Article pubs.acs.org/IECR
Preparation of Cellulose Hollow Fiber Membrane from Bamboo Pulp/ 1-Butyl-3-Methylimidazolium Chloride/Dimethylsulfoxide System Bomou Ma, Aiwen Qin, Xiang Li, and Chunju He* State Key Lab for Modification of Chemical Fibers and Polymer Materials, College of Material Science & Engineering, Donghua University, Shanghai 201620, People’s Republic of China ABSTRACT: In this paper, 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) was synthesized and used as solvent to dissolve bamboo pulp. Dimethylsulfoxide (DMSO) was selected as cosolvent to adjust the solution viscosity. As compared with the bamboo pulp/[Bmim]Cl system, the addition of DMSO decreases the solution viscosity dramatically. Afterward, cellulose hollow fiber membranes were prepared by dry-wet spinning process using 75 wt % [Bmim]Cl + 25 wt % DMSO as solvent. Scanning electron microscope (SEM), mechanical testing, pure water permeability, and retention rate were used to characterize its properties, respectively. The SEM result shows that the prepared cellulose hollow fiber membrane presents dense surface structure, which leads to a relative high tensile strength and retention rate to bovine serum albumin, i.e. 28 MPa and 98%, respectively, but a relative low pure water permeability, i.e. 83 L/m2·h. In the end, the [Bmim]Cl is recovered by azeotropy, and the recycling yield reaches 99 wt %.
1. INTRODUCTION Nowadays, the applications of membrane technologies cover almost every industrial areas, i.e. environment, electronics, energy, chemical engineering, and biotechnologies. Cellulose membranes, an important form of cellulose, have got extensive applications in separation, healthcare, and electrochemistry due to its relatively low cost, good biocompatibility, and excellent hydrophilicity.1−4 However, the strong inter- and intrahydrogen bonds between cellulose macromolecular chains make it difficult to be dissolved in general solvents. To date, xanthate viscose technology is still a popular way for cellulose processing, and has been widely used in commercial application. However, the carbon disulfide used in the process is hazardous to human health and environment, which restricts its further development. Therefore, many researchers have been committing to seek new solvent system. In recent years, Nmethylmorpholine-N-oxide (NMMO) attracts many researchers’ attention since it is environmentally friendly,5 and cellulose membranes prepared from NMMO have been reported in many references.1,2,6,7 Besides, ionic liquids (ILs), a kind of salts, are proposed as green solvents to replace the volatile organic compounds in various processing and synthetic industries. ILs hold many particular characteristics, i.e. thermal stability, negligible vapor pressure, and potential for recycling, which makes it take the honor of green solvent in 21st century. Especially, Swatloski et al.8 reported that cellulose can be dissolved directly in 1-butyl3-methylimidazolium chloride ([Bmim]Cl) to form a homogeneous solution up to 25 wt % concentration. Since then, more and more researchers have committed to the field, and some functional membranes were prepared using ionic liquids as solvent. Turner et al.9 prepared the bioactive cellulose films by introducing enzymes into cellulose/[Bmim]Cl solution. Yabuki et al.10 prepared a cellulose-based enzyme membrane as electrode materials for detecting glucose using 1-ethyl-3methylimidazolium acetate ([Emim]Ac) as solvent. Li et al.11 prepared high flux and antifouling cellulose nanofiltration © XXXX American Chemical Society
membrane using 1-allyl-3-methylimidazolium chloride ([Amim]Cl) as solvent. However, all the studies were carried out based on flat membranes. Polymeric hollow fiber membranes, one of the most important conformations, have been widely studied due to its self-mechanical support and large surface area to volume ratio. Date back to 1980, Bakunov et al.12 prepared the hollow cellulose acetate fiber from methylene chloride and studied its hemodialysis properties. Recently, Su et al.13 and Xing et al.14 prepared hollow cellulose acetate fiber membrane as well, where 1-ethyl-3-methylimidazolium thiocyanate and a mixture of acetone and formamide were used as solvent respectively. However, cellulose