Selective hydrogenation of benzene to cyclohexene over nanocomposite Ru-Mn/ZrO2 catalysts

Article abstract of DOI:10.1016/S1872-2067(11)60489-0

A series of Ru-Mn catalysts with different Mn contents were prepared by coprecipitation, and their catalytic performance, using nanoscale ZrO ₂ as a dispersant, for the selective hydrogenation of benzene to cyclohexene was investigated. The catalysts were characterized using X-ray diffraction, transmission electron microscopy, N₂ physisorption, X-ray fluorescence, atomic absorption spectroscopy, and Auger electron spectroscopy. The results confirmed that the Mn existed as Mn₃O ₄ on the Ru surface. The Mn₃O₄ reacted with ZnSO₄ to form an insoluble [Zn(OH)₂]₃(ZnSO ₄)(H₂O)₃ salt, which was readily chemisorbed on the Ru surface. This chemisorbed salt played a key role in improving the cyclohexene selectivity over the Ru catalyst. The cyclohexene yield of 61.3% was obtained over the Ru-Mn catalyst with the optimum Mn content of 5.4%. This catalyst had good stability and excellent reusability.

Full text of DOI:10.1016/S1872-2067(11)60489-0

Chinese Journal of Catalysis 34 (2013) 684–694
ava i l a b l e a t w w w. s c i e n c ed 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
Selective hydrogenation of benzene to cyclohexene over
nanocomposite Ru‐Mn/ZrO2 catalysts
SUN Haijie, JIANG Houbing, LI Shuaihui, WANG Hongxia, PAN Yajie, DONG Yingying,
LIU Shouchang, LIU Zhongyi*
College of Chemistry and Molecular Engnineering, Zhengzhou University, Zhengzhou 450001, Henan, 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‐Mn catalysts with different Mn contents were prepared by coprecipitation, and their
catalytic performance, using nanoscale ZrO2 as a dispersant, for the selective hydrogenation of ben‐
zene to cyclohexene was investigated. The catalysts were characterized using X‐ray diffraction,
transmission electron microscopy, N2 physisorption, X‐ray fluorescence, atomic absorption spec‐
troscopy, and Auger electron spectroscopy. The results confirmed that the Mn existed as Mn3O4 on
the Ru surface. The Mn3O4 reacted with ZnSO4 to form an insoluble [Zn(OH)2]3(ZnSO4)(H2O)3 salt,
which was readily chemisorbed on the Ru surface. This chemisorbed salt played a key role in im‐
proving the cyclohexene selectivity over the Ru catalyst. The cyclohexene yield of 61.3% was ob‐
tained over the Ru‐Mn catalyst with the optimum Mn content of 5.4%. This catalyst had good stabil‐
ity and excellent reusability.
Received 10 October 2012
Accepted 20 November 2012
Published 20 April 2013
Keywords:
Benzene
Selective hydrogenation
Cyclohexene
Ruthenium
Manganese
Zirconia
© 2013, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Zinc
1. Introduction
benzene were better than those of an Ru/SiO2 catalyst with the
same Ru loading. They also revealed that the B was present in
The production of nylon‐6 and nylon‐66 from benzene and
cyclohexene has attracted much attention because it is a safe,
environmentally benign, and atomic economic process [1–4].
Hydrogenation of benzene to cyclohexene is thermodynami‐
cally favored [5]. The development of a catalyst with high cy‐
clohexene selectivity in the hydrogenation of benzene is there‐
fore important for this technology.
Second metals or metal oxides could significantly improve
the cyclohexene selectivities and yields of Ru‐based catalysts.
Xie et al. [6] prepared an Ru‐B/SiO2 catalyst by impregnation
and chemical reduction methods, and found that the activity
and cyclohexene selectivity of this catalyst in H2 reduction of
two forms, namely oxidized and elemental B species, and modi‐
fication of their hydrophilicities on the surface of the Ru‐B/SiO2
catalyst gave this catalyst high activity and high cyclohexene
selectivity. Liu et al. [7] prepared an Ru‐La‐B/ZrO2 catalyst
using a chemical reduction method and obtained a cyclohexene
yield of 53.2% and a corresponding cyclohexene selectivity of
61.9%. They confirmed that the La in this catalyst existed as
La2O3. Fan et al. [8] prepared an Ru‐Co‐B/γ‐Al2O3 catalyst using
an impregnation reduction method, and achieved a cyclohex‐
ene yield of 28.8% without any additives. They discovered that
the Co in this catalyst was present as an oxide. Liu et al. [9]
prepared an Ru‐Ce/SBA‐15 catalyst using a “two‐solvent” im‐
*Corresponding author. Tel: +86‐371‐67783384; E‐mail: liuzhongyi@zzu.edu.cn
This work was supported by the National Nature Science Foundation of China (21273205), the Innovation Found for Technology Based Firms of
China (10C26214104505), the China Postdoctoral Science Fundation (2012M511125), and the Scientific Research Foundation of Graduate School of
Zhengzhou University.
