FTIR study of low-temperature CO adsorption on Mn-ZSM-5 and MnY zeolites. Effect of the zeolite matrix on the formation of Mn2+(CO)x geminal species

Abstract

Adsorption of CO on Mn-ZSM-5 zeolite at 85 K results in formation of physically adsorbed CO, several kinds of H-bonded CO and Mn2+ (CO)(x) geminal species (2202 cm(-1)). Decreasing the coverage during evacuation results in disappearance of the physically adsorbed CO and the H-bonded forms and in conversion of the dicarbonyls to linear Mn2+-CO Species (2214 cm(-1)). The latter are quite stable at 85 K. Coadsorption (CO)-C-12 and (CO)-C-13 reveals that the CO molecules in the geminal polycarbonyls behave as independent oscillators. In contrast, CO adsorption at 85 K on MnNaY zeolite only leads to formation of linear Mn2+-CO species (2210 cm-1) and mono- and di-carbonyls associated with residual sodium cations. The results are interpreted as evidence that site-specified geminal carbonyls are formed with cations possessing an ionic radius bigger than a critical value. This value is different for different positions in various zeolites and is bigger for cations in S-II positions in Y zeolites than is the case of cations in a ZSM-5 matrix. (C) 2002 Elsevier Science B.V. All rights reserved.