Localization and Coordination of Mg²⁺Cations in Ferrierite: Combined FTIR Spectroscopic and Computation Investigation of CO Adsorption Complexes

Localization and coordination of Mg 2+ cations in Mg-FER were investigated by a combination of IR spectroscopy of CO probe molecule and periodic DFT calculations employing the ω/ r correlation for calculations of CO stretching frequencies. Experimental data were interpreted on the basis of a very good agreement between theoretical and experimental results. The most intense band in the spectra centered at 2205 cm −1 was assigned to monocarbonyl complexes formed on Mg 2+ cations located in the six-member ring separating two adjacent FER cages (P6 site). Dicarbonyl complexes can be formed on these Mg 2+ sites (characterized by a band at 2194 cm −1 ) where each of the CO molecules approaches Mg 2+ from a different FER cage. The band at 2211 cm −1 observed for the intermediate CO coverage was assigned also to the monocarbonyl complexes on Mg 2+ sites inside the FER cage (P6 and P8 sites); the difference between 2205 and 2211 cm −1 bands is due to the different geometry of Al pair within the ring. At very low CO coverages, IR spectra show a band at 2222 cm −1 assigned to carbonyl complexes on Mg 2+ cations at the channel intersection and in the main channel. Because of a significant difference in the intensity of the bands at 2222 and 2211 as compared to that of the band at 2205 cm −1, it is concluded that Mg 2+ cations exchange preferably into the six-member ring between two FER cages. An unusually high site-specificity of ν CO observed for CO complexes formed in Mg-FER can be exploited in the investigation of preferable exchange sites of Mg 2+ in zeolites and in the investigation of the localization of Al pairs in zeolites.