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Porous materials with large, regular, accessible cages and tunnels are
increasingly in demand for many applications including chemical
separation or purification, catalysis, molecular sensors, electronics
and gas storage. Depending on their structure and pore size, these
materials allow molecules of only certain shapes and sizes to enter
the pores, a property known as shape selectivity. The environment
within the pores can be very different to that outside, thus promoting
chemical reactions that do not occur in the bulk material. Another
prospective use is as templates for forming calibrated, monodisperse
nanomaterials. In this respect, the larger the pores, the wider the
range of reactants that can be manipulated or stored.
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Starting
from simple assemblies and linking units, larger and larger
building blocks combine to form crystalline nanoporous materials
with more surface area than zeolites. The Zeotype architecture of
MIL-101 displays mesoporous cages with diameters of 29 Å (green)
and 34 Å (red), featuring 12 Å pentagonal and 15 Å hexagonal
openings. Credits: Science |
Férey and co-workers’ strategy combined three main ideas. First,
discrete multi-atom building units were designed and generated in
solution (Fig. 1). Second, with the aim of producing a compound with
large pores, the building units were combined to produce larger units.
For MIL-101 the key building unit is a supercluster of four smaller
clusters linked by difunctional organic components to make a large
tetrahedral assembly. The third idea involves being sure of what
you’ve actually made, i.e. how to determine the structure of the new
material. It is well known that it becomes increasingly difficult to
grow highly diffracting single crystals as structures grow larger.
When single crystals are unavailable, powder diffraction can provide
sufficient information for structure solution. Based on their
understanding of the ways the building units might combine, possible
structural models were predicted and assessed via a computational
strategy that calculated their relative stability. Favourable
solutions were then compared with the high quality powder diffraction
data collected from MIL-101 at ESRF. Once a good match between the
predicted and measured powder patterns was seen, the researchers could
be sure of the nature of their new material.
This breakthrough opens up a new field for targeted chemistry,
computational methods for structure prediction and most importantly
novel materials with useful applications. Férey and co-workers
describe the hydrid solid, MIL-101, as an excellent candidate for the
storage of gas, creation of nano-objects in a regular and monodisperse
mode with specific physical properties, or for drug delivery. Recent
studies on smaller porous materials carried out by various research
groups around the world leave open the possibility of successfully
creating hydrid materials with even larger pores and more complex
structures keeping always in mind that the most important goal should
be to incorporate useful functions.
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