Hydrothermal synthesis of V-doped hexagonal WO3 microspheres comprising of nanoblocks for catalytic ammoxidation of dichlorotoluene

Article abstract of DOI:10.1016/j.ica.2020.120173

V-doped hexagonal WO₃ microspheres comprising of nanoblocks were successfully prepared via a facile hydrothermal approach. Several techniques such as XRD, SEM, TEM, FTIR, EDS, TPR, BET and Raman have been performed and the characterization results reveal that V replaced W atoms into the hexagonal lattice-structure. The microspheres possess diameter of about 4.4 μm and the nanoblocks have thicknesses of 40–100 nm and widths of about 150 nm. In addition, the catalytic performance of the obtained V-doped WO₃ nanomaterial was investigated in the ammoxidation of dichlorotoluene. The catalytic results indicate that the as-prepared nanostructures show significantly improved selective performance with the selectivities of 3,4-DCBN and 2,6-DCBN reaching up to 89.8% and 86.2%, respectively.

Full text of DOI:10.1016/j.ica.2020.120173

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Hydrothermal synthesis of V-doped hexagonal WO microspheres comprising
3
of nanoblocks for catalytic ammoxidation of dichlorotoluene
Xiongjian Li, Chi Huang
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Reference:
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https://doi.org/10.1016/j.ica.2020.120173
ICA 120173
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17 November 2020
29 November 2020
Please cite this article as: X. Li, C. Huang, Hydrothermal synthesis of V-doped hexagonal WO microspheres
3
comprising of nanoblocks for catalytic ammoxidation of dichlorotoluene, Inorganica Chimica Acta (2020), doi:
https://doi.org/10.1016/j.ica.2020.120173
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Hydrothermal synthesis of V-doped hexagonal WO3
microspheres comprising of nanoblocks for catalytic
ammoxidation of dichlorotoluene
Xiongjian Lia Chi Huangb
aHubei Key Laboratory of Pollutant Analysis & Reuse Technology, College of
Chemistry and Chemical Engineering, Hubei Normal University, Huangshi, 435002,
P. R. China;
bCollege of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072,
P. R. China;
Abstract: V-doped hexagonal WO microspheres comprising of nanoblocks were
3
successfully prepared via a facile hydrothermal approach. Several techniques such as
XRD, SEM, TEM, FTIR, EDS, TPR, BET and Raman have been performed and the
characterization results reveal that V replaced W atoms into the hexagonal
lattice-structure. The microspheres possess diameter of about 4.4 m and the
nanoblocks have thicknesses of 40-100 nm and widths of about 150 nm. In addition,
the catalytic performance of the obtained V-doped WO3 nanomaterial was
investigated in the ammoxidation of dichlorotoluene. The catalytic results indicate
that the as-prepared nanostructures show significantly improved selective
performance with the selectivities of 3,4-DCBN and 2,6-DCBN reaching up to 89.8%
and 86.2%, respectively.
Key words: V-doped WO ; Microspheres; Dichlorobenzonitrile; Catalytic properties;
3
E-mail address: li_xiongjian@163.com

Hydrothermal synthesis
1
. Introduction
Recently, metal oxide nanostructures have received increasing attention since their
dimensions of nanometer-size magnitude exhibit specific physical and chemical
properties, which differ greatly from that of the bulk counterparts [1]. Among the
various metal oxides available, the high interest lies in the synthesis and
characterization of 3-dimensional (3D) hierarchical tungsten trioxide (WO3)
architectures due mainly to their intriguing physical and chemical properties and
wide-ranging technological applications, including sensors, optical devices, catalysts,
electrodes, ect [2-5]. For examples, Liu et al.[6] reported that Co-doped WO3
flower-like nanostructures assembled with nanoplates, were promising for acetone
sensors in environmental monitoring; Duan et al.[7] synthesized hexagonal WO3
mesocrystals with biconical morphology, exhibiting an excellent rate capability and
cycling performance; Song et al.[8] prepared Fe-doped WO ultrathin hollow spheres
3
with enhanced visible-light-driven photocatalytic performance. Despite the formation
of various WO nanostructures, the development of V-doped hexagonal WO
3
3
microspheres comprising of nanoblocks with excellent catalytic performance have not
yet been reported.
Ammoxidation of hydrocarbons to produce nitriles is considered to be the most
promising techniques considering the advantage of pollution-free. Ammoxidation of
dichlorotoluene (DCT) to dichlorobenzonitrle (DCBN) is of high significance due to
the increasing demand of DCBN for preparing lots of commercial chemicals [9,10].

