Review pubs.acs.org/CR
Homogeneous Functionalization of Methane Niles Jensen Gunsalus,† Anjaneyulu Koppaka,† Sae Hume Park,† Steven M. Bischof,† Brian G. Hashiguchi,† and Roy A. Periana*,† The Scripps Energy & Materials Center, The Scripps Research Institute, Jupiter, Florida 33458, United States ABSTRACT: One of the remaining “grand challenges” in chemistry is the development of a next generation, less expensive, cleaner process that can allow the vast reserves of methane from natural gas to augment or replace oil as the source of fuels and chemicals. Homogeneous (gas/liquid) systems that convert methane to functionalized products with emphasis on reports after 1995 are reviewed. Gas/solid, bioinorganic, biological, and reaction systems that do not specifically involve methane functionalization are excluded. The various reports are grouped under the main element involved in the direct reactions with methane. Central to the review is classification of the various reports into 12 categories based on both practical considerations and the mechanisms of the elementary reactions with methane. Practical considerations are based on whether or not the system reported can directly or indirectly utilize O2 as the only net coreactant based only on thermodynamic potentials. Mechanistic classifications are based on whether the elementary reactions with methane proceed by chain or nonchain reactions and with stoichiometric reagents or catalytic species. The nonchain reactions are further classified as CH activation (CHA) or CH oxidation (CHO). The bases for these various classifications are defined. In particular, CHA reactions are defined as elementary reactions with methane that result in a discrete methyl intermediate where the formal oxidation state (FOS) on the carbon remains unchanged at −IV relative to that in methane. In contrast, CHO reactions are defined as elementary reactions with methane where the carbon atom of the product is oxidized and has a FOS less negative than −IV. This review reveals that the bulk of the work in the field is relatively evenly distributed across most of the various areas classified. However, a few areas are only marginally examined, or not examined at all. This review also shows that, while significant scientific progress has been made, greater advances, particularly in developing systems that can utilize O2, will be required to develop a practical process that can replace the current energy and capital intensive natural gas conversion process. We believe that this classification scheme will provide the reader with a rapid way to identify systems of interest while providing a deeper appreciation and understanding, both practical and fundamental, of the extensive literature on methane functionalization. The hope is that this could accelerate progress toward meeting this “grand challenge.”
CONTENTS 1. Introduction 2. Classification Scheme 2.1. Classification Based on Practical Considerations 2.1.1. H2SO4/SO3 Systems 2.2. Classification Based on Mechanistic Considerations 2.2.1. Details of the Mechanistic Classification 3. Catalytic Systems That Utilize O2/O2-Regenerable Oxidants and Operate by Nonchain, CH Activation Reactions 3.1. Platinum 3.2. Mercury 3.3. Palladium 3.4. Rhodium 3.5. Iodine 4. Stoichiometric Systems That Utilize O2/O2-Regenerable Oxidants and Operate by Nonchain, CH Activation Reactions 4.1. Mercury 4.2. Palladium © 2017 American Chemical Society
5. Catalytic Systems That Utilize O2/O2-Regenerable Oxidants and Operate by Nonchain, CH Oxidation Reactions 5.1. Europium 5.2. Ruthenium 6. Stoichiometric Systems That Utilize O2/O2-Regenerable Oxidants and Operate by Nonchain, CH Oxidation Reactions 7. Chain Reactions That Utilize O2/O2-Regenerable Oxidants 7.1. Peroxide Initiators 7.2. Rhodium 7.3. Vanadium 7.4. Bromine 7.5. Cerium
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Chemical Reviews 8. Catalytic Systems That Utilize Non-O2-Regenerable Oxidants and Operate by Nonchain, CH Activation Reactions 8.1. Platinum 8.2. Palladium and Copper 8.3. Iridium 8.4. Scandium 8.5. Gold 9. Stoichiometric Systems That Utilize Non-O2Regenerable Oxidants and Operate by Nonchain, CH Activation Reactions 9.1. Thallium and Lead 9.2. Iodine 10. Catalytic Systems That Utilize Non-O2-Regenerable Oxidants and Operate by Nonchain, CH Oxidation Reactions 10.1. Palladium and Copper 10.2. Silver 10.3. Heteropoly Acids 10.4. Manganese and Cobalt 10.5. Vanadium and Copper 11. Stoichiometric Systems That Utilize Non-O2Regenerable Oxidants and Operate by Nonchain, CH Oxidation Reactions 11.1. Superacids 11.2. Osmium 11.3. Gas Phase Metal Ions 11.4. Chloride and Iodate 11.5. Dioxirane 12. Chain Reactions That Utilize Non-O2-Regenerable Oxidants 12.1. Osmium 12.2. Vanadium 12.3. Iron 12.4. Calcium 12.5. Copper 12.6. Manganese and Ytterbium 12.7. Mercury 13. Summary and Conclusion Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments References
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Figure 1. Various quantitative measures of reactivity of methane, ethylene, and methanol.
