Selective hydrogenation of benzene to cyclohexene over a Ru-Zn catalyst with diethanolamine as an additive

Article abstract of DOI:10.1016/S1872-2067(11)60351-3

A Ru-Zn catalyst was prepared by co-precipitation, and the effect of adding diethanolamine on benzene selective hydrogenation to cyclohexene was investigated. The catalyst was characterized by N₂ physisorption, transmission electron microscopy, X-ray diffraction, X-ray fluorescence, Fourier transform infrared spectroscopy, and temperature-programmed reduction. Diethanolamine reacted with ZnSO₄ in the slurry to form (Zn(OH) ₂)₃(ZnSO₄)(H₂O)₃ and diethanolamine sulfate. The amount of (Zn(OH)₂)₃(ZnSO ₄)(H₂O)₃ adsorbed on the catalyst surface increased with increasing diethanolamine amount. The synergism of (Zn(OH) ₂)₃(ZnSO₄)(H₂O)₃ and diethanolamine sulfate improved the cyclohexene selectivity of the Ru-Zn(4.9%) catalyst. With a diethanolamine dosage of 0.3 g, (Zn(OH)₂) ₃(ZnSO₄)(H₂O)₃ was highly dispersed on catalyst surface and the sample after hydrogenation was characterized. This catalyst exhibited the best performance with a cyclohexene selectivity and yield of 75.5% and 63.6%, respectively, at the benzene conversion of 84.3% in the third run. Moreover, the benzene conversion and cyclohexene selectivity were stable above 75% and cyclohexene yield was above 58% in the fourth run.

Full text of DOI:10.1016/S1872-2067(11)60351-3

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 4, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 610–620.
ARTICLE
Selective Hydrogenation of Benzene to Cyclohexene over a
Ru-Zn catalyst with Diethanolamine as an Additive
SUN Haijie, PAN Yajie, WANG Hongxia, DONG Yingying, LIU Zhongyi*, LIU Shouchang
Department of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan, China
Abstract: A Ru-Zn catalyst was prepared by co-precipitation, and the effect of adding diethanolamine on benzene selective hydrogenation
to cyclohexene was investigated. The catalyst was characterized by N
X-ray fluorescence, Fourier transform infrared spectroscopy, and temperature-programmed reduction. Diethanolamine reacted with ZnSO
in the slurry to form (Zn(OH) (ZnSO )(H O) and diethanolamine sulfate. The amount of (Zn(OH) (ZnSO )(H O) adsorbed on the
catalyst surface increased with increasing diethanolamine amount. The synergism of (Zn(OH) (ZnSO )(H O) and diethanolamine sulfate
improved the cyclohexene selectivity of the Ru-Zn(4.9%) catalyst. With a diethanolamine dosage of 0.3 g, (Zn(OH) (ZnSO )(H O) was
2
physisorption, transmission electron microscopy, X-ray diffraction,
4
)
2 3
4
2
3
)
2 3
4
2
3
2
)
3
4
2
3
2
)
3
4
2
3
highly dispersed on catalyst surface and the sample after hydrogenation was characterized. This catalyst exhibited the best performance
with a cyclohexene selectivity and yield of 75.5% and 63.6%, respectively, at the benzene conversion of 84.3% in the third run. Moreover,
the benzene conversion and cyclohexene selectivity were stable above 75% and cyclohexene yield was above 58% in the fourth run.
Key words: diethanolamine; ruthenium; zinc; benzene; selective hydrogenation; cyclohexene
The production of nylon-6 and nylon-66 from benzene and
cyclohexene has attracted much attention for its atomic
economy, mild reaction condition, and energy saving, and it is
environmentally benign and is superior to the use of benzene
and cyclohexane [1–3]. However, thermodynamics make it
difficult to get cyclohexene from benzene hydrogenation be-
cause the standard free energy change for cyclohexene forma-
tion from benzene hydrogenation is –23 kJ/mol, while that for
cyclohexane formation is –98 kJ/mol [1]. Hence, developing a
catalyst with high selectivity to cyclohexene from benzene
selective hydrogenation is the key in this technology.
