Cage Occupancy and Stability of N2O-Encaged Structure I and II Clathrate Hydrates

Abstract

The crystal structure and guest inclusion behaviors of nitrous oxide (N2O) hydrates with/without tetrahydrofuran (THF) were investigated through spectroscopic observations. The X-ray diffraction results showed that pure N2O hydrate has the formation of structure I (sI) hydrate and N2O-THF hydrate has the formation of structure II (sII) hydrate. For pure N2O hydrate, the Raman spectra revealed the occupation of N2O molecules in both the small 512 and large 51262 cages of the sI hydrate. There was a single Raman band in the v(1) and v(3) spectral regions for the N2O-THF (5.56 mol %) sII hydrate, indicating that the N2O molecules occupied only the small 512 cages of the sII hydrate. However, both sI and sII hydrate phases were monitored from the N2O THF (2 mol %) hydrate sample. From a combination of Raman results and the thermodynamic model, the occupancies of N2O molecules in the small 5(12) and large 5(12)6(2) cages of sI hydrate were estimated to be 0.746 and 0.978, respectively. The phase equilibrium conditions of pure N2O and N2O THF hydrates were measured and compared to those of pure CO2 and CO2-THF hydrates. The predictive Soave-Redlich-Kwong model predicted the phase stability boundary and cage occupancy of pure N2O hydrate, which agreed well with the experimental results. Temperature-dependent Raman spectra revealed that pure N2O hydrate shows the self-preservation behavior in the temperature region of 200-273 K.