Effect of (Zn(OH)2)3(ZnSO4)(H 2O)5 on the performance of Ru-Zn catalyst for benzene selective hydrogenation to cyclohexene

Article abstract of DOI:10.1016/j.apcata.2012.10.016

A series of Ru-Zn catalysts with different Zn contents were prepared by co-precipitation. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES)-Ar⁺ sputter, transmission electron micrographs (TEM)-energy dispersion scanning (EDS) and temperature-programmed reduction (TPR). The performances of the catalysts for benzene selective hydrogenation to cyclohexene were investigated in the presence of 0.6 mol/L of ZnSO₄. The results showed that the Ru and Zn in Ru-Zn catalyst were in metallic Ru and ZnO respectively and the ZnO was rich on the surface. The ZnO alone could not improve the selectivity to cyclohexene of Ru-Zn catalyst. However, the ZnO on the surface could react with ZnSO₄ to form a (Zn(OH) ₂)₃(ZnSO₄)(H₂O)₅ salt. The (Zn(OH)₂)₃(ZnSO₄)(H₂O) ₅ salt chemisorbed played a key role in improving the selectivity to cyclohexene of Ru-Zn catalysts. After Ru-Zn(8.6%) catalyst was pretreated 22 h in ZnSO₄ solution at 140 °C and 5 MPa H₂, a cyclohexene selectivity of 81.4% at a benzene conversion of 54.0% was achieved at 10 min and a maximum cyclohexene yield of 58.9% was reached. Moreover, the activity was stable above 50% and the cyclohexene selectivity and yield were steadily above 76% and 40% on this catalyst in the first six recycles, respectively.

Full text of DOI:10.1016/j.apcata.2012.10.016

Applied Catalysis A: General 450 (2013) 160–168
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Effect of (Zn(OH)2)3(ZnSO4)(H2O)5 on the performance of Ru–Zn
catalyst for benzene selective hydrogenation to cyclohexene
Hai-jie Suna, Hong-xia Wanga, Hou-bing Jianga, Shuai-hui Lia, Shou-chang Liua,
Zhong-yi Liua,∗, Xue-min Yuanb, Ke-jian Yangb
a Department of Chemistry, Zhengzhou University, Zhengzhou 450001, Henan, PR China
b China Tianchen Engineering Corporation, Tianjin 300400, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of Ru–Zn catalysts with different Zn contents were prepared by co-precipitation. The catalysts
were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron
spectroscopy (AES) – Ar+ sputter, transmission electron micrographs (TEM)–energy dispersion scanning
(EDS) and temperature-programmed reduction (TPR). The performances of the catalysts for benzene
selective hydrogenation to cyclohexene were investigated in the presence of 0.6 mol/L of ZnSO4. The
results showed that the Ru and Zn in Ru–Zn catalyst were in metallic Ru and ZnO respectively and the
ZnO was rich on the surface. The ZnO alone could not improve the selectivity to cyclohexene of Ru–Zn
catalyst. However, the ZnO on the surface could react with ZnSO4 to form a (Zn(OH)2)3(ZnSO4)(H2O)5
salt. The (Zn(OH)2)3(ZnSO4)(H2O)5 salt chemisorbed played a key role in improving the selectivity to
cyclohexene of Ru–Zn catalysts. After Ru–Zn(8.6%) catalyst was pretreated 22 h in ZnSO4 solution at
140 ◦C and 5 MPa H2, a cyclohexene selectivity of 81.4% at a benzene conversion of 54.0% was achieved at
10 min and a maximum cyclohexene yield of 58.9% was reached. Moreover, the activity was stable above
50% and the cyclohexene selectivity and yield were steadily above 76% and 40% on this catalyst in the
first six recycles, respectively.
Received 5 August 2012
Received in revised form
25 September 2012
Accepted 10 October 2012
Available online 8 November 2012
Keywords:
Benzene
Selective hydrogenation
Cyclohexene
Ruthenium
Zinc
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
organic [4,11–13] and inorganic [8,14,15]. Effective organic addi-
tives should contain a polar group such as hydroxyl or amine group.
There has been a growing interest in using heterogeneous cat-
alysts for the selective hydrogenation of benzene to cyclohexene,
a greener intermediate feedstock than cyclohexane for producing
nylons and fine chemicals [1–6]. However, it is thermodynami-
cally difficult to obtain cyclohexene in high selectivity. Therefore,
it has been long time that only cyclohexane was obtained dur-
ing the hydrogenation of benzene [7]. In 1989, the Asahi Chemical
Industry Co. industrialized the process for producing cyclohexene
from the partial hydrogenation of benzene employing an unsup-
ported Ru–Zn catalyst, and a cyclohexene yield of 32% at a benzene
conversion of 40% was obtained [8,9]. However, the yield of cyclo-
hexene is still relatively low. Therefore, it is necessary to study
catalyst preparation and modification in order to increase the yield
of cyclohexene.
However when using these organic additives the selectivity to
cyclohexene never exceeded 40%. Inorganic additives such as Zn, Fe,
Co, Ni, Cd, Ga, and In salts are more effective than organic additives
[14]. Among them, ZnSO4 has been regarded as the best additives.
Struijk et al. [14] used XPS to characterize Ru catalyst after hydro-
genation in the presence of ZnSO4 under the reaction conditions of
423 K and 5.0 MPa of H2, and found the majority of chemisorbed Zn
is present as Zn2+. They suggested that the chemisorbed Zn2+ could
enhance the hydrophilicity of Ru catalyst and selectively cover the
most reactive sites, which improved selectivity to cyclohexene of
Ru catalyst. Based on these, they specially pointed out the salts
as effective additives should have a high absorbability on ruthe-
nium surface and a difficult reduction ability under the reaction
conditions.
