Hydrogenation of commercial polystyrene on Pd/TiO2
monolithic ceramic foam catalysts: catalyticperformance and enhanced internal mass transfer
Kai-Yue Han • Gui-Ping Cao • Hao-Ran Zuo •
Wen-Ze Guo • Zhen-Wei Zhu • Chong Lu •
Yan-Hua Wang
Received: 1 April 2014 / Accepted: 9 October 2014
� Akademiai Kiado, Budapest, Hungary 2014
Abstract Monolithic TiO2 ceramic foam (CF) supported palladium nanoparticles,
possessing three-dimensional structure with open accessible pores, were developed
as effective catalysts for the hydrogenation of commercial polystyrene. The
monolithic CF was synthesized through uniform coating of TiO2 on synthetic
template and partial sintering. The mechanical and structural properties of CF were
investigated. The distribution and texture of Pd nanoparticles on the CF and their
hydrogenation performances were studied. Furthermore, the internal mass transfer
analysis of PS coils during the hydrogenation was evaluated. The results showed
that the TiO2 CF-0.652 possessed interconnected cell windows of 400–600 lm and
macropores of 200–300 nm on the struts. In 0.06 wt% Pd/CF-0.652, Pd nanopar-
ticles (5.0 nm) were located on the surface of the TiO2 ceramic base, the dispersion
of Pd was 34.3 %. As the Pd loading increased, the dispersion of Pd on the Pd/CF
significantly decreased. The activity of Pd for PS hydrogenation slightly increased
when the Pd loading increased from 0.06 to 0.45 wt%, which could be explained by
the existence of interaction between Pd atoms and TiO2 CF at low metal loadings.
For Pd/CF-0.652 catalysts, the Weisz modulus was calculated to be less than 0.30,
indicating that internal diffusion limitation inside the Pd/CF-0.652 catalysts could
be neglected for unique macropores on the struts.
Electronic supplementary material The online version of this article (doi:10.1007/s11144-014-0793-0)
contains supplementary material, which is available to authorized users.
K.-Y. Han � G.-P. Cao (&) � H.-R. Zuo � W.-Z. Guo � Z.-W. Zhu
UNILAB, State Key Laboratory of Chemical Engineering, School of Chemical Engineering, East
China University of Science and Technology, Shanghai 200237, China
e-mail: [email protected]
C. Lu � Y.-H. Wang
School of Materials Science ang Engineering, East China University of Science and Technology,
Shanghai 200237, China
123
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DOI 10.1007/s11144-014-0793-0
Keywords Polystyrene hydrogenation � Ceramic foam � Supported catalyst �Internal mass transfer
Introduction
With the growing global need for high performance polymers, inexpensive
commodity plastics, e.g. polyethylene (PE), polypropylene (PP), polystyrene (PS),
and polyvinylchloride (PVC) cannot satisfy all the combinations of physical
property requirements associated with the existing and emerging applications. The
hydrogenation of an existing commodity plastic is a quick and economical approach
to bring new polymers to the market, since it enhances the chemical, mechanical,
and thermal properties of parent polymer [1–10]. The heterogeneous hydrogenation
of PS to produce hydrogenated polystyrene (HPS), also known as polycyclohexyl-
ethylene (PCHE), has been proved to be a potential marketing process because of
the greatly improved heat, oxidation, and UV resistance of PCHE compared with PS
[11–13].
In PS hydrogenation studies, powder catalysts containing group VIII metals were
usually adopted using batch operation [14–16], such as platinum (Pt), palladium
(Pd), ruthenium (Ru), and rhodium (Rh) supported on charcoal [11], SiO2 [13, 17],
CaCO3 [18, 19], Al2O3 [14, 19], BaSO4 [15, 19, 20], and carbon nanotubes (CNT)
[10] etc. During the PS heterogeneous hydrogenation, the PS coils need to transport
from the bulk liquid phase to the external surface of the catalyst particle, and then
diffuse into the pores to access the active sites [17, 18]. It has been found that mass
transfer of PS coils in both the bulk liquid phase and the catalyst pores were the
challenging steps, because the reactant molecules have large size (10–70 nm) and
the solution has high viscosity (10-2–10 Pa s). The external diffusion of PS/PCHE
coils in the bulk liquid phase could be enhanced by simply increasing the agitation
rate. [15, 19, 21] However, the internal diffusion of the polymer coils in the
supported catalyst is a big problem due to the large size of PS coils. In the last
decade, many attempts have been made to reduce the pore diffusion resistance by
exploring some novel catalytic systems. G. W. Roberts and co-workers [22–24]
introduced supercritical CO2 (Sc-CO2) to the PS hydrogenation system and found
that the Sc-CO2 could reduce the pore diffusion resistance by facilitating faster
diffusion of the polymer molecules and reducing the size of polymer coils. J. S.
Ness and co-workers [17] utilized wide-pore silica (300–400 nm) to enable the
access of PS coils to the catalyst pores. Our previous study [10] utilized the Pd/CNT
catalyst with Pd nanoparticles deposited on the external surfaces of CNT to avoid
the pore diffusion of PS coils. Despite the important advances achieved, those
powder catalysts present some drawbacks from an engineering point of view. The
separation of powder catalyst from polymer solution after batch operation is a time-
consuming and energy-wasting process. What is more, powder catalysts could not
be utilized in the piston-oscillating fixed bed or rotating packed bed reactor [25–28]
owing to the high pressure drop. Therefore, further improvements should be made in
terms of catalyst preparation to obtain an industrially favorable catalyst, which
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possesses a monolith structure and could eliminate the pore diffusion resistance of
PS coils.
