Fabrication and Catalytic Properties of Palladium Nanoparticles

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Fabrication and Catalytic Properties of Palladium Nanoparticles Deposited on a Silanized Asymmetric Ceramic Support Rizhi Chen,* Yuanguo Jiang, Weihong Xing, and Wanqin Jin State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P.R. China ABSTRACT: We developed an improved fabrication technique for the deposition of palladium nanoparticles on a ceramic membrane support in which the support surface was silanized with amino-functional silane. In the present work, the ceramic membrane support was used for catalyst immobilization only. The as-fabricated Pd-loaded ceramic membrane support was extensively characterized by ICP emission spectroscopy, XRD, FESEM, EDS, XPS, HRTEM, and TPR, and its catalytic properties were tested in the liquid-phase hydrogenation of p-nitrophenol to p-aminophenol. A comparative study was also made with palladium nanoparticles deposited on the ceramic membrane support without silanization. Higher catalytic activity and stability were observed for the palladium nanoparticles deposited on the surface-silanized ceramic membrane support. The reason proposed for the higher catalytic activity is the higher dispersion of the palladium nanoparticles. The palladium nanoparticles were loaded onto the surface-silanized ceramic membrane support with chemical bonds; thus, it was not easy for the palladium nanoparticles to detach from the silanized membrane support, and a superior catalytic stability could be obtained.

1. INTRODUCTION Nanoscale metal particles often exhibit superior catalytic properties as compared to the corresponding bulk materials, because a significant volume of their microstructure is composed of interfaces/grain boundaries and a large fraction of the atoms reside in grain boundaries.1 These nanoscale-metal-particle catalysts can work in the form of powders suspended in a slurry, or they can be immobilized on various supports, such as glass, quartz, stainless steel, or membranes.2,3 It has been reported that catalysts in suspension have a better efficiency than immobilized ones.4,5 However, this slurry system generates other problems, such as the requirement to separate and recycle the catalysts from the reaction mixture, resulting in an inconvenient, time-consuming, and expensive process.6 Such problems can be avoided effectively if the catalyst particles are immobilized in or on membranes.7,8 In contrast to free powder catalysts, membrane catalysts can be easily reused in liquid-phase reactions. A similar study was reported by Alaoui and the co-workers.2 Generally, the catalyst particles are loaded onto membranes9,10 or entrapped in membranes to form composite membranes.11,12 Composite membranes generally show less activity in accordance with their low catalyst contents.13 Ceramic membranes such as Al2O3, TiO2, and ZrO2 are desirable for catalyst supports because of their advantages such as good chemical stability and favorable mechanical strength.9,10 The amount and quality of deposited metal particles depend on the surface properties of the substrate.14
17 Because of the absence of chemical bonding, the metal particles generally exhibit poor adhesion to the substrate,14 resulting in easy removal of the catalyst particles from the support surface during reactions. However, the ability of amino-functional silanes to increase the amounts of the metal particles loaded and improve the adhesion between the metal particles and the substrate has been reported.14
19 They are typically used as a pretreatment in electroless r 2011 American Chemical Society

Figure 1. Chemical structure of 3-APTS.

deposition.15
17 In electroless nickel plating on hollow glass microspheres, Zhang et al. found that, after a solution pretreatment using a coupling agent, hollow glass spheres adsorbed more palladium catalytic active centers on their surfaces during the activation process; hence, they obtained continuously and uniformly covered microspheres.15 In the present study, γ-aminopropyltriethoxy silane (3-APTS) was chosen as a common silane coupling agent with an amino group to enhance the loading and, thereby, to improve the catalytic properties of the catalyst loaded on the membrane. Palladium is a widely used metal catalyst in catalytic hydrogenation/dehydrogenation reactions.9,10,20 In this work, to investigate the feasibility of fabricating palladium nanoparticles deposited on surface-silanized membrane support with aminofunctional silane with superior catalytic performance, p-nitrophenol hydrogenation to p-aminophenol was taken as a model reaction for the following reasons: p-Aminophenol is of great commercial importance as an intermediate for the preparation of analgesic and antipyretic drugs.21,22 Traditionally, p-aminophenol is prepared by multistep iron-acid reduction of p-nitrochlorobenzene Received: November 8, 2010 Accepted: March 7, 2011 Revised: February 25, 2011 Published: March 16, 2011 4405