Some additives, which are directly put into the reaction system
together with the catalyst and sometimes are called co-catalyst
[8] and reaction modifier [10], can greatly improve the selectiv-
ity to cyclohexene. Generally, there are two kinds of additives:
The promoters also have great impacts on the yield of cyclo-
hexene. It was reported that K [16], Fe [17–19], Co [13,20], Ce
[21], Ba [22], La [15,23,24] and Zn [4,8,25–29] as a promoter or
co-promoters to modify the ruthenium by co-precipitation or co-
impregnation or other methods were beneficial for the increase of
the selectivity to cyclohexene. Among them, Zn has been consid-
ered to be the best promoter [4,8]. Wang et al. [26] and He et al.
[28] prepared Ru–Zn/ZrO2 catalysts using hydrogen reduction of
∗
Corresponding author. Tel.: +86 371 67783384.
E-mail address: liuzhongyi@zzu.edu.cn (Z.-y. Liu).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.10.016

H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
161
coprecipitation product of ruthenium trichloride and zirconium
2.2. Catalyst characterization
oxychloride in a ZnSO4 solution under the reaction conditions of
423 K and 453 K as well as H2 pressure of 4.28 MPa and 5.0 MPa
respectively. The catalysts were characterized by XPS, and it was
found that the BE of Zn 2p3/2 of the Zn in the catalyst was close
to that of metallic Zn. So they thought the chemisorbed Zn2+ could
be reduced by H atoms which spilled from metallic ruthenium sur-
face [26,28]. Wang et al. [26] suggested that the metallic Zn could
irreversibly occupy the most reactive sites unfavorable for ben-
zene selective hydrogenation. Yuan et al. [27] indicated that the
presence of Zn atoms on the surface of the Ru–Zn alloy resulted
in a direct decrease in sites for the chemisorption of cyclohexene
but also a depressed adsorption capability of the neighboring sur-
face Ru sites. Therefore, Ru-based catalyst modified by metallic
Zn gave a high selectivity to cyclohexene. However, the binding
energies (BEs) of metallic and oxidized Zn are very close and it is
difficult to assess the oxidation state of zinc by XPS measurements
[30].
As mentioned above, the ZnSO4 as an additive could not be
reduced. The Zn2+ as a promoter, however, could be reduced
to metallic state under the similar condition. Obviously, there
is a severe contradiction. Moreover, the roles of the promoter
and the additive were described in separate and the interac-
tions between them were ignored in all published documents
[15,21]. Motivated by these problems, a series of Ru–Zn cat-
alysts with different Zn contents were prepared. In order to
discern the oxidation state of Zn in Ru–Zn catalysts AES-Ar+
sputtering were employed since the Auger shift between Zn2+
and Zn0 is higher than 4.6 eV [31]. It is found that the Zn
in Ru–Zn catalyst exists as ZnO which is rich on the sur-
face. Only ZnO cannot improve the selectivity to cyclohexene
of Ru–Zn catalyst. ZnO on the surface can react with the addi-
tive ZnSO4 to form a insoluble (Zn(OH)2)3(ZnSO4)(H2O)5 salt. The
(Zn(OH)2)3(ZnSO4)(H2O)5 salt plays a key role in improving the
selectivity to cyclohexene of Ru–Zn catalysts and its modification
effects are discussed.
N2 physisorption (BET) was determined on a Quantachrome
Nova 100e apparatus at 77 K. The sample was heated at 423 K
under vacuum for 2 h before measurement. The pore size distri-
bution was calculated from the desorption branch of the isotherm
by the Barret–Joyner–Halenda (BJH) method. X-ray diffraction
(XRD) patterns were acquired on a PANalytcal X’Pert PRO instru-
ment using Cu K␣ (ꢀ = 1.541 A) with scan range from 5 to 90◦
at a step of 0.03◦. The atomic absorption spectroscopy (AAS) for
determining the Zn content of the catalyst was performed on a
Perkin Elmer AAnalyst 300 instrument, operating at ꢀ = 213.9 nm
and slit width = 0.20 nm. The Zn/Ru molar ratio was measured
by X-ray fluorescence (XRF) on a Bruker S4 Pioneer instru-
ment. Surface analysis of the catalysts was analyzed by X-ray
photoelectron spectroscopy (XPS) on a PHI Quantera SXM instru-
ment using a monochromatized Al K␣ radiation (Eb = 1486.6 eV)
at an energy resolution of 0.5 eV and at a base pressure of
3 × 10−8 Pa. The energy scale was calibrated and corrected for
charging using the C1s (284.8 eV) line as the binding energy ref-
erence. Auger electron spectroscopy (AES) and sputter profiles
were taken on a ULVAC PHI-700 Nano-canning Auger system
with on-axis scanning argon ion gun and CMA energy ana-
lyzer. The energy resolution ratio was 0.1%. The background
pressure of analysis room was less than 5.2 × 10−7 Pa. The stan-
dard sample was SiO2/Si. The sputtering rate was 9 nm/min.
Transmission electron micrographs (TEMs) and energy dispersion
scanning (EDS) were observed on a JEOL JEM-2100 instrument
using an accelerating voltage of 200 kV. Temperature-programmed
reduction (TPR) measurements were carried out in a U-shaped
◦
˚
quartz reactor, using a 5% H2/He gas flow of 50 cm3 min−1
thermal conductivity detector (TCD) and about 10 mg of
catalyst.
,
a
The electron density of benzene, cyclohexene and cyclohexane
was calculated by Gaussian 09 program under the level of B3LYP/6-
31g (d, p).
2.3. Activity testing
2. Experimental
The selective hydrogenation of benzene was performed in a
1 L autoclave lined the hastelloy. The autoclave was charged with
280 ml of H2O containing a share of Ru–Zn(x) catalyst, 49.2 g of
ZnSO4·7H2O and 9.8 g of ZrO2. Then heating commenced with H2
pressure of 5 MPa and stirring rate of 800 r/min. 140 ml of ben-
zene was fed and the stirring rate was elevated to 1400 r/min to
exclude the diffusion effect when the temperature reached 423 K.
