Promotional synergistic effect of Sn doping into a novel bimetallic Sn-W oxides/graphene catalyst for selective oxidation of alcohols using aqueous H2O2 without additives

Article abstract of DOI:10.1016/j.apcata.2018.03.017

A series of novel catalysts with bimetallic Sn-W oxides uniformly dispersed on reduced graphene oxide (denoted as Sn-W/RGO) were successfully prepared via a conventional one-pot hydrothermal method. Interestingly, it was found that the introduction of Sn could significantly improve the catalytic activities compared with single WO₃/graphene catalyst for selective oxidation of alcohols using H₂O₂ without organic solvents and additives. Subsequently, the catalysts were characterized by FT-IR, XRD, Raman, FESEM, EDS, HRTEM, XPS and N₂ adsorption-desorption to investigate the promotional synergistic effect of Sn among catalyst structure, surface properties and catalytic performance. The results revealed that the incorporation of Sn had induced the formation of hexagonal WO₃ with dominant exposed (001) and (200) planes, meanwhile, doping Sn could dramatically increase the specific surface area, change the structural properties and enhance the interactions between Sn-W oxides and graphene, which were beneficial to the improvement of catalytic performance. Additionally, reaction parameters (temperature, time, oxidant dosage and catalyst amount) were also explored. Both aryl and alkyl alcohols exhibited high conversion and selectivity to corresponding target products over the catalyst. And delightedly, the robust catalyst could be reused for five times without significant loss in catalytic efficiency. This work demonstrates the promotional synergistic effect of Sn in a novel Sn-doped WO₃/RGO catalyst and makes it a promising candidate in green oxidation of alcohols under mild reaction conditions.

Full text of DOI:10.1016/j.apcata.2018.03.017

Accepted Manuscript
Title: Promotional synergistic effect of Sn doping into a novel
bimetallic Sn-W oxides/graphene catalyst for selective
oxidation of alcohols using aqueous H2O2 without additives
Authors: Cai Xu, Lingling Zhang, Yu An, Xiaozhong Wang,
Gang Xu, Yingqi Chen, Liyan Dai
PII:
S0926-860X(18)30136-4
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.03.017
APCATA 16591
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
2-12-2017
21-2-2018
19-3-2018
Please cite this article as: Xu C, Zhang L, An Y, Wang X, Xu G,
Chen Y, Dai L, Promotional synergistic effect of Sn doping into a novel
bimetallic Sn-W oxides/graphene catalyst for selective oxidation of alcohols
using aqueous H2O2 without additives, Applied Catalysis A, General (2010),
https://doi.org/10.1016/j.apcata.2018.03.017
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Promotional synergistic effect of Sn doping into a novel
bimetallic Sn-W oxides/graphene catalyst for selective oxidation
of alcohols using aqueous H2O2 without additives
Cai Xu, Lingling Zhang, Yu An, Xiaozhong Wang, Gang Xu*, Yingqi Chen and
Liyan Dai*
Zhejiang Provincial Key Laboratory of Advanced Chemical Engineering Manufacture
Technology, College of Chemical and Biological Engineering, Zhejiang University,
Hangzhou 310027, China
*Corresponding authors.
(Xu) E-mail: xugang_1030@zju.edu.cn; Tel: +86 571 87952363.
(Dai) E-mail: dailiyan@zju.edu.cn; Tel: +86 571 87952693; Fax: +86 571 87952394.
Graphical abstract
1

Highlights
 A series of novel catalysts with bimetallic Sn-W oxides uniformly dispersed on
graphene were successfully prepared.
 Sn species act as an effective promoter for selective oxidation of alcohols using
H2O2 without organic solvents and any additives.
 The promotional synergistic effect of Sn was investigated in detail.
 The catalyst can catalyze the oxidation of both aromatic and aliphatic alcohols
efficiently with remarkable recyclability.
Abstract
A series of novel catalysts with bimetallic Sn-W oxides uniformly dispersed on
reduced graphene oxide (denoted as Sn-W/RGO) were successfully prepared via a
conventional one-pot hydrothermal method. Interestingly, it was found that the
introduction of Sn could significantly improve the catalytic activities compared with
single WO3/graphene catalyst for selective oxidation of alcohols using H2O2 without
organic solvents and additives. Subsequently, the catalysts were characterized by
FT-IR, XRD, Raman, FESEM, EDS, HRTEM, XPS and N2 adsorption-desorption to
investigate the promotional synergistic effect of Sn among catalyst structure, surface
properties and catalytic performance. The results revealed that the incorporation of Sn
had induced the formation of hexagonal WO3 with dominant exposed (001) and (200)
planes, meanwhile, doping Sn could dramatically increase the specific surface area,
change the structural properties and enhance the interactions between Sn-W oxides
2

