catalysts

Article
Selective Hydrogenation of Benzene to Cyclohexene
over Ru-Zn Catalysts: Investigations on the Effect of
Zn Content and ZrO2 as the Support and Dispersant
Haijie Sun 1 , Zhihao Chen 2, * , Lingxia Chen 1 , Huiji Li 1 , Zhikun Peng 3, *, Zhongyi Liu 3
and Shouchang Liu 3
1 Institute of Environmental and Catalytic Engineering, College of Chemistry and Chemical Engineering,
Zhengzhou Normal University, Zhengzhou 450044, Henan, China; sunhaijie406@163.com (H.S.);
clingxia@vip.163.com (L.C.); huijili@zznu.edu.cn (H.L.)
2 Zhengzhou Tobacco Research Institute of CNTC, Zhengzhou 450001, Henan, China
3 College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, Henan, China;
liuzhongyi406@163.com (Z.L.); liushouchang406@163.com (S.L.)
* Correspondence: chenzh@ztri.com.cn (Z.C.); Zhikunpeng@163.com (Z.P.);
Tel.: +86-371-6767-2762 (Z.C.); +86-158-3810-9080 (Z.P.)




Received: 15 October 2018; Accepted: 29 October 2018; Published: 2 November 2018


Abstract: m-ZrO2 (monoclinic phase) supported Ru-Zn catalysts and unsupported Ru-Zn
catalysts were synthesized via the impregnation method and co-precipitation method, respectively.
The catalytic activity and selectivity were evaluated for selective hydrogenation of benzene towards
cyclohexene formation. Catalyst samples before and after catalytic experiments were thoroughly
characterized via X-ray diffraction (XRD), X-ray Fluorescence (XRF), transmission electron microscopy
(TEM), N2 -sorption, X-ray photoelectron spectroscopy (XPS), H2 -temperature programmed reduction
(H2 -TPR), and a contact angle meter. It was found that Zn mainly existed as ZnO, and its content
was increased in Ru-Zn/m-ZrO2 by enhancing the Zn content during the preparation procedure.
This results in the amount of formed (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 increasing and the catalyst becoming
more hydrophilic. Therefore, Ru-Zn/m-ZrO2 with adsorbed benzene would easily move from the
oil phase into the aqueous phase, in which the synthesis of cyclohexene took place. The generated
cyclohexene then went back into the oil phase, and the further hydrogenation of cyclohexene would
be retarded because of the high hydrophilicity of Ru-Zn/m-ZrO2 . Hence, the selectivity towards
cyclohexene formation over Ru-Zn/m-ZrO2 improved by increasing the Zn content. When the
theoretical molar ratio of Zn to Ru was 0.60, the highest cyclohexene yield of 60.9% was obtained over
Ru-Zn (0.60)/m-ZrO2 . On the other hand, when m-ZrO2 was utilized as the dispersant (i.e., employed
as an additive during the reaction), the catalytic activity and selectivity towards cyclohexene synthesis
over the unsupported Ru-Zn catalyst was lower than that achieved over the Ru-Zn catalyst with
m-ZrO2 as the support. This is mainly because the supported catalyst sample demonstrated superior
dispersion of Ru, higher content of (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 , and a stronger electronic effect
between Ru and ZrO2 . The Ru-Zn(0.60)/m-ZrO2 was reused 17 times without any regeneration, and
no loss of catalytic activity and selectivity towards cyclohexene formation was observed.

Keywords: selective hydrogenation; benzene; cyclohexene; Ru; Zn; ZrO2

1. Introduction
Selective hydrogenation of benzene towards cyclohexene synthesis has been a significant reaction
in the field of catalysis research [1–5]. This is mainly attributed to the fact that the production of

Catalysts 2018, 8, 513; doi:10.3390/catal8110513 www.mdpi.com/journal/catalysts

Catalysts 2018, 8, 513 2 of 17

caprolactam and adipic acid via cyclohexene is environmentally friendlier, more energy preservation,
and might result in higher carbon atom economy in comparison to that via cyclohexane [6,7].
The first industrial plant for production of cyclohexene from selective hydrogenation of benzene
over unsupported Ru-Zn catalyst was manufactured by Asahi in 1989 [8]. However, some drawbacks
for the catalyst, such as high Ru content and the ease of being poisoned, are of great difficulty to
overcome. Therefore, the development of supported Ru catalysts with relatively low Ru loading
and high Ru dispersion have drawn great interest. Commonly, ZrO2 is selected as a proper support.
For instance, Zhou et al. [9] prepared Ru-Zn/ZrO2 catalyst via the deposition-precipitation method,
from which a 54% cyclohexene yield was obtained. Furthermore, Ru-Zn/ZrO2 catalyst was prepared
via a two-step impregnation method by Yan et al. [1], which gave a 48.5% cyclohexene yield. Moreover,
Peng et al. [10] applied a chemical reduction method to synthesize a Ru/m-ZrO2 /t-ZrO2 catalyst, from
which a 55.3% cyclohexene yield was achieved. Liu et al. [11] also used the chemical reduction method
to prepare a Ru-La-B/ZrO2 catalyst, and a 53.2% cyclohexene yield was shown. Other than ZrO2 ,
zeolite (i.e., SBA-15) [12] and γ-Al2 O3 [13] were also reported as the catalyst support for selective
hydrogenation of benzene over Ru-based catalysts. However, how supports affect the catalytic activity
and selectivity has been rarely addressed.
A promoter is one of the most effective ways to improve the cyclohexene yield over the Ru-based
catalytic system. Zn [14,15], Fe [16,17], Co [18], Mn [19], La [20,21], and Ce [22,23] have been
investigated as promoters for Ru catalysts on selective hydrogenation of benzene. Zn has been
widely studied and reported among all promoters due to its superior promotion performance [15].
However, the status of Zn (e.g., valence of Zn) in supported Ru-based catalysts has been controversial.
Zhou et al. [24] and Wang et al. [25] deemed that in Ru-Zn/ZrO2 , Zn existed as metallic Zn covering
some of the Ru active sites, with lower selectivity towards cyclohexene formation. They also suggested
that the Zn@Ru system could be achieved by doping metallic Zn into Ru, which spontaneously
modified the geometric and electronic structure of Ru, and thus improved the catalytic activity and
selectivity towards cyclohexene formation. On the other hand, Zhou et al. [9] and Yan et al. [1]
demonstrated that Zn existed as ZnO in Ru-Zn/ZrO2 . However, the mechanism of ZnO affecting the
formation of cyclohexene was not clearly explained. Thus, it is of great significance to investigate the
status of Zn in Ru-Zn/ZrO2 and how it affects the catalytic system, which could provide essential
guidance for the development of the supported Ru-based catalysts.
Based on the previous work, we prepared Ru-Zn/m-ZrO2 catalysts with different Zn content
via the impregnation method. In order to reveal the status of Zn in Ru-Zn/ZrO2 and how it affects
the catalytic system, all samples were evaluated for selective hydrogenation of benzene towards
cyclohexene formation. In addition, unsupported Ru-Zn catalyst with the same content of Ru and
Zn was synthesized via the co-precipitation method, which was tested under the same experimental
conditions by adding ZrO2 as a dispersant. In comparison to that observed over Ru-Zn/ZrO2 ,
the support effect of ZrO2 was proposed for the catalytic performance of Ru-Zn on the selective
hydrogenation of benzene towards cyclohexene generation.

2. Results

2.1. Effect of Zn Content

XRD patterns of Ru-Zn(x)/m-ZrO2 before (a) and after hydrogenation (b) are given in Figure 1.
In Figure 1a, ZrO2 of the monoclinic phase as well as metallic Ru can be observed for all fresh catalyst
samples, indicating that ZrO2 exists as the monoclinic phase and Ru is completely reduced. Notably,
when the theoretical molar ratio of Zn to Ru reaches 0.60, characteristic diffraction of ZnO starts to
be shown, suggesting that Zn mainly exists as ZnO in Ru-Zn(x)/m-ZrO2 . This is consistent with that
reported by Zhou et al. [9] and Yan et al. [1]. As long as the molar ratio of Zn to Ru is no higher than
0.47, ZnO reflections cannot be detected. This can be rationalized in terms that the amount of ZnO is
relatively low, and might not be able to aggregate into a phase. After hydrogenation (Figure 1b) on the
 Catalysts 2018, 8, 513 3 of 17

Catalysts 2018, 8, x FOR PEER REVIEW 3 of 17

other hand, instead of ZnO, reflections related to (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 can be detected when
the the
molar ratio
molar of Zn
ratio to to
of Zn RuRu
is is
higher
higherthan
than0.6.
0.6.This
Thisindicates
indicates that
that ZnO could react
ZnO could reactwith
withZnSOZnSO 4 during
4 during
the the
reaction
reactiontotoform
form the (Zn(OH)22))33(ZnSO
the (Zn(OH) (ZnSO44)(H
)(H22O) salt. Similarly,
O)33 salt. Similarly,no no(Zn(OH)
(Zn(OH) 2)3(ZnSO4)(H2O)3
2 )3 (ZnSO4 )(H2 O)3
diffraction cancan
diffraction be be
observed
observed when
when the
themolar
molarratio
ratioofofZn
ZntotoRu
Ru is
is less than 0.47,
less than 0.47, and
andthe thesame
samereason
reason
could be applied
could here.
be applied here.

(a) (b)
Figure
Figure 1. 1.
XRDXRD patternsofofRu-Zn(x)/ZrO
patterns Ru-Zn(x)/ZrO 2 before(a)
2 before (a)and
andafter
afterhydrogenation
hydrogenation (b).
(b).

