Hydroxyl radical reactions with 2-chlorophenol as a model for oxidation in supercritical water

Article abstract of DOI:10.1007/s11164-012-1015-x

To determine the detailed mechanism of 2-chlorophenol (2-CP) oxidation in supercritical water, both the experiments and theoretical calculations were conducted in this paper. A set of experiments was performed to oxidize 2-CP in supercritical water under temperatures of 380-420 °C, pressure of 25 MPa, residence times of 0-60 s, and H₂O₂ as oxidant. By determining the molar yields of products, the primary single-ring products were identified as chlorohydroquinone, 2,4-dichlorophenol (2,4-DCP), 2,6-DCP, and 4-CP. The trends for the molar yields of the four products were analyzed at various temperatures and residence times. And built upon the trends, the possible reaction pathways were conjectured. Subsequently, the reaction mechanism was further verified by theoretical calculations, in which density functional theory was adopted as the computational method. The calculated results have well illustrated the experimental results and ascertained the reaction paths we proposed. Springer Science+Business Media Dordrecht 2013.

Full text of DOI:10.1007/s11164-012-1015-x

Res Chem Intermed
DOI 10.1007/s11164-012-1015-x
Hydroxyl radical reactions with 2-chlorophenol
as a model for oxidation in supercritical water
Jiaming Zhang
Xiaohua Ren
•
Chunyuan Ma
•
Youmin Sun
•
Received: 5 September 2012 / Accepted: 29 December 2012
Springer Science+Business Media Dordrecht 2013
ꢀ
Abstract To determine the detailed mechanism of 2-chlorophenol (2-CP) oxida-
tion in supercritical water, both the experiments and theoretical calculations were
conducted in this paper. A set of experiments was performed to oxidize 2-CP in
supercritical water under temperatures of 380–420 ꢁC, pressure of 25 MPa, resi-
dence times of 0–60 s, and H O as oxidant. By determining the molar yields of
2
2
products, the primary single-ring products were identified as chlorohydroquinone,
,4-dichlorophenol (2,4-DCP), 2,6-DCP, and 4-CP. The trends for the molar yields
2
of the four products were analyzed at various temperatures and residence times. And
built upon the trends, the possible reaction pathways were conjectured. Subse-
quently, the reaction mechanism was further verified by theoretical calculations, in
which density functional theory was adopted as the computational method. The
calculated results have well illustrated the experimental results and ascertained the
reaction paths we proposed.
Keywords 2-Chlorophenol ꢀ Supercritical water ꢀ Hydroxyl radical ꢀ
Theoretical calculation
J. Zhang ꢀ C. Ma ꢀ X. Ren
School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
J. Zhang ꢀ C. Ma (&)
National Engineering Laboratory for Coal-fired Pollutants Emission Reduction,
Shandong University, Dongpeilou 601, No. 17923, Jingshi Road, Jinan 250061, China
e-mail: sdetechym@163.com
Y. Sun
School of Municipal and Environmental Engineering, Shandong Jianzhu University,
Jinan 250101, China
123

J. Zhang et al.
Introduction
Chlorophenols (CPs) are compounds widely used in the manufacture of wood
preservers, herbicides, insecticides, fungicides, biocides, dyes, and pharmaceuticals
[
1, 2]. As toxic and persistent pollutants, CPs are suspected of carcinogenicity and
mutagenicity [3]. They can be accumulated in tissues of organisms, and
biomagnified in food chain transfer [2]. CPs such as 2-CP, 4-CP, 2,4-dichlorophenol
(2,4-DCP), and 2,4,6-trichlorophenol are in the list of the 129 priority pollutants
compiled by USEPA [4].
Decomposition methods of CPs have been attracting a great deal of attention in
recent years. It has been reported that a wastewater stream containing 2-CP over
2
00 ppm cannot be treated effectively by direct biological methods [4], as the effect
of biological degradation was readily influenced by many environmental factors and
the microbes were usually very sensitive to the concentration change of CPs [5].
Many studies have adopted advanced oxidation processes (AOPs), such as
electrocatalytic degradation [6–8], photocatalytic degradation [9–12], Fenton’s
oxidation [13], and supercritical water oxidation (SCWO) [14–17].
Compared to other AOPs, SCWO is characterized by high treatment efficiency
and short reaction time [18–20]. At temperatures and pressures above the critical
point of water (374 ꢁC and 22.1 MPa), organic compounds, water, and oxygen or
other oxidants are completely mixed in a single homogeneous aqueous phase [21],
in which mass transfer is free of limitations [22, 23] and the organics were oxidized
by plenty of free radicals. SCWO has been successfully employed in treating toxic
and hazardous organic wastes [24–27], and has also gained good results in the
treatment of CPs [14–16]. Previous studies [17] showed that the decomposition
-
3
efficiency of 2,4-DCP in supercritical water at 420 ꢁC and 230 bar (6.135 9 10
mol/L 2,4-DCP and 0.221 mol/L H O ) was very high with the residence time of
2
2
1
.5 min. It was much more effective than the decomposition by biological treatment
-
0.307 9 10 mol/L 2,4-DCP in SBR system) [28] and photocatalyzed oxidation
3
(
with CdS (0.5 9 10 mol/L 2,4-DCP) [29], with residence time of 8 days and
-
3
4
0 min, respectively.
The studies of the decomposition mechanism for CPs have mainly focused on the
photodegradation [30, 31], and while several studies [14, 32] have reported the
decomposition mechanism of CPs in supercritical water. Those results referring to
the mechanism of CPs in SCWO gave rough estimations only derived from the
experimental data. For the short time of the SCWO process, the detailed reaction
pathways are as yet difficult to clearly identify from the inference of the
experimental results. Furthermore, as the previous studies [15] have shown, with
different reaction parameters, different oxidants, and sometimes additional material
participation (such as NaOH [33]), different intermediates and products can be
obtained, which indicates that the reaction mechanisms were also varied. Thus, it is
necessary to investigate the mechanism of CPs oxidation in supercritical water
under different reaction conditions, and the details need to be further clarified by the
application of more techniques, such as theoretical calculation.
In this paper, 2-CP was chose as the typical compound of CPs. Firstly, the
decomposition efficiency of 2-CP under different reaction conditions was studied;
1
23

