Liquid-phase oxidation of cyclohexane to cyclohexanone over cobalt-doped SBA-3

Article abstract of DOI:10.1016/j.catcom.2010.01.026

Liquid-phase oxidation of cyclohexane was carried out under mild reaction conditions over mesoporous Co/SBA-3 catalyst using aqueous hydrogen peroxide (30%) as oxidant and acetic acid as solvent without adding any initiator. The catalyst exhibited high substrate conversion (91.6%) and reasonable product (cyclohexanone) selectivity (64.3%). Fast hot catalyst filtration experiment proved that the catalyst acted as a heterogeneous one and it can be reused two times without losing its activity to a greater extent.

Full text of DOI:10.1016/j.catcom.2010.01.026

Catalysis Communications 11 (2010) 710–714
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Catalysis Communications
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Liquid-phase oxidation of cyclohexane to cyclohexanone over cobalt-doped SBA-3
Xiaochen Liu a, Jiao He a, Lijun Yang a,b, Yunan Wang a, Shihong Zhang a, Wei Wang a, Jiaqiang Wang a,
*
a Department of Applied Chemistry, Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education, Yunnan University, Kunming 650091, PR China
b College of Resource and Environment of Yuxi Normal University, Kunming 653100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Liquid-phase oxidation of cyclohexane was carried out under mild reaction conditions over mesoporous
Co/SBA-3 catalyst using aqueous hydrogen peroxide (30%) as oxidant and acetic acid as solvent without
adding any initiator. The catalyst exhibited high substrate conversion (91.6%) and reasonable product
(cyclohexanone) selectivity (64.3%). Fast hot catalyst filtration experiment proved that the catalyst acted
as a heterogeneous one and it can be reused two times without losing its activity to a greater extent.
Ó 2010 Elsevier B.V. All rights reserved.
Received 11 October 2009
Received in revised form 24 January 2010
Accepted 29 January 2010
Available online 20 February 2010
Keywords:
Selective oxidation of cyclohexane
Cyclohexanone
Cobalt-doped SBA-3
1. Introduction
the micro-mesoporous molecular sieves show improved perfor-
mance in cyclohexene epoxidation with tert-butyl hydroperoxide
The selective conversion of cyclohexane to cyclohexanone is an
industrially important reaction, because the latter is a starting
material used in the production of nylon-6 and nylon-66 [1,2].
Although homogeneous systems have been used for these pro-
cesses, many studies have illustrated the use of porous solids
including the titanosilicate TS-1, transition metal (Cr, V, Co, Ti,
etc.) doped MCM-41 [3], and Mo–MCM-41 [4] have been tested
for the selective oxidation of this hydrocarbon. In particular, Sakthi-
vel and Selvam used Cr–MCM-41 catalyst for oxidizing cyclohex-
ane, giving a conversion rate of 98.9% with a selectivity of 92.9%
towards cyclohexanol [5]. We showed that cerium-doped MCM-
41 has achieved very good conversions of 94.6% oxidizing cyclohex-
ane and 82.4% cyclohexanol [6]. Recently, Ag-substituted anatase
phase nano-TiO2 have been synthesized and used for the selective
photocatalytic oxidation of cyclohexane to cyclohexanone [7].
Although considerable efforts have been made, cyclohexane oxida-
tion continues to be a challenge [8]. Therefore, it is of great practical
interest to develop a more efficient, easily separable, reusable, and
environment-friendly catalyst for the oxidation process.
[14]. Besides, Nb-modified SBA-3 materials are very attractive cat-
alysts in both oxidative dehydrogenation of propane and epoxida-
tion of cyclohexene [15]. Moreover, we have demonstrated that Co/
SBA-3 was an efficient catalyst for the liquid-phase oxidation of 2-
methyl pyridine using H2O2 and gave 98% conversion, yielding 93%
selectivity towards 2-pyridinecarboxylic acid [16]. Thus, it was
suggested a good possibility to apply Co/SBA-3 as a catalyst for
the selective oxidation of cyclohexane.
Herein, Co/SBA-3 was used as a catalyst which showed high
turn over number (millimole of oxidized products per millimole
of metal in the catalyst, TON) for the selective oxidation of cyclo-
hexane to cyclohexanone. The effect of temperature, reaction time
and solvents were also studied.
2. Experimental
2.1. Synthesis of catalysts
Co/SBA-3 was synthesized with a process as mentioned in Refs.
[16,17]. For comparison, undoped SBA-3 was also prepared.
