Ind. Eng. Chem. Res. 2009, 48, 4933–4938
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Adding Microsized Silica Particles to the Catalysis/Ultrafiltration System: Catalyst Dissolution Inhibition and Flux Enhancement Zhaoxiang Zhong, Xin Liu, Rizhi Chen, Weihong Xing,* and Nanping Xu State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing UniVersity of Technology, Nanjing 210009, P.R. China
A system combining catalytic reaction with crossflow ultrafiltration (UF) was used to catalytic ammoximation of cyclohexanone to the oxime over titanium silicalite-1 (TS-1) catalysts. The effect of microsized silica particles on the performance of the catalysis/UF system was investigated in terms of catalytic activity and membrane filterability through dissolution and ultrafiltration experiments. Adding silica particles in the system inhibits the dissolution of TS-1 catalysts and increases both the reaction conversion and the selectivity significantly. Further characterizations (XRF, XRD, FTIR, etc.) indicated that the presence of silica particles remarkably limits the ammonia damage to the microstructure frame of TS-1 catalyst. In addition, silica particles play an important role in substantially removing the deposited TS-1 cake from the membrane surface, benefiting from the scouring effect. According to the results of visual observation to the tested membranes and the estimation of hydrodynamic forces acting on particles, microsized particles are hard to deposit on the membrane surface at the studied conditions and therefore a flux improvement has been achieved. 1. Introduction As a new kind of catalytic material, titanium silicalites has attracted extensive interest for their high catalytic activity and selectivity in the selective oxidation of organic compounds.1-6 ENICHEM developed a new method of catalytic direct ammoximation of cyclohexanone with NH3/H2O2 to the oxime on titanium silicalite-1 (TS-1) catalysts. However, one of the major challenges in the practical applications is the separation of TS-1 particles from the reaction slurry because TS-1 particles are too fine to be removed by gravity settling and porous tube filtration. Microfiltration (MF) and ultrafiltration (UF) have emerged as useful processes for separation of fine particles.7 A catalysis/ UF system was used to produce cyclohexanone oxime and then separate TS-1 catalysts from slurry at SINOPEC, where the recovered catalyst was recycled continuously back to the reactor and reused in the next batch of reaction.8 It was observed that the activity of TS-1 decreases rapidly after operating for a certain period.9,10 Petrini et al.11 pointed out that the main reason for catalyst deactivation is the basic reaction medium in the presence of ammonia, which leads to Si dissolution away from the titanium silicalite framework. It was further confirmed that the dissolution of Si is one of the primary factors, which lead to the deactivation of the catalyst during ammoximation of cyclohexanone. Although the dissolving process of Si in the reaction slurry is very slow, the longterm running will result in a continuous decrease in the amount of the titanium silicalites in the reaction system. The weight of catalyst recovered will be lower than that of the starting one. In some cases, the recovery of catalyst is only 35%.12 Sun10 tried to add a liquid Si-containing assistant to the reaction system to inhibit the Si dissolution, but the interaction between the liquid assistant and TS-1 particles played a significant role in the formation of dense cake layers at the membrane surface, leading to a greater flux decline during ultrafiltration of TS-1 particles.13 Membrane fouling is another challenge in the implementation of the catalysis/UF system. Fouling in membrane separation is * To whom correspondence should be addressed. Tel.: +86-25-83172288. Fax: +86-25-8317-2292. E-mail:
[email protected].