In this study, monolithic titanium dioxide (TiO2) ceramic foam (CF) supported
Pd nanoparticles, possessing three dimensional structure with open accessible pores,
were developed as the catalyst for commercial PS hydrogenation. The TiO2 CF with
good thermal and chemical stability were prepared through the synthetic template
replica method [29]. The TiO2 CF were characterized by strength test apparatus, N2-
physisorption, X-ray diffraction (XRD), and scanning electron microscopy (SEM).
The textual and mechanical properties of CF were adjusted by varying the
calcination temperature of CF. The distribution and texture of Pd nanoparticles on
the CF and their hydrogenation performances were studied by inductively coupled
plasma-atomic emission spectrometry (ICP-AES), SEM, energy-dispersive X-ray
analysis (EDX), transmission electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), and CO chemisorption. Based on the results, the internal mass
transfer of the PS coils was evaluated through theoretical Weisz modulus
calculation.
Experimental
Materials
Commercial titanium dioxide (TiO2) powder purchased from Lingfeng Chemical
Reagents Co., Ltd., was used as the raw material of CF, whose mean diameter of the
powder was 0.2 lm determined by both JSM-6360LV scanning electron microscope
(JEOL, Japan) and Mastersizer 2000 laser diffraction particle size analyzer
(Malvern Instruments Ltd, UK). Polyvinyl alcohol (PVA) used as a binder with
the polymerization degree of 1,750 ± 50 and n-octanol used as an antifoaming
agent were purchased from Lingfeng Chemical Reagents Co., Ltd. Tetramethyl-
ammonium hydroxide (TMAH) used as pH adjust agent was purchased from
Sinopharm Chemical Reagent Co., Ltd. A commercial polyurethane (PU) sponge
with an open porosity of 60 PPI (pores per inch) used as a template was purchased
from Qitai Sponge Material Co., Ltd., and washed with ethanol under ultrasonic
condition for at least 30 min. The Pd precursor Pd(NO3)2�2H2O was purchased from
Jiuling Chemical Co., Ltd.
Commercial PS (GPPS-123) with number-average molecular weight of 90 kg/
mol and weight-average molecular weight of 279 kg/mol was presented by
Shanghai SECCO Petrochemical Co., Ltd. Decahydronaphthalene (DHN) used as a
solvent was obtained from Sinopharm Chemical Reagent Co., Ltd. The size of PS
coils in PS-DHN solution at 304 K ranged from 10.6 nm to 64.3 nm with number
average diameter of 20.33 nm, as measured by an ALV/CGS-5022F dynamic light
scatting (DLS) at 632.8 nm with a 22 mW He–Ne laser. High-purity hydrogen was
purchased from Shanghai Yingjiang Chemical Co., Ltd. All the chemical reagents
were used without further purification.
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Preparation of CF
The TiO2 ceramic slurry was prepared by completely mixing TiO2 powder, binder,
antifoaming agent, pH adjust agent, and deionized water. The PU template with the
size of 40 9 40 9 8 mm was soaked into the ceramic slurry and then passed
through a rotating roller to remove the excess slurry. The slurry composition was
optimized in an earlier study to obtain suitable thixotropic behavior, which could
allow excess slurry to flow when passing through the roller and avoid the remaining
slurry to drip from the template. A slurry with a solid fraction of 0.42 was the
optimized option in order to get a uniform replication of the template. The PU
pieces coated with TiO2 slurry were dried at room temperature for 24 h, heated to
873 K to remove the template with a heating rate of 1.0 K/min, and subsequently
calcined at the desirable calcination temperature (Tcal) for 100 min with a heating
rate of 2.5 K/min.
Preparation of CF supported Pd catalysts
Pd was deposited by equilibrium impregnation of TiO2 CF supports with an aqueous
solution of Pd(NO3)2�2H2O. The samples were dried in stagnant air at ambient
temperature for 12 h, and the color of the CF supports surface changed from white
to yellowish. The obtained CF samples were reduced by adding the freshly prepared
NaBH4 solution (molar ratio of NaBH4 to Pd is 1:1) at 293 K. During the reduction
process, the color of the CF surface changed from yellowish to black, suggesting the
formation of metallic Pd nanoparticles on the surface. The Pd/CF was then washed
thoroughly with distilled water and then dried at 393 K for 12 h.
Characterization of the CF and the Pd supported CF catalysts
The morphology of CF was investigated with a JSM-6360LV SEM (JEOL, Japan)
with a point resolution of 3 nm. The textural properties of the CF were characterized
by N2 physisorption on an ASAP 2010C (Micromeritics, USA) at 77 K after out-
gassing the samples at 673 K for 6 h. X-ray diffraction was performed on a Rigaku
D/Max 2550 VB/PC diffractometer using Cu Ka radiation and a carbon
monochromator. The compression strength of the above prepared catalysts was
tested using FY-15 sheeting machine (Sichuang Technology Development Co. Ltd).
The Pd content of the catalyst was tested with IRIS 1000 inductively coupled
plasma - atomic emission spectroscopy (Thermo Elemental, USA). The distribution
of Pd on the monolith CF carrier was investigated by a NOVA Nano SEM450
(JEOL, Japan) and a TEAM EDS (EDAX, USA). The Pd/CF sample was grounded
and tested by a JEM-2010 transmission electron microscope (JEOL, Japan) operated
at 200 kV to study the morphology of Pd nanoparticles. Pulse CO chemisorption of
the catalysts was employed to determine the number of active metal sites using an
ASAP2020c chemisorption analyzer (Micromeritics, USA). The surface electronic
state of Pd was examined using ESCALAB 250 X-ray photoelectron spectroscope
(Thermo Fisher Scientific, USA) with Al-Ka radiation of 1,486.6 eV as incident
beam. The analyzer pass energy was 20 eV and the energy step was 0.05 eV.
