Novel WO3/SO42--ZrO2–TiO2 double bridge coordination catalyst hfor oxidation of cyclohexene

Article abstract of DOI:10.1016/j.jssc.2021.122239

A solid super acid WO₃/SO₄^2--ZrO₂–TiO₂ catalyst was prepared with adjustable acidity via double bridge connection strategy for oxidation of cyclohexene (CHE) to adipic acid (AA). XRD, SEM and N₂ adsorption-desorption isotherm indicated that WO₃ was successfully decorated and was highly dispersed on SO₄^2--ZrO₂–TiO₂ surface. An obvious stretching vibration peak (1125-1055 ?cm^?1) in FT-IR illustrated that connection effect between SO₄^2? and ZrO₂–TiO₂ was double bridge connection. NH₃-TPD profile appeared a strong acid center peak (516 ?°C), while this center of solid super acid catalyst could reduce decomposition rate of H₂O₂ directly, and increase reaction time between CHE and H₂O₂ meanwhile. The marked catalytic performance was attributed to the synergistic effect between WO₃ and SO₄^2--ZrO₂–TiO₂. DFT calculation was employed to further analyze reaction process and system energy.

Full text of DOI:10.1016/j.jssc.2021.122239

Journal of Solid State Chemistry 300 (2021) 122239
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Novel WO3/SO24--ZrO2–TiO2 double bridge coordination catalyst hfor
oxidation of cyclohexene
,
b
,
,*
Xiangxue Liu a, Ke Wang a, Baoquan Liu a, Zhenmei Guo a, Chao Zhang a **, Zhiguo Lv a
a State Key Laboratory Base for Eco-chemical Engineering, Key Laboratory of Multiphase Flow Reaction and Separation Engineering of Shandong Province, School of
Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
b Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin 300071, China
A R T I C L E I N F O
A B S T R A C T
A solid super acid WO3/SO24--ZrO2–TiO2 catalyst was prepared with adjustable acidity via double bridge
Keywords:
Adipic acid
Green oxidation
connection strategy for oxidation of cyclohexene (CHE) to adipic acid (AA). XRD, SEM and N2 adsorption-
desorption isotherm indicated that WO3 was successfully decorated and was highly dispersed on SO42-
-
Bridge double coordination
ZrO2–TiO2 surface. An obvious stretching vibration peak (1125-1055 cmꢀ1) in FT-IR illustrated that connection
effect between SO42ꢀ and ZrO2–TiO2 was double bridge connection. NH3-TPD profile appeared a strong acid center
peak (516 ꢁC), while this center of solid super acid catalyst could reduce decomposition rate of H2O2 directly, and
increase reaction time between CHE and H2O2 meanwhile. The marked catalytic performance was attributed to
the synergistic effect between WO3 and SO24--ZrO2–TiO2. DFT calculation was employed to further analyze re-
action process and system energy.
WO3/SO24--ZrO2–TiO2 catalyst
1. Introduction
oxidation because it could decrease H2O2 decomposition rate and
enhance reaction time between CHE and H2O2. In the traditional AA
Adipic acid (AA) had been widely used in plastic packaging, medical
and chemical preparation fields, as a vital aliphatic dicarboxylic acid [1,
2]. In previous work, AA was successfully synthesized by oxidizing of
cyclohexene, cyclohexanone or cyclohexanol with nitric acid. However,
there were still many problems and challenges, e.g., nitric acid served as
oxygen source could release nitric oxide, waste acid and so on. In addi-
tion, the system acidity couldn't be effectively controlled as well [3,4].
Therefore, it's necessary to develop a novel preparation strategy to ach-
ieve the green synthesis of AA.
It's reported that employing O2 or H2O2 as green oxygen sources was a
desirable candidate instead of nitric acid for CHE oxidation [5]. Partial
hydrogenation of benzene was a well-known reaction, which had been
studied for many years. The first industrial production of cyclohexene
was manufactured by Asahi in 1990 in Japan [6]. Up to 2015, China
became the second country to achieve industrialize CHE [7]. Therefore,
the price of CHE had appeared decrease in recent years. Generally, H2O2
served as oxygen source had the characteristics of moderate conditions,
low toxicity, high yield and short process route advantages [8–10].
