Development of Polyesters from Glycerol for Na Removal in Biodiesel Refining

摘要

Sodium hydroxide is the most commonly used basic homogeneous catalyst in biodiesel production. However, residual amounts of this catalyst in crude biodiesel may form sodium soaps during reaction and storage. In this paper, a dry washing process for biodiesel using polyesters synthesized from glycerol and adipic acid is proposed. Three formulations of polyesters were obtained, depending on the glycerol/adipic acid molar ratio. The resulting materials were characterized by FTIR analysis, nitrogen adsorption-desorption isotherms, as well as Na adsorption capacity. The FTIR spectra of the polyesters confirmed the presence of hydroxyl functional groups and glycerol esterification. The nitrogen adsorption-desorption isotherms confirmed that the polyesters are not porous materials. Adsorption experiments of sodium (present as soap and non-reacted catalyst) from crude biodiesel on polyester particles demonstrated that these polymers are able to remove sodium, being the polyester with a glycerol/adipic acid molar ratio of 1:1 the most powerful material in this regard, showing a Na adsorption capacity of 2.9 mg/g. Moreover, the obtained adsorption enthalpy of 10 kJ mol~(-1) indicates that the adsorption process was endothermic and dominated by H-bond interactions, possibly between Na soap carbonyl and polyester hydroxyl groups. Additionally, the gel type structure of the polyesters shows the ability to expand in crude biodiesel, allowing the accessibility of impurities. Finally, biodiesel column purification experiments using the developed polyesters show that quality control parameters of the resulting purified biodiesel were similar to those achieved by treatments with conventional adsorbents, having the polyester with glycerol/adipic acid molar ratio of 1:1 the best performance.