David Said Shihabi

Evidence of zeolite-solution equilibrium for isomorphous substitution using aqueous metal fluoride complexes

Abstract The reversibility of isomorphous substitution of tetrahedral framework metal ions using aqueous A1F 3 and (NH 4 ) 2 SiF 6 treatments on zeolite ZSM-5 is reported. 19 F NMR studies of A1F 3 and SiF 6 2− D 2 O solutions after reaction at 85 °C which NH 4 ZSM-5 showed formation of corresponding Si and A1 flouride species. Flouride material balances were reasonable within experimental error. These experiments qualitatively suggest an equilibrium condition exists between Al 3+ and Si 4+ using mild aqueous metal fluoride treatment and imply a general synthetic pathway to isomorphously substituted zeolites. As a further example, [Ga]ZSM-5 containing framework Ga 3+ was prepared by substitution of framework Al 3+ using aqueous GaF 3 treatment. The framework Ga 3+ was characterized by solid-state static and magic-angle spinning (MAS) NMR and the observed Ga chemical shift (155 ± 5 ppm) was consistent with tetrahedral framework Ga 3+ reported previously. Quantitation by Ga and Si NMR and temperature-programmed NH 3 desorption (TPAD) showed excellent agreement.