Palladium Decorated N-Doped Carbon Foam as a Highly Active and Selective Catalyst for Nitrobenzene Hydrogenation

Carbon foam was synthesized by the carbonization of 4-nitroaniline. The reaction is an alternative of the well-known “carbon snake” (or sugar snake) demonstration experiment, which leads to the formation of nitrogen-doped carbon foils due to its nitrogen content. The synthesized carbon foils were grinded to achieve an efficient catalyst support. Palladium nanoparticles were deposited onto the surface of the support, which showed continuous distribution. The prepared Pd nanoparticle decorated carbon foils showed high catalytic activity in nitrobenzene hydrogenation. By applying the designed catalyst, total nitrobenzene conversion, a 99.1 n/n% aniline yield, and an exceptionally high selectivity (99.8 n/n%) were reached. Furthermore, the catalyst remained active during the reuse tests (four cycles) even without regeneration.     

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).     

Effect of Water Vapor on Carbon Monoxide Oxidation over Promoted Platinum Catalysts

The catalytic behavior of the two kinds of Pd/SiO₂ catalysts prepared from dinitrodiamminepalladium and palladium nitrate are compared for ethene hydroformylation and carbon monoxide hydrogenation. The catalyst prepared from dinitrodiamminepalladium, on which very fine particles of Pd were observed, showed high activity for ethene hydroformylation. On the other hand, the catalyst prepared from palladium nitrate, on which relatively large particles of Pd were observed, showed high activity for CO hydrogenation.     

Three‐Phase Nitrobenzene Hydrogenation over Supported Glass Fiber Catalysts: Reaction Kinetics Study

The catalytic properties of Pd and Pt supported on woven glass fibers (GF) were investigated in the three‐phase hydrogenation of nitrobenzene (NB). Over all catalysts, a 100 % yield of aniline was attained. The catalytic activity for the best catalysts was two times higher than the activity of commercial Pt/C catalyst traditionally used for liquid–phase hydrogenation. The intrinsic reaction kinetics were studied and a reaction scheme is suggested. The direct formation of aniline from NB was observed over Pd/GF with traces of intermediates. Four intermediate products were detected during aniline formation over Pt/GF: nitrosobenzene, phenylhydroxylamine, azoxybenzene, and azobenzene. The Eley‐Rideal kinetic model fits the experimental data well. The parameters of the model were determined as a function of initial NB concentration and hydrogen pressure. Pt and Pd supported on GF in woven fabrics are suggested as suitable materials for reactors with a structured catalytic bed in multi‐phase reactor performance.     

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.     

Deactivation and coke formation on palladium and platinum catalysts in vegetable oil hydrogenation

Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating γ-alumina with either palladium or platinum salts, and by impregnating α-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/γ-Al₂O₃ catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/γ-Al₂O₃ catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/γ-Al₂O₃ catalyst. Virtually no coke was detected on the Pd/α-Al₂O₃ catalyst. The formation of coke on Pd/α-Al₂O₃ may be slower than on the Pd/γ-Al₂O₃ owing to the carrier’s smaller surface area and less acidic character. The absence of deactivation for the Pt/γ-Al₂O₃ catalyst may be explained by slower formation of coke precursors on platinum compared to palladium.     

Precious-Metal-Decorated Chromium(IV) Oxide Nanowires as Efficient Catalysts for 2,4-Toluenediamine Synthesis

The catalytic hydrogenation of 2,4-dinitrotoluene (DNT) to 2,4-toluenediamine (TDA) is a key step in the production of polyurethanes; therefore, the development of efficient hydrogenation catalysts for industrial use is of paramount importance. In the present study, chromium(IV) oxide nanowires were decorated by palladium and platinum nanoparticles in a one-step, simple, and fast preparation method to yield highly efficient hydrogenation catalysts for immediate use. The nanoparticles were deposited onto the surface of CrO₂ nanowires by using ultrasonic cavitation and ethanol as a reduction agent. Beneficially, the catalyst became catalytically active right at the end of the preparation and no further treatment was necessary. The activity of the Pd- and Pt-decorated CrO₂ catalysts were compared in the hydrogenation of 2,4-dinitrotoluene (DNT). Both catalysts have shown high activity in the hydrogenation tests. The DNT conversion exceeded 98% in both cases, whereas the 2,4-toluenediamine (TDA) yields were 99.7 n/n% and 98.8 n/n%, with the Pd/CrO₂ and Pt/CrO₂, respectively, at 333 K and 20 bar H₂ pressure. In the case of the Pt/CrO₂ catalyst, 304.08 mol of TDA formed with 1 mol Pt after 1 h hydrogenation. Activation energies were also calculated to be approximately 24 kJ∙mol⁻¹. Besides their immediate applicability, our catalysts were well dispersible in the reaction medium (methanolic solution of DNT). Moreover, because of their magnetic behavior, the catalysts were easy to handle and remove from the reaction media by using a magnetic field.     