DOI: 10.1016/S1872‐2067(11)60489‐0 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 34, No. 4, April 2013

SUN Haijie et al. / Chinese Journal of Catalysis 34 (2013) 684–694
pregnation method and achieved a cyclohexene yield of 53.8%
ma‐atomic emission spectroscopy (ICP‐AES) on an ICAT 6000
SERIES instrument of Heme Electron corporation. X‐ray dif‐
fraction (XRD) patterns were acquired using a PANalytcal
X′Pert PRO instrument with Cu Kα (λ = 0.1541 nm) radiation
and a scan range of 2θ = 5°–90° in steps of 0.03°. Transmission
electron microscopy (TEM) was performed using a JEOL
over the catalyst with an optimum Ce/Ru molar ratio of 0.25.
They found that the Ce existed as a Ce(Ⅲ) species. These studies
provide a good basis for the development of catalysts and cata‐
lytic systems for the selective hydrogenation of benzene to
cyclohexene.
In previous work, we developed nano‐amorphous
Ru‐M‐B/ZrO2 (M = Zn, Co, Fe, La) catalysts with Ru loadings
less than one‐third those of industrial Ru‐Zn catalysts and ob‐
tained a cyclohexene selectivity of 78.8% at a benzene conver‐
sion of 59.6%. This was much better than the cyclohexene se‐
lectivity of 80% at a benzene conversion of 40% obtained over
industrial Ru‐Zn catalysts [10–12]. We developed monolayer
dispersed Ru‐Si‐M (M = Zn, Mn, Fe, Ce, La) catalysts and
achieved a cyclohexene selectivity of 80% at a benzene conver‐
sion of 60%. The catalysts were used in a commercial unit and
were shown to be economical, safe, and environmentally
friendly. We have applied for national patents for these cata‐
lysts [13,14]. In this work, we prepared a series of Ru‐Mn cata‐
lysts with different Mn contents, investigated the influence of
different Mn contents on the performance of the Ru catalysts in
selective hydrogenation of benzene to cyclohexene, and deter‐
mined the role of the Mn promoter. The stability and reusabil‐
ity of the Ru‐Mn/ZrO2 catalyst with an optimum Mn content of
5.4% were also investigated.
JEM‐2100
instrument.
N2
physisorption
(Brunau‐
er‐Emmett‐Teller method) was determined using
a
Quantachrome Nova 100e apparatus. The compositions of the
catalysts were determined by X‐ray fluorescence (XRF), using a
Bruker S4 Pioneer instrument. Auger electron spectroscopy
(AES) of the Zn LMM transitions was performed using a ULVAC
PHI‐700 nanoscanning Auger system with an on‐axis scanning
Ar‐ion gun and a cylindrical mirror energy analyzer. The ener‐
gy resolution was 0.1%. The background pressure of the analy‐
sis room was less than 5.2 × 10−7 Pa. The standard sample was
SiO2/Si.
2.3. Activity tests
The selective hydrogenation of benzene was performed in a
1‐L Hastelloy‐lined autoclave. A sample of Ru‐Mn catalyst, 9.8 g
of ZrO2, and 49.2 g of ZnSO4 were charged in the autoclave.
Heating was begun with an H2 pressure of 5 MPa and a stirring
rate of 800 r/min. Benzene (140 ml) was fed into the system
and the stirring rate was increased to 1400 r/min to prevent
diffusion effects when the temperature reached 150 °C. The
reaction process was monitored by taking small samples of the
reaction mixture every 5 min. The products were analyzed by
gas chromatography using a GC‐1690 gas chromatograph with
a flame ionization detector (Hangzhou Kexiao Instrument Co.,
China). The benzene conversion and cyclohexene selectivity
were calculated from the product concentration obtained using
corrected peak area normalization. At the end of the reaction,
the organic phase was removed using a separating funnel. The
slurry containing the mixture of the catalyst and ZrO2 was re‐
used, according to the above operations, without any additions.
