Effect of size of catalytically active phases in the dehydrogenation of alcohols and the challenging selective oxidation of hydrocarbons

The size of the active phase is one of the most important factors in determining the catalytic behaviour of a heterogeneous catalyst. This Feature Article focuses on the size effects in two types of reactions, i.e., the metal nanoparticle-catalysed dehydrogenation of alcohols and the metal oxide nanocluster-catalysed selective oxidation of hydrocarbons (including the selective oxidation of methane and ethane and the epoxidation of propylene). For Pd or Au nanoparticle-catalysed oxidative or non-oxidative dehydrogenation of alcohols, the size of metal nanoparticles mainly controls the catalytic activity by affecting the activation of reactants (either alcohol or O(2)). The size of oxidic molybdenum species loaded on SBA-15 determines not only the activity but also the selectivity of oxygenates in the selective oxidation of ethane; highly dispersed molybdenum species are suitable for acetaldehyde formation, while molybdenum oxide nanoparticles exhibit higher formaldehyde selectivity. Cu(II) and Fe(III) isolated on mesoporous silica are highly efficient for the selective oxidation of methane to formaldehyde, while the corresponding oxide clusters mainly catalyse the complete oxidation of methane. The lattice oxygen in iron or copper oxide clusters is responsible for the complete oxidation, while the isolated Cu(I) or Fe(II) generated during the reaction can activate molecular oxygen forming active oxygen species for the selective oxidation of methane. Highly dispersed Cu(I) and Fe(II) species also function for the epoxidation of propylene by O(2) and N(2)O, respectively. Alkali metal ions work as promoters for the epoxidation of propylene by enhancing the dispersion of copper or iron species and weakening the acidity.     

Catalytic selective oxidation or oxidative functionalization of methane and ethane to organic oxygenates

Selective oxidation or oxidative functionalization of methane and ethane by both homogeneous and heterogeneous catalysis is presented concerning: (1) selective oxidation of methane and ethane to organic oxygenates by hydrogen peroxide in a water medium in the presence of homogeneous osmium catalysts, (2) selective oxidation of methane to formaldehyde over highly dispersed iron and copper heterogeneous catalysts, (3) selective oxidation of ethane to acetaldehyde and formaldehyde over supported molybdenum cat...     

乙烷部分氧化制C2氧化物用Fe－Al－P－O催化剂的研究Ⅱ．Fe－Al－P－O催化剂的吸附及催化性能

 用化学吸附－IR，化学吸附－TPD和微反技术研究了超细Fe－Al－P－O催化剂的化学吸附性能及对乙烷部分氧化反应的催化性能．结果表明，乙烷能够以－CH3中的H原子吸附于催化剂表面P＝O键的端氧上形成分子吸附态，并且随着吸附温度的升高，对乙烷的吸附强度逐渐增大；乙烯则主要是以C＝C双键吸附在催化剂的Lewis酸位Fe3＋上．乙烷部分氧化反应的主要产物为乙烯和COx，但在反应物中引入氢的条件下，乙烷部分氧化反应的性能大为改善，并可生成乙醇和乙醛等含氧化合物．     

稀土Pr_6O_(11)表面上低碳烃氧化反应的原位FT-IR研究

前文报道了CO与Pr_6O_(11)表面反应的结果.已发表的关于烃类与氧化物表面反应的研究,旨在了解烃类的活化和表面氧化机理,而且大部分工作都是在过渡金属氧化物上进行的.近年来的研究表明,稀土氧化物将是有潜力的甲烷氧化偶联制乙烯的催化剂,在消除环境污染的烃类完全氧化催化剂中,稀土氧化物也常被用作助剂或活性组分,但对于稀土氧化物本身的催化表面研究尚不多见.作者曾较系统地研究了CeO_2的表面催化性质,发现表面上存在多种氧物种,这些氧物种与烃类的反应活性差异很     

