A recyclable polymer-supported ruthenium catalyst for the oxidative degradation of bisphenol A in water using hydrogen peroxide

Abstract

A polypyridyl ruthenium(II) complex, cis-[Ru^II(2,9-Me₂phen)₂(H₂O)₂]²⁺, has been adsorbed onto the cation-exchange resins Dowex-50W and Chelex-100. The potential use of the supported ruthenium(II) complex as catalyst for the oxidative degradation of organic pollutants in water has been investigated using bisphenol A, an emerging endocrine disruptor, as substrate; and the environmentally friendly H₂O₂ as oxidant. These solid-supported catalysts are found to be efficient for the degradation of bisphenol A in aqueous solution by H₂O₂ under ambient conditions. The intermediates and products formed during the oxidative degradation of bisphenol A by these catalytic systems have been identified and a mechanism is proposed. The supported catalysts are easily recovered by simple filtration and display no loss of activity when recycled.

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Introduction

A variety of ruthenium complexes, especially those bearing polypyridyl or macrocyclic tertiary amine ligands, are active catalysts for the oxidation of various organic compounds including alkanes, alkenes, alkynes and alcohols; using terminal oxidants such as Ce(IV), tert-butyl hydroperoxide (TBHP), hydrogen peroxide or O₂.^1–12 Although these ruthenium complexes are among the most active catalysts for selective organic oxidation, their use as catalysts for the oxidative degradation of organic pollutants in water has not been explored. The amount of organic pollutants in wastewaters discharged by various industries is increasing every year and is causing serious global environmental problems. These pollutants include a wide range of persistent organic chemicals, such as pharmaceuticals and endocrine-disrupting compounds. We are now faced with the challenge of removing these compounds from their effluent before they are discharged. Although catalytic oxidation processes using Fenton (Fe²⁺ + H₂O₂) and photo-Fenton (Fe²⁺ + H₂O₂ + UV) reagents have been developed for organic pollutants removal from wastewaters, these methods are rather inefficient.^13–16 Moreover, the Fenton reaction is only effective at low pH (<4), since at higher pH Fe(OH)₃ precipitate will form which is inactive.^16,17Fenton reaction also results in inevitable flocs formation which requires time consuming further treatments. In general, ruthenium complexes are much more stable and active oxidation catalysts than iron complexes, it is anticipated that they should also be useful catalysts for the oxidative degradation of organic pollutants in water. Although ruthenium is more toxic and expensive than iron, if it can be made in the form of a robust solid-supported catalyst that can be readily recyclable without leaching into the environment, then its use is still a feasible process. We report herein the use of a ruthenium(II) complex, cis-[Ru^II(2,9-Me₂phen)₂(H₂O)₂](PF₆)₂ (2,9-Me₂phen = 2,9-dimethyl-1,10-phenanthroline), that is supported on a cation-exchange resin (Dowex-50W or Chelex-100), as efficient catalyst for the oxidative degradation of bisphenol A (2,2-bis(4-hydroxyphenyl)propane), using hydrogen peroxide as the oxidant under ambient conditions at various pH. Bisphenol A (structure f in Table 1) is a known endocrine disruptor which has been widely used for the production of polycarbonate (PC) and epoxy resins used in food containers.^18,19 Its concentration in wastewaters is increasing worldwide.²⁰ This is the first report on the use of a solid-supported ruthenium catalyst for environmental remediation.

Table 1 List of major intermediates (a–e) and main fragments (m/z) obtained by LC-MS.

Peak	Retention time/min	m/z (amu)	Molecular weight	MS/MS fragments (relative abundance)	Proposed structure
a	5.5	163	164	163(100)
				107(64)
b	7.2	163	164	163(100)
				107(64)
c	9.6	135	136	135(45)
				120(27)
				92(100)
d	12.0	257	258	257(100)
				242(18)
				229(97)
				214(50)
e	14.3	241	242	241(44)
				226(100)
f	16.3	227	228	227(100)
				212(63)
				133(20)

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Results and discussion

Preparation and characterization of supported catalysts

Upon adding Dowex-50W or Chelex-100 to a red solution of cis-[Ru(2,9-Me₂phen)₂(OH₂)₂]²⁺ (Ru^IIII) in water, the colour of the solution was rapidly discharged while the resin turned red, indicating adsorption of the ruthenium(II) complex onto the resin. Analysis of the ruthenium content by ICP-AES after leaching with 6 M HCl indicates that the maximum adsorption capacity of the resins are 8.2 μmol Ru^IIII/g Dowex-50W and 6.9 μmol Ru^IIII/g Chelex-100. Complete adsorption occurs if the amount of Ru^IIII used is less than these values. No Ru^IIII was detected in the solution after continuous stirring of a Ru^IIII-Dowex suspension in water or in 1 M HNO₃ for 4 h. On the other hand, although Ru^IIII-Chelex is stable in water or 0.1 M HNO₃, complete leaching of Ru^IIII occurs in 1.0 M HNO₃, as monitored by UV/Vis spectrophotometry (Ru^IIII: λ_max = 496 nm, ε = 7.1 × 10³ mol⁻¹ dm³ cm⁻¹).

