Selective oxidation of alkylbenzenes to aryl ketones with polymer-supported molybdenum catalyst

Article abstract of DOI:10.14233/ajchem.2015.18827

A new polymer-supported molybdenum-3,5-dimethyl pyrazole catalyst was synthesized and the structure was characterized by FT-IR and SEM. The resulting catalyst presented high activity for selective oxidation of alkyl aromatics to benzylic ketones with tert-butyl hydroperoxide and could be reused at least five times without loss of activity.

Full text of DOI:10.14233/ajchem.2015.18827

Asian Journal of Chemistry; Vol. 27, No. 10 (2015), 3555-3558
A
SIAN
J
OURNAL OF HEMISTRY
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http://dx.doi.org/10.14233/ajchem.2015.18827
Selective Oxidation of Alkylbenzenes to Aryl Ketones
with Polymer-Supported Molybdenum Catalyst
*
Q. XU, B.D. LI and L. CHEN
Department of Chemistry, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, P.R. China
*Corresponding author: Tel/Fax: +86 25 84315514; E-mail: libindong@mail.njust.edu.cn
Received: 29 October 2014;
Accepted: 13 January 2015;
Published online: 22 June 2015;
AJC-17301
A new polymer-supported molybdenum-3,5-dimethyl pyrazole catalyst was synthesized and the structure was characterized by FT-IR and
SEM. The resulting catalyst presented high activity for selective oxidation of alkyl aromatics to benzylic ketones with tert-butyl
hydroperoxide and could be reused at least five times without loss of activity.
Keywords: Polymer-supported, tert-Butyl hydroperoxide, Alkyl aromatics, Oxidation.
reusability limits their application. Hence many researchers
are constantly trying to find a suitable heterogeneous catalytic
system for the oxidation of alkyl aromatics.
INTRODUCTION
Aromatic ketone compounds are not only important
chemical raw materials, but also they are intermediates for the
generation of the useful primary, special chemicals and high
economic value fine chemicals, agrochemicals, pharmaceutical
and perfumes industry1. Many studies have been reported about
selective oxidation of alkyl aromatics to corresponding benzylic
ketones with O22-5, H2O26-8, tert-butyl hydroperoxide (TBHP)9,10
and KMnO411. Habibi and coworkers4 reported the aerobic oxi-
dation of ethyl benzene, cyclohexene and oximes catalyzed
by a SiO2/Al2O3-supported manganese catalyst and NHPI. The
reactions showed higher yields.Velusamy and Punniyamurthy8
reported the oxidation of alkylbenzenes to the corresponding
ketones with copper(II) complex in the presence of 30 % H2O2
with conversion higher than 80 %. Arshadi and coworkers9
reported the oxidation of ethyl benzene to acetophenone with
Co(II)-Schiff base complex in the presence of tert-butyl
hydroperoxide with conversion of 86 % and selectivity of 99 %.
And the following catalyst systems were used for the alkyl
aromatics to ketones, such as metalloporphyrins12, metal phtha-
locyanine13, metal complexes14, transition metal15, Schiff base
complexes16, heteropoly acid6, etc.
In this work, a recyclable polymer-supported molybdenum-
3,5-dimethyl pyrazole catalyst was synthesized in order to
solve the problem of resources waste. tert-Butyl hydroperoxide
is a kind of alkyl organic peroxide of hydrogen. This study
reported one catalytic system based on alkyl aromatics-catalyst-
tert-butyl hydroperoxide for alkyl aromatics oxidation. The
high conversion and selectivity were obtained under optimal
conditions.
EXPERIMENTAL
Chloromethylated polystyrene and ethyl benzene deri-
vatives were purchased from Aladdin. Other chemicals were
purchased from Sinopharm Chemical Reagent Co. Ltd. and
used without further purification. The constituents were
confirmed based on GC analysis. Gas chromatography (GC)
analysis was performed on anAgilent GC-6820 equipped with
a 30 m × 0.32 mm × 0.5 µm HP-Innowax capillary column
and a flame ionization detector. Fourier transform infrared
spectra (FT-IR) were recorded on NICOLET NEXUS870.
Scanning electron micrographs (SEM) of the samples were
taken with ZEISS-DSM 960A microscope with attached
camera.
In order to meet the demand of aromatic ketone compounds,
the new eco-friendly technique of the catalytic oxidation of
alkyl aromatics to ketones under mild conditions has been the
focus of researchers. Green oxidants such as hydrogen per-
oxide, alkyl hydroperoxides and molecular oxygen were used.
Transition metal complex can selectively oxidize organic
compounds under mild conditions17, but the problem of
Preparation of polymer-supported 3,5-dimethyl
pyrazole catalyst [PS-DVB(dmpz)]18: Chloromethylated
polystyrene (2.0 g) was allowed to swell in acetonitrile (10 mL)
for 2 h.A solution of 3,5-dimethyl pyrazole (3.05 g, 31.8 mmol)

