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A Dual-Membrane Airlift Reactor for Cyclohexanone Ammoximation over Titanium Silicalite‑1 Rizhi Chen, Honglin Mao, Xiangrong Zhang, Weihong Xing,* and Yiqun Fan State Key Laboratory of Materials-Oriented Chemical Engineering, National Engineering Research Center for Special Separation Membrane, Nanjing Tech University, Nanjing 210009, China ABSTRACT: A novel dual-membrane airlift reactor with a ceramic outer-membrane as a reactant ammonia distributor and another ceramic inner-membrane as a catalyst separator was developed for the cyclohexanone ammoximation over titanium silicalite-1(TS-1) without the addition of extra solvent such as tert-butanol, resulting in no need of separation step for solvent. It was interesting to find that the introduction of membrane distribution could increase the selectivity of cyclohexanone oxime by 18%, because many tiny ammonia bubbles could be produced with a membrane distributor to strengthen the mix of gas phase and organic phase. A blowing process was introduced into the membrane reactor to overcome the membrane fouling and decrease the filtration resistance. The operation conditions were optimized, and the conversion and selectivity remained stable at about 87% and 76% in a 25 h continuous run, respectively. Our research verifies the advantages of a dual-membrane reactor in a continuous ammoximation.

1. INTRODUCTION Cyclohexanone oxime is one of the most important intermediate products of ε- caprolactam that was used to produce Nylon-6.1,2 Titanium silicalite-1 (TS-1) was discovered to be an effective catalyst in the liquid-phase cyclohexanone ammoximation to prepare cyclohexanone oxime.3,4 However, it was difficult to separate TS-1 catalysts from the reaction products,5 because their particle size was too fine to be removed by gravity settling or porous tube filtration,6 which creates another problem to be solved in practical applications. The membrane reactor was introduced to solve this problem, in which the membrane was not selective and its major role was to filter the suspended catalysts.7 The coupling technology of reaction and ceramic membrane separation had been employed to realize in situ product removal.5 The liquid-phase cyclohexanone ammoximation to cyclohexanone oxime over TS-1 has achieved industrialization. However, some challenges still remain in the current industrial processes. For example, a large amount of tert-butanol was needed to recycle, which resulted in an increase of subsequent separation energy consumption. A number of studies showed the influence of solvent on the ammomation and proved that both methanol and water gave low oxime yield compared to the solvent consisting of water and tert-butanol.3,8,9 In the cases of nonsolvent, the mass transfer of reactants to the pore system would limit the ammomation reaction over TS-1 and lead to a low activity. Hence, in order to strengthen the mass transfer and increase the gas−liquid contact area in the cases of nonsolvent, some technologies such as membrane dispersion could be employed. Wang et al.10 designed a membrane dispersion minireactor and the mass transfer rate of cyclohexanecarboxylic acid (CCA) was significantly increased by membrane dispersion. Chen et al.11 employed a porous ceramic membrane as a distributor to control the supply of oxygen in phenol hydroxylation and considered that membrane distributor could produce numerous small oxygen bubbles, which © 2014 American Chemical Society

would increase the volumetric oxygen transfer coefficient and gas−liquid mass transfer. Airlift has many advantages such as good gas−liquid mass transfer performance,12−15 and it is an effective way to increase the permeate flux in the membrane filtration processes.16−18 Nitrogen was often used as a blowing source, on one hand, to supply three-phase cycle power, and on the other hand, to reduce the concentration polarization18 and control the membrane fouling. On the basis of extensive studies on the airlift membrane reactor and cyclohexanone ammoximation, this work developed a dual-membrane airlift reactor to carry out the cyclohexanone ammoximation over TS-1 without the addition of extra solvents such as tert-butanol. A ceramic outer-membrane was employed as a distributor controlling the supply of ammonia. Abundant tiny bubbles were produced and in favor of the mass transfer of ammonia, which would make the ammonia molecules reach easily the active sites of TS-1. Advantages of the membrane dispersion in enhancing the selectivity of phenol hydroxylation had been demonstrated.19 Another tubular porous ceramic membrane was employed as a membrane separator for in situ separation of catalysts from the products. Thus, the designed reactor not only improved the performance of cyclohexanone ammoximation, but also realized the in situ separation of catalysts and made the reaction continuous. To investigate the feasibility of the continuous cyclohexanone ammoximation reaction over TS-1 in this new proposed dual-membrane reactor, the effects of membrane distributor, blowing process, and operation conditions on the membrane reactor performance were studied in detail. Received: Revised: Accepted: Published: 6372