However, the reaction is a more difficult process compared to other ammoxidation of
alkyl aromatics and heteroaromatics [11]. There is a great deal of research that shows
the methyl aromatics with different substituents or same substituents on different
positions have different catalytic activities under identical conditions because of the
hindered accessibility of methyl group caused by bulky substituent atoms [9,12].
Therefore, the low yield of DCBN were often obtained over previously reported
catalysts (<72%) such as (NH ) [(VO) (P O ) ] [13], VPO/SiO [12,14], VPO/Al O
3
4
2
3
2
7 2
2
2
[
9], VPO/TiO [15], etc. Therefore, fabricating highly selective catalysts for
2
ammoxidation of DCT to DCBN remains a worthwhile goal.
Recently, a growing interest has been focused on the development of
tungsten-vanadium catalysts [7,16,17]. Tungsten oxides with medium to strong acid
strength are well known acidic co-catalysts of vanadium-based catalysts [18]. It is in
favor of the formation of surface acid sites which greatly influence the selective
performance [19]. On the other hand, the properties of catalysts are closely related to
the structure characteristics [20]. Based on these properties, the development of new
3
D hierarchical tungsten-vanadium oxide nanostructures to investigate their catalytic
ammoxidation properties is fascinating and significant. However, there are quite a few
efforts being devoted to the development of W–V binary oxides for the ammoxidation
applications [18].
In this communication, V-doped hexagonal WO microspheres comprising of
3
nanoblocks were fabricated via a hydrothermal approach and explored as catalyst for
the ammoxidation of DCT to DCBN. The composition and microstructure of the

as-prepared nanomaterial were investigated. The catalytic activity was also evaluated
in comparison with that of sample prepared by solid-state reaction.
2
. Experimental section
Synthesis of V-doped WO nanostructures
3
In a typical procedure, 0.03 mol of commercial V O powder, 0.005 mol of
2
5
(
NH ) W O and 0.270 mol of H C O ·2H O were dispersed into 400 mL of
4 10 12 41 2 2 4 2
redistilled water with magnetic stirring. The mixture was heated to 80 °C for about 2
h to get a homogeneous solution. Then, the above solution was transferred into a 500
mL stainless steel autoclave, which was sealed and maintained at 180 °C for 48 h and
then cooled to room temperature naturally. The products were filtered off, washed
with distilled water and absolute ethanol several times, and dried in the oven at 70 °C
for 12 h. In addition, another of the above mixed solution was slowly evaporated at
8
1
0 °C to dryness on a hot plate with stirring. The obtained solid was further dried at
10 °C for 8 h in an oven and calcined in air at 400 °C for 24 h (denoted as
SSR-V-W-O).
Characterization
Scanning electron microscopy (SEM, Quanta 200) and transmission electron
microscopy (TEM, JEM-2100) were used to study the morphologies of the samples.
The phase and crystal structure of the obtained samples were tested by X-ray powder
diffraction (XRD) using D8 X-ray diffractometer equipment with Cu Ka radiation,
λ=1.54060 Å. The chemical composition of the samples was confirmed by means of
the energy dispersive X-ray spectrometer (EDS) attached to the scanning electron

