Enclathration of CO2 as a co-guest of structure H hydrates and its implications for CO2 capture and sequestration

In this study, the thermodynamic behaviors, cage-specific guest distributions, structural transition, and dissociation enthalpies of sH hydrates with CO2 + N2 gas mixtures were investigated for their potential applications to hydrate-based CO2 capture and sequestration. The stability conditions of the CO2 + N2 + water systems and the CO2 + N2 + neohexane (2,2-dimethylbutane, NH) + water systems indicated that the gas mixtures in the range of flue gas compositions could form sH hydrates, thereby mitigating the pressure and temperature required for gas hydrate formation. Structure identification using powder X-ray diffraction (PXRD) revealed the coexistence of sI and sH hydrates in the CO2 (40%) + N2 (60%) + NH system and the hydrate structure transformed from sH into sI as the CO2 concentration increased. In addition, the Raman analysis clearly demonstrated that CO2 molecules were enclathrated into the cages of sH hydrates in the N2-rich systems. It was found from direct CO2 composition measurements that CO2 selectivity in the sH hydrate phase was slightly lower than that in the corresponding sI hydrate phase. Dissociation enthalpy (ΔHd) measurements using a high-pressure micro-differential scanning calorimeter (HP μ-DSC) indicated that the ΔHd values could also provide valuable information on the structural transition of sH to sI hydrates with respect to the CO2 concentration in the feed gas. This study provides a better understanding of the thermodynamic and physicochemical background for CO2 enclathration in the sH hydrates and its significance in gas hydrate-based CO2 capture and sequestration.