Cation exchange in RHO zeolite: the influence on the selective adsorption process of CO2 in relation to N2 and CH4

The use of RHO-type zeolites in adsorption processes has been attracting increasing attention from the scientific community due to their unique property, structural flexibility. In addition, their capability for cation exchange makes them suitable for a wide range of applications. In this work, the cation exchange of RHO zeolites with alkali metals (Na, Li, K) and alkaline earth metals (Ca, Sr) was investigated for CO 2 adsorption under high-pressure conditions (0.6 kPa from 1000 kPa in room temperature), and the selectivity toward N 2 and CH 4 was evaluated. The results showed that cation exchange did not significantly affect the crystalline phases of the material; however, morphological changes were observed for the samples exchanged with alkaline earth metals. After adsorption experiments, these materials also exhibited a reduction in CO 2 uptake, from 4.78 mmol/g to 0.83 mmol/g for SrRHO and 0.57 mmol/g for CaRHO, whereas Na-exchanged RHO showed an increased adsorption capacity of 5.24 mmol/g. The KRHO sample displayed enhanced adsorption up to 400 kPa, with CO 2 uptake increasing from 2.81 mmol/g (parent RHO) to 3.36 mmol/g at 101 kPa. Regarding selectivity, the type of cation in the framework proved to be crucial: for N 2 , LiRHO exhibited the highest selectivity, followed by NaRHO and the parent zeolite, while for CH 4 , KRHO presented the highest selectivity. Overall, these results indicate that RHO zeolites are promising materials for adsorption-based processes, provided that the specific cationic modification is carefully chosen according to the target application. • Cation positions in RHO zeolite framework are systematically analyzed. • Cation mobility in RHO studied under different pressure conditions. • Role of framework flexibility in CO 2 adsorption is evaluated. • Cation effects on selectivity in CO 2 /CH 4 and CO 2 /N 2 process calculated using IAST.