TheThe composition
composition andand textureproperties
texture propertiesof ofRu-Zn(x)/m-ZrO
Ru-Zn(x)/m-ZrO22 catalysts
catalysts before
before andandafter
aftercatalytic
catalytic
experiments, as well as pH values of the slurry after hydrogenation, are listed in Table1.1.As
experiments, as well as pH values of the slurry after hydrogenation, are listed in Table Ascancanbebe
seen, for the fresh catalysts, the specific surface area, pore diameter, and pore size slightly decreased
seen, for the fresh catalysts, the specific surface area, pore diameter, and pore size slightly decreased
with increasing Zn content. This implies that some of the macro pores of m-ZrO2 were blocked by
with increasing Zn content. This implies that some of the macro pores of m-ZrO2 were blocked by the
the impregnated Ru and ZnO. However, with enhancing the Zn content, pore diameter increased,
impregnated Ru and ZnO. However, with enhancing the Zn content, pore diameter increased, while
while the specific surface area and pore volume dropped after the reaction. This might be due to
the specific surface area and pore volume dropped after the reaction. This might be due to the fact
the fact that (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 was generated during the reaction, which covered some of
that (Zn(OH)2)3(ZnSO4)(H2O)3 was generated during the reaction, which covered some of the micro
the micro pores of m-ZrO2 . On the other hand, it is noticed that the molar ratio of Zn to Ru over
pores of m-ZrO2. On the other hand, it is noticed that the molar ratio of Zn to Ru over Ru-Zn (x)/m-
Ru-Zn (x)/m-ZrO2 after catalytic experiments is higher than that obtained over the fresh catalysts,
ZrOimplying
2 after catalytic2+
that Zn experiments
from ZnSO4 is orhigher
(Zn(OH) than that obtained over the fresh catalysts, implying that
2 )3 (ZnSO4 )(H2 O)3 was chemisorbed on the catalyst surface.
Zn Additionally,
2+ from ZnSO4no or obvious
(Zn(OH)variation
2)3(ZnSO4)(H2O)3 was chemisorbed on the catalyst surface. Additionally,
was noticed for n (Zr)/n (Ru), indicating that m-ZrO2 was barely
no obvious variation was noticed
lost during the catalytic experiments. for n (Zr)/n (Ru), the
Moreover, indicating
pH value that
of m-ZrO 2 was
the slurry afterbarely lost during
the reaction the
is less
catalytic
than 6, experiments.
suggesting thatMoreover, the pH
the slurry value of
is acidic. theisslurry
This mainlyafter the reaction
attributed is less
to the than 6, suggesting
hydrolysis of ZnSO4 .
thatMore
the slurry is acidic. This is mainly attributed to the hydrolysis of ZnSO . More
importantly, the pH value of the slurry after the reaction increased with increasing Zn content,
4 importantly, the
pH demonstrating
value of the slurry that after the reaction
the hydrolysis of increased with increasing
ZnSO4 is retarded. This canZnbecontent, demonstrating
rationalized in terms thatthatthe
the
hydrolysis
formationofof ZnSO(Zn(OH)4 is retarded.
2 )3 (ZnSO 4 )(H2This
O)3 ledcan be decrease
to the rationalized
in the in terms thatof ZnSO
concentration the formation
4. of
(Zn(OH)TEM 2)3(ZnSO 4)(H
images 2O)3the
and ledRu toparticle
the decrease in the concentration
size distribution of ZnSO4. 2 catalysts before (a,b)
of Ru-Zn (0.60)/m-ZrO
and after (c,d) catalytic experiments are shown in Figure 2. It can be observed from Figure 2a,b
that Ru is
Table 1. uniformly
Composition dispersed on m-ZrO
and texture 2 , and the
properties particle size of 2Rucatalysts
of Ru-Zn(x)/m-ZrO is around 4.5 nm.
before and After
after the
hydrogenation
hydrogenation,reaction,
as well as the
pHparticle
values size
of theofslurry
Ru remained at 4.5experiments.
after catalytic nm, indicating that no aggregation of
Ru happened during the reaction.
Catalyst SBET/(m2·g−1) 1 Vpore/(cm3·g−1) 1 dpore/(nm) 1 nZn/nRu 2 nZr/nRu 2 pH 3
ZrO2 34 0.13 16.0 - - -
Ru(0)/ZrO2 31 0.10 10.7 0 5.19 -
Ru-Zn(0.06)/ZrO2 30 0.09 9.8 0.06 5.30 -
Ru-Zn(0.33)/ZrO2 30 0.09 9.7 0.33 5.26 -
Ru-Zn(0.47)/ZrO2 30 0.10 9.6 0.47 5.24 -
Ru-Zn(0.60)/ZrO2 30 0.09 9.7 0.60 5.38 -
Ru-Zn(0.69)/ZrO2 28 0.08 9.6 0.69 5.26 -
Ru-Zn(0.86)/ZrO2 28 0.08 9.1 0.86 5.25 -
Ru-Zn(1.02)/ZrO2 29 0.08 9.1 1.02 5.16 -
Ru(0)/ZrO2 AH 30 0.11 9.5 0.20 5.21 4.25
Ru-Zn(0.06)/ZrO2 AH 30 0.10 9.8 0.25 5.29 4.25
Ru-Zn(0.33)/ZrO2 AH 30 0.09 9.9 0.37 5.38 5.38
Catalysts 2018, 8, 513 4 of 17

Table 1. Composition and texture properties of Ru-Zn(x)/m-ZrO2 catalysts before and after
hydrogenation, as well as pH values of the slurry after catalytic experiments.

Catalyst SBET /(m2 ·g−1 ) 1 Vpore /(cm3 ·g−1 ) 1 dpore /(nm) 1 nZn /nRu 2 nZr /nRu 2 pH 3
Catalysts 2018,
ZrO8,2 x FOR PEER REVIEW34 0.13 16.0 - - 4 -of 17
Ru(0)/ZrO2 31 0.10 10.7 0 5.19 -
Ru-Zn(0.06)/ZrO2 30 0.09 9.8 0.06 5.30 -
Ru-Zn(0.47)/ZrO
Ru-Zn(0.33)/ZrO2
2 AH
30
30 0.09
0.08 9.7
9.8 0.55
0.33
5.36
5.26
5.38
-
Ru-Zn(0.60)/ZrO
Ru-Zn(0.47)/ZrO2 2 AH 30 30 0.100.08 9.6 10.1 0.68
0.47 5.32
5.24 5.38
-
Ru-Zn(0.60)/ZrO
Ru-Zn(0.69)/ZrO2 AH 2 30 28 0.090.07 9.7 10.3 0.60
0.86 5.38
5.30 -
5.44
Ru-Zn(0.69)/ZrO2 28 0.08 9.6 0.69 5.26 -
Ru-Zn(0.86)/ZrO
Ru-Zn(0.86)/ZrO2 2 AH 28 29 0.080.08 9.1 10.6 1.10
0.86 5.26
5.25 5.82
-
Ru-Zn(1.02)/ZrO
Ru-Zn(1.02)/ZrO2 2 AH 29 29 0.080.06 9.1 11.4 1.25
1.02 5.31
5.16 5.73
-
Ru(0)/ZrO
1 Determined 2 AH 30 2 0.11 3 9.5 0.20 5.21 4.25
by N2-sorption; Determined by XRF; Determined by a pH meter at room temperature;
Ru-Zn(0.06)/ZrO2 AH 30 0.10 9.8 0.25 5.29 4.25
AH: after hydrogenation.
Ru-Zn(0.33)/ZrO2 AH 30 0.09 9.9 0.37 5.38 5.38
Ru-Zn(0.47)/ZrO2 AH 30 0.08 9.8 0.55 5.36 5.38
Ru-Zn(0.60)/ZrO
TEM images 2 AH
and the Ru30particle size distribution
0.08 10.1 (0.60)/m-ZrO
of Ru-Zn 0.68 2 catalysts
5.32 before5.38
(a,b)
Ru-Zn(0.69)/ZrO2 AH 28 0.07 10.3 0.86 5.30 5.44
and after (c,d) catalytic
Ru-Zn(0.86)/ZrO 2 AH
experiments
29 are shown
0.08in Figure 2. It can be observed
10.6 1.10 from Figure
5.26 2a,b that
5.82
RuRu-Zn(1.02)/ZrO
is uniformly2 AH dispersed on29 m-ZrO2, and0.06the particle size
11.4 of Ru is 1.25
around 4.5 5.31
nm. After 5.73the
hydrogenation
1 Determinedreaction, the particle
by N2 -sorption; size ofbyRu
2 Determined remained
XRF; at 4.5
3 Determined bynm,
a pHindicating thattemperature;
meter at room no aggregation
AH: of
after hydrogenation.
Ru happened during the reaction.

Figure 2.
Figure 2. TEM
TEM images
images and
and the
the Ru
Ruparticle
particlesize
sizedistribution
distributionofofRu-Zn
Ru-Zn(0.60)/m-ZrO
(0.60)/m-ZrO22 catalysts
catalysts before
before
(a,b) and
(a,b) and after
after (c,d)
(c,d) catalytic
catalytic experiments.
experiments.

X-ray photoelectron spectroscopy (XPS) profiles of the Ru-Zn (0.33)/m-ZrO2 catalyst and Ru-Zn
(0.60)/m-ZrO2 catalyst after catalytic experiments are presented in Figure 3. It can be observed from
Figure 3a that the peak of Ru3d includes Ru3d3/2 and Ru3d5/2, and the former one is partly overlapped
with the peak of C1s. Therefore, Ru3d5/2 is selected for further discussion. There are two peaks for
Ru3d5/2 with binding energy (BE) of 280.3 eV and 281.4 eV, which are attributed to Ru0 and Ruδ+,
Catalysts 2018, 8, 513 5 of 17

