Converting Mn/Al layered double hydroxide anion exchangers into cation exchangers by topotactic reactions using alkali metal sulfate solutions.

Layered double hydroxides (LDHs), traditional anion exchangers, can be converted into cation exchangers by topotactic reactions using aqueous solutions of alkali metal sulfates. LDHs with the composition [Mn2Al(OH)6](An-)1/n·yH2O (An- = Cl- or NO3-, CO32-) were reacted with aqueous solutions of Na2SO4. The anion carbonate could only be partially replaced by sulfate.

Cation Exchange Reactions in Layered Double Hydroxides Intercalated with Sulfate and Alkaline Cations (A(H2O)6)[M2+6Al3(OH)18(SO4)2]·6H2O (M2+ = Mn, Mg, Zn; A+ = Li, Na, K).

Layered double hydroxides (LDHs) with similar compositions to the minerals shigaite, natroglaucocerinite, and motukoreaite were synthesized by co-precipitation with increasing pH and characterized by several instrumental techniques. These minerals have previously been described to occur only with sodium and sulfate (Na(H2O)6)[M+26Al3(OH)18(SO4)2]·6H2O (M2+ = Mn, Mg, and Zn). These phases were synthesized successfully along with others containing lithium and potassium. Cation exchange reactions were performed in the presence of alkaline metal sulfates, and for the first time several instrumental techniques were employed to show that the cations can be totally exchanged without removing the intercalated sulfate anions. This class of compounds, traditionally considered to be anion exchangers, can also be considered cation exchangers, which opens new avenues for future scientific and industrial applications.