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Application of clean technologies using electrochemistry in ionic liquids.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
Published online 26 January 2011 in Wiley Online Library
(wileyonlinelibrary.com) DOI:10.1002/apj.529
Review article
Application of clean technologies using electrochemistry
in ionic liquids
A.P. Doherty,1 * L. Diaconu,1 E. Marley,1 P. L. Spedding,1 R. Barhdadi2 and M. Troupel2
1
2
School of Chemistry and Chemical Engineering, Queens’s University of Belfast, Belfast BT9 5AG, UK
East Paris Institute of Chemistry and materials Science, CNRS, 2-9 Rue Henri Dunant, 94320 Thiais, France
Received 9 August 2010; Revised 7 October 2010; Accepted 21 October 2010
ABSTRACT: Rising costs and green environmental concerns have focused attention on more efficient way of producing
chemical products. Room temperature ionic liquids (RTILs), especially in combination with electrochemical activation,
provide promise of reduction pollution in processing because of their recyclability and low vapour loss factors. The
fundamental and applied aspects of electrolytic processing in ionic liquid media are discussed using data from various
direct and catalytic redox processes. It is shown that ionic liquids are potentially very useful for performing important
redox transformations, for example alcohol oxidations, carboxylations and CO2 capture. These results indicated that
electrolytic transformations in RTIL media are feasible which present opportunities for developing new real chemical
processing applications. The opportunities and challenges for electrochemical engineers in this field are outlined and
discussed.  2011 Curtin University of Technology and John Wiley & Sons, Ltd.
KEYWORDS: ionic liquids; RTILs; electrochemistry; electrosynthesis; redox catalysis; green chemistry
INTRODUCTION
Electrons are considered to be clean reagents for
effecting oxidation and reduction reactions[1] ; reactions
which, under usual circumstances, require potentially
polluting metal-based reagents, which can result in
considerable waste streams which require downstream
remediation.[2] Electrochemistry has already shown
examples of direct electrochemical transformations
(e.g. electrohydrodimerisations[3,4] and electrocarboxylations[5,6] ) and indirect approaches which deploy
electro-generated redox reagents.[7 – 9] The latter approach can be used to effect a variety of chemical transformations using only small catalytic quantities which
are capable of replacing the traditional use of large-scale
stoichiometric amounts of reagents. Examples include
the oxidation of primary and secondary alcohols[7,8]
to the corresponding carbonyl products (aldehydes or
ketones) or the reduction of CO2 to oxalate.[9]
Despite the obvious advantages of preparative-scale
electrosynthesis, difficulties associated with the use of
conventional solvents, including water, as the reaction media severely limits the usefulness of the
approach.[10,11] Solvents are considered dangerous
*Correspondence to: A.P. Doherty, School of Chemistry and Chemical Engineering, Queens’s University of Belfast, Belfast BT9
5AG, UK. E-mail: a.p.doherty@qub.ac.uk
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Curtin University is a trademark of Curtin University of Technology
(toxic and explosion risk), while water is electrolysed to O2 and H2 at only moderate potentials. With
both organic solvent and aqueous media, there are
also potential risks of contaminated effluents. However, over the past decade the emergence of room
temperature ionic liquids (RTILs) provide electrochemists with new types of liquid media for performing molecular electrochemistry,[12 – 15] redox catalysis[16]
and preparative-scale electrosynthesis.[17]
Ionic liquids[18] are simply molten salts which are
ionically conduction (in the 10’s of mS cm range);
typical structures as shown in Fig. 1.
As the anion and cation structure can be varied
at will, the physical properties (e.g. viscosity, conductivity, hydrophobicity) can be tuned for specific
applications.[19] RTILs, as well as being inherently conduction, exhibit a number of properties that are useful
for electrosynthesis including a significant electrochemical stability, lack of appreciable vapour pressure and
the ability to dissolve a wide range of products[18,19]
(which can also be a disadvantage). Another advantage
of RTILs is that many examples are chemically stable
and therefore should not interfere with reactive electrogenerated species such as radical anions. However, there
is growing evidence that RTILs can influence the outcomes of reactions and overall reaction kinetics.[20]
In this article, the results from five different case
studies on electrochemistry in ionic liquids will be
Asia-Pacific Journal of Chemical Engineering
[Bmpy]
N
Me
[Bmim]
N + N
+
Me
APPLICATION OF CLEAN TECHNOLOGIES USING ELECTROCHEMISTRY
[NTf2]
n-Bu
O
O
F3C S N S CF3
O
O
n-Bu
Benzophenones
O
A
D
B
Figure 1. Structure of ionic liquid cations, [NTF2 ]
and benzophenone.
