Metal-organic frameworks (MOFs) are promising supports for heterogeneous catalysis due to their large surface area, high porosity, and stability. NU-1000 is a Zr-based MOF that is widely used because it can incorporate metallic ions in a binding site with catalytically available –OH and –OH₂ groups. Recent work from Hupp and coworkers at Northwestern has shown that residual organic moieties left in the MOF from the synthesis step, specifically formate ions, can occupy the catalyst anchoring sites during MOF synthesis and facilitate an unexpected reduction of Cu(II) to Cu(0) nanoparticles, which does not happen in formate-free systems. Therefore, to understand if it is thermodynamically favorable for formate to accomplish this under an inert atmosphere at 200 °C with the release of H₂, CO₂, CO, and H₂O, we carried out density functional calculations using the revM06-L density functional to study the thermodynamic properties of all possible reactions: both radical and nonradical processes with and without formate on the node. We used cluster models of the catalytic systems that contain up to 101 atoms. The calculations show that node-bound formate can assist in reducing Cu(II) to Cu(0) by a nonradical reaction that is thermodynamically favorable at both room temperature and 200 °C with a negative Gibbs free energy of reaction (ΔG) and negative enthalpy of reaction (ΔH). An alternative mechanism (“radical reaction”) involving the formation of a radical is unfavorable because of the first step in which Cu(II) must be reduced to Cu(I). We also found that both radical and nonradical reactions are unfavorable when formate acts as a free base. We conclude that the formate ion attached to the node behaves as a sacrificial two-electron donor reducing Cu(II) to Cu(0).

Siriluk Kanchanakungwankul

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