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Selective oxidation of light alkanes under mild conditions
Journal Pre-proof Selective oxidation of light alkanes under mild conditions Sunney I. Chan, Steve S.-F. Yu, Chih-Cheng Liu, Chung-Yuan Mou PII:
S2452-2236(19)30064-1
DOI:
https://doi.org/10.1016/j.cogsc.2019.12.003
Reference:
COGSC 311
To appear in:
Current Opinion in Green and Sustainable Chemistry
Received Date: 25 November 2019 Revised Date:
2 December 2019
Accepted Date: 3 December 2019
Please cite this article as: S.I. Chan, S.S.-F. Yu, C.-C. Liu, C.-Y. Mou, Selective oxidation of light alkanes under mild conditions, Current Opinion in Green and Sustainable Chemistry, https:// doi.org/10.1016/j.cogsc.2019.12.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
Selective oxidation of light alkanes under mild conditions Sunney I. Chan,a,b* Steve S.-F. Yu,a Chih-Cheng Liu,a Chung-Yuan Moub*
Addresses a
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan
b
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
Corresponding
authors:
Chan,
Sunney
I
([email protected]);
Mou,
C-Y
([email protected])
Keywords: methane oxidation; methane monooxygenases; tricopper cluster complex; dioxygenase model; heterogeneous catalysis
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Abstract
Methanotrophs mediate the conversion of methane (CH4) into methanol selectively and efficiently near ambient conditions so we can learn from microbes to develop biomimetic catalysts capable of performing this difficult chemistry. This review highlights the development of a tricopper cluster catalyst that functions just like the particulate methane monooxygenase enzyme in methanotrophic bacteria. The performance of this catalytic system formulated for quasi-heterogeneous catalysis is compared with other heterogeneous catalysts derived from Cuand Fe- based zeolites and Cu-mordenites known to activate CH4 stoichiometrically near 200 ºC. We also highlight a unique catalytic system, in which the oxidizing power of both O-atoms of the O2 molecule can be harnessed for oxidation of toluene to yield benzaldehyde at room temperature.
Introduction
Activation of the C–H bonds in methane (CH4) and other light alkanes is difficult chemistry [1]. The bond dissociation energy of the C–H bond in these hydrocarbons is very high (typically 100 kcal/mol; 104.5 kcal/mol in the case of CH4), so the C–H bond is extremely inert. The functionalization of CH4 to produce methanol (CH3OH) usually requires high temperatures and a catalyst [2,3]. Moreover, the process is non-selective. The product CH3OH is prone to further oxidation to formaldehyde (HCHO) and formic acid (HCOOH).
There is an impetus in recent years to develop a catalyst capable of efficient conversion of CH4 into CH3OH under mild conditions. The planet shall be running out of petroleum in the near future, but there is an abundance of natural gas, which is becoming an important alternative
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source of energy. CH4 is the principal component of natural gas. CH4 is a potential nextgeneration carbon feedstock for chemicals because of its abundance and low cost. The catalyst for CH4 is expected to work for the other light alkanes (ethane, propane, and butane) that are present in smaller amounts in natural gas. CH4 is also a greenhouse gas. While CO2 is the main culprit of global warming, CH4 accounts for about 20 % of the problem [4–6]. Most of the CH4 emissions are derived from natural gas seeps/leaks [7–11] and from human activities [12,13] especially the production of CH4 by methanogens [14] in livestock husbandry [15,16], landfills [17], decaying vegetation and soil [18]. Microbial CH4 now accounts for 400 million tons annually [19]. This source of CH4 is relatively low pressure and is best controlled by a catalyst that operates under mild conditions. Thus, for the control of CH4 emissions, the attention has been directed toward developing a catalyst that is capable of converting CH4 into CH3OH, and other light alkanes into their corresponding oxygenates near room temperature. Biological methane oxidation Two enzymes from methanotrophic bacteria are known to convert CH4 into CH3OH efficiently near ambient conditions [20]. These enzymes, the particulate methane monooxygenase (pMMO) and soluble methane monooxygenase (sMMO), can turn over at a frequency approaching one per second. They are the most efficient methane oxidizers known to date. Both of these enzyme systems have been under intense investigation over the past several decades. The hope has been that we can learn from these microbes to uncover the catalytic chemistry of these enzymes so that we can develop biomimetic catalysts capable of performing the same chemistry in the laboratory.
