ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
Abstract The presence of zeolitic water, with a reversible hydration behaviour, was determined by structural and kinetic studies on synthetic mixite BiCu6(OH)6(AsO4)3·nH2O (n≤3). X-ray diffraction and infrared-spectroscopic investigations were performed on single crystals. Isothermal thermogravimetric experiments were carried out to determine the reaction kinetics of the de- and rehydration processes. The single-crystal structure refinement of a fully hydrated crystal yielded five partially occupied Ow positions (Ow=oxygen atom of a H2O molecule) within the tube-like channels of the hexagonal [BiCu6(OH)6(AsO4)3] framework. For the partially dehydrated form, with n≈1, at least two of these sites were found to be occupied significantly. In addition, the structural investigations allowed two different intra-framework hydrogen bonds to be distinguished that are independent of the extra-framework water distribution and are responsible for the stability of the self-supporting framework. The kinetic analysis of the rate data in the 298–343K temperature range shows that the dehydration behaviour obeys a diffusion-controlled reaction mechanism with an empirical activation energy of E a dehyd=54±4 kJ mol–1. A two-stage process controls rehydration of which the individual steps were attributed to an initial surface-controlled (E a hyd-I=6±1 kJ mol–1) and subsequent diffusion-controlled reaction mechanism (E a hyd-II=12±1 kJ mol–1). The estimated hydration enthalpy of 42±5 kJ mol–1 supports the distribution model of molecular water within the channels based on a purely hydrogen-bonded network.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s002690050055
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