The influence of several phosphonium ionic liquids on CH4 hydrates formation and dissociation

The kinetic formation of CH 4 hydrates formation was investigated in the presence of [P 2444 ][PF 6 ], [P 4444 ][PF 6 ], [P 6444 ][PF 6 ], [P 4444 ]Br, [P 6444 ]Br and [P 12444 ]Br with concentrations of 0.25 wt%, 6.25 wt%, 10 wt% under the pressure of 4.0 and 5.0 MPa. The results show that [P 2444 ][PF 6 ], [P 4444 ][PF 6 ] and [P 6444 ][PF 6 ] increase the nucleation temperatures and shorten the induction time, where [P 4444 ][PF 6 ] improves the gas consumption and consumption rates of CH 4 hydrates. At the same time, [P 4444 ]Br, [P 6444 ]Br, and [P 12444 ]Br are unfavorable for increasing the nucleation temperature and shortening the induction time. This implies that [PF 6 ] − anion is more favorable for rapid nucleation than the Br − anion. However, in the presence of [P 4444 ]Br, [P 6444 ]Br and [P 12444 ]Br, the gas consumption was clearly enhanced, following an order of [P 12444 ]Br > [P 6444 ]Br > [P 4444 ]Br, indicating the superiority of long alkyl chains. The dissociation behaviors of CH 4 hydrates were also measured and the pre-dissociation time, dissociation temperature, and fast dissociation time were analyzed. It is found that [P 4444 ][PF 6 ] significantly enhanced the thermal stability of CH 4 hydrate due to a longer pre-dissociation time and a higher dissociation temperature. In contrast, ionic liquids with Br − anion exhibited the opposite effect. Despite having a greater ability to reabsorb CH 4 gas during the self-protection, [P 6444 ]Br and [P 12444 ]Br exhibit faster hydrate dissociation with larger gas-releasing amount compared to the symmetric [P 4444 ]Br and [P 4444 ][PF 6 ]. In general, [P 4444 ][PF 6 ] offers more advantages from the standpoint of safety regarding the storage and transportation of CH 4 hydrate.