SYNTHESIS, STABILITY AND OXIDATIVE ACTIVITY OF POLYOXOMETALATES PILLARED ANIONIC CLAYS ZNAL-SIW11 AND ZNAL-SIW(11)Z

Layered double hydrotalcites pillared by heteropolyoxometalates, ZnAl- SiW11 and ZnAl-SiW(11)Z (Z = Co2+, Ni2+ and CU2(+)) have been synthesi zed by an anion-exchange reaction. The powder X-ray diffraction spectr a and IR analysis show that the compounds are layered complexes with g allery heights of 0.99 nm and the anions entering the layer retain the ir Keggin structure. The decomposition temperatures of these complexes in air decrease in the following order: ZnAl-SiW,,Ni (620 K), ZnAl-Si W11Co (614 K), ZnAl-SiW11Cu (608 K), ZnAl-SiW11 (602 K), ZnAl-NO3 (563 K). The XRD patterns of the compounds treated in boiling solutions wi th different pH for 2 h reveal that the layer framework is stable in t he pH range from 3.6 to 7.5. The synthesized materials, especially ZnA l-SiW11Co, show high catalytic activities for the oxidation of benzald ehyde to benzoic acid using H2O2 in a liquid-solid biphase system.