Post on 26-Jan-2023
Catalytic activity of MnTUD-1 for liquid phase oxidationof ethylbenzene with tert-butyl hydroperoxide
Gaffar Imran • Muthusamy Poomalai Pachamuthu •
Rajamanickam Maheswari • Anand Ramanathan •
S. J. Sardhar Basha
� Springer Science+Business Media, LLC 2011
Abstract Manganese was incorporated into silica matrix
of TUD-1 using tetraethylene glycol (TEG) as the template.
Three samples with Si/Mn ratio of 115, 44 and 18 were
prepared and characterized by various techniques.
MnTUD-1 is shown to be mesoporous with tetrahedrally
coordinated Mn when Si/Mn = 115; and nano-particles of
manganese oxides are visible at higher loading of manga-
nese (Si/Mn = 44 and 18). The catalytic activity of
MnTUD-1 was explored in the liquid-phase oxidation of
ethylbenzene with tert-butylhydroperoxide (TBHP) as
oxidant. Influence of various reaction parameters such as
time, Si/Mn ratio, oxidant and solvent were studied. Finally
the catalytic activity also compared with well-known
microporous and mesoporous catalysts like MnAlPO-5, Mn
containing MCM-41, MCM-48 and SBA-15.
Keywords Manganese � TUD-1 � Mesoporous �Ethylbenzene � Oxidation � TBHP
1 Introduction
Hydrocarbon oxidation is one of the most fundamental
reactions in organic synthesis. The direct functionalization
of unactivated C–H bonds in saturated hydrocarbons usu-
ally requires drastic conditions such as high pressure and
high temperature. Hence, the catalytic oxidation of alkanes
under mild reaction conditions is an attractive alternative
for the fine chemical industries. Additionally, finding
suitable heterogeneous catalysts to work under these mild
reaction conditions still remains a challenge. Oxidation of
ethylbenzene has been studied with interest because of its
value added products like acetophenone and 1-phenyleth-
anol [1, 2]. Mesoporous materials with their large surface
area ([1,000 m2/g) and tunable pore size distribution
(2–12 nm) are suitable supports for immobilizing active
complex or incorporation of active metal [3, 4]. In partic-
ular manganese incorporated molecular sieves have drawn
considerable attention due to their remarkable activity as
catalyst in various oxidation reactions [5–8]. Recently,
Parida et al. [9], reported the Mn-MCM-41 with various
Mn loading, as catalyst for ethylbenzene oxidation reaction
with good conversion and selectivity. However, the syn-
thesis of the all the above said mesoporous materials
require long chain costly surfactants, difficult synthesis
procedure, poor thermal stability and mainly lower amount
of incorporated manganese due to its leaching in the
medium of synthesis. We have successfully incorporated
Mn into three dimensional amorphous mesoporous silicate,
TUD-1 by employing small, inexpensive and multifunc-
tional template triethanolamine (TEA). These catalysts are
shown to be active in epoxidation of styrene, trans-stilbene
and cyclohexane [10, 11]. Very recently the MnTUD-1 was
synthesized utilizing a similar bifunctional template tetra-
ethylene glycol (TEG) as a mesoporous structure directing
G. Imran � M. P. Pachamuthu
Department of Chemistry, Anna University, Chennai,
Tamil Nadu 600026, India
R. Maheswari (&) � A. Ramanathan
Center for Environmentally Beneficial Catalysis (CEBC),
The University of Kansas, Lawrence, KS 66047, USA
e-mail: maheswarianand@annauniv.edu
S. J. Sardhar Basha
Department of Chemistry, Anna University of Technology
Madurai, Ramanathapuram Campus, Ramanathapuram,
Tamil Nadu, India
123
J Porous Mater
DOI 10.1007/s10934-011-9519-0
agent [12]. A significant advantage of TEG is that higher
metal loadings in siliceous TUD-1 can be achieved [13]. In
this work we extended the application of MnTUD-1 (pre-
pared by TEG template) as catalyst for liquid phase
oxidation of ethylbenzene with TBHP. The effect of vari-
ous reaction parameters were studied in detail.
