Immobilized bis-layered ionic liquids/peroxotungstates as an efficient catalyst for selective oxidation of alcohols in neat water

Article abstract of DOI:10.1039/C6NJ01476C

An immobilized ionic liquids/peroxotungstates/SiO₂ catalyst was prepared and characterized. The catalyst was proved to be very efficient for the selective oxidation of primary and secondary alcohols to their corresponding carbonyls with benign H₂O₂ as an oxidant in neat water. A 1.0 mol% (based on tungstate) dose of the catalyst was found to be sufficient for the oxidation. The catalyst is easily recovered after reaction via simple filtration, and was reused at least six times without a noticeable loss of the activity. A notable feature of this novel protocol is avoiding any organic co-solvent.

Full text of DOI:10.1039/C6NJ01476C

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Immobilized bis-layered ionic liquids/peroxotungstates as an
efficient catalyst for selective oxidation of alcohols in neat water
Received 00th January 20xx,
Accepted 00th January 20xx
Juan Fan, Fan Pu, Man Sun, Zhong-Wen Liu, Xiao-Yan Han, Jun-Fa Wei and Xian-Ying Shi*
DOI: 10.1039/x0xx00000x
An immobilized ionic liquids/peroxotungstates/SiO
be very efficient for the selective oxidation of primary and secondary alcohols to their corresponding carbonyls with
benign H as an oxidant in a neat water. A 1.0 mol% (based on tungstate) dose of the catalyst was found to be sufficient
2
catalyst was prepared and characterized. The catalyst was proved to
www.rsc.org/
2
O
2
for the oxidation. The catalyst is easily recovered after reaction via a simple filtration, and was reused for at least six times
without a noticeable loss of the activity. Notable feature of this novel protocol is avoiding any organic co-solvent.
Significantly, homogeneous catalytic centers can be introduced
with the immobilized ILs via ion exchange and/or coordination.
However, the immobilized ionic liquids reported are exclusively
Introduction
The selective oxidation of alcohols to corresponding carbonyl
compounds is one of the most important and fundamental
transformation in organic synthesis because the carbonyl products
are important intermediates for synthesizing fine chemicals,
44-52
mono-layered ILs.
are very limited. To overcome this shortcoming, as given in Figure
, we designed bis-layered ionic liquids to bind covalently to silica,
As a result, the introduced catalytic centers
1
which have more imidazoliums per unit area, and more counter-
1
-5
particularly medicines, flavors, fragrances, and aniline dyes. The
traditional oxidation of alcohols are carried out with stoichiometric
amounts of inorganic oxidants such as Cr (VI) or Mn (VII)
compounds, which generates a large amount of heavy metal
anions are exchangeable with anionic reactants or catalytic
53-56
species.
Thus, employing this immobilized bis-layered ILs as a
scaffold, other catalytic components can be introduced via ion
exchange and/or coordination. Moreover, the bis-layered IL
framework can offer a deeper 3D environment that closely
resembles the liquid-phase character of ILs for both hydrophilic
and hydrophobic reactants to move into the brushes where the
active catalyst attached and for the product and by-product to
move out of the catalysts. We have proved their high efficiency
and recyclability for some organic reactions in neat water, such as
6
-7
waste. From the viewpoints of green sustainable chemistry, the
replacement of classic oxidants with benign counterparts is highly
desirable.
As an ideal oxidant, H O has attracted considerable attention in
2
2
recent years because it is environmentally friendly, inexpensive,
readily available, easy to handle, and gives water as the only
8
byproduct. Consequently, in the past decades, many catalytic
2 2
selective oxidation of olefins or sulfoxides with H O , Suzuki and
procedures including homogeneous and heterogeneous systems
have been developed for the selective oxidation of alcohols by
53-56
Heck coupling reaction.
Herein, in continuation of our ongoing research on the
development of novel catalytic systems for selective oxidation of
organic compounds in green media, we demonstrate the
application of the ILs/peroxotungstates/silica as a micro catalytic
reactor for the selective oxidation of primary and secondary
9
-18
using H O2 as a green oxidant.
