Oxidative coupling of methane over lithium doped magnesium oxide catalysts

Active sites are created on the surface of a Li/MgO catalyst used for the selective oxidation of methane, by the gradual loss of CO₂ from surface lithium carbonate species in the presence of oxygen. The sites created are not stable but disappear either as a result of reaction with SiO₂ to form Li₂SiO₃ or by the formation and subsequent loss of the volatile compound LiOH. The deactivation can be reversed, at least partially, by treating the catalyst in CO₂ under reaction conditions; it can be retarded if low concentrations of CO₂ are added to the reaction mixture.     

Oxidative coupling of methane over oxide catalysts with layered structure

The catalytic properties of complex oxides with a layered structure Bi₂GeO₅, Bi₂SiO₅, Bi₄Ti₃O₁₂, Bi₂CaSrCu₃O₈+x and YBa₂Cu₃O₆+δ in oxidative coupling of methane (OCM) have been studied. Bi₂EO₅ metastable compounds have been found to possess the high activity and C₂-selectivity 53–70%. It has been assumed that the intergrowth boundaries for the Bi₂EO₅ decomposition products, on which active catalytic sites may be located, play a specific role on the catalytic performance of the oxides. Bi₄Ti₃O₁₂ and Bi₂CaSrCu₃O₈+x have close catalytic activity values but Bi₂CaSrCu₃O₈+x as well as YBa₂Cu₃O₆+δ are not selective in OCM reaction.     

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.     

Partial oxidation of13C-labeled ethylene in methane over Ca/Ni/K catalysts

The relative reactivity of ethane and ethylene compared to methane over the Ca/Ni/K catalyst was determined. The reactivities are in the order of ethylene > ethane methane. The catalyst was also studied using temperature-programmed reaction, desorption and decomposition.     

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.     

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 alkali metal oxide promoted La2O3/BaCO3 catalysts

The oxidative coupling of methane (OCM) over various alkali metal oxide promoted La₂O₃/BaCO₃ catalysts and the effects of Na₂O content on the performance of Na₂O–La₂O₃/BaCO₃ catalysts have been studied. It was found that Na₂O promoted La₂O₃/BaCO₃ catalysts had the advantages of high CH₄ conversion, C₂ selectivity and C₂H₄/C₂H₆ ratio. Na₂O might affect the properties of the catalysts through electronic and geometric effects. The highest C₂ yield (19·0–20·6%) was obtained with Na₂O–9 wt% La₂O₃/BaCO₃ catalysts of 1·0–3·0% Na₂O. The effects of reaction conditions on OCM over 3 wt% Na₂O–9 wt% La₂O₃/BaCO₃ catalysts have also been investigated. The catalysts were characterized by BET, TPD and XRD. TPD studies on 3 wt% Na₂O–9 wt% La₂O₃/BaCO₃ catalysts demonstrated that CO₂, CH₄ and O₂ could be adsorbed strongly on the catalyst. This might be related to the activation of CH₄ and the formation and regeneration of active oxygen species.     

Oxidative coupling of methane over BaF2-TiO2 catalysts

The addition of F^– to Ba-Ti mixed oxide catalysts significantly improves the catalytic performances for the oxidative coupling of methane (MOC), which can achieve high C₂ yields at wide feed composition range and high GHSV. The effect is particularly marked for the BaF_2– TiO₂ catalysts containing more than 50 mol% BaF₂. The C₂ yield of 17% and the C₂ selectivity of > 60% were achieved over these catalysts at 700 ° C. After being on stream for 31 h, the 50 mol% BaF₂-TiO₂ catalysts showed only a 1–1.5% decrease in the C₂ yields. Results obtained by XRD show that various Ba-Ti oxyfluoride phases were formed due to the substitution of F^– to O^2–.     

