Cluster model studies of oxygen-bridged Cu pairs in Cu-ZSM-5 catalysts

Effects of the support environment on the existence of Cu ion pairs in Cu-exchanged ZSM-5 catalysts are examined using density functional theory. Results for the molecular and electronic structures of O- and O₂-bridged Cu oxocations ([CuOCu]²⁺ and [CuO₂Cu]²⁺) are presented, including two distinct isomers for the latter. Both types of oxocations are predicted to be strongly bound for conditions likely to occur in the zeolite. The zeolite framework is represented by a variety of cluster models, including a previously established single-T-site model and larger multi-T-site models specific to particular binding sites in ZSM-5. With the largest models, bent Cu-O_x-Cu structures are found with Cu-Cu distances consistent with X-ray absorption data for Cu-ZSM-5. Implications for catalytic chemistry, including a proposed pathway for O₂ formation and subsequent desorption from oxidized Cu sites in the zeolite, are discussed.