Determination of catalytic oxidation products of phenol by RP-HPLC

Article abstract of DOI:10.1007/s11164-011-0370-3

A reversed-phase high-performance liquid chromatography (RP-HPLC) with ultraviolet detection was established for the determination of phenol, catechol, hydroquinone, and p-benzoquinone in the reaction solution of catalytic oxidation of phenol using hydrogen peroxide as the oxidant and copper-doped FeSBA-15 zeolite as the catalyst. Separation was accomplished on a reversed-phase C₁₈ column, and the elution condition was optimized by changing the composition of the mobile phase. A good resolution of all of the relative components in the reaction solution was achieved when the mobile phase was methanol-water-1% acetic acid aqueous solution = 10:50:40 (v/v/v). The concentrations of phenol, catechol, hydroquinone, and p-benzoquinone were determined in 11 different reaction solutions by the external standard method. The proposed HPLC method was simple, accurate, reliable, and suitable for tracing the amount of target products during the catalytic oxidation reaction of phenol. The results can provide data support for evaluating the properties of catalysts, and, thus, guide the selection of catalysts for the industrial production of dihydric phenol. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11164-011-0370-3

Res Chem Intermed (2012) 38:549–558
DOI 10.1007/s11164-011-0370-3
Determination of catalytic oxidation products of phenol
by RP-HPLC
•
Jun-Qin Qiao Na Yuan Chang-Jin Tang
•
•
•
Jing Yang Jian Zhou Hong-Zhen Lian Lin Dong
•
•
Received: 16 July 2011 / Accepted: 23 August 2011 / Published online: 7 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A reversed-phase high-performance liquid chromatography (RP-HPLC)
with ultraviolet detection was established for the determination of phenol, catechol,
hydroquinone, and p-benzoquinone in the reaction solution of catalytic oxidation of
phenol using hydrogen peroxide as the oxidant and copper-doped FeSBA-15 zeolite
as the catalyst. Separation was accomplished on a reversed-phase C18 column, and
the elution condition was optimized by changing the composition of the mobile
phase. A good resolution of all of the relative components in the reaction solution
was achieved when the mobile phase was methanol–water–1% acetic acid aqueous
solution = 10:50:40 (v/v/v). The concentrations of phenol, catechol, hydroquinone,
and p-benzoquinone were determined in 11 different reaction solutions by the
external standard method. The proposed HPLC method was simple, accurate, reli-
able, and suitable for tracing the amount of target products during the catalytic
oxidation reaction of phenol. The results can provide data support for evaluating the
J.-Q. Qiao Á H.-Z. Lian Á L. Dong
Center of Materials Analysis, Nanjing University, 22 Hankou Road, Nanjing 210093, China
J.-Q. Qiao Á N. Yuan Á J. Yang Á J. Zhou Á H.-Z. Lian
State Key Laboratory of Analytical Chemistry for Life Science, Nanjing University,
22 Hankou Road, Nanjing 210093, China
N. Yuan Á C.-J. Tang Á J. Yang Á J. Zhou Á H.-Z. Lian (&) Á L. Dong (&)
School of Chemistry & Chemical Engineering, Nanjing University, 22 Hankou Road,
Nanjing 210093, China
e-mail: hzlian@nju.edu.cn
L. Dong
e-mail: donglin@nju.edu.cn
C.-J. Tang Á L. Dong
Key Laboratory of Mesoscopic Chemistry, Ministry of Education of China, Nanjing University,
22 Hankou Road, Nanjing 210093, China
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properties of catalysts, and, thus, guide the selection of catalysts for the industrial
production of dihydric phenol.
Keywords Reversed-phase high-performance liquid chromatography (RP-HPLC) Á
Phenol Á Catechol Á Hydroquinone Á p-Benzoquinone Á Catalytic oxidation
Introduction
The catalytic oxidation of phenol has attracted considerable attention in recent years
due to the important use of products generated from this reaction, namely, catechol
and hydroquinone. Catechol and hydroquinone are basic organic chemical raw
materials, intermediates, and additives of medicine, pesticide, dye, perfume, rubber,
and photographic material. In addition, they find uses as auto-oxidation inhibitor,
monomer polymerization inhibitor, oil anticoagulant, catalyst for ammonia, etc.
