Phosphotungstic acid encapsulated in USY zeolite as catalysts for the synthesis of cyclohexylbenzene

Article abstract of DOI:10.1007/s13738-020-02063-1

Abstract: In this work, a new type of catalyst, USY-HPW, was successfully prepared by encapsulating phosphotungstic acid (HPW) into ultra-stable Y zeolite (USY). The obtained catalyst USY-HPW was characterized by various techniques including N₂ adsorption/desorption isotherms, XRD, SEM, TG-DSC, XPS, NH₃-TPD, FT-IR, Py-IR. The catalytic properties of USY-HPW were evaluated by using a model alkylation reaction of benzene with cyclohexene to form cyclohexylbenzene. The conversion rate of cyclohexene can reach as high as 99.99%. Compared with USY alone, USY-HPW displayed markedly improved selectivity and yield for the target product of cyclohexylbenzene, ca. 5.41% and 8.73%, respectively. Besides, reusability tests indicated the high durability USY-HPW as the yield of cyclohexylbenzene can still reach to 83.50% after eight runs. All these results demonstrate that USY-HPW catalyst has good performances and holds good potential in acid catalyzed organic chemistry. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-020-02063-1

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-020-02063-1
SHORT COMMUNICATION
Phosphotungstic acid encapsulated in USY zeolite as catalysts
for the synthesis of cyclohexylbenzene
1
1
1
1
1,2
Yufei Yang · Yue You · Junyan Wu · Junbo Feng · Yadong Zhang
Received: 12 March 2019 / Accepted: 10 September 2020
©
Iranian Chemical Society 2020
Abstract
In this work, a new type of catalyst, USY-HPW, was successfully prepared by encapsulating phosphotungstic acid (HPW)
into ultra-stable Y zeolite (USY). The obtained catalyst USY-HPW was characterized by various techniques including N
2
adsorption/desorption isotherms, XRD, SEM, TG-DSC, XPS, NH -TPD, FT-IR, Py-IR. The catalytic properties of USY-
3
HPW were evaluated by using a model alkylation reaction of benzene with cyclohexene to form cyclohexylbenzene. The
conversion rate of cyclohexene can reach as high as 99.99%. Compared with USY alone, USY-HPW displayed markedly
improved selectivity and yield for the target product of cyclohexylbenzene, ca. 5.41% and 8.73%, respectively. Besides,
reusability tests indicated the high durability USY-HPW as the yield of cyclohexylbenzene can still reach to 83.50% after
eight runs. All these results demonstrate that USY-HPW catalyst has good performances and holds good potential in acid
catalyzed organic chemistry.
Graphical abstract
Keywords Ultra-stable Y zeolite (USY) · Phosphotungstic acid · Benzene · Cyclohexene · Cyclohexylbenzene
Introduction
Cyclohexylbenzene is an important chemical intermediate
with a wide range of applications, for example, as electro-
*
Yadong Zhang
lyte additive for overcharge protection of lithium ion battery
zhangyadong2016@163.com
[
1, 2] and the raw material in the synthesis of TFT liquid
1
2
crystal materials [3]. In addition, the oxidation decomposi-
tion of cyclohexylbenzene can aꢀord important chemical
substances, such as phenol and cyclohexanone [4], which
are critical raw materials for the production of bisphenol A,
School of Chemical Engineering and Energy, Zhengzhou
University, Zhengzhou 450001, China
Jiyuan Research Institute, Zhengzhou University,
Jiyuan 459000, China
Vol.:(0
1
123456789)
3

Journal of the Iranian Chemical Society
phenolic resin, adipic acid [5], as well as ε-caprolactone [6],
adipic acid [7] and caprolactam [8]. As a result, cyclohex-
ylbenzene has attracted more and more attention research-
ers, and the synthesis of cyclohexylbenzene has become a
research hotspot.