Compared with homogenous
2
catalysts, the heterogeneous catalysts are of significantly industrial
interest since they can be easily separated from the reaction
mixture and recycled. Although some heterogeneous catalysts
1
9-26
have been applied to oxidize alcohols,
many of them were
2 2
alcohols to their corresponding carbonyls with benign 30% H O as
carried out in toxic organic solvent such as CH CN, dioxane and
3
t
27-33
BuOH,
an oxidant in neat water. As a result of the hydrophilic
imidazoliums and hydrophobic hydrocarbon chains in the ILs, both
which can bring about environmental pollution. In a
sharp contrast, the green solvents have been seen considerably
3
4-37
2 2
the hydrophobic alcohols and the hydrophilic H O oxidant can be
less progress in the oxidation of alcohols.
Therefore, the major
readily diffused into the micro reactor, and give the carbonyls
catalyzed by the peroxotungstates. Thus, the homogeneous
challenge in this area is to accomplish oxidative process in a
nontoxic solvent employing an efficient solid catalyst.
Ionic liquids (ILs) with special solubility and flexible design reaction nature over the ILs/peroxotungstates/silica micro reactor
ability of cations and anions have been employed in modifying makes the oxidation of various alcohols highly efficient, and it can
3
8-43
mesoporous silica for supporting homogenous catalysts.
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Typical procedure for the selective oxidation of alcohols to
carbonyl compounds
In a typical experimental procedure, to a 10mL flask, alcohol (1
2 2
mmol) and 30 wt.% H O (1.4 mmol), the catalyst (1.0 mol% with
respect to the alcohol) and 2mL deionized water were added
successively. The resulting mixture was stirred electromagnetically
at 90 °C and monitored using TLC. After the completion of the
reaction, the catalyst was separated by filtration under reduced
pressure, was washed with EtOH (3 × 5mL) and dried in vacuum.
Fig. 1 The diagram for the ILs/peroxotungstates/silica micro
2
The excess hydrogen peroxide was destroyed using MnO . The
catalytic reactor and its possible working principle.
filtrate was concentrated under reduced pressure. Determination
of products in the reaction mixture was performed on a
QP2010UItra GC-MS instrument (EI) with HP-5 columns.
Quantitative analyses of the reaction products were performed
with an Agilent 6890 N using a capillary column (HP-5, 30m ×
be easily separated from the reaction mixture. Importantly, over
9
5% yield was obtained for the oxidation of benzyl alcohol, and the
catalyst was reused for six times without a noticeable loss of
activity.
0.25mm × 0.53 μm) with an FID detector.
Experimental
Results and discussion
Preparation and characterization of the catalysts
Characteristics of the catalyst
All commercial chemicals were used as received without further
purification. Hydrogen peroxide (30 wt.%) was of AR grade. Before
2
Following the preparation procedure, the formation of the SiO -
2 3 2 4
BisILs[W O (O ) ] catalyst was schematically shown in Scheme 1.
use, it was titrated using a standard KMnO
The SiO -BisILs[Cl] was synthesized via a 3-step route according
to the procedure we previously reported
4
solution.
From IR results (Fig. S1 in Supporting information), the v(C–H) at
-1
-1
2
3163, 2943, 2858cm and v(C–N) at 1639 cm clearly confirms the
[
56]
.
attachment of the ILs onto silica, leading to the formation of SiO
2
-
−
−
The exchange between Cl and PF
6
was realized by the direct
in CH CN at room
BisILs[Cl]. Moreover, 19.57, 2.88, and 7.39 wt.% for C, H, and N,
respectively, were determined by elemental analyses. From the TG
results (Fig. S2 in Supporting information), the amount of the
BisILs[Cl] was determined to be about 28.72 wt.%. Moreover, the
different slopes of the TG curves suggest that the ILs may be
immobilized on silica via a covalent bond as shown in Scheme 1.
reaction of the SiO -BiILs[Cl] and KPF
2
6
3
temperature for 24 h. Then, it was washed consecutively with
water and acetone. After drying under vacuum at 70 °C, the bis-
layered ionic liquids modified SiO
2
-BisILs[PF
], 10 mL 30% H
WO ·2H O (0.725, 2 mmol) in 10 mL
6
] was obtained.