Hydroxyl radical reactions with 2-chlorophenol
secondly, the temporal change pattern of the yields for four primary intermediates
was analyzed; finally, the mechanism details and decomposition pathways were
explored with quantum chemical methods, and the comparison of experimental
results with the theoretical calculated results was performed.
Experimental
Description of the experimental set-up and parametric conditions
All the oxidation reactions presented were conducted in a continuous-flow SCWO
reactor, which consisted of five major modules: pumps, preheaters, reactor, cooling
devices, and separator. As Fig. 1 shows, reactant feed was first handled into two
(
a)
(b)
Fig. 1 Schematic flow diagram of SCWO continuous-flow reactor system. a SCWO system, b reactor
123

J. Zhang et al.
separate lines and pressured into the reactor by high-pressure pumps, which
supplied a constant pressure of 25 MPa. The feed and oxidant (H O solution,
2
2
3
0 wt%) are first pumped by two separate electricity-driven liquid pumps into the
preheaters. The pumps are double entry liquid ring pumps and supply the flow at
.05–2 kg/h. The preheaters are high-pressure tubing about 360 cm length, 0.6 cm
inner diameter, and 1.5 cm outer diameter, made of Swagelok Stainless Steel (SS)
16L. The temperatures of every stream in the outlet of the preheaters are measured
0
3
using PT100 resistance thermometers (-200 to 500 ꢁC). The heating devices in the
preheating section are resistance wires. The H O decomposed to H O and O
2
2
2
2
completely in the preheater.
After pre-heating, the two lines were then mixed at the reactor inlet by a mixing
tee. The vessel of the reactor is made of stainless steel 321, with the outer and inner
diameter of 50 and 30 mm, respectively. The reaction chamber is 200 mL and
2
88 mm long. Six silicon carbide bars packed around the reactor (Fig. 1b) were
used to provide heat. It is insulated with materials containing asbestos to minimize
heat loss and allow isothermal operation of the flow reactor. A type K multipoints
thermocouple (0–1,000 ꢁC) was inserted at the top port of the mixing tee and into
the reactor to measure the stream temperature in the reactor inlet section. There is
also another similar thermocouple inserted into the end of the reactor to determine
the effluent temperature.
Then, the effluent is cooled by passing through a water-cooled, counter-flow heat
exchanger, and cooled to ambient temperature. The cooled products then pass into
the separator where gaseous and liquid effluent is removed for collection and/or
analysis.
The pressures of all streams are adjusted using pressure reducing valves
TESCOM, 44-1100 Series), and are then measured by pressure gauges and pressure
(
transducers (0–40 MPa). The flow rates of all streams are adjusted by plunger stroke
(
SLPMMV26V), which is measured by Coriolis-type mass flow meters.
To study the effect of residence time, initial COD concentrations, and O2
concentrations on COD removal efficiency, a set of SCWO experiments were
conducted in an isothermal, isobaric tubular reactor. The 2-CP feed solutions
(
0.002 mol/L) were prepared by mixing 2-CP with deionized water in a feed bank.
Three temperatures were set for the reactions: 380, 400, and 420 ꢁC, and the
residence time was 0–60 s.
Identification and quantification of organic species
The gaseous products were analyzed by the built-in methods with a Perkin Elmer
Clarus 580 GC, which is equipped with four gas valves, nine stainless steel packed
columns, and dual thermal conductivity detector. The products and 2-CP in the
aqueous effluent were identified and quantified using a Thermo ISQ II Gas
Chromatograph with TRACE GC Ultra with mass spectrometer (Thermo, USA).
Prior to the analysis, the liquid samples were first extracted with dichloromethane and
then acidified with sulfuric acid. The concentration of 2-CP that remained unreacted
and the products in the aqueous samples were determined by GC/MS according to the
following procedures: column, a DB-5 fused silica capillary (30 m long, 0.25 mm ID),
1
23

