electrochemical synthesis of peroxyacetic acid using
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1
Electrochemical Synthesis of Peroxyacetic Acid using 1
Conductive Diamond Electrodes 2
Salvador Cotillas, Ana Sánchez-Carretero, Pablo Cañizares, Cristina Sáez and Manuel 3
Andrés Rodrigo*. 4
Department of Chemical Engineering. Facultad de Ciencias Químicas. Universidad de 5
Castilla-La Mancha. Campus Universitario s/n. 13071 Ciudad Real. Spain. 6
7
8
Abstract 9
In this work, the synthesis of peroxyacetic acid and peroxoacetate salts by electrolysis 10
with conductive-diamond anodes is described, using three different raw materials: 11
ethanol, acetaldehyde and sodium acetate. Results show that peroxyacetic acid is 12
produced at significant concentrations during the electrolyses of the three raw materials, 13
although sodium acetate achieves the highest efficiency and, at the same time, the 14
smallest carbon mineralization. Acetaldehyde behaves as a good raw material during a 15
first stage but afterwards it catalyses the decomposition of peroxoacetate to the acetate 16
anion. Moderate current densities and slightly alkaline pH seems to promote the 17
synthesis of peroxyacetic acid and to restrain raw matter mineralization. 18
19
Keywords: Electrosynthesis, peroxyacetic acid, conductive diamond 20
*To whom all correspondence should be addressed. 21
Tel. +34 902204100 ext 3411 22
Fax. +34 926 295256. 23
e-mail: manuel.rodrigo@uclm.es 24
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2
1. Introduction 31
32
Peroxyacetic acid (PAA), also known as peracetic acid, is an organic peroxide in which 33
the carboxylic group of acetic acid (-COOH) is transformed into a peroxocarboxylic 34
group (- COOOH). It is a strong oxidant (E = 1.962 V vs SHE) with a reduction 35
potential larger than those of well- known oxidants, such as chlorine or chlorine 36
dioxide1. Depending on operating conditions, the PAA can present bleaching and 37
delignification properties2. For this reason, the PAA is used as bleaching agent and in 38
the industrial synthesis of epoxides2, 3. It is a very effective oxidant in water treatment4, 39
5. It is also used as a fowl sanitizer, and it is known to be a good disinfectant whose 40
action is based on the hydroxyl radical. PAA is a more potent antimicrobial agent than 41
hydrogen peroxide, being rapidly active at low concentrations against a wide spectrum 42
of microorganisms6-8. Thus, it has been found that hydrogen peroxide required much 43
larger doses than PAA for the same level of disinfection9. 44
45
PAA is commercially available in the form of a quaternary equilibrium mixture 46
containing acetic acid, hydrogen peroxide, PAA, and water. It can be produced from 47
acetic acid and also from acetaldehyde. The first process (eq 1) consists of the oxidation 48
of acetic acid by means of hydrogen peroxide10-12, catalysed by sulphuric acid. It 49
requires long reaction times. 50
51
CH3COOH + H2O2 → CH3COOOH + H2O (1) 52
53
The second process uses acetaldehyde as raw matter. The oxidation of acetaldehyde 54
with air or oxygen produces the initiation of the reaction by means of an acetyl radical 55
3
which forms the peroxide radical with oxygen. The reaction ends with PAA formation13 56
according to eq. 2. 57
58
CH3CHO + O2 → CH3COOOH (2) 59
60
However, the peroxyacetic acid can experience a homolysis of the peroxide group and 61
become to acetic acid. In addition, it is supposed that peroxyacetic acid reacts with 62
acetaldehyde giving α-hydroxiethylperacetate. Later, for a cyclical mechanism of 63
transition, it dissociates itself in two molecules of acetic acid (eq. 3). Therefore, PAA 64
can be the main product if the oxidation is realized in soft conditions, preferably without 65
catalyst and in a solvent such as ethyl acetate. 66
67
68
69
In recent years, electrochemical oxidation with conductive-diamond anodes has become 70
one of the most promising technologies in the treatment of industrial wastes polluted 71
with organics14-24 and in the electrosynthesis of oxidants25-33. Compared with other 72
electrode materials, conductive-diamond has shown higher chemical and 73
electrochemical stability as well as a higher current efficiency. In addition, the high 74
overpotential for water electrolysis is one of the most important properties of 75
conductive-diamond in the processing of aqueous solutions; its electrochemical window 76
is sufficiently large to produce hydroxyl radicals with high efficiency34, and this species 77
seems to be directly involved in the oxidation mechanisms that occur on diamond 78
surfaces. As a result, conductive-diamond electro-oxidation of waste is presently 79
CH3CHO + H-O-O-CCH3
O