Oxidative coupling of methane over LaF3/La2O3 catalysts

We studied the oxidative coupling of methane over the LaF₃/La₂O₃ (5050) catalyst. The catalyst was found active even at 873 K. At 1023 K, the C₂ yield was 12.7% at 26.0% CH₄ conversion and 49.1% C₂ selectivity. It was found to be stable and had a lifetime not less than 50 h at 1023 K. The catalyst was effective in C₂H₆ conversion to C₂H₄. XRD results indicated that the catalyst was mainly rhombohedral LaOF. It is suggested that the catalyst has ample stoichiometric defects and generates active oxygen sites suitable for methane dehydrogenation.     

Oxidative coupling of methane over Na---La2O3 catalysts

Adding sodium to lanthanum oxide increases appreciably the activity, the C⁺₂ selectivity and the stability of the catalyst. The variations of these catalytic properties when the main kinetic parameters (contact time, temperature etc) vary could find their explanation in the formation of new reactive centers. The selectivity of these centers would be the result mainly of the inhibition of the primary reaction of the oxidation of methane into CO₂.     

Oxidative coupling of methane on silver catalysts

Bulk silver catalysts were found to be active for the oxidative coupling of methane to ethane and ethylene if operated under oxygen-limited conditions at atmospheric pressure and at temperatures above 1020 K. The addition of small amounts of sodium phosphate as promoter increases markedly the C₂ selectivity (to values above 90%) and yield (>10%) by efficient suppression of reaction steps leading to total oxidation. Further improvement of the yields might be achieved by more appropriate reactor design.     

Oxidative coupling of methane to C2-hydrocarbons over La-promoted CaO catalysts

La₂O₃ promoted CaO [La/Ca (mol/mol) = 0.05] catalyst shows very high activity and selectivity (methane conversion: 25%, C₂-selectivity: 66% and C₂-space-time-yield: 864 mmol ·g^–1 (cat.)·h^–1) with no catalyst deactivation in oxidative coupling of methane to C₂-hydrocarbons at 800 ° C.     

Oxidative coupling of methane over strontium-doped neodymium oxide: Parametric evaluations

Since its discovery in 1982, oxidative coupling of methane (OCM) has been considered one of the most promising approaches for the on-purpose synthesis of ethylene. The development of more selective catalysts is essential to improve process economics. In this work, undoped neodymium oxide as well as neodymium oxide doped with high (20%) and low (2.5%) levels of strontium were tested in a high-throughput fashion covering a wide range of operating conditions. The catalysts were shown to be able to achieve greater than 18% C₂+ yield. Space velocity was shown to play a significant role in C₂+ selectivity. For a methane to oxygen feed ratio of 3.5, selectivity increased with increasing space velocity, reaching a maximum of 62% at a methane conversion of 30% at an optimal space velocity of ~250,000 ml/h/g. The difference in activity between the three samples was linked to the contribution of different oxygen centers.     

Oxidative coupling of methane on Ce/Na/CaO catalysts

Oxidative coupling of methane was investigated on Na/CaO and Ce/Na/CaO catalysts with different sodium and cerium contents. The reaction was carried out in a micro fixed-bed flow reactor operating at atmospheric pressure, at 700 and 750°C and molar ratio feed of CH₄ : O₂ : N₂ = 50 : 10 : 40. Catalysts were compared at isoconversion and were characterized by BET, XRD and XPS. The addition of cerium to Na/CaO catalyst increased the specific activity by a factor of eight. These catalytic and characterization results were related to the presence not only of Na₊O^- active sites, formed by the introduction of Na₊ in the CaO lattice, but also to peroxide active sites. A mechanism is proposed to explain the role of cerium in increasing the rate of the regeneration step of the O_s^– and Na₂O₂ sites.     

Oxidative coupling of methane over alkali-promoted Ti-La oxide catalysts. Effect of bulk and surface properties on catalytic performance

The reactivity of lanthanum titanate catalysts is investigated in the oxidative coupling of methane (OCM) under lean-oxygen conditions. The catalyst performances are influenced by the catalyst preparation method and by the amounts of the alkali dopant. A transient method was also used to study the mobility of lattice oxygen species under OCM reaction conditions. It is found that (i) in the absence of gas phase oxygen the sample does not possess lattice oxygen species which participate in the reaction; (ii) the alkali doping does not appreciably modify the mobility of lattice oxygen; and (iii) the role of the alkali promoter in the reaction is that of modifying the surface acid/base and oxidizing properties.     

