ZnBr2 supported on silica-coated magnetic nanoparticles of Fe3O4 for conversion of CO2 to diphenyl carbonate

Article abstract of DOI:10.1039/c5ra07431b

A magnetic Fe₃O₄@SiO₂-ZnBr₂ catalyst was prepared by supporting ZnBr₂ on silica-coated magnetic nanoparticles of Fe₃O₄ and used as a recoverable catalyst for the direct synthesis of diphenyl carbonate (DPC) from CO₂ and phenol in the presence of carbon tetrachloride. The as-prepared catalyst was characterized by infrared spectroscopy (IR), powder X-ray diffraction (XRD), a X-ray photoelectron spectrometer (XPS) and BET. Zn loading in the supported catalyst and leaching during the reaction process were determined by atomic absorption spectroscopy (AAS). It was found that Fe₃O₄@SiO₂-ZnBr₂ showed higher catalytic activity than homogenous ZnCl₂ and ZnI₂ as well as homogenous ZnBr₂. With this new catalyst under optimized conditions, a yield of DPC at 28.1% was obtained. The heterogeneous catalyst Fe₃O₄@SiO₂-ZnBr₂ can also be recovered by a permanent magnet after the reaction and reused up to 4 times without noticeable deactivation.

Full text of DOI:10.1039/c5ra07431b

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ZnBr2 supported on silica-coated magnetic
nanoparticles of Fe3O4 for conversion of CO2 to
diphenyl carbonate
Cite this: RSC Adv., 2015, 5, 56478
*
Guozhi Fan, Shanshan Luo, Qiang Wu, Tao Fang, Jianfen Li and Guangsen Song
A magnetic Fe3O4@SiO2–ZnBr2 catalyst was prepared by supporting ZnBr2 on silica-coated magnetic
nanoparticles of Fe3O4 and used as a recoverable catalyst for the direct synthesis of diphenyl carbonate
(DPC) from CO2 and phenol in the presence of carbon tetrachloride. The as-prepared catalyst was
characterized by infrared spectroscopy (IR), powder X-ray diffraction (XRD), a X-ray photoelectron
spectrometer (XPS) and BET. Zn loading in the supported catalyst and leaching during the reaction
process were determined by atomic absorption spectroscopy (AAS). It was found that Fe3O4@SiO2–
ZnBr2 showed higher catalytic activity than homogenous ZnCl2 and ZnI2 as well as homogenous ZnBr2.
With this new catalyst under optimized conditions, a yield of DPC at 28.1% was obtained. The
heterogeneous catalyst Fe3O4@SiO2–ZnBr2 can also be recovered by a permanent magnet after the
reaction and reused up to 4 times without noticeable deactivation.
Received 23rd April 2015
Accepted 22nd June 2015
DOI: 10.1039/c5ra07431b
www.rsc.org/advances
have been thought to be one of the most efficient ways to
overcome these problems.16,17 Heterogenization is generally
Introduction
achieved by graing the active sites on solid materials such as
inorganic particles, polymers and hybrid materials. Silica,18
alumina,19 active carbon,20,21 ceria,22 polystyrene23 and poly-
vinylpyrrolidone24 are typical examples. Recently magnetic
nanoparticles Fe3O4 (MNPs-Fe3O4) has attracted much atten-
tion due to their convenient isolation and recovery.16,17,25 It has
been reported that the heterogeneous catalysts supported on
MNPs-Fe3O4 reveal excellent performance in many reactions
including hydrolysis, hydrogenation, oxidation, carbon–carbon
coupling and reduction.16,17 For example, ionic liquid-coated
MNPs-Fe3O4 catalyst used in the reaction of CO2 with epox-
ides could be reused up to 11 times without obvious activity
loss.26
One of the most promising green reactions with CO2
involved is to produce carbonates.27 Many investigators have
used CO2 to couple with epoxide due to the atom economical
process and nearly no by-product formation.28,29 Diphenyl
carbonate (DPC), an important carbonate and precursor of
polycarbonate, is traditionally synthesized from phosgene
(extremely toxic) and phenol. Since the process creates severe
environmental pollution and equipment corrosion,30 it is
necessary to nd an alternative process.31 We previously repor-
ted the study on production of DPC from CO2, phenol and
tetrachloride carbon (CCl4) catalyzed by ZnCl2 alone and ZnCl2/
triuoromethanesulfonic acid (CF3SO3H).32–34 However, the
process still showed the problems including requiring a large
amount of catalyst and difficulty of recovering catalyst. There-
fore, there is a need to explore an efficient and effective catalyst
for direct synthesis of DPC from CO2.
