  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 pct. as compared to only 71 pct. on a
pristine SBA-15 sample with comparable pore size, pore volume and surface area.
The clathrate loading amounted to 14.8 g g-1. 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 pct. 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.
