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

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.