3556 Xu et al.
Asian J. Chem.
was prepared in acetonitrile in the presence of equivalent
amount of KI (5.28 g, 31.8 mmol) and triethylamine (3 mL).
This solution was added to the above suspension and the
reaction mixture was heated at 90 °C for 24 h with stirring.
After cooling to room temperature, the brown resins were sepa-
rated, washed several times with hot acetonitrile and dried in
air oven at 110 °C.
the absorptions at 3318 cm-1 was N-H group. In the spectra of
the polymer-supported molybdenum-dmpz catalyst [Fig. 1(b)],
the band at 3318 cm-1 upon N-H disappeared, which indicated
substitution reaction of -CH2Cl with -NH.
100
a
90
Synthesis of polymer-supported molybdenum-dmpz
catalyst PS-DVB[MoO(O2)2(dmpz)2]19: The polymer-
anchored 3,5-dimethylpyrazole PS-DVB (dmpz) (2.5 g) was
allowed to swell in DMF (10 mL) for 2 h. MoO3 (3.8 g, 26.5
mmol) was dissolved in 30 % H2O2 (265 mmol). The solution
was cooled in an ice-bath and then this solution was added in
PS-DVB (dmpz). A red suspension was obtained at this stage
which was stirred for 1 h and then left overnight at about 4 °C
to afford a lemon yellow coloured microcrystalline complex.
Oxidation of alkyl aromatics: In a typical procedure, to
a mixture of catalyst (0.5 mmol), acetonitrile (10 mL) and
alkyl aromatics (5 mmol) were added tert-butyl hydroperoxide
(15 mmol) in turn. Samples were withdrawn periodically.After
required hours, the reaction mixture was cooled to room tempe-
rature.
80
2920
3318
70
60
50
40
30
b
2920
941
941
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm–1)
Fig. 1. FT-IR spectra of (a) unsupported molybdenum-dmpz catalyst and
(b) the polymer-supported molybdenum-dmpz catalyst
RESULTS AND DISCUSSION
To further characterize the catalyst, SEM images were
obtained as shown in Fig. 2. It can be clearly shown that the
pure polymer beads have a smooth, flat surface and particle
diameter is also homogeneous in Fig. 2(a). The morphological
change in the polymer-anchored ligands and immobilized Mo
complexes is quite evident from Fig. 2(b). The image of the
metal complex shows blocky shapes, which may be due to
interaction of the Mo metal with the ligand changed the fixed
geometry of the complex.
Scheme-I shows the sequence of events in the functionali-
zation of PS-DVB with 3,5-dimethyl pyrazole and MoO3. First,
PS-DVB was allowed to swell in acetonitrile that can be used
in activating groups. It is easier to combine the secondary
nitrogen on 3,5-dimethyl pyrazole with -CH2Cl on PS-DVB
in the absence of KI and triethylamine. The brown resins
(Scheme I-2) were obtained. The second step, MoO3 dissolved
in aqueous H2O2 and reacted with the appropriate stoichio-
metric quantities of the brown resins (Scheme I-2), then
coordination polymer was isolated. Coordination polymer
(Scheme-I) was formed through coordination bond between
the imine on dmpz with Mo. The unsupported catalyst was
not considered because it can’t be reused.
Fourier-transform infrared (FT-IR) spectra analysis of
unsupported catalyst and the polymer-supported molybdenum-
dmpz catalyst were reported in Fig. 1. An absorption band at
about 2920 cm-1 attributed to the stretching vibrations of C-H
groups. Band at 1600 cm-1 was assigned to C-N stretching
vibration. Band at 941 cm-1 was assigned to Mo=O stretching
vibration. Band at 796 cm-1 was assigned to O-O stretching
vibration. In the spectra of unsupported catalyst [Fig. 1(a)],
Fig. 2. SEM image of PS-DVB and PS-DVB[MoO(O2)2(dmpz)2]
Ethyl benzene was selected as a model. The activities of
the catalytic system were measured at different molar ratio of
H C
H C
3
3
H C
3
CH
3
CH
CH
3
3
CH Cl
+
2
N
N
CH
CH
CH
2
2
HN
N
N
N
O
O
Mo O
O
O
N
HN
CH Cl
+
2
N
N
CH
N
2
N
2
H C
3
CH
CH
3
CH
3
3
H C
H C
3
3
1
2
3
Scheme-I: Preparation of PS-DVB[MoO(O2)2(dmpz)2]