THE ROLE OF LATTICE AND GASEOUS OXYGEN ON THE OXIDATIVE COUPLING OF METHANE STUDIED BY A PULSE REACTION TECHNIQUE

A comparative study of the role of lattice and gaseous oxygen in theoxidative coupling of methane over Na~ /Cao and Ca_xSr_(1-x)TiO_3 perovskiteoxide catalysts was performed by using a pulse reaction technique with CH_4,O_2 and mixtures of CH_4and O_2.It was found that there is an oxygen species onthe Surface of Cao which is active only for the total oxidation of methane at thereaction temperature.These oxygen species.once reacted with methane andconsumed,can be regenerated by the adsorption of oxygen molecules from thegas phase or by the migration of the lattice oxysen from the bulk onto thesurface.In contrast,no such oxygen species and no reaction can be detected bymethane pulsed over Ca_xSr_(1-x)TiO_3perovskite oxide catalysts.Na~ can heavilysuppress the non-selective oxygen species on the surface,and When 5mol%Na~ was incorporated onto the surface of Cao,the sample showed no reactivityto methane.When oxygen exists in the gas phase,or the pulse containsmethane and oxygen,the reaction over(Cao is mai     

First-Principles Evaluation of One-Dimensional Metal-Organic Frameworks for Electrocatalytic C−H Activation of Natural Gas

To replace the oxygen evolution reaction with thermodynamically more favorable and economically more profitable methane and ethane (the major components of natural gas) electrochemical partial oxidation, we employed constant electrode potential density functional theory calculations to screen 20 one-dimensional metal-organic frameworks containing heteroatom-substituted benzene as electrocatalysts. By computing the Pourbaix diagrams, O−H binding energies, and C−H activation barriers, we determined that although none of these catalysts were able to activate methane, one was able to hydroxylate ethane to ethanol with facile kinetics, making it a promising electrocatalyst for natural gas oxidation.     

Stepwise Oxidation of Thiophene and Its Derivatives by Hydrogen Peroxide Catalyzed by Methyltrioxorhenium(VII)

The oxidation of thiophene derivatives by hydrogen peroxide is catalyzed by methyltrioxorhenium(VII) (CH(3)ReO(3)). This compound reacts with hydrogen peroxide to form 1:1 and 1:2 rhenium peroxides, each of which transfers an oxygen atom to the sulfur atom of thiophene and its derivatives. Complete oxidation to the sulfone occurs readily by way of its sulfoxide intermediate. The rates for each oxidation step of dibenzothiophenes, benzothiophenes, and substituted thiophenes were determined. The rate constants for the oxidation of the thiophenes are 2-4 orders of magnitude smaller than those for the oxidation of aliphatic sulfides, whereas the rate constants are generally the same for the oxidation of the thiophene oxides and aliphatic sulfoxides. The rate constant for conversion of a sulfide to a sulfoxide (thiophene oxide) increases when a more electron-donating substituent is introduced into the molecule, whereas the opposite trend was found for the reaction that converts a sulfoxide to a sulfone (thiophene dioxide). Mechanisms consistent with this are proposed. The first trend reflects the attack of the nucleophilic sulfur atom of a thiophene center on a peroxide that has been electrophilically activated by coordination to rhenium. The second, more subtle, trend arises when both sulfoxide and peroxide are coordinated to rhenium; the inherently greater nucleophilicity of peroxide then takes control.     

Starch-Enhanced Synthesis of Oxygenates from Methane and Carbon Dioxide Using Dielectric-Barrier Discharges

In this work, starch has been used to enhance the oxygenate formation directly from methane and carbon dioxide using dielectric-barrier discharges (DBDs). The use of starch inhibits the formation of liquid hydrocarbons and significantly increases the selectivity of oxygenates. Oxygenates produced include primarily formaldehyde, methanol, ethanol, formic acid, and acetic acid. The total selectivity is about 10–40% with conversion of methane and carbon dioxide of about 20%. Lower methane feed concentration favors the production of oxygenates, and higher feed flow rate leads to higher selectivity of oxygenates in the presence of starch.     

Discrimination of Lattice Oxygen Species Involved in the Oxidative Coupling of Methane on a Mn Mixed-Mg Oxide

Two functional lattice oxygens on the surface of a Mn-mixed Mg oxide were distinguished in the oxidative coupling of methane: the oxygen of MgO, which was active for methyl radical formation, and that of a Mn-Mg oxide solid solution, which was active for the dehydrogenation of ethane.     