Global warming is a concern due to the emission of green-
house gases. It is known that CO2 is the main cause of global
warming because of overuse of petroleum, coal and natural
gas. CO2 is also regarded as a stable, safe and abundant C 1
resource since it is nontoxic and available. The trans-
formation of CO2 to value-added chemicals is promising in
organic synthesis from the chemical viewpoint.1 In recent
decades, much attention has been particularly paid to the
chemical xation of CO2.2–5 Direct synthesis of cyclic
compounds, including cyclic carbonates, cyclic carbamates
and cyclic ureas from CO2 is an alternative way of using CO2 as
a resource. From the environmental and practical viewpoints,
this alternative way can avoid using toxic and hazardous
reagents such as phosgene. Many compounds including
hydrogen, alkenes, acetals, epoxides, amines, phenol, etc.
have been explored to react with CO2 in the presence of metal
catalysts.6–14
CO2 is a highly oxidized and thermodynamically stable
compound with low chemical reactivity which restricts the
chemical conversion of CO2, leading to signicant challenges in
using CO2 as C 1 feedstock. Therefore, the effort to convert CO2
to useful chemicals is inevitably dependent on its activation via
catalysts.15 Although many homogenous catalysts such as salen-
complex, metal oxide and Lewis acid have been employed in the
reactions with CO2 involved,6–14 these catalysts oen suffer from
difficulty of separation. So far heterogeneous catalytic systems
School of Chemical and Environmental Engineering, Wuhan Polytechnic University,
Wuhan 430023, PR China. E-mail: fgzcch@whpu.edu.cn; Tel: +86 02783943956
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In this study, zinc halides including ZnCl2, ZnBr2 and ZnI2 determined by averaging three reaction runs based on the
were supported on silica-coated MNPs-Fe3O4 (SiO2@Fe3O4) charged phenol. The conversion, yield and selectivity were
and employed as catalysts for direct synthesis of valuable DPC calculated according to the following equations, respectively:
from CO2 and phenol in the presence of CCl4. The catalytic
performance of the magnetic supported catalysts was inves-
tigated, and the catalytic activity of heterogenized and
homologous zinc halide was compared. The effects of active
species, catalyst loading, amount of catalyst, reaction condi-
tions including CO2 pressure, reaction temperature and time
were investigated as well. The reusability of Fe3O4@SiO2–
ZnBr2 was also examined under the optimized reaction
conditions.
mphenol
m
Conversionð%Þ ¼
ꢁ 100%
mDPC
Yieldð%Þ ¼
ꢁ 100%
m
ꢁ 214
2 ꢁ 94
Yield
Conversion
Selectivityð%Þ ¼
ꢁ 100%
Experimental
Preparation of supported catalyst
where m is the mass of phenol taken for the reaction; mphenol
and mDPC are the mass of phenol and DPC remained and
detected in the reaction mixture; 94 and 214 are the molecular
weights (in g molꢂ1) of phenol and DPC, respectively.
In a typical experiment, 2.0 g MNPs-Fe3O4 (Supplied by Aladdin
Co. Ltd, Shanghai, China) was added to a mixture solvent of
ethanol and H2O (70 mL/10 mL). Aer the mixture was
dispersed by sonication for 20 min, 5 mL NH3$H2O and 5.0 mL
tetraethoxysilane (TEOS) were slowly added. The mixture was
vigorously stirred at 1200 rpm at room temperature for 24 h.
The formed magnetic Fe3O4@SiO2 was collected with a
permanent magnet, rinsed repeatedly with deionized water
until the lter became neutral, washed with ethanol and dried
Reuse of recovered catalyst
The residue containing Fe3O4@SiO2–ZnBr2 was collected by
centrifugation aer the reaction, followed by collecting with a
permanent magnet aer the addition of 10 mL dichloro-
methane (CH2Cl2), washing three times with CH2Cl2 (5 mL ꢁ 3)
ꢀ
and drying at 150 C under vacuum. Aer the Zn loading was
at 80 C under vacuum for 12 h.35
ꢀ
detected by AAS, the recovered catalyst was reused in the next
run without further pretreatment.
