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 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 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.     

Low temperature catalysts for oxidative coupling of methane

A simple review is given to the recent work of the oxidative coupling of methane at low temperature. Emphasis is laid on the different systems of low-temperature catalysts under conventional CH₄/O₂ co-feed conditions, and on the investigations of low-temperature oxidative coupling of methane in the presence of steam in the feed. Other approaches, e.g. oxidative coupling of methane at elevated pressure and moderate temperature, preparing ethylene by oxidative coupling reaction of methane on laser-activated solid surface, are also included.     

Effect of strontium doping on catalytic behaviour of lanthanum oxide on oxidative coupling of methane

The doping of lanthanum oxide with strontium maintains the good selectivity of the oxidative methane coupling for the catalysts prepared or calcined at high temperature (> 800 °C) by preserving the platelet shape of oxide particles.     

The oxidative coupling of methane by sodium promoted lanthanide catalysts

The characteristics of La₂O₃ catalysts with three different supports (BaCO₃, MgO, and ZnO), two different alkaline metal promoters (Na₂O and Na₂CO₃), and a china clay have been investigated. The relationship of the surface basicity of the support to the activity and selectivity of the catalyst were studied by TPD. The results showed that the surface basicity of these three supports decreased in the following order: BaCO₃ > MgO > ZnO. The choice of Na₂O as a promoter in La₂O₃/(BaCO₃ or MgO) had a better effect than the choice of Na₂CO₃. The Na₂O promoter could increase not only the catalytic activity and the C₂ selectivity, but also lower the optimal reaction temperature. Addition of 15% clay into the La₂O₃–alkaline metal catalysts could increase the strength of the catalyst and lower the optimum reaction temperature.     

Methane coupling catalysts based on zirconium combined with lanthanum

Two series of zirconium and lanthanum oxide based catalysts were synthesised, characterised (XRD, ESCA, BET) and tested for their effectiveness in the oxidative coupling of methane (OCM). Starting materials and the preparation method (ZrO₂ and wet impregnation, ZrO(NO3)₂·₄H₂O + La(NO3)₃·6H₂O and coprecipitation) determined the bulk composition of the catalysts (ZrO₂ + La₂O₃, La₂O₃ * 2ZrO₂). Doping with alkali (Li, Na, K) or alkaline earth metal (Mg, Ca, Sr) increased the selectivity to hydrocarbons. The best C₂-hydrocarbon yields were obtained with the Sr-doped ZrO₂ + La₂O₃ catalyst (16.1%) and the Li-doped La₂O₃ * 2ZrO₂ catalyst (17.3%).     

Oxidative dimerisation of methane on supported palladium oxide catalysts

Supported palladium oxide catalysts are able to convert CH₄ to C₂H₆, CO, CO₂, H₂ and H₂O at temperatures 315 °C. Catalysts did not show any support effect when TiO₂, Al₂O₃, ZrO₂, La₂O₃ and MgO were used as supports. With sequential O₂ pulsing the catalyst showed long term activity when used at temperatures below 400 °C. Addition of Pt increased selectivity whereas with Ga it decreased. Results indicate participation of lattice O₂ from catalyst in the reaction pathway.     

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.     

New efficient catalysts for the oxidative coupling of methane

During calcination of OCM catalyst precursors, Li...Cs spectacularly lower the amorphous silica -cristobalite phase transition temperature, shown here to be a critically important requirement for production of effective catalysts. Incorporation of W switches on OCM activity and newly discovered K/W and Rb/W formulations exhibit unsurpassed ethylene selectivity at high methane conversion. Addition of Mn significantly improves the performance of the former. An alkali-stabilised tungsten oxo species is thought to be the OCM active site.     

Cation effects in the oxidative coupling of methane on phosphate catalysts

The conversion of methane and the selectivities to the various products have been measured at 700 and 775 °C on a variety of phosphates of La(III), Zr(IV), V(V), Cr(III), Mn(III), Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Zn(II), Al(III), B(III), Pb(II), Bi(III) and Sm(III) in the presence and absence of carbon tetrachloride. The conversions reach as high as 30 and 49% at 700 and 775 °C, respectively, with methane and oxygen at partial pressures of 200 and 25 Torr, respectively. The highest C₂₊ selectivities (61 and 82%, respectively) were obtained for lead(II) phosphate at 700 and 775 °C, respectively. In general the conversions and C₂₊ selectivities are enhanced on addition of carbon tetrachloride (1.1 Torr) to the feedstream, although there are notable exceptions. Significantly high selectivities to formaldehyde are observed with a number of the catalysts, in particular 32% with boron(III)phosphate.     

