MgO/La2O2CO3催化剂上的低温甲烷氧化偶联

共沉淀法制备的MgO/La2O2CO3催化剂使甲烷氧化偶联(OCM)在炉温460℃时开始反应,且使反应在50℃炉温下至少24 h。助剂Ni的加入降低了起始和最低反应温度,使催化剂在380℃的炉温下开始反应,之后在无热源的情况下可使反应至少24 h。助剂Zn的加入提高了反应活性,使C2的选择性提高了6%,但同时对低温反应不利,反应在炉温100℃下6 h后自动停止。OCM体系中的强放热反应为OCM温和反应提供了热源。催化剂中的La2O2CO3是维持低温甲烷氧化偶联反应的关键H活性组分。     

沉淀剂对SrCO_3/La_2O_2CO_3上低温甲烷氧化偶联反应的影响及反应中的热点效应

用共沉淀法制备了系列SrCO3/La2O2CO3催化剂,制备中沉淀剂的选择影响催化剂的物理化学性质,并最终决定其在低温甲烷氧化偶联(OCM)中的催化性能,其中以n(NaOH)∶n(Na2CO3)=2∶1的NaOH/Na2CO3为复合沉淀剂效果最好,对应的催化剂中检测到两种La2O2CO3的组分,分别为四方晶相的(Ⅰ-)和六方晶相的(Ⅱ-)La2O2CO3。这两种晶相的共存为OCM的低温反应提供所需要的活性位。助剂SrCO3抑制了甲烷的过度氧化,提高了C2的选择性。所得到的最佳催化剂能在100℃炉温下维持OCM反应至少24 h,使CH4转化率达25.6%,C2选择性达43.4%。伴随OCM的甲烷氧化生成COx的副反应产生的热点效应为OCM温和反应提供了热源。     

低温燃烧法制备纳米La2O2CO3

采用低温燃烧合成法,以尿素为燃烧剂,以聚乙二醇为活化剂,加入少量的柠檬酸做引发剂,通过与硝酸镧反应制备碳酸氧镧,以XRD、TG-DTA、TEM等分析手段进行表征,系统考察焙烧时间、焙烧温度等工艺参数对最终产物的组成和粒径的影响,确定最佳的工艺条件.实验结果表明,以尿素为燃烧剂制备纳米碳酸氧镧是可行的,制备碳酸氧镧的最佳...     

甲烷氧化偶联NaWMnO/SiO_2类催化剂低温催化性能的研究进展

本文将甲烷氧化偶联制乙烯(Oxidative coupling of methane, OCM)反应的NaWMnO/SiO_2类催化剂中在700℃以下具有显著催化活性的称为NaWMnO/SiO_2低温催化剂,在催化剂的组成、制备方法及催化性能等方面综述了近年来的研究进展。同时对于其存在的潜在工业化应用前景做了概述。     

低温甲烷氧化偶联稀土氧化物催化剂的研究进展

在甲烷氧化偶联制乙烯(Oxidative coupling of methane,OCM)反应中,在700℃以下具有显著催化活性的催化剂称为低温催化剂。从低温催化剂的组成、制备方法及催化性能等方面综述了近年来Re_xO_y(Re=La、Y、Sm和Nd等稀土元素)稀土氧化物类低温催化剂的研究进展。对于具有潜在工业化应用前景的催化剂进行了简要分析。     

甲烷氧化偶联反应的研究进展

通过对于甲烷氧化偶联法制取乙烯的反应过程中的各方面性能研究,对其进行具体分析,探讨针对不同性能的催化剂对于反应产物的各方面影响来进行具体分析。同时对于不同反应条件下的不同反应方面的影响也进行细致的研究分析,通过对于甲烷氧化偶联反应过程中的催化剂的催化机理进行研究。     