The hydrophilic modification of the catalyst surface with
promoters or additives is an effective method to improve the
cyclohexene selectivity of a Ru-based catalyst. In 1989, Asahi
Chemical Industry Co. industrialized the process for producing
cyclohexene from benzene selective hydrogenation by em-
ploying an unsupported Ru-Zn catalyst and obtained a cyclo-
hexene yield of 32% at a benzene conversion of 40% [4,5]. Liu
et al. [6] prepared a Ce-promoted Ru/SBA-15 catalyst by a
Ce/Ru molar ratio of 0.25 in the presence of ZnSO
(0.42
4
mol/L). They achieved a cyclohexene yield of 57% and a se-
lectivity of 69% at the benzene conversion of 82% over a
Ru-La/SBA catalyst with CdSO and ZnSO as co-modifiers
4
4
[7]. Liu et al. [8] carried out the reaction over a Ru-Cu/ZnO
catalyst with the Ru/Cu ratio of 10:1 in 2 ml of 0.3 mol/L
NaOH solution and obtained a maximum cyclohexene yield of
49.4% at a benzene conversion of 72.3% and a cyclohexene
selectivity of 68.3%. Although the cyclohexene yields over
these promising catalysts were higher than that of the industrial
catalyst, the corresponding selectivity for cyclohexene was
lower than 70%, while a selectivity for cyclohexene close to
80% is required for industrial use to reduce the difficulties in
subsequent product (including benzene, cyclohexene, and
cyclohexane) separation.
In a previous work, we prepared a monolayer dispersed
Ru-Zn catalyst and achieved a cyclohexene selectivity of
69.8% at a benzene conversion of 84.8% at 10 min reaction
time [2]. In addition, we doped the Ru catalyst with Mn and Zn
as co-promoters and obtained a cyclohexene selectivity of
81.1% at a benzene conversion of 60.2% at 20 min reaction
“
two solvents” impregnation method and achieved a cyclo-
hexene yield of 53.8% over the Ru-Ce/SBA-15 catalyst with a
Received 21 October 2011. Accepted 21 November 2011.
*
Corresponding author. Tel: +86-371-67783384; E-mail: liuzhongyi@zzu.edu.cn
This work was supported by the Innovation Found for Technology Based Firms of China (02C26214100384, 10C26214104505).
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60351-3

SUN Haijie et al. / Chinese Journal of Catalysis, 2012, 33: 610–620
time over the Ru-Mn-Zn catalyst with the Mn/Zn ratio of 0.3
and the addition of 0.5 g NaOH. A maximum cyclohexene
yield of 59.9% was also achieved at 20 min [10]. We also
briefly reported cyclohexene yields of 61.4% and 58.9%, re-
spectively, with PEG-20000 and diethanolamine as additives
on a Ru-Zn catalyst [3]. In this work, the influence of the di-
ethanolamine doses on the performance of the Ru-Zn catalyst
for benzene selective hydrogenation to cyclohexene was in-
detector (TCD). The flow rate of 5% H
ml/min and the heating rate was 10 C/min.
2
-95% N was 30
2
o
1.3 Catalyst test
The selective hydrogenation of benzene was performed in a
1 L autoclave lined the Hastelloy. The autoclave was charged
with 280 ml H O containing 1.96 g catalyst, 9.8 g ZrO , 49.2 g
2
2
vestigated in the presence of ZnSO . The catalyst before and
ZnSO ·7H O, and a desired amount of diethanolamine. This
4
4
2
after hydrogenation was characterized and the role of dietha-
nolamine is discussed.
