Autocatalytic gas-phase dehydrogenation of ethane

Homogeneous gas-phase pyrolysis of ethane by continuous CO₂ laser irradiation was used in our experiments for bulk heating of the reaction mixture. Laser energy was absorbed by ethylene, the main product of ethane dehydrogenation, and transferred to the reaction medium via collisional relaxation. A mechanism of ethane dehydrogenation is suggested to describe the pyrolysis process. The mechanism is autocatalytic in respect of ethylene and includes ethane?Cethylene interaction with the formation of methyl and propyl radicals. Rate constants of elementary reactions, selectivity, and yields of pyrolysis products were determined. The composition of ethane dehydrogenation products determined in the experiments was substantially different from the calculated thermodynamic equilibrium composition.     

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。     

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.     

甲烷在CaO—Li—CeO2催化剂上氧化偶联反应宏观动力学

本文报导在CaO-Li-CeO_2催化剂上甲烷氧化偶联反应的宏观动力学。在893-973K温度范围内测出了甲烷氧化偶联反应产物的生成速度及其对甲烷及氧分压的反应级数。根据这些数据计算出了甲烷转化和相应产物生成的表观活化能E。在此基础上对反应机理进行了初步探讨。     

Electrochemical Reduction of CO2 to Ethane through Stabilization of an Ethoxy Intermediate

Electrochemical conversion of CO₂ into ethane is seldom observed because of the generally higher selectivity towards methane, ethylene, and ethanol. Consequently, little experimental evidence for its reaction mechanism exists and thus remains largely unknown. Now, by combining electrochemistry with in situ X-ray absorption fine-structure and in situ Raman techniques, iodide-derived copper (ID-Cu) and oxide-derived copper (OD-Cu) systems were studied to obtain a deeper understanding of the CO₂ to ethane mechanism. With trace iodine species on the surface and positively charged Cu species, production of ethane is significantly more favored on ID-Cu compared to OD-Cu, with higher selectivity and faster kinetics. For the first time, it is experimentally found that the formation of ethane follows the same pathway to ethylene and ethanol, and better stabilization of the late stage ethoxy intermediate can steer the reaction to ethane over ethanol.     

加拿大合成原油瓦斯油裂解反应规律与产物生成机理

利用小型固定流化床实验装置研究了加拿大合成原油重瓦斯油(HGO)和轻瓦斯油(LGO)的催化裂解性能和热裂解性能。HGO和LGO催化裂解总低碳烯烃(乙烯+丙烯+丁烯)产率在660℃附近达到最大值,分别为33.8%和35.6%。HGO和LGO热裂解反应程度很大,700℃的转化率分别为66.7%和76.3%。HGO热裂解总低碳烯烃的产率在680℃达到最大值27.9%。通过对比分析催化裂解与热裂解气体产物产率的比值发现,催化剂的加入促进了乙烯和液化气的生成,同时抑制了甲烷和乙烷的生成。研究结果揭示了小分子烃类的生成机理,甲烷和乙烷主要是自由基反应的产物,乙烯和液化气是自由基反应和正碳离子反应的共同产物。     

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.     

High pressure pyrolysis of n-heptane

Pyrolysis of n-heptane was investigated in a tubular reactor in the temperature range of 793–953 K and pressure range of 0.1–2.93 MPa. At all conditions, the main products were methane, ethylene, ethane, propylene, 1-butene, 1-pentene and 1-hexene. With an increase in pressure, the selectivities of hydrogen, methane, ethylene and propylene decreased and that of propane, n-butane and 1-butene increased. To explain the product distribution at high pressure, the Rice–Kossiakoff theory was modified by including the bimolecular reactions of alkyl radicals with the parent hydrocarbon. The initial product selectivities, calculated using the modified R–K mechanism, were in good agreement with the experimental selectivities. The overall kinetics of n-heptane pyrolysis was determined by non-linear analysis. The optimum values of the kinetic parameters at each pressure were determined by minimizing the difference between the calculated and experimental conversions. At each pressure, the reaction order was close to unity and the activation energy ranged between 209 and 219 kJ mol⁻¹.     

