Soluble Pd Species Research Articles

The formation of alluvial platinum-group minerals: present knowledge and the way ahead

AbstractThe weathering of platinum-group element (PGE) deposits presents unusual problems, especially in the very active environment of lateritic weathering under tropical conditions. There is clear evidence of the destruction of platinum-group minerals (PGM) to form PGE oxides, or fine intergrowths between relict PGM and iron oxides or hydroxides, as an intermediate stage during weathering. The PGE released by weathering are transported in solution with the more soluble Pd species remaining in solution and travelling further than the less soluble Pt species. The presence of PGM in the laterite differing in mineralogy, mineral assemblage and size from those in the primary rock is difficult to explain, especially when they show secondary textures. Differing interpretations have created controversy. Are alluvial PGM derived unaltered from the primary rock where they are rare and, therefore, not encountered by standard petrographic examination? Is it possible that they could have developed in the laterite by some process that we do not yet fully understand? Some favourable genetic conditions have been outlined and debated. For more than 100 years authors have reported secondary ore textures and recently proposed a biogenic origin. Frank Reith and his co-workers provided evidence of a process involving metallophillic bacteria which, for the first time, demonstrates PGM growth in the laboratory under supergene conditions. Their work shows that a mechanism for supergene growth (‘neoformation’) can occur, which offers a new field of study of the appropriate Eh, pH, $f_{{\rm O}_ 2}$ conditions and organic and bacterial reactions that could permit supergene growth.

Confirmation of Suzuki-Miyaura Cross-Coupling Reaction Mechanism through Synthetic Architecture of Nanocatalysts.

Despite widespread use of heterogeneous Pd catalysts in Suzuki-Miyaura coupling reactions, detailed roles of Pd, especially the nature of its active species, are still a topic of controversial debate. While some studies showed an active surface of Pd nanoparticles or nanoclusters acting heterogeneously, others claimed soluble Pd species leached from the metallic Pd to be active species which are homogeneous in nature. Besides, within the homogeneous mechanism, how the Pd leaches and promotes the cross-coupling reaction is then another question that needs to be addressed. It could be envisioned that if the soluble Pd species and solid-phase Pd are physically separated, the mechanism of Suzuki-Miyaura coupling could then be confirmed through examining the catalytic activity in different reaction regions. Herein we use microporous Stöber silica as a membrane to separate the soluble Pd species from solid Pd and conduct size-selective reactions which allow the passage of leaching Pd species, but not of reactants or products larger than the membrane aperture. With this strategy, we have been able to differentiate the surface reaction from the solution cross-coupling. We find that the leached Pd species are the only genuine catalytic intermediate in the cross-coupling reactions. We also confirm that oxidative addition of aryl halides to the solid Pd leads to leaching of the soluble Pd species which is necessary to promote Suzuki-Miyaura reactions.

Is the Suzuki-Miyaura Cross-Coupling Reaction in the Presence of Pd Nanoparticles Heterogeneously or Homogeneously Catalyzed? An Interfacial Surface-Enhanced Raman Spectroscopy Study.

Pd-catalyzed Suzuki-Miyaura C-C cross-coupling is very central in chemistry. The question of whether the catalysis by using Pd nanoparticles (NPs) is heterogeneous (on the Pd surface) or homogeneous (by soluble Pd released from the NP surface) remains under fundamental physicochemical debate. This work reports on the in situ characterization of the Suzuki-Miyaura cross-coupling reactions by using surface-enhanced Raman spectroscopy. We find clear evidence of heterogeneous catalysis on the Pd surface. In contrast, the soluble Pd species leaching into the solution cannot catalyze the reaction, indicating a direct contact of the aryl halides with the metal surface is a prerequisite. Accordingly, the surface ligands and charge of the Pd NPs, which determine the molecule-metal contact, are very important in the couplings. By a simple exchange of the surface ligand or a surface modification of the support material, the catalytic activity of Pd NPs is improved due to the enhanced electric attraction between the metal and the reactant molecules.