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The particulate methane monooxygenase (pMMO) is a multi-copper membrane-bound enzyme with a CuICuICuI tricopper cluster as the active site [1,21]. This enzyme has a limited substrate range (only C1 to C5 straight-chain alkanes and related alkenes), and the hydroxylation/epoxidation exhibits unusual regio-specificity and stereoselectivity. It is a relatively simple machine with 13 CuI ions anchored within the three subunits of the protein (PmoA, PmoB, and PmoC) to perform the various operations required for optimal catalytic turnover of the enzyme. The other MMO, called soluble methane monooxygenase (sMMO), is expressed under limited copper/biomass conditions only and is found in the cytosol of only certain strains of methanotrophic bacteria. It is a system of three protein components: the hydroxylase, the reductase, and a regulatory protein. The hydroxylase contains the non-heme diiron center as the active site [1,21,22]. The reductase provides the reducing equivalents required for the catalytic turnover. The regulatory protein controls the transfer of these reducing equivalents from the reductase to the hydroxylase as well as interactions between the substrates with the di-iron active site for the oxidation chemistry. Thus the molecular machinery of sMMO is much more intricate and sophisticated. This enzyme utilizes a system of three proteins, as well as protein-protein interactions and protein dynamics, to orchestrate the delivery of electrons, O2, protons, and the CH4 (or other substrates) to the catalytic site, so that the substrate is delivered in an orderly fashion, in the proper sequence and with precise timing, for kinetic control and optimal performance of the machinery. As sMMO is designed to oxidize a diverse range of organic substrates besides CH4, we can expect an individual organic substrate to feedback on to the hydroxylase near the catalytic site in its unique way for regulation of certain specifics of the catalytic mechanism to accomplish the desired molecular outcome [1,21]. A biomimetic catalyst for methane oxidation
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As a simple molecular machine, it is possible to describe the catalytic turnover of pMMO in concepts familiar to the chemists [21]. Based on this understanding of the catalytic mechanism, a biomimetic catalyst for CH4 oxidation that functions just like the enzyme has been developed [23-29]. A homogeneous catalyst that works well for CH4 oxidation in acetonitrile is shown in Scheme 1. It is based on the tricopper cluster complex [CuICuICuI(7-N-Etppz)]1+, where 7-NEtppz denotes the organic ligand 3,3’-(1,4-diazepane-1,4-diyl)bis[1-(4-ethylpiperazine-1yl)propan-2-ol]. This tricopper complex is capable of efficient conversion of CH4 into CH3OH (the oxidation of other light alkanes as well) upon activation by dioxygen (O2) under ambient conditions [23-25]. The turnover of the catalyst is shown in Scheme 2. To initiate the catalysis, the fully reduced tricopper complex is first activated by O2 to form the highly reactive [CuIICuII(µ-O)2CuIII(7-N-Etppz)]1+ intermediate, which harnesses the ”hot” oxene. The activation of the tricopper complex by O2 and the transfer of one of O-atoms to the alkane substrate, with the remaining O-atom embedded in the tricopper complex, is confirmed by 18O2 experiments. Since the solubility of CH4 (other light alkane gases as well) is very low in a typical solvent, a binding pocket with good affinity for CH4 is essential to ensure efficient productive cycling of the catalyst. When a CH4 molecule is associated with the hydrophobic alkane binding cleft at the base of the tricopper complex (see Scheme 1), the activated tricopper cluster can mediate facile O-atom transfer to one of the C−H bonds in the transition state. This binding pocket is selective to small straight-chain alkanes from C1−C6. There is no over-oxidation in the case of CH4 and ethane, but 2-propanone, 2-butanone, and 2-pentanone are also formed besides 2-propanol, 2-hutanol, and 2-pentanol in the case of propane, butane, and pentane respectively. To regenerate the catalyst and achieve multiple turnovers, the “spent” catalyst, namely, the [CuICuII(µ-O)CuII(7-N-Etppz)]1+ species, is reduced by one molecule of hydrogen peroxide
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(H2O2) to produce O2 and H2O. Thus, the catalyst functions as a methane monooxygenase, just like the pMMO enzyme, and one molecule of CH3OH is produced per molecule of H2O2 consumed in the case of CH4. The regeneration of the [CuICuICuI(7-N-Etppz)]1+catalyst after the oxene transfer step is the rate-limiting step in the catalytic turnover. The turnover frequency of the catalyst is proportional to the concentration of H2O2 regenerating the catalyst [26–28]. Both the turnover frequency and product turnover numbers are independent of the substrate. The Oatom transfer is significantly faster and is alkane substrate dependent [28].
Scheme 1. Space-filling model of the optimized structure of O2-activated tricopper complex [CuIICuII(µ-O)2CuIII(7-N-Etppz)]1+ showing the funnel-like opening or cleft at the bottom for a hydrocarbon substrate to access the “hot” oxene group harnessed. H, white; C, gray; N, blue; O, red; and Cu, brown.
The harnessed oxene in the activated tricopper catalyst is very reactive so that oxene transfer to substrates as well as its deactivation by reductants are very efficient. For example, H2O2 is also a substrate and can be oxidized to produce O2 and H2O with the assistance of two protons.
