Hydrocalumite and Its Polymer Derivatives. 1. Reversible Thermal Behavior of Friedel's Salt: A Direct Observation by Means of High-Temperature in Situ Powder X-ray Diffraction

The structure of the phases obtained upon dehydration and decomposition of Friedel's salt [Ca2Al(OH)6]Cl·2H2O, also known as hydrocalumite, was investigated by several experimental techniques, in particular, high-temperature in situ XRD measurements, which allowed the detection of a metastable intermediate phase. Thermogravimetric analyses show that Friedel's salt, like most of the layered double hydroxides, undergoes a three-step decomposition on heating (dehydration, dehydroxylation, and anion expulsion) over the following temperature ranges: 25−280, 280−400, and >400 °C. Sharp phase transitions are observed as a result of the ordered distribution of Ca and Al atoms in the hydroxide layer and the well-ordered interlayer structure. Upon cooling to room temperature and exposure to the atmosphere, the dehydrated phase obtained by calcination between 80 and 280 °C was found to recover the basal spacing characteristic of hydrated galleries (7.81 Å) within a few minutes. The structural determination of this thermally metastable phase based on X-ray powder diffraction data recorded at 116 °C reveals a quasi-pillared layer structure with chloride anions situated midway in the interlamellar space at only 2.904(3) Å from Ca atoms of adjacent hydroxide layers. Friedel's salt becomes amorphous at ca. 400 °C, and above 750 °C, it crystallizes into a mixture of CaO and mayenite Ca12Al14O33. Exposure of the amorphous residue obtained at 400 °C to aqueous solutions of KCl led to reconstruction−intercalation phenomena.