Article pubs.acs.org/IECR
Chitosan/Sulfonated Polyethersulfone−Polyethersulfone (CS/SPESPES) Composite Membranes for Pervaporative Dehydration of Ethanol Hong Wu,*,†,‡ Xianshi Li,† Cuihong Zhao,† Xiaohui Shen,† Zhongyi Jiang,† and Xuefen Wang§ †
Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P R China ‡ Tianjin Key Laboratory of Membrane Science & Desalination Technology, Tianjin 300072, P R China § State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Dong Hua University, Shanghai 201620, P R China ABSTRACT: CS/SPES-PES composite membrane was prepared by coating the chitosan (CS) active layer onto the sulfonated polyethersulfone (SPES)-polyethersulfone (PES) support layer. The SPES-PES support layer was prepared by blending with hydrophilic SPES into PES matrix in a wet phase inversion process. X-ray photoelectron spectroscopy (XPS) characterization and water static contact angle measurements confirmed the surface segregation of SPES on the support layer, which remarkably enhanced the adhesion between the active layer and the support layer. The effect of sulfonation degree and concentration of SPES on the structure and morphology of the resultant SPES-PES support layer was observed by field emission scanning electron microscope (FESEM). T-peel test results showed that the interaction strength at the interface of CS/SPES-PES composite membrane was nearly 5 times higher than that of CS/PES composite membrane. The effects of SPES presence in the support layer, annealing temperature, operating temperature, and feed concentration on the pervaporation performance for ethanol dehydration were investigated. A permeation flux of 1394 g/(m2 h) with a separation factor of 376 was obtained when using the CS/SPES(40-3)-PES composite membrane for pervaporation dehydration of a 90 wt.% ethanol aqueous solution at 353 K. The long-term stability study demonstrated that the structural stability and the performance stability of the as-prepared membrane were notably improved by hydrophilic modification of the support layer surface. Cross-linking of the active layer,5,6 interfacial polymerization,7 grafting on substrate surface,5 and introduction of an additional intermediate layer8−10 are commonly adopted to improve the interfacial compatibility. Liu et al. investigated the pervaporation performance of isopropyl alcohol−water mixture using composite membranes consisting of γ(glycidyloxypropyl)trimethoxysilane (GPTMS) cross-linked chitosan (CS) as the active layer and poly(styrene sulfuric acid) grafted poly(tetrafluoroethylene) as the support.5 The adhesion between the skin layer and the support was pretty good and survived after a long-term operation test in 45 days. Chitosan, a “green” polymer for its biodegradability and biocompatibility, is mainly obtained from the deacetylation of chitin extracted from shells of crustacean and mollusks.11 CSbased membranes are widely used in membrane separation of aqueous solution for its good performance and low cost.12 Polyethersulfone (PES) is used as a good support material for its chemical and mechanical stability. However, when these two materials are used to prepare composite membrane, the large difference between the solubility parameters of PES (24.2 (J/ mL)1/2) and CS (41.0 (J/mL)1/2) would cause trouble from the viewpoint of long-term operational stability.
1. INTRODUCTION Pervaporation has been considered to be a promising alternative to conventional energy intensive technologies such as extractive distillation and azeotropic distillation for the separation of azeotropes, close-boiling mixtures, thermally sensitive compounds, and species present in low concentrations.1,2 Pervaporation has found promising applications in three major areas: dehydration of aqueous−organic mixtures, removal of trace volatile organic compounds from aqueous solution,3 and separation of organic−organic solvent mixtures.4 Composite pervaporation membranes with a two-layer structure, a thin active layer and a support layer, are much more desirable than homogeneous membranes for practical applications due to their lower permeation resistance which always leads to a higher flux without sacrificing the selectivity. In addition, composite membranes allow selection of various support layer materials that are different from those used for active layers. However, the adhesion between the two layers is frequently the key issue that needs to be solved prior to application since the solubility parameter of the active layer and the support is usually quite different from each other. The composite structure seems to be easily disintegrated if the active layer and the underneath support do not swell in a coordinated manner.1 Therefore, the interfacial compatibility of the composite membrane is crucial to its structural stability and long-term operation performance. © 2013 American Chemical Society
Received: Revised: Accepted: Published: 5772
December 13, 2012 February 12, 2013 April 2, 2013 April 2, 2013 dx.doi.org/10.1021/ie303437r | Ind. Eng. Chem. Res. 2013, 52, 5772−5780
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Article
