Confining task-specific ionic liquid in silica-gel matrix by sol-gel technique: A highly efficient catalyst for oxidation of alcohol with molecular oxygen

Article abstract of DOI:10.1016/j.catcom.2010.10.020

A novel catalytic system was developed through confinement of TEMPO task-specific ionic liquid (TEMPO-IL) with CuCl₂ in a silica-gel matrix by sol-gel technique. The obtained TEMPO-IL/CuCl₂/silica-gel catalytic system was effective f

Full text of DOI:10.1016/j.catcom.2010.10.020

Catalysis Communications 12 (2011) 323–326
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Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Confining task-specific ionic liquid in silica-gel matrix by sol-gel technique: A highly
efficient catalyst for oxidation of alcohol with molecular oxygen
Lin Liu a, Juanjuan Ma a,b, Jiyin Xia a, Luodan Li a, Chunlong Li a, Xiaobo Zhang a,b
,
Junyan Gong a, Zhiwei Tong a,c,
⁎
a
Department of Chemical Engineering, Huaihai Institute of Technology, Lianyungang, 222005, China
Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing, 210094, China
SORST, Japan Science and Technology Agency (JST), Kawaguchi-shi, Saitama, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 June 2010
Received in revised form 24 October 2010
Accepted 27 October 2010
Available online 1 November 2010
A novel catalytic system was developed through confinement of TEMPO task-specific ionic liquid (TEMPO-IL)
with CuCl2 in a silica-gel matrix by sol-gel technique. The obtained TEMPO-IL/CuCl2/silica-gel catalytic system
was effective for the transformation of a series of benzylic, allylic and heterocyclic alcohols to the respective
carbonyl compounds under O2 atmosphere. Notably, the system facilitates ease separation of the catalyst and
product and shows good reusability in the reaction.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Alcohol selective oxidation
Supported ionic liquid
TEMPO
Sol-gel
1. Introduction
facilitation of catalyst separation from reaction system and more rapid
mass transfer, lower contamination of product, and the ability of using
In the past decade, ionic liquids (ILs) have attracted increasing
attention in organic synthesis and catalysis. The favorable character-
istics, such as low vapor pressure, high thermal stability, and peculiar
ion environment, make ILs “green” alternatives for replacing
traditional organic solvents. As both the anionic and the cationic
part of ILs can be easily varied, the physical and chemical properties of
ILs can be finely tuned. So ILs can be used as good solvents for a broad
spectrum of inorganic, organic and polymeric materials. However,
ionic liquids are still expensive even though being commercially
available. Furthermore, the high viscosity of ILs may induce mass
transfer limitations if the chemical reaction is fast, causing the
reaction to proceed in the diffusion layer of the catalyst, rather than in
the bulk solvent. Thus from economic and environmental viewpoint,
homogeneous catalysis in ILs is unattractive.
One solution to improve catalysis with ILs is the immobilization of
ionic liquid phase containing the transition metal catalysts on the
surface or pores of various porous solid materials via covalent
bonding, adsorption or sol-gel process[1]. Compared with pure ionic
liquids, these immobilized ionic liquids show additional advantages,
such as significantly decrease of the amounts of ionic liquid, the
it in gas phase reactions[2]. However, chemical bonding of ILs
fragments (anion or cation) onto a solid surface may limit the degrees
of freedom of the counterparts and even change the physicochemical
properties of ILs. As for physical adsorption, the main drawback is the
risk of leaching of ILs and transition metal catalysts under rigorous
reaction conditions.
Different from the former methods, the immobilization of ILs by
sol-gel process results in a more intimate biphasic system. The solid
matrix forms a porous prison that prevents ILs or transition metal
catalysts from leaching, but allows the free transportation of reactants
and products[3]. Recently, supported ionic liquid catalysts by sol-gel
method have been examined for carbonylation [4], hydroformylation
[5], and alcohol oxidation [6]. In these reactions, catalytic activities
were remarkably enhanced with decreased consumption of ILs
compared to bulk ionic-liquid catalyst system.
The selective oxidation of alcohols to the corresponding aldehydes
or ketones is the foundation of many important industrial and fine-
chemical processes[7]. Many highly efficient aerobic alcohol oxidation
systems either catalyzed by transition metal catalysts alone (such as
palladium[8], gold[9–13], ruthenium[14,15], copper[16,17], iron
[18,19], and vanadium[20]) or in combination with nitroxyl radical
2,2,6,6-tetramethylpiperidyl-1-oxy (TEMPO) have been developed.
