CO, H2 or C3H8 assisted catalytic combustion of methane over supported LaMnO3 monoliths

The effect of the addition of a second fuel such as CO, C₃H₈ or H₂ on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO₃/La-γAl₂O₃ catalyst. Results of autothermal ignition of different binary fuel mixtures characterised by the same overall heating value show that the presence of a more reactive compound reduces the minimum pre-heating temperature necessary to burn methane. The effect is more pronounced for the addition of CO and very similar for C₃H₈ and H₂. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H₂≥C₃H₈>CH₄. Under autothermal conditions, nearly complete methane conversion is obtained with catalyst temperatures around 800 °C mainly through heterogeneous reactions, with about 60–70 ppm of unburned CH₄ when pure methane or CO/CH₄ mixtures are used. For H₂/CH₄ and C₃H₈/CH₄ mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH₄ homogeneous oxidation promoted by H and OH radicals generated by propane and hydrogen pyrolysis at such relatively high temperatures.

Finally, a steady state multiplicity is found by decreasing the pre-heating temperature from the ignited state. This occurrence can be successfully employed to pilot the catalytic ignition of methane at temperatures close to compressor discharge or easily achieved in regenerative burners.     

Synthesis of C2H4 by partial oxidation of CH4 over LiCl/NiO

Alkali halide added transition metal oxides produced ethylene selectively in oxidative coupling of methane. The role of alkali halides has been investigated for LiCl-added NiO (LiCl/NiO). In the absence of LiCl the reaction over NiO produced only carbon oxides (CO₂ + CO). However, addition of LiCl drastically improved the yield of C₂ compounds (C₂H₆ + C₂H₄). One of the roles of LiCl is to inhibit the catalytic activity of the host NiO for deep oxidation of CH₄. The reaction catalyzed by the LiCl/NiO proceeds stepwise from CH₄ to C₂H₄ through C₂H₆ (2CH₄ → C₂H₆ → C₂H₄). The study on the oxidation of C₂H₆ over the LiCl/NiO showed that the oxidative dehydrogenation of C₂H₆ to C₂H₄ occurs very selectively, which is the main reason why partial oxidation of CH₄ over LiCl/NiO gives C₂H₄ quite selectively. The other role of LiCl is to prevent the host oxide (NiO) from being reduced by CH₄. The catalyst model under working conditions was suggested to be the NiO covered with molten LiCl. XPS studies suggested that the catalytically active species on the LiCl/NiO is a surface compound oxide which has higher valent nickel cations (Ni^(2+δ)+ or Ni³⁺). The catalyst was deactivated at the temperatures>973 K due to vaporization of LiCl and consumption of chlorine during reaction. The kinetic and CH₄---CD₄ exchange studies suggested that the rate-determining step of the reaction is the abstraction of H from the vibrationally excited methane by the molecular oxygen adsorbed on the surface compound oxide.     

C2H6/C3H8影响CH4爆炸极限参数及动力学特性研究

为研究C₂H₆/C₃H₈对甲烷爆炸极限参数及动力学特性的影响,采用标准的可燃气体爆炸极限测定装置测定了不同配比的C₂H₆/C₃H₈混合气体对甲烷爆炸极限的影响规律,同时得出了氮气惰化条件下甲烷爆炸临界参数的变化规律。此外,利用Chemkin软件模拟了C₂H₆/C₃H₈混合气体对甲烷爆炸过程中中间产物浓度的影响情况,并进行了敏感性分析。结果表明,C₂H₆/C₃H₈的存在降低了甲烷的爆炸上下限,增大了甲烷的爆炸危险度;在氮气惰化过程中甲烷的爆炸上限下降,爆炸下限上升,最终爆炸上下限重合,重合点处甲烷浓度和氮气临界浓度均随C₂H₆/C₃H₈的添加而逐渐减小;此外,C₂H₆/C₃H₈混合气体使甲烷爆炸过程中CO和·H的生成量逐渐增大,而CO₂、·O和·OH的生成量则有下降趋势,通过对爆炸过程中甲烷体积的敏感性分析,发现C₂H₆/C₃H₈的存在在某种程度上促进了甲烷爆炸。对比不同配比的C₂H₆/C₃H₈混合气体,发现C₃H₈含量越高,其对甲烷爆炸过程中相关参数的影响越大,这可为工矿企业的安全生产提供一定的理论依据。     

