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Effect of Catalyst Morphology on the Performance of Submerged Nanocatalysis/ Membrane Filtration System Rizhi Chen,† Yan Du,‡ Qinqin Wang,† Weihong Xing,† Wanqin Jin,*,† and Nanping Xu† State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China, and College of EnVironmental Sciences, Nanjing UniVersity of Technology, Nanjing 210009, People’s Republic of China
Coupling systems of nanocatalysis and membrane filtration (nanocatalysis/MF) are features of convenience for the in situ separation of nanocatalysts from the reaction mixture. In this work, a submerged nanocatalysis/ MF system with a tubular ceramic membrane as the separation unit was developed for the liquid-phase hydrogenation of p-nitrophenol to p-aminophenol over nickel nanoparticles with various particle morphologies obtained by hydrogen annealing at different temperatures. We extensively characterized the nickel nanoparticles using X-ray diffractometry (XRD), nitrogen adsorption, transmission electron microscopy (TEM), and highresolution transmission electron microscopy (HRTEM), and we determined that the annealing temperature significantly influenced the particle size, specific surface area, and crystalline morphology of the nickel nanoparticles. We then evaluated the catalytic performance and separation efficiency of the submerged nanocatalysis/MF system. The nickel nanoparticles annealed at different temperature showed remarkably different catalytic activity, because of their specific structural properties. There was an unexpected nonlinear relationship between the nickel particle size and the flux, which was due to the changed nature of the cake layer formed on the membrane surface and a pore blocking effect. The results from both aspects of catalysis and separation performance indicated that the nickel nanoparticles annealed at 100 °C displayed a best balanced catalytic performance and separation efficiency for the submerged nanocatalysis/MF system. 1. Introduction p-Aminophenol is a commercially important intermediate for the preparation of analgesic and antipyretic drugs such as paracetamol, acetanilide, and phenacetin.1-4 Traditionally, paminophenol is prepared via a multistep iron-acid reduction of p-nitrochlorobenzene or p-nitrophenol. The major disadvantage of the iron-acid reduction is the generation of large amount of Fe-Fe oxide sludge, which cannot be reused and causes severe disposal problems.2 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:5 (i) the quantitative formation of side products, such as aniline, via further hydrogenation of the intermediate phenylhydroxylamine; and (ii) the use of highly corrosive mineral acid. In view of the increasing demands for p-aminophenol, direct catalytic hydrogenation of p-nitrophenol to p-aminophenol becomes important, because this could be an efficient and environmental friendly route.5 Recently, many researchers reported the liquid-phase pnitrophenol hydrogenation over Raney nickel,6 nickel nanoparticles,6 and several types of noble-metal catalysts, e.g., Pt/C.5 In our previous work, the catalytic performance of nickel nanoparticles has been proven to be superior to that of Raney nickel for this reaction.6 Considering practical applications, the recovery of nickel nanoparticle catalysts from the reaction mixture becomes a critical issue, because it is directly associated with the product purity and the operation cost.7 A simple approach to overcome this problem is to attach catalysts to a suitable substrate; however, in that case, the drawbacks are * To whom correspondence should be addressed. Tel.: +86-258358-7211. Fax: +86-25-8358-7211. E-mail:
[email protected]. † State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering. ‡ College of Environmental Sciences.