Our recent work was focused on the oxidation of organic compounds
with molecular oxygen as oxidant[21,22]. Earlier, we disclosed a
catalytic system of TEMPO-IL/CuCl/MS3A (molecular sieve 3A) for the
aerobic oxidation of a wide range of benzylic alcohols in ionic liquids
⁎
Corresponding author. Department of Chemical Engineering, Huaihai Institute of
Technology, Lianyungang, 222005, China.
E-mail address: zhiweitong575@hotmail.com (Z. Tong).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.10.020

324
L. Liu et al. / Catalysis Communications 12 (2011) 323–326
[bmim]PF6, where MS3A serves as a heterogeneous Brønsted base
to enhance the reaction rate remarkably[23]. In light of the newly
developed supported ionic liquid catalyst systems, in this work, an
attempt has been made to entrap task-specific ionic liquid immobilized
TEMPO (TEMPO-IL) and transition metal salt into the pores of solid
silica-gel by sol-gel method (Scheme 1). After great effort, a silica-gel
supported task-specific ionic liquid catalyst system (TEMPO-IL/CuCl2/
silica-gel )was successfully prepared and shown to be effective for the
transformations of a wide range of alcohols, including benzylic, allylic
and heterocyclic alcohols, under mild conditions to the corresponding
aldehydes in high conversions and selectivities. The TEMPO-IL/CuCl2/
silica-gel catalyst can easily be separated from the oxidation system and
reused, which makes the system eco-friendly and cost-effective.
2. Experimental
2.1. General
Fig. 1. Illustration of the synthesis of silica-gel-confined TEMPO-IL with CuCl2.
All chemicals were obtained from commercial resources with
more than 99% purity and used without prior purification. Gas
chromatography (GC) analysis was performed on an Agilent GC-6820
equipped with a 30 m×0.32 mm×0.5 μm HP-Innowax capillary
column and a flame ionization detector. The elemental analysis of
copper was determined by ICP-AES (Thermo icap6300). Products
were all known compounds and were identified by comparing of their
physical and spectra data with those reported in the literature.
3. Results and discussion
3.1. Effects of catalysts on the oxidation of alcohol
The catalysts prepared with different transition-metal salts and
various amount of ionic liquid [bmim]PF6 and hydrochloric acid were
applied to the p-methoxybenzyl alcohol oxidation. The results were
listed in Table 1. First, the amount of [bmim]PF6 used in the preparation
of catalyst has an important effect on the catalytic activity (entries 1–3).
Only 18% of conversion was achieved in 3 h when the catalyst without
additional ionic liquid was used. The presence of additional ionic liquid
resulted in the increase of conversion. With the addition of 3.5 g [bmim]
PF6 in the preparation of catalyst, benzyl alcohol was completely
oxidized into benzaldehyde within 3 h. Next, increasing the amount of
acid used in the synthesis process of catalyst from 1 mL to 4 mL resulted
in decrease of the conversion of the alcohol and selectivity to aldehyde
(entries 3, 4). Lower conversion and selectivity may be ascribed to the
side reaction caused by the existence of excess acid. Finally, a series of
inexpensive transition-metal salts including CuCl2, Cu(NO3)2, Cu(OAc)2,
FeCl3, NiCl2, Mn(OAc)2 and Co(OAc)2 were tested as co-catalyst with
TEMPO-IL to oxidize p-methoxybenzyl alcohol to aldehyde. It can be
seen that among the salts tested, copper salts showed much higher
2.2. Catalyst preparation
The ionic liquid [bmim]PF6 was synthesized according to literature
[24]. TEMPO-IL was prepared according to literature[25]. Typically, to
a stirred solution of TEMPO-IL (0.5 g, 1.13 mmol) and [bmim]PF6
(3.5 g) in 8 mL of methanol, CuCl2·2H2O (0.193 g, 1.13 mmol) and
water (0.5 mL) were added. The solution was then heated to 60 °C,
and tetraethoxyorthosilicate (TEOS, 11 mL) was added. After the
formation of a clear and homogeneous liquid mixture, hydrochloric
acid (2 mol/L, 1 mL) was added dropwise and the mixture gradually
coagulated. After aging at 60 °C for 24 h, the resultant solid material
was dried in vacuum at 80 °C overnight and 6.3 g of catalyst (TEMPO-
IL/CuCl2/silica-gel) was obtained (Fig. 1).
