Direct synthesis of Mn-Ti-SBA-15 catalyst for the oxidation of ethylbenzene

Article abstract of DOI:10.1039/c3cy00306j

Bimetallic Mn-Ti-SBA-15 materials were prepared by direct hydrothermal method with appropriate pH adjustment. The materials were characterized by low and wide angle XRD, N₂ sorption studies, DRSUV-Vis, XPS, ESR, SEM and TEM. The XRD and BET results clearly demonstrated the incorporation of manganese and titanium species in the SBA-15 framework without affecting the mesoscopic nature of the material. The oxidation state and coordination environment of the active sites were confirmed by DRSUV-Vis, XPS and ESR. The results revealed that Mn²⁺ is in octahedral geometry whereas Mn ³⁺ and Ti⁴⁺ are in tetrahedral coordination in the SBA-15 framework. The O1s XPS spectrum well established the framework incorporation of manganese and titanium species. The catalytic activity of Mn-Ti-SBA-15 materials was evaluated in the oxidation of ethylbenzene with TBHP as the oxidant under optimized reaction condition. It was found that Mn-Ti-SBA-15(50) catalyst showed high catalytic activity with a rate constant of 7.155 × 10^-3 min^-1. The synergetic effect of Mn²⁺/Mn³⁺ and Ti⁴⁺ sites in Mn-Ti-SBA-15 is a promising catalyst for the oxidation of ethylbenzene and other alkylbenzenes. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cy00306j

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Direct synthesis of Mn-Ti-SBA-15 catalyst for the
oxidation of ethylbenzene
Cite this: Catal. Sci. Technol., 2013,
3, 2340
P. Visuvamithiran, K. Shanthi, M. Palanichamy and V. Murugesan*
Bimetallic Mn-Ti-SBA-15 materials were prepared by direct hydrothermal method with appropriate
pH adjustment. The materials were characterized by low and wide angle XRD, N2 sorption studies,
DRSUV-Vis, XPS, ESR, SEM and TEM. The XRD and BET results clearly demonstrated the incorporation of
manganese and titanium species in the SBA-15 framework without affecting the mesoscopic nature of
the material. The oxidation state and coordination environment of the active sites were confirmed by
DRSUV-Vis, XPS and ESR. The results revealed that Mn2+ is in octahedral geometry whereas Mn3+ and
Ti4+ are in tetrahedral coordination in the SBA-15 framework. The O1s XPS spectrum well established
the framework incorporation of manganese and titanium species. The catalytic activity of Mn-Ti-SBA-15
materials was evaluated in the oxidation of ethylbenzene with TBHP as the oxidant under optimized
reaction condition. It was found that Mn-Ti-SBA-15(50) catalyst showed high catalytic activity with a
rate constant of 7.155 Â 10À3 minÀ1. The synergetic effect of Mn2+/Mn3+ and Ti4+ sites in Mn-Ti-SBA-15
is a promising catalyst for the oxidation of ethylbenzene and other alkylbenzenes.
Received 1st May 2013,
Accepted 29th May 2013
DOI: 10.1039/c3cy00306j
www.rsc.org/catalysis
major challenge for catalytic applications. Since the incorpora-
tion of heteroatoms in SBA-15 is very difficult under strongly
1. Introduction
The selective oxidation of ethylbenzene to acetophenone is
an important reaction as the product is the raw material
for the production of fine chemicals and pharmaceuticals.1,2
Conventional oxidation reactions employ toxic solvents and
metal salts as the catalysts. Such drastic reaction condition,
and difficult separation of products from the reaction mixture
make the process tedious on the industrial scale. Transition
metal complexes are also used as the catalyst but the problem
of reusability limits their application. Hence researchers are
constantly trying to find a suitable heterogeneous catalytic
system for the oxidation of ethylbenzene. Mesoporous materials
possess well ordered arrangement of pores, high surface area,
large pore volume and uniform pore size distribution.3–6 Hence
these materials find a variety of applications in the field of
catalysis, adsorption, sensors, fuel cells and solar cells.7–11
In the family of mesoporous materials, SBA-15 possesses
large pore size and thicker walls compared with M41S family
members such as MCM-41 and MCM-48. However, siliceous
SBA-15 mesoporous materials do not find application in catalysis
due to lack of active sites in their framework.12,13 In view of this
deficiency, creation of active sites in mesoporous SBA-15 is a
acidic condition due to the hydrolysis of the M–O–Si network,
most researchers have introduced active sites on SBA-15 by a
post-synthetic grafting method.14,15 These grafted active sites in
mesoporous materials showed lot of inconvenience while in
use as catalyst due to reduction of surface area, pore diameter
and pore volume. Therefore, the introduction of active sites in
the silica framework by direct synthesis gained importance.
