Nature and Location of Copper Nanospecies in Mesoporous Molecular Sieves

Copper-mesoporous molecular sieves type MCM-41 have been successfully prepared by different direct incorporation methods of the metal in the initial synthesis gel. Various techniques including XRD, AAS, adsorption/desorption of N-2, UV-vis-DR, TPR, and XPS were employed for the materials characterization. All of the materials exhibited a good structural regularity, even for loadings of copper of similar to 10 wt %, besides high specific surface areas and pore volumes. An extended and detailed study about the nature of copper oxidation state as well as of the copper species distribution on the siliceous nanostructure has been made. It was found that copper is present on the mesoporous silica in the form of various species: isolated mononuclear Cu delta+ cations possibly in coordination with the lattice oxygen; linear oligonuclear [Cu delta+center dot center dot center dot O delta-center dot center dot center dot Cu delta+](n) clusters probably inserted into mesoporous channels; and bulky CuO oxide segregated of the structure. The distribution of copper species in the final solid depends on the preparation method used as well as of the copper content in the initial gel. Moreover, we infer the coexistence of the both copper oxidation states +1 and +2 (+delta) in the major species of these materials: isolated and oligonuclear copper species.