Effects of Cu2+ ions on the structure and reactivity of todorokite- and cryptomelane-type manganese oxide octahedral molecular sieves

The concentration effects on Cu uptake into the structures and reactivity of manganese oxide octahedral molecular sieves (OMS) were investigated. Two sets of 3 x S-tunnel structure OMS designated as OMS-1 were synthesized by I lydrothermal treatment at 160 degrees C for 48 h. The Cu-OMS-1 series of materials (tunnel substituted) were prepared by incorporating CU2+ ions into OL-l, which has a layered structure, at 60 DC for 24 h. [Cu]-OMS-1 materials (framework substituted) were prepared by, ion exchanging Cu2+ ions into the tunnels of OMS-1 under similar conditions. Cu-OMS-2 materials characterized by 2 x 2 tunnels were prepared by the reflux method. ICP analysis shows a Cu/Mn molar ratio of 0.278 for Cu-OMS-1 labeled C as the substitution limit for a pure sample based on its XRD pattern. The Cu(II) ions easily substitute for the divalent cations (Mg2+, Mn (2+)) either in tunnel or framework sites when exchange was done before hydrothermal treatment. The presence of CU2+ ions decreases the Mn2+/Mn4+ ratio in Cu-OMS-1 samples as suggested by an increasing trend in the average oxidation state of Mn and decreasing amounts of total Mn. XPS data reveal that Cu remains in the 2(+) state in all of the materials as indicated by the shake-up peaks associated with Cu in such valency. EPR spectra of Cu-containing OMS-1 materials show the six peaks of Mn2+ with a hyperfine splitting constant of 97 G, indicating an octahedral environment. The TGA profiles of[Cu]-OMS-1 show that these samples have the same thermal stability as OMS-1 but the amount of lattice oxygen evolved decreases slightly with the amount of CU2+ in, the tunnel. The thermal stability of the Cu- OMS-1 and Cu-OMS-2 structures decrease as the amount of Cu incorporated increases. The copper ions that substitute for Mn2+ in the framework are considered as defects in the structure of the crystallites which make them less stabile than OMS-1 containing Mg2+ in the framework where it exhibits a stabilizing effect. Resistivity measurements show a decrease and leveling off as the concentration of Cu increases in the Cu-OMS-1 series, which is a trend consistent with doping. The resistivity of the OMS-2 structure increases with the amount of copper incorporated. Cu(II) ions replacing the divalent cations in the framework during hydrothermal synthesis possibly account for the properties observed in the Cu-OMS-1 materials. An increased catalytic activity was observed for this set of materials as the Cu/Mn ratio increased for the oxidative dehydrogenation of ethylbenzene to styrene at 300 degrees C at 1 atm.