Cu/Zn/Co/Al/Cr-containing hydrotalcite-type anionic clays

Abstract

Compounds with the formula [M 6 2+M 2 3+(OH) 16CO 3 · 4H 2O [M 3+ = Cu, Zn, Co; M 3+ = Al, Cr] were prepared by precipitation of the metal nitrates with excess of NaHCO 3 in controlled conditions of temperature, stirring and pH. The characterization has been performed by X-ray powder diffraction (XRD), diffuse reflectance spectroscopy in the visible-UV region (DRS) and thermal analysis (TG-DTA). Cu/Zn/Co/Al and Cu/Zn/Co/Cr materials contain crystalline hydrotalcite as the predominant phase. Cu/Zn/Co/Cr samples show, in addition to hydrotalcite, the presence of some malachite whose amount decreases with increasing Co content. Cu/Zn/Co/Al/Cr precursors contain malachite as the only crystalline phase whereas hydrotalcite appears as a secondary amorphous phase. The thermal decomposition occurs, independently of cation composition, in four steps: the first three consisting essentially of dehydration and the fourth one of decarboxylation of the samples. The thermal treatment first produces a topotactic decomposition of the brucite-like layers giving some kind of oxycarbonates where the metal-oxygen framework remains essentially intact and the interlayered carbonate anions are substantially retained, and then, by release of carbon dioxide, the formation of mixed oxides. Dehydration temperature of samples is independent of both Co and Cr content. Decarboxylation for Cr-containing hydrotalcite-like, HT-Cr, precursors occurs at much lower temperature than for Al-containing, HT-Al, ones. The increasing amount of cobalt in HT-Al and HT-Cr compounds progressively lowers the crystallinity and the decarboxylation temperature of the samples. The effect of different cation composition on the properties, stability and phase composition of the hydroxycarbonate precursors is discussed.