CH4 enclathration in tetra-iso-amyl ammonium bromide (TiAAB) semiclathrate and its significance for natural gas storage

Tetra-iso-amyl ammonium bromide (TiAAB), one of the quaternary ammonium salts, was investigated as a semiclathrate former for its potential application in natural gas storage. The dissociation equilibrium temperature of pure TiAAB semiclathrate was the highest at a stoichiometric concentration (3.7 mol%) of TiAAB·26.0H2O. The dissociation enthalpy of pure TiAAB (3.7 mol%) semiclathrate was 253.0 ± 0.5 J/g using a differential scanning calorimeter (DSC). The thermodynamic stability of the CH4 + TiAAB semiclathrate was significantly higher than that of the CH4 gas hydrate. The CH4 + TiAAB (3.7 mol%) semiclathrate was thermodynamically stable at temperatures higher than 300 K for the pressure range of 1.5-8.5 MPa. 13C NMR and in situ Raman spectroscopy revealed that CH4 molecules were enclathrated in the small cages of the TiAAB semiclathrate and a CH4 inclusion did not affect the semiclathrate structure. The overall experimental results clearly indicate that the TiAAB semiclathrate can trap CH4 molecules at favorable conditions and thus it has a potential for natural gas storage.