Cooperation between Pt and Ru on RuPt/AC bimetallic catalyst in the hydrogenation of phthalates

Article abstract of DOI:10.1016/j.cclet.2020.04.016

RuPt/AC bimetallic catalysts were prepared by two-step incipient impregnation method and evaluated in the hydrogenation of phthalates. According to the characterization results, well dispersed RuPt bimetallic nanoparticles were formed on the catalyst, and

Full text of DOI:10.1016/j.cclet.2020.04.016

Journal Pre-proof
Cooperation between Pt and Ru on RuPt/AC bimetallic catalyst in the
hydrogenation of phthalates
Yan Xu, Chenguang Wu, Yan Wang, Ying Zhang, Han Sun, Haijun
Chen, Yujun Zhao
PII:
S1001-8417(20)30234-5
https://doi.org/10.1016/j.cclet.2020.04.016
CCLET 5595
DOI:
Reference:
To appear in:
Chinese Chemical Letters
Received Date:
Revised Date:
Accepted Date:
1 March 2020
14 March 2020
8 April 2020
Please cite this article as: Xu Y, Wu C, Wang Y, Zhang Y, Sun H, Chen H, Zhao Y,
Cooperation between Pt and Ru on RuPt/AC bimetallic catalyst in the hydrogenation of
phthalates, Chinese Chemical Letters (2020), doi: https://doi.org/10.1016/j.cclet.2020.04.016
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Communication
Cooperation between Pt and Ru on RuPt/AC bimetallic catalyst in the hydrogenation
of phthalates
Yan Xu a, Chenguang Wu a, Yan Wang a, Ying Zhang a, Han Sun b, Haijun Chen b, Yujun Zhao a,

