Oxidative Coupling of Methane: Perspective for High-Value C 2 Chemicals

: The oxidative coupling of methane (OCM) to C 2 hydrocarbons (C 2 H 4 and C 2 H 6 ) has aroused worldwide interest over the past decade due to the rise of vast new shale gas resources. However, obtaining higher C 2 selectivity can be very challenging in a typical OCM process in the presence of easily oxidized products such as C 2 H 4 and C 2 H 6 . Regarding this, different types of catalysts have been studied to achieve desirable C 2 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 (A 2 B 2 O 7 ) materials because of their promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding for OCM reaction. Conceptualization, Methodology, Investigation, Writing Review & Editing, Visualization, P.R.M.; Investigation, Methodology, Review & Editing, Visualization, Y.L.; Reviewing and Editing, G.W.; Reviewing and Editing; Y.D.; Conceptualization, Supervision, Writing—Reviewing and Editing, Funding acquisition, C.S. All authors have read and agreed to the published version of the manuscript.


Introduction
Methane, being an abundant hydrocarbon resource, provides a kind of comparably cheaper and environmentally friendly fuel [1,2]. The use of natural gas as an important industrial feedstock has increased as compared to petroleum over the past few decades due to the extraction of natural gas from unconventional sources, for example shale deposits [3]. Figure 1 reveals the recoverable shale gas reserves in the world in trillion cubic feet (TCF).
Ethylene is used as a raw material in the manufacture of polymers such as polyethylene, polystyrene, polyvinyl chloride and polyethylene terephthalate, as well as fibers, petrochemical products and organic chemicals. It is also used as the main feedstock (~50%) in the chemical industry. Hence, many global chemical companies are planning to develop economical and feasible processes to produce ethylene directly from methane [3,4].
In general, steam and the thermal cracking of naphtha processes are used for ethylene production in petroleum refineries. Global ethylene production is mainly dependent on these processes [5,6]. However, some of the key problems identified here are energy intensiveness (up to 40 GJ heat per tonne ethylene) and environmental unsustainability [7]. 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 CO 2 is released per ton of ethylene production [8]. Therefore, another way ethylene can be produced from natural gas is via indirect paths by the formation of syngas (CO/H 2 ) through the catalytic Fischer-Tropsch synthesis (FTS) process [9,10]. Alternatively, syngas can be transformed into methanol and then methanol to ethylene [11,12]. In addition to this, methane can also be converted directly to value-added chemicals and fuels through oxidative and nonoxidative catalytic reaction pathways. The non-oxidative processes involve the coupling of into methanol and then methanol to ethylene [11,12]. In addition to this, methane can also be converted directly to value-added chemicals and fuels through oxidative and non-oxidative 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 for large-scale applications [13]. Therefore, most of the research for direct conversion has emphasized a process called oxidative coupling of methane (OCM) [14][15][16][17][18][19]. The leading global countries are shown in different colors. Reproduced with permission from [3] Copyright 2016 Elsevier.
Zhao et al. [14] prepared hierarchically structured porous flower-like materials such as La2O3-H-and Sr-doped (Sr0.1LaOx-H) compounds and highly dense non-porous (La2O3-D and Sr0.1LaOx-D) microspheres. The porous materials showed higher catalytic performances in OCM than non-porous catalysts, presumably due to the creation of the active sites in porous materials, which could enhance the activation of oxygen. Sr0.1LaOx-H showed better OCM results than La2O3-H and reached the optimum selectivity (~48%) and C2 yield (~19%) at 550 °C due to Sr isolating the active sites of the catalyst, which can act as a strong basic site. However, deactivation occurred in the Sr0.1LaOx-H catalyst during OCM in ~30 h, but adding a small amount of Zr can produce better stability in OCM. Lopes et al. [15] prepared La2Ti2O7-based catalysts (LaTi1−xMgxO3+δ) with varied Ti/Mg molar ratios through the partial substitution of Ti by Mg to regulate the quantities of alkaline sites and surface oxygen species, properties crucial for OCM reaction. The characterization data testifies that alkalinity and the surface oxygen vacancy concentration increase as Mg increases. The presence of high amounts of Mg on the surface can increase the incidence of defective sites in the interface formed by La-O-Mg. In general, the rich defects are active and selective toward C2 production in the OCM reaction. Hence, the catalytic results showed that methane conversion and C2 selectivity increased as a function of the Mg amount, and also this catalyst exhibited high thermal stability after 24 h on stream. Zhao et al. [14] prepared hierarchically structured porous flower-like materials such as La 2 O 3 -H-and Sr-doped (Sr 0.1 LaO x -H) compounds and highly dense non-porous (La 2 O 3 -D and Sr 0.1 LaO x -D) microspheres. The porous materials showed higher catalytic performances in OCM than non-porous catalysts, presumably due to the creation of the active sites in porous materials, which could enhance the activation of oxygen. Sr 0.1 LaO x -H showed better OCM results than La 2 O 3 -H and reached the optimum selectivity (~48%) and C 2 yield (~19%) at 550 • C due to Sr isolating the active sites of the catalyst, which can act as a strong basic site. However, deactivation occurred in the Sr 0.1 LaO x -H catalyst during OCM in~30 h, but adding a small amount of Zr can produce better stability in OCM. Lopes et al. [15] prepared La 2 Ti 2 O 7 -based catalysts (LaTi 1−x Mg x O 3+δ ) with varied Ti/Mg molar ratios through the partial substitution of Ti by Mg to regulate the quantities of alkaline sites and surface oxygen species, properties crucial for OCM reaction. The characterization data testifies that alkalinity and the surface oxygen vacancy concentration increase as Mg increases. The presence of high amounts of Mg on the surface can increase the incidence of defective sites in the interface formed by La-O-Mg. In general, the rich defects are active and selective toward C 2 production in the OCM reaction. Hence, the catalytic results showed that methane conversion and C 2 selectivity increased as a function of the Mg amount, and also this catalyst exhibited high thermal stability after 24 h on stream. Sato et al. [16] proposed that, in the presence of an electric field, the CePO 4 nanorod catalysts showed high activity and stability even at low temperatures for OCM (C 2 yield = 18%) without any external heating. Because, 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 OCM, Schmack et al. proposed a meta-analysis method. It is often useful ascertain the links between a catalyst's physicochemical characteristics and its performance in an OCM reaction [17]. Pirro et al. [18] proposed the process simulator and illustrated it for the OCM to determine the catalyst-dependent kinetics for industrial implementation. For the case study, the feeding of oxygen and cooling were operated under a multi-stage adiabatic configuration. Three catalysts (Sn-Li/MgO, NaMnW/SiO 2 and Sr/La 2 O 3 ) were introduced to estimate the methane conversion and C 2+ yield compared to a single stage. They revealed that multi-phase configuration is advantageous compared to a single phase.
