Facile sub-/supercritical water synthesis of nanoflake MoVTeNbO:X-mixed metal oxides without post-heat treatment and their catalytic performance

Article abstract of DOI:10.1039/d0ra06877b

A fast and simple sub-/supercritical water synthesis method is presented in this work in which MoVTeNbOx-mixed metal oxides with various phase compositions and morphologies could be synthesized without post-heat treatment. It was demonstrated that the system temperature for synthesis had a significant influence on the physico-chemical properties of MoVTeNbOx. Higher temperatures were beneficial for the formation of a mixed crystalline phase containing TeVO4, Te3Mo2V2O17, Mo4O11 and TeO2, which are very different from the crystalline phases of conventional Mo-V-Te-Nb-mixed metal oxides. While at lower temperatures, Mo4O11 was replaced by Te. At high temperature, the as-prepared samples presented distinct nanoflake morphologies with an average size of 10-60 nm in width and exhibited excellent catalytic performances in the selective oxidation of propylene to acrylic acid. It is illustrated that the large specific surface area, presence of Mo4O11 and superficial Mo6+ and Te4+ ions are responsible for the high propylene conversion, while suitable acidic sites and superficial Nb5+ ions improved the selectivity to acrylic acid. This journal is

Full text of DOI:10.1039/d0ra06877b

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Facile sub-/supercritical water synthesis of
nanoflake MoVTeNbOx-mixed metal oxides
without post-heat treatment and their catalytic
performance
Cite this: RSC Adv., 2020, 10, 39922
Shuangming Li,ab Yongwei Liu,a Yaoxin Fan,a Zixuan Lu,a Yunong Yan,a Luyao Deng,a
ab
Zhe Zhanga and Sansan Yu
*
A fast and simple sub-/supercritical water synthesis method is presented in this work in which MoVTeNbOx-
mixed metal oxides with various phase compositions and morphologies could be synthesized without post-
heat treatment. It was demonstrated that the system temperature for synthesis had a significant influence
on the physico-chemical properties of MoVTeNbOx. Higher temperatures were beneficial for the formation
of a mixed crystalline phase containing TeVO4, Te3Mo2V2O17, Mo4O11 and TeO2, which are very different
from the crystalline phases of conventional Mo–V–Te–Nb-mixed metal oxides. While at lower
temperatures, Mo4O11 was replaced by Te. At high temperature, the as-prepared samples presented
distinct nanoflake morphologies with an average size of 10–60 nm in width and exhibited excellent
catalytic performances in the selective oxidation of propylene to acrylic acid. It is illustrated that the large
specific surface area, presence of Mo4O11 and superficial Mo6+ and Te4+ ions are responsible for the
high propylene conversion, while suitable acidic sites and superficial Nb5+ ions improved the selectivity
to acrylic acid.
Received 10th August 2020
Accepted 19th October 2020
DOI: 10.1039/d0ra06877b
rsc.li/rsc-advances
2 h at 600 ꢀC. Mazloom et al. prepared MoVTeNbOx using
a slurry method, in which continuous evaporation at 60 ꢀC was
1. Introduction
needed, rstly to remove the water in the precursor solution and
a subsequent calcination at 600 ꢀC for 2 h under nitrogen
atmosphere was then carried out.6 For the mixed oxides, most
notably, these aforementioned complicated synthesis processes
make complete reproduction difficult for other researchers.
Therefore, it is urgent to develop a simple and fast route to
prepare MoVTeNbOx-mixed oxides.
MoVTeNbOx-mixed metal oxides are the most signicant and
promising catalysts for the selective oxidation of propylene/
propane to acrylic acid. It is reported that the conversion of
feed and selectivity to products is closely related to the chemical
composition, structure and morphology of MoVTeNbOx-mixed
metal oxides, which depends greatly on the preparation
methods.1,2
Nowadays, sub-/supercritical water is widely used in the
preparation of metal- and non-metal-based nano/micro mate-
rials, due to the unique and adjustable physicochemical prop-
erties around its critical point.11-16 For instance, the low
dielectric constant of sub-/supercritical water is conducive to
the formation of nanoparticles of metal-based materials17-23 and
the hybridization of nanoparticles.24 More importantly, under
sub- and supercritical systems, the crystallization of metal-
based products can be achieved in a very short time.13,23-27
This makes the follow-up heat treatment of product no longer
necessary, simplifying the preparation process greatly. Diez-
Garcia et al. prepared highly crystalline brillar tobermorite in
just a few seconds under supercritical water.13 Lee et al.
