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 dich romate (Cr2O72-) have been synthesised and characterised. The parent chlori de 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 pri stine and pillared materials, following calcination between 100 to 450 degr ees C, was also thoroughly studied. It is shown that the intercalated guest s, chromate and dichromate anions, interact with the host hydroxylated shee ts at moderate temperatures. This so-called "grafting process" is character ised by a reorganisation of the anionic species within the interlamellar do main associated with a shrinking of the basal spacing. This phenomenon was clearly evidenced with powder X-ray diffraction (contraction of the interla mellar distance) and, more interestingly, by chromium K-edge EXAFS (variati on in the local environment of the probe element). On the other hand, therm ogravimetric studies indicated that the thermal stability was greatly enhan ced following pillaring with oxo-anions. This is reflected through the dehy droxylation step of the LDHs sheets which shifted to much higher temperatur es, from 280 degrees C in the [Ni-Al-Cl] precursor to over 450 degrees C in the dichromate intercalated compound. Rehydration and reverse exchanges wi th carbonate anions of the thermally treated materials were both unsuccessf ul, suggesting that the grafting phenomenon is irreversible. The specific s urface areas were also negatively affected following anion exchange and mod erate thermal treatment, indicating that the structural changes observed th rough the different analytical techniques also influence the microtextural properties.