Bimetallic Nanocatalyst Immobilized on Halloysite: Pd/Re Cooperative Effect in Fatty Acid Hydrogenations

The selective hydrogenation of fatty acids into fatty alcohols remains a key step in biomass valorization but poses significant challenges due to the high thermodynamic stability of the carboxylic acid group. Herein, we report an environmentally friendly Pd–Re bimetallic nanocatalyst supported on naturally occurring halloysite nanotubes (Pd–Re/HAL) for the efficient conversion of long-chain fatty acids under mild conditions. Characterization by TEM, PXRD, and XPS revealed Pd(0) nanoparticles and a distribution of Re species (Re(0)–Re(VII)) colocalized on the nanotubular support, with over 65% of Re species present in low oxidation states. Pd–Re/HAL demonstrated high activity (conversions in the range of 83–97%) compared to the monometallic counterparts (Pd/HAL and Re/HAL), and remarkable chemoselectivity (no over-reduction of alcohols toward alkanes by hydrodeoxygenation was observed) in the hydrogenation of C14–C22 fatty acids. Control experiments suggest a synergistic effect between Pd and Re, which is plausible, requiring both homolytic and heterolytic H2 activation mediated by Pd and ReOx species, respectively. The high surface area of the halloysite support further enhance dispersion, albeit electronic metal–metal interactions were ruled out. These results highlight halloysite-supported Pd–Re catalysts represent an efficient approach for fatty alcohol production.