A small amount of reaction mixture was sampled every 5 min and
sent for gas chromatographic analysis with a FID detector, and the
benzene conversion and cyclohexene selectivity were calculated.
After the reaction the organic was removed, the solid sample was
washed with distilled water until no Zn2+ and then was vacuum-
dried for characterization. The sample after reaction corresponding
to Ru–Zn(x) catalysts was denoted as Ru–Zn(x)AH, where AH stood
for After hydrogenation.
The effect of the pretreatment and the recyclability of
Ru–Zn(8.6%) catalyst were investigated according to the follow-
ing procedures. A share of Ru–Zn(8.6%) catalyst was pretreated for
22 h in the presence of 280 ml H2O containing 49.2 g of ZnSO4·7H2O
and 9.8 g of ZrO2 at 413 K and with a H2 pressure of 5.0 MPa as well
as a stirring rate of 800 r/min. Then the temperature was raised
to 423 K and hydrogenation was performed according to the above
hydrogenation procedures. At the end of the reaction, the autoclave
was cooled down and the organic phase was separated. The slurry
containing the mixture of the catalyst and ZrO2 was recycled in
accordance with the above hydrogenation procedures without the
pretreatment.
2.1. Catalyst preparation
Ru–Zn catalysts were prepared according to the procedure in
the literature [32]. 9.75 g RuCl3·H2O and a desired amount of
ZnSO4·7H2O were dissolved in 200 ml H2O with agitation. To the
stirred solution, 200 ml of a 20% NaOH solution was added instan-
taneously and the resulting mixture was agitated for an additional
4 h at 353 K. The mixture was left to stand and the black precipitate
was washed three times with an aqueous solution of 5% NaOH after
the supernatant had been removed by decantation. This black pre-
cipitate was dispersed in 400 ml of a 5% NaOH solution and charged
into a 1 L autoclave lined the Teflon. Hydrogen was introduced into
the autoclave to raise the total internal pressure to 5 MPa and the
reduction was conducted at 423 K and at 800 r/min stirring rate for
3 h. The reaction mixture was cooled and the obtained black pow-
der was washed three times with 5% NaOH, then with water until
neutrality, subsequently vacuum-dried and the desired Ru–Zn cat-
alysts were obtained. The catalyst was divided into two shares, one
share was used for activity test and the other for catalyst charac-
terization. This method ensured that the catalysts with different Zn
contents had the same Ru contents (about 1.8 g Ru). The amounts
of ZnSO4·7H2O were adjusted to give the catalyst with different
Zn contents which were denoted as Ru–Zn(x) catalysts, where x
denoted the weight percentage of Zn in the catalyst determined by
atomic absorption spectrometry.

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H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
small crystallite sizes of Ru. Two new diffraction peaks at 2ꢁ of
ZnO
Ru
8.1◦ and 16.2◦ belong to the phase of (Zn(OH)2)3(ZnSO4)(H2O)5
(JCPDS: 01-078-0246). This reveals that the ZnO on catalyst
surface reacted with the additive ZnSO4 to form a insoluble
(Zn(OH)2)3(ZnSO4)(H2O)5 salt. The diffraction peaks at 8.1◦ appear
and gradually increase with Zn content, indicating the increase
of the (Zn(OH)2)3(ZnSO4)(H2O)5 content. Combined with the
XPS and AES results below, it is concluded that the Zn on the
surface of Ru–Zn(x)AH exists as the (Zn(OH)2)3(ZnSO4)(H2O)5 salt.
However, the diffractions of the (Zn(OH)2)3(ZnSO4)(H2O)5 salt are
not detected on Ru–Zn(x)AH until the Zn content was 9.6 wt%. This
was because of the low amount of the (Zn(OH)2)3(ZnSO4)(H2O)5
salt below beyond the detection limit of the XRD instrument when
the Zn content was lower than 9.6 wt%.
(a)
Ru-Zn(29.1%)
Ru-Zn(14.9%)
Ru-Zn(12.4%)
Ru-Zn(9.6%)
Ru-Zn(8.6%)
Ru-Zn(7.7%)
Ru-Zn(5.2%)
Ru-Zn(2.6%)
Ru-Zn(0%)
(Zn(OH)2)3(ZnSO4)(H2O)5
Ru
(b)
Especially, it is the ZnO on the surface of Ru–Zn catalyst, not the
ZnO in the bulk, that can react with the additive ZnSO4 in the slurry
to form (Zn(OH)2)3(ZnSO4)(H2O)5 since it is very difficult for ZnSO4
in the slurry to enter into the bulk of the catalyst.
Ru-Zn(29.1%)AH
Ru-Zn(14.9%)AH
Ru-Zn(12.4%)AH
Ru-Zn(9.6%)AH
Ru-Zn(8.6%)AH
Ru-Zn(7.7%)AH
Ru-Zn(5.2%)AH
Ru-Zn(2.6%)AH
Ru-Zn(0%)AH
ZrO2
The Zn 2p3/2 binding energy (BE) of Ru–Zn(8.6%)
catalyst(Fig. 2(a)) is 1021.7 eV which is close to that of ZnO
(1021.6 eV) [33]. Its Zn LMM kinetic energies (KEs) are both
986.0 eV at the sputtering time of 0 s and 30 s, and it slightly
increases to 986.5 eV at the sputtering time of 1 min. However,
these values are a little lower than that (987 eV) of the Zn(II)
species in the PtZn/C catalyst [31] and that (988.1 eV) of the Zn(II)
species in the calcined un-reduced Pr/Cr–ZnO catalyst [30]. Based
on these, it is concluded that the Zn in the catalyst exists as ZnO
and there is no clear proof in favor of that ZnO can be reduced
by the spilled H atoms from the metallic Ru. The peak of Ru 3d
is overlapped by the peak of C 1s, moreover, Ru 3d is sensitive to
be influenced by reactant and product [14]. Therefore, Ru 3p3/2 is
employed for discussion. The profile of Ru 3p3/2 (Fig. 2(f)) shows
a shoulder, which is decomposed into two contributions. The one
at 461.9 eV is assigned to metallic Ru [34], whereas the other
corresponds to oxidized Ru in RuO2 [35]. This indicates that a small
part of Ru on the surface of the catalyst with the most activity can
be oxidized to RuO2 during the treatment of catalyst washing or
vacuum drying.
