Pd Leaching Research Articles

Polymer-Supported Pd Nanoparticles for Solvent-Free Hydrogenation.

Breaking the trade-off between activity and stability of supported metal catalysts has been a long-standing challenge in catalysis, especially for metal nanoparticles (NPs) with high hydrogenation activity but poor stability. Herein, we report a porous poly(divinylbenzene) polymer-supported Pd NP catalyst (Pd/PDVB) with both high activity and excellent stability for the solvent-free hydrogenation of nitrobenzene, even at ambient temperature (25 °C) and H2 pressure (0.1 MPa). Pd/PDVB gave a turnover frequency as high as 22,632 h-1 at 70 °C and 0.4 MPa, exceeding 5556 h-1 of the classical Pd/C catalyst under equivalent conditions. Mechanistic studies reveal that the polymer support benefits the desorption of the aniline product from the Pd surface, which is crucial for rapid hydrogenation under solvent-free conditions. In addition, the polymer support in Pd/PDVB efficiently hindered Pd leaching, resulting in good stability.

Deconvoluting Substrates, Support, and Temperature Effects on Leaching and Deactivation of Pd Catalysts: An In Situ Study in Flow.

Leaching behavior of three different Pd heterogeneous catalysts (PdEnCat 30, FibreCat FC1001, and Pd/Al2O3) during the Heck reaction of iodobenzene and methyl acrylate, in the presence of triethylamine, was compared using a tandem flow reactor. While leaching was observed in all three cases, Pd/Al2O3 appeared to be the most robust, showing little/no leaching at ambient temperature. The leached Pd species also appear to display different catalytic activities. With a slight modification of the reactor, the leaching caused by individual components of the reaction mixture can be assessed separately. For the polymer-supported catalysts, triethylamine caused the largest amount of leaching, even at 30 °C. In contrast, the leaching from Pd/Al2O3 was observed only in the presence of iodobenzene at 90 °C. Variations in leaching behavior were ascribed to differences in Pd species and immobilization methods.

Enhanced electroreduction of low-concentration nitrate using a Cu-Pd bimetallic cathode system: Performance evaluation, mechanism analysis, and potential applications

Copper (Cu) is a widely recognized cathode for the electrochemical reduction of nitrate (NO3-). Nevertheless, its practical applications are currently restricted due to limited activity and selectivity, particularly at low NO3- concentrations. Herein, a Cu-Pd bimetallic cathode system was designed for NO3- reduction reaction with low NO3- concentrations (≤40 mg-N L−1). In this system, a Cu nanowire array cathode was utilized for NO3- reduction, while a Pd/C particle cathode was employed for NO2- reduction with H2 generated in situ from a polypyrrole electrode. Due to the improved mass transfer and active sites derived from Pd/C particle cathode, along with the synergistic effects of NO3- reduction and NO2- reduction, the constructed Cu-Pd bimetallic cathode system achieved a 7.32-fold increase in NO3– removal rate and a 1.70-fold increase in NH4+ selectivity compared to the traditional Cu cathode. This performance surpassed that of many other state-of-the-art Cu-based cathodes under similar conditions. Moreover, the Cu-Pd bimetallic cathode system displayed high reusability, and extremely low Cu and Pd leaching. Additionally, it showed strong resistance to environmental factors and can adapt to a wide pH range (3−9) and various NO3- concentrations (10–40 mg-N L−1). The electron spin resonance and scavenger experiments confirmed that both direct electron transfer and indirect reduction by atomic H* were involved in the NO3- removal process, with indirect reduction playing a more significant role. Overall, our findings guide the design of superior catalytic systems for NO3- removal to enhance the activity and selectivity of NO3- reduction for efficient water purification.

The Influence of Ligands on the Pd-Catalyzed Diarylation of Vinyl Esters.

The impact of ligands on the palladium-catalyzed 1,2-diarylation reaction course is presented. The application of Pd-dmpzc as a catalyst provides an efficient, chemoselective and sustainable protocol for the synthesis of valuable 1,2-diphenylethyl acetates. The reaction is conducted in water under mild conditions. Reaction products can be easily separated from the reaction mixture and catalyst by simple extraction. What is more, the rational choice of catalyst significantly reduces the leaching of the metal into the product and its contamination (0.1 ppm). Efficient phase separation and ultralow Pd leaching enable the reuse of the water medium containing the Pd-dmpzc catalyst several times without a significant loss of activity and with even higher selectivity (from 95% to 100% in the third cycle). The recyclability of both the catalyst and the reaction medium together with high chemoselectivity and low palladium leaching reduces the amount of waste and the cost of the process, exhibiting an example of a sustainable and green methodology.

