Modification of CuCl2·2H2O by dielectric barrier discharge and its application in the hydroxylation of benzene

Article abstract of DOI:10.1039/c9nj03261d

CuCl₂·2H₂O powder was modified by a dielectric barrier discharge (DBD) plasma at room temperature and atmospheric pressure (CuCl₂-DBD). And the catalytic activity for the benzene hydroxylation reaction using CuCl₂

Full text of DOI:10.1039/c9nj03261d

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Modification of CuCl Á2H O by dielectric barrier
2
2
discharge and its application in the hydroxylation
Cite this:DOI: 10.1039/c9nj03261d
of benzene†
Lu Fang, Shiyang Dong, Lei Shi and Qi Sun
*
CuCl
and atmospheric pressure (CuCl
using CuCl –DBD as a catalyst was investigated. The CuCl
SEM, TEM, BET, XPS, FT-IR, and TG-DTA. The results demonstrate that after DBD plasma treatment,
crystal H O and about 50% of Cl in CuCl O were removed, and the amounts of Cu and O increased
Á2H
on the surface of CuCl –DBD. CuCl O with a large flat regular geometric shape was changed into
Á2H
small particles with the average particle diameter less than 50 nm; the specific surface area decreased
2
Á2H
2
O powder was modified by a dielectric barrier discharge (DBD) plasma at room temperature
–DBD). And the catalytic activity for the benzene hydroxylation reaction
–DBD samples were characterized by XRD,
2
2
2
2
2
2
2
2
2
2
À1
2
À1
from 16.1 m
crystal structure and the H3 type mesoporous structure of CuCl
treatment. The DBD plasma modified or etched the surface of CuCl
of elements on the surface. This surface change makes CuCl –DBD insoluble in polar solvents. For the
benzene hydroxylation reaction, CuCl –DBD exhibits a higher activity and selectivity than CuCl , in parti-
cular CuCl –DBD can be recycled and reused. Under the optimized conditions, the yield and the selec-
g
to 9.4 m
g
, and the pore volume increased approximately 45 times. However, the
Á2H O were still retained after plasma
Á2H O, and changed the amounts
2
2
2
2
Received 24th June 2019,
Accepted 17th July 2019
2
2
2
DOI: 10.1039/c9nj03261d
2
tivity of phenol were 19.1% and 95.6%, respectively. The activity of the CuCl
basically unchanged after repeated use four times.
2
–DBD catalyst remained
rsc.li/njc
plasma active species with the material opens some new reaction
channels, which makes the treated material have different properties
Introduction
The one-step hydroxylation of benzene to phenol has attracted compared to those from the traditional chemical method. DBD
much attention as an alternative method compared to the plasma treatment has been demonstrated to decompose some
three-step industrial cumene process from the viewpoints of carbonates, nitrates or hydroxide of transition metals to the corres-
1
7–20
the economy and environment. Many kinds of transition metal ponding metallic oxides,
species such as Cu, Co, Fe, Ti and V have been used in the the corresponding precious metals,
convert some precious metal salts into
21–23
remove organic templates in
1
–6
24,25
26–30
benzene hydroxylation reaction.
Taking into account the molecular sieves,
synthesise some advance materials,
recovery and reuse of the catalysts from the reaction mixture, surface properties of materials,
modify
31–37
and so on. In plasma surface
various supports such as metal oxide, zeolite, and carbon modification, energetic electrons, ions, and metastable species break
materials have been widely used for loading active components the molecular bonding, remove or create some functional groups on
7
–11
to prepare heterogeneous catalysts.
surfaces, depending on different plasma gases. Surface modification
Dielectric barrier discharge (DBD) plasma is a promising is usually a combination of a grafting process and etching. In this
technology for the surface treatment of materials and the respect, surface modification of polymeric materials (organics) by
1
2–16
preparation of advanced materials.
DBD plasma is a highly DBD plasma leads to a significant change in wettability. However,
reactive mixture, consisting of electrons, ions, radicals, photons, modification of copper dichloride powder (inorganic) to change its
molecules, and excited species and can be operated at room solubility in polar solvents is not yet reported.
