Hydrogenation of Nitrobenzene with Supported Transition Metal Catalysts in Supercritical Carbon Dioxide

Transition metal catalysts such as Pd, Pt, Ru, and Rh supported on carbon, silica and alumina have been examined for the hydrogenation of nitrobenzene (NB) in supercritical carbon dioxide (scCO₂) and in ethanol. The order of hydrogenation activity is Pt>Pd>Ru, Rh in scCO₂ and in ethanol. The effectiveness of the support is C>Al₂O₃, SiO₂ for either Pt or Pd in scCO₂. For all the catalysts, higher selectivity to aniline has been obtained in scCO₂ compared with ethanol. Hydrogenation of nitrobenzene catalyzed with Pd/C and Pt/C catalysts was successfully conducted in scCO₂ with a 100% yield to aniline at a lower reaction temperature of 35 °C. The product aniline (organic phase) can be easily separated from the side‐product water (aqueous phase), solvent (scCO₂), and catalyst (solid) by a simple phase separation process. The hydrogenation of NB is a structure‐sensitive reaction in ethanol as well as in scCO₂ except for a few Pt/C catalysts in which the degree of metal dispersion is small (<0.08).     

Development of High-Efficiency,Magnetically Separable Palladium-Decorated Manganese-Ferrite Catalyst for Nitrobenzene Hydrogenation

Aniline (AN) is one of the most important compounds in the chemical industry and is prepared by the catalytic hydrogenation of nitrobenzene (NB). The development of novel, multifunctional catalysts which are easily recoverable from the reaction mixture is, therefore, of paramount importance. Compared to conventional filtration, magnetic separation is favored because it is cheaper and more facile. For satisfying these requirements, we developed manganese ferrite (MnFe₂O₄)–supported, magnetically separable palladium catalysts with high catalytic activity in the hydrogenation of nitrobenzene to aniline. In addition to high NB conversion and AN yield, remarkable aniline selectivity (above 96 n/n%) was achieved. Surprisingly, the magnetic support alone also shows moderate catalytic activity even without noble metals, and thus, up to 94 n/n% nitrobenzene conversion, along with 47 n/n% aniline yield, are attainable. After adding palladium nanoparticles to the support, the combined catalytic activity of the two nanomaterials yielded a fast, efficient, and highly selective catalyst. During the test of the Pd/MnFe₂O₄ catalyst in NB hydrogenation, no by-products were detected, and consequently, above 96 n/n% aniline yield and 96 n/n% selectivity were achieved. The activity of the Pd/MnFe₂O₄ catalyst was not particularly sensitive to the hydrogenation temperature, and reuse tests indicate its applicability in at least four cycles without regeneration. The remarkable catalytic activity and other favorable properties can make our catalyst potentially applicable to both NB hydrogenation and other similar or slightly different reactions.     

Hydrogenation of Cinnamaldehyde over Pt‐Modified Molecular Sieve Catalysts

The catalytic properties of various Pt‐modified molecular sieves were tested in liquid‐phase hydrogenation of cinnamaldehyde and compared to Pt/MgO and commercial Pt/C catalysts. The type of support considerably influenced the catalytic properties. The superior selectivity performance of microporous catalysts was confirmed; the highest selectivity to allyl alcohol of about 40 % was obtained over the beta support whereas the mesoporous MCM‐41‐supported catalyst was unselective. The highest activity was obtained over the Pt/mordenite catalyst. In order to clarify the performance of catalysts, several characterization methods (XRD, XRF, FTIR, surface measurements) were employed.     

Hydrogenation of Nitrobenzene over a Pd/Al2O3 Catalyst – Mechanism and Effect of the Main Operating Conditions

The catalytic hydrogenation of nitrobenzene (NB) was studied in a three‐phase basket reactor with a commercial Pd/Al₂O₃ sample as catalyst. The kinetic experiments allowed a better understanding of the mechanism behind the formation of aniline (ANL) and by‐products, a topic not yet well comprehended. The effect of some operating conditions was studied and the existence of more by‐products than mentioned in the literature was stated; specifically, benzene formation was verified. Both the reaction kinetics and selectivity were found to be strongly dependent on the temperature, while the effect of total pressure is not that pronounced. Moreover, the high selectivity of the catalyst used in the present work was proved, and as such the deep hydrogenation of ANL to form by‐products only occurs in considerable extension when the NB concentration in the reaction mixture becomes negligible.     