The catalysts after hydrogenation were denoted by
Ru‐Mn(x)/ZrO2, where x is the weight percentage of Mn in the
catalyst, determined by atomic absorption spectrometry. The
Ru‐Mn(5.4%) catalyst without the addition of ZrO2 after hy‐
drogenation was denoted by Ru‐Mn(5.4%) AH, where AH rep‐
resents after hydrogenation.
2. Experimental
2.1. Catalyst preparation
RuCl3·H2O (9.75 g) and the desired amount of MnSO4·H2O
were dissolved in 200 ml of H2O with agitation. A 10% NaOH
solution was added to the stirred solution. After the reaction
was complete, the mixture was filtrated and the black precipi‐
tate was washed three times with distilled water. This black
precipitate was then dispersed in 400 ml of a 5% NaOH solu‐
tion and charged in a 1‐L Teflon‐lined autoclave. The reduction
conditions were as follows: H2 pressure 5 MPa, temperature
150 °C, stirring rate 800 r/min, and time 3 h. The obtained
black powder was washed with distilled water until neutrality
was achieved, and subsequently vacuum‐dried, giving the de‐
sired Ru‐Mn catalyst. The catalyst was divided into two por‐
tions. One portion was used for activity tests and the other was
used for catalyst characterization. This ensured the catalysts
with different Mn contents had the same Ru contents. The
amounts of MnSO4·H2O were adjusted to give catalysts with
different Mn contents, denoted by Ru‐Mn(x), where x denotes
the weight percentage of Mn in the catalyst, determined by
atomic absorption spectrometry. Ru‐Mn catalysts prepared
using different Mn precursors were obtained according to the
above procedure, except that MnSO4·H2O was replaced by
equal molar amounts of Mn(NO3)2 or MnCl2.
3. Results and discussion
3.1. Catalyst characterization results
Figure 1 shows the XRD patterns of the Ru‐Mn(x) catalysts
and the Ru‐Mn(x) catalysts with ZrO2 as a dispersant after hy‐
drogenation. Figure 1(a) shows that all the Ru‐Mn(x) catalysts
display the diffraction peaks of the hexagonal phases of metallic
Ru (JCPDS 01‐070‐0274) at 2θ = 38.5°, 42.3°, 44.0°, 58.3°, 69.2°,
78.4°, and 38.5°. The Ru‐Mn(8.0%) and Ru‐Mn(10.8%) cata‐
lysts show the diffraction peaks of Mn3O4 (JCPDS 00‐001‐1127)
at 2θ = 18.0°, 32.7°, 36.1°, 58.9°, and 69.3°. Morales et al. [15]
confirmed that the Mn in a Co‐Mn/TiO2 catalyst reduced below
2.2. Catalyst characterization
The weight percentages of Mn and the concentration of
Mn2+ and Zn2+ were analyzed by inductively coupled plas‐

SUN Haijie et al. / Chinese Journal of Catalysis 34 (2013) 684–694
Mn3O4
Ru
Ru-Mn(10.8%)
(c)
(a)
(b)
(Zn(OH)2)3(ZnSO4)(H2O)3
Ru
Ru
Ru-Mn(10.8%)/ZrO2
Ru-Mn(8.0%)
Ru-Mn(8.0%)/ZrO2
Ru-Mn(5.4%)/ZrO2
Ru-Mn(4.6%)/ZrO2
Ru-Mn(3.4%)/ZrO2
Ru-Mn(5.4%)
Ru-Mn(4.6%)
Ru-Mn(3.4%)
Ru-Mn(5.4%) AH
10 20 30 40 50 60 70 80
2 /( o
10 20 30 40 50 60 70 80
2 /( o
10 20 30 40 50 60 70 80 90
2 /( o
)
)
)
Fig. 1. XRD patterns of Ru‐Mn(x) (a), Ru‐Mn(x)/ZrO2 (b), and Ru‐Mn(5.4%) AH (c) samples. Ru‐Mn(x) denotes Ru‐Mn catalysts, where x is the mass
percentage of Mn in the catalyst; Ru‐Mn(x)/ZrO2 is Ru‐Mn(x) catalyst with ZrO2 as a dispersant after hydrogenation; Ru‐Mn(x) AH is Ru‐Mn(x) cata‐
lyst without ZrO2 after hydrogenation.
contents were in the range 3.4%–5.4%, the diffraction peaks of
Table 1
Mn3O4 were not observed in the XRD patterns of the Ru‐Mn(x)
catalysts as a result of the low amounts and small crystallites of
Mn3O4.