acetate is a kind of cellulose derivative, not the real cellulose. Moreover, in some conditions, cellulose acetate membrane is not a good candidate due to its poor endure ability to acid, alkali, and organic solvents. Therefore, to develop cellulose hollow fiber membrane is of great significance. In 2004, Jie et al.15 reported the preparation of cellulose hollow fiber membrane from cellulose/N-methylmorpholine-N-oxide system and studied its gas permeation performance. However, the system is instable due to the polonowski type reaction, which leads to cellulose degradation at the same time.16 After that, the patents CN10171646817 and JP201301906518 reported the preparation of cellulose hollow fiber membrane from different ionic liquids systems, respectively. Besides, there are few reports (i.e., those of Wang et al.19 and Liang et al.20) on this topic due to its processing complexity. In this paper, the cellulose hollow fiber membranes were prepared by dry-wet spinning from bamboo pulp/[Bmim]Cl/ dimethylsulfoxide system. First, [Bmim]Cl is synthesized and used to dissolve bamboo pulp, then, DMSO was introduced as Received: April 6, 2013 Revised: June 11, 2013 Accepted: June 12, 2013
A
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at 80 °C. The degassed cellulose solution was extruded through a hollow spinneret with orifice diameter of 1 mm and inner tube diameter of 0.6 mm under 0.4 MPa pressure to prepare cellulose hollow fiber membrane by dry-wet spinning process, and the air gap was 10 cm. Then, the extruded solution was immersed in 20 °C coagulation bath and precipitated into a hollow form. The primary hollow fibers were passed through a 50 °C washing bath to remove the solvent in fibers and collected using winder machine. In order to further remove the residual solvent in fibers, they were soaked in warm water (about 30 °C) for some time, and the water was changed every hour. Water was used as internal and external coagulation during the spinning process. The extrusion rate of spinning drops was about 2.5 mL/min, and the output of fibers was about 5 m/min. This process was stable and continuous until the solution was exhausted. The density and moisture content of prepared cellulose hollow fiber membrane are 1.44 g/cm3 and 8.9 wt % under equilibrium conditions at 25 °C and 65% relative humidity, especially, the density is measured by Ultraform 1000 true density analysis of solid (Quantachrome Instrument, USA). 2.4. Measurement. 2.4.1. Characterization of Rheology. For all the cellulose solutions, their dynamic rheology was performed on Ares-Rfs rotational rheometer (TA, USA) ranging from 70 to 120 °C at a fixed frequency and strain of 1 Hz and 10%, respectively. 2.4.2. Morphology Examination. The prepared cellulose hollow fiber membrane was immersed in a 30 wt % glycerol aqueous solution for 24 h and, subsequently, air-dried for the morphology study. Then, the dried membrane was immersed in liquid nitrogen, fractured, and sputtered with platinum. A field emission scanning electron microscope (FESEM) S-4800 (Hitachi, Tokyo) was used to observe the cross section and surface of the samples. 2.4.3. Mechanical Performance. The tensile strength of hollow fiber membrane at wet condition was performed on XQ1 fiber tensile tester (Shanghai New Fiber Instrument Co. Ltd., China) with an extension rate of 5 mm/min. The prepared hollow fiber membrane was immersed in distilled water at room temperature for 5 min, then gently blotted with tissue paper to remove excess water on the surface, the water content was about 70%. The statistical results came from 10 measurements for each sample. 2.4.4. Pure Water Permeability and Retention Rate. Ten pieces of cellulose hollow fiber membrane were put in a glass tube and sealed with epoxy resin. The effective length of the hollow fibers was 20 cm, the measurement of pure water permeability and retention rate was performed on self-made equipment. For the pure water permeability testing, precompaction on the hollow fibers was done under the pressure of 0.15 MPa for 1 h, then the water permeation was collected for 10 min interval under 0.1 MPa. The pure water permeability (PWP, L/m2·h) was calculated by the following equation:
