Tracking Active Site Formation during Oxidative Activation of Copper-Exchanged Zeolites for Methane-to-Methanol Conversion.

The evolution of active sites in Cu-zeolites for the CH4-to-CH3OH conversion has been investigated during oxidative treatment in O2. Three samples with different frameworks but comparable Cu loadings and Si/Al ratios have been prepared to assess the influence of topology on material oxidizability and the nature of the generated Cu(II) species. Complementary spectroscopic studies highlight that isomeric Cu(II) centers hosted within different topologies are characterized by distinct formation rates. In turn, the framework-specific kinetics of Cu(II) site generation regulate the overall oxidation potential of the individual zeolites. Apart from the topology, the formation rate of different Cu(II) species is governed by their specific structure, with dimeric Cu(II) centers ([Cu2(µ-O)]2+) being generated faster than monomeric ([CuOH]+, Cu2+) ones. Elevated temperatures accelerate the evolution of Cu(II) monomers but cause [Cu2(µ-O)]2+ to undergo autoreduction. The reversibility of this process is framework-dependent. Consequently, even though two types of [Cu2(µ-O)]2+ form at low temperatures in each material, only specific ones remain after high-temperature treatment. The autoreduction of [Cu2(µ-O)]2+ is accompanied by its transient reduction by hydrocarbon residues, originating from the preceding treatment in CH4. The oxidative decomposition of these impurities yields H2O, which adsorbs on [Cu2(µ-O)]2+ masks their spectroscopic fingerprints, and renders them inactive.