Pervaporation membranes were fabricated by blending different amount of zeolite NaA or NaX with three types of poly(amidesulfonamide) (PASA). The zeolite-filled membranes were characterized by IR spectroscopy; SEM, sorption measurements, and wide-angle X-ray diffraction. By adding the proper amount of NaA into the polymer casting solutions, the resultant zeolite-filled membranes exhibited improvement in both selectivity and permeability in the separation of 10% aqueous solutions of ethanol and propan-1-ol, as compared with the zeolite free membrane. (C) 2001 John Wiley & Sons, Inc

Chan, W. H.

He, X. M.

Ng, C. F.

何旭敏

Journal of Applied Polymer Science

Xiamen University Institutional Repository

Water–Alcohol Separation by Pervaporation ThroughZeolite-Modified Poly(Amidesulfonamide)XUMIN HE, WING-HONG CHAN, CHING-FAI NGDepartment of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong KongReceived 9 November 2000; accepted 16 November 2000ABSTRACT: Pervaporation membranes were fabricated by blending different amount ofzeolite NaA or NaX with three types of poly(amidesulfonamide) (PASA). The zeolite-filled membranes were characterized by IR spectroscopy, SEM, sorption measurements,and wide-angle X-ray diffraction. By adding the proper amount of NaA into the polymercasting solutions, the resultant zeolite-filled membranes exhibited improvement in bothselectivity and permeability in the separation of 10% aqueous solutions of ethanol andpropan-1-ol, as compared with the zeolite free membrane. © 2001 John Wiley & Sons, Inc.J Appl Polym Sci 82: 1323–1329, 2001Key words: poly(amidesulfonamide); zeolite; pervaporation; dehydration of alcoholINTRODUCTIONMembrane technology represents one of the mosteffective and energy-saving means for several sep-aration processes, including ultrafiltration (UF) re-verse osmosis (RO), pervaporation (PV), and gasseparation.1 Membrane materials play a pivotalrole in determining the effectiveness of the process.Therefore, the development of new membrane ma-terials with outstanding separation characteristicsis vital to sustain and expand the growth of mem-brane separation technology. In response to thisresearch opportunity, we have synthesized andcharacterized a series of homopolymers and copoly-mers of poly(amidesulfonamide)s (PASA).2,3 Fur-thermore, the potential use of these membrane ma-terials in RO and UF processes was demonstrated.4To further extend the utility of this new class ofmaterials, we have recently reported the pervapo-ration characteristics of some selected PASA in de-hydration of alcohols.5 Reminiscent to the pervapo-ration performance of other glassy polymers, mostof these materials exhibit a fairly high separationfactor albeit a permeation flux below 35 g m22 h21in the pervaporation separation of aqueous alcoholmixtures. For a true prospect of the application inpervaporation, the permeation flux through themembranes requires further improvement. Wehave some success in modifying the separationcharacteristics of the material by introducing or-ganic pending groups to the backbone of the poly-mer through N-alkylation of the sulfonamide moi-eties.4 By contrast, the transport properties of mem-branes for the separation of aqueous alcoholmixtures can be improved by the addition of zeolitesto the polymer matrix.6,7 The present work repre-sents our effort to upgrade the separation charac-teristics of PASA by blending with inert hydrophiliczeolites.EXPERIMENTALMaterialsPASA1-3 (Fig. 1) were synthesized by low-temper-ature solution polycondensation as described else-Contract grant sponsor: Research Committee of theHKBU; Contract grant number: FRG/97-98/I-25.Correspondence to: W-H. Chan (E-mail: whchan@hkbu.edu.hk).Journal of Applied Polymer Science, Vol. 82, 1323–1329 (2001)© 2001 John Wiley & Sons, Inc.1323where.2 N,N-Dimethylacetamide (DMAc) (Fluka)was dried on 4 Å molecular sieves (Merck) beforeuse. Absolute ethanol was purchased from Merck.Zeolite