Enhanced Activity of Heterogeneous Pd(II) Catalysts on Acid-Functionalized Metal–Organic Frameworks

Abstract

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.

Supported Single Atom Catalysts for C−H Activation: Selective C−H Oxidations, Dehydrogenations and Oxidative C−H/C−H Couplings

“…Oxidative coupling of a biphenyl moiety with an olefin was achieved by dispersing Pd single atoms in post‐synthetically sulfated or phosphated Hf 6 nodes of Hf‐MOF‐808 [45] . Characterization of the appended metal by XPS indicated the higher stability of Pd via immobilization at the acid sites of the SO 4 /PO 4 ‐functionalized MOF.…”

Pt/CeO2 with residual chloride as reusable soft Lewis acid catalysts: application to highly efficient isomerization of allylic esters

“…Lewis acidity can also be enhanced when the cation is isolated, as in frustrated Lewis pairs 24 . However, soft cations such as Au(I) and Pd(II) are easily reduced to zero valences and easily aggregate, which makes it di cult to maintain activity when developing them as immobilized soft Lewis acids 25,26 .…”

Single‐Site, Single‐Metal‐Atom, Heterogeneous Electrocatalyst: Metal–Organic‐Framework Supported Molybdenum Sulfide for Redox Mediator‐Assisted Hydrogen Evolution Reaction

“…Recently, chemists have realized the virtue of using metal‐organic frameworks (MOFs) as a heterogeneous support to yield single‐site, or near single‐site catalysts, owing to its periodic displacement of the two constituents, the inorganic node and the organic linker. In particular, we have recently reported a monometallic molybdenum disulfhydryl unit (Mo(SH) 2 ) post‐synthetically incorporated onto the hexanuclear oxozirconium‐based node of the MOF, NU‐1000 .…”