Co–MCM-41 and Co–SBA-15 with the similar concentration of
cobalt is prepared with processes described in Refs. [3,18,19],
respectively.
SBA-type silica materials are ordered mesoporous molecular
sieves with higher hydrothermal stability than MCM-41 [9,10].
Moreover, the microporosity in SBA-3 was significantly larger than
those in SBA-15 [11,12]. Thermal and mechanical properties of
SBA-3 are critical features for their application as catalysts and cat-
alyst supports [13], leading modified SBA-3 widely used in cataly-
sis fields. For instance, Ti-containing oxidation catalysts based on
2.2. Oxidation of cyclohexane
The oxidation reactions were carried out as follows: the catalyst
(100 mg), cyclohexane (AR grade, 2 g) and 15 mL of solvent (acetic
acid, trichloromethane, ethyl acetate, acetonitrile and acetone
* Corresponding author. Tel./fax: +86 871 5031567.
E-mail address: jqwang@ynu.edu.cn (J. Wang).
1566-7367/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.01.026

X. Liu et al. / Catalysis Communications 11 (2010) 710–714
711
were used as received without further purification) were added
successively into a temperature-controlled, round bottom, 3-
necked-flask having a reflux condenser. The aqueous H2O2 (30%
solution, approximately 6 mL, with a cyclohexane/H2O2 molar ratio
of 0.45) was added dropwise after the reaction mixture was heated
to the set temperature. The reaction mixture was filtered under re-
duced pressure after the set time. Anhydrous Na2CO3 (AR grade)
was used to remove more water from the extracted organic phase.
The obtained products were analyzed by gas chromatography
(Shimadzu GC-14C) with a packed column (/2 mm  3 M, 10%
PEG 20M/101) and a flame ionization (FID) detector. Reference
substances were used for the identification of the products.
composed of Si, Co, and O (Fig. 1a). The peak of Co2p can be fitted
into two smaller peaks at binding energies of 782.47 and
779.71 eV, which are attributed to Co2+ and Co3+ ions, respectively
[20–22]. They are also consistent with the results obtained by dif-
fuse reflectance UV–vis spectrum reported in our previous work
[16].
3.2. Catalytic performance
Using cobalt-doped SBA-3 as a catalyst, it was found that the
main products detected are cyclohexanone and cyclohexanol for
the oxidation of cyclohexane. Little cyclohexyl acetate could be de-
tected as by-products. For comparison, we have also studied the
oxidation of cyclohexane using SBA-3 without the incorporation
of Co as a catalyst under the same conditions with those used for
Co/SBA-3. As expected, undoped cobalt-free SBA-3 exhibited much
lower conversion and TON than Co/SBA-3. This indicates that in Co/
SBA-3, the active component was cobalt.
2.3. Catalyst characterization
XPS measurement was performed on a PHI5500ESCA analyzer.
The main parameters were as follows: Mg Ka, 200 W, vacuum
ꢀ10À7 Pa.
Furthermore, even incorporating Co into other mesoporous
materials, such as, Co–MCM-41 and Co–SBA-15 with the similar
concentration of cobalt, exhibited significantly lower activity than
Co/SBA-3. These comparisons are summarized in Table 1. Co/SBA-3
exhibits the highest conversion and TON. This feature is in good
agreement with the literature data [15] where Nb–SBA-3 was
found more effective than the corresponding Nb–MCM-41 and
Nb–SBA-15 catalysts. The surface areas of Co–SBA-15 and Co–
MCM-41 are 690.4 m2/g and 1041 m2/g, respectively, but they
are much less active than Co/SBA-3 (780.4 m2/g). This may imply
that pore size would play more important role since the pore size
3. Results and discussion
3.1. The characterization of Co/SBA-3
The surface area, pore volume, and the average pore size of Co/
SBA-3 were measured to be 780.4 m2/g, 0.65 cm3/g, and 4.5 nm,
respectively [16]. Co/SBA-3 was also characterized by X-ray dif-
fraction, diffuse reflectance UV–vis and FT-IR [16].
In order to investigate the cobalt phase of Co/SBA-3, the sample
was characterized with XPS. It was found that Co/SBA-3 is mainly
Fig. 1. (a) Typical XPS survey spectrum of Co/SBA-3 and (b) Co2p XPS spectra in Co/SBA-3.