For preparing K
2
[W
2
O
3
(O
2
)
4
2 2
O
was added under
stirring to the solution of K
2
4
2
-
-
After anion exchanging between Cl and PF
6
, a weight loss of
water at room temperature. Then, diluted HCl aqueous solution
was added until the solution was changed from yellow to colorless,
3
2.31 wt.% was obtained from the TG analysis (Fig. S2 in
Supporting information), which matches well with the substitute of
indicating the formation of K
SiO -BisILs[PF ] was added to the colorless solution for the anion
exchange. After stirring at room temperature for 36 h, it was
2 2 3 2 4
[W O (O ) ]. After this, 1 g of the
2
6
filtrated and dried. Finally, the SiO
obtained.
2 2 3 2 4
-BisILs[W O (O ) ] catalyst was
The porous properties of the catalyst were evaluated by the
nitrogen adsorption/desorption method at -190 °C on a Peking
Pioneer-2002 ST-03A instrument. The surface area of the catalyst
was determined by using the Brunauer–Emmett–Teller (BET)
method, and Pore size distributions were calculated from the
desorption branch using the BJH model. The SEM-EDAX was
performed on
a Philips-FEI Quanta 200 scanning electron
microscopy equipped with an EDAX energy dispersive X-ray
analysis. The elemental analyses of C, H and N were performed on
a Perkin-Elmer 2400 CHN elemental analyzer. IR spectra were
recorded on a Bruker Equinox-55 spectrophotometer (KBr pellets
−
1
in the range of 400–4000cm ). The thermogravimetric differential
scanning calorimetry (TG-DSC) was carried out on
a
Q1000DSC+LNCS+FACSQ600SDT thermogravimetric analyzer. The
sample was heated under air atmosphere from room temperature
o
o
to 800 C at a ramp of 10 C/min.
Scheme 1 Preparation of ILs/peroxotungstates/silica catalyst.
2
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-
-
PF
6
for Cl . This was also confirmed from the SEM-EDAX results (Fig. average pore size of about 8 nm, very narrowed pore size
distribution, and a reasonably high surface area.
S3 in Supporting information). Moreover, the absence of chloride
from the EDAX analyses reveals that the anion exchange between
-
-
Optimizing reaction conditions
Cl and PF
leading to the formation of SiO
The further formation of SiO
6
is completely finished under the conditions applied,
-BisILs[PF ].
2 3 2 4
-BisILs[W O (O ) ] was apparently
2
6
In the absence of the catalyst, control experiment indicates that
2
the oxidation of benzyl alcohol under the conditions of
seen from the TG results (Fig. S2 in Supporting information), in
which its weight loss of 24.64 wt.% is clearly smaller than that of
o
H
2
O
2
/alcohol molar ratio of 1.4, temperature of 90 C and reaction
time of 18 h affords a negligible amount of benzaldehyde (Table 1,
entry 1). When either SiO -IL[Cl] or SiO -IL[PF ] were used, the
yields of benzaldehyde were less than 20% (Table 1, entries 3, 4).
[W (O ] as a homogeneous catalyst, a 76% yield of
2
SiO -BisILs[Cl] (28.72 wt.%). Moreover, the IR spectrum (Fig. S1 in
Supporting information) shows the characteristic peaks of
2
2
6
2
-
-1
-1
[
W
2
O
3
(O
2
)
4
]
structure at 947cm for
v
(W=O), 839cm for
v
(O–O),
-
1
and 563 cm for
v
asym(WO
2
), which is similar to the previous By using K
2
2
O
3
2 4
)
58
report. The EDAX results (Fig. S4 in Supporting information) benzaldehyde was obtained under the same conditions. These
conclusively
indicate
the
successful
introduction
of results clearly indicate that the peroxotungstates is the effective
peroxotungstates. However, the presence P and F indicates that
catalytic site for the selective oxidation reactions. Under the same
2
-
-
the anion exchange between [W
2
O
3
(O
2
)
4
]
and PF
6
is not
conditions, a very high yield of 96% was achieved by using SiO
2
-
complete. Based on these results, the amount of the tungsten and
phosphorus are estimated to be 0.334 and 0.165 mmol/g,
respectively, revealing that the catalyst contains about 0.25 mmol
of imidazolium/g.
