Environmental Engineering Reference
In-Depth Information
Metal Organic Frameworks
Metal-organic frameworks (MOFs) are microporous crystalline solids
which are composed of organic bridging ligands or “struts” coordinated
to metal-based nodes to form three-dimensional extended networks with
uniform pore diameters, typically in the range 3 to 20 Å [6.11, 6.29, 6.30].
The nodes generally consist of one or more metal ions (e.g., Al
3+
, Cr
3+
,
Cu
2+
, or Zn
2+
) to which the organic bridging ligands coordinate via a
specifi c functional group (e.g., carboxylate, pyridyl). The past 20 years
have seen remarkable progress in the design, synthesis and characteri-
zation of metal-organic frameworks (MOFs) owing to their enormous
structural and chemical diversity and their potential applications in gas
storage, ion exchange, molecular separation and heterogeneous cataly-
sis. This effort is motivated in part by the unique structural properties of
the materials, which include: robustness, high thermal and chemical
stabilities, large internal surface areas (up to 5,000 m
2
/g), high void vol-
umes (55-90%) and low densities (from 0.21 to 1.00 g/cm
3
). Compared
to zeolites they are much simpler to systematically modify in terms of
pore dimensions and surface chemistry because their synthesis chemis-
try is based upon known organic reactions, as opposed to the hydrother-
mal synthesis of zeolites. In fact, from an experimental point of view
fi nding exactly the right solvent mixture and conditions for the MOF to
crystallize is often a very tedious process. For this high throughput,
experimentation is used very effi ciently (see
Movie 6.5.1
).
A sub-class of MOFs are zeolitic imidazolate frameworks (ZIFs). ZIFs
can adopt zeolite structure types based on the replacement of: (a) tetra-
hedral Si
4+
ions with tetrahedral transition metal ions such as Zn
2+
or
Co
2+
and (b) bridging O
2-
anions with bridging imidazolate-based ligands
[6.31]. Some examples of ZIF structures are given in
Figure 6.4.5
.
Here we illustrate how this fl exibility in the synthesis is used to arrive
at different strategies for carbon capture: open metal sites, interpene-
trated frameworks, fl exible frameworks, and surface-functionalized
frameworks.
Frameworks containing open metal sites
MOFs are often formed through crystallization from solution. For most
MOFs the ligands fully coordinate the metal, but for some MOFs the
solvent used for synthesis participates in the metal coordination. The
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