SnO2 Nanoparticle-Decorated Graphene Oxide Sheets Efficiently Catalyze Baeyer-Villiger Oxidation with H2O2

Article abstract of DOI:10.1007/s10562-015-1639-8

Uniform rutile SnO₂ nanoparticles with small size (ca. 3 nm) were highly dispersed on both sides of GO sheets through electrostatic interactions, giving pseudo-homogeneous catalysts of SnO₂/GO nanocomposites. The SnO₂-decorated GO nanocomposites, especially, SnO₂ (15 wt%)/GO was highly efficient and reusable in Baeyer-Villiger oxidation of ketones with H₂O₂. Graphical Abstract: SnO₂/GO nanocomposites, where uniform SnO₂ nanoparticles were tightly gripped on both sides of GO sheets by surface oxygenated functional groups through electrostatic interaction, have proved to be a versatile, efficient and reusable catalyst for the BV oxidation of ketones with H₂O₂.

Full text of DOI:10.1007/s10562-015-1639-8

Catal Lett
DOI 10.1007/s10562-015-1639-8
SnO Nanoparticle-Decorated Graphene Oxide Sheets Efficiently
2
Catalyze Baeyer–Villiger Oxidation with H O2
2
1
1
1
1
1,2
Weiguo Zheng
•
Rong Tan
•
Xuanfeng Luo
•
Chen Xing
•
Donghong Yin
Received: 9 August 2015 / Accepted: 9 October 2015
Ó Springer Science+Business Media New York 2015
Abstract Uniform rutile SnO nanoparticles with small
2
size (ca. 3 nm) were highly dispersed on both sides of GO
sheets through electrostatic interactions, giving pseudo-
homogeneous catalysts of SnO /GO nanocomposites. The
2
SnO -decorated GO nanocomposites, especially, SnO
2
2
(
15 wt%)/GO was highly efficient and reusable in Baeyer–
Villiger oxidation of ketones with H O .
2
2
Graphical Abstract SnO /GO nanocomposites, where
2
uniform SnO nanoparticles were tightly gripped on both
2
sides of GO sheets by surface oxygenated functional
groups through electrostatic interaction, have proved to be
a versatile, efficient and reusable catalyst for the BV oxi-
dation of ketones with H O .
2
2
Keywords Lewis acid catalysis Á Heterogeneous
catalysis Á Oxidation
1
Introduction
B–V oxidation of ketones with H O is of great importance
2
2
in organic chemistry, not only due to the environment
friendly oxygen source, but also because obtained lactones
or esters are valuable synthetic intermediates in agro-
chemical, chemical and pharmaceutical industries [1–9]. In
IV
&
&
Rong Tan
yiyangtanrong@126.com
2
001, Corma et al. reported the tetravalent tin (Sn )-
containing beta zeolite as a highly active and chemose-
lective catalyst for B–V oxidation of ketones with H O
IV
Donghong Yin
yindh@hunnu.edu.cn
2
2
[
10]. Sn centers, tetrahedrally coordinated in beta zeolite
1
Key Laboratory of Chemical Biology and Traditional
Chinese Medicine Research (Ministry of Education) and Key
Laboratory of the Assembly and Application for Organic
Functional Molecules, Hunan Normal University,
Changsha 410081, People’s Republic of China
framework, acted as Lewis acids to activate the carbonyl
group of ketones through coordination, facilitating the
nucleophilic attack on this group by H O . Despite high
2
2
yields of lactones, in some cases, microporosity of zeolite
hinders the diffusion of bulky molecules, resulting in lower
activity and faster deactivation of catalyst [11]. To elimi-
nate the diffusion limitation, mesoporous silica such as
2
Technology Center, China Tobacco Hunan Industrial
Corporation, NO. 426 Laodong Road,
Changsha 410014, Hunan, People’s Republic of China
123

W. Zheng et al.
IV
MCM-41, have been employed to incorporate the Sn
by the oxygenate groups on the GO layer, forming uniform
loading of SnO NPs on GO sheets. It has proved that SnO
species [2, 12, 13]. The mesoporous matrix indeed allowed
for the free diffusion of regents, but the amorphous struc-
IV
2
2
NPs were kept small in size and dispersed uniformly on the
ture of silica wall often incorporates Sn species disor-
GO sheets. The resultant SnO /GO nanocomposites were
2
IV
derly. As a result, not all Sn centers were equally active
demonstrated the efficient and selective solid Lewis acid
catalysts for B–V oxidation with H O . Quantitative yield
and/or accessible to the reactants for efficient B–V oxida-
tion [14–16]. Recently, layered double hydroxides (LDHs),
a class of two-dimensional anionic clays, have emerged as
2
2
of lactones was obtained within 2 h even in the case of
bulky 2-adamantanone. Furthermore, the catalyst could be
easily separated for reuse by centrifugation.
