On the Roles of Solid-Bound Ligand Scavengers in the Removal of Palladium Residues and in the Distinction between Homogeneous and Heterogeneous Catalysis

Abstract

We have studied the roles and behavior of typical resin- and silica-bound thiol scavengers in the removal of palladium (Pd) residues and in the determination of the true catalytic species in the Heck coupling of bromobenzene and styrene. The results of Pd scavenging and catalyst poisoning by elemental analysis (EA) and transmission electron microscopy (TEM) indicate that silica-bound thiols have an advantage over resin-bound thiols in residual Pd removal from a Heck reaction solution and that all of these scavengers poison effectively the catalytic species but hardly scavenge Pd clusters, even as small as 1 nm from solution presumably because of the steric barrier. Because of a smaller proportion of soluble Pd clusters, using a molecular Pd precatalyst results in a much higher Pd scavenging efficiency than using a supported Pd particle precatalyst. With the aid of catalyst poisoning by the scavengers, filtration testing and TEM studies further illustrate that Pd clusters are inactive for the Heck reaction over Pd(0)/SiO(2), with molecular Pd(0) being solely active. Studies through EA and thermogravimetric analysis suggest that the bound thiols are leached from the scavengers to a different extent at reaction temperatures of 90-135 °C, probably owing to base-catalyzed decomposition or based-promoted dissociation of functional groups from the surfaces, leading to interaction between leached thiols and a solid Pd precatalyst. Meanwhile, the effect of solid-bound thiol binding to soluble Pd on the efficiency of Pd scavenging and the impact of a scavenger on the Pd leaching from supported Pd particles are discussed.

Influence of graphene oxide supports on solution-phase catalysis of thiolate-protected palladium nanoparticles in water

“…47 The UV-vis spectra showed a very small peak around 400 nm for both PdNP and PdNP/GO catalysis samples indicating the formation of small amount of Pd (II) species. This suggested that the small leaching of Pd atoms could not be completely eliminated in water using either thiolate ligand protection or GO supporting strategy.…”

Mechanistic interpretation of selective catalytic hydrogenation and isomerization of alkenes and dienes by ligand deactivated Pd nanoparticles

“…On large supported bare PdNP catalysts, catalytic activity is heavily reduced over time because the alkene substrate forms too many strong di-σ-bonded species on the catalytic surface. 18,19 The thiolate capping agents on our PdNP catalyst help reduce substrate oversaturation, thereby limiting the poisonous effects. 27,28 The presence of partial PdS x layer on the surface of PdNP after thiolate monolayer formation, which has been observed by others, might also be the reason for deactivation of PdNP catalyst.…”

Nanocatalysts for Hiyama, Stille, Kumada, and Negishi C–C Coupling Reactions

“…Silica-bound thiols have also been used for the removal of soluble Pd from reaction mixtures, but are seen to be benign toward Pd clusters as small as 1 nm. 184 The so-called fractional poisoning experiments, where the number of equivalents of a strong "toxic" ligand (such as CS 2 and thiophene) per metal centre necessary for complete shutdown of the reaction is determined, is another poisoning test used for identification of a catalytic mechanism; for nanoparticle catalysts, where only a fraction of the metal atoms (the "surface-active sites") contribute to the actual catalysis, this number is much less than 1, while for monosite catalysts that operate via a molecular mechanism need 1 or more equivalent of the "toxic ligand" per metal site for deactivation. This test must, however, be carried out under conditions where the toxic ligand does not decompose.…”