|Different cages in the crystal structure of NOTT-112. The network in NOTT-112 is constructed by the packing of Cage A, Cage B and Cage C in the ratio of 1:2:1, respectively. Yan et al. (2009). Click to enlarge.|
Researchers at the University of Nottingham (UK), with participation from General Motors’s Chemical and Environmental Sciences Laboratory in Warren, Michigan, have developed a new metal–organic polyhedral framework that shows an excess hydrogen uptake of 7.07 wt% between 35 and 40 bar at 77 K (508-580 psi at -196 °C) and a total hydrogen uptake of 10 wt% at 77 bar (1,117 psi) and 77 K. An open-access paper on their work appears in the RSC journal Chemical Communications.
Metal-organic framework (MOF) compounds, consisting of metal-oxide clusters connected by organic linkers, are nano-porous materials that show promise for hydrogen storage applications because of their tunable pore size and functionality. (Earlier post.) MOFS were initially explored as mechanisms for simple hydrogen adsorption at nonmetal sites. However, researchers have found that enabling the direct binding of hydrogen to open metal coordination sites in MOFs allows the hydrogen molecules to pack together more closely and can provide a major boost in storage capacity. (Earlier post.)
The new Nottingham MOF (NOTT-112) is a blue octahedral-shaped single crystal ([Cu3(L)(H2O)3)]·8DMSO·15DMF·3H20) comprising three types of polyhedral cages (Cages A-C) in its three-dimensional space. Cage A has 12 open copper sites, making it “ideal for holding H2 molecules due to the high affinity of these open metal sites to gas molecules.”
The corresponding total volumetric storage density for NOTT-112 is 50.3 g L-1, similar to that reported for MOF-177 (48 g L-1 at 72 bar) with both NOTT-112 and MOF-177 (0.477 g cm-3) sharing similar crystallographic densities. At 1 bar NOTT-112 has a similar total H2 uptake (2.3%) to previously reported materials incorporating Cu(II) paddlewheel nodes (2.24–2.59%). However, NOTT-112 goes on to out-perform these phases at higher pressures of H2 and shows a total uptake of 7.8 wt% at 20 bar and 77 K. Interestingly, MOF-5, NOTT-112 and MOF-177 have similar BET surface areas of ca. 3800 m2 g-1 with the latter two also having similar pore volumes (1.6 cm3 g-1). Thus, it appears that this combination of high surface area and pore volume is necessary to achieve this level of H2 storage capacity.
...Modelling of H2 adsorption sites in metal–organic framework materials has revealed that aromatic rings on the organic linkers can play an important role in H2 adsorption and we believe this to be the case in the current study. Likewise, open Cu(II) sites are important in enhancing the affinity of H2 molecules in the pores. Thus, desolvated NOTT-112 incorporates and exploits both of these features, namely polyaromaticity of seven phenyl rings per linker and open metal sites at Cu(II), within a permanently porous framework comprising large and smaller polyhedral superstructures templated by the extended polyaromatic trigonal linker L6-.—Yan et al. (2009)
Yong Yan, Xiang Lin, Sihai Yang, Alexander J. Blake, Anne Dailly, Neil R. Champness, Peter Hubberstey and Martin Schröder (2009) Exceptionally high H2 storage by a metal–organic polyhedral framework. Chem. Commun., 2009, 1025 - 1027, doi: 10.1039/b900013e