Siting of Co2+ ions in cobalt-modified high-silica zeolites probed by low-temperature molecular hydrogen adsorption

Diffuse Reflectance Infrared Fourier Transform (DRIFT) spectroscopy was used to characterize the effects of introducing cobalt into the zeolites ZSM-5 and mordenite. Aqueous impregnation of the hydrogen form of the zeolite and subsequent in vacuo treatment at temperatures up to 920 K results in partial exchange of protons in bridging hydroxyl groups by Co2+ cations. These changes are evidenced by a decrease in intensity of the bands at 3606-3609 cm(-1) characteristic of Bronsted acid sites. DRIFT spectra of hydrogen adsorbed at 77 K also confirm the exchange of protons for Co2+ cations, as evidenced by a decrease in the intensity of the band at 4106 cm(-1) for H-2 adsorbed on protons. By contrast, the bands at similar to3905, similar to3965, and similar to4010 cm(-1) for H-2 adsorbed on the Co2+ cations increase in intensity. With increasing Si/Al ratio at a constant Co loading of 1 wt% the intensity of the band at 3905 cm(-1) for H-ZSM-5 strongly increases intensity relative to the other bands. This feature is attributed to Co2+ cations interacting with two adjacent cation-exchange sites located in a 10-membered ring. It is hypothesized that the Lewis acidity of Co2+ cations in such environments is higher than that of Co2+ cations associated with oxygen atom sin individual five- or six-membered rings containing tow Al atoms, because the Co2+ cation can interact with only two of the four basic oxygen anions located in the ring. It is proposed that Co2+ cations in the latter type of sites are identified by the bands at 3965 cm(-1) and 4010 cm(-1) for adsorbed H-2.