Investigation of IR vibrational band of C–O bond of carbonyl species in Cu+-MFI zeolites

Abstract

Interaction of CO molecules with Cu-MFI zeolites reduced in vacuum or CO atmosphere was investigated. The character of the absorption bands ascribed to intrazeolitic carbonyl species was explored in the dependence on the Cu loading, Si/Al ratio, degree of reduction and on the coverage of the Cu⁺ ions by CO molecules. The different character of individual Cu sites was revealed in the IR spectra of carbonyl species upon CO pressure. Some Cu ions are not able to form dicarbonyl species even at 7.5 kPa of CO. The band of residual monocarbonyl species strongly overlaps with the band of asymmetric vibration of dicarbonyl species, making the quantitative analysis of this band rather difficult. It was found that the position and the half-width of the monocarbonyl band, as well as the third and fourth statistical moment of the band are independent of the variable factors mentioned above. The main vibrational band of monocarbonyl species cannot be deconvoluted to individual bands corresponding to different Cu exchangeable sites. It means that IR spectroscopy of carbonyl species in the range of C–O stretching frequencies is not a suitable method for monitoring of the individual Cu⁺ sites in MFI zeolite framework. In addition, no correlation of –ΔH_ads and wavenumber of the fundamental vibration of C–O adsorbed on Cu⁺ ions in MFI zeolite matrix was found.