presented. The first study involves the direct reduction on substituted benzophenones (BPs)[12] in both
protic and aprotic ionic liquids. The second study
involves the mediated electrocatalytic oxidation of alcohols to carbonyl compounds (aldehydes and ketones)
using TEMPO (tetramethylpiperidinyl–N -oxyl),[16,17]
the third study involves the mediated and direct reduction of CO2 and the fourth investigation concerns the
dehalogenation of C1 and C2 freons.
Case study 1: electrochemistry of substituted
benzophenones in RTILs
The effect of medium on the electrochemistry of benzophenones (Fig. 1) is well known[21] such that a study
(a)
O
e-
(b)
O
of their reductive electrochemical behaviour generates
significant information concerning the nature of the
medium as well demonstrating the effect of the medium
on the eventual outcomes of electrolytic reactions.
In aprotic media, reduction involves two sequential
reversible one-electron processes leading to the radical
anion and dianion species (Scheme 1(a)), respectively.
Under such condition radical anion dimerisation usually occurs i.e. C–C bond formation reactions leading
to a diol. However, in protic media, reduction is via a
single process involving the transfer of two electrons
coupled with two protons which leads to saturation of
the carbonyl giving the corresponding alcohol product
(Scheme 1(b)).
Also, the electrochemical reduction potentials (Ep )
are very sensitive to the electronic nature of substituents,[21,22] as described by the Hammett substituent
constants (σ , given in Table 1), which, in turn, are very
sensitive to the polarity of the medium via the well
know Hammett relation;
Ep,x − Ep,s = ρ
C·
C-
e-
O
–
C·
pKa = 9.2 (DMF)
+ Bmim
E02
OH
+
e-
+ Bmim
(1)
–
O
+e-
σ
where Ep,x is the reduction potential of benzophenone and Ep,s is the reduction potentials of substituted
benzopheneones. Significantly, the slope ρ is dependent on the polarity of the medium (e.g. ρ = 0.33 for
acetonitrile[22] ). As such, simple electrochemical experiments using these probe molecules can reveal significant
information about the protic (or otherwise) nature of
RTILs and also their overall effective polarity. In this
O–
E01
pKa = 21-23
OH
C·
E02 > E01
Scheme 1. Consecutive one-electron transfers (a) and electron/proton transfer (b).
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
DOI: 10.1002/apj
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A. P. DOHERTY et al.
Asia-Pacific Journal of Chemical Engineering
Table 1. Hammett substituent constants for various
benzophenones.
Species
4,4-Dimethoxy benzophenone
4,4-Dimethyl benzophenone
3-Methyl benzophenone
Benzophenone
4-Bromo benzophenone
4,4-Dichloro benzophenone
4-Cyano benzophenone
BP in [Bmim] at 0.005 V s-1
0.0
Hammett No (σ )
-5.0e-6
−0.54
−0.34
−0.07
0.00
+0.23
+0.46
+1.00
-1.0e-5
-1.5e-5
-2.0e-5
-2.5e-5
-3.0e-5
-3.5e-5
-2.0
-1.8
-1.6
-1.4
E / V vs. Ag/AgCl
-1.2
Figure 3. Cyclic voltammograms of benzophenone at a
glassy carbon electrode at 0.005 V s−1 in [Bmim][NTf2 ]
ionic liquid.
(a)
Figure 2. Cyclic voltammograms of benzophenone at a
glassy carbon electrode at (a) 10 V s−1 and (b) 1 V s−1 in
[Bmpy][NTf2 ] ionic liquid.
work, the electrochemistry of BP and six of its derivatives (Table 1) was investigated.[12]
Cyclic voltammograms (CVs) for the reduction on
BP in [Bmpy][NTf2 ] are shown in Fig. 2 where two
reversible sequential one-electron reductions are evident.