在Li-Nd-Zn-Mg氧化物上甲烷氧化偶联

天然气资源丰富,其中主要成分为甲烷.以甲烷为原料开发化工产品一直引起人们的重视.1982年Keller首先提出甲烷氧化偶联制乙烯、乙烷的催化过程,为开拓廉价乙烯原料路线提供了新的可能途径,从而引起各国催化工作者的重视.近年来,此催化过程的乙烯及乙烷的收率(y_(02))已提高到10—30%之间.各国都在努力开发活性高及选择性好的催化剂,促使此催化过程工业化。本文研究了Li-Nd-Zn-Mg氧化物甲烷氧化偶联催化剂,对反应条件进行了初步考察,乙烯及乙烷的收率y_(02)达到30.8%。     

Kinetic Simulation of Methane and Ethane Hydroxylation by Methane Monooxygenase

According to the mechanism of alkane hydroxylation, whose main postulate is the formation of an intermediate complex containing pentacoordinated carbon, the hydroxylation of methane and ethane by methane monooxygenase was kinetically simulated by the numerical method. The published data on the kinetic isotope effects of oxidation of deuterium-substituted methane molecules (CHD₃, CH₂D₂, and CH₃D) and the distribution of products of chiral ethane (R- and S-MeCHDT) oxidation by methane monooxygenase were examined. The kinetic models proposed for the oxidation of isotopically substituted methane and ethane are in good agreement with experimental data.     

甲烷催化氧化制含氧化合物的研究*

本文对甲烷催化氧化制含氧化合物的研究, 从多相催化和液相催化两个方面进行了综述; 对多相催化的研究从催化剂的选择、反应温度、反应压力、氧源、反应添加物、反应器及反应机理等方面进行了总结, 液相氧化的研究则对不同的研究体系进行了详细的综述。     

Conversion of methane into C1 oxygenates by deep-UV photolysis on solid surfaces: influence of the nature of the solid and optimization of photolysis conditions

Deep-UV photolysis (either 165 or 185 nm) of surface hydroxy groups leads to homolytic O-H bond-cleavage with the generation of oxyl radicals that can initiate the room-temperature radical-chain methane activation. Whilst in the absence of oxygen, radical coupling reactions to give low-molecular-weight alkanes are observed in the gas phase, the presence of some oxygen quenches these radicals and increases the selectivity towards C1 oxygenates (methanol, formaldehyde, and formic acid species). The nature of the solid influences the efficiency of the photochemical process and the distribution between products in the gas and solid phases. Using Beta-, delaminated ITQ2 and ITQ6, and medium-pore ZSM5 zeolites, mesoporous MCM41 silicates, and non-porous TiO(2), we observed that confinement and porosity increased the proportion of C1 oxygenates adsorbed onto the solid and reduced the contribution of the gas-phase products. In addition, the presence of aluminum in the zeolite framework, which is responsible for the generation of acid sites, increased overoxidation of methanol and methoxy groups into formaldehyde and formic acids. For a given amount of methane and unchanged photolysis conditions, the conversion increased with the amount of the solid used as photocatalyst. In this way, methane conversions of up to 7% were achieved for the 185 nm photolysis of methane for 1 h with a 76 MJ mol(-1) energy consumption.     

Conversion of Methane and Carbon Dioxide in a DBD Reactor: Influence of Oxygen

A continuous plug flow reactor supported by a dielectric barrier discharge (DBD) is used to study the conversion of methane, carbon dioxide, and oxygen at different compositions. The three studied gases were diluted with helium to 3 % with an overall flow rate of 200 sccm. The 13.56 MHz plasma was ignited at atmospheric pressure. The product stream and the inlet flow were analyzed by a FTIR spectrometer equipped with a White-cell and by a quadrupole mass spectrometer. The DBD reactor generates hydrogen, carbon monoxide, ethane, ethene, acetylene, formaldehyde, and methanol. Additional oxygen in the feed has positive effects on the yield of methanol, formaldehyde and carbon monoxide and reduces the total consumed energy. The hydrogen yield reaches its maximum at medium amounts of oxygen in the inlet flow. The conversion of methane increases to a limiting value of about 35 %. Methane rich feeds increase the yield of hydrogen, ethane and methanol. On the other hand, additional oxygen has a negative influence on the produced amount of C2 hydrocarbons. The conversion of methane and carbon dioxide as well as the yield of synthesis gas components and C2 hydrocarbons increases by changing the plasma power to higher values.     

