result of the noncontiguous Pd pairs, which provide Pd atoms within the two bridge sites across the four-fold hollow site.…”

“…6, there is a relatively rapid reduction in the intensity of the acetate infrared feature (at ~1414 cm -1 ) when pressurized by ~1×10 -4 Torr of ethylene at 327 K. The decrease in the acetate signal intensity confirms that acetate species react with ethylene and the plot (•) shows the variation in intensity of the 1414-cm -1 acetate feature as a function of time. Another peak appears at ~1320 cm -1 as the acetate species have been removed assigned to ethylidyne formation on Pd(111) [19]. Calibration experiments show that the intensity of the 1414-cm -1 feature is proportional to the acetate coverage so that this plot represents a change in coverage of the acetate species as a function of time.…”

“…In order to explore the reaction pathway for the synthesis of vinyl acetate by a reaction between ethylene and surface acetate species, titration experiments were carried out in which an acetatecovered surface was pressurized with ethylene and the intensity of the strong OCO asymmetric stretching mode followed as a function of time [18,19]. It should be noted that adsorbing ethylene on an acetate-covered surface did not lead to reaction in TPD and merely resulted in the desorption of ethylene and decomposition of the acetate species.…”

Reaction Between Ethylene and Acetate Species on Clean and Oxygen-Covered Pd(100): Implications for the Vinyl Acetate Monomer Formation Pathway

“…It is found that the reaction proceeds on Pd(111) via the insertion of adsorbed ethylene into the O-Pd bond of the surface acetate species [6] to form an acetoxyethyl-palladium intermediate, which decomposes via a b-hydride elimination reaction to yield VAM [7,8]. This reaction route, first proposed by Samanos [9] was confirmed using density functional theory (DFT) calculations which were able to rather precisely calculate the energies of the elementary steps in the reaction [8].…”

“…This implies that, if adsorbed acetate and ethylene do react on Pd(100), this will ultimately result in VAM being formed on a relatively uncrowded surface. The surface chemistry of VAM has been explored on clean and oxygen-covered Pd(100) surfaces [13] and indeed it is found that it thermally decomposes on the surface at *300 K implying that it will decompose on Pd(100) surfaces under the conditions (temperatures greater than 300 K) that the titration experiments and catalytic reactions [6][7][8] are carried out.…”