Chitosan-functionalized poly(3-hydroxybutyrate) bead as a novel biosupport for palladium in the Suzuki cross-coupling reaction

The utilization of environmentally friendly biopolymer poly(3-hydroxybutyrate) (PHB) as a support for metal catalysts is an unexplored area. Herein, exploring a stable and recyclable heterogeneous palladium (Pd) catalyst based on PHB for the Suzuki cross-coupling reaction is an attractive topic. In this study, a novel hydrophobic and hydrophilic core-shell bead supported Pd catalyst was developed by using PHB particle as a hydrophobic rigid framework and hydrophilic chitosan (CS) as the surface of the supporting matrix to fix Pd nanoparticles (NPs). The results of structural characterization showed that the Pd NPs with a small average size of 2.3±0.82 nm were uniformly distributed on the surface of the PHB-CS bead with a Pd content of 0.12 wt%. This demonstrated that the PHB-CS bead created a stable coordinated environment for Pd fixation, preventing Pd leaching. Furthermore, the Pd@PHB-CS catalyst exhibited high catalytic efficiency with 99% conversion in the Suzuki reaction under mild conditions, using a solvent mixture of EtOH and H2O (1:1) for 0.5 h. It is worth noting that the catalyst exhibits excellent stability, and reusability. It can be reused five times without a significant reduction in activity while maintaining its structural integrity. The successful construction of Pd@PHB-CS will further expand the application of PHB in the field of metal catalysts and may help establish a green and environmentally friendly multifunctional platform for the development of highly recyclable heterogeneous catalysts containing metal nanoparticles.

Effect of I- ligand and surface oxygen-containing groups on catalytic activity and stability of activated carbon-supported Pd catalysts in acetylene dicarbonylation

The support and ligand play a crucial role in regulating the catalytic performance, selectivity, and stability of heterogeneous catalysis. Herein, activated carbon (AC) supported palladium (Pd) catalysts (2.0 wt%) were prepared by impregnation for acetylene dicarbonylation. The oxygen-containing groups on the surface of the AC support were modulated by nitric acid (HNO3) to investigate metal–support interactions on the catalytic performance and stability of the Pd/AC. The characterization of the chemical composition and structure of the supports and catalysts demonstrates that high concentrations of oxygen-containing groups on the surface of the carbon support lead to the fine dispersion of Pd nanoparticles. Notably, the finely dispersed Pd/AC70 catalyst exhibited good activity and stability for acetylene dicarbonylation under high-pressure CO conditions after several rounds of regeneration. Furthermore, operational parameters including the type of support, co-catalyst (KI), and reaction time and temperature were identified for their impact on the reaction. It was found that I- anions serve as a co-catalyst and can ligate with Pd species, thereby preventing the leaching of Pd and improving reaction selectivity. This study presents a straightforward and sustainable modulation strategy to improve the dispersion of active metals, as well as to prevent the leaching and sintering of these metals during high-pressure CO reactions.

Unveiling the potential of Pd(II)‐covalent organic polyimide framework in Suzuki coupling reactions: A study on synthesis, characterization, and catalytic efficiency

In this research, a highly effective heterogeneous Pd catalytic system was conceived by immobilizing Pd(II) onto a substrate composed of covalent organic polyimide frameworks (PI‐COF). The synthesized catalyst underwent comprehensive characterization using techniques such as transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2 adsorption analysis (Brunauer–Emmett–Teller [BET]), Fourier transform infrared (FT‐IR), thermogravimetric analysis (TGA), and inductively coupled plasma atomic emission spectroscopy (ICP‐AES). Subsequently, the catalytic activity of this immobilized Pd catalyst was explored in the context of the Suzuki reaction. The reactions at 70°C up to 1 h under a normal atmosphere in water result in biaryls at good to excellent yields (>90%). More importantly, the leaching of Pd(II) during the reaction process could be negligible (about 0.5 ppm), which is crucially important when preparing bioactive species. The loading of Pd(II) ions on the PI‐COF was 0.49 mmol/g. The catalyst exhibited exceptional performance, demonstrating remarkable efficacy in catalyzing a wide range of aryl halides (excluding aryl chlorides) when reacted with phenylboronic acid under environmentally friendly reaction conditions. Notably, the catalyst exhibited robust recyclability, retaining its catalytic activity throughout 5 reaction cycles without any significant decrease.

