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.     

Influence of Pd precursors on the catalytic performance of Pd–H4SiW12O40/SiO2 in the direct oxidation of ethylene to acetic acid

The effects of palladium precursors (PdCl₂, (NH₄)₂PdCl₄, Pd(NH₃)₂Cl₂, Pd(NO₃)₂ and Pd(CH₃COO)₂) on the catalytic properties in the selective oxidation of ethylene to acetic acid have been investigated for 1.0 wt% Pd–30 wt% H₄SiW₁₂O₄₀/SiO₂. The structures of the catalysts were characterized using X-ray diffraction, N₂ adsorption, H₂-pulse chemical adsorption, infrared spectrometry of the adsorbed pyridine, H₂ temperature-programmed reduction and X-ray photoelectron spectroscopy. The present study demonstrates that the different palladium precursors can lead to the significant changes in the dispersion of palladium. It is found that Pd dispersion decreases as follows: PdCl₂ > (NH₄)₂PdCl₄ > Pd(NO₃)₂ > Pd(NH₃)₂Cl₂ > Pd(C₂H₃O₂)₂, which is nearly identical to the catalytic activity. This indicates that the dispersion of palladium plays an important role in the catalytic activity. Furthermore, density of Lewis (L) and Brönsted (B) acid sites are also strongly dependent on the palladium precursors. It is also demonstrated that an effective catalyst should possess a well combination of Brönsted acid sites with dispersion of palladium.     

Amidophosphine‐stabilized palladium complexes catalyse Suzuki–Miyaura cross‐couplings in aqueous media

We report a simple and efficient procedure for Suzuki–Miyaura reactions in aqueous media catalysed by amidophosphine‐stabilized palladium complexes trans‐{L³PPh₂}₂PdCl₂ ( 3 ), trans‐{L³PPhtBu}₂PdCl₂ ( 4 ), [Pd(η³‐C₃H₅)(L³PPh₂)Cl] ( 5 ) and {Pd[2‐(Me₂NCH₂)C₆H₄](L³PPh₂)Cl} ( 6 ). The acidity of the NH proton in complexes 3 , 4 , 5 , 6 plays an important role in their catalytic activity. In addition, the palladium complexes cis‐{L¹PPh₂}PdCl₂ ( 1 ) and trans‐{L²PPh₂}₂PdCl₂ ( 2 ) stabilized by phosphines containing Y,C,Y‐chelating ligands L^1,2 have also been found to be useful catalysts for Suzuki–Miyaura reactions in aqueous media. The method can be effectively applied to both activated and deactivated aryl bromides yielding high or moderate conversions. The catalytic activity of couplings performed in pure water increases when utilizing a Pd complex with more acidic NH protons. A decrease of palladium concentration from 1.0 to 0.5 mol% does not lead to a substantial loss of conversion. In addition, Pd complex 1 can be efficiently recovered using two‐phase system extraction. Copyright © 2016 John Wiley & Sons, Ltd.     

Preparation of the Pd/C catalysts: A molecular-level study of active site formation

This review summarizes the results of molecular-level studies on the mechanism of Pd/C catalyst formation from the PdCl₂ precursor. Two processes occur in acidic media during the contact of H₂PdCl₄ with carbon: (a) adsorption of palladium chloride to form surface complexes and (b) redox interaction between PdCl₂ and carbon with the formation of palladium metal particles. The ratio between these adsorbed palladium species depends on the conditions of adsorption and especially on the size of carbon support grains and the oxidative atmosphere. The observations are explained by the fact that carbon support exhibits electrochemical and ligand properties. X-ray diffraction, X-ray scattering, XPS, and high-resolution electron microscopy revealed that the nanostructure of carbon materials, in particular the extent of their three-dimensional ordering, is crucial for the ligand properties. The presence of two forms, metallic and ionic, of sorbed palladium determines the bimodal size distribution of the metal. After the reduction of ionic species, metal particles are “blocked” with support. The nature of the ionic forms of palladium (mostly (PdCl₂)_n) clusters chemically and epitaxially bound to the carbon surface suggests the mechanisms of the bimodal distribution of the supported metal particles on the surface and the methods for the control of the ratio between “blocked,” low-dispersed, and highly-dispersed particles in the catalyst. One of these methods is the use of palladium polynuclear hydroxo complexes (PHCs) with low oxidation potentials as starting compounds for catalysts preparation. The data on the PHC structure in a solution and its change upon the adsorption of PHC on the surface of the carbon material obtained by the¹⁷O,²³Na,¹³³Cs, and³⁵Cl NMR techniques are discussed. PHCs are shown to be a clew of the [Pd(OH)₂]_n polymeric filament, whose fractions are bound with alkali metal ions. When PHC is adsorbed on the surface of the carbon support and then dried, palladium oxide is formed from which highly dispersed metal particles are formed during reduction. The nature of alkali metal ions in PHC affects the activity of the Pd/C catalyst. An important role of the ligand, electrochemical, and lyophilic properties of carbon material during the formation of the species of the active catalyst component is discussed.     

