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Original article
13 (
1
); 357-365
doi:
10.1016/j.arabjc.2017.05.001

Electrochemical oxidation of vanillic acid by electro-Fenton process: Toward a novel route of protocatechuic acid electrosynthesis

, , ,
 Laboratory of Electrochemistry and Environment, National Engineering School of Sfax, Sfax University, BP1173, 3038 Sfax, Tunisia

⁎Corresponding author. Ridha.Abdelhedi@enis.rnu.tn (Ridha Abdelhedi)

Received: , Accepted: ,
© The Author(s) 2017
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The electrochemical oxidation of vanillic acid (VA) in acetone–water mixtures, was studied by electro-Fenton (EF) process in an undivided electro-chemical cell equipped with a carbon fiber cloth as cathode and Pt anode. The objective was the conversion of VA into protocatechuic acid (PCA) well known by its high added value. To our knowledge, the EF method was not used in organic electrosynthesis. The influence of several operating parameters, such as applied current, initial VA concentration and volumic percentage of acetone was investigated. The evolution of the concentrations of VA and its main oxidation product PCA during electrolyses was monitored by means of high performance liquid chromatography (HPLC). The experimental data indicated that the kinetics of VA disappearance follow a pseudo first and zero orders for initial VA concentrations respectively equals to 6.5 and (10, 20, 30) mmol L−1. The maximum PCA concentration increases with volumic percentage of acetone and initial VA concentration. Under optimal experimental conditions of applied current (20 mA) and volumetric percentage of acetone (20%), the increase in the initial VA concentration leads to a linear increase in the maximum of PCA concentration. The very good selectivity rate of the oxidation of VA to PCA by the EF process, was interpreted by the stabilizing effects of acetone and especially the complexation of PCA by the iron ions.

Keywords

Vanillic acid
Protocatechuic acid
Electro-Fenton
Iron complex
Carbon fiber
1

1 Introduction

Phenolic compounds are a complex group of substances that have attracted a considerable attention thanks to their roles in providing flavor and color characteristics of food in human health (Visioli and Galli, 1998). Among these compounds, the orto-dihydroxylated aromatic products exhibit higher antioxidant activity (Cotoras et al., 2014; Dridi Gargouri et al., 2013; Farkas et al., 2004; Ponomarenko et al., 2015; Simić et al., 2007). For example, PCA (3,4-dihydroxybenzoic acid) exists in most edible plants and fruits (Herrmann, 1989; Hudson et al., 2000; Kayano et al., 2002; Liu, 2004). In addition, this compound has been reported for its potential actions as an antioxidant, antibacterial, anticancer, antiulcer, antidiabetic, antiaging, antifibrotic, antiviral, anti-inflammatory, analgesic, antiatherosclerotic, a cardiac, an hepatoprotective, a neurological and nephro protective activities (Kakkar and Bais, 2014).

Diverse procedures for the PCA production have been developed. However, the methods so far proposed are expensive, complex and/or produce low yields. In a very old work, it has been shown that vanillin could be converted into PCA (Irwin and Pearl, 1946) with a good yield (60–65%). However, the used procedure is complex and involves the use of hazardous chemicals such as concentrated hydrochloric acid and sulfur dioxide at elevated temperatures reaching 245 °C. Furthermore, it has been reported that PCA can be biologically obtained by demethylation of VA with poor yield (3.57%) (Priefert et al., 1997). Moreover, PCA could be isolated by extraction from a large variety of plants (Abida et al. 2015). But, the extraction methods are generally characterized by their lengthy extraction procedures, their consumption of large amounts of energy and solvent and their low extraction yields (Yang et al. 2013). Additionally, organic solvents can be quite harmful to the environment and human health. Furthermore, we have shown in a previous work, that the anodic oxidation of VA has been converted into PCA at gold oxide and lead dioxide anodes (Chaabane Elaoud et al., 2001). In this context, Bellakhal et al. (Bellakhal et al., 2006) have shown that electro-oxidation of VA by EF process using a carbon felt cathode and Fe2+ leads to its complete mineralization.

Following our works on organic synthesis (Abdelhedi and Bouguerra, 1991a,b; Savall et al., 1990; Dridi Gargouri et al., 2013; Kallel Trabelsi et al., 2015), we have sought to study in the present work, the conversion of VA into PCA by EF process using a carbon fiber cloth cathode and Fe3+.