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Catalytic hydrogenation of CF supported catalysts and analytical procedure
The hydrogenation reactions were performed in a 500 mL batch reactor. 3 wt% of
PS-DHN solution was loaded into the reactor with four baffles. The Pd/CF catalysts
with weight ratio of 1 gcat/gPS were placed in a catalyst basket, which had three
vertical paddles on it and was fixed on the axle of the stirrer. The reactor was sealed
and flushed with N2 to remove air. After subsequently flushing with H2, the reactor
was heated to the reaction temperature with agitation (500 rpm). When the
temperature reached reaction temperature and the agitation was adjusted to
1,000 rpm, the reactor was charged with H2 to about 5.8 MPa to initiate
hydrogenation. After reaction, the reactor was cooled to room temperature and
the hydrogen in the reactor was purged, the catalyst basket could be easily removed
from the polymer solution without any other treatment. The concentration of
aromatic rings was determined by the absorption of UV-light at 261.5 nm. The
hydrogenation of PS during the reactor heating was negligible. The original and
hydrogenated PS solution were diluted with DHN and tested with a UV–Vis
spectrophotometer (UV7504C, China). The degree of hydrogenation (HD),
which was the conversion of the aromatic rings, was calculated by the aromatic
rings concentration of the initial and the hydrogenated PS solution (c0andc),
HD = 1 - c/c0.
Results and discussion
Preparation of monolithic CF support
The profile and the porous structure of the Pd/CF are shown in Fig. 1a, b. The
obtained TiO2 CF had a homogeneous open-pore structure, and the closed cell
almost did not exist. Three types of pores can be distinguished in the TiO2 CF after
calcination [30], the large pores (400–600 lm), which are also called ‘‘windows’’,
the triangular pores (10–40 lm) in the center of the struts resulting from the
pyrolysis of polymer template, and the macropores on the struts owning to the
partial sintering of the ceramic particles. Calcination enabled the TiO2 particles in
the green body to melt, migrate, react, and connect with each other. The mechanical
and structural properties of CF were adjusted by the degree of partial sintering, i.e.
Tcal. A series of CF samples were prepared at Tcal over the range of 0.543 and 0.730
Tm (designated as from CF-0.543 to CF-0.730), where Tm is the melting point of
TiO2, 2,093 K. The SEM image of CF-0.652 was shown in Fig. 1c, the mean length
and diameter of rod-like TiO2 particles was about 1.5 lm and 0.6 lm, respectively,
and the formation of necks between TiO2 touching particles could be clearly
observed. The irregularly shaped macropores on the struts were about 200–300 nm.
The SEM images of other CF samples are shown in Fig. S1 in the Supplementary
Information. The loose agglomerations of TiO2 powders were observed in CF-0
(without calcination), which did not have a certain shape. When the Tcal was
elevated to 0.609 Tm, the agglomerations of TiO2 powders transformed to rod-like
particles with the mean length of 1.0 lm and diameter of 0.3 lm, and the neck
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between touching particles occurred. Further elevation of Tcal from 0.652 to 0.730
Tm caused further surface diffusion and sublimation-condensation of TiO2, resulting
in a compact and sleeked strut. The XRD patterns of CFs calcined at various
temperatures (Fig. S2, Supplementary Information) showed that TiO2 underwent an
anatase–rutile phase transformation when the Tcal was elevated to 0.565 Tm, since
the characteristic peaks for rutile emerged at 2h = 27.5�, 36.2�, 39.3�, 41.3�, and
44.2�. When Tcal was higher than 0.609 Tm, the anatase totally transferred to the
rutile.
The mechanical properties of CF were tested and are shown in Fig. 2. Fig. 2
shows that the compressive strength of the CF was very poor when Tcal was lower
than 0.565 Tm. The compressive strength increased from 0.50 to 2.06 MPa over the
range of Tcal from 0.587 to 0.652 Tm. The formation of necks between TiO2
touching particles could enhance particle bonding and improve the strength of
porous ceramics, which was consistent with the observation in SEM. Afterwards,
the further increase of Tcal from 0.652 to 0.730 Tm, the compressive strength
increased from 2.06 to 2.27 MPa.
The specific surface area and the pore volume of CFs calcined at Tcals over the
range of 0.609 to 0.730 Tm were shown in Table 1. The surface area decreased when
elevating the Tcal. The CF-0.652 showed a typical II type isotherm. A slight increase
of N2 adsorption quantity was observed when p/p0 was from 0 to 0.20, and the
adsorbed quantity of N2 almost remained flat while p/p0 increased from 0.20 to 0.70.
N2 adsorption quantity began to increase obviously when p/p0 was higher than 0.8.
Hence, the CF-0.652 had no micro- and mesopores, revealing that the surface areas
came from the external surface. When the Tcal was higher than 0.696 Tm, the struts
of CF were almost sintered and the specific surface areas were very limited. The
macropores on the struts can be measured from SEM images (Fig. S1,
Supplementary Information) and listed in Table 1.
Therefore, the mechanical property and surface structure of CF could be adjusted
by varying the Tcal. The Tcal should be high enough to obtain good mechanical
strength and prevent the catalysts from attrition and being broken. On the other
hand, the Tcal had to be suitable to prevent densification in order to provide
sufficient surface area as catalyst carrier. Considering the mechanical strength and
the surface morphology of CF, CF-0.652 was utilized as catalyst support for PS
hydrogenation.
