Synthesis of anionic clay minerals (mixed metal hydroxides, hydrotalcite)

A number of approaches to the synthesis of a class of anionic clay minerals (Ma2+Mb3+ (OH)2a+2b(X−)2b ·xH2O; M2+ = Mg, Ni, Co, Zn, Cu, etc.; M3+ = Al, Cr, Fe, Sc; M2+/M3+ ∼ 1−5; X− = water and base stable anion; x = 0−6) have been sumarized. The most general method involves the mixing of a concentrated, aqueous solution of M2+ and and M3+ with aqueous hydroxide-carbonate to yield an amorphous gel followed by crystallization at 60–325°. For a number of these materials, the synthetic latitudes with respect to the nature of M2+, M3+, the M2+/M3+ ratio, the solution pH and the crystallization temperature has been detailed. The crystallization temperature and time influences the particle size, morphology, surface area and the appearance of foreign phases. The incorporation of various interstitial anions by exchange or synthesis is discussed.