Produção de biodiesel via esterificação e transesterificação utilizando catalisadores heterogêneos bifuncionais

Abstract

The use of biodiesel as an alternative fuel has been a promising option since it is less aggressive to the environment. It may be produced by reactions of esterification and/or transesterification. The present work aimed to synthesize bifunctional heterogeneous calcium oxide and tungsten-based catalysts supported by silica-alumina to be applied on esterification and transesterification reactions in order to provide a better conversion of triglycerides and free fatty acids, simultaneously, to the corresponding methyl esters. Thereunto, eight catalysts were prepared with different proportions of calcium oxide (5, 10, and 20% in mass) and tungsten oxide (5% by of) supported by silica-alumina. The calcination temperature for each catalyst was determined by thermogravimetric and differential thermal analyses (TGA and DTA). The catalysts were called SiAl, 5Ca/SiAl, 10Ca/SiAl, 20Ca/SiAl, 5Ca5W/SiAl, 10Ca5W/SiAl, 20Ca5W/SiAl and 5W/SiAl, which were characterized using the techniques of X-ray diffraction and Fourier transform infrared spectroscopy (FTIR). Initially, the catalysts were tested in simultaneous esterification and transesterification reactions of soy oil plus oleic acid (acid oil) at a proportion of 75 and 25%, respectively. The catalytic tests were conducted with different proportions of catalysts and molar ratio acid oil/methanol. Posteriorly, new catalytic tests were conducted in the best conditions, however varying the content of oleic acid in the acid oil, and the contribution of each reaction (esterification and transesterification) was determined. The results of characterization indicated that there was an incorporation of calcium and tungsten oxide to silica-alumina. The catalytic tests showed that synthesized catalysts were efficient on converting the fatty acids and triglycerides to the methyl esters simultaneously. The 5Ca/SiAl was the best catalyst with 96.1% of yield in the reaction conducted with 5% of catalyst and molar ratio acid oil/methanol 1:20. Regarding the catalytic tests varying the content of oleic acid, the addition of oleic acid favored the reaction, thus demonstrating that bi-functional catalysts promoted, simultaneously, the reactions of esterification and transesterification. It was observed the existence of a limit for the addition of oleic acid to the acid oil since values higher than 25% decreased the conversion into the bi-functional catalysts. Increasing the oleic acid content provided high yield for the acid catalysts (SiAl and 5W/SiAl) obtaining 99.9% for the catalyst 5W/SiAl in a reaction conducted with 100% of oleic acid. The catalysts synthesized in the present work were active and tolerant to the presence of free fatty acids, what represents an alternative for the production of biodiesel, which may be applied in reactions with a low-quality raw material.