Hydrophobic Modification of Microenvironment of Highly Dispersed Co3O4 Nanoparticles for the Catalytic Selective Oxidation of Ethylbenzene

Article abstract of DOI:10.1002/cctc.201901771

Microenvironments in enzymes play crucial roles in controlling the activities of active centers. Here, hexagonal mesoporous silicas (HMS) with hydrophobic microenvironment was used to mimic the enzymes on the hydrocarbon oxidation with cobalt oxides incor

Full text of DOI:10.1002/cctc.201901771

Accepted Article
Title: Hydrophobic Modification of Microenvironment of Highly
Dispersed Co3O4 Nanoparticles for the Catalytic Selective
Oxidation of Ethylbenzene
Authors: Li Zhao, Song Shi, Meng Liu, Chen Chen, Guozhi Zhu, Jin
Gao, and Jie Xu
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ChemCatChem
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Hydrophobic Modification of Microenvironment of Highly
Dispersed Co Nanoparticles for the Catalytic Selective
3
O
4
Oxidation of Ethylbenzene
Li Zhao,[a,b] Song Shi,* Meng Liu,[a,b] Chen Chen,[a,c] Guozhi Zhu,[a,b] Jin Gao, and Jie Xu*[a]
[a]
[a]
Abstract: Microenvironments in enzymes play crucial roles in
controlling the activities of active centers. Here, hexagonal
mesoporous silicas (HMS) with hydrophobic microenvironment was
used to mimic the enzymes on the hydrocarbon oxidation with cobalt
oxides incorporated as active sites. The variation of the structure and
surface properties of the materials with phenyl group incorporation
and the state of the supported cobalt oxides were carefully studied. It
or products.[5] With this inspiration, the microenvironment aspects
[6]
such as distal hydrogen bond donors, hydrophobic binding
cavities,[7] and confined environments have been researched in
[8]
[
9]
small molecule activation,[10]
the asymmetric synthesis,
[
11]
hydrocarbon cracking, etc. While, during the selective oxidation
of hydrocarbons, water is an unavoidable product. The
competitive adsorption caused by the polar difference between
weak polar hydrocarbons and strong polar products (including
3 4
showed that cobalt oxides presented as Co O distributed evenly
among the support, and the catalytic microenvironment was modified
by the neighboring phenyl groups to be hydrophobic. This novel
catalyst showed enhancement in the adsorption of ethylbenzene and
in turn promoted the catalytic conversion than the hydrophilic one.
2
H O) could show obvious effect on the catalytic performance.
With this special characteristic, it would be possible to design high
efficient catalyst for the hydrocarbon oxidation by precisely
controlling the hydrophobic microenvironment.
5
5.1% conversion of ethylbenzene and 86.1% selectivity for
acetophenone could be obtained at 393 K for 6 h under 1.0 MPa of
with free solvent. Hydrophobic microenvironment of pore surface
In our previous works, cobalt-silicon nanocomposites
modified with oganic groups has shown good performance in the
hydrocarbon oxidation.[12] However, the state of cobalt was
ambiguous and the reason for the promotion effect was unclear.
Recently, mesoporous silicas have been applied to mimic the role
of the scaffold in enzymes, and with supported metal complex as
the active center.[4b] With this model catalyst, the effect of the
microenvironment and active center could be researched
seperately.
O
2
was considered as an important factor for the better catalytic activity.
Introduction
Catalytic selective oxidation of hydrocarbons to oxygen-
containing compounds such as ketone, alcohol, aldehyde and
acid is an important industrial process, which supplies the
essential fundamental chemicals to manufacture polyester, fibre,
paint, etc. for our daily life.[1] For its inert characteristic of C-H bond,
the activation and catalytic selective conversion of hydrocarbons
was considered as a challenging work in science.[2] To solve this
problem, learning from nature, i.e. the bio-mimic process is an
efficient way. Enzymes is a kind of natural catalyst [3] and a lot of
high efficient catalysts for hydrocarbon oxidation have been
Herein, in combination with our previous cobalt-silicon
nanocomposite works, hexagonal mesoporous silicas (HMS) was
used to mimic the enzymes’ backbone and the highly dispersed
3 4
Co O nanoparticles was applied as active center. With the HMS
skeleton modified with phenyl organic group, the
hydrophilicity/hydrophobicity microenvironment of the active
center could be changed. And with the increasement of the
hydrophobicity of the micromentvironment, the catalytic
performance on ethylbenzene oxidtion was promoted. Detailed
research proved that the pore hydrophobic microenvironment
enhanced the adsorption of ethylbenzene and in turn improved
the catalytic activity.