The AES depth profiles of Ru–Zn(8.6%) catalyst (Fig. 2(d)) show
the concentration of Ru increases, and the Zn concentration as well
as the O concentration decreases with the sputtering time, indicat-
ing the enrichment of ZnO on catalyst surface. Moreover, the atom
ratio of Zn to O is 0.93 on the top surface of the catalyst, indicating
the oxidation of a small part of metallic Ru with the most activity,
which is in accordance with the XPS results. This ratio increases
gradually with the sputtering time and reaches 1 at the sputter-
ing time of 1 min (the depth of 9 nm), indicating that the content
of RuO2 decreases with the sputtering time and the bulk phase is
composed of the metallic Ru and ZnO.
The Zn 2p3/2 BE of Ru–Zn(8.6%)AH (Fig. 2 (b)) is 0.7 eV higher
than that for the Ru–Zn(8.6%) catalyst, while the chemical shift of
metallic Zn relative to the Zn in ZnO is about 0.5 eV. Moreover, the
Zn LMM KE (983.6 eV) of Ru–Zn(8.6%)AH (Fig. 2(e)) is 2.9 eV lower
than that (986.5 eV) of Ru–Zn(8.6%) catalyst. All of these indicate
that a new Zn(II) species, which is the (Zn(OH)2)3(ZnSO4)(H2O)5
salt as confirmed by XRD, is formed during the hydrogenation pro-
cess. And the Zn(II) species are electron-deficient.
Unfortunately, the peak of Ru 3p3/2 for Ru–Zn(8.6%)AH (Fig. 2(g))
becomes too dispersive to discriminate between the metallic Ru
and RuO2 due to the dispersion of ZrO2. However, the peak of Ru
3p3/2 becomes complex, indicating the variation of atomic environ-
ment for Ru due to the formation of the (Zn(OH)2)3(ZnSO4)(H2O)5
salt on the surface of catalyst. For this reason, it is difficult to
directly judge whether the Zn(II) species donated some electrons
to metallic Ru or sulfate since the sulfate is more effective than Ru
in drawing electrons. However, the composition of Zn(II) species
10
20
30
40
2
50
(degree)
60
70
80
90
Fig. 1. XRD patterns of (a) Ru–Zn(x) catalysts and (b) Ru–Zn(x)AH.
3. Results and discussion
3.1. Structural and electronic properties
BET surface area, pore volume and pore diameter of Ru–Zn(x)
catalysts are listed in Table 1. It can be seen that BET surface area
decreases, and pore volume and average pore diameter increase
with Zn content generally. Combined with TEM results, the forma-
tion of big ZnO particle on the surface of Ru–Zn(x) catalysts might
be responsible for this. Table S1 shows the BET surface area, pore
volume and pore diameter of Ru–Zn(x)AH. It is found the BET sur-
face area, pore volume and pore diameter of Ru–Zn(x)AH are bigger
than that of ZrO2. This is because Ru–Zn(x)AH are composed by
a fine mechanical mixture of Ru–Zn(x) catalyst and ZrO2, as con-
firmed by the TEM results. The BET surface area, pore volume and
pore diameter of Ru–Zn(x)AH changes little with the Zn contents.
This probably relates to the weight ratio of ZrO2 to the catalyst (5:1).
XRD patterns of Ru–Zn(x) catalysts (Fig. 1(a)) show that the
diffraction peaks observed at 2ꢁ of 38.4◦, 44.0◦, 58.3◦, 69.4◦, 78.4◦
and 84.7◦ are assigned to the metallic Ru phase (JCPDS: 01-070-
0274). Evaluation of the Ru crystallite size from XRD using the
Scherrer formula (Table 1) indicates Zn species addition has little
influence on the Ru crystallite sizes of Ru–Zn(x) catalysts. However,
Hu et al. [25] prepared a series of Ru–Zn/SiO2 catalysts with various
Zn contents by an incipient wetness co-impregnation method and
found that increasing Zn species addition increased the Ru crystal-
lite size. Obviously, this difference was caused by different catalyst
preparation methods. Seven new diffraction peaks of Ru–Zn(29.1%)
catalyst at 2ꢁ of 31.8◦, 34.4◦, 36.3◦, 47.5◦, 56.6◦, 62.9◦ and 68.0◦
are ascribed to the hexagonal phases of ZnO (JCPDS: 01-070-2551),
indicating the existence of ZnO in the catalyst. This implies that at
least a part of Zn in Ru–Zn(x) catalysts exists as ZnO.
The comparison of the diffraction peaks of ZrO2 and Ru–Zn(x)AH
in Fig. 1(b) shows all the diffraction peaks of Ru–Zn(x)AH are
attributed to the monoclinic phase of ZrO2 (JCPDS: 00-024-1165)
except for the reflections at 2ꢁ of 8.1◦, 16.2◦ and 44.0◦. The wide
and weak diffraction peaks at 2ꢁ of 44◦ of Ru–Zn(x)AH correspond
to the metallic Ru phase (JCPDS: 01-070-0274), indicating the

H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
163
Table 1
Physical properties and crystallite size of the Ru–Zn(x) catalysts.
