Low Palladium Loading Research Articles

Sustainable catalytic hydrogenation of high concentration nitrate to ammonia over Ruthenium-based catalysts

Elevated nitrate levels in natural water bodies significantly disrupt the global nitrogen cycle and pose a substantial environmental and human health risk. This study investigates the selective conversion of nitrate to ammonia via catalytic hydrogenation. This highly efficient and selective process offers a sustainable alternative to traditional ammonia production methods while simultaneously addressing nitrate pollution. Ruthenium (Ru) catalysts supported on ceria (CeO2), titania (TiO2), and tin dioxide (SnO2) were synthesized using a simple impregnation method, incorporating oxygen vacancies and low Palladium (Pd) loadings (0.25–0.5 wt%) to enhance catalytic performance. Ru-Pd/CeO2 exhibited superior activity and selectivity, maintaining near 100 % ammonia selectivity across various nitrate concentrations. At 75 °C, complete conversion of 2000 ppm nitrate to ammonia was achieved within 80 min using Ru-Pd/CeO2, and the yield of ammonia can reach up to 16440 mg•h−1•gRu-1. Subsequent ammonia enrichment through evaporation yielded pure ammonia solutions up to 3.6 wt%. Characterization and theoretical calculations suggest that abundant oxygen vacancies on the CeO2 surface and efficient hydrogen spillover from Pd nanoparticles synergistically contribute to the Ru-Pd/CeO2 catalyst’s superior performance. The catalyst exhibited excellent stability and reusability, maintaining high activity and selectivity over four cycles. This research presents a promising strategy for addressing nitrate pollution in water bodies while offering a sustainable and environmentally friendly route for ammonia synthesis. The findings of this study lay the foundation for further advancing and potentially implementing catalytic hydrogenation technology for nitrate pollution remediation and ammonia synthesis.

Elucidating the Synergistic Promotional Mechanism of Water and Oxygen in the Aerobic C-C Homocoupling Reaction Catalyzed by Visible-Light-Derived Core-Shell Pd@A-CQDs Nanostructures.

Rational design and precise synthesis of biogenic noble-metal-based catalysts possessing distinctive structure and composition play a crucial role in the chemical industry, enabling sustainable construction of an inclusive range of chemical resources. In this study, we have effectively fabricated Pd@A-CQDs through a straightforward one-pot aqueous protocol assisted by visible light employing renewable biomass-derived amine-rich carbon quantum dots (A-CQDs). The remarkable visible light harnessing capability (bandgap, ca. 2.81 eV), high density (35.7 × 1018 cm-3), and long lifetime (25 ps) of photocharge carriers and amine-rich surface in A-CQDs make them ideal candidates as both reducing and stabilizing agents, thereby facilitating the in situ construction of metallic Pd(0) nanoparticles. Comprehensive physicochemical characterizations have provided compelling evidence for the spherical morphology of Pd@A-CQDs core-shell nanostructures, with ultrathin A-CQDs shells of ca. 1.9 nm and an average diameter of 14 ± 1 nm. The effectiveness of the synthesized Pd@A-CQDs catalysts was assessed in the ligand- and base-free homocoupling reaction of arylboronic acids in water at ambient temperature. The catalytic tests demonstrated the selective production of the homocoupled compound over protodeboronation products with excellent yield and high catalyst recyclability under ambient conditions. The protocol employed exhibited a high TOF (1.05 × 10-2 mol g-1 min-1) and a low E-factor, with a remarkably low palladium loading. XPS analysis confirmed the retention of the metallic nature of the palladium core within the catalysts during the reaction. The catalytic function of the palladium core in conjunction with the A-CQDs shell, along with the promotional effects provided by water and oxygen for the formation of nucleophilic tetravalent boron, was conclusively recognized by 11B NMR and O2-TPD measurements. The obtained experimental results deliver valuable insights into the probable reaction pathway for the homocoupling reaction catalyzed by the Pd@A-CQDs catalysts. Through a comprehensive and sustainable evaluation, the current methodology exhibits superior performance compared to previously documented techniques in relation to estimated circularity and adherence to good manufacturing practices (GMP).