X-ray photoelectron spectroscopy (XPS) profiles of the Ru-Zn (0.33)/m-ZrO2 catalyst and Ru-Zn
(0.60)/m-ZrO2 catalyst after catalytic experiments are presented in Figure 3. It can be observed from
Figure 3a that the peak of Ru3d includes Ru3d3/2 and Ru3d5/2 , and the former one is partly overlapped
with the peak of C1s. Therefore, Ru3d5/2 is selected for further discussion. There are two peaks for
Ru3d5/2 with binding energy (BE) of 280.3 eV and 281.4 eV, which are attributed to Ru0 and Ruδ+ ,
respectively [26]. The presence of Ruδ+ implies that some Ru lost electrons. Furthermore, it is found
that n (Ruδ+ )/n (Ru0 ) over Ru-Zn (0.60)/m-ZrO2 after hydrogenation (AH) is slightly higher than
that Catalysts
obtained over Ru-Zn (0.33)/m-ZrO2 AH (i.e., 1.18 vs. 1.09), indicating that more electrons
2018, 8, x FOR PEER REVIEW 5 of 17
were
transferred out of Ru with the enhancement of Zn content. Moreover, as can be seen from Figure 3b,
the BE of Zn2p
obtained over over Ru-Zn
3/2Ru-Zn (0.33)/m-ZrO
(0.33)/m-ZrO 2 AH (i.e., 2 AH
1.18 and Ru-Zn
vs. 1.09), (0.60)/m-ZrO
indicating that more2 AH is 1022.0
electrons wereeV and
transferred out of Ru with the enhancement of Zn content. Moreover, as
1021.1 eV, respectively. However, it is very difficult to justify whether it is metallic Zn or Zn , sincecan be seen from Figure 2+
3b,
the BEsthe BE of Zn2p3/2Zn
of metallic over
andRu-Zn
Zn2+(0.33)/m-ZrO
for Zn2p3/22 AH and Ru-Zn (0.60)/m-ZrO
are extremely close [27]. Hence,2 AH is 1022.0 eV and 1021.1
the kinetic energy (KE) of
Zn LMM is considered to judge the valence of Zn. It was shown that theZn
eV, respectively. However, it is very difficult to justify whether it is metallic or of
ZnZn2+, since the BEs
KE LMM for Ru-Zn
of metallic Zn and Zn2+ for Zn2p3/2 are extremely close [27]. Hence, the kinetic energy (KE) of Zn LMM
(0.33)/m-ZrO2 AH and Ru-Zn (0.60)/m-ZrO2 AH is 988.6 eV and 989.2 eV, respectively (Figure 3d).
is considered to judge the valence of Zn. It was shown that the KE of Zn LMM for Ru-Zn (0.33)/m-
This suggests that the valence of Zn in Ru-Zn/m-ZrO2 AH is positive 2, since the KE of metallic Zn is
ZrO2 AH and Ru-Zn (0.60)/m-ZrO2 AH is 988.6 eV and 989.2 eV, respectively (Figure 3d). This
992.1suggests
eV [28].that
This theisvalence
in good of agreement
Zn in Ru-Zn/m-ZrOwith the XRD results. In addition, it is noticed that the BE of
2 AH is positive 2, since the KE of metallic Zn is 992.1
Zn2peV 1/2 over Ru-Zn (0.60)/m-ZrO AH is
[28]. This is in good agreement with the XRD results.
2 lower than In that observed
addition, over Ru-Zn
it is noticed that the(0.60)/m-ZrO
BE of Zn2p1/2 2 AH
(i.e., over
1021.1 eV vs. 1022.0 eV), but the former’s KE is higher than
Ru-Zn (0.60)/m-ZrO2 AH is lower than that observed over Ru-Zn (0.60)/m-ZrO2 AH (i.e., that obtained over the 1021.1
latter. This
demonstrates
eV vs. 1022.0 that more
eV), electrons
but the former’swereKE istransferred
higher than that to Zn 2+ withover
obtained increasing
the latter.Zn content.
This In contrast,
demonstrates
the BE thatofmore
Zr3delectrons
5/2 over were
Ru-Zn transferred
(0.33)/m-ZrOto Zn 2+ with increasing Zn content. In contrast, the BE of Zr3d5/2
2 AH and Ru-Zn (0.60)/m-ZrO 2 AH is 180.7 eV and 180.6
over Ru-Zn (0.33)/m-ZrO 2 AH and Ru-Zn (0.60)/m-ZrO2 AH is 180.7 eV and 180.6 eV, respectively,
eV, respectively, which means that the addition of Zn is not able to modify the electronic structure of
which means
Zr. However, it isthat the addition
worth mentioning of Znthat
is notthe able
BEtoofmodify
Zr3d5/2 thefor
electronic structure of
ZrO2 observed inZr.
thisHowever,
work is itclearly
is worth mentioning that the BE of Zr3d5/2 for ZrO2 observed in this work is clearly lower than that
lower than that reported in literature (i.e., 182.2 eV) [29], which might be due to the fact that some of
reported in literature (i.e., 182.2 eV) [29], which might be due to the fact that some of the electrons
the electrons from Ru were transferred to Zr as well. This reveals that there is a strong electronic effect
from Ru were transferred to Zr as well. This reveals that there is a strong electronic effect between
between the active
the active component component
(Ru) and (Ru)
the and the (m-ZrO
support support 2).
(m-ZrO2 ).

Figure
Figure 3. X-ray
3. X-ray photoelectronspectroscopy
photoelectron spectroscopy (XPS)
(XPS) profiles
profilesofofRu-Zn
Ru-Zn(0.33)/m-ZrO
(0.33)/m-ZrO 2 catalyst and Ru-Zn
2 catalyst and Ru-Zn
(0.60)/m-ZrO
(0.60)/m-ZrO 2
2 catalyst
catalyst after
after catalytic
catalytic experiments.
experiments. (a)
(a)Ru3d
Ru3d3/2 and
3/2
Ru3d
and 5/2; (b) Zn2p
Ru3d 5/2 ; (b) Zn2p
1/2 and Zn2p
1/2
3/2; Zn2p
and (c) 3/2 ;
Zr3d3/2 and
(c) Zr3d and Zr3d
Zr3d5/2; (d) Zn LMM.
; (d) Zn LMM.
3/2 5/2

The hydrophilicity of Ru-Zn(x)/m-ZrO2 after catalytic experiments was further examined, and
the corresponding water droplets on each catalyst are illustrated in Figure 4. It is obvious that the
water contact angle declined along with raising the Zn content; that is, 139° over Ru/m-ZrO2 AH
versus 6° over Ru-Zn (1.02)/m-ZrO2 AH. This indicates that the hydrophilicity of the catalysts is
drastically improved by increasing the Zn content. The same assumption can be made from Figure 5,
Catalysts 2018, 8, 513 6 of 17

The hydrophilicity of Ru-Zn(x)/m-ZrO2 after catalytic experiments was further examined, and
the corresponding water droplets on each catalyst are illustrated in Figure 4. It is obvious that the
water contact angle declined along with raising the Zn content; that is, 139◦ over Ru/m-ZrO2 AH
versus 6◦2018,
Catalysts
Catalysts over
2018, 8,8,xxRu-Zn
FOR
FORPEER
PEER(1.02)/m-ZrO
REVIEW
REVIEW 2 AH. This indicates that the hydrophilicity of the catalysts 66 of 17 is
of 17
drastically improved by increasing the Zn content. The same assumption can be made from Figure 5,
inin
inwhich
which
which fewer
fewer
fewer catalysts suspended
catalystssuspended
catalysts suspendedin in
inthe
theorganic
the organicphase
organic phasewere
phase wereobserved
were observedby
observed by
byenhancing
enhancing
enhancing the
the Zn
theZnZncontent
content
content
of
of ofthe
thethe catalyst.
catalyst.
catalyst. TheThe
The improvement
improvementofofthe
improvement of the wettability
thewettability
wettabilityof of Ru-Zn(x)/m-ZrO
of Ru-Zn(x)/m-ZrO catalysts
Ru-Zn(x)/m-ZrO2 2catalysts
2
catalysts can
can be
can rationalized
bebe
rationalized
rationalized
asas
asfollows:
follows:
follows: (1)
(1)(1) When
WhenWhen ZnO
ZnO content
ZnOcontent was
contentwas increased,
wasincreased,
increased,moremore
more(Zn(OH)
(Zn(OH)222))3)3(ZnSO
(Zn(OH) (ZnSO44)(H)(H22O)
O)33would
wouldbe begenerated
generated
3 (ZnSO4 )(H2 O)3 would be generated
and
and chemisorbed
chemisorbed on
on the
the Ru
Ru surface,
surface, which
which directly
directly led
led to
tomore
more Ru
Ru δ+being
δ+
being formed
formed
δ+ during
during the
the reaction.
reaction.the
and chemisorbed on the Ru surface, which directly led to more Ru being formed during
Since
Since the
the lone
lone pair
pair of
of electrons
electrons of
ofoxygen
oxygen in
inwater
water molecules
molecules are
are easily
easily linked
linked to
tothe
the empty
empty ddorbitals
orbitals
reaction. Since the lone pair of electronsδ+ of oxygen in water molecules are easily linked to the empty
of
of Ru,
Ru, the
the higher
higher concentration
concentration of
of Ru
Ru δ+ resulted
resulted in
δ+in more
more water
water molecules
molecules being
being linked
linked to
to the
the Ru
Ru
d orbitals of Ru, the higher concentration of Ru resulted in more water molecules being linked
surface, and thus there was an improvement in the
surface, and thus there was an improvement in the hydrophilicity. (2) The chemisorbed hydrophilicity. (2) The chemisorbed
to the Ru surface, and thus there was an improvement in the hydrophilicity. (2) The chemisorbed
(Zn(OH)
(Zn(OH)22))33(ZnSO
(ZnSO44)(H )(H22O)O)33 mainly
mainly existed
existed as as hydrated
hydrated ions ions onon thethe RuRu surface,
surface, which
which caused
caused the the
(Zn(OH)
formation 2 ) 3 (ZnSO 4 )(H 2 O) 3 mainly existed as hydrated ions on the Ru surface, which caused the
formation of of thethe stagnant
stagnant waterwater layer.
layer. ThisThis also
also leads
leads to to the
the increase
increase in in hydrophilicity
hydrophilicity for for thethe
formation
catalysts. of the stagnant water layer. This also leads to the increase in hydrophilicity for the catalysts.
catalysts.

Figure
Figure 4.4.4.
Figure Water
Water contact
Watercontact angle
angleof
contactangle ofRu-Zn(x)/m-ZrO
of Ru-Zn(x)/m-ZrO22after
Ru-Zn(x)/m-ZrO catalytic
2 after
after experiments:
catalytic
catalytic (a)
(a)Ru/m-ZrO
experiments:
experiments: 22AH;
(a) Ru/m-ZrO
Ru/m-ZrO AH;(b) AH;
2(b)
(b)Ru-Zn(0.06)/m-ZrO
Ru-Zn(0.06)/m-ZrO
Ru-Zn(0.06)/m-ZrO 22 AH;
AH;
2
(c)
AH;(c) Ru-Zn(0.33)/m-ZrO
(c)
Ru-Zn(0.33)/m-ZrO 22 AH;
Ru-Zn(0.33)/m-ZrO AH;
2
(d)
AH;(d) Ru-Zn(0.60)/m-ZrO
(d) Ru-Zn(0.60)/m-ZrO
Ru-Zn(0.60)/m-ZrO 22 AH;
AH;
2
(e)
AH;
(e) Ru-Zn
(e) Ru-Zn
Ru-Zn
(0.86)/m-ZrO
(0.86)/m-ZrO 2 AH; (f)
(f)Ru-Zn
AH;(f) Ru-Zn(1.02)/m-ZrO
(1.02)/m-ZrO22AH.
(0.86)/m-ZrO 2 2AH; Ru-Zn (1.02)/m-ZrO 2AH.
AH.

Oil
Oil

Water
Water

Figure
Figure
Figure5.5.5.The
Thesuspend
The suspend situation
suspend situation
situation of Ru-Zn(x)/m-ZrO
of Ru-Zn(x)/m-ZrO catalysts
Ru-Zn(x)/m-ZrO22 2catalysts
catalysts inin
in the the
the oiloil
oil phase
phase
phase after
after
after catalytic
catalytic
catalytic
experiments:
experiments: (a)
(a) Ru/m-ZrO
Ru/m-ZrO AH; (b)
AH; Ru-Zn(0.06)/m-ZrO
(b) Ru-Zn(0.06)/m-ZrO AH; (c)
AH;Ru-Zn(0.33)/m-ZrO
(c) AH;
Ru-Zn(0.33)/m-ZrO
experiments: (a) Ru/m-ZrO2 2AH; (b) Ru-Zn(0.06)/m-ZrO2 AH;2 (c) Ru-Zn(0.33)/m-ZrO2 AH; (d)2Ru-Zn
2 2 2 (d) Ru-Zn
AH; (d)
(0.47)/m-ZrO
Ru-Zn (0.47)/m-ZrO
(0.47)/m-ZrO 2 AH; (e)
2 AH; (e) Ru-Zn(0.60)/m-ZrO
2 AH; AH;
(e) Ru-Zn(0.60)/m-ZrO
Ru-Zn(0.60)/m-ZrO 2 (f) Ru-Zn(0.69)/m-ZrO AH;
2 AH; (f) Ru-Zn(0.69)/m-ZrO
2 AH; (f) Ru-Zn(0.69)/m-ZrO 2 (g) Ru-Zn
2 AH; (g) Ru-Zn (0.86)/m-ZrO
AH; (g) Ru-Zn
2 (0.86)/m-ZrO 22

AH; (h)
(0.86)/m-ZrORu-Zn
AH; (h) Ru-Zn 2 AH;(1.02)/m-ZrO AH.
(h) Ru-Zn2 (1.02)/m-ZrO
(1.02)/m-ZrO 2 AH. 2 AH.