Non-oxidative methane coupling over Co-Pt/NaY bimetallic catalysts

Methane activation and coupling of the CH_k species formed from methane into higher hydrocarbons over NaY, Pt/NaY, Co/NaY, Co-Pt/NaY and Co-Pt/Al₂O₃ have been compared. Co-Pt/NaY and Co-Pt/Al₂O₃ showed exceptionally high yields (100%) referred to the adsorbed CH_x species and high selectivity in the formation of C_2k hydrocarbons (83.6 and 92.6%, respectively) in the two-step reaction using 523 K for chemisorption and 523 K for hydrogenation. However, the amount of CH_x is fourfold higher on Co-Pt/NaY than Co-Pt/Al₂O₃. The synergistic effect can be interpreted by insertion of Co into Pt inside the zeolite cages which causes a preferential coupling of CH _k species vs. its hydrogenation into methane. Separate experiments carried out on the removal of CH _k species with deuterium show that deep dissociation of methane does not occur on bimetallic catalyst and the weakly bonded CH _k species can easily participate in chain building reaction on the surface.     

Oxidative coupling of methane over CaO catalysts promoted with alkali and alkaline earth oxides

CaO catalysts promoted with various elements were examined systematically for the oxidative coupling of methane. 10 mol% Li⁺-CaO and 10 mol% Ba²⁺-CaO gave high C₂ yield (23 to 26% at 1023 to 1073 K for 10% Li⁺-CaO) under CH₄ and O₂ partial pressure of 2.8 and 1.4 kPa respectively. A three components catalyst, 10 mol% Li⁺-5 mol% Ba²⁺-CaO, was also found to be an effective catalyst, given that the reaction temperature to give the maximum C₂ yield was lower than those for the two components catalysts under the same reaction condition. Quenching the flow down stream the reactor improved C₂ yield.     

Oxidative dehydrodimerization of methane over complex oxide catalysts prepared by self-propagating high-temperature synthesis

Oxidative dehydrodimerization of methane to C₂ - hydrocarbons has been examined for complex oxide ceramics prepared by using method of self-propagating high-temperature synthesis (SHS) in a combustion mode. The novel catalysts containing rare-earth, alkali-earth metals and copper showed sufficiently high level of activity, C₂ selectivity and stability in the presence of oxygen at temperature 700–800C, atmospheric pressure, the mole ratio CH₄/O₂= 5–8 and contact time 0.6–9.0 s.     

Oxidative coupling of methane over sodium-salt-promoted zirconia catalysts prepared by the mixed solution method

Sodium-salt-promoted zirconia catalysts prepared from mixed solutions of zirconyl chloride and sodium salts (chloride, carbonate, nitrate or sulfate) are found to exhibit effective catalytic performances in the oxidative coupling of methane. The highest C₂₊ yield (16.6%) and ethylene-to-ethane ratio are obtained over the Na₂CO₃-added catalyst having an appropriate Na/Zr ratio; however, the active promoting species in this catalyst are actually identified to be Na⁺ and Cl⁻. The mixed solution method is considered to be an effective preparation method when proper precursor compounds of zirconia and promoters are chosen, giving a desirable interaction between them. The reaction and characterization results indicate that the chlorine ion plays a more prominent role than the sodium ion, and the X-ray photoelectron spectroscopy results show the presence of multiple chlorine species on the surface. It is suggested that the Cl⁻ bound to Na⁺ is capable of activating both methane and ethane whereas the Cl⁻ bound to Zr⁴⁺ is only capable of activating ethane. The sodium ion is considered to play an important role in stabilizing the chlorine ion.