Ti-phenyl nanoparticles encapsulated in mesoporous silica as active and selective catalyst for the oxidation of alkenes

Article abstract of DOI:10.1016/j.catcom.2013.12.012

Ti-phenyl nanoparticles encapsulated in mesoporous silica (Ti-phenyl@SiO₂) were synthesized and used as catalysts in the oxidation of styrene by aqueous H₂O₂. The Ti-phenyl@SiO₂(2) with Si/Ti mol ratio of two exhibited the best catalytic performance for the oxidation of styrene with 92% conversion and 99% selectivity towards benzaldehyde. This superior catalytic activity shown Ti-phenyl@SiO₂ catalyst was attributed to the presence of phenyl-group and SiO₂ on Ti and the ease in accessibility to the active sites by the porous SiO₂.

Full text of DOI:10.1016/j.catcom.2013.12.012

Catalysis Communications 46 (2014) 150–155
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ti–phenyl nanoparticles encapsulated in mesoporous silica as active and
selective catalyst for the oxidation of alkenes
Syamsi Aini a,b, Jon Efendi a,b, Hendrik Oktendy Lintang a, Sheela Chandren a, Hadi Nur a,
⁎
a
Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia UTM, 81310 Skudai, Johor, Malaysia
b
Department of Chemistry, Universitas Negeri Padang, Jln. Prof. Dr. Hamka, Air Tawar, Padang 25131, West Sumatera, Indonesia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 October 2013
Received in revised form 7 December 2013
Accepted 9 December 2013
Available online 18 December 2013
Ti–phenyl nanoparticles encapsulated in mesoporous silica (Ti–phenyl@SiO2) were synthesized and used as cat-
alysts in the oxidation of styrene by aqueous H2O2. The Ti–phenyl@SiO2(2) with Si/Ti mol ratio of two exhibited
the best catalytic performance for the oxidation of styrene with 92% conversion and 99% selectivity towards
benzaldehyde. This superior catalytic activity shown Ti–phenyl@SiO2 catalyst was attributed to the presence of
phenyl-group and SiO2 on Ti and the ease in accessibility to the active sites by the porous SiO2.
© 2013 Elsevier B.V. All rights reserved.
Keywords:
Ti–phenyl nanoparticles
Mesoporous silica
Oxidation of styrene
Aqueous H2O2
1. Introduction
oxidation of different alkenes, namely 1-octene, 1-dodecene and sty-
rene by aqueous H2O2 in order to prove the selectivity of the SiO2's
Metal nanoparticles have grown to be increasingly popular in catal-
ysis. The main advantage of metal nanoparticles in catalysis is their large
surface area and high efficiency under mild condition, when compared
with micro or macro catalysts [1,2]. It has been shown that the catalytic
activity and selectivity of metal nanoparticles are strongly dependent
upon their size, substrate and active metals sites [3,4].
pores. The catalytic activity and selectivity of Ti–phenyl@SiO2 catalysts
in the oxidation alkenes were compared to those of commercial TiO2.
2. Experimental
2.1. Preparation of Ti–phenyl and Ti–phenyl@SiO2
For catalytic applications, hydrophobic substrates can only be
strongly adsorbed on hydrophobic surfaces of the catalysts. The hydro-
phobic moiety on the metal can be improved by modification with or-
ganic moieties through covalent bonding to the active sites on metals
(M–C). These organic groups are also useful to prevent agglomeration
of the metal nanoparticles [5]. However, metals nanoparticles are less
stable in solutions [6]. To overcome this problem, encapsulation of
metal nanoparticles in oxides such as, silica (SiO2), alumina (Al2O3)
and zirconium oxide (ZrO2) has been carried out [7]. Silica is a good ad-
sorbent and catalyst support due to its hydrophilicity and the ease of ac-
cessibility by organic substrates through the pores of SiO2 [8]. Therefore,
the synthesis of Ti–phenyl encapsulated by mesoporous SiO2 shell could
be an ideal catalyst to oxidize alkenes [9]. Here, the activity of Ti–phenyl
nanoparticles encapsulated in mesoporous silica was evaluated in the
Ti–phenyl nanoparticles encapsulated in mesoporous SiO2 were
synthesized in three steps by following the original procedure reported
[10,13], with some minor modifications. Typically, aniline (50 mmol)
was dissolved in ice cold 50% fluoroboric acid (20 ml) and then the so-
lution was placed in an ice bath with stirring for 30 min. Then, a cold so-
lution of sodium nitride (50 mmol) was added to deionized water
(10 ml). The solution was then allowed to stir for 1 h. The prepared
phenyldiazonium fluoroborate was washed with cold ethanol. For the
second step, phenyldiazonium fluoroborate (5 mmol) was dispersed
in tetrahydrofurane (20 ml). After 1 h of vigorous stirring, 2.5 mmol
of TiCl4 was added directly into phenyldiazonium suspension. Typically,
the reduction of TiCl4 (5 mmol) and phenyldiazonium fluoroborate
(10 mmol) with sodiumborohydride (12 mmol) was done to produce
titanium–phenyl nanoparticles (Ti–phenyl), followed by coating of the
Ti–phenyl using 5 to 20 mmol of tetraethyl orthosilicate (TEOS Fluka).
The Ti–phenyl@SiO2 formed has Si/Ti mol ratio of 1 to 4, and labeled
as Ti–phenyl@SiO2(x), where x corresponds to the Si/Ti mol ratio. The
amount of Ti was constant for all of the samples.
⁎
Corresponding author. Tel.: +60 7 5536162; fax: +60 7 5536080.
E-mail address: hadi@kimia.fs.utm.my (H. Nur).
URL: http://www.hadinur.com (H. Nur).
1566-7367/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2013.12.012

