Cu-Exchanged Ferrierite Zeolite for the Direct CH4 to CH3OH Conversion: Insights on Cu Speciation from X-Ray Absorption Spectroscopy

Abstract

The direct stepwise transformation of CH4 to CH3OH over Cu-exchanged zeolites has been an intensively researched reaction as it can provide a solution for the utilization of this abundant feedstock. Up to date a commercial process is far from realization, which is why an understanding of the Cu speciation in zeolites as a function of reaction conditions as well as the development of a mechanistic view of the reaction are necessary to further advance the field. Herein we study Cu-exchanged ferrierite zeolite for the direct CH4 to CH3OH conversion by utilizing X-ray absorption spectroscopy (XAS), in order to assess the local structure and electronic properties of Cu through the reaction. A Cu-FER sample with a Cu/Al = 0.20 and Si/Al = 11 was subjected to three reaction cycles yielding ultimately 96 µmol $$_{{{\text{C}}{{\text{H}}_3}{\text{OH}}}}/{{\text{g}}_{{\text{zeolite}}}}$$ . Normalized to the Cu loading, this accounts for 0.33 mol $$_{{{\text{C}}{{\text{H}}_3}{\text{OH}}}}$$ /molCu, making the sample comparable to very active Cu-MOR materials reported in the literature. During O2 activation, a transient self-reduction regime of CuII to CuI ions was identified; eventually leading to mostly framework interacting CuII species. CH4 loading leads to a reduction of these CuII containing species; which are finally partially reoxidized during H2O-assisted CH3OH extraction. The speciation after CH4 activation as well as H2O-assisted CH3OH extraction was assessed via linear combination fitting analysis of the XAS data.