Mixite, MCu6(OH)6(AsO4,AsO3OH)33H2O, M= (Bi3+,Ca), is a microporous framework structure with similarities to common zeolites, and therefore could be considered for similar applications. The complex [MCu6(OH)6(AsO4)3]0 framework consists of not only tetrahedral units but also of BiO9 and CuO5 polyhedra. Therefore it differs from the purely tetrahedral frameworks of aluminosilicate zeolites. The mixite framework defines tubular channels, directed along the c-axis, which house molecular H2O on sites that have not previously been determined. The effective aperture diameter of these channels is approximately 7.4 Å, an appropriate size for zeolitic behaviour as has been indicated by previous observations of variable water contents under ambient conditions.
Particular attention has been paid to the stereochemistry of the Cu2+ atom in this study. The planar-square bases of the pyramidal Cu2+O5 polyhedra are aligned to form parts of the framework's channel walls and, hence, the Cu2+ cation can be expected to have an influence on the distribution of the molecular water within the channels due to formation of weak Cu-O bonds. In order to investigate the possible influence of the Cu2+ cation on the extra-framework atomic distribution a kinetic study of the hydration, and its reversibility, was performed on synthetic mixite, BiCu6(OH)6(AsO4)3 xH2O, x ≤ 3. Isothermal thermo-gravimetric investigations in the 25 to 70 °C range were carried out to determine dehydration (in nitrogen) and rehydration (in air) as a function of time.
Dehydration was found to obey a diffusion-controlled mechanism, whereas rehydration is a two-step process of which the initial and the subsequent steps are distinguished by grain-boundary and diffusion controlled mechanisms respectively. Analysis of the rate data yielded activation energies from which an estimated hydration enthalpy of 46 ± 4 kJ mol-1 was derived. This value, which is close to the vaporization enthalpy of liquid water, is much lower than hydration enthalpies known for various zeolites. This indicates that no significant bonding between the molecular H2O and cations, including the copper cations, can be expected. The location of the water positions by single-crystal X-ray diffraction studies on a crystal embedded in nitrogen/water support these results: Cu-Ow distances (Ow = oxygen atom of the relevant water molecules in the channels) of at least 2.9 Å indicate that these H2O molecules do not participate in the Cu coordination. The located Ow sites, as well as the results of the kinetic study, lead us to expect that a purely hydrogen-bonded network is responsible for the configuration of H2O in mixite. This could explain the high variability of the water content of mixite, even under ambient conditions.