and graphene, which were beneficial to the improvement of catalytic performance.
Additionally, reaction parameters (temperature, time, oxidant dosage and catalyst
amount) were also explored. Both aryl and alkyl alcohols exhibited high conversion
and selectivity to corresponding target products over the catalyst. And delightedly, the
robust catalyst could be reused for five times without significant loss in catalytic
efficiency. This work demonstrates the promotional synergistic effect of Sn in a novel
Sn-doped WO3/RGO catalyst and makes it a promising candidate in green oxidation
of alcohols under mild reaction conditions.
Keywords: Sn-W oxides, graphene, synergistic effect, alcohols oxidation.
1. Introduction
Highly selective oxidation of alcohols to corresponding carbonyl compounds,
emerging as one of the most essential chemical transformations in both academic and
industrial research, because it produces valuable intermediates in pharmaceuticals,
dyestuffs, agrochemicals, and fragrances industries [1–3]. Many homogeneous
catalysts have been used to promote this oxidation process but inevitably bring some
environmental problems, and the efficient recycling also remains a great challenge
[4–8]. Hence, increasing efforts have been devoted in developing heterogeneous
catalysts. Therein, noble metals (e.g., gold [9,10], palladium [11], ruthenium [12] and
platinum [13]) exhibit excellent catalytic activities for this chemical transformation,
but it is imperative to reduce their usage due to the low abundance and precious
nature. Likewise, other non-noble transitional metals (e.g., cobalt [14], manganese
3

[15], iron [16], bismuth [17] and copper [18]) have been explored for alcohols
oxidation, but their efficiencies are usually lower in comparison with the noble metals
based on substrate scope and catalytic performance, and non-green base additives
(such as NaNO2, Na2CO3) are commonly added, served as active promoters to achieve
higher selectivity to desirable products [19,20]. On the other hand, expensive and
toxic oxidants such as chromium trioxide [21], tert-butyl hydroperoxide (TBHP)
[22,23], and TEMPO [24] have been widely used in alcohols oxidation. However,
from the environmental point of view, green oxidants such as O2 and H2O2 have
proved to be more promising [25–27]. Particularly in aqueous solution, H2O2 is
greatly attractive for releasing active oxidative species and producing water as the
only by-product [28–30]. In this context, employing non-noble metals to fabricate
efficient catalysts for alcohols oxidation with green oxidant is highly desirable.
Traditionally, tungsten oxide (WO3) has been used in various oxidation reaction
due to its attractive features, such as strong Lewis acidity, robust deprotonation
capacity and highly hydrophilic properties [31–33]. Nevertheless, slow charge
transfer and inadequate catalytic activities of pure WO3 confine it to a great extent in
catalysis. To achieve better catalytic performances, it is suggested to turn from single
metal to bimetallic composites, since bimetallic catalysts have proved to be more
superior in catalytic activities compared to their monometallic counterparts [34,35].
In recent years, tin oxides have attracted considerable research interest in gas
sensor, electrochromic devices and heterogeneous catalysis due to its appealing
4

features, such as oxygen vacancies defects and variations in valence state [36,37].
More recently, Dai et al. prepared WO3/SnO2 catalyst for selective oxidation of 1,
2-benzenedimethanol with H2O2 and investigated the effect of calcination temperature
of the support [38]. Kamata et al. synthesized heterogeneous W-Sn based catalyst for
efficient oxidation of amines using H2O2 and achieved satisfying yield [39]. These
reports have confirmed the fascinating potential of Sn-W oxides in the area of
catalytic oxidation. Besides, considering that loss of active species and agglomeration
of nanoparticles usually occur during the reaction process, it is urgent and meaningful
to develop proper supports to immobilize metal components. Currently, graphene has
attracted widespread attention in heterogeneous catalysis for its high surface area for
dispersion of metal oxides and strong conductivity for electron transfer [26,40]. It was
verified that when metal catalysts were anchored on graphene sheets, the composite
displayed higher catalytic activity and longer durability due to the strong interaction
between graphene and metals [41,42]. These remarkable performances make graphene
an excellent support for metal oxides to construct high-efficiency catalysts. Therefore,
the combination of binary Sn-W oxides anchored on graphene sheets is reasonably
believed to possess much better catalytic performance.
Herein, a series of novel bimetallic Sn-W oxides/graphene catalysts were firstly
prepared via a conventional one-pot hydrothermal method. Then the catalytic
activities for selective oxidation of alcohols using H2O2 without toxic solvents and
additives were evaluated. And the effects of reaction parameters on catalytic activities
5

