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 H-2 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 H-2 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 Bronsted-Evans-Polanyi relationship relating the barrier for the second step, CO2 hydrogenation, with the H-2 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.