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X. Liu et al. / Catalysis Communications 11 (2010) 710–714
Table 1
Effects of catalysts on the cyclohexane reaction.a
Catalysts
Conversion (%)
TON
Selectivity (%)
Cyclohexanone
Cyclohexanol
By-products
Co/SBA-3
91.6
91.5
91.2
53.4
42.8
8.1
667
666
664
339
312
41
64.3
64.1
63.9
46.9
68.0
–
35.6
35.8
36.0
52.9
31.8
–
0.1b
0.1
0.1
0.2
0.2
–
First recycled (Co/SBA-3)
Second recycled (Co/SBA-3)
SBA-3
Co–SBA-15
Co–MCM-41 [3]
a
Reaction conditions: substrate/oxidant, 0.45; reaction time, 8 h; reaction temperature, 373 K; catalyst, 100 mg; solvent, acetic acid; TON, turn over number (millimole of
oxidized products per millimole of metal in the catalyst).
b
Cyclohexyl acetate.
of Co/SBA-3 (4.5 nm) is bigger than those of Co–SBA-15 (3.6 nm)
and Co–MCM-41 (2.8 nm). Moreover, even compared with those
catalysts for the production of cyclohexanone used in Refs. [3–7],
the high TON and reasonable product (cyclohexanone) selectivity
(64.4%) of Co/SBA-3 make it an efficient catalyst for the oxidation
of cyclohexane to cyclohexanone. This would be the highest selec-
tivity for cyclohexanone as we know. For example, V–MCM-41 [3],
Cr–MCM-41 [5] and Ce–MCM-41 [6] have achieved very good
conversions oxidizing cyclohexane but the selectivities of cyclo-
hexanone are very low (<30%). Ag-substituted anatase phase
nano-TiO2 achieved a good selectivity of 63% in selective photocat-
alytic oxidation of cyclohexane to cyclohexanone but the substrate
conversion (9.0%) is very low [7]. In our previous work [23], using
acetic Co-doped mesoporous titania with anatase crystalline
structure as the catalyst, cyclohexane would be oxidized into
cyclohexanol and cyclohexanone in the ratio of 1.1:1 (ꢀ44.5%
cyclohexanone selectivity) with excellent conversion and good
selectivity. Due to the good performance of Co/SBA-3, in the fol-
lowing we concentrate on the study of the influence of various
parameters on the cyclohexane conversion and selectivity of cyclo-
hexanone over Co/SBA-3.
100
90
80
3.3. Oxidation of cyclohexane with different amount of catalysts
70
After reaction
60
The effect of catalyst concentration on the reaction was also
investigated (Fig. 3). An initial steep increase in the conversion of
cyclohexane and a distinctly increase in selectivity of cyclohexa-
none were observed when the amount of the catalyst was in-
creased up to 100 mg. Beyond this amount, the conversion of
cyclohexane and selectivity of cyclohexanone only decreased par-
tially. These results indicate that in the system catalyzed by Co–
SBA-3 only small amount of catalyst is active in the oxidation of
cyclohexane. This could be also attributed to the competent inter-
action of metal oxo-species in Co–SBA-3 sample with alkylperoxy
species and cyclohexane [24]. Therefore, 100 mg was selected as
the suitable amount of the catalyst for oxidizing cyclohexane.
50
Co/SBA-3
40
30
20
10
0
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber(cm)-1
Fig. 2. FT-IR spectra of Co/SBA-3 and used Co/SBA-3.
100
100
Conversion of cyclohexane
Conversion of cyclohexane
80
80
60
40
20
0
60
Selectivity of cyclohexanone
Selectivity of cyclohexanone
40
20
0
0
40
80
120
160
200
240
Catalyst (mg)
4
6
8
10
12
Time (h)
Fig. 3. Effects of the amount of catalyst on the conversion and selectivity over Co/
SBA-3.
Fig. 4. Effects of reaction time on the conversion and selectivity over Co/SBA-3.

X. Liu et al. / Catalysis Communications 11 (2010) 710–714
713
3.4. Influence of reaction time
3.6. Effect of solvents
As the reaction time was increased, the conversion of the cyclo-
hexane and selectivity of cyclohexanone increased, respectively.
And it passed through a maximum at 8 h and shown in Fig. 4. A fur-
ther increase in the reaction time resulted in slightly decrease in
the conversion rate of cyclohexane and selectivity of cyclohexa-
none. It is probably attributed to the weak inhibition of radicals
caused by cyclohexane and cyclohexanol which acted as a free rad-
ical scavenger and led to some quenching of the catalytic activity
[25,26], even though the decrease in the conversion rate of cyclo-
hexane and selectivity of cyclohexanone are not understood clearly
yet, further experiments are underway. Therefore, 8 h is a suitable
reaction time for oxidizing cyclohexane. Moreover, the FT-IR spec-
tra of fresh and recycled Co/SBA-3 reacted at 373 K for 12 h was
obtained to compare the characterization of fresh and used catalyst
in the relevant sites. It was found that they are very similar (Fig. 2),
which indicated that the catalyst changed little after use.
