Electrosynthesis of 2,3-butanediol and methyl ethyl ketone from acetoin in flow cells

Article abstract of DOI:10.1039/c8gc03028f

Acetoin could shortly become a platform molecule due to current progress in fermentation technology, the megatrend for shifting from an oil-based economy to the one based on biomass, the quest for green manufacturing processes and its two highly reactive carbonyl and hydroxyl moieties. In this paper, the successful electro-conversion of acetoin into two valuable chemicals, 2,3-butanediol (2,3-BD) and methyl ethyl ketone (MEK), at a constant electrical current in an aqueous phase at room temperature using both divided and undivided 20 cm² filter-press flow cells under experimental conditions suitable for industrial production is reported. Cathode material is the key parameter to drive the electroreduction towards one or another chemical. 2,3-BD is the major chemical produced by electrohydrogenation when low hydrogen overvoltage cathodes, such as Pt and Ni, of high surface areas obtained by PVD coating on a carbon gas diffusion layer are used, while MEK is the principal product produced by electrohydrogenolysis when high hydrogen overvoltage cathodes, such as graphite, Pb and Cd foils, are employed. 2,3-BD and MEK can be obtained, respectively, in 92.8% and 85.7% selectivities, 71.7% and 80.4% current efficiencies, with 1.21 and 1.08 kg h^-1 m^-2 productivities and power consumptions of 2.94 and 4.1 kWh kg^-1 using undivided cells and aqueous K₂HPO₄ electrolysis media at pH values of 3.6 and 5.5. The reported electroconversion of acetoin is highly flexible because 2,3-BD and MEK can be produced by changing just the cathode but using the same cell, with the same electrolyte at the same current density.

Full text of DOI:10.1039/c8gc03028f

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PAPER
Electrosynthesis of 2,3-butanediol and methyl
ethyl ketone from acetoin in flow cells†‡
Cite this: Green Chem., 2019, 21, 164
a
José R. Ochoa-Gómez, *a Francisco Fernández-Carretero,
Francisca Río-Pérez, b Alberto García-Luis, a Tomás Roncal a and
a
Eduardo J. García-Suárez
Acetoin could shortly become a platform molecule due to current progress in fermentation technology,
the megatrend for shifting from an oil-based economy to the one based on biomass, the quest for green
manufacturing processes and its two highly reactive carbonyl and hydroxyl moieties. In this paper, the
successful electro-conversion of acetoin into two valuable chemicals, 2,3-butanediol (2,3-BD) and
methyl ethyl ketone (MEK), at a constant electrical current in an aqueous phase at room temperature
using both divided and undivided 20 cm2 filter-press flow cells under experimental conditions suitable for
industrial production is reported. Cathode material is the key parameter to drive the electroreduction
towards one or another chemical. 2,3-BD is the major chemical produced by electrohydrogenation when
low hydrogen overvoltage cathodes, such as Pt and Ni, of high surface areas obtained by PVD coating on
a carbon gas diffusion layer are used, while MEK is the principal product produced by electrohydrogenoly-
sis when high hydrogen overvoltage cathodes, such as graphite, Pb and Cd foils, are employed. 2,3-BD
and MEK can be obtained, respectively, in 92.8% and 85.7% selectivities, 71.7% and 80.4% current efficien-
cies, with 1.21 and 1.08 kg h−1 m−2 productivities and power consumptions of 2.94 and 4.1 kWh kg−1
using undivided cells and aqueous K2HPO4 electrolysis media at pH values of 3.6 and 5.5. The reported
electroconversion of acetoin is highly flexible because 2,3-BD and MEK can be produced by changing just
the cathode but using the same cell, with the same electrolyte at the same current density.
Received 26th September 2018,
Accepted 28th November 2018
DOI: 10.1039/c8gc03028f
rsc.li/greenchem
high-volume-acetoin-derived chemicals to act as a driving force
for increasing its production, a key issue to reducing its price.
Introduction
Acetoin (3-hydroxy-2-butanone) is currently a fine chemical
The first barrier has been overcome by means of new very
used in foods and cosmetics as a flavoring agent. Its reactive effective fermentation processes,2 and highly efficient downstream
carbonyl and hydroxyl moieties, together with its biomass procedures for acetoin recovery.3 Thus, our group has developed a
origin, led to the NREL of the U.S. Department of Energy to fermentation process to acetoin using the newly isolated non-
classify acetoin as one of the 30 potential platform chemicals pathogenic Risk Group 1 (and GRAS, Generally Recognized As
for a biobased economy in 2004.1 As an example, some chemi- Safe) mutant strain Lactococcus lactis subsp. lactis CML B4 and
cals that could be obtained from acetoin are depicted in glucose as the carbon source.4 Acetoin titers of 59 g L−1 and pro-
Scheme 1. However, since then two barriers have prevented its ductivities exceeding 2 g L−1 h−1, the highest ever reported among
general use as a platform chemical: its high price, ranging non-pathogenic bacteria, are obtained. As far as we know, this pro-
between 6 $ kg−1 for synthetic acetoin and 10 $ kg−1 for ductivity is only surpassed by Enterobacter cloacae,4 which is classi-
natural acetoin, and the lack of effective processes to produce fied as a Risk Group 2 (RG2) pathogenic bacterium, similar to
Serratia marcescens, another acetoin producer with productivities
close to 2 g L−1 h−1. Pathogenic bacteria increase the production
aTecnalia R&I, Parque Tecnológico de Álava, Leonardo da Vinci 11, 01510 Miñano,
costs due to the strict biosafety and containment requirements.
Spain. E-mail: jramon.ochoa@tecnalia.com
bBiokemik, Parque Científico y tecnológico de Bizkaia, Astondo Bidea Ed. 612,
The second barrier is being continuously and strongly weak-
ened by some market disturbances and world megatrends.
Thus, in the last few years a disturbance in the huge 1,3-buta-
diene (BDE) market has been occurring. BDE is currently key to
manufacture synthetic rubber for tires. It has been a co-product
in the ethylene production by steam cracking of naphtha, but
crackers are shifting the raw material from naphtha to ethane,5
48160 Derio, Spain. E-mail: paqui.rio@biokemik.eu
†Part of this article was presented as an oral communication in the 3rd Green &
Sustainable Chemistry Conference, Berlin (Germany), 2018, May 14–16th. Oral
presentation O3.2 (May 15th). Session
Synthesis.
3 – Recent Developments in Green
‡Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc03028f
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Scheme 1 An acetoin-derived chemical platform.
resulting in a BDE yield drop from about 16 to 2 kg per 100 kg
Fedoroňko et al.16 showed that acetoin could be polarogra-
of ethylene. Therefore, a shortage in the BDE supply is expected, phically electroreduced to 2,3-BD and methyl ethyl ketone
and consequently new on-purpose processes to BDE are being (MEK) but, their work, while important for analytical chem-
developed.6 Moreover, sustainability is a strong and unstop- istry, has little interest for synthetic purposes. As far as we
pable current megatrend requiring the use of renewable know, the only paper valuable for synthetic purposes is that of
resources in production processes. Therefore, it is understand- the great organic electrochemist M. M. Baizer.17 He and his co-
able that one of the new on-purpose BDE processes more workers developed a process for converting 2,3-BD in aqueous
studied is the catalytic dehydration of 2,3-butanediol (2,3-BD), solution to MEK at room temperature and pH 7 by passing it
for which yields as high as 91.9% have been reported.7
through a porous graphite anode at which it was oxidized to
2,3-BD is an oil-derived chemical with a potentially high acetoin in the electrolyte by anodically electrogenerated NaBrO
volume market for manufacturing polyesters and poly- from NaBr, and then pumping to a porous cathode (Pb/Hg or
urethanes provided its manufacturing cost is reduced, which Zn/Hg) at which acetoin was reduced to MEK. The overall
can be done by glucose fermentation.8–12 Another economi- current yield for 2,3-BD conversion into MEK was 45%.
cally efficient approach developed through the collaboration of
This process is an interesting example of a combination of
Tecnalia and Biokemik is the catalytic hydrogenation of indirect electrosynthesis leading to acetoin and a paired elec-
biomass-derived acetoin both under batch13 and continuous trosynthesis leading to a chemical (MEK) with a large market
conditions.14 Using a carbon-supported ruthenium catalyst, as a solvent. However, it has no industrial feasibility due to its
yields higher than 99% are obtained in both cases, with pro- very low current density of 20 A m−2, far below the useful
ductivities suitable for large production plants. However, the industrial one ranging from 500–1000 A m−2 to 5000 A m−2
.
