Discrimination of the roles of CdSO4 and ZnSO4 in liquid phase hydrogenation of benzene to cyclohexene

Article abstract of DOI:10.1016/j.jcat.2009.09.007

CdSO₄ and ZnSO₄ as co-modifiers of RuLa/SBA-15 lead to improved catalysts for the partial hydrogenation of benzene to cyclohexene. Based on the experimental results and theoretical calculations, it is shown that CdSO₄ acts as surface modification, suppressing more the adsorption of cyclohexene than that of benzene, while the function of ZnSO₄ is mainly the stabilization of cyclohexene in the liquid phase, accelerating the desorption as well as hindering the re-adsorption of cyclohexene.

Full text of DOI:10.1016/j.jcat.2009.09.007

Journal of Catalysis 268 (2009) 100–105
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Discrimination of the roles of CdSO and ZnSO in liquid phase hydrogenation
4
4
of benzene to cyclohexene
a
a
a
a
a,*
b
b
Jian-Liang Liu , Yuan Zhu , Jun Liu , Yan Pei , Zhen Hua Li , Hui Li , He-Xing Li ,
a,
a
*
Ming-Hua Qiao , Kang-Nian Fan
a
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, PR China
Department of Chemistry, Shanghai Normal University, Shanghai 200234, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
CdSO and ZnSO as co-modifiers of RuLa/SBA-15 lead to improved catalysts for the partial hydrogenation
4
4
Received 12 June 2009
Revised 1 September 2009
Accepted 6 September 2009
Available online 2 October 2009
of benzene to cyclohexene. Based on the experimental results and theoretical calculations, it is shown
that CdSO acts as surface modification, suppressing more the adsorption of cyclohexene than that of
benzene, while the function of ZnSO is mainly the stabilization of cyclohexene in the liquid phase, accel-
erating the desorption as well as hindering the re-adsorption of cyclohexene.
Ó 2009 Elsevier Inc. All rights reserved.
4
4
Keywords:
Benzene
Cyclohexene
Hydrogenation
CdSO
ZnSO
4
4
1
. Introduction
gen-bonded cyclohexene would desorb at a higher rate than cyclo-
hexene bonded directly to the catalyst. However, since the bonding
between organic modifiers and cyclohexene is very weak (enthalpy
of hydrogen bonding is about ꢀ1 kcal mol ), the selectivity to
cyclohexene never exceeded 40% when using these organic
modifiers.
There has been a growing interest in using heterogeneous cata-
ꢀ1
lysts for the partial hydrogenation of benzene to cyclohexene, a
greener intermediate feedstock than cyclohexane for producing
nylons and fine chemicals [1–3]. However, it is thermodynamically
difficult to obtain cyclohexene in high selectivity, since the stan-
dard free energy change for cyclohexene formation from benzene
Inorganic modifiers such as Zn, Fe, Co, Ni, Cd, Ga, and In salts are
more effective than organic modifiers [6]. Among them, ZnSO
been regarded as the best modifier, and Asahi Chemical has indus-
trialized a process based on Ru as the catalyst and ZnSO as the
modifier for partial hydrogenation of benzene to cyclohexene [3].
However, there is no consensus on the role of ZnSO in improving
the selectivity. For example, Fukuoka and coworkers [3,12] attrib-
uted the promoting effect of ZnSO to the stabilization of the
4
has
ꢀ1
hydrogenation is ꢀ5.5 kcal mol , while that for cyclohexane
ꢀ1
formation is ꢀ23.4 kcal mol . Therefore, it has been a long time
that only cyclohexane was obtained during the hydrogenation of
benzene [4].
