pervaporation of benzene/cyclohexane mixtures through aromatic polyamide membranes

Journal of Membrane Science 185 (2001) 193–200

Pervaporation of benzene/cyclohexane mixturesthrough aromatic polyamide membranes

Yi-Chieh Wanga, Chi-Lan Lia, James Huangb, Chi Linc,Kueir-Rarn Leea,∗, Der-Jang Liawd, Juin-Yih Laic

a Department of Chemical Engineering, Nanya Institute of Technology, Chung Li 32034, Taiwan, ROCb Department of Chemistry, Chung Yuan University, Chung Li 32023, Taiwan, ROC

c Department of Chemical Engineering, Membrane Research Laboratory, Chung Yuan University, Chung Li 32023, Taiwan, ROCd Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10772, Taiwan, ROC

Received 1 June 2000; accepted 26 October 2000

Abstract

A series of novel aromatic polyamides based on the 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane (BAPPH) and2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPPP) with various aromatic diacids were investigated for pervaporation sepa-ration of benzene/cyclohexane binary system. The aromatic polyamide membranes exhibited a benzene-permselectivity for allthe feed compositions and the permeation rate increased with increasing benzene in the feed solution. The fluorine-containingpolyamide membranes were swollen in benzene but only slightly in cyclohexane. A separation factor toward benzene, apermeation rate, and a pervaporation separation index (PSI) value through the fluorine-containing polyamide membrane (F-3)for a 50 wt.% feed benzene concentration were 4.0, 1470 and 5903 g m−2 h−1, respectively. The permeation rate and the PSIvalue of fluorine-containing polyamide membranes based on the BAPPH were higher than that of the aromatic polyamidemembranes based on the BAPPP. © 2001 Elsevier Science B.V. All rights reserved.

Keywords:Fluorine-containing aromatic polyamides; Benzene/cyclohexane; Pervaporation; Membrane

1. Introduction

The membrane separation technique has beenfound applicable to liquid mixtures which are dif-ficult to separate or purify by solvent extraction,distillation and other traditional separation pro-cesses. Pervaporation is a membrane based unitoperation in which separation of the desired com-ponent takes place through a dense membrane bya solution-diffusion mechanism. It has been widely

∗ Corresponding author.E-mail address:[email protected] (K.-R. Lee).

considered as an alternative separation process forazeotropic mixtures, close-boiling point mixtures andisomers [1–3]. Separation of aromatic hydrocarbonfrom aliphatic is a most important target in chemicalindustry. Specifically, separation of the mixture ofbenzene/cyclohexane is difficult by a conventionaldistillation process because these components fromclose boiling points mixtures at the entire range ofcompositions. Further, these components also forman azeotrope. It is know that the selectivity of liquidmixtures through a membrane is a function of thedifference in solubility of the solvents toward a mem-brane and diffusivity of a solvent in a membrane.From this, the development of novel membranes

0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S0376-7388(00)00655-4

194 Y.-C. Wang et al. / Journal of Membrane Science 185 (2001) 193–200

for benzene/cyclohexane mixtures has been givenmuch attention [4–10]. The first attempts to separatebenzene/cyclohexane mixtures by polyethylene mem-brane were performed by Huang and Lin [11]. A suc-cessful attempt was performed by McCandless et al.who obtained a selectivity of 20 using poly(vinylidinefluoride) membrane [12]. Cabasso et al. tried out alloymembranes of polyphosphonates and acetyl cellulosefor benzene/cyclohexane separation [13]. Recently,methacrylate series of polymers has become popu-lar because of its favorable characteristics towardsbenzene/cyclohexane separation [14–17]. In addition,Uragami et al. have reported the separation of thebenzene/cyclohexane mixtures by benzoylchitosanmembranes [18]. Yamasaki and Mizoguchi discusseda comparison of permselectivity of homogeneousand asymmetric poly(vinyl alcohol) membranes forthe benzene/cyclohexane mixtures in pervaporation[19]. Hao and Okamoto used a sulfonyl-containingpolyimide membrane for benzene/cyclohexane sep-aration [20]. Yamaguchi et al. filled the pores of aHDPE film with poly(methyl acrylate) by plasmagrafting and used it for separation of benzene from itsmixture with cyclohexane [21]. Aromatic polyamideis a potential thermoplastics material for perva-poration because of its attractive combination ofchemical, physical, and mechanical properties. Thepurpose of this article is to study the effects of poly-mer structure on the pervaporation performances ofbenzene/cyclohexane mixtures through the aromaticpolyamide membranes. In addition, the effects offeed solution concentration and feed solution temper-ature on the pervaporation performances were studiedsystematically.

2. Experimental

2.1. Monomer synthesis

The monomer of 2,2-bis[4-(4-amionphenoxy)-phenyl]hexafluoropropane, BAPPH, was preparedby following the procedures described in [22].The final product was a brown–white crystal (mp,162–163◦C) in 98% yield. Elemental analysis resultsof C27H20N2O2F6 agreed well with theoretical val-ues. Calculated: C, 62.55%; H, 3.89%; N, 5.40%.Found: C, 62.56%; H, 3.84%; N, 5.39%.

Table 1The reaction pairs of polyamides

Polamide Diamine Diacid

H-1 BAPPPa iso-Phthalic acidH-2 BAPPPa 2,6-Naphthalic acidH-3 BAPPPa 5-t-Butyl-iso-phthalic acidF-1 BAPPHb iso-Phthalic acidF-2 BAPPHb 2,6-Naphthalic acidF-3 BAPPHb 5-t-Butyl-iso-phthalic acid

a 2,2-Bis[4-(4-amionphenoxy)phenyl]propane.b 2,2-Bis[4-(4-amionphenoxy)phenyl]hexafluoropropane.

