Calibration of Raman Quantification Factors of Guest Molecules in Gas Hydrates and Their Application to Gas Exchange Processes Involving N2

Methane-dominated natural gas hydrate deposits have been considered as a potential hydrocarbon resource and as long-term storage reservoirs for the anthropogenic greenhouse gas CO2 via CH4–CO2–N2 replacement in gas hydrates. In this study, N2-hydrates of structure type I (sI) were formed, characterized, and quantified in terms of N2 cage occupancies using synchrotron X-ray diffraction. Pure sI CH4- and N2-hydrates with known cage occupancies were used to calibrate the relative Raman quantification factors (F-factors) of N2 to its H2O framework and to CH4 in sI hydrate phase. The F-factors of CO2/CH4, CO2/H2O, and CH4/H2O in the hydrate cavities were corrected for the presence of ice Ih. Using these empirical ratios of F-factors, the absolute cage occupancies, the bulk guest composition, and hydration number of gas hydrates containing CH4, CO2, N2, and C2H6 molecules can now be determined by Raman spectroscopy without additional thermodynamic assumptions. In this way, one can gain insight into details of the gas composition in mixed hydrates, for example, during the N2-assisted CH4–CO2 exchange reaction, as well as into the preference of certain gas species for small or large cages.