Probing the Gas-Host Interactions in Metal Organic Frameworks
Jason M. Simmons1,2, Shengqian Ma3, Xi-Sen Wang3, Yun Liu1,4, Craig M. Brown1, Taner Yildirim1,2, Hong-Cai Zhou3
1NIST Center for Neutron Research, 2University of Pennsylvania, 3Miami University 4University of Maryland
Metal Organic Frameworks (MOFs), a class of nano-porous organic-inorganic compounds, are actively studied as host systems for storage and separation of technologically important gases. In light of recent economic and environmental pressures, particular emphasis has been placed on the storage of gaseous fuels such as hydrogen and methane for mobile energy applications. Indeed, the Department of Energy has set targets for technologically relevant gas storage. To reach these ambitious goals, further understanding of the gas-host interaction is of utmost importance.
In order to probe the gas-host interaction, we use a home-made, fully computer controlled Sievert apparatus which can operate continuously in a wide range of temperatures (30K to 500 K) and pressures (0 atm. to 60 atm.). Adsorption measurements from this instrument are used to determine the maximum adsorption and the heat of absorption, and can be used to better understand adsorption kinetics and hysteresis. Further detailed information, including sequential adsorption sites and binding energies, are determined using elastic and inelastic neutron scattering measurements which are closely coupled with first-principles total energy calculations. Combining these three approaches allows us to thoroughly characterize a large number of MOFs materials.
Here we show the effect of narrow constrictions in large-pore MOFs and the role of metal coordination in increasing gas-host binding. MOFs that contain small channels between pores show an “activated absorption” where no adsorption is observed below a blocking temperature Tb. Such “activated absorption” suggests that it may be possible to load H2 at high pressure, store it at ambient pressure and release it simply by increasing temperature. An alternative approach for improved gas storage is to increase the binding energy of the adsorbent. For that, we have investigated the effect of the metal coordination in MOFs on the binding energy. Those MOFs with unsaturated metal centers show significant hydrogen saturation capacity and initial heat of adsorption at 77K. In addition, some of the MOFs that we have studied are among the first materials that exceed DOE targets for useful methane storage at room temperature.