Oxidative Coupling of Methane over Lithium Doped (Mn+W)/SiO_2 Catalysts

Modification and performance of Li induced silica phase transition of （Mn＋W）/SiO2 catalyst, under reaction conditions of oxidative coupling of methane （OCM）, have been investigated employing textural characterizations and redox studies. Stability and precrystalline form of fresh Li induced silica phase transition catalyst depend on the Li loading. A catalyst, with high lithium loading, destabilizes on OCM stream. This destabilization is not due to Li evaporation at OCM reaction conditions, α-cristobalite is proposed to be an intermediate in the crystallization of amorphous silica into quartz in the Li-induced silica phase transition process. However, the type of crystalline structure was found to be unimportant with regard to the formation of a selective catalyst. Metal-metal interactions of Li-Mn, Li-W and Mn-W, which are affected during silica phase crystallization, are found to be critical parameters of the trimetallic catalyst and were studied by TPR. Role of lithium in Li doped （Mn＋W）/SiO2 catalyst is described as a moderator of the Mn-W interaction by involving W in silica phase transition. These interactions help in the improvement of transition metal redox properties, especially that of Mn, in favor of OCM selectivity.

Oxidative coupling of methane in a fixed bed reactor over perovskite catalyst:A simulation study using experimental kinetic model

The oxidative coupling of methane （OCM） to ethylene over a perovskite titanate catalyst in a fixed bed reactor was studied experimentally and numerically. The two-dimensional steady state model accounted for separate energy equations for the gas and solid phases coupled with an experimental kinetic model. A lumped kinetic model containing four main species CH4, O2, COx （CO2, CO）, and C2 （C2H4 and C2H6） was used with a plug flow reactor model as well. The results from the model agreed with the experimental data. The model was used to analyze the influence of temperature and feed gas composition on the conversion and selectivity of the reactor performance. The analytical results indicate that the conversion decreases, whereas, C2 selectivity increases by increasing gas hourly space velocity （GHSV） and the methane conversion also decreases by increasing the methane to oxygen ratio.

Influence of alkali metal doping on surface properties and catalytic activity/selectivity of CaO catalysts in oxidative coupling of methane

Surface properties （viz. surface area, basicity/base strength distribution, and crystal phases） of alkali metal doped CaO （alkali metal/Ca= 0.1 and 0.4） catalysts and their catalytic activity/selectivity in oxidative coupling of methane （OCM） to higher hydrocarbons at different reaction conditions （viz. temperature, 700 and 750 ℃; CH4/O2 ratio, 4.0 and 8.0 and space velocity, 5140-20550 cm^3 ·g^-1·h^-1） have been investigated. The influence of catalyst calcination temperature on the activity/selectivity has also been investigated. The surface properties （viz. surface area, basicity/base strength distribution） and catalytic activity/selectivity of the alkali metal doped CaO catalysts are strongly influenced by the alkali metal promoter and its concentration in the alkali metal doped CaO catalysts. An addition of alkali metal promoter to CaO results in a large decrease in the surface area but a large increase in the surface basicity （strong basic sites） and the C2＋ selectivity and yield of the catalysts in the OCM process. The activity and selectivity are strongly influenced by the catalyst calcination temperature. No direct relationship between surface basicity and catalytic activity/selectivity has been observed. Among the alkali metal doped CaO catalysts, Na-CaO （Na/Ca = 0.1, before calcination） catalyst （calcined at 750 ℃）, showed best performance （C2＋ selectivity of 68.8% with 24.7% methane conversion）, whereas the poorest performance was shown by the Rb-CaO catalyst in the OCM process.

