Structure, distribution, and properties of Co ions in ferrierite revealed by FTIR, UV-Vis, and EXAFS

Ion-exchanged Co ions and nitrosyl complexes in CoH- and CoNaK-ferrierites with a Co loading up to Co/Al = 0.4 were investigated by diffuse reflectance UV-Vis-NIR, FTIR in the region of the skeletal and NO vibrations, and EXAFS measurements. Co(II) ion coordination, cation-induced perturbations of the framework T-O bonds of the hosted cationic sites, and Co-O distances of three typical ion-exchanged alpha-, beta-, and gamma-type Co ions in ferrierite are described. The alpha-type Co ions, coordinated to the rectangle of framework oxygens in the wall of the main ten-member ring channel, exhibit an open coordination sphere, weak bonding to the framework oxygens, and a high tendency to form dinitrosyl complexes upon NO adsorption. The B-type Co ions, the most populated ones in the whole concentration range, are coordinated to four framework oxygens of the deformed six-member ring of the ferrierite cavity at a distance of 1.99 Angstrom. They exhibit medium-strength bonding to framework oxygens and, compared to the alpha-type Co ions, a substantially suppressed ability to bind dinitrosyls. The gamma-type Co ions provide the highest perturbation of the hosted framework T-O bonds and thus the strongest bonding to framework oxygens attributed to the "boat-shaped" site of fenierite. The tendency of the Co ions to relocalize under severe thermaYhydrothermal calcination of Co-ferrierite increased in the sequence gamma < beta much less than alpha, in agreement with the strength of bonding of the Co ions to framework oxygens at cationic sites. (C) 2000 Academic Press.