limited mass transfer of reactants to the catalyst surface and a decrease in the effective surface area of the catalyst particles. It has been reported that catalysts in suspension have better catalytic activity than immobilized ones.8,9 One of promising methods to solve the aforementioned problem is to couple the heterogeneous catalytic reaction with ceramic membrane separation,10 in which the membrane facilitates the in situ separation of fine catalysts from the reaction mixture. Catalysis with membrane filtration (catalysis/MF) systems can be assembled in two distinct designs: either as a submerged catalysis/MF system, with the reaction and separation zone combined in a single unit of equipment, or as a side-stream catalysis/MF system, where the reaction occurs in a stirred reaction vessel and the separation of the product is performed in a separate cross-flow membrane filtration unit.10 In our previous work, a side-stream nanocatalysis and membrane filtration (nanocatalysis/MF) system has been developed for the hydrogenation of p-nitrophenol to p-aminophenol over nickel nanoparticles, in which the nickel nanoparticles could be recovered completely, using a ceramic membrane.7 However, it was determined that the nickel nanoparticles frequently adsorbed on the surface of the pipeline, the tank, and the membrane, which caused a decrease in catalyst concentration in the reaction slurry and led to a consequent decline in the reaction rate and the permeate flux of the membrane.7 To overcome this problem, in the present work, we introduced a submerged nanocatalysis/MF system for the hydrogenation of p-nitrophenol over nickel nanoparticles, where the ceramic membrane was submerged in the reactor. The preliminary experiments confirmed that the decrease in reaction rate in the submerged nanocatalysis/MF system was much lower than that in the side-stream nanocatalysis/MF system during the continuous reaction cycles, because the latter provided more areas for the adsorption of nickel nanoparticles. In fact, the submerged hybrid system has attracted more attention recently, because of
10.1021/ie900033m CCC: $40.75 2009 American Chemical Society Published on Web 06/16/2009
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Figure 1. Schematic diagram of the submerged nanocatalysis/MF system used for hydrogenation experiments. Legend: 1, autoclave; 2, liquid outlet valve; 3, nitrogen inlet valve; 4, exhaust valve; 5, hydrogen inlet valve; 6, stirrer; M, membrane module; PR, pressure regulator; PI, pressure indicator; and TI, thermocouple. (For inset: a, glaze sealer; and b, ceramic membrane.)
its lesser loss of fine catalysts and energy costs, compared to the side-stream hybrid system.11-13 With respect to the hybrid system, the catalytic properties of the nickel nanoparticles and the separation efficiency of the ceramic membrane are the two main points when we evaluate the performances of the catalysis/separation coupling system, which must be taken into account integrally. It is well-known that the physical characteristics of catalyst influence not only the catalytic properties but also the membrane efficiency, especially the filtration flux. In the present work, we first annealed nickel nanoparticle catalysts in hydrogen at different temperatures to obtain nickel nanoparticles with different morphologies and extensively characterized nickel catalysts. We then evaluated the influence of nickel catalyst morphology on the performance of the submerged hybrid system including the hydrogenation reaction and the membrane separation aiming to find nickel catalysts having better membrane filtration performance and comparable catalytic properties. 2. Experimental Section 2.1. Catalyst Preparation and Characterization. The nickel nanoparticles were prepared by the improved chemical reduction method in a continuous reactor, according to Du et al.,4 and marked as Ni-0. In addition, some nickel samples were annealed in a H2/He mixture (10% H2 by volume) for 2 h at 100, 200, 300, and 500 °C and were designated as Ni-100, Ni-200, Ni300, and Ni-500, respectively. Powder X-ray diffraction (XRD) patterns were obtained on a Bruker D8 instrument with nickelfiltered Cu KR radiation (λ ) 0.154 nm) at 40 kV and 30 mA, employing a scanning rate of 0.05°/s in the 2θ range of 20°-80°. Brunauer-Emmett-Teller (BET) surface areas of the samples were measured by N2 adsorption on a BELSORP II adsorption apparatus. Before