2 Experimental
2.1 Catalyst preparation
The synthesis and characterization of MnTUD-1 catalysts
under study was discussed in detail in our earlier paper
[12]. Briefly, a solution of manganese (II) acetate tetrahy-
drate is mixed with tetraethyl orthosilicate (TEOS) fol-
lowed by addition of TEG and water. After stirring for
30 min, tetraethyl ammonium hydroxide (TEAOH) was
added drop wise to the above mixture to yield final gel
composition of 1 SiO2: (0.01–0.05) Mn2O3: 1 TEG: 0.5
TEAOH: 11 H2O. The gel obtained was aged at room
temperature for 24 h, dried at 100 �C for 24 h and hydro-
thermal treatment was carried out 180 �C for 8 h in Teflon
lined SS autoclave. Template was removed by calcination
at 600 �C for 10 h. MnTUD-1 with Si/Mn ratios 115, 44
and 18 were synthesized and denoted as MnTUD-1(115),
MnTUD-1(44) and MnTUD-1(18).
For the comparative evaluation of catalytic activity
microporous MnAlPO-5 and mesoporous MnMCM-41 and
MnMCM-48 and MnSBA-15 were also synthesized as per
the well reported procedures [7, 14–16].
2.2 Catalyst characterization
Powder X-ray diffraction patterns of samples were recor-
ded on Rigaku instrument with Cu-Ka (k = 1.54A�) radi-
ation in the 2h range of 10�–80�. Low angle XRD were
carried out on a Brucker D8 instrument. The specific sur-
face area, total pore volume and average pore diameter
were measured by N2 adsorption–desorption method using
Micromeritics ASAP 2020 porosimeter. FTIR spectra of
MnTUD-1 samples were measured on a Bruker (Tensor)
spectrometer in KBr pellets with a resolution of 4 cm-1.
Diffuse reflectance UV–Vis spectra were recorded with
Thermoscientific (Evolution 600) spectrometer with a dif-
fuse reflectance attachment, using BaSO4 as the reference.
Transmission electron microscopy (HR-TEM) was carried
out with HRTEM JEOL 3010 with a UHR pole piece
operates at an accelerating voltage 300 kV. For ESR
measurements, samples were loaded into 3 mm 9 92 mm
suprasil quartz tubes and ESR spectra were recorded at
X-band at room temperature on a Brucker ESP 300 spec-
trometer. The magnetic field was calibrated with a Varian
E-500 gaussmeter. The microwave frequency was mea-
sured by a Hewlett-Packard HP 5342A frequency counter.
Elemental analysis was carried out on ICP-OES (Perkin
Elmer OES Optima 5300 DV spectrometer).
2.3 Ethylbenzene oxidation with MnTUD-1
The catalytic activity of MnTUD-1 catalyst was conducted
in a 50 mL glass round bottom (RB) flask placed in ther-
mostatic oil bath fitted with water cooled condenser. In a
typical reaction, ethylbenzene (10 mmol), chlorobenzene
(2 mmol, internal standard), 10 mL solvent (acetonitrile)
and 100 mg catalyst (activated at 200 �C for 4 h) was
placed in RB flask. Finally, 10 mmol TBHP (70% in water)
was added drop wise to the above mixture. The whole
mixture was continuously stirred at 80 �C. Samples were
withdrawn periodically and analyzed on a GC equipped
with DB-5 capillary column (60 m 9 0.32 mm ID) and
flame ionization detector (FID, 280 �C, N2 as carrier gas
and injector temperature 250 �C). The products were fur-
ther confirmed by GC–MS (HP-5 column).
