Search Results (6)

C₂₊ hydrocarbons, especially C₂₊ olefins, as important basic chemical raw materials, mainly come from petroleum cracking. With the increasing scarcity of petroleum resources, the search for new olefins production routes has become the focus of research, and the production of olefins by the oxidative coupling of methane (OCM) process has attracted extensive attention. The OCM route is an important alternative to the production of olefins from petroleum resources and is also an important direction for the development of efficient and clean utilization of natural gas. In this paper, the mechanism, catalysts, and other key factors for the production of olefins by methane oxidative coupling are reviewed. The mechanism of OCM, including the reaction pathway and the formation of intermediate products, is introduced. Then, commonly used catalysts, such as alkali metal/alkaline earth metal oxides, rare earth metal oxides, composite metal oxides with special structures, and classical catalysts Mn/Na₂WO₄/SiO₂, and their mechanisms of action in the reaction are discussed. In addition, the application of chemical looping oxidative coupling of methane (CLOCM) in olefin production is also investigated, which is a promising alternative way due to the high selectivity of olefins and the low cost of catalysts owing to the excellent performance of the catalyst recycling. These studies will help to further understand the mechanism of OCM for olefin production and provide guidance and support for applications in related fields. Full article

The Mn-Na₂WO₄/SiO₂ catalyst is regarded as the most promising catalyst for the oxidative coupling of methane (OCM). Despite its remarkable performance, the Mn-Na₂WO₄/SiO₂ catalyst requires a high reaction temperature (>750 °C) to show significant activity, a temperature regime that simultaneously causes quick deactivation. In the current work, we show that the benefits of this catalyst can be leveraged even at lower reaction temperatures by a using a stacked catalyst bed, which includes also a small amount of 5% La₂O₃/MgO on-top- of the Mn-Na₂WO₄/SiO₂ catalyst. The simple stacking of the two catalysts provides >7-fold higher activity and ~1.4-fold higher C₂ yield at 705 °C compared to Mn-Na₂WO₄/SiO₂ and La₂O₃/MgO, respectively. We specifically show that the enhanced OCM performance is associated with synergistic interactions between the two catalyst domains and study their origin. Full article

The paper presents the research results obtained in the process of oxidative coupling of methane, in which unpurified biogas was used as the feedstock. Biogas obtained from two kinds of biomass materials, i.e., plant materials (potato and beet pulp, Corn-Cob-Mix—biogas 1) and animal waste (waste from fish filleting—biogas 2) was considered. The influence of temperature, the ratio of methane/oxygen and total flows of feedstock on the catalytic performance in oxidative coupling of methane process was investigated. Comparative tests were carried out using pure methane and a mixture of methane-carbon dioxide to simulate the composition of biogas 2. The process was carried out in the presence of an Mn-Na₂WO₄/SiO₂ catalyst. Fresh and used catalysts were characterised by means of powder X-ray diffraction, X-ray photoelectron spectroscopy, and low-temperature nitrogen adsorption techniques. In oxidative coupling of methane, the type of raw material used as the source of methane has a small effect on methane conversion (the differences in methane conversion are below 3%), but a significant effect on the selectivity to C₂. Depending on the type of raw material, the differences in selectivity to C₂ reach as high as 9%. However, the Mn-Na₂WO₄/SiO₂ catalyst operated steadily in the tested period of time at any feedstock composition. Moreover, it was found that CO_2, which is the second main component of biogas in addition to methane, has an effect on catalytic performance. Comparative results of catalytic tests indicate that the CO₂ effect varies with temperature. Below 1073 K, CO₂ exerts a small poisoning effect on methane conversion, while above this temperature the negative effect of CO₂ disappears. In the case of selectivity to C₂₊, the negative effect of CO₂ was observed only at 1023 K. At higher temperatures, CO₂ enhances selectivity to C₂₊. The effect of CO₂ was established by correlating the catalytic results with the temperature programmed desorption of CO₂ investigation. The poisoning effect of CO₂ was connected with the formation of surface Na₂CO₃, whose concentration depends on temperature. Full article

Biogas is a promising renewable energy source; however, it needs to be upgraded to increase its low calorific value. In this study, oxidative coupling of methane (OCM) was selected to convert it to a higher fuel standard. Prior to establishing the scaled-up OCM process, the effect of organic/inorganic binders on catalytic activity was examined. The selection of the binders and composition of the catalyst pellet influenced the pore structure, fracture strength, and catalytic activity of the catalyst pellets. It was also observed that the O₂ supply from the inorganic binder is a key factor in determining catalytic activity, based on which the composition of the catalyst pellets was optimized. The higher heating value increased from 39.9 (CH₄, Wobbe index = 53.5 MJ/Nm³) to 41.0 MJ/Nm³ (OCM product mixture, Wobbe index = 54.2 MJ/Nm³), achieving the fuel standard prescribed in many countries (Wobbe index = 45.5–55.0 MJ/Nm³). The reaction parameters (temperature, gas hourly space velocity, size of the reaction system, and the CH₄/O₂ ratio) were also optimized, followed by a sensitivity analysis. Furthermore, the catalyst was stable for a long-term (100 h) operation under the optimized conditions. Full article

Novel reactor configurations for the oxidative coupling of methane (OCM), and in particular membrane reactors, contribute toward reaching the yield required to make the process competitive at the industrial scale. Therefore, in this work, the conventional OCM packed bed reactor using a Mn-Na₂WO₄/SiO₂ catalyst was experimentally compared with a membrane reactor, in which a symmetric MgO porous membrane was integrated. The beneficial effects of distributive feeding of oxygen along the membrane, which is the main advantage of the porous membrane reactor, were demonstrated, although no significant differences in terms of performance were observed because of the adverse effects of back-permeation prevailing in the experiments. A sensitivity analysis carried out on the effective diffusion coefficient also indicated the necessity of properly tuning the membrane properties to achieve the expected promising results, highlighting how this tuning could be addressed. Full article

The oxidative coupling of methane (OCM) is operated at high temperatures and is a highly exothermic reaction; thus, hotspots form on the catalyst surface during reaction unless the produced heat is removed. It is crucial to control the heat formed because surface hotspots can degrade catalytic performance. Herein, we report the preparation of Mn₂O₃-Na₂WO₄/SiC catalysts using SiC, which has high thermal conductivity and good stability at high temperatures, and the catalyst was applied to the OCM. Two Mn₂O₃-Na₂WO₄/SiC catalysts were prepared by wet-impregnation on SiC supports having different particle sizes. For comparison, the Mn₂O₃-Na₂WO₄/SiO₂ catalyst was also prepared by the same method. The catalysts were analyzed by nitrogen adsorption–desorption, X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The transformation of SiC into α-cristobalite was observed for the Mn₂O₃-Na₂WO₄/SiC catalysts. Because SiC was completely converted into α-cristobalite for the nano-sized SiC-supported Mn₂O₃-Na₂WO₄ catalyst, the catalytic performance for the OCM reaction of Mn₂O₃-Na₂WO₄/n-SiC was similar to that of Mn₂O₃-Na₂WO₄/SiO₂. However, only the surface layer of SiC was transformed into α-cristobalite for the micro-sized SiC (m-SiC) in Mn₂O₃-Na₂WO₄/m-SiC, resulting in a SiC@α-cristobalite core–shell structure. The Mn₂O₃-Na₂WO₄/m-SiC showed higher methane conversion and C₂₊ yield at 800 and 850 °C than Mn₂O₃-Na₂WO₄/SiO₂. Full article