In the recent years, authoritative reviews on electrochemical technologies have been published. In this context, the EF process was well described and considered as an emerging technology for oxidizing organic compounds (Brillas et al., 2009; Brillas and Martínez-Huitle 2015; Brillas and Sirés 2015; Martínez-Huitle and Brillas 2009; Martínez-Huitle et al., 2015; Sirés et al., 2014; Stasinakis, 2008; Suty et al., 2004).

The performance of this process is based on the electrochemical in situ generation of H2O2 from the continuous aeration on the cathode Eq. (1). The Fe2+ present in the electrolyte, catalyses the generation of homogeneous hydroxyl radicals from H2O2 Eq. (2). The formation of these radicals in Fenton's process is commonly recognized. It has been confirmed using probes or spin trapping coupled with spectroscopic techniques (Lindsey and Tarr, 2000; Peralta et al., 2014; Vandjelovic et al., 2012; Walling 1998). Furthermore, Fe2+ is continuously recycled by its cathodic reduction as shown by Eq. (3).

(1)
O 2 ( g ) + 2 H + + 2 e - → H 2 O 2
(2)
Fe 2 + ( aq ) + H 2 O 2 → Fe 3 + ( aq ) + OH - ( aq ) + HO •
(3)
Fe 3 + + e - → Fe 2 +

According to its non-selectivity oxidation action, the EF process has been thoroughly and successfully applied to the mineralization of toxic and persistent organic pollutants (Bounab et al., 2015; Stasinakis, 2008). To our knowledge, this technique has not been used in the field of organic electrosynthesis. Moreover, it has been shown that the reactivity of hydroxyl radicals weakens in the presence of an organic solvent such as acetone, acetonitrile, methanol and ethanol (Dhaouadi et al., 2009; Kozlov et al., 2005; Lunak and Sedlak, 1992). In this case, the oxidation of organic compounds becomes partial. So, the EF process could be tested as method for electrosynthesis, by converting VA into a high-added-value product such as PCA.

2

2 Materials and methods

2.1

2.1 Chemicals

VA, PCA and acetone were analytical grade from Aldrich. Sodium nitrate (NaNO3) and iron (III) nitrate (Fe(NO3)3; 9 H2O) were of analytical grade from Fluka. Acetonitrile used as organic solvent for the liquid chromatography mobile phase was HPLC grade from Aldrich. All the solutions were prepared with pure water obtained from a Millipore Milli-Q system (Millipore, Direct –Q® 3 with Pump) with resistivity >18 MΩ cm at 25 °C.

2.2

2.2 Electrolysis

The electro-Fenton experiments were performed in an undivided electrolytic cell. It was equipped with a double jacket for circulating water at 20 ± 0.5 °C and was surmounted by a condenser allowing the recycling of the evaporated acetone (Fig. 1). The cathode was a carbon fiber cloth (l = 5 cm, Ø = 3.5 cm) and was fed with compressed oxygen at about 1 dm3 min−1, starting 10 min before electrolysis. The anode was a cylindrical Pt grid (l = 4.5 cm, Ø = 5 cm), surrounded by the cathode. Applied current control was achieved using a direct current (DC) power supply (model ATTEN TPR 3303-3C-LG precision Co. Ltd, Korea). The electrolytic solutions (Vs = 130 mL) initially, contained VA with 0.05 mmol L−1 NaNO3 as background electrolyte and 0.5 mmol L−1 (Fe (NO3)3; 9H2O) as homogeneous catalyst. The pH 3 of the solution was adjusted, before and over the course of the electrolysis, by adding either 0.5 mol L−1 HNO3 or 0.5 mol L−1 NaOH. This pH was selected as the optimum value to carry out Fenton’s reaction (Eq. (2)), according to the litterature data on EF process (Muruganandham and Swaminathan, 2004; Umar et al., 2010).

Scheme of electrolytic cell. (1) power supply; (2) anode (Pt); (3) carbon fiber cloth cathode; (4) magnetic bar; (5) refrigerant; (6) magnetic stirrer; (7) oxygen tank; (8) bubbling oxygen.
Figure 1 Scheme of electrolytic cell. (1) power supply; (2) anode (Pt); (3) carbon fiber cloth cathode; (4) magnetic bar; (5) refrigerant; (6) magnetic stirrer; (7) oxygen tank; (8) bubbling oxygen.

