Ru-Zn catalysts for selective hydrogenation of benzene using coprecipitation in low alkalinity

Article abstract of DOI:10.1016/S1872-2067(14)60231-X

Several unsupported Ru-Zn catalysts were successfully prepared using the coprecipitation method under low alkaline conditions, and their catalytic performance was evaluated for the selective liquid-phase hydrogenation of benzene. The effect of the amount of ZnCl₂ added to the coprecipitation solution on the physical and catalytic properties of the Ru-Zn catalysts was studied whilst keeping the amount of the NaOH precipitant constant. The properties of the resulting catalysts were characterized by N₂ adsorption, X-ray diffraction, and temperature-programmed reduction. The effects of the stirring rate and the amount of ZnSO₄ additive on the catalytic properties of the Ru-Zn catalysts were investigated using the optimal Zn content. The recyclability of the optimal Ru-Zn catalyst was also explored. The results revealed that the optimal Zn content for the Ru-Zn catalysts was 16.7 wt%, and the selectivity for cyclohexene could reach up to 80% (yield > 45%) when the benzene conversion was 57% in an aqueous solution of ZnSO₄ (0.45 mol/L) under the optimal reaction conditions (i.e., hastelloy reactor, 1200 r/min, 150 °C and 5 MPa of H₂ pressure). The presence of ZnO crystals in the Ru catalysts was found to be critical to obtaining high selectivity for cyclohexene (>80%). The Ru-Zn catalysts prepared under the low alkaline conditions also showed good stability, which indicates that they could potentially be used for industrial application.

Full text of DOI:10.1016/S1872-2067(14)60231-X

Chinese Journal of Catalysis 36 (2015) 400–407
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Ru‐Zn catalysts for selective hydrogenation of benzene using
coprecipitation in low alkalinity
Zhengbao Wang*, Qi Zhang, Xiaofei Lu, Shuangjia Chen, Chunjie Liu
College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, Zhejiang, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Several unsupported Ru‐Zn catalysts were successfully prepared using the coprecipitation method
under low alkaline conditions, and their catalytic performance was evaluated for the selective liq‐
uid‐phase hydrogenation of benzene. The effect of the amount of ZnCl2 added to the coprecipitation
solution on the physical and catalytic properties of the Ru‐Zn catalysts was studied whilst keeping
the amount of the NaOH precipitant constant. The properties of the resulting catalysts were charac‐
terized by N2 adsorption, X‐ray diffraction, and temperature‐programmed reduction. The effects of
the stirring rate and the amount of ZnSO4 additive on the catalytic properties of the Ru‐Zn catalysts
were investigated using the optimal Zn content. The recyclability of the optimal Ru‐Zn catalyst was
also explored. The results revealed that the optimal Zn content for the Ru‐Zn catalysts was 16.7
wt%, and the selectivity for cyclohexene could reach up to 80% (yield > 45%) when the benzene
conversion was 57% in an aqueous solution of ZnSO4 (0.45 mol/L) under the optimal reaction con‐
ditions (i.e., hastelloy reactor, 1200 r/min, 150 °C and 5 MPa of H2 pressure). The presence of ZnO
crystals in the Ru catalysts was found to be critical to obtaining high selectivity for cyclohexene
(>80%). The Ru‐Zn catalysts prepared under the low alkaline conditions also showed good stability,
which indicates that they could potentially be used for industrial application.
Received 22 August 2014
Accepted 24 September 2014
Published 20 March 2015
Keywords:
Coprecipitation
Low alkalinity
Benzene
Selective hydrogenation
Ru‐Zn catalyst
© 2015, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
towards the development of new methods for the selective
hydrogenation of benzene to cyclohexene, and Ru‐based cata‐
Cyclohexene is an important intermediate in the fine chem‐
icals industry, where it is used for the construction of many
organic chemicals, and this compound is therefore considered
to be of significant commercial importance. The production of
cyclohexanol from benzene via cyclohexene is a green process,
but the commercialization of this process is considered by
many scientists in the fine chemicals industry to be difficult
because the partial hydrogenation of benzene is thermody‐
namically unfavorable. Several innovative techniques, however,
have been developed to allow for the efficient partial hydro‐
genation of benzene. Numerous studies have been conducted
lysts have been being used in the majority of these cases be‐
cause they give the best yield of cyclohexene. The first reported
example of the Ru‐based catalyst for the hydrogenation of
benzene was provided by Hartog et al. [1] in 1963. In this par‐
ticular case, benzene was hydrogenated over a Ru‐black cata‐
lyst in the presence of an aliphatic alcohol, but the yield of cy‐
clohexene was reported to be as low as 2.2%. The first encour‐
aging result for this reaction was obtained by Drinkard et al.