BisILs[W O (O ) ] as a catalyst. Thus, the hydrophobic and
2
3
2 4
hydrophilic environment makes the SiO -BisILs[W O (O ) ] an
2
2
3
2 4
efficient micro catalytic reactor for the selective oxidation of
hydrophobic benzyl alcohol with hydrophilic H O . Since the anion
2
2
The porous properties of the SiO
important for its catalytic functions as a result of the liquid-phase
reaction. From the N adsorption-desorption isotherms of the
catalyst (Fig. 3, ), a type IV isotherm together with an H4-type
hysteresis loop was observed according to the IUPAC
2 2 3 2 4
-BisILs[W O (O ) ] are
2-
-
exchange between [W O (O ) ] and PF6 is more efficient than
2
3
2
2 4
-
-
that between [W O (O ) ] and Cl , the catalyst prepared by direct
2
3
2 4
2
-
exchanging Cl with peroxotungstates also gave a lower yield of
product (Table 1, entry 5), which can be reasonably ascribed to the
lower loading of peroxotungstates.
a
57
classification. The BET surface of the catalyst was determined to
2
The oxidation of alcohols to carbonyls was studied in detail by
optimizing the reaction variables such as the molar ratio of
alcohol/H O , alcohol/catalyst, and reaction temperature. Benzyl
be 136.8 m /g. The parallel part of the hysteresis loop in the
regions of P/P
pore size of about 8.06 nm was determined from the pore size
distribution calculated by the BJH method (Fig. 3, ). Moreover,
0
＞ 0.7 implies open and cylindrical pores. A peak
2
2
alcohol was selected as a model substrate to optimize reaction
conditions and water was used as a solvent. In view that the choice
of reaction temperature is vital to the oxidation reaction, the
influence of the reaction temperature on the selective oxidation of
benzyl alcohol was firstly optimized under the conditions of 1
mol% catalyst, H O /alcohol molar ratio of 1.4, and reaction time
b
the mesoporous structure of the catalyst was clearly reflected
from the sharp pore size distribution. Thus, high catalytic
2 2 3 2 4
performance of SiO -BisILs[W O (O ) ] for the titled reaction can
be reasonably expected from its mesoporous structure with an
2
2
a
o
Ad so r p to i
n
of 18 h. With increasing the reaction temperature from 60 to 95 C,
the yield of benzaldehyde was firstly increased almost linearly, and
250
200
150
100
Desorption
o
the highest yield of 96% was obtained at 90 C (Fig. 3). The further
o
increasing of the reaction performed to 95 C afforded a sharp
Table 1 Selective oxidation of benzyl alcohol to benzaldehyde
a
employing different catalysts
50
CH OH
o
CHO
2
3
0 wt% H O , 90 C
0
2 2
0.0
0.2
0.4
0.6
0.8
1.0
2
H O, catalyst, 18 h
Relative Pressure
(
P/Po)
b
b
Entry
Catalyst
No
SiO -BisILs[W O (O ) ]
Yield(%)
0.05
0.04
0.03
0.02
0.01
0.00
1
2
trace
96
13
16
33
2
2
3
2 4
c
3
4
5
SiO
2
-BisILs[Cl]
d
-
SiO -BisILs[PF ]
2
6
e
-
2-
SiO -BisILs[(2m-2k)Cl ]{k[W O (O ) ] }
2
2
3
2 4
6
K [W O (O ) ]
76
2
2
3
2 4
a
Condition: alcohols (1 mmol), H
2
O
2
(1.4 mmol), H
2
O (2 mL), catalyst (1mol%,
b
based on the content of tungsten), were stirred at 90℃ for 18 h. GC yields.
c
d
-
e
0
20
40
60
80
100
nm
120
140
160
SiO
2 2 6
-BisILs[Cl] as catalyst. SiO -BisILs[PF ] as catalyst. Catalyst prepared by
Pore Diameter
(
)
the chloride anions directly exchanged with peroxotungstates.