Asymmetric Hydrogenation of Ethyl Pyruvate using Layered Double Hydroxides–Supported Nano Noble Metal Catalysts

Layered double hydroxide (LDH)–supported nano noble metal heterogeneous catalysts are synthesized by ion exchange of K₂PtCl₆, Na₂PdCl₄ and impregnation of RhCl₃ ^.3H₂O followed by reduction with H₂. The LDH–Rh, Pt, and Pd catalysts are tested in the enantioselective hydrogenation of ethyl pyruvate to ethyl lactate with very good yields and enantiomeric excess's (e.e.'s) of up to 72% were obtained with Pt. The catalyst was recovered and reused for several cycles with consistent activity.     

Neutral and cationic (pyrazolylmethyl)pyridine palladium(II) complexes: kinetics and chemoselectivity studies in hydrogenation of alkenes and alkynes

Reactions of (3,5-dimethylpyrazolylmethyl)pyridine (L1) and (3,5-diphenylpyrazolylmethyl)pyridine (L2) with either [PdCl₂(NCMe)₂] or [PdClMe(COD)] afforded the respective neutral palladium complexes, [PdCl₂(L1)] (1), [PdCl₂(L2)] (2) and [PdClMe(L1)] (3). Treatment of complex 1 with equimolar amounts of PPh₃ or PPh₃/NaBAr₄ produced the corresponding cationic complexes [Pd(L1)ClPPh₃]Cl (4) and [Pd(L1)ClPPh₃]BAr₄ (5), respectively. Complexes 1–5 formed active catalysts in hydrogenation of alkenes and alkynes. Isomerization reactions were predominant in the hydrogenation reactions of terminal alkenes, while hydrogenation of alkynes involved a two-step process via alkene intermediates prior to the formation of the respective alkenes. The lack of induction periods in the hydrogenation reactions in addition to pseudo-first-order kinetics with respect to the substrates established the homogeneous nature of the active species.     

Hydrogenation of crotonaldehyde and cinnamaldehyde catalyzed by water-soluble palladium complex

The selective hydrogenations of crotonaldehyde and cinnamaldehyde in the aqueous-benzene biphasic system were investigated using water-soluble palladium complex PdCl₂(TPPTS)₂ as catalyst. The hydrogenation rate of crotonaldehyde was higher than that of cinnamaldehyde under similar reaction conditions. The palladium complex selectively catalyzed the hydrogenation of CC bond in crotonaldehyde to form butanal (100%). On the contrary, hydrogenation of both CC and CO bonds in cinnamaldehyde occurred simultaneously, with the amount of phenylpropanal only slightly higher than that of phenylpropanol. However, the reduction of CO bond of cinnamaldehyde could be inhibited by the addition of Na₂CO₃ solution. Therefore, high selectivity to form phenylpropanal (91%) could be obtained by using Na₂CO₃ solution at pH 12.2. Other factors affecting the hydrogenation conversion and selectivity of crotonaldehyde and cinnamaldehyde were also discussed.     

