Influence of the basicity of zeolite for CO2 capture

https://doi.org/10.1088/1468-6996/9/1/013007

“The basic properties of zeolites brought by cations allow a strong capitation of acidic molecules by enhancing the electron density of the framework oxygen [22, 23]. The basic strength of these sites increases with the electropositivity of exchangeable cations [24]. More specifically, certain works have indicated that the basic strength of cationic zeolites containing the cations of Group 1A increases as following: Li⁺<Na⁺<K⁺<Rb⁺<Cs⁺ [25, 26]. Study of CO₂ adsorption on three zeolites, including natural hershelite-sodium chabazite (sodium aluminosilicate), clinoptilolite (sodium aluminosilicate) and clinoptilolite (potassium calcium sodium aluminosilicate), have also revealed that the capacities and rate of CO₂ adsorption of natural hershelite-sodium chabazite and clinoptilolite (sodium aluminosilicate) are greater than clinoptilolite (potassium calcium sodium aluminosilicate) [27]. This difference of adsorption has been attributed to the higher basicity of the surface of natural hershelite-sodium chabazite and clinoptilolite (sodium aluminosilicate) as compared to that of clinoptilolite (potassium calcium sodium aluminosilicate), resulting from a greater amount of sodium ions on the natural hershelite-sodium chabazite and clinoptilolite (sodium aluminosilicate). Moreover, it has also been reported that the basicity of oxygen atoms of framework of zeolites NaX and NaY is strongly decreased by the substitution of Na⁺ cations by the Ba²⁺ cations [22]. This phenomenon might be caused by a decrease of partial negative charge of oxygen atoms adjacent to Ba²⁺ cations. In this matter, an analysis of the CO₂ adsorption on zeolites KX, BaX and LaX has effectively indicated that the zeolite KX was the most basic among these three zeolites [28]. In contrast, a study of CO₂ adsorption on the clinoptilolite has indicated that the substitution of Na⁺ and K⁺ by Ca²⁺ caused a rise of the basicity of the framework oxygen acting as basic center [7]. The basic strength and the capacity of CO₂ adsorption of zeolites might also be increased significantly by occlusion of alkali metal oxides. The basic sites of basic metal oxides such as rare earth oxides and alkaline earth oxides, are considered to be more strongly basic that those of ion-exchanged zeolites [26]. In this matter, it has been observed that the occlusion of oxides of cesium (CsO_x) in the zeolites NaX and NaY caused a rise of the basicity and capacity of CO₂ adsorption on these zeolites, and the increase of the capacity of CO₂ adsorption was proportional to amount of occluded CsO_x [29].”