Article pubs.acs.org/IECR
A Side-Stream Catalysis/Membrane Filtration System for the Continuous Liquid-Phase Hydrogenation of Phenol over Pd@CN to Produce Cyclohexanone Zhengyan Qu, Shuo Hu, Hong Jiang, Yefei Liu, Jun Huang, Weihong Xing, and Rizhi Chen* State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, China S Supporting Information *
ABSTRACT: A catalysis/membrane filtration system combining a catalytic reaction and a separation process can realize the in situ separation of ultrafine catalysts from the reaction mixture and make the production continuous. In this study, a side-stream catalysis/membrane filtration system was developed for the first time for the continuous liquid-phase hydrogenation of phenol over Pd@CN to produce cyclohexanone. The operating parameters including the reaction and filtration conditions were optimized by balancing their influences on the catalytic and separation properties. It was found that the properties of the side-stream catalysis/membrane filtration system depended strongly on the operating conditions. Continuous phenol hydrogenation over Pd@CN was performed under the optimized operating conditions. A stable operation of 30 h was achieved with both a phenol conversion and a cyclohexanone selectivity of greater than 85%, and the ceramic membrane showed excellent stability. This study is a contribution to the development of green cyclohexanone production processes.
1. INTRODUCTION Cyclohexanone is widely used for the industrial manufacture of nylon-6 and nylon-66.1 Three routes have been commercialized for the production cyclohexanone, namely, cyclohexene hydration, cyclohexane oxidation, and phenol hydrogenation.2−5 Cyclohexene hydration typically has a low reaction rate and low conversion.2 Cyclohexane oxidation is often operated under harsh reaction conditions, and the resulting byproducts increase the purification costs.3 There are two-step and one-step routes for phenol hydrogenation.4 In the two-step approach, cyclohexanol is first obtained by phenol hydrogenation, and then cyclohexanone is produced by cyclohexanol dehydrogenation.6 In the one-step method, the reaction can be performed in the gas or liquid phase.7 A high temperature is required for the gas-phase hydrogenation, which inevitably leads to the coke-based deactivation of the catalyst. 8 Alternatively, in the liquid-phase hydrogenation, the temperature is quite low, which moderates the catalyst deactivation and makes the process more energy-efficient.5,9 However, in the liquid-phase route, it is very challenging to obtain a high cyclohexanone selectivity greater than 95% at a high phenol conversion of at least 80%, because the obtained cyclohexanone can be further hydrogenated to cyclohexanol. Recently, a variety of catalysts have been developed to increase the yield of cyclohexanone.10−15 Among them, the Pd@carbon nitride (Pd@CN) catalyst was found to exhibit better catalytic performance and to promote the manufacture of cyclohexanone under mild conditions in aqueous solution without ionic additives, in addition to simplifying the purification of the product.4,14,15 However, the reuse of ultrafine catalysts is still a © XXXX American Chemical Society
key problem limiting the continuous production of cyclohexanone.16 By coupling heterogeneous catalysis with membrane separation, the membrane reactor is very promising for separating catalysts in situ from the reaction mixture and making the heterogeneous catalysis process continuous.17 Previously, polymeric membrane reactors were mainly used for biochemistry, photocatalysis, and wastewater treatment.18−23 Recently, with the development of inorganic membranes, especially ceramic membranes, membrane reactors have attracted considerable attention in the production of chemicals.24 Many studies have focused on oxidation reactions in ceramic membrane reactors.25−30 For example, a ceramic membrane reactor was developed for the continuous hydroxylation of phenol to dihydroxybenzene, in which the phenol conversion and dihydroxybenzene selectivity remained at 11% and 95%, respectively, for 20 h.28 Similarly to oxidation, hydrogenation is also an important reaction for the production of chemicals.31 However, few experiments have been performed on membrane reactors being used to carry out hydrogenation processes, especially continuous hydrogenation reactions. This work aimed to design and construct a catalysis/ membrane filtration system for the continuous liquid-phase hydrogenation of phenol over Pd@CN to produce cyclohexanone. The in situ separation of the catalyst with the Received: Revised: Accepted: Published: A
July 23, 2017 September 25, 2017 September 27, 2017 September 27, 2017 DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Ceramic membrane reactor system developed for the continuous liquid-phase hydrogenation of phenol to cyclohexanone over Pd@CN.
China) was used to circulate the reaction mixture between the reactor and membrane module. The membrane module was made of stainless steel, and a perfluoroelastomer O-ring (Dupont, Wilmington, DE) was used as the sealing material. The tubular ceramic membrane was a single tube with an outer diameter of 12 mm, an inner diameter of 8 mm, a length of 33 cm, a separation layer thickness of 10 μm, and a filtration area of 83 cm2 (Jiangsu Jiuwu High-Tech Co., Ltd., Nanjing, China). The membrane had an asymmetric structure, having a top layer of α-Al2O3 with a pore size of 200 or 500 nm on the inner wall of the support layer (α-Al2O3, pore size of 2000 nm). During the hydrogenation process, the liquid-phase reaction mixture partly permeated out of the membrane under the transmembrane pressure, and the mode of permeation was insideout. A constant flow pump provided by Beijing Chuangxin Tongheng Science & Technology Co., Ltd., Beijing, China, was employed to pump phenol solution into the stirred tank. 2.4. Continuous Phenol Hydrogenation over Pd@CN. The continuous hydrogenation of phenol over Pd@CN (Scheme 1) was performed in the side-stream catalysis/ membrane filtration system depicted in Figure 1.
ceramic membrane was realized. This work focused on the effects of the operating parameters on the hydrogenation reaction and membrane separation to achieve optimized operating conditions. Furthermore, a 30-h test of the hydrogenation of phenol in the developed side-stream catalysis/membrane filtration system under the optimized conditions was performed to assess the feasibility of continuous cyclohexanone production.