S. Aini et al. / Catalysis Communications 46 (2014) 150–155
151
2.2. Characterization of catalysts
shows the 13C NMR spectra of aniline, phenyldiazonium and Ti–phenyl.
From Fig. 1a, it can be seen that five phenyl carbons of aniline can be
clearly identified at 146.42, 129.13, 118.60 and 114.74 ppm. The four
phenyl carbon peaks of phenyldiazonium also appeared at 155.44,
130.03, 120.70 and 115.44 ppm (Fig. 1b). Peak broadening and chemi-
cal shifting of carbon most adjacent to nitrogen can be seen at chemical
shift of 146.42 to 155.44 ppm in the 13C NMR of aniline (Fig. 1a) and
phenyldiazonium (Fig. 1b). This might be due to the effect of substituent
N`N bonding to phenyl. For the 13C NMR spectrum of Ti–phenyl nano-
particles shown in Fig. 1c, three types of phenyl carbon can be identified
at chemical shift of 162.33, 161.17 and 103.74 ppm. Chemical shifts and
broadening occurred for all the carbon of Ti–phenyl, which is caused by
heteronuclear interaction of the bonding between Ti and phenyl. This is
the results of fast spin–spin relaxation of carbon close to a metal core
[11].
The structure of Ti–phenyl was confirmed by means of 13C and 1H nu-
clear magnetic resonance (NMR) measurements. Liquid 13C and 1H NMR
experiments were carried using Bruker Avance 300 MHz spectrometer.
The 13C of Ti–phenyl and 1H of Ti–phenyl spectra were analyzed with
numbers of scan of 512 and 32, respectively. The lattice fringes, core
particles size, and the existence of the SiO2 shell of Ti–phenyl and Ti–
phenyl@SiO2 were characterized using a Philips CM200/FEG high resolu-
tion transmission electron microscope (HRTEM), with acceleration
voltage of 200 kV. The pore size distribution and the surface areas of
Ti–phenyl@SiO2 formed were determined by nitrogen (N2) sorption
analysis utilizing the BET method with a Quantachrome instrument.
2.3. Catalytic testing
Fig. 2 shows the 1H NMR spectra of aniline, phenyldiazonium tetra-
fluoroborate, and Ti–phenyl. In the Fig. 2a, the existence of five phenyl
protons can be proven by the peaks at 6.8 (3H) and 7.2 (2H) ppm, and
one of amine proton is at 6.8 ppm huddle with the phenyl proton
peaks. As shown in Fig. 2b, for phenyldiazonium tetrafluoroborate, the
five phenyl protons peaks have been broadening and shifting due to
the N`N functional group lying closer to titanium nanoparticles. Five
of these peaks can be identified at 6.9, 7.3 7.9, 8.3 and 8.6 ppm; the
same case for 1H NMR spectra of Ti–phenyl. The fact that there are no
sharp peaks identified in the range 6.8–8.6 for 1H NMR spectra indicates
that there is a bond between phenyl with titanium nanoparticles, and the
absence of any excessive free phenyl. The additional peak at 3.7 and
1.7 ppm was identified for existence of THF solvent residuals. Based on
the 13C NMR, and 1H NMR spectra results, it can be concluded that Ti–
phenyl were successfully synthesized, with no excessive diazonium [12].
The existence of titanium metal nanoparticles on the resulting
Ti–phenyl and Ti–phenyl@SiO2 is clearly shown in the TEM image in
The catalytic activity of the Ti–phenyl@SiO2 being made was carried
out in the oxidation of styrene with 30% H2O2 as an oxidant. The reac-
tions were carried out with mixtures containing styrene (5 mmol),
H2O2 (8 mmol), 50 mg of catalysts and acetonitrile (3.61 ml) in a
spherical flask attached to a condenser at 80 °C with stirring for 24 h.
The products from the reactions were separated by centrifugation and
analyzed by gas chromatography (GC) Shidmazu GC-2014 equipped
with BPX-5 column, with length of 30 m, inner diameter of 0.25 mm,
film thicknesses of 0.25 μm and a flame ionization detector (FID).
3. Results and discussion
3.1. Characteristics of Ti–phenyl and Ti–phenyl@SiO2
The formation of phenyldiazonium and Ti–phenyl bonding were
characterized by 13C and 1H nuclear magnetic resonance (NMR). Fig. 1
114.74
129.32
118.60
146.42
a
b
130.03
115.4
120.70
155.44
162.33
161.171
c
103.74
100
180
160
140
120
80
60
40
20
Chemical shift / ppm
Fig. 1. 13C NMR spectra of (a) aniline, (b) phenyl diazonium and (c) Ti–phenyl.

