Electrochemical Synthesis of Peroxyacetic Acid Using Conductive

1 Electrochemical Synthesis of Peroxyacetic Acid using

2 Conductive Diamond Electrodes

3 Salvador Cotillas, Ana Sánchez-Carretero, Pablo Cañizares, Cristina Sáez and Manuel 4 Andrés Rodrigo*. 5 Department of Chemical Engineering. Facultad de Ciencias Químicas. Universidad de 6 Castilla-La Mancha. Campus Universitario s/n. 13071 Ciudad Real. Spain. 7 8 9 Abstract

10 In this work, the synthesis of peroxyacetic acid and peroxoacetate salts by electrolysis

11 with conductive-diamond anodes is described, using three different raw materials:

12 ethanol, acetaldehyde and sodium acetate. Results show that peroxyacetic acid is

13 produced at significant concentrations during the electrolyses of the three raw materials,

14 although sodium acetate achieves the highest efficiency and, at the same time, the

15 smallest carbon mineralization. Acetaldehyde behaves as a good raw material during a

16 first stage but afterwards it catalyses the decomposition of peroxoacetate to the acetate

17 anion. Moderate current densities and slightly alkaline pH seems to promote the

18 synthesis of peroxyacetic acid and to restrain raw matter mineralization.

20 Keywords: Electrosynthesis, peroxyacetic acid, conductive diamond

21 *To whom all correspondence should be addressed. 22 Tel. +34 902204100 ext 3411 23 Fax. +34 926 295256. 24 e-mail: [email protected] 25

31 1. Introduction 32

33 Peroxyacetic acid (PAA), also known as peracetic acid, is an organic peroxide in which

34 the carboxylic group of acetic acid (-COOH) is transformed into a peroxocarboxylic

35 group (- COOOH). It is a strong oxidant (E = 1.962 V vs SHE) with a reduction

36 potential larger than those of well- known oxidants, such as chlorine or chlorine

37 dioxide1. Depending on operating conditions, the PAA can present bleaching and

38 delignification properties2. For this reason, the PAA is used as bleaching agent and in

39 the industrial synthesis of epoxides2, 3. It is a very effective oxidant in water treatment4,

40 5. It is also used as a fowl sanitizer, and it is known to be a good disinfectant whose

41 action is based on the hydroxyl radical. PAA is a more potent antimicrobial agent than

42 hydrogen peroxide, being rapidly active at low concentrations against a wide spectrum

43 of microorganisms6-8. Thus, it has been found that hydrogen peroxide required much

44 larger doses than PAA for the same level of disinfection9.

46 PAA is commercially available in the form of a quaternary equilibrium mixture

47 containing acetic acid, hydrogen peroxide, PAA, and water. It can be produced from

48 acetic acid and also from acetaldehyde. The first process (eq 1) consists of the oxidation

49 of acetic acid by means of hydrogen peroxide10-12, catalysed by sulphuric acid. It

50 requires long reaction times.

52 CH3COOH + H2O2 → CH3COOOH + H2O (1)

54 The second process uses acetaldehyde as raw matter. The oxidation of acetaldehyde

55 with air or oxygen produces the initiation of the reaction by means of an acetyl radical

56 which forms the peroxide radical with oxygen. The reaction ends with PAA formation13

57 according to eq. 2.

59 CH3CHO + O2 → CH3COOOH (2)

61 However, the peroxyacetic acid can experience a homolysis of the peroxide group and

62 become to acetic acid. In addition, it is supposed that peroxyacetic acid reacts with

63 acetaldehyde giving α-hydroxiethylperacetate. Later, for a cyclical mechanism of

64 transition, it dissociates itself in two molecules of acetic acid (eq. 3). Therefore, PAA

65 can be the main product if the oxidation is realized in soft conditions, preferably without

66 catalyst and in a solvent such as ethyl acetate.

OC H3C H O

CHCH3CHO3CHO + + H H-O-O-O-O-CCH-CCH33 C C CH3 22 CH CH3COOH3COOH (3) (3) HO O O 68

70 In recent years, electrochemical oxidation with conductive-diamond anodes has become

71 one of the most promising technologies in the treatment of industrial wastes polluted

72 with organics14-24 and in the electrosynthesis of oxidants25-33. Compared with other

73 electrode materials, conductive-diamond has shown higher chemical and

74 electrochemical stability as well as a higher current efficiency. In addition, the high

75 overpotential for water electrolysis is one of the most important properties of

76 conductive-diamond in the processing of aqueous solutions; its electrochemical window

77 is sufficiently large to produce hydroxyl radicals with high efficiency34, and this species

78 seems to be directly involved in the oxidation mechanisms that occur on diamond

79 surfaces. As a result, conductive-diamond electro-oxidation of waste is presently

80 considered as an advanced oxidation technology. The advantageous properties described

81 above have led to great improvements (in both efficiency and electrode stability) in the

82 use of conductive diamond in the electrosynthesis of oxidants. A number of recent

83 studies have been published in which the generation of peroxodisulphates35,

84 peroxodiphosphate31, percarbonates36, ferrates32, 33 and perchlorates29 by electrochemical

85 techniques are described. Significant improvements are reported as compared to more

86 commonly used synthesis methods.

