Department of Chemical and Biological Engineering, Northwestern University
June 2009 -- present.
Adviser: Randall Q. Snurr
Metal-organic frameworks (MOFs) are permanently porous, crystalline materials comprised of metal or metal oxide nodes connected via organic "linker" molecules. They exhibit high surface areas, e.g. over 6000 m2/g, and are useful gas sorbents. They are used in gas storage, separations, and catalysis, amongst other things.
We are currently investigating MOFs for hydrogen storage. Hydrogen has many desirable properties as a fuel. It is gravimetrically energy dense and non-toxic, and its oxidation product is water. However, storage is difficult due to its small volumetric density. The US Department of Energy has set targets for hydrogen storage materials for personal vehicles. The entire storage system must produce 2.5 kWh/kg and 2.3 kWh/L. This translates into 7 wt% H2 and 0.070 kg/L for the entire storage system. This is quite challenging considering the density of liquid H2 is 0.0678 kg/L at its normal boiling point of -250oC!
MOFs typically interact physically with hydrogen, and binding energies on the order of 5 kJ/mol are common. This is strong enough to generate significant storage at cryogenic temperatures, but storage near ambient conditions is quite small. We are currently working on modifying MOF linkers to increase the H2 binding energy. We have all the functionality of organic chemistry to work with!
We are also designing MOFs for gas separations. A classic application is CO2 removal from air (which we model with N2.) To separate one component from another, the MOF must exhibit a stronger preference to bind one component over another. The component that binds more strongly thus adsorbs in the MOF, taking up most of the potential binding sites and leaving none for the weaker binding component. If a gas mixture is passed through the MOF material, the MOF should then take up most of the strongly binding component, thus separating it out of the mixture.
The physical binding energies between gas molecules and MOF atoms are caused by perturbations in the gas molecule electronic structure due to point charges, dipoles, induced dipoles, and other phenomena. These binding energies are typically greatest in dipolar molecules. However, both CO2 and N2 are quadrupolar, and thus designing MOFs that distinguish the binding energies well enough to drive separation at ambient conditions is challenging. One strategy is to maximize the electronic features of the MOF itself.
Other projects are either in the early stages or confidential until published. Check back again soon!