Interaction of divalent metal cations with ferrierite: Insights from density functional theory

The extraframework locations of many metal cations in pentasil zeolites, such as ferrierite, are not known, but new infrared bands in the 800-1000 cm^-1 region are experimentally observed upon exchange. To investigate the origin of these bands and to better understand cation siting, density functional theory calculations were performed on ferrierite cluster models for seven divalent metals: magnesium, manganese, iron, cobalt, nickel, copper, and zinc. The calculated properties, such as the metal-zeolite binding energy, follow a U-shaped trend with respect to the number of metal d electrons. The calculated infrared bands are consistent with the occupation of the B and G ferrierite sites by the metal cations. However, a definitive assignment of the published experimental infrared bands to particular extraframework sites was not possible. Despite this limitation, all seven metals were shown to perturb the ferrierite framework similarly. All of the cations coordinate to four framework oxygen atoms and the framework T-O bonds associated with these oxygen atoms are elongated by the metal. This elongation is shown to be proportional to the wavenumber of the infrared bands.