Targeted Synthesis of\nSilicomolybdic Acid (Keggin\nAcid) inside Mesoporous Silica Hollow Spheres for Friedel–Crafts\nAlkylation

Abstract

Herein we report an <i>inside-out</i> preinstallation-infusion-hydration\nmethod for targeted synthesis of Keggin heteropoly acids (silicomolybdic\nacid, H₄SiMo₁₂O₄₀) within mesoporous\nsilica (SiO₂) hollow spheres. In this process, discrete\nmolybdenum dioxide (MoO₂) nanoparticles with diameter size\nranging from 25 to 60 nm were first prepared by a one-pot hydrothermal\nroute in water/ethanol mixed solvents at 180 °C, which were then\nused as cores to grow the shell of supramolecular templated silica\nwith tetraethyl orthosilicate (TEOS) and hexadecyltrimethyl- ammonium\nchloride (CTACl) in alkaline solution. By thermal treatment of as-synthesized\nMoO₂@SiO₂ core–shell spheres, the organic\ntemplate was burned off and mesoporous shell was formed (BET specific\nsurface area was as high as 872 m²/g). Meanwhile, the encapsulated\nMoO₂ was oxidized to Mo⁶⁺ and infused to the\nmesoporous silica shells, forming heptamolybdate species (Mo₇O₂₄^6–) uniformly dispersed on the mesopore\nsurfaces of silica, while generating void space at the center of spheres.\nAfter hydration with water, H₄SiMo₁₂O₄₀ was formed by reaction between the surface Mo₇O₂₄^6– and silica species in the presence of water.\nThe prepared H₄SiMo₁₂O₄₀ @<i>m</i>SiO₂ hollow spheres were tested for Friedel–Crafts\nalkylation of toluene by benzyl alcohol. The H₄SiMo₁₂O₄₀@<i>m</i>SiO₂ catalysts\nfabricated via this novel route exhibited excellent catalytic activity\ntoward benzylation of toluene, which was approximately 2.6 times as\nhigh as that of commercial Amberlyst-15 catalyst. In addition, the\nH₄SiMo₁₂O₄₀@<i>m</i>SiO₂ catalyst was very robust and could be reused after regeneration.