Study on the unsteady state oxidative coupling of methane: effects of oxygen species from O2, surface lattice oxygen, and CO2 on the C2+ selectivity

Metal Ions (Li, Mg, Zn, Ce) Doped into La2O3 Nanorod for Boosting Catalytic Oxidative Coupling of Methane

A series of La2O3 nanorod catalysts with doping of active metal ions (Li, Mg, Zn and Ce) were synthesized successfully by the hydrothermal method. The La2O3 nanorods show a uniform size with the length of 50–200 nm and the width of 5–20 nm, and the {110} crystal facet is a preferentially exposed surface. The active metal ions (Li, Mg, Zn and Ce) doped into the lattice of La2O3 nanorods enhance the selectivity of the desired products during oxidative coupling of methane (OCM) and decrease the reaction temperature. Among these catalysts, the Mg-La2O3 catalyst exhibits the best catalytic performance during the OCM reaction, i.e., its selectivity and yield of C2 products at 780 °C is 73% and 21%, respectively. The effect of doped metal ions on catalytic activity for OCM was systematically investigated. Insight into the fabrication strategy and promoting factors of the OCM reaction indicates the potential to further design a high-efficient catalyst in the future.

Raw Biogas as Feedstock for the OCM Process

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 CO2, 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.

Na2WO4/Mn/SiO2 Catalyst Pellets for Upgrading H2S-Containing Biogas via the Oxidative Coupling of Methane

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.

Alkali-Added Catalysts Based on LaAlO3 Perovskite for the Oxidative Coupling of Methane

In this study, we aimed to enhance the catalytic activity of perovskite catalysts and elucidate their catalytic behavior in the oxidative coupling of methane (OCM), using alkali-added LaAlO3 perovskite catalysts. We prepared LaAlO3_XY (X = Li, Na, K, Y = mol %) catalysts and applied them to the OCM reaction. The results showed that the alkali-added catalysts’ activities were promoted compared to the LaAlO3 catalyst. In this reaction, ethane was first synthesized through the dimerization of methyl radicals, which were produced from the reaction of methane and oxygen vacancy in the perovskite catalysts. The high ethylene selectivity of the alkali-added catalysts originated from their abundance of electrophilic lattice oxygen species, facilitating the selective formation of C2 hydrocarbons from ethane. The high COx (carbon monoxide and carbon dioxide) selectivity of the LaAlO3 catalyst originated from its abundance of nucleophilic lattice oxygen species, favoring the selective production of COx from ethane. We concluded that electrophilic lattice oxygen species play a significant role in producing ethylene. We obtained that alkali-adding could be an effective method for improving the catalytic activity of perovskite catalysts in the OCM reaction.

Exploring the formation of carbonates on La2O3 catalysts with OCM activity

Two series of La2O3 samples with identical bulk structures but different morphologies indicate substantially different carbonate forming pathways, which provides insight into the related oxidative coupling of methane (OCM) reaction.