Enantioselective C-H Arylation for Axially Chiral Heterobiaryls.

An enantioselective palladium-catalyzed C-H arylation of functionalized pyrazoles/triazoles/imidazoles is developed, affording a variety of axially chiral ortho-nitro/formyl-substituted heterobiaryls with excellent enantioselectivities and good yields. The method features a deuterated P-chiral phosphorus ligand CD3-AntPhos, a broad substrate scope of functionalized heterobiaryls, mild reaction conditions, and low palladium loadings.

Pyrazine Derivative as Supramolecular Host for Immobilization of Palladium Nanoparticles for Efficient Suzuki Coupling

AbstractPalladium nanoparticles (NPs) have been extensively explored as unique catalyst for carbon‐carbon coupling reactions. Nonetheless, because of extreme tendency of nanoparticles to undergo agglomeration, the immobilization of these metal NPs on organic frameworks is an important area of research. The present investigation demonstrates the synthesis of pyrazine derivative PYZ‐TA as a supramolecular host for holding co‐released Pd NPs derived from the original catalyst (Pd(II)) under standard Suzuki coupling. Unprecedent, physical bars are not required to capture Pd NPs within the pores of supramolecular host. The as obtained catalyst PYZ‐TA@Pd exhibits high potential to undergo self‐assembly in solid as well as in liquid state. The PYZ‐TA@Pd ensemble shows high catalytic activity and recyclability (up to seven cycles) in Suzuki‐Miyaura coupling reactions using low palladium loading and provides the corresponding products in excellent yields (up to 98 %). Therefore, this study provides an efficient strategy to develop an easy to synthesize palladium centered solid catalyst through coordination between organic host and Pd NPs.

Palladium nanoparticles supported on reduced graphene oxide (Pd@rGO): an efficient heterogeneous catalyst for Suzuki–Miyaura, Heck–Matsuda and Double Suzuki–Miyaura cross-coupling reactions

Synthesis of biaryls, terphenyls and cinnamates using a reduced graphene oxide-supported palladium nanoparticle (Pd@rGO)-based nanocatalyst with low palladium loadings.

Protoboration of Alkynes and Miyaura Borylation Catalyzed by Low Loadings of Palladium.

The versions of Miyaura borylation and protoboration of alkynes catalyzed by low loadings of palladium (400 mol ppm = 0.04 mol %) have been developed. These transformations have a broad substrate scope, good functional-group compatibility, and gram-scale synthetic ability.

Hydroarylation of terminal alkynes with arylboronic acids catalyzed by low loadings of palladium

The hydroarylation reaction of terminal alkynes with arylboronic acids catalyzed by low (400 ppm) loadings of palladium has been developed. The reaction is broad in scope and high-yielding, even on multi-gram scale. It is suitable for the synthesis of alkenes labeled with deuterium, and for the late-stage modification of bioactive molecules.

Magnetically Recoverable Nanoparticulate Catalysts for Cross-Coupling Reactions: The Dendritic Support Influences the Catalytic Performance.

Carbon-carbon cross-coupling reactions are among the most important synthetic tools for the preparation of pharmaceuticals and bioactive compounds. However, these reactions are normally carried out using copper, phosphines, and/or amines, which are poisonous for pharmaceuticals. The use of nanocomposite catalysts holds promise for facilitating these reactions and making them more environmentally friendly. In the present work, the PEGylated (PEG stands for poly(ethylene glycol) pyridylphenylene dendrons immobilized on silica loaded with magnetic nanoparticles have been successfully employed for the stabilization of Pd2+ complexes and Pd nanoparticles. The catalyst developed showed excellent catalytic activity in copper-free Sonogashira and Heck cross-coupling reactions. The reactions proceeded smoothly in green solvents at low palladium loading, resulting in high yields of cross-coupling products (from 80% to 97%) within short reaction times. The presence of magnetic nanoparticles allows easy magnetic separation for repeated use without a noticeable decrease of catalytic activity due to the strong stabilization of Pd species by rigid and bulky dendritic ligands. The PEG dendron periphery makes the catalyst hydrophilic and better suited for green solvents. The minor drop in activity upon the catalyst reuse is explained by the formation of Pd nanoparticles from the Pd2+ species during the catalytic reaction. The magnetic separation and reuse of the nanocomposite catalyst reduces the cost of target products as well as energy and material consumption and diminishes residual contamination by the catalyst. These factors as well as the absence of copper in the catalyst makeup pave the way for future applications of such catalysts in cross-coupling reactions.