The OCM process follows the value-added transformation of natural gas through the catalytic reaction between methane and oxygen for ethylene production. Although more than four decades have passed since the initial work on OCM, this reaction has still not been able to be used as a successful industrial process for ethylene production because of the lower yield (<30%) of C 2 products.
In the reaction mechanism of OCM, the first step of abstraction of a hydrogen atom from methane with the help of surface-active oxygen results in the formation of hydroxyl groups, which react with each other to produce H 2 O, and at the same time create an oxygen vacancy on the catalyst surface. Meanwhile, methyl radicals will combine to form ethane which further undergoes a dehydrogenation reaction to produce the desired product of ethylene, as shown in Figure 2. According to previous reports, active oxygen species on the surface of catalyst consist of peroxide (O 2 2− ), lattice oxygen (O 2− ), carbonate (CO 3 2− ), superoxide (O 2 − ) and hydroxide (OH − ) ions. Among all, surface lattice oxygen (O 2− ) anions play a crucial role in attaining the C 2 product selectively, whereas surface electrophilic anions (O − , O 2 − and O 2 2− ) usually result in an over-oxidation of the CO x product [20,21].
Sato et al. [16] proposed that, in the presence of an electric field, the CePO4 nanorod catalysts showed high activity and stability even at low temperatures for OCM (C2 yield = 18%) without any external heating. Because, 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 OCM, Schmack et al. proposed a meta-analysis method. It is often useful ascertain the links between a catalyst's physicochemical characteristics and its performance in an OCM reaction [17]. Pirro et al. [18] proposed the process simulator and illustrated it for the OCM to determine the catalyst-dependent kinetics for industrial implementation. For the case study, the feeding of oxygen and cooling were operated under a multi-stage adiabatic configuration. Three catalysts (Sn-Li/MgO, NaMnW/SiO2 and Sr/La2O3) were introduced to estimate the methane conversion and C2+ yield compared to a single stage. They revealed that multi-phase configuration is advantageous compared to a single phase.
The OCM process follows the value-added transformation of natural gas through the catalytic reaction between methane and oxygen for ethylene production. Although more than four decades have passed since the initial work on OCM, this reaction has still not been able to be used as a successful industrial process for ethylene production because of the lower yield (<30%) of C2 products.
In the reaction mechanism of OCM, the first step of abstraction of a hydrogen atom from methane with the help of surface-active oxygen results in the formation of hydroxyl groups, which react with each other to produce H2O, and at the same time create an oxygen vacancy on the catalyst surface. Meanwhile, methyl radicals will combine to form ethane which further undergoes a dehydrogenation reaction to produce the desired product of ethylene, as shown in Figure 2. According to previous reports, active oxygen species on the surface of catalyst consist of peroxide (O2 2− ), lattice oxygen (O 2− ), carbonate (CO3 2− ), superoxide (O2 − ) and hydroxide (OH − ) ions. Among all, surface lattice oxygen (O 2− ) anions play a crucial role in attaining the C2 product selectively, whereas surface electrophilic anions (O − , O2 − and O2 2− ) usually result in an over-oxidation of the COx product [20,21].  [19]. Copyright 2018 Elsevier.
From the pioneering work of Keller and Bhasin [22] and Ito and Lunsford [23], various catalysts have been investigated to attain adequate methane conversion and C2 selectivity/yield. Most of the metal oxides in the periodic table, such as transition metal oxides, rare earth metal oxides, alkali or alkaline earth metal oxides, perovskites and pyrochlore compounds, have been individually or in various combinations tested for OCM reaction [3,[24][25][26][27][28]. Zavyalova et al. [29] conducted a comprehensive statistical analysis of about ~1000 research articles related to OCM. They pointed out that both rare-earth metal oxides and alkaline-earth metal oxides, along with alkali metal oxides, are good candidates for OCM reaction. In addition, binary and ternary combinations of metal oxides are also crucial for high and sustainable catalytic activity in the OCM reaction. The selectivity of the From the pioneering work of Keller and Bhasin [22] and Ito and Lunsford [23], various catalysts have been investigated to attain adequate methane conversion and C 2 selectivity/yield. Most of the metal oxides in the periodic table, such as transition metal oxides, rare earth metal oxides, alkali or alkaline earth metal oxides, perovskites and pyrochlore compounds, have been individually or in various combinations tested for OCM reaction [3,[24][25][26][27][28]. Zavyalova et al. [29] conducted a comprehensive statistical analysis of about 1000 research articles related to OCM. They pointed out that both rare-earth metal oxides and alkaline-earth metal oxides, along with alkali metal oxides, are good candidates for OCM reaction. In addition, binary and ternary combinations of metal oxides are also crucial for high and sustainable catalytic activity in the OCM reaction. The selectivity of the catalysts can be further improved by using various dopants such as alkali (Cs, Na) and alkaline earth (Sr, Ba) metals, whereas Mn, W and the Cl − anion dopants can be used to enhance the catalytic activity. Figure 3 represents the most promising OCM catalytic systems with C 2 yields greater than 25%. catalysts can be further improved by using various dopants such as alkali (Cs, Na) and alkaline earth (Sr, Ba) metals, whereas Mn, W and the Cl − anion dopants can be used to enhance the catalytic activity. Figure 3 represents the most promising OCM catalytic systems with C2 yields greater than 25%.
This review presents three types of catalysts for OCM reaction and highlights the features that play key roles in the OCM reactions, such as active oxygen species, the mobility of the lattice oxygen, and the surface alkalinity of the catalysts. The catalyst compositions and their catalytic functions are also discussed. Special attention has been focused on pyrochlore (A2B2O7) materials with promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding, these being essential for OCM reaction.

Catalysts for Oxidative Coupling of Methane
Oxide catalysts are the main catalysts used for OCM reaction. These can be a pristine form or modified with group alkali/alkaline earth metals or transitional metals. These catalytic systems are established under various synthesis methods such as hydrothermal, solgel, precipitation, impregnation, thermal decomposition and flame spray pyrolysis to yield tunable catalyst compositions by varying basicity and oxygen vacancies, which are the most important characteristics of OCM catalysts and also key factors in activating methane. These catalysts can be coupled with tunable reaction conditions such as space, velocity and temperature to enhance methane conversion, selectivity and yield [19,30].

