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

Abstract

A ligand-free Pd catalyst system derived from Pd(acac) 2 /SiO 2 and its catalytic properties toward the Heck coupling of bromobenzene and styrene have been studied. Combined studies by X-ray photoelectron spectroscopy (XPS), ultraviolet and visible absorption spectroscopy (UV-vis), thermogravimetric analysis (TGA) and powder X-ray diffraction (XRD) indicate that calcination of Pd(acac) 2 /SiO 2 in air at 400℃ produces Pd II /SiO 2 that contains isolated Pd 2+ ions coordinated to the lattice oxygen as the dominant component and PdO particles as the minor component. XPS and UV-vis observations together with catalytic results reveal that most of Pd II is reduced to Pd 0 on SiO 2 by the reaction media during the Heck reaction and that the resulting catalyst system is highly active. Transmission electron microscopy measurements show that the in situ prepared Pd 0 /SiO 2 has much finer Pd particles than conventional Pd 0 /SiO 2 . The rapid growth of the supported Pd particles during the highly efficient Heck reaction suggests that the catalysis occurs via an Ostwald ripening mechanism. Resulting in excellent catalytic properties, Pd II /SiO 2 possesses incomparable advantages over conventional Pd 0 /SiO 2 as a precatalyst in catalytic activity, catalyst recycling and product contamination control.

Synthesis of 4-Methoxybiphenyl Using Pd-Containing Catalysts Based on Polymeric Matrix of Functionalized Hypercrosslinked Polystyrene

“…Water is an environmentally friendly solvent; besides, polar solvents are preferable due to the possibility of enhancing of the catalyst stability [13]. However the rate of cross-coupling in pure water is extremely low in comparison with organic solvent-water mixtures [26,27]. This fact can be explained by insufficient solubility of substrates in water and also by the difficulty of Pd(II) reduction that is an essential step of the Suzuki reaction mechanism.…”

Au Core–Pd Shell Bimetallic Nanoparticles Immobilized within Hyper-Cross-Linked Polystyrene for Mechanistic Study of Suzuki Cross-Coupling: Homogeneous or Heterogeneous Catalysis?

“…However, they cannot be reused, resulting in a loss of expensive Pd metal and organic ligands . In the last decades, great attention was given to so-called ligandless palladium catalysts based on zeolites, , metal–organic frameworks, , metal oxides, − resins, , activated carbon, ,, carbon nanotubes, and graphenes . Their main advantage is easy separation from the reaction mixture and possibility of reuse .…”

Characterization of palladium species after γ-irradiation of a TBP–alkane–Pd(NO3)2system

“…The reaction between 25 C and 110 C, and between 110 C and 350 C can be attributed to water vaporization 63 and release of organic functions, 67,68 respectively. The weight loss between 350 C and 650 C may correspond to the release of (CN) 2(g) .…”