These observation are consistent with the reaction
sequence in Scheme 1(a). This indicates that both the
radical anion and dianion species are stable in this
medium – a stability which is similar to that described
for BP reduction in liquid NH3 at −60 ◦ C.[23]
In contrast, the electrochemistry of BP at low potential sweep rates in [Bmim][NTf2 ] (Fig. 3) shows a
single irreversible reduction, the shape of which is characteristic of a two-electron process. Upon increasing the
potential sweep rate (ν) the reduction process begins to
split into two overlapping processes which appears to
indicate a transition from a single two-electron event
into two one electron reductions as the time scale of
the measurement is decreased.
A plot of the peak current vs. ν 1/2 is shown in
Fig. 4(a) where two distinct regions of behaviour
(slopes) are observed.
At low ν, the slope is largest and from which the
diffusion coefficient (D) of BP in this medium can
be calculated using the Randles–Sevchk equation[24]
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
(b)
Figure 4. (a) Plots of peak current vs. υ 1/2 for the
reduction on 4,4 dimethoxybenzophenone recorded in
1
[Bmim][NTf2 ]. (b)Plots of peak current vs. υ /2 for the
reduction on 4,4 dimethylbenzophenone recorded in
[Bmpy][NTf2 ].
(at 298 K, Eqn (2));
Ip = 2.69 × 105 n A D 1/2 ν 1/2 C∞
(2)
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
APPLICATION OF CLEAN TECHNOLOGIES USING ELECTROCHEMISTRY
-1.68
3-methylBP in [Bmim]
Epc / V vs. Ag/AgCl
-1.69
-1.70
-1.71
-1.72
-1.73
slope = -27mV decade-1
-1.74
-1.75
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
log10 ν (V s )
-1
Scheme 2. Catalytic cycle for TEMPO-mediated oxidation
where n is the number of electrons (n = 2 is assumed
at low sweep rate), A is the area of the electrode
(cm2 ) and C∞ is the concentration of BP (mol cm−3 ).
At low ν, assuming n = 2, D was determined to
be 1.43 × 10−7 cm2 s−1 . Upon increasing ν, the slope
decreases which, assuming that n = 1 at the high sweep
rates, yields a value for D of 1.46 × 10−7 cm2 s−1 . This
clearly indicates the transition from n = 2 to n = 1 by
altering the time scale of the measurement.
By way of contrast, similar plots of Ip vs ν for BP
reduction in [Bmpy][NTf2 ] (Fig. 4(b)) exhibits as single
slope thus indicating a single one-electron transfer
process occurs over all time scales.
Collectively, these observations (in [Bmim][NTf2 ])
are consistent with the reaction sequence shown in
Scheme 1(b) where the proton source is likely to be
from the C2 position of the [Bmim] cation, which
is known to be acidic.[25] In terms of mechanism for
the two-electron reduction (in [Bmim][NTf2 ]), plots
of peak potential vs. log ν (Fig. 5) for various benzophenones yields a slope of 29 ± 2 mV per decade
ν, which is indicative of an electrochemical-chemicalelectrochemical (ECE) mechanism[26] which is in accordance with Scheme 1(a).
Information concerning the polarity of the ionic liquid
media can be obtained
using the Hammett relation by
plotting E1/2 vs.
σ as shown in Fig. 6 for [Bmpy]
where the ρ value of 0.32 is observed, while a value of
0.31 was obtained for [Bmim]. These values are typical
for polar solvents such as acetonitrile.[22]
Case study 2: TEMPO-mediated oxidation
of alcohols in RTILs, the fundamentals
Oxidation of alcohols to the corresponding carbonyl
compounds (aldehydes or ketones) is the most frequently performed reaction in preparative-scale organic
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
of alcohols to carbonyl products.
2.0
Hammett realtionship in [Bmpy]
1.9
- E1/2 / V vs. Ag/AgCl
Figure 5. Plot of reduction peak potential as a function of
log10 υ.
1.8
1.7
1.6
1.5
b[0] = 1.7217918453
1.4
b[1] = -0.3195174553
r2 = 0.9763023494
1.3
-0.8 -0.6 -0.4 -0.2 0.0
0.2
0.4
0.6
0.8
1.0
1.2
Σσx
Figure 6. Hammett plot for the reduction on benzophenones in [Bmpy][NTf2 ].
synthesis and is of immense importance. Traditionally,
metal-based (e.g. Cr or Mn) oxidation reagents are
employed. However, over recent years, the use of
organic redox catalysts such as TEMPO have become
of interest because of their more green credentials,[27]
the use of which is shown in Scheme 2.