Preparation of ZnBr2 supported on Fe3O4@SiO2
Typically, aer 1.0 g Fe3O4@SiO2 was added to a solution of
1.1 g ZnBr2 (4.9 mmol) in 20 mL methanol, the mixture was
heated under reux for 3 h.36 The formed Fe3O4@SiO2–ZnBr2
was collected with a permanent magnet and dried at 150 ꢀC
under vacuum for 10 h. The supported catalyst with particle size
below 75 mm was collected by passing through a 200 mesh Tyler
screen. The Zn loading was determined by atomic absorption
spectroscopy (AAS).
Measurements
Infrared spectroscopy (IR) was measured on an EQUINOX 55
spectrometer in the range from 4000 to 500 cmꢂ1 with reso-
lution of 3.875 cmꢂ1 and scan number of 32. The solid samples
were ground with dried KBr powder, and compressed into a
disc prior to analysis. Powder X-ray diffraction (XRD)
measurements were performed on a D/MAX-RB RU-200BRU-
200B diffractometer with Cu Ka radiation at 40 kV and 40
mA in the range of 2q from 10 to 80ꢀ with scanning rate of 5ꢀ
minꢂ1, respectively. The specic surface area of the sample
was determined by nitrogen adsorption–desorption isotherm
at 77 K using the one-point modied BET method on a Gemini
2360 analyzer. XPS were recorded on a Kratos XSAM800 spec-
trometer with Mg Ka radiation (1253.6 eV) operated at 12 kV
and 10 mA. The energy scale was calibrated and corrected
using the C 1s (284.8 eV) line as the binding energy reference.
The Zn loading of either fresh or leached catalyst from the
reaction was determined by AAS with a Perkin-Elemer Analyst
300 using acetylene ame. The conversion of phenol, the yield
and the selectivity towards DPC were analyzed using GC2020
gas chromatograph with HP-5 capillary column (30 m ꢁ 0.32
mm ꢁ 0.25 mm, 5% phenyl methyl-siloxane) and ame ioni-
zation detector (FID). GC-MS analysis was performed using
Agilent 7890A/5975C GC equipped with HP-5 capillary column
and EI source. The detection was performed in the scan mode
from m/z 20 to 400. 1.0 mL minꢂ1 helium was used as the
Catalytic test
Typically, 12 mmol phenol, 40 mmol CCl4 and as-prepared
Fe3O4@SiO2–ZnBr2 (containing 1.2 mmol ZnBr2) were charged
into a 100 mL stainless steel autoclave. The autoclave was sealed
and ushed with 2 MPa CO2 three times to wash out the air in it.
Aer the mixture was heated to the desired reaction tempera-
ture with stirring at 1200 rpm, CO2 was then introduced into the
autoclave to the desired pressure using a high-pressure pump.
Aer a certain reaction time, the autoclave was cooled to room
temperature and the pressure was gradually released. The
mixture was centrifuged aer the addition of 10 mL ethanol,
followed by quantitatively analysis by gas chromatography (GC)
using biphenyl as internal standard. The formation of DPC was
also qualitatively identied by gas chromatography mass spec-
trometry (GC-MS).
Determination of conversion, yield and selectivity
All reactions were performed in triplicate. The conversion of carrier gas. The ionization voltage and source temperature
ꢀ
phenol, the yield as well as the selectivity towards DPC were were 70 eV and 230 C, respectively.
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Results and discussion
Characterization of catalyst
Fig. 1 shows the IR spectra of SiO2, Fe3O4, Fe3O4@SiO2 and
Fe3O4@SiO2–ZnBr2 with 15.1 wt% Zn loading. The band at
3448 cmꢂ1 is assigned to the symmetrical stretching vibration of
hydroxyl groups (–OHs). The band at 1628 cmꢂ1 is attributed to
the bending vibration of adsorbed water.35 The bands at 1090
and 795 cmꢂ1 in the IR spectra of SiO2, Fe3O4@SiO2 and
Fe3O4@SiO2–ZnBr2 are related to the asymmetric stretching
vibration and symmetric stretching vibration of Si–O–Si,
respectively.37 The band at 960 cmꢂ1 is assigned to symmetric
stretching vibration of Si–OH.35 In addition, the band at
565 cmꢂ1 is attributed to the vibration of Fe–O bond.38 It can be
concluded from Fig. 1 that Fe3O4 is coated with silica because
all the characteristic bands related to SiO2 as well as Fe3O4 are
shown in the spectrum of Fe3O4@SiO2.