where Y was the weight fraction of SPES in SPES-PES support (wt.%) and Z was the sulfonation time of SPES (h). Unless otherwise specified, Z was 3 h in all the subsequent experiments. 2.3. Characterizations. 2.3.1. Characterizations of the Composite Membranes. The morphology of the composite membranes was observed using a field emission scanning electron microscope (Nanosem 430, FEI, USA). The water contact angles of the SPES-PES support layers were measured by a contact angle meter (JC2000C, Shanghai Powereach Digital Technology Co., China). Each membrane sample was cut into a ∼7 cm × 1 cm strip, and the contact angles at more than seven positions along the strip were measured and averaged with an error of about 1 degree. The 1H NMR spectra were recorded on an INOVA 500 MHz spectrometer (Varian Medical Systems Inc. Palo Alto, US). The chemical composition of the SPES-PES asymmetric support layer surface was analyzed by XPS (Mg Kα 1254.0 eV, PHI-1600, USA). Attenuated total reflectance (ATR) infrared spectra of the support layer surface and the active layer surface were acquired using a Nicolet-560 spectrophotometer. 2.3.2. T-Peel Adhesion Strength Test. The interfacial adhesion strength of CS/PES and CS/SPES-PES composite membranes was measured by T-peel adhesion strength tests using a Micro-Uniaxial Fatigue Testing system (MUT-1020, CMC). The membrane sample was cut into 8 mm (width) × 30 mm (length) strips. The measurement was carried out at a loading rate of 0.5 mm/s at room temperature, and the displacement-dependent load curve was simultaneously recorded based on ASTM test standard. 2.4. Pervaporation Experiments. All the pervaporation experiments were conducted in a laboratory pervaporation apparatus as previously described in ref 15. The effective membrane area in contact with the feed was about 25.6 cm2. The downstream pressure was maintained at 0.3 kPa. The feed flow rate was kept at 60 L/h. The permeate vapor was condensed in a liquid nitrogen cold trap and weighed. The permeation flux (J, g/(m2 h)) was calculated by eq 1
This work presented a new approach to support layer (PES) modification by segregation of hydrophilic polymer molecules (SPES) to the support layer surface during wet phase inversion process. The resultant SPES-enriched surface was expected to lead to an improved interfacial compatibility with the CS active layer. To demonstrate the effect of the SPES on the composite membrane performance, the pervaporation dehydration of aqueous ethanol solution was investigated over a wide range of operating temperature and feed concentration.
2. EXPERIMENTAL SECTION 2.1. Materials and Chemicals. Chitosan (CS) (90.2% Ndeacetylation degree with a viscosity-average molecular weight of 450 000) was purchased from Jinan Haidebei Marine Bioengineering Co. Ltd. (Jinan, China). Polyethersulfone (PES Ultrasonic 6020P with Mw = 58,000 g/mol) was provided by BASF Co. (Germany) and dried at 60 °C for 24 h before use. Dimethylformamide (DMF), acetic acid glacial, glutaraldehyde (GA) (50 wt.%), hydrochloric acid (HCl), chlorosulfonic acid, chloroform, and sulfuric acid (H2SO4) were obtained from Tianjin Kewei Ltd. (Tianjin, China). Absolute alcohol was supplied by Tianjin Guangfu Fine Chemical Research Institute (Tianjin, China). All chemicals were of analytical grade and used without any further purification. Deionized water was used throughout the experiments. Sulfonated polyethersulfone (SPES) was synthesized using chlorosulfonic acid as sulfonating agent and chloroform as solvent via aromatic nucleophilic substitution polycondensation.13 In a typical procedure, 10 g of PES was added into a flask and dissolved in 100 mL of chloroform under stirring at room temperature. Then, the flask was chilled in an ice−water bath. Six g of chlorosulfonic acid in 50 mL of chloroform was added dropwise (1 drop per second) into the above solution. The mixture was under stirring for a certain period of time. The product precipitate was then collected by repeated filtration and rinsing with deionized water. Finally, the product was dried at 60 °C for 24 h under vacuum before use. 2.2. Membrane Preparation. 2.2.1. Preparation of SPESPES Asymmetric Support Layer. SPES-PES asymmetric support layers were prepared by phase inversion. First, SPES and PES with various weight ratios were dissolved in DMF at 60 °C for 3 h to get a 15 wt.% polymer solution. The solution was filtered to remove any undissolved residues. Then, the solution was cast on a glass substrate and immersed into a coagulation bath of deionized water for 12 h. Finally, the membrane was peeled off, rinsed with water, and dried at room temperature for 24 h before use. 2.2.2. Fabrication of CS/SPES-PES Composite Membrane. CS/SPES-PES composite membranes were fabricated via spin coating. CS was dissolved in 2.5 wt.% acetic acid glacial under stirring at 60 °C for 2 h to get a 2.5 wt.% CS solution. The solution was filtered to remove undissolved residue and then spun onto the SPES-PES support layer at a suitable spin velocity. The as-prepared composite membranes were dried at room temperature for 24 h and then immersed into 0.8 M sodium hydroxide in a 50 v/v% aqueous ethanol solution for 24 h in order to convert cationic amine groups (NH3+) into free amine groups (NH2).14 The membranes were immersed into 0.05 M sulfuric acid in 50 v/v% aqueous acetone solution for 6 h to allow cross-linking. Finally, the membranes were washed repeatedly with water and dried at a certain temperature in the range of 25−140 °C for 2 h. The prepared composite membrane samples were designated as CS/SPES(Y-Z)-PES,
J=
Q At
(1)
where Q (g) was the weight of the permeate collected during the experimental time t (h), and A (m2) was the effective membrane area. The collected permeate was analyzed by HP4890 gas chromatography (GC) equipped with a thermal conductivity detector (TCD) and a column packed with GDX103 (Tianjin Chemical Reagent Co., China). The separation factor was calculated as follows
α=
yw /ye x w /xe
(2)
where yw and ye were the weight fractions of water and ethanol on the permeate side, respectively, and xw and xe were the weight fractions of water and ethanol on the feed side. Unless otherwise specified, all the experiments was performed using 90 wt.% ethanol aqueous solution at 353 K with a flow rate at 60 L/h.