of benzene with cyclohexene over SiO -grafted AlCl cata-
2
3
lyst and found that the catalyst was active but unsatisfac-
tory reusability [13]. Song has developed a novel reusable
catalytic system, Sc(OTf) with [Bmim][SbF ] ionic liquids,
3
6
in the Friedel–Crafts alkylation of aromatic compounds
with alkenes [14]. Alexander has found that in apolar reac-
Basically, synthesis cyclohexylbenzene is mainly divided
into three broad categories, including the selective hydro-
genation of biphenyl [9], the hydroalkylation of benzene
+
tion media the nitrosonium cation (NO ) activated alkenes
toward electrophilic substitution of arene substrates to yield
the alkylated arene [15]. Karshtedt has described a novel
methodology for the hydroarylation of unactivated oleꢃns
utilizing a series of Pt complexes activated by Ag(I) salts
[16]. However, these catalysts described above were expen-
sive and just displayed moderate catalytic activity. At this
time, solid acid catalyst stands out among many catalysts
because of its high activity, good selectivity, reutilization
and environmental friendliness [32].
[10–12] and the alkylation reaction of benzene and cyclohex-
ene [13–16], which are summarized in Table 1. Among
them, the hydroalkylation of benzene is the most simple and
important method. But most of catalysts used in this method
are precious metal compounds, and their catalytic conver-
sion eꢁciencies still remain to be improved. The process of
the selective hydrogenation of biphenyl is simple, but it is
disappointing that the catalysts for this reaction are imma-
ture. In particular, the biphenyl is obtained via high-temper-
ature dehydrocoupling or oxidative coupling of benzene with
a low conversion and yield. Therefore, the research on the
synthesis of cyclohexylbenzene via the alkylation of benzene
and cyclohexene has become particularly urgent.
Zeolites have good thermal and hydrothermal stabili-
ties because of their special porous structure and adjustable
acidity. Those make zeolites have good catalytic ability and
high selectivity participating in Friedel–Crafts alkylation
reactions [33]. USY zeolites have a large speciꢃc surface
area and three-dimensional channels, containing a 1.30 nm
super-cages which are connected by a twelve-membered ring
channel with a pore diameter of 0.74 nm. The unique shape
framework and acidic sites of the zeolites provide catalytic
activity for the main catalytic processes inside the crystal
[34].
The traditional synthesis of cyclohexylbenzene utilizing
sulfuric acid or hydroꢂuoric acid as catalyst can be con-
ducted at mild reaction conditions with good yield. On the
other hand, the reaction has many shortcomings such as
large catalyst consumption, non-reusability, serious equip-
ment corrosion and environmental pollution [17]. With the
increasing requirements for environmental protection and
resource utilization, the search for pollution-free, easily
separated and recyclable catalysts has become a research
hotspot [18–31]. Sato has studied the liquid-phase alkylation
Heteropoly acids (HPAs) can be easily dissolved in polar
solvents and exhibit excellent catalytic performance in many
reactions owing to its unique structures, strong acidity and
oxidative properties. Phosphotungstic acid H PW O .
3
12 40
Table 1 List of synthesis methods of cyclohexylbenzene
Production methods
Synthetic process
Catalyst
Operation Condition
Results
Conversion
Selectivity
Hydrogenation
of biphenyl [9]
Skeletal Ni
Pd/HY
343 K,
100%
1.5 MPa
95%–99%
Hydroalkylation of benzene
423 K,
42%
77%
and H [10–12]
2 MPa, H /C H 1
2
2
6
6
Pt/Ni/Re-
443 K,
20%
75%
1
3X
3.5 MPa
Mo(CO)/Y
363 K,
Conversion20%
0
.1 MPa
Alkylation of cyclohexene and
benzene [13–16]
SiO –AlCl
253 K,
83.8%
97.2%
2
3
0.1 MPa
Sc(OTf) [Bmim]
293 K,
99%
3
[
SbF ]
Mol=20:1
90%
6
+
NO
303 K
78%
86%
Mol=9:1
AgBF4
353 K
Yield:65%
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Journal of the Iranian Chemical Society
xH O (HPW) is one of the most widely used HPAs. But on
sample was dried at 90 °C for 12 h and calcined at 400 °C
for 3 h to aꢀord the USY-HPW catalyst.
