Highly active Ga promoted Co-HMS-X catalyst towards styrene epoxidation reaction using molecular O2

Article abstract of DOI:10.1016/j.apcata.2014.05.024

Atom efficient synthesis of various value added products has been focused as an intense research area after Kyoto protocol. Styrene epoxidation is a challenging reaction as styrene epoxide is an important monomer for large number of polymers. It has been found that surface acidity and redox property of the catalyst has a major contribution to the catalytic activity for oxidation reaction. In this study Co-HMS-X catalyst was prepared and characterized by BET surface area and porosity measurement, SAXS, SANS, FESEM, HRTEM, UV-vis spectra, FTIR, ²⁹Si NMR, H₂-TPR and NH₃-TPD techniques. It is observed that 5 mol% Co-HMS-X showed highest catalytic activity for styrene epoxidation reaction using molecular O₂ as an oxidant. Interestingly it is found that among three different dopant (Al, Ga, Tl), the Ga promoted Co-HMS-X catalyst (Si:Co:Ga = 100:5:1.25) showed highest catalytic activity in terms of styrene conversion (100%) and styrene epoxide selectivity (68%). The unusual trend of Al, Ga and Tl towards the activity of Co-HMS-X (5 mol%) catalyst has been discussed from the results obtained in the H ₂-TPR and NH₃-TPD study.

Full text of DOI:10.1016/j.apcata.2014.05.024

Applied Catalysis A: General 482 (2014) 61–68
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Highly active Ga promoted Co-HMS-X catalyst towards styrene
epoxidation reaction using molecular O2
Sumbul Rahmana, Chiranjit Santraa, Rawesh Kumara, Jitendra Bahadurb, Asima Sultanac,
Ralf Schweinsd, Debasis Senb, Sudip Maitye, S. Mazumdarb, Biswajit Chowdhurya,∗
a Department of Applied Chemistry, Indian School of Mines, Dhanbad, India
b Solid State Physics Division, Bhabha Atomic Research Center (BARC), Mumbai, India
c National Institute of Advanced Industrial Science & Technology (AIST), Tsukuba, Japan
d Institut Laue-Langevin, Grenoble, France
e Central Institute of Mining and Fuel Research, Dhanbad, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Atom efficient synthesis of various value added products has been focused as an intense research area
after Kyoto protocol. Styrene epoxidation is a challenging reaction as styrene epoxide is an important
monomer for large number of polymers. It has been found that surface acidity and redox property of the
catalyst has a major contribution to the catalytic activity for oxidation reaction. In this study Co-HMS-X
catalyst was prepared and characterized by BET surface area and porosity measurement, SAXS, SANS,
FESEM, HRTEM, UV–vis spectra, FTIR, 29Si NMR, H2-TPR and NH3-TPD techniques. It is observed that
5 mol% Co-HMS-X showed highest catalytic activity for styrene epoxidation reaction using molecular O2
as an oxidant. Interestingly it is found that among three different dopant (Al, Ga, Tl), the Ga promoted Co-
HMS-X catalyst (Si:Co:Ga = 100:5:1.25) showed highest catalytic activity in terms of styrene conversion
(100%) and styrene epoxide selectivity (68%). The unusual trend of Al, Ga and Tl towards the activity of
Co-HMS-X (5 mol%) catalyst has been discussed from the results obtained in the H2-TPR and NH3-TPD
study.
Received 20 December 2013
Received in revised form 13 May 2014
Accepted 21 May 2014
Available online 29 May 2014
Keywords:
Mesoporous silica
Cobalt
Molecular oxygen
Epoxidation
Promoter
Gallium
© 2014 Published by Elsevier B.V.
1. Introduction
complexes is venerable in biological system as well as organic syn-
thesis. Uses of molecular oxygen as an oxidant with cobalt based
In the 21st century, sustainable chemistry has been playing a key
role in organic synthesis [1]. The atom efficient route for chemical
reaction could motivate the chemist for replacing stoichiometric
reactions using simple catalytic reactions. Among several reaction
selective oxidations mainly epoxidation reactions deserve more
attention as epoxides are important starting material for several
value added products [2]. The direct epoxidation of ethylene using
molecular oxygen over silver catalyst has opened a new horizon
for the development of catalyst system which can demonstrate
direct epoxidation of higher carbon number, e.g. propylene [3,4],
1-3 butadiene [5], cyclohexene, styrene as revealed in the literature
[6,7].
catalysts are most desirable due to environment and economi-
cal consideration. Development of catalyst for styrene epoxidation
using molecular oxygen as oxidant was started by cobalt salen
complexes with need of sacrificial co-reductant such as isobu-
tyraldehyde [9]. Though several heterogeneous cobalt catalysts
have been reported for alkene epoxidation [9,10], however devel-
oping simple cobalt based catalyst without using co-reductant
might be an interesting study from green chemistry perspective.
