Autocatalytic gas-phase ethane dehydrogenation in a wall-less reactor

The thermal gas-phase pyrolysis of ethane was studied under conditions of the bulk heating of the reaction mixture with IR-laser radiation. The concentrations of ethane pyrolysis products as functions of reaction time were calculated in accordance with standard kinetic schemes; they showed that a classical radical chain mechanism corresponded to only highly dilute mixtures of ethane with an inert gas. As found by calculations, the experimental data on the kinetics of consumption of the initial substance and on the kinetics of buildup of pyrolysis products in undiluted mixtures of ethane and its conversion products were adequately described by an autocatalytic (with respect to ethylene) mechanism of ethane dehydrogenation. This mechanism involved the step of ethane interaction with ethylene to form methyl and propyl radicals.     

Autocatalytic dehydrogenation of propane

Homogeneous gas-phase pyrolysis of propane was performed by using continuous CO₂ laser irradiation for bulk heating of the reaction mixture. Laser energy was absorbed by ethylene, the main product of propane dehydrogenation, and transferred to the reaction medium via collisional relaxation. A mechanism of propane dehydrogenation is suggested to describe the pyrolysis process. The mechanism involves autocatalysis by ethylene and includes propane–ethylene interaction with the formation of ethyl and propyl radicals.     

Catalytic Pyrolysis of Methane

Methane decomposition over metal oxides/SiO₂ surface was investigated. At 1400 K obtained product distribution of this decomposition varied with metal oxide used. The effectiveness of these catalysts has been discussed in terms of activity and C₂ selectivity. ThO₂/SiO₂ was found to be the most effective catalyst for the catalytic decomposition of methane. Positive catalytic effect of ThO₂/SiO₂ on the pyrolysis has also been confirmed at 1073 K. At low reaction conversions, ethane and ethylene are found as major products. Yields of ethylene and other unsaturated products are sensitively inhibited by NO impurities in the methane. A reaction mechanism has been proposed to account observed experimental results.     

Multiphoton infrared laser-induced degradation of polydimethylsiloxane and hexamethyldisiloxane

The IR laser-induced degradations of liquid polydimethylsiloxane (PDMS) vapor and of hexamethyldisiloxane vapor have been studied in order to determine whether the high temperature thermal properties of the dimethylsiloxane unit is best represented by Si? O bond rearrangement (the conventional pyrolysis mechanism) or Si? C bond cleavage (the thermodynamic reaction pathway). The volatile products of these pulsed laser experiments with various viscosities of PDMS are methane, ethane, ethylene, and hydrogen. These results are consistent with Si? C bond cleavage to form methyl radicals, which can then recombine to form ethane or abstract a hydrogen atom from a matrix molecule to form methane. The presence of ethylene and hydrogen can be explained by the decomposition of hot ethane molecules. No evidence of Si? O bond cleavage was observed. Reaction temperatures are estimated with computer modeling using heat capacity data.     

Effect of methane co-feeding on the selectivity of ethylene produced from oxidative dehydrogenation of ethane with CO2 over a Ni-La/SiO2 catalyst

A Ni-La/SiO₂ catalyst was prepared through the incipient wetness impregnation method and tested in the oxidative dehydrogenation of ethane (ODHE) with CO₂. The fresh and used catalysts were characterized by XRD and SEM techniques. The Ni-La/SiO₂ catalyst exhibited catalytic activity for the oxidative dehydrogenation of ethane, but with low ethylene selectivity in the absence of methane. The selectivity to ethylene increased with increasing molar ratio of methane in the feed. The carbon deposited on the catalyst surface in the sole ODHE with CO₂ was mainly inert carbon, while much more filamentous carbon was formed in the presence of methane. The filamentous carbon was easy to be removed by CO₂, which might play a role in improving the conversion of ethane to ethylene. The introduction of methane might affect the equilibrium of the CO₂ reforming of ethane and the ODHE with CO₂. As a consequence, the synthesis gas produced from CO₂ reforming of methane partly inhibited the reaction of ethane and promoted the ODHE with CO₂, thus increasing the selectivity of ethylene.     