2
+
temperature and atmospheric pressure. The interaction of these
The Cu –H
homogeneous and heterogeneous systems.
experiment, we found that the CuCl /H O system was effective
2 2
O system shows good hydroxylation activity in
2,4,6
In the preliminary
2
2 2
Faculty of Chemistry and Chemical Engineering, Liaoning Normal University,
Dalian 116029, China. E-mail: sunqils@163.com; Fax: +86-0411-82156989;
Tel: +86-0411-84259069
for benzene hydroxylation. But, like many homogeneous systems,
it is difficult to separate CuCl from the reaction system. Herein,
2
we are trying to design a simple method in which the catalyst can
be recovered while maintaining its high homogeneous activity.
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9nj03261d
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We directly treated CuCl
the partial Cu–Cl bond for transforming CuCl
2
Á2H
2
O powder using DBD plasma to break Benzene (1 mL), acetonitrile (5 mL) and catalyst (10 mg) were
2 2
into an insoluble added to a flask reactor with a reflux condenser. Then, H O
2
substance. When CuCl Á2H O was exposed to a DBD plasma (30%, 2 mL) was added to the stirred mixture in the reactor. The
2
2
atmosphere, the crystal H O and about 50% of the chlorine in reaction temperature and time were 65 1C and 4 h, respectively.
2
CuCl Á2H O were removed under our experimental conditions. After the reaction, the mixture was centrifuged to remove the
2
2
The formation of Cu–OH and Cu–O bonds was observed. The solid catalyst, and then the liquid was analyzed using a gas
CuCl
O have the same crystal construction and H3 mesoporous none in the products were confirmed by GC-MS (Agilent 6890N-
structure. The CuCl –DBD sample that is insoluble in polar sol- 5975B) and also by comparing the retention time of known
2
Á2H
2
O treated with DBD plasma (CuCl
2
–DBD) and CuCl
2
Á
chromatograph (GC126). Phenol, hydroquinone, and benzoqui-
2H
2
2
vents showed a higher catalytic activity and good reuse perfor- commercial pure samples. Yield of phenol = mmol phenol/
mance in the benzene hydroxylation reaction. Moreover, the mmol initial benzene. Selectivity of phenol = mmol phenol/
catalytic properties of CuCl –DBD and CuCl Á2H O were compared. mmol all products. For data presented carbon balances are
2
2
2
observed, i.e. 100% Æ 5%.
Experimental
Preparation and characterization
Results and discussion
The DBD apparatus used in this work was composed of a high Catalyst characterization
voltage generator (CTP-2000K), two steel electrodes (DBD-50), and a
quartz reactor chamber (DBD-100A), which were purchased from
The XRD patterns of CuCl -pure and CuCl –DBD are shown in
2
2
Fig. 1. The diffraction peaks of CuCl
2
-pure at 2y values of
Corona Laboratory, Nanjing, China. The high voltage generator can
supply a voltage from 0 to 30 kV with a sinusoidal waveform in the
frequency range of 5–20 kHz. DBD-100A was sandwiched in the
middle of the two electrodes. The space between the inner surfaces
of the quartz reactor chamber was the discharge gap, ca. 8 mm. The
1
4
6.191, 21.921, 26.611, 28.841, 32.721, 33.951, 35.391, 38.011,
0.821, 43.021, 44.731, 49.081, 51.461, 54.371, 57.361 and 68.581
correspond, respectively, to the (110), (020), (101), (111), (220),
201), (130), (310), (221), (131), (040), (400), (112), (420), (241)
(
38
and (440) lattice planes of CuCl
After DBD plasma treatment, some major peaks of CuCl still exist
at 2y values of 16.191, 21.921, 28.841, and 33.951, implying that
CuCl
new peaks are detected. All diffraction peaks of CuCl
greatly weakened, indicating that CuCl –DBD is obviously smaller
in particle size. The average particle size of CuCl –DBD calculated
using the Scherrer formula is about 38 nm.
Fig. 2 shows the different magnifications of SEM images of
CuCl -pure (Fig. 2a and b) and CuCl –DBD (Fig. 2c–f). CuCl -
2
Á2H
2
O (JCPDS Card No. 33-0451).
