Fabrication of Ultrafine Palladium Phosphide Nanoparticles as Highly Active Catalyst for Chemoselective Hydrogenation of Alkynes

Monodisperse palladium phosphide nanoparticles (Pd–P NPs) with a smallest size ever reported of 3.9 nm were fabricated using cheap and stable triphenylphosphine as phosphorous source. After the deposition and calcination at 300 °C and 400 °C, the resulting Pd–P NPs increased in size to 4.0 nm and 4.8 nm, respectively. Notably, the latter NPs probably crystallized with a single phase of Pd₃P_0.95, which acted as a highly active catalyst in semi‐ and stereoselective hydrogenation of alkynes. X‐ray photoelectron spectroscopy analysis determined a positive shift of binding energy for Pd(3d) in Pd–P NPs compared to that in Pd on carbon. It indicated the electron flow from metal to phosphorus and the larger electron deficiency of Pd in Pd–P NPs, which suppressed palladium hydride formation and subsequently increased the selectivity. Thus, this result may also indicate the applications of Pd–P and other metal–P NPs in various selective hydrogenation reactions.     

Pd embedded N,S co‐doped graphene wrapped core‐shell magnetic nanospheres: Engineered stable nanocatalyst for Suzuki couplings

Designed nitrogen and sulfur co‐doped graphene wrapped magnetic core‐shell supported Pd nanoparticles were synthesized through the following steps. Firstly, Fe₃O₄ was prepared, coated with silica and then functionalized with amine groups to create a positive charge on the structure for enhancing the interaction of the Fe₃O₄@SiO₂ with graphene oxide. Secondary, the pre‐catalyst wrapped with graphene to enhance adsorption of aromatic substrates through π–π stacking. Thirdly, graphene was doped with nitrogen and sulfur to increase the grafting of Pd in hybrid. Finally, Pd NPs were attached on the surface of pre‐engineered structure to produce Fe₃O₄@SiO₂@N,S‐wG@Pd which exhibited high performance in Suzuki reactions. This superior activity can be indexed to the incorporation of N and S atoms into graphene led to high anchoring and well‐dispersion of Pd NPs on the nanocomposite surface offering large amounts of active centers, that strongly increased the interaction between Pd and substrates to decreases Pd leaching.     

Periodic mesoporous organosilica grafted palladium organometallic complex: efficient heterogeneous catalyst for water‐medium organic reactions

Water‐medium organic reactions were studied over periodic mesoporous silica (PMO) containing Pd(II) organometallic complex. This heterogeneous catalyst was achieved by Pd(II) compound coordinated with the PPh₂‐ligand onto the pore surface of phenylene‐bridged PMO support. This catalyst displayed ordered mesoporous channels, which ensured the high dispersion of Pd(II) active sites and the convenient diffusion of reactant molecules into the pore channels. Meanwhile, the phenyl group in the pore wall of PMO could enhance the surface hydrophobicity which promoted the adsorption of organic reactant molecules on the catalyst in aqueous environment. As a result, this elaborated catalyst exhibited comparable activity and selectivity with the corresponding PdCl₂(PPh₃)₂ homogeneous catalyst in the water‐medium organic reactions, and could be used repeatedly, showing a good potential in industrial applications. Copyright © 2010 John Wiley & Sons, Ltd.     

CuPd Nanoparticles as a Robust Catalyst for Electrochemical Allylic Alkylation

An efficient CuPd nanoparticle (NP) catalyst (3 nm CuPd NPs deposited on carbon support) is designed for catalyzing electrochemical allylic alkylation in water/isopropanol (1:1 v/v) and 0.2 m KHCO₃ solution at room temperature. The Pd catalysis was Pd/Cu composition‐dependent, and CuPd NPs with a Pd/Cu ratio close to one are the most efficient catalyst for the selective cross‐coupling of alkyl halides and allylic halides to form C?C hydrocarbons with product yields reaching up to 99 %. This NP‐catalyzed electrochemical allylic alkylation expands the synthetic scope of cross‐coupling reactions and can be further extended to other organic reaction systems for developing green chemistry electrosynthesis methods.     

