A combination of DFT and kMC to study on the formation mechanism of ethyl acetate during the gas-phase synthesis of vinyl acetate from ethylene on PdAu(100) surface

Abstract

Vinyl acetate (VAc) is an important chemical raw material for the production of polyvinyl alcohol and polyvinyl acetate, among others. The preparation of vinyl acetate over PdAu catalysts using ethylene, oxygen and acetic acid as raw materials is one of the mainstream processes for the production of vinyl acetate in the world. The by-product ethyl acetate in this process is more difficult to be completely removed, and it is an impurity component in vinyl acetate products that needs to be strictly controlled in content. However, studies on the mechanism of ethyl acetate generation have not been reported, which limits the improvement of the purity of vinyl acetate products and the optimization of the ethyl acetate separation process. In this paper, the mechanism of ethyl acetate generation during ethylene vinyl acetate reaction on PdAu catalysts was systematically investigated using a combination of density functional theory (DFT) and kinetic Monte Carlo (kMC) simulations.The results show that the species tend to adsorb to the Pd-Au bridge sites, diagonal Pd-Pd vacancies, and diagonal Pd-Au vacancies of PdAu(100), and the Pd on the catalyst surface interacts more strongly with the adsorbed species. The dominant pathway for the generation of ethyl acetate on PdAu(100) is CH2CH2* + O*→ CH2CH* + H* → CH3CH* + CH3COO* → CH3COOCHCH3* + H* → CH3COOCH2CH3*, where the hydrogenation of CH2CH* to CH3CH* is the rate-control step in the dominant pathway for the generation of ethyl acetate. This step has an energy barrier of 1.09 eV and a reaction heat of 0.90 eV. The study of the generation mechanism of the by-product ethyl acetate in this paper is hoped to provide more targeted guidance and suggestions for the modification of Pd-Au catalysts, so as to reduce the generation of ethyl acetate as a by-product, to reduce the energy consumption required for the separation, and to enhance the purity of vinyl acetate products.