Effect of Metal Coordination on the Charge Distribution over the Cation Binding Sites of Zeolites. A Combined Experimental and Theoretical Study

Simplified cluster models representing cation binding sites in a zeolite of ferrierite structure have been optimized with a series of divalent (MgII, MnII, CoII, NiII) and monovalent (NaI, CuI, AgI) metal cations. It has been found that the coordination of metal cations significantly perturbs the charge distribution over the cation binding sites. The perturbation is of electronic origin and arises from the strong electric field concentrated on the metal cation and from framework to metal charge-transfer effects. Its extent is specific to the type of cation and can be expressed with an empirical formula, which contains the formal charge of the cation and bond distance parameters. A sound correlation has been found between the extent of perturbation and the measured infrared vibrational wavenumbers of the shifted T−O−T antisymmetric skeletal vibrations, appearing in the 1020−780 cm-1 region of the FTIR spectra of ferrierite exchanged with metal cations.

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