2 CH3COOH (3)CH3CHO + H-O-O-CCH3
C
C
H3C
HO O O
C
OH
CH3 2 CH3COOH (3)
4
considered as an advanced oxidation technology. The advantageous properties described 80
above have led to great improvements (in both efficiency and electrode stability) in the 81
use of conductive diamond in the electrosynthesis of oxidants. A number of recent 82
studies have been published in which the generation of peroxodisulphates35, 83
peroxodiphosphate31, percarbonates36, ferrates32, 33 and perchlorates29 by electrochemical 84
techniques are described. Significant improvements are reported as compared to more 85
commonly used synthesis methods. 86
87
With this background in mind, the goal of the work described here was to investigate 88
the feasibility of generating peroxoacetic acid and its salts by electrolyses with 89
conductive-diamond electrodes. In an effort to achieve this aim, bench-scale 90
electrolysis assays of different raw material solutions on conductive diamond anodes 91
were carried out in order to characterize the mechanism of the process and to clarify the 92
role of the most significant process parameters (pH and current density). 93
94
2. Experimental. 95
96
2.1. Analytical procedures. Acetate standard solutions were prepared by dissolving the 97
appropriate amount of CH3COONa in water. Peroxyacetic acid was determined by 98
titration with Na2S2O3. The titration is based on the next procedure37: 4 ml of a 25% 99
(v/v) H2SO4 solution were added to a l0 ml PAA sample. An excess of solid KI was 100
added and the iodide was oxidized by PAA to brownish iodine, then it was titrated with 101
0.01 M Na2S2O3 until only a pale brown color remained (eqs. 4 and 5). One drop of 1% 102
(w/v) starch solution was added and the titration was continued until complete 103
decolorization occurred. 104
5
2I- + CH3COOO- + 2H+ → CH3COO- + I2 + H2O (4) 105
I2 + 2Na2S2O3 → 2NaI + Na2S4O6 (5) 106
107
To dismiss the other oxidants and hydrogen peroxide presence, it was carried out 108
different analytical techniques such as gas chromatography and permanganate addition. 109
Gas chromatographic analyses were carried out in a column SUPELCOWAX 10 (30 m 110
x 0.25 mm) (macroporous particles with 0.25 µm diameter). Volume injection was set 111
to 10 μL. To oxidize hydrogen peroxide, 0.02 M KMnO4 solution was added until the 112
color of the solution turned pale rose. Oxidant stability rules out the hydroxyl radicals, 113
ozone and other unstable oxidants presence. Therefore the oxidant electrogenerated 114
comes from the partial oxidation of raw material forming a peroxo group. 115
116
2.2. Electrochemical cell. The electrosynthesis was carried out in a double-117
compartment electrochemical flow cell. A cationic exchange membrane (STEREOM L-118
105) was used to separate the compartments. Diamond-based material was used as 119
anode and stainless steel (AISI 304) as cathode. Both electrodes were circular (100 mm 120
diameter) with a geometric area of 78 cm2 each and an electrode gap of 15 mm. Boron-121
doped diamond (BDD) films were provided by Adamant (Switzerland) and synthesised 122
by the hot filament chemical vapour deposition technique (HF CVD) on single-crystal 123
p-type Si <100> wafers (0.1 cm, Siltronix).The anolyte and the catholyte were stored 124
in dark glass tanks and circulated through the electrolytic cell by means of a centrifugal 125
pump. A heat exchanger was used to maintain the temperature at the desired set point. 126
The pH was monitored by means of the WTW-InoLab pHmeter. 127
128
6
2.3. Experimental Procedures. Bench scale electrolyses under galvanostatic conditions 129
were carried out to determine the influence of the main parameters in the process. The 130
anolyte and the catholyte consisted of 1 M solutions of ethanol (C2H6O), aceltadehyde 131
(C2H4O) or sodium acetate (CH3COONa). The range of current densities employed was 132
300 to 1000 A m-2. The range of pH studied was 1 to 13. 133
134
3. Results and discussion. 135
136
Figure 1 shows the influence of the raw matter with the applied electric charge, on the 137
production of PAA into the anodic chamber of a double compartment electrochemical 138
cell equipped with conductive-diamond anodes. This Figure also shows the 139
mineralization of carbon obtained during the same electrolyses. The experiments were 140
carried out with three different organic raw materials (ethanol, acetaldehyde and sodium 141
acetate in 1M aqueous solutions) at a current density of 300 A m-2, regulating the 142
temperature at 25oC during the process. As it can be observed, PAA can be obtained 143
from the three species tested with a significant production rate (C2H6O: 0.061 mmol/Ah; 144
C2H4O: 0.203 mmol/Ah; CH3COONa: 0.033 mmol/Ah). However, acetate behaves as 145
