Pervaporation dehydration of ethanol-water mixture using crosslinked poly ( vinyl alcohol ) membranes

Crosslinked poly(vinyl alcohol) (PVA) composite membranes were synthesized by casting selective crosslinked PVA films on the polyacrylonitrile (PAN) porous substrates. The PVA films were prepared by in-situ crosslinking technique using four different crosslinking agents, such as glutaraldehyde, fumaric acid, maleic acid and malic acid. The separation performance in terms of permeation flux and separation factor of prepared membranes were evaluated for pervaporation dehydration of ethanol/water mixture of 80/20 wt% at 60 C. The prepared membranes were also characterized by FTIR, SEM, swelling and sessile drop contact angle measurements. It was found that the chemical structure of the PVA membrane was changed via crosslinking reaction. The physicochemical properties (hydrophilicity and swelling degree) and separation performance of the prepared membranes were affected by the chemical structures of the crosslinking agents. Furthermore, there was a trade-off between permeation flux and selectivity of the resulting membranes. When the flux increased, the separation factor decreased. The results of this study contributed to enrich the data of the crosslinking reaction of PVA membranes, and expected to help researcher in suitable choosing crosslinking agent for producing pervaporation PVA membrane for dehydration of ethanol solutions.

Pervaporation dehydration of ethanol-water mixture using crosslinked poly(vinyl alcohol) membranes 6], but the PVA crosslinked with dicarboxylic acids need higher temperature (>100 o C) [2,3,7,8].However, to our knowledge, the effect of different chemical structures of crosslinking agent on the physicochemical properties and separation performance of modified membrane was not concerned sufficiently.In this study, we developed the composite membranes by casting a crosslinked PVA layer on a PAN porous substrate.The PAN porous could suppress the swelling of the PVA selective film at the interface and thus retain the dense skin.
Accordingly, the high permeation flux and good selectivity as well as the durability of the pervaporation membranes could be achieved [1][2][3]8].In-situ crosslinking technique was employed to prepared crosslinked PVA selective layer using four different crosslinking agents such as glutaraldehyde (GA), fumaric acid, maleic acid and malic acid in the presence of HCl as catalyst.The fumaric and maleic acids had the same number of carbon atoms and also the carbon double bond.The difference of the two acids was that the fumaric acid had the trans configuration while the maleic acid owned the cis configuration.The trans structure of fumaric acid was expected to much more pack the polymer chain and give better selectivity for the modified membrane.The malic acid had an additional hydroxyl group which was hypothesized to provide more hydrophilic property for the crosslinked membrane, whereas GA with the reduced oxygen content was looked forward to produce the tightest crosslinked membrane.The effects of crosslinker's chemical structure on the physicochemical properties (i.e., chemical structure, hydrophilicity and swelling degree) of the crosslinked membranes were characterized using FTIR, SEM, swelling and sessile drop contact angle measurements.The separation performance of the prepared membrane in terms of flux and separation factor was evaluated for pervaporation dehydration of the ethanol/water mixture of 80/20 (wt%) at 60 o C.

Membrane preparation
PVA (Mw 146-186 kDa, 99%, Sigma) solutions with the concentration of 10 wt% were prepared by dissolving PVA in deionized (DI) water at 90 o C under agitation for 6 h.Next, PVA solution was cooled to room temperature and the crosslinkers with the concentration of 5 wt% per weight of PVA was added under continuous stirring for 24 h.Four crosslinkers used in this study were glutaraldehyde (GA, 25% in water), fumaric acid, maleic acid and malic acid (>98%, Sigma, USA).The 2M HCl solution as catalyst was subsequently added under vigorous stirring for 3h to produce PVA casting solution.Then the resulting solution was degassed before casting.The composite membranes were prepared by casting PVA aqueous solution on the PAN support membranes (PAN200, Ultura, USA) by using a casting knife with a constant gap of 70 µm.The resulting membranes were dried at room temperature for 48h.After that, the prepared membranes were crosslinked at elevated temperature in 2 h at 120 o C for the three dicarboxylic acids [2,3,7], while the ambient temperature (30 o C) was fixed for GA [2][3][4][5][6].Finally, the derived membranes were immersed in 80wt% ethanol solution for 3 h and dried at room temperature for 24 h before pervaporation tests.

Membrane characterization
The surface and thickness of the PVA selective layers was determined from SEM images by using Scanning Electron Microscopy (S4800, FE-SEM) and digital micrometer (accuracy ± 1 µm, Series 293, Mitutoyo).The chemical structure of the membranes was characterized using ATR-FTIR spectroscopy.
The hydrophilicity of the membrane surfaces was investigated using sessile drop water contact angle measurement.