4
4
In order to improve the selectivity to cyclohexene, the addition
of a modifier to the reaction system (the so-called reaction or pro-
cess modifier [5]) is indispensable [6,7]. Generally, there are two
kinds of reaction modifiers: organic and inorganic. Effective organ-
ic modifiers should contain a polar group such as hydroxyl or
amine group [8–11]. IR studies revealed the adsorbate–adsorbate
4
hydrogenated intermediate by forming adduct with cyclohexene,
which weakens the adsorption of cyclohexene on Ru and increases
the rate of desorption. On the other hand, Struijk et al. [6,13] as-
4
cribed the promoting effect of ZnSO to its chemisorption on the
interaction between the organic modifier (e.g.
col, or aliphatic alcohols) and cyclohexene through hydrogen
bonding [8,9]. As a consequence, overlapping of -electrons of
e
-caprolactam, gly-
surface of the Ru catalyst that results in a hydrophilic surface. Thus,
the Ru particles are surrounded by a stagnant water layer that im-
pedes the adsorption of H and cyclohexene onto the Ru surface
2
p
the double bond in cyclohexene with d-orbitals of Ru, the most
selective catalyst for this reaction, is diminished, and the hydro-
and consequently, slows down the further hydrogenation of cyclo-
hexene. An important reason for such controversy is the poor
understanding of the phenomenon due to its unforeseen complex-
ity and to the unavailability of in situ characterization results [5].
Moreover, all published documents have focused on using one
inorganic modifier in the partial hydrogenation of benzene to
*
Corresponding authors. Fax: +86 21 65641740.
E-mail addresses: lizhenhua@fudan.edu.cn (Z.H. Li), mhqiao@fudan.edu.cn
(
M.-H. Qiao).
0
021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2009.09.007

J.-L. Liu et al. / Journal of Catalysis 268 (2009) 100–105
101
cyclohexene. In the present work, we employed for the first time a
combination of two inorganic modifiers, CdSO and ZnSO , for the
target reaction, and found that two modifiers performed much bet-
ter than either of them alone. In order to elucidate the respective
Table 1
Results of the hydrogenation of benzene over the RuLa/SBA-15 catalyst with different
concentrations of CdSO .
4
4
4
a
CdSO
4
Conversionb
(%)
Selectivityb
(%)
Yieldb
(%)
Timeb
(min)
ꢀ
3
(
10 M)
role of CdSO
4 4
and ZnSO in the reaction, we fell to theoretical cal-
2
+
culation to obtain the information about the interactions of Cd
0
1.17
1
1
2
76
82
84
76
55
9
32
33
35
27
7
26
28
27
15
3
5
6
11
45
2
+
and Zn ions with benzene and cyclohexene. Based on the exper-
imental data and theoretical results, the modification effects of
.56
.95
.34
4 4
CdSO and ZnSO on selectivity enhancement were discriminated.
a
2
Reaction conditions: 1.0 g of catalyst, 50 ml of benzene, 100 ml of H O, tem-
2
. Experimental methods
perature of 413 K, H2 pressure of 4.0 MPa, stirring rate of 1000 rpm.
b
Values recorded at the maximum yield of cyclohexene.
2.1. Catalyst preparation
sion of benzene reached 76% within 3 min. However, the maxi-
mum yield of cyclohexene was only 7% with the corresponding
selectivity to cyclohexene of 9%. With the increase of the concen-
The mesoporous siliceous SBA-15 was synthesized according to
the procedure proposed by Zhao et al. [14]. The catalyst was pre-
pared by a ‘‘two solvents” method [15,16]. Namely, 30 ml of cyclo-
hexane was added to 1.0 g of SBA-15 under stirring at 298 K. After
ꢀ3
tration of CdSO
4
to 1.56 ꢂ 10 M, cyclohexene reached a maxi-
mum yield of 28%. For comparison, by using Ru black as the
catalyst, Struijk et al. found that the yield of cyclohexene maxi-
all SBA-15 was well dispersed, 3.0 ml of 0.04 g Ru per ml of RuCl
solution containing 0.40 g La(NO O was added drop by drop
ꢁ6H
with stirring. The supernatant was decanted, and the solid residue
was dried at 373 K overnight, before it was reduced by 5 vol.% H
3
3
)
3
2
ꢀ3
mized at 1.48 ꢂ 10 M of CdSO
4
[6], which is in good agreement
with our observation, although their yield was only 16.9% and de-
creased more drastically than our case on both the lower and high-
2
/
ꢀ1
Ar gas at 573 K for 4 h at a ramping rate of 2 K min . This as-pre-
pared catalyst was designated RuLa/SBA-15.