use of hydrogen as a reactant makes them unsuitable for small Additionally, Baizer et al. stated that working above 20 A m−2
production plants in which the construction of an in situ caused more H2 evolution resulting in poor current efficiency.
hydrogen production facility by the steam reforming of natural Therefore, the process of Baizer et al. results in very poor
gas is not economically feasible.
specific productivities (kg h m−2) requiring a huge capital
Organic electrosynthesis provides a green and economically investment. Additional strong drawbacks from an environ-
feasible solution. As reported by Frontana-Uribe et al.,15 mental standpoint are that Hg-based cathodes are used and
“…organic electrosynthesis has become recognized as one of the presence of NaBrO within the electrolyzed solution.
the methodologies to develop environmentally compatible pro-
In this paper, the direct batch electro-conversion of acetoin
cesses. It can be used to replace toxic or dangerous oxidizing into 2,3-BD (eqn (1)) and MEK (eqn (2)) at a constant electrical
or reducing reagents, reduce energy consumption, and can be current in the aqueous phase at room temperature using filter-
used for the in situ production of unstable and hazardous press flow cells is reported.
reagents...”. For electrohydrogenations, as that of acetoin into
C4H8O2 þ 2eꢀ þ 2Hþ ! C4H10O2
C4H8O2 þ 2eꢀ þ 2Hþ ! C4H8O þ H2O
ð1Þ
ð2Þ
2,3-BD, hydrogen is cathodically generated in situ at ambient
pressure and temperature through the electroreduction of
water or protons present in the reaction medium, and it is sim-
ultaneously consumed during electrolysis. This inherent
The influence of cathode nature, electrolyte, pH, cell type
characteristic of electrohydrogenations paves the way for per- (divided or undivided), acetoin concentration, current density
forming hydrogenations in an economically and safe manner and circulated electric charge is studied. The challenge is to
in both large and small production plants.
develop industrially feasible green electroreduction processes
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for 2,3-BD and MEK production. Whenever possible, this tant as 2,3-BD is obtained by hydrogenation of the carbonyl
requires working under the following conditions: (a) Use of an moiety of acetoin. In contrast, electroreduction to MEK
undivided cell instead of a divided one provided the usual involves the hydrogenolysis of the C–O bond of the C–OH
decrease in current yield is counterbalanced by the decrease in moiety which requires that the applied electrical energy is
power consumption; (b) use of water as a solvent; (c) working used in breaking the C–O bond instead of electrogenerating
at ambient pressure and temperature; (d) current densities hydrogen. Consequently, HHO cathodes are required to obtain
≥1000 A m−2. If properly handled all these conditions lead to MEK by preventing or minimizing hydrogen electrogeneration
lower CAPEX and OPEX as well as to a safer process.
from water or protons in an acidic medium because in the
presence of hydrogen the reduction of the carbonyl group is
favored relative to that of the hydroxyl one, as shown by us.13
We reported the selective reduction of acetoin to 2,3-BD by
hydrogenation using pressures up to 5 MPa and temperatures
up to 125 °C. No MEK was found.
Results and discussion
Cathode influence and reaction mechanisms
When in an electrosynthesis carried out in aqueous media the
The reaction mechanisms leading to 2,3-BD and MEK are
anodic reaction has no synthetic interest, as is the case of the depicted in Scheme 2(a) and (b), respectively. The electro-con-
present work, the choice of the anode material is in most version of acetoin into 2,3-BD using LHO metal cathodes is an
cases obvious: a low oxygen overvoltage anode for both divided electrohydrogenation whose mechanism involves hydrogen
and undivided cells. In the first case to decrease the power electrogeneration and reaction between the nascent hydrogen
consumption while in the second one also to avoid or mini- atoms and substrate both adsorbed onto the electrode
mize the anodic electrooxidation of both the substrate and the surface.19 In the acidic medium used, the first step of the reac-
target chemical obtained in the cathode. Therefore, a low tion pathway is the electroreduction of protons to hydrogen.
oxygen overvoltage DSA®-O2 anode based on iridium oxide The nascent hydrogen is adsorbed and dissociated onto the
supported on titanium has been used in all experiments catalytic metal cathodic surface while an acetoin molecule is
reported herein.
also adsorbed onto the metal surface. Then, the hydrogen
The results obtained with different cathode materials in atom with a positive charge density adds to the oxygen atom of
both undivided and divided cells are given in Table 1 under the carbonyl group in acetoin activating the carbon atom of
the experimental conditions reported therein. Reported cath- the said group. Next, the hydrogen atom with a negative
odes can be classified into low hydrogen overvoltage (LHO) charge density attacks the carbon atom of the carbonyl moiety
cathodes (entries 1–7) and high hydrogen overvoltage (HHO) leading to 2,3-BD which is subsequently desorbed from the
cathodes (entries 8–14) with hydrogen overvoltages being electrode surface. However, when HHO cathodes are used, the
reported by Popp and Schultz.18 As can be seen LHO cathodes electroreduction mechanism corresponds to a hydrogenolysis.
lead to a high 2,3-BD selectivity while selectivity moves The electrical energy supplied via the cathode is not used in
towards MEK when HHO cathodes are used. LHO cathodes producing hydrogen because the applied cathode potential is
studied are based on Pt and Ni, typical hydrogenation cata- below that needed for hydrogen evolution. Instead, the
lysts, which explains that 2,3-BD is the main chemical resul- carbon–oxygen bond is broken leading to one hydroxyl ion and
Table 1 Cathode influence on acetoin conversion and product selectivities
Entry#
Cell
ECa
Cathode
C (%) = εactb (%)
S2,3-BD (%)
SMEK (%)
S2-BuOH (%)
H2 overvoltage (V)
1
2
3
4
5
6
7
8
Uc
U
U
U
U
U
U
U
Dd
D
D
D
D
D
A
A
A
A
A
A
A
A
B
B
B
B
B
B
Pt/Ti (sheet)
Ni (sheet)
Pt-30/GDL
Pt3Co-30/GDLe
Pt3Ni-30/GDL
Pt-47/GDL
PtOx-80/GDL
GDL
29.9
50.2
33.7
45.7
49.9
57.9
69.0
75.6
74.1
77.8
65.6
88.1
74.3
91.8
52.4
34.8
59.2
62.8
60.3
74.7
74.6
9.0
11.0
8.3
17.6
30.4
9.7
—
—
—
—
—
—
—
64.2
74.1
68.2
73.0
49.7
77.2
55.4
—
—
—
—
—
—
—
—
6.5
2.1
2.0
6.5
3.2
7.9
0.29
0.56
0.29
0.29 (Pt)
0.29 (Pt)-0.56 (Ni)
0.29
0.29 (Pt)
0.77
0.77
0.77
0.77
0.94
1.0
1.22
9
GDL
10
11
12
13
14
RVC-4000
Graphite (sheet)
Zn
Pb
Cd
16.6
a Experimental conditions: A: 1000 A m−2, anode: DSA-O2, electrolyte: 5 wt% KH2PO4 + 100 g L−1 acetoin, room temperature, electric charge:
100% of theoretical one = 2 F mol−1 of acetoin; B: 1500 A m−2, anode: DSA-O2, anolyte: 5 wt% aqueous sulfuric acid, Nafion 324 membrane, cath-
olyte: 5 wt% KH2PO4 adjusted at pH 5.5 with 20 wt% KOH + 100 g L−1 acetoin, room temperature, electric charge: 100% of the theoretical one.
b Acetoin conversion matches the acetoin current efficiency because the electric charge used was the theoretical one: 2 F mol−1 of acetoin.
c Undivided cell. d Divided cell. e GDL: porous gas diffusion layer Sigracet® 24-BC.