2.3

2.3 Analysis

According to the litterature data, the analysis of organic compounds has been performed by the implementation of several analytical techniques such as liquid chromatography (Li et al., 2004; Lin et al., 2015; Priefert et al., 1997; Wójciak-Kosior et al., 2006; Yang et al., 2013), gas chromatography-mass spectrometry (GC-MS) (Khoddami et al., 2013), and electrochemical methods (Bavandpour et al., 2015; Kallel Trabelsi et al. 2004; Karimi-Maleh et al., 2013; Karimi-Maleh et al., 2014; Karimi-Maleh et al., 2016). In the present work, the quantitative analysis of VA and its oxidation intermediate product were made by high performance liquid chromatography (HPLC) using a Hewlett-Packard 1100 HPLC apparatus. The used internal standard is a solution equals to 0.1 mmol L−1 of maleic acid. The products are separated on a Hamilton PRP X300 column (250 mm × 4.1 mm). Samples were monitored with a UV–vis diode array detector (DAD) at a wavelength of 220 nm for the first 6 min then at 261 nm up to 18 min.

The mobile phase was a mixture of acetonitrile and 0.05 mol L−1sulphuric acid with the percentage by volume of acetonitrile varying linearly with time as following: from 5% to 25% for the first 5 min then from 25% to 40% up to 10 min, and finally from 40% to 5% up to 18 min. The mobile flow rate was fixed at 1.3 mL min−1.

The yield (Y) of the oxidation reaction of VA into PCA, is evaluated by the equation Eq. (4):

(4)
Y = [ PCA ] t [ VA ] o - [ VA ] t × 100 where [VA]o and [VA]t are the VA concentrations at times 0 and t, respectively and [PCA]t is the PCA concentration at time t.

The energy consumption per mole of produced PCA (ξ, in kWh (mol PCA)−1) at a given electrolysis time (t) was determined by the equation Eq. (5) (Hamza et al., 2011).

(5)
ξ = E cell It [ PCA ] V s where Ecell is the average cell voltage (V), t is the electrolysis time (min) and I is the applied constant current (A).

3

3 Results and discussion

At the beginning of this work, the electrolysis of 6.5 mmol L−1 VA was carried out in aqueous solution using the EF process at 20 mA. It is important to note that the electrolyte which is initially colorless takes a green color that intensifies during the electrolysis time.

Fig. 2 shows the chromatogram of the electrolyte recorded after 3h of electrolysis. This last shows the appearance of four peaks 1, 2, 3 and 4 at the respective retention times to 2, 3.5, 7.5 and 9.3 min. These peaks were attributed to ferric and/or ferrous ions, maleic acid, PCA and VA compared with their authentic chromatograms recorded under the same experimental conditions.

HPLC chromatograms after 3 h VA oxidation by EF process. 1: ferric and/or ferrous ions; 2: maleic acid; 3: PCA; 4: VA.
Figure 2 HPLC chromatograms after 3 h VA oxidation by EF process. 1: ferric and/or ferrous ions; 2: maleic acid; 3: PCA; 4: VA.

Fig. 3 presents the VA and PCA concentration profiles during the electrolysis. It shows that the PCA maximum concentration (3.6 mmol L−1) corresponds to a yield of 62%. Furthermore, the accumulation of PCA, despite its lower oxidation potential than the VA, could be explained by its possible complexation by the ferric and/or ferrous ions. It is worth noting that the complexation of catechol with iron has been intensively studied (Qian et al., 2002; Yamahara et al., 2002; Perron and Brumaghim, 2009). However, the complexes are pH and iron/catechol ratios dependent. They often exhibit variable coordination modes (mono-, di-, or tri-coordinated). At slightly acidic pH (5–6.5), iron is bound by two catecholate ligands per metal ion, giving blue-purple colored Fe3+ complexes (Jovanovic et al., 1998; Schweigert et al., 2001). At more acidic pH (<4), the Fe3+ is bound by monocatecholate that is characterized by a dark green color (Binbuga et al., 2005; Mijangos et al., 2006). This green complex is unstable in the pH range 0–2 (Contreras et al., 2006; Hider et al., 1983). This would be the reason why the PCA-iron complex is converted into PCA during its analysis by HPLC using an acidic eluant (Fig. 2).

