Immobilization of cobalt porphyrin on CeO2@SiO2 core-shell nanoparticles as a novel catalyst for selective oxidation of diphenylmethane

Article abstract of DOI:10.1016/j.molcata.2012.11.005

In this study, CeO₂@SiO₂ core-shell nanoparticles have been synthesized and used as supports to graft cobalt porphyrin via an amide bond. The catalyst was characterized using techniques such as FT-IR, UV-vis, SEM, TEM and BET. The re

Full text of DOI:10.1016/j.molcata.2012.11.005

Journal of Molecular Catalysis A: Chemical 367 (2013) 7–11
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Immobilization of cobalt porphyrin on CeO2@SiO2 core–shell nanoparticles as a
novel catalyst for selective oxidation of diphenylmethane
Xiang Guo, Yuan-Yuan Li, Dan-Hua Shen, Yuan-Yuan Song, Xiao Wang, Zhi-Gang Liu∗
School of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan 410082, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 August 2011
Received in revised form 30 May 2012
Accepted 1 November 2012
Available online 16 November 2012
In this study, CeO2@SiO2 core–shell nanoparticles have been synthesized and used as supports to graft
cobalt porphyrin via an amide bond. The catalyst was characterized using techniques such as FT-IR,
UV–vis, SEM, TEM and BET. The results show that the catalyst was composed of regular nanoparticles
(around 50 nm) with a core–shell structure. In addition, the catalyst exhibits an excellent activity, selec-
tivity and stability for solvent-free selective oxidation of diphenylmethane with atmospheric pressure of
oxygen. The conversion of diphenylmethane was as high as 41.6% with selectivity to diphenyl ketone of
96.3%. Even after reused up to 6 times, the catalyst maintained stable working ability.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Core–shell
Cobalt porphyrin
Ceria
Diphenylmethane
Selective oxidation
1. Introduction
reported to be negligible and cannot be used as promoter of cata-
lysts [12].
Supported metalloporphyrin catalysts have been used as
effective catalysts for a wide range of selective oxidations of hydro-
carbons, for the micro-environment of the support can bring higher
selectivity and prevent catalyst self-oxidation [1,2]. Therefore,
there has been a wide-spread interest in the synthesis of metal-
loporphyrin catalysts supported on inorganic, organic, or hybrid
materials, especially those with a core–shell structure [3–6].
Because of the advantages of core–shell structured nanocom-
posites resulting in non-aggregated nanoparticles with a controlled
surface and/or coating properties [7–10], the prospect of materi-
als with core–shell structure used as catalyst carriers has attracted
more attention. Huang et al. [11,12] employed magnetic polymer
nanospheres (with Fe3O4 core and polystyrene shell) as a support of
metalloporphyrin. It was found that these nanospheres have good
magnetic responsiveness and thus can be recycled by employing an
external magnetic field. Liu et al. [13] adopted silica-coated Fe3O4
nanoparticles as a carrier and synthesized a series of supported
metalloporphyrin catalysts with different saturation magnetiza-
tion. The influence of the saturation magnetization of magnetic
Fe3O4 nanoparticles on the catalytic abilities of supported metallo-
porphyrin was also investigated. Unfortunately, the effect of Fe3O4
embedded in the nanospheres on catalysis of the nanospheres was
CeO2 is a well-known promoter additive because of its oxygen
storage and releasing capacity [14,15]. Herein, CeO2 was expected
to be used as the core in the core–shell materials since CeO2 pos-
sesses catalytic activity and cooperates with metalloporphyrin in
redox reactions.
However, the surface precipitation of ceria on silica is inade-
quate when precipitant such as ammonium solution is used [16],
because the pH of the precipitate is 10, surface of silica is nega-
tively charged, and the pH of ceria is above 8. As a result, silica
could not coat the ceria as core due to the expected electrostatic
repulsion between both particles. To induce the attraction interac-
tion between the silica and ceria, Grasset et al. [17] moved the pH
of the silica–ceria suspension to pH 4, where the silica and ceria
were oppositely charged with respect to each other.
