Structure effect of activated carbon in Ru/AC catalysts for hydrogenation of phthalates

Article abstract of DOI:10.1016/j.catcom.2019.105825

Activated carbon with different microporous surface area were used to prepare a series of Ru/AC catalysts for the hydrogenation of dioctyl phthalate (DOP) to dioctyl 1,2-cyclohexanedicarboxylate (DOCH). The presence of micropores benefit the dispersion of Ru, but Ru particles encapsulated into the micropores can be hardly accessed by DOP molecule. An appropriate microporous surface area of the support should be crucial important for the preparation of highly efficient catalyst for phthalates hydrogenation. The as-prepared 0.45%Ru/AC-2 catalyst (Micropore surface area = 604 m²/g) exhibited the best activity in DOP hydrogenation with a TOF of 1406 h^?1 due to its suitable micropore surface area.

Full text of DOI:10.1016/j.catcom.2019.105825

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Structure effect of activated carbon in Ru/AC catalysts for
hydrogenation of phthalates
Yan Xu, Yan Wang, Chenguang Wu, Huijiang Huang, Ying
Zhang, Jun Nan, Haibin Yu, Yujun Zhao
PII:
S1566-7367(19)30287-0
DOI:
https://doi.org/10.1016/j.catcom.2019.105825
CATCOM 105825
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
14 May 2019
26 August 2019
15 September 2019
Please cite this article as: Y. Xu, Y. Wang, C. Wu, et al., Structure effect of activated carbon
in Ru/AC catalysts for hydrogenation of phthalates, Catalysis Communications (2018),
https://doi.org/10.1016/j.catcom.2019.105825
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2018 Published by Elsevier.

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Structure effect of activated carbon in Ru/AC catalysts for hydrogenation of
phthalates
a
a
a
a
a
b
b
Yan Xu , Yan Wang , Chenguang Wu , Huijiang Huang , Ying Zhang , Jun Nan , Haibin Yu ,
a,*
Yujun Zhao yujunzhao@tju.edu.cn
a
Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative
Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and
Technology, Tianjin University, Tianjin 300072, China
b
Tianjin Chemical Research and Design Institute Co., Ltd., Tianjin 300131, China
*
Corresponding author.
Abstract:
Activated carbon with different microporous surface area were used to prepare a series of Ru/AC
catalysts for the hydrogenation of dioctyl phthalate (DOP) to dioctyl 1,2-cyclohexanedicarboxylate
(DOCH). The presence of micropores benefit the dispersion of Ru, but Ru particles encapsulated into the
micropores can be hardly accessed by DOP molecule. An appropriate microporous surface area of the
support should be crucial important for the preparation of highly efficient catalyst for phthalates
2
hydrogenation. The as-prepared 0.45%Ru/AC-2 catalyst (Micropore surface area=604 m /g) exhibited
-1
the best activity in DOP hydrogenation with a TOF of 1406 h due to its suitable micropore surface area.
Keywords: Plasticizer, DOP, hydrogenation, Ruthenium, activated carbon
1
. Introduction
Nowadays, plastic has been widely used in agriculture, industry and every field of daily life. In
order to improve the forming and using performance, some additives will be added in the plastic
processing, such as plasticizers, stabilizers, colorants, lubricants and foaming agents. Phthalate is a kind
of common plasticizer[1], which can improve the softness and elongation of polymer materials, making
them maintain good performance on insulation and cold resistance.
However, phthalate-based plasticizer may migrate and be released to the environment during the
application[2][3], which is potentially harmful to human health[4][5]. These plasticizers may induce
hepatocellular carcinoma through oxidative damage mediated mechanisms due to the presence of

