Kinetics of Carboxylation of Styrene Using a Homogeneous Palladium Complex Catalyst

Kinetics of carboxylation of styrene to methyl 2-phenylpropionate and methyl 3-phenylpropionate has been investigated using a homogeneous Pd(OAc)2/PPh3/p-toluenesulfonic acid catalyst system. The effect of concentration of styrene, catalyst, water, and partial pressure of CO on the activity, selectivity, and rate of reaction has been studied in a temperature range of 338−358 K. The activity of the catalyst as well as the selectivity of the products was sensitive to all the reaction parameters studied. The selectivity of 2-phenylpropionate was found to decrease substantially with an increase in temperature and was enhanced by an increase in the partial pressure of CO and beyond a styrene concentration of 3.84 kmol/m3. The rate of carboxylation varied linearly with the catalyst and was zero order with respect to styrene concentration in the range of 0.567−3.84 kmol/m3. An unusual trend of an increase in activity was observed beyond a styrene concentration of 3.84 kmol/m3. The rate also increased with an increase in the partial pressure of CO initially and was independent beyond 3.4 MPa. Water showed a promoting effect up to a concentration of 9.244 × 10-2 kmol/m3. On the basis of the initial rate data obtained, a rate equation was proposed and the kinetic parameters evaluated. The activation energy was found to be 65.49 kJ/mol. Also, a plausible reaction mechanism for the carboxylation of styrene has been proposed.