Till the discovery of MCM family mesoporous silica by Mobil
researcher several types of mesoporous silica, e.g. SBA-15, TUD-1,
KIT-6, FDU-16 have been developed [2,11] which could incorporate
transition metal ions in its matrix demonstrating high activity for
oxidation reaction [12,13]. Among several factor the control of
acidic sites and redox property of the doped silica remained as a key
factor for determining their catalytic activity [14]. It is often found
that use of promoter has a significant contribution to improve the
catalytic activity in the epoxidation of olefins. For the ethylene
epoxidation calcium carbonate was used as a promoter for silver
catalyst [15] whereas barium nitrate promoter was responsible
Cobalt ions and their complexes are well known conventional
homogeneous catalysts for the selective oxidation of alkenes using
TBHP and O2 as oxidizing agents [8]. Interaction of oxygen by cobalt
∗
Corresponding author. Tel.: +91 326 2235663; fax: +91 326 2296563.
E-mail address: biswajit chem2003@yahoo.com (B. Chowdhury).
http://dx.doi.org/10.1016/j.apcata.2014.05.024
0926-860X/© 2014 Published by Elsevier B.V.

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S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
for improving the catalytic activity of titanosilicate supported
gold nanoparticle catalyst towards propylene epoxidation reaction
[3,16]. The reason for the role of promoters varies in different cases
but catalyst systems are much simpler as found in the system.
The newly developed two dimensional mesoporous silica HMS-
X material has an advantage over conventional mesoporous silica
because of having ordered hexagonal porous structure which can
effectively help reactants to diffuse on the catalyst surface [17].
In the present study the three different loadings of Co-HMS-X
(Si/Co = 20, 10 and 7) catalyst were prepared by sol–gel method and
catalytic activities have been tested for styrene epoxidation reac-
tion. It is found that fine tuning of acid–base and redox properties
of ordered cobalt containing mesoporous silica catalyst might be
helpful to get better activity towards direct epoxidation of styrene.
Al, Ga and Tl (0.25 mol%) are used as promoters in cobalt con-
taining mesoporous silica (Co-HMS-X 5 mol%) catalyst for styrene
epoxidation reaction using molecular O2. Ga promoted Co-HMS-X
catalyst (Si:Co:Ga = 100:5:1.25) demonstrated 100% styrene con-
version with 68% styrene oxide selectivity in the optimum reaction
condition. The catalysts were characterized by several techniques,
i.e. BET surface area and porosity measurements, SAXS, SANS,
FESEM, HRTEM, UV–vis, FT-IR, 29Si-NMR, H2-TPR and NH3-TPD
techniques in order to elucidate the reason for high catalytic activ-
ity. The unusual trend of Al, Ga, and Tl towards the surface acidity of
Co-HMS-X (5 mol%) catalyst resulting in different catalytic activity
has been explained.
size distributions were calculated using NLDFT (Non linear Density
Functional Theory) model of cylindrical pore approximation.
2.2.2. Small angle X-ray scattering (SAXS)
Small-angle X-ray scattering (SAXS) measurements were per-
formed using a laboratory based SAXS instrument with Cu K␣ X-ray
source. Variation of scattering intensity with wave vector transfer
[q = 4␲ sin(ꢀ)/ꢁ] was measured for the powder samples.
2.2.3. Small angle neutron scattering (SANS)
Small-angle neutron scattering (SANS) experiments have been
carried out at ILL, Grenoble, France using D11 instrument.
2.2.4. Field emission scanning electron microscope (FESEM)
Field emission scanning electron microscope (FESEM) was car-
ried out by using Supra 55, Carl (Zeiss, Germany) microscope.
Sample was supported on lacey carbon and then coated with plat-
inum by plasma prior to measurement.
2.2.5. High resolution transmission electron microscope (HRTEM)
The HRTEM investigation was done on JEOL JEM 2100 micro-
scope operated at 200 kV acceleration voltage using lacey carbon
coated Cu grid of 300-mesh size.
2.2.6. Ultraviolet–visible spectroscope (UV–vis)
DRUV-Visible measurement was carried out by using Var-
ian Cary 500 (Shimadzu) spectrophotometer. The spectra were
recorded in the range of 190–500 nm wavelength.