However, it could not look forward to lower the pH with
the addition of acid when ammonium solution was used as
precipitant during the synthesis of CeO2@SiO2. To achieve the elec-
trostatic attraction between the silica–ceria, the surface of the silica
particles was modified with amine group via the hydrolysis of 3-
aminopropyl-trieth-oxysilane [18].
As illustrated in Scheme 1, we used citrate sodium to mod-
ify the surface properties of ceria and synthesized the CeO2@SiO2
core–shell structured support. Cobalt porphyrin was then immobi-
lized on these synthesized CeO2@SiO2 structures for the first time.
The catalysts were characterized using FT-IR, UV–vis, SEM, TEM,
XRD and BET, and the catalytic performance for solvent-free oxi-
dation of diphenylmethane to diphenyl ketone was measured. The
∗
Corresponding author. Tel.: +86 731 88821314; fax: +86 731 88821667.
E-mail addresses: liuzhigang@hnu.edu.cn, cerialiu@gmail.com (Z.-G. Liu).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.11.005

8
X. Guo et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 7–11
2.3.2. Synthesis of surface modified CeO2
Surface modified CeO2 was synthesized according to literature
methods [20,21]. 5.996 g of Ce(NO3)3·6H2O was added into volu-
metric flask and prepared for 250 mL of solution with deionized
water. Under mechanical stirring, 250 mL 0.36 M of NaOH solution
was slowly dropped into Ce(NO3)3 solution until the solution pH
was 13. After stirring for 30 min, the precipitate was filtered and
washed with deionized water twice and then dispersed in 200 mL
0.3 M of citrate sodium solution. After stirring at 90 ◦C for 6 h, the
product was filtered, washed with ethanol and water, respectively,
and dried. The modified CeO2 was achieved.
Scheme 1. Illustration of the major steps for preparation of CoCPTPP grafted on
CeO2@SiO2 nanoparticles.
results show that the catalysts exhibit excellent catalytic activity,
selectivity and stability.
2.3.3. Synthesis of CeO2@SiO2 with core–shell structure
CeO2@SiO2 nanoparticles were prepared as described in the
literature [22,23]. Namely, 0.30 g of CeO2 was well dispersed in
the mixture of ethanol (160 mL), deionized water (40 mL) and
concentrated ammonia aqueous solution (5.0 mL, 28 wt.%), fol-
lowed by addition of tetraethyl orthosilicate (TEOS, 1 mL) and
3-aminopropyltriethoxysilane (APTES, 0.2 mL). After stirring for 6 h
at room temperature, the product was filtered and washed with
ethanol and water. The obtained CeO2@SiO2 core–shell nanoparti-
cles were denoted as CS, with the specific surface area 50.8 m2/g.
Nanoparticles without coating CeO2 were synthesized by the sim-
ilar method and denoted as NCS.
2. Experimental
2.1. Materials
5-(4-Carboxyphenyl)-10,15,20-triphenyl porphyrin (CPTPP)
was synthesized and purified in our laboratory. The metal oxides
used in this work were prepared with coprecipitation method
from corresponding metal nitrates. Water used in all experiments
was deionized and doubly distilled prior to use. 4-Carboxy benz-
aldehyde, benzaldehyde and pyrrole were redistilled before use.
All other reagents were obtained commercially and used without
further purification.
2.3.4. Synthesis of CoCPTPP anchored on CeO2@SiO2
According to literature [24], 0.05 g of CoCPTPP, 3.50 g of CS and
150 mL N,N-dimethylformamide (DMF) were loaded into a three-
neck flask, then heated to reflux for 12 h. After cooling, washing
with deionized water and filtering, dichloromethane was used to
extract the sample in a Soxhlet extractor till the extracted solution
was turned to clarify. The product was designated as CS-IM-CP. The
specific surface area of CS-IM-CP was 25.9 m2/g and CoCPTPP load-
ing in CS-IM-CP was 7.9 mg/g-cat. Following the similar procedure,
CoCPTPP was immobilized on NCS, and the product was denoted
as CS-IM-CP with CoCPTPP loading of 8.2 mg/g. The CoCPTPP load-
ing in CS-IM-CP was determined by UV–vis quantitative analysis
method.