a key factor affecting the economic and commercial viability of a membrane system that essentially depends on the stable permeation flux obtained. In this system, adsorption of organic matter, deposition of TS-1 particles, and formation of dense filter cakes on the membrane surface are suggested as causes for fouling. Backflushing was used to maintain flux, but a relatively small amount of membrane cleaning was achieved. In our previous studies, microsized alumina particles were used to remove the cake layer from the membrane surface effectively after filtration, because of their scouring effect.13 In another nanocatalysis/UF system, we found that addition of microsized alumina particles in nanosized nickel suspension can effectively inhibit the adhesion of nanosized nickel to the contact surfaces, and these inert particles had no evident effect on reaction rate.14,15 Chen et al.16 reported that addition of inert particles in the reaction system of nanocatalysts could increase the reaction conversion significantly. They concluded that inert particles can act as a dispersant to uniformly disperse nanometals in the reaction system and to prevent the agglomeration of nanometals. Meanwhile, inert particles can enhance the absorption rate of reactant gas in the solvent, which may accelerate the reaction rate. Some researchers mixed micrometer- or millimeter-sized particles (such as silica, glass, metal, and powdered activated carbon particles) with macrosolute (such as proteins, dextran, biopolymers, and colloids) to improve the filtration flux.17-20 However, there is no report addressing the enhancement effect of adding particles on both catalytic activity and filtration performance in the catalysis/UF system. In this research, solid silica particles were added into the catalysis/UF system and circulated with TS-1 catalysts. The effect of silica particles on the performances of TS-1 and membrane flux was investigated. The purpose of the investigation is to develop a method that can not only reduce the dissolution of catalysts but also enhance the membrane flux, thus making a contribution to the catalysis/UF system applied in the industrial production of cyclohexanone oxime.
10.1021/ie801774a CCC: $40.75 2009 American Chemical Society Published on Web 04/01/2009
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Figure 1. Schematic diagram of the catalysis/UF system: (1) reaction tank; (2) feeding pump; (3, 4) rotameter; (5) membrane module; (P1-P3) pressure gauges; (V1-V7) valves.
2. Experimental Section 2.1. Materials. Spherical TS-1 samples with a particle size of 0.1-0.3 µm were supplied by SINOPEC (Beijing, China). Solid silica with mean diameter of 35 µm was obtained from Shantou Xilong Chemical Factory (Guangdong Province, China). A particle-size distribution of the samples was measured using a particle-size analyzer (Malvern MasterSizer 2000, UK). The ultrafiltration membrane was a multichannel tubular membrane (0.5 m long, 19 channels with a 4.0 mm inner diameter) supplied by Nanjing Jiusi High-Tech Co. Ltd., P.R. China. Its nominal pore size was 50 nm, and the filtration area was 0.12 m2. The membrane was composed of a top layer of ZrO2 and a support layer of R-Al2O3. 2.2. Catalysis/UF System. The experimental setup was shown in Figure 1. The recirculation loop was composed of a 20 L reaction tank (sealed with a glass cap and jacketed for temperature control), two flowmeters, a centrifugal pump, a membrane module, and the accompanying pressure gauges, valves, and tubes. 2.3. Dissolution Test. To study the effect of silica on the structure stability of the TS-1 catalyst, a dissolution test was conducted under reaction conditions by immersing TS-1 in the solution that contained desired amount of t-butanol, hydrogen peroxide, cyclohexanone, and ammonia at the conditions of with/ without silica particles. The solution was stirred to prevent precipitation at 75 °C. Because of reaction consumption, ammonia needed to be added to keep concentration constant. So after specific period, the solution mainly contained cyclohexanone, cyclohexanone oxime, ammonia, water, and solvent t-butanol. The TS-1 was separated from the solution without silica using a ceramic membrane with an average pore size of 50 nm. As to the