measurement, each sample was degassed at 100 °C for 3 h. The nickel samples were examined using a conventional transmission electron microscopy (TEM) system (JEOL, Model JEM-1010) operated at 100 kV and a highresolution transmission electron microscopy (HRTEM) system (Phillips, Model CM20T) operated at 200 kV to observe the particle size and the crystalline morphology of the nickel catalysts. Fourier transform infrared (FTIR) spectra were recorded with a Nexus Model 670 spectrometer. All the spectra were obtained at room temperature after the samples were pressed into wafers, with the help of KBr. 2.2. Submerged Nanocatalysis/MF System. The submerged nanocatalysis/MF system, as shown in Figure 1, was designed and constructed on a laboratory scale, which consisted mainly of a slurry stirred autoclave, a ceramic membrane module, a hydrogen resource, and nitrogen resource. The autoclave was comprised of stainless steel with a working volume of 2 L,
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which was equipped with an external heater and an internal thermocouple for temperature monitoring. The configuration of ceramic membrane module was also shown in Figure 1. A tubular ceramic membrane, which was provided by Nanjing Jiusi High-Tech Co. Ltd., PRC, with an outer diameter of 12 mm, an inner dimaeter of 8 mm inner diameter and a filtration area of 38 cm2 was used in this work. The membrane has an asymmetric structure in which a thin layer of R-Al2O3 with a nominal pore size of 0.5 µm was coated on the outer wall of a tubular R-Al2O3 porous support. The membrane tube was connected with the liquid outlet valve at one end, while the other one was sealed by glaze. 2.3. Hydrogenation Experiments. The catalytic hydrogenation of p-nitrophenol to p-aminophenol over nickel catalyst, depicted in Scheme 1, was performed in the submerged nanocatalysis/MF system. After 2 g of catalysts and 35 g of p-nitrophenol in 815 mL of ethanol solution were introduced into the autoclave, the reactor was sealed and purged with nitrogen through the membrane tube six times to remove air and then was purged six times with hydrogen. The reactor was heated to a desired temperature under low stirring. After the temperature reached the set value, the hydrogen was introduced into the reactor to a set level, and the reaction mixture was stirred at 800 rpm. The preliminary experiments proved that the hydrogenation reaction at stirring speeds of >800 rpm was not influenced by external diffusion. Finally, the hydrogenation reaction was performed at 102 °C and 1.65 MPa. After the hydrogenation reaction ended, the reactor was cooled to 60 °C and the hydrogen was discharged. The reactor then was purged with nitrogen through the membrane tube six times, to remove hydrogen. Subsequently, the membrane filtration was performed at a nitrogen pressure of 0.4 MPa and a temperature of 60 °C. After the membrane filtration was finished, the ceramic membrane tube was backflushed with nitrogen at 0.3 MPa, removed from the reactor and then cleaned with 3% (v/v) nitric acid solution at 80 °C. Some membrane samples without the backflushing and cleaning procedure were checked using field-emission scanning electron microscopy (FESEM) (Model S-4800). The hydrogenation products were analyzed by a high-performance liquid chromatography (HPLC) system (Agilent 1100 Series, USA) 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/min was used. In this paper, the hydrogenation rate is expressed by the amount of hydrogen consumed per minute and per gram of catalyst.14 For further detailed discussion, the hydrogenation rate is normalized by the catalyst surface area to give a specific hydrogenation rate defined by the amount of hydrogen consumed per minute and per surface area of catalyst, which can be calculated according to eq 1. rn )
r s×m
(1)
where rn is the normalized hydrogenation rate (given in units of mmol H2/(min m2)), r the hydrogenation rate (given in units Scheme 1. Catalytic Hydrogenation of p-Nitrophenol to p-Aminophenol
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Figure 2. XRD patterns of the fresh nickel treated in hydrogen at different temperatures. Table 1. BET Surface Areas of the Fresh and Used Nickel Specific Surface Area (m2/g) sample
fresh nickel
used nickel
Ni-0 Ni-100 Ni-200 Ni-300 Ni-500
17.2 16.9 15.2 14.9 11.3
13.5 12.4 11.1 10.6 9.7
Figure 3. TEM pictures of different nickel samples of (a) Ni-0 and (b) Ni-500.