[1–3]. The catalytic oxidation of phenol using hydrogen peroxide as an oxidant over
catalyst (Fig. 1) has become a main method for the large-scale production of dihydric
phenols, owing to its good reputation of ‘‘clean technology’’. Obviously, the catalyst
plays a key role in this technology. The incorporation of transition metal ions into
framework sites of SBA-15, a new type of mesoporous silica molecular sieve, has
become a popular procedure for the preparation of catalysts in the oxidation of
phenol [4–6]. Commonly, the conversion rate of phenol and yields of dihydric
phenols are used for estimating the extent of the reaction. Therefore, an indispensable
part of the catalytic oxidation of phenol is to develop an effective method for the
determination of products and related compounds in this reaction.
The analytical methods for phenol and dihydric phenols include ultraviolet–
visible (UV) spectrophotometry [7], thin-layer chromatography (TLC) [8], and
fluorescence [9–12], while the most frequently used methods are gas chromatog-
raphy (GC) [13–16], high-performance liquid chromatography (HPLC) [17–20],
and electrochemical methods [21–24]. The UV spectra of phenol, catechol, and
hydroquinone overlap with one another when using spectrophotometry, so software
analysis such as the successive approximation method [25], multi-wavelength linear
OH
OH
OH
OH
OH
O
O
oxidation further
H2O2
tar
catalyst
Fig. 1 Catalytic oxidation reaction of phenol
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Determination of catalytic oxidation
551
regression-differential coefficient spectrophotometry [26], and factor analysis-
differential coefficient spectrophotometry [27] must be used in order to distinguish
them. However, these analyses are complex and of low sensitivity. The selection of
developing solvent is time-consuming in TLC analysis. Conventional fluorometry
cannot determine dihydric phenols simultaneously. Yang et al. [12] obtained a better
result when the synchronous-polarization-derivative technique was used. The
common electrochemical method to determine phenol and dihydric phenols is to
employ a modified electrode. However, chemical modification to prepare electrodes
is complex, expensive, and of low reproducibility. The accuracy of using GC to
determine phenols is low, due to the poor stability and reliability of GC combined
with the oxidation property of phenols under operation temperature. Comparatively,
reversed-phase (RP)-HPLC is simple, rapid, and accurate. However, the previously
reported RP-HPLC methods focused on a single determination or two or three
simultaneous determinations of phenol and dihydric phenols. There has been no
report on the simultaneous determination of phenol, catechol, hydroquinone, and
p-benzoquinone to date.
Based on the actual situation of impurities in samples, an RP-HPLC–UV method
was established, which can accurately determine phenol, catechol, hydroquinone,
and p-benzoquinone simultaneously. The obtained data intuitively reflected the
extent of the reaction; thereby, it provided the basis for evaluating the catalytic
performance of catalysts, and then it directed the technology optimization of the
catalytic oxidation of phenol to dihydric phenols.
Experimental
Instruments and reagents
The HPLC instrumentation included a Waters Alliance 2695 Separations Module
equipped with a vacuum degasser, a quaternary pump, and an auto–sampler, and
was connected to a 996 UV–Vis photodiode–array (PDA) detector and an Empower
chromatography manager system (Waters, Milford, MA, USA).
Reference substances (RSs) of phenol (C98.0%), catechol (C98.0%), hydroqui-
none (C98.0%), and p-benzoquinone (C99.0%) were purchased from Shanghai
Lingfeng Chemical Reagent Co., Ltd. (Shanghai, China), Sinopharm Chemical
Reagent Co., Ltd. (Shanghai, China), Nanjing Chemical Reagent Co., Ltd. (Nanjing,
China), and Shanghai Yuming Industrial Co., Ltd. Yuanhang Reagent Factory
(Shanghai, China), respectively. Acetic acid (C99.5%, analytical reagent) was
purchased from Sinopharm Chemical Reagent Co., Ltd. Methanol (HPLC grade)
was purchased from Merck (Darmstadt, Germany). Wahaha purified water (Wahaha
Group Co., Ltd., Hangzhou, China) was used throughout the experiment.
The samples used in the present experiment were the reaction solutions of phenol
oxidized by hydrogen peroxide with FeSBA-15 zeolite doped with different
amounts of copper as catalysts.
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Chromatographic conditions
HPLC separation was carried out on an Agela Venusil XBP C18 column
(250 9 4.6 mm i.d., 5 lm) (Agela Technology Inc., USA). The column temperature
was maintained at 30 °C. An isocratic elution with methanol, water, and 1% (vol.%)
acetic acid solution (10:50:40, v/v/v) was employed at a flow rate of 1.0 mL/min.
The injection volume was 10 lL. The effluent was monitored by the PDA detector
set at 244 nm for p-benzoquinone, and 275 nm for phenol, catechol, and
hydroquinone.