2.2. Characterization of catalysts
UV–vis patterns were recorded on
a Shimadzu UV-2450
spectrophotometer using BaSO4 as reference with a range of
200–800 nm and a scan step of 0.5 nm. FT-IR spectra were measured
on an Agilent Perkin-Elmer 783 infrared spectrometer in KBr disc.
The specific surface areas of samples were determined by analyz-
ing the results of N2 adsorption at −196 ◦C in a Micromeritics ASAP
2020 apparatus and applying the Brunaumer–Emmett–Teller (BET)
method to the experimental values. All samples were dehydrated at
150 ◦C for 24 h prior to N2 adsorption. SEM (JSM 6700F) was used
to observe the particle sizes and shapes of samples. Prior to the
measurements, the samples were mounted on a carrier made from
glassy carbon and coated with a film of gold. TEM (Hitachi 800,
operated at 175 kV) images were obtained by dispersing samples
in ethanol using an ultrasonication bath and then depositing a drop
of liquid containing the particles onto a copper grid.
2.4. Measurement of catalytic performance
The catalytic performance of catalysts for solvent-free oxida-
tion of diphenylmethane was measured in a 100 mL four-neck flask
with a magnetic stirrer. In a typical experiment, 30 mL of diphenyl-
methane and 100 mg of catalyst were loaded into the glass reactor.
The reactor was sealed and bubbled with oxygen at atmospheric
pressure. The reactor was then heated to desired reaction tempera-
ture in an oil bath under stirring. Every hour, 0.5 mL of samples were
withdrawn with a syringe and analyzed by gas chromatography
with internal standard method using chlorobenzene as reference
substance. After the reaction, the reactor was cooled to room tem-
perature and the product was filtered through a millipore filter.
Then, the catalyst was recovered and dried at 80 ◦C for 12 h. The
regenerated catalyst was used for cycling studies.
2.3. Synthesis of catalysts
2.3.1. Synthesis of cobalt(II)
5-(4-carboxyphenyl)-10,15,20-triphenyl porphyrin
According to literature [19], 200 mL of propanoic acid,
0.0525 mol of benzaldehyde and 0.0175 mol of 4-carboxy benzalde-
hyde were loaded into a three-neck flask and heated to reflux under
stirring, then 0.07 mol of pyrrole was slowly dropped through
a funnel within 20 min. Under refluxing condition, the mixture
was stirred for 30 min. After cooling the reaction solution in a
refrigerator overnight the mixture was then filtered and purified
by column chromatography and 5-(4-carboxyphenyl)-10,15,20-
triphenyl porphyrin was obtained. 0.50 g of obtained porphyrin
was dissolved in 100 mL of N,N-dimethylformamide (DMF), and
2.54 g of CoCl2·6H2O was added under stirring, and then heated to
reflux until porphyrin detected by TLC was exhausted. After cool-
ing overnight, this mixture was filtered and washed repeatedly
with hot water, cobalt(II) 5-(4-carboxyphenyl)-10,15,20-triphenyl
porphyrin (CoCPTPP) was achieved.
3. Results and discussion
3.1. Characterization of catalysts
Solid-state diffuse-reflectance UV–vis spectroscopy was typi-
cally used to analyze metalloporphyrin species because metallo-
porphyrin species have characteristic peaks, called as Soret band
and Q-band, respectively [25]. In Fig. 1, the characteristic bands
of CS-IM-CP in the UV–vis spectra can be found at 428 nm and
541 nm. The support CS was measured by UV–vis spectra without
characteristic peaks. As for CS-IM-CP, the characteristic peaks of

X. Guo et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 7–11
9
CS-IM-CP
428 nm
Fresh CS-IM-CP
Used CS-IM-CP
CS
541 nm
CS
3450cm-1
1550cm-1
1660cm-1
2000
Wavenumber
cm-1
4000
3500
3000
2500
1500
1000
500
300
400
500
Wavelength(nm)
600
700
(
)
Fig. 1. UV–vis spectra of CS-IM-CP and CS.