Hydroxyl radical reactions with 2-chlorophenol
5
0 ꢁC hold for 3 min, and then ramped to 250 ꢁC at a rate of 10 ꢁC/min. The injector,
transfer line, and ion source were kept at 250, 250, and 200 ꢁC, respectively; helium
was used as the carrier gas with the flow rate of 1 mL/min. When chromatographic
separation was finished, the samples were identified by mass spectrometry that was
operated at 70 eV and scanned from 60 to 400 amu at 1 scan/s.
Computational details
•
High-accuracy quantum chemical calculations were carried out for the OH-initiated
SCWO reaction of 2-CP. Geometry optimizations of the reactants, the product
radicals, pre-reactive and the transitions state complexes were performed with the
density functional theory (DFT) method within the GAUSSIAN 03 package [34] on an
SGI Origin 2000 supercomputer. The choice of the basis sets and levels is considered
on the basis of the computational accuracy and feasibility as well as economic
computational time. The geometrical parameters of the reactants, the product radicals,
pre-reactive and transition state complexes were optimized at MPWB1K/
6
-31?G(d,p) level without any symmetry restriction. The MPWB1K method [35] is
a hybrid DFT model with excellent performance for thermo-chemistry, thermo-
chemical kinetics, hydrogen bonding, and weak interactions. In order to determine the
nature (local minima or first-order saddle points) of stationary points, the vibration
frequencies were also calculated at the same level, from which the zero point energies
(
ZPEs), and the thermal contributions to the free energy of activation, have also been
derived. The single-point energy of all structures involved in the reaction were
calculated at the MPWB1K level with a standard 6-311?G(3df,2p) basis set. All
reaction pathways were traced by employing intrinsic reaction coordinate [36]
calculations. The profiles of the potential energy surface were constructed at the
MPWB1K/6-311?G(3df,2p) level including ZPE correction.
Results and discussion
Conversion efficiency of 2-CP
The experimental results of the SCWO of 2-CP solutions (0.002 mol/L) with 160 %
excess H O at 25 MPa and 380, 400, and 420 ꢁC are shown in Fig. 2. The
conversion efficiency was displayed as 2-CP disappearance calculated with Eq. (1):
2
2
½
2CPꢂ ;
X ¼ 1 ꢁ
ð1Þ
½
2CPꢂ0
where [2CP] is the initial concentration of 2-CP in the reactor, and [2CP] is the
0
residual 2-CP concentration of the liquid product effluent when the reaction
completed.
Overall, the conversion efficiency ranged from 18.2 to 99.1 % (5.3–58.2 s residence
time at 380 ꢁC) and from 23.8 to 99.9 % (3–51.2 s at 420 ꢁC), depending on the
residence time, the temperature, and the initial reactant concentration. The conversion
123

J. Zhang et al.
1
00
8
0
0
0
0
6
4
2
3
4
4
80°C
00°C
20°C
0
0
10
20
30
40
50
60
Residence Time(s)
Fig. 2 Conversion efficiency of 2-CP (0.002 mol/L) with pressure (25 MPa), temperature (380, 400, and
20 ꢁC), and residence time at 0.086 mol/L H
4
2 2
O
of 2-CP in supercritical water can reach a considerable value in a short time. The
conversion was above 80 % at 420 ꢁC and 16.8 s, and oxidation of 2-CP in the
supercriticalwatercouldbeobtainedat90 % ata temperaturenear420 ꢁCin25 s, while
the conversion efficiency maximized at around 75 and 65 % with 380 and 400 ꢁC at
2
5 s. In related studies, Li et al. [14] obtained a conversion efficiency of 8.6–99.7 % for
2
-CP in SCWO, and Lee et al. [33] obtained values of conversion efficiency ranging
from 9 to 99 % in experiments of 2-CP oxidation in supercritical water with the addition
of NaOH. The conversion values in our experiments were around the range of these
studies.
As seen from Fig. 2, the conversion efficiency increased rapidly with the increase
of temperature, which is consistent with the property of a typical irreversible
reaction. The temperature increase favors the irreversible reaction performance.
However, when the residence time was long enough (after about 45 s), the
conversion efficiency did not show a remarkable difference with the increase of
residence time, which was because the reactions were approximately complete at
that time. From this point, one could see that the complete conversion of 2-CP could
be gained in a short time and that SCWO is an effective approach for 2-CP
wastewater treatment.
Compared with the study of Li et al. [14], a high reaction rate in our experiments
was observed. In the study of 2-CP oxidation in supercritical water by Li et al., the
conversion efficiency was about 99.7 % at 67.5 s, 380 ꢁC, 27.8 MPa. In our
experiments, the conversion efficiency has reached 99.1 % at 58.2 s, 380 ꢁC,
2
5 MPa. Although the pressure of our experiments was lower than that in the study
of Li et al., higher conversion efficiency was obtained, which resulted from the
different oxidants. Rice et al. [37] considered the H O was completely converted to
O in supercritical water. More studies [38, 39] have shown that the incomplete
2
2
2
1
23

