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Acknowledgements
We greatly appreciate financial support from Yantai Science &
Technology Bureau (Project 2008155) and the Department of
Science & Technology of Shandong Province (Project
2006GG2205033). Additionally, we are grateful to Prof. Dr. Wenzhong
Wu for his helpful discussions in our works.
References
[1] F. Alonso, I.P. Beletskaya, M. Yus, Chem. Rev. 102 (2002) 4009.
[2] K.V.R. Chary, P.V. Ramana Rao, V. Venkat Rao, Catal. Commun. 9 (2008) 886.
[3] X.G. Liu, J.X. Chen, J.Y. Zhang, Catal. Commun. 8 (2007) 1905.
[4] M.A. Keane, R. Larsson, Catal. Commun. 9 (2008) 333.
[5] E. Lopez, S. Ordonez, F.V. Diez, Appl. Catal. B: Environ. 62 (2006) 57.
[6] C. Schueth, N.A. Kummer, C. Weidenthaler, H. Schad, Appl. Catal. B: Environ. 52
(2004) 197.
[7] S. Zinovyev, A. Shelepchikov, P. Tundo, Appl. Catal. B: Environ. 72 (2007) 289.
[8] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, A. Marinas, J.M. Marinas, F.J.
Urbano, Appl. Catal. B: Environ. 43 (2003) 71.
[9] G.V. Lowry, M. Reinhard, Environ. Sci. Technol. 34 (2000) 3217.
[10] Y. Ukisu, S. Kameoka, T. Miyadera, Appl. Catal. B: Environ. 27 (2000) 97.
[11] J.M. Moreno, M.A. Aramendia, A. Marinas, J.M. Marinas, F.J. Urbano, Appl. Catal.
B: Environ. 76 (2007) 34.
[12] B. Leger, A. Nowicki, A. Roucoux, J.P. Rolland, J. Mol. Catal. A: Chem. 266 (2007)
221.
[13] G. Yuan, M.A. Keane, Catal. Commun. 4 (2003) 195.
[14] J.B. Hoke, G.A. Gramiccioni, E.N. Balko, Appl. Catal. B: Environ. 1 (1992) 285.
[15] Y. Ukisu, T. Miyadera, J. Hazard. Mat. 122 (2005) 1.
[16] U.M. Lindstrom, Chem. Rev. 102 (2002) 2751.
[17] L. Calvo, M.A. Gilarranz, J.A. Casas, A.F. Mohedano, J.J. Rodriguez, Appl. Catal. B:
Environ. 78 (2008) 259.
[18] G.V. Lowry, M. Reinhard, Environ. Sci. Technol. 33 (1999) 1905.
[19] B.W. Zhu, T.T. Lim, Environ. Sci. Technol. 41 (2007) 7523.
[20] S. Zinovyev, P. Tundo, Appl. Catal. B: Environ. 75 (2007) 124.
[21] C.H. Xia, J. Xu, W.Z. Wu, Q. Luo, J.P. Chen, Q. Zhang, X.M. Liang, Appl. Catal. B:
Environ. 45 (2003) 281.
[22] C.H. Xia, J. Xu, W.Z. Wu, X.M. Liang, Catal. Commun. 5 (2004) 383.
[23] Z.J. Yang, C.H. Xia, Q. Zhang, J.P. Chen, W.Z. Wu, X.M. Liang, Waste Manage. 27
(2007) 588.
[24] Z.J. Yang, C.H. Xia, Q. Zhang, J.P. Chen, W.Z. Wu, X.M. Liang, A. Kettrup, Fresen.
Environ. Bull. 15 (2006) 86.
[25] F. Zhang, J.P. Chen, H. Zhang, Y.W. Ni, X.M. Liang, Chemosphere 68 (2007)
1716.
[26] V. Felis, C. De Bellefon, P. Fouilloux, D. Schweich, Appl. Catal. B: Environ. 20
(1999) 91.
[27] G. Yuan, M.A. Keane, Chem. Eng. Sci. 58 (2003) 257.
