Expanded CO2 Capture Properties of Semiclathrate Hydrates by Structure-Designed Ions

The gas capacity and CO2 separation performance of semiclathrate hydrates formed with structure-designed asymmetric quaternary alkylammonium salts─tri-n-butyl, n-hexylammonium bromide (N4446Br) and chloride (N4446Cl)─were compared with those of tetra-n-butylammonium bromide (TBAB), tri-n-butylphosphine oxide (TBPO), and tetrahydrofuran (THF). Single-gas capacity measurements with CH4 and CO2, combined with our previously reported data, showed that THF hydrates have the highest CH4 capacity, whereas N4446Br hydrates have the highest CO2 capacity on molar and mass bases and the highest capacities per mole of promoter for both gases. CO2 capture tests with binary gas mixtures revealed that N4446Br and N4446Cl achieved the highest CO2 uptake in the CH4 + CO2 system and at elevated pressures in the N2 + CO2 system, with the most stable separation performance. The ionic promoters (TBAB, N4446Br, N4446Cl) exhibited markedly higher CO2 selectivity than the non-ionic TBPO and THF. The superior performance is attributed to the cage geometry of HS-I or its related structures, which favors CO2 encapsulation; previously reported phase equilibrium and crystal structure data suggest that the N4446 salts form these structures. These findings demonstrate that designing ionic promoters is an effective strategy for tuning hydrate structures to enhance CO2 capture.