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

Abstract

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=0.951nm, b=0.993nm, c=1.459nm, 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.