February 10, 2014 March 25, 2014 March 27, 2014 March 27, 2014 dx.doi.org/10.1021/ie500573d | Ind. Eng. Chem. Res. 2014, 53, 6372−6379

Industrial & Engineering Chemistry Research

Article

Figure 1. Diagram of a novel dual-membrane airlift reactor system.

tubular Al2O3 porous support (nominal pore size of 4 μm), and one end was sealed with glazing compound while the other end was kept open, which was employed as an ammonia distributor. The temperature was controlled with a heating water bath. The reaction pressure was controlled by adjusting the gas outlet of the reactor. 2.3. Ammoximation Experiments. The cyclohexanone ammoximation over TS-1 was carried out in the dual membrane airlift reactor according to the equation20 as shown in Figure 2. After a defined amount of cyclohexanone

2. EXPERIMENTAL SECTION 2.1. Materials. TS-1 catalyst (average particle size, 260 nm; specific surface area, 408 m2·g−1; Si/Ti molar ratio, 33) was provided by SINOPEC. Cyclohexanone and 30% H2O2 were analytical grade and purchased from Shanghai Lingfeng Chemical Reagent Co., Ltd. China, and SINOPHARM Chemical Reagent Co., Ltd. China, respectively. Ammonia gas (purity is 99.999%) was supplied by Nanjing Maikesi Nanfen Special Gas Co., Ltd. China. Deionized water with an electrical conductivity below 10 μs·cm−1 was homemade through a reverse osmosis (RO) system. 2.2. Dual-Membrane Airlift Reactor System. Figure 1 shows a schematic diagram of a novel dual-membrane airlift reactor system that mainly consisted of the following parts: a feed system, an airlift reactor, two porous ceramic tubular membrane modules, and a reaction mixture receiver. Two constant flow pumps (Beijing Chuangxin Tongheng Science & Technology Co., Ltd. China) were used to feed cyclohexanone and hydrogen peroxide into the reactor, respectively. The pure ammonia gas flowed into the reactor was controlled by a mass flow controller (Beijing Sevenstar Electronics Co., Ltd. China). The reactor was made from stainless steel with 1.5 L of working volume, which consisted of the riser and the comedown. One tubular ceramic membrane module was a part of the new riser, which distinguished it from the traditional riser in the external loop reactors. The reactor had no stirring paddle, therefore, the circulation of gas−liquid−solid three phases relied on the power provided by nitrogen which was sparged from a gas aerator under a ceramic membrane module. Tubular ceramic membranes were provided by Nanjing Jiuwu High-Tech Co., Ltd., China. The effective length, outer diameter, and inner diameter were 60, 12, and 8 mm, respectively. The membrane separator was composed of a thin layer of ZrO2 with a nominal pore size of 0.2 μm coated on the internal surface of a tubular Al2O3 porous support (nominal pore size of 4 μm). The membrane distributor was made up of a fine layer of Al2O3 (nominal pore size of 0.5 μm) coated on the outer surface of a

Figure 2. Chemical equation of the cyclohexanone ammoximation reaction.

and TS-1 catalysts were added into the airlift reactor, the airlift reactor was heated to a desired temperature through circulating hot water. Nitrogen was sparged from the aerator by the membrane module to drive the circulation of gas−liquid−solid three phases and make TS-1 particles suspended. When the temperature reached the setting value, hydrogen peroxide was fed continuously into the reactor at a constant flow rate. Ammonia gas was injected into the lumen of the ceramic membrane distributor and pressed through the micropores of the distributor into the reaction system. The required pressure of 0.3−0.5 MPa should be gradually raised, which increased the solubility of ammonia and made the experiments safe. The ammoximation reaction started and was run for 100 min. After that, cyclohexanone, hydrogen peroxide, and ammonia gas were fed continuously into the reactor at constant flow rates. Meanwhile, the product stream was extracted through the membrane separator at a flow rate equal to the total feed, and the continuous ammoximation reaction started and the first 6373

