Dispersed Palladium Particles Research Articles

Roles of metal and acid sites in the reductive depolymerization of concentrated lignin over supported Pd catalysts

Lignin is a natural polymer contained in lignocellulose and is a potential feedstock for the production of phenolics or small aromatic molecules. However, lignin depolymerization is typically performed using low lignin concentrations and insufficiently active catalysts, which hinders industrial applications. To address this issue, we herein investigated the depolymerization of concentrated kraft lignin and oak-extracted organosolv lignin promoted by supported bifunctional metal (palladium)-acid catalysts and elucidated the roles of acid and metal sites. In addition to acid sites, which are known to be critical for lignin degradation, metal sites were found to be required to initiate lignin depolymerization. Palladium atoms promoted depolymerization even in the absence of acid sites, while acid site–promoted depolymerization required the presence of surface palladium atoms. Catalysts with bulk palladium particles were much more active than those with highly dispersed palladium particles, which suggested that efficient depolymerization required the bulky lignin polymer to be adsorbed on the palladium surface in a multidentate fashion. Thus, our work provides valuable insights into the mechanism of catalytic lignin depolymerization and paves the way to the industrial-scale application of this process.

Modification of a phase-inhomogeneous alumina support of a palladium catalyst. Part II: the effect of palladium dispersion on the formation of hydride forms, electronic state, and catalytic performance in the reaction of partial hydrogenation of unsaturated hydrocarbons

It was shown for the first time that amorphous phase in an alumina support promotes the formation of palladium particles in a wide size range. This catalyst has a low selectivity to butenes in the 1,3-butadiene hydrogenation. It was suggested that surface palladium aluminates contribute to an increase in butene selectivity up to 99.5% at a hydrogenation temperature of not more than 65 °C. At higher reaction temperatures, the catalyst based on phase-homogeneous γ-Al2O3 has the highest activity and butene selectivity. This catalyst was obtained by the traditional impregnation method and contains highly dispersed palladium particles with a sufficiently high electron density. It was shown that the formation of hydride forms on palladium particles with a size of less than 1 nm was detected by temperature-programmed reduction with hydrogen.

Cellular Carbon Foam as a New Type of Supports for Palladium Catalysts

The applicability of a new carbon material—carbon foam with a cellular structure synthesized from the pyrolysis products of hydrocarbon gases—as a catalyst support was demonstrated. The modification of the carbon foam by the deposition of nanoglobular carbon particles on its surface made it possible to obtain a nonporous material with a specific surface area of <1 m2/g and an open-cell structure. Dispersed palladium particles uniformly distributed over the support surface were formed using a hydrolytic deposition method. It was found that the resulting catalyst with palladium immobilized on cellular carbon is promising for the selective hydrogenation of acetylene to ethylene.

Structural and Electronic Properties of Highly Dispersed Particles of the Active Components of Pd/Al2O3 Catalysts of Butadiene-1,3 Hydrogenation

The effect of the acidic characteristics of an alumina support on the properties of formed palladium particles is studied to improve the activity of catalysts for the hydrogenation of unsaturated hydrocarbons of the pyrogasoline fraction. High catalytic activity is characteristic of highly dispersed palladium particles, but the surfaces of palladium particles are blocked by unsaturated hydrocarbons, due to their electron deficiency. In this work, palladium/alumina catalysts with supports of different acidities due to chemical modification with various reagents are studied via NH3 temperature-programed desorption, transmission electron microscopy, and X-ray photoelectron spectroscopy. The samples are subjected to catalytic tests in the butadiene-1,3 hydrogenation reaction under laboratory conditions. The catalysts on supports with acidic modifiers display low butadiene-1,3 conversion and higher selectivity toward butene-1, relative to an unmodified sample. The catalysts on the supports treated with basic additives displayed high butadiene-1,3 conversion while retaining their selectivities toward butene-1 and butane.

An Experimental Approach on Industrial Pd-Ag Supported α-Al2O3 Catalyst Used in Acetylene Hydrogenation Process: Mechanism, Kinetic and Catalyst Decay

The current research presents an experimental approach on the mechanism, kinetic and decay of industrial Pd-Ag supported α-Al2O3 catalyst used in the acetylene hydrogenation process. In the first step, the fresh and deactivated hydrogenation catalysts are characterized by XRD, BET (Brunauer–Emmett–Teller), SEM, TEM, and DTG analyses. The XRD results show that the dispersed palladium particles on the support surface experience an agglomeration during the reaction run time and mean particle size approaches from 6.2 nm to 11.5 nm. In the second step, the performance of Pd-Ag supported α-Al2O3 catalyst is investigated in a differential reactor in a wide range of hydrogen to acetylene ratio, temperature, gas hourly space velocity and pressure. The full factorial design method is used to determine the experiments. Based on the experimental results ethylene, ethane, butene, and 1,3-butadiene are produced through the acetylene hydrogenation. In the third step, a detailed reaction network is proposed based on the measured compounds in the product and the corresponding kinetic model is developed, based on the Langmuir-Hinshelwood-Hougen-Watson approach. The coefficients of the proposed kinetic model are calculated based on experimental data. Finally, based on the developed kinetic model and plant data, a decay model is proposed to predict catalyst activity and the parameters of the activity model are calculated. The results show that the coke build-up and condensation of heavy compounds on the surface cause catalyst deactivation at low temperature.

