Online Kinetics Study of Oxidative Coupling of Methane over La₂O₃ for Methane Activation: What Is Behind the Distinguished Light-off Temperatures?

Abstract

Oxidative coupling of methane (OCM) is a catalytic partial oxidation process that converts methane directly to valuable C2 products (ethane and ethylene). The main difficulties from further investigation of this reaction are due to the nature of its high operating temperature and the severe reaction exothermicity. In this work, an especially designed online characterization setup is applied for this reaction, which achieved both precise bed temperature control and real-time product measurement. High-resolution temperature-dependent product rates of OCM (CO2, ethane, and ethylene) were obtained, and their behavior vs reaction conditions as well as the activation energy barriers above their onset temperatures are clearly differentiated over a recently reported high performance nanorod La2O3 catalyst. Different from general expectation, CO2, resulting from full methane oxidation, dominates all the products in the lower temperature region, whereas the C2 species are only formed at much higher temperatures. Further analysis indicates that the selectivity and apparent activation energy for both CO x and C2 products are strongly influenced by the oxygen concentration and temperature. In combination with density functional theory calculations and additional experimental measurements, significant insights are brought to this high-temperature reaction of wide interest.

An In Situ Temperature-Dependent Study of La2O3 Reactivation Process

“…It is considered as one of the promising ethylene production processes (Galadima and Muraza, 2016;Taifan and Baltrusaitis, 2016). Previous publications (Le Van, 1993;Lacombe et al, 1995;Xu et al, 1995), including our recent in situ studies, indicated that La 2 O 3 is not stable in this reaction (Liu et al, 2018;Li et al, 2019;Guan et al, 2021;Pang et al, 2021;Zhou et al, 2021). In our recent studies, it was strongly suggested that a high temperature (800°C) precleaned sample surface, which is free of carbonates and hydroxyls, is enriched with a specific subsurface peroxide species (Zhou et al, 2021).…”

“…The SEM result indicates the sample is in the shape of nanorods with 10 nm diameter in average. BET measurement (N 2 ) yields specific surface area of 3.4 m 2 /g (Liu et al, 2018).…”

Catalyst screening for the oxidative coupling of methane: from isothermal to adiabatic operation via microkinetic simulations

“…The minor impact of temperature on catalyst ranking is encouraging in terms of comparing isothermal experimental data from different sources. Pronounced deviations from ideal isothermal operation 37,59 , which undoubtedly influence the catalytic performance, might not be the most influential factor in the ranking and, hence, OCM catalyst selection. Afterwards, the impact of the methane-to-oxygen ratio at a fixed operating temperature was assessed (Figure 3/D).…”

“…In fact, the consecutive oxidation of the products was found to be much less significant in an oxygen-lean environment (high CH4/O2) thanks to the reduced availability of the oxidant. Mainly, because the limited amount of oxygen in the gas phase is already consumed in the unavoidable and undesired oxidation of CH3 radicals 37 .…”

Surface coupling of methyl radicals for efficient low-temperature oxidative coupling of methane

“…Since the pioneering work of Keller and Bhasin in the early 1980s [18], hundreds of catalytic materials composed of 68 elements have been investigated for OCM reaction [19]. Due to the high binding energy of CH3−H bond (435 kJ mol −1 ), oxidative coupling of methane generally proceeds at high temperatures (≥ 700 °C) [20]. Because the high temperature process requires special materials for construction, high energy consumption and great capital input, the minimum criteria of C2 yield for industrial application is 30% [21].…”