High Performance of Palladium Nanoparticles Supported on CarbonNanotubes for the Hydrogenation of Commercial PolystyreneKai-Yue Han,† Hao-Ran Zuo,† Zhen-Wei Zhu,† Gui-Ping Cao,*,† Chong Lu,‡ and Yan-Hua Wang‡

†UNILAB, State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237,China‡School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China

*S Supporting Information

ABSTRACT: The carbon nanotube (CNT) supported palladium catalysts were synthesized by the impregnation method andapplied in the hydrogenation of commercial polystyrene (PS) for the first time. The Pd/CNT catalysts displayed a quite excellenthydrogenation activity compared with those of traditional catalysts, e.g., Pd/AC or Pd/BaSO4, and allowed the reaction to becarried out under milder reaction conditions. The physical and chemical properties of Pd/CNTs were characterized byinductively coupled plasma-atomic emission spectrometry, N2 physisorption, transmission electron microscopy, X-ray diffraction,CO chemisorption, and kinetics analysis. The catalyst characterization results showed that the active metal deposited on theexternal surface of CNTs with good dispersion. Kinetics analysis showed that the activation energy of Pd/CNTs was similar tocomparison catalysts, while the TOF of Pd/CNTs (0.102 s−1) was much higher. The high external surface area and theinteraction between polymer and CNTs could be the reason for the high performance of Pd/CNTs.

1. INTRODUCTION

Hydrogenation is an economic process to improve thechemical, mechanical, and thermal properties of unsaturatedpolymer.1−8 Usually, the hydrogenation of unsaturatedpolymers could be carried out over either homogeneous orheterogeneous catalysts.6,9 Homogeneous hydrogena-tion3−6,8,10−13 adopts ruthenium (Ru), rhodium (Rh), osmium(Os), and palladium (Pd) complexes as catalysts, which allowsthe hydrogenation of diene-based polymer to be carried out atmilder operating conditions and without diffusion problems.The recovery of homogeneous catalysts is usually difficult andexpensive, and the residual catalyst in the product polymerscould result in polymer degradation. However, the heteroge-neous catalysts, involving supported metal as active sites, couldbe easily filtered out from the polymer solution after thereaction, which could avoid the metal contamination of thepolymer products.The hydrogenation of commercial polystyrene (PS) is a

representative example to produce hydrogenated polystyrene(HPS), also known as polycyclohexylethylene (PCHE), apolymer that shows greatly improved heat and UV resistancecompared with PS.14 Generally, the PS hydrogenation is carriedout over heterogeneous supported group VIII metal catalysts,such as platinum (Pt), Pd, Ru, and Rh supported on charcoal,15

SiO2,14,16 CaCO3,

17,18 Al2O3,18,19 BaSO4,

7,18,20 etc. However,the PS hydrogenation over traditional catalysts shows a veryslow reaction rate. High temperature, high catalyst concen-tration, and low PS concentration are usually employed in thehydrogenation, which would result in the depolymerization ofPS/PCHE, increment of catalyst separation cost, and waste ofenergy. Research is urgently needed to remove those seriousimpediments to industrialize the heterogeneous PS hydro-genation process.

It has been well-known that the characteristics of polymerhydrogenation is quite different from that of hydrogenation ofsmall molecules. In the PS hydrogenation, the unsaturated PScoils need to transport from the bulk liquid phase to theexternal surface of the catalyst particle, and then diffuse into thepores to access the active sites on the pore walls. Only a part ofthe aromatic rings on the PS chains could be adsorbed andcatalyzed at one time, and the partially hydrogenated PS coilsneed to conformationally rearrange the chains for many timesin order to get all the aromatic rings to be saturated.17,21 Theproduct PCHE coils need to desorb from the sites and diffuseout of the pores to the bulk liquid phase. Because of the largedimension of PS and PCHE macromolecules and the highviscosity of the solution, the mass transfer of PS and PCHEcoils in both the bulk of the solution and the pores of thecatalysts proved extremely challenging.20,21 The externaldiffusion of PS/PCHE coils could be enhanced by increasingthe agitation rate or space velocity. However, the enhancementof pore diffusion of PS/PCHE coils is still a big problem.Moreover, the adsorption and hydrogenation of aromatic ringson active metal sites are restrained from the steric hindrance ofa large size of PS coils.In order to enhance the pore diffusion of PS coils, several

approaches were usually proposed, including introduction ofsupercritical CO2 (Sc-CO2)

22−24 and optimal design of thecatalysts.14,21,25 G. W. Roberts and co-workers22 introduced Sc-CO2 to the PS hydrogenation system and found that theaddition of Sc-CO2 could effectively reduce the size of polymercoils and enable faster diffusion of the polymer molecules in the

Received: February 2, 2013Revised: November 8, 2013Accepted: November 17, 2013

Article

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pores of catalyst. However, the deactivation of catalyst oftenoccurred owing to the existence of CO which was formed byreverse water-gas-shift reaction in the presence of CO2 andH2.

