Selective benzylic oxidation of alkyl substituted aromatics to ketones over Ag/SBA-15 catalysts

Article abstract of DOI:10.1016/j.catcom.2012.02.023

Benzylic and cycloalkane CH bonds have selectively been oxidized into corresponding ketones with t-BuOOH over Ag/SBA-15 catalysts, which were prepared by varying the loading of Ag (2, 4, 6 and 8% by weight) on SBA-15 support using an impregnation method. The retention of mesoporous structural ordering and crystalline behavior of Ag have been confirmed by N₂ adsorption and XRD studies. 4Ag/SBA-15 catalyst is found to be the best catalyst among the Ag/SBA-15 series. The influence of various parameters such as oxidant, solvent, temperature and time of reaction etc. have been systematically studied on Ag/SBA-15 catalyst.

Full text of DOI:10.1016/j.catcom.2012.02.023

Catalysis Communications 23 (2012) 5–9
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Selective benzylic oxidation of alkyl substituted aromatics to ketones over
Ag/SBA-15 catalysts
Narani Anand, Kannapu Hari Prasad Reddy, Ganjala Venkata Siva Prasad,
Kamaraju Seetha Rama Rao, David Raju Burri ⁎
Catalysis Laboratory, Indian Institute of Chemical Technology, Hyderabad-500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzylic and cycloalkane C\H bonds have selectively been oxidized into corresponding ketones with t-
BuOOH over Ag/SBA-15 catalysts, which were prepared by varying the loading of Ag (2, 4, 6 and 8% by
weight) on SBA-15 support using an impregnation method. The retention of mesoporous structural ordering
and crystalline behavior of Ag have been confirmed by N adsorption and XRD studies. 4Ag/SBA-15 catalyst is
2
found to be the best catalyst among the Ag/SBA-15 series. The influence of various parameters such as oxi-
Received 8 December 2011
Received in revised form 17 February 2012
Accepted 22 February 2012
Available online 3 March 2012
dant, solvent, temperature and time of reaction etc. have been systematically studied on Ag/SBA-15 catalyst.
Keywords:
©
2012 Elsevier B.V. All rights reserved.
Ag/SBA-15 catalysts
Selective oxidation
Alkyl aromatics
Cycloalkanes
1
. Introduction
SBA-15 supported silver catalysts for the oxidation of benzylic C\H
bonds have not yet been studied. Instead, other supported silver cat-
alysts have been extensively studied as oxidizing catalysts and have
been applied industrially to the epoxidation of ethylene [13,14]. It
has also been recognized to show high activity in several reactions,
such as hydrogenation of unsaturated aldehydes [15], partial oxida-
tion of methanol to formaldehyde [16], and oxidative coupling of
methane to ethane and ethylene [17], CO oxidation etc. [18,19].
Being an eco-friendly and high performance catalyst for the ben-
zylic and cycloalkane C\H bonds oxidation, herein, the details of cat-
alyst preparation, characterization and evaluation studies of Ag/SBA-
15 has been delineated.
Oxidation of benzylic C\H bond into corresponding ketone is one
of the important transformations in organic synthesis [1], since, the
oxidation products are essential intermediates for the manufacture
of high-value fine chemicals, agrochemicals, pharmaceutical and
high-tonnage commodity chemicals [2,3]. Generally, benzylic C\H
bond oxidations are carried out with a large excess of metal based
oxidants such as chromium reagents and manganese reagents [1].
Discharging of toxic metal residues, waste disposal and involvement
of tedious workup procedures are the main disadvantages often en-
countered in the classical benzylic oxidations. At this juncture, devel-
opment of eco-friendly supported catalysts those can be operated
at mild conditions in combination with appropriate stoichiometric
oxidant is the subject of interest.
2. Experimental
Of late, mesoporous material based supported catalysts have been
proven to be the most ideal catalysts because of their high surface
area, high porosity, large and uniform channel size etc. [4–8]. These
structural and textural characteristics of mesoporous supports pro-
vide a consistent and well-isolated environment for the fine deposi-
tion of active components and free access to the reactants [4].