synthesized multivariate LiFePO4 composite metal oxides by
sub- and supercritical water in 1 h.20 Zheng et al. synthesized
pure-phase BaTeMo2O9 polycrystals in a supercritical water
system.28 Moreover, this technology can easily achieve scaled-up
To date, MoVTeNbOx-mixed oxides have been prepared by
different synthesis routes, including slurry, co-precipitation,
hydrothermal, dry-up and solid state reactions.3-7 Among
them, slurry and hydrothermal methods are the most common
options; however, both of the processes need long reaction
times that can take dozens of hours.8-10 Furthermore, in order to
improve the crystallinity and stability of products, a necessary
ꢀ
post-heat treatment at 550–600 C under a given atmosphere,
lasting for several hours, is also involved in current routes. For
instance, Ueda et al.5 synthesized three Mo–V–O based oxides
ꢀ
with single crystals by using a hydrothermal method at 175 C
for 48 h followed by calcination under nitrogen atmosphere for
aCollege of Chemical Engineering, Shenyang University of Chemical Technology, No. 9,
11 St., Shenyang Economic & Technological Development Zone, Shenyang 110142,
China. E-mail: ssyu@syuct.edu.cn; Fax: +86 24 89383760; Tel: +86 24 89381190
bKey Laboratory of Chemical Separation Technology of Liaoning Province, Shenyang
University of Chemical Technology, Shenyang 110142, China
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production through continuous reaction with a simple ow type system at an accelerating voltage of 200 kV. The X-ray photo-
system. Nugroho and Hong et al. synthesized CoOx, MnOx electron spectroscopy (XPS) was carried out on an ESCALAB250
and Li4Ti5O12
, LiFePO4 nanoparticles by a continuous spectrometer (Thermo Electron, USA). Hydrogen temperature-
sub-/supercritical water process.21,22,29-32 To the best of our programmed reduction (H2-TPR) and ammonia temperature-
knowledge, however, there is no report on the synthesis of programmed desorption (NH3-TPD) were carried out in
MoVTeNbOx-mixed metal oxides using sub- and supercritical a ChemBET system (Quantachrome, USA) equipped with
water techniques at present.
a thermal conductivity detector. For H2-TPR, 0.1 g catalyst was
Herein, a sub-/supercritical water technique is employed to reduced in 10% H2/Ar with a ow rate of 30 mL minꢁ1 from
fabricate MoVTeNbOx-mixed metal oxides directly. A series of room temperature to 800 ꢀC at a heating rate of 10 ꢀC minꢁ1. For
products are obtained by adjusting the synthesis system NH3-TPD, catalysts were carried out in 7% NH3/He from room
temperature within 1 h and without post-heat treatment. It is temperature to 700 ꢀC with the same heating rate. The specic
noteworthy that the products do not contain the M1/M2 crystal- surface area was recorded by the BET method using nitrogen
line phase, which exists in the traditional Mo–V–Te–Nb-mixed adsorption–desorption isotherms at liquid nitrogen tempera-
metal oxides. Moreover, it is very interesting that certain as- ture of ꢁ196 ꢀC on an Autosorb-IQ gas adsorption analyzer
prepared products present distinctive nanoake structures with (Quantachrome, America).
large specic surface areas. Furthermore, the catalysis results
reveal that the as-prepared products can catalyze the oxidation of
propylene to acrylic acid efficiently with the maximum conver-
sion of 89% and selectivity of 59%. It is demonstrated that sub-/
supercritical water is a simple, rapid and promising method to
2.3 Catalytic tests
The catalytic performance of MoVTeNbOx-mixed oxides was
evaluated under different temperatures ranging from 380 to
ꢀ
440 C at atmospheric pressure. The experiments were carried
synthesize efficient MoVTeNbOx catalysts.
out in a xed-bed stainless steel tubular reactor (i.d. 8 mm;
length 400 mm). Aer being tableted, crushed and sieved, 1.0 g
of each catalyst was diluted with silica sands of the same quality
in order to achieve homogeneous heat distribution within the
catalyst bed, and then introduced in the middle of the reactor.