2. Experimental
2.2.7. Fourier transformation infrared spectroscopy (FT-IR)
The FT-IR measurements were carried out by using Perkin Elmer
GX spectrophotometer. The spectra were recorded in the range of
400–4000 cm−1 using KBr pellet.
2.1. Materials and catalyst preparation
Hexagonally ordered mesoporous Co-HMS-X was synthesized
by using triblock copolymer as a template under acidic con-
dition [17]. In a typical synthesis of 2D ordered mesoporous
Co-HMS-X catalyst, Pluronic P123 (Aldrich) (4 g) was dissolved
in 144 mL distilled water and 7.4 g 35% HCl (Merck) by contin-
uous stirring. After stirring for 4 h a clear solution was obtained
and then 4 g n-butanol (Merck) was added to this solution. The
stirring was continued for 1 h. Tetraethyl orthosilicate (8.4 g)
(TEOS, Acros) and aqueous solution of cobalt nitrate (0.81 g)
(Aldrich) was added to this solution. The resulting gel composition
of the mixture is P123:H2O:HCl:n-butanol:TEOS:Co(NO3)2·6H2O
equal to 0.017:200:5.4:1.325:1:0.05–0.15 (molar ratio) and it
was stirred for 24 h at room temperature. To prepare the
metal nitrate promoted Co-HMS-X the gel composition is fixed
as P123:H2O:n-butanol:TEOS:Co(NO3)2·6H2O:M(NO3)2 equal to
0.017:200:5.4:1.325:1:0.05–0.15:0.0025 (molar ratio). Besides Al,
Ga and Tl nitrate salts are also used for catalyst preparation. After
stirring the mixture was taken in a closed polypropylene bottle and
aged at 100 ◦C temperature for 24 h under static hydrothermal con-
dition. After hydrothermal treatment the material was filtered in
hot condition without washing and then dried at 100 ◦C for 12 h in
air. Finally drying the material was calcined at 540 ◦C for 24 h. The
material obtained after calcinations was labelled as Co-HMS-X and
M-Co-HMS-X (M = Al, Ga, Tl).
2.2.8. Magic-angle spinning nuclear magnetic resonance (MAS
NMR)
29Si MAS NMR was recorded at 500 MHz on Bruker advanced II-
500 spectrometer equipped with a magic angle spin probe at room
temperature.
2.2.9. Temperature programmed reduction (TPR)
TPR profiles of the samples are recorded with ChemiSorb 2720
(Micrometrics, USA) equipped with a TCD detector. The TPR profiles
are obtained by reducing the catalyst samples by a gas mixture of
10% H2 in Ar with a flow rate of 20 mL/min while the temperature is
increased from ambient to 700 ◦C at a rate of 10 ◦C/min. Hydrogen
consumptions in the profiles were evaluated by peak area of CuO
TPR calibrations.
2.2.10. Temperature programmed desorption (TPD)
The acidity of the samples was measured by ammonia
temperature-programmed desorption (NH3-TPD) technique. A
quadrapole mass spectrometer (BEL JAPAN) is used to detect des-
orbed NH3, ca. 100 mg of the sample was outgassed at 500 ◦C for 1 h
in a helium flow followed by ammonia adsorption at 100 ◦C for 1 h.
Subsequently, the sample was flushed with helium for 30 min at
100 ◦C to remove physically adsorbed ammonia. Ammonia desorp-
tion was carried out by raising the temperature to 700 ◦C with a
heating rate of 10 ◦C/min.
2.2. Catalyst characterization
2.2.1. BET surface area and porosity measurement
2.3. Styrene epoxidation reaction
The nitrogen adsorption–desorption isotherm of the samples
were measured at liquid nitrogen temperature with a Quanta-
chrome Nova-3200e at −196 ◦C. Pre-treatment of the samples were
carried out at 300 ◦C for 3 h under high vacuum. The surface area
was determined by Brunauer–Emmett–Teller (BET) equation. Pore
Styrene epoxidation was carried out following the procedure
reported in the literature [9]. In a typical procedure 10 mmol
styrene (Acros), 10 mL DMF (Merck) and 0.2 mL internal standard
(dodecane) were taken in a two neck 50 mL round bottle flask fitted

S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
63
Fig. 1. Surface area and pore size of the catalyst.
with water condenser. Subsequently, 0.2 g catalyst was added to the
3.2. Structural characterization of the catalyst by small angle
X-ray scattering (SAXS) and small angle neutron scattering (SANS)
reaction mixture followed by an appropriate oxygen flow (Aalborg
mass flow controller) under constant stirring at 540 rpm using Tar-
son spinot digital magnetic stirrer. The reaction is carried for 4 h at
100 ◦C. The product analysis was done using a gas chromatograph
(CIC-India) through SE-30 column.
