Ru catalyst supported on bentonite for partial hydrogenation of benzene to cyclohexene

Article abstract of DOI:10.1016/j.molcata.2011.12.013

A ruthenium catalyst supported on the cheap bentonite with high activity has been prepared by impregnation-hydrothermal method and characterized by TEM, XRD, and XPS. The catalytic performance for selective hydrogenation of benzene to cyclohexene was investigated. The effect of additives NaOH and ZnSO ₄ on the catalytic performances was studied. It was demonstrated that both NaOH and ZnSO₄ could significantly enhance the selectivity to cyclohexene over Ru/bentonite catalyst. The influences of reaction temperature, pressure, amounts of catalyst and additives, and reaction time on the selective hydrogenation of benzene were studied in detail.

Full text of DOI:10.1016/j.molcata.2011.12.013

Journal of Molecular Catalysis A: Chemical 355 (2012) 174–179
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ru catalyst supported on bentonite for partial hydrogenation of benzene to
cyclohexene
Weitao Wang, Huizhen Liu, Tianbin Wu, Peng Zhang, Guodong Ding, Shuguang Liang,
Tao Jiang∗, Buxing Han∗
Beijing National Laboratory for Molecular Sciences (BNLMS), Centre for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A ruthenium catalyst supported on the cheap bentonite with high activity has been prepared by
impregnation-hydrothermal method and characterized by TEM, XRD, and XPS. The catalytic performance
for selective hydrogenation of benzene to cyclohexene was investigated. The effect of additives NaOH
and ZnSO4 on the catalytic performances was studied. It was demonstrated that both NaOH and ZnSO4
could significantly enhance the selectivity to cyclohexene over Ru/bentonite catalyst. The influences of
reaction temperature, pressure, amounts of catalyst and additives, and reaction time on the selective
hydrogenation of benzene were studied in detail.
Received 28 September 2011
Received in revised form 7 December 2011
Accepted 8 December 2011
Available online 17 December 2011
Keywords:
Benzene
Cyclohexene
Hydrogenation
Ru
© 2011 Elsevier B.V. All rights reserved.
Bentonite
1. Introduction
support, such as SBA-15, gave the higher yield of cyclohexene [8,9].
A colloidal ruthenium catalyst stabilized by silica was prepared
Cyclohexene with the active C C bond is commonly used as
the chemical intermediates for the synthesis of adipic acid, nylon
6, nylon 66 and other fine chemicals. Conventionally, the produc-
tion of cyclohexene involved dehydrogenation of cyclohexanol,
dehydrohalogenation of cyclohexane halide and dehydrogenation
of cyclohexane. But the efficiencies are usually poor for these
processes. Alternatively, cyclohexene can be produced by partial
hydrogenation of benzene [1–6]. The great interest in this new
route is intrigued for the merits of that (1) it is a green route to
produce cyclohexene compared to the traditional methods since it
consumes less hydrogen and needs less steps; (2) both the product
cyclohexene and cyclohexane are useful chemical materials, which
lowers the production cost. Therefore, the route for selective hydro-
genation of benzene to cyclohexene is superior over other routes
in terms of inexpensive starting feedstock, energy-saving, lower
amounts of undesirable products [7].
through microemulsion processing for selective hydrogenation of
benzene to cyclohexene. Much higher activity and selectivity was
obtained compared with its supported counterparts [10]. Fígoli and
coworkers studied the effect of the support Al2O3 and SiO2 on the
selective hydrogenation of benzene over ruthenium catalysts [11].
They found that more electron-deficient Ru species are present on
Al2O3 than on SiO2. The electronic state of Ru affects the selec-
tivity to cyclohexene. The properties of Ru/Al2O3 catalysts were
studied using different techniques by the same authors. Differ-
ences were found in the Ru species (Ru0, ruthenium oxides and
ruthenium oxychloride), metal dispersion and chlorine content in
the catalysts as well as in their catalytic activity and selectivity
according to the preparation procedure used [12]. The colloidal
RuB/Al2O3·xH2O catalyst exhibited high activity and selectivity
over the RuB/␥-Al2O3 catalyst in selective hydrogenation of ben-
zene to cyclohexene. The higher selectivity was attributed to the
existence of more structural water and surface hydroxyl groups on
the RuB/Al2O3·xH2O catalyst [13]. In addition, the cordierite mono-
lithic catalysts showed potential application in fix-bed reactor for
this reaction, which gave much higher selectivity than particulate
catalyst [5,14,15].