Refinery oil upgrading under methane environment over PdOx/H-ZSM-5: Highly selective olefin cyclization

Olefin content minimization is critical for the long-term storage and long-distance transportation of oil product, which is commonly achieved through hydrogen saturation at high pressure to form paraffin. In this study, a novel catalytic process is reported to convert olefin to cycloalkane under methane environment with high selectivity. The main components of the product oil collected over Pd/ZSM-5 catalysts during olefin model compound (i.e., 1-decene) upgrading are cyclopentane, 1-methylbutyl- and cyclopentane, hexyl- while there are more than 46 species identified in the liquid product from the run with H-ZSM-5 support engaged. The total selectivity of the aforementioned cycloalkane products is up to 91.3% when 0.27wt% Pd/ZSM-5 catalyst is charged, thus making the product oil more profitable. The HRTEM and XPS data reveal that the highly dispersed palladium particles are in the form of PdOx. The results from pyridine FT-IR and NH3-TPD indicate that these PdOx species anchored onto the surface acidic sites exhibit the moderate acidic feature of Lewis acidic sites, which enhances the formation and stabilization of carbenium ion intermediates generated from 1-decene cracking. The strong interaction between PdOx species and surface acidic sites could not only stabilize the particle size and modify the electronic structure of Pd species but also modulate the acidic properties of ZSM-5 support, leading to promoted methane activation which would not only release sufficient hydrogen donors for preventing olefin aromatization, but also inhibit the over-cracking of carbenium ions. As a result, the selectivity of catalyst assisted olefin cyclization under methane environment is greatly enhanced.

Highly Dispersed Palladium–Polypyrrole Nanocomposites: In‐Water Synthesis and Application for Catalytic Arylation of Heteroaromatics by Direct C–H Bond Activation

AbstractPd@PPy hybrid catalytic materials are synthesized in water via redox polymerization reaction of pyrrole with [Pd(NH3)4Cl2]. The nanocomposites formed are composed of highly dispersed palladium particles which are either zerovalent or easily reducible, and are embedded in spherical polypyrrole globules. A unique combination of high palladium dispersion (NP size: 2.4 nm) and elevated palladium content (35 wt%) is obtained. The components of these novel nanocomposites are characterized by means of FTIR, XPS, XRD, SEM, and TEM microscopy techniques. The process of formation in solution is also monitored using UV‐visible and DLS techniques. The application of these novel hybrid nanomaterials in the palladium‐catalyzed direct arylation of heteroaromatics is reported. High efficiency in C–C bond formation is obtained using these materials. Furans and thiophenes are arylated by using bromoarenes. Pd@PPy nanocomposites can efficiently couple n‐butyl furan and n‐butyl thiophene with bromobenzene and bromoquinoline, as well as with activated or deactivated electron‐poor and electron‐rich functionalized bromoarenes. Thus, a clean reaction process is developed that combines the absence of organic ligand in catalytic reactions and easy recovery of Pd@PPy nanocomposite via simple filtration. Preliminary kinetic and post‐catalysis studies suggest a molecular or colloidal soluble active species. These very active species are efficiently delivered by the nanocomposites and susceptible to a surprisingly uniform back redeposition within the polypyrrole support.

Electrochemical properties of composite films based on poly-3,4-ethylenedioxythiophene with inclusions of metallic palladium

The electrochemical behavior of composite Pd-PEDOT films is studied. These films are obtained by chemical deposition of Pd particles in the polymeric matrix of PEDOT (poly-3,4-ethylenedioxythiophene). Characteristics of the films are determined by means of cyclic voltammetry, faradaic impedance, microgravimetry, and energy-dispersive x-ray fluorescence analysis. Impedance spectra of composite Pd-PEDOT films, compared to the original PEDOT film, reveal a new response at potentials of −0.3 and −0.4 V in the form of a distinct semicircle, which results from processes in the electrochemical sorption-desorption of hydrogen. Weight gain during the chemical deposition of palladium in the polymer structure is estimated by mircrogravimetry. It is shown that the mass of palladium loaded in the film depends on the time of synthesis and the initial concentration of palladium ions in solution. The size of the actual surface and the average radius of dispersed palladium particles in the film are also estimated.

Effect of the Structure of Carbon Nanofibers on the State of an Active Component and on the Catalytic Properties of Pd/C Catalysts in the Selective Hydrogenation of 1,3-Butadiene

The state of highly dispersed palladium particles supported on filamentous carbon was studied using high-resolution electron microscopy, XPS, and X-ray diffraction analysis. Three types of filamentous carbon were used, in which the basal planes of graphite were arranged along, across, and at an angle to the nanofiber axis. The amount of supported palladium was 0.25–5.8 wt %. The structure of the carbon support was found to affect the properties of the active component. Highly dispersed palladium particles exhibited the strongest interaction with a carbon surface formed by the butt ends of graphite (002) layers. This interaction resulted in electron transfer from the metal to the support and in the stabilization of palladium in the most dispersed state. A change in the properties of palladium particles caused a change in the catalytic properties of Pd/C catalysts in the reaction of selective 1,3-butadiene hydrogenation to butenes. The strong interaction of Pd2+ with the butt ends of graphite resulted in the stabilization of palladium in an ionic state. An increase in the fraction of Pd2+ in the catalysts was responsible for a decrease in both the overall activity and selectivity of Pd/C catalysts in the reaction of 1,3-butadiene hydrogenation to butenes.