À1
air flow rate was 30 mL min as a discharge atmosphere. A sample
2
of CuCl
2
Á2H
2
O (2.0 g) was introduced into the quartz reactor
chamber. The sample was treated with DBD plasma for 5 min
per treatment at a frequency of about 9.1 kHz, and was then
manually stirred. The sample was treated 5 times. After DBD
treatment, the sample was collected in a test tube, washed three
2
–DBD retains the basic crystal structure of CuCl
2
. And no
2
–DBD are
2
2
times with deionized water to remove unreacted CuCl
at 100 1C for 12 h. The sample treated with DBD was labeled CuCl
DBD. CuCl O was labeled CuCl -pure.
2
, and dried
39
2
–
2
Á2H
2
2
2
2
2
X-ray diffraction (XRD) patterns were collected on a Bruker D8
pure shows a regular geometric shape with a flat surface and
Advance powder diffractometer using a Cu Ka radiation source
bigger size (120 mm  31 mm). After DBD treatment, the CuCl
2
–
À1
from 101 to 801 with a scan rate of 21 min . Scanning electron
DBD sample is composed of small particles with a particle
diameter less than 50 nm. These particles are dispersed and
only a few particles are agglomerated. This is the result of DBD
microscopy (SEM) was performed on a Hitachi SU8010 field-
emission scanning electron microscope equipped with an attach-
ment of energy-diffusive spectroscopy (EDS). TEM micrographs of
the prepared samples were obtained using a JEOL JEM-2100.
Brunauer–Emmett–Teller (BET) surface area and pore volume
2
studies were carried out by degassing the samples under N flow
at 120 1C using a AUTOSORB-1MP analyzer. The surface chemical
compositions of samples were confirmed by X-ray photoelectron
spectroscopy (XPS) on a Thermo Escalab 250Xi apparatus with a
monochromatic X-ray source of Al Ka. Fourier transform infrared
(
4
FT-IR) spectra were recorded on a TENSOR-27 in the region of
À1
000–400 cm . Thermogravimetric (TG) analysis was carried out
À1
with a STA449 F3 instrument at a heating rate of 5 1C min . The
UV-Vis spectra of the reaction system were recorded in the region
of 200–600 nm using the PerkinElmer Lambda 35 Spectrometer.
Catalytic reaction
The catalytic activities of the samples were tested for direct
oxidation of benzene to phenol with H
2
O
2
2 2
as the oxidizer. Fig. 1 XRD pattern of CuCl -pure and CuCl –DBD.
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2 2
Fig. 2 SEM and EDS images of CuCl -pure and CuCl –DBD.
plasma etching of CuCl ÁH O. The EDS results of CuCl -pure
Fig. 3 shows the TEM images of CuCl -pure (Fig. 3a, c and e)
2
2
2
2
and CuCl –DBD show that the wt% of Cu, Cl, and O are and CuCl –DBD (Fig. 3b, d and f). CuCl -pure has a neat regular
2
2
2
2
Cu : Cl : O = 1 : 3 : 1 for CuCl -pure and Cu : Cl : O = 1 : 0.8 : 1.37 ‘slice-like’ shape. The interplanar spacings are 0.33 nm,
for CuCl
about 50% of the chloride is removed, and the Cu and O of CuCl
contents increase in the CuCl –DBD sample. morphology of CuCl
2
–DBD, respectively (Fig. 2g and h). This indicates that 0.55 nm, corresponding to the (201) and (220) lattice planes
2
, respectively (Fig. 3e). After DBD treatment, the surface
2
2
–DBD was changed, and it was composed
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of well-sized small particles (o50 nm). The interplanar spa- the BJH method are illustrated in Fig. 4. Both samples show a
cings are 0.19 nm, and 0.33 nm, corresponding to the (110) type IV adsorption–desorption isotherm with the mesoporous
and (201) lattice planes of CuCl , respectively (Fig. 3f). This H3 type hysteresis loops in the relative pressure (P/P ) range of
2
0
4
1
indicates that the main structure of CuCl –DBD is still CuCl
0.5–1.0. CuCl -pure has a wide pore size distribution in the
2
2
2
crystal. From Fig. 3g, the particle size distribution of CuCl2– range of 2–50 nm, and CuCl –DBD is in the range of 2–6 nm,
2
DBD is obtained by counting the particle sizes of 100 or more which is a narrow pore size distribution and more uniform. In
4
0
particles. The average particle diameter obtained by TEM is addition, in comparison with CuCl
2 nm (38 nm measured by XRD). Therefore, the DBD plasma area of CuCl –DBD decreases slightly, but the pore volume
greatly reduces the particle sizes and modifies the surface of increases about 45 times as shown in Table S1 (ESI†). The
CuCl -pure. obvious increase in pore volume is likely due to the etching of
The N adsorption–desorption isotherm diagrams of CuCl - the CuCl Á2H O surface by DBD plasma breaking some of the
2
-pure, the specific surface
4
2
2
2
2
2
2
pure and CuCl –DBD and the pore size distribution analyzed by copper–chlorine bonds and the conversion of large particles
2
2 2
Fig. 3 TEM images of CuCl -pure and CuCl –DBD.
