Fabrication of highly active Sn/W mixed transition-metal oxides as solid acid catalysts

Article abstract of DOI:10.1007/s11243-011-9465-3

Catalytically active Sn/Wmixed transition-metal oxides were prepared by calcination of the corresponding Sn/W hydroxide precursors at different temperatures. The obtained mixed oxides were characterized by physicochemical and spectroscopic methods. With variation of the molar ratios of Sn/W, the prepared Sn/W mixed oxide catalysts had different reaction activities. Thus, the Sn/W-2-800 oxide acted as an effective heterogeneous catalyst for the Baeyer-Villiger oxidation of ketones and the Friedel-Crafts reaction. Many ketones, as well as benzyl alcohol and acetic anhydride, were transformed into the corresponding products with high conversion and selectivity. The catalysts can be easily separated from the reaction mixtures and can be reused for at least five cycles without significant loss of activity. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9465-3

Transition Met Chem (2011) 36:269–274
DOI 10.1007/s11243-011-9465-3
Fabrication of highly active Sn/W mixed transition-metal oxides
as solid acid catalysts
•
Zhiwang Yang Lengyuan Niu Zhenhong Ma
•
•
•
Hengchang Ma Ziqiang Lei
Received: 20 November 2010 / Accepted: 24 January 2011 / Published online: 8 March 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Catalytically active Sn/W mixed transition-metal
oxides were prepared by calcination of the corresponding
Sn/W hydroxide precursors at different temperatures. The
obtained mixed oxides were characterized by physico-
chemical and spectroscopic methods. With variation of the
molar ratios of Sn/W, the prepared Sn/W mixed oxide cat-
alysts had different reaction activities. Thus, the Sn/W-2-800
oxide acted as an effective heterogeneous catalyst for the
Baeyer–Villiger oxidation of ketones and the Friedel–Crafts
reaction. Many ketones, as well as benzyl alcohol and acetic
anhydride, were transformed into the corresponding prod-
ucts with high conversion and selectivity. The catalysts can
be easily separated from the reaction mixtures and can be
reused for at least five cycles without significant loss of
activity.
by-products are often obtained, resulting in the need for
product separation and the waste of solvent. Second, such
homogeneous catalysts cannot be recycled. Hence, these
systems will potentially cause environmental problems.
Therefore, the discovery of environmentally benign catalysts
is a very important area of current research. In principle,
heterogeneous solid acids can overcome these problems.
Many solid acids are mineral oxides, such as sulfated
zirconia [1–3], Cs-exchanged heteropoly acids [4], acidic
polymers [5, 6], and zeolites [7], which contain both Lewis
and Brønsted acid sites. These materials have been used in
the isomerization of alkanes and other petrochemical pro-
cesses such as alkylation of aromatics and hydrocarbon
cracking reactions [8]. As important solid acids, transition-
metal oxides are currently attracting considerable attention
due to their unique properties and broad range of potential
applications. During the last decades, monometallic oxides
such as MgO [9–11], SnO2 [12–14], WO3 [12, 14], ZrO2
[12, 14, 15], TiO2 [12, 14, 16], Al2O3 [17], Nb2O5 [12, 14,
18], Ta2O5 [12, 14, 19], and bimetallic oxides such as
Nb–Ta oxides [12, 14, 20, 21], Mg–Ta oxides [21, 22], Nb-W
oxides [23], Sn/W oxides [24], Zr-W oxides [25, 26], and
WO3-TiO2 [27] have all been studied. The transition-metal
centers in the inorganic frameworks of porous structures also
possess variable oxidation states and empty d-orbitals, which
allow electron transfer to occur between the reactants and
active sites during the catalytic process [28].
Introduction
Traditionally, inorganic acidssuch as H2SO4 and H3PO4, and
Lewis acids such as AlCl3, ZnBr2, FeCl3, and BF3 etherate
have been used to promote acid-catalyzed reactions.
Although these catalytic processes are generally simple and
efficient, several problems have been encountered. First,
selective transformations of the substrates are often difficult;
Z. Yang Á L. Niu Á Z. Ma Á H. Ma Á Z. Lei
In this paper, we report the synthesis of Sn/W mixed
oxides and their application in some acid-catalyzed reac-
tions including Baeyer–Villiger oxidation and the Friedel–
Crafts reaction. A series of Sn/W oxides were synthesized
through coprecipitation and calcination methods. The obtained
Sn/W oxides, especially for the oxide of Sn/W-2-800, exhibit
the advantages of high conversion, high selectivity, and recy-
clability in these reactions.
