Nanoparticulate Palladium Supported by Covalently Modified Silicas:  Synthesis, Characterization, and Application as Catalysts for the Suzuki Coupling of Aryl Halides

Abstract

We present a study of the use of amine-functionalized, mesoporous silicas as supports for nanoparticulate palladium, and the use of the composite materials as heterogeneous catalysts for the Suzuki coupling of aryl bromides. Upon modification of the silica, via attachment of N-functionalized aminopropylsilyl ethers to surface silanol groups, only a small reduction in surface area and average pore diameter is observed. The mesoporosity and high surface area are also maintained after introduction of nanoparticulate palladium, as evidenced by the measurement of BET nitrogen sorption isotherms. Electron microscopy shows that the palladium particles are well-dispersed and of typical diameter 3-6 nm. Catalysis was initially tested using the coupling of phenylboronic acid with 4-bromoanisole in the presence of K 2 CO 3 and with toluene as solvent. This revealed that the choice of organic modification has a crucial role in determining the activity and recyclability of the catalyst: optimum behavior was found for diamine-and triamine-containing systems, while quaternary alkylammonium salts showed poor activities. The optimized catalysts are also active in the coupling of a range of aryl bromides and phenylboronic acids, and after three catalytic runs they show virtually no drop in activity. Upon further cycling, however, and after six catalytic runs, we do observe a drop in activity, and this is accompanied by some leaching of palladium and pore-blocking by reaction products and byproducts.

Silica-supported palladium: Sustainable catalysts for cross-coupling reactions

“…Similar to mercaptopropyl ligands, aminopropyl ligands were grafted to amorphous silica by reaction with the corresponding aminopropyltriethoxysilane [91]. These modified silica were loaded with Pd nanoparticles by treating with Pd(OAc) 2 and initially tested in Suzuki reaction of phenylboronic acid with 4-bromoanisole in the presence of K 2 CO 3 and with toluene as solvent.…”

Catalytic studies of palladium nanoparticles immobilized on alumina synthesized by a simple physical precipitation method

“…However, such nanoparticles require a suitable support to prevent aggregation during the reaction to be catalyzed. For applications in solutions and at moderate temperatures, polymers [12][13][14][15][16], dendrimers [17], microgels [18] and colloids [19][20][21][22] have been used as carrier systems. If the activity of the bare nanoparticles is to be studied in a quantitative manner, then the support should be totally inert.…”

Nanocatalysts for the Suzuki Coupling Reactions

“…Other study shows the formation of Pd-NPs when using Pd(OAc) 2 with organosiloxane-modified silicas containing amine functionalities ranging from primary to quaternary nitrogen centers. 37 Zhang and coworkers 38 have also characterized the formation of Pd-NPs by using Pd(OAc) 2 with an amino-modified silica. The catalyst is highly efficient in the Suzuki reaction and could be recovered and recycled by simple filtration and successfully reused for more than 15 consecutive trials without significant loss of catalytic activity.…”