Effects of Cu2+ Ions on the Structure and Reactivity of Todorokite- and Cryptomelane-Type Manganese Oxide Octahedral Molecular Sieves
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- 作者：Elaine Nicolas-Tolentino ; Zheng-Rong Tian ; Hua Zhou ; Guanguang Xia ; and Steven L. Suib
- 刊名：Chemistry of Materials
- 出版年：1999
- 出版时间：July 1999
- 年：1999
- 卷：11
- 期：7
- 页码：1733 - 1741
- 全文大小：116K
- 年卷期：v.11,no.7(July 1999)
- ISSN：1520-5002
文摘
The concentration effects on Cu uptake into the structures and reactivity of manganeseoxide octahedral molecular sieves (OMS) were investigated. Two sets of 3 × 3-tunnel structureOMS 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 Cu2+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 Cu2+ ions into the tunnels ofOMS-1 under similar conditions. Cu-OMS-2 materials characterized by 2 × 2 tunnels wereprepared 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. TheCu(II) ions easily substitute for the divalent cations (Mg2+, Mn 2+) either in tunnel orframework sites when exchange was done before hydrothermal treatment. The presence ofCu2+ ions decreases the Mn2+/Mn4+ ratio in Cu-OMS-1 samples as suggested by anincreasing 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 theshake-up peaks associated with Cu in such valency. EPR spectra of Cu-containing OMS-1materials show the six peaks of Mn2+ with a hyperfine splitting constant of 97 G, indicatingan octahedral environment. The TGA profiles of [Cu]-OMS-1 show that these samples havethe same thermal stability as OMS-1 but the amount of lattice oxygen evolved decreasesslightly with the amount of Cu2+ in the tunnel. The thermal stability of the Cu- OMS-1and Cu-OMS-2 structures decrease as the amount of Cu incorporated increases. The copperions that substitute for Mn2+ in the framework are considered as defects in the structure ofthe crystallites which make them less stable than OMS-1 containing Mg2+ in the frameworkwhere it exhibits a stabilizing effect. Resistivity measurements show a decrease and levelingoff as the concentration of Cu increases in the Cu-OMS-1 series, which is a trend consistentwith doping. The resistivity of the OMS-2 structure increases with the amount of copperincorporated. 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. Anincreased catalytic activity was observed for this set of materials as the Cu/Mn ratio increasedfor the oxidative dehydrogenation of ethylbenzene to styrene at 300 C at 1 atm.
C for 48 h.The Cu-OMS-1 series of materials (tunnel substituted) were prepared by incorporating Cu2+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 Cu2+ ions into the tunnels ofOMS-1 under similar conditions. Cu-OMS-2 materials characterized by 2 × 2 tunnels wereprepared 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. TheCu(II) ions easily substitute for the divalent cations (Mg2+, Mn 2+) either in tunnel orframework sites when exchange was done before hydrothermal treatment. The presence ofCu2+ ions decreases the Mn2+/Mn4+ ratio in Cu-OMS-1 samples as suggested by anincreasing 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 theshake-up peaks associated with Cu in such valency. EPR spectra of Cu-containing OMS-1materials show the six peaks of Mn2+ with a hyperfine splitting constant of 97 G, indicatingan octahedral environment. The TGA profiles of [Cu]-OMS-1 show that these samples havethe same thermal stability as OMS-1 but the amount of lattice oxygen evolved decreasesslightly with the amount of Cu2+ in the tunnel. The thermal stability of the Cu- OMS-1and Cu-OMS-2 structures decrease as the amount of Cu incorporated increases. The copperions that substitute for Mn2+ in the framework are considered as defects in the structure ofthe crystallites which make them less stable than OMS-1 containing Mg2+ in the frameworkwhere it exhibits a stabilizing effect. Resistivity measurements show a decrease and levelingoff as the concentration of Cu increases in the Cu-OMS-1 series, which is a trend consistentwith doping. The resistivity of the OMS-2 structure increases with the amount of copperincorporated. 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. Anincreased catalytic activity was observed for this set of materials as the Cu/Mn ratio increasedfor the oxidative dehydrogenation of ethylbenzene to styrene at 300 C at 1 atm.