Sample
BET surface area (m2/g), SSA
Pore volume (cm3/g)
Average pore diameter (nm)
Crystallite size (nm)a
Ru–Zn(0%)
88
77
67
74
77
55
66
64
57
0.18
0.18
0.15
0.17
0.16
0.15
0.27
0.27
0.22
4.1
4.8
4.6
4.5
4.0
5.3
8.2
8.4
7.8
4.1
4.3
3.8
3.7
3.9
4.3
4.0
3.8
3.8
Ru–Zn(2.6%)
Ru–Zn(5.2%)
Ru–Zn(7.7%)
Ru–Zn(8.6%)
Ru–Zn(9.6%)
Ru–Zn(12.4%)
Ru–Zn(14.9%)
Ru–Zn(29.1%)
a
Determined by XRD.
was measured by the XRF instrument and it is found that the
molar ratios of Zn/Ru and S/Ru are 0.44 and 0.06 respectively. This
suggests that the amount of sulfate is much lower that of Zn(II)
species on the surface of Ru–Zn(8.6%)AH. Moreover, Struijk et al.
[14] found that the BE of S 2p for Ru catalyst after hydrogenation
in the presence of ZnSO4 was 169.9 eV, which was higher than
the standard value (169.2 eV for ZnSO4) [36]. All of these indicates
that the sulfate gets few electrons from the Zn(II) species. These
(a)
1021.7
Zn 2p
986.5 eV
(c)
Ru-Zn(8.6%)
Ru-Zn(8.6%)
986.0 eV
1044.8
Zn 2p
986.0 eV
1022.4
Zn 2p
(b)
1045.1
Zn 2p
Ru-Zn(8.6%)AH
984
987
990
993
996
999
1015 1020 1025 1030 1035 1040 1045 1050 1055
Binding energy (ev)
Kinetic energy (eV)
Ru-Zn(8.6%)
89
88
87
86
85
(d)
(e)
Ru-Zn(8.6%) AH
984.0 eV
Ru
Ru-Zn(8.6%)
O
7.5
7.0
6.5
6.0
Zn
984
987
990
993
996
999
0.0
0.2
0.4
0.6
0.8
1.0
Kinetic energy (eV)
Ar sputter time (min)
461.9
Ru
(f)
Ru3p
464.2
RuO
Ru-Zn(8.6%)
(g)
Ru-Zn(8.6%)A
Ru3p
453
456
459
462
465
468
471
Binding energy (ev)
Fig. 2. (a) Zn 2p, (c) Zn LMM, (f) Ru 3p3/2 and (d) AES depth profiles of the Ru–Zn(8.6%) catalyst; (b) Zn 2p, (e) Zn LMM and (g) Ru 3p3/2 of the Ru–Zn(8.6%)AH sample.

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H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
Fig. 3. TEM images of (a) Ru–Zn(0%) catalyst; (b) Ru–Zn(0%)AH; (c) Ru–Zn(8.6%) catalyst; (d) Ru–Zn(8.6%)AH; (e) Ru–Zn(29.1%) catalyst; (f) Ru–Zn(29.1%)AH.
prompt us to suggest that partial electrons might transfer from the
Zn(II) species to the metallic Ru.
of the presence of metallic Zn observed in the XPS and AES
results.
In addition, Table 2 shows that the pH values of the liq-
uid phase after the reaction at room temperature are around
6.0 due to the hydrolysis of ZnSO4. It is well known that
increasing temperature favors the hydrolysis. This means that
the acidity of liquid phase is much higher at the reaction tem-
perature of 423 K due to the increase of hydrolysis degree of
ZnSO4. As we know, it is difficult for the metallic Zn to exist
in the acid solution, which is consistent with no clear evidence
TEM images in Fig. 3 show that the Ru crystallite sizes of
Ru–Zn(0%) catalyst, Ru–Zn(8.6%) catalyst and Ru–Zn(29.1%) cata-
lyst center on about 4 nm, which is consistent with the XRD results.
It should be specially noted that the small particle in Fig. 3(c) and
(d) is mainly composed of metallic Ru besides small amounts of
Zn species. Thus they are generally denoted as Ru–Zn(8.6%) and
Ru–Zn(29.1%) catalysts, respectively. A piece of thing is found on
the surface of Ru–Zn(8.6%) catalyst in Fig. 3(c), which is confirmed

H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
165
Table 2
Benzene conversion, cyclohexene selectivity and yield in 5 min as well as the maximum yield in 25 min over Ru–Zn(x) catalysts and pH value of liquid phase after the reaction.a
Catalyst
Conversionb (%)
Selectivityb (%)
Yieldb (%)
Maximum yieldc (%)
pH valued
Ru–Zn(0%)
57.9
68.1
54.4
50.5
46.0
26.4
20.7
13.1
6.8
38.2
47.2
69.5
71.1
76.2
84.1
86.0
87.5
90.4
0
22.1
32.1
37.8
35.9
35.1
22.2
17.8
11.5
6.2
23.1
32.1
48.0
47.4
50.9
50.4
49.8
34.2
19.0
0
5.7
5.8
5.9
5.9
6.1
5.9
6.1
5.8
6.0
7.2
7.3
Ru–Zn(2.6%)
Ru–Zn(5.2%)
Ru–Zn(7.7%)
Ru–Zn(8.6%)
Ru–Zn(9.6%)
Ru–Zn(12.4%)
Ru–Zn(14.9%)
Ru–Zn(29.1%)
Ru–Zn(0%)e
Ru–Zn(8.6%)e
100
86.7
0
1.6
1.9
1.6
a
Reaction conditions: a share of catalyst, 49.2 g ZnSO4·7H2O, 9.8 g ZrO2, 280 ml H2O, 5 MPa H2, 423 K, stirring rate of 1400 r/min.
