Design of Functionalized Metal Organic Frameworks for CO2 Hydrogenation
Jingyun Ye1, J. Karl Johnson1
1Department of Chemical & Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, 15261, USA
Efficient conversion of CO2 into valuable chemicals has the potential to reduce net CO2 emissions while generating high-energy density fuels and other commodities. However, hydrogenation (reduction) of CO2 is very challenging due to its chemical inertness and thermodynamic stability. Perhaps the most simple and direct route for CO2 reduction is the addition of a proton (to the oxygen atom) and a hydride (to the carbon atom) to produce formic acid. Conceptually, the required protic and hydridic hydrogens can be provided by Lewis bases and acids, respectively. It has been experimentally shown that frustrated Lewis pairs (FLPs), which are molecules having both Lewis acid and base sites but that are sterically hindered to prevent mutual quenching, can both bind CO2 and heterolytically dissociate H2. However, FLPs are homogeneous catalysts and therefore have considerable disadvantages compared with heterogeneous catalysts, such as difficulty with catalyst recycling and product separation.
In this work we design novel CO2 hydrogenation catalysts in silico that combine the advantages of both homogeneous and heterogeneous catalysts. Our approach is to functionalize metal organic frameworks (MOFs) with a series of different Lewis pairs (LPs) to create heterogeneous catalysts. We employ UiO-66 as our starting base MOF because it is chemically and thermally stable, is highly selective toward CO2 and can be readily functionalized. We have designed a family of eight LP functional groups that can be bound to the linkers of UiO-66. We have evaluated these materials by computing binding energies and reaction pathways for CO2 reduction from density functional theory (DFT). We found that all of these materials have qualitatively similar reaction pathways. We found that CO2 reduction proceeds via a two-step process, with the first step being heterolytic H2 dissociation and the second step being a 2-electron reduction of CO2 via concerted hydride and proton attachment. We have identified linear energy relationships for the CO2 hydrogenation barriers and H2 dissociation barriers that allow for efficient screening of the catalytic activity of potential catalysts. We have also designed catalysts based on UiO-67 based on insight derived from our UiO-66 studies. Our DFT calculations indicate that these materials should be stable and that in some cases the catalysts are predicted to have significant activity at moderate temperatures and pressures for producing formic acid from CO2 and H2. We have identified materials that are good candidates for synthesis and experimental verification of catalytic activity.