SYNTHESIS AND CHARACTERIZATION OF OCTAHEDRAL MOLECULAR-SIEVES (OMS-2) HAVING THE HOLLANDITE STRUCTURE

Hollandite and cryptomelane materials have been prepared using two different methods. Octahedral molecular sieve (OMS) having the 2 x 2 hollandite structure with a one-dimensional pore diameter of 4.6 angstrom. Synthetic cryptomelane or OMS-2 can be formed by refluxing or autoclaving an acidic solution of KMnO4 and Mn2+. Temperature, pH, and countercation are important synthetic parameters. The hollandite formed shows thermal stability up to 600-degrees-C. OMS-2 formed by oxidation of Mn2+ under basic conditions and calcined at higher temperature (200-800-degrees-C) is thermally stable up to 800-degrees-C. X-ray powder diffraction and electron diffraction patterns have been simulated that show good agreement with experimental data supporting a tetragonal crystal system in the I4/m space group. Hollandites were also prepared in the presence of other transition-metal ions such as Cu2+ and Fe3+. The Cu2+ and Fe3+-doped OMS-2 materials were prepared by refluxing a solution of MnO4- and Mn2+ with Cu2+ or Fe3+. Electron paramagnetic resonance (EPR) data show that OMS-2 materials synthesized in the presence of Cu2+ and Fe3+ contain nonexchangeable Mn2+. EPR data for Cu-OMS-2 showed a characteristic six-line pattern with a g value of 2.0 and an A value of 85 G indicative of octahedral Mn2+ coordination. The Mn2+ EPR peaks in Fe-OMS-2 showed similar g and A values. EPR spectra, ion-exchange data, X-ray diffraction patterns, and theoretical simulations of diffraction data suggest that Cu2+ and Fe3+ are located in the tunnels of OMS-2.