over the bimetallic catalysts were also explored. Moreover, it was interesting to
observe that introducing a certain amount of Sn species into the catalyst could
dramatically enhance the catalytic performance for alcohols oxidation. Therefore,
promotional synergistic effect of Sn was investigated in detail, and various
characterizations were used to better understand the correlations between catalyst
structure, surface properties and the enhanced catalytic activity resulting from the
incorporation of Sn.
2. Experimental
2.1. Catalysts preparation
All chemical reagents with analytical grade were used as received and without
further purification. Graphene oxide (GO), the precursor of graphene, was synthesized
based on a modified Hummer’s method from natural graphite powder [43]. Reagents
and detailed preparation procedure of GO were presented in Supporting Information.
Additionally, the Sn-doped WO3/RGO catalysts were prepared through a conventional
one-pot hydrothermal reaction. Typically, the obtained GO was homogeneously
dispersed in deionized water and sonicated for 2 h with the concentration of 0.5
mg/mL. Then, 5 mmol Na2WO4·2H2O, a certain molar amount of SnCl2·2H2O (the
molar ratios of Sn to W = 0.1, 0.2, 0.3, 0.4, respectively) were dissolved in 50 ml
above GO solution and kept stirring for 3 h. Afterwards, 37% hydrochloric acid
solution was added dropwise to the above mixture under vigorous stirring until PH
value of the solution was adjusted to 2, and then kept stirring for 3 h. Subsequently,
6

the suspension was transferred into a 100 ml Teflon-lined autoclave to undergo a
hydrothermal process at 140 ℃ for 14 h. After cooling to ambient temperature, the
sample was collected and washed with deionized water and ethanol for several times,
and then dried at 60 ℃ overnight in vacuum oven to give the bimetallic Sn-W
oxides/graphene catalyst. For convenience, the resultant samples were labeled as
ySn-W/RGO, where ‘y’ represents the Sn/W molar ratio. For comparison, WO3/RGO,
SnO2/RGO were prepared through the same method above in the absence of Sn, W
precursor, respectively.
2.2. Materials characterization
Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a
Nexus 670 spectrometer using KBr pellets in the range of 500-4000 cm-1. X-ray
diffraction (XRD) patterns were collected on a PANalytical X’Pert PRO
diffractometer using Cu Kα radiation (λ = 1.5406 Å). Raman spectra were tested by a
ThermoFisher DXR SmartRaman spectrometer with the excitation wavelength of 532
nm. Field emission scanning electron microscopy (FESEM) images were taken on a
Hitachi SU-70 microscope, and samples were analyzed by spreading them on a
carbon tape. Energy dispersive X-ray spectrometer (EDS) connected with FESEM
was used for elements analysis and elemental distribution mapping. Transmission
electron microscopy (TEM) and high-resolution TEM (HRTEM) images were
obtained from a FEI Tecnai G2 F20 S-TWIN microscope operated at 200 kV. X-ray
photoelectron spectra (XPS) were measured on Thermo Scientific Escalab 250Xi
7

spectrometer using Al Kα radiation at 20 kV, and the binding energies were calibrated
with C 1s peak at 284.6 eV. The surface area and pore volume of the catalysts were
calculated by using BET and BJH methods from nitrogen adsorption-desorption
isotherms at 77 K determined with a Micromeritics Belsorp-mini II (BEL Japan, Inc.).
2.3. Catalytic oxidation of alcohols
The selective oxidation of benzyl alcohol using aqueous H2O2 without organic
solvents and additives was chosen as the probe reaction to test the activities of
as-prepared catalysts. Typically, benzyl alcohol (0.5 mmol), H2O2 (30 wt%, 0.6 mmol),
deionized water (1 ml), catalyst (10 mg) were added sequentially into a sealed glass
tube with vigorous magnetic stirring at 80 ℃ for 3 h. After the reaction, the solid
catalyst was separated by centrifugation, and then washed with deionized water and
ethanol thoroughly, followed by drying at 60 ℃ in vacuum oven overnight for reuse.
The filtrate was extracted by ethyl acetate and then analyzed by GC (Agilent 6820,
FID detector, 30 m hp-5 capillary column) with external standard method, and the
products were identified through comparison with known standards. Additionally, to
ensure the reproducibility of all activity tests, each result had been repeated at least
three times and found to be within acceptable error ranges (±2.0%).
3. Results and discussion
3.1. Catalysts characterization
3.1.1. Structure and morphology characterization
The FT-IR results of various Sn-W/RGO samples are depicted in Fig. 1, for
8