Platinum group metals as catalysts in enantioselective 1-phenylpropane-1,2-dione hydrogenation

Different γ-Al₂O₃ supported Ir, Pd, Ru, Rh and Pt catalysts were tested in enantioselective 1-phenylpropane-1,2-dione hydrogenation using cinchona alkaloid modifiers. Activity and enantioselectivity over Ir and Ru catalysts were low. Pd catalyst was active in the hydrogenation of 1-phenylpropane-1,2-dione, however, the enantioselectivity over this catalyst was almost negligible. Over Pd hydrogenation proceeded mainly via hydrogenation of the C₁O₁ carbonyl group, which is attached to the phenyl ring. Hydrogenation over Pd did not proceed in the second hydrogenation step via an enol form as found for ethyl pyruvate hydrogenation over Pd. The structure-selectivity relationship and solvent effects are similar over Pt and Rh in the first hydrogenation step. However, in the second hydrogenation step of hydroxyketones to diols large mechanistical differences between Pt and Rh were observed. Although the activity over Rh catalysts was lower than over Pt after optimization the best result obtained with Rh/γ-Al₂O₃ (5754 Lancaster) was 60% ee in toluene at maximum yield of 28%, which makes Rh a promising metal for enantioselective hydrogenation.     

Hydrodesulfurization of 4,6-dimethyldibenzothiophene over Pt, Pd, and Pt–Pd catalysts supported on amorphous silica–alumina

The activities and selectivities of Pt, Pd, and Pt–Pd supported on amorphous silica–alumina (ASA) in the hydrodesulfurization (HDS) of 4,6-dimethyldibenzothiophene (4,6-DM-DBT) were investigated. The ASA-supported catalysts had much higher activities than alumina-supported catalysts, due to the creation of electron-deficient metal particles. Pd had a high hydrogenation activity for 4,6-DM-DBT, but the removal of sulfur from 4,6-DM-DBT and its HDS intermediates occurred faster over Pt than over Pd. Comparison of two Pt/ASA catalysts with different Pt loadings showed that the metal dispersion strongly influenced the product selectivity. Larger metal particles led to relatively faster hydrogenation and slower C–S bond breaking. Bimetallic Pt–Pd catalysts were much more active than the monometallic constituents, indicating that the metal particles were alloyed. Acid-catalyzed cracking and isomerization occurred especially over Pt/ASA.     

Pd/C催化剂催化邻硝基苯胺加氢制备邻苯二胺

卤代芳胺是重要的有机中间体,广泛应用于合成染料、农药、医药、香料及橡胶助剂等。卤代芳香硝基化合物通过液相催化加氢制备卤代芳胺的技术以其环境友好、产品质量稳定和工艺先进而受到重视。用负载型贵金属催化剂催化芳香硝基化合物选择加氢制备相应的芳胺有广泛的应用价值。采用邻硝基苯胺为原料,Pd/C为催化剂,低压催化加氢还原合成邻苯二胺,考察不同溶剂、反应压力、反应温度和反应时间对产物收率的影响。结果表明,在甲醇为溶剂、反应温度100 ℃、反应压力0.8 MPa和反应时间100 min条件下,邻苯二胺平均收率为97%。与传统硫化碱还原或铁粉化学法还原工艺相比,以甲醇为溶剂,Pd/C催化剂催化加氢法在减少废水和降低成本等方面有较大优势。     

Structured but not over-structured: Woven active carbon fibre matt catalyst

Catalytic hydrogenation of citral was studied on a Pt on active carbon cloth (ACC) catalyst, with a Pd in ionic liquid on ACC and a commercial Pt on active carbon powder catalysts. The metal was supported on active carbon either by direct impregnation or utilizing the ionic liquid as the intermediate phase on the carbon. The influence on selectivity and activity, of the most important variables, such as temperature and pressure, was investigated in a batch reactor. Four consecutive experiments were carried out with each catalyst. The aim with the reuse of catalysts in the batch reactor was to elucidate eventual catalyst deactivation. The decrease in activity was very notable in the case of traditional impregnated catalysts, whereas the novel SSIL-TM (structured supported ionic liquid-transition metal) or Pd in ionic liquid on active carbon essentially maintained its activity in four consecutive batches. The catalysts were characterized with scanning electron microscopy, N₂ physisorption, and inductively coupled plasma analysis combined with mass spectroscopy. With the Pt on active carbon fibre catalyst, 80–100% selectivity of carbonyl group hydrogenation was achieved at 15% conversion, whereas the Pd in ionic liquid on ACC catalyst displayed an impressive metal efficiency (citral-to-Pd ratio of 156, mol:mol), selectivity (45%) and activity (92% conversion at 140 min) as well as tolerance towards catalyst deactivation. Supported ionic liquids provide a new reaction environment for catalytic transformations.     