4 4
er concentration sides of CdSO . At higher concentrations of CdSO ,
the reaction slowed down further, and the maximum yield of
ꢀ3
cyclohexene declined continuously (Table 1). At 2.34 ꢂ 10 M of
CdSO , the reaction rate was less than 1/15 of that without CdSO
2
.2. Activity testing
4
4
,
and the yield of cyclohexene was decreased to 15%.
Liquid phase hydrogenation of benzene was carried out in a
5
00 ml stainless steel autoclave with a mechanical stirrer. The
autoclave was charged with 100 ml of distilled water containing
a desired amount of the modifier(s), 1.0 g of catalyst, and 50 ml
3.2. The synergistic effect of CdSO and ZnSO
4
4
ꢀ3
of benzene, then sealed and purged with H
air. The reaction was performed at a reaction temperature of 413 K,
pressure of 4.0 MPa, and stirring rate of 1000 rpm to exclude the
2
for 4 times to exclude
Since the addition of 1.56 ꢂ 10 M of CdSO afforded the opti-
4
mal modification effect, and ZnSO has been testified as the most
4
H
2
effective modifier for this reaction [3,6], we additionally modified
diffusion effect. The reaction process was monitored by gas chro-
matographic analysis using a PEG-20 M packed column and a ther-
mal conductivity detector (TCD).
the reaction system containing CdSO with ZnSO aiming to further
4
4
improve the selectivity to cyclohexene. As shown by the data given
ꢀ
3
in Table 2, in the presence of 1.56 ꢂ 10 M of CdSO , an increment
4
4
of the concentration of ZnSO from zero to 0.42 M resulted in an in-
2
.3. Theoretical calculation
crease of the selectivity to cyclohexene from 33% to 69% measured
at the maximum yield of cyclohexene. Accordingly, the yield of
cyclohexene was increased to 57%, which is among the best results
The B3PW91 density functional method [17–19] in conjunction
with the 6-31+G(d) basis set was used to optimize the complexes
formed between benzene or cyclohexene and the metal ions of
CdSO and ZnSO to simulate the interaction between the reactant
4 4
4
reported so far. Further increasing of the concentration of ZnSO to
0.56 M led to lower selectivity and yield of cyclohexene. Fig. 1 pre-
sents the courses of the hydrogenation of benzene in the presence
ꢀ
3
or intermediate with the metal ions in the liquid phase during the
hydrogenation of benzene to cyclohexene. We have considered the
interaction of both bare and hydrated metal ions with benzene and
cyclohexene. Two possible binding configurations have been con-
sidered: binding of one bare or hydrated metal ion to one and
two benzene or cyclohexene molecules. The calculations were all
performed with the Gaussian 03 software package [20]. Default
convergence criteria were used for the geometry optimization.
Vibrational frequency analyses were performed for the optimized
geometries.
of 1.56 ꢂ 10 M of CdSO
4
alone and a combination of 1.56 ꢂ
and 0.42 M of ZnSO for comparison.
In addition, we optimized the concentration of ZnSO
modifier under the same reaction condition over the same catalyst.
The optimal concentration of ZnSO is also 0.42 M. As indicated in
entry 7 of Table 2, this concentration of ZnSO afforded a maxi-
ꢀ
3
10 M of CdSO
4
4
4
as the sole
4
4
mum yield of cyclohexene of 39%, and the corresponding selectiv-
ity to cyclohexene is 51%. It should be mentioned that the optimal
concentration of ZnSO
of CdSO . In addition, ZnSO
effect on the hydrogenation activity. In the presence of 0.42 M of
ZnSO , the conversion of benzene is 76% within 10 min. A compa-
4
is two orders of magnitude higher than that
4
4
has a much less remarkable negative
4
3
. Results
.1. The modification effect of CdSO
Although the effect of the concentration of ZnSO
ꢀ3
rable activity was observed using only 1.95 ꢂ 10 M of CdSO
4
as
the modifier (entry 4, Table 1).
