半焦炭催化甲烷裂解及动力学特性

在平推流反应器上,考察了非催化和半焦炭催化条件下甲烷裂解.利用气相色谱分析研究了甲烷裂解的规律.结果表明,半焦对甲烷裂解具有明显的催化作用,不同种类的炭催化剂表现出相似的催化活性.半焦炭催化剂条件下,甲烷裂解转化率同时受两方面的影响:一方面甲烷裂解产牛的积炭沉积在半焦表面及孔内,覆盖大部分的活性位,同时堵塞半焦内的孔道阻碍甲烷向半焦孔内扩散,使甲烷的裂解率降低;另一方面甲烷裂解生成的新物种反过来又对甲烷的裂解起催化作用,促进甲烷裂解.研究还表明,半焦中的灰分对甲烷裂解没有明显的作用,甲烷裂解主要受温度控制.采用简单的平推流模型对甲烷热分解动力学参数进行了计算,计算得到甲烷非催化和半焦催化裂解的表观活化能分别为154.02 kJ/mol和82.06 kJ/mol.     

Influence of the deposition parameters on the texture of pyrolytic carbon layers deposited on planar substrates

Pyrolytic carbon layers were deposited from methane on planar substrates (pyrolytic boron nitride) at various residence times, methane pressures and deposition temperatures. The depositions were performed in a cavity oriented perpendicular to the gas flow. The small surface area/reactor volume ratio of the reactor geometry allows depositions in the growth and nucleation mechanism. Transmission electron microscopy was applied to study the texture and microstructure of the carbon layers. A texture transition from medium- to high-textured pyrolytic carbon occurs as a result of increasing residence times, methane pressures and temperatures. Improved textures are generally correlated with increasing deposition rates, which are not necessarily constant during long-term depositions. Lower textures are observed in the vicinity of the substrate interface that are attributed to the influence of the substrate morphology and microstructure.     

The pyrolysis of natural gas: A study of carbon deposition and the suitability of reactor materials

This study of methane pyrolysis was designed to look at carbon deposition on the internal reactor and wafer surface during CH₄ pyrolysis. The rate of carbon deposition on the internal reactor surfaces could be reduced with: lower methane/oxygen ratios, shorter residence times, and lower temperatures. The type of carbon formed appeared to have a significant effect on the pyrolysis process. Pyrolytic carbon with a lower order structure produces a higher selectivity for carbon formation compared to carbon with a higher order structure. Form a process perspective, there are two obvious means of addressing this: deposited carbon could be regularly removed; and/or pyrolysis conditions are selected that produce carbon with a higher order structure. From the results, it is very clear that any development of a commercial process for natural gas pyrolysis in ceramic reactor systems would have to carefully address the selection of reactor material. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1035–1046, 2019     

A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts

Catalytic partial oxidation of methane has been reviewed with an emphasis on the reaction mechanisms over transition metal catalysts. The thermodynamics and aspects related to heat and mass transport is also evaluated, and an extensive table on research contributions to methane partial oxidation over transition metal catalysts in the literature is provided.Presented are both theoretical and experimental evidence pointing to inherent differences in the reaction mechanism over transition metals. These differences are related to methane dissociation, binding site preferences, the stability of OH surface species, surface residence times of active species and contributions from lattice oxygen atoms and support species.Methane dissociation requires a reduced metal surface, but at elevated temperatures oxides of active species may be reduced by direct interaction with methane or from the reaction with H, H₂, C or CO.The comparison of elementary reaction steps on Pt and Rh illustrates that a key factor to produce hydrogen as a primary product is a high activation energy barrier to the formation of OH. Another essential property for the formation of H₂ and CO as primary products is a low surface coverage of intermediates, such that the probability of O–H, OH–H and CO–O interactions are reduced.The local concentrations of reactants and products change rapidly through the catalyst bed. This influences the reaction mechanisms, but the product composition is typically close to equilibrated at the bed exit temperature.     

Formic acid decomposition on Au catalysts: DFT,microkinetic modeling,and reaction kinetics experiments

A combined theoretical and experimental approach is presented that uses a comprehensive mean‐field microkinetic model, reaction kinetics experiments, and scanning transmission electron microscopy imaging to unravel the reaction mechanism and provide insights into the nature of active sites for formic acid (HCOOH) decomposition on Au/SiC catalysts. All input parameters for the microkinetic model are derived from periodic, self‐consistent, generalized gradient approximation (GGA‐PW91) density functional theory calculations on the Au(111), Au(100), and Au(211) surfaces and are subsequently adjusted to describe the experimental HCOOH decomposition rate and selectivity data. It is shown that the HCOOH decomposition follows the formate (HCOO) mediated path, with 100% selectivity toward the dehydrogenation products (CO₂ + H₂) under all reaction conditions. An analysis of the kinetic parameters suggests that an Au surface in which the coordination number of surface Au atoms is ≤4 may provide a better model for the active site of HCOOH decomposition on these specific supported Au catalysts. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1303–1319, 2014     

