Azeotropic clathrate: Compelling similarity of CO2 and N2O uptake in an organic crystalline host

Both CO2 and N2O are greenhouse gases (GHGs) notorious for their impact on climate change. Because they threaten the future of human beings, they must be captured and recovered from the perspective of environmental protection. Herein, we evaluate the potential feasibility of using hydroquinone (HQ) clathrate formed from CO2N2O gas mixtures for storage and recovery of GHGs. Structural characteristics and guest-occupation behaviors of the HQ samples reacted with CO2-N2O gas mixtures at room temperature were investigated using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, infrared spectroscopy, and solid-state NMR. The combined results via these measurements indicate that all the HQ samples formed from CO2-N2O gas mixtures were fl-HQ clathrate structures. The gas storage capacity of the fl-HQ clathrates and the cage occupancy of guest molecules were found to be 60.2-62.3 cm3 g-1 and 0.82-0.84, respectively. Selectivity measurements indicate that the fl-HQ clathrate reveals azeotropic-like behavior for all gas compositions, showing no preferential occupation by CO2 or N2O in the cages of fl-HQ clathrate. The compositions of CO2 and N2O in the fl-HQ clathrates are very close to the initial gas compositions. This indicates that this CO2-N2O-loaded fl-HQ clathrate is the first example of an all-composition azeotropic clathrate. Kinetics experiments demonstrate that the azeotropic fl-HQ clathrates are attributable to very similar formation kinetics of fl-HQ clathrates with CO2 and with N2O guest molecules. These results provide useful knowledge of host-guest and guest-guest interactions in clathrate compounds and novel applications of fl-HQ clathrates. These will be adopted to develop further a clathrate-based GHG storage and recovery technology.