[2], who reported that a much higher yield (30%) of cyclohex‐
ene could be achieved when benzene was hydrogenated over a
Ru‐based catalyst in the presence of water, and this basic idea
*Corresponding author. Tel: +86‐571‐87952391; Fax: +86‐571‐87951227; E‐mail: zbwang@zju.edu.cn
This work was supported by the National Natural Science Foundation of China (U1162129).
DOI: 10.1016/S1872‐2067(14)60231‐X | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 36, No. 3, March 2015

Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
401
was subsequently developed further by several other re‐
during these processes.
searchers [3–30]. The effects of numerous promoters and
co‐catalysts on this reaction have also been studied in detail
[5–8].
In this study, we have developed a new process for prepar‐
ing Ru‐Zn catalysts using the coprecipitation method under low
alkaline conditions, whilst also avoiding the need to wash the
resulting catalyst with an alkaline solution. Furthermore, the
catalysts produced in this way showed high selectivity towards
cyclohexene during the selective hydrogenation of benzene.
The effect of the amount of ZnCl2 added to the coprecipitation
solution was investigated, as well as the impact of several other
reaction conditions (e.g., stirring rate) and the amount of
ZnSO4. The stability of the optimal Ru‐Zn catalyst was also
evaluated.
In 1988, Nagahara et al. [8] of Asahi‐Kasei Chemical Co., Ltd
reported the synthesis of an efficient catalyst composed of a
Ru‐black promoted with ZnO that provided a high yield of cy‐
clohexene (48%) from benzene with a selectivity of 80%. This
particular process was carried out in a mechanically agitated
tetraphase reactor (i.e., oil phase: benzene; aqueous phase:
ZnSO4 aqueous solution; gas phase: H2; and solid phase:
Ru‐based catalyst) at 150 °C under 5 MPa of H2 pressure in the
presence of suspended ZrO2, which was added to avoid the
agglomeration of the catalyst. Based on this process, Asa‐
hi‐Kasei developed the first commercial plant for the product of
cyclohexene in 1990 that produced 60 000 t of this material per
year via the selective hydrogenation of benzene.
2. Experimental
2.1. Catalyst preparation
Supported [1–24] and unsupported [25–33] Ru‐based cata‐
lysts have been investigated extensively. Although numerous
reports have been published in the literature pertaining to the
development and application of supported Ru‐based catalysts,
there have been no reports, to the best of our knowledge, con‐
cerning the commercial application of these catalysts for the
selective hydrogenation of benzene. Considerable research
efforts have been devoted to the development of unsupported
Ru‐based catalysts by Liu and his team in China [16–24]. To
date, Ru‐black promoted with ZnO (designated as Ru‐Zn) has
been widely used by numerous researchers for the selective
hydrogenation of benzene, where it has been reported to ex‐
hibit a high level of reactivity [6–11,20,22]. Ru‐Zn catalysts
have been prepared by a variety of different methods, including
coprecipitation [6–11,20,22] and chemical reduction [17]. The
process involved in the coprecipitation method is operationally
simple, and the commercial Ru‐Zn catalyst used in Asahi‐Kasei
process is also prepared by this method. Liu et al. [20,22] have
conducted detailed research pertaining to the preparation of
Ru‐Zn catalysts using the coprecipitation method and the sub‐
sequent evaluation of their catalytic activity. However, the
Ru‐Zn catalysts generated in these studies were coprecipitated
under high alkaline conditions and washed with an alkaline
solution following their preparation by the coprecipitation
method, which is similar to the process used by Asahi‐Kasei [8].
Using processes of this type, it can be difficult to control the Ru
and Zn contents in the final Ru‐Zn catalysts because Zn and
even Ru atoms can be lost during the coprecipitation and
washing processes under the highly alkaline conditions. Fur‐
thermore, a large amount of alkaline waste water is produced
2.1.1. Low alkaline conditions
The Ru‐Zn catalysts were prepared as follows. A NaOH solu‐
tion (4% (m/V), 35 mL) was quickly added into a stirred solu‐
tion of RuCl3·xH2O (2.5 g; Shenyang Nonferrous Metal Research
Institute, Shenyang, China; Ru content: 36–38 wt%) and the
desired amount of ZnCl2 (Sinopharm Chemical Reagent Co., Ltd,
Shanghai, China) in water (250 mL), and the resulting solution
was stirred for 2 h at 80 °C. The reaction mixture was then
cooled to ambient temperature and stirred overnight. The su‐
pernatant (~135 g) was decanted, and the remaining mixture
was placed into a 250 mL Teflon‐lined autoclave. H2 was fed
into the autoclave after the system had been purged five times,
and the purged mixture was reduced under 5 MPa of H2 pres‐
sure at 150 °C and a stirring rate of 1000 r/min for 3 h. The
mixture was then cooled to ambient temperature, and the re‐
sulting Ru‐Zn black powder was washed with water until no Cl–
could be detected. The resulting Ru‐Zn catalysts were stored in
water, and a series of Ru‐Zn catalysts (Ru‐Zn‐0 to Ru‐Zn‐4) was
prepared by changing the amount of ZnCl2 added to the co‐
precipitation solution. The details of these catalysts are shown
in Table 1.