was heated at H pressure of 5 MPa and stirring rate of 800
2
r/min. Benzene (140 ml) was fed in and the stirring rate was
increased to 1400 r/min to exclude diffusional effects when the
o
1
Experimental
temperature reached 150 C. The reaction products were
monitored by taking a small amount of the reaction mixture
every 5 min for analysis using a GC-1690 gas chromatograph
with an FID detector, which purchased from Hangzhou Kexiao
Instrument Co. Benzene conversion and cyclohexene selectiv-
ity were calculated based on the product concentration calcu-
lated using corrected peak area normalization. At the end of the
reaction, the organic phase was removed using a separator
funnel. The slurry containing the mixture of the catalyst and
1
.1 Catalyst preparation
The Ru-Zn catalysts were prepared according to the proce-
dure in the literature [11]. The required amounts of RuCl
and ZnSO ·7H O were dissolved in 200 ml H O with agitation.
To the stirred solution, 200 ml of 30% NaOH solution was
added rapidly and the resulting mixture was agitated for an
additional 4 h at 80 C. The mixture was left to stand and the
3
·H
2
O
4
2
2
o
ZrO was reused using the above operation without any addi-
2
black precipitate obtained was washed three times with an
aqueous solution of 5% NaOH after the supernatant had been
removed by decantation. This black precipitate was dispersed
in 400 ml of 5% NaOH solution and charged into a 1 L auto-
clave lined with Teflon. Hydrogen was introduced into the
autoclave to a pressure of 5 MPa and the reduction was con-
tion. After the reaction, the organic liquid was removed and the
solid sample was washed for characterization. The samples
after reaction and recycle were denoted Ru-Zn(x)-y and
Ru-Zn(x)-R, respectively, where y and R represent the mass of
diethanolamine and recycle time, respectively.
o
ducted at 150 C and 800 r/min stirring for 3 h. The reaction
2 Results and discussion
mixture was cooled and the black powder obtained was washed
three times with 5% NaOH and then with water until neutral,
and the desired Ru-Zn catalyst was obtained. The amounts of
ZnSO ·7H O were adjusted to give the desired Zn loadings.
2.1 Catalyst characterization
4
2
2.1.1 N physisorption
2
The catalysts with different Zn loadings were denoted
Ru-Zn(x), where x is the Zn loading determined by atomic
absorption spectroscopy (AAS).
Table 1 shows the textural properties of ZrO2, Ru-Zn cata-
lyst, and the Ru-Zn(4.9%) catalyst with different diethanola-
mine dosages as an additive after hydrogenation. The
Ru-Zn(4.9%) catalyst has a specific surface area of 62 m2/g,
1
.2 Catalyst characterization
Table
1
2
Textural properties of ZrO , Ru-Zn catalyst and the
AAS for determining the Zn loading was performed on a
Perkin Elmer AAnalyst 300 instrument operated at ꢀ = 213.9
Ru-Zn(4.9%) catalyst with different diethanolamine dosages as an addi-
tive after hydrogenation
nm and slit width = 0.20 nm. N physisorption (BET) was
2
2
3
Sample
ZrO
A/(m /g)
d/nm
15.99
5.74
V/(cm /g)
determined on a Quantachrome Nova 100e apparatus. Trans-
mission electron microscopy (TEM) were carried out on a
JEOL JEM-2100 instrument. X-ray diffraction (XRD) patterns
were acquired on a PANalytcal XꢀPert PRO instrument using
2
34
62
37
35
36
37
42
0.130
0.089
0.179
0.092
0.159
0.124
0.168
Ru-Zn(4.9%)
Ru-Zn(4.9%)-0
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.4
Ru-Zn(4.9%)-R
19.88
10.58
17.78
13.26
15.74
o
o
Cu K (ꢀ = 0.1541 nm) in a scan range of 5 –90 at a scan step
ꢁ
o
of 0.03 . The elemental analysis was measured by X-ray fluo-
rescence (XRF) on a Bruk S4 Pioneer instrument. Fourier
transform infrared (FT-IR) spectra were recorded using a KBr
The x in Ru-Zn(x) catalyst represents the Zn content determined by
atomic absorption spectroscopy; Ru-Zn(x)-y is the Ru-Zn(x) catalyst
pellet on a Thero Nicolet Nexus-470 spectrophotometer. H2
temperature-programmed reduction (H -TPR) was carried out
sample after hydrogenation and
y is the diethanolamine dosage;
2
on a homemade microreactor with a thermal conductivity
Ru-Zn(4.9)-R is Ru-Zn(4.9%) catalyst sample after 5 recycles.