designed
through
mimicking
the
active
center
of
[
4]
metalloenzymes. Except for the active center, enzymes achieve
high catalytic activities and selectivities by creating favorable
microenvironments such as hydrophobic pockets around the
active centers, which could have interactions with the substrates
Results and Discussion
Characterization of materials
[
a]
L. Zhao, Dr. S. Shi, M. Liu, Dr. C. Chen, G. Zhu, Prof. Dr. J. Gao,
Prof. Dr. J. Xu
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics
Chinese Academy of Sciences
Dalian National Laboratory for Clean Energy
Dalian 116023 (P.R. China)
E-mail: shisong@dicp.ac.cn; xujie@dicp.ac.cn
L. Zhao, M. Liu, G. Zhu
University of Chinese Academy of Sciences
Beijing100049 (P.R. China)
The present address of C. Chen is School of Materials Science &
Chemical Engineering, Ningbo University, Ningbo, Zhejiang, China.
First, the small-angle XRD patterns of HMS and Ph-HMS both
exhibited a single diffraction peak corresponding to the (100)
plane at 2θ of 1-3 o, which was typical characteristic of HMS
materials with thick framework wall (Figure 1A). And this single
low-angle peak was considered to possess short-range
[
[
b]
c]
[13]
hexagonal symmetry with uniform mesoporous diameter.
Compared with the diffraction peak position in the pattern of HMS,
a shift to higher angle for Ph-HMS appeared, manifesting that a
lattice contraction occurred with the introduction of phenyl groups.
At the same time, the diffraction peak intensity also decreased,
manifesting that the ordered mesoporous structure was partly
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201901771
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destroyed for Ph-HMS. This situation usually happened during the
preparation of organic-inorganic hybrid silica with higher content
of organic groups.[12b, 14] These demonstrated that the phenyl
groups were successfully introduced into the materials.
Moreover, through
N
2
adsorption-desorption isotherm
measurements (Figure S1a, S1c, S2a, S2c and Table 1), HMS
exhibited a type IV isotherm with H1 hysteresis as defined by
IUPAC for mesoporous materials.[15] The isotherm exhibited the
sharp inflection characteristic of capillary condensation with the
uniform mesopores. However, with the introduction of phenyl
group, the relative pressure range of P/P
of Ph-HMs broaden and the two branches remained nearly
horizontal and parallel over a wide range of P/P , which exhibited
0
of the hysteresis loops
0
H4 type, indicating a decrease of mesoporosity caused by phenyl
loading. Wahab and co-workers observed a similar phenomenon
for hybrid periodic mesoporous organosilica materials.[16] Table 1
Figure 2. A) FT-IR spectra of (a) HMS and (b) Ph-HMS; B) FT-IR spectra of (a)
3 4
5Co/HMS, mCo/Ph-HMS (m= (b) 1.5, (c) 3, (d) 5 and (e) 7), (f) Co O .
lists the detailed structural parameters of the materials, it could
found that, HMS and Ph-HMS both had high surface area and
well-distributed pore size, and also big pore volume, which would
be good candidates for catalysis application. Further, with phenyl
modified Ph-HMS, the properties of pore diameter and pore
volume were decreased, these verified that the phenyl was
successfully introduced into the inner pore and the pore
environment was changed.
higher wave numbers. This may be caused by the partial
substitution of Si-O with Si-Ph, and the bending vibration of Si-O
was affected by the neighboring phenyl groups. Therefore, these
results can be considered as supplementary proof of the
successful immobilization of phenyl groups in Ph-HMS.
