Magnetically-separable Fe3O4@SiO2@SO4-ZrO2 core-shell nanoparticle catalysts for propanoic acid esterification

Monodispersed, sulfated zirconia encapsulated magnetite nanoparticles were synthesized as magnetically-separable solid acid catalysts. Catalyst nanoparticles are prepared via coating preformed 80 nm Fe2O3 particles with a 15 nm SiO2 protective coating prior to growth of a uniform 28 nm ZrO2 shell. The thickness of the ZrO2 shell in resulting Fe3O4@SiO2@ZrO2 nanoparticles was controlled by adjusting the zirconium butoxide to Lutensol AO5 ratio, with 1:10 found as the optimal ratio to produce monodispersed ZrO2 coated nano-spheres. Sulfation using an ammonium sulfate precursor is less corrosive towards the core-shell structure of Fe3O4@SiO2@ZrO2 nanoparticles leading to superior sulfated materials compared to those obtained using H2SO4. Resulting Fe3O4@SiO2@SO4-ZrO2 solid acid catalysts exhibit high activity for propanoic acid esterification with methanol, far exceeding that of conventional sulfated zirconia nanoparticles, while being amenable to facile magnetic separation.