Article pubs.acs.org/IECR
Preparation of Palladium Nanoparticles Deposited on a Silanized Hollow Fiber Ceramic Membrane Support and Their Catalytic Properties Rizhi Chen,*,†,‡ Yuanguo Jiang,†,‡ Weihong Xing,† and Wanqin Jin† †
State Key Laboratory of Materials-Oriented Chemical Engineering, and ‡Jiangsu Key Laboratory of Industrial Water-Conservation and Emission Reduction, Nanjing University of Technology, Nanjing 210009, P.R. China ABSTRACT: An efficient and reusable catalyst was developed by depositing palladium nanoparticles on a hollow fiber ceramic membrane support in which the support surface was silanized with aminofunctional silane. In the present work, the hollow fiber ceramic membrane support was used for catalyst immobilization only. The as-prepared Pd-loaded hollow fiber ceramic membrane support was extensively characterized by ICP, FESEM, EDS, XPS, and HRTEM, and the hydrogenation of pnitrophenol to p-aminophenol was used as a model reaction to evaluate its catalytic properties. For comparison the palladium nanoparticles were also deposited on a tubular ceramic membrane support. The catalytic activity of Pd-loaded hollow fiber ceramic membrane support is significantly higher than that of Pd-loaded tubular ceramic membrane support. The hollow ceramic fibers can provide more membrane area for the deposition of palladium nanoparticles at the same volume of membrane module, and a higher palladium loading is obtained, and their pore structure is beneficial for the reactants to diffuse onto the catalyst surface, leading to a higher catalytic activity.
1. INTRODUCTION Nanosized metal particles are a focus of extensive study due to their superior catalytic properties compared to that of bulk materials.1 These nanosized metal particles catalysts can be applied in the form of powder suspended in slurry or they can be immobilized on various supporting media, such as glass, quartz, stainless steel, or membrane.2,3 Although the catalysts in suspension have better catalytic properties compared to those of immobilized ones, the suspended catalysts need to be separated from the products in practical applications.4−6 An efficient route to solve the above problem is to immobilize the catalyst particles in or on membranes.7,8 The surface area/ volume ratio of membrane supports cannot compete with the normal powder supports. However, contrary to free powder catalysts, the supported metal catalysts can be easily reused in the liquid-phase reactions. A similar study has been reported.2 Compared to polymeric membranes, ceramic membranes such as Al2O3 and ZrO2 are a desirable candidate as catalyst supports because of their advantages such as superior chemical stability, long life, and excellent mechanical strength.9,10 The surface properties of the support have a great influence on the amounts and quality of deposited metal particles.11,12 Generally, the metal particles have poor adhesion to the support due to the absence of chemical conjunction,11 which will lead to the easy leaching of the catalyst particles from the support surface during the reactions. It is well-known that the aminofunctional silanes can be applied to increase the loaded amounts of the metal particles and improve the adhesion between the metal particles and the support.11−14 In our previous work, γ-aminopropyltriethoxy silane (3-APTS) was used to modify the ceramic membrane surface and thereby to improve the catalytic properties of palladium nanoparticles deposited on the silanized ceramic membrane support surface.15 3-APTS can react with the −OH groups on the © 2013 American Chemical Society
ceramic membrane support surface through condensation, forming the covalent bonds such as −Si−O−Al− between the ceramic membrane substrate and 3-APTS. Because the amine group (−NH2) in 3-APTS is a strong electron donor having a strong capability for chelating to transition metal ions, Pd2+ can react with 3-APTS to form a complex and be bound to the ceramic membrane support surface. After the reduction step, palladium nanoparticles can be formed and uniformly distributed on the silanized ceramic membrane support surface. As a result, the Pd-loaded surface-silanized ceramic membrane support showed superior catalytic properties in the liquid-phase hydrogenation of p-nitrophenol to p-aminophenol.15 Membrane configuration is also one of the key factors affecting the amounts and quality of deposited metal particles. Recently, hollow fiber ceramic membranes have attracted considerable interest due to the much higher surface area/ volume ratio over most of the disk/tubular counterparts.16−18 When hollow fiber ceramic membranes are used as supports, higher surface area can be provided at the same volume for the deposition of nanosized particles, leading to higher loading and better catalytic performance. For example, in the work of Wu et al., a hollow fiber catalytic ceramic membrane with a surface area/volume ratio of approximately 2770 m2/m3 could be achieved, and the catalytic ceramic membrane showed better catalytic performance in the partial oxidation of methane to syngas.17 Thus, in the study, fabrication of hollow fiber ceramic membranes was attempted for supporting palladium nanoparticles. Received: Revised: Accepted: Published: 5002
