High Performance of Palladium Nanoparticles Supported on Carbon

Article pubs.acs.org/IECR

High Performance of Palladium Nanoparticles Supported on Carbon Nanotubes for the Hydrogenation of Commercial Polystyrene Kai-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 and applied in the hydrogenation of commercial polystyrene (PS) for the first time. The Pd/CNT catalysts displayed a quite excellent hydrogenation activity compared with those of traditional catalysts, e.g., Pd/AC or Pd/BaSO4, and allowed the reaction to be carried out under milder reaction conditions. The physical and chemical properties of Pd/CNTs were characterized by inductively 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 the external surface of CNTs with good dispersion. Kinetics analysis showed that the activation energy of Pd/CNTs was similar to comparison catalysts, while the TOF of Pd/CNTs (0.102 s−1) was much higher. The high external surface area and the interaction between polymer and CNTs could be the reason for the high performance of Pd/CNTs.

1. INTRODUCTION Hydrogenation is an economic process to improve the chemical, mechanical, and thermal properties of unsaturated polymer.1−8 Usually, the hydrogenation of unsaturated polymers could be carried out over either homogeneous or heterogeneous catalysts. 6,9 Homogeneous hydrogenation3−6,8,10−13 adopts ruthenium (Ru), rhodium (Rh), osmium (Os), and palladium (Pd) complexes as catalysts, which allows the hydrogenation of diene-based polymer to be carried out at milder operating conditions and without diffusion problems. The recovery of homogeneous catalysts is usually difficult and expensive, and the residual catalyst in the product polymers could result in polymer degradation. However, the heterogeneous catalysts, involving supported metal as active sites, could be easily filtered out from the polymer solution after the reaction, which could avoid the metal contamination of the polymer products. The hydrogenation of commercial polystyrene (PS) is a representative example to produce hydrogenated polystyrene (HPS), also known as polycyclohexylethylene (PCHE), a polymer that shows greatly improved heat and UV resistance compared with PS.14 Generally, the PS hydrogenation is carried out 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 very slow reaction rate. High temperature, high catalyst concentration, and low PS concentration are usually employed in the hydrogenation, which would result in the depolymerization of PS/PCHE, increment of catalyst separation cost, and waste of energy. Research is urgently needed to remove those serious impediments to industrialize the heterogeneous PS hydrogenation process. © XXXX American Chemical Society

It has been well-known that the characteristics of polymer hydrogenation is quite different from that of hydrogenation of small molecules. In the PS hydrogenation, the unsaturated 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 on the pore walls. Only a part of the aromatic rings on the PS chains could be adsorbed and catalyzed at one time, and the partially hydrogenated PS coils need to conformationally rearrange the chains for many times in order to get all the aromatic rings to be saturated.17,21 The product PCHE coils need to desorb from the sites and diffuse out of the pores to the bulk liquid phase. Because of the large dimension of PS and PCHE macromolecules and the high viscosity of the solution, the mass transfer of PS and PCHE coils in both the bulk of the solution and the pores of the catalysts proved extremely challenging.20,21 The external diffusion of PS/PCHE coils could be enhanced by increasing the agitation rate or space velocity. However, the enhancement of pore diffusion of PS/PCHE coils is still a big problem. Moreover, the adsorption and hydrogenation of aromatic rings on active metal sites are restrained from the steric hindrance of a large size of PS coils. In order to enhance the pore diffusion of PS coils, several approaches were usually proposed, including introduction of supercritical CO2 (Sc-CO2)22−24 and optimal design of the catalysts.14,21,25 G. W. Roberts and co-workers22 introduced ScCO2 to the PS hydrogenation system and found that the addition of Sc-CO2 could effectively reduce the size of polymer coils and enable faster diffusion of the polymer molecules in the Received: February 2, 2013 Revised: November 8, 2013 Accepted: November 17, 2013