2
the other hand, the small speciꢃc surface area, low thermal
stability and diꢁculty in recovery restrict the use of HPW in
zeolite catalysts. In order to exert its catalytic ability, HPW
is usually incorporated in zeolites by impregnation or in situ
synthesis methods which oꢀer applications as Brønsted acid
catalyst [35–39]. For example, HPW clusters with diameters
Characterization of catalysts
BET surface area and pore structure of USY and USY-HPW
were characterized by measuring N adsorption/desorption
2
1
nm can be introduced inside the USY cages of 1.3 nm
isotherms with Quantachrome NOVA4200 distribution
analyzer. SEM images were obtained using a Zeiss Auriga
FIB-SEM. TG-DSC test was used STA 449 F3 Jupiter syn-
chronous thermal analyzer to study the structural weight
loss of substances and the decomposition of USY-HPW.
The crystal structures of the catalyst were recorded on
X-ray Power Diꢀraction (XRD) using Rigaku ULTIM III
X-ray diꢀractometer. High resolution X-ray photoelectron
spectroscopy (XPS) was carried out on a VG ESCALAB
250 to measure the composition of elements in the catalyst.
and 0.74 nm of channels, allowing for a signiꢃcant increase
in the surface area of the HPW with a possible increase in
its thermal stability, in addition to creating Brønsted acid
sites within the USY support. So the preparation of catalysts
by encapsulating Zeolite with heteropoly acids is the main
direction for the current research of heteropoly acid catalyst
[
40, 41].
In this article, we report a composite catalyst, USY-HPW,
which was prepared by the encapsulation of HPW inside
USY for catalyzing benzene alkylation with cyclohexene.
USY-HPW displayed high catalytic eꢁciency as well as
selectivity for the production of cyclohexylbenzene. The
reusability tests showed that the USY-HPW still have high
activity after running eight times. These results suggest that
highly eꢁcient USY-HPW catalyst can provide signiꢃcant
advances in the development of environmentally benign pro-
cesses in the chemical industry.
NH -TPD experiments were characterized by auto-catalytic
3
adsorption system Quantachrome Autosorb IQ 2.02 to per-
form the acidity of the catalyst. The infrared spectroscopies
of the catalysts were performed on Fourier transform infra-
red spectrometer (FT-IR) using Nicolet Nexus 470. Lewis
acid or Brønsted acid sites presented on the surface of the
catalyst adsorbed with pyridine were carried out using Nico-
let IR 200 spectrometer.
Evaluation of catalysts
Experimental
The activity of the catalyst was evaluated by the alkylation
reaction of benzene and cyclohexene in a 100-mL autoclave
reactor at desired temperature. The catalyst, a certain amount
of benzene and cyclohexene were placed into the autoclave
which was pressurized to 3 MPa and heated to 150 °C. The
reactor was naturally cooled to room temperature after the
reaction was completed. Then, the catalyst was filtered
and separated to aꢀord a reaction mixture which was ana-
lyzed with GC7900 gas chromatograph. Conversion rates
of cyclohexene and selectivity of cyclohexylbenzene were
calculated with an FID detector using n-decane as an inter-
nal standard.
Chemical reagent and materials
Dibasic sodium phosphate (Na HPO ·12H O) and sodium
2
4
2
tungstate dihydrate (Na WO ·2H O) were purchased from
2
4
2
Tianjin Kemiou Chemical Reagent Co. Ltd., China. Hydro-
chloric acid (HCl) was purchased from Luoyang Chemical
Reagent Co., Inc., China. USY zeolite was supplied by Tian-
jin Yuanli Chemical Co. Ltd., China. Benzene, cyclohexene,
cyclohexylbenzene and decane were obtained from Shanghai
Macklin Biochemical Technology Co. Ltd., China. Ethanol
was purchased from Tianjin Sailboat Chemical Reagent
Technology Co. Ltd., China.
