(241g) Triethyl and Tributyl Citrate Production from Dicalcium Citrate Salt: Esterification Kinetics

Plasticizer global demand has experimented a significant increase during last years, due to the large consumption of plastic materials. Phthalates are the worldwide leader plasticizers, however some of them have demonstrated negative impacts in public health and in the environment because of their toxicity and low biodegradability. Therefore, the demand for more benign, ecofriendly, and biobased plasticizers has been growing, and it is expected that the market of non-phthalates plasticizers will substantially increase in the coming years.

Citric acid esters are classified as a bio-based, biocompatible, and renewable plasticizers, suitable to be used as functional substitutes for phthalates in different applications. The production process of most commercial citrate esters involves the semi batch esterification with low molecular weight alcohols (ethanol or butanol), using large excess of alcohol and long processing times to overcome equilibrium limitations. High purity citric acid (i. e. higher cost) is mostly used as reactant because any remaining water could limit the reaction conversion. However, alternative sources of the citric backbone could with lower cost could be used as alternative feedstock for citrate plasticizers production. During the citric acid fermentation, a precipitation with calcium is used to recover the acid from the broth. The calcium salt is then filtrated, dispersed in water, acidified with sulfuric acid, and finally, a citric acid solution is obtained for further refining, concentration, and crystallization.

As an alternative process, this work proposed the direct use of the calcium salts as raw materials for the synthesis of citrate plasticizers. By avoiding most traditional purification steps involved in the citric acid production, the use of calcium citrate is envision as a feasible alternative for the production of citrates under reduced costs. In this process, a solid-solid-liquid reaction involving the acidification of the citrate salts with sulfuric acid in an anhydrous alcoholic medium, allows the citric acid recovery and its simultaneous esterification. As byproduct, calcium sulfate forms, allowing water removal by capturing hydration water, but creating a solid shell that can limit the acidification and the esterification processes. In this process, ethyl or butyl citrates can be directly obtained from the calcium salt, avoiding the citric acid purification.

In order to create a reaction model, preliminary experiments to evaluate the esterification kinetics were developed. The esterification kinetics of citric acid with ethanol and butanol, with concentrated homogeneous sulfuric acid as catalyst, was studied through a Box-Behnken experimental design. Here, reaction temperature, catalyst loading, and acid/alcohol feed ratio on the reaction kinetics were evaluated. With the aim to simplify the parameters regression, chemical equilibrium evaluations were performed as independent experiments. Determination of reactants concentration were performed by using HPLC analysis, where only citrate species were quantified. The kinetic parameters were adjusted by a minimum squares method implemented in MatLab, and the obtained model agrees well with the experimental observations. The obtained kinetics can further used for the construction of a solid-solid-liquid model to describe the simultaneous acidification-esterification of calcium citrate.