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benzene group in these molecule[6][7].
It has been found that cyclohexane dicarboxylic acid ester produced by phthalates hydrogenation
gives better performance on safety and plasticity[8][9][10]. These compounds have been considered as
the most likely plasticizer to replace phthalates for children's toys, medical supplies, food packaging and
other products. For the synthesis of cyclohexane dicarboxylic acid ester from phthalates hydrogenation,
some results have been reported in the catalyst preparation and reaction mechanism. Zhao et al.[11]
prepared three different catalysts 60%Ni/Al O , 60%Ni/MgO-B and 60%Ni/Al O -B, on which the
2
3
2
3
conversion of DOP can achieve 34%, 55% and 72%, respectively. The results showed that highly
dispersed nickel catalyst has high conversion and selectivity to DOCH and the adsorption of DOP was
not influenced by surface acid/base property of support. Jiang et al.[12] synthesized a kind of
mesoporous carbon nitride (MCN) material for the preparation of Pd/MCN catalyst. They found that the
interaction between N and Pd plays an important role in the aqueous-phase hydrogenation of dimethyl
phthalate (DMP).
Activated carbon (AC) is broadly used as the carrier of heterogeneous catalyst because of its high
specific surface area and bargain price[13][14]. According to the literature, carbon structure is one of the
most important factors affecting the dispersion of Ru particles[15]. There are two important methods for
obtaining high metallic dispersion: well developed porosity and high surface area. Usually, the catalytic
activity could be significantly promoted by increasing Ru dispersion on AC. Zanutelo[16] studied the
pretreatment of the AC and its effect on the performance of Ru/C catalyst in partial hydrogenation of
benzene. Different treatment methods will lead to different kinds of oxygen-containing surface groups
on the AC carrier. The acidic surface groups could improve the hydrophilicity of AC surface and reduce
the adsorption of nonpolar benzene, which is beneficial to increase the selectivity of cyclohexene.
Although, Ru/C catalysts have been extensively studied in aromatic ring hydrogenation
system[17][18][19], the structure-activity relationship of Ru/AC catalysts in phthalates hydrogenation is
not yet clear. Therefore, how to rationally design an effective catalyst for phthalates hydrogenation is
still a big challenge.
In this work, to provide meaningful instruction for developing efficiently an industrial catalyst for

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phthalates hydrogenation, a series of highly dispersed Ru/AC catalysts with different structure properties
was prepared by incipient wetness impregnation method, and the structure effect of Ru/AC in
hydrogenation of DOP was well discussed. In addition, the hydrogenation of dibutyl phthalate (DBP)
and diamyl phthalate (DAP) over Ru/AC were studied as well for examining the applicability of the
Ru/AC catalyst in phthalate hydrogenation.
2
2
. Experimental
.1 Materials
Ruthenium(III) chloride hydrate (99.9%, J&K Scientific), activated carbon (powder, 100-200 mesh,
Chengde Huayu Activated Carbon Manufacturing Corporation), bis(2-ethylhexyl) phthalate (99.2%,
Tianjin Bohai Chemical Industry Group CO.,LTD), dibutyl phthalate (99.2%, Kaifeng Jiuhong Chemical
Corporation) and diamyl phthalate (98%, Shanghai Aladdin Biochemical Technology), anhydrous
ethanol (HPLC, Tianjin Concord Technology Corporation) were used as received without any further
purification. The supplier used steam activation method under different activation time and temperature
to obtain different kind of AC from the same raw material. The specifications of activated carbon are
listed in Table S1.
2
.2 Catalyst preparation
Activated carbon were firstly dried at 393 K for 3 h as support. Ru/AC catalysts were prepared by
incipient wetness impregnation through an aqueous solution of the precursor RuCl . After dried at room
3
temperature for 10-12 h, the catalyst was dried at 393 K for 12 h in an oven. The catalyst was then
treated at 573 K in N for 4 h. Prior to the evaluation of catalytic performance, the catalysts were ex-situ
2
reduced in a tube reactor at 473 K for 3 h in a flow of 90 mL/min H under ambient pressure.
2
2
.3 Catalyst characterization
The N adsorption-desorption, X-ray diffraction (XRD), transmission electron microscope (TEM),
2
scanning electron microscope (SEM), CO pulse chemisorption, Fourier-transform infrared spectroscopy
FTIR) and inductively coupled plasma optical emission spectrometer (ICP-OES) were used to
(
characterize the physicochemical properties of the samples. The details of catalyst characterizations have
been provided in the supporting information.