Kinetics of methane hydrate decomposition

The kinetics of methane hydrate decomposition was studied using a semibatch stirred-tank reactor. The decomposition was accomplished by reducing the pressure on a hydrate slurry in water to a value below the three-phase equilibrium pressure at the reactor temperature. The data were obtained at temperatures from 274 to 283 K and pressures from 0.17 to 6.97 MPa. The stirring rates were high enough to eliminate mass-transfer effects. Analysis of the data indicated that the decomposition rate was proportional to the particle surface area and to the difference in the fugacity of methane at the equilibrium pressure and the decomposition pressure. The proportionality constant showed an Arrhenius temperature dependence. An estimate of the hydrate particle diameters in the experiments permitted the development of an intrinsic model for the kinetics of hydrate decomposition.     

Decomposition and regeneration of methane in the absence and the presence of a hydrogen-absorbing alloy CaNi5

Suitable catalysts for the decomposition of methane into carbon and hydrogen and for the regeneration of methane by hydrogenation of the carbon have been looked for in a series of transition metals and precious metals supported on various carriers. The most active catalyst for both reactions was Ni/SiO₂. The carbon formed on this catalyst was thermodynamically less stable than graphite. The different rate equations for the decomposition of methane obtained for the fresh and carbon deposited Ni/SiO₂ suggest that the rate-determining steps are different for the two catalysts. The highest number of carbon atoms deposited per one Ni atom was 31 at 773 K. However, the number of methane molecules recovered easily at 773 K was limited to 1.5 per Ni atom.

A physical mixture of Ni/SiO₂ and CaNi₅, a hydrogen-absorbing alloy, enhanced the decomposition rate of methane, enabling the complete conversion of methane at 773 K beyond the thermodynamic limitation. The presence of CaNi₅ at 273 K separated from the catalyst in a reaction system further enhanced the decomposition of methane due to an increased hydrogen-absorbing capacity of the CaNi₅ at low temperatures. The carbon deposited on Ni/SiO₂ in this case was reactive to be hydrogenated back to methane at 773 K, giving an average 7.5 CH₄ molecules per one Ni atom.     

Conversion of nitric oxide and methane over Pd/ZSM-5 catalysts in the absence of oxygen

We have studied the conversion of nitric oxide and methane on several H- and Na-ZSM-5 zeolite catalysts in the absence of oxygen. Our results suggest that the NO-CH₄ reaction can be explained in terms of a mechanism that starts with a nitric oxide decomposition step followed by the surface reaction of methane with the product oxygen regenerating the active site. We have found that reduced Pd/ZSM-5 catalysts are active for the nitric oxide decomposition reaction but deactivate rapidly due to self-poisoning by product oxygen. By contrast, in the presence of methane these catalysts can exhibit high activity and stability under certain conditions. For instance, when the nitric oxide decomposition and the reaction of methane with the surface oxygen proceed at comparable rates the catalyst is stable but when the methane conversion is lower than that required to remove all the oxygen produced (stoichiometric methane conversion) the catalyst rapidly deactivates. Under some conditions the methane conversion may be higher than the stoichiometric requirement leading to the deposition of carbonaceous species. These carbonaceous deposits can promote the reaction by helping to remove the product oxygen.     

Catalytic characteristics of various rubber-reinforcing carbon blacks in decomposition of methane for hydrogen production

Carbon black has recently been reported to act as an effective catalyst for methane decomposition and to exhibit stable catalytic behavior despite carbon deposition, and thus it can be used for CO₂-free production of hydrogen from natural gas. In this work, various carbon blacks with different primary particle size were investigated with respect to methane decomposition under atmospheric pressure from 1123 to 1223 K. Catalytic characteristics, such as activity, activation energy and reaction order, were investigated and compared. It was observed that with decreasing primary particle size (or increasing specific surface area), the specific activity increased and the activation energy decreased. The reaction orders for various pelletized, rubber-reinforcing carbon blacks were 0.6–0.7, about the same regardless of the primary particle size, while they were near 1 for fluffy carbon blacks. Fluffy carbon black showed higher activity and activation energy than the pelletized carbon black of the same primary particle size. Changes of the surface morphology during carbon deposition were observed by TEM. Variations of the number of active sites were discussed in regard of the primary particle size, carbon deposition and binder. The presence of different types of active sites was also suggested.     

