Support Effects in Hydrogenation Catalysis Using Low-Loading Pd and Rh Catalysts

A sustainable and scalable one-pot impregnation protocol, avoiding high-temperature calcination/activation, was employed to prepare Pd/Al₂O₃ (0.24 wt%), Pd/TiO₂ (0.18 wt%), Pd/ZrO₂ (0.21 wt%), Pd/SiO₂ (0.37 wt%), Rh/Al₂O₃ (0.18 wt%), and Rh/TiO₂ (0.15 wt%). Support effects on activity, selectivity, and recyclability of these low-metal content heterogeneous catalysts were investigated, using (E)-cinnamaldehyde and levulinic acid as probe molecules. In cinnamaldehyde hydrogenation, Pd catalysts were highly effective for chemoselective C=C reduction to 3-phenylpropanal under mild conditions, with Pd/TiO₂ displaying the highest activity and robust performance over several recycles. However, the Lewis acidity of TiO₂ promoted a solvent-involving side reaction in 2-propanol, with hemiacetal and ether formation, highlighting that apparent selectivity is strongly shaped by support acidity and product residence time. Rh/Al₂O₃ exhibited lower activity than Pd analogues but near-quantitative selectivity to the saturated aldehyde, whereas Rh/TiO₂ again favored hemiacetal formation. In levulinic acid hydrogenation, Pd catalysts were essentially inactive toward ketone hydrogenation even at elevated temperature and H₂ pressure, while Rh catalysts achieved high productivity with exclusive formation of γ-valerolactone, Rh/Al₂O₃ being the most active at comparatively mild pressures.