microscope (SEM). FT-IR spectra were recorded on a Nicolet 60-SXB spectrometer
-1
-1
using KBr pellet technique from 4000 to 400 cm with a resolution of 4 cm . Raman
spectra were carried out by a Renishaw RM-1000 spectrometer (Confocal Raman
Microspectroscopy) equipped with an excitation laser wavelength of 514.5 nm. The
specific surface area was examined by Brunauer-Emmett-Teller (BET) N gas
2
adsorption method at 77 K and carried out on JW-BK equipment. Pore size
distributions were calculated by Barrett-Joyner-Halenda (BJH) formula from the
adsorption branch. The redox properties of catalysts were studied by temperature
programmed reduction (TPR) on a TP-5080 adsorption instrument (TianJin XianQuan
Co., gas composition: H (3 mL/min) and N (27 mL/min), heating rate: 10 °C/min).
2
2
Catalytic activity evaluation
Ammoxidation runs were conducted in a fixed-bed micro reactor equipped with a
0 mm inside diameter quartz tube, loaded with 5g of catalyst. DCT was injected to
3
the vaporizer by a micro-pump, and mixed with NH and air measured by gas flow
3
meter. Test runs were carried out at different conditions by varying the temperature,
space velocity of the reactant (GHSV), and the molar ratios of ammonia and air to
DCT. The product stream was cooled and collected every 1/2 h after attaining steady
state conditions and then analyzed off-line by a gas chromatograph (Lunanruihong,
China; column: SE-30) equipped with FID module. Every test was repeated by three
times and then taken the average with a margin of error of 0.5%. The reaction
equations of the ammoxidation of 3,4-DCT, and 2,6-DCT to the corresponding DCBN
could be expressed in Scheme 1.

Scheme 1 The ammoxidation of 3,4-DCT and 2,6-DCT to the corresponding DCBN
over V-W-O catalysts.
3
. Results and discussion
Fig. 1 XRD patterns of the prepared V-doped WO nanomaterial.
3
The phase and composition of the prepared V-doped WO products were
3
investigated by XRD, as depicted in Fig. 1. It can be seen that all the reflection peaks
can be readily indexed to the hexagonal tungsten bronze (JCPDS: 33-1387) with the
main reflections at 2θ = 13.97, 22.75, 24.35, 26.86, 28.19, 33.61, 36.61, 49.99, 55.55,
5
8.33 and 63.55° [19,21]. No characteristic peaks related to the impurities of
vanadium or other tungsten oxides were detected. Furthermore, the peaks shift
slightly to larger reflection angles in comparison to standard JCPDS: 33-1387,
indicating the reduced adjacent interplanar distance d(h k l). These results reveal that
the smaller V ions substituted W doped into hexagonal structure of WO by the
3
present hydrothermal synthetic route.

Fig. 2 FT-IR spectra of (a) prepared V-doped WO , (b) pure V O and (c) pure WO .
3
2
5
3
FT-IR spectra of the prepared V-doped WO sample, pure V O and WO are
3
2
5
3
-1
-1
shown in Fig. 2. The peaks at 1620 cm and 1401 cm are the characteristics of
absorbed H O and residual impurities of C–H, respectively [22]. The signals at 795
2
-1
cm for prepared V-doped WO catalyst is assigned to W–O vibrations in hexagonal
3
-1
tungsten bronze; in addition, the broad band at approximately 621 cm is related to
the overlaps of W–O–W and W–O–V rocking vibrations [19].
Fig. 3 Raman spectrum of (a) as-obtained V-doped WO sample and (b) pure WO .
3
3
Further information about the composition of the obtained V-doped WO sample
3
was studied by Raman spectroscopy, as shown in Fig. 3. The band at 966 cm-1
indicates the higher presence of W-O bonds generated by a structural defect as a result
of vanadium incorporation and/ or V-O bonds associated to polymeric V–O–W chains
-1
-1
[
19,23], while the bands at around 807 cm and 684 cm are related to two different