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Figure 2. Schematic representation of the reaction steps in the preparation of palladium deposited on ceramic membrane support.

or p-nitrophenol. The major disadvantage of the iron-acid reduction is the generation of large amounts of iron
iron oxide sludge that cannot be reused and causes severe disposal problems.22 The catalytic hydrogenation of nitrobenzene in the presence of strong acids such as sulfuric acid is another important commercial method, but it also has two major drawbacks:23 the quantitative formation of side products such as aniline through further hydrogenation of the intermediate phenylhydroxylamine and the use of highly corrosive mineral acid. To meet the growing demand for p-aminophenol, the direct catalytic hydrogenation of p-nitrophenol is considered to be an alternative green process for the preparation of p-aminophenol.23 The purpose of this investigation was to fabricate palladium nanoparticles deposited on a ceramic membrane support silanized by 3-APTS with enhanced catalytic properties in the hydrogenation of p-nitrophenol to p-aminophenol. The asfabricated Pd-loaded ceramic membrane support was characterized by inductively coupled plasma (ICP) emission spectroscopy, X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), highresolution transmission electron microscopy (HRTEM), and temperature-programmed reduction (TPR). A comparative study with the palladium nanoparticles deposited on unsilanized ceramic membrane support was also performed.

2. EXPERIMENTAL SECTION 2.1. Preparation of Pd-Loaded Ceramic Membrane Supports. A tubular ceramic membrane support (designated as CM)

with a 12-mm outer diameter, an 8-mm inner diameter, and a 6-cm length was used as the starting material. The membrane support (provided by Nanjing Jiusi High-Tech Co. Ltd., Nanjing, China) had an asymmetrical structure in which a thin layer of Al2O3 with nominal pore size of 0.2 μm was coated on the outer wall of a tubular R-Al2O3 porous support layer. These tubes were sealed at their ends during the preparation of Pd-loaded ceramic membrane supports. γ-Aminopropyltriethoxysilane (3-APTS, industrial product; purchased from Nanjing Capatue Chemical Co. Ltd., Nanjing, China) was used as a silane coupling agent, and its structure is shown in Figure 1. The procedure for preparing Pd-loaded ceramic membrane supports is shown in Figure 2. First, the tubular ceramic membrane

support was immersed in 50 mL of a 6 g 3 L
1 solution of silane coupling agent in dichloromethane at room temperature for 48 h. 3-APTS can react with the OH groups on the ceramic membrane support surface through condensation as shown in step (1) in Figure 2,17 forming the covalent bonds of strong
Si
O
Al
between the ceramic membrane substrate and 3-APTS. After silanization, the support was rinsed thoroughly with ethanol and dried at room temperature. Then, the silalized ceramic membrane support (designated as CMS) was impregnated with 25 mL of a 0.02 M solution of Pd(OAc)2 in acetone at 40 °C for 12 h. Because the amine group (
NH2) is a strong electron donor having a strong capability for chelating to transition metal ions, Pd2þ can react with 3-APTS to form a complex (step 2 in Figure 2) and be bound to the ceramic membrane support surface. After impregnation, the support surface changed to a yellowish color. Finally, Pd2þ ions were reduced by 11 mL of an alkaline solution (pH 9.5) hydrazine hydrate (N2H4 3 H2O, 0.015 M) at room temperature to form metallic Pd particles (step 3 in Figure 2). During the reduction reaction, the color of the support surface changed from yellowish to black, suggesting the formation of metallic palladium nanoparticles on the surface. The sample was kept in alkaline hydrazine hydrate solution for 30 min and was then rinsed thoroughly with distilled water and dried at room temperature; this sample is denoted as Pd-CMS. For comparison, the ceramic membrane support without silanization was impregnated with an acetone solution of Pd(OAc)2 and reduced with N2H4 3 H2O under the same conditions; this Pdloaded ceramic membrane support is denoted as Pd-CM. 2.2. Characterization of Pd-Loaded Ceramic Membrane Supports. Pd content was analyzed by inductively coupled plasma (ICP) emission spectroscopy using an Optima 2000 DV system. For ICP analyses, the samples were digested in 10% (v/v) nitric acid solution at 60 °C for 1 h. ICP measurements were performed at 340.458 nm, with a Pd standard. X-ray powder diffraction (XRD) patterns were obtained on a Bruker D8 instrument with Ni-filtered Cu KR radiation (λ = 0.154 nm) at 40 kV and 40 mA, with a scanning rate of 0.05o/s in the 2θ range from 30° to 70°. The surface morphology of the support was observed by field-emission scanning electron microscopy (FESEM, Hitachi S-4800), and the composition was determined using energy-dispersive X-ray spectroscopy (EDS, NORAN System Six). XPS characterization was conducted on a Thermo ESCALAB 250 system with monochromatized Al KR radiation (hν = 1486.6 eV) at 15 kV. The residual pressure in the 4406

