Selective hydrogenation of benzene to cyclohexene over Ru-Zn/ZrO2 catalysts prepared by a two-step impregnation method

Article abstract of DOI:10.1016/j.molcata.2015.12.013

Supported Ru-Zn/ZrO₂ catalysts are successfully prepared by a two-step impregnation method (Zn deposition-calcination-Ru deposition), and their catalytic performances for selective liquid-phase hydrogenation of benzene are investigated. Physical properties of the catalysts are characterized by N₂ adsorption, atomic absorption spectroscopy, X-ray diffraction, temperature-programmed reduction, transmission electron micrographs, dynamic light scattering and X-ray photoelectron spectroscopy. Effects of the Zn content, concentration of alkaline treatment (NaOH) after hydrogen reduction, and hydrogen reduction temperature on the properties of Ru-Zn/ZrO₂ catalysts with a fix Ru loading of 10.0 wt% are studied. It is found that the optimal ZnO content is 10.5 wt% in the first-step impregnation (ZnO-ZrO₂), and that the alkaline treatment after hydrogen reduction is very important for obtaining the optimal Zn content in the ultimate catalyst. Over the optimal Ru-Zn/ZrO₂ catalyst with 10.0 wt% Ru and 2.78 wt% Zn, the selectivity to cyclohexene can reach 80% at the benzene conversion of 50% under the reaction condition (1200 r/min, 150 °C, 5 MPa H₂). The specific activity of the catalyst is about 4 times higher than those of Ru-Zn catalysts prepared by coprecipitation. The recycle catalytic performance of the optimal Ru-Zn/ZrO₂ catalyst is also explored. There is no significant decrease of catalytic performance after four recycles, indicating good stability of the catalyst prepared by the two-step impregnation method.

Full text of DOI:10.1016/j.molcata.2015.12.013

Accepted Manuscript
Title: Selective hydrogenation of benzene to cyclohexene over
Ru-Zn/ZrO2 catalysts prepared by a two–step impregnation
method
Author: Xuhua Yan Qi Zhang Mingqiao Zhu Zhengbao Wang
PII:
S1381-1169(15)30176-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.12.013
MOLCAA 9715
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
26-10-2015
15-12-2015
18-12-2015
Please cite this article as: Xuhua Yan, Qi Zhang, Mingqiao Zhu, Zhengbao Wang,
Selective hydrogenation of benzene to cyclohexene over Ru-Zn/ZrO2 catalysts prepared
by a twondashstep impregnation method, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.12.013
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Selective hydrogenation of benzene to cyclohexene over Ru-Zn/ZrO2 catalysts
prepared by a two–step impregnation method
*
Xuhua Yan, Qi Zhang, Mingqiao Zhu, Zhengbao Wang zbwang@zju.edu.cn
College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027,
China
*
Corresponding author. Tel: +86-571-87952391; Fax: +86-571-87951227.
1

Graphical Abstract
2

Highlights
►
►
►
►
Ru-Zn/ZrO
2
catalysts are prepared by two-step impregnation.
Alkali treatment of Ru-Zn/ZrO
2
is necessary after reduction.
is obtained.
Optimal composition of Ru-Zn/ZrO
2
Specific activity of Ru-Zn/ZrO
2
catalysts is 4 times that of Ru-Zn catalysts.
3

Abstract
2
Supported Ru-Zn/ZrO catalysts are successfully prepared by a two–step impregnation
method (Zn deposition--calcination--Ru deposition), and their catalytic performances for
selective liquid-phase hydrogenation of benzene are investigated. Physical properties of
2
the catalysts are characterized by N adsorption, atomic absorption spectroscopy, X-ray
diffraction, temperature-programmed reduction, transmission electron micrographs,
dynamic light scattering and X-ray photoelectron spectroscopy. Effects of the Zn content,
concentration of alkaline treatment (NaOH) after hydrogen reduction, and hydrogen
reduction temperature on the properties of Ru-Zn/ZrO
2
catalysts with a fix Ru loading of
1
0.0 wt% are studied. It is found that the optimal ZnO content is 10.5 wt% in the
2
first-step impregnation (ZnO-ZrO ), and that the alkaline treatment after hydrogen
reduction is very important for obtaining the optimal Zn content in the ultimate catalyst.
Over the optimal Ru-Zn/ZrO catalyst with 10.0 wt% Ru and 2.78 wt% Zn, the selectivity
to cyclohexene can reach 80% at the benzene conversion of 50% under the reaction
condition (1200 r/min, 150 ºC, 5 MPa H ). The specific activity of the catalyst is about 4
times higher than those of Ru-Zn catalysts prepared by coprecipitation. The recycle
catalytic performance of the optimal Ru-Zn/ZrO catalyst is also explored. There is no
2
2
2
significant decrease of catalytic performance after four recycles, indicating good stability
of the catalyst prepared by the two–step impregnation method.
Keywords: Two–step impregnation; Alkaline treatment; Benzene; Selective
2
hydrogenation; Ru-Zn/ZrO catalyst.
4