2.1.2. High alkalinity conditions
The Ru‐Zn‐5 catalyst was prepared according to the proce‐
dures from the literature [8,20,22]. Briefly, a stirred solution of
RuCl3·xH2O (2.5 g) and ZnCl2 (3.25 g) in H2O (250 mL) was
treated with a NaOH solution (30% (m/V), 35 mL), and the
resulting mixture was agitated for 2 h at 80 °C. The mixture was
then cooled to ambient temperature to give a black precipitate,
which was washed three times with a NaOH solution 4% (m/V)
Table 1
Zn contents in the Ru‐Zn catalysts and their textural properties.
ZnCl2
(g)
pH of reduction
Zn/Rua
(m/m)
0.00
0.08
0.16
0.24
0.32
1.73
Zn contentb
(wt%)
0.0
Zn/Rub
(m/m)
0.00
0.10
0.20
0.26
0.31
0.13
Surface area
(m2/g)
71.7
Pore volume
(cm3/g)
0.34
Pore sizec
(nm)
19.2
17.0
16.0
17.0
15.3
—
Catalyst
solution
>12
>11
~10
~6
~6
Ru‐Zn‐0
Ru‐Zn‐1
Ru‐Zn‐2
Ru‐Zn‐3
Ru‐Zn‐4
Ru‐Zn‐5d
0.00
0.15
0.30
0.45
0.60
3.25
7.6
13.4
16.7
19.7
72.8
68.9
58.7
59.8
0.32
0.29
0.24
0.24
>13
9.6
—
—
a Theoretical value. b Measured by ICP. c BJH desorption average pore diameter (4V/A). d 30% NaOH aq. solution.

402
Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
after the supernatant had been removed by decantation. The
resulting black precipitate was dispersed in a NaOH solution
(5% (m/V), 150 mL) and charged into a 250 mL Teflon‐lined
autoclave. The reduction process used in this particular case
was similar to the method described above, except the reduc‐
tion time was increased to 12 h. The reduction mixture was
then cooled to ambient temperature, and the resulting Ru‐Zn
black powder was washed three times with a NaOH solution
5% (m/V), and then washed with water until no Cl– could be
detected. The resulting Ru‐Zn catalyst was stored in water.
of benzene per hour over 1 g of Ru at a conversion of 40%.
For experiments concerning the recyclability of the Ru‐Zn‐3
catalyst, the organic phase was removed from the autoclave by
suction, and the remaining slurry containing the Ru‐Zn catalyst,
ZrO2, and ZnSO4 was used in the next experiment. Experiments
involving the recycled catalyst were conducted under the same
conditions as those described above.
3. Results and discussion
3.1. Catalyst characterization results
2.2. Catalyst characterization
As shown in Table 1, five unsupported Ru‐Zn catalysts with
different Zn contents (i.e., Ru‐Zn‐0 to 4) were prepared by the
coprecipitation method under low alkaline conditions. The
amount of the NaOH precipitant (4%) was kept constant in all
of the experiments involving the addition of different amounts
of ZnCl2. The pH of the precipitation solution therefore de‐
creased gradually as the amount of ZnCl2 added to the mixture
increased. The pH value of the reduction solution was less than
7 when the amount of ZnCl2 added to the solution was greater
than 0.45 g. The Ru and Zn contents in the precipitation and
reduction solutions were measured by ICP, and found to be
lower than 0.25 ppm in both cases, which indicated that very
little Ru and Zn were lost to the solution during the precipita‐
tion and reduction processes. This result was confirmed by the
similarity in the Ru/Zn ratios between the ICP results and the
theoretical values.
If Ru and Zn existed in their respective atomic states in the
catalysts, then the theoretical total contents of Ru and Zn in
Ru‐Zn‐0 to 4 would be 0.91, 0.98, 1.06, 1.13, and 1.20 g, respec‐
tively. In practice, the final mass of Ru and Zn in the catalysts
synthesized in the current study (i.e., Ru‐Zn‐0 to 4) was 0.90,
0.96, 1.11, 1.31, and 1.41 g, respectively. When the Zn content
in the catalysts was higher than 16.7 wt% (e.g., Ru‐Zn‐3 and 4),
the final mass of the catalysts was higher than the theoretical
values. These results therefore suggested that these catalysts
could also be made up of some Ru and Zn oxide species. Fig. 1
shows the XRD patterns of all five catalysts. When the Zn con‐
tent was lower than 13.4 wt% (e.g., Ru‐Zn‐0 to 2), the XRD pat‐
tern only contained diffraction peaks that could be assigned to
The Ru‐Zn catalysts prepared above were stored in water
immediately after their preparation and were subsequently
vacuum‐dried at 40 °C prior to being characterized. The Ru and
Zn contents in the catalysts were measured on an Optima 7000
DV ICP‐OES Spectrometer (PerkinElmer, USA). X‐ray powder
diffraction (XRD) patterns were collected on an Ultima IV dif‐
fractometer (Ultima, Rigaku, Japan) using Cu K radiation. N2
physisorption experiments were performed on a Micromeritics
ASAP 2020 system (USA). The total surface area of each cata‐
lyst was obtained using the BET equation, and the pore size
was determined by BJH desorption analysis.
