Co²⁺ Ion Siting in Pentasil-Containing Zeolites. I. Co²⁺ Ion Sites and Their Occupation in Mordenite. A Vis−NIR Diffuse Reflectance Spectroscopy Study

Abstract

VIS−NIR diffuse reflectance spectra of Co2+H−mordenites and co-cation (K, Cs, Ca, Ba)-containing Co−mordenites with Co/Al varying from 0.0015 to 0.35 were studied. Three different sites-coordinations of the Co2+ ions in dehydrated mordenite were found from interpretation of optical spectra, and the populations of these sites depending on Co-ion concentration were established. The assignment of characteristic absorption bands in the VIS spectrum to the Co2+ ions located at individual cationic sites was based on knowledge of siting of K, Cs, Ca, and Ba ions in mordenite structure. Co2+ ions are located in sites A and C of the mordenite pocket and in site E of the main channel. In site A, occupied preferentially in CoH−mordenite, the Co2+ ion is coordinated to six framework oxygens of the twisted eight-ring. The number of coordinated framework oxygens and detail geometry of the “boat-shape” site C is not known. This site is occupied only at high Co loading. Site E, consisted of four oxygens forming a rectangle, is located in the main channel. This site is occupied at high Co loadings. The occupation of individual sites A, C, and E by the Co ions is also affected by the presence of co-cations, the way of the Co incorporation, and conditions of the dehydration procedure.