Effect of organic additives on the performance of nano-sized Ru-Zn catalyst

Article abstract of DOI:10.1002/cjoc.201190092

A novel Ru-Zn catalyst was prepared by coprecipitation. The catalyst was characterized by XRF, XRD and TEM. The effects of organic additives on the performance of the Ru-Zn catalyst for benzene selective hydrogenation to cyclohexene were investigated. The results showed that the catalyst was composed of Ru and Zn in molar ratio of 33.8:1, and the most probable value of the Ru crystallite size in the catalyst was 5.1 nm. The modification of Ru with Zn and the small size effect were the main cause why the catalyst exhibited the high activity and the excellent cyclohexene selectivity. When PEG (polyethylene glycol) was used as an additive, the activity of the catalyst decreased, and the cyclohexene selectivity increased with the increase of the PEG molecular weight. With the addition of PEG-20000, a cyclohexene selectivity of 78.9% at a benzene conversion of 68.7% and a maximum cyclohexene yield of 61.4% were obtained. With diethanolamine and triethanolamine as additives, cyclohexene yields were as high as 58.9% and 58.2%, respectively. Copyright

Full text of DOI:10.1002/cjoc.201190092

NOTE
Effect of Organic Additives on the Performance of Nano-sized
Ru-Zn Catalyst
Sun, Haijie(孙海杰)
Chen, Zhihao(陈志浩)
Liu, Zhongyi*(刘仲毅)
Guo, Wei(郭伟)
Zhou, Xiaoli(周小莉)
Liu, Shouchang(刘寿长)
Department of Chemistry, Zhengzhou University, Zhengzhou, Henan 450001, China
A novel Ru-Zn catalyst was prepared by coprecipitation. The catalyst was characterized by XRF, XRD and
TEM. The effects of organic additives on the performance of the Ru-Zn catalyst for benzene selective hydrogena-
tion to cyclohexene were investigated. The results showed that the catalyst was composed of Ru and Zn in molar ra-
tio of 33.8∶ 1, and the most probable value of the Ru crystallite size in the catalyst was 5.1 nm. The modification of
Ru with Zn and the small size effect were the main cause why the catalyst exhibited the high activity and the excel-
lent cyclohexene selectivity. When PEG (polyethylene glycol) was used as an additive, the activity of the catalyst
decreased, and the cyclohexene selectivity increased with the increase of the PEG molecular weight. With the addi-
tion of PEG-20000, a cyclohexene selectivity of 78.9% at a benzene conversion of 68.7% and a maximum cyclo-
hexene yield of 61.4% were obtained. With diethanolamine and triethanolamine as additives, cyclohexene yields
were as high as 58.9% and 58.2%, respectively.
Keywords benzene, hydrogenation, ruthenium, zinc, cyclohexene
Introduction
inorganic additive. Employing ZnSO4 as an additive and
an unsupported Ru-Zn catalyst, Asahi Chemical Indus-
try Co. has industrialized the process for producing
cyclohexene from the selective hydrogenation of ben-
zene.10 However, the cyclohexene yield is just about
32% at the selectivity of about 80% in the industrial
units.21 A great deal of research has been focused on
organic additives. Alcohols, amines, β-alanine, glysine
and some other soluble polymers have ever been used as
additive for this reaction.22-25 Fan et al.25 used ethyl-
enediamine as an additive and achieved a cyclohexene
of 34.8% over a Ru-Co-B/γ-Al2O3, which was an en-
couraging development in this field. However, cyclo-
hexene selectivity was only about 48.0%.
Based on the forgoing research, a novel Ru-Zn cata-
lyst was prepared and characterized, the effects of or-
ganic additives on the performance of the catalyst for
benzene selective hydrogenation to cyclohexene were
investigated and the roles of organic additives were
discussed.
The production of nylon-6 and nylon-66 from ben-
zene and cyclohexene has drawn much attention be-
cause this is an environmentally benign process with
atomic economy.1-5 Improving the cyclohexene yield
and selectivity has been the focus of research. In recent
years, great progress has been made in this field. Ning et
al.6 prepared Ru-SiO2 catalysts by microemulsion proc-
essing and obtained a maximum cyclohexene yield of
42%. Liu et al.7 prepared a Ce-promoted Ru/SBA-15
catalyst by a “two solvents” impregnation method and
achieved a cyclohexene yield of 53.8%. Then they per-
formed the reaction with a RuLa/SBA-15 catalyst, and
found that cyclohexene yield could reach 57% by using
ZnSO4 and CdSO4 as co-modifiers.8 Liu et al.9 reported
that a Ru-Fe-B/ZrO2 catalyst was prepared by the
chemical reduction and gave a cyclohexene yield of
57.3%. These are the best level reported. However, the
selectivity (only about 70%) corresponding to the max-
ium yield is low, resulting in difficulties in product
separation. Hence, scientific studies are still needed.
Modification of Ru catalyst with promoters is an
important method for improving the cyclohexene selec-
tivity. It was reported that the promoters such as K, Fe,
Co, Ce, Ba, La and Zn can remarkably enhance the
cyclohexene selectivity, and Zn has been regarded as
the best promoter.7,10-20 Another important method is
addition of some organic and inorganic additives into
reaction slurry. ZnSO4 has been considered as the best
Experimental
Catalyst preparation
The Ru-Zn catalysts were prepared according to the
procedure in the literatures.26,27 A desired amount of
RuCl3•H2O and ZnSO4•7H2O were dissolved in 200 mL
of H2O with agitation. To the stirred solution, 200 mL
of 20% NaOH solution was added instantaneously and
*
E-mail: liuzhongyi@zzu.edu.cn; Tel.: 0086-0371-67783384
Received August 20, 2010; revised October 13, 2010; accepted October 22, 2010.
Chin. J. Chem. 2011, 29, 369— 373
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
369

