Synthesis of esters: Development of the rate expression for the Dowex 50 Wx8-400 catalyzed esterification of propionic acid with 1-propanol

The kinetics of the esterification reaction of propionic acid with 1-propanol over the ion-exchange resin Dowex 50Wx8-400 has been studied in this investigation. Kinetic experiments were conducted using a 1 L Lab-Max system at a stirrer speed of 900 rpm over a temperature range of 303.15 –333.15 K. The catalyst loading was varied from 10 to 60 g dry cat/L and acid to alcohol molar ratios of 1:1, 1:2, 1:4, 2:1 and 4:1 were employed. The equilibrium constants for this reaction were determined in separate experiments at 303.15, 313.15 and 323.15 K. The values were equal to 33.18, 30.62 and 28.37, respectively, with a standard enthalpy change of reaction of 6.4 kJ/mol. These values show the reaction to be mildly exothermic. It was found that both external and internal diffusion limitations did not affect the overall reaction rate. The conversion of propionic acid increased with increasing temperature and catalyst loading and decreased with increasing initial mole fraction of acid. The increase in chain length of acid or alcohol or branching had a retarding effect on the conversion. Several kinetic models were tested to correlate the kinetic data, the pseudo-homogeneous (P-H) model, the Eley–Rideal (E–R) model, the Langmuir–Hinshelwood (L–H) model, the modified Eley–Rideal (M-E–R) model and the modified Langmuir–Hinshelwood (M-L–H) model. In all models, the activity coefficients were estimated using UNIFAC to account for the non-ideal thermodynamic behavior of reactants and products. A correction factor for the resin affinity for water (α)(α) was used in both M-E–R and M-L–H models. The above models predicted the kinetic behavior of the studied system with an overall error ranging from 1.65% to 13.32%. Water was found to be more strongly adsorbed than other species present in the system. The M-E–R model between adsorbed 1-propanol and non-adsorbed propionic acid which assumes surface reaction as the rate controlling step, with αα equal to 2, was found to be the best model with the least overall error (1.65%). The activation energy for the esterification was estimated to be 67.3 kJ/mol by this model.