Silver and palladium nanoparticles-containing products of the low-temperature wave conversion of an energetic material: Catalytic activity in piperylene hydrogenation

The hydrogenation of 1,3-pentadiene into pentenes over the commercial 0.5% Pd/Al₂O₃ catalyst and over a new catalyst containing 1.0% Pd and 3.7% Ag (μ-catalyst) has been investigated. The new catalyst has been prepared via the flameless wave conversion of cyclotrimethylenetrinitramine in a porous composite. The catalytic properties of the new composite in the hydrogenation reaction depend on the hydrogen/1,3-pentadiene ratio and on the catalyst activation temperature. The reaction conditions for selective 1,3-pentadiene hydrogenation have been optimized. The pentenes yield as a function of temperature passes through a maximum at any H₂/C₅H₈ ratio between 1 and 2. The 2-pentene/1-pentene ratio in the reaction products increases as the temperature is raised.     

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(Pd3d_5/2) = 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.     

Supported catalysts based on Pd–In nanoparticles for the liquid- phase hydrogenation of terminal and internal alkynes: 2. catalytic properties

Pd–In/Al₂O₃ and Pd–In/MgAl₂O₄ catalysts prepared from dinuclear Pd–In acetate complexes were studied in the hydrogenation of alkyne compounds with different structures. The Pd–In catalysts demonstrate high selectivity in the hydrogenation of internal alkynes comparable with that of the Lindlar catalyst. Similar activity/selectivity characteristics are reached at a significantly lower Pd content. For terminal alkynes, the favorable effect of Indium introduction is considerably less pronounced. An analysis of the In effect on the selectivity and the ratio between the rates of the first and second hydrogenation steps suggests that the reaction selectivity is determined to a large extent by a thermodynamic factor (adsorption–desorption equilibrium between the reactants and the reaction products).     

Pd/Cu(111)双金属表面催化糠醛脱碳及加氢的反应机理

采用密度泛函理论(DFT)研究糠醛在最稳定Pd/Cu(111)双金属表面上的吸附构型和糠醛脱碳及加氢的反应机理。结果表明,当糠醛初始吸附于O_3-Pd-top、O_7-Cu-hcp位时,吸附构型最稳定,其吸附能为73.4 kJ/mol。糠醛在Pd/Cu(111)双金属表面上更易发生脱碳反应。对于糠醛脱碳反应,所需活化能较低,各个基元反应均为放热反应,糠醛更易先失去支链上的H形成(C_4H_3O)CO,然后中间体脱碳加氢得到呋喃,其中,C_4H_3O加氢生成呋喃所需活化能(72.6 kJ/mol)最高,是反应的控速步骤。对于加氢反应,糠醛与首个氢原子的反应需要最大的活化能(290.4 kJ/mol),是反应的限速步骤。     

Influence of Metallic Modification on Ethylbenzene Dealkylation over ZSM‐5 Zeolites

A series of metal‐modified HZSM‐5 catalysts were prepared by impregnation and were used for ethylbenzene dealkylation of the mixed C8 aromatics (ethylbenzene, m‐xylene and o‐xylene). The effects of different supported metals (Pt, Pd, Ni, Mo) on catalytic performance, including reaction conditions, were investigated. The physicochemical properties of catalysts were characterized by means of XRD, BET, TEM and NH₃‐TPD. Experimental results showed that metallic modification obviously increased the ethylbenzene conversion and reduced the coke deposition, greatly improving the catalyst stability. The distinction of ethylbenzene conversion depended on the interaction between hydrogenation reactivity and acidic cracking of bifunctional metal‐modified zeolites. Compared with Pt and Ni, Pd and Mo were easier to disperse into HZSM‐5 micropores during loading metals. The acidic density of different metal‐modified HZSM‐5 declined in the following order: HZSM‐5>Pt/HZSM‐5>Pd/HZSM‐5>Ni/HZSM‐5>Mo/HZSM‐5. The activity of ethylene hydrogenation decreased with Pt/HZSM‐5>Pd/HZSM‐5>Ni/HZSM‐5>Mo/HZSM‐5. In comparison, Pd/HZSM‐5 showed the best catalytic performance with both high activity and high selectivity, with less cracking loss of m‐xylene and o‐xylene. Moreover, the following reaction conditions were found to be preferable for ethylbenzene dealkylation over Pd/HZSM‐5: 340°C, 1.5 MPa H₂, WHSV 4 h^?1, H₂/C8 4 mol/mol.     