Hydroxyl radical reactions with 2-chlorophenol
conversion of a small amount of H O , which built up a radical pool during the
2
2
•
reaction process, and the OH radicals were of low concentration but the reactions
•
were of high rate constants. It is suggested that the OH radical pool led to an
accelerated decay in 2-CP concentration.
Yields of organic single-ring products
The products of 2-CP incomplete oxidation in supercritical water under different
conditions were identified and quantified. The final gaseous products for the
experiments were CO and H O. The organics in the aqueous solution obtained
2
2
were composed of compounds of comparative high molecular weight (dichloro-
phenoxyphenol and a trace amount of chlorodibenzodioxin) and comparative low
molecular weight single-ring products (2-CP, chlorohydroquinone, 4-CP, 2,4-DCP,
2
,6-DCP) and other dimers. These products were similar to the related studies of Li
et al. [14] and Lee et al. [33]. A trace amount of dichlorophenoxyphenol was also
found in the experiments accomplished by Lin and Wang [40]. As for the single-
ring products, in the related published reports [23, 28] considering their thermal
stability, no effort was made to check the various yields of single-ring products at
different temperatures and residence times. Hence, in this study, we only focused on
the single-ring intermediates determined in appreciable yields: chlorohydroquinone,
4
-CP, 2,4-DCP, and 2,6-DCP.
The temporal variations for the yields of the four above products were analyzed
under three different temperature ranges (Fig. 3). These data were obtained by
operating the flow reactor at 380–420 ꢁC, 25 MPa, and using a feed stream with an
initial 2-CP concentration of 0.002 mol/L and 160 % excess H O . As Fig. 3
2
2
displays, among the four products, the yield of the chlorohydroquinone was the
highest, and the yield of 4-CP was the lowest. The reactions at the temperature of
3
80 ꢁC (Fig. 3a), 400 ꢁC (Fig. 3b), and 420 ꢁC (Fig. 3c) have the similar temporal
pattern of product yield, and under higher temperatures the reactions proceeded
more rapidly. As residence time increased, the yields of the four products all first
increased and then decreased. As Fig. 3a presents, the yields of chlorohydroquinone
and 4-CP increased slowly with the reaction time before 45 s and then decreased.
The yield of 2,4-DCP increased rapidly with time until it reached the maximum. As
time continuously increased, however, the concentration of the 2,4-DCP decreased
until it was present in very low concentrations at the long reaction time. The
concentration of 2,4-DCP showed a trend similar to 2,6-DCP.
The data in Fig. 3 indicate that 2,4-DCP was oxidized on roughly the same time
scale as 2,6-DCP at short residence time. Nevertheless, as the residence time
increased, the yield of 2,4-DCP got lower than that of 2,6-DCP. The phenomenon
can be attributed to the para-Cl atom, which is easier to react than the Cl atoms on
the other sites because the steric hindrance for the para-Cl is smaller than the other
sites. Furthermore, the attraction product, chlorohydroquinone, was easy to be
formed and not sensible to the temperature variance. So, in the middle interval of
the whole reaction process, no significant change for chlorohydroquinone yield was
observed at temperatures of 380, 400, and 420 ꢁC.
123

J. Zhang et al.
(
a) 30
chlorohydroquinone
2
2
4
,4-dichlorophenol
,6-dichlorophenol
-chlorophenol
2
1
0
0
0
0
10
20
30
40
50
60
Residence Time (s)
(
b) 30
chlorohydroquinone
2
2
4
,4-dichlorophenol
,6-dichlorophenol
-chlorophenol
2
1
0
0
0
0
10
20
30
40
50
60
Residence Time (s)
(
c) 30
chlorohydroquinone
2
2
4
,4-dichlorophenol
,6-dichlorophenol
-chlorophenol
2
1
0
0
0
0
10
20
30
40
50
60
Residence Time (s)
Fig. 3 Time evolution of the concentrations of the products treated with pressure (25 MPa) at
.086 mol/L H
. a–c represent the reaction at the temperature of 380, 400, and 420 ꢁC, respectively
0
2 2
O
1
23

Hydroxyl radical reactions with 2-chlorophenol
The data in Fig. 3 also showed that a large fraction of the original 2-CP went into
the formation of the organic single-ring products. For example, at a residence time
of 13 s, and temperatures of 380, 400, and 420 ꢁC, the total concentrations of the
four organic single-ring products were 28.5, 36.0, and 42.4 ppm, respectively, and
the 2-CP concentrations were 1,276, 928, and 620 ppm, respectively. Thus, one can
estimate that approximately 3.9, 3.3, and 3.1 % of the 2-CP went into the formation
of these organic single-ring products.
Reaction pathways for 2-CP oxidation conversion to single-ring products
On the basis of these identified intermediate products and time evolution of the
concentrations of organic single-ring products, a possible scheme of the primary
•
OH and 2-CP reaction pathways and the secondary reactions is proposed in
Scheme 1. For the formation of the main product P1 (chlorohydroquinone), four
•
reaction pathways were inferred, including OH addition to the para-C atom of
•
2
-CP, the para-H abstraction of 2-CP, OH addition to the para-C of one product
2
pathway for the para-H abstraction of 2-CP and then the Cl addition to the para-C
,4-DCP, and the para-Cl abstraction of 2,4-DCP. For forming P2 (2,4-DCP), one
•
•
atom was proposed. The ortho-H atom abstraction of 2-CP and the Cl addition to
the ortho-C atom can explain the formation of the product P3 (2,6-DCP). Finally,
•
•
OH abstracted the ortho-Cl atom of the product P2 and then one H can add to the
ortho-C site to form the product P4 (4-CP). According to the experiment results, P1
was the primary product. However, four pathways have been conjectured for the
formation of P1. To identify which pathway was the first step and explain the
degradation mechanism, theoretical calculations were carried out on the following.
Similarly, the reaction mechanisms for the formations of other products were
calculated.
Reaction mechanism with theoretical calculations
To further explain the reaction mechanism, theoretical calculations with the DFT
method were carried out. The reactions were simulated in gas as representative for
studying the mechanism. According to the experimental results, two types of
reactions including the addition reaction (add.) and the abstraction reaction (abs.)
•
for the OH and 2-CP reactions are considered. The reaction schemes are presented
•
in Scheme 1. The structures and atom labels of 2-CP and OH are shown in Fig. 4.
The optimized geometries of intermediates involved in the reactions are displayed
in Fig. 5, and the transition states are shown in Fig. 6. The potential energy surface
profiles along the reaction coordinates are given in Fig. 7. All the relative energies
including the energy barriers and the reaction heats are shown in Table 1.
•
For OH attacking 2-CP, three reactions are identified, denoted as add. (a), abs.
b) and abs. (c) in Scheme 1. As illustrated in Figs. 5, 6, and 7, for the addition
(
-
1
reaction, a pre-complex IM1, the energy of which is 1.33 kcal mol lower than the
original reactants, was first found. Then, an adduct IM2 was formed via a transition
•
state TS1, where OH was almost perpendicularly attacking the molecular 2-CP, and
•
˚
the distance of C4 and O of OH was 0.711 A shorter than that in IM1. Compared
123