The SAXS patterns of various catalysts are shown in Fig. 2. A
highly long range ordered mesoporous silica structure is evident
from SAXS studies. Each sample exhibits an intense diffraction peak
corresponding to the (1 0) plane at 0.64 nm−1, typical characteristic
of HMS material [19–21]. The intensity of the reflection correspond-
ing to the (1 0), (1 1) and (2 0) planes decreases with the increase
in cobalt loading as compared with the undoped siliceous HMS-X.
This decrease in peak intensity upon higher Co loading compared
to HMS-X analogue confirms change of the pore diameter in the
mesoporous silica at higher cobalt loading. While comparing the
results of Al, Ga and Tl, the ordered structure of HMS-X diminished
3. Results and discussion
3.1. Textural characterization of the catalyst by BET surface area
and porosity measurement
The BET surface area and porosity measurement results are
depicted in Fig. 1. It is found that all materials has typical type IV
adsorption/desorption isotherms with H1 hysteresis loop, which
is characteristic of typical mesoporous materials. The sharp steps
occurred at a relative partial pressure of 0.6–0.8, corresponding to
the capillary condensation of N2, which indicates the uniformity of
the pores [2]. Pore structure parameters of HMS-X and Co-HMS-
X calculated from the desorption branch are shown in Table S1
(supporting information). Compared with pure silica HMS-X, the
introduction of cobalt made the surface area and pore volume sig-
nificantly decrease. For Co-HMS-X, with increase of cobalt content,
both the surface area and pore volume are gradually reduced. On
increasing Co loading from 5 to 15 mol%, surface area of catalyst
decreases from 674 to 621 m2/g, this may be due to the sub-
nanometer level cobalt species in the pores of silica as found in
our recent study [17]. Interestingly it is observed that doping of Al,
Ga and Tl has remarkably changed the surface area and pore size
distribution. Doping of Tl destroyed the mesoporosity of Co-HMS-
X material as revealed from adsorption isotherm and the surface
area was quite low among the material (Table S1). A fine tuning
of Co:Ga ratio could result in uniform pore size distribution, larger
pore size and pore volume. The nature of interaction between PEO
based templates and alkoxides controls the folding of templating
agent which forms the ordered mesophase during synthesis [18].
May be higher basicity of Tl compared to others are responsible for
collapse of mesophase in the present case.
Fig. 2. The SAXS profiles for the catalyst.

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S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
Fig. 3. (a) The SANS profiles for the catalyst. (b) The contrast match Co-HMS-X (5 mol%) and (c) the evolutions of scattering profiles for the contrast match Co-HMS-X (5 mol%)
catalyst at 130 ◦C. “CM”, contrast match; detector distance: 5 m.
after incorporation of Tl strengthening the results of BET surface
area and porosity measurements.
high dispersion of cobalt oxide species on the silica surface (Fig.
S5, Supporting information). The EDS analyses (Fig. S6, Supporting
information) confirm the presence of cobalt, silica and oxygen in
Co-HMS-X catalyst.
Small angle neutron scattering experiment was found as a newly
developed tool for characterizing the ordered mesoporous silica
[22]. To know the accessibility of the pores in the HMS-X cata-
lyst SANS characterization was carried out. Diffraction intensity
analysis of small-angle scattering measurements of dry catalysts
confirms the ordered mesopore structure. Fig. 3(a) shows the scat-
3.4. Chemical characterization of the catalyst by UV–vis
spectroscopy and FTIR spectroscopy
−1
˚
tering curve over the 0.005–0.5 A
range due to the coherent
The UV–vis spectra of Co-HMS-X catalysts show two absorp-
tion peaks 529 nm and 590 nm (Fig. S7, Supporting information)
scattering from the hexagonal array of primary mesopores results
in the (1 0), (1 1)−a1nd (2 0) planes. Bragg diffraction peaks are 0.06,
4
which can be unambiguously assigned to the 4A2(F) → T1(P) tran-
˚
0.104 and 0.13 A respectively for the HMS-X and Co-HMS-X cat-
sition of Co(II) ions in tetrahedral environments [26]. The blue
colour of the catalyst is also a fingerprint of tetrahedral Co(II)O4
sites. No such peak is observed in case of undoped HMS-X. The
absorbance in the range of 500–700 nm is characteristic of Co(II)
in an isolated state, the cobalt species in Co-HMS-X mainly exists
in the state of single-site Co(II). However at higher loading there
is appearance of absorption band around 250 nm region which
is due to charge transfer between O2− to Co2+ species [27]. The
absence of clear peaks in the 500–700 nm region in Co-HMS-X
(15 mol%) is observed on increasing the loading. This may be due
to clustering of highly dispersed Co species in the silica matrix. The
black spots in the HRTEM image and decrease in porosity for Co-
HMS-X (15 mol%) (Fig. S4) strengthen the observation of UV–vis
spectra.