2. EXPERIMENTAL SECTION 2.1. Synthesis of Pd@CN. The synthesis of Pd@CN was performed as reported in the literature.4 Citric acid and dicyandiamide in a molar ratio of 2:3 were dissolved in methanol with stirring (where the ratio of dicyandiamide to methanol was 1 g:25 mL). After about 4 h, activated carbon (in a weight ratio of 9:2 with respect to citric acid) was added to this solution, and the resulting mixture was stirred for 12 h. Then, the CN material was obtained by rotary evaporation of the mixture, dried at 60 °C under a vacuum overnight, and calcined at 550 °C under nitrogen for 4 h. After that, a certain amount of CN material was added to a palladium acetate acetone solution (where the ratio between CN and acetone was 1 g:25 mL and the typical Pd loading was 2 wt %) with stirring for about 12 h. Finally, the Pd(OAc)2@CN material was obtained by removing the acetone through rotary evaporation and calcined at 300 °C under a hydrogen atmosphere for 4 h to produce the Pd@CN catalyst. 2.2. Characterization of Pd@CN. To measure the particle size, a particle size analyzer (Malvern Mastersizer 2000) was used. To detect the crystal structure of the Pd@CN catalyst, Xray diffraction (XRD) patterns were acquired at 2θ values of 20°−80° using a Rigaku MiniFlex 600 diffractometer. To investigate the distribution of Pd nanoparticles on the support, transmission electron microscopy (TEM) images were obtained using a Tecnai 12 microscope operated at 120 kV. To obtain the Pd loading on the support, inductively coupled plasma atomic emission spectroscopy (ICP-AES, Optima 7000 DV) was employed. A Micromeritics ASAP 2020 adsorption apparatus was used to determine the specific surface area of Pd@CN catalyst. Nitrogen sorption measurements were conducted after the sample had been evacuated at 130 °C for 12 h. 2.3. Side-Stream Catalysis/Membrane Filtration System. A side-stream catalysis/membrane filtration system (Figure 1) was developed in this work. The working volume of the stirred-tank reactor was 5 L, and the reaction temperature was controlled with an external water jacket. A centrifugal pump (Hangzhou Jianbeng Co., Ltd., Hangzhou,
Scheme 1. Reaction Pathways for Phenol Hydrogenation
Typically, after the addition of 3 L of deionized water and the required amount of Pd@CN catalyst, the reactor was sealed and pumped with 0.3 MPa hydrogen several times to remove the air. Then, the centrifugal pump and the reactor heating system were turned on. When the temperature reached the required value, a phenol aqueous-phase solution with a concentration of 1 wt % was added to the reactor. The phenol hydrogenation started after the hydrogen pressure and stirring rate had been tuned to the expected values. The flow rate of product on the permeate side was controlled by a bleeder value and was the same as the feeding flow rate of the phenol solution, so that the liquid level in the tank reactor was constant. With the help of the ceramic membrane, the suspended Pd@CN catalyst particles were kept in the system. B
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research The products were collected from the permeate side and analyzed by gas chromatography.32 The continuous reaction− separation process was operated for 8 h for the investigation of the operating conditions. After each reaction process, we discharged the hydrogen, cooled the reactor to room temperature, and then purged the side-stream catalysis/membrane filtration system with nitrogen twice. After that, the system was discharged and cleaned with deionized water. As the pure-water flux of the used membrane recovered more than 95% of the value of the fresh membrane, the same ceramic membrane was applied in the next phenol hydrogenation cycle. Specifically, in 30 h of continuous operation of the catalysis/membrane filtration system, the membrane was not back-flushed or cleaned to reverse the fouling. The phenol conversion (X), cyclohexanone selectivity (S), and cyclohexanone yield (Y) were calculated according to the equations32−34 X=
S=
0 − Cphenol Cphenol 0 Cphenol
Ccyclohexanone 0 Cphenol − Cphenol
Y=X×S
Figure 2. Effects of residence time on the reaction and filtration performance (stirring rate, 800 rpm; hydrogen pressure, 0.1 MPa; reaction temperature, 70 °C; catalyst concentration, 9 g·L−1).
phenol conversion was obtained. Cyclohexanone is often further hydrogenated to produce cyclohexanol (Scheme 1).32,36 Thus, with increasing residence time, the concentration of cyclohexanone increased, and the concentration of cyclohexanol also increased, leading to a decrease in the cyclohexanone selectivity. The yield of cyclohexanone first increased obviously until 3 h and then increased only slightly. The filtration resistance decreased as the residence time increased. The residence time could be controlled by varying the flow rates of the feed and discharge. An increase ni the residence time could be obtained by decreasing the feed flow rate. Correspondingly, to make the liquid level in the system stable, the membrane flux/discharge flow rate needed to be reduced, resulting in a decreased transmembrane pressure, as shown in Table 1. As a result, a thin filtration cake formed, leading to a decrease of the filtration resistance.37 Hence, a residence time of 3 h was found to be suitable.