3
4
2
+
2+
4
on the perfor-
3.3. Interactions of Cd and Zn ions with benzene and cyclohexene
mance of Ru catalysts has been extensively investigated in liquid
phase hydrogenation of benzene to cyclohexene [6,15,16,21], only
To gain a better insight into the roles of Cd2+ and Zn2+ ions in
liquid phase hydrogenation of benzene to cyclohexene, we fell to
one work on CdSO
ification effect to that of ZnSO
reaction proceeded very fast in the absence of CdSO
4
was reported, probably due to its inferior mod-
[6]. As summarized in Table 1, the
. The conver-
2+
4
theoretical calculation to investigate the interactions of Cd and
2
+
4
Zn ions with benzene and cyclohexene molecules. The optimized

1
02
J.-L. Liu et al. / Journal of Catalysis 268 (2009) 100–105
Table 2
Results of the hydrogenation of benzene over the RuLa/SBA-15 catalyst at different concentrations of ZnSO
.a
4
d
d
ZnSO
0
4
(M)
Conversionb (%)
Selectivityb (%)
Yieldb (%)
Timeb (min)
xCd2þ (%)
xZn2þ (%)
84
77
81
82
83
72
76
33
59
64
69
64
41
51
28
45
52
57
53
30
39
5
28
32
34
39
9
–
–
0.14
0.28
0.42
0.56
0
.14c
0
.42c
13
11
12
11
–
0.23
0.36
0.54
0.58
–
10
–
–
a
Reaction conditions: 1.0 g of catalyst, 50 ml of benzene, 100 ml of H
2
O, temperature of 413 K, H
2
pressure of 4.0 MPa, stirring rate of 1000 rpm, and CCdSO4 of
ꢀ
3
1
.56 ꢂ 10 M.
b
Values recorded at the maximum yield of cyclohexene.
Without CdSO .
4
x = (the amount of cation in the catalyst)/(the amount of cation added in the aqueous phase) ꢂ 100%. The catalysts after reaction were washed with water for 6 times
c
d
before being subjected to the elemental analysis.
of each benzene molecule. When binding to cyclohexene, both cat-
(
a) 100
ions form normal cation-
molecule.
p bonding with the cyclohexene
The formation energies of the complexes, i.e. the relative poten-
tial energies between the complexes and their corresponding com-
ponents (cation plus benzene or cyclohexene) are presented in
Table 3. Both benzene and cyclohexene can form stable complexes
8
6
4
2
0
0
0
0
cyclohexane
2
+
2+
with Cd and Zn ions, and one important conclusion that can be
2
+
drawn from the data given in Table 3 is that the Zn ion binds
2+
more tightly with benzene and cyclohexene than the Cd ion.
2
+
The formation energies of the complexes of the Zn ion formed
with one and two benzene or cyclohexene molecules are about
cyclohexene
benzene
ꢀ1
2+
3
0–50 kcal mol more negative than those of the Cd ion.
We have also explicitly taken the solvent effect of water into
consideration and optimized the geometries of the complexes
formed by one hydrated metal cation and one and two benzene
or cyclohexene molecules. Fig. 3 illustrates the optimized geome-
tries, and Table 4 presents the formation energies of the complexes
formed by the hydrated metal cations. The formation energies are
defined as the energy changes to form the complexes from the
0
0
2
4
6
8
10
12
Reaction Time (min)
(
b) 100
2
þ
2+
MðH
2
OÞ (M = Zn, Cd) ions, since the first hydration shell of Cd
6
2
+
and Zn contains six water molecules [22,23]. The results of the
complexes formed between one hydrated metal cation and two
benzene or cyclohexene molecules are not shown here, because
their formation energies are positive. It is found that when includ-
ing the effect of water, the binding energies of the complexes be-
tween the hydrated metal cations and one benzene or
cyclohexene molecule are much less negative than those without
water. However, the conclusion is still valid that the hydrated
8
6
4
2
0
0
0
0
cyclohexene
cyclohexane
2
+
Zn ion binds more tightly with benzene and cyclohexene than
2+
benzene
the hydrated Cd ion.