The presence of phenyl organic group was also detected by
Si MAS-NMR characterization (Figure 3). For the unmodified
Further, the FT-IR spectra of HMS and Ph-HMS are shown
in Figure 2A. The absorption bands at around 1100, 800, and 470
cm-1 were corresponding to Si-O-Si asymmetric stretching
vibrations, symmetric stretching vibrations and bending vibrations,
respectively. Typical bands associated with the formation of a
condensed siloxane network were present in both cases. For the
spectra of the Ph-HMS, typical bands associated with aromatic
2
9
HMS, obvious resonances characteristics of the silica network
n
4
[
[
Q =Si(OSi)
n
(OH)4-n, n = 2-4] were observed, among which Q
4
3
3
Q = Si(OSi)
4
] and Q [Q = Si(OSi)
3
(OH)] species were the main
components, showing characteristic peaks at around -110 and -
02 ppm, respectively. In addition to the Q4 and Q3 signals,
resonances characterizing of the organosiloxane network T3
1
-
1
C=C ring stretching at around 1437 and 1600 cm , mono-
substituted benzene out-of-plane bending vibration at around 698
and 740 cm-1 could be observed.[12a] These results further
confirmed the presence of phenyl groups in Ph-HMS. Moreover,
[
T3=PhSi(OSi)
3
] was observed for the Ph-HMS material at around
3
-
78 ppm. The appearance of T organosilane signal was the
phenyl immobilizations caused the monotonic blue shift of the
band at around 470 cm-1 assigned to Si-O bending vibration to
Table 1. The physical properties of the samples.
Sample
Co
Co
3
O
4
S
BET
2
Dpore
Vpore
(mL/g)
Water
[b]
[c]
[d]
content
content
(m /g)
(nm)
contact
[
a]
[a]
o [e]
(
wt%)
(wt%)
angle ( )
Co
HMS
Co/HMS
Ph-HMS
3
O
4
-
-
83
17.3
5.7
5.7
3.4
3.2
3.5
3.5
3.5
0.40
2.33
2.48
1.66
1.86
1.84
1.54
1.98
13
-
-
811
19
5
5.24
-
7.14
-
842
18
1399
1374
1318
1315
1312
148
147
147
138
138
1.5Co/Ph-HMS
3Co/Ph-HMS
5Co/Ph-HMS
7Co/Ph-HMS
1.52
3.07
5.27
7.30
2.07
4.18
7.18
9.94
[
a] Co content determined by ICP-OES. [b] Surface area calculated from nitrogen
Figure 1. A) Small-angle XRD patterns of (a) HMS and (b) Ph-HMS; B) Small-
angle XRD patterns of (a) 5Co/HMS, mCo/Ph-HMS (m= (b) 1.5, (c) 3, (d) 5 and
adsorption isotherms at 77 K using BET equation. [c] Average pore width calculated
from nitrogen adsorption isotherms at 77 K using DFT methods. [d] Total pore volume
(
e) 7); C) XRD patterns of (a) 5Co/HMS, mCo/Ph-HMS (m= (b) 1.5, (c) 3, (d) 5
calculated from nitrogen adsorption isotherms at 77 K at P/P
by sessile water contact angle measurements.
0
= 0.99. [e] Determined
and (e) 7), (f) Co
O .
3 4
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Figure 3. 29Si MAS NMR of (a) HMS and (b) Ph-HMS.
Figure 4. A) The adsorption isotherms of water on (a) HMS and (b) Ph-HMS;
B) The adsorption isotherms of ethylbenzene on (a) HMS and (b) Ph-HMS.
characteristic of fully crosslinked organosiloxane species,[17]
demonstrating the successful introduction of the phenyl organic
groups, which is crucial for increasing the hydrophobic property of
the material surface to change the pore environment. Really,
sessile water contact angles were measured and the results
showed that HMS had a low water contact angle with 19 , which
was apparently hydrophilic. While Ph-HMS had a much higher
water contact angle with 148 o with hydrophobic property (Figure
S3), indicating the phenyl modification can change the
hydrophilic/hydrophobic property.