IV
the alternative support for Sn species through ion-ex-
II
change [17, 18]. The key structural characteristic that M
III
and M cations were distributed in a uniform manner in
IV
the hydroxide layers resulted in the formation of Sn /LDH
2 Experimental
with specific morphology/surface structure and high dis-
persion [19]. Furthermore, ‘flexible’ interlayer spaces of
IV
2.1 Materials and Methods
the layered Sn /LDH catalyst not only fitted substrates
IV
either bulky or less, but also allowed the free access of Sn
Cyclopentanone and high-purity graphite (99.9999 %,
200 mesh) were purchased from Alfa Aesar. Cyclohex-
anone and adamantanone were obtained by J&K. Other
commercially available chemicals were laboratory grade
reagents from local suppliers. All of the solvents were
purified by standard procedures.
center during reaction. It thus represented a fascinating
strategy for developing highly efficient and stable Tin-
based catalysts for B–V oxidation with H O .
2
2
Graphene, a novel layered carbon material with a tight
packing of honeycomb lattice, has become one of the rising
stars in material science [20]. Its intriguing properties, such
as unique layered structure, high surface area, high flexi-
bility and mechanical strength, made it very attractive in
heterogeneous catalysis [21–24]. Although many SnO2/-
graphene nanocomposites have been recently proposed as
an anode material for Li-ion batteries [25–28], the
Surface structure of the samples was measured using a
TEM (JEOL JEM-3010). X-ray diffraction (XRD) patterns
were recorded on a Philips X’PERT-Pro-MPD diffrac-
˚
tometer using Cu Ka radiation (k = 1.542 A). A continu-
o
o
ous scan mode was used to collect the 2h from 5 to 80 .
X-ray Photoelectron Spectroscopy (XPS) data were
obtained with an ESCALab220i-XL electron spectrometer
from VG Scientific using 300 W Al Ka radiation. The base
employment of SnO /graphene nanocomposites in catalysis
2
was scarcely reported. For higher catalytic performance, it
was desirable to have smaller particle size and higher
-
7
pressure was about 3 9 10 Pa. SnO NPs contents were
2
dispersion of SnO NPs in the composite. However, it was
2
analyzed by TGA using a NETZSCH STA 449C thermal
difficult to control the dispersion state of loaded NPs on
graphene surfaces, mainly due to the lack of strong inter-
actions between them [29]. Furthermore, layer graphene
sheets tended to stack with each other in solution, thus
losing their high surface area and intrinsic chemical and
physical properties. An ideal solution to above problems
analyzer. Samples (ca. 0.01 g) were heated from room
o
temperature up to 800 C with 10 K/min under air flow
using alumina sample holders. Thin layer chromatography
(TLC) was conducted on glass plates coated with silica gel
GF254. The conversion and ee values were measured by a
6890 N gas chromatograph (Agilent Co.) equipped with a
capillary column (HP19091G-B213, 30 m 9 0.32 mm 9
0.25 lm).
was supporting SnO NPs on a graphene oxide (GO) sheet
2
instead of a graphene sheet. Different from graphene, GO
sheet consisted of intact graphitic regions interspersed with
3
sp -hybridized carbons containing carboxyl, hydroxyl, and
2.2 Preparation of SnO (x)/GO (Where x is
2
epoxide functional groups on the edge, top, and bottom
surface of each sheet. The abundant surface oxygenated
the Mass Ratio of SnO NPs)
2
functional groups ensured GO to grip SnO NPs tight on
2
GO was prepared by the oxidation of high-purity graphite
powder (99.9999 %, 200 mesh) with H SO /KMnO
4
the surface through electrostatic interactions, which pre-
2
4
vented not only the aggregation of SnO NPs, but also the
2
according to the Hummers method [30], and then was
subjected to dialysis for 7 days to completely remove metal
ions and acids. The resulting product was dried at room
temperature under vacuum overnight, giving GO as yel-
lowish-brown powder. FT-IR (KBr): 3187, 3132, 1735,
aggregation of graphene sheets. In addition, all the SnO2
NPs located on the GO surface were exposed to the
reagents in catalysis, which may further enhance the cat-
alytic efficiency of SnO /GO nanocomposites.
2
-
1
Herein, SnO NPs were grown on the GO surfaces via a
2
1621, 1224, 1050 and 581 cm .
hydrothermal method using SnCl Á5H O as a metal pre-
The dried GO (1.0 g) was sonicated in deionized water
(100 mL) for 0.5 h to ensure most GO being fully exfoliated.
