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

Article abstract of DOI:10.1039/c0nj00583e

A polypyridyl ruthenium(ii) complex, cis-[Ru^II(2,9-Me ₂phen)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. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011.

Full text of DOI:10.1039/c0nj00583e

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PAPER
A recyclable polymer-supported ruthenium catalyst for the oxidative
degradation of bisphenol A in water using hydrogen peroxidew
Zongmin Hu,abc Chi-Fai Leung,a Yat-Kwai Tsang,a Hongxia Du,abc
Haojun Liang,bc Yongfu Qiua and Tai-Chu Lau*ab
Received (in Montpellier, France) 25th July 2010, Accepted 19th August 2010
DOI: 10.1039/c0nj00583e
A polypyridyl ruthenium(II) complex, cis-[RuII(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.
since at higher pH Fe(OH)3 precipitate will form which is
inactive.16,17 Fenton reaction also results in inevitable flocs
Introduction
A variety of ruthenium complexes, especially those bearing
formation which requires time consuming further treatments.
polypyridyl or macrocyclic tertiary amine ligands, are active
In general, ruthenium complexes are much more stable and
catalysts for the oxidation of various organic compounds
active oxidation catalysts than iron complexes, it is anticipated
including alkanes, alkenes, alkynes and alcohols; using term-
that they should also be useful catalysts for the oxidative
inal oxidants such as Ce(IV), tert-butyl hydroperoxide (TBHP),
degradation of organic pollutants in water. Although ruthe-
hydrogen peroxide or O2.1–12 Although these ruthenium com-
nium is more toxic and expensive than iron, if it can be made
plexes are among the most active catalysts for selective organic
in the form of a robust solid-supported catalyst that can be
oxidation, their use as catalysts for the oxidative degradation
readily recyclable without leaching into the environment, then
of organic pollutants in water has not been explored. The
its use is still a feasible process. We report herein the use of a
ruthenium(II) complex, cis-[RuII(2,9-Me2phen)2(H2O)2](PF6)2
amount of organic pollutants in wastewaters discharged by
various industries is increasing every year and is causing
(2,9-Me2phen = 2,9-dimethyl-1,10-phenanthroline), that is
serious global environmental problems. These pollutants in-
supported on
Chelex-100), as efficient catalyst for the oxidative degradation
of bisphenol (2,2-bis(4-hydroxyphenyl)propane), using
a cation-exchange resin (Dowex-50W or
clude a wide range of persistent organic chemicals, such as
pharmaceuticals and endocrine-disrupting compounds. We
are now faced with the challenge of removing these com-
pounds 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 More-
over, the Fenton reaction is only effective at low pH (o4),
A
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.
a Department of Biology and Chemistry, City University of
Hong Kong, Centre for Marine Environmental Research and
Innovative Technology (MERIT), Tat Chee Avenue, Kowloon,
Hong Kong, P. R. China. E-mail: bhtchlau@cityu.edu.hk
b Advanced Laboratory of Environmental Research and Technology
(ALERT), Joint Advanced Research Center, USTC-CityU, Suzhou,
Jiangsu 215123, P. R. China
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+ (RuII) in water, the colour of
the solution was rapidly discharged while the resin turned red,
indicating adsorption of the ruthenium(II) complex onto the
c Department of Polymer Science and Engineering, University of
Science and Technology of China, Hefei, Anhui 230026, P. R. China
w Electronic supplementary information (ESI) available: mass spectra
of intermediates. See DOI: 10.1039/c0nj00583e
c
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New J. Chem., 2011, 35, 149–155 149

