Structure and Guest Distribution of the Mixed Carbon Dioxide and Nitrogen Hydrates As Revealed by X-ray Diffraction and 13C NMR Spectroscopy

Abstract

In this contribution, X-ray diffraction and 13C NMR spectroscopy were used to identify structure and guest distribution of the mixed N2 + CO2 hydrates. X-ray diffraction results of the mixed N2 + CO2 hydrates confirmed that the unit cell parameter was ∼11.8 A 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) A. 13C cross-polarization (CP) NMR spectroscopy was used to examine the distribution of carbon dioxide molecules in small 512 and large 51262 cages of structure I. From NMR spectra of the mixed N2 + 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 512 cages of structure I....