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.

Olivine has been shown to have a good catalytic effect on biomass tar, however, carbon deposition is still one of the adverse conditions that affect its activity. A highly carbon-resistant olivine-supported metallic nickel catalyst, which was synthesized using thermal fusion (TF), was tested in a fixed bed reactor for tar removal. Toluene was selected as a tar destruction model molecule. The current work evaluated three catalysts, namely, raw olivine, TF-olivine (olivine calcined at 1400 °C in air) and TF-Ni/olivine (TF-olivine loaded with 5% wt Ni calcined at 1400 °C in Ar), for toluene steam reforming and investigated the effects of reaction temperature, steam to carbon molar ratio (S/C) and weight hourly space velocity (WHSV) on toluene conversion, carbon deposition and gas composition. Catalytic activity and resistance against carbon deposition were discussed based on physiochemical properties of the catalyst, including BET surface area, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), temperature programmed reduction (TPR) and Laser Raman spectroscopy (LRS). The TF-Ni/olivine catalyst showed the best catalytic performance in terms of both toluene conversion and carbon deposition. A very low carbon deposition (less than 1%) was obtained when the S/C ratio was 0.88 and 1.02. Relative amounts of reducible Ni, Fe and Mg on the surface played a significant role in the catalytic behavior. Thermal fusion at a high temperature made the interaction between active metal and olivine structure stronger. The toluene conversion increased from 73.7% to 99.6% with the reaction temperature ranged from 750 °C to 950 °C, however it decreased form 92.4% to 82.6% when the WHSV increased from 0.77 h⁻¹ to 1.35 h⁻¹, similarly it decreased slightly (96.9% to 86.2%) with the S/C ratio ranging from 0.29 to 1.02. When comparing the characteristics between fresh and spent TF-Ni/olivine, the main structure and surface topography remained unchanged.

In this manuscript, glycerol was used in corncobs’ pretreatment to promote levoglucosan production by fast pyrolysis first and then was further utilized as raw material for chemicals production by microbial fermentation. The effects of glycerol pretreatment temperatures (220–240 °C), time (0.5–3 h) and solid-to-liquid ratios (5–20%) were investigated. Due to the accumulation of crystalline cellulose and the removal of minerals, the levoglucosan yield was as high as 35.8% from corncobs pretreated by glycerol at 240 for 3 h with a 5% solid-to-liquid ratio, which was obviously higher than that of the control (2.2%). After glycerol pretreatment, the fermentability of the recovered glycerol remaining in the liquid stream from glycerol pretreatment was evaluated by Klebsiella pneumoniae. The results showed that the recovered glycerol had no inhibitory effect on the growth and metabolism of the microbe, which was a promising substrate for fermentation. The value-added applications of glycerol could reduce the cost of biomass pretreatment. Correspondingly, this manuscript offers a green, sustainable, efficient and economic strategy for an integrated biorefinery process.

A plasma–catalyst hybrid system has been developed for the direct conversion of methane to C₂₊ hydrocarbons in dielectric barrier discharge (DBD) plasma. TiO₂ presented the highest C₂₊ yield of 11.63% among different dielectric materials when integrated with DBD plasma, which made us concentrate on the TiO₂-based catalyst. It was demonstrated that MnTi catalyst showed the best methane coupling performance of 27.29% C₂₊ yield with 150 V applied voltage, without additional thermal input. The catalytic performance of MnTi catalyst under various operation parameters was further carried out, and different techniques, such as X-ray diffraction, X-ray photoelectron spectroscopy, transmission electron microscopy, and H₂-temperature-programmed reduction were used to explore the effect of Mn loading on methane oxidative coupling (OCM) performance. The results showed that applied voltage and flow rate had a significant effect on methane activation. The dielectric particles of TiO₂ loaded with Mn not only synergistically affected the coupling reaction, but also facilitated charge deposition to generate a strong local electric field to activate methane. The synergy effects boosted the OCM performance and the C₂₊ yield became 1.25 times higher than that of the undoped TiO₂ under identical operating conditions in plasma, which was almost impossible to occur even at 850 °C on the MnTi catalyst in the absence of plasma. Moreover, the reaction activity of the catalyst was fully recovered by plasma regeneration at 300 °C and maintained its stability in for at least 30 consecutive cyclic redox tests. This work presents a new opportunity for efficient methane conversion to produce C₂₊ at low temperatures by plasma assistance.

Raw syngas conditioning using molten salts was carried out in a fixed-bed reactor. The effects of the reaction conditions, including temperature, gas velocity, bubble diameter, molten salt static liquid heights, and inlet gas composition, on the composition of the syngas product and the properties of the spent molten salts were investigated. The molten salt absorbed CO₂ in all of the experiments (at temperatures from 350 °C to 500 °C) and decreased its concentration down to 1%. The H₂/CO increased from 0.94 to a maximum of 11.0, which may meet most of the synthetic process’ requirements. The temperature, gas velocity, and molten salt static liquid heights had significant effects on the H₂ and CO concentrations in the gas product. Molten salt raw syngas conditioning can be a suitable follow-up procedure for gasification and pyrolysis.