Aerward, the reactor was heated to the desired temperature
2. Experimental section
2.1 Preparation
A MoVTeNbOx-mixed oxide with a nominal Mo/V/Te/Nb atomic
ratio of 1/0.31/0.37/0.065 was prepared by a sub-/supercritical
water method. First, an aqueous solution containing
under the reaction gas feed ow composed of C3H6, H2O, N2
and O2 in the mole ratio of 1.0 : 3.0 : 8.0 : 1.8. The reaction
(NH4)6Mo7O24$4H2O and NH4VO3 was prepared at room
products were collected at a given time interval aer being
temperature. In parallel, TeO2 and Nb(HC2O4)5$xH2O were
condensed with ice water and then analyzed by a gas chro-
dispersed/dissolved in deionized water, respectively. Next, the
matograph (GC7890, Agilent, USA) equipped with a thermal
aqueous suspension of TeO2 and Nb(HC2O4)5$xH2O solution
was transferred into a Mo–V-mixed solution in sequence to form
a light yellow slurry. The slurry was stirred at 80 ꢀC for 1 h and
conductivity detector (TCD) and Porapak-Q column.
3. Results and discussion
3.1 Characterization
then introduced into a SUS 316 tubular reactor with an inner
volume of 10 mL. Aer being tightly sealed, the reactor was
immersed into the molten salt bath (KNO3, NaNO3, and
Ca(NO3)2 in the weight ratio of 46ꢀ: 24 : 30), which was heated to
a desired temperature (250–450 C) beforehand. Aer 1 h, the
reactor was quenched in cold water. The resulting powders were
ltered and washed with water several times. Then, the prod-
Fig. 1 shows the XRD patterns of as-prepared MoVTeNbOx-
mixed oxide samples under different temperatures in a sub-/
supercritical water system. For S250 and S300, the diffraction
peaks are indexed to mixed phases of TeVO4, Te3Mo2V2O17,
ꢀ
ucts were dried overnight at 90 C under vacuum.
For the nomenclature, the nal dark powder products ob-
ꢀ
tained at 250, 300, 350, 400 and 450 C were labeled as S250,
S300, S350, S400 and S450, respectively. All of the above
reagents were analytical reagent (AR) grade and purchased from
Sinopharm Chemical Reagent Co, Ltd. (Shanghai, China).
2.2 Characterization
The powder X-ray diffraction (XRD) patterns were recorded on
an Empyrean X-ray generator (PANalytical B.V., Netherlands).
Diffraction intensities were measured from 20 to 50ꢀ with a 2q
step of 0.01ꢀ for 8 s per point using Cu Ka radiation. The
transmission electron microscopy (TEM) and energy dispersive
X-ray spectroscopy (EDX) were carried out by using a TECNAI G2
Fig. 1 XRD patterns of MoVTeNbOx samples. S250 (i), S300 (ii), S350
F20 microscope (FEI, USA) equipped with an EDX analyzer (iii), S400 (iv) and S450 (v).