of mmol H2/(min g)), s the specific surface area of nickel (given in units of m2/g), and m the nickel loading (in grams). In this study, the filtration flux is expressed by the amount of the permeate per hour. 3. Results and Discussion 3.1. Effect of Annealing Temperature on the Structure Features of Nickel Nanoparticles. The XRD patterns of nickel annealed in hydrogen at different temperature are shown in Figure 2. Only three characteristic peaks of face-centered cubic (fcc) nickel (2θ ) 44.5°, 51.8°, and 76.4°), corresponding to the Miller indices (1 1 1), (2 0 0), and (2 2 0), respectively, can be observed in the 2θ range of 20°-80°, which suggests that the as-prepared samples are pure fcc nickel. When the nickel sample is annealed in hydrogen, the diffraction peak of crystalline nickel becomes sharp, and the trend is more obvious at higher temperature. The results indicate that the grain size of nickel particles increases as the annealing temperature increases. The specific surface areas of the nickel that has been annealed in hydrogen at different temperatures are listed in Table 1. It can be observed that the specific surface area decreases gradually with the annealing temperature. It is reasonable to assume that the particle size varies according to the reverse order of the change of specific surface area with annealing temperature, because, at higher temperature, fine particles have a stronger tendency to coalesce with each other to form larger particles.14 This was confirmed by TEM observation on the fresh nickel samples (Ni-0) and the sample annealed at 500 °C (Ni-500). As shown in Figure 3, the average particle size of Ni-0 is ∼25 nm, which is clearly smaller than that of Ni-500, which has a diameter in the range of 25-250 nm. We further used HRTEM to examine and compare the crystalline morphology of Ni-0 and Ni-500. Figure 4 clearly
Figure 4. HRTEM pictures of different nickel samples of (a) Ni-0 and (b) Ni-500.
shows that many defects are present on the surface of Ni-0, whereas the surface of Ni-500 is much smoother and seems better crystallized with fewer defects. 3.2. Catalytic Performances of Different Nickel Nanoparticles. Figure 5 shows the hydrogenation rates of p-nitrophenol catalyzed by nickel samples annealed in hydrogen at different temperatures. Apparently, the annealing temperature significantly influences the catalytic activity of the nickel. The catalytic activity of Ni-100 is almost identical to that of Ni-0, which indicates that the catalytic activity of nickel is not affected when it was annealed at relatively low temperature (for instance, 100 °C). When the annealing temperature further increases, the catalytic activity evidently decreases. For example, the catalytic activity of nickel (which is expressed as the hydrogenation rate at a reaction time of 20 min) decreases from 6.83 mmol min-1 g-1 to 3.16 mmol min-1 g-1 as the annealing temperature increases from 100 °C to 200 °C. As the annealing temperature
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Figure 5. Change of the hydrogenation rate of p-nitrophenol with reaction time.
increases further to 300 °C, the catalytic activity of nickel is only ∼0.4 mmol min-1 g-1. Furthermore, we found that the hydrogenation reaction with Ni-300 as the catalyst required ∼10 h to complete, which is much longer than that of the other nickel catalysts shown in Figure 5, which further reveals the extremely low catalytic activity of Ni-300. When the Ni-500 sample was used as the catalyst, no noticeable hydrogenation occurred during the reaction period of 90 min, suggesting as the annealing temperature is as high as 500 °C the nickel catalyst is almost completely deactivated. Figure 5 shows that, when Ni-0, Ni100, or Ni-200 was used as the catalyst, the hydrogenation rate first increased with time, then leveled off, and finally decreased. The first stage corresponds to the activation of nickel, whereas the second and third stages are related to the concentration of p-nitrophenol; namely, the hydrogenation rate is not affected by p-nitrophenol concentration, except at low concentration.5 