(337e) Design of Functionalized Metal Organic Frameworks for CO2 Hydrogenation

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

Abstract:

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.