Wells–Dawson polyoxometelates [HP2W18–nMonO62]Fe2.5, xH2O; n = 0, 6: Synthesis, spectroscopic characterization and catalytic application for oxidation of dyes

Ouahiba Bechiri; Mostefa Abbessi; Robila Belghiche; Lahcène Ouahab

doi:10.1016/j.crci.2013.07.002

Wells–Dawson polyoxometelates [HP₂W_18–nMo_nO₆₂]Fe_2.5, xH₂O; n = 0, 6: Synthesis, spectroscopic characterization and catalytic application for oxidation of dyes

Ouahiba Bechiri ¹ ; Mostefa Abbessi ¹ ; Robila Belghiche ¹ ; Lahcène Ouahab ²

¹ Laboratoire de recherche génie de l’environnement, département de génie des procédés, faculté des sciences de l’ingénieur, université Badji Mokhtar, BP 12, Annaba, Algeria
² Organométalliques et matériaux moléculaires, UMR 6226 CNRS – université de Rennes-1, sciences chimiques de Rennes, avenue du Général-Leclerc, 35042 Rennes cedex, France

Comptes Rendus. Chimie, Volume 17 (2014) no. 2, pp. 135-140.

Résumé

The synthesis, IR, ³¹P NMR and cyclic voltammetry characterizations of new Wells–Dawson-type heteropolyanions that contain iron, HFe_2.5P₂W₁₈O₆₂, 23H₂O and HFe_2.5P₂W₁₂Mo₆O₆₂, 22 H₂O, are reported. The catalytic activity of these compounds was evaluated through the oxidation of methyl violet dye, by hydrogen peroxide. The influence of different parameters such as the initial pH, the initial H₂O₂ concentration, the catalyst mass, and the initial dye concentration have been studied.

Métadonnées

Reçu le : 2013-03-26
Accepté le : 2013-07-08
Publié le : 2013-09-03

DOI : 10.1016/j.crci.2013.07.002

Mots clés : Heteropolyanions, Wells–Dawson-type, Synthesis, Characterizations, Dyes, Hydrogen peroxide, Oxidation

Affiliations des auteurs :

Ouahiba Bechiri ¹ ; Mostefa Abbessi ¹ ; Robila Belghiche ¹ ; Lahcène Ouahab ²

¹ Laboratoire de recherche génie de l’environnement, département de génie des procédés, faculté des sciences de l’ingénieur, université Badji Mokhtar, BP 12, Annaba, Algeria
² Organométalliques et matériaux moléculaires, UMR 6226 CNRS – université de Rennes-1, sciences chimiques de Rennes, avenue du Général-Leclerc, 35042 Rennes cedex, France

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     author = {Ouahiba Bechiri and Mostefa Abbessi and Robila Belghiche and Lahc\`ene Ouahab},
     title = {Wells{\textendash}Dawson polyoxometelates {[HP\protect\textsubscript{2}W\protect\textsubscript{18{\textendash}\protect\emph{n}}Mo\protect\textsubscript{\protect\emph{n}}O\protect\textsubscript{62}]Fe\protect\textsubscript{2.5},} {\protect\emph{x}H\protect\textsubscript{2}O;} \protect\emph{n}\hspace{0.25em}=\hspace{0.25em}0, 6: {Synthesis,} spectroscopic characterization and catalytic application for oxidation of dyes},
     journal = {Comptes Rendus. Chimie},
     pages = {135--140},
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     volume = {17},
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     doi = {10.1016/j.crci.2013.07.002},
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%A Mostefa Abbessi
%A Robila Belghiche
%A Lahcène Ouahab
%T Wells–Dawson polyoxometelates [HP2W18–nMonO62]Fe2.5, xH2O; n = 0, 6: Synthesis, spectroscopic characterization and catalytic application for oxidation of dyes
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Ouahiba Bechiri; Mostefa Abbessi; Robila Belghiche; Lahcène Ouahab. Wells–Dawson polyoxometelates [HP₂W_18–nMo_nO₆₂]Fe_2.5, xH₂O; n = 0, 6: Synthesis, spectroscopic characterization and catalytic application for oxidation of dyes. Comptes Rendus. Chimie, Volume 17 (2014) no. 2, pp. 135-140. doi : 10.1016/j.crci.2013.07.002. https://comptes-rendus.academie-sciences.fr/chimie/articles/10.1016/j.crci.2013.07.002/

