Chemistry Reference
In-Depth Information
7.7.3 Binding of Carbon Dioxide upon Mechanochemical
Treatment
Manual grinding of solid silver carbonate (Ag
2
CO
3
) with PPh
3
in air and in
the presence of ethanol leads to the absorption of one equivalent of water
(present in the form of air moisture) and one equivalent of CO
2
gas to yield
bis(triphenylphosphino)silver(
I
) hydrogen-carbonate, which can again re-
lease CO
2
upon heating.
99
Monitoring of the reaction through solid-state
infrared spectroscopy suggests a two-step mechanism involving the for-
mation of the solid tetrakis(triphenylphosphino)silver(
I
) carbonate as the
intermediate. Although this particular mechanochemical product is not
an extended metal-organic structure, the ability to chemically bind carbon
dioxide through a mechanochemical transformation of a metal-organic
system is fascinating, especially when considering the growing importance
of metal-organic materials for sequestration of waste CO
2
.
7.8 Mechanistic Studies of Mechanochemical
MOF Synthesis
7.8.1 Stepwise Analysis
The mechanochemical synthesis of coordination polymers and MOFs does
not necessarily need to proceed in a single step. The simplest approach to
detect crystalline reaction intermediates in mechanosynthesis is by step-by-
step analysis, i.e. by periodically halting the mechanochemical milling and
analyzing the composition of the reaction mixture. Such analysis revealed
the stepwise transformation of ZIF topologies in ILAG synthesis of zinc
2-ethylimidazolate from ZnO, which was tentatively associated to relative
thermodynamic stabilities of porous and non-porous frameworks.
54
Another
example of a stepwise mechanochemical reaction mechanism is the syn-
thesis of the 1D coordination polymer zinc fumarate tetrahydrate
(Figure 7.13a).
52,71
Whereas milling together of ZnO, fumaric acid and three
equivalents of water for ca. 45 min leads to the specific formation of the
tetrahydrate phase, stopping the milling earlier reveals the presence of an
intermediate pentahydrate phase. This stepwise process is explained by the
rapid, liquid-assisted formation of the crystalline pentahydrate at early
stages of the reaction in which water is abundant. The rapid formation of the
pentahydrate phase, however, eciently depletes the water in the reaction
mixture and transforms the fast liquid-assisted reaction into a slower neat
grinding process.
71
The resulting neat grinding process is speculated to
proceed through an amorphous intermediate, deduced by the spontaneous
formation of different coordination polymers by ageing of the partially
reacted reaction mixture (Figure 7.13a). A similar stepwise mechanism was
observed in the mechanochemical reaction of CuO and acetic acid, where
the abundance of acetic acid early in the reaction leads to the formation of
Search WWH ::
Custom Search