Impact of impurities in covalent organic frameworks on catalytic properties of supported isolated Pd complexes

Abstract

Supported single-atom catalysts (SACs) have garnered significant interest due to their distinct electronic and geometric properties compared to metal nanoparticles. Covalent organic frameworks (COFs), owing to their high surface area and tunable functionalities, serve as promising supports for stabilizing isolated metal atoms. In this work, we synthesized a PdCl₂-functionalized pyrene-based COF (PdCl₂@Py-1P COF) that offers uniform binding sites ideal for immobilizing Pd single atoms. However, the synthetic route for pristine Py-1P COF could incorporate Pd impurities during synthesis, resulting from the incomplete removal of the homogeneous Pd(PPh₃)₄ catalyst used in the synthesis of the 1,3,6,8-tetrakis(4-aminophenyl)pyrene (PyTTA) monomer via Suzuki–Miyaura cross-coupling reaction. To our knowledge, no study has been reported on the role of Pd impurities in pristine COFs, complicating the distinction between the activity of Pd single atoms and the pristine COF itself. To address this, we used a combination of X-ray photoelectron spectroscopy (XPS), and CO-DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) to systematically investigate different purification strategies to minimize the impact of Pd impurities on ethylene hydrogenation kinetics. The PdCl₂ complexes deposited on the COF (PdCl₂@Py-1P COF) were isolated and structurally uniform. However, the Pd impurities in the pristine COF exhibit significant activity for ethylene hydrogenation, masking the activity of PdCl₂@Py-1P COF. To minimize Pd carryover, the PyTTA monomer was purified prior to COF synthesis by triphenylphosphine (PPh₃) treatment or acid, lowering the Pd content in the resulting COFs from 0.35 wt% (untreated) to 0.23 wt% (PPh₃) and 0.04 wt% (acid). Acid purification removed the vast majority of Pd impurities from the pristine (pre-deposition) COF, thereby enabling measurement of the intrinsic kinetics of subsequently deposited PdCl₂ complexes. The results emphasize the importance of the monomer purification process to accurately measure the intrinsic properties of Pd single atoms within the COF structure.