(701f) Esterification of Citric Acid with Ethanol Using Homogeneous and Heterogeneous Catalysts

Citric acid esters are widely used as plasticizers for polymers with applications in the food, cosmetics, and health industries. Generally, production of alkyl citrates is carried out in batch reactors using homogeneous or heterogeneous acid catalysts. Operating temperature during reaction is constrained to be less than 150 °C to avoid citric acid polymerization or decomposition. The minimum acid to alcohol molar ratio is defined by the solubility of the acid, but in general large excess of alcohol is required.

Even though citric acid esters are well known, few literature reports present kinetic models to describe the process. In this regard, this work summarizes the experimental evaluation of a kinetic model for the esterification of citric acid with ethanol using both, Methane Sulfonic Acid and Amberlyst 70 as catalyst.

A Box-Behnken experimental design was developed to evaluate the kinetic models of reaction, varying reaction temperature (80 to 120 °C). Acid to alcohol feed ratio (1:7 to 1:15) and catalyst loading (0.5 to 1.5% w/w). Experiments were carried out in a 100 mL jacketed stirred SS reactor. Temperature was maintained by a thermal oil circulating bath. Reaction samples were withdrawn periodically and they were analyzed using HPLC, (previously calibrated). Reaction time was about 8 hours.

To reduce the amount of parameters to be fitted with the kinetic experiments, some independent chemical equilibrium experiments were carried out (80 to 120 °C). A set of 10 ml SS-hermetic tubes were loaded with a defined amount of the reactive mixture and the catalyst. These tubes were placed within an oven at given temperatures, and maintained in continuous agitation. After at least three days, tubes were sampled and the equilibrium concentrations were measured. Experimental equilibrium constants were obtained as the ratio of concentrations of products and reactants.

Regression of experimental data was performed in MatLabTM using a pseudo-homogeneous, and a heterogeneous model. Both agreed reasonably well with experimental kinetics ad can be used confidently for further process design.