Spectroscopic analysis of carbon dioxide and nitrogen mixed gas hydrates in silica gel for CO2 separation

In this study solid-state NMR spectroscopy was used to identify structure and guest distribution of the mixed N2 + CO2 hydrates. These results show that it is possible to recover CO2 from flue gas by forming a mixed hydrate that removes CO2 preferentially from CO2/N2 gas mixture. Hydrate phase equilibria for the ternary CO2–N2–water system in silica gel pores were measured, which show that the three-phase H–Lw–V equilibrium curves were shifted to higher pressures at a specific temperature when the concentration of CO2 in the vapor phase decreased. 13C cross-polarization (CP) NMR spectra of the mixed hydrates at gas compositions of more than 10 mol% CO2 with the balance N2 identified that the crystal structure of mixed hydrates as structure I, and that the CO2 molecules occupy mainly the abundant 51262 cages. This makes it possible to achieve concentrations of more than 96 mol% CO2 gas in the product after three cycles of hydrate formation and dissociation.