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TeO2 and Te. These main peaks at 2q ¼ 23.0, 25.5, 26.1, 26.8, form aggregates of small particles (Fig. 2a–c), whereas S350,
29.8, 34.4, 44.4 and 47.2ꢀ are related to TeVO4 (JCPDS # 25-0930), S400 and S450 display distinct nanoake-like morphologies
the peaks located at 2q ¼ 23.0, 25.5, 26.1, 29.8, and 45.4ꢀ are (Fig. 3a–c). On the one hand, such a morphology change of
attributable to Te3Mo2V2O17 (JCPDS # 35-0129), the peaks at 2q ¼ MoVTeNbOx can be explained by the crystal re-growth resulting
26.1, 29.8, 37.3 and 48.5ꢀ are associated with TeO2 (JCPDS # 74- from the improved solubility of products at the higher
1666) and the peaks at 27.5, 38.2 and 40.4ꢀ correspond to Te (JCPDS temperature. Small crystalline nuclei were rst formed in
# 36-1452). For S350, another phase (2q ¼ 22.1, 23.6 and 25.5ꢀ) a supersaturated solution and then followed by subsequent
corresponding to orthorhombic Mo4O11 (JCPDS # 65-2473) emerges crystal growth, which is just like the Ostwald ripening
in the pattern. Moreover, it is apparent that the relative peak process.36,37 On the other hand, according to the XRD patterns
intensity of Mo4O11 increases obviously when the temperature was in Fig. 1, orthorhombic Mo4O11 arose when the preparation
ꢀ
elevated from 350 to 450 ꢀC (see pattern iv and v). In contrast, the temperature increased from 300 to 350 C. It was documented
peaks belonging to Te become very weak from pattern iii to v. that the orthorhombic crystal system is inclined to anisotropic
It is clear that the sub-/supercritical water system tempera- growth and easily forms akes.38 Therefore, the existence of
ture had a substantial inuence on the crystal phase composi- orthorhombic Mo4O11 in these samples might be another
tion of MoVTeNbOx-mixed oxides. Four phases including reason for the change of morphology. Furthermore, it is note-
TeVO4, Te3Mo2V2O17, Te and Mo4O11 could be synthesized, in worthy that with the increase of synthesis system temperature,
which the former two were the essential constituents, while Te the dimension of these akes evolves gradually from slender
and Mo4O11 were formed only under certain conditions. Higher (average size of 10 nm in width and 200 nm in length for S350)
temperatures were favorable for the fabrication of Mo4O11, (Fig. 3a) to short and thick (average size of 60 nm in width and
whereas the formation of Te occurred at lower temperatures. It 100 nm in length for S450) (Fig. 3c). According to crystal growth
should be noted that the diffraction peaks of TeO2 existed in all theory, the crystal morphology is usually determined by the
samples, suggesting some tellurium dioxide did not participate relative growth rate of the different crystal faces.39 Therefore, it
in the reaction, but the morphology changed in the process. was inferred that the surface energy of each crystal face of the
Moreover, Mo4O11 was an oxygen defect shear structure of product was different under the given sub-/supercritical water
MoO3.33 Based on this, it was possible that MoO3 was rst system, leading to various growth rates between crystal faces.
formed via the decomposition of (NH4)6Mo7O24$4H2O and then Consequently, nanoakes with different widths and lengths
further converted to Mo4O11 through phase evolution under the were formed along with the elimination of fast growing crystal facets
sub-/supercritical water system with a rapid heating and and the preservation of slow ones. EDX analyses in Fig. 2b and c
quenching process.34 No phase formed by Nb was observed, and 3a–c further indicate the presence of Mo, V, Te and Nb in
which is probably due to the insertion of Nb into the lattice of MoVTeNbOx samples. Moreover, the detailed HRTEM charac-
other mixed oxide systems (TeVO4 and Te3Mo2V2O17) as terizations show that various interplanar spacing d values are
a substitution element for V.35 The above results reveal that observed in S350–S450 (Fig. 3d–i), among which 0.352, 0.333
MoVTeNbOx-mixed oxides with different crystal phase compo- and 0.337 nm can be attributed to the (ꢁ111), (ꢁ112) and (102)
sitions can be facilely and controllably fabricated by adjusting lattice planes of TeVO4, 0.406 nm corresponds to the (101)
the synthesis system temperature.
lattice plane of TeO2 or (600) of Mo4O11, while 0.397, 0.398,
Fig. 2 and 3 show the TEM and HRTEM images and EDX 0.400, and 0.336 nm are assigned to the (211) and (002) lattice
analyses of as-synthesized MoVTeNbOx-mixed oxides at planes of Mo4O11, respectively.
different synthesis system temperatures. As shown in Fig. 2,
XPS was applied to investigate the element valence state on
S250 and S300 present obvious assembled morphologies that the sample surface. Fig. 4 presents the XPS survey spectrum and
Fig. 2 TEM, HRTEM images and EDX analyses of MoVTeNbOx samples, S250 (a and b) and S300 (c). ((b) is the detailed morphology of the zone
marked in (a), and the insets are the corresponding EDX).
