Liquid-phase oxidation of toluene by molecular oxygen over copper manganese oxides

Copper manganese oxides (Cu–Mn oxides) were prepared by coprecipitation method and characterized by several techniques, such as X-ray powder diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy and Temperature-programmed reduction (TPR). Catalytic activities of the Cu–Mn oxides were tested by the oxidation of toluene with molecular oxygen in liquid phase and solvent-free conditions. The molar ratio of Cu:Mn in catalyst was optimized to be 1:1 and thus the corresponding crystalline material was designated as Cu_1.5Mn_1.5O₄.     

锰卟啉-醋酸钴复合催化体系对甲苯氧气氧化的催化作用

研究了在无溶剂体系中,对氯四苯基锰卟啉［T(p-Cl)PPMnCl］和醋酸钴［Co(OAc)2］复合催化下,空气氧化甲苯制苯甲醛、苯甲醇和苯甲酸的新工艺。实验发现,T(p-Cl)PPMnCl/Co(OAc)2为复合催化剂时比单独使用T(p-Cl)PPMnCl 或Co(OAc)2时有更高的甲苯转化率和苯甲醛、苯甲醇、苯甲酸的收率,表现出明显的复合催化作用。研究表明,反应温度、反应时间和催化剂比例对T(p-Cl)PPMnCl/Co(OAc)2的复合催化效果有影响。     

Ammoxidation of toluene over Y-Ba-Cu-Co-O perovskites

Ammoxidation of toluene over the perovskites YBa₂Cu₃O_6.1, YBa₂Cu₂CoO_6.7 and YBaCuCoO_4.9 was investigated at 400 °C. At low partial pressures of O₂ benzonitrile was selectively formed, while CO₂ was the main product at high pressures of O₂. Systematic differences in activity were observed for the three phases and are related to the crystal contents of Cu and Co. At low O₂ pressures, Cu-sites are active for nitrile formation, while Co-sites give CO₂. At high O₂ pressures, the activity for CO₂ of Cu-sites increases more than that of Co-sites due to filling of near-surface oxygen vacancies.     

Catalytic reduction of NO by propene in the presence of oxygen over mechanically mixed metal oxides and Ce-ZSM-5

The mechanical mixing of Mn₂O₃ or CeO₂ to Ce-ZSM-5 considerably enhanced the rate of the reduction of NO by propene in the low to medium temperature region, although Mn₂O₃ or CeO₂ itself was much less active for this reaction. In contrast, Mn₂O₃ was highly active and CeO₂ was moderately active for the oxidation of NO to NO₂. On the basis of the comparison of the rates of the C₃H₆ + O₂, NO + C₃H₆ + O₂ and NO₂ + C₃H₆ + O₂ reactions over these catalysts, a bifunctional mechanism is proposed, in which Mn₂O₃ and CeO₂ accelerate the oxidation of NO and the subsequent reaction steps between NO₂ and propene proceed on Ce-ZSM-5.     

Selective liquid-phase oxidation of toluene with molecular oxygen catalyzed by Mn-TiO2 under solvent-free conditions

In this paper, we reported the preparation, characterization, and catalytic performance of TiO₂ doped with Mn for the selective oxidation of toluene to benzyl alcohol and benzaldehyde without any solvent. The structure of the catalyst was determined by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption–desorption analysis (Brunauer–Emmet–Teller [BET]), scanning electron microscope (SEM), transmission electron microscope (TEM), and mapping. The results demonstrated that the catalytic properties of Mn-TiO₂ were higher than those of pure Mn₃O₄ and TiO₂. The effects of reaction temperature, reaction time, oxygen pressure, and the amount of catalyst were studied. Under the optimal conditions, Mn-TiO₂ could afford 6.4% toluene conversion at a combined benzyl alcohol/benzaldehyde selectivity of 58.6%. Moreover, the catalyst can be repeated in the experiment for five times without significant loss of activity.     

Selective catalytic oxidation of aniline to azoxybenzene over titanium silicate molecular sieves

The oxidation of aniline using aqueous H₂O₂ and titanium silicates, TS-1 and TS-2 as catalysts was carried out in a batch reactor in the temperature range 333–353 K. TS-1 catalyzes aniline selectively to azoxybenzene and is superior to TS-2. The influence of different solvents, concentration of H₂O₂ and the catalyst in the reaction mixture on the conversion and product distribution has been studied. Acetonitrile is a suitable solvent in this reaction, while acetone is not. For the TS-1 catalyzed oxidation reaction,t-butyl hydroperoxide is not a suitable oxidant. At optimum conditions, a H₂O₂ efficiency of about 100% for aniline conversion is obtained with a selectivity of 97% to azoxybenzene in the product.     