Among the mesoporous material based catalysts, SBA-15 supported
catalysts are under intensive investigation recently [9–12]. However,
2.1. Catalyst preparation
The siliceous SBA-15 has been synthesized in accordance with the
literature procedures [20]. A solution of EO20PO70EO20:2M HCl: TEOS:
H O=2:60:4.25:15 (mass ratio) was prepared, stirred for 12 h at
2
40 °C and then hydrothermally treated at 100 °C under static condi-
tion for 12 h, subsequently filtered, dried at 100 °C and calcined at
5
00 °C for 8 h to get the parent SBA-15 mesoporous silica support.
The Ag/SBA-15 catalysts with the silver loading of 2, 4, 6 and 8 wt.%
were deposited by impregnating the SBA-15 support with an aqueous
solution of silver nitrate followed by evaporation to dryness at 100 °C
for 12 h, calcination in air at 500 °C for 3 h and reduced at 300 °C in H
2
⁎
Corresponding author. Tel.: +91 40 27191712; fax: +91 40 27160921.
E-mail address: david@iict.res.in (D.R. Burri).
flow for 1 h.
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.02.023

6
N. Anand et al. / Catalysis Communications 23 (2012) 5–9
Table 1
Textural and structural characteristics of SBA-15 Ag/SBA-15 catalysts derived from N
adsorption–desorption isotherms and low-angle XRD analysis respectively.
2
Catalyst
S
BET
2
V
t
D
(nm)
BJH
d
100
(nm)d
a
0
t(nm)f
a
3
b
c
e
(m /g)
(cm /g)
(nm)
SBA-15
665
552
534
527
454
1.01
0.88
0.83
0.86
0.78
5.9
5.8
5.9
6.1
6.2
9.7
9.5
9.1
9.7
9.2
11.2
11.0
10.5
11.2
10.5
7.1
6.9
6.4
7.1
6.6
2
4
6
8
Ag/SBA-15
Ag/SBA-15
Ag/SBA-15
Ag/SBA-15
a
b
c
d
e
f
BET surface area.
The total pore volume.
BJH average pore diameter.
Periodicity of SBA-15 derived from low angle XRD.
The unit cell parameter (a =2 d100/√3).
The pore wall thickness (t=a −DBJH).
0
0
3
. Results and discussion
.1. N adsorption studies
The N adsorption–desorption isotherms for the Ag/SBA-15 cata-
2
Fig. 1. N adsorption–desorption isotherms of Ag/SBA-15 catalysts and their pore size
distribution curves as in set.
3
2
2
lysts are shown in Fig. 1, which are of type IV adsorption isotherms
with a H1 hysteresis loops, according to the IUPAC classification.
These adsorption isotherms are having three well-distinguished
2
.2. Catalyst characterization
adsorption–desorption isotherms were recorded for calcined
N
2
regions P/P
0
=0–0.61 (monolayer multilayer adsorption), P/P
0
=
catalysts using an ASAP 2020 V3.01 H adsorption unit (Micromeritics,
USA). The X-ray diffraction (XRD) patterns were recorded at room
temperature using a Rigaku, Multiflex, diffractometer with a nickel
filtered CuKα radiation. Transmission electron microscope (TEM)
analysis was made using a Philips Technai G2 FEI F12 at an accelerat-
ing voltage of 80–100 kV.
0.61–0.75 (capillary condensation) and P/P
0
=0.75–1.0 (multilayer
adsorption on the external surface). The textural parameters like
BET surface area, total pore volume, and average pore diameter deter-
mined from the BJH method are displayed in Table 1. There is a
diminishing trend both in BET surface area and total pore volume
with increase in the loading of Ag in Ag/SBA-15 catalysts series,
which are expected due to the agglomeration of Ag particles and
thereby partial blockage of pores. The pore size distribution curves
are depicted in Fig. 1 as an inset, which are narrowly distributed
with the distribution maxima at about 7.1 nm.