comparison, FT-IR spectra of GO and WO3/RGO are also presented. The band at
around 3410 cm-1 was clearly found in all six spectra which could be ascribed to the
stretching vibration mode of O–H bond or physically adsorbed water. In the spectrum
of GO in Fig 1a, the characteristic absorption bands were distinctly observed at 1049
cm-1 (C–O), 1224 cm-1 (C–O–C), 1621 cm-1 (C
C) and 1736 cm-1 (C
O),
respectively [44,45]. The broad absorption bands in the range of 500-1000 cm-1 were
assigned to stretching vibration of W–O and Sn–O, but the difference was not quite
obvious and thus XRD was used to further characterize the samples. Additionally, it
should be noted that the peaks at 1224 cm-1 (C–O–C), 1736 cm-1 (C
O)
disappeared completely and the peaks at 3410 cm-1 (O–H) became weak in these
spectra of Sn-W/RGO, implying that the pristine GO has been partially reduced to
RGO through the hydrothermal process and simultaneously the Sn-W/RGO
composites have been successfully synthesized.
The crystalline phase and structure of the obtained samples were characterized
by XRD, with the results shown in Fig. 2. A strong characteristic diffraction peak at
around 9° of GO could be clearly observed, which was attributed to (002) plane with
interplanar spacing of approximately 0.98 nm [26]. After hydrothermal process, the
diffraction peaks of GO disappeared completely, but no obvious RGO peaks were
found in the spectra of other samples due to its low content [46]. Additionally, the
XRD patterns of WO3 shown in Fig. 2b could correspond to well-crystallized
orthorhombic structure (JCPDS No. 20-1324). But with the addition of Sn from 0.1 to
9

0.4, the diffraction peaks of WO3 gradually changed. And notably, hexagonal WO3
(JCPDS No. 33-1387) emerged when the Sn/W molar ratio reached to 0.3, indicating
that more Sn species had been incorporated into the lattice of WO3 crystallites and
eventually caused this structure transformation. As regards these Sn-W/RGO catalysts
(as shown in Fig. 2c, d, e, and f), the peak at 51.2° might ascribe to (211) plane of
tetragonal SnO2 nanocrystals (JCPDS No. 41-1445) [38]. Moreover, the diffraction
peaks of 0.3Sn-W/RGO were considerably broadened compared with those of
WO3/RGO, suggesting that a smaller grain size was obtained, which also meant that
doping Sn into the catalyst could enhance nucleation and inhibit size growth of the
crystallites. The equivalent particle size of WO3 in WO3/RGO and 0.3Sn-W/RGO are
45.6 and 14.6 nm, respectively, calculated from the dominated (120) plane in Fig. 2b
and (200) plane in Fig. 2e using Scherrer’s equation (Table 1).
Raman spectroscopy was used to characterize the structure of obtained catalysts.
Herein, it should be noted that the D band at around 1350 cm-1 and the G band at
around 1595 cm-1 represent the disorder induced features caused by lattice defects and
the first-order scattering of the E2g vibrational mode within sp2-hybridized graphene
domains, respectively [26]. As shown in Table 1, the calculated ID/IG intensity ratios
of WO3/RGO and 0.3Sn-W/RGO were 1.17 and 1.22, respectively. And both of them
were larger than that of bare GO (0.73), demonstrating that unrepaired defects still
remained after the removal of oxygen-containing functionalities by this hydrothermal
synthesized process [47]. Notably, ID/IG ratio of 0.3Sn-W/RGO was slightly larger
10

than that of WO3/RGO, indicating that the presence of Sn could decrease the
graphitization degree and enhance the intrinsic disorder of graphene matrix. On the
other hand, for WO3/RGO in Fig. 3b, the peak at 270 and 320 cm-1 associated with the
bending of dangling oxygen, δ (O–W–O), were clearly observed. Additionally, typical
W–O stretching vibration located at 717 cm-1 and the vibrational mode of the O–W–O
stretching located at 810 cm-1 were also observed [48]. With the introduction of Sn,
the dominant bands of 0.3Sn-W/RGO in Fig. 3c slightly shifted to low frequencies,
which demonstrated the existence of strong interactions between Sn and W species.
Besides, a new band at around 960 cm-1 appeared in spectrum of 0.3Sn-W/RGO,
implying that the size of WO3 grains was quite small and in nanoscale, as reported by
Pagnier et al. [49]. No obvious peaks of Sn–O were detected which might attribute to
its weak Raman activity and low content, but the Raman analysis could further
confirm the great impacts on WO3 and graphene arising from the introduction of Sn.
FESEM and TEM were taken to investigate the morphological structure of the
as-prepared catalysts. Fig. 4a presents the FESEM image of WO3/RGO, it was clear
that uniform WO3 nanoplates formed on the graphene sheets tightly with an average
thickness of 60 nm. Nevertheless, for 0.3Sn-W/RGO in Fig. 4b, the particle size
significantly decreased and the homogeneous nanoplates broke down to irregular
shapes with the introduction of Sn. TEM images in Fig. 4c and d also suggested that
doping Sn could remarkably refine grain size, which was in good accordance with the
broadened peaks in XRD patterns and Raman analysis. The morphological changes
11