Continuous flow toluene methylation over AlPO4 and AlPO4-Al2O3 catalysts

Toluene methylation with methanol over AlPO₄ (AP) and AlPO₄-Al₂O₃ (APAl) catalysts, obtained through different methods, was carried out in a continuous down-flow fixed bed reactor. The main products were xylenes (XYL), although trimethylbenzenes (TMB) were also found over APAl catalysts. The benzene and ethylbenzene selectivities increased slightly with time on stream at the expense of XYL and TMB selectivities. Isomer distribution was approximately 50, 24 and 26 mol% foro-, m- andp-XYL, and 72, 27 and 0 mol% for 1,2,3-, 1,2,4- and 1,3,5-TMB. The initial reaction rate constants were higher on APA1 catalysts and, furthermore, APAl catalysts exhibited similar catalytic activities, although those obtained in ethylene or propylene oxide are the most active ones. The same occurs on AP catalysts. Moreover, the changes in catalytic activity are similar to the changes in the acidic characteristics measured, in gas phase, versus pyridine. Furthermore, the activity decreased with time on stream due to coke deposition according to the expressionk = k₀ exp(-t). The rate of deactivation, evaluated from the deactivation coefficients (), was greater for APAl than for AP catalysts.     

Mechanistic study of the reduction of oxygen in air electrode with manganese oxides as electrocatalysts

Electrochemical reduction of oxygen (O₂) in air electrode with manganese oxides (MnOx) as electrocatalysts was studied with MnOx/Nafion-modified gold (Au) electrodes using cyclic voltammetry, potential-controlled amperometry and rotating ring-disk electrode (RRDE) voltammetry in alkaline aqueous solution. At Nafion-modified (MnOx free) Au electrode, O₂ reduction undergoes two successive two-electron processes with HO₂⁻ as intermediate. The presence of MnOx, including Mn₂O₃, Mn₃O₄, Mn₅O₈ and MnOOH, on Nafion-modified Au electrodes obviously increases the first reduction peak current of O₂ to hydrogen peroxide (HO₂⁻ in this case) and decreases the second one of HO₂⁻ to OH⁻, while does not shift the reduction potential. MnOx was found to show catalytic activity for the disproportionation reaction of HO₂⁻ to O₂ and OH⁻ and thus, the O₂ reduction in air electrode was considered to include an initial two-electron reduction of O₂ to HO₂⁻ followed by a disproportionation reaction of HO₂⁻ into O₂ and OH⁻ catalyzed by MnOx. The excellent activity of MnOx for the follow-up disproportionation reaction substantially results in an overall four-electron reduction of O₂ at MnOx/Nafion-modified Au electrodes in the first reduction step, depending on potential scan rate and the kind of MnOx. The present work provides a scientific significance of the mechanism of O₂ reduction in air electrode using MnOx as electrocatalysts to effect a four-electron reduction of O₂ to OH⁻.     

A molecular level mechanism ofn-butane oxidation to maleic anhydride over vanadyl pyrophosphate

A molecular level mechanism is proposed for the highly selective 14-e^– oxidative transformation ofn-butane to maleic anhydride on the surface of vanadyl pyrophosphate. The mechanism suggests that the dimeric active sites assume at any given time, one of four possible interconvertible states which differ from each other in the number of available oxygen atoms and the formal oxidation states of the individual vanadium atoms. The relative ratios of active sites in each of the four possible states are dictated by the reaction conditions, the redox properties of the reacting gases and the structure of the vanadyl pyrophosphate active surface. A crucial feature of the mechanism is a pseudo-ozonide surface species formed by the interaction of a chemisorbed dioxygen molecule and an adjacent metal-oxo group. This unusual species is responsible for the initial activation of then-butane, which occurs when the chemisorbed dioxygen abstracts an H-atom from the alkane and the adjacent metal-oxo group reacts with the incipient alkyl radical to form an alkoxy group. The proposed mechanism is entirely consistent with literature reports describing the behaviour of (VO)₂P₂O₇ in flow, pulse and TAP reactors.     

Partial oxidation of methane to synthesis gas over corundum supported mixed oxides: One channel studies

Catalytic partial oxidation of methane (POM) over the monolithic catalyst LaNiO_x/CeO₂–ZrO₂/α-Al₂O₃ has been studied. Experiments were conducted with one channel of a monolith at a varied channel length, contact time (1–6 ms) and temperature using the diluted gas mixture (1% CH₄ + 0.5% O₂ in He). At increasing temperature and contact time, CO selectivity rises within the whole temperature range whereas the contact time dependence of H₂/CO ratio varies with the temperature. These results support the POM reaction scheme including primary formation of CO and H₂ followed by their oxidation in the presence of gas-phase O₂. Steam and dry methane reforming reactions occur in the part of monolithic channel where oxygen is absent, thus increasing syngas yield.     

Effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation under dynamic oxygen storage and release measurements over Pd/CeO2 catalysts

A kinetic mathematical model has been applied to investigate for the first time the effects of Pd particle size on the rates of oxygen back-spillover and CO oxidation during Oxygen Storage Capacity (OSC) measurements under dynamic conditions over Pd/CeO₂ catalysts in the 500–700 °C range. The dependence of the intrinsic rate constant k₁ of the CO oxidation reaction on PdO, and that of k ₂ ^app of the oxygen back-spillover from ceria to Pd/PdO on the palladium particle size was estimated by performing curve-fitting of the experimental CO and CO₂ pulse transient responses obtained. Activation energies of 8.0, 9.5 and 21.1 kJ/mol were calculated for the Eley–Rideal step of CO oxidation for the 1.3, 1.8 and 16.4 nm Pd particles, respectively, supported on CeO₂. The transient rates of CO oxidation and oxygen back-spillover were found to decrease with increasing Pd particle size.     

Solid-State Construction of CuOx/Cu1.5Mn1.5O4 Nanocomposite with Abundant Surface CuOx Species and Oxygen Vacancies to Promote CO Oxidation Activity

Carbon monoxide (CO) oxidation performance heavily depends on the surface-active species and the oxygen vacancies of nanocomposites. Herein, the CuO_x/Cu_1.5Mn_1.5O₄ were fabricated via solid-state strategy. It is manifested that the construction of CuO_x/Cu_1.5Mn_1.5O₄ nanocomposite can produce abundant surface CuO_x species and a number of oxygen vacancies, resulting in substantially enhanced CO oxidation activity. The CO is completely converted to carbon dioxide (CO₂) at 75 °C when CuO_x/Cu_1.5Mn_1.5O₄ nanocomposites were involved, which is higher than individual CuO_x, MnO_x, and Cu_1.5Mn_1.5O₄. Density function theory (DFT) calculations suggest that CO and O₂ are adsorbed on CuO_x/Cu_1.5Mn_1.5O₄ surface with relatively optimal adsorption energy, which is more beneficial for CO oxidation activity. This work presents an effective way to prepare heterogeneous metal oxides with promising application in catalysis.     

Metal oxides nanoparticles on SBA-15: Efficient catalyst for methane combustion

Catalysts based on crystalline nanoparticles of Mn and Co metal oxides supported on mesoporous silica SBA-15 have been developed. These materials were characterized by XRD, BET and transmission electron microscopy (TEM) techniques. SBA-15 mesoporous silica was synthesized by a conventional sol–gel method using a tri-block copolymer as surfactant. Supported Mn₃O₄ and Co₃O₄ nanoparticles were obtained after calcination of as-impregnated SBA-15 by a metal salt precursor. The catalytic activity was evaluated in the combustion of methane at low concentration.Co₃O₄/SBA-15 (7 wt.%) exhibits the highest performance among the different oxides. Furthermore, this novel generation of catalysts appeared as active as conventional LaCoO₃ perovskite, usually taken as reference for this reaction. Thanks to its organized meso-structures, SBA-15 material creates peculiar diffusion conditions for reactants and/or products.     

Partial oxidation of methane over nickel catalysts supported on various aluminas

Partial oxidation of methane (POM) was systematically investigated in a fixed bed reactor over 12 wt% Ni catalysts supported on α-A1₂O₃, γ-A1₂O₃ and θ-A1₂O₃ which were prepared at different conditions. Results indicate that the catalytic activity toward POM strongly depends on the BET surface area of the support. All the Ni/ θ-Al₂O₃ catalysts showed high activity toward POM due to the less formation of inactive NiAl₂O₄ species, the existence of NiO, species and stable θ-Al₂O₃ phase. Although Ni/γ-Al₂O₃ showed the highest activity toward POM, long-time stability cannot be expected as a result of the deterioration of the support at higher temperature, which is revealed from BET results. From the reaction and characterization results, it is inferred that the optimal conditions for the preparation of θ-Al₂O₃ are 1,173 K and 12 h.     

