Hydrogen Storage in Zeolite-Like Hexacyanometallates: Role of the Building Block

Hydrogen storage in zeolite-like hexacyanometallates, Zn(3)A(2)[M(CN)(6)](2) with A = K, Rb, Cs, and M = Fe, Ru, Os, was studied. In a previous article, we have reported the role of the exchangeable metal (A) for M = Fe on the H-2 adsorption. This contribution concerns to the effect of the molecular block, [M(CN)(6)], on the recorded H2 adsorption isotherms and the corresponding adsorption heats. This family of porous materials can be considered as octahedral anionic blocks, [M(CN)6](4-), assembled by zinc (2+) atoms linked at their nitrogen ends. The porous framework topology was described from the refined crystal structures. In the resulting 3D network the zinc atom is found with a tetrahedral coordination. This leads to formation of ellipsoidal cavities, of about 12.5 x 9 x 8 angstrom, which remain communicated by elliptical windows. The H-2 adsorption heat was estimated using the isosteric method from isotherms recorded at 75 and 85 K. The estimated values for the adsorption heats follow the order: Os > Ru > Fe. The building block contribution to the H2 adsorption potential takes place through the charge delocalization from the inner metal (M) to increase the electric field gradient at the cavity surface. The CO2 adsorption isotherms are also conclusive on the contribution of the building block to the cavity adsorption potential. All of the studied samples were characterized from X-ray diffraction, infrared, and thermogravimetric data.