alysts [23,24]. The intensities of the higher order (1 1) and (2 0)
diffraction peaks show that the material has highly ordered struc-
ture. Contrast matching-small angle neutron scattering (CM-SANS)
measurements have been carried out to probe the accessibility of
the microspores using mixture of H2O and D2O. Typical survey
scans are shown in Fig. 3(b) and (c). Here, diffraction intensity
decreased below detection limit for the (1 0) peak at the con-
trast matching point and at the contrast match point, diffraction
was eliminated completely, which implies that any micropores are
larger than the water molecules are completely accessible [22,25].
Thus, all the pores in the present probed length scale are open in
nature. Therefore, it can be said that agglomerated cobalt species
are highly dispersed keeping the pores accessible to the reactant
molecule.
The FTIR spectra of HMS-X and three differently loaded Co-
HMS-X material are shown in Fig. S8 (Supporting information). The
peaks at 1085 cm−1 and 1195 cm−1 are attributed to the asymmet-
3.3. Morphological study of the catalyst by FESEM and HRTEM
characterization
ric stretching vibration of framework Si
at 817 cm−1 is attributed to Si
Si symmetric stretching. The
gradual decrease of peak intensity at 817 cm−1 might be signifying
the successive formation of Si
Co moiety. The peak at 960 cm−1
has been interpreted for stretching vibration of Si OH and Si
O Si bridges. The peak
O
From the FESEM image it is observed that the catalyst has worm-
like morphology (Figs. S2 and S3, Supporting information) even at
higher cobalt loading. Interestingly the morphology of HMS-X com-
pletely changed while Tl was doped in the Co-HMS-X which was
unchanged in presence of dopant, e.g. Al and Ga in this present case.
The HRTEM images (Fig. 4) of Co-HMS-X (Co/Si = 5/100) exhibits
highly ordered hexagonal array of uniform channels. The ordered
structure was not destroyed even at higher cobalt loading (Fig.
S4, Supporting information). The XRD analysis could not detect
any cobalt oxide species even at higher loading emphasizing the
O
O
␦-groups. As found in literature the peak ascribed in the region
960 cm−1 can be considered as an evidence of the isomorphous
substitution of Si by Co metal ions, i.e. as Si
O Co connectivity
[28]. This peak is also widely used to characterize the presence of
transition metal atoms near the silica framework as the stretching
Si O vibration mode perturbed by the neighbouring metal ions.
At higher loading this peak ramify, probably due to more diffusion

S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
65
Fig. 4. HRTEM images of 5 mol% Co-HMS-X catalyst.
of cobalt oxide nanoparticles into the pores of silica matrix [17].
The bands appeared around 3500 cm−1 and 1633 cm−1 can be
attributed to O H and H H respectively. As such there is no
change in FTIR pattern for metal (Al, Ga and Tl) doped Co-HMS-X
catalyst which is quite rational.
3.6. Redox and acid–base characterization
O
The occurrence of multiple reduction peaks in TPR (Tempera-
ture Programmed Reduction) profile (Fig. 6) indicates the presence
of a number of reducible cobalt species. The reduction of supported
cobalt oxide was generally divided into two stages, a lower temper-
ature region located between 254 and 487 ◦C could be assigned to
the two step reduction of Co3O4 (Co3O4 → CoO → Co). The reduc-
tion behaviour of the high temperature region implies the degree of
the interaction between the cobalt oxide and the support, forming
3.5. Characterization by 29Si NMR spectroscopy
The 29Si NMR spectra of Co-HMS-X depicted in Fig. 5, shows
three different signals at −91, −101 and −110 ppm, similar to that
obtained for doped mesoporous silica as reported in literature [17].