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Scheme 1. Catalytic Hydrogenation of p-Nitrophenol to p-Aminophenol

analysis chamber was about 10
9 mbar. The analyzer was operated in constant-analyzer-energy (CAE) mode with a pass energy of 20 eV and a 0.05 eV step. The C 1s signal (284.6 eV) was used to calibrate the binding energies. The morphology and particle size of the palladium nanoparticles were examined by high-resolution transmission electron microscopy (HRTEM, JEM-2010) at 200 kV. For HRTEM observations, the samples were prepared by scraping off the top layer of the membrane, applying sonication in ethanol for 10 min, and then depositing the material onto carbon-coated copper grids. Temperature-programmed reduction (TPR) was conducted using a BELCAT-A analyzer using a 150 mg sample for each measurement; the top membrane layer was scratched off for the analysis. Prior to the reduction, the sample was pretreated in an Ar stream at 300 °C for 2 h and then cooled to 50 °C. After that, a H2/ Ar mixture (10% H2 by volume) was switched on, and the temperature was raised to 750 °C at a rate of 10 °C 3 min
1. The consumption of H2 in the reactant stream was detected with a thermal conductivity cell. 2.3. Hydrogenation Experiments. The catalytic hydrogenation of p-nitrophenol to p-aminophenol, depicted in Scheme 1, was carried out in a 300 mL stainless steel autoclave equipped with a magnetically driven impeller. For each experiment, 14 g of p-nitrophenol dissolved in 163 mL of ethanol was first charged into the autoclave reactor, and the prepared Pd-loaded ceramic membrane support was fixed inside the autoclave. The reactor was sealed, purged with hydrogen five times to remove air, and then heated to the desired temperature under slow stirring (80 rpm). Once the temperature had reached the set value, hydrogen was introduced into the reactor to a set level, and the stirring rate was increased to 250 rpm. Finally, the hydrogenation reaction was performed at 102 °C under a pressure of 1.65 MPa. After 1 h of reaction, the support was removed from the reactor, thoroughly washed with ethanol, and dried at room temperature for the next run. The hydrogenation products were analyzed with a high-performance liquid chromatography (HPLC) system (Agilent 1200 Series) equipped with a diode array detector (DAD) and an autosampler. Chromatographic separations were performed at 35 °C using a ZORBAX Eclipse XDB-C18 column (5 μm, 4.6 mm  250 mm). A mobile phase composed of 80% methanol and 20% water at a flow rate of 1 mL 3 min
1 was used. In this work, the hydrogenation rate is expressed by the amount of hydrogen consumed per hour per surface area of the membrane support.