1
. Introduction
Caprolactam and adipic acid are important industrial materials for production of
nylon-6 and nylon-66, and they can be produced by two processes. One process includes
selective oxidation of cyclohexane to cyclohexanone and cyclohexanol, and another one
includes selective hydrogenation of benzene to cyclohexene. However, for selective
oxidation of cyclohexane, the per-pass yield is as low as ~4% [1], and the oxidation
process of cyclohexane is prone to explosion. In contrast, the selective hydrogenation of
benzene usually affords higher per-pass yield (~40%), and the process is safer.
Cyclohexene, as an intermediate product in the hydrogenation of benzene, is a
thermodynamically less favorable product. To obtain cyclohexene with high selectivity,
the reaction should be carried out in a tetraphase reaction system (gas (hydrogen), oil
(benzene), water and solid (catalyst)), and the catalyst with high performance is needed.
There are many research reports about the selective hydrogenation of benzene [2–6].
Preparation of effective catalysts taking both activity and selectivity into consideration is
critical to this reaction. To date, ruthenium is widely accepted as the most effective metal
for selective hydrogenation of benzene to cyclohexene, and many different metals as
promoters are reported [7–23]. Ruthenium catalysts for selective hydrogenation of
benzene can be divided into two major types: unsupported Ru nanoparticles [24–30] and
supported Ru catalysts [7–23,31–33]. The latter is highly desirable because the noble
metal has high dispersion on the support surface and shows high activity. The Ru-Zn
catalyst prepared by the coprecipitation method has been used in the commercial plant
with the disperser of ZrO
2 2
by Asahi-Kasei Corporation. Therefore, Ru-Zn supported ZrO
catalysts have been extensively investigated.
5

2 3
Fan et al. [31] prepared Ru-Zn/m-ZrO catalysts by coprecipitation of RuCl and
ZrOCl
2
with NH
3
·H
2
O, followed by hydrogen reduction of the product in an ZnSO
4
o
aqueous solution (0.14 M) at 180 C for 40 min under 4.28 MPa H
2
pressure. The highest
catalysts was 43.4% (conversion 69.3% and
selectivity 62.7%), and the specific activity was 994 gBenzene/(gRu·h). Yuan et al. [9,10]
cyclohexene yield over Ru-Zn/m-ZrO
2
prepared Ru-Zn/ZrO
followed by reduction in different concentration ZnSO
.0 MPa H
The best cyclohexene yield up to 44% was obtained at the Zn content of 2.72 wt%.
2
catalysts by coprecipitation of RuCl
3
and ZrOCl
2
with KOH
o
4
solution at 180 C for 8 h, under
5
2
pressure, and obtained the catalysts with the Zn content of 2.42~2.95 wt%.
Huang and Liu [32] prepared Ru-Zn/ZrO
and ZrO
under 5.0 MPa H
8.5% and selectivity 60.1%) was obtained. Han et al. [23] prepared Ru/ZnO-ZrO
catalysts by coprecipitating a certain amount of ZnO, ZrO(NO and RuCl
, and reducing the obtained solid in a hydrogen flow at 180
2
catalysts by coprecipitation of RuCl
3 4
, ZnSO
o
2
with NaOH and reduction in the autoclave at 150 C and 1000 r/min for 3 h
2
pressure. The highest cyclohexene yield up to 41.2% (conversion
(OH)
in the
6
x
y
3
)
2
3
solution of NaOH and NaBH
4
o
C for 2 h. The Ru loading in the catalysts was 3 wt%. When the Zn/Zr molar ratio was
1
0:1, the highest cyclohexene yield was 56.0% (conversion 77.5% and selectivity 72.3%),
and the specific activity was 258 gBenzene/(gRu·h).
2
Ru-M-B/ZrO catalysts were prepared using a chemical reduction method (also
called reduction-impregnation method) and the effects of transition metals (M=La, Cr,
Mn, Fe, Co, Ni, Cu, and Zn) on the catalytic performance of the catalysts were
2
investigated by Liu et al. [17,18,33]. It was found that the Ru-Co-B/ZrO catalyst
exhibited excellent selectivity for cyclohexene (82.8% at the conversion of 75.8%) and
6