Catalytic decomposition of light alkanes, alkenes and acetylene over Ni/SiO2

The decomposition of different hydrocarbons (CH₄, C₂H₆, C₂H₄, C₂H₂, C₃H₈, and C₃H₆) over Ni (5 wt.%)/SiO₂ catalysts was carried out. The initial rates of decomposition of the hydrocarbons, the kinetic curves of the decomposition and the kinetic curves of the hydrogenation of deposited carbon into methane depended on the types of hydrocarbons. In addition, the catalytic life of the Ni/SiO₂ catalyst was also dependent on the types of hydrocarbons, i.e. the life was longer according to the order, alkanes>alkenesacetylene.

The carbons deposited on the catalyst were characterized by SEM and Raman spectroscopy. The appearances of the deposited carbons were different among alkanes, alkenes, and acetylene, i.e. a zigzag fiber structure from methane, and a rolled fiber structure from alkenes and acetylene. From Raman spectra of the deposited carbons, it was found that the degree of graphitization of deposited carbon was higher in the order, alkanes>alkenes>acetylene. These results suggest that the mechanism of decomposition of hydrocarbons and the growth mechanism of carbon fibers on the catalyst were different among alkanes, alkenes and acetylene.     

High temperature catalytic combustion of methane and propane over hexaaluminate catalysts: NOx emission characteristics

Several hexaaluminate-related materials were prepared via hydrolysis of alkoxide and powder mixing method for high temperature combustion of CH₄ and C₃H₈, in order to investigate the effect of the concentration of the fuels, O₂ and H₂O on NO_x emission and combustion characteristics. Among the hexaaluminate catalysts, Sr_0.8La_0.2MnAl₁₁O₁₉₋ prepared by the alkoxide method exhibited the highest activity for methane combustion and low NO_x emission capability. NO_x emission at 1500 °C was increased linearly with O₂ concentration, whereas water vapor addition decreased NO_x emission in CH₄ combustion over the Sr_0.8La_0.2MnAl₁₁O₁₉₋ catalyst. In the catalytic combustion of C₃H₈ over the Sr_0.8La_0.2MnAl₁₁O₁₉₋ catalyst, the amount of NO_x emitted was raised in the temperature range between 1000 and 1500 °C when the C₃H₈ concentration increased from 1 to 2 vol.%. It was found that NO_x emission in this temperature range was reduced effectively by adding water vapor.     

Chemistry of surface oxygen formed from N2O on ZSM-5 at moderate temperatures

Conversion of CH₄, C₂H₆, C₃H₈, benzene and their binary mixtures over H-NaZSM-5 catalyst in the presence of N₂O was studied. It was found that under experimental conditions methane alkylates benzene to give toluene and xylenes. Acidity of the catalyst had no effect on the reactivity of active oxygen formed from N₂O towards methane and benzene, but affected their secondary transformation. Acidic samples favored the reaction of aromatic ring methylation with methane whereas deep oxidation of CH₄ prevailed on NaHZSM-5. Based on the relative reactivities and ¹³C label distribution in the products of ¹³CH₄+C₆H₆+N₂O feed conversion, the scheme of hydrocarbon transformation was proposed.     

The directed relation graph method for mechanism reduction in the oxidative coupling of methane

A detailed mechanism comprising the GRI-Mech natural gas combustion mechanism and recent updates to it that included soot formation and reactions involving acetylene by Frenklach and coworkers was chosen to describe the gas phase reactions in the oxidative coupling of methane (OCM). The complete mechanism was reduced for use under OCM conditions with the directed relation graph method. The mechanism reduction achieved reduction ratios of 0.55 for the number of species and 0.57 for the number of reactions when the differences in the concentrations of CH₄, O₂, C₂H₆, C₂H₄, and C₂H₂ between the original and reduced mechanisms were required to be less than 5%.     

