Palladium Catalysts For Hydrogenation Research Articles

Dynamic structural evolution of MgO-supported palladium catalysts: from metal to metal oxide nanoparticles to surface then subsurface atomically dispersed cations.

Supported noble metal catalysts, ubiquitous in chemical technology, often undergo dynamic transformations between reduced and oxidized states-which influence the metal nuclearities, oxidation states, and catalytic properties. In this investigation, we report the results of in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, and other physical characterization techniques, bolstered by density functional theory, to elucidate the structural transformations of a set of MgO-supported palladium catalysts under oxidative treatment conditions. As the calcination temperature increased, the as-synthesized supported metallic palladium nanoparticles underwent oxidation to form palladium oxides (at approximately 400 °C), which, at approximately 500 °C, were oxidatively fragmented to form mixtures of atomically dispersed palladium cations. The data indicate two distinct types of atomically dispersed species: palladium cations located at MgO steps and those embedded in the first subsurface layer of MgO. The former exhibit significantly higher (>500 times) catalytic activity for ethylene hydrogenation than the latter. The results pave the way for designing highly active and stable supported palladium hydrogenation catalysts with optimized metal utilization.

Synthesis and Catalytic Activity in the Hydrogenation Reaction of Palladium-Doped Metal-Organic Frameworks Based on Oxo-Centered Zirconium Complexes

Metal-nanocluster-doped porous composite materials are attracting considerable research attention, due to their specific catalytic performance. Here we report a simple, cheap, and efficient strategy for the preparation of palladium hydrogenation catalysts based on metal-organic frameworks (MOFs). It has been shown that the synthesis of Pd/MOF results in the formation of palladium nanoparticles uniformly fixed on the surface. The composition and structure of the resulting composites were studied using elemental analysis, DSC, TGA, XRD, TEM, and IR spectroscopy. Pd nanoparticles with an average diameter of 8–12 nm were successfully confined in the UiO-type MOFs, and the obtained nanocomposites exhibited abundant porosity, high stability, and a large surface area. It has been shown that the resulting catalytic systems with high activity, selectivity, and stability reduce phenylacetylene and allyl alcohol to styrene and propanol, respectively, in liquid-phase hydrogenation reactions.

The effect of crystallinity on selectivity of palladium catalysts in hydrogenation of 2-ethyl-9,10-anthraquinone

The catalytic properties of palladium crystallites obtained from various precursors were studied in the hydrogenation of 2-ethyl-9,10-anthraquinone. The highest yield of H2O2 (88%) is achieved on unmodified Pd crystallites obtained from Pd(dba)2 at 50°C. A sigmoidal relationship between the coherent domain size (CDS) of Pd particles and the content of anthrones, which are by-products of the eAQ transformation, is established. Small Pd clusters, along with the reduction of eAQ carbonyl groups, accelerate predominantly the hydrogenation of the eAQH2 aromatic rings. The smallest contribution of the side processes is observed for a palladium catalyst with a CDS of 3–5 nm.

When the nature of surface functionalities on modified carbon dominates the dispersion of palladium hydrogenation catalysts

Commercial carbon nanofibers with different graphitic structure and commercial multiwall carbon nanotubes (CNT) were chemically modified in order to introduce specific alkyl ligands on their surface. Palladium catalysts have been prepared using these modified supports and subsequently tested in the partial hydrogenation of 1,3-butadiene under conditions of excess hydrogen. Herein, we used thermogravimetry (TG), temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and nitrogen adsorption at 77K techniques in order to characterize both supports and catalysts. We focus on testing the effects of support surface functionalities either on morphology of supported palladium (Pd) nanoparticles (NPs) or on their catalytic performances. High selectivity to butenes was obtained with the catalysts prepared over supports containing alkyl chains, while over-hydrogenation to butane took place over oxygen-containing functional groups. Nicely the catalysts with modified supports minimize the secondary hydrogenation of butenes even at high conversions. Therefore, Pd NPs on modified nano-carbon catalysts may open up more opportunities to optimize the activity and the selectivity for partial hydrogenation reactions.