× 100% (1)
× 100% (2) (3)
where X, S, and Y are the phenol conversion, cyclohexanone selectivity, and cyclohexanone yield, respectively. C0 and C are the concentrations (mol·L−1) in the feed and in the permeate, respectively. The filtration resistance is also an important factor reflecting the properties of the catalysis/membrane filtration system, and it was estimated by Darcy’s law35
R=
ΔP Jμ
(4)
Table 1. Effects of Residence Time on the Membrane Flux, Transmembrane Pressure, and Filtration Resistance
where R, ΔP, J, and μ are the filtration resistance (m−1), transmembrane pressure (Pa), membrane flux (m·s−1), and permeate viscosity (Pa·s), respectively. A S-4800 II field-emission scanning electronic microscopy (FESEM) system was used to estimate the stability of the membrane material during the continuous phenol hydrogenation.
3. RESULTS AND DISCUSSION A catalysis/membrane filtration system couples a catalysis reaction with a membrane separation process, and the operating parameters including the reaction and filtration conditions can affect both the catalytic and separation properties. In this study, the operating parameters were optimized by balancing their influences on the system properties. In addition, a 30-h run was performed under the optimized conditions in the developed side-stream catalysis/ membrane filtration system to assess the feasibility of the continuous hydrogenation of phenol to cyclohexanone over Pd@CN. 3.1. Optimization of Reaction Conditions. Figure 2 shows the influence of the residence time on the system performance. The phenol conversion gradually increased as the residence time was increased, whereas the cyclohexanone selectivity gradually decreased. As expected, the contact time among reactants increased with the residence time, and a high
residence time (h)
J (L·m−2·h−1)
ΔP (103 Pa)
R (1011 m−1)
2 2.5 3 4
180.9 144.8 120.6 90.5
10.2 7.2 5.8 4.1
4.97 4.42 4.23 4.05
The influence of the stirring rate on the system performance is presented in Figure 3. As the stirring rate increased, the phenol conversion increased slightly, and the cyclohexanone selectivity first increased until the stirring rate reached 800 rpm and then decreased slightly. It is well-known that heterogeneous catalytic reactions depend strongly on internal and external diffusion.38 The particles of the Pd@CN catalyst were tiny, with a particle size of 0.2−2 μm.32 Therefore, according the Mears criterion,39 the internal diffusion limitations could be ignored. Thus, the mass-transfer resistance could be attributed to external diffusion limitations. As the stirring rate increased, the external diffusion was enhanced, and the phenol conversion and the cyclohexanone selectivity increased.30 Meanwhile, the rate of cyclohexanone converted into cyclohexanol through deep hydrogenation also increased. Once the deep hydrogenation rate surpassed a specific value, the cyclohexanone selectivity would decrease.40 The yield of cyclohexanone first increased gradually and then decreased slightly with increasing C
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
catalyst particles generated a thicker filtration cake, so the filtration resistance increased. A high yield and a low filtration resistance were preferred. Therefore, a hydrogen pressure of 0 MPa (gauge pressure) was selected. The influence of the reaction temperature on the phenol hydrogenation was investigated from 60 to 90 °C. As shown in Figure 5, increasing the reaction temperature was beneficial for
Figure 3. Effects of stirring rate on the reaction and filtration performance (residence time, 3 h; hydrogen pressure 0.1 MPa; reaction temperature, 70 °C; catalyst concentration, 9 g·L−1).
stirring rate. The filtration resistance decreased with increasing stirring rate. Organic matter can adsorb on the particle surfaces in the filtration cake and vary its nature and permeability.41 In this work, with increasing stirring rate, the formation of byproducts and their adsorption on the surfaces of the Pd@CN catalyst particles decreased, leading to the formation of a thinner filtration cake and the reduction of the filtration resistance.25 In addition, as the stirring rate was increased, the turbulence of the reaction solution was enhanced, resulting in a decrease of the cake thickness and filtration resistance. According to the above results, a stirring rate of 800 rpm was selected. Figure 4 shows the influence of the hydrogen pressure on the coupling process. With increasing hydrogen pressure, the
Figure 5. Effects of reaction temperature on the reaction and filtration performance (residence time, 3 h; stirring rate, 800 rpm; hydrogen pressure, 0 MPa; catalyst concentration, 9 g·L−1).
the phenol conversion but not for the cyclohexanone selectivity. As expected, the reaction rate increased at higher reaction temperature, thereby resulting in a higher phenol conversion. The reduction of the cyclohexanone selectivity occurred because of high cyclohexanone concentration.36 When the reaction temperature was 90 °C, the highest cyclohexanone yield was obtained. The filtration resistance was found to decrease with increasing reaction temperature. The fluid viscosity decreased when the temperature was increased.25 Thus, according to Darcy’s law, to make the filtration flux steady, a lower transmembrane pressure was needed. As a consequence, a thinner filtration cake was formed, resulting in a lower filtration resistance. Hence, the appropriate temperature was determined to be 90 °C. Figure 6 illustrates the influence of the catalyst concentration on the system performance. The phenol conversion clearly increased with increasing catalyst concentration, and the cyclohexanone selectivity gradually decreased. Because a higher catalyst concentration was beneficial for contact of the reactants, the rates of phenol conversion and deep hydro-
Figure 4. Effects of hydrogen pressure on the reaction and filtration performance (residence time, 3 h; stirring rate, 800 rpm; reaction temperature, 70 °C; catalyst concentration, 9 g·L−1).
