Silica tethered with poly(ethylene and/propylene) oxide as supports for polyoxometalates in catalytic oxidation

Use of the sol-gel technique has allowed the preparation of silicates with polyfunctionalized surfaces. In this way, silicates containing combinations of hydrophilic poly(ethylene oxide) (PEO), hydrophobic poly(propylene oxide) (PPO) and cationic quaternary ammonium (Q⁺) groups were prepared. These silicates coupled with polyoxometalates led to active catalytic assemblies. In one application, H₅PV₂Mo₁₀O₄₀ was complexed to PEO-SiO₂ and used to catalyze the oxydehydrogenation of dihydroanthracene with improved activity and selectivity compared to the non-supported catalyst. Recycle of the assembly was possible. In a further use, [ZnWMn₂/(II)(ZnW₉O₃₄)₂]^12-, bound by a Q⁺ moiety to the silicate surface modified also with polyethers formed an assembly catalytically active, with excellent recycle for the epoxidation of alkenes with aqueous 30% H₂O₂. A silicate with a hydrophobic (PPO)/hydrophilic (PEO) balance showed the highest activity due to the optimal contact of the apolar hydrocarbon and the aqueous oxidant. Dispersion of the silicate in a solvent depended much on the polyether tether. Hydrophilic PEO-SiO₂ formed a homogeneous gel in water but was not dispersed in chloroform. For hydrophobic PPO-SiO₂, the opposite was observed. Maximal dispersion together with ease of recovery was possible with intermediate 10% PEO, 10% PPO-SiO₂.