Table 1 presents the Ru crystallite sizes of the catalysts cal‐
culated from the strongest peak broadening at 2θ = 44° using
the Scherrer equation. As can be seen, the Ru crystallite sizes of
the Ru‐Mn(x) catalysts were distributed in the narrow range
3.7–4.5 nm, indicating that the introduction of Mn3O4 had little
effect on the Ru crystallite sizes.
Textural properties and crystallite sizes of Ru‐Mn(x) catalysts.
Surface area Pore diame‐ Pore volume Crystallite size
Catalyst
(m²/g)
72
71
62
68
ter (nm)
12.2
11.8
11.6
9.4
(ml/g)
0.22
0.21
0.18
0.16
0.16
(nm)
3.7
Ru‐Mn(3.4%)
Ru‐Mn(4.6%)
Ru‐Mn(5.4%)
Ru‐Mn(8.0%)
Ru‐Mn(10.8%)
4.3
3.8
4.0
4.5
56
11.2
Figure 1(b) shows that all the Ru‐Mn(x)/ZrO2 catalysts after
hydrogenation have the weak diffraction peaks of metallic Ru
at 2θ = 44.0°, indicating small Ru crystallite sizes. All the other
300 °C existed as Mn3O4. The reduction temperature was only
150 °C. These results prompted us to suggest that the Mn in the
Ru‐Mn(x) catalysts was mainly present as Mn3O4. When the Mn
(a)
Ru-Zn(5.4%)
(b)
Ru-Mn(5.4%)/ZrO2
Ru-Mn(5.4%)/ZrO2
(c)
Ru-Mn(5.4%)
(d)
12
10
8
6
4
2
0
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 3.5 4.0 4.5 5.0 5.5 6.0
Ru crystallite size (nm)
Ru crystallite size (nm)
Fig. 2. TEM images (a, b) and Ru crystallite size distributions (c, d) of Ru‐Mn(5.4%) and Ru‐Mn(5.4%)/ZrO2.

SUN Haijie et al. / Chinese Journal of Catalysis 34 (2013) 684–694
diffraction peaks corresponded to the monoclinic phases of
the surface areas, pore volumes, and pore diameters. The tex‐
tural properties of the Ru‐Mn(x) catalysts after hydrogenation
were similar to that of ZrO2 because the mass ratio of ZrO2 to
catalyst was 5:1 [19]. N2 physisorption studies of the
Ru‐Mn(x)/ZrO2 after hydrogenation were therefore not neces‐
sary.
ZrO2 (JCPDS 00‐024‐1165). Figure 1(c) shows that the
Ru‐Zn(5.4%) catalyst without the addition of ZrO2 after hydro‐
genation displayed not only the diffraction peaks of metallic Ru
but also the diffraction peaks of the [Zn(OH)2]3(ZnSO4)(H2O)3
salt (JCPDS 01‐078‐0247). This indicated that the
[Zn(OH)2]3(ZnSO4)(H2O)3 salt had formed on the surface of the
Ru‐Mn(5.4%) catalyst after hydrogenation in the presence of
ZnSO4. However, the diffraction peaks of this salt were not ob‐
served on the surface of the Ru‐Mn(x) catalysts after hydro‐
genation with the addition of ZrO2, suggesting that this salt was
highly dispersed on the surface of the Ru‐Mn(x) catalysts and
the ZrO2 dispersant. Xie et al. [16] confirmed that many salts
could spontaneously disperse on the support surfaces.
Figure 2 shows the TEM images of the Ru‐Mn(5.4%) catalyst
and the Ru‐Mn(5.4%) catalyst after hydrogenation, with ZrO2
as a dispersant, and the corresponding Ru crystallite size dis‐
tributions. Figure 2(a) shows that the Ru‐Mn(5.4%) catalyst
consisted of nanoscale spherical and ellipsoidal Ru crystallites.
Figure 2(c) shows that the Ru crystallite size of the
Ru‐Mn(5.4%) catalyst was concentrated at around 4.8 nm,
which was consistent with the XRD results. Figure 2 shows that
the ZrO2 dispersant mainly consisted of ZrO2 crystallites of size
about 20 nm. The catalyst particles were separated and isolat‐
ed by ZrO2 after the first hydrogenation, indicating that a suita‐
ble amount of ZrO2 could significantly suppress agglomeration,
which could have occurred as a result of collisions among dif‐
ferent catalyst particles under high agitation [17]. Figure 2(d)
shows that the Ru crystallite sizes of the Ru‐Mn(5.4%)/ZrO2
catalyst after hydrogenation were mainly distributed at around
4.5 nm, which is much smaller than the crystallites of the
Ru‐Mn(5.4%) catalyst before hydrogenation. This indicated
that the ZrO2 played an important role in dispersing the cata‐
lyst particles.