Version originale du texte intégral

1 Introduction

Polyoxometalates (POMs) constitute a diverse class of inorganic oxo-metal clusters composed of early transition metals in their highest oxidation state [1]. They have a great deal of structural diversity and various applications in different areas, including catalysis, material sciences, medicine, and biology. Several types have been characterized, and each of them is defined by the M/X ratio used in the polycondensation under acidic conditions. The two best-known heteropolyanions are the Keggin-type [Xⁿ⁺M₁₂O₄₀]^(8–n)– [2] and the Wells–Dawson type [(Xⁿ⁺)₂M₁₈O₆₂]^(16–2n)– [3] (Fig. 1).

Fig. 1
Polyhedral representation of the polyanion Dawson structure.

Polyoxometalltes with Dawson structure may be promising catalysts in homogeneous and heterogeneous systems because their redox and acidic properties can be controlled at atomic and molecular levels. They are used not only in acid-catalyzed [4] reactions, but also in many oxidations of organic compounds [5,6]. Recently, the Dawson-type Fe(III)-substituted heteropolyanion (α₂P₂W₁₂Mo₅O₆₁Fe)^7– was successfully used as a catalyst for the oxidation of methyl orange dye (MO) by H₂O₂ in aqueous solution [7]. This compound was synthesized by addition of iron on the lacunary heteropolyanion (α₂P₂W₁₂Mo₅O₆₂)^10– [8]. In this work, we have synthesized and characterized new heteropolyanions that contain iron HFe_2.5P₂W₁₈O₆₂, 23H₂O and HFe_2.5P₂W₁₂Mo₆O₆₂, 22H₂O, by addition of Fe³⁺ ions to the Dawson acid forms H₆P₂W₁₈O₆₂24H₂O or H₆P₂W_12-Mo₆O₆₂24H₂O.

The catalytic activity of these compounds was evaluated through the oxidation of an aqueous dye, methyl violet (its structure is shown in Fig. 2) [9,10] by hydrogen peroxide.

Fig. 2
Chemical structure of methyl violet.

Methyl violet (MV), of molecular formula C₂₄H₂₈ClN₃, is a triphenylmethane dye, soluble in water, ethanol, methanol, diethylene glycol and dipropylene glycol. It is a dark green powder [11]. Methyl violets are the mixture of tetramethyl, pentamethyl, hexamethyl, and pararosanilines. These dyes are chiefly used in heterogray and printing inks. They impart a deep violet colour in paint and printing ink. They are also used to obtain shades of deep colours that can be applied for the dyeing of cotton, silk, paper, bamboo, weed, straw and leather. Methyl violet dye is widely used in analytical chemistry laboratories as a pH indicator to test pH ranges from 0 to 1.6. The toxic information reveals that the dye may cause hard skin and eye irritation – it means that in case of physical contact with the dye, this causes an irritation with redness and pain. The dye is harmful if it is swallowed. Also, the inhalation of methyl violet may cause an irritation to the respiratory tract, whereas its ingestion causes an irritation to gastrointestinal tract [12]. Therefore, it is necessary to remove it from the waste water. The influence of different parameters such as initial pH, initial H₂O₂ concentration, catalyst mass, and initial dye concentration has also been studied.

2 Experimental part

2.1 Preparation and characterization of catalysts

The heteropolyanions precursors H₆P₂W₁₈O₆₂ 24H₂O and H₆P₂W₁₂Mo₆O₆₂ 24H₂O, as well as their acids forms, were synthesized according to the published procedures [13,14], and their purity was confirmed by infrared and NMR ³¹P spectroscopies. IR spectra were recorded on KBr pellets using a spectrophotometer Shimadzu FTIR-8400s. ³¹P NMR spectra were recorded on a Bruker 2000 apparatus operating at 110 MHz in the Fourier-transform mode. The ³¹P shifts were measured in a 10⁻³ M solution of polyanions in a D₂O solution and were referenced to H₃PO₄ 85%.

