Catalytic activity of Ag-Co-MCM-41 for liquid-phase selective oxidation of styrene to benzaldehyde

Article abstract of DOI:10.1166/jnn.2020.17137

A series of Ag-Co-MCM-41 with different metal loadings have been synthesized through the hydrothermal method. All the prepared catalysts were characterized by N₂ adsorption-desorption, X-ray diffraction analysis, transmission electron microscop

Full text of DOI:10.1166/jnn.2020.17137

Article
Journal of
Nanoscience and Nanotechnology
Vol. 20, 1670–1677, 2020
Copyright © 2020 American Scientific Publishers
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Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase
Selective Oxidation of Styrene to Benzaldehyde
Tingshun Jiang∗, Guofang Gao, Cheng Yang, YuLin Mao, Minglan Fang, and Qian Zhao
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, Jiangsu, China
A series of Ag-Co-MCM-41 with different metal loadings have been synthesized through the
hydrothermal method. All the prepared catalysts were characterized by N2 adsorption–desorption,
X-ray diffraction analysis, transmission electron microscopy. The results revealed that the structure
of MCM-41 was well preserved and Ag and Co have been introduced successfully into the meso-
porous channels of MCM-41. The liquid-phase catalytic oxidation of styrene on these catalysts was
investigated using H2O2 as an oxidizing agent and acetone as a solvent in thermostatic water bath.
The influence of metal loading, the catalyst dose, temperature, time and styrene/oxidant molar ratio
on the conversion of styrene and yield and selectivity of benzaldehyde was investigated. Also, the
reusability of the catalysts was evaluated.
Keywords: MCM-41 Mesoporous Material, Ag-Co-MCM-41, Styrene, Oxidation, Benzaldehyde.
IP: 95.181.217.151 On: Sun, 29 Dec 2019 09:10:32
Copyright: American Scientific Publishers
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1. INTRODUCTION
reaction mixture [13], much attention has been focused
on immobilizing the complexes on solid supports such as
ordered mesoporous silicates [20–25], polymers [26–29],
zeolite [30–32] and nanoparticles [33] to afford hetero-
geneous catalysts. For example, Clara et al. [34] pre-
pared a series of different metal cations incorporated
ZSM-5 for the oxidation of styrene to benzaldehyde with
H2O2 as oxidant. They found that Cr-ZSM-5 exhibited
the good catalytic activity, but the conversion of styrene
and selectivity for benzaldehyde were only 48.9% and
60.9%, respectively. Islam et al. [28] reported a reusable
polymer-anchored Cu(II) complex as the catalyst for this
reaction. The conversion of styrene reached 53%, how-
ever, the selectivity for benzaldehyde was 52%, and exces-
sive by-product caused the product difficult to recover.
In summary, the result was inferior catalytic activity and
recoverability [28].
In current work, we prepared a series of Ag-Co-
MCM-41 mesoporous materials with different Ag and Co
contents. The binding ability of oxygen and Ag is weak,
it is easy to break out, and the reduced Ag is relatively
difficult to oxidize. Co, on the contrary, binds strongly
to oxygen and is not easy to detach, but easy to oxi-
dize. Therefore, the co-introduction of silver and cobalt
into MCM-41 mesoporous molecular sieves can assist the
catalysis together. The resulting materials were employed
Oxidation of styrene is a reaction of great interest because
its products, mainly benzaldehyde and styrene epoxide,
act as versatile and useful intermediates [1–4]. Certainly,
benzaldehyde is widely used for the perfumery, pharma-
ceutical intermediates, dyestuffs and agrochemicals [5].
Therefore, a variety of methods for preparing benzalde-
hyde have been reported [1, 2, 6–9]. Traditionally, it
is produced by hydrolysis of benzal chloride and liq-
uid phase oxidation of toluene [10]. These processes will
make the product with chlorine and cause harm to the
environment. Recently, much attention has focused on
developing eco-friendly aerobic catalytic processes based
on efficient catalysts with easy recoverability [11]. The
liquid phase selective oxidation of styrene for providing
chlorine-free benzaldehyde was developed which H2O2 as
oxidant. H2O2 has many advantages as an oxidant because
water is only by-product [12].