Phase engineering of Pd–Te nanoplates via potential energy trapping

Phase modulation of noble metal alloys (NMAs) is critically important in nanoscience since the distinct atomic arrangements can largely determine their physicochemical properties. However, the precise modulation of NMAs is formidably challenging, because thermodynamically stable phases are generally preferential compared to those metastable ones. Herein, we proposed a potential energy trapping strategy for phase modulation of Pd–Te alloys with solvents. Thereinto, ethylene glycol can increase the energy barrier for both Pd leaching and Te introduction, forming metastable Pd20Te7 phase. Inversely, N, N-dimethylformamide is unable to trap metastable phase, inducing the phase evolution to thermodynamically stable PdTe phase, and the precise phase modulation was realized including Pd20Te7, PdTe and PdTe2 phases. The Pd–Te alloys displayed phase-dependent formic acid oxidation catalytic performance with PdTe phase showing the best. This work proposes a strategy for creating metastable phase with potential energy trap, which may deepen the understanding of phase engineering for noble metal-based nanocrystals.

Palladium Catalysts Supported in Microporous Phosphine Polymer Networks.

A new set of microporous organic polymers (POPs) containing diphosphine derivatives synthesized by knitting via Friedel-Crafts has been attained. These amorphous three-dimensional materials have been prepared by utilizing diphosphines, 1,3,5-triphenylbenzene, and biphenyl as nucleophile aromatic groups, dimethoxymethane as the electrophilic linker, and FeCl3 as a promoting catalyst. These polymer networks display moderate thermal stability and high microporosity, boasting BET surface areas above 760 m2/g. They are capable of coordinating with palladium acetate, using the phosphine derivative as an anchoring center, and have proven to be highly efficient catalysts in Suzuki-Miyaura coupling reactions involving bromo- and chloroarenes under environmentally friendly (using water and ethanol as solvents) and aerobic conditions. These supported catalysts have achieved excellent turnover numbers (TON) and turnover frequencies (TOF), while maintaining good recyclability without significant loss of activity or Pd leaching after five consecutive reaction cycles.

Pd on composite chitosan bead containing a star-like hybrid metal-organic framework: A promising catalyst for hydrogenation of lubricants

A star-like hybrid composite of metal-organic framework with amino-functionalized Fe-based metal-organic framework and zeolitic imidazolate framework components was synthesized, characterized and then embedded in chitosan beads. The beads were cross-linked with glutaraldehyde to furnish robust beads, which were then utilized as supporting materials for stabilizing Pd nanoparticles. The obtained catalytic composite was characterized via XRD, FTIR, TGA, ICP, SEM, EDS, elemental mapping and utilized for the hydrogenation of polyalphaolefin oil. The effects of temperature, catalyst loading and hydrogen pressure were investigated to find the best reaction conditions. According to the results, the present catalyst promoted hydrogenation reaction under relatively mild conditions (P = 8 bar, T = 130 °C and 5 wt% catalyst loading) to give the product in 98% yield and bromine index of 50 mg-Br/100 g. Besides, the catalyst exhibited acceptable recyclability and insignificant Pd leaching, which is very promising for large-scale applications. Hot filtration test also affirmed heterogeneous catalysis.