Maintaining an ideal acid condition was extremely important for CHE
synthesis process, PH value was adjusted by introducing inorganic acid
(e.g., H2SO4) or organic acids (e.g., AA) [11,12]. For example, Jin et al.
discussed the effect of acidity environment for AA yield, proving that
adding H2SO4 could significantly improve CHE conversion rate [13].
However, this strategy had not been applied in industrialization due to
some subsistent problems (such as corrode equipment and waste acid
treatment) [14–16]. Up to now, a lot of efforts had been devoted to solve
this problem. From recent researches, it was found that metal oxides with
mechanical stability and easier separation could be employed in the
catalysts for preparation of AA [17,18]. Moreover, metal oxides showed
the unique ability for fixing acids as well, which could combined with
SO42ꢀ to structure SO42ꢀ/MxOy solid super acid [19].
In this paper, a novel WO3/SO42--ZrO2–TiO2 solid super acid catalyst
had been established successfully via impregnation precipitation method
for AA preparation. The main advantages involved: (i) Designing a solid
super acid catalyst could regulate acidity. (ii) Comparing to traditional
technology process, the introduction of the other acidic chemicals was
avoided and a simplified the experiment pathway was also realized. The
synthesis process of catalyst was shown in Scheme 1. This study provided
* Corresponding author.
** Corresponding author. State Key Laboratory Base for Eco-chemical Engineering, Key Laboratory of Multiphase Flow Reaction and Separation Engineering of
Shandong Province, School of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China.
E-mail addresses: chaozhangchem@qust.edu.cn (C. Zhang), lvzhiguo@qust.edu.cn (Z. Lv).
https://doi.org/10.1016/j.jssc.2021.122239
Received 20 February 2021; Received in revised form 16 April 2021; Accepted 27 April 2021
Available online 1 May 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

X. Liu et al.
Journal of Solid State Chemistry 300 (2021) 122239
Scheme 1. The preparation process diagram of WO3/SO24--ZrO2–TiO2.
a new strategy for green preparation of AA by H2O2 oxidation of CHE
without any other reagents.
programmed to rise to 600 ꢁC at a rate of 10 ꢁC⋅minꢀ1. The electrical
signal was detected by the TCD thermal conductivity detector. In order to
test the thermal stability of the tungsten multiphase catalyst, TGA-DTG
measurements were performed on a METTLER TGA/DSC1. The temper-
ature was increased from 30 to 800 ꢁC with heating rate of 10 ꢁC⋅minꢀ1
under nitrogen atmosphere. FT-IR spectra were carried out on HGCS
spectrometer from 500 to 4000 cmꢀ1 using material diluted with KBr.
SEM pictures were obtained by JEM-2100 electron microscope.
2. Experimental
2.1. Catalysts preparation
2.1.1. Preparation of SO24--ZrO2–TiO2 support
The SO24--ZrO2–TiO2 was manufactured by the impregnation precip-
itation method [20]. Firstly, 1.2 mol TiCl4 and 1.2 mol ZrOCl2⋅8H2O
were diluted by 200 ml deionized water. PH of the mixture was
controlled at around 9–10 with NH3∙H2O and then stirred for 1 h. Then
the solution was placed at room temperature for 10 h, and the filter cake
was repeatedly washed until all Clꢀ were washed away (detected with
0.1 mol Lꢀ1 AgNO3). Subsequently, the dried precipitate solid was
immersed in 0.5 mol Lꢀ1 H2SO4 aqueous solution (proportion of 15 ml
2.4. Catalysts test
110 mmol H2O2 (50%) and 0.28 g catalyst were placed in a three-
mouth flask with an isobaric drop funnel and reflux unit of the ther-
mometer. When reaction temperature reached 75 ꢁC, 25 mmol CHE was
added until no reflux occurred in the reaction system, and the reaction
temperature was finally raised to 95 ꢁC for 6 h. The quantitative and
qualitative analysis of products was respectively carried out using gas
chromatography (GC, SE-30 Capillary column) and gas chromatograph-
mass spectrometer (GC-MS) under hot conditions (FID detector temper-
ature: 280 ꢁC; Injection temperature: 280 ꢁC; Heating procedure: 40–280
ꢁC with a heating rate of 10 ꢁC and stayed at 120 ꢁC for 5min).
