Abstract
The oxidative coupling of methane (OCM) to C2 hydrocarbons (C2H4 and C2H6) has aroused worldwide interest over the past decade due to the rise of vast new shale gas resources. However, obtaining higher C2 selectivity can be very challenging in a typical OCM process in the presence of easily oxidized products such as C2H4 and C2H6. Regarding this, different types of catalysts have been studied to achieve desirable C2 yields. In this review, we briefly presented three typical types of catalysts such as alkali/alkaline earth metal doped/supported on metal oxide catalysts (mainly for Li doped/supported catalysts), modified transition metal oxide catalysts, and pyrochlore catalysts for OCM and highlighted the features that play key roles in the OCM reactions such as active oxygen species, the mobility of the lattice oxygen and surface alkalinity of the catalysts. In particular, we focused on the pyrochlore (A2B2O7) materials because of their promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding for OCM reaction.
Highlights
Methane, being an abundant hydrocarbon resource, provides a kind of comparably cheaper and environmentally friendly fuel [1,2]
In the electric field, these catalysts can be produced and regenerate the surface-active oxygen species even at low temperatures, which are mainly responsible for oxidative coupling methane (OCM), Schmack et al proposed a meta-analysis method
In order to improve the catalytic stability of these catalysts, various modifications have been made during the last few decades via the addition of rare-earth oxides or doping with earth alkaline metals or through coordination of multiple ions
Summary
Methane, being an abundant hydrocarbon resource, provides a kind of comparably cheaper and environmentally friendly fuel [1,2]. High temperatures (>1200 ◦ C) are required for breaking the C-H bond (bond energy, 440 kJ/mol) in this process and approximately three tons of CO2 is released per ton of ethylene production [8] Another way ethylene can be produced from natural gas is via indirect paths by the formation of syngas (CO/H2 ) through the catalytic FischerTropsch synthesis (FTS) process [9,10]. Methane can be converted directly to value-added chemicals and fuels through oxidative and nonoxidative catalytic reaction pathways. The non-oxidative processes involve the coupling of methane to olefins; this process suffers from intrinsic thermodynamic limitations and needs higher energy large-scale [13].
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