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Fig. 3 TEM, HRTEM images and EDX analyses of MoVTeNbOx samples, S350 (a, d, e), S400 (b, f, g), and S450 (c, h, i) (the insets are the cor-
responding EDX).
high resolution spectra of Mo 3d, V 2p, Te 3d, Nb 3d and O 1s of seen from Fig. 4b and e that more low valence metal ions such
MoVTeNbOx, in which XPS spectra were calibrated to the as Mo5+ and Nb4+ exist on the surface of S250 than that of S300–
adventitious C 1s peak at a binding energy of 284.6 eV. In the S450, which is probably due to the weaker oxidation of sub-
XPS survey spectra (Fig. 4a), ve peaks centered at around 232.8, critical water at low temperatures. Furthermore, the peaks of
516.0, 575.9, 207.1 and 530.0 eV correspond to Mo 3d5/2, V 2p3/2
,
O 1s can be t into two peaks centered at binding energies of
Te 3d5/2, Nb 3d5/2 and O 1s, respectively.40-44 Fig. 4b shows that 530.0 and 531.1 eV, which belong to the typical metal–oxygen
the Mo 3d5/2 BE peak of S250 can be split into two peaks at 232.8 bonding and lattice oxygen, respectively.44,51
and 232.0 eV, indicating that the surface Mo species consist of
The acid sites of as-synthesized samples were determined by
Mo6+ and Mo5+.45,46 For S300 and S350, there are also both Mo6+ NH3-TPD and the proles are presented in Fig. 5a. According to
and Mo5+. However, considering that the peak values are closer the literature, the desorption peaks centered at 250 ꢀC and
to 232.8 eV, the amount of Mo6+ largely outweighs that of Mo5+
.
624 ꢀC in the prole of sample S250, marked as a and b, can be
Split peak tting further veries this. On the surface of S400 and attributed to weak and strong acidic sites, respectively.52,53 With
S450, only Mo6+ is detected, illustrating that higher synthesis the increase of synthesis temperature from 250 to 400 C, the
ꢀ
temperatures are favorable for the enrichment of Mo6+ on the area of peak a is continuously enlarged, meanwhile, an obvious
sample surface. For V, two valence states of +4 (516.0 eV) and +5 shi of peak b to higher temperatures (from 624 ꢀC for S250 to
(517.0 eV)43,46 exist on the surface of all MoVTeNbOx samples 710 ꢀC for S450) is observed. In addition, another desorption
(Fig. 4c), and it is clear that V4+ has the highest concentration. peak g, also corresponding to strong acidic sites, emerges at
ꢀ
In Fig. 4d, the BE value around 575.9 eV (Te 3d5/2) can be 577 C in the curve of S400, which is enlarged greatly in S450.
attributed to Te4+,42,,47,48 demonstrating tellurium is in Unlike the former four samples, S450 possesses peak d (326 ꢀC)
a uniform state on the sample surfaces. As Fig. 4e shown, the Nb related to medium acidic sites52,53 instead of peak a. According
3d peak of S250 is split into two peaks at 206.3 and 207.1 eV, to the results of XRD and XPS, it could be seen that the amount
which are ascribed to Nb4+ and Nb5+, respectively.48-50 Unlike of the Mo4O11 phase, Mo6+ and Nb5+ increased evidently with
S250, only Nb5+ exists in the samples of S300–S450. It can be the increase of synthesis system temperature. It was reported
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Fig. 4 XPS full survey spectra (a), high resolution spectra XPS spectra of Mo 3d (b), V 2p (c), Te 3d (d), Nb 3d (e) and O 1s (f) of MoVTeNbOx. S250
(i), S300 (ii), S350 (iii), S400 (iv) and S450 (v).
that metallic oxides with high valence states could behave as Te4+) species in mixed metal oxides54-57 while type IV can be
acidic oxides.33 Therefore, the increase of Mo6+, Nb5+ and the attributed to the reduction of TeMoOx.58 Moreover, the peak
corresponding phase could be responsible for the shi of acid type V and VI at high temperatures are likely related to the
sites to high temperature and abundance.
monometallic oxide. According to the study, the reduction
The reduction behaviors of MoVTeNbOx are examined by H2- temperature can reect the migration of lattice oxygen from the
TPR. As Fig. 5b shows, six types of reduction peaks at 495 (I), bulk to the surface.57 Therefore, the S250 sample exhibited the
533–549 (II), 586–633 (III), 660–693 (IV), 703–733 (V) and 728 ꢀC greatest reducibility of lattice oxygen. In addition, these
(VI) are observed in the H2-TPR proles of samples. It is docu- samples display changes in the TPR patterns depending on the
mented that the peak type I, II and III at low temperature synthesis system temperature and there is an obvious tendency
originate from the reduction of metal cation (Mo6+, V5+ and for reduction peaks to move towards high temperatures from
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Fig. 6 N2 adsorption–desorption isotherms (a) and the BET specific
surface areas (b) of MoVTeNbOx.