Efficient HCl leaching of platinum group metals from waste three-way catalysts: A study on kinetics and mechanisms

Waste three-way catalysts (TWCs) have attracted much attention due to the presence of platinum group metals (PGMs) and hazardous substances such as heavy metals and organic matter. The extraction of PGMs from waste TWCs using hydrochloric acid (HCl) has been extensively researched. However, the addition of oxidizing agents like H2O2 and aqua regia is necessary to facilitate PGMs dissolution, which poses significant environmental and operational hazards. Hence, developing a green PGMs recovery process without oxidants is imperative. Previously, we investigated the process of Li2CO3 calcination pretreatment to enhance the leaching of PGMs from waste TWCs by HCl, focusing on the process and mechanism of Li2CO3 calcination pretreatment. In this study, we focused on the leaching process of HCl after pretreatment. Our investigation includes a detailed examination of leaching kinetics and mechanisms. The optimal leaching conditions were: leaching temperature of 150 °C, leaching time of 2 h, HCl concentration of 12 M, and liquid-solid ratio of 10 mL/g. The experiments resulted in maximum leaching rates of about 96%, 97%, and 97% for Pt, Pd, and Rh, respectively. However, given the presence of heavy metals, attention needs to be paid to the harmless treatment of waste acids and leaching residues. The Pt and Pd leaching process is controlled by a mixture of interfacial chemical reactions and internal diffusion, and dominated by internal diffusion, while the leaching process of Rh is controlled by interfacial chemical reactions. Li+ in Li2PtO3, Li2PdO2, and Li2RhO3 preferentially leached and underwent ion-exchange reactions with H+, promoting the dissolution of Pt, Pd, and Rh in HCl.

Immobilization of Pd(0) nanoparticles on polyaza-ligand-functionalized polyacrylonitrile fiber as highly active catalyst for the Heck reaction

An novel aza-ligand functionalized polyacrylonitrile fiber (PANPALF) with amino groups and quaternary ammonium salt moieties was synthesized, and then a highly active catalyst for the Heck reaction was obtained by immobilizing Pd(0) nanoparticles on PANPALF through chelation and in situ reduction. The functionalized fibers were characterized by appropriate instruments, such as inductively coupled plasma atomic emission spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), etc., in order to further confirm that the modification process has been successfully carried out. The catalytic properties of the prepared fiber were evaluated in Heck reaction. The experimental results show that PANPALF-Pd(0) can catalyze Heck reactions and achieving 42 kinds of products with yields ranging from 43% to 99% under solvent-free condition, and verifies that the reactive activation of iodized aromatics is better than that of brominated aromatics. Noteworthily, the TON value of the reaction catalyzed by PANPALF-Pd(0) can reach up to 89000, showing that the fiber catalyst has excellent catalytic activity. Moreover, after PANPALF-Pd(0) reused 11 times, the isolated yield was still as high as 93%, and the average single Pd(0) leaching is only 0.075%, indicating that PANPALF-Pd(0) has satisfactory recyclability. Finally, a rational reaction mechanism was proposed to explain the role of fiber catalyst in the reaction.

Simultaneous leaching and separating of palladium from spent palladium catalyst by tribromide ionic liquids as non-volatile oxidizing solvents

The recovery of palladium (Pd) from secondary resources has attracted much attention because of its unique environmental effects and economic value. A novel technique for the oxidation, dissolution, and separation of precious metals is presented in this paper. Tribromide ionic liquids (ILs) with low viscosity, high stability, and strong oxidation were synthesized and applied for the one-step leaching and separation of Pd from spent Pd-Al2O3 catalysts. Through the preliminary performance test, [C1C12PYR]Br3 was selected as the optimal leaching agent from three kinds of tribromide ILs, and the reasons for the obvious difference in leaching ability were analyzed from the perspective of quantum chemistry.The optimum leaching conditions were determined as follows: leaching time was 12 h, leaching temperature was 45 °C, and the dosage of [C1C12PYR]Br3 was 0.8 g (the mass of fixed Pd was 20 mg). Under these conditions, the leaching efficiency (η%) can reach 94.82%. By optimizing the response surface methodology (RSM) model, the leaching temperature in the actual system was determined to be 50.28 °C, and the dosage of [C1C12PYR]Br3 was 0.74 g, which achieved the best economic value of the system. Using XPS, HPLC, and EDS, the leaching mechanism of Pd was demonstrated to be a process that combined redox with coordination and complexation. The kinetic study shows that the rate control step of the leaching process was a chemical reaction process. The 20% hydrazine hydrate can achieve efficient (99%) stripping of Pd in the IL phase. As a result of the leaching agent's good selectivity and reusability when used to recover spent Pd-Al2O3 catalyst, [C1C12PYR]Br3 is regarded as a promising precious metal leaching agent.

