Oxidative dimerization of CH4/CD4 mixtures: Evidence for methyl intermediate

Isotope tracer experiments prove the role of methyl in the oxidative coupling of methane and disprove a recently proposed mechanism involving methylene. The ethane product from oxidative coupling of CH₄/CD₄ mixtures over a Li/MgO catalyst consists of C₂H₆, C₂D₆ and CH₃CD₃ thus proving that ethane is formed by combination of methyl intermediates. The co-reaction of labelled CH₄ (D and¹³C) with C₂H₄ produces propylene labelled predominantly at the methyl (3) position, thus proving C₃ formation by terminal addition of methyl to ethylene rather than via a cyclic intermediate as has been proposed.     

Study of the oxidative methylation of acetonitrile to acrylonitrile with CH4 over Li/MgO catalysts

The oxidative methylation of acetonitrile to acrylonitrile with methane for temperatures in the range 550–730°C over Li/MgO follows a radical mechanism. The reaction proceeds via the formation of radicals at the α-carbon of acetonitrile and methyl radicals from methane. The coupling of these radicals leads to propionitrile which is further transformed to acrylonitrile via oxidative dehydrogenation. Experimental evidences indicate that the reaction is Langmuir–Hinselwood. The Li⁺O⁻ surface sites of Li/MgO are the active centers for the activation of both methane and acetonitrile. Oxygen is absolutely necessary for the formation of the corresponding radicals from methane and acetonitrile but it must be provided at a controllable manner in order to avoid undesired oxidation reaction of nitriles. The decomposition of acetonitrile which would lead to CH₄ and HCN does not take place. However, the increase of the nitrile chain length favors the breaking of the C–C bond between the cyanide group and the α-carbon of the corresponding nitrile.     

CH4/CD4 isotope effects in the carbon dioxide reforming of methane to syngas over SiO2-supported nickel catalysts

By performing the CH₄ + CO₂ and CD₄ + CO₂ reactions alternately over SiO₂-supported nickel catalysts in a pulse micro-reactor, normal deuterium isotope effects on both the methane conversion reaction and on the CO formation reaction have been observed in the process of CO₂ reforming of methane. Based on the observed CH₄/CD₄ isotope effects, the pathways for the formation of CO are discussed.     

Segregation of Pt and Re During CO2 Reforming of CH4

Pt–Re supported on Ce_0.52 Zr_0.48 O₂ was studied for the carbon dioxide reforming of methane at 800 °C. Diffuse reflectance fourier transform infrared spectroscopy and temperature programmed reduction studies suggest that Pt and Re segregation occurs during the reaction. The segregation results in an increase in the Pt sites available for CH₄ decomposition and results in the bimetallic catalyst exhibiting an increase in the conversion of methane with time on stream. After 20 h of reaction, the CH₄ conversion observed for the bimetallic catalyst was the same as the CH₄ conversion observed for the monometallic catalyst.     

Impact of agriculture on soil consumption of atmospheric CH4 and a comparison of CH4 and N2O flux in subarctic, temperate and tropical grasslands

Increasing concentrations of methane (CH₄) in the atmosphere are projected to account for about 25% of the net radiative forcing. Biospheric emissions of CH₄ to the atmosphere total approximately 400 Tg C y^-1. An estimated 300 Tg of CH₄-C y^-1 is oxidized in the atmosphere by hydroxyl radicals while about 40 Tg y^-1 remains in the atmosphere. Approximately 40 Tg y^-1 of the atmospheric burden is oxidized in aerobic soils. Research efforts during the past several years have focused on quantifying CH₄ sources while relatively less effort has been directed toward quantifying and understanding the soil sink for atmospheric CH₄. Recent research has demonstrated that land use change, including agricultural use of native forest and grassland systems has decreased the soil sink for atmospheric methane. Some agricultural systems consume atmospheric CH₄ at rates less than 10% of those found in comparable undisturbed soils. While it has been necessary to change land use practices over the past centuries to meet the required production of food and fiber, we need to recognize and account for impacts of land use change on the biogeochemical nutrient cycles in the biosphere. Changes that have ensued in these cycles have and will impact the atmospheric concentrations of CH₄ and N₂O. Since CH₄ and N₂O production and consumption are accomplished by a variety of soil microorganisms, the influence of changing agricultural, forest, and, demographic patterns has been large. Existing management and technological practices may already exist to limit the effect of land use change and agriculture on trace gas fluxes. It is therefore important to understand how management and land use affect trace gas fluxes and to observe the effect of new technology on them. This paper describes the role of aerobic soils in the global CH₄ budget and the impact of agriculture on this soil CH₄ sink. Examples from field studies made across subarctic, temperate and tropical climate gradients in grasslands are used to demonstrate the influence of nutrient cycle perturbations on the soil consumption of atmospheric CH₄ and in increased N₂O emissions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Marked Role of Carbon Monoxide in the Formation of Benzene During CH4-CO Reaction Over Rh/SiO2 Catalysts