J. Zhang et al.
OH
OH
OH
Cl
Cl
TS2
+
OH (d)
H2O
Cl
add. (a)
TS1
-
+
OH(e)
H
OH
H
OH
OH
P1
IM1
OH
IM2
OH
OH
OH
Cl
OH
Cl
TS3
Cl
Cl
abs. (b)
+ Cl (f)
Cl
+
OH
H
Cl
P2
OH
H
H
O
IM3
OH
IM4
para-2-ClHP
OH
OH
O
H
OH
Cl
OH
H
H
Cl
TS4
Cl
Cl (f)
Cl
Cl
+
abs. (c)
IM5
P3
IM6
ortho-2-ClHP
OH
Cl
add. (h)
TS5
+ OH (k)
- HOCl
OH
Cl
Cl
OH
IM7
OH
OH
OH
Cl
Cl
OH
P1
abs. (i)
TS6
+ OH (l)
Cl
+
OH
Cl
Cl
H
O
IM8
para-2-ClHP
OH
H
OH
OH
Cl
O
OH
OH
Cl
Cl
abs. (j)
TS7
+ H (m)
Cl
Cl
Cl
IM9
IM10
ClHP
P4
Scheme 1 Possible reaction pathways for 2-CP oxidation in supercritical water
˚
with TS1, the distance of C4 and O was 0.559 A shorter and the C4–H2 bond was
˚
.019 A longer in IM2, meaning that the C4–O bond was being formed and the
0
C4–H2 bond was easier to break. The energy barrier of add. (a) is 2.98 kcal mol
-
1
.
Then, OH could abstract the H2 atom to form the product P1 (chlorohydroquinone)
•
1
23

Hydroxyl radical reactions with 2-chlorophenol
Fig. 4 Structures and atom
labels of 2-CP and OH
•
via a transition state TS2 (denoted as (d)). This reaction had a negative barrier,
meaning that the abstraction was a barrierless process. From Table 1, it is noted that
the whole reaction including two steps add. (a) and (d) had the lowest energy barrier
•
among all the reactions. This indicates that OH adding to the para-C site was most
•
favorable, which was probably the first step of the OH and 2-CP reaction. This was
also in good agreement with the experiment result that P1 (chlorohydroquinone)
was the primary product. In additional, several studies [41, 42] have shown that
chlorohydroquinone had been detected during the 2-CP degradation and found to be
the major intermediate.
•
For abs. (b), OH could abstract a para-H atom to form a para-chlorohydrox-
ycyclohexadienyl radical, which would form P1 (chlorohydroquinone) and P2 (2,4-
•
•
DCP) through adding OH and Cl, respectively. As shown in Figs. 5, 6, and 7, a pre-
1
complex IM3 was first found on the potential energy surface. It was -1.32 kcal mol
-
•
more stable than the separate reactants. Then, a transition state TS3, where OH was
abstracting the H2 atom denoted by the longer C4-H2 bond, was formed. After that, an
adduct IM4 was formed in the process of producing the para-chlorohydroxycyclo-
-
1
hexadienyl radical. The energy barrier of this reaction was 7.29 kcal mol . As
illustrated in Table 1, the activation energy of this process was relatively low,
indicating that the reaction could occur readily. It matched well with the above
experimental results that the molar yield of P2 was increasing in the beginning 5 min.
For abs. (c), an ortho-H atom was abstracted to produce an ortho-chlorohydrox-
ycyclohexadienyl radical, which could be converted into P3 (2,6-DCP) through
•
adding a Cl . In this pathway, a pre-complex IM5, the energy of which was
-
1
-
2.67 kcal mol compared to the original reactants, was first formed. In IM5, two
hydrogen bonds, O–H4 and H–O1, were formed with the bond lengths of 2.729 and
˚
1.945 A, respectively. TS4 was the transition state for the reaction, which showed an
obvious H-atom transfer from C2 to O atom denoted by the C2–H4 bond was
123

J. Zhang et al.
IM1
IM2
IM3
IM4
IM5
IM6
Fig. 5 Optimized geometries of the intermediates for 2-CP at MPWB1K/6-31?G(d,p) level (the gray,
white, red, and green balls denote carbon, hydrogen, oxygen, and chlorine atoms, respectively, and bond
lengths are in angstroms). (Color figure online)
elongated by 17.79 % and the O–H4 distance was shortened by 55.70 %. Then, an
adduct IM6, in which the formed H O was leaving the main body, was found in the
2
potential energy surface. The decomposition of IM6 yielded P3 and H O. The energy
2
1
23

