Pyrazine Analogues on Clathrate Hydrates in Methane–Water Systems

This paper reports the change in equilibrium pressure–temperature (pT) boundaries in a methane (CH₄)–water system by adding three pyrazine analogues: 2-methylpyrazine, 2,3-dimethylpyrazine, and 2,3,5-trimethylpyrazine. The 2-methylpyrazine, 2,3-dimethylpyrazine, and 2,3,5-trimethylpyrazine showed no effect on the clathrate hydrate structure that formed in the CH₄–water system upon analysis via Raman spectroscopy. Raman spectra revealed that clathrate crystals that formed in the CH₄–pyrazine analogue–water system were of a structure-I CH₄ hydrate. Phase equilibrium pT boundaries in the three CH₄–pyrazine analogue–water systems shifted to a higher p and lower T condition than that for the CH₄–water system with no additive. The higher p/lower T shift confirmed that 2-methylpyrazine, 2,3-dimethylpyrazine, and 2,3,5-trimethylpyrazine act as thermodynamic inhibitors for hydrate formation. The order of ΔT, a lower T shift from the CH₄–water system, was as follows: 2-methylpyrazine–water > 2,3-dimethylpyrazine > 2,3,5-trimethylpyrazine. This order is in agreement with the number of −CH₃ groups in the molecules.