Swelling experiments
The pieces of dried membranes with the dimension of 3×3 cm were immersed in both water and ethanol/water solution of 80/20 wt% at 30 o C for 48h to reach an equilibrium swelling.The swollen membranes were wiped carefully using tissue paper for removing residual solution on the membrane surfaces.
Then, the swollen membranes were weighted by a mass balance (accuracy ± 0.0001 g).The degree of swelling was defined as (1) Wherein, WS (g) and WD (g) were the mass of the swollen membrane and the mass of the dried membrane, respectively.The data of swelling degree were collected from four replicate experiments.

Pervaporation performance
The separation performance of prepared membranes was investigated for ethanol/water mixture of 80/20 wt%, using a lab scale pervaporation unit.The schematic diagram of the pervaporation setup was illustrated in Figure 1.A module membrane designed with the membrane area of 19 cm 2 with a channel height of 2 mm.The feed solution was circulated from the feed tank through the membrane cell with a flow rate of 90 L/h by a pump.The temperature of the feed solution was maintained at 60 ± 1 o C by a laboratory recirculating heater.The pressure of the feed side was at atmosphere pressure and the pressure on the permeate side was maintained lower than 1 mbar by using a vacuum pump (Robinair, USA).The separation process was conducted for 2 h, and the permeate vapor was condensed in a cold trap by a laboratory chiller and heat exchanger using pure ethanol liquid (-15 o C ÷ -20 o C).The collected permeate was weighed using a mass balance (accuracy ± 0.0001 g) for determining the permeation flux.The permeation flux (J) was determined using the following equation Where, Q (g), A (m 2 ) and t (h) represented the weight of permeate, effective membrane area and the operation time, respectively.Figure 2.For the non-modified membrane, the broad peak at 3000-3600 cm -1 attributed to the hydroxyl group and the peak at 1578 cm -1 was characteristic of acetate group of PVA.For the crosslinked membranes, the intensity of the broad peak at 3000-3600 cm -1 was reduced and the peak at 1578 cm -1 was disappeared.In the spectra of the membrane crosslinked by three dicarboxylic acids, a new small peak at 1725 cm -1 attributed to the ester group was observed as compared to the non-crosslinked membrane [7].Whereas, the PVA membrane crosslinked with GA exhibited the stretch of 1050 cm -1 and 1140 cm -1 for acetal and ether linkages [4][5][6].
The changes in the FTIR spectra implied that the chemical structure of the PVA membrane was altered by the crosslinking linkages compared to the plain PVA membrane.Figure 4. presented the SEM images of the surface and cross-section structure of uncrosslinked and crosslinked PVA membranes.There was no observed difference of surface and cross-section morphology between the virgin and the modified membranes.The derived membranes had a composite structure, involving a dense and uniform PVA film on the top of the PAN porous layer, and the non-woven fabric substrate at the bottom of the membrane.It was observed that macro voids or defects were not found on and in the selective PVA film.Moreover, the interface between the PVA film and PAN support was difficult to discriminate for the crosslinked membrane, implying the good bond formed between the PAN support layer and PVA selective film.The thickness of the derived membranes was approximately 10 m via maintaining the gap of the casting knife at 70 m.

Separation performance of the crosslinked PVA membranes
Figure 5 presented the permeation flux and separation factor of prepared membranes, which was evaluated for the pervaporation dehydration of 80 wt% ethanol solution at 60 o C. The permeation flux of the derived membrane was observed to decrease in the order of plain PVA > GA-PVA > Malic acid-PVA > maleic acid-PVA > fumaric acid-PVA.Meanwhile, the separation factor showed the opposite trend.The plain PVA membrane was easily swelled in aqueous solution during pervaporation test due to the high affinity of the hydroxyl group with water and hence, the permeation flux was high but the selectivity was insignificant.The results showed that the permeation flux of the crosslinked membranes was lower than that of the plain PVA membrane.The crosslinking in the PVA layer resulted in the rigid structure.
The rigid structure of the polymer chains had less mobility and hence, the solubility and diffusive ability of the water and ethanol molecules were hindered.Accordingly, the crosslinked membranes obtained a lower permeation flux but higher selectivity.For PVA membrane crosslinked by dicarboxylic acids, the permeation flux and separation factor varied with the chemical structure of the crosslinker.
The membrane crosslinked by malic acid showed the highest flux but the lowest separation factor in comparison with that by fumaric and maleic acid due to the higher steric hindrance of hydroxyl functionality of malic acid.Despite having the same carbon double bond, fumaric acid owned trans isomerism with smaller steric hindrance than cis isomerism structure of maleic acid.Therefore, the fumaric acid crosslinked membrane exhibited the lowest flux but the highest separation factor.For crosslinking PVA membrane with GA at ambient temperature, the separation factor was observed to significantly enhance while the flux was declined partially as compared to the plain PVA membrane.From Figure 6 and Figure 7, it was found that the permeation flux and separation factor were closely related to the swelling degree of the derived membrane.The higher swelling degree gave the lower separation factor but the higher flux.Both flux and selectivity were the essential parameters for the pervaporation process.It was observed that there was a tradeoff between the permeation flux and selectivity of the membrane.When the flux increased, the separation factor decreased.Therefore, the term of pervaporation separation index (PSI) was employed for measuring the separation capability of the pervaporation membrane.The PSI of the prepared membranes were showed in Figure 8.Among the prepared membranes, GA crosslinked membrane exhibited the highest PSI.In comparison, GA was a good crosslinker for modifying membrane due to good flux and good selectivity, while the crosslinking condition was not required high temperature.

