Guest Commentary pubs.acs.org/JPCL
Methane to Light Hydrocarbons via Oxidative Methane Coupling: Lessons from the Past to Search for a Selective Heterogeneous Catalyst
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that exhibited the classic selectivity conversion plot for a series reaction (A → B → C). In such reactions, high selectivity is obtained at low conversion, decreasing as conversion increases, resulting in a maximum C2 yield of about 30−35%. Lunsford and co-workers,4 using matrix isolation, demonstrated that on Li/MgO, one of the most studied catalysts for OMC, the main role of the catalyst was to produce methyl radicals (CH3•), which recombined to yield C2H6. This led to a free radical unselective reaction pathway in which ethane was further dehydrogenated to form C2H4 and oxidized to form carbon oxides that limited the C2 yield. Such a pathway was generic for most oxide catalysts and their mixtures, which under reaction conditions often resulted in complex multiple phases. As a result, the presence of multiple phases further complicated the mechanistic interpretation of the role of each phase. A summary of the reaction pathway on oxide catalyst mixtures is presented in a cartoon in Scheme 1, which includes possible phases and reaction steps (adapted from ref 1). Scheme 1 shows a generic oxide catalyst “BO” (such as MgO or La2O3) promoted by an alkali or alkaline earth metal oxide “A” (such as Li2O or SrO), referred hereafter as A/BO for simplicity. While in most studies the surface was assumed to be homogeneous, in reality, such catalysts can form various oxide phases (AO, A2O, BO), mixed oxides (A2BO3), or a solid solution with B, and/or carbonates might also be present. At the time, most surface characterization studies were done ex situ; therefore, phase identification was done before and after reaction, using techniques such as XRD at room temperature and with the catalyst exposed to air. For that reason, in Scheme 1, the surface is shown as an undefined line over the various hypothetical phases that could exist under reaction conditions. In addition, some phases can exist in semiliquid form and can volatilize or sinter during the reaction, thus changing in composition with time on stream (see ref 1). Thus, the surface composition depends on various factors such as the nature and the loading of A onto B, the reactants partial pressure and temperature, and the time on stream. Therefore, clearly, the surface structure and composition of these catalysts is complex and not well understood under reaction conditions, and it was difficult to correlate the activity with material properties. The gas-phase pathway shows that methyl radicals recombine to form other hydrocarbons and hydroperoxy radicals, which dimerize to become the desirable C2 products but can also undergo further oxididation to undesirable carbon oxides. One of the early contributions from our group was to differentiate between gas-phase and surface reactions.1,5a We found that at the temperatures at which methyl radicals form on these oxides (above 700 °C), the gas-phase reactions (in an empty reactor) were significant, and in some reports, they were as significant as the catalytic reactions because there was more gas volume in
ith the use of the new fracking methods to extract natural gas (and oil) from shale (and old oil wells), the availability of natural gas has increased significantly, and the conversion of gas to liquids has become, once again, a topic of renewed research interest. The conversion of methane, one of the main components of natural gas, to liquids is a process operating at industrial scale in several parts of the world (i.e., South Africa, Malaysia, Qatar). It is carried out via the steam reforming of methane to synthesis gas followed by Fischer− Tropsch synthesis. This process is profitable despite the many thermodynamic limitations that such transformations entail. The conversion of methane to ethane and ethylene or C2 hydrocarbons via oxidative methane coupling (OMC, 2CH4 + 1/2O2 → C2H6 + H2O) is a reaction that in principle offers a more energetically favorable pathway. The search for a catalyst for OMC was the subject of intensive research in the 1980s− 1990s, and the author of this Guest Commentary was involved in such efforts and edited a book that summarized much of the work done by the main players involved.1 The purpose of this Guest Commentary is to summarize the key results of such efforts so that new research focuses on new avenues rather than rediscovering what was done in the past Essentially, prior research served to establish that the catalysts studied at the time were not selective enough to produce a yield of C2 hydrocarbons that was economically viable; hence, funding, and ultimately research in this field, was significantly curtailed. The initial drivers for this research were linked to the exothermic nature of the OMC, which, although thermodynamically advantageous, poses an engineering challenge. The heat generated by the reaction must be removed, but this challenge is not an unsurmountable one and can be addressed using engineering ingenuity. The most challenging aspect of the reaction lies in the stability of the methane molecule that requires significant energy to break the C−H bond (∼438 KJ/mol). Additionally, once the first hydrogen is removed from methane, the energies required to abstract the subsequent hydrogen are similar, resulting in the abstraction of more hydrogen atoms from methane, ultimately leading to undesirable carbon formation. Hence, the search for a selective catalyst that could lower the activation energy barrier, resulting in a significant C2 yield in the presence of oxygen, became the objective of the research previously carried out. Much of the work in the early 1980s consisted primarily of catalyst screening, which quickly turned the focus to oxide catalysts. Apparently, the first report on the conversion of methane to higher hydrocarbons came from the patent literature,2 followed by the paper by Keller and Bhasin.3 These authors used reducible oxides to yield C2 hydrocarbons via lattice oxygen and later in the presence of gas-phase oxygen. Their work led the field to the search for oxide catalysts and their combinations often promoted by alkaline oxides, driving the exploration of the periodic table, which uncovered catalysts © 2014 American Chemical Society
Published: March 20, 2014 986
dx.doi.org/10.1021/jz500197h | J. Phys. Chem. Lett. 2014, 5, 986−988
The Journal of Physical Chemistry Letters
Guest Commentary
Scheme 1. Schematic Model of the OMC Reaction Occurring on an Alkali (A) Promoted Oxide (BO), Showing Possible Phases That Could Exist under Reaction Conditions
transition metals and noble metals often abstract all hydrogen atoms from methane, leading to carbon formation, which in the presence of adsorbed oxygen leads to carbon oxides. It follows that materials that can trap or adsorb methane without abstracting hydrogen and that do not adsorb oxygen should be good candidates. Carbides or other phases such as carbonates or sulfides of complex materials should be explored via theoretical and experimental methods. Materials such as special exchanged zeolites or metal−organic frameworks (MOFs) used as supports could also be investigated along these lines to trap or absorb methane. The functionalization of homogeneous catalysts in a heterogeneous substrate is also an area that needs to be investigated. The oxygen reaction pathway is another critical area to consider. Its main role should be one to catalyze a single hydrogen abstraction from adsorbed or trapped methane via an Eley−Rideal pathway and form water. We must explore whether oxygen should be added to the gas phase or be part of the solid structure, which today would be called “chemical looping”. However, the catalyst’s integrity could be a problem during the regeneration process. Yet, this strategy is a promising one, and knowledge of oxygen transport and mechanical properties in solids should be studied in more detail by theory. The above suggestions are but a few of the many routes that can be explored. Much needs to be done, and new reviews on the subject already available8 should be thoroughly studied before going back to what has already been done. Clearly, the challenges are significant, and it is unlikely that new results will be produced by trial and error only. Unless a rational approach is used, the wheel of methane activation will be rediscovered several more times in the future.
