Structural aspects and thermal properties of takovite-like layered double hydroxides pillared with chromium oxo-anions

Intercalated layered double hydroxides (LDHs) of chromate (CrO42–) and dichromate (Cr2O72–) have been synthesised and characterised. The parent chloride material ([Ni–Al–Cl]) was obtained by coprecipitation at constant pH and further anion-exchanged to incorporate the desired oxo-anions. The physico-chemical properties of all compounds were investigated using X-ray powder diffraction, infrared spectroscopy, thermogravimetric analyses and extended X-ray absorption spectroscopy (EXAFS). The structural evolution of the pristine and pillared materials, following calcination between 100 to 450 °C, was also thoroughly studied. It is shown that the intercalated guests, chromate and dichromate anions, interact with the host hydroxylated sheets at moderate temperatures. This so-called "grafting process" is characterised by a reorganisation of the anionic species within the interlamellar domain associated with a shrinking of the basal spacing. This phenomenon was clearly evidenced with powder X-ray diffraction (contraction of the interlamellar distance) and, more interestingly, by chromium K-edge EXAFS (variation in the local environment of the probe element). On the other hand, thermogravimetric studies indicated that the thermal stability was greatly enhanced following pillaring with oxo-anions. This is reflected through the dehydroxylation step of the LDHs sheets which shifted to much higher temperatures, from 280 °C in the [Ni–Al–Cl] precursor to over 450 °C in the dichromate intercalated compound. Rehydration and reverse exchanges with carbonate anions of the thermally treated materials were both unsuccessful, suggesting that the grafting phenomenon is irreversible. The specific surface areas were also negatively affected following anion exchange and moderate thermal treatment, indicating that the structural changes observed through the different analytical techniques also influence the microtextural properties.