Grafting and anchoring of molecular complexes as metal precursors for alumina-supported Pd catalysts

Abstract

The surface coordination chemistry of Pd complexes on alumina has been studied in the framework of synthesizing Pd/γ-Al₂O₃ catalytic materials. Two methodologies were explored: the direct grafting of Pd complexes on hydroxyl functions present at the alumina surface and the anchoring of the precursors via amine-bearing silanes previously grafted on the support. Suitable conditions to graft and anchor Pd complexes on alumina surface were found and experimental proofs of grafting and anchoring processes are provided. The results show that covalent grafting indeed took place for samples prepared in acetonitrile with [Pd(CF₃CO₂)₂(bipy)] and [PdCl₂(PhCN)₂] complexes or with [Pd(OAc)₂] and [Pd(CF₃CO₂)₂] in acetone. The anchoring was successful for catalysts prepared in acetone with 1 wt.% of [Pd(CF₃CO₂)₂] loading. Grafting and anchoring were found to stabilize palladium in its Pd(II) oxidation state. This has an adverse effect on the activation step that should lead to reduction of the complex to give the metallic catalytic supported active phase.     

Mechanism of the formation of palladium complexes serving as catalysts in hydrogenation reactions : II. Reactivity of palladium phosphine halides towards molecular hydrogen

[(PPh₃)₃(PPh₂)₂Pd₃Cl] Cl, benzene and aniline hydrochloride were isolated as products of the reactions of (PPh₃)₂PdCl₂]₂ or [(PPh₃)PdCl₂]₂ with H₂ in organic amines (Am). Similar products were obtained when (Ph₃P)₂Pd(Ph)Br was treated with H2₃ Both in amines and aromatic solvents. The reaction between H₂ and [(PBu₃)PdCl₂]₂ resulted in the formation of [(PBu₃(PBu_2)PdCl2 ·. 2 Am The kinetic data for H₂ absorption by solutions of palladium(II) complexes are consistent with the heterolytic mechanism of cleavage fo hte HH bond in the coordination sphere of palladium(II); the function of the H⁺ acceptor being performed by the bases (e.g. Am or Ph). The reaction between the palladium complexes and H₂ is autocatalytic. Reduction of the initial Pd^II complexes leads to lower oxidation state palladium complexes, which catalyse the reduction of Pd^II complexes. In the coordination sphere of the lower oxidation state palladium complexes, the oxidative addition of PR₃ to Pd takes place with formation of compounds containing a Pd-R bond. It is the reaction between these complexes and H₂ that yields palladium compounds with PR₂ ligands.     

Preparation and catalytic properties of NR-stabilized palladium colloids

Palladium colloids revealing narrow particle size distributions can be obtained by chemical reduction using tetra–alkylammonium hydrotriorganoborates. Combining the stabilizing agent [NR] with the reducing agent [BEt₃H^?] provides a high concentration of the protecting group at the reduction centre. Alternatively, NR₄X (X = halogen) may be coupled to the metal salt prior to the reduction step: addition of N(octyl)₄Br to Pd(ac)₂ in THF, for example, evokes an active interaction between the stabilizing agent and the metal salt. Reduction of NR-stabilized palladium salts with simple reducing agents such as hydrogen at room temperature yields stable palladium organosols which may be isolated in the form of redispersible powders. The anion of the palladium salt is crucial for the success of the colloid synthesis. Electron microscopy shows that the mean particle size ranges between 1.8 and 4.0 nm. An X–ray–photoelectron spectrscopic examination demonstrated the presence of zerovalent palladium. These palladium colloids may serve as both homogeneous and heterogeneous hydrogenation catalysts. Adsorption of the colloids onto industrially important supports can be achieved without agglomeration of palladium particles. The standard activity of a charcoal catalyst containing 5% of colloidal palladium determined through the cinnamic acid standard test was found to exceed considerably the activity of the conventional technical catalysts. In addition, the lifespan of the catalyst containing a palladium colloid, isolated from the reduction of [N(octyl)₄]₂PdCl₂Br₂ with hydrogen, is superior to conventionally prepared palladium/charcoal (Pd/C) catalysts. For example, the activity of a conventional Pd/C catalyst is completely suppressed after 38×10³ catalytic cycles per Pd atom, whereas the colloidal Pd/C catalyst shows activity even after 96times;10³ catalytic cycles.     

