Dynamics of Copper-Containing Porous Organic Framework Catalysts Reveal Catalytic Behavior Controlled by the Polymer Structure

The structure of enzyme catalytic centers often guides the synthesis of heterogeneous catalysts. These structures are dynamic and provide changes in the chemical environment and redox state of transition metal centers that result in drastic changes in the catalytic behavior. Inspired by these catalytic traits, porous polymers represent a versatile platform for preparing catalysts by anchoring metals to organic functional groups. Following this inspiration, we studied the structural evolution of a series of catalysts containing atomically dispersed Cu ions grafted onto imine-linked porous organic frameworks (IPOFs) during synthesis and reaction. Cu species were found to be bound to the imine or diiminopyridine units of IPOFs and to be active for oxidation of carbon monoxide. Combining X-ray photoelectron spectroscopy (XPS) and in situ X-ray absorption spectroscopy (S) showed that the metal centers could gradually activate oxygen and change their oxidation state. Interestingly, depending on the type of functional group responsible for binding Cu, the active sites either reach a balanced redox turnover between Cu(I) and Cu(II) states or get poisoned by CO and locked at a Cu(I) state during reaction. Furthermore, samples with agglomerated Cu species were found to be redispersed upon contact with the reaction mixture, which promoted a high initial activity not seen in the original samples. Overall, our results show that catalytic sites in porous polymers can have very dynamic structures that are highly dependent on the polymer functional groups as well as reaction conditions and show how this strategy can be used to produce active catalysts with a wide range of possible compositions and chemistries.