Ionic liquids promoted the CH oxidation of alcohols to carbonyl compounds using a new polysiloxane-supported (salen)chromium(III) catalyst

Article abstract of DOI:10.1016/j.catcom.2012.03.038

An ion-exchangeable layered polysiloxane was first used as support for immobilizing a sulphonato-(salen)chromium(III) complex, and the resulted catalyst exhibited moderate to great reactivities in the CH oxidation of different alcohols into carbonyl compounds when aqueous hydrogen peroxide were employed as oxidant. And then three room temperature ionic liquids were loaded respectively for the purpose of recycling, and indeed the ionic liquid phases containing catalyst could be recycled effectively. At last, a novel layered structure of the supported catalyst was afforded and briefly discussed after X-ray powder diffraction analysis.

Full text of DOI:10.1016/j.catcom.2012.03.038

Catalysis Communications 25 (2012) 69–73
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Ionic liquids promoted the C\H oxidation of alcohols to carbonyl compounds using a
new polysiloxane-supported (salen)chromium(III) catalyst
Guojun Zhou a, Zhaorui Zhang a, Xin Feng a,b, Bobo Dang a, Xiaoyong Li a, Yang Sun a,
⁎
a
Department of Applied Chemistry, School of Science, Xi'an Jiaotong University, Xi'an, No. 28, Xianning West Road 710049, China
b
North Institute of Information Engineering, Xi'an Technological University, Xi'an, No. 4, Jinhua North Road 710025, China
a r t i c l e i n f o
a b s t r a c t
Article history:
An ion-exchangeable layered polysiloxane was first used as support for immobilizing a sulphonato-(salen)
chromium(III) complex, and the resulted catalyst exhibited moderate to great reactivities in the C\H oxida-
tion of different alcohols into carbonyl compounds when aqueous hydrogen peroxide were employed as
oxidant. And then three room temperature ionic liquids were loaded respectively for the purpose of recycling,
and indeed the ionic liquid phases containing catalyst could be recycled effectively. At last, a novel layered
structure of the supported catalyst was afforded and briefly discussed after X-ray powder diffraction analysis.
© 2012 Elsevier B.V. All rights reserved.
Received 2 February 2012
Received in revised form 13 March 2012
Accepted 31 March 2012
Available online 11 April 2012
Keywords:
Ionic liquid
Ion-exchange
(salen)Chromium(III)
Oxidation
Recycling
Structure
1. Introduction
do a great job in fresh catalysis, but usually suffered from poor product
separation and catalyst recovery [12]. Therefore, some groups have
The chemoselective oxidation of alcohols into carbonyl compounds
is one of the fundamental organic transformations not only from syn-
thetic point of view, but it also aroused significant biological interests
[1,2]. Therefore, there has been much research along this direction, espe-
cially those for versatile and selective reagents. Among the widely used
are chromium compounds, including Jones reagent (chromiumtrioxide/
H2SO4), Collins reagent (chromiumtrioxide/pyridine complex), and PCC
(pyridinium chloro-chromate) [3], which already appeared as commer-
cially available oxidants for this transformation. However, the lack of
generality, stoichiometric amounts of loading, and environmentally
hazardous issues limit their use for large-scale and industrial applica-
tions [4]. Currently, some homogeneous systems making use of copper
[5], palladium [6], ruthenium [7], or cobalt [8] catalysts, and facilitating
clean terminal oxidant, were reported to overcome these drawbacks.
Even so, there is still need for a wide scope of alcohol substrates, inex-
pensive catalysts, and environmental friendly conditions.
tried to immobilize metallosalen complexes onto various supports,
such as silica [13] or MCM-41 [14]. Remarkably, both Anderson's [15]
and Choudary's [16] groups had successfully intercalated a sulphonato-
Mn(salen) complex into Zn(II)/Mg(II)–Al(III) hydrotalcite to produce a
stable and active catalyst, which indeed meant a lot about synergetic
effects of metal centers with local environments. We supposed a basic
structure of (salen)chromium(III) complex might do work in C\H oxi-
dation of some aromatic and aliphatic alcohols. Fine immobilization of
(salen)chromium(III) backbone also seemed to be effective for the pro-
motion of catalytic outputs. In this study, an ion-exchangeable layered
polysiloxane developed by Kaneko et al. [17,18] was selected as can-
didate carrier, and electrostatic linkage between two sulfonate substit-
uents of (salen)chromium(III) with this polysiloxane was anticipated.
