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 α-, β-, and γ-type Co ions in ferrierite are described. The α-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 β-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 Å. They exhibit medium-strength bonding to framework oxygens and, compared to the α-type Co ions, a substantially suppressed ability to bind dinitrosyls. The γ-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 ferrierite. The tendency of the Co ions to relocalize under severe thermal/hydrothermal calcination of Co–ferrierite increased in the sequence γ<β⪡α, in agreement with the strength of bonding of the Co ions to framework oxygens at cationic sites.