Pd nanoparticles confined within triazine-based carbon nitride NTs: An efficient catalyst for Knoevenagel condensation-reduction cascade reactions

Abstract

Pd nanoparticles (NPs) were immobilized within triazine-based g-C3N4 nanotubes (NTs) to obtain a novel bifunctional catalyst, which was used as an efficient catalyst in the Knoevenagel condensation-reduction tandem reaction. The results of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), UV–vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) demonstrated that Pd NPs were successfully immobilized within triazine-based g-C3N4 NTs. The base amounts of the catalysts were determined by CO2 temperature programmed desorption (CO2-TPD). The textural and morphology of Pd/triazine-based g-C3N4 NTs were characterized by N2 adsorption-desorption, scanning electronic micrograph (SEM), transmission electron microscope (TEM) and CO adsorption. The characterization results indicated that Pd NPs were mainly immobilized on the inner surface of triazine-based g-C3N4 NTs. Pd/triazine-based g-C3N4 NTs showed excellent catalytic performance in the tandem reaction, in which the conversion of benzaldehyde and selectivity to benzylmalononitrile were 100% and 99.2%, respectively. The result was attributed to the high base amounts (especially -NH2 and -NH groups) and the high catalytic activities of Pd NPs within Pd/triazine-based g-C3N4 NTs. Furthermore, the high catalytic activity of Pd/triazine-based g-C3N4 NTs is also attributed to the confinement effect of the nanotube structure. The structure of nanotube not only tune the Pd particle size of the catalysts, but also provide spatial restriction on metal catalyst encapsulated in their channels, which hampered particles aggregation during the reaction, thus maintaining good catalytic activity and stability. Moreover, the anchor and stabilization effects of pyridine N atoms of triazine-based g-C3N4 NTs inhibited the aggregation and the leak of active sites, thereby achieving a good catalytic recyclability with almost unchanged catalytic activity for more than eight times in the tandem reaction.