Formation of LTA zeolite crystals with multi-hollow polycrystalline core–shell structure via aggregation–recrystallization route in presence of emulsion droplets

Abstract

Na-A zeolite crystals consisting of thin crystalline polyhedral shell and multi-hollow polycrystalline core were formed for the first time following reverse crystallization mechanism. These extraordinary crystals were formed via two step crystallization from a system which included n-dodecane droplets stabilised with cetyltrimethylammonium bromide. A systematic investigation of the crystal growth over different crystallization stages indicates that at the first stage of crystal growth aggregates of nanocrystallites are formed which then undergo surface to core crystallization. At the start, n-dodecane droplets surrounded by a thin layer of cetyltrimethylammonium bromide enhance the formation of aggregates of amorphous nanoparticle which later transform into polycrystalline aggregates by local crystallization of each nanoparticle into a crystallite. Crystallized islands are built on the surface of the polycrystalline aggregates which are then extended and merged resulting in typical cubic morphology of LTA zeolite. Surface recrystallization continues towards the core via Oswald ripening process increasing the thickness of the shell. During the entire growth process, the n-dodecane droplets are retained inside the polycrystalline aggregates, probably due to slow crystallization rate at room temperature. The removal of the n-dodecane droplets and cetyl trimethylammonium bromide molecules results in Na-A zeolite particles with core–shell structure. The shell is highly crystalline Na-A zeolite and the core comprises of multi-hollow polycrystalline aggregates. The product shows mesoporosity with large size distribution attributed to polycrystalline aggregates that are free of the crystallized shell. Characterization of the samples was performed using transmission electron microscopy, scanning electron microscopy, cryogenic temperature transmission electron microscopy, X-ray diffraction, thermogravimetric analyzer, solid state NMR, FTIR, and N2 adsorption–desorption analysis.