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2
.4 Evaluation of catalytic performance
Ru catalysts supported on activated carbon were used for DOP hydrogenation. In order to evaluate
the catalytic performance, the hydrogenation was carried out in a 100 mL high pressure autoclave. In a
typical experiment, about 10.0 g of DOP, 0.15 g of the reduced catalyst and 30 mL of anhydrous ethanol
(
as solvent) were added to the autoclave. The autoclave was then purged five times with hydrogen to
remove the remaining air inside. The initial pressure was 4 MPa. The reaction was performed for 4 h at
93 K with a stirring rate of 800 rpm. The liquid products were diluted by ethanol and analyzed at the
same conditions by GC (BEIFEN SP 2100 gas chromatograph) with an Agilent HP-5 capillary column
3
(60 m×0.32 mm×0.25 m) and an FID detector.
The formulas for calculating the conversion (X), selectivity (S), yield (Y) and turnover frequency
TOF) are as follows. The by-product is only tetrahydrophthalate.
(
Amount of substrate after reaction (mol)
Xsubstrate= [1-
] ×100%
Total amount of substrate before reaction (mol)
Amount of substrate consumed by the formation of product (mol)
Total amount of substrate converted (mol)
Sproduct
=
×100%
Amount of substrate consumed by the formation of by-product (mol)
Sby-product
=
×100%
Total amount of substrate converted (mol)
Yproduct=Xsubstrate×Sproduct×100%
Total amount of DOP before reaction (mol)-Amount of DOP after reaction (mol)
CO uptake(mol/g)×1×Total amount of catalyst (g)×reaction time (h)
TOFDOP
=
3
. Results and discussion
The textual properties of the four activated carbon materials and corresponding Ru/AC-x catalysts
2
are listed in Table 1 and Fig. 1. The microporous surface areas grow gradually from 505 m /g for sample
2
AC-1 to 947 m /g for sample AC-4, and Ru/AC-x catalysts present the same trend. The high surface area
of AC is mainly provided by micropores, and mesoporous only contribute to a part of that. As listed in
Table 1, The average pore size and pore volume of Ru/AC-x catalysts are almost similar to that of the
AC support. The N adsorption-desorption isotherm and micropore size distribution curve of the samples
2
are shown in Fig. 1A. All the samples for both AC-x and Ru/AC-x exhibit type IV isothermals with H4
hysteretic loop, which illustrates the coexistence of micropores and mesopores on activated carbon. In

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addition, the shape of the channel is narrow slit-like. The micropore size distribution curves of both the
support and catalyst show a peak in the range of 0.6 to 0.7 nm (Fig. 1B), and the mesopore size
distribution curves of all the samples show a peak in the range of 3.5 to 4.5 nm (Fig. S1), indicating that
the pore size distribution of AC and Ru/AC-x is all relatively concentrated. Obviously, the porous
structure of Ru/AC catalysts is also well preserved after loading Ru nanoparticles.
The dispersion of Ru was characterized by CO pulse chemisorption. As listed in Table 2, Ru/AC-4
presents the highest Ru dispersion, and the average diameter of ruthenium particles ranges from 1.5 to
4
.4 nm for Ru/AC-x, in agreement with the results of TEM relatively (Fig. S2). When the dispersion of
Ru was correlated with micropore surface area of the activated carbon (Fig. 2), interestingly, it was
found that the dispersion of Ru increases linearly with the surface area of micropores. It indicates that
the presence of more micropore will benefit the dispersion of Ru.
It is well-known that part of the precursor of Ru in the micropore will migrate and stack at the
entrance of micropore during the drying process. As a result, most of the metallic Ru particles might be
located in mesoporous or at the edge of micropores, and others may be encapsulated in micropores as
ultra-fine particles (Fig. 3). It can be also explained by the truth that particle size of Ru metal in all the
catalysts are more than a few nanometers [21], which are much larger than the diameter of the
micropores (0.6 to 0.7 nm). The higher proportion of highly dispersed ultra-fine nanoparticles may lead
to the increase of Ru dispersion detected by CO pulse chemisorption method.
The XRD patterns obtained for the AC-x and Ru/AC-x are shown in Fig. S3. The characteristic
diffraction peak of Ru (ICDD-JCPDS Card No. 06-0663) cannot be observed on Ru/AC-x catalysts,
because the Ru loading is too low and the Ru nanoparticles supported on AC are too small to be detected
[
21][22][23]. Two broad peaks appear in all the samples at around 23°and 44°, corresponding to (002)
and (100) diffractions of typical amorphous carbon, respectively[24][25]. The characteristic peaks of
Ru/AC-x catalysts on the FTIR spectrum are the same (Fig. S4), illustrating that the four different
activated carbons have the same functional groups on the surface. The SEM images for the AC-x are
shown in Fig. S5. No apparent difference can be found on the SEM images for various activated carbon.
In order to insight the structure effect of Ru/AC in hydrogenation of DOP, a set of catalytic