Evaluation of oxygen storage profile on CeO2–ZrO2 mixed oxides by periodic injections of O2 pulse and their reduction behavior

Oxygen storage profile on CeO₂–ZrO₂ mixed oxide (CZ) has been observed by periodic injections of O₂ pulse. The reduction behavior of oxygen after the O₂ pulse injection was also investigated using Temperature Programmed Reduction (TPR) method. The oxygen storage profiles of the CZ catalyst with κ-CeZrO₄ phase indicate that the solid solution phase facilitates to diffuse oxygen into the bulk, and TPR profiles suggest that oxygen is preferentially stored by the reaction with Ce³⁺ species derived from CeO₂ phase compared with those from the κ-CeZrO₄ phase, especially at low temperatures.     

Photocatalytic oxidation of toluene to benzaldehyde over anatase TiO2 hollow spheres with exposed {001} facets

A new series of anatase TiO₂ hollow structures were prepared by a facile hydrothermal process. When the hydrothermal time was increased from 20 min to 72 h, the resulting TiO₂ solid spheres gradually transformed into TiO₂ hollow spheres with higher surface crystallinity and exposed {001} facets. The as-prepared TiO₂-72 h sample exhibited the highest activity comparing to other TiO₂-based samples and commercial product Degussa P-25 towards the selective photocatalytic oxidation of toluene to benzaldehyde. Such great photocatalytic performance was mainly attributed to enhanced UV-adsorption and better charge separation efficiency due to higher surface crystallinity of TiO₂-72 h.     

Effect of MgF2 and Al2O3 supports on the structure and catalytic activity of copper–manganese oxide catalysts

Structure and catalytic activity of double copper–manganese oxide catalysts supported on MgF₂ and Al₂O₃ have been studied. All samples were calcined at 400 °C and those supported on Al₂O₃ also at 550 and 950 °C. The properties of surface species have been characterized by low temperature adsorption of nitrogen, XRD and TPR-H₂. The catalytic activities have been tested in low-temperature CO oxidation and in NO reduction by propene. The supported oxides react with each other during calcination to form CuMn₂O₄ spinel. The spinel seems to be responsible for the catalytic activity of the double copper–manganese catalysts. The temperature of calcination changes the strength of interaction between the active phase and the supports influencing the catalytic activity.     

The influence of preparation conditions on electrochemical properties of LiNi0.5Mn1.5O4 thin film electrodes by PLD

LiNi_0.5Mn_1.5O₄ thin films were prepared by pulsed laser deposition (PLD) on stainless steel substrates. The growth of the films has been studied as a function of substrate temperature and oxygen partial pressure in deposition, using X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM). Electrochemical properties of LiNi_0.5Mn_1.5O₄ thin film cathodes were investigated using cyclic voltammetry and galvanostatic charge/discharge against a lithium anode. The initial capacity and capacity retention of the films are highly dependent on the crystallinity and purity of the films. LiNi_0.5Mn_1.5O₄ thin films grown at 600 °C in an oxygen partial pressure of 200 mTorr are well crystallized with high purity, exhibiting excellent capacity retention between 3 and 5 V with a LiPF₆-based electrolyte.     

Partial oxidation of methane over ruthenium catalysts

The catalytic partial oxidation of methane with oxygen to produce synthesis gas was studied under a wide range of conditions over supported ruthenium catalysts. The microreador results demonstrated the high activity of ruthenium catalysts for this reaction. A catalyst having as little as 0.015% (w/w) Ru on Al₂O₃ gave a higher synthesis gas selectivity than a catalyst having 5% Ni on SiO₂. XANES measurements for fresh and used catalyst samples confirmed that ruthenium is reduced from ruthenium dioxide to ruthenium metal early during the experiments. Ruthenium metal is thus the active element for the methane partial oxidation reaction.     

Preferential CO oxidation over Pt–SnO2/Al2O3 in hydrogen rich streams containing CO2 and H2O (CO removal from H2 with PROX)

The effects of reaction gases including CO₂ and H₂O and temperature on the selective low-temperature oxidation of CO were studied in hydrogen rich streams using a flow micro-reactor packed with a Pt–SnO₂/Al₂O₃ sol–gel catalyst that was initially designed and optimized for operation in the absence of CO₂ and H₂O. 100% CO conversion was achieved over the 1 wt% Pt–3 wt% SnO₂/Al₂O₃ catalyst at 110 °C using a feed composition of 1.0% CO, 1.5% O₂, 25% CO₂, 10% H₂O, 58% H₂ and He as balance at a space velocity of 24,000 cm³/(g h). CO₂ in the feed was found to decrease CO conversion significantly while the presence of H₂O in the feed increased CO conversion, balancing the effect of CO₂.