Palladium Nanoparticles Immobilized on the Amine-Functionalized Lumen of Halloysite for Catalytic Hydrogenation Reactions

Supported Pd-based catalysts have been widely applied in the hydrogenation of specific functional groups. Recent trends have focused on employing Pd-based heterogeneous catalysts supported on inorganic nanotubes, wherein inner surface functionalization modulates both palladium nanoparticle (Pd-NP) dispersion and the interaction between reactants and the catalyst surface, thereby influencing catalytic properties. This study aims to develop a catalytic system using amine-lumened halloysite nanotubes immobilizing Pd-NPs (Pd/HNTA) as catalysts for hydrogenation reactions. The formation of Pd-NPs within the organo-functionalized lumen—modified by 3-aminopropyltrimethoxysilane—is confirmed by transmission electron microscopy (TEM) imaging, which reveals a particle size of 2.2 ± 0.4 nm. For comparison, Pd-NPs supported on pristine halloysite (Pd/HNTP) were used as control catalysts, displaying a metal particle size of 2.8 ± 0.8 nm and thereby demonstrating the effect of organic functionalization on the halloysite nanotubes. Both catalysts were employed as model substrates for the hydrogenation of furfural (FUR) and nitrobenzene (NB). Pd/HNTA demonstrated superior catalytic performance for both substrates, with TOF values of 880 h⁻¹ for FUR and 946 h⁻¹ for NB, and selectivities exceeding 98% for tetrahydrofurfuryl alcohol (THFOH) and aniline (AN), respectively. However, recyclability studies displayed an opposite trend, where Pd/HNTA was deactivated at the 10 catalytic cycles during the hydrogenation of FUR, whereas, in the hydrogenation of NB, ten catalytic cycles were achieved with maximum conversion and selectivity at 360 min. These results revealed that the liquid-phase environment plays a pivotal role in catalyst stability. In the hydrogenation of NB, the coproduction of H₂O adversely affects the interaction between the Pd particles and the inner amine-modified surface, increasing the deactivation of the catalyst with reuse. Thus, the Pd/HNTA catalyst holds significant promise for the development of noble-metal-based catalysts and their application in reducing other reducible organic functional groups via hydrogenation.