Fig. 2. FT-IR spectra of catalysts (CS and CS-IM-CP).
cobalt porphyrin were expected to appear at 428 nm and 541 nm,
respectively. Because no peaks were seen in the CS spectrum, the
peaks at 428 nm and 541 nm must be the synthesized cobalt por-
phyrin supported on CS. In addition, CS-IM-CP was extracted with
dichloromethane in a Soxhlet extractor until the extracted solution
was turned to clarify. This indicates that no physical absorption
cobalt porphyrin existed in CS-IM-CP. Therefore, we can confirm
that that cobalt porphyrin has been immobilized via covalent bond
on the silica surface.
dehydrolysis reaction among the precursors with functional groups
of COOH and NH2. In Fig. 2 the FTIR spectra of CS-IM-CP is shown.
The absorption bands observed at 1040–1260 cm−1 were ascribed
to stretching vibrations of Si
tion bands which appeared at 1690–1630 cm−1 due to the C
group in amide (N O) demonstrate that the cobalt porphyrin
O Si and O H in silanols. The absorp-
O
C
was covalently bound to the silica support. In addition, both CS
and CS-IM-CP have the characteristic absorption of the N H group
observed near 1550 cm−1. However, It verified that CoCPTPP has
been immobilized in CS through an amide bond. The peak of
the N H group in CS-IM-CP, compared with CS, has a blue shift,
which was assumed to the influence of C O group in amide [26].
Many kinds of covalent bonds such as C
can be employed to anchor cobalt porphyrin on the surface
of supports [13,26,27]. In this study, N O was formed via
O C, C N and N C O
C
Fig. 3. SEM patterns of catalysts (a) CS and (b) CS-IM-CP, TEM patterns of catalysts (c) CS and (d) CS-IM-CP.

10
X. Guo et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 7–11
Table 1
Selective oxidation of diphenylmethane to diphenyl ketone.a
28.5
33.0
47.4
Catalyst
Time (h)
Conversion (%)
Diphenyl ketone selectivity (%)
Surface modified CeO2
CS-IM-CP
CeO2
CS
CS-IM-CP
NCS-IM-CP
24
24
24
24
17.0
15.6
41.6
37.3
97.0
97.8
96.3
96.1
a
Reaction condition: catalyst: 100 mg; diphenylmethane: 30 mL; flow of O2:
200 mL/min; temperature: 130 ◦C; pressure: 1 atm; reaction time: 24 h.
50
45
CS-IM-CP
CoCPTPP
CeO2
40
35
30
25
20
15
10
5
10
20
30
θ(degree)
40
50
2
Fig. 4. Wide-angle XRD patterns of CeO2, surface modified CeO2 and CS-IM-CP.
It indicates cobalt porphyrin was anchored on the surface of silica
via amide.
SEM micrographs of CS and CS-IM-CP are presented in Fig. 3(a)
and (b). In both cases the nanoparticles consisted of an arrangement
of relatively uniform particles with a particle size of around 50 nm.
This means the morphology of CeO2@SiO2 has changed little dur-
ing the immobilization of CoCPTPP. Fig. 3(c) and (d) displays TEM
micrographs of samples, respectively. The presence of CeO2@SiO2
core–shell nanoparticles is clearly shown and the CeO2 can easily
be seen at the center of the structure. However, the aggregation
of CeO2 nanoparticles was observed and seemed to be dispersed
beside CeO2@SiO2 core–shell nanoparticles. Typically, the concen-
tration of CeO2 is 25% in the CeO2@SiO2 core–shell nanoparticles or
even lower as reported in other studies [16,17]. Here, the concen-
tration of CeO2 in the CeO2@SiO2 core–shell nanoparticles is 53%,
much larger than 25%. The overloading may be the cause of the
appearance of bulk CeO2 nanoparticles.
In Fig. 4, it can be seen that the distinct diffraction peaks of CeO2
were observed at 28.5, 33.0 and 47.4, respectively [28]. This result
indicates that the CeO2 in CeO2@SiO2 composite particles is in cubic
flourite structure. No peaks of SiO2 were observed in the pattern
of CeO2@SiO2 composite particles, which may be due to the fact
that the as-prepared SiO2 particles exist as an amorphous phase at
this temperature [18]. In addition, no new phases were detected,
which demonstrate that CeO2 and SiO2 are independent phases in
the composite particles.