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.     

Variation in Catalytic Activity of Carbon Black during Methane Decomposition: Active Site Estimations from Surface Structural Characteristics

Abstract  

The variation in the catalytic activity of carbon black (CB) during methane decomposition was investigated by considering the number of active sites of CB. We demonstrated that the activity variation could be well estimated by assuming the edge length of graphitic sheets evolving from the CB surface. The results suggested that the activity variation originated from surface structural changes due to carbon deposition.     

The Heterogeneity of Glass Surfaces Revealed by Temperature Programmed Desorption

Adsorption of butanol and pyridine on E‐glass fibers with three different compositions, as well as on powders of silica and the crushed fibers, was studied by temperature programmed desorption (TPD) with a mass‐sensitive detector. In the case of butanol, there are two types of desorbing molecules: at lower temperatures butanol desorbs, but in the range 450°C–600°C, 1‐butene desorption is also observed. It is shown that 1‐butene desorption is due to thermal decomposition of butanol chemisorbed to OH groups on both the glass and silica surfaces. The binding energy distributions of adsorption sites for butanol and pyridine are similar on all three glass compositions, but they are much more heterogeneous compared to silica; this difference is most evident for pyridine and is attributed to the presence Al and B in the glasses. The decomposition temperature of chemisorbed butanol is highest for silica and depends on glass composition for the fibers and powders. Interestingly, the glass which does not contain boron shows a well‐defined peak for the decomposition of chemisorbed butanol, suggestive of unique adsorption sites on this boron‐free surface; but they are much less temperature stable than the chemisorption sites on silica. In situ exposure to water vapor increased the number of active sites for chemisorption.     

The synthesis of ethene during the oxidative coupling of methane

The effect of residence time on the selectivity of a MgO catalyst for the formation of ethene in the methane coupling reaction has been studied. It is found that the variation in ethene selectivity with this time follows opposite trends depending on the experimental conditions. In both cases the results show a positive intercept at zero residence time which may indicate that ethene is a primary product. However, calculations show that gas phase radical reactions just above the catalyst surface could account for the ethene observed even at very short residence times. It is concluded that experiments such as these cannot reliably distinguish between primary and secondary product formation. A SmOCl catalyst was studied and it was found that HCl was released when wet gases were passed over the catalyst at relatively low temperatures. It is concluded that much of the ethene produced over oxychloride catalysts may be produced by gas phase reactions involving chlorine radicals.Based in part on a paper given at the Royal Society-U.S.S.R. Academy Workshop on Catalysis and Surfaces, Oxford, April 8–10, 1989.     

Transformation of LPG into Aromatic Hydrocarbons and Hydrogen over Zeolite Catalysts

A way to increase the value of LPG cut from petroleum feedstocks is its direct transformation to H2 and aromatic products; these aromatic products, BTX—essentially benzene (B), toluene (T), and C, -aromatics (X)—can be used as raw material for the petrochemical industry or as a blending mixture to enhance the octane number of gasoline. However, these transformations require high temperatures. Thermodynamic data show that the conversion of paraffins into aromatics is favored by increasing the length of the chain, and that aromatics are favored in relation to olefins (Table 1) [1,2]. Whereas aromatization of propane and higher paraffins can be carried out at temperatures lower than 500°C, transformation of ethane, and especially that of methane, requires much higher temperatures. This is experimentally supported by the transformation of various hydrocarbons, at constant temperature and space velocity. For instance, over H-[All-ZSM-5, butane and isobutane react four times faster than propane and 100 times faster than ethane [3].     

Effect of carbon deposition over carbonaceous catalysts on CH4 decomposition and CH4-CO2 reforming

An investigation was made using a continuous fixed bed reactor to understand the influence of carbon deposition obtained under different conditions on CH₄-CO₂ reforming. Thermogravimetry (TG) and X-ray diffraction (XRD) were employed to study the characteristics of carbon deposition. It was found that the carbonaceous catalyst is an efficient catalyst in methane decomposition and CH₄-CO₂ reforming. The trend of methane decomposition at lower temperatures is similar to that at higher temperatures. The methane conversion is high during the initial of stage of the reaction, and then decays to a relatively fixed value after about 30 min. With temperature increase, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decomposition, whereas the carbon deposition does not affect methane decomposition significantly. Different types of carbon deposition were formed at different methane decomposition reaction temperatures. The carbon deposition Type I generated at 900°C has a minor effect on CH₄-CO₂ reforming and it easily reacts with carbon dioxide, but the carbon deposition Type II generated at 1000°C and 1100°C clearly inhibits CH₄-CO₂ reforming and it is difficult to react with carbon dioxide. The results of XRD showed that some graphite structures were found in carbon deposition Type II.     

