Beneath the Appalachian mountains in the United States is one of
the world’s great gas fields, as yet undeveloped. The methane
gas it contains could act as a transport fuel, and indeed in some
countries compressed natural gas, which is mainly methane, is used
as fuel for various forms of transport, including cars. The U.S.
Department of Energy (DOE) would also like to promote it as a
vehicle fuel, ideally by absorbing it into an inert material so
that it can be stored at normal temperatures and pressures. The DOE
has set a target ratio for such a material of 180, in other words
the absorbing system must store a volume of methane 180 times
larger than its own volume. This would be almost like storing
Carbon nanotubes, activated carbon, and zeolites have the capacity
to absorb gases but they do not meet the DOE target. Metal-organic
frameworks (MOFs) are much more promising, and researchers in the
past decade have been focusing on these. Crystal structures based
on 9,10-anthracene-dicarboxylate have enormous cavities, and
theoretical calculations suggested these could meet the DOE
requirement. However, when the group headed by Hong-Cai (Joe) Zhou
at the Department of Chemistry at Miami University made this
ultra-porous material, it fell far short of the 180 target.
Research Front Map of Graphene
from the Special Topic of
Zhou, who is now at Texas A&M University, was not deterred by
this setback and looked at an alternative ligand,
5,5’-(9,10-anthracenediyl)di-isophthalate, which he suspected
would produce crystals with much larger pore size—and he was
right. The crystals had nano-sized cavities that provided 0.87
cm3 of empty space per gram of material. His remarkable
paper occupies position #11, just below the current Hot Ten (S. Ma,
et al., J. Am. Chem. Soc., 130: 1012-6, 23
January 2008; 18 citations this period, 47 overall). The new
crystals were grown overnight from a solution of dimethylformamide.
They absorbed methane in excess of the DOE’s target and in
fact achieved a storage capacity ratio of 230 after activation.
However, this storage success does not in itself solve the problem
of using methane as a transport fuel. As Zhou tells Science
Watch: "For one, the DOE storage goal refers to a systems
goal. The system contains not only the absorbent but also the tank
and necessary pipes. In addition, we have to lower the production
cost of the new material as much as possible."
Zhou’s research group is now working on MOFs with even higher
adsorption capacities, and not only for methane, but for hydrogen
also. The latter is the other gaseous fuel that might one day drive
vehicles and is potentially the greenest fuel of all since the
product of combustion is entirely H2O. However, while
the hydrogen economy has been hyped for more than 20 years, there
still exist significant challenges to its implementation. Hydrogen
storage is also of vital importance and a challenge for which MOFs
might also provide the answer.
Recently Zhou has widened his groundbreaking work to include
studies involving this gas. He and his co-workers have investigated
the potential of lanthanide-based MOFs to absorb hydrogen (J. Luo,
et al., J. Am. Chem. Soc., 130;
9626, 2008), and found a way of
enhancing hydrogen uptake in copper-containing MOFs (X. Wang,
et al.,Angew. Chem. Int. Ed., 47:
7263-6, 2008), as well as investigating the effect of framework
catenation (S. Ma, et al., J. Am. Chem. Soc.,
130: 15896-902, 2008).
His group have also been concentrating on the MOFs themselves,
looking at the way these can be modified to increase their capacity
to hold gaseous molecules, as described in three recent Journal
of the American Chemical Society reports: J. Li, et
al., 131(18): 6368, 2009; S. Ma, et al., 131(18):
6445, 2009; and D. Zhao, et al., 131(26): 9186, 2009.
MOFs are a material of the future, not only useful in gas storage
but for separating mixtures of gases. Says Zhou: "MOF-based
mesh-adjustable molecular sieves allow selective adsorption to be
tuned by temperature adjustment. This new direction toward
molecular sieves can be used in highly efficient and energy-saving
separation procedures." His group is now concentrating on this
aspect as well as on other things, such as the potential of MOFs to
act as catalysts.
One day MOFs might well play a part in CO2 capture. We
might even envisage a car of the future, which is fuelled by
methane stored in MOFs, capturing the CO2 of the exhaust
gases in MOF cavities that have become vacated as the methane is
Dr. John Emsley is based at the Department of Chemistry,
Cambridge University, U.K.
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