Wu et al.16 reported the incorporation of heteroatoms such as
Al and Ti into SBA-15 by direct synthesis with suitable pH
adjustment. Recently several reports are available on the incor-
poration of metal ions such as Al, Ti, V, Cr, Mn and Fe into SBA-15
framework by direct method under suitable pH condition.17–23
Manganese is particularly attractive as a donor because
many of its oxidation states possess redox potential that are
suitable to drive demanding organic transformations.24 Porous
titanosilicates continue to attract considerable attention due to
their remarkable catalytic efficiency in the oxidation of a wide
variety of small chain hydrocarbons.25,26 Recently bimetals
such as Ti–Co and Ti–V incorporated AlPO-5 molecular sieves
have been found to exhibit good catalytic activity due to
oxophilic centre of Ti(IV) active sites located in close proximity
to another species, thus facilitated synergistic influence both
from structural and catalytic perspective.27,28 The incorporation
of two different metal ions into mesoporous materials may
create new functional material with different or new redox and
Department of Chemistry, Anna University, Chennai-600 025, India.
E-mail: v_murugu@hotmail.com; Fax: +91-44-2220066/22201213;
Tel: +91-44-22358641
c
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acidic properties. However, only a few reports are available on isotherms were recorded at 77 K on a Micromeritics ASAP 2020
bimetal incorporated mesoporous materials, demonstrating volumetric adsorption analyzer. Prior to each adsorption
higher catalytic activity than monometal incorporated meso- measurement, the sample was degassed at 350 1C for 3 h under
porous materials.29–38 Based on this useful previous report, it vacuum (p o 10À5 mbar) in the degas port of adsorption
was attempted to incorporate both Mn and Ti into SBA-15 analyzer. The specific surface area was calculated by Brunauer–
framework. It was presumed that bimetal incorporated SBA-15 Emmett–Teller (BET) method, and the pore size distribution
can create new redox sites compared to mono metal substituted was calculated using Barrett–Joyner–Halenda (BJH) method.
SBA-15, and hence it was expected to display different catalytic The DRSUV–Vis absorption spectra were recorded between
activity.
190 and 800 nm on a UV–Vis spectrophotometer (Shimadzu
In the present study, Mn-Ti-SBA-15 material was synthesized model 2450) using BaSO4 as the reference. The X-ray photo-
by direct hydrothermal method with careful pH adjustment. electron (XPS) spectrum was recorded using MS Omicron
The catalyst was thoroughly characterized by various techniques nanotechnology with XM-1000 monochromator and AlKa as
as discussed in the following sections. The catalytic activity of the radiation source operated at 200 W. The ESR spectrum
Mn-Ti-SBA-15 was evaluated in the oxidation of ethylbenzene. was recorded on a varian E-112 at room temperature with a
The effects of reaction temperature, time, solvent and amount modulation frequency of X-band (8.5–9.5 GHz). The morphology
of catalyst on conversion and products’ selectivity were also of the materials was examined by scanning electron microscope
examined and discussed.
(SEM) after gold coating using SEM-JEOL, JSM-5600 model with
an accelerating voltage of 20 keV. High-resolution transmission
electron microscopic (HR-TEM) images were captured using
JEOL JEM 2100 with an accelerating voltage of 200 keV.
