Synthesis and Raman spectroscopic characterisation of hydrotalcite with CO32- and (MoO4)2- anions in the interlayer

Abstract

Raman spectroscopy has been used to characterise synthetic mixed carbonate and molybdate hydrotalcites of formula Mg6Al2(OH)16((CO3)2-,(MoO4)2-).4H2O. The spectra have been used to assess the molecular assembly of the cations and anions in the hydrotalcite structure. The spectra may be conveniently subdivided into spectral features based upon the carbonate anion, the molybdate anion, the hydroxyl units and water units. Bands are assigned to the hydroxyl stretching vibrations of water. Three types of carbonate anions are identified: (a) carbonate hydrogen-bonded to water in the interlayer, (b) carbonate hydrogen-bonded to the hydrotalcite hydroxyl surface, (c) free carbonate anions. It is proposed that the water is highly structured in the hydrotalcite as it is hydrogen bonded to both the carbonate and the hydroxyl surface. The spectra have been used to assess the contamination of carbonate in an open reaction vessel in the synthesis of a molybdate hydrotalcite of formula Mg6Al2(OH)16((CO3)2-,(MoO4)2-).4H2O. Bands are assigned to carbonate and molybdate anions in the Raman spectra. Importantly the synthesis of hydrotalcites from solutions containing molybdate provides a mechanism for the removal of this oxy-anion.