Clean and selective Baeyer-Villiger oxidation of ketones with hydrogen peroxide catalyzed by Sn-palygorskite

Article abstract of DOI:10.1016/j.jorganchem.2006.09.038

An environmentally benign and selective Baeyer-Villiger oxidation system is introduced. Palygorskite-supported Sn complexes were prepared by a simple procedure. Cyclic ketones and acyclic ketones were oxidized by hydrogen peroxide in a reaction catalyzed by palygorskite-supported Sn complexes, affording corresponding lactones or esters with selectivity for the product of 90-99%. The influence of the solvents, reaction temperature, the amount of catalyst used and the reaction time on the catalytic activity and product selectivity were investigated in detail. The catalyst is cheap, easy to be prepared in large scale and can be recycled.

Full text of DOI:10.1016/j.jorganchem.2006.09.038

Journal of Organometallic Chemistry 691 (2006) 5767–5773
www.elsevier.com/locate/jorganchem
Clean and selective Baeyer–Villiger oxidation of ketones with
hydrogen peroxide catalyzed by Sn-palygorskite
*
Ziqiang Lei , Qinghua Zhang, Rongmin Wang, Guofu Ma, Chengguo Jia
Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University,
Lanzhou 730070, China
Received 26 June 2006; received in revised form 27 August 2006; accepted 15 September 2006
Available online 29 September 2006
Abstract
An environmentally benign and selective Baeyer–Villiger oxidation system is introduced. Palygorskite-supported Sn complexes were
prepared by a simple procedure. Cyclic ketones and acyclic ketones were oxidized by hydrogen peroxide in a reaction catalyzed by paly-
gorskite-supported Sn complexes, affording corresponding lactones or esters with selectivity for the product of 90–99%. The influence of
the solvents, reaction temperature, the amount of catalyst used and the reaction time on the catalytic activity and product selectivity were
investigated in detail. The catalyst is cheap, easy to be prepared in large scale and can be recycled.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Baeyer–Villiger oxidation; Palygorskite; Ketone oxidation; Sn-palygorskite catalyst
1. Introduction
potentially produce large amounts of harmful wastes.
Much recent effort has been devoted to find the methods
that use chemically green oxidants such as low concentra-
tion hydrogen peroxide and molecular oxygen along with
the recyclable catalysts [15–18].
Baeyer–Villiger oxidation was first reported by Baeyer
and Villiger in 1899 when they converted menthone, cam-
phor and carvone into the corresponding lactones with
Caro’s acid as an oxidant [1]. Usually, this transformation
is carried out by peroxy compounds such as peracids [2,3]
and hydroperoxides [4–9]. Acids, bases, enzymes, and
metal-containing reagents are known to catalyze Baeyer–
Villiger oxidations [10–13].
Baeyer–Villiger oxidation is now a frequently used syn-
thetic tool for the conversion of cycloalkanones to lac-
tones. The reaction can also be applied in the synthesis of
a wide variety of other chemicals, ranging from simple
monomers used in the polyester industry to the more com-
plex molecules that applied for the synthesis of pharmaceu-
tical products [14]. However, the oxidants used in the
traditional Baeyer–Villiger oxidation are organic peroxy
acids which generally prepared by the utilization of excess
hazardous concentration levels of hydrogen peroxide and
Supported catalysis, in particular, accorded with green
chemistry by allowing easy separation of the products
and permitting the recycling and reuse of the catalysts with
operational and economical advantages. Polymer-
anchored metal complexes [19–21], solid acids [4], Sn-
MCM-41 [22], sulphonated resins [23], titanium silicalites
[24], transition metal-functionalized hydrotalcites [25],
hydrotalcite-supported Sb catalyst [26] and Sn-synthesized
hydrotalcites [27] are examples of some of the heteroge-
neous catalysts used to perform the Baeyer–Villiger reac-
tion. The best catalytic system for the Baeyer–Villiger
reaction was reported by Corma and his colleagues (see
Strukul’s comment [28]). Corma et al. [29] found that Sn-
zeolite beta was a very active and selective catalyst for
the Baeyer–Villiger oxidation of cycloketones with environ-
mentally benign hydrogen peroxide as an alternative to
organic peroxy acids. The Sn-zeolite beta was synthesized
using dealuminated nanocrystalline (20 nm) zeolite as seeds
*
Corresponding author. Tel./fax: +86 931 7970359.
