Tunable water vapor adsorption properties on ion exchange microporous zeolite Na-X derived from natural kaolin

Abstract

The adsorption of water vapor on K+, Ca2+and Mg2+ exchanged zeolite Na-X is studied by gravimetric method. As reported by previous studies, the samples were modified by impregnation and characterized by physicochemical and thermal analysis methods. The analysis of the X-ray fluorescence data obtained on the samples confirmed the effectiveness of cation exchange. XRD, FTIR, SEM, and NMR analyses show good zeolite structure and texture conservation after ion exchange. It is determined that the Na-X zeolite had a BET surface area, SBET, of 764 cm2/g and pore volume of 0.39 cm3 /g when nitrogen (N2) was adsorbed at 77 K. After the ion exchange, these values decreased to 0.11 cm3/g and 211 cm2/g for the zeolite exchanged with K+ ions, 0.25 cm3/g and 482 cm2/g for that exchanged with Mg2+ ions, and 0.23 cm3/g and 433 cm2/g for that exchanged with Ca2+ ions. Water vapor adsorption properties were evaluated by dynamic vapor sorption highlighting the dependence of this process on the nature of the compensating cations. This exothermic process experiences a significant improvement when the zeolite is exchanged with Mg2+ and Ca2+ cations. The differences between the adsorption capacities of the samples are mainly due to the location, size, and charge of the zeolite X extraframework cations. Experimental data were fitted to the Langmuir, Freundlich, Sips, Toth, and Dual-site Langmuir (DSL) isotherm models. The coefficient of determination (R2) and the chi-square error (X2) were used to estimate the accuracy of the data obtained from these isothermal models. According to error function results, it can be seen that all five models fit the experimental data well depending on the temperature. The mean isosteric enthalpy of adsorption values for the various samples are -123.93, -280.22, -36.55 and -29.35 kJ/mol for Zeo-Na-X, Zeo-K-X, ZeoMg-X and Zeo-Ca-X, respectively. As a result of examining the reusability of magnesium zeolite, it was seen that the material was reusable even after being used 5 times.