Temperature‐programmed reduction (TPR) experiments
were performed on a PX200 Multi‐sorption equipment (Tianjin
Golden Eagle Technology Co., Ltd, China). In a typical run, a
sample of the catalysts (without reduction) that was kept in
water (~5 mg dry basis) was added with water directly to the
reactor where it was heated at 300 °C for 2 h under Ar stream
(10 mL/min). The H2‐TPR curves were subsequently deter‐
mined by passing 5% H2‐95% Ar stream (20 mL/min) through
the preheated sample whilst the temperature was increased
from 25 to 500 °C at a rate of 10 °C/min. A thermal conductivity
detector (TCD) was used to determine the amount of H2 con‐
sumed by this process.
2.3. Reactivity testing
The selective hydrogenation of benzene was performed in a
250 mL autoclave (316L stainless steel or hastelloy C276, Bei‐
jing Century Senlong Experimental Apparatus Co., Ltd, China)
with a magnetic stirrer. In a typical reaction, the autoclave was
charged with water (70 mL), Ru‐Zn catalyst (0.12 g, dry basis),
ZnSO4·7H2O (8.4 g), and ZrO2 powder (0.6 g; RC‐100, Daiichi
Kigenso Kagaku Kogyo Co., Ltd, Japan). H2 gas was fed into the
autoclave (1.0 MPa) immediately after it had been purged 5
times with N2, and the autoclave was then heated at a stirring
rate of 300 r/min. Benzene (35 mL) was added to the autoclave
when the temperature reached 150 °C, and the H2 pressure and
the stirring rate were adjusted to 5 MPa and 1200 r/min, re‐
spectively. The reaction was then stirred at 150 °C for 10–60
min. The products in the organic phase were analyzed by GC on
a GC‐1690 system (Hangzhou Kexiao) equipped with an FID
detector. The resulting GC data were used to calculate the ben‐
zene conversion and cyclohexene selectivity. The specific activ‐
ity of the catalyst (40) was defined as the converted amount (g)
Ru-Zn-0
Ru-Zn-1
Ru-Zn-2
Ru-Zn-3
Ru-Zn-4
10
20
30
40
50
60
70
80
90
2/(o )
Fig. 1. XRD patterns of the Ru‐Zn catalysts with different Zn contents.

Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
403
the metallic Ru phase (2 = 38.4°, 44.0°, 58.3°, 69.4°, 78.4°, and
84.7°). Evaluation of the Ru crystallite size from the XRD pat‐
terns using the Scherrer formula indicated that the nature of
the Zn species had very little impact on the Ru crystallite size
(~4.0 nm) in the Ru‐Zn catalysts. When the Zn content was
higher than 16.7 wt%, the XRD patterns contains seven diffrac‐
tion peaks that could be assigned to the hexagonal phases of
ZnO (2 = 31.8°, 34.4°, 36.3°, 47.5°, 56.6°, 62.9°, and 68°), which
indicated the presence of ZnO in the Ru‐Zn‐3 and Ru‐Zn‐4 cat‐
alysts. The XRD patterns of the Ru‐Zn catalysts before the re‐
duction did not contain any peaks corresponding to crystals
(data not shown).
The XRD peaks corresponding to the ZnO phase were also
observed for the Ru‐Zn‐3 catalyst after the reduction time was
extended to 6 h (data not shown), which indicated that the ZnO
was difficult to reduce at 150 °C under 5 MPa of H2 pressure.
These results also indicated that the ZnO particles were highly
dispersed in the Ru‐Zn catalysts when the Zn content was low‐
er than 13.4 wt% (e.g., Ru‐Zn‐1 and Ru‐Zn‐2). The XRD peaks
corresponding to Ru oxide were not observed for these cata‐
lysts, which indicated that they did not contain Ru oxides or the
Ru oxides were highly dispersed in all of the catalysts.