utilization of the CH4 in natural gas from remote locations or as a transportation fuel requires its chemical conversion into a liquid product because of the high costs of transporting a gas that cannot be liquefied at practically accessible lower temperatures (critical temperature of methane is −82.3 °C). The most ideal liquid products, such as CH3OH, (CH3)2O, longer alkanes, diesel, gasoline, etc., would retain most all of the energy content of CH4. Methanol or dimethyl ether would also serve as an efficient, flexible chemical feedstock in place of petroleum as commercial technology exists to produce most bulk chemicals, olefins, aromatics, and saturated hydrocarbons either directly or indirectly from these precursors. The central challenge to methane functionalization is the molecule’s low intrinsic reactivity, coupled with the higher reactivity of desired products such as ethylene or methanol. This can lead to overoxidized products such as CO2 or coke in the conversion of methane. This is particularly relevant to typical commercial oxidation reactions as these processes generally involve radicals.11,12 The fundamental basis for this relative reactivity can be understood by comparing various qualitative measures of reactivity for methane with those of desired products, such as ethylene and methanol, as shown in Figure 1. As can be seen, by any of these measures, methane would be expected to be substantially less reactive than either ethylene or methanol. Thus, the homolytic bond dissociation energy (BDE) of the CH bonds of methane is ∼11 kcal/mol higher than those of methanol and 41 kcal/mol higher than the π-bond of ethylene. As reactions proceeding by free-radical, H-abstraction mechanisms strongly correlate with BDE, this would explain the much higher rate of oxidation of methanol relative to methane in freeradical, autoxidation processes. Similarly, the rate of addition of radicals to the π-bond of ethylene would be expected to be much higher than that of H atom abstraction from methane. Most commercial oxidation chemistry involving the direct use of O2 proceeds by radical reactions. This is the primary reason no process for the direct conversion of methane and O2 to generate methanol has yet been developed. All attempts at such a reaction have led to impractically low selectivity for methanol. This also provides a basis for the well-established challenges with low selectivity at high methane conversion in the extensively examined area of so-called “oxidative coupling” of methane to ethylene.13−16 The reactions, with nucleophiles (qualitatively measured by electron affinity), with protons (measured by the
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1. INTRODUCTION One of the most abundantly available carbon-based feedstocks, methane (CH4), is the smallest saturated hydrocarbon and the major component in natural gas.1−6 Comparable to oil on a per energy content basis (CH bonds per carbon atom),7,8 natural gas is available throughout the world. A remaining “grand challenge”9,10 in chemistry is the development of a next generation process based on methane conversion that could economically and cleanly generate all products currently generated from petroleum (perhaps with the exception of tar). Importantly, only large volume products such as fuels or commodity chemicals will make any significant impact in meeting this grand challenge. While natural gas located in developed areas can be transported by pipeline, efficient 8522
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more expensive oxidants such as hydrogen peroxide. However, the most impactful process, the “grand challenge”,9,10 is a largescale process that utilizes methane to augment or replace petroleum. Consequently, given the low cost of fuels and commodity chemicals compared to the relatively high costs of oxidants such as H2O2, any large-scale process must utilize only O2 (or ideally air) as the only coreactant.
