Hydrogenation Catalysts Based on Polynuclear Palladium Complexes with Organophosphorus Ligands

A new procedure is proposed for the preparation of hydrogenation catalysts. This procedure includes the synthesis of cyclic tetranuclear palladium complexes with bridging diphenylphosphide ligands followed by a reaction with Pd(CH₃COO)₂ in the presence of hydrogen to form nanosized particles. In the test catalysts, the ensembles of palladium atoms (or palladium hydrides) immobilized on supramolecular structures formed by the association of phosphinidene and phosphide complexes of palladium are responsible for the catalytic activity.     

Nature of the modifying action of white phosphorus on the properties of nanosized hydrogenation catalysts based on bis(dibenzylideneacetone)palladium(0)

The catalytic properties and nature of the nanoparticles forming in the system based on Pd(dba)₂ and white phosphorus are reported. A schematic mechanism is suggested for the formation of nanosized palladium-based hydrogenation catalysts. The mechanism includes the formation of palladium nanoclusters via the interaction of Pd(dba)₂ with the solvent (N,N-dimethylformamide) and substrate and the formation of palladium phosphide nanoparticles. The inhibiting effect exerted by elemental phosphorus on the catalytic process is due to the conversion of part of the Pd(0) into palladium phosphides, which are inactive in hydrogenation under mild conditions, and the formation of mainly segregated palladium nanoclusters and palladium phosphide nanoparticles. By investigating the interaction between Pd(dba)₂ and white phosphorus in benzene, it has been established that the formation of palladium phosphides under mild conditions consists of the following consecutive steps: Pd(0) → PdP₂ → Pd₅P₂ → Pd₃P. It is explained why white phosphorus can produce diametrically opposite effects of on the catalytic properties of nanosized palladium-based hydrogenation catalysts, depending on the nature of the palladium precursor.     

膦化聚2,6—二甲基1,4—苯醚负载钯催化剂的表征及其催化性能

报道了四种不同P/Pd摩尔比的膦化聚2,6-二甲基1,4-苯醚负载把催化剂的加氢和异构化性能;通过XPS、电镜和远红外对催化剂进行了表征;并考察了溶剂和温度对催化剂活性的影响.     

Properties of Pd–Ag/C catalysts in the reaction of selective hydrogenation of acetylene

Samples of Pd/C and Pd–Ag/C, where C represents carbon nanofibers (CNFs), are synthesized by methane decomposition on a Ni–Cu–Fe/Al₂O₃ catalyst. The properties of Pd/CNF are studied in the reaction of selective hydrogenation of acetylene into ethylene. It is found that the activity of the catalyst in hydrogenation reaction increases, while selectivity decreases considerably when the palladium content rises. The obtained dependences are caused by the features of palladium’s interaction with the carbon support. At a low Pd content (up to 0.04 wt %) in the catalyst, the metal is inserted into the interlayer space of graphite and the catalytic activity is zero. It is established by EXAFS that the main share of palladium in catalysts of 0.05–0.1 wt % Pd/CNF constitutes the metal in the atomically dispersed state. The coordination environment of palladium atoms consists of carbon atoms. An increase in the palladium content in a Pd/CNF catalyst up to 0.3 wt % leads to the formation of highly dispersed (0.8–1 nm) Pd particles. The Pd/CNF samples where palladium is mainly in the atomically dispersed state exhibit the highest selectivity in the acetylene hydrogenation reaction. The addition of silver to a 0.1 wt % Pd/CNF catalyst initially probably leads to the formation of Pd–Ag clusters and then to alloyed Pd–Ag particles. An increase in the silver content in the catalyst above 0.3% causes the enlargement of the alloyed particles and the palladium atoms are blocked by a silver layer, which considerably decreases the catalytic activity in the selective hydrogenation of acetylene.     

