Effect of additives in the nucleation and growth of methane hydrates confined in a high-surface area activated carbon material

A high-surface area activated carbon material (PPAC) prepared from a petroleum residue has been used as a host structure to promote the nucleation and growth of confined methane hydrates after the incorporation of additives. Sodium dodecyl sulfate (SDS), Leucine and Tetrahydrofuran (THF) have been evaluated either dissolved in the pre-impregnation media (ultrapure water) or incorporated in the carbon surface using a mechanochemical approach. High-pressure methane adsorption isotherms show that the mechanochemical approach does not improve the storage performance of the original carbon material due to the steric effects by the bulkier organic functionalities. Furthermore, the incorporation of the additives shifts the nucleation in narrow pores to higher pressures. On the contrary, when the additives are dissolved in water, high-pressure methane isotherms anticipate a modification in the kinetics and/or thermodynamics of the nucleation process. Whereas gas hydrates grown in large pores and/or the external surface exhibits a significant reduction in the induction time, the nucleation process in narrow micropores becomes more uniform and occurs in a narrower pressure window, slightly shifted to higher pressures. However, no changes in the total uptake at 10 MPa could be found. The situation is different when THF is dissolved in water. In this case, the nucleation takes place at pressures below the natural process (<3–4 MPa), the total amount of methane stored at 10 MPa being slightly limited. In-situ synchrotron X-ray diffraction studies anticipate a sI structure for the pure water methane hydrates, while a combination of sI and sII is identified for the THF-based system.