Effect of Mo loading on 2-naphthaldehyde formation from vapor phase oxidation of 2-methylnaphthalene with V2O5/TiO2 catalysts

Article abstract of DOI:10.1016/j.catcom.2018.12.019

Mo-modified V₂O₅/TiO₂ catalysts were prepared by wetness impregnation method and investigated for the selective oxidation of 2-methylnaphthalene to 2-naphthaldehyde. The catalysts were characterized by XRD, BET, XPS, and Raman to investigate the promotional effect of Mo on catalyst structure, surface property, and catalytic performance. The introduction of molybdenum decreases the amount of KVO₃ phase and enhances the dispersion of the vanadium phase. XPS and Raman results indicate that Mo prevents Sn and K to interact with V species and bonds with Ti or V through bridge oxygen. V1Mo1 sample demonstrates a remarkable improvement in conversion and selectivity compared with others.

Full text of DOI:10.1016/j.catcom.2018.12.019

Accepted Manuscript
Effect of Mo loading on 2-naphthaldehyde formation from vapor
phase oxidation of 2-methylnaphthalene with V2O5/TiO2
catalysts
Xiaoqiang Gao, Fang Zhang, Yi Yu, Yonghui Dou, Li Xu, Guoji
Liu
PII:
S1566-7367(18)30592-2
DOI:
Reference:
https://doi.org/10.1016/j.catcom.2018.12.019
CATCOM 5582
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
7 October 2018
16 December 2018
28 December 2018
Please cite this article as: Xiaoqiang Gao, Fang Zhang, Yi Yu, Yonghui Dou, Li Xu, Guoji
Liu , Effect of Mo loading on 2-naphthaldehyde formation from vapor phase oxidation of
2-methylnaphthalene with V2O5/TiO2 catalysts. Catcom (2018), https://doi.org/10.1016/
j.catcom.2018.12.019
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ACCEPTED MANUSCRIPT
Effect of Mo loading on 2-naphthaldehyde formation from vapor phase oxidation
of 2-methylnaphthalene with V2O5 /TiO2 catalysts
Xiaoqiang Gao1, Fang Zhang2, Yi Yu1, Yonghui Dou1, Li Xu1*, Guoji Liu1*
1 School of Chemical Engineering and Energy, Zhengzhou University, No.100 Science
Avenue, Zhengzhou 450001, China
2 School of Environmental and Municipal Engineering, North China University of
Water Resources and Electric Power, No.36 Beihuan Road, Zhengzhou 450045, China
Corresponding author: Li Xu, xuli@zzu.edu.cn; Guoji Liu, guojiliu@zzu.edu.cn

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Abstract
Mo-modified V2O5/TiO2 catalysts were prepared by wetness impregnation method
and investigated for the selective oxidation of 2-methylnaphthalene to
2-naphthaldehyde. The catalysts were characterized by XRD, BET, XPS, and Raman
to investigate the promotional effect of Mo on catalyst structure, surface property, and
catalytic performance. The introduction of molybdenum decreases the amount of
KVO3 phase and enhances the dispersion of the vanadium phase. XPS and Raman
results indicate that Mo prevents Sn and K to interact with V species and bonds with
Ti or V through bridge oxygen. V1Mo1 sample demonstrates a remarkable
improvement in conversion and selectivity compared with others.
Keywords: V2O5/TiO2; Mo; 2-methylnaphthalene; 2-naphthaldehyde; vapor phase
oxidation

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1. Introduction
Supported vanadium oxide has been widely used as catalysts for methyl
aromatics oxidation over the past few decades, such as oxidation of toluene, o-xylene,
and 2-methylnaphthalene [1-4]. The transformation of 2-methylnaphthalene to
2-methyl-1,4-naphthoquinone and 2-naphthaldehyde under oxygen atmosphere has
been developed over silica [4], alumina [5], or titania [6] supported vanadium oxide
catalysts. Both of them are valuable chemicals and widely used in the field of plant
regulators, antitumor drugs, vitamins of the K group and so on [7-9]. The synthesis of
2-methyl-1,4-naphthoquinone on V2O5-K2SO4-SiO2 catalyst with air as oxidant under
high temperature has achieved a pretty good yield [4]. However, there still requires
much effort to be done for catalytic synthesis of 2-naphthaldehyde. Otherwise, in the
traditional manufacture of 2-naphthaldehyde, chlorine-containing wastewater
produced costs a large amount of treatment fee. Therefore, it is urgent to develop a
new clean production process and novel catalysts to solve this problem.