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 College of Electronic Information and Optical Engineering, Nankai University, Tianjin 300350, China
 Corresponding author.
E-mail address: yujunzhao@tju.edu.cn (Y. Zhao).
Graphical Abstract
The charge transfer between Ru and Pt significantly promotes the hydrogenation of phthalates and the selectivity.
ARTICLE INFO
ABSTRACT
RuPt/AC bimetallic catalysts were prepared by two-step incipient impregnation method and
evaluated in the hydrogenation of phthalates. According to the characterization results, well
dispersed RuPt bimetallic nanoparticles were formed on the catalyst, and the strong interaction
between the two metals resulted in the formation of RuPt alloy. It was found that Ru can
donate electrons to Pt on RuPt alloy nanoparticles, leading to the formation of electron-
deficient Ru which significantly promotes the hydrogenation rate of dioctyl phthalate and
improves the selectivity of dioctyl di-2-ethylhexylcyclohexane-1,4-dicarboxylate by
accelerating the further hydrogenation of intermediate products. The bimetallic RuPt catalyst
also presented excellent stability and versatility in the hydrogenation of phthalates,
demonstrating its prospective future in the hydrogenation of aromatic ring contained
compounds.
Article history:
Received 1 March 2020
Received in revised form 14 March 2020
Accepted 29 March 2020
Available online
Keywords:
Ru
RuPt alloy
Bimetallic catalyst
Phthalates
Hydrogenation
In recent years, the effect of phthalates has become one of the major public health concerns [1-3]. Cyclohexane dicarboxylic acid
ester is benzol-free ester, which can be produced by phthalates hydrogenation. Some research showed that cyclohexane dicarboxylic
acid ester has better performance on plasticity and safety, and it is becoming an environment-friendly plasticizer to replace phthalates
[4,5].
Bimetallic catalyst is always used in some reaction system as a result of its higher activity and selectivity. Qu et al. [6] designed
bimetallic RuRe catalyst for selective hydrogenation of dimethyl terephthalate (DMT) to 1,4-cyclohexane dicarboxylate (DMCD). The
distribution of active metal species on the surface was greatly improved by the addition of Re, at the same time Re promoted the charge
transfer between Ru and Re and strengthened the RuRe synergistic interaction. Zhang et al. [7-9] reported that bimetallic RuPd
prevented the nanoparticle aggregation and the structure of NiAlRu-layered double hydroxide endowed the higher metal dispersion and
Ru facilitate the reduction of Ni2+ because of the hydrogen spillover.
Li et al. [10] found that RuSn/Al2O3 catalyst was not a suitable catalyst to produce 1,4-cyclohexanedimethanol (CHDM) via one-pot
hydrogenation of DMT, because the introduction of Sn metal restrained the hydrogenation capability of Ru catalyst. However, when Pt
metal is introduced into the RuSn/Al2O3 catalyst, the ability of Ru in the hydrogenation of phenyl group is enhanced obviously,
resulting a higher yield to the designed alcohol CHDM. They suggested that the introduction of Pt into the bimetallic RuSn catalyst
may enhance the reducibility and dispersion of the metal particles.
In previous work [11], we found that monometallic Ru/AC catalyst with appropriate microporous structure exhibited excellent
activity in the hydrogenation of various phthalates. However, under mild reaction conditions the Ru/AC catalyst has lower selectivity
of dioctyl di-2-ethylhexylcyclohexane-1,4-dicarboxylate (DEHCH) due to its lower activity in the further hydrogenation of the bis(2-
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ethylhexyl) cyclohexene-1,2-dicarboxylate (4H-DOP), which is an intermediate product of dioctyl phthalate (DOP) hydrogenation.
Therefore, how to improve the hydrogenation activity of Ru is the key for developing highly efficient catalyst. The addition of second
metal such as Pt could help improving the hydrogenation ability of Ru in DOP hydrogenation to DEHCH. An insight into the possible
synergistic effect between Pt and Ru could give some rational direction for this reaction system.
Herein, a series of bimetallic RuPt/AC catalysts were prepared by two-step incipient impregnation with the activated
carbon as the support and applied for DOP hydrogenation (details can be found in Supporting information). It was found that bimetallic
RuPt catalyst showed much higher activity and selectivity than monometallic Ru or Pt catalysts due to the electron transfer between Ru
and Pt.
Pore structure distribution curves and N2 adsorption-desorption isotherm are shown in Figs. S1A-C (Supporting information). All the
samples show type IV adsorption isotherm and type H4 hysteresis loop, in accordance with the categorization of IUPAC, which are
typical characteristics of micro-mesoporous materials [12,13]. As listed in Table S1 (Supporting information), all catalysts present the
identical pore volume, specific surface area and the average pore size as a result of the lower metal loading (both Pt and Ru) [14]. The
similar structure of various catalysts excludes the difference in mass transfer.
Fig. S1D (Supporting information) shows the X-ray diffraction (XRD) patterns of the RuPtx/AC with various Pt/Ru mole ratio (x =
0, 0.3, 0.6, 1.2). No significant difference in the diffraction peaks of these samples. All the XRD patterns of these samples show two
broad peaks near 23° and 44°, belonging to (002) and (100) planes of typical amorphous carbon [15]. It indicates that the structure of
the initial activated carbon is basically maintained during the preparation of the catalysts. The characteristic diffraction peaks of Ru
[16,17] and Pt [18] particles are not observed, implying that both Ru and Pt are highly dispersed on AC carriers [19,20].
CO pulse chemisorption was used to characterize the Ru dispersion and particle sizes for all the catalysts. As listed in Table 1, with
the increase of Pt content, the metal dispersion gives a slight decrease. During the two-step incipient impregnation process for the
catalyst preparation, Ru particles could deposit around Pt particles, and finally formed RuPt nanoparticles after reduction. Therefore,
the increase of Pt content resulted in larger RuPt nanoparticles.
Table 1
Dispersion, particle diameter and Ru Pt binding energy of the catalysts.
Binding energyc (eV)
a
b
Metal
DA
(nm)
DA
Catalysts
Dispersiona (%)
(nm)
Pt 4f5/2
75.01
-
74.87
74.83
74.79
Pt 4f7/2
71.69
-
71.43
71.37
71.37
Ru 3d5/2
-
280.52
280.52
280.57
280.61
Pt/AC
-
-
-
Ru/AC
30.6
27.9
24.2
19.2
2.94
3.43
4.08
5.35
2.51
3.44
3.96
6.60
RuPt0.3/AC
RuPt0.6/AC
RuPt1.2/AC
aDetermined by CO pulse chemisorption. bDetermined by transmission electron microscope (TEM). cDetermined by X-ray photoelectron spectroscopy (XPS).
The size of RuPtx nanoparticle was measured by transmission electron microscope (TEM). It can be seen from Fig. 1 that all these
catalysts show the disordered structure of amorphous carbon, corresponding to the above XRD results. The overview images of
Ru/AC, RuPt0.3/AC, RuPt0.6/AC and RuPt1.2/AC (Fig. 1) reveal that the catalysts with a Pt/Ru ratio of less than 1.2 can ensure the high
dispersion of RuPt nanoparticles on the carbon support, while much higher ratio of Pt/Ru led to the severe aggregation of RuPt
nanoparticles (eg., RuPt1.2/AC catalyst). Similarly, the distribution histograms of RuPt nanoparticle size show that the nanoparticle size
of RuPt increases with the content of Pt. The average nanoparticle sizes of Ru/AC, RuPt0.3/AC, RuPt0.6/AC and RuPt1.2/AC are 2.51,
3.44, 3.96 and 6.60 nm, respectively, which agrees well with the results of CO pulse chemisorption. Energy dispersive spectroscopy
(EDS) mapping images of the RuPt0.6/AC catalyst are shown in Fig. 1F and Fig. S2 (Supporting information), both Pt (green) and Ru
(red) exhibit almost the similar element distribution, allowing the possible formation of RuPt alloy [21,22].

