Crystal structure, characterization and thermal stability of NH4+-exchanged –LIT-type zeolite

We investigated the change in the crystal structure and thermal stability of the synthetic –LIT type zeolite [Si8.74Al3.26O26H2K5.26] by ion exchange with ammonia. In the NH4+ ion exchange, the crystal structure was considerably changed accompanied by a large framework deformation. The crystal structure of –LIT type zeolite has a 10 T-site membered ring and the abw-type composite building unit (CBU). In this topology, the framework comprises the hydrogen bond of SiOH···OSi between neighboring silanol groups. Potassium ions are included into the abw-type CBU and the pseudo micropore accompanied with hydrogen bonds. The crystal structure of the NH4+ treated –LIT zeolite (NH4LIT) has a monoclinic symmetry with a space-group of P21/c, and lattice constants of a = 0.951 nm, b = 0.993 nm, c = 1.459 nm, and β = 89.95°, which indicates a pseudo orthorhombic symmetry. The chemical formula of the treated zeolite was estimated to be Si8.74Al3.26O22(OH)4·5.3(NH4/H2O), where the NH4/H2O ratio was approximately 0.5. Characteristic hydrogen bonds disappeared, as confirmed by 1H MAS NMR spectroscopy. The structural collapse of NH4LIT occurred easily by heating up to 120 °C, whereas the thermal stability of the as-made –LIT zeolite reached over 700 °C.BRIEFCrystal structure and thermal stability of synthetic –LIT zeolite was dramatically changed via. NH4+ ion exchange. Hydrogen bond in the framework structure of synthetic –LIT disappeared after ion exchange.

Graphical abstractA structural change in synthetic –LIT type zeolite by the ion exchange with ammonia was investigated in detail. The hydrogen bond of SiOH⋅⋅⋅OSi between neighboring silanol groups in as-synthesized –LIT zeolite disappeared after ion exchange. The pore shape and the pore size of a pseudo 10MR changed with deformation of the framework structure.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A structural change in synthetic –LIT zeolite occurred by the ion exchange with ammonia. ► The hydrogen bonds of SiOH···OSi in framework disappeared. ► The –LIT-type framework structure was remarkably deformed by the NH4+ ion exchange. ► The thermal stability of the –LIT zeolite was lowered to ca. 120 °C by the ion exchange.