E-mail address: leizq@nwnu.edu.cn (Z. Lei).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.09.038

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Z. Lei et al. / Journal of Organometallic Chemistry 691 (2006) 5767–5773
and a crystallization procedure was performed in Teflon-
lined stainless steel autoclaves, which were heated to
140 ꢁC and rotated over a period of 20 days.
gorskite (Fig. 1), Sn-palygorskite was obtained. This mate-
rial acts as an efficient and stable heterogeneous catalyst for
the Baeyer–Villiger oxidation. The effect of solvent, the
reaction temperature, the amount of catalyst used and
the reaction time on the catalytic activity and product
selectivity were investigated.
Low concentration of hydrogen peroxide (<30%) is a
highly attractive oxidant for a number of reasons: it is a
cheap, mild and an environmentally benign reagent with
a high content of active oxygen, and water is formed as the
only by-product. However, expensive catalysts are normally
required to effect oxygen transfer from hydrogen peroxide to
the substrate. Many transition metal complexes have been
used for the activation of hydrogen peroxide [30–33].
2. Experimental
SnCl2 Æ 2H2O, 2-adamantanone, cyclohexanone, 2-meth-
ylcyclohexanone, 4-methylcyclohexanone, 2-tert-butylcy-
In a previous article [34], we report the catalytic Baeyer–
Villiger oxidation of ketones with a recyclable catalytic sys-
tem using palygorskite as a catalytic supporter and 30%
hydrogen peroxide as oxidant. We reported here the
detailed experimental results of this interesting oxidation
system. The procedure for the preparation of the Sn-paly-
gorskite is much simpler. Furthermore, palygorskite is a
biocompatible and environmentally friendly natural min-
eral abundant in deposits all over the world. The catalyst
can be prepared in large scale and can be recycled.
Palygorskite is a natural clay mineral characterized by a
porous crystalline structure containing tetrahedral layers
alloyed together by longitudinal sideline chains. A typical
unit cell consists of (Mg,Al)5Si8O20(OH)2(H2O)4 Æ 4H2O,
with Mg preferentially located in octahedral sites. These
mineral clays possess Mg2+ and Al3+ cations that can be
easily exchanged [35]. Due to its sorptive and rheological
properties, palygorskite is widely used in different indus-
trial fields. In spite of numerous studies on the catalytic
properties of palygorskite, organic reactions catalyzed by
palygorskite supported catalyst, especially for the oxida-
tion reactions have been scarcely reported. We reported
that cyclic ketones and acyclic ketones were oxidized by
30% hydrogen peroxide in a reaction catalyzed by paly-
gorskite-supported Sn complexes. We showed that upon
incorporation of 2.41% of tin into the framework of paly-
clohexanone,
4-tert-butylcyclohexanone,
3-methyl-
2-pentanone, 4-methyl-2-pentanone, and cyclopentone
were obtained from commercial sources and used as
received without further purification.
Sn-palygorskite was prepared according to the following
procedure. Palygorskite (1 g) (received from Gansu Kaixi
Ecological Environment Material Company Limited, pow-
dered to 0.26–2.93 lm) was leached with 1N HCl (20 mL)
at room temperature for 1 h. SnCl2 Æ 2H2O (2 g) was then
added to the mixture and the resultant slurry was stirred
at room temperature for 24 h. The products were collected
by filtration, washed with copious amounts of ethanol and
water, dried at 333 K under vacuum for 24 h. The content
of Sn in the Sn-palygorskite is 2.41 wt% according to ICP
analysis.
Oxidation of ketones was carried out in a 25 mL glass
reactor. A typical procedure for the Baeyer–Villiger oxida-
tion is as follows: 2-adamantanone (15 mg, 0.1 mmol) and
30% hydrogen peroxide (1.5 equiv.) were dissolved in nitro-
benzene or 1,4-dioxane (3 mL). Sn-palygorskite (3 mg,
6.09 · 10À4 mmol) was added, the mixture was heated to
90 ꢁC and stirred for 24 h. The products were identified
by GC–MS analysis. Other cyclic ketones and acyclic
ketones were also oxidized in this oxidation system to give
the corresponding lactones or esters. The results are sum-
marized in Tables 1–6.
