Estimation of gas composition and cage occupancies in CH4-C2H6 hydrates by CP-MAS 13C NMR technique

CP-MAS C-13 NMR measurements were carried out on mixed gas hydrates containing CH4-C2H6. The changes in NMR chemical shift values for CH4 and C2H6 Clearly corresponded to the structural changes in the hydrate structure. The encaged gas compositions estimated by the integrated C-13 NMR signal intensities agreed well with the dissociated gas compositions measured by gas chromatography. Therefore, the gas composition in mixed gas hydrates can be directly estimated from the C-13 NMR spectra. The cage occupancies of the small and large cages of the hydrates were estimated from the C-13 NMR spectra on the basis of a statistical thermodynamic model. The large cages were almost fully occupied with guest molecules, whereas small cage occupancy decreased with increasing C2H6 concentration. Therefore, large cages are highly preferentially occupied by C2H6 molecules rather than CH4 molecules.