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Scheme 2 Acetoin electro-conversion mechanisms: (a) via electrohydrogenation to 2,3-BD, (b) via electrohydrogenolysis to MEK. E: cathode
potential (V), I: current intensity (A), t: time (h), E × I × t: Wh.
a carbocation 1 which takes an electron to be converted into to use porous materials to increase the electrode surface area
the radical 2 which, after taking another electron, is converted while keeping constant the geometric one. In this way, it is
into the carbanion 3, which eventually reacts with a proton in possible to decrease the actual current density while working
the acidic medium resulting in MEK formation. Evidence of at the same current intensity, i.e. which in turn leads to a
this mechanism is provided by the fact that 3,4-dimethyl-2,5- lower cathode potential resulting in a lower hydrogen current
hexanedione was detected by GC-MS (see Fig. S1 in ESI‡), yield favoring a higher substrate conversion and product
which can only be obtained by a coupling of two radicals 2. selectivity. Thus, in entries 3–7 Pt, Ni and mixtures of Pt–Co
The formation of 2-butanol detected in most reactions is a and Pt–Ni were deposited by physical vapor deposition on a
result of MEK electrohydrogenation.
porous gas diffusion layer (Sigracet® GDL-24BC) as described
Regarding the LHO cathodes (entries 1–7) it can be in the Experimental section. In entry 7 platinum oxide was de-
observed that the use of bidimensional (flat sheet) Pt and Ni posited on GDL but it is reduced to Pt during electrolysis. As
(entries 1 and 2) leads to poor acetoin current efficiencies as can be seen, these so-called decorated cathodes lead to an
shown by the low conversions of 29.9% and 50.2%, respect- increase in both acetoin conversion and 2,3-BD selectivity with
ively, as well as low selectivities to 2,3-BD, mainly with Ni due both values increasing as the metal load increases. Thus, a
to its higher hydrogen overvoltage than Pt which increases the metal load of 30 µg cm−2 (entry 3) can be considered as the
electrode potential window for electroreducing acetoin to threshold load for improving the cathode efficiency because both
chemicals other than 2,3-BD. According to the results reported conversion and 2,3-BD selectivity increase slightly relative to a Pt
in Table 1 for HHO cathodes (entries 8–14) presumably MEK is bidimensional cathode (entry 1). Above a load of 30 µg cm−2
the other main chemical produced. The low conversions both conversion and selectivity increase (entries 6 and 7)
obtained mean that energy is being wasted in producing reaching values as high as 69.0% and 74.6%, respectively, at a
hydrogen. This can be avoided or minimized by decreasing the load of 80 µg cm−2. This increase must be ascribed to the
current density, i.e. by decreasing the current intensity, but noble metal deposition on the porous GDL support and not to
this results in a decreased productivity and, therefore, in an the support porosity as shown by the results in entry 8 in
increased CAPEX. A typical way for overcoming this problem is which, although a high 75.6% acetoin conversion is reached, a
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low 9.0% 2,3-BD selectivity is obtained with MEK being the cathodes based on Pb and Cd were employed for studying the
main product.
MEK electrosynthesis.
Regarding the HHO cathodes (entries 8–14) studied, they
can be divided into porous (entries 8–10) and non-porous
(entries 11–14) cathodes. All porous cathodes are based on
carbon. There is no appreciable difference among them with
respect to both acetoin conversions and MEK selectivities. The
higher selectivity obtained in entry 9 relative to entry 8 must be
ascribed to the use of a divided cell which prevents MEK anodic
oxidation. However, in comparison with a non-porous material
of the same chemical composition such as graphite (entry 11)
using the same type of divided cell, conversions are about 10
points higher while selectivities to MEK are similar (selectivities
of 74.1, 68.2 and 73.0% for entries 9, 10 and 11, respectively, are
within the analytical experimental error; the mean value is
71.8% with a standard deviation of 3.1%). The reduced cathode
potentials in porous electrodes in comparison with sheet elec-
trodes at the same current intensity explain the higher conver-
sions obtained with the former electrodes. Acetoin electroreduc-
tion is favored relative to proton electroreduction to hydrogen.
On the other hand, with non-porous flat electrodes of
increasing hydrogen overvoltage there is a clear correlation
between acetoin conversion and H2 overvoltage. The former
increases when the latter increases except for Zn. The higher
conversion with Zn than with Pb despite its lower H2 overvol-
tage can be attributed to the fact that Zn is a well-known redu-
cing agent while Pb and Cd are not. The use of Zn as a catalyst
in the reduction of ketones to alcohols has been reported.20–22
Therefore, Zn can have a dual behavior in the electroreduction
of acetoin acting both as a hydrogenation cathode due to its
Electrosynthesis of 2,3-BD
Influence of pH. Acetoin conversion and 2,3-BD selectivity
as a function of pH (average between the initial and final pH
values) between 2.9 and 3.8 using a 5 wt% KH2PO4 aqueous
electrolyte and an undivided cell fitted with a decorated PtOx-
80/GDL cathode at 1000 A m−2, room temperature, 100 g L−1
acetoin and 100 Qt% are depicted in Fig. 1. 2,3-BD selectivity
is practically constant in the pH range studied. The high 85%
value at a pH of 2.9 is a consequence of the low 39.5% conver-
sion as it is a typical pattern of most (electro)chemical pro-
cesses that selectivity is higher at low conversions than at high
ones. On the other hand, conversion decreases dramatically
below a pH of 3.2 as the proton concentration increases favor-
ing the competing reaction of hydrogen evolution.
Consequently, a pH between 3.2 and 3.9 is suitable for acetoin
electro-conversion to 2,3-BD.
Influence of the electrolyte concentration. Acetoin conver-
sion and 2,3-BD selectivity as a function of KH2PO4 concen-
tration in water using an undivided cell fitted with a decorated
PtOx-80/GDL cathode at 1000 A m−2, room temperature, 100 g
L−1 acetoin and 100 Qt% are given in Fig. 2. pH was ranged
from 3.7 to 4.0. Selectivity is practically constant and high
(76–80%) when the electrolyte concentration increases while
conversion decreases strongly above
a concentration of
2.5 wt%. It is well known that in aqueous solutions the thick-
ness of an electrical double layer in which the electrode–sub-
strate electronic transference occurs is typically of a few nano-
meters and it decreases with increasing electrolyte concen-
tration.23 Therefore, the increase in KH2PO4 concentration
should decrease the mass transfer resistance from the bulk
solution to the double layer favoring the acetoin electroreduc-
tion, which really does not occur. Consequently, as sketched in
the box inserted in Fig. 2, a plausible explanation could be
hydrogenation catalytic activity and as
a hydrogenolysis
cathode thanks to its high hydrogen overvoltage. This dual be-
havior of Zn can explain the higher acetoin conversion which
is equal to that with a Cd cathode (entry 14), the material with
the highest hydrogen overvoltage, and at the same time the
high 30.4% 2,3-BD selectivity in comparison with the other
HHO sheet cathodes. MEK selectivity increases from graphite
to lead and then decreases sharply with Cd, most probably
because its high hydrogen overpotential favors acetoin
reduction to other chemicals. This explains both the 7.9%
2-butanol (obtained by MEK electroreduction in a two-electron
process) selectivity which is the highest of the whole series,
and the sum of 2,3-BD, MEK and 2-BuOH selectivities (79.9%
while values of 92.6%, 86.6% and 90.1% are obtained for
graphite, Zn and Pb, respectively) which is the lowest of the
whole series. Cd leads to a higher number of unidentified
chemicals. Pb leads to the best results for MEK electrosynth-
esis based on MEK selectivity providing also a good acetoin
conversion, while Cd results in the best results based on
acetoin conversion although with a low MEK selectivity.
However, the high 91.8% current efficiency for acetoin conver-
sion obtained with Cd supports further studies with this
material to determine whether under other experimental con-
ditions MEK selectivity can be significantly enhanced or not.
Accordingly, porous LHO cathodes based on Pt and PtOx
Fig. 1 Influence of pH. Undivided cell, anode: DSA-O2, electrolyte:
aqueous 5 wt% KH2PO4, cathode: PtOx-80/GDL, 1000 A m−2, 100 g L−1
acetoin, charge: 100 Qt%, room temperature. pH of aqueous 5 wt%
KH2PO4 = 3.8. pH was adjusted to the desired value with H3PO4 in the
were used for studying the 2,3-BD electrosynthesis while HHO other reactions.