The diffuse reflectance spectrum of Ru^IIII-Dowex shows a broad peak with λ_max at around 500 nm (Fig. 1), which is similar to that of Ru^IIII in water (496 nm). On the other hand, the diffuse reflectance spectrum of Ru^IIII-Chelex shows λ_max at around 520 nm, slightly red-shifted compared with the unsupported Ru^II complex.

Fig. 1 Diffuse reflectance spectra of (a) Ru^IIII-Dowex (b) Ru^IIII-Chelex (c) Ru^IIII-Dowex after catalytic oxidation.

Oxidative degradation of bisphenol A

The potential of the polymer-supported ruthenium complex as catalyst for oxidative degradation of organic pollutants in water was investigated using bisphenol A (BPA) as the substrate and the environmentally friendly hydrogen peroxide as oxidizing agent. Upon adding Ru^IIII-Dowex or Ru^IIII-Chelex to a solution of bisphenol A in water containing H₂O₂, the concentration of bisphenol A decreased with time, as monitored by LC and LC/MS (Fig. 2). A 0.50 mM solution of bisphenol A in water can be completely degraded by 25 mM of H₂O₂ using the unsupported Ru^IIII, Ru^IIII-Dowex or Ru^IIII-Chelex. Theoretically 36 moles of H₂O₂ are required to completely oxidize 1 mole of BPA to CO₂ and H₂O. As expected, Ru^IIII has the highest activity, while Ru^IIII-Chelex is the least active catalyst, although it has a faster initial rate than Ru^IIII-Dowex. RuCl₃·xH₂O, which is a common ruthenium complex, is much less effective. No degradation of BPA occurred in the absence of ruthenium catalysts. Notably the solutions containing the supported catalysts remained colourless throughout the catalytic oxidation, the absorbances at 496 nm (λ_max of Ru^II) were <0.005, indicating that [Ru^IIII] in the solutions were <10⁻⁷ M. In an independent experiment, no oxidation of bisphenol A was found to occur when a 1 × 10⁻⁷ M Ru^IIII was used as catalyst. These results strongly suggest that the active catalyst is the supported one.

Fig. 2 Time course for the ruthenium catalyzed degradation of BPA (0.50 mM) by H₂O₂ (25 mM) in water (50 mL). T = 25 °C.

Since Ru^IIII-Dowex is more active than Ru^IIII-Chelex, it was selected for further investigations. Fig. 3 shows the effect of H₂O₂ concentration on the degradation of bisphenol A. As expected, lowering the H₂O₂ concentration leads to a decrease in reaction rate. Also, complete degradation of bisphenol A is not achieved using 5 mM or 10 mM H₂O₂.

Fig. 3 Effect of H₂O₂ concentration on degradation of BPA. Ru^IIII-Dowex (8.2 μmol Ru^IIII/g Dowex), 1.0 g; bisphenol A, (0.50 mM) in 50 mL water. T = 25 °C.

The decrease in [BPA] at various [H₂O₂] was found to obey first-order kinetics. Plots of ln(C₀/C) versus time t are linear (C₀ = initial concentration, C = concentration at time t), and the slopes give the pseudo-first-order rate constants k_obs. The plot of 1/k_obsversus 1/[H₂O₂] is linear (Fig. 4), which is consistent with equilibrium formation of a precursor complex between Ru^IIII-Dowex and H₂O₂ prior to the oxidation of BPA, as shown in eqn (1) and (2). The rate law is shown in eqn (3); k and K are found to be (3.24 ± 0.45) × 10⁻¹s⁻¹ and (430 ± 10) M⁻¹, respectively at 25 °C.

The effect of the amount of catalyst on the degradation of bisphenol A is illustrated in Fig. 5.

Fig. 4 Plot of 1/k_obsversus 1/[H₂O₂] for the Ru^IIII-Dowex catalyzed oxidative degradation of BPA by H₂O₂. Slope = (7.12 ± 0.88) × 10⁻³, y-intercept = 3.09 ± 0.11, r = 0.9924. T = 25 °C.