Fig. 1 The profile and SEM images of CF-0.652. a Profile of CF-0.652; b Porous structure of CF-0.652;c Surface morphology of CF-0.652
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Pd supported monolithic CF catalysts
To obtain an appropriate dispersion and grain size of Pd, the loadings of Pd should
be optimized for the CF support with certain surface area and morphology. In this
work, Pd/CF catalysts with various Pd loadings of 0.06–0.45 wt% were prepared.
The SEM images of Pd/CF (Fig. S3, Supplementary Information) showed that the
Pd nanoparticles deposited on the geometric surface of the CF supports. At low Pd
loading of 0.06 wt%, only a few Pd clusters could be seen on the surface. When the
Pd loading increased, more and larger Pd agglomerates on the CF surface. The
SEM-EDX analysis (Fig. S4, Supplementary Information) showed that the Pd
particles dispersed uniformly on the outer wall and cross section of the struts, which
was probably attributed to the macro-porous structure of the CF-0.652 struts.
Fig. 2 Effects of Tcal on compressive strength of CF
Table 1 Summary of characterization results of CF support calcined at different Tcal
Sample BET surface
areaa (m2/g)
Pore volume a
(cm3/g)
Diameter of macropore
on the strutsb (lm)
CF-0.609 16.35 0.022 0.3–0.5
CF-0.652 14.47 0.012 0.2–0.3
CF-0.696 4.42 0.002 0.1–0.2
CF-0.730 0.74 nd –
nd means the value could not be detected using N2 adsorption–desorption method, – means the pore on
struts did not exista BET surface area, pore volume, and mesopore diameter are calculated using BET and BJH method,
respectively, according to the N2 adsorption–desorption isothermsb Macropore size on the struts of CF are measured by SEM images
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The TEM image of 0.06–0.22 wt% Pd/CF-0.652 in Fig. 3 confirmed that no
micro- or mesopores could be observed in the Pd/CF-0.652 catalysts, and the Pd
grains only deposit on the external surface of TiO2 particles. At low Pd loading
(0.06 wt%), the Pd grain dispersed as semi-sphere particles with the Pd diameters
within the range of 4.5–5.5 nm. At higher Pd loading (0.22 wt%), the concentration
of Pd on the TiO2 CF surface greatly increased and composite a thin layer with
thickness of 5.5 nm. We can also imply that the thickness of the layer would
increase when the Pd loading further increased to 0.45 wt%.
The XPS spectra of 0.06 and 0.22 wt% Pd/CF-0.652 catalyst are presented in
Fig. 4, which shows that the binding energy (BE) range corresponds to the Pd 3d5/2
and Pd 3d3/2 doublets. In both samples, the obvious XPS signals at about 335.0 and
340.4 eV were observed, which means that most of Pd element existed as metallic
Pd(0). The BE of Pd 3d5/2 of 0.06 wt% Pd/CF-0.652 was 335.3 eV, which was
0.3 eV higher than that of 0.22 wt% Pd/CF-0.652 (335.0 eV). The higher shift of
BE might be contributed to the interaction between Pd and TiO2 surface. The signal
at about 336.2 and 341.5 eV were a consequence of the presence of PdOx on the
surface of Pd, which was probably due to oxidation in the air.
The number of active Pd sites, NPd (defined as Pd sites per gram of catalyst), and
the dispersion of Pd, DPd (defined as the fraction of surface Pd atoms in the whole
Pd atoms), on the catalysts with Pd loadings of 0.06, 0.12, 0.22 and 0.45 wt% were
calculated from the adsorbed volume of CO per gram of catalyst, VCO, measured by
pulse CO chemisorption, as shown in Eqs. 1–3.
SPd ¼RavVCONAAPd
Vm
ð1Þ
NPd ¼RavVCONA
Vm
ð2Þ
DPd ¼RavVCOMPd
VmwPd
� 100% ð3Þ
Here, Rav is the average chemisorption stoichiometry (=1, i.e. average number of
Pd atoms linked to one probe molecule CO); NA is Avogadro’s number
(6.02 9 1023 1/mol); APd is the specific surface area of Pd atom (0.0787 nm2),
Vm is the molar volume (22,414 cm3/mol); MPd is the mole weight of Pd (106.4 g/
mol). The VCO, SPd, NPd, and DPd are listed in Table 2. The active surface area of
Pd/CF-0.652 increased over 36 %, from 0.098 to 0.134 m2/g, when the Pd loading
increased 2 times from 0.06 to 0.12 wt%. When the Pd loading was 0.22 and
0.45 wt%, the SPd was only 0.148 and 0.168 m2/g, respectively. As the Pd loading
increased, the dispersion of Pd on the Pd/CF significantly decreased.
Catalytic performance of monolithic CF supported Pd catalysts
PS hydrogenation performances over Pd/CFs with various Pd loadings ranged from
0.06–0.45 wt% were carried out under following conditions. The reaction temper-
ature was 453 K, initial H2 pressure was 5.8 MPa, agitating rate was 1,000 rpm,
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catalyst concentration was 1.00 gcat/gPS, and PS concentration was 3 wt% with
DHN as solvent. The external diffusion of PS coils was determined to be negligible
in the earlier test when conducting the experiments at an agitation rate of 1,000 rpm.