dx.doi.org/10.1021/ie500573d | Ind. Eng. Chem. Res. 2014, 53, 6372−6379


Article

data point was taken. Each continuous ammoximation was carried out at least 8 h in order to investigate the effects of operation conditions. The products were collected in a 1 L measuring cylinder and analyzed by using a gas chromatograph (GC-2014 SHIMADZU). To estimate the rejection of TS-1 catalysts by the membrane separator, the concentration of TS-1 catalysts in the permeate was analyzed by inductively coupled plasma emission spectroscopy (ICP, Optima 2000DV). Cyclohexanone conversion and cyclohexanone oxime selectivity were calculated on the basis of the starting amount of cyclohexanone, according to eqs 1 and 2, respectively. X(%) =

S(%) =

Ccyclohexanone(in) − Ccyclohexanone(out) Ccyclohexanone(in)

Table 1. Effect of Ammonia Feeding Mode on the Conversion and Selectivity of Cyclohexanone Ammoximation Reactiona

conversion/% selectivity/%

89.6 52.4

91.0 70.5

Operation conditions: reaction temperature = 343 K, reaction time = 100 min, n(ammonia)/n(cyclohexanone) = 1.4, n(hydrogen peroxide)/n(cyclohexanone) = 1.2, m(TS-1catalyst)/n(cyclohexanone) = 8 g/mol, stirring rate = 300 rpm.

feeding mode of ammonia had an important effect on the conversion and selectivity. Compared to mode A, higher conversion and selectivity could be obtained with mode B, indicating that the introduction of the ceramic membrane distributor in the cyclohexanone ammoximation over TS-1 was beneficial for the enhancement of reaction performance. It was mainly because the porous ceramic membrane distributor owned many microchannels to satisfy the dispersion of ammonia in cyclohexanone in the way of tiny bubbles,11 which increased effectively the contact area between the gas and liquid phase, and as a result, improved the conversion and selectivity. 3.2. Effect of Airlift. As shown in Figure 3, when the flow rate of nitrogen increased, the conversion and selectivity

(1)

(2)

where X is the cyclohexanone conversion, S is the selectivity of cyclohexanone oxime, Ccyclohexanone(in) is the initial concentration of cyclohexanone in the feed, C cyclohexanone(out) is the concentration of cyclohexanone in the permeate, and CAO is the concentration of cyclohexanone oxime in the outlet of the membrane module. To determine the actual amount of NH3, excess ammonia was absorbed by water, and the solution was titrated with a diluted solution of sulfuric acid of known concentration with use of a methyl red indicator.4 The methyl red indicator was made up by dissolving 0.1 g of methyl red in 60% alcohol (mass fraction). 2.4. Filtration Resistance. Filtration performance of ceramic membrane is limited by the membrane fouling. However, the membrane fouling is an unavoidable phenomenon in the membrane separation process, so a key problem is how to control the membrane fouling. So as to solve this problem, an airlift membrane reactor coupling the membrane separation and airlift technology was used, which could reduce the membrane fouling and energy consumption.21,22 Membrane fouling could be controlled by changing the transmembrane pressure and represented through the filtration resistance. On the bases of Darcy’s law,23 the filtration resistance was calculated by the following equation:

R=

ammonia gas was fed through membrane distributor

a

100

CAO 100 Ccyclohexanone(in) − Ccyclohexanone(out)

ammonia gas was fed through stainless steel inlet

Figure 3. Effect of nitrogen blowing on the conversion and selectivity of cyclohexanone ammoximation continuous reaction and the filtration resistance (Ccyclohexanone = 4.42 mol·L−1, CH2O2 = 5.31 mol·L−1, catalyst concentration = 30 g·L−1, blowing(ammonia) = 800 mL·min−1, residence time = 100 min, reaction pressure = 0.3 MPa, reaction temperature = 343 K).

