(600ae) Kinetics of Hydroesterification of Ethylene Using Homogeneous Palladium Complex Catalyst

The hydroesterification of olefins with CO and alcohols is an emerging technology for producing valuable carboxylic acid esters, and it is attracting continuous and increasing academic and industrial interest. Studies on the kinetics of hydroformylation of olefins are not only instructive for improvement of the catalytic system but also provide relevant information for further engineering study.

In this research, the kinetics of hydroesterification of ethylene to methyl propionate has been investigated using a homogeneous Pd(OAc)₂/PPh₃/p-toluenesulfonic acid (TsOH) catalyst system. The hydroesterification reaction was carried out in a flat-bottomed, magnetically stirred 150 mL steel autoclave equipped with a 120 mL teflon tube. The effect of concentration of catalyst, the partial pressure of CO and ethylene on the activity and selectivity has been studied in a temperature range of 80-100 °C. Under the conditions of Pd concentration of 4mmol/L, Pd(OAc)₂:PPh₃:TsOH (mole ratio)=1:9:10, C₂H₄: CO=1:1, 2 MPa and 100 °C, the selectivity for methyl propionate was greater than 92%. Under these typical hydroesterification conditions, the kinetic order of the mixed gas of ethylene and CO was found to be zero order, and the activation energies was 52.87 kJ/mol in the range of 80-100 °C. The selectivity for methyl propionate was low when the concentration of ethylene was high in the mixed gas, so the kinetic orders below were evaluated under the low ethylene concentration. When the molar ratio of C₂H₄: COwas 1:10, the kinetic order of C₂H₄ was 1.23, so the kinetic order of CO was -1.23, and when the concentration of C₂H₄and CO is constant, the kinetic order of methanol was 1.36. On the basis of the initial rate data obtained, a rate equation was proposed. Also, a plausible reaction mechanism for the carboxylation of styrene has been proposed.