Cyclic voltammetry experiments were performed on an EDAQ e-corder 401 potentiostat. All experiments were carried out using a three-electrode cell configuration with a glassy carbon working electrode, a saturated calomel reference electrode (SCE), and a platinum auxiliary electrode.

All experimental solutions were de-aerated thoroughly by bubbling pure N₂ into the solutions for 10 min. All cyclic voltammograms were recorded at a scan rate of 50 mV s⁻¹. All experiments were performed at room temperature.

HFe_1,5P₂W₁₈O₆₁ 23H₂O (1): 5 g (1.088 mmol) of H₆P₂W₁₈O₆₂ were dissolved in 20 ml of water at room temperature and 0.767 g of solid FeCl₂, 6H₂O (3.26 mmol) was then added. The mixture was then stirred for 10 min. A yellow powder of (1) was obtained after five days by slow evaporation. IR (KBr pellet, cm⁻¹): 1092(s), 1025(w), 960(s), 909(s).NMR of (1): ³¹P δ = –12.43 ppm. Anal. calcd. (found): P 1.27 (1.20); W 68.10 (62.05); Fe 1.72 (2.01).

HFe_1,5P₂W₁₂Mo₆O₆₁22H₂O (2): 5 g (1.2 mmol) of H₆P₂W₁₂Mo₆O₆₂ were dissolved in 20 ml of water at room temperature and 0,541 g (3.56 mmol) of solid FeCl₂ 6H₂O was then added. The mixture was stirred for 10 min. A dark yellow powder of (2) was obtained after five days by slow evaporation. IR (KBr pellet, cm⁻¹): 1084(s), 1025(w), 953(s), 912(s). NMR of (2): ³¹P δ = –8.82 ppm. Anal. calcd. (found): P 2.13(1.87); W 76.08(74.03); Mo 19.80(18.70); Fe 1.92(3.01).

2.2 Procedure for catalytic oxidations

2.2.1 Reagents

Methyl violet (MV) was supplied by Merck. H₂O₂ was purchased from Aldrich. All other reagents (NaOH or H₂SO₄) used in this study were of analytical grade.

2.2.2 Procedure – analysis

The initial concentration of the MV solution was 10 mg/L for all experiments, except for those carried out to examine the effect of the initial dye concentration. In all experiments, 100 mL of MV solution containing the appropriate quantity of catalyst and H₂O₂ was magnetically stirred at room temperature.

The pH of the reaction was adjusted by using 0.1 N H₂SO₄ or NaOH aqueous solutions.

The MV concentration was measured by a Jenway UV–vis spectrophotometer. The wavelength that corresponds to the maximum absorbance is λ_max = 585 nm. The resolutions of the wavelength and the bandwidth were 1 nm and 0.5 nm. The cell used during the experiments was made of 1-cm-thick quartz.

3 Results and discussion

3.1 Preparation and characterization of catalysts

The addition of Fe³⁺ ions to Dawson acid forms H₆P₂W₁₈O₆₂, 24H₂O or H₆P₂W₁₂Mo₆O₆₂, 24H₂O in an aqueous medium, with a stoichiometric (3:1) amount, leads to a substitution of some protons by Fe³⁺ ions yielding powder compounds HFe_2.5P₂W₁₈O₆₂, 22 H₂O (1) and HFe_2.5P₂W₁₂Mo₆O₆₂, 23H₂O (2).

The IR spectrum of the acid Wells–Dawson compound H₆P₂W₁₈O₆₂24H₂O is characterized by the elongation bands of P–O at 1090 cm⁻¹ and W–O terminal band, inter and intra W–O–W at 960, 914 and 769 cm⁻¹, respectively [15]. However, the incorporation of transition ions in the structure of the two Dawson acids H₆P₂W₁₈O₆₂, 24H₂O and H₆P₂W_12-Mo₆O₆₂, 24H₂O generates a small displacement of these bands. The selected IR data are presented in Table 1.