3 Result and discussion
3.1 Characterizations of Mn-containing TUD-1
The meso-structured character of MnTUD-1 samples was
ascertained by the presence of an intense peak around
0.5�–1� (2h) in the low angle powder XRD (Fig. 1a). No
extraframework Mn-oxides were identified in the high
angle XRD when Si/Mn C 44 (Fig. 1b). Mesoporous nat-
ure of these materials were established by nitrogen sorption
isotherms which showed a typical type IV isotherm with a
H2 type hysteresis loop characteristic of mesoporous
materials (Fig. 2). The BET surface area, pore volume and
Fig. 1 Powder XRD of Mn-TUD-1 samples in a low angle and
b high angle
J Porous Mater
123
pore diameter measured from N2 sorption (Table 1) did not
vary very much with an increase in manganese content.
The disordered worm-hole like arrangement of pores in
MnTUD-1(44) sample is evident from HR-TEM image
(Fig. 3). It has been reported earlier that MnTUD-1(18)
sample have 2–3 nm sized Mn2O3 species from HR-TEM
measurements [12]. Two bands around 270 and 500 nm
were observed for all MnTUD-1 samples in the Diffuse
Reflectance UV–VIS spectra (Fig. 4). The band at 270 nm
is due to the ligand to metal [O2- ? Mn3?] charge transfer
for manganese atoms in tetrahedral coordination. The band
at 500 nm is assigned to 6A1g ? 4T2g crystal field transi-
tions of Mn2? as observed for Mn3O4 or MnO [17]. The
presence of manganese in ?2 and ?3 oxidation states were
evident from these results. Similar observations were also
made for manganese incorporated mesoporous materials
[5, 7, 18]. Six hyperfine lines centered on g value of 2.0093
and with a hyperfine coupling constant (a) of 91G were
observed in EPR spectra of MnTUD-1(44) sample recorded
at room temperature (Fig. 5). A similar environment for
Mn2? species can be suggested from the identical shape of
signal [5, 18, 19].
3.2 Catalytic activity of MnTUD-1 for the oxidation
of ethylbenzene with TBHP
The main products observed during the oxidation of eth-
ylbenzene with TBHP (Scheme 1) are acetophenone (AP),
1-Phenylethanol (1-PE) and Benzaldehyde (PhCHO). The
choice of the oxidant was tert-butylhydroperoxide (TBHP)
as aqueous hydrogen peroxide (H2O2) did not show any
appreciable conversion of the ethylbenzene. The studies on
various reaction parameters show that oxidation rate and
product selectivity in the liquid-phase oxidation of ethyl-
benzene is greatly dependent on a cooperative effect of
the substrate, solvent, oxidant, time and temperature.
Fig. 2 Nitrogen sorption isotherm of MnTUD-1(44) and its pore size
distribution (inset picture)
Table 1 Elemental analysis and sorption characteristics of Mn-TUD-
1 (Si/Mn)
Catalyst (Si/Mn)a SBETb (m2/g) Vp, BJH
c (cc/g) dP, BJHd (nm)
MnTUD-1 (115) 487 0.71 4.7
MnTUD-1 (44) 433 0.67 4.1
MnTUD-1 (18) 493 0.61 4.1
a ICP-OES analysisb SBET = Specific surface areac VP,BJH = Pore volumed dP,BJH = Average adsorption pore diameter
Fig. 3 HR-TEM image of MnTUD-1(44)
Fig. 4 Diffuse reflectance UV–VIS spectra of MnTUD-1 samples
J Porous Mater
123
The influence of the reaction parameters on EB conversion
in the presence of MnTUD-1 catalyst are as follows.
Figure 6 shows catalytic performance of MnTUD-1cat-
alyst in the ethylbenzene oxidation for a time period of 8 h.