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Recently, we have found that V2O5/TiO2 catalyst has good selectivity for
2-naphthaldehyde. However, conversion of 2-methylnaphthalene was about 45% for
V2O5/TiO2 catalyst. According to Martin [10,11], the oxidation of methyl aromatics to
their aldehydes strongly depends on the nature, strength, and concentration of acidic
and basic surface sites of the catalysts and the acid-base properties of the reaction
mixture. Early studies [12-14] indicates that addition of SO42- and alkali cations (Li+,
Na+, K+, Rb+, and Cs+) increases the acidity and basicity of vanadia-titania catalysts,
respectively. The amounts of K2SO4 have an important effect on transformation of
2-methylnaphthalene to 2-naphthaldehyde for vanadia-titania catalysts.
MoO3 is also an important catalytic agent for oxidation of hydrocarbons.
Bertinchamps et al. [15] demonstrated that the incorporation of MoOx into V2O5/TiO2
catalyst enhances the activity for the oxidation of benzene, and MoOx phases induce a
synergetic effect with VOx. The simultaneous impregnation of Mo and V brings
beneficial effect than two-step impregnation. However, Matralis et al. [16] reported
that the addition of molybdenum decreases the activity and selectivity of V2O5/TiO2
catalyst for the oxidation of toluene, and inhibits the interaction between vanadium
and anatase TiO2. So in the present investigation, for understanding the promotional
effect on the activity of the catalyst, different amount of Mo was introduced into
V2O5/TiO2 catalyst using ammonium heptamolybdate by impregnation. The
promotional effect of Mo is examined by different characterization techniques
including X-ray diffraction (XRD), Raman spectrum, BET (Brunauer–Emmett–Teller)
surface area, and X-ray photoelectron spectroscopy (XPS) and activities of the

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catalysts are tested by choosing oxidation of 2-methylnaphthalene as a model
reaction.
2. Experimental
2.1 Catalyst preparation
Molybdenum and vanadium were deposited on the TiO2 support using the wet
impregnation method. In a typical procedure, a certain amount of H2C2O4 (Kermel,
China, AR) was dissolved in 10 mL deionized water, then NH4VO3 (Aladdin, China,
AR) and (NH4)6Mo7O24·4H2O (Sinopharm, China, AR) were added simultaneously to
react for a while till no bubbles came out, and the required amount of K2SO4, K2S2O7,
SnSO4, KH2PO4, and Na2B4O7·10H2O were added in sequence subsequently. Finally,
9.5 g TiO2 support powder (Aladdin, China, AR) was introduced into the solution and
stirred for half an hour. Excess water was rotary evaporated on the water bath. The
obtained sample was oven dried at 110 ◦C for 10 h and calcined at 450 ◦C in air for 2 h.
A series of Mo-V2O5/TiO2 catalysts was prepared by tuning V/Mo molar ratio. The
catalysts was denoted as V2O5/TiO2 (without molybdenum) and VxMoy (where x:y =
6:1, 3:1, 1:1, 2:3, 1:2). The vanadium loading and K/V molar ratio were fixed at
5.77% (wt) and 1.97 for each sample. The detail amounts of compounds were
presented in ESI.
2.2. Catalyst characterization
Powder X-ray diffraction (XRD) patterns were determined on a Bruker AXS D8
Advance diffractometer with Cu-Kα radiation (λ= 0.15406 nm). The BET surface area