Fig. 2 shows the XRD patterns of SiO2, ZnBr2 (JCPDS:
75-1331), Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2–ZnBr2 with
15.1 wt% Zn loading. Only one wide and weak dispersion peak
at 23.2ꢀ ascribed to the amorphous structure is observed in the
pattern of pure SiO2 (Fig. 2a). The peaks at 13.7ꢀ, 21.1ꢀ, 27.5ꢀ,
46.1ꢀ and 53.4ꢀ (Fig. 2b) are related to the characteristic
diffraction of ZnBr2. The peaks at 30.4ꢀ, 35.6ꢀ, 43.3ꢀ, 53.7ꢀ, 57.1ꢀ
and 62.8ꢀ in the pattern of Fe3O4 (Fig. 2c) are associated with
Miller indices values [hkl] of [220], [311], [400], [422], [511] and
[440], respectively. These peaks are assigned to the inverse cubic
spinel structure of Fe3O4.39 All the characteristic diffraction
peaks related to Fe3O4 also present in the pattern of
Fe3O4@SiO2 (Fig. 2d), indicating almost no change occurs in the
structure of Fe3O4 aer coating by SiO2. Peaks assigned to the
typical diffraction of ZnBr2 are observed at 13.7ꢀ, 21.1ꢀ, 27.4ꢀ,
46.0ꢀ and 53.5ꢀ in the pattern of Fe3O4@SiO2–ZnBr2 (Fig. 2e). In
addition, the characteristic diffraction peaks of Fe3O4 in the
pattern of Fe3O4@SiO2–ZnBr2 become weaker than those
observed in the patterns of Fe3O4 and Fe3O4@SiO2, indicating
that the cubic spinel structure of Fe3O4 could be slightly
affected by introduction of ZnBr2.
Fig. 2 XRD patterns of (a) SiO2, (b) ZnBr2, (c) Fe3O4, (d) Fe3O4@SiO2
and (e) Fe3O4@SiO2–ZnBr2.
The XRD patterns of Fe3O4@SiO2–ZnBr2 with various Zn
loadings are presented in Fig. 3. Fig. 3 reveals that the typical
peaks ascribed to Fe3O4 become weaker while the intensity of
the typical peaks assigned to ZnBr2 is enhanced with increasing
Zn loading. This can be ascribed to the decrease in the amount
of Fe3O4 and increase in ZnBr2. No typical peaks of ZnBr2 are
observed in the pattern of Fe3O4@SiO2–ZnBr2 with 4.5 wt% Zn
loading while the intensity of such diffraction peaks become
stronger in samples with higher ZnBr2 content (curves b and c).
It probably suggests that ZnBr2 is uniformly distributed in the
support of Fe3O4@SiO2, and thus no characteristic peak could
be observed with a lower Zn loading.
Fig.
4 shows the XPS signals of Fe3O4@SiO2 and
Fe3O4@SiO2–ZnBr2 with 15.1 wt% Zn loading. Three peaks at
530.5, 532.5 and 533.4 eV in the XPS spectrum of O 1s (Fig. 4a)
Fig. 1 IR spectra of (a) SiO2 (b) Fe3O4, (c) Fe3O4@SiO2 and (d) Fig. 3 XRD patterns of Fe3O4@SiO2–ZnBr2 with various Zn loadings (a)
Fe3O4@SiO2–ZnBr2.
4.5 wt% Zn loading, (b) 15.1 wt% Zn loading and (c) 17.5 wt% Zn loading.