solution with silica, for silica particles were much larger than the TS-1, silica was removed with mesh screen, and then the TS-1 was separated from the solution by the ceramic membrane. These TS-1 samples were characterized through different techniques to investigate the dissolution content. The element (Si and Ti) contents of the TS-1 were determined by X-ray fluorescence (XRF) by an ARL-9800 spectrometer under an input power of 1 kW. X-ray diffraction (XRD) patterns were obtained on a Bruker D8-Advance X-ray diffractometer using Cu KR radiation and a Ni filter to determine the crystallinity. Samples of 1000 mg were pressed into pellets before the XRF and XRD analysis. Fourier transform infrared spectroscopy (FTIR) was performed on a Nexus 870 FTIR spectrometer. For FTIR measurements, 3 mg TS-1 sample and 400 mg KBr were ground with an agate mortar and pressed into a pellet. The concentration of dissolved silica was measured
with inductively coupled plasma (ICP; Optima2000 DV, Perkin Elmer, USA). 2.4. Reaction Test. A reaction test was carried out to investigate the activity of catalysts. The reaction test was taken as following reaction conditions: cyclohexanone/H2O2 ) 1:1.1 (mol), cyclohexanone/NH3 ) 1:1.7 (mol), cyclohexanone/tbutanol ) 1:2.5 (w/w), reaction temperature of 75 °C, catalyst concentration of 30 g/L, and silica concentration of 10 g/L. The mixture consisting of cyclohexanone, TS-1 (with and without silica particles), and t-butanol solvent was stirred and heated up to 75 °C by circulating hot water through the jacket of reactor used. The reaction was started by feeding the ammonia and H2O2 into the reaction mixture from the inlet of the reactor. After 2 h, the ammonia and H2O2 were stopped and then the feeding pump was initiated to start the membrane separation. The crossflow filtration was run at a constant temperature of 75 °C, crossflow velocity of 3 m/s, and transmembrane pressure of 0.1 MPa. The products on the permeation side were then analyzed using a gas chromatograph (CE8000) equipped with a capillary column (OV-1) and a flame ionization detector (FID). The reaction was described as follows:
The conversion of cyclohexanone and the selectivity to cyclohexanone oxime can be calculated by the equations below, respectively: Xcyo )
Soxime )
c0cyo - ccyo c0cyo coxime c0cyo
- ccyo
× 100%
(2)
× 100%
(3)
c0 and c denote the initial and the final mole fraction. The subscripts cyo and oxime represent cyclohexanone and cyclohexanone oxime, respectively. 2.5. Characterization of the Membrane. In some cases, membranes after filtration test were examined by a JSM-6300 scanning electron microscope (SEM; JEOL, Japan). Prior to the SEM analysis, the membrane specimens were carefully taken from the middle of the elements (lengthwise) and sputter-coated with gold. 3. Results and Discussion 3.1. Effect of Silica Addition on Ammoximation Reaction. The evolution of conversion and selectivity of ammoximation reaction with dissolution time is shown in Figure 2. For the first 100 h, the dissolution can be considered small for the fresh TS-1 catalyst; furthermore, no negative influence on the performance of catalyst is observed after adding the silica particles. The conversion of cyclohexanone and the selectivity of cyclohexanone oxime were both over 95%, which is consistent with the previous studies.21-23 After 100 h, the conversation of cyclohexanone in the system without silica decreased quickly from 97.5% to 75.9%, while it decreased from 97.5% to 88.1% in the comparative system with silica. At the same time, the selectivity of the cyclohexanone oxime decreased from 99.2% to 87.8% and from 99.2% to 93.4% in the reaction system without and with silica, respectively. The results indicated that silica decreased the dissolution of TS-1 catalysts
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Figure 2. Conversion and selectivity vs dissolution time.
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Figure 4. XRD patterns of fresh and used TS-1 catalyst: (1) fresh catalyst; (2) catalyst dissolved with silica particles for 480 h; (3) catalyst dissolved without silica particles for 480 h. Table 1. Data Calculated from XRD Spectra of TS-1 Catalysts
Figure 3. Variation of the Si and Ti content of TS-1 catalyst.