26,27 Recent research of R. G. Carbonell28,29 found that theutilization of bimetallic catalysts contained Ni and Ru and thecomplete replacement of CO2 with propane could alleviate theCO poisoning of hydrogenation catalysts. F. S. Bates and co-workers21 used a platinum supported wide-pore (average poresizes between 300 and 400 nm) silica catalyst in PShydrogenation in order to facilitate the pore diffusion of PScoils. According to their internal mass transfer study, even PScoils with a number-average molecular weight of 276 kg/molcan access the active sites in the wide pores, as the PS coils hada gyration radius of around 42 nm. However, the initial reactionrate became much lower for the hydrogenation of PS with highmolecular weight, e.g., Mn > 190 kg/mol. The surface area ofwide-pore silica catalyst was about 15 m2/g, and the dispersionof Pt was about 12%. K. A. Almusaiteer18 found out 5 wt % Pd/BaSO4 with a surface area of 5 m2/g and 5 wt % Pd/CaCO3with a surface area of 8.3 m2/g exhibited better activities than 5wt % Pd/Al2O3 with a surface area of 340 m

2/g, which could beexplained by the inaccessibility of the PS coils into the micro-and mesopores of the Pd/Al2O3 catalysts.The above studies give a very important enlightenment for us

to design and optimize the catalyst structure for highermolecular weight PS hydrogenation. It seems that traditionalporous catalysts with high surface areas and small pore diametermight not be suitable for PS hydrogenation. Catalysts with highspecific surface area and abundant micro- or mesopores lead tobetter dispersion of active metal, but most of the active metalgrains deposit on the surface of internal pores, resulting in thedifficult access of PS coils to the active sites. On the other hand,the non-porous catalysts, such as CaCO3 and BaSO4 catalysts,possess active sites on the external surface and could avoid porediffusion, but the surface area is limited and the dispersion ofactive metal on the carrier is poor. Therefore, the aim of ourstudy is to explore a conveniently prepared catalyst for PShydrogenation, which could not only eliminate inner porediffusion of PS coils but also have a high external surface area atthe same time.Carbon nanotubes (CNTs) are one-dimensional tube-like

carbon materials where the carbon layer is rolled upcylindrically with diameters in the nanoscale range. CNTshave been considered to be promising supports forheterogeneous catalysts, such as selective hydrogenation30−32

and electro-oxidation reactions.33−35 Due to the one-dimen-sional tubular structures, CNTs have a surprisingly high aspectratio, little microporosity, and a very large external surface area.These specific properties of CNTs may shed interesting lighton the catalyst exploitation of PS hydrogenation, which hasnever been reported in the literature. In this work, we reportthat CNT supported Pd catalyst with Pd grains depositedhomogeneously on the external surface showed a much highercatalytic performance compared with Pd/AC and commercialPd/BaSO4 catalyst containing the same content of Pd. Pd/CNTs, Pd/AC, and Pd/BaSO4 were characterized byinductively coupled plasma-atomic emission spectrometry(ICP-AES), N2-physisorption, transmission electron micros-copy (TEM), X-ray diffraction (XRD), and CO chemisorptionto investigate the properties of the catalysts. Both character-ization of catalysts and further kinetics analysis were employedto understand the reason for the high performance of Pd/CNTs.

2. EXPERIMENTAL SECTION2.1. Materials. Carbon nanotubes (CNTs) having an inner

diameter of 5−10 nm and an outer diameter of 20−30 nm werepurchased from Chengdu Organic Chemicals Co. Ltd. TheCNTs were prepared by the chemical vapor deposition method(CVD) and highly purified with a purity of 95%. The 4.8 wt %of impurities was the amorphous carbon and 0.2 wt % was Niand Cl. The inner diameter and outer diameter of CNTs werealso measured according to the TEM image (Figure 1), and the

results were consistent with the numbers provided by thesupplier. The palladium precursor Pd(NO3)2·2H2O waspurchased from Jiuling Chemical Co., Ltd., with a Pd contentof 39.5 wt %. The activated carbon (AC) was purchased fromHuage Chemical Engineering Co., Ltd., and the content ofacidic oxygen-containing groups on the surface of AC was 1.56mmol/g tested by Bohem titration. Commercial 5 wt % Pd/BaSO4 catalyst was purchased from Xi’an Kaili CatalystChemical Co., Ltd.Commercial PS (GPPS-123) was presented by Shanghai

SECCO Petrochemical Co., Ltd., with a number average,weight average, and viscosity average molecular weight of 90,263, and 279 kg/mol, respectively, measured by Waters1515 gelpermeation chromatography (Waters Co., Ltd., USA) withtetrahydrofuran as the solvent at 35 °C. The solvent,decahydronaphthalene (DHN), was purchased from Sino-pharm Chemical Reagent Co., Ltd. The size distribution of PScoils in 3 wt % PS-DHN solution was characterized using alaser light scattering spectrometer (ALV-laser Vertriebsgesell-schaftm.b.H.) at 632.8 nm and a scattering angle of 90° with a22 mW He−Ne laser. High purity hydrogen (H2) was suppliedby Shanghai Zhongyuan Chemical Co., Ltd. All the reactantswere used without further treatment.

2.2. Preparation of Pd/CNTs Catalysts. Pd/CNTcatalysts were prepared by the wetness impregnationmethod.36−38 The typical procedure for preparing 5 wt %Pd/CNTs was as follows. 4.5 g of CNTs were immersed in 60 gof deionized water and dispersed for 2 h in an ultrasonic bath. A5 g portion of Pd(NO3)2 solution with a certain amount of Pdwas added to the suspension, stirred vigorously for 6 h, and leftstanding for 12 h. The color of supernatant solution convertedfrom brown to achromatic and transparent. Then, the CNTsadsorbing Pd(NO3)2 were filtered and dried at 100 °C for 2 h.Finally, the mixture was calcinated under a stream of N2 at 400°C for 4 h and reduced by H2 at 350 °C for 5 h (120 mL/min).The 5 wt % Pd/AC catalyst was synthesized via a similar

Figure 1. TEM image of CNT carrier.