Since the 1970s, the research interests are increased year
by year. Among the literatures, the heterogeneous catalysts are
widely employed due to the easy separation after reaction. For
design of the heterogeneous catalysts, ruthenium metal is recorded
as the most active [4]. The catalyst supports are often sin-
gle or binary hydrophilic oxides. Ruthenium on the SiO2-based
Clays, as one of the SiO2-based material, are considered to
be a good support not only because of containing SiO2 but also
including several metal oxides since the metal oxides, even the
binary metal oxides, are also good supports for the partial hydro-
genation of benzene to cyclohexene. Bentonite is a layered clay
∗
Corresponding authors. Tel.: +86 10 62562821; fax: +86 10 62562821.
E-mail addresses: Jiangt@iccas.ac.cn (T. Jiang), Hanbx@iccas.ac.cn (B. Han).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.12.013

W. Wang et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 174–179
175
mineral in the aluminosilicate smectite family. Water is thus able
to enter into the interlayer to facilitate swelling and expanding the
layers. Its porosity can be further improved by exchanging with
the polymeric metal or organic cations [16]. The tunable surface
and structural properties make bentonite widely used in cataly-
sis such as hydrogenation and selective hydrogenation [17–20].
Furthermore, bentonite is widespread, easily available, low-cost
and environmentally friendly. However, the bentonite supported
ruthenium catalysts for the partial hydrogenation of benzene to
cyclohexene are rarely documented to the best of our knowledge.
Ruthenium over bentonite support is expected to have high activ-
ity considering the interlayer structure of bentonite giving the high
dispersion of ruthenium [21]. It is no doubt that exploration of
highly efficient, economical and simple catalysts for the selective
hydrogenation of benzene is of great importance, but is challenging.
In the present work, the partial hydrogenation of benzene to
cyclohexene was studied over the Ru/bentonite catalysts prepared
by an impregnation-hydrothermal method. The catalyst was char-
acterized by powder X-ray diffraction (XRD), transmission electron
spectroscopy (TEM), and X-ray photoelectron spectroscopy (XPS).
The catalyst showed the high activity for hydrogenation of benzene.
Both the addition of ZnSO4 and NaOH to the reaction system can
improve the selectivity to cyclohexene. Meanwhile, the influences
of reaction temperature, pressure, concentration of the additives,
and reaction time were studied.
Tianchen Electronic Company), and the temperature fluctuation of
the air bath was 0.1 ◦C. Hydrogen with desired pressure was fed
into the reactor after desired temperature was reached. The hydro-
genation reaction began by starting the agitation. The reaction was
monitored with the consuming of the hydrogen. The total uptake of
hydrogen which is related to the conversion was used to estimate
the conversion of benzene. When desired conversion was reached,
the reaction was stopped by quickly cooling down the reactor in
ice water and the reaction time was recorded. After reaction the
hydrogen was released slowly. The liquid products were extracted
with dichloromethane. The products were analyzed using a gas
chromatograph (Agilent 6820) equipped with a flame ionization
detector (FID) and a PEG-20M capillary column (0.25 mm in diam-
eter, 30 m in length). Identification of the products and reactant
was done using a GC–MS (SHIMADZU-QP2010) as well as by com-
paring the retention time of the standards in GC trace. Conversion
of benzene (C), selectivity (S) and yield (Y) of cyclohexene were
calculated according to the following equations:
mole of reacted benzene
C =
S =
Y =
× 100%
mole of initial benzene
mole of cyclohexene formed
mole of rewacted benzene
× 100%
× 100%
mole of cyclohexene formed
mole of initial benzene
2. Experimental
2.4. Characterization
2.1. Materials
The catalysts were characterized by transmission electron
microscopy (TEM), powder X-ray diffraction (XRD) and X-ray pho-
toelectron spectroscopy (XPS) techniques. The TEM observations
were carried out on JEOL JEM 1011 instrument with 100 kV. XRD
was performed on a X’PERT SW X-ray diffractometer operated at
30 kV and 100 mA with Cu K␣ radiation. XPS data were obtained
with an ESCALab220i-XL electron spectrometer from VG Scien-
tific using 300 W Mg K␣ radiation. The base pressure was about
3 × 10−9 mbar. The binding energies were referenced to the C 1s line
at 284.8 eV from adventitious carbon. FT-IR spectra were recorded
on Bruker Tensor 27 spectrometer with a resolution of 1 cm−1 and
32 scans. The thickness of the liquid film was 0.015 mm.
