Synthesis of hollow and rattle-type mesoporous silica spheres by treating layered mesoporous silica with a basic solution, and using the spheres as microreactors for two-phase reactions

Hollow silica spheres with radially oriented mesopores were synthesized by treating core-shell structured mesoporous silica spheres (pure silica core and propyl group-grafted silica shell) with an ammonia solution in the presence of alkyltrimethylammonium chloride. The pure silica cores dissolved in the basic solution but the propyl groups and the surfactant adsorbed on the shells protected the mesopore walls in the shells from the basic solution. Treating mesoporous silica spheres with three-layered structures (silica with grafted propyl groups, pure silica, and then silica with grafted propyl groups) led to mesoporous spheres with rattle structures being formed. The hollow and rattle-type spheres acted as microreactors when they were added to two-phase reactions, halogen exchange reaction, hydroformylation and esterification, and the reaction rates were increased. Stirring caused the hollow spheres containing the aqueous phase to be transferred into the organic phase, causing the reaction rate to be increased. Modifying the mesopore walls with phenyl groups to make the spheres highly hydrophobic increased the amount of spheres that was present in the organic phase, increasing the reaction rate. A higher reaction rate was achieved using hollow spheres than that using non-hollow mesoporous silica spheres, indicating that the hollow structure was an important factor in increasing the reaction rate.