3. RESULTS AND DISCUSSION 3.1. Characterization of Pd-Loaded Ceramic Membrane Supports. XRD patterns of the prepared Pd-loaded ceramic

membrane supports all include a weak broad peak with a maximum at 2θ ≈ 40° indexing to face-centered cubic palladium marked by Miller indices (111) (Joint Committee on Powder Diffraction Standards card 05-0681) (data not shown here).24 The results indicate that the palladium salts were reduced to

Figure 3. FESEM images of the (a
c) top view and (d
f) base substrate below the top layer of (a,d) CM, (b,e) Pd-CMS, and (c,f) Pd-CM.

elemental Pd that deposited on the membrane support as nanoparticles. Similar observations were also found in the literature.14,25 FESEM images of the ceramic membrane support and the asfabricated Pd-loaded ceramic membrane supports are presented in Figure 3. Significant changes in the surface morphology can be observed for the Pd-loaded ceramic membrane supports as compared to the smooth surface layer of the ceramic membrane support. The roughening of the surface layer can be attributed to the deposition of palladium nanoparticles on the surface. By comparing parts b and c of Figure 3, one can see that the palladium distributions on the ceramic membrane supports with and without silanization by 3-APTS are different. For Pd-CMS, the entire membrane support surfaces are loaded with palladium nanoparticles, whereas the membrane support surfaces were only partially loaded for Pd-CM. Therefore, when the palladium loading levels are essentially the same for CMS and CM, as discussed later, the dispersion of palladium nanoparticles on CMS is better than that on CM. This result indicates that silanization with the silane coupling agent 3-APTS can be beneficial to improve the dispersion of palladium nanoparticles. From EDS analysis, it was found that the palladium was located on the ceramic membrane surface layer but not in the large pore support layer. This was verified by FESEM characterization of the base substrate below the top layer. For the observation, the sample was prepared by scraping off the membrane top layer. For consistency with Figure 3a
c, the FESEM image of the base substrate is also shown with a magnification factor of 50000 in Figure 3d
f. With such a magnification factor, only a single particle on the base substrate can be observed according to the 4407

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Figure 4. XPS survey spectra of the powder taken from the top membrane layer of Pd-CMS, Pd-CM, and CM.

FESEM characterization of the base substrate with a low magnification factor (data not shown). Some cracks as observed in Figure 3d
f might come from the particle itself. No obvious difference was found in the FESEM images of the ceramic membrane support and the Pd-loaded ceramic membrane supports when their membrane top layers were scraped off, indicating that no palladium was located in the large pore support layer. Bottino et al. also found that palladium was located only in a selective layer (γ-Al2O3 layer) owing to the higher surface area of γ-Al2O3 compared to the R-Al2O3 support.26 According to the ICP analysis, the palladium content was 0.32 mg per cm2 of membrane support area (corresponding to 7.23 mg mounted in the reactor for the measurement of catalytic properties) for Pd-CMS, and almost the same palladium content, 0.31 mg per cm2 of membrane support area was determined for the Pd-CM. In contrast to the results reported by Zhang et al.,15 silanization with 3-APTS did not noticeably increase the amount of palladium nanoparticles loaded on the ceramic membrane support surface. X-ray photoelectron spectroscopy (XPS) analysis was performed to confirm the presence of silane coupling agent on the silanized ceramic membrane support surface. The top membrane layer was scratched off for the analysis. Figure 4 presents the XPS survey spectra of the powder taken from the top membrane layer of the Pd-loaded ceramic membrane supports, and the result for ceramic membrane support is also presented for comparison. Si was detected in both types of Pd-loaded ceramic membrane supports, whereas N was found only in Pd-CMS, and the content of Si was 2.6 at. % in Pd-CMS, significantly higher than the value of 1.47 at. % in Pd-CM, which confirms that the silane coupling agent was present on the silanized ceramic membrane support surface. The Si in the sample of Pd-CM should come from the ceramic membrane support, as shown in Figure 4. As shown in Figure 5, the Pd 3d5/2 and Pd 3d3/2 electronic states for Pd-CM were observed at 334.80 and 340.05 eV, respectively, which are the characteristic values for Pd(0) species.24 In contrast, Pd-CMS showed photoelectron peaks corresponding to both Pd(0) and Pd(II) species. The Pd 3d5/2 and Pd 3d3/2 electronic states for Pd(0) were observed at 334.95 and 340.05 eV, respectively. Peaks were also detected at 337.60 and 342.95 eV corresponding to the 3d5/2 and 3d3/2 electronic states, respectively, of Pd(II) species, which suggests that the reduction process as presented in section 2.1 led to only a partial reduction of Pd(II) to Pd(0) for

