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
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 surface area.
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."
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