[28] G. Yuan, M.A. Keane, Catal. Today 88 (2003) 27.
[29] F.J. Urbano, J.M. Marinas, J. Mol. Catal. A 173 (2001) 329.
[30] M.A. Aramendia, R. Burch, I.M. Garcia, A. Marinas, J.M. Marinas, B.W.L.
Southward, F.J. Urbano, Appl. Catal. B: Environ. 31 (2001) 163.
[31] G. Yuan, M.A. Keane, J. Catal. 225 (2004) 510.
[32] G. Yuan, M.A. Keane, Appl. Catal. B: Environ. 52 (2004) 301.
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Furthermore, it was also observed that the dechlorination rates
of 2-CP and 4-CP were different in aqueous solutions and in ethanol
systems when the bases were different in the reactions. The
rates of 4-CP and 2-CP were much higher when using NaOH than
those with Et3N in aqueous solutions. However, in ethanol, the
rates of 4-CP and 2-CP were much higher when using Et3N as a proton
scavenger. The reasons of these results were possibly relevant
to the solubility and basicity of the bases. Contrast to triethylamine
(Et3N), a weak organic base, NaOH is a stronger base and much easier
to react with HCl produced in the dechlorination of chlorophenols
than Et3N. However, in ethanol, Et3N, as an organic base, is
easier to dissolve than NaOH. Therefore, it was more advantageous
to eliminate the poison of HCl to Pd/C for Et3N in ethanol.
Additionally, both dechlorination rates of 2-CP and 4-CP were
similar when using NaOH and Et3N in ethanol, which was completely
different to the results in water (Table 2). These variations
of the rates illustrated that the bases could affect the reactivity of
the dechlorination of organic chlorides in liquid phases. It also implied
that the roles of the bases were not limited to neutralize
hydrochloric acid produced in the hydrodechlorination of monochlorophenols.
The bases possibly had some influences on the
activity of the catalyst and the selectivity of the dechlorination
[30–32].
It was also found in Table 2 that the change of the solvent system
had an obvious influence on the dechlorination reactivity of 2-
CP and 4-CP. Using water as the solvent, it was advantageous to the
dechlorination of 4-CP and 2-CP. However, in ethanol, the dechlorination
rates of 2-CP and 4-CP were much lower than those in
water. We thought that sodium chloride (NaCl) formed by the neutralization
of HCl and NaOH could accumulate on the surface of Pd/
C in ethanol thereby resulting in a decline in catalytic activity.
However, in water, NaCl is easily dissolved and ionized to form
Na+ and Cl. So, water had some function to clean the surface of
Pd/C during the hydrodechlorination in aqueous solutions. Ukisu
et al. [10] in their studies on hydrogenation of aromatic chlorides
over Rh–Pt/C have observed the activity of the catalyst gradually
decreased due to accumulation of sodium chlorides on its surface
and water could wash away these salts.
Thus, the nature of solvents was considerably important to the
hydrodechlorination of organic chlorides in liquid phase. These
facts encourage us to believe that accurately choosing proper reaction
environments is essential, where both the reaction rate and
reactivity may be favored. In this way, a useful hydrodechlorination
method could be coupled to the destruction of chlorinated organic
wastes, such as PCBs and dioxins.
In conclusion, monochlorophenols (CPs) were completely hydrodechlorinated
with hydrogen gas over 5% Pd/C in aqueous solutions
at room temperature under atmospheric pressure. 2-CP, 3-CP
and 4-CP showed different dechlorination reactivity and the rates
of the dechlorination followed the order of 4-CP 3-CP > 2-CP in
aqueous solutions. Furthermore, it was found that the natures of
bases and solvents also had obvious influences on the hydrodechlorination
reactivity and selectivity of monochlorophenols as
well as the activity of the catalyst. In aqueous solutions, it was
more advantageous to the hydrodechlorination of monochlorophenols
during the hydrogenation due to the cleaning function of
water. Furthermore, the presence of bases, especially for sodium
hydroxide, also ensured the high activity of the catalyst in water.