2
.3. Evaluation of catalytic activity
The catalytic activity was determined in the liquid phase under sol-
vent free conditions. In a typical experiment, 1 mmol of substrate and
0 mg of catalyst were taken in a 25 ml RB flask and added 3 mmol of
3.2. X-ray diffraction analysis
5
aqueous t-BuOOH slowly under constant stirring at 90 °C for 5 h. The
influence of solvents on the reaction was also studied taking 2 ml of
solvents. The products were identified and analyzed by GCMS–QP-
The low-angle X-ray diffraction patterns of Ag/SBA-15 catalysts
along with parent siliceous SBA-15 are shown in Fig. 2, which exhibited
three typical diffraction lines at 0.91–0.97°, 1.60–1.66 and 1.86–1.92°
respectively on the 2θ scale that are indexable as (100), (110) and
(200) reflections associated with p6mm hexagonal symmetry. The in-
corporation of the different amounts of Ag on the hexagonally ordered
mesoporous SBA-15 did not considerably alter the structural ordering.
5
050 (M/s. Shimadzu Instruments, Japan) with ZB-5 capillary column
(
25 m×0.32 mm) supplied by M/s. J & W Scientific, USA. Toluene was
used as an external standard for the quantification of the products.
Fig. 2. Low-angle XRD patterns of Ag/SBA-15 catalysts.
Fig. 3. Wide-angle XRD patterns of Ag/SBA-15 catalysts.

N. Anand et al. / Catalysis Communications 23 (2012) 5–9
7
Fig. 4. TEM image and Ag particle size distribution (inset) of 4Ag/SBA-15 catalyst.
The wide-angle XRD spectra for the Ag/SBA-15 catalysts are
shown in Fig. 3, which reveals that there are four well-distinguished
diffraction lines on the 2θ scale at 38.11, 44.25, 64.46 and 77.35° re-
spectively, these are indexable as (111), (200), (220) and (311), cor-
responding to cubic Ag metallic (JCPDS No. 4-07830) crystals.
of high selectivity (100%) has also been reported in the biphasic oxi-
dation with perfluoroalkyl phthalocyanine complexes [23]. Compared
to 4Ag/SBA-15 catalyst the remaining two higher loading catalysts
exhibited lower selectivity toward acetophenone, which may be due
to agglomeration of Ag particles. The increased particle size with in-
crease in Ag loading and decreased acetophenone selectivity can be
seen from Table 2. In the subsequent study, other reaction parameters
were optimized using 4Ag/SBA-15 catalyst.
Wide range of literature reveals that the selective production of
acetophenone is a challenge on heterogeneous catalysts. High selec-
tivity towards acetophenone is mainly hindered due to the formation
other oxygenates via different oxidation sites depending on the type
of catalyst used [24]. The other products that are generally formed
from ethylbenzene oxidation are benzaldehyde, benzoic acid, pheny-
lacetic acid, styrene, 1-phenylethane-1,-2-diol p-acetophenone,
m-acetophenone etc. [25,26].
As evidenced from Table 3, the conversion of ethylbenzene and
the selectivity of acetophenone are substantially increased with reac-
tion time from 1 to 5 h, whereas the selectivity of 1-phenyl ethanol
decreased, which implies that the major product (i.e. acetophenone)
has been obtained via 1-phenyl ethanol. The impact of reaction tem-
perature in the oxidation of ethylbenzene can also be seen from
Table 3. At room temperature, the conversion of ethylbenzene is
around 31%, which has been rose up to 63% when the reaction tem-
perature increased to 60 °C. Surprisingly, the selectivity of acetophe-
none and 1-phenyl ethanol are in the same molar ratio (71:29) at
25 and 60 °C. Similar trend is observed at lower conversion levels,
i.e., at 1 and 2 h reaction time. At 90 °C the conversion has
The morphology of Ag nanoparticles (4Ag/SBA-15) obtained from
TEM analysis was displayed in Fig. 4, which demonstrates that most
of the Ag nanoparticles are in prolate ellipsoidal shape. The others
are in the form of spheres, which are lower in number. Due to se-
quential assemblage of prolate ellipsoidal shaped Ag nanoparticles
the rope/wire like morphology was generated. It is evident that nano-
wires/nonorods of different metal nanoparticles are produced within
the hexagonally ordered pores of SBA-15 support [21,22]. In the pre-
sent study the size of nanoparticles that are seen from TEM image is
bigger than that of the pore channels of SBA-15 support. The deposi-
tion of certain amount of Ag nanoparticles within the pores of SBA-15
may not be excluded. The histogram shown in Fig. 4 as an inset de-
scribes the particle size distribution. According to particle size distri-
bution histogram most of the nanoparticles of Ag deposited on SBA-
1
5 support are in the range of 30–50 nm.