Hydroxyl radical reactions with 2-chlorophenol
IM7
IM8
IM9
IM10
Fig. 5 continued
-1
barrier of this reaction was 8.63 kcal mol , and the reaction heat was
-2.88 kcal mol . The relatively lower activation energy suggested that the reactions
-1
could occur, which agreed well with the experiment result that P3 was steadily
increasing at the start.
•
In Fig. 3, the molar yield of P2 decreased after 5 s. We assumed that OH could
also attack P2 (2,4-DCP). Therefore, we have calculated the possible reactions of
•
2
,4-DCP and OH. The relative energies are also summarized in Table 1.
From Scheme 1, it can be seen that three possible reactions including add. (h),
•
abs. (i) and abs. (j) were identified for the 2,4-DCP and OH reaction. For the
addition reaction, no pre-complex was first formed. A transition state TS5 was
found for the reaction. In the structure of TS5, the C4–Cl bond and the benzene ring
were no longer in a plane, and the O–C4–Cl angle was 94.9ꢁ, a nearly right angle.
Then, an intermediate IM7 was formed. Significant changes could be found in the
˚
˚
C4ꢀꢀꢀO distance, which became shorter, from 1.983 A in TS5 to 1.395 A in IM7, and
˚
˚
the C4–Cl bond, which was elongated from 1.726 A in TS5 to 1.838 A in IM7. The
shortened C4–O bond and the elongated C4–Cl bond in the IM7 indicated that the
C4–O bond would be formed and the C4–Cl bond would be broken. Finally, another
•
OH could abstract the Cl atom to form P1 (chlorohydroquinone), which could
123

J. Zhang et al.
TS1
TS2
TS3
TS4
TS5
TS6
TS7
Fig. 6 Optimized geometries of the transition states for 2-CP at MPWB1K/6-31?G(d,p) level (the gray,
white, red, and green balls denote carbon, hydrogen, oxygen, and chlorine atoms, respectively, and bond
lengths are in angstroms). (Color figure online)
1
23

Hydroxyl radical reactions with 2-chlorophenol
4
6.41 ClHP
+
HOCl
Erel
kcal mol-1
4
4.36 IM10
4.04 IM8
4
4.09 TS7
4
43.73 TS6
add. (a)
abs. (b)
abs. (c)
add. (h)
abs. (i)
abs. (j)
5
.97TS3
5
.96 TS4
1
.65 TS1
0
.00
-CP + OH
P2 + OH
1
.59 TS5
-
1.32 IM3
-
1.54 ortho-2-ClHP
+
+
2
H O
2
-
1.33 IM1
-2.10 IM9
(
)
-2.49 para-2-ClHP
H2O
-2.71 IM4
-
2.67 IM5
-2.88 IM6
-16.80 IM2
-20.03 IM7
Fig. 7 Calculated potential energy surface profiles of the reactions
Table 1 Calculated relative
-1
Reactions
DE
2.98
DH
energies (kcal mol ) for 2-CP
with different reaction
pathways
a
b
c
d
e
f
-16.80
7.29
8.63
–
-2.71
-2.88
-95.05
-109.28
-97.45
-97.22
-20.03
44.04
–
–
g
h
i
–
1.59
43.73
44.09
j
44.36
explain that the molar yield of P1 was still increasing when P2 was decreasing.
-
1
Moreover, the energy barrier of the add. (h) was 1.59 kcal mol , indicating the
reaction was kinetically favorable and occurred easily.
•
Along pathway abs. (i), a transition state TS6, in which OH was abstracting the
˚
para-Cl atom denoted by the being broken C4–Cl bond with a distance of 2.458 A,
was first found on the potential energy surface. Then, IM8, where the being-formed
HOCl was leaving the main body, is formed. The unimolecular decomposition of
•
IM8 could produce para-chlorohydroxycyclohexadienyl radical and H O. OH
2
could add to the para-C atom of the para-chlorohydroxycyclohexadienyl radical to
form P1 (chlorohydroquinone). On the potential energy surface of abs. (j), IM9 with
˚
a standard hydrogen bond of 1.922 A was initially formed. Then, IM9 could evolve
123

J. Zhang et al.
˚
into IM10 via TS7, where the distance of C2 and Cl being abstracted was 2.505 A
˚
and the O–Cl bond was 1.714 A, indicating that the C2–Cl bond would be broken
and the O–Cl bond would be formed. Then, the IM10 could be decomposed into
ortho-chlorohydroxycyclohexadienyl radical and H O. Finally, the ortho-chlorohy-
2
•
droxycyclohexadienyl radical could form 4-CP by adding H on the ortho-C site.
Interestingly, a pre-complex was found for the abs. (j) but no pre-complex was
found for the abs. (i). Possibly the formed hydrogen bond between the H5 atom and
the O atom could play an important role in stablizing the IM9 for the abs. (j). As
illustrated in Table 1, the energy barriers of abs. (i) and abs. (j) were 43.73 and
-
1
4
4.09 kcal mol , respectively. The high energy barriers indicated that the two
abstraction reactions were energetically very unfavorable and impossible at low
energy conditions, which was in agreement with the theoretical study of Han et al.
[
barrier.
43] that the para-Cl of 4-CP was difficult to be abstracted with a high energy
•
The theoretical calculations showed that OH adding to the para-C atom of 2-CP
with the lowest energy barrier was the easiest to occur, and was the most favorable
process for forming chlorohydroquinone. The ortho-Cl atom was difficult to be
abstracted by OH with the highest activation energy, thus 4-CP was found to the
smallest molar yields. And 2,4-DCP and 2,6-DCP were also verified as the
important intermediates at the start of the reactions.
•
Conclusions
The oxidation of 2-CP was conducted in supercritical water using H O as the
2
2
oxidant, and the conversion efficiency was gained varying from 18.2 to 99.9 %,
depending on the reaction conditions. The maximum conversion occurred at a
temperature of 420 ꢁC, pressure of 25 MPa, and residence time of 51.2 s. The
SCWO is concluded to be an effective method for 2-CP wastewater treatment.
The primary single-ring products of the 2-CP oxidation in supercritical water
were determined to be chlorohydroquinone, 2,4-DCP, 2,6-DCP, and 4-CP, among
which chlorohydroquinone had the highest molar yields.
The theoretical results were in good agreement with the experimental findings,
and help to better understand the detailed mechanism of 2-CP decomposition
•
initiated by the OH radical in supercritical water.
Acknowledgments This research is supported by the National High Technology Research and
Development Program of China (863 Program, the Project No. 2007AA05Z235). We acknowledge the
research foundation of Shandong Environmental Protection Bureau for support. We also thank the
independent innovation project for the University Institutes of Jinan Technology Bureau (No. 201 102 046).
References
1. P.M. Armenante, D. Kafkewitz, G.A. Lewandowski, C.J. Jou, Anaerobic–aerobic treatment of hal-
ogenated phenolic compounds. Water Res. 33, 681–692 (1999)
2. U.G. Ahlborg, T.M. Thunberg, H.C. Spencer, Chlorinated phenols: occurrence, toxicity, metabolism,
and environmental impact. CRC Crit. Rev. Toxicol. 7, 1–35 (1980)
1
23