Direct formation of H2O2 from H2 and O2 and decomposition/hydrogenation of H2O2 in aqueous acidic reaction medium over halide-containing Pd/SiO2 catalytic system

Formation of H₂O₂ from H₂ and O₂ and decomposition/hydrogenation of H₂O₂ have been studied in aqueous acidic medium over Pd/SiO₂ catalyst in presence of different halide ions (viz. F⁻, Cl⁻ and Br⁻). The halide ions were introduced in the catalytic system via incorporating them in the catalyst or by adding into the reaction medium. The nature of the halide ions present in the catalytic system showed profound influence on the H₂O₂ formation selectivity in the H₂ to H₂O₂ oxidation over the catalyst. The H₂O₂ destruction via catalytic decomposition and by hydrogenation (in presence of hydrogen) was also found to be strongly dependent upon the nature of the halide ions present in the catalytic system. Among the different halides, Br⁻ was found to selectivity promote the conversion of H₂ to H₂O₂ by significantly reducing the H₂O₂ decomposition and hydrogenation over the catalyst. The other halides, on the other hand, showed a negative influence on the H₂O₂ formation by promoting the H₂ combustion to water and/or by increasing the rate of decomposition/hydrogenation of H₂O₂ over the catalyst. An optimum concentration of Br⁻ ions in the reaction medium or in the catalyst was found to be crucial for obtaining the higher H₂O₂ yield in the direct synthesis.     

Hydrodechlorination of 1,1-Dichlorotetrafluoroethane on Supported Palladium Catalysts. A Static-Circulation Reactor Study

Supported palladium catalysts were studied in CF₃CFCl₂ hydrodechlorination at 100°C using a static-circulation system. In order to minimize catalyst's deactivation a large excess of hydrogen was employed (H₂/CF₃CFCl₂ ratio 54/1). In spite of this precaution significant inhibition of the process occurred, associated with blocking palladium surface by hydrogen chloride species. Differences in the catalytic behavior of alumina-supported and unsupported palladium are discussed. A mild dependence between the catalytic activity and Pd dispersion was found. The Pd/Al₂O₃ catalyst characterized by low metal dispersion was more active than highly dispersed catalysts, showing the overall activity and selectivity to CF₃CFH₂ comparable with those observed by other authors for palladium single crystals. It is speculated that the most active sites for hydrodechlorination are plane atoms, whereas low coordination sites (on edges and corners of metal crystallites) are less suitable.     

二氧化硅接枝黑荆树单宁负载钯催化剂的制备及其性能

以二氧化硅接枝黑荆树单宁为载体,制备了一种新型的负载型钯催化剂,并将其用于硝基苯的液相催化加氢。结果表明,二氧化硅接枝黑荆树单宁负载钯催化剂(SiO2-BT-Pd)对硝基苯的液相加氢具有很高的催化活性。当以甲醇为溶剂,在温度为50℃和H2压力1.0MPa的条件下进行反应时,SiO2-BT-Pd对硝基苯的初始催化转化频率(TOF)高达2061.5mol/(mol·h),转化率在80min达到100%,转化为苯胺的选择性为99.5%。该催化剂重复使用4次后,其催化活性及选择性没有明显降低。这表明SiO2-BT-Pd是一种具有高催化活性和高选择性的负载型催化剂。     

SiO_2负载高分子钯配合物催化剂的制备及其加氢性能研究

以SiO2为载体,三聚氰胺与甲醛的缩聚物为高分子配体,制备出一类新的SiO2负载含氮杂环的高分子配体配合钯催化剂,并用XPS对其进行了结构表征,结果表明高分子配体与活性中心钯进行了配位作用;并以硝基苯的加氢反应为研究对象,考察了其对硝基苯的催化加氢活性。在复合载体中,较佳的氮含量为1.49%;在氮气保护下,用乙醇还原时制备的催化剂具有较高的催化活性;并考察了反应温度及催化剂用量对反应的影响。在0.3g(0.00523mmolPd/0.1g催化剂)催化剂作用下,1mL硝基苯在乙醇溶剂中于313K的反应温度下,在0.1MPa压力下加氢2h,可使硝基苯的转化率达98%,而产物仅有苯胺。     

