Kinetic and catalytic aspects in melt transesterification of dimethyl terephthalate with ethylene glycol in the presence of different catalytic systems

Abstract

The kinetics of transesterification of dimethyl terephthalate with ethylene glycol performed in the presence of catalysts such as: Pb, Zn, Mg, Co, and Mn acetates and a mixture of Mg, Mn, and Zn acetates has been studied in semibatch conditions. The catalytic behavior of Sb2O3 has been proven, too. The performance of the different catalytic systems has been investigated following both the amount of methanol released during time and the evolution with time of the concentration of any kind of oligomer formed as consequence of the reaction. The oligomers obtained were separated, identified, and quantitatively determined by HPLC analysis. In this way, information have been achieved on both the activities and selectivities of the different catalysts. The experimental data have been interpreted through a classic kinetic model based on a complex reaction scheme. Despite the complexity of the model, only two kinetic parameters and two equilibrium constants are necessary to simulate the kinetic behavior of all the oligomers. A kinetic constant (K1) is related to the reaction of a methyl group with a hydroxyl of ethylene glycol, while the other (K2) corresponds to the reaction of a methyl group with a terminal hydroxyl of a growing chain. The Mn, Pb, and Zn acetates have shown comparable high catalytic activities; however, the Mn selectivity to give oligomers with hydroxyl-hydroxyl terminal groups is better and similar to that shown by Co and Mg acetate, at a lower activity. Sb2O3 has a very low activity in transes-terification but this activity could be important to eliminate the residual terminal methyl group during the polycondensation step. The catalytic activity of the mixture of Mg, Mn, and Zn acetate was greater than that shown by each component the mixture, while its selectivity was comparable with that of Mn and Mg acetate. © 1996 John Wiley & Sons, Inc.