2.3. Typical experimental procedure for oxidation of alcohols
Table 1
Effects of catalysts on the oxidation of p-methoxybenzyl alcohol.a
Alcohol (0.5 mmol) and catalyst (100 mg) were mixed with
n-octane (5 mL) and stirred at 50 °C under O2 (10 mL/min, 1 atm.)
for given time. After completion (monitored by TLC and GC), the
mixture was centrifuged to separate the catalyst, which was washed
several times with n-octane, dried and recharged with fresh substrate
and solvent for the next catalytic cycle. The combined solvent phase
was concentrated under vacuum, the crude product was purified
by column chromatography (petroleum ether/ethyl acetate=10/1)
to provide the analytically pure aldehyde which was characterized
by 1HNMR and IR analysis.
Entry Catalyst
[bmim] Acid/ Time/ Conv./ Select./
PF6/g
mL
h
%
%
1
2
3
4
5
6
7
8
TEMPO-IL/CuCl2/silica-gel
-
1
1
1
4
1
1
1
1
1
1
-
3
3
3
3
3
3
12
12
12
12
3
18
N99
N99
N99
85
N99
N99
72
TEMPO-IL/CuCl2/silica-gel
TEMPO-IL/CuCl2/silica-gel
TEMPO-IL/CuCl2/silica-gel
TEMPO-IL/Cu(NO3)2/silica-gel
TEMPO-IL/Cu(OAc)2/silica-gel
TEMPO-IL/FeCl3/silica-gel
TEMPO-IL/NiCl2/silica-gel
TEMPO-IL/Mn(OAc)2/silica-gel
TEMPO-IL/Co(OAc)2/silica-gel
TEMPO-IL/CuCl2
1.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
3.5
-
84
100
81
95
90
63
42
51
55
58
52
89
83
85
9
10
11b
12c
N99
N99
O
acetamido-TEMPO/CuCl2/DMAP
-
-
5
OH
R2
O
N
N
O
TEMPO-IL/CuCl2/silica-gel
a
Catalyst preparation: TEMPO-IL (0.5 g, 1.13 mmol), [bmim]PF6, metal salts
(1.13 mmol), TEOS (11 mL), hydrochloric acid (2 mol/L), water (0.5 mL) and methanol
(8 mL); oxidation reaction conditions: p-methoxybenzyl alcohol (0.5 mmol), catalyst
(100 mg) and n-octane (5 mL) under O2 at 50 °C.
PF6
O2, n-octane, 50οC
R1
R1
R2
N
O
R1= aryls, alkyls; R2= H,aryls, alkyls
b
p-methoxybenzyl alcohol (0.5 mmol), TEMPO-IL (0.0175 mmol) and CuCl2
TEMPO-IL
(0.0175 mmol).
c
Reference [26], in [bmim]PF6, 5 mol% acetamido-TEMPO, 5 mol% CuCl2, 10 mol%
Scheme 1. Aerobic oxidation of alcohols catalyzed by TEMPO-IL/CuCl2/silica-gel.
DMAP under O2 at r.t.

L. Liu et al. / Catalysis Communications 12 (2011) 323–326
325
100
80
60
40
20
Run1
Run2
Run3
Run4
Run5
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Reaction time /h
Fig. 3. Recycling of the catalytic system for the oxidation of p-methoxybenzyl alcohol.
Reaction conditions: p-methoxybenzyl alcohol (0.5 mmol), TEMPO-IL/CuCl2/silica-gel
(100 mg), and n-octane (5 mL), under O2 at 50 °C.
Fig. 2. Effects of solvents on the oxidation of alcohol. Reaction conditions: p-methoxybenzyl
alcohol (0.5 mmol), TEMPO-IL/CuCl2/silica-gel (100 mg), and solvent (5 mL), under O2 at
50 °C, 3 h.