Fig. 5 Effect of the amount of Ru^IIII-Dowexcatalyst on the oxidative degradation of BPA. H₂O₂, 25 mM; bisphenol A, 0.50 mM in 50 mL water. T = 25 °C.

As expected, the rate of oxidation of bisphenol A increases with the amount of catalyst (2 g > 1 g > 0.5 g). The decrease in [BPA] obeys first-order-kinetics, and the pseduo-first-order rate constants are found to increase linearly with the amount of catalyst (Fig. 6). Complete degradation of bisphenol A was not achieved when 0.5 g of catalyst was used. On the other hand, when 2 g of catalyst with half the ruthenium loading (4.1 μmol Ru^IIII/g) was used, the rate is similar to using 1 g of catalyst with ruthenium loading of 8.2 μmol Ru^IIII/g, although the former catalyst should have a higher surface area.

Fig. 6 Plot of k_obsversus amount of Ru^IIII-Dowex in the oxidative degradation of BPA by H₂O₂. Slope = (2.86 ± 0.14) × 10⁻¹ g⁻¹s⁻¹, r = 0.9986. T = 25 °C.

The catalytic activity of Ru^IIII-Dowex was also investigated at various pH. As shown in Fig. 7, Ru^IIII-Dowex is an efficient catalyst at pH 4–9, which is the range of pH found in most natural and polluted waters. The activity follows the order pH 8 > 9 > 7 > 4. In general, the catalyst exhibits higher activity at high pH. In this regard, this system is better than the Fenton reagent, which is ineffective at pH > 4 due to the precipitation of Fe(OH)₃.¹⁷

Fig. 7 Oxidative degradation of BPA (0.5 mM) catalyzed by Ru^IIII-Dowex at various pH. H₂O₂, 25 mM; Ru^IIII-Dowex, 1.0 g (8.2 μmol Ru^IIII); 50 mL water. T = 25 °C.

Recycling of the polymer-supported Ru^IIIIcatalyst

After catalytic reaction, the solution was centrifuged. The Ru^IIII-Resin was collected, washed with deionized water and then used again for oxidation degradation of bisphenol A under the same conditions as before. It was found that there was virtually no loss of activity after three successive catalyst recyclings. The ruthenium content of the resin after three recyclings remained the same as analyzed by ICP. There was also no decomposition nor leaching of the ruthenium complex into the solution, as monitored by UV-Vis spectrophotometry. Moreover, the diffuse reflectance spectra of Ru^II-resin before and after catalytic oxidation are the same. In addition, little or no decomposition of Ru^II was also found to occur in homogeneous catalytic oxidation, as illustrated in Fig. 8. The UV-Vis spectra of Ru^II before and after catalytic oxidation are very similar, the difference in the UV region is probably due to the presence of oxidized organic products.

Fig. 8 UV-Vis spectra of Ru^IIII in homogeneous catalytic oxidation. (a) Before adding H₂O₂, [Ru^IIII] = 0.07 mM, [BPA] = 0.5 mM. (b) Immediately after adding H₂O₂, [H₂O₂] = 25 mM, (c) After 5 h, excess Na₂SO₃ added to remove H₂O₂.

Analysis of the intermediates and products formed in the oxidative degradation of bisphenol A

The intermediates formed during the degradation of bisphenol A were identified by LC/MS and LC/MS/MS experiments. Fig. 9 shows the total ions chromatogram (TIC) from LC/MS for the intermediates formed during bisphenol A degradation by Ru^IIII-Dowex after 7 h. Five major intermediates were identified, as listed in Table 1.

Fig. 9 Liquid chromatogram of the reaction mixture after 7 h at 23 °C. BPA, 0.5 mM; H₂O₂, 25 mM; Ru^IIII-Dowex, 1.0 g (8.2 μmol Ru^IIII); 50 mL water.

In the liquid chromatogram, the peak f is bisphenol A with a m/z of 227. The m/z value for each peak corresponds to the [M–H]⁻ ion that arises from deprotonation of a hydroxyl group. Since a reverse-phase LC column and a polar mobile phase were used, the products were eluted in decreasing order of polarity. Each peak was identified by interpretation of the MS/MS spectra. Similar intermediates have also been reported for the degradation of bisphenol A using Fenton's reagent¹⁵ and TiO₂ photocatalyst.²¹ The compounds in peak a and b have the same molecular weight and fragment ions, indicating that they are closely related isomers. The compound in peak a should be more polar, hence it is assigned as the ortho-quinone derivative, while that in peak b is assigned as the para-quinone derivative.

