Copper Sites in Cu-ZSM-5 Zeolites. Part II. An Identification of Defective AlOCu+ Sites by FTIR

Abstract

By in-situ FTIR study using CO as a probe molecule, a new type of Cu+ site, identified as defective (AlOδ-)Cu+, was found to exist mainly in activated solution-exchanged Cu-ZSM-5, and was thought to derive from mixed oxo-bridged Cu+ ions, (SiAlO)Cu+, due to high-temperature (400 °C) calcination. At room temperature the CO adsorption on this sample produced three infrared carbonyl bands at 2176, 2156, and 2138 cm-1. The first two bands, previously ascribed to the CO adsorbed at Cu2+ sites and mixed oxo-bridged (SiAlO)Cu+ ions, respectively, vanished upon Ar purge at room temperature whereas the last band remained unaffected. The 2138 cm-1 band was assigned to the CO adsorbed at defective (AlOδ-)Cu+ site, and its desorption energy was higher than those at oxo-bridged Cu+ and Cu2+ sites. The stronger Cu+-CO bonding and weaker C-O stretching are explained by a stronger π back-donation due to the defective structure. The 2138 cm-1 band was found to convert to the 2156 cm-1 band during heating between 100 and 280 °C, with an estimated activation energy of the transition approximately equal to 4.8 kcal/mol. A reaction scheme is proposed to depict the mechanism of this transition.