CO₂ enclathration in hydrates of peralkyl-(Ammonium/Phosphonium) salts: Stability conditions and dissociation enthalpies

The present work investigates equilibrium conditions and dissociation enthalpies of semiclathrate hydrates formed from CO₂ + tetra-n-butylammonium chloride (TBACl) + water, CO₂ + tetra-n-butylammonium nitrate (TBANO3) + water, and CO₂ + tetra-n-butylphosphonium bromide (TBPB) + water mixtures. Differential scanning calorimetry (DSC) was used for the determination of hydrate-liquid-vapor (H-L-V) equilibrium conditions in the presence of TBACl, TBANO₃, and TBPB solutions at ammonium salt mass fractions of 0.3618, 0.3941, and 0.3707, respectively, and at CO₂ pressure in the range of (0.5 to 2.0) MPa. Results reveal that semiclathrate hydrates of TBACl, TBANO₃, and TBPB are able to incorporate carbon dioxide in their structure and that the resulting mixed hydrates have significantly lower formation pressures than those of pure CO₂ hydrate. The dissociation enthalpies of semiclathrate hydrates of TBACl, TBANO₃, and TBPB with CO₂ were determined by both DSC and the Clausius-Clapeyron equation. The DSC experiments demonstrate that mixed hydrates of TBANO₃, TBACl, and TBPB with CO₂ have higher melting enthalpies than single hydrates. From our measurements, it appears that mixed TBPB + CO₂ hydrate has appropriate stability conditions (p, T) and latent heat content for secondary refrigeration applications. DSC measurements combined with the Clausius-Clapeyron equation show that mixed TBPB + CO₂ hydrate can store large amounts of CO₂ and thus could be attractive for gas capture and storage applications.