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

Abstract

The dehydration and hydration behaviour of a natural zeolite with YUG framework type was investigated as a function of water vapour pressure (PH2O) and temperature (T) by in situ single crystal X-ray diffraction. The chemical composition of the sample is CaAl2Si6O16∙nH2O, with n varying as a function of T and PH2O. Under ambient conditions (T = 302 K and relative humidity RH = 60%) the structure contains 4 H2O per formula unit. After exposure to dry air for 30 min, 1 H2O is released. Prolongation of the exposure time to 60 min does not induce further dehydration. The H2O can easily be up taken by exposing the crystal to ambient conditions for few minutes.The temperature-dependent experiments were conducted under N2 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 H2O 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.