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 × 3-tunnel structure OMS designated as OMS-1 were synthesized by hydrothermal treatment at 160 °C for 48 h. The Cu−OMS-1 series of materials (tunnel substituted) were prepared by incorporating Cu 2+ ions into OL-1, which has a layered structure, at 60 °C for 24 h. [Cu]−OMS-1 materials (framework substituted) were prepared by ion exchanging Cu 2+ ions into the tunnels of OMS-1 under similar conditions. Cu−OMS-2 materials characterized by 2 × 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 (Mg 2+, Mn 2+ ) either in tunnel or framework sites when exchange was done before hydrothermal treatment. The presence of Cu 2+ ions decreases the Mn 2+ /Mn 4+ 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 Mn 2+ 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 Cu 2+ 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 Mn 2+ in the framework are considered as defects in the structure of the crystallites which make them less stable than OMS-1 containing Mg 2+ 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 °C at 1 atm.