Membrane‐Occluded Gold‐Palladium Nanoclusters as Heterogeneous Catalysts for the Selective Oxidation of Alcohols to Carbonyl Compounds

Pre‐formed polyvinylpyrrolidone‐stabilized gold‐palladium clusters, consisting of 80 mol% gold and with a mean size of 1.9 nm, were immobilized quantitatively in a porous polyimide membrane via the process of phase inversion, without loss of metal nanodispersion. The obtained gold‐palladium/polyimide membrane emerged as a highly active heterogeneous metal catalyst for the amide‐phase and solvent‐free oxidation of aliphatic, allylic and benzylic alcohols with full selectivity to the corresponding aldehydes and ketones, and could be recycled with excellently preserved catalytic activity and product selectivity. Occlusion of the optimized bimetallic clusters in the polyimide structure proved beneficial in view of their superior catalytic performance compared to the analogous colloidal gold‐palladium clusters.     

On the Nature of the Active Species in Palladium Catalyzed Mizoroki–Heck and Suzuki–Miyaura Couplings – Homogeneous or Heterogeneous Catalysis,A Critical Review

A wide array of forms of palladium has been utilized as precatalysts for Heck and Suzuki coupling reactions over the last 15 years. Historically, nearly every form of palladium used has been described as the active catalytic species. However, recent research has begun to shed light on the in situ transformations that many palladium precatalysts undergo during and before the catalytic reaction, and there are now many suggestions in the literature that narrow the scope of types of palladium that may be considered true “catalysts” in these coupling reactions. In this work, for each type of precatalyst, the recent literature is summarized and the type(s) of palladium that are proposed to be truly active are enumerated. All forms of palladium, including discrete soluble palladium complexes, solid‐supported metal ligand complexes, supported palladium nano‐ and macroparticles, soluble palladium nanoparticles, soluble ligand‐free palladium, and palladium‐exchanged oxides are considered and reviewed here. A considerable focus is placed on solid precatalysts and on evidence for and against catalysis by solid surfaces vs. soluble species when starting with various precatalysts. The review closes with a critical overview of various control experiments or tests that have been used by authors to assess the homogeneity or heterogeneity of catalyst systems.     

Investigation of palladium interaction with cerium oxide and its state in catalysts for low-temperature CO oxidation

Palladium catalysts supported on nanosized CeO₂ supports were synthesized by different methods. The catalysts showed high low-temperature activity (LTA) in CO oxidation. The synthesized palladium–ceria catalysts for low-temperature CO oxidation were investigated by a complex of physicochemical methods, and their catalytic performance was determined in the light-off regime. It was shown using high-resolution transmission electron microscopy (HRTEM) and EDX microanalysis that the catalysts with high LTA are characterized by exceptionally high dispersity of palladium on the surface of the supports. Two different states of palladium were observed by XPS. They correspond to the surface interaction phases (SIPs) as Pd_xCeO_2−δ and small metal clusters (<10 Å). According to diffraction images obtained by HRTEM, the latter have flattened shape due to epitaxial binding between (1 1 1) facets of palladium and CeO₂. Two types of CO adsorption sites (Pd²⁺ and Pd⁰) were distinguished by FTIR. They can be attributed to SIP (Pd²⁺) and palladium in flat metal clusters (Pd^δ+ and Pd⁰). The drop of LTA in CO oxidation is related to the loss of the palladium chemical interaction with the surface of the support and palladium sintering to form PdO nanoparticles. The formation of PdO particles is stimulated by crystallization of CeO₂ particle surface due to the calcination of support at temperatures above 600 °C. The XPS, HRTEM and FTIR data give reliable evidence that PdO particles are not responsible for LTA in CO oxidation.In this work, the structure of the active sites consisting of two phases: atomically dispersed palladium within the SIP and palladium metal nanoclusters is proposed. The catalyst pretreatment in hydrogen was found to improve significantly its catalytic (LTA) properties. The effect of the hydrogen pretreatment was supposed to be related to the formation of hydroxyl groups and their effect on the electronic and geometrical state of the surface active sites and their possible direct participation in CO oxidation.     

