Structure and guest distribution of the mixed carbon dioxide and nitrogen hydrates as revealed by X-ray diffraction and C-13 NMR spectroscopy

Abstract

In this contribution, X-ray diffraction and C-13 NMR spectroscopy were used to identify structure and guest distribution of the mixed N-2 + CO2 hydrates. X-ray diffraction results of the mixed N-2 + CO2 hydrates confirmed that the unit cell parameter was similar to11.8 Angstrom over the gas mixture composition range of 3-20 mol % CO2 and the formed hydrates were identified as structure I. When the composition of the gas mixture was reduced to 1 mol % CO2, the structure of the mixed hydrate was transformed to structure II showing the unit cell parameter of 17.26(2) Angstrom. C-13 cross-polarization (CP) NMR spectroscopy was used to examine the distribution of carbon dioxide molecules in small 5(12) and large 5(12)6(2) cages of structure I. From NMR spectra of the mixed N-2 + CO2 hydrate formed from gas mixture of 20 and 10 MOl % CO2, powder patterns having chemical shift anisotropy of -54.5 and -53.8 ppm were observed, respectively. There was no clear isotropic line indicating carbon dioxide molecules in the small 5(12) cages of structure I. These NMR spectra showed that the carbon dioxide molecules occupied mainly the large 5(12)6(2) cages of structure I when the mixed N-2 + CO2 hydrate was formed at a vapor-phase composition range of 10-20 mol % CO2. In addition, from the analysis of the gases collected from a dissociating hydrate sample, the amount of carbon dioxide in the mixed N-2 + CO2 hydrates increased greatly with a small increase of carbon dioxide in the vapor phase. Accordingly, the carbon dioxide molecules seemed to occupy both the small and large cages of structure I above the vapor-phase composition of 33.3 mol % CO2.