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are assigned to lattice oxygen, adsorbed oxygen and oxygen consistent with the observation of characteristic peaks of ZnBr2
species in –OHs on the surface, respectively.40,41 It can be seen in the XRD patterns, as shown in Fig. 2 and 3. The results in
that these peaks shi to higher binding energies in the XPS Table 1 reveal that the heterogeneous catalyst still possesses
resolution of Fe3O4@SiO2–ZnBr2 (Fig. 4a). In addition, the peak acceptable surface area although it decreases aer the intro-
of Zn 2p at 1022.9 eV is lower than that of ZnBr2 at 1023.5 eV duction of ZnBr2. The surface areas change from 75 to 86 m2 gꢂ1
(Fig. 4b). The change in the binding energies of O 1s and Zn 2p indicates that Zn loading gives a negligible effect on the surface
reveals that there is a possible electronic interaction between possibly due to its relatively low content.
Zn2+ and hydroxyl or surface oxide species, in which O atom
donates electron to Zn2+. As a result, the binding energy of Zn
atom in ZnBr2 shis towards lower values while the O atom
shis to higher values.42
Catalytic reaction
It has been reported that simple Lewis acids are effective
Table 1 shows the values of the BET surface area for Fe3O4, catalysts for synthesis of DPC from CO2, phenol and CCl4.15
Fe3O4@SiO2 and Fe3O4@SiO2–ZnBr2 with various Zn loading. It The results in Table 2 showed that zinc halides displayed
can be seen that the surface area of Fe3O4@SiO2–ZnBr2 is lower similar activity with respect to the total conversion of phenol,
than that of Fe3O4@SiO2, which can be ascribed to the disper- yield and selectivity towards DPC in the presence of zinc
sion and deposition of ZnBr2 on the surface of Fe3O4@SiO2. It is halides (entries 1–3), which is consistent with our previous
work.32 However, with 0.1 molar ratio of ZnCl2 to phenol, the
yield of DPC was only 5.8%. When a higher molar ratio (ZnCl2/
phenol ¼ 0.5) or CF3SO3H was used, much higher yields of
DPC at 22 and 25% were obtained.32,34 Thus Lewis acids sup-
ported on Fe3O4@SiO2 were further investigated based on the
excellent performance of the magnetic catalyst in many reac-
tions.16 Although no activity was observed in the presence of
Fe3O4@SiO2 alone (entry 4), the catalytic performance of Fe3-
O4@SiO2–ZnBr2 was signicantly enhanced as compared to
that of ZnBr2, giving 9.8% yield of DPC (entry 5). It has been
known that the reactions with CO2 involved can be activated by
–OHs contained on the surface of support.43 So it is logical to
speculate that Fe3O4@SiO2 may also play the role of a
promoter besides support because the surface of Fe3O4 and
SiO2 contain
a large number of accessible –OHs. The
improvement in the catalytic performance of supported ZnBr2
may also be ascribed to the possible interaction between Zn2+
and hydroxyl or surface oxide species, which was conrmed by
XPS analysis (see Fig. 4). In addition, it can be seen from Table
2 that the catalytic performance of simple physical mixture of
ZnBr2 and Fe3O4@SiO2 was far poorer than that of Fe3O4@-
SiO2–ZnBr2 obtained via impregnation (entries 5, 6). The
results further revealed that there is a possible interaction
between Zn2+ and hydroxyl or surface oxide species, which may
occur during the supported catalyst preparation.
The results in Table 2 also show that there is a difference in
the yield of DPC with various zinc halides in heterogeneous
system (entries 5, 7, 8). The yield of DPC with Fe3O4@SiO2–
ZnCl2 or Fe3O4@SiO2–ZnI2 was lower than that with
Fe3O4@SiO2–ZnBr2. It may be attributed to variation in the
Lewis acidity as well as the steric hindrance of halide anions. It
is generally accepted that increasing acidity gives a positive
effect on the performance but the steric hindrance shows the
opposite. The order of acidity is as following: ZnCl2 < ZnBr2 <
ZnI2 while the steric hindrance is on the contrary. These con-
icting factors can compensate each other, thus generating
better activity for Fe3O4@SiO2–ZnBr2. The tendency towards
DPC yield in the supported catalytic systems (entries 5, 7, 8) is
inconsistent with the results obtained in the presence of
homogeneous zinc halides (entries 1–3), further indicating that
the chemical environment in supported catalyst is different
Fig. 4 XPS spectra of O 1s and Zn 2p.