Na12[(AlO2)12(SiO2)12] 3 XH2O (4 Å) (NaA)(Aldrich) and Na86[(AlO2)86(SiO2)106] 3 XH2O (10Å) (Fluka) were used as received.Membrane PreparationPure polymer membranes were prepared as re-ported previously.5 The zeolite-filled membraneswere prepared by adding a calculated amount ofzeolite into the polymer solution (Table I). Themixture was stirred for 2 days to obtain a homo-geneous casting solution. The solutions were caston a glass plate with an applicator, and the sol-vent was allowed to evaporate 70°C for 24 h. Theywere then immersed in deionized water to yieldzeolite-filled membranes.Membrane CharacterizationScanning electron microscopy (SEM) (Jeol T330)was used to investigate the morphology of themembranes. Zeolite-filled membranes were alsocharacterized by infrared (IR), Raman spectros-copy, and wide-angle X-ray diffraction (WAXD).The degree of swelling (DS; %) of the membranedefined byDS 5 Sweight of swollen membraneweight of dry membrane 2 1D3 100%was determined as described previously.5Pervaporation ExperimentsThe pervaporation apparatus used in this inves-tigation was fabricated in-house, as reported inthe previous study,5 with an effective membranearea in the cell of 33.18 cm2. The membrane wassupported by a filter paper over a perforatedmetal disc 6.5 cm in diameter. The cell was loadedwith 250 mL of 90 wt % aqueous ethanol at 20°C.A constant downstream pressure was maintainedat 0.5 mm Hg by a vacuum pump (Edwards E2M-15). The permeate was condensed in a cold trapFigure 1 Structure of PASA.Table I Formulation of Various Zeolite-Filled PASA Casting Solutions and Degree of Swelling(%) of Some Selected Zeolite-Filled MembranesPolymer[h]intrinsic(dL g21)a Type of ZeoliteConcentration of Zeoliteb(wt %)DSc (%)Water EthanolPASA1 0.85 — — 9.18 16.36PASA1 0.85 NaA 2 — —PASA1 0.85 NaA 5 9.51 13.94PASA1 0.85 NaA 8 — —PASA1 0.85 NaX 2 — —PASA2 1.36 — — 9.84 19.31PASA2 1.36 NaA 2 12.17 18.02PASA2 1.36 NaA 4 — —PASA2 1.36 NaA 6 13.22 21.43PASA2 1.36 NaX 2 15.37 23.62PASA2 1.36 NaX 3.5 — —PASA3 0.75 NaA 2 — —a Viscosity in DMAC solution at 25.00 6 0.05°C.b The concentration of PASAs in all casting solutions is 13.5% by wt.c Average value over three determinations, with a RSD below 5%.1324 HE ET AL.cooled by liquid nitrogen, and the permeate rate(J) was obtained by measuring the weight of thepermeate. A Hayesep DB column (1 m) in combi-nation with a thermal conductivity detector(TCD) was used to analyze both the feed andpermeate composition by means of a Shimadzugas chromatography. The separation factor aH2O/EtOH was calculated from the following equation:aH2O/EtOH 5YH2O/YEtOHXH2O/XEtOHwhere Xi and Yi are the weight fractions of speciesin the feed and permeate, respectively.RESULTS AND DISCUSSIONMorphology of the Zeolite-Filled MembranesSEM was employed to study the morphology ofthe zeolite-filled membranes. Typical SEMs of 5%NaA-filled PASA1 membrane are shown in Figure2. Several features of the pictures are worth com-menting on. The surface view of the micrographshows that there are no macroscopic voids withinthe membrane matrix. The low permeation fluxobserved for the zeolite-filled membranes sup-ports the absence of macroscopic voids. Under theprescribed fabricated conditions, the zeolite par-ticles disperse uniformly within the polymer ma-trix. The size of the zeolite was found to be around5 mm. In contrast, the appearance of empty poresobserved in the cross-sectional view of the mem-brane indicates that the adhesion between thepolymer and the zeolite particle is fairly weak.The pore formation as observed is presumably theresult of the peeling of zeolite particles during thesample preparation of the cross section, using theliquid nitrogen cracking technique. Attempts tostrengthen the adhesion between zeolites andpolymer by thermal treatment proved fruitless.IR and Raman spectra of some selected zeolite-filled membranes appeared to be identical beforeand after heat treatment at #150°C.SorptionThe DS of both PASA and zeolite-filled mem-branes in pure water and ethanol was quite sub-stantial, and in all cases exceeded 9% (Table I,column 5 and 6). Reminiscent to the sorptionproperties of PASA membranes, the extent of DSof zeolite-filled PASA membranes was found to begreater in pure ethanol than in pure water, eventhough both NaX and NaA zeolite are known to behydrophilic. By contrast, upon the addition of ze-olite, the DS of the modified PASA membranes inpure water increased (Table I, column 5). Theresults were consistent with the fact that both Atype and X type zeolite are hydrophilic and pos-sessing pores much greater than the kinetic di-ameter of water molecule (i.e., ;2.65 Å).When the sorption process attained an equilib-rium, additional amount of water will fill up someof the pores of the zeolite, resulting in an overallincrease in DS. The sorption behavior of zeolite-filled PASA membranes in pure ethanol wasmuch complicated. NaX with larger pore sizecould accommodate additional amount of ethanol(kinetic diameter ;4.7 Å) (Table I). The sorptionbehavior of zeolite-filled PASA membranes will beused to rationalize their PV performance (see be-low).CrystallinitiesThe presence of crystallites in glassy polymerssuch as PASA would deter the permeability of thepenetrants in the PV process which in turn mayreduce its permeation flux. To monitor the effectof the zeolite additives on the degree of crystal-linity of the polymer system is of great signifi-cance in understanding the separation character-istics of the membranes in the PV process. Thedegree of crystallinity for the parent and zeolite-filled PASA membranes was investigated by X-ray (2u 5 5–40°). The results obtained for thePASA1 and PASA2 series are shown in Figure 3.As shown by XRD study and structural consider-ation of the polymer backbone, PASA2 possessesa higher degree of crystallinity than that ofPASA1. For the loosely packed PASA1, consider-able amount of free volume may exist within thepolymer matrix. Thus, the general shape of theXRD pattern of zeolite-filled PASA1 membranesis very similar. The sharp diffraction maxima ofzeolite is enhanced as its content in the mem-brane is increased. In contrast, Figure 3b showsthe crystallinity of zeolite-filled PASA2 mem-branes decreased significantly compared withthat of the parent membrane; 2% of zeolite addi-tive amounting to about 13% by weight of thefinal membrane content, would be sufficient tocause a sharp reduction of the crystallinity in thepolymer matrix. This, in turn, would have a pro-nounced effect on changing the permeability ofthe membranes (see below).PERVAPORATION THROUGH ZEOLITE-MODIFIED PASA 1325Thickness of the MembranesFor the glassy type symmetric nonporous polymermembranes, a limiting membrane thicknessvalue of ;15 (m was required for pervaporationmembranes to exhibit stable performance.8 Fur-ther increase the thickness of the membranewould enhance the separation factor in the ex-penses of suppressing the permeation flux. Thefirst question warranting our attention is to de-termine the desired thickness of the zeolite-filledFigure 2 Scanning electron microscopy of 5% zeolite (NaA)-filled PASA1 membrane.(a) Surface view; (b) cross-sectional view.1326 HE ET AL.membranes that would exhibit good compromiseof separation characteristics. A 5% NaA-modifiedPASA1 membrane was chosen as a representativemembrane, which was subjected to the thicknessstudy. Results of such study are shown in TableII. Drastic improvement in separation factor ofthe membranes took place when the thickness ofthe membrane was changed from 24 to 33 mm.Further increase in the thickness of the mem-brane will only reduce the flux and with no effecton the separation factor of the membranes. Thus,the thickness of all subsequent zeolite-filled mem-branes would be kept at ;30–35 mm.Pervaporation PropertiesIt has been widely accepted that under reducedpressure conditions the transport of a