Catalytic properties of nanostructured Pd–Ag catalysts in the liquid-phase hydrogenation of terminal and internal alkynes

A comparative catalytic study of Pd–Ag bimetallic catalysts and the commercial Lindlar catalyst (Pd–Pb/CaCO₃) has been carried out in the hydrogenation of phenylacetylene (PA) and diphenylacetylene (DPA). The Pd–Ag catalysts have been prepared using the heterobimetallic complex PdAg₂(OAc)₄(HOAc)₄ supported on MgAl₂O₄ and aluminas (α-Al₂O₃ and γ-Al₂O₃). Physicochemical studies have demonstrated that the reduction of supported Pd–Ag complex with hydrogen results in homogeneous Pd–Ag nanoparticles. Equal in selectivity to the Lindlar catalyst, the Pd–Ag catalysts are more active in DPA hydrogenation. The synthesized Pd–Ag catalysts are active and selective in PA hydrogenation as well, but the unfavorable ratio of the rates of the first and second stages of the process makes it difficult to kinetically control the reaction. The most promising results have been obtained for the Pd–Ag₂/α-Al₂O₃ catalyst. Although this catalyst is less active, it is very selective and allows efficient kinetic control of the process to be carried out owing to the fact that, with this catalyst, the rate of hydrogenation of the resulting styrene is much lower than the rate of hydrogenation of the initial PA.     

Nanoscale Magnetic Stirring Bars for Heterogeneous Catalysis in Microscopic Systems

Nanometer‐sized magnetic stirring bars containing Pd nanoparticles (denoted as Fe₃O₄‐NC‐PZS‐Pd) for heterogeneous catalysis in microscopic system were prepared through a facile two‐step process. In the hydrogenation of styrene, Fe₃O₄‐NC‐PZS‐Pd showed an activity similar to that of the commercial Pd/C catalyst, but much better stability. In microscopic catalytic systems, Fe₃O₄‐NC‐PZS‐Pd can effectively stir the reaction solution within microdrops to accelerate mass transfer, and displays far better catalytic activity than the commercial Pd/C for the hydrogenation of methylene blue in an array of microdroplets. These results suggested that the Fe₃O₄‐NC‐PZS‐Pd could be used as nanoscale stirring bars in nanoreactors.     

Chemoselective hydrogenation method catalyzed by Pd/C using diphenylsulfide as a reasonable catalyst poison

While Pd/C is one of the most useful catalysts for hydrogenation, the high catalyst activity of Pd/C causes difficulty in its application to chemoselective hydrogenation between different types of reducible functionalities. In order to achieve chemoselective hydrogenation using Pd/C, we investigated catalyst poison as a controller of the catalyst activity. We found that the addition of Ph₂S (diphenylsulfide) to the Pd/C-catalyzed hydrogenation reaction mixture led to reasonable deactivation of Pd/C. By the use of the Pd/C-Ph₂S catalytic system, olefins, acetylenes, and azides can be selectively reduced in the coexistence of aromatic carbonyls, aromatic halides, cyano groups, benzyl esters, and N-Cbz (benzyloxycarbonyl) protecting groups. The present method is promising as a general and practical chemoselective hydrogenation process in synthetic organic chemistry.     

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over Pd-supported HNb<Subscript>3</Subscript>O<Subscript>8</Subscript> metal oxide nanosheet catalyst