Factors determining the chemoselectivity of phosphorus-modified palladium catalysts in the hydrogenation of chloronitrobenzenes

The precursor nature effect on the state of the Pd–P surface layer in palladium catalysts and on their properties in the liquid-phase hydrogenation of chloronitrobenzenes under mild conditions has been investigated. A general feature of the Pd–P-containing nanoparticles obtained from different precursors and white phosphorus at P/Pd = 0.3 (PdCl2 precursor) and 0.7 (Pd(acac)2 precursor) is that their surface contains palladium in phosphide form (BE(Pd3d 5/2) = 336.2 eV and BE(Р2р) = 128.9 eV) and Pd(0) clusters (BE(Pd3d5/2) = 335.7 eV). Factors having an effect on the chemoselectivity of the palladium catalysts in chloronitrobenzenes hydrogenation are considered, including the formation of small palladium clusters responsible for hydrogenation under mild conditions.

Palladium hydrogenation catalyst based on a porous carbon material obtained upon the dehydrochlorination of a chloro polymer

The applicability of a porous carbon material obtained as a result of the “chemical” dehydrochlorination of chlorinated polyvinyl chloride as a support for palladium hydrogenation catalysts was demonstrated. The efficiency of the catalyst was evaluated in the liquid-phase reactions of nitrobenzene hydrogenation and chlorobenzene hydrodechlorination. Although the specific activity of the catalyst was lower by a factor of 3–4 than that of the samples based on Sibunit and carbon nanotubes, the complete conversion of the initial compounds with the selective formation of end products under mild conditions was achieved at a relatively low palladium content (1.5%).

A study of palladium hydrogenation catalysts based on nanodiamonds and activated carbon

Palladium hydrogenation catalysts based on nanodiamonds and activated carbon with a metal content of 1 wt % have been studied by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). It has been shown that the nature of the support has an effect on the electron binding energy of palladium and the stability of metal sites formed during supporting. The resulting catalysts exhibit activity in the model nitrobenzene and cyclohexene hydrogenation reactions under mild conditions (T = 318 K; $${P_{{H_2}}}$$ = 0.1 MPa; solvent, ethanol); the nanodiamond-based catalyst is significantly more active and efficient than the counterpart based on activated carbon.

From the Lindlar catalyst to supported ligand-modified palladium nanoparticles: selectivity patterns and accessibility constraints in the continuous-flow three-phase hydrogenation of acetylenic compounds.

Site modification and isolation through selective poisoning comprise an effective strategy to enhance the selectivity of palladium catalysts in the partial hydrogenation of triple bonds in acetylenic compounds. The recent emergence of supported hybrid materials matching the stereo- and chemoselectivity of the classical Lindlar catalyst holds promise to revolutionize palladium-catalyzed hydrogenations, and will benefit from an in-depth understanding of these new materials. In this work, we compare the performance of bare, lead-poisoned, and ligand-modified palladium catalysts in the hydrogenation of diverse alkynes. Catalytic tests, conducted in a continuous-flow three-phase reactor, coupled with theoretical calculations and characterization methods, enable elucidation of the structural origins of the observed selectivity patterns. Distinctions in the catalytic performance are correlated with the relative accessibility of the active site to the organic substrate, and with the adsorption configuration and strength, depending on the ensemble size and surface potentials. This explains the role of the ligand in the colloidally prepared catalysts in promoting superior performance in the hydrogenation of terminal and internal alkynes, and short-chain alkynols. In contrast, the greater accessibility of the active surface of the Pd-Pb alloy and the absence of polar groups are shown to be favorable in the conversion of alkynes containing long aliphatic chains and/or ketone groups. These findings provide detailed insights for the advanced design of supported nanostructured catalysts.

Effects of functionalization and catalyst treatments on selective behavior of multi-walled carbon nanotube-supported palladium catalysts in hydrogenation of acetylene

Two nanocatalysts of palladium over multi-walled carbon nanotubes (MWCNT) were synthesized and their catalytic properties evaluated in selective hydrogenation of acetylene. Different procedures were applied to synthesize catalysts which resulted in two distinct average particle sizes of palladium (Pd) over the MWCNT. The resulting catalysts displayed dissimilar NH3-TPD behaviors as well as different selectivity performances at lower temperatures. While enhancement of the hydrogen transfer mechanism occurred upon the temperature increase, similar behaviors for the aforementioned materials were obtained. Furthermore, it was revealed that the catalytic performance at higher temperatures did not depend upon the Pd particle size or heterogeneous energetic sites formed during functionalizing of the MWCNT material.