Table 2 shows the compositions of the Ru‐Mn(x)/ZrO2 and
Ru‐Mn(x) AH, the concentrations of metallic ions in the aque‐
ous phase, and the pH values of the aqueous phase at room
temperature. The Mn/Ru, Zn/Ru, and Zr/Ru atomic ratios
clearly reflect the variations in the catalyst compositions before
and after hydrogenation. Ru‐Mn(x) catalysts with different Mn
contents were prepared using MnSO4·H2O as the precursor. As
can be seen from Table 2, the molar ratios of the
Ru‐Mn(x)/ZrO2 catalysts after hydrogenation were all 0.02,
indicating trace amounts of Mn in the Ru‐Mn(x)/ZrO2 after
hydrogenation. The Zn/Ru atomic ratios of Ru‐Mn(x)/ZrO2
after hydrogenation increased, and the concentrations of Mn2+
in the aqueous phase increased and the concentrations of Zn2+
decreased with Mn content. In particular, the colorless slurries
after hydrogenation prompted us to propose that the Mn in the
slurries existed as Mn2+. All of these results indicated that the
Mn3O4 on the surfaces of the Ru‐Mn(x) catalysts had reacted
with ZnSO4 to form a new Zn species and an Mn(Ⅱ) species. The
Zn species were chemisorbed on the Ru surfaces and the Mn(Ⅱ)
species were dissolved in the slurries. A trace amount of Mn
was also detectable in the Ru‐Mn(5.4%) catalyst without the
addition of ZrO2 after hydrogenation. Its Zn/Ru molar ratio was
similar to that with ZrO2. It was also found that the S/Ru molar
ratio for the catalyst without ZrO2 was 0.06, which was con‐
sistent with the XRD results, and showed that the Zn species
existed as the [Zn(OH)2]3(ZnSO4)(H2O)3 salt. The addition of
ZrO2 lowered the S contents of the Ru‐Mn(x) catalysts after
hydrogenation to below the detection limit of the XRF instru‐
ment. All these results implied that the Zn species existed as the
Table 1 shows the textural properties of the Ru‐Mn(x) cata‐
lysts. As can be seen, the surface areas, pore volumes, and pore
diameters generally decreased with the Mn content of the cat‐
alysts. Zhang et al. [18] found that the specific surface area of a
Co‐Mn/TiO2 catalyst was less than that of TiO2, and they sug‐
gested that amorphous MnOx was dispersed on the TiO2 surface
and blocked some of the catalyst pores. Similarly, it is proposed
that Mn3O4 was dispersed on the surface of the catalysts and
could block some of the catalyst pores, resulting in decreases in
[Zn(OH)2]3(ZnSO4)(H2O)3
salt.
The
amount
of
[Zn(OH)2]3(ZnSO4)(H2O)3 salt increased with the Mn3O4 con‐
tent, which resulted in an increase in the Zn/Ru atomic ratio. It
was concluded that the Mn3O4 was mainly present on the Ru
surface because the ZnSO4 in the slurry could only react with
Mn3O4 on the Ru surface, i.e., the [Zn(OH)2]3(ZnSO4)(H2O)3 salt
formed could only be chemisorbed on the Ru surface. Table 2
shows that the pH values of the aqueous solutions after hydro‐
Table 2
Composition of Ru‐Mn(x)/ZrO2 catalysts after hydrogenation and concentrations of the metallic ions in the aqueous phase as well as their pH values
at room temperature.
Elemental ratioa (mol/mol)
Ion concentrationb (mol/L)
Sample
Mn source
pH valuec
Mn/Ru
Zn/Ru
0.34
0.42
0.46
0.52
0.55
0.45
0.47
0.49
Zr/Ru
Zn2+
0.44
0.43
0.42
0.41
0.37
—
Mn2+
3.11×10–3
4.11×10–3
6.36×10–3
9.82×10–3
1.0×10–2
—
Ru‐Mn(3.4%)/ZrO2
Ru‐Mn(4.6%)/ZrO2
Ru‐Mn(5.4%)/ZrO2
Ru‐Mn(8.0%)/ZrO2
Ru‐Mn(10.8%)/ZrO2
Ru‐Mn(5.2%)/ZrO2
Ru‐Mn(5.6%)/ZrO2
Ru‐Mn(5.4%) AH
MnSO4
MnSO4
MnSO4
MnSO4
MnSO4
Mn(NO3)2
MnCl2
0.02
0.02
0.02
0.02
0.02
0.01
0.02
0.02
5.16
5.12
4.98
5.07
5.26
5.04
5.17
0
5.6
5.4
6.0
5.9
6.2
—
—
—
—
6.0
MnSO4
—
—
a Measured by X‐ray fluorescence (XRF).
b Determined by ICP‐AES.