A robust polyfunctional Pd(II)-based magnetic amphiphilic nanocatalyst for the Suzuki\u2013Miyaura coupling reaction

Herein, a robust Pd(II)-based polyfunctional magnetic amphiphilic artificial metalloenzyme was prepared by anchoring a Pd(2,2′-dipyridylamine)Cl2 bearing hydrophilic monomethyl ether poly(ethylene glycol) (mPEG) chains on the surface of amino-functionalized silica-coated magnetic nanoparticles. The 2,2′-dipyridylamine (dpa) has shown excellent complexation properties for Pd(II) and it could be easily anchored onto functionalized magnetic support by the bridging nitrogen atom. Moreover, the bridging nitrogen atom at the proximity of Pd(II) catalytic center could play an important role in dynamic suppramolecular interactions with substrates. The leaching, air and moisture resistant [Pd(dpa)Cl2] complex endow the dynamic and robust structure to the designed artificial enzyme. Moreover, the water dispersibility of designed artificial metalloenzyme raised from mPEG chains and the magnetic nanoparticles core which could function as protein mimics endow it other necessary characters of artificial enzymes. The prepared artificial metalloenzyme displayed remarkable activity in Suzuki–Miyaura cross-coupling reaction employing low-palladium loading under mild conditions, with the exceptionally high turnover frequency, clean reaction profile, easy work-up procedure, good to excellent products yields and short reaction times. The designed air- and moisture-stable artificial metalloenzyme could recycle more than fifteen times with easy separation procedure in aqueous solution under aerobic conditions without any noticeable loss in activity.

Monodisperse CuPd alloy nanoparticles supported on reduced graphene oxide as efficient catalyst for directed C−H activation

The palladium-catalyzed selective activation of CH bonds is of great importance for the construction of diverse bioactive molecules. Herein, monodisperse CuPd alloy nanoparticles supported on reduced graphene oxide have been used as high efficient and recyclable catalyst for C(sp3)−H acyloxylation of 8-methylquinolines in good yields under mild conditions. The developed catalytic system proceeds with low palladium loading, good catalyst recyclability, high step and atom efficiency, excellent chemo and regioselectivity, good substrate and functional group compatibility, and applicable for large scale up synthesis. Preliminary studies were carried out to gain insights into the heterogeneous catalysis mechanism.

Asymmetric Hydroesterification of Diarylmethyl Carbinols

AbstractAn efficient asymmetric hydroesterfication of diarylmethyl carbinols is developed for the first time with a Pd‐WingPhos catalyst, resulting in a series of chiral 4‐aryl‐3,4‐dihydrocoumarins in excellent enantioselectivities and good yields. The method features mild reaction conditions, a broad substrate scope, use of easily accessible starting materials, and low palladium loadings. A plausible stereochemical model is also proposed with the Pd‐WingPhos catalyst. This method has enabled a 4‐step asymmetric synthesis of (R)‐tolterodine from readily available starting materials.

Asymmetric Hydroesterification of Diarylmethyl Carbinols.

An efficient asymmetric hydroesterfication of diarylmethyl carbinols is developed for the first time with a Pd-WingPhos catalyst, resulting in a series of chiral 4-aryl-3,4-dihydrocoumarins in excellent enantioselectivities and good yields. The method features mild reaction conditions, a broad substrate scope, use of easily accessible starting materials, and low palladium loadings. A plausible stereochemical model is also proposed with the Pd-WingPhos catalyst. This method has enabled a 4-step asymmetric synthesis of (R)-tolterodine from readily available starting materials.