Hydroxyl radical reactions with 2-chlorophenol
3
4
. L. Keith, Compilation of Sampling Analysis Methods (US Environmental Protection Agency, Boca
Raton, 1991), pp. 389–486
. M.A. Callahan et al., Water-Related Environmental Fate of 129 Priority Pollutants (Office of Water
Planning and Standards, Office of Water and Waste Management, U.S. Environmental Protection
Agency, Washington, DC, 1979)
5
6
7
8
9
. A.O. Olaniran, E.O. Igbinosa, Chlorophenols and other related derivatives of environmental concern:
properties, distribution and microbial degradation processes. Chemosphere 83, 1297–1306 (2011)
. D. Sun, H. Zhang, Electrochemical determination of 2-chlorophenol using an acetylene black film
modified glassy carbon electrode. Water Res. 40, 3069–3074 (2006)
. J.D. Rodgers, W. Jedral, N.J. Bunce, Electrochemical oxidation of chlorinated phenols. Environ. Sci.
Technol. 33, 1453–1457 (1999)
. A.M. Polcaro, S. Palmas, Electrochemical oxidation of chlorophenols. Ind. Eng. Chem. Res. 36,
1791–1798 (1997)
. J. Yang, J. Dai, C. Chen, J. Zhao, Effects of hydroxyl radicals and oxygen species on the 4-chlo-
2
rophenol degradation by photoelectrocatalytic reactions with TiO -film electrodes. J. Photochem.
Photobiol. A 208, 66–77 (2009)
0. N. Rao, A. Dubey, S. Mohanty, P. Khare, R. Jain, S. Kaul, Photocatalytic degradation of 2-chlo-
rophenol: a study of kinetics, intermediates and biodegradability. J. Hazard. Mater. 101, 301–314
1
(
2003)
1. M. Czaplicka, Photo-degradation of chlorophenols in the aqueous solution. J. Hazard. Mater. 134,
5–59 (2006)
2. Y. Cheng, H. Sun, W. Jin, N. Xu, Photocatalytic degradation of 4-chlorophenol with combustion
synthesized TiO under visible light irradiation. Chem. Eng. J. 128, 127–133 (2007)
1
1
1
4
2
3. M. Munoz, Z.M. de Pedro, J.A. Casas, J.J. Rodriguez, Assessment of the generation of chlorinated
byproducts upon Fenton-like oxidation of chlorophenols at different conditions. J. Hazard. Mater.
190, 993–1000 (2011)
1
1
1
4. R. Li, P.E. Savage, D. Szmukler, 2-Chlorophenol oxidation in supercritical water: global kinetics and
reaction products. AlChE J. 39, 178–187 (1993)
5. G. Lee, T. Nunoura, Y. Matsumura, K. Yamamoto, Effects of a sodium hydroxide addition on the
decomposition of 2-chlorophenol in supercritical water. Ind. Eng. Chem. Res. 41, 5427–5431 (2002)
6. Z. Sun, F. Takahashi, Y. Odaka, K. Fukushi, Y. Oshima, K. Yamamoto, Effects of potassium alkalis
and sodium alkalis on the dechlorination of o-chlorophenol in supercritical water. Chemosphere 66,
151–157 (2007)
1
1
1
2
2
2
7. H.C. Lee, J.H. In, J.H. Kim, K.Y. Hwang, C.H. Lee, Kinetic analysis for decomposition of 2,4-
dichlorophenol by supercritical water oxidation. Korean J. Chem. Eng. 22, 882–888 (2005)
8. P.E. Savage, S. Gopalan, T.I. Mizan, C.J. Martino, E.E. Brock, Reactions at supercritical conditions:
applications and fundamentals. AlChE J. 41, 1723–1778 (1995)
9. W.R. Killilea, K. Swallow, G.T. Hong, The fate of nitrogen in supercritical-water oxidation. J.
Supercrit. Fluids 5, 72–78 (1992)
0. T.J. Park, J.S. Lim, Y.W. Lee, S.H. Kim, Catalytic supercritical water oxidation of wastewater from
terephthalic acid manufacturing process. J. Supercrit. Fluids 26, 201–213 (2003)
1. H. Pi n´ kowska, P. Wolak, A. Złoci n´ ska, Hydrothermal decomposition of alkali lignin in sub- and
supercritical water. Chem. Eng. J. 187, 410–414 (2012)
2. M. Goto, T. Nada, A. Ogata, A. Kodama, T. Hirose, Supercritical water oxidation for the destruction
of municipal excess sludge and alcohol distillery wastewater of molasses. J. Supercrit. Fluids 13,
277–282 (1998)
23. P.E. Savage, Organic chemical reactions in supercritical water. Chem. Rev. 99, 603–622 (1999)
24. T.D. Thornton, P.E. Savage, Phenol oxidation pathways in supercritical water. Ind. Eng. Chem. Res.
31, 2451–2456 (1992)
2
5. V. Marulanda, G. Bola n˜ os, Supercritical water oxidation of a heavily PCB-contaminated mineral
transformer oil: laboratory-scale data and economic assessment. J. Supercrit. Fluids 54, 258–265
(2010)
2
2
2
6. G. Anitescu, L.L. Tavlarides, Oxidation of Aroclor 1248 in supercritical water: a global kinetic study.
Ind. Eng. Chem. Res. 39, 583–591 (2000)
7. Q. Guan, C. Wei, X.-S. Chai, Pathways and kinetics of partial oxidation of phenol in supercritical
water. Chem. Eng. J. 175, 201–206 (2011)
8. X. Quan, H. Shi, J. Wang, Y. Qian, Biodegradation of 2,4-dichlorophenol in sequencing batch
reactors augmented with immobilized mixed culture. Chemosphere 50, 1069–1074 (2003)
123

