NEW SYNTHETIC ROUTES TO HYDROTALCITE-LIKE COMPOUNDS - CHARACTERIZATION AND PROPERTIES OF THE OBTAINED MATERIALS

Hydrotalcite-like anionic clays of general formula [M(II)(1-x) M(III)( x)(OH)(2)](x+)[CO3x/12-](x-). m H2O with M(III) = Al and M(II) = Mg, N i, Zn have been prepared by precipitation of the hydroxycarbonates fro m the ''homogeneous'' solution after the thermally induced hydrolysis of urea. The effect of the temperature of precipitation, of the total metal cations concentration, of the molar fraction M(III)/M(III) + M(I I) and of the molar fraction urea/M(II) + M(III) in solution on the co mposition and on the crystallinity degree of the samples has been inve stigated. The optimum conditions are reported to obtain micro-crystall ine powders with a narrow distribution of particle size in a short tim e with a simple procedure. The compounds obtained have been characteri sed for chemical composition, thermal behaviour, particle-size distrib ution and BET-surface area. In addition, the crystal structure of Mg0. 67Al0.33(OH)(2) (CO3)(0.165) . 0.4 H2O has been refined by X-ray diffr action powder methods. The carbonate form has been converted into the chloride form by letting gaseous HCl flow over the hydrotalcite-like c ompounds, heated at 150 degrees C. The exchange of Cl- anions with som e alkoxide anions in the presence of the respective alkanols has been investigated. The exchange reaction was driven by the segregation of N aCl crystals poorly soluble in alkanols and led to the cointercalation of the alkoxide ions together with the alkanol with the formation of a bi-layer of extended alkyl chain in the interlayer region of the Mg- Al hydrotalcite. The intercalation compound, washed with water, produc es a hydrotalcite with Cl- and OH- as balancing anions.