The adsorption isotherms of water and ethylbenzene on HMS
and Ph-HMS were measured using the IGA at 298 K, respectively
of ethylbenzene on HMS was higher than Ph-HMS because of the
high pore volume of HMS. Furthermore, compared with the curves
of the adsorption amount of ethylbenzene on HMS and Ph-HMS
at different time (Figure S5), the adsorption rate of ethylbenzene
on Ph-HMS was higher than HMS, which suggested that
ethylbenzene was more easily and more quickly to be adsorbed
on Ph-HMS. In conclusion, HMS was easier to adsorb water than
Ph-HMS, while the Ph-HMS was easier to adsorb ethylbenzene
relative to HMS, which verified the hydrophobic and lipophilic
property of Ph-HMS. As a consequence, Ph-HMS could be used
as a support to investigate the effect of the hydrophobic pore
environment for the catalytic reactions and might have a higher
catalytic activity.
More than above, HMS and Ph-HMS introduced with cobalt
oxides were firstly tested by small angle XRD (Figure 1B). The
patterns showed that 5Co/HMS and the different Co content of
Co/Ph-HMS all owned a kind of short-range symmetry of
mesoporous structure with the diffraction peaks at 2θ of 1-3 o,
suggesting the ordered mesoporous structure of the materials
were maintained after the introduction of cobalt oxides. Inevitably,
with the increasing of Co content, partial destroy of the
mesoporous structure appeared and caused the diffraction peak
of Co/Ph-HMS in the small-angle region slightly decreased. Yet,
o
(
(
Figure 4). For the isotherms of water on HMS and Ph-HMS
Figure 4A), which could both be characterized as type III,
exhibiting gradually increasing with the pressures.[15a, 18] While the
phenyl modified Ph-HMS showed a much lower adsorption
amount at the low pressure and at high pressure mostly exhibited
physical condensation. In addition, HMS always exhibited higher
adsorption amounts than Ph-HMS over the range of test pressure,
which indicates higher water adsorption capacity. The results
showed that water sorption capacity on HMS is 2.35 times to that
of Ph-HMS. The reason would be, on the one hand, HMS had a
bigger pore volume (Table 1), 2.33 mL/g for HMS and 1.66 mL/g
for Ph-HMS, which is 1.40 times as large as Ph-HMS; on the other
hand, HMS was hydrophilic with much stronger interaction with
water relative to Ph-HMS (Figure S3). Combining with the curves
of the adsorption amount of water on HMS and Ph-HMS at
different time (Figure S4), we can see that the adsorption amount
of water on HMS was higher than that of Ph-HMS over the range
of time, deducing the adsorption rate of water on HMS was higher
than that of Ph-HMS. As a result, water was much easier
adsorbed on HMS than Ph-HMS. Meanwhile, the corresponding
isotherms of ethylbenzene on HMS and Ph-HMS were also
characterized (Figure 4B). The adsorption amounts of both were
increased with the pressure. However, the adsorption amount of
ethylbenzene on Ph-HMS was increased faster than on HMS at
the low pressure and reached approximately to pseudo-
equilibrium quickly, while the adsorption amount of ethylbenzene
on HMS was increased steadily. Finally, the adsorption amounts
2
through N adsorption-desorption measurement results (Table 1),
it could found that, these materials still remained a high specific
surface area and big pore size after the cobalt oxides species
being immobilized, almost the same with the corresponding
supports HMS and Ph-HMS, which were advantageous factors for
good catalytic performance.
Through TG-DSC analysis of the as-synthesized 5Co/Ph-
HMS (Figure S6), it could be acknowledged that after the weight
loss of crystal water with a small amount of adsorbed water and
residual ethylene glycol before 530 K, the decomposition
3 2
temperature of Co(NO ) being supported on Ph-HMS gradually
occurred at about 573 K and the dissociation of Si-Ph was above
12a, 19]
6
23 K.[
Here, the calcinations temperature was set as 603 K
to cobalt oxides and
avoid the cleavage of Si-Ph at the same time. Cobalt contents
were determined by ICP-OES, and the results with Co
loadings are listed in Table 1. The real cobalt contents in different
3 2
to assure the full decomposition of Co(NO )
3
O
4
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samples were almost the same with the theoretical calculation in
the initial precursor.