Alkali/Alkaline Earth Metal Doped/Supported on Metal Oxide Catalysts
Pristine or unmodified metal oxide catalysts generally exhibit low C2 selectivity/yield and rapid deactivation in OCM reaction. Modified systems are considered to achieve higher yield at lower temperature regions due to the altered either basicity or oxygen vacancies and/or other defects, which are crucial for CH4 activation [31][32][33][34][35][36]. Ferreira et al., [32] developed CeO2 catalysts modified with earth alkaline metals (M = Mg, Ca, and Sr) and demonstrated that among these three dopants, Ca-doped CeO2 catalyst showed the best performance in the OCM reaction. This is mainly due to the similar ionic radii of Ca 2+ and Ce 4+ ions and the higher ratio of the oxygen species O2 − and O2 2− to lattice oxygen and a high amount of surface basic sites. Yıldız et al. [33] proved that mesoporous TiO2-rutile This review presents three types of catalysts for OCM reaction and highlights the features that play key roles in the OCM reactions, such as active oxygen species, the mobility of the lattice oxygen, and the surface alkalinity of the catalysts. The catalyst compositions and their catalytic functions are also discussed. Special attention has been focused on pyrochlore (A 2 B 2 O 7 ) materials with promising properties such as high melting points, thermal stability, surface alkalinity and tunable M-O bonding, these being essential for OCM reaction.

Catalysts for Oxidative Coupling of Methane
Oxide catalysts are the main catalysts used for OCM reaction. These can be a pristine form or modified with group alkali/alkaline earth metals or transitional metals. These catalytic systems are established under various synthesis methods such as hydrothermal, sol-gel, precipitation, impregnation, thermal decomposition and flame spray pyrolysis to yield tunable catalyst compositions by varying basicity and oxygen vacancies, which are the most important characteristics of OCM catalysts and also key factors in activating methane. These catalysts can be coupled with tunable reaction conditions such as space, velocity and temperature to enhance methane conversion, selectivity and yield [19,30].

Alkali/Alkaline Earth Metal Doped/Supported on Metal Oxide Catalysts
Pristine or unmodified metal oxide catalysts generally exhibit low C 2 selectivity/yield and rapid deactivation in OCM reaction. Modified systems are considered to achieve higher yield at lower temperature regions due to the altered either basicity or oxygen vacancies and/or other defects, which are crucial for CH 4 activation [31][32][33][34][35][36]. Ferreira et al. [32] developed CeO 2 catalysts modified with earth alkaline metals (M = Mg, Ca, and Sr) and demonstrated that among these three dopants, Ca-doped CeO 2 catalyst showed the best performance in the OCM reaction. This is mainly due to the similar ionic radii of Ca 2+ and Ce 4+ ions and the higher ratio of the oxygen species O 2 − and O 2 2− to lattice oxygen and a high amount of surface basic sites. Yıldız et al. [33] proved that mesoporous TiO 2 -rutile supported Mn x O y -Na 2 WO 4 catalysts' better performance than commercial TiO 2 -rutileand TiO 2 -anatase-supported catalysts. In addition, this catalyst is stable during 16 h on stream in OCM. Ivanov et al. [34] synthesized Mg, Al, Ca, Ba and Pb-substituted titanates SrTi 1−x A x O 3 (A = Mg, Al, Ca, Ba, Pb, x = 0.1) and Sr 2 Ti 1−x A x O 4 (A = Mg, Al, x = 0.1) catalysts by using a mechanochemical method and tested these catalysts for OCM reaction at 850 and 900 • C. Finally, they demonstrated that among these dopants, Mg-and Al-doped SrTiO 3 and Sr 2 TiO 4 catalysts showed the best performance in the OCM reaction to provide a higher C 2 yield up to 25% and C 2 selectivity around 66%, presumably due to (Sr, Mg)O oxide segregate on the surface, which has active oxygen ion-radicals. Peng et al. [35] developed SnO 2 modified with alkaline metals and demonstrated that the Li-doped SnO 2 catalyst showed the best performance in the OCM reaction among these dopants. This is mainly due to the higher surface intermediate alkaline sites and electrophilic oxygen species. Moreover, this catalyst showed stable catalytic performance up to 100 h without deactivation. Finally, they proposed that the catalytic activity and selectivity towards the desired products were higher for the optimized catalyst (Sn 5 Li 5 ) than Mn-Na 2 WO 4 /SiO 2 at a lower temperature (750 • C), which is considered a state-of-the-art catalyst for OCM. Kim et al. [37] carefully explored the role of oxygen species using lanthanum-based perovskite catalysts, which are prepared by a citrate sol-gel method. Their study found that the active surface lattice oxygen species and a facile filling of lattice oxygen vacancies by gas-phase oxygen are responsible for the production of C 2 hydrocarbons in OCM.
Elkins et al. [38] studied rare earth oxides (Sm 2 O 3 , TbO x , PrO y and CeO 2 ) doped with alkali (Li and Na) and alkaline earth metal (Mg and Ca) supported on MgO. These metal dopants can alter the acid-base property of catalysts, which has an impact on catalysts activity, selectivity and stability. Hence, doped catalysts showed higher C 2 yields than undoped catalysts owing to the presence of strong basic sites (see Figure 4). Among all catalysts, Li-TbO x /n-MgO showed better activity, C 2 selectivity and C 2 yield at 650 • C. However, undoped catalysts showed higher yield at a lower temperature (<600 • C) than the doped catalysts because the active sites in doped catalysts are blocked with by-product CO 2 at low temperatures.
supported MnxOy-Na2WO4 catalysts' better performance than commercial TiO2-rutile-and TiO2-anatase-supported catalysts. In addition, this catalyst is stable during 16 h on stream in OCM. Ivanov et al. [34] synthesized Mg, Al, Ca, Ba and Pb-substituted titanates SrTi1−xAxO3(A = Mg, Al, Ca, Ba, Pb, x = 0.1) and Sr2Ti1−xAxO4(A = Mg, Al, x = 0.1) catalysts by using a mechanochemical method and tested these catalysts for OCM reaction at 850 and 900 °C. Finally, they demonstrated that among these dopants, Mg-and Al-doped SrTiO3 and Sr2TiO4 catalysts showed the best performance in the OCM reaction to provide a higher C2 yield up to 25% and C2 selectivity around 66%, presumably due to (Sr, Mg)O oxide segregate on the surface, which has active oxygen ion-radicals. Peng et al. [35] developed SnO2 modified with alkaline metals and demonstrated that the Li-doped SnO2 catalyst showed the best performance in the OCM reaction among these dopants. This is mainly due to the higher surface intermediate alkaline sites and electrophilic oxygen species. Moreover, this catalyst showed stable catalytic performance up to 100 h without deactivation. Finally, they proposed that the catalytic activity and selectivity towards the desired products were higher for the optimized catalyst (Sn5Li5) than Mn-Na2WO4/SiO2 at a lower temperature (750 °C ), which is considered a state-of-the-art catalyst for OCM. Kim et al. [37] carefully explored the role of oxygen species using lanthanum-based perovskite catalysts, which are prepared by a citrate sol-gel method. Their study found that the active surface lattice oxygen species and a facile filling of lattice oxygen vacancies by gas-phase oxygen are responsible for the production of C2 hydrocarbons in OCM.