SUN Haijie et al. / Chinese Journal of Catalysis, 2012, 33: 610–620
3
pore diameter of 5.74 nm, and pore volume of 0.089 cm /g. The
specific surface area, pore diameter, and pore volume of the
samples after reaction and recycles were close to those of ZrO2,
which may be related to the dosage of ZrO (mass ratio of ZrO
dispersion effect of ZrO .
2
2.1.3 XRD results
2
2
and catalyst was 5:1).
.1.2 TEM results
Figure 1 gives the TEM images of the Ru-Zn(4.9%) catalyst
and the samples after reaction and recycles using 0.3 g di-
ethanolamine as an additive, and the crystallite size distribution
of the corresponding catalyst. From Fig. 1, the crystallite size
of the Ru-Zn(4.9%) catalyst was 3.5 nm. Figure 1 shows that
the catalyst particles were separated and isolated. This suggests
Figure 2 shows XRD patterns of the Ru-Zn(4.9%) catalyst
and the catalyst after hydrogenation with different dietha-
o
2
nolamine dosages as additives. The reflections at 2ꢂ = 38.4 ,
o
o
o
o
o
4
2.4 , 43.9 , 58.3 , 69.3 , and 78.3 were assigned to metallic
Ru (JCPDS 01-070-0274). Sun et al. [2] has used XRD and
XPS to confirm that most of the Zn species in Ru-Zn catalysts
exist as ZnO. Part of the Zn in the catalyst was present in the
form of metallic Zn. However, no ZnO or metallic Zn diffrac-
tions were detected in the XRD patterns, indicating their high
dispersion and that their small crystallite size was smaller than
the detectable limit of the instrument. When the diethanola-
mine dosage was increased from 0 to 0.3 g, besides the weak
that a suitable amount of ZrO can remarkably suppress ag-
2
glomeration that could have occurred due to the collision of
o
different particles under high agitation. The added ZrO also
diffraction at 2ꢂ = 43.9 from the Ru-Zn(4.9%) catalyst after
2
reduced catalyst adhesion to the metallic surface of the reactor
hydrogenation due to metallic Ru (JCPDS 01-070-0274), the
other diffractions corresponded to the monoclinic phase of
[
12]. In addition, a suitable amount of ZrO improved the
2
activity and selectivity of the catalyst [13]. As can be seen from
Fig. 1, the crystallite size of the catalyst after the first hydro-
genation run was 3.5 nm, which agrees with that of the fresh
catalyst. The crystallite size of the catalyst after five recycles
was slightly increased to 3.8 nm. The main component of the
Ru-Zn catalyst is metallic Ru, which is difficult to disperse and
prone to agglomerate to form bigger particles. The very slight
increase of the catalyst size after five recycles confirmed the
ZrO (JCPDS 00-024-1165). When the diethanolamine dosage
2
was 0.4 g, in addition to the diffractions of metallic Ru and
o
ZrO , the diffraction at 2ꢂ = 9.8 from the Ru-Zn(4.9%) cata-
2
lyst after hydrogenation was attributed to the
(Zn(OH) ) (ZnSO )(H O) phase (JCPDS 01-078-0247). This
2
3
4
2
3
revealed that the diethanolamine added reacted with ZnSO in
4
the slurry to form an insoluble (Zn(OH) ) (ZnSO )(H O) salt
2
3
4
2
3
and diethanolamine sulfate. This is shown as reactions (1) and
Ru-Zn(4.9%)-0.3-R
ZrO2
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)
ZrO2
ZrO2
1
0 nm
10 nm
1
0 nm
1
2
0
8
6
4
2
0
Ru-Zn(4.9%)
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.3-R
1
2.0
2.5
3.0
3.5
4.0
4.5
3.0
3.5
4.0
4.5
5.0
3.0
3.5
4.0
4.5
5.0
5.5
Diameter of crystallite (nm)
Diameter of crystallite (nm)
Diameter of crystallite (nm)
Fig. 1. TEM images and the crystallite size distribution of different catalysts. (a, b) Ru-Zn(4.9%); (c, d) Ru-Zn(4.9%)-0.3; (e, f) Ru-Zn(4.9%)-0.3-R.