significantly. The lifetime of the TS-1 catalyst was prolonged, and the stable operation time was increased. 3.2. Effect of Silica on Stability of Ti-Silicate Catalyst Framework. Generally, both amorphous and crystalline silica were soluble in alkaline medium.24 Accordingly, the Ti-silicalite framework can dissolve in the reaction environment because of the ammonia. According to the results from Petrini et al.11 and Liu et al.,25 Si in the framework of TS-1 can be removed from the solid due to the dissolution in the alkaline reaction medium, while almost all Ti remains on the catalyst. Therefore, the Ti content of the catalyst tends to increase and the Si content tends to decrease. Figure 3 shows the variation of Si and Ti content of TS-1 catalyst. With dissolution time, the Ti content increased and Si content decreased in both cases, just as the literature reported.11 However, Si and Ti content in the reaction with silica particles were more stable than that in the reaction without silica particles. The result indicated that presence of silica particles inhibited the dissolution of TS-1 catalyst. The dissolution process of silica in aqueous solutions is mainly due to hydrolysis of Si-O-Si bonds, resulting in the liberation of silicic acid into the aqueous phase.25 The actual solubility of silica is strongly pH dependent, where silicic acid (Si(OH)4) is formed at pH values e 9. At pH > 9, monosilicates (SiO(OH)3-, Si(OH)22-) and polysilicates (Si2O2(OH)5-, Si2O3(OH)42-) form.24 In the ammoximation system, the dissolution of silica particles can increase the concentration of monosilicates and polysilicates in the solution, according to common ion effect, which will decrease the solubility of TS-1 catalysts in the presence of common ions. The XRD patterns of fresh and used TS-1 catalyst are shown in Figure 4. It can be seen from the figure that all three samples have the typical MFI structure. The relative crystallinities of the samples were calculated based on the intensity of the five strong reflections and assuming that of the fresh catalyst is 1.26
sample
relative crystallinity
1 2 3
1.00 0.88 0.67
The results (given in Table 1) demonstrated that the Ti-silicate framework had undergone some changes during the experiments. After treatment with the ammonia solution, the crystallinity decreased, indicating that ammonia damaged the framework of the zeolite. With the presence of silica particles, the crystallinity of TS-1 decreased but to a lesser extent. These results indicate that the damaging effect of ammonia can be limited by adding silica particles. The IR band at 960 cm-1 is attributed to the presence of titanium in the Ti-silicate framework, i.e. existing in the form of -Ti-O-Si- species.11,26 The intensity of the band illustrates the amount of this species. To study the effects of ammonia and the silica on the structure stability of the catalysts, TS-1 samples were further studied by FTIR. The results illustrated in Figure 5 show that, after treatment in ammonia solution without silica, the intensity of the peak at 960 cm-1 decreased markedly. In the literature,11 the decrease of the intensity of this band was attributed to the interaction of ammonia with, and then the destruction of, -Ti-O-Si- species. However, with the presence of silica, the treatment with ammonia solution did not cause much decrease of the intensity of the peak at 960 cm-1. The dissolution also influenced the size of particles. As shown in Figure 6, the size and size distribution of catalyst parti-
Figure 5. FTIR spectra of fresh and used TS-1 catalyst: (1) fresh catalyst; (2) catalyst dissolved with silica particles for 480 h; (3) catalyst dissolved without silica particles for 480 h.
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Figure 6. Particle size distribution of fresh TS-1catalyst and TS-1catalyst dissolved for 480 h.
Figure 7. Variation of flux at conditions of with/without silica particles.
cles in the ammonia solution were changed after 480 h of dissolution. In the mixture solution, particles had a bimodal distribution with smaller TS-1 particles and larger silica particles. The average size of TS-1 used with silica particles shifted slightly to a lower value. As a contrast, in the TS-1 alone solution, TS-1 presented the smallest mean diameter due to Si dissolution away from the particles. Meanwhile, the size variation between fresh and used silica particles was also observed. Those used silica particles became smaller due to dissolution. The content of silica dissolved in reaction medium was measured with ICP. After 480 h of dissolution, the silica concentration was about 1237 mg/L in the solution without silica particles. As a contrast, it was about 1855 mg/L in the solution with silica particles. This result demonstrated that silica particles addition introduced more silica into the reaction medium because of dissolution. This increased the silica concentration in a way different from that in ref 10, in which liquid silica was added directly and was proven to inhibit the dissolution erosion of the TS-1 catalyst. In industrial production, TS-1 was used for a longer time, hence it can be reasonably inferred that the ammonia gradually dissolved the TS-1 to a greater extent than that in this experiment. During filtration of TS-1 catalysts with membranes, the deposition of smaller particles will increase the specific resistance of cake layer and lead to the greater flux decline.27 3.3. Flux Enhancement with Silica Addition. 3.3.1. Effect of Silica on the Filtration of TS-1. Figure 7 gives UF flux for different combinations of feed solutions including TS-1 slurry and a mixture of TS-1 and silica. When UF was carried out with only TS-1 slurry, there was a significant decline in flux of approximately 52% after 8 h of operation. However, when TS-1 and silica were mixed together, an obvious flux enhancement was observed. With an increase of silica dosages of up to 10
Figure 8. SEM pictures of membrane surface after filtration of TS-1 solution: (a) without silica; (b) with silica, top view.