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With the low solubility of CH4 and other light alkanes in an organic solvent, this abortive cycling of the catalyst can limit the product turnover numbers (TON) of the alkane oxidation when one tries to drive the catalytic turnover using sufficiently high concentrations of H2O2. To overcome this problem, the tricopper cluster complex has been immobilized in mesoporous silica nanoparticles (MSN) [26–28], where the solubility of CH4 gas within the nano-confined liquid is several hundred fold higher than the solubility of the gas in the bulk solvent [30]. This enhancement in the solubility of an inert gas within the nano-confined liquid is referred to as the “oversolubility” phenomenon. In Figure 1, we summarize the TON obtained for the conversion of CH4, ethane, and propane into CH3OH, ethanol, and propanol, in this formulation of the catalyst (CuEtp@MSN). In the heterogenized catalytic system, the overall catalytic efficiency of the process approaches 90% with essentially one molecule of alkane oxidized per molecule of H2O2 consumed to drive the catalysis [26,27]. The contrast when the tricopper cluster complex is deployed as a homogeneous catalyst in a solvent versus when it is reformulated as a quasiheterogeneous catalyst in the MSN is dramatic. Interestingly, while the TONs are essentially the same for CH4, ethane and propane when these gases are subjected to the catalyst in individual experiments on the pure gases, there is competitive oxidation of these light alkanes in a 1:1:1 mixture of the three gases (Figure 2) [28]. This kinetic fractionation reflects the significantly stronger affinity of the hydrophobic binding pocket for propane and ethane versus CH4 at the catalytic site.
Scheme 2. Catalytic cycle: Comparison of a productive cycle that mediates the oxidation of CH4 to CH3OH versus an abortive cycle that merely leads to the oxidation of H2O2 to O2 and H2O
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Figure 1. Time course of the TON of (a) CH3OH in CH4 oxidation, (b) ethanol in ethane oxidation, and (c) isopropanol and acetone obtained in propane oxidation, catalyzed by the CuEtp@MSN-TP catalyst at room temperature using 200 equiv. of H2O2 to initiate the catalytic turnover. [Taken from Reference 28. Reproduced with permission of The Royal Society of Chemistry.]
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Figure 2. Time course of the individual product TON for oxidation of the light alkanes in a 1:l:1 mixture of CH4, ethane, and propane (total (C1+C2+C3) = 500 equiv.) catalyzed by the CuEtp@MSN-TP catalyst at room temperature initiated by 200 equiv. of H2O2 to drive the catalytic turnovers. [Taken from Reference 28. Reproduced with permission of The Royal Society of Chemistry.]
Methane activation by heterogeneous catalysts based on zeolites and mordenites Zeolites loaded with copper and iron ions, including Cu-exchanged zeolites [31-35], Fe– ZSM-5 [36] and Cu-promoted Fe-ZSM-5 [37,38], and Cu-mordenites [39-42], have also been shown to be potential heterogeneous materials for selective oxidation of CH4 to CH3OH under mild conditions. Cu-exchanged zeolite, Cu-ZSM-5, activated either by N2O at 100 °C or by O2 at 175 °C, was the first reported Cu-based zeolite with the capability to convert CH4 into CH3OH [31]. Based on spectroscopic evidence (in-situ X-ray absorption), the catalytic species was identified as the mono-(µ-oxo)-bridged dicopper core, namely, [Cu2O]2+ [32]. Grundner et al. [42] has recently developed a copper-based mordenite with uniform reactive sites that showed high reactivity toward the selective oxidation of CH4. A study of the conversion versus Cu2+
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concentration for the Cu-exchanged MOR indicates that 3 Cu atoms are involved in the oxidation of one CH4 molecule, with the active species a tricopper species [Cu3(µ–O)3]2+. All these metal oxide systems can only convert CH4 into CH3OH stoichiometrically and produce CH3OH in yields that are dependent on the levels of the di- or tri-copper−oxo clusters in the Cu-MOR. To sustain the turnover, the activation of the copper sites needs to be combined with a strategy to remove the product CH3OH and to regenerate the reducing equivalents. Moreover. unlike the biomimetic Cu-complex approach, high temperature treatments at ~450 °C are required to reactivate the Cu-zeolite catalyst. In addition to Cu-based catalysts, Fe-based catalysts [43-44] mimicking sMMO have received recent attention. In Fe-ZSM-5, it has been proposed that a Fe(II)−Fe(II) active species activates O2 to produce a high-valent metal−oxo intermediate that is responsible for the activation of the C−H bond in CH4 via hydrogen abstraction. More recently, similar di-iron O2 activation chemistry has also been reported for Fe-sites incorporated in metal organic framework (MOF) [45]. Discovery of a dicopper dioxygenase model All of the above processes, homogeneous or heterogeneous for the conversion of CH4 into CH3OH, are based on monooxygenase chemistry when O2 is used as the oxidant to mediate the CH4 oxidation. In other words, only one of the O atoms in the O2 is harnessed for CH4 oxidation and the other O-atom consumed is reduced to H2O by two electrons with the assistance of two protons. It would be an advance in the field if we could harness the oxidizing power of both Oatoms of the O2 molecule and exploit it for the hydrocarbon oxidation process. The copper complex would then function as a dioxygenase, which uses both O-atoms of the O2 to mediate the oxidation of the alkane. Recently, a dicopper system (CuIII(µ–O)2CuIII complex immobilized
