An elementary-step kinetic model for ethyl acetate synthesis by direct addition of acetic acid to ethylene on a silicotungstic acid catalyst

Abstract

An elementary-step kinetic model for ethyl acetate synthesis by direct addition of acetic acid to ethylene over a silica supported silicotungstic acid catalyst is developed. A reaction network comprising 5 reactions, i.e. 3 direct additions, an esterification and a condensation, between 6 major components (ethylene, acetic acid, water, ethyl acetate, ethanol and diethyl ether) is accounted for. The reactions are described by means of Langmuir-Hinshelwood-Hougen-Watson and Eley-Rideal type rate expressions, including the adsorptions of all components, except diethyl ether, on the active sites of the catalyst. The model can quantitatively capture the effects of space time, temperature, pressure and feed composition, on conversions and also qualitatively reproduces the observed selectivities. However, it is necessary to include two physicochemical phenomena, i.e. i) silicotungstic acid solvation by water for the adequate description of the pressure effect on kinetics and ii) the proton-ethanol cluster formation to improve the fit between the experimental and model predicted values. Estimates for activation energies, adsorption enthalpies, solvation and cluster formation enthalpies and entropies are obtained through regression. At a temperature of 442 K, pressure equal to 1.2 MPa, an ethylene:acetic acid:water:dinitrogen feed molar ratio of 78.2:6.5:5.3:10.0 and relative space time amounting to 0.5, ethyl acetate is for 84 % produced by direct addition of acetic acid to ethylene, while the esterification between acetic acid and ethanol contributes for 16 %. Hence, besides reproducing conversions and selectivities, the kinetic model provides insights on fundamental level, which are essential for the optimization and innovation of production plants utilizing direct addition technology.