the best raw material, because it allows obtaining the best faradaic efficiencies in the 146
production of PAA and, at the same time, it obtains the lower mineralization 147
percentage. Initially, this can be easily interpreted in terms of the hydroxylation of the 148
carboxylic group by means of hydroxyl radicals produced on the surface of the diamond 149
surface (eq 6-9). 150
151
H2O (OH)· + H+ + e- (6) 152
CH3-COOH + OH· (CH3-COO) · + H2O (7) 153
7
(CH3-COO)· + OH· CH3-COOOH (8) 154
155
In this point, it is worth to mention that this process has been previously reported and 156
extensively explained for the production of inorganic peroxosalts such as 157
peroxosulphates and peroxophosphates, obtained from the electrolyses with diamond of 158
sulphates or phosphates solutions25- 35. In those cases, processes are directly related to 159
the production of hydroxyl radicals (or, at least, significantly contributed), which 160
produces the formation of radicals of the anions that in later stages yield the peroxoacid 161
or peroxosalt (depending on the operation pH) as shown in eqs. 9-10 for 162
peroxodisulphate, eqs. 11-12 for peroxodiphosphate and eqs. 11 and 13 for 163
monoperoxophosphoric acid. 164
165
HSO4– + OH· SO4
– · + H2O (9) 166
SO4– · + SO4
– S2O82– (10) 167
HPO43- + OH· (PO4
2-) · + H2O (11) 168
(PO42-) · + (PO4
2-) · P2O84- (12) 169
(H2PO4)· + OH· H3PO5 (13) 170
171
The greater mineralization obtained in the case of the acetaldehyde, and especially in 172
the case of the ethanol, indicates the small selectivity of the conductive-diamond 173
electrolyses towards the direct transformation of the hydroxyl and the aldehyde groups 174
into the peroxocarboxylic one, by the direct attack of oxygenated groups in the organic 175
molecules, and prevent against their use as raw materials. 176
177
8
At this point, it is interesting to observe the changes of the main intermediates found 178
during electrolyses of ethanol and acetaldehyde (Figure 2). As it is shown, during 179
ethanol electrolysis both acetaldehyde and acetic acid are produced, although the 180
concentrations of acetic acid measured were very small. In addition, two other 181
intermediates were also monitored during the electrolyses but their concentrations were 182
negligible and they were not identified (they do not correspond to any C1 or C2 183
alcohols, acids or aldehydes). 184
185
The changes observed for the electrolyses of acetaldehyde are of a great interest, 186
because initially this raw matter shows to be the most efficient in the electrochemical 187
production of the organic peroxoacid, but the trend in the production of PAA meets a 188
maximum and then, it decreases to finally meet a constant value. A possible explanation 189
for such behaviour is the decomposition of PAA catalyzed by acetaldehyde (eq 14), 190
because the plateau is obtained just in the moment in which acetaldehyde is completely 191
depleted from solution. 192
193
194 (14)
195
At this respect, during the electrolyses of acetaldehyde, significant amounts of acetate 196
and smaller concentrations of the two other intermediates were found. This supports the 197
less efficient production of PAA at long reaction times. 198
199
Concerning the other raw matter studied, no intermediates were found by HPLC during 200
the electrolyses of acetate solutions, indicating that mineralization and PAA formation 201
CH3CHO + H-O-O-CCH3
O
2 CH3COOH
9
are the lone processes occurring in the anodic chamber of the electrochemical cell, and 202
that acetate is the best choice as raw matter for PAA production. 203
204
Figure 3 shows the effect of the pH (range studied from 1 to 13) during the electrolyses 205
of solutions containing 1 M CH3COONa. Initially, the pH was expected to have a great 206
influence on the species formed during the electrosynthesis, but surprisingly no 207
intermediates different of PAA were detected during the electrolyses of acetate at any 208
pH range. In addition, it can be observed that the effect is not very important on the 209
PAA production (merely a small increase for both slightly alkaline pH and highly acidic 210
pHs). However it is very significant for mineralization which it is promoted at strongly 211
acidic and alkaline pHs. This advises against the use of extreme pHs in the synthesis of 212
PAA and suggests to use slightly alkaline pHs (range from pH 7 to 10) in order to 213
optimize the yield and simultaneously not promoting carbon mineralization. 214
215
Figure 4 shows the effect of the current density on the sodium acetate electrolysis. In 216
these experiments, the pH is not regulated but simply monitored, and the temperature 217
