Revealing the role of hydrogen bond, mechanism and kinetic for hydroesterification of ethylene to methyl propionate

Hydroesterification of ethylene with carbon monoxide (CO) and methanol (MeOH) for preparing methyl propionate (MP) has stimulated researcher's attention in recent years. Herein, the palladium-phosphine catalyzed ethylene hydroesterification with the promotion of acidic poly ionic liquids (APILs) for direct MP synthesis was developed. Series of APILs samples having different anions and acid densities were prepared and their structural properties were characterized by employing NMR, FTIR, TG and XPS. The anion type and acid density in APILs shown great influence on the catalytic performance due to the electronic effect and proton supply capacity related to Pd-H species formation. The hydrogen bond formed through the interaction between the anion of APILs and methanol could effectively promote the ethylene hydroesterification. The substrate extension experiments revealed the polarity and steric hindrance of alcohol would impact ethylene conversion. The mechanistic studies using in-situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) unraveled that the hydroesterification started with the activation of palladium complex to Pd-H species by CO, followed by the sequential formation of Pd-C2H4, Pd-alkyl and Pd-acyl intermediate species and consequential product MP. As a result, 94.4 % ethylene conversion could be achieved with 100 % selectivity toward MP at 80 °C and 2 MPa. No obvious deactivation behavior was observed for this kind of palladium catalyst after ten recycling experiments. The kinetic studies revealed the activation energy of ethylene hydroesterification is 59.3 kJ/mol.