July 8, March March March
2012 13, 2013 14, 2013 14, 2013
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The procedure outlined in our previous research can be referred to for preparing Pd-loaded hollow fiber ceramic membrane support. First, in order to have enough −OH groups for the silanization reaction, the hollow fiber ceramic membrane support was immersed in distilled water for 6 h and then dried at 100 °C overnight prior to the silanization. After the pretreatment, the hollow fiber ceramic membrane support was immersed in 50 mL of a 3 g·L−1 solution of silane-coupling agent in dichloromethane at room temperature for 12 h and rinsed thoroughly with ethanol and dried at room temperature. Then, the silanized membrane support was impregnated with 25 mL of a 0.04 M solution of Pd(OAc)2 in acetone at 40 °C for 12 h. After impregnation, the support surface changed to a yellowish color. Finally, Pd2+ ions were reduced by 60 mL of a 0.015 M alkaline solution (pH = 9.5) of hydrazine hydrate (N2H4·H2O) in water at 0 °C to form metallic Pd particles. 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 hydrazine hydrate alkaline solution for 30 min, followed by rinsing thoroughly with distilled water and subsequent drying at room temperature; the sample is denoted as Pd-HFCM. For comparison, the tubular alumina ceramic membrane support (outer diameter,12 mm; inner diameter, 8 mm; length, 6 cm; and nominal pore size, 0.2 μm; provided by Nanjing Jiusi High-Tech Co. Ltd., Nanjing, China) was modified by 3-APTS, impregnated with Pd(OAc)2, and reduced by N2H4·H2O under the same conditions; this Pd-loaded tubular ceramic membrane support is denoted as Pd-TCM. 2.2. Characterization of Pd-Loaded Ceramic Membrane Supports. Pd content was analyzed by inductively coupled plasma emission spectroscopy (ICP, Optima 2000DV). 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 Pd standard. The surface morphology of the support was observed using field-emission scanning electron microscopy (FESEM, Hitachi S-4800), and the composition was determined using energy-dispersive X-ray spectroscopy (EDS, NORAN System Six). The XPS characterization was conducted on a ULVAC PHI 5000 VersaProbe system with a monochromatized Al Kα radiation (hν = 1486.6 eV) at 15 kV. The residual pressure in the analysis chamber was about 10−10 mbar. The C1s signal (284.6 eV) was used to calibrate the binding energies. The morphology and particle size of the palladium nanoparticles were examined by a highresolution transmission electron microscope (HRTEM, JEM2100) operated at 200 kV. For HRTEM observations, the samples were prepared by sonication in ethanol for 10 min and then deposited onto carbon-coated copper grids. For the XPS, or TEM analysis, the membrane was broken into powders with respect to Pd-HFCM, while the top membrane layer was scratched off for Pd-TCM. 2.3. Catalytic Properties Measurement. 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 membrane module, as shown in Figure 2. The membrane module was fixed on the inlet pipeline, and the length was 6 cm, and the diameter was 12 mm. In this work, twelve Pd-HFCMs and one Pd-TCM were used, and the volume of membrane module was the same for the two Pdloaded membrane supports. The membrane area was about 45 and 23 cm2 for HFCMs and TCM, respectively. Thus, the
In the present work, and in continuation of our research interest on the supported catalysts, we have attempted to fabricate palladium nanoparticles deposited on the hollow fiber ceramic membrane support silanized by 3-APTS with enhanced catalytic properties in the hydrogenation of p-nitrophenol to paminophenol. The as-prepared Pd-loaded hollow fiber ceramic membrane support was characterized by ICP, FESEM, EDS, XPS, and HRTEM. A comparative study was also carried out with palladium nanoparticles deposited on the tubular ceramic membrane support. The present work focuses on investigating the effect of membrane configuration on the performance of Pd-loaded ceramic membrane support.
