Theoretical assessment of the elastic constants and hydrogen storage capacity of some metal-organic framework materials

Metal-organic frameworks (MOFs) are promising materials for applications such as separation, catalysis, and gas storage. A key indicator of their structural stability is the shear modulus. By density functional theory calculations in a 106-atom supercell, under the local density approximation, we find c(11)=29.2 GPa and c(12)=13.1 GPa for Zn-based MOF 5. However, we find c(44) of MOF-5 to be exceedingly small, only 1.4 GPa at T=0 K. The binding energy E-ads of a single hydrogen molecule in MOF-5 is evaluated using the same setup. We find it to be -0.069 to -0.086 eV/H-2 near the benzene linker and -0.106 to -0.160 eV/H-2 near the Zn4O tetrahedra. Substitutions of chlorine and hydroxyl in the benzene linker have negligible effect on the physisorption energies. Pentacoordinated copper (and aluminum) in a framework structure similar to MOF-2 gives E-ads approximate to-0.291 eV/H-2 (and -0.230 eV/H-2), and substitution of nitrogen in benzene (pyrazine) further enhances E-ads near the organic linker to -0.16 eV/H-2, according to density functional theory with local density approximation. (c) 2006 American Institute of Physics.