The
The
The catalytic
catalytic
catalytic activity
activity andand
activity and selectivity
selectivity towards
selectivity towards cyclohexene
cyclohexene
towards formation
cyclohexene formation over
over Ru-Zn/m-ZrO
over Ru-Zn/m-ZrO
formation Ru-Zn/m-ZrO2 catalysts
22

catalysts
with with
different
catalysts withZndifferent
contentZn
different Zniscontent isis illustrated
illustrated
content in Figurein
illustrated in6.Figure
It can6.6.be
Figure ItItclearly
can
canbe be clearly
seen
clearly seen
that the
seen that the
the catalytic
catalytic
that activity
catalytic
activity
towards towards
activitybenzene benzene
benzene conversion
towardsconversion conversion
over Ru-Zn/m-ZrOover
over Ru-Zn/m-ZrO
Ru-Zn/m-ZrO
2 catalysts 22 catalysts
catalysts
was was
was
inhibited inhibited
inhibited
by by
by
increasing increasing
increasing
the Zn the
the
content;
Zn content;
Znis,
that content; that
when that is, when
is, when
the molar the molar
the of
ratio molar
Zn toratio
ratio of Zn
Ruofgrew to Ru
Zn tofrom grew
Ru grew from
0 to from 0 to 1.02,
1.02, 0catalytic catalytic
to 1.02, catalytic activity
activity activity towards
towardstowardsbenzene
benzene
benzene conversion
conversion conversion
dropped from dropped
dropped99.8%from
fromto 99.8%
99.8% to
14.9% in 14.9%
to 14.9%
20 min in
inof20 min
min of
20reaction of reaction
reaction
time. On time.theOn
time. On the
the contrary,
contrary, contrary,
catalytic
catalytic
catalyticselectivity
selectivity selectivity
towards towards
towardscyclohexene
cyclohexene formationformation
cyclohexene increasedincreased
formation increased
along with alongZn with
along withZn
content.Zncontent.
Notably,Notably,
content. Notably,
when the when
when
molar
the
the molar
molar ratio
ratio of
of Zn
Zn to
to Ru
Ru exceeded
exceeded 0.6,
0.6, catalytic
catalytic activity
activity towards
towards benzene
benzene conversion
conversion was
was
ratio of Zn to Ru exceeded 0.6, catalytic activity towards benzene conversion was drastically suppressed
drastically
drastically suppressed (e.g.,
(e.g., 88.8% ofof benzene conversion
conversion over Ru-Zn
Ru-Zn (0.60)/m-ZrO
(0.60)/m-ZrO22 vs. vs. 21.7%
21.7% of
(e.g., 88.8% ofsuppressed
benzene conversion 88.8% over benzene
Ru-Zn (0.60)/m-ZrO over
2 vs. 21.7% of benzene conversion over
of
benzene conversion over Ru-Zn (1.02)/m-ZrO after 40 min of
benzene conversion over Ru-Zn (1.02)/m-ZrO2 after 40 min of catalytic experiments), while no
2 catalytic experiments), while no
significant
significantimprovement
improvementfor forthe
theselectivity
selectivitytowards
towardscyclohexene
cyclohexenesynthesis synthesiswas wasobserved
observed(e.g.,(e.g.,68.0%
68.0%
of cyclohexene selectivity over Ru-Zn (0.60)/m-ZrO
of cyclohexene selectivity over Ru-Zn (0.60)/m-ZrO2 vs. 88.8% of benzene conversion over Ru-Zn
2 vs. 88.8% of benzene conversion over Ru-Zn
(1.02)/m-ZrO
(1.02)/m-ZrO22 after
after 40
40 min
min ofof catalytic
catalytic experiments).
experiments). When When n(Zn)/n(Ru)
n(Zn)/n(Ru) isis 0.6,0.6, aa 60.9%
60.9% cyclohexene
cyclohexene
yield
yieldwas
wasachieved
achievedwithin
within35 35min,
min,which
whichisisthe thehighest
highestyield yieldof ofcyclohexene
cyclohexeneever everreported
reportedover overRu-Ru-
Catalysts 2018, 8, 513 7 of 17

Ru-Zn (1.02)/m-ZrO2 after 40 min of catalytic experiments), while no significant improvement for the
selectivity towards cyclohexene synthesis was observed (e.g., 68.0% of cyclohexene selectivity over
Ru-Zn (0.60)/m-ZrO2 vs. 88.8% of benzene conversion over Ru-Zn (1.02)/m-ZrO2 after 40 min of
catalytic experiments). When n(Zn)/n(Ru) is 0.6, a 60.9% cyclohexene yield was achieved within 35
min, which is the highest yield of cyclohexene ever reported over Ru-Zn/ZrO2 [1,9,25]. Combined
Catalysts
with the 2018, 8, x FOR PEER
characterization REVIEW
results, the effect of Zn content can be concluded as follows: If the surface 7 of 17 of

Ru is hydrophobic, the catalyst mainly stays in the oil phase, in which the adsorbed benzene tends
Zn/ZrO2 [1,9,25]. Combined with the characterization results, the effect of Zn content can be
to be completely hydrogenated into cyclohexane, since desorption of the formed cyclohexene hardly
concluded as follows: If the surface of Ru is hydrophobic, the catalyst mainly stays in the oil phase,
proceeds in thethe
in which oil adsorbed
phase. Therefore,
benzene tendsWhentoZn be content
completelyincreases, more (Zn(OH)
hydrogenated 2 )3 (ZnSO4since
into cyclohexane, )(H2 O)3
woulddesorption
be generated and chemisorbed on the Ru surface, improving the wettability
of the formed cyclohexene hardly proceeds in the oil phase. Therefore, When Zn content of the catalyst.
The result of this
increases, more is that the 2hydrophilic
(Zn(OH) )3(ZnSO4)(H2O) catalyst
3 wouldeasily movesand
be generated from the oil phase
chemisorbed on theintoRuthe aqueous
surface,
phase improving
and stays the there. It is wellofknown
wettability that the
the catalyst. Thesolubility
result of of cyclohexene
this in water is catalyst
is that the hydrophilic weaker easily
than that
moves[30],
of benzene from thus
the oilthephase into the aqueous
synthesized phase and
cyclohexene stays there.
in aqueous It is well
phase wouldknown be that the solubility
transferred into the
of cyclohexene in water is weaker than that of benzene [30], thus
oil phase spontaneously. The formed cyclohexene is difficult to be re-adsorbed on the hydrophilic the synthesized cyclohexene in
aqueous phase would be transferred into the oil phase spontaneously.
Ru surface, leading to the inhibition of its further hydrogenation, and thus improving the selectivity The formed cyclohexene is
difficult to be re-adsorbed on the hydrophilic Ru surface, leading to the inhibition of its further
towards cyclohexene formation. Besides, when the hydrogenation of benzene takes place in the
hydrogenation, and thus improving the selectivity towards cyclohexene formation. Besides, when
aqueous phase, the generated cyclohexene can be stabilized by the hydrogen bond formed between
the hydrogenation of benzene takes place in the aqueous phase, the generated cyclohexene can be
cyclohexene
stabilized and bywater molecules
the hydrogen [31].formed
bond As demonstrated in Figureand
between cyclohexene 7, twowatertypes of hydrogen
molecules [31]. As bond
between cyclohexene
demonstrated and water
in Figure 7, two molecules could be
types of hydrogen bond formed
between in cyclohexene
the aqueous andphase. However, as
water molecules
the hydrophilicity
could be formed further
in theincreases
aqueouswith phase. increasing
However,n as (Zn)/n (Ru), there isfurther
the hydrophilicity less retention
increases time
with that
Ru-Zn/ZrO2 spends in the oil phase. This leads to a fall in benzene adsorption of Ru-Zn/ZrO2 catalysts,
increasing n (Zn)/n (Ru), there is less retention time that Ru-Zn/ZrO 2 spends in the oil phase. This
leads to a fall in benzene adsorption of Ru-Zn/ZrO catalysts, and catalytic
and catalytic activity towards benzene conversion declines (Figure 6a). Therefore, there is an optimum
2 activity towards benzene
molarconversion
ratio of Zn declines
to Ru (Figure 6a). Therefore,
for Ru-Zn/ZrO there is an optimum molar ratio of Zn to Ru for Ru-
2 catalysts; that is, n (Zn)/n (Ru) = 0.6 (Figure 7, “catalyst
Zn/ZrO
B”). When Zn 2 catalysts; that is, n (Zn)/n (Ru) = 0.6 (Figure 7, “catalyst B”). When Zn content is very
content is very insufficient, as in “catalyst A” in Figure 7, the generated cyclohexene
insufficient, as in “catalyst A” in Figure 7, the generated cyclohexene is likely hydrogenated into
is likely hydrogenated into cyclohexane, leading to a relatively low selectivity towards cyclohexene.
cyclohexane, leading to a relatively low selectivity towards cyclohexene. In contrast, when Zn content
In contrast, when Zn content is excessive (Figure 7 “catalyst C”), adsorption of benzene becomes quite
is excessive (Figure 7 “catalyst C”), adsorption of benzene becomes quite challenging, which causes
challenging,
a drasticwhich
decrease causes a drastic
of catalytic decrease
activity towardsof catalytic activity towards benzene conversion.
benzene conversion.