g
ꢀ1) for 12 h. Finally, the dried filter cake was calcined in a muffle
furnace at 550 ꢁC for 4 h and the resulting product was named
SO24--ZrO2–TiO2. A similar method for preparing CuO–TiO2, La2O3–TiO2,
ZrO2–TiO2 by replacing ZrOCl2⋅8H2O with Cu(NO3)2⋅H2O and
La2(NO3)3⋅6H2O.
2.1.2. Synthesis of WO3/SO24--ZrO2–TiO2 sample
3. Results and discussion
The synthesis of the WO3/SO24--ZrO2–TiO2 sample was prepared via
the complexation method. First of all, 3.75 g (NH4)10W12O41~xH2O was
dissolved in an aqueous solution containing 0.5 g C2H2O4⋅2H2O and was
stirred for 4 h at 25 ꢁC. Subsequently, 8.01 g SO24--ZrO2–TiO2 was added
to above solution. And then the resulting mixed solution (A) was heated
to 85 ꢁC. They were maintaining reaction temperature until all water
evaporated. In the end, the dried solid was calcined and treated in muffle
furnace at 450 ꢁC for 4 h. The product obtained was named X WO3/SO24--
ZrO2–TiO2 (X was the mass fraction of WO3 in the catalyst). WO3/
CuO–TiO2, WO3/La2O3–TiO2 and WO3/ZrO2–TiO2 were also prepared in
the same way, except to replace SO24--ZrO2–TiO2 with CuO–TiO2,
La2O3–TiO2 and ZrO2–TiO2, respectively.
3.1. Characterization of the catalyst
SEM images of ZrO2–TiO2, SO24--ZrO2–TiO2 and WO3/SO42--ZrO2–TiO2
catalysts were presented in Fig. 1. The ZrO2–TiO2 exhibited irregular
lumps structures in Fig. 1a–c. However, when SO24ꢀ was introduced, the
sulfated ZrO2–TiO2 (SO24--ZrO2–TiO2) showed
a smooth surface.
Furthermore, the ZrO2–TiO2 sample size also became relatively uniform
due to the existence of SO24ꢀ (Fig. 1d–f). After loading with WO3, the
WO3/SO24--ZrO2–TiO2 catalyst exhibited a rough surface (Fig. 1g–i),
which demonstrated that the WO3 had been successfully decorated on
SO24--ZrO2–TiO2 surface.
The crystal structure of pure WO3 and a series of composite samples
were characterized by X-ray diffraction analysis (XRD) in Fig. 2b. It could
be clearly seen that the SO24--ZrO2–TiO2 presented similar curves [21,22],
compared to the pristine ZrO2–TiO2. Furthermore, the characteristic
peak appeared at ca. 54ꢁ completely disappeared, and the other peak at
31ꢁ showed a significant shift (31 to 22ꢁ) [23,24]. This phenomenon
illustrated that the synergistic effect between ZrO2–TiO2 and SO24ꢀ could
lightly eliminate the crystal structure of the ZrO2–TiO2 sample and
reduce the influence caused by various crystal types. Interestingly, the
diffraction peaks located at 23, 33 and ca.50ꢁ were identified over the
complexation WO3/SO24--ZrO2–TiO2 in Fig. 2a. With the increase of WO3,
the flat peaks were gradually strengthened to be sharp peaks, and peak
position was almost unchanged. It indicated the WO3/SO24--ZrO2–TiO2
was successfully synthesized.