2. Experimental
2.1. Chemicals
2.4. Catalytic studies
Tetraethylorthosilicate (TEOS) (Aldrich) and triblockcopolymer
(poly(ethylene glycol)-poly(propylene glycol)-block-poly(ethylene Mn-Ti-SBA-15 (75 mg) was dispersed in 5 ml acetonitrile taken
glycol) (Pluronic P123) of molecular weight 5800 (Aldrich) were in a 100 ml round bottom flask. Ethylbenzene (3 mmol) and
used as silicon source and structure directing agent respectively. TBHP (5 mmol) were added to it dropwise. The reaction mixture
Titanium tetraisoproxide (Aldrich) and manganese nitrate was stirred magnetically at 80 1C for 2 h. The product was filtered,
(Aldrich) were used as Ti and Mn sources respectively. tert-Butyl and the residue washed thoroughly with water and extracted with
hydroperoxide (TBHP) (Merck) was used as the oxidant. Ethyl- ether. The products so collected were analyzed by a gas chromato-
benzene (Fluka) and all other solvents (Merck) were used without graph (Shimadzu GC-17A model) equipped with DB-5 capillary
further purification.
column (30 m  0.25 mm  0.25 mm) and a flame ionization
detector. The products were also identified by a gas chromatograph
coupled with mass spectrometer (Perkin Elmer Clarus 500) using
2.2. Synthesis of bimetallic Mn-Ti-SBA-15
The bimetallic Mn-Ti-SBA-15(25) was prepared as follows: helium as the carrier gas at flow rate of 1 mL minÀ1
.
Pluronic P123 (4 g) was dissolved in 30 ml distilled water and
stirred for 2 h. Then 2 M HCl (120 ml) was added to it and the
stirring continued for another 2 h after the pH was adjusted to
3 by adding dilute ammonium hydroxide (solution A). Subse-
3. Results and discussion
3.1. Mechanism for the formation of Mn-Ti-SBA-15
quently, TEOS (9 g), manganese nitrate (0.867 g) and titanium The incorporation of an heteroatom (transition metal) in
tetraisoproxide (0.982 g) were added to deionized water (5 ml) mesoporous SBA-15 is very difficult under highly acidic condition
and stirred for 30 min to form a clear solution (solution B). The due to easy hydrolysis of the M–O–Si network. However, the rate of
solution B was added dropwise to solution A and stirred at hydrolysis of the M–O–Si bond was decreased by lowering the
40 1C for 24 h. The resulting mixture was aged at 100 1C for acidity of the solution. The XRD patterns of Mn-Ti-SBA-15(25) at
24 h. The solid product was recovered by filtration and dried at pH 2, 2.5 and 3 are shown in Fig. 1. The intensity of the XRD peaks
100 1C overnight. The template was removed by calcination at increased with increase of pH from 2 to 3. The results revealed that
550 1C for 6 h at a heating rate of 2 1C minÀ1. The prepared high amount of manganese and titanium species were incorpo-
sample was denoted as Mn-Ti-SBA-15(25).
rated in SBA-15 at moderately low acidic pH. It is found that pH 3 of
the gel mixture is higher than the isoelectric point of silica and
hence it formed negatively charged silica species. The resulting
2.3. Characterization of the catalyst
The small angle X-ray diffraction (XRD) pattern of the material negatively charged silica species interacted with M–OH species via
was recorded on a Bruker D8 advanced powder X-ray diffracto- hydroxyl group condensation catalyzed by nitrate ion.23,39 It was
meter using Cu Ka (l = 1.54 Å) as the radiation source in the observed that pH 3 of the gel is found to incorporate high metal ion
2y range 0.5–61 with a step size of 0.011 and a step time of 1s. species into SBA-15 without loss of its mesoporous nature.
The wide angle X-ray diffraction patterns were recorded on a
3.2. Characterization
PANalytical X pert PRO diffractometer equipped with Cu Ka (l =
1.54 Å) as the radiation source in the 2y range of 5–801 with a
3.2.1. XRD. The small angle XRD patterns of SBA-15 and
step size of 0.021 and a step time of 1s. The nitrogen sorption Mn-Ti-SBA-15 with Si/(Mn + Ti) ratios of 25, 50, 75 and 100 are
c
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to (110) and (220) respectively. These characteristic diffraction
patterns well supported the hexagonal ordered mesoporous
structure with space group of P6mm.40 The small angle XRD
patterns of Mn-Ti-SBA-15 exhibited similar characteristics of
SBA-15. The intensity of the peaks increased with increase of
metal content. It was also noticed that the unit cell parameters
increased with increase of metal content from 100 to 25, and
the results are presented in Table 1. The increase of unit cell
parameters is due to higher ionic radii of Ti4+ and Mn2+/Mn3+
than Si4+ as well as higher bond length of Ti–O and Mn–O than
Si–O. This suggested that manganese and titanium species
were incorporated into SBA-15 framework. The wide angle
XRD patterns of Mn-Ti-SBA-15 are shown in Fig. 3. There was
no distinct diffraction pattern corresponding to metal oxides
such as TiO2 and MnO2 or Mn3O4. This confirmed the incor-
poration of manganese and titanium species into SBA-15
framework without affecting the mesoporous silica.
