Bonding of Co ions in ZSM-5, ferrierite, and mordenite:: An X-ray absorption, UV-Vis, and IR study

Coordination and bonding of Co ions at cationic sites in zeolites of ZSM-5, FER, and MOR structures were investigated by X-ray absorption and by UV-Vis and FTIR spectroscopy. Co(II) ions, exchanged under conditions in which no hydrolysis took place, were coordinated exclusively to framework oxygen atoms; no Co-O-Co bridging structures were found. Three types of Co ion coordination and three types of perturbation of the framework T-O bonds at the corresponding cationic sites were distinguished by Vis spectra of the Co(II) ions and IR spectra of skeletal vibrations. EXAFS and FTIR showed that Co ion bonding at exchangeable sites of pentasil ring zeolites requires substantial deformation of the adjacent framework T-O bonds to attain coordination to four or three framework oxygens with Co-O bond distances of 1.99 to 2.06 Angstrom. Such coordination is possible only with certain cationic site structures alpha,beta, and gamma, which are very similar for ZSM-5, ferrierite, and mordenite. Slight differences in these local framework structures lead to different perturbation of the framework bonds at the individual cationic sites. To satisfy the bonding requirements of the Co ions, the sequence of the extent of perturbation of framework bonds is MOR > ZSM-5 > FER.