2. EXPERIMENTAL SECTION 2.1. Preparation of Pd-Loaded Ceramic Membrane Supports. An alumina hollow fiber ceramic membrane support (marked as HFCM, provided by University of Science and Technology of China) with a 2-mm outer diameter, a 1.4-mm inner diameter, and a 6-cm length was used as the starting material. The alumina hollow fiber was prepared using the reported phase inversion method19 and has an average pore size of 2 μm according to the pore size distribution curve (data not shown here). An asymmetric pore structure is observed for the alumina hollow fiber as presented in Figure 1a, which consists
Figure 1. (a) Cross-sectional FESEM image of HFCM, (b) surface FESEM image of HFCM, (c) surface FESEM image of Pd-HFCM, (d) surface FESEM image of TCM, (e) surface FESEM image of Pd-TCM, and (f) EDS analysis of cross section of Pd-HFCM.
of an inner sponge-like region, fingerlike voids, and a region close to the outer surface which may be occupied either by a spongelike structure or by fingerlike voids.20 These tubes were sealed at their ends during the preparation of Pd-loaded hollow fiber 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. 5003
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Scheme 1. Catalytic Hydrogenation of p-Nitrophenol to pAminophenol
surface. By comparing parts c and e of Figure 1, one can see that the dispersions of palladium nanoparticles on the membrane support surfaces are different. For Pd-HFCM some palladium nanoparticles agglomerate together, whereas the palladium nanoparticles are uniformly dispersed on the membrane support surface with respect to Pd-TCM, indicating the dispersion of palladium nanoparticles using the hollow fiber membranes as supports is worse. There is one possible explanation for this observation. The surface of the hollow fiber membrane support is composed of some larger particles as shown in Figure 1b, resulting in providing a lower surface area for the deposition of palladium nanoparticles and thus an unsatisfactory dispersion. It is seen from the EDS analysis of the cross section of Pd-HFCM in Figure 1f that the entire membrane support cross section is loaded with palladium nanoparticles (white dots). Furthermore, more palladium nanoparticles are loaded in the regions close to the inner and outer surfaces compared to those loaded in the middle region, which should be related to the asymmetric pore structure of the hollow fiber ceramic membrane support. As presented in Figure 1a, in the middle region are fingerlike voids, and the surface area is lower for depositing the palladium nanoparticles. Thus, the loaded palladium nanoparticles are few. Our previous research indicated that the palladium nanoparticles were only located in the tubular ceramic membrane surface layer,15 owing to the higher area of the surface layer compared to that of the large pore support layer.22 ICP analysis shows that the weight percentage of Pd is 0.50 wt % (corresponding to 9.8 mg of palladium mounted in the reactor for the catalytic reactions) with respect to Pd-HFCM, significantly higher than the value of 0.072 wt % determined for Pd-TCM (corresponding to 8.7 mg of palladium mounted in the reactor for the catalytic reactions). The significantly higher weight percentage of Pd for Pd-HFCM should be mainly due to the very thin wall of alumina hollow fiber as presented in section 2.1. The result indicates that it is beneficial to increase the loading of palladium nanoparticles at the same volume as that of the membrane module with the hollow ceramic fibers as supports because a larger membrane area is provided for the deposition of palladium nanoparticles. XPS was performed to confirm the presence of silane coupling agent and palladium in the as-prepared Pd-loaded ceramic membrane supports and for the semiquantitative analysis of the relative atomic concentration of different elements. From the XPS survey scan spectra, shown in Figure 3, the XPS peaks of Si 2p, Pd 3d, and N 1s can be found for both Pd-loaded ceramic membrane supports. The N 1s peak is
Figure 2. Schematic diagram of the reactor system used for hydrogenation experiments. Legend: 1, hydrogen inlet valve; 2, stirrer; 3, exhaust valve; 4, heater; 5, inlet pipeline; M, membrane module; PR, pressure regulator; P, pressure indicator; and T, thermocouple.
membrane area-to-volume ratio for HFCMs was about 2 times higher than that for TCM. In each hydrogenation run, 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 supports were fixed inside the autoclave. The reactor was sealed, purged with hydrogen five times to remove air, and then heated to a desired temperature under slow stirring (80 rpm). As the temperature 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 and 1.65 MPa. After 1 h reaction, the supports were removed from the reactor, thoroughly washed with ethanol, and dried at room temperature for the next run. The hydrogenation products were analyzed by a high-performance liquid chromatography (HPLC) system (Agilent 1200 series, U.S.A.) equipped with a diode array detector (DAD) and an autosampler. Chromatographic separations were performed at 35 °C using a ZORBAX Eclipse XDB-C18, 5 μm, 4.6 mm × 250 mm column. A mobile phase composed of 80% methanol and 20% water at a flow rate of 1 mL·min−1 was used. In this present work, the hydrogenation rate is expressed by the amount of hydrogen consumed per hour per liquid phase volume.21 1H NMR spectra were acquired using an AV400D nuclear magnetic resonance spectrometer to further analyze the formation of p-aminophenol with DMSO-d6 as the solvent and TMS as the internal standard.