Effect of carbon deposition over carbonaceous catalysts on CH<Subscript>4</Subscript> decomposition and CH<Subscript>4</Subscript>-CO<Subscript>2</Subscript> reforming

An investigation was made using a continuous fixed bed reactor to understand the influence of carbon deposition obtained under different conditions on CH₄-CO₂ reforming. Thermogravimetry (TG) and X-ray diffraction (XRD) were employed to study the characteristics of carbon deposition. It was found that the carbonaceous catalyst is an efficient catalyst in methane decomposition and CH₄-CO₂ reforming. The trend of methane decomposition at lower temperatures is similar to that at higher temperatures. The methane conversion is high during the initial of stage of the reaction, and then decays to a relatively fixed value after about 30 min. With temperature increase, the methane decomposition rate increases quickly. The reaction temperature has significant influence on methane decomposition, whereas the carbon deposition does not affect methane decomposition significantly. Different types of carbon deposition were formed at different methane decomposition reaction temperatures. The carbon deposition Type I generated at 900°C has a minor effect on CH₄-CO₂ reforming and it easily reacts with carbon dioxide, but the carbon deposition Type II generated at 1000°C and 1100°C clearly inhibits CH₄-CO₂ reforming and it is difficult to react with carbon dioxide. The results of XRD showed that some graphite structures were found in carbon deposition Type II.     

Pyrolysis of Natural Gas: Effects of Process Variables and Reactor Materials on the Product Gas Composition

High‐temperature pyrolysis of natural gas is the basis of the standard method for the manufacture of acetylene. The study of methane pyrolysis was designed to find optimum process conditions that would produce high yields of acetylene with minimal carbon formation. High temperatures and short residence times enhanced the selectivity for acetylene, while hydrogen dilution was found to suppress the generation of carbonous products. Carbon formation on reactor surfaces over time may be mainly responsible for the misalignment of predicted and measured product gas compositions, as the mechanisms reported do not consider the surface chemistry. In essence, the pyrolysis system favors the highest possible temperature and shortest possible residence time, suggesting that the selection of reactor materials is the key for pyrolysis process optimization. The operating temperature is likely dictated by the physical properties of the reactor materials rather than the selection of optimal pyrolysis conditions.     

Pyrolysis of methane in a single pulse shock tube

The thermal decomposition of methane has been studied in a chemical shock tube at pressures up to 20 atm over a temperature range of between 1750 and 2700 K and for reaction times up to 2.5 ms. Attention is drawn to some of the experimental features of the shock tube and to the fact that reaction temperatures were measured. Optimum conditions for the production of acetylene from methane are suggested, and the relatively small effect of pressure on the acetylene yields is noted. Values for activation energy (93.6 kcal/mol) and frequency factor (3.8 × 10¹³ s^?1 for methane decomposition are given. The experimental results obtained are discussed in connection with suggested mechanisms of decomposition. In the discussion attention is drawn to the difficulty of predicting acetylene yields arising from the incomplete understanding of the mechanism of acetylene decomposition and of “carbon” formation under the conditions employed.     

Promoter action of KCl on CuCl2/SiO2 catalysts used for the oxyhydrochlorination of methane

The effect of KCl addition on the catalytic activity and product distribution of SiO₂-supported CuCl₂ catalysts has been investigated. The activity of these K-containing catalysts can be related to the release of chlorine atoms from the surface which initiate a gas phase chain reaction. The active phase of these catalysts is molten under reaction conditions (700 Torr, 670 K). In this case, the presence of KCl in the melt increases the catalytic activity and favors the regeneration of the catalyst. However, overdoses of KCl have a negative effect on both, catalyst activity and stability.When KCl is not added to the CuCl₂ catalyst, the reaction can still proceed by an alternative path over the solid surface, probably involving the dissociative chemisorption of methane. As opposed to the effect observed at 670 K, at lower temperatures, e.g., 500 K, the addition of KCl reduces the catalytic activity by blocking copper chloride active sites.     

Crystallization behavior of polyphenylenesulfide

The crystallization behavior of poly(phenylene sulfide) (PPS) has been examined by differential scanning calorimetry as a function of melt temperature, residence time in the melt, and the presence of a liquid crystal polymer. The results suggest that the thermal history of the sample plays an important role in determining the crystallization kinetics. Short residence times at low melt temperatures and high melt temperatures alone resulted in a low value for the Avrami exponent n. The former effect was ascribed to incomplete melting of the polymer while the latter effect was attributed to thermal degradation. Blending a liquid crystal polymer, Vectra A950, with PPS had only a minor effect on the crystallization behavior.