Structured water in hydrotalcites of formula Mg_<i>x</i>Zn_6−<i>x</i>Al₂(OH)₁₆(CO₃)·4H₂O: a Raman microscopic study

Abstract

Abstract Raman microscopy was used to characterize synthesized hydrotalcites of formula Mg x Zn 6− x Al 2 (OH) 16 (CO 3 )·4H 2 O. The Raman spectra are conveniently subdivided into spectral features based on (a) the carbonate anion, (b) the hydroxyl units and (c) metal–oxygen units. A model is proposed based on a tripod of M 3 OH units in the hydrotalcite structure. In a simplified model, Raman spectra of the hydroxyl‐stretching region enable bands to be assigned to the Mg 3 OH, Zn 3 OH and Al 3 OH units. Bands are also assigned to the hydroxyl stretching vibrations of water. Three types of water are identified: (a) water hydrogen bonded to the interlayer carbonate ion, (b) water hydrogen bonded to the hydrotalcite hydroxyl surface and (c) interlamellar water. A model of water in the hydrotalcite structure is proposed in which water is in a highly ordered structure as it is hydrogen bonded to both the carbonate anion and the hydroxyl surface and also forms bridges between the MOH surface and the carbonate anion. Copyright © 2002 John Wiley &amp; Sons, Ltd.