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Table 1 BET surface area for support and heterogeneous catalysts with various Zn loading
Fe3O4@SiO2–ZnBr2 (Zn wt%)
4.5 10.6 12.7
86 79 80
Sample
Fe3O4
63
Fe3O4@SiO2
158
15.1
77
17.5
79
20.2
75
BET surface area (m2 gꢂ1
)
Table 2 Synthesis of DPC with various catalysta
the yield of DPC were dependent on the molar ratio of ZnBr2 to
phenol in the range between 0.05 and 0.25 but the selectivity
changed insignicantly. The maximum conversion and yield
were observed at a molar ratio of 0.1 (entry 2) and then a
decrease was observed. The yield of DPC was found to be 4.4%
with a molar ratio of 0.25 (entry 5). The results are consistent
with those previously obtained using CF3SO3H as co-catalyst34
but not in accordance with those in the absence of co-catalyst.
Both the conversion and the yield were enhanced with
increasing the amount of zinc halide and no optimal ratio of
catalyst to substrate was observed in the latter.32 These results
further indicate that Fe3O4@SiO2 may not only play a role of
support but also act a promoter in the present work. It has been
reported that two phases including liquid and gas phase were
present in the reaction mixture using pressured CO2 as raw
material as well as solvent for the synthesis of DPC, in which the
reaction usually proceeds in the liquid phase mainly composed
of CO2.32 As the molar ratio of ZnBr2 to phenol increased,
Fe3O4@SiO2 increased more markedly due to relatively low
content of ZnBr2 in the heterogeneous catalyst. Thus catalyst
and substrate may not be covered fully by liquid phase in the
presence of excessive heterogeneous catalyst, leading to a
decrease in the conversion of phenol and yield of DPC.
Conversion
Selectivity
(%)
Yield
(%)
Entry
Catalyst
(%)
1
2
3
4
ZnCl2
ZnBr2
ZnI2
Fe3O4@SiO2
Fe3O4@SiO2–ZnBr2
Fe3O4@SiO2/ZnBr2
Fe3O4@SiO2–ZnCl2
Fe3O4@SiO2–ZnI2
12.0
12.2
11.7
2.4
16.8
1.3
48.1
46.8
48.5
—
58.3
47.4
48.3
43.6
5.8
5.7
5.7
—
9.8
0.6
1.9
4.1
5b
6c
7b
8b
3.9
9.4
a
Reaction conditions: phenol
¼
12 mmol, CCl4
¼
40 mmol,
temperature ¼ 100 ꢀC, CO2 pressure ¼ 8 MPa, reaction time ¼ 6 h,
catalyst (containing zinc halide 1.2 mmol). b Zn loading was 15.1 wt%.
c
Simple physical mixture of ZnBr2 and Fe3O4@SiO2.
from that of simple zinc halide, which also support the possible
interaction between Zn2+ and hydroxyl or surface oxide species.
Table 3 shows the effects of heterogenized catalysts with
different Zn loadings on the synthesis of DPC from CO2. It can
be seen that the yield and the selectivity towards DPC are
signicantly dependent on the Zn content. Both were enhanced
by increasing Zn loading in the range from 4.5 to 15.1 wt%
(entries 1–4) and a maximum yield of DPC at 9.8% was obtained
in the presence of 15.1 wt% Zn loading. Then the conversion of
phenol slightly dropped but the selectivity showed nearly no
change when the Zn loading was increased to 20.2 wt%. It is
possibly ascribed to the aggregation of the active sites with
higher Zn content, thus leading to poor dispersion37 and giving
the negative effect on the reaction.