3. RESULTS AND DISCUSSION 3.1. Characterization of Pd-Loaded Ceramic Membrane Supports. Parts b−e of Figure 1 show the FESEM images of the ceramic membrane supports and the as-prepared Pd-loaded ceramic membrane supports. As compared to the bare surface of the ceramic membrane supports, significant changes in the surface morphology are observed for the two types of Pd-loaded ceramic membrane supports, which are attributed to the deposition of palladium nanoparticles on the
Figure 3. XPS survey spectra of Pd-HFCM and Pd-TCM. 5004
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not obvious, especially for Pd-HFCM, which should be due to the low content of the N element as presented in Table 1. This Table 1. Atomic Concentration of Different Elements for Pd-Loaded Ceramic Membrane Supports atomic concentration (%) sample
C 1s
N 1s
O 1s
Al 2p
Si 2p
Pd 3d
Pd-HFCM Pd-TCM
23.44 27.26
0.84 1.33
50.02 46.28
19.09 20.43
5.44 3.11
1.17 1.58
confirms the presence of the silane coupling agent on the silanized ceramic membrane support surface and the loading of palladium nanoparticles on the surface with chemical bonds. As shown in Figure 4, the Pd 3d5/2 and Pd 3d3/2 electronic states Figure 5. TEM images of (a,b) Pd-HFCM and (c,d) Pd-TCM.
are nanosized palladium particles distributed on the Al2O3 particle surface taken from the Pd-loaded hollow fiber ceramic membrane support; however, these nanosized palladium particles trend to aggregate together. Compared to PdHFCM, the palladium nanoparticles can uniformly distribute in Pd-TCM shown in Figure 5c, in good agreement with the FESEM characterization. The mean diameter of the palladium nanoparticles in Pd-HFCM is about 5 nm, while it is 3 nm in Pd-TCM as was the reported value in our previous work.15 The characteristic lattice fringes of 2.2 Å spacing confirms the (111) planes of the face-centered cubic Pd(0) structure as shown in the HRTEM images.23 One can see from Figure 5b and d that there is a thin amorphous layer on the surface of the Al2O3 particle, which might be attributed to the silane coupling agent of 3-APTS, in agreement with the analysis results of XPS. 3.2. Hydrogenation of p-Nitrophenol on the PdLoaded Ceramic Membrane Supports. In continuing our work in the area of nanosized catalysts loaded on ceramic membrane supports,15 we tested the catalytic properties of the as-prepared Pd-loaded ceramic membrane supports in the pnitrophenol hydrogenation in an autoclave and not in membrane mode. Figure 6 presents the catalytic activity
Figure 4. Pd 3d XPS spectra of Pd-HFCM and Pd-TCM.
for Pd-HFCM are observed at 335.2 and 340.4 eV, respectively, which are the characteristic values for Pd(0) species.23 Similar results are also observed for Pd-TCM. The results indicate that Pd(II) can be completely reduced to Pd(0) in the present work. In contrast, Pd(II) was only partly reduced to Pd(0), and Pd(II) species were still found in our previous research.15 The difference is related to the reduction conditions. In this work 60 mL of a 0.015 M hydrazine hydrate solution was used to reduce Pd(II), obviously higher than the value of 11 mL adopted in previous work. The coordination between Pd(II) and 3-APTS makes the reduction of Pd(II) difficult. A similar result was also found in the preparation of palladium nanoparticles stabilized by dendritic phosphine ligand-silanized silica.24 The XPS spectra seem to suggest that Pd-TCM has a higher Pd loading than Pd-HFCM, which should be related with the experimental conditions. As presented in section 2.3, the membrane area was about 45 and 23 cm2 for HFCM and TCM, respectively. According to the ICP analysis, the total Pd loading is 9.8 mg for Pd-HFCM and 8.7 mg for Pd-TCM. Thus, the Pd loading per membrane area for Pd-HFCM is about 0.22 mg·cm−2, lower than the value of 0.38 mg·cm−2 for Pd-TCM, because the pore size of HFCM (2 μm) is larger than that of TCM (0.2 μm) and as a result the former provides a lower surface area for the deposition of palladium nanoparticles compared to the latter.15 XPS is used to measure the elemental composition of the surface, and thus, in the present work the XPS results can indicate that Pd-TCM has a higher Pd loading compared to that of Pd-HFCM. TEM was used to further examine and compare the morphology and particle size of the palladium nanoparticles loaded on the two types of ceramic membrane supports. The TEM image of Pd-HFCM in Figure 5a clearly shows that there
Figure 6. Variation of the hydrogenation rate of p-nitrophenol with time.
