Behavior of transition metals ferricyanides as microporous materials

The properties of divalent transition metal ferricyanides as microporous materials remain poorly documented. This family of compounds crystallizes with a cubic cell (Mn, Co, Ni, Cu, Zn, Cd) and also in a rhombohedral (R-3c) modification (Zn). The porous nature of the cubic complexes is related to the occurrence of systematic vacancies of the octahedral building unit [Fe(CN)(6)]. The system of vacancies forms a porous framework with polar surface. The rhombohedral modification for the zinc complex salt also has a porous structure but free of vacancy and with a practically non-polar surface. Such structural information was complemented with adsorption isotherms of H2O, N-2 and CO2, which were evaluated according to Dubinin model for the volume pore filling. For CO2 the maximum adsorbed amount was calculated using the Langmuir-Freundlich isotherm. From these adsorption data, a characterization of the studied ferricyanides according to pore volume, pore accessibility and guest-host interactions is given. Information on the guest-host interaction was also derived from Mossbauer spectra recorded on samples with N2, CO2, ethanol, methanol and water as adsorbed species. The quadrupole splitting (Delta) of the Mossbauer spectrum appears as an excellent sensor for the interaction between the adsorbed species and the metal at the pore surface. The Mossbauer spectra were also used as sensor for the pore accessibility to light hydrocarbons. The surface properties of the studied ferricyanides result modulated by the metal used to form the 3D structure from the octahedral block. In this sense, the studied materials behave as tunable zeolites. The materials behavior during the dehydration process and their thermal stability were evaluated from X-ray powder patterns, thermogravimetric and spectroscopic (IR and Mbssbauer) data. For all the studied compositions the crystal structure was refined from the corresponding X-ray powder patterns using the Rietveld method. (c) 2006 Elsevier Inc. All rights reserved.