AmericanChemicalSociety.com

A range of coordination framework materials incorporating carboxylate-, pyridyl- and cyanometallate-based polytopic ligands have been synthesized and their structures and physical properties characterized. In targeting both high gravimetric and volumetric dihydrogen uptakes at non-extreme conditions our synthetic efforts have been driven by a number of considerations: 1) the achievement of high dihydrogen physisorption binding enthalpies through the generation of high energy surfaces achieved both directly through the synthetic process and through post-synthetic modification; 2) the achievement of both structural robustness and high surface areas through the use of large, rigid molecular tectons; 3) the achievement of low-density framework architectures through the use of relatively light elements.

In addition to the synthesis and characterization of new materials, extensive efforts have been devoted to the post-synthetic surface modification of existing metal-organic framework phases in an effort to increase the physisorptive dihydrogen binding enthalpy and therefore favor hydrogen storage under non-extreme conditions. Efforts here have notably seen an increase from 6 to over 8 kJ mol-1 in a well-known metal-organic framework, resulting in an increase in the dihydrogen sorption at ambient conditions despite a reduction in the surface area (as measured by standard N2 sorption methods at 77 K). More recent efforts have uncovered complete metal-ion exchange within a porous metal-organic framework, with the modified, metastable phase having substantially reduced crystallinity but retaining the high surface area of the parent phase and sorbing considerable amounts of dihydrogen.

Investigation of hydrogen storage by neutron powder diffraction (D2) and inelastic neutron scattering (H2), accompanied by DFT modeling of this process, has been applied to a number of high symmetry framework phases in collaboration with Dr Craig Brown at NIST. These studies, which have focused on a range of frameworks with different metal ions, have notably demonstrated a strong propensity for dihydrogen to sorb preferentially at bare metal sites. Measurement of the dihydrogen binding enthalpies for these systems through use of the Claussius-Clapeyron relation have yielded systematic information on H2…metal interactions. A parallel investigation of methane sorption into a number of these systems has shown a similar although less pronounced behavior, with methane…metal interaction enthalpies being comparable to interactions of methane to surfaces with high curvatures (i.e., within small pores).