Photooxygenation of hydrocarbons over efficient and reusable decatungstate heterogenized on hydrophobically-modified mesoporous silica

Article abstract of DOI:10.1039/b821987g

Decatungstate W₁₀O₃₂^4- heterogenized on a novel hydrophobically-organomodified SBA-15 was proven an efficient, green and reusable catalyst for photooxygenation by oxygen of a range of aryl alkanes to the corresponding ketones under mild conditions.

Full text of DOI:10.1039/b821987g

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Photooxygenation of hydrocarbons over efficient and reusable
decatungstate heterogenized on hydrophobically-modified
mesoporous silicaw
Lingli Ni,a Ji Ni,b Yuan Lv,a Ping Yang*a and Yong Cao*b
Received (in Cambridge, UK) 9th December 2008, Accepted 30th January 2009
First published as an Advance Article on the web 25th February 2009
DOI: 10.1039/b821987g
Decatungstate W10O324À heterogenized on a novel hydrophobically-
organomodified SBA-15 was proven an efficient, green and
reusable catalyst for photooxygenation by oxygen of a range
of aryl alkanes to the corresponding ketones under mild
conditions.
limited activity or stability during the reaction.12 In view of
the intrinsic hydrophilic nature of the mesoporous silica
surface (the active sites in which may not be easily accessible
to the hydrocarbon molecules), it is conceivable that intro-
duction of additional hydrophobic organic fragments onto the
silica surface may provide unique opportunity for fabricating
4À
new efficient heterogeneous W10O32
system. Herein, we
Direct conversion of hydrocarbons into highly desirable
oxygenates is one of the most challenging reactions in
synthetic chemistry.1 Although a number of methods have
been developed, the search for new facile, cost-effective, energy
efficient and green procedures has attracted substantial
interest.2 An attractive alternative is the photocatalytic
oxygenation with dioxygen or air as the sole terminal oxidant,
which represents a key strategy towards the development of
new sustainable methods for ‘‘greener’’ chemical synthesis.3 In
particular, the development of soluble polyoxometalates
(POMs) as efficient catalysts that can allow solar-
photoassisted oxygenation of alkanes under mild conditions
has attracted special attention.4 One distinct advantage of
POMs is that a wide range of redox potentials can be
readily attained,5 which enables a remarkable array of homo-
geneous oxidation reactions.4–7 Among them, decatungstate
4À
report for the first time heterogenization of W10O32 on a
novel hydrophobically-modified SBA-15, which was proven
an efficient, green and reusable photocatalyst for oxygenation
of a range of aryl alkanes under ambient conditions.
The organomodified SBA-15 was prepared by an initial
insertion of alkyl groups via refluxing with Cn-Si(OEt)3,
followed by grafting 3-aminopropyl groups on to the parent
support.13 The resultant material was denoted as Cn-AP-SBA,
where Cn is the alkyl group (n is the number of the carbon
atoms of the alkyl group) and AP is the 3-aminopropyl
groups. A series of photocatalysts having different alkyl-
chain length was prepared by acidification of Cn-AP-SBA
using trifluoromethanesulfonic acid and ion exchange by
decatungstate,14 denoted as W10/Cn-AP-SBA (experimental
details are shown in the ESIw). In this process, the
3-ammoniumpropyl groups, –(CH2)3NH3+, immobilize the
[W10O32]4À polyanions on the pore walls while the alkyl
groups form hydrophobic regions around the decatungstate.
A decrease of the BET surface areas is observe after the post-
grafted process; the BET surface areas of SBA-15, AP-SBA,
C8-AP-SBA and W10/C8-AP-SBA are 757, 613, 461 and
4À
W10O32 appears to be particularly suitable for the catalytic
functionalization of inert C–H bonds owing to its powerful
oxidizing capacity.7 From a practical point of view, however,
it is more attractive to develop
a solid and reusable
photocatalyst that is efficient for this transformation.
Heterogenization of POMs has attracted tremendous recent
attention by providing an easily adaptable technique for
fabrication of new recoverable photocatalytic materials with
well-defined and tailored molecular structure.8–11 So far,
several technical procedures including simple impregnation
of a solid carrier8 and immobilization with amino-functionalized
amorphous silica9 have been developed to prepare hetero-
376 m2
g
À1, respectively. Infrared spectra, UV-Vis spectra
and ICP analysis confirmed that the molecular structure of
the decatungstate anions was maintained in the catalysts
(see ESIw). TEM, N2 adsorption isotherms confirmed that
the catalysts maintained the mesostructure of SBA-15
(see ESI). Elemental analysis of the heterogenized
W10/Cn-AP-SBA using ICP gave ca. 12 wt% loading of
decatungstate.
4À
genized W10O32 catalysts for various applications. Other
methods like embedding in polymeric membranes10 or inside
the silica network11 have also been used. However, these
methods have presented general problems which include
To start our investigations, we examined the photo-
oxygenation of ethylbenzene using dioxygen (O2) as the oxidant
at ambient temperature (see ESI for experimental detailsw).