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method except that the catalyst carrier was replaced by activecarbon.2.3. Hydrogenation Reaction and Measurement. The 3

wt % PS solution and catalyst with a concentration of 1.00 gcat/gPS were placed into a 500 mL autoclave with four verticalbaffles. Before being heated to the reaction temperature, thereactor was flushed with H2 to remove air. The reactor washeated to the reaction temperature, and then, the reaction wasinitiated by flushing H2 to 5.8 MPa and adjusting the agitationrate to 1000 rpm. During the hydrogenation, the consumptionof hydrogen was monitored and recorded until the assumptionof hydrogen leveled off. After the hydrogenation, the catalystwas removed from the hydrogenated polymer solution byfiltration. The samples of initial PS solution and hydrogenatedreaction mixture were taken and analyzed by UV−visspectrophotometer (UV7504C, China) at 261.5 nm todetermine the concentration of aromatic rings.20 Theconversion of the aromatic rings, also named the degree ofhydrogenation (HD), was calculated as HD = (1 − cA/cA,0) ×100%, in which the concentrations of aromatic rings at initialand time t were designated as cA,0 and cA, respectively.2.4. Characterization of the CNT Carriers and

Supported Catalysts. Nitrogen physisorption was adoptedto investigate the specific surface area and pore volume of theCNTs using an ASAP2010 static volumetric instrument(Micromeritics, USA). The samples were first degassed at300 °C for 3 h, and then, the adsorption−desorption isothermswere measured at −197 °C. The pore size distribution and porevolume were calculated using the BJH method from thedesorption isotherm, and the total pore volume was calculatedfrom the cumulative volume adsorbed at a relative pressure lessthan 0.98 to avoid the inaccuracy of the measurement at highrelative pressure. IRIS 1000 ICP-AES (Thermo Elemental,USA) was used to determine the Pd content of the catalyst. Aknown amount of the catalyst sample was first calcinated undera steam of air at 650 °C for 300 min to remove the carbon.Aqua regia was added to the remainder, and the mixture washeated until the remainder was fully dissolved. The solution wasdiluted to 50 mL in volumetric flasks and analyzed by IRIS1000. The XRD patterns of CNTs and Pd/CNTs wererecorded on a Rigaku D/MAX2550VB/PC diffractometer usingCu Kα radiation and a carbon monochromator. The dispersionof Pd particles was investigated by TEM and COchemisorption, respectively. The sample of TEM was preparedas follows. A 1 mg of catalyst sample was sonicated in 10 mL ofethanol for 20 min, and a drop of the above suspension wasplaced on a copper grid and dried in air for 2 h. Themicroscopic images of the copper grid were tested with a JEM-2010 electron microscope (JEOL, Japan) operated at 200 kV.CO chemisorption of the catalysts was employed to determinethe number of active metal sites using an ASAP2020cchemisorption analyzer (Micromeritics, USA). The procedureof CO chemisorption was as follows. A sample (about 0.100 g)was placed into a U-tube quartz, evacuated to 10−5 mmHg at350 °C for 60 min, and reduced by a flow of H2 at 300 °C for180 min. The sample was then evacuated at 300 °C to desorbH2. After cooling to 40 °C, CO was introduced to the systemand the isotherm was measured with pressure ranging from15.0 to 450.0 mmHg. The isotherm of CO adsorption over thecatalyst was tested. The chemisorbed adsorbed volumes pergram of catalyst, VCO, could be determined by extrapolating tozero pressure of the linear part of the adsorption isotherm. Thenumber of active Pd sites NPd (defined as sites per gram of

catalyst) and the average metal particle size dPd/CO werecalculated from VCO.

3. RESULTS3.1. Catalytic Performance of Pd/CNTs Catalysts in PS

Hydrogenation. First, a series of PS hydrogenations wereperformed over catalysts whose sizes were smaller than 200mesh in the batch reactor with agitation rate ranging from 800to 1200 rpm and initial H2 pressure ranging from 4.0 to 6.0MPa. The conversion of PS had no significant change when theinitial H2 pressure was higher than 5.5 MPa and the agitationrate was higher than 1000 rpm. The external diffusion could beprecluded, and the surface of catalysts was saturated by H2under those conditions. Therefore, PS hydrogenation perform-ances over 5 wt % Pd/CNTs, 5 wt % Pd/AC, and commercial 5wt % Pd/BaSO4 were performed under the same conditions,which had a reaction temperature of 150 °C, initial H2 pressureof 5.8 MPa, agitating rate of 1000 rpm, catalyst concentration of1.00 gcat/gPS, and PS concentration of 3 wt %, using DHN asthe solvent. Since PCHE is the only product of PShydrogenation, the relationship between HD and reactiontime (t) was tested during the PS hydrogenation process toevaluate the catalytic activities of catalysts. In order to find outwhether the CNT carrier was catalytically active in PShydrogenation, the experiment using the CNT carrier wascarried out. No HD of PS could be detected in the absence ofPd, suggesting that Pd was the only active site for PShydrogenation. As shown in Figure 2, 5 wt % Pd/CNTs

