Chemistry Reference
In-Depth Information
Generally, in both the in-cell and ex-cell cases, if
reaction is slow then the cell must include a separa-
tor between the anode and cathode to prevent the
counter-electrode from causing an electrochemical
back-reaction of the redox agent. A strategy that
has been used to try to eliminate the need for a cell
separator (and thus reduce the cost) is to introduce
a phase-transfer agent into the emulsion phase,
which selectively removes the active redox agent
after regeneration.
methane sulfonic acid for the oxidation because of
its high selectivity to many of the desired products,
high solubility, low cost and fast reaction.
Another interesting example is the Electriciteé de
France process for the conversion of sugar beet
extract to mucic acid using indirect oxidation with
bromine, i.e. bromine-mediated oxidation of galac-
turonic acid to galactaric acid. The strategy adopted
here was to base the process around a modified Grig-
nard reactor, familiar to the industry to which elec-
trochemistry was being introduced, and achieve
simple cell design and installation and low cost.
Another bromine-mediated synthesis approach-
ing commercialisation at BASF (Germany) is the
conversion of hydroxypivalic aldehyde to methyl
hydroxypivalate, which subsequently is converted to
hydroxypivalic acid using an electrodialysis step.
Redox catalysts
The ability, in terms of reaction rate and selectivity,
of a redox agent to carry out a particular indirect syn-
thesis will depend largely on the required oxidation
or reduction power of the agent, which is effectively
measured by the standard potential of the redox
couple. Although inorganic mediators often involve
slow electron transfer in regeneration, often they can
offer the advantage of moderating the reactivity of
intermediates to enable selective reaction. Typical
inorganic species used to carry out oxidations
include several metal ions, e.g. Fe(III), Co(II),
Mn(III), Ce(IV) and Cr(VI). The use of mediated
electrochemical reduction of organic compounds is
not as popular in industry as that of oxidations. The
use of alkali-metal amalgams in the reduction of
organics, notably nitro compounds, is practised.
The use of homogeneous reducing agents such as
Eu(II), Cr(II), Ti(II) , Sn(II) and V(II) also could be
considered [24].
The wide variety of indirect mediated electro-
organic reactions has been reviewed by Steckhan
[25]. There has been significant research on the oxi-
dation of toluenes using electrochemically generated
mediators. The method is particularly useful due to
the low solubilities of the organic species in the
aqueous electrolyte. For the electrosynthesis of ben-
zaldehydes, the mediators frequently used are Ce(IV)
and Mn(III). An important example (adopted by
W. R. Grace and Co.) of mediated synthesis is the
oxidation of aromatic precursers to aldehydes,
ketones and quinones. The products of the oxidation
are used as intermediates, flavours, fragrances and
dyes. The non-electrochemical processes, using
sodium dichromate, MnO
2
or oxygen in air, have
limitations in terms of disposal of spent metal oxi-
dants or poor selectivity in the case of air oxidation.
The Grace process [22] uses Ce(IV) in a solution of
Non-aqueous electrosynthesis
Electrosynthesis can be carried out in non-aqueous
solution by methods that include the use of organic
solvents, micelles and solid polymer electrolytes.
Solid polymer electrolyte.
Solid polymer elec-
trolytes, or proton-conducting membranes, can be
used to perform hydrogenations. One example is the
partial hydrogenation of the unsaturated fatty acid
constituent of edible oil—triglyceride—at palladium
(or platinum) catalyst supported on carbon:
H
+
+ e
-
Æ H(adsorbed)
2 H(ads.) + -RHC=CHR- Æ -RH
2
C-CH
2
=R-
where -RH
2
C-CH
2
=R- is an unsaturated fatty acid.
The catalyst is bonded to the membrane (Nafion) to
produce a PEM fuel cell type of structure [26].
Electrosynthesis as a means of organic chemical
and intermediates manufacture is an area that has
fulfilled its promise. It is being seen widely as a
cleaner alternative to wet chemistry and catalysis.
Innovative electrochemical syntheses can be per-
formed, which reduce the number of reaction steps,
say, in the multistep routes to pharmaceuticals. One
such example is in the electrocarboxylation of
chlorodiphenylether as part of the route to fenopro-
fen [22]. The process is an example of synthesis in
non-aqueous media, i.e. dimethylformamide (DMF),
and uses a novel cell design based on a consumable