Effect of the reduction conditions of the supported palladium precursor on the activity of Pd/C catalysts in hydrogenation of sodium 2,4,6-trinitrobenzoate

The study addresses the effect of the reduction conditions of palladium polynuclear hydroxo complexes (PHC) supported on the Sibunit carbon material on the dispersion of the metal particles and the activity of 0.5%Pd/Sibunit catalysts in the selective hydrogenation of sodium 2,4,6-trinitrobenzoate to 1,3,5-triaminobenzene in an aqueous solution (temperature of 323 or 343 K, pressure of 0.5 MPa). The palladium PHC were reduced using the most common methods pertaining to catalyst preparation: liquid-phase reduction with sodium formate and reduction in a hydrogen flow at elevated temperature. It was found that high-temperature reduction in the gas phase gives rise to Pd particles with a markedly lower dispersion compared with the sample obtained under mild liquid-phase reduction conditions. The catalytic activity of the sample containing large Pd particles proved to be higher than the activity of the catalyst obtained by reduction with sodium formate.     

聚苯乙烯邻氨基苯甲酸负载钯催化剂的加氢性能研究

含有双配位基的聚苯乙烯邻氨基苯甲酸与钯络合物反应制得螫合的配位高分子钯催化剂。用甲醇-水(pH=12)还原可得晶粒分布均匀的胶态钯催化剂,它们对烯烃的加氢具有良好的催化作用。研究表明,钯络合物上原有配体的给予性常数(E_n)与负载后络合催化剂的加氢活性具有良好的线性关系,金属粒径为40—50A的催化剂对己烯-1的加氢活性最高。本文还研究了溶剂极性,载体孔结构及底物对催化剂活性的影响。     

Preparation of Polyoxometalate-Stabilized Colloidal Solutions of Palladium Metal and Catalysts Supported on Them

Sols containing Pd(0) clusters with polyoxo anions are prepared by the reduction of colloidal solutions of polyhydroxo complexes of Pd(II) in the presence of Mo(VI), W(VI), V(V), and Nb(V) polyoxo anions. The cluster sizes varied within the limits of 1–10 nm depending on the nature of the polyoxo anion. The stability of sols toward coagulation depends on the ratio between the palladium and polyoxo anion amounts in solution and on the composition of the solvent. Supported Pd catalysts are obtained by the adsorption of particles from sols; Pd can exist in these catalysts as individual particles or associates or form filamentary structures.     

Application of Laser Induced Breakdown Spectroscopy as a Novel Approach for Monitoring of the Activity of Nano Palladium Catalyst as Compared to Two Well-known Methods

Catalyst deactivation is an unavoidable process that occurs in catalytic chemical reactions. Laser Induced Breakdown Spectroscopy (LIBS) is used here as a novel approach to investigate the activity of palladium supported with carbon catalyst (Pd/C) over the hydrogenation of cinnamic acid with tetralin. Their outputs for four catalyst samples are reported for different time intervals of 0, 5, 10, 15 min during the reaction. The results of LIBS analysis are compared to Inductively Coupled Plasma Mass Spectrometry (ICP-MS), which shows a good agreement. Experimental data specify that line intensities of palladium (Pd) are decreased significantly with an increment of the reaction time. Moreover, the Field Emission Scanning Electron Microscope with energy dispersive spectroscopy (FESEM-EDS) of catalysts samples show aggregation of palladium particles for some places in the catalyst surface. The changes of Pd content and sintering of Pd particles in the catalyst during the reaction play substantial roles in catalyst deactivation.     

高分子钯络合物催化的亚胺化合物的加氢反应

制备了一系列以二氧化硅为载体的侧链上含有各种氮配位基团的聚硅氧烷-钯络合物,这些高分子钯络合物能够在常温常压下催化亚胺化合物的加氢反应并表现出不同程度的催化活性.对其中聚-(N,N-二乙酰基)-γ-氨丙基硅氧烷-钯络合物催化的亚苄基苯胺的加氢反应做了比较深入的研究,发现该催化剂在反应中是稳定的并给出唯一的加氢产物,络合物中的N/Pd原子比、反应温度以及不同底物对反应速度的影响也被报道.     

