Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Claveau, Emily E.; Sader, Safaa; Jackson, Benjamin A.; Khan, Shahriar N.; Miliordos, Evangelos

doi:10.1039/d2cp05480a

Cited by 19 publications

(20 citation statements)

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“…While the abundance of natural gas has motivated the valorization of methane as an energy feedstock, direct use of methane for the production of higher-value chemicals like methanol is limited by the lack of successful strategies for selective partial methane oxidation. , A limiting factor is the high C–H bond dissociation energy in methane compared to the partially oxidized products, which leads to overoxidation. − Instead, energy-intensive routes that first produce syngas from methane at very high operating temperatures and pressures are customarily used in industry. − Living systems leverage metalloenzymes such as soluble methane monooxygenase (sMMO) to convert methane selectively into methanol under ambient conditions using molecular oxygen as the oxidant − by forming, for example, a mononuclear Fe(IV)=O species that activates methane. − These metalloenzymes have inspired the design of synthetic homogeneous and heterogeneous catalysts for the direct conversion of methane to methanol via more energy-efficient methods. − Nevertheless, no synthetic catalyst to date is capable of simultaneously achieving high conversions and selectivities as enzymes have, motivating a wider search of candidate catalysts.…”

Section: Introductionmentioning

confidence: 99%

Computational Discovery of Stable Metal–Organic Frameworks for Methane-to-Methanol Catalysis

et al. 2023

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The challenge of direct partial oxidation of methane to methanol has motivated the targeted search of metal–organic frameworks (MOFs) as a promising class of materials for this transformation because of their site-isolated metals with tunable ligand environments. Thousands of MOFs have been synthesized, yet relatively few have been screened for their promise in methane conversion. We developed a high-throughput virtual screening workflow that identifies MOFs from a diverse space of experimental MOFs that have not been studied for catalysis, yet are thermally stable, synthesizable, and have promising unsaturated metal sites for C–H activation via a terminal metal-oxo species. We carried out density functional theory calculations of the radical rebound mechanism for methane-to-methanol conversion on models of the secondary building units (SBUs) from 87 selected MOFs. While we showed that oxo formation favorability decreases with increasing 3d filling, consistent with prior work, previously observed scaling relations between oxo formation and hydrogen atom transfer (HAT) are disrupted by the greater diversity in our MOF set. Accordingly, we focused on Mn MOFs, which favor oxo intermediates without disfavoring HAT or leading to high methanol release energiesa key feature for methane hydroxylation activity. We identified three Mn MOFs comprising unsaturated Mn centers bound to weak-field carboxylate ligands in planar or bent geometries with promising methane-to-methanol kinetics and thermodynamics. The energetic spans of these MOFs are indicative of promising turnover frequencies for methane to methanol that warrant further experimental catalytic studies.

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Section: Introductionmentioning

confidence: 99%

Computational Discovery of Stable Metal–Organic Frameworks for Methane-to-Methanol Catalysis

et al. 2023

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“…[14][15][16] These metalloenzymes have inspired the design of synthetic homogeneous and heterogeneous catalysts for the direct conversion of methane to methanol via more energy-efficient methods. [17][18][19][20][21][22][23][24][25] Nevertheless, no synthetic catalyst to date is capable of simultaneously achieving high conversions and selectivities as enzymes have, motivating a wider search of candidate catalysts.…”

Section: Introductionmentioning

confidence: 99%

Computational Discovery of Stable Metal-Organic Frameworks for Methane-to-Methanol Catalysis