152
S. Aini et al. / Catalysis Communications 46 (2014) 150–155
a
5
3
7
6
4
9
8
2
1
7.9
8.6
8.3
6.9
7.3
b
9
8
7
6
5
4
3
2
1
c
10
9
8
7
6
5
4
3
2
1
0
Chemical shift / ppm
Fig. 2. 1H NMR spectra of (a) aniline, (b) phenyldiazonium tetrafluoroborate and (c) Ti–phenyl.
Fig. 3. According to the TEM image and enlarged image of the square
area on the right, all of the Ti in Ti–phenyl and in Ti–phenyl@SiO2 is in
metal form with lattice spacing of 2.2 Å for Ti (011) and lattice spacing
of 1.8 Å for Ti (012) [13]. From these images it is also observed that the
size and thickness of the SiO2 shell is directly proportionate to the
amount of TEOS used. The SiO2 shell for Ti–phenyl@SiO2(4) seemed
denser than that of Ti–phenyl@SiO2(1). The amount of Ti in Ti–phe-
nyl@SiO2 calculated by XRF were ca. 15–21% (w/w%) (see Table 1).
In order to examine the pores of the SiO2 shell in the samples, the Ti–
phenyl@SiO2 samples, nitrogen (N2) sorption analysis was utilized. As
shown in Fig. 4, N2 isotherm Ti–phenyl@SiO2 sample gave a type IV
isotherm, characteristics for mesoporous particles [14]. The BET surface
area are 99, 103, 229 and 250 m2 g−1 for Ti–phenyl@SiO2(1), Ti–
phenyl@SiO2(2), Ti–phenyl@SiO2(3), and Ti–phenyl@SiO2(4), respec-
tively, while the average pore width calculated from the desorption
branch (inset) are 69, 61, 100 and 102 Å, respectively. The porosity
and surface area of the Ti–phenyl@SiO2 increased with increasing mol
ratio of Si/Ti. This shows that the SiO2 shell of the Ti–phenyl@SiO2 is po-
rous and contributes significantly to the surface area of the catalysts.
3.2. Catalytic oxidation of alkenes
The catalytic results of the oxidation of styrene using (a) TiO2, (b) Ti–
phenyl, (c) Ti–phenylSiO2(1), (d) Ti–phenyl@SiO2(2), (e) Ti–phenyl@
SiO2(3) and (f) Ti–phenyl@SiO2(4) as catalysts are summarized in
Table 1. As can be seen from the table, Ti–phenyl and Ti–phenyl@SiO2
catalysts were found to show the highest conversion and selectivity as
compared to TiO2.
Table 1 and Fig. 5 show the conversion and product selectivity of sty-
rene oxidation with 30% H2O2 during the first run and also the second
run using different catalysts. As shown in Table 1 and Fig. 5, the lowest
conversion was shown by TiO2 while Ti–phenyl@SiO2 has the highest
conversion for styrene oxidation. The Ti–phenyl@SiO2 showed high re-
usability of only ca. 10% decrease in conversion. The highest conversion
of styrene (92%) were shown by Ti–phenyl@SiO2(2), while the lowest
conversion of styrene (44%) for Ti–phenyl@SiO2 samples were obtained
when Ti–phenyl@SiO2(4) was used, albeit still much higher than those
shown by TiO2 (1%). GC analysis indicated that benzaldehyde is the
majority product in the oxidation of styrene by all of the catalysts.