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Fig. 1. High resolution TEM (HRTEM) images of Ru/AC (A), RuPt0.3/AC (B), RuPt0.6/AC (C), RuPt1.2/AC (D) catalysts and EDS mapping image of the
RuPt0.6/AC catalyst (F).
In order to insight into the chemical nature of the metal phases on the catalysts surface, X-ray photoelectron spectroscopy (XPS)
analysis was carried out on the RuPtx/AC catalysts, as well as the Pt/AC and Ru/AC monometallic catalysts. The binding energies are
listed in Table 1. As shown in Fig. 2A, the peak intensity of RuPtx/AC is lower than that of Pt/AC, which should be ascribed to the
lower content of Pt and the confinement of Pt in the lattice of Ru. Moreover, the binding energies of Pt0 on RuPtx/AC shift to lower
binding energy (4f7/2 at 71.37-71.43 eV) in comparison with that (4f7/2 at 71.69 eV) on Pt/AC. The negative migration of binding
energy of Pt 4f7/2 in RuPtx/AC is because of the electron contribution of Ru to Pt. Because Ru 3d3/2 region can overlap with C ls region
at around 284.5 eV, the 3d5/2 region of Ru are investigated for carbon supported Ru catalyst. The Ru 3d5/2 region in Fig. 2B identify
that ruthenium is presented in a state of both Ru0 (3d5/2 at 280.53 eV) and Ru4+ (3d5/2 at 281.14 eV, oxidized during sample preparation
at room temperature in air [23]) on the catalyst surface. Compared with the Ru/AC (280.52 eV), the binding energies of the Ru0 in
RuPtx/AC shift slightly to higher energies (280.61 eV), suggesting Ru donates electrons to Pt. The interaction between the two metals
indicates the possible formation of RuPt alloy on the surface of bimetallic catalyst.
Fig. 2. XPS spectra of Pt 4f (A) and Ru 3d (B) for RuPtx/AC catalysts.
The H2 temperature programmed reduction (TPR) results of various catalysts are shown in Fig. 3. No obvious reduction peak can be
found in the TPR pattern of Pt/AC, implying that most of Pt species on Pt/AC has been reduced by the surface reductive species on AC
o
o
during the calcination in N2 environment. However, there are two reduction peaks at 283 C and 163 C for Ru/AC catalyst, which
should belong to the RuOx having strong interaction with support and highly dispersed RuOx, respectively [24-26]. Moreover, in the
case of RuPtx/AC catalysts, the second reduction peak of RuOx is disappeared, demonstrating that the interaction between Ru
nanoparticles and support has changed due to the presence of Pt. The close contact between Pt and Ru could promote the reduction of
RuOx since metallic Pt can accelerate the hydrogen activation and spillover to adjacent RuOx. Thus, the formation of RuPt alloy
nanoparticles can be enhanced during this reduction process [27]. This result agrees well with the XPS and HRTEM results.