It was interesting to test whether a layered structure of polysiloxane
could bring about better reactivity or chemoselectivity. And accordingly,
structural variation of supported catalyst would be illustrated through
X-ray powder diffraction (XRD), using polysiloxane as reference.
Room temperature ionic liquids (called RTILs or ILs), composed
entirely of ions with melting points below 100 °C, have attracted
renewed interests in a variety of reactions, like Heck reactions [19],
hydrogenation [20], Suzuki coupling reactions [21], and oxidation
[22]. As known, ionic liquids are normally nonvolatile and often envi-
ronmental friendly for chemical reactions. On the other, since cata-
lysts featuring polar or ionic properties could be fixed in ionic
liquids, it was anticipated the mixture containing both catalysts and
ionic liquids become recyclable. Moreover, a proper choice of cation
Transition metal salen (N,N′-bis(salicylidene)ethylenediaminato)
complexes had attracted continuous attentions for almost two de-
cades owing to their great catalytic performance and synthetic acces-
sibility [9]. This system included some basic oxygen transferring
processes, such as Jacobsen epoxidation [10] and sulfide oxidation
[11]. In general, homogeneous transition metal–salen complexes can
⁎
Corresponding author. Tel.: +86 29 82663914; fax: +86 29 82668559.
E-mail address: sunyang79@mail.xjtu.edu.cn (Y. Sun).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.03.038

70
G. Zhou et al. / Catalysis Communications 25 (2012) 69–73
and anion may provide a fine-tuning of their miscibility with organic
the stated values. The X-ray diffraction (XRD) patterns of samples were
recorded on a Shimadzu XRD-6000 X-ray diffractometer (Cu-Ka), and
diffraction data were collected for 2θ angles of 4°–30° at 0.02° intervals.
solvents or water. In this work, three ionic liquids BMImX (BMIm+
=
1-n-butyl-3-methylimidazolium; X−=PF6−, BF4−, NO3−) were employed
for the purpose of recycling, and aqueous hydrogen peroxide were
loaded as a clean terminal oxidant.
2.2. Catalysis
2. Experimental
Catalytic oxidation reactions of benzyl alcohol, cyclohexanol, and
n-hexanol to corresponding carbonyl compounds were carried out
for evaluating catalyst generality. At room temperature, 0.1 mol of
alcohol, catalyst 1 or 2 (loading of Cr was 3 mol% based on the mole
of substrate), and ionic liquid (for entries 1, 2, 6, 7, 11 and 12, none;
for others, 20 mL) were combined in a 1000 mL, two-neck, round bottom
flask with a condenser, then vigorously stirred for two hours. Next, 28 mL
commercial aqueous hydrogen peroxide (30%) was added dropwise. The
two outlets of flask were sealed with balloons. Heated to 313 K with stir-
ring, reaction was monitored once an hour by TLC (GF254 silica gel, color-
ation in phosphomolybdic acid/ethanol for aromatic alcohol, or in KMnO4
solution for aliphatic alcohols).
When reaction was completed, the mixture was concentrated
under reduced pressure to remove water. Since both catalysts 1 and
2 were soluble in ionic liquids, there were two approaches for product
extraction and recycling. For entries 1, 2, 6, 7, 11 and 12, the upper
liquid layer (containing alcohols and products) can be collected and
analyzed by 1H NMR for conversion and selectivity, the solid catalysts
precipitated at bottom were recovered after washing by a small
amount of n-hexane. For other entries, the mixture was diluted with
n-hexane (100 mL). After stirring for a short while, the n-hexane layer
was decanted, washed with water and brine, dried over anhydrous
Na2SO4, concentrated, and characterized by 1H NMR. Ionic liquid phase
containing catalyst was reloaded with alcohols and hydrogen peroxide
for recycling.