Self-assembled three-dimensional Pd/MoS2/reduced graphene oxide nanocatalyst: A case for homogeneous leaching mechanism

In this work, a new design of three-dimensional (3D) molybdenum disulfide -reduced graphene oxide nanosheets supported palladium (Pd/MoS2-rGO) catalyst was prepared by a facile one-pot self-assembled procedure. The existence of MoS2 not only succeeded in preventing the restacking of rGO nanosheets and increasing the specific surface area, but also afforded an additional transport platform for Pd nanoparticles to facilitate its catalytic properties. Because of the specific structural and different functional components, the as-prepared Pd/MoS2-rGO showed superior catalytic performance and reusability towards the cross-coupling reactions and the reduction of 4-nitrophenol. Moreover, it was confirmed that the catalytic nature of Pd catalyst is a kind of similar homogeneous leaching mechanism with the hot filtration test, the solid-phase poisoning and the three-phase test, etc. This means that leaching of soluble Pd species promotes the reaction process in the liquid phase, and the leaching Pd can return to the carrier of catalyst after completion of transformation. Therefore, the rational design of 3D MoS2-rGO hydrogel material supported highly active Pd nanoparticles, combined with a facile one-pot self-assembled procedure, provides a universal strategy to construct desirable 3D multifunctional nanocatalysts that can be used to research the catalytic nature of active Pd.

Single-Walled Carbon Nanotube–Polyamidoamine Dendrimer Hybrids for Heterogeneous Catalysis

We report the synthesis and catalytic properties of single-walled carbon nanotube-polyamidoamine dendrimers hybrids (SWCNT-PAMAM), prepared via a convergent strategy. The direct reaction of cystamine-based PAMAM dendrimers (generations 2.5 and 3.0) with pristine SWCNTs in refluxing toluene, followed by immobilization and reduction of [PdCl4](2-), led to the formation of highly dispersed small palladium nanoparticles homogeneously confined throughout the nanotube length. One of these functional materials proved to be an efficient catalyst in Suzuki and Heck reactions, able to promote the above processes down to 0.002 mol % showing a turnover number (TON) of 48 000 and a turnover frequency (TOF) of 566 000 h(-1). In addition, the hybrid material could be recovered and recycled for up to 6 times. No leaching of the metal has been detected during the Suzuki coupling. Additional experiments carried out on the spent catalyst permitted to suggest that a "release and catch" mechanism is operative in both reactions, although during Heck reaction small catalytically active soluble Pd species are also present.

Application of “Boomerang” Linear Polystyrene-Stabilized Pd Nanoparticles to a Series of C-C Coupling Reactions in Water

The application of a catch-and-release system for soluble Pd species between water (reaction medium) and polystyrene (polymer support) was examined in the Suzuki coupling reaction with 2-bromothiophene and the Heck reaction with styrene or bromobenzene. Although a slight increase in particle size was observed by TEM after re-stabilization of the Pd species on linear polystyrene, no agglomeration was observed.

Activation of Aryl Chlorides in Water under Phase-Transfer Agent-Free and Ligand-Free Suzuki Coupling by Heterogeneous Palladium Supported on Hybrid Mesoporous Carbon

In this study, heterogeneous palladium catalysts supported on ordered mesoporous cobalt oxide–carbon nanocomposites were applied to the water-mediated Suzuki coupling reaction of chlorobenzene and phenylboronic acid and exhibited a high yield of biphenyl (49%) under mild reaction conditions free of phase-transfer agents and ligands. Product yields in the reaction of aryl chlorides containing electron-withdrawing groups attached to their benzene ring can reach approximately 90%. Thiol-functionalized mesoporous silica, which can trap soluble Pd species, was used to confirm the negligible leaching in solution and therefore heterogeneous reaction. This heterogeneous catalyst is stable, showing unobvious activity loss after 10 catalytic runs. Characterization by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption fine structure analysis, and N2 sorption techniques revealed intercalated CoO nanoparticles inside a carbon matrix with uniform mesopore sizes (∼3 n...

Pd2(dba)3 as a Precursor of Soluble Metal Complexes and Nanoparticles: Determination of Palladium Active Species for Catalysis and Synthesis

Tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) is ubiquitously used as a source of soluble Pd species for catalysis and as a precursor in the synthesis of more complex Pd structures. In spite of the massive usage of this convenient Pd complex, its nature in solution has not been revealed in detail and the applications rely on the assumed state and purity of the compound. In the present study we have developed a convenient NMR procedure to reveal the nature of Pd2(dba)3 and to determine the purity of the complex. Surprisingly, it was found that commercially available samples of Pd2(dba)3 may readily contain up to 40% of Pd nanoparticles in a wide range of sizes (10–200 nm). The study has shown that the routinely accepted practice of utilization of Pd2(dba)3 without analysis of the purity (both commercially available and prepared by common procedures) can introduce significant errors in the estimation of catalyst efficiency and lead to incorrect values of TON, TOF, and reported mol % values in the catalytic procedures. The presence of Pd nanoparticles in the catalyst precursor provides an opportunity for heterogeneous catalytic systems of different nature to be directly accessible from Pd2(dba)3. In the present study we report a modified procedure for the synthesis of Pd2(dba)3·CHCl3 with 99% purity.

