A comprehensive review of semi-clathrate hydrates for CO2 capture: Characterizations, mechanism and role of promoters

•Semi-clathrates have some special thermodynamic properties in contrast to gas hydrates that work at low pressure.•Promoters speed up hydrate formation by raising the rate of hydrate production and lowering the equilibrium condition.•TBAB exhibited the greatest CO2 selectivity even at modest pressure compared to TBAC, TBPB, and TBPC.•TBAB and TBPB in combination increased CO2 absorption because of acceleration of hydrate formation.•Hydrate-based capture can reduce energy costs by 50–75 % compared to membranes and cryogenic distillation. Recently, clathrate hydrate-based CO2 separation is considered as one of the most attractive processes for reducing CO2 emissions because of its effective energy utilization, environmental friendliness, and economic viability. The ionic semi-clathrate hydrate, a quaternary salt used to facilitate hydrate formation, is particularly shown interest because it can boost CO2 capture by improving the physical and chemical interactions between host lattice and guest molecules for hydrate formation. A reduced pressure of 1 MPa or less is high enough for effective gas trapping at 280 K using a semi-clathrate hydrate. The operating parameters, ionic hydrate structure, and promoter concentration affect CO2 capture. The efficiency of the CO2 separation process can be significantly reduced by inhibitory effects at a particular salt concentration. Research has been conducted using tetra-n-butyl-ammonium and phosphonium salts because their crystal structure and morphology are favorable to form semi-clathrate hydrates. However, only a few lookups on environment-friendly and appropriate characterization strategies of the hydrates and novel promoters, and their design, and operability have been performed. This review addresses the mechanisms involving the size of CO2 molecules in an ionic hydrate network, the characterization methods of the hydrates, promoter integration and their overall performance analysis. In addition to that, operational strategies of the semi-clathrate hydrate-based CO2 capture processes, the drawbacks and future routes to research CO2 capture using semi-clathrate hydrates have been addressed. [Display omitted]