2.1. 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
22− to lattice oxygen and a high amount of surface basic sites. Yıldız et al. [
33] proved that mesoporous TiO
2-rutile supported Mn
xO
y-Na
2WO
4 catalysts’ better performance than commercial TiO
2-rutile- and 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−xA
xO
3(A = Mg, Al, Ca, Ba, Pb, x = 0.1) and Sr
2Ti
1−xA
xO
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
2TiO
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
5Li
5) than Mn-Na
2WO
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
2O
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.
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
+O
−] centers. [Li
+O
−] catalyzes the CH
4 dehydrogenation in the following steps:
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
4c2+ 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
2CaSiO
4 than Mn-Na
2WO
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
2CaSiO
4 is likely to result from the combination of multiple cations in the crystal lattice. The structure of Li
2CaSiO
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
2O
3 supported on MgO and they found that the Li-Tb
2O
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
2O
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.
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
2O
3 supported on MgO and they found that the Li-Tb
2O
3/n-MgO catalyst exhibited higher stability and lower deactivation rate after 30 h on stream [
49], while Li
2CaSiO
4 showed the better catalytic activity and selectivity over Mn-Na
2WO
4/SiO
2 and also exhibited higher catalyst durability for 50 h on stream without deactivation and structural changes for the OCM reaction [
48].
2.2. 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]. Li S-B. et al., J. H. Lunsford et al. and R. M. Lambert et al. groups established Mn-NaWO
4/SiO
2 and the optimized chemical composition of this catalyst is 1.9 wt% Mn-5 wt% NaWO
4/SiO
2 with Na: W: Mn atom ratio of 2:1:2 [
55,
61,
62]. Ghose et al. [
54] prepared different nanostructured complex metal oxides such as Sr–Al complex oxides, La
2O
3, La–Sr–Al complex oxides, and Na
2WO
4-Mn/SiO
2 by using the solution combustion synthesis (SCS) method. Among these catalysts, Sr
3Al
2O
6 (double perovskite phase) in the Sr–Al oxides is active for OCM. The La
2O
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
2WO
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
2WO
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
2WO
4/Mn/Mg
0.05Ti
0.05Si
0.90O
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
2WO
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.
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
2O
3/Na
2WO
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
2WO
4/SiO
2 catalyst of the corresponding OCM reaction temperature by using different analytical techniques. The Mn-Na
2WO
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
2WO
4/SiO
2, Na
2WO
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
2WO
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
2WO
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
2WO
4/SiO
2 catalysts for OCM.
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
2WO
4/SiO
2 with other metal oxides such as La
2O
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
xZr
1−xO
2 catalysts using a sol-gel method with varied Ce/Zr molar ratios and then modified with Mn
2O
3-Na
2WO
4 doping. Among them, Ce
0.15Zr
0.85O
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
2WO
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
2O
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.
2.3. Pyrochlore Catalysts
Pyrochlore (A
2B
2O
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
2B
2O
7 compounds can be tuned with the ionic radius ratio of r
A and r
B 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 (A
2B
2O
7) structure belongs to the space group Fd-3m. It can be written as A
2B
2O
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
2B
2O
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
2B
2O
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
2B
2O
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
2Sn
2−xBi
xO
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
2Sn
2O
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
rA/
rB 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
2Ce
2O
7 (Ln = La, Pr, Sm and Y) catalysts have been developed and demonstrated as the defective cubic fluorite phase. Among them, La
2Ce
2O
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
2B
2O
7 (B-Ti, Zr and Ce) pyrochlore compounds with different crystal phases have been prepared. The crystalline phase of La
2B
2O
7 differs from monoclinic layered perovskite (La
2Ti
2O
7) to ordered cubic pyrochlore (La
2Zr
2O
7) and defective cubic fluorite (La
2Ce
2O
7) by declining the
rA/
rB ratios. These catalytic activities and C
2 yields follow the order of La
2Ce
2O
7 > La
2Zr
2O
7 > La
2Ti
2O
7, being consistent with higher surface active oxygen species and moderate surface alkaline sites [
78]. Aimed to optimize the La
2Ce
2O
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
2Ce
1.5Ca
0.5O
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
2Ce
2O
7 pyrochlore catalysts have been synthesized in our group using the sol-gel 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
2Ce
2O
7, especially allowing strontium to enter into the crystal lattice (La
1.5Sr
0.5Ce
2O
7), significantly improved the mobility of lattice oxygen compared to the undoped pyrochlore (La
2Ce
2O
7) and impregnated Sr sample (8% Sr/La
2Ce
2O
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
1.5Sr
0.5Ce
2O
7 > 8% Sr/La
2Ce
2O
7 > La
2Ce
2O
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
1.5Sr
0.5Ce
2O
7 catalysts at 800 °C (see
Figure 11). Moreover, this catalyst showed stable catalytic performance up to 30 h without deactivation (see
Figure 12).
In another study, we substitute the B site of La
2Ce
2O
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
2Ce
1.5Sr
0.5O
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).