Single-site heterogeneous catalysts (SSHCs) play important roles in fundamental science and technology, owing to the molecular level control of structure-support interactions that is possible in these systems. Recently, SSHCs supported by acidic oxides have attracted particular interest because catalytically active metal centers can be formed at the surface sites. Here, we incorporated a palladium SSHC in phosphated and sulfated metal-organic frameworks (MOFs), hafnium-based MOF-808 (Hf-MOF-808-PO4 and Hf-MOF-808-SO4). The structural and electronic properties of the Pd(II) sites coordinated to the acidic sites in these MOFs were investigated through X-ray photoelectron spectroscopy, vibrational spectroscopy, X-ray crystallographic techniques, catalytic studies, and quantum mechanical electronic structure calculations employing density functional theory. We demonstrated that the presence of node-bound acidic functional groups stabilizes the Pd(II) site in these MOFs, resulting in enhanced catalytic activities (compared to in the nonacid functionalized Hf-MOF-808) in the oxidative Heck reaction where Pd(II) is the active species. The density functional calculations support the interpretation that the acid functionalization of the MOF node can stabilize the Pd(0) intermediate state during the catalytic reactions, thereby suppressing Pd(0) aggregation leading to catalyst deactivation. These findings offer insights and methodology for the catalytic investigation of SSHCs in MOFs.
Bibliographical noteFunding Information:
This work was supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences under Award No. DESC0012702. This work made use of the J.B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR-1720139) at the Materials Research Center of Northwestern University. This work made use of the EPIC and Keck-II facilities of the NUANCE Center at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205); the 16MRSEC program (NSF DMR- 1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois through the IIN.
© 2019 American Chemical Society.
- metal-organic frameworks
- oxidative Heck reaction
- palladium catalyst
- single-crystal X-ray diffraction
- single-site heterogeneous catalyst