4
2
cursor. Then SnO NPs were formed, in situ, and anchored
2
1
23

2
SnO Nanoparticle-Decorated Graphene Oxide Sheets Efficiently Catalyze Baeyer–Villiger...
Various amount of SnCl Á5H O (0.125, 0.25, 0.375, 0.50 and
hydrothermal method was employed for the hybridization, as
shown in Scheme 1. GO sheets were readily dispersed in
waterduetosurfaceoxygenatespecies, formingauniformGO
nanosheets suspension. When GO solution was mixed with
4
2
0
.75 g) in deionized water (50 mL) was then added into the
homogeneous dispersion. After further ultrasonic treatment
for 0.5 h, the mixtures were refluxed at 100 °C for 24 h,
resulting in a black suspension. The precipitate was collected
by centrifugation and washed with deionized water. The
products were dried at room temperature overnight under
IV
SnCl Á5H O solution, the Sn cation was selectively bonded
4
2
with the negative oxygenated groups by electrostatic force.
IV
After incubated at 100 °C for 24 h, anchored Sn cation was
vacuum, giving SnO /GO nanocomposites with various
2
in situ converted to SnO NPs, and gripped by the oxygenated
2
SnO loading (ca. 7.5, 15, 20, 30 or 40 wt%).
2
functional groups through electrostatic interaction. The driv-
ing force for the gripping of SnO NPs in the nanocomposite
2
2
.3 B–V Oxidation of Ketones with H O
2
structures may be subdivided into the following categories:
2
(I) dative bonds between oxygen atom in oxygenated func-
SnO /GO nanocomposites (0.05 g) and ketones (1.0 mmol)
2
tional groups and Sn atoms of SnO , (II) hydrogen-bond
2
were mixed in 1,2-dichloroethane (15 mL) under vigorously
stirring. H O (30 wt%, 2.0 mmol) was then added dropwise
between oxygen atom of SnO-2 NPs and hydrogen atom in
hydrogen-containing oxygenated functional groups. As a
2
2
at 90 °C. The reaction progress was monitored by GC. After
the reaction, catalyst was separated by centrifugation. The
liquid reaction mixture was extracted by ethyl ether and then
quantitatively analyzed by a 6890 N gas chromatograph
result, uniform SnO NPs with small size should be highly
2
dispersed on both sides of GO sheets, as proposed in
Scheme 1. This unique hybrid architecture gave rise to an
efficient and stable Lewis acid catalyst for B–V oxidation.
(
(
Agilent Co.) equipped with the capillary column
HP19091G-B213, 30 m 9 0.32 mm 9 0.25 lm). The
3.2 Characterization of Samples
recovered catalyst was washed with ethanol and distilled
o
water repeatedly, and dried at 40 C overnight for reused.
3.2.1 XPS
XPS was employed as an optimal characterization technique
to provide the direct evidence for the conjugation of SnO2
NPs and GO nanosheets (Fig. 1). Figure 1a shows the XPS
3
Results and Discussions
3
.1 Preparation of SnO /GO Nanocomposites
2
survey spectra of GO and typical SnO (15 wt%)/GO
2
nanocomposite, which were normalized with respect to the
respective C 1s peaks. Obviously, besides C (C 1s, 286.1 ev)
and O (O 1s, 532.0 ev) peaks [34, 35], the nanocomposite
exhibited typical Sn (Sn 3p, 3d, 4s, 4p, 4d) signals in the
survey spectrum [36], suggesting the existence of Tin species
on GO sheets. The Sn 3d XPS spectrum of composite was
presented in Fig. 1b, where the peaks of Sn 3d5/2 (487.20 eV)
and Sn 3d3/2 (495.53 eV) were distinct. They had a spin
energy separation of 8.33 eV and an area ratio of 1:1.5,
which were in good accordance with the reported XPS data
of SnO [37, 38]. It suggested that SnO NPs were in situ
GO, an oxygenated derivative of graphene, contains abundant
oxygen-containing groups such as carboxyl, carbonyl,
hydroxyl and ether on the surface. The oxygen-containing
groups were capable of gripping metal oxide NPs through
electrostatic interactions, which thus facilitated high disper-
sion of metal oxide NPs on both sides of GO sheet, and also
prevented them from aggregation [31–33]. With this point in
mind, we decided to locate SnO NPs on the surface of GO
2
sheets through electrostatic interactions between the oxy-
genated functional groups and SnO NPs. A convenient
2
2
2
2
Scheme 1 Schematic representation of SnO NPs loaded on GO through hydrothermal method
123