Engineered mesoporous ionic‐modified γ‐Fe2O3@hydroxyapatite decorated with palladium nanoparticles and its catalytic properties in water

A new mesoporous organic–inorganic nanocomposite was formulated and then used as stabilizer and support for the preparation of palladium nanoparticles (Pd NPs). The properties and structure of Pd NPs immobilized on prepared 1,4‐diazabicyclo[2.2.2]octane (DABCO) chemically tagged on mesoporous γ‐Fe₂O₃@hydroxyapatite (ionic modified (IM)‐MHA) were investigated using various techniques. The synergistic effects of the combined properties of MHA, DABCO and Pd NPs, and catalytic activity of γ‐Fe₂O₃@hydroxyapatite‐DABCO‐Pd (IM‐MHA‐Pd) were investigated for the Heck cross‐coupling reaction in aqueous media. The appropriate surface area and pore size of mesoporous IM‐MHA nanocomposite can provide a favourable hard template for immobilization of Pd NPs. The loading level of Pd in the nanocatalyst was 0.51 mmol g^?1. DABCO bonded to the MHA surface acts as a Pd NP stabilizer and can also lead to colloidal stability of the nanocomposite in aqueous solution. The results reveal that IM‐MHA‐Pd is highly efficient for coupling reactions of a wide range of aryl halides with olefins under green conditions. The superparamagnetic nature of the nanocomposite means that the catalyst to be easily separated from solution through magnetic decantation, and the catalytic activity of the recycled IM‐MHA‐Pd showed almost no appreciable loss even after six consecutive runs.     

Multifunctional Tubular Organic Cage‐Supported Ultrafine Palladium Nanoparticles for Sequential Catalysis

The imine condensation reaction of 5,5′‐(benzo[c][1,2,5]thiadiazole‐4,7‐diyl)diisophthalaldehyde with cyclohexanediamine resulted in a shape‐persistent multifunctional tubular organic cage (MTC1). It exhibits selective fluorescence sensing towards divalent Pd ions with a very low detection limit (38 ppb), suggesting effective complexation between these two species. Subsequent reduction of MTC1 and Pd(OAc)₂ with NaBH₄ afforded a cage‐supported catalyst with well‐dispersed ultrafine Pd nanoparticles (NPs) in a narrow size distribution (1.9±0.4 nm), denoted as Pd@MTC1‐1/5. Such ultrafine Pd NPs in Pd@MTC1‐1/5, in cooperation with photocatalytically active MTC1, enable efficient sequential reactions involving visible light‐induced aerobic hydroxylation of 4‐nitrophenylboronic acid to 4‐nitrophenol and the following hydride reduction with NaBH₄. This is the first example of a multifunctional organic cage capable of sensing, directing nanoparticle growth, and catalyzing sequential reactions.     

Palladium–cadmium sulfide nanopowder at oil–water interface as an effective catalyst for Suzuki–Miyaura reactions

A simple and effective strategy is described for the synthesis of Pd–CdS nanopowder by the reduction of an organopalladium(II) complex, [PdCl₂(cod)] (cod = cis ,cis ‐1,5‐cyclooctadiene), in the presence of CdS quantum dots (QDs) at a toluene–water interface. We investigated the impact of addition of CdS QDs on catalytic activity of Pd nanoparticles (NPs). The Pd–CdS nanopowder functions as an efficient catalyst for Suzuki–Miyaura reactions for the formation of carbon–carbon bonds. There is a high electron density on Pd NPs and due to their high electron affinity they behave as an electron scavenger from CdS increasing the rate of oxidative addition, which is the rate‐determining step of the catalytic cycle, and, just as we expect, the C─C coupling reaction with the Pd–CdS nanopowder is faster and occurs in less time than that with Pd nanocatalysts. Compared to classical reactions, this method consistently has the advantages of short reaction times, high yields in a green solvent, reusability of the catalyst without considerable loss of catalytic activity and low cost, and is a facile method for the preparation of the catalyst.     

Triazole‐based ligands functionalized silica: Effects of ligand denticity and donors on catalytic oxidation activity of Pd nanoparticles

Triazole‐based ligands, tris (triazolyl)methanol (Htbtm), bis (triazolyl)‐phenylmethanol (Hbtm), and phenyl (pyridin‐2‐yl)(triazolyl)methanol (Hpytm), with differences in ligand denticity (i.e., bidentate and tridentate) and type of N donors (i.e., triazole and pyridine) were functionalized onto a silica support to produce the corresponding SiO₂‐ L ( L  = tbtm, btm, pytm). Subsequent reactions with Pd (CH₃COO)₂ in CH₂Cl₂ yielded Pd/SiO₂‐ L . ICP‐MS reveals that Pd loadings are higher with increased N loadings, resulting in the following trend: Pd/SiO₂‐tbtm (0.83 mmol Pd g^?1) > Pd/SiO₂‐btm (0.65 mmol Pd g^?1) ~ Pd/SiO₂‐pytm (0.63 mmol Pd g^?1). Meanwhile, TEM images of the used Pd/SiO₂‐ L catalysts after the first catalytic cycle show that the mean size of Pd NPs is highest with Pd/SiO₂‐pytm (8.5 ± 1.5 nm), followed by Pd/SiO₂‐tbtm (6.4 ± 1.6 nm) and Pd/SiO₂‐btm (4.8 ± 1.3 nm). Based on TONs, catalytic studies toward aerobic oxidation of benzyl alcohol to benzaldehyde at 60 °C in EtOH showed that Pd/SiO₂‐pytm possessed the most active surface Pd(0) atoms, most likely as a result of more labile properties of the pyridine–triazole ligand compared to tris‐ and bis (triazolyl) analogs. ICP‐MS and TEM analysis of Pd/SiO₂‐btm indicate minimal Pd leaching and similar average Pd NPs sizes after 1^st and 5^th catalytic runs, respectively, confirming that SiO₂‐btm is an efficient Pd NPs stabilizer. The Pd/SiO₂‐btm catalyst was also active toward aerobic oxidation of various benzyl alcohol derivatives in EtOH and could be reused for at least 7 reaction cycles without a significant activity loss.     