Preparation of Palladium Metal Intercalated between Graphite Layers with Low Palladium Loading for Cinnamaldehyde Hydrogenation

Abstract Spherical palladium nanoparticles between graphite layers were prepared by the intercalation of palladium chloride followed by its reduction between graphite layers. The palladium particles intercalated between graphite layers were more active for cinnamaldehyde hydrogenation to hydrocinnamaldehyde and hydrocinnamyl alcohol in n-heptane solvent than those supported on graphite surface.

Improvement in the Palladium-Catalyzed Miyaura Borylation Reaction by Optimization of the Base: Scope and Mechanistic Study.

Aryl boronic acids and esters are important building blocks in API synthesis. The palladium-catalyzed Suzuki-Miyaura borylation is the most common method for their preparation. This paper describes an improvement of the current reaction conditions. By using lipophilic bases such as potassium 2-ethyl hexanoate, the borylation reaction could be achieved at 35 °C in less than 2 h with very low palladium loading (0.5 mol %). A preliminary mechanistic study shows a hitherto unrecognized inhibitory effect by the carboxylate anion on the catalytic cycle, whereas 2-ethyl hexanoate minimizes this inhibitory effect. This improved methodology enables borylation of a wide range of substrates under mild conditions.

Cross-Coupling between Hydrazine and Aryl Halides with Hydroxide Base at Low Loadings of Palladium by Rate-Determining Deprotonation of Bound Hydrazine.

Reported here is the Pd-catalyzed C-N coupling of hydrazine with (hetero)aryl chlorides and bromides to form aryl hydrazines with catalyst loadings as low as 100 ppm of Pd and KOH as base. Mechanistic studies revealed two catalyst resting states: an arylpalladium(II) hydroxide and arylpalladium(II) chloride. These compounds are present in two interconnected catalytic cycles and react with hydrazine and base or hydrazine alone to give the product. The selectivity of the hydroxide complex with hydrazine to form aryl over diaryl hydrazine was lower than that of the chloride complex, as well as the catalytic reaction. In contrast, the selectivity of the chloride complex closely matched that of the catalytic reaction, indicating that the aryl hydrazine is derived from this complex. Kinetic studies showed that the coupling process occurs by rate-limiting deprotonation of a hydrazine-bound arylpalladium(II) chloride complex to give an arylpalladium(II) hydrazido complex.

Cross‐Coupling between Hydrazine and Aryl Halides with Hydroxide Base at Low Loadings of Palladium by Rate‐Determining Deprotonation of Bound Hydrazine

AbstractReported here is the Pd‐catalyzed C–N coupling of hydrazine with (hetero)aryl chlorides and bromides to form aryl hydrazines with catalyst loadings as low as 100 ppm of Pd and KOH as base. Mechanistic studies revealed two catalyst resting states: an arylpalladium(II) hydroxide and arylpalladium(II) chloride. These compounds are present in two interconnected catalytic cycles and react with hydrazine and base or hydrazine alone to give the product. The selectivity of the hydroxide complex with hydrazine to form aryl over diaryl hydrazine was lower than that of the chloride complex, as well as the catalytic reaction. In contrast, the selectivity of the chloride complex closely matched that of the catalytic reaction, indicating that the aryl hydrazine is derived from this complex. Kinetic studies showed that the coupling process occurs by rate‐limiting deprotonation of a hydrazine‐bound arylpalladium(II) chloride complex to give an arylpalladium(II) hydrazido complex.

The Heck synthesis of β‐arylated ketones catalyzed by palladium immobilized on functional polysiloxane microspheres

Palladium catalysts, obtained by the impregnation of Pd(OAc)2 on aminopropyl‐ or pyridine‐functionalized polysiloxane microspheres, were used in the Heck reaction of iodobenzene with 3‐buten‐2‐one and 3‐buten‐2‐ol at 120°C using an oil bath or microwave heating. The synthesis of 4‐phenyl‐3‐buten‐2‐one was one‐step arylation of ketone while 4‐phenyl‐2‐butanone was formed in two‐step arylation–isomerization sequential transformation of alcohol. A very low palladium loading, 0.05 mol%, was sufficient to obtain a yield of ketones higher than 90%. In recycling experiments, an effect of the functional group present in polysiloxane was observed and much better results were obtained for the aminopropyl‐modified polymer. The catalyst was easily retrieved and reused in eight consecutive runs in the reaction of 3‐buten‐2‐one, while with 3‐buten‐2‐ol 11 subsequent cycles were performed with practically the same yield.