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trolyte but 4 wt% Na2SO4 was also added to increase the con-
ductivity of the reaction medium to avoid a strong increase in
power consumption. Conversions with Pt and Pt/Ru (66.6/33.3
w/w) decorated cathodes decrease at a low metal load relative
to that in the GDL support, most probably because the metal
coverage of the GDL support is low resulting in a low amount
of adsorbed acetoin so that hydrogen evolution is the main
reaction. As the metal load increases, the metal coverage of the
porous support increases, thereby favoring the adsorption of
higher amounts of acetoin on the metal catalytic sites which
favors its electrohydrogenation to 2,3-BD, as shown by the
increase in the selectivities with the metal load for the three
electrodes studied. Although conversions of 72–77% and selec-
tivities of around 90% are achieved with the three electrodes
studied at different metal loads, it must be concluded that the
best electrode is that decorated with PtOx because both values
Fig. 2 Influence of KH2PO4 concentration. Undivided cell, anode:
DSA-O2, cathode: PtOx-80/GDL, 1000 A m−2, 100 g L−1 acetoin, charge:
100 Qt%, room temperature.
are obtained with a low load of 80 µg cm−2
It is obvious that during electrolysis PtOx is reduced to Pt(0)
that an increasingly salting-out effect with the increase in elec- by nascent hydrogen as shown by the results obtained by EDX
(see Table S1 in the ESI‡) in which the oxygen content
.
trolyte concentration is produced within the electrical double
layer, which results in the expulsion of acetoin from the said decreased from 23.02% in the PtOx-GDL-24BC as deposited to
layer to the bulk aqueous solution. Thus, the depleted acetoin 0% after electrohydrogenation. Therefore, the process is that
of the activation of Adams’s catalyst, PtO2, which is reported to
concentration in the proximity of the LHO cathode favors the
competing reaction of protons electroreduction to hydrogen lead to Pt(0) nanoparticles of about 3 nm.26,27 This small par-
leading to a decrease in acetoin current efficiency, i.e., in ticle size explains the good performance of the PtOx/
GDL-24BC relative to the other herein reported decorated
acetoin conversion. In fact, the decrease in water acetoin and
2,3-BD solubilities when a salt is added has been largely porous cathodes because it leads to an increase of the actual
exploited to develop effective extraction procedures of the said electrode surface area and therefore to a decrease in the actual
chemicals from fermentation broths.4,24,25
current density which, in turn, causes a decrease in the
Influence of the metal load in decorated cathodes. cathode potential favoring the acetoin electrohydrogenation to
Variations of conversions and selectivities as a function of 2,3-BD. This conclusion is also supported by the fact that PtOx
deposited by PVD on GDL-24BC shows a dendritic structure
metal load deposited by PVD on porous Sigracet® GDL-24BC
for three different coatings (Pt, PtOx and Pt/Ru (66.6/33.3 w/w)) (see Fig. S2 in the ESI‡) which is held after electrohydrogena-
are plotted in Fig. 3 under the experimental conditions given tion to Pt as shown by SEM (see Fig. S3 in the ESI‡). This den-
dritic structure leading to a high surface area has been pre-
in the legend. According to the best results obtained in the
above section, a 2.5 wt% KH2PO4 solution was used as the elec- viously reported by Zhensheng et al.28 and Yamada et al.29 for
Fig. 3 Influence of metal load in decorated cathodes based on a porous Sigracet® GDL-24BC support on conversion and 2,3-BD selectivity.
Undivided cell, anode: DSA-O2, electrolyte: aqueous 2.5 wt% KH2PO4 + 4 wt% Na2SO4, pH 3.6, 1000 A m−2, 100 g L−1 acetoin, charge: 100 Qt%,
■
▲
room temperature, cathode: ( ) PtOx; (♦) Pt; ( ) Pt/Ru (66.6/33.3 w/w).
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Adam’s catalyst. These last authors have shown that a Pt cata- aldol condensation into higher hydrocarbons in a basic
lyst film with a dendritic microcrystalline structure prepared medium.31 As can be seen, there is not a great influence of pH
by reducing PtO2 deposited by reactive sputtering onto a gas within the studied range although the higher acetoin conver-
diffusion layer shows a superior performance as a cathode sions and MEK selectivities are obtained between 4.10 and
catalyst of a polymer electrolyte fuel cell compared to a conven- 5.50. This pH range was chosen for further studies.
tional sputtered Pt film, with the activity based on the unit of
Influence of quaternary ammonium salts (QAS). The
the electrochemical surface of the dendritic Pt being higher addition of tetraethylammonium ethylsulphate to the catholyte
than that of conventionally sputtered Pt in which the agglom- was a key factor for the successful development of the electro-
eration of large sphere-like Pt particles is observed.
hydrodimerization of acrylonitrile to adiponitrile on the indus-
On the other hand, it has been reported that platinum trial scale.32,33 The hydrophobic aliphatic groups in QAS
minimizes the hydrogenolysis of functional groups such as promote, even at low concentrations, their preferential adsorp-
alcohols,30 which can also explain the high 90–92% selectiv- tion on the cathode relative to protons or water preventing the
ities obtained with both PtOx and Pt at low loads.
proton or water electrolysis to hydrogen by apparently increas-
Power consumption. This is one of the key parameters on ing the hydrogen overvoltage. Therefore, the current efficiency
which the industrial feasibility of an electrochemical process to adiponitrile was increased in two ways: by preventing the
underlies because it is one of the main components of OPEX. energy waste in hydrogen electrogeneration and by avoiding
The best result obtained corresponds to that reported in Fig. 3 the hydrogenation of acrylonitrile to propionitrile by the
for a PtOx/Sigracet® GDL-25BC cathode: 77.6% acetoin conver- nascent hydrogen. To ascertain whether the same effect could
sion, 92.8% 2,3-BD selectivity (ε2,3-BD: 71.7%). The average occur in the electrohydrogenolysis of acetoin to MEK, the
voltage was 5 V (initial and final voltages were 4.9 and 5.1 V, influence of tetraethyl ammonium bromide (TEABr) and tetra-
respectively) and power consumption was 4.2 kWh kg-2,3- butyl ammonium bromide (TBABr) at a concentration of
BD−1. However, a typical interelectrode gap in industrial 0.5 wt% relative to the total weight of the catholyte was
electrochemical cells is 2 mm instead of 8 mm used herein. studied. Experimental conditions and results obtained with a
Consequently, considering the conductivity of the aqueous Pb cathode in a divided cell are given in Table 3. As can be
solution it can be calculated that the expected industrial seen, the effect of both QAS is opposite to the expected one.
average voltage is 3.5 V leading to a power consumption of Despite the fact that current efficiencies for acetoin conversion
2.94 kWh kg-2,3-BD−1. This is really a low value which can be increase notably indicating the suppression of hydrogen evol-
further reduced because this process is not optimized.