The first at around ı = −110 ppm can be assigned to (–O–)4Si with
no OH group attached to the silicon atom (Q4). The second peak
at ı = −101 ppm is assigned to (–O–)3Si(OH) with one OH group
(Q3). The last peak at ı = −91 ppm, is assigned to (–O–)2Si (OH)2
with two OH groups (Q2). A close look on the 29Si NMR of different
cobalt loaded HMS-X reveals the decrease of Q3 peak intensity at
higher loading. It confirms that most of the cobalt species are not in
the silica network. This kind of trend is also reported in literature
[29].
Si
O Co species which are difficult to be reduced [30]. The inset
image in Fig. 6 shows that total hydrogen consumption is increased
after addition of small amount of promoter (i.e. Al, Ga and Tl) in
Co-HMS-X material. The promoter enhanced the reducibility of the
catalysts which might be due to shift of reduction temperature at
lower value compared with un-promoted Co-HMS-X catalyst.
NH3-temperature programmed desorption (NH3-TPD) was used
to measure the total acidity and relative acid strength of the
catalysts. Two-stage desorption of NH3 over the promoted and
unpromoted Co-HMS-X catalyst is observed from the TPD profiles
as shown in Fig. 7. The first maxima in the temperature range of
200–340 ◦C can be assigned to weak chemisorptions of ammonia
Fig. 5. 29Si NMR spectra of the catalyst: (a) Co-HMS-X (5 mol%), (b) Co-HMS-X
(10 mol%) and (c) Co-HMS-X (15 mol%).
Fig. 6. H2-TPR profile of the catalyst.

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S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
Table 1
Catalytic activity of the catalyst towards styrene epoxidation reaction.
Catalyst
Conversion (%) Selectivity (%)
Styrene oxide Benzaldehyde
Co-HMS-X (5 mol%)
90
100
75.7
100
100
100
100
45
94
27
28
67
59.8
35.3
30.1
61.6
60.8
67.5
59.9
62.0
66.0
45.0
17.0
79.0
40.2
64.7
69.9
38.4
39.2
32.5
40.1
Others
Others
55.0
Others
Others
Al-Co-HMS-X (Co/Al = 20)
Tl-Co-HMS-X (Co/Tl = 20)
Ga-Co-HMS-X (Co/Ga = 20)
Ga-Co-HMS-X (Co/Ga = 7)
Ga-Co-HMS-X (Co/Ga = 4)
Ga-Co-HMS-X (Co/Ga = 3)
Co-MCM-41 (0.78 wt.%) [8]
Co-SBA-15 (20 wt.%) [2]
Ga-TUD-1 (4 mol%) [14]
La-KIT-6 (0.04 wt.%) [33]
Co-ZSM-5 (4 mol%) [34]
Reaction conditions: solvent = DMF (20 mL); reactant = styrene (10 mmol); cata-
lyst = 0.2 g; temperature = 120 ◦C; O2 flow = 15 mL/min; time = 4 h.
3.7.3. Effect of O2 flow rate
Fig. 7. NH3-TPD profile of the catalyst.
The influence of flow rate of O2 on the epoxidation of styrene
over Co-HMS-X at 120 ◦C has also been carried out (Table S11, Sup-
porting information). The styrene conversion was 45.6% when flow
rate was 5 mL/min, and then slowly increased to 90% with the rise
of flow rate of O2 to 15 mL/min. The styrene conversion is decreased
when the flow rate of O2 was higher than 15 mL/min which may be
due to over oxidation of the substrate as reported in the literature
[32].
inside the porous framework, possibly interacting with Lewis acidic
site [14]. Desorption profile at ∼660 ◦C might be due to chemisorp-
tion of ammonia on stronger acid sites [31]. A comparison of relative
amount of acid sites shows that incorporation of Al in M-Co-HMS-X
catalyst contribute to increase in acidity in the weak range, while
the Ga incorporation in the strong range. The incorporation of Tl
was found to decrease the acidity of Co-HMS-X catalyst in both
the weak and strong range. Among the different percentage Ga
loaded Co-HMS-X catalyst, the increase in Ga loading decreased
the weak acidity and increased the strong acidity. The catalyst with
Co/Ga ratio = 4 showed maximum number of acid sites in the strong
acidity range. Increase of acidity in the strong range is expected to
enhance the styrene epoxidation reaction as revealed in our previ-
ous studies [14].