0
0
5
10
15
20
25
Time(h)
Fig. 5. Influence of reaction time for oxidation of diphenylmethane over CS-IM-CP
and CoCPTPP; reaction condition: catalyst: 100 mg; diphenylmethane: 30 mL; flow
of O2: 200 mL/min; temperature: 130 ◦C; pressure: 1 atm; reaction time: 24 h.
promoter, cooperating with metalloporphyrins and enhancing the
activity of CS-IM-CP.
The stability of CS-IM-CP for oxidation of diphenylmethane is
shown in Fig. 5. In Fig. 5, the conversion of diphenylmethane over
CoCPTPP is a little higher than that of CS-IM-CP in 24 h. This can be
explained by the fact that the active center can more easily interact
with the substrate in the monophase. Alternatively, when immobi-
lized on CeO2@SiO2, the stability of CoCPTPP is evidently improved
and has a high reusability, though the activity of CS-IM-CP may be
a little lower than that of CoCPTPP.
Fig. 6 demonstrates the reusability of CS-IM-CP for oxidation
of diphenylmethane. After the reaction CS-IM-CP was recycled by
simple filtration of the catalyst, the solid remnants with MeOH
98
Conversion of diphenylmethane
3.2. Solvent-free oxidation of diphenylmethane by dioxygen at
ambient pressure
Selectivity of diphenyl ketone
20
Diphenylmethanone is a very important feed stock used in fra-
grance, as an initiator and so on. Here we consider the reaction
of diphenylmethane to diphenylmethanone as the model reac-
tion. Molecular oxygen was used as an oxidant and the reaction
pressure was kept at ambient pressure. The catalytic performance
of the catalysts for oxidation of diphenylmethane is shown in
Table 1. Compared with CS-IM-CP and CoCPTPP, CeO2 and CS
have lower conversion (15.6% and 17.0%) with a similar selectivity.
When metalloporphyrin was immobilized on the surface of CS, the
diphenylmethane conversion over CS-IM-CP reaches 41.6%. This
result means that the catalytic activity of catalysts is remarkably
improved after the immobilization of CoCPTPP. In addition, the con-
version of NCS-IM-CP is lower than that of CS-IM-CP by 4.3%, which
agrees with our previous studies [24] showing that ceria can act as a
96
19
94
1
2
3
4
5
6
Recycle times
Fig. 6. Stability of CS-IM-CP for selective oxidation of cyclohexane. Reaction
condition: catalyst: 100 mg; diphenylmethane: 30 mL; flow of O2: 200 mL/min;
temperature: 130 ◦C; pressure: 1 atm; reaction time: 8 h.

X. Guo et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 7–11
11
were washed off and dried at 80 ◦C for 12 h. Diphenylmethane and
Acknowledgment
oxygen were then introduced to the catalytic material and the cat-
alyzed reaction restarted. In Fig. 6, it can be clearly observed that
CS-IM-CP, which was reused up to six times, only slightly decreases
and shows high stabilization. In fact, supported metalloporphyrin
deactivation is a very common occurrence in oxidation reactions
of alkane [12,13]. In these systems, the deactivation was believed
to be a result of decomplexation of the metalloporphyrin bound
to the silica support. The precursor to the decomplexation is a
␮-oxo-bridged dimer which is also inactive in diphenylmethane
catalysis.
Meanwhile, the UV–vis spectra of the used catalyst demonstrate
the characteristic peaks of cobalt porphyrin ring and is almost the
same as that of the fresh one. This indicates the structure of por-
phyrin ring still exists. Furthermore, no new peaks appear which
indicates no presence of new species. It also disagrees with the
opinion of the formation of a ␮-oxo-bridged dimer [27]. However,
the intensity of the peaks of the used catalyst was lowered than that
of the fresh one. It can be explained by some leaching or decom-
posing of the cobalt porphyrin grafted on the CS which happened
during the reaction. All in all, though the supported cobalt por-
phyrin may suffer from the leaching and decomposing during the
reaction, CS-IM-CP has a higher stability and reusability than their
encounter homogeneous cobalt porphyrin.
The authors gratefully acknowledge the financial support
from Hunan Province Natural Science Foundation of China (No.
10JJ4008).
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