Formation Of Palladium Nanoparticles Research Articles

Pincer‐Type Heck Catalysts and Mechanisms Based on PdIV Intermediates: A Computational Study

Pincer-type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain Pd(II) centers, controversial Pd(II)/Pd(IV) cycles have been often proposed as potential catalytic mechanisms. However, pincer-type Pd(IV) intermediates have never been experimentally observed, and computational studies to support the proposed Pd(II)/Pd(IV) mechanisms with pincer-type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving Pd(IV) intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine-, phosphine-, and phosphite-based pincer-type Heck catalysts with styrene and phenyl bromide as substrates and (E)-stilbene as coupling product. The potential-energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that Pd(II)/Pd(IV) mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)] [X=NH, R=piperidinyl (1 a); X=O, R=piperidinyl (1 b); X=O, R=iPr (1 c); X=CH(2), R=iPr (1 d)] to yield cationic, three-coordinate, T-shaped 14e(-) palladium intermediates of type [{2,6-C(6)H(3)(XPR(2))(2)}Pd](+) (2). An alternative reaction path to generate complexes of type 2 (relevant for electron-poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Cl)(CH(2)=CHPh)] (4) and consecutive dissociation of the chloride ligand to yield cationic square-planar styrene complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(CH(2)=CHPh)](+) (6) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate Pd(IV) complexes of type [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))](+) (11), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(C(6)H(5))(CH(2)=CHPh)](+) (12). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes [{2,6-C(6)H(3)(XPR(2))(2)}Pd(Br)(CHPhCH(2)Ph)](+) (13). Subsequent beta-hydride elimination induces direct HBr liberation to yield cationic, square-planar (E)-stilbene complexes with general formula [{2,6-C(6)H(3)(XPR(2))(2)}Pd(CHPh=CHPh)](+) (14). Subsequent liberation of (E)-stilbene closes the catalytic cycle.

The biosynthesis of palladium nanoparticles by antioxidantsin Gardenia jasminoides Ellis: long lifetime nanocatalysts forp-nitrotoluene hydrogenation

Gardenia jasminoides Ellis’ water crude extract was used for the bioreductionof palladium chloride in this paper. The UV–vis spectrum, x-ray diffractionspectrum measurement, the Fourier transform infrared spectroscopy and TEMtechnique confirmed the formation of palladium nanoparticles and identifiedantioxidants including geniposide, chlorogenic acid, crocins and crocetin werereducing and stabilizing agents for synthesizing palladium nanoparticles in watercrude extract. The particle size and dispersity were temperature-dependent.The particle sizes ranged from 3 to 5 nm and revealed the best dispersity at70 °C. Catalyticperformance of the biosynthetic Pd nanoparticles with good dispersity was investigated by hydrogenationof p-nitrotoluene. The catalysts showed a conversion of 100% under conditions of 5 MPa,150 °C for 2 h. Theselectivity of p-methyl-cyclohexylamine achieved 26.3%. The catalyst was recycled five times with noagglomeration and maintained activity, which was attributed to the appropriateprotection of the antioxidants. On the basis of the study, it appears to be a newpromising biosynthetic nanocatalyst for the development of an industrial process.

Effect of composition of sucrose fatty acid esters on formation of palladium nanoparticles in reverse micelles

Palladium (Pd) nanoparticles were prepared in the reverse micellar system containing sucrose fatty acid esters with various esterification degrees. The TEM showed that Pd nanoparticles were of spherical and relatively uniform. The size of Pd nanoparticles strongly depended upon the composition of sucrose fatty acid esters. The resultant Pd colloid could be preserved for at least 2 months without precipitation.

Palladium nanoparticle seeded electroless metallization of Al/Al 2O 3 surfaces

A simple method for the formation of palladium nanoparticles on aluminum and aluminum oxide surface is demonstrated. In the present method, the palladium nanoparticles obtained directly on the solid surface by immersing the specimen in palladium(II)acetate solution followed by reduction to metallic palladium by using sodium hypophosphite. In the investigation, as-received, boiled, electropolished and anodized aluminum was used as substrate. Additionally, the method is combined with the electroless nickel plating, which facilitates two steps metallization technique on the respective surfaces. In these cases, specimens were first immersed in palladium(II)acetate solution followed by immersion in electroless nickel plating solution, where, palladium reduced on the specimen surface providing catalytic site for the subsequent electroless nickel deposition. Using the technique, about 6 to 8.5 µm thick Ni–P metallic layer was deposited on the specimen surface.

