Sono-emulsion electrosynthesis: Electrode-insensitive Kolbe reactions

Article abstract of DOI:10.1039/b007037h

The electro-oxidation of water-immiscible liquid aliphatic acids (RCO₂H) leading to decarboxylation to afford a hyrocarbon (R-R) may be achieved using an emulsion formed via insonation so that the organic phase continuously extracts the products; in complete contrast to conventional monophasic electrolyses, the type and yield of products obtained from this biphasic Kolbe electrolysis are independent of the electrode material used.

Full text of DOI:10.1039/b007037h

Sono-emulsion electrosynthesis: electrode-insensitive Kolbe reactions
a
a
a
b
b
Jay D. Wadhawan, Frank Marken,† Richard G. Compton,* Steven D. Bull‡ and Stephen G. Davies
a
Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK OX1 3QZ.
E-mail: compton@ermine.ox.ac.uk
Dyson Perrins Laboratory, Oxford University, South Parks Road, Oxford, UK OX1 3QY
b
Received (in Cambridge, UK) 30th August 2000, Accepted 16th November 2000
First published as an Advance Article on the web 14th December 2000
The electro-oxidation of water-immiscible liquid aliphatic
acids (RCO H) leading to decarboxylation to afford a
was stopped, excess hydroxide neutralised with acid, and the
2
organic products extracted with ethyl acetate and analysed by
GC–MS and NMR spectroscopy. Data summarised in Table 1
describe the conditions used and the yields of the main product,
decane—the Kolbe dimer, generated in this electrolysis process.
At platinum electrodes, it can be seen that a threshold current
density (or applied potential) exists below which the Kolbe
process cannot compete against background processes such as
oxygen evolution. As in the monophasic case,14 increasing the
temperature at which the reaction takes place causes a decrease
in current efficiency; decreasing the aqueous electrolyte
concentration also has this effect. The effect of the current
density can be rationalised as follows. Initially, increasing the
current density causes the current efficiency to improve, and
good yields of decane are observed at current densities in the
hydrocarbon (R–R) may be achieved using an emulsion
formed via insonation so that the organic phase continuously
extracts the products; in complete contrast to conventional
monophasic electrolyses, the type and yield of products
obtained from this biphasic Kolbe electrolysis are independ-
ent of the electrode material used.
The introduction of power ultrasound into homogeneous
solutions has a considerable effect upon mass transport
processes due to macroscopic streaming and microscopic
1
interfacial cavitational events; the sonication of biphasic media
can result in the formation of an in situ emulsion,2 primarily
24
due to cavitational events that occur preferentially at the phase
3
22
22
boundary. These mechanical forces act to divide droplets again
range 0.2–0.3 A cm ; at values of 0.35 A cm and higher, the
current efficiency decreases. This can be rationalised in terms of
the flux of hexanoate towards the electrode surface. The
ultrasound-determined mass transport limiting current (Ilim) in
and again forming microdroplets and effectively ‘homogenis-
ing’ the heterogeneous system.
5,6
Electrosynthesis in emulsion media is long established.
1
Sono-emulsion systems have considerable potential for applica-
tion in electrosynthesis because (i) there is no need for emulsion
stabilising reagents, (ii) the separation of products is facile, (iii)
aqueous solutions may be co-emulsified with a sparingly
soluble liquid depolariser, resulting in a medium of high