cosolvent to adjust the solution viscosity. Afterward, the cellulose hollow fiber membranes were prepared by dry-wet spinning. In the end, the recovery of [Bmim]Cl is studied as well. As compared with other references, this paper puts emphasis on the feasibility of cellulose hollow fiber membrane preparation from ionic liquid and the effect of DMSO on this system.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Bamboo pulp with degree of polymerization 500 was provided by Fujian Nanping Paper Co. Ltd. and crushed before use. Industrial-grade N-methylimidazole and 1-chlorobutane were purchased from Shanghai Jiachen Chemical Co. Ltd. and distilled before use. To be specific, Nmethylimidazole is purified through vacuum distillation, and 1chlorobutane is purified through air distillation. Other reagents were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received. 2.2. Synthesis of [Bmim]Cl and Preparation of Spinning Solution. [Bmim]Cl was synthesized according to ref 21. Purified N-methylimidazole and 1-chlorobutane with a molar ratio of 1:1.2 were added to a round-bottomed flask fitted with reflux condenser and magnetic stirring, kept at 80 °C for 24 h, and then ethyl acetate (EA) was added to extract the untreated reactants. After purification, [Bmim]Cl presents white solid state. Its 1H NMR spectra is shown as follows: [Bmim]Cl: 1H NMR (400 MHz, D2O, TMS), δ0.88−0.92 (3H, t), 1.23−1.28 (2H, m), 1.75−1.79 (2H, t), 3.87 (3H, s), 4.17−4.20 (2H, t), 7.75 (1H, d), 7.83 (1H, d), 9.35 (1H, s). A certain amount of bamboo pulp was mixed with [Bmim]Cl and mechanically stirred at 80 °C until a transparent solution was obtained, it needs about 1 h. Then, a certain amount of DMSO was added under mechanical stirring until the solution was homogeneous. The weight ratio of DMSO and [Bmim]Cl was controlled at 1:3, the cellulose concentration was fixed at 9 wt %. In order to compare the effect of DMSO on solution rheology, 5, 7, and 9 wt % cellulose solution was prepared from pure [Bmim]Cl, respectively. 2.3. Fabrication of Cellulose Hollow Fiber Membrane. In order to satisfy the spinning requirements, it is necessary to filtrate the solution with 400 mesh filter and to remove the bubble under vacuum. The apparatus and procedures for the hollow fiber spinning are shown in Figure 1, which is according to refs 22 and 23 with some modifications. In previous reports,24,25 the degradation of cellulose macromolecule chains will be more serious at elevated temperature, especially higher than 100 °C. In view of this, the spinning temperature was fixed
PWP =
Q At
Where Q is the water volume of permeation (L), A is the effective filtration area (m2), and t is the interval time (h). Bovine serum albumin (BSA) aqueous solution with a concentration of 0.5g/L was used for the measurement of retention rate (R), and the operating pressure is 0.1 MPa. The retention rate was calculated by the following equation:
Figure 1. Schematic of the spinning process. B
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Figure 2. Rheological properties of different cellulose solutions.
Figure 3. Morphology of the prepared cellulose hollow fiber membrane.
R=1−
2C2 0.5 + C1
pure [Bmim]Cl. For wet or dry-wet spinning process, a good spinning solution should possesses low viscosity and high polymer concentration since low viscosity contributes to the processability, and high polymer concentration guarantees the high strength of product.27 Here, the spinning solution, prepared from 75 wt % [Bmim]Cl + 25 wt % DMSO, possess high cellulose concentration (i.e., 9 wt %) and low viscosity (i.e., similar to 5 wt % cellulose solution). Therefore, it is a good candidate for spinning. 3.2. Properties of the Prepared Cellulose Hollow Fiber Membrane. Figure 3 displays the surface morphology of cellulose hollow fiber membrane using 75 wt % [Bmim]Cl + 25 wt % DMSO as solvent. It can be seen that the cross section presents a porous structure, while the inner surface and outer surface present a dense structure. This is concerned with the coagulation characteristic of the cellulose/[Bmim]Cl/DMSO system. In the spinning process, when the cellulose solution contacts with the nonsolvent (water), a delayed demixing takes
Where C1 and C2 are the concentrations of retention solution and permeation solution, respectively.
