The role of la in improving the selectivity to cyclohexene of Ru catalyst for hydrogenation of benzene

Article abstract of DOI:10.1016/j.molcata.2012.12.001

Ru-La catalysts with different La/Ru molar ratios were prepared by co-precipitation. Characterizations revealed that the promoter La existed as La(OH)₃ on the Ru surface. The La(OH)₃ itself could not enhance the selectivity to cyclohexene of Ru catalyst. However, the La(OH) ₃ could react with ZnSO₄ in slurry to form an insoluble (Zn(OH)₂)₃(ZnSO₄)(H₂O)₃ salt. The chemisorbed (Zn(OH)₂)₃(ZnSO₄)(H ₂O)₃ salt on Ru surface played a key role in improving the selectivity to cyclohexene of Ru catalyst. Ru-La catalyst with the optimum La/Ru molar ratio of 0.14 gave a maximum cyclohexene yield of 59.5%. Besides, Ru-La(0.14) catalyst had a good reusability and an excellent stability.

Full text of DOI:10.1016/j.molcata.2012.12.001

Journal of Molecular Catalysis A: Chemical 368–369 (2013) 119–124
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The role of La in improving the selectivity to cyclohexene of Ru catalyst for
hydrogenation of benzene
Hai-Jie Sun, Ying-Ying Dong, Shuai-Hui Li, Hou-Bing Jiang, Yuanxin Zhang, Zhong-Yi Liu∗,
Shou-Chang Liu
College of Chemistry and Molecular Engineering, 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:
Received 8 October 2012
Received in revised form 1 December 2012
Accepted 3 December 2012
Available online 11 December 2012
Ru–La catalysts with different La/Ru molar ratios were prepared by co-precipitation. Characterizations
revealed that the promoter La existed as La(OH)3 on the Ru surface. The La(OH)3 itself could not enhance
the selectivity to cyclohexene of Ru catalyst. However, the La(OH)3 could react with ZnSO4 in slurry to
form an insoluble (Zn(OH)2)3(ZnSO4)(H2O)3 salt. The chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt on Ru
surface played a key role in improving the selectivity to cyclohexene of Ru catalyst. Ru–La catalyst with
the optimum La/Ru molar ratio of 0.14 gave a maximum cyclohexene yield of 59.5%. Besides, Ru–La(0.14)
catalyst had a good reusability and an excellent stability.
Keywords:
Benzene
Cyclohexene
© 2012 Elsevier B.V. All rights reserved.
Ru
La
Selective hydrogenation
1. Introduction
the combination of the promoters and the reaction modifier ZnSO4
could remarkably improve the selectivity to cyclohexene of the
Cyclohexene is commercially important for the production of
adipic acid, nylon 6, nylon 66 and many other fine chemicals [1–4].
Selective hydrogenation of benzene to cyclohexene is superior due
to inexpensive products, lower amounts of undesirable products
and simplified operation, compared with traditional methods, such
as dehydrogenation of cyclohexane, dehydration of cyclohexanol,
and the Birch reduction [5]. However, it is difficult to obtain a
high yield of cyclohexene through this route, because cyclohexane,
the complete hydrogenation product, is thermodynamically more
favorable.
It has been proved that the addition of one or two reaction
modifiers to the reaction system is one of the simplest meth-
ods to enhance the selectivity to cyclohexene of Ru catalysts.
Various substances have been used as the reaction modifiers
including inorganic salts [6], organic compounds [7–10], and
ionic liquids [11]. ZnSO4 has been regarded as the best modifiers
[12]. It has been also found the modification of Ru catalysts with
the promoters such as Fe [13,14], Zn [15–18], Co [19], Cu [20],
Ba [21], La [22–24], Ce [4], and K [25] by the co-precipitation
method, the impregnation method, or the chemical reduction
method could enhance the selectivity to cyclohexene. Specially,
Ru catalysts. Liu et al. [23,24] obtained a cyclohexene yield of 53%
over a Ru–La–B/ZrO2 catalyst in the presence of ZnSO4. Liu et al. [5]
utilized ZnSO4 as a modifier and achieved a cyclohexene yield of
53.8% over a Ru–Ce/SBA-15 catalyst. Liu et al. [22] used ZnSO4 and
CdSO4 as co-modifiers and obtained a cyclohexene yield of 57%
over a Ru–La/SBA-15 catalyst. Sun et al. [9,10] employed ZnSO4
and amines (or alcohols) as co-modifiers and got a cyclohexene
yield of above 60% over a Ru–Zn catalyst. Asahi Chemical has
industrialized the process of selective hydrogenation of benzene to
cyclohexene using a Ru catalyst and ZnSO4 as the reaction modifier
[12]. It has been accepted that the promoter and ZnSO4 were
acting respectively. For example, Liu et al. [5] suggested that the
Ce(III) species could enhance the hydrophilicity of the catalyst and
denote some electrons to metallic Ru, which increased the selec-
tivity to cyclohexene of Ru/SBA-15 catalyst. On the other hand,
they proposed that the Zn2+ of ZnSO4 could form adducts with
cyclohexene and hinder the readsorption of cyclohexene, which
suppressed the raid consecutive hydrogenation of cyclohexene
and improved the selectivity to cyclohexene of the catalyst.