Vol. 27, No. 10 (2015)
Selective Oxidation of Alkylbenzenes to Aryl Ketones 3557
ETB:catalyst:TBHP and the results were shown in Table-1.
The tests with catalyst were better in conversion compared to
blank test. With increased amount of tert-butyl hydroperoxide,
the conversion of ethyl benzene was enhanced (Table-1, entries
3-6). However, more by-products are produced such as PEA
and BA because of over-oxidation. The separation difficulties
and the cost of experiments are increased. With double amount
of catalyst,AcPO was not easy to be produced at lower tempe-
rature. Instead, more by-products are produced. With prolon-
ging of the reaction time, conversion of ethyl benzene and
selectivity of AcPO were increased at 80 °C (Table-1, entries
7-10). So, with a molar ratio of ETB:catalyst:TBHP at 1:0.2:3,
94 % selectivity of acetophenone and 99 % conversion of ethyl
benzene were obtained at 80 °C for 36 h.
Table-2 summarized the reaction of alkyl aromatics in
the presence of the prepared catalyst, alkyl aromatics were
oxidized to their corresponding ketones under mild conditions.
As can be seen in Table-2, the oxidations of some ethyl benzene
derivatives proceed smoothly and show good selectivity to
acetophenone. Significantly, an electron donating substituent
(e.g. methoxy group, entry 1) at the para-position of ethyl
benzene has a positive effect on the oxidation yield while an
TABLE-1
EFFECT OF AMOUNT OF tert-BUTYLHYDROPEROXIDE AND CATALYST ON ETHYL BENZENE OXIDATIONa
Selectivity (%)b
Conversion
Entry
Ethyl benzene:Catalyst:Oxidant
Temp. (°C)
Time (h)
(%)b
AcPO
37
96
95
77
84
89
92
70
PEA
21
2
2
10
5
1
1
14
7
4
Others
42
2
Blank
1
2
3
4
5
6
7
8
1:0:3
1:0.1:3
1:0.1:3
1:0.1:4.5
1:0.1:4.5
1:0.1:4.5
1:0.1:4.5
1:0.2:3
1:0.2:3
1:0.2:3
1:0.2:3
80
80
80
70
70
80
80
70
70
80
80
24
24
36
24
36
24
36
24
36
24
36
51
89
88
87
93
95
96
87
92
94
99
3
13
11
10
7
16
6
87
93
94
9
10
3
3
3
aReaction conditions: ethyl benzene (5 mmol), acetonitrile (5 mL).
bDetermined by GC and GC-MS analysis based on the internal standard (n-decane).
TABLE-2
OXIDATION OF VARIOUS ALKYL AROMATICS CATALYZED BY
PS-DVB[MoO(O2)2(dmpz)2] UNDER THE INVESTIGATED CONDITIONSa
Entry
1
Substrate
Conversion (%)b
94
Selectivity (%)b
94
Product
O
H CO
3
H CO
3
O
O
2
3
4
5
75
78
68
65
89
89
80
79
Br
Br
O2N
O2N
O
Br
Br
O
NO
2
NO
2
O
6
7
8
92
87
75
87
85
81
O
O
aReaction conditions: substrate (5 mmol), tert-butylhydroperoxide (50 mmol), catalyst (1 mmol), acetonitrile (5 mL), 80 °C, 24 h.
bDetermined by GC and GC-MS analysis based on the internal standard (n-decane).

