Mechanistic study of the production of ethylene glycol diacetate as a byproduct of the ethylene vapor phase process for vinyl acetate on PdAu(100) surface

Abstract

With the rapid development of the photovoltaic industry, the demand for new materials such as EVA resin is increasing, and the study of its synthetic monomer vinyl acetate is of great significance. The ethylene vapor phase process is the current mainstream method for producing ethylene acetate. This method mainly uses a PdAu bimetallic catalyst to catalyze the production of ethylene acetate and water from ethylene, acetic acid and oxygen. The reaction process produces ethylene glycol diacetate, a high boiling point byproduct, which not only increases the energy consumption for separating the byproduct and reduces the selectivity of the target product but also may lead to catalyst deactivation. Therefore, reducing its production is of great importance in industrial production. The production mechanism of ethylene glycol diacetate is not clear due to the large variety of surface species and the complexity of the reaction in the ethylene vapor phase method, which makes it challenging to investigate the production mechanism of ethylene glycol diacetate directly by experimental means without interference. Molecular simulation techniques, especially Density Functional Theory (DFT) simulations and kinetic Monte Carlo (kMC) simulations, allow for a targeted study of the catalyst surface reaction mechanism to obtain the mechanism of ethylene glycol diacetate production.In this paper, the elementary reaction in the possible formation path of ethylene glycol diacetate on PdAu(100) surface has been studied by the DFT simulations, and the energy barrier and reaction heat of each elementary reaction have been obtained. Moreover, based on the DFT simulation data, the generation process of ethylene glycol diacetate on PdAu(100) surface is investigated by the kMC simulation, including the processes of reactant adsorption, intermediate species diffusion, surface reaction, and product desorption. The calculation results show that the species are mainly adsorbed on the Pd-Au bridge site or Pd top site; ethylene can firstly generate CH3COOCH2CH2, CH3COOCHCH2 or CH3COOCHCH species and then be combined with CH3COO species to produce ethylene glycol diacetate, where the energy barrier of the reaction of CH3COOCH2CH2 species with CH3COO species is lower than other two species with CH3COO species. The advantage generation path for the ethylene glycol diacetate on PdAu(100) surface is CH2CH2→CH3COOCH2CH2→CH3COOCH2CH2OCOCH3, and the rate-determining step is the reaction of ethylene with CH3COO species to form CH3COOCH2CH2 species.