The homogeneous gas phase oxidation of methane and the retarding effect of basic/inert surfaces

The homogeneous gas phase O₂-based oxidation of methane was studied in the temperature range, from 500°C to 750°C at methane partial pressures ranging from 3 bar to 40 bar. At the lower end of the temperature range methanol, formaldehyde, and CO represent the main products, while at temperatures exceeding 650° C/C-coupled products, C₂H₆, C₂H₄, C₃H₆ and C₃H₈ predominate. The change in selectivity as function of the temperature is well explained based on a free radical chain mechanism with degenerate branching, initiated by the gas phase reaction, CH₄+O₂→CH^·₃+HO^·₂. Bringing in basic catalysts known to catalyze the system at low methane partial pressures, in the reactor e.g. SrCO₃, BaCO₃, and 7% Li/MgO resulted in reduced rates of methane and oxygen conversions, and only minor changes in the selectivity to coupled products were observed.     

H2 production by ethanol decomposition with a gliding arc discharge plasma reactor

A gliding arc discharge (GRD) reactor was used to decompose ethanol into primarily H₂ and CO with small amounts of CH₄, C₂H₂, C₂H₄, and C₂H₆. The ethanol concentration, electrode gap, input voltage and Ar flow rate all affected the conversion of ethanol with results ranging from 40.7% to 58.0%. Interestingly, for all experimental conditions the S_H2/S_CO selectivity ratio was quite stable at around 1.03. The mechanism for the decomposition of ethanol is also described.     

Non-oxidative reaction of CBrF3 with methane over NiZSM-5 and HZSM-5

Catalytic hydrodehalogenation of CBrF₃ with methane was studied over NiZSM-5 and HZSM-5 in tubular reactor between 573 and 873 K and at ambient pressure. It was found that the incorporation of nickel into HZSM-5 significantly enhanced the activity of the zeolite. A variety of products were formed during reaction, including CH₃Br, CHF₃, CH₂Br₂, C₂F₆, C₂H₄, C₂H₂, C₂H₂F₂, CHBrF₂, CH₂BrF, and C₂H₃Br. XRD analysis showed that these two zeolite catalysts did not suffer any loss in their crystallinity during use. Deactivation of both NiZSM-5 and HZSM-5 may, in part, be due to poisoning of the zeolite by halogens. Coking is another cause of the deactivation of HZSM-5, but appears to play a minor role in NiZSM-5 deactivation. A series of methylated silicone oils was detected during reaction over NiZSM-5.     

基于分子动力学模拟的轮胎橡胶气相热解产物反应机理

运用分子动力学的方法,对轮胎橡胶的热解过程进行了模拟,并结合模拟结果和密度泛函数对其气相产物的反应路径进行推测计算。模拟结果表明,热解过程主要分为两个阶段,低温热解阶段发生的主要反应是橡胶长链断裂形成单体,主要产物为异戊二烯、苯乙烯和1,3-丁二烯;高温热解阶段发生的主要反应是单体进一步生成小分子气体,产物中CH₄、H₂、C₂H₄占大部分,还有少量C₂H₆、C₃H₆。其中CH₃·攻击异戊二烯和苯乙烯单体夺取特定位置的H·是生成CH₄的最优路径,H·攻击苯乙烯单体夺取特定位置的H·是生成H₂的最优路径,CH₂CH·攻击1,3-丁二烯单体夺取特定位置的H·是生成CH₂CH₂的最优路径。本文还将热解产物分别跟天然橡胶单独热解和天然橡胶与丁苯橡胶共热解的热解产物做对比,为废旧轮胎橡胶热解得到特定的气相产物和催化热解提供理论依据。     

