Methane Oxidation by H2O2 over Different Cu-Species of Cu-ZSM-5 Catalysts

Abstract

Cu(Fe)-containing zeolites are promising catalysts for biomimetic functionalization of light hydrocarbons. Catalysts with various copper states have been elaborated and studied in the peroxide oxidation of methane to methanol and formic acid. ICP-AES, UV–vis DR, EPR, NH3-TPD, H2-TPR, and N2 adsorption techniques were used for the characterization of the Cu(II) states, the textural, acidic and redox properties of the catalysts. The prepared zeolites contained the following Cu species: (1) isolated Cu2+ ions, (2) square planar binuclear oxo/hydroxo complexes ([Cu2(OH)2]2+) in the zeolite channels, (3) CuO- and/or Cu(OH)2-like clusters and nanoparticles on the zeolite external surface. UV–vis DR spectroscopy also was used for studying the ability of different Cu-ZSM-5 to form Cu peroxocomplexes with H2O2. Three types of peroxocomplexes were detected. These were terminal Cu–OOH originated from the isolated Cu2+ ions, binuclear Cu(II) with a terminal OOH group and the binuclear Cu(II) with a bridging OO-group formed from the Cu-species with extra lattice oxygen. 1H and 13C NMR analysis revealed the formation of methylhydroperoxide, formaldehyde, and CO2 as side products during peroxide oxidation of methane. Two pathways of methane oxidation with hydrogen peroxide involving both binuclear Cu-species with extra-lattice oxygen and isolated Fe-sites and/or oligomer oxo-clusters of Fe(III) ions were supposed.