good stability, indicating its good prospect for industrial applications. The specific
activity of the Ru-Co-B/ZrO catalyst was 448 gBenzene/(gRu·h).
Many Ru supported catalysts with other hydrophilic supports (SiO
20], SBA-15 [11,12,16], Nb [13], TiO [14] and hydroxyapatite (HAP) [15]) have
also been investigated. For example, the Ru-Ce/SBA-15 catalyst exhibited a maximum
cyclohexene yield of 53.8% at the ZnSO concentration of 0.42 M in the reactor [12]. The
2
2
2 3
[7,8], γ-Al O
[
2
O
5
2
4
bimetallic Ru-Zn catalysts supported on HAP with different Ru/Zn molar ratios were
prepared by the ion-exchange method [15]. The yield of cyclohexene can reach 33.0%
(
conversion 69.8%, selectivity 47.3%) over Ru-Zn/HAP with a Ru/Zn molar ratio of 1:1
and Ru loading of 2.5 wt% at 150 °C and 5 MPa of hydrogen. Overall, the catalytic
performances of Ru supported SiO , -Al , Nb catalysts were lower than that of Ru
supported ZrO catalysts.
2
2
O
3
2 5
O
2
Although Ru supported catalysts have been extensively investigated, only
unsupported Ru-Zn catalysts with dispersant of zirconia are being used in industrial
plants. The Ru-Zn supported ZrO
commercial utilization because it has similar properties with the unsupported Ru-Zn
catalyst dispersed in ZrO aqueous suspension. Based on commercial estimations, a
selectivity of greater than 80% is required for the selective hydrogenation of benzene to
cyclohexene at the conversion of 50%. However, the selectivity of many Ru-Zn/ZrO
catalysts reported in the literature was lower than 80% at the benzene conversion of 50%.
In this study, we demonstrate that Ru-Zn/ZrO catalysts can be prepared by a two–step
2 2
(Ru-Zn/ZrO ) catalyst should be a good candidate for
2
2
2
impregnation method with further treatment in an alkaline solution, which show a high
selectivity to cyclohexene (>80% at the conversion of 50%) and high activity in the
7

3 2 2
selective hydrogenation of benzene. Effects of the Zn(NO ) ·6H O amount in the
first-step impregnation, concentration of NaOH for alkaline treatment of catalysts after
hydrogen reduction, and hydrogen reduction temperature are investigated. Finally, the
2
stability of the optimal Ru-Zn/ZrO catalyst is evaluated.
2
2
. Experimental
.1. Materials
RuCl
Daiichi Kigenso Kagaku Kogyo Co., Ltd., Japan), ZnSO
Industries, Ltd.). Other chemicals including Zn(NO ·6H
2
3
·xH
2
O (Kunming Sino-platinum Metals Co., Ltd, Ru≥37%), ZrO
·7H O (Wako Pure Chemical
O, NaOH and benzene were
2
powder (RC-100,
4
2
3
)
2
purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used in all
experimental processes.
2
.2. Catalyst preparation
Ru-Zn/ZrO catalysts were prepared by a two–step impregnation method, and the
detailed procedure is as follows.
1) Preparation of ZnO-ZrO
Introduction of Zn species was achieved through the impregnation method. A desired
amount of Zn(NO •6H O was dissolved in 3 mL deionized water in a glass beaker, then
2
(
2
supports with various ZnO contents
3
)
2
2
2
.25 g ZrO
2
powder was added and stirred for 1 h. The glass beaker was sealed with
o
plastic wrap and placed at 60 C in a convection oven for 2 h, then the sample was left to
o
dry off at 100 C after removing the plastic wrap. The obtained solid was transferred to a
o
o
tube furnace for calcination at 250 C for 1 h, and 300 C for 1 h at the air flow of 20
8

mL/min. The resulting ZnO-ZrO
2
samples with various ZnO contents in ascending order
were denoted as ZnO-ZrO
2
-n (n=0-5) and the detailed amount of Zn(NO ·6H O is listed
3
)
2
2
in Table 1.
(
2) Preparation of Ru-Zn/ZrO
Introduction of Ru species was also achieved through the impregnation method. 0.60
g RuCl ·xH O was dissolved in 3 mL water, and the obtained ZnO-ZrO support (2.00 g)
was added to the RuCl
2
with various Zn contents
3
2
2
3
·xH
2
O solution in a beaker. After stirred for 2 h, the beaker was
o
sealed with plastic wrap and placed at 60 C in an oven for 1 h. Then the sample was left
o
to dry off in a water bath at 90 C without the plastic wrap, and transferred to a tube
furnace for gas reduction in a hydrogen flow of 30 mL/min at a certain temperature
o
(
typically 200 C) for 3 h. The reduction product was treated with 100 mL of NaOH
aqueous solution (typically 0.01 M) for 3 times, and then washed with 100 mL of
-
deionized water for 7 times until no Cl ions were detected. The catalysts with different
Zn contents were kept in water and their solid contents were determined, which were
2
denoted as Ru-Zn/ZrO -n (n=0-5) (Table 2).
2
.3. Catalyst characterization
The obtained Ru-Zn/ZrO catalysts kept in water were vacuum-dried at 60 C prior to
characterization. Zinc contents were measured by the atomic absorption spectroscopy
AAS). X-ray powder diffraction (XRD) patterns were collected on an Ultima IV
diffractometer (Rigaku, Japan) using Cu K radiation. N physisorption was performed
o
2
(
2
on a Micromeritics ASAP 2020 equipment at 77 K, and the sample was heated at 423 K
under vacuum for 2 h before measurement. The total surface area was obtained by the
9