Fig. 5 NH3-TPD (a) and H2-TPR (b) profiles of MoVTeNbOx. S250 (i),
S300 (ii), S350 (iii), S400 (iv) and S450 (v).
(from S250 to S450) for all catalytic temperatures (see line chart
in Fig. 7), and the conversion reaches a maximum of 89% at
420 ꢀC for sample S450 (Fig. 7c). Furthermore, the selectivity to
acrylic acid ranges from 22.07% to 54.16%, in which sample
S250 and S300 exhibit the minimum and maximum at catalytic
S250 to S450. The total peak areas of S300, S350, S400 and S450
are larger than that of S250, indicating that the number of
lattice oxygens in samples increased dramatically59 when the
ꢀ
ꢀ
temperatures of 400 C and 440 C, respectively. It is apparent
that the catalytic temperature had a great inuence on the
selectivity to acrylic acid, and almost all samples exhibit the
ꢀ
synthesis system temperature was higher than 250 C.
The N2 adsorption–desorption isotherms and specic
surface area of the samples are shown in Fig. 6. The isotherm
proles of all the samples exhibit a type III isotherm based on
the IUPAC classication, featuring an H3-type hysteresis loop in
the relative pressure (P/P0) of 0.2–1.0 (Fig. 6a). In addition, with
the increase of synthesis system temperature, the specic
surface area of samples is increased progressively. As shown in
Fig. 6b, S250 shows the lowest specic surface area of 10.93 m2
ꢀ
maximum selectivity at 400 C.
gꢁ1
possesses the maximal specic surface area of 66.52 m2 gꢁ1
, while S450 as-prepared at the highest temperature
.
S300, S350 and S400 exhibit that of 18.49, 28.27 and 62.39 m2
gꢁ1, respectively. In view of the close relationship between the
specic surface area and morphology, it was supposed that the
low specic surface areas of S250 and S300 could be ascribed to
the agglomeration of particles.
3.2 Catalytic performance
The catalytic performance of as-prepared MoVTeNbOx was
tested for the selective oxidation of propylene to acrylic acid at
different catalytic temperatures with an interval of 20 C (380,
400, 420 and 440 ꢀC) and the results are shown in Fig. 7. It can
be observed that there is an apparent rising trend in propylene
Fig.
7 The catalytic performance of MoVTeNbOx for selective
ꢀ
oxidation of propylene to acrylic acid at test temperatures of 380 (a),
400 (b), 420 (c) and 440 ꢀC (d), respectively. Line and column charts
correspond to the conversion of propylene and selectivity to acrylic
conversion with the increase of synthesis system temperature acid, respectively.
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It is well known that the activity of a catalyst is greatly shown in Fig. 8. It is revealed that the conversion of propylene
affected by the specic surface area. In this work, the trend of and selectivity to acrylic acid is well-maintained during a 30 h
propylene conversion was completely consistent with that of the reaction test.