Green synthesis of palladium nanoparticles using Pistacia atlantica kurdica gum and their catalytic performance in Mizoroki–Heck and Suzuki–Miyaura coupling reactions in aqueous solutions

A one‐pot green method for the synthesis of palladium nanoparticles (Pd‐NPs) supported on Pistacia atlantica kurdica (P. a. kurdica) gum is described. This natural gum is used as a reducing and stabilising agent. The formation of the Pd‐NPs/P. a. kurdica gum catalyst was verified using several techniques, such as Fourier transform infrared spectroscopy, ultraviolet–visible spectrophotometry, scanning and transmission electron microscopies, X‐ray diffraction, energy‐dispersive X‐ray spectroscopy, dynamic light scattering and wavelength‐dispersive X‐ray spectroscopy. The Pd‐NPs stabilised by P. a. kurdica gum were employed as a heterogeneous catalyst in Mizoroki–Heck and Suzuki–Miyaura cross‐coupling reactions at low palladium loading (0.1 mol%) under aerobic, phosphine‐free and ligand‐free conditions in water. Product yields of up to 98%, a facile work‐up, no evidence of leached palladium from the catalyst surface and smooth recovery of the catalyst, which can be reused at least eight times, confirm the efficiency of the catalysts in the reactions investigated. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of α‐MoC1−x Nanoparticles with a Surface‐Modified SBA‐15 Hard Template: Determination of Structure–Function Relationships in Acetic Acid Deoxygenation

Surface modification of mesoporous SBA‐15 silica generated a hydrophobic environment for a molybdenum diamine (Mo‐diamine) precursor solution, enabling direct growth of isolated 1.9±0.4 nm α‐MoC_1?x nanoparticles (NPs) inside the pores of the support. The resulting NP catalysts are bifunctional, and compared to bulk α‐MoC_1?x and β‐Mo₂C, the NPs exhibit a greater acid‐site:H‐site ratio and a fraction of stronger acid sites. The greater acid‐site:H‐site ratio results in higher decarbonylation (DCO) selectivity during acetic acid hydrodeoxygenation (HDO) reactions, and the stronger acid sites lead to higher activity and ketonization (KET) selectivity at high temperatures. The hard‐templating synthetic method could be a versatile route toward carbide NPs of varying size, composition, and phase, on a range of mesoporous oxide supports.     

Preparation,characterization and application of silica‐supported palladium complex as a new and heterogeneous catalyst for Suzuki and Sonogashira reactions

An efficient heterogeneous Pd catalytic system has been developed, based on immobilization of Pd nanoparticles (PNPs) on a silica‐bonded propylamine–cyanuric–cysteine (SiO₂–pA–Cyan–Cys) substrate. The synthesized catalyst was characterized by transmission electron microscopy, scanning electron microscopy, FT‐IR, N₂ adsorption analysis (BET), TGA and inductively coupled plasma/atomic emission spectroscopy, and catalytic activity of this catalyst was investigated in the Suzuki and Sonogashira cross‐coupling reactions. The catalysts showed excellent performance in these two reactions, including various aryl halide derivatives (except aryl chloride derivatives) with phenylboronic acid and phenylacetylene under green conditions. Moreover, the catalyst was recycled for several runs without any significant loss of catalytic activity. Copyright © 2014 John Wiley & Sons, Ltd.     