Structure–Activity Studies on Highly Active Palladium Hydrogenation Catalysts by X-ray Absorption Spectroscopy

Functionalized carbon nanotubes were used to produce Pd-based hydrogenation catalysts. Pd/CNT with small (1–2 nm) Pd particles showed classical catalytic behavior in propyne hydrogenation, with high propene selectivity at moderate conversion levels and propane formation near full conversion. Pd/CNT with larger (∼15 nm) nanoparticles, however, was selective (88%) toward propene even at practically full propyne conversion. An additionally prepared Pd2Ga/CNT catalyst exhibited even higher propene selectivity at full conversion. All of these materials were studied in situ by X-ray absorption spectroscopy at the Pd K-edge. Pd2Ga/CNT was stable under all conditions examined without variation in XANES or in the derived EXAFS parameters. Both Pd/CNT samples formed β-hydride under hydrogen, as assessed from the calculated lattice expansion and the characteristic red shift of the XANES maxima. The minor spectroscopic difference between the monometallic catalysts observed at high propyne conversion suggests the decisive role of a Pd–C (subsurface C) contribution in the structure of larger Pd particles, being absent with ultrasmall nanoparticles. In general, all factors (intermetallic phase formation, subsurface C, etc.) that reduce the surface H coverage will give rise to enhanced partial hydrogenation selectivity of palladium when secondary alkene hydrogenation at late bed segments or diffusion issues in the pores are avoided.

Effect of the nature of the acido ligand in the precursor on the properties of nanosized palladium-based hydrogenation catalysts modified with elemental phosphorus

The effect of the nature of the acido ligand in the precursor and the modifying action of elemental phosphorus on palladium catalysts for hydrogenation are reported. The large turnover frequency (TOF) and turnover number (TON) values observed for styrene hydrogenation on the Pd blacks prepared in situ by PdCl2 reduction with hydrogen in DMF are due to the formation of fine-particle catalyst with a base particle size of 6–10 nm. This is explained by the high PdCl2 reduction rate and by the formation of a palladium cluster stabilizer—dimethylammonium chloride—in the reaction system via the catalytic hydrolysis of the solvent (DMF). The modifying action of elemental phosphorus on the properties of the palladium catalysts depends on the nature of the acido ligand in the precursor. In the case of oxygen-containing precursors at small P/Pd ratios, elemental phosphorus exerts a promoting effect, raising the TON and TOF values by a factor of about 9. In the case of palladium dichloride as the precursor, white phosphorus exerts an inhibiting effect. At the same time, it enhances the stability of the catalyst, raising the TON value at P/Pd = 0.3. The causes of these distinctions are considered.

Formation and properties of hydrogenation catalysts based on palladium bisacetylacetonate and sodium tetrahydroborate

Properties of the Pd(acac)2-nNaBH4 system in catalysis of the reactions of hydrogenation of alkenes, alkynes, and carbonyl and nitro groups were studied. A number of spectral methods (NMR, UV spectroscopy) and X-ray phase analysis were used to examine the main stages of formation of palladium hydrogenation catalysts produced in the interaction of Pd(acac)2 with sodium tetrahydroborate, and reasons for the bimodal nature of the dependence of the catalytic activity on the B/Pd ratio were considered.

Understanding Palladium Hydrogenation Catalysts: When the Nature of the Reactive Molecule Controls the Nature of the Catalyst Active Phase

Die prinzipiellen Unterschiede in der Pd-katalysierten Hydrierung von C-C-Doppel- und -Dreifachbindungen sind auf deutliche Unterschiede in der Zusammensetzung der Katalysatoroberfläche zurückzuführen. Die In-situ-Röntgenphotoelektronenspektroskopie des oberflächennahen Bereichs aktiver Pd-Katalysatoren enthüllt eine starke Korrelation zwischen der chemischen Natur (Alkin/Alken) der reagierenden Moleküle und dem Oberflächenzustand des Katalysators (I–IV).