(a)
(a’)
The crystalline phase of the cobalt oxides was determined by
the wide angle XRD measurements (Figure 1C). In the patterns
of all the samples, typical diffraction peak of amorphous SiO
2
o
emerged at 2 θ of 22 . In addition, with the increasing of the cobalt
o
o
o
content, the diffraction peaks located at around 37 , 45 , 59
and 65 o gradually appeared, corresponding to (311), (400), (511)
and (440) diffractions of Co (ICDD-JCPDS: 00-042-1467),
revealing the formation of compound probably as Co form.
3
O
4
20 nm
3
O
4
(
b)
(b’)
(c’)
(d’)
(e’)
While weak diffraction intensity and broad diffraction peaks
suggested that cobalt oxides owned small nanoparticles and
presented as a high dispersion state on the materials.[20]
The TEM images showed that Co/HMS and Co/Ph-HMS
possessed the typical wormhole structure of HMS material
assembled from long alkyl chain neutral amine as templates
(
Figure 5). From the low resolution of the TEM graphs, cobalt
nanoparticles could not be seen clearly, which might be existed
with too small size and the weak contrast difference between the
2
0 nm
support SiO
2
and Co
3
O
4
nanoparticles, consistent with the results
(c)
of weak and broad diffraction peaks of XRD. While from the HR-
TEM micrographs, we can see the randomly oriented lattice
fringes, including (222), (311) and (400), further verified the cobalt
3 4
oxides presented as Co O . And the particle size was about 2-3
nm, which was easily to locate in the pore of the silicas.
The coordination geometries of cobalt species supported on
HMS and Ph-HMS were carefully studied by UV-Vis DRS method
(
Figure 6). A broad absorption band around 700 nm was attributed
20 nm
to Co (Ⅱ) in tetrahedral environments and the absorption band
emerging at 420 nm was attributed to Co (Ⅲ), which were similar
(d)
3 4
O
(Figure 6f).[20a, 21] These results
with the spectrum of Co
indicated that the cobalt oxides supported on HMS or Ph-HMS
presented as Co phase. This was consistent with the
3
O
4
characterization results of wide angle XRD and TEM. At the same
time, it can be deduced that the presence of phenyl groups did
not change the crystal phase of cobalt oxides because that the
coordination geometries of cobalt species were same for Co/HMS
and Co/Ph-HMS. Moreover, two intense bands in the UV region
centered at 212 and 262 nm exhibited characteristic vibration
structure for all the samples with Ph-HMS as support (Figures 7b-
e), which were assigned to π-π* transitions of benzene ring.[12b] It
can be taken as a strong evidence for the presence of phenyl
groups in these materials.
20 nm
(
e)
The FT-IR spectra of the samples are shown in Figure 2B.
For Co/HMS and Co/Ph-HMS, like the supports HMS and Ph-
HMS, typical bands associated with formation of a condensed
siloxane network were present in all cases (Si-O-Si bands around
20 nm
-1
1100, 800, and 470 cm ). For the spectra of all the Co/Ph-HMS,
(f)
(f’)
typical bands associated with aromatic C=C ring stretching (weak
-1
peaks around 1437 and 1600 cm ), mono-substituted benzene
-1
bending vibration (around 698 and 740 cm ) could be observed.
These results further confirmed the presence of phenyl groups in
Co/Ph-HMS. The introduction of hydrophobic phenyl groups on
the surface of these Co/Ph- HMS materials might change their
pore environment of surface hydrophilicity/hydrophobicity.
Actually, through the sessile water contact angle measurement
20 nm
(
Table 1), it could be observed that phenyl-containing materials
o
all owned a high water contact angel between 138-147 , which
Figure 5. TEM images of (a, a’) 5Co/HMS, mCo/Ph-HMS (m= (b, b’) 1.5, (c, c’)
, (d, d’) 5 and (e, e’) 7) and (f, f’) 5Co/Ph-HMS after reaction.