Several tungsten-based catalyst systems were investigated,
including the homogeneous POM system (Table 1, entry 1),
solid WO3 (Table 1, entry 2), and the heterogenized
decatungstate system with various surface modification
(Table 1, entries 6–10). Of all catalysts tested, the octyl-grafted
catalyst showed the highest reactivity (Table 1, entry 9). All
other tested catalysts showed lower or no reactivity. It is
interesting to note that the corresponding catalytic activities
a Department of Chemistry, East China Normal University, Shanghai,
200062, PR China. E-mail: pyang@chem.ecnu.edu.cn;
Fax: +86 21 62232414; Tel: +86 21 62238536
b Department of Chemistry & Shanghai Key Laboratory of Molecular
Catalysis and Innovative Material, Fudan University, Shanghai,
200433, PR China. E-mail: yongcao@fudan.edu.cn;
Tel: +86 21 55665287
w Electronic supplementary information (ESI) available: Experimental
section, TEM, UV-Vis and FTIR data. See DOI: 10.1039/b821987g
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Table 1 Results on ethylbenzene photooxidation to acetophenone
with O2 over different catalysts in CH3CN as solventa
primary products during the oxygenation of alkanes by
illuminated (Bu4N)4W10O32.16 While the reaction in pure
water showed moderate selectivity toward ketone products
(Table 2, entry 8), the use of cosolvent of CH3CN–H2O
mixture can afford the formation of acetophenone with a high
selectivity well above 90%. The ratio of water to acetonitrile
was varied (Table 2, entries 5–7) which revealed that a
1 : 1 mixture provided the most active and selective catalyst.
Presumably, appropriate amount of water can favour the
decomposition of hydroperoxides to give the formation of
acetophenone in the presence of W10/C8-AP-SBA under the
photooxygenation reaction conditions.17
Conversion Selectivity
(mol%)
Entry Catalyst
S/Cb (mol%)
TOF/hÀ1
1c
2
(Bu4N)4W10O32 300
44
1.6
—
6.3
4.7
64.4
96
—
77.6
83.6
84
88
90
22
0.8
—
3.2
2.4
23.5
24.5
26.5
30
WO3
AP-SBA
300
—
3
4d
5e
6
PW/AP-SBA
SiW/AP-SBA
W10/AP-SBA
300
300
300
One important issue when working with heterogenized
systems is the recyclability of the final material during the
course of the reaction. The W10/C8-AP-SBA catalyst was
stable and can be easily reused during reactions in
CH3CN–H2O (1 : 1 v/v) mixture. After the first ethylbenzene
oxidation, the catalyst was separated from the reaction
mixture by filtration, thoroughly washed with acetone, and
then reused as catalyst for the next run under the same
conditions. As shown in Table 3, the acetophenone yields
remained essentially constant for the five successive cycles,
reflecting high stability and reusability of the catalyst. This is
consistent with the characterization results for this catalyst.
TEM (Fig. S2w) and IR (Fig. S3w) results show essentially no
changes in the nanostructure of the SBA-15 support and
decatungstate species, respectively, after the five successive
runs. Moreover, it was confirmed by ICP that the W content
of the used W10/C8-AP-SBA catalyst was the same as that of
the fresh catalyst and that no W was in the filtrate.
47
7
8
9
10
W10/C2-AP-SBA 300
W10/C4-AP-SBA 300
W10/C8-AP-SBA 300
W10/C16-AP-SBA 300
49
53
60
52
90.1
91
26
a
Reaction conditions: ethylbenzene = 1.7 mmol, 13 mL solvent,
b
oxygen flow = 3 mL minÀ1, reaction for 6 h. S/C is the molar ratio
c
of ethylbenzene to decatungstate. (Bu4N)4W10O32 was dissolved in
CH3CN. Reported values are the mean of two repeated experiments
d
3À
e
4À
and are Æ 3%. PW means PW12O40
.
SiW means SiW12O40 .
of the organomodified catalysts comprising ethyl, butyl, and
octadecyl groups were all lower than those of the octyl-grafted
catalyst. The reason for such alkyl length-dependence
behavior is probably due to the similar dimension of the octyl
4À
groups relative to that of the decatungstate W10O32 species
(ca. 1 nm), which can afford more suitable conditions for the
highly active photooxygenation.15
To examine the scope of the photooxygenation reaction
with the W10/C8-AP-SBA system, we extended our studies to
various aryl alkanes. The results are summarized in Table 4. It
was found that the photooxygenation of alkylbenzenes with
varying alkyl chain length (C2–C4) all proceeded efficiently,
giving the corresponding phenones as mainly products
(Table 4, entries 1–3). The decreasing selectivity toward ketone
products with increasing chain length is indicative of a
preferential oxidation of the C–H bond at a-methylene group.
