Hydrogen storage in zeolite-like hexacyanometallates: Role of the building block

Hydrogen storage in zeolite-like hexacyanometallates, Zn₃A 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 H₂ 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 × 9 × 8 Å, which remain communicated by elliptical windows. The H₂ 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 derealization 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.