For the p-nitrophenol hydrogenation over Ni-300, the increasing trend of hydrogenation rate is not observed, because of its lower catalytic activity. When we correlate the catalytic performances of the nickel particles with their particle structure (size and surface area), we observe that a larger catalyst particle size leads to lower catalytic activity, because of its lower surface area for the hydrogenation, as shown in Table 1. Clearly, the hydrogenation of p-nitrophenol to p-aminophenol is structure-sensitive, and the particle size has a great influence on the catalytic activity of nickel. Specifically, a smaller nickel particle size is beneficial to obtain higher catalytic activity, which is in agreement with our previous work.14 Ermakova et al. also found that the hydrogenation rate increased as the average size of nickel in the hydrogenation of benzene decreased.15 To determine whether only the surface area contributes to the catalytic activity, we interpret the obtained reaction results in the way of specific hydrogenation rate (see the Experimental Section for the definition of specific hydrogenation rate). As shown in Figure 6, the normalized hydrogenation rate after surface area correction still follows the trends as observed in Figure 5. However, there are differences that originate from surface area in Figures 5 and 6. At the same time, the differences are not obvious, because no significant difference is observed in the specific surface areas of the nickel samples, as shown in Table 1. For example, the hydrogenation rate on Ni-300 is ∼6% of that on Ni-0, whereas the specific hydrogenation rate on Ni-300 is ∼7% of that on Ni-0. The results indicate that, excluding the surface area effect, other parameters such as surface structure, shape, etc., of the nickel particles also have some certain roles in determining the catalytic activity of the nickel (similar to the results reported by Hwang et al.16). As discussed previously, there are fewer defects in nickel nanoparticles annealed at 500 °C, compared
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Figure 6. Change of the specific hydrogenation rate of p-nitrophenol with reaction time.
Figure 7. X-ray diffraction (XRD) patterns of the used nickel.
with the fresh untreated nickel nanoparticles as shown in Figure 4. The result shows that, as the nickel sample is annealed in hydrogen at higher temperature, the surface defects will decrease, resulting in fewer amounts of active sites and poor catalytic activity.6 Fewer surface defects should be one of the main reasons for the lower catalytic activity of Ni-200, Ni-300, or Ni-500. Moreover, the used nickel samples were characterized by XRD, and the results are shown in Figure 7. It is interesting to note that, for the used nickel samples of Ni-0, Ni-100, and Ni-200, the composition is almost elementary nickel, whereas, with respect to the used Ni-300 and Ni-500, the main composition of used nickel is still elementary nickel, but, at the same time, some other peaks (2θ ) 23.8°, 33.2°, and 59°) possibly originated from contaminations have also been observed. The contaminations strongly adsorbed on the surface of nickel should be partially attributed to the extremely low catalytic activity of Ni-300 and Ni-500 in the p-nitrophenol hydrogenation. The used nickel samples were also characterized by FTIR, and the results are shown in Figure 8. Compared to the used nickel samples of Ni-0, Ni-100, and Ni-200, some new adsorption bands are observed, with respect to the used Ni-300 and Ni-500, further indicating the existence of contaminations on the surface of used Ni-300 and Ni-500. The band observed at 470 cm-1 can be assigned to the out-of-plane CO vibration coupled with the phenyl ring torsion.17 The band at 648 cm-1 might be assignable to the in-plane ring deformation coupled with NO2 scissoring vibration.17 These 1117, 1167, 1296, and 1488 cm-1 bands are
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Figure 9. Catalytic properties of nickel in p-nitrophenol hydrogenation. Figure 8. Fourier transform infrared (FTIR) spectra of the used nickel.