Application of palladium nanoparticle‐decorated Artemisia abrotanum extract‐modified graphene oxide for highly active catalytic reduction of methylene blue,methyl orange and rhodamine B

A green palladium‐based catalyst supported on Artemisia abrotanum extract‐modified graphene oxide (Pd NPs/RGO‐A. abrotanum) hybrid material has been used as a recoverable and heterogeneous nanocatalyst for the catalytic reduction of various dyes, including methylene blue, methyl orange and rhodamine B, in the presence of NaBH₄ as reducing agent in aqueous medium at room temperature. With the help of UV–visible spectroscopy, the catalytic reactions were investigated. According to the results, these reactions followed the pseudo‐first‐order rate equation.     

Highly dispersed palladium nanoclusters incorporated in amino‐functionalized silica spheres for the selective hydrogenation of succinic acid to γ‐butyrolactone

Highly dispersed palladium nanoclusters incorporated on amino‐functionalized silica sphere surfaces (Pd/SiO₂‐NH₂) were fabricated by a simple one‐pot synthesis utilizing 3‐(2‐aminoethylamino)propyltrimethoxysilane (AAPTS) as coordinating agent. Uniform palladium nanoclusters with an average size of 1.1 nm can be obtained during the co‐condensation of tetraethyl orthosilicate and AAPTS owing to the strong interaction between palladium species and amino groups in AAPTS. The palladium particle size can be controlled by addition of AAPTS and plays a significant role in the catalytic performance. The Pd/SiO₂‐NH₂ catalyst exhibits high catalytic activity for succinic acid hydrogenation with 100% conversion and 94% selectivity towards γ‐butyrolactone using 1,4‐dioxane as solvent at 240°C and 60 bar for 4 h. Moreover, the Pd/SiO₂‐NH₂ catalyst is robust and readily reusable without loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

An improved preparation of mesoporous silica‐supported Pd as sustainable catalysts for phosphine‐free Suzuki–Miyaura and Heck coupling reactions

An improved and eco‐friendly procedure has been developed to generate mesoporous silica‐supported palladium nanoparticles (SiO₂@PdNP) that could be used as a sustainable heterogeneous Pd catalyst for phosphine‐free Suzuki–Miyaura and Heck coupling reactions with excellent turnover number and turnover frequency. The presence of Pd on the silica surface was detected by X‐ray diffraction and the structural morphology of SiO₂@PdNP was obtained by transmission electron microscopy. The heterogeneous catalytic system is recyclable and leaching of the metal after the reaction is not apparently observed. Copyright © 2013 John Wiley & Sons, Ltd.     

A Non‐Phosgene Route Synthesis of Carbamate in Continuous Fixed Bed Reactor

A non‐phosgene route synthesis of carbamate was carried out in a continuous fixed‐bed reactor through oxidative carbonylation of aniline using palladium catalysts and sodium iodide as promoter. The activity, selectivity and stability of both carbon and alumina‐supported palladium catalysts were evaluated. It was found that the alumina‐supported catalyst system exhibited a higher activity and selectivity than that of the carbon‐supported system, and an average aniline conversion of 95.6% and carbamate selectivity of 74.6% were achieved for the Se‐Pd/Al₂O₃ catalyst after 91 h on stream. Reclamation analysis of the spent Pd/C catalyst suggested that the deactivation was mainly due to the leaching and sintering of palladium metal and the accumulation of insoluble chemicals on catalyst support also aggravated the decline of catalyst activity. When small amounts of selenium were added to the Pd/Al₂O₃ catalyst, its activity, selectivity and stability were significantly improved which indicated that a promotional effect existed for carbamate formation on a Pd‐Se catalyst system.     

Directing Reaction Pathways through Controlled Reactant Binding at Pd–TiO2 Interfaces

Recent efforts to design selective catalysts for multi-step reactions, such as hydrodeoxygenation (HDO), have emphasized the preparation of active sites at the interface between two materials having different properties. However, achieving precise control over interfacial properties, and thus reaction selectivity, has remained a challenge. Here, we encapsulated Pd nanoparticles (NPs) with TiO₂ films of regulated porosity to gain a new level of control over catalyst performance, resulting in essentially 100 % HDO selectivity for two biomass-derived alcohols. This catalyst also showed exceptional reaction specificity in HDO of furfural and m-cresol. In addition to improving HDO activity by maximizing the interfacial contact between the metal and metal oxide sites, encapsulation by the nanoporous oxide film provided a significant selectivity boost by restricting the accessible conformations of aromatics on the surface.     