Benzene conversion, cyclohexene selectivity and yield in 5 min.
Maximum yield of cyclohexene in 25 min.
pH value of liquid phase after the reaction at room temperature.
In the absence of ZnSO4.
b
c
d
e
to be ZnO by EDS. This also indicates that ZnO is rich on the surface
thus the shoulders consist of two peaks or three peaks. Besides,
the absence of any additional peaks indicates that the promoter
ZnO cannot be reduced to metallic Zn within 500 K. Although
bulk ZnO reduction is thermodynamically feasible, temperatures as
high as 923 K are required. All the H2-TPR profiles for Ru–Zn(x)AH
(Fig. 4(b)) also appear peak shoulders between 300 K and 400 K
which are also assigned to the step-by-step reduction of RuO2 to
metallic Ru. Obviously, the temperatures of complete reduction for
all Ru–Zn(x)AH are lower than the reaction temperature of 423 K,
indicating only the existence of metallic Ru under the conditions of
423 K and 5 MPa H2.
and cannot be highly dispersed. Although the results of XPS, AES-
Ar+ and TEM all show the existence of ZnO, no ZnO phase is detected
in XRD pattern of the Ru–Zn(8.6%) catalyst. This indicates that ZnO
exists in the form of amorphous. Eo et al. [37] found that ZnO with
heat treatment at 373–473 K for 60 min still showed an amorphous
phase. Thus it is usual that no crystalline ZnO phases are detected
when Zn content is lower than 29.1% since the catalyst reduction
temperatures were only 423 K. A columnar ZnO (EDS confirmed)
is displayed on the surface of Ru–Zn(29.1%) catalyst in Fig. 3(e),
indicating its oriented growth under the catalyst preparation. This
also accounts for the hexagonal phases of ZnO on Ru–Zn(29.1%)
catalyst. Surprisingly, neither ZnO images nor the images of the
(Zn(OH)2)3(ZnSO4)(H2O)5 salt are detected on Ru–Zn(8.6%)AH in
Fig. 3(d) and Ru–Zn(29.1%)AH in Fig. 3(f). This is probably due to the
uniform dispersion of the (Zn(OH)2)3(ZnSO4)(H2O)5 salt. Xie et al.
[38] had confirmed that many salts could disperse spontaneously
onto the surface of supports. More importantly, the different cat-
alytic performances between Ru–Zn(8.6%) catalysts in the presence
and in the absence of ZnSO4 (as described below) imply that only
when the (Zn(OH)2)3(ZnSO4)(H2O)5 salt uniformly disperses on Ru
surface can the catalytic performance of Ru catalyst be significantly
improved. Besides, the catalyst particles are separated and isolated
by ZrO2, indicating that ZrO2 can reduce the chance of the collision
of different catalyst particles under reaction condition of high agita-
tion and suppress the agglomeration of the catalyst. Moreover, the
added ZrO2 also reduces catalyst adhesion to the metallic surface
of the reactor. These are beneficial for the catalyst life [32].
3.2. Catalytic performance
The performances of Ru–Zn(x) catalysts with different Zn
contents for selective hydrogenation of benzene to cyclohexene
were investigated without the pretreatment. Benzene conversion,
cyclohexene selectivity and yield in 5 min as well as the maximum
yield in the reaction time over Ru–Zn(x) catalysts are summa-
rized in Table 2. Fig. S1 gives benzene conversion and cyclohexene
selectivity in 20 min. In Table 2 and Fig. S1, benzene conversion
decreased and cyclohexene selectivity increased continuously with
Zn content. Obviously, the increase of cyclohexene selectivity is at
the expense of catalyst activity. Thus cyclohexene yield in 5 min
and the maximum cyclohexene yield in reaction time are both
first increased and then decreased with Zn content. A maximum
cyclohexene yield of 50.9% was achieved on Ru–Zn(8.6%) catalyst.
The selectivity and the yield of cyclohexene reached 38.2% and
22.1% respectively at benzene conversion of 57.9% on Ru–Zn(0%)
catalyst at 5 min in the presence of ZnSO4. While in the absence
of ZnSO4 within only 5 min benzene was totally consumed on
this catalyst. This indicates that the reaction of cyclohexene
The H2-TPR profiles of Ru–Zn(x) catalysts (Fig. 4(a)) show peak
shoulders between 300 K and 400 K. The shoulders are assigned to
the reduction of RuO2 to metallic Ru with the bid of XPS. More-
over, the process may experience the reduction Ru2O3 and/or RuO,
Ru-Zn(29.1%)
Ru-Zn(14.9%)
Ru-Zn(12.4%)
Ru-Zn(9.6%)
Ru-Zn(8.6%)
(a)
(b)
Ru-Zn(29.1%)AH
Ru-Zn(14.9%)AH
Ru-Zn(12.4%)AH
Ru-Zn(9.6%)AH
Ru-Zn(8.6%)AH
Ru-Zn(7.7%)AH
Ru-Zn(5.2%)AH
Ru-Zn(2.6%)AH
Ru-Zn(7.7%)
Ru-Zn(5.2%)
Ru-Zn(2.6%)
Ru-Zn(0%)AH
Ru-Zn(0%)
300
350
400
450
500
300
350
400
450
500
Temperature(K)
Temperature(K)
Fig. 4. H2-TPR profiles of Ru–Zn(x) catalysts and Ru–Zn(x)AH.

166
H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
Fig. 5. Electronic interaction between Ru and Zn(II) species and electron cloud den-
sity of benzene and cyclohexene.