2.2. Preparation of WO3/SO24--ZrO2–TiO2–H sample
Solution A was aged 24 h at room temperature, then it was main-
tained at 110 ꢁC for 24 h in hydrothermal synthesis reactor. After filtering
and drying, the precipitate was calcined in muffle furnace at 450 ꢁC for 4
h, named WO3/SO24--ZrO2–TiO2–H.
2.3. Characterization of catalyst
The X-ray diffraction (XRD) patterns were recorded on a RINT2000
vertical goniometer X-ray diffractometer (using CuK
α radiation (λ ¼
0.15418 nm) in the range of 10–80ꢁ, at a speed of 5ꢁ⋅minꢀ1). N2
adsorption-desorption isotherm was carried out on ASAP 2020V4.01
sorptometer at ꢀ196 ꢁC. All samples were outgassed at 100 ꢁC for 8 h
before the measurement. NH3-TPD was used to reflect NH3 adsorption on
the catalyst surface. It was fully automated by Zhejiang Pantai Instrument
FINESORB-3010 Temperature-rising chemisorption to test. The test
sample was 70 mg, the particle size was 40–60 mesh, and the TPD process
was pretreated at 200 ꢁC for 1h. At room temperature of 25 ꢁC, NH3 was
adsorbed for 1 h and N2 was purged for 15 min. The temperature was
FT-IR spectra was employed to further study on the chemical con-
struction of ZrO2–TiO2 and SO24--ZrO2–TiO2 in Fig. 3a. The absorption
bands at around 3444 and 1630 cmꢀ1 were attributed to the character-
istic peaks of water (O–H). Similarly, compared to the pristine
ZrO2–TiO2, for SO42--ZrO2–TiO2 and WO3/SO42--ZrO2–TiO2, the absorp-
tion peaks between 1055 and 1125 cmꢀ1 were belonged to the charac-
–
teristic peaks of S O and S–O, which was well indexed to the double
–
2

X. Liu et al.
Journal of Solid State Chemistry 300 (2021) 122239
Fig. 1. SEM images of the TiO2–ZrO2 (a–c), SO24--ZrO2–TiO2 (d–f) and WO3/SO42--ZrO2–TiO2 (g-i)
Fig. 2. XRD patterns of pure WO3 and X WO3/SO24--ZrO2–TiO2 (a) and SO24--ZrO2–TiO2 and ZrO2–TiO2 (b).
bridge connection type of solid super acid [25]. This results illustrated
that SO24ꢀ had been successfully loaded on ZrO2–TiO2. Furthermore, the
peaks at near 810 cmꢀ1 was the vibration absorption peaks of W–O,
corresponding to the characteristic peaks of WO3, demonstrating that
WO3 was resoundingly decorated on the surface of SO24--ZrO2–TiO2 [26,
27].
ascribed to NH3 species desorbed at medium-strong acid sites [28]. As
presented in Fig. 3b, two distinct desorption peaks were observed over
WO3/SO24--ZrO2–TiO2 catalyst in the entire temperature range. The
desorption peaks at 103 ꢁC could be attributed to the NH3 desorbed at the
weak acid sites. However, the desorption peaks were exhibited at 516 ꢁC,
which confirmed that induction of SO24ꢀ brought about the strong acidity
for WO3/SO24--ZrO2–TiO2 [29]. FT-IR spectra and NH3-TPD of catalysts
illustrated the successful construction of solid super acid.
Fig. 3b showed the NH3-TPD profiles of WO3/SO24--ZrO2–TiO2 and
WO3/ZrO2–TiO2. Curve of the WO3/ZrO2–TiO2 was observed over a wide
temperature range, suggesting that there were two types acid sites. For
WO3/ZrO2–TiO2, the broad desorption peaks appeared at 113 ꢁC was
weak acid sites, while the NH3 desorption peaks at 370 ꢁC could be
N2 adsorption-desorption isotherm and pore size distribution of cat-
alysts were shown in Fig. 4a. It displayed IV-type curves with the H1
hysteresis loops [3], which indicated that mesoporous materials had the
3

X. Liu et al.
Journal of Solid State Chemistry 300 (2021) 122239
Fig. 3. FT-IR spectra of WO3, ZrO2–TiO2, WO3/SO42--ZrO2–TiO2 and SO24--ZrO2–TiO2 catalysts (a) NH3-TPD profiles of WO3/SO24--ZrO2–TiO2 and WO3/ZrO2–TiO2 (b).