proton affinity), by loss of an electron (measured by ionization potential), with electrophiles (measured by highest occupied molecular orbital (HOMO) energy), and by deprotonations (measured by pKa) all predict substantially higher reactivity of methanol and ethylene relative to methane (Figure 1).17 The gaseous property of methane also manifests challenges given the low solubility relative to the complete miscibility of methanol in most reaction solvents. Thus, if methane can be converted to methanol in any practical system, the methanol concentration will be much higher than that of methane as the reaction proceeds to any practical extent. Consequently, maintaining high selectivity requires that methane react much faster than methanol. Assuming a reactor with a 1:1 gas to liquid ratio and 500 psi methane pressure, kinetic models show that the relative rate constants for the conversion of methane to methanol and methanol to CO2 must be at least 20:1 to achieve >90% methanol selectivity at >15% conversion of methane.18 Given this requirement to reverse the typical reactivity of methane and methanol, it would be ideal for next generation catalysts to operate by non free radical reactions. Due to the challenges associated with the direct conversion of methane to methanol, current commercial processes operate by initial conversion of methane to syngas (CO/H2) at high temperatures (>800 °C generated directly or indirectly by combustion of methane with O2 or air) followed by a separate lower temperature conversion of the syngas to methanol. Selectivity is high in this process as the conversion of methane to syngas is under thermodynamic control. The commercial methanol process operates at very high carbon yields of ∼70%,19 but the high temperatures, large temperature variations, and multiple steps lead to a high capital, emissions, and an energy intensive process. This results in prohibitively high costs that limit the use of methanol in the fuel industry. Significantly, capital costs contribute more than 60% of the expense currently associated with methanol production.1,20,21 Given the relatively high carbon yield of the commercial process, the key challenge for any new process, in addition to maintaining or exceeding this high yield (and therefore requiring high selectivity), is to substantially reduce capital costs and, ideally, energy consumption and emissions. One key to dramatically reducing the capital cost is to design new processes that operate at temperatures below 250 °C, which would allow the use of less expensive reactors and heat management systems. To provide a thermodynamic driving force for the direct, chemical conversion of CH4 to liquid products at lower temperatures, a coreactant (typically an oxidant) is required. The hypothetical chemical processes, the dehydrogenative couplings of methane to ethane and H2, eq 1, and of methane and H2O to generate methanol and H2, eq 2, are not thermodynamically feasible at lower temperatures and only proceed at temperatures above 800 °C. However, as can be seen, the generation of these products becomes viable at lower temperatures with the use of a coreactant such as O2, eqs 3 and 4, respectively. In developing any new large scale process for conversion of CH4, it is important to note that the existing high temperature, commercialized syngas-based processes utilize only O2 or air as the co-oxidant. This is because O2 (ideally air) is the only economically viable coreactant for the large scale chemical conversion of CH4 to high volume products such as methanol or fuels.22 Even the use of pure O2 versus air can add undesirable expense given the significant costs for the O2-separation plant. It is possible that smaller scale products such as methanesulfonic acid,23 methyl chloride, methyl bromide, etc., could utilize other
2CH4 → CH3CH3 + H 2
ΔG200 ° C = +70 kJ/mol