Fig. 3 TEM images of Pd/CF-0.652 with various Pd loadings. a, b 0.06 wt% Pd/CF-0.652; c,d 0.22 wt% Pd/CF-0.652
(a)
(b)
Fig. 4 The peak-fit analysis of the Pd 3d XPS spectrum for a 0.06 wt% Pd/CF-0.652; b 0.22 wt% Pd/CF-0.652; Solid lines are experimental data, and dashed lines represent curve fitted data
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123
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Reac Kinet Mech Cat
123
The relationship between HD and reaction time (t) was recorded to investigate the
catalytic activities of catalysts, and shown in Fig. 5.
The initial rates of PS hydrogenation, r0, could be calculated and listed in
Table 3. The initial rates, r0, for 0.06, 0.12, 0.22, and 0.45 wt% Pd/CF-0.652 were
3.12, 4.76, 5.83, and 7.34 9 10-6 mol/L s. A power law rate equation was used to
fit the experimental data of PS hydrogenation over the Pd/CF-0.652 with various Pd
loadings, and the lines in Fig. 5 are the model values. The fitting method can be
referred to in Han et al’s paper [10]. The non-linear fitting results showed that the
reaction order with respect to the concentration of aromatic rings was close to 1
(0.94–1.06), the reaction orders with respect to the concentration of hydrogen was
close to 0 (10-6–10-5). The apparent reaction constant k0 was calculated and shown
in Table 3. When the Pd loading increased from 0.06 to 0.12 wt%, the k0 increased
from 0.43 9 10-6 to 0.67 9 10-6 L/g s. As the Pd loading increased to 0.22 and
0.45 wt%, the k0 was 0.84 9 10-6 and 1.09 9 10-6 L/g s, respectively. The
number of Pd sites participated in PS hydrogenation could be calculated as mcatNPd
and shown in Table 3, where mcat was the weight of catalyst. Combining the results
of mcatNPd, r0, and k0 for 0.06 and 0.45 wt% Pd/CF-0.652, it could be seen that r0
and k0
increased about 2.36 and 2.53 times, respectively, when the number of Pd
sites participated in the reaction increase about 1.73 times. The Pd sites seem to be
more active when the catalyst had higher Pd loading (0.22 and 0.45 wt%).
The hydrogenation of ethyl benzene (EB) over 0.06 and 0.45 wt% Pd/CF-0.652
were conducted for comparison, where the experimental conditions were the same
as those for PS hydrogenation. It was found that the r0 for EB hydrogenation over
0.06 and 0.45 wt% Pd/CF-0.652 was 1.38 9 10-5 and 6.62 9 10-5 mol/L s,
respectively. The r0 for EB hydrogenation increased about 4.79 times, when the
number of Pd sites increased about 1.73 times. The difference in intrinsic
hydrogenation activity for aromatic rings could be attributed to the metal-support
interaction effect [31, 32] between Pd and TiO2 CF at low Pd loading (0.06 wt%).
In the case of PS hydrogenation, our previous study [10] found that the catalytic
performance in PS hydrogenation was not only determined by the intrinsic activity
of active metal but also influenced by the adsorption behavior of PS coils on the
catalysts. Ness [17] found that during the adsorption of PS coils the fraction of
styrene units present as trains could be influenced by the size of polymer coils
relative to the size of a metal crystallite. The time to hydrogenate a PS coil consisted
of the time for the hydrogenation of the aromatic rings and the time for the
conformational arrangement of PS coils due to the different affinity of saturated and
unsaturated unit on the active sites. In this study, the CF catalysts with various Pd
loading had same macro-porous ceramic structure, therefore the polymer chains
adsorbed on the surface of CF catalysts in the same blocky manner, which was
described by Scheutjens and Fleer [33]. Hence, for Pd/CFs with various Pd loadings,
the different activities in PS hydrogenation are related to the different intrinsic
hydrogenation activities for the aromatic rings and the adsorption behavior of PS
coils on different size of Pd nanoparticles.
Considering the dispersion of Pd and catalytic performances in PS hydrogenation,
the 0.06 wt% Pd/CF-0.652 was an economic catalyst for PS hydrogenation. In the
catalyst reuse experiments, the PS hydrogenation activity of 0.06 wt% Pd/CF
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decreased slightly after the five times reuse. The r0 for the catalysts reused five times
was 2.89 9 10-6 mol/L s, which was slightly lower than the r0 for fresh catalyst
(3.12 9 10-6 mol/L s). The SPd of 0.06 wt% Pd/CF-0.652 reused once decreased
slightly from 0.098 to 0.094 m2/gcat, and the slight deactivation during the reaction
almost had no effect on the form of experimental curve and the calculation of reaction
constant. The catalyst had good mechanical strength and thermal stability. After
heating the CF catalyst to 600 �C and cooling in air, no cracking could be found on the
monolith. In order to get high rate of PS hydrogenation, the 0.06 wt% Pd/CF-0.652
could be packed in the fixed bed or rotating bed reactor, where the weight ratio of
catalyst were much higher than the batch operation.
Internal diffusion behavior of the PS coils in CF catalyst
The internal diffusion of the PS coils in the macropores of the struts could to be
evaluated using the Weisz modulus (U) [22, 34, 35], expressed in Eq. 4.
Fig. 5 PS hydrogenation activities over Pd/CF catalysts with various Pd loading ranged from 0.06 to0.45 wt%. (453 K, 1.00 gcat/gPS, 3 wt% PS-DHN, 5.8 MPa initial H2 pressure, 1,000 rpm agitation rate.)