ascribed to the CH in-plane bending vibrations. The band at 1346 cm-1 corresponds to the symmetric NO2 stretching vibration. The band at 1587 cm-1 can be attributed to the CdC aromatic stretching vibrations. The FTIR results indicate that the contaminations contain a CO group, a NO2 group, and a phenyl ring. In addition, the contaminations cannot form at a higher apparent initial solution pH, because there are no Ni ions in the mother solutions, according to our previous work.18 Therefore, we believe that the contaminations might be the complex compounds of p-nitrophenol and Ni ions. During the hydrogenation of p-nitrophenol to p-aminophenol over nickel catalysts, the p-nitrophenol will be competitively involved in two reactions. The first is the reaction with hydrogen on the nickel surface to form p-aminophenol. The second is that the p-nitrophenol might interact with Ni ions to form complex compounds. The Ni ions are formed due to the dissolution of some nickel particles under the weak acid reaction conditions.18 For the hydrogenation reaction that uses Ni-0, Ni-100, or Ni200 as the catalyst, because of the higher catalytic activity of nickel, the p-nitrophenol will preferentially interact with hydrogen to form p-aminophenol and the first reaction might be predominant, and the complex compounds may not be able to form on the nickel surface. However, for the hydrogenation reaction using Ni-300 or Ni-500 as the catalyst, because of the lower catalytic activity of nickel, the p-nitrophenol will have more chances to interact with Ni ions to form complex compounds. Through the aforementioned analyses, we believe that the lower catalytic activity of nickel annealed in hydrogen at higher temperature is a combined effect of the lower specific surface area and fewer surface defects, and the strong adsorption of contaminations on the surface of nickel also makes the catalytic activity of Ni-300 and Ni-500 decrease. Figure 9 presents the conversion and selectivity of pnitrophenol hydrogenation over Ni-0, Ni-100, Ni-200, and Ni300. Note that the p-aminophenol selectivity reaches 100% for all cases, which indicates that the annealing temperature has no obvious influence on the selectivity. The p-nitrophenol conversion is Ni-100 > Ni-0 > Ni-300 > Ni-500 When the membrane filtration was finished, the ceramic membrane tube was first backflushed with nitrogen and then cleaned with nitric acid. After the cleaning operation, the pure water flux (PWF) of the membrane was measured using
Ni-200 > Ni-100 > Ni-0 > Ni-300 > Ni-500 Obviously, there is no linear relationship between the flux and the nickel particle size, and the straightforward assumption that a larger particle size will lead to higher flux is not true. This is an unexpected phenomenon, because the nickel sample with larger particle size is anticipated to improve the membrane flux. Generally, there are two main reasons for the higher flux for the membrane filtration of suspension that has large particles. On one hand, large particles have a higher lift force, so it is not easy for them to adhere onto the membrane surface and only a thin cake layer can be formed.19,20 On the other hand, the vacancies between large particles are larger and the corresponding cake layer is looser, in comparison with the cake layer composed of fine particles, leading to a lower specific resistance.21 However, in this study, larger particle size did not always result in higher flux, as mentioned previously. The unusual results should be related to the specific characteristics of the hydrogenation of p-nitrophenol over nickel particles. During the hydrogenation process, a complex compound composed of p-nitrophenol and Ni ions will form and adsorb on the nickel surface when the catalytic activity of nickel is lower, as discussed previously. Therefore, here, the nature of the formed cake layer is determined by two factors: the nickel particle size and the complex compound. For the system of Ni0, Ni-100, and Ni-200 suspensions, because of the higher catalytic activity of nickel, we believe that the complex compound is not likely to form; therefore, the cake layer that is mainly composed of nickel particles is porous and thin, as illustrated in Figure 12a. Therefore, as the nickel particle size increases, the specific resistance decreases and the average flux consequently increases. However, for the system of Ni-300 and Ni-500 suspensions, because of the lower catalytic activity of nickel, the complex compound can form, as shown in Figures 7 and 8. Therefore, when the reaction mixture passed through the membrane surface, the pores in the cake layer gradually filled with absorbed or precipitated complex compound, and a dense and thick cake layer was formed on the membrane surface
Figure 13. Field-emission scanning electron microscopy (FESEM) photomicrographs of the membrane used to treat the reaction mixture with Ni-0 as the catalyst: (a) top view and (b) side view.
