Environmental Engineering Reference
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For example, a recent study has examined the effects of structural modi-
fications on the evolution of the crystal structure, pore characteristics, and
H
2
capacities of MOF-5s and found that structural modifications can signifi-
cantly influence the pore characteristics, and the specific surface areas of the
MOF-5s decreased with the evolution of an ultrafine porosity [47]. These
changes were correlated with an increase in the H
2
storage capacity of the
MOF-5 (from 1.2 to 2.0 wt% at 196°C and 1 bar). The structural modifica-
tions also enhanced the thermal stability of the MOF-5s. In another study,
porous carbon with hierarchical pore structure derived from highly crystal-
line MOFs (denoted as MOF-derived carbon or MDC) without any carbon
source was found to display hierarchical pore structures with high ultrami-
croporosity, high specific surface area, and very high total pore volume, and
thereby to exhibit reversible H
2
storage capacities at certain conditions that
were better than those of previously reported porous carbons and MOFs, with
a 3.25 wt% uptake for MDC-1 at 77 K and 1 bar [48]. This exceeded that
of the benchmark materials and surpassed the performance of all other mate-
rials characterized to date. Figure 7.8 shows a comparison of the hydrogen
storage capacity of the MDCs and isoreticular MOFS (IRMOFs) with some
benchmark materials such as PCN-12 and ZTC.
7.3 CLATHRATE HYDRATES
Clathrate hydrates are a class of solid inclusion compounds in which guest
molecules occupy cages formed from hydrogen-bonded water molecule net-
works. The usually unstable empty cages can be stabilized by inclusion of
appropriately sized molecules. Clathrate hydrates of hydrogen often possess
two different-sized cages to meet the necessary storage requirements.
However, the higher pressures required, around 2 kbar, to produce the mate-
rial makes it impractical. The synthesis pressure can be decreased by filling
the larger cavity with tetrahydrofuran (THF) to stabilize the material, with
the compromise of the potential storage capacity of the material [49]. In a
related study, reversible hydrogen storage capacities in THF-containing
binary-clathrate hydrates have been increased to ∼4 wt% at 270 K and
modest pressures (12 MPa) by tuning their composition to allow the hydro-
gen guests to enter both the larger and the smaller cages, while retaining low
pressure stability. The tuning mechanism is quite general and convenient,
using water-soluble hydrate promoters and various small gaseous guests [50].
Figure 7.9 shows the hydrogen content as a function of THF concentration
and a schematic diagram of hydrogen distribution in the cages of THF+H
2
hydrates. A subsequent study found that the use of inexpensive hydrogels as
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