Alkali cations exchange in MCM-22

Ion exchange of alkali ions Li+, K+, and Cs+ into a Na-MCM-22 zeolite has been studied in aqueous solution at 303K, aiming at determining their exchange isotherms. Values of the thermodynamic equilibrium constant K-a and the standard free energy of exchange Delta G degrees were calculated for reversible systems. Except for lithium, the degree of exchange of metal ions reached 100%. A thermodynamic affinity sequence Cs+> K+> Li+ was established. The Rietveld method of structure refinement was used to determine the crystallographic positions of alkali cations after the calcination step at 623 K. The results showed the presence of six different cationic sites located in the hexagonal prism (S1), inside the pocket (S2 and S5), in the supercages (S3 and S4) and in the pore system defined by the two-dimensional sinusoidal channels (S6).