Inactive Pd Research Articles

Deconvoluting the Individual Effects of Nanoparticle Proximity and Size in Thermocatalysis.

Nanoparticle (NP) size and proximity are two physical descriptors applicable to practically all NP-supported catalysts. However, with conventional catalyst design, independent variation of these descriptors to investigate their individual effects on thermocatalysis remains challenging. Using a raspberry-colloid-templating approach, we synthesized a well-defined catalyst series comprising Pd12Au88 alloy NPs of three distinct sizes and at two different interparticle distances. We show that NP size and interparticle distance independently control activity and selectivity, respectively, in the hydrogenation of benzaldehyde to benzyl alcohol and toluene. Surface-sensitive spectroscopic analysis indicates that the surfaces of smaller NPs expose a greater fraction of reactive Pd dimers, compared to inactive Pd single atoms, thereby increasing intrinsic catalytic activity. Computational simulations reveal how a larger interparticle distance improves catalytic selectivity by diminishing the local benzyl alcohol concentration profile between NPs, thus suppressing its readsorption and consequently, undesired formation of toluene. Accordingly, benzyl alcohol yield is maximized using catalysts with smaller NPs separated by larger interparticle distances, overcoming activity-selectivity trade-offs. This work exemplifies the high suitability of the modular raspberry-colloid-templating method as a model catalyst platform to isolate individual descriptors and establish clear structure-property relationships, thereby bridging the materials gap between surface science and technical catalysts.

Overcoming Pd Catalyst Deactivation in the C-H Coupling of Tryptophan Residues in Water Using Air as the Oxidant.

The Pd-catalyzed C-H activation of natural tryptophan residues has emerged as a promising approach for their direct synthetic modification. While using water as the solvent and harnessing air as the oxidant is enticing, these conditions induce catalyst deactivation by promoting the formation of inactive Pd(0) clusters. In this work, we have studied optimized Pd-based catalytic systems via nonsteady state kinetic analysis and in situ X-ray absorption spectroscopy (XAS) to overcome catalyst deactivation, which enables the effective use of lower Pd loadings.

Unlocking High-Efficiency Methane Oxidation with Bimetallic Pd-Ce Catalysts under Zeolite Confinement.

Catalytic complete oxidation is an efficient approach to reducing methane emissions, a significant contributor to global warming. This approach requires active catalysts that are highly resistant to sintering and water vapor. In this work, we demonstrate that Pd nanoparticles confined within silicalite-1 zeolites (Pd@S-1), fabricated using a facile in situ encapsulation strategy, are highly active and stable in catalyzing methane oxidation and are superior to those supported on the S-1 surface due to a confinement effect. The activity of the confined Pd catalysts was further improved by co-confining a suitable amount of Ce within the S-1 zeolite (PdCe0.4@S-1), which is attributed to confinement-reinforced Pd-Ce interactions that promote the formation of oxygen vacancies and highly reactive oxygen species. Furthermore, the introduction of Ce improves the hydrophobicity of the S-1 zeolite and, by forming Pd-Ce mixed oxides, inhibits the transformation of the active PdO phase to inactive Pd(OH)2 species. Overall, the bimetallic PdCe0.4@S-1 catalyst delivers exceptional outstanding activity and durability in complete methane oxidation, even in the presence of water vapor. This study may provide new prospects for the rational design of high-performance and durable Pd catalysts for complete methane oxidation.

Heterogenizing palladium tetraiodide catalyst for carbonylation reactions

We report the first example of successful heterogenization of the classical PdI42- carbonylation catalyst, achieved in two simple steps from ionic liquid-functionalized multi-walled carbon nanotubes (MWCNTs). The newly developed materials (PdI4@MWCNT-imi-X, X = Br, I) present the PdI42- anion supported on an imidazolium network (imi) grown on MWCNTs and have been fully characterized.The activity of PdI4@MWCNT-imi-X has been successfully tested in a paradigmatic carbonylation reaction, the oxidative monoaminocarbonylation of 1-alkynes with amines to give high value added 2-ynamides (obtained in good yields, 50–84%, starting from various substrates). The heterogeneous catalyst could be easily recycled and showed a good efficiency even after the fourth recycle, when deactivation began to occur owing to the formation of inactive Pd(0) species, as confirmed by XPS analysis. Hot and cold filtration tests were compatible with an essentially heterogeneous catalytic process, and limited metal contamination in the final organic compounds occurred, as assessed by ICP-MS analysis of representative carbonylation products.

