Changes in Porous Parameters of the Ion Exchanged X Zeolite and Their Effect on CO2 Adsorption

Abstract

Zeolite 13X (NaX) was modified through ion-exchange with alkali and alkaline earth metal cations. The degree of ion exchange was thoroughly characterized with ICP, EDS and XRF methods. The new method of EDS data evaluation for zeolites was presented. It delivers the same reliable results as more complicated, expensive, time consuming and hazardous ICP approach. The highest adsorption capacities at 273 K and 0.95 bar were achieved for materials containing the alkali metals in the following order K < Na < Li, respectively, 4.54, 5.55 and 5.94 mmol/g. It was found that it is associated with the porous parameters of the ion-exchanged samples. The Li0.61Na0.39X form of zeolite exhibited the highest specific surface area of 624 m2/g and micropore volume of 0.35 cm3/g compared to sodium form 569 m2/g and 0.30 cm3/g, respectively. The increase of CO2 uptake is not related with deterioration of CO2 selectivity. At room temperature, the CO2 vs. N2 selectivity remains at a very high stable level prior and after ion exchange in co-adsorption process (XCO2 during adsorption 0.15; XCO2 during desorption 0.95) within measurement uncertainty. Additionally, the Li0.61Na0.39X sample was proven to be stable in the aging adsorption-desorption tests (200 sorption-desorption cycles; circa 11 days of continuous process) exhibiting the CO2 uptake decrease of about 6%. The exchange with alkaline earth metals (Mg, Ca) led to a significant decrease of SSA and micropore volume which correlated with lower CO2 adsorption capacities. Interestingly, the divalent cations cause formation of mesopores, due to the relaxation of lattice strains.