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

Abstract

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”.