To compare the properties of the Ru‐Zn catalysts prepared
under low alkaline conditions with those of the corresponding
catalyst prepared under high alkaline conditions, we prepared
Ru‐Zn‐5 by adding a 30% NaOH solution to the precipitation
solution, and washing the resulting catalyst in NaOH aqueous
solution (4% (m/V)) before reducing it in a NaOH solution (5%
(m/V)). As shown in Table 1, the final Zn content of Ru‐Zn‐5
was only 9.6 wt% with a Zn/Ru ratio of 0.13 (as determined
from the ICP data). This Zn/Ru ratio was much lower than that
of the theoretical value (1.73) even though ZnCl2 (3.25 g) was
added to the precipitation solution. The XRD pattern of Ru‐Zn‐5
did not contain any diffraction peaks corresponding to the ZnO
phase (data not shown), which indicated that most of the Zn
species must have been lost to the solution and that it is there‐
fore difficult to control the Zn content in Ru‐Zn catalysts pre‐
pared under high alkaline conditions.
the Ru‐Zn‐3 catalyst, which was 139.3 °C. This difference in the
reduction temperatures of two catalysts indicated that the ad‐
dition of Zn species effectively inhibited the reduction of the Ru
species. The H2‐TPR profile of the Ru‐Zn‐3 catalyst contained a
peak with shoulders between 100 and 170 °C, which were at‐
tributed to the step‐by‐step reduction of RuO2 to metallic Ru
[22]. Similar H2‐TPR profiles have been reported in the litera‐
ture for Ru‐Zn catalysts containing different Zn contents by Liu
and his coworkers [22]. The temperatures required for the
complete reduction of all of the Ru‐Zn catalysts were close to
the reaction temperature 150 °C, which indicated that metallic
Ru was the only Ru species present in the catalyst when it was
heated at 150 °C under 5 MPa of H2 pressure.
The H2‐TPR profiles of the reduced Ru‐Zn catalysts were al‐
so measured. The reduced Ru‐Zn catalysts that had been kept
in water were added directly to the H2‐TPR reactor. The sam‐
ples were then dried and preheated at 300 °C in Ar gas for 2 h.
The H2‐TPR profiles of the resulting materials are shown in Fig.
3. These results revealed that the reduction peak appeared at
room temperature for the catalysts with low Zn contents (e.g.,
Ru‐Zn‐1 and Ru‐Zn‐2), and that the reduction process started
from 30 °C and peaked at temperatures greater than 56 °C for
the catalysts with high Zn contents (e.g., Ru‐Zn‐3 and Ru‐Zn‐4).
Furthermore, the reduction peak of the catalyst with the lowest
Zn content (i.e., Ru‐Zn‐1) was much larger than that of a cata‐
lyst with a high Zn content (e.g., Ru‐Zn‐3) (Fig. 4), which indi‐
cated that the catalysts with low Zn contents could be readily
reduced and oxidized. In fact, the H2‐TPR reduction peaks of
the catalysts reduced in the autoclave and subsequently vacu‐
um‐dried were much larger than those of the catalysts reduced
in the autoclave, stored in water and directly added to the TPR
reactor, especially for the catalysts with low Zn contents. Taken
together, these results indicated that the reduced Ru‐Zn cata‐
lysts could be readily oxidized, especially those with low Zn
contents.
Interestingly, it was found that catalysts prepared with the
same amount of RuCl3·xH2O but different amounts of ZnCl2
exhibited different sedimentation behavior in water. As shown
in Fig. 5, the dispersion degree of the catalyst in water in‐
creased with increasing Zn content. Thus, catalysts with high
Zn contents (e.g., Ru‐Zn‐3 and Ru‐Zn‐4) showed good disper‐
Fig. 2 shows the H2‐TPR profiles of the Ru‐Zn‐0 and Ru‐Zn‐3
catalysts without reduction. The reduction temperature for the
Ru‐Zn‐0 catalyst was 111.3 °C, clearly much lower than that of
Ru-Zn-0
Ru-Zn-3
Ru-Zn-4
Ru-Zn-3
Ru-Zn-2
Ru-Zn-1
100
200
300
400
500
0
100
200
300
400
500
Temperature (oC)
Temperature (oC)
Fig. 3. H2‐TPR profiles of Ru‐Zn catalysts with different Zn contents
Fig. 2. H2‐TPR profiles of catalysts Ru‐Zn‐0 and Ru‐Zn‐3 before reduc‐
after reduction.
tion.