comparison of the two types of Pd-loaded ceramic membrane supports. As expected, the hydrogenation rate of Pd-HFCM is significantly higher compared to that of Pd-TCM under the same reaction conditions. For example, at about 60 min, the hydrogenation rate of Pd-HFCM is about 44% higher than that of Pd-TCM. Combined with the foregoing characterization 5005
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results of ICP, one can find that the increase of palladium loading is one of the key factors for the higher catalytic activity of Pd-HFCM. However, compared to that of Pd-TCM the palladium loading in Pd-HFCM only increases 13% (from 8.7 to 9.8 mg), obviously lower than the increased degree of catalytic activity. In addition, according to the above results of FESEM and TEM in Figures 1 and 5, for Pd-HFCM the dispersion of palladium nanoparticles on the membrane support surface is worse, and the corresponding mean particle size is larger. Our previous work showed that a smaller catalyst particle size and a higher dispersion could lead to higher catalytic activity.15 These results indicate that, excluding the loading effect, other parameters such as membrane pore structure have some certain roles in determining the catalytic activity of the palladium. As discussed previously, for PdHFCM, the entire membrane support cross section is loaded with palladium nanoparticles, the thickness of cross-section is only about 300 μm, and the average membrane pore size is 2 μm, obviously higher than the value of 0.2 μm for the tubular ceramic membrane support. As a result, compared to Pd-TCM, the reactants can easily diffuse into the catalyst surface for PdHFCM, resulting in the enhanced catalytic activity. According to the above discussion, we can find that there are two main reasons for the higher catalytic activity of Pd-HFCM. On one hand, higher palladium loading is obtained when the hollow fiber ceramic membranes are used as supports. On the other hand, the thin cross-section and larger membrane pore size of the hollow fiber ceramic membrane support are beneficial to the mass transfer of reactants into the catalyst surface. In order to investigate the catalytic stabilities of the asprepared Pd-loaded ceramic membrane supports, a number of catalytic reaction cycles were carried out. The correlations between the hydrogenation rate (at about 30 min), and the number of catalytic reaction cycle are presented in Figure 7.
mon is serious, resulting in more palladium leaching and a worse catalytic stability. This can be verified by ICP analyses of the used Pd-loaded ceramic membrane supports. ICP analysis shows that, after six continuous hydrogenation cycles, the palladium in Pd-HFCM amounts to 4.8 mg which is only 49% of the initial palladium content in fresh Pd-HFCM, while the palladium content decreases to 6.8 mg in Pd-TCM, corresponding to 78% of the initial content in fresh Pd-CM. Therefore, we can preliminarily conclude that more palladium leaching should be responsible for the worse catalytic stability of Pd-HFCM. The ICP analysis results also suggest that the palladium leaching should be one of the main reasons for the decrease of catalytic activity of the Pd-loaded ceramic membrane support. Catalyst leaching in the reaction medium was the main cause of deactivation in liquid-phase reactions.25 Jayamurugan et al. also reported that a significant reduction in palladium loading resulted in the consistent catalytic activity of the recycled catalyst.24 However, a correlation between catalyst deactivation and palladium leaching in the present work shows that the two do not match. For example, only 78% of the initial palladium is present in the used Pd-TCM, but the catalyst maintains 95% of its initial catalytic activity. This result indicates that, excluding palladium leaching, other parameters, such as the morphology evolution of membrane and palladium nanoparticles, also play certain roles in determining the catalytic stability of the Pd-loaded ceramic membrane support. To investigate the influence of the morphology evolution of membrane and palladium nanoparticles, the recycled Pd-loaded ceramic membrane supports after six catalytic reaction cycles were examined by FESEM and TEM as shown in Figures 8 and
Figure 7. Catalytic stability investigations of as-prepared Pd-loaded ceramic membrane supports. Figure 8. (a, b) surface FESEM images of fresh and recycled PdHFCM, respectively; (c, d) cross-sectional FESEM images of fresh and recycled Pd-HFCM, respectively; (e, f) surface FESEM images of fresh and recycled Pd-TFCM, respectively; (g, h) cross-sectional FESEM images of fresh and recycled Pd-TFCM, respectively.