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and pore size were measured with a Quantachrome Autosorb-iQ-2 instrument by
◦
nitrogen adsorption-desorption at -196 C. The samples were degassed in vacuum at
◦
300 C for 4 h before N2 physisorption. Raman spectra were measured by JY Horiba
LabRam HR800 Raman spectrometer under ambient conditions using 532 nm laser
excitation. The spectra were recorded with a resolution of 2.5 cm-1, a collection time
of 100 s and an accumulation of 1 scans. The X-ray photoelectron spectroscopy was
measured with AXIS Supra (XPS) surface analysis instrument (Kratos Analytical Ltd,
monochromated Al Kα radiation). The binding energies were calibrated by using the
C1s peak at 284.8 eV. The obtained spectra were fitted by Gaussian-Lorentzian
functional shape (80:20) and Shirley-type background subtraction. X-ray fluorescence
(XRF) and temperature programmed desorption (NH3-TPD) experiments were carried
out and the details were shown in the ESI material.
2.3. Catalytic activity measurement
The catalytic gas-phase oxidation reactions were performed in a fixed-bed
stainless steel reactor under atmospheric pressure. The scheme of the experimental set
up was shown graphically in ESI. About 0.50 g catalyst (65-80 mesh，pressed and
sieved) diluted with 2.50 g same particle size quartz was filled into the tube reactor
(10 mm i.d.) between two quartz wool plugs. Then the reactant gas mixture (2-MN/O2
molar ratio=1:160) obtained by passing air through a heated flask with
2-methylnaphthalene was fed into the reactor using mass flow controllers. The
effluent stream was absorbed by 1,4-dioxane solvent in a cold trap and analyzed
offline by gas chromatography (GC7890B, Agilent) equipped with flame-ionization

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detector every two hours. All the lines connecting the reactor were heated to 140 ◦C to
prevent condensation of the products. Prior to the measurements, the catalysts were
◦
activated at 450 C with an air flow (100 ml/min) for 2 h. The temperature was
◦
decreased to 340 C and the flow was then switched to the reaction mixture. The
activity test duration was about 120 h for all the catalysts. The feed flow and gas
hourly space velocity (GHSV) was 0.05 g/h and 10000 h-1, respectively.
2-Methylnaphthalene conversion (X/mol-%) and 2-naphthaldehyde yield (Y/mol-%)
were determined after the catalyst has achieved steady-state and given as an average
of four runs. The carbon balance was maintained at (100 ± 10) %. Blank tests showed
that the reactor walls were inert in terms of 2-methylnaphthalene oxidation over the
temperature range studied.
3. Results and discussion
3.1. Structure and surface study of catalysts
3.1.1. XRD
The XRD patterns of the catalysts were shown in Fig. 1. The diffraction peaks
representing anatase TiO2 (JCPDS, 21-1272) and very weak rutile TiO2 (JCPDS,
21-1276) existed in all the samples, but there were no diffraction peaks related to
MoO3 or V2O5 species. This indicates that Mo and V species were well dispersed or
presented in amorphous phase. Notably, the peaks related to KVO3 species in the
VxMoy catalyst samples decreases or disappears with increasing Mo content,
suggesting Mo introduction affects the interaction between K and V.

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3.1.2. BET surface area and pore size
Table 1 lists the textural parameters of each catalyst sample. From Table 1, the
introduction of Mo species into V2O5/TiO2 catalyst increases the specific surface area
first, then decreases it seriously. The average pore size and pore volume of VxMoy
samples decrease with the increasing amount of Mo, which could be caused by
gradually entering of molybdenum ion into the pore of TiO2 support. VxMoy catalysts
possess smaller pore size than V2O5/TiO2 catalyst, which could be beneficial for
desorption and diffusion of products in the catalytic gas-phase oxidation reaction.
Compared to the fresh catalyst, the used one’s pore size and pore volume
declined seriously, which was probably due to coke deposits produced during the
catalytic reaction. After a long time running, coke deposits could be accumulated on
the surface or interior of the catalyst. It could hinder the reaction process, causing the
decrease in conversion and selectivity.
3.1.3. Raman
The Raman spectra of the V2O5/TiO2 sample and of the V1Mo1 catalyst are
shown in Fig. S1 (ESI). Both of them shows five bands at 139, 193, 392, 510, and 634
cm-1, due to the anatase TiO2[17,18]. The band at 475 and 632 cm-1 can be assigned to
the SnO2 species [19] and the latter one is covered by the strong peaks at 634 cm-1
belonging to TiO2. No bulk MoO3 and V2O5 bands in the Raman spectra are observed
[20], consistent with the result of XRD. Additional weak scattering is observed in the
spectral region 1100-800 cm-1, due to molybdenum oxide or vanadium oxide species.