SUN Haijie et al. / Chinese Journal of Catalysis 34 (2013) 684–694
could be reduced to metallic Zn by the hydrogen atoms that
spilled from the surface of the Ru catalyst. Yuan et al. [21] and
He et al. [22] also indicated that Zn atoms could be introduced
into Ru‐based catalysts by the reduction of Zn2+, based on XPS
results. However, Struijk et al. [24] indicated that the Zn2+ in
the adsorption of ZnSO4 could not be reduced on the surface of
an Ru catalyst, also on the basis of XPS results. An important
reason for such differences is the closeness of the Zn 2p3/2 BEs
corresponding to Zn(II) and metallic Zn. It is therefore very
difficult to assess the valences of Zn based on the Zn 2p3/2 BEs
[25]. This drawback can be overcome by using the Zn LMM
Auger transition, as the Auger shift between Zn(II) and metallic
Zn is higher than 4.6 eV [26]. Figure 3 shows the AES Zn LMM
spectra of the Ru‐Mn(5.4%) catalyst with ZrO2 after hydro‐
genation. The spectra were recorded after Ar+ sputtering for 1
min to avoid interruptions of the surface oxidation of the cata‐
lyst. As can be seen, the kinetic energy of Zn LMM for
Ru‐Mn(5.4%)/ZrO2 after hydrogenation was 984.2 eV, which
was in agreement with that of Zn(II) species [27,28]. This was
also consistent with the XRD result that the Zn species existed
as the [Zn(OH)2]3(ZnSO4)(H2O)3 salt. This also indicates that the
Zn species chemisorbed on the Ru surface were still present as
Zn2+ even under reaction conditions of 150 °C and an H2 pres‐
sure of 5.0 MPa. The AES measurements did not identify any
additional contributions from metallic Zn (commonly appear‐
ing in the 991–995 eV range), indicating that the Zn2+ chemi‐
sorbed on the Ru surface could not be reduced to metallic Zn.
The results in Table 2 show that the aqueous solutions after
hydrogenation at room temperature were acidic, as a result of
the hydrolysis of ZnSO4. It is well known that higher tempera‐
tures favor hydrolysis. This means that the acidity of the liquid
phase is much higher at a reaction temperature of 150 °C as a
result of the increase in the degree of hydrolysis of ZnSO4. As is
known, it is difficult for metallic Zn to exist in acidic solutions,
and this is consistent with there being no clear evidence in the
AES results of the presence of metallic Zn on the surface of the
Ru catalysts.
984.2
975
980
985
990
995
1000
Kinetic energy (eV)
Fig. 3. AES Zn LMM spectrum of Ru‐Mn(5.4%)/ZrO2 sample.
genation generally increased with Mn content of the catalyst.
More Mn3O4 reacted with more ZnSO4 to form the
[Zn(OH)2]3(ZnSO4)(H2O)3 salt, resulting in a decrease in the
Zn2+ concentrations. The degree of hydrolysis of ZnSO4 there‐
fore decreased and the pH values of the aqueous solutions in‐
creased. The molar ratios of the Ru‐Mn(x)/ZrO2 catalysts after
hydrogenation were stable, at around 5.00, as a result of the
fixed amounts of ZrO2.
Ru‐Mn(x) catalysts were prepared from different precur‐
sors, namely MnSO4, Mn(NO3)2, and MnCl2, in equimolar quan‐
tities. The Mn mass percentages in the catalysts, measured by
atomic absorption spectroscopy, were 5.4%, 5.2%, and 5.6%,
respectively. Table 2 shows that the Mn/Ru, Zn/Ru, and Zr/Ru
atomic ratios of the Ru‐Mn(x)/ZrO2 catalysts after hydrogena‐
tion were similar, indicating similar chemical compositions of
the Ru‐Mn(x)/ZrO2 catalysts after hydrogenation.
