The effect of topology in Lewis pair functionalized metal organic frameworks on CO₂ adsorption and hydrogenation

We have used density functional theory and classical grand canonical Monte Carlo simulations to identify two functionalized metal organic frameworks (MOFs) that have the potential to be used for both CO₂ capture from flue gas and catalytic conversion of CO₂ to valuable chemicals. These new materials based on MIL-140B and MIL-140C functionalized with Lewis pair (acid and base) moieties, which are integrated into the framework linkers. We show that the Lewis pair functional groups are capable of catalyzing heterolytic dissociation of H₂ and subsequent hydrogenation of CO₂ through concerted 2-H addition. We have examined the effect of pore size and framework topology on the competitive binding of H₂ and CO₂. We show that the small pore size of functionalized MIL-140B stabilizes the formation of a pre-activated CO₂ species and that this pre-activated CO₂ has a lower overall reaction barrier for hydrogenation of CO₂ to formic acid than a competing pathway in the same material that does not go through a pre-activated complex. We demonstrate that steric hindrance can potentially break energy scaling relationships, which limit the ability to optimize traditional heterogeneous catalysts, by independently changing one part of the CO₂ hydrogenation pathway, without negatively impacting other parts of the pathway. Specifically, we show that steric effects can reduce the CO₂ hydrogenation barrier without impacting the H₂ dissociation barrier or binding energy.