Studies on the structure and catalytic performance of S and P promoted Na-W-Mn-Zr/SiO_2 catalyst for oxidative coupling of methane

A series of Na-W-Mn-Zr/SiO2 catalysts promoted by different contents of S or/and P were prepared and their catalytic performance for oxidative coupling of methane was investigated to clarify the effect of S and P on the Na-W-Mn-Zr/SiO2 catalyst. The catalysts were characterized by X-ray diffraction （XRD） and X-ray photoelectron spectroscopy （XPS）. From the characterization results, it is found that the addition of S and P to the Na-W-Mn-ZffSiO2 catalyst helps the formation of active phases, such as α-cristobalite, Na2WO4, ZrO2, and Na2SO4. Moreover, the addition of S and P increases the concentration of surface-active oxygen species by improving the migration of active components from the bulk phase to the surface of the catalyst. According to the activity test, impressive methane conversion and C2 hydrocarbons yield were obtained at a low temperature of 1023 K over the six-component Na-W-Mn-Zr-S-P/SiO2 catalyst, which contained 2 wt% S and 0.4 wt% P simultaneously. The deactivation of Na-W-Mn-Zr-S-P/SiO2 was due to the loss of surface active components.

Active oxygen center in oxidative coupling of methane on La_(2)O_(3) catalyst

La_(2)O_(3) catalyzed oxidative coupling of methane(OCM) is a promising process that converts methane directly to valuable C_(2)(ethylene and ethane) products. Our online MS transient study results indicate that pristine surface without carbonate species demonstrates a higher selectivity to C_(2) products, and a lower light-off temperature as well. Further study is focused on carbonate-free La_(2)O_(3) catalyst surface for identification of active oxygen species associated with such products behavior. XPS reveals unique oxygen species with O 1 s binding energy of 531.5 e V correlated with OCM catalytic activity and carbonates removal. However, indicated thermal stability of this species is much higher than the surface peroxide or superoxide structures proposed by earlier computation models. Motivated by experimental results,DFT calculations reveal a new more stable peroxide structure, formed at the subsurface hexacoordinate lattice oxygen sites, with energy 2.18 e V lower than the previous models. The new model of subsurface peroxide provides a perspective for understanding of methyl radicals formation and C_(2) products selectivity in OCM over La_(2)O_(3) catalyst.

Kinetic studies of the oxidative coupling of methane over the Mn/Na_2WO_4/SiO_2 catalyst

Oxidative coupling of methane is a direct way to obtain C2 hydrocarbon, and Mn-Na-W/SiO2 catalyst is the most promising among all the catalysts. The 2%Mn/5%Na2WO4/SiO2 catalyst was prepared by the incipient wetness impregnation method. A 7-step heterogeneous reaction model of the oxidative coupling of methane to C2 hydrocarbons was conducted by co-feeding methane and oxygen at a total pressure of 1 bar over the catalyst. The kinetic measurements were carried out in a micro-catalytic fixed bed reactor. The kinetic data were obtained at the appropriate range of reaction conditions （4 kPa〈Po2 〈20 kPa, 20 kPa〈PcH4〈80 kPa, 800 ℃〈T〈900℃）. The proposed reaction kinetic scheme consists of three primary and four consecutive reaction steps. The conversions of hydrocarbons and carbon oxides were evaluated by applying Langmuir-Hinshelwood type rate equations. Power-law rate equation was applied only for the water-gas shift reaction. In addition, the effects of operating conditions on the reaction rate were studied. The proposed kinetic model can predict the conversion of methane and oxygen as well as the yield of C2 hydrocarbons and carbon oxides with an average accuracy of ± 15%.

Surface coupling of methyl radicals for efficient low‐temperature oxidative coupling of methane

Selective coupling of methyl radicals to produce C_(2) species(C2H4 and C2H6)is a key challenge for oxidative coupling of methane(OCM).In traditional OCM reaction systems,homogeneous transformation of methyl radicals in O_(2)‐containing gases are uncontrollable,resulting in limited C_(2) selectivity and yield.Herein,we demonstrate that methyl radicals generated by La_(2)O_(3)at low reaction temperature can selectively couple on the surface of 5 wt%Na2WO4/SiO_(2).The controllable surface coupling against overoxidation barely changes the activity of La_(2)O_(3)but boosts the C_(2)selectivity by three times and achieves a C_(2)yield as high as 10.9%at bed temperature of only 570℃.Structure‐property studies suggest that Na_(2)WO_(4) nanoclusters are the active sites for methyl radical coupling.The strong CH_(3)·affinity of these sites can even endow some methane combustion catalysts with OCM activity.The findings of the surface coupling of methyl radicals open a new direction to develop OCM catalyst.The bifunctional OCM catalyst system,which composes of a methane activation center and a CH_(3)·coupling center,may deliver promising OCM performance at reaction temperatures below the ignition temperature of C2H6 and C2H4(~600℃)and is therefore more controllable,safer,and certainly more attractive as an actual process.

