(700f) Impact of Structural Factors On the Adsorption Properties of Metal Organic Frameworks (MOFs)

A better understanding of how key structural features affect adsorption properties of guest molecules is necessary for the development of simple heuristics that can lead to the design of new MOFs with high gas adsorption capacities and improved selectivities for specific gas separations. This study is an effort to understand the interplay of different factors (pore size, heat of adsorption, open metal sites, electrostatics and ligand functionalization) contributing to adsorption in MOFs.

Two MOFs, Cu-BTC and Zn₂[bdc]₂[dabco] were synthesized and characterized using powder X-ray diffraction experiments and nitrogen adsorption at 77K. Adsorption isotherms for CO₂ and CO were measured gravimetrically at room temperature. GCMC simulations were performed to calculate adsorption of CO and CO₂ for the synthesized MOFs and two other MOFs IRMOF-1 and IRMOF-3. Heats of adsorption for each component in all four MOFs were also computed. Binary mixture (CO₂/CO) simulations were performed for 5%, 50%, and 95% CO₂ mixtures and adsorption selectivities were calculated. Simulations show that all the MOFs are selective for CO₂ over CO. Cu-BTC displays higher selectivities for CO₂ over CO at lower pressures for all mixtures due to the increase in electrostatic interactions of CO₂ with the exposed copper sites. IRMOF-3 shows surprisingly higher selectivities for CO₂ over CO for 50% and 95% mixtures at higher pressures due to the presence of amine functionalized groups. These results and their implications for enhancement of adsorption separation systems will be discussed.