CH_4 Gas Extraction by CO_2: 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 5^126^2 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 (\sim 44%) while conserving the structure of the hydrate framework. First-principles calculations indicate that CO_2 binds more strongly to the 5^126^2 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.