Catalytic behaviour of Pt or Pd metal nanoparticles–zeolite bifunctional catalysts for n-pentane hydroisomerization

Bifunctional catalysts based on acidified Mordenite or ZSM-5 and platinum or palladium as metal function, were tested for n-pentane hydroisomerization. Two methods were used to introduce platinum: wetness impregnation and microemulsion; palladium was introduced via an organometallic complex. A lower catalytic activity was obtained for palladium catalyst in comparison with platinum samples which is explained by the lower activity of Pd in the dehydrogenation reaction step. Different catalytic behaviour of systems based on Mordenite and ZSM-5 was attributed to zeolite pore structure. The uncompleted removal of surfactant during calcination could explain the lower activity showed by catalysts prepared by microemulsion.     

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

The 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.     

Membrane reactor immobilized with palladium‐loaded polymer nanogel for continuous‐flow Suzuki coupling reaction

A catalytic membrane reactor, which was immobilized with palladium‐loaded nanogel particles (NPs), was developed for continuous‐flow Suzuki coupling reaction. Palladium‐loaded membranes were prepared by immobilization of NPs, adsorption of palladium ions, and reduction into palladium(0). The presence of palladium in the membrane was confirmed by the scanning electron microscopy; palladium aggregation was not observed. The catalytic activity of the membrane reactor in continuous‐flow Suzuki coupling reaction was approximately double that of a comparable reactor in which palladium ions were directly adsorbed onto an aminated membrane. This was attributed to the formation of small palladium particles. The reusability in the continuous‐flow system was higher than that in a batch system, and the palladium‐loaded membrane reactor had high long‐term stability. © 2014 American Institute of Chemical Engineers AIChE J, 61: 582–589, 2015     

Palladium nanocatalysts protected by polyacids

Several colloidal palladium nanocatalysts prepared by the in situ reduction of palladium chloride PdCl₂, ammonium tetrachloropalladate (NH₄)₂PdCl₄, and palladium acetate Pd(CH₃COO)₂ were protected by various water-soluble polymers, with special emphasis on polyacids. The particle sizes, morphologies, and size distributions of the palladium nanoparticles were determined by transmission electron microscopy (TEM), and their catalytic activities were qualitatively tested by the hydrogenation of cyclohexene. The type of the polymer (for example, polyacid versus a nonionic, water-soluble polymer) can influence the nanoparticle sizes and morphologies, as well as colloidal stabilities. For the catalytic activities of these metal–polymer systems, the choice of the protective polymer can be equally important. Lower catalytic activities have been mostly found if polyacids were used as protective matrices for these palladium nanocatalysts. It was found to be important to consider several influences, such as the particle size and morphology, as well as the interaction between the polymer and the catalyst nanoparticle. Thus, the selection of the protective polymer is crucial for the development of tailored metal–polymer catalyst systems. Additional influences may stem from the presence of ions, for example, those from the metal precursor, or the counterions of the polymer side groups. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1209–1219, 1998     

Catalytically active poly(amideimide) nanofibre mats with high activity tested in the hydrogenation of methyl-cis-9-octadecenoate

Nanofibres of poly(amideimide) (PAI) were prepared by electrospinning from solutions of 11 wt% PAI in dimethylformamide. Addition of small amounts of citric acid to the spinning solution improved the fibre size and fibre uniformity. Catalytic activation of the electrospun nanofibres was performed by coating the fibres with an organic solution of palladium diacetate containing citric acid onto the fibres and subsequent thermally induced conversion to nanosized palladium clusters. The catalyst was durably fixed on the fibres by this procedure. The presence of palladium (Pd) nanoparticles was confirmed by R-ray-diffractometry. These fibres were applied in the hydrogenation of methyl-cis-9-octadecenoate as a model reaction. The palladium-doped nanofibres showed an almost seven times higher hydrogenation rate than a commercial palladium catalyst supported on alumina.     