404
Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
genation of benzene to cyclohexene. The performance of the
600
500
400
300
200
100
0
Ru‐Zn catalysts was evaluated in terms of the reaction time,
benzene conversion, cyclohexene selectivity and yield, and the
results are summarized in Table 2. It is noteworthy that the
reaction time was adjusted according to the reactivity over the
catalyst to obtain similar levels of benzene conversion. The
Ru‐Zn‐0 catalyst, which did not contain any Zn, gave the lowest
selectivity for cyclohexene over all the catalysts tested, despite
also providing the lowest benzene conversion. Based on the
reaction time required over the different catalysts to obtain
similar levels of benzene conversion, it became clear that the
activity of the catalysts decreased with increasing Zn content,
whereas the selectivity and yield of cyclohexene increased. In
other words, the addition of Zn led to significant improvements
in the selectivity and yield of cyclohexene. As proposed in the
literature [11,22], the main reason for this improvement in the
selectivity and yield of cyclohexene is that the Zn component in
the catalysts inhibits the resorption of the cyclohexene product
onto the surface of the catalyst, and therefore suppresses the
hydrogenation of cyclohexene to cyclohexane. As shown in
Table 2, the Ru‐Zn‐0 catalyst provided lower levels of benzene
conversion and cyclohexene selectivity than the Ru‐Zn‐1 cata‐
lyst after the same reaction time. As described above in Section
3.1, these differences in the performance of the catalysts could
be attributed to the poorer dispersion of the Ru‐Zn‐0 catalyst in
water. No significant increase was observed in the selectivity of
the catalyst Ru‐Zn‐4 although a longer reaction time of 60 min
was needed. The cyclohexene selectivity over the Ru‐Zn‐3 cat‐
alyst was higher than 80% with benzene conversion of 47%,
and it should be possible to separate the cyclohexene and cy‐
clohexane products by solvent extraction because of their sim‐
ilar boiling points. Naturally, the separation costs associated
with processes with high cyclohexene selectivity will be lower
than those incurred for processes with low cyclohexene selec‐
tivity. With this in mind, a cyclohexene selectivity of greater
than 80% is required by the fine chemicals industry [11]. The
performance characteristics of the Ru‐Zn‐3 catalyst clearly
meet this need, indicating that the Ru‐Zn‐3 catalyst could po‐
tentially be used in commercial application. The Ru‐Zn‐3 cata‐
lyst was used to investigate the effects of several other factors
on the performance of the hydrogenation reaction, and the
results are discussed in detail below. According to the literature
[20,22], the best Zn content for the hydrogenation of benzene
to cyclohexene is 8–10 wt%. The catalysts described in those
studies were prepared under highly alkaline conditions and
Ru-Zn-1
Ru-Zn-3
0
500 1000 1500 2000 2500 3000 3500
Time (s)
Fig. 4. H2‐TPR profiles of catalysts Ru‐Zn‐1 and Ru‐Zn‐3 after reduction.
Fig. 5. Photos of Ru‐Zn catalysts with different Zn contents. (a) Ru‐Zn‐0;
(b) Ru‐Zn‐1; (c) Ru‐Zn‐2; (d) Ru‐Zn‐3; (e) Ru‐Zn‐4.
sion in water. There are three possible reasons for these dif‐
ferences in the dispersion behavior, including (1) the total
amount of the catalyst increases with increasing Zn content; (2)
the hydrophilicity of the catalyst increases with increasing Zn
content; and (3) the Ru species can be readily reduced in the
catalysts with low Zn contents, and the resulting catalyst can
readily aggregate after reduction.
The textural properties of the Ru‐Zn catalysts were charac‐
terized by N2 physisorption, and the results are shown in Table
1. As mentioned above, the Ru‐Zn catalysts were composed of
Ru nanoparticles (~4 nm in diameter). The nanoparticles read‐
ily aggregated, which meant that there were mesopores in the
catalysts after they had been dried. It was found that the BET
surface areas and pore volumes of the catalysts with high Zn
contents (e.g., Ru‐Zn‐3 and Ru‐Zn‐4) decreased significantly
compared with the catalysts with low Zn contents. This result
was consistent with the appearance of a ZnO phase in these two
catalysts. Another reason for this decrease in the BET surface
areas and pore sizes of these catalysts could be a decrease in
the Ru nanoparticle contents in these catalysts. The pore sizes
of catalysts containing Zn were slightly smaller than that of the
catalyst without Zn. Despite these minor differences, there was
no clear trend between the pore size and the Zn content in the
catalysts.
Table 2
Effects of the Zn content on the reaction properties.
Reaction time
Conversion
(%)
Selectivity
(%)
47.4
58.8
75.0
80.7
81.9
65.8
Yield
(%)
Catalyst
(min)
10
10
20
45
Ru‐Zn‐0
Ru‐Zn‐1
Ru‐Zn‐2
Ru‐Zn‐3
Ru‐Zn‐4
Ru‐Zn‐5
41.0
55.2
47.3
47.2
47.9
50.4
19.4
32.4
35.5
38.1
39.2
33.2
3.2. Catalytic properties
60
20
3.2.1. Effects of the Zn content
Reaction conditions: catalyst 0.12 g, ZrO2 0.6 g, ZnSO4·7H2O 8.4 g, C6H6
35 mL, H2O 70 mL, 150 °C, 5 MPa of H2 pressure, stirring rate 1200
r/min.
The catalytic performance of the Ru‐Zn catalysts with dif‐
ferent Zn contents was investigated for the selective hydro‐

Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
405
had to be pretreated in the aqueous solution of ZnSO4·7H2O
100
80
60
40
20
0
and suspended ZrO2 solution before the reaction. The Ru‐Zn‐3
catalyst was prepared under low alkaline conditions and did
not require a pretreatment process. As shown in Table 2, the
Ru‐Zn‐5 catalyst, which was prepared under highly alkaline
conditions, showed lower selectivity as well as a lower yield of
cyclohexene than the other catalysts prepared under low alka‐
line conditions (e.g., Ru‐Zn‐2 to 4). The XRD pattern of the
Ru‐Zn‐5 catalyst did not contain any peaks that could be as‐
signed to ZnO crystals (data no shown). Based on the results of
the XRD analysis (Fig. 1) and the outcome of the reactions (Ta‐
ble 2), it was proposed that the presence of ZnO crystals in the
Ru‐based catalysts is critical to obtaining high selectivity for
cyclohexene (>80%).