The effect of CCl4 was further investigated. Table 4 shows
that increasing CCl4 increased the conversion of phenol, yield
and selectivity towards DPC. A lower yield of DPC with less CCl4
(5 mmol, entry 6) can be related to the formation of smaller
+
amount of CCl3 , which is believed to be an important inter-
mediate for the synthesis of DPC from phenol and dense phase
CO2.44 A maximum yield of DPC at 11.9% was obtained by the
Table 4 shows the effects ZnBr2 and CCl4 on the reaction
between CO2 and phenol. Both the conversion of phenol and
a
Table 4 Synthesis of DPC with various amounts of ZnBr2 and CCl4
ZnBr2/phenol
(molar ratio)
CCl4
(mmol)
Conversion
(%)
Selectivity
(%)
Yield
(%)
Entry
Table 3 Synthesis of DPC with various Zn loadinga
1
2
3
4
5
6
7
8
9
10
0.05
0.1
0.15
0.2
0.25
0.1
0.1
0.1
0.1
0.1
40
40
40
40
40
5
10
20
30
50
10.1
16.8
16.6
14.3
8.4
12.0
18.8
17.1
12.5
8.8
50.6
58.3
56.5
51.8
52.2
55.8
63.3
61.9
59.9
58.1
5.1
9.8
9.4
7.4
4.4
Zn loading
(%)
Conversion
(%)
Selectivity
(%)
Yield
(%)
Entry
1
2
3
4
5
6
4.5
10.6
12.7
15.1
17.5
20.2
11.1
15.6
16.3
16.8
15.5
13.1
9.8
33.5
41.1
58.3
54.8
55.1
1.1
5.2
6.7
9.8
8.5
7.2
6.7
11.9
10.6
7.5
5.1
a
a
Reaction conditions: phenol
¼
12 mmol, CCl4
¼
40 mmol,
Reaction conditions: phenol ¼ 12 mmol, temperature ¼ 100 ꢀC, CO2
temperature ¼ 100 ꢀC, CO2 pressure ¼ 8 MPa, reaction time ¼ 6 h, pressure ¼ 8 MPa, reaction time ¼ 6 h, Fe3O4@SiO2–ZnBr2 (15.1 wt%
Fe3O4@SiO2–ZnBr2 (containing ZnBr2 1.2 mmol) was employed.
Zn loading) was employed.
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addition of 10 mmol CCl4 (entry 7). Further increase in CCl4
resulted in a drop in both the conversion and the yield but
nearly no change in selectivity. Our previous investigation also
indicated that two phases were always presented under the
present reaction conditions and the reactions mainly occurred
in the liquid phase.32 Either concentration of phenol or ZnBr2 in
the liquid phase became smaller in the presence of a larger
amount of CCl4, and thus might reduce the conversion of
phenol.32 The results in Table 4 also show that a slightly
excessive amount of CCl4 is essential for the synthesis of DPC
from CO2, phenol and CCl4. The optimal molar ratio of CCl4 to
phenol was 1 : 1.2 (entry 7), which is higher than the stoichio-
metric molar ratio of 1 : 4.32
Table 5 shows the effects of reaction variables including the
CO2 pressure, reaction temperature and time. Temperature was
rst tested over a range from 90 to 140 ꢀC. It can be seen that the
conversion of phenol was progressively improved with
increasing temperature but the selectivity was low at lower
and/or higher temperature. The selectivity of 21.1 and 38.1%
Fig.
5
Reusability of Fe3O4@SiO2–ZnBr2. Reaction conditions:
phenol ¼ 12 mmol, CCl4 ¼ 10 mmol, temperature ¼ 130 ꢀC, CO2
pressure 8 ¼ MPa, reaction time ¼ 4 h, Fe3O4@SiO2–ZnBr2 (Zn loading
15.1 wt%, containing ZnBr2 1.2 mmol for the first run) was employed.
ꢀ
were observed at 90 and 140 C, respectively (entries 1, 6). The
maximum yield of 27.2% was obtained at medium temperature
ꢀ
of 130 C. It has been reported that higher temperature favors
(entries 9, 10). The unique properties appearing near the critical
point are probably responsible for the positive effect observed
around 8 MPa which is close to the critical pressure of pure CO2.
