(703g) Catalytic Hydrogenation of Carbon Dioxide in Functionalized Metal Organic Frameworks

Metal Organic frameworks (MOFs) are a class of porous materials having potential for both gas adsorption and catalysis. Functionalization of MOFs with catalytic moieties results in heterogeneous catalysts with active sites similar to those of molecular catalysts. In this work, we will present how the capture and conversion of CO₂ to C1 products can be enhanced in MOFs by incorporating Lewis pairs (LPs) and chelated metal centers. In contrast to CO₂ reduction on metal surfaces, the pathways here involve concerted transfer of a hydride and proton to the adsorbed or co-adsorbed CO₂. The density functional theory results presented here were calculated using the CP2K code, with the PBE functional to describe the exchange correlation energy. Girmme’s D3 dispersion correction was used to account for van der Waals interactions. We computed reaction pathways for H₂ dissociation, CO₂ chemisorption, and CO₂ hydrogenation.

We have shown that inclusion of Lewis pair (LP) moieties into UiO-66 and UiO-67 gives effective catalysts for hydrogenation of CO₂ to formic acid and methanol [1, 2]. The reaction pathway in this system has a lower barrier than the barriers reported for the reduction of CO₂ on traditional heterogeneous catalysts [3]. MOFs have a similar structure topology to zeolites, where the effects of confinement on the transition states and intermediates are well documented [4]. Here, we focus on how the effects of pore size could be applied to improving catalysis in MOFs. We do so by comparing and contrasting results from UiO-66 (largest pore), MIL-140C, and MIL-140B (smallest pore).

We have also investigated the reduction of CO₂ to formic acid via a formate intermediate on chelated metal centers. The pathway for Cu active centers are formic acid desorption limited, with low (<0.2 eV) barriers for the bond breaking/formation steps. This system allows for further tuning of the activity by changing the identity of the metal atom, as well as the electron affinities of the chelating ligands.

1. Ye, J.; Johnson, J. K. ACS Catal. 5, 6219-6229 (2015)
2. Ye, J.; Johnson, J. K. Catal. Sci. Technol. 6, 8392-8405 (2016)
3. Ye, J.; Johnson, J. K. ACS Catal. 5, 2921-2928 (2015)
4. Jones, A. J.; Zones, S. I.; Iglesia, E. J. Phys. Chem. C 118, 17787-17800 (2014)