exhibited extremely higher PS hydrogenation activity comparedwith 5 wt % Pd/AC and 5 wt % Pd/BaSO4 catalysts whichcontained the same content of Pd. 99.83% of HD was achievedover 5 wt % Pd/CNTs within 40 min, while 78.05% of HD wasachieved over 5 wt % Pd/BaSO4 and only 18.60% of HD wasachieved over 5 wt % Pd/AC within 200 min. The amount ofreacted aromatic rings instantaneously was calculated via theamount of hydrogen consumption. The initial rate ofhydrogenation (r0) was obtained by dividing the amount ofreacted aromatic rings instantaneously by reaction time, asshown in Table 1. The initial rates, r0, for 5 wt % Pd/CNTs, 5wt % Pd/BaSO4, and 5 wt % Pd/AC were 2.745 × 10−4, 0.331× 10−4, and 0.048 × 10−4 mol/(L·s), respectively. It should benoted that the initial PS hydrogenation rate over 5 wt % Pd/CNTs was almost 8 times higher than that over 5 wt % Pd/BaSO4, and was nearly 50 times larger than 5 wt % Pd/AC.Figure 2 also showed that the Pd/CNTs catalysts with Pdcontent of 2 wt % had a much higher catalytic activitycompared with 5 wt % Pd/BaSO4, and 0.5 wt % Pd/CNTs evenshowed a similar catalytic activity to 5 wt % Pd/BaSO4.

Figure 2. Catalytic PS hydrogenation over different catalysts (reactionconditions: 150 °C reaction temperature, 1.00 gcat/gPS, 3 wt % PS-DHN, 5.8 MPa initial H2 pressure, 1000 rpm agitation rate).

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A 2.54 g portion of hydrogenated polymer solution aftercatalyst filtration was taken and tested by ICP-AES in order toquantify potential metal residue. The possible palladium residueleaching into polymer solution could not be detected. Thehydrogenated product was precipitated by adding an excessamount of methanol. The precipitated polymer product wasdried in a vacuum oven at 80 °C for 48 h. The viscosity averagemolecular weight of PCHE product over 5 wt % Pd/CNTscatalyst and PS raw material were measured using theviscosimetry method at 35 °C in THF. The Mark−Howinkconstants of PCHE were obtained by gel permeationchromatography as K = 0.03735 mL/g and α = 0.574, whileMark−Howink constants of PS were obtained as K = 0.011mL/g and α = 0.725. The viscosity average molecular weight ofPCHE and PS were 302 and 292 kg/mol, respectively,indicating that no depolymerization of polymer occurredduring the hydrogenation over Pd/CNTs. In the catalystreuse experiments, the activity of Pd/CNTs decreased slightlyafter the seven times reuse. The r0 of aromatic rings for thecatalysts reused seven times was 2.640 × 10−4 mol/L·s, whichwas slightly lower than the r0 for fresh catalyst (2.745 × 10−4

mol/L·s).Catalytic performances of 5 wt % Pd/CNTs for PS

hydrogenation under different reaction conditions were testedand shown in Figure 3. The 5 wt % Pd/CNTs showed

remarkable activity at higher PS concentration, e.g., 5 and 8 wt%. As shown in Figure 3, the PS hydrogenation fulfilled within70 min when the PS concentration increased to 5 wt %. 97.7%of HD was achieved within 141 min when the PS concentrationincreased to 8 wt %. The r0 for 5 and 8 wt % PS solution were1.638 × 10−4 and 1.241 × 10−4 mol/L·s, respectively, whichwere lower than the r0 for the diluted PS solutions (3 wt %).This is probably due to the increased viscosity affecting the gas/liquid and liquid/solid mass transfer under those conditions.Pd/CNT catalyst also exhibited a high hydrogenation ratewhen decreasing the concentration of catalyst. 94.50% of HD

was achieved within 101 min, when the catalyst concentrationdecreased from 1.00 gcat/gPS to 0.25 gcat/gPS. The HD of 63.6%was achieved within 160 min, when the reaction temperaturedecreased to 120 °C. Therefore, utilization of CNTs as acatalyst carrier could enable the PS hydrogenation to be carriedout efficiently under milder reaction conditions.

3.2. The Textural Properties. The textural properties ofCNTs, 5 wt % Pd/CNTs, 5 wt % Pd/BaSO4, and 5 wt % Pd/AC, were characterized by N2 physisorption, as shown in Figure4. Figure 4a showed a typical type II adsorption isotherm

according to the IUPAC nomenclature, and indicated thatCNTs and 5 wt % Pd/CNT catalyst had similar pore structures.The low N2 adsorption at low relative pressures (p/p0) between0 and 0.2 indicated that few micropores were available and theadsorption of N2 might occur on the defects of the CNTs and 5wt % Pd/CNTs. An obvious increase of N2 adsorption wasobserved at p/p0 between 0.2 and 0.8, revealing the adsorptionof N2 on the outer surface of the tube wall. A very small H1type hysteresis loop was also observed, suggesting that only asmall amount of cylindrical pores existed and the ends of mostnanotubes remained sealed in the CNT and Pd/CNT samples.Therefore, the Pd nanoparticles are likely to deposit on theouter surface of the CNTs. Calculated by the BET and BJHmethods, the specific surface area (ST) and pore volume (Vm)of the CNT carrier and 5 wt % Pd/CNT were 156.86 m2/g,0.7766 cm3/g, 159.32 m2/g, and 0.8344 cm3/g, respectively.Since 90% of PS coils have a diameter larger than 8 nm (Figure

