Effects of Cu²⁺ Ions on the Structure and Reactivity of Todorokite- and Cryptomelane-Type Manganese Oxide Octahedral Molecular Sieves

Abstract

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.

Reaction of aqueous Cu–Citrate with MnO2 birnessite: Characterization of Mn dissolution, oxidation products and surface interactions

“…This phenomenon, also known as ''isomorphic substitution'', has been encountered by other researchers (Kanungo et al, 2004). Divalent cations, such as Ni(II) or Cu(II), can readily substitute for Mn(II) in the manganese oxide lattice to counterbalance the charge that develops during dissolution of Mn(IV) to Mn(III) and (II) (Nicolas-Tolentino et al, 1999). Although divalent heavy metals may follow this rule, the plots of Cu-Citrate complexes were non-linear.…”

Structural and chemical disorder of cryptomelane promoted by alkali doping: Influence on catalytic properties

“…For example, partial exchange of K + ions in the OMS-2 tunnels with H 3 O + ions yielded excellent acid catalysts for selective oxidation reactions [12], while doping the framework with Co(II), Zr(II), Cu(II), W, Fe(III), and Cr(III), produced cryptomelane with novel morphologies and catalytic properties [13][14][15][16][17][18]. Incorporation pathways reported include solid-state conversion from metal doped birnessite [15], aqueous a priori incorporation using redox reactions [19], and ion-exchange techniques [20]. The first two methods lead to the formation of by-products, namely hausmannite, bixbytes, pyrolusite and nsutite, while the ion-exchange technique has the disadvantage of a lower replacement degree of K + ions in the tunnel structure.…”

CuxM1–x(HCOO)2·2H2O, (M=Mn, Co, Ni, Cd): Crystal Structures and Thermal Behavior

“…The systematically lower dehydration temperatures of the Cu}M mixed formates as compared with those of the pure Mn, Co, and Ni formates from which they derive shows that incorporation of copper decreases the thermal stability of the structures: the copper ions that substitute for M> in these compounds could be considered as defects in the structure; a decrease in thermal stability has been recently observed in the manganese-containing oxides todorokite and cryptomelane, where it has been shown that the thermal stability of the crystal structures decreases as the amount of Cu that substitutes for Mn in the structure increases (10). On the other hand, the opposite tendency is observed when copper substitutes for zinc or cadmium in the structures of the pure cadmium or zinc dihydrated formates: the stability increases with the incorporation of copper in these structures, in good agreement with previous results for the Cu}Zn case (11).…”