Fig. 1 Small angle XRD patterns of Mn-Ti-SBA-15(25) at pH (a) 2, (b) 2.5 and (c) 3.
3.2.2. N2 sorption studies. The nitrogen adsorption–
desorption isotherms of SBA-15 and Mn-Ti-SBA-15 are shown in
Fig. 4. The isotherms of Mn-Ti-SBA-15 samples exhibited type IV
adsorption with H1 hysteresis loop in the relative pressure range
of 0.6 and 0.8, which is similar to that of parent SBA-15. The
textural properties such as surface area, pore volume and pore
diameter of SBA-15 and Mn-Ti-SBA-15 with Si/(Mn + Ti) ratios of
25, 50, 75 and 100 are given in Table 1. All textural parameters
increased with increased incorporation of metal content, as a
consequence adsorption of nitrogen increased with increase
of metal content. The surface area increased from 692 to
729 m2 gÀ1 and pore volume increased from 0.74 to 0.93 cm3 gÀ1
due to expansion of mesopores and increase of pore diameter.
The pore size distribution of SBA-15 and Mn-Ti-SBA-15 materials
is shown in Fig. 5. Although SBA-15 showed very narrow pore
size distribution, metal ions incorporated SBA-15 showed a
slightly broad distribution which indicated well dispersion of
metal ions into SBA-15 framework, and thus increased the pore
diameter.
3.2.3. DRSUV-Vis spectroscopy. DRSUV-Vis spectra (Fig. 5)
was useful to understand the coordination environment of
Mn2+/Mn3+ and Ti4+. The DRSUV-Vis spectra of Mn-Ti-SBA-15
materials showed four distinct absorption bands at 210, 270,
305 and 513 nm. The bands at 270 and 513 nm were assigned to
Mn2+/Mn3+ ions in SBA-15 framework.41–43 The band at 270 nm
Fig. 2 Small angle XRD patterns of (a) SBA-15, (b) Mn-Ti-SBA-15(25), (c) Mn-Ti-SBA-
15(50), (d) Mn-Ti-SBA-15(75) and (e) Mn-Ti-SBA-15(100).
shown in Fig. 2. The small angle XRD patterns of SBA-15
showed characteristic reflections in the 2y range of 0.5–5, indicated the charge transfer transition of O2À–Mn3+ with
exhibiting a sharp intense peak below 11 (2y) corresponding tetrahedral coordination. The broad band centered at 513 nm
6
to (100) and small humps at 1.651 and 1.81 (2y) corresponding was assigned to the d–d transition of Mn2+ in A1g–4T2g crystal
Table 1 Textural properties of Mn-Ti-SBA-15
Unit cell parameter ao
(nm)
Surface area BET
Pore volume Vp
Pore diameter Dp
(nm)
Pore wall thickness
(nm)
Catalyst
(m2 gÀ1
)
(cm3 gÀ1
)
SBA-15
11.01
11.32
11.22
11.20
11.08
685
729
721
715
692
0.74
0.93
0.87
0.81
0.74
7.09
7.29
7.23
7.21
7.14
3.92
4.03
3.99
3.99
3.94
Mn-Ti-SBA-15(25)
Mn-Ti-SBA-15(50)
Mn-Ti-SBA-15(75)
Mn-Ti-SBA-15(100)
ao calculated from a = 2d100/31/2. Pore wall thickness = ao À Dp.
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Fig. 3 XRD patterns of (a) Mn-Ti-SBA-15(25), (b) Mn-Ti-SBA-15(50), (c) Mn-Ti-
SBA-15(75) and (d) Mn-Ti-SBA-15(100).
field transition.44,45 This confirmed that Mn2+ as well as Mn3+
ions coexist in SBA-15 framework. The peaks at 210 and 305 nm
were assigned to Ti4+ in tetrahedral coordination.46–48 The first
band is due to the charge transfer between O2À and Ti4+, and
the latter band at 305 nm was assigned to the tetrahedral
coordination with tripodal nature of Ti4+ in (SiO)3TiQO.49,50
The results confirmed the octahedral coordination of Mn2+ and
tetrahedral coordination of Mn3+ and Ti4+ ions in the meso-
porous SBA-15.