Variation of (Δ) VA, (▴) PCA concentration and (o) the yield Y (%) versus electrolysis time. Experimental conditions: [VA]o = 6.5 mmol L−1; [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA.
Figure 3 Variation of (Δ) VA, (▴) PCA concentration and (o) the yield Y (%) versus electrolysis time. Experimental conditions: [VA]o = 6.5 mmol L−1; [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA.

Based on the large stability constants of iron–catecholate complexes at pH 3 (Hider et al., 1983; Tofan-Lazar et al.,2013), it would seem, that these complexes are more difficult to destroy by hydroxyl radical than free PCA and VA molecules. As this complexation does not occur with the para-dihydroxylated derivatives, these last ones degrade more easily than the ortho-dihydroxy derivatives. This is probably why the ortho-diphenols are the oxidation products that predominate in the presence of iron ions. Indeed Belhadj Tahar et al. (Belhadj Tahar et al. 2009), have shown that the anodic oxidation of phenol at Ta/PbO2 in the absence of iron ions, leads to the formation of hydroquinone and catechol, the percentages of which are respectively 47.6 and less than 0.47%. It is important to note that in this case, the electrochemical oxidation occurs by the intermediary of hydroxyl radicals adsorbed on the surface of the lead dioxide anode, resulting from the oxidation of water (Comninellis et al., 2008). It has shown that these heterogeneous hydroxyl radicals have a lower oxidizing power than that of the homogeneous hydroxyl radicals resulting from the Fenton reagent (Martínez-Huitle et al., 2015).

The preceding results allow to schematize the oxidation mechanism of VA in Fig. 4. VA molecules undergo a first bioelectronic discharge leading to the formation of phenoxonium carbocations for which the electronic charge distribution can be represented by three mesomeric forms (A), (B) and (C). By considering the electronic effects of methoxy and carboxy groups, the carbocation (A) which would be the most stable gives simultaneously the 3,4-dioxobenzoic acid (A′) and a methanol molecule by hydrolysis. At the cathode the reduction of (A′) leads to the formation of PCA which is rapidly converted into its complexed form by the action of the iron ions. The higher current intensities accelerate the degradation of this complex.

Reaction pathway of VA oxidation by EF process.
Figure 4 Reaction pathway of VA oxidation by EF process.

In the following the oxidation of VA was investigated by EF at different operating conditions (volumic fractions of acetone, current intensities and initial concentrations of VA).

3.1

3.1 Effect of volumic percentage of acetone

The influence of the solvent nature on the EF process was investigated by electrolysis of 6.5 mmol L−1VA in mixtures of water–acetone at different volumic fractions of acetone i.e., 0%, 20% and 40%. The current intensity was fixed at 20 mA. The variations of VA and PCA concentrations during this electrolysis are shown in (Fig. 5a and b). These results show, firstly, that VA concentration decreases exponentially during electrolysis time. This could be interpreted by assuming that reaction Eq. (6) is of a first-order relating to VA concentration and the hydroxyl radical concentration is quasi steady-state during EF process. Thus, the oxidation rate (r) can be described by the following equation:

(6)
r = - d[VA] dt = K [ HO • ] α [ VA ] = K app [ VA ] where α is the reaction order related to the hydroxyl radicals concentration, K is the real rate constant and Kapp is the apparent observed pseudo first-order rate constant of the VA oxidation. The above differential equation can be written as follows:
(7)
Ln [ VA ] o [ VA ] t = K app t where [VA]o and [VA]t are the VA concentrations at times 0 and t, respectively.
Variation of (♢, □) VA; (♦, ■) PCA concentrations and (o) the yield Y (%) versus electrolysis time. Experimental conditions: [VA]o = 6.5 mmol L−1; [Fe(NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA. %Vol acetone: (a) 20 and (b) 40.
Figure 5 Variation of (♢, □) VA; (♦, ■) PCA concentrations and (o) the yield Y (%) versus electrolysis time. Experimental conditions: [VA]o = 6.5 mmol L−1; [Fe(NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA. %Vol acetone: (a) 20 and (b) 40.