Results and discussion
Standard curves and detection limits
The individual stock solutions of phenol, catechol, hydroquinone, and p-benzoqui-
none used for calibration purposes were prepared by separately weighing a certain
amount of RS into four different 10-mL volumetric flasks, among which phenol,
catechol, and hydroquinone were dissolved and made up to the mark with water and
p-benzoquinone with methanol. A proper amount of each stock solution was
transferred into a 10-mL volumetric flask, mixed, and water was added to the
volume. The concentrations of phenol, catechol, hydroquinone, and p-benzoquinone
in mixed stock solution were 1.03, 0.154, 0.101, and 0.0112 mg/mL, respectively.
Mixed standard solutions were obtained by diluting the mixed stock solution step by
step with water and storing in the dark at low temperature. Each mixed standard
solution was injected two times consecutively. The linearity of the calibration curve
was obtained from the regression of the peak area versus the concentration of the
standard. The correlation coefficient (r) of p-benzoquinone was lower presumably
because of the lower concentration level than the others. The limit of detection
(LOD) was defined as the concentration where the peak height was three times the
background level (Table 1).
Chromatograms of mixed standard solution are shown in Fig. 2. The elution
order of the four compounds was hydroquinone, p-benzoquinone, catechol, and
Table 1 Standard curves and detection limits
Component
Linear range
(mg/mL)
Standard curve
Correlation Detection
coefficient (r) limit (mg/mL)
Hydroquinone
6.00 9 10-4 *0.10 A = -1,403.17 ? 8.51 9
0.9999
0.9845
1.0000
1.0000
2.00 9 10-4
5.00 9 10-5
3.00 9 10-4
2.50 9 10-3
106C
p-Benzoquinone 1.50 9 10-4 *0.01 A = 5,525.63 ? 5.72 9
106C
Catechol
Phenol
9.00 9 10-4 *0.15 A = -357.26 ? 1.19 9
107C
7.50 9 10-3 *1.03 A = -2,996.75 ? 6.99 9
106C
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Determination of catalytic oxidation
553
Fig. 2 Chromatograms of mixed standards. Column: Agela Venusil XBP C18, 250 9 4.6 mm i.d.,
5 lm; column temperature: 30 °C; mobile phase: methanol–water–acetic acid (1%) aqueous
solution = 10:50:40 (v/v/v); flow rate: 1.0 mL/min; injection volume: 10 lL; wavelength for UV
detection: 244 nm (a) and 275 nm (b). Peaks: 1 hydroquinone (0.0101 mg/ml), 2 p-benzoquinone
(0.00112 mg/mL), 3 catechol (0.0154 mg/mL), 4 phenol (0.103 mg/mL)
phenol, which is associated with the hydrophobic interaction mechanism of RP-
HPLC. The hydrophobic group is the same for the four substances, that is, benzene
ring, therefore, the elution order is determined by the strength of the hydrophilic
group. Hydroquinone has two hydrophilic –OH groups which are in para-position,
and hydroxyl oxygen apt to form hydrogen bonds with water in the mobile phase.
The two carbonyl oxygen of p-benzoquinone also can form hydrogen bonds with
water, which is a little weaker than that of hydroxyl oxygen. The intra-molecular
hydrogen bond between two ortho-position –OH in catechol reduces the hydroxyl
oxygen number to form hydrogen bonds with water. Phenol has only one hydroxyl
oxygen to form hydrogen bonds. So, the hydrophobic interaction of hydroquinone
with the stationary phase is the weakest and it was eluted first, while the retention of
phenol on the hydrophobic stationary phase is the strongest.
Determination of related substances in samples
A quantity of 1.00 mL of catalytic oxidation reaction solution 1 was accurately
transferred to 10-mL volumetric flasks in five duplicates, for the intra-day
measurement. The first sample solution was injected five times consecutively, and
the determined concentrations of hydroquinone, catechol, and phenol were 0.151,
0.240, and 1.94 mg/mL, respectively, with relative standard deviations (RSDs) of
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1.6%, 1.8%, and 1.2%, respectively. p-Benzoquinone was not detected. The rest of
the four sample solutions were all injected twice. Inter-day measurement was
performed by the analysis of five duplicates of catalytic oxidation reaction solution
1 diluted ten times in five consecutive days. The results with intra- and inter-day
RSDs listed in Table 2 displays a good precision of the established RP-HPLC
method for all of the substances determined. On this basis, the concentrations of the
four substances in the other ten catalytic oxidation reaction solutions were
determined subsequently, and the results are listed in Table 3. For the 11 reaction
solutions, p-benzoquinone was always not detected. From Tables 2 and 3, it can be
seen that the concentration of phenol is the lowest and that of dihydric phenols is the
highest in catalytic oxidation reaction solution 8. That is to say, a higher yield of
dihydric phenols can be produced when using the catalyst of solution 8.