Table 3 Initial rate and apparent reaction constant for PS hydrogenation over Pd/CF catalysts with
different Pd loadings
Catalyst r0a 9 10-6
(mol/L s)
k0
9 106
(L/g s)mcatNPd � 1018
sites
0.06 wt% Pd/CF-0.652 3.12 0.43 4.46
0.12 wt% Pd/CF-0.652 4.76 0.67 6.08
0.22 wt% Pd/CF-0.652 5.83 0.84 6.77
0.45 wt% Pd/CF-0.652 7.34 1.09 7.70
a Reaction conditions: 453 K, 1.00 gcat/gPS, 3 wt% PS-DHN, 5.8 MPa initial H2 pressure, 1,000 rpm
agitation rate
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U ¼ ðnþ 1Þl2cRv
2DeffCs
ð4Þ
Here, n is the order of the reaction with respect to aromatic rings, lc is the
thickness of the CF struts wall [34, 35], Rv is reaction rate of aromatic rings per
geometric catalyst volume, Deff is the effective diffusion coefficient of PS coils in
the macropores of the struts, Cs is the concentration of aromatic rings on the catalyst
surface. Internal diffusion in PS hydrogenation catalyzed by 0.45 wt% Pd/CF-0.652
was evaluated as an example. The above fitting results of the experimental data
showed that n was close to 1. As measured in the cross section SEM image of
0.45 wt% Pd/CF-0.652, lc was 40 lm. Since external diffusion was precluded, the
surface concentration of aromatic rings, Cs, could be determined using the mole
concentration of aromatic rings in the bulk liquid at initial conditions, i.e.,
3.24 9 10-4 mol/mL. Some structural properties of 0.45 wt% Pd/CF-0.652 was
listed in Table 4, which are required to calculate the Weisz modulus.
The Rv on the CF catalyst could be calculated via r0, in Eq. 5.
Rv ¼ r0ðmcat
qcat
Þ�1 ð5Þ
Gere r0 and mcat are defined in the catalytic performance section, i.e. Table 3.
mcat was 3.60 g and the qcat was 0.63 mL/g in this study. Therefore, Rv was
calculated to be 1.73 9 10-7 mol/(mLcat s).
In order to calculate a value of U, the effective diffusivity of the PS coils in the
pores of the catalyst, Deff , needed to be estimated. The Deff values could be
calculated by
Deff ¼Des
ð6Þ
Here, D is the diffusion coefficient of PS coils in the pores, and s is the catalyst
tortuosity which is typically described as the ratio of the real length of the
connecting pore channels to the thickness of the porous medium [37]. In order to
model the three-dimensional flow in the ceramic foams, Du Plessis and Masliyah
[38] suggested modeling of the porous structure by a representative unit cell, which
had three short square duct sections that oriented mutually perpendicular. Smit and
Du Plessis [36] suggested that s could be calculated as Eq. 7, from which s for the
0.45 wt% Pd/CF-0.652 was calculated to be 3.89.
s ¼ 2þ 2 cos� 4p
3þ 1
3cos�1ð2e� 1Þ
�ð7Þ
The hydrodynamic radii of PS coils at reaction temperature, RPS, can be
evaluated as Eq. 8.
RPS ¼kBT
6pgD0
ð8Þ
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Here kB is the Boltzmann constant, g is the viscosity of solvent DHN, T is the
absolute temperature, and D0 represents the infinite dilution diffusion coefficient of
PS coils in DHN. L. B. Dong et al. [24] tested the diffusion coefficient of
monodispersed PS ( �Mn = 308 kg/mol, PDI = �Mw= �Mn = 1.04) in DHN using the
dynamic light scattering method with temperature ranging from 297 to 392 K. The
activation energy of PS diffusion was reported to be 12.7 kJ/mol. In this research, the
D0 of monodispersed PS at 453 K could be extrapolated to be 5.05 9 10-7 cm2/s via
the linear relationship between ln(D0) and 1/T. Therefore, the D0 of commercial PS
( �Mw = 279 kg/mol) used could be simplified and scaled from the D0 value for
monodispersed PS using the relationship D0 / ðMwÞ�0:5, based on the bead-spring
theory of a polymer diffusing in a dilute solution. The D0 of commercial PS in this
study could be obtained to be 5.30 9 10-7 cm2/s. Viscosities of the DHN at 304 and
453 K were 1.9 and 0.3 mPa s, as predicted from the study of R. E. Whittier et al. [39].
We tested the average radius of commercial PS in this study at 304 K using DLS
method, and the average radius of commercial PS was about 10.2 nm. Therefore,
according to the Stokes–Einstein equation (Eq. 8), the average radius of commercial
PS coils was calculated to be 13.77 nm at 453 K in the hydrogenation system. The
average radius of the macropores on the struts was 0.15 lm. Therefore, the ratio of the
PS coils radius to the average radius of the macropores on the struts, k ¼ RPS=RCF,
was 0.092 in the catalytic system. For k\ 0.5, the value of D could be predicted using
the Renkin equation [40] (Eq. 9).
D ¼ Dbð1� kÞ2½1� 2:1044kþ 2:0888k3 � 0:948k5� ð9Þ
Here, the Db is the diffusion coefficient of PS coils in the bulk solution of
catalytic system. The value of Db at 453 K in the 3 wt% PS/DHN solution could be
predicted to be 1.50 9 10-6 cm2/s according to the linear dependence relationship
of diffusion coefficient Db on polymer concentration [24]. D was calculated to be
1.00 9 10-6 cm2/s. Therefore, according to Eq. 6, Deff could be obtained and the
value was 2.18 9 10-7 cm2/s, and the Weisz modulus U for the PS hydrogenation
catalyzed by 0.45 wt% Pd/CFs-0.652 was computed to be 0.039. Similarly, the Ufor 0.06, 0.12, and 0.22 wt% Pd/CFs-0.652 catalysts were 0.016, 0.025, and 0.030.