Sun et al.
NOTE
the resulting mixture was agitated for an additional 4 h
at 80 ℃. The mixture was left to stand and the black
precipitate was washed three times with an aqueous so-
lution of 5% NaOH after the supernatant had been re-
moved by decantation. This black precipitate was dis-
persed in 400 mL of 5% NaOH solution and charged
into a 1 L autoclave lined the Teflon. Hydrogen was
introduced into the autoclave to raise the total internal
pressure to 5 MPa and the reduction was conducted at
150 ℃ and 800 r/min stirring for 3 h. The reaction
mixture was cooled and the obtained black powder was
washed three times with 5% NaOH, then with water
until neutrality, and the desired Ru-Zn catalysts were
obtained.
the most probable value of the Ru crystallite size in the
catalyst was 5.1 nm, which is in agreement with the
XRD results.
Catalyst characterization
The bulk compositions of the catalysts were ana-
lyzed by a Bruker S4 Pioneer X-ray fluorescence (XRF).
Powder X-ray diffraction (XRD) patterns were acquired
on a PANalytcal X′Pert X-ray diffractometer using
Ni-filtered on a Cu Kα radiation in the range of 5°—85°.
The surface morphology and particle size were observed
by transmission electron microscopy (TEM; JEOL
JEM-2100) operating at 200 kV.
Figure 1 XRD pattern of Ru-Zn catalyst.
Catalytic test
The selective hydrogenation of benzene was per-
formed in a 1 L autoclave lined the hastelloy. The de-
sired amount of organic additive, 1.96 g of the catalyst,
9.8 g of ZrO2 and 280 mL of distilled water were intro-
duced. The above mixture was agitated for 2 h on the
conditions of 800 r/min and H2 pressure of 5.0 MPa (to
dissolve the organic additives completely). 140 mL of
benzene was then charged into the reactor at 423 K, the
stirring rate was increased to 1400 r/min to exclude the
external diffusion effect and the reaction timing began.
A small amount of reaction mixture was sampled every
5 min and sent for gas chromatographic analysis with a
FID detector, and the benzene conversion and the
cyclohexene selectivity were calculated.
Results and discussion
The XRF results reveal that the catalyst consists of
Ru and Zn in molar ratio of 33.8∶1. Figure 1 shows the
XRD pattern of the Ru-Zn catalyst. As can be seen, the
pattern contains six characteristic peaks of Ru at 2θ of
38.4°, 42.2°, 44.0°, 58.3°, 69.4° and 78.4°, ascribed to
100, 002, 101, 102, 110 and 103 diffraction planes of
hexagonal Ru (JCPDS No.01-070-0274), respectively.
The crystallite size of Ru in the catalyst estimated from
the strongest peak broadening of 44.0° using the
Scherrer equation is 4.7 nm. Figure 2 displays the TEM
image (a), the corresponding lattice Fourier-transform
pattern (b) and the particle size distribution histogram
(c). It is found that the metallic Ru in the catalyst has a
hexagonal structure, the particle size distribution of the
catalyst was concentrated in the range of 3—8 nm and
Figure 2 TEM image (a), the corresponding lattice Fou-
rier-transfrom pattern (b) and the particle size distribution histo-
gram (c) of the Ru-Zn catalyst.
Table 1 shows the effect of various organic additives
on the performance of the catalyst for selective hydro-
genation of benzene to cyclohexene. As can be seen, a
cyclohexene yield of 55.2% at benzene conversion of
85.1% was obtained over the Ru-Zn catalyst as-prepared
without the addition of any additives. This indicates that
the Ru-Zn catalyst itself exhibits the good activity and
the excellent cyclohexene selectivity. It is found that the
organic additives can be divided into three groups ac-
370
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© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2011, 29, 369— 373