Conversion
Selectivity
Yield
0.0
0.2
0.4
0.6
0.8
Concentration of ZnSO
4
(mol/L)
Fig. 6. Effect of the ZnSO4 concentration on the outcome of the reaction.
Reaction conditions: Ru‐Zn‐3 catalyst 0.12 g, ZrO2 0.6 g, ZnSO4·7H2O
0.0–12.3 g, C6H6 35 mL, H2O 70 mL, 150 °C, 5 MPa of H2 pressure, 45
min, stirring rate 1200 r/min.
3.2.2. Effects of the stirring rate
The catalytic performance of the Ru‐Zn‐3 catalyst was in‐
vestigated under a variety of different stirring rates, and the
results are shown in Table 3. The results showed that the stir‐
ring rate had very little impact on the benzene conversion and
yield of cyclohexene when it was between 1100 and 1300
r/min. Struijk et al. [13] reported a strong increase in the initial
rate of hydrogen uptake with the stirring rate as the stirring
rate was below 1000 r/min, and that the rate of hydrogen up‐
take increased only gradually at stirring rates above 1000
r/min. At stirring rates above 1000 r/min, the diffusional re‐
tardation of the reaction rate due to the mass transport of hy‐
drogen at the gas/liquid interface and of benzene at the liq‐
uid/liquid interface is minimized. Liu’s group [19] also found
that the reaction selectivity for cyclohexene increased with
increasing stirring rate when it was slower than 800 r/min.
Struijk et al. [13] reported that stirring rates of higher than
2000 r/min led to the attrition of the catalyst, with the catalyst
also adhering to the inner wall of the reactor. A stirring rate of
1400 r/min was used by Liu et al. [22] in their study of the ef‐
fect of zinc contents. The results of the current study revealed
that a stirring rate of 1100–1300 r/min was optimal for the
production rate of cyclohexene, at least when a magnetic stirrer
was used with our reactor system.
clohexene increased significantly from about 23.6% up to
89.2%, when the ZnSO4 concentration was increased from 0 to
0.3 mol/L. However, a further increase in the ZnSO4 concentra‐
tion from 0.30 to 0.60 mol/L led to a slight decrease in the se‐
lectivity for cyclohexene although the benzene conversion and
yield of cyclohexene increased significantly. Although the yield
increased with increasing ZnSO4 concentration, the selectivity
for cyclohexene was lower than 80% when the concentration of
ZnSO4 was higher than 0.5 mol/L. The optimal concentration of
ZnSO4 was therefore determined to be 0.42–0.45 mol/L. When
the concentration of ZnSO4 was 0.45 mol/L, the yield of cyclo‐
hexene reached 45% with a selectivity of greater than 80%.
The specific activity (40) and selectivity (S40) of the Ru‐Zn‐3
catalyst were found to be 166 g/(g·h) and 85.5% at ZnSO4 con‐
centration of 0.38 mol/L, and 185 g/(g·h) and 83.0% at ZnSO4
concentration of 0.42 mol/L, respectively. Based on commercial
estimations, a specific activity (40) of greater than 100 g/(g·h)
is required for the reduction of benzene to cyclohexene togeth‐
er with a selectivity (S40) of greater than 80%, and the perfor‐
mance characteristics of the Ru‐Zn‐3 catalyst clearly meet these
requirements. Furthermore, the results of the current study are
comparable with data from the literature, which gave 40 and
3.2.3. Effect of the amount of ZnSO4
S40 values of 155 g/(g·h) and 85.5%, respectively [34].
The zinc sulphate (ZnSO4·7H2O) concentration was varied in
a series of separate experiments designed to develop a deeper
understanding of the influence of salts on the performance of
the Ru‐Zn catalyst, and the results are shown in Fig. 6. The re‐
sults show that the addition of a small amount of ZnSO4 sup‐
pressed the reaction rate, but also led to a significant increase
in the selectivity for cyclohexene. The initial selectivity for cy‐
It has been shown that the presence of an aqueous salt solu‐
tion is essential for obtaining a high yield of cyclohexene in the
selective hydrogenation of benzene over Ru‐based catalysts
[13], and the results of the current study are similar to those
reported elsewhere in the literature [14]. Liu et al. [16] inves‐
tigated the influence of different Zn ion concentrations and pH
values on the performance of the Ru‐Fe‐B/ZrO2 catalyst, and
found that the optimal concentration of Zn2+ in the slurry was
in the range of 0.5–0.6 mol/L with a pH value of 5.4–5.5. The
impact of the salt solution can be explained as follows. ZnSO4
would be chemisorbed onto the surface of the Ru catalyst,
which would result in the Ru becoming hydrophilic. The Ru
catalyst particles would therefore be surrounded by a layer of
water, which would lead to a strong diffusional resistance to‐
wards mass transfer of hydrogen and cyclohexene to the Ru
surface. The presence of such a water layer would also slow
Table 3
Effect of stirring rate on the reaction properties over the Ru‐Zn‐3 cata‐
lyst.
