Metastable structure II methane hydrate: A bridge to obtain high methane uptake in the presence of thermodynamic promoter.

A new synergistic strategy, based on simultaneously encapsulating CH 4 molecules in some 51264 cages (structure II hydrates) and facilitating structure I CH 4 hydrate formation, was found can place more CH 4 in the hydrate phase and facilitate more water-to-hydrate conversion, thereby increasing significantly the final gas uptake. Relative to the LCU, the highest increase in CH 4 uptake obtained in this work is up to 232.75 %. [Display omitted] • CH 4 hydrate formation mechanism in the presence of a promoter are illustrated. • Metastable Str.II pure CH 4 hydrates play a bridge to obtain mixture hydrate phase. • A CH 4 uptake, as high as 141.21 mmol/molH 2 O, is obtained from this work. • A new strategy that can significantly increase hydrate gas uptake is proposed. The addition of thermodynamic promoters is a promising and perhaps the only effective method to reduce hydrate formation conditions. However, this is achieved at the cost of the gas storage capacity reduction since the thermodynamic promoter will occupy some hydrate cages. To date, there is no effective method that can simultaneously limit this negative effect and obtain high water-to-hydrate conversion. In this work, we determine the methane (CH 4) hydrate formation mechanism in the presence of a thermodynamics promoter, Cyclopentane (CP), and find that structure II (Str.II) CH 4 hydrate, as a metastable intermediate structure of the reaction, plays a quite significant role in encapsulating CH 4 molecules in some 51264 cages (Str.II hydrates) and facilitating structure I CH 4 hydrate formation, thereby increasing gas uptake significantly. A CH 4 uptake, as high as 141.21 mmol/molH 2 O, is obtained from this work. Relative to the acknowledged limit of CH 4 uptake of Str.II hydrates formed commonly in the presence of same mole fractions of thermodynamics promoter (8 (P r o m o t e r) · 16 (C H 4) · 136 H 2 O)), the highest increase in CH 4 uptake is up to 232.75 %. These results would stimulate the development of the hydrate reaction theory and provide a new direction for promoter design. [ABSTRACT FROM AUTHOR]