3. RESULTS AND DISCUSSION 3.1. Rheology of the Spinning Solution. Figure 2a shows the rheology property of spinning solution. As compared with the 9 wt % cellulose solution prepared from pure [Bmim]Cl, the addition of DMSO decreases the viscoelasticity of solution dramatically since the DMSO slows down the kinetics of dissolution by decreasing the ionic strength of [Bmim]Cl and the solvation of DMSO molecules.26 From Figure 2b, it can be seen that the storage modulus G′ and loss modulus G″ increase quickly with increasing cellulose concentration. And the G′ and G″ value of spinning solution approaches that of 5 wt % cellulose solution prepared from C
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Figure 4. Mechanical properties and pure water permeability of the cellulose hollow fiber membrane.
can be separated by separating funnel, and the EA can be reused. Through this process, the concentration of [Bmim]Cl reaches up to 95 wt %. And then, the concentrated [Bmim]Cl was distilled under vacuum to obtain purified [Bmim]Cl. It needs further separation and purification in the case of using DMSO as cosolvent, that is because DMSO cannot be removed easily by distillation. But, DMSO can be dissolved in EA, which is incompatible with [Bmim]Cl, therefore, extraction is a feasible way to remove the residual DMSO in [Bmim]Cl. Recycling was repeated three times and the yield for each time reached 99 wt %. The recycled [Bmim]Cl still presents strong dissolving power for cellulose, and its 1H NMR spectra have no difference with the fresh one.
place since the high viscosity of spinning solution retards the diffusion between solvent and nonsolvent. Therefore, the cellulose chains have enough time to move close to each other and form a dense structure for the inner and outer surface. At the same time, the dense surface structure as a barrier further retards the diffusion between inner solvent and outer nonsolvent. This causes a two phases separation in the cellulose solution: one is a cellulose-rich phase and the other is a solventrich phase. In the following solidification process, the celluloserich phase forms the membrane structure and the solvent-rich phase forms the micropores as presented in SEM photos. At the same time, the porous structure contributes to the pure water permeability of cellulose hollow fiber membrane, which will be discussed as follows. As shown in Figure 4a, the prepared cellulose hollow fiber membrane presents excellent mechanical property at wet condition, i.e. 28 MPa, which is far higher than that of cellulose membranes reported by Wang19 and Liang.20 This is due to its dense structure and high cellulose concentration of spinning solution. As shown in Figure 4b, pure water permeability of cellulose hollow fiber membrane is about 83 L/m2·h, which is lower for separation purpose. This is mainly due to the dense structure of inner and outer surface of cellulose hollow fiber membrane. If a certain amount of porogen, i.e. polyethyleneglycol or polyvinylpyrrolidone was added, the property of pure water permeability could be improved. On the other hand, the dense surface structure also makes the cellulose hollow fiber membrane holds relative high retention rate, i.e. 98%, which makes it can be used as a ultrafitration membrane. Besides, degree of polymerization of the prepared hollow fiber membrane was measured by viscosimetry method. The results show that it is decreased 24% from 500 to 380 as compared with the raw bamboo pulp. This is due to the cellulose thermal degradation during dissolving process. 3.3. Recovery of [Bmim]Cl. Distillation under vacuum is a common way to separate [Bmim]Cl from its aqueous solution; however, this method is high in energy consumption, especially when the concentration of [Bmim]Cl is low. Here, a new method is proposed. It is well-known that water is easy to form azeotropy with EA, toluene, N-butyl alcohol, etc., which can be used to remove the water in aqueous [Bmim]Cl. However, the boiling point of toluene and N-butyl alcohol is high, which is unfavorable for further purification. Therefore, EA is adopted. A certain amount of spinning wastewater with 20 wt % [Bmim]Cl was fetched to remove the solid particles through filtering. Then, EA was added and distilled under air pressure until no products were collected. EA and water in the collected mixture
