CH₄ Gas Extraction by CO₂ : Substitution in Clathrate Hydrate through Bimolecular Iteration

Methane clathrate hydrate (MCH) is a promising energy resource, but controllable extraction of CH₄ from MCH remains a challenge. Gradually replacing CH₄ in MCH with CO₂ is an attractive scheme, as it is cost efficient and mitigates the environmentally harmful effects of CO₂ by sequestration. However, the practicable implementation of this method has not yet been achieved. In this study, using in situ neutron diffraction, we confirm that CH₄ in the 5¹²6² cages of bulk structure-I (sI) MCH can be substituted by gaseous CO₂ under high pressure and low temperature with a high substitution ratio (∼44%) while conserving the structure of the hydrate framework. First-principles calculations indicate that CO₂ binds more strongly to the 5¹²6² cages than methane does, and that the diffusion barrier for CH₄ is significantly lowered by an intermediate state in which one hydrate cage is doubly occupied by CH₄ and CO₂. Therefore, exchange of CO₂ for CH₄ in MCH is not only energetically favorable but also kinetically feasible. Experimental and theoretical studies of CH₄/CO₂ substitution elucidate a method to harness energy from these combustible ice resources.