Effect of interlayer anions on the physicochemical properties of zinc-aluminium hydrotalcite-like compounds

Zinc-aluminium hydrotalcite-like compounds (ZnAlAn--HT) with a Zn/Al atomic ratio 2.0 and An- = CO2-3, Cl-, NO-3 and SO2-4, were synthesized by coprecipitation under low supersaturation. Their physicochemical properties were studied using powder X-ray diffraction (PXRD), infrared (IR) and laser Raman (LR) spectra, thermogravimetry (TG), differential scanning calorimetry (DSC), evolved gas analysis (EGA), 27Al MAS NMR, BET surface area and pore-size determination. The PXRD of the synthesized samples showed that the crystallinity was affected by the nature of the anions present in the interlayer space. The IR and LR studies revealed that except the NO3 ion, the symmetry of these interlayer anions was reduced upon intercalation. The TG, DSC and EGA results showed two or three stages of weight loss corresponding to the removal of the interlayer water, structural water and the anion, respectively. The activation energy, Ea, for the decomposition process was found to decrease in the order ZnAlCO3-HT > ZnAlSO4-HT > ZnAlCl-HT > ZnAlNO3-HT. Formation of a pentacoordinated Al (AlV) in addition to the octahedral (AlVI) and tetrahedral Al (AlIV) was the special feature noticed in the 27Al MAS NMR of the calcined samples. Thermal calcination around 500°C resulted in the formation of non-stoichiometric ZnO whose crystallinity decreased in the order ZnAlNO3-CHT > ZnAlCl-CHT > ZnAlSO4-CHT > ZnAlCO3-HT while their extent of solid solubility was found to be the reverse. The crystallinity of the calcined samples was also correlated with surface area and pore-size determination.