Green chemistry advancement in methane storage: revealing the role of amino acid types and side chain characteristics in the formation and decomposition of methane hydrates

Methane hydrate (MH) plays a crucial role in energy storage, but its slow formation kinetics limits its wide application. In this study, the effects and mechanisms of six amino acids on the formation and dissociation kinetics of MH were deeply investigated around the hydrophobicity and side-chain characteristics of amino acids. The results showed that hydrophilic amino acids could not promote MH formation due to their strong interaction with H₂O. On the contrary, hydrophobic amino acids could effectively promote MH formation, but their effects were affected by side-chain characteristics and concentration. In particular, methionine and phenylalanine showed the most significant promotion, with 0.5 wt% phenylalanine having a gas storage capacity of 143.02 V/V, attributed to the benzene ring structure on its side chain. Moreover, the induction time of methionine and tryptophan initially increased and then decreased with increased concentration, while the opposite trend were observed for isoleucine and phenylalanine. This phenomenon was explained by hydrophobic effects and side-chain characteristics. In terms of MH dissociation, an innovative combination of infrared thermal imaging techniques revealed similarities in the dissociation patterns of hydrophobic amino acids, but differences existed in the main dissociation stages, which were concentrated within 20–120 min and 20–60 min after thermal stimulation, respectively. This difference was mainly due to the differences in intermolecular force between side-chain structure and H₂O and steric hindrance. Three consecutive experiments on the formation and dissociation of MH confirmed that methionine was better than phenylalanine in terms of stability and there was no foaming in the whole dissociation process. This study provides important theoretical support for the application of amino acids in MH.