Identifying Different Types of Catalysts for CO2 Reduction by Ethane through Dry Reforming and Oxidative Dehydrogenation

The recent shale gas boom combined with the requirement to reduce atmospheric CO₂ have created an opportunity for using both raw materials (shale gas and CO₂) in a single process. Shale gas is primarily made up of methane, but ethane comprises about 10 % and reserves are underutilized. Two routes have been investigated by combining ethane decomposition with CO₂ reduction to produce products of higher value. The first reaction is ethane dry reforming which produces synthesis gas (CO+H₂). The second route is oxidative dehydrogenation which produces ethylene using CO₂ as a soft oxidant. The results of this study indicate that the Pt/CeO₂ catalyst shows promise for the production of synthesis gas, while Mo₂C‐based materials preserve the C? C bond of ethane to produce ethylene. These findings are supported by density functional theory (DFT) calculations and X‐ray absorption near‐edge spectroscopy (XANES) characterization of the catalysts under in situ reaction conditions.     

逆水煤气变换耦合乙烷脱氢制备乙烯反应的研究

综述了近年来国内外有关利用逆水煤气变换耦合乙烷脱氢制备乙烯反应的研究状况。SiO2和Silicalie-2分子筛担载的铬、锰、铁及稀土氧化物催化剂具有较好的催化活性。催化剂表面碱性位的存在有利于CO2的活化,消除乙烷脱氢产物H2和表面积炭,提高反应活性及催化剂的稳定性。进一步提高活性的关键是选择适且的催化剂,以增加CO2的加氢活性。该耦合反应也是充分利用低碳烷烃和CO2资源的新途径。     

Oxidative Dehydrogenation of Ethane to Ethylene over LiCl/MnOx/PC Catalysts

The catalytic stability of LiCl/MnO_x/PC catalyst have been investigated, the deactivation mechanism was discussed. The experimental results show that ethane conversion decreases and ethylene selectivity keeps about 90% as reaction time increases. The main deactivation reasons of LiCl/MnO_x/PC catalyst for oxidative dehydrogenation of ethane (ODHE) to ethylene are the transition of active species Mn₂O₃ to MnO species and the loss of active component Cl in catalyst. Instead of ethane with FCC tailed‐gas, the stability of LiCl/MnO_x/PC catalyst has been largely improved.     

乙烷在氧化镍上催化氧化脱氢的反应机理研究

应用O~2-TPD,脉冲实验,原位Weiss磁测量以及TAP(temporalanalysisofproducts)技术对NiO上的乙烷氧化脱氢制乙烯催化作用机理进行了研究。结果表明,NiO中的非化学计量氧表现出与气相氧交换的可逆性,其中在较低温度下脱附的α氧(很可能是O~2^-,O~2^2^-)仅存在于催化剂表面,与气相氧交换迅速,而较高温下脱附的β氧(很可能是O^-)不仅存在于催化剂表面,还存在于催化剂体相。β氧较α氧表现出更高的乙烯选择性。在反应条件下,Ni均应处于高氧化态(Ni^(^2^+^δ^)^+,0≤δ≤1),一旦催化剂中有微量Ni^0生成,乙烷便发生裂解反应,乙烯选择性立即降为零。乙烷在NiO上的氧化脱氢(ODHE)的可能反应机理为:首先乙烷与NiO中的非化学计量氧O~n(~s~)作用脱除一个α-H生成乙基自由基,然后进一步脱除一个β-H生成乙烯,乙烯生成的整个过程是在催化剂表面上进行的;副产物CO~2是由表面乙烯进一步氧化(很可能是与O~2^-,O~2^2^-作用)生成的。失去O~n~(~s~)的NiO在反应体系(一定的氧分压)中,重新生成含非化学计量氧的NiO。     

Processing of Oil Refinery Gases: Oxidative Dehydrogenation of the Ethane–Ethylene Fraction

Oxidative transformations of the ethane–ethylene fraction of oil refinery gases, containing 20 vol % C₂H₄, on VMoTeNb oxide catalyst in the temperature interval 330–450°C were studied. Comparison with oxidative transformations of the individual components (oxidative dehydrogenation of C₂H₆ and oxidation of C₂H₄) shows that ethylene does not noticeably influence the ethane conversion, whereas ethane strongly suppresses the ethylene conversion. The maximal yield of ethylene from the ethane–ethylene fraction is close to that reached in oxidative dehydrogenation of ethane under similar conditions and amounts to 70–72%.     

