Effect of extra-framework cation in ion-exchanged ZSM-5 from rice husk ash on CO2 adsorption

Abstract ZSM-5 zeolite with high aluminum content (Si/Al = 25) was synthesized and then cation-exchanged with alkali metal, alkaline earth metal, transition metal and rare earth metal cations (Li+, Mg2+, Ca2+, Cu2+, Zn2+, La3+ and Ce3+). Crystal structure and physicochemical properties were characterized by XRD, FTIR, TGA/DTG, XRF and nitrogen adsorption-desorption measurement. CO2 adsorption isotherms of all samples were recorded approximately 0–120 kPa at 0 °C, 30 °C and 60 °C, and they were all successfully fitted by the Toth model. The N2 adsorption isotherm presented an approximate straight rising line, and it was perfectly fitted by the Langmuir model. In addition, the CO2/N2 adsorption selectivity, Henry's law constant and the working adsorption quantity were calculated by single component CO2 and N2 adsorption testing. Transition metal cations Cu2+ and Zn2+ type ZSM-5 had the maximum adsorption quantity of 0 °C (70.24 and 74.38 cm3/g), and they had the maximum adsorption selectivity under high pressure and adsorption saturation conditions. The adsorption heat of all ion-exchanged samples was lower than Na-ZSM-5, and it had the approximate value at high loadings (in the range of 15.46–17.65 kJ/mol).