Selectivity Control in the Direct CO Esterification over Pd@UiO-66: The Pd Location Matters

The selectivity control of Pd nanoparticles (NPs) in the direct CO esterification with methyl nitrite toward dimethyl oxalate (DMO) or dimethyl carbonate (DMC) remains a grand challenge. Herein, Pd NPs are incorporated into isoreticular metal–organic frameworks (MOFs), namely UiO-66-X (X=-H, -NO₂, -NH₂), affording Pd@UiO-66-X, which unexpectedly exhibit high selectivity (up to 99 %) to DMC and regulated activity in the direct CO esterification. In sharp contrast, the Pd NPs supported on the MOF, yielding Pd/UiO-66, displays high selectivity (89 %) to DMO as always reported with Pd NPs. Both experimental and DFT calculation results prove that the Pd location relative to UiO-66 gives rise to discriminated microenvironment of different amounts of interface between Zr-oxo clusters and Pd NPs in Pd@UiO-66 and Pd/UiO-66, resulting in their distinctly different selectivity. This is an unprecedented finding on the production of DMC by Pd NPs, which was previously achieved by Pd(II) only, in the direct CO esterification.     

Selective Hydrogenation of Citral over a Carbon‐titania Composite Supported Palladium Catalyst

A novel carbon‐titania composite material, C/TiO₂, has been prepared by growing carbon nanofibers (CNFs) on TiO₂ surface via methane decomposition using Ni‐Cu as a catalyst. The C/TiO₂ was used for preparing supported palladium catalyst, Pd/C/TiO₂. The support and Pd/C/TiO₂ catalyst were characterized by BET, SEM, XRD and TG‐DTG. Its catalytic performance was evaluated in selective hydrogenation of citral to citronellal, and compared with that of activated carbon supported Pd catalyst. It was found that the Pd/C/TiO₂ catalyst contains 97% of mesopores. And it exhibited 88% of selectivity to citronellal at citral conversion of 90% in citral hydrogenation, which was much higher than that of activated carbon supported Pd catalyst. This result may be attributed to elimination of internal diffusion limitations, which were significant in activated carbon supported Pd catalyst, due to its microporous structure.     

Active and recyclable ordered mesoporous magnetic organometallic catalyst as high‐performance visible light photocatalyst for degradation of organic pollutants

A novel light‐active magnetic Pd complex as a photocatalyst was prepared through bonding organometallics to mesoporous silica channels formed on the surface of silica‐coated iron oxide nanoparticles. The nanocomposite (denoted as Fe₃O₄@SiO₂@m‐SiO₂@PDA‐Pd(0); PDA = 1,10‐phenanthroline‐2,9‐dicarbaldehyde) is more efficient and has higher photocatalytic capability in the degradation of 2,4‐dichlorophenol under visible light irradiation compared with virgin Pd complex (PDA‐Pd). This noteworthy photodegradation activity can be due to the high dispersion of Pd nanoparticles. High yield, low reaction time and non‐toxicity of the catalyst are the main merits of this protocol. Also magnetic separation is an environmentally friendly alternative method for the separation and recovery of the catalyst, since it minimizes the use of solvents and auxiliary materials, reduces operation time and minimizes catalyst loss by preventing mass loss and oxidation. The produced Pd catalyst was characterised using various techniques. Furthermore, transmission electron microscopy characterization was used for determining the structural properties of the Pd nanocatalyst.     

Periodic Mesoporous Organosilicas: A Type of Hybrid Support for Water‐Mediated Reactions

Hybrid mesoporous periodic organosilicas (Ph‐PMOs) with phenylene moieties embedded inside the silica matrix were used as a heterogeneous catalyst for the Ullmann coupling reaction in water. XRD, N₂ sorption, TEM, and solid‐state NMR spectroscopy reveal that mesoporous Ph‐PMO supports and Pd/Ph‐PMO catalysts have highly ordered 2D hexagonal mesostructures and covalently bonded organic–inorganic (all Si atoms bonded with carbon) hybrid frameworks. In the Ullmann coupling reaction of iodobenzene in water, the yield of biphenyl was 94 %, 34 %, 74 % and for palladium‐supported Ph‐PMO, pure silica (MCM‐41), and phenyl‐group‐modified Ph‐MCM‐41 catalysts, respectively. The selectivity toward biphenyl reached 91 % for the coupling of boromobenzene on the Pd/Ph‐PMO catalyst. This value is much higher than that for Pd/Ph‐MCM‐41 (19 %) and Pd/MCM‐41 (0 %), although the conversion of bromobenzene for these two catalysts is similar to that for Pd/Ph‐PMO. The large difference in selectivity can be attributed to surface hydrophobicity, which was evaluated by the adsorption isotherms of water and toluene. Ph‐PMO has the most hydrophobic surface, and in turn selectively adsorbs the reactant haloaryls from aqueous solution. Water transfer inside the mesochannels is thus restricted, and the coupling reaction of bromobenzene is improved.