Comparison of the surface and catalytic properties of rare earth‐promoted CaO catalysts in the oxidative coupling of methane

Rare earth (viz. La, Ce, Sm, Nd and Yb) promoted CaO catalysts have been investigated, comparing their surface properties (viz. surface area and basicity/base strength distribution) and catalytic activity/selectivity in the oxidative coupling of methane at different reaction conditions (temperatures, 650–800 °C, CH₄/O₂ ratios, 2.0–8.0 and space velocity, 51 360 cm³ g^?1 h^?1). The surface properties and catalytic activity/selectivity are strongly influenced by the rare earth promoter and its concentration. Apart from the Sm‐promoted CaO catalyst, both the total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) are decreased due to the promotion of CaO by rare earth metals (viz. La, Ce, Nd and Yb). The catalytic activity/selectivity is strongly influenced by the temperature, particularly below ?700 °C, whereas at higher temperature no further effect is seen. The La₂O₃? CaO, Nd₂O₃? CaO and Yb₂O₃? CaO catalysts showed high activity and selectivity, and also their results are comparable. Among the catalysts, Nd‐promoted CaO (with Nd/Ca = 0.05) showed the best performance (19.5% CH₄ conversion with 70.8% C₂₊ selectivity) in the oxidative coupling of methane. A close relationship between the surface density of total and strong basic sites (measured in terms of CO₂ chemisorbed at 50° and 500 °C respectively) and the C₂₊ selectivity and/or C₂₊ yield has been observed. Copyright © 2005 Society of Chemical Industry     

Transient isotopic labeling studies using12CH4/13CH4 over alkali-promoted molybdate catalysts in the oxidative coupling of methane

The methane coupling reaction was investigated over MnMoO₄ and alkali (Li, Na, K)-promoted MnMoO₄ at 700°C using¹²CH₄/¹³CH₄ transient isotopic labeling experiments under steady-state reaction conditions. The variations observed in the surface residence times of CH₄, CO, and CO₂, ethane and ethylene were used to help explain selectivity differences previously reported in the oxidative coupling of methane over alkali-promoted molybdate catalysts.     

Methane oxidative coupling over fluoro-oxide catalysts

LaF₃ modified ZrO₂, CeO₂ and ThO₂ catalysts for the oxidative coupling of methane indicate that ZrO₂/LaF₃, CeO₂/LaF₃ and ThO₂/LaF₃ catalysts have high activity and high selectivity at low temperature. In the temperature region 480–650 °C, the methane conversion is 24.38–30.8% and the C ₂ ⁼ selectivity is 40–55.38%. The characterization of oxygen species on the catalysts indicates that, because of the modification of LaF₃ to ZrO₂, CeO₂ and ThO₂, it is favourable for the activation of O₂.Patent application No. CN 92105258.8 inventors: Xiaoping Zhou, Huilin Wan and K.R. Tsai.     

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 over some titanates based perovskite oxides

A series of perovskites of the formula Ca_1–xSr_xTi_1–yM_yO_3– (M = Fe or Co,x = 0–1,y = 0–0.6 for Fe,y = 0–0.5 for Co) were prepared and tested as the catalyst for the oxidative coupling of methane. The catalysts were stable under the reaction conditions. The catalysts of high p-type and oxide ionic conductivity afforded the high selectivity. Some catalysts containing Co on B-sites are thermally unstable and decomposed to metal oxide components at high temperature, giving rise to synthesis gas production.     

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–.     

MnOx/Mg-Al2O3催化甲烷燃烧反应的研究

在Al2O3上浸渍Mg（NO3）2溶液,焙烧后制得Mg改性Al2O3（Mg-Al2O3）。以Mg-Al2O3为载体,制备了MnOx/Mg-Al2O3系列催化剂,测试了这些催化剂对甲烷燃烧反应的催化活性。结果表明：Mg的加入有效抑制了Al2O3在高温焙烧时发生γ相变,提高了Al2O3的热稳定性,MnOx/10%Mg-Al2O3对甲烷燃烧的催化活性较高,Mg的适宜加入量为10%。     

Influence of the presence of chlorine on the selectivity to ethane and ethene during methane coupling over samarium-based catalysts

The oxidative coupling of methane to ethane and ethene has been investigated on chlorine-containing catalysts. The sensitivity of the product distribution to temperature, gas composition, flow-rate, catalyst mass, and reactor dimensions has been demonstrated. It has been found that a long contact time is an important factor in raising the C₂H₄/C₂H₆ ratio, and that reactions within the catalyst bed are important for the conversion of ethane to ethene. Back-mixing of the reagents into the gas space above the catalyst bed tended to lead to combustion, but appeared to have little influence on enhancing the C₂H₄/C₂H₆ ratio. Experiments have been performed with Sm₂O₃, SmOCl and SmCl₃ and with various pairs of these catalysts when separated by a gas space. These experiments have demonstrated the importance, under certain circumstances, of gas phase chlorine species, especially radicals, in the conversion of ethane to ethene. However, the results also show that these chlorine-induced gas phase reactions occur within the voids between the particles in the catalyst bed and not in the free gas space outside the catalyst bed.