87 88 With this background in mind, the goal of the work described here was to investigate

89 the feasibility of generating peroxoacetic acid and its salts by electrolyses with

90 conductive-diamond electrodes. In an effort to achieve this aim, bench-scale

91 electrolysis assays of different raw material solutions on conductive diamond anodes

92 were carried out in order to characterize the mechanism of the process and to clarify the

93 role of the most significant process parameters (pH and current density).

95 2. Experimental.

97 2.1. Analytical procedures. Acetate standard solutions were prepared by dissolving the

98 appropriate amount of CH3COONa in water. Peroxyacetic acid was determined by

37 99 titration with Na2S2O3. The titration is based on the next procedure : 4 ml of a 25%

100 (v/v) H2SO4 solution were added to a l0 ml PAA sample. An excess of solid KI was

101 added and the iodide was oxidized by PAA to brownish iodine, then it was titrated with

102 0.01 M Na2S2O3 until only a pale brown color remained (eqs. 4 and 5). One drop of 1%

103 (w/v) starch solution was added and the titration was continued until complete

104 decolorization occurred.

- - + - 105 2I + CH3COOO + 2H → CH3COO + I2 + H2O (4)

106 I2 + 2Na2S2O3 → 2NaI + Na2S4O6 (5)

107 108 To dismiss the other oxidants and hydrogen peroxide presence, it was carried out

109 different analytical techniques such as gas chromatography and permanganate addition.

110 Gas chromatographic analyses were carried out in a column SUPELCOWAX 10 (30 m

111 x 0.25 mm) (macroporous particles with 0.25 µm diameter). Volume injection was set

112 to 10 μL. To oxidize hydrogen peroxide, 0.02 M KMnO4 solution was added until the

113 color of the solution turned pale rose. Oxidant stability rules out the hydroxyl radicals,

114 ozone and other unstable oxidants presence. Therefore the oxidant electrogenerated

115 comes from the partial oxidation of raw material forming a peroxo group.

116

117 2.2. Electrochemical cell. The electrosynthesis was carried out in a double-

118 compartment electrochemical flow cell. A cationic exchange membrane (STEREOM L-

119 105) was used to separate the compartments. Diamond-based material was used as

120 anode and stainless steel (AISI 304) as cathode. Both electrodes were circular (100 mm

121 diameter) with a geometric area of 78 cm2 each and an electrode gap of 15 mm. Boron-

122 doped diamond (BDD) films were provided by Adamant (Switzerland) and synthesised

123 by the hot filament chemical vapour deposition technique (HF CVD) on single-crystal

124 p-type Si <100> wafers (0.1  cm, Siltronix).The anolyte and the catholyte were stored

125 in dark glass tanks and circulated through the electrolytic cell by means of a centrifugal

126 pump. A heat exchanger was used to maintain the temperature at the desired set point.

127 The pH was monitored by means of the WTW-InoLab pHmeter.

128

129 2.3. Experimental Procedures. Bench scale electrolyses under galvanostatic conditions

130 were carried out to determine the influence of the main parameters in the process. The

131 anolyte and the catholyte consisted of 1 M solutions of ethanol (C2H6O), aceltadehyde

132 (C2H4O) or sodium acetate (CH3COONa). The range of current densities employed was

133 300 to 1000 A m-2. The range of pH studied was 1 to 13.

134

135 3. Results and discussion.

136

137 Figure 1 shows the influence of the raw matter with the applied electric charge, on the

138 production of PAA into the anodic chamber of a double compartment electrochemical

139 cell equipped with conductive-diamond anodes. This Figure also shows the

140 mineralization of carbon obtained during the same electrolyses. The experiments were

141 carried out with three different organic raw materials (ethanol, acetaldehyde and sodium

142 acetate in 1M aqueous solutions) at a current density of 300 A m-2, regulating the

143 temperature at 25oC during the process. As it can be observed, PAA can be obtained

144 from the three species tested with a significant production rate (C2H6O: 0.061 mmol/Ah;

145 C2H4O: 0.203 mmol/Ah; CH3COONa: 0.033 mmol/Ah). However, acetate behaves as

146 the best raw material, because it allows obtaining the best faradaic efficiencies in the

147 production of PAA and, at the same time, it obtains the lower mineralization

148 percentage. Initially, this can be easily interpreted in terms of the hydroxylation of the

149 carboxylic group by means of hydroxyl radicals produced on the surface of the diamond

150 surface (eq 6-9).

