Selective Encaging of N₂O in N₂O–N₂ Binary Gas Hydrates via Hydrate-Based Gas Separation

Abstract

The crystal structure and guest inclusion behaviors of nitrous oxide-nitrogen (N₂O-N₂) binary gas hydrates formed from N₂O/N₂ gas mixtures are determined through spectroscopic analysis. Powder X-ray diffraction results indicate that the crystal structure of all the N₂O-N₂ binary gas hydrates is identified as the structure I (sI) hydrate. Raman spectra for the N₂O-N₂ binary gas hydrate formed from N₂O/N₂ (80/20, 60/40, 40/60 mol %) gas mixtures reveal that N₂O molecules occupy both large and small cages of the sI hydrate. In contrast, there is a single Raman band of N₂O molecules for the N₂O-N₂ binary gas hydrate formed from the N₂O/N₂ (20/80 mol %) gas mixture, indicating that N₂O molecules are trapped in only large cages of the sI hydrate. From temperature-dependent Raman spectra and the Predictive Soave-Redlich-Kwong (PSRK) model calculation, we confirm the self-preservation of N₂O-N₂ binary gas hydrates in the temperature range of 210-270 K. Both the experimental measurements and the PSRK model calculations demonstrate the preferential occupation of N₂O molecules rather than N₂ molecules in the hydrate cages, leading to a possible process for separating N₂O from gas mixtures via hydrate formation. The phase equilibrium conditions, pseudo-pressure-composition (P-x) diagram, and gas storage capacity of N₂O-N₂ binary gas hydrates are discussed in detail.