Carbon dioxide chemistry during oxidative coupling of methane on a Li/MgO catalyst

Reactor tests, temperature-programmed reaction/desorption (TPR/D) carried out in a thermogravimetric balance and FT-IR were employed to investigate the course of CO₂ formed during oxidative coupling of methane (OCM) on a 7-wt.-% Li/MgO catalyst between 600–800°C. Initially, the carbonate free Li/MgO catalyst showed good OCM activity even at 600°C. However, its activity diminished considerably after about 20 min on stream. This coincided with the appearance of CO₂ in the OCM products and the disappearance of ethylene. During OCM, a substantial amount of the available lithium was converted to a stable carbonate which did not decompose easily even at 800°C. FT-IR and TPR revealed that carbonate formation began at 400°C and that the lithium carbonate existed in the form of monodentate (LiOCO₂) and perhaps mixed bridged [Mg(Li)O₂CO] carbonates. Simultaneous existence of a commonly accepted Li₂CO₃ cannot be excluded. The role of CO₂ produced during OCM in modifying the catalytic performance of Li/MgO is discussed.     

Pretreatment effects on the active site for methane activation in the oxidative coupling of methane over MgO and Li/MgO

The effects of high temperature pretreatments on the activity of MgO and Li/MgO catalysts for the oxidative coupling of methane have been studied. The MgO powder catalyst exhibited a turnover frequency of 3.0×10^–3 molecules/sites, at 990K, whereas the Li/MgO catalyst showed a turnover frequency of 7.0×10^–2 molecules/sites, under the same reaction conditions. The initial C₂ formation rate was observed to increase with pretreatment temperature over the MgO catalyst, supporting our previous proposal that F-type defects are responsible for methane activation.     

Oxidative coupling of methane over doped Li/MgO catalysts

A series of zirconia doped Li/MgO catalysts with a fixed amount of zirconia and varying concentrations of lithium was used for the oxidative coupling of methane. It was found that an increase in lithium concentration resulted in a decrease in initial activity, while the selectivity was not affected. The life-time of Zr doped Li/MgO catalysts with a fixed concentration of ZrO₂ is a function of the lithium concentration. Previous results have shown that Li₂Mg₃ZrO₆ is active and selective but it is now shown to be instable under reaction conditions.     

The role of heterogeneous reactions during the oxidative coupling of methane over Li/MgO catalysts

The role of heterogeneous and homogeneous reactions during the oxidative coupling of methane over Li/MgO was investigated by carrying out experiments at reduced pressure (60 Pa). The conclusion is that the radical reactions occurring in the gas phase play an essential role in the C₂₊ selectivity of the process. Our reaction model of consecutive reactions is confirmed. The selectivity of the active catalyst is mainly due to very high rates of methyl radical production which couple in the gas phase.     

Effect of Li/MgO methane coupling catalyst on alonized steel reactors

Alonized 316 stainless steel reactors were used in life tests of a Li/MgO methane coupling catalyst. Severe corrosion of the portion of the reactor in contact with the catalyst bed was observed. Loss of physical integrity and chemical passivity resulted from layer formations of differing elemental compositions from the bulk metal. Chromium was leached from the stainless steel alloy and deposited on the catalyst. There were no adverse changes in those portions of the reactor not in direct contact with the catalyst. These results demonstrate that Alonized 316 stainless steel is unsuitable for use in methane coupling service with this lithium-containing catalyst.     

The effect of fixed-bed reactor configuration on the oxidative coupling of methane over a Li/Pb/Ca catalyst

Introduction of additional O₂ at the midpoint of the catalyst bed of a methane oxidative coupling, fixed bed reactor, increases the C₂ STY more than the CO _x . STY over a Li/Pb/Ca catalyst. This observation is not only a consequence of kinetics but may also be attributed to increased methyl radical generation on the O₂ replenished catalyst surface.     

Oxidative coupling of methane using Li/MgO catalyst: Re-appraisal of the optimum loading of Li

The effect of the level of lithium carbonate doping on MgO, prepared by thermal decomposition of the basic carbonate, is re-examined. A low, sub monolayer, loading, ie. 0.2% Li₂CO₃-MgO is shown to significantly enhance both the specific activity for methane activation and the total C₂ hydrocarbon selectivity. The study indicates that the optimal loading of alkali promoters on MgO prepared in this way is considerably lower than indicated in previous studies.     

The role of carbon dioxide in the oxidative dimerization of methane over Li/MgO

CO₂ is strongly adsorbed on Li/MgO as a surface carbonate and desorbs concomitantly with Li with an activation energy of desorption of 210 kJ/mol. The C₂ product is strongly influenced by the presence of CO₂,0.5 Torr being sufficient to substantially lower the rate of C₂ production and to establish an activation energy for reaction of 210 kJ/mol. In the absence of CO₂, the activation energy of C₂ production falls to 105 kJ/mol.     

