Magnetic Exchange Coupling in Zeolite Copper Dimers and Its Contribution to Methane Activation

Abstract

The highly reactive binuclear [Cu₂O]²⁺ active site in copper zeolites activates the inert C-H bond of methane at low temperatures, offering a potential solution to reduce methane flaring and mitigate atmospheric methane levels. While substantial progress has been made in understanding the activation of methane by this core, one critical aspect, the active site's spin, has remained undetermined. In this study, we use variable-temperature, variable-field magnetic circular dichroism spectroscopy to define the ground state spin of the [Cu₂O]²⁺ active sites in Cu-CHA and Cu-MFI. This novel approach allows for site-selective determination of the magnetic exchange coupling between the two copper centers of specific [Cu₂O]²⁺ cores in a heterogeneous mixture, circumventing the drawbacks of bulk magnetic techniques. These experimental findings are coupled to density functional theory calculations to elucidate magnetostructural correlations in copper zeolites that are different from those of homogeneous binuclear Cu(II) complexes. The different spin states for the [Cu₂O]²⁺ cores have different reactivities governed by how methane approaches the active site. This introduces a new understanding of zeolite topological control on active site reactivity.