types of bridging oxygen of O–W–O vibrations in tungsten bronzes [24,25]. These
results particularly sheds light on the vanadium doped into hexagonal tungsten
bronzes [23], which are in agreement well with the XRD and FT-IR results.
Fig. 4 (a) SEM, (b) TEM and (c) HRTEM images of prepared V-doped WO3
sample. The inset of (b) gives the SAED pattern.
The morphology and particle size of V-doped WO sample were investigated by
3
SEM, TEM and SAED technologies, as shown in Fig. 4. The SEM image (Fig. 4a)
exhibits an individual microsphere with a diameter of about 4.4 m, composed of
numerous zero-dimensional nanoblocks. The TEM image (Fig. 4b) shows that the
typical nanoblocks possess thicknesses of approximately 40−100 nm and widths of
about 150 nm. Distinctly visual lattice fringes of 0.315 nm, matching well with the
[
200] crystal planes of hexagonal WO , is observed in the HRTEM image (Fig. 4c)
3
revealing that the nanoblocks are of monocrystalline nature. In addition, The SAED
pattern (top right corner in Fig. 4b) gives partial garbled light spots, further
confirming that the obtained V-doped WO catalyst is monocrystalline and the V was
3
doped into hexagonal WO bronzes.
3
The typical EDS spectrum (Fig. 5) corroborates that the nanoblocks composed of
only C, O, V and W elements, where the C peak is owing to residual impurities
absorbed on the surface of sample [26]. The molar ratio of V:W atom for V-doped

WO catalyst is 0.25. In addition, Fig. 6 clearly shows V, W and O elements are
3
uniformly distributed.
Fig. 5 EDS spectrum of prepared V-doped WO sample.
3
Fig.6 EDS elemental maps of obtained V-doped WO products.
3
Fig. 7 SEM images of the products prepared at 180 °C for different reaction times: (a)
1
2 h, (b) 24 h.
A series of time-dependent experiments were performed to reveal the formation
mechanism of nanoblock-built microspheres, as shown in Fig. 7. The SEM

observation (Fig. 7a) of the precursor, which were collected after reacting for 12 h,
exhibits that lots of solid nanoblocks were formed and aggregated into irregular
spheres to reduce the surface energy. As the reaction time prolonging to 24 h, the later
formed nanoblocks grew preferentially at the surface defect sites of the spheres and
thereby inhibited the accumulation (Fig. 7b). Finally, the nanoblock-built
microspheres were formed (Fig. 4a) [27].
Fig. 8 H -TPR profiles of (a) prepared V-doped WO , (b) WO and (c) V O .
2
3
3
2
5
The redox properties of the V-doped WO sample were investigated by H -TPR, as
3
2
shown in Fig. 8. In contrast to the TPR profiles of pure WO and V O , the profile of
3
2
5
prepared nanostructures shows three reduction peaks at 525, 630-700, and 734 °C,
5
+
4+
3+
which could be attributed to reduction of V → V → V in hexagonal structure,
and W6+ → W , respectively. Similar reduction peaks were observed by Chieregato
5+
et al. when tungsten-vanadium oxide catalysts were treated in 10% H /Ar mixture up
2
to 800 °C [23]. These further prove that vanadium is successfully incorporated into
hexagonal WO framework.
3
The N adsorption-desorption isotherms of the V-doped hexagonal WO
2
3
architecture are depicted in Fig. 9. It exhibits a typical hysteresis loop, inferring the
nanostructures having mesoporous characteristics [28,29]. The pore size distribution

(inset in Fig. 9) shows a peak centered at around 3.4 nm. The BET surface area of the
2
-1
nanomaterial is 4.3 m g .
Fig. 9 The N adsorption-desorption isotherms of V-doped WO nanostructures.
2
3
Inset is pore size distribution curve.
Table 1 shows the catalytic performances of the prepared V-doped WO3
nanostructures and SSR-V-W-O sample in the ammoxidation of 3,4-DCT and
2
,6-DCT to the corresponding DCBN. In comparison to SSR-V-W-O, although
V-doped hexagonal WO catalyst shows lower catalytic activities, it exhibits much
3
higher selective performance and yields of DCBN under identical conditions. In the
process of ammoxidation reaction,
V
as active component undergoes
5
+
oxidation-reduction reaction; V=O catalyzes -CH to aldehyde intermediates; then
3
the acid sites of W with medium to strong acid strength in close proximity to the
redox sites (V) adsorb NH and insert nitrogen into the intermediates. On the other
3
hand, WO as support promotes the dispersion of vanadium beneficial to the
3
formation active sites. For SSR-W-V-O catalyst, V is in bulk with stronger oxidizing
properties [30,31,32], and shows improper distances to the acid sites of W, thus
leading to decreasing efficiency of nitrogen insertion into oxidized aldehyde
intermediates. In contrast, the vanadyl species of prepared V-doped WO3