phenol conversion increased slightly, and the cyclohexanone selectivity decreased obviously. According to Henry’s law, the hydrogen concentration in solution increased with increasing hydrogen pressure, which was responsible for the enhancement of the hydrogen adsorption on the catalyst surface and the hydrogenation activity. However, as the maximum value of the hydrogen adsorption was reached, the conversion increased slightly.42 Meanwhile, the large concentration of cyclohexanone increased the rate of deep hydrogenation, resulting in a decrease of the cyclohexanone selectivity.43 Figure 4 also demonstrates that the filtration resistance increased gradually when the hydrogen pressure was increased. The increased amount of byproducts adsorbed on the surfaces of the Pd@CN
Figure 6. Effects of catalyst concentration on the reaction and filtration performance (residence time, 3 h; stirring rate, 800 rpm; hydrogen pressure, 0 MPa; reaction temperature, 90 °C). D
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research genation grew.44 The cyclohexanone yield increased slightly when the catalyst concentration was higher than 5 g·L−1. The filtration resistance increased with increasing catalyst concentration. A thicker filtration cake was formed at higher catalyst concentration, thereby increasing the filtration resistance.45 In addition, a decrease in selectivity should also be a reason for the increase in filtration resistance, as presented above. Therefore, the optimal catalyst concentration was greater than 5 g·L−1. 3.2. Optimization of Filtration Conditions. The dependence of the system performance on the membrane pore size was also investigated. In comparison with the membrane with a pore size of 200 nm (Table 2), the phenol conversion and
results indicate that, for the membrane with a pore size of 500 nm, the membrane fouling was mainly caused by pore blocking, leading to serious membrane fouling and a greater decrease in the flux. This decrease was mainly associated with the particle size of the catalyst. The main particle size of the Pd@CN catalyst was 0.2−2 μm. Obviously, some of the tiny catalyst particles with particle sizes of less than 500 nm could easily enter the membrane pores and deposit on the pore walls, leading to pore narrowing or pore blocking, which caused the membrane flux to decrease significantly and increased the complexity of the membrane filtration operation and membrane regeneration. Considering the above analyses, a ceramic tubular membrane with a pore size of 200 nm was applied in the subsequent studies. The effect of the cross-flow velocity was examined in the range from 1.0 to 3.5 m·s−1, as presented in Figure 7. An
Table 2. Effects of Membrane Pore Size on the Reaction and Filtration Performancea pore size (nm)
C (%)
S (%)
Y (%)
R (1011 m−1)
200 500
85.4 83.5
92.4 92.1
78.9 76.9
4.90 4.50
a
Reaction conditions: residence time, 3 h; stirring rate, 800 rpm; hydrogen pressure, 0 MPa; reaction temperature, 90 °C; catalyst concentration, 5 g·L−1; cross-flow velocity, 2.5 m·s−1.
cyclohexanone selectivity were slightly lower for the membrane with a pore size of 500 nm, which might be due to the deposition of some catalyst particles in the membrane pores. To verify this assumption, the cross sections of the ceramic membranes were analyzed by energy-dispersive spectroscopy (EDS). As shown in Table 3, the Pd element coming from the Table 3. EDS Analysis of the Cross Section of the Ceramic Membrane
Figure 7. Effects of cross-flow velocity on the reaction and filtration performance (membrane pore size, 200 nm).