A Study on Applications of N-Substituted Main-Chain NHC-Palladium Polymers as Recyclable Self-Supported Catalysts for the Suzuki–Miyaura Coupling of Aryl Chlorides in Water

The preparation and characterization of a number of main-chain organometallic polymers (NHC-Pd MCOP) with different N-alkyl substituted groups such as benzyl (3a), n-hexyl (3b), and n-dodecyl (3c) are described. Among these polymers, 3c bearing the more lipophilic group n-dodecyl was found to be a more reactive and recoverable catalytic system in the Suzuki-Miyaura cross-coupling reaction of chloroarenes, including both deactivated and hindered aryl chlorides with different types of arylboronic acids under aqueous conditions. While the catalysts seem to be highly recyclable, on the contrary, we have provided much compelling evidence, such as kinetic monitoring, poisoning experiments, and average molecular weight determination before and after catalysis, that shows that the described organometallic polymers might be indeed the source of production of active soluble Pd species in the form of either Pd nanoparticles or fragmented NHC-Pd complexes. Our studies showed that in order to assess whether the catalysts are functioning in a heterogeneous pathway or they are simply a source of production of active Pd species, it is crucial to devise a suitable and highly efficient poison that could capture essentially soluble catalytic species. In this regard, we interestingly found that among a variety of well-known catalyst poisons such as Hg(0), SBA-15-PrSH, and cross-linked poly(4-vinylpyridine) (PVP), only PVP could efficiently quench catalysis, thus providing clear evidence of the formation soluble Pd species in our protocol. In addition, several experiments such as bright-field microscopy, dynamic light scattering (DLS) of the reaction mixture, and kinetic monitoring of the reaction at an early stage confirm not only that the described organometallic polymers could be a source of production of trace amounts of Pd nanoparticles but the capsular structures of these lipophilic polymers in water provides a means of entrapment of nanoclusters in a hydrophobic region, thus accelerating the reaction in pure water in the absence of any co-organic solvent.

Identification of the Active Species Generated from Supported Pd Catalysts in Heck Reactions: An in situ Quick Scanning EXAFS Investigation

Quick scanning extended X-ray absorption fine structure (QEXAFS) studies in the subsecond time scale have been performed to gain insight into the reaction mechanism of Heck-type C-C coupling reactions in the presence of supported Pd-based catalysts. Using a specially designed in situ EXAFS cell, both the solid catalyst and the liquid reaction mixture during the reaction of phenyl bromide (PhBr) with styrene were monitored. Soluble Pd species were only, but rapidly, detected in the liquid reaction phase once the reaction temperature of 150 °C was reached. At the same time, the conversion of PhBr started, and during the following "active phase" of the catalyst hardly any changes in the corresponding EXAFS and XANES spectra were observed. The present species could be identified as colloidal Pd(0) clusters with a size of ∼2 nm estimated from the corresponding EXAFS spectra. The QEXAFS mode not only allowed monitoring rapid changes in the second time scale but also permitted minimization of effects caused by the heterogeneity of the systems. When the reaction rate started to decrease, pronounced changes in the EXAFS spectra were observed, which were attributed to an increased formation of bromo-palladates ([PdBr(4)](2-), [Pd(2)Br(6)](2-)). In addition to the liquid-phase species, significant changes were observed for the solid catalyst that was also probed in situ during the reaction. The originally oxidized Pd catalyst was efficiently reduced upon heating. Additionally, growth of the supported Pd particles was observed by both EXAFS and STEM. The above results confirm the role of the soluble molecular Pd species as the catalytically active species and clarify their conjunction with the in situ formed Pd colloids. Furthermore, the investigation demonstrates the potential of the QEXAFS not only for monitoring rapid changes during catalysis but also for gaining deeper insight into the mechanism of such complex industrially important systems under relevant reaction conditions.