Elkins et al. [38] studied rare earth oxides (Sm2O3, TbOx, PrOy and CeO2) doped with alkali (Li and Na) and alkaline earth metal (Mg and Ca) supported on MgO. These metal dopants can alter the acid-base property of catalysts, which has an impact on catalysts activity, selectivity and stability. Hence, doped catalysts showed higher C2 yields than undoped catalysts owing to the presence of strong basic sites (see Figure 4). Among all catalysts, Li-TbOx/n-MgO showed better activity, C2 selectivity and C2 yield at 650 °C. However, undoped catalysts showed higher yield at a lower temperature (<600 °C) than the doped catalysts because the active sites in doped catalysts are blocked with by-product CO2 at low temperatures. Li-doped MgO is one of the best systems for OCM reaction [39][40][41][42][43][44][45] because, in this system, an additional electron is produced when each Li + cation replaces an Mg +2 cation. This excess of an electron is compensated by the formation of electron holes strongly bonded to lattice oxygen, and O − centers are formed with a strong radical character local-  Li-doped MgO is one of the best systems for OCM reaction [39][40][41][42][43][44][45] because, in this system, an additional electron is produced when each Li + cation replaces an Mg +2 cation. This excess of an electron is compensated by the formation of electron holes strongly bonded to lattice oxygen, and O − centers are formed with a strong radical character localized to the O atom, as in the case of Li-doped MgO forming [Li + catalyzes the CH 4 dehydrogenation in the following steps: 3 (1) Here, 'V' denotes an oxygen vacancy. The [Li + O − ] centers are generated in Equation (3) again to fulfill a redox cycle [46]. Ali Farsi et al. [41] reported that 7% Li/MgO catalysts were prepared with two different preparation methods, such as incipient wetness impregnation (IWI) and sol-gel (SG). The catalyst prepared by using the sol-gel method exhibited better methane conversion, higher C 2 selectivity, and yield for the desired product than the catalyst prepared by the IWI method. The better performance of the catalyst prepared through the sol-gel method was suggested to relate to the following two factors: (i) it has a significantly larger surface area, and (ii) it comprises a greater quantity of Li incorporated in the MgO matrix. Both of these effects contribute to an increase in the number of active sites in the catalyst. However, at 850 • C, the selectivity of Li/MgO-SG catalyst decreased because of the evaporation of some of the Li from the MgO surface. Qian et al. [42] reported that Li can restructure the MgO surface to expose high-indexed facets such as {110}, {111} and {100} facets; among them, the Mg 4c 2+ sites of MgO {110} facets are highly active and selective in catalyzing the OCM reaction, which produces high C 2 selective products by the formation of methyl radical intermediate without other less-stable carbon-containing radicals (CH 2 and CH), which will reduce the methyl radical dissociation and subsequent combustion reaction. Li-doped MgO catalysts with different Li loadings (1.3% and 5.6%) have been prepared by Luo et al. [47]. Different Li contents were observed over both the as-prepared catalysts, but similar Li was present in the used catalysts with different Li surface distribution. Hence, higher CH 4 conversion and C 2 selectivity were observed over Li (5.6%)/MgO than the Li (1.3%)/MgO catalyst because the loss of Li occurred during the reaction process through the movement of these ions from the bulk material to the catalyst surface and successive desorption was responsible for asymmetrical and coarse structures with bare MgO {110} and {111} facets observed in Li (5.6%)/MgO catalysts but not in the Li (1.3%)/MgO. Hence, the main role of Li in MgO-based catalyst during OCM reaction acts as a structural modifier of the active MgO component rather than as an active center. Due to the poor stability of these Li-supported catalysts, some researchers have recently focused on depressing Li-ions' evaporation at high temperatures. Matsumoto et al. [48] proposed that the catalytic activity and selectivity towards desired products were higher over crystalline Li 2 CaSiO 4 than Mn-Na 2 WO 4 /SiO 2, which is considered a state-of-the-art catalyst for OCM. The CH 4 conversion and C 2 selectivity were obtained at 28.3% and 77.5%, respectively, at the temperature of 750 • C for the OCM reaction. In addition, they exhibited higher catalyst durability for 50 h on stream without deactivation and structural changes. The improvement of OCM activity in Li 2 CaSiO 4 is likely to result from the combination of multiple cations in the crystal lattice. The structure of Li 2 CaSiO 4 consists of a single oxygen site, neighboring with Li, Ca and Si (two Li, two Ca, and one Si atoms). The resulting coordination around oxygen provides a dual character such as strong basicity and lattice stability, which are mainly responsible for CH 4 conversion and C 2 selectivity, respectively. Elkins et al. [49] reported two different types of metal oxide catalysts such as Li-doped TbOx and Sm 2 O 3 supported on MgO and they found that the Li-Tb 2 O 3 /n-MgO catalyst exhibited high activity and selectivity towards desired products at low reaction temperature i.e 650 • C. In addition, the catalyst showed higher stability and lower deactivation rate after 30 h on stream, because the Li addition prompted the reduction of TbO x phase to Tb 2 O 3 and also shifted the Tb 3d core-level electrons to higher binding energy that caused strong Li-TbO x interactions, which in turn became more active toward C 2 product formation than the SmOx. Table 1 summaries the available data in literature on various Li doped/impregnated catalysts for OCM. *GHSV-gas hourly space velocity.