Essentially, TEMPO is oxidised at electrodes (one
electron) to the oxoammonium species (T + ) which, in
the presence of a base (B), oxidises primary and secondary alcohols while returning TEMPO. At electrodes
TEMPO is immediately oxidised regenerating T + thus
giving rise to a catalytic cycle. Such voltammetric
behaviour can be observed in Fig. 7(a)–(d). Figure 7(a)
is a cyclic voltammogram of TEMPO in [Bmpy][NTf2 ],
while Fig. 7(b) and (c) shows that the addition of alcohol (7b) or lutidine (7c) has no effect on the voltammetry. However, in the presence of both lutidine (a
base) and alcohol the voltammetry is characteristic of a
catalytic process occurring at the electrode–electrolyte
interface.
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
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A. P. DOHERTY et al.
Asia-Pacific Journal of Chemical Engineering
Figure 7. Cyclic voltammograms for the oxidation of TEMPO
in [Bmpy][NTf2 ] (a), TEMPO in the presence of bezylic alcohol
(b), TEMPO in the presence of lutidine (c) and TEMPO in the
presence of both benzylic alcohol and lutidine (d).
The overall catalytic reaction is as follows [for
primary and secondary alcohols (Eqns (3) and (4)),
respectively];
where a = initial conjugate acid concentration, x =
TEMPO concentration, [T + ]0 and [B ]0 are the initial
concentration of T + and B . The left hand term of the
equation can be calculated iteratively by electrochemically monitoring the (TEMPO) (= x ) as a function
of time. This was achieved using a rotating disk electrode as the reaction proceeded. The calculated left
hand term is then plotted vs. time, the slope of which
is k .
Typical kinetic plots are shown in Fig. 8(a)–(c). From
these data the product k2 Keq (global rate constant) can
be extracted from the slopes. The effect of the nature
of the base on k2 Keq is shown in Table 2 where it
can be seen that the global rate constant k2 Keq is
a function of pKa as expected for a pH sensitive
reaction.
Also, for all alcohols/bases combinations examined
k2 was found to be ∼
= 108 s−1 , therefore the rate
of the reaction is controlled by Keq , i.e. through the
initial alcohol/alcoholate equilibrium with the base.
Comparing rates obtained in [Bmpy][NTf2 ] with those
obtained in molecular solvent (acetonitrile) (Table 3)
demonstrated the reaction is ca. a factor of six higher
in the ionic liquid medium.
2T + + 2B + RCH2 OH = 2TEMPO + 2HB+ RCHO
(3)
2T + + 2B + R2 CHOH = 2TEMPO + 2HB+ R2 CO
(4)
The mechanism for this reaction is shown in Scheme 3
where the initial step (Step 1) in the reaction is the
generation of the alcoholate species through acid–base
equilibrium with B . The alcoholate reacts with T +
forming intermediate X (Step 2) which, in turn reacts
with another T + and B to complete the reaction
(Step 3).
Under pseudo-first order conditions with respect to
T + , the kinetic expression given in Eqn (5) can derived
where k ∼
= 2 k2 Eeq (alcohol)0 (B)0 .[16]
(5)
Figure 8. Kinetic plots for benzylic alcohol oxidation
with lutidine [curves (a) and (b)] and with pyridine [curve
(c)]. The difference between curves (a) and (b) is due to
varying the initial concentration of the conjugate acid, term
a in Eqn (5).
STEP 1
Table 2. Effect of base pKa on the global rate constant
(k2 Keq ).
STEP 2
Base
[a + (T + )0 ] ln[(T + )0 − x ] + x = −k t
+ [a + (T + )0 ] ln[(T + )0 ]
R2CHOH + B
R2CHO-
+
X + T+
T+
Keq
k2
R2CHO-
+ BH+
X
R2C=O + BH+ + 2 TEMPO
STEP 3
Reaction mechanism for the base catalysed
redox catalysed oxidation of alcohols.
Scheme 3.
