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

• A cage-specific guest exchange during the CH 4 –CO 2 replacement was quantitatively observed. • A multi-methodological method was used to examine the time-dependent guest exchange behaviors. • The guest exchange in the large cages had a greater effect on the extent of replacement. • The replacement kinetics was faster at a higher PCO 2 because of the higher CO 2 occupancy in the large cages. CH 4 –CO 2 replacement in naturally occurring gas hydrates has been considered a promising method for both energy recovery and CO 2 sequestration. In this study, the time-dependent guest exchange behaviors and guest distributions during CH 4 –CO 2 replacement were closely examined at two different CO 2 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 13C NMR spectra confirmed that the cage occupancy ratio of the CH 4 molecules in the large 51262 and small 512 cages (θ L /θ S,CH4) after the replacement was significantly smaller than that before the replacement because of the preferential occupation of CO 2 in the large 51262 cages. The time-dependent Raman spectra revealed that the rate of CO 2 inclusion and the resultant CH 4 depletion in the hydrate phase during the replacement was faster at a higher CO 2 injecting pressure. The Rietveld refinement of the PXRD patterns offered a quantitative cage occupancy of CH 4 and CO 2 molecules before and after the replacement. The time-dependent cage occupancy values of CH 4 and CO 2 during the replacement obtained from a multi-methodological approach, which is a combination of PXRD analysis and in-situ Raman measurement, demonstrated that a significant guest exchange in the large 51262 cages had a greater effect on the extent of replacement and that the kinetics of the CH 4 –CO 2 replacement was accelerated at a higher CO 2 injecting pressure. The results provide a better understanding of the kinetics and mechanism of the cage-specific CH 4 –CO 2 replacement occurring in the sI hydrates for CH 4 recovery and CO 2 sequestration. [ABSTRACT FROM AUTHOR]