3
.3. Oxidation of benzylic C\H bond of ethylbenzene
Initially, the catalytic activity of various Ag/SBA-15 catalysts to-
ward the oxidation of ethylbenzene to acetophenone has been inves-
tigated and depicted in Table 2, which reveals that the conversion of
ethylbenzene is >90% on all Ag/SBA-15 catalysts series except over
2
Ag/SBA-15 catalyst. The lower conversion of 2Ag/SBA-15 catalyst
may be due to insufficient amount of Ag. The conversions of ethylben-
zene on all the three higher loading catalysts (4Ag/SBA-15, 6Ag/SBA-
Table 3
1
5 and 8Ag/SBA-15) are more or less equal, but 4Ag/SBA-15 catalyst
Effect of reaction time and temperature on the oxidation of ethyl benzene over
4Ag/SBA-15 catalysts at 90 °C.
exhibited superior acetophenone selectivity (99%). The similar kind
Reaction time (h)
Conversion (%)
ACP selectivity (%)a
PE selectivity (%)b
Table 2
1
2
3
4
5
48
60
73
81
92
31
63
69
68
84
94
99
71
71
31
32
16
6
1
29
29
Effect of Ag loading on the oxidation of ethyl benzene over Ag/SBA-15 catalysts at 90 °C
for 5 h using t-BuOOH as oxidant under solvent free conditions.
Crystallite size (nm)a
ACP selectivity (%)b
Catalyst
Conversion (%)
c
2
4
6
8
Ag/SBA-15
Ag/SBA-15
Ag/SBA-15
Ag/SBA-15
39
37
57
65
73
92
93
93
93
99
77
80
5
5
d
a
ACP = Acetophenone.
PE = 1-phenyl ethanol.
Reaction at 25 °C.
b
c
a
Determined from Scherrer equation.
Acetophenone.
b
d
Reaction at 60 °C.

8
N. Anand et al. / Catalysis Communications 23 (2012) 5–9
Table 4
Effect of solvent on the oxidation of ethyl benzene over 4Ag/SBA-15 catalysts using 70%
t-BuOOH as oxidant at 90 °C.
Solvent
Conversion (%)
ACP selectivity (%)a
PE selectivity (%)b
Toluene
DMF
DMSO
Acetonitrile
Water
DCE
25
15
52
85
65
52
92
92
80
77
86
89
62
99
8
20
32
12
10
38
1
No solvent
a
ACP = Acetophenone.
PE = 1-phenyl ethanol.
b
significantly increased from 63 to 92%, but there is dramatic change
in the selectivity ratio (99:1), which reinforces transformation of
1
-phenyl ethanol into acetophenone at higher conversion levels.
To optimize the oxidant, different oxidants such as aqueous 30% H
2 2
O ,
4 2 2 8
urea hydrogen peroxide, NaIO , K S O , molecular oxygen and t-BuOOH
have been used. When the reaction was conducted using t-BuOOH as ox-
idant both conversion and selectivity are 92% and 99% respectively, but
with other oxidants no appreciable conversion. Hence, t-BuOOH is the
most appropriate oxidant among the oxidants used in this study.