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Compared with the XPS results of CuCl
the content of Cl in CuCl
2
-pure (Fig. S1, ESI†),
–DBD decreases from 53.0% (CuCl -
2
2
pure) to 27.7%, implying that nearly 50% of Cu–Cl bonds are
broken after DBD treatment. Meanwhile, the contents of copper
and oxygen increase by about 20% and 100%, respectively,
which is consistent with the results of EDS analysis. In addi-
tion, a new peak appears at 529.8 eV in the O 1s spectrum of
CuCl
2
–DBD, corresponding to Cu–O. These results indicate
-pure by break-
that DBD plasma modifies the surface of CuCl
2
ing the Cu–Cl bond and leading to the formation of the new
bonds (Cu–O), resulting in a change in the number of surface
elements.
The FT-IR spectra of CuCl -pure and CuCl –DBD are shown
2
2
À1
À1
in Fig. 6. For CuCl
2
-pure, the peaks at 3390 cm , 3276 cm
À1
and 3151 cm
are attributed to v(–OH) of crystal water in
4
9
À1
CuCl
2
-pure. The peak at 1596 cm is due to d(H O). The peaks
2
À1
À1
appearing at 1039 cm and 1110 cm are assigned to the
hydroxyl–metal bond (Cu–O–H) formed by Cu and crystal
2
+
5
0,51
À1
À1
H O.
2
The peaks at 578 cm and 435 cm are attributed
49
to v(Cu–O)
at 3314 cm , 3354 cm and 3452 cm are red-shifted and
obviously split compared with CuCl -pure, illustrating that the
.
After DBD treatment, for CuCl –DBD the v
peaks
2
À1
(–OH)
À1
À1
2
surface hydroxyl groups are decomposed and separated, i.e. the
À1
crystal H
2
O in CuCl
2
Á2H
2
O is removed. The 1625 cm peak of
d
(H O) greatly weakens, and shows a red shift, implying that the
2
crystal water is removed. And the v(Cu–O) peaks in the range of
À1
4
50–590 cm also show a red shift. When the particle size is
decreased to the nanometer level, lattice expansion occurs,
resulting in an increase in the average bond length and a
Fig. 4
N
2
adsorption–desorption isotherm and pore-size distribution of
5
2,53
decrease in the vibration frequency of the bond.
In short,
CuCl
2
-pure and CuCl –DBD.
2
the corresponding infrared absorption peak may be red-shifted.
In addition, there is a blue shift in the v(Cu–O–H) peak in the
À1
into small particles. The above experimental results show that range of 830–990 cm , implying that Cu–O exhibits a higher
2 2
CuCl –DBD has both the textural properties of CuCl , and the vibration frequency and shorter bond length. This can be
crystal structure of CuCl . Obviously, DBD plasma mainly attributed to the transition of H O–Cu (the coordination
2
2
2
+
changes the surface structure of CuCl -pure.
between crystal water and Cu ) to Cu–OH after DBD treatment.