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Figure 5. Pd 3d XPS spectra of the powder taken from the top membrane layer of Pd-CMS and Pd-CM.

Table 1. Binding Energies of Some Elements in the Ceramic Membrane Support and Pd-Loaded Ceramic Membrane Supports element

CM

Pd-CM

Pd-CMS

Al 2p

73.80

73.89

73.90

O 1s

530.65

530.54

530.57

Si 2p

102.0

102.29

102.03

N 1s







399.64

Pd-CMS. The presence of Pd(II) species should be due to a 3-APTS
Pd(II) complex. As in this investigation, Jayamurugan et al. also reported a similar result in the preparation of palladium nanoparticles stabilized by dendritic phosphine ligand-silanized silica.27 Table 1 lists the binding energies of some elements. For CM, Pd-CM, and Pd-CMS, no obvious differences were found in the binding energy of Al 2p. Similar phenomena were observed for O 1s and Si 2p. The binding energy of the N 1s peak in Pd-CMS was 399.64 eV higher than that of the
CH2
NH2 peak (at 399.4 eV), which might be related to N
Pd(0) coordinate bonds.17 Figure 6 shows TEM images of palladium nanoparticles on Al2O3 particles taken from the top layer of an asymmetrical R-Al2O3 membrane support with deposited palladium. Clearly, the palladium nanoparticles distributed in Pd-CMS uniformly (Figure 6a), whereas they formed larger aggregates in Pd-CM (Figure 6b), in good agreement with the FESEM characterization. The mean diameter of the palladium nanoparticles was about 3 nm in Pd-CMS and 7 nm in Pd-CM. Shang et al. also found that the use of a coupling agent effectively reduced the silica particle size in the hybrids.28 The characteristic lattice fringes of 2.2 Å spacing confirm the (111) planes of the facecentered cubic Pd(0) structure,24 as shown in the HRTEM images. From Figure 6c, it can be determined that there was a thin amorphous layer on the surface of Al2O3 particle, which can be attributed to the silane coupling agent of 3-APTS, in agreement with the XPS analysis results. Therefore, we concluded that, when the ceramic membrane support is silanized with a silane coupling agent of 3-APTS, small palladium nanoparticles with good dispersion can be obtained. TPR profiles of the powder taken from the top membrane layer are presented in Figure 7. The profiles of Pd-CMS and 4408

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Figure 8. Variation of the hydrogenation rate of p-nitrophenol with time.

Figure 6. TEM images of the powder taken from the top membrane layer of (a,c) Pd-CMS and (b,d) Pd-CM.

Figure 9. Catalytic stability investigations of prepared Pd-loaded ceramic membrane supports.

Figure 7. TPR profiles of the powder taken from the top membrane layer of (a) CM, (b) CMS, (c) Pd-CM, and (d) Pd-CMS.