2.1. General
All chemicals were purchased from Sigma-Aldrich Corporation
without further purification. Poly(3-aminopropyl)siloxane hydro-
chloride (denoted as PAPS-Cl, Cl·NH3(CH2)3SiO1.5) was synthesized
according to the literature [17,18]. Sulphonato-(salen)chromium(III)
complex (catalyst 1) was prepared by coordination of a sulphonato-
salen ligand [23] with chromium(II) chloride that derived from the
zinc amalgam reduction of chromium(III) chloride [24]. Polysiloxane-
sulphonato-(salen)chromium(III) complex (catalyst 2), as shown in
Fig. 1, was obtained by mixing 10 mmol catalyst 1 with 20 mmol PAPS-
Cl in distilled water (150 mL) under continuous stirring for three days.
The dark brown precipitates were collected and washed by water and
dichloromethane carefully. Ionic liquids BMImX (BMIm+=1-n-butyl-3-
methylimidazolium; X−=PF6−, BF4−) were prepared in acetone [25],
and similarly BMImX (X−=NO3−) was synthesized by ion exchange reac-
tion of commercial BMImX (X−=Cl−) with AgNO3 in aqueous solution.
Chromium contents in catalysts 1 and 2 were 8.07×10−4 and
2.77×10−4 mol g−1, respectively, which recorded on an inductive
coupled high frequency plasma (ICP) instrument, IRIS Advantage, Thermo
Scientific. Elemental analyses were carried out on an Elementar VarioEL III
instrument. The ideal formula of PAPS-Cl is Cl·NH3(CH2)3SiO1.5 (repeti-
tive monomer, FW 146.5). Anal. Calcd. for PAPS-Cl: C, 24.6; H, 6.1;
N, 9.6. Found: C, 23.7; H, 6.3; N, 8.8. The ideal formula of catalyst 2 is
[NH3(CH2)3SiO1.5]2∙C16H12N2O8S2CrCl·9H2O (FW 895.5). Anal. Calcd. for
catalyst 2: C, 24.1; H, 4.6; N, 6.2. Found: C, 24.3; H, 4.7; N, 5.6. FT-IR
data were collected in potassium bromide pellets on a Bruker Tensor
27 spectrometer. 1H NMR spectra were reported in CDCl3 on a Bruker
ADVANCE III instrument (400 MHz), using tetramethylsilane (TMS) as
reference standard: sample/(CDCl3 and TMS) 10% (v/v), CDCl3/TMS
25/1 (v/v), spectral width in Hz (SWH) 8223.6 Hz, dwell time (DW)
60.8 μs, temperature 295.7 K, chemical shifts of aldehydic hydrogen
and α-carbon hydrogen of alcohols as criteria in association with 1H
NMR spectra of pure by-products; ratios of integration value offered
conversion and selectivity after normalization, relative error 5% of
3. Results and discussion
Synthetic route of catalyst 2 was shown in Fig. 1. Because PAPS-Cl
has ammonium groups around polysiloxane core and Cl− as counterions,
and both PAPS-Cl and catalyst 1 are miscible in water, ion-exchange re-
action was performed in aqueous solution. The precipitation occurred
gradually, and FT-IR curves of PAPS-Cl, catalyst 1 and catalyst 2 (precip-
itates) were consecutively listed in Fig. 2. It could be seen that PAPS-Cl
(curve a) exhibited characteristic bands at 3445 and 1129 cm−1 due to
stretching modes of N\H and Si\C bonds, together with 912 and
556 cm−1 assigned to two different Si\O bonds [26]. In the meanwhile,
O
O
OH
Si
O
Si
y
x
N
O
N
O
NH3Cl
NH3Cl
PAPS-Cl
Cr
Cl
NaO3S
SO3Na
Distilled water, room temperature, 3 days
Catalyst 1
N
O
N
Cr
Cl
Si OH
O
O
O
NH
3 O3S
O
SO3 H3N
SO3 H3N
O
Si
x
y
x
N
N
O
Cr
Cl
Si
NH
3 O3S
O
O
O
HO Si
y
Catalyst 2
Fig. 1. Ion-exchange reaction of monomer sulphonato-(salen)chromium(III) complex (catalyst 1) with PAPS-Cl.