In addition, at volumic fractions of acetone 0%, 20% and 40%:

  • the Kapp values of VA oxidation by HO • radicals are respectively equal to 6 × 10−5, 3 × 10−5 and 4 × 10−5 min−1;

  • and the maximum concentrations of the accumulated PCA are respectively equal to 3.54, 4.18 and 4.35 mmol L−1. In addition, the corresponding yields reached at 300 min of electrolysis, are respectively 48.7%, 77.8% and 87%.

  • According to the literature data, the increase in the volumic fraction of acetone should lead to two antagonist phenomena:

  • the increase of oxygen solubility (Battino et al., 1983) which results in greater production of HO • and therefore a faster disappearance of VA;

  • the loss of the hydroxyl radicals by their action on acetone Eq. (8) which leads to the formation of 2-oxopropyl radical • CH 2 COCH 3 which is less reactive then HO • (Stefan et al., 1996).

(8)
HO • + CH 3 COCH 3 → • CH 2 COCH 3 + H 2 O

Globally, this latter phenomenon appears to be the most dominant for the obtained results.

3.2

3.2 Effect of current intensity

The effect of applied current on the conversion rate of VA into PCA during the oxidation of 6.5 mmol L−1VA was investigated by electrolysis at different currents, i.e., 20, 50 and 100 mA. The electrolyte is a mixture of acetone–water (20:80, v/v). As shown in Fig. 6a, the VA disappearance rate increases with the current intensity. Moreover, the decay of concentration of this latter exhibits an exponential behavior with all the applied currents indicating a first-order reaction kinetics for the oxidation reaction as depicted in Fig. 6b. Thus, apparent rate constant values of VA oxidation by HO • radicals at 20, 50 and 100 mA are respectively equal to 3 × 10−5, 9 × 10−5 and 14 × 10−5 min−1. This kinetic acceleration can be explained by a greater production of HO • from Fenton's reagent Eq. (2).

Variation of (a) VA concentration and (b) Ln[VA]o/[VA]t versus electrolysis time. Experimental conditions: water–acetone mixture (80/20, v/v); [VA]o = 6.5 mmol L−1;[Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL. I: (♢) 20, (□) 50 and (Δ) 100 mA.
Figure 6 Variation of (a) VA concentration and (b) Ln[VA]o/[VA]t versus electrolysis time. Experimental conditions: water–acetone mixture (80/20, v/v); [VA]o = 6.5 mmol L−1;[Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL. I: (♢) 20, (□) 50 and (Δ) 100 mA.

On the other hand, Fig. 7a shows that the yield of the VA oxidation into PCA decreases when the current intensity increases from 20 to 50 mA and stabilizes beyond this value. For example, after 180 min of electrolysis the yield (Y) is by about 91%, 47% and 48% for 20, 50 and 100 mA, respectively. The enhancement of the current intensity from 20 to 50 mA leads to a greater production of HO • and consequently accelerates the degradation kinetic of the PCA-iron complex. For current intensities 50 and 100 mA, the production of HO • and thereafter the degradation kinetic of the PCA-iron complex, becomes limited by the oxygen transfer to the cathode. This is the reason for which the PCA yield stabilizes. Furthermore, the PCA yield versus specific charge (Fig. 7b) increases with applied current intensity.

Variation of the yield Y (%) versus (a) electrolysis time and (b) specific charge (Q). Experimental conditions: water–acetone mixture (80/20, v/v); [VA]o = 6.5 mmol L−1; [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL. I: (♢) 20, (□) 50 and (Δ) 100 mA.
Figure 7 Variation of the yield Y (%) versus (a) electrolysis time and (b) specific charge (Q). Experimental conditions: water–acetone mixture (80/20, v/v); [VA]o = 6.5 mmol L−1; [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL. I: (♢) 20, (□) 50 and (Δ) 100 mA.

This trend could be accounted for by the high enhancement of the parasitic reaction of water formation starting at HO • Eq. (9) on the one hand and mass transport limitation of reactions (1) on the other hand.

(9)
2 HO • → 1 2 O 2 + H 2 O

3.3

3.3 Effect of initial VA concentration

Fig. 8a shows the effect of the initial VA concentration (6.5, 10, 20 and 30 mmol L−1) on the trend of VA concentration during the electrolysis of VA solution in a water–acetone mixture (80/20, v/ v) under a current intensity of 20 mA.

