Dynamic Temperature-Rate Dependent and Isothermal Time-Dependent In-Situ Dehydration of Cavansite (Ca(VO)(Si₄O₁₀)·4H₂O)

Non-equilibrium dehydration dynamics of cavansite (Ca(VO)(Si₄O₁₀)·4H₂O), a zeolite-like material, were investigated with fast time-resolved high-temperature (HT) synchrotron powder X-ray diffraction (XRD). In-Situ dehydration experiments were done with both distinct heating rates (3 K/min and 10 K/min) and particle sizes (crystals <25 μm, and between 25 and 50 μm). In addition, time-dependent isothermal runs at 381, 490, and 726 K were performed for fine and coarse grained samples to track dehydration accompanied by structural evolution. The non-equilibrium dynamic dehydration is more influenced by the heating rate than by the crystal size. In general, dehydration steps involve less pronounced volume modifications and are shifted to higher temperature than found for “equilibrated” single-crystals. Isothermal release of 1H₂O at 381 K is characterized by a first rapid increase of cell volume related to thermal motion of extra framework H₂O, then the internal pressure is released by H₂O liberation and the cell volume continuously decreases. The coarse fraction reacts time-delayed compared to the fine one. At 490 K 2H₂O are expelled. This process converges to a low volume plateau, which is reached for the fine sample in a broad time span between 770 and 1880s. Surprisingly, the coarse sample reacts faster reaching the slightly lowered volume plateau after ca. 300 s. The faster reaction of the coarse sample is assigned to lower partial H₂O pressure due to differences of coarse and fine particle packing in the capillary. At 726 K we expected release of 3H₂O. However, the coarse sample rapidly decreased to a volume plateau characteristic of the 2H₂O modification and remained crystalline until the end of the experiment (2200 s). The fine sample dehydrated faster, continuously lost crystallinity, and became amorphous after already 490 s.