However, synthesizing an efficient catalyst is the key to
the success of the reaction [13]. Various catalysts, such as
metal oxides [14, 15], metals [16], metal doped zeolites,
solid acids [17], and metal Schiff bases or Salen com-
plexes [18, 19], have been discussed. Since these homoge-
nous catalysts are usually difficult to separate from the
∗Author to whom correspondence should be addressed.
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1533-4880/2020/20/1670/008
doi:10.1166/jnn.2020.17137

Jiang et al.
Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase Selective Oxidation of Styrene to Benzaldehyde
as catalysts on the styrene selective oxidation using hydro-
gen peroxide as oxidant under mild reaction conditions,
obtaining benzaldehyde as the main product. Key reac-
tion conditions such as metal loading, the catalyst dose,
temperature and time and styrene/oxidant molar ratio were
investigated. The reusability of the catalysts was also
evaluated.
2.3. Catalyst Characterization
XRD patterns were recorded on a powder XRD instru-
ment (Rigaku D/max 2500PC) with Cu Kꢁ radiation
(ꢂ = 0.15418 nm) operating at 40 kV and 50 mA in
the 2ꢃ range from 1 to 10ꢀ. N2 adsorption–desorption
isotherms at 77 K were recorded with a NOVA2000e ana-
lytical system made by Quantachrome Corporation (USA).
ꢀ
Before measuring, all samples were outgassed at 300 C
for 30 min. Calculation of specific surface area use BET
method, while pore size distribution and pore volume
were calculated using BJH method. Transmission elec-
tron microscopy (TEM) morphologies of samples were
observed on a Philips TEMCNAI-12 with an acceleration
voltage of 100–120 kV. The Ag and Co content in sample
was determined by inductive coupled plasma (ICP) tech-
nique (Vista-MPX, Australia), and the detector is MPX
CCD. The results are listed in Table I.
2. EXPERIMENTAL DETAILS
2.1. Chemicals
Styrene (CR), cetyl trimethyl ammonium bromide
(CTAB, AR), concentrated sulfuric acid and sodium
silicate nonahydrate (Na2SiO3 · 9H2O) were purchased
from Sinopharm Chemical Reagent Co., Ltd. Hydrogen
peroxide (30%H2O2, AR) was purchased from Nanjing
Chemical Reagent Co., Ltd. Acetone (AR), cobalt nitrate
hexahydrate (Co(NO3ꢀ2 · 6H2O AR) was purchased from
Shanghai Chemical Reagent Co., Ltd. Silver nitrate
(AgNO3 AR) was purchased from China Pharmaceutical
(Group) Shanghai Chemical Reagent Company.
2.4. Catalyst Test
The oxidation reaction was performed in a 250 ml round
bottom flask with reflux device where vigorous stirring in
a thermostat water bath for several hours, the amount of
catalyst is 3 wt% of the reactants. Typically, according
to a certain proportion, styrene and hydrogen peroxide as
raw materials, acetone as solvent, were added to the flask.
In the oxidation reaction of styrene, we chose acetone as
2.2. Catalyst Preparation
The pure siliceous MCM-41 was synthesized using
Na2SiO3 · 9H2O as silicon source and CTAB as tem-
plate by hydrothermal method, according to the procedure
solvent owing to acetone is relatively stable. If there are
IP: 95.181.217.151 On: Sun, 29 Dec 2019 09:10:32
in the literatures [35, 36]. The typical synthetic process
Copyright: American SncoiesnptiefciciaPl ucibrcliusmhesrtasnces, acetone does not normally react
was n(SiO2ꢀ:n(CTAB):n(H2O) = 1:0.2:70. First, 7.29 g of
Delivered by Ingenta
with hydrogen peroxide. After the reactions, we chose the
CTAB was dissolved in 40 ml distilled water until a gelati-
nous solution was obtained, named solution A. 28.42 g of
Na2SiO3 ·9H2O was dispersed into 40 ml distilled water to
form solution B. The B solution was poured into a three-
catalyst with the best catalytic activity for reusability test.
The catalyst was collect, washed with ethanol for 3 times,
ꢀ
dried at 60 C overnight, and then reused [37]. Excess
hydrogen peroxide is quenched by sodium thiosulfate dur-
ing the reaction.