catalytic activity than other transition metal salts. All three copper salts
(Cl−, NO−3 and OAc− salts) gave similar catalytic activity and identical
selectivity (entries 3, 5, 6). In the case of Fe, Ni, Mn and Co salts,
the oxidation of p-methoxybenzyl alcohol was relatively slow with only
42–63% conversion even after 12 h and the formation of carboxylic acid
and its esters was observed (entries 7–10). Compared with acetamido-
Table 2
Catalytic aerobic oxidation of various alcohols.a
Entry
1
Substrate
Product
Time/h
3
Conv./Select.b
%
Yieldc/%
92
100/N99
OH
O
2
4.5
100/N99
92
OH
O
O2N
O2N
3
4
4
3
100/N99
100/N99
91
93
OH
O
Cl
Cl
OH
O
H3CO
H3CO
H3CO
H3CO
H3CO
H3CO
5d
8
97/N99
92
OH
OH
O
O
6e
7
2.5
3.5
100/N99
100/N99
93
92
OH
O
H3C
CH3
H3C
CH3
8
9
4
8
100/N99
99/N99
91
90
OH
O
OH
O
10
11
5.5
3.5
96/N99
84
85
OH
O
97/97
O
O
OH
O
12
15
35/N99
-
n-C11H23-CH2OH
n-C11H23-CHO
a
Reaction conditions: alcohol (0.5 mmol), TEMPO-IL/CuCl2/silica-gel (100 mg), and n-octane (5 mL) under O2 at 50 °C.
GC conversion and selectivity.
Isolated yield.
Reaction temperature was 30 °C.
Reaction temperature was 85 °C.
b
c
d
e

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L. Liu et al. / Catalysis Communications 12 (2011) 323–326
TEMPO/CuCl2/DMAP homogeneous system (entry 12), the present
TEMPO-IL/CuCl2/silica-gel system showed favorable activity.
bonding between TEMPO-IL and silica gel. The leaching of TEMPO-IL
under drastic reaction conditions is inevitable, which might be the
main reason for the loss of activity.
3.2. Effects of solvents on the oxidation of alcohol
4. Conclusions
The solvents employed were crucial for the successful oxidation of p-
methoxybenzyl alcohol (Fig. 2). In n-octane, p-methoxybenzyl alcohol
was completely oxidized into corresponding aldehyde within 3 h. High
catalytic activities in the n-octane may be attributed to the stability of
the catalytic system in the solvent and high-concentration of ionic
liquids in the pores of silica matrix [4]. In solvents such as toluene and
[bmim]PF6, the rate of oxidation was significantly slower. In DMF, the
reaction almost stopped. The decrease in catalytic activity in these
solvents may be due to the dissolution of TEMPO-IL. In these cases, the
reaction was not limited in the pores of silica matrix but extend to the
whole solvent. Consequently, they are actually homogeneous catalyst
systems, intrinsically different from those in n-octane. Other factors
such as the solubility of O2 in solvent, the polarity and viscosity of
solvent may have influence on the reaction, too.
In conclusion, we have successfully developed a novel catalytic
system by the confinement of task-specific ionic liquid (TEMPO-IL)
and transition metal salt within a silica-gel matrix through sol-gel
method. It was found that TEMPO-IL/CuCl2/silica-gel catalyst system
was highly efficient for the aerobic oxidation of various alcohols. In
addition, the catalyst does not lose catalytic activity and selectivity for
at least five runs.
Acknowledgement
This work was supported by National Natural Science Foundation
of China (Grant No. 50873042), the Natural Science Foundation of
Jiangsu Province (BK2009651) and the Scientific Research Program of
the HuaiHai Institute of Technology (Z2009015, Z2009021).
3.3. Aerobic oxidation of alcohols promoted by TEMPO-IL/CuCl2/silica-gel
Based on the results above, we studied oxidation of different
alcohols catalyzed by TEMPO-IL/CuCl2/silica-gel in n-octane and the
results were listed in Table 2. All benzylic alcohols can be converted
into corresponding aldehydes in high conversions within 8 h (entries
1–9). Similar reactivity was observed in the oxidation of substrates
with electron-withdrawing or donating groups in the aromatic ring.
Benzyl alcohols with methyl substitution on the o- or p- position all
served as good substrates, which indicated that the steric effect of the
substitution did not apparently affect the efficiency of the oxidation.
To further probe the potential of the new catalyst system, we carried
out the reaction at 30 °C. With prolonging the reaction time to 8 h,
97% of p-methoxybenzyl alcohol was transformed into aldehyde,
demonstrate that the catalytic system of TEMPO-IL/CuCl2/silica-gel
was highly effective. When the temperature was increased to 85 °C, a
full conversion was obtained within only 2.5 h. Moreover, allylic and
heterocyclic primary alcohols were also very smoothly oxidized to the
corresponding aldehydes with high conversions and selectivities
(entries 10 and 11). Double bond was unaffected. Compared to
benzylic, allylic and heterocyclic alcohols, aliphatic alcohol was rather
difficult to oxidize under the same conditions and the conversion was
quite low even after 15 h (entry 12).