might result from the penetration of Sn into WO3 lattice and the strong interactions
between W and Sn. Additionally, the corresponding SAED pattern (Fig. 4e) confirmed
that the 0.3Sn-W/RGO was close to polycrystalline structure in nature. Moreover, the
lattice fringes could be observed clearly from the high-resolution TEM (HRTEM)
images shown in Fig. 4f and g, indicating that these as-prepared samples were highly
crystalline. The measured lattice fringe spacing of 0.335 nm in Fig. 4f exactly
corresponds to the (120) plane of orthorhombic WO3 in WO3/RGO. In addition, Fig
4g presents the lattice distances of 0.315 nm and 0.392 nm which are in good
agreement with the dominant exposed (200) and (001) planes of hexagonal WO3 in
0.3Sn-W/RGO. The chemical composition and elemental distribution of
0.3Sn-W/RGO were also examined by using energy dispersive X-ray spectrometer
(EDS), as presented in Fig. 4h and Fig. S6, only four elements of C, O, W and Sn
were identified in the spectrum. These results evidently demonstrated that Sn species
could play important roles in changing the catalyst structure and reducing the grain
size, which associated with the better catalytic performance in alcohols oxidation.
3.1.2. XPS characterization
XPS was perform to elucidate the surface components and chemical states of
elements in GO, WO3/RGO and 0.3Sn-W/RGO samples. The overall XPS survey
spectrum of 0.3Sn-W/RGO in Fig. 5a confirmed the existence of elements C, O, W
and Sn, meanwhile, no impurity element was found indicating the high purity of it.
Additionally, the C 1s spectrum of GO (Fig. 5b) could be divided into three peaks,
12

including C C (284.4 eV) derived from graphitic matrix SP2 carbons, C–O (286.5
eV) and C O (288.7 eV) originated from oxygenated groups attached to carbon
atoms [26,50]. Besides, C 1s spectra of WO3/RGO and 0.3Sn-W/RGO (Fig. 5b) were
similar to that of GO, but intensity of the peaks related to oxygen-containing groups
(such as C–O and C O) were much weaker than those of GO and C C was the
dominated peak, demonstrating that the hydrothermal reaction could remove
oxygenated groups and make GO reduced to RGO.
In the high resolution spectra of W 4f in WO3/RGO (Fig. 5c), two obvious peaks
at 34.8 eV and 37.0 eV corresponding to two characteristic W 4f7/2 and W 4f5/2 peaks
of W6+ in WO3/RGO were observed [51,52]. Notably, the binding energy values of W
4f7/2 and W 4f5/2 doublets in 0.3Sn-W/RGO were slightly larger than those of
WO3/RGO, and both two peaks moved 0.2 eV to higher binding energy. Such a shift
could be ascribed to the electron transfer and strong interactions between Sn and W
species. In terms of XPS spectra of Sn 3d (Fig. 5d), it can be seen that the bands at
486.5 eV and 495.0 eV related to Sn 3d5/2 and Sn 3d3/2, respectively [53]. And the
binding energies both shifted to a lower position compared to those of pure SnO2,
implying that there were excess electrons around Sn and part of the low valence Sn
species existed over 0.3Sn-W/RGO. Furthermore, the surface Sn/W molar ratio of
0.3Sn-W/RGO was lower than nominal ratio as shown in Table 1, demonstrating that
a certain amount of Sn had penetrated into the WO3 lattice, which was well consistent
with XRD analysis. And finally, based on these XPS results, it was reasonable to
13

conclude that the introduction of Sn markedly changed the surface composition and
enhanced the interactions between Sn-W oxides and graphene.
3.1.3. N2 adsorption-desorption analysis
It is known that a larger surface area can offer more adsorption and reactive sites
for catalytic reactions, leading to an improvement in catalytic activity. And thus the
obtained samples were also measured by N2 adsorption-desorption, with the results
shown in Table 1 and Fig. 6. It is clear that WO3/RGO exhibits the typical type II N2
adsorption-desorption isotherm according to the IUPAC classification [26]. The
obscure hysteresis and high adsorption at high P/P0 pressure indicated the formation
of large mesopores and macropores, which might arise from the self-assembly of the
interconnected nanosheets, these results were consistent well with the corresponding
pore size distribution curve in Fig. 6b. However, with the introduction of Sn, the
adsorption-desorption profiles of 0.3Sn-W/RGO were found to be different from
those of WO3/RGO, displaying type IV isotherm with obvious H3 hysteresis loop
based on the IUPAC classification [54]. And it implied the presence of slit-shape
pores and small mesopores which were associated with the hierarchical structure, the
pore size distribution centered at the range of 1-10 nm and the TEM image in Fig. 4d
could also corroborate it. Correspondingly, the 0.3Sn-W/RGO has a BET surface area
of 52.9 m2 g-1, which is several times higher than that of WO3/RGO (6.3 m2 g-1).
Obviously verifying that doping Sn could greatly improve the specific surface area,
which matched well with the small grain size obtained from XRD and TEM analysis.
14