Temperature-programmed desorption (He-TPD) and temperature-programmed reaction with hydrogen (H₂-TPR), carbon monoxide (CO-TPR) or methane (CH₄-TPR) were carried out to elucidate the benzene formation mechanism as well as the role of CO during CH₄-CO reaction over SiO₂-supported Rh catalysts. The steady-state surface for the CH₄-CO reaction was different from that of the CH₄ decomposition reaction. The existence of benzene-like adsorbed species as building blocks was demonstrated on the CH₄-CO reaction surface, while no such higher hydrocarbon adsorbed species was detected in the case of the CH₄ decomposition surface. On the contrary, in CO-TPR experiments various unsaturated hydrocarbons were released from the steady-state CH₄ decomposition surface, which was not the case from the CH₄-CO reaction surface. It is concluded that adsorbed CO may play an important role to enhance the C-C bond formation of carbonaceous species, which correlates deeply with the novel phenomenon of selective benzene formation in the CH₄-CO reaction.     

Thermodynamic-kinetic synergistic separation of CH4/N2 on a robust aluminum-based metal-organic framework

A robust aluminum-based metal–organic framework (Al-MOF) MIL-120Al with 1D rhombic ultra-microporous was reported. The nonpolar porous walls composed of para-benzene rings with a comparable pore size to the kinetic diameter of methane allow it to exhibit a novel thermodynamic-kinetic synergistic separation of CH₄/N₂ mixtures. The CH₄ adsorption capacity was as high as 33.7 cm³/g (298 K, 1 bar), which is the highest uptake value among the Al-MOFs reported to date. The diffusion rates of CH₄ were faster than N₂ in this structure as confirmed by time-dependent kinetic adsorption profiles. Breakthrough experiments confirm that this MOF can completely separate the CH₄/N₂ mixture and the separation performance is not affected in the presence of H₂O. Theoretical calculations reveal that pore centers with more energetically-favorable binding sites for CH₄ than N₂. The results of pressure swing adsorption (PSA) simulations indicate that MIL-120Al is a potential candidate for selective capture coal-mine methane.     

Superhydrophobic zeolitic imidazolate framework with suitable SOD cage for effective CH4/N2 adsorptive separation in humid environments

Various adsorbents for CH₄/N₂ separation were developed to enrich low-concentration coal-mine methane. Most are hydrophilic and cannot treat moist coal-mine methane. We report for the first time a microporous zeolitic imidazolate framework Co(dcIm)₂ (TUT-100) with superhydrophobic properties for CH₄/N₂ separation. The CH₄ adsorption capacity and CH₄/N₂ selectivity were as high as 45.29 cm³/cm³ and 6.3 (298 K, 1 bar), respectively, which results from the suitable SOD cage size (0.80 nm). The H₂O adsorption was lower than 6.3 cm³/g at 298 K and near saturated pressure due to the hydrophobic group  Cl. Breakthrough experiments were carried out to indicate the significant potential for CH₄/N₂ adsorption separation in a humid environment. The adsorption behavior of the gas mixture on the TUT-100 was investigated by the Grand Canonical Monte Carlo method and coupled with the experimental data.     