4. CONCLUSIONS In this paper, DMSO was confirmed to be a good cosolvent for cellulose/[Bmim]Cl system, and preparation of cellulose hollow fiber membrane from cellulose/1-butyl-3-methylimidazolium chloride/dimethylsulfoxide system was reported. SEM results show that the prepared cellulose hollow fiber membrane presents a porous cross section, but a dense surface. The porous section contributes to its pure water permeability, i.e. 83 L/m2·h, and the dense surface contributes to its excellent tensile strength at wet condition and high retention rate, i.e. 28 MPa and 98%, respectively, which makes it usable as an ultrafitration membrane. On the other hand, it is feasible to recover the [Bmim]Cl by azeotropy, and the recycling yield reaches 99 wt %. Although the pure water flux of this kind of hollow fiber membrane is relatively low, we firmly believe that cellulose hollow fiber membrane will present fascinating application in future as the said problem is solved.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by grants from the National High-tech Research and Development Projects (863, 2012AA03A605), the National Science Foundation of China (No. 51103019 and No. 21174027), Program for New Century Excellent Talents in University (No. NCET-12-0827), Program of Introducing Talents of Discipline to Universities (No. 111-2-04) and D
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membranes structure and properties. J. Funct. Polym. 2011, 24, 308− 313. (20) Liang, Y.; Song, J.; Cheng, B. W.; Lu, F. Influence of air gap length and bore liquid concentration on cellulose hollow fiber membranes structure and properties. Membrane Sci. Technol. 2011, 31, 83−87. (21) Huddleston, J. G.; Rogers, R. D. Room temperature ionic liquids as novel media for ‘clean’ liquid−liquid extraction. Chem. Commun. 1998, 16, 1765−1766. (22) Deshmukh, S. P.; Li, K. Effect of ethanol composition in water coagulation bath on morphology of PVDF hollow fibre membranes. J. Membr. Sci. 1998, 150, 75−85. (23) Hou, D.; Wang, J.; Qu, D.; Luan, Z.; Ren, X. Fabrication and characterization of hydrophobic PVDF hollow fiber membranes for desalination through direct contact membrane distillation. Sep. Purif. Technol. 2009, 69, 78−86. (24) He, C. J.; Ma, B. M. Degradation of Bamboo Pulp in Imidazole Ionic Liquid. Proceedings of the Fiber Society 2009, Spring Conference; Shanghai, May 27−29, 2009; pp 179−183. (25) Chen, X.; Zhang, Y. M.; Cheng, L. Y.; Wang, H. P. Rheology of Concentrated Cellulose Solutions in 1-Butyl-3-methylimidazolium Chloride. J. Polym. Environ. 2009, 17, 273−279. (26) Cuissinat, C.; Navard, P.; Heinze, T. Swelling and dissolution of cellulose. Part IV: Free floating cotton and wood fibres in ionic liquids. Carbohyd. Polym. 2008, 72, 590−596. (27) Shen, X. Y.Gao Fen Zi Cai Liao Jia Gong Yuan Li (The Principle of Polymer Fabrication), second ed.; China Texile & Apparel Press: Beijing, 2000. (28) Liu, L. Y.; Chen, H. Z. Study on the Rheological Behavior of Cellulose Materials/Ionic Liquid Solutions. J. Cellulose Sci. Technol. 2006, 14, 8−12.
Innovation Funds for PhD Students of Donghua University. Besides, we also need to thank the previous works from Cuissinat,26 Jie,15 and Liu,28 which inspired us to do the present work.
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