The Li-supported systems, especially Li-MgO catalysts, are among the most studied OCM Catalysts. Because they show a C 2 yield of 20% with higher methane conversion rates at lower temperatures, however, these catalysts suffer from deactivation due to Li vaporization. [Li + O − ] is specified as the active site for the formation of methyl radical intermediate in OCM reaction, even though many features are still unclear, such as stability of the catalyst, structure activity relationship, and active center; hence, proficient materials are needed to stabilize Li to overcome these issues, and these systems must be stable at high temperatures, where OCM usually occurs. Thus, the synthesis of efficient and Li stable catalysts for OCM is highly challenging. To overcome these issues, 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. As per the previous results, we understand that the mixed-phase oxides have multiple elements with an interface that are believed to show higher activity and stability due to the synergistic effect between the metal oxides. For example, when used Li doped TbO x and Sm 2 O 3 supported on MgO and they found that the Li-Tb 2 O 3 /n-MgO catalyst exhibited higher stability and lower deactivation rate after 30 h on stream [49], while Li 2 CaSiO 4 showed the better catalytic activity and selectivity over Mn-Na 2 WO 4 /SiO 2 and also exhibited higher catalyst durability for 50 h on stream without deactivation and structural changes for the OCM reaction [48].

Modified Transition Metal Oxide Catalysts
Mn-NaWO 4 /SiO 2 catalyst is widely accepted as the most applicable catalyst, with a higher C 2 yield (~27%) in OCM [33,[52][53][54][55][56][57][58][59][60] [54] prepared different nanostructured complex metal oxides such as Sr-Al complex oxides, La 2 O 3 , La-Sr-Al complex oxides, and Na 2 WO 4 -Mn/SiO 2 by using the solution combustion synthesis (SCS) method. Among these catalysts, Sr 3 Al 2 O 6 (double perovskite phase) in the Sr-Al oxides is active for OCM. The La 2 O 3 catalyst showed the highest C 2 yields (~13.5%) as compared to similar catalysts presented in the literature. However, the addition of La exhibited a higher C 2 yield even at lower temperatures of 750 • C. The Na 2 WO 4 -Mn/SiO 2 catalyst was stable and showed a higher C 2 yield (25%) in OCM, which is one of the best results in the literature. Wang et al. [57] carefully explored the effect of Na 2 WO 4 -Mn/SiO 2 catalysts prepared by different methods such as incipient wetness impregnation (IWI), mixture slurry and the sol-gel method for OCM reaction. As per the different analytical methods, IWI showed that Na, W and Mn are mainly distributed on the catalyst's surface. However, the other two ways are produced more uniform between the surface and bulk on the catalysts. Hence, the mixture slurry method has excellent stability and is stable during 500 h on stream at 800 • C in the OCM, and during the stream, methane conversion, and C 2 selectivity are maintained at 27-31% and 68-71%, respectively. Yunarti et al. [58] developed promising oxide composite material Na 2 WO 4 /Mn/Mg 0.05 Ti 0.05 Si 0.90 O n , prepared using a one-pot sol-gel method. This catalyst showed a 23.1% C 2 yield at 800 • C, an 18% higher C 2 selectivity, and a 35% higher C 2 yield than the conventional Na 2 WO 4 /Mn/SiO 2 catalyst, presumably due to Mg and Ti, which are doped in the α-cristobalite SiO 2 . These ions can inhibit incorporating Na/W/Mn compounds into the α-cristobalite SiO 2 structure. As a result, Na/W/Mn compounds are exposed to the catalyst's surface, which is more accountable for the higher C 2 selectivity and yield in the OCM reaction. The catalytic results of Mn-NaWO 4 /SiO 2 indicate synergistic interactions between the various catalyst components (Na, Mn and W), as displayed in Figure 5.
°C. The Na2WO4-Mn/SiO2 catalyst was stable and showed a higher C2 yield (25%) in OCM, which is one of the best results in the literature. Wang et al. [57] carefully explored the effect of Na2WO4-Mn/SiO2 catalysts prepared by different methods such as incipient wetness impregnation (IWI), mixture slurry and the sol-gel method for OCM reaction. As per the different analytical methods, IWI showed that Na, W and Mn are mainly distributed on the catalyst's surface. However, the other two ways are produced more uniform between the surface and bulk on the catalysts. Hence, the mixture slurry method has excellent stability and is stable during 500 h on stream at 800 °C in the OCM, and during the stream, methane conversion, and C2 selectivity are maintained at 27-31% and 68-71%, respectively. Yunarti et al. [58] developed promising oxide composite material Na2WO4/Mn/Mg0.05Ti0.05Si0.90On, prepared using a one-pot sol-gel method. This catalyst showed a 23.1% C2 yield at 800 °C, an 18% higher C2 selectivity, and a 35% higher C2 yield than the conventional Na2WO4/Mn/SiO2 catalyst, presumably due to Mg and Ti, which are doped in the α-cristobalite SiO2. These ions can inhibit incorporating Na/W/Mn compounds into the α-cristobalite SiO2 structure. As a result, Na/W/Mn compounds are exposed to the catalyst's surface, which is more accountable for the higher C2 selectivity and yield in the OCM reaction. The catalytic results of Mn-NaWO4/SiO2 indicate synergistic interactions between the various catalyst components (Na, Mn and W), as displayed in Figure 5. If the catalyst contains only Na or W-oxide, it is inactive and unselective. However, catalysts consisting only of Mn-oxide are quite active towards methane combustion. Na and W-oxide catalysts doping enhance their conversion and C2 selectivity, presumably due to synergy between Na and W components with structural effects and the introduction of surface basic sites. The addition of Na makes it easy to reduce the W-oxide component, which has a positive impact on OCM. Catalysts having both Mn and W oxides If the catalyst contains only Na or W-oxide, it is inactive and unselective. However, catalysts consisting only of Mn-oxide are quite active towards methane combustion. Na and W-oxide catalysts doping enhance their conversion and C 2 selectivity, presumably due to synergy between Na and W components with structural effects and the introduction of surface basic sites. The addition of Na makes it easy to reduce the W-oxide component, which has a positive impact on OCM. Catalysts having both Mn and W oxides showed better C 2 selectivity but were slightly less active. The bi-component system (Na and Mn oxide) is 100% selective but has poor conversion, while, the three-component system (Mn-Na-W oxide supported on SiO 2 ) shows significantly higher selectivity than bi-component (Na-W, Na-Mn and, and Mn-W) catalysts, although it is slightly less active than the Na-W oxide catalyst. Each component has a specific role in the reaction, and the Na species are generally known to convert SiO 2 support from its primary amorphous phase to crystalline SiO 2 (α-cristobalite) phase, which plays an important role in the OCM reaction and also favors in the migration of Mn and W species to the surface of the catalyst [55,56,63]. The common role of Mn is promoting oxygen mobility between surface-adsorbed and lattice oxygen atoms. The surface Mn sites are responsible for OCM activity and C 2 hydrocarbon selectivity [53]. OCM reaction mechanism for the supported Mn 2 O 3 /Na 2 WO 4 /SiO 2 catalyst is shown in Figure 6.