Liquid‐Phase Catalytic Hydrogenation of p‐Chlorobenzophenone to p‐Chlorobenzhydrol over a 5 % Pd/C Catalyst

The kinetics of the liquid‐phase catalytic hydrogenation of p‐chlorobenzophenone have been investigated over a 5 % Pd/C catalyst. The effects of hydrogen partial pressure (800–2200 kPa), catalyst loading (0.4–1.6 gm dm^–3), p‐chlorobenzophenone concentration (0.37–1.5 mol dm^–3), and temperature (303–313 K) were studied. A stirring speed > 20 rps has no effect on the initial rate of reaction. Effects of various catalysts (Pd/C, Pd/BaSO₄, Pd/CaCO₃, Pt/C, Raney nickel) and solvents (2‐propanol, methanol, dimethylformamide, toluene, xylene, hexane) on the hydrogenation of p‐chlorobenzophenone were also investigated. The reaction was found to be first order with respect to hydrogen partial pressure and catalyst loading, and zero order with respect to p‐chlorobenzophenone concentration. Several Langmuir‐Hinshelwood type models were considered and the experimental data fitted to a model involving reaction between adsorbed p‐chlorobenzophenone and hydrogen in the liquid phase.     

Synthesis of p‐aminophenol from the hydrogenation of nitrobenzene over metal–solid acid bifunctional catalyst

BACKGROUND: A single‐step conversion of nitrobenzene (NB) to p‐aminophenol (PAP) through catalytic hydrogenation is a widely used synthesis route for PAP. The main shortcoming of this route is the use of sulfuric acid for rearrangement of the phenylhydroxylamine (PHA) intermediate. In this paper, S₂O₈^2?/ZrO₂ (PSZ) solid acid and Pt‐S₂O₈^2?/ZrO₂ (Pt‐PSZ) bifunctional catalysts were prepared for the synthesis of PAP in non‐acid medium. RESULTS: Calcination temperature has a substantial effect on the acidity, structure and activity for PHA rearrangement of PSZ. The highest PAP yield was 33.8% over PSZ calcined at 823 K when the reaction was carried out in water at 423 K. A high PAP yield of 23.9% was achieved by a single‐step reaction of nitrobenzene over Pt‐PSZ bifunctional catalysts. CONCLUSION: PSZ solid acid exhibits high activity for PHA rearrangement. Perfect tetragonal ZrO₂ and much stronger acid sites play important roles in catalytic activity. Inhibiting the hydrogenation activity by reducing the amount of Pt loading on Pt‐PSZ can improve the competition of PHA rearrangement on acid sites with hydrogenation of PHA on metal active sites, resulting in better selectivity to PAP. Copyright © 2008 Society of Chemical Industry     

Effect of the nature of the support on the catalytic performance of noble metal catalysts for the water–gas shift reaction

The catalytic activity of supported noble metal catalysts (Pt, Rh, Ru, and Pd) for the WGS reaction is investigated with respect to the physichochemical properties of the metallic phase and the support. It has been found that, for all metal-support combinations investigated, Pt is much more active than Pd, while Rh and Ru exhibit intermediate activity. The turnover frequency (TOF) of CO conversion does not depend on metal loading, dispersion or crystallite size, but depends strongly on the nature of the metal oxide carrier. In particular, catalytic activity of Pt and Ru catalysts, is 1-2 orders of magnitude higher when supported on “reducible” (TiO₂, CeO₂, La₂O₃, and YSZ) rather than on “irreducible” (Al₂O₃, MgO, and SiO₂) metal oxides. In contrast to what has been found in our previous study over Pt/TiO₂ catalysts, catalytic activity of dispersed Pt does not depend on the structural and morphological characteristics of CeO₂, such as specific surface area or primary crystallite size.     