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Fig. 3. H2-TPR profiles of the Ru/AC, Pt/AC and RuPtx/AC catalysts.
In order to clarify the catalysis nature of RuPt alloy nanoparticles in the DOP hydrogenation on various catalysts, a series of
experiments were carried out in a stirred tank reactor. As listed in Table 2, Pt/AC shows much lower conversion (2.5%) than Ru/AC
catalyst (39.9%), indicating the less activity of Pt than Ru in DOP hydrogenation. However, the introduction of Pt significantly
improved the DOP conversion of Ru catalyst, and a much higher conversion of about 56.7% is achieved on RuPt0.6/AC bimetallic
catalyst. Therefore, we believe that Ru instead of Pt is the active specie of the catalyst at the given reaction conditions. Meanwhile, as
shown in Fig. S3 (Supporting information), an excess amount of Pt will also result in a decrease of activity, probably due to the
agglomeration of RuPt nanoparticles. Therefore, the highest activity can be obtained at the optimized Pt/Ru ratio of 0.6. In addition, the
selectivity of DEHCH is significantly improved from 63.7% on Ru/AC to 83.2% on RuPt0.6/AC, implying the promoting effect of PtRu
alloy on the further hydrogenation of the intermediate product 4H-DOP with only single C=C bond. Their TOF value further
demonstrates that the formation of PtRu alloy significantly improves the activity of Ru active site on the conversion of DOP (Fig. S4 in
Supporting information shows that the reaction rate is constant at the conversion below 60%). On basis of the XPS and TPR results, the
electronic transfer from Ru to Pt leads to the formation of electron-deficient Ru, which could promote the hydrogenation rate of 4H-
DOP and improves the DEHCH selectivity.
Table 2
Hydrogenation of DOP on the RuPtx/AC Catalysts.
Catalysts
ACc
Conversion (%)
Selectivitya (%)
Selectivityb (%)
TOF (h-1)
0.0
21.5
2.5
0.0
0.0
-
Pt+Ru/ACd
57.1
44.3
62.7
82.6
83.2
47.9
42.9
55.7
37.3
17.4
16.8
20.4
-
Pt/AC
-
Ru/AC
39.9
50.6
56.7
43.9
3015
4349
5142
5194
RuPt0.3/AC
RuPt0.6/AC
RuPt1.2/AC
a DEHCH selectivity. b 4H-DOP selectivity. c Only active carbon support. d Mechanical mixture of Pt/AC and Ru/AC.
Hydrogenation of various phthalates including DOP, dipentyl phthalate (DPP) and dimethyl phthalate (DMP) were examined on
RuPt0.6/AC and Ru/AC catalysts. As listed in Table S2 (Supporting information), the two catalysts show different activity in the
hydrogenation of various model reactants, and their TOFs exhibit a decrease trend with the increase of the reactant molecular size. In
addition, RuPt0.6/AC is much more active than Ru/AC in the hydrogenation of all phthalates due to the Ru-Pt interaction.
Figs. S5 and S6 (Supporting information) show the products distribution obtained on the Ru/AC and RuPt0.6/AC catalysts. For both
the two catalysts, the concentration of 4H-DOP is first increased and then decreased to 0% with the time (Fig. S5A in Supporting
information), while the total hydrogenation product DEHCH presents an increasing trend until its selectivity reaches 100% (Fig. S6B
in Supporting information). It strongly evidences that the liquid-phase hydrogenation of DOP follows a consecutive reaction route as
illustrated in Scheme 1. This result agrees well with the findings reported by Duc Ha et al. [28] on the hydrogenation of DMP. For the
Ru-based catalyst, the further hydrogenation of the intermediate 4H-DOP could be the determining step for this consecutive reaction.
The bimetallic RuPt catalyst is more active than the monometallic one due to the superior activity in the hydrogenation of intermediate.
On this catalyst, the Ru site decorated by Pt is much more active than single Ru or Pt site in the hydrogenation of aromatic ring,
especially in the saturation of 4H-DOP with single C=C bond. Thus, the cooperation between Pt and Ru significantly enhances the
reaction rate of 4H-DOP hydrogenation, accompanied by the formation of DEHCH.
Scheme 1. Reaction route for DOP hydrogenation.

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The bimetallic RuPt catalyst exhibits a stable activity during the reusage in DOP hydrogenation (Fig. S7 in Supporting information).
Moreover, it also presents excellent catalytic performance and versatility in various phthalates’ hydrogenation (Table S3 in Supporting
information). These results demonstrate the prospective future of the bimetallic RuPt catalyst in the synthesis of environment-friendly
plasticizer via phthalates hydrogenation.
In summary, the hydrogenation of phthalates to environment-friendly plasticizers is becoming an attractive issue from both the
academic and industrial fields. This work prepared a series of RuPtx/AC catalysts for the DOP hydrogenation and the mechanism of
RuPt alloy nanoparticle is well discussed and proposed. It was found that both the metals were highly dispersed on the AC support and
the interaction between Ru and Pt led to formation of RuPt alloy nanoparticles during the reduction. However, an excess amount of Pt
can result in the agglomeration of RuPt nanoparticles. The as-prepared RuPt/AC bimetallic catalyst exhibited a superior catalytic
activity than monometallic Ru/AC or Pt/AC catalyst, which is ascribed to the formation of RuPt alloy in the RuPt bimetallic catalyst. It
was proposed that Ru can donate electron to Pt in RuPt nanoparticles, resulting in electron-deficient Ru sites which significantly
promote the hydrogenation rate of DOP and improve the DEHCH selectivity by enhancing the deep hydrogenation of intermediate
products. The RuPt bimetallic catalyst presented excellent stability during the reusage in batch reactor and exhibited promising
versatility in phthalates hydrogenation. These findings demonstrate a prospective future of the RuPt/AC bimetallic catalyst in the
selective hydrogenation of aromatic ring contained compounds.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Acknowledgements
We appreciate the National Natural Science Foundation of China (Nos. 21878227, 21276186) for the financial support
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