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in MSN (CuImph@MSN-TP-3) is developed that can mediate the catalytic conversion of toluene into benzaldehyde by O2, in which the oxidizing power of both O-atoms is harnessed for catalytic turnover under mild conditions [46]. No reductant is needed for the removal of the second O-atom. This is the first example of a CuIII(µ–O)2CuIII complex capable of functioning like a “dioxygenase” in hydrocarbon oxidation. A mechanistic study was undertaken to clarify how this catalytic conversion is accomplished without the input of sacrificial reductants. While the first O atom in the CuIII(µ–O)2CuIII complex can actively participate in the functionalization of the aliphatic C–H bond, the second O atom left in the CuII(µ–O)CuII complex is inert. It was shown that a second molecule of O2 is involved in activating the dicopper catalyst, forming an O2 complex with the CuII(µ–O)CuII intermediate to give a species with the [Cu2O3]2+ core, which then mediates the transfer of the remaining O atom of the original O2 molecule to the organic substrate to complete the turnover cycle [47]. The proposed catalytic cycle for the aliphatic oxidation of toluene to benzyl alcohol and benzaldehyde is shown in Scheme 3. The significance of the finding is that, without any sacrificial reductant, an oxidation catalyst using O2 as the oxidant only with unprecedented catalytic performance and atom economy is available. Also importantly, the O2 activation mechanism in Scheme 3 offers an analogue of the heterogeneous metal oxide catalyst that oxidizes substrates with the lattice oxygens by the Marsvan-Krevelen (MvK) mechanism. Hybrid materials bearing organic and inorganic motifs may be an excellent enzyme-mimic approach for atomically resolved catalytic sites within confined space. In solution, the above dicopper catalyst, after C−H bond activation, will often dissociate into monomeric species and act as stoichiometric reagents rather than as catalytic sites. However, when immobilized in MSN, the pore-confinement effect can be used to stabilize the metal cluster in the catalytic cycle and
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operate in conjunction with the oversolubility effect of nonpolar gases (CH4, O2) within the mesopores. TGA studies indicate that immobilization of the copper complex into the silica nanochannels leads to better thermal stability than mere adsorption on to the silica external surface.[46] Furthermore, heterogenization of the metal complex catalyst allows reuse of the catalyst many times with easy separation of the catalysts.[26,46]
Scheme 3. Reaction scheme for the aliphatic oxidation of toluene mediated by the CuImph@MSN-TP-3 catalytic system.
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The challenge for the future would be to adapt the above O2 activation chemistry to new copper cluster catalysts to develop an efficient catalyst for conversion of CH4 to yield two molecules of CH3OH or one molecule of HCHO per catalytic cycle. If O2 could be the sole oxidant for converting CH4 to CH3OH under mild condition, much of the flared natural gas at remote fields may be recovered as CH3OH and transported economically.
Conclusions
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A biomimetic tricopper cluster complex capable of efficient and selective CH4 oxidation under ambient temperature and pressure is highlighted in this review. With three reducing equivalents readily available in the CuICuICuI cluster for the activation of O2, the O–O bond is efficiently and irreversibly broken to harness the highly reactive oxene for facile electrophilic insertion across one of the C–H bonds of CH4 when the latter is bound to the hydrophobic hydrocarbon binding pocket at the base of the tricopper complex. When this tricopper catalyst is immobilized in the nanochannels of MSN, this quasi-heterogeneous formulation of the catalyst functions like the active site of pMMO in methanotrophic bacteria with comparable turnover frequency and overall catalytic efficiency. This catalytic system is capable of multiple turnovers when H2O2 is employed as a reducing agent to regenerate the catalyst, unlike the stotichiometric activation of CH4 by heterogeneous catalysts based on Fe- and Cu-zeolites and Cu-mordenites at somewhat higher temperatures. We also highlight the discovery of a dicopper system capable of harnessing the oxidizing power of both O-atoms of O2 for oxidation of the aliphatic C–H bonds of toluene to produce benzyl alcohol and benzaldehyde. The challenge in the future would be to develop a similar dioxygenase model capable of CH4 oxidation without the consumption of sacrificial reductants. Acknowledgments. This work is supported by Academia Sinica’s “Taiwan’s Deep Decarbonization Pathways towards a Sustainable Society” Research Program (AS-KPQ-106DDPP). CCL acknowledges a postdoctoral fellowship from Academia Sinica, Taiwan. Conflict of interest statement: The authors declare no competing interest.