conductivity, (iv) high rates of mass transport can be achieved,
and (v) the electrode surface is continually activated.7
homogeneous solution is given by eqn. (1):
nFADc
Ilim
=
(1)
d
where n refers to the number of electrons transferred at the
21
electrode, F is the Faraday constant (96485 C mol ), A is the
2
area of the platinum working electrode (1.1 cm ), D and c refer
We report the use of sono-emulsion media to study the
to the diffusion coefficient and the concentration of hexanoate
in the emulsion respectively, and the Nernstian diffusion layer
electro-oxidation of hexanoic acid (Kolbe reaction). Previous
sono-electrosynthetic studies8
212
have focussed upon the
1
15
thickness, d is ca. 5 mm. Using the Wilke–Chang method for
estimation of diffusion coefficients, and further assuming the
viscosity of the emulsion to be not too different from that of
water, a hypothetical mass transport controlled limiting current
effects of ultrasound on Kolbe electrolysis, exclusively under
monophasic conditions, and with seemingly contradictory data.
In this work it is shown that under biphasic conditions, although
Kolbe electrolysis is conducted at a potential far beyond that
required for solvent decomposition, an efficient conversion of
hexanoic acid into decane is achieved at both platinum and
boron-doped CVD (chemical vapour deposited) diamond film
anodes. Surprisingly, the anode material does not affect the type
or ratio of products formed.
Table 1 Results from galvanostatic electrolysis experimentsa
2
2 4 2 2 8
Me(CH ) CO H ? Me(CH ) Me
Mass of
hexanoic Concentration
acid/g of NaOH/M
Current
density/
A cm
An electrochemical cell equipped with an ultrasonic probe§
Yield (%)
b
13
22
was used as reported earlier. With an ultrasound intensity of
Tenperature/K
of decane
2
2
1
90 W cm , this configuration leads to forceful mixing and
2
1
.1 cm platinum disc anode
emulsification of two-phase systems such as water | decane. The
biphasic emulsion systems of hexanoic acid (ca. 5 mL) in
aqueous 1.0 M NaOH (ca. 15 ml) had a conductivity of ca. 20
1.72
1.72
1.83
1.0
1.0
1.0
1.0
0.1
1.0
0.08
0.13
0.18
0.35
0.18
0.18
293
293
293
293
293
313
0
24 ± 3
45 ± 5
40 ± 5
17 ± 2
3 ± 1
2
1
mS cm and pH of 6.1. These values were kept more or less
constant throughout the electrolysis by the reduction of water at
the platinum counter electrode, and the gradual dissolution of
the organic acid into the aqueous solution phase.
1
1
1
.82
.67
.87
2
0
.25 cm free-standing polycrystalline boron-doped CVD diamond anode
Power ultrasound was used to emulsify ca. 15 mmol of
hexanoic acid in an aqueous 1.0 M NaOH biphasic system.
After the passage of 1 Faraday (per mole of hexanoic acid) of
charge (ca. 1500 C, sufficient to quantitatively convert hexanoic
acid assuming a one-electron oxidation process) the reaction
1.80
1.0
1.0
0.35
0.70
293
293
40 ± 5
14 ± 5
1
.80
a
Reaction conditions: oxidation of hexanoic acid at a 1.1 cm2 platinum
anode (or at a 0.25 cm2 free-standing polycrystalline boron-doped CVD
diamond electrode) in an aqueous NaOH/hexanoate emulsion system in the
presence of power ultrasound (7 mm electrode-to-horn distance, 190 W
cm ). Charge equivalent to 1 Faraday per mol of hexanoic acid was passed
before analysis (ca. 1500 C). Yields based upon quantitative analysis of H
†
Present address: Department of Chemistry, Loughborough University,
Loughborough, UK LE11 3TU.
Present address: School of Chemistry, University of Bath, Claverton
22
b
1
‡
NMR signals.
Down, Bath, UK BA2 7AY.
DOI: 10.1039/b007037h
Chem. Commun., 2001, 87–88
This journal is © The Royal Society of Chemistry 2001
87