However, we found that a large part of the promoter Zn in Ru–Zn
catalyst existed as ZnO. The ZnO on catalyst surface could react
with ZnSO4 in the slurry to form a ZnSO4·3Zn(OH)2·7H2O salt dur-
ing hydrogenation. The ZnSO4·3Zn(OH)2·7H2O salt played a key
role in improving the selectivity to cyclohexene [26,27]. Thus there
was a question what the roles of the other promoters were. In
∗
Corresponding author. Tel.: +86 371 67783384.
E-mail address: liuzhongyi@zzu.edu.cn (Z.-Y. Liu).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.12.001

120
H.-J. Sun et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 119–124
Table 1
this work, we prepared Ru–La catalysts with different La/Ru molar
ratios by a simple co-precipitation method. The optimum La/Ru
molar ratio was determined to be 0.14, on which a cyclohexene
yield of 59.5% was obtained. Moreover, Ru–La(0.14) catalyst had a
good reusability and an excellent stability. The role of the promoter
La in improving the selectivity to cyclohexene of Ru catalyst was
investigated.
Physicochemical properties and Ru crystallite sizes of Ru–La(x) and Ru–La(x) AH.
Sample
BET surface
Pore volume Pore diameter
Ru crystallite
size (nm)
area (cm2/g) (cm3/g)
(nm)
Ru(0)
59
52
53
57
56
0.18
0.15
0.15
0.17
0.16
0.14
0.16
0.09
10.63
11.67
10.96
10.89
10.44
9.13
4.7
4.5
4.7
3.6
4.5
4.4
4.7
3.8
Ru–La(0.14)
Ru–La(0.19)
Ru–La(0.30)
Ru(0) AH
Ru–La(0.14) AH 55
Ru–La(0.19) AH 52
Ru–La(0.30) AH 43
2. Experimental
9.76
8.72
2.1. Catalyst preparations
Ru–La catalysts were prepared according to the following pro-
cedure. 9.75 g RuCl3·H2O and a desired amount of LaCl3·nH2O were
dissolved in 400 ml H2O with agitation. To the stirred solution,
200 ml of a 10% NaOH solution was added instantaneously and
the resulting mixture was agitated for an additional 4 h at 353 K.
This black precipitate was dispersed in 400 ml of a 5% NaOH solu-
tion and charged into a 1 L autoclave lined the Teflon. Hydrogen
was introduced into the autoclave to raise the total internal pres-
sure to 5 MPa and the reduction was conducted at 423 K and at
800 r/min stirring rate for 3 h. The reaction mixture was cooled and
the obtained black powder was washed with water until neutral-
ity, subsequently vacuum-dried and the desired Ru–La catalysts
were obtained. The catalyst was divided into two shares, one share
was used for activity test and the other was used for catalyst
characterization. This method ensured that the catalysts with dif-
ferent La contents had the same Ru contents (about 1.8 g Ru).
The monometallic Ru catalyst was denoted as Ru(0) catalysts. The
amounts of LaCl3·nH2O were adjusted to give the catalysts with dif-
ferent La contents which were denoted as Ru–La(x) catalysts, where
x denoted the La/Ru molar ratio measured by X-ray fluorescence
(XRF).
The reaction products were monitored by taking a small amount
of the reaction mixture every 5 min and analyzed using a GC-
1690 Gas Chromatograph with FID detector. After the reaction the
organic was removed and the solid sample was vacuum-dried at
60 ◦C for characterization. The sample after reaction correspond-
ing to Ru–La(x) catalyst was denoted as Ru–La(x) AH, where AH
stood for After Hydrogenation. The monometallic Ru catalyst after
hydrogenation was denoted as Ru(0) AH.
