|A simplified rendering of the structure of the MOF with 6.9 wt % hydrogen uptake. Details in paper referenced at end. Click to enlarge. Source: Jeffrey Long.|
Chemical and Engineering News highlights three papers published in its sister publication, the Journal of the American Chemical Society, that report the first definitive evidence for hydrogen binding to open metal coordination sites in nanoporous metal-organic frameworks (MOFs).
Developing hydrogen storage systems of sufficient density for vehicles is proving to be a difficult problem. Direct binding, however, allows the hydrogen molecules to pack together more closely and may provide a major boost in storage capacity over simple hydrogen adsorption at nonmetal sites in previously prepared materials.
A team of researchers from University of California, Berkeley; General Motors (which funded the research); Purdue; NIST; University of Maryland; and Indiana University synthesized a manganese benzenetristetrazolate MOF with a previously unknown cubic topology. Crystals of the compound remain intact upon desolvation and show a total H2 uptake of 6.9 wt % at 77 K and 90 bar—at 60 g H2/L, this provides a storage density that is 85% that of liquid hydrogen. This capacity exceeds the Department of Energy’s 2010 targets of 6.0 wt % and 45 g/L.
The material exhibits a maximum isosteric heat of adsorption of 10.1 kJ/mol, the highest yet observed for a metal-organic framework. Neutron powder diffraction data demonstrate that this is directly related to H2 binding at coordinatively unsaturated Mn2+ centers within the framework. The data also showed H2 adsorption at several nonmetal pore sites.
A team of researchers in Australia and the US developed a copper benzenetricarboxylate MOF and used neutron powder diffraction to study D2 adsorption. The data revealed six distinct D2 adsorption sites in the framework, with sites at Cu2+ atoms occupied first, followed by nonmetal sites in smaller pores and then in larger pores.
Finally, a team from UC Santa Barbara, Stony Brook University and the Korea Research Institute of Chemical Technology prepared a nickel sulfoisophthalate MOF and used neutron scattering spectroscopy to identify strong metal-H2 binding sites and weaker nonmetal adsorption sites in the material.
The overall capacity for hydrogen of this material as well as the much stronger binding of hydrogen than in typical porous material represent an important step toward a possible utilization of porous media for hydrogen storage, according to the team.
“Hydrogen Storage in a Microporous Metal-Organic Framework with Exposed Mn2+ Coordination Sites”; Mircea Dincă, Anne Dailly, Yun Liu, Craig M. Brown, Dan. A. Neumann, and Jeffrey R. Long; J. Am. Chem. Soc., 128 (51), 16876 -16883, 2006. 10.1021/ja0656853 S0002-7863(06)05685-X
“Neutron Powder Diffraction Study of D2 Sorption in Cu3(1,3,5-benzenetricarboxylate)2”; Vanessa K. Peterson, Yun Liu, Craig M. Brown, and Cameron J. Kepert; J. Am. Chem. Soc., 128 (49), 15578 -15579, 2006. 10.1021/ja0660857 S0002-7863(06)06085-9
“Adsorption of Molecular Hydrogen on Coordinatively Unsaturated Ni(II) Sites in a Nanoporous Hybrid Material”; Paul M. Forster, Juergen Eckert, Brandon D. Heiken, John B. Parise, Ji Woong Yoon, Sung Hwa Jhung, Jong-San Chang, and Anthony K. Cheetham; J. Am. Chem. Soc., 128 (51), 16846 -16850, 2006. 10.1021/ja0649217 S0002-7863(06)04921-3