Key Laboratory of Eco-Environment-Related Polymer Materials,
Ministry of Education, College of Chemistry and Chemical
Engineering, Northwest Normal University, Lanzhou, China
Z. Yang Á L. Niu Á Z. Ma Á H. Ma Á Z. Lei (&)
Key Laboratory of Polymer Materials of Gansu Province,
College of Chemistry and Chemical Engineering,
Northwest Normal University, Lanzhou 730070, China
e-mail: leizq@nwnu.edu.cn
123

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Transition Met Chem (2011) 36:269–274
Experimental
reaction of anisole was performed by stirring a mixture of
20 mg of catalyst, 10 mmol of anisole and 1 mmol of
benzyl alcohol or 1 mmol of acetic anhydride at 100 °C for
3–5 h. All the products in these reactions were determined
by GC analysis. Some of the products from the Baeyer–
Villiger oxidations were purified by silica gel column
chromatography with hexane/ethyl acetate (10:1) as eluent.
The products of alkylation were first separated by silica gel
column chromatography with petroleum ether as eluent and
then further purified by silica gel column chromatography
with CCl4 as eluent, similar to the first step.
SnCl4Á5H2O and Na2WO4Á2H2O were obtained from
Shanghai Zhongtai Chemical Co. 2-Adamantanone (98%),
2-methylcyclohexanone (95%), 4-methylcyclohexanone
(98%), and 4-tert-butylcyclohexanone (98%) were pur-
chased from Meryer Chemical Co. Ltd., Shenzhen.
Cyclopentanone (99%), cyclohexanone (98%), anisole,
acetic anhydride, and benzyl alcohol were obtained from
Tianjin Guangfu Fine Chemical Technology Co. Ltd. Other
solvents were all obtained from commercial sources and
were used as received without further purification.
GC analysis was carried out on a Shimadzu GC-2010 gas
chromatography instrument equipped with a 15 m 9
Product separation and identification
1
0.53 mm 9 1.50 lm RTX-1 capillary column. H and 13C
The products were purified by a silica gel column chro-
matography with hexane/ethyl acetate (10:1) or petroleum
ether as eluent. NMR data are given below.
NMR spectra were recorded on a Varian Mercury 400 plus
instrument in CDCl3 using TMS as internal standard. X-ray
diffraction (XRD) patterns were collected on a Bruker D8
Product 1b: 1H NMR: d 4.489 (1H), 3.076 (1H), 2.114
(2H), 2.004 (2H), 1.957 (1H), 1.921 (2H), 1.856 (2H),
1.739 (2H), 1.280 (1H). 13C NMR: d 178.960, 73.136,
41.151, 35.862, 35.679, 33.713, 30.863, 29.628, 25.856,
25.650.
˚
Advance instrument using CuKa radiation (k = 1.5406 A).
FT/IR spectra were recorded on a Nicolet Nexus 670 Fourier
transform infrared spectrometer using KBr tablets. The SEM
images were taken on a Hitachi S-4800 scanning electron
microscope. BET results as well as the Nitrogen adsorption–
desorption isotherms were recorded at 77 K on a Quanta-
chrome Autosorb-3B instrument.
Product 2b: 1H NMR: d 4.282 (2H), 2.49 (2H), 1.798
(4H). 13C NMR: d 171.339, 69.331, 29.636, 22.091,
18.875.
Catalyst preparation
Product 3b: 1H NMR: d 4.235 (2H), 2.643 (2H), 1.867
(2H), 1.795 (4H). 13C NMR: d 171.339, 69.331, 29.636,
22.091, 18.875.
The Sn/W oxides were synthesized according to the liter-
ature [24]. In typical synthesis, 2.5 mmol of Na2WO4Á
2H2O (dissolved in 5 mL of deionized water) and a mmol
of SnCl4Á5H2O (a = 2.5, 5, 10 or 20) were mixed and
stirred for 1 h at room temperature, and then 20 mL of
deionized water was added. After stirring for 24 h at room
temperature, the resulting white precipitate was filtered off
and washed with deionized water until no chloride ion was
detected. The precipitate was dried in vacuum to afford
hydroxide precursors with different molar ratios of Sn/W.