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dx.doi.org/10.1021/ie401184h | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Materials. Carbon nanotubes (CNTs) having an inner diameter of 5−10 nm and an outer diameter of 20−30 nm were purchased from Chengdu Organic Chemicals Co. Ltd. The CNTs 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 Ni and Cl. The inner diameter and outer diameter of CNTs were also measured according to the TEM image (Figure 1), and the

pores of catalyst. However, the deactivation of catalyst often occurred owing to the existence of CO which was formed by reverse water-gas-shift reaction in the presence of CO2 and H2.26,27 Recent research of R. G. Carbonell28,29 found that the utilization of bimetallic catalysts contained Ni and Ru and the complete replacement of CO2 with propane could alleviate the CO poisoning of hydrogenation catalysts. F. S. Bates and coworkers21 used a platinum supported wide-pore (average pore sizes between 300 and 400 nm) silica catalyst in PS hydrogenation in order to facilitate the pore diffusion of PS coils. According to their internal mass transfer study, even PS coils with a number-average molecular weight of 276 kg/mol can access the active sites in the wide pores, as the PS coils had a gyration radius of around 42 nm. However, the initial reaction rate became much lower for the hydrogenation of PS with high molecular weight, e.g., M̅ n > 190 kg/mol. The surface area of wide-pore silica catalyst was about 15 m2/g, and the dispersion of 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/CaCO3 with a surface area of 8.3 m2/g exhibited better activities than 5 wt % Pd/Al2O3 with a surface area of 340 m2/g, which could be explained by the inaccessibility of the PS coils into the microand mesopores of the Pd/Al2O3 catalysts. The above studies give a very important enlightenment for us to design and optimize the catalyst structure for higher molecular weight PS hydrogenation. It seems that traditional porous catalysts with high surface areas and small pore diameter might not be suitable for PS hydrogenation. Catalysts with high specific surface area and abundant micro- or mesopores lead to better dispersion of active metal, but most of the active metal grains deposit on the surface of internal pores, resulting in the difficult 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 pore diffusion, but the surface area is limited and the dispersion of active metal on the carrier is poor. Therefore, the aim of our study is to explore a conveniently prepared catalyst for PS hydrogenation, which could not only eliminate inner pore diffusion of PS coils but also have a high external surface area at the same time. Carbon nanotubes (CNTs) are one-dimensional tube-like carbon materials where the carbon layer is rolled up cylindrically with diameters in the nanoscale range. CNTs have been considered to be promising supports for heterogeneous catalysts, such as selective hydrogenation30−32 and electro-oxidation reactions.33−35 Due to the one-dimensional tubular structures, CNTs have a surprisingly high aspect ratio, little microporosity, and a very large external surface area. These specific properties of CNTs may shed interesting light on the catalyst exploitation of PS hydrogenation, which has never been reported in the literature. In this work, we report that CNT supported Pd catalyst with Pd grains deposited homogeneously on the external surface showed a much higher catalytic performance compared with Pd/AC and commercial Pd/BaSO4 catalyst containing the same content of Pd. Pd/ CNTs, Pd/AC, and Pd/BaSO 4 were characterized by inductively coupled plasma-atomic emission spectrometry (ICP-AES), N2-physisorption, transmission electron microscopy (TEM), X-ray diffraction (XRD), and CO chemisorption to investigate the properties of the catalysts. Both characterization of catalysts and further kinetics analysis were employed to understand the reason for the high performance of Pd/ CNTs.

Figure 1. TEM image of CNT carrier.

results were consistent with the numbers provided by the supplier. The palladium precursor Pd(NO3)2·2H2O was purchased from Jiuling Chemical Co., Ltd., with a Pd content of 39.5 wt %. The activated carbon (AC) was purchased from Huage Chemical Engineering Co., Ltd., and the content of acidic oxygen-containing groups on the surface of AC was 1.56 mmol/g tested by Bohem titration. Commercial 5 wt % Pd/ BaSO4 catalyst was purchased from Xi’an Kaili Catalyst Chemical 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 gel permeation chromatography (Waters Co., Ltd., USA) with tetrahydrofuran as the solvent at 35 °C. The solvent, decahydronaphthalene (DHN), was purchased from Sinopharm Chemical Reagent Co., Ltd. The size distribution of PS coils in 3 wt % PS-DHN solution was characterized using a laser light scattering spectrometer (ALV-laser Vertriebsgesellschaftm.b.H.) at 632.8 nm and a scattering angle of 90° with a 22 mW He−Ne laser. High purity hydrogen (H2) was supplied by Shanghai Zhongyuan Chemical Co., Ltd. All the reactants were used without further treatment. 2.2. Preparation of Pd/CNTs Catalysts. Pd/CNT catalysts were prepared by the wetness impregnation method.36−38 The typical procedure for preparing 5 wt % Pd/CNTs was as follows. 4.5 g of CNTs were immersed in 60 g of deionized water and dispersed for 2 h in an ultrasonic bath. A 5 g portion of Pd(NO3)2 solution with a certain amount of Pd was added to the suspension, stirred vigorously for 6 h, and left standing for 12 h. The color of supernatant solution converted from brown to achromatic and transparent. Then, the CNTs adsorbing 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 B