CH4 + H 2O → CH3OH + H 2
ΔG200 ° C = +107 kJ/mol
2CH4 + 1/2O2 → CH3CH3 + H 2O
CH4 + 1/2O2 → CH3OH
(1) (2)
ΔG200 ° C = −139 kJ/mol (3)
ΔG200 ° C = −103 kJ/mol
(4)
As capital costs are primarily related to reactor expense, heat management, and separations, the five key requirements for any new process are (I) high selectivity of >90% (sets the level of emissions), (II) space-time yield (STY), also referred to as “volumetric productivity”, of ∼10−6 mol cm−3 s−1 (sets the size and cost of reactors), (III) temperatures of .8. Efficient oxygenation of methane and other lower alkanes in acetonitrile. Tetrahedron 1997, 53, 3603−3614. (244) Süss-Fink, G.; Nizova, G. V.; Stanislas, S.; Shul’pin, G. B. Oxidations by the reagent < O2−H2O2 − vanadate anion − pyrazine-2carboxylic acid>.: Part 10 - Oxygenation of methane in acetonitrile and water. J. Mol. Catal. A: Chem. 1998, 130, 163−170. (245) Gonzalez Cuervo, L.; Kozlov, Y. N.; Süss-Fink, G.; Shul’pin, G. B. Oxidation of saturated hydrocarbons with peroxyacetic acid catalyzed by vanadium complexes. J. Mol. Catal. A: Chem. 2004, 218, 171−177. (246) Shul’pina, L. S.; Kirillova, M. V.; Pombeiro, A. J. L.; Shul’pin, G. B. Alkane oxidation by the H2O2−NaVO3−H2SO4 system in acetonitrile and water. Tetrahedron 2009, 65, 2424−2429. (247) Süss-Fink, G.; Stanislas, S.; Shul’pin, G. B.; Nizova, G. V.; Stoeckli-Evans, H.; Neels, A.; Bobillier, C.; Claude, S. Oxidative functionalisation of alkanes: synthesis, molecular structure and catalytic implications of anionic vanadium(V) oxo and peroxo complexes containing bidentate N, O ligands. J. Chem. Soc., Dalton Trans. 1999, 3169−3175. (248) Süss-Fink, G.; Stanislas, S.; Shul’pin, G. B.; Nizova, G. V. Catalytic functionalization of methane. Appl. Organomet. Chem. 2000, 14, 623−628. (249) Romakh, V. B.; Süss-Fink, G.; Shul’pin, G. B. Vanadate ioncatalyzed oxidation of methane with hydrogen peroxide in an aqueous solution. Pet. Chem. 2008, 48, 440−443. Published in original Russian text in: Romakh, V. B.; Süss-Fink, G.; Shul’pin, G. B. Neftekhimiya 2008, 48, 437−440. (250) Taniguchi, Y.; Hayashida, T.; Shibasaki, H.; Piao, D. G.; Kitamura, T.; Yamaji, T.; Fujiwara, Y. Highly Efficient VanadiumCatalyzed Transformation of CH4 and CO to Acetic Acid. Org. Lett. 1999, 1, 557−560. 8572
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(269) Annese, C.; D’Accolti, L.; Fusco, C.; Curci, R. Selective Hydroxylation of Methane by Dioxiranes under Mild Conditions. Org. Lett. 2011, 13, 2142−2144. (270) Chan, S. I.; Lu, Y.-J.; Nagababu, P.; Maji, S.; Hung, M.-C.; Lee, M. M.; Hsu, I.-J.; Minh, P. D.; Lai, J. C.-H.; Ng, K. Y.; Ramalingam, S.; Yu, S. S.-F.; Chan, M. K. Efficient Oxidation of Methane to Methanol by Dioxygen Mediated by Tricopper Clusters. Angew. Chem., Int. Ed. 2013, 52, 3731−3735. (271) Press releases on various industrial activities related to United States shale natural gas: General Articles: Chevron Phillips Chemical Advances Plans for USGC Petrochemicals Project. http://www. cpchem.com/en-us/news/pages/chevron-phillips-chemical-advancesplans-for-usgc-petrochemicals-project.aspx (accessed March 3, 2017). Chevron Phillips Chemical Co. LP. Methanex Proceeds with Louisiana Project. http://methanex.mwnewsroom.com/press-releases/ methanex-proceeds-with-louisiana-project-tsx-mx201207250807944001?page=newsroom (accessed March 3, 2017). Methanex Corp., July 25, 2012. Shell Plans World-Scale Chemical Plant in USA. http://www.shell.com/chemicals/aboutshell/mediacentre/media-releases/2011-media-releases/pr-plan-chemical-plantusa.html (accessed March 3, 2017). Shell Chemical, June 6, 2011. Formosa Plastics Announces Long-Term Capital Investments. http:// www.fpcusa.com/company/news/releases/FPCUSAInvestmentPlans_022712.pdf (accessed March 3, 2017). Formosa Plastics Corp., USA, Feb 27, 2012.
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