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Pyridine
3-Picoline
2,6-Lutidine
2,4,6-Collidine
pKa in H2 O
k2 Keq (dm3 mol−1 s−1 )
5.25
5.70
6.75
7.43
9.5 × 10−3
1.4 × 10−2
1.0 × 10−1
∼
=1(estimation)
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
APPLICATION OF CLEAN TECHNOLOGIES USING ELECTROCHEMISTRY
Table 3. Comparison of global rate constant between acetonitrile and [Bmpy][NTf2 ].
(Alcohol)0 (mol dm−3 )
Base and concentration (mol dm−3 )
0.12
Pyridine (0.42)
0.12
2,6-Lutidine (0.39)
0.24
2,6-Lutidine (0.14)
Solvent
k2 K2 dm3 mol−1 s−1
bmpyNTf2
MeCN
bmpyNTf2
MeCN
bmpyNTf2
MeCN
9.5 × 10−3
1.6 × 10−3
9.0 × 10−2
1.4 × 10−2
9.0 × 10−2
1.4 × 10−2
Case study 3: TEMPO-mediated oxidation
of alcohols in RTILs, bulk electrolysis,
electrolysis data
From the preceding discussion, it is evident that
TEMPO/T + is an effective redox catalyst for alcohol oxidation in ionic liquid media, the efficiency of
which was assessed via bulk electrolysis using catalytic
quantities of TEMPO under various conditions. Electrolysis was carried out galvanostatically under divided
cell operation where either sintered glass frits and
Nafion exhibited essentially identical performance.
Although protons generated from the anodic reaction
could, in principle, have been reduced at the cathode as
a counter electrode reaction under undivided conditions,
the reductive electro-activity of the carbonyl products
prevents this possibility. For this study, product was
recovered from the RTIL medium by solvent extraction
into hexane followed by GC analysis. Figure 9 shows
the stoichiometry (as percentage expected) of various
species during the bulk electrolysis of benzylic alcohol
as a function of electrolytic charge (Coulombs). It is evident that as the benzylic alcohol is consumed there is a
corresponding gain in the benzyaldehyde product. It can
be seen that the conversion is close to 100% stoichiometrically and coulombically efficient. Also shown in
the plot is the loss of lutidine which become protonated
during the reaction.
Table 4 shows the results of bulk electrolysis of
various alcohols over a range of (TEMPO)/(Alcohol)
and (Base)/(Alcohol) ratios where the products were
analysed at 1 and 2 coulomb equivalents with the
exception of the last three entries which were terminated before 2 coulombs had passed. It is obvious from the data presented that highly efficient electrolysis is possible using TEMPO redox catalyst in
RTIL medium. However, it is also evident that enolisable aldehyde products (entries 5–7 in Table 4) exhibit
low efficiencies when the charge passed becomes
greater than 1 coulomb equivalent; this is due to the
acid catalysed enolisation process. This effect will
occur irrespective of which ever reaction medium is
used. However, should proton conducting membranes
be deployed to divide the anode and cathode, the
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 9. Compositional plots for the bulk electrolysis of
benzylic alcohol as a function of charge passed.
proton generated at the anode could be transferred efficiently to the catholyte thus inhibiting the parasitic
reaction.
Case study 4: CO2 reduction in RTILs
Efficient electrochemical reduction of CO2 has been
a target for decades[28 – 30] with the main motivation
being the synthesis of small organic molecules (e.g.
alkane, formate and oxalate) from freely available waste
(CO2 ). More recently, the motivation has developed
to encompass the need for CO2 capture. The electrochemistry of CO2 reduction is quite complex and is
dependent on the electrode material used (e.g. Cu generates hydrocarbons,[28] indium generated oxalate[29] ) and
the electrolyte deployed. The electrochemical reaction
occurs at very negative potentials which are required
to effect the reduction (−2.21 V[31] ). Extreme potentials lead to electrolysis of the solvent medium, e.g.
parasitic H2 and O2 evolutions in aqueous media. As
many RTILs exhibit extreme electrochemical stability
(e.g. ±3.5 V[32] ) the opportunity for direct and indirect reduction of CO2 has become evident. A small
number of reports have already appeared describing
the direct reduction of CO2 at solid electrodes[33] and
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
DOI: 10.1002/apj
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A. P. DOHERTY et al.
Asia-Pacific Journal of Chemical Engineering
Table 4. Bulk electrolysis data for various alcohols and bases.