Table 1. Summary of Initial Reaction Rate and the Properties of Pd/CNTs, Pd/AC, and Pd/BaSO4

catalyst r0 × 104 a (mol/L·s) wPdb (wt %) Sex

c (m2/g) STc (m2/g) VCO

d (mL/gcat) NPd × 1019 d (sites/gcat) dPd/COd (nm)

5 wt % Pd/CNTs 2.745 5.03 126.46 159.32 1.90 7.65 3.95 wt % Pd/AC 0.048 5.05 130.54 967.96 1.41 7.20 4.15 wt % Pd/BaSO4 0.331 4.97 4.67 7.03 0.98 5.26 5.6

ar0 is the initial reaction rate. Reaction conditions: 150 °C, 1.00 gcat/gPS, 3 wt % PS-DHN, 5.8 MPa initial H2 pressure, 1000 rpm agitation rate. bwPdis the content of Pd in the catalyst determined by ICP-AES. cST is the specific surface area and calculated using the BET method; Sex is external usingthe BJH method. The data for calculating Sex and ST were obtained through N2 physisorption.

dVCO is the chemisorbed adsorbed volume per gram ofcatalyst tested by CO chemisorption. NPd is the number of active metal surface sites per gram of catalyst. dPd/CO is the average size of Pd particles.NPd and dPd/CO were calculated according to VCO.

Figure 3. Catalytic PS hydrogenation using 5 wt % Pd/CNTs withdifferent conditions. (Except otherwise indicated in the figure, reactionconditions: 150 °C reaction temperature, 1.00 gcat/gPS, 3 wt % PS-DHN, 5.8 MPa initial H2 pressure, 1000 rpm agitation rate.)

Figure 4. N2 adsorption−desorption isotherms: (a) CNTs and 5 wt %Pd/CNTs; (b) 5 wt % Pd/BaSO4; (c) 5 wt % Pd/AC.

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S1, Supporting Information), the pores with a pore size smallerthan 8 nm were not accessible by PS coils, and the surface areaof those pores was considered as the internal surface area (Sin).The external surface area (Sex) represented the surface area thatPS coils could access. The relationship of pore area withaverage pore diameter was calculated by the BJH method

(Table S1, Supporting Information); Sex and Sin of 5 wt % Pd/CNTs were 126.46 and 32.77 m2/g, respectively. ST and Sexwere shown in Table 1. The adsorption isotherm of 5 wt % Pd/BaSO4 in Figure 4b also displayed a typical structure containingfew micropores or mesopores, with a specific surface area andpore volume of 7.03 m2/g and 0.0313 cm3/g, respectively.

Figure 5. TEM images of catalysts: (a) 0.5 wt % Pd/CNTs; (b) 2 wt % Pd/CNTs; (c) 5 wt % Pd/CNTs; (d) 5 wt % Pd/AC; (e) 5 wt % Pd/BaSO4.

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Figure 4c showed a type I isotherm and type H4 hysteresis loopfor 5 wt % Pd/AC, representing the abundance of microporeswith a specific surface area of 964.84 m2/g, pore volume of0.5474 cm3/g, and average pore diameter of 3.5 nm. The Sexvalues of 5 wt % Pd/BaSO4 and 5 wt % Pd/AC were alsocalculated and shown in Table 1.3.3. The Dispersion of Pd. Typical TEM images of CNT

carriers, Pd/CNT catalysts with Pd content of 0.5, 2, and 5 wt%, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4 were shown in Figure5. In Figure 1, it was found that the CNT carriers had a tubularstructure and most of the ends of CNTs were sealed. In Figure5a, b, and c, the Pd grains on the CNTs were found to have asize range of 2−7 nm and the average sizes were 4.2, 4.4, and4.1 nm (by statistical calculation) in 0.5 wt % Pd/CNTs, 2 wt %Pd/CNTs, and 5 wt % Pd/CNTs, respectively. Noticeably,considering the fact that the nanotube caps of CNTs weremostly sealed, most of the Pd grains were visible on the externalsurface of CNTs, which was in agreement with the N2physisorption results. Figure 5d showed that 5 wt % Pd/ACalso had a wide Pd grain size distribution of 2−10 nm, whichmight be attributed to the high microporosity of AC. The Pdgrains deposited on the inner walls of the micropores of AChad a diameter of 2−4 nm, and other Pd grains deposited onthe external surface had a diameter of 7−10 nm. As shown inFigure 5e, the 5 wt % Pd/BaSO4 had a wide Pd grain size rangeof 4−10 nm, and most of the Pd grains located on the externalsurface of BaSO4 particles.Figure 6 displayed the XRD patterns of the CNT carrier, Pd/