Variations of structure and active species in mesoporous Cr-MSU-x catalyst during the dehydrogenation of ethane with CO<Subscript>2</Subscript>

The dehydrogenation of ethane to ethylene under CO₂ over mesoporous Cr-MSU catalyst was investigated with respect to the time on-stream behavior. When ethane was allowed to react for about 240 min, the meso-structure of catalyst remained nearly unchanged in spite of some decrease of surface area. The Cr(VI) species in tetrahedral coordination in fresh Cr-MSU were reduced to Cr(III) species in octahedral coordination, that was expected to cause the activity decrease of catalyst, together with the structure change. Cr(VI) is more active than Cr(III) for ethane dehydrogenation with CO₂, but Cr(III) represent fairly stable active centers for the reaction. The article is published in the original.     

Ethanol conversion and coke formation on TiO2 supported molybdovanadophosphoric acids

The catalytic conversion of ethanol on H_3+xPMo_12–xV_xO₄₀/TiO₂ catalysts (x=0,1,2) yielding both the products of alcohol dehydration (ethylene, diethyl ether) and dehydrogenation (acetaldehyde) but also ethane as secondary product is accompanied by the formation of coke. The amount and composition of coke was estimated on the basis of mass balance. The fact that the hydrogen/carbon atom ratio was low (0.6) supports the conclusion that coke is involved in the hydrogenation of primary ethylene.     

Catalytic performance of silica-supported chromium oxide catalysts in ethane dehydrogenation with carbon dioxide

The oxidative dehydrogenation of ethane into ethylene by CO₂ over a series of silica-supported chromium oxide catalysts was investigated. The results showed that the catalysts were effective for the reaction and CO₂ in the feed promoted the catalytic activity. The 5%Cr/SiO₂ catalyst exhibited the excellent performance with 30.7% ethane conversion and 96.5% ethylene selectivity at 700^oC. ESR and UV-DRS were used to probe the active sites and the species with high valent states (Cr⁵⁺ and/or Cr⁶⁺) were found to be important for the reaction.     

Formation of active phases of Fe/C,Cr/C and Fe–Cr/C catalysts in oxidative dehydrogenation of ethane

The oxidative dehydrogenation of ethane into ethylene using CO₂ as an oxidant at temperatures of 650–750 °C was carried out over Fe/C, Cr/C and Fe–Cr/C catalysts deposited on a carbon support. Before and after the reaction the catalysts were investigated by X-ray powder diffraction (XRD), in situ magnetometry and transmission electron microscopy methods. The correlation between activity of Fe/C, Cr/C and Fe–Cr/C catalytic systems and their phase composition was established.     

Deactivation of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the reaction of oxidative ethane dehydrogenation

The operational stability of a mixed oxide catalyst of Mo–V–Te–Nb–O composition in the oxidative dehydrogenation of ethane (ratio of C₂H₆: O₂ = 3: 1) is studied in a flow reactor at temperatures of 340–400°C, a pressure of 1 atm, and a WHSV of the feed mixture of 800 h^?1. It is found that the selectivity toward ethylene is 98% at 340°C, but the conversion of ethane at this temperature is only 6%; when the temperature is raised to 400°C, the conversion of ethane is increased to 37%, while the selectivity toward ethylene is reduced to 85%. Using physical and chemical means (XPS, SEM), it is found that the lack of oxidant in the reaction mixture leads to irreversible changes in the catalyst, i.e., reduced selectivity and activity. Raising the reaction temperature to 400°C allows the reduction of tellurium by ethane, from the +6 oxidation state to the zerovalent state, with its subsequent sublimation and the destruction of the catalytically active and selective phase; in its characteristics, the catalyst becomes similar to the Mo–V–Nb–O system containing no tellurium.     

油浆高温快速裂解过程的气固相产物研究

采用高频炉快速热解装置研究油浆的高温快速热解特性,考察了热解温度、氮气流量对气固相产物的组成和产率的影响。温度是影响气相产物产率的关键因素,气相产物主要为甲烷、氢气和乙烯,升高温度可提高甲烷和氢气的产率,而乙烯产率受高温下二次反应的影响在800℃到达最大值后逐渐降低,乙烷、丙烯产率较小且受二次反应的影响在700℃到达最大值后逐渐降低,温度高于800℃时会有少量乙炔生成且升温可提高乙炔产率。增加氮气流量可降低甲烷、氢气分压,缩短乙烯、丙烯等在高温区的停留时间,从而增加气相产物的产率。积炭产率随热解温度升高迅速增加,氮气流量的增加能够削弱二次反应从而降低积炭产率。     

