Reaction Conditions‐Dependent Formation of Catalytically Active Palladium Complexes or Palladium Nanoparticles on a Silica Support (original) (raw)
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ACS Omega
The organic moiety plays an essential role in the design of homogeneous catalysts, where the ligands are used to tune the catalytic activity, selectivity, and stability of the transition metal centers. The impact of ligands on the catalytic performance of metal nanoparticle catalysts is still less understood. Here, we prepared supported nanoparticle (NP) catalysts by the immobilization of preformed Pd NPs on the ligand-modified silica surfaces bearing amine, ethylenediamine, and diethylenetriamine groups. After excluding any size effect, we were able to study the influence of the ligands grafted on the support surface on the catalytic activity of the supported nanoparticles. Higher activity was observed for the Pd NPs supported on propylaminefunctionalized support, whereas the presence of ethylenediamine and diethylenetriamine groups was detrimental to the activity. Upon the addition of excess of these amine ligands as surface modifiers, the hydrogenation of alkene to alkane was fully suppressed and, therefore, we were able to tune Pd selectivity. The selective hydrogenation of alkynes into alkenes, although a considerable challenge on the traditional palladium catalysts, was achieved here for a range of alkynes by combining Pd NPs and amine ligands.
Molecules, 2015
An insight into the nano-and micro-structural morphology of a polymer supported Pd catalyst employed in different catalytic reactions under green conditions is reported. The pre-catalyst was obtained by copolymerization of the metal-containing monomer Pd(AAEMA)2 [AAEMA − = deprotonated form of 2-(acetoacetoxy) ethyl methacrylate] with ethyl methacrylate as co-monomer, and ethylene glycol dimethacrylate as cross-linker. This material was used in water for the Suzuki-Miyaura cross-coupling of aryl bromides, and for the reduction of nitroarenes and quinolines using NaBH4 or H2, as reductants. TEM analyses showed that in all cases the pristine Pd(II) species were reduced in situ to Pd(0), which formed metal nanoparticles (NPs, the real active species). The dependence of their average size (2-10 nm) and morphology on different parameters (temperature, reducing agent, presence of a phase transfer agent) is discussed. TEM and micro-IR analyses showed that the polymeric support retained its porosity and stability for several catalytic cycles in all reactions and Pd NPs did not aggregate after reuse. The metal nanoparticle distribution throughout the polymer matrix after several recycles provided precious information about
2018
New classes of N-heterocyclic carbene-palladium(II) complexes containing either a vinyl group in different positions or an anthracene tag in the backbone of the N-heterocycle were synthesized and fully characterized. The vinyl based complexes were copolymerized with divinylbenzene to fabricate robust polymer-supported NHC-Pd(II) systems, while the anthracene based complexes were π-stacked onto the surface of reduced graphene oxide (rGO) producing immobilized catalysts with interesting features. Advanced solid-state characterization techniques focused on the structure and oxidation state of the Pd(II) center of the all the supported complexes and showed the presence of Pd(II) species in the native catalysts without any trace of Pd(0) nanoparticles, confirming the success of the preparation pathways. The supported catalysts were applied as heterogeneous catalysts in C-H activation of arenes using different methodologies. In the undirected C-H acetoxylation of simple, methylated, and p...
Dissimilar catalytic behavior of molecular or colloidal palladium systems with a new NHC ligand
Dalton transactions (Cambridge, England : 2003), 2017
In this work, we describe the synthesis of a new N-heterocyclic carbene (NHC) ligand, derived from a hybrid pyrazole-imidazolium scaffold, namely 1-[2-(3,5-dimethylpyrazol-1-yl)ethyl]-3-((S)-1-phenylethyl)-3H-imidazol-2-ylidene (L). This ligand has been used as a stabilizer for the organometallic synthesis of palladium(0) nanoparticles (Pd NPs). L presents a better stabilizing effect than its pre-carbenic HLCl counterpart, allowing the formation of isolated Pd NPs while HLCl yields aggregated ones. Additionally, molecular Pd(ii) coordination compounds of L and HLCl were synthesized and characterized to better understand the coordination modes of these ligands. Both molecular and colloidal Pd systems have been further tested in catalytic C-C coupling processes. Three different types of reactions have been observed depending on the catalytic system: (i) the Suzuki-Miyaura reaction takes place with Pd molecular complexes; (ii) a secondary reaction, the dehalogenation of the substrate, ...
Shape-dependent catalytic activity of palladium nanoparticles embedded in SiO2 and TiO2
Catalysis Today, 2012
Catalytic studies have shown that the activity of metal nanoparticles depends on their size and shape. Palladium nanoparticles with different morphologies have been prepared and embedded in a SiO 2 and TiO 2 matrix, and their catalytic performance in the solvent-free aerobic oxidation of 1-phenylethanol has been studied. The observed activity is strongly related to the exposed surface sites, being some of the active sites embedded in the matrix or blocked by residual organic species. (1 1 1)/(1 0 0) edge sites are involved in the oxidative dehydrogenation of 1-phenylethanol. The amount of these sites depends on particle morphology. Under our conditions, Pd bars -SiO 2 (Pd nanobars embedded in SiO 2 ) were found to be the most active.
Metals
In the heterogeneous catalyst i-Pd(OAc)2(TPPTS)2, Pd(II) was reduced to Pd(0) by using different alcohol solvents, and the catalyst’s activity was studied in the aerobic oxidation of benzyl alcohol. We studied the effects of the impregnation time in ethanol as a solvent and the use of various alcoholic solvents on the size of palladium nanoparticles. We found that the reduction of palladium by the various alcohols yielded palladium nanoparticles that were active in the aerobic oxidation of benzyl alcohol. As determined by DLS, TEM, and zeta potential analyses, both the impregnation time in ethanol and the type of alcohol used were observed to affect nanoparticle formation, particle size distribution, and agglomeration, as well as the conversion rate. The palladium nanoparticles’ hydrodynamic diameter sizes obtained during the 24 h of impregnation time were in the range of 10–200 nm. However, following 24 h of impregnation in ethanol the nanoparticles tended to form aggregates. The c...
Journal of Nanostructures, 2019
An efficient procedure and green route has been developed by using eco-friendly green synthesized palladium nanoparticles (Pd-NPs) as reusable heterogeneous nanocatalyst (Pd@nanocat) for the synthesis of diaryl ethers from the cross coupling reaction of the aryl halides with the phenol in the presence of dimethylsulfoxide (DMSO) as a solvent at 110 oC under natural ligand condition capped Pd-NPs. The generated Pd-NPs were characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) techniques. Structural characterization of diaryl ether was done by nuclear magnetic resonance spectroscopy (NMR). The nanoparticles exhibited stability, reusability with excellent catalytic activity and the results of this study demonstrate the possible application of biosynthesized Pd-NPs as heterogeneous nanocatalyst in environmental remediation. Application of the cheaper base, shorter reaction time, cost-effective, high yield and environme...