POM	ν_as(P–O_a)	ν_as(W–O_d)	M–Ob–M	M–OcM
H₆P₂W₁₈O₆₂	1090	962	914	769

HFe_2.5P₂W₁₈O₆₂	1092	960	909	784

H₆P₂ W₁₂ Mo₆O₆₂	1084	955	914	789

HFe_2.5P₂W₁₂Mo₆O₆₂	1084	953	912	789

It is well known that phosphorus NMR is an appropriate and powerful way to check the purity of the product. Phosphorous NMR spectra of (1) and (2) (Fig. 3) reveal a virtually pure product with a single resonance peak at δ = –12.43 ppm and δ = –8.82 ppm, respectively. This means that the two phosphorus atoms are equivalent.

Fig. 3
³¹P NMR spectra of a: compound (1); b: compound (2).

According to the voltammogram of the compound HFe_2.5P₂W₁₈O₆₂, 22 H₂O, we observe five redox systems. The first peak is situated at +0.77 V/ECS; it appears in different voltammograms of heteropolyanionic complexes that contain Fe³⁺. Example: (α₂-P₂W₁₇O₆₁Fe)^7–and (α₁-P₂W₁₇O₆₁Fe)^7–[16], (Fe₃P₂W₁₅O₅₉)^9–[17]. It is attributed to the reduction of an electron iron (III) into Fe (II): (Fe³⁺ + 1e^– → Fe²⁺). The other peaks that appeared in 0,1, –0,07, –0,4, and 0,7 V/ECS are attributed to tungsten reduction.

According to the voltammogram of the compound HFe_2.5P₂W₁₂Mo₆O₆₂, 23H₂O, we can deduce that the introduction of iron in the compound H₆P₂W_12-Mo₆O₆₂, 24H₂O makes the last stage of tungsten reduction disappear.

3.2 Catalytic oxidation

Factors that influence the oxidation reaction of methyl violet are:

• the initial pH of the solution;
• the mass of the catalyst, Cat 1: (HFe_2.5P₂W₁₈O₆₁, 23H₂O) or Cat 2: (HFe_2.5P₂W₁₂Mo₆O₆₁, 22H₂O);
• the concentration of H₂O₂;
• the initial concentration of the solution of MV.

The oxidation efficiency (discolouration), which was determined as shown below [18]:

DE = (C_{i} - C_{f}) / C_{i} \times 100

(1)

DE: discolouration efficiency, C_i: initial concentration of MV, C_f: final concentration of MV.

3.2.1 Effect of solution pH

The effect of pH on the discolouration efficiency was investigated by varying the pH from 3 to 10. For more acidic pHs, there is a risk of dimerization of the catalyst, while for pHs above 10, the catalyst is likely to deteriorate. The obtained results are shown in Fig. 4.

Fig. 4
Voltammograms of HFe_2.5P₂W₁₈O₆₂, 22 H₂O at pH = 3 and pH = 5.

The optimum results were of found to be obtained at a pH of about 3 using the two catalysts. ED= 40% using catalyst Cat1 and ED = 49% using catalyst Cat2.

This result can be explained by the stability of the catalyst at this pH. Also, it has been shown that the catalytic efficiency of the Fe³⁺/H₂O₂ system towards the oxidation of organic dyes is better at pH = 3 than at other pH values [19]. H₂O₂ molecules are unstable in alkaline solution, and therefore dye degradation decreases in alkaline solution [20]. The stability of heteropolyanions is also affected by a modification of the pH and they can decompose at alkaline pHs [21].

3.2.2 Effect of the mass of the catalyst

The effect of the mass of the catalyst on MV oxidation by H₂O₂, using Cat1 or Cat2, was investigated. The results are displayed in Fig. 5.

Fig. 5
Voltammograms of HFe_2.5P₂W₁₂Mo₆O₆₂, 23H₂O at pH = 3 and pH = 5.

These results show that the best discolouration efficiency (ED = 48%) is obtained for a mass of catalyst equal to 0.02 g using catalyst Cat1 and 0.01 g (ED = 61%) using catalyst Cat2. Beyond these masses, discolouration efficiency decreases.

This result is mentioned in the literature [22]. It can be explained by the presence of a secondary reaction consuming the hydroxyl radicals.

3.2.3 Effect of H₂O₂ concentration

Experiments were conducted to determine the effect of H₂O₂ concentration on the MV oxidation using Dawson type heteropolyanions as catalysts; the obtained results are displayed in Figs. 6 and 7.