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

method except that the catalyst carrier was replaced by active carbon. 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 vertical baffles. Before being heated to the reaction temperature, the reactor was flushed with H2 to remove air. The reactor was heated to the reaction temperature, and then, the reaction was initiated by flushing H2 to 5.8 MPa and adjusting the agitation rate to 1000 rpm. During the hydrogenation, the consumption of hydrogen was monitored and recorded until the assumption of hydrogen leveled off. After the hydrogenation, the catalyst was removed from the hydrogenated polymer solution by filtration. The samples of initial PS solution and hydrogenated reaction mixture were taken and analyzed by UV−vis spectrophotometer (UV7504C, China) at 261.5 nm to determine the concentration of aromatic rings. 20 The conversion of the aromatic rings, also named the degree of hydrogenation (HD), was calculated as HD = (1 − cA/cA,0) × 100%, in which the concentrations of aromatic rings at initial and time t were designated as cA,0 and cA, respectively. 2.4. Characterization of the CNT Carriers and Supported Catalysts. Nitrogen physisorption was adopted to investigate the specific surface area and pore volume of the CNTs using an ASAP2010 static volumetric instrument (Micromeritics, USA). The samples were first degassed at 300 °C for 3 h, and then, the adsorption−desorption isotherms were measured at −197 °C. The pore size distribution and pore volume were calculated using the BJH method from the desorption isotherm, and the total pore volume was calculated from the cumulative volume adsorbed at a relative pressure less than 0.98 to avoid the inaccuracy of the measurement at high relative pressure. IRIS 1000 ICP-AES (Thermo Elemental, USA) was used to determine the Pd content of the catalyst. A known amount of the catalyst sample was first calcinated under a steam of air at 650 °C for 300 min to remove the carbon. Aqua regia was added to the remainder, and the mixture was heated until the remainder was fully dissolved. The solution was diluted to 50 mL in volumetric flasks and analyzed by IRIS 1000. The XRD patterns of CNTs and Pd/CNTs were recorded on a Rigaku D/MAX2550VB/PC diffractometer using Cu Kα radiation and a carbon monochromator. The dispersion of Pd particles was investigated by TEM and CO chemisorption, respectively. The sample of TEM was prepared as follows. A 1 mg of catalyst sample was sonicated in 10 mL of ethanol for 20 min, and a drop of the above suspension was placed on a copper grid and dried in air for 2 h. The microscopic images of the copper grid were tested with a JEM2010 electron microscope (JEOL, Japan) operated at 200 kV. CO chemisorption of the catalysts was employed to determine the number of active metal sites using an ASAP2020c chemisorption analyzer (Micromeritics, USA). The procedure of CO chemisorption was as follows. A sample (about 0.100 g) was placed into a U-tube quartz, evacuated to 10−5 mmHg at 350 °C for 60 min, and reduced by a flow of H2 at 300 °C for 180 min. The sample was then evacuated at 300 °C to desorb H2. After cooling to 40 °C, CO was introduced to the system and the isotherm was measured with pressure ranging from 15.0 to 450.0 mmHg. The isotherm of CO adsorption over the catalyst was tested. The chemisorbed adsorbed volumes per gram of catalyst, VCO, could be determined by extrapolating to zero pressure of the linear part of the adsorption isotherm. The number of active Pd sites NPd (defined as sites per gram of