RuCl3·3H2O, ZnSO4·7H2O, NaOH, dichloromethane, cyclohex-
ane and benzene were analytical grade and purchased from Beijing
Chemical Reagent Company. Cyclohexene (99%) was from Alfa
Aesar. Bentonite was obtained from Shanghai Fourth Chemical
Reagent Factory. H2 (99.99%) was provided by Beijing Analytical
Instrument Company.
2.2. Catalyst preparation
The Ru catalyst supported on bentonite was prepared by
an impregnation-hydrothermal method, which was designated
as Ru/BEN (IP-HY). Typically, a desired amount of commercial
bentonite (500 mg) without any treatment was added to the
RuCl3·3H2O aqueous solution (0.038 g of RuCl3·3H2O, 30 mL of H2O)
under stirring for 30 min, and then the slurry was transferred to
a stainless steel autoclave and sealed. Afterward, the autoclave
underwent 120 ◦C for 10 h. The obtained precipitation was col-
lected on a ceramic filter and washed thoroughly with distilled
water. The sample was then dried in vacuum at 60 ◦C overnight. The
as-prepared Ru/BEN (IP-HY) samples were reduced in H2 (99.99%)
at 300 ◦C for an hour. The nominal Ru loading of the catalyst was
3.0 wt.%.
3. Results and discussions
3.1. Characterization
The TEM image of Ru/BEN (IP-HY) catalyst is shown in Fig. 1. It
can be seen that the ruthenium are well dispersed on the bentonite.
The XRD patterns of the catalyst and support are shown in Fig. 2.
There is no Ru or RuO2 peaks appeared in the pattern of Ru/BEN (IP-
HY) compared with that of bentonite. Only the intensity of the peaks
was attenuated after the Ru was loaded on the bentonite, indi-
cating that the Ru was highly dispersed on the bentonite support
[8], which was consistent with the result of TEM image. Moreover,
based on the XRD analysis, the basal spacing of the pristine ben-
tonite was 0.97 nm, while the spacing of Ru/bentonite was enlarged
to d001 = 1.43 nm due to the intercalation of Ru nanoparticles in the
interlayers of bentonite [19,21].
The electronic state of the Ru element in the catalyst was ana-
lyzed by XPS. As shown in Fig. 3, in spite of the interference of the
adventitious carbon, the XPS spectrum can be resolved fairly well
with two spin-orbit-split doublets from two chemically different
Ru entities and the C1s peak. The peaks at 280.9 eV (Ru 3d5/2) and
284.5 eV (Ru 3d3/2) can be assigned to metallic Ru, while the peaks
at 281.6 eV and 286.6 eV showed the presence of Ru oxide which
2.3. Activity test
The reactor and the procedures used for the hydrogenation were
similar to those described previously [22]. Typically, the hydro-
genation was carried out in a Teflon-lined stainless-steel reactor of
6 mL in capacity with a magnetic stirrer. The reactor was connected
to a hydrogen cylinder of the reaction pressure, so that hydrogen of
fixed pressure could be supplied continuously. In the experiment,
a desired amount of catalyst, benzene and ZnSO4 aqueous solu-
tion was introduced. Then the reactor was sealed and purged with
hydrogen to remove the air. After checking for leaks, the reactor was
then placed in an air bath at the desired temperature which was
controlled by a PID temperature controller (model SX/A-1, Beijing

176
W. Wang et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 174–179
Table 1
The results of benzene hydrogenation over different amounts of Ru/BEN (IP-HY).