3.7.4. Effect of solvent
The effect of solvent was studied in the optimum reaction tem-
perature and oxygen flow which is also interesting (Table S12,
Supporting information). There are several studies on the coordina-
tion of DMF or DMA to cobalt (II) complexes, and such coordination
may affect the ability of O2 to bind the cobalt complexes and their
redox potential [9]. Although a certain styrene conversion was
obtained in dimethyl sulfoxide (DMSO) and cyclohexanone, the
epoxide selectivity was low, and the main product was benzalde-
hyde. So, it can be concluded that DMF is the best solvent for styrene
oxidation reaction of cobalt based catalytic system [10].
3.7. Catalytic activity studies for styrene epoxidation reaction
using molecular O2
3.7.1. Effect of Co loading
3.7.5. Optimum condition
Catalytic activities of 5, 10 and 15 mol% Co-HMS-X catalyst
for styrene epoxidation reaction using molecular O2 have been
studied (Table S9, Supporting information). On increasing the
cobalt loading from 5 to 15 mol%, a decrease in styrene conver-
sion (i.e. 75.2–30.9%) and styrene oxide selectivity (i.e. 46.1–29.1%)
is observed. It can be correlated from the surface area and poros-
ity measurement data, where with increasing loading surface area
and pore size were decreased. May be due to sub-nanometer cobalt
oxide nanocluster species in the pores induce the lower accessibil-
ity for the active sites on the catalyst surface.
As discussed above by varying different conditions over 5 mol%
Co-HMS-X catalyst, it is found that high oxygen flow rate, i.e.
15 mL/min, reaction temperature 120 ◦C, solvent DMF and 4 h time
on stream are the optimum parameters to get best catalytic activity
for this reaction (90% styrene conversion and 59.8% styrene oxide
selectivity) as shown in Table 1.
3.7.6. Effect of different promoters
To improve the catalytic activity, the three different promoters
Al, Ga and Tl are added to Co-HMS-X catalyst (5 mol%). Group III A
metals are well known for their acidity where the acidity decreases
in the order Al > Ga > Tl. Addition of promoter showed significant
impact on styrene conversion and selectivity. The styrene conver-
sion leads to 100% when Al and Ga are added as promoters but in
case of Tl conversion decreases to 75.7%. The selectivity of epox-
ide followed up the trend of the styrene conversion; it reached
to 61.5% in case of Ga and least in case of Tl. Again changing the
Co/Ga ratio from 20 to 4, increase in styrene oxide selectivity has
been observed, i.e. styrene oxide selectivity increases from 61.5% to
68% (Table 1), which is quite satisfactory with the literature report.
After addition of promoter the change in catalytic activity of Co-
HMS-X can be correlated with the H2-TPR and NH3-TPD results.
The H2-TPR study shows that among the three promoters (Al, Ga
and Tl), Ga-Co-HMS-X (Si:Co:Ga = 100:5:0.25) has the highest H2
3.7.2. Effect of temperature
The influence of reaction temperature on the epoxidation of
styrene with O2 over Co-HMS-X have been studied (Table S10, Sup-
porting information). The conversion of styrene increased sharply
with the increasing of reaction temperature. For example, styrene
conversion was 9.8% at 80 ◦C, quickly rose to 75.2% at 100 ◦C and
81.6% at 120 ◦C. But styrene conversion started to decrease with
further increase of temperature. The rise in styrene conversion is
decelerated to 72.5% at 140 ◦C. As reported in the literature [9] at
temperature higher than 90 ◦C are probably rate determined by the
diffusion of reactants to the active sites. It seems that an appropri-
ate reaction temperature is favourable to the formation of epoxide
for the reaction [2].

S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
67
Table 2
Catalytic activity of the catalyst during recycling and substrate variation towards epoxidation reaction.
Substrate
Catalyst
Conversion (%)
Selectivity (%)
Oxided
Aldehydee
Styrene
Co-HMS-X (5 mol%)
Co-HMS-X (5 mol%)
Co-HMS-X (5 mol%)
Co-HMS-X (5 mol%)
90.0
90.0
81.7
80.1
–
17
13.4
15
59.8
45.1
32.9
30.1
–
60.8
100
67.5
40.2
54.9
67.1
69.9
–
39.2
–
32.5
Styrenea
Styreneb
Styrenec
Cyclooctene
Cyclooctene
Cyclooctene
Stilbene
Co-HMS-X (5 mol%)
Ga-Co-HMS-X (Co/Ga = 20)
Ga-Co-HMS-X (Co/Ga = 4)
Ga-Co-HMS-X (Co/Ga = 20)
Reaction conditions: solvent = DMF (20 mL); reactant = styrene 10 mmol; catalyst = 0.2 g; temperature = 120 ◦C; O2 flow = 15 mL/min; time = 4 h. ‘a’, 1st recycle; ‘b’, 2nd recycle;
‘c’, 3rd recycle.
d
Corresponding oxide.