Synthesis and characterization of Sibunit-supported Pd–Ga,Pd–Zn,and Pd–Ag catalysts for liquid-phase acetylene hydrogenation

Pd/Sibunit and Pd–M/Sibunit (M = Ga, Zn, or Ag) catalysts have been synthesized, and their catalytic properties in liquid-phase acetylene hydrogenation have been investigated. Doping of the palladium catalyst with a metal M leads to the formation of the Pd₂Ga, PdZn, or Pd_0.46Ag_0.54 bimetallic compound. The bimetallic particles are much smaller (1.6–2.0 nm) than the monometallic palladium particles (4.0 nm). Doping with zinc raises the ethylene selectivity by 25% without affecting the activity of the catalyst. Specific features of the effect of each of the dopants on palladium are reported.     

硅胶负载聚乙烯吡啶-聚[苯乙烯-顺丁烯二酸]-钯催化剂的结构研究

利用X-光电子能谱、紫外-可见光谱、电子显微镜等技术研究了硅胶负载的聚乙烯吡啶-聚[苯乙烯-顺丁烯二酸]-钯催化剂的结构.发现催化剂中活性组分除钯(0)外,还存在少量钯(Ⅱ).第二种高分子的存在可以影响钯(Ⅱ)的相对含量.催化剂中钯以很小的粒子均匀分布在载体上,因此具有良好的催化加氢性能.     

Selective hydrogenation by Pd nanoparticles embedded in polyelectrolyte multilayers

Alternating adsorption of poly(acrylic acid) and a polyethylenimine-Pd(II) complex on alumina and subsequent reduction of Pd(II) by NaBH4 yield catalytic Pd nanoparticles embedded in multilayer polyelectrolyte films. The polyelectrolytes limit aggregation of the particles and impart catalytic selectivity in the hydrogenation of alpha-substituted unsaturated alcohols by restricting access to catalytic sites. Hydrogenation of allyl alcohol by encapsulated Pd(0) nanoparticles can occur as much as 24-fold faster than hydrogenation of 3-methyl-1-penten-3-ol. Additionally, the nanoparticle/polyelectrolyte system suppresses unwanted substrate isomerization, when compared to a commercial palladium catalyst. Selective diffusion through poly(acrylic acid)/polyethlyenimine membranes suggests that hydrogenation selectivities are due to different rates of diffusion to nanoparticle catalysts. First-order kinetics are also consistent with a diffusion-limited mechanism. Further exploitation of the versatility of polyelectrolyte films should increase selectivity in hydrogenation as well as other reactions.     

Synthesis,properties, and activity of nanosized palladium catalysts modified with elemental phosphorus for hydrogenation

The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogenation catalysis were optimized. The modifying effect of elemental phosphorus depends on the P/Pd ratio; it is associated with changes in the catalyst dispersity and the nature of the formed nanoparticles containing various palladium phosphides (PdP₂, Pd5P₂, and Pd₆P) and Pd(0) clusters. The main stages of the formation of palladium catalysts for hydrogenation were determined, and a model of an active catalyst, in which the Pd₆P phosphide is the core of a nanoparticle and Pd(0) clusters form a shell, was proposed.     

Factors determining the chemoselectivity of phosphorus-modified palladium catalysts in the hydrogenation of chloronitrobenzenes

The precursor nature effect on the state of the Pd–P surface layer in palladium catalysts and on their properties in the liquid-phase hydrogenation of chloronitrobenzenes under mild conditions has been investigated. A general feature of the Pd–P-containing nanoparticles obtained from different precursors and white phosphorus at P/Pd = 0.3 (PdCl₂ precursor) and 0.7 (Pd(acac)₂ precursor) is that their surface contains palladium in phosphide form (BE(Pd3d _5/2) = 336.2 eV and BE(Р2р) = 128.9 eV) and Pd(0) clusters (BE(Pd3d_5/2) = 335.7 eV). Factors having an effect on the chemoselectivity of the palladium catalysts in chloronitrobenzenes hydrogenation are considered, including the formation of small palladium clusters responsible for hydrogenation under mild conditions.     