BET equation and the pore size was determined by BJH desorption. Dynamic light
scattering (DLS) was performed with Malvern ZEN 3600 to detect the particle size
distribution of catalysts. Surface analysis of the catalysts was analyzed by X-ray
photoelectron spectroscopy (XPS) on a PHI Quantera SXM instrument using a
b
monochromatized Al Kradiation (E =1486.6 eV) at an energy resolution of 0.5 eV and
-8
at a base pressure of 3×10 Pa.
The surface morphology and particle sizes were measured by transmission electron
microscopy (TEM) on a Tecnai G2 F30 microscope operated at 200 kV. The catalysts
were dispersed in anhydrous ethanol, sonicated for 10 min, and dropped onto a
carbon-film-coated copper grid. Particle-size-distribution histograms were obtained by
randomly measuring at least 100 nanoparticles.
The temperature-programmed reduction (TPR) was performed on a PX200
Multi-sorption equipment (Tianjin Golden Eagle Technology Co., Ltd.). The sample of
ZnO-ZrO
installed into the reactor and pretreated at 200 ºC for 2 h under Ar (20 mL/min). The
-TPR curve was determined by passing a stream of 5% H in Ar (20 mL/min) through
2 3
after impregnation of RuCl without reduction (~30 mg dry base) was directly
H
2
2
the pretreated samples and the temperature was increased from 30 ºC to 500 ºC at a
linearly programmed rate of 5 ºC/min. A thermal conductivity detector (TCD) was used
2
to measure the amount of H consumed.
2
.4. Reactivity testing
The selective hydrogenation of benzene was performed in a hastelloy autoclave (250
mL) equipped with a magnetic stirrer. In a typical experiment, water (70 mL),
10

4 2
ZnSO ·7H O (8.4 g) and catalyst (0.3 g, dry base) were put into the autoclave; after being
purged by hydrogen (2 MPa) five times to remove the air, the autoclave was heated with
a stirring rate of 300 r/min. When the temperature was increased to 150 ºC, benzene (35
2
mL) was introduced into the autoclave, and the H pressure and the stirring rate were
adjusted to 5 MPa and 1200 r/min, respectively. For the catalysts with different Zn
contents, the reaction was carried out at 150 ºC for different periods to obtain the
conversion of ca. 40% and 50%. The products were analyzed using a gas chromatography
(Hangzhou Kexiao) with a flame ionization detector (FID). The benzene conversion and
cyclohexene selectivity were calculated by the GC result. The S50 is defined as the
selectivity of cyclohexene at the conversion of 50%. The specific activity of the catalyst
(
g/g·h) is defined as the converted benzene amount (g) per hour for 1 g Ru, and the 50 is
the specific activity at the conversion of 50%.
2
For the recycle activity measurement of Ru-Zn/ZrO -3 catalyst, the organic phase was
removed from the autoclave by suction and the remaining catalyst slurry containing the
Ru-Zn/ZrO catalyst and ZnSO was used for next activity evaluation. The recycle
activity measurement was the same with the above procedure but the catalyst amount was
2
4
0
.9 g instead of 0.3 g.
3
3
. Results and discussion
.1. Catalyst characterization
As shown in Table 1, six ZnO-ZrO
2
2
supports with different Zn contents, ZnO-ZrO -0
o
to ZnO-ZrO
2
-5, were prepared by impregnation method and calcined at 250 C for 1 h,
o
and 300 C for 1 h. The amount of ZrO
2
(2.25 g) was kept constant when the
11

Zn(NO
ZnO-ZrO
Zn(NO
are only diffraction peaks assigned to monoclinic phase of ZrO
When the Zn(NO ·6H
Other peaks are covered by the peaks from ZrO
ZnO (JCPDS: 01-070-2551) indicates the existence of ZnO crystals in the ZnO-ZrO
ZnO-ZrO -5 supports. Evaluation of the ZnO crystallite size from XRD using the
3
)
2
·6H
supports with different Zn contents (ZnO-ZrO
·6H O adding amount is lower than 0.97 g (ZnO-ZrO
2
O adding amount was varied. Fig. 1 shows the XRD patterns of the
-0 to ZnO-ZrO -5). When the
-0 to ZnO-ZrO -2), there
2
2
2
3
)
2
2
2
2
2
(JCPDS: 00-024-1165).
o
3
)
2
2
O adding amount is ≥ 0.97 g, a diffraction peak at 2of 36.3
(
2
.) ascribed to the hexagonal phases of
-3 to
2
2
Scherrer formula indicates that the ZnO crystallite sizes are 45.7 nm, 46.7 nm and 43.6
o
nm, respectively. For the ZnO-ZrO
indicating that the ZnO species are highly dispersed in these two supports [34].
The XRD patterns of six Ru-Zn/ZrO catalysts with different Zn contents,
2
-1 and ZnO-ZrO
2
-2, there is no peak at 2of 36.3 ,
2
Ru-Zn/ZrO
2
-0 to Ru-Zn/ZrO
2
-5, are shown in Fig. 2. These catalysts have the same Ru
o
content of 10 wt%. Compared with Fig. 1, there are no peaks at 2of 36.3 in Fig. 2a,
indicating that the ZnO crystals vanished away in the second impregnation and washing
steps. It can also be confirmed by measuring the Zn content using AAS (Table 2). The
measured Zn content is much lower than the incorporated amount. Because RuCl
3
solution is acidic (pH 1~2), some ZnO crystals will be dissolved into ZnCl , and the
2
dissolved Zn species will be washed away during washing process using alkaline solution
and water. It makes their measured Zn contents lower than the incorporated amounts
o
(
Table 2). A weak diffraction peak at 2of 44.0 assigned to the metallic Ru phase
(JCPDS: 01-070-0274) can be observed in Fig. 2a. To have a better characterization, we
o
o
o
achieved XRD patterns by slow scan of 0.5 /min between 38 and 48 , and subtracted
12