CH4/C2H6/C3H8在MOF-505@5GO上吸附相平衡和选择性

应用溶剂热法合成了不同氧化石墨烯（GO）负载量的MOF-505@GO复合材料,分别采用全自动表面积吸附仪、P-XRD、SEM和Raman对材料进行了性能表征,测定了CH₄、C₂H₆和C₃H₈在MOF-505@GO上的吸附等温线,并进行Langmuir-Freundlich方程拟合,依据IAST理论模型计算了C₂H₆/CH₄和C₃H₈/CH₄二元混合气在MOF-505@5GO上的吸附选择性。研究结果表明,随着GO负载量增大,MOF-505@GO复合材料的孔容及BET比表面积先增大后减小,当GO负载量为5%（质量）时,复合材料MOF-505@5GO的孔容及BET比表面积达到最大,当GO负载量进一步增大至8%（质量）和10%（质量）时,复合材料的孔容及BET比表面积逐渐降低。在0.1 MPa和298 K条件下,MOF-505@5GO对CH₄、C₂H₆和C₃H₈的吸附容量分别为0.88、4.81和5.17 mmol·g^-1,相比MOF-505分别提高了14.9%、30.7%和13.1%。MOF-505@5GO对C₂H₆/CH₄和C₃H₈/CH₄的吸附选择性分别为40.1和3056.1,其对C₂H₆/CH₄和C₃H₈/CH₄具有极高的吸附选择性。     

Free-radical reactions involved in C1-C3 hydrocarbons interaction with oxide catalysts

Experimental proofs of a free radical mechanism in methane oxidative coupling, with homolytic rupture of the C---H bond are given. High concentrations of free radical sites are produced by mechanical milling of SiO₂. A study of C₁---C₃alkanes interaction with these sites allows to simulate the, processes of alkanes oxidation and oxidative dehydrogenation. The reactivity of ethane and propane is higher than that of methane in accordance with the Polanyi-Semenov rule. Oxidative dehydrogenation of ethane is studied on Cd-containing zeolites. CH₄, C₂H₆ and C₃H₈ oxidative dehydrogenation by O₂ or CO₂ is studied on a MNO/SiO₂ catalyst. The initiation of radical reactions of hydrocarbons on Cl-containing catalysts proceeds via chlorine atoms generation.     

A pillared-layer metal-organic framework for efficient separation of C3H8/C2H6/CH4 in natural gas

Metal-organic frameworks (MOFs) have great potentials as adsorbents for natural gas purification. However, the trade-off between selectivity and adsorption capacity remains a challenge. Herein, we report a pillared-layer metal-organic framework Ni(HBTC)(bipy) for efficiently separating the C₃H₈/C₂H₆/CH₄ mixture. The experimental results show that the adsorption capacity of C₃H₈ and C₂H₆ on Ni(HBTC)(bipy) are as high as 6.18 and 5.85 mmol·g^-1, while only 0.93 mmol·g^-1 for CH₄ at 298 K and 100 kPa. Especially, the adsorption capacity of C₃H₈ at 5 kPa can reach an unprecedented 4.52 mmol·g^-1 and for C₂H₆ it is 1.48 mmol·g^-1 at 10 kPa. The ideal adsorbed solution theory predicted C₃H₈/CH₄ selectivity is as high as 1857.0, superior to most of the reported materials. Breakthrough experiment results indicated that material could completely separate the C₃H₈/C₂H₆/CH₄ mixture. Therefore, Ni(HBTC)(bipy) is a promising material for separation of natural gas.     