Recovery test
Catalytic oxidation reaction solution 1 was prepared in triplicate. Each sample was
spiked with different amounts of mixed standard solution and diluted to the mark
with water. The recoveries were in the range of 97.6–104.5% (Table 4).
Optimization of the chromatographic conditions
Products in the 11 catalytic oxidation reaction solutions are almost the same, owing
to the similar reaction conditions, except for the catalysts used. Therefore, only one
of them, spiked with p-benzoquinone reference, was chosen as the test solution in
the optimization of the chromatographic condition.
Selection of the mobile phase
Because of the weak acidity of phenol, catechol, and hydroquinone, acetic acid was
chosen as the ion suppressor in the mobile phase. The proportion of 1% acetic acid
aqueous solution was kept at 40%, and separation was optimized by changing the
content of methanol and water. The separation effect at different methanol
proportions is shown in Fig. 3. A good separation with a suitable run time was
obtained when the proportion of methanol was 10%. Figure 4 shows the
chromatograms of the test solution spiked with p-benzoquinone reference under
Table 2 Determination of the four related substances in catalytic oxidation reaction solution 1
Component
Hydroquinone
p-Benzoquinone
Catechol
Phenol
Intra-day (n = 5)
Concentration (mg/mL)
RSD (%)
0.151
1.6
ND
–
0.240
1.8
1.94
1.2
Inter-day (n = 5)
Concentration (mg/mL)
RSD (%)
0.154
2.1
ND
–
0.235
2.6
1.91
2.4
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Table 3 Determination of the four related substances in the other ten catalytic oxidation reaction
solutions
Reaction solution
Concentration (mg/mL)
Hydroquinone
p-Benzoquinone
Catechol
Phenol
2
0.161
0.186
0.190
0.185
0.204
0.203
0.207
0.206
0.167
0.185
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
0.241
0.273
0.279
0.268
0.301
0.294
0.305
0.304
0.255
0.294
1.90
1.87
1.87
1.89
1.80
1.81
1.79
1.80
1.88
1.86
3
4
5
6
7
8
9
10
11
Table 4 Recoveries of the four related substances in catalytic oxidation reaction solution 1
Component
Original Added
Concentration (mg/mL)
Detected
Recovery (%)
Hydroquinone
0.01510
ND
0.00507
0.02005
0.02502
0.03578
0.00058
0.00117
0.00219
0.03161
0.03930
0.05589
0.2450
97.6
97.8
0.01014
0.02028
0.00056
0.00112
0.00224
0.00772
0.01544
0.03088
0.0516
102.0
103.6
104.5
97.8
p-Benzoquinone
Catechol
0.02403
0.1942
98.2
98.9
103.2
98.4
Phenol
0.1031
0.2995
102.1
97.8
0.2062
0.3958
the mobile phase of methanol–water–acetic acid (1%) aqueous solution = 10:50:40
(v/v/v).
Detection wavelength
The UV absorption spectra of the four compounds obtained by the online PDA
detector are shown in Fig. 5. Hydroquinone, catechol, and phenol have similar
absorption, with characteristics approximately in 220 and 270 nm, due to their
similarity in structure. p-Benzoquinone, which is largely different to the others
in structure, displays a unique absorption with characteristic of 244 nm. However,
in this wavelength, the other three compounds have very low absorption. It is
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40
35
30
25
20
15
10
5
D
C
B
A
0
0
10
20
30
40
50
CH OH / %
3
Fig. 3 Effect of the methanol proportion on the retention factor. A hydroquinone, B p-benzoquinone,
C catechol, D phenol
Fig. 4 Chromatograms of the test solution, added by p-benzoquinone reference, under 244 nm (a) and
275 nm (b). Peaks: 1 hydroquinone, 2 p-benzoquinone, 3 catechol, 4 phenol
difficult to choose a proper wavelength suitable for all of the compounds monitored.
So, dual-wavelength detection was selected, in order to increase the detection
sensitivity. Finally, p-benzoquinone was detected under 244 nm, and hydroquinone,
catechol, and phenol under 275 nm.