CH4 emission and oxidation in Chinese rice paddies

In the paper, the characteristics of CH₄ emission from the rice paddies, its temporary and spatial variations as well as factors regulating CH₄ emission and oxidation are reviewed with an emphasis on CH₄ emission from rice paddies in China. The observed four types of diel variation and two type of seasonal variation can be explained by the variations of methane production in the soil and the transport efficiencies of the three transport routs. The inter-annual variation of CH₄ emission from rice fields is significant, but the process causing this change is very complicated and unclear based on the available data at present. The large special variation, more than 10 times difference, of the total season methane emissions observed in various rice fields in China, is largely attributed to soil type difference although both soil physics and chemistry are important. Rice growing activities regulate the diel and seasonal variation patterns of the methane emissions. Drainage of flooded water may significantly reduce the emission. Organic fertilizer may enhance the emission, while some of the chemical fertilizers may reduce the emission. Local climate conditions, average temperature and annual rainfall, may be responsible for part of the observed year to year differences of the total season emission. Estimates of total emissions of CH₄ from Chinese rice fields, based on field measurement and model calculation, are 9.7–12.7 Tg/year and 8.17–10.52 Tg/year respectively, for the year of 1994. Oxidation of CH₄ reduces the emission of CH₄ produced in the soil of rice field to the atmosphere. The most likely sites for CH₄ oxidation in rice fields are the water–soil interface and the rhizosphere. When the flood water dries up in irrigated fields, the oxidation of CH₄ in the soil is more important and can partially explain the lower emission rates during the last period before harvest in most experiments. The magnitude of oxidation in the rhizosphere is not well known. Good correlation between methane reduction and O₂ mixing ratio in the soil has been found in most soil types. Methane oxidation rate is mainly controlled by the gas transport resistance in the soil. The oxidation rate increases with the increase of temperature in the temperature range of 5–36 °C.     

Formation and reactions of CH3 species over Mo2C/Mo(111) surface

The reaction pathways of adsorbed CH₃ on the Mo₂C/Mo(111) surface were investigated by means of temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS). CH₃ fragments were produced by the dissociation of the corresponding iodo-compound. CH₃I adsorbs molecularly on Mo₂C at 90 K and dissociates at and above 140 K. The main products of the reaction of adsorbed CH₃ are hydrogen, methane and ethylene. The coupling into ethane was not observed. The results are discussed in relevance to the conversion of methane into benzene on Mo₂C deposited on ZSM-5. This revised version was published online in August 2006 with corrections to the Cover Date.     

CO2 reforming of CH4 in coke oven gas to syngas over coal char catalyst

The CO₂ reforming of methane (in coke oven gas) on the coal char catalyst was performed in a fixed bed reactor at temperatures between 800 and 1200 °C under normal pressure. The effects of the coal char catalyst pretreatment and the ratio of CO₂/CH₄ were studied. Experimental results showed that the coal char was an effective catalyst for production of syngas, and addition of CO₂ did not enhance the CH₄ reforming to H₂. It was also found that the product gas ratio of H₂/CO is strongly influenced by the feed ratio of CO₂/CH₄. The modified coal char catalyst was more active during the CO₂–CH₄ reforming than the coal char catalyst based on the catalyst volume, furthermore the modified catalyst exhibited high activity in CO₂–CH₄ reforming to syngas. The conversion of methane can be divided into two stages. In the first stage, the conversion of CH₄ gradually decreased. In the second stage, the conversion of methane maintained nearly constant. The conversion of CO₂ decreased slightly during the overall reactions in CO₂–CH₄ reforming. The coal char catalyst is a highly promising catalyst for the CO₂ reforming of methane to syngas.     

Separation of CH4/N2 by an ultra-stable metal–organic framework with the highest breakthrough selectivity

Selectively separating CH₄ from N₂ in natural gas purification is extremely important, but challenging. Herein, a copper-based metal–organic framework (MOF) NKMOF-8-Me with inert pore environment was reported for efficient CH₄/N₂ separation. Adsorption results show that this material owns the highest CH₄ uptake (1.76 mmol/g) and initial adsorption heat (Q_st⁰) of CH₄ (28.0 kJ/mol) as well as difference in Q_st⁰ (9.1 kJ/mol) among all materials with good water stability. Breakthrough experiments confirm that this MOF can completely separate the CH₄/N₂ mixture with the highest CH₄/N₂ breakthrough selectivity (7.8) reported so far. Theoretical calculations reveal the separation mechanism is the short average distance between CH₄ and pore wall, resulting in a stronger adsorption affinity for CH₄. In addition, this MOF exhibits highly structural stability and regeneration. These results guarantee this MOF as a promising adsorbent for the recovery of CH₄ from coalbed methane.     

Experimental and numerical study of CH4/CH3Cl/O2/N2 premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH₃Cl addition on CH₄/O₂/N₂ premixed flames under oxygen enrichment. The laminar flame speeds of CH₄/CH₃Cl/O₂/N₂ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH₃Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH₃Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH₃Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H₂ → H + H₂O reaction is reduced because of the reduction of OH concentration. However, the function of CH₃Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 103–111, November–December, 2006.     