SUN Haijie et al. / Chinese Journal of Catalysis, 2012, 33: 610–620
Table 2 Contents of metal elements in the catalyst samples after the
hydrogenation with different diethanolamine dosages as an additive
(Zn(OH) ) (ZnSO )(H O)
2 3 4 2 3
Ru
Contents (%)
Sample
Ru
Zn
Zr
Hf
Ru-Zn(8.4%)-0.3
ZrO
2
—
—
94.65
76.44
76.47
74.90
74.25
82.81
73.98
5.35
3.51
3.25
3.26
2.98
3.45
2.47
Ru-Zn(4.9%)-R
Ru-Zn(4.9%)-0.4
Ru-Zn(4.9%)-0
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.4
Ru-Zn(4.9%)-R
Ru-Zn(8.4%)-0.3
17.15
16.67
17.28
16.62
9.32
2.91
3.60
4.56
6.15
3.42
6.23
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0
17.32
Ru-Zn(4.9%)
different diethanolamine dosages as additives. The sample
after hydrogenation mainly contains four metallic elements,
Ru, Zn, Zr and Hf. Hf comes from the ZrO sample that con-
10
20
30
40
50
60
70
80
2
o
2
ꢀ/( )
tains HfO . The amount of Zn in the Ru-Zn(4.9%) catalyst after
2
hydrogenation increased with increasing diethanolamine dos-
ages. XRD confirmed that diethanolamine reacted with ZnSO4
in the slurry to form an insoluble (Zn(OH) ) (ZnSO )(H O)
Fig. 2. XRD patterns of the Ru-Zn(4.9%) catalyst and the catalyst sam-
ples with different diethanolamine dosages as an additive after the hy-
drogenation.
2
3
4
2
3
salt that was easily adsorbed on the catalyst surface. This re-
sulted in the increase of the Zn content in the samples. It is to be
(
2):
+
noted that the (Zn(OH)
2
)
3
(ZnSO
4
)(H
2
O) salt was formed in the
3
H
HO
OH
HO
OH
+
H O
+
OH
-
(1)
(2)
N
H
2
hydrogenation process and so can only be dispersed on the
catalyst surface. It was found that both Ru and Zn contents
decreased after five recycles, which indicated the loss of
catalyst in the recycles. This was consistent with the XRD
results. The Zn contents in the Ru-Zn(4.9%) catalyst with 0.4 g
diethanolamine and the Ru-Zn(8.4%) catalyst with 0.3 g di-
ethanolamine after hydrogenation were 6.15% and 6.23%,
respectively, which showed the high content of the
N
H
6
OH-
+
4Zn2+ + SO42- + 3H O
(Zn(OH) ) (ZnSO )(H O)
2 3 4 2 3
2
This also indicated that the (Zn(OH) ) (ZnSO )(H O) salt
2
3
4
2
3
was highly dispersed on the Ru-Zn(4.9%) catalyst and ZrO2
particles when the dosage of diethanolamine was 0.3 g. It was
found that the diffraction at 2ꢂ = 43.9 corresponding to me-
o
tallic Ru was remarkably weakened after five recycles, indi-
cating a loss of catalyst in the oil phase when this phase was
(Zn(OH)
2
)
3
(ZnSO
4
)(H
2
O) salt on the catalyst surface. This
3
was in agreement with the XRD results.
