Synthesis, characterization, and application of phosphotungstic acid supported on iron-based magnetic nanoparticles coated with silica

Magnetic nanoparticles based on magnetite (MNP) are important materials for different research applications, particularly when they are coated with inert phases. For instance, coating them with silica forms MNP@SiO2, which has various uses, including as a support for catalysts because it has surface functional groups that form bonds to a coating layer; the material presents itself with high magnetic susceptibility, turning it easily separable from the reaction in a heterogeneous medium. Herein, magnetite was synthesized and coated with tetraethylorthosilicate (TEOS) and then used to support phosphotungstic acid (H3PW12O40, HPW). The X%HPW/MNP@SiO2 materials were characterized by several techniques, such as EDXRF, XRD, SEM, FT-IR, low temperature N2 physisorption, and pyridine gas adsorption/desorption (Py-TPD) and then applied to the esterification reaction of oleic acid with ethanol. The results successfully indicated the synthesis of the catalysts, keeping the structural and magnetic characteristics of the MNP@SiO2 and providing a good dispersion and integrity of HPW on its surface. In the model reaction, high activity, selectivity (100%), and stability were observed, with yields of 98%, 66% and 54% of ethyl oleate when using 30%, 20% and 10%HPW/MPN@SiO2, respectively, for a reaction time of one hour and an acid to ethanol molar ratio of 1:6. No leaching of the active phase was detected up to four recycles of the best catalyst (30%HPW/MPN@SiO2).