Study of the Use of Hydrotalcite–Hydroxyapatite as Heterogeneous Catalysts for Application in Biodiesel Using By-Product as Raw Material

The materials hydrotalcite—HT, hydroxyapatite—HAp and hydrotalcite–hydroxyapatite—HT–HAp were synthesized using the co-precipitation method at constant pH (pH 10), in which first the materials of HT and HAp were synthesized and later a reaction mixture for the formation of HT–HAp material. The materials were characterized by X-ray diffraction (XRD), N2 adsorption and desorption, infrared spectral analysis (FT-IR), thermogravimetric analysis (TG) and morphology. The results indicated that the analysis by XRD confirmed that all materials (HT, HAp, and HT–HAp) presented the typical basal peaks of HT and HAp and that in the HT–HAp material it presented a better crystalline ordering. The analysis of specific surface area (BET) showed that the mixture of materials (HT–HAp) caused a loss of the specific area, indicating a possible interaction between the particles of both materials. The pore diameters showed mesoporosity. In addition, the materials HT, HAp and HT–HAp had their catalytic activity determined through transesterification reactions using different lipid raw materials (commercial soy oil and vegetable oil deodorization distillate-VODD) obtaining ethyl esters (biodiesel) as a product. The results for the raw material commercial soybean oil showed that the catalysts HT and HAp obtained incomplete reactions, that is, they presented low conversions in ethyl esters (about 39–65% in 6 h of reaction). On the other hand, the study of the influence of temperature on the calcination of the HT–HAp catalyst obtained high conversions in ethyl esters, about 98% for the catalyst calcined at 700 °C, under conditions of 45:1 ethanol/oil molar ratio, temperature of 70 °C, catalyst concentration of 5% and 6 h of reaction. With the transesterification of soybean oil as a comparative form, the catalyst in its best conditions was used to transesterify the VODD by-product. Which results of conversion into considerable ethyl esters were found (about 88%, in just 6 h of reaction), thus confirming that the chemical catalyst was able to form the main esters of fatty acids even using different lipid sources.