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

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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

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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

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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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