Influence of the Si/Al ratio of zeolite for CO2 capture

“It has been shown that the adsorption capacity and selectivity of zeolites for the polar molecules increases when the Si/Al ratio decreases [10]. This effect is more important when the quadrupole moment of molecules is great. This phenomenon could be due to an increase of electric field in the zeolites pores induced by increasing number of charged sites present at the surface of zeolites. Moreover, the basicity of zeolites framework enhances with the content of Al3+ ions due to the presence of a greater amount of exchangeable cations [42]. Hence, at low pressure, the zeolites that possess the more small Si/Al ratios should have the best adsorption capacity and selectivity for the polar molecules such as CO2. This phenomenon might be explained by the fact that the polarity of adsorbed molecules plays a more important role in their adsorption when the electric field into the zeolites pores is great. This is supported by the results of Calleja et al [10] and Harlick and Tezel [11] that have indicated that the CO2 adsorption capacity of H-ZSM-5 vary from 1.869 to 1.279 mmol of CO2 g−1 (temperature =309 K; pressure =88.75 and 84.67 kPa) when the Si/Al ratio of zeolite enhances from 15 to 60, as well as 1.24 mmol of CO2 g−1 (temperature =313.15 K; pressure = 101.3 kPa) with a Si/Al ratio of 280 (table 1). A study of CO2 adsorption on the erionite (ZAPS), mordenite (ZNT) and clinoptilolite (ZN-19) has also indicated that the adsorption of CO2 is greater on erionite than on the mordenite and clinoptilolite while the erionites: 3Si/Al3.5, mordenites: 4.17Si/Al5.0 and for the clinoptilolites: 4.25Si/Al5.25 (table 2) [1243].”

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