Boosting selective C2H2/CH4, C2H4/CH4 and CO2/CH4 adsorption performance via 1,2,3-triazole functionalized triazine-based porous organic polymers

Nitrogen-rich porous organic polymers have shown great potentials in gas adsorption/separation, photocatalysis, electrochemistry, sensing and so on. Herein, 1,2,3-triazole functionalized triazine-based porous organic polymers (TT-POPs) have been synthesized by the copper-catalyzed azide-alkyne cycloaddition (Cu-AAC) polymerization reactions of 1,3,5-tris(4-azidophenyl)-triazine with 1,4-diacetylene benzene and 1,3,5-triacetylenebenzene, respectively. The characterizations of N₂ adsorption at 77 K show TT-POPs possess permanent porosity with BET surface areas of 666 m²·g^-1 (TT-POP-1) and 406 m²·g^-1 (TT-POP-2). The adsorption capacities of TT-POPs for CO₂, CH₄, C₂H₂ and C₂H₄, as well as the selective separation abilities of CO₂/N₂, CO₂/CH₄, C₂H₂/CH₄ and C₂H₄/CH₄ were evaluated. The gas selective separation ratio of TT-POPs was calculated by the ideal adsorbed solution theory (IAST) method, wherein the selective separation ratios of C₂H₂/CH₄ and C₂H₄/CH₄ of TT-POP-2 was 48.4 and 13.6 (298 K, 0.1 MPa), which is comparable to other adsorbents (5.6-120.6 for C₂H₂/CH₄, 10-26 for C₂H₄/CH₄). This work shows that the 1,2,3-triazole functionalized triazine-based porous organic polymer has a good application prospect in natural gas purification.     

Comparison between methane and propylene as reducing agents in the SCR of NO over Pd supported on tungstated zirconia

It is now well known that when Pd is supported on acidic supports, it becomes highly selective for the reduction of NO by methane in the presence of excess oxygen. It is also known that this promoting effect not only occurs with acidic zeolite supports, but also with acidic zirconia supports, such as sulfated zirconia (SZ) and tungstated zirconias (WZ). However, this promoting effect has not been investigated for the SCR with other hydrocarbons as reducing agents. In this contribution, we have investigated the behavior of a series of Pd/WZ catalysts and compared them using methane and propylene as reducing agents. The results show some important differences when the reducing agent is changed. For example, while with CH₄ the addition of W to the catalyst results in an increase in both NO and hydrocarbon conversion, with C₃H₆ it results in a decrease in activity. At the same time, while the presence of NO accelerates the activation of CH₄, it inhibits the activation of C₃H₆, moving its light-off to higher temperatures. Finally, an important difference between CH₄ and C₃H₆ as reducing agents is regarding the selectivity towards N₂ as opposed to N₂O. Using CH₄ resulted in much lower production of N₂O than using C₃H₆, over the entire temperature range investigated.     

SCR of NOx with CH3OH on H-mordenite: mechanism and reaction intermediates

The selective reduction of NO_x over H-mordenite (H-m) was studied using CH₃OH as reducing agent. Results are compared with those obtained with other conventional reducing agents (ethylene and methane), with gas-phase reactions, and with other metal-exchanged mordenites (Cu-mordenite (Cu-m) and Co-mordenite (Co-m)). H-m was found to be an effective catalyst for the SCR of NO_x with CH₃OH. When different reducing agents were compared over H-m, CH₃OH > C₂H₄ > CH₄ was the order according to the maximum NO conversion obtained using 1% of oxygen in the feed. Instead, if selectivity is considered, the order results CH₄ > CH₃OH > C₂H₄. In reaction experiments, two distinct zones defined by two maxima with NO to N₂ conversion are obtained at two different temperatures. A correlation exists between the said zones and the CO : CO₂ ratio. At low temperatures, CO prevails whereas at high temperatures CO₂ prevails. These results indicate that there exist different reaction intermediates. Evidence from reaction experiments, FTIR results, and transient experiments suggest that the reaction mechanism involves formaldehyde and dimethyl ether (DME) as intermediates in the 200–500°C temperature range. The surface interaction between CH₃OH (or its decomposition products) and NO is negligible if compared with NO₂, indicating that the oxidation of NO to NO₂ on acid sites is a fundamental path in this system. Different from other non-oxygenated reductants (methane and ethylene), a gas-phase NO_x initiation effect on hydrocarbon combustion was not observed.     