An Imidazole‐Rich Pd(II)‐Polymer Pre‐catalyst for the Suzuki‐Miyaura Coupling: Stability Influenced by Dissolved Oxygen and Reactants Concentration

AbstractHerein a novel Pd(II)‐polymeric pre‐catalyst was prepared by coordinating Pd(II) ions to a low cost imidazole/carboxylate‐rich polymer. The material displays good activity in the Suzuki‐Miyaura coupling towards a range of aryl bromides and iodides in iPrOH/H2O mixtures at 25 or 60 °C. Catalyst longevity and recyclability proved to be sensitive to the concentration of the cross‐coupling partners. When the concentration of PhBr is high ([PhBr]=250 mmol L−1), inactive Pd(0) aggregates were detected. On the other hand, when the reaction was performed at 20‐fold dilution ([PhBr]=12.5 mmol L−1) the material could be reused up to 12 times without significant loss of catalytic activity. In this condition, minimum amount of Pd(0) was detected by XPS analysis and no Pd(0) aggregates were observed by XRPD. Importantly, it was found that the presence of oxygen is fundamental for avoiding formation of inactive Pd(0) aggregates.

Pd(NHC)(μ-Cl)Cl]2: Versatile and Highly Reactive Complexes for Cross-Coupling Reactions that Avoid Formation of Inactive Pd(I) Off-Cycle Products.

SummaryThe development of more reactive, general, easily accessible, and readily available Pd(II)–NHC precatalysts remains a key challenge in homogeneous catalysis. In this study, we establish air-stable NHC–Pd(II) chloro-dimers, [Pd(NHC)(μ-Cl)Cl]2, as the most reactive Pd(II)–NHC catalysts developed to date. Most crucially, compared with [Pd(NHC)(allyl)Cl] complexes, replacement of the allyl throw-away ligand with chloride allows for a more facile activation step, while effectively preventing the formation of off-cycle [Pd2(μ-allyl)(μ-Cl)(NHC)2] products. The utility is demonstrated via broad compatibility with amide cross-coupling, Suzuki cross-coupling, and the direct, late-stage functionalization of pharmaceuticals. Computational studies provide key insight into the NHC–Pd(II) chloro-dimer activation pathway. A facile synthesis of NHC–Pd(II) chloro-dimers in one-pot from NHC salts is reported. Considering the tremendous utility of Pd-catalyzed cross-coupling reactions and the overwhelming success of [Pd(NHC)(allyl)Cl] precatalysts, we believe that NHC–Pd(II) chloro-dimers, [Pd(NHC)(μ-Cl)Cl]2, should be considered as go-to precatalysts of choice in cross-coupling processes.

Substrate co-doping modulates electronic metal-support interactions and significantly enhances single-atom catalysis.

Transitional metal nanoparticles or atoms deposited on appropriate substrates can lead to highly economical, efficient, and selective catalysis. One of the greatest challenges is to control the electronic metal-support interactions (EMSI) between the supported metal atoms and the substrate so as to optimize their catalytic performance. Here, from first-principles calculations, we show that an otherwise inactive Pd single adatom on TiO2(110) can be tuned into a highly effective catalyst, e.g. for O2 adsorption and CO oxidation, by purposefully selected metal-nonmetal co-dopant pairs in the substrate. Such an effect is proved here to result unambiguously from a significantly enhanced EMSI. A nearly linear correlation is noted between the strength of the EMSI and the activation of the adsorbed O2 molecule, as well as the energy barrier for CO oxidation. Particularly, the enhanced EMSI shifts the frontier orbital of the deposited Pd atom upward and largely enhances the hybridization and charge transfer between the O2 molecule and the Pd atom. Upon co-doping, the activation barrier for CO oxidation on the Pd monomer is also reduced to a level comparable to that on the Pd dimer which was experimentally reported to be highly efficient for CO oxidation. The present findings provide new insights into the understanding of the EMSI in heterogeneous catalysis and can open new avenues to design and fabricate cost-effective single-atom-sized and/or nanometer-sized catalysts.

Comparison of the catalytic activity for the Suzuki-Miyaura reaction of (η(5)-Cp)Pd(IPr)Cl with (η(3)-cinnamyl)Pd(IPr)(Cl) and (η(3)-1-t-Bu-indenyl)Pd(IPr)(Cl).