3. RESULTS 3.1. Catalytic Performance of Pd/CNTs Catalysts in PS Hydrogenation. First, a series of PS hydrogenations were performed over catalysts whose sizes were smaller than 200 mesh in the batch reactor with agitation rate ranging from 800 to 1200 rpm and initial H2 pressure ranging from 4.0 to 6.0 MPa. The conversion of PS had no significant change when the initial H2 pressure was higher than 5.5 MPa and the agitation rate was higher than 1000 rpm. The external diffusion could be precluded, and the surface of catalysts was saturated by H2 under those conditions. Therefore, PS hydrogenation performances over 5 wt % Pd/CNTs, 5 wt % Pd/AC, and commercial 5 wt % Pd/BaSO4 were performed under the same conditions, which had a reaction temperature of 150 °C, initial H2 pressure of 5.8 MPa, agitating rate of 1000 rpm, catalyst concentration of 1.00 gcat/gPS, and PS concentration of 3 wt %, using DHN as the solvent. Since PCHE is the only product of PS hydrogenation, the relationship between HD and reaction time (t) was tested during the PS hydrogenation process to evaluate the catalytic activities of catalysts. In order to find out whether the CNT carrier was catalytically active in PS hydrogenation, the experiment using the CNT carrier was carried out. No HD of PS could be detected in the absence of Pd, suggesting that Pd was the only active site for PS hydrogenation. As shown in Figure 2, 5 wt % Pd/CNTs

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

exhibited extremely higher PS hydrogenation activity compared with 5 wt % Pd/AC and 5 wt % Pd/BaSO4 catalysts which contained the same content of Pd. 99.83% of HD was achieved over 5 wt % Pd/CNTs within 40 min, while 78.05% of HD was achieved over 5 wt % Pd/BaSO4 and only 18.60% of HD was achieved over 5 wt % Pd/AC within 200 min. The amount of reacted aromatic rings instantaneously was calculated via the amount of hydrogen consumption. The initial rate of hydrogenation (r0) was obtained by dividing the amount of reacted aromatic rings instantaneously by reaction time, as shown in Table 1. The initial rates, r0, for 5 wt % Pd/CNTs, 5 wt % 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 be noted 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 Pd content of 2 wt % had a much higher catalytic activity compared with 5 wt % Pd/BaSO4, and 0.5 wt % Pd/CNTs even showed a similar catalytic activity to 5 wt % Pd/BaSO4. C



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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 %)

Sexc (m2/g)

STc (m2/g)

VCOd (mL/gcat)

NPd × 1019 d (sites/gcat)

dPd/COd (nm)

5 wt % Pd/CNTs 5 wt % Pd/AC 5 wt % Pd/BaSO4

2.745 0.048 0.331

5.03 5.05 4.97

126.46 130.54 4.67

159.32 967.96 7.03

1.90 1.41 0.98

7.65 7.20 5.26

3.9 4.1 5.6

r0 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. bwPd is 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 using the BJH method. The data for calculating Sex and ST were obtained through N2 physisorption. dVCO is the chemisorbed adsorbed volume per gram of catalyst 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. a

was achieved within 101 min, when the catalyst concentration decreased from 1.00 gcat/gPS to 0.25 gcat/gPS. The HD of 63.6% was achieved within 160 min, when the reaction temperature decreased to 120 °C. Therefore, utilization of CNTs as a catalyst carrier could enable the PS hydrogenation to be carried out efficiently under milder reaction conditions. 3.2. The Textural Properties. The textural properties of CNTs, 5 wt % Pd/CNTs, 5 wt % Pd/BaSO4, and 5 wt % Pd/ AC, were characterized by N2 physisorption, as shown in Figure 4. Figure 4a showed a typical type II adsorption isotherm