Recoverable Palladium Catalysts for Suzuki–Miyaura Cross‐ Coupling Reactions Based on Organic‐Inorganic Hybrid Silica Materials Containing Imidazolium and Dihydroimidazolium Salts

AbstractThe systems formed by palladium acetate [Pd(OAc)2] and hybrid silica materials prepared by sol‐gel from monosilylated imidazolium and disilylated dihydroimidazolium salts show catalytic activity in Suzuki–Miyaura cross‐couplings with challenging aryl bromides and chlorides. They are very efficient as recoverable catalysts with aryl bromides. Recycling is also possible with aryl chlorides, although with lower conversions. In situ formation of palladium nanoparticles has been observed in recycling experiments.

Reduction of PdCl2 by Emeraldine Base; Synthesis of Palladium Nanoparticles

In situ reduction of PdCl2by Emeraldine base (EB) of polyaniline leads to formation of palladium nanoparticles in N-methyl-2-pyrrolidone (NMP) and N′, N-dimethylformamide (DMF). So-generated nanoparticles of Pd in polyaniline (PAni), in its UV-visible absorption spectra, showed λ(max) at about 320 nm and 420 nm due to polyaniline along with complete suppression of band at 620 nm initially present in Emeraldine base (EB). X-ray diffraction pattern (XRD) of powder showed broad peaks for nanosized Pd and the particle size from Scherrer's equation was found to be less than 15 nm. Scanning electron microscopy (SEM) showed that the composite has a granular and/or fluffy agglomerated morphology. The elemental composition was studied by EDAX.

Gas phase hydrogenation reaction using a ‘metal nanoparticle–polymer’ composite catalyst

A facile synthesis route is described here for the preparation of a poly (anthranilic acid)-palladium nanoparticle composite material by polymerization of anthranilic acid (AA) monomer using palladium acetate (PA) as the oxidant. It was found that oxidative polymerization of AA leads to the formation of poly-AA (PAA), while the reduction of PA results in the formation of palladium nanoparticles with an average size of ~2 nm. The palladium nanoparticles were uniformly dispersed and highly stabilized within the macromolecular matrix resulting in a uniform metal–polymer composite material. The resultant composite material was characterized by means of different techniques, such as IR and Raman spectroscopy, which yielded information about the chemical structure of polymer, whereas electron microscopy images provided information concerning the morphology of the composite material and the distribution of the metal particles in the composite material. The composite material was tested as a catalyst for ethylene hydrogenation reaction and showed a catalytic activity at higher temperatures.

Reduction of PdCl2 by Emeraldine Base; Synthesis of Palladium Nano-Particles

In-situ reduction of PdCl2 by Emeraldine base (EB) of polyaniline leads to formation of palladium nano-particles in N-methyl-2-pyrrolidone (NMP) and N',N-dimethylformamide (DMF). So-generated nanoparticles of Pd in polyaniline (PAni), in its UV-visible absorption spectra, showed λ(max) at about 320 nm and 420 nm due to polyaniline along with complete suppression of band at 620 nm initially present in Emeraldine base (EB). X-ray diffraction pattern (XRD) of powder showed broad peaks for nano-sized Pd, and the particle size from Scherrer's equation was found to be less than 15 nm. Scanning electron microscopy (SEM) showed that the composite has granular and/ or fluffy agglomerated morphology. The elemental composition was studied by EDAX.

Reductive dechlorination of 3,3′,4,4′-tetrachlorobiphenyl (PCB77) using palladium or palladium/iron nanoparticles and assessment of the reduction in toxic potency in vascular endothelial cells

Palladium-based nanoparticles immobilized in polymeric matrices were applied to the reductive dechlorination of 3,3',4,4'-tetrachlorobiphenyl (PCB77) at room temperature. Two different dechlorination platforms were evaluated using (1) Pd nanoparticles within conductive polypyrrole films; or (2) immobilized Fe/Pd nanoparticles within polyvinylidene fluoride microfiltration membranes. For the first approach, the polypyrrole film was electrochemically formed in the presence of perchlorate ions that were incorporated into the film to counter-balance the positive charges of the polypyrrole chain. The film was then incubated in a solution containing tetrachloropalladate ions, which were exchanged with the perchlorate ions within the film. During this exchange, reduction of tetrachloropalladate by polypyrrole occurred, which led to the formation of palladium nanoparticles within the film. For the second approach, the membrane-supported Fe/Pd nanoparticles were prepared in three steps: polymerization of acrylic acid in polyvinylidene fluoride microfiltration membrane pores was followed by ion exchange of Fe(2+), and then chemical reduction of the ferrous ions bound to the carboxylate groups. The membrane-supported iron nanoparticles were then soaked in a solution of tetrachloropalladate resulting in the deposition of Pd on the Fe surface. The nanoparticles prepared by both approaches were employed in the dechlorination of PCB77. The presence of hydrogen was required when the monometallic Pd nanoparticles were employed. The results indicate the removal of chlorine atoms from PCB77, which led to the formation of lower chlorinated intermediates and ultimately biphenyl. Toxicity associated with vascular dysfunction by PCB77 and biphenyl was compared using cultured endothelial cells. The data strongly suggest that the dechlorination system used in this study markedly reduced the proinflammatory activity of PCB77, a persistent organic pollutant.