新型合成气甲烷化催化剂La2O3-ZrO2-Ni/Al2O3的制备与性能

针对常规合成气甲烷化催化剂高热结构稳定性差、活性低、适应性差等不足,本文创新地引用稀土金属氧化物La₂O₃复配过渡金属氧化物ZrO₂作为多功能复合助剂,利用反向沉淀法制备了新型合成气甲烷化催化剂La₂O₃-ZrO₂-Ni/Al₂O₃,同时制备催化剂Cr₂O₃-Ni/Al₂O₃作为参照组。采用X射线衍射（XRD）、透射电子显微镜（TEM）表征了催化剂的微观结构,并利用N₂吸附仪（BET）测量催化剂经高温水热处理前后的微孔结构参数,以考察催化剂的高热结构稳定性。结合国内某大型煤制天然气项目工艺特征和运行实践,应用Aspen Plus软件模拟了四段甲烷化工艺理论平衡值。基于自主固定床合成气甲烷化评价实验装置,考察了反应压力、空速和原料气H₂O(g)含量等因素对La₂O₃-ZrO₂-Ni/Al₂O₃催化性能的影响,并开展了1000h长周期寿命评价实验。结果表明,La₂O₃-ZrO₂-Ni/Al₂O₃比Cr₂O₃-Ni/Al₂O₃具有更优的高热结构稳定性;可使CO和CO₂反应达到或接近催化剂床层出口温度下的理论平衡状态,呈现显著的宽温活性;活性组分NiO晶粒尺寸介于7~10nm,分散度较高;对反应压力、空速和原料气H₂O(g)含量的变化不敏感,具有良好的操作弹性;1000h反应后仍能保持较高的活性和稳定性。     

锶掺杂的La2Ce2O7烧绿石催化甲烷氧化偶联性能研究

基于烧绿石类材料热稳定性高、晶格氧流动性可调控的特点,通过溶胶-凝胶法制备了La₂Ce₂O₇催化剂,向其中掺杂碱土金属Sr,研究了其在甲烷氧化偶联（OCM）反应中的催化性能。通过一系列表征确认了La₂Ce₂O₇样品的缺陷立方萤石结构;锶掺杂不仅显著提高了晶格氧的活动性,还提升了催化剂表面强碱位点的比例;与OCM反应活性关联发现,晶格氧流动性增强以及表面强碱位点的增加是提高OCM反应活性的关键因素;在优化的La_1.5Sr_0.5Ce₂O₇催化剂上,800℃下可以获得57%的C₂选择性和14%以上的C₂收率,连续反应30 h性能保持稳定。     

助剂MgO对CO甲烷化Ni-Al2O3催化剂结构和性能影响

采用溶液燃烧法制备CO甲烷化Ni基催化剂,考察了助剂MgO添加量对Ni-Al_2O_3催化剂结构和性能的影响,并初步探讨了MgO添加量-催化剂结构-CO甲烷化性能的构效关系。结果显示,质量分数6%MgO添加量的催化剂具有适宜的MgO晶体含量、适当的还原温度和较高的比表面积,其CO转化率、CH_4选择性和收率分别高达99%,97%和94.5%。寿命实验表明:在24 h反应时间内,质量分数6%MgO添加量的催化剂CO转化率和CH_4选择性分别高达97%和95%以上,表现出较高的活性、选择性和稳定性。     

LiCl/MnO_2-H_3BO_3体系甲烷氧化偶联催化剂的研究

由于石油资源的日见枯竭 ,天然气在化工原料中的地位日益突出。对天然气的主要成分甲烷、氧化偶联 (OCM)制备乙烯的催化剂—— Li Cl/Mn O2 - H3 BO3 进行了系统研究并探讨了其组分及外界反应条件对该催化剂性能的影响 ,发现了可能的活性组分 :立方晶型的 Li4 B7O12 Cl。     

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.     

Photoluminescence properties of La2O3 methane coupling catalysts

La₂O₃ catalysts prepared at 923 K (La₂O₃-LT) and 1073 K (La₂O₃-HT) exhibit different photoluminescence properties due to notably different concentrations of ions in position of low coordination at the surface or coordinatively unsaturated surface sites (cus). The catalyst which exhibits the higher yields of photoluminescence due to the higher concentration of cus corresponds to the one which gives the higher C₂₊ selectivity in the oxidative methane coupling reaction. On leave of absence from Laboratoire de Réactivité de Surface, Université Pierre et Marie Curie, URA 1106-CNRS, 75252 Paris Cedex, France.     

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.     

A study of the oxidative coupling of methane over SrO-La2O3/CaO catalysts by using CO2 as a probe