3.2.4. XPS. The XPS spectra (Fig. 6) clearly revealed the
oxidation state and chemical environment of the metal ions in
SBA-15. All the binding energies are with reference to 1s
spectrum of elemental carbon with a binding energy of 287.5 eV.
The survey spectrum of Mn-Ti-SBA-15 (Fig. 6a) showed binding
energy peaks for 2p of Mn and Ti and also O1s. Fig. 6b shows the
XPS spectrum of the doublet spin states of Mn2p with their
binding energies of 641.53 and 653.04 eV for Mn2p3/2 and
Mn2p1/2 respectively.51 The FWHM of the Mn doublet state is
found to be 1.4 eV, which provides good evidence for the presence
Fig. 4 (a) N2 adsorption–desorption isotherms of (a) SBA-15, (b) Mn-Ti-SBA-15(100),
(c) Mn-Ti-SBA-15(75), (d) Mn-Ti-SBA-15(50) and (e) Mn-Ti-SBA-15(25). (b) Pore size
distribution of (a) SBA-15, (b) Mn-Ti-SBA-15(100), (c) Mn-Ti-SBA-15(75), (d) Mn-Ti-SBA-
15(50) and (e) Mn-Ti-SBA-15(25).
of both Mn2+ and Mn3+ in the silica framework. The corresponding The surface composition of the elements in wt% is O (68.4), Si
spin orbit coupling energy difference is found to be 11.51 eV.52 The (23.9), Mn (1.3) and Ti (1.1). These results also confirmed that
XPS spectrum of Ti 2p of Ti4+ is shown in Fig. 6c. The binding manganese and titanium ions are present in the framework of
energies of 464.11 and 458.39 eV are attributed to Ti 2p1/2 and Ti SBA-15.
2p3/2 signal respectively, which is in good agreement with the
binding energies of Ti4+ incorporated SBA-15.53,54
3.2.5. ESR spectrum. The oxidation state and coordination
environment of Mn and Ti in Mn-Ti-SBA-15 were analyzed by its
The framework incorporation of metal ions was verified by ESR spectrum. The ESR spectrum of Mn2+ showed weak sextet
the O1s XPS spectrum shown in Fig. 6d. The XPS spectrum of splitting with a g value of 2.0031 and hyperfine coupling
O1s showed three distinct deconvolute peaks which are constant around 97 G (Fig. 7). This provides good evidence
assigned to Si–O, Mn–O and Ti–O with their corresponding for the presence of Mn2+ with octahedral coordination in Mn-
binding energies of 533.2, 532.1 and 533.92 eV respectively.55–57 Ti-SBA-15.41 The observed weak ESR sextet line may be due to
This result confirmed the incorporation of manganese and the presence of Mn3+. It is reported that Mn3+ is more opt than
titanium species in SBA-15 framework. It was also noticed that Mn2+ to be substituted for Si4+ in the framework position of
O1s binding energies of manganese and titanium are different zeolite and MCM-41.58 It is therefore concluded that Mn2+/Mn3+
from TiO2 (anatase), MnO2 and Mn3O4. The framework incor- ions coexist in SBA-15 framework, and not as Mn4+ ion. This is
poration of metal ions is well established by the O1s spectrum. in good agreement with the DRSUV-Vis spectra.
c
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Fig. 5 UV-DRS spectra of (a) Mn-Ti-SBA-15(25), (b) Mn-Ti-SBA-15(50), (c) Mn-Ti-
SBA-15(75) and (d) Mn-Ti-SBA-15(100).
3.2.6. SEM and TEM. The morphology of Mn-Ti-SBA-15
materials was revealed by the SEM images as shown in Fig. 8.
The morphology of the materials with different amount of
metal ions loading at moderately low acidic condition (i.e. at
pH 3) showed a worm-like structure.59,60 All the materials
showed rope-like hexagonal structures of several hundred
micrometers bundled together. The TEM images of SBA-15
and Mn-Ti-SBA-15 with various Si/(Mn + Ti) ratios are shown
in Fig. 9. The images clearly demonstrated the uniform pores
formed even after incorporation of the metal ions, and the
estimated diameter of the pores is around 7 nm. It is inferred
that the mesoporous structure was not affected during the
synthesis under low acidic condition. This is in good agreement
with the results of XRD and BET analysis.
