EFFECT OF INTERLAYER ANIONS ON THE PHYSICOCHEMICAL PROPERTIES OF ZINC-ALUMINUM HYDROTALCITE-LIKE COMPOUNDS

Zinc-aluminium hydrotalcite-like compounds (ZnAlA(n-)-HT) with a Zn/Al atomic ratio 2.0 and A(n-)=CO32-, Cl-, NO3- and SO42-, were synthesiz ed by coprecipitation under low supersaturation. Their physicochemical properties were studied using powder X-ray diffraction (PXRD), infrar ed (IR) and laser Raman (LR) spectra, thermogravimetry (TG), different ial scanning calorimetry (DSC), evolved gas analysis (EGA), Al-27 MAS NMR, BET surface area and pore-size determination. The PXRD of the syn thesized samples showed that the crystallinity was affected by the nat ure of the anions present in the interlayer space. The IR and LR studi es revealed that except the NO3- ion, the symmetry of these interlayer anions was reduced upon intercalation. The TG, DSC and EGA results sh owed two or three stages of weight loss corresponding to the removal o f the interlayer water, structural water and the anion, respectively. The activation energy, E(a), for the decomposition process was found t o decrease in the order ZnAlCO3-HT > ZnAlSO4-HT > ZnAlCl-HT > ZnAlNO3- HT. Formation of a pentacoordinated Al (Al-V) in addition to the octah edral (Al-VI) and tetrahedral Al (Al-IV) was the special feature notic ed in the Al-27 MAS NMR of the calcined samples. Thermal calcination a round 500 degrees C resulted in the formation of non-stoichiometric Zn O 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.