Selective Hydrogenation of Polyunsaturated Fatty Acid Methyl Esters over Bifunctional Ligand-Modified Pd/MIL-101(Cr) Catalysts

Highly dispersed palladium nanoparticles (Pd NPs) were incorporated into MIL-101(Cr) frameworks functionalized with bifunctional ligands for the liquid-phase hydrogenation of polyunsaturated fatty acid methyl esters (FAMEs). A series of amino- and carboxylic-acid-containing ligands─ethylenediamine (en), diethylenetriamine (DET), alanine (AN), 4-aminobutyric acid (ABA), 5-aminovaleric acid (AVA), glutamic acid (GA), and adipic acid (AA)─were grafted onto MIL-101(Cr), followed by Pd loading (0.5 wt %). Spectroscopic and structural analyses confirmed ligand coordination to both Cr nodes and Pd species. Catalysts bearing ABA, AVA, and GA exhibited Pd0 dispersion (<1 nm), yielding high turnover frequencies (up to ∼15,400 h-1) and >94% selectivity for monounsaturated FAMEs. In contrast, strong Pd-N interactions in en- and DET-grafted materials suppressed Pd0 formation, reducing activity. Hot filtration and recyclability tests confirmed high catalyst stability and negligible Pd leaching. The bifunctional ligand architecture effectively tunes Pd speciation and activity, providing a robust platform for selective and reusable hydrogenation catalysts.