3
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Table 2. Catalytic oxidation of ethylbenzene over the catalysts[a]
.
Products distribution (%)
Conv.
%)
TOF
(h )
Catalysts
-
1 [b]
(
2
3
4
5
6
Co
3
O
4
21.6
40.8
47.8
49.2
55.1
54.4
56.8
80.9
82.4
76.9
86.1
77.7
28.2
15.1
11.6
15.2
7.0
1.7
0.5
0.9
1.0
0.6
0.5
3.4
3.3
4.7
6.6
5.9
4.8
9.9
0.2
0.4
0.3
0.4
4.6
14
5
1
3
5
7
Co/HMS
376
1519
774
.5Co/Ph-HMS
Co/Ph-HMS
Co/Ph-HMS
Co/Ph-HMS
505
12.4
360
[
2
a] Reaction conditions: 10 mL of ethylbenzene, 50 mg of catalyst, 1.0 MPa of O ,
6
Co
h, 393 K. [b] TOF numbers were calculated based on the total molar quantity of
Figure 6. UV-Vis DRS of (a) 5Co/HMS, mCo/Ph-HMS (m= (b) 1.5, (c) 3, (d) 5
and (e) 7), and (f) Co O .
3 4
[22]
3
O
4
.
was far bigger than that of Co/HMS (18 o) (Figure S3),
demonstrating the materials changed from hydrophilic to
hydrophobic with the phenyl group introduced. Otherwise, the
increasing of cobalt content did not change the surface
hydrophobicity apparently. In the distribution test, it could be
observed that 5Co/HMS showed hydrophilicity and it distributed
in the water phase, while 5Co/Ph-HMS distributed in the
organicphase of ethylbenzene for its hydrophobicity (Figure S7).
In addition, in the FT-IR spectra of Co/Ph-HMS, the absorption
bands at around 580 and 660 cm-1 were similar with that of Co-O
hydrophobic catalysts of mCo/Ph-HMS, 5Co/Ph-HMS showed a
best catalytic activity. The conversion of ethylbenzene reached
55.1%, which were higher than that with the hydrophilic catalyst
of 5Co/HMS. The pore environment of surface hydrophobicity
might be the main reason for this catalytic activity enhancement.
It can be explained: for the catalytic selective oxidation of
ethylbenzene, water is the inevitable by-product accompanying
the formation of the oxygenated products, and the polarity is very
different for ethylbenzene and water molecules, with the
[23]
molecular dipole moment of 0.59 and 1.85 D, respectively. With
vibration of Co
presented as Co
characterization results.
3
O
4
, which further indicated that the cobalt oxides
, consistent with UV-Vis DRS and XRD
the dispersion test (Figure S7), the weak polar of ethylbenzene
and strong polar of water had quite different adsorption effect on
the catalysts. On the hydrophobic microenvironment modified
catalyst, the weak polar ethylbenzene molecule was more
inclined to be adsorbed, while the strong polar molecule such as
water molecule was more inclined to be adsorbed on the
hydrophilic catalyst. In addition, with the IGA measurements in
Figure 4 and the adsorption amount of ethylbenzene and water
versus time in Figures S4 and S5, we can deduce that
ethylbenzene was easier and faster adsorbed on the sample with
hydrophobic microenvironment, reflected by the higher adsorption
amount of ethylbenzene on Ph-HMS at the same time, while water
was easier and faster adsorbed on the sample with hydrophilicity
3
O
4
Catalytic performances
Selective oxidation of ethylbenzene was chosen as a model
reaction to study the improvement of supported cobalt oxides
catalysts and the pore environment effect of surface
hydrophilicity/hydrophobicity on the catalytic performance. The
oxidation products were mainly including acetophenone (2), 1-
phenethyl alcohol (3), benzaldehyde (4), benzoic acid (5) and
phenylethylhydroperoxide (6). Through the temperature test on
(
Figure S4, S5). As a result, for hydrophilic sample who preferred
5
Co/Ph-HMS catalyst, 393 K was selected as the best reaction
temperature (Figure S8). The catalytic reactions were all operated
at 393 K with O as oxidant in the absence of any solvent and
additives, and the results are listed in Table 2. Pure Co was
to adsorb water than hydrophobic one, the active center was
occupied by water. While for the hydrophobic microenvironment
catalyst, ethylbenzene was easier and faster to be adsorbed
which lead to the inhibition of water to occupy the active site than
on the hydrophilic sample. Thus the hydrophobic
microenvironment modified catalyst promoted the adsorption of
ethylbenzene and in turn improved the catalytic activity.
