Co 2+ ion siting in pentasil-containing zeolites, part 3. : Co 2+ ion sites and their occupation in ZSM-5: a VIS diffuse reflectance spectroscopy study

Abstract

Siting of the Co 2+ ions in dehydrated Co–ZSM-5 is suggested based on a VIS optical study, the MFI topology and parallelism between the spectra of the Co 2+ ions and local framework structures in ZSM-5, mordenite and ferrierite. The proposed Co 2+ ion sites represent a first attempt to describe the cationic sites for transition metal ions in the MFI structure. Co ions exchanged in CoMeH–ZSM-5 zeolites (Me=Na +, K +, Ca 2+, Ba 2+) with compositions of Co 2+/Al molar ratios up to 0.5 and Si/Al molar ratios between 12 and 22 have been studied by diffuse reflectance VIS spectroscopy. Three characteristic spectral components, α, β and γ, i.e. a single band at 15 100 cm −1 (α), four bands at 16 000, 17 150, 18 600 and 21 200 cm −1 (β), and a doublet at 20 100 and 22 000 cm −1 (γ), have been ascribed to three Co ion types located at different cationic sites. The α-type Co ions are suggested to be coordinated to four oxygen atoms of the straight channel wall. The β-site is located at the deformed six-member ring at the intersection of straight and sinusoidal channels. The β-type Co ions predominate at all the Co loadings in Co–ZSM-5. The Co ions at ‘boat-shaped’ γ-site are located in the sinusoidal channel, and they represent a minor concentration of the Co ions in ZSM-5. A distribution of Co ions among those cationic sites was established.