A 2.54 g portion of hydrogenated polymer solution after catalyst filtration was taken and tested by ICP-AES in order to quantify potential metal residue. The possible palladium residue leaching into polymer solution could not be detected. The hydrogenated product was precipitated by adding an excess amount of methanol. The precipitated polymer product was dried in a vacuum oven at 80 °C for 48 h. The viscosity average molecular weight of PCHE product over 5 wt % Pd/CNTs catalyst and PS raw material were measured using the viscosimetry method at 35 °C in THF. The Mark−Howink constants of PCHE were obtained by gel permeation chromatography as K = 0.03735 mL/g and α = 0.574, while Mark−Howink constants of PS were obtained as K = 0.011 mL/g and α = 0.725. The viscosity average molecular weight of PCHE and PS were 302 and 292 kg/mol, respectively, indicating that no depolymerization of polymer occurred during the hydrogenation over Pd/CNTs. In the catalyst reuse experiments, the activity of Pd/CNTs decreased slightly after the seven times reuse. The r0 of aromatic rings for the catalysts reused seven times was 2.640 × 10−4 mol/L·s, which was 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 tested and shown in Figure 3. The 5 wt % Pd/CNTs showed

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

according to the IUPAC nomenclature, and indicated that CNTs and 5 wt % Pd/CNT catalyst had similar pore structures. The low N2 adsorption at low relative pressures (p/p0) between 0 and 0.2 indicated that few micropores were available and the adsorption of N2 might occur on the defects of the CNTs and 5 wt % Pd/CNTs. An obvious increase of N2 adsorption was observed at p/p0 between 0.2 and 0.8, revealing the adsorption of N2 on the outer surface of the tube wall. A very small H1 type hysteresis loop was also observed, suggesting that only a small amount of cylindrical pores existed and the ends of most nanotubes remained sealed in the CNT and Pd/CNT samples. Therefore, the Pd nanoparticles are likely to deposit on the outer surface of the CNTs. Calculated by the BET and BJH methods, 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

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

remarkable activity at higher PS concentration, e.g., 5 and 8 wt %. As shown in Figure 3, the PS hydrogenation fulfilled within 70 min when the PS concentration increased to 5 wt %. 97.7% of HD was achieved within 141 min when the PS concentration increased to 8 wt %. The r0 for 5 and 8 wt % PS solution were 1.638 × 10−4 and 1.241 × 10−4 mol/L·s, respectively, which were 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 rate when decreasing the concentration of catalyst. 94.50% of HD D



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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.

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

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



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particles were considered as “cubic shape”. Several kinds of CO adsorption states exist on the surface of Pd, including linear, bridged, and multibonded.39−42 One adsorbed CO molecule may correspond to one, two, or three Pd atoms. A. GuerreroRuiz41 and K. Zorn42 reported that the bridge-bonded state was the main adsorption state of CO on a Pd surface.40 G. Fagherazzi40 reported that the average Pd particle sizes from TEM and SAXS were in good agreement with those obtained from the CO chemisorption when a stoichiometry Pd/CO of 2 was assumed. In this work, the average chemisorption stoichiometry Pd/CO was assumed to be 1.5, 1.9, and 2.0 for 5 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 TEM and 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 Pd dispersion and deposited location. The Pd nanoparticles of 5 wt % Pd/CNTs and 5 wt % Pd/BaSO4 deposited on the external surface of the carrier, while a large amount of Pd nanoparticles on 5 wt % Pd/AC deposited inside the micropores. The Pd/ CNTs had Pd grains dispersed uniformly on the external surface of CNTs, which may result from the high surface area and the absence of microporosity of CNTs.

Figure 4c showed a type I isotherm and type H4 hysteresis loop for 5 wt % Pd/AC, representing the abundance of micropores with a specific surface area of 964.84 m2/g, pore volume of 0.5474 cm3/g, and average pore diameter of 3.5 nm. The Sex values of 5 wt % Pd/BaSO4 and 5 wt % Pd/AC were also calculated 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 Figure 5. In Figure 1, it was found that the CNT carriers had a tubular structure and most of the ends of CNTs were sealed. In Figure 5a, b, and c, the Pd grains on the CNTs were found to have a size range of 2−7 nm and the average sizes were 4.2, 4.4, and 4.1 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 were mostly sealed, most of the Pd grains were visible on the external surface of CNTs, which was in agreement with the N2 physisorption results. Figure 5d showed that 5 wt % Pd/AC also had a wide Pd grain size distribution of 2−10 nm, which might be attributed to the high microporosity of AC. The Pd grains deposited on the inner walls of the micropores of AC had a diameter of 2−4 nm, and other Pd grains deposited on the external surface had a diameter of 7−10 nm. As shown in Figure 5e, the 5 wt % Pd/BaSO4 had a wide Pd grain size range of 4−10 nm, and most of the Pd grains located on the external surface 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/