W. Zheng et al.
O 1s
SnO (15 wt%)/GO
(
a)
2
GO
O 1s
C 1s
0
200
400
600
800
1000
1200
Binding Energy (eV)
2
SnO (15 wt%)/GO
SnO
2
(15 wt%)/GO
SnO (15 wt%)/GO
2
(b)
(c)
(d)
Sn 3d5/2
2
84.6(C-C/C=C)
5
32.62
487.2
Sn 3d3/2
286.83 (C-O-C)
SnO
2
5
32.2 eV
531.28
(O1s)
287.68 (C=O)
88.88 (O-C=O)
4
95.53
2
2
85.37(C-O)
5
25 530 535 540
291.46 (Sn-O-C)
Sn 3d5/2
SnO
GO
86.97 (C-O-C)
2
GO
2
84.61(C-C/C=C)
2
5
32.74
Sn 3d3/2
486.98
2
85.6(C-O)
495.40
2
87.68 (C=O)
88.91(O-C=O)
2
480
485
490
495
500 525
530
535
540
280
285
290
295
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
Fig. 1 XPS survey spectra of SnO (15 wt%)/GO and GO (a), Sn3d XPS spectra of SnO
2
2
(15 wt%)/GO and commercial SnO
2
NPs (b), O1s XPS
spectra of SnO (15 wt%)/GO and GO (c), and C1s XPS spectra of SnO
2
2
(15 wt%)/GO and GO (d)
grown on GO surfaces during hydrothermal synthesis.
While, we noticed that the obtained nanocomposite showed
slight increase in binding energy of Sn 3d, as compared with
binding energy of oxygenated functional groups in SnO /GO
2
composite, shifted from 532.74 to 532.62 eV, compared
with that in GO (Fig. 1c). Furthermore, an additional O1 s
IV
commercial SnO NPs (Fig. 1b). Higher binding energy
2
signal (531.28 eV) associated with O-Sn species of SnO2
should be related with the coordination between oxygenated
[39] was also observed in the O1s XPS spectrum of
functional groups of GO and Sn atoms of SnO . The
2
SnO (15 wt%)/GO composite. This was unambiguous evi-
2
deduction could also be drawn from the evidence that the O1s
dence for a SnO /GO composite structure. Notably, the O 1s
2
1
23

SnO
2
Nanoparticle-Decorated Graphene Oxide Sheets Efficiently Catalyze Baeyer–Villiger...
IV
binding energy (531.28 eV, Fig. 1c) of O-Sn species in
nanocomposite slightly decreased as compared with that in
#
#
*
#
commercial SnO NPs (532.20 eV, Fig. 1c inset). It was
2
#
probably due to a hydrogen-bonding between oxygen atom
of the SnO NPs and the hydrogen atom of hydrogen-con-
e
d
c
-2
taining oxygenated functional groups on GO sheets. We
could thus deduce that SnO NPs were tightly bonded to the
2
GO layer by oxygenate groups through dative bond and/or
hydrogen bond. Actually, similar interactions have been
reported in other metal oxide/GO hybrids, such as CuO/GO
[
40], MnO /GO [41], and Fe O /GO [31] etc. These facts
2 3 4
b
a
were consistent with the synthesis design, and confirmed the
successful dispersion of SnO NPs on the basal planes of GO
2
nanosheets, as shown in Scheme 1.
Figure 1d showed the C1s deconvolution spectra of
1
0
20
30
40
50
60
70
80
SnO (15 wt%)/GO composite and pristine GO. As com-
2
2
-theat, degrees
pared with GO, the C1s XPS spectrum of SnO /GO
2
nanocomposite exhibited an additional C1s signal at
Fig. 2 XRD patterns of GO (a), SnO
wt%)/GO (c), SnO (30 wt%)/GO (d), and SnO
2
(15 wt%)/GO (b), SnO
(40 wt%)/GO (e)
2
(20
2
91.46 eV, which was probably associated with the Sn–O–
2
2
C carbonaceous bonds due to the interaction between Sn
atom and oxygen-containing functional groups of GO as
mentioned above. Moreover, decreased intensity of carbon
binding to oxygenated functional groups was observed in
SnO in typical SnO (15 wt%)/GO was estimated using
2 2
Scherrer equation of d = Kk/bcosh and was found to be
2.6 nm at 2h = 26.56°, where d is the average crystal size,
K = 0.89 is the Scherrer constant, k = 0.154 nm is the
wavelength of X-ray, b is the width (in radian)of the XRD
peak at half maximum intensity and h = 26.56°/2 is the
Bragg diffraction angle found from XRD data.
the C1s XPS spectrum of SnO /GO. Such results indicate
2
that partial oxygen-containing functional groups were
eliminated during the hydrothermal process. It thus tuned a
hydrophobic surrounding for SnO NPs, suppressing the
2
undesired competitive coordination of water to the Lewis
acid center. The remaining oxygenated groups helped to
grip metal oxide NPs tight on the GO surface through
electrostatic interactions, which were crucial to the high
3.2.3 Morphological Analyses
dispersion of SnO NPs on GO sheets.
