Mechano-chemistry of Ionic and Molecular Materials
Personnel: V.K. Pecharsky, V.P. Balema, K. Hosokawa, R. Fleming
Abstract:
Mechanical processing, also known as mechanical alloying, is conventionally
used in materials science for the non-thermal structural modification
of metals and metal alloys. By contrast, the use of mechanochemistry
for the processing of ionic and molecular materials, such as complex
metal hydrides and organic solids, is virtually non-existent, however,
it has been recently discovered by us to be a viable synthesis route.
The principal basic issues of this research effort are to develop both
the new science and relevant models to enable understanding of mechanically
induced solid-state transformations in non-metallic systems, especially
complex ionic and molecular solids.
Recent Results:
Mechanically induced transformations of complex aluminohydrides
Exploratory work on mechanochemistry of complex aluminum-based hydrides
spawned from our discovery of an unusual mechanically induced transformation
of lithium aluminohydride (LiAlH4) into Li3AlH6, Al and H2 in the presence
of a small amount of TiCl4. The solid state rearrangement of tetrahedral
[AlH4]- into octahedral [AlH6]3- is promoted by Al3Ti, which forms during
the first stage of the mechanical processing from LiAlH4 and TiCl4 (Eqs.
1, 2), and is the true catalyst in the process (Eq. 3-5).
Pure LiAlH4 is stable in the absence of a catalyst during long-term (35 h) mechanical processing and the ability of different catalysts to promote the mechanochemical transformations in LiAlH4 gradually decreases in the series TiCl4 > Al3Ti >> Al22Fe3Ti8 > Al3Fe > Fe, and TiCl4 >> FeCl2 > PdCl2 >> PtCl2 = NiCl2. Remarkably, the well-known hydriding-dehydriding catalysts, Ni and Pt, are practically inactive in the mechanochemical transformations of LiAlH4.
Mechanochemistry of organic solids
As a result of our most recent effort, we found that mechanical processing
of some solid organic materials results in their chemical transformations,
which were previously known to occur exclusively in solution. These
are shown schematically below.
Several solvent-free processes have been successfully carried out in this study by mechanochemical means, including the preparation of phosphonium salts, generation of phosphorus ylides and synthesis of unsaturated organic materials by the solvent-free Wittig reaction. These newly discovered mechanochemical transformations occur in the solid state and are exceptionally selective: clear discrimination between thermodynamically and kinetically preferred products has been observed in all cases. The existing knowledge about similar processes in solution is inapplicable to explain the mechanism of the mechanochemical organic transformations.
For the first time solid-state nuclear magnetic resonance (solid-state NMR) has been successfully utilized for the investigation of materials formed during mechanical processing of ionic and molecular solids. Solid-state NMR gives us unique data that support the solid-state character of the discovered processes, and as a result several working hypotheses describing the mechanisms of solid-state reactions in non-metallic solids have been formulated. At present we are carrying out additional experiments to verify and improve our models.
Significance:
Research on mechanochemical transformations of ionic and molecular
solids uncovered several solid-state processes, which were either unknown
before (mechanically induced solid-state transformations of LiAlH4)
or were considered to be only possible exclusively in a solution (mechanochemical
transformations of molecular materials). This creates exceptional opportunities:
(1) to extend current knowledge about solid-state transformations in
non-metallic materials, and (2) to obtain basic understanding of chemical
interactions in molecular and ionic solids in response to varying input
of mechanical energy. Furthermore, the results available to date clearly
demonstrate shortcomings and limitations of certain fundamental approaches
to chemical reactivity and highlight the importance of expanding basic
understanding of phase transformations in molecular solids. Finally,
we showed that solid-state NMR, a non-destructive analytical technique
normally used in conventional chemistry, can be effectively applied
to the solution of complex materials science problems.
Future Work:
The basic questions to be addressed in our future research on mechanochemistry
of complex hydrides are as follows: (1) How do the metal catalysts enable
solid state transformations in complex aluminohydrides? (2) As a model
case, what is the mechanism of the solid state rearrangement of the
tetrahedral [AlH4]-1
anion into the octahedral [AlH6]-3
anion during mechanochemical processing? (3) What is the difference
between mechanically induced and thermally induced catalytic transformations
in complex aluminohydrides? The existing models explaining hydriding-dehydriding
processes in organic compounds in the presence of heterogeneous metal
catalysts, are inapplicable to complex inorganic hydrides. As a consequence,
new models must be developed and experimentally validated.
Similar to complex aluminohydrides, studies of the fundamentals of
mechanochemical processes in molecular solids are virtually non-existent
and major basic questions which should be addressed are: (1) Do all
mechanochemical reactions proceed in a solid-state or is there an intermediate
formation of a liquid in the case of low melting temperature materials?
(2) What are the driving forces of the mechanochemical processes and
how are they related to the corresponding transformations in a solution?
(3) What are the mechanisms of the mechanochemical reactions? (4) What
is the role of the transformation kinetics? (5) How can input of mechanical
energy be quantified and related to the chemistry and physics of the
processes to explain both mass and charge transfer without solvent(s)?
Interactions:
Marek Pruski (Scientist), Jerzy W. Wiench (Assistant Scientist) both
in the Chemical and Biological Sciences Program; and Kevin W. Dennis
(Assistant Scientist) in the Materials and Engineering Physics Program in the
Ames Laboratory.
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Last update: February 19, 2008