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The band at 872 and 932-942 cm-1 can be assigned to the amorphous KVO3 [21]. The
bands at 957 cm-1 are due to the symmetric stretch of the V=O groups in the
polymeric vanadium oxide [22]. The band at 983 and 1000-1016 cm-1 can be assigned
to K-doped species (K–O–V=O sites) and K-perturbed species (K…O=V sites)
[21,23]. After doping molybdenum, the intensity of the band at 139 cm-1 increases,
indicating that molybdenum introduction affects the surface of the TiO2 support and
the interaction of TiO2 with V2O5. There may exist interaction between Mo and SnO2
causing the decrease of peak intensity at 475 cm-1. The intensities of KVO3 and
K–O–V=O sites decrease due to the interaction of Mo with V2O5, probably by
substituting K to form new bridge bonds with V.
3.1.4. XPS
The V2O5/TiO2 and V1Mo1 catalysts, before and after catalytic tests, were
characterized by XPS spectroscopy to elucidate the influence of Mo on V2O5/TiO2
catalyst. The binding energies of V 2p3/2, Ti 2p3/2, O 1s, Sn 3d5/2, and Mo 3d5/2 of all
samples were listed in Table 2.
Figure S2 (ESI) shows the spectra of Ti 2p. The peak at ca. 458.7-459.0 eV of all
the samples was ascribed to Ti 2p3/2 for Ti4+ [17], without Ti3+ species existing on the
surface. Notably, the peak for 2p3/2 of Ti4+ shifted to higher values with about ΔE =
0.3 eV after Mo introduction. This indicated the electron transfer between Mo and
TiO2, namely interaction between Mo and TiO2 for V1Mo1catalyst.
The Mo 3d spectra in Fig. S3 (ESI) shows the two peaks at 233.0 (232.7) and
236.1 (235.8) eV, corresponding to 3d5/2 and 3d3/2 of Mo6+ [24]. After catalytic tests,

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the peak moved to lower values with about ΔE = 0.3 eV, implying Mo participated in
the catalytic reaction. As for Sn 3d5/2 shown in Fig. S4 (ESI), the peaks at 487.4 and
486.3 eV corresponded to Sn4+ and Sn2+, respectively [25]. After addition of Mo, there
was only one peak at 486.8 eV for Sn 3d5/2, indicating that Mo inhibited the
transformation of Sn2+ to Sn4+.
The V 2p3/2 spectra in Fig. S5 (ESI) were fitted with two peaks at 517.7 and
516.6 eV, which were the typical binding energy of V5+and V4+ [18]. After the
introduction of Mo, Ammonium heptamolybdate consumed part of oxalic acid,
reducing the amount used to reduce V5+, and decreased the ratio of V4+/V5+. Besides,
the introduction of Mo inhibited the conversion of V5+ to V4+ via Sn (2V5+
+
Sn2+→2V4+ + Sn4+), which could affect oxygen vacancies and unsaturated chemical
bonds over the catalyst surface, thus leading to a change of the surface chemisorbed
oxygen.
Figure 2 shows the O 1s XPS spectra. All samples exhibit two peaks around
532.0 and 530.0 eV. The peak at binding energies (531–533 eV) corresponded to the
surface adsorbed oxygen species (labeled Oa) relating to Ti-OH or Ti-O-V bond, and
the band at binding energies (529–531 eV) was attributed to the lattice oxygen species
(labeled Oc) relating to Ti-O-Ti bond [26,27]. In the case of V1Mo1 catalysts, the new
peak at ca. 531.1 eV mainly corresponded to the surface adsorbed species (labeled Ob)
relating to Ti-O-Mo or V-O-Mo bond, demonstrating that the introduction of Mo
changed the state of oxygen species and interacted with Ti or V species. The
percentage of lattice oxygen species (Oc) of V2O5/TiO2 catalyst was the same to