The nature of solvents was known to have a major influence on
reaction kinetics and product selectivity in the oxidation of cyclo-
hexane. Therefore, the effects of various solvents on the reaction
are summarized in Table 2. It was found that Co/SBA-3 catalyst
had the best performance in acetic acid. This is due to possible par-
tial decomposition of H2O2 because it was reported that the
decomposition of H2O2 are faster in these solvent than acetic acid
[27]. Moreover, it has been reported that acetic acid does not only
act as a solvent, but also serves as a good oxidizing agent because
of the formation of the framework titanium–peracetic acid com-
plex when it reacts with hydrogen peroxide in the presence of tita-
nium-containing zeolites (TS-1) [28]. And in this work,
a
framework silica–peroxy acetic acid complex probably formed in
the pores of Co–SBA-3 then it would be more hydrophobic and sta-
ble, as compared with hydrogen peroxide. Therefore, a better inter-
action of this complex with cyclohexane can be expected. In
general, it is considered that acetic acid does not only facilitate
homogeneity of the liquid phase, but also be responsible for the in-
crease in overall oxidation activity.
3.5. Effect of reaction temperature
As shown in Fig. 5, the catalytic performances were affected by
temperature. The conversion of cyclohexane increased by increas-
ing the reaction temperature and passed through a maximum at
373 K while the percentage selectivity to cyclohexanone increased.
A further increase in the reaction temperature resulted in decrease
both conversion and selectivity, probably owing to a quicker
decomposition of H2O2 at higher temperature which resulted in a
relatively low activity [18]. Thus, 373 K was chosen as the suitable
temperature for the oxidation of cyclohexane.
3.7. The reusability and fast hot catalyst filtration experiment
The recyclability of the catalyst was studied by running the
reaction successively (under the optimum reaction conditions)
over two recycles as shown Table 1. In three successive trials the
conversion and selectivity did not show any significant change.
These results indicate that the catalyst could be used several times.
In order to prove whether the catalyst is a heterogeneous one,
experiments with fast hot catalyst filtration and studying the reac-
tivity of the filtrate had been done by a modified process as de-
scribed in Ref. [29,16]: 100 mg catalyst, 2 g cyclohexane, 15 mL
HAc and 2 mL H2O2 was stirred at the temperature of 373 K for
1 h (conversion of cyclohexane was 33.4%), then quickly filtrate
the catalyst. Let the filtration react further by adding 4 mL H2O2.
We found that after hot filtration the filtrate reacted little further
comparing that observed when the catalyst was not filtered. This
observation was also well supported by ICP–AES analysis of the fil-
trates obtained from the fast hot catalyst filtration where negligi-
ble amount of leaching of active cobalt and silicium species
(60.001 wt.%) was detected. These indicated that the active com-
ponent (cobalt) did not leach to the solution and the Co/SBA-3
did act as a heterogeneous catalyst.
100
90
Conversion of cyclohexane
80
70
60
Selectivity of cyclohexanone
50
40
30
20
10
0
4. Conclusions
In summary, it can be concluded that Co/SBA-3 was an efficient
and high substrate conversion catalyst for the oxidation of the
cyclohexane under relatively mild reaction conditions without
adding any initiator. Among the various solvents, acetic acid was
found to be the most suitable for the high conversion and high
320
330
340
350
360
370
380
390
Temperature (K)
Fig. 5. Effects of reaction temperature on the conversion and selectivity over Co/
SBA-3.
Table 2
Effects of solvents on the cyclohexane reaction.a
Solvents
Conversion (%)
TON
Selectivity (%)
Cyclohexanone
Cyclohexanol
By-products
Acetic acid
Acetone
Acetonitrile
Ethyl acetate
Trichloromethane
91.6
29.0
12.3
8.9
667
211
90
65
5
64.3
67.2
55.5
53.4
54.3
35.6
32.7
44.4
46.4
45.5
0.1b
0.1
0.1
0.2
0.2
0.7
a
Reaction conditions: substrate/oxidant, 0.45; reaction time, 8 h; catalyst, 100 mg; reaction temperature, boiling point of solvents, except for acetic acid at 373 K.
Cyclohexyl acetate.
b

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X. Liu et al. / Catalysis Communications 11 (2010) 710–714
[10] F. Kleitz, W. Schmidt, F. Schüth, Micropor. Mesopor. Mater. 65 (2003) 1–29.