Preparation of catalysts
Results and discussion
An aqueous solution of a certain molar ratio of dibasic
sodium phosphate and sodium tungstate dihydrate was added
to the USY zeolites, and the reaction mixture was stirred at
room temperature for 12 h, then the pH was adjusted to 2–3
with hydrochloric acid. After stirring at 80 °C for 24 h, the
sample was ꢃltered and washed with hot deionized water
several times until neutral in order to remove the phospho-
tungstic acid anions on the external surface. Finally, the
Characterization
Figure 1 displays the XRD patterns of zeolite samples
USY (A) and USY-HPW (B). The sample USY-HPW
depicts the similar typical characteristic crystalline diꢀrac-
tion peaks with USY, which conꢃrming that the framework
structure of the zeolite is preserved after encapsulation.
However, the peaks intensity in XRD also indicates the
crystallinity of the zeolite. Obviously, the peaks intensity
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Journal of the Iranian Chemical Society
the encapsulation has a signiꢃcant etching eꢀect on the
zeolite, which aꢀects the pore and textural property of
USY-HPW.
The N adsorption/desorption isotherms of USY and
2
USY-HPW are shown in Fig. 3a. The isotherm slope of the
USY-HPW is larger than that of the zeolite USY, indicat-
ing the formation of mesopores after encapsulating. And
the range of the hysteresis loop is slightly expanded, which
means that the pore size of catalyst becomes larger after
encapsulating in phosphotungstic acid. The pore sizes dis-
tribution of the catalysts exhibited in Fig. 3b are around 3.82
and 3.89 nm for USY and USY-HPW. Apparently, the total
pore volume of USY-HPW is larger than showed that of
USY (Fig. 3b and Table 2). In addition, the characteriza-
tion shows that the BET surface area and total pore volume
have greatly improved after encapsulating phosphotungstic
acid, which can facilitates the alkylation reaction of ben-
zene and cyclohexene. After reaction, the isotherm slope
and range of the hysteresis loop of catalyst become smaller,
which indicates the pore size same situation. The reason
for the decrease in BET surface area and total pore volume
(Table 2) is that catalyst sintered at high temperature or some
organic compounds deposited on the surface and block the
pores of USY-HPW, which make the activity loss and the
catalytic performance of USY-HPW decline.
Fig. 1 XRD patterns of USY (a) and USY-HPW (b)
of USY-HPW decrease compared with that of USY. The
reason is that the relative strong acid used in the synthe-
sis of USY-HPW makes the aluminum species dissolved
which leading to the collapse of structure and decrease in
relative crystallinity.
SEM images of USY and USY-HPW are shown in
Fig. 2. USY-HPW maintained the basic shape of the USY.
In detail, as presented in Fig. 2a and 2b, the surfaces of
USY are intact and smooth, no obvious pores appears.
From Fig. 2c and 2d, it can be signiꢃcantly rough. The
amount of small particles adsorbed on the surface of the
USY increase slightly after the treatment. Therefore,
In order to get more deep insight of the USY-HPW cata-
lyst, the XPS analysis is carried out (Fig. 4). The survey
spectrum (Fig. 4a and 4b) indicates the presence of phospho-
rus and tungsten after encapsulating phosphotungstic acid
into USY. As shown in Fig. 4c, the peaks at 33.35 eV and
35.5 eV correspond to W4f and W4f , respectively. The
7
/2
5/2
Fig. 2 SEM images of USY(a
and b) and USY-HPW(c and d)
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Journal of the Iranian Chemical Society
Fig. 3 N adsorption/desorption isotherm (a) and pore volume distribution (b) of USY and USY-HPW
2
Table 2 Textural properties of diꢀerent catalysts
change of USY-HPW and the framework collapse occur,
2
a
3
b
D(nm)c
which may cause a decrease in catalyst activity.
