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1.
A perovskite material of Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−δ (BSCF), with both electronic and ionic conductivity, was synthesized by a combined citrate–EDTA complexing method. The dense membrane tube made of BSCF was fabricated using the plastic extrusion method. The partial oxidation of methane (POM) to syngas was performed in the tubular BSCF membrane reactor packed with a LiLaNiO/γ–Al 2O 3 catalyst. The reaction performance of the membrane reactor was investigated as functions of temperature, air flow rate in the shell side and methane concentration in the tube side. The mechanism of POM in the membrane reactor was discussed in detail. It was found that in the tubular membrane reactor, combustion reaction of methane with permeated oxygen took place in the reaction zone close to the surface of the membrane, then followed by steam and CO 2 reforming of methane in the middle zone of the tube side. The membrane tube can be operated steadily for 500 h in pure methane with 94% methane conversion and higher than 95% CO selectivity, and higher than 8.0 ml/cm 2 min oxygen permeation flux. 相似文献
2.
In order to examine the importance of the further oxidation of the desired C 2 products in the oxidative coupling of methane, ethylene and ethane have been added to the feed (containing methane and oxygen) to a Li/MgO or Ca/Sm 2O 3 catalyst. The results of these measurements show that neither of these C 2 molecules is stable under these conditions with either of the catalysts. Additionally, the rates of the oxidation of ethane and of ethylene alone have been measured using a gradientless reactor for both catalysts as well as for a quartz bed. It was found that the Ca/Sm 2O 3 material had higher activities for the oxidation of C 2H 6 and C 2H 4 (and also of CH 4) than had the Li/MgO material. These higher activities result in a lower optimal reaction temperature for the oxidative coupling of methane and are (at least partially) responsible for the lower selectivity to C 2 products observed with the Ca/Sm 2O 3 catalyst compared to that with the Li/MgO catalyst. 相似文献
3.
Coprecipitated catalyst systems containing BaCO 3 and La 2O n(CO 3) m (n≥1.5) with La/Ba = 0.05-10 were tested for catalytic activity, selectivity and stability in the oxidative coupling of methane reaction (OCM). Maximum C 2+ selectivities of 78% and C 2+ yields of 11% were obtained. The results show that La is the more active cation component of the system. The presence of BaCO 3 in the system leads to decreasing crystal size of the La phases, and to higher C 2+ selectivities at equal methane conversions. Life time tests showed that the Ba- La-containing catalysts were quite stable. Na impurities in the system lead to larger crystals in the La phases, and to less selective and less stable catalysts for the OCM reaction. Na is lost during reaction. 相似文献
4.
The effect of the addition of a second fuel such as CO, C 3H 8 or H 2 on the catalytic combustion of methane was investigated over ceramic monoliths coated with LaMnO 3/La-γAl 2O 3 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 C 3H 8 and H 2. Order of reactivity of the different fuels established in isothermal activity measurements was: CO>H 2≥C 3H 8>CH 4. 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 CH 4 when pure methane or CO/CH 4 mixtures are used. For H 2/CH 4 and C 3H 8/CH 4 mixtures, emissions of unburned methane are lower, probably due to the proceeding of CH 4 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. 相似文献
5.