2
XRD patterns of ZrO from each to gain particular XRD patterns (Fig. 2b). Evaluation of
Ru crystal sizes from XRD patterns (Fig. 2b) using the Scherrer formula showed that Ru
crystal sizes were 4.2 nm, 4.8 nm, 4.6 nm, 5.1 nm, 4.8 nm and 4.6 nm, respectively. This
indicates that there are no significant differences for the Ru crystal size in Ru-Zn/ZrO
2
catalysts with different Zn contents.
2 2
Fig. 3 shows the H -TPR profiles of six Ru-Zn/ZrO catalyst precursors (without
reduction). For the Ru-Zn/ZrO
2
-0 catalyst precursor without Zn, there is a wide reduction
o
o
peak at 112 C, whereas the main reduction peak is at 160-204 C for other catalyst
precursors. According to the literature [35-37], unsupported RuCl
3
is reduced at lower
o
o
temperature (~142 C) and supported Ru oxide at higher temperature (160-205 C).
o
Therefore, we deduce that the peak of 112 C is attributed to the reduction of RuCl
3
species. The reduction temperature of RuCl
lower than the literature value. This is probably due to the highly dispersed RuCl
ZnO-ZrO -0 catalyst precursor. As the RuCl solution is acidic in the process of
impregnation, some ZnO particles were dissolved into ZnCl , and RuCl hydrolysed into
Ru(OH) and then dehydrated into Ru during drying for catalyst precursors
3
in the ZnO-ZrO
2
-0 catalyst precursor is
3
in the
2
3
2
3
3
x y
O
Ru-Zn/ZrO
2
2
-1 to Ru-Zn/ZrO
-1 to Ru-Zn/ZrO
or Ru
2
-5. Therefore, the reduction peaks of catalyst precursors
o
Ru-Zn/ZrO
2
-5 are near 160-204 C (Fig. 3), attributed to the reduction
o
o
of Ru(OH)
3
x
O
y
. The reduction peak temperature increased from 160 C to 175 C
with the increase of Zn content in the catalysts (Ru-Zn/ZrO
2
2
-1 to Ru-Zn/ZrO -3),
o
o
whereas the reduction peak temperature is higher than 185 C (even 204 C) for the
2 2
catalysts Ru-Zn/ZrO -4 and Ru-Zn/ZrO -5. This indicates that the interaction between Ru
species and Zn species increases with the increase of Zn content. When the incorporated
13

Zn content is as high as 10.9 wt% and 16.5 wt% in the catalysts Ru-Zn/ZrO
Ru-Zn/ZrO -5, respectively, the high content of Zn species results in the smaller BET
specific surface area, pore volume and pore size of the ZnO-ZrO support as shown in
2
-4 and
2
2
Table 1. It is difficult to obtain high dispersion of Ru species in the support with smaller
pore volume and pore size, resulting in a very high reduction peak temperature.
3
3
.2. Catalytic properties
.2.1. Effects of Zn content
2
The catalytic performance of Ru-Zn/ZrO catalysts with different Zn contents for
selective hydrogenation of benzene to cyclohexene was investigated. The reaction time,
benzene conversion, cyclohexene selectivity and yield over the Ru-Zn/ZrO catalysts are
summarized in Table 3. In order to obtain a benzene conversion around 40~50%, the
reaction time was adjusted according to the activity of the catalyst. The selectivity (S50
2
)
and specific activity (γ50) were calculated from two different experimental points. From
the specific activity or γ50 value, it is known that the incorporation of Zn species into the
catalyst decreased the activity of the catalyst. At the same benzene conversion, the
activity of the catalysts decreased (e.g., γ50), and the selectivity of cyclohexene (e.g., S50
increased with increasing the Zn content from the catalyst ZnO-ZrO -0 to the catalyst
-3, whereas the selectivity decreased with increasing the Zn content from
-3 to ZnO-ZrO -5.
)
2
ZnO-ZrO
2
2
ZnO-ZrO
2
2 2
For the first three catalysts (Ru-Zn/ZrO -1 to Ru-Zn/ZrO -3), the addition of Zn can
significantly improve the selectivity and yield of cyclohexene. As proposed in the
literature [25], the main reason is considered to be that the zinc component inhibits the
14