the reactor than catalysts sites. Oxygen from the gas phase could also substitute with lattice oxygen, which we found to occur at much lower temperatures than expected.5b In addition, heat and mass transfer limitations further complicate the kinetic analysis of the available results. A key issue in this reaction pathway is that methane did not adsorb significantly on these oxides as in a classical catalytic sequence but rather collided with the surface, forming free radicals. The free radicals further reacted unselectively with the desired C2 products and other gas-phase species and could get oxidized in the gas phase or on the surface by either lattice oxygen or adsorbed oxygen, lowering C2 selectivity. Oxygen can be adsorbed or dissociate and exchange with the lattice, and a great deal of research and discussion has focused on the oxygen exchange capacity, vacancies, oxygen mobility in the oxides, and the nature of the oxygen species (O− or O). The hydrogen abstracted from methane combines with surface oxygen to form OH− species that eventually produced H2O. A reactor design that can potentially reduce the role of oxygen was the use of a membrane reactor in which oxygen was fed to one side of the membrane and methane was on the other side. As such, the available gas-phase oxygen in the reaction was minimized, but the throughput through solid membranes were very small. Furthermore, the higher temperatures required by the OCM reaction prevented such membranes from being effective in the long term as they sintered or pin holes developed, which diminished their effectiveness.6 In essence, what past research has taught us is that future research in the OCM should aim at discovering a solid catalyst in which the reactants follow a catalytic pathway instead of one involving free radical reactions. Combinatorial tools have been used recently7 to broaden the search space, but so far, they ended up selecting similar combinations of oxides catalysts as in the past, with similar selectivity limitations. As in many areas involving the conversion of gas to liquids, new tools are or will be used, but they can yield similar results as those previously obtained, as outlined above for the OMC reaction, unless a new approach is taken. What is needed is a true rational approach to catalyst design and not strategies that often turn out to be buzz words disguising old approaches. Therefore, what are the key ideas that should inform future research for the OMC reaction? Ultimately, we must search for materials that activate methane through a true catalytic pathway involving adsorption, surface reaction, and desorption. Solid oxides in general do not adsorb methane below 600 °C, the temperature at which methyl radical formation starts in the gas phase. Reduced
E. E. Wolf
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Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States
REFERENCES
(1) Wolf, E. E., Ed. Methane Conversion by Oxidative Processes, Fundamental and Engineering Aspects; Van Nostrand-Reinhold: New York, 1992. (2) Mitchell, H. L.; Waghorne, R. H. Catalysts for the Conversion of Relatively Low Molecular Weight Hydrocarbons to Higher Molecular Weight Hydrocarbons and the Regeneration of the Catalysts. U.S. Patent 4,239,658 A, 1980.
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Guest Commentary
(3) Keller, G. F.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane, Determination of Active Catalysts. J. Catal. 1982, 73, 9−19. (4) (a) Driscoll, D. J.; Lunsford, J. H. Gas-Phase Radical Formation during the Reactions of Methane, Ethane, Ethylene and Propylene. J. Phys. Chem. 1985, 89, 4415−4418. (b) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. Formation of Gas-Phase Methyl Radicals over MgO. J. Am. Chem. Soc. 1985, 107, 58−63. (5) (a) Lane, G.; Wolf, E. E. Methane Utilization by Oxidative Coupling. J. Catal. 1988, 113, 144. (b) Wolf, E. E.; Kalenik, Z. Temperature Programmed Isotopic Exchange of Lattice Oxygen during Methane Oxidative Coupling. Catal. Lett. 1991, 11, 309−18. (6) Sanches-Marcano, J. G.; Tsotsis, T. T. Catalytic Membrane and Membrane Reactors; Wiley-VCH, Verlag GmbH: Weinheim, Germany, 2002. (7) Olivier, L.; Haag, S.; Pennemann, H.; Hofman, C.; Mirodatos, C.; van Veen, A. C. Recent Developments in Combinatorial Catalysis Research and High-Throughput Technologies. Catal. Today 2008, 137, 80−89. (8) Hammonds, C.; Conrad, S.; Hermans, I. Oxidative Methane Upgrading. ChemSusChem 2012, 1668−1686.
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