Synthesis of palladium clusters with surface initiator for polymerization of 2-methyl-2-oxazoline

Summary Palladium clusters were synthesized by reduction of palladium(II) acetate in the presence of a bipyridyl ligand (1) with an initiator for polymerization of 2-methyl-2-oxazoline. The 1-protected palladium clusters were soluble in CHCl₃ and CH₂Cl₂. The 1-protected palladium clusters were combined with 2-methyl-2-oxazoline in chloroform. After polymerization, the reacted bipyridyl ligand was removed from the palladium cluster by suspending the obtained palladium clusters with pyridine. The ¹H NMR measurement indicated that the reacted bipyridyl ligand was formed via surface polymerization of 2-methyl-2-oxazoline. Received: 19 April 2001/Accepted: 2 May 2001     

Macrocyclic Palladium(II) Complexes in C?C Coupling Reactions: Efficient Catalysis by Controlled Temporary Release of Active Species

Stabilization of palladium species against agglomeration is essential for reasonable catalytic activity in C C coupling reactions. In contrast to common methods of palladium(0) complex or particle stabilization, a new concept is introduced here: it is demonstrated that a controlled release of palladium from an inactive precatalyst provides stability, too, and leads to high catalytic activity. This paper presents surprising catalytic results for Heck and Suzuki reactions with aryl chlorides and bromides, using three highly stable macrocyclic palladium complexes as catalyst precursors. Three different behaviour patterns for the macrocyclic complexes can be deduced from the evaluation of catalytic activities, UV‐Vis spectroscopy, recycling studies of immobilized complexes, and ligand addition experiments. (i) Palladium tetraphenylporphyrin reversibly releases only extremely low amounts of palladium during the reactions, and low coupling activities are observed. (ii) Release of palladium from its phthalocyanine complex is irreversible; cumulative release of palladium into the reaction mixtures leads to high catalytic activity. (iii) Extraordinary results were obtained with a Robson‐type complex of palladium, which reversibly releases effectual amounts of palladium into solution under reaction conditions. This controlled release prevents the formation of inactive palladium agglomerates under harsh conditions and leads to high catalytic performances. Even strongly deactivated electron‐rich aryl chlorides (4‐chloroanisole) can be completely and selectively converted by the in situ formed anionic palladium halide complexes; the addition of typical stabilizing additives (TBAB) was found to be unnecessary. The bimetallic palladium complex is regenerated at the end of the reaction. These results contribute to the current understanding of the active species in C C coupling reactions of Heck and Suzuki types.     

Palladium nanoparticles embedded chitosan/poly(vinyl alcohol) composite nanofibers as an efficient and stable heterogeneous catalyst for Heck reaction

In this study, palladium nanoparticles were successfully embedded into modified chitosan/poly(vinyl alcohol) composite nanofibers (Pd-CS/PVA nanofibers) by electrospinning. Then, the Pd-CS/PVA nanofibers were treated at evaluated temperature to improve its solvent resistance and in situ reduce Pd²⁺ cations into Pd⁰ active species. The incorporated palladium nanoparticles with ultra small mean diameter of 3.73 ± 1.04 nm are evenly distributed inside the Pd-CS/PVA nanofiber. The resulting Pd-CS/PVA nanofiber mat exhibits high catalytic activity for Heck reaction of aromatic iodides with alkenes and can be recycled for 18 times without loss of initial activity. The high catalytic activity and stability of Pd-CS/PVA nanofiber mat can be attributed to the ultra small diameter nanofibers, strong chelating ability of chitosan, and fine embedment of palladium species inside the nanofiber. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48026.     

Activity Enhancement of Bimetallic Rh-Ag/Al2O3 Catalysts for Selective Catalytic Reduction of NO by C3H6

The catalytic performance of bimetallic rhodium–silver clusters for the selective catalytic reduction of NO by propylene has been examined over 1%(Rh-Ag)/A₂O₃ catalysts of variable Ag content. It was found that substitution of small amounts of Rh by Ag results in a significant increase of the catalytic activity, which goes through a maximum for catalysts with metal compositions of ca. 95%Rh-5%Ag. This behaviour is tentatively attributed to the formation of rhodium-rich phase alloys, which may stabilize Rh in its reduced state at reaction conditions.     

