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: gpcao@ecust.edu.cn

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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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.

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