Experimental and computational investigation of methane hydrate inhibition in the presence of amino acids and ionic liquids

Amino acids (glycine and alanine) and ionic liquids ([BMIM][BF4] and [BMIM][I]) were examined for their inhibition effects on CH4 hydrates with experimental and computational approaches. Both amino acids and ionic liquids functioned well as thermodynamic hydrate inhibitors, by shifting equilibrium curves of CH4 hydrates toward harsh conditions. However, powder X-ray diffraction patterns indicated that amino acids and ionic liquids did not affect the hydrate structure because they were not included in the hydrate cages due to their large molecular size. Gas uptake measurements showed that the conversion of water into gas hydrates and the formation rates of CH4 hydrate were significantly influenced by inhibitors. Density functional theory calculations indicated that [BMIM][BF4] had a larger potential than glycine to inhibit gas hydrate formation by giving a more negative interaction energy between a cage and an inhibitor molecule. The time-dependent Raman spectra collected during CH4 hydrate formation demonstrated that [BMIM][BF4] hindered CH4 molecules from occupying small (512) cages, whereas glycine had a strong influence on large (51262) cages of sI hydrates. The experimental and computational results provide a better understanding of inhibition mechanisms of gas hydrates and thus can contribute to the improved control of hydrate formation in oil and gas pipelines