G. Zhou et al. / Catalysis Communications 25 (2012) 69–73
71
Fig. 2. FT-IR spectra of PAPS-Cl (a), monomer sulphonato-(salen)chromium(III) complex
(catalyst 1, b), and polysiloxane-sulphonato-(salen)chromium(III) complex (catalyst 2, c).
Fig. 3. XRD patterns of the samples: (a) powered PAPS-Cl and (b) powered catalyst 2.
the broad absorption band from 3500 to 3700 cm−1 can be ascribed to
hydroxyl groups of polymeric silanols in PAPS-Cl. Whereas catalyst 1
showed IR bands at 1035 and 1123 cm−1 associated with the symmetric
and anti-symmetric stretching modes of SO3− moiety, together with
others at 1636 cm−1 (CH_N), 1538 cm−1 (Ar\O), 616 cm−1 (Cr\O),
and 554 cm−1 (Cr\N) [16]. Shown in curve c of Fig. 2, the above charac-
teristic bands were all retained in catalyst 2 with slight shifts, confirming
the effective linkage of catalyst 1 with PAPS-Cl, and featuring broad band
of crystal water at 3000–3400 cm−1 as well. Elemental analyses further
supported the [PAPS-Cl]/[catalyst 1] molar ratio for catalyst 2 was 2.
Blank experiments using PAPS-Cl (3 mol% based on substrate) as only
catalyst, 30% H2O2 as terminal oxidant for oxidations of benzyl alcohol,
cyclohexanol, and n-hexanol had been attempted, before regular catalysis
was summarized in Table 1. After eight hours, no considerable oxidations
appeared under TLC and 1H NMR monitoring. In general, however, all
oxidation reactions proceeded smoothly, and satisfactory selectivities
were achieved within 5 h, but conversions of benzyl alcohol were much
higher than those of n-hexanol and cyclohexanol. Catalyst 1 did not afford
pronounced fresh catalysis under 30% H2O2 (entries 1, 6, 11; deposited as
green Cr(III) salts, FT-IR verified), neither did the combination of catalyst
1 with ionic liquids (some of selectivities below 90% for benzyl alcohol,
conversions less than 3% for n-hexanol and cyclohexanol). Catalyst 2,
on the contrary, showed improved and recycling conversions for all
substrates (salen framework retained, FT-IR tested), probably not only
owing to electrostatic linkage of PAPS-Cl with Cr complex, but to lay-
ered structure of PAPS-Cl that enwrapped salen backbone.
Catalytic data also revealed ionic liquids play as a phase-transfer re-
agent to accelerate reaction [27], and in practice faster color darkening
of reaction solution occurred in entries 3 to 5, compared with entry 2.
In recycling outputs of benzyl alcohol (entries 4 and 5), catalyst 2, to-
gether with BMImBF4 or BMImNO3, showed good recyclable conver-
sions which were equal or comparable to the hydrotalcite-supported
(salen)Cr(III) [28], vapor-phase catalysis [29], or organic oxidant system
[4]. Comparatively higher conversions of n-hexanol were obtained in
the presence of BMImNO3 (17% to 22%, entry 15), which might afford
a plausible approach for oxidation of aliphatic alcohol at atmospheric
pressure [3]. But catalytic conversions of cyclohexanol were substantially
retained below 17% (entries 6–10). In addition, commercial aqueous
hydrogen peroxide (30%) was an appropriate terminal oxidant to avoid
over-oxidation.
Table 1
Catalytic C\H oxidation of alcohols to carbonyl compounds using different chromium systemsa.