The reusability and stability of Ru–La(0.14) catalyst were inves-
tigated according to the following procedures. At the end of the
first reaction, the autoclave was cooled down and the organic phase
was separated. The slurry containing the catalyst was recycled in
accordance with the above hydrogenation procedures without any
addition.
3. Results and discussion
Fig. 1(a) and (b) shows that Ru–La(x) catalysts and Ru–La(x) AH
all showed the type-IV adsorption properties. All the closed hys-
teresis loops were of type H3, according to IUPAC classification.
Fig. 1(c) and (d) shows that most of the pores of Ru–La(x) catalysts
and Ru–La(x) AH were mainly distributed in the range of 2–50 nm.
Besides, there were some macropores in the range of 60–120 nm
in these catalysts. Table 1 shows that BET surface areas, pore vol-
umes and pore diameters of Ru–La(x) catalysts slightly changed
with the increase of the molar ratios of La/Ru, indicating that the
addition of La could not alter the texture structure of the catalysts.
The BET surface areas, pore volumes and pore diameters of Ru–La(x)
AH generally decreased with the increase of the molar ratios of
La/Ru. Combined with the characterization results, it is proposed
that the (Zn(OH)2)3(ZnSO4)(H2O)3 salt formed in hydrogenation
might block some of the pores of the catalysts, which resulted in the
decrease of the BET surface areas, pore volumes and pore diameters
of Ru–La(x) AH.
Fig. 2(a) shows that all the Ru–La(x) catalysts showed the diffrac-
tion peaks of metallic Ru (JCPDS 01-070-0274), indicating the Ru
in the catalysts mainly existed as metallic Ru. Besides, the diffrac-
tion peaks of La(OH)3 (JCPDS 00-006-0585) were present in the
XRD patterns of Ru–La(x) catalyst, indicating the promoter La
mainly existed as La(OH)3. Moreover, the intensity of the diffrac-
tion peaks of La(OH)3 increased with the molar ratio of La/Ru,
indicating the increment of the La(OH)3 contents. Fig. 2(b) shows
that all the Ru–La(x) AH gave the diffraction peaks of metallic
Ru. Besides, the diffraction peaks of the (Zn(OH)2)3(ZnSO4)(H2O)3
salt (JCPDS 00-078-0247) were clearly observed on Ru–La(x) AH.
Moreover, the intensity of the diffraction peak at 11.2◦ of this salt
increased with the molar ratios of La/Ru, indicating the increase
of its amount. Interestingly, the diffraction peaks of La(OH)3 dis-
appeared. All of these indicated that the promoter La(OH)3 on
the surface of the catalyst had reacted with the reaction modi-
fier ZnSO4 and H2O to form an insoluble (Zn(OH)2)3(ZnSO4)(H2O)3
salt, which was shown in reaction (1). Moreover, the amount of the
formed (Zn(OH)2)3(ZnSO4)(H2O)3 salt increased with the loading
2.2. Catalyst characterization
N2 physisorption was determined on a Quantachrome Nova
100e apparatus at 77 K. The specific surface areas were calculated
using the Brunauer–Emmett–Teller (BET) method, and the pore size
distributions were obtained by the Barrett–Joyner–Halenda (BJH)
method according to the desorption branches. X-ray diffraction
(XRD) patterns were acquired on a PANalytcal X’Pert PRO instru-
◦
ment using Cu K␣ (ꢀ = 1.541 A) with scan range from 5 to 90◦ at
a step of 0.03◦. The crystallite sizes of metallic Ru in the catalysts
were estimated from the strongest peak broading at 44.0◦ using the
Scherrer equation. Transmission electron micrographs (TEM) and
energy dispersion scanning (EDS) were observed on a JEOL JEM-
2100 instrument using an accelerating voltage of 200 kV. Auger
Electron Spectroscopy (AES) and sputter profiles were taken on
a ULVAC PHI-700 Nano-canning Auger system with on-axis scan-
ning argon ion gun and CMA energy analyzer. The energy resolution
ratio was 0.1%. The background pressure of analysis room was less
than 5.2 × 10−7 Pa. The standard sample was SiO2/Si. The sputter-
ing rate was 9 nm/min. The La/Ru molar ratios and the compositions
of Ru–La(x) catalysts after hydrogenation were measured by X-ray
fluorescence (XRF) on a Bruker S4 Pioneer instrument.