Synthesis of HFC-134a by isomerization and hydrogenation

For the preparation of HFC-134a, the isomerization of CFC-114 and the hydrogenation of CFC-114a were investigated. Both reactions were catalyzed by AlC₃ and supported Pd catalysts, respectively. For the comparison purpose, the isomerization of CFC-113 was carried out also. With virgin AICI₃ catalyst, both isomerization reactions proceeded after a certain induction period probably because the catalyst needed the activation by the halogen exchange. The catalyst deactivated gradually with the time on stream. However, the deactivation rate could be reduced by removing impurities from the reactants. Isomerization rate of CFC-114 was much slower than that of CFC-113. Palladium supported on carbon catalyzed the hydrogenation reaction quite selectively while the selectivity declined when the support was replaced with different supports. The catalytic activity and selectivity to desired products increased in the following order. Pd/kieselguhr Pd/silica-alumina ≅Pd/silica gel Pd/TiO₂Pd/Al₂O₃Pd/C     

Characterization of Pd catalysts supported on USY zeolites with different SiO2/Al2O3 ratios for the hydrogenation of naphthalene in the presence of benzothiophene

A series of ultra-stable Y-type (USY) zeolites with different SiO₂/Al₂O₃ ratios in the range of 10–80 were used as supports for preparing Pd/USY at 2 wt% Pd loading. The FT-IR of hydroxyl groups of USY zeolites, the n-butylamine chemisorption and the temperature-programmed desorption were used in combination to characterize the zeolite acidity. TPR, H₂-TPD and chemisorption using H₂ were used to characterize the Pd reduction and dispersion. The hydrogenation of naphthalene was conducted at 200 °C in the presence of benzothiophene at different sulfur/metal ratios. The hydrogenation activity, selectivity, and the sulfur tolerance strongly depended on the SiO₂/Al₂O₃ ratio (thus the acidity) of the zeolites. The activity decreased with increasing SiO₂/Al₂O₃ in this range. The IR and n-butylamine TPD showed that both the amount and strength of Brönsted acidity decreased with the increase of the SiO₂/Al₂O₃ ratio. The good relationship between the acidity modification and catalytic performance suggests that the sulfur tolerance of Pd/USY zeolite might be due to the desired metal-support interaction, which resulted in larger amount of electron-deficient Pd. However, as shown in TGA and TPO-IR studies, the higher hydrogenation performance on more acidic zeolite also caused higher amount of carbonaceous species on the catalyst.     

Effect of promotion with cobalt or zinc on the hydrogenating and oligomerizing activities of the Pd/Al2O3 catalyst in the hydrogenation of the BTX fraction

The promoter nature and content effects on the catalytic activity and stability of Pd-Co/δ-Al₂O₃ and Pd-Zn/δ-Al₂O₃ bimetallic catalysts in the hydrogenation of dienic and vinyl aromatic hydrocarbons in the BTX fraction have been investigated by IR spectroscopy and temperature-programmed reduction. The Pd : Co (Zn) molar ratio in the catalysts prepared is 1.0 : 0.5, 1.0 : 1.0, or 1.0 : 1.5, and their Pd content is 0.5 wt %. The support is δ-Al₂O₃ doped with sodium (0.5 wt %). Promotion of the palladium catalyst with zinc and cobalt causes the disappearance of cationic palladium species, thereby reducing the oligomerizing capacity of the active component, and, as was demonstrated by 100-h-long catalytic tests, enhances the stability of the catalyst. The Pd-Co/δ-Al₂O₃(Na) catalyst with Pd : Co = 1.0 : 1.0 mol/mol is recommended for the hydrogenation of the BTX fraction under industrial conditions. The expected service life of this catalyst between regenerations is 16 months.     

A Novel Synthesis Route of Butyric Acid from Hydrogenation of Maleic Anhydride over Pd/TiO2 Catalysts

A highly efficient Pd/TiO₂ catalyst for the liquid phase hydrogenation of maleic anhydride has been prepared by sol–gel method, and super critical fluid of ethanol drying (SCFE) was applied. The catalyst exhibited excellent activity and high yield to butyric acid. The structural properties of TiO₂ supported Pd catalyst were investigated by BET, TEM, XRD, XPS and TPR techniques with the aim of finding a correlation between the structure parameters and the catalytic activities.     