ꢀ
necked flask at 35 C, stirring constantly until completely
dissolved, then added the A solution into the flask and
continued to stir 30 min. After that, the reaction solution
was adjusted to pH = 11 by dropwise addition of sulfuric
acid (50 vol.%), and the reaction solution was continu-
ously stirred for another 3 h. Finally, the mixed solutiꢀon
was poured into the autoclave and crystallized at 120 C
for 24 h in an oven. After cooling to room temperature,
the sample was filtered,ꢀ washed with deionized water to
neutral, and dried atꢀ60 C overnight. At last, the samples
The products were analyzed by gas chromatograph (SP-
2000) fitted with a SE-54 capillary column coupled with
FID and calculated using ꢀnormalization method. The col-
umnꢀ temperature is 120 C, the detector temperature is
ꢀ
250 C and the injector is 200 C.
In the oxidation of styrene, the conversion of styrene,
the yield and selectivity of benzaldehyde were calcu-
lated by the following formula: Cstyrene = ꢄnꢅstyreneꢆinitialꢀ
ꢅstyreneꢆbalanceꢀꢇ/nꢅstyreneꢆinitialꢀ (1); Ybenzaldehyde
ꢄnꢅstyreneꢆinitialꢀ nꢅstyreneꢆbalanceꢀꢇ/nꢅstyreneꢆinitialꢀ
−
n
=
ꢀ
−
(2);
were heated to 550 C at a heating rate of 2 C/min and
kept at 550 ꢀC for 6 h, the calcined sample was designated
as MCM-41.
Sbenzaldehyde = nbenzaldehyde/ꢄnꢅstyreneꢆinitialꢀ −nꢅstyreneꢆbalanceꢀ] (3),
where is the quantity of matter, and Cstyrene
n
,
Ybenzaldehyde and Sbenzaldehyde stand for the conversion
The Ag-Co-MCM-41 was directly synthesized by
hydrothermal method. A given amount of Co(NO3ꢀ2 ·6H2O
and AgNO3 (the molar ratio of M/Si = 0.005, 0.01, 0.015
and 0.02) were dispersed in B. Other steps are the same as
above. For comparison, the 0.01-Ag-MCM-41 and 0.01-
Co-MCM-41 catalysts were prepared in the same manner.
Then the samples were pressed tablets at 10 mpa. After
grinded, we screened 40–60 mesh particles as catalysts.
Table I. The Ag and Co element content in sample.
Co-Ag-MCM- Co-Ag-MCM- Co-Ag-MCM- Co-Ag-MCM-
Sample
41-0.005
41-0.001
41-0.015
41-0.02
W(Ag)%
W(Co)%
0.41
0.22
0.79
0.42
1.02
0.64
1.48
0.85
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Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase Selective Oxidation of Styrene to Benzaldehyde
Jiang et al.
of styrene, the yield and selectivity of benzaldehyde,
respectively.
at about 2ꢃ = 24.8ꢀ, and several diffraction peaks repre-
senting metal oxides appeared between 2ꢃ = 30 and 60ꢀ.
With the increase of metal loading, the intensity of diffrac-
tion peak will be stronger and the diffraction peak of silica
will be weaker, which further shows that the metal ions
doped on the mesoporous molecular sieve MCM-41.
3. RESULTS AND DISCUSSION
3.1. Catalytic Properties of Ag-Co-MCM-41
3.1.1. XRD Analysis
The low-angle patterns of MCM-41 and n-Ag-Co-MCM-
41 are depicted in Figure 1(a). The pattern of MCM-41
shows an intense diffraction peak at 2ꢃ = 2ꢈ3ꢀ, which is
MCM-41 mesoporous molecular sieve crystal plane peak
(100). At the same time, we can see two clearly identifiable
(110), (200) diffraction peaks at 2ꢃ = 3–5ꢀ are attributed
to its characteristic hexagonal pore structure [38]. As com-
pared with the pure silicon MCM-41, the intensity of
the (100) diffraction peak of the Ag-Co-MCM-41 sample
slightly weakened when the doped metal content is less
than or equal to 0.01. When the doped content of metal
in Ag-Co-MCM-41 sample is between 0.01 and 0.02, the
intensity of diffraction peak decreased obviously, and the
diffraction peak became wider and weaker, suggesting that
the metal (Ag and Co) doped into the framework of MCM-
41 molecular sieve [39]. On the other hand, we found
that diffraction peaks shifted to low angle, showing that
increasing the spacing of molecular sieves. The reason
3.1.2. N2 Adsorption–Desorption Analysis
The N2 adsorption–desorption isotherms of the synthe-
sized samples are shown in Figure 2. According to IUPAC
classification, the prepared samples are IV type isotherm
from the Figure 2. All isotherms exhibit a sharp step at
a relative pressure of ca. 0.2–0.4, further showing that all
the samples have typical mesostructure with uniform pore
size distribution and large pore volumes [40]. The hystere-
sis loops of the low pressure stage became smaller with
the metal loaded increasing. It indicates that the ordering
of these materials deteriorated. It also proves the partial
pore collapse and obstruction and the samples regularity
decreased.
