Oxidative coupling of methane (OCM): An overview of the challenges and opportunities for developing new technologies
Graphical abstract
Introduction
The exploitation of unconventional reservoirs such as shale, coalbed, and tight formations by hydraulic fracturing (fracking) has increased worldwide reserves of natural gas in the past decade (Kerr, 2010; McFarland, 2012). In this regard, methane, the main component of natural gas, has gained attention as a primary source of fuels and chemicals (Kerr, 2010; McFarland, 2012; Horn and Schlögl, 2015; Taifan and Baltrusaitis, 2016). Unfortunately, most of these reservoirs are localized in remote areas, making methane transportation economically inviable. Because methane cannot be converted on site into easily transportable hydrocarbons or chemicals, it becomes an underused byproduct that is burned, releasing greenhouse gases (COx) (Horn and Schlögl, 2015; Taifan and Baltrusaitis, 2016; Schwach et al., 2017). Therefore, developing efficient methane upgrading technologies to utilize natural gas economically and make the oil industry more environmentally friendly is needed. The quest for such technology has renewed interest in catalytic methods and reactor configurations for methane upgrading. Nowadays, methane can be converted via indirect or direct routes, as shown in Fig. 1.
The industrial conversion of methane typically proceeds via indirect routes, which require the intermediate step of producing synthesis gas (syngas, CO + H2) (Horn and Schlögl, 2015; Schwach et al., 2017). Here, methane is initially reformed to syngas and then converted to olefins, gasoline, diesel, or oxygenates via the Fischer-Tropsch synthesis (FTS). Furthermore, syngas can be converted to methanol and then to olefins (methanol-to-olefins), gasoline (methanol-to-gasoline), or aromatics (methanol-to-aromatics) (Horn and Schlögl, 2015; Schwach et al., 2017). However, the sequence of initially oxidize methane to syngas and then reduce CO to a desirable product has several drawbacks. Firstly, methane reforming processes are energy-intensive and require high temperatures and pressures (15–40 atm, 900–1200 K) (Iulianelli et al., 2016; Jang et al., 2019). Secondly, the FTS requires H2 or CO to reduce CO, which results in an efficiency of carbon-atom utilization lower than 50% (Guo et al., 2014). Furthermore, the H2 used here is typically obtained by cracking naphtha, which emits enormous CO2 amounts. Finally, methane reforming and FTS processes have operating problems, such as carbonaceous deposit formations and sintering, which lead to catalyst deactivation (Rytter and Holmen, 2015; Jang et al., 2019).
The conversion of methane also proceeds via direct routes, which are cheaper and simpler because they do not involve syngas production. These routes can be either nonoxidative, such as methane to olefins, aromatics, and hydrogen (MTOAH) (Guo et al., 2014) and methane dehydroaromatization (MDA) (Wang et al., 1993) or oxidative, such as oxidative coupling of methane (OCM) (Keller and Bhasin, 1982). Although the nonoxidative routes offer some advantages, such as potential operation in remote areas since no reagents are needed and no explosion risks or COx emissions because of the absence of oxygen, they present several drawbacks (Spivey and Hutchings, 2014; Schwach et al., 2017; Huang et al., 2018). While MTOAH suffers from high-temperature requirements and narrow operating conditions, MDA suffers from coke formation, catalyst stability issues, and low methane conversion (Schwach et al., 2017). Conversely, the OCM route occurs at wider operating conditions, lower temperatures, and higher methane conversions. The OCM directly converts methane into ethane and ethylene (C2 hydrocarbons), with ethylene being an important building block to produce various chemicals and polymers (Fan et al., 2013; Schwach et al., 2017; Gao et al., 2019). Since the unprecedented work of Keller and Bhasin (1982), the OCM route has attracted attention as a technology to supply the chemical industry with raw materials and minimize the crude-oil dependency. However, after decades of research efforts, the industrial deployment of the OCM route remains limited (Kuo et al., 1989; Ortiz-Espinoza et al., 2017; Spallina et al., 2017; Cruellas et al., 2019). This is because the OCM route suffers from selective issues, with undesirable deep oxidation compounds (COx) being more thermodynamically stable and producing faster than C2 hydrocarbons (Farrell et al., 2016).
In this perspective, we present a concise but critical overview of the challenges and opportunities for developing efficient methane upgrading technologies via OCM. We highlight the obstacles that hinder the industrial deployment of the OCM route: the high stability of methane and the thermodynamic and kinetic limitations. From heterogeneous catalytic methods to reactor configurations, we analyze some potential and promising strategies to overcome these limitations. Through a systematic analysis of the literature, we discuss the current knowledge of the OCM catalysts, emphasizing the relationship between the catalyst properties and the activation of methane and gas-phase oxygen. The topics cover some fundamentals to be considered to design improved OCM catalysts, such as C–H bond activation, tuning of electronic properties of oxide-based catalysts, and active oxygen species. Additionally, we point out some common insights about the Mn–Na2WO4/SiO2 catalyst by contrasting the well-consolidated literature with recent fast-developing in situ/operando studies. Finally, we discuss some new reactor configurations and the possibility of incorporating highly selective catalysts into them.
