In general, metal organic frameworks (MOFs) can be categorized into two categories: Those with open metal sites and those without. Compared to the fully coordinated metal centers, the structurally unsaturated metal sites show much larger affinities to guest gas molecules, such as CO2. The exposed metal cations create strong electrostatic fields, which interact with the guest molecules via both electrostatic and induction effects, resulting in increased adsorption energies.
Despite the resulting significantly enhanced loading capacities, these open metal sites are also vulnerable to various contaminant species. While previous work generally has not considered molecules like SOx, NOx, (and their hydrated acid forms), due to their low concentrations in the flue gas, recent studies illustrate the profound effects of these contaminants on using such MOFs for CO2 sequestration process. It is shown that in spite of their low concentrations, these contaminants can gradually reduce the CO2 loading capabilities of MOFs over several adsorption/purging cycles. Despite this important issue, studies concerning flue gas contaminants are quite limited, both experimentally and theoretically.
In our group, we employ density functional theory (DFT) and a simple Langmuir model to study the flue gas contaminant effects in two important MOFs with open metal sites: Mg/MOF-74 and MIL-101. We have constructed cluster models of these two MOFs and investigated their interactions with various contaminant molecules. We have obtained adsorption enthalpies and free energies for all the contaminants, and using this data, in conjunction with simple adsorption models, we can better understand the roles of these molecules in the MOF columns. Through our calculations, we have identified the most detrimental contaminant species. This work highlights the importance of considering the entire application environment when choosing optimal MOF structures for desired applications.