Host-Guest Chemistry and Catalysis of Layered Metal Hydroxides with Hydrotalcite Structure

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As to the host-guest chemistry of layered metal hydroxide, intercalation of isopolyanions or metal-halide anions into the interlayer of the hydroxide was examined. For example, decavanadate anions were intercalated by anion exchange into the interlayer gallery of a layered double hydroxide (Mg_2Al(OH)_6NO_3) and the chemical transformation of the isopolyanions by thermal treatment was examined by XRD and IR techniques. The isopolyanions are stable only up to 473 K and transformed into smaller species. The calcination at 573 - 623 K leads to the formation of polymeric vanadate of the type [ -VO3- ]_3^<n->. The overall layered structure is maintained up to 623 K. At 623-673 K, the host layers are turned into a MgO structure. The vanadate reacts with the host oxide to form alpha-Mg_2V_2O_7 around 873 K. Thermal treatment also results in the reduction of part of V(V) to V(IV) species as indicated by ESR.Metal-halide anions such as NiCl_4^<2-> can intercalate into the interlayer of the hydroxide. The anions intercalated can undergo a halide exchange with alkyl bromides such as butyl bromide. Electronic spectrum study showed that the NiCl_4^<2-> anion has tetrahedral symmetry and that the symmetry is retained even after the halide-exchange reaction, the bromide anions being coordinated to Ni cations.On the catalysis of layered metal hydroxide, the catalytic ability of the Cl^-- or NiCl_4^<2->-intercalated hydroxide to a halide-exchange reaction between alkyl chloride and alkyl bromide was examined. Using these hydroxides, alkyl bromides such as benzyl bromide can be easily synthesized. The effect of solvent and of the kind of alkyl group on the rate of the reaction was also examined.