EXAFS wavelet transform analysis of Cu-MOR zeolites for the direct methane to methanol conversion

Cu-exchanged zeolites have been shown to possess Cu-oxo species active towards the direct methane to methanol (DMTM) conversion, carried out through a chemical-looping approach. Different Cu-zeolites have been investigated for the DMTM process, with Cu-mordenite (Cu-MOR) being among the most active. In this context, an accurate determination of the local structure and nuclearity of selective Cu-oxo species responsible for an efficient DMTM conversion still represents an ongoing challenge for characterization methods, including synchrotron-based X-ray absorption spectroscopy (XAS). Herein, we explore the potential of an alternative analysis of Extended X-ray Absorption Fine Structure (EXAFS) data using wavelet transform (WT) to enhance the technique sensitivity to multimeric Cu species hosted in the MOR framework. Combiningex situXAS measurements under model red-ox conditions within situdata collected after the key steps of the DMTM process, we demonstrate how EXAFS-WT enables unambiguous detection of Cu-Cu scattering contributions from multimeric Cu-species. As also confirmed by complementaryin situIR spectroscopy results, these are observed to dynamically respond to the chemical environment over the different conditions probed. We finally report a proof-of-concept EXAFS fit using the WT representation, applied to the structural refinement of O-2-activated Cu-MOR. The fitting results reveal a Cu local coordination environment consistent with mono-(mu-oxo) di-copper cores, with Cu-Cu separation of similar to 3.1 angstrom, paving the way to future applications and developments of the method in the field of Cu-zeolite research and beyond.