Complexes of the type (η3-allyl)Pd(L)(Cl) and (η3-indenyl)Pd(L)(Cl) are highly active precatalysts for the Suzuki–Miyaura reaction. Even though allyl and indenyl ligands are similar to cyclopentadienyl (Cp) ligands, there have been no detailed comparative studies exploring the activity of precatalysts of the type (η5-Cp)Pd(L)(Cl) for Suzuki–Miyaura reactions. Here, we compare the catalytic activity of (η5-Cp)Pd(IPr)(Cl) (IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene, Cp) with two commercially available catalysts (η3-cinnamyl)Pd(IPr)(Cl) (Cin) and (η3-1-t-Bu-indenyl)Pd(IPr)(Cl) (tBuInd). We show that Cp gives slightly better catalytic activity than Cin, but significantly inferior activity than tBuInd. This order of activity is rationalized by comparing the rates at which the precatalysts are activated to the monoligated Pd(0) active species along with the tendency of the starting precatalysts to comproportionate with monoligated Pd(0) to form inactive Pd(I) dimers. As part of this work the Cp supported Pd(I) dimer (μ-Cp)(μ-Cl)Pd2(IPr)2 (CpDim) was synthesized and crystallographically characterized. It does not readily disproportionate to form monoligated Pd(0) and consequently CpDim is a poor catalyst for the Suzuki–Miyaura reaction.

Effects of water vapor and temperature upon combustion stability of CeZrO2-modified Pd-Pt/Al2O3 catalyst

Two bimetallic Pd-Pt catalysts were prepared by wash-coating the composites of Pd-Pt/CeZrO2-Al2O3 onto cordierite monolith and they were evaluated in a tubular reactor for stability in combustion of SOFC stack flue gas at GHSV 29,280 h−1 and 1 atm. The Pd-rich catalyst showed better stability than the Pt-rich catalyst for combustion of the wet feed. Water vapor in the stack flue gas affected the catalyst activity adversely via the formation of inactive Pd(OH)2 species at the reaction temperatures of 723 and 823 K, but this effect was eliminated by raising the temperature to 923 K or above. Catalyst properties such as specific surface area, pore size and metal dispersion did not deteriorate significantly after combustion of the wet feed in the temperature range of 723-1,023 K. The presence of CeZrO2 in Al2O3 material was considered to affect the support properties favorably, leading to a kind of metal-support interaction beneficial for the combustion of the wet feed. This interaction helped to enable the Pd-Pt catalysts to maintain good stability for combusting the wet SOFC stack flue gas with the stoichiometric air-to-fuel ratio at 923 K or above.

Palladium nanoparticles generated from allylpalladium chloride in situ: A simple and highly efficient catalytic system for Mizoroki–Heck reactions

The Mizoroki–Heck reactions of aryl halides catalyzed by palladium nanoparticles generated in situ from a simple allyl palladium precursor were investigated in argon. The high turnover numbers of 9,300,000 have been obtained with 4-bromobenzonitrile as substrate and 3500 with 4-nitrochlorobenzene. When the reaction was performed in air, a low yield was given, but it could be improved obviously by addition of PEG-400. The main reason was that inactive Pd(II) species could be rapidly reduced to the active Pd(0) by PEG. In other word, the existence of air and PEG led to a synergistic effect which the oxidation by air prevents the aggregation of Pd NPs and the reduction by PEG maintains the high activity of Pd(0) species.

Development of safe and scalable continuous-flow methods for palladium-catalyzed aerobic oxidation reactions

The synthetic scope and utility of Pd-catalyzed aerobic oxidation reactions has advanced significantly over the past decade, and these reactions have potential to address important green-chemistry challenges in the pharmaceutical industry. This potential has been unrealized, however, because safety concerns and process constraints hinder large-scale applications of this chemistry. These limitations are addressed by the development of a continuous-flow tube reactor, which has been demonstrated on several scales in the aerobic oxidation of alcohols. Use of a dilute oxygen gas source (8% O(2) in N(2)) ensures that the oxygen/organic mixture never enters the explosive regime, and efficient gas-liquid mixing in the reactor minimizes decomposition of the homogeneous catalyst into inactive Pd metal. These results provide the basis for large-scale implementation of palladium-catalyzed (and other) aerobic oxidation reactions for pharmaceutical synthesis.