The Zn(II) species chemisorbed on the Ru surface played an
important role in improving the cyclohexene selectivities of the
Ru catalysts [20–23]. However, there are different opinions
about the Zn valence. Wang et al. [20] characterized
Ru‐Zn/m‐ZrO2 catalysts using XPS and found that the Zn 2p3/2
binding energy (BE) of the Zn in the catalyst was close to that of
metallic Zn. They suggested that chemisorbed Zn2+ cations
60
100
(a)
(b)
(c)
90
50
40
30
20
10
0
80
60
40
20
0
80
70
60
50
40
30
Ru-Mn(3.4%)/ZrO2
Ru-Mn(4.6%)/ZrO2
Ru-Mn(5.4%)/ZrO2
Ru-Mn(8.0%)/ZrO2
Ru-Mn(10.8%)/ZrO2
Ru-Mn(3.4%)/ZrO2
Ru-Mn(4.6%)/ZrO2
Ru-Mn(5.4%)/ZrO2
Ru-Mn(8.0%)/ZrO2
Ru-Mn(10.8%)/ZrO2
Ru-Mn(3.4%)/ZrO2
Ru-Mn(4.6%)/ZrO2
Ru-Mn(5.4%)/ZrO2
Ru-Mn(8.0%)/ZrO2
Ru-Mn(10.8%)/ZrO2
5
10
15
20
25
0
20
40
60
80
100
5
10
15
20
25
Time (min)
Time (min)
Benzene conversion (%)
Fig. 4. Performance of Ru‐Mn(x)/ZrO2 catalysts with different Mn contents for selective hydrogenation of benzene to cyclohexene. Reaction condi‐
tions: a share of Ru‐Mn(x) catalyst, 49.2 g ZnSO4·7H2O, 9.8 g ZrO2, 280 ml H2O, 5 MPa H2, 150 oC, stirring rate 1400 r/min.

SUN Haijie et al. / Chinese Journal of Catalysis 34 (2013) 684–694
90
60
(a)
(c)
(b)
75
60
45
30
15
85
80
75
70
50
40
30
20
MnSO4
Mn(NO3)2
MnCl2
MnSO4
Mn(NO3)2
MnCl2
MnSO4
Mn(NO3)2
MnCl2
5
10
15
Time (min)
20
25
20 30 40 50 60 70 80
Benzene conversion (%)
5
10
15
Time (min)
20
25
Fig. 5. Performence of Ru‐Mn(5.4%)/ZrO2 catalysts prepared by different Mn precursors. Reaction conditions: a share of Ru‐Mn(x) catalyst, 49.2 g
ZnSO4·7H2O, 9.8 g ZrO2, 280 ml H2O, 5 MPa H2, 150 oC, stirring rate 1400 r/min.
To summarize, the Mn promoter in the Ru‐Mn(x) catalysts
existed as Mn3O4 on the Ru surface. The Mn3O4 reacted with
ZnSO4 to form the [Zn(OH)2]3(ZnSO4)(H2O)3 salt in the hydro‐
genation process. This salt could be readily chemisorbed on the
Ru surface and directly affected the performance of the Ru cat‐
alyst in the selective hydrogenation of benzene to cyclohexene.
desorption and hinder re‐adsorption of cyclohexene for further
hydrogenation to cyclohexane [13,30]. (2) The Zn2+ of the
chemisorbed [Zn(OH)2]3(ZnSO4)(H2O)3 salt could selectively
cover most reactive sites of the catalyst, which could reduce the
adsorption enthalpies of cyclohexene and benzene. A decrease
in the adsorption enthalpy of cyclohexene could result in an
increase in the cyclohexene desorption rate, and hence an in‐
crease in the cyclohexene selectivity. However, a decrease in
the adsorption enthalpy of benzene would lead to a decrease in
the catalytic activity [24,30]. (3) Electronic interactions be‐
tween Zn2+ of the chemisorbed [Zn(OH)2]3(ZnSO4)(H2O)3 salt
and the active Ru components could modify the electronic
structure of Ru, which might be favorable for the formation of
cyclohexene [13,20,30]. (3) The Zn2+ of the chemisorbed
[Zn(OH)2]3(ZnSO4)(H2O)3 salt could form loosely bound ad‐
ducts with cyclohexene, which could stabilize the cyclohexene
formed on the surface of the Ru catalyst and improve the cy‐
clohexene selectivity [9,20,29]. The activities of the Ru‐Mn(x)
catalysts therefore decreased and the cyclohexene selectivities
increased with increasing amounts of chemisorbed
[Zn(OH)2]3(ZnSO4)(H2O)3 salt. The [Zn(OH)2]3(ZnSO4)(H2O)3
3.2. Performances of Ru‐Zn(x) catalysts in selective
hydrogenation of benzene to cyclohexene
Figure 4 shows the performances of Ru‐Mn(x) catalysts with
different Mn contents, using ZrO2 as a dispersant, for the selec‐
tive hydrogenation of benzene to cyclohexene. As can be seen
from Fig. 4(a) and (b), the catalytic activity increased and the