Fig. 1. AFM patterns of palygorskite used for the preparation of Sn-palygorskite catalyst. The palygorskite particle is about 0.3–6 lm · 0.2–5 lm with the
thickness of 25–50 nm.

Z. Lei et al. / Journal of Organometallic Chemistry 691 (2006) 5767–5773
5769
Table 1
Table 4
Catalytic oxidation of 2-adamantanone with different amount of Sn-
palygorskite
Catalytic oxidation of 2-adamantanone catalyzed by Sn-palygorskite at
different temperature using nitrobenzene as solvent
Sn-playgorskite(3mg) , 30%H O (1.5eq)
Sn-playgorskite , 30%H O (1.5eq)
2 2
2
2
24h, Nitrobenzene
1,4-dioxane, 70oC, 24h
O
O
O
O
O
O
Entry
Temperature (ꢁC)
Conv. (%)
Sele. (%)
Entry
Cat (mmol)
Conv. (%)
Sele. (%)
2.03 · 10À4
4.06 · 10À4
6.09 · 10À4
8.12 · 10À4
10.15 · 10À4
27
38
61
57
55
>99
>99
>99
>99
>99
1
2
3
4
5
6
30
50
70
90
110
130
28
33
71
100
62
47
>99
>99
>99
>99
>99
>99
1
2
3
4
5
Reaction conditions: 2-adamantanone 0.1 mmol, H2O2 (30%) 1.5 equiv.
1,4-dioxane 3 mL, 70 ꢁC, 24 h.
Reaction conditions: Sn-palygorskite 3 mg (6.09 · 10À4 mmol), 2-ada-
mantanone 0.1 mmol, H2O2 (30%) 1.5 equiv. nitrobenzene 3 mL, 24 h.
Percentage of conversion and product selectivity were determined using
GC analysis.
Percentage of conversion and product selectivity were determined using
GC analysis.
Table 2
Catalytic oxidation of 2-adamantanone catalyzed by Sn-palygorskite with
different solvents
Table 5
Catalytic oxidation of 2-adamantanone by Sn-palygorskite with different
time
Sn-playgorskite , 30%H O (1.5eq)
2 2
o
Sn-playgorskite(3mg) , 30%H O (1.5eq)
2
2
70
C, 24h
O
90oC, Nitrobenzene
O
O
O
O
O
Entry
Solvent
Conv. (%)
Sele. (%)
Entry
Time (h)
Conv. (%)
Sele. (%)
1
2
3
4
5
CH3CN
C6H5Cl
C6H5NO2
1,4-Dioxane
C6H5F
15
17
71
61
31
>99
>99
>99
>99
>99
1
2
3
4
5
6
7
4
6
15
35
42
49
53
>99
>99
>99
>99
>99
>99
>99
10
14
16
20
24
Reaction conditions: Sn-palygorskite 3 mg (6.09 · 10À4 mmol), 2-ada-
mantanone 0.1 mmol, H2O2 (30%) 1.5 equiv. solvent 3 mL, 70 ꢁC, 24 h.
Percentage of conversion and product selectivity were determined using
GC analysis.
74
100
Reaction conditions: Sn-palygorskite 3 mg (6.09 · 10À4 mmol), 2-anda-
mantanone 0.1 mmol, H2O2 (30%) 1.5 equiv., nitrobenzene 3 mL, 90 ꢁC.
Percentage of conversion and product selectivity were determined using
GC analysis.
Table 3
Catalytic oxidation of 2-adamantanone catalyzed by Sn-palygorskite at
different temperature using 4-dioxane as solvent
with a Shimadzu ICPV-5600 spectrophotometer under
the standard conditions.