ution, selectivity of MEK decreases in both cases while selectiv-
ities for hydrogenation products, i.e. 2,3-BD and 2-BuOH,
increase. This fact is more apparent when TBABr is used indi-
cating that the increase in the size of the QAS cation, i.e. the
increase of its hydrophobic character, has a remarkable influ-
ence on the reaction mechanism. The decrease in MEK selec-
tivities could be explained by the fact that QAS have been
reported to promote the cathodic dimerization of activated
olefins and carbonyl compounds.32–36 However, the increase
in hydrogenation products is difficult to explain. Considering
that hydrogen formation is not avoided as shown by the
acetoin current efficiency below 100%, one possibility could be
that the QAS cation interacts with the oxygen in the carbonyl
moiety of acetoin and MEK, thereby polarizing the carbon–
Electrosynthesis of MEK
Influence of pH. The results obtained at room temperature,
100 Qt%, 1500 A cm−2, in a divided cell fitted with a DSA-O2
anode, a Nafion 324 cation exchange membrane and a Pb
cathode, using an aqueous 5 wt% KH2PO4/100 g L−1 acetoin
solution adjusted to the corresponding pH with diluted H3PO4
or KOH as a catholyte and an aqueous 5 wt% H2SO4 solution
as an anolyte, are given in Table 2. A minimum pH of 3.1 was
chosen based on 2,3-BD electrosynthesis in which below that
pH acetoin conversion decreases strongly because of proton
electroreduction to hydrogen. A maximum pH of 7 was
selected as the upper limiting one to avoid losses of acetoin by
Table 2 Influence of pH on MEK electrosynthesis in a divided cella
Table 3 Influence of QAS addition (0.5 wt%) on MEK electrosynthesis
in a divided cella
C (%) =
εMEK
(%)
SMEK
(%)
S2,3-BD
(%)
S2-BuOH
(%)
Entry# pH
εacetb (%)
C (%) =
εMEK
(%)
SMEK
(%)
S2,3-BD
(%)
S2-BuOH
(%)
Entry# QAS
εacetb (%)
15
16
13
17
3.10 78.4
4.10 71.0
5.50 74.3
7.00 77.1
50.6
53.1
57.3
54.3
70.4
74.9
77.2
70.4
7.8
10.3
9.7
2.5
0.0
3.2
4.5
16
18
19
None
71.0
TEABr 84.2
TBABr 87.8
53.1
54.3
49.7
74.9
64.5
56.6
10.3
12.9
24.4
0.0
4.6
6.8
12.0
a Experimental conditions: A divided cell fitted with a DSA-O2 anode, a a Experimental conditions: A divided cell fitted with a DSA-O2 anode, a
Nafion 324 cation exchange membrane and a Pb cathode; anolyte: Nafion 324 cation exchange membrane and a Pb cathode; anolyte:
aqueous 5 wt% H2SO4; catholyte: aqueous 5 wt% KH2PO4/100 g L−1 aqueous 5 wt% H2SO4; catholyte: aqueous 0.5 wt% QAS/5 wt%
acetoin solution adjusted to the corresponding pH with diluted H3PO4 KH2PO4/100 g L−1 acetoin; average reaction pH: 4.10–4.27; 100 Qt%;
or KOH; 100 Qt%; 1500 A m−2; room temperature. b Acetoin conversion 1500 A m−2; room temperature. b Acetoin conversion matches the
matches the acetoin current efficiency because the electric charge used acetoin current efficiency because the electric charge used was the
was the theoretical one: 2 F mol−1 of acetoin.
theoretical one: 2 F mol−1 of acetoin.
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oxygen bond and favoring its hydrogenation by the nascent
hydrogen.
Influence of current density. Variations of conversions and
selectivities as a function of current density in the 750–1500
A m−2 range for Cd and Pb cathodes in a divided cell are
plotted in Fig. 4 under the experimental conditions given in
the legend. Acetoin conversion (= acetoin current efficiency
because the circulated charge was 100 Qt%) remains constant
for Cd at 92% within the current density range studied while
for Pb it remains constant also at 87.5% up to 1000 A m−2 and
then decreases noticeably to 74% at 1500 A m−2. Conversely,
the MEK selectivity remains constant for Pb at 75% in all the
current density intervals studied while it decreases constantly
for Cd from 70.6% at 750 A m−2 to 55.4% at 1500 A m−2. The
Fig. 5 Influence of the initial acetoin concentration. A divided cell
fitted with a DSA-O2 anode, a Nafion 324 cation exchange membrane
and a Pb cathode; anolyte: aqueous 5 wt% H2SO4; catholyte: aqueous
same patterns for 2,3-BD and 2-BuOH selectivities are 5.0 wt% KH2PO4 adjusted at pH 5.5 with 20 wt% KOH/X g L−1 acetoin;
1500 A m−2; 100 Qt%; room temperature. (♦) Acetoin conversion =
acetoin current efficiency because the electric charge was the theore-
observed.
The results can be explained based on the much higher
■
▲
tical one; ( ) SMEK; ( ) S2,3-BD; (•) S2-BuOH; (×): SMEK + S2,3-BD + S2-BuOH
.
hydrogen overpotential of Cd compared to Pb. This allows one
to obtain a high 92% constant current efficiency for acetoin
conversion in the current density range studied but since the
cathode potential increases with current density, and hence conditions given in the legend. As is known, the limiting
the cathode reduction power, the MEK selectivity decreases current density increases with the substrate concentration
continuously at the expense of the increase in 2,3-BD and which explains the increase in the acetoin current efficiency
2-BuOH selectivities. For Pb, however, with a much lower from 74% at 100 g L−1 to 90.5% above 200 g L−1. However,
hydrogen overvoltage than Cd, above 1000 A m−2 the electricity MEK selectivity decreases steadily when the acetoin concen-
is increasingly used in hydrogen generation as shown by the tration increases despite the fact that 2,3-BD and 2-BuOH
strong decrease in acetoin conversion above that current selectivities remain constant. One possibility is that losses of
density. The reduction power of Pb does not increase above acetoin and MEK could occur from the catholyte to the anolyte
1000 A m−2 because at this current density the cathode poten- by diffusion through the membrane resulting in “apparent”
tial matches or it is close to the lead hydrogen overpotential. conversions and selectivities, higher and lower, respectively,
In other words, the hydrogen electrogeneration “protects” than the actual ones. However, this possibility was rejected
acetoin and MEK from being converted into 2,3-BD and after carrying out several tests without electric current at
2-BuOH, respectively, leading to a constant MEK selectivity in acetoin and MEK concentrations of 100, 200 and 300 g L−1
.
,
the current density interval studied. Therefore, Pb is the more Acetoin and MEK diffusion rates were 16.1 and 17.8 g h−1 m−2
suitable cathode for this electrosynthesis for working at independently of the concentration. Considering the durations
current densities required at the industrial level.
of the electrolysis, this means that losses of acetoin and MEK
Influence of the initial acetoin concentration. Variations of would be 0.65% and 1.72% relative to the initial acetoin mass
conversions, selectivities and the sum of selectivities as a func- and final MEK mass, respectively, at most in the worst case.
tion of initial acetoin concentration in a divided cell fitted
This means that chemicals other than MEK, 2,3-BD and
with a Pb cathode are plotted in Fig. 5 under the experimental 2-BuOH are being formed in increasing yields as the initial
Fig. 4 Influence of current density for cadmium and lead cathodes. A divided cell fitted with a DSA-O2 anode, a Nafion 324 cation exchange mem-
brane and a Cd or Pb cathode; anolyte: aqueous 5 wt% H2SO4; catholyte: aqueous 5.0 wt% KH2PO4 adjusted at pH 5.5 with 20 wt% KOH/100 g L−1
■
▲
acetoin; 100 Qt%; room temperature. (♦): acetoin conversion; ( ) SMEK; ( ) S2,3-BD; (•) S2-BuOH
.
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acetoin concentration increases which is confirmed by means stopping the electrolysis when an electrical charge below the
of the sum of MEK, 2,3-BD and 2-BuOH selectivities which theoretical one for full acetoin conversion has been circulated.
decreases from 90.1% at 100 g L−1 to 84.1% at 200 g L−1, and Variations of acetoin current efficiency, selectivities and the
to 77.9% at 300 g L−1. As previously mentioned one of these sum of selectivities as a function of the circulated electric
chemicals was 3,4-dimethyl-2,5-hexanedione (see Scheme 2). charge for an initial acetoin concentration of 200 g L−1 in a
When the initial acetoin concentration increases the average divided cell fitted with a Pb cathode are plotted in Fig. 6 under
concentrations of species such as 2 and 3 in Scheme 2b also the experimental conditions given in the legend.
increase during the electrolysis, as well as that of MEK. This
The only difference relative to the experimental conditions
fact would favor coupling reactions such as coupling of two for the results depicted in Fig. 5 is that the KH2PO4 concen-
radicals 2 to yield 3,4-dimethyl-2,5-hexanedione, and coupling tration was doubled to decrease the cell voltage and hence
of the carbanion 3 with the ketone moieties of acetoin and power consumption. Some herein not reported comparative
MEK leading to the pinacol 4 and the beta-hydroxyketone 5, experiments at both concentrations showed that this increase
respectively, as shown in Scheme 3. Another possibility is that in the supporting electrolyte concentration had no influence
the enhanced acetoin concentration favors the formation of 6 on the results.
through the well-known electroreduction mechanism of
As can be seen, εMEK is above 96% up to 85 Qt% and then
ketones leading to pinacols, also shown in Scheme 3. The for- decreases to 88.7% at 100 Qt%. Likewise, SMEK and the sum of
mation of pinacols in an acidic medium by electrochemical MEK, 2,3-BD and 2-BuOH selectivities remain high up to 75
reductive coupling of ketones and aldehydes is a long time Qt% (80.3% and 84.3%, respectively) and then decrease
known reaction.37–41 GC-MS analysis was only carried out in
one reaction with a low sum of acetoin, MEK and 2-BuOH
selectivities and the only byproduct detected with a significant
area was 2. However, the GC-MS was carried out after the
extraction of the aqueous catholyte with dichloromethane so
that the highly hydrophilic alcoholic species 4, 5 and 6 could
very likely not be extracted.