In general, ethylbenzene conversion and acetophenone
selectivity increased linearly with time, whereas selectivity
for 1-PE and benzaldehyde decreases with time. It is
interesting to note that formation of 1-PE was considerably
higher over MnTUD-1(18) catalyst and it passes through a
maximum. This trend suggests that 1-PE must be primarily
formed product [20–22]. Distant oxygen of TBHP is acti-
vated by its chemisorption on MnTUD-1 which then reacts
with ethylbenzene yielding 1-PE. Two pathways are now
possible: (a) abstraction –OH hydrogen and –CH hydrogen
by the activated t-butylhydroperoxide oxygen yielding
acetophenone and (b) abstraction of –OH hydrogen of
1-phenylethanol by the activated t-butyl hydroperoxide
yielding benzaldehyde [9, 22]. Since AP was obtained as
the major product, it can be established that route (a) pre-
dominates. The conversion also increased with increase in
manganese content. MnTUD-1(115) showed EB conver-
sion of 18.5% after 8 h, whereas MnTUD-1(18) displayed
higher EB conversion (40%) during the same time. Apart
from the presence of manganese in isolated tetrahedral
sites, finely dispersed Mn2O3 nanoparticles coexists in
MnTUD-1(18) catalyst which may be responsible for the
observed higher conversion of ethylbenzene. Similar trends
were observed for these catalysts in cyclohexane oxidation
reaction [12]. However, selectivity of AP decreased with
increase in Mn content, suggesting that isolated tetrahedral
Mn sites are responsible for conversion of 1-PE to AP
selectively. Even though MnTUD-1(18) catalyst showed
better performance, in order to understand the effect of
other reaction parameters, MnTUD-1(44) was chosen as
Scheme 1 Ethylbenzene oxidation over MnTUD-1 catalyst
Fig. 5 EPR spectra of MnTUD-1(44)
Fig. 6 EB oxidation and
product selectivites over
MnTUD-1 catalyst at 80 �C.
Reaction conditions:
EB = 10 mmol,
TBHP = 10 mmol,
acetonitrile = 10 mL and
MnTUD-1 = 100 mg
J Porous Mater
123
this catalyst has higher amount of isolated Mn sites and no
extraframework Mn2O3 species.
In general, the role of solvent is very complex in liquid
phase reactions especially in the product distribution.
Commonly used solvents such as acetonitrile, ethylacetate,
DMSO, DMF and ethanol were employed and their role as
solvent was examined (Table 2). Acetonitrile displayed
significantly higher EB conversion (19.8%) compared to
other solvents under study. Based on the EB conversion, the
solvents were arranged in the decreasing order as follows:
acetonitrile [ ethyl acetate [ DMSO [ DMF [ ethanol.
This can be attributed to competitive adsorption between
the EB and solvent molecules for the active Mn sites.
Interestingly, 1-PE was produced as major project when
DMSO is used as solvent. Selectivity towards AP in dif-
ferent solvents follows the order, ethanol [ DMF [ ace-
tonitrile [ ethylacetate [ DMSO. Similar solvent effects
were also observed for Co-MCM-41 [23].
The amount of TBHP was varied and its effect on EB
conversion and product selectivity is listed in Table 3. EB
conversion increased with time for various amount of
TBHP under study. With an increase in TBHP/EB ratio
from 1 to 3, EB conversion was nearly doubled (*28%)
after 5 h suggesting that higher EB conversion can be
achieved in shorter time by employing higher concentra-
tion of TBHP. On the other hand no reasonable difference
was observed in product selectivites.
The activity of MnTUD-1 is also compared with other
well-known ordered microporous MnAlPO-5 and Mn
containing ordered mesoporous MCM-41, MCM-48 and
SBA-15 materials and the results are listed in Table 4. The
EB conversion is also normalized for total amount of Mn
present in the sample. The isolated framework incorporated
Mn sites are highly active irrespective of amorphous or
ordered mesoporous material as noticed form TON (turn
over number). MnAlPO-5 showed relatively poor EB
conversion compared to other Mn catalysts. This could be
due to lack of reactant diffusion into the active sites of
microporous catalyst. It has also been reported that for
similar loading of metal, mesoporous supports is found to
be superior to that of a microporous support [24]. However,
the activities of Mn containing ordered mesoporous mate-
rials were slightly lower than disordered MnTUD-1 cata-
lyst. A significantly higher amount of benzaldehyde was
produced over Mn containing ordered mesoporous silicas.