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performance experiments was carried out over Ru/AC-x catalysts. The blank experiment with AC as the
catalyst sample confirmed that AC exhibited no catalytic activity in the hydrogenation of DOP. As listed
in Table 3, when increasing the surface area of AC’s micropore, the Ru/AC catalyst firstly presented an
increasing trend in the conversion of DOP, as well as the selectivity to cyclohexane dicarboxylic acid
ester. It can be ascribed to the increasing Ru dispersion, which was greatly affected by the increasing
surface area of micropore. According to the CO-chemisorption results and structure properties, the
higher micropore surface area can significantly improve Ru dispersion. Thus, the apparent activity of the
catalyst was significantly increased. However, although the dispersion of Ru was further promoted when
2
the surface area of AC’s micropore increased up to 947 m /g, both the conversion and selectivity to
cyclohexane dicarboxylic acid ester markedly decreased. The highest performances were achieved on
Ru/AC-3, whose AC support had appropriate amount of the surface area of micropores. It can be
deduced that the presence of much more micropores in the support will enhance the Ru dispersion, but it
will also lead to the encapsulation of more Ru nanoparticles inside the micropores. These Ru NPs should
be inaccessible for DOP molecule because of its larger molecular kinetic diameter, which is about 2.15
nm on basis of DFT calculations (Fig. S6). According to the literature, the molecular kinetic diameter of
H is only about 0.289 nm[26]. Based on the pore size distribution and molecular simulation results, it
2
can be concluded that DOP instead of H is difficult to diffuse into the micropores smaller than 2.15 nm.
2
Therefore, those ultra-fine Ru nanoparticles encapsulated in the micropores (0.6-0.7 nm in pore diameter)
of Ru/AC could not be well accessed by the DOP molecule. Moreover, the amount of the Ru sites in
mesopores could be similar for Ru/AC-2 and Ru/AC-3, so similar DOP conversions were obtained on
these two samples.
As listed in Table 3, the variation trend of selectivity for DOCH is consistent with that of conversion.
As is well known, the hydrogenation of DOP is a consecutive reaction, and the intermediate product is
only di(2-ethylhexyl) tetrahydrophthalate (Scheme 1). The further complete hydrogenation of
intermediates can be promoted if the resident time is longer enough or the activity of the catalyst is high
enough. On the contrary, lower catalytic activity or shorter resident time will lead to the reserve of more
intermediate products.

