Synthesis of ZSM-5 zeolite hollow spheres with a core/shell structure

Hollow and core/shell ZSM-5 spheres were simply synthesized through in situ transformation of mesoporous silica spheres (MSS) into MFI-type zeolite with the assistant of isopropylamine (IPA) as a structure-directing agent (SDA). IPA, with a mild structure-directing ability for the construction of MFI structure, triggered a stepwise crystallization of MSS after Al addition. First, the sphere surface was crystallized to a shell of ZSM-5 crystals. The silica and alumina species in the core condensed and crystallized to ZSM-5 thereafter, forming zeolite crystal aggregates inside the spheres. The obtained ZSM-5 spheres were highly crystalline materials and maintained the shape of pristine MSS, but exhibited a hollow and core/shell structure. The techniques of XRD, FT-IR, SEM, N₂ adsorption-desorption, ²⁹Si and ²⁷Al MAS NMR were employed to trace the crystallization process, which allowed us to propose a "templating and surface to core" crystallization mechanism. This strategy may serve as an alternative way for synthesizing hollow zeolite spheres.