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

摘要

We investigated the change in the crystal structure and thermal stability of the synthetic -LIT type zeolite [Si_(8.74)Al_(3.26)O_(26)H2K_(5.26)] 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 =SiO-H···O-Si= 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 P = 89.95°, which indicates a pseudo orthorhombic symmetry. The chemical formula of the treated zeolite was estimated to be Si_(8.74)Al_(3.26)O_(22)(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. BRIEF: Crystal 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.