炭材料负载Pt催化肉桂醛选择性加氢性能的研究

采用液相还原法分别制备了碳纳米管(MWCNT)、活性炭(AC)、碳纳米纤维(CNF)和炭气凝胶(CA)负载质量分数3%的Pt催化剂,并对催化剂的结构和形貌进行了XRD和TEM等表征。以肉桂醛加氢作为探针反应,研究了其催化肉桂醛加氢的活性和产物选择性。结果表明,炭材料的结构对其催化肉桂醛加氢行为具有重要影响,纳米炭材料催化剂（Pt/MWCNT、Pt/CNF）表现出较高的CO选择性加氢行为,而无定形碳催化剂表现为C=C选择性加氢,同时Pt/MWCNT的催化活性最高。     

Enantioselective hydrogenation of carbonyl compounds over Pd-Pt/alumina catalysts

A series of Pd-Pt/alumina catalysts were prepared by consecutive deposition of Pd onto supported Pt particles. Electrochemical model studies indicated that under very mild conditions in aqueous acetic acid the reduction of Pd²⁺ ions occurred partly via the ionization of hydrogen adsorbed on Pt and partly by the slow oxidation of acetic acid. There was only a moderate change in the surface Pd/Pt atomic ratio during heat treatment in flowing hydrogen at 400°C, as determined by XPS analysis. The catalytic performance of the bimetallic catalysts was tested in the enantioselective hydrogenation of ethyl pyruvate and ketopantolactone, in the presence of cinchonidine. Pd was virtually inactive and acted as a site blocker, which decreased the size of Pt ensembles and hindered the formation of the bulky transition complex between the reactant and the chiral modifier. The intrinsically low activity and selectivity of Pd are discussed in the light of H/D exchange studies in deuterated ethanol. This revised version was published online in July 2006 with corrections to the Cover Date.     

EXAFS study on Pd–Pt catalyst supported on USY zeolite

Local structure around Pd and Pt in the bimetallic Pd–Pt catalysts supported on ultra stable Y (USY) zeolite (SiO₂/Al₂O₃=680) was investigated by an extended X-ray absorption fine structure (EXAFS) method during oxidation, reduction, and sulfidation. The Pt L III-edge EXAFS spectra showed that a new bond that was significantly different from Pt–Pt to Pt–Pd metallic bonds was formed in the bimetallic Pd–Pt (4:1) reduced catalysts supported on USY zeolite. This new bond may reflect the ionic properties of Pt through the Pt–Pd interaction. Furthermore this new bond survived sulfidation indicating that the bond has a cationic property and sulfur-tolerance property. The Pt–Pd ionic interaction in these catalysts allows some of the Pd metal to survive as metallic phase. The existence of this metallic phase under sulfidation condition may result in high activity of Pd–Pt (4:1) catalyst supported on USY zeolite in the aromatics hydrogenation.     

Activity and Selectivity Control in Reductive Amination of Butyraldehyde over Noble Metal Catalysts

Approaches to control selectivity and activity in the catalytic reductive amination of butyraldehyde with ammonia over carbon supported noble metal catalysts (Ru, Rh, Pd, and Pt) were explored. Detailed analysis of the reaction network shows that the Schiff base N-[butylidene]butan-1-amine is the most prominent initial product and, only after nearly all butyraldehyde had been converted to N-[butylidene]butan-1-amine, amines are detected in the product mixture. From this intermediate, good hydrogenolysis catalysts (Ru, Rh) produce mostly butylamine, while catalysts less active in hydrogenolysis (Pd, Pt) lead to the hydrogenation of N-[butylidene]butan-1-amine to mostly dibutylamine.     

Characteristics of Al-MCM-41 supported Pt catalysts: effect of Al distribution in Al-MCM-41 on its catalytic activity in naphthalene hydrogenation

Naphthalene hydrogenation was carried out in a high-pressure batch reactor over platinum catalysts supported on Al-MCM-41 where aluminum was incorporated through two different methods: a direct sol–gel method (Pre) and post-synthetic grafting method (Post). The catalytic reaction was also performed in the presence of dibenzothiophene to investigate the sulfur tolerance. The hydrogenation activity, selectivity and the sulfur tolerance strongly depended on the acidic nature of Al-MCM-41 support. It was suggested that the acid sites of Al-MCM-41-Post be more accessible than those of Al-MCM-Pre due to different aluminum distribution within the pore wall. The naphthalene and tetralin conversion increased with the acid amount of the supports in Pt/Al-MCM-41 catalysts. The acid sites in bifunctional catalysts seemed to contribute to alternative pathway by the spillover hydrogen in the acid–metal interfacial region for naphthalene hydrogenation, since the metal dispersions were kept constant for Pt/Al-MCM-41 catalysts. The trans-decalin selectivity generally increased with temperature or acid amount. The acid sites seemed to enhance the sulfur tolerance of supported platinum catalysts due to the electron-deficient state of metal.     

