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 degrees C at w(TBPC) = 0.36, where wTBPC denotes the mass fraction of TBPC, (or the mole fraction of TBPC, x(TBPC) = 0.034). The TBAAc system was 18.2 degrees C at w(TBAAc) = 0.36, where wTBAAc 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 x 23.7 x 23.7 angstrom 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 x 33.1 x 33.1 angstrom 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.