Formation of active sites in copper-exchanged zeolites for the direct methane-to-methanol conversion.

Cu-exchanged zeolites are extensively studied for their promising properties in the direct and selective conversion of CH4 to CH3OH at low temperatures. Their performance has been attributed to the presence of multiple oxygenated Cu(II) active sites, which can co-exist in a given zeolite framework. Most emphasis has been placed on identifying these Cu-oxo centers, understanding their structural development during reaction with CH4, and establishing a correlation between the Cu speciation and the zeolite's Cu loading, Si/Al ratio, and topology. On the contrary, the processes underlying the oxidative formation of Cu(II) active sites have received comparatively little attention. This critical review presents an overview of the current understanding of the generation of Cu(II) active centers and highlights the impact of the oxidant type and the reaction conditions on the nature of the formed Cu(II) species. Key knowledge gaps are identified by addressing prevailing misconceptions and conflicting reports in the literature. To remedy these issues, strategies are proposed to resolve discrepancies and to gain new insight into the processes preceding the CH4-to-CH3OH conversion in Cu-exchanged zeolites, aiming to better understand and control the generation of these Cu(II) active sites in zeolites.