3. RESULTS AND DISCUSSION 3.1. Membrane Characterizations. 3.1.1. 1H NMR. The 1 H NMR spectra of PES and SPES with different sulfonation 5773
dx.doi.org/10.1021/ie303437r | Ind. Eng. Chem. Res. 2013, 52, 5772−5780
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3.1.2. Membrane Morphology. The surface and crosssection morphology of the membranes were observed by FESEM. Figure 2(A) showed the surface morphology of the SPES-PES support layer with various SPES contents. The surface roughness increased slightly with the SPES content and became severe when the SPES content reached as high as 70 wt.%. The addition of the hydrophilic SPES enhanced the affinity of the SPES-PES-DMF casting solution to the water coagulation bath compared to pure PES-DMF solution, leading to a lower mutual diffusion rate between the solvent DMF and nonsolvent water. This prolonged mutual diffusion allowed longer time for phase inversion and the segregation of SPES from the bulk casting solution up to the solution-water interface.17 The more SPES added, the more SPES segregated to the surface, resulting in a rougher surface. Figure 2(B) illustrated the typical cross-section morphology of the composite membranes. The composite membrane consisted of a dense and thin active CS layer with a thickness of only ∼260 nm and an SPES-PES support with a thickness of ∼150 μm. The support had a porous finger-like asymmetric structure induced by wet phase inversion. The thin active layer and the porous support layer would help to increase the permeation flux. No defects between the active layer and the support layer were observed from the amplified image. 3.1.3. Static Contact Angles. Water contact angles were measured to evaluate the effect of the SPES content on the hydrophilicity of the support layer surface. Figure 3 showed that the addition of SPES in the casting solution led to a slight reduction in contact angle due to the synergy effect of the
times were shown in Figure 1. The intensity of the peak at 8.3 ppm was ascribed to the proton on −SO3H and increased with
Figure 1. 1H NMR spectra of PES and SPES with various sulfonation time (1.5 to 6 h).
sulfonation time. The sulfonation degree of SPES was determined from the spectrum according to eq 3 DS =
4I(b″) I(b , b′) + I(b″)
(3)
where I(b″) and I(b,b′) were the integrated areas of the peak for H-b″ and H-b,b′, respectively.16 The sulfonation degrees of SPES corresponding to the sulfonation time of 1.5 h, 3 h, 4.5 h, and 6 h were 3.2%, 6.9%, 12.3%, and 14.5%, respectively.
Figure 2. (A) Surface morphology of SPES(Y-3)-PES support layer with SPES content of 10 wt.% (a), 30 wt.% (b), 50 wt.% (c), and 70 wt.% (d). (B) Cross-section morphology of the CS/SPES(40)-PES composite membrane. 5774
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Figure 3. Static contact angles of SPES(Y)-PES support layer with various SPES contents.
enrichment of the hydrophilic SPES on the surface and the resultant higher surface roughness. This enhanced hydrophilicity was favorable to improve the interfacial compatibility and stability upon further casting of a hydrophilic active layer like chitosan on it. 3.1.4. FT-IR. Comparing the FT-IR spectra of the membranes with and without chitosan layer (Figure 4), it could be seen that
Figure 4. FT-IR spectra of SPES (40)-PES support layer and CS/ SPES (40)-PES membranes.
all the characteristic bands originated from SPES and PES (1025 cm−1, 1180 cm−1, 1470 cm−1, and 1584 cm−1)18 disappeared after casting of CS layer, while the characteristic bands from chitosan (1650 cm−1, 1550 cm−1, and 1090 cm−1) appeared, confirming a full coverage of CS layer on the SPESPES support. 3.1.5. XPS. XPS analysis was employed to quantitatively determine the chemical composition of the SPES-PES support layer surface, and the result was shown in Figure 5. The peaks at 168 eV, 300 eV, and 550 eV in Figure 5(a) were attributed to S2p, C1s, and O1s, respectively. The peak intensity of S2p (168 eV) increased with the increase of SPES content as indicated in Figure 5(b). The S2p spectrum was further deconvoluted as shown in Figure 5(c). Two peaks were identified, the one at 167.6 eV was assigned both to the sulfur in the sulfonyl group
Figure 5. High-resolution XPS spectra: SPES(40)-PES and PES (a), S2p of SPES(Y)-PES (b), and deconvoluted S2p of SPES(70)-PES (c).
(OSO), while the other one at 168.6 eV was attributed to the sulfur in the sulfonic acid group (−SO3H). The nearsurface coverage of SPES (φ) was thereby calculated 5775
dx.doi.org/10.1021/ie303437r | Ind. Eng. Chem. Res. 2013, 52, 5772−5780
Industrial & Engineering Chemistry Research φ=
S2 × 100% S1
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was increased by about 4.5-fold compared with the CS/PES membrane (
