Generalized synthesis of core–shell structured nano-zeolite@ordered mesoporous silica composites

Novel core–shell structured composite molecular sieves comprising mono-dispersed nano-sized zeolite single-crystals (i.e., nano-zeolite Y) as cores and ordered mesoporous silica as shells were synthesized via a surfactant-directed sol–gel process in basic media by using cetyltrimethylammonium bromide (CTAB) as a template and tetraethyl-orthosilicate (TEOS) as a precursor. Uniform mesoporous silica shells are coated on the mono-dispersed nano-zeolites to form the hierarchical porous structures, the thickness of which can be tailored depending on the adding amount of TEOS. The composite molecular sieves with the thickness of 20 nm possess ultra high surface area of 1198 m2/g, ordered mesopores (∼3.5 nm in diameter) from the silica shell and uniform micropores (∼0.9 nm) from nano-zeolite core. The ordered mesopore channels in the shells are annularly vertical to the nano-zeolite crystal-faces. On the other hand, the nano-zeolite@mesoporous silica composite molecular sieves with such high surface area and opened hierarchical pores, can provide sufficient voids for capturing reactant molecules and also promote molecule diffusion from mesopores to micropores. Thus, the composite molecular sieves show greatly enhanced adsorption capacity (4.7 mmol/g) for large molecules such as benzene relative to that of pristine nano-zeolites (3.0 mmol/g), ascribing to the large contribution from mesopores in the shell.