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44. Snyder BER, Vanelderen P, Bols ML, Hallaert SD, Böttger LH, Ungur L, Pierloot K, Schoonheydt RA, Sels BF, Solomon EI: The active site of low-temperature methane hydroxylation in iron-containing zeolites. Nature 2016, 536:317−321. *45. Osadchii DY, Olivos-Suarez AI, Szécsényi A, Li G, Nasalevich MA, Dugulan IA, Crespo PS, Hensen EJM, Veber SL, Fedin MV, Sankar G, Pidko EA, Gascon J: Isolated Fe sites in metal organic frameworks catalyze the direct conversion of methane to methanol. ACS Catal 2018, 8:5542−5548. Fe-containing MOF that comprises an antiferromagnetically coupled high-spin species in a coordination environment resembling sMMO is synthesized and tested for methane oxidation to methanol. 46. Liu C-C, Lin T-S, Chan SI, Mou C-Y: A room temperature catalyst for toluene aliphatic C−H bond oxidation: tripodal tridentate copper complex immobilized on mesoporous silica. J Catal 2015, 322:139−151. **47. Liu C-C, Tsai Y-F, Mou C-Y, Yu SSF, Chan SI: A dicopper dioxygenase model immobilized in mesoporous silica nanoparticles for toluene oxidation: a mechanism to harness both “O” atoms of O2 for catalysis. J Phys Chem C 2019, 123:11032−11043. Through the pore-confinement effect, various stages of the oxygen-bridged dinuclear copper complex during catalytic oxidation of toluene to benzyl alcohol and benzaldehyde are stabilized and identified.
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S2452-2236(19)30064-1
DOI:
https://doi.org/10.1016/j.cogsc.2019.12.003
Reference:
COGSC 311
To appear in:
Current Opinion in Green and Sustainable Chemistry
Received Date: 25 November 2019 Revised Date:
2 December 2019
Accepted Date: 3 December 2019
Please cite this article as: S.I. Chan, S.S.-F. Yu, C.-C. Liu, C.-Y. Mou, Selective oxidation of light alkanes under mild conditions, Current Opinion in Green and Sustainable Chemistry, https:// doi.org/10.1016/j.cogsc.2019.12.003. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.
Selective oxidation of light alkanes under mild conditions Sunney I. Chan,a,b* Steve S.-F. Yu,a Chih-Cheng Liu,a Chung-Yuan Moub*
Addresses a
Institute of Chemistry, Academia Sinica, Nankang, Taipei 11529, Taiwan
b
Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan
Corresponding
authors:
Chan,
Sunney
I
([email protected]);
Mou,
C-Y
([email protected])
Keywords: methane oxidation; methane monooxygenases; tricopper cluster complex; dioxygenase model; heterogeneous catalysis
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Abstract
Methanotrophs mediate the conversion of methane (CH4) into methanol selectively and efficiently near ambient conditions so we can learn from microbes to develop biomimetic catalysts capable of performing this difficult chemistry. This review highlights the development of a tricopper cluster catalyst that functions just like the particulate methane monooxygenase enzyme in methanotrophic bacteria. The performance of this catalytic system formulated for quasi-heterogeneous catalysis is compared with other heterogeneous catalysts derived from Cuand Fe- based zeolites and Cu-mordenites known to activate CH4 stoichiometrically near 200 ºC. We also highlight a unique catalytic system, in which the oxidizing power of both O-atoms of the O2 molecule can be harnessed for oxidation of toluene to yield benzaldehyde at room temperature.
Introduction
Activation of the C–H bonds in methane (CH4) and other light alkanes is difficult chemistry [1]. The bond dissociation energy of the C–H bond in these hydrocarbons is very high (typically 100 kcal/mol; 104.5 kcal/mol in the case of CH4), so the C–H bond is extremely inert. The functionalization of CH4 to produce methanol (CH3OH) usually requires high temperatures and a catalyst [2,3]. Moreover, the process is non-selective. The product CH3OH is prone to further oxidation to formaldehyde (HCHO) and formic acid (HCOOH).
There is an impetus in recent years to develop a catalyst capable of efficient conversion of CH4 into CH3OH under mild conditions. The planet shall be running out of petroleum in the near future, but there is an abundance of natural gas, which is becoming an important alternative
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source of energy. CH4 is the principal component of natural gas. CH4 is a potential nextgeneration carbon feedstock for chemicals because of its abundance and low cost. The catalyst for CH4 is expected to work for the other light alkanes (ethane, propane, and butane) that are present in smaller amounts in natural gas. CH4 is also a greenhouse gas. While CO2 is the main culprit of global warming, CH4 accounts for about 20 % of the problem [4–6]. Most of the CH4 emissions are derived from natural gas seeps/leaks [7–11] and from human activities [12,13] especially the production of CH4 by methanogens [14] in livestock husbandry [15,16], landfills [17], decaying vegetation and soil [18]. Microbial CH4 now accounts for 400 million tons annually [19]. This source of CH4 is relatively low pressure and is best controlled by a catalyst that operates under mild conditions. Thus, for the control of CH4 emissions, the attention has been directed toward developing a catalyst that is capable of converting CH4 into CH3OH, and other light alkanes into their corresponding oxygenates near room temperature. Biological methane oxidation Two enzymes from methanotrophic bacteria are known to convert CH4 into CH3OH efficiently near ambient conditions [20]. These enzymes, the particulate methane monooxygenase (pMMO) and soluble methane monooxygenase (sMMO), can turn over at a frequency approaching one per second. They are the most efficient methane oxidizers known to date. Both of these enzyme systems have been under intense investigation over the past several decades. The hope has been that we can learn from these microbes to uncover the catalytic chemistry of these enzymes so that we can develop biomimetic catalysts capable of performing the same chemistry in the laboratory.