Ortiz-Bravo et al. [64] systematically elucidate the electronic and molecular structure of the W and Mn sites on the Mn-Na 2 WO 4 /SiO 2 catalyst of the corresponding OCM reaction temperature by using different analytical techniques. The Mn-Na 2 WO 4 /SiO 2 catalyst's structure is highly temperature-dependent; thus, the association of any OCM activity with crystalline phases observed at room temperature is inadequate. In situ TPO-XRD technique clearly displays that the crystalline phases presented at room temperature in the Mn-Na 2 WO 4 /SiO 2 , Na 2 WO 4 /SiO 2 and WO 3 /SiO 2 catalysts are absent at OCM temperatures (>700 • C). Upon heating in oxidizing conditions, Na and W-oxide's crystalline phases altered to α to β then γ-WO 3 ; cubic to orthorhombic, then molten Na 2 WO 4 ; and the support SiO 2 phase can be changed α to β-cristobalite. TPO-Raman spectra clearly validate that the bond order of W sites with tetrahedral (T d ) and octahedral (O h ) symmetry varies during the phase transformation. Because all samples retain essentially W 6+ valence and O h -Mn 3+ sites are always present on Mn-Na 2 WO 4 /SiO 2 catalyst, TPO-XANES spectra expose that bond order differences are due to distortion degree variations. Finally, they established that O h -W 6+ sites are inactive in the steady-state OCM tests, but T d -W 6+ sites are more active in the presence of O h -Mn 3+ sites, which can be helpful for the activation of methane. Table 2 lists the available data in the literature on MnOx-Na 2 WO 4 /SiO 2 catalysts for OCM. showed better C2 selectivity but were slightly less active. The bi-component system (Na and Mn oxide) is 100% selective but has poor conversion, while, the three-component system (Mn-Na-W oxide supported on SiO2) shows significantly higher selectivity than bicomponent (Na-W, Na-Mn and, and Mn-W) catalysts, although it is slightly less active than the Na-W oxide catalyst. Each component has a specific role in the reaction, and the Na species are generally known to convert SiO2 support from its primary amorphous phase to crystalline SiO2 (α-cristobalite) phase, which plays an important role in the OCM reaction and also favors in the migration of Mn and W species to the surface of the catalyst [55,56,63]. The common role of Mn is promoting oxygen mobility between surface-adsorbed and lattice oxygen atoms. The surface Mn sites are responsible for OCM activity and C2 hydrocarbon selectivity [53]. OCM reaction mechanism for the supported Mn2O3/Na2WO4/SiO2 catalyst is shown in Figure 6. Ortiz-Bravo et al. [64] systematically elucidate the electronic and molecular structure of the W and Mn sites on the Mn-Na2WO4/SiO2 catalyst of the corresponding OCM reaction temperature by using different analytical techniques. The Mn-Na2WO4/SiO2 catalyst's structure is highly temperature-dependent; thus, the association of any OCM activity with crystalline phases observed at room temperature is inadequate. In situ TPO-XRD technique clearly displays that the crystalline phases presented at room temperature in the Mn-Na2WO4/SiO2, Na2WO4/SiO2 and WO3/SiO2 catalysts are absent at OCM temperatures (>700 °C). Upon heating in oxidizing conditions, Na and W-oxide's crystalline phases altered to α to β then γ-WO3; cubic to orthorhombic, then molten Na2WO4; and the support SiO2 phase can be changed α to β-cristobalite. TPO-Raman spectra clearly validate that the bond order of W sites with tetrahedral (Td) and octahedral (Oh) symmetry varies during the phase transformation. Because all samples retain essentially W 6+ valence and Oh-Mn 3+ sites are always present on Mn-Na2WO4/SiO2 catalyst, TPO-XANES spectra expose that bond order differences are due to distortion degree variations. Finally, they established that Oh-W 6+ sites are inactive in the steady-state OCM tests, but Td-W 6+ sites are more active in the presence of Oh-Mn 3+ sites, which can be helpful for the activation of  *WHSV-Weight hourly space velocity.
Due to their excellent reaction and thermal stability, various supports such as Mg-Ti mixed oxides, SiC, COK-12, SBA-15 were used instead of SiO 2 support to improve the catalyst's performance [60,[68][69][70]. In addition, the promotion of Mn/Na 2 WO 4 /SiO 2 with other metal oxides such as La 2 O 3 , SnO 2 , and CeO 2 have also been conducted in OCM [71][72][73]. Aimed to optimize the catalytic material for OCM, Sun et al. [74] prepared a series of Ce x Zr 1−x O 2 catalysts using a sol-gel method with varied Ce/Zr molar ratios and then modified with Mn 2 O 3 -Na 2 WO 4 doping. Among them, Ce 0.15 Zr 0.85 O 2 containing catalyst exhibited better conversion (25%) and selectivity (67%) even at 660 • C due to generate more O 2 − species, which are responsible for enhanced activity and selectivity at a lower reaction temperature. Moreover, this catalyst showed stable catalytic performance up to 100 h at 660 • C without decreasing the methane conversion (25%) and C 2 -C 3 selectivity (67%). Recently, Geo Jong Kim et al. proved that TiO 2 support showed better performance than SiO 2 in Mn/Na 2 WO 4 -based catalysts. The authors have modified the conventional OCM catalyst (Mn-NaWO 4 /SiO 2, MNWSi) with TiO 2 (MNWTi) as support and CeO 2 (MNWCeTi) as a promoter. Here they varied the reaction parameters and compared the activities with conventional catalyst. They found that MNWTi catalyst improved methane conversion and C 2 yield. The methane conversion and C 2 yield of this catalyst enhanced from 28.5 to 46.0% and from 12.1 to 25.9% at 775 • C, respectively, as compared to with conventional OCM catalyst, presumably due to having MnTiO 3 and Mn 2 O 3 , with higher Mn species on the surface which are helpful to activate the O 2 at low temperature [75]. Temperatures higher than 700 • C are usually required to initiate this reaction, and higher C 2 yields could be achieved around 800 • C [52,76]. Therefore, catalyst stability and selectivity of hydrocarbons are limited due to the suppression of active sites at very high temperatures and increased non-selective oxidation products. Therefore, there is still a strong motivation to explore novel catalysts with high reaction performance, particularly at low temperatures, to execute this reaction industrially.