Air-Stable, Recyclable, and Regenerative Phosphine Sulfide Palladium(0) Catalysts for C−C Coupling Reaction

Mononuclear phosphine sulfide Pd(0) complexes and a polymer-supported triphenylphosphine sulfide Pd(0) complex were prepared as new air-stable Pd(0) catalysts for C−C coupling reactions. The phosphine sulfide Pd(0) complexes are not decomposed after completion of Suzuki−Miyaura coupling, and the polymer-supported Pd(0) catalyst is practically recyclable, while phosphine Pd(0) complexes are decomposed into inactive Pd(0) black after consuming the substrates. New catalytic activity of Pd(0) that promotes chalcogen atom replacement of phosphine chalcogenides (R3P═X, X = O, S, Se) is reported. A mechanistic study revealed that the new catalytic chalcogen replacement results from activation of the P═X bond as well as promotion of the oxidative chalcogenide formation. The intermediate phosphine was successfully trapped as a phosphine Pd(II) complex, and the P═X bond activation is applicable to regeneration of phosphine or phosphine sulfide from oxidized phosphine.

High Catalytic Activity of Palladium(II)‐Exchanged Mesoporous Sodalite and NaA Zeolite for Bulky Aryl Coupling Reactions: Reusability under Aerobic Conditions

Exchange for the better: Mesoporous sodalite and NaA zeolite exchanged with Pd(2+) exhibit remarkably high activity and reusability in C-C coupling reactions under aerobic atmosphere. It is proposed that the catalytic reactions are mediated by a molecular Pd(0) species generated in situ within the pores (see picture), which is oxidized back to Pd(2+) by O(2), preventing the formation of catalytically inactive Pd(0) agglomerates.

Catalytic Combustion of Effluents from Methane-Based MCFC Device over Cordierite Supported Pd/La-Al2O3 Catalyst

Influence of steam, loading amounts of Pd (viz., 0.7, 2.1, 5.4 wt %) and other process parameters (viz. GHSV and temperature) on methane-based MCFC anodeeffluents over Pd-based La2O3-Al2O3 supported on honeycomb substrate have been investigated. TGA study reveals precise temperature of decomposition of Pd-O in catalytic combustion at high temperature. An increase in methane conversion at elevated temperature is attributable to decomposition of PdO to Pd active phase. The XRD analysis of spent catalysts suggests that the reflections corresponding to PdO gradually become weaker with increase in the Pd loading. The addition of 4% water in the feed decreases the methane conversion due to the reversible reaction of PdO with water adduct resulting in the formation of inactive Pd(OH)2 phase. However, the methane conversion remains unaffected and increases monotonously with 100% water concentration in the feed.

Human Papillomavirus E7 Requires the Protease Calpain to Degrade the Retinoblastoma Protein

Cervical cancers transformed by high risk human papilloma virus (HPV) express the E7 oncoprotein, which accelerates the degradation of the retinoblastoma protein (Rb). Here we show that the E7-mediated degradation of Rb requires the calcium-activated cysteine protease, calpain. E7 bound and activated mu-calpain and promoted cleavage at Rb(810), with mutation of this residue preventing E7-mediated degradation. The calpain cleavage product, Rb(1-810), was unable to mediate cell cycle arrest but retained the ability to repress E6/E7 transcription. E7 also promoted the accelerated proteasomal degradation of Rb(1-810). Calpain inhibitors reduced the viability of HPV-transformed cells and synergized with cisplatin. Calpain, thus, emerges as a central player in E7-mediated degradation of Rb and represents a potential new drug target for the treatment of HPV-associated lesions.

A Practical Synthesis of Highly Functionalized Aryl Nitriles Through Cyanation of Aryl Bromides Employing Heterogeneous Pd/C: In Quest of an Industrially Viable Process.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A practical synthesis of highly functionalized aryl nitriles through cyanation of aryl bromides employing heterogeneous Pd/C: in quest of an industrially viable process

Preparation of aryl nitrile 2a through classical Rosenmund-von Braun reaction of aryl bromide 1a resulted in a poor yield (61%) due to a high reaction temperature (165 °C) and a lack of efficient procedure for separating 2a from a large quantity of heavy metal waste (Cu salts). To address these issues, a practical synthesis of multifunctional aryl nitriles through cyanation of aryl bromides has been developed with heterogeneous Pd/C used as the catalyst. Treatment of aryl bromides 1 with Zn(CN) 2 in the presence of Pd/C, Zn, ZnBr 2 and PPh 3 in DMA provided aryl nitriles 2 involving those carrying sterically demanding electron-rich substituent in good yields and in highly reproducible manner. The activity of Pd/C is highly dependent on the properties of the Pd/C. Oxidic thickshell type catalyst Pd/C D5 was found to furnish the highest rate acceleration and yield. The use of heterogeneous Pd/C might anchor and disperse Pd over the solid support of the catalyst, at least in the initial stage of the reaction, to assure the formation of monomeric Pd complex without precipitating to inactive Pd black. The use of a slightly excess of Zn(CN) 2 (0.6 equiv) and air oxidation of phosphine ligand, after end of the reaction, converted Pd species to insoluble phosphine-free Pd cyanides, from which Pd was recovered in high yield through simple filtration followed by usual recovery process involving combustion.

