Mechanism of zeolite A nanocrystal growth from colloids at room temperature.

Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA.
Science (Impact Factor: 31.48). 03/1999; 283(5404):958-60. DOI: 10.1126/science.283.5404.958
Source: PubMed

ABSTRACT The formation and growth of crystal nuclei of zeolite A from clear solutions at room temperature were studied with low-dose, high-resolution transmission electron microscopy in field emission mode and with in situ dynamic light scattering. Single zeolite A crystals nucleated in amorphous gel particles of 40 to 80 nanometers within 3 days at room temperature. The resulting nanoscale single crystals (10 to 30 nanometers) were embedded in the amorphous gel particles. The gel particles were consumed during further crystal growth at room temperature, forming a colloidal suspension of zeolite A nanocrystals of 40 to 80 nanometers. On heating this suspension at 80 degrees C, solution-mediated transport resulted in additional substantial crystal growth.

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    • "The increase in the chemical shift in the 29 Si MAS NMR spectrum demonstrates that the Si/Al ratio of the gel skeleton decreased as the geopolymerization process continued. These results confirm the same coordination in the Al 2 O 3 –2SiO 2 –Na 2 O geopolymer and the NaA zeolite structures and the existence of NaA zeolite seeds in the geopolymer, which can be grown into NaA zeolite crystals under proper crystallization conditions [15] [16] [22] [23] "
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    • "Although the physical and chemical properties of zeolites largely depend on their structures, the morphology and size distribution of zeolite crystals also have a significant effect on their properties and applications. Recently, a lot of work has been performed to investigate the nucleation mechanism [3] [4] and the hydrothermal synthesis parameters [5] that may influence the crystal size and morphology of NaA zeolites, such as the solvent [6], Si/Al ratio in the gel [7] [8], solid/ liquid ratio [9], alkalinity [9] [10] [11] [12], crystallization time [13] and temperature [14] used during synthesis. The results showed that the overall chemical composition of the reaction mixture, especially its alkalinity, determined the morphology of the crystalline end products. "
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    ABSTRACT: The influences of the crystallization temperature, time, and alkalinity of the reaction system and aluminum sources (aluminum chloride and aluminum sulfate) on the synthesis of NaA zeolite have been systematically investigated using the dry-gel conversion method (DGC). The phase and morphology of the synthesis products were characterized using X-ray diffraction and SEM, respectively. This study showed that the alkalinity and temperature affected the NaA zeolite crystallization process and the size distribution of the crystalline end products. Furthermore, as the reaction time increased, the metastable phase transformed into a more stable phase. It was worth mentioning that the final NaA zeolite product contained a small quantity of NaX zeolite when aluminum chloride was used as the aluminum source while using aluminum sulfate as the aluminum source, the final product was mostly sodalite, indicating that the anions in the different aluminum sources have a significant effect on the crystallization process, most likely due to their electrostatic and steric interactions with the zeolite framework. To obtain NaA zeolite crystals with uniform sizes and high crystallinity, the optimal synthesis parameters for the two aluminum sources were determined.
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    • "They have widely been used as catalysts, adsorbents, and ion exchangers [1] [2]. During the past decade, decreasing of the crystallization time at moderate temperature in the preparation of zeolite-type materials has successfully been achieved [3] [4]. Indeed, natural zeolites are often formed at low temperature in closed alkaline and saline lake systems [5]. "
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    ABSTRACT: Nanozeolite NaA was synthesized by the hydrothermal method with silica extracted from rice husk as silica source. Amorphous silica with 87.988 wt%   SiO 2 was extracted from rice husk ash by a suitable alkali solution. The effect of the crystallization time and the ratio of Na 2 O / SiO 2 on the properties of the final product was investigated. The synthesized nanozeolite was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) techniques, and Brunauer-Emmett-Teller (BET) method. Results revealed that the crystallization time and alkalinity have significant effects on the structural properties of nanozeolite. Nanocrystals NaA with crystal sizes ranging from 50 to 120 nm were synthesized at room temperature with 3 days aging, without adding any organic additives.
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