Tempo/ROH
mol.%
[LU]/[ROH]
mol/mol
1
4.8
2.9
2
5.3
3
4
Entry
Substrate ROH
Product
Charge
(Fmol−1 )
Conversion
(%)a
Chemical
yield (%)a
Selectivity
(%)a
1.0
2.1
50
98
50
98
100
100
3.0
1.0
2.0
50
100
50
97
100
97
5.1
2.9
1.0
2.0
50
100
46
91
92
91
4.9
2.8
1.0
2.0
50
100
50
95
100
95
5
10
5.0
1.0
1.37b
50
69
43
60
86
77
6
10
5.0
1.0
1.39b
50
70
45
54
90
77
7
12.2
3.0
1.0
1.41b
36
46
35
35
97
72
a
b
determined by gas chromatography analysis.
Incomplete electrolysis.
indirect reduction via electron transfer from the electrogenerated super-oxide species.[34]
We have also investigated both approached in [Bmpy]
[NTf2 ] at Au and glassy carbon electrodes and the alternative approach where CO2 is ‘fixed’ via reaction with
electro-generated species (electrocarboxylation[30] ).
Figure 10 shows the electrochemistry of benzophenone in [Bmpy][NTf2 ] under N2 atmosphere where the
two one-electron reversible reductions are observed.
Also shown in Fig. 10 is the same reduction under a
CO2 atmosphere (1 bar) where a single two-electron
irreversible reduction occurs which is characteristic
of an electrocarboxylation reaction at the α-carbon.
Effectively, the electro-generated radical anion of
benzophenone reacts with CO2 generating the corresponding α-hydroxyl acid. Not only is this a useful
synthetic approach but also provides a use of otherwise
waste CO2 .
In terms of direct CO2 reduction, Au is particularly
valuable as it is electrocatalytic towards the reaction.
Despite this, CO2 reduction at Pt microelectrodes in
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 10.
Cyclic voltammograms of 4-cyanobenzophenone in [Bmpy][NTf2 ] in the absence and presence
of CO2 .
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
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Asia-Pacific Journal of Chemical Engineering
APPLICATION OF CLEAN TECHNOLOGIES USING ELECTROCHEMISTRY
Figure 11. Cyclic voltammograms for the direct reduction
of CO2 at Au electrodes in [Bmpy][NTf2 ]. Featureless
voltammogram is in the absence of CO2 .
[Bmim][Ac] (Ac = acetate) ionic liquid has appeared[33]
where the reduction process occurs in the same potential region as the electrolytic decomposition of the ionic
liquid. In this work, Au electrodes have been used.
Figure 11 shows CO2 reduction at Au in [Bmpy][NTf2 ]
recorded at 10 mV s−1 ; the single one-electron irreversible reduction process observed corresponds to the
formation of the radical anion of CO2 and a subsequent post-electron transfer reaction leading to oxalate,
formate or CO. Significantly, direct CO2 reduction can
be achieved with this system without the electrolytic
decomposition of the supporting ionic liquid medium.
Compton et al . have already shown that CO2 can be
reduced via reaction with electro-generated superoxide
(at Au microelectrodes) to form peroxodicarbonate.[34]
Here we have investigated an aromatic ester-based
redox mediator[35] to activate CO2 reduction.
Figure 12(a) and (b) shows cyclic voltammograms of
methylbenzoate in dimethylformamide (DMF) organic
solvent and [Bmpy][NTf2 ] under N2 atmosphere conditions.
In both cases, the results suggest that the reduction is
a reversible single-electron event leading to the highly
reactive radical anion.[36] Significantly, the currents
recorded in the ionic liquid are much smaller than those
in the DMF, which is due to the significantly higher
viscosity of [Bmpy][NTf2 ]. In contrast, substituting the
N2 for a CO2 atmosphere (Fig. 12(c) (DMF) and 12(d)
([Bmpy][NTf2 ]) leads to an enhanced reduction current which has been interpreted as being because of the
mediated (redox catalytic) reduction of CO2 . The current enhancement in DMF was by a factor of 12 (similar to that reported by Saveant et al .[35] ), whereas the
enhancement in the [Bmpy][NTf2 ] was much smaller
at ca. 2.3. This difference cannot be explained by the
viscosity difference as the currents should scale proportionately. It has been shown recently[37] that quaternary
 2011 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 12.
