A novel in situ green method for synthesizing a palladium NPs-based catalyst aimed at the selective hydrogenation of aldehydes in cinnamaldehyde

Abstract

In the first study, Palladium nanoparticles (NPs) were synthesized via wet impregnation of hybrid hydroxyapatite nanomaterials using a sodium tetrachloropalladate dihydrate (Na₂PdCl₄·2 H₂O) precursor. Citrate served as active sites to facilitate the selective and "in situ" reduction of palladium NPs. To investigate the role of citrate, hybrid hydroxyapatite (nCit-HAp) nanomaterials were prepared by varying the molar ratio of citrate to HAp (n = 0.2, 0.4, and 0.8). The resulting (PdNPs/nCit-Hap) catalysts were characterized using techniques such as N₂ adsorption-desorption, ICP-OES, IR-ATR, X-ray diffraction, TEM and XPS. The second study, focused on the effects of parameters like the Cit/HAp molar ratio and heat treatment on the selective hydrogenation of. The Pd/0.8Cit-HAp catalyst demonstrates remarkable performance in the selective hydrogenation of cinnamaldehyde, achieving approximately 90 % selectivity toward cinnamyl alcohol and 100 % conversion. This enhanced selectivity is attributed to the presence of small palladium nanoparticles (∼2.7 nm) with surface electropositive Pd^2 + sites, as identified by X-ray photoelectron spectroscopy (XPS). These Pd²⁺ sites, stabilized by citrate functionalization, promote preferential adsorption and polarization of the carbonyl (CO) group, thereby directing the reaction pathway toward the desired unsaturated alcohol. The observed structure sensitivity in the hydrogenation of CO versus CC bonds, along with the intrinsic activities (TOF), was attributed to size-dependent effects of palladium particles, which were controlled by the citrate groups on the hydroxyapatite surface.