Methane to Methanol Conversion in Small Pore Zeolites

One of the major challenges in catalysis today is the direct conversion of methane to methanol. Due to the similarities to enzymatic catalysis and the low activation barriers, most recently transition metal exchanged zeolites have drawn significant amounts of attention. However, a thorough understanding of the nature of the active sites in many of them and the impact of the confined environment is still missing. In this contribution we address two major points in this context, i.e. the nature of the active site for methane-to-methanol conversion in Cu exchanged SSZ-13 and the impact of the confining environment on enthalpy barriers in the conversion of methane to methanol over Fe-oxo centers in zeolites.

Cu_xO_y clusters embedded in different zeolite frameworks have shown significant activity in converting methane to methanol. However, depending on the zeolite structure and the exact synthesis and preparation protocols, different sites have been claimed in the literature to be most favorable. Here we develop a thermodynamic model to identify the most stable Cu_xO_y under various conditions in the zeolite SSZ-13. We find Cu₂O₂ clusters anchored at defect sites in zeolites to be most stable. We discuss the impact on several parameters on their formation and give guidelines what parameters might influence the Cu_xO_y distribution in the material. Furthermore we compare modeled UV-vis and Raman spectra to experiment and find excellent agreement for the suggested site with experimental measurements.

Additionally several studies indicate that confinement influences the activation enthalpies for this reaction in zeolites. To simplify the problem we focus on a more simple and better-understood Fe-oxo center, that also shows activity for the given reaction. We find that confinement stabilizes a methyl radical significantly more than the initially adsorbed methane molecule, which significantly lowers activation energies. We furthermore generalize these results to arrive at statements about the impact of confinement methane to methanol conversion and also other reactions.