Conformational change induced by hydration: trans‐dichloro­bis­[(1R,2R,3R,5S)‐(–)‐isopinocampheylamine]palladium(II) and trans‐dichloro­bis[(1S,2S,3S,5R)‐(+)‐isopinocampheylamine]palladium(II) hemihydrate

The title compounds, trans‐dichloro­bis[(1R,2R,3R,5S)‐(−)‐2,6,6‐trimethyl­bicyclo­[3.1.1]heptan‐3‐amine]palladium(II), [PdCl₂(C₁₀H₁₉N)₂], and trans‐dichloro­bis[(1S,2S,3S,5R)‐(+)‐2,6,6‐trimethyl­bicyclo­[3.1.1]heptan‐3‐amine]palladium(II) hemihydrate, [PdCl₂(C₁₀H₁₉N)₂]·0.5H₂O, present different arrangements of the amine ligands coordinated to Pd^II, viz. antiperiplanar in the former case and (−)anticlinal in the latter. The hemihydrate is an inclusion compound, with a Pd coordination complex and disordered water mol­ecules residing on crystallographic twofold axes. The crystal structure for the hemihydrate includes a short Pd⋯Pd separation of 3.4133 (13) Å.     

Temperature-programmed desoprtion of H2 from the surfaces of pd/support and Pd-Ag/support catalysts (support = Al2O3, SiO2)

Thermal desorption of H₂ from the surface of Pd/support and Pd-Ag/support (support = Al₂O₃, SiO₂) catalysts has been investigated. Two wide desorption peaks can be observed for the 5% Pd/support catalyst. The presence of these peaks in the thermogram indicates that several adsorption states exist, which is the result of occurrance of different adsorption centers of specific bond strengths for hydrogen. The addition of silver to the palladium catalysts causes a considerable decrease in the size of the high temperature desorption peak. It is also worth noting that the temperature of the maximum of the desorption rate remains practically constant for all bimetallic catalysts studied. This means that the activation energy of the hydrogen desorption process does not change after the introduction of silver to the palladium catalyst.     

Continuous‐Flow Suzuki‐Miyaura and Mizoroki‐Heck Reactions under Microwave Heating Conditions

Microwave‐assisted continuous‐flow reactions have attracted significant interest from synthetic organic chemists, especially process chemists from practical points of view, due to a less complicated shift to large‐scale synthesis based on simple and continuous access to products with low energy requirements. In this personal account, we focused on the Suzuki‐Miyaura and Mizoroki‐Heck reactions, both of which are significantly important cross‐coupling reactions for the synthesis of various functional materials. Microwave power is effective for heating. Typical homogeneous palladium catalysts, such as PdCl₂(PPh₃)₂, Pd(PPh₃)₄, and Pd(OAc)₂, as well as heterogeneous palladium catalysts, such as Pd‐film, Pd/Al₂O₃, Pd/SiO₂, and Pd supported on polymers, can be used for these reactions.     

Palladium salicylaldimine complexes containing boronate esters

Reactions of salicylaldehydes with boronate ester derivatives of aniline have been examined. Addition of these Schiff base ligands to palladium acetate or Na₂PdCl₄ afforded novel boron-containing trans-bis(N-arylsalicylaldiminato) palladium complexes.Condensation of salicylaldehyde (2-HOC₆H₄C(O)H) with H₂NC₆H₄Bpin (pin=1,2-O₂C₂Me₄) afforded the boron-containing Schiff bases, 2-HOC₆H₄C(H)=NC₆H₄Bpin (1–3a). Similar reactivity with 2-hydroxy-5-nitrobenzaldehyde and 2-hydroxy-1-naphthaldehyde gave the corresponding Schiff bases (1-3b) and (1-3c), respectively. Reaction of Schiff bases (2) and (3) with palladium acetate or Na₂PdCl₄ afforded complexes of the type PdL₂ (4,5), where L=deprotonated Schiff base. The molecular structure of the nitro-salicylaldehyde 4-Bpin palladium complex (5b) was characterized by an X-ray diffraction study. All new palladium compounds have been characterized fully and tested for their antifungal activity against Aspergillus niger and Aspergillus flavus.     

