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.