From Table II, the specific surface areas and pore vol-
umes of all the samples synthesized decrease with the
increase of the amount of metal added. Combined with the
results of XRD and N2 adsorption–desorption isotherms,
it is reasonable to conclude that the ordering of the syn-
thesized mesoporous molecular sieves decreases with the
is ionic radius of the metal (rCo+2 = 0.065 nm; rAg+1
0.115 nm) is larger than silicon (rSi+4 = 0.04 nm). The
IP: 95.181.217.151 On: Sun, 29 Dec 2019 09:10:32
=
increase of the amount of metal added.
Copyright: American Scientific Publishers
other one is probably due to the metal ion replaced Si
in framework of the molecular sieve. Further, the char-
acteristic diffraction peak weakened with increasing of
metal content. This indicated that the ordering of meso-
porous molecular sieves gradually declined, and hexago-
nal structure disappeared. Perhaps metal ions may lead to
channel blocking and tunnel collapse. Moreover, the 0.01-
Ag-Co-MCM-41 catalyst used reusability test was col-
lected (named as 0.01-Ag-Co-MCM-41 after five runs) and
the XRD pattern was shown in Figure 1(a). It is noted that
the intensities of the characteristic peaks slightly weak-
ened and the position was almost unchanged. Figure 1(b)
shows the wide-angle XRD patterns of the sample. It can
be seen that a strong peak is the diffraction peak of silica
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3.1.3. Transmission Electron Microscopy Analysis
Figure 3 shows TEM images of the prepared samples.
As shown in Figure 3, all the four samples exhibit a
hexagonal mesoporous structure of MCM-41. Showing
that these samples have mesoporous structure and Ag-Co-
MCM-41 mesoporous molecular sieves were successfully
synthesized. Materials still retains mesostructure when the
loaded metal content is less than or equal to 0.01, but the
mesoporous ordering decreases. While the metal content
continues to increase, the pore structure disappeared. We
can conclude that metal loaded changed the order of the
samples.
(b)
(a)
(100)
MCM-41
(110)
(200)
0.005-Ag-Co-MCM-41
0.01-Ag-Co-MCM-41
MCM-41
0.005-Ag-Co-MCM-41
0.01-Ag-Co-MCM-41
0.01-Ag-Co-MCM-41(after five runs)
0.015-Ag-Co-MCM-41
0.015-Ag-Co-MCM-41
0.02-Ag-Co-MCM-41
0.02-Ag-Co-MCM-41
10
20
30
40
50
60
70
80
2
4
6
8
10
2Theta(deg)
Figure 1. XRD patterns of Ag-Co-MCM-41 samples with different M/Si molar ratios: (a) the low-angle patterns; (b) the wide-angle patterns.
J. Nanosci. Nanotechnol. 20, 1670–1677, 2020
2Theta(deg)
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Jiang et al.
Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase Selective Oxidation of Styrene to Benzaldehyde
enhances the catalytic activity of catalyst, therefore, the
1000
(1)
ads
des
oxidation process eased and resulted in higher amounts of
by-products.
(2)
750
500
250
3.2.2. Effect of Catalyst Dose
(3)
0.01-Ag-Co-MCM-41 molecular sieve was used as cata-
lyst for the reaction. The amounts of catalyst were 1 wt%,
2 wt%, 3 wt%, 4 wt%, 5 wt%, respectively. Other reac-
tion conditions remain unchanged. The result of the reac-
tion is shown in Figure 5. It can be seen that with the
increasing amount of catalyst, there is a maximum conver-
sion of styrene, because when the catalyst content is low,
increasing the amount of catalyst can increase the amount
of hydrogen peroxide adsorbed by the catalyst, resulting in
the formation of many more active, therefore, the conver-
sion of styrene increased [41]. When the amount of cata-
lyst exceeded a certain amount, the conversion of styrene
decreased. This is due to the increase in the amount of cat-
alyst used to aggravate the decomposition of part of hydro-
gen peroxide, resulting in a decrease in the conversion of
styrene. Further, when the catalyst weight was 3 wt%, the
conversion of styrene and the yield of the benzaldehyde
were 45.2% and 24.1% respectively, and the selectivity of
benzaldehyde was 53.3%.
