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Context

CH₄ emissions account for ~30% of global warming (IEA, 2022), and natural sources may increase (Zhang, 2023). Technologies for oxidizing atmospheric CH₄, area CH₄ emissions, and unavoidable point sources could substantially mitigate climate change. While CH₄ above ~44,000 ppm can be flared, lower concentrations require catalyzed oxidation. However, ~75% of CH₄ pollution is atmospheric (2 ppm) or area emissions below 1000 ppm that are too dilute to be oxidized in reactors at scale using existing photocatalyst, thermocatalyst, or electrocatalyst technologies, for which the required energy input is infeasible (Abernathy, 2023). Adequate efficiency improvements to those technologies are not anticipated (Abernathy, 2023).

Additionally, incentives and utility would be strongest if even flareable point sources could be used as resources instead of oxidizing them to CO₂. However, the high CapEx of CH₄ transport or oxidation to methanol prevents the use of many small or distant CH₄ point sources. A one-step CH₄-to-methanol oxidation catalyst could enable a simpler process compared to the leading two-step, high-temperature high-pressure process, thereby improving economic viability of many methane sources and reducing overall methane emissions (Haynes, 2014).

Thus, an efficient methane oxidation catalyst could mitigate warming from methane via multiple avenues.

Significance

One-pot CH₄ oxidation at ambient conditions occurs biologically (Tucci, 2024). While methanotroph-based technologies are being developed for dilute CH₄ oxidation (La, 2018; Lidstrom, 2023) and methanol manufacturing (Bjorck, 2018), cell-free technologies may be advantageous. Cell-free systems could be more readily rationally engineered and adapted to designs optimized for mass transport. In methanol manufacturing, cell-free systems could avoid toxicity of feedstock impurities; growth inhibition by concentrated methanol; and costly methanol oxidation inhibitors (Bjorck, 2018). Cell-free systems may achieve lower CapEx by obviating an on-site bioreactor for methanotroph cultivation.

Methane monooxygenase enzymes (MMOs) are candidates for cell-free methane oxidation systems, but they require reductants, which are cost-prohibitive at scale, to activate O₂ into a reactive oxidant (Tucci, 2024). A more economically feasible biocatalyst system would use a reactive oxidant generated electrochemically or by the CH₄ oxidation process without additional inputs, or use electrons directly.

Goals

A cell-free system should be developed to oxidize methane without the input of additional small-molecule reductants. Possible solutions could involve direct electron transfer to MMO from an electrode, or catalysis of CH₄ oxidation by a reactive oxidant generated electrochemically or by the CH₄ oxidation process.

H₂O₂ is a reactive oxidant that might work for CH₄ oxidation, if an efficient catalyst were developed. Methanol manufacturing would favor electrochemically-generated H₂O₂, while oxidation of dilute methane to CO₂ could favor H₂O₂ generated through methane oxidation reactions. Candidate enzyme families for engineering catalysis of CH₄ oxidation by H₂O₂ include soluble methane monooxygenases (sMMO), the hydroxylase component (MMOH) of which catalyzes CH₄ oxidation by H₂O₂ at a low rate (Jiang, 1993); the unspecific peroxidases (Hofrichter, 2021) that oxidize short-chain alkanes but did not act on CH₄ in the one study we are aware of (Sebastian, 2013); and the P450 peroxidases, which use H₂O₂ as an oxidant (Munn, 2018).

We note that techno-economic modeling (TEA) to estimate possible economic advantages of cell-free methane oxidation in various use cases will be an essential guide for engineering (Reginato, 2014), though proof-of-principle demonstration of reductant-free systems can begin now.

Open Questions

The significance could be made more clear by answering:

• Quantitatively, what are the expected economic benefits of a cell-free methane oxidation system compared to one based on methanotrophs (Reginato, 2014)?

The goals could be made more granular by answering:

• What overall CH4 oxidation rate is necessary for a cell-free methane oxidation system to enable a small enough reactor for economically-viable CapEx (Reginato, 2014)?

What total turnover number (Bommarius, 2023) would need to be achieved by cell-free enzymes to accomplish the necessary oxidation rate with economically-viable input costs?

Other information

• Chen et. al engineered P450 to oxidize methane using iodosylbenzene as an oxidant (Chen, 2012; Chen, 2011).
• Astier et. al (Astier, 2013) hypothesized that H₂O₂ may play a role in CH₄ oxidation by electrode-immobilized soluble methane monooxygenase (sMMO) (see second half of their Discussion)
• A high-specificity methane-to-methanol oxidation catalyst that operates at ambient temperatures could reduce process emissions of methanol manufacturing by up to ~0.25 Gt CO₂/yr. (2.3 t CO₂/t methanol for ~98 Mt of methanol manufactured from methane. IRENA, 2021 specifies emissions of ~0.1 t CO₂/GJ, which converts to CO₂/t by: 0.1 t CO2/GJ * 17.8 GJ/m³ methanol * 1/0.792 m³ methanol/t = 2.3 CO₂/t methanol (Neutrium, 2023).