Sn-playgorskite (3mg), 30%H O (1.5eq)
2
2
1,4-dioxane, 24h
O
O
O
3. Results and discussion
Entry
Temperature (ꢁC)
Conv. (%)
Sele. (%)
1
2
3
4
30
50
70
90
–
–
3.1. Preparation and characterization of the catalyst
27
61
85
>99
>99
53
Fig. 2 shows the infrared spectra of natural palygorskite
and Sn-palygorskite. The bands at 1026 and 469 cmÀ1 were
attributed to in-layered Si–O–Si bonds. The band near
800 cmÀ1 may correspond to the Al–O–Si bond. The Lewis
and Bronsted acid sites were situated at 1653 and
1448 cmÀ1, respectively, as identified by the adsorption of
an n-butylamine probe molecule [36]. There are obvious
changes in the IR spectra of Sn-palygorskite compared to
that of palygorskite. In the IR spectra of palygorskite-Sn,
the peak near 1448 cmÀ1 almost disappears. The strong
absorption at 469 cmÀ1 is shifted to 539 cmÀ1. These may
be attributed to the palygorskite inlaid with tin through
the formation of Sn–O bond.
Reaction conditions: Sn-palygorskite 3 mg (6.09 · 10À4 mmol), 2-ada-
mantanone 0.1 mmol, H2O2 (30%) 1.5 equiv. 1,4-dioxane 3 mL, 24 h.
Percentage of conversion and product selectivity were determined using
GC analysis.
FT-IR spectra were recorded on a Bio-Rad Win-IR
spectrometer using KBr disks. GC/MS analyses were car-
ried out on a trace HP GC6890/MS5973 equipped with a
25 m · 0.25 mm SE-54 column and a Shimadzu GC-16A
gas chromatograph with a 3 m · 3 mm OV-17 column.
The metal content of the Sn-palygorskite was determined

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Z. Lei et al. / Journal of Organometallic Chemistry 691 (2006) 5767–5773
Table 6
Baeyer–Villiger oxidation of several ketones catalyzed by Sn-palygorskite
Substrate
Temperature
(ꢁC)
H2O2 (30%)
Solvent
Conversion
(%)
Selectivity
(%)
Turnover
number
Product
90
1.5 equiv.
Nitrobenzene
100
>99
164
O
O
O
O
O
70
1.5 equiv.
1.5 equiv.
1,4-Dioxane
1,4-Dioxane
81
>99
132
O
O
O
70
16
90
26
O
O
C
70
90
1.5 equiv.
1.5 equiv.
1,4-Dioxane
1,4-Dioxane
37
80
>99
>99
61
O
C
CH3
O
CH3
O
197
O
O
CH3
CH3
O
O
O
90
1.5 equiv.
1,4-Dioxane
44
>99
108
CH3
O
CH3
90
1.5 equiv.
1,4-Dioxane
25
>99
31
O
O
C(CH3)3
C(CH3)3
O
O
C
90
90
1.5 equiv.
1.5 equiv.
1,4-Dioxane
1,4-Dioxane
100
100
>99
>99
123
123
CH3 CH2 CH
CH3
C
CH3
CH3 CH2 CH
CH3
O
CH3
O
O
C
CH3 CH CH2
CH3
C
CH3
CH3 CH CH2
CH3
O
CH3
Reaction conditions: Sn-palygorskite 3 mg (6.09 · 10À4 mmol), ketone 0.1 mmol, H2O2 (30%) 1.5 equiv., solvent 3 mL.
Percentage of conversion and product selectivity were determined using GC analysis.
3.2. Catalytic oxidation properties of the Sn-palygorskite
1.5 equiv. hydrogen peroxide were used for each batch of
the experiment. In order to investigate the effect of the tem-
perature on the catalytic conversion and selectivity, both
nitrobenzene and 1,4-dioxane were used as solvents. In
other cases only 1,4-dioxane was used. Having established
an effective oxidation protocol a series of ketones including
Oxidation of ketones was carried out in a 25 mL glass
reactor. We chose 2-adamantanone as a model compound
for the investigation of catalytic activity and product selec-
tivity. In most of the cases 15 mg of adamantanone and

Z. Lei et al. / Journal of Organometallic Chemistry 691 (2006) 5767–5773
5771
Fig. 2. The IR spectra of palygorskite and Sn-palygorskite.
