ABSTRACT
Zeolite rho was prepared by hydrothermal synthesis using an 18-crown-6 ether (18C6) as a structure-directing agent, and the effects of the calcination temperature for removal of 18C6 on the physicochemical properties and CO(2)-adsorption properties were investigated. CO(2) adsorption on zeolite rho calcined at 150°C was lower than that on samples calcined at temperatures above 300°C. For samples calcined above 300°C, CO(2) adsorption increased with increasing calcination temperature up to 400°C. It is thought that the pore volume for adsorption of CO(2) increased as a result of 18C6 removal, resulting in increasing CO(2) adsorption. A decrease in CO(2) adsorption for calcination from 400°C to 500°C was observed. The particle size of zeolite rho increased with increasing 18C6 molar ratio. Particle sizes of 1.0-2.1 µm and 1.4-2.6 µm were found by field-emission scanning electron microscopy and dynamic light-scattering, respectively. The particle size is controlled in these regions by adjusting the 18C6 molar ratio. XRD showed that zeolite rho samples with 18C6 molar ratios of 0.25-1.5 had high crystallinity. The adsorbed amount of CO(2) is almost constant, at 3.4 mmol-CO(2)g(-1), regardless of the 18C6 molar ratio. However, CO(2) selectivity, which is the CO(2)/N(2) adsorption ratio, decreased. The amount of CO(2) adsorbed on zeolite rho is lower than that on zeolite NaX, but higher than that on SAPO-34. The CO(2)/N(2) adsorption ratio for zeolite rho was higher than those for SAPO-34 and zeolite NaX.
ABSTRACT
There are many viewpoints on the formation mechanisms for zeolites, but the details are not clear. An understanding of the elementary steps for their formation is important for the development of large-scale membranes and efficient manufacturing processes. In this study, the effects of silicon, aluminum, and the incorporation of 18-crown-6 (18C6) ether, on the formation of zeolite rho, using 18C6 as the structure directing agent (SDA) have been investigated by using field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray fluorescence spectrometry (EDX), nuclear magnetic resonance spectroscopy (NMR), thermo gravimetric analysis (TGA), and the pH measurement. These results suggested that a zeolite rho has four synthesis steps; (1) 0-3 h, the dehydration and condensation reaction between the silica and alumina to form amorphous aluminosilicates; (2) 3-20 h, the particle growth and aggregation process for the amorphous aluminosilicates; (3) 20-48 h, the crystallization and crystal growth of zeolite rho, with the incorporation of 18C6; and (4) 48-96 h, gentle growth with an increase in Na/Si ratio and a change in rate for the bounding state between the silica- and the alumina-based species. We consider the above to reflect the four steps for the formation of zeolite rho.
