The core challenge in atmospheric methane (CH₄) removal is selectively extracting CH₄ amidst the significantly more abundant nitrogen (N₂). Current technologies, such as photo- oxidation, microbial conversion, and solid sorption, exhibit drastic performance decline at low CH₄% [1]. Solid capture (some followed by in situ oxidation) is considered promising due to good capacity (mostly 0.5-1 mol/kg, the best: ~3 mol/kg in pure CH₄) and CH₄/N₂ selectivity (mostly 2-6, the best: 12 at a 1:1 molar ratio) [2-6]. However, both values would decrease by ~6 orders of magnitude in ppm-level CH₄ based on sorption mechanisms [7]. Liquid capture was deemed impractical due to CH₄'s low solubility in most known liquid solvents. Yet, many organic solvents exhibit decent CH₄/N₂ selectivity >5 (defined by Henry’s constant), comparable to most solid sorbents [2]. For atmospheric removal, we argue that the evaluation criteria should emphasize selectivity over capacity. This is justified by the scarcity of CH₄, where even material with moderate capacity could process a significant air volume. Hence, we advocate reassessing the strategy for the liquid capture of atmospheric CH₄.

The core problem of atmospheric methane removal by solid sorbents lies in the competitive binding of CH₄ and N₂ molecules on the surfaces due to their similar physical and chemical properties, with CH₄ disadvantaged by extreme scarcity. Dissolutions of N₂ and CH₄ in a liquid solvent are independent and eliminate such competition, as a result, the scarcity of CH₄ prolongs equilibrium yet does not affect its selectivity. This converts the challenge of selectivity under ppm-level CH₄ into a gas-to-liquid mass transfer issue, which we propose to address by converting bulk liquid into high-surface area, gas-permeable geometries, or “structured liquid”.

The primary goals by which success will be the delivery of the best solvent geometries, optimal reactor prototypes and quantitative characterization of sorption performance as a function of solvent geometries and process conditions. If the concept is validated, the knowledge gained in the studies will help fill the gap and stimulate investigations of methane solvents and process developments, which in turn could benefit more efficient atmospheric methane removal.

This proposal involves high risks and promises high rewards and impacts, including,

1. Pioneering a mechanistically distinct and unprecedented approach in methane removal.
2. Introducing a novel concept and method for creating "structured liquid solvents."
3. Generating new knowledge to comprehend CH₄ gas-to-liquid mass transfer at ppm-level concentrations in unconventional solvents.
4. Catalyzing the development of innovative methane solvents.
5. Establishing the first-of-its-kind reactor prototypes and advancing process development.
6. First-hand experimental data including quantitative determination of CH₄ sorption rate, selectivity, and capacity by systematically varying the geometry, thickness, and surface area of liquid-on-solid sorbents.