Figure 6. Cont.
 Catalysts 2018, 8, x FOR PEER REVIEW 8 of 17
Catalysts 2018, 8, 513 8 of 17
Catalysts 2018, 8, x FOR PEER REVIEW 8 of 17

Figure
Figure 6.6.Catalytic
FigureCatalytic activity
6. Catalytic
activity towards
activity towards
towards selective
selective
selective hydrogenation
hydrogenation of
hydrogenation ofofbenzene
benzene
benzene overover
over Ru-Zn/m-ZrO
Ru-Zn/m-ZrO
Ru-Zn/m-ZrO 2 catalysts
2 catalysts
2 catalysts
with different
with Zn
different content
Zn (m
content (m = 1.2
= g,
1.2 g,mm == 50.0
50.0 g,
g, v
v == 280
280 cm
cm 3, 3
with different Zn content (mcat = 1.2 g, mZnSO4 = 50.0 g, vH2O = 280 cm , vbenzene = 140 cm , T
cat cat ZnSO4
ZnSO4 H2O
H2O v , v 3 =
benzene
benzene =
140140
cm 3cm
, T 3=, 423
T = 423
3
K, K,423
pH2
= pH2
= 5.0
K, = 5.0
pH2MPa). MPa).
= 5.0(a) (a) Benzene
Benzene
MPa). conversion
conversion
(a) Benzene as function
as function
conversion of reaction
of reaction
as function time; (b) Cyclohexene
time; (b)time;
of reaction Cyclohexene selectivity
selectivity
(b) Cyclohexene as function
as function
selectivity as
of benzene conversion;(c) (c) Cyclohexeneyield yieldasas function
function ofofreaction
of benzene
function conversion;
of benzene Cyclohexene
conversion; (c) Cyclohexene yield reaction
as function oftime.
time. time.
reaction

Figure 7. Two types of hydrogen bond formed between cyclohexene and water molecules, as well as
the hydrogenation scheme both in the oil phase and aqueous phase, over Ru-Zn/ZrO2 catalysts with
Figure 7.7.Two
Figure Twowettability.
different typesof
types ofhydrogen
hydrogenbond
bondformed
formedbetween
betweencyclohexene
cyclohexeneandandwater
watermolecules,
molecules,as aswell
wellas
as
thehydrogenation
the hydrogenationscheme
schemeboth
bothininthe
theoil
oilphase
phaseand
andaqueous
aqueousphase,
phase,over
overRu-Zn/ZrO catalystswith
Ru-Zn/ZrO22 catalysts with
2.2. Effectwettability.
different
different of ZrO2 as Support and Dispersant
wettability.
Figure 8 demonstrates the XRD patterns of the supported Ru-Zn (0.60)/m-ZrO2 catalyst and the
2.2.
2.2. Effect
Effect of
of ZrO
ZrO22 as
unsupported as Support
Support
Ru-Zn
and
and
(0.60)
Dispersant
Dispersant
catalyst before (a) and after (b) catalytic experiments. As with m-ZrO2
supported samples, metallic
Figure Ru and ZnO reflections were observed for(0.60)/m-ZrO
the unsupported Ru-Zn and
Figure 88 demonstrates
demonstrates the the XRD
XRD patterns
patterns ofthe
of thesupported
supported Ru-Zn
Ru-Zn 2 catalyst
(0.60)/m-ZrO2 catalyst and the
the catalyst,
unsupported which again
Ru-Zn proves
(0.60)that the valence
catalyst of
before Ru and Zn
(a) after
and (b)is 0 and positive
aftercatalytic
(b) catalytic 2, respectively.
experiments.Moreover,
As with2
unsupported Ru-Zn (0.60) catalyst before (a) and experiments.
the characteristic diffraction of (Zn(OH)2)3(ZnSO4)(H2O)3 was also pronounced over the unsupported
As with m-ZrO
m-ZrO
supported supported samples, metallic Ru and ZnO reflections were observed for the unsupported
sample,samples, metallic
that Ru and ZnO thereflections
support wereplays observed
no role for in thetheunsupported Ru-Zn
2
indicating ZrO 2 as formation of
Ru-Zn
catalyst,catalyst,
which whichproves
again again that
proves
the that the of
valence valence
Ru and ofZn
Ruisand
0 andZnpositive
is 0 and 2, positive 2, respectively.
respectively. Moreover,
(Zn(OH)2)3(ZnSO4)(H2O)3.
Moreover, the characteristic
the characteristic diffraction ofdiffraction
(Zn(OH)2)of (Zn(OH) )3 (ZnSO
3(ZnSO4)(H22O) 4 )(Hpronounced
3 was also 2 O)3 was also overpronounced over
the unsupported
the unsupported sample, indicating
sample, indicating that ZrO2 as the support that ZrO 2 as the support
plays no role in the formation of
plays no role in the formation of
(Zn(OH) ) (ZnSO )(H
(Zn(OH)22)33(ZnSO44)(H22O)33. O) .
 Catalysts 2018, 8, 513 9 of 17
Catalysts 2018, 8, x FOR PEER REVIEW 9 of 17

Figure
Figure8.8.XRD
XRDpatterns
patterns of
of the
the supported
supported Ru-Zn(0.60)/m-ZrO
Ru-Zn(0.60)/m-ZrO2 2catalyst
catalystand
andthe
theunsupported
unsupportedRu-Zn
Ru-Zn
(0.60)
(0.60)catalyst
catalystbefore
before(a)
(a)and
andafter
after(b)
(b)catalytic
catalyticexperiments.
experiments.

Thetexture
The textureproperties
propertiesand andcomposition
composition of of the
the supported
supported Ru-Zn
Ru-Zn (0.60)/m-ZrO
(0.60)/m-ZrO2 2catalyst
catalystand andthethe
unsupported Ru-Zn (0.60), as well as the pH values of the slurry, after catalytic
unsupported Ru-Zn (0.60), as well as the pH values of the slurry, after catalytic experiments are given experiments are given
ininTable
Table2.2.ItItisisfound
foundthat thatunsupported
unsupportedRu-ZnRu-Zn(0.60)
(0.60)catalyst
catalystdemonstrated
demonstratedsimilar
similartexture
textureproperties
properties
totothat
thatobtained
obtainedover overparent
parentm-ZrO
m-ZrO22after
afterthe
thecatalytic
catalyticexperiment.
experiment.ThisThiscan
canbe beattributed
attributedtotothe thefact
fact
that the added
that the added m-ZrO was
22 was 10 times the amount of catalyst used and was mechanically
times the amount of catalyst used and was mechanically mixed with mixed with the
catalyst
the during
catalyst during thethe
reaction. Moreover,
reaction. Moreover, an an
increase
increasein the molar
in the ratio
molar of Zn
ratio of Znto Ru was
to Ru was also shown
also shown for
the unsupported Ru-Zn catalyst after catalytic experiments,
for the unsupported Ru-Zn catalyst after catalytic experiments, indicating indicating that (Zn(OH) )
2 3 (ZnSO 4 )(H O)
that
2 3
was formed
(Zn(OH) as well.
2)3(ZnSO 4)(HInterestingly,
2O)3 was formed it was
asnoticed that n (Zn)/nit(Ru)
well. Interestingly, wasover Ru-Zn
noticed (0.60)/ZrO
that n (Zn)/n 2(Ru) is slightly
over
higher (0.60)/ZrO
Ru-Zn than that obtained over higher
2 is slightly the unsupported
than thatsample afterover
obtained the reaction; that is, 0.68
the unsupported versus after
sample 0.65. This
the
implies that m-ZrO as the support benefits the adsorption
reaction; that is, 0.682versus 0.65. This implies that m-ZrO2 as the support of (Zn(OH) ) (ZnSO )(H O) on
2 3benefits4 the 2adsorption
3 the Ru
of
surface.2)Furthermore,
(Zn(OH) 3(ZnSO4)(H2O) the
3 onpH
thevalue of the Furthermore,
Ru surface. slurry with application
the pH value of Ru-Zn (0.60)/ZrO
of the slurry 2 is slightly
with application
ofhigher
Ru-Zn than that observed
(0.60)/ZrO with higher
2 is slightly unsupported Ru-Zn
than that (0.60),with
observed suggesting that more
unsupported Zn2+(0.60),
Ru-Zn was chemisorbed
suggesting
on the catalyst surface as well.
that more Zn was chemisorbed on the catalyst surface as well.
2+

Textureproperties
Table 22.Texture
Table propertiesandand composition
composition of
of the
the supported
supported Ru-Zn(0.60)/ZrO
Ru-Zn(0.60)/ZrO2 2catalyst
catalystand
andthe
the
unsupportedRu-Zn
unsupported Ru-Zn(0.60),
(0.60),as
aswell
wellas
asthe
thepH
pHvalues
valuesofofthe
theslurry,
slurry, after
after hydrogenation.
hydrogenation.
2 ·g2−1−1
Catalyst
Catalyst SBET /(m
SBET /(m ·g ) )1 1 VVpore /(cm3 ·3g·g−−11)) 11
pore/(cm ddpore
pore /(nm) 11 nnZn
/(nm) /nRu
Zn/n Ru2
2
nnZrZr/n/nRuRu2 2 pH
pH33
Ru-Zn(0.60)/ZrO
Ru-Zn(0.60)/ZrO 2 2 3030 0.09
0.09 9.7
9.7 0.60
0.60 5.38
5.38 --
Ru-Zn(0.60)/ZrO2 AH
Ru-Zn(0.60)/ZrO 2 AH
3030 0.08
0.08 10.1
10.1 0.68
0.68 5.33
5.33 5.38
5.38
Ru-Zn(0.60) 65 0.19 11.7 0.61 0 -
Ru-Zn(0.60)AH
Ru-Zn(0.60)+ZrO 3365 0.19
0.12 11.7
14.6 0.61
0.65 0
5.35 -
5.29
2
Ru-Zn(0.60)+ZrO
ZrO2 2 AH 3433 0.12
0.13 14.6
16.0 0.65 - 5.35 - 5.29-
ZrO2 by N2 -sorption;
1 Determined 34
2 Determined 0.13
3 16.0 -
by XRF; Determined by a pH meter at room temperature; AH:- -
1 after
hydrogenation.
Determined by N2-sorption; 2 Determined by XRF; 3 Determined by a pH meter at room temperature;
AH: after hydrogenation.
Figure 9 shows the TEM images and Ru particle size distribution of the unsupported Ru-Zn (0.60)
catalyst before and after the reaction. As can be observed from Figure 9a,b, fresh Ru-Zn (0.60) displays
Figure 9 shows the TEM images and Ru particle size distribution of the unsupported Ru-Zn
a circular or elliptical shape, of which the particle size is around 4.5 nm. In addition, analogous to that
(0.60) catalyst before and after the reaction. As can be observed from Figure 9a,b, fresh Ru-Zn (0.60)
observed over the supported sample, no obvious change in the particle size of Ru-Zn (0.6) after the
displays a circular or elliptical shape, of which the particle size is around 4.5 nm. In addition,
catalytic experiment is demonstrated (Figure 9c). However, unlike Ru-Zn (0.60)/m-ZrO2 , it is noticed
analogous to that observed over the supported sample, no obvious change in the particle size of Ru-
that Ru-Zn (0.6) is clearly not uniformly dispersed by adding m-ZrO2 as the dispersant (Figure 9d).
Zn (0.6) after the catalytic experiment is demonstrated (Figure 9c). However, unlike Ru-Zn (0.60)/m-
This suggests that in comparison with using m-ZrO as the dispersant, m-ZrO could contribute better
ZrO2, it is noticed that Ru-Zn (0.6) is clearly not 2uniformly dispersed by 2adding m-ZrO2 as the
to the dispersion of Ru when it is utilized as the support.
dispersant (Figure 9d). This suggests that in comparison with using m-ZrO2 as the dispersant, m-
ZrO2 could contribute better to the dispersion of Ru when it is utilized as the support.
Catalysts 2018, 8, 513 10 of 17
Catalysts 2018, 8, x FOR PEER REVIEW 10 of 17

