Selectivity and Reactivity of Alkylamine- and Alkanethiolate-Stabilized Pd and PdAg Nanoparticles for Hydrogenation and Isomerization of Allyl Alcohol

Abstract

Here, we describe the stability of solutions of various Pd and PdAg organic-protected nanoparticles (NPs) in the presence of H2 and their selectivity and reactivity as catalysts for hydrogenation or isomerization of allyl alcohol. Pd and Pd91Ag9 NPs stabilized with hexadecylamine (C16NH2) ligands are stable against H2-induced aggregation, whereas those stabilized with octylamines (C8NH2) and dodecylamines (C12NH2) precipitate within 1 h. The stability of C16NH2 Pd NPs is comparable to that of hexanethiolate (C6S)-protected Pd NPs and mixed monolayer C6S/C8NH2 (1/1) Pd NPs that were studied previously. The stability of C16NH2 Pd NPs decreases as the alkylamine/PdII ratio used in the synthesis decreases from 12:1 to 6:1 to 3:1. A bilayer or partial bilayer of C16NH2 ligands forms around the Pd core for ratios greater than 6:1, which explains the higher stability of these NPs against aggregation. The various Pd and PdAg NPs catalyzed the hydrogenation and isomerization of allyl alcohol in the presence of H2 with various selectivities and reactivities. C6S Pd NP catalysts are >95% selective toward the isomer; C8NH2/C6S Pd NPs are 60–75% selective toward the isomer, depending on the ligand ratio; and CnNH2-coated Pd NPs generally produce a 1:1 or 3:2 ratio of the hydrogenation/isomerization products, with a few exceptions. The catalytic turnover frequency (TOF) is low for C6S Pd NPs becaue of the strong thiolate–Pd bond. The TOF increases with increasing chain length in the order C16NH2 Pd > C12NH2 Pd > C8NH2 Pd and increases for Pd91Ag9 alloys compared with pure Pd. The mixed ligand C8NH2/C6S Pd NPs exhibit TOFs similar to pure C8NH2 Pd for low thiol content and similar to C6S Pd NPs for high thiol content. The 130/150 C8NH2/C6S Pd exhibits the optimal TOF for the mixed monolayer Pd NPs. C16NH2 Pd91Ag9 has the highest TOF of all the NPs studied due to the high stability afforded by the bilayer structure of the C16 chain and the high reactivity due to very little interference from the weak metal–amine interaction. Several of the Pd NPs that are stable in the presence of H2 are not stable during the catalysis reaction (H2 plus allyl alcohol), showing that the substrate also plays a role in NP stability.