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Determination of catalytic oxidation
557
Fig. 5 UV absorption spectra for the four relative substances: a hydroquinone, b p-benzoquinone,
c catechol, d phenol
Conclusions
A reversed-phase high-performance liquid chromatography (RP-HPLC) method
was developed for the detection of phenol, catechol, hydroquinone, and p-benzo-
quinone based on the actual situation of impurities in the oxidation reaction solution
of phenol with copper-doped FeSBA-15 zeolite as the catalyst. Two detection
wavelengths were employed, i.e., 244 nm for p-benzoquinone and 275 nm for
phenol, catechol, and hydroquinone. The four related compounds in 11 different
reaction solutions were analyzed, respectively. The method is accurate and reliable.
It can effectively evaluate the extent of the catalytic oxidation reaction and provide
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the basis for the selection of catalyst, thereby, guiding the optimization of the
process of the catalytic oxidation of phenol.
Acknowledgment This study was supported by the National Natural Science Foundation of China
(90913012, 20873060, 20973091), the National Basic Research Program of China (973 program,
2009CB421601, 2011CB911003, 2009CB623500), National Science Funds for Creative Research Groups
(20821063), and the Analysis & Test Fund and Starting Fund for Young Teachers of the Center of
Materials Analysis, Nanjing University.
References
1. F. Adam, J. Andas, I. Ab Rahman, Chem. Eng. J. 65, 658 (2010)
2. Y.C. Hu, D.H. Han, C.H. Qi, P. Yin, T. Liang, Chem. React. Eng. Tech. 22, 544 (2006)
3. V. Rives, O. Prieto, A. Dubey, S. Kannan, J. Catal. 220, 161 (2003)
4. L.P. Wang, A.G. Kong, B. Chen, H.M. Ding, Y.K. Shan, M.Y. He, J. Mol. Catal. A 230, 143 (2005)
5. H.C. Xin, J. Liu, F.T. Fan, Z.C. Feng, G.Q. Jia, Q.H. Yang, C. Li, Microporous Mesoporous Mater.
113, 231 (2008)
6. B.P. Nethravathi, K.N. Mahendra, J. Porous Mater. 107, 17 (2010)
7. W.H. Wang, X.F. Yu, L. Wen, Chin J. Mod. Med. 12, 86 (2002)
8. S.P. Huang, R.J. Xu, Q. Wang, Chin. J. Anal. Chem. 27, 331 (1999)
9. M. Bosco, M.P. Callao, M.S. Larrechi, Talanta 72, 800 (2007)
10. S.Y. Bao, Y.Q. Cheng, S.Y. Shi, Chin. J. Anal. Chem. 31, 357 (2003)
11. B. Tang, W.H. Li, X.W. He, Acta Pharm. Sinic. 31, 700 (1996)
12. J.D. Yang, S.R. Zhang, S.P. Liu, Acta Chim. Sinica 65, 2309 (2007)
13. K.M. Parida, S. Mallick, J. Mol. Catal. A 279, 104 (2008)
14. M.R. Maurya, S.J.J. Titinchi, S. Chand, J. Mol. Catal. A 193, 165 (2003)
15. G.P. Cao, W. Sun, M.H. Zhang, H.H. Le, Z.N. Zhong, Chem. React. Eng. Technol. 17, 322 (2007)
16. K. Bielicka-Daszkiewicz, A. Voelkel, M. Szejner, J. Osypiuk, Chemosphere 890, 62 (2006)
17. S. Shahrzad, I. Bitsch, J. Chromatogr. A 741, 223 (1996)
18. R. Klaewkla, S. Kulprathipanja, P. Rangsunvigit, T. Rirksomboon, W. Rathbun, L. Nemeth, Chem.
Eng. J. 129, 21 (2007)
19. O. Fiehn, M. Jekel, J. Chromatogr. A 769, 189 (1997)
20. A. Asan, I. Isildak, J. Chromatogr. A 988, 145 (2003)
21. R.M. de Carvalho, C. Mello, L.T. Kubota, Anal. Chim. Acta 420, 109 (2000)
22. X.Y. Sun, S. Hu, L.F. Li, J. Xiang, W. Sun, J. Electroanal. Chem. 65, 94 (2011)
23. M.E. Rueda, L.A. Sarabia, A. Herrero, M.C. Ortiz, Anal. Chim. Acta 479, 173 (2003)
24. H. Zhang, A. Galal, J.F. Rubinson, I. Marawi, T.H. Ridgway, S.K. Lunsford, H. Zimmer, H.B. Mark
Jr., Electrochim. Acta 3511, 43 (1998)
25. X.W. He, D.G. Zhang, Chin. J. Anal. Chem. 7, 179 (1979)
26. C.X. He, Z.X. Wang, L.S. Chen, Anal. Lab. 15, 25 (1996)
27. P. Liu, Y.Z. Su, Anal. Lab. 18, 50 (1999)
123