Compared with the previous reported catalytic system for the
solvent-free oxidation of ethylbenzene under oxygen in the
references (Table S1),[
2
3
O
4
taken as a referenced catalyst, which gave a 21.6% conversion
of ethylbenzene under the present conditions. Compared with the
pure Co O , the supported hydrophilic catalyst of 5Co/HMS
3 4
showed a higher catalytic activity with 40.8% conversion of
ethylbenzene. This result manifested that the catalytic active
species supported on a porous material could show a better
catalytic performance. Among the different supported
20a,
24]
most of which belong to the
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hydrophilic catalyst, the present hydrophobic microenvironment
modified 5Co/Ph-HMS catalyst showed predominance.
the reaction time increased, the two catalysts both had the ability
to accumulate high concentration of ethylbenzene, the conversion
of ethylbenzene would both increase smoothly. This was
consistent with the IGA measurement results of the adsorption
amount of ethylbenzene on Ph-HMS and HMS at different time
(Figure S5). And the adsorption rate of ethylbenzene on Ph-HMS
was higher than that of HMS at the initial period, which was
reflected by the higher adsorption amount at the same time. Thus
it was quickly to reach adsorption pseudo -equilibrium on Ph-HMS,
which could promote phenyl modified catalyst have higher
concentration of ethylbenzene at the initial period and then have
higher conversion increasing from 1 h to 2 h in the initial reaction
period. We also got a suitable reaction time of 6 h to make a
sufficient conversion of ethylbenzene.
The Co content of the solution was only 3.5 ppm after
reaction, suggesting trace Co was lost in the reaction. In addition,
the conversion of ethylbenzene with homogeneous catalyst with
similar Co content of cobalt acetate was only 9.2 % under the
same condition,[20a] which indicated that cobalt aqueous made
3 4
little contribution to the reaction and supported Co O was the
actual active composition. Besides, the FT-IR spectrum and UV-
Vis DRS of 5Co/Ph-HMS after the reaction were shown in Figure
S9 and Figure S10. From the FT-IR spectrum we can see the
mono-substituted benzene out-of-plane bending vibration peaks
at around 698 and 740 cm , suggesting the phenyl group was
remained during the reaction. At the same time, UV-Vis DRS of
-1
5Co/Ph-HMS after the reaction showed two intense bands
exhibited in the UV region centered at 212 and 262 nm, which
were assigned to π-π* transitions of benzene ring. It can be taken
as a strong evidence for the presence of phenyl groups in the
catalyst, which was consistent with the result of FT-IR. In addition,
TEM characterization showed that cobalt oxide after the reaction
Conclusions
Hexagonal
mesoporous
silicas
with
hydrophobic
microenvironment to mimic the enzymes was realized to achieve
high efficiency catalyst for the liquid phase oxidation of
ethylbenzene with oxygen. Carefully characterizations revealed
that phenyl groups were successfully introduced into the
frameworks of HMS through co-condensation method and had
made a difference on the structure properties compared with non-
modification material, providing a hydrophobic pore environment
around the cobalt oxide nanoparticles, of which dispersed evenly
3 4
was still highly dispersed and kept the Co O structure (Figure 5(f,
f’)), in agreement with the result of UV-Vis DRS, of which a broad
absorption band around 700 nm was attributed to Co (Ⅱ) in
tetrahedral environments and the absorption band emerging at
420 nm was attributed to Co (Ⅲ). All these indicated the active
centers were relatively stable during the reaction process.