Hydrierung von p‐Nitrophenol zu p‐Aminophenol als Testreaktion für die katalytische Aktivität von Pt‐Trägerkatalysatoren

The hydrogenation of p‐nitrophenol (PNP) to p‐aminophenol (PAP) using NaBH₄ as a reducing agent was studied as a test reaction for determining the catalytic activity of supported Pt catalysts. The initial reaction rate, which is accessible within less than 10 minutes via online UV‐vis spectroscopy at room temperature, ambient pressure and in water as a solvent, was used as measure for the catalytic activity. For three Pt catalysts supported on porous SiO₂, porous glass and Al₂O₃, respectively, significant differences in the catalytic activity were observed. However, especially in case of very active catalysts, limitations of the reaction by internal or external mass transfer have to be considered.     

载体对Pd和Pt催化剂加氢脱硫反应性能的影响

以质量分数为0.8%的二苯并噻吩(DBT)的十氢萘溶液为模型化合物,考察了SiO_2,Si-MCM-41和Al-MCM-41负载的Pd和Pt催化剂加氢脱硫(HDs)反应性能,并与传统的γ-Al_2O_3负载的催化剂进行了对比.反应结果表明,负载型Pd和Pt催化剂在DBT的HDS反应中表现出不同的反应特点.Pd催化剂具有较高的加氢反应路径(HYD)选择性,而Pt催化剂则表现出较高的直接脱硫路径(DDS)选择性.载体结构和表面酸性显著影响其负载的Pd和Pt贵金属催化剂的HDS活性以及HYD选择性和稳定性.提高载体比表面积和酸性有利于提高负载型贵金属催化剂HYD选择性.Al-MCM-41具有规整的介孔结构、较高比表面积和较强酸性,其负载的Pd和Pt催化剂表现出较高的HYD选择性和稳定性.研究还发现,催化剂加氢裂化反应活性随载体酸性的提高而增加.     

Facile synthesis of Pd@Pt octahedra supported on carbon for electrocatalytic applications

Due to the scarcity of Pt, it is highly desirable to construct core‐shell structures with ultrathin Pt shells while maintaining their high electrocatalytic activities. However, it is necessary to preferentially synthesize a core with a specific structure before further formation of core‐shell catalysts with specific morphologies. This prerequisite greatly increases the complexity of the synthesis process. This article describes a synthetic method of core‐shell Pd@Pt octahedra catalysts from Pd nanocubes, truncated nanocubes or truncated octahedra. The formation of octahedral core‐shell structures involves two key factors: (1) the oxidative etching process of Pd atoms at the corner sites; (2) the different reduction rates between Pt and Pd precursors. This mechanism can be extended to synthesize carbon‐supported sub‐8 nm Pd@Pt octahedra from commercial Pd/C catalysts. The derived carbon‐supported Pd@Pt octahedra catalysts performed comparable activity and durability for methanol oxidation reaction with state‐of‐art PtRu/C catalysts. This synthetic method provides an innovative path for large‐scale production of well‐controlled catalysts. © 2017 American Institute of Chemical Engineers AIChE J, 63: 2528–2534, 2017     

Palladium and platinum catalysts supported on carbon nanofiber coated monoliths for low-temperature combustion of BTX

In this work carbon nanofiber (CNF)-coated monoliths with a very thin, homogeneous, consistent and good adhered CNF layer were obtained by means of catalytic decomposition of ethylene on Ni particles.The catalytic behaviour of Pt and Pd supported on the CNF-coated monoliths was studied in the low-temperature catalytic combustion of benzene, toluene and m-xylene (BTX) and compared with the performance of Pt and Pd supported on γ-Al₂O₃ coated monoliths.The catalysts supported on CNF-coated monoliths were the most active, independent of the metal catalyst or the type of the tested aromatic compound. TPD experiments showed that the γ-Al₂O₃ phase retained important amounts of the water molecules produced during the reaction. When water vapour was supplied to the reactant flow, the activity of Pd catalysts decreased much stronger than the Pt ones, and the activity of the Pt catalysts supported on the γ-Al₂O₃ was more affected than that of the catalysts supported on CNF.BTX combustion reactions seem to be catalyzed by Pt and Pd through different kinetic mechanisms, explaining why Pt catalysts always were more active than the Pd ones deposited on the same type of support. Pd catalyzed combustion of benzene is strongly inhibited by oxygen and by water.Catalysts supported on CNF-coated monoliths showed a selectivity to burn benzene better than toluene or m-xylene, attributed to a better aromatic-CNF surface interaction.