Fig. 4. N2 adsorption-desorption isotherm and pore size distribution (Inset) of heterogeneous catalysts (a) TG spectra of SO24--ZrO2–TiO2 and ZrO2–TiO2 (b).
characteristic of typical uniform pore structure [30]. As displayed in
almost unchanged and the total weight loss about 2.5%.
Fig. 4a and Table 1, it showed large specific surface area of ca. 299.71 m2
gꢀ1 and pore volume of ca. 0.18 cm3
g
ꢀ1. After combining SO24ꢀ with
3.2. Catalytic performance test
ZrO2–TiO2, the specific surface area and pore volume increased signifi-
cantly to 347.28 m2 gꢀ1 and 0.84 cm3 gꢀ1 respectively. What's more,
when WO3 was introduced, the specific surface area and pore volume of
3.2.1. Influence of catalyst type
Major difference in CHE conversion between the different catalysts
types were observed. Compared to WO3/CuO–TiO2 and WO3/
La2O3–TiO2, catalytic properties of WO3/ZrO2–TiO2 was enhanced
dramatically. As shown in Fig. 5a, the activity of WO3/SO24--ZrO2–TiO2 in
SO24--ZrO2–TiO2 decreased to 201.77 m2 gꢀ1 and 0.49 cm3 ꢀ1, which
g
suggested the WO3 might occupy the part of channel of support. In
addition, the pore size distribution of ZrO2–TiO2, SO24--ZrO2–TiO2 and
WO3/SO24--ZrO2–TiO2 centered at ca. 3.96, 7.15 and 7.15 nm, respec-
tively (the inner of Fig. 4a) further illuminated the excellent synergistic
effect between SO24ꢀ and ZrO2–TiO2. Those results proved that SO24ꢀ was
successfully loaded on the surface of ZrO2–TiO2.
CHE oxidation reaction was
4 times higher than catalyst WO3/
ZrO2–TiO2, which might be ascribed to appropriate acid sites of WO3/
SO24--ZrO2–TiO2. Interestingly, the strength of acid sites in WO3/SO24--
ZrO2–TiO2 might be much greater than WO3/ZrO2–TiO2 (Fig. 3b), which
suggested that the catalyst's acidity also played an important role in
oxidation reaction of cyclohexene. The stronger acid could inhibit the
H2O2 decomposition rate, which could enhance the contact time between
CHE and H2O2. Therefore, reducing the decomposition rate of H2O2
could increase oxygen molecule utilization rate, thus enhancing the
conversion of CHE.
The thermal stability of WO3/SO24--ZrO2–TiO2 and SO42--ZrO2–TiO2
were shown in Fig. 4b. The sample SO24--ZrO2–TiO2 weight loss process
could be roughly divided into two parts. The first part occurred at
100–200 ꢁC was ascribed to the gradual evaporation of physically
adsorbed water from sample. The largest weight loss occurred at about
515 ꢁC and the total weight loss was 20.0%. The appearance of this
weightlessness peak could be attributed to decomposition of sulfur
components on the sample surface [31]. Interestingly, when WO3 was
introduced into the support SO42--ZrO2–TiO2, the mass of the catalyst was
3.2.2. Effect of the preparation method
Fig. 5b displayed the influence of different preparation methods on
catalyst activity. It could be found that (NH4)10W12O41~xH2O as a
tungsten source was conducive to oxidation reaction of CHE. Further-
more, compared to the hydrothermal method, catalyst had predominant
performance via complexation method. At the moment, AA yield and
CHE conversion rate could reach up to 42.5 and 99.6%, respectively.