Ce-promoted Mn/Na_2 WO_4/SiO_2 catalyst for oxidative coupling of methane at atmospheric pressure

A series of Ce-promoted Mn-Na2WO4/SiO2 catalysts were prepared by incipient wetness impregnation method, and their catalytic performance for oxidative coupling of methane （OCM） was investigated at atmospheric pressure in a micro-quartz-tube reactor. The catalysts were characterized by X-ray diffraction （XRD）, temperature program reduction （TPR） and BET surface area. Ce promoter increased surface area and Na2WO4 species dispersion, which enriched the amount of the surface species. In addition, Ce promoter increased the Na/W species reduction, but the reduction peak shifted to higher temperature. Stability test of 5wt%Ce catalyst indicated suitable performance and stability. The selectivity and yield of C^2＋ hydrocarbons after 50 h operation reached 65.5% and 19.6%, respectively, at 840 ℃ over 5wt%Ce-2wt%Mn5wt%Na2WO4/SiO2 catalyst.

Oxidative coupling of methane over (Na_2WO_4+Mn or Ce)/SiO_2 catalysts:In situ measurement of electrical conductivity

The effects of manganese oxide or ceria promoters on the performance of Na2WO4/SiO2 catalysts for oxidative coupling of methane （OCM） are reported. The OCM reaction was performed in a continuous-flow microreactor at 800℃, atmospheric pressure and under GHSV = 13200 ml·gCat^-1·h^-1.Catalysts were characterized by in situ conductivity measurement, FT-IR spectroscopy, XRD, SEM and temperature programmed reduction analysis. Manganese oxide promoted Na2WO4/SiO2 is considered as one of the active and selective catalysts for OCM reaction. Ceria with high oxygen storage capacity is selected as a proper oxygen activator, providing a higher concentration of the oxy-anion species which is suitable for OCM reaction and compared with manganese oxide. Electrical conductivity of the catalysts was measured in OCM reaction under oxidizing atmosphere, i.e. in the absence of methane. It was found that the trimetallic catalysts, i.e. the catalysts having sodium, tungsten and Mn or Ce species, exhibited similar crystalline structures and morphologies, which lead to suitable bulk properties for the formation of an active and selective catalyst. However, tungsten had significant effect on the texture and redox properties of the catalysts. It was also shown that the crystalline structure of the bimetallic （Na＋Mn or Ce）/SiO2 samples was quite different. This reveals that the metal oxides have significant effect on the extent of crystallization, taking place in the course of interaction of sodium with silica support. Similar conductivities and catalytic performances of （Na2WO4＋Mn or Ce）/SiO2 catalysts propose that the ability of Na2WO4/SiO2 for utilizing oxy-anions formed in presence of different metal oxides is limited.

Kinetic simulation of oxidative coupling of methane over perovskite catalyst by genetic algorithm:Mechanistic aspects

The reaction kinetics of oxidative coupling of methane catalyzed by perovskite was studied in a fixed bed flow reactor.At atmospheric pressure,the reactions were carried out at 725,750 and 775℃,inlet methane to oxygen ratios of 2 to 4.5 and gas hourly space velocity （GHSV） of 100 min^-1.Correlation of the kinetic data has been performed with the proposed mechanisms.The selected equations have been regressed with experimental data accompanied by genetic algorithm （GA） in order to obtain optimized parameters.After investigation the Langmuir-Hinshelwood mechanism was selected as the best mechanism,and Arrhenius and adsorption parameters of this model were obtained by linear regression.In this research the Marquardt algorithm was also used and its results were compared with those of genetic algorithm.It should be noted that the Marquardt algorithm is sensitive to the selection of initial values and there is possibility to trap in a local minimum.