These results inferred that in the Pd/CF catalytic system, U for PS hydrogenation
were less than 0.30, which ensured the elimination of significant internal diffusion
limitations of PS coils in the Pd/CF catalysts. The existence of macropores in the CF
struts results in the elimination of the pore diffusion resistance of PS coils.
Table 4 Structural properties of 0.45 wt% Pd/CF-0.652
Catalyst 0.45 wt% Pd/CF-0.652
Catalyst porosity, e (%) 0.85
Bulk density of the catalyst, qcat (g/mL) 0.63
Average thickness of the CF struts wall, lc (lm) 40
Catalyst tortuosity, s 3.89 a
a Catalyst tortuosity was calculated according to the study of Smit and Du Plessis [36]
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Therefore, the utilization of Pd/CF catalyst could not only enhance the pore
diffusion of PS coils but also facilitate catalyst separation, which might provide a
promising system for industrial fixed bed or rotating bed reactor.
Conclusions
A series of TiO2 CF supported Pd catalysts were prepared using the synthetic
template replica method. Considering the mechanical strength and the surface area
of CF, CF-0.652 was utilized as catalyst support for PS hydrogenation. Monolithic
Pd/CFs with different Pd loadings were prepared by an equilibrium impregnation
method and utilized in PS hydrogenation. The Pd nanoparticles were located on the
surface of the TiO2 ceramic base. As the Pd loading increased, the dispersion of Pd
on the Pd/CF significantly decreased. The activity of Pd for PS hydrogenation
slightly increased when the Pd loading increased from 0.06 to 0.45 wt%, which
could be explained by the existence of interaction between Pd atoms and TiO2 CF at
low metal loading, resulting in a decreased hydrogenation activity for aromatic
rings. The Weisz modulus U for PS hydrogenation over all Pd/CF catalysts were
calculated to be less than 0.30. These results showed that the Pd/CF-0.652 was an
effective catalyst for the heterogeneous catalytic reactions, especially for the PS
hydrogenation usually controlled by internal diffusion.
Acknowledgments This research was supported by STCSM Non-governmental International Science
and Technology Cooperation Program (No. 10520706000), Specialized Research Fund for the Doctoral
Program of Higher Education of China (No. 20110074110012), and State Key Laboratory of Chemical
Engineering Open Fund (No. SKL-ChE-09C07). The author thanks for the support from National College
Students Innovative Entrepreneurial Training Plan Program (No.091025105 and No.1210251101) and the
assistance of Meng Wang and Lei Lu.
References
1. McManus N, Rempel G (1995) Chemical modification of polymers: catalytic hydrogenation and
related reactions. J Macromol Sci Part C Polym Rev 35:239–285
2. Durham JE, Kovacic P (1977) Polymerization of aromatic nuclei. XVII. Catalytic hydrogenation of
poly(p-phenylene). J Polym Sci Polym Chem Ed 15:2701–2706
3. Bruner H, Bailar JC Jr (1973) Heterogeneous selective catalysis of the hydrogenation of olefins by
polymeric palladium(II) and platinum(II) complexes. Inorg Chem 12:1465–1470
4. Hinchiranan N, Charmondusit K, Prasassarakich P, Rempel GL (2006) Hydrogenation of synthetic
cis-1, 4-polyisoprene and natural rubber catalyzed by [Ir (COD) py (PCy3)] PF6. J Appl Polym Sci
100:4219–4233
5. Hinchiranan N, Prasassarakich P, Rempel G (2006) Hydrogenation of natural rubber in the presence
of OsHCL (CO)(O2)(PCY3) 2: kinetics and mechanism. J Appl Polym Sci 100:4499–4514
6. Mahittikul A, Prasassarakich P, Rempel G (2006) Hydrogenation of natural rubber latex in the
presence of OsHCl (CO)(O2)(PCy3) 2. J Appl Polym Sci 100:640–655
7. Wang H, Pan Q, Rempel GL (2012) Organic solvent-free catalytic hydrogenation of diene-based
polymer nanoparticles in latex form: part I. Preparation of nano-substrate. J Polym Sci Part A Polym
Chem 50:4656–4665
8. Wang H, Pan Q, Rempel GL (2012) Diene-based polymer nanoparticles: preparation and direct
catalytic latex hydrogenation. J Polym Sci Part A Polym Chem 50:2098–2110
Reac Kinet Mech Cat
123
9. Wang H, Yang L, Scott S, Pan Q, Rempel GL (2012) Organic solvent-free catalytic hydrogenation of
diene-based polymer nanoparticles in latex form. Part II. Kinetic analysis and mechanistic study.
