Preparação e caracterização de catalisadores magnéticos superácidos do tipo Core-Shell a base de Fe3O4@ZrO2-SO4 com potencial aplicação na obtenção de biodiesel

Abstract

In this work, was studied the esterification reaction of oleic acid with ethanol catalyzed by superacid magnetic nanoparticles based on core-shell structure as Fe3O4@SiO2@ ZrO2-SO4 and Fe3O4@ZrO2-SO4 catalyst. The magnetic core Fe3O4 was prepared by the solvothermal method and the shell of SiO2 and/or ZrO2 was obtained based on the sol-gel method from precursor alkoxides. The materials were characterized by TG/DTA, FTIR, XRD, VSM, FEG-SEM and EDX-TEM technics, checking the formation of sulfated zirconia supported on surface of magnetite nanoparticles with well defined core-shell structures. The results of thermal analysis showed that the zirconia shell promotes a thermal protection to the magnetite core, preventing oxidation of magnetite to hematite, well as found in the TG-DTA curves of the pure magnetite. The DTA curves of Fe3O4@ZrO2 and Fe3O4@SiO2@ZrO2 catalysts showed the exothermic peak characteristic of crystallization of zirconia around 500 °C. The FTIR data showed the presence of characteristic bands of Zr–O of zirconia, of silica Si–O as well as the characteristic bands of the sulfate groups. The characterization by XRD showed the formation of magnetite on solvothermal synthesis, in which, after heat treatment at 500°C, was converted to hematite. In the case of materials core-shell treated at 500 °C it was observed the crystallization of zirconia in the tetragonal phase and that magnetite core was converted to maghemite. Data of magnetization showed which core-shell materials exhibited superparamagnetic behavior at room temperature. FEG-SEM data observed a spherical morphology for magnetite particles with a mean size around 100- 200 nm. The results of EDX-TEM showed the formation of core-shell structures, confirming the core diameter around 100–200 nm and identifying the shell thickness between 6–10 nm. The EDX spectra of the core-shell particles showed the iron present in the core, and typical peaks of the presence of silica, zirconia, and sulfur of the shell. The core-shell catalysts were applied in the esterification reaction observing that the core-shell catalyst Fe2O3@ZrO2-SO4 treated at 500°C presented much higher conversion than obtained in the reaction with Fe3O4, showing the use potential of the catalyst for application in the biodiesel production.