Esterification of dicarboxylic acids over amberlyst ion-exchange resins

Abstract

Diester can be synthesized via esterification of dicarboxylic acid with alcohol over various strong acid Amberlyst ion-exchange resins i.e., Amberlyst 15, Amberlyst 16 wet and Amberlite IR-120 to compare the catalytic efficiency. Effects of types of the resin, catalyst amount, molar ratio of adipic acid/2-ethyl-1-hexanol reaction temperature, reaction time, chain length of the diacids (adipic acid, azelaic acid and sebacic acid) and structure of alcohols (n-octanol and 2-ethyl-1-hexanol) on the diester yield and the product distribution have been examined. Amberlyst 15 is the most active catalyst in the esterification of adipic acid with 2-ethyl-1-hexanol. The activity was related to the pore size and specific surface area of the resin catalyst. The diester yield was increased with the increasing catalyst amount. A decrease in the adipic acid/2-ethyl-1-hexanol molar ratio resulted in an improvement of the diester yield and the diester selectivity. The adipic acid conversion and the ester yield were increased with the reaction temperature and reaction time. The size of diacid was significantly affected. When the number of carbon in the linear alkyl chain of dicarboxylic acid was increased, the diester yield and the diester selectivity were decreased due to the enhancement of steric effect. Amberlyst 15 was used for ten cycles to examine its reusability. It was indicated that the catalyst can be reused without losing catalytic activity when using acetone as solvent. The activation energy (Ea) of this reaction was found to be 63.7 kJ mol-1. The reaction was found to follow Eley–Rideal mechanism that appeared adsorption of 2-ethyl-1-hexanol and water molecules and was also observed the inhibiting effect of water. The rate constant (k) can be given by k = 2.51 × 10²⁵ exp (- 21964/T) L² g⁻¹ mol⁻¹ h⁻¹.