Time-dependent observation of a cage-specific guest exchange in sI hydrates for CH4 recovery and CO2 sequestration

CH4-CO2 replacement in naturally occurring gas hydrates has been considered a promising method for both energy recovery and CO2 sequestration. In this study, the time-dependent guest exchange behaviors and guest distributions during CH4 -CO2 replacement were closely examined at two different CO2 injecting pressures (2.2 and 3.5 MPa) using nuclear magnetic resonance (NMR), in-situ Raman spectroscopy, powder X-ray diffraction (PXRD), and gas chromatography. The C-13 NMR spectra confirmed that the cage occupancy ratio of the CH4 molecules in the large 5(12)6(2) and small 5(12) cages (theta(L)/theta(S,CH4)) after the replacement was significantly smaller than that before the replacement because of the preferential occupation of CO2 in the large 5(12)6(2) cages. The time-dependent Raman spectra revealed that the rate of CO2 inclusion and the resultant CH4 depletion in the hydrate phase during the replacement was faster at a higher CO2 injecting pressure. The Rietveld refinement of the PXRD patterns offered a quantitative cage occupancy of CH4 and CO2 molecules before and after the replacement. The time-dependent cage occupancy values of CH4 and CO2 during the replacement obtained from a mull-methodological approach, which is a combination of PXRD analysis and in-situ Raman measurement, demonstrated that a significant guest exchange in the large 5(12)6(2) cages had a greater effect on the extent of replacement and that the kinetics of the CH4-CO2 replacement was accelerated at a higher CO2 injecting pressure. The results provide a better understanding of the kinetics and mechanism of the cage-specific CH4-CO2 replacement occurring in the sI hydrates for CH4 recovery and CO2 sequestration.