Fig. 6. Stagnant water layer formed by the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)5
salt around Ru–Zn catalyst.
formation only happens on the surface of the Ru catalyst which
can contact with ZnSO4. For Ru–Zn(8.6%) catalyst, the conver-
sion of benzene reached 86.7% within 5 min in the absence of
ZnSO4. The yield to cyclohexene was only 1.6% with the corre-
sponding selectivity to cyclohexene of 1.9%. This implies that the
promoter ZnO alone cannot enhance the selectivity to cyclohexene
of Ru–Zn(8.6%) catalyst. However, this catalyst afforded a selec-
tivity to cyclohexene of 76.2% and a cyclohexene yield of 35.1%
with the corresponding benzene conversion of 46.1% at 5 min in
the presence of ZnSO4, indicating that the synergistic effect of ZnO
and ZnSO4 enhances the selectivity to cyclohexene of the catalyst.
Namely, the (Zn(OH)2)3(ZnSO4)(H2O)5 salt formed by the ZnO on
the surface of the catalyst reacting with ZnSO4 plays a key role in
improving the selectivity to cyclohexene of the catalyst. Based on
the catalyst characterization and the previous works, the roles of
the (Zn(OH)2)3(ZnSO4)(H2O)5 salt in improving the selectivity to
cyclohexene of Ru–Zn catalyst can be attributed to the following
reasons.
ZnSO4 also contains Zn2+, however, it is a soluble salt and the
Zn2+ chemisorbed on the surface of Ru active sites is very little.
Therefore, ZnSO4 can only improve the selectivity to cyclohex-
ene to some extent. Although ZnO also has Zn2+, it is difficult
for it to be uniformly dispersed on Ru active sites (as confirmed
by TEM). Thus Ru–Zn(8.6%) catalyst shows very little selectivity
to cyclohexene in the absence of ZnSO4.
(3) The (Zn(OH)2)3(ZnSO4)(H2O)5 salt is abundant with crystal
water. The crystal water can form a stable stagnant water layer
around the Ru catalyst particles, as shown in Fig. 6. The dif-
fusion rate of cyclohexene in water is very low due to its low
solubility, indicating the low re-adsorption rate of cyclohexene
through the stagnant water layer onto the surface of the cata-
lyst [39]. This means it is hard for the desorbed cyclohexene to
be re-adsorbed on the surface of the catalyst and hydrogenated
to cyclohexane. Thus the selectivity to cyclohexene of the cat-
alyst is improved. Meanwhile it is also difficult for the formed
cyclohexene to diffuse through the stagnant water layer into the
oil phase. However, the small cyclohexene droplets, mixed up
with cyclohexane, can break through the stagnant water layer
directly into the oil phase [39].
(1) It has been confirmed by XPS that the Zn(II) species of the
(Zn(OH)2)3(ZnSO4)(H2O)5 salt donates some electrons to Ru
species and Ru is rich in electrons. The electron density of car-
bon atoms in benzene and cyclohexene is shown in Fig. 5. As can
be seen, the electron densities of six carbon atoms of benzene
are −0.239e × 6, while that of cyclohexene are −0.230e × 2,
−0.494 × 2 and −0.464 × 2, respectively. This indicates that the
electron densities of carbon atoms for cyclohexene are much
higher than that for benzene. Thus the repulsive forces between
the electrons around Ru and carbon atoms of cyclohexene are
stronger than that between the electrons around Ru and car-
bon atoms of benzene, which is beneficial for the desorption of
cyclohexene from the surface of the catalyst and improvement
of the selectivity to cyclohexene. Meanwhile the activity of the
catalyst decreases for the repulsive force between the electrons
around Ru and carbon atoms of benzene.
(2) It has been demonstrated that the (Zn(OH)2)3(ZnSO4)(H2O)5
salt can be uniformly dispersed on the active Ru sites
(as confirmed by TEM). The Zn2+ of the chemisorbed
(Zn(OH)2)3(ZnSO4)(H2O)5 salt can selectively cover the most
reactive sites of the catalyst, which can reduce the active sites
for the chemisorption of cyclohexene and suppress the fur-
ther hydrogenation of cyclohexene to cyclohexane. Therefore,
increasing the content of the promoter increased the formation
of the (Zn(OH)2)3(ZnSO4)(H2O)5 salt, resulting in the decrease
of activity and the increase of the selectivity to cyclohexene of
Ru–Zn catalysts. Struijk et al. [14] suggested the salts which act
as the effective additive should have enough absorbability on
Ru to cover 50% Ru active sites.
Above all, the (Zn(OH)2)3(ZnSO4)(H2O)5 salt plays a key role
in improving the selectivity to cyclohexene for Ru–Zn(9.9%) cat-
alyst. The formation of the (Zn(OH)2)3(ZnSO4)(H2O)5 salt is closely
related to the ZnO content, especially its surface content, and the
concentration of ZnSO4. Thus the optimum Zn content has a close
relationship with the concentration of the additive ZnSO4. He et al.
[28] also found this relationship, however, they could not give a
good explanation for this based on the existence of Zn atoms.
The influence of pretreatment on the selectivity to cyclohex-
ene of the Ru–Zn(8.6%) catalyst has also been investigated. Fig. 7(a)
shows the pretreated catalyst afforded a maximum cyclohexene
yield of 58.9%, which is among the best results reported so far
[15,21]. Moreover, a selectivity to cyclohexene of 81.4% at a benzene
conversion of 54.0% was achieved over the pretreated catalyst as
shown in Fig. 7(b). The products of hydrogenation, benzene, cyclo-
hexene and cyclohexane, were separated by solvent extraction due
to their close boiling point. The higher the selectivity to cyclohex-
ene is, the cheaper the cost of separation is. Thus the cyclohexene
selectivity of more than 80% is requested by industry. Obviously,
the performance of Ru–Zn(8.6%) catalyst meets this need.
