https://doi.org/10.1016/j.jmrt.2020.02.061
“Adsorption and desorption isotherms measured at pressure up to 115 kPa are presented in Fig. 6. The high degree of coincidence of the adsorption and desorption curves indicates that physical adsorption occurs in zeolite X. It is also demonstrated by the fact that the amount of adsorption decreases significantly with increasing temperature. In fact, the adsorption capacity of zeolite X for CO2 is derived from the permanent quadrupole distance of the extra-framework cations. The resulting electrostatic field has a strong CO2 attraction. The adsorption amount of the samples at 101 kPa is listed in Table 3, wherein the adsorption amounts of NaX and LaLiX are 6.14 and 4.36 mmol/g, respectively. After ion exchange, a decrease in charge density results in electrostatic interaction and a decrease in quadrupole. Therefore, it has been observed that the CO2 adsorption capacity of the ion-exchanged zeolite is lowered, whereas the four modified zeolites still had appreciable adsorption capacities of about 4 mmol/g. Similar results are reported in the literature of Ca and K ion-exchanged zeolite X [42]. Na or Li ions are replaced by La and Ce ions with the same charge number, but due to the different charges they carry, the number of ions and the density of the ions are different. Modified samples of La have a higher degree of substitution and a higher ionic density than Ce. Thus the effect of La modification on CO2 adsorption performance is more obvious.”
“Fig. 6. Adsorption and desorption isotherms of CO2 on (a) NaX, (b) LaNaX, (c) CeNaX, (d) LaLiX and (e) CeLiX.”