Structure I methane hydrate confined in C₈-grafted SBA-15: A highly efficient storage system enabling ultrafast methane loading and unloading

Confinement of water and methane in mesopores of hydrophobized SBA-15 is demonstrated to promote methane hydrate formation. In comparison to as-synthesized SBA-15, hydrophobization by C₈ grafting accelerates the kinetics of methane storage in and delivery from the hydrate. C₈ grafting density was determined at 0.5 groups nm⁻² based on TGA and quantitative NMR spectroscopy. Multinuclear ¹H-¹H DQSQ and ¹H-¹H RFDR NMR provided spectroscopic evidence for the occurrence of C₈ chains inside the mesopores of SBA-15, by showcasing close spatial proximity between the grafted C₈ chains and pore-intruded water species. X-ray diffraction demonstrates formation of Structure I hydrate on SBA-15 C₈. At 7.0 MPa and 248 K, the water-to-hydrate conversion on hydrophobized SBA-15 C₈ reaches 96% as compared to only 71% on a pristine SBA-15 sample with comparable pore size, pore volume and surface area. The clathrate loading amounted to 14.8 g/g. 2D correlation NMR spectroscopy (¹H-¹³C CP-HETCOR, ¹H-¹H RFDR) reveals hydrate formation occurs within pores of SBA-15 C₈ as well as in interparticle volumes. Following the initial crystallization of SBA-15 C₈-supported methane hydrate taking several hours, a pressure swing process at 248 K allows to desorb and re-adsorb methane from the structure within minutes and without thawing the frozen water structure. Fast loading and unloading of methane was achieved in 19 subsequent cycles without losses in kinetics. The ability to harvest the gas and regenerate the structure without the need to re-freeze the water represents a 50% energy gain with respect to melting and subsequently recrystallizing the hydrate at 298 K and 248 K, respectively. After methane desorption, a small amount of residual methane hydrate in combination with an amorphous yet locally ordered ice phase is observed using ¹³C and ²H NMR spectroscopy. This effect offers an explanation for the enhanced hydrate formation kinetics in adsorption-desorption cycles. These findings open new perspectives for clathrate hydrate-based methane storage.