A new nano‐(2Li2O/MgO) catalyst/porous alpha‐alumina composite for the oxidative coupling of methane reaction

The present work discloses a new methodology for the production of detached nanorods of 2Li₂O/MgO catalyst particles on the internal surface of α‐Al₂O₃ porous supports to be used as efficient catalysts for the oxidative coupling of methane reaction (OCM). The peculiarity of our preparatory recipe is the success in producing “detached” nanosized entities on the support surface. The performance of the new catalyst/support system for the OCM reaction has been evaluated using a special reactor assembly with cross flow of methane and oxygen gas streams. Under the optimum process conditions, the yield of C product is 25% at an average reaction temperature of 750°C. Under the optimum conditions, the yield of ethylene reaches 8%. It is shown that the enhanced catalytic properties of the new catalyst/support composite may be attributed to nanoeffects. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Kinetic studies of the oxidative dimerization of methane on thin-film MgO and Li/MgO

The oxidative dimerization of methane to ethane over thin-film MgO and Li/MgO catalysts prepared under well-controlled, ultrahigh vacuum (UHV) conditions has been studied using a combination UHV/reactor cell system that allows elevated pressure kinetic studies and surface science techniques to be employed. Reactions investigating temperature, pressure and pretreatment activation have been performed to determine ethane dependence on each reaction parameter. Agreement between turnover frequencies, activation energies and lithium promotional effects of the thin-film MgO and previously reported results for powdered MgO indicates that the thin-film catalyst is an excellent model catalyst for the oxidative coupling of methane.     

Investigations on a Ce/Li/MgO-Catalyst for the oxidative coupling of methane

The heterogeneously catalyzed oxidative coupling of methane into ethane and ethylene has been investigated using a Ce/Li/MgO catalyst. A tentative catalytic mechanism, which explains the role of cerium, is proposed and confirmed by pulsing experiments. The loss of lithium during pretreatment and under reaction conditions was determined. A correlation between the lithium loading of the catalyst and its C₂ production rate was found. The reaction scheme has been partly elucidated using C₂H₄/O₂N₂ as feed gas. It is shown that both homogeneous and heterogeneously catalyzed reaction steps are of great importance.     

Kinetics and mechanism of oxidative coupling of methane over sodium-manganese oxide catalyst

The kinetics of methane oxidative coupling (OCM) was studied using 1 g of Na-Mn₂O₃ catalyst at 1073 to 1123 K, in an integral flow reactor (I.d. = 10 mm), at atmospheric pressure with methane and oxygen partial pressures of 0.27 and 0.13 bar, respectively, so that the ratio of CH₄ to O₂ was 2. The flow rate range was 50 to 200 ml/min. the kinetic data were analyzed by the Rideal-redox type of rate equation assuming the methyl radical and active surface oxygen to be the steady-state intermediates. Oxidation and reduction rate constants (K_ox, K_red) for methane consumption were calculated from experimental catalysis results by computer simulation using the multiple least squares method. The activation energies at rate constants K_ox and K_red for this type of catalyst were reported as 43.26 and 62.2 kcal/mol, respectively.     

Effect of steam in the oxidative coupling of methane over MgO and CaO based catalysts

Influence of the initial methane-oxygen mixture dilution with steam on the parameters of the oxidative coupling of methane is studied. Catalytic tests of Pb---Mg, Sm---Mg, Bi---Ca, Zr---Mg and Na---Ca oxide systems are carried out with steam content 50–70% at 700–850°C and P(CH₄)/P(O₂)= 1,5–4,9. It is shown that the addition of steam gives increase of ethylene yield up to 12–16 mol%. The role of steam is investigated.     

The effect of Nb2O5 and ZrO2 additions on the behaviour of Li/MgO and Li/Na/MgO catalysts for the oxidative coupling of methane

Incorporation of Nb₂O₅ or ZrO₂ into both Li/MgO and Li/Na/MgO systems produced ternary and quaternary catalysts, respectively, capable of attaining optimal C₂ yields and selectivities at lower temperatures relative to the unpromoted materials. The degree of enhancement effected by these metal oxide additives was compared to that produced by Li/MgO and Li/Na/MgO catalysts promoted with SnO₂ or Co₃O₄. At reaction temperatures < 700°C, the Li/Co/MgO ternary system showed marked differences in behaviour compared to the other ternary catalysts tested. This was particularly evident in the variation in C₂ selectivity with time on stream during ageing studies of (i) untreated materials, (ii) materials pretreated in CO₂, and (iii) materials dosed periodically with CHCI₃.     

Carbon dioxide reforming of methane over MgO promoted Ni/CNT catalyst

Carbon dioxide reforming of methane to syngas was investigated over a series of MgO promoted Ni/CNT catalysts. MgO played a critical role in improving the catalytic performance of Ni/CNT. The results showed that the addition of MgO strengthened the interaction of Ni and interior surface of CNT. Highly dispersed nickel particles with small size (less than 4.5 nm) were also observed in MgO modified CNT. Otherwise, the NiO nanoparticles were facilely reduced over the catalyst prepared with a narrow size of CNT, as shown in H₂-TPR. The reaction tests demonstrated that the Ni-based catalyst with an addition of MgO and narrow size of CNT exhibited better catalytic activity. Furthermore, the lifetime of Ni-based catalyst was prolonged effectively after adding MgO, attributed to the stabilization and dispersion of Ni particles and the effective restraint on the gasification of CNT.     

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.     

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.     

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.