S. Aini et al. / Catalysis Communications 46 (2014) 150–155
153
a
b
c
d
e
Fig. 3. TEM images of (a) Ti–phenyl, (b) Ti–phenyl@SiO2(1), (c) Ti–phenyl@SiO2(2), (d) Ti–phenyl@SiO2(3) and (e) Ti–phenyl@SiO2(4).

154
S. Aini et al. / Catalysis Communications 46 (2014) 150–155
Table 1
Catalytic performances of TiO2 and Ti–phenyl@SiO2 catalysts in the oxidation of styrene by aqueous hydrogen peroxide.
Catalyst
Ti w/w%a
First run
Second run (reusability testing)
Conversion/%
Selectivity of benzaldehyde/%
TONb
Conversion/%
Selectivity of benzaldehyde/%
TONb
TiO2
Ti–phenyl
Ti–phenyl@SiO2(1)
Ti–phenyl@SiO2(2)
Ti–phenyl@SiO2(3)
Ti–phenyl@SiO2(4)
–
2.3
62.6
46.1
92.2
50.4
44.3
1.0
96.8
94.7
98.8
97.5
42.8
0.1
2.9
6.6
18.9
11.8
13.2
–
–
–
61.4
31.8
22.2
19.5
15.3
–
–
–
41.9
79.7
48.2
39.5
99.2
99.5
99.0
98.8
5.9
16.3
11.2
11.7
a
Weight/wt.% of Ti in the sample calculated by XRF.
TON = the amount of total product (mmol)/the amount of active site (mmol), where the amount of active site in Ti (mmol) was obtained from XRF data calculation.
b
2000
1500
1000
(d)
500
0
100
100
150
150
200
200
1000
500
0
(c)
500
0
(b)
(a)
400
300
200
100
0
100
100
150
150
200
200
500
0
Pore diameter / Å
d
0
0.2
0.4
0.6
0.8
1.0
400
300
200
100
0
c
0
0.2
0.2
0.4
0.4
0.6
0.6
0.8
0.8
1.0
100
b
0
1.0
100
a
0
1.0
0
0.2
0.4
0.6
0.8
Relative pressure / P/P0
Fig. 4. Nitrogen adsorption–desorption isotherm, and pore size distribution calculated by BJH method (inset) of (a) Ti–phenyl@SiO2(1), (b) Ti–phenyl@SiO2(2), (c) Ti–phenyl@SiO2(3) and
(d) Ti–phenyl@SiO2(4).

