Superior aqueous-phase catalytic hydrogenation activity of palladium modified with nitrogen-containing ligands compared with the TPPTS benchmark modifier in micellar nanoreactors

• High activities of palladium nanoparticle catalysts stabilized by nitrogen ligands in micellar nanoreactors (TOF = 110,000 h−1).
• Superior activity over water-soluble Pd/TPPTS benchmark catalyst.
• Hydrogenation of renewable polyunsaturated methyl esters of soybean oil.
• Upgraded biodiesel of improved oxidative stability, energy and environmental performance.
• Environmentally friendly aqueous medium.

Exceptionally high catalytic activities (TOF = 110,000 h−1) have been achieved by water-dispersible palladium(0) nanoparticle catalysts stabilized by water-soluble nitrogen-containing ligands in the hydrogenation of renewable polyunsaturated methyl esters of soybean oil (MESBO) to their monounsaturated counterparts in aqueous/organic micellar systems. Palladium(II) chloride catalyst precursors exhibited a superior aqueous-phase catalytic activity with various nitrogen-containing ligands inter alia bathophenanthrolinedisulfonic acid disodium salt [(BPhDS) (TOF = 71,000 h−1)], diethylenetriaminepentakis (methylphosphonic acid) (TOF = 68,000 h−1), ethylenediaminetetraacetic acid tetrasodium salt (TOF = 50,000 h−1) compared with the TPPTS benchmark ligand modifier (TOF = 34,000 h−1) in this environmentally friendly biphasic catalytic hydrogenation reaction. Mercury poisoning experiments of preformed Pd/BPhDS catalysts proved the heterogeneous nature of this catalytic hydrogenation system. Dynamic light scattering experiments evidenced the presence of micellar nanoreactors with in situ prepared [Pd(OAc)2]3/BPhDS catalyst possessing an average hydrodynamic radius of 36 nm and with preformed and recycled PdCl2/BPhDS catalyst with an average hydrodynamic radius of 57 nm. The apparent activation energy of preformed palladium(0) nanoparticle hydrogenation catalysts stabilized by BPhDS in micellar systems was calculated to 23.7 kJ/mol which is of the same order of magnitude as reported earlier for palladium(0) nanoparticles stabilized by dendritic core-multishell architectures. A recycling experiment at 120 °C showed that the activity of palladium(0) nanoparticle catalyst stabilized by BPhDS in micellar systems in aqueous media remained high in a consecutive run indicating a stable palladium(0) catalytic nanoparticle system which is remarkable when one considers that transition metal(0) catalytic nanoparticles are usually applied at much lower temperatures due to their lower stabilities.

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