4. DISCUSSION By correlating the catalytic performances, textural properties, and the Pd dispersion of the catalysts, a relation between the carrier structures and the catalytic activities could be established. It could be found that the activity of PS hydrogenation not only depended on the dispersion of Pd crystalline grains but also on the Pd geometric location on the carrier. During PS heterogeneous catalysis processes, the macromolecules must be able to diffuse into the catalyst pores in order to get access to interior active sites and the hydrogenated product, PCHE, must also be able to diffuse out of the pores. M. Kawaguchi and his co-workers43,44 found that, when the size ratio (Dp/2Rg) was more than 2.5, the polymer coil could deform itself in order to enter the pore. Among all the catalysts in this work, the Pd/AC with a large amount of micropores had the lowest conversions. The r0 of 5 wt % Pd/ BaSO4 which had few micro- or mesopores was much higher than that of 5 wt % Pd/AC. In this study, most of the Pd grains deposited in micropores could not be reached by PS macromolecules, therefore leading to the poor reactivity of 5 wt % 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 surfaces of those two carriers and were accessible to the PS macromolecules. As shown in Table 1, 5 wt % Pd/CNTs had active sites of 7.65 × 1019 sites/gcat, which was 1.5 times larger than that of 5 wt % Pd/BaSO4 (5.26 × 1019 sites/gcat). However, the r0 of 5 wt % Pd/CNTs was almost 8 times larger than that of 5 wt % Pd/BaSO4. In order to gain more information on the reason for the high performance of 5 wt % Pd/CNTs compared with 5 wt % Pd/BaSO4, kinetics analysis were 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 carried out at various reaction temperatures ranging from 120 to 170

Figure 6. XRD patterns of CNTs, Pd/CNTs, Pd/AC, and Pd/BaSO4 catalysts: (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.

CNT catalyst, characteristic diffraction peaks at 26.5° were observed, 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, which could be attributed to the (111) and (110) reflections of Pd crystalline grains on the carriers. The Scherrer equation was employed to calculate the average size of Pd grains, using the full width at half maxima (FWHMs) of the major diffraction peak of Pd(111) planes according to XRD spectra. The average sizes 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) planes were 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 5 wt % Pd/BaSO4 were calcuated from CO chemisorption data VCO. The VCO, NPd, and dPd/CO are shown in Table 1. The Pd F



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

catalysts was calculated by the Arrhenius equation and shown in Table 2. According to Table 2, PS hydrogenations over 5 wt % Table 2. The TOF and Kinetics Parameter Results of PS Hydrogenation over Pd/CNTs, Pd/AC, and Pd/BaSO4 catalyst

TOFa (s−1)

Eb (kJ/mol)

nAb

nHb

5 wt % Pd/CNTs 5 wt % Pd/AC 5 wt % Pd/BaSO4

0.102 0.011 0.027

54.4 49.2 51.0

0 1 1

0 0 0

TOF is the turnover frequency. Reaction conditions: 150 °C, 1.00 gcat/gPS, 3 wt % PS-DHN, 5.8 MPa initial H2 pressure, 1000 rpm agitation rate. bE is the activation energy of the reaction. nA is the reaction order with respect to the concentration of aromatic rings. nH is 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. a

Pd/CNTs, 5 wt % Pd/AC, and 5 wt % Pd/BaSO4 shared a similar activation energy (51.5 ± 2.3 kJ/mol), which indicated that Pd active sites on the three catalysts had almost the same intrinsic activity for the hydrogenation of aromatic rings on PS chains. The TOF was also calculated, as follows, to determine the number of reacted aromatic rings per Pd site per second. The results were shown in Table 2

