Synthesis, characterization and thermal-property measurements of ionic semi-clathrate hydrates formed with tetrabutylphosphonium chloride and tetrabutylammonium acrylate

This paper reports an experimental study on the formation of the two new semi-clathrate hydrates with tetrabutylphosphonium chloride (TBPC) and tetrabutylammonium acrylate (TBAAc). The hydrate formation was demonstrated by the measurements of temperature-composition phase diagrams and dissociation heat of the hydrates, visual observations of the hydrate crystals, and single-crystal X-ray diffraction analyses. The highest equilibrium temperature for the TBPC system was 10.3 °C at w _TBPC = 0.36, where w _TBPC denotes the mass fraction of TBPC, (or the mole fraction of TBPC, x _TBPC = 0.034). The TBAAc system was 18.2°C at w _TBAAc = 0.36, where w _TBAAc is the mass fraction of TBAAc, (or the mole fraction of TBAAc, x _TBAAc = 0.031). The greatest dissociation heat for TBPC system was 194 kJ kg ^-1 at w _TBPC = 0.37 and the TBAAc system was 195 kJ kg ^-1 at w _TBAAc = 0.33. For visual observations of the hydrate crystals, the major morphology in both systems was a columnar shape, but hexagonal plate crystals were observed at w _TBPC = 0.10 in the TBPC system. It was also confirmed that the hydrate crystals grown at higher subcooling are finer than those at lower subcooling. The crystallographic structure of TBPC hydrate formed at w _TBPC = 0.36 was identified to be tetragonal with 12.5 × 23.7 × 23.7 Å lattice parameters by the single-crystal X-ray diffraction analysis. Similarly, the crystallographic structure of TBAAc hydrate formed at w _TBAAc = 0.36 was tetragonal with 12.2 × 33.1 × 33.1 Å lattice parameters. The above findings indicate that TBPC and TBAAc hydrates are promising for applications in hydrate based technologies, such as cool energy storage, gas storage and gas separation.