Note that, throughout six continuous hydrogenation cycles, the hydrogenation rate of Pd-HFCM decreases from 399.4 to 213.3 mol·h−1·m−3 and suffers about 47% deactivation, whereas the Pd-TCM system suffers about 5% deactivation corresponding to the decrease of hydrogenation rate from 237.0 to 225.2 mol·h−1·m−3. These results indicate that the catalytic stability of Pd-HFCM system is worse compared to that of the Pd-TCM system, which might be related to the membrane pore structure. Compared to that over Pd-TCM, during the hydrogenation of p-nitrophenol over Pd-HFCM the reaction solution can easily come into contact with the palladium nanoparticles as discussed above, and the scouring phenome-
9. For both Pd-HFCM and Pd-TCM, the surface and cross section of the ceramic membrane support do not change obviously after six catalytic reaction cycles as shown in Figure 8, indicating the excellent thermal and chemical stability of ceramic membrane support.26 By comparing Figures 5 and 9, one can find that, with respect to Pd-HFCM or Pd-TCM, the particle size of palladium is almost the same; however, the 5006
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by NMR spectroscopic analysis: 1H NMR (DMSO-d6, 400 MHz) δ (ppm): 8.34 (s, 1H, −OH), 6.49 (d, 2H, ArH), 6.40 (d, 2H, ArH), 4.38 (s,2H, NH2).27
4. CONCLUSION In this work, a hollow fiber ceramic membrane silanized with aminofunctional silane was used as a support to fabricate a Pdloaded hollow fiber ceramic membrane support with enhanced catalytic properties and without additional separation steps for the catalyst particles. A higher catalytic activity is observed for the Pd-loaded hollow fiber ceramic membrane support compared to that for the Pd-loaded tubular ceramic membrane support in the liquid-phase hydrogenation of p-nitrophenol to p-aminophenol, which is mainly caused by the higher palladium loading and the easy diffusing of the reactants onto the catalyst surface. Unfortunately, the catalytic stability of the Pd-loaded hollow fiber ceramic membrane support is not satisfying, and further research is in progress to improve its catalytic stability by optimizing the preparation conditions and the operation mode of the membrane reactor.
Figure 9. TEM images of (a, b) Pd-HFCM and (c, d) Pd-TCM after six catalytic reaction cycles.
palladium dispersion improves especially for Pd-TCM, resulting in higher catalytic activity and a lower degree of deactivation compared to that of leaching. These results indicate that the decrease of catalytic activity of Pd-loaded ceramic membrane support during the continuous hydrogenation cycles is mainly caused by the palladium leaching and the improvement of palladium dispersion can restrain the decrease of catalytic activity. One also can find from Figure 7 that, although the decrease in the degree of hydrogenation rate for Pd-HFCM is higher than that for Pd-TCM, the hydrogenation rate of PdHFCM is higher than that of Pd-TCM until the fourth cycle and the hydrogenation rate of the fifth or sixth cycle is almost the same for Pd-HFCM and Pd-TCM, indicating that adopting the hollow fiber ceramic membrane as a support is beneficial to enhancing the catalytic activity of palladium nanoparticles. The catalytic performance of Pd-loaded tubular ceramic membrane support prepared in the present work is different from that in the reported results in our previous work,15 which should be related to the different preparation conditions as presented in the Experimental Section. The hydrogenation products were analyzed by HPLC, as shown in Figure 10. The retention times at 2.4 and 2.8 min are for p-aminophenol and p-nitrophenol, respectively. No impurities are found, suggesting that the as-prepared Pd-loaded ceramic membrane supports have high catalytic selectivity in the catalytic hydrogenation of p-nitrophenol to p-aminophenol. The selective formation of p-aminophenol is further confirmed
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AUTHOR INFORMATION
Corresponding Author
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[email protected] Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial supports from the National Key Science and Technology Program (2011BAE07B05), the National High Technology Research and Development Program (2012AA03A606), the National Natural Science Foundation (20990222, 21106061) and the Natural Science Foundation of Jiangsu Province (BK2010549, BK2009021) of China are gratefully acknowledged.
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REFERENCES
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Figure 10. HPLC analyses of (a) analytical grade p-aminophenol, (b) analytical grade p-nitrophenol, and the reaction products catalyzed by (c) Pd-TCM and (d) Pd-HFCM. 5007
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dx.doi.org/10.1021/ie303104m | Ind. Eng. Chem. Res. 2013, 52, 5002−5008
