Oriented Aggregation of Silicalite-1 Subcrystals into MFI Zeolite Crystals

Precursor nanoparticles that form spontaneously on hydrolysis of silica source in aqueous solutions of tetrapropylammonium (TPA) hydroxide evolve to (TPA)- silicalite-1, a molecular-sieve crystal that serves as a model for the self-assembly of porous inorganic materials in the presence of organic structure-directing agents. The structure and role of these nanoparticles are of practical significance for the fabrication of 3-dimensional (3D) ordered porous materials and molecular-sieve films, but still remain elusive. Here we show experimental findings of nanoparticle and crystal evolution by controllable manipulation of silicalite-1 synthesis. We have investigated detailed structural features of MFI precursors, which involves precise temperature control during synthesis, fine purification of 6–10 nm silicalite-1 subcrystals (SCs) as starting materials, and subsequent compression via an ice-templating method. SCs aggregate into MFI crystals with unidirectional orientation─indicating that SCs possess at least partial MFI structure and a slab-like morphology. This synthetic route is not only facile and environmentally friendly but also highly reproducible. We believe this method holds great promise for advancing the application of membrane-based separation processes in the natural gas industry and organic liquid separation.