The low-temperature oxidative coupling of methane over zirconium oxide

Zirconium oxide is shown to be capable of catalyzing the conversion of methane to ethane at temperatures as low as 530 °C. The lowest temperature at which ethane is produced is found to be dependent upon the method employed for the preparation of the catalyst. The presence of surface hydroxyl groups appears to be necessary for the production of ethane at these low temperatures.     

The oxidative coupling of methane and the oxidative dehydrogenation of ethane over a niobium promoted lithium doped magnesium oxide catalyst

The promoting effect of niobium in a Li/MgO catalyst for the oxidative coupling of methane (OCM) and for the oxidative dehydrogenation of ethane (ODHE) has been studied in some detail. It has been found that a Li/Nb/MgO catalyst with 16 wt % niobium showed the highest activity for the C₂ production in the OCM reaction; the activity at 600 °C was ten times that of the Li/MgO catalyst at the same temperature. The Li/Nb/MgO catalyst was also slightly more active for the ODHE reaction than was the Li/MgO catalyst. However, the Li/Nb/MgO catalyst produced considerably more carbon dioxide in the both reactions. Structural investigation of the catalyst showed that the addition of niobium to the Li/MgO catalyst increased the surface area and gave an increase in the lithium content of the calcined catalysts. Two niobium phases, LiNbO₃ and Li₃NbO₄, were formed; it is shown that the first of these probably causes the increased activity. Ageing experiments showed that the activity of the catalyst was lost if the catalyst was used above 720 °C, the melting point of the lithium carbonate phase. The catalyst showed a decrease of surface area after ageing and a sharp decrease of the amount of the two niobium phases. The addition of carbon dioxide to the feed could not prevent the deactivation of the Li/Nb/MgO catalyst.     

Kinetics, mathematical modeling, and optimization of the oxidative coupling of methane over a LiMnW/SiO2 catalyst

A phenomenological kinetic model is developed, and the numerical values of the kinetic parameters of the oxidative coupling of methane that is catalyzed by a new LiMnW/SiO₂ composite material are determined. The mathematical modeling of the process is performed, and the optimal conditions and approaches to the apparatus-technological design of the process are determined.     

Doped zinc oxide as methane coupling catalyst

The alkali metal distribution in a variety of alkali doped ZnO catalysts has been studied and is shown to affect total methane conversion and C2 selectivity. When Na₂CO₃ precursors are used, CO₃²⁻ groups decompose upon calcination and sodium penetrates into the ZnO matrix, even at high loading. In the case of lithium, layers of Li₂CO₃, proportional to the Li concentration, segregate on the ZnO and reduce the oxide active surface available for gas reaction.

In the aim of determining the best preparation and reaction parameters a correlation between XPS evidences and catalytic data collected during the initial phase of reaction was attempted.     

Numerical simulation of fixed bed reactor for oxidative coupling of methane over monolithic catalyst

A three-dimensional geometric modelwas set up for the oxidative coupling of methane (OCM) fixed bed reactor loaded with Na₃PO₄-Mn/SiO₂/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 CH₄/O₂ 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 CH₄ and the selectivity of products (C₂H₆, C₂H₄, CO,CO₂ and H₂) in the reactor outlet with an error range of ±4%. The mass fractions of CH₄ and O₂ 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 C₂H₆, C₂H₄, CO and CO₂ 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.     

Effect of reaction temperature on the selectivity of oxidative methane coupling over lead oxide

In the oxidative coupling of methane over lead oxide, reaction temperature is a critical variable which determines the reaction pathway of methyl intermediates on the surface. Low temperature favors oxidation to carbon oxides, while high temperature favors desorption as methyl radicals.     

