Optimizing the Reaction Performance of La₂Ce₂O₇‐Based Catalysts for Oxidative Coupling of Methane (OCM) at Lower Temperature by Lattice Doping with Ca Cations

Abstract

To optimize the reaction performance of La2Ce2O7 for OCM, catalysts with varied La/Ce molar ratios and doped by Ca additive have been synthesized and characterized by different techniques. It is discovered that a La2Ce2O7 compound having a disordered cubic defective fluorite phase is predominantly formed in all the samples. Varying the La/Ce ratio influences the fine crystalline structure of the catalysts. As a result, the abundance of the surface facile oxygen and alkaline sites increase by increasing the La/Ce ratio. Moreover, the Ca additive exists on the surface of Ca0.5La2Ce2O7 and Ca0.5La1.5Ce2O7 as dispersed CaCO3, which covers the active sites and is harmful to the OCM reaction. However, for La2Ce1.5Ca0.5O7, the major part of the Ca additive has entered into the lattice of La2Ce2O7 phase as Ca2+ cations, thus creating more oxygen vacancies and improving the surface alkalinity of the catalyst, which is favorable to the OCM reaction. Both the active surface oxygen and alkaline sites are vital for OCM, and their synergism determines the reactivity. By increasing La/Ce ratio and doping Ca2+ into La2Ce2O7 lattice, the quantities of both kinds of active sites can be significantly improved, thus obtaining catalysts with much improved reaction performance. La2Ce1.5Ca0.5O7 owns the largest amount of both types of active sites, hence depicting the best reaction performance, over which the highest C2 yield of 22.5 % can be achieved even at 750 °C.

Nanosheet-Like Ho2O3 and Sr-Ho2O3 Catalysts for Oxidative Coupling of Methane

“…As evidenced in Figure 10, the C 2 -C 3 yield obtained at 700 • C correlates well with the number of moderate basic sites present on the Ho 2 O 3 -based catalysts. This finding is in accordance with some previous reports that the surface basic sites with moderate strength are more favorable for the C 2 product formation in the OCM process [25,26,44,55,[57][58][59][60].…”

Oxidative Coupling of Methane for Ethylene Production: Reviewing Kinetic Modelling Approaches, Thermodynamics and Catalysts

“…According to these authors, the superior activity of the monolith-like structured catalytic reactor would relate to its organised flow pattern, which can contribute to more efficient interaction between reactants and the catalyst surface, as also concluded by others [56]. More recently, La2Ce1.5Ca0.5O7 oxides have also been reported to reach similar catalytic performances, with demonstrated thermal stability for up to 100 h [38].…”

Oxidative Coupling of Methane: Perspective for High-Value C2 Chemicals

“…Moreover, a typical pyrochlore (A 2 B 2 O 7 ) compound containing rare-earth A-and B-site can offer surface alkalinity. In addition, all the characteristics such as thermal stability, surface alkalinity and inherent oxygen vacancies can be altered by replacing or partly substituting of the A and B sites with alkali or alkaline earth metals [85].…”

“…Recently, Wang and co-workers have developed different types of pyrochlore catalysts and examined their properties', such as r A /r B ratios, effect on crystalline structures, surface active oxygen sites, intrinsic oxygen vacancies and surface alkaline sites, being essential for OCM reaction [77,78,85]. A series of Ln 2 Ce 2 O 7 (Ln = La, Pr, Sm and Y) catalysts have been developed and demonstrated as the defective cubic fluorite phase.…”

“…Aimed to optimize the La 2 Ce 2 O 7 catalytic material for OCM, a series of catalysts were fabricated by a sol-gel method with varied La/Ce molar ratios and being doped with Ca, which had a close ionic radius with La and Ce ions. Among them, La 2 Ce 1.5 Ca 0.5 O 7 showed better yields (22.5%) at 750 • C due to its enhanced surface alkalinity and oxygen mobility with doping of Ca additive [85]. Table 3 lists the available data in the literature on pyrochlore catalysts for OCM.…”