Aerobic Ligand‐Free Suzuki Coupling Reaction of Aryl Chlorides Catalyzed by In Situ Generated Palladium Nanoparticles at Room Temperature

AbstractAn aerobic, ligand‐free Suzuki coupling reaction catalyzed by in situ generated palladium nanoparticles in polyethylene glycol with an average molecular weight of 400 Da (PEG‐400) at room temperature has been developed. This catalytic system is a very simple and highly active protocol for the Suzuki coupling of aryl chlorides with arylboronic acids, which proceed smoothly in excellent yields in short times using low catalyst loadings. Control experiments demonstrated that the Suzuki reaction catalyzed by the in situ generated palladium nanoparticles can be carried out much quicker than that using the preprepared particles under the same conditions. The formation of palladium nanoparticles in PEG‐400 was promoted by arylboronic acids.

Kinetic- or Thermodynamic-Growth Formation of Pd Nanocrystals: a combined TEM-DLS Approach

AbstractThe kinetics of formation of palladium nanoparticles with well-defined morphologies (rods, cubes, MTP icosahedra, bipyramids) was studied using a combination of TEM and dynamic light scattering (DLS). The present study was focused on the role of the concentration and the size of seeds on both the kinetics of growth and the morphology distribution. Results clearly emphasize the role of the seed concentration (and not the size) on the growth kinetics in agreement with a collision theory model.

Stabilization of Palladium Nanoparticles by Polyoxometalates Appended with Alkylthiol Tethers and their Use as Binary Catalysts for Liquid Phase Aerobic Oxydehydrogenation

A polyoxometalate with two alkyl thiol appendages, Q4[SiW11O40(SiCH2CH2CH2SH)2] (Q=ammonium salt) stabilized the formation of palladium nanoparticles. This tethered polyoxometalate–palladium binary catalyst was effective for the aerobic oxydehydrogenation of vinylcyclohexene and vinylcyclohexane to styrene as the major product via activation of the allylic, tertiary carbon-hydrogen bond.

A combined small-angle scattering study of a chemical reaction at specific sites and reaction-induced self-assembly as a problem in open non-equilibrium phenomena

As a problem in open non-equilibrium phenomena, small-angle scattering (SAS) studies of chemical reactions at specific sites and reaction-induced self-assembly of a system which is obtained by mixing two stable solutions of palladium acetate [Pd(OAc)2] in N,N-dimethylformamide and the second-generation polyamidoamine dendrimer in methanol are presented. The self-assembly was studied using a combination of neutron and X-ray SAS. The results revealed that the self-assembly involves the initial formation of aggregates of an average radius of 20 nm composed of the dendrimers and Pd(OAc)2 followed by formation of palladium nanoparticles of a radius of 2.0 nm inside the aggregates. The aggregates were found to provide a special field for a chemical reaction for reduction of Pd(II) ions with methanol and for the self-assembly of the reduction products of Pd(0) atoms into nanoparticles. The nanoparticles are found to be trapped and stabilized in the aggregates.

Formation of palladium nanoparticles in olefin oxidation with iron(III) aqua ions in the presence of the Pd/ZrO2/SO4 metallic catalyst

The reactions of the Pd/ZrO2/SO4-catalyzed oxidation of ethylene, propene, and but-1-ene in a 0.1–1.5 M solution of perchloric acid with iron(III) aqua ions to carbonyl compounds, viz., acetaldehyde, acetone, and methyl ethyl ketone, respectively, were studied. The formation of palladium nanoparticles (5 nm) in solution on contact of the initial heterogeneous Pd/ZrO2/SO4 catalyst with perchloric acid was proved by transmission electron microscopy. The palladium nanoparticles are assumed to play the key role in olefin oxidation with the iron(III) aqua ions.

Calorimetric investigation on the formation of palladium nanoparticles in water/AOT/ n-heptane microemulsions

The formation enthalpy of palladium nanoparticles in water/sodium bis(2-ethylhexyl) sulfosuccinate ( AOT) n- heptane microemulsions as a function of the water AOT molar ratio ( R = [ water] [ AOT] ) was measured by a calorimetric technique. The results indicate that at R < 10 the energetic state of the palladium nanoparticles compartmentalized within the reversed AOT micelles is signficantly different from that in bulk water. Effects due to the small size of the palladium nanoparticles and to interactions between nanoparticles and the water AOT interface are discussed.