Room temperature catalytic synthesis of biaryl and benzimidazole derivatives – Triggered by nano-sized Pd-loaded-modified chitosan polyurethane foams

Though Pd-based catalysts have been the most efficient in catalytic C-C coupling reactions, but the leaching of Pd is one of the serious limitations. To shed more light on the Pd-based catalysts to overcome the existing disadvantages, four different new functionalized polyurethane foams, namely, PUCS-Pd, PUCS-CA-Pd, PUCS-TA-Pd and PUCS-GA-Pd (where PU = polyurethane; CS = chitosan, CA = citric acid, TA = tartaric acid and GA = gallic acid) were prepared. Further, these materials were employed as an effective catalytic system in C-C coupling reactions yielding a series of biaryl products (80–98%) and benzimidazole derivatives (80–98%) at room temperature. The structures and compositions of these catalysts were well characterized to understand the morphology and physicochemical properties. The BET surface area of all these chitosan-based PU foams can be altered and was achieved as high as 12.156 m2g−1. The TEM data demonstrate the presence of 3 to 10 nm sized nano-Pd species grafted on the surface of polymer support. The catalyst, PUCS-CA-Pd facilitated the cross-coupling reactions even at a low Pd-loading of 0.624 mg in methanol-water. Because of the pore structure toward the captured Pd species, PUCS-CA-Pd displayed exceptional stability, recyclability and reusability at least 8 times without loss of activity.

Electrocatalytic dechlorination of florfenicol using a Pd-loaded on blue TiO2 nanotube arrays cathode

Electrocatalytic hydrodechlorination technology provides an attractive strategy to effectively eliminate the biotoxicity of halogenated organic pollutants (HOPs). Herein, we demonstrated a highly dispersed Pd loaded on blue TiO2 nanotube arrays (Pdx-BTNA) cathode that can achieve efficient dechlorination and detoxification toward florfenicol (FLO). Characterization results indicated that strong interaction between Pd and BTNA was achieved through Pd-O bonds, which facilitated the electron transfer rate. Pdx-BTNA significantly outperformed other cathodes (i.e., BTNA, Pdx-TNA and Pdx-Ti) in the electrocatalytic dechlorination of FLO due to enhanced utilization of active H*. The dechlorination performances of FLO well followed the pseudo-first-order kinetic, and the reaction constant rate on Pdx-BTNA cathode (0.035 min−1) was 17.5, 1.94, 7.0 times higher than that on BTNA, Pdx-TNA and Pdx-Ti cathodes, respectively. Quenching experiments and electrochemical characterization indicated that both direct electron transfer and indirect reduction process mediated by H* were involved in the dechlorination of FLO. The optimal dechlorination efficiency of 98.79% was obtained by Pdx-BTNA cathode within 120 min at 1 mM PdCl2, applied cathode potential of −1.2 V. In addition, Pdx-BTNA exhibited superior stability and durability with almost no Pd leaching (0.0011 mg/L) during five dechlorination cycles. Our research proposed a novel strategy to design low noble Pd-loaded cathode material for efficient dechlorination and detoxification of HOPs.

Improved Palladium Extraction from Spent Catalyst Using Ultrasound-Assisted Leaching and Sulfuric Acid–Sodium Chloride System

This paper presents a process for efficiently recovering palladium (Pd) from spent Pd/Al2O3 catalysts used for hydrogenation reactions, using ultrasound-assisted leaching (UAL). A system composed of H2SO4 and NaCl was investigated under ultrasound-enhanced conditions and compared to regular leaching methods to demonstrate the superiority of UAL. Single-factor experiments were conducted to determine the optimal conditions for leaching, which included an ultrasound power of 200 W, a liquid–solid ratio of 5:1, a leaching time of 1 h, a leaching temperature of 60 °C, H2SO4 concentration of 60%, and 0.1 mol of NaCl. The leaching rate under these conditions was found to be 99%. Additionally, kinetic analysis of the UAL process showed that the apparent activation energy of the Pd leaching reaction was 28.7 kJ/mol, and it was found that Pd leaching from spent catalysts was controlled by diffusion. The tailings were analyzed by SEM, revealing that during ultrasonic leaching, the specific surface area of the spent catalyst increased, the mass transfer rate of the solution was accelerated, the passivation film on the surface of the spent catalyst was peeled off, and a new reaction interface was formed. This improved the leaching rate of Pd and provided a new approach to efficiently leach precious metals such as Pd from spent catalysts.