Figure 9.
Figure 9. TEM
TEM images
images and
and Ru
Ru particle
particle size
size distributions
distributions of
of the
the unsupported
unsupported Ru-Zn
Ru-Zn (0.60)
(0.60) catalyst
catalyst with
with
m-ZrO22 as
m-ZrO as the
the dispersant
dispersant before
before (a,b)
(a,b) and
and after
after (c,d)
(c,d) catalytic
catalytic experiments.
experiments.

profilesofofthethe
XPS profiles unsupported
unsupported Ru-ZnRu-Zn (0.60)(0.60)
catalyst catalyst with m-ZrO
with m-ZrO as the dispersant
2 as the2 dispersant after
after catalytic
catalytic experiments
experiments are demonstrated
are demonstrated in Figurein10. Figure
As can 10.be Asobserved
can be observed
from Figurefrom10a, Figure
two10a,
peakstworelated
peaks
related
to the BE to of
theRu3d
BE of 5/2Ru3d were observed
were5/2observed at 280.3 ateV280.3
andeV andeV,
281.4 281.4 eV, which
which to Ru0to
are attributed
are attributed andRuRu δ+ ,
0 and

Ru , respectively.
respectively.
δ+ This isThis is consistent
consistent with that with that obtained
obtained over theover the corresponding
corresponding supported supported sample
sample (Ru-Zn
(Ru-Zn (0.60)/m-ZrO
(0.60)/m-ZrO 2 ). ).
Moreover,
2 Moreover,
from from
Figure Figure
10b, 10b,
the BEthe ofBE
Zn2pof Zn2p
3/2 over
3/2 over unsupported
unsupported Ru-ZnRu-Zn (0.60)
(0.60) is
is observed
observed to be
to be 1021.5
1021.5 eV, eV,
which which is slightly
is slightly higher higher
than than that demonstrated
that demonstrated over Ru-Zn
over Ru-Zn (0.60)/m-2
(0.60)/m-ZrO
ZrO21021.1
(i.e., (i.e., 1021.1 eV). Additionally,
eV). Additionally, the KE of theZnKE LMM of ZnoverLMM over unsupported
unsupported Ru-Zn (0.60) Ru-Zn (0.60) lower
is slightly is slightly
than
lower
that than that
observed over observed over Ru-Zn (0.60)/m-ZrO
Ru-Zn (0.60)/m-ZrO 2 ; that is, 989.02 ; that
eV is,
versus989.0 eV
989.2 versus
eV. This 989.2
can beeV. This can
rationalized be
in
rationalized
terms that morein terms
(Zn(OH)that2more
)3 (ZnSO (Zn(OH)
4 )(H2 O) 2)33(ZnSO 4)(H2O)3 was on
was chemisorbed chemisorbed on Ru-Zn (0.60)/m-ZrO
Ru-Zn (0.60)/m-ZrO 2 than that on2
than that on unsupported
unsupported Ru-Zn (0.60),Ru-Zn (0.60), thus
thus leading to moreleading to more
electrons electrons
being being transferred
transferred from Ru to Zn. fromThis
Ru to is
Zn.a good
in This agreement
is in a good withagreement
the XRF results.with the XRF results.
Additionally, the BE Additionally,
of Zr3d5/2 over the unsupported
BE of Zr3d5/2Ru-Zn over
unsupported
(0.60) with m-ZrO Ru-Zn 2 as(0.60) with m-ZrO
the dispersant 2 as
was the dispersant
normalized to bewas normalized
182.0 eV, which to be 182.0
is close eV, reported
to that which is
close
in to that reported
literature (i.e., 182.2ineV) literature (i.e.,suggests
[29]. This 182.2 eV)that [29].there
Thisissuggests that there
no obvious is no effect
electronic obvious electronic
between the
effect between
active component the (Ru)
active andcomponent
m-ZrO2 as (Ru)
theand m-ZrO2 It
dispersant. asisthe dispersant.
therefore deemedIt is that
therefore deemedwere
no electrons that
no electronsfrom
transferred wereRu transferred from Ru tomixing
to Zr by mechanical Zr by mechanical
of Ru-Zn (0.60) mixingwithof m-ZrO
Ru-Zn 2(0.60)
. with m-ZrO2.
 Catalysts 2018, 8, 513 11 of 17
Catalysts
Catalysts2018,
2018,8,8,xxFOR
FORPEER
PEERREVIEW
REVIEW 1111ofof1717

Figure
Figure10.
10.XPS
XPSprofiles
profilesofofthe
theunsupported
unsupportedRu-Zn
Ru-Zn(0.60)(0.60)catalyst
catalystwith
withm-ZrO
m-ZrO2 22asasthethedispersant
dispersantafterafter
catalytic experiments.
catalytic experiments. (a)
experiments. (a) Ru3d
(a)Ru3d 3/2 and
3/2 and
Ru3d3/2 Ru3d5/2
and Ru3d5/2; (b)
5/2; ;(b)Zn2p
(b)Zn2p1/2 and
Zn2p1/21/2andZn2p
andZn2p3/2
Zn2p ; (c)
; (c)
3/23/2 Zr3d
Zr3d
; (c) 3/2
Zr3d and Zr3d
andand
3/2 3/2 Zr3d5/2
Zr3d ; ;(d)
5/2 (d)Zn
; Zn
(d)
5/2
LMM.
LMM.
Zn LMM.

The
The HH2-TPR
-TPRprofiles
2-TPR profiles ofof Ru-Zn
Ru-Zn (0.60)/m-ZrO
Ru-Zn (0.60)/m-ZrO2 22and
(0.60)/m-ZrO andunsupported
and unsupportedRu-Zn
unsupported Ru-Zn(0.60)
Ru-Zn (0.60)after
(0.60) after catalytic
catalytic
experiments
experiments
experiments are
aregiven
given in
in Figure
Figure 11.
11.
Figure 11. The
The reduction
reduction peak
peak of
of Ru
Ru over
over Ru-Zn
Ru-Zn (0.60)/m-ZrO
peak of Ru over Ru-Zn (0.60)/m-ZrO22can
(0.60)/m-ZrO 2 canbe be
observed
observedatataround
at around360
around 360K,
360 K,which
K, higher
whichisishigher
higherthanthan that
thanthat shown
thatshown
shown over
over unsupported
unsupported
over unsupported Ru-Zn
Ru-Zn
Ru-Zn (0.60)
(0.60) (i.e.,
(i.e.,
(0.60) 344
(i.e.,344
K).
344
K). This
This indicates
indicates thatthat
the the interaction
interaction between between
Ru andRu and
the the
support
K). This indicates that the interaction between Ru and the support (m-ZrO support
(m-ZrO (m-ZrO
2 ) is ) is
stronger
2 stronger
than than
that that
between
2) is stronger than that
between Ru
Ruand
Ru and the
between andthethedispersant.
dispersant. This is inThis
dispersant. isisininagreement
agreement
This with thewith
agreement withthe
XPS theXPS
XPSresults.
results. results.

360K
360K
Ru-Zn(0.60)/ZrO AH
signal/a.u.

2 AH
Ru-Zn(0.60)/ZrO
TCD signal/a.u.

344K
344K
TCD

Ru-Zn(0.60)+ZrO
Ru-Zn(0.60)+ZrO AH
2 AH
2

300
300320
320340
340360
360380
380400
400420
420440
440460
460480
480
Temperature/K
Temperature/K
Figure11.
Figure
Figure H2-temperature-programmed
11.HH
11. -temperature-programmed reduction
reductionprofiles
22-temperature-programmedreduction
profiles of
profilesof Ru-Zn(0.60)/m-ZrO
ofRu-Zn(0.60)/m-ZrO andunsupported
Ru-Zn(0.60)/m-ZrO2 22and
and unsupported
unsupported
Ru-Zn
Ru-Zn (0.60)
(0.60) after
after catalytic
catalytic experiments.
experiments.
Ru-Zn (0.60) after catalytic experiments.
Catalysts 2018, 8, 513 12 of 17
Catalysts 2018, 8, x FOR PEER REVIEW 12 of 17

The wettability
The wettability ofof unsupported
unsupported Ru-Zn(0.60)
Ru-Zn(0.60) after
after catalytic
catalytic experiments
experiments waswas also
also examined,
examined, and and
the water droplet on the catalyst is illustrated in Figure 12. As presented, the contacted
the water droplet on the catalyst is illustrated in Figure 12. As presented, the contacted angle over angle over
unsupported Ru-Zn(0.60) ◦ 40◦ ).
unsupported Ru-Zn(0.60) is is 52
52°,, which
which isis higher
higher than
than that
that observed
observed overover Ru-Zn(0.60)/ZrO (i.e., 40°).
Ru-Zn(0.60)/ZrO22 (i.e.,
This demonstrates
This demonstrates thatthat hydrophilicity
hydrophilicity of of Ru-Zn
Ru-Zn withwith m-ZrO
m-ZrO22 as as the
the dispersant
dispersant is
is weaker
weaker than
than that
that
with m-ZrO as the support. This is mainly due to the fact that no electron was transferred
with m-ZrO22 as the support. This is mainly due to the fact that no electron was transferred from Ru from Ru to
Zr,Zr,
causing lessless δ+
Ru Ruto bebegenerated. This leads to the factfact
thatthat
lessless
water molecules are are
linked to the
to causing δ+ to generated. This leads to the water molecules linked to
Ru surface, thus decreasing the hydrophilicity of the catalyst
the Ru surface, thus decreasing the hydrophilicity of the catalyst surface.surface.

Figure
Figure 12.
12. Water
Water contact
contact angle
angle of
of unsupported
unsupported Ru-Zn (0.60) after
Ru-Zn (0.60) after catalytic
catalytic experiments.
experiments.