[11] P.A. Albouy, A. Ayral, Chem. Mater. 14 (2002) 3391–3397.
[12] R. Ryoo, C.H. Ko, M. Kruk, V. Antochshuk, M. Jaroniec, J. Phys. Chem. B 104
(2000) 11465–11471.
cyclohexanone selectivity. The catalyst can be reused with almost
the same activity.
[13] A. Galarneau, D. Desplantier-Giscard, F.D. Renzo, F. Fajula, Catal. Today 68
(2001) 191–200.
Acknowledgements
[14] F. Chen, H.K. Luo, Y.F. Han, C. Wang, G.J. Gan, Catal. Today 131 (2008) 76–81.
[15] B. Kilos, A. Tuel, M. Ziolek, J.C. Volta, Catal. Today 118 (2006) 416–424.
[16] B. Li, L. Li, L. Sun, F. Xia, S. Wang, J. Wang, Catal. Commun. 9 (2008) 2287–2290.
[17] F. Chen, X. Xu, S. Shen, S. Kawi, K. Hidajat, Micropor. Mesopor. Mater. 75 (2004)
231–235.
The authors thank the National Basic Research Program of China
(Project 2008CB617514), National Natural Science Foundation of
China (Project 20863009) and Yunnan Province (Projects
2007B018M, 2008CD065), and Specialized Research Fund for the
Doctoral Program of Higher Education for financial support.
[18] F. Chang, W. Li, F. Xia, Z. Yan, J. Xiong, J. Wang, Chem. Lett. 34 (2005) 1540–
1541.
[19] F. Xia, E. Ou, L. Wang, J. Wang, Dyes Pigments 76 (2008) 76–81.
[20] J.S. Girardon, A.S. Lermontov, L. Gengembre, P.A. Chernavskii, A. Griboval-
Constant, A.Y. Khodakov, J. Catal. 230 (2005) 339–352.
[21] A.Yu. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, P.
Chaumette, J. Catal. 168 (1997) 16–25.
[22] Z. Yan, Z. Wang, D.B. Bukur, D.Y. Goodman, J. Catal. 268 (2009) 196–200.
[23] W. Yao, H. Fang, E. Ou, J. Wang, Z. Yan, Catal. Commun. 7 (2006) 387–390.
[24] S.E. Dapurkar, A. Sakthivel, P. Selvam, J. Mol. Catal. A: Chem. 223 (2004) 241–
250.
[25] R. Atkinson, S.M. Aschmann, J. Arey, B. Shorees, J. Geophys. Res. 97 (1992)
6065–6073.
[26] R. Atkinson, J. Arey, S.M. Aschmann, S.B. Corchnop, Y. Shus, Int. J. Chem. Kinet.
27 (1995) 941–955.
References
[1] J. Tao, D. Tang, Q. Li, Z. Yu, E. Min, J. Nat. Gas Chem. 10 (2001) 295–307 (in
Chinese).
[2] K. Weissermel, H.J. Horpe, Industrial Organic Chemistry, second ed., VCH Press,
Weinheim, 1993.
[3] W.A. Carvalho, P.B. Varaldo, M. Wallau, U. Schuchardt, Zeolites 18 (1997) 408–
416.
[4] R.K. Rana, A.C. Pulikottil, B. Viswanathan, Stud. Surf. Sci. Catal. 113 (1998) 211–
217.
[5] A. Sakthivel, P. Selvam, J. Catal. 211 (2002) 134–143.
[6] W. Yao, Y. Chen, L. Min, H. Fang, Z. Yan, H. Wang, J. Wang, J. Mol. Catal. A:
Chem. 246 (2006) 162–166.
[27] H. Mimoun, L. Saussine, E. Daire, M. Postel, J. Fischer, R. Weiss, J. Am. Chem.
Soc. 105 (1983) 3101–3110.
[7] R. Vinu, G. Madras, Appl. Catal. A 366 (2009) 130–140.
[8] U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, M.C. Guerreiro, D. Mandellif,
E.V. Spinacé, E.L. Pires, Appl. Catal. A 211 (2001) 1–17.
[9] Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A.
Firouzi, B.F. Chmelka, F. Schüth, G.D. Stucky, Chem. Mater. 6 (1994) 1176–
1191.
[28] T. Sooknoi, J. Limtrakul, Appl. Catal. A 233 (2002) 227–237.
[29] R. Sheldon, M. Wallau, I. Arends, U. Schuchardt, Acc. Chem. Res. 31 (1998) 485–
493.