Oxidative coupling of methane in Ba0.5Sr0.5Co0.8Fe0.2O3−δ tubular membrane reactors

A dense membrane tube made of Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) was prepared by plastic extrusion from BSCF oxide synthesized by the complexing EDTA-citrate method. The membrane tube was used in a catalytic membrane reactor for oxidative coupling of methane (OCM) to C₂ without an additional catalyst. At high methane concentration (93%), about 62% C₂ selectivity was obtained, which is higher than that achieved in a conventional reactor using the BSCF as a catalyst. The dependence of the OCM reaction on temperature and methane concentration indicates that the C₂ selectivity in the BSCF membrane reactor is limited by high ion recombination rates. If an active OCM catalyst (La-Sr/CaO) was packed in the membrane tube, C₂ selectivity and CH₄ conversion increased compared to the blank run. The highest C₂ yield in the BSCF membrane reactor in presence of the La-Sr/CaO catalyst was about 15%, similar to that in a packed-bed reactor with the same catalyst under the same conditions. However, the ratio of C₂H₄/C₂H₆ in the membrane reactor was much higher than that in the packed-bed reactor, which is an advantage of the membrane reactor.     

Synthesis of an active quaternary phosphonium salt and its application to the Wittig reaction: Kinetic study

The synthesis of liquid crystal methyl [4-(nonyloxy) styryl] benzoate was achieved in four stages. First, toluic acid [1] was brominated with N-bromosuccinimide in refluxing CCl₄ to form 4-bromotoluic acid [2] (BTA); second, BTA was esterified with methanol in SOCl₂/CH₂Cl₂ to afford methyl (4-bromomethyl) benzoate [3] (BME); third, BME was condensed with triphenylphosphine (PPh₃) in CH₂Cl₂ to produce the active Wittig reagent methyl (4-methylbenzoate) triphenylphosphonium bromide [4] (MBPB) and fourth, MBPB reacted with 4-nonyloxybenzaldehyde to produce MNSB in high yield. Each of these four reactions was carried out in a separate homogeneous organic solution and the effects of the reaction conditions in each reaction as well as a kinetic model for each individual reaction were studied. Furthermore, the active reagent was applied to the reaction of 4-nonyloxybenzaldehyde (n-C₉H₁₉O(C₆H₄)CHO) to synthesise cis-(E) and trans-(Z)methyl[4-(nonyloxy)styryl]benzoate (n-C₉H₁₉(C₆H₄)CHCH(C₆H₄)COOCH₃) from a two-phase medium alkaline solution of NaOH/organic solvent. High conversion and high selectivity were obtained via this reaction.     

载体对Li-Mn氧化物催化甲烷氧化偶联反应作用

采用浸渍法制备不同金属氧化物载体负载的Li-Mn/MO_x(M=Mg,La,Ti,Si,Zr,Ta)催化剂,对其甲烷氧化偶联反应活性进行评价。结果表明,以TiO_2为载体制备的Li-Mn/TiO_2催化剂具有较高的CH_4转化率和C2烃选择性,C_2烃产率显著提高,金属氧化物TiO_2是Li-Mn复合氧化物的优良催化剂载体。n(Li)∶n(Mn)=1.0∶2.0形成的Li-Mn/TiO_2催化剂具有最高的CH_4转化率和C_2烃选择性,n(C_2H_4)∶n(C_2H_6)的增加有助于提高反应产物中C_2H_4的相对浓度,W元素的添加未能进一步提高Li-Mn/TiO_2催化剂的催化活性。Li-Mn/TiO_2催化剂在n(Li)∶n(Mn)=1.0∶2.0、反应温度775℃、反应压力0.1 MPa、V(CH_4)∶V(O_2)=2.5、空速7 200 m L·(h·g)~(-1)和催化剂用量0.5 g条件下,CH_4转化率达31.9%,C_2选择性达52.7%,表现出最佳催化效果。