A New Bis‐(NHC)‐P(II) Complex Supported on Magnetic Mesoporous Silica: An Efficient Pd(II) Catalyst for the Selective Buchwald‐Hartwig Monoarylation of Ammonia

AbstractWe report the synthesis and identification of a new bis(NHC)‐Pd(II) catalyst supported on magnetic SBA‐15 (Fe3O4@SBA‐15). The surface of magnetic SBA‐15 was subsequently treated with (3‐aminopropyl) triethoxysilane (APTES), cyanuric chloride (CC), imidazole, and 2‐bromopyridine. The modified magnetic mesoporous material was then further treated with separately prepared trans‐[Pd(Cl)2(SMe2)2] complex to give Fe3O4@SBA‐AP‐CC‐bis(NHC)‐Pd(II) as a supported bis(NHC)‐Pd(II) complex. The catalyst was fully characterized by common spectroscopic methods. X‐ray photoelectron spectroscopy (XPS) revealed that some imine nitrogens interact with palladium atoms. The supported Pd(II) complex catalyzed the direct monoarylation of ammonia with iodo‐, bromo‐, and chloroarenes, selectively. The effects of reaction components and parameters were investigated to establish the optimal reaction conditions. The catalyst was separated by a magnet, washed, and applied in the subsequent run. It exhibited noticeable stability and reused over seven catalytic cycles with negligible deactivation. A small Pd leaching (1.04 %) was observed in the first run.

Recovery of Platinum and Palladium from Spent Automotive Catalysts: Study of a New Leaching System Using a Complete Factorial Design

The recovery of materials and energy from end-of-life products is increasingly a fundamental factor in the sustainable development of various countries. Recovering metals from different types of waste is not only a practice in support of the environment, but is also a profitable economic activity. For this reason, exhausted automotive catalysts can become renewable sources of critical raw materials such as Pt, Pd, and Rh. However, recovering Pt and Pd from spent catalysts through an efficient, economical, and green method remains a challenge. This article presents a new leaching process for the hydrometallurgical recovery of Pt and Pd from exhausted automotive catalysts. The leaching solution consists of an aqueous mixture of hydrochloric acid, two organic acids (citric acid and acetic acid) and hydrogen peroxide. A complete factorial plan on two levels (2k) was performed in order to evaluate the main effects of the analyzed factors and their interactions. The factors that were presumed to be the most influential on the leaching of Pt and Pd were the concentrations of the different reagents and the reaction time. The optimal circumstances for achieving the largest recovery (over 80% Pt and 100% Pd) were achieved using the following conditions: a concentration of HCl of 5 M, a concentration of H2O2 of 10% wt./vol., a concentration of C2H4O2 of 10%vol./vol., and a reaction time of 3 h.

Impregnation of KOAc on PdAu/SiO2 causes Pd-acetate formation and metal restructuring

During catalyst preparation, the simple wet impregnation of KOAc on bimetallic PdAu/SiO2 catalysts forms various Pd-acetate complexes, particularly at increasing concentrations of KOAc and AcOH, causing Pd leaching and metal restructuring.

Pd nanoparticles decorated on a porous Co(BDC-NH2) MOF as an effective heterogeneous catalyst for dye reduction.

Herein, a new catalytic nanocomposite [Co(BDC-NH2)-Pd NPs] composed of a Co(BDC-NH2) MOF has been developed. The catalyst was prepared by modifying the synthesized porous Co(BDC-NH2) MOF with decorated Pd nanoparticles. This nanocatalyst was used as a heterogeneous catalyst in the reductive degradation of organic dyes Rhodamine B and methyl orange with NaBH4. The kinetic and thermodynamic parameters of the reactions were evaluated. The results showed that the low catalyst content could successfully catalyze the dye reduction reaction quickly (1 min). The metal-organic frameworks unique porous morphology of the Co(BDC-NH2) MOF appears to increase dye adsorption and achieve effective dye reduction. Additionally, recyclability studies of the catalyst confirmed that it could be recovered and reused for 10 consecutive reaction cycles with negligible Pd leaching and reduction in catalytic activity.