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The particulate methane monooxygenase (pMMO) is a multi-copper membrane-bound enzyme with a CuICuICuI tricopper cluster as the active site [1,21]. This enzyme has a limited substrate range (only C1 to C5 straight-chain alkanes and related alkenes), and the hydroxylation/epoxidation exhibits unusual regio-specificity and stereoselectivity. It is a relatively simple machine with 13 CuI ions anchored within the three subunits of the protein (PmoA, PmoB, and PmoC) to perform the various operations required for optimal catalytic turnover of the enzyme. The other MMO, called soluble methane monooxygenase (sMMO), is expressed under limited copper/biomass conditions only and is found in the cytosol of only certain strains of methanotrophic bacteria. It is a system of three protein components: the hydroxylase, the reductase, and a regulatory protein. The hydroxylase contains the non-heme diiron center as the active site [1,21,22]. The reductase provides the reducing equivalents required for the catalytic turnover. The regulatory protein controls the transfer of these reducing equivalents from the reductase to the hydroxylase as well as interactions between the substrates with the di-iron active site for the oxidation chemistry. Thus the molecular machinery of sMMO is much more intricate and sophisticated. This enzyme utilizes a system of three proteins, as well as protein-protein interactions and protein dynamics, to orchestrate the delivery of electrons, O2, protons, and the CH4 (or other substrates) to the catalytic site, so that the substrate is delivered in an orderly fashion, in the proper sequence and with precise timing, for kinetic control and optimal performance of the machinery. As sMMO is designed to oxidize a diverse range of organic substrates besides CH4, we can expect an individual organic substrate to feedback on to the hydroxylase near the catalytic site in its unique way for regulation of certain specifics of the catalytic mechanism to accomplish the desired molecular outcome [1,21]. A biomimetic catalyst for methane oxidation
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As a simple molecular machine, it is possible to describe the catalytic turnover of pMMO in concepts familiar to the chemists [21]. Based on this understanding of the catalytic mechanism, a biomimetic catalyst for CH4 oxidation that functions just like the enzyme has been developed [23-29]. A homogeneous catalyst that works well for CH4 oxidation in acetonitrile is shown in Scheme 1. It is based on the tricopper cluster complex [CuICuICuI(7-N-Etppz)]1+, where 7-NEtppz denotes the organic ligand 3,3’-(1,4-diazepane-1,4-diyl)bis[1-(4-ethylpiperazine-1yl)propan-2-ol]. This tricopper complex is capable of efficient conversion of CH4 into CH3OH (the oxidation of other light alkanes as well) upon activation by dioxygen (O2) under ambient conditions [23-25]. The turnover of the catalyst is shown in Scheme 2. To initiate the catalysis, the fully reduced tricopper complex is first activated by O2 to form the highly reactive [CuIICuII(µ-O)2CuIII(7-N-Etppz)]1+ intermediate, which harnesses the ”hot” oxene. The activation of the tricopper complex by O2 and the transfer of one of O-atoms to the alkane substrate, with the remaining O-atom embedded in the tricopper complex, is confirmed by 18O2 experiments. Since the solubility of CH4 (other light alkane gases as well) is very low in a typical solvent, a binding pocket with good affinity for CH4 is essential to ensure efficient productive cycling of the catalyst. When a CH4 molecule is associated with the hydrophobic alkane binding cleft at the base of the tricopper complex (see Scheme 1), the activated tricopper cluster can mediate facile O-atom transfer to one of the C−H bonds in the transition state. This binding pocket is selective to small straight-chain alkanes from C1−C6. There is no over-oxidation in the case of CH4 and ethane, but 2-propanone, 2-butanone, and 2-pentanone are also formed besides 2-propanol, 2-hutanol, and 2-pentanol in the case of propane, butane, and pentane respectively. To regenerate the catalyst and achieve multiple turnovers, the “spent” catalyst, namely, the [CuICuII(µ-O)CuII(7-N-Etppz)]1+ species, is reduced by one molecule of hydrogen peroxide
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(H2O2) to produce O2 and H2O. Thus, the catalyst functions as a methane monooxygenase, just like the pMMO enzyme, and one molecule of CH3OH is produced per molecule of H2O2 consumed in the case of CH4. The regeneration of the [CuICuICuI(7-N-Etppz)]1+catalyst after the oxene transfer step is the rate-limiting step in the catalytic turnover. The turnover frequency of the catalyst is proportional to the concentration of H2O2 regenerating the catalyst [26–28]. Both the turnover frequency and product turnover numbers are independent of the substrate. The Oatom transfer is significantly faster and is alkane substrate dependent [28].
Scheme 1. Space-filling model of the optimized structure of O2-activated tricopper complex [CuIICuII(µ-O)2CuIII(7-N-Etppz)]1+ showing the funnel-like opening or cleft at the bottom for a hydrocarbon substrate to access the “hot” oxene group harnessed. H, white; C, gray; N, blue; O, red; and Cu, brown.