2
The high dispersion of uniform SnO
sheets was further proved by morphological analyses. Fig-
ure 3 shows the TEM images of SnO /GO nanocomposites
with various SnO2 loading. Obviously, TEM image of
SnO (15 wt%)/GO showed thin layered GO sheets, on
which uniform SnO NPs were distributed homogeneously
2
NPs on exfoliated GO
3
.2.2 XRD
2
The crystal structure of SnO NPs placed on GO sheets was
2
2
determined by XRD. Figure 2 shows powder XRD patterns
2
of SnO /GO nanocomposites and pristine GO material.
2
with the size of ca. 3 nm (Fig. 3b). The size obtained from
OB-Viously, pristine GO displayed a strong (002) peak (*)
o
centered at 10.5 , suggesting the layered structure with an
TEM was approach to that determined by XRD (2.6 nm). It
was direct visual evidence that SnO NPs were evenly
2
average interlayer distances of ca. 0.85 nm (Fig. 2a).
While, the sharp diffraction peak disappeared, accompa-
anchored by the oxygenated groups on GO surface rather
than disordered physical adsorption on the basal planes. The
strong interaction between them prevents the NPs from
nied by the appearance of several new peaks (#) at 2h of
o
6.56, 34.22, 51.50, and 65.42 in XRD patterns of
2
aggregating and increases the stability of SnO
oxygenate groups are uniformly distributed on two sides of
the GO sheets, SnO NPs should be gripped on both sides of
GO sheets, resulting in a sandwich-like structure where layer
of GO sheet alternated by SnO NPs sheet. Aggregation
2
NPs. As the
SnO (x)/GO nanocomposites (Fig. 2b–e), which were
2
attributed to (100), (101), (211), and (112) planes of
tetragonal rutile SnO2 (JCPDS Card No. 41-1049),
2
respectively [28, 42]. Therefore, rutile SnO crystals were
2
2
deposited on GO sheets during the hydrothermal process,
and layer-stacking regularity of GO sheets was disturbed
by the decorated rutile SnO2 NPs. The exfoliated GO
problem of GO could be also prevented or at least minimized,
which should be beneficial for mass transfer during the
heterogeneous catalysis. Increasing the mass ratio of SnO
2
sheets should be beneficial for free access of SnO NPs to
2
from 15 to 40 wt% led to the aggregation or even accumu-
lation of NPs on the basal planes (Fig. 3b–e). It was probably
reagents during the oxidation. An average crystal size of
123

W. Zheng et al.
0
Fig. 3 TEM images of SnO
wt%)/GO (d), and SnO (40 wt%)/GO (e)
2
(7.5 wt%)/GO (a), SnO
2
(15 wt%)/GO fresh (b) and recovered after 10 times (b ), SnO
2 2
(20 wt%)/GO (c), SnO (30
2
Fig. 4 Thermogravimetric
a) and differential
thermogravimetric (b) profiles
of the pristine GO (a), SnO (15
wt%)/GO (b), SnO (20 wt%)/
GO (c), SnO (30 wt%)/GO (d),
and SnO (40 wt%)/GO (e), and
110
100
e
(
(a)
(b)
f
2
38
2
90
d
2
2
80
2
236
70
60
50
40
30
20
10
0
commercial SnO
air atmosphere
2
NPs (f) under
c
e
231
b
d
c
b
223
a
a
205
-
10
0
100 200 300 400 500 600 700 800
0
100 200 300 400 500 600 700 800
o
o
Temperature( C)
Temperature( C)
due to that SnO Nps were too much to be gripped by the
2
3.2.4 TGA
surface oxygenated groups. Sparse SnO Nps was observed
2
on the GO sheets when mass ratio of SnO decreased to
2
Thermal analysis was performed to assess the thermal
decomposition behaviour of SnO /GO nanocomposites, as
well as to quantify the mass percentage of SnO in the
7
.5 wt% (Fig. 3a). Low local concentration of active sites
2
was detrimental to efficient catalysis. Therefore, SnO (15 -
2
2
wt%)/GO composite has the most reasonable structure that
sufficient uniform SnO2 NPs were homogeneously dis-
tributed onto GO sheets, which led to highly efficient solid
Lewis acid catalyst for B–V oxidation with H O .
nanocomposites. The results obtained are depicted in
Fig. 4. Pristine GO sheets exhibited three distinct steps of
weight loss in the combined TG–DTG curves (Fig. 4a).