resorption of cyclohexene generated on the catalyst surface to suppress hydrogenation of
cyclohexene to cyclohexane. However, when the Zn content further increased from
Ru-Zn/ZrO
catalyst Ru-Zn/ZrO
discussed in section 3.1, the much higher reduction peak temperature of catalysts
2
2
-3 to Ru-Zn/ZrO -5, the selectivity of cyclohexene declined. That is, the
2
-3 showed the highest selectivity and yield of cyclohexene. As
Ru-Zn/ZrO
2
-4 and Ru-Zn/ ZrO
-3) indicates the bigger particle size of Ru species in catalysts Ru-Zn/ZrO
-5. That is, some of Ru crystals (relatively large Ru particles) with less
Zn species around them existed in the Ru-Zn/ZrO catalysts with too high Zn species,
resulting in the decrease of the cyclohexene selectivity of catalysts Ru-Zn/ZrO -4 and
Ru-Zn/ZrO -5. The XRD patterns of ZnO-ZrO (Fig. 1) showed that ZnO crystals (40~50
nm) became increasingly accumulated from ZnO-ZrO -4 to ZnO-ZrO -5, whereas the
ZnO particles highly dispersed without ZnO diffraction peaks for ZnO-ZrO -1 and
ZnO-ZrO -2. The accumulation of ZnO particles may lead to the decrease of Zn species
around Ru species, resulting in the decrease of the selectivity of the catalyst.
Under the optimal conditions, the selectivity to cyclohexene is higher than 80% at the
benzene conversion of 50% over the Ru-Zn/ZrO -3 catalyst. In industrial process, the
2
-5 than that of other catalysts (Ru-Zn/ZrO
2
-1 to
Ru-Zn/ZrO
2
2
-4
and Ru-Zn/ ZrO
2
2
2
2
2
2
2
2
2
2
reaction products of benzene, cyclohexene and cyclohexane should be separated by
solvent extraction due to their close boiling point. The higher the selectivity to
cyclohexene is, the cheaper the cost of separation is. Therefore, the cyclohexene
selectivity of more than 80% is requested by industry. Obviously, the performance of
2 2
Ru-Zn/ZrO -3 catalyst meets this need. That is, the Ru-Zn/ZrO -3 catalyst showed a
potential in commercial application. It is worth mentioning that the most common
15

unsupported catalysts for selective hydrogenation of benzene to cyclohexene were
prepared by coprecipitation and the value of 50 was usually about 100~200
gBenzene/(gRu·h) [29,30] at the conversion of 50%, but here the value of 50 for the
Ru-Zn/ZrO
2
-3 catalyst can reach higher than 800 gBenzene/(gRu·h). This is the big
catalysts prepared by the two–step
superiority for the supported Ru-Zn/ZrO
2
impregnation method as compared with the unsupported Ru-Zn catalysts prepared by the
coprecipitation method.
To have a better comparison with the two–step impregnation method, we also
prepared a catalyst (Ru-Zn/ZrO
same amount Zn(NO )·6H O and RuCl
dissolved in 3 mL water, and then impregnated with 2.25 g ZrO
and alkaline treatment were the same as the two–step impregnation method. The catalytic
performance of the catalyst (Ru-Zn/ZrO -3a) was also shown in Table 3. The selectivity
is 57.8% at the conversion of 56.8%, which is much lower than that of Ru-Zn/ZrO -3
prepared by the two–step impregnation method. This indicates that the two–step
2
-3a) by a one–step impregnation method, that is, the
·xH O as Ru-Zn/ZrO -3 were simultaneously
. The hydrogen reduction
3
2
3
2
2
2
2
2
impregnation method is very important for the preparation of supported Ru-Zn/ZrO
2
catalysts with high catalytic performance.
3
.2.2. Effects of NaOH concentration
After hydrogen reduction of the Ru-Zn/ZrO
2
catalysts, they were treated 3 times with
1
00 mL NaOH (0.01 M) and washed 7 times with 100 mL deionized water. In order to
catalysts prepared by our
method, the effects of NaOH concentration were investigated. The results are shown in
further elucidate the performance principle of the Ru-Zn/ZrO
2
16

Table 4. With increasing the NaOH concentration from 0.0 to 0.01 M, S50 enhanced from
7.1% to 81.6%, and 50 decreased by 42%. With increasing the NaOH concentration
7
from 0.01 to 0.025 M, S50 enhanced lightly and 50 decreased by 24%. In other words, the
increase degree of S50 decreased with the further increase of NaOH concentration from
0
.01 to 0.025 M. The Zn content was 0.05, 2.78 and 3.48 wt%, respectively, when the
NaOH concentration was 0.00, 0.01 and 0.025 M. We measured the pH of the solution for
the first 3 times washings with water when the NaOH concentration was 0.00 M and
found that the pH was about 3.5 for all 3 times. This is because RuCl
3
solution is acidic
(
pH 1~2) in the process of impregnation, and part of ZnO will dissolve into ZnCl ;
2
therefore, the sample was also weak acidic after hydrogen reduction. Zn species in the
catalyst would be dissolved in acidic solutions; therefore, the Zn content measured was as
low as 0.05 wt%. When treated 3 times with 0.01 M NaOH, the measured pH of the
filtrate solution was about 6.5, 11.5 and 12.0, respectively, while treated 3 times with
0
.025 M NaOH, the measured pH value was about 6.5, 12.0 and 12.5, respectively.
Mixing the filtrate solutions of 3 times treatments, white precipitate of Zn(OH) in the
2
solution was observed, indicating that part zinc species were in the filtration solution.
Because the pH values of filtration solutions from three alkaline treatments were all
2
higher than 6.0, some Zn species could be fixed in the solid catalyst as Zn(OH) or ZnO.
The higher the treating NaOH concentration, the more Zn species remained in the solid
catalyst. Therefore, the Zn content increased with the increase of the NaOH concentration.
The catalyst activity decreases with the increase of the Zn content. Therefore, the 50
value decreased with increasing the NaOH concentration of alkaline treatment.
To the best of our knowledge, this is the first time to use the alkaline treatment
17