2-adamantanone, acetophenone, cyclohexanone, 2-methyl-
cyclohexanone, 2-tert-butylcyclohexanone, 4-methylcyclo-
hexanone, 3-methyl-2-pentanone, 4-methyl-2-pentanone,
and cyclopentanone were treated with Sn-palygorskite to
give the corresponding lactones or esters. Formation of
products and consumption of substrates were monitored
by GC. The identity of products was determined either
by comparison with authentic samples using gas chroma-
tography or by GC/MS analysis. The conversion and prod-
uct selectivity were determined using GC analysis. The
results are summarized in Tables 1–6.
indicates that the optimum ratio of catalyst to substrates is
3.0 mg (6.09 · 10À4 mmol):0.1 mmol.
Also as shown in Table 1, the selectivity to the product is
higher than 99% in all cases.
3.2.2. Effect of organic solvent
In order to investigate the effect of the solvents on the
reaction, adamantanone was oxidized with hydrogen per-
oxide. The reactions were carried out at 70 ꢁC for 24 h.
Several organic solvents were examined for the oxidation
of 2-adamantanone using Sn-palygorskite as catalyst.
Table 2 shows that the reaction works in several organic
medium such as acetonitrile, chlorobenzene, nitrobenzene,
1,4-dioxane and fluorobenzene. Although in all the exam-
ined solvents the product selectivity remains 100%, only
in nitrobenzene and 1,4-dioxane, the conversion over 60%
can be obtained. In the following experiments, we chose
nitrobenzene or 1,4-dioxane as the organic medium to ful-
fill the oxidation.
3.2.1. Effect of the ratios of catalyst to substrate
The reactions were carried out for 24 h at 70 ꢁC to inves-
tigate the implications of the ratios of catalyst to substrate
on the Baeyer–Villiger oxidation reactions, adamantanone
was chosen as test substrate for the oxidation with hydro-
gen peroxide. Different ratios of catalyst to substrate were
used to investigate the corresponding variations of the oxi-
dation result. We stabilized the value of the substrate 2-
adamantanone, and changed the milligram (mmol) of the
catalyst. The oxidation results were changed with the ratio.
The results are shown in Table 1.
It can be concluded from Table 1 that when the used
amount of catalyst is in the range of 1.0 mg
(2.03 · 10À4 mmol)–3.0 mg (6.09 · 10À4 mmol), the conver-
sion increases with the amount of the catalyst. But when the
amount of catalyst exceeds 3.0 mg (6.09 · 10À4 mmol), the
volume of conversion decreases with the amount of catalyst.
As we know, the numbers of the activity center of the catalyst
generally increase with the amount of the catalyst used. The
fact that when the amount of the catalyst used exceeds 3 mg
(6.09 · 10À4 mmol) in this oxidation system, the conversion
decreases with the amount. This may suggests that when the
amount of the catalyst used is over certain amount, the cat-
alyst catalyze the decomposition of the hydrogen peroxide
which is not consumed for the substrate oxidation. The result
3.2.3. Effect of temperature
The reactions were carried out for 24 h at different tem-
peratures to investigate the influence of reaction temperature
on the Baeyer–Villiger oxidation reactions, adamantanone
was also chosen as test substrate for the oxidation with
hydrogen peroxide. The effect of the reaction temperature
is summarized in Tables 3 and 4. Table 3 shows that in 1,4-
dioxane the conversion increased as the reaction tempera-
ture was increased from 30 to 90 ꢁC Although relatively
higher conversion can be obtained at higher temperatures,
the selectivity to the product was decreased. In a different sol-
vent such as in nitrobenzene, we obtain different results.
Table 4 shows that increasing the reaction temperature from
30 to 90 ꢁC obviously results in a significant increase in the
conversion, but increasing the reaction temperature from
90 to 130 ꢁC leads to a decrease in the conversion. Our exper-
iment reveals that in nitrobenzene the oxidation will not

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Z. Lei et al. / Journal of Organometallic Chemistry 691 (2006) 5767–5773
Table 7
occur when we use hydrogen peroxide with the concentra-
tion of less than 20%. Although at higher temperature the
oxidation will carried on in higher speed, at the same time
hydrogen peroxide decomposes more quickly and this lead
to the concentration of hydrogen peroxide become lower
than needed concentration for the oxidation in a shorter per-
iod of time and thus result in the decrease of the conversion at
temperature higher than 90 ꢁC. The advantage of this cata-
lytic system in nitrobenzene is that in all temperature range
from 30 to 130 ꢁC, the product selectivity is higher than 99%.