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in good agreement with the aforementioned results shown in Figure 11, possibly because that the nickel particles in the looser cake layer can be easily removed from the membrane by the backflushing with nitrogen. 4. Conclusions
Figure 14. FESEM pictures of the membrane used to treat the reaction mixture with Ni-500 as the catalyst: (a) top view and (b) side view.
In this study, a submerged nanocatalysis and membrane filtration (nanocatalysis/MF) system was designed for the hydrogenation of p-nitrophenol to p-aminophenol over nickel nanoparticles. Nickel nanoparticles were annealed in hydrogen at different temperatures, and the particle size increased with annealing temperature while the specific surface area decreased. The performances of the submerged hybrid system using different nickel nanoparticles were investigated. There was no noticeable change of the catalytic selectivity for nickel nanoparticles annealed at different temperatures; however, nickel nanoparticles annealed at lower temperature showed better catalytic activity, and the one annealed at 500 °C was completely deactivated, mainly because nickel annealed at higher temperature had a lower specific surface area and fewer surface defects. In addition, the easy formation of complex compounds between p-nitrophenol and Ni ions that strongly adsorbed onto the catalyst surface further deteriorated the catalytic activity of nickel nanoparticles that were annealed at 300 and 500 °C. Membrane efficiency was directly dependent on nickel particle size, and there was a nonlinear relationship between filtration flux and nickel particle size: the average flux first increased (fresh nickel particles and nickel particles annealed at 100 and 200 °C) and then decreased (nickel particles annealed at 300 and 500 °C) with particle size. This can be understood: with respect to the hydrogenation of p-nitrophenol over bigger nickel particles, the p-nitrophenol will have more chances to interact with Ni ions to form complex compounds, because of the lower catalytic activity of nickel. The complex compounds will adsorb on the nickel surface and result in the formation of a dense and thick cake layer during the membrane filtration. As a result, the corresponding filtration flux will be lower. Meanwhile, a pore blocking effect might also contribute to the observed flux. As we balance the catalytic properties and separation performances of the submerged nanocatalysis/MF system, we have determined that nickel nanoparticles annealed in hydrogen at 100 °C are the most optimized catalysts for the hydrogenation of p-nitrophenol to p-aminophenol. Acknowledgment
Figure 15. Pure water flux for different membranes.
deionized water, and the results are given in Figure 15, together with the PWF of the fresh membrane for comparison. It is evident that the backflushing cannot recover the membrane flux completely, while, as the membrane was cleaned in nitric acid, the flux of the membrane almost can be restored to the original PWF, because the nickel particles adsorbed on the membrane surface or in the membrane pores can be dissolved.19 Interestingly, the PWF of the membrane used after backflushing with nitrogen varies for different nickel suspensions, and, again, it follows the order Ni-200 > Ni-100 > Ni-0 > Ni-300 > Ni-500
Financial support from the National Basic Research Program (2009CB623406), the National High Technology Research and Development Program (2007AA06A402), and the National Natural Science Foundation (20636020) of China is gratefully acknowledged. Literature Cited (1) Du, Y.; Chen, H. L.; Chen, R. Z.; Xu, N. P. Poisoning effect of some nitrogen compounds on nano-sized nickel catalysts in p-nitrophenol hydrogenation. Chem. Eng. J. 2006, 125, 9. (2) Rode, C. V.; Vaidya, M. J.; Jaganathan, R.; Chaudhari, R. V. Hydrogenation of nitrobenzene to p-aminophenol in a four-phase reactor: reaction kinetics and mass transfer effects. Chem. Eng. Sci. 2001, 56, 1299. (3) Rode, C. V.; Vaidya, M. J.; Chaudhari, R. V. Synthesis of p-Aminophenol by catalytic hydrogenation of nitrobenzene. Org. Process Res. DeV. 1999, 3, 465.
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ReceiVed for reView January 9, 2009 ReVised manuscript receiVed May 9, 2009 Accepted June 1, 2009 IE900033M
