Co2+ ion siting in pentasil-containing zeolites, part 3.: Co2+ ion sites and their occupation in ZSM-5:: a VIS diffuse reflectance spectroscopy study

Siting of the Co2+ ions in dehydrated Co-ZSM-5 is suggested based on a VIS optical study, the MFI topology and parallelism between the spectra of the Co2+ ions and local framework structures in ZSM-5, mordenite and ferrierite. The proposed Co2+ 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+, Ki, Ca2+, Ba2+) with compositions of Co2+/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, alpha, beta and gamma, i.e. a single band at 15100 cm(-1) (alpha), four bands at 16 000, 17 150, 18 600 and 21 200 cm(-1) (beta), and a doubler at 28 100 and 22 000 cm(-1) (gamma), have been ascribed to three Co ion types located at different cationic sites. The alpha-type Co ions are suggested to be coordinated to four oxygen atoms of the straight channel wall. The beta-site is located at the deformed six-member ring at the intersection of straight and sinusoidal channels. The beta-type Co ions predominate at all the Co loadings in Co-ZSM-5. The Co ions at 'boat-shaped' gamma-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. (C) 2000 Elsevier Science B.V. All rights reserved.