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density of up to ca. 1 A cm2 may be inferred. Hence, the
oxidation is a mass transport controlled process and not limited
by electrode kinetics. Increasing the current density beyond the
mass transport limit gives rise to additional oxygen evolution
rather than the desired Kolbe process, resulting in loss of
efficiency. Decreasing the current density below a critical value
2
We thank the Royal Society for financial support through a
University Research Fellowship (F. M.), and DeBeers Industrial
Diamond Division, UK and the EPSRC (GR/N 12051) for
supporting this work.
2
2
of ca. 0.1 A cm causes the Kolbe process to stop as the anode
potential falls below the threshold required for Kolbe elec-
trolysis.
Notes and references
The ultrasound generator employed was a VCX400 model immersion
§
horn (Sonics and Materials, USA) equipped with a 3 mm diameter stepped
titanium-alloy tip (electrically insulated) emitting 20 kHz sound with power
The Kolbe dimer is not produced exclusively: GC–MS and
NMR analysis of the electrolytic products permits the identi-
fication of the ester, amyl caproate, formed in < 5% yield. The
absence of any pentenes and pentanols is novel and surprising.
This is indicative of a trapping of the initial reaction
intermediates at the electrode surface; the pentyl carbocation is
unlikely to be formed as a free intermediate during elec-
trolysis.
22
level set to 190 W cm
(calorimetrically determined). The horn-to-
electrode distance was maintained at 7 mm. Galvanostatic electrosynthesis
3
was undertaken in the thermostatted cell (volume ca. 20 cm ) using a PAR
173 (EG&G) galvanostat fitted with a PAR 178 (EG&G) digital coulometer.
The cathode employed was a coil of platinum wire; the anode was either a
1
2 mm diameter platinum disc (Aldrich) or a 5 3 5 mm free-standing
polycrystalline boron-doped CVD diamond plate (DeBeers Industrial
Diamond Division, UK). Chemical reagents and NaOH (Aldrich) were of
the highest commercially available purity.
It is widely known that ultrasound damages the surface of
platinum electrodes. Recently, boron-doped CVD diamond
1
electrodes have been employed in the presence of ultrasound
with negligible damage to the electrode surface.16 Furthermore,
diamond surfaces are chemically inert under hostile conditions
1
For a review see F. Marken, J. C. Eklund and R. G. Compton,
Electroanalysis, 1997, 7, 509.
2 F. Marken, R. G. Compton, S. D. Bull and S. G. Davies, Chem.
Commun., 1997, 955.
3 F. Marken and R. G. Compton, Electrochim. Acta, 1998, 43, 2157.
4
5
17
and after long-term electrolysis at very positive potentials.
The level of boron doping is high, typically ca. 1020 cm ,
3
O. Behrend, K. Ax and H. Schubert, Ultrason. Sonochem., 2000, 7,
77.
H. Fees and H. Wednt, in Techniques of Electroorganic Chemistry Part
III, ed. N. L. Weinberg, Wiley, New York, 1981, p. 81.
J. F. Rusling and D. L. Zhou, J. Electroanal. Chem., 1997, 439, 89.
R. P. Akkermans, S. L. Roberts and R. G. Compton, Chem. Commun.,
corresponding to a B+C atom ratio of 1+1000, allowing a
resistivity of 0.3 mΩ m to be achieved.18
Galvanostatic electrosynthesis experiments employing a
boron-doped CVD diamond anode were conducted at different
current densities, and the products analysed as before. The
products observed in conventional Kolbe electrolyses suggest
that the electrode material exerts a strong control, with products
6
7
1
999, 1115.
8 H. Fujiwara, M. Atobe, H. Kanetsuna and T. Nonaka, J. Chin. Chem.
Soc., 1998, 45, 175.
9 M. Tashiro, H. Tsuzuki, H. Goto and S. Makata, Chem. Exp., 1991, 4,
predominantly derived from carbocation intermediates detected
at carbon anodes.14 Surprisingly, electrosynthesis at a boron-
4
1.
doped CVD diamond anode under sono-emulsion conditions
again gives rise to the detection of predominately the Kolbe
dimer (see Table 1). Current efficiencies and yields are only
slightly lower than those observed at platinum electrodes.
Interestingly, the ester, amyl caproate is again the sole by-
product, suggesting that a mechanism similar to that at platinum
electrodes is operative.
1
1
1
0 D. J. Walton, S. S. Phull, U. Geissler, A. Chyla, A. Durham, S. Ryley,
T. J. Mason and J. P. Lorimer, Electrochem. Commun., 2000, 2, 431.
1 A. Chyla, J. P. Lorimer, T. J. Mason, G. Smith and D. J. Walton,
J. Chem. Soc., Chem. Commun., 1989, 603.
2 D. J. Walton, A. Chyla, J. P. Lorimer and T. J. Mason, Synth. Commun.,
1
990, 1843.
13 A. N. Blythe, R. P. Akkermans and R. G. Compton, Electroanalysis,
2000, 12, 16.
14 C. J. Brockman, Electroorganic Chemistry, Wiley, New York, 1926.
5 C. R. Wilke and P. Chang, AIChE J., 1955, 1, 264.
6 C. H. Goeting, J. S. Foord, F. Marken and R. G. Compton, Diamond Rel.
Mater., 1999, 8, 824.
7 P. A. Michaud, E. Mahe, W. Haenni, A. Perret and C. Comninellius,
Electrochem. Solid State Lett., 2000, 3, 77.
In summary, Kolbe electrolysis has been accomplished under
the novel conditions of an emulsion generated in situ by power
ultrasound. This method of electrosynthesis renders good yields
of product and is highly charge efficient. For the first time in
1
1
1
50 years,19 the electrode material used and conditions
1
1
employed are observed to have only little effect upon the type of
products formed. Although the mechanism of the reaction is
unclear, it may involve encapsulation of intermediates within
the organic component of the emulsion. This type of electro-
synthetic methodology shows promise for wider application.
8 R. G. Compton, F. Marken, C. H. Goeting, R. A. J. McKeown, J. S.
Foord, G. Scarsbrook, R. S. Sussmann and A. J. Whitehead, Chem.
Commun., 1998, 1961.
19 H. Kolbe, Ann. Chim., 1849, 69, 257.
88
Chem. Commun., 2001, 87–88