Phase equilibrium data for semiclathrate hydrates formed with tetra-n-butylammonium (bromide or chloride) and tetra-n-butylphosphonium (bromide or chloride) under hydrogen + carbon dioxide pressure

Abstract

Semiclathrate hydrates are host–guest materials that can be used for CO2 capture due to their unique gas selectivity. CO2 capture from H2 + CO2 mixed gas is possible by semiclathrate hydrates formed by tetra-n-butylammonium (TBA) and tetra-n-butylphosphonium (TBP) salts. TBA bromide (TBAB), TBA chloride (TBAC), TBP bromide (TBPB) and TBP chloride (TBPC) are widely available ionic substances for semiclathrate hydrate formation. In order to understand their gas capture properties, phase equilibrium data of these materials are necessary. Here we report experimental three phase (gas–hydrate–aqueous phase) equilibrium data measured in (TBAB, TBAC, TBPB or TBPC) + H2 + CO2 + H2O systems. The comparison with the literature data suggested that H2 can be incorporated in the TBAB and TBAC hydrates together with CO2, but TBPB and TBPC hydrates reject H2. The hydrate phases were identified by single crystal X-ray diffraction analyses. The TBAB, TBPB and TBPC form the same or similar hydrate phases, i.e., orthorhombic and hexagonal phases. The TBAC formed the tetragonal hydrate phase. The present results suggest that different CO2 capture properties are obtained by these semiclathrate hydrates.