Researchers Report Highest Hydrogen Capacity Yet for MOF-5 at Warmer Temperatures

19 December 2007

Structure of the hydrogen-storing metal-organic framework MOF-5. Click to enlarge. Source: (2007) Proc. Natl. Acad. Sci. USA 104, 20145-20146

Metal-organic framework (MOF) compounds, which consist of metal-oxide clusters connected by organic linkers, are a relatively new class of nanoporous material that show promise for hydrogen storage applications because of their tunable pore size and functionality. (Earlier post.) The hydrogen sorption processes in these systems display good reversibility and fast kinetics.

One of the challenges of MOFs, however, is that the weak dispersive interactions that hold H₂ molecules require low operations temperatures and/or high pressures. A research team led by Professor Rajeev Ahuja at Uppsala University (Sweden) has now reported what it believes is the highest hydrogen capacity yet for metal-organic framework-5 (MOF-5) at warmer temperatures (200 K to 300 K, or -73°C to +27°C). These findings are being published in Proceedings of the National Academy of Sciences (PNAS).

MOF-5 consists of 1,4-benzenedicarboxylate linkers joining Zn₄O clusters to form a periodic cubic framework. Prior work on MOF-5 has shown that the material can store 1.3 wt% of hydrogen at 78 K (-195°C) and approximately 1 atmosphere of pressure. The researchers calculated that the adsoprtion of lithium ions on the organic linkers would increase the strength of the hydrogen molecule binding to the framework, increasing the temperature at which hydrogen can be stored in MOFs. This discovery makes MOFs more suitable for being used as hydrogen storage materials.

We have found that two Li atoms are strongly adsorbed on the surfaces of the six-carbon rings, one on each side, carrying a charge of +0.9e per Li atom. Each Li can cluster three H₂ molecules around itself with a binding energy of 12 kJ (mol H₂)^–1. Furthermore, we show from ab initio molecular dynamics simulations with a hydrogen loading of 18 H₂ per formula unit that a hydrogen uptake of 2.9 wt % at 200 K and 2.0 wt % at 300 K is achievable. To our knowledge, this is the highest hydrogen storage capacity reported for metal–organic framework 5 under such thermodynamic conditions.

The authors believe the storage capacity would increase under applied pressure.

Resources

• A. Blomqvist, C. Moysés Araújo, P. Srepusharawoot, and R. Ahuja. “Li-decorated metal–organic framework 5: A route to achieving a suitable hydrogen storage medium” Proceedings of the National Academy of Sciences 104: 20173-20176