Entryb
1
Major productc
Catalyst (media)
Timed (h)
5
Conversione (%)
19
Selectivityf (%)
99
1
2
3
4
5
6
2
5
2
2
2
5
69 (75, 82, 85)
71 (70, 82, 95)
79 (82, 95, 93)
86 (90, 77, 88)
4
99
97
98
99
99
2 (BMImPF6)
2 (BMImBF4)
2 (BMImNO3)
1
7
8
9
10
11
12
13
14
15
2
5
3
3
3
5
5
3
3
3
10 (8, 10, 10)
11 (11, 11, 9)
15 (16, 17, 15)
12 (13, 15, 6)
3
17 (17, 17, 18)
9 (10, 10, 7)
24 (25, 26, 20)
17 (19, 19, 22)
99
100
100
99
100
100
100
100
100
2 (BMImPF6)
2 (BMImBF4)
2 (BMImNO3)
1
CH3(CH2)4CHO
2
2 (BMImPF6)
2 (BMImBF4)
2 (BMImNO3)
a
Reaction conditions: substrate (0.1 mol), catalyst (3 mol%, based on the mole of substrate), ionic liquid (20 mL), H2O2 (28 mL, 30%, w/w), and temperature was maintained at
313 K.
b
c
d
e
f
Substrates: benzyl alcohol (entries 1–5), cyclohexanol (entries 6–10), n-hexanol (entries 11–15).
Determined by 1H NMR.
Monitored by TLC.
Determined by 1H NMR, conversions of alcohols to corresponding carbonyl compounds, formatted as cycle fresh (cycle 1, cycle 2, cycle 3).
Determined by 1H NMR in the product of cycle fresh, selectivities of major products. By-products: benzoic acid and benzyl benzoate (from benzyl alcohol, entries 1–5), adipic
acid (from cyclohexanol, entries 6, 7, 10), none (other entries).

72
G. Zhou et al. / Catalysis Communications 25 (2012) 69–73
NH2
NH3Cl
+ HCl
MeO Si OMe
OMe
MeO Si OMe
OMe
Self-organization
Stacking
(Si-O-Si framework)
N
O
N
O
PAPS-Cl
(hexagonal phase)
Cr
Cl
O3S
SO3
N
N
O
N
O
Cr
Cl
O3S
O3S
O
N
SO3
SO3
Cr
Cl
N
O
N
O
O
N
Cr
Cl
NaO3S
SO3Na
N
O
Cr
Cl
Catalyst 1
Rearrangement
O3S
O3S
O3S
O
SO3
SO3
SO3
N
O
N
O
N
O
Cr
Cl
N
O
Cr
Cl
Catalyst 2
Fig. 4. Proposed formation mechanism of catalyst 2.
Fig. 3 shows the XRD patterns of powered PAPS-Cl and catalyst 2.
hydrogen peroxide (30%) was an appropriate terminal oxidant accord-
ing with green chemistry point of view. In addition, a novel structure of
polysiloxane-supported catalyst was put forward to help the design of
new metal salen catalysts.
Spectra (a) in Fig. 3 exhibited two characteristic peaks at 2θ=7° and
2θ=12° that can be ascribed to the hexagonal phase of PAPS-Cl [18].
Additionally, the broad peak centered at 2θ=22°–24° should be at-
tributed to aminopropyl chains packing within layered framework,
which previously explained as a distinctive peak of polysiloxane com-
pounds [30]. In spectra (b), two original peaks (2θ=7° and 12°) disap-
peared, which revealed the dissociation of hexagonal stacking mode of
PAPS-Cl during the formation of catalyst 2. The original broad peak cen-
tered at 2θ=22°–24° was still retained but shifted to lower diffraction
angles, confirming successful grafting of (salen)Cr(III) onto aminopro-
pyl chains of PAPS-Cl and new electrostatic linkages packing within
the layers at a larger gallery spacing. Most likely, during the process of
immobilization, units of PAPS-Cl should have a rearrangement and a
new layered distribution of ladder-typed planes appeared. This pro-
posed formation mechanism was depicted in Fig. 4.
Acknowledgments
This study was supported by Fundamental Research Funds for the
Central University of China.
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