The oxidation activity data of 4Ag/SBA-15 catalyst with different sol-
vents is presented in Table 4, which reveals that the catalyst is highly ef-
fective under solvent-free conditions rather than with solvents. With all
the solvents used the catalyst is active, comparatively acetonitrile seems
to be better solvent. Interestingly, even in the water medium, the con-
versions and selectivities are significantly higher.
Scheme 1. Proposed C\H bond oxidation mechanism in the presence of t-BuOOH.
of 4Ag/SBA-15 towards the oxidation of halogen substituted ethyl-
benzenes at p-position is excellent, but little lower compared to eth-
ylbenzene (entries 2–4). The catalyst exhibited outstanding
performance in the case of electron donating group at p-position i.e.
p-methoxyethylbezene (entry 5), similar trend has already been
reported [27]. With increase in annelated aromatic rings, the decrease
in activity is observed (entries 1, 7 and 8). In the case of diarylmethy-
lenes like diphenylmethane and fluorine including indane are selec-
tively oxidized at benzylic position with high yields (entries 9–11),
which is well matched with the literature [28]. For the oxidation of
cycloalkanes the catalyst is not so effective in producing cyclic ke-
tones (entries12–14).
3
.4. Oxidation of benzylic and cycloalkane C\H bonds
The optimized conditions have been applied for the other sub-
strates and the details are depicted in Table 5. The catalytic activity
Table 5
Selective oxidation of different benzylic and cycloalkane C\H bonds over 4Ag/SBA-15 catalyst under solvent-free conditions.
Entry
1
Substrate
Product
Time (h)
5
Conversion (%)
92
Selectivity (%)
99
2
3
4
5
6
5
5
5
5
5
88
87
85
97
90
95
94
95
99
93
7
8
9
7
24
5
90
45
95
99
99
96
73
99
96
95
1
1
0
1
5
5
1
2
3
24
24
43
40
72
69
1
14
24
48
70
Substrate (1 mmol), t-BuOOH (3 mmol), catalyst (50 mg), temperature (90 °C).

N. Anand et al. / Catalysis Communications 23 (2012) 5–9
9
[
[
[
[
10] N. Anand, K.H.P. Reddy, V. Swapna, K.S. Rama Rao, D.R. Burri, Microporous and
The benzylic oxidation of C\H bond into corresponding ketone in
the presence t-BuOOH followed the free radical mechanism. To verify
this, an experiment was conducted by adding 4-t-butyl catechol
to the reaction mixture. Due to addition of a free radical scavenger
Mesoporous Materials 143 (2011) 132–140.
11] N. Anand, K.H.P. Reddy, K.S. Rama Rao, D.R. Burri, Catalysis Letters 141 (2011)
1355–1363.
12] G. Saidulu, N. Anand, K.S. Rama Rao, B. Abhishek, S.-E. Park, D.R. Burri, Catalysis
Letters 141 (2011) 1865–1871.
13] D.J. Guo, H.L. Li, Carbon 43 (2005) 1259–1264.
(
4-t-butyl catechol) the conversion of ethylbenzene dropped signifi-
cantly from 92 to 20%, which provides some information in support
of a free radical mechanism for the benzylic C\H bond oxidation.
On the basis of published literature [29–31] and the present results
a mechanism is proposed as detailed in Scheme 1.
[14] J.Q. Lu, J.J. Bravo-Suarez, A. Takahashi, M. Haruta, S.T. Oyama, Journal of Catalysis
232 (2005) 85–95.
[
15] A.K. Datye, D.J. Smith, Catalysis Reviews: Science and Engineering 34 (1992)
29–178.
[16] X. Bao, M. Muhler, B. Pettinger, R. Schlogl, G. Ertl, Catalysis Letters 22 (1993)
15–225.
1
2
[
[
17] X. Bao, M. Muhler, R. Schlogl, G. Ertl, Catalysis Letters 32 (1995) 185–194.
18] Z.P. Qu, M.J. Cheng, W.X. Huang, X.H. Bao, Journal of Catalysis 229 (2005)
4
. Conclusions
446–458.