volatilesubstance through a PV membrane comprises ofthree steps: a sorption step at the membrane up-stream face, followed by a diffusion through thedense polymer film and a desorption into the vac-uum. Under high vacuum, on the downstreamside of the polymer film, the desorption is consid-ered a fast step. Therefore, the overall separationcharacteristics of a membrane depends on a deli-cate balance of three intrinsic properties of thepenetrants of the mixture: affinity to the mem-brane, the size of the penetrating molecules andtheir vapor pressure. Considering the pervapora-tion separation of aqueous alcohol solutions, therelative affinity of alcohol and water molecules tothe membrane could be assessed experimentallyby sorption measurements. To match our antici-pation, by blending the PASA with hydrophiliczeolites (i.e., either NaA or NaX type), the perme-ation flux of all zeolite-filled PASA membraneswas found to be enhanced proportional to theamount of the zeolite added (Table III, column 3).The results of sorption measurements describedabove corroborated to this observation. Thehigher content of zeolite added, the greater theDS of the filled PASA membranes and the fasterthe permeation flux of the membranes. Under-standably, the effect of the flux enhancement wasmore pronounced if NaX was used in place of NaA(Table III, entry 2 vs 5 and entry 7 vs 10). TheTable II Effect of Membrane Thickness of 5%NaA-Modified PASA1 Membranes onPervaporation Separation of 90% wt %Ethanol–H2O MixtureThicknessof theMembrane(mm)PermeationFlux J(g/m2h)SeparationFactor aPervaporationSeparationIndex (PSI)(g/m2h)20 138.54 16.9 2,34124 131.37 19.1 2,50933 32.09 70.8 2,27337 17.80 76.2 1,355Figure 3 X-ray diffraction diagrams of zeolite-filledPASA membranes. (a) PASA1 series; (b) PASA2 series.PERVAPORATION THROUGH ZEOLITE-MODIFIED PASA 1327large pore size (10 Å) present in NaX would ac-commodate more water and facilitate the diffu-sion of the penetrants. At the outset of this inves-tigation, as indicated by the XRD pattern and thestructural consideration, PASA2 and PASA3 areregarded as densely packed glassy polymers,whereas the intermolecular interaction withinPASA1 matrix is considerably weaker. In the de-hydration of 90% ethanol-water mixture by PVthrough PASA2, the separation factor was explic-itly established as4aH2O/EtOH 5 asorptionadiffusion191 5 ~8.01! 3 ~23.87!In other words, using PASA2 as the membranebarrier, the PV separation of ethanol–water mix-tures is diffusion controlled. In a relative sense,we believe that the separation process of PASA1will be controlled with equal importance by sorp-tion and diffusion.When 4 Å NaA zeolite was added to PASA1 andPASA2 at a level of 5% and 2%, respectively, theDS of NaA zeolite-filled PASA in water increased,whereas it decreased in ethanol. With such a lowlevel of additives, presumably, the zeolitic crys-tallites spread in the polymer bulk forming iso-lated zeolite islands and the membrane behavesessentially as that of the unmodified PASA mem-branes. By contrast, the small pore size of NaAwould also favor the preferential permeation ofwater molecules. The complementary effects ofthe zeolite additive on sorption and diffusionwould improve membrane performance in termsof both flux rate and separation factor. As mea-sured by the pervaporation separation index(PSI), substantial improvement of membranes inseparation characteristics (i.e., entry 3, 7, 13 vs 1,6, 12 in Table III) was achieved by the zeolitemodification approach. Clearly, such extent of im-provement will not take place if NaX was usedbecause its large pore size (10 Å) would not dis-criminate the diffusion of both water and ethanolmolecules. It is worth noting that when theamount of NaA zeolite in PASA1 and PASA2 wasfurther increased to 8% and 4% (i.e., 37% and 23%by weight of the modified membrane content),respectively, the separation factor was abruptlydiminished. At this relatively high level of