o
The first weight loss before 100 C corresponded to
2
2
1
23

2
SnO Nanoparticle-Decorated Graphene Oxide Sheets Efficiently Catalyze Baeyer–Villiger...
removal of the surface-adsorbed and/or interlayer water
o
molecules. A second large weight loss centered at 205 C,
Table 1 Results of the B–V oxidation of 2-adamantanone with H O2
2
over different catalysts
which was assigned to the pyrolysis of the labile oxygen-
containing functional groups [43]. The third step assigned
to the successive cleavage of carbon sketch appeared at
o
o
a
Conversion (%)
a
Selectivity (%)
Entry
Catalyst
5
50 C. The weight loss extended up to ca. 730 C until the
GO was completely decomposed under air flow. A similar
1
2
3
4
5
6
7
8
SnO
2
Trace
Trace
56
/
thermal behaviour was observed for the SnO /GO samples
2
GO
/
(
Fig. 4b–e). Three mass loss steps associated with the
SnO
SnO
SnO
SnO
SnO
SnO
2
2
2
2
2
2
(7.5 wt%)/GO
(15 wt%)/GO
(20 wt%)/GO
(30 wt%)/GO
(40 wt%)/GO
[99
[99
[99
[99
[99
removal of water molecules, oxygen-containing functional
groups and carbon sketch are distinguished in the com-
97
70
bined TG–DTG curves of SnO /GO samples. The non-re-
2
55
movable residue belonged to incorporated SnO NPs which
2
48
c
3
was quite stable with negative mass loss during the tem-
perature range (Fig. 4f). Thus, the mass percentages of
b
c
-MCM-41
96
2 2
Catalyst (0.025 g), 2-adamantanone (1.0 mmol), H O (30 wt%,
SnO in corresponding composites could be accordingly
2
o
.0 mmol), 1,2-dichloroethane (2 mL), 2 h, 90 C
2
estimated to be 15 (Fig. 4b), 20 (Fig. 4c), 30 (Fig. 4d) and
a
Determined by GC
4
0 (Fig. 4e) wt%, respectively. Furthermore, we noticed
b
Catalyst (0.66 mol% of Sn with respect to the ketone), 2-adaman-
(50 wt%, 1.5 mmol), dioxane (3 g), 16 h, 75
that the decomposition temperature of oxygenated groups
tanone (1 mmol), H
o
C
O
2 2
gradually increases with the increase of SnO content in
2
composites (Fig. 4b–e). The observation suggested the
interaction between SnO2 NPs and oxygenated groups,
which enhanced thermal stability of the oxygen-containing
functional groups.
c
Data from the Ref. [5]
importantly, the synergistic effect between SnO NPs and
2
GO support effectively promoted further enhancement of
catalytic activity. The deduction can be drawn from the
evidence that SnO2 directly grafted onto mesoporous
MCM-41 yielded negligible 2-adamantanone conversion
under analogous reaction conditions (Table 1, entry 8), as
3
.3 Catalytic Performances
The featured structure that small sized SnO NPs are uni-
2
formly dispersed on the surface of GO sheets turns the
SnO /GO nanocomposite into the efficient solid Lewis acid
2
reported by Rom a´ n-Leshkov et al. [5]. Notably, SnO
/GO
2
catalysts for the B–V oxidation of ketones with H O . The
2
nanocomposite containing 15 wt% SnO NPs showed the
2
2
catalytic efficiency of the resultant SnO /GO nanocom-
2
most remarkable activity in the B–V oxidation. Almost
quantitative conversion of 2-adamantanone with excellent
selectivity to lactone ([99 %) was achieved over 0.025 g
posite catalyst was investigated in B–V of bulky
2
-adamantanone with H O (30 wt%) in 1,2-dichlor-
2 2
o
oethane at 90 C. The results are presented in Table 1. GO
of SnO
ing catalytic Tin sites) within 2 h (Table 1, entry 4). While,
the conversion gradually decreased as SnO loading
increased (Table 1, entries 4–7). Only 48 % conversion of
2-adamantanone was observed when SnO loading was
increased to 40 wt% (Table 1, entry 7). Higher loading of
SnO resulted in the lower dispersion state of SnO NPs, as
(15 wt%)/GO nanocomposite (2.0 mol%, regard-
2
material and commercial SnO NPs were also examined for
2
comparison purposes.