Stirring rate (r/min) Conversion (%) Selectivity (%) Yield (%)
1100
1200
1300
47.8
48.3
47.7
83.9
82.3
84.2
40.1
39.8
40.1
Reaction conditions: Ru‐Zn‐3 catalyst 0.12 g, ZrO2 0.6 g, ZnSO4·7H2O 8.4
g, C6H6 35 mL, H2O 70 mL, 150 °C, 5 MPa of H2 pressure, 45 min.

406
Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
method under low alkaline conditions. The incorporation of Zn
100
80
60
40
20
0
species led to a decrease in the activity of the Ru catalyst alt‐
hough the selectivity was improved significantly. The optimal
Zn content in the Ru‐Zn catalysts was determined to be 16.7
wt% (i.e., Ru‐Zn‐3). The use of a stirring rate in the range of
1100 to 1300 r/min had very little impact on the catalytic reac‐
tion. The addition of small amount of ZnSO4 to the reaction
solution led to a decrease in the benzene conversion and a sig‐
nificant improvement in the selectivity, with a ZnSO4 concen‐
tration of >0.3 mol/L leading to an increase in the benzene
conversion. The optimal concentration of ZnSO4 was found to
be 0.42–0.45 mol/L. The selectivity for cyclohexene reached
80% (yield > 45%) when the conversion of benzene was 57%
in ZnSO4·7H2O solution of 0.45 mol/L under the optimal reac‐
tion conditions (i.e., hastelloy reactor, 150 °C, 5 MPa of H2
pressure, 1200 r/min). The presence of ZnO crystals in the
Ru‐based catalysts was very important to obtain a high selec‐
tivity for cyclohexene (>80%). Taken together, these results
demonstrate that Ru‐Zn catalysts prepared under low alkaline
conditions could be used for industrial application.
Conversion
Selectivity
Yield
1
2
3
4
5
Recycle times
Fig. 7. Recyclability of the Ru‐Zn‐3 catalyst for the selective hydrogena‐
tion of benzene to cyclohexene. Reaction conditions: Ru‐Zn‐3 catalyst
0.12 g, ZrO2 0.6 g, ZnSO4·7H2O 8.4 g, C6H6 35 mL, H2O 70 mL, 150 °C, 5
MPa of H2 pressure, stirring rate 1200 r/min, 45 min.
down the hydrogenation of cyclohexene to cyclohexane be‐
cause it would suppress the direct hydrogenation of the ad‐
sorbed cyclohexene. The presence of a water layer would also
slow down the rate of cyclohexene re‐adsorption because of the
poor solubility of cyclohexene in water. Liu’s group [22] re‐
cently proposed that the synergistic effect of ZnO and ZnSO4
enhanced the selectivity for cyclohexene. Namely, the
(Zn(OH)2)3(ZnSO4)(H2O)5 salt formed by ZnO on the surface of
the catalyst would react with ZnSO4 and play a key role in im‐
proving the selectivity of the catalyst for cyclohexene. However,
no explanation has been provided to date in the literature to
account for the observed increase in the benzene conversion
with increasing ZnSO4 concentration. We propose that the pH
value of the reaction solution decreases with increasing ZnSO4
concentration, which would result in the dissolution of the ZnO
on the surface of the catalyst, leading to an increase in the ben‐
zene conversion. However, further research would be required
to investigate this hypothesis in detail.
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have been successfully prepared using the coprecipitation

Zhengbao Wang et al. / Chinese Journal of Catalysis 36 (2015) 400–407
407
Graphical Abstract
Chin. J. Catal., 2015, 36: 400–407 doi: 10.1016/S1872‐2067(14)60231‐X
Ru‐Zn catalysts for selective hydrogenation of benzene using coprecipitation in low alkalinity
Zhengbao Wang*, Qi Zhang, Xiaofei Lu, Shuangjia Chen, Chunjie Liu
Zhejiang University
Oil phase
H2
Reduction (H2 5 MPa,
Zn
Ru
NaOH
80 oC, 2 h
1000 r/min, 150 oC)
Ru(OH)3 Zn(OH)2
Water washing
Water phase
Ru3+
Zn2+
The Ru‐Zn catalysts did not need to be washed after the coprecipitation and could just be washed with water (decantation) after the re‐
duction. The Ru‐Zn catalysts with a ZnO crystal phase showed high catalytic performance in the selective hydrogenation of benzene.
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