Effect of Organic Additives on the Performance of Nano-sized Ru-Zn Catalyst
Table＋1 Performance of the Ru-Zn catalyst in the presence of the organic additives, the pH value of the additives and the concentration
of Zn2 in the slurry after the reaction
＋
－1
Additive
Conversiona/%
85.1
Selectivitya/%
64.9
Yeilda/% Maximum yield/%
pH valueb [Zn2 ]c/(mol•L )
No
55.2
57.5
56.0
60.4
57.2
54.2
10.2
43.4
52.8
50.6
51.8
55.0
55.8
42.2
40.3
47.2
55.2
57.5
56.0
60.4
59.4
61.4
21.2d
58.9
58.2
50.6
54.1
55.0
55.8
42.2
40.3
47.2
6.5
5.7
5.7
5.8
7.0
7.2
9.5
10.6
8.7
8.0
6.0
5.5
7.4
7.4
5.2
5.1
0.4908
0.4709
0.4826
0.5051
0.5108
0.4842
0.5027
0.4634
0.4784
0.4683
0.3941
0.4581
0.4722
0.4737
0.4550
0.4901
0.2 g PEG-400
83.6
68.8
0.2 g PEG-600
88.2
63.5
0.2 g PEG-6000
0.2 g PEG-10000
0.2 g PEG-20000
0.2 g 1,2-diaminoethane
0.2 g diethanolamine
0.2 g Triethanolamine
0.2 g PVAAH-26
0.02 g PVA-1750
0.2 g PVPK-30
92.6
65.2
78.6
72.8
68.7
78.9
11.8
86.5
55.1
78.7
71.6
73.7
93.3
54.2
75.7
68.4
90.8
60.6
0.02 g PAA-Na
0.2 g CMC
86.3
64.7
81.7
51.7
0.2 g Acacia Gum
82.7
48.7
0.2 g EDTA
77.9
60.6
a Benzene conversion, cyclohexene selectivity and yield at 10 min. b Measuring method of pH value: The additive corresponding to the
weight in Table 1 was completely dissolved in 280 mL of distilled water, and the pH value was measured with a pH meter. c The concen-
＋
tration of Zn2 in the slurry after the reaction was measured by chemical titration. d The yield was obtained by detecting the last sampling
at 25 min.
cording to the performance of the catalyst in presence of
them. The first group containing PEG (polyethylene
glycol), 1,2-diaminoethane, diethanolamine and
triethanolamine can greatly improve the cyclohexene
selectivity and yield, and decrease the activity. It is in-
teresting to note that when PEG is used as an additive
the activity decreases and the cyclohexene selectivity
increases with the increase of the PEG molecular weight.
With PEG-20000 as an additive, a cyclohexene selectiv-
ity of 78.9% at a benzene conversion of 68.7% is
achieved over the catalyst, indicating not only a high
cyclohexne yield, but also a reduction of product sepa-
ration load. At the same time a maximum cyclohexene
yield of 61.4% is also obtained, which represents the
best level reported in the open literatures.7-10 When
diaminoethane is added, the highest cyclohexene se-
lelectivity of 86.5% is reached over this catalyst, while
the activity (only 11.8% at 10 min) is the lowest. The
second group including PVA (polyvinyl alcohol), PVP
(polyvinypyrrolidone) and PAA-Na has no obvious ef-
fect on the performance of the catalyst, and the activity
and cyclohexene selectivity both change very little. The
others included in the last group can not only lower the
activity, but also especially decrease the cyclohexene
selectivity.
number of the active sites favorable for the production
of cyclohexene is the highest. According to previous
investigations, the roles of the organic additives are
proposed to attribute to the following reasons. (1)
Compared with PVA, CMC (carboxymethyl) and Aca-
cia Gum with multi-hydroxyl group, it is found that
PEG with ether linkages or aminated compounds with
amine groups is closely related with the increment of
the cyclohexene selectivity of the catalyst. It is clear that
O (N) in the ether linkage (the amine group) and water
can generate the intermolecular hydrogen bonds.
Therefore, after these additives adsorbed on the surface
of the catalyst, the hydrophilicity of the catalyst is en-
hanced, which is beneficial to the selectivity to cyclo-
hexene.28,29 Obviously, when the weight is constant, the
higher the molecular weight of PEG, the more the ether
linkage. This indicates the increment of the intermo-
lecular hydrogen bonds, which results in the increase of
the cyclohexene selectivity. Among 1,2-diaminoethane,
diethanolamine and triethanolamine, when the weight is
constant, 1,2-diaminoethane can provide the most amine
groups, which can form the most intermolecular hydro-
gen bonds with water. Therefore, with the addition of it
the catalyst exhibits the highest cyclohexene selectivity.
At the same time 1,2-diaminoethane occupies the most
of Ru active sites and the catalyst presents the lowest
activity. Besides, due to steric hindrance (as showed in
Scheme 1), it is difficult for the N of triethanolamine to
form intermolecular hydrogen bonds with water and
with the addition of triethanolamine, the catalyst shows
the lowest cyclohexene selectivity. (2) Considering the
It was reported that the modification of Ru with Zn
is one of main reasons for the excellent performance of
the catalyst in the literatures.17-20 Another important
reason is the nano size effect. The most probable value
of the Ru crystallite size of the catalyst is 5.1 nm, and
Bu et al.13 found that on Ru crystallites of such size the
Chin. J. Chem. 2011, 29, 369— 373
© 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
371