CONCLUSIONS
The PVA membrane crosslinked with various crosslinking agents was successfully prepared by in-situ crosslinking technique.The results showed that crosslinking PVA membrane improved the separation performance of the resulting membrane.The chemical structure of the crosslinkers affected the physicochemical properties and separation performance of the crosslinked membrane.Particularly, the swelling degree exhibited a significant relationship with the separation performance of the prepared membrane.Moreover, elevating the crosslinking temperature was observed to improve the selectivity, but decrease the permeation flux of the resulting membrane.Crosslinking PVA membrane with dicarboxylic acids at high temperature was found to achieve higher separation factor but lower flux as compared to that with GA.The GA exhibited as a proper crosslinker for modifying PVA membrane due to good flux and good selectivity, while the crosslinking condition was not required high temperature.

Acknowledgment
City University of Technology, VNU-HCM(Manuscript Received on October 19th, 2016, Manuscript Revised November 28th, 2016)    ABSTRACT Crosslinked poly(vinyl alcohol)  (PVA) composite membranes were synthesized by casting selective crosslinked PVA films on the polyacrylonitrile (PAN) porous substrates.The PVA films were prepared by in-situ crosslinking technique using four different crosslinking agents, such as glutaraldehyde, fumaric acid, maleic acid and malic acid.The separation performance in terms of permeation flux and separation factor of prepared membranes were evaluated for pervaporation dehydration of ethanol/water mixture of 80/20 wt% at 60 o C. The prepared membranes were also characterized by FTIR, SEM, swelling and sessile drop contact angle measurements.It was found that the chemical structure of the PVA membrane was changed via crosslinking reaction.The physicochemical properties (hydrophilicity and swelling degree) and separation performance of the prepared membranes were affected by the chemical structures of the crosslinking agents.Furthermore, there was a trade-off between permeation flux and selectivity of the resulting membranes.When the flux increased, the separation factor decreased.The results of this study contributed to enrich the data of the crosslinking reaction of PVA membranes, and expected to help researcher in suitable choosing /water separation is PERVAP membrane, which is formed by Sulzer (Germany).It is a composite membrane with a PVA active layer coated on a polyacrylonitrile (PAN) porous support membrane.The main advantage of the composite membrane is the higher permeation flux, stability and good selectivity as compared to traditional one-layer membrane [2,3].So far, poly(vinyl alcohol) (PVA) has been attracting for development of pervaporation membranes for ethanol dehydration [1-8].It is due to the inherent hydrophilicity, excellent thermal, mechanical, chemical stability and good film-forming ability of PVA.However, because of the high hydrophilicity, the PVA films is not stable in aqueous solutions, leading to low separation performance of the derived membranes.Therefore, PVA membranes should be modified to offer long-term durability in a pervaporation process.Some techniques usually used for modifying PVA membrane are heating, grafting, crosslinking, irradiating and blending [2].Among them, crosslinking is more efficient because the physicochemical properties and separation performance of the prepared membranes are easily controlled by varying the crosslinking agents, crosslinker concentration and reaction conditions.PVA membrane could crosslink by dicarboxylic acids, dialdehydes, and alkoxysilanes [2,3].Many crosslinkers such as fumaric acid, maleic acid and glutaraldehyde are investigated for crosslinking PVA membrane.Previous studies showed that glutaraldehyde reacted with PVA at ambient temperature to produce crosslinking linkages [2-

Figure 1 .
Figure 1.The schematic diagram of the lab scale pervaporation unit The composition of permeated product was specified by measuring the refractive indices with a digital differential refractometer (Reichert, AMETEK GmbH, Germany) with the aid of a calibration curve for ethanol/water mixture prepared using known quantities of the two components.The refractometry

Figure 3
Figure 3 showed the contact angle and swelling degree of the prepared membranes.The contact angle, indicating the hydrophilicity of the crosslinked membrane surfaces was between 50 o and 57 o .It indicated that the hydrophilicity of crosslinked membranes was lower than that of non-crosslinked membrane.

Figure 2 .
Figure 2. FTIR spectra of non-crosslinked and crosslinked PVA membranes

Table 1 .
The characteristics of original PAN200 support substrate 1 Including the non-woven support of thickness approximately 100 µm 2 Based on above 70% rejection of PEG

Table 2 .
Chemical formula and structure of investigated crosslinking agents