Pyrochlore Catalysts
Pyrochlore (A 2 B 2 O 7 ) catalysts are promising catalysts for OCM reaction due to their high melting points, thermal stability, tunable M-O bonding, surface alkalinity, and oxygen vacancies [77][78][79][80][81][82]. The crystal structure of A 2 B 2 O 7 compounds can be tuned with the ionic radius ratio of r A and r B sites cations. In detail, pyrochlore structure exists when r A /r B is between 1.46 and 1.78, where A and B are distributed in order. The disordered defective fluorite phase formed when r A /r B is less than 1.46 and the arrangement of A and B ions are disordered; if r A /r B is higher than 1.78, the monoclinic phase will be formed [77,78]. The crystal structure diagram of pyrochlore and defective fluorite can be seen in Figure 7. be achieved around 800 °C [52,76]. Therefore, catalyst stability and selectivity of hydrocarbons are limited due to the suppression of active sites at very high temperatures and increased non-selective oxidation products. Therefore, there is still a strong motivation to explore novel catalysts with high reaction performance, particularly at low temperatures, to execute this reaction industrially.

Pyrochlore Catalysts
Pyrochlore (A2B2O7) catalysts are promising catalysts for OCM reaction due to their high melting points, thermal stability, tunable M-O bonding, surface alkalinity, and oxygen vacancies [77][78][79][80][81][82]. The crystal structure of A2B2O7 compounds can be tuned with the ionic radius ratio of rA and rB sites cations. In detail, pyrochlore structure exists when rA/rB is between 1.46 and 1.78, where A and B are distributed in order. The disordered defective fluorite phase formed when rA/rB is less than 1.46 and the arrangement of A and B ions are disordered; if rA/rB is higher than 1.78, the monoclinic phase will be formed [77,78]. The crystal structure diagram of pyrochlore and defective fluorite can be seen in Figure 7. The ideal pyrochlore (A2B2O7) structure belongs to the space group Fd-3m. It can be written as A2B2O6 O' form, and the formula denotes the existence of two different types of lattice oxygen ions in its crystal structure. Each lattice oxygen ion (O) is coordinated to two A and B sites cations, and the left lattice oxygen ion (O') is coordinated to four A sites cation. Compared to the ordered cubic fluorite structure, the pyrochlore compounds contain less of a lattice oxygen ion thus forming inherent oxygen vacancies, which increases the oxygen mobility of the pyrochlore catalysts [80,83,84]. Moreover, a typical pyrochlore (A2B2O7) compound containing rare-earth A-and B-site can offer surface alkalinity. In ad- The ideal pyrochlore (A 2 B 2 O 7 ) structure belongs to the space group Fd-3m. It can be written as A 2 B 2 O 6 O' form, and the formula denotes the existence of two different types of lattice oxygen ions in its crystal structure. Each lattice oxygen ion (O) is coordinated to two A and B sites cations, and the left lattice oxygen ion (O') is coordinated to four A sites cation. Compared to the ordered cubic fluorite structure, the pyrochlore compounds contain less of a lattice oxygen ion thus forming inherent oxygen vacancies, which increases the oxygen mobility of the pyrochlore catalysts [80,83,84]. 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].
Based on these properties, the A 2 B 2 O 7 type of materials has shown some promising catalytic properties in OCM reaction. Petit et al. [81] reported that the catalytic activity of pyrochlore (A 2 B 2 O 7 ; A-rare earth metal, B-Ti, Sn, Zr) on OCM mainly depended on the B-O bond strength. They found that lower B-O binding energy compounds provided higher C 2 yield. C. A. Mims et al., [86] prepared Bi-Sn pyrochlore catalysts and then expanded the series of Bi 2 Sn 2−x Bi x O 7−x/2 (0 ≤ x ≤ 0.86) with substitution of Bi cation in the B site of the pyrochlore. These catalysts were used for OCM and have shown improvement in C 2 selectivity as the number of Bi cations at B sites increased. In addition, A. C. Roger et al. [87] synthesized Sn-deficient pyrochlore (Sm 2 Sn 2 O 7 ) catalysts by using a sol-gel method, and these catalysts improved the C 2 yield in the OCM reaction.
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. Among them, La 2 Ce 2 O 7 exhibited the highest C 2 yield of 16.6% at 750 • C due to the moderated alkaline sites and surface active oxygen species [77]. To examine the relationship between phase structure and reactivity of pyrochlore catalysts for the OCM, three model La 2 B 2 O 7 (B-Ti, Zr and Ce) pyrochlore compounds with different crystal phases have been prepared. The crystalline phase of La 2 B 2 O 7 differs from monoclinic layered perovskite (La 2 Ti 2 O 7 ) to ordered cubic pyrochlore (La 2 Zr 2 O 7 ) and defective cubic fluorite (La 2 Ce 2 O 7 ) by declining the r A /r B ratios. These catalytic activities and C 2 yields follow the order of La 2 Ce 2 O 7 > La 2 Zr 2 O 7 > La 2 Ti 2 O 7 , being consistent with higher surface active oxygen species and moderate surface alkaline sites [78]. 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. La 2 Ce 2 O 7 pyrochlore catalysts have been synthesized in our group using the solgel method and further doped with Sr through optimization on Sr loading [89]. The defective cubic fluorite phase remained after Sr doping. Introduction of Sr in La 2 Ce 2 O 7 , especially allowing strontium to enter into the crystal lattice (La 1.5 Sr 0.5 Ce 2 O 7 ), significantly improved the mobility of lattice oxygen compared to the undoped pyrochlore (La 2 Ce 2 O 7 ) and impregnated Sr sample (8% Sr/La 2 Ce 2 O 7 ) (Figure 8). The relative contents of O 2− lattice oxygen of the three prepared catalysts, i.e., doped, impregnated and undoped, followed the order of La 1.5 Sr 0.5 Ce 2 O 7 > 8% Sr/La 2 Ce 2 O 7 > La 2 Ce 2 O 7 as shown in XPS results (see Figure 9, Table 4); these obtained results were in line with H 2 -TPR results and the OCM reaction performance. A sample containing a large amount of lattice oxygen sites showed better reaction performance (see Figure 11). CO 2 -TPD results (see Figure 10, Table 5) revealed that introduction of Sr enhanced the ratio of strong basic sites, which is usually considered as a beneficial factor for producing C 2 products. As a result, the selectivity and yield of C 2 -based products could reach 57% and 14%, respectively, over La 1.5 Sr 0.5 Ce 2 O 7 catalysts at 800 • C (see Figure 11). Moreover, this catalyst showed stable catalytic performance up to 30 h without deactivation (see Figure 12). order of La1.5Sr0.5Ce2O7 > 8% Sr/La2Ce2O7 > La2Ce2O7 as shown in XPS results (see Figure 9, Table 4); these obtained results were in line with H2-TPR results and the OCM reaction performance. A sample containing a large amount of lattice oxygen sites showed better reaction performance (see Figure 11). CO2-TPD results (see Figure 10, Table 5) revealed that introduction of Sr enhanced the ratio of strong basic sites, which is usually considered as a beneficial factor for producing C2 products. As a result, the selectivity and yield of C2based products could reach 57% and 14%, respectively, over La1.5Sr0.5Ce2O7 catalysts at 800 °C (see Figure 11). Moreover, this catalyst showed stable catalytic performance up to 30 h without deactivation (see Figure 12).  order of La1.5Sr0.5Ce2O7 > 8% Sr/La2Ce2O7 > La2Ce2O7 as shown in XPS results (see Figure 9, Table 4); these obtained results were in line with H2-TPR results and the OCM reaction performance. A sample containing a large amount of lattice oxygen sites showed better reaction performance (see Figure 11). CO2-TPD results (see Figure 10, Table 5) revealed that introduction of Sr enhanced the ratio of strong basic sites, which is usually considered as a beneficial factor for producing C2 products. As a result, the selectivity and yield of C2based products could reach 57% and 14%, respectively, over La1.5Sr0.5Ce2O7 catalysts at 800 °C (see Figure 11). Moreover, this catalyst showed stable catalytic performance up to 30 h without deactivation (see Figure 12).              [89].