Selective Oxidative Dehydrogenation of Ethane and Propane over Copper-Containing Mordenite: Insights into Reaction Mechanism and Product Protection

Copper(II)-containing mordenite (CuMOR) is capable of activation of C−H bonds in C₁-C₃ alkanes, albeit there are remarkable differences between the functionalization of ethane and propane compared to methane. The reaction of ethane and propane with CuMOR results in the formation of ethylene and propylene, while the reaction of methane predominantly yields methanol and dimethyl ether. By combining in situ FTIR and MAS NMR spectroscopies as well as time-resolved Cu K-edge X-ray absorption spectroscopy, the reaction mechanism was derived, which differs significantly for each alkane. The formation of ethylene and propylene proceeds via oxidative dehydrogenation of the corresponding alkanes with selectivity above 95 % for ethane and above 85 % for propane. The formation of stable π-complexes of olefins with Cu^I sites, formed upon reduction of Cu^II-oxo species, protects olefins from further oxidation and/or oligomerization. This is different from methane, the activation of which proceeds via oxidative hydroxylation leading to the formation of surface methoxy species bonded to the zeolite framework. Our findings constitute one of the major steps in the direct conversion of alkanes to important commodities and open a novel research direction aiming at the selective synthesis of olefins.     

HCl存在下CeO2基催化剂上乙烷氧化脱氢制乙烯

报道了一种HCl存在时温和条件下的乙烷氧化脱氢制乙烯催化转化新途径. 研究发现,在多种金属氧化物催化剂中,CeO₂呈现最佳乙烯生成的催化性能. 与纳米粒子相比,具有棒状和立方体状形貌的CeO₂纳米晶具有较高的乙烷转化率和乙烯选择性. 以MnO_x修饰CeO₂可进一步提高催化性能. 在8 wt% MnO_x-CeO₂催化剂上,723K反应2 h时乙烷转化率和乙烯选择性分别为94%和69%. 该催化剂性能稳定,反应100 h乙烯收率可保持在65%-70%. HCl的存在对乙烯的选择性生成起着至关重要的作用,一部分乙烯来自于氯乙烷的脱HCl反应.     

Reaction of a diaryldigermyne with ethylene

Reaction of the stable digermyne BbtGeGeBbt (Bbt = 2,6-[CH(SiMe₃)₂]₂-4-[C(SiMe₃)₃]-C₆H₂) with ethylene initially afforded the corresponding 1,2-digermacyclobutene. Depending on the reaction conditions applied, further reaction of this 1,2-digermacyclobutene with ethylene furnished two different reaction products: a 1,4-digermabicyclo[2.2.0]hexane or a bis(germiranyl)ethane. Combined experimental and theoretical results suggested that the 1,4-digermabicyclo[2.2.0]hexane and the bis(germiranyl)ethane are the thermodynamic and kinetic reaction products, respectively. A reaction mechanism in agreement with these results was proposed.     

Mathematical modeling of self-oscillations in ethane oxidation over nickel

The methodology of constructing a phenomenological model for complex heterogeneous catalytic reactions is described in detail. The proposed approach is applicable to development of mathematical models describing the onset of self-oscillations in hydrocarbon oxidation on the transition metal surface. The approach is based on construction of a microkinetic scheme taking into account the formation of main reaction products and intermediates, on estimation of the heat of reaction, activation energy, and preexponential factor for elementary steps and includes development and a subsequent analysis of the corresponding mathematical model. Catalytic reactions are considered in the ideal adsorption layer approximation without taking into account the relationship between coverages and spatial coordinates. Accordingly, the mathematical model is an independent system of ordinary differential equations. This methodology is used to develop a point (lumped) model for ethane oxidation over nickel, which is based on a 36-step microkinetic scheme taking into account the oxidation and reduction of nickel and the formation of total (CO₂ and H₂O) or partial (CO and H₂) ethane oxidation products, as well as the dehydrogenation of ethane into ethylene. The proposed model predicts the onset of self-oscillations in this reaction at atmospheric pressure in the temperature range from 850 to 1400 K. The kinetic oscillations are caused by the cyclic oxidation and reduction of nickel. The self-oscillations of the reaction rate are accompanied by oscillations of the catalyst temperature. The results of modeling are compared with experimental data.