To investigate the thermal stability of the prepared sample,
2
2
The surface elements of CuCl –DBD were analyzed by X-ray
electron spectroscopy (XPS). The data of the binding energy TG-DTA analysis was conducted (Fig. S2, ESI†). For CuCl
2
-pure,
(B.E.) have been corrected as per the C 1s (284.8 eV) reference there are three weightless platforms within the measured
peak. The XPS survey scans in the full range of 0–1200 eV are temperature range. The first weight loss (21.3%) between
shown in Fig. 5 and Cu 2p (935.0 eV), Cl 2p (199.2 eV), and O 1s 50 1C and 100 1C is due to the weight loss of physically
(
531.7 eV) were measured. There are two intense peaks at adsorbed water and transformation of partially crystalline
2
+
9
2
9
35.05 eV and 954.91 eV (Fig. 5b) assignable to Cu 2p3/2 and water. The second stage of weight loss (16.4%) (320–440 1C)
2
+
p1/2, along with corresponding Cu satellite peaks at 942.1 eV, and the third weight loss (26.0%) (440–540 1C) are attributed to
4
2,43
44.4 eV, and 962.8 eV,
with large peak areas. When the the complete decomposition of CuCl2 to form CuO. Con-
2
+
2+
satellite peak of Cu has a large area, Cu does not correspond sequently, according to weight loss the amount of Cu in
to the oxide of copper. In the Auger spectrum of copper (inset CuCl
of Fig. 5a), the only peak located at 570.9 eV ascribes to Cu , stages of weight loss. The first and second stages in the total
and the Auger peaks of Cu and Cu are not observed, which weight loss (9.0%) of CuCl
confirms there are no Cu and Cu in CuCl
weak peak at 932.0 eV is attributed to Cl–Cu–Cl. The content chemisorbed water may be removed after DBD treatment. The
4
4
2
-pure is 36.3%. Similarly, CuCl
2
–DBD exhibits three
2
+
+
0
45
2
–DBD occur between 0 1C and
+
0
2
–DBD. Thus, the 310 1C. The results indicate that physically adsorbed and
4
6
of Cl–Cu–Cl is about 6% based on the peak area. The spectrum third stage of weight loss occurs between 400 1C and 600 1C
of Cl 2p was fitted to two peaks at 198.5 eV and 200.3 eV, (46.6%) due to the complete decomposition of CuCl –DBD to
2
4
7
corresponding to Cl. The spectrum of O 1s was de-convoluted form CuO. The Cu content is about 44.4% in CuCl
into three peaks at 529.8 eV, 531.6 eV and 532.9 eV, corres- comparison with TG results of CuCl -pure, the actual Cu
–DBD increases by about 22.3%.
2
–DBD. In
2
4
8
ponding to Cu–O bond, –OH and O atom, respectively.
content in CuCl
2
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2
Fig. 5 XPS spectra of CuCl –DBD.
experimental results and literature, the scheme of plasma treat-
ment CuCl
2
Á2H
2
O is proposed in Scheme 1.
The literature reports that Cu and Cl in CuCl2 form a
covalent network of edge-shared CuCl , i.e. polymeric CuCl
4
2
5
4,55
chains. These chains are arranged in layer.
After DBD
plasma treatment, large particles of CuCl
2
Á2H
2
O are converted
to small particles of CuCl –DBD because the DBD plasma
2
breaks some of the Cu–Cl bonds. Cu–Cl bonds on the surface
of the small particles are replaced by Cu–OH and Cu–O bonds.
The transformation of the bonds makes the small particles
(
CuCl –DBD) insoluble in the solvent.
2
Catalytic activity of CuCl
2
–DBD
The catalytic performance of CuCl
2
–DBD was evaluated by the
yield and selectivity of phenol for benzene hydroxylation. In
this experiment, some reaction parameters which can poten-
tially impact the catalytic properties, such as reaction time,
2 2
Fig. 6 FT-IR spectrum of CuCl -pure and CuCl –DBD.
2 2
reaction temperature, amount of catalyst, and H O /benzene
Further, we detected the exhaust gas by a chemical method molar ratios were examined (Fig. 7). The effect of reaction time
during the discharge process. The presence of Cl and HCl has on the yield and selectivity of phenol was studied and the
2
been confirmed. Namely, the DBD plasma breaks some of the findings are shown in Fig. 7a. With the increase of reaction
Cu–Cl bonds in CuCl
at the same time, which produce Cl
The Cu–O and Cu–OH bonds were formed. According to the time (8 h), phenol peroxidation leads to the decrease of
2
Á2H
2
O and decomposes the crystal water time, both yield and selectivity increased. The yield reached the
2
and HCl leaving the system. maximum at 4 h (19.1%). With the prolongation of reaction
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Scheme 1 Schematic illustration of CuCl
2
Á2H
2 2
O to CuCl –DBD by DBD plasma.
selectivity. Therefore, the optimum reaction time is 4 h. The of catalyst is 10 mg (Fig. 7c). The phenol yield and selectivity is
yield of phenol increased with reaction temperatures improving significantly affected by the H O /benzene molar ratio (Fig. 7d).