Different Sn/W oxide catalysts were prepared by calcining
the corresponding hydroxide at different temperatures
(ranging from 400 to 1,000 °C) in a muffle furnace for 3 h
under air. The as-prepared products are denoted as Sn/W-x-T
(x = a/2.5, representing the different molar ratios of Sn/W in
the products).
Product 4b: 1H NMR: d 4.449 (1H), 2.653 (2H), 1.930
(4H), 1.627 (2H), 1.352 (3H). 13C NMR: d 175.60, 76.80,
36.21, 35.00, 28.28, 22.88, 22.57.
Product 5b: 1H NMR: d 4.188 (2H), 2.630 (2H), 1.912
(2H), 1.862 (1H), 1.498 (1H), 1.343 (1H), 1.002 (3H). 13C
NMR: d 175.995, 67.984, 37.097, 35.116, 33.081, 30.634,
22.008.
Product 6b: 1H NMR: d 4.335 (1H), 4.158 (1H), 2.585
(1H), 2.569 (1H), 2.085 (2H), 1.527 (1H), 1.369 (2H),
0.898 (9H). 13C NMR: d 176.285, 68.579, 50.667, 33.386,
32.936, 30.261, 27.472, 27.373, 27.304, 23.669.
Para-benzylanisole: 1H NMR: d 7.256 (2H), 7.182 (3H),
7.092 (2H), 6.831 (2H), 3.922 (2H), 3.770 (3H). 13C NMR:
d 157.888, 141.549, 133.219, 129.828, 128.776, 128.395,
125.941, 113.817, 55.219, 40.984.
General procedures for the catalytic reactions
Oxidation of ketones was carried out in a 10-mL flask by
stirring a mixture of 0.1 mmol of the ketone required,
3 mL of 1, 2-dichloroethane, 0.3 mmol of 30% H2O2, and
6 mg of Sn/W- x -T at 75 °C for 10 h. Friedel–Crafts
Ortho-benzylanisole: 1H NMR: d 7.214 (6H), 7.065 (1H),
6.869 (2H), 3.971 (2H), 3.807 (2H). 13C NMR: d 157.279,
140.977, 130.270, 129.607, 128.929, 128.220, 127.367,
125.736, 120.416, 110.326, 55.303, 35.809.
123

Transition Met Chem (2011) 36:269–274
Results and discussion
271
for SnO2, suggesting that a SnO2-like phase was formed,
and the polytungstate species were highly dispersed on
SnO2 and/or incorporated into the SnO2 frameworks [24].
Temperature is a major factor that affects the crystal form
of the oxides. Thus, XRD patterns similar to those of the
hydroxide precursor were obtained for the oxides prepared
at low calcination temperatures (\800 °C). On increasing
the temperature to 800 °C, signals of triclinic WO3 phase
were observed in the products. On increasing the Sn con-
tent and keeping the temperature at 800 °C, similar pat-
terns to Sn/W-2-800 were detected. Finally, most of the
tungstate species are converted into WO3 crystallites at
1,000 °C.
FTIR spectra and XRD patterns and SEM images
The FTIR spectra of the Sn/W oxides are shown in Fig. 1,
and the peak at 954 cm-1 is attributed to the W=O
stretching vibration. The W–O–W band at 740–890 cm-1
and Sn–O–Sn band at 610–700 cm-1 were also observed
for all of the samples, suggesting the presence of poly-
tungstate species. As shown in Fig. 2, the XRD patterns of
the Sn/W hydroxide showed broad signals as also observed
The crystal morphologies of the Sn/W oxides and Sn/W-
2 hydroxide were investigated by scanning electron
microscopy (SEM). As shown by the representative SEM
images (Fig. 3), the samples of Sn/W-2 hydroxide con-
sisted of lump-shaped particles with diameters of about
40 lm (Fig. 3a), while the corresponding Sn/W oxides
consisted of smaller corallite-type uniform crystal powders
(Fig. 3b–g). The particle diameters of the Sn/W oxides
tend to become smaller with increasing calcining temper-
atures. When the temperature was raised to 1,000 °C,
the particles were aggregated and the framework was
collapsed (Fig. 3b–d). On the other hand, the images also
reveal that the particles are inclined to aggregate when a
high content of Sn component was present in the oxides
(Figs. 3f, g).