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V1Mo1 sample. After catalytic tests, the percentage of lattice oxygen species of
V2O5/TiO2 catalyst increased, while the V1Mo1 sample decreased, implying that Mo
reacted with Ti-O-Ti bond to form Ti-O-Mo bond during the catalytic process.
3.2. Catalytic activity
The selective oxidation of 2-methylnaphthalene to 2-naphthaldehyde was
conducted as a model reaction to evaluate the activities of as-prepared catalysts. The
catalytic test results indicated that the main products of the gas oxidation reaction
were 2-naphthaldehyde, phthalic anhydride, 4-methylphthalic anhydride,
2-methyl-1,4-naphthoquinone, and 6-methyl-1,4-naphthoquinone. Only a small total
oxidation to COX was observed under some conditions and cumulative selectivity to
other detected compounds did not exceed 5%. The catalytic activity of all tested
samples was stable during tests.
As shown in Fig. 3(a), with the increase of Mo introduction amount, conversion
of 2-methylnaphthalene was first increased, then decreased. The optimal value was
achieved for the V1Mo1 sample (V/Mo molar ratio, 1:1). The similar phenomenon
can be seen in Fig. S6 (ESI) and Fig. 3(b) for selectivity and yield to
2-naphthaldehyde. Importantly, the influence of Mo introduction on selectivity to
2-naphthaldehyde was more obvious than that on conversion of 2-methylnaphthalene
for V2O5/TiO2 catalyst. Selectivity to 2-naphthaldehyde decreased with the increase in
conversion of 2-methylnaphthalene, quite noticeable for the V1Mo2 sample. Another
phenomenon is that selectivity and yield to 2-naphthaldehyde decreased seriously for
most catalysts once the reaction temperature reached 400◦C. Furthermore, the

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catalytic performance of V2Mo3 sample (V/Mo molar ratio, 2:3) was very close to
V1Mo1 sample because of the approximate V/Mo molar ratio.
3.3. Possible mechanism of Mo promotional effect
The addition of Mo can significantly alter the activity of V2O5/TiO2 catalyst.
Only with the appropriate amount of Mo, V2O5/TiO2 catalyst can acquire the best
performance for catalytic oxidation of 2-methylnaphthalene to 2-naphthaldehyde.
This result indicates that Mo can enhance or weaken the performance of V2O5/TiO2
catalyst depending on the loading amount. One possible reason is that Mo interacts
with V or Sn, even with Ti element. According to the XPS analysis of V2O5/TiO2 and
V1Mo1 catalysts, the chemical state and composition of V and Sn in the catalyst were
changed after the addition of Mo. From the Raman spectra result, Mo interacts with
vanadium species and Sn species and decreases the intensity of K–O–V=O sites and
SnO2. These subtle variations affect the activity of V2O5/TiO2 catalysts largely. Here
we propose a mechanism to explain the enhancing effect of Mo, shown in Fig. 4 and
Fig. S7-S8 (ESI). Assuming the polymeric vanadium species as the active agent,
vanadium species contain V=O bonds, bridging Ti–O–V bonds and one bridging
K–O–V bond. With the addition of Mo, Mo attacks the bridging oxygen and then
replaces K to form a new bridging Mo–O–V bond, or reacts with Ti to form a new
Mo–O–Ti bond. As is shown in Fig S7-S8 (ESI), the terminal V=O bond and the
bridge V-O-Ti bond participates in the oxidation of 2-methylnaphthalene by
transferring of electrons. Moreover, the formation of bridging Mo–O–V and
Mo–O–Ti bond would decrease the amounts of Ti–O–V bond, causing the decrease of