The harnessed oxene in the activated tricopper catalyst is very reactive so that oxene transfer to substrates as well as its deactivation by reductants are very efficient. For example, H2O2 is also a substrate and can be oxidized to produce O2 and H2O with the assistance of two protons.
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With the low solubility of CH4 and other light alkanes in an organic solvent, this abortive cycling of the catalyst can limit the product turnover numbers (TON) of the alkane oxidation when one tries to drive the catalytic turnover using sufficiently high concentrations of H2O2. To overcome this problem, the tricopper cluster complex has been immobilized in mesoporous silica nanoparticles (MSN) [26–28], where the solubility of CH4 gas within the nano-confined liquid is several hundred fold higher than the solubility of the gas in the bulk solvent [30]. This enhancement in the solubility of an inert gas within the nano-confined liquid is referred to as the “oversolubility” phenomenon. In Figure 1, we summarize the TON obtained for the conversion of CH4, ethane, and propane into CH3OH, ethanol, and propanol, in this formulation of the catalyst (CuEtp@MSN). In the heterogenized catalytic system, the overall catalytic efficiency of the process approaches 90% with essentially one molecule of alkane oxidized per molecule of H2O2 consumed to drive the catalysis [26,27]. The contrast when the tricopper cluster complex is deployed as a homogeneous catalyst in a solvent versus when it is reformulated as a quasiheterogeneous catalyst in the MSN is dramatic. Interestingly, while the TONs are essentially the same for CH4, ethane and propane when these gases are subjected to the catalyst in individual experiments on the pure gases, there is competitive oxidation of these light alkanes in a 1:1:1 mixture of the three gases (Figure 2) [28]. This kinetic fractionation reflects the significantly stronger affinity of the hydrophobic binding pocket for propane and ethane versus CH4 at the catalytic site.
Scheme 2. Catalytic cycle: Comparison of a productive cycle that mediates the oxidation of CH4 to CH3OH versus an abortive cycle that merely leads to the oxidation of H2O2 to O2 and H2O
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Figure 1. Time course of the TON of (a) CH3OH in CH4 oxidation, (b) ethanol in ethane oxidation, and (c) isopropanol and acetone obtained in propane oxidation, catalyzed by the CuEtp@MSN-TP catalyst at room temperature using 200 equiv. of H2O2 to initiate the catalytic turnover. [Taken from Reference 28. Reproduced with permission of The Royal Society of Chemistry.]
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Figure 2. Time course of the individual product TON for oxidation of the light alkanes in a 1:l:1 mixture of CH4, ethane, and propane (total (C1+C2+C3) = 500 equiv.) catalyzed by the CuEtp@MSN-TP catalyst at room temperature initiated by 200 equiv. of H2O2 to drive the catalytic turnovers. [Taken from Reference 28. Reproduced with permission of The Royal Society of Chemistry.]
Methane activation by heterogeneous catalysts based on zeolites and mordenites Zeolites loaded with copper and iron ions, including Cu-exchanged zeolites [31-35], Fe– ZSM-5 [36] and Cu-promoted Fe-ZSM-5 [37,38], and Cu-mordenites [39-42], have also been shown to be potential heterogeneous materials for selective oxidation of CH4 to CH3OH under mild conditions. Cu-exchanged zeolite, Cu-ZSM-5, activated either by N2O at 100 °C or by O2 at 175 °C, was the first reported Cu-based zeolite with the capability to convert CH4 into CH3OH [31]. Based on spectroscopic evidence (in-situ X-ray absorption), the catalytic species was identified as the mono-(µ-oxo)-bridged dicopper core, namely, [Cu2O]2+ [32]. Grundner et al. [42] has recently developed a copper-based mordenite with uniform reactive sites that showed high reactivity toward the selective oxidation of CH4. A study of the conversion versus Cu2+
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concentration for the Cu-exchanged MOR indicates that 3 Cu atoms are involved in the oxidation of one CH4 molecule, with the active species a tricopper species [Cu3(µ–O)3]2+. All these metal oxide systems can only convert CH4 into CH3OH stoichiometrically and produce CH3OH in yields that are dependent on the levels of the di- or tri-copper−oxo clusters in the Cu-MOR. To sustain the turnover, the activation of the copper sites needs to be combined with a strategy to remove the product CH3OH and to regenerate the reducing equivalents. Moreover. unlike the biomimetic Cu-complex approach, high temperature treatments at ~450 °C are required to reactivate the Cu-zeolite catalyst. In addition to Cu-based catalysts, Fe-based catalysts [43-44] mimicking sMMO have received recent attention. In Fe-ZSM-5, it has been proposed that a Fe(II)−Fe(II) active species activates O2 to produce a high-valent metal−oxo intermediate that is responsible for the activation of the C−H bond in CH4 via hydrogen abstraction. More recently, similar di-iron O2 activation chemistry has also been reported for Fe-sites incorporated in metal organic framework (MOF) [45]. Discovery of a dicopper dioxygenase model All of the above processes, homogeneous or heterogeneous for the conversion of CH4 into CH3OH, are based on monooxygenase chemistry when O2 is used as the oxidant to mediate the CH4 oxidation. In other words, only one of the O atoms in the O2 is harnessed for CH4 oxidation and the other O-atom consumed is reduced to H2O by two electrons with the assistance of two protons. It would be an advance in the field if we could harness the oxidizing power of both Oatoms of the O2 molecule and exploit it for the hydrocarbon oxidation process. The copper complex would then function as a dioxygenase, which uses both O-atoms of the O2 to mediate the oxidation of the alkane. Recently, a dicopper system (CuIII(µ–O)2CuIII complex immobilized
