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. 19 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 26 27 28 29 30 1 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. 45 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. 51 52 CH3COOH + H2O2 → CH3COOOH + H2O (1) 53 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 2 56 which forms the peroxide radical with oxygen. The reaction ends with PAA formation13 57 according to eq. 2. 58 59 CH3CHO + O2 → CH3COOOH (2) 60 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. 67 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 69 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 3 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). 94 95 2. Experimental. 96 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. 4 - - + - 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 5 129 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).