New insight into the preferred valency of interlayer anions in hydrotalcite-like compounds: The effect of Mg/Al ratio

Magnesium-aluminum hydrotalcite-like compounds with molar ratios of Mg/Al ranging from 1.93 to 6.63 were synthesized and characterized with elemental analysis, XRD, SEM, TG/MS, XPS, 27Al solid state MAS NMR, 13C solid state CP MAS NMR, and vibrational spectroscopy (FTIR and Raman). The results of physico-chemical studies show that the materials are free of impurities and consistently point to the steady evolution of the interlayer anion composition upon increase of Mg/Al ratio. In particular, it is observed that the growing content of magnesium within the hydrotalcite layer is accompanied by the gradual substitution of dinegative interlayer carbonate anions with mononegative bicarbonates and/or nitrates. The observation contradicts the generally accepted view that the hydrotalcite-like structures have greater affinities for multivalent anions compared with monovalent ones. It is argued that in materials with low degree of Al for Mg substitution, hence larger distance between the charge generating sites, compensation by monovalent anions enhances Coulombic interactions within the hydrotalcite-like structure.