A three-dimensional geometric modelwas set up for the oxidative coupling of methane (OCM) fixed bed reactor loaded with Na 3PO 4-Mn/SiO 2/cordierite monolithic catalyst, and an improved Stansch kinetic model was established to calculate the OCMreactions using the computational fluid dynamicsmethod and Fluent software. The simulation conditions were completely the same with the experimental conditions that the volume velocity of the reactant is 80 ml·min -1 under standard state, the CH 4/O 2 ratio is 3 and the temperature and pressure is 800 ℃ and 1 atm, respectively. The contour of the characteristic parameters in the catalyst bed was analyzed, such as the species mass fractions, temperature, the heat flux on side wall surface, pressure, fluid density and velocity. The results showed that the calculated valuesmatchedwell with the experimental values on the conversion of CH 4 and the selectivity of products (C 2H 6, C 2H 4, CO,CO 2 and H 2) in the reactor outlet with an error range of ±4%. The mass fractions of CH 4 and O 2 decreased from 0.600 and 0.400 at the catalyst bed inlet to 0.445 and 0.120 at the outlet, where the mass fractions of C 2H 6, C 2H 4, CO and CO 2 were 0.0245, 0.0460, 0.0537 and 0.116, respectively. Due to the existence of laminar boundary layer, the mass fraction contours of each species bent upwards in the vicinity of the boundary layer. The volume of OCM reaction was changing with the proceeding of reaction, and the total moles of products were greater than reactants. The flow field in the catalyst bed maintained constant temperature and pressure. The fluid density decreased gradually from 2.28 kg·m -3 at the inlet of the catalyst bed to 2.18 kg·m -3 at the outlet of the catalyst bed, while the average velocity magnitude increased from 0.108 m·s -1 to 0.120 m·s -1. 相似文献
6.
Pulse reaction method and in situ IR spectroscopy were used to characterize the active oxygen species for oxidative coupling of methane (OCM) over SrF 2/Nd 2O 3 catalyst. It was found that OCM activity of the catalyst was very low in the absence of gas phase oxygen, which indicated that lattice oxygen species contributed little to the yield of C 2 hydrocarbons. IR band of superoxide species (O 2−) was detected on the O 2-preadsorbed SrF 2/Nd 2O 3. The substitution of 18O 2 isotope for 16O 2 caused the IR band of O 2− at 1128 cm −1 to shift to lower wavenumbers (1094 and 1062 cm −1), consistent with the assignment of the spectra to the O 2− species. A good correlation between the rate of disappearance of surface O 2− and the rate of formation of gas phase C 2H 4 was observed upon interaction of CH 4 with O 2-preadsorbed catalyst at 700 °C. The O 2− species was also observed on the catalyst under working condition. These results suggest that O 2− species is the active oxygen species for OCM reaction on SrF 2/Nd 2O 3 catalyst. 相似文献
7.
Oxidative coupling of methane to higher hydrocarbons was investigated using alkali and rare earth cuprates such as YBa 2Cu 3O 7-x, La 1.8Ba 0.2CuO and La 2CuO 4. Oxygen and methane in helium were fed to the reactor in a cofeed mode. Approximately, 5 g of catalyst in powder form was placed in a quartz flow reactor in all experiments. Oxygen partial pressure was changed as a parameter (P 0 = 0.5 to 9.1 kPa) while keeping methane partial pressure and temperature constant at 18kPa and 1023 K respectively. Investigation of catalytic activity in terms of overall methane conversion and C 2 + (C 2H 4+C 2H 6) product selectivity indicated that higher conversions and lower selectivities were obtained as O 2 partial pressure was increased at a constant methane partial pressure of 8kPa. In comparing the performance of the two catalysts, La 1.8Ba 0.2CuO and La 2CuO 4; the selectivity results indicated a positive influence of incorporation of Ba into La 2CuO 4 structure. Similarly, selectivity values substantially increased teaching 86.3% at a reaction temperature of 1023 K and at P CH4./P O3, = 6 when La was replaced by Y and Ba. 相似文献
8.
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 (CO 2 + CO). However, addition of LiCl drastically improved the yield of C 2 compounds (C 2H 6 + C 2H 4). One of the roles of LiCl is to inhibit the catalytic activity of the host NiO for deep oxidation of CH 4. The reaction catalyzed by the LiCl/NiO proceeds stepwise from CH 4 to C 2H 4 through C 2H 6 (2CH 4 → C 2H 6 → C 2H 4). The study on the oxidation of C 2H 6 over the LiCl/NiO showed that the oxidative dehydrogenation of C 2H 6 to C 2H 4 occurs very selectively, which is the main reason why partial oxidation of CH 4 over LiCl/NiO gives C 2H 4 quite selectively. The other role of LiCl is to prevent the host oxide (NiO) from being reduced by CH 4. 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 Ni 3+). The catalyst was deactivated at the temperatures>973 K due to vaporization of LiCl and consumption of chlorine during reaction. The kinetic and CH 4---CD 4 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. 相似文献
9.