References
[1] Y.L. Gu, G.X. Li, Adv. Synth. Catal. 351 (2009) 817–847.
[2] H.H. Zhao, N.Y. Yu, J.Q. Wang, D.Y. Zhuang, Y. Ding, R. Tan, D.H. Yin, Microporous
and Mesoporous Materials 122 (2009) 240–246.
[3] P. Wasserscheid, T. Welton, Ionic liquids in synthesis, second ed.Wiley-VCH,
Weinheim, 2008.
[4] F. Shi, Q.H. Zhang, D.M. Li, Y.Q. Deng, Chemistry-a European Journal 11 (2005)
5279–5288.
[5] HamzaK. , BlumJ. , Eur. J. Org. Chem. (2007) 4706–4710.
[6] R. Ciriminna, P. Hesemann, J.J.E. Moreau, M. Carraro, S. Campestrini, M. Pagliaro,
Chemistry-a European Journal 12 (2006) 5220–5224.
[7] M. Hudlický, Oxidations in Organic Chemistry, American Chemical Society
Science, Washington, DC, 1990.
[8] B. Feng, Z. Hou, H. Yang, X. Wang, Y. Hu, H. Li, Y. Qiao, X. Zhao, Q. Huang, Langmuir
26 (2009) 2505–2513.
[9] V.R. Choudhary, D.K. Dumbre, Catal. Commun. 10 (2009) 1738–1742.
[10] V.R. Choudhary, D.K. Dumbre, S.K. Bhargava, Ind. Eng. Chem. Res. 48 (2009)
9471–9478.
[11] V.R. Choudhary, A. Dhar, P. Jana, R. Jha, B.S. Uphade, Green Chem. 7 (2005)
768–770.
[12] D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing, M.
Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362–365.
[13] V.R. Choudhary, R. Jha, P. Jana, Green Chem. 9 (2007) 267–272.
[14] M.P. Checinski, A. Bruckner, J. Radnik, A. Kockritz, Appl. Catal. A-gen. 366 (2009)
212–219.
[15] H. Yu, Y. Zhang, X.B. Fu, F. Peng, H.J. Wang, J. Yang, Catal. Commun. 10 (2009)
1752–1756.
[16] E.T.T. Kumpulainen, A.M.P. Koskinen, Chem-eur. J. 15 (2009) 10901–10911.
[17] P.J. Figiel, A. Sibaouih, J.U. Ahmad, M. Nieger, M.T. Raisanen, M. Leskela, T. Repo,
Adv. Synth. Catal. 351 (2009) 2625–2632.
3.4. Recycling of the catalytic system
The recyclability of the catalyst system was examined for the
aerobic oxidation of p-methoxybenzyl alcohol in n-octane (Fig. 3).
After reuse of the TEMPO-IL/CuCl2/silica-gel catalyst for five times, the
conversion of the reactant decreased gradually from 100% to 87%,
while the selectivity to aldehyde kept unchanged. To investigate the
stability of different species confined in silica-gel, a series of analysis
on the supernatant fraction were carried out after the reaction. In the
supernatant fraction, Cu was not detected by ICP analysis, whereas
trace TEMPO-IL species was detected by HPLC analysis. There is no
[18] F. Shi, M.K. Tse, M.M. Pohl, A. Bruckner, S.M. Zhang, M. Beller, Angew. Chem. Int.
Edit. 46 (2007) 8866–8868.
[19] W.L. Yin, C.H. Chu, Q.Q. Lu, J.W. Tao, X.M. Liang, R.H. Liu, Adv. Synth. Catal. 352
(2010) 113–118.
[20] Q.H. Tang, Y.T. Chen, Y.H. Yang, J. Mol. Catal. A-chem. 315 (2010) 43–50.
[21] L. Liu, L.Y. Ji, Y.Y. Wei, Catal. Commun. 9 (2008) 1379–1382.
[22] L. Liu, L.-Y. Ji, Y.-Y. Wei, Monatsh. Chem. 139 (2008) 901–903.
[23] L. Lin, M. Juanjuan, J. Liuyan, W. Yunyang, J. Mol. Catal. A-chem. 291 (2008) 1–4.
[24] S.V. Dzyuba, R.A. Bartsch, Chem. Commun. (2001) 1466–1467.
[25] X.E. Wu, L. Ma, M.X. Ding, L.X. Gao, Synlett (2005) 607–610.
[26] N. Jiang, A.J. Ragauskas, Org. Lett. 7 (2005) 3689–3692.