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Scheme 1. The reaction scheme of hydrogenation of DOP to DOCH
To further reveal the structure-activity relationship of the catalyst, the TOF of DOP was normalized
by the total amount of Ru sites obtained by the CO-chemisorption. As listed in Table 3, the TOF first
increased and then decreased. The Ru/AC-2 catalyst exhibits the highest TOF. It can be suggested that
appropriate amount of micropores could promote the dispersion of Ru in mesopores of the AC support,
however, redundant micropores will lead to a decrease of the Ru dispersion in mesopores due to the
encapsulation of more Ru NPs inside the micropores. As discussed above, those Ru sites inside the
micropores could be difficult for the DOP molecule to contact with. Thus, these parts of Ru site should
be inactive for DOP hydrogenation, resulting in a lower TOF value. It can be well interpreted by the
schematic diagram in Fig 3. For Ru/AC-3 and Ru/AC-4, excessive Ru nanoparticles exists in micropores
in the form of ultra-fine nanoparticles due to the presence of abundant micropores in their AC supports.
The less distribution of Ru in mesopores lead to the lower TOF of these two catalysts. These results can
confirm that the hydrogenation of phthalate is structure sensitive reaction on the as-prepared Ru/AC
catalyst.
The as-prepared Ru/AC catalyst also exhibited excellent activity in the hydrogenation of various
phthalates. As listed in Table 3, both the conversion and selectivity can reach 100% over Ru/AC-4
catalyst for DBP (Entry 10) and DAP (Entry 11) hydrogenation in 6 h. While DOP hydrogenation need
another four more hours to reach almost completely conversion (Entry 12, 13) because DOP (2.15 nm)
has a larger molecule than DBP (1.37 nm) and DAP (1.51 nm) (Fig. S6). These results can further
confirm the structure effect of the catalyst on phthalates hydrogenation.
The catalyst stability of Ru/AC-3 was also evaluated to test the reusability of Ru/AC for the
hydrogenation of DOP (Fig. S7). The mixture after reaction was centrifuged and washed by ethanol
before the next run. The yield of DOCH remained at a stable level of 40% after the fourth cycle over the
Ru/AC-3, illustrating that the Ru/AC could be a prospective catalyst for phthalate hydrogenation from
an industrial aspect.

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4
. Conclusions
As a kind of environmentally friendly plasticizer, cyclohexane dicarboxylic acid ester is believed to
be the most likely replacement for traditional phthalate plasticizer in the near future. The selective
hydrogenation of phthalates on Ru-based catalyst is a promising route for the synthesis of cyclohexane
dicarboxylic acid ester due to its high activity and stability. In this work, a series of highly dispersed
Ru/AC catalysts was prepared by using various activated carbon with different microporous surface area
as the supports. And, the structure effect of Ru/AC on DOP hydrogenation was well elucidated. It was
found that the increase of micropores promoted the dispersion of Ru in mesopores of the AC support,
and the presence of more micropores in the support will also lead to the encapsulation of more ultra-fine
Ru particles in the micropores. As the Ru dispersion increases, the performance of catalysts showed a
trend of increasing first and then decreasing. It was suggested that part of the ultra-fine Ru nanoparticles
encapsulated inside the micropores of AC could be difficultly accessed by DOP molecule, whose kinetic
diameter (2.15 nm) is rather bigger than the pore mouth of those micropores. Therefore, part of ultra-fine
particles is no longer work on DOP hydrogenation. Anyway, the Ru/AC catalyst with appropriate
microporous structure exhibited excellent activity in the hydrogenation of various phthalates. These
findings could be meaningful instructions for developing efficiently industrial catalyst for the
hydrogenation of various aromatic compounds with bigger molecule.
Acknowledgements
We are grateful to the financial support from the National Natural Science Foundation of China
(21878227, 21276186).
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Tables and Figure Captions:
Table 1. Textual properties of the support and catalyst.
Table 2. Characteristics of the Ru/AC-x catalysts.
Table 3. Hydrogenation of phthalate on Ru/AC-x catalysts.
Fig. 1. N isothermal adsorption-desorption and micropore size distribution curves of samples.
2
Fig. 2. Correlation between Ru dispersion and AC’s micropore surface area.
Fig. 3. Schematic diagram of the metallic Ru particles distribution in AC’s pore.
Table 1. Textual properties of the support and catalyst.
Surface
Surface
Surface
c
d
c
d
Volume
Volume
Diametermicro
Diametermeso
a
b
b
micro
meso
Catalyst
area
area
area
meso
total
2
micro
2
3
3
(cm /g)
(cm /g)
(nm)
(nm)
2
(m /g)
(m /g)
(m /g)
AC-1
AC-2
AC-3
567
593
997
1046
615
671
1072
1105
505
527
846
947
535
604
920
1015
62
66
151
99
80
67
0.22
0.13
0.12
0.25
0.22
0.17
0.15
0.25
0.25
0.67
0.64
0.68
0.71
0.66
0.63
0.69
0.69
4.38
4.00
4.06
3.69
4.28
4.30
4.07
3.77
0.23
0.39
0.41
0.25
0.26
0.42
0.43
AC-4
Ru/AC-1
Ru/AC-2
Ru/AC-3
Ru/AC-4
152
90
a
Total surface area calculated by BET method
b
Micropore surface area calculated by T-Plot method
c
Micropore properties determined by HK method
d
Mesopore properties determined by BJH method
Table 2. Characteristics of the Ru/AC-x catalysts.
b
b
c
d
CO uptake
Dispersion
Particle diameter
Particle diameter
a
Catalyst
Ru wt.%
(μmol/gcat)
(%)
30
49
83
87
(nm)
4.4
(nm)
3.6
Ru/AC-1
Ru/AC-2
Ru/AC-3
Ru/AC-4
0.51
0.45
0.47
0.52
15
22
38
45
2.7
3.0
1.6
2.3
1.5
1.9
a
Determined by ICP-OES
b
Determined by CO pulse chemisorption.
c
Particle diameter was obtained using the equation: Particle diameter=1.33/Dispersion.[20]

