State and coordination of metal ions in high silica zeolites Incorporation, development and rearrangement during preparation and catalysis

Cu and Co ions in Na- and H-forms of ZSM-5 and ferrierite were used to elucidate siting-coordination-bonding of ban divalent cations and cation-extra-framework ligand complexes (with NO, CO, NO2, H2O and NH3) in high silica zeolite matrices. By using a multi-spectroscopic approach involving VIS-NIR diffuse reflectance spectra of the Cu2+ and Co2+ ions, Cu+ luminescence, FTIR spectra of skeletal T-O-T vibrations and vibrations of adsorbed molecules (ligands), information on the bonding of the bare cations and cation-guest-ligand complexes to the framework oxygen ligands was obtained. Based on our previous results on characteristic spectral features of the Cu siting in dehydrated ZSM-5, a correlation between coordination of the Cu ions in hydrated and dehydrated ZSM-5 zeolites at various Cu/Al/Si ratios was established, and the importance of Al distribution in the framework (so called 'Al pairs' and single Al atoms) for the Cu ion exchange and siting in zeolites was evidenced. Owing to the stable divalency of Co2+ in zeolites, this cation was used to monitor coordination-bonding of bare divalent cations and cation-ligand complexes in ferrierite. It has been found that the Co2+ ions induce local perturbations of the T-O-T bonds adjacent to the cation reflected in three characteristic ('deformation') shifts of the T-O-T framework vibrations. Simultaneously measured d-d transition spectra of the Co2+ ions, providing information on the symmetry of the framework oxygen atoms bonding a bare cation, indicated that the changes in T-O-T vibrations are accompanied by changes of VIS spectra. Depending on the strength and number of the extra-framework ligands, this local framework deformation induced by the Co2+ ion is partly, or eventually completely, removed (T-O-T vibration 'relaxation' shift) upon adsorption of molecules with formation of cation-guest-ligand complexes (also detected via ligand vibration itself). (C) 1998 Elsevier Science B.V. All rights reserved.