Thermodynamic phase equilibria and cage occupancy of NF3 hydrate

Abstract

F-gases are man-made gases that are utilized mainly in the semiconductor industry and in refrigeration systems. Because F-gases have a high potential of contributing to global warming, various methods, including gas hydrate-based F-gas separation, for separating and recovering F-gases have been widely studied. However, gas hydrate formation with NF3 has not been well studied in spite of its extremely high global warming potential (12,700) and its long atmospheric lifetime (740 years). In this study, the enclathration of NF3 in gas hydrate lattices was investigated, with a focus on phase equilibria and guest distribution. The three-phase (gas hydrate (H) liquid water (Lw) vapor (V)) equilibria of NF3 hydrate were measured to observe the stability conditions for NF3 hydrate. The crystal structure of NF3 hydrate was identified as cubic sl (Pm3n) with a lattice parameter of 11.87 angstrom through powder X-ray diffraction (PXRD). In addition, the cage-filling behavior of NF3 hydrate was examined through both PXRD and in situ Raman spectroscopy. This revealed that the large (5(12)6(2))cages were fully occupied by NF3 molecules whereas the small (5(12)) cages were less populated. The results obtained in this study would be helpful for understanding the cage-specific occupation of F-gas molecules in sl hydrate and devising possible gas hydrate-based F-gas separation methods.