Oxidative coupling of methane and oxidative dehydrogenation of ethane over strontium-promoted rare earth oxide catalysts

Sr-promoted rare earth (viz. La, Ce, Pr, Nd, Sm, Eu, Gd, Dy, Er and Yb) oxide catalysts (Sr/rare earth ratio = 0·1) are compared for their performance in the oxidative coupling of methane (OCM) to C₂ hydrocarbons and oxidative dehydrogenation of ethane (ODE) to ethylene at different temperatures (700 and 800°C) and CH₄ (or C₂H₆)/O₂ ratios (4–8), at low contact time (space velocity = 102000 cm³ g⁻¹ h⁻¹). For the OCM process, the Sr–La₂O₃ catalyst shows the best performance. The Sr-promoted Nd₂O₃, Sm₂O₃, Eu₂O₃ and Er₂O₃ catalysts also show good methane conversion and selectivity for C₂ hydrocarbons but the Sr–CeO₂ and Sr–Dy₂O₃ catalysts show very poor performance. However, for the ODE process, the best performance is shown by the Sr–Nd₂O₃ catalyst. The other catalysts also show good ethane conversion and selectivity for ethylene; their performance is comparable at higher temperatures (≥800°C), but at lower temperature (700°C) the Sr–CeO₂ and Sr–Pr₆O₁₁ catalysts show poor selectivity. © 1998 SCI.     

Oxidative coupling of methane over CaO-MgO mixed oxide

Mixed oxide catalyst prepared by co-precipitating magnesium oxide and calcium oxide showed an excellent activity for the oxidative coupling of methane. The high performances were presumed to arise from the high basicity of the mixed oxide.     

Kinetics and mechanism of the vapour phase nitroxidation of toluene over nickel oxide-aluminium oxide catalyst

The kinetics of the oxidation of toluene by nitric oxide over nickel oxide-aluminium oxide catalyst has been studied in the temperature range 280-380°C. A rate equation R_n= k_tp_tk_nP_n I (k₁P₁ + k_nP_n) was deduced, assuming a steady state involving a two-stage irreversible oxidation-reduction process. The model represented the data satisfactorily for the conversion of toluene to benzonitrile.     

Kinetics of methane combustion over CVD-made cobalt oxide catalysts

The present investigation provides the required kinetic parameters to evaluate and to predict the rate of the catalytic combustion of methane over cobalt oxide. For this purpose, monolithic cordierites with low specific surface area were uniformly coated with cobalt oxide thin films of controlled thickness using the chemical vapor deposition (CVD) process. The obtained catalysts were tested in the catalytic combustion of methane in oxygen-deficient and -rich conditions. Catalysts with loadings above 0.46 wt.% are active starting at a temperature of 250 °C and completely convert methane to CO₂ below 550 °C where the conversion rate reaches 35 μmol (CH₄)/g_cat s. The involvement of the bulk-oxide-ions in the catalytic reaction was supported by the constant value of the normalized reaction rate to the weight of deposited cobalt oxide. The experimental data fit well to the Mars–Van Krevelen redox model and can be approximated with a power rate law in oxygen-rich mixtures. The resulting activation energies and frequency factors allow the identification of the rate-limiting step and accurately reproduce the effect of the temperature and partial pressure of the reactants on the specific reaction rate.     

Kinetic modelling of the oxidative coupling of methane over lanthanum oxide in connection with mechanistic studies

A kinetic model of the oxidative coupling of methane (OCM) over lanthanum oxide is developed on the basis of mechanistic conclusions obtained elsewhere. A reaction scheme is proposed, and the kinetic parameters corresponding to the different kinetic equation are optimized. Experimental and calculated reaction rates are compared with the aim of testing the proposed scheme and validating the basic mechanistic assumptions.     

The importance of heterogeneous and homogeneous reactions in oxidative coupling of methane over chloride promoted oxide catalysts

The formation of ethene from ethane and methane in a silica reactor has been studied both in the presence and in the absence of chloride-containing catalysts. Some homogeneous conversion of ethane to ethene occurs in the gas phase through direct dehydrogenation, oxidative dehydrogenation, and, when HCl is present, chlorine radical induced reactions. Methyl chloride is detected in the gas phase but has no influence on the conversion of ethane to ethene. It is shown that under typical catalytic conditions, when a chloride-modified catalyst is used, ethane is mostly produced in the catalyst bed.     