˚
2.3. Activity test
The selective hydrogenation of benzene was performed in a
1 L autoclave lined the hastelloy. The autoclave was charged with
280 ml of H2O containing a share of Ru–La catalyst (comprising 1.8 g
Ru) and 49.2 g of ZnSO4·7H2O. Then heating commenced with H2
pressure of 5 MPa and stirring rate of 800 r/min. 140 ml of ben-
zene was fed and the stirring rate was elevated to 1400 r/min to
exclude the diffusion effect when the temperature reached 150 ◦C.

H.-J. Sun et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 119–124
121
(c)
(a)
Ru-La(0.30)
Ru-La(0.30)
Ru-La(0.19)
Ru-La(0.19)
Ru-La(0.14)
Ru(0)
Ru-La(0.14)
Ru(0)
(d)
(b)
Ru-La(0.30) AH
Ru-La(0.19) AH
Ru-La(0.30) AH
Ru-La(0.19) AH
Ru-La(0.14) AH
Ru(0) AH
Ru-La(0.14) AH
Ru(0) AH
0.0
0.2
0.4
0.6
0.8
1.0
10
100
Pore diameter (nm)
Relative Pressure (p/p0)
Fig. 1. N2 adsorption–desorption isotherms of (a) Ru–La(x) catalysts and (b) Ru–La(x) AH as well as pore size distribution of (c) Ru–La(x) catalysts and (d) Ru–La(x) AH.
of La(OH)3.
the Zn in the catalyst was close to that of metallic Zn. Thus they
suggested that the Zn2+ cations chemisorbed could be reduced to
metallic Zn by the hydrogen atoms that spilled from the surface
of the Ru catalyst. Yuan et al. [17] and He et al. [18] also indicated
the Zn atoms could be introduced into the Ru-based catalyst by the
reduction of Zn2+ according to the XPS results. However, Struijk
et al. [6] indicated that the Zn2+ of the adsorption of ZnSO4 could
not be reduced on the surface of Ru catalyst also on the basis of the
XPS results. But the assessment of the oxidation state of Zn from
the binding energy of the Zn 2p3/2 level is not very useful due to
the close appearance on XPS of the transitions corresponding to
Zn(II) and metallic Zn [28]. This drawback can be overcome using
the Zn LMM Auger transition, as the Auger shift between Zn(II) and
metallic Zn is higher than 4.6 eV [29]. Fig. 4 shows the AES Zn LMM
spectra of Ru–La(0.14) AH at the different sputtering time (0 min,
0.5 min and 1 min). As can be seen, the Kinetic energies (KEs) of Zn
LMM of Ru–La(0.14) AH at the depth of 0 nm, 4.5 nm 9 nm were
983.7 eV, 983.7 eV and 983.6 eV respectively, these contributions
being previously assigned to oxidized Zn [30,31]. This finding sug-
gested that the Zn was present on the surface of Ru–La(0.14) AH
mainly as oxidized Zn even under the reaction conditions of 150 ◦C
and H2 pressure of 5.0 MPa, which was in accordance with that
the Zn on the surface of Ru–La(0.14) AH presented in the form
of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt. Unfortunately, AES measure-
ments did not allow discerning any additional contribution from
metallic Zn (commonly appearing in the 991–995 eV range). More-
over, Table 2 shows that the pH values of the aqueous solutions
after hydrogenation in the presence of ZnSO4 at room temperature
were around 6.0, indicating the acidity of aqueous phase due to the
hydrolysis of ZnSO4. It is well known that increasing temperature
2La(OH)3 + 4ZnSO4 + 3H2O → (Zn(OH)2)3(ZnSO4)(H2O)3↓
+ La2(SO4)3
(1)
Table 1 also shows when the La/Ru molar ratio was below 0.30,
the crystallite sizes of Ru–La(x) catalyst and Ru–La(x) AH were in
range of 4.4–4.7 nm, indicating the promoter La(OH)3 had little
effect on the crystallite sizes of Ru catalyst and the crystallites of
Ru–La(x) catalysts had changed little during the hydrogenation pro-
cess under the reaction condition of high agitation. The crystallite
sizes of Ru–La(0.30) and Ru–La(0.30) AH significantly decreased to
3.6 nm and 3.8 nm respectively. This probably was due to the dis-
persion effect of abundant La(OH)3 in Ru–La(0.30) catalyst on the
Ru crystallites.
