Cubes of Zeolite A with an Amorphous Core

Abstract

The syntheses of zeolites involve very complex nucleation and growth processes. During the past decade, significant progress has been made towards understanding zeolite crystallization mechanisms. This progress has been made possible by advanced analytical techniques, such as high-resolution transmission electron microscopy (HRTEM), small-angle X-ray scattering, and atomic force microscopy. [1][2][3][4][5] A number of zeolite growth mechanisms were proposed based on the respective synthesis of the zeolites. For instance, by monitoring the crystallization of silicalite-1 from silica sols in tetrapropylammonium ion (TPA) at room temperature, an oriented aggregation mechanism was proposed.[4] In the growth mechanism of zeolite A evolving from the nuclei inside the amorphous gel, the particles gradually grow into larger crystals by consuming the surrounding amorphous gels.[2] The gel was formed by using aluminosilicate solutions and tetramethylammonium hydroxide as the structure-directing agent (SDA).[2] For zeolite A formation, evidences of nucleation at the solid-liquid interface of the gel cavities were also found in sodium aluminosilicate gels without organic SDA.[3] In addition, a reversed crystal growth process from the surface to the core of nanocrystallite aggregates was observed in the crystal growth of zeolite analcime icositetrahedra.[6] These studies have undoubtedly provided new insights into zeolite crystallization processes.As non-structure-directing agents, organic polymers have significant effects on zeolite nucleation and growth. The confinement of sodium aluminosilicate zeolite gels in thermoreversible methylcellulose hydrogels resulted in zeolite A and X nanocrystals under hydrothermal treatment.[7] Crosslinked polyacrylamide hydrogels was used to reduce SAPO-34 crystal sizes in vapor-phase transport synthesis. [8] In all these cases, the small crystal sizes is due to space confinement of the polymer hydrogel networks. Hollow sodalite spheres and zeolite A crystals were also synthesized hydrothermally in the presence of crosslinked polyacrylamide hydrogels. It was suggested that the scaffolds of polyacrylamide hydrogels were the preferential sites for zeolite nucleation, and promoted the direction of nanoparticle aggregation subsequent to the surface-to-core growth.[9] These results suggest that the roles of polymer hydrogels in zeolite synthesis are complex, and syntheses of the zeolites depend on the microstructure of the polymer hydrogels and the interaction between the polymer chains and the zeolite gels.Herein we report the formation of cubes of zeolite A with a single crystalline shell and an amorphous core by in-situ crystallization of sodium aluminosilicate gel inside the polymer networks of uncrosslinked chitosan hydrogel. This work provides further direct evidence for the surface-to-core reversed-growth mechanism. Chitosan is a biopolymer derived from chitin that is found in a wide range of natural sources, such as crab, lobster, and shrimp shells. Chitosan, containing abundant amino and...

What Can Electron Microscopy Tell Us Beyond Crystal Structures?

“…[58] Since then, this phenomenon has been observed in many other materials, including zeolite A, CaTiO 3 perovskite, ZnO, CuPt alloy, metal-organic frameworks, and even organic crystals. [59][60][61] Figure 7 (c) is a TEM image of a zeolite A particle with the corresponding SAED pattern viewed from the [001] projection. Both the regular shape and the diffraction pattern are normal and indicate a single crystal state.…”

“…an amorphous core in a very thin single crystal shell (Figure 7, d). [61] Under electron beam exposure, it was seen that the core material appeared more disordered, with a mobile fluid-like appearance and had separated from the thin shell. The almost identical SAED pattern in Figure 7 (d) was actually from the shell of 7 nm in thickness.…”

Growth Mechanism of Dendritic Hematite via Hydrolysis of Ferricyanide

“…This is the so-called reversed crystal growth mechanism, which has been observed from several crystal growth systems. [7][8][9][10] To synthesise low dimensional materials, e.g. nanowires, some special conditions are normally applied to partially block crystal growth directions.…”

On the reversed crystal growth of BaZrO3 decaoctahedron: shape evolution and mechanism

“…Evidence of such novel crystal growth phenomenon has been found in materials such as zeolites and CaTiO 3 perovskite. [43][44][45][46][47][48] However, this mesoscale transformation process and its mechanism are still challenging because current interests are more focused on the investigation of its role rather than on comprehensive insight into the mechanism. Concomitant mechanisms lead to increased difficulty in understanding this growth process, which is important and must be the focus of detailed studies of the nanocrystal growth mechanism.…”