Due to phase change of CO2 from gas to supercritical uid, the
variation of density around the critical point generally causes
changes in chemical or physical equilibrium, possibly
promoting the dissolution of phenol in liquid and the inter-
solubility between supercritical CO2 and CCl4. Therefore, the
rate and the selectivity were remarkably dependent on the
pressure of CO2 since it acts as both reactant and solvent in the
present reaction. Similar results with a maximum selectivity at a
pressure near the critical point of CO2 were also reported
elsewhere.15,32,33
the formation of phenoxide which would further transfer into
DPC,32 explaining why the selectivity to DPC increased with
temperature as shown in Table 5 below 130 C (entries 1–5). A
ꢀ
further increase in temperature may be favorable to the
formation of another intermediate p-hydroxybenzoic acid-like
compound which would not change into the objective
product. Thus temperatures above 130 ꢀC led to the reduced
yield and selectivity towards DPC.45
The CO2 pressure has been considered one of the most
crucial factors for the reactions using CO2 as reactant as well as
reaction medium. The conversion of phenol, yield and selec-
tivity towards DPC were improved with an enhancement in the
pressure of CO2 below 8 MPa (entries 5, 7, 8). Although the
conversion was slightly changed, a negative effect was observed
in terms of selectivity with further increase in the pressure
Table 5 also indicates that the synthesis of DPC was depen-
dent on the reaction time. The conversion of phenol and the
Table 5 Optimization of reaction conditiona
Entry
Temperature (ꢀC)
CO2 pressure (MPa)
Time (h)
Conversion (%)
Selectivity (%)
Yield (%)
1
2
3
4
5
6
7
8
90
100
110
120
130
140
130
130
130
130
130
130
130
130
8
8
8
8
8
8
6
7
9
10
8
8
8
8
6
6
6
6
6
6
6
6
6
6
2
4
8
8.5
18.8
32.8
38.8
42.8
51.5
38.8
36.2
44.3
46.6
37.9
42.9
45.9
44.1
21.1
63.3
59.8
61.1
63.6
38.1
45.6
67.1
55.3
35.0
53.8
65.5
60.8
62.6
1.8
11.9
19.6
23.7
27.2
19.6
17.7
24.3
24.5
16.3
20.4
28.1
27.9
27.6
9
10
11
12
13
14
10
a
Reaction conditions: phenol ¼ 12 mmol, CCl4 ¼ 10 mmol, Fe3O4@SiO2–ZnBr2 (Zn loading 15.1 wt%, containing ZnBr2 1.2 mmol) was employed.
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yield of DPC were increased from 2 to 4 h (entries 11, 12) and
then kept nearly constant (entries 5, 13, 14). This possibly
suggests the reaction of CO2 with phenol in the presence of CCl4
reached equilibrium at 4 h.
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Fe3O4@SiO2–ZnBr2 with 15.1 wt% Zn loading can be easily
recovered with a permanent magnet aer the reaction and
reused in the next run without further treatment. The recyclable
performance was shown in Fig. 5. It can be seen that
Fe3O4@SiO2–ZnBr2 possessed excellent stability at the initial 4
runs, in which the yield of DPC changed in a small range from
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aer each cycle. These results revealed that the drop in the 12 P. Unnikrishnan, P. Varhadi and D. Srinivas, RSC Adv., 2013,
catalytic activity could be ascribed to ZnBr2 leaching. The 3, 23993–23996.
decrease in Zn content aer the fourth run can be ascribed to 13 T. Iijima and T. Yamaguchi, Appl. Catal., A, 2008, 345, 12–17.
the following reason: the active species ZnBr2 supported on 14 H. Kawanami, A. Sasaki, K. Matsui and Y. Ikushima, Chem.
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adhere to the surface of the support under high pressure and 15 G. Z. Fan, Z. G. Wang, B. Zou and M. Wang, Fuel Process.
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ZnBr2 supported on MNPs-Fe3O4 coated by SiO2 was developed
as an effective and recoverable catalyst for the synthesis of DPC
from CO2 and phenol in the presence of CCl4. It was found that
the catalytic performance of Fe3O4@SiO2–zinc halides was
dependent on the kind of zinc halides. Fe3O4@SiO2–ZnBr2
showed better catalytic performance than that of the hetero-
genized ZnCl2 and ZnI2 as well as homologous ZnBr2. Under the
optimized conditions, 28.1% of DPC yield was obtained using
Fe3O4@SiO2–ZnBr2 as the catalyst. The XPS result and the
activity comparison between simple mixing ZnBr2 with
Fe3O4@SiO2 and Fe3O4@SiO2–ZnBr2 revealed that there is a
possible interaction between Zn2+ and hydroxyl or surface oxide
species in support Fe3O4@SiO2. Fe3O4@SiO2–ZnBr2 can be
easily recovered by using an external magnet and reused
without signicant loss in activity for 4 runs. The yield of DPC
showed little change in the range 27.6–28.1%.
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´
´
´
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`
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The authors acknowledge the nancial support from the
Scientic Research Project from Hubei Provincial Department
of Education (no. D20141704; T201407) and the Hubei Provin-
cial Natural Science Foundation of China (no. 2014CFB890).
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