Although these aromatic intermediates could still have endocrine disrupting effects, they all eventually disappeared with time. In the LC of the reaction mixture after 24 h (Fig. 10), there is only one major peak. The main species in this peak was identified as butenedioic acid (HCOOCH=CHCOOH). GC-MS was also used to identify any volatile products not detected by LC-MS. Direct injection of the reaction mixture into GC-MS gave only one major peak which consists of N₂, O₂ and CO₂ with relative abundance of 69, 100 and 75, respectively. The amount of carbon dioxide detected is far greater than the background level in the atmosphere, indicating that it is derived from oxidative degradation of bisphenol A. Additional CO₂ could have escaped into the atmosphere since the oxidation reactions were done in the open air. The reaction solution was also extracted with ether (2 × 1 mL) and then analyzed by GC-MS (Fig. 11). In this case three major peaks were observed, which were identified as formic acid (x), oxalic acid (y) and acetic acid (z).

Fig. 10 Liquid chromatogram of the mixture containing BPA, H₂O₂ and Ru^IIII-Dowexcatalyst after 24 h reaction.

Proposed degradation pathways of bisphenol A

Based on the observed intermediates and products, it is evident that the degradation pathways involve a Fenton mechanism which produces hydroxyl radicals (˙OH) according to eqn (4).

Ru^IIII + H₂O₂ → Ru^IIIIIIOH + ˙OH (4)

The proposed oxidative degradation pathways for bisphenol A are shown in Fig. 12. The first step is the addition of ˙OH to an aromatic ring of bisphenol A to give I. The formation of hydroxylated derivatives by the reaction of hydroxyl radicals with aromatic rings is well-known.^22–25 The radical species I may then undergo hydrogen-atom abstraction (HAT) by Ru^IIIIII–OH to give the dihydroxylated derivative II, which can be further oxidized to give the trihydroxylated derivative III by the same mechanism. II and III are then oxidized viaHAT, first by ˙OH and then by Ru^IIIIIIOH, to give the quinones d and e. These phenolic and quinone derivatives of bisphenol A then undergo oxidative C–C bond cleavage by ˙OH to give benzophenone, substituted benzophenones and substituted phenols; which undergo further oxidation and ring degradation by ˙OH to finally give butenedioic acid, formic acid, acetic acid, oxalic acid and carbon dioxide.

Fig. 11 Gas chromatogram of the mixture (after extraction with diethyl ether) containing BPA, H₂O₂ and Ru^IIII-Dowexcatalyst after 24 h reaction.

Fig. 12 Proposed oxidative degradation pathways of BPA by Ru^IIII-Dowex/H₂O₂.

This catalytic system is reasonably efficient. In our studies we found that 50 equivalents of H₂O₂ are required for complete disappearance of bisphenol A. Complete conversion to CO₂ and H₂O would require 36 equivalents, whereas degradation to butenedioic acid requires 14 equivalents. Since a mixture of aliphatic acids and CO₂ were produced, we estimate that around 25 equivalents of H₂O₂ are used for the degradation of bisphenol A, the remaining H₂O₂ probably decomposes to H₂O and O₂.

Ru^IIII versus Fe^II/Fe^III as catalyst

The highly catalytic activity of polypyridyl ruthenium complexes such as [Ru^II(2,9-Me₂phen)₂(H₂O)₂]²⁺ compared to simple iron(II)/(III) salts may arise from the following factors. First, because of strong π-backbonding between Ru(II) and polypyridyl ligands, the geometry of Ru(II) is very similar to that of Ru(III), hence inter-conversion between the two oxidation states is rapid. For example, the self-exchange rate of [Ru(bpy)₃]^2+/3+ is 2 × 10⁹ M⁻¹s⁻¹,²⁶ while that of [Fe(OH₂)₆]^2+/3+ is 4 M⁻¹s⁻¹.²⁷ Hence the redox reaction between Ru^IIII and H₂O₂ to generate ˙OH is expected to be fast. On the other hand, Ru(III) aqua and hydroxo complexes bearing polypyridyl ligands are strong oxidants that can readily oxidize organic substrates via one-electron transfer or HAT.^5,28 This means Ru^IIIIII–OH can be readily converted back to Ru^IIII which can then react with H₂O₂ to generate more ˙OH. Moreover polypyridyl ruthenium(II) complexes are usually very stable at room temperature in aqueous solutions.⁵