elements (wt %) membrane cross section
C
O
Al
Pd
fresh, 500 nm used, 500 nm fresh, 200 nm used, 200 nm
15.15 21.85 30.40 27.20
59.68 51.20 48.50 50.28
25.17 26.91 21.20 22.53
− 0.04 − 0.01
increase in phenol conversion and a decrease in cyclohexanone selectivity were observed as the cross-flow velocity increased to 2.5 m·s−1, and then both the phenol conversion and the cyclohexanone selectivity remained almost constant. Higher cross-flow velocities were beneficial for the removal of the catalyst particles deposited on the membrane surface. Thus, at higher cross-flow velocities, the filtration cake thickness decreased, and the catalyst concentration in the suspension increased.27 As a result, the phenol conversion increased in the reaction system, whereas the cyclohexanone selectivity decreased with the catalyst concentration, similar to the results in Figure 6. Higher cross-flow velocities decreased the thickness of the filtration cake, resulting in a decrease of the filtration resistance. Considering the cyclohexanone yield and energy consumption of the circulating pump, 2.5 m·s−1 was chosen as the appropriate cross-flow velocity. 3.3. Feasibility of Continuous Cyclohexanone Production. The continuous operation of the side-stream catalysis/membrane filtration system was tested for 30 h under the above optimized operating conditions. A minor decrease in the conversion was observed in the first 12 h, followed by an obvious decrease (Figure 8). During the primary stage of the continuous operation, the Pd@CN catalyst particles easily adsorbed on the clean membrane surface, leading to a decrease of the catalyst concentration in the reaction system and a decrease in the reaction rate,25 which was responsible for the slight decrease in the phenol conversion. This explanation could be verified by FESEM analyses of the fresh and used membranes. As shown in Figure 9, compared to
Pd@CN catalyst was observed in the cross section of the used membrane with a pore size of 500 nm, whereas the Pd content in the cross section could be ignored for the membrane with a pore size of 200 nm. In addition, the significant differences in the carbon and oxygen contents between the fresh and used membranes might be caused by surface porosity, roughness, and so on. The used membrane was rinsed with deionized water to remove the pollutants from the membrane surface. The pure-water fluxes of the fresh and used membranes are reported in Table 4. It was found that the flux of the membrane with a pore size of 200 nm basically returned to the value for the fresh membrane, whereas that of the membrane with a pore size of 500 nm returned to only 60.9% of the initial flux. These Table 4. Pure-Water Fluxes of Fresh Membrane and Used Membrane after Cleaninga pore size (nm)
J1 (L·h−1·m−2·bar−1)
J2 (L·h−1·m−2·bar−1)
J2/J1
200 500
2915 4325
2825 2635
96.9 60.9
J1, flux of fresh membrane; J2, flux of used membrane after cleaning; temperature, 60 °C. a
E
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
CN material, which was quite similar to the appearance of the fresh catalyst (Figure S2). It can thus be inferred that the aggregation of the Pd particles was not the reason for deactivation.46 The recovered Pd@CN had a Pd loading of 1.62 wt %, similar to the value of 1.78 wt % for the fresh catalyst. The slight decrease might be due to the fact that organic matter deposited in/on the Pd@CN catalyst, which reduced the specific weight of Pd in the catalyst. Furthermore, the amount of Pd in the permeate was negligible according to the ICP-AES analysis, indicating that no obvious leaching of palladium occurred during the continuous phenol hydrogenation process. Figure 10 shows the nitrogen sorption Figure 8. Operation stability of the continuous side-stream catalysis/ membrane filtration system (residence time, 3 h; stirring rate, 800 rpm; hydrogen pressure, 0 MPa; reaction temperature, 90 °C; catalyst concentration, 9 g·L−1; membrane pore size, 200 nm; cross-flow velocity, 2.5 m·s−1).
the fresh membrane, the used membrane exhibited some Pd@ CN catalysts deposited on the membrane surface. The further decrease of the phenol conversion was likely caused by catalyst deactivation, as discussed further below. The cyclohexanone selectivity first decreased and then increased through the process, which should be related to the concentration of cyclohexanone.36 As the reaction proceeded, the filtration resistance first obviously increased and then remained almost constant. During the initial stage of phenol hydrogenation, the Pd@CN particles could rapidly adsorb on the clean membrane surface, leading to the formation of a filtration cake (Figure 9b,e) and an obvious increase in the filtration resistance. With increasing reaction time, the adsorption and desorption of Pd@ CN particles reached a balance, making the filtration resistance stable.25 To explore the reasons for catalyst deactivation, XRD, TEM, N2 adsorption−desorption, and ICP-AES were applied to characterize the fresh and recovered Pd@CN catalysts. The XRD pattern of the recovered Pd@CN catalyst (Figure S1) showed the same peaks as that of the fresh catalyst, suggesting that the used catalyst had the same crystal structure as the fresh one. For the recovered Pd@CN catalyst, Pd nanoparticles with particle sizes of 1−4 nm were homogeneously dispersed on the
Figure 10. Nitrogen adsorption and desorption isotherms of fresh and recovered Pd@CN catalysts. (Solid and open symbols represent adsorption and desorption branches, respectively.)
isotherms of the two Pd@CN catalysts. Both of the isotherms show clear hysteresis loops. According to the IUPAC classification of pores, the presence of micropores and mesopores was confirmed.47 After 30 h of reaction, the specific surface area of the Pd@CN catalyst decreased from 607 to 433 m2·g−1 as calculated by the Brunauer−Emmett−Teller (BET) model. The pores were clogged with organic compounds, and therefore, the deactivation of the Pd@CN catalyst occurred.48 Moreover, no obvious change was observed upon comparison of the fresh and cleaned membranes after 30 h of continuous hydrogenation reaction (Figure 9a,c,d,f), indicating
Figure 9. FESEM images of ceramic membranes: (a−c) top and (d−f) side views of (a,d) fresh membrane, (b,e) used membrane, and (c,f) used membrane after cleaning. F
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research that the ceramic membrane had excellent mechanical and chemical stability and would be suitable for the continuous liquid-phase hydrogenation of phenol over Pd@CN. The results in Figure 8 highlight the fact that the developed side-stream catalysis/membrane filtration system is feasible for the continuous liquid-phase hydrogenation of phenol to cyclohexanone over Pd@CN. Thirty hours of continuous operation could be achieved, and both the phenol conversion and the cyclohexanone selectivity remained greater than 85%. Unfortunately, the obtained phenol conversion and cyclohexanone selectivity were lower than previously reported values, possibly because of the significantly lower reaction volumes in the literature.11,49−52 For instance, the reaction volume was only 20 mL in the work of Zhou et al.,49 significantly lower than the value of 3000 mL in our work. Smaller reaction volumes favor mass transfer, thereby resulting in higher phenol conversions and cyclohexanone selectivities.11,49−52 To further improve the feasibility of continuous cyclohexanone production in the side-stream catalysis/membrane filtration system, a great deal of work is urgently needed, such as the improvement of mass transfer of the catalysis/ membrane filtration system and the enhancement of the catalytic performance of the Pd@CN catalyst.
