Structural changes in –LIT zeolites related to cation-exchange treatments under aqueous and non-aqueous conditions

Abstract

Reversible structural changes in synthetic lithosite (–LIT)-type zeolites were achieved through cation exchange treatments. –LIT is a rare and unique zeolite that requires solvothermal conditions for synthesis. The structural feature of as-synthesized –LIT zeolite was a pseudo 10-membered ring window that included orderly terminal silanols with hydrogen bonds between them. This made it difficult to achieve cation exchange at room temperature. Thus, the cation exchange treatment was performed at 60°C. It was found that prolonged ion exchange treatments with ammonium or lithium cations removed potassium cations from the zeolite micropores. This resulted in a structural change from type-A (as-synthesized –LIT zeolite) to type-B (ammonium exchanged form), with the breakage of the hydrogen bonds. The zeolitic micropores in the type-B zeolite samples were accessible to cations larger than potassium ions (e.g., cesium cations). The cation-exchange treatment for the type-B samples conducted using an aqueous potassium chloride solution induced the simultaneous adsorption of water molecules with potassium cations, which inhibited the reversible change to the type-A structure. However, when the type-B zeolite samples were treated with potassium ethoxide at 60°C, their structure could be transformed to one similar to that of the type-A zeolite samples through the regeneration of hydrogen bonds. This was owing to a decrease in the influence of the water molecules.