The capture of CO2 from power plant flue streams is critical for the reduction of greenhouse gas emissions. We are currently investigating metal-organic frameworks with exposed metal cation sites for this application. Owing to its greater quadrupole moment and polarizability, CO2 is expected to interact more strongly with these sites than N2. Indeed, in the framework Mg2(dobdc), a much higher uptake is recorded for CO2 compared to N2. We are also investigating the effects functionalizing pore walls with alkylamines. Here, the basic amines chemically adsorb CO2, dramatically enhancing the capacity and selectivity of metal-organic frameworks for CO2 at conditions relevant to CO2 capture from flue gas.
Find more details about this project here.
Find more details about this project here.
Our group is also focusing on the development of new metal-organic frameworks for the storage of H2. The preparation of new frameworks derived from lightweight main group ions, such as Be2+, B3+, Mg2+, and Al3+, is expected to yield significant advantages in the gravimetric H2 uptake compared to their heavier transition metal counterparts. Recently, we succeeded in obtaining the first beryllium-based framework, Be12(OH)12(BTB)4 (Be-BTB, BTB3-= 1,3,5-benzenetrisbenzoate). This compound has an unusual structure consisting of inorganic and organic channels, and has one of the highest surface areas recorded in a metal-organic framework (SABET = 4020 m2/g). The high surface area translates to exceptional cryogenic hydrogen storage properties, wherein 9.1 wt % or 43 g/L of H2 is adsorbed at 77 K and 95 bar.
We are also developing materials that have a greater storage capacity at room temperature. For this to be feasible, the enthalpy of adsorption (Qst) must be increased from the 5-7 kJ/mol often observed in frameworks, to approximately -15 kJ/mol over the entire adsorption range. By employing the 1,3,5-benzenetristetrazolate (BTT3-) ligand, we have succeeded in isolating a series of cubic, sodalite-like metal-organic frameworks, M3[(M4Cl)3(BTT)8]2 (M-BTT, M = Mn, Fe, Cu). These materials possess open coordination sites, which prove to be polarizing towards H2. This is reflected in the higher Qst observed in these materials compared to those without open coordination sites.
The energy costs associated with the separation of light hydrocarbons could potentially be lowered through development of selective solid adsorbents. We have recently synthesized a metal-organic framework, Fe2(dobdc), featuring channels lined with a high concentration of soft Fe2+ cation sites, which exhibits excellent performance characteristics for the separation of ethylene/ethane and propylene/propane mixtures at 318 K. Breakthrough data obtained for these mixtures provide experimental validation of simulations, which are then used to show that the material can be expected to exhibit higher selectivities and capacities compared to other known adsorbents for: the fractionation of ethane/ethane/ethylene/acetylene mixtures, removing acetylene impurities from ethylene, and membrane-based olefin/paraffin separations.
Fig. 3 Left: Neutron powder diffraction data confirm a side-on coordination of unsaturated ethylene to the iron(II) centers in Fe2(dobdc). Right: Calculated methane, ethane, ethylene, and acetylene breakthrough curves for an equimolar mixture of the gases flowing through a bed of Fe2(dobdc) and a schematic representation of the separation of a mixture of these gases using just three packed beds of Fe2(dobdc).
While metal-organic frameworks have been widely investigated for their gas sorption properties, and to a lesser extent possible applications in catalysis and sensing there have been very few investigations of charge transport in these materials. We have recently shown that the incorporation of lithium alkoxides into a metal-organic framework with open metal cation sites can produce solids with an ionic conductivity of 3 x 10-4 S/cm at 300 K. We are exploring similar strategies to get higher conductivities and explore the conduction of other ions. Our group is also actively exploring methods to engender electronic conductivity into metal-organic frameworks have traditionally been made with redox-inactive metals and hard ligands that form bonds with ionic character, minimizing orbital overlap and electronic communication. We are exploring systems with redox-active metals and softer ligands that form bonds with greater covalency and π-overlap with the metal orbitals. One framework synthesized in our lab, Cu[Ni(pdt)2], upon I2 doping, displays a conductivity of greater than 10-4 S/cm at 295K.