was maintained in 25ºC. As it can be observed, the initial rate of the oxidants generation 218
does not depend of the current density applied. However, from applied current charges 219
around 10-15 A h dm-3, there is a significant change in the slope (production rate of 220
PAA) for the larger current densities, decreasing significantly the efficiency of the 221
process (although the trend remains linear at any time). At the same time, the 222
mineralization rate is maintained in every case in a very low value and it is not 223
increased by the current density but just maintained (Figure 4b). This can only be 224
explained in terms of the decomposition of PAA to yield oxygen and acetic acid, which 225
seems to be promoted working under harder oxidation conditions. 226
10
4. Conclusions. 227
228
From this work the following conclusions can be drawn: 229
230
Conductive-diamond electrolysis can be successfully used to synthesize PAA 231
from sodium acetate, acetaldehyde and ethanol solutions, although the efficiency 232
and the intermediates are largely affected by the choice of the raw matter. 233
Sodium acetate is the best raw matter because its electrolysis does not promote 234
carbon mineralization and yield merely PAA. Efficiency of the process improves 235
working at current densities close to 300 A m-2 (enough to produce hydroxyl 236
radicals in the electrolytic cell used). Concerning the pH, it does not affect 237
significantly to the efficiency but it can be advised to work under slightly 238
alkaline pHs in order optimize the yield and simultaneously not to promote 239
carbon mineralization. 240
Acetaldehyde seems to behave as a good raw matter for low applied current 241
charges but a side reaction prevents against its use because of the decomposition 242
of the PAA formed. 243
Ethanol electrolysis yields many intermediates and promotes a high 244
mineralization of carbon. 245
246
247
248
249
250
11
Acknowledgements 251
252
This work was supported by the JCCM (Junta de Comunidades de Castilla La Mancha, 253
Spain) through the project PEII11-0097-2026. 254
255
References 256
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361
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365
366
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368
369
370
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Figure captions 371
372
Fig 1. Variation of the PAA concentration (part a) and carbon mineralization (part b) 373
during the electrolyses with BDD anodes (j 300 Am-2, T 25ºC) of three different raw 374
materials: 1 M C2H6O (▲), 1 M C2H4O (□), 1 M CH3COONa(♦). 375
376
Fig. 2. Main intermediates found during the electrolyses with BDD anodes (j 300 Am-2, 377
T 35ºC) of C2H4O (part a) and C2H6O (part b). acetic acid; acetaldehyde; 378
ethanol; * non identified product 1; + non identified product 2 379
380
Fig. 3. Variation of oxidants concentration (part a) and carbon mineralization (part b) 381
with pH during the electrolysis of CH3COONa solutions at different current charge 382
passed. (1 M CH3COONa, j 300 Am-2, T 25ºC). 383
384
Fig. 4. Variation of oxidants concentration (part a) and total organic carbon (part b) with 385
current density during the electrolysis of CH3COONa solutions. (□) 300 Am-2, (■) 600 386
Am-2, (▲) 1000 Am-2 (1 M CH3COONa, T 25ºC). 387
388
389
390
391
392
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395
16
396
397
398
399
Figure 1 400
401
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Figure 2 408
409
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411
0
0.2
0.4
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0.8
1
0 5 10 15 20 25 30
PA
A /
mM
Q/ Ah dm-3
0
20
40
60
0 5 10 15 20 25 30
Car
bo
n m
iner
aliz
ed
/%
Q/ Ah dm-3
a
b
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0 5 10 15 20 25 30
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a
Q / Ah dm-3
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10000
15000
20000
0 5 10 15 20 25 30
Chro
mat
ogra
phic
ar
ea
Q / Ah dm-3
a b
17
412
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415
Figure 3 416
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Figure 4 425
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0
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0 2 4 6 8 10 12 14
PA
A /
mM
pH/ units
0
10
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50
0 2 4 6 8 10 12 14
min
eral
izat
ion
/ %
pH/ units
5 kA h m-3
25 kA h m-3
50 kA h m-3
5 kA h m-3
25 kA h m-3
50 kA h m-3
a
b
0
0.1
0.2
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0 20 40 60 80 100 120
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C /
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Co
Q / Ah dm-3
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