cyclohexene selectivity at the same benzene conversion de‐
creased with Mn content of the catalyst. Obviously, the increase
in cyclohexene selectivity was achieved at the expense of cata‐
lytic activity. As can be seen from Fig. 4(c), the cyclohexene
yields continuously increased when the Mn content of the cat‐
alyst increased from 3.4% to 5.4%. However, with further in‐
creases in the Mn content, the cyclohexene yields increased
gradually. The Ru‐Mn(5.4%) catalyst gave a cyclohexene yield
of 61.3%, which is among the best results reported so far
[9,29]. It should also be noted that the Ru‐Mn(5.4%) catalyst
without ZrO2 gave a cyclohexene selectivity of 82.7% at a ben‐
zene conversion of 59.4% at 15 min. The activity was lower
(64.4%) and the cyclohexene selectivity was slightly higher
(80.7%) than those with ZrO2. This implied that the ZrO2 played
an important role in dispersing the Ru catalyst particles; this is
consistent with the TEM results.
The results described above show that the amount of
[Zn(OH)2]3(ZnSO4)(H2O)3 salt formed in the hydrogenation
process increased with the Mn3O4 content of the Ru‐Mn(x) cat‐
alysts. The chemisorbed [Zn(OH)2]3(ZnSO4)(H2O)3 salt played a
key role in improving the cyclohexene selectivity of the Ru cat‐
alyst. (1) The chemisorbed [Zn(OH)2]3(ZnSO4)(H2O)3 salt was
rich in crystallization water. Chemisorption of the salt on the
surface of the catalyst therefore resulted in the Ru catalyst be‐
ing surrounded by a stagnant water layer. The existence of a
stagnant water layer on the catalyst surface could accelerate
120
Benzene conversion
Cyclohexene selectivity
Cyclohexene yield
100
80
60
40
20
0
1
2
3
4
5
6
7
8
Recycle time
Fig. 6. Reusability of the Ru‐Mn(5.4%)/ZrO2 catalyst for selective hy‐
drogenation of benzene to cyclohexene. Reaction conditions: a share of
Ru‐Mn(x) catalyst, 49.2 g ZnSO4·7H2O, 9.8 g ZrO2, 280 ml H2O, 5 MPa H2,
150 oC, stirring rate 1400 r/min.

SUN Haijie et al. / Chinese Journal of Catalysis 34 (2013) 684–694
Graphical Abstract
Chin. J. Catal., 2013, 34: 684–694 doi: 10.1016/S1872‐2067(11)60489‐0
Selective hydrogenation of benzene to cyclohexene over nanocomposite Ru‐Mn/ZrO2 catalysts
SUN Haijie, JIANG Houbing, LI Shuaihui, WANG Hongxia, PAN Yajie, DONG Yingying, LIU Shouchang, LIU Zhongyi*
Zhengzhou University
An Ru‐Mn catalyst with an optimum Mn content of 5.4% gave a cyclohexene yield of 61.3%. The chemisorbed [Zn(OH)2]3(ZnSO4)(H2O)3
salt, which was formed by the reaction of Mn3O4 with ZnSO4 in the slurry, improved the selectivity of the Ru catalyst.
salt chemisorbed on the Ru surface therefore played a decisive
role in the enhancement of the cyclohexene selectivities of the
Ru catalysts.
salt chemisorbed on the Ru surface played a key role in im‐
proving the cyclohexene selectivity of the Ru catalyst. These
results indicate that some catalyst promoters could react with
additives to form new substances. These new substances might
significantly affect the selectivity and activity of the catalyst.
This might provide new ideas for understanding the action
mechanism of promoters in catalysis.
Figure 5 shows the performances of the Ru‐Mn(x) catalysts
prepared using different precursors, with ZrO2 as a dispersant,
for selective hydrogenation of benzene to cyclohexene. As can
be seen, the activities, the cyclohexene selectivities, and the
cyclohexene yields of these catalysts were all very similar. The
characterization results confirmed that these catalysts had
similar chemical compositions. This could be responsible for
the very similar performances of these catalysts. The results
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