3558 Xu et al.
Asian J. Chem.
electron withdrawing substituent including the NO2 or Br group
at the para-position suppresses the oxidation (entries 2, 3).
Due to steric effects, all substituents at ortho-position have a
negative effect on the oxidation reaction (entries 4, 5). Alkyl
benzenes containing open chain alkyl group such as n-propyl
benzene and n-butyl benzene underwent the reaction with 92
and 87 % conversion providing corresponding benzylic ketones
with 87 and 85 % selectivity (entries 6, 7). Indane is oxidized
to 1-indanone effectively and selectively, which gives 75 %
conversion and 81 % selectivity (entry 8). These results show
the substrate compatibility of this protocol.
other alkyl aromatics to the corresponding ketones under mild
conditions.As a supported catalyst, it is environmentally friendly
and reusable.
ACKNOWLEDGEMENTS
This project is funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD).
REFERENCES
1. M. Hudlicky, Oxidations in Organic Chemistry, American Chemical
Society, Washington (1990).
PS-DVB[MoO(O2)2(dmpz)2] is slightly soluble.The catalyst
was separated simply by filtering and the resulting clear
supernatant can be decanted.After being washed by methanol,
it was subjected to another reaction with identical substrate.
In this experiment, the supported catalyst can be reused for at
least 5 times at the same reaction conditions without loss of
activity (Fig. 3).
2. F. Rajabi, R. Luque, J.H. Clark, B. Karimi and D.J. Macquarrie, Catal.
Commun., 12, 510 (2011).
3. X.M. Yang, Y. Wang, C.F. Zhang, T. Fang, L.P. Zhou, W. Zhang and J.
Xu, J. Phys. Org. Chem., 24, 693 (2011).
4. D. Habibi, A.R. Faraji, M. Arshadi, H. Veisi and A. Gil, J. Mol. Catal.
A, 382, 41 (2014).
5. Q.H. Zhang, C. Chen, H. Ma, H. Miao, W. Zhang, Z.Q. Sun and J. Xu,
J. Chem. Technol. Biotechnol., 83, 1364 (2008).
6. G.X. Li, Y.Z. Li, R.N. Mu, Y.D. Xu and P. Dong, React. Kinet. Mech.
Catal., 109, 199 (2013).
7. C.L. Lu, Z.H. Fu, Y.C. Liu, F.L. Liu, Y.Y. Wu, J.W. Qin, X.L. He and
D.L. Yin, J. Mol. Catal. A, 331, 106 (2010).
8. S. Velusamy and T. Punniyamurthy, Tetrahedron Lett., 44, 8955 (2003).
9. M. Arshadi, M. Ghiaci, A.A. Ensafi, H. Karimi-Maleh and S.L. Suib,
J. Mol. Catal. A, 338, 71 (2011).
10. S. Gago, S.M. Bruno, D.C. Queirós, A.A. Valente, I.S. Gonçalves and
M. Pillinger, Catal. Lett., 141, 1009 (2011).
11. A. Shaabani, A. Bazgir, F. Teimouri and D.G. Lee, Tetrahedron Lett.,
43, 5165 (2002).
12. R.X. Wang, H.L. Zhu and B.J. Gao, React. Kinet. Mech. Catal., 103,
431 (2011).
13. A. Shaabani, E. Farhangi and A. Rahmati, Appl. Catal. A, 338, 14
(2008).
14. D. Habibi and A.R. Faraji, Appl. Surf. Sci., 276, 487 (2013).
15. J. Luo, H. Yu, H.J. Wang and F. Peng, Catal. Commun., 51, 77 (2014).
16. L. Chen, B.D. Li, Q.X. Xu and D.B. Liu, Chin. Chem. Lett., 24, 849
(2013).
17. M.X. Zhu, X. Wei, B.D. Li and Y.Z. Yuan, Tetrahedron Lett., 48, 9108
(2007).
18. M.R. Maurya, M. Kumar andA.Arya, Catal. Commun., 10, 187 (2008).
19. S. Das, T. Bhowmick, T. Punniyamurthy, D. Dey, M.K. Chaudhuri and
J. Nath, Tetrahedron Lett., 44, 4915 (2003).
Conversion
Selectivity
100
90
80
70
60
50
40
30
20
10
0
1
2
3
4
5
Runs
Fig. 3. Recycling study of PS-DVB[MoO(O2)2(dmpz)2]
Conclusion
The prepared catalyst PS-DVB[MoO(O2)2(dmpz)2]
showed highly activity for the oxidation of ethyl benzene and