Promotional effect of Ca on the Pd/Ce–Zr/Al2O3 catalyst for low-temperature catalytic combustion of methane

Promotional effect of Ca on the catalytic property of Pd/Ce–Zr/Al₂O₃ catalyst towards methane combustion is examined. The surface properties and the oxidation/reduction behavior of these catalysts are investigated by BET, TEM, XPS, TPR, TPO and TPSR techniques. Activity tests in methane combustion show that addition of Ca to Pd/Ce–Zr/Al₂O₃ can promote remarkably its low-temperature activity. The thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst to the exposure at high temperature is also enhanced by Ca loading. XPS and TEM results show that the addition of Ca to Pd/Ce–Zr/Al₂O₃ catalyst generates well-dispersed PdO particles on support. H₂–TPR, O₂–TPO and CH₄/O₂–TPSR experiments show that the addition of Ca improves the reduction/reoxidation properties and thermal stability of the active PdO species, which increases the catalytic activity and thermal stability of the Pd/Ce–Zr/Al₂O₃ catalyst.     

Mixed oxides as catalysts for the selective reduction of nitrobenzene

The catalytic behaviour of the PbO-Mn₃O₄ and the Bi₂O₃-MoO₃ systems was investigated in the selective reduction of nitrobenzene to nitrosobenzene. Lead compounds appeared to be good catalysts, and co-catalysts when used with Mn₃O₄. Different from oxidations by di-oxygen, Bi₃O₃ alone is a good catalyst and formation of mixed Bi-Mo-O compounds does not enhance the catalytic activity. It is suggested that the difference between these catalysts in the mentioned reaction is related to the way in which the oxygen vacancy is represented by the oxygen donor.     

A study on the dispersion and catalytic activity of gamma alumina-supported palladium catalysts

Electron spectroscopy for chemical analysis (ESCA) and carbon monoxide adsorption techniques have been applied to study the percent exposed (i.e., dispersion) and Pd deposition in the pores of highly porous gamma alumina-supported Pd catalysts. A correlation has been found between Pd dispersion and its extent of penetration into the pores: more edge-coated catalysts are less dispersed. The dispersion of Pd is controlled by a carrier-catalyst interaction that originates in part from electron transfer from the support to the supported Pd. This electronic interaction is demonstrated by the broadening of the ESCA peaks. The activity of the catalysts, measured by the hydrogenation of nitrobenzene to aniline, is dependent on the dispersion of palladium.     

Activation of Supported Pd Metal Catalysts for Selective Oxidation of Hydrogen to Hydrogen Peroxide

Catalytic activity of supported Pd metal catalysts (Pd metal deposited on carbon, alumina, gallia, ceria or thoria) showing almost no activity in the liquid-phase direct oxidation of H₂ to H₂O₂ (at 295 K) in acidic medium (0.02 M H₂SO₄) can be increased drastically by oxidizing them using different oxidizing agents, such as perchloric acid, H₂O₂, N₂O and air. In the case of the Pd/carbon (or alumina) catalyst, perchloric acid was found to be the most effective oxidizing agent. The order of the H₂-to-H₂O₂ conversion activity for the perchloric-acid-oxidized Pd/carbon (or alumina) and air-oxidized other metal oxide supported Pd catalysts is as follows: Pd/alumina < Pd/carbon < Pd/CeO₂ < Pd/ThO₂ < Pd/Ga₂O₃. The H₂ oxidation involves lattice oxygen from the oxidized catalysts. The catalyst activation results mostly from the oxidation of Pd metal from the catalyst producing bulk or sub-surface PdO. It also caused a drastic reduction in the H₂O₂ decomposition activity of the catalysts. There exists a close relationship between the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity in the oxidation process and the H₂O₂ decomposition activity of the catalysts; the higher the H₂O₂ decomposition activity, the lower the H₂-to-H₂O₂ conversion activity and/or H₂O₂ selectivity.     

CO2 Hydrogenation to Methanol Over Cu/ZnO/Al2O3 Catalysts Prepared by a Novel Gel-Network-Coprecipitation Method

A novel gel-network-coprecipitation process has been developed to prepare ultrafine Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from CO₂ hydrogenation. It is demonstrated that the gel-network-coprecipitation method can allow the preparation of the ultrafine Cu/ZnO/Al₂O₃ catalysts by homogeneous coprecipitation of the metal nitrate salts in the gel network formed by gelatin solution, which makes the metallic copper in the reduced catalyst exist in much smaller crystallite size and exhibit a much higher metallic copper-specific surface area. The effect of the gel concentration of gelatin on the structure, morphology and catalytic properties of the Cu/ZnO/Al₂O₃ catalysts for methanol synthesis from hydrogenation of carbon dioxide was investigated. The Cu/ZnO/Al₂O₃ catalysts prepared by the gel-network-coprecipitation method exhibit a high catalytic activity and selectivity in CO₂ hydrogenation to methanol.