A two-dimensional layered niobium oxide and its exfoliated nanosheet were examined as potential solid acid supports for direct synthesis of hydrogen peroxide from hydrogen and oxygen under intrinsically safe and noncorrosive reaction conditions. The catalytic performance strongly depended on the acid strength of the support material. The Pd-supported protonated niobium oxide nanosheet catalyst (Pd/HNb₃O₈-NS) with remarkably enhanced acidity was superior to layered Pd/KNb₃O₈ or Pd/HNb₃O₈ to promote the reaction. Hydrogen peroxide decomposition testing revealed that, although HNb₃O₈ was comparable to its exfoliated counterpart, HNb₃O₈-NS, in suppressing hydrogen peroxide decomposition without hydrogen, HNb₃O₈ was virtually ineffective in preventing hydrogen peroxide hydrogenation in the presence of hydrogen. However, compared with HNb₃O₈, HNb₃O₈-NS was found to be still effective at suppressing hydrogen peroxide hydrogenation. The different efficiency observed between HNb₃O₈ and HNb₃O₈-NS in the prevention of hydrogen peroxide hydrogenation implies that use of a highly acidic support is advantageous to effectively suppress faster and therefore more unfavorable hydrogen peroxide hydrogenation compared with decomposition. This result clearly demonstrates that the highly acidic HNb₃O₈ nanosheet can serve as an efficient solid acid support for direct synthesis of hydrogen peroxide from hydrogen and oxygen.     

H+−H− Pairs in Partially Oxidized MAX Phases for Bifunctional Catalytic Conversion of Furfurals into Linear Ketones

Currently, less favorable C=O hydrogenation and weak concerted acid catalysis cause unsatisfactory catalytic performance in the upgrading of biomass-derived furfurals (i.e., furfural, 5-methyl furfural, and 5-hydroxymethyl furfural) to ketones (i.e., cyclopentanone, 2,5-hexanedione, and 1-hydroxyl-2,5-hexanedione). A series of partially oxidized MAX phase (i.e., Ti₃AlC₂, Ti₂AlC, Ti₃SiC₂) supporting Pd catalysts were fabricated, which showed high catalytic activity; Pd/Ti₃AlC₂ in particular displayed high performance for conversion of furfurals into targeted ketones. Detailed studies of the catalytic mechanism confirm that in situ hydrogen spillover generates Frustrated Lewis H⁺−H⁻ pairs, which not only act as the hydrogenation sites for selective C=O hydrogenation but also provide acid sites for ring opening. The close intimate hydrogenation and acid sites promote bifunctional catalytic reactions, substantially reducing the reported minimum reaction temperature of various furfurals by at least 30–60 °C.     

Development of magnetic,ferrite supported palladium catalysts for 2,4-dinitrotoluene hydrogenation

Cobalt, copper, and nickel ferrite spinel nanoparticles have been synthesized by using a combination of sonochemical treatment and combustion. The magnetic nanoparticles have been used as supports to prepare ~4 wt% palladium catalysts. The ferrites were dispersed in an ethanolic solution of Pd(II) nitrate by ultrasonication. The palladium ions were reduced to metallic Pd nanoparticles, which were then attached to the surface of the different metal oxide supports. Thus, three different catalysts (Pd/CoFe₂O₄, Pd/CuFe₂O₄, Pd/NiFe₂O₄) were made and tested in the hydrogenation of 2,4-dinitrotoluene (DNT). A possible reaction mechanism, including the detected species, has been envisaged based on the results. The highest 2,4-diaminotoluene (TDA) yield (99 n/n%) has been achieved by using the Pd/NiFe₂O₄ catalyst. Furthermore, the TDA yield was also reasonable (84.2 n/n%) when the Pd/CoFe₂O₄ catalyst was used. In this case, complete and easy recovery of the catalyst from the reaction medium is ensured, as the ferrite support is fully magnetic. Thus, the catalyst is very well suited for applicationy in the hydrogenation of DNT or other aromatic nitro compounds.     

Intermetallic Pd<Subscript>1</Subscript>–Zn<Subscript>1</Subscript> nanoparticles in the selective liquid-phase hydrogenation of substituted alkynes

A comparative study of the catalytic characteristics of monometallic Pd/α-Al₂O₃ and bimetallic Pd–Zn/α-Al₂O₃catalysts in the liquid-phase hydrogenation of structurally different substituted alkynes (terminal and internal, symmetrical and asymmetrical) was carried out. It was established that an increase in the reduction temperature from 200 to 400 and 600°C led to a primary decrease in the activity of Pd–Zn/α-Al₂O₃ due to the formation and agglomeration of Pd1–Zn1 intermetallic nanoparticles. The Pd–Zn/α-Al₂O₃ catalyst containing Pd1–Zn1 nanoparticles exhibited increased selectivity to the target alkene formation, as compared with that of Pd/α-Al₂O₃. Furthermore, the use of the Pd–Zn/α-Al₂O₃ catalyst made it possible to more effectively perform the kinetic process control of hydrogenation because the rate of an undesirable complete hydrogenation stage decreased on this catalyst.     

