Structural Evolution of Copper-Oxo Sites in Zeolites upon the Reaction with Methane Investigated by Means of Cu K-edge X-ray Absorption Spectroscopy

The structure of copper sites formedunder an oxidative environmentand their evolution in the course of the reaction with methane atelevated temperature was investigated by means of Cu K-edge X-rayabsorption spectroscopy for a series of copper-containing MFI, MOR,and FAU zeolites. The pretreatment in oxygen at 723 K leads to theformation of copper-(II)-oxo sites, whose nature depends on the frameworktype. Dimeric species are formed in CuMFI material, dimeric and monomericsites coexist in CuMOR, and agglomerated copper-oxo nanoclusters arefound in large-pore copper-containing faujasite (CuFAU). For all studiedmaterials, the reaction with methane resulted in the exclusive formationof copper-(I) species; no formation of metallic copper was detectedeven at 748 K. The nature of formed copper-(I) species is governedby the structure of corresponding copper-(II) centers. In particular,monomeric and dimeric copper-(II)-oxo sites hosted in CuMOR and CuMFIare transformed into isolated copper-(I) cations coordinated to ion-exchangepositions of the zeolite. Contrarily, copper-(II)-oxo clusters presentin CuFAU undergo restructuring with only a partial loss of extra-frameworkoxygen and form aggregated species with a structure similar to thatof bulk copper-(I) oxide.