ΔP Jμ

where R is the filtration resistance (m−1), ΔP is the transmembrane pressure (Pa), J is the membrane flux (m· s−1), and μ is the viscosity of permeate (Pa·s). 2.5. Membrane Characterization. The ceramic membranes were characterized by field-emission scanning electronic microscopy (FESEM) (S-4800 II, Hitachi) and an energy dispersive X-ray spectrometer (INCAXSIGHT, UK).

remained almost stable, while the filtration resistance first dropped sharply and then remained stable. The blowing process was mainly effective in the control of concentration polarization16 and reducing the thickness of the filter cake to a certain degree;24,25 however, it had little effect on removing pore blocking. Therefore, when the concentration polarization was basically eliminated, the cake resistance played a key role, and then the blowing had no obvious effect on the filtration resistance. The 800 mL/min value would be chosen as the optimal flow rate for the nitrogen blowing because the lowest filtration resistance was obtained with high conversion and selectivity. 3.3. Effect of Operation Conditions. The catalytic process of cyclohexanone ammoximation without the addition

3. RESULTS AND DISCUSSION 3.1. Effect of Ceramic Membrane Distributor. Ammonia gas was added to the reactor in two different ways for the cyclohexanone ammoximation over TS-1 in order to study the influence of the ceramic membrane distributor. These were (A) adding ammonia gas directly into the reactor through a stainless steel inlet and (B) allowing the ammonia gas to flow through the ceramic outer membrane. Table 1 shows the cyclohexanone conversion and cyclohexanone oxime electivity using the different feeding modes of ammonia gas. Obviously, the 6374



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of extra solvents over TS-1 in the dual-membrane airlift reactor was studied in detail by investigating the effects of operating parameters including residence time, temperature, catalyst concentration, and molar ratio between the reactants on the cyclohexanone conversion, cyclohexanone oxime selectivity, and the filtration resistance. 3.3.1. Effect of Residence Time. By controlling the flow rates of the feed and the discharge, the effect of residence time from 40 to 120 min on the conversion, selectivity, and filtration resistance was investigated, and the results are shown in Figure 4. An increase of conversion was observed with an increase of

Figure 5. Effect of temperature on the conversion and selectivity of cyclohexanone ammoximation continuous reaction and the filtration resistance (Ccyclohexanone = 4.42 mol·L−1, CH2O2 = 5.31 mol·L−1, catalyst concentration = 30 g·L−1, blowing(ammonia) = 800 mL·min−1, blowing(nitrogen) = 800 mL·min−1, residence time = 100 min, reaction pressure = 0.3 MPa).

reduced at constant membrane flux, which led to the decrease of filter cake thickness and filtration resistance.28 For these reasons, the suitable temperature should be 343 K for this continuous coupling reaction. 3.3.3. Effect of Catalyst Concentration. Figure 6 illustrates the influence of catalyst concentration on the conversion,

Figure 4. Effect of residence time on the conversion and selectivity of cyclohexanone ammoximation continuous reaction and the filtration resistance (Ccyclohexanone = 4.42 mol·L−1, CH2O2 = 5.31 mol·L−1, catalyst concentration = 30 g·L−1, blowing(ammonia) = 800 mL·min−1, blowing(nitrogen) = 800 mL·min−1, reaction temperature = 343 K, reaction pressure = 0.3 MPa).

residence time until a maximum value at the residence time of 100 min and then remained almost stable, while the selectivity did not change appreciably. At the same time, the filtration resistance decreased with the residence time. When the feed flow rate decreased, the discharge flow rate also must be decreased to keep the liquid level in the reactor constant. So when the residence time increased, both feed flow rate and discharge flow rate (membrane flux) must be decreased. The decrease of the membrane flux resulted in the reduction of the needed transmembrane pressure, which would make the cake layer thickness and filtration resistance decrease.26 All this suggested that the suitable residence time should be 100 min, and the corresponding transmembrane pressure was 0.08 MPa. 3.3.2. Effect of Temperature. The effect of temperature on the conversion, selectivity, and filtration resistance was studied. As can be seen from Figure 5, with increasing the temperature, the conversion increased first, reached its maximum at 343 K, and then decreased, while the selectivity increased first and then almost remained constant. Due to the increase of temperature, the molecular Brownian movement enhanced,27 which gave rise to the increase of reaction rate and conversion. However, the increase of temperature also could speed up the decomposition reaction of the H2O2 and the volatilization of dissolved ammonia, which brought about the decrease of conversion. The temperature had a positive effect on not only the main reaction but also the side reaction, and as the former was dominant the selectivity of cyclohexanone oxime would increase. In the meantime, the filtration resistance decreased gradually with the temperature. As the temperature increased, the fluid viscosity reduced. As a result, the needed transmembrane pressure was

