Cation composition during recrystallization of layered double hydroxides from mixed (Mg, Al) oxides

摘要

Changes in cation composition and M~(2+)/M~(3+) ratio during hydrotalcite regeneration were studied. Regenerated hydrotalcites were obtained by recrystallization of mixed (Mg, Al) oxides in solutions of divalent (Mg, Zn, Co, Ni, Cu) or trivalent (Al, Fe) cations. Heating Mg―Al―CO_3 hydrotalcites with Mg/Al = 2, 3 and 3.7, at 600℃ for 2 h yielded periclase-like mixed (Mg, Al) oxides (HT-P). The hydrotalcite structure was restored by dispersing oxides 48 h in water or aqueous solutions of different cations. The presence of Mg~(2+), Zn~(2+), Ni~(2+), Co~(2+), Cu~(2+) salts or of low soluble hydromagnesite increased the M(2+)/Al ratio, reaching a maximum value of 3.8. An incorporation of Zn~(2+), Ni~(2+), Co~(2+) and Cu~(2+) cations in the newly formed hydrotalcite was detected, while Mg~(2+) remained in solution. In the presence of soluble Al salts or freshly precipitated Al(OH)_3, the M~(2+)/Al ratio approximated the minimal possible value of 2. The Mg/Al ratio of a hydrotalcite crystallized from a mixture of two HT-P samples with different Mg/Al ratios is equal to the weighted average value. The results obtained support the conception of the dissolution―crystallization mechanism of hydrotalcite regeneration from mixed (Mg, Al) oxides contrary to the widely accepted concept of topotactic processes.