The State and Catalytic Properties of Platinum and Palladium in Faujasite-type Zeolites

The extensive body of literature published on the physicochemical and catalytic properties of metal zeolites has been reviewed in detail by Minachev and Isakov [1]. Industrial processes in petroleum chemistry involving metal zeolite systems have been examined by Bolton [2], Quite recently Rudham and Stockwell [3] presented a review covering many aspects of catalysis by faujasites. The scope of the present review is far more limited since only platinum and palladium faujasites will be considered and the subject will be further restricted by deliberately ignoring the important domain of bifunctional catalysis. So far little attention has been paid to the rapid progress made in the knowledge of very small particles supported on zeolites. During the past few years improved methods of particle size measurements, and new methods of evaluating the location, the structure, and the electronic properties of metal, and reliable measurements of intrinsic rates of reaction have provided much information on highly dispersed metal in zeolites which deserves to be known not only by people in the zeolite club but also by all those concerned with very small supported particles. Indeed, the choice of focusing our attention on palladium and platinum in faujasites was governed by the fact that these materials are the best known and the best suited to investigate the properties of highly divided metals. Thus, even with high loading, platinum in faujasites can be obtained in very homogeneous states of dispersion with particle sizes ranging down to nearly atomic scale. This is not true for most of the metal-zeolite systems where encaged species are always found together with unreduced cations and/or larger particles. This is not true either for platinum on other supports where wider size distribution and larger sizes are present and where minute loadings prevent detailed structural investigations.     

Iridium Clusters for Catalytic Partial Oxidation of Methane—An In Situ Transmission and Fluorescence XAFS Study

In situ transmission and fluorescence EXAFS combined with on-line gas analysis have provided new insight in the structural changes and the catalytic properties of Ir clusters during the catalytic partial oxidation (CPO) of methane. A novel in situ fluorescence XAFS setup with a multi element silicon drift detector allows time-resolved in situ studies on catalysts with small noble metal concentrations. Significant structural differences were found between a 0.5 wt% Ir/Al₂O₃ and a 2.5 wt% Ir/Al₂O₃ catalyst upon treatment in He and H₂. After He treatment metallic clusters form on high loading Ir catalysts but not when the loading is small. Upon H₂ treatment metallic Ir clusters are detected in both catalysts, but the particle size is smaller when a low loading is used. The smaller clusters appear to be more sensitive to oxidation. The CPO reaction is found to ignite at 320°C, nearly independent of the residence time and the Ir cluster size. The structure of the clusters changes significantly at the ignition point. Below the ignition point they are partly (2.5 wt% Ir) or nearly fully (0.5 wt% Ir) oxidized and above the ignition temperature they are abruptly reduced. The catalytic and structural changes are reversible.     

Desorption and catalytic properties of palladium, supported on Al2O3-La2O3, prepared by the sol-gel method

Desorption and catalytic properties of 0.3 wt% Pd catalysts supported on alumina-lanthana, prepared by the sol-gel method, were studied. A large excess of consumed hydrogen was observed during TPR experiments on these catalysts due to the partial reduction of lanthana. The enhanced hydrogen adsorption (H/Pd varies from 8 to 15) detected by TPD for these catalysts is supposed to be a consequence of spillover. Spillover of hydrogen is favored by a high dispersion of palladium on the support surface and the presence of lanthana reduced species. Palladium on alumina-lanthana catalysts show a higher catalytic activity than palladium on alumina catalysts because reduced species of lanthana stabilize palladium particles. Similarity of ammonia selectivity at high temperatures allows one to suggest that reduced lanthana is involved in the reaction on pure alumina-lanthana support and palladium on alumina-lanthana catalysts at these temperatures. A synergetic effect between small palladium particles and reduced species of lanthana is suggested to be responsible for observed behavior of the lanthana-promoted palladium catalysts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Palladium Nanoparticles in Suzuki Cross‐Couplings: Tapping into the Potential of Tris‐Imidazolium Salts for Nanoparticle Stabilization

Inspired by the proclivity of various palladium sources to form nanoparticles in imidazolium‐based ionic liquids, we now report that tris‐imidazolium salts bearing hexadecyl chains and a bridging mesitylene moiety are potent stabilizers of palladium nanoparticles efficiently prepared via a Chaudret‐type hydrogenation of the bis(dibenzylideneacetone)palladium(0). The palladium nanoparticles have been isolated in pure form and characterized by ¹H nuclear magnetic resonance, transmission electron microscopy, electron diffraction and dynamic light scattering. The new materials proved effective in Suzuki cross‐coupling at a loading of 0.2% palladium. Thus, using a tris‐imidazolium iodide‐palladium material, a series of biaryl products has been prepared starting from aryl bromides and some activated chlorides. The possibility that this catalytic activity might be due to the formation of palladium N‐heterocyclic carbenes has been addressed through solid state ¹³C NMR and the synthesis of an imidazolium analogue in which the acidic 2‐H was replaced with a methyl group.     