Formation and Properties of Hydrogenation Catalysts Based on Palladium Complexes with Primary Phosphines

The formation and catalytic properties of hydrogenation catalysts based on palladium(II) complexes with primary phosphines were studied. With the use of IR and UV spectroscopy, XRD analysis and GLC, it was found that the interaction of bis(acetylacetonato)palladium(II) or palladium(II) acetate with primary phosphines in an inert atmosphere resulted in the formation of polynuclear palladium complex associates mainly containing μ³-PR and a coordinated phosphine. Polynuclear palladium complexes and the palladium phosphide Pd₆P, which is formed from these complexes in an atmosphere of hydrogen, serve as supports for Pd(0) clusters. The effects of the ratio between initial components and the nature of the acido ligand at the palladium atom on the optimum conditions of catalyst formation were considered.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 609–614.Original Russian Text Copyright © 2005 by Belykh, Goremyka, Gusarova, Sukhov, Shmidt.     

Transfer hydrogenation using recyclable polyurea-encapsulated palladium: efficient and chemoselective reduction of aryl ketones

A robust and recyclable palladium catalyst [Pd0EnCat] has been prepared by ligand exchange of polyurea-encapsulated palladium(II) acetate with formic acid, resulting in deposition of Pd(0) in the support material; Pd0EnCat is shown to be a highly efficient transfer hydrogenation catalyst for chemoselective reduction of a wide range of aryl ketones to benzyl alcohols.     

Hydrogenation of olefins in supercritical CO(2) catalyzed by palladium nanoparticles in a water-in-CO(2) microemulsion

Nanometer-sized metallic palladium particles can be synthesized by hydrogen reduction of Pd2+ ions dissolved in the water core of a water-in-CO2 microemulsion. The Pd nanoparticles, stabilized by the micromeulsion and uniformly dispersed in the supercritical fluid phase, are effective catalysts for hydrogenation of olefins. Examples of rapid and efficient hydrogenation of water-soluble and CO2-soluble olefins catalyzed by the Pd nanoparticles in supercritical CO2 are given.     

Synthesis, structure, theoretical studies, and Ligand exchange reactions of monomeric, T-shaped arylpalladium(II) halide complexes with an additional, weak agostic interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.     

The irreversible formation of palladium carbide during hydrogenation of 1-pentyne over silica-supported palladium nanoparticles: in situ Pd K and L3 edge XAS

The catalytically active phase of silica-supported palladium catalysts in the selective and non-selective hydrogenation of 1-pentyne was determined using in situ X-ray absorption spectroscopy at the Pd K and L(3) edges. Upon exposure to alkyne, a palladium carbide-like phase rapidly forms, which prevents hydrogen to diffuse into the bulk of the nano-sized particles. Both selective and non-selective hydrogenation occur over carbided particles. The palladium carbide-like phase is stable under reaction conditions and only partially decomposes under high hydrogen partial pressure. Non-selective hydrogenation to pentane is not indicative of hydride formation. The palladium carbide phase was detected in the EXAFS analysis and the K edge XANES showed representative features.     

Simple preparation and catalytic activity of Pd particles dispersed mesoporous carbons from poly(VDC/MA) containing Pd and Y compounds

We report a simple preparation of Pd particles dispersed mesoporous carbons. The carbons were prepared by steam activation of carbonized vinylidene chloride/methyl acrylate copolymer (poly(VDC/MA)) containing yttrium acetylacetonate (Y(acac)(3)) and palladium acetylacetonate (Pd(acac)(2)). The resulting carbons consist of high contents of mesopore and uniformly dispersed fine Pd particles. We measured the catalytic activities of the carbons obtained for hydrogenation of methyl linoleate. The Pd particles dispersed in mesoporous activated carbons obtained from poly(VDC/MA) containing both Y(acac)(3) and Pd(acac)(2) showed high catalytic activities, compared with the microporous activated carbon obtained from poly(VDC/MA) containing only Pd(acac)(2). Especially Pd particles dispersed in mesoporous carbons exhibited excellent selectivity for hydrogenation of diene (methyl linoleate) to monoene (methyl oleate).