2
2
from 55 1C to 65 1C. But with a further increase in temperature, The yield increased with the increase of the H
a clear decrease in yield and selectivity of phenol could be molar ratios from 1 : 1 to 6 : 1. The maximum yield of phenol
observed, owing to the over-oxidation of phenol to benzoqui- was 18.5% when the H /benzene molar ratio was 6 : 1. But
none. The appropriate reaction temperature is 65 1C (Fig. 7b). with a further increase in the H /benzene molar ratio, the
2 2
O /benzene
2 2
O
2 2
O
The phenol yield increased from 11.5% to 18.8% when the yield and selectivity of phenol fall dramatically, because too
amount of catalyst was increased from 5 mg to 10 mg. More- much H O may result in excessive oxidation of phenol. There-
2
2
over, the increase of the amount of catalyst to 20 mg caused an fore, the best H O /benzene molar ratio for this reaction is 6 : 1.
2
2
obvious decrease in the yield and selectivity of phenol which Thus, under the optimized reaction conditions, the yield and
may be attributed to the excessive oxidation of phenol or selectivity of benzene hydroxylation over CuCl
benzene caused by the excessive catalyst. So, the best amount and 95.6%, respectively.
2
–DBD are 19.1%
2
Fig. 7 Effect of reaction conditions on benzene hydroxylation activity and selectivity over CuCl –DBD.
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To investigate the stability of CuCl
NJC
2
2 2
–DBD, a cycling test was benzene on its surface. Upon further adding H O to the above
conducted under the same conditions for four runs. When each system to obtain curve (3), both catalysts show nearly identical
catalytic run was completed, the retrievable catalyst was sepa- absorption peaks with a peak at 235 nm corresponding to H O .
2
2
rated by filtering, rinsed with acetonitrile and deionized water
For quantitative determination of adsorbed benzene on
respectively, and eventually dried. The results reveal that the CuCl –DBD, we investigated the relationship between the
2
activity remains steady along the durability testing procedure initial concentration of benzene and the equilibrium concen-
5
7
and there was only a slight decrease in the activity after the tration in the CuCl
fourth run as shown in Fig. S3 (ESI†). The decrease in the included CuCl –DBD, acetonitrile, and benzene. A calculated
benzene conversion is due to the inevitable loss of the catalyst amount of benzene (1.1, 2.2, 3.3 and 5.5 mmol) and 5 mL
2
–DBD system. The experimental system
2
during the transfer process. These results indicate that CuCl
DBD could remain stable in the reaction system.
2
–
acetonitrile were uniformly mixed and placed in a test tube.
Then 20 mg of CuCl –DBD was added into each solution and
2
the mixture was stood for 24 h at room temperature. The initial
and equilibrium concentrations of benzene were analyzed by
GC. The results are shown in Fig. S4 (ESI†). The equilibrium
concentration of benzene is lower than the initial concen-
Comparison between CuCl -pure and CuCl –DBD
2
2
The UV-Vis spectra of CuCl
2
2
-pure and the CuCl –DBD catalyst
in the same reaction system are shown in Fig. 8. When only
mixing catalyst and acetonitrile in the system, curve (1), the
tration, indicating that benzene is adsorbed on CuCl
2
–DBD.
CuCl -pure is easily soluble in polar solvents, such as water
2
UV-Vis spectrum of CuCl -pure shows a peak at 310 nm corres-
2
2
+ 56
2+
and acetonitrile, as a homogeneous catalyst in the reaction
ponding to Cu . Contrarily, no corresponding Cu peak is
observed in the CuCl –DBD system, indicating that there is no
free Cu in the solution, and CuCl –DBD is a heterogeneous
system. Conversely, CuCl –DBD is insoluble. The activity of
2
2
2
+
both catalysts for hydroxylation of benzene is shown in Table S2
2
(
ESI†). CuCl –DBD shows better phenol yield and higher
2
catalyst. Upon adding benzene to the above system (benzene,
acetonitrile and catalyst), curve (2). A new absorption peak
appears at 254 nm corresponding to benzene. The absorption
selectivity.