20%SrO-20%La₂O₃/CaO catalyst (SLC-2), prepared by impregnation, has shown 18% CH₄ conversion and 80% C₂-selectivity for the oxidative coupling of methane (OCM) at 1073–1103 K with CH₄O₂ molar ratio=91 and total flow rate of 100 ml/min. Addition of SrO onto La₂O₃/CaO (LC) catalyst strengthens the surface basicity and leads to an increase in CH₄ conversion and C₂-selectivity. Meanwhile, the reaction temperature required to obtain the highest C₂-yield increases with increasing SrO content. The formation of carbonate on the catalyst surface is the main reason for the deactivation of LC and SLC catalysts. If the amount of CO₂ added into the feed is appropriate and the reaction temperature is high enough, there is no deactivation at all. In such case, the added CO₂ will suppress the formation of CO₂ produced via the OCM reaction, therefore, improves the C₂-selectivity. The FT-IR spectra of CO₂ adspecies recorded at different temperatures show that CO₂ interacts easily with the catalyst surface to form different carbonate adspecies. Unidentate carbonate is the main CO₂ adspecies formed on the catalyst surface. On the LC catalyst surface, the unidentate carbonate was first formed on Ca²⁺ cations at room temperature. If the temperature is higher than 473 K, it will form on La³⁺ cations. On the SLC catalyst surface, if the temperature is lower than 573 K, only the unidentate carbonate formed on Ca²⁺ cations could be observed. When the temperature is higher than 673 K, it will then form on Sr²⁺ cations. This suggests that the unidentate carbonate can migrate on the LC and SLC catalyst surface on one hand, and on the other hand, that the surface composition of SLC catalysts is dynamic in nature. On the basis of both the decomposition temperatures of the carbonate species, and the temperature dependence of the value which is the difference of symmetric and asymmetric stretching frequencies of surface carbonates, the in situ FT-IR technique offered two approaches to measure the surface basicity of the SLC catalyst. The results thus obtained are in good agreement with that of CO₂-TPD. The role of the surface basicity of the SLC catalyst is also discussed.     

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.     

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.     

Ce-Doped La2O3 based catalyst for the oxidative coupling of methane

The effect of ceria was studied on the oxidative coupling of methane (OCM) with Ce-doped La₂O₃ in a La:Ce molar ratio of 75:25 using two preparation methods. The characterisation techniques used were XRD and XPS. The results revealed high concentration of oxygen vacancies. Different types of ions (Ce^3 + + Ce^4 +) were detected. More surface Ce^3 + and higher ratio [(O₂^2― + O^—)/ O^2―] were obtained in the oxide synthesised by the solvothermal method, affecting the OCM reaction in terms of higher C₂ hydrocarbons selectivity. This was ascribed to the higher relative amount of O^― species on the catalyst surface.     

Deactivation of NaCl/B2O3/Fe2O3 catalysts and their improvement for the oxidative coupling of methane

In the present work, the deactivation of the NaCl/B₂O₃/Fe₂O₃ catalysts was studied for the oxidative coupling of methane. Several techniques, such as XPS, XRD, SEM, H₂-TPR, and flow-reaction, were employed to examine the function of each catalyst component, and its change during the catalytic reaction. NaCl and B₂O₃ show a synergistic effect on the Fe₂O₃ surface. B₂O₃ modifies the oxidative activity of Fe₂O₃ and makes the first reduction peak of Fe₂O₃ shift from 490 to 750°C. The NaCl modified B₂O₃/Fe₂O₃ catalyst has high reduction rate, high activity and selectivity at about 750°C. It is demonstrated that the deactivation of the NaCl/B₂O₃/Fe₂O₃ catalysts is a complicated process, consisting of chloride loss, sodium change, B₂O₃ loss, silica deposition and catalyst sintering. The chloride loss enhances the surface basicity, which causes the silica deposition and sodium change, and aggravates the catalyst sintering. The silica deposition and catalyst sintering cause permanent deactivation. The B₂O₃ loss is not a direct reason for catalyst deactivation. NaCl crystal diluted NaCl/B₂O₃/Fe₂O₃ catalysts have a better stability. The deactivated catalyst has a more stable structure. When it is regenerated by impregnating with NaCl again, a more stable catalyst can be obtained.     

Scale-up analysis of autothermal operation of methane oxidative coupling with La2O3/CaO catalyst

The exothermicity of oxidative coupling of methane (OCM) renders a cooled packed-bed reactor impractical or impossible. Recently, we proposed an adiabatic autothermal reactor as a solution to this problem and reported the first results for stable autothermal operation (AO) with feed at ambient temperature. AO on the ignited branch is possible only in the region of steady-state multiplicity. High per-pass conversion and productivity requirements demand a stable ignited branch at the lowest possible feed temperature and high flow rate. To achieve OCM scale-up, many conditions must be satisfied simultaneously. Using a kinetic model for La₂O₃/CaO catalyst, we examine the impact of space time, feed methane to oxygen ratio, feed temperature, particle size, inter-phase heat and mass transfer gradients, pore-diffusion, bed scale heat/mass dispersion on the region of AO for large scale adiabatic packed-bed reactors. We show that while it is possible to achieve CH₄ conversion of about 20% and C₂ selectivity of about 80% in scaled-up reactors, these values are sensitive to the design and operating parameters.