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In order to further evaluate the dechlorination reactivity of
monochlorophenols, the equal amounts of 2-CP, 3-CP and 4-CP
were mixed in aqueous solution and were catalytic hydrogenated
with hydrogen gas over 5% Pd/C. The results are shown in Fig. 1.
For the mixtures of monochlorophenols, the dechlorination rates
of 2-CP, 3-CP and 4-CP show the same order as that in the former
experiments: 4-CP 3-CP > 2-CP. So, 2-CP was the most difficult
dechlorination in the catalytic hydrogenation of monochlorophenols
with H2 over 5% Pd/C in aqueous solutions. In other words,
chlorine atoms, which are away from hydroxyl group in 2-CP, 3-
CP and 4-CP, are much easier to be hydrogenated than that in
the neighboring position in aqueous solutions over Pd/C. It was
indicated that the steric hindrance effect of the hydroxyl group
possibly affected the hydrogenolysis of ortho position CACl bond
[13,27–28]. Recently, Aramendia et al. [8] also found that the steric
effect of the methyl group could inhibit the dechlorination rate
during the hydrogenation of o-chlorotoluene.
Additionally, we also research the effects of bases and solvents
on the reactivity of monochlorophenols in the catalytic hydrodechlorination
over 5% Pd/C. Here the mixtures of 2-CP and 4-CP
were used as models and different bases were added into aqueous
solutions. The results are listed in Table 2.
During the hydrodechlorination of organic chlorides, hydrochloric
acid (HCl) is generated and can cause catalytically active
metal to poison [29]. In the liquid-phase system the presence of
a base acts as a proton scavenger, maintaining the catalytic metal
in a reduced state and limiting Cl interaction(s) [14,30].Therefore,
when there was no base in the dechlorination reaction, because
of the inhibitory effect of HCl on Pd/C catalyst, the dechlorination
rates of 2-CP and 4-CP were lower than those using bases, such
as NaOH, Et3N (Table 2).
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(60 m in length, U0.25 mm, 0.25 lm film thickness). The compositions
of the reaction system were analysed by GC-FID (HP-4890)
with a column of HP-5 (30 m in length, U0.53 mm, 1.5 lm film
thickness). For the dechlorination of the mixtures of 2-CP, 3-CP
and 4-CP, the compositions of the dechlorination reaction were
analysed by HPLC with a PU-1580 pump (JASCO, Japan), a UV-
1575 detector (JASCO, Japan) and Rheodyne 7725i injector with a
20 ll loop (Rheodyne, Cotati, CA, USA). The stationary phase was
TECHSPHERE ODS column (5 lm, 250 4.6 mm). The mobile
phase was 60/40 (v/v) MeOH/50 mM sodium phosphate buffer
(pH 3.0). All of the analyses were performed isocratically at a flow
rate of 0.8 ml/min and recorded the absorbance at 254 nm.
3. Results and discussion
At room temperature, monochlorophenols (2-CP, 3-CP and 4-
CP) were catalytically hydrogenated with hydrogen gas over 5%
Pd/C in aqueous solutions. 2-CP, 3-CP and 4-CP were completely
dechlorinated within 60 min (Table 1). In the catalytic dechlorination,
2-CP, 3-CP and 4-CP were hydrogenated and transformed to
phenol, cyclohexanone, cyclohexanol and hydrochloric acid (HCl).
Phenol, cyclohexanone and cyclohexanol are lowly toxic and useful
as intermediates in chemical manufacturing. Hydrochloric acid
immediately reacted with sodium hydroxide (NaOH) to produce
sodium chloride (NaCl). Phenol was the main product, and only a
small amount of cyclohexanone and cyclohexanol were formed.
The hydrodechlorinated process of monochlorophenols is shown
in Scheme 1.