Sun et al.
NOTE
Pauling electronegativities of Ru (2.2), N (3.0) and O
(3.4), metallic Ru may donate an electron to O in the
ether linkages (or N in the amine groups) and a lot of
Conclusions
The nano-sized catalyst was prepared by
co-precipitation. The modification of Ru with Zn and
the nano-sized effect are responsible for the excellent
performance of the catalyst. With the addition of
PEG-20000, a cyclohexene selectivity of 78.9% at a
benzene conversion of 68.7% was achieved over this
catalyst, implying a high cyclohexene yield and a sig-
nificant reduction of the separation load. Moreover, a
maximum cyclohexene yield of 61.4% was also ob-
tained, which is the best result reported. With the addi-
tion of diethanolamine and triethanolamine, cyclohex-
ene yield reached 58.9% and 58.2%, respectively. The
increment of the selecitivity of catalyst may attribute to
the ether linkages or the amine groups in the organic
additives, rather than to the hydroxyl groups. Instead,
CMC, Acacia Gum and EDTA with hydroxyl groups or
carboxyl groups can result in the emulsification of the
slurry and the decrease of the cyclohexene selectivity.
The interaction among each organic additive, the cata-
＋
δ
Ru species may be formed. Mazzieri et al.30 found
that the cyclohexene＋selectivity increased with the de-
crease of the Ru0/Ru ratio. They suggested that cyclo-
δ
hexene might be more weakly adsorbed on the most
electron-deficient Ru species and easily desorbed by
avoiding its further hydrogenation to cyclohexane and
the cyclohexene selectivity increased. Fan et al.14 also
＋
δ
found that the existence of Ru in the RuCoB/Al2O3
catalyst could enhance the yield of cyclohexene.
Meanwhile, the absorption of benzene is also decreased
and the activity of the catalyst decreases. 1,2-Diamino-
ethane possesses the mo＋st amine groups and with
δ
addition of it the most Ru species are formed and the
activity of the catalyst decreases remarkably. (3) It was
proposed the formation of a hydrogen bond (－3—－5
kJ/mol) between cyclohexene and modifier molecular
can weaken the overlap of the π-electrons of C＝C
double bond in cyclohexene molecular with the d orbital
of ruthenium. This can cause the hydrogen-bonded
cyclohexene to be desorbed rapidly on the surface of
ruthenium, and improve the cyclohexene selectivity.25
(4) As can be seen in Table 1, the pH value of every
＋
lyst and Zn2 in the slurry also can affect the perform-
ance of the catalyst.
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