Hidrogenação seletiva de ésteres metílicos insaturados de ácidos graxos

Abstract

Biodiesel is a clean, renewable fuel, formed by a mixture of saturated and unsaturated methyl or ethyl esters, named FAMEs (Fatty Acid Methyl Ester), which has been replacing fossil fuels because it releases fewer polluting gases in atmosphere. Biodiesel is produced either by the transesterification reaction of the triacylglycerol present in vegetable oils or animal fats or by the reaction of the esterification of fatty acids. Among the biodiesels produced worldwide, soybean biodiesel occupies a privileged position for its high world production. Its composition consists of 12% methyl palmitate (C16:0), 4% methyl stearate (C18:0), 24% methyl oleate (O.M., C18:1), 52% methyl linoleate (L.M., C18:2) and 7% methyl linolenate (LL.M., C18:3). The presence of L.M. and LL.M. in high proportions makes it more susceptible to oxidation and one of the ways to reduce this susceptibility is its partial hydrogenation. However, this hydrogenation must occur in a selective way, since the increase of C18:0 or compounds with carbon-carbon double bond in the trans position introduces undesirable properties in biodiesel. In this work, a study was carried out on the selective hydrogenation of soybean biodiesel, using three types of palladium catalysts: palladium supported on alumina (0.7% Pd / Al2O3), palladium supported on charcoal (0,5% Pd/C) and palladium doped with silver and supported on alumina (Pd-Ag / Al2O3). After careful selection of the catalyst, the reaction temperature, the hydrogen pressure and the reaction time, it was possible to determine the conditions for a selective hydrogenation where FAME chains containing predominately C18:1 were formed. However, a detailed study of the products in a polar and selective chromatographic column that allowed separating cis and trans isomers, as well as using a derivatization technique (ethenolysis by metathesis), it was possible to demonstrate that, although selective for C18:1, the conditions and catalysts employed led to an intense isomerization both of position and stereochemistry, leading to trans products, which are unwanted. Finally, new reaction conditions were developed where not only the di- and tri-unsaturated compounds are selectively reduced to monounsaturated, without the excessive formation of saturated (C18:0), but also this process occurs without the unwanted isomerization of the double C-C double bound position or its conversion to the most stable trans isomer.