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

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.