Liquid-phase hydrogenation of alkenes and aromatic compounds with Pd-loaded oxidized diamond catalyst

Hydrogenation reactions of alkenes (cyclohexene, ethyl acrylate, styrene and 1,5-cyclooctadiene) and aromatic compounds (o-, m- and p-xylene) were carried out in order to examine the activity of palladium-loaded surface-oxidized diamond (Pd/O-Dia) catalyst in liquid-phase hydrogenation. The catalytic performance was compared to commercial palladium-loaded activated carbon (Pd/C) catalyst. The catalyst activities were evaluated by conversions of reactants and H₂ uptake rates in the early stage of the reaction. In all the hydrogenation reactions of alkenes and aromatic compounds, the activity of Pd/O-Dia was almost the same as or slightly higher than that of Pd/C. Dispersion of Pd metal was measured by a CO-pulsed adsorption technique and TEM observations of the catalysts. Pd dispersions were on the same order of magnitude according to the CO-pulsed adsorption technique, although the Pd/C catalyst had a higher surface area (718 m²/g) than that of Pd/O-Dia (23 m²/g). The Pd particle sizes on O-Dia measured by TEM observation were slightly smaller than those on the activated carbon. Such highly dispersed Pd particles on O-Dia would contribute to higher activity for the hydrogenation reaction of alkenes and aromatic compounds.     

Catalytic deoxygenation of waste soybean oil over hybrid catalyst for production of bio-jet fuel: in situ supply of hydrogen by aqueous-phase reforming (APR) of glycerol

A one-pot reaction was performed to produce oxygen-free saturated hydrocarbons via the catalytic deoxygenation and hydrogenation of waste soybean oil over a hybrid catalyst (Pd/C and NiO/γ-Al₂O₃). We utilized in situ hydrogen generated from a reforming reaction of glycerol, a byproduct of triglyceride hydrolysis, for the one-pot reaction to produce hydrocarbons. When NiO/γ-Al₂O₃ (2 g) was used along with Pd/C (1 g), most of the unsaturated free fatty acids (FFAs) were hydrogenated into saturated FFAs, and the percentage of desirable hydrocarbons in the liquid product increased, in contrast to the case when only Pd/C (1 g) was used. This result means that using a hybrid catalyst is better for promoting the catalytic deoxygenation reaction than increasing the degree of loading of Pd/C, and suggests that it should be possible to decrease the amount of precious metal catalysts to be used for deoxygenation reaction.     

Synthesis,properties, and activity of nanosized palladium catalysts modified with elemental phosphorus for hydrogenation

The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogenation catalysis were optimized. The modifying effect of elemental phosphorus depends on the P/Pd ratio; it is associated with changes in the catalyst dispersity and the nature of the formed nanoparticles containing various palladium phosphides (PdP₂, Pd5P₂, and Pd₆P) and Pd(0) clusters. The main stages of the formation of palladium catalysts for hydrogenation were determined, and a model of an active catalyst, in which the Pd₆P phosphide is the core of a nanoparticle and Pd(0) clusters form a shell, was proposed.     

Monolithic catalysts for hydrogenation of nitrobenzene to aniline: influence of aluminum suspension properties

Aluminum suspensions with different properties were prepared and characterized by UV–Vis spectra, BET, XRD and NH₃-TPD. A layer of Al₂O₃ was coated onto the cordierite monolith by sol–gel method. Pd/Al₂O₃/cordierite monolith catalysts with the same Pd loading and particle size were prepared, and tested by nitrobenzene hydrogenation under solvent-free conditions. The adsorption of nitrobenzene on acid sites might be the primary effect under the reaction conditions, and a good linear relationship between hydrogenation rate and the acid content of the Al₂O₃ layer was observed. Furthermore, the stronger acid sites were responsible for the lower aniline selectivity.