XRF results reveal that the Zn/Ru and S/Ru molar ratios of
Ru–Zn(8.6%) catalyst after the pretreatment slightly increase to
0.48 and 0.07 respectively, which is higher than those values
(0.44 and 0.06) before the pretreatment. XRD (not shown) con-
firms that there are no other diffraction peaks detected besides the

H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
167
100
80
60
40
20
0
(a)
85
80
75
70
65
(b)
benzene
(54.0, 81.4)
Cyclohexene
58.9
cyclohexane
30
40
50
60
70
80
90
0
5
10
15
20
25
Benzene conversion (%)
Reaction Time (min)
Fig. 7. (a) The course of the hydrogenation of benzene over the pretreated Ru–Zn(8.6%) catalyst and (b) cyclohexene selectivity as a function of benzene conversion over the
pretreated catalyst. Pretreatment conditions: a share of catalyst, 280 ml H2O, 49.2 g ZnSO4, 5 MPa H2, stirring rate of 800 r/min, temperature of 413 K. Reaction conditions:
stirring rate of 1400 r/min, 140 ml benzene, and temperature of 423 K.
reflections of metallic Ru and ZrO2. Struijk et al. [14] investigated
the influence of the pretreatment on unsupported Ru catalysts and
found that the metal cations chemisorbed on the catalyst surface
improve the selectivity to cyclohexene and decrease the activity,
which originate from corrosion of the stainless steel wall of the
reactor. However, there is no Fe or other metallic elements detected
after the pretreatment of the catalyst since our hydrogenation is
performed in a hastelloy autoclave, which has a high resistance
to acid. Milone et al. [40] carried out the pretreatment of Ru/␥-
Al2O3 catalysts and suggested that the pretreatment gave a more
reduced catalyst, which exhibited the higher selectivity to cyclo-
hexene. However, TPR results have confirmed that the ruthenium
oxide is completely reduced to metallic Ru under our hydrogena-
tion conditions. Some of authors [5] also suggested hydrogen was
preferentially adsorbed on the most active sites and benzene could
only adsorbed on the moderately active sites, which benefited for
the activation of benzene and also the desorption of cyclohexene.
However, when the catalyst was pretreated in the absence of ZnSO4
under the same conditions, the pretreated catalyst gave a very
poor selectivity (5.01% at 5 min) to cyclohexene. Thus based on the
increase of Zn content in the pretreated catalyst, we suggests that a
small part of ZnO on catalyst surface, which is difficult to react with
ZnSO4 in the short time of direct hydrogenation, can continue to
react with ZnSO4 to form the (Zn(OH)2)3(ZnSO4)(H2O)5 salt in the
process of pretreatment. In addition, the (Zn(OH)2)3(ZnSO4)(H2O)5
salt can more stably be chemisorbed on catalyst surface after the
catalyst is pretreated in the presence of ZnSO4. Therefore, the pre-
treatment makes the catalyst exhibit the excellent selectivity to
cyclohexene.
The stability of Ru–Zn(8.6%) catalyst was investigated. The cat-
alyst was first pretreated for 22 h in the presence of ZnSO4, and
then the pretreated catalyst was recycled seven times without
any more pretreatment and additions, and the results are listed
in Fig. 8. As can be seen, the benzene conversions are stable above
50%, and the cyclohexene selectivity and yields are kept above 76%
and 40% in the first six recycles respectively, indicating a good
stability of the catalyst. The activity of the catalyst remarkably
decreases in 7th recycle due to the inevitable catalyst loss during
the recycles and six recycles without regeneration, however, the
selectivity to cyclohexene and the yield are still as high as 81.4% and
26.0% respectively. Thus Ru–Zn(8.6%) catalyst has good prospects
for industrial application.
4. Conclusions
The Ru and Zn of Ru–Zn catalyst exist as metallic Ru and ZnO,
and ZnO is rich on the surface. The ZnO alone cannot improve
the selectivity to cyclohexene since it is difficult for it to be
highly dispersed on the surface of Ru–Zn catalyst. However, the
ZnO on the surface of Ru–Zn catalyst can react with the addi-
tive ZnSO4 to form a insoluble (Zn(OH)2)3(ZnSO4)(H2O)5 salt. The
(Zn(OH)2)3(ZnSO4)(H2O)5 salt can highly disperse on the surface of
Ru–Zn catalyst. The (Zn(OH)2)3(ZnSO4)(H2O)5 salt chemisorbed on
surface of Ru–Zn catalyst plays a key role in improving the selectiv-
ity to cyclohexene. In the pretreatment, more ZnO can react with
ZnSO4 to form more the (Zn(OH)2)3(ZnSO4)(H2O)5 salt. Moreover,
the (Zn(OH)2)3(ZnSO4)(H2O)5 salt can more stably be chemisorbed
on catalyst surface after the catalyst is pretreated in the presence
of ZnSO4. Therefore, the pretreatment makes Ru–Zn(8.6%) catalyst
exhibit the excellent selectivity to cyclohexene.
100
benzene conversion
cyclohexene selectivity
cyclohexene yield
80
60
40
20
0
Acknowledgments
1
2
3
4
5
6
7
Recycle times
This work was supported by the National Nature Science
Foundation of China (21273205), the Innovation Found for Tech-
nology Based Firms of China (10C26214104505) and the Scientific
Research Foundation of Graduate School of Zhengzhou University.
The authors thank Professor Lu Zhu for XRD analysis.
Fig. 8. Benzene conversion, cyclohexene selectivity and yields over the pretreated
Ru–Zn(8.6%) catalyst in seven recycles. Pretreatment conditions: a share of catalyst,
280 ml H2O, 49.2 g ZnSO4, 5 MPa H2, stirring rate of 800 r/min, temperature of 413 K.
Reaction conditions: stirring rate of 1400 r/min, 140 ml benzene, and temperature
of 423 K.

168
H.-j. Sun et al. / Applied Catalysis A: General 450 (2013) 160–168
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