S. Aini et al. / Catalysis Communications 46 (2014) 150–155
155
100
80
thin or none existence, the adsorption of the reactants may be reduced.
The optimum Si/Ti mol ratio of this catalyst in the oxidation of styrene
with H2O2 determined was two.
Styrene oxide
Phenylacetaldehyde
4. Conclusions
Benzaldehyde
60
Ti–phenyl nanoparticles encapsulated in mesoporous SiO2 shell
(Ti–phenyl@SiO2) catalysts with different amounts of SiO2 were pre-
pared and used as catalysts in the oxidation of styrene with H2O2 as
the oxidant. The porosity and surface area of SiO2 shell increased with
increasing Si/Ti mol ratio in Ti–phenyl@SiO2. The Ti–phenyl showed
high catalytic activity for styrene oxidation, but without reusability.
On the other hand, the Ti–phenyl@SiO2 catalysts are highly reusable
and displayed very high catalytic activity for styrene oxidation, as the
phenyl group in Ti–phenyl@SiO2 allows strong interaction between
the catalysts and styrene, which also possesses a benzene ring in its
structure. The optimum Si/Ti mol ratio determined was two, showing
highest turnover number, highest conversion of styrene and selectivity
towards benzaldehyde.
40
20
0
(a)
(b) (b*)
(c) (c*)
(d) (d*)
(e) (e*)
Catalyst
Fig. 5. The conversion and product selectivity of styrene oxidation with 30% H2O2 at 80 °C,
by using (a) TiO2, (b) Ti–phenyl@SiO2(1), (c) Ti–phenyl@SiO2(2), (d) Ti–phenyl@SiO2(3),
and (e) Ti–phenyl@SiO2(4) as catalysts. The bars labeled as (b*), (c*), (d*) and (e*) were
the conversion and product selectivity of the same catalysts during the second run.
Acknowledgments
The authors would like to acknowledge the following parties for fi-
nancial supports: the Ministry of Science, Technology and Innovation
Malaysia (MOSTI) through Science Fund, Ministry of Higher Education
Malaysia (MOHE) through Fundamental Research Grant Scheme and
Universiti Teknologi Malaysia through Research University Grant.
The highest selectivity (99%) towards benzaldehyde was shown by
Ti–phenyl@SiO2(2), with a relatively high turnover number (TON)
(19), while the lowest selectivity of benzaldehyde (95%) by Ti–
phenyl@SiO2(1), which was still very high. Other minor products
detected were styrene oxide and phenylacetaldehyde. Another note-
worthy point is that, as mentioned earlier and shown in Fig. 4, the sur-
face area of the catalysts increased with increasing amount of SiO2 in
the sample. However, the catalytic activity results suggest that the cat-
alytic activity was independent of the surface area of the catalysts.
The high catalytic activity shown by Ti–phenyl@SiO2 catalysts in the
oxidation of styrene can be explained on the basis of the similarity in
structure of the phenyl group present at Ti–phenyl@SiO2 catalysts
with the structure of styrene. The phenyl groups in the Ti–phenyl@
SiO2 catalysts might have enhanced the adsorption of styrene onto the
catalysts and thus enhancing the catalytic activity. The relationships be-
tween catalytic activity and Si/Ti mol ratio in the oxidation of styrene
are shown in Fig. 5. In order for the catalyst to be really efficient in
this reaction, the appropriate thickness of SiO2 synthesized with differ-
ent Si/Ti mol ratio is needed. If the SiO2 is too thick, it will hinder the dif-
fusion of the products from the catalyst. However, if the SiO2 layer is too
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