Catalysts
S(m /g)
V(cm /g)
The NH -TPD tests of the USY and USY-HPW are shown
3
USY
USY-HPW
After Reaction
304.17
768.30
237.77
0.20
0.37
0.17
3.82
3.89
3.83
in Fig. 6, the two kinds of acid sites are corresponding to the
weak acid peak around 160 °C and the strong intensity acid
site appears at a high temperature around 450 °C, respec-
tively. The result indicates that the acid center has been
changed and relatively reduced after phosphotungstic acid
being encapsulated in the USY, which may be the ascribed to
interaction between the HPW and the USY. USY-HPW not
only maintains the large speciꢃc surface area and the regular
pore structure of USY, but also changes its acid amount and
strength, which will facilitate the reaction between benzene
and cyclohexene.
a
BET surface area
Total pore volume calculated by single point method at P/Po=0.99
BJH desorption average pore diameter
b
c
diꢀerence of binding energy between W4f and W4f is
7
/2
5/2
2
.15 eV. In the high-resolution P2p spectra (Fig. 4d), the
peaks at 131.37 eV and 132.15 eV are related to P2p3/2 and
P2p1/2, respectively. Combined with the XPS analysis, it
can be conꢃrmed that Keggin-type heteropoly acid (HPW)
formed between the phosphorus and tungsten encapsulated
in USY during the preparation of the USY-HPW catalyst.
According to the TG-DSC characterization analysis of
HPW-USY catalyst (Fig. 5), the TG curve corresponds to
the change of sample mass at diꢀerent temperatures and the
mass decreases with increasing temperature. It is observed
that its weight loss is mainly divided into three processes. It
is physically adsorbed water below 100 ℃ and bound water
of the catalyst between 100 ℃ and 260 ℃. The mass loss
is 12.88% in the above temperature range, which shows a
slightly bulging exothermic peak on the DSC curve. There is
an endothermic peak at 260 ℃. At the same time, the sinter-
ing reaction on the surface of the sample starts to proceed,
and the weight loss rate is very fast. At this stage, the mass
loss is 39.71%. When it is above 350 ℃, there is a 39.71%
mass loss. It may be the structural water of phosphotung-
stic acid. After 460 ℃, the phosphotungstic acid begins to
decompose, so the DSC curve shows a large exothermic pro-
cess and reach a peak at 670 °C. At that time, the structure
The FT-IR was also tested in order to get more details of
the USY-HPW catalysts (Fig. 7), there are two signiꢃcant
diꢀerences between USY-HPW and USY in the area around
−
1
−1
3500 cm and 1050 cm . Skeleton vibration spectrum of
−
1
zeolite appears in the range of 200–1300 cm , obviously
−
1
−1
at two spectral bands 800 cm and 1020 cm . The distinct
decrease in absorption indicates the change of the skeleton
in the USY-HPW catalyst. The broad stretching intense char-
−1
acteristic peak appearing around 3500 cm is slightly wider
in USY-HPW than in USY, which may be owing to a com-
bination of vibrations related to hydroxyl groups in the pore
−
1
structure. An absorption band at 555–560 cm is the signal
of vibrations of the structural double ring in the USY zeolite.
At last, the Py-IR (infrared spectra of pyridine adsorbed)
test of USY-HPW catalysts was conducted. The absorption
−
1
spectrum of coordination bond with pyridine at 1450 cm
corresponds to the Lewis acid center and the absorption peak
−
1
of pyridine ion at 1540 cm can be used as a measure of
the Brønsted acid site. In general, Lewis acid and Brønsted
acid has a large amount of acid site at 100 °C. It can be seen
that the amount of acid decreases rapidly with the increase
in temperature in the Py-IR spectra patterns of USY-HPW
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Journal of the Iranian Chemical Society
Fig. 4 XPS analysis: (a and b) XPS survey spectra of (a) USY and (b) USY-HPW; XPS spectra of (c) W4f, (d) P2p of USY-HPW
Fig. 5 TG-DSC curves of the sample USY-HPW
catalysts (Fig. 8). The lower temperature corresponds to
the weak acid site, but the strong acid site corresponding
to 400 °C is not detected, which means that there are no
strong acid centers in the catalyst. Therefore, the weak acid
site of the catalyst is the active center of the reaction. It
can be deduced that the acid strength is weakened by the
Fig. 6 NH -TPD of USY (a) and USY-HPW(b)
3
interaction between phosphotungstic acid and USY zeolite,
which is mutually conꢃrmed with the results of NH -TPD
3
characterization (Fig. 6).
