having similar pore volumes (1.6 cm3 gꢀ1). Thus, it appears
that this combination of high surface area and pore volume is
necessary to achieve this level of H2 storage capacity.
P. Hubberstey, N. R. Champness and M. Schroder, Chem. Commun.,
¨
2008, 359–361; D. Britt, D. Tranchemontagne and O. M. Yaghi,
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High BET surface areas and large pore volumes tend to lead
to low H2 uptake at low pressure because large pores can have
weak potential overlaps to bind H2 molecules. Low pressure
gravimetric H2 sorption isotherms up to 1 bar were recorded at
78 K and 88 K (Fig. 3b). Interestingly, NOTT-112 can adsorb
a respectable total 2.3 wt% of H2 at 78 K and 1 bar.3,14,21
Thus, the high pore volume, which defines the high uptake at
high pressure, does not prejudice the performance of this
polyhedral material at low pressures. This low pressure effi-
ciency can be attributed to the high affinity of H2 to exposed
Cu(II) centres and the presence of the relatively small Cage A.
A similar cage structure has been found in the framework
PCN-1221 which exhibits a high gravimetric H2 uptake of
3.05 wt% at 77 K and 1 bar, probably due to alignment of the
vacant Cu(II) sites within the pore. With no open metal sites,
MOF-5 and MOF-177 only reach about 1.32 wt% and
1.25 wt% at 1 bar and 77 K, respectively.10–12 However, the
overall enhanced H2 adsorption capacity of NOTT-112 at both
high and low pressures suggests that the mix of small and large
polyhedral cages with open Cu(II) sites represents a powerful
strategy to optimise H2 uptake over extended pressure ranges.
The presence of a relatively strong interaction between H2
and the framework of NOTT-112 was revealed by analysis of
the isosteric heats of adsorption (Qst). Qst was calculated by
fitting the gravimetric H2 adsorption isotherms at 78 K and
88 K to a virial-type expression (see ESIw). The value of Qst
at low coverage is estimated to be 5.64 kJ molꢀ1 gradually
reducing to 4.74 kJ molꢀ1 as H2 coverage increases. Modelling
of H2 adsorption sites in metal–organic framework materials26
has revealed that aromatic rings on the organic linkers can
play an important role in H2 adsorption and we believe this to
be the case in the current study. Likewise, open Cu(II) sites are
important in enhancing the affinity of H2 molecules in the
pores.14,21,27 Thus, desolvated NOTT-112 incorporates and
exploits both of these features, namely polyaromaticity of
seven phenyl rings per linker and open metal sites at Cu(II),
within a permanently porous framework comprising large and
smaller polyhedral superstructures templated by the extended
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polyaromatic trigonal linker L6ꢀ
.
In summary, NOTT-112 exhibits a high apparent BET
surface area of 3800 m2 gꢀ1 with an excess H2 uptake of
7.07 wt% at 35–40 bar at 77 K and an excellent total H2
uptake of 10.0 wt% at 77 bar and 77 K. Ongoing work is
focused on the mechanisms of H2 adsorption within the three
different cages in NOTT-112 coupled to the search for higher
H2 storage capacity materials.
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We thank the EPSRC (UKSHEC) and the University of
Nottingham for support. We thank the EPSRC National
Crystallography Service and the CCLRC station 9.8,
Daresbury SRS for data collection. MS gratefully acknowledges
receipt of a Royal Society Wolfson Merit Award and an ERC
Advanced Grant. SY thanks EPSRC for DHPA funding.
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ꢂc
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