Figure 6. Effect of catalyst concentration on the conversion and selectivity of cyclohexanone ammoximation continuous reaction and the filtration resistance (Ccyclohexanone = 4.42 mol·L−1, CH2O2 = 5.31 mol· L−1, blowing(ammonia) = 800 mL·min−1, blowing(nitrogen) = 800 mL· min−1, residence time = 100 min, reaction pressure = 0.3 MPa, reaction temperature = 343 K).

selectivity, and filtration resistance. Catalyst concentration played a key role in the reaction of cyclohexanone ammoximation. The results in Figure 6 showed that the conversion and selectivity increased obviously until a catalyst concentration of 35 g/L, and then almost remained stable. In addition, the increased catalyst concentration resulted in an increase in the filtration resistance. The catalyst role of the claybased TS-1 composite was highlighted in the cyclohexanone ammoximation, which had a positive effect on the reaction rate.29 Further increasing the catalyst concentration had little effect on the conversion and selectivity. However, the further increased catalyst concentration resulted in further deposition 6375



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of the catalyst particles on the membrane surface, which made the filtration resistance greater. From the above, the suitable concentration was 35 g/L. 3.3.4. Effect of Molar Ratio of H2O2/Cyclohexanone. To illustrate the effect of the molar ratio of H2O2/cyclohexanone on the reaction, the cyclohexanone ammoximation was carried out in five molar ratios (0.6, 0.8, 1, 1.2, 1.4), and the results are shown as Figure 7. The conversion and selectivity increased

Figure 8. Effect of ammonia blowing on the conversion and selectivity of cyclohexanone ammoximation continuous reaction (Ccyclohexanone = 4.42 mol·L−1, CH2O2 = 5.31 mol·L−1, catalyst concentration = 30 g·L−1, blowing(nitrogen) = 800 mL·min−1, residence time = 100 min, reaction pressure = 0.3 MPa, reaction temperature = 343 K).

ammonia gave rise to the side reactions such as the decomposition of hydrogen peroxide.30 To determine the actual amount of NH3, excess ammonia was absorbed by water and analyzed by a titration method as presented in the Experimental Section. The actual molar ratio of NH3/cyclohexanone under different flow rates of ammonia is shown in Table 2. When the flow rate of ammonia ranged from

Figure 7. Effect of molar ratio of H2O2/cyclohexanone on the conversion and selectivity of cyclohexanone ammoximation continuous reaction and the filtration resistance (Ccyclohexanone = 4.42 mol·L−1, catalyst concentration = 35 g·L−1, blowing(ammonia) = 800 mL·min−1, blowing(nitrogen) = 800 mL·min−1, residence time = 100 min, reaction pressure = 0.3 MPa, reaction temperature = 343 K).