Working at the highest possible concentration is indust-
rially highly desirable. MEK selectivity decreases from 77% at
100 g L−1 of acetoin to 67.5% at 200 g L−1. The decrease is sig-
nificant, but not dramatic, mainly because the decrease in
selectivity is caused by the increase in acetoin conversion with
its initial concentration and not by a decrease in MEK current
efficiency. In fact, εMEK increases slightly from 57.3% at 100
Fig. 6 Influence of the electrical charge at 200 g L−1 acetoin. A divided
cell fitted with a DSA-O2 anode, a Nafion 324 cation exchange mem-
g L−1 of acetoin to 60.9% at 200 g L−1. If the above hypothesis
is correct, a way of increasing both MEK selectivity and MEK
brane and a Pb cathode; anolyte: aqueous 5 wt% H2SO4; catholyte:
current efficiency by minimizing some of the above cited side
reactions would be to keep the MEK concentration in the
catholyte below a threshold value. At first, this can be done by
aqueous 10 wt% KH2PO4 adjusted at pH 5.5 with 20 wt% KOH/200 g L−1
acetoin; 1500 A m−2; room temperature. (♦) Acetoin current efficiency;
■
▲
(
) SMEK; ( ) S2,3-BD; (•) S2-BuOH; (×) SMEK + S2,3-BD + S2-BuOH
.
Scheme 3 Possible byproducts formed mainly at high initial acetoin concentrations.
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quickly up to values of 64.1% and 72%, confirming the above it has been much lower. In other words, under the experi-
hypothesis. εMEK at 75 Qt% was 77% and 56.8% at 100 Qt%. mental conditions employed the electrolyte concentration has
That is, the negative effect of increasing the initial acetoin con- little or no influence when using HHO cathodes, i.e. when
centration from 100 to 200 g L−1 can be overcome by decreas- MEK is the target chemical. Obviously, further research is
ing the circulated charge from 100% to 75 Qt% leading to needed to confirm this hypothesis.
better results. A comparison between the results under these
Influence of the cell type. The use of undivided flow cells is
concentration-charge conditions is given in Table 4. It can be highly desirable from an industrial standpoint because it leads
seen how by decreasing the circulated charge to 75 Qt% at to reduced CAPEX and OPEX provided similar results to those
200 g L−1 of acetoin, the MEK current efficiency, MEK selecti- in divided cells are achieved. Variations of acetoin current
vity and MEK productivity are higher than those obtained at efficiency and selectivities as a function of current density at
200 and 100 g L−1 for a 100 Qt%. Even, results are improved by 75 Qt% and an acetoin initial concentration of 200 g L−1 in a
decreasing the charge to 50 Qt% but the MEK final concen- divided cell fitted with a Pb cathode are plotted in Fig. 7 under
tration, although higher than that at 100 g L−1 and 100 Qt%, is the experimental conditions given in the legend. 2-BuOH was
lower than that obtained at 200 g L−1 and 75 Qt%. A MEK con- not detected. εMEK are close to 100% for all the current den-
centration of 95–100 g L−1 appears as the maximum one to sities studied but SMEK are much lower remaining constant at
achieve a good selectivity and current efficiency. Above it, side 80% up to 1000 A m−2 and then decreasing to 71.8% at 1500
reactions become increasingly important. The absence of A m−2. At 1500 A m−2, εact are equal in both undivided and
2-BuOH, the product of MEK electroreduction, below 75 Qt% divided (see Fig. 6) cells but SMEK is lower in an undivided cell
confirms this statement.
(71.8%) than in a divided cell (80.3%). However, at 1000
Influence of the electrolyte concentration. This variable has A m−2, both acetoin current efficiency and MEK selectivity are
not been studied. However, it is worth mentioning that the equal within the experimental analytical error: 96% and
results depicted in Fig. 6 were obtained at 10 wt% KH2PO4, the 80.3%, respectively, in the divided cells and 98.8% and 79.5%
concentration where the current efficiency for acetoin conver- in the divided one.
sion was as low as 25% when 2,3-BD was the main product
A new set of experiments were carried out to determine if
obtained using a decorated PtOx-80/GDL cathode at 1000 the results could be improved at 1000 A m−2 by decreasing
A m−2, room temperature, 100 g L−1 acetoin and 100 Qt% as slightly the circulated charge under the other experimental
depicted in Fig. 2. In fact, εact decreased dramatically with the conditions given in the legend of Fig. 7. The results are plotted
electrolyte concentration above 2 wt%. This was attributed to in Fig. 8. 2-BuOH was not detected. As can be seen, SMEK
an increased hydrogen evolution because proton electroreduc-
tion is favored due to an increasing salting-out effect as the
electrolyte concentration increases. However, this does not
occur when MEK is the main product: εact is 88.7% at 100 Qt%
using 10 wt% KH2PO4, as shown in Fig. 6. There are some
differences which can explain this result for MEK. First, the
acetoin concentration in the experiments depicted in Fig. 6 is
200 g L−1, two times that in the experiments shown in Fig. 2.
This means that the acetoin concentration in the double layer
after the salting-out effect could be higher in the former case
than in the latter one. Second, in the experiments depicted in
Fig. 6 a HHO cathode (Pb) was used instead of a LHO cathode.
Therefore, the limiting current density for acetoin conversion
is higher because hydrogen evolution is not favored. Both
differences taken together allow the hypothesis that in the
experiments depicted in Fig. 6 the working current density has
Fig. 7 Influence of current in an undivided cell fitted with a DSA-O2
anode and a Pb cathode; electrolyte: aqueous 5.0 wt% KH2PO4 adjusted
at pH 5.5 with 20 wt% KOH/200 g L−1 acetoin; 75 Qt%; room tempera-
■
▲
been close to the limiting one while in those depicted in Fig. 2 ture. (♦) Acetoin current efficiency; ( ) SMEK; ( ) S2,3-BD; (•) S2-BuOH
.
Table 4 Comparison of the results obtained at several acetoin concentration-circulated charge combinations in a divided cell at 1500 A m−2 and
room temperaturea
[Acetoin] g L−1
Charge, Qt%
εMEK (%)
SMEK (%)
[MEK]final, g L−1
PMEK
b
Entry#
20
21
22
23
100
200
200
200
100
100
75
57.3
60.9
77.0
84.3
77.2
67.5
80.3
83.8
47.0
99.8
94.7
69.0
1.16
1.23
1.55
1.70
50
a For experimental conditions see the legends of Fig. 5 and 6. b MEK productivity, kg h−1 m−2
.
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increases up to 85.7% at 70 Qt% confirming the above con-
clusion that a careful adjustment of the circulated charge can
decrease side reactions significantly.
A comparison of the best results in both types of cells is
given in Table 5. Although a higher MEK productivity is
obtained in a divided cell at 1500 A m−2, the higher MEK
current efficiency and MEK selectivity together with 41.4%
lower power consumption at 1000 A m−2 and the simplicity of
an undivided cell compared to a divided one makes the
undivided cell configuration a better choice for industrial
upscaling.