CNTs, Pd/AC, and Pd/BaSO4. For CNTs and 5 wt % Pd/

CNT catalyst, characteristic diffraction peaks at 26.5° wereobserved, corresponding to the (002) reflection of graphite.Diffraction peaks at 40.0 and 46.5° were also observed in 5 wt% Pd/CNTs, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4, whichcould be attributed to the (111) and (110) reflections of Pdcrystalline grains on the carriers. The Scherrer equation wasemployed to calculate the average size of Pd grains, using thefull width at half maxima (FWHMs) of the major diffractionpeak of Pd(111) planes according to XRD spectra. The averagesizes of Pd grains of 5 wt % Pd/CNTs, 5 wt % Pd/AC, and 5 wt% Pd/BaSO4 were calculated to be 4.3, 4.0, and 5.0 nm,respectively. Weak peaks corresponding to Pd(111) planeswere observed in 2 wt % Pd/CNTs and 0.5 wt % Pd/CNTs,possibly due to the low content of Pd in the samples.The number of active Pd sites NPd and the average metal

particle size dPd/CO on 5 wt % Pd/CNTs, 5 wt % Pd/AC, and 5wt % Pd/BaSO4 were calcuated from CO chemisorption dataVCO. The VCO, NPd, and dPd/CO are shown in Table 1. The Pd

particles were considered as “cubic shape”. Several kinds of COadsorption states exist on the surface of Pd, including linear,bridged, and multibonded.39−42 One adsorbed CO moleculemay correspond to one, two, or three Pd atoms. A. Guerrero-Ruiz41 and K. Zorn42 reported that the bridge-bonded state wasthe main adsorption state of CO on a Pd surface.40 G.Fagherazzi40 reported that the average Pd particle sizes fromTEM and SAXS were in good agreement with those obtainedfrom the CO chemisorption when a stoichiometry Pd/CO of 2was assumed. In this work, the average chemisorptionstoichiometry Pd/CO was assumed to be 1.5, 1.9, and 2.0 for5 wt % Pd/CNTs, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4,respectively. The dPd/CO of 5 wt % Pd/CNTs, 5 wt % Pd/AC,and 5 wt % Pd/BaSO4 was 3.9, 4.1, and 5.6 nm, respectively,which were in good agreement with those obtained from TEMand XRD.According to the results of N2 physisorption, TEM, XRD,

and CO chemisorption, it could be known that 5 wt % Pd/CNTs, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4 had different Pddispersion and deposited location. The Pd nanoparticles of 5 wt% Pd/CNTs and 5 wt % Pd/BaSO4 deposited on the externalsurface of the carrier, while a large amount of Pd nanoparticleson 5 wt % Pd/AC deposited inside the micropores. The Pd/CNTs had Pd grains dispersed uniformly on the externalsurface of CNTs, which may result from the high surface areaand the absence of microporosity of CNTs.

4. DISCUSSIONBy correlating the catalytic performances, textural properties,and the Pd dispersion of the catalysts, a relation between thecarrier structures and the catalytic activities could beestablished. It could be found that the activity of PShydrogenation not only depended on the dispersion of Pdcrystalline grains but also on the Pd geometric location on thecarrier. During PS heterogeneous catalysis processes, themacromolecules must be able to diffuse into the catalystpores in order to get access to interior active sites and thehydrogenated product, PCHE, must also be able to diffuse outof the pores. M. Kawaguchi and his co-workers43,44 found that,when the size ratio (Dp/2Rg) was more than 2.5, the polymercoil could deform itself in order to enter the pore. Among allthe catalysts in this work, the Pd/AC with a large amount ofmicropores had the lowest conversions. The r0 of 5 wt % Pd/BaSO4 which had few micro- or mesopores was much higherthan that of 5 wt % Pd/AC. In this study, most of the Pd grainsdeposited in micropores could not be reached by PSmacromolecules, therefore leading to the poor reactivity of 5wt % Pd/AC.As for 5 wt % Pd/CNTs and 5 wt % Pd/BaSO4, pore

diffusion resistances of PS macromolecules could be negligible,since almost all of the Pd grains deposited on the outer surfacesof those two carriers and were accessible to the PSmacromolecules. As shown in Table 1, 5 wt % Pd/CNTs hadactive sites of 7.65 × 1019 sites/gcat, which was 1.5 times largerthan that of 5 wt % Pd/BaSO4 (5.26 × 1019 sites/gcat).However, the r0 of 5 wt % Pd/CNTs was almost 8 times largerthan that of 5 wt % Pd/BaSO4. In order to gain moreinformation on the reason for the high performance of 5 wt %Pd/CNTs compared with 5 wt % Pd/BaSO4, kinetics analysiswere performed and turnover frequency (TOF) was calculated.The kinetics experiments of PS hydrogenations over 5 wt %Pd/CNTs, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4 were carriedout at various reaction temperatures ranging from 120 to 170

Figure 6. XRD patterns of CNTs, Pd/CNTs, Pd/AC, and Pd/BaSO4catalysts: (a) CNTs; (b) 0.5 wt % Pd/CNTs; (c) 2 wt % Pd/CNTs;(d) 5 wt % Pd/CNTs; (e) 5 wt % Pd/AC; (f) 5 wt % Pd/BaSO4.

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°C, and the rates of the reaction were shown in Figure 7. Thereaction rate can be expressed as

− = − = = = ′rct

kc c c kK c p c k c p cdd

( )n n n n n n nA

Acat H A H cat H A cat H A

H A H H A H A

(1)

where t was the reaction time, s; k was the reaction constant; cAwas the concentration of aromatic rings in the solution, mol/L;cH was the concentration of hydrogen in the solution, mol/L;pH was the hydrogen pressure, MPa; ccat was the concentrationof catalyst, g/L; nH and nA were the reaction orders with respectto the concentration of hydrogen and aromatic rings,respectively; k′, k′ = (kKH

nH), was the apparent reactionconstant; and KH was the Henry law constant of hydrogen inthe solution of DHN. The experimental data of pH wererecorded during hydrogenation. At any time t, cA was calculatedwith respect to the consumption of hydrogen. The reaction rate(−rA) was calculated by taking derivatives of cA. The kineticparameters nH, nA, and k′ were regressed by non-linearregression analysis against the experimental data (Figure 7).The red lines in Figure 7 were the model values.The non-linear fitting results showed that the nH for all three

catalysts was between 10−14 and 10−12. The nA for 5 wt % Pd/CNTs was between 10−9 and 10−7, and the nA for 5 wt % Pd/AC (0.90−1.08) and 5 wt % Pd/BaSO4 (0.93−1.13) was closeto 1. The estimated parameters k′ and standard deviation σwere shown in Tables S2 and S3 in the Supporting Information.The activation energy (E) of PS hydrogenation over different

catalysts was calculated by the Arrhenius equation and shown inTable 2. According to Table 2, PS hydrogenations over 5 wt %