Table 2. Actual Molar Ratio of NH3/Cyclohexanone at Different Ammonia Blowing

with the molar ratio of H2O2/cyclohexanone, and reached a maximum at the molar ratio of H2O2/cyclohexanone of 1.2. Although the ammoximation of cyclohexanone required equimolar cyclohexanone and hydrogen peroxide in analytical chemometrics, excess hydrogen peroxide was needed to achieve high conversion and selectivity. It was mainly because H2O2 was an active oxidant and could be easily decomposed at high temperatures. Hence, when the concentration of H2O2 was insufficient, the conversion and selectivity were very low; however, when the dosage of H2O2 was excessive, the decomposition of hydrogen peroxide would be boosted30 and made the conversion and selectivity decrease. The filtration resistance changed slightly with the molar ratio of H2O2 to cyclohexanone until 1.2 and then increased. Organic substances can be adsorbed on to the surfaces of the catalyst particles in the cake layer and subsequently change the permeability of the cake layer. In this work, when the selectivity was low, some byproducts were formed and absorbed on the catalyst particles in the cake layer (verified by the TG results of fresh TS-1 particles and used TS-1 particles taken from the membrane surface, data not shown here), resulting in a compact cake layer and a higher filtration resistance. Taking into account all these factors, the suitable molar ratio of H2O2/cyclohexanone was 1.2. 3.3.5. Effect of Molar Ratio of NH3/Cyclohexanone. Figure 8 exhibits the optimal molar ratio of NH3 to cyclohexanone by using a porous ceramic outer-membrane as a membrane distributor, in which the cyclohexanone concentration was kept at 4.42 mol L−1. It can be seen that the conversion increased with the ammonia blowing, while the selectivity decreased. The increase of ammonia concentration could obtain a high conversion.31 However, the selectivity decreased when increasing the dosage of ammonia, owing to that excess

ammonia blowing (mL/min)

actual molar ratio of NH3/cyclohexanone

200 400 600 800 1000

0.61 0.86 1.20 1.59 3.53

200 to 1000 mL/min, the actual molar ratio of NH3/ cyclohexanone were 0.61, 0.86, 1.2, 1.59, and 3.53, respectively. The 800 mL/min value would be chosen as the optimal flow rate for the ammonia blowing because the maximum yield could be obtained at the optimal value, and the actual molar ratio of NH3/cyclohexanone was 1.59. 3.4. Feasibility of the Continuous Reaction Process. The continuous ammoximation of cyclohexanone was run for 25 h to investigate the feasibility of the reaction and the results are shown in Figure 9. We found that the conversion and selectivity mainly remained at ∼87% and ∼76%, respectively, in the whole 25 h. The filtration resistance increased significantly within the first 5 h and held constant at ∼2.2 × 1011 m−1 in the following 15 h, and then increased obviously. ICP analysis result showed that the catalyst particles were completely retained inside the reactor by the membrane separator, i.e., the rejection of TS-1 catalysts was 100%. However, considering the size distribution of catalyst and membrane, few catalyst particles having smaller particle size might enter the membrane pore during the reaction, resulting in the lose of TS-1 catalysts, which will be estimated in future work. The feasibility study showed that the airlift membrane system for continuous cyclohexanone ammoximation over TS-1 could maintain a steady reaction process of cyclohexanone oxime over 25 h; however, it could not maintain a steady continuous separation 6376



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hexanone. The fresh and used membranes were characterized by FESEM. As shown in Figures 10 and 11, for the membrane

Figure 9. Stability of the continuous reaction and separation process (Ccyclohexanone = 4.42 mol·L−1, CH2O2 = 5.31 mol·L−1, catalyst concentration = 35 g·L−1, blowing(ammonia) = 800 mL·min−1, blowing(nitrogen) = 800 mL·min−1, residence time = 100 min, reaction pressure = 0.3 MPa, reaction temperature = 343 K). Figure 10. FESEM pictures of the membrane distributor: (a1) top view of fresh membrane; (b1) top view of used membrane; (a2) side view of fresh membrane; and (b2) side view of used membrane.

because of the membrane fouling. As the reaction progressed, organic byproducts were adsorbed on the surfaces of particles in the filter cake and changed the character of the filter cake,32 which led to the increase of resistance. Further works will be carried out to control the membrane fouling. Table 3 lists the typical results from this work and compares them with data from different reactors under their respective optimal operation conditions as found in the literature.8,33−35 As seen in Table 3, the obtained results in the present work are among those with high conversion and selectivity during the cases using the solvent of water, due to the introduction of membrane distribution that could strengthen the mix of the gas and organic phases. Compared to the solvent of water−tertbutanol, the conversion and selectivity in the present study are lower, possibly because the existence of organic solvents such as tert-butanol can enhance the mass transfer of reactants to the pore system of TS-1. An important consideration is the life of the ceramic membranes used in the continuous ammoximation of cyclo-

distributor or membrane separator, no obvious difference was observed between the fresh membrane surface and the used membrane surface, and the top membrane layer still remained well intact with the support after 25 h of continuous use. The results suggested that the ceramic membranes used in the cyclohexanone ammoximation experiments had excellent thermal stability and chemical stability, in good agreement with our previous work.5