Suzuki Coupling Reactions in Pure Water Catalyzed by Supported Palladium – Relevance of the Surface Polarity of the Support

Heterogeneous (supported) palladium catalysts like palladium on carbon and a variety of metal oxides have been shown to be highly active for Suzuki coupling reactions in neat water under mild reaction conditions (T=65 °C). It has been demonstrated for the first time that hydrophobic effects of the catalyst surface play an important role for the catalyst activity in water. Catalysts possessing hydrophobic surfaces (e.g., palladium on carbon) show higher activity for Suzuki coupling reactions in water than their hydrophilic counterparts (palladium on metal oxides). Tuning of the surface polarity of metal oxide supports (by silylation) results in higher activity under these conditions. Stronger alkaline conditions (three‐fold excess of base) compensate the effect of hydrophobic supports and result in high activity of the catalysts also with hydrophilic supports. The addition of tetrabutylammonium bromide to generate, activate and stabilize the catalytic species (dissolved palladium complexes) is necessary for the conversion of more demanding substrates. The reaction is considered to be homogeneous taking place near the catalyst surface inside a droplet or layer of the reactant.     

Modular Palladium Bipyrazoles for the Isomerization of Allylbenzenes – Mechanistic Considerations and Insights into Catalyst Design and Activity,Role of Solvent,and Additive Effects

The catalytic activity of novel bidentate N,N‐chelated palladium complexes derived from electron excessive, backbone fused 3,3′‐bipyrazoles in the selective isomerization of terminal arylpropenoids and 1‐alkenes is described. The catalysts are easily modified by appropriate wing tip substitution, while maintaining the same bulky, rigid unreactive aliphatic backbone. Eleven novel palladium complexes with different electronic and steric properties were investigated. Their performance in the palladium(II)‐catalyzed isomerization of a series of substituted allylbenzenes was evaluated in terms of electronic as well as steric effects. Besides the clear finding of a general trend towards higher catalyst activity with more electron‐donating properties of the coordinated N,N‐bidentate ligands, we found that the catalytic process strongly depends on the choice of solvents and additives. Extensive solvent screening revealed that reactions run best in a 2:1 toluene‐methanol mixture, with the alcohol employed being a crucial factor in terms of electronic and steric factors. A reaction mechanism involving a hydride addition–elimination mechanism starting with a palladium hydride species generated in situ in alcoholic solutions, as corroborated by experiments using deuterium labeled allylbenzene, seems to be most likely. The proposed mechanism is also supported by the observed reaction rate orders of κ_obs[cat.]≈1 (0.94), κ_obs [substrate]=0.20→1.0 (t→∞) and κ_obs [methanol]=−0.51 for the isomerization of allylbenzene. Furthermore, the influence of acid and base, as well as the role of the halide coordinated to the catalyst, are discussed. The system catalyzes the isomerization of allylbenzenes very efficiently yielding high E:Z selectivities under very mild conditions (room temperature) and at low catalyst loadings of 1 mol% palladium even in unpurified solvents. The integrity and stability of the catalyst system were confirmed by multiple addition reaction cycles, successive filtration and isolation experiments, and the lack of palladium black formation.     

低温油相合成金钯纳米粒子及其催化性能研究

采用低温油相法合成了不同金钯量比的双金属纳米催化剂,对其形貌和电催化性能进行了分析。透射电镜表征结果显示,金钯的量比对催化剂的分散性和纳米颗粒的尺寸有一定的影响。当金钯等量比时,分散性最好、纳米颗粒的粒径最小。而电化学测试表明,当金钯等量比时,高分散、小尺寸的金钯纳米催化剂更易于裸露有效活性位点,从而对甲酸和乙醇显示出较高的催化活性和稳定性。