The influence of ionic liquids with heterocyclic cations on CO2 hydrates formation

Kinetic characteristics of CO2 hydrates formation in the presence of eight ionic liquids with heterocyclic cations and [PF6]- anion were experimentally investigated here. It is found that the imidazolium ionic liquids with short substituted alkyl chains can inhibit CO2 hydrates formation. Increasing the alkyl chain length can alleviate the nucleation inhibition effect, and 1-hexadecyl-3-methylimidazolium hexafluorophosphate([C16mim][PF6]) with a concentration of 10 wt% even shows a slight promotion effect comparing to the pure water. In contrast, adding pyrrolidinium, piperidinium and pyridinium ionic liquids increases the nucleation temperatures, shortens the induction times and enhances the CO2 consumption. Based on DFT calculations and MD simulations, it is inferred that the strong binding energy between H2O and 1-ethyl-imidazolium hexafluorophosphate([Emim][PF6]) plays a main role on its inhibition behavior during CO2 hydrates formation. The H2O-H2O RDF curves of N-butyl pyridinium hexafluorophosphate([OPy][PF6]), N-butyl-N-methylpiperidinium hexafluorophosphate([PP14][PF6]) and N-butyl-N-methylpyrrolidinium hexafluorophosphate([PY14][PF6]) aqueous systems exhibit three peaks which become intensified and narrow with time, while [Emim][PF6]-containing system only has a single peak due to its strong interaction with water and thus inhibits the water-water attraction. In addition, RDF peaks of H2O–[PF6]- become weak gradually with time, implying that the anion [PF6]- does not engage in the structures of water cages.