It is especially noteworthy that W10/C8-AP-SBA was highly
active and selective for the photooxygenation of ethylbenzene,
with an almost exclusive formation of acetophenone corres-
ponding to a turnover frequency (TOF) of ca. 47 hÀ1 for
chemoselective conversion of ethylbenzene can be achieved
(Table 4, entry 1). In contrast, a previously reported decatung-
state-based system is found to be moderately active for
photoconversion of this substrate.18 Moreover, the
W10/C8-AP-SBA showed high activity even when the reaction
was carried out using air in place of O2, although a longer
reaction time is required (Table 4, entry 7).
Next, we investigated the influence of the solvent on the
performance of W10/C8-AP-SBA in more detail (Table 2).
Remarkably, an equal volume mixture of CH3CN–H2O
(1 : 1 v/v) is revealed as the solvent of choice. Changing the
solvent to dichloromethane and 1,2-dichloroethane lowered
conversion of ethylbenzene to 40% and 46% (Table 2,
entries 1 and 2), respectively. An even lower conversion was
found when the solvent was changed to acetone or water
(Table 2, entries 3 and 8). It is noted that about 9% of
oxygenated product is hydroperoxides using pure CH3CN as
solvent after reaction for 6 h, which are found to be the
Table 2 Results of ethylbenzene photooxidation to acetophenone
with O2 over W10/C8-AP-SBA in various solventsa
Conversion Selectivity
(mol%)
Entry S/C
Solvent
(mol%)
TOF/hÀ1
1
2
3
4
5
300
300
300
300
300
CH2Cl2
40
46
35
60
67
74.8
69.7
88.6
90.1
93.6
20
23
18
30
34
ClCH2CH2Cl
CH3COCH3
CH3CN
CH3CN–H2O
(3 : 1 v/v)
Table 3 Photooxidation of ethylbenzene with O2 catalyzed by
W10/C8-AP-SBAa
6
7
300
300
300
CH3CN–H2O
(1 : 1 v/v)
CH3CN–H2O
(1 : 3 v/v)
H2O
87
78
8
96.5
97.4
87.5
44
39
4
Reuse number
%Yield of acetophenone
1
85
2
84
3
83
4
83
5
83
8
a
Reaction conditions: ethylbenzene
=
1.7 mmol, 13 mL
,
a
CH3CN–H2O (1 : 1 v/v) as solvent, oxygen flow = 3 mL minÀ1
Reaction conditions: ethylbenzene = 1.7 mmol, 13 mL solvent,
oxygen flow = 3 mL minÀ1, reaction for 6 h at 5–10 1C.
2172 | Chem. Commun., 2009, 2171–2173
reaction for 6 h at 5–10 1C.
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Table 4 Results for the photooxidation of various alkanes catalyzed by W10/C8-AP-SBA using dioxygena
Entry
Substrate
Reaction time/h
Conv. (mol%)
Major product (selectivity, mol%)
1
2
3
4
Ethylbenzene
Propylbenzene
Butylbenzene
Cumene
Diphenylmethane
Cyclohexyl-Benzene
Ethylbenzene
7
8
6
8
12
6
97
81
44
87
78
28
89
Acetophenone (99)
Propiophenone (90.8)
Butyrophenone (50.5)
Acetophenone (51), 1,1-dimethylbenzyl alcohol (47)
Benzophenone (95.3)
5
6
3-Phenylcyclohexanone (64), 4-phenylcyclohexanone (25)
Acetophenone (97)
7b
7
a
b
Reaction conditions: S/C = 300, 13 mL CH3CN–H2O (v/v) = 1 : 1 as solvent, oxygen flow = 3 mL minÀ1, temperature 5–10 1C. Using air as
oxidant.
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4À
Given the excellent chemoselectivity of heterogenized W10O32
for ethylbenzene oxygenation, it was decided to investigate
whether W10/C8-AP-SBA could provide a viable solution to
this challenge. The result showed that W10/C8-AP-SBA
mediated photooxygenation can afford highly efficient photo-
catalytic oxidation of cyclohexane to cyclohexanone (Scheme 1).
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hexanone production can be achieved for cyclohexane
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In conclusion, we have demonstrated that W10O324À hetero-
genized on hydrophobically modified SBA-15 has great
potential for photooxygenation of a range of aryl alkanes to
corresponding ketones under mild conditions. This catalyst
has been proven to be highly efficient and chemoselective in
the photooxygenation of ethylbenzene and cyclohexane. The
efficiency and stability of the catalyst has also been demon-
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appreciable drop in the reaction rate.
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Financial support by the National Natural Science Founda-
tion of China (20633030, 20721063 and 20873026), the Shanghai
Science & Technology Committee (07QH14003) and Shanghai
Education Committee (06SG03) is kindly acknowledged.
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Chem. Commun., 2009, 2171–2173 | 2173