3.3. Catalytic activity of Mn-Ti-SBA-15
The catalytic activity of Mn-Ti-SBA-15 was evaluated in the
oxidation of ethylbenzene using TBHP as the oxidant. The
influence of reaction parameters such as solvent, ratio of Si/
(Mn + Ti), temperature and time of the reaction was also
studied to optimize the reaction conditions to obtain high
conversion with high selectivity of the product. The results
are presented in Table 2. The entries 1 to 4 in Table 2 revealed
the influence of the solvent in the oxidation of ethylbenzene.
Solvents such as dichloromethane, acetone, methanol and
acetonitrile were used as the reaction medium for ethylbenzene
oxidation. Among these solvents, acetonitrile exhibited the
highest conversion with high selectivity of acetophenone. This
is due to the high polar nature of the solvent that led to a partial
decomposition of TBHP. The entries 5 to 7 in Table 2 show the
results of Mn-SBA-15, Ti-SBA-15 and a physical mixture of these
two monometallic SBA-15 as catalysts in the same oxidation
Fig. 6 XPS spectrum of Mn-Ti-SBA-15(50): (a) survey spectrum, (b) Mn 2p
spectrum, (c) Ti 2p spectrum and (d) O 1s spectrum.
which resulted low conversion. It contrast Mn-Ti-SBA-15 catalytic activity of Mn2+ and Ti4+ centers. The synergistic
showed higher activity than monometallic substituted SBA-15 activity ascertained the simultaneous incorporation of Mn2+
as well as physically mixed Mn-SBA-15 and Ti-SBA-15. The high and Ti4+ species in close proximity into the SBA-15 framework.
catalytic activity of Mn-Ti-SBA-15 is ascribed to the synergetic The synergetic effect of Mn-Ti-SBA-15 is attributed to the
c
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Fig. 7 ESR spectrum of Mn-Ti-SBA-15(50).
Fig. 9 TEM images of (a) SBA-15 (b) Mn-Ti-SBA-15(25), (c) Mn-Ti-SBA-15(50),
(d) Mn-Ti-SBA-15(75) and (e) Mn-Ti-SBA-15(100).
in Fig. 10. The ethylbenzene conversion increased with increase
of metal content from 100 to 50, and further increase of metal
content decreased the conversion due to limitation of reactant
molecule diffusion caused by the pore blocking effect, which
resulted small reduction of micropores in mesoporous SBA-15.
So the reactants could not freely access the reactive sites, and
hence a higher amount of Mn and Ti incorporated SBA-15
showed relatively low conversion. The order of activity for
different ratios of Si/(Mn + Ti) in Mn-Ti-SBA-15 catalyst is
Fig. 8 SEM images of (a) Mn-Ti-SBA-15(25), (b) Mn-Ti-SBA-15(50), (c) Mn-Ti-
SBA-15(75) and (d) Mn-Ti-SBA-15(100).
oxophilic centre of Ti4+ which easily coordinates with TBHP, 50 > 25 > 75 > 100. The rate of reaction for the oxidation of
and Mn2+/Mn3+ species may be involved in the redox activity, ethylbenzene was evaluated by the apparent rate constant, and
which enhanced the catalytic activity. The entries 8 and 9 in the order for Si/(Mn + Ti) ratios of 50, 25, 75 and 100 is 7.155 Â
Table 2 show the results of other oxidants used in the oxidation 10À3 minÀ1 > 3.815 Â 10À3 minÀ1 > 2.667 Â 10À3 minÀ1
>
of ethylbenzene. When molecular O2 was used as the oxidant, 2.325 Â 10À3 minÀ1
.
the conversion was found to be very low due to easy expulsion
3.3.1. Effect of temperature. Fig. 11 depicts the influence
of O2 from the reaction mixture. Similarly when H2O2 was used of the reaction temperature on ethylbenzene oxidation and
as the oxidant, it was decomposed easily and evolved oxygen acetophenone selectivity over Mn-Ti-SBA-15(50) using TBHP
rapidly due to the high exothermic nature, which resulted in a as the oxidant. The results revealed that the increase of the
low conversion of ethylbenzene. In the case of TBHP as the reaction temperature from 50 to 80 1C increased the ethylbenzene
oxidant, it generated active oxygen species on the redox sites conversion and selectivity of acetophenone. This may be attrib-
and formed transition state on the catalyst which prevented uted to an enhanced decomposition of TBHP with the increase of
expulsion of O2 from the reaction mixture.