Entry Catalyst (mg) Selectivity (%) Conversion (%) Yield (%) Time (min)
1
2
3
4
5
20
15
10
5.0
2.5
32.2
35.4
41.6
51.0
51.4
49.3
37.5
39.3
46.9
44.0
15.9
13.3
16.3
23.9
22.6
26
33
41
65
120
Reaction conditions: benzene, 1.0 mL; reaction temperature, 150 ◦C; H2 pressure,
4.0 MPa; ZnSO4 solution, 0.4 mol/L, 2.0 mL.
to 5.0 mg. Although the entries 4 and 5 had the similar selectivity,
the conversion of entry 4 was larger than that of entry 5, which
indicated that the yield of entry 4 was larger than that of entry 5.
Therefore, with the decreased amount of the catalysts, the selec-
tivity first increased and then decreased as well as the yield with
the threshold of catalyst was 5.0 mg. It is worth noting that the
molar ratio of substrate to ruthenium was more than 7500 in entry
4, which corresponded to the TOF of 1648 h−1 (TON = mol of cyclo-
hexene product/mol of Ru, TOF = TON/time). The TOF is higher than
that of the nanoscale ruthenium catalysts in ionic liquids for the
partial hydrogenation of benzene to cyclohexene [24] and the col-
loidal ruthenium catalyst stabilized by silica [10]. The high activity
of the Ru/bentonite was ascribed to the high dispersion, and mainly
due to the Ru particles in the interlayers of bentonite [21]. In addi-
tion, it can be found in Table 1 that the time of 26 min at a conversion
of 49.3% with 20 mg catalyst increased to 120 min at a conversion
of 44.0% with 2.5 mg catalyst, which indicated that at the high cat-
alyst load the high reaction rate was obtained, and vice versa. And
the similar behavior of the cyclohexene selectivity was found from
32.2% to 51.4% with the decrease of the catalyst load. Due to the
ZnSO4 concentration was kept constant, the decrease of the cata-
lyst load led to the increase of the ruthenium coverage by ZnSO4
resulting in the increased selectivity but the decreased reaction
rate, which was consistent with the literature [25].
Fig. 1. TEM image of Ru/BEN (IP-HY) catalyst.
Fig. 2. Wide-angle XRD patterns of (a) bentonite and (b) Ru/BEN (IP-HY) catalyst.
3.2.2. The effect of additives on the hydrogenation of benzene
indicated that the presence of a passivated Ru nanoparticles upon
exposure to air [23].
Addition of ZnSO4 aqueous solution to the reaction slurry has
shown to increase the selectivity of hydrogenation of benzene to
cyclohexene. Table 2 illustrates the influence of aqueous ZnSO4
solution on the yield and selectivity of cyclohexene. The selectiv-
ity was only 0.7% without water and the ZnSO4. When only water
was added in the hydrogenation process, the reaction proceeded
so fast that the conversion reached 58.1% after 17 min, and the
selectivity of cyclohexene was increased to 10.5%, indicating that
addition of water can greatly enhance the selectivity to cyclohex-
ene. Due to the hydrophilicity of the catalyst, the water can stay on
its surface to form a stagnant water film, which results in that the
3.2. Catalytic performance in selective hydrogenation of benzene
3.2.1. The effect of amount of the catalysts
The influence of catalyst amount on the reduction of benzene
was carried out in the range of 2.5–20 mg, while the amount of the
benzene, hydrogen pressure and the temperature were kept con-
stant. As shown in Table 1, the selectivity of cyclohexene increased
from 32.2% to 51.0% with a decrease in catalyst loading from 20
Fig. 3. XPS spectra of Ru/BEN (IP-HY) catalyst.

W. Wang et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 174–179
177
Table 2
Table 4
The results of benzene hydrogenation under different ZnSO4 concentration.
Hydrogenation of cyclohexene with additives.
Concentration (mol/L) Selectivity (%) Conversion (%) Yield (%) Time (min)
Entry
Additives
Concentration (mol/L)
Time (min)
Conversion (%)
Without additives
0
0.7
10.5
13.4
39.3
43.9
44.7
44.9
44.8
40.1
58.1
58.9
58.2
58.5
59.6
59.7
57.3
0.3
6.1
7.9
22.9
25.7
26.6
26.8
25.7
165
17
17
33
47
65
66
58
1
2
3
H2O
ZnSO4
NaOH
–
0.4
0.4
65
65
69
11.9
1.1
0.4
0.01
0.05
0.1
0.2
0.4
0.6
Reaction conditions: Ru/BEN (IP-HY) catalyst, 5.0 mg; cyclohexene, 1.0 mL; water,
1.0 mL; reaction temperature, 150 ◦C; H2 pressure 4.0 MPa.
cyclohexene to absorb on the catalyst surface and result in the
disfavor of hydrogenation of cyclohexene to cyclohexane.