Synthesis of Pd(II) and Pt(II) complexes possessing bicyclo[3.2.0]heptanyl phosphinite ligands: Identification of a novel Pd(II) precatalyst for 1,6-diene cycloisomerisation

The synthesis of monodentate and bidentate phosphinite ligands, possessing the unusual bicyclo[3.2.0]heptane framework, are reported. A convenient, tin-free synthesis of a key intermediate, namely 3- endo-6- endo-dihydroxybicyclo[3.2.0]heptane, is described. The air-sensitive phosphinite ligand 1 was either protected as the borane adduct, which is air-stable, or reacted directly with bis(acetonitrile)palladium(II)chloride to give the novel air and moisture stable palladium(II) complex 11. A platinum(II) relative 12 has also been synthesised by reaction of phosphinite 1 with bis(benzonitrile)platinum(II)chloride. Each complex has been thoroughly characterised and their molecular structures confirmed by X-ray diffraction studies. In catalytic applications, such as cross-coupling reactions of organometallic reagents with organohalides, an unexpectedly poor activity has been established for 11. For example, Suzuki–Miyaura cross-coupling of activated and deactivated aryl bromides with aryl boronic acids, in the presence of catalytic quantities of 11, proceed in low yield, accompanied by substantial homocoupling. Palladium agglomeration, to produce catalytically inactive Pd black, is rapid in these reactions, under both aqueous and non-aqueous conditions. The poor reactivity is proposed to arise through an unfavourable near tetrahedral ‘(PP′)Pd(0)’ geometry, which slows the oxidative addition step in the catalytic cycle with either activated or deactivated aryl halides. The steric bulk of the ligand and the associated large P–M–P′ bite angle, particularly at the palladium zero oxidation state, is proposed to account for the poor reactivity. However, we have established that cationic derivatives of 11 promote the cycloisomerisation of diallylmalonate in a regioselective fashion.

Is Li2Pd3B a self-doped hole superconductor?

We propose that the electrons responsible for superconductivity in Li2Pd3B come from the palladium 4d-electrons. So, its electronic properties are likely to be dominated by strong electronic correlations. The basic unit in this material are Pd6B octahedra which share vertices to form a 3-dimensional network. Due to the highly distorted nature of the Pd6B octahedron, one far stretched Pd atom per octahedra becomes almost inactive for electronic conduction. Thus, the material escapes the fate of becoming a half-filled insulating Mott antiferromagnet by hiding extra charges at these inactive Pd sites and becomes a self-doped correlated metal. We propose a 3-dimensional single band t–J model which could be the correct minimal model for this material.

Highly active [Pd(AcO)2(dppp)] catalyst for the CO-C2H4 copolymerization in H2O-CH3COOH solvent [dppp = 1,3-bis(diphenylphosphino)propane

Abstract In MeOH, [Pd(AcO)2(dppp)] becomes an efficient catalyst for the perfectly alternated CO-C2H4 copolymerization when used in combination of a relatively large amount of CH3COOH (CH3COOH/Pd, ca. 2.0×104). Under 4.56 MPa (CO/C2H4 = 1/1), at 90 °C, 7.5 kg polyketone/g Pd∗h are obtained. A significantly higher productivity is obtained in H2O-CH3COOH, in place of MeOH (27.5 kg polymer/g Pd∗h under 4.56 MPa (CO/C2H4 = 1/1), 90 °C, H2O=37% (mol/mol)). Under these conditions the catalyst undergoes only a minor deactivation with time as after 3 h the productivity decreases only by ca. 10%. The 13 C NMR analysis shows that [Pd(AcO)2(dppp)] in CH3COOH-H2O yields a perfectly alternated polyketone of ca. 8000 g/mol bearing only ketonic end groups. During the course of the copolymerization reaction CO2 forms in significant amount. These findings suggest that the insertion of C2H4 into PdH bond starts the catalytic cycle, and that alternating insertions of the monomers followed by protonolysis of a PdC bond of a Pd-CH2-CH2-(CO-C2H4)n-H growing chain yields the polymer with only keto ending groups and a Pd-OH species; a reaction closely related to the WGS reproduces the starting Pd-hydride; deprotonation of this species to inactive Pd(0) is prevented by the acid.