ACKNOWLEDGMENTS
■
REFERENCES
(1) Bart, J. C. J.; Cavallaro, S. Transiting from adipic acid to bioadipic acid. 1, Petroleum-based processes. Ind. Eng. Chem. Res. 2015, 54, 1− 46. (2) Qiu, T.; Kuang, C. H.; Li, C. G.; Zhang, X. W.; Wang, X. D. Study on feasibility of reactive distillation process for the direct hydration of cyclohexene to cyclohexanol using a cosolvent. Ind. Eng. Chem. Res. 2013, 52, 8139−8148. (3) Wang, Y.; Zhang, J. S.; Wang, X. C.; Antonietti, M.; Li, H. R. Boron- and fluorine-containing mesoporous carbon nitride polymers: Metal-free catalysts for cyclohexane oxidation. Angew. Chem., Int. Ed. 2010, 49, 3356−3359. (4) Wang, Y.; Yao, J.; Li, H. R.; Su, D. S.; Antonietti, M. Highly selective hydrogenation of phenol and derivatives over a Pd@carbon nitride catalyst in aqueous media. J. Am. Chem. Soc. 2011, 133, 2362− 2365. (5) Zhong, J. W.; Chen, J. Z.; Chen, L. M. Selective hydrogenation of phenol and related derivatives. Catal. Sci. Technol. 2014, 4, 3555− 3569. (6) Cheng, H. Y.; Liu, R. X.; Wang, Q.; Wu, C. Y.; Yu, Y. C.; Zhao, F. Y. Selective reduction of phenol derivatives to cyclohexanones in water under microwave irradiation. New J. Chem. 2012, 36, 1085−1090. (7) Chen, J. Z.; Zhang, W.; Chen, L. M.; Ma, L. L.; Gao, H.; Wang, T. J. Direct selective hydrogenation of phenol and derivatives over polyaniline-functionalized carbon-nanotube supported palladium. ChemPlusChem 2013, 78, 142−148. (8) Chary, K. V. R.; Naresh, D.; Vishwanathan, V.; Sadakane, M.; Ueda, W. Vapour phase hydrogenation of phenol over Pd/C catalysts: A relationship between dispersion, metal area and hydrogenation activity. Catal. Commun. 2007, 8, 471−477. (9) Liu, H. Z.; Jiang, T.; Han, B. X.; Liang, S. H.; Zhou, Y. X. Selective phenol hydrogenation to cyclohexanone over a dual supported Pd−Lewis acid catalyst. Science 2009, 326, 1250−1252. (10) Xu, X.; Li, H. R.; Wang, Y. Selective hydrogenation of phenol to cyclohexanone in water over Pd@N-doped carbon derived from ionicliquid precursors. ChemCatChem 2014, 6, 3328−3332. (11) Cheng, L.; Dai, Q. G.; Li, H.; Wang, X. Y. Highly selective hydrogenation of phenol and derivatives over Pd catalysts supported on SiO2 and γ-Al2O3 in aqueous media. Catal. Commun. 2014, 57, 23− 28. (12) Zhang, Q. S.; Li, H. F.; Gao, P.; Wang, L. L. PVP-NiB amorphous catalyst for selective hydrogenation of phenol and its derivatives. Chin. J. Catal. 2014, 35, 1793−1799. (13) Xu, G. Y.; Guo, J. H.; Zhang, Y.; Fu, Y.; Chen, J. Z.; Ma, L. L.; Guo, Q. X. Selective hydrogenation of phenol to cyclohexanone over Pd-HAP catalyst in aqueous media. ChemCatChem 2015, 7, 2485− 2492. (14) Li, Y.; Xu, X.; Zhang, P. F.; Gong, Y. T.; Li, H. R.; Wang, Y. Highly selective Pd@mpg-C3N4 catalyst for phenol hydrogenation in aqueous phase. RSC Adv. 2013, 3, 10973−10982. (15) Xiang, Y. Z.; Kong, L. N.; Xie, P. Y.; Xu, T. Y.; Wang, J. G.; Li, X. N. Carbon nanotubes and activated carbons supported catalysts for phenol in situ hydrogenation: Hydrophobic/hydrophilic effect. Ind. Eng. Chem. Res. 2014, 53, 2197−2203.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03043.
■
■
Financial support from the National Key R&D Program (2016YFB0301503), the Jiangsu Province Natural Science Foundation for Distinguished Young Scholars (BK20150044), the National Natural Science Foundation (91534110, 21606124), the Jiangsu Province Natural Science Foundation (BK20160978), the Foundation of the State Key Laboratory of Materials-Oriented Chemical Engineering (ZK201402, ZK201407), and the Jiangsu Province Technology Innovation Foundation for Science and Technology Enterprises (BC2015008) of China is gratefully acknowledged.