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in MSN (CuImph@MSN-TP-3) is developed that can mediate the catalytic conversion of toluene into benzaldehyde by O2, in which the oxidizing power of both O-atoms is harnessed for catalytic turnover under mild conditions [46]. No reductant is needed for the removal of the second O-atom. This is the first example of a CuIII(µ–O)2CuIII complex capable of functioning like a “dioxygenase” in hydrocarbon oxidation. A mechanistic study was undertaken to clarify how this catalytic conversion is accomplished without the input of sacrificial reductants. While the first O atom in the CuIII(µ–O)2CuIII complex can actively participate in the functionalization of the aliphatic C–H bond, the second O atom left in the CuII(µ–O)CuII complex is inert. It was shown that a second molecule of O2 is involved in activating the dicopper catalyst, forming an O2 complex with the CuII(µ–O)CuII intermediate to give a species with the [Cu2O3]2+ core, which then mediates the transfer of the remaining O atom of the original O2 molecule to the organic substrate to complete the turnover cycle [47]. The proposed catalytic cycle for the aliphatic oxidation of toluene to benzyl alcohol and benzaldehyde is shown in Scheme 3. The significance of the finding is that, without any sacrificial reductant, an oxidation catalyst using O2 as the oxidant only with unprecedented catalytic performance and atom economy is available. Also importantly, the O2 activation mechanism in Scheme 3 offers an analogue of the heterogeneous metal oxide catalyst that oxidizes substrates with the lattice oxygens by the Marsvan-Krevelen (MvK) mechanism. Hybrid materials bearing organic and inorganic motifs may be an excellent enzyme-mimic approach for atomically resolved catalytic sites within confined space. In solution, the above dicopper catalyst, after C−H bond activation, will often dissociate into monomeric species and act as stoichiometric reagents rather than as catalytic sites. However, when immobilized in MSN, the pore-confinement effect can be used to stabilize the metal cluster in the catalytic cycle and
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operate in conjunction with the oversolubility effect of nonpolar gases (CH4, O2) within the mesopores. TGA studies indicate that immobilization of the copper complex into the silica nanochannels leads to better thermal stability than mere adsorption on to the silica external surface.[46] Furthermore, heterogenization of the metal complex catalyst allows reuse of the catalyst many times with easy separation of the catalysts.[26,46]
Scheme 3. Reaction scheme for the aliphatic oxidation of toluene mediated by the CuImph@MSN-TP-3 catalytic system.
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The challenge for the future would be to adapt the above O2 activation chemistry to new copper cluster catalysts to develop an efficient catalyst for conversion of CH4 to yield two molecules of CH3OH or one molecule of HCHO per catalytic cycle. If O2 could be the sole oxidant for converting CH4 to CH3OH under mild condition, much of the flared natural gas at remote fields may be recovered as CH3OH and transported economically.
Conclusions
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A biomimetic tricopper cluster complex capable of efficient and selective CH4 oxidation under ambient temperature and pressure is highlighted in this review. With three reducing equivalents readily available in the CuICuICuI cluster for the activation of O2, the O–O bond is efficiently and irreversibly broken to harness the highly reactive oxene for facile electrophilic insertion across one of the C–H bonds of CH4 when the latter is bound to the hydrophobic hydrocarbon binding pocket at the base of the tricopper complex. When this tricopper catalyst is immobilized in the nanochannels of MSN, this quasi-heterogeneous formulation of the catalyst functions like the active site of pMMO in methanotrophic bacteria with comparable turnover frequency and overall catalytic efficiency. This catalytic system is capable of multiple turnovers when H2O2 is employed as a reducing agent to regenerate the catalyst, unlike the stotichiometric activation of CH4 by heterogeneous catalysts based on Fe- and Cu-zeolites and Cu-mordenites at somewhat higher temperatures. We also highlight the discovery of a dicopper system capable of harnessing the oxidizing power of both O-atoms of O2 for oxidation of the aliphatic C–H bonds of toluene to produce benzyl alcohol and benzaldehyde. The challenge in the future would be to develop a similar dioxygenase model capable of CH4 oxidation without the consumption of sacrificial reductants. Acknowledgments. This work is supported by Academia Sinica’s “Taiwan’s Deep Decarbonization Pathways towards a Sustainable Society” Research Program (AS-KPQ-106DDPP). CCL acknowledges a postdoctoral fellowship from Academia Sinica, Taiwan. Conflict of interest statement: The authors declare no competing interest.
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