Adsorption of ethane, ethylene, propane, and propylene on a magnesium-based metal-organic framework

Separation of olefin/paraffin is an energy-intensive and difficult separation process in petrochemical industry. Energy-efficient adsorption process is considered as a promising alternative to the traditional cryogenic distillation for separating olefin/paraffin mixtures. In this work, we explored the feasibility of adsorptive separation of olefin/paraffin mixtures using a magnesium-based metal-organic framework, Mg-MOF-74. Adsorption equilibria and kinetics of ethane, ethylene, propane, and propylene on a Mg-MOF-74 adsorbent were determined at 278, 298, and 318 K and pressures up to 100 kPa. A dual-site Sips model was used to correlate the adsorption equilibrium data, and a micropore diffusion model was applied to extract the diffusivities from the adsorption kinetics data. A grand canonical Monte Carlo simulation was conducted to calculate the adsorption isotherms and to elucidate the adsorption mechanisms. The simulation results showed that all four adsorbate molecules are preferentially adsorbed on the open metal sites where each metal site binds one adsorbate molecule. Propylene and propane have a stronger affinity to the Mg-MOF-74 adsorbent than ethane and ethylene because of their significant dipole moments. Adsorption equilibrium selectivity, combined equilibrium and kinetic selectivity, and adsorbent selection parameter for pressure swing adsorption processes were estimated. The relatively high values of adsorption selectivity suggest that it is feasible to separate ethylene/ethane, propylene/propane, and propylene/ethylene pairs in a vacuum swing adsorption process using Mg-MOF-74 as an adsorbent.     

Primary mechanism of the thermal decomposition of tricyclodecane

To better understand the thermal decomposition of polycyclanes, the pyrolysis of tricyclodecane has been studied in a jet-stirred reactor at temperatures from 848 to 933 K, for residence times between 0.5 and 6 s and at atmospheric pressure, corresponding to a conversion between 0.01% and 25%. The main products of the reaction are hydrogen, methane, ethylene, ethane, propene, 1,3-cyclopentadiene, cyclopentene, benzene, 1,5-hexadiene, toluene, and 3-cyclopentylcyclopentene. A primary mechanism containing all the possible initiation steps, including those involving diradicals, as well as propagation reactions has been developed and allows experimental results to be satisfactorily modeled. The main reaction pathways of consumption of tricyclodecane and of formation of the main products have been derived from flow rate and sensitivity analyses.     

A concerted hydrogen-transfer reaction in the gas phase

The kinetics of the thermal reaction of mixtures of ethylene and cyclopentene has been examined for the occurrence of a concerted hydrogen-transfer reaction, . The main products of the reaction were ethane and cyclopentadiene, and the rate of formation of ethane was first order in each reactant over a 2500-fold change in the ratio of concentrations of the reactants. An increased surface-to-volume ratio of the reaction vessel or additions of oxygen and nitric oxide had little effect on the rate of formation of ethane, and it was concluded that the dominant reaction in the system was the concerted hydrogen-transfer process. The rate constant for the reaction, measured over the temperature range of 325–505°C, was represented as      

Mathematical simulation of the oxygen–ethane reaction

Previous experimentation on the initial stages of the oxygen-ethane reaction at temperatures near 600°C demonstrated the existence of a significant induction period under homogeneous conditions. Below 0.13% conversion, the rate with oxygen is less than the rate of ethane pyrolysis in the absence of oxygen. Experiments were conducted with mixtures of ～1% ethane and ～0.4% oxygen in nitrogen using the “wall-less” technique. Mathematical simulations of the oxidative and simple pyrolyses of ethane were established by a numerical analysis. The experimentally observed induction period and the mechanisms previously proposed are supported by these computations. Steady-state conditions are much more slowly attained in the presence of oxygen than in its absence.     

