(532b) Competitive Binding of Ethylene, Water, and Carbon Monoxide in Metal-Organic Framework Materials with Open Cu Sites

Metal-organic frameworks (MOFs) with open metal sites (OMS) are known to have selectivity in olefin/paraffin separations. One challenge associated with these separations is that other species that may bind strongly to OMS may also be present in feed streams, causing competition for these sites. We used Density Functional Theory (DFT) to assess the binding energy of ethylene, water and carbon monoxide on a set of more than- 60 MOFs with open Cu sites in the form of Cu dimers. One useful observation from our results is that the relative binding energies of pairs of molecules (e.g. ethylene and water) can be calculated accurately from calculations that hold the MOF structure rigid and only relax the positions of the adsorbing molecules. These kinds of calculations are far more numerically efficient than calculations that relax all degrees of freedom in the system, so this observation will be useful in future efforts to screen larger collections of materials. A second observation is that the binding energies of each molecule in the 60 MOFs are quite similar to the binding energies in CuBTC, an exemplar MOF with open Cu sites in the form of Cu dimers. Analysis of the variations that do exist in the binding energies among materials points to possible avenues for controlling either the absolute binding energies or the relative binding energies of species associated with OMS in these materials.