(4)
(5)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure P/P0
Figure 2. N2 adsorption–desorption isotherms of samples. (1) MCM-
41; (2) 0.005-Ag-Co-MCM-4; (3) 0.01-Ag-Co-MCM-41; (4) 0.015-Ag-
Co-MCM-41; (5) 0.02-Ag-Co-MCM-41.
3.2. Optimization of Reaction Conditions
The results of styrene oxidation over Ag-Co-MCM-41 in
acetone solution are carried out as following experiment.
The main product of the reaction is benzaldehyde, pos-
sibly, the others could be found, such as styrene oxide,
phenylacetaldehyde, benzoic acid and so on.
3.2.1. Effect of Ag and Co Loaded
0.1 moL styrene was added to 250 mL flask, the amount
of catalyst was 3% of the mass fraction of styrene, oxi-
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dant and solvent were added according to the mole ratio
Copyright: American Scientific Publishers
of styrene:hydrogen peroxide:acetone = 1:3:2, in which
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hydrogen peroxide was used as oxidant ꢀand acetone as
solvent. The reaction temperature was 60 C and the reac-
tion time was 7 h. The catalytic reaction takes place in
a water bath pot with constant temperature and constant
speed stirring. The catalyst was n-Ag-Co-MCM-41 (n =
0.005, 0.01, 0.015, 0.02). The results are as shown in
Figure 4. It can be seen that the conversion of styrene
gradually increased with the increasing of metal load-
ing amount, suggesting that the catalytic activity of Ag-
Co-MCM-41 catalyst enhanced with the increasing of
metal loading amount. This may be due to an increase
in the amount of active sites in samples. On the other
hand, we found that selectivity of benzaldehyde gradually
decreased with the increasing of metal loading amount.
This is probably due to the increase of metal loading
Table II. Surface areas and pore sizes of MCM-41 and Ag-Co-MCM-
41 samples with different M/Si molar ratios.
Surface
area/
(m2/g)
Average
pore size/
(nm)
Total
pore volume/
(cm3/g)
Samples
MCM-41
1102ꢈ80
908ꢈ30
794ꢈ17
642ꢈ92
551ꢈ75
3.46
3.56
3.93
3.99
4.55
0.97
0.89
0.71
0.64
0.63
0.005-Ag-Co-MCM-41
0.01-Ag-Co-MCM-41
0.015-Ag-Co-MCM-41
0.02-Ag-Co-MCM-41
Figure 3. TEM images of various samples: (A) MCM-41; (B) 0.005-
Ag-Co-MCM-41; (C) 0.01-Ag-Co-MCM-41; (D) 0.015-Ag-Co-MCM-
41; (E) 0.02-Ag-Co-MCM-41.
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Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase Selective Oxidation of Styrene to Benzaldehyde
Jiang et al.
60
50
40
30
20
10
0
60
50
40
30
20
10
0
conversion of styrene
yield of benzaldehyde
selectivity of benzaldehyde
conversion of styrene
yield of benzaldehyde
selectivity of benzaldehyde
0.005
0.010
0.015
0.020
5h
6h
7h
8h
9h
amount of metal loaded
reaction time
Figure 4. Effect of amount of metal loaded (reaction condition: Cat-
alyst dose = 3 wt% of the reactants; temperature = 60 ꢀC; time = 7 h;
Figure 6. Effect of reaction time (reaction condition: Catalyst dose =
3 wt% of the reactants; temperature = 60 ꢀC; the loaded amount of the
nstyrene/nH = 1:3).
O
2
2
metal = 0.01; nstyrene/nH2 = 1:3).