Table 1
Physical properties of heterogeneous catalysts.
Sample
SBET (m2⋅gꢀ1
)
VT (cm3⋅gꢀ1
)
DBJH (nm)
ZrO2–TiO2
299.71
347.28
201.77
0.18
0.84
0.49
2.73
8.15
8.14
SO24--ZrO2–TiO2
30%WO3/SO24--ZrO2–TiO2
4

X. Liu et al.
Journal of Solid State Chemistry 300 (2021) 122239
Fig. 5. Influence of catalyst preparation method.
(A-E preparation method in supporting).
Fig. 6. Influence of the molar ratio of cyclohexene to hydrogen peroxide (a), reaction temperature (b), reaction time (c) and catalyst amount (d).
5

X. Liu et al.
Journal of Solid State Chemistry 300 (2021) 122239
3.2.3. Influence of calcination temperature of the catalyst
Table 2
Oxidation of CHE to AA catalyzed by optimal conditions.
As shown in Fig. 5c, when calcination temperature was set as 450 ꢁC,
the AA yield reached up to 42.5%. Increasing the calcination temperature
would reduce the yield of AA. This phenomenon was mainly attributed to
the reduction of catalytic activity, which was due to the decomposition of
SO24ꢀ bonded to metal oxides. In addition, TG (Fig. 4b) test results further
proved our conclusion.
Catalyst
amount (g)
Conversion
(%)
Yield (%)
Selectivity (%)
–
CHE
AA
1, 2-
others
AA
1, 2-
diol
40.2
61.6
Others
diol
0.28
0.41
92.5
85.7
51.2
16.7
37.2
52.8
4.6
12.3
55.3
19.5
4.9
14.4
3.2.4. Effect of nWO3: nSO42-
nWO3: nSO24ꢀ had an important influence on the activity of difference
catalysts. With an increase of nWO3: nSO24ꢀ, the catalytic performance
was enhanced gradually (Fig. 5d). When the molar ratio of tungsten
trioxide to sulfate reached 1 : 1.88, the AA yield was as high as 42.5%.
What's more, with a further increase of nWO3: nSO24ꢀ, the yield of AA
increased slightly. Therefore, from an economic point of view, 1:1.88
(30% WO3/SO24--ZrO2–TiO2) was chosen as the best molar ratio.
3.2.5. Influence of the molar ratio of CHE to H2O2
As shown in Fig. 6a, AA yield on WO3/SO24--ZrO2–TiO2 increased with
the mole ratio CHE to H2O2, and then descend. When the molar ratio of
CHE to H2O2 was changed from 1 : 4 to 1 : 4.4, the AA yield increased
from 24.8% to 42.5%. The AA selectivity slightly decreased with the
increasing H2O2 amount.
3.2.6. Effect of reaction temperature
The effect of reaction temperature on catalyst activity was illustrated
in Fig. 6b. The AA selectivity increased steadily with the reaction tem-
perature rising from 80 to 95 ꢁC. However, it would began to decrease as
the temperature reached 110 ꢁC. This phenomenon might be attributed
to the fact that lower temperatures (<95 ꢁC) were conducive to the H2O2
decomposition. Moreover, the reaction system at higher temperature
would lead to the rapid decomposition of H2O2. These results indicate
that the optimal temperature was 95 ꢁC for CHE oxidation.
Fig. 7. The stability test results of WO3/SO24--ZrO2–TiO2.
could be roughly divided into three categories according to the methods
of the connection. A large number of studies by researchers had found
that FT-IR spectrum was an effective way to identify the three binding
types (Scheme 2a-c). The anti-symmetric stretching vibration absorption
3.2.7. Influence of reaction time
Fig. 6c presented the influence of reaction time on the oxidation of
CHE with H2O2. The yield and selectivity of AA had an approximately
double increase from 5 h to 6 h, after that, the AA yield decreased to
47.5% (8 h). This manifested that the CHE oxidation reaction reached to
a chemical equilibrium state when the reaction time was maintained for
ca. 6 h.