Catalysis Letters - Oxidative coupling of methane (OCM) over SrO/La2O3 catalyst was studied in a densely perovskite-type Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) mixed-conducting membrane reactor at... 相似文献
10.
A study of the selective oxidative coupling of methane (OCM) to C 2 hydrocarbons (ethane and ethylene) in a solid-state electrochemical reactor made from yttria-stabilized zirconia (YSZ) has been made. Three different catalyst–electrode systems based on silver and two trimetallic formulations of Mn modified alkali (Na and K) tungstates supported on silica were used. A comparison is made between co-fed and electrochemically-supplied oxygen. The electrochemically-supplied oxygen gave higher overall C 2 selectivities than the co-fed method under low current conditions, which was attributed to differences in local methane to oxygen ratios at the catalyst surface. The potassium tungstate supported catalyst gave the best overall C 2 selectivity (86% at 4% C 2 yield). 相似文献
11.
The homogeneous gas phase O 2-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, C 2H 6, C 2H 4, C 3H 6 and C 3H 8 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, CH 4+O 2→CH ·3+HO ·2. Bringing in basic catalysts known to catalyze the system at low methane partial pressures, in the reactor e.g. SrCO 3, BaCO 3, 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. 相似文献
12.
Oxidative coupling of methane to higher hydrocarbons (C 2+) using NaOH/CaO and pure CaO as catalyst was studied in fluidized- and packed-fluidized-bed reactors at 700°C to 800°C, partial pressures of methane from 0.5 to 0.7 bar and oxygen from 0.05 to 0.25 bar and a total pressure of ca 1 bar; oxygen conversion amounted generally to 50 to 100 %. C 2+ selectivity depends for both reactors markedly on temperature and oxygen partial pressure. The optimum temperature ranges between 750 and 800°C. Highest selectivities (ca 76 %) were achieved at the lowest oxygen partial pressure (ca 0.06 bar); maximum yields (ca 13.5 %), however, were obtained at an oxygen partial pressure of ca 0.14 bar. The application of the fluidized-bed reactor is more favourable than the packed-fluidized-bed reactor with respect to operability and C 2+ selectivity. 相似文献
13.
The decomposition of different hydrocarbons (CH 4, C 2H 6, C 2H 4, C 2H 2, C 3H 8, and C 3H 6) over Ni (5 wt.%)/SiO 2 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/SiO 2 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. 相似文献
14.
Oxygen permeation fluxes through dense disk-shaped Ba 0.5Sr 0.5Co 0.8Fe 0.2O 3−δ (BSCFO) membranes were investigated as a function of temperature (973–1123 K), pressure (2–10 atm), and membrane thickness (1–2 mm) under an air/helium gradient. A high oxygen permeation flux of 2.01 ml/cm 2 min was achieved at 1123 K and 10 atm under an air/He oxygen partial pressure gradient. Based on the dependence of the oxygen permeation flux on the oxygen partial pressure difference across the membrane and the membrane thickness, it is assumed that bulk diffusion of oxygen ions was the rate-controlling step in the oxygen transport across the BSCFO membrane disk under an air/He gradient. The partial oxidation of methane (POM) to syngas using LiLaNiO x/γ-Al 2O 3 as catalyst in a BSCFO membrane reactor was successfully performed at high pressure (5 atm). Ninety-two percent methane conversion, 90% CO selectivity, and 15.5 ml/cm 2 min oxygen permeation flux were achieved in steady state at a temperature of 1123 K and a pressure of 5 atm. A syngas production rate of 79 ml/cm 2 min was obtained. Characterization of the membrane surface by SEM and XRD after reaction showed that the surface exposed to the air side preserved the Perovskite structure while the surface exposed to the reaction side was eroded. 相似文献
15.