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d
Determined by TEM.
Table 3. Hydrogenation of phthalate on Ru/AC-x catalysts.
*
-1
Entry
Substrate
DOP
DOP
DOP
DOP
DOP
DOP
DOP
DOP
DOP
DBP
DAP
DOP
DOP
Catalyst
Ru/AC-1
Ru/AC-2
Ru/AC-3
Ru/AC-4
Ru/AC-1
Ru/AC-2
Ru/AC-3
Ru/AC-4
AC-3
Time/h Xsubstrate/% Sproduct/%
Sby-product /%
Yproduct/% TOFDOP/h
a
1
1
1
4
16
17
14
24
71
76
59
0
49
49
46
46
59
72
75
65
0
51
51
54
54
41
28
25
35
0
2
8
1342
3879
2329
1659
668
1406
848
567
-
a
2
a
3
1
8
a
4
1
7
b
5
4
14
52
57
39
0
b
6
4
b
7
4
b
8
4
b
9
4
c
1
0
Ru/AC-4
Ru/AC-4
Ru/AC-4
Ru/AC-4
6
100
100
90
99
100
100
88
99
0
100
100
79
99
-
c
1
1
1
1
6
0
-
c
c
2
3
6
12
1
-
10
-
Reaction conditions: Temperature=393 K, Pressure=4 MPa, Stirring rate=800 rpm, 30 mL ethanol;
a
1
0 g substrate, 0.05 g catalyst;
b
1
0 g substrate, 0.15 g catalyst;
c
2
g substrate, 0.05 g catalyst;
*
The by-product is only tetrahydrophthalate.

Journal Pre-proof

Journal Pre-proof
Fig. 1. (A) N isothermal adsorption-desorption of supports and catalysts; (B) HK micropore size
2
distribution curves of supports and catalysts
Fig. 2. Correlation between Ru dispersion and AC’s micropore surface area

Journal Pre-proof
Fig. 3. Schematic diagram of the metallic Ru particles distribution in AC’s pore
Highlights




Highly dispersed Ru/AC catalysts with different structure properties were prepared.
The Ru dispersion increased linearly with micropore’s surface area.
Appropriate amount of micropores benefit the activity in DOP hydrogenation.
Ru/AC catalyst exhibited excellent activity in various phthalates hydrogenation.