Kinetic Promotion and Inhibition of Methane Hydrate Formation by Morpholinium Ionic Liquids with Chloride and Tetrafluoroborate Anions

Ionic liquids of 1-hydroxyethyl-1-methylmorpholinium chloride (HEMM-Cl) and 1-hydroxyethyl-1-methylmorpholinium tetrafluoroborate (HEMM-BF4) were synthesized to investigate the different effects of anions in the ionic liquids on the methane hydrate formation kinetics. HEMM-Cl and HEMM-BF4 acted as the kinetic hydrate promoter and inhibitor, respectively. The induction time of HEMM-BF4 solutions increased in proportion to the HEMM-BF4 concentration, and both ionic liquids showed thermodynamic hydrate inhibition effect on methane hydrate formation. The X-ray diffraction pattern of hydrates formed in the presence of both ionic liquids showed that there was no influence on, and no incorporation of ionic liquids into, the crystal structure. Using the dynamic light scattering (DLS) technique, HEMM-Cl was revealed not to form micelles, which implies that HEMM-Cl is nonamphiphilic and its hydrate promotion mechanism is different from that of amphiphilic surfactant promoters. HEMM-Cl might distort the rigid hydrate host framework at the surface by the hydrogen bonds of the ions with water molecules, which could promote the methane penetration or inclusion into the growing clathrate hydrate structures, thereby improving hydrate formation. In contrast, the kinetic hydrate inhibition behavior of HEMM-BF4 might be attributed to the hypothesis that BF4 could act as a mobile pseudoguest because the anion cannot be the guest of the hydrate cage owing to the charge imbalance even though it could fit into the structure I hydrate cages.