Chemistry Reference
In-Depth Information
Raman spectroscopy enables monitoring of the reaction progress, including
the transformations of different reaction intermediates and products.
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7.8.3 Electron Microscopy
There have been very few studies of the morphology of MOF particles ob-
tained by mechanosynthesis. For example, scanning electron microscopy
images of zinc fumarate pillared MOFs obtained by LAG from ZnO, fumaric
acid and bipy reveal well developed rectangular crystals with approximate
size of 200 nm.
1
A similar observation was also made for mechanochemically
prepared microcrystals of zinc isonicotinate tetrahydrate, the monomeric
precursor to an interpenetrated diamondoid zinc isonicotinate framework,
which exhibited well-developed needle- and rod-like morphologies.
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Over-
all, the several existing scanning electron microscopy (SEM) studies indicate
that coordination polymers and MOFs prepared by LAG or ILAG will exhibit
morphologically well-developed crystals with sizes in the range 100 nm-
1 mm. The, so far, only systematic SEM study of particle development
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during mechanochemical synthesis of MOFs has been conducted for slow
(up to 240 h milling) neat grinding of ZnO and 2-methylimidazole to form
ZIF-8. The study revealed that the transformation of ZnO particles into ZIF-8
can penetrate as far as 10 nm below the surface of the reactant oxide particle.
Consequently, small particles of ZnO can be completely converted into ZIF-8.
For larger ZnO particles, or their agglomerates, the mechanochemical re-
action leads to the formation of particles consisting of a core of zinc oxide
surrounded by a shell of ZIF-8 (Figure 7.14d).
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7.9 Outlook
The topics outlined in this brief overview of metal-organic mechano-
chemistry confirm the notion that this is a highly dynamic and rapidly de-
veloping area. Over a decade of intense research has provided sucient
information to begin establishing a systematic understanding of mechano-
chemical techniques and types of chemical reactivity most appropriate for
the synthesis of metal-organic compounds. Several benefits arising from the
application of mechanochemistry for the synthesis of functional metal-
organic materials have been recognized and embraced by the research
community, above all the ability to synthesize microporous MOFs rapidly
and without solvent, as well as to activate normally inert mineral-like
materials, such as metal oxides, into versatile precursors for MOF synthesis.
Recent work indicates a growing interest for mechanochemistry as a means
to prepare solid solutions
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of coordination polymers or amorphous, non-
crystalline forms of MOFs.
107,108
These areas are likely to become more
visible in the further development of metal-organic mechanochemistry, as
well as applications of mechanochemistry for organometallic synthesis,
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which are still very rare. The understanding of reaction kinetics and mech-
anisms of mechanochemical reactions is still in its early stages and further
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