4. CONCLUSIONS This work put forward a novel dual-membrane airlift reactor for the continuous cyclohexanone ammoximation over TS-1 without the addition of extra solvents such as tert-butanol. The membrane distribution could obviously improve the selectivity. The membrane for separation could effectively

Table 3. Comparison of Cyclohexanone Conversion and Cyclohexanone/Oxime Selectivity reactor slurry bed reactor

dual- membrane airlift reactor

catalyst

catalyst amount /(g cat/g ketone)

residence time/h

TS-1

0.05 0.15

2

clay- based TS-1

0.33

2.5

TS-1

0.01 0.20

1 5

Ti-MWW

0.01

1

TS-1

0.10

4

TS-1

0.08

1.67

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solvent H2O H2O−tertbutanol H2O methanol H2O−tertbutanol H2O H2O−tertbutanol H2O−tertbutanol H2O−tertbutanol methanol H2O toluene H2O

conversion/%

selectivity/%

yield/%

46.8 96.5

93.3 99.3

46.7 95.8

8

ref

43.0

33

100.0

97.0

46.0 97.0

16.2 97.0

72.8 99.9

11.9 96.9

99.4

99.9

99.3

99.4

95.6

95.0

35

89.0 99.2 99.8 87

73.8 80.1 84.0 76

65.7 79.4 83.8 66.1

this work

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silicalite-1 composite in a semibatch process. Ind. Eng. Chem. Res. 2011, 50, 13703−13710. (5) Lu, C.; Chen, R.; Xing, W.; Jin, W.; Xu, N. A submerged membrane reactor for continuous phenol hydroxylation over TS-1. AIChE J. 2008, 54, 1842−1849. (6) Zhong, Z. X.; Xing, W. H.; Jin, W. Q.; Xu, N. P. Adhension of nanosized nickel catalysts in the nanocatalysis/UF system. AIChE J. 2007, 52, 1204−1210. (7) Fu, J. F.; Ji, M.; Wang, Z.; Jin, L. N.; An, D. N. A new submerged membrane photocatalysis reactor (SMPR) for fulvic acid removal using a nano-structured photocatalyst. J. Hazard. Mater. 2006, B131, 238−242. (8) Xu, H.; Zhang, Y. T.; Wu, H. H.; Liu, Y. M.; Li, X. H.; Jiang, G. J.; He, M. Y.; Wu, P. Postsynthesis of mesoporous MOR-type titanosilicate and its unique catalytic properties in liquid-phase oxidations. J. Catal. 2011, 281, 263−272. (9) Zhao, S.; Xie, W.; Yang, J. X.; Liu, Y. M.; Zhang, Y. T.; Xu, B. L.; Jiang, J. G.; He, M. Y.; Wu, P. An investigation into cyclohexanone ammoximation over Ti-MWW in a continuous slurry reactor. Appl. Catal., A 2011, 394, 1−8. (10) Wang, K.; Lu, Y. C.; Xu, J. H.; Gong, X. C.; Luo, G. S. Reducing side product by enhancing mass-transfer rate. AIChE J. 2006, 52, 4207−4213. (11) Chen, R. Z.; Bao, Y. H.; Xing, W. H.; Jin, W. Q.; Xu, N. P. Enhanced phenol hydroxylation with oxygen using a ceramic membrane distributor. Chin. J. Catal. 2013, 34, 200−208. (12) Kouakou, E.; Salmon, T.; Toye, D.; Marchot, P.; Crine, M. Gasliquid mass transfer in a circulating jet-loop nitrifying MBR. Chem. Eng. Sci. 2005, 60, 6346−6353. (13) Xu, Z.; Yu, J. Hydrodynamics and mass transfer in a novel multiairlifting membrane bioreactor. Chem. Eng. Sci. 2008, 63, 1941−1949. (14) Cui, Z. F.; Chang, S.; Fane, A. G. The use of gas bubbling to enhance membrane processes. J. Membr. Sci. 2003, 221, 1−35. (15) Liu, C.; Tanaka, H.; Zhang, L.; Jing, Z. G.; Xia, H. C.; Jin, M.; Matsuzawab, Y. Fouling and structural changes of Shirasu porous glass (SPG) membrane used in aerobic wastewater treatment process for microbubble aeration. J. Membr. Sci. 2012, 421, 225−231. (16) Bellara, S. R.; Cui, Z. F.; Pepper, D. S. Gas sparging to enhance permeate flux in ultrafiltration