2-CP, 3-CP and 4-CP were taken as reactive reference models,
which show three representative CACl positions on the phenol
ring: ortho, meta and para positions of the hydroxyl group. In Table
1, 2-CP, 3-CP and 4-CP exhibited different dechlorination reactivity
in aqueous solutions over 5% Pd/C. It can be clearly observed that
the turnover frequencies (TOFs) of the dechlorination of monochlorophenols
followed the order of 4-CP 3-CP > 2-CP, which
was adverse to the results reported by Felis et al. [26]. Felis found
the ortho and para chlorine atoms were equally reactive whereas
the meta position chlorine was more difficult to hydrogenolysis
in monochlorophenols. This difference was possibly relevant to
the different catalyst used in the dechlorination of chlorophenols.
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a b s t r a c t
Monochlorophenols (CPs) were completely hydrodechlorinated with 5% Pd/C in aqueous solutions at
room temperature under atmospheric pressure. In the hydrogenation process, 2-CP, 3-CP and 4-CP
showed different dechlorination reactivity. It was found the CACl bond in the ortho position of hydroxyl
group was the most difficult dechlorination in the hydrogenation of monochlorophenols in aqueous solutions
compared to those in meta and para positions. Additionally, bases and solvents also had influences
on the hydrodechlorination reactivity and selectivity of monochlorophenols as well as the activity of the
catalyst. In aqueous solutions, it was more advantageous to the hydrodechlorination of monochlorophenols
during the hydrogenation due to the cleaning function of water. Furthermore, the presence of bases,
especially for sodium hydroxide, also ensured the high activity of the catalyst in water.
2008 Elsevier B.V. All rights reserved.
1. Introduction
Heterogeneously catalytic hydrogenation is a convenient method
for the dehalogenation of aryl halides in organic synthesis because
of its experimental simplicity, good yields and high purity
of products [1]. In recent years, because the disposal of chlorinated
organic waste becomes a serious environmental problem, catalytic
hydrodehalogenation methods have been developed in the treatment
of chlorinated organic waste [2–9] and a wide variety of hydrodehalogenation
systems have been used in the environmental
catalysis [10–18].
In most hydrogenation reactions of aromatic halides in liquid
phase, organic solvents are usually used as the reaction medium.
However, water, being cheaper, safer and more environmentally
amicable than organic solvents, is seldom utilized in the hydrodechlorination
of aryl halides [17–20]. We have reported the heterogeneously
catalytic hydrodechlorination of aromatic halides in the
liquid phase. It was found that the addition of water greatly improved
the dechlorination rates of the hydrogenation reactions
[21–22]. Recently, the hydrodehalogenation of aromatic halides,
PCBs and dioxins in aqueous solution has been studied in ourgroup [23–25]. Here, we reported the hydrodechlorination reactivity
of monochlorophenols (2-CP, 3-CP and 4-CP) in aqueous solutions
at room temperature under atmospheric pressure.
Additionally, it was also found that bases and solvents also had
influences on the hydrodechlorination reactivity of monochlorophenols.
2. Experimental section
5% Pd/C catalyst used in experiments was purchased from C&P
Chemical Co., China. The catalyst was not pre-treated before experiments
and only hermetically-sealed kept in a desiccator. 2-Chlorophenol
(2-CP), 3-chlorophenol (3-CP) and 4-chlorophenol (4-CP)
(purityP98%) were obtained from Aldrich. Water used in the
reaction was deionized. The purities of hydrogen and nitrogen used
in the experiments were more than 99.99%.
The reaction was carried out in a three-necked flask, which was
attached with a thermometer, a condenser and a hydrotreater
(including a hydrogen cylinder, hydrogen flowmeter, three-way
valve and a nitrogen cylinder), with a magnetic stirrer at room
temperature. Under atmospheric pressure, 50 ml aqueous solution
of monochlorophenols (4.0 mmol), stoichiometric NaOH (160 mg,
4.0 mmol) and 5% Pd/C (30 mg, 14.2 103 mmol) were added
into the flask. At room temperature, monochlorophenols were
treated with hydrogen gas and 5% Pd/C in aqueous solutions.
The intermediate products in the hydrodechlorination reaction
of monochlorophenols (2-CP, 3-CP and 4-CP) were determined by
GC/MS (Finnigan trace 2000 GC/MS-Trap) with a column of HP-5
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没事以后就来写写咯
好久不用这个了
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