4. CONCLUSIONS A side-stream catalysis/membrane filtration system was developed for the continuous synthesis of cyclohexanone by the one-step liquid-phase hydrogenation of phenol over Pd@ CN. The phenol conversion, cyclohexanone selectivity, and membrane filtration resistance were greatly affected by the operating conditions. The optimal operating conditions were obtained by considering the effects on the cyclohexanone yield and the membrane filtration performance. A cyclohexanone production process could be continuously and stably operated for 30 h. The adsorption of Pd@CN catalyst particles on the clean membrane surface was responsible for the slight decrease in the phenol conversion in the primary stage, and the deactivation of the Pd@CN catalyst due to the adsorption of organic compounds was the reason for the further decrease in the phenol conversion. This study could provide some references for one-step continuous cyclohexanone production, and further work is underway to improve the mass transfer of the catalysis/membrane filtration system and enhance the catalytic performance of the Pd@CN catalyst.
■
Article
XRD and TEM characterizations of fresh and recovered Pd@CN catalysts (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Rizhi Chen: 0000-0003-4298-483X Notes
The authors declare no competing financial interest. G
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research (16) Molinari, R.; Argurio, P.; Lavorato, C. Review on reduction and partial oxidation of organics. Curr. Org. Chem. 2013, 17, 2516−2537. (17) Bodalo, A.; Gomez, J. L.; Gomez, E.; Bastida, J.; Maximo, M. F.; Montiel, M. C. Ultrafiltration membrane reactors for enzymatic resolution of amino acids. Enzyme Microb. Technol. 2001, 28, 355−361. (18) Wang, X. H.; Chang, V. W. C.; Tang, C. Y. Osmotic membrane bioreactor (OMBR) technology for wastewater treatment and reclamation: Advances, challenges, and prospects for the future. J. Membr. Sci. 2016, 504, 113−132. (19) Molinari, R.; Lavorato, C.; Argurio, P. Photocatalytic reduction of acetophenone in membrane reactors under UV and visible light using TiO2 and Pd/TiO2 catalysts. Chem. Eng. J. 2015, 274, 307−316. (20) Huang, L. Y.; Lee, D. J. Membrane bioreactor: A mini review on recent R&D works. Bioresour. Technol. 2015, 194, 383−388. (21) Neoh, C. H.; Noor, Z. Z.; Mutamim, N. S. A.; Lim, C. K. Green technology in wastewater treatment technologies: Integration of membrane bioreactor with various wastewater treatment systems. Chem. Eng. J. 2016, 283, 582−594. (22) Ding, Y. W.; Sun, W.; Cao, L. M.; Yang, J. A spontaneous catalytic membrane reactor to dechlorinate 2,4,6-TCP as an organic pollutant in wastewater and to reclaim electricity simultaneously. Chem. Eng. J. 2016, 285, 573−580. (23) Molinari, R.; Argurio, P.; Poerio, T. Vanadyl acetylacetonate filled PVDF membranes as the core of a liquid phase continuous process for pure phenol production from benzene. J. Membr. Sci. 2015, 476, 490−499. (24) Hashim, S. M.; Mohamed, A. R.; Bhatia, S. Catalytic inorganic membrane reactors: Present research and future prospects. Rev. Chem. Eng. 2011, 27, 157−178. (25) Lu, C. J.; Chen, R. Z.; Xing, W. H.; Jin, W. Q.; Xu, N. P. A submerged membrane reactor for continuous phenol hydroxylation over TS-1. AIChE J. 2008, 54, 1842−1849. (26) Chen, R. Z.; Mao, H. L.; Zhang, X. R.; Xing, W. H.; Fan, Y. Q. A dual-membrane airlift reactor for cyclohexanone ammoximation over titanium silicalite-1. Ind. Eng. Chem. Res. 2014, 53, 6372−6379. (27) Jiang, H.; She, F.; Du, Y.; Chen, R. Z.; Xing, W. H. One-step continuous phenol synthesis technology via selective hydroxylation of benzene over ultrafine TS-1 in a submerged ceramic membrane reactor. Chin. J. Chem. Eng. 2014, 22, 1199−1207. (28) Jiang, X. L.; She, F.; Jiang, H.; Chen, R. Z.; Xing, W. H.; Jin, W. Q. Continuous phenol hydroxylation over ultrafine TS-1 in a sidestream ceramic membrane reactor. Korean J. Chem. Eng. 2013, 30, 852−859. (29) Zhong, Z. X.; Xing, W. H.; Jin, W. Q.; Xu, N. P. Adhesion of nanosized nickel catalysts in the nanocatalysis/UF system. AIChE J. 2007, 53, 1204−1210. (30) 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. (31) Gong, Y. T.; Li, M. M.; Li, H. R.; Wang, Y. Graphitic carbon nitride polymers: Promising catalysts or catalyst supports for heterogeneous oxidation and hydrogenation. Green Chem. 2015, 17, 715−736. (32) Jiang, H.; Qu, Z. Y.; Li, Y.; Huang, J.; Chen, R. Z.; Xing, W. H. One-step semi-continuous cyclohexanone production via hydrogenation of phenol in a submerged ceramic membrane reactor. Chem. Eng. J. 2016, 284, 724−732. (33) Wang, Y.; Wang, X. C.; Antonietti, M. Polymeric graphitic carbon nitride as a heterogeneous organocatalyst: From photochemistry to multipurpose catalysis to sustainable chemistry. Angew. Chem., Int. Ed. 2012, 51, 68−89. (34) Wang, X. C.; Blechert, S.; Antonietti, M. Polymeric graphitic carbon nitride for heterogeneous photocatalysis. ACS Catal. 