Screening Lewis Pair Moieties for Catalytic Hydrogenation of CO₂ in Functionalized UiO-66

The capture and reuse of CO₂ as a liquid fuel could reduce the overall anthropogenic carbon footprint but requires a catalytic pathway for CO₂ hydrogenation under mild conditions, coupled with a renewable source of H₂ or another reducing agent. We have computationally designed eight functional groups having both Lewis acid and base sites for inclusion inside a porous metal-organic framework (MOF) and have evaluated these functionalized MOFs for their catalytic activity toward CO₂ hydrogenation. We have used density functional theory to compute reaction energies, barriers, and geometries for the elementary steps of CO₂ reduction. The reaction pathways involve two elementary steps for each of the eight functional groups, consisting of heterolytic dissociation of H₂ on the Lewis acid and base sites followed by concerted addition of a hydride and a proton to CO₂ in a single step. Our analysis of the reaction energetics reveals that the reaction barrier for hydrogen dissociation can be correlated as a function of the chemical hardness of the Lewis acid site. Furthermore, we have identified a Brønsted-Evans-Polanyi relationship relating the barrier for the second step, CO₂ hydrogenation, with the H₂ adsorption energy on the Lewis sites. Surprisingly, this linear relationship also holds for correlating the hydrogenation barrier with the hydride attachment energy for the gas-phase Lewis acid site. These correlations provide a computationally efficient method for screening functional groups for their catalytic activity toward CO₂ hydrogenation. These relationships are further utilized to carry out a Sabatier analysis on a simplified model of the reaction to generate contour plots of the Sabatier activity that can be used to identify properties of the functional groups for maximizing the reaction rate.