Linear nanoscale clusters of CuO in Cu-ZSM-5 catalysts

The electron states of copper have been studied by ESR and UV-Vis as a function of the method used for preparation of Cu-ZSM-5 (ion-exchange, wet impregnation, and deposition), synthesis conditions (pH of copper solution, copper precursor, temperature, etc.) and copper loading. For calcined Cu-ZSM-5 samples treated under vacuum at 400 deg. C, axial ESR spectra of O- ion radicals (gperpendicular=2.05, gparallel=2.02), which are presumably assigned to linear -O- -Cu+ -O- -Cu+ -O-chains in zeolite channels, are observed. The same samples treated under vacuum at 150-400 deg. C, exhibit UV-Vis absorption bands at the unexpected region of 18,000-23,000 cm-1. They are assigned to the ligand-metal charge transfer bands in -O2- -Cu2+ -O2- -Cu2+ -O2-like chains in the zeolite channels. Along with these states, isolated octahedral Cu2+ ions with different extents of tetragonal distortion (absorption band (a.b.) 12,500-14,000 cm-1) and clustered copper oxide species (charge transfer band (CTB) 27,000-32,000 cm-1) are also observed