Pd-CM are characterized by a negative peak at 105 and 103 °C, respectively, that can be attributed to hydrogen evolution due to decomposition of the β-PdH phase.29 For the Pd-CMS sample, the reduction peak observed at around 275 °C can be ascribed to the reduction of Pd(II) species, in agreement with the XPS characterization. For the Pd-CM sample, only Pd(0) was found on the ceramic support by XPS, as shown in Figure 5, so no reduction peaks originating from palladium oxide were observed. The broad hydrogen consumption band between 300 and 750 °C in the profiles of Pd-CM and Pd-CMS might be due to the reduction of ceramic membrane support, as shown in Figure 7a,b, respectively. 3.2. Hydrogenation of p-Nitrophenol on the Pd-Loaded Ceramic Membrane Supports. The catalytic properties of the as-fabricated Pd-loaded ceramic membrane supports were tested in the hydrogenation of p-nitrophenol to p-aminophenol in an

autoclave and not in membrane mode. A comparison of the catalytic activities of the two types of Pd-loaded ceramic membrane supports is shown in Figure 8. It can be seen that the hydrogenation rate of Pd-CMS was higher than that of Pd-CM under the same reaction conditions. Correlation of the catalytic activities of the palladium particles with their particle structures (size and dispersion) indicated that a smaller catalyst particle size and a higher dispersion led to higher catalytic activity. Our experimental findings suggest that the surface property of the ceramic membrane support silanized with silane coupling agent of 3-APTS is beneficial to improving the dispersion and obtaining smaller particle sizes, leading to a higher catalytic activity. To investigate the catalytic stabilities of the as-fabricated Pdloaded ceramic membrane supports, a number of catalytic reaction cycles were carried out. In this study, the catalytic stability is expressed by the ratio of the hydrogenation rate after a certain number of reaction cycles to that during the first reaction cycle, as displayed in Figure 8. Figure 9 shows a comparison of the catalytic stabilities of the two types of Pd-loaded ceramic membrane supports. Combined with the results in Figure 8, one can see that, for each cycle, the hydrogenation rate of Pd-CMS is higher than that of Pd-CM, indicating that silanization with 3-APTS is beneficial in enhancing the catalytic activity of palladium nanoparticles. During each continuous p-nitrophenol hydrogenation cycle, both prepared Pd-loaded membrane supports 4409

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Figure 10. TEM images of the powder taken from the top membrane layer of (a,c) Pd-CMS and (b,d) Pd-CM after six catalytic reaction cycles.

underwent almost-complete deactivation, but the deactivation trends were completely different: For the Pd-CMS system, the catalytic activity decreased until the fourth cycle and then remained constant for the fifth and sixth cycles, whereas for the Pd-CM system, the catalytic activity continued to decrease throughout the six catalytic reaction cycles. Note that, after six continuous hydrogenation cycles, the Pd-CMS system suffered about 16% deactivation, whereas the Pd-CM system suffered about 28% deactivation. These results indicate that the catalytic stability of the Pd-CMS system is superior to that of the Pd-CM system. For the former, the ceramic membrane support was silanized with the amino-functional silane 3-APTS before the conventional impregnation process, and thus, the palladium nanoparticles could be loaded onto the membrane support surface with chemical bonds, as presented in Figure 2. For the latter, the palladium nanoparticles were deposited on the membrane support surface through van der Waals forces. Because the binding force of a covalent bond is much stronger than the van der Waals force,17 it was harder for the palladium nanoparticles to detach from the silanized membrane support, and a superior catalytic stability could be obtained. This can be verified by ICP analysis of the used Pd-loaded ceramic membrane supports. ICP analysis showed that, after six continuous hydrogenation cycles, the palladium in Pd-CMS amounted to 0.24 mg per cm2 of membrane support area, which was 75% of the initial palladium content in fresh Pd-CMS, whereas the palladium content decreased significantly to 0.11 mg per cm2 of membrane support area in Pd-CM, corresponding to 35% of the initial content in fresh Pd-CM. Therefore, we can preliminarily conclude that the catalytic stability of Pd-CMS system is better than that of the PdCM system. More reaction cycles for the investigation of the catalytic stabilities of the prepared Pd-loaded ceramic membrane supports, especially for Pd-CMS, will be further carried out. The ICP analysis results also suggest that palladium leaching should be one of the main reasons for the decrease of the catalytic

