Angew. Chem. Int. Ed. 2005, 44 (01),
Nested metal-organic frameworks as possible novel hydrogen storage materials
The success of hydrogen technology
for driving vehicles depends on the storage of hydrogen, for which a truly
satisfying solution has yet to be found. A team of scientists from the
University of North Carolina and the United States Department of Energy has now
developed a metal-organic material whose cavities keep hydrogen molecules
"trapped"—this may be a new prototype for the design of new storage media.
The team led by Wenbin Lin works
with compounds of the metal zinc and special organic molecules with six to
eight aromatic six-membered rings as their central structural element. Aromatic
rings are important because they strongly attract hydrogen molecules.
It turns out that these
metal-organic building blocks crystallize in the form of a three-dimensional
grid with very large cubic cavities. What is unusual in this case is that four
of these grids are partially pushed into each other, which causes them to
overlap. The cubic cavities thus get correspondingly smaller. These tiny
"caves" are accessible from the outside by means of open channels. When the
crystal is freshly formed, the cavities are first unevenly occupied by solvent
molecules. These "guests" can easily be completely removed without causing the
framework to collapse.
The empty cavities can take up
hydrogen molecules. At a pressure of 48 bar, it was possible to store 1.12 (for
the compound with six rings) to 0.98 (compound with eight rings) percent by
weight of hydrogen—and to release it. This storage capacity is about equivalent
to that of carbon nanotubes, another material being considered for hydrogen
storage. In comparison with record holders in their own class of metal-organic
porous frameworks, the two newcomers are only slightly inferior. The best of
the class owe their superiority to their five- to ten-fold higher interior
How is it that these two new
metal-organic frameworks can store hydrogen so well, without an especially high
surface area or a particularly large pore volume? Because of the multiple
nested grids, the hydrogen molecules in the cavities come into contact with a
larger number of aromatic rings than they do in pores of ordinary single grids.
The hydrogen is well and truly trapped. "The trapping mechanism of our highly aromatic,
strongly interlocking grid structure," says Lin," could point to a new path for
the development of effective metal-organic hydrogen storage materials."