O
2
3.2.3. Effect of Reaction Time
3.2.4. Effect of Reaction Temperature
According to the above experiments, 0.01-Ag-Co-MCM-
41 was selected as the catalyst, the reaction time was 5,
6, 7, 8 and 9 h respectively, and the other conditions were
unchanged. The results are shown in Figure 6. As shown in
Figure 6, when the reaction time increased from 5 to 6 h,
the conversion of styrene and the yield of benzaldehyde are
slightly increased. When the reaction time is 7 h, the con-
version of styrene and the yield of benzaldehyde increased
According to the above experiments, 0.01-Ag-Co-MCM-
41 was selected as the catalyst, the amount of catalyst
was 3 wt%, the reaction time was 7 h, and the reaction
ꢀ
temperature was 50, 60, 70, 80 and 90 C, respectively.
The other experimental conditions were not changed. The
experimental results are showed in Figure 7. We found
from Figure 7 that with the increase of reaction tempera-
IP: 95.181.217.151 On: Sunt,u2re9, Dtheecco2n0v1e9rs0io9n:1o0f:3st2yrene, the yield and selectivity of
rapidly. This may be because the reaction time is relatively
Copyright: American Scientific Publishers
benzaldehyde increased first and then decreased. The max-
short, only part of the reactant reaction. When the reaction
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imum conversion of styrene was 65.2% at 80 ꢀC, the max-
time continued to increase, the conversion of styrene was
almost unchanged, while the yield of benzaldehyde began
to decrease, because with the reaction time prolonged, the
product continued to take place deep oxidation, resulting
in a decrease in selectivity. This suggested that the longer
time is not more conducive to the occurrence of the reac-
tion. Therefore, 7 h was chosen as the reaction time for
the follow-up reaction.
ꢀ
imum yield of benzaldehyde was 28.9% at 70 C, and the
maximum selectivity of benzaldehyde was 53.5% at 60 ꢀꢀC.
However, when the reaction temperature reaches 90 C,
the conversion of styrene increased. This may be attributed
to the decomposition of some oxidants and the partial
volatilization of acetone caused by the high reaction tem-
perature. The reduction of acetone content is not conducive
70
60
50
40
30
60
50
40
30
20
20
conversion of styrene
conversion of styrene
yield of benzaldehyde
10
10
yield of benzaldehyde
selectivity of benzaldehyde
selectivity of benzaldehyde
0
0
50
1wt%
2wt%
3wt%
4wt%
5wt%
60
70
80
90
catalyst dose
reaction temperature
Figure 5. Effect of catalyst dose (reaction condition: Time = 7 h;
temperature = 60 ꢀC; the loaded amount of the metal = 0.01;
Figure 7. Effect of reaction temperature (reaction condition: Catalyst
dose = 3 wt% of the reactants; time = 7 h; the loaded amount of the
nstyrene/nH = 1:3).
metal = 0.01; nstyrene/nH2 = 1:3).
O
O
2
2
2
1674
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Jiang et al.
Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase Selective Oxidation of Styrene to Benzaldehyde
Table III. Results of the catalytic oxidation of styrene over different
catalysts.
to the mass transfer between styrene and hydrogen perox-
ide, which makes styrene and hydrogen peroxide not easily
adsorbed on the active site on the surface of the catalyst,
so that the conversion of styrene decreases.
Conversion of Selectivity of
Yield of
styrene
(%)
benzaldehyde benzaldehyde
Sample
(%)
(%)
3.2.5. Effect of Molar Ratio of H2O2 to Styrene
0.01-Co-MCM-41
0.01-Ag-MCM-41
0.01-Ag-Co-MCM-41
35.6
28.9
45.2
43.8
37.7
53.3
15.6
10.9
24.1
Figure 8 shows the effect of different molar ratios of
styrene to H2O2 on the oxidation of styrene onto 0.01-
Ag-Co-MCM-41 catalyst with the reaction temperature of
Notes: Reaction condition: Catalyst dose = 3 wt% of the reactants; temperature =
ꢀ
60 ꢀC; the loaded amount of the metal = 0.01; time = 7 h, nstyrene/nH
= 1:3.
O
2
2
60 C and the reaction time of 7 h, the molar ration of
styrene to H2O2 was 1:2, 1:2.5, 1:3, 1:3.5 and 1:4, respec-
tively. The results showed that the conversion of styrene
increased with the increase of the ratio of hydrogen perox-
ide, and the smooth trend was observed when the ratio is
over 1:3. This may be due to when the amount of hydrogen
peroxide increases, the amount of H2O2 adsorbed in the
pores of catalyst also increases. There are sufficient active
oxygen sources in the oxidation process of styrene, thus
improving the conversion of styrene. In addition, when
the amount of hydrogen peroxide is lower than the chem-
ical reaction metering ratio, the hydrogen peroxide con-
tent is less, the concentration is lower, and the effect of
increasing the conversion of styrene is not very obvious.