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vanadium species sites. Under the overdosed Mo introduction, vanadium species sites
may decrease badly. This inhibits the activity of V2O5/TiO2 catalyst, leading to a poor
performance for V1Mo2 sample. Further investigations will be done to study and
verify this reaction mechanism.
4. Conclusion
Introduction of molybdenum to V2O5/TiO2 catalyst has a noticeable effect on the
vanadium surface structure. XRD experiment indicates KVO3 phase gradually
disappears with the increase of Mo loadings. The surface states of Ti, V, O, and Sn
characterized by XPS are altered by Mo introduction. Catalytic data on the selective
oxidation of 2-methylnaphthalene reveals the influence of molybdenum on catalytic
performance of V2O5/TiO2 catalyst. The addition of molybdenum (V1Mo1 and
V2Mo3) shows an accelerating effect, while other addition shows no promotional
effect on the catalytic properties of V2O5/TiO2 catalyst. Molybdenum modified
catalysts demonstrate different surface structure and state of vanadium species due to
the addition amount. These changes determine the high activity and selectivity in the
oxidation of 2-methylnaphthalene into 2-naphthaldehyde for the V1Mo1 sample.
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Table Captions
Table 1 Textural properties of the different samples (V/Mo ratios =1:0, 6:1, 3:1, 1:1,
2:3, 1:2)

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Table 2 Binding energies and ratios of V, Ti, O, Sn and Mo obtained from XPS
spectroscopy
Table 1 Textural properties of the different samples (V/Mo ratios =1:0, 6:1, 3:1, 1:1,
2:3, 1:2)
Surface
Area(m2·g-1)
5.41
Average
size(nm)
37.22
Pore Pore
Volume(cm3·g-1)
Samples
V2O5/TiO2
V6 Mo1
0.050
0.034
0.027
0.011
0.014
0.009
5.56
24.51
V3 Mo1
3.16
35.42
V1 Mo1
4.33
28.36
V2 Mo3
3.42
17.40
V1 Mo2
1.88
19.68
V2O5/TiO2
used
3.04
13.92
0.011
V1 Mo1 used
V2 Mo3 used
4.23
3.24
7.55
8.05
0.008
0.006
Table 2 Binding energies and ratios of V, Ti, O, Sn and Mo obtained from XPS
spectroscopy

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V2O5/TiO2
used
V1Mo1
used
V2O5/TiO2
V1Mo1
Relative species
517.7
(92)
517.7 (80) 517.5 (68)
516.6 (20) 516.4 (32)
517.5 (54)
516.4 (46)
V5+
V4+
V 2p3/2
516.6 (8)
459.0
Ti 2p3/2 459.1
458.9
458.9
Ti4+
532.2
(52)
Ti-OH, Ti-O-V
532.3 (76) 532.2 (68)
532.1 (48)
531.0 (37)
530.2 (15)
531.1
(24)
Ti-O-Mo,
Mo-O-V
O 1s
-
-
530.3 (24) 530.2 (32)
530.3
(24)
Ti-O-Ti
487.8 (50) 487.6 (31)
486.7 (50) 486.5 (69)
-
-
Sn4+
Sn2+
Sn
3d5/2
486.8
486.5
Mo
-
-
233.0
232.7
Mo6+
3d5/2
Figure captions
Fig. 1. XRD patterns of the VxMoy samples: (◆) anatase phase TiO2; (○) KVO3; (●)
rutile phase TiO2.
Fig. 2. XPS spectra of O 1s over various catalysts.

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Fig. 3. Conversion of 2-methylnaphthalene (a) and selectivity to 2-naphthaldehyde (b)
over VxMoy-TiO2 catalysts.
Fig. 4. Proposed mechanism of Mo introduction over VxMoy-TiO2 catalysts.
Fig. 1. XRD patterns of the VxMoy samples: (◆) anatase phase TiO2; (○) KVO3; (●)
rutile phase TiO2.

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Fig. 2. XPS spectra of O 1s over various catalysts.

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Fig. 3. Conversion of 2-methylnaphthalene (a) and yield to 2-naphthaldehyde (b) over
VxMoy-TiO2 catalysts.
Fig. 4. Proposed mechanism of Mo introduction over VxMoy-TiO2 catalysts.

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Highlights
 Mo-modified V2O5/TiO2 catalysts were prepared by wetness impregnation
method.
 Catalysts were tested via oxidizing 2-MN to 2-naphthaldehyde by fixed bed
reactor.
 Mo introduction affects the conversion and selectivity of V2O5/TiO2 catalysts.