2
Obviously, commercial SnO NPs were inactive in B–V
2
oxidation with H O giving practically no lactone of
2,
2
2
4
-oxatricyclo [4. 3. 1. 1 ]-undecan-9-one even if double
,8
2
H O was used (Table 1, entry 1). While, the catalytic
2
2
2
2
efficiency benefited from dispersing SnO2 NPs on GO
sheets, although GO itself was also inactive (Table 1, entry
shown in TEM images, which was detrimental to the
catalysis. These results thus prompted us to further
2
). 48-97 % Conversion of 2-adamantanone was obtained
decrease SnO
2
loading from 15 to 7.5 wt%. Unexpectedly,
(7.5 wt%)/GO nanocom-
over the obtained SnO /GO nanocomposites (Table 1,
2
although uniform NPs, SnO
2
entries 3-7). The results demonstrated the positive effect of
posite was unsatisfied for the B–V oxidation of
2-adamantanone, likely due to the insufficient active sites
(Table 1, entry 3). Interestingly, the selectivity to lactone
was almost 100 % irrespective of the Tin concentration
with no formation of any side-products.
GO sheets on catalytic performance of SnO (x)/GO
2
nanocomposites. Abundant oxygenate groups on GO sheets
ensured high dispersion of uniform SnO NPs on both sides
2
of the GO layer with small size. Delamination of the
nanocomposites allowed catalytic reactions to be carried
out under pseudo-homogeneous reaction conditions,
thereby significantly increasing the reaction rate. More
Apart from enhanced catalytic efficiency, the reusability
of SnO
/GO composites in B–V oxidation was also
(15 wt%)/GO nanocomposite was
2
enjoyable. Typical SnO
2
123

W. Zheng et al.
1
00
SnO NPs was also avoided by the electrostatic interaction
2
between SnO NPs and oxygenated groups on GO sheets,
2
as indicated by the identical SnO content in recovered
2
9
8
7
6
5
0
0
0
0
0
catalyst (15 wt% loading). More importantly, the organic
deposits, a main reason for poisoning the Tin-containing
catalyst [11, 12], can also be avoided because of the free
traffic of reactants and products in the layered SnO /GO
2
nanocomposite. Therefore, the novel SnO (15 wt%)/GO
2
nanocomposite was efficient and reusable in the typical B–
V oxidation of 2-adamantanone with H O .
2
2
It should be more attractive if the nanocomposite can
be applied to various ketones. Table 2 summarized the B–
V oxidation of various ketones in the presence of
Conversion
Selectivity
SnO (15 wt%)/GO. Besides 2-adamantanone, cyclobu-
2
tanone was also successfully oxidized to lactone with high
efficiency and excellent selectivity under identical con-
dition (Table 2, entry 2). The oxidation of other cyclic
ketones (5- or 6- membered rings) also occurred smoothly
to afford the corresponding lactones with excellent
selectivity, although longer reaction time was required
0
1
2
3
4
5
6
7
8
9
10 11
Run times
Fig. 5 Reusability of SnO
2 2
-adamantanone with H O
2
(15 wt%)/GO in the B–V oxidation of
2
(Table 2, entries 4, 6, and 8). Interestingly, the substituent
employed to investigate the reusability in B–V oxidation of
-adamantanone with H O , and the results were listed in
in six-membered cyclic ketone influenced the conversion
of ketone in B–V oxidation (Table 2, entries 7 and 8).
Cyclohexanone with methyl substituent at para-position
was less reactive than cyclohexanone, due to the steric
effect, as well as electron-donating inductive effect of
methyl group (Table 2, entry 8 vs 6). Notably, although
the reaction rate depends on the chemical structure of
2
2
2
Fig. 5. Simply by centrifugation, the SnO (15 wt%)/GO
2
catalyst could be facilely recovered for reuse. To our
delight, it could be reused for at least ten times with no
appreciable decrease in catalytic activity and selectivity.
Therefore, the SnO (15 wt%)/GO catalyst was quite
2
stable in the oxidation. The stability was further identified
ketones, it can be concluded that the SnO (15 wt%)/GO
2
by TEM image of recovered SnO (15 wt%)/GO
2
nanocomposite could oxidize the various ketones to cor-
responding lactones with excellent selectivity ([99 %)
(Table 2, entries 1, 2, 4, 6, and 8). The high selectivity
obtained is attributed to the carbonyl group activation
mechanism proposed by Corma et al., which involves
initial selective activation of the carbonyl group, followed
by reaction with non-activated H O [10].