Ordered Mesoporous Organosilica with Ionic‐Liquid Framework: An Efficient and Reusable Support for the Palladium‐Catalyzed Suzuki–Miyaura Coupling Reaction in Water

The preparation of a novel palladium-supported periodic mesoporous organosilica based on alkylimidazolium ionic liquid (Pd@PMO-IL) in which imidazolium ionic liquid is uniformly distributed in the silica mesoporous framework is described. Both Pd@PMO-IL and the parent PMO-IL were characterized by N(2)-adsorption-desorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), TEM, and solid-state NMR spectroscopy. We have demonstrated that Pd@PMO-IL is an efficient and reusable catalyst for the Suzuki-Miyaura coupling reaction of various types of iodo-, bromo-, and even deactivated aryl chlorides in water. It was also found that although the PMO-IL nanostructure acts as reservoir for soluble Pd species, it can also operate as a nanoscaffold to recapture the Pd nanoparticles into the mesochannels thus preventing extensive agglomeration of Pd. This observation might be attributed to the isolated ionic liquid units that effectively control the reaction mechanism by preventing Pd agglomeration and releasing and recapturing Pd nanoparticles during the reaction process. The catalyst can be recovered and reused for at least four reaction cycles without significant loss of activity.

Preparation of a nonleaching, recoverable and recyclable palladium-complex catalyst for Heck coupling reactions by immobilization on Au nanoparticles

The loading of a palladium complex immobilized on the surface of Au nanoparticles (NPs) was controlled through coordination to a supported spacer ligand (SL = S(CH 2) 11NHP(O)(2-py) 2) to yield catalyst particles of the composition (RS) x Au(SL) y (SL–PdCl 2) z (SR = S(CH 2) 7CH 3; x: y: z = (0.31–2.3):1:(0.45–3.3)) with diameters of 3.1–4.9 nm. A fraction of the supported spacer ligands (23–69%) was left uncapped to capture any soluble Pd species resulting from leaching. These surface-bound Pd(II)-complex catalysts were highly effective for Heck reactions of iodobenzene and alkylacrylates, yielding a maximum turnover frequency (TOF) of 4.87 × 10 4 h 1. Their catalytic activity was three- to tenfold higher than that of their unbound counterparts. These hybrid catalysts could be dissolved and precipitated. They could be quantitatively recovered and effectively recycled 15 times without significant loss of reactivity. The recovered Au NP-supported palladium could be also dissolved, precipitated and isolated. Various spectroscopic analyses were performed to determine a surface-structure composition of (RS) x Au(SL) 1(SL–Pd 0) 0.53.

Pd catalysts immobilized onto gel-supported ionic liquid-like phases ( g-SILLPs): A remarkable effect of the nature of the support

New polymeric materials containing functional sites analogous to ionic liquids have been synthesized. Those materials are a supported reservoir for the active Pd species. Their catalytic activity has been evaluated for the Heck reaction proving an excellent performance in terms of both activity and recyclability. Although soluble Pd species seem to participate in the catalytic cycle, as for many other supported Pd-systems, the g-SILLPs present the ability to efficiently release and recapture those soluble species. This allows to dramatically reduce or eliminate the amount of Pd leached to the final solution, in particular at higher temperatures, and opens the way, based on a release and catch strategy, for the development of active-supported Pd catalytic systems, easily recoverable and reusable for a large number of catalytic cycles. The exact nature of the polymer (SILLPs) has a remarkable influence on the overall process. The appropriate design of the g-SILLPs is a key factor for the optimization of the release and catch system. The functionalized polymers prepared can be reused for a significant number of catalytic cycles without any loss in performance.

Pd/MOx Materials Synthesized by Sol‐Gel Coprecipitation as Catalysts for Carbon–Carbon Coupling Reactions of Aryl Bromides and Chlorides

AbstractIn order to control the interaction of palladium species with oxide supports, a sol‐gel coprecipitation synthesis has been developed for Pd/MOx. The resulting Pd/MOx catalysts are characterized by a rather strong interaction between the highly dispersed Pd2+ species and the alumina and silica lattices. During coupling reactions of the Heck type the catalytically active species is generated by partial dissolution of Pd from the support surface. Due to the strong bonding of Pd to the support this dissolution occurs to significant extent only at comparatively high temperatures (≥160 °C). These temperatures are exactly necessary for the activation of less‐reactive substrates like bromobenzene and aryl chlorides. In this way we are able to synthesize supported catalysts that facilitate a controlled release of soluble Pd species under reaction conditions that are required for the activation of aryl bromides and chlorides. The Pd/MOx catalysts show excellent activity in the Heck reaction of bromobenzene (TON up to 10000; TOF up to 5000 h–1) and 4‐chloroacetophenone (TON up to 9300; TOF up to 1550 h–1).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)