Pd/CNTs, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4 shared asimilar activation energy (51.5 ± 2.3 kJ/mol), which indicatedthat Pd active sites on the three catalysts had almost the sameintrinsic activity for the hydrogenation of aromatic rings on PSchains.The TOF was also calculated, as follows, to determine the

number of reacted aromatic rings per Pd site per second. Theresults were shown in Table 2

=r VN

m N S STOF

/0 A

cat Pd ex T (2)

where V was the volume of PS solution in the PShydrogenation process, L; NA was Avogadro’s constant,mol−1; mcat was the weight of catalyst, g; r0VNA referred tothe number of reacted aromatic rings instantaneously at thebeginning of reaction; and mcatNPdSex/ST represented thenumber of Pd sites deposited on the external surface ofcatalysts. It could be seen that the 5 wt % Pd/CNTs had areally high TOF (0.102 s−1), while those of 5 wt % Pd/AC and5 wt % Pd/BaSO4 were only 0.011 and 0.027 s−1, respectively,at a reaction temperature of 150 °C.Usually, TOF was employed to evaluate the intrinsic activity

of active metal in the hydrogenation of small molecules.However, in the case of polymer hydrogenation, TOF was notonly determined by the intrinsic activity of active metal but alsoinfluenced by the behavior of polymer coils on the catalysts.Scheutjens and Fleer45 had investigated the adsorption forms ofmacromolecules on the surface through an improved matrixmodel, and found that only a small fraction of the segments ofmacromolecules could adsorb on the surface, named as trains.The rest of the chains were adsorbed on the surface with one orboth ends, named as tails and loops, respectively, while themiddle parts of the chains were still in the solution. J. H.Rosedale and F. S. Bates17 advocated the mechanism ofpolymer hydrogenation for the first time, in which chainconformational rearrangement and more than one adsorptionstep of each polymer coils were needed to get all segmentssaturated. F. S. Bates and co-workers21 found out the TOFdecreased significantly while increasing the molecular weight ofPS, because only a small fraction of aromatic rings could adsorbon the catalyst for the PS coils with high molecular weight. For5 wt % AC and 5 wt % Pd/BaSO4, it could be known that onlya part of the PS chains could actually be adsorbed and catalyzedby the active sites owing to the steric hindrance of PS coils atthe beginning of hydrogenation. Then, the partially hydro-

Figure 7. Rate of PS hydrogenation over different catalysts at reactiontemperatures ranging from 120 to 170 °C: (a) 5 wt % Pd/CNTs; (b)5 wt % Pd/AC; (c) 5 wt % Pd/BaSO4.

Table 2. The TOF and Kinetics Parameter Results of PSHydrogenation over Pd/CNTs, Pd/AC, and Pd/BaSO4

catalyst TOFa (s−1) Eb (kJ/mol) nAb nH

b

5 wt % Pd/CNTs 0.102 54.4 0 05 wt % Pd/AC 0.011 49.2 1 05 wt % Pd/BaSO4 0.027 51.0 1 0

aTOF is the turnover frequency. Reaction conditions: 150 °C, 1.00gcat/gPS, 3 wt % PS-DHN, 5.8 MPa initial H2 pressure, 1000 rpmagitation rate. bE is the activation energy of the reaction. nA is thereaction order with respect to the concentration of aromatic rings. nHis the reaction order with respect to the concentration of hydrogen.The approximate values nA and nH are shown to facilitate comparison.All kinetic parameters were calculated by a non-linear fitting method.

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genated PS chains would occupy those active sites andconformationally rearrange for many times on the surface ofcatalyst until the rest of the aromatic rings on the chains werehydrogenated. Actually, the hydrogenation rates of monomerunits were much higher than the rate of PS hydrogenation. Ther0 values of ethylbenzene hydrogenation catalyzed by Pd/CNTsand Pd/BaSO4 under the same conditions were 0.0015 and0.0006 mol/L·s, respectively, which means that the conforma-tional rearrangement of polymer coils cost a long time duringhydrogenation. Hence, the TOF was also influenced by thetime needed for the PS chains to do conformationalrearrangement.The coating or wrapping of polymer chains around the