Reaction conditions: Ru/BEN (IP-HY) catalyst, 5.0 mg; benzene, 1.0 mL; ZnSO4 solu-
tion, 1.0 mL; reaction temperature, 150 ◦C; H2 pressure 4.0 MPa.
To investigate the promotion effects of salt additives on the
selective hydrogenation of benzene, cyclohexene hydrogenation to
cyclohexane was studied under the same conditions as the hydro-
genation of benzene. As shown in Table 4, by adding ZnSO4 and
NaOH the hydrogenation of cyclohexene can be greatly suppressed.
It appeared that NaOH suppressed the hydrogenation of cyclohex-
ene more strongly than ZnSO4. Actually, the additives suppressed
the hydrogenation of benzene to cyclohexene as well as hydrogena-
tion of cyclohexene to cyclohexane. However, the hydrogenation of
cyclohexene to cyclohexane was more suppressed, therefore giving
the high selectivity [32].
formation rate of cyclohexene is higher than that of hydrogenation
of cyclohexene, and subsequently improve the selectivity because
benzene is more soluble than cyclohexene in water [25]. Because of
the different solubility of benzene and cyclohexene, the step mech-
anism is predominant since the hydrogenation of benzene is two
consecutive reactions and a parallel one [26]. Furthermore, water
competes with cyclohexene to adsorb on the surface, which inhibits
the re-adsorption of cyclohexene and thus the hydrogenation of
cyclohexene is suppressed [26].
The FT-IR was employed to study the effects of ZnSO4 and
NaOH. Aqueous solution of ZnSO4 (0.4 mol/L) or NaOH (0.4 mol/L)
were mixed separately with cyclohexene in the volume ratio of
1:1 and then stirred for an hour. Then the organic phase was used
to record the FT-IR spectra. Compared with the pure cyclohex-
ene, the interaction between the additives and cyclohexene were
not observed from the FT-IR spectra of cyclohexene treated with
ZnSO4 or NaOH aqueous solution, which implied that the inter-
action between the cyclohexene and the additives was weak in
this condition. Similar experiments were also conducted to inves-
tigate the interaction between benzene and ZnSO4 or NaOH and
the same results were obtained. Actually, Li et al. have theoreti-
When the concentration of ZnSO4 was higher than 0.05 mol/L,
the selectivity and yield of cyclohexene can be dramatically
increased. It is noted that with the increasing concentration of
ZnSO4, the selectivity and yield of cyclohexene greatly enhanced
with the maximum of 44.9% and 26.8%, respectively, being observed
at the ZnSO4 concentration of 0.4 mol/L. However, the hydro-
genation rate was decreased with the increase of the ZnSO4
concentration. As a matter of fact, various salts as additives have
been studied such as zinc, iron, cobalt, nickel, cadmium, gallium,
and indium etc. [4]. Among them, the strong acid salts of zinc, in
which zinc sulphate was widely used, were particularly effective
[2,27]. The zinc sulphate can be chemisorbed by the ruthenium
catalyst to enhance the hydrophilicity and light poison the catalyst
to increase the selectivity of cyclohexene [27].
The NaOH as the additive for the partial hydrogenation of ben-
zene has been studied with metal oxides as the support of catalysts.