     

Influence of phosphorus concentration on the state of the surface layer of Pd–P hydrogenation catalysts

Phase composition and surface layer state of the Pd–P hydrogenation catalyst formed at various P/Pd ratios from Pd(acac)₂ and white phosphorus in a hydrogen atmosphere were determined. Palladium on the catalyst surface is mainly in two chemical states: as Pd(0) clusters and as palladium phosphides. As the P/Pd ratio increases, the fraction and size of palladium clusters decrease, and also the phase composition of formed palladium phosphides changes: Pd₃P_0.8 → Pd₅P₂ → PdP₂. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.     

Metal–organic framework derived Pd/ZrO2@CN as a stable catalyst for the catalytic hydrogenation of 2,3,5‐trimethylbenzoquinone

Metal–organic frameworks (MOFs) have recently been identified as versatile sacrificing templates to construct functional nanomaterials for heterogeneous catalysis. Herein, we report a thermal transformation strategy to directly fabricate metal Pd nanoclusters inlaid within a ZrO₂@nitrogen‐doped porous carbon (Pd/ZrO₂@CN) composite using Pd@NH₂‐UiO‐66(Zr) as a precursor that was pre‐synthesized by a one‐pot hydrothermal method. The developed Pd/ZrO₂@CN as a robust catalyst delivered remarkable stability and activity to the catalytic hydrogenation of 2,3,5‐trimethylbenzoquinone (TMBQ) to 2,3,5‐trimethylhydroquinone (TMHQ), a key reaction involved in vitamin E production. The hydrogenation was carried out at 110 °C with 1.0 MPa H₂, and it resulted in 98% TMHQ yield as the sole product over five consecutive cycles, outperforming the analogue Pd/ZrO₂@C without nitrogen doping templated from Pd@UiO‐66(Zr). The excellent catalytic properties of Pd/ZrO₂@CN likely originated from the highly stable ultrafine Pd nanoclusters inlaid within ZrO₂@CN matrix on account of the strong interaction between N and Pd, as well as on the Lewis acidity of ZrO₂, which was beneficial to the hydrogenation.     

Ionic liquid [BMIM]PF6 as a medium for the selective hydrogenation of 1,4-diacetoxybut-2-yne on the Pd-containing catalysts

A possibility of using a ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM]PF₆), as a reaction medium in the liquid-phase hydrogenation of 1,4-diacetoxybut-2-yne was examined. Two types of catalysts were studied: Pd(10%)/C and the palladium-containing catalytic system based on the biopolymer chitosan supported on silica gel (Pd(1%)/chitosan/SiO2). The data obtained indicate high selectivity of hydrogenation of 1,4-diacetoxybut-2-yne to cis-1,4-diacetoxybut-2-ene under selected conditions.     

Properties of Pd–Ag/C catalysts in the reaction of selective hydrogenation of acetylene

Samples of Pd/C and Pd–Ag/C, where C represents carbon nanofibers (CNFs), are synthesized by methane decomposition on a Ni–Cu–Fe/Al₂O₃ catalyst. The properties of Pd/CNF are studied in the reaction of selective hydrogenation of acetylene into ethylene. It is found that the activity of the catalyst in hydrogenation reaction increases, while selectivity decreases considerably when the palladium content rises. The obtained dependences are caused by the features of palladium’s interaction with the carbon support. At a low Pd content (up to 0.04 wt %) in the catalyst, the metal is inserted into the interlayer space of graphite and the catalytic activity is zero. It is established by EXAFS that the main share of palladium in catalysts of 0.05–0.1 wt % Pd/CNF constitutes the metal in the atomically dispersed state. The coordination environment of palladium atoms consists of carbon atoms. An increase in the palladium content in a Pd/CNF catalyst up to 0.3 wt % leads to the formation of highly dispersed (0.8–1 nm) Pd particles. The Pd/CNF samples where palladium is mainly in the atomically dispersed state exhibit the highest selectivity in the acetylene hydrogenation reaction. The addition of silver to a 0.1 wt % Pd/CNF catalyst initially probably leads to the formation of Pd–Ag clusters and then to alloyed Pd–Ag particles. An increase in the silver content in the catalyst above 0.3% causes the enlargement of the alloyed particles and the palladium atoms are blocked by a silver layer, which considerably decreases the catalytic activity in the selective hydrogenation of acetylene.