Optimization of process variables in the synthesis of butyl butyrate using amino acid-functionalized heteropolyacids as catalysts

Abstract

Response surface methodology was successfully applied to optimize esterification of butanol with butyric acids over amino acid-functionalized heteropolyacids, namely, [GlyH]xH3-xPW12O40 (x=1.0-3.0). A series of catalysts were prepared by incorporating tungstophosphoric acid (TPA) with varied amounts of glycine (Gly). The [GlyH]1.0H2.0PW12O40 catalyst was found to exhibit the best activity with an optimal butyl butyrate yield of 97.9%, which was in excellent agreement with the predicted value by the Box-Behnken design (BBD) model. These superior esterification activities observed for the organic TPA salt catalysts might be due to their highly acidic nature, weak molecular transport resistance, and "pseudoliquid" catalysis modes. Furthermore, the catalytic activity of [GlyH]1.0H2.0PW12O40 catalyst was observed to have no appreciable change in the conversion after five consecutive runs. A kinetic study for the esterification was also performed under optimal reaction conditions. The results revealed that the reaction followed second-order kinetics, and the activation energy was found to be 81.90 kJ/mol.