151

· + 152 H2O  (OH) + H + e- (6)

· · 153 CH3-COOH + OH  (CH3-COO) + H2O (7)

· · 154 (CH3-COO) + OH  CH3-COOOH (8)

155

156 In this point, it is worth to mention that this process has been previously reported and

157 extensively explained for the production of inorganic peroxosalts such as

158 peroxosulphates and peroxophosphates, obtained from the electrolyses with diamond of

159 sulphates or phosphates solutions25- 35. In those cases, processes are directly related to

160 the production of hydroxyl radicals (or, at least, significantly contributed), which

161 produces the formation of radicals of the anions that in later stages yield the peroxoacid

162 or peroxosalt (depending on the operation pH) as shown in eqs. 9-10 for

163 peroxodisulphate, eqs. 11-12 for peroxodiphosphate and eqs. 11 and 13 for

164 monoperoxophosphoric acid.

165

– · – · 166 HSO4 + OH  SO4 + H2O (9)

– · – 2– 167 SO4 + SO4  S2O8 (10)

3- · 2- · 168 HPO4 + OH  (PO4 ) + H2O (11)

2- · 2- · 4- 169 (PO4 ) + (PO4 )  P2O8 (12)

· · 170 (H2PO4) + OH  H3PO5 (13)

171

172 The greater mineralization obtained in the case of the acetaldehyde, and especially in

173 the case of the ethanol, indicates the small selectivity of the conductive-diamond

174 electrolyses towards the direct transformation of the hydroxyl and the aldehyde groups

175 into the peroxocarboxylic one, by the direct attack of oxygenated groups in the organic

176 molecules, and prevent against their use as raw materials.

177

178 At this point, it is interesting to observe the changes of the main intermediates found

179 during electrolyses of ethanol and acetaldehyde (Figure 2). As it is shown, during

180 ethanol electrolysis both acetaldehyde and acetic acid are produced, although the

181 concentrations of acetic acid measured were very small. In addition, two other

182 intermediates were also monitored during the electrolyses but their concentrations were

183 negligible and they were not identified (they do not correspond to any C1 or C2

184 alcohols, acids or aldehydes).

185

186 The changes observed for the electrolyses of acetaldehyde are of a great interest,

187 because initially this raw matter shows to be the most efficient in the electrochemical

188 production of the organic peroxoacid, but the trend in the production of PAA meets a

189 maximum and then, it decreases to finally meet a constant value. A possible explanation

190 for such behaviour is the decomposition of PAA catalyzed by acetaldehyde (eq 14),

191 because the plateau is obtained just in the moment in which acetaldehyde is completely

192 depleted from solution.

193 O 194 CH3CHO + H-O-O-CCH3 2 CH3COOH (14)

195

196 At this respect, during the electrolyses of acetaldehyde, significant amounts of acetate

197 and smaller concentrations of the two other intermediates were found. This supports the

198 less efficient production of PAA at long reaction times.

199

200 Concerning the other raw matter studied, no intermediates were found by HPLC during

201 the electrolyses of acetate solutions, indicating that mineralization and PAA formation

202 are the lone processes occurring in the anodic chamber of the electrochemical cell, and

203 that acetate is the best choice as raw matter for PAA production.

204

205 Figure 3 shows the effect of the pH (range studied from 1 to 13) during the electrolyses

206 of solutions containing 1 M CH3COONa. Initially, the pH was expected to have a great

207 influence on the species formed during the electrosynthesis, but surprisingly no

208 intermediates different of PAA were detected during the electrolyses of acetate at any

209 pH range. In addition, it can be observed that the effect is not very important on the

210 PAA production (merely a small increase for both slightly alkaline pH and highly acidic

211 pHs). However it is very significant for mineralization which it is promoted at strongly

212 acidic and alkaline pHs. This advises against the use of extreme pHs in the synthesis of

213 PAA and suggests to use slightly alkaline pHs (range from pH 7 to 10) in order to

214 optimize the yield and simultaneously not promoting carbon mineralization.