addi-tives, a drastic change in the bulk of polymer isenvisaged. Many pores within the zeolite/modi-fied polymer interface will be inevitably createdsince the cohesion between the zeolite and thepolymer is weak.Use of the zeolite-filled PASA1 and PASA2membranes was extended to the pervaporationseparation of other water alcohol mixtures. Theresults are summarized in Table IV. For the sep-aration of water–methanol mixture, no improve-ment was found when zeolite-filled PASA mem-Table III Characteristics of Membranes Derived from the Parent Polymer and the Zeolite-ModifiedPolymers in Pervaporation of 90% Wt Ethanol–Water Mixture at 20°CEntry PolymerZeolite Contentby Wt in theModifiedMembrane (%)PermeationFlux J(g/m2h)SeparationFactor aPervaporationSeparation Index(PSI) (g/m2h)1 PASA1 0 19.59 51.0 9992 12% NaA (4 Å) 12.9 32.87 42.51 1,3973 15% NaA 28.5 32.09 70.84 2,2734 18% NaA 37.2 96.01 18.21 1,7485 12% NaX (10 A) 12.9 53.41 26.37 1,4086 PASA2 0 4.52 191.20 864.27 12% NaA 12.9 5.91 777.10 4592.78 14% NaA 22.8 8.65 59.28 512.89 16% NaA 30.8 15.02 21.80 327.410 12% NaX 12.9 18.61 30.10 560.211 13.5% NaX 20.6 82.08 4.73 388.212 PASA3 0 3.70 355.80 1316.513 12% NaA 12.9 7.23 753.40 5313.31328 HE ET AL.branes were employed. In contrast, in comparisonwith the parent polymer membrane, both 5%NaA-modified PASA1 membrane and 2% NaA-modified PASA2 again exhibited a much im-proved separation characteristics in the separa-tion of water-propanol mixture. In addition, thepresent finding agreed with the separation char-acteristics observed for other glassy polymericmaterials in PV processes.9 For the homologousseries of methanol, ethanol and 1-propanol, TableIV shows that the permeation flux of each of thezeolite-filled PASA membranes increases as thesize of the penetrating molecules decreases. Mem-brane selectivity is found to depend on the bulk-iness of the alcohol molecules.CONCLUSIONSIn summary, we have demonstrated that modifi-cation of pervaporation membranes by blendingwith suitable amount of hydrophilic zeolite suchas NaA (4A) is an effective means to improve theirseparation characteristics (i.e., selectivity andpermeation flux) in the separation of 10% aque-ous ethanol and propan-1-ol, as compared withthe zeolite free membrane.REFERENCES1. Ho, W. S.; Sirka, K. K. Eds. Membrane Handbook;Van Nostrand-Reinhold: New York, 1992.2. Chan, W. H.; Lam-Leung, S. Y.; Ng, C. F.; Ding, J.;Xi, S. J Polym Sci Polym Chem Ed 1995, 33, 2525.3. Chan, W. H.; Lam-Leung, S. Y.; Ng, C. F.; Ding, J.;Xi, S. Polymer 1995, 36, 4503.4. Chan, W. H.; Ng, C. F.; Lam-Leung, S. Y.; He, X. JMembr Sci 1999, 160, 77.5. Chan, W. H.; Ng, C. F.; Lam-Leung, S. Y.; He, X.;Cheung, O. C. J Appl Polym Sci 1997, 65, 1113.6. Goldman, M.; Fraenkel, D.; Levin, G. J Appl PolymSci 1989, 37, 1791.7. Z.; Gao, Yue, Y.; Lu, W. Zeolites 1996, 16, 70.8. Koops, G. H.; Nolter, J. A. M.; Mulder, M. H. V.;Smolders, C. A. J Appl Polym Sci 1994, 53, 1639.9. Huang, R. Y. M.; Jarvis, N. R. J Appl Polym Sci1970, 14, 2341.Table IV Characteristics of Membranes Derived from the Parent Polymer and the Zeolite-ModifiedPolymers in Pervaporation of Water-Alcohol Mixtures Containing 90% Wt % Alcohol at 20°CPolymerWater–Methanol Water–Ethanol Water–PropanolPermeationFlux J(g/m2h)SeparationFactor aPermeationFlux J(g/m2h)SeparationFactor aPermeationFlux J(g/m2h)SeparationFactor aPASA1 26.45 6.10 16.17 51.50 6.92 `15% NaA 164.3 1.68 32.09 70.84 14.15 `18% NaA 311.2 1.53 96.01 18.21 31.27 1294PASA2 13.06 24.08 4.51 191.2 2.17 `12% NaA 14.40 11.95 5.91 777.1 4.60 `14% NaA 22.20 9.51 8.65 59.28 6.91 277416% NaA 58.60 4.18 15.02 21.79 9.49 100.6PERVAPORATION THROUGH ZEOLITE-MODIFIED PASA 1329

Water-alcohol separation by pervaporation through zeolite-modified poly(amidesulfonamide)

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Water-alcohol separation by pervaporation through zeolite-modified poly(amidesulfonamide)

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