Fig. 2 Nitrogen adsorption-desorption isotherm (a) and the
BJH pore size distribution of the SiO -BisILs[W (O ] (b).
2
2
O
3
2 4
)
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o
decrease of the yield due to the formation of over-oxidation 1 mol% catalyst at 90 C for 18 h (Fig. 5). The highest yield of the
o
products. Thus, the optimal temperature of 90
C
was product was obtained at 1.4 equiv. H
than the stoichiometric value. The reason may be that the
occurs in the oxidation process and
effect of amount of catalyst on the yield was studied by using 1.4 the excess of oxidant could increase the reaction rate.
2 2
O , which is slightly higher
recommended.
2 2
To continue with the study of the reaction optimization, the decomposition of some H O
o
equiv. H
2
O
2
at 90 C for 18 h, and the results are given in Fig. 4.
mol% catalyst is sufficient for this The generality of the catalytic system
Noticeably, only
1
transformation. However, when the amount of the catalyst was
further increased from 1 to 2 mol%, the yield of benzaldehyde was
continuously decreased, which can be reasonably ascribed as the
over-oxidation of the benzaldehyde as confirmed from GC analysis
results.
With the optimized conditions in hand, a variety of alcohols
including aromatic and aliphatic alcohols were employed to
investigate the generality of this oxidative system with neat water
as a solvent. From the results given in Table 2, although
substituted-benzylic alcohols were readily selectively oxidized to
the desired products in satisfied yields, a significant electronic
effect of substituted groups on the activity was observed. The
electron-rich benzylic alcohols such as methyl and methoxy, in
generally, gave better yields (Table 2, entries 2, 3). Benzylic
alcohols bearing weak electron-withdrawing halogen substituents
led to a decreased efficiency (Table 2, entries 4-6), which may due
to the formation ofthe acid as a by-product. The presence of a
strong electron withdrawing group of NO2 in the phenyl ring
afforded the desired product in a lower yield of 75% (Table 2, entry
The influence of the amount of H O was also investigated with
2
2
100
80
60
40
20
0
7). Moreover, the steric effect could be observed by comparing the
60
70
80
Reaction temperature/
90
100
results with 2-bromobenzylic alcohol and 4-bromobenzylic alcohol
as a substrate (Table 2, entries 5, 6). Although this oxidation
tolerated dual-substituted benzylic alcohol, the lowest yield of 67%
was achieved with 3,4-dimethoxylbenzylic alcohol (Table 2, entry
o
C
Fig. 3 Effect of reaction temperature on the selective oxidation of
benzyl alcohol to benzylaldehyde.
8).
100
Apart from aromatic primary alcohols, aliphatic primary alcohol
9
9
8
8
7
7
5
0
5
0
5
0
is also active in this transformation, and provides the aldehyde in
an excellent yield (Table 2, entry 9). Moreover, aromatic secondary
Table 2 The selective oxidation of different alcohols catalyzed by
a
SiO -BisILs[W O (O ) ] with H O
2
2
2
3
2 4
2
OH
H O
O
2
2
R1
R2
H O, catalyst
R1
R2
2
b
c
0
.5
1.0
1.5
2.0
1
2
R
Time Selectivity
Yield
(%)
96
94
88
81
80
73
75
67
90
91
58
88
84
89
Amount of catalyst/mol%
Entry
R
(
h)
(%)
98
95
96
87
85
88
82
97
93
99
100
99
98
99
Fig. 4 Effect of the amount of catalyst on the selective oxidation of
1
2
3
4
5
6
7
8
9
C6H5
6 4
4-CH OC H
H
H
H
H
H
H
H
H
H
18
18
18
22
22
22
30
26
24
26
24
18
26
18
benzyl alcohol to benzylaldehyde.
