Oxidative dehydrogenation of ethane over some titanates based perovskite oxides

A series of perovskite catalysts have been tested for the oxidative dehydrogenation of ethane. The composition of these catalysts covered CaTi_1–x Fe _x O_3–, with 0 x 0.4, SrTi_1–x Fe _x O_3–, with 0 x 1.0, as well as mixtures of these. The latter catalysts containing more basic Sr metal showed higher selectivity to ethene than the former catalysts containing Ca. A few catalysts with Co on B-sites in the lattice were tested, but lost their stability above 923 K, resulting in a substantial change in the product selectivity. The perovskites gained activity when Fe was introduced in the lattice to form hypervalent ions (Fe⁴⁺) which are believed to play a role in the catalytic activity of these materials.     

Oxidative coupling of methane over NbO (p-type) and Nb2O5 (n-type) semiconductor materials

Oxidative coupling of methane to higher hydrocarbons was investigated using two types of semiconductor catalysts, NbO (p-type) and Nb₂O₅ (n-type) at 1 atm pressure. The ratio of methane partial pressure to oxygen partial pressure was changed from 2 to 112 and the temperature was kept at 1023 K in the experiments conducted in a cofeed mode. The results indicated a strong correlation between C₂₊ selectivity performance and the electronic properties of the catalyst in terms of p-vs. n-type conductivity. The p-type semiconductor catalyst, NbO, had a larger selectivity (e.g. 95.92%) over the n-type Nb₂O₅ catalyst (23.08%) both at the same methane conversion of 0.64%. Catalyst characterization via X-ray diffraction, TGA and reaction studies indicated that NbO was transformed to Nb₂O₅ during the course of the reaction which limits catalyst life.     

Partial oxidation of methane to synthesis gas over some titanates based perovskite oxides

Ca_1–x - _x Sr _x TiO₃-based mixed oxide catalysts containing chromium, iron, cobalt or nickel were prepared and used in the oxidation of methane. The catalyst containing cobalt or nickel showed high activity for the synthesis gas production from methane. In the case of nickel containing catalyst, nickel oxide originally separated from the perovskite structure was easily reduced to nickel metal, which showed synthesis gas production activity. In the case of the cobalt containing catalyst, pretreatment with methane was required for high activity. Reduced metallic cobalt was formed from the perovskite structure, which revealed relatively high selectivity for the oxidative coupling of methane, and afforded synthesis gas production. Both the catalysts also catalyzed carbon dioxide reforming of methane and especially both high activity and selectivity were observed over the nickel containing catalyst.     

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 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 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 over alkali metal chloride promoted zirconia: Effect of the preparation method

Promoting ZrO₂ with Na⁺Cl⁻ via a sol-gel process, an effective catalytic system is obtained which is more active, selective and stable with time-on-stream than unpromoted ZrO₂, or any other alkali chloride promoted ZrO₂ in the oxidative coupling of methane at 750°C, atmospheric pressure, CH₄/O₂ = 4 and a space velocity of 7500 cm³g⁻¹h⁻¹. Operating at a low partial pressure of the reactants (CH⁴+O₂ = 91.2 Torr, 1 Torr = 133.3 Pa) and the aforementioned conditions, a C₂ yield of 14.1% is obtained. The performance of the alkali metal chloride promoted ZrO₂ is found to depend strongly on the nature of the cation, the method of catalyst preparation, the promoter content and the reaction conditions (temperature, partial pressure, CH₄/O₂ ratio and space velocity). The relationship between the catalytic performance and the physico-chemical properties of Na⁺-ZrO₂-Cl⁻ revealed by X-ray diffraction and X-ray photoelectron spectroscopy is explored.     

EXAFS and FTIR characterization of tetrarhodium carbonyl clusters attached on tris-(hydroxymethyl)phosphine grafted silica catalytically active for olefin hydroformylation

A series of perovskites of the formula Ca_1–x Sr _x Ti_1–y M _y O_3–, M=Fe, Co, Cr or Ni,x = 0–1,y = 0–0.6, has been synthesized by a modified sol-gel method using citrate. Several of these materials were proved to be stable under operating conditions in reducing atmospheres of air and hydrocarbons. An outline of the synthesis procedure is given, together with the results of XRD, SEM, BET, TG, DTA and IR characterization before and after catalytic testing. The solubility of Ni and Cr in this perovskite was very limited, and the solubility of Co decreased abruptly above 1173 K. The solubility range of Ca and Sr on alkaline earth sites is 100%.     