A larger surface area of 0.3Sn-W/RGO could provide more active sites for adsorption
of the reactant molecules, making the catalytic oxidation process more efficient.
3.2. Catalytic oxidation of alcohols
3.2.1. Effect of reaction parameters
The selective oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) with
H2O2 was carried out as a model reaction to estimate the activities of as-prepared
catalysts. Firstly, a blank experiment without using catalysts was carried out, the listed
result in Table S1 suggests that no significant BzOH conversion was achieved.
Likewise, more detailed reaction parameters, such as reaction temperature, reaction
time, H2O2 dosage and catalyst amount were also investigated to get the optimal
reaction conditions, with results shown Table S1 and Fig. S1-4. It was found that
reaction temperature and H2O2 dosage were key factors responsible for BzOH
conversion and BzH selectivity. For example, the conversion increased significantly
from 11.1% to 94.0% with the temperature increased from 40 ℃ to 80 ℃, which was
attributed to the endothermic nature of this reaction and relatively high temperature
could facilitate the reaction process. H2O2 dosage greatly affected the selectivity to
desirable product and which reached a maximum of 94.3% when H2O2/BzOH molar
ratio was only 1.2, excess oxidant would conversely decrease the selectivity. It is
proposed that the reaction pathway of W-based catalytic system with H2O2 follows
this: the main active component (W species) firstly reacts with H2O2 to form a peroxo
complex, and then the complex further reacts with BzOH to give an intermediate,
15

which finally gives BzH and regenerates the catalyst [55]. It also means the catalyst
has little promotion in H2O2 self-decomposition into inactive oxygen species, the
relatively low H2O2 dosage also corroborates it. And effect of reaction time on BzOH
oxidation was also investigated with the results shown in Fig S3, prolonging the
reaction time just achieved a little increase in BzOH conversion and inversely lowered
the selectivity to BzH after the reaction ran for 3 h. Furthermore, Fig. S4 presents the
influence of catalyst dosage used in this model reaction, and only 10 mg catalyst
could obtain the maximum yield of BzH, indicating the high catalytic efficiency of
this Sn-W/RGO catalyst. And thus the optimized reaction conditions for selective
oxidation of benzyl alcohol with H2O2 in water were summarized as follows:
H2O2/BzOH molar ratio was 1.2, the catalyst dosage was 10 mg and the reaction was
performed at 80 ℃ for 3 h.
Besides, with the optimized reaction conditions, both monometallic and a series
of bimetallic Sn-W/RGO catalysts were tested for the selective oxidation of BzOH to
BzH. Correlations between catalytic activity and Sn/W molar ratio were shown in Fig
S5. It was found that the SnO2/RGO exhibited low BzOH conversion of 35.6%, but
the WO3/RGO showed relatively high BzOH conversion of 78.5%, which indicated
that WO3 were the main active components in the bimetallic catalysts. Interestingly,
with the addition of Sn, much better catalytic activities were achieved over the
Sn-W/RGO bimetallic catalysts. Both BzOH conversion and BzH selectivity
maintained at around or above 90%, implying that the addition of Sn could
16

significantly improve the catalytic performance and directly demonstrating the
promotional synergistic effect of Sn. Among the Sn-W/RGO catalysts with different
Sn/W molar ratios, 0.3Sn-W/RGO exhibited the optimal catalytic activity with the
maximum yield to desirable product, and doping an excess of Sn would lead to a
slight decrease in the yield of target product. Probably because overmuch Sn species
dispersed on graphene sheets would occupy more sites for WO3 and then hinder the
formation of peroxo complex with strong oxidizability, and thus decreasing the
catalytic activity slightly. But overall, Sn species could act as an effective promoter
among Sn-W/RGO to achieve higher catalytic performance compared with single
WO3/RGO. Furthermore, comparison of this catalytic system with other catalytic
systems for selective oxidation of benzyl alcohol to benzaldehyde was presented in
Table 2, and we could see the present catalysts exhibited remarkable catalytic
activities in benzyl alcohol oxidation without using organic solvents and additives.
3.2.2. Catalyst reusability
As a heterogeneous catalyst, stability and durability are of great importance in
further industrial applications. Hence, recycling experiments over 0.3Sn-W/RGO for
selective oxidation of benzyl alcohol with H2O2 was carried out. After the reaction,
the catalyst was separated by simple centrifugation, then washed with ethanol
thoroughly and dried overnight for next use. The results in Fig. 7 indicated that the
heterogeneous catalyst could be successfully recycled for five times without
significant loss in catalytic activities, both BzOH conversion and BzH selectivity
17

