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-e xchanged ZSM-5 catalysts are examined using density functional theory. Resu lts for the molecular and electronic structures of O- and O-2-bridged Cu ox ocations ([CuOCu](2+) and [CuO2Cu](2+)) are presented, including two distin ct isomers for the latter. Both types of oxocations are predicted to be str ongly bound for conditions likely to occur in the zeolite. The zeolite fram ework 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 propo sed pathway for O-2 formation and subsequent desorption from oxidized Cu si tes in the zeolite, are discussed.