Reversible hydration in synthetic mixite, BiCu6(OH)(6)(AsO4)(3)center dot nH(2)O (n<=3): hydration kinetics and crystal chemistry

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) . nH(2)O (n less than or equal to 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 approximate to 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.