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Journal of the Iranian Chemical Society
Table 3 Reactivity of USY and USY-HPW catalysts
Catalysts
Conversion (%)
Selectivity (%)
Yield (%)
USY
USY-HPW
96.32
99.99
90.27
95.68
86.95
95.68
Reaction conditions: benzene (0.144 mol), cyclohexene (0.012 mol),
catalyst (0.3 g), reaction pressure ꢃlled with N (3 MPa), reaction
2
temperature (150 °C) and reaction time (3 h)
Fig. 7 FT-IR patterns of USY (a) and USY-HPW (b)
Fig. 9 Reusability of USY-HPW
was tested by carrying out repeated runs of the reaction at
the same conditions. After each reaction, the catalyst is sepa-
rated by centrifugation, then dried and calcined for the next
reaction. It’s worth mentioning that the yield of cyclohex-
ylbenzene is up to 83.50% after the running of eighth times
(
Fig. 9). These results demonstrate that phosphotungstic acid
Fig. 8 Py-IR spectra patterns of USY-HPW
encapsulated in the USY zeolite (USY-HPW) can improve
the reactivity and durability in synthesizing cyclohexylben-
zene from benzene and cyclohexene.
Catalytic performance
The catalytic performance of these catalysts has been exam-
ined in the synthesis of cyclohexylbenzene from benzene
and cyclohexene, and the reaction results are shown in
Table 3. It can be found that USY-HPW catalyst is more eꢁ-
cient than USY. The detected conversion rates of cyclohex-
ene can reach up to 99.99% by using USY-HPW. Compared
with USY catalyst, the selectivity and yield of cyclohexylb-
enzene corresponding to USY-HPW catalysts are increased
by 5.41% and 8.73%, respectively. Therefore, USY-HPW
catalyst presents higher conversion, selectivity and yield
than USY catalyst. Durability is an important aspect of any
industrial process, and it is also an aspect of investigating
the performance of catalysts. Thus, reusability of the catalyst
Conclusions
A new USY-HPW catalyst was successfully prepared
through encapsulating the phosphotungstic acid in USY
zeolite. The performance of USY-HPW catalyst in facilitat-
ing the alkylation reaction of benzene and cyclohexene to
synthesize cyclohexylbenzene was studied. It was found that
USY-HPW catalyst had Brønsted acid sites and showed good
catalytic activity. Especially, the conversion rate of cyclohex-
ene reached 99.99%, and the selectivity of cyclohexylben-
zene was 95.68%. Moreover, USY-HPW exhibited excel-
lent durability as the yield of cyclohexylbenzene can still
reach 83.50% during the eighth use of catalyst. All in all,
the HPW-USY catalyst presents higher activity and stability
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Journal of the Iranian Chemical Society
than USY zeolite in the synthesis of cyclohexylbenzene from
the reaction between benzene and cyclohexene.
18. A. Maleki, Z. Hajizadeh, P. Salehi, Sci. Rep. 9, 5552 (2019)
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9. A. Maleki, R.H. Firouzi, Sci. Rep. 8, 17303 (2018)
0. A. Maleki, Z. Hajizadeh, R.H. Firouzi, MicroporousMesoporous
Mate. 259, 46 (2018)
Acknowledgments This work was supported by the National Natural
Science Foundation of China [grant number 21706240], NSFC-Henan
Joint Fund General Project [grant number 162300410253].
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