3
3
6 4
2-CH C H
1
00
4-ClC H
6
4
90
80
70
60
50
40
30
20
4-BrC H
6
4
2-BrC H
6
4
2
6 4
4-NO C H
3,4-(CH O) C H
3
2
6
3
cyclohexyl
1
1
0
1
n
-hexyl
CH3
C6H5
CH3
C6H5
C H
12
13
6
5
1
2
R =R = (CH )
2
5
1
.0
1.1
1.2
1.3
1.4
1.5
1
4
C6H
5
H
Amount of hydrogen peroxide/equiv.
a
Conditions: alcohols (1 mmol), H
2
O
2
(1.4mmol), H
2
O (2 mL), catalyst (1 mol%)
Fig. 5 Effect of the amount of hydrogen peroxide on the selective
b
based on content of tungstate, were stirred at 90℃. Select.=selectivity. The
oxidation of benzyl alcohol to benzylaldehyde.
c
selectivity of product was calculatedbasedon GC. GC yields.
4
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alcohols and aliphatic alcohols could also be applied in this any organic co-solvent, and high yields of carbonyl compounds
were achieved. The catalyst can be easily recovered and reused at
least for six times without significant loss of activity. Because of its
easy recyclability, the use of hydrogen peroxide as an oxidant, and
neat water as a medium, it distinguishes itself by green and
economical advantages from other supported catalysts.
oxidation system and provided ketones in good yields (Table 2,
entries 10-13).
To gain an insight into the applicability of this catalytic system,
the scaled-up experiment was carried out employing benzyl
alcohol as a substrate under the optimized conditions. When the
amount of benzyl alcohol increases from 1to 5 mmol, only a
limited decrease in the yield of benzaldehyde was observed(Table
Acknowledgements
2
, entry 14), which demonstrates that this heterogeneous
The authors are grateful to the National Science Foundation of
China (Grant No. 21572122 and 21272145), Shaanxi Innovative
Team of Key Science and Technology (2013KCT-17), the
Fundamental Research Funds for the Central Universities (Grant
GK201503030 and GK 201601005) for providing financial supports.
oxidation has a potentially scaled-up application.
2 2 3 2 4
The recycling of SiO -BisILs[W O (O ) ]
One of the main advantages of heterogeneous catalysts is the
convenience of separation and the successive catalytic cycles. The
recyclability of the developed SiO -BisILs[W O (O ) ] was evaluated
2
2
3
2 4
by performing consecutive cycles of the oxidation of benzyl alcohol Notes and references
under the optimized conditions. After aprevious catalytic run, the
1
2
3
G.-J. ten Brink, I.W.C.E. Arends and R.A. Sheldon, Science, 2000,
87, 1636–1639.
catalyst was separated from the reaction solution, washed several
times with ethanol to remove any physically absorbed molecules,
dried and then reused in the next catalytic run. As shown in Fig. 6,
after 6 cycles, a steady yield of above 90% was still kept, indicating
that the activity of the catalyst did not change significantly
compared with the fresh sample. This is clearly supported by the IR
results (Fig. S5 in Supporting information), in which similar IR
patterns are obtained for the fresh and the recovered catalyst
2
R.A. Sheldon, I.W.C.E. Arends, G.-J. ten Brink and A. Dijksman,
Acc. Chem. Res., 2002, 35, 774–781.
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 and
G.J. Hutchings, Science,2006, 311, 362–365.
4
5
T. Mallat and A. Baiker, Chem. Rev.,2004, 104, 3037–3058.
V.R. Choudhary, R. Jha and P. Jana, Green Chem., 2007,
72.
9, 267–
2
after used for six times. Moreover, the BET surface area (130 m /g)
2
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100
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DOI: 10.1039/C6NJ01476C
Graphical Abstract
Immobilized bis-layered ionic liquids/peroxotungstates as an
efficient catalyst for selective oxidation of alcohols in neat water
Juan Fan, Fan Pu, Man Sun, Zhong-Wen Liu, Xiao-Yan Han, Jun-Fa Wei, Xian-Ying Shi
Key Laboratory for Macromolecular Science of Shaanxi Province, School of Chemistry and Chemical Engineering,
Shaanxi Normal University, Xi’an 710062 China
An environmentally benign and reusable immobilized bifunctional ionic
liquid/peroxotungstate catalyst has been developed for the selective oxidation of alcohols to
2 2
corresponding carbonyls with aqueous H O in neat water without any co-organic solvent. The
catalyst can be reused at least six times without noticeable loss of the catalytic activity.