Anode of steel. DSA anodes are very expensive and constitute
one of the major contributions to the cell cost. Their substi-
Fig. 8 Influence of the circulated electrical charge at 1000 A m−2 in an
undivided cell fitted with a DSA-O2 anode and a Pb cathode; electrolyte: tution for cheap materials such as steel is highly desirable.
aqueous 5.0 wt% KH2PO4 adjusted at pH 5.5 with 20 wt% KOH/200
g
Although not possible in most cases due to severe corrosion, it
L−1 acetoin; room temperature. (♦) Acetoin current efficiency;
has been carried out successfully in the industrial manufactur-
ing of adiponitrile by cathodic electrohydrodimerization of
acrylonitrile wherein a stainless steel anode is used. The
addition of additives such as EDTA and borax in small
amounts to the electrolyte inhibits largely the anode cor-
rosion.33 We have studied this possibility substituting the DSA
anode by carbon steel, the cheaper type of steel. The reactions
were carried out in an undivided cell under the experimental
conditions leading to the best results (see Table 5). The results
are given in Table 6. As can be seen the addition of EDTA
alone (entry 26) and borax alone (entries 27 and 28) does not
inhibit the anode corrosion rate relative to the blank experi-
ment (entry 25) as can be seen by the high corrosion rates
ranging between 61 and 64.2 mm per year. However, the com-
bination of both additives decreases dramatically the carbon
steel corrosion rate, with the lowest one of 2.1 mm per year
being achieved when EDTA and borax are combined in 1 wt%
and 2 wt%, respectively (entry 34). However, this strong 96.7%
reduction in the corrosion rate has a penalty: the reduction in
both MEK current efficiency and selectivity. SMEK decreases
between 9.9 points (compare entries 24 and 29) and 19.7
points (compare entries 24 and 33). More byproducts are being
formed when carbon steel is used as the anode as shown by
the strong decrease in the sum of MEK, 2,3-BD and 2-BuOH
selectivities compared to the use of a DSA-O2 anode. The intro-
■
▲
(
) SMEK; ( ) S2,3-BD
.
Table 5 Electrosynthesis of MEK.a Comparison of the best results for
divided and undivided cells at 200 g L−1 acetoin
Divided
Undivided
j (A m−2
Electrical charge (Qt%)
C (%)
)
1500
75
72
77.0
80.3
6.0
1.55
5.8
1000
70
65.8
80.4
85.7
4.5
εMEK (%)
SMEK (%)
Average voltageb (V)
MEK productivity (kg h−1 m−2
)
1.08
4.1
Power consumptionb (kWh kg−1
)
a Difference in the experimental conditions between divided and un-
divided cells was because the former was fitted with a Nafion 324
cation exchange membrane to separate the anolyte from the catholyte,
and the anolyte was a 5 wt% sulfuric acid aqueous solution. The rest
of the conditions were the same for both cells: anode: DSA-O2;
cathode: Pb; electrolyte (catholyte for a divided cell): aqueous 10 wt%
KH2PO4 adjusted at pH 5.5 with aqueous 20 wt% KOH; room tempera-
ture; interelectrode gap: 8 mm for an undivided cell and 17 mm for a
divided one. b Calculated for a typical industrial interelectrode gap of
4.25 mm and 2 mm for divided and undivided cells, respectively.
Table 6 Results in an undivided cell
Entry#
Anode
EDTA,a wt%
Borax,a wt%
Corrosion rate, mm per year
εMEK, %
SMEK, %
∑Sb, %
24
25
26
27
28
29
30
31
32
33
34
35
DSA-O2
CSc
CS
CS
CS
CS
CS
CS
CS
0
0
1.5
0
0
1.0
1.5
2.0
2.5
1.0
1.0
1.0
0
0
0
0
80.4
73.8
76.1
78.9
82.9
75.1
74.2
71.1
71.3
72.5
72.9
73.5
85.7
72.3
75.8
73.5
83.6
75.8
72.6
71.0
71.2
66.0
67.5
67.9
88.0
77.4
80.5
74.1
90.3
82.9
79.5
77.6
76.7
73.2
74.5
75.9
61.0
64.0
64.2
69.3
2.9
6.4
3.9
3.3
2.9
2.1
2.2
0.5
1.0
1.0
1.0
1.0
1.0
1.5
2.0
2.5
CS
CS
CS
a Relative to the total amount of electrolyte. b Sum of MEK, 2,3-BD and 2-BuOH selectivities. c Carbon steel.
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Table 7 Summary of the best results in an undivided cell
trolysis is reduced to Pt(0). It is thought that this is because
PtOx deposited by PVD on a gas diffusion layer shows a dendri-
tic structure of a high surface area which is held after electro-
reduction to Pt(0).
2,3-BDa
MEKb
C (%)
S (%)
ε (%)
77.6
92.8
71.7%
3.5
65.8
85.7
80.4
4.5
The best results for MEK electrosynthesis are obtained
when Pb is used as a cathode. The use of Cd increases the
current efficiency for acetoin conversion but its higher poten-
tial window before the onset for hydrogen evolution (higher
reduction power) leads to a decrease in MEK selectivity.
pH must be adjusted between 3 and 4 for 2,3-BD electro-
synthesis, and 3–5.5 for MEK electrosynthesis. Below a pH of
3 hydrogen evolution by electroreduction of protons is the
major reaction. The fact that both electrosyntheses can be
done in overlapping pH ranges and with the same electrolyte
is an important finding because it means that both processes
can be operated using the same feeding aqueous solution
making the process highly flexible.
By carefully selecting the circulated electrical charge and
the current density both reactions can be advantageously
carried out in undivided cells. The best results and related
experimental conditions for both processes are summarized in
Table 7.
Flexible production of 2,3-BD or MEK by a simple change of
the cathode using the same undivided cell type and electrolyte,
mild experimental conditions, good productivities and low
power consumptions make these processes suitable for indus-
trial upscaling, mainly considering that there is room for
further improvement as further discussed in the ESI.‡
Average voltagec (V)
Productivity (kg h−1 m−2
)
1.21
1.08
Power consumptionc (kWh kg−1
)
2.94c
4.1c
a Experimental conditions: Undivided cell, anode: DSA-O2, cathode:
PtOx-80/Sigracet® GDL-24BC, electrolyte: aqueous 2.5 wt% KH2PO4
+
4 wt% Na2SO4, pH 3.6, 1000 A m−2, 100 g L−1 acetoin, charge: 100
Qt%, room temperature, interelectrode gap: 8 mm. b Experimental con-
ditions: Undivided cell, anode: DSA-O2, cathode: Pb, electrolyte:
aqueous 10 wt% KH2PO4 adjusted at pH 5.5 with KOH, 1000 A m−2
,
200 g L−1 acetoin, charge: 70 Qt%, room temperature, interelectrode
gap: 8 mm. c Calculated for an industrial interelectrode gap of 2 mm.
duction of ions such as Fe3+ (the corrosion of carbon steel pro-
duces in the first step primarily Fe2+ but it is oxidized quickly to
Fe3+ in the reaction medium) in the electrolyte resulting from
the anode corrosion favors side reactions. Consequently, under
the experimental conditions reported herein, it is not advisable
to use a carbon steel anode. However, taking into account that
the decrease in MEK selectivity has not been catastrophic, it is
worth carrying out further research to find conditions able to
reduce the corrosion rate much more beyond 2.1 mm per year
due to the great economic advantages obtainable.
Conclusions
It has been shown that acetoin can be successfully electroche-
mically reduced in an aqueous medium to 2,3-BD and MEK at
room temperature in both divided and undivided cells using
KH2PO4 as the supporting electrolyte and a DSA-O2 anode
based on IrO2 supported on titanium. The key parameter to
drive the electroreduction towards one or another chemical is
the cathode material. 2,3-BD is the major chemical produced
when low hydrogen overvoltage cathodes, such as Pt and Ni,
are used, while MEK is the principal product produced when
high overvoltage cathodes, such as graphite, Pb and Cd, are
employed. The cause of such an effect is related to the
different electroreduction mechanisms leading to both chemi-
cals. 2,3-BD is obtained by electrohydrogenation and conse-
quently those metals typically used as hydrogenation catalysts
are most suitable to be used as cathodes. However, MEK is syn-
thesized by electrohydrogenolysis and therefore those
materials avoiding or minimizing the hydrogen production by
electrolysis, with a broad potential window before the onset of
hydrogen evolution, are more suitable as cathodes.
Experimental
Materials
All chemicals used were from Sigma-Aldrich, synthetic grade
with a purity higher than 98%, except for acetoin whose purity
was higher than 96%. DSA®-O2 anodes based on iridium oxide
supported on titanium were supplied by Ercros (Spain). The
Pt/Ti anode and Nafion 324 membranes were supplied by
ElectroCell A/S (Denmark). 99.9% pure, 1 mm thick, Ni, Zn, Pb
and Cd sheets used as cathodes were purchased from
Goodfellow GmbH (Germany). The gas diffusion layer used as
the support of decorated cathodes obtained by physical vapor
deposition was Sigracet® GDL-24-BC from the SGL Group
(Germany). 2 mm thick graphite JP 900 plates were supplied by
Mersen Ibérica.
