Adsorption of carbon dioxide and nitrogen on zeolite rho prepared by hydrothermal synthesis using 18-crown-6 ether

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 CO2-adsorption properties were investigated. CO2 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, CO2 adsorption increased with increasing calcination temperature up to 400 °C. It is thought that the pore volume for adsorption of CO2 increased as a result of 18C6 removal, resulting in increasing CO2 adsorption. A decrease in CO2 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 CO2 is almost constant, at 3.4 mmol-CO2 g−1, regardless of the 18C6 molar ratio. However, CO2 selectivity, which is the CO2/N2 adsorption ratio, decreased. The amount of CO2 adsorbed on zeolite rho is lower than that on zeolite NaX, but higher than that on SAPO-34. The CO2/N2 adsorption ratio for zeolite rho was higher than those for SAPO-34 and zeolite NaX.

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► The optimum calcination temperature of zeolite rho for 18C6 removal and CO2 adsorption is 400 °C.
► The particle size of zeolite rho is controlled in these regions by adjusting the 18C6 molar ratio.
► The highest CO2 selectivity was obtained with zeolite rho prepared at an 18C6 molar ratio of 0.25.
► This selectivity was higher than that of commercial NaX.