N2 adsorption–desorption experiments
Fig. 1 FT-IR spectra for a Sn/W-2 hydroxide, b Sn/W-2-400, c Sn/
W-2-600, d Sn/W-2-800, e Sn/W-2-1000 oxides
The pore properties of all samples were characterized by
nitrogen adsorption–desorption methods. Table 1 shows
the specific surface areas and pore properties of the syn-
thesized materials. The surface areas were estimated using
the Brunauer–Emmett–Teller (BET) method, and pore
volumes were obtained by the Barrett–Joyner–Halenda
(BJH) method. Different results were observed for Sn/W
mixed oxides obtained at different calcination tempera-
tures. The data clearly showed that the surface area grad-
ually decreased from 137 to 13 m2/g as the calcining
temperature increased from 400 to 1,000 °C, while the pore
volumes increased drastically due to the formation of void
spaces between particles and the increasing pore size.
Figure 4 displays the N2 adsorption–desorption iso-
therms of the Sn/W oxides. The nitrogen adsorption iso-
therms of the three oxides obtained at different calcination
temperatures from 400 to 800 °C are of classical type IV,
mesoporous materials, according to the IUPAC classifica-
tion [29]. However, for Sn/W-2-1000, high calcination
temperature resulted in a collapse of the framework leading
to the formation of an amorphous structure without any
long-range order.
Fig. 2 XRD patterns for a Sn/W-1-800, b Sn/W-2-400, c Sn/W-2-
600, d Sn/W-2-800, e Sn/W-2-1000, f Sn/W-4-800, g Sn/W-8-800,
h Sn/W-2 hydroxide
123

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Transition Met Chem (2011) 36:269–274
Fig. 3 SEM images for a Sn/W-2 hydroxide, b Sn/W-2-400, c Sn/W-2-600, d Sn/W-2-800, e Sn/W-2-1000, f Sn/W-4-800, g Sn/W-8-800
Table 1 Properties of the as-prepared Sn/W mixed transition-metal
oxides
Application as catalysts for Baeyer–Villiger oxidations
The catalytic properties of all the synthesized Sn/W oxides
were first tested for the Baeyer–Villiger oxidation of 4-tert-
butylcyclohexanone, with the results listed in Table 2.
Variation of the Sn and W content resulted in remarkably
different conversions of the substrate. Of all the oxides, the
series of Sn/W-2 type possessed the highest catalytic
activity under an identical calcining temperature. A maxi-
mum conversion of 81% was reached for the oxide of Sn/
W-2-800. These results indicate that the structure of the
oxides plays an important role in the oxidation.
Sample
Surface area
(m2/g)
Pore size
(nm)
Pore volume
(mL/g)
Sn/W-2-400
Sn/W-2-600
Sn/W-2-800
Sn/W-2-1000
137
84
1.54
1.92
3.25
16.7
0.048
0.114
0.205
0.228
63
13
We then went on to explore the Baeyer–Villiger oxi-
dation of 4-tert-butylcyclohexanone catalyzed by different
amounts of Sn/W-2-800. As shown in Table 3, the con-
version of 4-tert-butylcyclohexanone is accelerated with
increasing amounts of catalyst, and the highest conversion
was obtained when 6 mg of Sn/W-2-800 was used.
Therefore, the optimum catalyst amount used in the Bae-
yer–Villiger oxidation system was fixed at 6 mg for further
experiments.
According to the above results, the Sn/W-2-800 oxide
bears the highest catalytic activity in Baeyer–Villiger
oxidation, so it was selected and was applied for the oxi-
dation of other ketones with a range of different structures.
As shown in Table 4, several of the cyclic ketones,
including 2-adamantanone, cyclopentanone, cyclohexa-
none, and its derivatives, were oxidized to the corre-
sponding lactones with high conversion and selectivity.
