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 CH ₄ from MCH remains a challenge. Gradually replacing CH ₄ in MCH with CO ₂ is an attractive scheme, as it is cost efficient and mitigates the environmentally harmful effects of CO ₂ 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 CH ₄ in the 5 ¹²6² cages of bulk structure-I (sI) MCH can be substituted by gaseous CO ₂ 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 CO ₂ binds more strongly to the 5 ¹²6² cages than methane does, and that the diffusion barrier for CH ₄ is significantly lowered by an intermediate state in which one hydrate cage is doubly occupied by CH ₄ and CO ₂. Therefore, exchange of CO ₂ for CH ₄ in MCH is not only energetically favorable but also kinetically feasible. Experimental and theoretical studies of CH ₄/CO ₂ substitution elucidate a method to harness energy from these combustible ice resources.