specic surface area of samples (Fig. 6b and 7). S450 possessed
In addition, the structure and performance of the catalysts in
the largest specic surface area, which is one of the main this work were compared with the MoVTeNbOx-mixed metal
reasons for its maximum propylene conversion. In addition, oxides prepared conventionally. It was documented that the
S450 and S400 contained more of the Mo4O11 phase (Fig. 1), an most important factor affecting the performance of
oxygen defect shear structure of MoO3, which has shown MoVTeNbOx catalyst is the phase composition. The phases of
excellent catalytic activity for the oxidation of propylene.33,60 The conventional MoVTeNbOx catalysts for propylene oxidation to
XPS results showed that there were abundant supercial Mo6+ acrylic acid consist of M1, M2, Mo5ꢁx(V/Nb)xO14, V0.95Mo0.97Ox,
ions in the samples, which also played an important role in such TeMo5O16, TeMo4O13 and V0.33Mo0.67O2. It was reported that the
a reaction. On the one hand, Mo6+ could strengthen the binding conversion of propylene and selectivity to acrylic acid range
of the catalyst with the organic fragment, facilitating the hydro- from 4.5–86.9% and 0–89.6%, respectively.7,61,63,64 Among which,
lysis reaction of this surface complex on its path towards acrylic the highest yield of 56.1% could be obtained when the crystal
acid formation.61 On the other, the synergism of Mo6+ and Te4+ phase of the catalyst was pure M1.63 In this work, distinct phases
could abstract an allylic hydrogen from propylene, improving the consisting of TeVO4, Te3Mo2V2O17, Mo4O11 and TeO2 were
conversion of propylene.62 For the same reason, the lowest ratio detected. Nevertheless, the results showed that these catalysts
of Mo6+/Mo5+ of S250 might be another reason for its lowest also exhibit good performance, in which the highest conversion
propylene conversion. Although S250 possessed the greatest and selectivity achieved 88.9% and 54.2%, respectively.
reducibility of lattice oxygen (Fig. 5b), it did not seem to play
a signicant role in activating propylene.
4. Conclusions
Compared with the other four catalysts, S250 showed the
worst selectivity to acrylic acid at all catalytic temperatures,
Five MoVTeNbOx-mixed metal oxides were fabricated simply
which might be attributed to the less weak acidic sites and more
and rapidly by using a sub-/supercritical water technique
strong acidic sites (see Fig. 5a). It is documented that the
without a post-heat treatment and their catalytic performance
supercial acid–base properties of a catalyst have an important
were investigated. It is concluded that the crystalline phase,
morphology, surface properties, as well as acidic and redox
properties were inuenced greatly by the synthesis system
effect on the selectivity of partial oxidation reactions.8,46 The
ideal acidity of the catalyst should be able to interact properly
with the reactant and target product to facilitate desorption of
temperature. Nanoake-like MoVTeNbOx consisting of TeVO4,
target products, avoiding over-oxidation. Besides S250, the average
selectivity of S450 was also signicantly lower than that of S300,
S350 and S400, which was ascribed to the over-oxidation caused by
the more strong acidic sites. In addition, Nb was also considered to
improve the selectivity to acrylic acid by reducing the over-
oxidation rate of acrylic acid.5,35 Based on this, it was speculated
that the more supercial niobium ions with low valence states of
S250 (Fig. 4e) had an adverse impact on its selectivity.
Time-on-stream experiments were also carried out to inves-
tigate the stability of the prepared catalyst. Taking S400 catalyst
as an example, the time-on-stream results of selective oxidation
of propylene to acrylic acid over MoVTeNbOx at 400 ꢀC are
ꢀ
Te3Mo2V2O17, Mo4O11 and TeO2 were obtained at 400–450 C,
showing large specic surface areas of ca. 62–66 m2 gꢁ1
.
Compared with the others, the samples prepared at 250 ꢀC
possessed more supercial Mo5+ and Nb4+. In the process of
catalytic reaction, there was a signicant positive correlation
between the conversion of propylene and the specic surface
area of the samples. A certain amount of Mo4O11 and supercial
Mo6+, Te4+ ions also contributed to the conversion. Samples
prepared at 300–400 ꢀC owning suitable acidic sites and
adequate supercial Nb5+ ions showed excellent selectivity to
acrylic acid, in which the maximum yield of acrylic acid reached
43%. For the synthesis of a MoVTeNbOx catalyst with excellent
catalytic performance, it is demonstrated that sub-/supercritical
water is a simple, rapid and promising method.
Conflicts of interest
There are no conicts of interest to declare.
Acknowledgements
The authors gratefully acknowledge the nancial supports by
the National Natural Science Foundation of China (Grant No.
21706165), Scientic Research Fund of Liaoning Provincial
Education Department, China (Grant No. LQ2019007) and the
Natural Science Foundation of Liaoning Province, China (Grant
No. 20170540714).
Fig. 8 Time-on-stream of selective oxidation of propylene to acrylic
acid over MoVTeNbOx (S400 catalyst) at 400 ꢀC.
39928 | RSC Adv., 2020, 10, 39922–39930
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