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Indeed, in situ monitoring of mechanochemical reactions has been
largely limited to temperature- and pressure-measurements on mechanic-
ally induced self-sustaining reactions, or on monitoring pressure changes
in reactions adsorbing or releasing a gas. An example of the latter is the
reactions kinetically analyzed by Urakaev and Boldyrev.
104
Very recently,
however, the application of highly penetrating synchrotron X-ray radiation
enabled an unprecedented technique for the real time in situ study of
mechanochemical synthesis of porous metal-organic frameworks by pow-
der X-ray diffraction.
15,17
The excellent penetrating power of X-rays with a
short wavelength (0.1 Å) enabled recording of the powder diffraction pat-
terns of reaction mixtures as they were undergoing milling, with a time
resolution in seconds. Such real-time structural studies have painted an
unexpectedly dynamic picture of mechanochemical milling reaction, in-
volving highly labile intermediate phases whose lifetime is less than a
minute, dependent on milling conditions and environment, and whose
chemical and particle size evolution could be measured in real time and
in situ (Figure 7.14a-c). For the mechanochemical synthesis of the porous
framework material ZIF-8 in situ studies have enabled the quantification of
product formation rates for reactions conducted by LAG and by ILAG.
17
Assuming Avrami-Erofe'ev kinetics, switching from LAG to ILAG by salt
addition led to a ca. 15-fold increase in the rate constant for product
formation.
The in situ monitoring technique readily captured the formation and
interconversion of different ZIF topologies upon mechanochemical ILAG
reaction of ZnO and 2-ethylimidazole, and plotting the time-dependent in-
tensities of the characteristic X-ray reflections for all involved materials
provided the first insight into the kinetics of ZIF mechanosynthesis and
transformation (Figure 7.14d).
17
The real-time in situ monitoring of the
mechanochemical milling reaction between imidazole (Him) and ZnO re-
vealed a strong dependence of the reaction behavior on reaction conditions.
Neat grinding of the two reactants was observed to proceed in a single step to
provide the discrete coordination tetramer of composition Zn
4
(im)
8
(Him). In
contrast, ILAG with ethanol as the grinding liquid led to the rapid, one-step
formation of the non-porous 3D zinc imidazolate framework with zni
topology. Replacing ethanol with DMF as the grinding liquid led to for-
mation of the open framework structure known as ZIF-4, containing in-
cluded DMF guests. However, reducing the amount of DMF led to a stepwise
process in which the initially formed ZIF-4 is subsequently, within minutes,
replaced by a low porosity structure ZIF-6 (Figure 7.14d).
17
The latest development in the synchrotron X-ray diffraction technique for
monitoring mechanochemical reactions has been the introduction of an
internal diffraction standard, in particular crystalline silicon.
101
The first
studies using the internal standard technique allowed the quantitative
analysis of mechanochemical reaction mechanisms using procedures of
Rietveld refinement, which revealed not only the pseudo-fluid behavior first
proposed by the James group, but also suggested the presence of a
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