separated from the catalyst during the recycling. It was ob-
o
served that the diffraction at 2ꢂ
=
9.8 from the
2.1.5 FT-IR results
(
Zn(OH) ) (ZnSO )(H O) phase also appeared with the
2
3
4
2
3
Ru-Zn(8.4%) catalyst after hydrogenation when the dosage of
diethanolamine was 0.3 g. It was reported that part of the Zn
species in the Ru-Zn(8.6%) catalyst existed as ZnO and ZnO
Figure 3 shows the FT-IR spectra of the Ru-Zn(4.9%) cata-
Ru-Zn(4.9%)-0.4
Ru-Zn(4.9%)-0.3
can react with ZnSO in the slurry to form an insoluble
4
ZnSO ·3Zn(OH) ·7H O salt (JCPDS 00-009-0204), which can
4
2
2
be dispersed on catalyst surface to form a monolayer [2]. Di-
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0
ethanolamine also can react with ZnSO in the slurry to form a
4
(
Zn(OH) ) (ZnSO )(H O) salt, with properties close to those
2
3
4
2
3
of the ZnSO ·3Zn(OH) ·7H O salt since their difference is
4
2
2
Diethanolamine
only four water molecules of crystallization. Thus, with the
addition of diethanolamine, the (Zn(OH) ) (ZnSO )(H O)
2
3
4
2
3
formed can constitute a phase with the ZnSO ·3Zn(OH) ·7H O
4
2
2
salt on the Ru-Zn(8.4%) catalyst surface.
.1.4 XRF results
4
000 3500 3000 2500 2000 ꢁ11500 1000 500
2
Wavenumber (cm )
Fig. 3. FT-IR spectra of the Ru-Zn(4.9%) catalyst with different di-
ethanolamine dosages as an additive after the hydrogenation.
Table 2 gives the amounts of the metallic elements of ZrO
and the Ru-Zn catalysts before and after hydrogenation with
2

SUN Haijie et al. / Chinese Journal of Catalysis, 2012, 33: 610–620
fect. Therefore, with increasing dithanolamine dosage, the
catalyst particles after hydrogenation became smaller, which
led to the reduction peak shifting to lower temperatures.
However, when the dithanolamine dosage was increased to 0.4
g, too much (Zn(OH) ) (ZnSO )(H O) salt was formed and
Ru-Zn(4.9%)-0.4
2
3
4
2
3
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.2
deposited on the catalyst surface, which made the reduction of
RuO more difficult. It is also found that the temperatures for
x
the full reduction of the samples were all lower than the reac-
o
tion temperature of 150 C, implying the existence of metallic
Ru during the reaction.
Ru-Zn(4.9%)-0
2
2
.2 Catalytic performance
0
50
100
Temperature ( C)
150
200
o
.2.1 Effect of dithanolamine dosages on catalytic activity
Figure 5 shows the activity of the Ru-Zn(4.9%) catalyst with
Fig. 4. TPR profiles of the Ru-Zn(4.9%) catalyst after the hydrogena-
tion with different diethanolamine dosages as an additive.
different dithanolamine dosages as additives. With increasing
dithanolamine dosage from 0 to 0.3 g, the activity of the cata-
lyst decreased, the cyclohexene selectivity increased for the
same benzene conversion, and the maximum yield of cyclo-
hexene increased to 59.4% from 55.2% with reaction time.
Increasing dithanolamine dosage to 0.4 g, the activity of the
catalyst was significantly decreased, but the cyclohexene se-
lectivity was slightly increased at the same benzene conversion
and the maximum yield of cyclohexene decreased to 41.9%.
The Ru-Zn(4.9%) catalyst exhibited the best performance for
benzene selective hydrogenation to cyclohexene at the opti-
mum dithanolamine dosage of 0.3 g. As known from the above
lyst after hydrogenation with different dithanolamine dosages
as additives and that of dithanolamine as a reference. With the
increase of dithanolamine dosages, no absorbance from
dithanolamine was observed on the catalysts after hydrogena-
tion, which indicated that dithanolamine was not adsorbed on
the catalyst surface and the dithanolamine existed in the slurry.