Understanding Palladium Hydrogenation Catalysts: When the Nature of the Reactive Molecule Controls the Nature of the Catalyst Active Phase

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany, Institute of Isotopes, Hungarian Academy of Sciences, Post Office Box 77, Budapest H-1525, Hungary WestCHEM, Department of Chemistry, University of Glasgow, Glasgow G128 QQ, Scotland, UK Universite de Lyon, Institut de Chimie, Laboratoire de Chimie, Ecole Normale Superieure de Lyon and CNRS 46 Allee d’Italie, 69364 Lyon, Cedex 07 (France)

Identification of Non-Precious Metal Alloy Catalysts for Selective Hydrogenation of Acetylene

The removal of trace acetylene from ethylene is performed industrially by palladium hydrogenation catalysts (often modified with silver) that avoid the hydrogenation of ethylene to ethane. In an effort to identify catalysts based on less expensive and more available metals, density functional calculations were performed that identified relations in heats of adsorption of hydrocarbon molecules and fragments on metal surfaces. This analysis not only verified the facility of known catalysts but identified nickel-zinc alloys as alternatives. Experimental studies demonstrated that these alloys dispersed on an oxide support were selective for acetylene hydrogenation at low pressures.

Preparation of high-performance nanosized palladium hydrogenation catalysts using white phosphorus and phosphine

The nature and properties of nanosized palladium hydrogenation catalysts modified with elemental phosphorus and phosphine (PH3) were studied.

Palladium hydrogenation catalysts modified with aluminum-and phosphorus-containing compounds and with alcohols: Effect of modifiers

The catalytic properties of nanosize catalysts derived from Pd(acac)2 and triethylaluminum in hydrogenation of organic substrates were studied, and the optimal conditions for the catalyst preparation were found. The maximum observed on the plot of the catalyst specific activity vs. Al/Pd ratio was explained by experiments.

Strontium as an efficient promoter for supported palladium hydrogenation catalysts

The effect of strontium promotion is studied for a series of supported palladium catalysts such as Pd/zeolite-β, Pd/Al 2O 3, Pd/SiO 2, Pd/hydrotalcite and Pd/MgO. Strontium is found to be an effective promoter for enhancing the metal area, percentage dispersion of the metal and therefore the hydrogenation activity of the different supported palladium catalysts with varying acid/base properties. The effect of addition of Sr and Pd onto the support by simultaneous and sequential impregnation methods is studied besides the effect of Sr/Pd ratio. It is revealed that when sequential addition of Sr and Pd by impregnation is advantageous for alumina, silica, magnesia and hydrotalcite supported palladium catalysts, simultaneous impregnation of Sr and Pd is found to be more effective for zeolite-β supported catalyst. This may be attributed to the microporous nature of the zeolite support that could cause the migration of Sr to its micropores thereby making them non-available for the promotional effect if added first. Addition of strontium is also found to enhance the basicity of the supported palladium catalysts. TPR studies suggest a change in the metal environment upon Sr-promotion indicating an electronic effect responsible for the improvement in the hydrogen adsorption capacity and hydrogenation activity. The promotional effect of Sr on the supported palladium catalysts is explained to be both electronic and physical in nature.

Aryl couplings with heterogeneous palladium catalysts

The coupling reaction of diverse phenylboronic acids with aryl halogenides and -triflates has been performed by the use of simple palladium hydrogenation catalysts. Evidence for a heterogeneous type of catalysis has been obtained.

Preparation and Characterization of Montmorillonite-Supported Palladium Hydrogenation Catalysts Possessing Molecular Sieving Properties

Highly dispersed palladium metal has been deposited in the interlayer space of montmorillonite clay by the reduction of intercalated palladium complexes. The use of the highly electrophilic Pd(Il) precursor complexes [Pd(PPh3)(NCMe)3]2+ and [PdL4]2+ (L = NCMe, (CH3)2CO) is shown to facilitate the reduction step under very mild conditions (0°C; MeOH as reaction medium). TEM analysis of reduced [Pd(PPh3)(NCMe)3]2+/clay and [Pd(NCMe)4]2+/clay samples reveals that the palladium is present largely in the interlamellar regions of the clay, in the latter case as particles of platelet morphology. A procedure for depositing palladium metal exclusively in the interlamellar region of the clay particles is also described. Transmission electron micrographs of the resulting material (containing 1 wt% Pd) reveal a very narrow size distribution for the palladium crystallites formed, the median particle diameter being ca. 8 Å. Consistent with this is an observed basal spacing of 8-10 Å for the intercalated regions of the clay. Molecular sieving effects are observed when this material is used as a catalyst for the hydrogenation of cyclic and monosubstituted alkenes of varying critical dimension. In contrast, catalysts prepared by simple reduction of Pd(II) complexes homogeneously dispersed in montmorillonite show significantly less shape selectivity towards olefinic reactants.