The amorphous three-dimensional pore structure of TUD-1
with wide pore size distribution decreases the pore diffu-
sion resistance and enhances greater accessibility of the
reactants to the manganese active sites present in the pore
walls.
4 Conclusion
We have demonstrated that manganese containing amor-
phous three dimensional mesoporous silicate MnTUD-1, as
a good catalyst for ethylbenzene oxidation with TBHP. The
co-existence of Mn2? and Mn3? in MnTUD-1 samples was
evident from diffuse reflectance UV–Vis study. All Mn2?
Table 2 Effect of various solvent in EB conversion and product selectivity over MnTUD-1(44) catalyst
Solvent Conv. EB Selectivity (%)
AP 1-PE PhCHO Others
Acetonitrile 19.8 60.8 26.6 12.6 0.0
Ethyl acetate 13.7 65.8 21.3 10.3 2.6
DMSO 11.4 35.6 44.6 19.8 0.0
DMF 1.4 68.8 0.0 31.3 0.0
Ethanol 0.6 100.0 0.0 0.0 0.0
Others = mainly benzoic acid
Reaction conditions: MnTUD-1 (44) = 100 mg, T = 80 �C, EB:TBHP = 1:1, solvent = 10 mL, time = 8 h
Table 3 Effect of amounts of TBHP in EB conversion and product
selectivity over MnTUD-1(44) catalyst
TBHP/EB
mole ratio
Time (h) Conv. EB Selectivity (%)
AP 1-PE PhCHO
0.5 1 6.1 57.6 28.8 13.6
3 9.3 59.2 28.4 12.4
5 11.5 60.0 28.7 11.3
1 1 6.2 57.0 28.5 14.5
3 12.2 57.4 28.0 14.6
5 15.9 58.7 27.8 13.5
2 1 6.5 54.2 24.5 21.3
3 13.0 57.8 24.9 17.3
5 20.1 58.5 25.2 16.3
3 1 12.5 53.3 29.7 17.0
3 20.9 51.9 25.7 22.4
5 27.8 53.1 24.9 22.0
Reaction conditions: MnTUD-1 (44) = 100 mg, T = 80 �C,
acetonitrile = 10 mL
J Porous Mater
123
were found to exist in similar environment as shown by
EPR characterization. MnTUD-1 exhibits different cata-
lytic sites; at lower Mn-loading only isolated species could
be detected while at high Mn-loading Mn2O3 embedded
inside the mesopores of the TUD-1 could be easily iden-
tified. It was shown that EB conversion increased with
increase in Mn content, time and oxidant amount. Primarily
produced 1-PE was immediately converted to AP via –OH
hydrogen and –CH hydrogen by the activated t-butylhy-
droperoxide oxygen which is the predominant route over
MnTUD-1 catalysts. Acetonitrile is found to be suitable
solvent for EB oxidation and it was also demonstrated that
the MnTUD-1 showed better catalytic activity when com-
pared with other ordered mesoporous and microporous
materials.
Acknowledgments We gratefully acknowledge support from DST
SERC Fast Track Schemes (CS-41 and CS-44). The authors G. Imran
and M.P. Pachamuthu are thankful for JRF support from DST SERC
Fast Track Scheme.