Catalytic activity and selectivity

selectivity towards
towards cyclohexene
cyclohexene formationformation over over unsupported
unsupported Ru-ZnRu-Zn (0.60),
(0.60),
as well as Ru-Zn
Ru-Zn (0.60)/m-ZrO
(0.60)/m-ZrO22, ,are arepresented
presented in in Figure
Figure 13. 13. AsAs can can bebe seen,
seen, both
both catalytic activity
towards benzene conversion,
conversion, and selectivity
selectivity towards
towards cyclohexene,
cyclohexene, over over unsupported
unsupported Ru-Zn Ru-Zn (0.60),
(0.60),
are lower
lower than
thanthatthatachieved
achievedover overRu-Zn
Ru-Zn(0.60)/m-ZrO
(0.60)/m-ZrO 2; that is, only
2 ; that 69.5%
is, only 69.5%of benzene conversion
of benzene and
conversion
71.5%
and 71.5%of cyclohexene
of cyclohexene selectivity
selectivitywerewere obtained
obtained over
overunsupported
unsupportedRu-Zn Ru-Zn(0.60),
(0.60), while
while Ru-Zn
(0.60)/m-ZrO22gave
(0.60)/m-ZrO gave84.6%
84.6%of ofbenzene
benzeneconversion
conversion and and 71.5%
71.5% of of selectivity
selectivity to to cyclohexene
cyclohexene after 35 min
of reaction
reaction time.
time.This
Thiscan canbebeattributed
attributed toto
twotwomainmain reasons:
reasons: (1) (1)
Ru Ru can can be highly
be highly dispersed
dispersed on m-2
on m-ZrO
ZrO2 when
when m-ZrO m-ZrO
2 is is utilized
utilized
2 as as
the the support,
support, while while
the the dispersion
dispersion of of
Ru Ru
is is
lessless uniform
uniform whenwhen m-ZrO
m-ZrO 2 is2
is applied
applied as as
thethe dispersant.
dispersant. ThisThis results
results in more
in more active active
Ru Rusitessites
being being available
available during
during the reaction,
the reaction, and
and further
further improves
improves the catalytic
the catalytic activityactivity
towards towards
benzene benzene
conversion. conversion.
(2) m-ZrO (2)2 m-ZrO 2 supported
supported Ru-
Ru-Zn (0.60)
Zn (0.60) benefits the adsorption of (Zn(OH) ) (ZnSO )(H O) , enhancing
benefits the adsorption of (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 , enhancing the hydrophilicity of the catalyst
2 3 4 2 3 the hydrophilicity of the
catalyst
surface. surface.
Therefore, Therefore,
more Ru-Zn more(0.60)/m-ZrO
Ru-Zn (0.60)/m-ZrO2 would2stay would in the stay in the aqueous
aqueous phase than phase
thatthan that
happens
happens for the unsupported
for the unsupported sample, whichsample, which
helps thehelps
desorptionthe desorption of the generated
of the generated cyclohexenecyclohexene and
and inhibits
inhibits its hydrogenation.
its further further hydrogenation. Hence, the Hence, the highest
highest cyclohexene cyclohexene
yield ofyield 60.9% ofwas
60.9% was achieved
achieved over
over Ru-Zn
Ru-Zn (0.60)/m-ZrO
(0.60)/m-ZrO 2 , while 2 , while
only 50% only
of 50%
the of
maximum the maximum
cyclohexene cyclohexene
yield was yield
obtained was
over obtained over
unsupported
unsupported
Ru-Zn (0.60). Ru-Zn (0.60).
The reusability of Ru-Zn (0.60)/m-ZrO2 was investigated under the same reaction conditions
without further regeneration (Figure 14). It can be observed that benzene conversion as well as
cyclohexene selectivity is maintained above 80% and 70%, respectively, after 17 iterations of the catalytic
experiments. Thus, the catalytic system over Ru-Zn (0.60)/m-ZrO2 shows a good reusability, indicating
that this catalyst possesses great potential for industrial application in selective hydrogenation of
benzene towards cyclohexene production.

Figure 13. Catalytic activity towards selective hydrogenation of benzene over unsupported Ru-Zn
(0.60) as well as Ru-Zn(0.60)/m-ZrO2 (m(Ru-Zn (0.60)/m-ZrO2) = 1.2 g, or m(Ru-Zn (0.60)) = 0.2 g and
m(ZrO2) = 1.0 g; mZnSO4 = 50.0 g; vH2O = 280 cm3; vbenzene = 140 cm3; T = 423 K; pH2 = 5.0 MPa). (a) Benzene
conversion as function of reaction time; (b) Cyclohexene selectivity as function of benzene conversion;
(c) Cyclohexene yield as function of reaction time.
 Zn (0.60) benefits the adsorption of (Zn(OH)2)3(ZnSO4)(H2O)3, enhancing the hydrophilicity of the
catalyst surface. Therefore, more Ru-Zn (0.60)/m-ZrO2 would stay in the aqueous phase than that
happens for the unsupported sample, which helps the desorption of the generated cyclohexene and
inhibits its further hydrogenation. Hence, the highest cyclohexene yield of 60.9% was achieved over
Ru-Zn2018,
Catalysts (0.60)/m-ZrO
8, 513 2, while only 50% of the maximum cyclohexene yield was obtained over
13 of 17
unsupported Ru-Zn (0.60).

Catalysts 2018, 8, x FOR PEER REVIEW 13 of 17

The reusability of Ru-Zn (0.60)/m-ZrO2 was investigated under the same reaction conditions
without
Figure further
Figure13.
regeneration
13.Catalytic
Catalyticactivity
(Figure
activitytowards
14). It can
towardsselective
be observed that
selectivehydrogenation
hydrogenationofofbenzene
benzene
benzeneover
conversion Ru-Zn
overunsupported
unsupportedRu-Zn
as well as
cyclohexene
(0.60)
(0.60)asaswellselectivity is maintained
wellasasRu-Zn(0.60)/m-ZrO above 80% and 70%, respectively, after 17 iterationsandof the
Ru-Zn(0.60)/m-ZrO 2 2(m(Ru-Zn
(m(Ru-Zn(0.60)/m-ZrO
(0.60)/m-ZrO22))== 1.21.2g,
g,ororm(Ru-Zn
m(Ru-Zn(0.60))
(0.60))==0.2
0.2ggand
catalytic
m(ZrO
m(ZrO experiments.
22))== 1.0
Thus,
1.0 g; mZnSO4
ZnSO4 = =
theg;catalytic
50.0
50.0 g; vH2O
vH2O = system
= 280280
cm3cm 3over =Ru-Zn
; vbenzene
; vbenzene 140 =cm 3(0.60)/m-ZrO
140 ; Tcm 3 ; TK;
= 423 = 423 shows
pH22=K; pH2
5.0 =a5.0
MPa).good reusability,
MPa).
(a) (a)
Benzene
indicating
Benzene that this
conversion catalyst
as function possesses
of reaction great
time; potential
(b) for
Cyclohexene industrial
selectivity as application
function
conversion as function of reaction time; (b) Cyclohexene selectivity as function of benzene conversion; of in selective
benzene
hydrogenation
conversion;
(c) Cyclohexene ofCyclohexene
(c) benzene towards
yield as
yield as function ofcyclohexene
function
reaction of production.
reaction
time. time.

Figure14.14.Reusability
Figure ReusabilityofofRu-Zn
Ru-Zn(0.60)/m-ZrO
(0.60)/m-ZrO2 2for
forselective
selectivehydrogenation
hydrogenationofofbenzene,
benzene,including
including
benzeneconversion,
benzene conversion, cyclohexene
cyclohexene selectivity,
selectivity, andand yield
yield (vH2O
(vH2O = 280
= 280 3, v
cmcm 3, vbenzene = 140 cm
= 140 cm 3 ,3,TT==423
423K,K,
benzene
pH2==5.0
pH2 5.0MPa,
MPa,ttreaction
reaction ==40
40min).
min).

3.3.Materials
Materialsand
andMethods
Methods
3.1. Chemicals
3.1. Chemicals
All chemicals were directly used without any further purification. RuCl3 ·3H2 O was delivered
All chemicals were directly used without any further purification. RuCl3·3H2O was delivered
from Sino-Platinum Co., Ltd. (Kunming, China). ZnSO4 ·7H2 O was purchased from the Fuchen
from Sino-Platinum Co., Ltd. (Kunming, China). ZnSO4·7H2O was purchased from the Fuchen
Chemical Reagent Factory (Tianjin, China). NaOH and benzene were commercially obtained from
Chemical Reagent Factory (Tianjin, China). NaOH and benzene were commercially obtained from
the Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). m-ZrO2 was synthesized according to the
the Kemiou Chemical Reagent Co., Ltd. (Tianjin, China). m-ZrO2 was synthesized according to the
literature [32]. Distilled water was applied in all experiments.
literature [32]. Distilled water was applied in all experiments.
3.2. Preparation of Catalysts
3.2. Preparation of Catalysts
Ru-Zn/m-ZrO2 catalysts were synthesized as follows. First, 0.45 g of RuCl3 ·3H2 O and 0.28 g
Ru-Zn/m-ZrO2 catalysts were synthesized as follows. First, 0.45 g of RuCl3·3H2O and 0.28 g of
of ZnSO4 ·7H2 O were dissolved in 1 cm3 of deionized water. With continuous stirring, the aqueous
ZnSO4·7H2O were dissolved in 1 cm3 of deionized water. With continuous stirring, the aqueous
solution was added dropwise onto 1.0 g of m-ZrO2 powder. Subsequently, the solid was dried at 323 K
solution was added dropwise onto 1.0 g of m-ZrO2 powder. Subsequently, the solid was dried at 323
for 3 h, and then calcined at 423 K in a Muffle furnace for another 3 h. After that, the calcined sample,
K for 3 h, and then calcined at 423 K in a Muffle furnace for another 3 h. After that, the calcined
together with 200 cm3 of 5 wt. % NaOH aqueous solution, was transferred into a 1000 cm3 Hastelloy
sample, together with 200 cm3 of 5 wt. % NaOH aqueous solution, was transferred into a 1000 cm3
autoclave at 423 K under 5.0 MPa of hydrogen and a stirring speed of 800 min−1 for 1 h. Ru(OH)3
Hastelloy autoclave at 423 K under 5.0 MPa of hydrogen and a stirring speed of 800 min−1 for 1 h.
was generated when the NaOH was mixed with the calcined sample in the autoclave during the
Ru(OH)3 was generated when the NaOH was mixed with the calcined sample in the autoclave during
the temperature rising procedure. Then the black powder was cooled down to room temperature,
washed with distilled water until neutral, and vacuum-dried. Subsequently, 1.2 g of fresh Ru-Zn/m-
ZrO2 catalyst was obtained with a theoretical molar ratio of Zn to Ru of 0.06, and was denoted as Ru-
Zn (0.60)/m-ZrO2. Additionally, Ru-Zn (x)/m-ZrO2 catalysts were synthesized via the same procedure
by modifying the usage of ZnSO4·7H2O, where x refers to the theoretical molar ratio of Zn to Ru. The
Catalysts 2018, 8, 513 14 of 17

temperature rising procedure. Then the black powder was cooled down to room temperature, washed
with distilled water until neutral, and vacuum-dried. Subsequently, 1.2 g of fresh Ru-Zn/m-ZrO2
catalyst was obtained with a theoretical molar ratio of Zn to Ru of 0.06, and was denoted as Ru-Zn
(0.60)/m-ZrO2 . Additionally, Ru-Zn (x)/m-ZrO2 catalysts were synthesized via the same procedure
by modifying the usage of ZnSO4 ·7H2 O, where x refers to the theoretical molar ratio of Zn to Ru.
The preparation
Catalysts procedure
2018, 8, x FOR of Ru-Zn (x)/m-ZrO2 catalysts is illustrated in Scheme 1.
PEER REVIEW 14 of 17

OH-
Zn2+
RuCl3 Ru3+
ZnSO4 calcination OH-
ZrO2 ZrO2 ZrO2
impregnation

ZnO Zn(OH)2
Ru Ru(OH)3
H2
ZrO2 ZrO2

Scheme 1.
Scheme 1. Preparation
Preparation procedure
procedure of
of Ru-Zn
Ru-Zn (x)/m-ZrO
(x)/m-ZrO22 catalysts.
catalysts.

