Core-shell Fe3O4@SiO2@HNbMoO6 nanocomposites: new magnetically recyclable solid acid for heterogeneous catalysis

A new type of magnetically recyclable solid acid catalyst was designed and retro-synthesized via a two-step template method. Magnetic Fe3O4 cores were synthesized by a solvothermal approach, and then coated with a silica layer by a sol-gel process in order to improve their stability in acid environments. HNbMoO6 nanosheets used as outer shell precursors were obtained by exfoliating a layered tri-rutile transition metal oxide (LiNbMoO6) via proton exchange and intercalation of tetrabutylammonium cations. Negatively charged nanosheets were assembled upon the Fe3O4@SiO2 particles using poly(diallyldimethylammonium chloride) as an opposite polyelectrolyte via a layer-by-layer sequential adsorption process. Protonation of the outer layer was realized by acid exchange treatment. The protonated core-shell nanocomposites showed effective catalytic properties for Friedel-Crafts alkylation reaction and high magnetic recyclability. These results highlight that the rational design and controllable synthesis of multifunctional catalysts is a promising strategy in green chemistry and green technologies.