Scale up and stability test for oxidative coupling of methane over Na_2WO_4-Mn/SiO_2 catalyst in a 200 ml fixed-bed reactor

The study of scale up for the oxidative coupling of methane （OCM） has been carried out in a 200 ml stainless steel fixed-bed reactor over a 5wt% Na2WO4-1.9wt% Mn/SiO2 （W-Mn/SiO2） catalyst. The effects of reaction conditions were investigated in detail. The results showed that, with increasing reaction temperature, the gas-phase reaction was enhanced and a significant amount of methane was converted into COx; with the CH4/O2 molar ratio of 5, the highest C2 （ethylene and ethane） yield of 25% was achieved; the presence of steam （as diluent） had a positive effect on the C2 selectivity and yield. Under lower methane gaseous hourly space velocity （GHSV）, higher selectivity and yield of C2 were obtained as the result of the decrease of released heat energy. In 100 h reaction time, the C2 selectivity of 66%-61% and C2 yield of 24.2%-25.4% were achieved by a single pass without any significant loss in catalytic performance.

Oxidative coupling of methane over BaCl_2-TiO_2-SnO_2 catalyst

The performance of BaC12-TiO2-SnO2 composite catalysts in oxidative coupling of methane reaction has been investigated. A series of BaC12-TiO2, BaC1E-SnO2, TiO2-SnO2, and BaC12-TiO2-SnO2 catalysts were prepared, and characterized by BET, XRD, XPS, CO2-TPD and H2-TPR, respectively. The synergistic effect among BaC12, SnO2 and TiO2 compositions enhances the catalytic performance. The best C2 selectivity and ethylene yield are obtained on the catalyst with the equal molar amount of the three compositions （BaC12 ： TiO2 ： SnO2 molar ratio of 1 ： 1 ： 1）. The optimal reaction conditions are as follows： 800 ℃, 44 mL.min-1 for methane, 22 mL.min-1 for oxygen and a space velocity of 5000 mL-h-1 .g-1, and the C2H4 yield over the catalyst is 20.1% with the CH4 conversion of 43.8% and C2 selectivity of 53.3%.

Effect of CO_2 on the structural variation of Na_2WO_4/Mn/SiO_2 catalyst for oxidative coupling of methane to ethylene

In this work,the influence of CO2 on the structural variation and catalytic performance of Na2WO4/Mn/Si O2 for oxidative coupling of methane to ethylene was investigated. The catalyst was prepared by impregnation method and characterized by XRD,Raman and XPS techniques. Appropriate amount of CO2 in the reactant gases enhanced the formation of surface tetrahedral Na2WO4 species and promoted the migration of O in MOx,Na,W from the catalyst bulk to surface,which were favorable for oxidative coupling of methane. When the molar ratio of CH4/O2/CO2 was 3/1/2,enriched surface tetrahedral Na2WO4 species and high surface concentration of O in MOx,Na,W were detected,and then high CH4 conversion of 33.1% and high C2H4 selectivity of 56.2% were obtained. With further increase of CO2 in the reagent gases,the content of active surface tetrahedral Na2WO4 species and surface concentration of O in MOx,Na,W decreased,while that of inactive species（Mn WO4 and Mn2O3） increased dramatically,leading to low CH4 conversion and low C2H4 selectivity. It could be speculated that Na2WO4 crystal was transformed into Mn WO4 crystal with excessive CO2 added under the reaction conditions. Pretreatment of Na2WO4/Mn/Si O2 catalyst by moderate amount of CO2 before OCM also promoted the formation of Na2WO4 species.