References
1. H. Siegel, M. Eggersdorfer, ‘‘Ketones’’, Ullmann’s Encyclopediaof Industrial Chemistry (Wiley-VCH, Weinheim, 2005)
2. A.E. Shilov, G.B. Shulpin, Chem. Rev. 97, 2879 (1997)
3. A. Sayari, Chem. Mater. 8, 1840 (1996)
4. A. Corma, Chem. Rev. 97, 2373 (1997)
5. Q. Zhang, Y. Wang, S. Itsuki, T. Shishido, K. Takehira, J. Mol.
Catal. A: Chem. 188, 189 (2002)
6. S. Gomez, O. Giraldo, L.J. Garces, J. Villegas, S.L. Suib, Chem.
Mater. 16, 2411 (2004)
7. G. Satish Kumar, M. Palanichamy, M. Hartmann, V. Murugesan.
Microporous Mesoporous Mater. 112, 53 (2008)
8. N.Z. Logar, N.N. Tusar, G. Mali, M. Mazaj, I. Arcon, D. Arcon,
A. Recnik, A. Ristic, V. Kaucic, Microporous Mesoporous Mater.
96, 386 (2006)
9. K.M. Parida, S.S. Dash, J. Mol. Catal. A: Chem 306, 54 (2009)
10. A. Ramanathan, T. Archipov, R. Maheswari, U. Hanefeld, E.
Roduner, R. Glaser, J. Phys. Chem. C. 112, 7468 (2008)
11. R. Anand, M.S. Hamdy, R. Partone, T. Maschmeyer, J.C. Jansen,
R. Glaser, F. Kapteijn, U. Hanefeld, Aust. J. Chem. 62, 360
(2009)
12. R. Maheswari, R. Anand, G. Imran, J. Porous Mater. doi
10.1007/s10934-011-9472-y
13. C. Simons, U. Hanefeld, I.W.C.E. Arends, R.A. Sheldon, Th.
Maschmeyer, Chem. Eur. J. 10, 5829 (2004)
14. H. Nur, H. Hamdan, Mater. Res. Bull. 36, 315 (2001)
15. K.M. Parida, S.S. Dash, S. Singha, Appl. Catal. A: Gen. 351, 59
(2008)
16. J. Xu, Z. Luan, M. Hartmann, L. Kevan, Chem. Mater. 11, 2928
(1999)
17. S. Velu, N. Shah, T.M. Jyothi, S. Sivasanker, Microporous
Mesoporous Mater. 33, 61 (1999)
18. K.M. Parida, S.S. Dasha, S. Singh, Appl. Catal. A: Gen. 351, 59
(2008)
19. S. Vetrivel, A. Pandurangan, J. Mol. Catal. A: Chem. 246, 223
(2006)
20. K. George, S. Sugunan, Catal. Commun. 9, 2149 (2008)
21. M. Zhu, X. Wei, B. Li, Y. Yuan, Tetrahedron Lett. 48, 9108
(2007)
22. T. Radhika, S. Sugunan, Catal. Commun. 8, 150 (2007)
23. S.S. Bhoware, A.P. Singh, J. Mol. Catal. A: Chem. 266, 118
(2007)
24. X.Y. Quek, Q. Tang, S. Hu, Y. Yang, Appl. Catal. A: Gen. 361,
130 (2009)
Table 4 Effect of different supports on catalytic activity of ethylbenzene oxidation
Catalyst EB conversion (%) Selectivity (%) TON 9 102
AP 1-PE PhCHO
MnTUD-1(114) 18.5 62.6 27.8 9.6 230.07
MnTUD-1(44) 19.8 60.8 26.6 12.6 95.04
MnTUD-1(18) 39.6 56.4 22.6 21.0 77.76
MnSBA-15(50) 16.1 51.2 17.6 31.2 87.82
MnMCM-48(100) 18.7 55.1 21.2 23.7 204.00
MnMCM-41(50) 17.1 46.2 17.4 36.4 93.27
MnAlPO-5 (2%) 10.6 61.0 22.9 16.1 57.82
Reaction condition: ethylbenzene:TBHP = 1:1 mol ratio (each 10 mmol); catalyst = 100 mg, acetonitrile = 10 mL, temperature = 80 �C,
time = 8 h
TON turn over number
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123