0
4. Discussion
0
10
20
30
40
50
Reaction Time (min)
The theoretical calculation results have two important implica-
4 4
tions for the understanding of the roles of CdSO and ZnSO in par-
Fig. 1. The reaction course of benzene hydrogenation over the RuLa/SBA-15
catalyst. Reaction conditions: 1.0 g of catalyst, 100 ml of H O, 50 ml of benzene, T
of 413 K, P of 4.0 MPa, stirring rate of 1000 rpm, (a) 1.56 ꢂ 10 M CdSO
2+
tial hydrogenation of benzene to cyclohexene. (1) The Cd ion
2
2+
ꢀ
3
binds more weakly with benzene than the Zn ion. Therefore, a
4
, (b)
ꢀ
3
more drastic decrement in the hydrogenation activity when CdSO
was used as the modifier cannot be attributed to a stronger inter-
1
.56 ꢂ 10 M CdSO
4 4
and 0.42 M ZnSO .
4
2
+
action of the Cd ion with benzene that can impede the adsorption
2
+
geometries of the complexes between one bare metal cation and
one and two organic molecules are illustrated in Fig. 2. In the com-
plexes formed by the bare metal cation and one benzene molecule,
both Cd2+ and Zn ions sit above the ring plane and both the com-
of benzene on the catalyst. (2) The Zn ion forms a more stable
2
+
complex with cyclohexene than the Cd ion. Such a stabilization
effect can accelerate the desorption of the intermediate product
cyclohexene from the catalyst surface, or retard the re-adsorption
of the desorbed cyclohexene, thus improving the selectivity to
cyclohexene [3]. As a result, it is unlikely that the selectivity
2+
plexes have a C6v symmetry. On the other hand, Cd2 and Zn ions
+
2+
have different bonding configurations when binding to two ben-
2
+
ꢀ3
zene molecules. The Cd ion bonds to just one carbon atom of each
benzene molecule, while the Zn2 ion bonds to two carbon atoms
enhancement at 1.56 ꢂ 10 M of CdSO (entry 3, Table 1) should
4
+
be mainly attributed to such a stabilization effect, because a com-

J.-L. Liu et al. / Journal of Catalysis 268 (2009) 100–105
103
Fig. 2. Optimized geometries of the complexes formed by bare Cd2+ and Zn2+ ions. Left: binding of metal cation to one benzene or cyclohexene molecule; Right: binding of
metal cation to two benzene or cyclohexene molecules.
more negative than that of the Cd2
+
ion (ꢀ56.8 vs.
Table 3
Formation energies (kcal mol 1) of the complexes formed between Cd2+ and Zn2+ ions
ꢀ
ꢀ1
2+
ꢀ48.0 kcal mol ). This indicates that the Zn ion, when adsorbed
on the catalyst, will be more effective in improving the hydrophi-
licity of the catalyst. If such an effect dominates the selectivity
and benzene or cyclohexene, where
corresponding complex is more stable.
a more negative value indicates that the
a
Ligand
Cd2+
Zn2+
enhancement in the present case, a concentration of ZnSO
4
lower
than that of CdSO would be expected to achieve a similar selectiv-
ity to cyclohexene, which is in sharp contrast to the experimental
results reported in Tables 1 and 2.
4
Benzene
ꢀ125.9
ꢀ205.0
ꢀ140.6
ꢀ212.6
ꢀ165.9
ꢀ250.8
ꢀ174.3
ꢀ256.1
(
Benzene)
Cyclohexene
Cyclohexene)
2
(
2
These findings prompt us to conclude that CdSO
ence the selectivity via directly modifying the active sites of the
catalyst, whereas ZnSO plays a more effective role in stabilizing
4
tends to influ-
a
Zero-point vibrational energy corrected.
4
the intermediate product cyclohexene in the liquid phase, while
affecting less markedly the activity of the catalyst even at much
higher concentrations. The synergistic effect of these two modifiers
results in a higher selectivity to cyclohexene than using either of
them alone. Moreover, elemental analysis data show that more
parable selectivity enhancement was found at as high as 0.14 M of
ZnSO (entry 6, Table 2) that is more efficient in stabilizing cyclo-
hexene as predicted by theoretical calculation.