C−H Arylation of Thiophenes with Aryl Bromides by a Parts-per-Million Loading of a Palladium NNC-Pincer Complex

A palladium NNC-pincer complex efficiently catalyzed the direct arylation of thiophene derivatives with extremely low palladium loadings of the order of parts per million. Thus, the reaction of various thiophenes with aryl bromides in the presence of 25–100 mol ppm of chlorido[(2-phenyl-κ-C 2)-9-phenyl-1,10-phenanthroline-κ2-N,N′]palladium(II) NNC-pincer complex, K2CO3, and pivalic acid in N,N-dimethyl­acetamide afforded the corresponding 2- or 5-arylated thiophenes in good to excellent yields. A combination of the present C–H arylation and Hiyama coupling with the same NNC-pincer complex provides an efficient synthesis of unsymmetrical 2,5-thiophenes with catalyst loadings at mol ppm levels.

The oxygen reduction reaction on palladium with low metal loadings: The effects of chlorides on the stability and activity towards hydrogen peroxide

Hydrogen peroxide is considered one of the most important commodity chemicals worldwide but its main production method, the anthraquinone process, poses serious logistical, environmental and safety challenges. Electrocatalytic synthesis through the reduction of molecular oxygen is a promising H2O2 production route. However, the reduction of molecular oxygen is kinetically hindered and stable electrocatalysts with a high activity and selectivity towards the 2-electron transfer reaction are needed. In this work, we evaluated the influence of chloride on catalysts with low palladium loadings on the ORR selectivity towards H2O2. We report the factors and dynamics that influence H2O2 production and highlight synthesis strategies to obtain close to 100% selectivity. By probing the electrode surface after various degradation cycles, we evaluate the role of adsorbing species and the catalysts oxidation states on the hydrogen peroxide selectivity. We systematically modified the catalyst synthesis using different Pd-precursors that were reduced and supported on high surface area graphene nanoribbons. Identical location transmission electron microscopy was used to probe catalyst dynamics during reaction and the activities and selectivities were measured by a rotating ring disk electrode. We probe the potential boundary conditions that lead to catalyst degradation during accelerated stress tests and potentiostatic polarisation and demonstrate how the catalytically active surface can be revived after degradation. The obtained insights can be used as guideline for the development of active, selective and stable catalysts with low noble metal loadings.

Fabrication of Nanoreactors Based on End-Functionalized Polymethacrylate and Their Catalysis Application

Polymer-based nanoreactor, which can be achieved by self-assembling of amphiphilic polymers in water, has attracted more and more attention in recent years. In this study, a kind of Pd-nanoreactor was fabricated by self-assembling of chiral palladium complex-containing amphiphilic polymers in water. Firstly, chain transfer reagent (CTA) CEPA-PyOx was obtained by condensation reaction of (S,S)-4-hydroxymethyl-5-phenyl-2-(2′-pyridinyl)-1,2-oxazoline (PyOx) and 4-cyano-4-[((ethylthio)carbonothioyl)thio] pentanoic acid (CEPA) in the presence of EDC·HCl. Next, polymethacrylate with a terminal chiral oxazoline ligand (CEPA-POEGMA36-PyOx) was synthesized through Reversible Addition-Fragmentation Chain Transfer (RAFT) polymerization. After removal of the thioester bond and coordination with palladium trifluoroacetate, the polymer self-assembled in water affording Pd-nanoreactor, which possess core-shell structure and with chiral palladium complex in the core. At last, the nanoreactor was successfully applied in the asymmetric catalytic synthesis of flavanone in aqueous phase, and the catalyst loading was greatly reduced compared to unsupported palladium complex. In the presence of low palladium loading (0.5 mol%), flavanone was successfully synthesized with excellent yield (98%) and high enantioselectivity (82%).