CuCl -pure and CuCl
benzene hydroxylation reaction. Therefore, the catalytic decom-
position of H by these two catalysts was investigated. The
rate constant k was measured in a system of CH CN, H and
2
2
–DBD show good activity for the
2
peak obtained for the CuCl –DBD system is much smaller than
2 2
O
that of the CuCl -pure system, so CuCl –DBD may adsorb
2
2
3
2 2
O
catalyst at 50 1C, 55 1C, 60 1C, and 65 1C, respectively. The
specific data are summarized in Table S3 (ESI†), The decom-
position of H O accords with the first-order kinetic model with
2
2
2
58,59
R 4 0.98 in the fitting equation.
The activation energy of
the catalytic reaction was calculated using the Arrhenius
empirical formula, plotted as ln k to 1/T. The activation energy
À1
À1
is 64.4 kJ mol for the CuCl
2
-pure system and 141.0 kJ mol
for the CuCl –DBD system, respectively. The activation energy
2
of the CuCl -pure system is smaller, indicating that the rate of
2
decomposition of H O is faster. This can form more active
2
2
species in a relatively shorter time leading to a decrease in
phenol selectivity. Simultaneously, due to the slower reaction
rate of benzene oxidation, some activated H
consumed before taking part in the hydroxylation reaction in
the CuCl -pure system. So the conversion of benzene was not
2 2
O has been
2
2
further improved. The results explain that CuCl –DBD shows
better yield and selectivity than CuCl -pure.
2
Conclusions
Modification of CuCl
method to prepare a heterogeneous catalyst, CuCl
room temperature and atmospheric pressure. When CuCl
H O is exposed to a DBD plasma atmosphere, about 50% of
2
Á2H
2
O by DBD plasma is an effective
2
–DBD, at
2
Á
2
2
Cl and crystal H O in CuCl Á2H O are removed and a small
2
2
2
amount of new Cu–O bonds are formed. DBD plasma mainly
treats the surface of CuCl O, resulting in a change in the
Á2H
number of surface elements. The H3 type mesoporous structure
and the crystal structure of CuCl O are not changed after
Á2H
2
2
Fig. 8 UV-Vis spectra of CuCl
solution.
2 2
-pure and CuCl –DBD in the same
2
2
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DBD plasma treatment. In comparison with CuCl
2
Á2H
2
O, the 15 C. J. Liu, M. Y. Li, J. Q. Wang, X. T. Zhou, Q. T. Guo,
particle size of CuCl
2
–DBD is greatly reduced to an average size
J. M. Yan and Y. Z. Li, Chin. J. Catal., 2016, 37, 340–348.
less than 50 nm, and the specific surface area is slightly 16 O. Goossens, E. Dekempeneer, D. Vangeneugden, R. Van de
decreased, and the pore volume is increased 45 times. CuCl2–
DBD is insoluble in polar solvents and can be recycled for reuse
Leest and C. Leys, Surf. Coat. Technol., 2001, 142–144,
474–481.
after the reaction. CuCl
for benzene as a heterogeneous catalyst. The CuCl
catalyst gives almost the same benzene conversion (20.0%) as 18 W. Hua, L. J. Jin, X. F. He, J. H. Liu and H. Q. Hu, Catal.
2
–DBD has a good adsorption capacity 17 X. L. Yan, B. R. Zhao, Y. Liu and Y. N. Li, Catal. Today, 2015,
2
–DBD
256, 29–40.
CuCl
only a slight decrease in the catalytic activity of CuCl
2
Á2H
2
O, but higher phenol selectivity (95.6%). There is
Commun., 2010, 11, 968–972.
–DBD 19 P. Y. Kuai, C. J. Liu and P. P. Huo, Catal. Lett., 2009, 129,
2
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without additional reagents.
63, 188–190.
21 J. F. Sauvageau, S. Turgeon, P. Chevallier and M. A. Fortin,
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Conflicts of interest
There are no conflicts to declare.
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4 M. Rivallan, I. Yordanov, S. Thomas, C. Lancelot, S. Mintova
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Acknowledgements
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2
This work was supported by the National Natural Science
Foundation of China (21576128, 21173110).
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