A systematically generated, pressure-dependent mechanism for high-conversion ethane pyrolysis. 1. Pathways to the minor products

The deposition of carbon during hydrocarbon pyrolysis is part of many industrial processes. The rate and nature of deposition depend, in part, on the gas-phase chemistry of the minor pyrolysis products, which serve as deposition precursors. But the specific reaction pathways governing the formation and destruction of these minor gas-phase products are only partially known. We apply an updated version of our automated mechanism generation tool XMG-PDep to the high-conversion, pyrocarbon-depositing ethane pyrolysis system of Glasier and Pacey, to systematically uncover the likely reaction pathways governing the observed minor products acetylene, propylene, 1,3-butadiene, and benzene. Thorough examination by means of sensitivity, equilibrium, and reaction-pathway analyses reveals an extremely complex, intertwined set of reaction pathways controlling these deposition precursors, some of which are not often considered in the wider pyrolysis literature. Large, aggregated sets of disproportionation reactions, for example, appear to play an important role in the formation of benzene. The analyses motivate a companion paper (following paper in this issue) which explores in greater depth the effects of large groups of radical disproportionation reactions, omitted reaction families, and the possibility that pressure changes in the reactor could alter the distribution of the deposition precursors.     

Kinetic modeling of propane oxidation and pyrolysis

Propane oxidation in jet-stirred reactor was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C₁? C₄ hydrocarbons. The present detailed mechanism is able to reproduce experimental species concentration profiles obtained in our high-pressure jet-stirred reactor (900 ? T/K ? 1200; 1 ? P/atm ? 10; 0.15 ? ? ? 4) and in a turbulent flow reactor at 1 atm; ignition delay times measured in shock tube (1200 ? T/K ? 1700; 2 ? P/atm ? 15; 0.125 ? ? ? 2); H-atoms concentrations measured in shock tube during the pyrolysis of propane and burning velocities of freely propagating premixed propane-air laminar flames. The computed results are discussed in terms of pressure and equivalence ratio (?) effects on propane oxidation. The same detailed kinetic reaction mechanism can also be used to model the oxidation of methane, ethylene, ethane, and propene in similar conditions. © John Wiley & Sons, Inc.     

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。     

Influences of ClH and BrH on the pyrolyses of neopentane and ethane at small extents of reaction

A symbolic mechanism “μH, YH” has been proposed to account for the homogeneous chain pyrolysis of an organic compound μH in the presence of a hydrogenated additive YH at small extents of reaction. An analysis of this mechanism leads to two limiting cases: the thermal decomposition of neopentane corresponds to the first one (A), that of ethane to the second one (B). Previous experimental work has shown that this mechanism seems to account for a number of experimental observations, especially the inhibition of alkane pyrolyses by alkenes. Experimental investigations were extended by examining the influences oftwo hydrogen halides (ClH and BrH) upon the pyrolyses of neopentane (at 480°C) and ethane (around 540°C). The experiments have been performed in a conventional static Pyrex apparatus and reaction products have been analyzed by gas-liquid chromatography. The study shows that ClH and BrH accelerate the pyrolysis of neopentane (into i-C₄H₈ + CH₄). The experimental results are interpreted by reaction schemes which appear as examples of the mechanism “μH, YH” in the first limiting case (A). The proposed schemes enable one to understand why the accelerating influence of ClH is lower or higher than that of BrH, depending on the concentration of the additive. An evaluation of the rate constant of the elementary steps neo-C₅H₁₁ · → i-C₄H₈ + CH₃ · is discussed. In the case of ethane pyrolysis, BrH inhibits the formation of the majorproducts (C₂H₄ + H₂) and, even more, that of n-butane traces. The experimental results are interpreted by a reaction scheme which appears as an example of the mechanism “μH, YH” in the second limiting case (B). On the contrary, ClH has no noticeable influence on the reaction kinetics. This result inessentially due to the fact that the bond dissociation energy of Cl? H(?103 kcal/mol) is higher than that of C₂H₅—H (?98 kcal/mol), whereas that of Br—H (?88 kcal/mol) is lower.     

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.     

膜反应器中Ni-Cu催化剂上CO加氢合成乙烯的研究

 将硅藻土－TiO2－聚乙烯（或聚丙烯）疏水复合膜反应器应用于\r\nCO加氢合成乙烯的反应过程，以Ni－Cu／TiSiO和Ni－Cu／MgSiO为催化\r\n剂，考察了反应温度、空速和进料组成等因素对CO转化率和乙烯选择性\r\n的影响．结果表明：膜催化反应的产物与常规催化反应相同，催化反应\r\n机理没有区别，在适宜条件下，膜催化反应的CO转化率和乙烯选择性较\r\n之常规催化反应提高了10％左右，二氧化碳和甲烷的选择性降低，而乙\r\n烷的选择性略有增加．     

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。