Fig. 6
Effect of the solution pH on MV oxidation using Cat1 as the catalyst (C₀ = 10 mg/L, m_cat = 0.05 g, [H₂O₂] = 0.25 mM, T = 25 °C).

Fig. 7
Effect of the solution pH on MV oxidation using Cat2 as the catalyst (C₀ = 10 mg/L, m_cat = 0.05 g, [H₂O₂] = 0.25 mM, T = 25 °C).

It was found that the best discolouration efficiency is obtained for a concentration of H₂O₂ equal to 0.25 mM using catalyst Cat1 (ED = 48%) and 0.4 mM using catalyst Cat2 (ED = 63%).

Generally, the degradation rate of organic compounds increases as H₂O₂ concentration increases until a critical H₂O₂ concentration is achieved. However, when a concentration higher than the critical one is used, the degradation rate of organic compounds decreases; this result is called the scavenging effect [23,24]. The reaction of H₂O₂ and OH^• in aqueous solution can be expressed by the following equation [25]:

H_{2} O_{2} + {OH}^{} \to H_{2} O + H {O_{2}}^{}

(2)

According to the above results, the critical H₂O₂ concentration for the degradation of 10 mg/L of MV dye is about 0.25 mM using Cat1 as the catalyst and 0.4 mM using Cat2 as the catalyst.

3.2.4 Effect of methyl violet concentration

The study of the methyl violet dye initial concentration effect on discolouration efficiency was carried out for concentrations ranging from 2 mg/L to 30 mg/L. The results are presented in Figs. 8 and 9. The best result was found for a MV concentration equal to 5 mg/L using catalyst Cat1 (ED = 80%) and equal to 2 mg/L using catalyst Cat2 (ED = 91%) (Figs. 10–13).

Fig. 8
Effect of the mass of the catalyst on MV oxidation using Cat1 as the catalyst (C₀ = 10 mg/L, [H₂O₂] = 0,25 mM, pH = 3, T = 25 °C).

Fig. 9
Effect of the mass of the catalyst on MV oxidation using Cat2 as the catalyst (C₀ = 10 mg/L, [H₂O₂] = 0.25 mM, pH = 3, T = 25 °C).

Fig. 10
Effect of the initial H₂O₂ concentration on MV oxidation using Cat1 as the catalyst (C₀ = 10 mg/L, m_Cat1 = 0.02 g, pH = 3, T = 25 °C).

Fig. 11
Effect of the initial H₂O₂ concentration on MV oxidation using Cat2 as the catalyst (C₀ = 10 mg/L, m_Cat2 = 0.01 g, pH = 3, T = 25 °C).

Fig. 12
Effect of the initial dye concentration on MV oxidation using Cat1 as the catalyst (m_Cat1 = 0.02 g, pH = 3, [H₂O₂] = 0.25 mM, T = 25 °C).

Fig. 13
Effect of the initial dye concentration on MV oxidation using Cat2 as the catalyst (m_Cat2 = 0.01 g, pH = 3, T = 25 °C).

4 Conclusion

A new iron substituted heteropolyanion with general formula [HP₂W_18–nMo_nO₆₂]Fe_2.5, xH₂O, n = 0, 6 have been synthesized and characterized by IR and ³¹P RMN spectroscopies.

Elementary analysis confirms the mentioned metals’ insertion into the polytunstic and polymolybdotunstic matrix.

The Dawson entity was confirmed by IR spectroscopy for all synthesized heteropolyanions through the appearance of the characteristic peaks.

RMN confirms the synthesized compounds’ purity.

Cyclic voltammetry shows that the iron is present in the Dawson structure.

Approximately 80% of the dye has been eliminated after 40 minutes under the following operating conditions: pH: 3, [MV]₀ = 5 mg/L, catalyst (Cat 1) mass: 0.02 g, [H₂O₂]₀ = 0.25 mM. And approximately 91% of the dye has been eliminated after 80 min with the following operating conditions: pH: 3, [MV]₀ = 2 mg/L, catalyst (Cat 2) mass: 0.01 g, [H₂O₂]₀ = 0.4 mM.

This system constitutes a simple and effective method compared to those previously reported for the oxidation of methyl violet using a commercial catalyst [26].

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