process after the hydrogen reduction of the supported Ru catalyst solid particles in the
impregnation method. The two–step impregnation method was used to prepare Ru-Zn
2 2 2 3 2 2
supported catalysts on SiO , SiO -Al O , SiO -ZrO by Liu et al. [38], however, the
alkaline treatment was not employed and the catalysts were reduced at very high
o
temperature (e.g., 400 C). The selectivity to cyclohexene of those catalysts was just
around 40% at the conversion of 30-50%.
XPS spectra of O 1s region peak fitting for lattice oxygen (530 eV) and oxygen in
surface hydroxyl (532 eV) [39] for the catalysts washed with water (a) and treated with
0
.01 M NaOH (b) are shown in Fig. 4. There is a large amount of surface hydroxyl
groups in monoclinic ZrO itself, and we found that oxygen of surface hydroxyl in the
2
catalyst treated with the NaOH solution (Fig. 4b) was more than that in the catalyst
washed with water (Fig. 4a). This indicates that the Zn species existed in the form of
Zn(OH)
Qiao et al. [40] prepared Ru-Zn/ZrO
method, and the reduced Ru-Zn/ZrO catalysts in solution were then subjected to alkaline
2
for the catalyst treated with NaOH.
2
catalysts using a deposition-precipitation
2
post-treatment by using NaOH aqueous solution (5-30 wt%) for five times. They
concluded that upon treatment with NaOH solutions, dealloying of the binary
2 2
Ru-Zn/ZrO catalysts occurred, resulting in Ru/ZrO catalysts with smaller Ru
nanoparticles. The concentration of 5-30 wt% is equal to 1.25-7.5 M, which is much
higher than the concentration used in this study (e.g. 0.01 M). The Zn species was almost
completely dissolved out after alkaline treatment in the case of ref. 40. The alkaline
treatment in this study was used to neutralize the acidity of the catalyst precursors.
Therefore, with respect to this function, the alkaline treatment in this study is different
18

from ref. 40. It is also concluded in ref. 40 that alkaline post-treatment was able to
introduce more hydroxyl groups on the catalyst surface, rendering improved
hydrophilicity. This is consistent with our above result that oxygen of surface hydroxyl in
the catalyst treated with the NaOH solution was more than that in the catalyst washed
with water. The increase of hydrophilicity of the catalysts improved their selectivities in
benzene selective hydrogenation.
Recently, Liu and coworkers [25] proposed that the synergistic effect of ZnO and
ZnSO
salt formed by the reaction of ZnO on the catalyst surface with ZnSO
improving the selectivity to cyclohexene of the catalyst. Based on the above results, we
proposed a schematic plot of Ru-Zn/ZrO catalysts for benzene selective hydrogenation
as shown in Fig. 5. Herein, we proposed that the (Zn(OH) (ZnSO )(H O) salt formed
during the reaction in the autoclave from Zn(OH) instead of ZnO. Due to tremendous
solubility difference between benzene and cyclohexene in water, the stagnant water layer
formed by (Zn(OH) (ZnSO )(H O) causes a strong diffusion resistance for
4
enhances the selectivity to cyclohexene. Namely, the (Zn(OH)
2 3 4 2 5
) (ZnSO )(H O)
4
plays a key role in
2
2
)
3
4
2
5
2
2
)
3
4
2
5
cyclohexene to the ruthenium surface as compared with benzene. That is, the presence of
the stagnant water layer slows down the further hydrogenation of cyclohexene to
cyclohexane due to the suppression of the direct hydrogenation of adsorbed cyclohexene.
This makes the decline of catalyst activity and the increase of selectivity to cyclohexene
with the increase of Zn(OH)
after hydrogen reduction.
2
by enhancing NaOH concentration for alkaline treatment
3
.2.3. Effects of reduction temperature
The preparation of Ru-Zn/ZrO -3 catalyst under different reduction temperatures was
2
19