The role of Cl− in a Li+-ZnO-Cl− catalyst on the oxidative coupling of methane and the oxidative dehydrogenation of ethane

The effect of Cl^? ion addition to a Li⁺-ZnO catalyst has been studied with respect to the oxidative coupling of CH₄ and the oxidative dehydrogenation (OXD) of C₂H₆. Increasing the Cl/Li ratio from 0.65 to 0.90 had relatively little effect on the CH₄ conversion, whereas the C₂H₄/C₂H₆ ratio was enhanced significantly as a result of an increase in the OXD reaction rate. Conversely, loss of Cl^? from the catalyst during the reaction had a much more deleterious effect on ethane OXD activity than on methane coupling activity. Addition of Cl^? ions at a Cl/Li ratio of 0.9 caused a decrease both in the number of basic sites and in the basic strength of these sites, as determined by temperature-programmed desorption of CO₂. The similarities between the results obtained over Li⁺-ZnO-Cl^? catalysts and those previously reported for Li⁺-MgO-Cl^? catalysts confirm that basicity of the host oxide plays only a minor role in determining the properties of these chlorided catalysts.     

Catalytic reaction of CH4 with CO2 over alumina-supported Pt metals

The dissociation of CH₄ and CO₂, as well as the reaction between CH₄ and CO₂ at 723–823 K have been studied over alumina supported Pt metals. In the high temperature interaction of CH₄ with catalyst surface small amounts of C₂H₆ were detected. In the reaction of CH₄+CO₂, CO and H₂ were produced with different ratios. The specific activities of the catalysts decreased in the order: Ru, Pd, Rh, Pt and Ir, which agreed with their activity order towards the dissociation of CO₂.This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged.     

Critical Influence of Mn on Low-Temperature Catalytic Activity of Mn/Na2WO4/SiO2 Catalyst for Oxidative Coupling of Methane

Oxidative coupling of methane (OCM) was investigated in the temperature range 370-775 °C over Mn/Na₂WO₄/SiO₂ catalysts with different loadings of manganese in integral-mode conditions. Na₂WO₄/SiO₂ shows no activity at low temperature (370 °C), whereas Mn-doped catalyst exhibits 14% C²⁺ yield under similar reaction conditions, indicating that manganese plays a critical role in low-temperature methane coupling reaction. Partial pressure of oxygen in the feed also influences the low-temperature OCM activity of the catalysts.     

NO Reduction by CH4in the Presence of O2over Pd-H-ZSM-5

An investigation of the interaction of NO and NO₂with Pd-H-ZSM-5, as well as the reduction of NO by CH₄, has been conducted using mass spectrometry andin situinfrared spectroscopy. Prior to reaction most of the Pd in Pd-H-ZSM-5 (Pd/Al=0.048) is present as Pd²⁺cations. NO reduction by CH₄in the absence of O₂results in the progressive reduction of Pd²⁺cations above 610 K and the formation of small Pd particles. Reduction of Pd²⁺cations is significantly suppressed when O₂is added to the feed of NO and CH₄.In situinfrared spectroscopy reveals the presence of NO⁺and NO as the principal adsorbed species. NO⁺is present as a charge-compensating cation (e.g., Z⁻NO⁺) and is believed to be formed via the reaction 2 Z⁻H⁺+2 NO+1/2 O₂=2 Z⁻NO⁺+H₂O. NO⁺does not react with CH₄at temperatures up to 773 K. Adsorbed NO reacts with CH₄above 650 K and CN species are observed as intermediates. The latter species react with both NO, O₂, and presumably NO₂. Based on the accumulated data, a mechanism is proposed to explain the reduction of NO by CH₄both in the presence and absence of O₂.     

Adsorption of CO2, CH4, CO2/N2 and CO2/CH4 in novel activated carbon beads: Preparation,measurements and simulation