The recycling of Sn-palygorskite
Sn-playgorskite (3mg), 30%H O (1.5eq)
2 2
90˚C, nitrobenzene, 24h
O
O
O
Recycle times
Conv. (%)
Sele. (%)
1
2
3
4
100
95
85
>99
>99
>99
>99
61
Reaction conditions: Sn-palygorskite 3 mg (6.09 · 10À4 mmol), 2-and-
amtanone 0.1 mmol, H2O2 (30%) 1.5 equiv., nitrobenzene 3 mL.
Percentage of conversion and product selectivity were determined using
GC analysis.
3.2.4. Effect of reaction time
The conversion is increased with the reaction time and
the selectivity to the product is higher than 99% in all reac-
tion time ranged from 2 to 24 h. Table 5 shows that the
reactions generally begin with relatively fast rates in the
first 4–6 h but tend to slow down in 10–16 h. After this time
interval the reaction again turned fast until 24 h.
ther treatment. The activity is lowered to 95%, 85% and 61%
from the original 100% after 2, 3, and 4 cycles, respectively.
As is shown in Table 7, Sn-palygorskite as heterogeneous
catalyst can be recycled although the catalytic activity is low-
ered about 39% compared to that of the starting reactivity.
3.2.5. Oxidation of other ketones
A variety of cyclic and acyclic ketones were also oxi-
dized using 3 mg (6.09 · 10À4 mmol) of Sn-palygorskite
catalyst at 70–90 ꢁC in this oxidation system. As shown
in Table 6 Sn-palygorskite are active and highly selective
for the Baeyer–Villiger oxidation of 2-adamantanone, 2-
methylcyclohexanone, cyclopentone, 3-methyl-2-penta-
none and 4-methyl-2-pentanone. The TONs obtained are
generally encouraging for these five ketones. The outstand-
ing of this catalytic system is that besides cyclo-ketones, the
catalyst is also active for the oxidation of chain aliphatic
ketones like 3-methyl-2-pentanone and 4-methyl-2- penta-
none. For these two aliphatic ketones the TONs are over
100, the conversions are 100% and the selectivity to the cor-
responding esters remained higher than 99%. The results
show that this oxidation procedure is promising for cyclic
and acyclic ketones and much cleaner than the traditional
BV oxidation as it gives relatively higher TON and involves
no use of peracids which would produce harmful by-prod-
ucts. The catalyst preparation is very simple and does not
involve the use of any expensive materials.
4. Conclusions
In summary, Sn-palygorskite was prepared by a simple
procedure and shown to act as a highly active catalyst
for the Baeyer–Villiger oxidation of ketones with high
TONs and almost 100% selectivity using environmentally
friendly 30% hydrogen peroxide as oxidant. Palygorskite
is a biocompatible and environmentally friendly natural
mineral abundant in deposits all over the world. The cata-
lyst can be prepared in large scale and can be recycled. Sn-
palygorskite is an efficient and relatively cheap catalysts for
the BV oxidation reactions.
Acknowledgements
This work was financially supported by National Natural
Science Foundation of China (Nos. 20174031 and
20474052), National Basic Research Pre-Program of China
(2005CCA06000), Ministry of Science and Technology of
China and University Doctoral Foundation (20050736001)
from Ministry of Education. We also thank Key Laboratory
of Eco-Environment-Related Polymer Materials (North-
west Normal University), Ministry of Education, for finan-
cial support.
Sn-palygorskite possesses high catalytic activity for the
Baeyer–Villiger oxidation of cyclic ketones and acyclic
ketones. In particular, 2-adamantanone, 3-methyl-2-penta-
none and 4-methyl-2-pentanone were converted into the
corresponding lactone or esters almost quantitatively. We
confirmed that the oxidation reaction did not occur in
the absence of the catalyst. Moreover, the reaction did
not proceed in the presence of untreated palygorskite.
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