In another study, we substitute the B site of La2Ce2O7 by Ca 2+ (0.1 nm) and Sr 2+ (0.126 nm), respectively, which have an identical atomic radius to Ce 4+ (0.097 nm). These catalysts showed better selectivity and yield than host compounds. XRD results ( Figure 13) reveal that all doped samples presented well-resolved peaks analogous to their host pyrochlore materials with only a slight shift of the 2θ values, signifying those doped compounds have similar phase structures to their host compounds. The quantitative analysis of H2-TPR results (see Figure 14, Table 6) revealed that the reduction temperature and reducible lattice oxygen species of doped samples were significantly enhanced than the un-doped sample; consequently, this rise in the reduction temperature indicated higher M-O bonding strength. Among these doped catalysts, La2Ce1.5Sr0.5O7 showed a higher reduction temperature and contained a larger amount of lattice oxygen, which was primarily responsible for the higher C2 selectivity in the OCM reaction. Therefore, this catalyst exhibited better yield at 800 °C (see Figure 15). In another study, we substitute the B site of La 2 Ce 2 O 7 by Ca 2+ (0.1 nm) and Sr 2+ (0.126 nm), respectively, which have an identical atomic radius to Ce 4+ (0.097 nm). These catalysts showed better selectivity and yield than host compounds. XRD results ( Figure 13) reveal that all doped samples presented well-resolved peaks analogous to their host pyrochlore materials with only a slight shift of the 2θ values, signifying those doped compounds have similar phase structures to their host compounds. The quantitative analysis of H 2 -TPR results (see Figure 14, Table 6) revealed that the reduction temperature and reducible lattice oxygen species of doped samples were significantly enhanced than the un-doped sample; consequently, this rise in the reduction temperature indicated higher M-O bonding strength. Among these doped catalysts, La 2 Ce 1.5 Sr 0.5 O 7 showed a higher reduction temperature and contained a larger amount of lattice oxygen, which was primarily responsible for the higher C 2 selectivity in the OCM reaction. Therefore, this catalyst exhibited better yield at 800 • C (see Figure 15).

Conclusions and Perspective
Natural gas has attracted considerable attention because of fluctuating fuel prices around the world and continuous depletion of energy reserves of crude oil, and also due to its unique features such as a clean source of fossil energy and as a feedstock of various other chemicals including ethylene. Methane is a major component of natural gas, and its deposits are expected to be higher than crude oil in the coming future. Ethylene is considered as a key building block in the field of industrial chemical production. Therefore, a strong economic interest is developing in the processes that allow the conversion of methane to high value-added products such as numerous intermediate products such as polymers, i.e., polyethylene, polystyrene; and hydrocarbons i.e., ethylene. In this regard, oxidative coupling methane (OCM) is an important reaction process for the catalytic upgrading of methane in natural gas and/or shale gas to ethylene for meeting its industrial production demand. This review article mainly consolidated recent literature that captured and evaluated the modifications during the catalyst development progress made regarding the OCM reaction.
The development of commercially viable catalysts for OCM reaction is still a crucial challenge for researchers. In order to obtain the desired goal, recently, various catalysts have been used based on the metal oxides such as reducible metal oxides, rare earth oxides, alkaline earth oxides, alkali-promoted metal oxides or mixed oxides, and pyrochlore compounds individually or in various combinations. In the current review, we have explored a variety of catalysts such as alkali/alkaline earth metals-based catalysts viz., Li supported catalysts, Mn-NaWO 4 /SiO 2 and pyrochlore catalysts. It has been reviewed that catalyst performance depends on several factors such as their preparation method, structural composition and the basicity of catalyst. From the reports, we have understood that active sites, facile oxygen abundance, surface alkalinity and metal-support interactions are crucial for catalytic performance. However, by using these catalysts, a low yield of C 2 products still was achieved (<30%) and could not match the commercial requirement.

Perspective
Li-based catalysts have shown higher activity and selectivity in OCM reaction due to strong basicity and M-O bond. However, the catalysts suffer deactivation during the reaction, being related to the loss of Li at high operation temperatures. Therefore, how to stabilize Li and restrain its evaporation at high temperatures by forming strong metal support interactions might be a possible solution for this. Meanwhile, it is noted that pyrochlore (A 2 B 2 O 7 ) catalysts have shown some promising properties for OCM reaction due to high melting points, thermal stability, tunable M-O bonding and moderate surface alkalinity. Using pyrochlore as the stable support to load Li might generate strong interactions between Li and pyrochlore, forming new phases of Li compounds, such as Li 2 SnO 3 and LiLaO 2 , which provides a feasible way to stabilize Li. Some preliminary results have been obtained in our group to show its promising perspectives. In addition, optimization of experimental conditions for the synthesis of pyrochlore catalysts, especially for those with specific morphology, might improve the catalytic performance at lower temperatures. Discrimination of the key factors caused by A/B regulation or substitution and their interactions with the doped Li give the opportunity to further enhance the catalytic activity and selectivity as well as the stability by stabilizing Li with optimized pyrochlore.

Conflicts of Interest:
The authors declare no conflict of interests.