The purpose of this overview is not summarizing the extensive OCM literature, which is well discussed in excellent review articles covering the chemistry of methane activation (Schwarz, 2011; Schwach et al., 2017), heterogeneous catalytic methane conversion (Arndt et al., 2011a, b; Horn and Schlögl, 2015; Galadima and Muraza, 2016; Olivos-Suarez et al., 2016; Taifan and Baltrusaitis, 2016; Gambo et al., 2018; Kiani et al., 2019), the use of alternative oxidants (Arinaga et al., 2020), and reactor technologies (Farrell et al., 2016; Karakaya and Kee, 2016). Conversely, we aim to give the reader an updated perspective of methane upgrading technologies by contrasting the classic literature with recent studies and providing our opinion on future research directions.
Section snippets
Thermodynamic limitations
To understand the inherent challenges of the direct methane conversion, we initially discuss the thermodynamic feasibility of some potential reactions in this section. Considering the MDA route as a set of separate reactions, the direct conversion of methane to benzene (C6H6), naphthalene (C10H8), ethane (C2H6), and ethylene (C2H4) may be described as follows:
Kinetic limitations
The complete analysis of the proposed OCM kinetic models goes beyond the scope of the present overview. For this purpose, the literature presents previous excellent review articles (Sinev, 2003; Lomonosov and Sinev, 2016; Kiani et al., 2019). Rather, we aim to discuss OCM kinetic limitations to understand the methane conversion pathways and find potential improvements. The analysis of the OCM kinetic limitations is, however, challenging because the complete set of the catalytic reaction
Methane activation
The methane activation occurs by abstracting a hydrogen atom via C–H bond cleavage. However, the methane molecule is highly stable, exhibiting strong (first bond dissociation energy = 439.3 kJ mol−1) (Schwach et al., 2017) and weakly polarized (2.84 × 10−40 C2 m2 J−1) C–H bonds (Amos, 1979). This is due to the geometrical structure of the methane molecule, where a central carbon atom is tetrahedrally coordinated (point group, Td) to four hydrogen atoms (sp3 hybridization) with C–H bond lengths
Catalysts for OCM
As discussed before, OCM requires high temperatures (973 − 1123 K) and an oxidant to cleave the strong C–H bond of methane. At these conditions, however, COx compounds are more stable thermodynamically and form faster than C2 hydrocarbons. Thus, the main challenge in developing OCM catalysts is to activate methane by oxidation reactions and stop combustion reactions towards COx. To rationalize the design of improved catalysts, in this section, we discuss the current knowledge of the OCM
Engineering approaches to overcome the OCM limitations
As discussed before, the inherent thermodynamic and kinetic limitations of the OCM route lead to low C2 yields, hampering its industrial deployment. The thermodynamic limitations can be summarized as follows: i) the energetic preference to form COx compounds and ii) the high exothermicity that leads to the appearance of zones of elevated temperature (hot spots) where deep oxidation reactions are favored. In addition, the kinetic limitations are i) the faster formation of undesired COx compounds
Summary
The increasing reserves of natural gas have renewed interest in developing technologies to upgrade methane. Direct methane conversion routes are more economical and simpler than indirect ones because they do not need the intermediate step of producing syngas. OCM is a promising route to directly convert methane into C2 hydrocarbons, but its industrial deployment is still limited for low C2 yields. This is due to inherent thermodynamic and kinetic limitations. The thermodynamic limitations are i
Research perspectives
The development of improved catalysts and reactors for OCM still has several challenges. Further advancement in this field will require the following:
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The deactivation mechanism and the structure-activity/selectivity relationship of the Li–MgO catalyst should be elucidated. Further improvements of the Li–MgO catalyst will require in situ/operando studies to fully understand the temperature dependence of the catalyst structure and deactivation mechanism. For this, co-doping MgO with Li and an
Concluding remarks
The OCM route is of great interest as a means of converting methane directly into ethylene, a key building block in the chemical industry. This review, however, shows that OCM has not achieved industrial application due to inherent thermodynamic and kinetic limitations. The unselective COx compounds are thermodynamically more stable and produce faster than C2 hydrocarbons, which lead to low, not economically viable C2 yields. From heterogeneous catalysis to reactor engineering, we have
Supplementary material
The Supplementary Material contains the thermodynamic calculations of a set of possible reactions that can occur in the OCM and MDA routes.
Notes
The authors declare no competing financial interest.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. C.A.O–B gratefully acknowledge the financial support of the Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ) (process No. E-26/200.785/2019) for scholarship grant.
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