Mixed-Metal Cu-BTC Metal-Organic Frameworks as a Strong Adsorbent for Molecular Hydrogen at Low Temperatures

The advancement of hydrogen and fuel cell technologies hinges on the development of hydrogen storage methods. Metal-organic frameworks (MOFs) are one of the most favorable materials for hydrogen storage. In this study, we synthesized a series of isostructural mixed-metal metal-organic frameworks (MM-MOFs) of 1,3,5-benzenetricarboxylate (BTC), M-Cu-BTC, where M = Zn2+, Ni2+, Co2+, and Fe2+ using the post-synthetic exchange (PSE) method with metal ions. The powder X-ray diffraction patterns of MM-MOFs were similar with those of single-metal Cu-BTC. Scanning electron microscopy indicates the absence of amorphous phases. Inductively coupled plasma mass spectroscopy of the MM-MOFs shows successful metal exchanges using the PSE method. The N2 adsorption measurements confirmed the successful synthesis of porous MM-MOFs. The metal exchanged materials Ni-Cu-BTC, Zn-Cu-BTC, Fe-Cu-BTC, and Co-Cu-BTC were studied for hydrogen storage and showed a gravimetric uptake of 1.6, 1.63, 1.63, and 1.12 wt %; respectively. The increase in hydrogen adsorption capacity for the three metal exchanged materials is about 60% relative to that of the parent MOF (Cu-BTC). The improvement of gravimetric uptake in M-Cu-BTC (where M = Ni2+, Zn2+, and Fe2+) is probably due to the increase in binding enthalpy of H2 with the unsaturated metal sites after the partial exchange from Cu2+ to other metal ions. The higher charge density of metal ions strongly polarizes hydrogen and provides the primary binding sites inside the pores of Cu-BTC and subsequently enhances the gravimetric uptake of hydrogen.