TOF =

(2)

where V was the volume of PS solution in the PS hydrogenation process, L; NA was Avogadro’s constant, mol−1; mcat was the weight of catalyst, g; r0VNA referred to the number of reacted aromatic rings instantaneously at the beginning of reaction; and mcatNPdSex/ST represented the number of Pd sites deposited on the external surface of catalysts. It could be seen that the 5 wt % Pd/CNTs had a really high TOF (0.102 s−1), while those of 5 wt % Pd/AC and 5 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 not only determined by the intrinsic activity of active metal but also influenced by the behavior of polymer coils on the catalysts. Scheutjens and Fleer45 had investigated the adsorption forms of macromolecules on the surface through an improved matrix model, and found that only a small fraction of the segments of macromolecules could adsorb on the surface, named as trains. The rest of the chains were adsorbed on the surface with one or both ends, named as tails and loops, respectively, while the middle parts of the chains were still in the solution. J. H. Rosedale and F. S. Bates17 advocated the mechanism of polymer hydrogenation for the first time, in which chain conformational rearrangement and more than one adsorption step of each polymer coils were needed to get all segments saturated. F. S. Bates and co-workers21 found out the TOF decreased significantly while increasing the molecular weight of PS, because only a small fraction of aromatic rings could adsorb on the catalyst for the PS coils with high molecular weight. For 5 wt % AC and 5 wt % Pd/BaSO4, it could be known that only a part of the PS chains could actually be adsorbed and catalyzed by the active sites owing to the steric hindrance of PS coils at the beginning of hydrogenation. Then, the partially hydro-

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

−rA = −

r0VNA mcat NPdSex /ST

dcA = kccatc HnHcAnA = (kKHnH)ccatpHnH cAnA = k′ccatpHnH cAnA dt (1)

where t was the reaction time, s; k was the reaction constant; cA was 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 concentration of catalyst, g/L; nH and nA were the reaction orders with respect to the concentration of hydrogen and aromatic rings, respectively; k′, k′ = (kKHnH), was the apparent reaction constant; and KH was the Henry law constant of hydrogen in the solution of DHN. The experimental data of pH were recorded during hydrogenation. At any time t, cA was calculated with respect to the consumption of hydrogen. The reaction rate (−rA) was calculated by taking derivatives of cA. The kinetic parameters nH, nA, and k′ were regressed by non-linear regression 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 close to 1. The estimated parameters k′ and standard deviation σ̂ were shown in Tables S2 and S3 in the Supporting Information. The activation energy (E) of PS hydrogenation over different G



Article

other catalysts; therefore, more aromatic rings on those adsorbed segments could be activated by the active metal loaded on the external surface of CNTs, and the time needed for PS coils to conformationally rearrange would be certainly reduced. The CNTs also possess a high external surface area (126.46 m2/g), which could make the adsorption of PS coils on the CNTs less crowed than the Pd/BaSO4 catalyst with low external surface area (4.67 m2/g). Therefore, the interaction between CNTs and PS coils and high external surface area of CNTs might be the reasons for the high TOF of CNT catalysts compared with Pd/BaSO4. As can be seen from Figure 7 and Table 2, the PS hydrogenation catalyzed by 5 wt % Pd/CNTs could be described by a zero-order equation with respect to aromatic rings of PS and H2 concentration, while PS hydrogenation catalyzed by 5 wt % Pd/AC and 5 wt % Pd/BaSO4 could be described by a pseudo-first-order equation with respect to aromatic rings of PS and zero order with respect to H2 concentration. It was found that the reaction order with respect to H2 concentration for the three catalysts was zero, indicating that the surfaces of active metal in those three catalysts were all saturated by H2 under the conditions in this study. However, the reaction orders with respect to aromatic rings 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 aromatic rings, which was consistent with the assumption that the interaction between CNTs and PS coils could increase the amount of adsorbed PS segments on the CNTs and therefore provide more aromatic rings available for activation. In a word, the results of kinetics analysis and catalyst characterization demonstrated that the nanoscale tubular structure of CNTs could not only eliminate pore diffusion of PS coils but also allow more segment to physically adsorb on the CNTs; therefore, more aromatic rings on those adsorbed segments could be activated.