Cyclic voltammograms of methylbenzoate
in DMF, methylbenzoate in [Bmpy][NTf2 ], methylbenzoate in DMF with CO2 and methylbezoate in [Bmpy][NTf2 ]
with CO2 . This figure is available in colour online at
www.apjChemEng.com.
Figure 13. Detailed cyclic voltammogram of methybenzoate
in [Bmpy][NTf2 ].
ammonium cations stabilise the electro-generated radical anions of methylbenzoate. As [Bmpy] is merely a
cyclic quaternary ammonium species, such stabilisation
should, in principle, occur in the [Bmpy][NTf2 ] ionic
liquid environment. A closer examination of the voltammetry of methylbenzoate in [Bmpy][NTf2 ] is shown
in Fig. 13 recorded at 0.5 V s−1 where the reoxidation is not a simple Nernstian process but is a series
of discrete reoxidation events occurring over the potential region form −2.4 V to higher than −1.0 V. This
observation suggests that the electro-generated radical
anions are stabilised in discrete various forms in the
[Bmpy][NTf2 ] medium. This supposition is supported
by the theoretical framework forwarded by Saveant[38]
where it is predicted that the reoxidation potentials of
Asia-Pac. J. Chem. Eng. 2012; 7: 14–23
DOI: 10.1002/apj
21
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A. P. DOHERTY et al.
Figure 14. CVs of reduction on F114B2 in (a) [Bmim][NTF2 ],
(b) [Bmpyr][NTF2 ], (c) [C6 C6 C6 C14 P][NTF2 ] and (d) DMF
media at GC electrodes at a potential sweep rate of
0.2 V s−1 .
stabilised reductively generated species are displaced
from the thermodynamic reversible potential by values
corresponding to the ion–ion interaction energy.
Case study 5: dehalogenation reactions
Globally, there are large stocks of freons which are
no longer in use because of national legislation and
international agreement. These compounds need to be
rendered harmless and electrolytic dehalogenation is
an attractive route, in particular their conversion to
valuable products. We have shown recently that ionic
liquids are good media for dehalogenation of C1 and C2
freons.[39,40] Cyclic voltammograms for Freon F114B2
in various ionic liquids (DMF) are shown in Fig. 14. It
is clear that the reductive electrolytic dehalogenations
occur in the ionic liquid medium.
Asia-Pacific Journal of Chemical Engineering
can interfere with reductive electrochemistry. The redox
catalytic oxidation of alcohols in [Bmpy][NTf2 ] is particularly advantageous as highly efficient processing can
be carried out catalytically in the absence of metal-based
oxidation reagents and volatile molecular solvents. It is
also significant the global rate constant for the oxidation reaction (k2 Keq ) is a factor of six larger than that
observed in molecular solvent (acetonitrile). It is also
important that ionic liquids exhibit extreme redox stability such that direct (or indirect) electrochemistry can be
performed at extreme potentials thus facilitating access
to otherwise inaccessible electrochemical processes, for
example direct and mediated CO2 reduction.
To date, the electrochemical engineering solutions
necessary to harness the useful properties of RTILs
for electrolytic processing have not been investigated
to any extent. As RTILs eliminate the two most frequently cited reasons for the general lack of interest
in commercial-scale bulk electrosynthesis (which are
(1) the dangers of volatile solvents and (2) the difficulty with separating extraneous salts from reaction
products), and the recent demonstration of the applicability of RTILs for electrochemical transformations,
it seems that it is now opportune for involvement of
chemical engineers in this field, in particular to design
new reactor/process technologies to overcome some of
the challenges presented by RTIL media. These challenges include maximise mass transport rates to/from
electrode surfaces, designing and constructing electrochemical flow through cells that accommodate heterogeneous processing relatively viscous (RTIL) media, the
operation of membrane-divided cells and the new materials sciences necessary to achieve whole processes.
Acknowledgements
EM thanks the Department of Education, Northern
Ireland, for a Studentship, and LD thanks QUB for a
Studentship.
SUMMARY AND ENGINEERING CHALLENGES
REFERENCES
As pointed out by Hapiot and Lagrost,[41] and observed
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relatively low viscosity aprotic salt, [Bmpy][NTf2 ] is
useful for generating highly reactive species for further
reaction, whereas the [Bmpy] derivative’s protic nature
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