maintained at above 90% during these recycled tests, demonstrating the highly
excellent recyclability of this bimetallic catalyst. In addition, intensity of the
diffraction peaks almost remained unchanged even after the catalyst was reused for
five times as the XRD results shown in Fig. 8. Moreover, inductively coupled plasma
optical emission spectrometry (ICP-OES, Varian 730-ES) was used to detect the
content of metal ions in the filtrate solution, the results suggested that no metal
species were found under the detection limit of the apparatus. And thus these results
indicated the extraordinary recyclability of this catalyst and revealed its application
potential in future industrial scale.
3.2.3. General applicability
In order to extend the general applicability of this bimetallic catalyst, the green
and selective oxidation of various aromatic and aliphatic alcohols were investigated
under the similar reaction conditions. And the results in Table 3 revealed that almost
all the derivatives could favorably transform into corresponding aldehydes or ketones
with high conversion and good selectivity. Taking benzyl alcohol for an example,
bearing an electron-donating group (Entries 2 and 3) would increase the conversion to
96.3% and 98.3%, respectively. Nevertheless, the reaction rate and activity decreased
slightly when adding electron-withdrawing groups on it (Entries 4 and 5). That might
because the reduction of electron cloud density on aromatic rings caused by negative
induction effect, which was detrimental to the improvement of catalytic activity in
alcohols oxidation. With respect to the typical secondary aromatic alcohols (Entries
18

6-8), both high conversion and excellent selectivity were obtained under the moderate
reaction conditions, and the selective oxidation of other primary aromatic alcohols
(Entries 9 and 11) also proceeded efficiently. It was noteworthy that even cinnamyl
alcohol (Entry 10) bearing activated double bonds worked well and afforded the target
product in relatively high yield of 77.3%. Additionally, the present catalytic system
was also applicable to the oxidation of secondary aliphatic alcohols (Entries 12-15),
relatively appealing conversion and satisfying selectivity were achieved respectively
by prolonging the reaction time. However, as regards the typical long-chain primary
aliphatic alcohol (Entry 16), n-octanol presented relatively low yield to desired
product though increasing the oxidant dosage, that might result from the effects of
steric hindrance of its long-chain alkyl group and the absence of activation with the
conjugated system [56]. Therefore, in general, a wide range of alcohols including aryl
and alkyl alcohols were greatly suitable for this catalytic oxidation system, indicating
the versatility of 0.3Sn-W/RGO catalyst for selective transformation of alcohols to
corresponding carbonyl compounds with H2O2.
4. Conclusions
In summary, a series of novel Sn-doped WO3/RGO catalysts were successfully
prepared using a conventional one-pot hydrothermal method. Compared with single
WO3/RGO catalyst, the bimetallic Sn-W/RGO catalysts (especially 0.3Sn-W/RGO)
exhibited dramatic enhanced catalytic activities for selective oxidation of various
alcohols to corresponding carbonyl compounds. The increased activities should be
19

ascribed to the promotional synergistic effect of Sn and the strong interactions among
the composites, which were confirmed by the above characterization results.
Detailedly, the FT-IR, XRD, Raman and BET analysis revealed that the superior
catalytic performance was attributed to better structure properties, such as higher
surface area, preferential crystal configuration and strong interactions between Sn-W
oxides and graphene. FESEM and TEM images verified that the incorporation of Sn
could promote nucleation and dramatically decrease the grain size. XPS analysis
further confirmed that the strong interactions among the catalysts and the great
changes of chemical composition resulting from the introduction of Sn. All these
advantages led to the outstanding improved catalytic performance of this bimetallic
Sn-W/RGO catalyst. Besides, the Sn-promoted catalyst could catalyze the oxidation
of both aryl and alkyl alcohols efficiently with H2O2 under moderate reactions
conditions, which accorded well with the requirements of sustainable development
and green chemistry. And its remarkable reusability also provided the great
application potential in future industrial catalysis.
Acknowledgements
This work was supported by Zhejiang Provincial Key Laboratory of Advanced
Chemical Engineering Manufacture Technology (No. 2017E10001), Zhejiang
Province (China).
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Figure captions:
22

