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.

A 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 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.