2
.1.6
H -TPR results
2
Figure 4 gives the H
2
-TPR profiles of the Ru-Zn(4.9%)
catalyst after hydrogenation with different dithanolamine
dosages as additives. All the samples have one reduction peak
characterization, dithanolamine can react with ZnSO
an insoluble (Zn(OH) (ZnSO )(H O) salt. This adsorbed salt
to form
4
due to the reduction of RuO , indicating the oxidation of the
2
)
3
4
2
3
x
most active Ru sites during catalyst washing. With the increase
of the dithanolamine dosage, the reduction peak shifted to
lower temperatures. However, on further increasing the
dithanolamine dosage, the reduction peak shifted back. Al-
though the FT-IR spectra revealed that little dithanolamine was
adsorbed on the catalyst surface, dithanolamine in the slurry
can suppress agglomeration of the catalyst due to a space ef-
changed the composition of the catalyst surface, which is a
vital factor in the performance of the catalyst. Based on pre-
vious works [7], the insoluble (Zn(OH)
)
(ZnSO
)(H
O)
may
O)
2
3
4
2
3
play four roles as follows. (1) The (Zn(OH)
)
3
(ZnSO
)(H
2
4
2
3
2
+
2+
salt contains large amounts of Zn ions. The Zn ions can
form a stable complex with cyclohexene, which can accelerate
the desorption of cyclohexene and retard the re-adsorption of
1
00
9
0
0
6
5
4
3
2
0
0
0
0
0
8
6
4
2
0
0
0
0
0
8
70
60
50
40
Ru-Zn(4.9%)-0
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.4
Ru-Zn(4.9%)-0
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.4
Ru-Zn(4.9%)-0
Ru-Zn(4.9%)-0.2
Ru-Zn(4.9%)-0.3
Ru-Zn(4.9%)-0.4
10
5
10
15
20
25
20
40
60
80
100
5
10
15
20
25
Reaction time (min)
Benzene conversion (%)
Reaction time (min)
Fig. 5. Reaction performance of benzene hydrogenation over Ru-Zn(4.9) catalysts samples with different diethanolamine dosages as an additive. Reac-
o
tion conditions: 1.96 g catalyst, 9.8 g ZrO
2
, 49.2 g ZnSO
4
·7H
2
O, p(H
2
) = 5 MPa, 150 C.

SUN Haijie et al. / Chinese Journal of Catalysis, 2012, 33: 610–620
9
8
8
8
2
8
4
0
6
5
4
0
0
0
80
70
60
50
40
30
20
10
30
20
10
Ru-Zn(4.9%)
Ru-Zn(8.4%)
Ru-Zn(4.9%)
Ru-Zn(8.4%)
Ru-Zn(4.9%)
Ru-Zn(8.4%)
76
5
10
15
20
25
10 20 30 40 50 60 70 80
Benzene conversion (%)
5
10
15
20
25
Reaction time (min)
Reaction time (min)
Fig. 6. Catalytic performance of the Ru-Zn catalyst with different Zn contents for hydrogenation of benzene to cyclohexene. Reaction conditions: 1.96 g
o
catalyst, 9.8 g ZrO
2
, 49.2 g ZnSO
4
·7H
2
O, 0.3 g diethanolamine, p(H
2
) = 5 MPa, 150 C..