Unsupported Ru-Zn
Ru-Zn catalyst
catalyst was prepared using a reported co-precipitation
co-precipitation method
method [8]. First,
First,
3
0.45 gg of
ofRuCl 3 ·3H
RuCl3·3H 2O2O precursor
precursor andand 0.28
0.28 g ofgZnSO
of ZnSO
4·7H 42·O
7Hwere
2 O were dissolved
dissolved in 100incm
100ofcm
3 of distilled
distilled water.
water. Then 3 of NaOH (5 wt.%) aqueous solution was added at 353 K with continuous stirring
Then 100 cm100
3 of cm
NaOH (5 wt.%) aqueous solution was added at 353 K with continuous stirring for 2
forfollowed
h, 2 h, followed by moving
by moving all solids
all solids andand solutions
solutions intointo a 1000
a 1000 mLHastelloy
mL Hastelloyautoclave
autoclavefor
foraa reduction
reduction
procedure
procedure at 423 K with 5.0 MPa of hydrogen and a stirring speed of 800 rpm for 3 h. h. After
After reduction,
reduction,
the sample was cooled, and the fresh catalyst was gained by washing with deionized water to neutral
and vacuum-drying. The prepared Ru-Zn catalysts were denoted as Ru-Zn (0.60), where 0.60 stands
for the theoretical molar ratio of Zn to Ru.

3.3. Catalytic
3.3. Catalytic Experimental
Experimental Procedure
Procedure
All hydrogenation
All hydrogenation reactions
reactions took
took place
place inin aa 1000
1000 mLmL GS-1
GS-1 type
type Hastelloy
Hastelloy autoclave.
autoclave. In In aa typical
typical
hydrogenation reaction, 1.2 g Ru-Zn (x)/m-ZrO catalyst, 50.0 g ZnSO · 7H O (0.62 mol L −1 ), as well as
hydrogenation reaction, 1.2 g Ru-Zn (x)/m-ZrO2 catalyst, 50.0 g ZnSO4·7H2O (0.62 mol L ), as well as
2 4 2 −1

280 cm 3 distilled water, were added in the reactor. Then the autoclave was charged using N to 4 MPa 4
280 cm3 distilled water, were added in the reactor. Then the autoclave was charged using N 2 2 to 4 MPa
4 times, which was followed by purification with H2 to 4 MPa for another 4 times. Then, reactor
times, which was followed by purification with H 2 to 4 MPa for another 4 times. Then, the was
the reactor
heated to 423 K under 5.0 MPa of H with a stirring speed of 800 min −1 , followed by pouring 140 cm3
was heated to 423 K under 5.0 MPa2of H2 with a stirring speed of 800 min , followed by pouring 140 −1

of benzene
cm and and
3 of benzene modifying the stirring
modifying speed
the stirring to 1400
speed rpm rpm
to 1400 (to get
(torid
getofrid
theofmass transfer
the mass limitation)
transfer [33].
limitation)
After that, that,
[33]. After each each
liquidliquid
sample was taken
sample from from
was taken the reactor everyevery
the reactor 5 min.5 Allmin. withdrawn samples
All withdrawn were
samples
analyzed using GC-FID from the Hangzhou Kexiao Chemical Instrument
were analyzed using GC-FID from the Hangzhou Kexiao Chemical Instrument and Equipment Co., and Equipment Co., Ltd.
(Hangzhou,
Ltd. (Hangzhou, China). As with
China). As the
withevaluation of the of
the evaluation unsupported
the unsupportedRu-Zn Ru-Zn
catalyst, 0.2 g of0.2
catalyst, catalyst sample
g of catalyst
as well as 1 g of m-ZrO were individually added instead of 1.2 g of Ru-Zn
sample as well as 1 g of m-ZrO2 were individually added instead of 1.2 g of Ru-Zn (x)/m-ZrO2 catalyst,
2 (x)/m-ZrO 2 catalyst,
and the
and the rest
rest of
of the
the procedure
procedure waswas the the same.
same. TheThe catalytic
catalytic activity
activity towards
towards benzene
benzene conversion
conversion and and
selectivity towards
selectivity towards cyclohexene
cyclohexene were were calculated
calculated using
using thethe calibration
calibration area area normalization
normalization method,method,
and the 2 of all compounds was higher than 0.99. After each reaction, the
and the correlation
correlation coefficient
coefficient (R(R2)) of all compounds was higher than 0.99. After each reaction, the
organic phase
organic phasewaswasremoved
removed viavia
a separating
a separatingfunnel, and other
funnel, and parts
otherwereparts recharged into the autoclave
were recharged into the
to investigate the reusability of the catalysts through identical experimental
autoclave to investigate the reusability of the catalysts through identical experimental procedures. procedures. Statistical
validation was evaluated
Statistical validation by conducting
was evaluated catalytic experiments
by conducting over Ru-Zn
catalytic experiments over(0.60)/m-ZrO 2 in three2
Ru-Zn (0.60)/m-ZrO
separate
in runs under
three separate runsthe samethe
under reaction
same conditions, and the standard
reaction conditions, deviationdeviation
and the standard for cyclohexene yield was
for cyclohexene
yield was calculated at 1.3% after 35 min of reaction time. For the used catalysts that needed to be
characterized, samples were filtered and washed until the filtrate became neutral and no Zn2+ could
be detected. Then solid samples were dried in Ar flow at 373 K and stored in ethanol, ready for further
characterization.
Catalysts 2018, 8, 513 15 of 17

calculated at 1.3% after 35 min of reaction time. For the used catalysts that needed to be characterized,
samples were filtered and washed until the filtrate became neutral and no Zn2+ could be detected. Then
solid samples were dried in Ar flow at 373 K and stored in ethanol, ready for further characterization.

3.4. Catalysts Characterization

X-ray diffraction (XRD) patterns for the fresh and spent catalysts were measured using an X’Pert
Pro instrument from Philips (Almelo, The Netherland) at room temperature. The diffracted intensity
of Cu-Kα radiation (λ = 0.154 nm) was recorded in the range of 2θ from 5◦ to 90◦ , with a step size of
0.03◦ . In addition, textural properties were analyzed using the Nova 1000 e-Physisorption Analyzer
(Quantachrome Instruments, Boynton Beach, FL, USA). Before measurements, all the samples were
evacuated at 523 K under vacuum pressure for 2 h, then the isotherms were taken at 77 K. The specific
surface area (SBET ) was determined using the Brunauer-Emmett-Teller (BET) model. Furthermore,
elemental analysis was conducted via X Ray Fluorescence (XRF) using a S4 Pioneer instrument
(Bruker AXS, Karlsruhe, Germany). Additionally, X-ray photoelectron spectroscopy (XPS) using a PHI
Quantera SXM instrument from Ulvac-Phi (Kangawa, Japan) was utilized for analyzing the valence
state of Ru and Zn on the catalyst surface. Al Kα (Eb = 1486.6 eV) was selected as the source of
radiation and the vacuum degree was set to 6.7 × 10-8 Pa. The C1s (Eb = 284.8 eV) line as the binding
energy reference was used for calibrating and correcting the energy scale. Furthermore, JEOL JEM
2100 transmission electron microscopy (TEM) combined with an energy dispersive spectrometer (EDS)
(Akishima, Tokyo, Japan) was applied to investigate the dispersion of the catalysts, as well as the
particle size. To investigate the hydrophilicity of the catalyst surface, a contact angle meter (JC2000 C1,
Powereach, Shanghai, China) was used to measure water contact angle values (CAs) at ambient
temperature for each sample. Moreover, temperature programmed reduction (TPR) was conducted
with an Autosorb-IQ from Quantachrome (Boynton Beach, FL, USA). Typically, prior to reduction,
a 10 mg sample was oxidized in flowing synthetic air (flow rate: 30 cm3 min−1 ) while being heated
to 423 K and held for 1 h. After cooling in an Argon stream (flow rate: 30.0 cm3 min−1 ) to 293 K, the
sample was treated for another 2 h. Then an Ar stream containing 10 Vol % H2 was introduced instead
(30 cm3 min-1 ), while being heated to 573 K (10 K min-1 ) and held for 1 h. The hydrogen consumption
was recorded and determined using a standard CuO calibration.

4. Conclusions
m-ZrO2 supported Ru-Zn catalysts, as well as unsupported Ru-Zn catalyst with m-ZrO2
as the dispersant, were evaluated for selective hydrogenation of benzene towards cyclohexene
formation. Zn mainly exists as ZnO in Ru-Zn catalysts. Moreover, by increasing the Zn content
in the Ru-Zn/m-ZrO2 catalyst, more (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 was generated and chemisorbed
on the surface of the catalysts. This decreases the catalytic activity towards benzene (99.8% over
Ru/m-ZrO2 vs. 14.9% over Ru-Zn (1.02)/m-ZrO2 after 20 min of reaction) and improves the selectivity
to cyclohexene formation (68.0% over Ru-Zn (0.06)/m-ZrO2 vs. 88.8% over Ru-Zn (1.02)/m-ZrO2
after 40 min of reaction). When the molar ratio of Zn to Ru reached 0.6, the highest cyclohexene
yield of 60.9% was achieved. In addition, when m-ZrO2 was applied as the dispersant instead of
being utilized as the support, both catalytic activity towards benzene conversion and selectivity to
cyclohexene were suppressed; that is, only 69.5% of benzene conversion and 71.5% of cyclohexene
selectivity were obtained over unsupported Ru-Zn (0.60), while Ru-Zn (0.60)/m-ZrO2 gave 84.6% of
benzene conversion and 71.5% of selectivity to cyclohexene after 35 min of reaction time. This can
be rationalized in terms that m-ZrO2 as the support has a strong interaction with the Ru-Zn catalyst,
benefiting the dispersion of Ru and the chemisorption of (Zn(OH)2 )3 (ZnSO4 )(H2 O)3 . Notably, the
reusability of Ru-Zn (0.60)/m-ZrO2 was evaluated for selective hydrogenation of benzene, and no
obvious decrease in catalytic activity towards cyclohexene formation was observed after 17 reaction
iterations, over which at least a 59.3% cyclohexene yield can be achieved.
Catalysts 2018, 8, 513 16 of 17

Author Contributions: H.S., Z.C., and Z.P. conceived and designed the experiments; H.L. performed the
experiments; Z.L. and S.L. analyzed the data; L.C. contributed reagents, materials, and analysis tools; H.S.
wrote the paper.
Funding: This research received no external funding.
Acknowledgments: This work was supported by the National Nature Science Foundation of China (21273205),
the Key Scientific Research Project of Henan Province (18A180018), the Environmental Catalysis Innovative
Research Team of Zhengzhou Normal University (702010), and the Student Innovation Program of Zhengzhou
Normal University (DCZ2017014).
Conflicts of Interest: The authors declare no conflict of interest.

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