posted on 2020-12-03, 15:43authored byYing Yang, Xuan Zhang, Siriluk Kanchanakungwankul, Zhiyong Lu, Hyunho Noh, Zoha H. Syed, Omar K. Farha, Donald G. Truhlar, Joseph T. Hupp
A desirable
feature of metal–organic frameworks (MOFs) is
their well-defined structural periodicity and the presence of well-defined
catalyst grafting sites (e.g., reactive −OH and −OH2 groups) that can support single-site heterogeneous catalysts.
However, one should not overlook the potential role of residual organic
moieties, specifically formate ions that can occupy the catalyst anchoring
sites during MOF synthesis. Here we show how these residual formate
species in a Zr-based MOF, NU-1000, critically alter the structure,
redox capability, and catalytic activity of postsynthetically incorporated
Cu(II) ions. Single-crystal X-ray diffraction measurements established
that there are two structurally distinct types of Cu(II) ions in NU-1000:
one type with residual formate and one without. In NU-1000 with formate,
Cu(II) solely binds to the node via the formate-unoccupied, bridging
μ3–OH, whereas in the formate-free case, it
displaces protons from two node hydroxo ligands and resides close
to the terminal −OH2. Under an inert atmosphere,
node-bound formate facilitates the unanticipated reduction of isolated
Cu(II) to nanoparticulate Cu(0)a behavior which is essentially
absent in the formate-free analogue because no other sacrificial reductant
is present. When the two MOFs were tested as benzyl alcohol oxidation
catalysts, we observed that residual formate boosts the catalytic
turnover frequency. Density functional calculations showed that node-bound
formate acts as a sacrificial two-electron donor and assists in reducing
Cu(II) to Cu(0) by a nonradical pathway. The negative Gibbs free energy
of reaction (ΔG) and enthalpy of reaction (ΔH) indicate that the reduction is thermodynamically favorable.
The work presented here highlights how the often-neglected residual
formate prevalent in nearly all zirconium-based MOFs can significantly
modulate the properties of supported catalysts.