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 SiO 2. A study of C 1---C 3alkanes 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. CH 4, C 2H 6 and C 3H 8 oxidative dehydrogenation by O 2 or CO 2 is studied on a MNO/SiO 2 catalyst. The initiation of radical reactions of hydrocarbons on Cl-containing catalysts proceeds via chlorine atoms generation. 相似文献
16.
A disk-type Sm 0.4Ba 0.6Co 0.2Fe 0.8O 3 − δ perovskite-type mixed-conducting membrane was applied to a membrane reactor for the partial oxidation of methane to syngas (CO + H 2). The reaction was carried out using Rh (1 wt%)/MgO catalyst by feeding CH 4 diluted with Ar. While CH 4 conversion increased and CO selectivity slightly decreased with increasing temperature, a high level of CH 4 conversion (90%) and a high selectivity to CO (98%) were observed at 1173 K. The oxygen flux was increased under the conditions for the catalytic partial oxidation of CH 4 compared with that measured when Ar was fed to the permeation side. We investigated the reaction pathways in the membrane reactor using different membrane reactor configurations and different kinds of gas. In the membrane reactor without the catalyst, the oxygen flux was not improved even when CH 4 was fed to the permeation side, whereas the oxygen flux was enhanced when CO or H 2 was fed. It is implied that the oxidation of CO and H 2 with the surface oxygen on the permeation side improves the oxygen flux through the membrane, and that CO 2 and H 2O react with CH 4 by reforming reactions to form syngas. 相似文献
17.
Conversion of CH 4, C 2H 6, C 3H 8, benzene and their binary mixtures over H-NaZSM-5 catalyst in the presence of N 2O 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 N 2O towards methane and benzene, but affected their secondary transformation. Acidic samples favored the reaction of aromatic ring methylation with methane whereas deep oxidation of CH 4 prevailed on NaHZSM-5. Based on the relative reactivities and 13C label distribution in the products of 13CH 4+C 6H 6+N 2O feed conversion, the scheme of hydrocarbon transformation was proposed. 相似文献
18.
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 CH 4, O 2, C 2H 6, C 2H 4, and C 2H 2 between the original and reduced mechanisms were required to be less than 5%. 相似文献
19.
The influences of calcination temperatures and additives for 10 wt.% Cu/γ-Al 2O 3 catalysts on the surface properties and reactivity for NO reduction by C 3H 6 in the presence of excess oxygen were investigated. The results of XRD and XPS show that the 10 wt.% Cu/γ-Al 2O 3 catalysts calcined below 973 K possess highly dispersed surface and bulk CuO phases. The 10 wt.% Cu/γ-Al 2O 3 and 10 wt.% Mn–10 wt.% Cu/γ-Al 2O 3 catalysts calcined at 1073 K possess a CuAl 2O 4 phase with a spinel-type structure. In addition, the 10 wt.% La–10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K possesses a bulk CuO phase. The result of NO reduction by C 3H 6 shows that the CuAl 2O 4 is a more active phase than the highly dispersed and bulk CuO phase. However, the 10 wt.% Mn–10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K possesses significantly lower reactivity for NO reduction than the 10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K, although these catalysts possess the same CuAl 2O 4 phase. The low reactivity for NO reduction for 10 wt.% Mn–10 wt.% Cu/γ-Al 2O 3 catalyst calcined at 1073 K is attributed to the formation of less active CuAl 2O 4 phase with high aggregation and preferential promotion of C 3H 6 combustion to CO x by MnO 2. The engine dynamometer test for NO reduction shows that the C 3H 6 is a more effective reducing agent for NO reduction than the C 2H 5OH. The maximum reactivity for NO reduction by C 3H 6 is reached when the NO/C 3H 6 ratio is one. 相似文献
20.
A gliding arc discharge (GRD) reactor was used to decompose ethanol into primarily H 2 and CO with small amounts of CH 4, C 2H 2, C 2H 4, and C 2H 6. 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. 相似文献
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