genated PS chains would occupy those active sites and conformationally rearrange for many times on the surface of catalyst until the rest of the aromatic rings on the chains were hydrogenated. Actually, the hydrogenation rates of monomer units were much higher than the rate of PS hydrogenation. The r0 values of ethylbenzene hydrogenation catalyzed by Pd/CNTs and Pd/BaSO4 under the same conditions were 0.0015 and 0.0006 mol/L·s, respectively, which means that the conformational rearrangement of polymer coils cost a long time during hydrogenation. Hence, the TOF was also influenced by the time needed for the PS chains to do conformational rearrangement. The coating or wrapping of polymer chains around the CNTs was found to be an interesting phenomenon occurring between polymer chains and CNTs,46−50 which was quite different from the adsorption behavior of polymers on solid surface. CNTs have a nanotubular electron-rich structure, and therefore, polymer chains containing many C−H groups or aromatic groups could interact with the CNT surface, leading to the conformational changes around CNTs51,52 during the physical adsorption. J. N. Coleman51 studied the interaction between conjugated polymer and CNTs by the microscopic and spectroscopic method, and observed that the polymer molecules conformed dramatically to wrap the CNTs. Michael Zaiser53 studied the physisorption behavior of polymer on CNTs, e.g., PS, poly(phenylacetylene), and poly(p-phenylenevinylene), via force-field-based molecular dynamics simulation. The intermolecular interactions between two polymers and between CNTs and polymers were stimulated. During the adsorption process, the interaction between two polymers gradually decreases; therefore, the segments of PS chains could separate from each other and began to coat the CNTs. M. A. Pasquinelli54,55 stimulated the interaction between CNTs and polymer with different backbone and side chain structures, and found out that the morphology of polymer chains adsorbed on CNTs was influenced by the chemical composition and structure of the polymer. According to their study, PS chains are likely to move translationally along the length of CNTs. Therefore, we could infer the physical adsorption behavior of PS coils on CNTs in the solvent conditions based on the above studies. As shown in Figure 8, during the reaction, the PS coils might tend to conformationally change and wrap or stretch along the surface of CNTs. The interaction between PS coils and CNTs could allow much more segments of PS chains to adsorb on the CNT surface compared to the adsorption on

5. CONCLUSIONS The Pd/CNT catalyst displayed an excellent hydrogenation activity in the hydrogenation of commercial PS with a weight number molecular weight of 263 kg/mol to produce PCHE with high properties. Pd/CNTs allowed the hydrogenation of PS to be carried out efficiently at the mild reaction temperature (120 °C), with a low amount of catalyst (0.25 gcat/gPS) and high concentration of PS (8 wt % PS-DHN solution). The Pd grains deposited on the external surface of CNTs with an average size of 3.9 nm, which could eliminate the pore diffusion of PS coils to Pd grains. The kinetics analysis showed that the activation energies for Pd/CNTs (54.4 kJ/mol) were similar to those of other catalysts. The interaction between PS coils and the CNT surface could allow more segments to physically adsorb on the CNTs; therefore, more aromatic rings could be activated on the Pd/CNTs. The interaction between polymer and CNTs and the high external surface area could be the reason for the high performance of Pd/CNTs.

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Figure 8. Simulative view of Pd/CNT catalyst preparation and interaction between PS coils and catalyst. PS with a polymerization degree of ∼3000 was used in this study. Only a fraction of PS coils was shown in this scheme; the excess polymer coils are not shown for clarity.

ASSOCIATED CONTENT

S Supporting Information *

Figure of size distribution of PS coils in polymer solution, table of the relationship of pore area with average pore diameter for the 5 wt % Pd/CNTs, 5 wt % Pd/BaSO4, and 5 wt % Pd/AC H



Article

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

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AUTHOR INFORMATION

Corresponding Author

*Phone: 0086-21-6425 3934. Fax: +86-21-6425 3934. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support provided by the Non-governmental International Science and Technology Cooperation Program (10520706000) from the Science and Technology Commission of Shanghai Municipality, the Ph.D. Programs Foundation of Ministry of Education of China (20110074110012), and the State Key Laboratory of Chemical Engineering open fund (SKL-ChE-09C07). We are also thankful to Professor G. W. Roberts for his generous support.

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High Performance of Palladium Nanoparticles Supported on Carbon

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