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activity of Pd-loaded ceramic membrane supports. Besson and Gallezot suggested that catalyst leaching in the reaction medium was the main cause of deactivation in liquid-phase reactions.30 In the study of Jayamurugan et al., a significant reduction in palladium leaching was also considered to contribute to the consistent catalytic activity of the recycled catalyst.27 However, a correlation between catalyst deactivation and palladium leaching in this work shows that the two do not match. For instance, only 35% of the initial palladium was present in the used Pd-CM, but the catalyst maintained more than 70% of its initial catalytic activity. This result indicates that, excluding palladium leaching, other parameters such as morphology evolution and species change of the palladium nanoparticles also play certain roles in determining the catalytic stability of the Pd-loaded ceramic membrane support. To investigate the influence of the evolution of the morphology of the supported palladium nanoparticles, the palladium nanoparticles on the membrane supports after six catalytic reaction cycles were examined by TEM as shown in Figure 10. For the Pd-CMS, the palladium dispersion and particle size did not change obviously as, presented in Figure 6a,c and Figure 10a,c. By comparing Figure 6b,d and Figure 10b,d, it can be seen that, for Pd-CM, the palladium dispersion improved, and the particle size decreased, possibly because of more palladium leaching, resulting in a higher catalytic activity and a lower deactivation degree compared to the leaching degree. Compared to Pd catalysts supported on powders,31 the hydrogenation rate of the prepared Pd-loaded ceramic membrane support is obviously lower (about 80 mmol 3 min
1 3 gPd
1) under similar reaction conditions, because it is much more difficult for the liquid to contact the palladium particles in the membrane support than in powder catalysts. However, the catalytic stability is superior to that of Pd catalysts supported on powders. Throughout six continuous hydrogenation cycles, the Pd
B/ TiO2 system suffered 42% deactivation in the p-nitrophenol hydrogenation,31 obviously higher than the value in this work. This can be explained as follows: The hydrogenation reaction was carried out in a slurry reactor with vigorous stirring to eliminate the influence of external diffusion. Compared to powder catalysts, vigorous stirring could result in constant interparticle collisions and collisions of the catalyst particles with the impeller, resulting in the leaching of active metal from the support and, thereby, deactivation. For the Pd-loaded ceramic membrane support, the active metal was bound to the membrane support, and no collisions occurred, leading to a higher catalytic stability. In addition, the leaching of palladium nanoparticles from the support could also be affected by the aggregate change in H2 contact. Moreover, the Pd-loaded ceramic membrane support could be easily removed from the reactor and repeatedly used in the liquid-phase hydrogenation of p-nitrophenol, in contrast to free powder catalysts. HPLC analysis showed that the selectively to p-aminophenol reached 100% in all cases (data not shown here), meaning that the prepared Pd-loaded ceramic membrane supports had a high catalytic selectivity in the p-nitrophenol hydrogenation.

4. CONCLUSIONS The surface of an alumina tubular membrane support was functionalized with a silane coupling agent, and Pd nanoparticles were then deposited on the silanized membrane support surface to fabricate Pd-loaded ceramic membrane support with improved catalytic properties and without additional separation 4410

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Industrial & Engineering Chemistry Research steps for the catalyst particles. The distribution of Pd nanoparticles deposited on silanized membrane supports was much more uniform and the particle size was smaller, compared to those deposited on membrane supports without silanization. As a consequence, the Pd nanoparticles deposited on silanized membrane supports showed a higher reaction rate and enhanced stability in the liquid-phase hydrogenation of p-nitrophenol to paminophenol. The results of this explorative study are encouraging, and further research is in progress to improve the performance of the as-fabricated Pd-loaded ceramic membrane supports by optimizing the preparation conditions and the operation mode of membrane reactor.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the National Basic Research Program (2009CB623406), the National Natural Science Foundation (20636020, 20806038), the Natural Science Foundation of Jiangsu Province (BK2010549, BK2009021), and the Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (09KJB530006) of China is gratefully acknowledged. ’ REFERENCES


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