Chen and Hu found that NaOH aqueous solution was prerequisite
for ZnO based binary oxides [28–30]. Liu et al. studied the effects of
NaOH on the ZnO supported ruthenium catalyst [31]. In this work,
the effect of NaOH on the performance of hydrogenation of Ru/BEN
(IP-HY) catalyst was investigated, as shown in Table 3. It is inter-
esting that the NaOH can greatly improve the selectivity and gave
a yield of cyclohexene (entry 3) similar to that in the presence of
ZnSO4, which was rarely reported with the SiO2 based support. In
the presence of NaOH, the rate of hydrogenation decreased with the
increasing amount of NaOH when the amount of catalyst was kept
constant (entries 1 and 2, 3 and 4, 5 and 6). Ronchin and Toniolo
found that the rate of cyclohexene hydrogenation was depressed
whenever the unreduced catalyst was treated with NaOH [26]. It
can be proposed that the NaOH and water synergistically enhance
the selectivity. NaOH improves the hydrophilicity of Ru cata-
lyst while water and hydroxide anion combine to compete with
2+
cally proved that the formation of M(H2O)n(cyclohexene)m and
M(H2O)n(benzene)m2+complexes from Zn2+ in water is energeti-
cally unfavorable [33]. Therefore the additives may not enhance the
selectivity to cyclohexene through their interactions with cyclo-
hexene or benzene, and probably through their interactions with
the catalyst. Struijk et al. have proved that the additives covered the
ruthenium not only to afford hydrophilicity of the catalyst but also
to temper the intrinsic activity of the catalyst to lower the adsorp-
tion enthalpy of benzene and of cyclohexene [27]. Furthermore,
water afforded the stagnant water film around the catalyst which
brought the difference in solubility of benzene and cyclohexene.
All of these effects combined together resulted in the enhanced
selectivity.
3.2.3. The effect of reaction temperature
The effects of temperature on cyclohexene yield and benzene
conversion were investigated in the temperature range from 120
to 175 ◦C over the Ru/BEN (IP-HY) catalyst. As shown in Table 5,
the yields of cyclohexene increased firstly, and slightly decreased
at the temperatures higher than 160 ◦C when the temperature
Table 3
The results of benzene hydrogenation in NaOH solution.
Entry
Catalyst (mg)
Solvent (mL)
Concentration (mol/L)
Selectivity (%)
Conversion (%)
Yield (%)
Time (min)
1
2
3
4
5
6
20
20
10
10
5
2.0
1.0
1.0
1.0
1.0
1.0
0.4
0.4
0.4
0.1
0.4
0.1
51.4
30.0
45.5
25.9
59.3
39.3
43.1
55.0
56.6
63.9
37.3
54.8
22.1
16.5
25.8
16.5
22.1
21.5
38
31
105
32
418
86
5
Reaction conditions: Ru/BEN (IP-HY) catalyst; benzene, 1.0 mL; NaOH solution; reaction temperature, 150 ◦C; H2 pressure, 4.0 MPa.

178
W. Wang et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 174–179
Table 5
The results of benzene hydrogenation under different temperatures.
Temperature (◦C)
Selectivity (%)
Conversion (%)
Yield (%)
Time (min)
170
165
160
155
150
145
140
130
120
45.5
46.9
45.9
44.3
44.9
44.5
41.2
41.3
29.6
56.6
54.1
59.2
59.9
59.7
59.2
60.3
52.7
53.4
25.8
25.4
27.2
26.5
26.8
26.3
24.8
21.8
15.8
66
66
66
66
66
66
66
66
114
Reaction conditions: Ru/BEN (IP-HY) catalyst, 5.0 mg; benzene, 1.0 mL; H2 pressure,
4.0 MPa; ZnSO4 solution, 0.4 mol/L, 1.0 mL.
Table 6
Fig. 4. The reaction course of benzene hydrogenation. Reaction conditions: Ru/BEN
(IP-HY) catalyst, 5.0 mg; benzene, 1.0 mL; ZnSO4 solution, 1.0 mL, 0.4 mol/L; reaction
temperature, 150 ◦C; H2 pressure, 4.0 MPa.
The results of benzene hydrogenation under different pressures.
Pressure (MPa)
Selectivity (%)
Conversion (%)
Yield (%)
Time (min)
8
7
6
5
4
3
2
43.7
45.2
46.6
46.0
44.9
39.8
11.4
59.5
61.5
58.8
58.0
59.7
58.4
50.7
26.0
27.8
27.4
26.7
26.8
23.2
5.8
39
66
47
60
66
which indicated the formation and hydrogenation of cyclohexene
temporarily reached balance.