Fig. 3(a) and (b) displays that both of Ru–La(0.14) catalyst and
Ru–La(0.14) AH were consist of spherical and ellipsoidal crys-
tallites. Fig. 3(c) and (d) shows that the Ru crystallite sizes of
Ru–La(0.14) catalyst and Ru–La(0.14) AH were mainly distributed
around 4.2 nm, which was consistent with the XRD results. Only
lattice fringes of the (1 0 1) plane of the metallic Ru with an
average spacing of 0.21 nm was detectable on the images of
Ru–La(0.14) catalyst and Ru–La(0.14) AH. This might be due to the
uniform dispersion of La(OH)3 on Ru–La(0.14) catalyst and of the
(Zn(OH)2)3(ZnSO4)(H2O)3 salt on Ru–La(0.14) AH [5,27].
It has been extensively reported that the presence of Zn in Ru
catalysts could significantly enhance the selectivity to cyclohex-
ene of the Ru catalyst [15–18,26,27]. However, there were different
opinions about the valence of Zn. Wang et al. [16] characterized the
Ru–Zn/m-ZrO2 catalyst by XPS and found that the Zn 2p3/2 BE of
Ru
La(OH)3
(Zn(OH)2)3(ZnSO4)(H2O)3
Ru-La(0.30)
Ru
Ru-La(0.30) AH
Ru-La(0.19)
Ru-La(0.14)
Ru-La(0.19) AH
Ru-La(0.14) AH
Ru(0) AH
Ru(0)
10
20
30
40
50
60
70
80
90
10
20
30
40
50
60
70
80
90
2 /( o
)
2 /( o
)
Fig. 2. XRD patterns of Ru–La(x) catalysts and Ru–La(x) AH.

122
H.-J. Sun et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 119–124
Fig. 3. TEM images of (a) Ru–La(0.14) and (b) Ru–La(0.14) AH as well as their Ru crystallite size distribution.
favors the hydrolysis. This means that the acidity of liquid phase
is much higher at the reaction temperature of 423 K due to the
increase of hydrolysis degree of ZnSO4. As we know, it is difficult
for the metallic Zn to exist in the acid solution, which is consistent
with no clear evidences of the presence of metallic Zn on the surface
of Ru catalysts observed in the AES results.
that almost all the La(OH)3 in the catalyst had reacted with ZnSO4
in the slurry to form the (Zn(OH)2)3(ZnSO4)(H2O)3 salt. Especially,
this also implied that the promoter La(OH)3 could not enter into the
Ru lattices and mainly existed on the Ru surface. The molar ratios of
Zn/Ru and S/Ru increased with the loading of La(OH)3, suggesting
the increment of the (Zn(OH)2)3(ZnSO4)(H2O)3 salt in consistent
with the XRD results. The insoluble (Zn(OH)2)3(ZnSO4)(H2O)3 salt
was readily chemisorbed on the surface of the Ru particles. Based
on these, the structures of Ru–La(x) catalyst and Ru–La(x) AH were
sketched as shown in Fig. 5. Therefore, (Zn(OH)2)3(ZnSO4)(H2O)3
salt chemisorbed on the surface of Ru particles might directly influ-
ence the performance of Ru catalyst.
Table 2 gives the composition of Ru–La(x) AH. As can be seen,
only trace amounts of La were detected on Ru–La(x) AH, indicating
983.6 eV
983.7 eV
Before comparing the selectivity over these catalysts, Carberry
number (Ca) and Wheeler–Weisz number (ꢁϕ2) were calculated
Table 2
983.7 eV
Composition of Ru–La(x) AH and pH values of liquid phase at room temperature
after hydrogenation.
Catalyst
La/Ru AHc
Zn/Ru AHc
S/Ru AHc
pH valued
Ru(0)a
0
0.0313
0.2542
0.3885
0.5789
0
0.0026
0.0163
0.0381
0.0673
0
5.53
6.18
6.35
6.37
6.99
Ru–La(0.14)a
Ru–La(0.19)a
Ru–La(0.30)a
Ru–La(0.14)b
0.0017
0.0018
0.0032
0.1427
a
975 980 985 990 995 1000 1005 1010
Kinetic energy (eV)
In the presence of 0.6 mol/L ZnSO4.
In the absence of 0.6 mol/L ZnSO4.
Determined by XRF.
b
c
d
Measured by pH meter.
Fig. 4. AES Zn LMM spectra of Ru–La(0.14) AH.

H.-J. Sun et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 119–124
123
(Zn(OH)2)3(ZnSO4)(H2O)3 salt formed by La(OH)3 reacting with
ZnSO4 played a key role in improving the selectivity to cyclohexene
of the Ru catalyst.