4
On the other hand, it has been proposed that the water layer
surrounding the catalyst can impede the re-adsorption of cyclo-
hexene, so the influence of the metal cation on the hydrophilicity
of the catalyst also affects the selectivity to cyclohexene [6,13].
2
+
than 10% of the initial Cd ions remain on the used catalyst after
being separated and washed with water for 6 times (Table 2).
When the concentration of ZnSO
0.56 M, the amount of Cd ions left on the catalyst maintained
4
was increased from 0.14 to
Although the effect of the adsorbed Zn2 and Cd ions on the
hydrophilicity of the catalyst is not known exactly due to complex-
ity of the practical catalyst, our theoretical results suggest that the
binding energy between the Zn2 ion and six water molecules is
+
2+
2+
2
+
nearly unaltered, implying that Zn ions compete less favorably
2
+
than Cd ions with the active sites. According to the ‘‘Maxted
rule”, electrons of cations that have a fully or partially occupied
+

1
04
J.-L. Liu et al. / Journal of Catalysis 268 (2009) 100–105
Fig. 3. Optimized geometries of the complexes formed by hydrated Cd2+ and Zn2+ ions. Left: binding of the hydrated metal cation to one benzene molecule; Right: binding of
the hydrated metal cation to one cyclohexene molecule.
Table 4
face in an aqueous solution, and found that the Cd2+ ion deposition
is thermodynamically driven by the formation of Cd–Ni bond.
When Ni is covered by a monolayer of Cd, the Ni surface is not
accessible by the reactant anymore. The same is true for the pres-
ent case. Table 1 reveals the progressive deactivation of the RuLa/
Formation energies (kcal molꢀ1) of the complexes formed by CdðH
2þ
2
OÞ
6
and
2
6
þ
ZnðH
2
OÞ ions and benzene or cyclohexene, where a more negative value indicates
a
that the corresponding complex is more stable.
Ligand
2þ
2þ
CdðH
2
OÞ
ZnðH OÞ
2
6
6
Benzene
Cyclohexene
ꢀ13.8
ꢀ12.7
ꢀ15.7
ꢀ13.7
4
SBA-15 catalyst with the increment of the concentration of CdSO .
4
At even higher concentration of CdSO , the complete deactivation
a
Zero-point vibrational energy corrected.
of the catalyst is expected.
However, the decrement in the hydrogenation activity after the
2
+
adsorption of Cd ions cannot be simply attributed to the physical
2
+
4
d shell are more weakly bound to the nucleus of the cation than
blocking of the active sites by the catalytically inactive Cd ions. It
is known that the adsorption of benzene on Ru surface requires
more contiguous active sites than that of cyclohexene [26,27].
Thus, the site blocking effect would influence more remarkably
the adsorption and hydrogenation of benzene than that of cyclo-
hexene, which should lower the selectivity to cyclohexene. This
2
+
electrons of cations that have a 3d shell [24]. Therefore, Cd ions
with a fully occupied 4d shell have a higher electron-donating
power in the direction of the metal surface than Zn2 ions with a
fully occupied 3d shell, resulting in a stronger cation bonding [6].
Godard et al. [25] studied the adsorption of Cd2 ions on a Ni sur-
+
+

J.-L. Liu et al. / Journal of Catalysis 268 (2009) 100–105
105
is not consistent with the results in Table 1, which shows that the
decreased hydrogenation activity after the modification by Cd
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5
. Conclusions
The synergistic effect of two metal cations on selective hydroge-
nation has been seldom investigated and interpreted. We found
that CdSO and ZnSO as co-modifiers are highly efficient in the
partial hydrogenation of benzene to cyclohexene. CdSO plays a
4
4
4
more important role in surface modification, suppressing more
the adsorption of cyclohexene than that of benzene, while the
function of ZnSO is mainly to stabilize cyclohexene in the liquid
4
phase, accelerating the desorption as well as hindering the re-
adsorption of cyclohexene. This finding may open a new avenue
for selectivity enhancement for other hydrogenation reactions.
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This work was supported by the National Basic Research Pro-
gram of China (2006CB202502), the Fok Ying Tong Education Foun-
dation (104022), the NCET, the NSF of China (20673025), the
Science & Technology Commission of Shanghai Municipality
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