4. Conclusions
91
283
The Ru/BEN (IP-HY) catalyst prepared by impregnation-
hydrothermal method exhibited high activity for benzene hydro-
genation with good selectivity to cyclohexene. The presence of
an additive is essential for getting good cyclohexene yields. Both
ZnSO4 and NaOH can suppress the hydrogenation of cyclohexene
and improve the selectivity and yield of cyclohexene. Water played
an important role in this system to enhance the selectivity. This
catalyst is easily prepared, cheap and environmentally friendly.
Reaction conditions: Ru/BEN (IP-HY) catalyst, 5.0 mg; benzene, 1.0 mL; ZnSO4 solu-
tion, 0.4 mol/L, 1.0 mL; reaction temperature, 150 ◦C.
was increased. With the increasing temperature, the desorption of
cyclohexene is promoted and the hydrogen coverage on the surface
of catalyst is lower, which reduces the further hydrogenation of cyl-
cohexene [25]. However, the solubility of cyclohexene and benzene
are increased in the stagnant water film around the catalysts when
elevating the reaction temperature [34], which lead to the enhance-
ment of the cyclohexene hydrogenation. The counteracting of the
opposite effects gives the maximum yield of cyclohexene.
Acknowledgements
The authors are grateful to National Natural Science Founda-
tion of China (20932002), Ministry of Science and Technology
of China (2011CB808600) and Chinese Academy of Sciences
(KJCX2.YW.H30.02) for financial supports.
3.2.4. The effect of the pressure
The influences of the hydrogen pressure on benzene conversion
and on cyclohexene yield were studied by a series of experiments
under the varying pressure from 2 MPa to 8 MPa. The relationship
between the yield and hydrogen pressure is presented in Table 6.
The yield of cyclohexene increased with elevating pressure from
2 MPa to 7 MPa, and then decreased slightly when the pressure
increased to 8 MPa. The result indicates that there is an optimum
pressure which gives the highest yield of cyclohexene. At low pres-
sures, the step mechanism is predominant [10]. Furthermore, the
hydrogenation of cyclohexene to cyclohexane is less dependent on
the pressure than the hydrogenation of benzene to cyclohexene
[35], which gave the high yield of cyclohexene. In addition, the
hydrogen coverage is increased with the increasing hydrogen of
pressure, which leads to the gradual increase of the hydrogenation
rate. However, at high pressures, the rate decreases for that the
adsorption of benzene is the relatively slow step in the reactants
due to the excessive coverage of hydrogen on the surface [28]. The
hydrogenation of cyclohexene to cyclohexane becomes easier at
high pressures [10,36]. As a result, the yield of cyclohexene declined
after a certain high hydrogen pressure was reached, corresponding
to 7 MPa in the pressure range studied.
References
[1] S. Liu, Z. Liu, Z. Wang, Y. Wu, P. Yuan, J. Chem. Eng. 139 (2008) 157–164.
[2] H. Nagahara, M. Ono, M. Konishi, Y. Fukuoka, Appl. Surf. Sci. 121–122 (1997)
448–451.
[3] S. Hu, Y. Chen, J. Chin. Inst. Chem. Eng. 29 (1998) 387–396.
[4] P. Kluson, L. Cerveny, Appl. Catal. A: Gen. 128 (1995) 13–31.
[5] Y. Zhao, J. Zhou, J. Zhang, S. Wang, J. Mol. Catal. A: Chem. 309 (2009) 35–39.
[6] H. Sun, W. Guo, X. Zhou, Z. Chen, Z. Liu, S. Liu, Chin. J. Catal. 32 (2011) 1–16.
[7] P.T. Suryawanshi, V.V. Mahajani, J. Chem. Technol. Biotechnol. 69 (1997)
154–160.
[8] J. Liu, L. Zhu, Y. Pei, J. Zhuang, H. Li, H.X. Li, M. Qiao, K.N. Fan, Appl. Catal. A: Gen.
353 (2009) 282–287.
[9] J. Bu, J.L. Liu, X.Y. Chen, J.H. Zhuang, S.R. Yan, M.H. Qiao, H.Y. He, K.N. Fan, Catal.
Commun. 9 (2008) 2612–2615.
[10] J. Ning, J. Xu, J. Liu, F. Lu, Catal. Lett. 109 (2006) 175–180.
[11] V. Mazzieri, N. Fígoli, F. Pascual, P. L’Argentière, Catal. Lett. 102 (2005) 79–82.