using hollow fibre membranes. J. Membr. Sci. 1996, 121, 175−184. (17) Cabassud, C.; Laborie, S.; Laine, J. M. How slug flow can improve ultrafiltration flux in organic hollow fibres. J. Membr. Sci. 1997, 128, 93−101. (18) Zhang, F.; Jing, W. H.; Xing, W. H. Modeling of cross-flow filtration processes in an airlift ceramic membrane reactor. Ind. Eng. Chem. Res. 2009, 48, 10637−10642. (19) Jiang, H.; Meng, L.; Chen, R. Z.; Jin, W. Q.; Xing, W. H.; Xu, N. P. A novel dual-membrane reactor for continuous heterogeneous oxidation catalysis. Ind. Eng. Chem. Res. 2011, 50, 10458−10464. (20) Saronno, P. R.; Mailan, M. P.; Arese, E. M.; Besnate, G. D. A. Catalytic process for preparing cyclohexanone oxime. US4745221, 1988. (21) Berube, P. R.; Lin, H.; Watai, Y. Fouling in air sparged submerged hollow fiber membranes at sub- and super-critical flux conditions. J. Membr. Sci. 2008, 307, 169−180. (22) Wicaksana, F.; Fane, A. G.; Chen, V. Fibre movement induced by bubbling using submerged hollowed fibre membranes. J. Membr. Sci. 2006, 271, 186−195. (23) Li, W. X.; Xing, W. H.; Xu, N. P. Modeling of relationship between water permeability and microstructure parameters of ceramic membranes. Desalination 2006, 192, 340−345. (24) Hwang, K. J.; Wu, Y. Flux enhancement and cake formation in air-sparged cross-flow microfiltration. Chem. Eng. J. 2008, 139, 296− 303. (25) Hwang, K. J.; Hsu, C. E. Effect of gas-liquid flow pattern on airsparged cross-flow microfiltration of yeast suspension. Chem. Eng. J. 2009, 151, 160−167. (26) Kumar, S. M.; Madhu, G. M.; Roy, S. Fouling behaviour, regeneration options and on-line control of biomass-based power plant

Figure 11. FESEM pictures of the membrane separator: (a1) top view of fresh membrane; (b1) top view of used membrane; (a2) side view of fresh membrane; (b2) side view of used membrane.

prevent TS-1 passing the micropores to realize the coupled reaction and separation. The blowing process was introduced into the coupled process to overcome the membrane fouling. The conversion, selectivity, and filtration resistance were all closely related to the operation conditions. The continuous cyclohexanone ammoximation over TS-1 in the dual-membrane airlift reactor could run stably for 25 h, and the conversation of 87% and selectivity of 76% were obtained under the optimized operation conditions. Meanwhile, the used ceramic membranes had excellent thermal stability and chemical stability. However, the continuous cyclohexanone ammoximation reaction could not run stably for longer time under the optimized operation conditions due to the obviously increased membrane resistance. Further works will be carried out to prolong the long-term operation stability of the membrane reactor system such as adding back-flushing and decreasing the operation pressure for the membrane filtration.

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*E-mail: [email protected]. Tel.: +86-25-8317-2288. Fax: +86-25-8317-2292. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National High-Tech Research and Development Program of China (2012AA03A606) and the National Natural Science Foundation of China (21125629, 21076102) is gratefully acknowledged.

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REFERENCES

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