Fig. 1. FT-IR spectra of the as-prepared (a) GO, (b) WO3/RGO, (c) 0.1Sn-W/RGO, (d)
0.2Sn-W/RGO, (e) 0.3Sn-W/RGO and (f) 0.4Sn-W/RGO.
Fig. 2. XRD patterns of the as-prepared (a) GO, (b) WO3/RGO, (c) 0.1Sn-W/RGO, (d)
0.2Sn-W/RGO, (e) 0.3Sn-W/RGO and (f) 0.4Sn-W/RGO. o-WO3 and h-WO3
represent orthorhombic WO3 and hexagonal WO3, respectively.
Fig. 3. Raman spectra of (a) GO, (b) WO3/RGO and (c) 0.3Sn-W/RGO. Inset is the
partial enlarged diagram of D and G band.
Fig. 4. FESEM and TEM images of WO3/RGO (a and c, respectively) and
0.3Sn-W/RGO (b and d, respectively), (e) Selected area diffraction (SAED) pattern
recorded at the marked area of (d), HRTEM images of (f) WO3/RGO and (g)
0.3Sn-W/RGO, (f) EDS spectrum of 0.3Sn-W/RGO sample.
Fig. 5. (a) XPS survey spectrum of 0.3Sn-W/RGO sample, high-resolution XPS
spectra of (b) C 1s, (c) W 4f and (d) Sn 3d.
Fig. 6. (a) N2 adsorption-desorption isotherms and (b) pore size distributions of
WO3/RGO and 0.3Sn-W/RGO sample.
Fig. 7. Catalytic performance of recycled 0.3Sn-W/RGO catalyst in selective
oxidation of benzyl alcohol to benzaldehyde. Reaction conditions: benzyl alcohol (0.5
mmol), H2O2/benzyl alcohol molar ratio (1.2), H2O (1 ml), reaction temperature
(80 ℃), reaction time (3 h).
Fig. 8. XRD patterns of fresh and used 0.3Sn-W/RGO catalyst after catalytic reaction.
23

Fig. 1
Fig. 2
24

Fig. 3
Fig. 4
25

Fig. 5
Fig. 6
26

Fig. 7
Fig. 8
27

Table captions:
Table 1 Physicochemical properties of GO, WO3/RGO and 0.3Sn-W/RGO.
Particle
Relative atomic
percentage (%)
Sn/W molar
ratio (%)
SBET
VP
size (nm)
Sample
ID/IG
(m2 g-1) (cm3 g-1)
XRD TEM C C C–O C O EDX XPS
GO
/
/
/
/
40.7 55.5 3.8
/
/
0.73
1.17
WO3/RGO
6.3
0.195 45.6 150.1 70.4 21.6 8.0
0
0
0.3Sn-W/RGO 52.9
0.108 14.6 16.2 86.1 8.3
5.6 28.7 27.0 1.22
28

Table 2 Catalytic performance of Sn-W/RGO catalysts and the comparison with other
published works in benzyl alcohol oxidation to benzaldehyde a.
Reaction
Conv. Sel. Yield
Entry
Catalyst
Oxidant Solvent
Ref.
conditions
(%)
78.5
88.2
90.6
94.0
90.1
86
(%)
90.4
89.2
95.2
94.3
93.8
89
(%)
80 , 3 h
℃
1
2
3
4
5
6
7
8
9
WO3/GO
0.1Sn-W/RGO
0.2Sn-W/RGO
0.3Sn-W/RGO
0.4Sn-W/RGO
Au/Al2O3
H2O2
H2O2
H2O2
H2O2
H2O2
H2O
H2O
H2O
H2O
H2O
71.0 Herein
78.7 Herein
86.3 Herein
88.6 Herein
84.5 Herein
80 , 3 h
℃
80 , 3 h
℃
80 , 3 h
℃
80 , 3 h
℃
100
90
, 2h
1MPa O2 Toluene
℃
76.5
71.1
51.7
34.8
[9]
, 6h
Au-Pd/TiO2
Au-Pd/MgO
Mn(OAc)2
O2
O2
None
H2O
℃
74.2
52
95.8
99.5
52
[10]
[11]
[15]
80 , 4 h
℃
80 , 8 h
℃
TBHP
CH3CN
67
irradiation,
r.t., 8 h
10
11
Bi2WO6/RGO
VPO-Co
O2
H2O
90
93
83.7
[17]
90 , 8 h
℃
TBHP
TBHP
CH3CN
No
57
93
53
[22]
[23]
125
, 5 h
12 Au/Li2O/Al2O3/PF6
℃
89.9
89.2
80.2
80 , 6 h,
℃
13
Ce (IV)/TEMPO 0.3 MPa O2 H2O
61.1
89.5
54.7
[19]
NaNO2
100 , 7 h
℃
14
15
Co3O4/RGO-N
Fe3O4-ECH-D
O2
DMF
H2O
93.9
36.6
99.9
93.5
93.8
34.2
[26]
[30]
100 , 1.5 h
℃
H2O2
irradiation,
16 Zn3In2S6/10%ZnO
O2
BTF
91.3
97
88.6
[54]
50
, 2 h
℃
a Reaction conditions: benzyl alcohol (0.5 mmol), H2O2/benzyl alcohol molar ratio (1.2), H2O (1
ml), catalyst amount (10 mg), reaction temperature (80 ℃), reaction time (3 h) .
29