Targeted Synthesis of Silicomolybdic Acid (Keggin Acid) inside Mesoporous Silica Hollow Spheres for Friedel-Crafts Alkylation

Herein we report an inside-out preinstallation-infusion-hydration method for targeted synthesis of Keggin heteropoly acids (silicomolybdic acid, H4SiMo12O40) within mesoporous silica (SiO2) hollow spheres. In this process, discrete molybdenum dioxide (MoO2) nanoparticles with diameter size ranging from 25 to 60 nm were first prepared by a one-pot hydrothermal route in water/ethanol mixed solvents at 180 degrees C, which were then used as cores to grow the shell of supramolecular templated silica with tetraethyl orthosilicate (TEOS) and hexadecyltrimethyl- ammonium chloride (CTACl) in alkaline solution. By thermal treatment of as-synthesized MoO2@SiO2 core-shell spheres, the organic template was burned off and mesoporous shell was formed (BET specific surface area was as high as 872 m(2)/g). Meanwhile, the encapsulated MoO2 was oxidized to Mo6+ and infused to the mesoporous silica shells, forming heptamolybdate species (Mo7O246-) uniformly dispersed on the mesopore surfaces of silica, while generating void space at the center of spheres. After hydration with water, H4SiMo12O40 was formed by reaction between the surface Mo7O246- and silica species in the presence of water. The prepared H4SiMo12O40@mSiO(2) hollow spheres were tested for Friede-Crafts alkylation of toluene by benzyl alcohol. The H4SiMo12O40@mSiO(2) catalysts fabricated via this novel route exhibited excellent catalytic activity toward benzylation of toluene, which was approximately 2.6 times as high as that of commercial Amberlyst-15 catalyst. In addition, the H4SiMo12O40@mSiO(2) catalyst was very robust and could be reused after regeneration.