Conclusions

The oxidative degradation of bisphenol A by H₂O₂ catalyzed by a polypyridyl ruthenium(II) complex, cis-[Ru^II(2,9-Me₂phen)₂(H₂O)₂]²⁺, that is supported on the cation-exchange resin Dowex-50W or Chelex-100, has been studied. The supported catalyst can decompose bisphenol A in aqueous solution with H₂O₂ as oxidant to give carbon dioxide and biodegradable aliphatic acids. The Ru^IIII-Resin is a much more active catalyst than simple iron(II)/(III) salts. Moreover it is active at a wide range of pH values, whereas the Fenton reagent can only function at pH < 4. The Ru^IIII-Resincatalyst can be readily reused without loss of activity. In conclusion, Ru^IIII-Dowex/H₂O₂ is potentially a useful catalytic system for the degradation of organic pollutants in water at a wide range of pH.

Experimental

Materials

Bisphenol A, H₂O₂ (35% by weight) and 2,9-dimethyl-1,10-phenanthroline (2,9-Me₂phen) were obtained from Sigma-Aldrich. Chelex-100 resin (sodium form, with iminodiacetate functional group) with 1% cross-linking and of 50–100 dry mesh size was also purchased from Sigma-Aldrich. Dowex-50W cation-exchange resin (with sulfonate functional group) with 8% cross-linking and of 50–100 dry mesh size was obtained from Dow Chemical Company. The resins were first washed with 1.0 M HCl, then with doubly distilled deionized water and dried at 100 °C for 2 h before use. Acetonitrile (HPLC grade) was obtained from Lab-Scan. All other chemicals were of analytical grade and were used without further purification. cis-[Ru^II(2,9-Me₂phen)₂(H₂O)₂](PF₆)₂ was prepared by a literature procedure.⁸ Found: C, 39.71; H, 3.49; N, 6.54. Calc. for C₂₈F₁₂H₂₈N₄O₂P₂Ru: C, 39.86; H, 3.35; N, 6.64. λ_max/nm (ε, dm³ mol⁻¹ cm⁻¹) in water: 496 (1.02 × 10⁴).

Immobilization of ruthenium catalyst onto resins

Typically 1 g of resin (Dowex-50W or Chelex-100) was added to a red solution of cis-[Ru^II(2,9-Me₂phen)₂(H₂O)₂](PF₆)₂ (Ru^IIII, 3–8 mg) in water (5 ml) at room temperature with continuous stirring. The colour of the resin gradually turned red, indicating adsorption of the ruthenium complex onto its surface. After 1 h, the solution was filtered and the Ru^IIII-Resin was washed with water and dried in vacuo.

Characterization of the supported catalysts

Diffuse reflectance UV-vis spectra of the Ru^IIII-Resins before and after catalytic oxidation were obtained using a Perkin Elmer Lambda 900 spectrophotometer. Since Ru^IIII-Resins after catalytic reaction were turned from pellet into powder form due to the stirring process, the Ru^IIII-Resins before catalysis were also ground into powder form for direct comparison.

The amount of catalyst adsorbed onto the resin was determined by adding 6 M HCl to Ru^IIII-Resin at room temperature with stirring for 1 h. The resin turned back to its original color and the HCl solution became red. The ruthenium content in the HCl solution was determined by a Perkin-Elmer Plasma 1000 inductively-coupled plasma atomic emission spectrometer (ICP-AES).

Oxidative degradation of bisphenol A

Ru^IIII-Resin was added to an aqueous solution containing bisphenol A and hydrogen peroxide, and the mixture was stirred at 500 rpm at 25 °C. Aliquots were withdrawn at various time intervals and centrifuged. Analysis of bisphenol A, intermediates and products were carried out by an Agilent 1200 series liquid chromatograph equipped with an Agilent C18 column, a UV detector and a API 2000 mass spectrometer (electrospray ionization). The mass spectrometer was operated at the anionic mode in the m/z range 50–500 for LC/MS and 50–300 for LC/MS/MS. The mobile phase was acetonitrile–water (55/45, v/v) with a flow rate of 200 μL min⁻¹. Final degradation products were also identified by GC-MS using an Agilent 6890 GC equipped with a DB-FFAP capillary column and an Agilent 5975 MSD mass selective detector.

Acknowledgements

The work described in this paper was supported by grants from the University Grants Committee of Hong Kong Special Administrative Region (CityU 2/06C, AoE/P-04/04). H.L. is also grateful for financial support provided by the Outstanding Youth Fund (No. 20525416) and National Natural Science Foundation of China (No. 20874094).

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Footnote

† Electronic supplementary information (ESI) available: mass spectra of intermediates. See DOI: 10.1039/c0nj00583e

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