A series of high performance carbonaceous mesoporous materials: activated carbon beads (ACBs), have been prepared in this work. Among the samples, ACB‐5 possesses the BET specific surface area of 3537 m² g^?1 and ACB‐2 has the pore volume of 3.18 cm³ g^?1. Experimental measurements were carried out on the intelligent gravimetric analyzer (IGA‐003, Hiden). Carbon dioxide adsorption capacity of 909 mg g^?1 has been achieved in ACB‐5 at 298 K and 18 bar, which is superior to the existing carbonaceous porous materials and comparable to metal‐organic framework (MOF)‐177 (1232 mg g^?1, at 298 K and 20 bar) and covalent‐organic framework (COF)‐102 (1050 mg g^?1 at 298 K and 20 bar) reported in the literature. Moreover, methane uptake reaches 15.23 wt % in ACB‐5 at 298 K and 18 bar, which is better than MOF‐5. To predict the performances of the samples ACB‐2 and ACB‐5 at high pressures, modeling of the samples and grand canonical Monte Carlo simulation have been conducted, as is presented in our previous work. The adsorption isotherms of CO₂/N₂ and CO₂/CH₄ in our samples ACB‐2 and 5 have been measured at 298 and 348 K and different compositions, corresponding to the pre‐ and postcombustion conditions for CO₂ capture. The Dual‐Site Langmuir‐Freundlich (DSLF) model‐based ideal‐adsorbed solution theory (IAST) was also used to solve the selectivity of CO₂ over N₂ and CH₄. The selectivities of ACBs for CO₂/CH₄ are in the range of 2–2.5, while they remain in the range of 6.0–8.0 for CO₂/N₂ at T = 298 K. In summary, this work presents a new type of adsorbent‐ACBs, which are not only good candidates for CO₂ and CH₄ storage but also for the capture of carbon dioxide in pre‐ and postcombustion processes. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Parameter optimization for the oxidative coupling of methane in a fixed bed reactor by combination of response surface methodology and computational fluid dynamics

A resource and time saving method is introduced for optimizing fixed bed reactors by the combination of response surface methodology (RSM) and CFD simulation. This is demonstrated for the oxidative coupling of methane (OCM) based on the reaction kinetics by Stansch et al. (1997). Firstly, a parameter screening is performed to identify the power factors modified reaction time, heating temperature and CH₄ to O₂ ratio. Secondly, utilizing Central Composite Design, meta models for the most interesting responses are developed, i.e. C₂ selectivity and yield as well as CH₄ conversion. The statistical models describe the characteristics of the responses over a wide range. Thirdly, an optimization of C₂ yield is carried out using RSM. The maximum is detected to be approx. 14% and validated by three dimensional CFD simulations. In the investigated parameter space the optimized parameter conditions are found for a feed composition of 20% nitrogen, 26.7% oxygen, 53.3% methane (CH₄ to O₂ ratio of 2), a modified reaction time of 61 kg s/m³ and a heating temperature of 801.5 °C.     

Effect of Land Management in Winter Crop Season on CH4 Emission During the Following Flooded and Rice-Growing Period

A greenhouse pot experiment was carried out to study the effect of land management during the winter crop season on methane (CH₄) emissions during the following flooded and rice-growing period. Three land management patterns, including water management, cropping system, and rice straw application time were evaluated. Land management in the winter crop season significantly influenced CH₄ fluxes during the following flooded and rice-growing period. Methane flux from plots planted to alfalfa (ALE) in the winter crop season was significantly higher than those obtained with treatments involving winter wheat (WWE) or dry fallow (DFE). Mean CH₄ fluxes of treatments ALE, WWE, and DFE were 28.6, 4.7, and 4.1 mg CH₄ m^–2 h^–1 in 1996 and 38.2, 5.6, and 3.2 mg CH₄ m^–2 h^–1 in 1997, respectively. The corresponding values noted with continuously flooded fallow (FFE) treatment were 6.1 and 5.2 times higher than that of the dry fallow treatment in 1996 and 1997, respectively. Applying rice straw just before flooding the soil (DFL) significantly enhanced CH₄ flux by 386% in 1996 and by 1,017% in 1997 compared with rice straw application before alfalfa seed sowing (DFE). Land management in the winter crop season also affected temporal variation patterns of CH₄ fluxes and soil Eh after flooding. A great deal of CH₄ was emitted to the atmosphere during the period from flooding to the early stage of the rice-growing season; and CH₄ fluxes were still relatively high in the middle and late stages of the rice-growing period for treatments ALE, DFL, and FFE. However, for treatments DFE and WWE, almost no CH₄ emission was observed until the middle stage, and CH₄ fluxes in the middle and late stages of the rice-growing period were also very small. Soil Eh of treatments ALE and DFL decreased quickly to a low value suitable for CH₄ production. Once Eh below –150 mV was established, the small changes in Eh did not correlate to changes in CH₄ emissions. The soil Eh of treatments DFE and WWE did not decrease to a negative value until the middle stage of the rice-growing period, and it correlated significantly with the simultaneously measured CH₄ fluxes during the flooded and rice-growing period.