Comparative study of kinetic modeling for the oxidative coupling of methane by genetic and marquardt algorithms

Overall kinetic studies on the oxidative coupling of methane,OCM,have been conducted in a tubular fixed bed reactor,using perovskite titanate as the reaction catalyst.The appropriate operating conditions were found to be：temperature 750-775 ℃,total feed flow rate of 160 ml/min,CH4 /O2 ratio of 2 and GHSV of 100·min-1 .Under these conditions,C 2 yield of 28% was achieved.Correlations of the kinetic data have been performed with lumped rate equations for C2 and COx formation as functions of temperature,O2 and CH4 partial pressures.Six models have been selected among the common lumped kinetic models.The selected models have been regressed with the experimental data which were obtained from the Catatest system by genetic algorithm in order to obtain optimized parameters.The kinetic coefficients in the overall reactions were optimized by different numerical optimization methods such as：the Levenberg-Marquardt and genetic algorithms and the results were compared with one another.It has been found that the Santamaria model is in good agreement with the experimental data.The Arrhenius parameters of this model have been obtained by linear regression.It should be noted that the Marquardt algorithm is sensitive to the first guesses and there is possibility to trap in the relative minimum.

Synthesis and characterization of the flower-like La_(x)Ce_(1-x)O_(1.5+δ) catalyst for low-temperature oxidative coupling of methane

Lanthanum-based oxides are promising candidates for low-temperature oxidative coupling of methane(OCM).To further lower the OCM reaction temperature,the Ce doped flower-like La_(2)O_(2)CO_(3)microsphere catalysts were synthesized,achieving a significantly low reaction temperature (375℃) while maintaining high C_(2) hydrocarbon selectivity (43.0%).Doping Ce into the lattice of La_(2)O_(2)CO_(3)created more surface oxygen vacancies and bulk lattice defects,which was in favor of the transformation and migration of oxygen species at 350–400℃.The designed H_(2) temperature-programmed reduction (H_(2)-TPR) experiments provided strong evidence that the low reaction temperature of La_(x)Ce_(1-x)O_(1.5+δ)can be attributed to the transformation and migration of oxygen species,which dynamically generated surface oxygen vacancies for continuous oxygen activation to selectively convert methane.Moreover,designed temperatureprogrammed surface reaction (TPSR) clarified that two kinds of surface oxygen species in La_(x)Ce_(1-x)O_(1.5+δ)catalysts were concerned with catalytic performance,that is,the surface chemisorbed oxygen species for the activation of CH_(2)and the formation of CH_(2)·intermediates,surface La-Ce-O lattice oxygen species that caused the excessive oxidation of CH_(2)·intermediates.Finally,the factors affecting the transformation and migration of oxygen species were explored.

Reaction Chemistry of W-Mn/SiO2 Catalyst for the Oxidative Coupling of Methane

Reaction chemistry of the OCM reaction on W-Mn/SiO_2 catalyst has beenreviewed in this account. Initial activity and selectivity, stability in a long-term reaction,reaction at elevated pressures and a modelling test in a stainless-steel fluidized-bed reactor showthat W-Mn/SiO_2 has promising performance for the development of an OCM process that directlyproduces ethylene from natural gas. A study on surface catalytic reaction kinetics and used catalyststructure characterization revealed a possible reason why C_2 and CO_x selectivity changed duringthe long-term reaction. Further improvement of the catalyst composition and preparation methodshould be a future direction of study on OCM reaction over W-Mn/SiO_2 catalyst.

Effects of operating parameters on oxidative coupling of methane over Na-W-Mn/SiO_2 catalyst at elevated pressures

The effects of operating parameters on oxidative coupling of methane （OCM） over Na-W-Mn/SiO2 catalyst have been studied at elevated pressures of 0.2, 0.3 and 0.4 MPa under low gaseous hourly space velocity （GHSV） and low temperature conditions. Experimental results show that when the operating pressure is increased, C2＋ yield slightly decreases, while the maximum ratio of ethylene to ethane remains unchanged. Moreover, it has been found empirically that increase of pressure does not affect the catalyst behavior permanently, the catalyst recovers its original low pressure performance without hysteresis behavior by reducing the pressure. Under the investigated conditions, when oxygen is completely consumed, the increase of GHSV leads to improvement in C2 selectivity, while C3＋ and COx selectivities decrease slightly. The C2＋ selectivity increases by increase of nitrogen diluent in the feed, but the C3＋ hydrocarbons selectivities decrease with increase of nitrogen since it is possible that further dilution at high pressure may reduce the probability of collision between CH3 and C2＋ hydrocarbons. During the stability test at high pressure, the catalyst performance remains unchanged throughout the 20 h running. The fresh and used catalysts were characterized using XRD, SEM and N2 adsorption-desorption methods. It was found that the phase transformation of the support from α-cristobalite to tridymite and quartz does not have obvious effect on catalyst performance at high pressure.