2012, 2, 1596−1606. (35) Masoodi, R.; Tan, H.; Pillai, K. M. Darcy’s law-based numerical simulation for modeling 3D liquid absorption into porous wicks. AIChE J. 2011, 57, 1132−1143. (36) Scirè, S.; Minicò, S.; Crisafulli, C. Selective hydrogenation of phenol to cyclohexanone over supported Pd and Pd−Ca catalysts: An
investigation on the influence of different supports and Pd precursors. Appl. Catal., A 2002, 235, 21−31. (37) Kumar, S. M.; Madhu, G. M.; Roy, S. Fouling behaviour, regeneration options and on-line control of biomass-based power plant effluents using microporous ceramic membranes. Sep. Purif. Technol. 2007, 57, 25−36. (38) Liu, H.; Lu, G. Z.; Guo, Y. L.; Guo, Y.; Wang, J. S. Chemical kinetics of hydroxylation of phenol catalyzed by TS-1/diatomite in fixed-bed reactor. Chem. Eng. J. 2006, 116, 179−186. (39) Han, L.; Zhou, Z. Q.; Bollas, G. M. Heterogeneous modeling of chemical-looping combustion. Part 1: Reactor model. Chem. Eng. Sci. 2013, 104, 233−249. (40) Pérez, Y.; Fajardo, M.; Corma, A. Highly selective palladium supported catalyst for hydrogenation of phenol in aqueous phase. Catal. Commun. 2011, 12, 1071−1074. (41) Lee, S.; Choo, K.; Lee, C.; Lee, H.; Hyeon, T.; Choi, W.; Kwon, H. Use of ultrafiltration membranes for the separation of TiO2 photocatalysts in drinking water treatment. Ind. Eng. Chem. Res. 2001, 40, 1712−1719. (42) Brennan, R. A.; Nirmalakhandan, N.; Speece, R. E. Comparison of predictive methods for Henry’s law coefficients of organic chemicals. Water Res. 1998, 32, 1901−1911. (43) Wang, W. Y.; Zhang, K.; Li, L.; Wu, K.; Liu, P. L.; Yang, Y. Q. Synthesis of highly active Co−Mo−S unsupported catalysts by a onestep hydrothermal method for p-cresol hydrodeoxygenation. Ind. Eng. Chem. Res. 2014, 53, 19001−19009. (44) Wang, C. C.; Jiang, H.; Chen, C. L.; Chen, R. Z.; Xing, W. H. A submerged catalysis/membrane filtration system for hydrogenolysisof glycerol to 1,2-propanediol over Cu−ZnO catalyst. J. Membr. Sci. 2015, 489, 135−143. (45) Jiang, H.; Jiang, X. L.; She, F.; Wang, Y.; Xing, W. H.; Chen, R. Z. Insights into membrane fouling of a side-stream ceramic membrane reactor for phenol hydroxylation over ultrafine TS-1. Chem. Eng. J. 2014, 239, 373−380. (46) Zhou, Y.; Dai, M. Z.; Ma, W. H.; Chen, X. M. Characterization and catalytic activity of lead-promoted palladium nanoparticle catalysts. Chem. Eng. J. 2009, 150, 237−241. (47) Shao, X. H.; Feng, Z. H.; Xue, R. S.; Ma, C. C.; Wang, W. C.; Peng, X.; Cao, D. P. Adsorption of CO2, CH4, CO2/N2 and CO2/CH4 in novel activated carbon beads: Preparation, measurements and simulation. AIChE J. 2011, 57, 3042−3051. (48) Simescu-Lazar, F.; Meille, V.; Pallier, S.; Chaînet, E.; De Bellefon, C. Regeneration of deactivated catalysts coated on foam and monolith: Example of Pd/C for nitrobenzene hydrogenation. Appl. Catal., A 2013, 453, 28−33. (49) Zhou, H.; Han, B. B.; Liu, T. Z.; Zhong, X.; Zhuang, G. L.; Wang, J. G. Selective phenol hydrogenation to cyclohexanone over alkali-metal-promoted Pd/TiO2 in aqueous media. Green Chem. 2017, 19, 3585−3594. (50) Guan, Q. Q.; Wang, B.; Chai, X. S.; Liu, J.; Gu, J. J.; Ning, P. Comparison of Pd-UiO-66 and Pd-UiO-66-NH2 catalysts performance for phenol hydrogenation in aqueous medium. Fuel 2017, 205, 130− 141. (51) Zhang, F. W.; Yang, H. Q. Multifunctional mesoporous silicasupported palladium nanoparticles for selective phenol hydrogenation in the aqueous phase. Catal. Sci. Technol. 2015, 5, 572−577. (52) Lin, C. J.; Huang, S. H.; Lai, N. C.; Yang, C. M. Efficient roomtemperature aqueous-phase hydrogenation of phenol to cyclohexanone catalyzed by Pd nanoparticles supported on mesoporous MMT-1 silica with unevenly distributed functionalities. ACS Catal. 2015, 5, 4121−4129.
H
DOI: 10.1021/acs.iecr.7b03043 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