the desorbed cyclohexene [7]. This improved the selectivity for
cyclohexene. (2) The (Zn(OH) ) (ZnSO )(H O) salt adsorbed
results are shown in Fig. 6. Both the activity and cyclohexene
yield of the Ru-Zn(4.9%) catalyst were higher than the
Ru-Zn(8.4%) catalyst, but the selectivity of the latter was
higher although by less than 2%. Obviously, the performance
of the Ru-Zn(4.9%) catalyst was superior to that of the
Ru-Zn(8.4%) catalyst. With the addition of 0.3 g dithanola-
mine, the (Zn(OH) ) (ZnSO )(H O) salt agglomerated on the
2
3
4
2
3
on the catalyst surface can block active sites unsuitable for the
formation of cyclohexene. Struijk et al. [17] suggested that the
salt can selectively cover the most reactive sites, which lowers
the adsorption enthalpies of benzene. This resulted in an in-
crease of the cyclohexene desorption rate, which enhances the
selectivity for cyclohexene. (3) The (Zn(OH) ) (ZnSO )(H O)
2
3
4
2
3
Ru-Zn(8.4%) catalyst, which led to a decrease of the activity of
the catalyst and a slight increase of the selectivity for cyclo-
hexene. When the dithanolamine dosage was 0.3 g, the
(Zn(OH) ) (ZnSO )(H O) salt was highly dispersed on the
2
3
4
2
3
salt adsorbed on the catalyst surface comprised plentiful water
of crystallization, which can enhance the hydrophilicity of the
catalyst and improve the selectivity to cyclohexene. (4) An
interaction between the (Zn(OH) ) (ZnSO )(H O) salt and the
2
3
4
2
3
Ru-Zn(4.9%) catalyst and this catalyst gave the best perform-
ance for benzene selective hydrogenation to cyclohexene. A
selectivity for cyclohexene of 78.9% and a yield of 56.3% at a
benzene conversion of 71.3% were obtained at 20 min reaction
time under this condition. This not only meets the industrial
requirement but also exceeds those of the industrial catalyst
[5,9]. In addition, this also confirmed that the optimum Zn
content of the Ru-Zn catalyst is closely related to the kind of
additives.
2
3
4
2
3
active Ru component can modify the electronic structure of Ru,
which makes the Ru catalyst more suitable for benzene selec-
tive hydrogenation to cyclohexene [2]. Therefore, with the
increase of dithanolamine dosage, the amount of the
(
Zn(OH) ) (ZnSO )(H O) salt increased, resulting in the in-
2
3
4
2
3
crease of the activity and the decrease of the selectivity for
cyclohexene. When the dithanolamine dosage was 0.3 g, the
(
Zn(OH) ) (ZnSO )(H O) salt was highly dispersed on the
2
3
4
2
3
surface of the Ru-Zn(4.9%) catalyst and the catalyst exhibited
good activity and excellent selectivity for cyclohexene. FT-IR
confirmed that little dithanolamine was adsorbed on the cata-
lyst surface. The isolated electron pair of O (or N) of ditha-
nolamine sulfate can form a hydrogen bond, which can sig-
nificantly stabilize cyclohexene and reduce the re-adsorption of
desorbed cyclohexene onto the catalyst and its further hydro-
genation from cyclohexene to cyclohexane [18]. Thus the
synergism of (Zn(OH) ) (ZnSO )(H O) and diethanolamine
2.2.3 Catalyst stability
Figure 7 shows the performance of Ru-Zn(4.9%) during five
recycles with 0.3 g dithanolamine as an additive. Benzene
conversion and cyclohexene selectivity were stable above 75%
and cyclohexene yields were kept above 58% over the
Ru-Zn(4.9%) catalyst in first four recycles. This demonstrated
the good activity and selectivity as well as the excellent sta-
bility of the catalyst. However, the benzene conversion,
cyclohexene selectivity, and yield decreased to 59.8%, 80.4%,
and 48.1% in the fifth recycle, respectively. XRD and XRF
results revealed that the decrease was caused by the loss of the
catalyst in the recycling. It is worth noting that the benzene
conversion, cyclohexene selectivity, and cyclohexene yield
reached 84.3%, 75.5%, and 63.6%, respectively, which is better
2
3
4
2
3
sulfate improved the cyclohexene selectivity of the Ru-Zn
catalyst.
2
.2.2 Effect of Zn content on catalytic performance
With 0.3 g dithanolamine, the effect of the Zn content on the
performance of the Ru-Zn catalysts was investigated. The

SUN Haijie et al. / Chinese Journal of Catalysis, 2012, 33: 610–620
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