215

216 Figure 4 shows the effect of the current density on the sodium acetate electrolysis. In

217 these experiments, the pH is not regulated but simply monitored, and the temperature

218 was maintained in 25ºC. As it can be observed, the initial rate of the oxidants generation

219 does not depend of the current density applied. However, from applied current charges

220 around 10-15 A h dm-3, there is a significant change in the slope (production rate of

221 PAA) for the larger current densities, decreasing significantly the efficiency of the

222 process (although the trend remains linear at any time). At the same time, the

223 mineralization rate is maintained in every case in a very low value and it is not

224 increased by the current density but just maintained (Figure 4b). This can only be

225 explained in terms of the decomposition of PAA to yield oxygen and acetic acid, which

226 seems to be promoted working under harder oxidation conditions.

227 4. Conclusions.

228

229 From this work the following conclusions can be drawn:

230

231  Conductive-diamond electrolysis can be successfully used to synthesize PAA

232 from sodium acetate, acetaldehyde and ethanol solutions, although the efficiency

233 and the intermediates are largely affected by the choice of the raw matter.

234  Sodium acetate is the best raw matter because its electrolysis does not promote

235 carbon mineralization and yield merely PAA. Efficiency of the process improves

236 working at current densities close to 300 A m-2 (enough to produce hydroxyl

237 radicals in the electrolytic cell used). Concerning the pH, it does not affect

238 significantly to the efficiency but it can be advised to work under slightly

239 alkaline pHs in order optimize the yield and simultaneously not to promote

240 carbon mineralization.

241  Acetaldehyde seems to behave as a good raw matter for low applied current

242 charges but a side reaction prevents against its use because of the decomposition

243 of the PAA formed.

244  Ethanol electrolysis yields many intermediates and promotes a high

245 mineralization of carbon.

246

247

248

249

250

251 Acknowledgements

252

253 This work was supported by the JCCM (Junta de Comunidades de Castilla La Mancha,

254 Spain) through the project PEII11-0097-2026.

255

256 References

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359 liquid chromatographic method for the determination of peroxyacetic acid. J. 360 Chromatogr. A 1996, 730, 203. 361 362 363 364 365 366 367

368

369

370

371 Figure captions

372

373 Fig 1. Variation of the PAA concentration (part a) and carbon mineralization (part b)

374 during the electrolyses with BDD anodes (j 300 Am-2, T 25ºC) of three different raw

375 materials: 1 M C2H6O (▲), 1 M C2H4O (□), 1 M CH3COONa(♦).

376

377 Fig. 2. Main intermediates found during the electrolyses with BDD anodes (j 300 Am-2,

378 T 35ºC) of C2H4O (part a) and C2H6O (part b).  acetic acid;  acetaldehyde; 

379 ethanol; * non identified product 1; + non identified product 2

380

381 Fig. 3. Variation of oxidants concentration (part a) and carbon mineralization (part b)

382 with pH during the electrolysis of CH3COONa solutions at different current charge

-2 383 passed. (1 M CH3COONa, j 300 Am , T 25ºC).

384

385 Fig. 4. Variation of oxidants concentration (part a) and total organic carbon (part b) with

-2 386 current density during the electrolysis of CH3COONa solutions. (□) 300 Am , (■) 600

-2 -2 387 Am , (▲) 1000 Am (1 M CH3COONa, T 25ºC).

388

389

390

391

392

393

394

395

1 60

0.8 a 40 0.6

0.4 b PAA /mM PAA 20

0.2 Carbon mineralized /% mineralized Carbon

0 0 0 5 10 15 20 25 30 0 5 10 15 20 25 30 Q/ Ah dm-3 396 Q/ Ah dm-3

397

398

399

400 Figure 1

401

402

403

6000 800 20000 700 a b 700  5000 600 600 15000 500 4000 500 400 3000 400 10000 300 300 2000 200 200 5000

Chromatographic area Chromatographic 1000 100 area Chromatographic 100 0 0 0 5 10 15 20 25 30 0 0 0 5 10 15 20 25 30 Q / Ah dm-3 Q / Ah dm-3 404

405

406

407

408 Figure 2

409

410

411 16

0.6 a 50 kA h m-3 50 40 b 50 kA h m-3 25 kA h m-3 0.4 30 25 kA h m-3 5 kA h m-3 20

PAA /mM PAA 0.2 10 -3

mineralization % / mineralization 5 kA h m

0 0 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 pH/ units pH/ units 412

413

414

415

416 Figure 3

417

418

419

420

1.2 0.7 0.6 1

0.5 o 0.8 0.4 0.6 b 0.3

TOC / TOC TOC/ 0.4 Oxidants mM Oxidants / 0.2 a 0.1 0.2 0 0 0 20 40 60 80 100 120 0 20 40 60 80 100 120 Q / Ah dm-3 Q / Ah dm-3 421

422

423

424

425 Figure 4

426

427