REVERSIBLE HYDRATION IN SYNTHETIC MIXITE, BICU6(OH)(6)(ASO4)(3)CENTER-DOT-NH(2)O (N-LESS-THAN-OR-EQUAL-TO-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 diff raction and infrared-spectroscopic investigations were performed on si ngle crystals. Isothermal thermogravimetric experiments were carried o ut to determine the reaction kinetics of the de- and rehydration proce sses. The single-crystal structure refinement of a fully hydrated crys tal 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 ap proximate 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 responsi ble for the stability of the self-supporting framework. The kinetic an alysis of the rate data in the 298-343K temperature range shows that t he dehydration behaviour obeys a diffusion-controlled reaction mechani sm 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 mecha nism (E-a(hyd-II) = 12 +/- 1 kJ mol(-1)). The estimated hydration enth alpy of 42 +/- 5 kJ mol(-1) supports the distribution model of molecul ar water within the channels based on a purely hydrogen-bonded network .