N-Formylation of Amines with CO₂ and H₂ Using Pd–Au Bimetallic Catalysts Supported on Polyaniline-Functionalized Carbon Nanotubes

Bimetallic Pd–Au catalyst was prepared by depositing the Pd–Au alloy nanoparticles on polyaniline-functionalized carbon nanotubes (PANI-CNT) and the resulting Pd–Au/PANI-CNT catalyst exhibited excellent catalytic activity for the N-formylation of pyrrolidine using CO2/H2. The structural and electronic properties of the Pd–Au/PANI-CNT was characterized by X-ray powder diffraction (XRD), nitrogen adsorption–desorption, transmission electron microscopy (TEM), high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and X-ray photoelectron spectroscopy (XPS). Under optimized conditions, an N-formylpyrrolidine yield of 98.3% was obtained from pyrrolidine and CO2/H2 at 125 °C by using Pd–Au/PANI-CNT with a Pd/Au molar ratio of 1:1. Our research further reveals that Pd atoms should be the true active sites for the hydrogenation reaction and the N-formylation reaction might occur mainly over Pd atoms or over the interface between Pd atoms and Au atoms for the bimetallic Pd–Au/PANI-CNT catalyst. The enhanced catalytic performance of bimetallic Pd–Au/PANI-CNT is mainly related to beneficial interactions between Pd atoms and Au atoms, leading to changes of the electronic properties of the formed bimetallic Pd–Au nanoparticles.