investigated and the results are shown in Table 5. It can be seen that the benzene
conversion and yield of cyclohexene significantly changed when the reduction
o
o
temperature increased from 150 C to 250 C. That is, the catalyst activity decreased
when the reduction temperature increased, whereas the selectivity of cyclohexene got an
o
optimal value at the reduction temperature of 200 C. For the reduction temperature of
o
2
50 C, the catalyst activity is very low. As mentioned in the introduction section, most
o
Ru-Zn/ZrO
2
catalysts were reduced at 150-180 C [9,10,23,31,32].
o
To investigate the reason of low activity of the catalyst reduced at 250 C, we
o
o
achieved XRD patterns of the three catalysts by slow scan of 0.5 /min between 38 and
o
4
6
8 , and subtracted XRD pattern of ZrO
2
from each to gain particular XRD patterns (Fig.
). Evaluation of Ru crystal sizes from XRD using the Scherrer formula indicates that Ru
crystal sizes are 4.3, 5.1 and 4.8 nm, respectively. As shown in Fig. 7, TEM images and
particle-size-distribution histograms with Gaussian analysis fittings of the Ru particles
o
o
o
(
Fig. 7) in the catalysts with different reduction temperatures (150 C, 200 C and 250 C)
indicate that Ru crystal sizes are 3.71, 3.80 and 4.66 nm, respectively. This means that the
Ru crystallite size in Ru-Zn/ZrO catalysts slightly increased with the increase of
2
reduction temperature. This is probably one of the reasons for the lower activity of the
catalysts reduced at higher temperature.
As shown in Table 5, the Zn content of the catalyst increased with the increase of
reduction temperature, that is, the measured values of Zn content were 1.34 wt%, 2.78
wt% and 3.57 wt%, respectively. The higher Zn content will decrease the catalyst activity,
therefore, this might be one reason for the lower activity of catalysts reduced at higher
temperatures. From the increase of Zn content in the catalyst, we guess the interaction
20

between Ru species and Zn species also increased with the reduction temperature. The
strong interaction may also decrease the catalytic activity.
We measured the size distributions of catalysts obtained from different reduction
temperatures by DLS (Fig. 8), and average secondary particle sizes are 574.3 nm, 638.1
nm and 933.1 nm, respectively; in other words, the average secondary particle size
o
o
increased by 1.46 times when the reduction temperature increased from 200 C to 250 C.
This is probably one of the reasons for the sharp activity decrease of catalysts reduced at
o
o
2
50 C as compared with that reduced at 200 C, because the dispersion of catalyst in
water is very important to the tetraphase catalytic reaction system.
3
.2.4. Stability of the catalyst
2
The stability of Ru-Zn/ZrO -3 (2.78 wt% Zn, 10.0 wt% Ru) catalyst was investigated.
The catalyst was recycled four times without any addition after every reaction, and the
results are shown in Fig. 9. As can be seen, the benzene conversions were stable above
5
3%, and the cyclohexene selectivity and yields were kept above 78% and 42% in the
four recycles, respectively, indicating a good stability of the catalyst. The benzene
conversion was 54.6%, and the cyclohexene selectivity and yields was 80.1% and 43.8%
th
in the first recycle, and slightly decreased in the 4 recycle after three recycles without
regeneration. However, the benzene conversion, the selectivity to cyclohexene and the
yield were still as high as 54.3%, 78.4% and 42.4%, respectively. The specific activity
was sustained at about 740 g/(g·h) when the catalyst amount was 0.9 g. Thus
2
Ru-Zn/ZrO -3 catalyst has good prospect for industrial application.
21

4
. Conclusions
2
Supported Ru-Zn/ZrO catalysts with different Zn contents were successfully
prepared by a two–step impregnation method and alkaline treatment after reduction. The
incorporation of zinc species can decrease the activity of the ruthenium catalyst, whereas
the highest selectivity can be obtained at an optimal Zn content. The optimal ZnO content
in ZnO-ZrO
2
2
support is 10.5 wt% for Ru-Zn/ZrO catalysts with Ru content of 10.0 wt%
(
i.e., Ru-Zn/ZrO
2
-3), and the final Zn content in the catalyst is 2.78 wt%. We obtained the
optimal S50 up to 81.6% and the specific activity (50) > 800 g/(g·h), four times higher
than unsupported Ru-Zn catalysts preparation by coprecipitation method. Alkaline
treatment of the catalysts after hydrogen reduction with 0.01 M NaOH aqueous solution
significantly decreased the catalyst activity and increased the selectivity of cyclohexene
as compared with just washing the catalyst with water, and further increasing the NaOH
concentration could only improve the selectivity to cyclohexene slightly. The reduction
temperature showed significant effect on the catalytic properties of the catalyst, and the
o
optimal reduction temperature is 200 C. The cost of the catalyst for selective
hydrogenation of benzene is expected to decrease, implying good prospect for industrial
application.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(U1162129) and China National Petroleum Corporation.
22

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Figure Captions
2
Fig. 1. XRD patterns of ZnO-ZrO samples with different Zn contents.
26

2
Fig. 2. XRD patterns of Ru-Zn/ZrO catalysts with different Zn contents. (b) XRD
patterns of ZrO were subtracted from data in (a).
2
27

2 2
Fig. 3. H -TPR profiles of Ru-Zn/ZrO catalysts before reduction, (b) is minizone of (a).
28

Fig. 4. XPS spectra of O 1s for catalysts (a) washed with water, (b) treated with 0.01 M
NaOH.
29