Humidity- and temperature-dependent study of YUG type zeolite. A new dehydrated topology.

The dehydration and hydration behaviour of a natural zeolite with YUG framework type was investigated as a function of water vapour pressure (P H2O) and temperature (T) by in situ single crystal X-ray diffraction. The chemical composition of the sample is CaAl 2 Si 6 O 16 ∙ n H 2 O, with n varying as a function of T and P H2O. Under ambient conditions (T = 302 K and relative humidity RH = 60%) the structure contains 4 H 2 O per formula unit. After exposure to dry air for 30 min, 1 H 2 O is released. Prolongation of the exposure time to 60 min does not induce further dehydration. The H 2 O can easily be up taken by exposing the crystal to ambient conditions for few minutes. The temperature-dependent experiments were conducted under N 2 flow from 299 to 673 K, in steps of 50 K. In the investigated T range, the unit-cell volume is reduced by 13% and the space group Pc is maintained. Two main structural changes were observed: i) at 523 K, the release of 2.5 H 2 O is accompanied by the doubling of one unit-cell parameter (a -axis); ii) at 573 K, the structure becomes anhydrous, the c -axis triples, and two T-O-T connections of the framework break, leading to the formation of an interrupted framework. This topology (called HT-B phase) was never reported before. Moreover, the breaking process takes place by a different mechanism with respect that reported for other zeolites, and it is here described for the first time. [Display omitted] • The H 2 O content of the investigated YUG zeolite at RH = 60%, T = 26 °C is 4H 2 O pfu. • The structure immediately loses up to 1H 2 O when exposed to dry air. • Further H 2 O is not absorbed after exposure to high-humidity conditions for 15 days. • The YUG zeolite was investigated from RT to 673K in situ by SC-XRD. • A new phase with an interrupted framework was detected at 573 K. [ABSTRACT FROM AUTHOR]