g/L, continuous flux improvement was observed and a steady state with a decline in flux of approximately 7% was obtained. When silica dosages increased to 15 g/L, no more flux enhancement was noticed. This result indicated that silica particles had a significant influence on the permeate flux. They might have a “scouring ball” effect that removes the already deposited TS-1 away from the membrane surface. This explanation can be confirmed by SEM analysis of the membrane. Figure 8 shows the surface of the used membrane after UF of the two different feed solutions. As shown in Figure 8a, a cake layer of TS-1 particles less than 1 µm was formed on the membrane surface after TS-1 slurry was filtered. For comparison, Figure 8b shows that few particles were on the membrane surface after the mixture was filtered. Theoretical analysis of deposition behavior of TS-1 particles on the membrane surface is needed to further clarify the role of silica in the filtration process. 3.3.2. Theoretical Approach to the Deposition of TS-1. To understand the deposition behavior of TS-1 particles on the membrane surface, the forces acting on a single particle should
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Figure 9. Estimation of the forces on a particle: fluid density, 950 kg/m3; fluid viscosity, 0.0012 Pa; initial flux, 700 L/(m2 h).
be considered. There are three normal forces acting upon a particle on the membrane surface, which are drag force of the filtrate flow, Fy; the adhesive force of neighboring particles, Fa; and the inertial lift force, Fl. The sum of first two forces moves the particles toward the membrane surface (negative direction), while the last shifts the particles away from the membrane surface (positive direction). A detailed description of these forces is given in other references.28-30 The drag force of filtrate flow Fy can be estimated with the Stockes equation: Fy ) 3πµdPVF
(4)
The lift force Fl is caused by the shear flow. It can be calculated as follows: Fl ) 0.761
τw1.5dP3F0.5 µ
(5)
The adhesive force of interacting particles is typically van der Waals forces (FvdW).28,29 The van der Waals forces FvdW between two ideal spheres can be calculated according to Fa ) FvdW )
pdP 32πa2
(6)
In eqs 4-6, µ is the dynamic fluid viscosity, dP, particle size, VF, filtration rate, τw, shear stress, F, fluid density, p, Lifschitz-van der Waals constant (10-20 J), and a, adhesive distance (0.4 nm). The balance between the negative forces and positive forces determines whether the particle will be swept off or remain stable on the cake layer. Figure 9 shows an estimation of forces in the direction of the filtrate flow. It indicates that the sum of drag force and adhesion force are higher than the lift force in a particle range smaller than several microns. This means a small single particle is irreversibly attached to the layer and cannot return into the crossflow. Only large particles can be removed from the layer. This phenomenon was also described elsewhere.30,31 As shown in Figure 7, when filtration of the TS-1 solution was performed, the deposition of TS-1 particles and formation of filter cakes were suggested as the main causes for the fouling. Organic matter involved in this reaction system had little effect on membrane fouling, because most of the organic matter was small molecular monomers compared to those macromolecular polymers that greatly increased the specific cake resistance.31 Because silica particles were several tens of micrometers in diameter, they had a
Figure 10. SEM picture of membrane after filtration of TS-1 solution with silica, side view.