Corresponding aldehyde.
e
consumption as well as NH3-TPD profile indicates the presence of
strong acidic sites into the catalyst surface. Lewis acid sites have
an impact on metal–support interaction, perhaps influencing the
cobalt reducibility. Thus, Ga-Co-HMS-X exhibits high reducibility
in nature hence possesses high oxidizing property in compari-
son to the Al-Co-HMS-X and Tl-Co-HMS-X. A higher temperature
desorption of ammonia is related to stronger acid sites. Among the
Ga-Co-HMS-X, Co/Ga = 4 have the highest strengths of acid sites
followed by Co/Ga = 20 > 7 > 3.
reaction of different substrate, e.g. cyclooctene, stilbene to the cor-
responding epoxide (Table 2).
3.7.8. Proposed reaction mechanism
It is interesting to note that in co-doped (M = Ga) catalyst,
Co/Ga = 4 is needed to get 100% conversion and improved styrene
oxide selectivity up to 68%. Based on the catalytic activity and lit-
erature survey a proposed mechanism for epoxidation of styrene
with molecular oxygen at DMF solvent system over Co2+ catalyst
has been depicted in Fig. 8 [10,35]. However the role of co-doped
metal on styrene epoxidation reaction needs to be explained. The
ionic radius of Co2+ and Ga3+ are more comparable (Co2+ = 65 pm,
Ga3+ = 62 pm) rather than Al3+ (53 pm) or Tl3+ (150 pm). In case of
gallium, it might be possible that Co2+ ion can be replaced by Ga3+
and hold into an interstitial site (like Frenkel defect). This causes
defects as well as co-ordinatively unsaturated interstitial cobalt
sites. The resultant cobalt complex [DMF-LCo(II)] was coordinated
to molecular oxygen to form a cobalt superoxo complex DMF-
LCo(III)OO•, which led to oxidative addition with styrene to give
3.7.7. Catalytic regeneration and substrate variation
The catalyst recyclability is one of the important parameter to
commercialize the catalyst. Catalytic activity of the catalysts almost
unaffected up to three cycles as presented in Table 2. The marginal
decrease observed in conversion may be due to the loss of cata-
lyst during filtration and drying. A sharp decrease in selectivity of
styrene oxide, probably due to formation of Co(II) → Co3O4 species
led to increase in the formation of benzaldehyde. In addition,
the Ga-Co-HMS-X (Co/Ga = 4) catalyst induced several epoxidation
Fig. 8. Proposed reaction mechanism.

68
S. Rahman et al. / Applied Catalysis A: General 482 (2014) 61–68
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plex (A) was formed. This complex collapsed to the cyclic peroxide
radical (B). The intermediate (B) underwent thermal decomposi-
tion to benzaldehyde or react with another styrene molecule to
generate styrene oxide.
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Co-HMS-X catalyst of different loading is synthesized by
sol–gel method in which 5 mol% loading is found as optimum.
Small angle X-ray scattering and small-angle neutron scattering
results revealed that catalysts were mesoporous in nature as well
as hexagonal pore structure. The HRTEM images of Co-HMS-X
(5 mol%) shows presence of well-shaped uniform mesoporous
surface. UV–vis results confirms that tetrahedral cobalt (II) was
highly dispersed inside the HMS-X mesopores. It is the key factor
for the epoxidation of styrene by molecular oxygen. 29Si NMR
result indicated the presence of three different types of silica
species and Si
O Co linkage, confirming the Co incorporation
into Si framework. Both H2-TPR and NH3-TPD results revealed that
Ga-Co catalyst is more reducible and acidity increases increasing
Ga loading. Thus, the catalytic activity primarily depends on the
acidity of the catalyst which in turn strongly depends on the
gallium content in the sample. Interestingly, 100% conversion
efficiency was found in case of Ga-Co-HMS-X (Co/Ga = 4) with 68%
selectivity for styrene oxide.
Acknowledgments
SR and CS would like to acknowledge UGC, Govt. of India for
research fellowship. BC would like to acknowledge UGC, Govt of
India for funding in the research project [Project No. F.39-802/2010
(SR)]. RK acknowledge ISM for providing senior research fellow-
ships.
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Appendix A. Supplementary data
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