temperature. It is concluded that higher temperature favored high
The effect of Si/(Mn + Ti) ratios of 25, 50, 75 and 100 conversion and selective oxidation. The activation energy was
incorporated into SBA-15 on the catalytic activity is depicted calculated from the Arrhenius equation, and it was found to be
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Table 2 Ethylbenzene oxidation under different reaction conditionsa
Selectivitye (%)
Acetophenone
Conversione (%)
1-phenyl ethanol
2-phenyl ethanol
Entry
Catalyst
Solvent
1
2
3
4
5
6
7
8
Mn-Ti-SBA-15(50)
Mn-Ti-SBA-15(50)
Mn-Ti-SBA-15(50)
Mn-Ti-SBA-15(50)
Mn-SBA-15(50)
Ti-SBA-15(50)
CH2Cl2
Acetone
CH3OH
ACN
ACN
ACN
ACN
ACN
—
ACN
59
52
34
92
52
63
67
02
21
16
21
29
39
47
87
62
68
53
10
47
52
31
59
46
22
09
31
21
29
58
36
37
41
12
15
31
04
07
11
18
32
17
11
28
Mn-SBA-15 + Ti-SBA-15b
—
9
10
11
Mn-Ti-SBA-15(50)
Mn-Ti-SBA-15(50)c
Mn-Ti-SBA-15(50)d
ACN
a
b
Reaction conditions: Catalyst (100 mg), ethylbenzene (3 mmol), TBHP (5 mmol), time 2 h, temperature 80 1C, and solvent (5 ml). Physical
c
d
e
mixture. O2 as oxidant. H2O2 as oxidant. Determined by GC.ACN = Acetonitrile.
Fig. 10 Effect of Si/(Mn + Ti) ratios in the oxidation of ethylbenzene.
Fig. 12 Arrhenius plot.
3.3.2. Effect of reaction time. The influence of reaction
time on conversion and selectivity was studied between 1 and
3 h over Mn-Ti-SBA-15(50) catalyst using TBHP as the oxidant at
80 1C in acetonitrile, and the results are depicted in Fig. 13. The
ethylbenzene conversion increased with the increase of reac-
tion time up to 2 h whereas the selectivity of acetophenone did
not change. The conversion and selectivity of the product were
not alerted after 2 h reaction time. The results revealed that the
reaction was almost completed in about 2 h, when the TBHP
efficiency reached a maximum. This result also clearly revealed
that the catalytic activity of Mn-Ti-SBA-15 was not affected by a
longer reaction time. Based on the results, it is concluded that
the optimal reaction time for high conversion of ethylbenzene
is 2 h.
3.3.3. Reusability. The recyclability of Mn-Ti-SBA-15(50)
catalyst was examined in ethylbenzene oxidation using TBHP
as the oxidant in acetonitrile medium at 80 1C for 2 h. The used
catalyst in the first cycle was recovered by filtration, activated at
Fig. 11 Effect of the reaction temperature on the oxidation of ethylbenzene.
45.14 kJ molÀ1 (Fig. 12). The results revealed that ethylbenzene 300 1C and reused in the second cycle. This was repeated for
oxidation over Mn-Ti-SBA-15 (50) at 80 1C showed high conver- 5 cycles and the results are depicted in Fig. 14. The results clearly
sion with high selectivity of the product.
showed that the reused catalyst did not show a significant loss in
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Catalysis Science & Technology
recyclability of the catalyst up to 5 cycles indicated that man-
ganese and titanium species are intact in the mesoporous
framework without leaching.
Acknowledgements
The authors gratefully acknowledge the financial support from
the Department of Science and Technology (DST) (Sanction No.
SR/S1/PC-10/2009, Dated 28.10.2009). One of the authors, Visu-
vamithiran is thankful to DST for the award of SRF position in
this project. The authors are also thankful to UGC-DRS scheme
and DST-FIST programme for creating infrastructural facilities.
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