J. Zhang et al.
2
3
3
9. W.Z. Tang, C. Huang, Photocatalyzed oxidation pathways of 2,4-dichlorophenol by CdS in basic and
acidic aqueous solutions. Water Res. 29, 745–756 (1995)
0. P. Boule, C. Guyon, J. Lemaire, Photochemistry and environment IV—photochemical behaviour of
monochlorophenols in dilute aqueous solution. Chemosphere 11, 1179–1188 (1982)
1. N. Akai, S. Kudoh, M. Takayanagi, M. Nakata, Photoreaction mechanisms of 2-chlorophenol and its
multiple chloro-substituted derivatives studied by low-temperature matrix-isolation infrared spec-
troscopy and density-functional-theory calculations. J. Photochem. Photobiol. A 146, 49–57 (2001)
2. C. Wei, D. Li, H. Shi, Q. Guan, Y. Hu, Dechlorination kinetics and mechanism of p-chlorophenol in
supercritical water. Zhongguo Keji Lunwen Zaixian/Sci. Pap. Online 5, 441–447 (2010)
3. G. Lee, T. Nunoura, Y. Matsumura, K. Yamamoto, Comparison of the effects of the addition of
NaOH on the decomposition of 2-chlorophenol and phenol in supercritical water and under super-
critical water oxidation conditions. J. Supercrit. Fluids 24, 239–250 (2002)
3
3
3
4. M. Frisch, G. Trucks, H. Schlegel, G. Scuseria, M. Robb, J. Cheeseman, J. Montgomery Jr.,
T. Vreven, K. Kudin, J. Burant, Gaussian 03, Revision D. 01 (Gaussian, Wallingford, 2004)
5. Y. Zhao, D. Truhlar, Hybrid meta density functional theory methods for thermochemistry, thermo-
chemical kinetics, and noncovalent interactions: the MPW1B95 and MPWB1K models and com-
parative assessments for hydrogen bonding and van der Waals interactions. J. Phys. Chem. A 108,
3
6908–6918 (2004)
3
3
6. K. Fukui, The path of chemical reactions—the IRC approach. Acc. Chem. Res. 14, 363–368 (1981)
˚
7. S.F. Rice, T.B. Hunter, A.C. Ryd e´ n, R.G. Hanush, Raman spectroscopic measurement of oxidation in
supercritical water. 1. Conversion of methanol to formaldehyde. Ind. Eng. Chem. Res. 35, 2161–2171
(
1996)
8. E. Croiset, S.F. Rice, Direct observation of H
water. Ind. Eng. Chem. Res. 37, 1755–1760 (1998)
3
3
4
4
2 2 2
O during alcohol oxidation by O in supercritical
9. E. Croiset, S.F. Rice, R.G. Hanush, Hydrogen peroxide decomposition in supercritical water. AlChE
J. 43, 2343–2352 (1997)
2
0. K.S. Lin, H.P. Wang, Supercritical water oxidation of 2-chlorophenol catalyzed by Cu cations and
?
copper oxide clusters. Environ. Sci. Technol. 34, 4849–4854 (2000)
1. D. Shchukin, S. Poznyak, A. Kulak, P. Pichat, TiO
sations and activity for 2-chlorophenol degradation in water. J. Photochem. Photobiol. A 162,
23–430 (2004)
2 2 3
–In O photocatalysts: preparation, characteri-
4
4
4
2 2 2
2. M. Bertelli, E. Selli, Reaction paths and efficiency of photocatalysis on TiO and of H O photolysis
in the degradation of 2-chlorophenol. J. Hazard. Mater. 138, 46–52 (2006)
3. Z. Han, D. Zhang, Y. Sun, C. Liu, Reexamination of the reaction of 4-chlorophenol with hydroxyl
radical. Chem. Phys. Lett. 474, 62–66 (2009)
1
23