Catalytic oxidative coupling of methane over hydroxyapatite modified with lead

Hydroxyapatite modified with lead catalyzes the oxidative dehydrogenation of methane with high selectivity to C₂ compounds at reaction temperatures as low as 700 °C. The activity is stabilized after reduction in the surface area of the catalyst during the reaction.     

Oxidative coupling of methane with participation of oxide catalyst lattice oxygen

Redox properties of the supported Li₂O/MgO, K₂O/Al₂O₃ and PbO/Al₂O₃ catalysts are studied. New mechanism of the catalyst re-oxidation is suggested. Re-oxidation of the catalyst in the course of steady-state reaction can proceed as an oxidative dehydrogenation of surface OH groups.     

Catalytic reduction of nitric oxide by methane over CaO catalyst

The selective catalytic reduction of nitric oxide by methane was studied over CaO catalyst in a bubbling fluidized bed in the temperature range of 800–900 °C, in which NO cannot be reduced by CH₄ without CaO catalyst. The nitric oxide conversion was found to depend on oxygen and CH₄ feed concentration, and also on temperature. In addition, the presence of water vapors in the flue gas enhanced the NO reduction admirably well in the absence of O₂. But water vapor has an inhibiting effect on the reaction while O₂ is present in the flue gas. The addition of CO₂ poisoned the CaO catalyst and exhibited a detrimental effect on NO conversion at the working temperature range, 800–900 °C. However, with a temperature rise to 900 °C the CO₂ poisoning effect on NO reduction was weakened. The mechanism was studied and discussed according to the references in the paper. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

The influence of various parameters on the oxidative coupling of methane reaction over lithium doped lanthanum oxide

The influence of reaction temperature, space velocity and methane and oxygen partial pressures were studied for the oxidative coupling of methane reaction (OCM) over a lithium doped lanthanum oxide catalyst. The kinetic data obtained with this catalyst, at temperatures between 650 and 750°C, indicate that oxygen is adsorbed non-dissociatively and non-competitively. Product selectivities extrapolated to zero percent methane conversion are similar to those obtained with Li/MgO, suggesting that both rare earth and alkaline earth based catalysts involve similar mechanisms. Carbon monoxide and ethylene were found to be secondary products exclusively.     

The investigation of individual reaction steps in the oxidative coupling of methane over lithium doped magnesium oxide

To elucidate the importance of various reaction steps in the oxidative conversion of methane, experiments were carried out with three reaction products: ethane, ethylene and carbon monoxide. These products were studied separately in oxidation experiments with and without a catalyst. Moreover, the effect of admixing them to a methane/oxygen feed was investigated. All experiments were carried out in a micro flow tubular quartz reactor which was either empty or filled with catalyst at a temperature of 800 °C. The ethane and ethylene experiments showed that the conversion of ethane to ethylene is much more rapid than ethane combustion, irrespective of the presence of a catalyst. The main combustion path goes via ethylene. Ethane is converted much more rapidly than methane and this imposes serious constraints on the maximum attainable yields. The principal combustion product in the absence of a catalyst is CO but with a catalyst, CO₂ dominates, in agreement with rapid catalytic oxidation observed with CO/O₂ feeds.

The conclusions are summarized in a simplified overall reaction scheme.     

Low temperature oxidative conversion of methane to syngas over NiO-CaO catalyst

Catalytic partial oxidation of methane over NiO-CaO (with or without its prereduction by H₂) at low temperatures ( 973 K) under non-equilibrium conditions yields syngas (H₂/CO ratio 2.0) with high conversion/selectivity and extremely high productivity. If required, the H₂/CO ratio can be increased by adding water vapour to the feed. Product selectivity is controlled by the process kinetics and also the reaction path is different from that observed for the high-temperature non-catalytic and catalytic processes operating at equilibrium.