Anion Exchange Properties and Application of Layr Structured Basic Metal Salts

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In contrast to a variety of cation-exchangeable layred crystals, anion-exchangeable ones are very rare. This is probably because anions are used as building units constructing the framework of crystal structures, and strongly bound to cations. Hydrotalcite-type layred double hydroxides (LDHs) with a general formula of[M^<ii>_<1-x>M^<iii>^x (OH) _2]^x+Y^<z->_<x/z> nH_2O are exceptionally rare examples of anion-exchangeable crystals. As often referred to as anionic clays, LDHs are structural complements of smectite cationic clays ; the positive charge of the layrs are created by the substitution of divalent cations with trivalent ones in the octahedral layrs. The charge-balancing anions are located between the hydroxy layrs, and easily exchanged with various anions.In a previous study, we showed that basic copper salt Cu_2(OH)_3(OCOCH_3)・H_2O with the botallackite type layr structure exhibited interesting anion exchange properties ; the acetate ions directly coordinated to the copper ions were exchangeable with various anions. In this study, a new type of anion-exchangeable layred mixed basic salts have been developed. The layred compound with the title composition(0.15<x<0.25)was prepared by a hydrothermal hydrolysis of a mixed acetate solution of nickel and zinc. The structure can be characterized as the Zn_5 (OH) _8Cl_2・H_2O type ; Ni^2+ions are in the octahedral sites of Ni(OH)_2 layr with vacancies, and Zn^2+ions are in tetrahedral sites above and below the vacancies of the hydroxide layrs. The interlayr acetate ions of the new layred compound were topochemically exchanged with a variety of anions such as Cl-, Br-, I-, NO_3-, ClO_4-, CO_3^<2->, and SO_4^<2->. The reactions were reversible. Exchanging with[Si_8O_<20>]^8-ions resulted in the formation of two-dimensional silicate layrs similar to a 2 : 1 type clay mineral with swelling properties.