A palladium-anchored covalent organic framework for cyclic carbonate and one-pot oxazolidinone synthesis

The development of heterogeneous catalysts that efficiently transform carbon dioxide into value-added chemicals under mild conditions offers a sustainable approach towards chemical synthesis. Nevertheless, achieving high catalytic activity, excellent selectivity, and broad applicability within a single catalyst remains a persistent challenge. In this work, an imide-linked covalent organic framework (TAPA-NDA COF) was synthesized and employed as a support for the immobilization of Pd nanoparticles, yielding the Pd@TAPA-NDA COF as an efficient heterogeneous catalyst. The resulting COF-based catalyst was thoroughly characterized by PXRD, FR-IR, FE-SEM, HRTEM, TGA, BET and XPS analyses, confirming the successful formation of the imide-linked framework and the incorporation of Pd species within its porous COF architecture. Furthermore, PXRD analysis provided structural validation of the Pd₆ nanoclusters@TAPA-NDA COF, while theoretical calculations identified the thermodynamically preferred adsorption site of the Pd₆ nanocluster within the COF. The catalytic activity of the Pd@TAPA-NDA COF was first assessed for the cycloaddition of CO₂ with epoxides to form cyclic carbonates, delivering good to excellent yields (i.e. 72% to 96%) with up to 99% selectivity under mild and solvent-free conditions. Subsequently, the catalyst was applied to the cyclization of propargylic alcohols with phenyl isocyanate for the synthesis of 2-oxazolidinones, an important class of bioactive heterocycles, affording the desired products in yields up to 92% with 99% selectivity under the optimized conditions. The outstanding catalytic efficiency is attributed to the synergistic interaction between Pd(0)/Pd(II) active sites and the porous COF. Moreover, the Pd@TAPA-NDA COF could be readily recovered and reused over five consecutive catalytic cycles without any appreciable loss in activity, as evidenced by FT-IR and SEM analyses of the recycled catalyst. Overall, the Pd@TAPA-NDA COF serves as a versatile, recyclable, and efficient heterogeneous catalyst for CO₂ fixation and one pot oxazolidinone synthesis, offering a sustainable platform for value-added chemical synthesis.