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.
Bibliographical noteFunding Information:
This work was supported as part of the Inorganometallic Catalyst Design Center, an EFRC funded by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (DE-SC0012702). H.N. gratefully acknowledges support from the Ryan Fellowship program of the Northwestern University International Institute of Nanotechnology. Z.L. acknowledges support from the National Natural Science Foundation of China (Grant 21601047). Z.H.S. acknowledges support from the National Science Foundation Graduate Research Fellowship under Grant DGE-1842165. This work made use of EPIC and KECK II facilities of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR1720139) at the Materials Research Center; the Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. This work made use of the IMSERC facility at Northwestern University, which has received support from the NSF (CHE-1048773 and DMR-0521267); SHyNE Resource (NSF NNCI-1542205); and the State of Illinois and IIN. This work made use of the REACT facility at Northwestern University, which has received support from the DOE (DE-FG02-03ER15457). This work made use of Advanced Photon Source of Argonne National Lab (ANL), at DND-CAT (Sector 5), which is supported by E.I. DuPont de Nemours & Co., Northwestern University, and The Dow Chemical Co. ANL is supported by DOE under Grant DE-AC02-06CH11357. We thank Qing Ma at DND-CAT and Dr. Neil M. Schweitzer at Northwestern University for assisting with the XAS measurement. The authors thank Timothy A. Goetjen for providing the organic linker for NU-1000 single crystal synthesis.
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