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Table 1 List of major intermediates (a–e) and main fragments (m/z) obtained by LC-MS.
MS/MS fragments
Peak
Retention time/min
m/z (amu)
Molecular weight
(relative abundance)
Proposed structure
163(100)
107(64)
a
5.5
163
164
163(100)
107(64)
b
c
7.2
9.6
163
135
164
136
135(45)
120(27)
92(100)
257(100)
242(18)
229(97)
214(50)
d
12.0
257
258
241(44)
226(100)
e
f
14.3
16.3
241
227
242
228
227(100)
212(63)
133(20)
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 mmol RuII/g Dowex-50W
and 6.9 mmol RuII/g Chelex-100. Complete adsorption
occurs if the amount of RuII used is less than these
values. No RuII was detected in the solution after continuous
stirring of a RuII-Dowex suspension in water or in 1 M HNO3
for 4 h. On the other hand, although RuII-Chelex is
stable in water or 0.1 M HNO3, complete leaching of
RuII occurs in 1.0 M HNO3, as monitored by UV/Vis
spectrophotometry (RuII: lmax = 496 nm, e = 7.1 ꢀ
103 molꢁ1 dm3 cmꢁ1).
The diffuse reflectance spectrum of RuII-Dowex shows a
broad peak with lmax at around 500 nm (Fig. 1), which is
similar to that of RuII in water (496 nm). On the other hand,
the diffuse reflectance spectrum of RuII-Chelex shows lmax at
around 520 nm, slightly red-shifted compared with the
unsupported RuII complex.
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 RuII-Dowex or RuII-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 RuII, RuII-Dowex or
RuII-Chelex. Theoretically 36 moles of H2O2 are required to
completely oxidize 1 mole of BPA to CO2 and H2O. As
expected, RuII has the highest activity, while RuII-Chelex is
the least active catalyst, although it has a faster initial rate
than RuII-Dowex. RuCl3ꢂxH2O, which is a common ruthenium
complex, is much less effective. No degradation of BPA
Fig. 1 Diffuse reflectance spectra of (a) RuII-Dowex (b) RuII-Chelex
(c) RuII-Dowex after catalytic oxidation.
Fig. 2 Time course for the ruthenium catalyzed degradation of BPA
(0.50 mM) by H2O2 (25 mM) in water (50 mL). T = 25 1C.
c
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Fig. 3 Effect of H2O2 concentration on degradation of BPA.
RuII-Dowex (8.2 mmol RuII/g Dowex), 1.0 g; bisphenol A, (0.50 mM) in
50 mL water. T = 25 1C.
Fig. 4 Plot of 1/kobs versus 1/[H2O2] for the RuII-Dowex catalyzed
oxidative degradation of BPA by H2O2. Slope = (7.12 ꢃ 0.88) ꢀ 10ꢁ3
,
y-intercept = 3.09 ꢃ 0.11, r = 0.9924. T = 25 1C.
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 (lmax of RuII) were o0.005, indicating that [RuII] in
the solutions were o10ꢁ7 M. In an independent experiment,
no oxidation of bisphenol A was found to occur when a
1 ꢀ 10ꢁ7 M RuII was used as catalyst. These results strongly
suggest that the active catalyst is the supported one.
Since RuII-Dowex is more active than RuII-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. 5 Effect of the amount of RuII-Dowex catalyst on the oxidative
degradation of BPA. H2O2, 25 mM; bisphenol A, 0.50 mM in 50 mL
water. T = 25 1C.
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 kobs. The
plot of 1/kobs versus 1/[H2O2] is linear (Fig. 4), which is
consistent with equilibrium formation of a precursor complex
between RuII-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 ꢃ 0.45) ꢀ 10ꢁ1 sꢁ1 and
(430 ꢃ 10) Mꢁ1, respectively at 25 1C.
K
RuII þ H2O2
Ð
RuII ꢂ H2O2
ð1Þ
ð2Þ
k
RuII ꢂ H2O2 þ BPA ꢁ! products
d½RuIIꢄ
kK½H2O2ꢄ
1 þ ½H2O2ꢄ
ꢁ
¼ kobs½RuIIꢄ ¼
½RuIIꢄ
ð3Þ
Fig. 6 Plot of kobs versus amount of RuII-Dowex in the oxidative
dt
degradation of BPA by H2O2. Slope = (2.86 ꢃ 0.14) ꢀ 10ꢁ1 gꢁ1 sꢁ1
,
r = 0.9986. T = 25 1C.
The effect of the amount of catalyst on the degradation of
bisphenol A is illustrated in Fig. 5.
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 mmol RuII/g) was used, the rate is similar to using 1 g of
catalyst with ruthenium loading of 8.2 mmol RuII/g, although
the former catalyst should have a higher surface area.
The catalytic activity of RuII-Dowex was also investigated at
various pH. As shown in Fig. 7, RuII-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
c
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Fig. 9 Liquid chromatogram of the reaction mixture after 7 h at
23 1C. BPA, 0.5 mM; H2O2, 25 mM; RuII-Dowex, 1.0 g (8.2 mmol RuII);
50 mL water.
Fig.
7
Oxidative degradation of BPA (0.5 mM) catalyzed by
RuII-Dowex at various pH. H2O2, 25 mM; RuII-Dowex, 1.0
(8.2 mmol RuII); 50 mL water. T = 25 1C.
g
Fig. 9 shows the total ions chromatogram (TIC) from LC/MS
for the intermediates formed during bisphenol A degradation
by RuII-Dowex after 7 h. Five major intermediates were
identified, as listed in Table 1.
reagent, which is ineffective at pH > 4 due to the precipitation
of Fe(OH)3.17
Recycling of the polymer-supported RuII catalyst
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.
After catalytic reaction, the solution was centrifuged. The