Effect of the nature of the acido ligand in the precursor on the properties of nanosized palladium-based hydrogenation catalysts modified with elemental phosphorus

The effect of the nature of the acido ligand in the precursor and the modifying action of elemental phosphorus on palladium catalysts for hydrogenation are reported. The large turnover frequency (TOF) and turnover number (TON) values observed for styrene hydrogenation on the Pd blacks prepared in situ by PdCl₂ reduction with hydrogen in DMF are due to the formation of fine-particle catalyst with a base particle size of 6–10 nm. This is explained by the high PdCl₂ reduction rate and by the formation of a palladium cluster stabilizer—dimethylammonium chloride—in the reaction system via the catalytic hydrolysis of the solvent (DMF). The modifying action of elemental phosphorus on the properties of the palladium catalysts depends on the nature of the acido ligand in the precursor. In the case of oxygen-containing precursors at small P/Pd ratios, elemental phosphorus exerts a promoting effect, raising the TON and TOF values by a factor of about 9. In the case of palladium dichloride as the precursor, white phosphorus exerts an inhibiting effect. At the same time, it enhances the stability of the catalyst, raising the TON value at P/Pd = 0.3. The causes of these distinctions are considered.     

Selective Semihydrogenation of Alkynes on Shape‐Controlled Palladium Nanocrystals

A systematic study on the selective semihydrogenation of alkynes to alkenes on shape‐controlled palladium (Pd) nanocrystals was performed. Pd nanocrystals with a cubic shape and thus exposed {100} facets were synthesized in an aqueous solution through the reduction of Na₂PdCl₄ with L ‐ascorbic acid in the presence of bromide ions. The Pd nanocubes were tested as catalysts for the semihydrogenation of various alkynes such as 5‐decyne, 2‐butyne‐1,4‐diol, and phenylacetylene. For all substrates, the Pd nanocubes exhibited higher alkene selectivity (>90 %) than a commercial Pd/C catalyst (75–90 %), which was attributed to a large adsorption energy of the carbon–carbon triple bond on the {100} facets of the Pd nanocubes. Our approach based on the shape control of Pd nanocrystals offers a simple and effective route to the development of a highly selective catalyst for alkyne semihydrogenation.     

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 hydrothermal treatment on the physicochemical characteristics of Pd/C composites prepared <Emphasis Type="Italic">via</Emphasis> pyrolysis of sawdust impregnated with palladium nitrate

Palladium supported on carbon (Pd/C) catalysts (0.55–0.65 wt.% of Pd) were synthesized by pyrolysis of birch sawdust under inert atmosphere proceeded by prolonged impregnation of sawdust in aqueous solution of palladium nitrate. In some cases, hydrothermal treatment (HT) of the pristine sawdust was conducted to modify the specific surface area of the final carbon material applied as a catalyst support. Based on low-temperature nitrogen adsorption technique, it was postulated that HT of sawdust in the liquid phase increases. while HT in the gaseous phase decreases the specific surface area of Pd/C. The obtained catalysts contained Pd particles (size ranged from 2 to 10 nm) both coated and not coated with carbon shell as evidenced by XPS and TEM techniques. The synthesized Pd/C composites provide high conversion of chlorobenzene and high selectivity in respect to benzene in hydrodechlorination reaction performed in a flow fixed-bed reactor in the presence of H₂. XPS data for Pd/C composites tested in the catalytic reaction indicate their high resistance to HCl. A minor part of metal Pd was found to transform into PdCl₂ and PdO.     

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.     

Palladium‐catalyzed carbonylative cyclization of 3‐bromoallyl alcohols leading to furan‐2(5H)‐ones

3‐Bromoallyl alcohols are carbonylatively cyclized under carbon monoxide pressure in toluene in the presence of a catalytic amount of Pd(OAc)₂ and PPh₃ along with Na₂CO₃ to give furan‐2(5H)‐ones in good yields. Copyright © 2012 John Wiley & Sons, Ltd.