Methods
Fabrication and characterization of decorated electrodes by
PVD. Pt, PtCo and PtNi coatings on Sigracet® GDL-24-BC were
The current efficiency for acetoin conversion and 2,3-BD prepared by PVD magnetron sputtering using a commercial
selectivity increase significantly when catalytic metals are de- equipment (CemeCon® CC800/8) provided with four rectangu-
posited by PVD on a porous high specific surface matrix such lar targets (200 mm × 88 mm × 5 mm). The original PVD was
as a carbon-based gas diffusion layer due to the decrease in fitted with 4 DC power sources and after a modification two AC
the electrode potential, with the metal type and metal load power sources were incorporated. Two different deposition pro-
having also a significant influence. The best results are cedures were followed: DC mode for PtM (M = Ni, Co) and AC
obtained with an 80 µg cm−2 PtOx deposit which during elec- mode for PtOx. Depositions of PtCo and PtNi were carried out
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in DC mode using two magnetrons with the Pt and Co/Ni cath- were attached to stainless steel plates acting as current collec-
odes. Substrates (Sigracet® GDL-24BC supports and silicon tors as previously described.
wafers) for catalyst deposition were vertically fixed and aligned
Electrosynthesis experimental set up
at 350 mm from the cathodes. After a degassing process
including heating under vacuum, argon was introduced into The electrolysis were carried out at a constant current using a
the chamber up to a working pressure from 0.16 to 0.6 Pa. parallel-plate filter press assembly as a flow cell and a Promax
Previously the minimum power required for the Ar plasma FAC-376, 5A, 30 V as a DC power supply. Experiments were
ignition for Ni and Co targets was determined to be similar done in both undivided and divided cells fitted with a DSA®-
and limiting for the deposition process. Therefore, to prepare O2 anode and a cathode, both of 20 cm2 of the effective geo-
catalysts with different stoichiometries, the power applied to metric area. In divided cells anodic and cathodic compart-
the Pt target was modified between 280 and 540 W, fixing the ments were separated by a Nafion 324 cation exchange mem-
power to the Ni or Co target to 200 W. The process time ranged brane and the anolyte was always a 5 wt% aqueous solution of
from 30 seconds to 2 minutes, depending on the desired cata- H2SO4. Interelectrode gaps were 8 mm and 17 mm for undi-
lyst loading.
vided and divided cells, respectively. The electrolyte compart-
PtOx cathodes were prepared using the mid-frequency alter- ments were defined by polypropylene frames and sealing was
nating current (AC) dual technology based magnetron sputter- carried out with Viton gaskets. No turbulence promoter was
ing system under reactive sputtering conditions by employing used. Aqueous solutions consisting of acetoin and a support-
a mixture of oxygen/argon (9 : 1 v/v) sputtering gas in a PVD ing electrolyte were contained in 0.25 L jacketed closed glass
chamber comprising a planar magnetron with a Pt cathode. reservoirs fitted with a condenser to avoid evaporation of vola-
The sample holder was rotated during the deposition process.
tile chemicals such as methyl ethyl ketone. In operation, these
Metals deposited on Sigracet® GDL-24BC supports were aqueous solutions were pumped through their respective com-
used as electrodes in the electrosynthesis of 2,3-butanediol partments (cathodic one in the divided cell and the interelec-
while those deposited on silicon wafers were used to deter- trode one in the undivided cell) in the electrochemical cell by
mine the catalyst mass loading by measuring the weight differ- means of magnetic pumps (lwaki MD-10-R) to provide flow
ence before and after the deposition, and for Raman, SEM and rates of 2 L min−1. Temperature was controlled by recirculating
EDX analyses.
water through the jackets of the reservoirs from a bath a room
The composition of the mixed electrodes PtCo and PtNi was temperature.
determined by EDX. Scanning electron microscopy (SEM)
Analytical procedures
images and the energy dispersive X-ray spectroscopy (EDX)
spectrum for PtxO/Sigracet® GDL-24BC were acquired using Acetoin, 2,3-butanediol, methyl ethyl ketone and 2-butanol
an SEM Inspect™ F50 with an EDX system INCA PentaFETx3 were analyzed by HPLC with a Varian equipment model
(FEI Company, Eindhoven, The Netherlands) in an energy 920-LC fitted with a refractive index detector and a 300 ×
range between 0 and 30 keV. The O/Pt ratio as determined by 7.8 mm Bio-Rad Aminex HPX-87H column under the following
EDX was 2.8 (see Table S1 in the ESI‡) indicating that it con- conditions: mobile phase, 0.01 N H2SO4; mobile phase flow
sisted of a mixture of Pt oxides (PtOx). Nevertheless, Raman rate, 0.7 mL min−1; column temperature, 65 °C; detector temp-
spectra obtained with a inViaTM Raman microscope equipped erature, 50 °C; injection volume, 10 μL. Samples for analysis
with an optical microscope LEICA DMLM confirmed the exist- were prepared by diluting aliquots of the electrolyte into the
ence of α-PtO2 bands (see Fig. S4 in the ESI‡).
mobile phase. The dilution factor was 1 : 50. Before analysis,
Finally, before being fitted in the filter press electro- the samples were filtered through 0.45 μm nylon syringe
chemical cell, decorated GDL-24BC with a geometric area of filters. Quantification was carried using calibration lines
20 cm2 were attached to stainless steel plates acting as current obtained with 50–1000 mg L−1 aqueous solutions of the corres-
collectors by gluing with an electrically carbon conductive ponding standards in the mobile phase.
cement (CCC Carbon Adhesive ref. 12664, Aname
Instrumentación Científica, Spain).
10 mL of a few aqueous electrolytes were extracted with 3 ×
3.5 mL of dichloromethane. The combined organic phases
Fabrication of PtRu (66.6/33.3 w/w)/Sigracet® GDL-24-BC were dried over anhydrous sodium sulfate, filtered, concen-
electrodes. A commercial Pt(20 wt%)–Ru(10 wt%) on a carbon trated by air stripping and analyzed by GC-MS for the identifi-
black powder catalyst (HiSPEC®-5000 from Johnson Matthey, cation of byproducts using an Agilent GC (7890B)-MS detector
32 mg) was suspended in 20 mL of iso-propyl alcohol. Then, (5977A) apparatus with a 30 m × 0.25 mm × 0.25 mm HP-5 ms
64 mg of a 5 wt% Nafion® 117 solution in a mixture of lower capillary GC column under the following experimental con-
aliphatic alcohols and water (Sigma-Aldrich) was added. The ditions: injector temperature: 300 °C; detector temperature:
suspension was homogenized by combining magnetic stirring 230 °C; injection volume: 1 μL; split ratio: 1 : 20; oven tempera-
with ultra-sonication at room temperature until a homo- ture ramp: 40 °C for 5 min, 10 °C min−1 up to 300 °C, then
geneous ink was obtained. PtRu (66.6/33.3 w/w) electrodes 5 min at 300 °C, total time for analysis: 36 min; acquisition
with different loads were prepared by spraying this ink on a mode: SCAN, with masses from 26 to 250 being scanned for
Sigracet® GDL-24-BC support placed on a hot plate at 70 °C– the first 2 minutes and masses from 30 to 500 for the remain-
80 °C for drying. Electrodes with a geometric area of 20 cm2 ing analysis time.
176 | Green Chem., 2019, 21, 164–177
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Paper
13 J. R. Ochoa-Gómez, S. Gil-Río, F. Río-Pérez, L. Lorenzo-
Ibarreta, C. Diñeiro-García and T. Roncal Martínez, US Pat,
Abbreviations
C (%)
Si (%)
Acetoin conversion
9975827B2, 2018.
Selectivity for i = 2,3-BD (2,3-butanediol), MEK 14 J. R. Ochoa-Gómez, S. Pérez-Gil, M. M. Díaz De Guereñu
(methyl ethyl ketone), 2-BuOH (2-butanol)
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2-BuOH (2-butanol)
Current density relative to the electrode geometric
area.
Theoretical charge for full acetoin conversion assum-
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