Li-Nd-Mg氧化物上甲烷氧化偶联

自从1982年Keller首先报导了甲烷氧化偶联制乙烯、乙烷的催化反应以来,引起国内外广泛的重视,若甲烷的转化率C达35%,C_2的选择性S(C_2)达88%,乙烯/乙烷较大时,则甲烷氧化偶联催化过程可以工业化.当前国内外的工作重点是寻求甲烷转化率高,C_2选择性好的催化剂.Li和烯土复合氧化物催化剂引起人们注意,C_2的收率Y(C_2)可达24.6%,本文采用高温灼烧而成的Li-Nd-Mg复合氧化物,Y(C_2)可达31.3%. 实验部分Li-Nd-Mg复合氧化物由高温灼烧而成,实验采用小型固定床石英反应器,反应器的直     

Recent Results on Fast Flow Gas-Phase Partial Oxidation of Lower Alkanes

Recent experimental results and kinetic modeling of fast flow gas-phase oxidation of methane and other lower alkanes to methanol and other oxygenates are discussed,alongside with prospects and possible areas for applications of the processes.     

Contemporary Methods for the Direct Catalytic Conversion of Methane

The principal methods for the conversion of methane into useful chemical compounds are discussed. Promising methods include direct nonoxidative dehydrocondensation of methane to aromatic hydrocarbons, oxidative coupling of methane to ethylene, and partial oxidation of methane to oxygenates. In the case of the last reaction the proposed approach makes it possible to compare precisely the selective action of heterogeneous catalysts and to predict that a maximum yield will be obtained in a flow-type reactor with recycling.     

Light paraffin oxidative conversion in a silent electric discharge

Methane, ethane, and their mixtures with oxygen have been passed through the silent electric discharge at various operating conditions Beside.carbon oxides and water, formaldehyde and methane, and acetaldehyde and ethanol from ethane were the other major products. The effect of temperature, pressure, discharge voltage, contact time, and oxygen concentration have been examined. The rate of oxygen consumption is almost independent of oxygen concentration and depends mainly on the discharge voltage. The selectivities of methanol formation from methane and ethanol/formation from ethane were 20 and 15%, respectively.     

Oxidative processing of light alkanes: State-of-the-art and prospects

Recent literature data on partial oxidation of light alkanes into syngas and oxidative coupling of methane into C₂ hydrocarbons are reviewed. The problems of these processes (high cost of pure oxygen; safety; activity, selectivity and stability of catalysts; temperature regime; coke formation and other by-products; insufficient level of methane transformation into ethane and ethylene) are considered. Possible solutions of these problems and prospects of practical use of light alkanes processing are discussed.     

Hydrogen production from a combination of the water-gas shift and redox cycle process of methane partial oxidation via lattice oxygen over LaFeO3 perovskite catalyst

A redox cycle process, in which CH4 and air are periodically brought into contact with a solid oxide packed in a fixed-bed reactor, combined with the water-gas shift (WGS) reaction, is proposed for hydrogen production. The sole oxidant for partial oxidation of methane (POM) is found to be lattice oxygen instead of gaseous oxygen. A perovskite-type LaFeO3 oxide was prepared by a sol-gel method and employed as an oxygen storage material in this process. The results indicate that, under appropriate reaction conditions, methane can be oxidized to CO and H2 by the lattice oxygen of LaFeO3 perovskite oxide with a selectivity higher than 95% and the consumed lattice oxygen can be replenished in a reoxidation procedure by a redox operation. It is suggested that the POM to H2/CO by using the lattice oxygen of the oxygen storage materials instead of gaseous oxygen should be possibly applicable. The LaFeO3 perovskite oxide maintained relatively high catalytic activity and structural stability, while the carbonaceous deposits, which come from the dissociation of CH4 in the pulse reaction, occurred due to the low migration rate of lattice oxygen from the bulk toward the surface. A new dissociation-oxidation mechanism for this POM without gaseous oxygen is proposed based on the transient responses of the products checked at different surface states via both pulse reaction and switch reaction over the LaFeO3 catalyst. In the absence of gaseous-phase oxygen, the rate-determining step of methane conversion is the migration rate of lattice oxygen, but the process can be carried out in optimized cycles. The product distribution for POM over LaFeO3 catalyst in the absence of gaseous oxygen was determined by the concentration of surface oxygen, which is relevant with the migration rate of lattice oxygen from the bulk toward the surface. This process of hydrogen production via selective oxidation of methane by lattice oxygen is better in avoiding the deep oxidation (to CO2) and enhancing the selectivity. Therefore, this new route is superior to general POM in stability (resistance to carbonaceous deposition), safety (effectively avoiding accidental explosion), ease of operation and optimization, and low cost (making use of air not oxygen).