0
nanocomposite (Fig. 3b vs b). No significant change in
layered structure of catalyst, as well as dispersed state and
particle size of SnO NPs was observed even after reused
2
for ten times. The results indicated that tightly pinning
SnO NPs on GO sheets could effectively enhance the
2
dimensional stability of NPs during cycling. Leaching of
2
2
Table 2 Results of B–V
a
Conversion (%)
a
Selectivity (%)
Entry
1
Substrate
Product
Time (h)
2
oxidation of various ketones
with H over SnO (15 wt%)/
GO nanocomposite
O
2 2
2
97
[99
[99
O
O
O
O
O
O
2
2
99
O
O
O
O
O
3
4
2
6
73
92
[99
[99
O
O
5
6
2
6
56
91
[99
[99
O
O
7
8
2
43
95
[99
[99
16
2 2 2
SnO (15 wt%)/GO (0.025 g, 2 mol% of Sn with respect to the ketone), substrate (1 mmol), H O
o
30 wt%, 2 mmol), 1,2-dichloroethane (2 mL), 2 h, 90 C
(
a
The same as Table 1
1
23

2
SnO Nanoparticle-Decorated Graphene Oxide Sheets Efficiently Catalyze Baeyer–Villiger...
Table 3 Comparison of results
obtained over different catalysts
a
Conv. (%)
a
Sel. (%)
Entry
Catalyst
SnO (15 wt%)/GO
Substrate
Ref.
2 2
in the B–V oxidation with H O
O
1
2
3
4
5
6
7
8
2
92
45
81
22
37
97
49
89
[99
100
100
100
75
This work
b
Ref.
Sn-Montmorillonite
Sn-palygorskite
Sn/PS
c
Ref.
d
Ref.
e
Silica-AlCl
3
Ref.
SnO (15wt%)/GO
2
[99
91
This work
f
Ref.
Sn/MCM-41
Sn/MSNSs
O
f
98
Ref.
2 2
Catalyst (0.025 g), 2-adamantanone (1.0 mmol), H O (30 wt%, 2.0 mmol), 1,2-dichloroethane (2 mL),
o
h, 90 C
2
a
Determined by GC
b
d
c
e
f
Data from the Ref. [44]
Data from the Ref. [18]
Data from the Ref. [45]
Data from the Ref. [46]
Data from the Ref. [10]
Superiority of the SnO (15 wt%)/GO nanocomposite
2
loading of SnO NPs on both sides of the exfoliated GO
2
over other reported catalysts, such as Sn-Montmorillonite
sheets through electrostatic interactions. Benefiting from
[
44], Sn-palygorskite [18], Sn/PS [45], and Silica-AlCl , is
3
the flexible GO support, high dispersion of SnO NPs, as
2
seen in Table 3. Obviously, SnO (15 wt%)/GO was far
2
well as the intimate interaction between SnO NPs and GO,
2
more efficient than these reported catalysts either layered
or porous in B–V oxidation of cyclopentanone with H O ,
the resultant SnO /GO composites, especially SnO
2
2
(15 wt%)/GO, exhibited excellent performance as the solid
Lewis acid catalysts in B–V oxidation of ketones with
H O . Furthermore, catalyst poisoning by organic deposits
2
2
due to the pseudo-homogeneous reaction conditions, as
well as synergistic effect between SnO2 NPs and GO
support (Table 3, entry 1 vs entries 2-5). Especially, silica-
2
2
could also be effectively avoid by free traffic of reactants
and products in the layered SnO2/GO nanocomposite. The
heterogeneous catalyst was stable in the reaction system
and could be easily recovered for efficient reuse. The
AlCl gave only 37 % conversion of cyclopentanone with
3
low selectivity (70 %) to lactone (Table 3, entry 5). Fur-
thermore, SnO (15 wt%)/GO nanocomposite also exhib-
2
ited much higher activity than tin containing mesoporous
silicas even in the case of bulky 2-adamantanon (Table 3,
entry 6 vs entries 7, 8) [10]. The results demonstrated the
design and controlled synthesis of SnO /GO nanocom-
2
posites provided a facile, economic, and versatile approach
to promote the catalytic performance of other metal oxides
and can be scaled up easily for industrial production.
advantages of SnO /GO nanocomposite used for baeyer-
2
villager oxidation reaction. Aapart from higher efficiency,
Acknowledgments The project was financially supported by the
National Natural Science Foundation of China (Grant No. 21476069),
the Scientific Research Fund of Hunan Provincial Education
Department (13B072), the Program for Excellent Talents in Hunan
Normal University (ET14103), the Hunan Provincial Innovation
Foundation for Postgraduate (CX2013B209) and the Program for
Science and Technology Innovative Research Team in Higher Edu-
cational Institutions of Hunan Province.
the reusability of SnO (15 wt%)/GO was also superior to
2
that of the compared catalysts. Blocking of the active sites
by either products or intermediates, a main reason for
deactivation of the Tin-containing catalyst, was avoided by
the flexible layered structure of SnO /GO nanocomposite.
2
Furthermore, leaching of SnO NPs was also minimized
2
due to the electrostatic interaction between SnO NPs and
2
oxygenated groups on GO sheets.
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