nanostructures are highly dispersed and incorporated into the framework of hexagonal
tungsten bronze; this makes that the acid sites of W are in close proximity to the redox
sites of V, facilitating nitrogen insertion [33]. Therefore, the prepared V-doped WO3
exhibited remarkable improvement of selectivity. The yield and selectivity of
2
,6-DCBN obtained over the nanostructural catalyst reach up to 67.6% and 86.2%,
respectively. As for 3,4-DCT, it exhibits 72.5% yield with selectivity of 89.9%.
a
b
Table 1 Catalytic results in the ammoxidation of 3,4-DCT and 2,6-DCT .
Substrate
Catalyst
T (°C)
380
Conversion (%)
Yield (%)
72.5
Selectivity (%)
V-doped WO3
SSR-W-V-O
V-doped WO3
SSR-W-V-O
80.7
89.8
78.4
83.2
89.8
76.2
86.2
75.8
3
,4-DCT
,6-DCT
380
68.4
390
67.6
2
390
63.1
a
-1
Conditions: 3,4-DCT: air:NH molar ratio 1:25:7, T = 380 °C, GHSV = 363 h ;
3
b
-1
Conditions: 2,6-DCT:air:NH molar ratio 1:20:9, T = 390 °C, GHSV = 315 h .
3
4
. Conclusion
In conclusion, V-doped WO3 microspheres comprising of numerous
zero-dimensional nanoblocks were fabricated via a hydrothermal method. SEM and
TEM observations suggest the typical nanoblocks have thicknesses of 40-100 nm and
widths of about 150 nm. XRD, Raman and SAED confirms the hexagonal lattice
structure of WO with vanadium species incorporated inside the framework. The
3
yields of 3,4-DCBN and 2,6-DCBN obtained over the prepared V-doped WO3

microspheres are 72.5% and 67.6%, corresponding to the selectivities of 89.8% and
6.2%, respectively.
8
Acknowledgments
This work was partially supported by the National Natural Science Foundation of
China (Grant 51572201).
Reference
Author statement
I have made substantial contributions to the conception or design of the work; or
the acquisition, analysis, or interpretation of data for the work; And I have drafted the
work or revised it critically for important intellectual content; And I have approved
the final version to be published; And I agree to be accountable for all aspects of the
work in ensuring that questions related to the accuracy or integrity of any part of the
work are appropriately investigated and resolved.
All persons who have made substantial contributions to the work reported in the
manuscript, including those who provided editing and writing assistance but who are
not authors, are named in the Acknowledgments section of the manuscript and have
given their written permission to be named. If the manuscript does not include
Acknowledgments, it is because the authors have not received substantial
contributions from nonauthors.
Xiongjian Li

E-mail: li_xiongjian@163.com
Hubei Normal University, Huangshi, 435002, P. R. China
No conflict of interest exits in the submission of this manuscript, and manuscript
is approved by all authors for publication. I would like to declare on behalf of my
co-authors that the work described was original research that has not been published
previously, and not under consideration for publication elsewhere, in whole or in part.
All the authors listed have approved the manuscript that is enclosed.
Graphical Abstract
This paper investigated the formation of V-doped hexagonal WO microspheres
3
comprising of nanoblocks and the nanomaterial showed significantly improved
selective performance in the ammoxidation of dichlorotoluene.
Highlights
1
2
. V-doped hexagonal WO microspheres comprising of nanoblocks were prepared.
3
. The prepared microspheres showed significantly improved selective

performance.
3
. The selectivities of 3,4-DCBN and 2,6-DCBN reached up to 89.8% and 86.2%,
respectively.
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