Variation of (a) VA and (b) PCA concentrations versus electrolysis time. Experimental conditions: water–acetone mixture (80/20, v/v); [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA. [VA]o: (♢) 6.5, (□) 10, (Δ) 20 and (o) 30 mmol L−1. (c) Evolution of maximum of PCA concentration with initial VA concentration.
Figure 8 Variation of (a) VA and (b) PCA concentrations versus electrolysis time. Experimental conditions: water–acetone mixture (80/20, v/v); [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA. [VA]o: (♢) 6.5, (□) 10, (Δ) 20 and (o) 30 mmol L−1. (c) Evolution of maximum of PCA concentration with initial VA concentration.

For initial VA concentration equal to 6.5 mmol L−1, we have already shown that the kinetics of disappearance of the VA are of pseudo first order and are under the mass transport control.

However, for initial VA concentrations equal to 10, 20 and 30 mmol L−1, VA concentration decreases almost linearly with time according to a zero order kinetics Eqs. (10) and (11). In these conditions, the VA disappearance kinetic is under the current control.

(10)
r = - d[VA] dt = K [ HO • ] α [ VA ] o = K app

The integration of the differential Eq. (10) leads to (11):

(11)
[ VA ] t = kt + [ VA ] o

Figs. 8b and c, show that the maximum of PCA concentration increases linearly with the initial VA concentration. In addition, Fig. 9 shows that the yield as a function of the consumed specific charge increases with the initial concentration of VA. For example, after 10 h of electrolysis, corresponding to the conception of 1.54 Ah L−1, the PCA yields (Y) are about 26%, 73%, 85% and 92% for initial VA concentration equal to 6.5, 10, 20 and 30 mmol L−1, respectively. At this electrolysis time, Fig. 10 shows that the energy consumption decreases by increasing the initial VA concentration. These results can be attributed to the decrease of the loss of hydroxyl radicals by the secondary reaction (Eq. (9)) which leads to an improvement of the process efficiency.

Variation of the Yield Y (%) versus specific charge (Q). Experimental conditions: water–acetone mixture (80/20, v/v); [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA. [VA]o: (♢) 6.5, (□) 10, (△) 20 and (o) 30 mmol L−1.
Figure 9 Variation of the Yield Y (%) versus specific charge (Q). Experimental conditions: water–acetone mixture (80/20, v/v); [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA. [VA]o: (♢) 6.5, (□) 10, (△) 20 and (o) 30 mmol L−1.
Variation of energy consumption versus initial VA concentration. Experimental conditions: electrolysis time: 10 hours, water–acetone mixture (80/20, v/v); [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA.
Figure 10 Variation of energy consumption versus initial VA concentration. Experimental conditions: electrolysis time: 10 hours, water–acetone mixture (80/20, v/v); [Fe (NO3)3] = 0.5 mmol L−1; [NaNO3] = 0.05 mol L−1; T = 20 °C; pH = 3; Vs = 130 mL; I = 20 mA.

As an indication, for an initial concentration of VA equal to 30 mmol L−1, the maximum concentration of PCA was obtained from the disappearance of 50% VA (Fig. 8a and b) with a conversion yield (Y) equal to 93% (Fig. 9). Consequently, the oxidation of VA to PCA by the EF process occurs with a very good selectivity rate.

4

4 Conclusions

The electrochemical oxidation of vanillic acid (VA) has been investigated by electro-Fenton process (EF). The main product protocatechuic acid (PCA) is well known by its biological properties. In this context, we have firstly studied the VA disappearance kinetics. Moreover, we have shown that the maximum PCA concentration obtained during electrolysis increases (i) by decreasing the current intensity from 100 to 20 mA (ii) with the enhancement of the volumic percentage of acetone from 0% to 20% (iii) and linearly with the initial VA concentration from 6.5 to 30 mmol L−1. The very good selectivity of the EF process with respect to PCA production, was interpreted by the stabilizing effect of acetone and especially the stabilization of this latter by complexation. Finally, a mechanistic schema for the VA oxidation by EF process has been proposed.

Acknowledgment

The authors would like to thank the Ministry of Higher Education and Scientific Research of Tunisia (Contrat programme LR14ES08), for their support of this research work.

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