Synthesis and Size-Selective Application of Palladium Metal Particles Intercalated in Mesopore-Size Controlled Smectite

Mesoporous smectite-type materials (SM) were synthesized with a hydrothermal method from water glass, magnesium chloride and alkyl ammonium salt. The mesopore diameter of SM was controlled from 30 to 130 Å by changing the reaction temperature, pH and the template materials during hydrothermal treatment. Palladium metal particles were intercalated in the mesopores of SM with an ion-exchanging of [Pd(NH3)4Cl2] on them and hydrogen reduction treatment. The size-selective hydrogenation of butadiene-acrylonitrile rubbers (NBR) in carbon tetrachloride was studied using palladium loaded smectite catalysts (Pd-SM). The reaction was controlled by the sizes of mesopore of Pd-SM, which determine the diffusion of polymer molecules onto the dispersed palladium particles within the pores.

Size-selective hydrogenation of NBR polymers catalyzed by pore-size controlled smectites loaded with palladium

The size-selective hydrogenation of butadiene–acrylonitrile rubbers (NBR) in carbon tetrachloride was studied using palladium particles dispersed in pores of different sizes. Synthetic porous smectite materials (SM) were used as supports and their pore sizes were changed with the synthetic conditions from 30 to 130 Å compared with the average size of NBR polymers of 42 Å in the solvent. At atmospheric pressure and at 50 atm of hydrogen, the hydrogenation of carbon–carbon double (CC) bonds of NBR polymers proceeded by the Pd/SM catalysts with mesopores larger than 60 Å. When the mesopores of catalysts are smaller than 40 Å, in contrast, the reaction very slightly proceeds at atmospheric pressure and very slowly proceeds at 50 atm of hydrogen, respectively. The reaction is controlled by the pore sizes of catalysts, which determine the diffusion of the polymer molecules onto the dispersed palladium particles within the pores.

Palladium-doped screen-printed electrodes for monitoring formaldehyde

Screen printing of palladium-doped carbon strips results in the construction of disposable amperometric sensors for formaldehyde. These sensors rely on the strong electrocatalytic action of the dispersed palladium particles contributing to the reduction of formaldehyde. The electrocatalytic behaviour, sensor optimization and analytical performance are reported. The low detection limit (2 × 10–6 mol l–1) is accompanied by a wide linear range and high selectivity. Application for assays of various samples is demonstrated. The new single-use sensor strips should facilitate on-site environmental and industrial monitoring of formaldehyde.

Effects of the Interaction of Dispersed Metal Particles with the Support in Catalysts Prepared Using Organometallic Compounds : VI. Study of the State of Components in (Pd+Zr)/SiO2

Fourier transform infrared, X-ray photoelectron and extended X-ray absorption fine structure spectroscopy applied were to study the formation and state of components and forms of carbon monoxide adsorption with (Pd+Zr)/SiO2 catalysts obtained from organometallic precursors. Based on the results obtained, a model of active sites assuming direct interaction of highly dispersed palladium particles with surface zirconium ions is proposed. Taking into account the similarity of the adsorptive and catalytic properties of (Pd+Zr)/SiO2 and Pd/ZrO2 systems, which reveal the effects of a strong metal-support interaction (SMSI), the possibility of the application of this model to explain the mechanism of SMSI is discussed.

Effects of the Interaction of Dispersed Metal Particles with the Support in Catalysts prepared using Organometallic Compounds. : V. Influence of Group IVB Ions on Adsorption and Catalytic Properties of Pd/SiO2

Results are reported of a systematic study of the effect of Group IVB transition metal (titanium, zirconium, hafnium) ions attached to the silica surface on the properties of dispersed palladium particles. Catalysts have been obtained from organometallic precursors. The following effects of the metal ions have been observed: (i) an increase in stability of palladium particles to sintering in hydrogen atmosphere at up to 873 K; (ii) a decrease in palladium surface coverage with chemisorbed hydrogen and carbon monoxide; (iii) an increase in TON activity and selectivity of palladium towards methanol synthesis from carbon monoxide and hydrogen; and (iv) a decrease in palladium activity in benzene hydrogenation and ethane hydrogenolysis. The data obtained have been compared with those for palladium supported on bulk TiO2 and ZrO2 and are discussed in terms of different hypotheses of the nature of a strong metal-support interaction.

X-Ray diffraction studies of hydride formation ? decomposition in dispersed palladium particles

Dispersed palladium particles disintegrate when subjected to multiple hydride formation-decomposition cycling. The effect is much more pronounced for palladium supported on gamma alumina.