J Polym Sci Part A Polym Chem 50:4612–4627
10. Han K-Y, Zuo H-R, Zhu Z-W, Cao G-P, Lu C, Wang Y-H (2013) High performance of palladium
nanoparticles supported on carbon nanotubes for the hydrogenation of commercial polystyrene. Ind
Eng Chem Res 52:17750–17759
11. Taylor G, Davison S (1968) Glass temperature of hydrogenated polystyrene. J Polym Sci Part B
Polym Lett 6:699–705
12. Hucul DA, Hahn SF (2000) Catalytic hydrogenation of polystyrene. Adv Mater 12:1855–1858
13. Bates FS, Fredrickson GH, Hucul D, Hahn SF (2001) PCHE-based pentablock copolymers: evolution
of a new plastic. AIChE J 47:762–765
14. Chang JR, Huang SM (1998) Pd/Al2O3 catalysts for selective hydrogenation of polystyrene-block-
polybutadiene-block-polystyrene thermoplastic elastomers. Ind Eng Chem Res 37:1220–1227
15. Xu D, Carbonell RG, Kiserow DJ, Roberts GW (2003) Kinetic and transport processes in the
heterogeneous catalytic hydrogenation of polystyrene. Ind Eng Chem Res 42:3509–3515
16. Xu JJ, Bates FS (2003) Synthesis and thermal properties of hydrogenated poly (styrene-co-1,
1-diphenylethylene) copolymers. Macromolecules 36:5432–5434
17. Ness JS, Brodil JC, Bates FS, Hahn SF, Hucul DA, Hillmyer MA (2002) Molecular weight effects in
the hydrogenation of model polystyrenes using platinum supported on wide-pore silica. Macro-
molecules 35:602–609
18. Rosedale J, Bates F (1988) Heterogeneous catalytic hydrogenation of poly (vinylethylene). J Am
Chem Soc 110:3542–3545
19. Almusaiteer KA (2012) Effect of supports on the catalytic hydrogenation of polystyrene. Top Catal
55:498–504
20. Gehlsen MD, Weimann PA, Bates FS, Harville S, Mays JW, Wignall GD (1995) Synthesis and
characterization of poly(vinylcyclohexane) derivatives. J Polym Sci Part B Polym Phys
33:1527–1536
21. Bussard A, Dooley KM (2008) Polymer hydrogenation in pulsed flow systems with extrusion. AIChE
J 54:1064–1072
22. Dong LB, Turgman-Cohen S, Roberts GW, Kiserow DJ (2010) Effect of polymer size on hetero-
geneous catalytic polystyrene hydrogenation. Ind Eng Chem Res 49:11280–11286
23. Xu D, Carbonell RG, Roberts GW, Kiserow DJ (2005) Phase equilibrium for the hydrogenation of
polystyrene in CO2-swollen solvents. J Supercrit Fluids 34:1–9
24. Dong LB, Carbonell RG, Roberts GW, Kiserow DJ (2009) Determination of polystyrene-carbon
dioxide-decahydronaphthalene solution properties by high pressure dynamic light scattering. Polymer
50:5728–5732
25. Zhang L-L, Wang J-X, Xiang Y, Zeng X-F, Chen J-F (2011) Absorption of carbon dioxide with ionic
liquid in a rotating packed bed contactor: mass transfer study. Ind Eng Chem Res 50:6957–6964
26. Luo Y, Chu G-W, Zou H-K, Wang F, Xiang Y, Shao L, Chen J-F (2012) Mass transfer studies in a
rotating packed bed with novel rotors: chemisorption of CO2. Ind Eng Chem Res 51:9164–9172
27. Chen Y-S (2011) Correlations of mass transfer coefficients in a rotating packed bed. Ind Eng Chem
Res 50:1778–1785
28. Waghmare YG, Bussard AG, Forest RV, Knopf FC, Dooley KM (2010) Partial hydrogenation of
soybean oil in a piston oscillating monolith reactor. Ind Eng Chem Res 49:6323–6331
29. Studart AR, Gonzenbach UT, Tervoort E, Gauckler LJ (2006) Processing routes to macroporous
ceramics: a review. J Am Ceram Soc 89:1771–1789
30. Haugen H, Will J, Kohler A, Hopfner U, Aigner J, Wintermantel E (2004) Ceramic TiO2-foams:
characterisation of a potential scaffold. J Eur Ceram Soc 24:661–668
31. Li Y, Xu B, Fan Y, Feng N, Qiu A, He JMJ, Yang H, Chen Y (2004) The effect of titania polymorph
on the strong metal-support interaction of Pd/TiO2 catalysts and their application in the liquid phase
selective hydrogenation of long chain alkadienes. J Mol Catal A Chem 216:107–114
32. Tapin B, Epron F, Especel C, Ly BK, Pinel C, Besson M (2013) Study of monometallic Pd/TiO2
catalysts for the hydrogenation of succinic acid in aqueous phase. ACS Catal 3:2327–2335
33. Scheutjens J, Fleer G (1980) Statistical theory of the adsorption of interacting chain molecules. 2.
Train, loop, and tail size distribution. J Phys Chem 84:178–190
34. Zhu J, Li M, Lu M, Zhu J (2013) Effect of structural properties on catalytic performance in citral
selective hydrogenation over carbon–titania composite supported Pd catalyst. Catal Sci Technol
3:737–744
Reac Kinet Mech Cat
123
35. Boldrini DE, Sanchez MJF, Tonetto GM, Damiani DE (2012) Monolithic stirrer reactor: performance
in the partial hydrogenation of sunflower oil. Ind Eng Chem Res 51:12222–12232
36. Smit G, Du Plessis J (1999) Modelling of non-Newtonian purely viscous flow through isotropic high
porosity synthetic foams. Chem Eng Sci 54:645–654
37. Habisreuther P, Djordjevic N, Zarzalis N (2009) Statistical distribution of residence time and tor-
tuosity of flow through open-cell foams. Chem Eng Sci 64:4943–4954
38. Du Plessis JP, Masliyah JH (1988) Mathematical modelling of flow through consolidated isotropic
porous media. Transp Porous Media 3:145–161
39. Whittier RE, Xu D, van Zanten JH, Kiserow DJ, Roberts GW (2006) Viscosity of polystyrene
solutions expanded with carbon dioxide. J Appl Polym Sci 99:540–549
40. Renkin EM (1954) Filtration, diffusion, and molecular sieving through porous cellulose membranes.
J Gen Physiol 38:225–243
Reac Kinet Mech Cat
123