Oxidative coupling of methane over Sm-promoted MgO: Influence of composition and preparation conditions

Influences of promoter concentration (or Sm/Mg ratio), precursor for MgO (viz. Mg-acetate, Mg-carbonate and Mg-hydroxide), calcination temperature of Sm-promoted MgO catalyst on the catalytic activity/selectivity in the oxidative coupling of methane (OCM) at different temperatures (650–850°C) and CH₄/O₂ ratios in feed (2·0–8·0) at a high space velocity (51600 cm³ g⁻¹ h⁻¹) have been investigated. The catalytic activity/selectivity of Sm–MgO catalysts in the OCM are found to be strongly influenced by the Sm/Mg ratio, precursor used for MgO and catalyst calcination temperature. The catalyst with Sm/Mg ratio of 0·11, prepared using magnesium acetate and magnesium carbonate as a source of MgO and calcining at 950°C, is found to be highly active and selective in the OCM process. A drastic reduction in catalytic activity/selectivity is observed when the catalyst is supported on low surface area porous catalyst carriers, indicating strong catalyst–support interactions. ©1997 SCI     

Oxidative coupling of methane catalyzed by Li, Na and Mg doped BaSrTiO3

The XBaSrTiO₃ (X = Li, Na, Mg) with perovskite structure was prepared by impregnation of LiCl, NaCl or MgCl₂ solution on BaSrTiO₃ (BST) surface. The products were characterized with XRD, SEM, UV, FT-IR and Raman spectroscopy. Determination of band gap, conduction and basicity of the prepared catalysts revealed that NaBST exhibits the lowest band gap and the most conduction and basicity. The catalytic performances of XBSTs for the oxidative coupling of methane (OCM) in a tubular continuous flow reactor were evaluated. The NaBST showed the maximum catalytic effect on methane conversion (47%), C₂₊ selectivity (51%) and ethylene yield (24%) at the temperature of 800 °C.     

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 by carbon dioxide: a highly C2 selective La2O3/ZnO catalyst

In the oxidative coupling of methane by carbon dioxide, La₂O₃/ZnO catalysts were found to have high C₂ selectivity and good stability. The coupling selectivity on La₂O₃/ZnO is about 90%, which is much higher than that on pure La₂O₃ or ZnO. The consumption ratio of carbon dioxide to methane is approximately one. X-ray diffraction analysis reveals that the structural forms of the oxides are unchanged during the reaction. The reaction mechanism for C₂ formation is discussed.     

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.     

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.     

Oxidative coupling of methane at moderate (600–650°C) temperatures

The presented results of methane oxidative coupling indicate that the lowering of the reaction temperature below 700°C, without any loss of its effectiveness, requires a much longer contact time than the times applied in a great majority of studies reported so far.     

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 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 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.     

Studies on oxidative coupling of methane using Sm2O3-based catalysts

The effects of Mn/Na₂WO₄, Li, and CaO loading on the monoclinic Sm₂O₃ catalyst were investigated for the oxidative coupling of methane using O₂ or N₂O as an oxidant. The catalysts were prepared by wet impregnation method and characterized by XRD, BET, CO₂-TPD, and XPS analysis. Impregnation of Mn/Na₂WO₄ on monoclinic Sm₂O₃ resulted in the formation of Sm_2?xMn_xO₃ phase, decreasing the catalytic performance. Li impregnation increased the C₂ selectivity but decreased the catalytic activity. The SmLiO₂ formation increased the catalytic activity and selectivity. High amounts of CaO impregnation increased the C₂ selectivity of monoclinic Sm₂O₃ without a loss in catalytic activity. 6Li/m-Sm₂O₃ were found unstable due to the Li loss from the catalyst. The 15CaO/m-Sm₂O₃ was quite stable and showed 8.2% ethylene yield with N₂O use, much higher than that was obtained with the well-known 2Mn/5Na₂WO₄/SiO₂ and 4Li/MgO catalysts. N₂O was more selective than O₂ as an oxidant and enhanced ethylene formation.     

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–.