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 Fe 2 O 3 particles with a 15 nm SiO 2 protective coating prior to growth of a uniform 28 nm ZrO 2 shell. The thickness of the ZrO 2 shell in resulting Fe 3 O 4 @SiO 2 @ZrO 2 nanoparticles was controlled by adjusting the zirconium butoxide to Lutensol AO5 ratio, with 1:10 found as the optimal ratio to produce monodispersed ZrO 2 coated nano-spheres. Sulfation using an ammonium sulfate precursor is less corrosive towards the core-shell structure of Fe 3 O 4 @SiO 2 @ZrO 2 nanoparticles leading to superior sulfated materials compared to those obtained using H 2 SO 4 . Resulting Fe 3 O 4 @SiO 2 @SO 4 -ZrO 2 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.