The effect of reaction time on the conversion of ethylbenzene
is shown in Figure 7. At the first 1 h, the conversion of
ethylbenzene was both very low, 7.8% and 11.9% with 5Co/Ph-
HMS and 5Co/HMS, respectively. With time going on, the
conversion of ethylbenzene increased uniformly and then
remained stable over 5Co/HMS, while with 5Co/Ph-HMS, the
conversion of ethylbenzene increased steeply at the initial 1-2 h,
from 7.8% to 34.5% and then increased linearly till to reach steady.
Compared with non-phenyl modified 5Co/HMS catalyst, the initial
reaction of ethylbenzene was relatively faster with phenyl
modified 5Co/Ph-HMS catalyst. The reason may be that phenyl
modified catalyst had hydrophobic and lipophilic properties, it can
adsorb and accumulate substrate ethylbenzene, which is a weak
polar molecular, more quickly than non-phenyl catalyst, thus had
a higher concentration of substrate and a faster conversion. As
3 4
on the support presented as Co O . As a result, phenyl
hydrophobic modification made the catalyst showed
enhancement in the adsorption of ethylbenzene and in turn
promoted the catalytic conversion than non-phenyl modification
one. This research makes us believe that carefully tune the
hydrophobicity of catalytic microenvironment will open a new
avenue for heterogeneous catalysis.
Experimental Section
Material preparation
Tetraethyl orthosilicate (TEOS, 98%), cobalt nitrate (Co(NO
9%), and ethylene glycol ((CH OH) , 99%) were obtained from Tianjin
Kermel Chemical Reagent Development Center, China.
3 2 2
) ·6H O,
9
2
2
Phenyltriethoxysilane (PTES, 98%) and hexadecylamine (HDA, 90%)
were purchased from Aladdin, China. Deionized water used in all
experiments was obtained from a Milli-Q system (Millipore). Other
reagents were used as received.
Phenyl modified hexagonal mesoporous silicas (Ph-HMS) was
synthesized based on the previous report by a modified method.[13]
A
typical material was synthesized from the gels with the following total molar
composition: 0.85 TEOS: 0.15 PTES: 0.27 HDA: 10.78 C OH: 60.89
O. The template was removed from the as-synthesized material by
NH NO /ethanol extraction. A pure siliceous HMS was synthesized as
2 5
H
H
2
4
3
above method without PTES.
Co/Ph-HMS and Co/HMS were prepared by quantitative impregnation
method with Ph-HMS and HMS as support and Co(NO
precursor in ethanol solution. Typically, 0.3 g support was pre-treated with
0.5 g ethylene glycol for 1 h at room temperature and then filtrated, dried
at oven under 120 oC. Next, appropriate amount of Co(NO
·6H O was
added into 5 mL ethanol solution and stirred homogeneously, then the
3 2 2
) ·6H O as metal
2
Figure 7. Influence of reaction time on catalytic performances at 393 K over (a)
5Co/ HMS and (b) 5Co/Ph-HMS.
3
)
2
2
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ChemCatChem
10.1002/cctc.201901771
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support was added and stirred overnight under 40 oC. The solution was
Keywords: hydrophobic microenvironment • adsorption •
catalytic oxidation • Co • hexagonal mesoporous silicas
o
evaported and then dried under 80 C. The as-synthesized catalysts were
calcined at 603 K for 10 min in muffle furnace. The obtained catalysts were
denoted as mCo/Ph-HMS and mCo/HMS (m is the initial mass fraction of
3
O
4
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
Hexagonal mesoporous silicas with
Li Zhao, Song Shi,* Meng Liu, Chen
Chen, Guozhi Zhu, Jin Gao, and Jie
Xu*
hydrophobic
modified by phenyl group was used to
mimic the enzymes with Co O
4
microenvironment
3
incorporated as active sites, the pore
hydrophobic environment had an
enhancement in the adsorption of
ethylbenzene and in turn improved
the catalytic selective oxidation
activity than the hydrophilic one.
Page No. – Page No.
Hydrophobic Modification of
Microenvironment of Highly
Dispersed Co O Nanoparticles for
3 4
the Catalytic Selective Oxidation of
Ethylbenzene
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