One-pot synthesis of molybdenum oxide nanoparticles encapsulated in hollow silica spheres: an efficient and reusable catalyst for epoxidation of olefins

Abstract

We report a one-pot synthesis of MoO_x NPs encapsulated inside hollow silica spheres (MoO_x@HSS) which act as an efficient and reusable heterogeneous catalyst in the epoxidation of olefins. A yolk–shell nanostructured MoO_x–SiO₂ composite, consisting of core MoO_x NPs and permeable porous hollow silica spheres, was fabricated via a self-assembly approach using poly(acrylic acid) as an organic template and dodecyltrimethoxysilane as a pore-directing agent. On the basis of comprehensive structural analyses by FE-SEM, TEM, elemental mapping, N₂ physisorption and Mo K-edge XAFS, we show that MoO_x NPs of ca. 5.8 nm diameter are encapsulated within hollow silica spheres having randomly arranged mesopores of ca. 2.1 nm diameter, while addition of an excess amount of the pore-directing agent leads to a deformation of the yolk–shell nanostructures. The obtained yolk–shell nanostructured MoO_x–SiO₂ composite exhibits prominent reusability with a modest catalytic activity in the liquid-phase epoxidation of olefins with TBHP owing to the presence of the protective silica shell, making it an effective heterogeneous catalyst for the transformation of olefins into their corresponding epoxides.