[
[
19] D. Tian, G.P. Yong, Y. Dai, X.Y. Yan, S.M. Liu, Catalysis Letters 130 (2009) 211–216.
20] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky,
Science 279 (1998) 548–552.
Ag/SBA-15 catalysts are proved to be useful eco-friendly catalysts
for the oxidation of benzylic and cyclic C\H bond into corresponding
ketones with t-BuOOH as an oxidant under solvent-free conditions.
[21] Y.-J. Han, J.M. Kim, G.D. Stucky, Chemistry of Materials 12 (2000) 2068–2069.
[
22] Y. Wu, T. Livneh, Y.X. Zhang, G. Cheng, J. Wang, J. Tang, M. Moskovits, G.D. Stucky,
Nano Letters 4 (2004) 2337–2342.
References
[23] T. Qiu, X. Xu, X. Qian, Journal of Chemical Technology and Biotechnology 84
2009) 1051–1055.
(
[
[
[
1] M. Hudlicky, Oxidations in Organic Chemistry, ACS Monograph No.186, American
Chemical Society, Washington DC, 1990.
2] R.A. Sheldon, H. Bekkum, Fine chemical through heterogeneous catalysis
[24] T.H. Bennur, D. Srinivas, S. Sivasanker, Journal of Molecular Catalysis A 207 (2004)
163–171.
[25] T.K. Das, K. Chaudhari, E. Nandanan, A.J. Chandwadkar, A. Sudalai, T.
Ravindranathan, S. Sivasanker, Tetrahedron Letters 38 (1997) 3631–3634.
[26] M.R. Maurya, M. Kumar, U. Kumar, Journal of Molecular Catalysis A 273 (2007)
133–143.
(Chapter 9), Wiley-VCH, Weinheim, 2001.
3] H.A. Wittcoff, B.G. Reuben, J.S. Plotkin, Industrial organic chemical, Wiley, New
Jersey, 2004 (ed.).
[
[
4] A. Taguchi, F. Schuth, Microporous and Mesoporous Materials 77 (2005) 1–45.
5] D.R. Burri, Ki-W. Jun, Y.-H. Kim, J.M. Kim, S.-E. Park, J.S. Yoo, Chemistry Letters
[27] Y. Bonvin, E. Callens, I. Larrosa, D.A. Henderson, J. Oldham, A.J. Burton, A.G.M.
Barrett, Organic Letters 7 (2005) 4549–4552.
(
2002) 212–213.
[28] S.-I. Murahashi, N. Komiya, Y. Oda, T. Kuwabara, T. Naota, Journal of Organic
Chemistry 65 (2000) 9186–9193.
[
[
[
[
6] D.J. Kim, B.C. Dunn, P. Cole, G. Turpin, R.D. Ernst, R.J. Pugmire, M. Kang, J.M. Kim, E.M.
Eyring, Chemical Communications (2005) 1462–1464.
7] S.S. Reddy, D.R. Burri, V.S. Kumar, A.H. Padmasri, S. Narayanan, K.S. Rama Rao,
Catalysis Communications 8 (2007) 261–266.
[29] P. Lignier, F. Morfin, L. Piccolo, J.-L. Rousset, V. Caps, Catalysis Today 122 (2007)
284–291.
[30] P.P. Toribio, A.G. Gargallo, M.C. Capel-Sanchez, M.P. de Frutos, J.M. Campos-
Martin, J.L.G. Fierro, Applied Catalysis A: General 363 (2009) 32–39.
[31] I. Hermans, J. Peeters, P.A. Jacobs, Journal of Organic Chemistry 72 (2007)
3057–3064.
8] D.R. Burri, I.R. Shaikh, K.-M. Choi, S.-E. Park, Catalysis Communications 8 (2007)
731–735.
9] S.S. Reddy, D.R. Burri, A.H. Padmasri, P.K.S. Prakash, K.S. Rama Rao, Catalysis
Today 141 (2009) 61–65.