RuII-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 RuII-resin before
and after catalytic oxidation are the same. In addition, little or
no decomposition of RuII was also found to occur in
homogeneous catalytic oxidation, as illustrated in Fig. 8.
The UV-Vis spectra of RuII before and after catalytic oxidation
are very similar, the difference in the UV region is probably
due to the presence of oxidized organic products.
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
(HCOOCHQCHCOOH). 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
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. 8 UV-Vis spectra of RuII in homogeneous catalytic oxidation.
(a) Before adding H2O2, [RuII] = 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.
Fig. 10 Liquid chromatogram of the mixture containing BPA, H2O2
and RuII-Dowex catalyst after 24 h reaction.
c
152 New J. Chem., 2011, 35, 149–155
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RuII versus FeII/FeIII as catalyst
The highly catalytic activity of polypyridyl ruthenium
complexes such as [RuII(2,9-Me2phen)2(H2O)2]2+ compared
to simple iron(II)/(III) salts may arise from the following
factors. First, because of strong p-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 Mꢁ1 sꢁ1 26
,
while that of
[Fe(OH2)6]2+/3+ is 4 Mꢁ1 sꢁ1 27
Hence the redox reaction
.
between RuII and H2O2 to generate ꢅOH 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 RuIII–OH can be readily converted
back to RuII which can then react with H2O2 to generate
more ꢅOH. Moreover polypyridyl ruthenium(II) complexes
are usually very stable at room temperature in aqueous
solutions.5
Fig. 11 Gas chromatogram of the mixture (after extraction with
diethyl ether) containing BPA, H2O2 and RuII-Dowex catalyst after
24 h reaction.
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).
Conclusions
The oxidative degradation of bisphenol A by H2O2 catalyzed
by a polypyridyl ruthenium(II) complex, cis-[RuII(2,9-Me2-
phen)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 RuII-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 o 4. The RuII-Resin catalyst
can be readily reused without loss of activity. In conclusion,
RuII-Dowex/H2O2 is potentially a useful catalytic system for
the degradation of organic pollutants in water at a wide
range of pH.
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 (ꢅOH)
according to eqn (4).
RuII + H2O2 - RuIIIOH + OH
(4)
ꢅ
The proposed oxidative degradation pathways for bisphenol A
are shown in Fig. 12. The first step is the addition of ꢅOH 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
RuIII–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 via HAT,
first by ꢅOH and then by RuIIIOH, to give the quinones
d and e. These phenolic and quinone derivatives of bisphenol
A then undergo oxidative C–C bond cleavage by ꢅOH to give
benzophenone, substituted benzophenones and substituted
phenols; which undergo further oxidation and ring degradation
by ꢅOH to finally give butenedioic acid, formic acid, acetic
acid, oxalic acid and carbon dioxide.
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 1C for 2 h before use.
Acetonitrile (HPLC grade) was obtained from Lab-Scan. All
other chemicals were of analytical grade and were used with-
out further purification. cis-[RuII(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. lmax/nm (e, dm3 molꢁ1 cmꢁ1) in water:
496 (1.02 ꢀ 104).
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.
c
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Fig. 12 Proposed oxidative degradation pathways of BPA by RuII-Dowex/H2O2.
Immobilization of ruthenium catalyst onto resins
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).
Typically 1 g of resin (Dowex-50W or Chelex-100) was added
to a red solution of cis-[RuII(2,9-Me2phen)2(H2O)2](PF6)2
(RuII, 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 RuII-Resin
was washed with water and dried in vacuo.
Oxidative degradation of bisphenol A
RuII-Resin was added to an aqueous solution containing
bisphenol
A and hydrogen peroxide, and the mixture
was stirred at 500 rpm at 25 1C. 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 mL minꢁ1. 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.
Characterization of the supported catalysts
Diffuse reflectance UV-vis spectra of the RuII-Resins
before and after catalytic oxidation were obtained using a
Perkin Elmer Lambda 900 spectrophotometer. Since
RuII-Resins after catalytic reaction were turned from pellet
into powder form due to the stirring process, the RuII-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 RuII-Resin at room
temperature with stirring for 1 h. The resin turned back
c
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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
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