Adamji

Nandy

Kevlishvili

et al. 2023

Preprint

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The challenge of direct partial oxidation of methane to methanol has motivated the targeted search of metal-organic frameworks (MOFs) as a promising class of materials for this transformation because of their site-isolated metals with tunable ligand environments. Thousands of MOFs have been synthesized, yet relatively few have been screened for their promise in methane conversion. We developed a high-throughput virtual screening (HTVS) workflow that identifies MOFs from a diverse space of experimental MOFs that have not been studied for catalysis, yet are thermally stable, synthesizable, and have promising unsaturated metal sites for C–H activation via a terminal metal-oxo species. We carried out density functional theory (DFT) calculations on the radical rebound mechanism for methane to methanol conversion on models of the secondary building units (SBUs) from 87 selected MOFs. While we showed that oxo formation favorability decreases with increasing 3d filling, consistent with prior work, previously observed scaling relations between oxo formation and hydrogen atom transfer (HAT) are disrupted by the greater diversity in our MOF set. Accordingly, we focused on Mn MOFs, which favor oxo intermediates without disfavoring HAT or leading to high methanol release energies—a key feature for methane hydroxylation activity. We identified three Mn MOFs comprising unsaturated Mn centers bound to weak-field carboxylate ligands in planar or bent geometries with promising methane to methanol kinetics and thermodynamics. The energetic spans of these MOFs are indicative of promising turnover frequencies for methane to methanol that warrant further experimental catalytic studies.

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“…A critical review on the performance of metal oxide molecular complexes, and ways to improve it, has recently been published by Claveau, Miliordos, and co-workers . Among the various suggestions, a major conclusion was that hydrogen bonds between the OH group of methanol and hydrophilic regions of the ligands can perturb the transition state structure for methanol significantly, increasing the activation barrier and slowing its oxidation.…”

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confidence: 99%

“…This considerably increases the Rh–O bond length from 1.697 to 1.867 Å, which is only 0.014 Å shorter than the transition state Rh–O distance (1.881 Å). The quasi-t 2g 5 electronic configuration of (CH 4 )(NH 3 ) 4 RhO 2+ maximizes the exposure of the coordinated C–H bond to the Rh 4+ center (see ref and below), which leads to an elongation of 0.04 Å for the C–H bond (from 1.087 to 1.125 Å) and a contraction of 0.03 Å (on average) for the Rh–N bonds. Therefore, the coordination of methane to (NH 3 ) 4 RhO 2+ affects dramatically the electronic structure of the latter, which in turn stretches a H 3 C–H bond.…”

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confidence: 99%

“…Various molecular species (such as transition metal oxides) and materials (such as metal oxide surfaces, metal-exchanged zeolites, metal−organic frameworks, and others) have been probed to catalyze the conversion of methane to methanol. 3 Despite meticulous efforts, no reliable or practical method has been identified to date. The current industrial approach (breaking up methane to syngas) occurs under harsh conditions, and its replacement would be welcome.…”

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confidence: 99%

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Methane against Methanol: The Tortoise and the Hare of the Oxidation Race

Claveau,

Heller,

Richardson

et al. 2023

J. Phys. Chem. Lett.

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The selective partial oxidation of methane to methanol has been a major chemistry challenge over the past several decades. The reason for this is that the weaker C−H bond of the desired product (methanol) is readily activated by the same catalyst used to activate the stronger C−H bond of methane. Quantum chemical calculations reveal how hydrogen-bonding interactions with the catalyst as well as other electronic and geometric effects slow the unwanted methanol oxidation reaction. Thus, the oxidation of methane (the tortoise in Aesop's fable) becomes faster than methanol (Aesop's hare), increasing the selectivity toward the desired product. Activation barriers are calculated for two different mechanisms (2+2 and radical), and reaction rates for the oxidation of the two molecules are obtained using semiclassical instanton theory to include tunneling effects for the proton transfers. The tunneling effects are shown to accelerate all reactions substantially but do not dramatically affect the selectivity.

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Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Abstract: Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance...

Cited by 19 publications

References 130 publications

Computational Discovery of Stable Metal–Organic Frameworks for Methane-to-Methanol Catalysis

Computational Discovery of Stable Metal–Organic Frameworks for Methane-to-Methanol Catalysis

Computational Discovery of Stable Metal-Organic Frameworks for Methane-to-Methanol Catalysis

Methane against Methanol: The Tortoise and the Hare of the Oxidation Race

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