Studies on the Nature of Catalysis: Suppression of the Catalytic Activity of Leached Pd by Supported Pd Particles during the Heck Reaction

Abstract

In the Heck coupling of bromobenzene and styrene over conventionally prepared Pd particles supported on SiO2 at 135 °C under Ar with Na2CO3 as the base, the catalysis of 2−4 ppm of leached Pd in solution is considerably affected by the supported Pd particles. Although such low concentration leached Pd alone is very active for the Heck reaction, its catalytic activity is suppressed in the presence of the supported Pd particles. Despite this, the variation of TON with leached Pd concentration follows the same trend of a homogeneous Pd catalyst. Homogeneous catalysis in nature is suggested based on the fact that the solid−liquid reaction rate is dependent on the leached Pd concentration rather than the supported Pd loading. The change of homogeneous catalysis relating to the variation of active Pd fraction in the presence and absence of the supported Pd particles is discussed.

Heck Chemistry - a Highly Active Ligand-Free Metal Catalyst System in Situ Generated from Pd<SUP>II</SUP> Supported on SiO<SUB>2</SUB>

“…28,30 With Pd/SiO 2 , such a Heck reaction likely occurs in solution following a "release and catch" mechanism. 16 Catalytic Pd species are leached out of Pd/SiO 2 , then catalyze the coupling reaction in solution, and redeposit on Pd/SiO 2 at the end of the catalytic cycle.…”

“…Nevertheless, little attention has been paid to on amorphous SiO 2 In recent years, we have focused our research on catalysis by the Pd/SiO 2 system for the Heck coupling of bromobenzene (PhBr) and styrene shown in Scheme 1. [27][28][29][30] We have ever reported a preliminary study relating to a highly active Pd II /SiO 2 system for the Heck reaction. 27 Pd II /SiO 2 exhibited an incomparable advantage over conventional Pd 0 /SiO 2 in catalytic activity.…”

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

“…After 30 min of the Heck reaction over Pd(acac) 2 , Pd(acac) 2 has been transformed into molecular Pd 0 , Pd clusters, and Pd black, with Pd clusters remaining as the minor part. 15 Of all Pd species derived from Pd compounds such as palladium acetate and Pd(acac) 2 , only molecular Pd 0 is deemed to be the active species for C-C coupling reactions. 5,6,8,15,68 Figure 4 and Table 3 show the poisoning effect of the scavengers on the Heck reaction over Pd(acac) 2 and the related EA data during the poisoning processes, respectively.…”

“…15 Of all Pd species derived from Pd compounds such as palladium acetate and Pd(acac) 2 , only molecular Pd 0 is deemed to be the active species for C-C coupling reactions. 5,6,8,15,68 Figure 4 and Table 3 show the poisoning effect of the scavengers on the Heck reaction over Pd(acac) 2 and the related EA data during the poisoning processes, respectively. The Pd-(acac) 2 system, which showed a high catalytic activity for the Heck reaction (Figure 4a), was completely deactivated once any of the scavengers was added (Figure 4b-d).…”

“…Our recent work has demonstrated that under Heck reaction conditions, soluble Pd contains a higher fraction of Pd clusters over Pd 0 /SiO 2 than over Pd(acac) 2 and that the fraction of Pd clusters in solution increases slowly as a Heck reaction proceeds over Pd 0 /SiO 2 . 15 Figure 6 and Tables 4 and 5 show the poisoning effect of the scavengers on catalysis, the soluble Pd scavenging, and the mutual impact between scavenger and the supported Pd particles in leaching in the whole process. After the addition of any of the scavengers, a low activity that the Pd 0 /SiO 2 system had for the Heck reaction (Figure 6a) was rapidly quenched (Figure 6b-d).…”