higher lift force in the hydrodynamic conditions of this study, which made them difficult to deposit on the membrane. When silica was added to TS-1 solution, the scouring effect of suspended silica particles enhanced the shearing stress on the membrane surface,32 which led to the increase of the lift force based on the analysis of eq 5. At the same time, the collision of silica with deposited TS-1 provided them enough kinetic energy to overcome adhesion energy between interacting particles. In the initial operation time, flux decreased due to the deposition of TS-1 particles, but in the flowing operation time with particles swept off from the membrane surface, flux recovered slowly and then reached a steady state. The result indicated that microsized particles could enhance flux and thus minimize the fouling problems. 3.3.3. Membrane Material Stability. The used membrane was characterized by scanning electron microscopy. Figure 10 shows SEM pictures taken from the membrane used for ammoximation of cyclohexanone with silica addition. It can be seen that the top membrane remains well intact with the support after several rounds of filtration. The results indicate that the ceramic membrane used in the experiments has excellent physical and chemical stability. 4. Conclusion A catalysis/UF system was applied for the catalytic direct ammoximation of cyclohexanone on TS-1 catalysts. A method that can both slow the catalyst deactivation and improve the filtration rate was proposed. Our research suggests that addition of microsized silica particles in the system could inhibit the dissolution of TS-1 catalysts due to common ion effect and thus slow the deactivation of catalysts. Meanwhile, presence of microsized silica particles in the system could provide a mechanical scouring effect that removes the already deposited TS-1 away from the membrane surface and results in a flux enhancement. Addition of microsized particles in the catalysis/UF system was proven to be an effective method to increase the whole operating efficiency of the system. It made a contribution to the industrial application of the system for the ammoximation of cyclohexanone. Acknowledgment This work is supported by the National Basic Research Program of China (No. 2009CB623400), the National Natural Science
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Foundation of China (No. 20806038), the Natural Science Foundation of Jiangsu (No.BK2008504), China Postdoctoral Science Foundation (No.20070421005), Jiangsu Planned Projects for Postdoctoral Research Funds (No. 0702020B), and Program for New Century Excellent Talents in University (NCET) of China. Literature Cited (1) Taramasso, M.; Perego, G.; Notari, B. Preparation of porous crystalline synthetic material comprised of silicon and titanium oxides. US Patent 4,410,501, 1993. (2) Esposito, A.; Taramasso, M.; Neri, C.; Buonomo F. Hydroxylation of Aromatic Hydrocarbons. DE Patent 3,135,559, 1985. (3) Clerici, M. G.; Bellussi, G.; Romano, U. Synthesis of propylene oxide from propylene and hydrogen peroxide catalyzed by titanium silicalite. J. Catal. 1991, 129, 159. (4) Huybrechts, D. R. C.; De Bruycker, L.; Jacobs, P. A. Oxyfunctionalization of alkanes with hydrogen peroxide on titanium silicalite. Nature (London) 1990, 345, 240. (5) Huybrechts, D. R. C.; Buskens, P. L.; Jacobs, P. A. Alkane oxygenations by hydrogen peroxide on titaniumsilicalite. Stud. Surf. Sci. Catal. 1992, 72, 21. (6) Rpffia, P.; Padovan, M.; Moretti, E.; De Albetti, G. Catalytic Process for Preparing Cyclohexanone-Oxime. European Patent 0,208,311, 1987. (7) Baker, R. W. Membrane Technology and Applications; John Wiley & Sons: Chichester, 2004. (8) Fu, S. B.; Wang, H. B.; Xu, F. H.,; Zhu, Z. H. Cyclic separation of titanium silicalite-1 catalysts in their catalytic reactions. Chinese Patent 00113447.7, 2003. (9) Jiang, F.; Fu, S. B.; Tang, Q.; Zhu, Z. H.; Sun, B.; Li, Y. X.; Wu, W. Research on the Loss of Ti - Si Molecular Sieve in the Cyclohexane Ammonium - Oximation Process. Chem. Ind. Eng. Prog. (In Chinese) 2003, 22, 1116. (10) Sun, B. Study on dissolution erosion of titanium silicalite zeolite in cyclohehanone ammoximation. Pet. Process. Petrochem. 2005, 36, 54. (11) Petrini, G.; Cesana, A.; De Alberti, G.; Genoni, F.; Leofanti, G.; Padovan, M.; Paparatto, G.; Roffia, P. Deactivation phenomena on TiSilicalite. Stud. Surf. Sci. Catal. 1991, 69, 761. (12) Centi, G.; Cavani, F.; Trifiro`, F. SelectiVe Oxidation by Heterogeneous Catalysis; Kluwer Academic/Plenum Publishers: New York, 2001. (13) Zhong, Z. X.; Xing, W. H.; Liu, X.; Jin, W. Q.; Xu, N. P. Fouling and regeneration of ceramic membranes used in recovering titanium silicalite-1 catalysts. J. Membr. Sci. 2007, 301, 67. (14) Zhong, Z. X.; Xing, W. H.; Jin, W. Q.; Xu, N. P. Adhesion of nano-sized nickel catalysts in the nanocatalysis/UF system. AIChE J. 2007, 53, 1204. (15) Zhong, Z. X.; Chen, R. Z.; Xing, W. H.; Xu, N. P. Recovery of nanometer nickel catalyst with ceramic membrane. Chin. J. Chem. Ind. Eng. 2006, 57, 849.
