(479e) Molecular-Level Insight Into Unusual Low Pressure CO2 Affinity in Pillared Metal-Organic Frameworks

Fundamental insight into how low pressure adsorption properties are affected by chemical functionalization is critical to the development of next generation porous materials for post-combustion CO₂ capture. In this work, we present a systematic approach to understanding low pressure CO₂ affinity in isostructural metal-organic frameworks (MOFs) using molecular simulations and apply it to obtain quantitative, molecular-level insight into interesting experimental low pressure adsorption trends in a series of pillared MOFs.  Our experimental results show that increasing the number of non-polar functional groups on the benzene dicarboxylate (BDC) linker in the pillared DMOF-1 [Zn₂(BDC)₂(DABCO)] structure is an effective way to tune the CO₂ Henry’s coefficient in this isostructural series.  These findings are contrary to the common scenario where polar functional groups induce the greatest increase in low pressure affinity through polarization of the CO₂ molecule.  Instead, MOFs in this isostructural series containing nitro, hydroxyl, fluorine, chlorine, and bromine functional groups result in little increase to the low pressure CO₂ affinity.  Strong agreement between simulated and experimental Henry’s coefficient values is obtained from simulations on representative structures, and a powerful yet simple approach involving the analysis of the simulated heats of adsorption, adsorbate density distributions, and minimum energy 0 K binding sites is presented to elucidate the intermolecular interactions governing these interesting trends.  Through a combined experimental and simulation approach we demonstrate how subtle, structure-specific differences in CO₂ affinity induced by functionalization can be understood at the molecular-level through classical simulations.  This work also illustrates how structure-property relationships resulting from chemical functionalization can be very specific to the topology and electrostatic environment in the structure of interest. Given the excellent agreement between experiments and simulation, predicted CO₂ selectivities over N₂, CH₄, and CO are also investigated to demonstrate that methyl groups also provide the greatest increase in CO₂ selectivity relative to the other functional groups. These results indicate that methyl ligand functionalization may be a promising approach for creating both water stable and CO₂ selective variations of other MOFs for various industrial applications.