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 o f 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 int o OL-l, which has a layered structure, at 60 DC for 24 h. [Cu]-OMS-1 materi als (framework substituted) were prepared by, ion exchanging Cu2+ ions into the tunnels of OMS-1 under similar conditions. Cu-OMS-2 materials characte rized by 2 x 2 tunnels were prepared by the reflux method. ICP analysis sho ws 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 su bstitute for the divalent cations (Mg2+, Mn (2+)) either in tunnel or frame work sites when exchange was done before hydrothermal treatment. The presen ce of CU2+ ions decreases the Mn2+/Mn4+ ratio in Cu-OMS-1 samples as sugges ted by an increasing trend in the average oxidation state of Mn and decreas ing 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 s ix 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 samp les have the same thermal stability as OMS-1 but the amount of lattice oxyg en evolved decreases slightly with the amount of CU2+ in, the tunnel. The t hermal stability of the Cu- OMS-1 and Cu-OMS-2 structures decrease as the a mount of Cu incorporated increases. The copper ions that substitute for Mn2 + in the framework are considered as defects in the structure of the crysta llites which make them less stabile than OMS-1 containing Mg2+ in the frame work 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 t he OMS-2 structure increases with the amount of copper incorporated. Cu(II) ions replacing the divalent cations in the framework during hydrothermal s ynthesis possibly account for the properties observed in the Cu-OMS-1 mater ials. An increased catalytic activity was observed for this set of material s as the Cu/Mn ratio increased for the oxidative dehydrogenation of ethylbe nzene to styrene at 300 degrees C at 1 atm.