peak of the highest S O bond appearing below 1200 cmꢀ1 belonged to
–
–
the bridge double coordination [34]. In other words, the connection
mode between SO24ꢀ and the metal oxide was a bridge double coordi-
nation connection. In this experiment, bridge double coordination
(Scheme 2b) solid super acid was successfully fabricated. It could be
clearly seen in Scheme 2d that, when SO24ꢀ coordinated and adsorbed on
the metal oxide surface, the electron cloud on the metal-oxygen bond
underwent an obvious electron shift and then formed two acid centers
(Brønsted acid and Lewis acid) [35]. Subsequently, the resulting acidic
centers attracted electrons from surrounding water molecules to restore
neutrality. Meanwhile, a large amount of Hþ would float around in re-
action system, which could further inhibit the H2O2 decomposition rate
and improve a suitable acidic environment for hydrolysis reaction.
Hydrolysis and oxidation reaction were involved in the conversion of
cyclohexene to adipic acid (Scheme 3a). Generally, tungsten base catalyst
was beneficial to the ring opening of 1, 2-epoxycyclohexane (2). The
acidic condition was believed to be required in the two hydrolysis steps
[19]. The acidic condition could be provided by SO24ꢀ/MxOy catalyst
(Scheme 2d). What's more, DFT calculation was used to further examine
the reaction mechanism by the energy change and the energy of each
substance (Scheme 3b).
3.2.8. Effect of catalyst amount
In Fig. 6d, the dependency of AA yield on the catalyst amount was
illustrated. It was found that the amount of catalyst was achieved
increased from 0.20 to 0.28 g. At this juncture, the AA yield was the
highest (51.2%), which was due to an increase in the number of active
sites, leading to higher catalyst activity [32,33]. Nevertheless, AA
selectivity significantly decreased with the increase of catalyst amount,
which was due to the fact that the excessive amount of catalyst might
aggravate the by-product formation. GC-MS analysis showed the yield
and selectivity of 1, 2-cyclohexanediol (1, 2-diol) was enhanced with the
increase of amount of catalyst from 0.28 to 0.41g (Table 2).
3.3. Reusability of the catalyst
The catalytic performance and stability of the WO3/SO24--ZrO2–TiO2
were evaluated under the optimal conditions. The WO3/SO24--ZrO2–TiO2
catalyst could be recycled by filtration and washing with ethanol. After
testing five times, it could be found that the activity and selectivity of the
catalyst decreased only slightly in Fig. 7. This proved that WO3/SO24--
ZrO2–TiO2 catalyst had superior activity and stability.
4. Conclusions
In summary, composite WO3/SO24--ZrO2–TiO2 solid super acid cata-
lyst was fabricated successfully by fixing acid strategy and was firstly
applied in production of adipic acid. The bridge double coordination
occurred between ZrO2–TiO2 and SO24ꢀ with the introduction of SO24ꢀ
.
3.4. Analog computation and catalytic mechanism analysis
Building solid super acid might restrict the decomposition rate of H2O2
and might simplify the reaction pathway. Under optimal reaction
As presented in Scheme 2, the solid super acids of SO24ꢀ/MxOy type
6

X. Liu et al.
Journal of Solid State Chemistry 300 (2021) 122239
Scheme 2. Single coordination (a) bridge double coordination (b) chelating double (c) Coordination exhibited the acid center structure of SO24ꢀ/MxOy solid super acid
catalyst (d).
Zhang: Validation, Resources, Writing – original draft. Zhiguo Lv:
Project administration, Resources, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgments
This work was supported by the National Science Foundation of Na-
tional (NSFC21978141), the China Pakistan Science Foundation funded
project (2020M672015), the Open Project of Nankai University Key
Laboratory of Advanced Energy Materials Chemistry (Ministry of
Education).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jssc.2021.122239.
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CRediT authorship contribution statement
Xiangxue Liu: Experimental Design; Experimental Operation,
Writing – original draft. Ke Wang: Software, Data curation. Baoquan
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