Oxidative coupling of methane in a dual-bed reactor comprising of particle/cordierite monolithic catalysts

A dual-bed reactor was constructed comprising of a 5%Na2WO4-2%Mn/SiO2 particle catalyst and a 4%Ce-5%Na2WO4-2%Mn/SiO2 /cordierite monolithic catalyst.The reaction performance of the oxidative coupling of methane （OCM） over the dual-bed reactor system was evaluated.The effects of the bed height and operation mode,as well as the reaction parameters such as reaction temperature,CH4/O2 ratio and flowrate of feed gas,on the catalytic performance were investigated.The results indicated that the suggested dual-bed reactor exhibited a good performance for the OCM reaction when the feed gases firstly passed through the particle catalyst bed and then to the monolithic catalyst bed.A CH4 conversion of 38.2% and a C2H4 selectivity of 43.3% could be obtained using the dual-bed reactor with a particle catalyst bed height of 10 mm and a monolithic catalyst bed height of 50 mm.Both the CH4 conversion and C2H4 selectivity have increased by 2.5% and 12.8%,respectively,as compared with the 5%Na2WO4-2%Mn/SiO2 particle catalyst in a conventional single-bed reactor and by 12.9% and 23.0%,respectively,as compared with the 4%Ce-5%Na2WO4-2%Mn/SiO2 /cordierite monolithic catalyst in a single-bed reactor.The catalytic performance of the OCM in the dual-bed reactor system has been improved remarkably.

Characterization and catalytic behavior of Na-W-Mn-Zr-S-P/SiO_2 prepared by different methods in oxidative coupling of methane

Na-W-Mn-Zr-S-P/SiO2 catalysts for oxidative coupling of methane （OCM） were prepared by incipient wetness impregnation, sol-gel and mixture slurry methods. The catalyst prepared by mixture slurry method showed the best catalytic performance among all samples. In addition, the effects of different addition sequences of Na, W, Mn, Zr, S and P on the catalytic performance were studied. The absence of Na before the addition of Mn and Zr in the catalysts preparation depressed the formation of the active phases of Mn2O3 and ZrO2 and decreased the activities of the catalysts significantly.

Experimental investigation of fluidized-bed reactor performance for oxidative coupling of methane

Performance of the oxidative coupling of methane in fluidized-bed reactor was experimentally investigated using Mn-Na2WO4/SiO2,La2O3/CaO and La2O3-SrO/CaO catalysts.These catalysts were found to be stable,especially Mn-Na2WO4/SiO2 catalyst.The effect of sodium content of this catalyst was analyzed and the challenge of catalyst agglomeration was addressed using proper catalyst composition of 2%Mn2.2%Na2WO4/SiO2.For other two catalysts,the effect of Lanthanum-Strontium content was analyzed and 10%La2O 3-20%SrO/CaO catalyst was found to provide higher ethylene yield than La2O3/CaO catalyst.Furthermore,the effect of operating parameters such as temperature and methane to oxygen ratio were also reviewed.The highest ethylene and ethane （C2） yield was achieved with the lowest methane to oxygen ratio around 2.40.5% selectivity to ethylene and ethane and 41% methane conversion were achieved over La2O3-SrO/CaO catalyst while over Mn-Na2WO4 /SiO2 catalyst,40% and 48% were recorded,respectively.Moreover,the consecutive effects of nitrogen dilution,ethylene to ethane production ratio and other performance indicators on the down-stream process units were qualitatively discussed and Mn-Na2WO4/SiO2 catalyst showed a better performance in the reactor and process scale analysis.