Early Stage Reversed Crystal Growth of Zeolite A and Its Phase Transformation to Sodalite

Abstract

Microstructural analysis of the early stage crystal growth of zeolite A in hydrothermal synthetic conditions revealed a revised crystal growth route from surface to core in the presence of the biopolymer chitosan. The mechanism of this extraordinary crystal growth route is discussed. In the first stage, the precursor and biopolymer aggregated into amorphous spherical particles. Crystallization occurred on the surface of these spheres, forming the typical cubic morphology associated with zeolite A with a very thin crystalline cubic shell and an amorphous core. With a surface-to-core extension of crystallization, sodalite nanoplates were crystallized within the amorphous cores of these zeolite A cubes, most likely due to an increase of pressure. These sodalite nanoplates increased in size, breaking the cubic shells of zeolite A in the process, leading to the phase transformation from zeolite A to sodalite via an Ostwald ripening process. Characterization of specimens was performed using scanning electron microscopy and transmission electron microscopy, supported by other techniques including X-ray diffraction, solid-state NMR, and N(2) adsorption/desorption.

Reversed Crystal Growth of Calcite in Naturally Occurring Travertine Crust

“…Instead, they underwent aggregation to form polycrystalline particles with a rhombohedral appearance (Figure 7d). Despite the short history of this reversed crystal growth route it has been used to explain the formation of many hollow and core-shell crystals in several inorganic systems including zeolites [48,74,75], perovskites [76], metal oxides [26,77], plus more recently organic resorcinarene hexamer [78], alloy nanoparticles [79] and metal organic frameworks [80,81]. There is a need for further extensive studies on the complex interactions between organic structure directing agents and metal cations to establish the role EPS plays in biomineralisation.…”

Electron microscopic studies of growth of nanoscale catalysts and soot particles in a candle flame

“…This so-called reversed crystal-growth route was later found in other systems, such as zeolite A [5,6], perovskites CaTiO 3 and BaTiO 3 [7,8], metal [9] and alloy particles (F. J. Yu et al paper under preparation). Recently, it was also found in organic crystals [10].…”

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“…[29,30] In those systems, large organic molecules were intro duced into traditional synthetic methods, strengthening the interaction between mole cules or small particles, leading to the forma tion of disordered spherical particles. During extended hydrothermal treatment, recrystal lization takes place on the particle surface to form a thin monocrystalline polyhedral shell, followed by recrystallization from the surface to the core, gradually approaching to a single crystal state.…”