[12] V. Mazzieri, F. Coloma-Pascual, A. Arcoya, P.C. L’Argentière, N.S. Fígoli, Appl.
Surf. Sci. 210 (2003) 222–230.
[13] J. Wang, P. Guo, S. Yan, M. Qiao, H. Li, K. Fan, J. Mol. Catal. A: Chem. 222 (2004)
229–234.
[14] Y. Zhao, J. Zhou, J. Zhang, S. Wang, Catal. Lett. 131 (2009) 597–605.
[15] Y. Zhao, J. Zhou, J. Zhang, D. Li, S. Wang, Ind. Eng. Chem. Res. 47 (2008)
4641–4647.
[16] C.H. Zhou, Appl. Clay Sci. 53 (2011) 87–96.
[17] R. Zhang, F. Li, N. Zhang, Q. Shi, Appl. Catal. A: Gen. 239 (2003) 17–23.
[18] M. Mazzel, W. Marconi, M. Riocci, J. Mol. Catal. 9 (1980) 381–387.
[19] D. Divakar, D. Manikandan, G. Kalidoss, T. Sivakumar, Catal. Lett. 125 (2008)
277–282.
[20] D. Divakar, D. Manikandan, T. Sivakumar, J. Chem. Technol. Biotechnol. 83
(2008) 1472–1478.
[21] S. Miao, Z. Liu, B. Han, J. Huang, Z. Sun, J. Zhang, T. Jiang, Angew. Chem. Int. Ed.
45 (2006) 266–269.
[22] H. Liu, T. Jiang, B. Han, S. Liang, Y. Zhou, Science 326 (2009) 1250–1252.
3.2.5. The effect of reaction time
The course of hydrogenation of benzene over Ru/BEN (IP-HY)
catalyst in ZnSO4 solution is presented in Fig. 4. With the increasing
of the conversion, the selectivity decreased while the yield of cyclo-
hexene first increased then declined gradually. In the reaction time
range of 66–85 min, the yield of cyclohexene was almost constant,

W. Wang et al. / Journal of Molecular Catalysis A: Chemical 355 (2012) 174–179
179
[23] H. Dai, X. Kang, P. Wang, Int. J. Hydrogen Energy 35 (2010) 10317–10323.
[24] E.T. Silveira, A.P. Umpierre, L.M. Rossi, G. Machado, J. Morais, G.V. Soares,
I.J.R. Baumvol, S.R. Teixeira, P.F.P. Fichtner, J. Dupont, Chem. Eur. J. 10 (2004)
3734–3740.
[25] J. Struijk, M. d’Angremond, W. Regt, J. Scholten, Appl. Catal. A: Gen. 83 (1992)
263–295.
[26] L. Ronchin, L. Toniolo, Appl. Catal. A: Gen. 208 (2001) 77–89.
[27] J. Struijk, R. Moene, T. Van der Kamp, J. Scholten, Appl. Catal. A: Gen. 89 (1992)
77–102.
[28] S. Hu, Y. Chen, Ind. Eng. Chem. Res. 36 (1997) 5153–5159.
[29] S. Hu, Y. Chen, J. Chem. Technol. Biotechnol. 76 (2001) 954–958.
[30] S. Hu, Y. Chen, Ind. Eng. Chem. Res. 40 (2001) 3127–3132.
[31] H. Liu, S. Liang, W. Wang, T. Jiang, B. Han, J. Mol. Catal. A: Chem. 341 (2011)
35–41.
[32] P. Yuan, B. Wang, Y. Ma, H. He, Z. Cheng, W. Yuan, J. Mol. Catal. A: Chem. 309
(2009) 124–130.
[33] Z. Li, J. Liu, M. Qiao, K. Fan, Mol. Phys. 107 (2009) 1271–1282.
[34] J. Wang, Y. Wang, S. Xie, M. Qiao, H. Li, K. Fan, Appl. Catal. A: Gen. 272 (2004)
29–36.
[35] C.U.I. Odenbrand, S.T. Lundin, J. Chem. Technol. Biotechnol. 30 (1980) 677–687.
[36] H. Liu, T. Jiang, B. Han, S. Liang, W. Wang, T. Wu, G. Yang, Green Chem. 13 (2011)
1106–1109.