(16) Chen, Y.; Hsieh, T.-Y. Effects of inert particles on liquid phase hydrogenation over nano-sized catalysts. J. Nanopart. Res. 2002, 4, 455. (17) Van der Waal, M. J.; Van der Velden, P. M.; Koning, J.; Smolders, C. A.; Van Swaay, W. P. M. Use of fluidized beds as turbulence promotors in tubular membrane systems. Desalination 1977, 22, 465. (18) Rios, G. M.; Rakotoarisoa, H.; Tarodo de la Fuente, B. J. Membr. Sci. 1987, 34, 331. (19) Fane, A. G. Ultrafiltration of suspensions. J. Membr. Sci. 1984, 20, 249. (20) Park, H. S.; Choo, K. H.; Lee, C. H. Flux Enhancement with Powdered Activated Carbon Addition in the Membrane Anaerobic Bioreactor. Sep. Sci. Technol. 1999, 34, 2781. (21) Taramasso, M.; Perego, G.; Notari, B. Preparation of porous crystalline synthetic materials comprised of Si and titanium oxides. US Patent 4410501, 1983. (22) Thangaraj, A.; Sivasanker, S.; Ratnasamy, P. Catalytic properties of crystalline titanium Silicalites IIIsAmmoximation of cyclohexanone. J. Catal. 1991, 131, 394. (23) Zhang, X. J.; Wang, Y.; Xin, F. Coke deposition and characterization on titanium silicalite-1 catalyst in cyclohexanone ammoximation. Appl. Catal., A 2006, 307, 222. (24) Sjo¨berg, S. Silica in aqueous environments. J. Non-Cryst. Solids. 1996, 196, 51. (25) Liu, N.; Guo, H.; Wang, X.; Chen, L.; Chen, Y. Hydrothermostability of Titanium Silicate TS-1 Zeolite in Environment of Cyclohexanone Ammoxidation. Chin. J. Catal. 2003, 24, 441. (26) Wu, C. T.; Wang, Y. Q.; Mi, Z. T.; Xue, L.; Wu, W.; Min, E. Z.; Han, S.; He, F.; Fu, S. B. Effects of Organic Solvents on the Structure Stability of TS-1 for the Ammoximation of Cyclohexanone. React. Kinet. Catal. Lett. 2002, 77, 73. (27) Choo, K.-H.; Lee, C.-H. Hydrodynamic behavior of anaerobic biosolids during crossflow filtration in the membrane anaerobic bioreactor. Water Res. 1998, 32, 3387. (28) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic: New York, 1992. (29) Altmann, J.; Ripperger, S. Particle deposition and layer formation at the crossflow microfiltration. J. Membr. Sci. 1997, 124, 119. (30) Ripperger, S.; Altmann, J. Crossflow microfiltration- state of the art. Sep. Purif. Technol. 2002, 26, 19. (31) Lee, S.-A.; Choo, K.-H.; Lee, C.-H.; Lee, H.-I.; Hyeon, T.; Choi, W.; Kwon, H.-H. Use of ultrafiltration Membranes for the separation of TiO2 photocatalysts in drinking water treatment. Ind. Eng. Chem. Res. 2001, 40, 1712. (32) Huang, X.; Wei, C. H.; Yu, K. C. Mechanism of membrane fouling control by suspended carriers in a submerged membrane bioreactor. J. Membr. Sci. 2008, 309, 7.
ReceiVed for reView November 19, 2008 ReVised manuscript receiVed March 4, 2009 Accepted March 13, 2009 IE801774A
