Effects of exposed metal cations on high pressure hydrogen storage in metal-organic frameworks

Hydrogen is often regarded as a viable clean burning alternative to our current petroleum based fuel sources as a result of its high gravimetric energy d. and renewable nature. Furthermore, H2 has received significant attention as a transportation fuel. A severe hurdle in the implantation of H2 as a fuel resides in the difficulty of storing it in sufficient quantities for vehicles to span the same range as those currently powered by gasoline. Porous materials can be used to increase the volumetric d. of H-2 inside fuel tanks at a range of pressures and temps. For instance, microporous metal-org. frameworks have drawn much interest for room temp. H2 storage due to their high surface areas and tunable pore chem. In particular, the presence of coordinatively unsatd. metal centers can increase H2 binding enthalpy. In an effort to systematically study H2 storage in an isostructural series of metal-org. frameworks contg. exposed metal cations, we synthesized four new analogs of M2(dobpdc) (dobpdc4- = 4, 4'-dioxidobiphenyl-3,3'-dicarboxylate ; M = Mn, Fe, Co, Ni). These materials feature expanded pores as compared to the widely studied M2(dobdc) (dobdc4- = 2, 5 - dioxido-1,4-benzenedicarboxylate). These new materials were studied by measuring low-pressure H-2 adsorption isotherms at 77K and high-pressure isotherms up to 300 bar at 25, 0, and -25 °C. Isosteric heats of adsorption and volumetric capacities were calcd. to study how the interplay of open metal sites and pore size effects H2 capacity and binding enthalpy. In addn., powder neutron diffraction expts. were employed to identify H2 binding sites in these expanded materials.