CNTs was found to be an interesting phenomenon occurringbetween polymer chains and CNTs,46−50 which was quitedifferent from the adsorption behavior of polymers on solidsurface. CNTs have a nanotubular electron-rich structure, andtherefore, polymer chains containing many C−H groups oraromatic groups could interact with the CNT surface, leadingto the conformational changes around CNTs51,52 during thephysical adsorption. J. N. Coleman51 studied the interactionbetween conjugated polymer and CNTs by the microscopicand spectroscopic method, and observed that the polymermolecules conformed dramatically to wrap the CNTs. MichaelZaiser53 studied the physisorption behavior of polymer onCNTs, e.g., PS, poly(phenylacetylene), and poly(p-phenyl-enevinylene), via force-field-based molecular dynamics simu-lation. The intermolecular interactions between two polymersand between CNTs and polymers were stimulated. During theadsorption process, the interaction between two polymersgradually decreases; therefore, the segments of PS chains couldseparate from each other and began to coat the CNTs. M. A.Pasquinelli54,55 stimulated the interaction between CNTs andpolymer with different backbone and side chain structures, andfound out that the morphology of polymer chains adsorbed onCNTs was influenced by the chemical composition andstructure of the polymer. According to their study, PS chainsare likely to move translationally along the length of CNTs.Therefore, we could infer the physical adsorption behavior ofPS coils on CNTs in the solvent conditions based on the abovestudies. As shown in Figure 8, during the reaction, the PS coilsmight tend to conformationally change and wrap or stretchalong the surface of CNTs. The interaction between PS coilsand CNTs could allow much more segments of PS chains toadsorb on the CNT surface compared to the adsorption on

other catalysts; therefore, more aromatic rings on thoseadsorbed segments could be activated by the active metalloaded on the external surface of CNTs, and the time neededfor PS coils to conformationally rearrange would be certainlyreduced. The CNTs also possess a high external surface area(126.46 m2/g), which could make the adsorption of PS coils onthe CNTs less crowed than the Pd/BaSO4 catalyst with lowexternal surface area (4.67 m2/g). Therefore, the interactionbetween CNTs and PS coils and high external surface area ofCNTs might be the reasons for the high TOF of CNT catalystscompared with Pd/BaSO4.As can be seen from Figure 7 and Table 2, the PS

hydrogenation catalyzed by 5 wt % Pd/CNTs could bedescribed by a zero-order equation with respect to aromaticrings of PS and H2 concentration, while PS hydrogenationcatalyzed by 5 wt % Pd/AC and 5 wt % Pd/BaSO4 could bedescribed by a pseudo-first-order equation with respect toaromatic rings of PS and zero order with respect to H2concentration. It was found that the reaction order withrespect to H2 concentration for the three catalysts was zero,indicating that the surfaces of active metal in those threecatalysts were all saturated by H2 under the conditions in thisstudy. However, the reaction orders with respect to aromaticrings of PS for the three catalysts were different. For 5 wt %Pd/AC and 5 wt % Pd/BaSO4, the value of nA was close to 1,revealing that the amount of adsorbed aromatic rings was low.For 5 wt % Pd/CNTs, the value of nA was close to zero,suggesting that the Pd active sites were saturated by aromaticrings, which was consistent with the assumption that theinteraction between CNTs and PS coils could increase theamount of adsorbed PS segments on the CNTs and thereforeprovide more aromatic rings available for activation. In a word,the results of kinetics analysis and catalyst characterizationdemonstrated that the nanoscale tubular structure of CNTscould not only eliminate pore diffusion of PS coils but alsoallow more segment to physically adsorb on the CNTs;therefore, more aromatic rings on those adsorbed segmentscould be activated.

5. CONCLUSIONSThe Pd/CNT catalyst displayed an excellent hydrogenationactivity in the hydrogenation of commercial PS with a weightnumber molecular weight of 263 kg/mol to produce PCHEwith high properties. Pd/CNTs allowed the hydrogenation ofPS to be carried out efficiently at the mild reaction temperature(120 °C), with a low amount of catalyst (0.25 gcat/gPS) andhigh concentration of PS (8 wt % PS-DHN solution). The Pdgrains deposited on the external surface of CNTs with anaverage size of 3.9 nm, which could eliminate the pore diffusionof PS coils to Pd grains. The kinetics analysis showed that theactivation energies for Pd/CNTs (54.4 kJ/mol) were similar tothose of other catalysts. The interaction between PS coils andthe CNT surface could allow more segments to physicallyadsorb on the CNTs; therefore, more aromatic rings could beactivated on the Pd/CNTs. The interaction between polymerand CNTs and the high external surface area could be thereason for the high performance of Pd/CNTs.

■ ASSOCIATED CONTENT*S Supporting InformationFigure of size distribution of PS coils in polymer solution, tableof the relationship of pore area with average pore diameter forthe 5 wt % Pd/CNTs, 5 wt % Pd/BaSO4, and 5 wt % Pd/AC

Figure 8. Simulative view of Pd/CNT catalyst preparation andinteraction between PS coils and catalyst. PS with a polymerizationdegree of ∼3000 was used in this study. Only a fraction of PS coils wasshown in this scheme; the excess polymer coils are not shown forclarity.

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calculated using the BJH method, and tables of the estimatedparameters k′ at different reaction temperatures over 5 wt %Pd/BaSO4, 5 wt % Pd/AC, and 5 wt % Pd/CNTs. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

Corresponding Author*Phone: 0086-21-6425 3934. Fax: +86-21-6425 3934. E-mail:gpcao@ecust.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We gratefully acknowledge the financial support provided bythe Non-governmental International Science and TechnologyCooperation Program (10520706000) from the Science andTechnology Commission of Shanghai Municipality, the Ph.D.Programs Foundation of Ministry of Education of China(20110074110012), and the State Key Laboratory of ChemicalEngineering open fund (SKL-ChE-09C07). We are alsothankful to Professor G. W. Roberts for his generous support.

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