When the amount of hydrogen peroxide is close to the
chemical reaction metering ratio, the conversion of styrene
increased obviously. When the amount of hydrogen per-
oxide continued to increase, due to the dilution of water
in 30% hydrogen peroxide, the increase of oxidant con-
centration in the reaction system was not obvious, so that
the conversion of styrene tended to change smoothly. Fur-
ther, it is noted from Figure 8 that when the molar ratio of
styrene to H2O2 was 1:3, the benzaldehyde selectivity and
benzaldehyde yield reached the maximum, respectively.
Continue to increase the styrene/H2O2 molar ratio, and
the reaction concentration would be diluted, resulting in
the decrease in catalytic activity and benzaldehyde yield.
In this case, more styrene oxide as the main byproduct
was produced, leading to the decrease of the selectivity of
benzaldehyde.
3.2.6. Effect of Different Catalysts
Table III listed the catalytic results obtained onto different
catalysts. According to the Table III, the 0.01-Ag-Co-
MCM-41 catalyst exhibits high catalytic activity as com-
pared with the 0.01-Co-MCM-41 and 0.01-Ag-MCM-41
catalysts. This is probably attributed to the co-introduction
of silver and cobalt into MCM-41 mesoporous molecular
sieves could assist the catalysis together.
3.2.7. Catalyst Reusability
In the experiment of catalytic oxidation of styrene to ben-
zaldehyde, the stability of the catalyst was studied. Under
IP: 95.181.217.151 On: Sun, 29 Dec 2019 09:10:32
the optimum reaction conditions, the catalyst used in the
Copyright: American Scientific Publishers
first time was recovered, and then the impurities were
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removed by washing anhydrous ethanol for many times.
Put in the oven dry, the resulting samples continue to
use, repeat five times and observe the catalytic effect. The
results summarized in Figure 9 show that the 0.01-Ag-Co-
MCM-41 could be successfully used five times without
significant loss of the activity. The slight decrease in selec-
tivity for recovered catalyst is probably due to residual
organics on catalyst impeding desorption of benzaldehyde
from the catalytic centers, leading to over oxidation to
benzoic acid.
60
50
60
50
40
30
20
10
0
conversion of styrene
selectivity of benzaldehyde
yield of benzaldehyde
40
conversion of styrene
yield of benzaldehyde
30
selectivity of benzaldehyde
20
10
0
1:2
1:2.5
1:3
1:3.5
1:4
molar ratio styrene to H2O2
1
2
3
4
5
Figure 8. Effect of molar ratio of styrene to H2O2 (reaction condition:
repeating time
ꢀ
Catalyst dose = 3 wt% of the reactants; temperature = 60 C; the loaded
amount of the metal = 0.01; time = 7 h).
Figure 9. Catalyst reusability.
J. Nanosci. Nanotechnol. 20, 1670–1677, 2020
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Catalytic Activity of Ag-Co-MCM-41 for Liquid-Phase Selective Oxidation of Styrene to Benzaldehyde
Jiang et al.
4. CONCLUSIONS
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In this paper, different proportions of Ag-Co-MCM-41
were prepared by direct synthesis. The catalytic oxida-
tion of styrene on Ag-Co-MCM-41 catalyst was studied
in details. The optimum reaction conditions are as fol-
lows: ꢀcatalyst dose is 3 wt% of the reactants, temperature
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is 60 C, time = 7 h, nstyrene/nH O is 1:3, and the loaded
amount of the metal is 0.01. A2s compared with the Ag-
MCM-41 and Co-MCM-41, the catalytic activity of Ag-
Co-MCM-41 catalyst in oxidation of styrene enhanced.
The Ag-Co-MCM-41 catalyst could be used five times
without significant loss, suggesting that the Ag-Co-MCM-
41 catalyst had good stability. Also, the Ag-Co-MCM-41
catalyst is a promising catalyst for the catalytic oxidation
reaction of styrene.
2
Acknowledgments: Financial support from Project
supported by the National Natural Science Foundation of
China (51572115) is gratefully acknowledged.
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Received: 6 November 2018. Accepted: 15 February 2019.
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1677