Based on the previous works, the roles of the chemisorbed
(Zn(OH)2)3(ZnSO4)(H2O)3 salt on Ru surface might be attributed
to one or a combination of the following reasons. Firstly, the
chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt was rich in crystal
water. Therefore, the salt chemisorbed on the surface of the cat-
alyst caused Ru catalyst to be surrounded by a firm stagnant water
layer, as shown in Fig. 5. The existence of the stagnant water layer
on the surface of the catalyst could accelerate the desorption and
hinder the re-adsorption of cyclohexene for further hydrogenation
to cyclohexane due to the very low solubility of cyclohexene [33],
resulting in the improvement of the selectivity to cyclohexene. Sec-
ondly, the Zn2+ of the chemisorbed (Zn(OH)2)3(ZnSO4)(H2O)3 salt
can selectively cover the most reactive sites of the catalyst, which
can reduce the active sites for the chemisorption of cyclohexene
and suppress the further hydrogenation of cyclohexene to cyclo-
hexane [6,16,26,27]. Struijk et al. [6] suggested the salts which act
as the effective additive should have enough absorbability on Ru
to cover 50% Ru active sites. Finally, the Zn2+ of the chemisorbed
(Zn(OH)2)3(ZnSO4)(H2O)3 salt could form loosely bound adducts
with cyclohexene, which could stabilize the formed cyclohexene
on the surface of Ru catalyst and improve the selectivity to cyclo-
hexene of Ru catalyst [5,16,22]. Therefore, increasing the loading of
La(OH)3 increased the formation of the (Zn(OH)2)3(ZnSO4)(H2O)5
salt, resulting in the decrease of activity and the increase of the
selectivity to cyclohexene of Ru catalyst. Above all, the chemisorbed
(Zn(OH)2)3(ZnSO4)(H2O)3 salt formed by La(OH)3 reacting with
ZnSO4 in the slurry directly enhanced the selectivity to cyclohexene
of Ru catalyst.
Fig. 5. Structure sketches of Ru–La(x) catalyst and Ru–La(x) AH.
according to previous works [32]. It was found that Carberry num-
bers and Wheeler–Weisz numbers were smaller than 0.05 and 0.1
respectively, indicating that external mass transport limitation and
pore diffusion limitation could be neglected. This indicates that the
reaction was always under kinetic control. Fig. 6(a) and (b) shows
that the activity decreased and the selectivity to cyclohexene of
the catalysts increased with the molar ratio of La/Ru in the pres-
ence of ZnSO4. Fig. 6(c) shows the maximum yield of cyclohexene
increased with the increment of the molar ratio of La/Ru in the pres-
ence of ZnSO4, and then declined at higher doping amounts of La,
with the optimal La/Ru molar ratio being 0.14. Ru–La(0.14) catalyst
gave the highest cyclohexene yield of 59.5%, which was among the
best results reported so far [5,12,22]. However, Ru–La(0.14) catalyst
gave the poorest selectivity of 2.3% and the weakest yield of 1.9% in
the absence of ZnSO4, suggesting that the promoter La(OH)3 alone
could not enhance the selectivity to cyclohexene and the cyclo-
hexene yield of Ru catalyst. This indicated that the chemisorbed
100
90
(a)
(b)
Ru(0)
80
75
Ru-La(0.14)
60
60
Ru(0)
Ru-La(0.14)
45
Ru-La(0.19)
Ru-La(0.19)
40
20
Ru-La(0.30)
30
Ru-La(0.30)
15
5
10
15
20
25
20
40
60
80
100
Time (min)
Benzene conversion (%)
59.5
60
50
40
30
20
10
(c)
Ru-La(0.14)
Ru-La(0.19)
37.1
36.4
Ru(0)
33.0
Ru-La(0.30)
20
5
10
15
Time (min)
25
Fig. 6. (a) Benzene conversion and (b) cyclohexene selectivity as well as (c) cyclohexene yield over Ru–La(x) catalysts. Reaction conditions: a share of Ru–La(x) catalyst, 49.2 g
of ZnSO4·7H2O, 280 ml of H2O, 140 ml of benzene, reaction temperature of 150 ◦C, H2 pressure of 5.0 MPa and stirring rate of 1400 r/min.

124
H.-J. Sun et al. / Journal of Molecular Catalysis A: Chemical 368–369 (2013) 119–124
105
90
75
60
45
30
15
0
Acknowledgments
Conversion (%)
Selectivity (%)
Yield (%)
Maximum yield
This work was supported by the National Nature Science
Foundation of China (21273205), the Innovation Found for Tech-
nology Based Firms of China (10C26214104505) and the Scientific
Research Foundation of Graduate School of Zhengzhou University.
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