The results also suggest that the reactivity of different
Fig. 4 N2 adsorption–desorption isotherms of a Sn/W-2-400, b Sn/
W-2-600, c Sn/W-2-800, d Sn/W-2-1000
123

Transition Met Chem (2011) 36:269–274
273
Table 2 Baeyer–Villiger oxidation of 4-tert-butylcyclohexanone
with various Sn/W oxides catalysts
Table 4 Baeyer–Villiger oxidation of ketones catalyzed by Sn/W-2-
800
Conversion
(%) a
Selectivity
(%) a
Entry
1
Substrate
Product
100
100
Entry
Catalysts
Conversion (%)a
Selectivity (%)a
2
3
4
72
100
1
Sn/W-1-400
Sn/W-2-400
Sn/W-4-400
Sn/W-8-400
Sn/W-1-600
Sn/W-2-600
Sn/W-4-600
Sn/W-8-600
Sn/W-1-800
Sn/W-2-800
Sn/W-4-800
Sn/W-8-800
Sn/W-1-1000
Sn/W-2-1000
Sn/W-4-1000
Sn/W-8-1000
Sn/W-2 hydroxide
None
19
26
13
15
46
73
23
12
51
81
19
14
10
16
5
100
79
2
88
65
87
82
3
100
100
92
4
5
6
89
7
81
8
100
100
100
100
100
100
63
9
5
6
97
81
84
10
11
12
13
14
15
16
17
18
100
100
100
0
6
0
Reaction conditions: substrate 0.1 mmol, catalyst 6 mg, 30%H2O2
3.0 eq., 1,2-dichloroethane 3 mL, 10 h, 75 °C
0
0
a
Reaction conditions: 4-tert-butylcyclohexanone 0.1 mmol, catalyst
6 mg, 30%H2O2 3.0 eq., 1,2-dichloroethane 3 mL, 10 h, 75 °C
Conversion and selectivity were determined by GC analysis
a
Conversion and selectivity were determined by GC analysis
Table 3 Baeyer–Villiger oxidation of 4-tert-butylcyclohexanone
catalyzed by different amounts of Sn/W-2-800
among the investigated series under identical conditions.
On the other hand, ketones that possess a substituent near
the carbonyl group, such as 2-methyl-cyclohexanone,
always show lower reactivity in the oxidation. This is
because Baeyer–Villiger oxidation is a two-step reaction:
nucleophilic addition of an oxidant to give the so-called
Criegee adduct is followed by rearrangement to the ester or
lactone [30, 31]. A substituent neighboring the carbonyl
bond hinders the formation of the Criegee adduct. How-
ever, as for 4-methylcyclohexanone and 4-tert-butylcy-
clohexanone, promising conversions (97 and 81%,
respectively) were obtained because of the long distance
between the alkyl and carbonyl group.
Entry
Catalyst (mg)
Conversion (%)a
Selectivity (%)a
1
2
3
4
2.0
4.0
6.0
8.0
61
72
81
80
99
99
100
98
Reaction conditions: 4-tert-butylcyclohexanone 0.1 mmol, 30% H2O2
3.0 eq., 1,2-dichloroethane 3 mL, 10 h, 75 °C
Application as catalysts for Friedel–Crafts reactions
a
Conversion and selectivity were determined by GC analysis
The high catalytic performance of the Sn/W-2-800 oxide
was further demonstrated as a heterogeneous catalyst for
the Friedel–Crafts reaction of anisole with benzyl alcohol
or acetic anhydride. Anisole was efficiently converted into
the corresponding benzylanisole in excellent yields
ketones depends on the ring size and the migratory ability
of the substituent group. Ketones with a highly strained
ring, such as 2-adamantanone, show the highest conversion
123

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Transition Met Chem (2011) 36:269–274
Acknowledgments The research was financially supported by
NSFC (20774074), Gansu provincial Natural Science Foundation of
China (3ZS061-A25-018), and Scientific Research Fund of NWNU
(NWNU-KJCXGC-03-56). We also thank Key Laboratory of Eco-
Environment-Related Polymer Materials (Northwest Normal Uni-
versity), Ministry of Education, for financial support.
Scheme 1 Friedel–Crafts reaction of anisole catalyzed by Sn/W-2-
800
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para-methoxyacetophenone was exclusively obtained and
no ortho-product was formed (Scheme 1). All these results
suggest that the prepared Sn/W mixed oxides are promising
solid acid catalysts for Friedel–Crafts reactions.
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catalyzed reactions, including Baeyer–Villiger oxidation
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advantages of high conversion, high selectivity, and recy-
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