2.2. Polymerization

A serious of aromatic polyamides were preparedby direct polymerization of BAPPH and BAPPP withvarious aromatic diacids by using triphenyl phosphiteand pyridine inN-methyl-2-pyrrolidone (NMP), asshown in Table 1.

A typical example (F-3) of polycondensationis shown below. A mixture of diamine (1.296 g,2.5 mmol), 5-t-butyl-iso-phathlic acid (0.555 g,2.5 mmol), calcium chloride (0.36 g), NMP (5 ml),pyridine (1.4 ml) and triphenyl phosphite (1.4 ml)was heated with stirring at 100◦C for 3 h. Thepolymer solution was poured into a large amountof methanol with constant stirring. The precipitatedpolymer was washed thoroughly with methanol andhot water and then dried at 100◦C under vacuum.The yield was 1.777 g (96%). The inherent viscos-ity of polymer in N,N-dimethylacetamide (DMAc)was 0.54 dl g−1. The IR spectrum exhibited absorp-tion at 3314 cm−1 (N–H) and 1659 cm−1 (C=O).In addition, Table 2 shows the results of polymer-ization. In all cases, the inherent viscosities of thearomatic polyamides are greater than 0.54 dl g−1. Themolecular weights were high enough to cast flexibleand tough films.

2.3. Measurement

X-ray diffractograms were recorded with an X-raydiffractometer (Philips Model PW1710). The spe-cific volume was measured by using a micromeritricsAccupyc 1330 Pycnometer. This instrument mea-sures the volume of the solid by the gas displacementmethod.

Y.-C. Wang et al. / Journal of Membrane Science 185 (2001) 193–200 195

Table 2Synthesis of aromatic polyamidesa

Polymer ηinh (dl g−1) Tg (◦C) Decompositiontemperature (◦C)b

Tensile strength(MPa)

Elongation atbreak (%)

Yield (%)

H-1 0.67 217 488 67 5 99H-2 0.79 220 505 70 6 98H-3 0.55 244 487 65 7 98F-1 0.59 224 500 61 6 96F-2 0.88 231 512 73 6 98F-3 0.54 253 474 62 8 96

a Heating rate: 10◦C min−1 in nitrogen.b Temperature at which 10% mass loss was recorded with TGA.

2.4. Membrane preparation

The membrane was prepared from a casting solutioncontaining 10 wt.% of aromatic polyamide in DMAc.The membrane was formed by casting the solution ona glass plate to a predetermined thickness. The glassplate was then heated at 70◦C for 40 min. It was foundthat the average thickness of the membranes is about20mm.

2.5. Pervaporation experiment

The apparatus for pervaporation is depicted in theprevious paper [23]. The feed solution temperaturemaintained in the range of 30–60◦C and in direct con-tact with the membrane of which the effective mem-brane area was 10.2 cm2. The permeation rate wasdetermined by measuring the weight of the permeate.The compositions of the feed solutions, the perme-ates, and the solutions adsorbed in the membranewere analyzed by gas chromatography (GC ChinaChromatography 8700T). Each data presented in thiswork is an average value of at least three experiments.The separation factorαA/B was calculated from

αA/B = YA/YB

XA/XB

whereXA, XB andYA, YB are the weight fraction of A(benzene) and B (cyclohexane) in the feed and perme-ate (A is the more permeative species), respectively.

2.6. Sorption measurement

A piece of membranes (weight= 0.07 g, area=50 cm2, thickness = 20mm) was immersed in

benzene/cyclohexane mixtures at 50◦C. When thesorption reached equilibrium, it was subsequentlyblotted between tissue paper to remove excess solventand placed in the left tube of a twin tube set-up. Thesystem was evacuated while the left tube was heatedwith hot water and the right tube was cooled in liquidnitrogen. The composition of the condensed liquid inthe right tube was determined by GC.

3. Results and discussion

3.1. Influence of polyamide structure on thepervaporation performances

Table 3 shows the pervaopration performance for a50 wt.% feed benzene/cyclohexane solution through aseries of aromatic polyamide membranes. It indicatesthat the fluorine-containing aromatic polyamide mem-branes (F-1–F-3) has higher permeation rate and lower

Table 3Performances of the aromatic polyamide membranes for ben-zene/cyclohexane by pervaporationa

Membranes Permeation rate(g m−2 h−1)

Separationfactor

PSIb

H-1 –c – –H-2 205 7.4 1482H-3 817 4.9 3752F-1 440 6.8 2998F-2 299 5.8 1734F-3 1470 4.0 5903

a Feed (benzene/cyclohexane): 50/50 (wt.%), feed temperature:50◦C, membrane thickness: 20mm.

b PSI= separation factor× permeation rate.c No permeates were measured.


separation factor than that of the unfluorine-containingaromatic polyamide membranes (H-1–H-3), respec-tively. For example, compared with the H-3 membranewith a propane group in the polymer backbone, theF-3 membrane with a hexafluoropropane group in thepolymer backbone had a higher permeation rate. Thesephenomena might be due to the fact that the introduc-tion of a bulky hexafluoropropane group into the poly-mer backbone resulted in an aromatic polyamide withan amorphous structure. Thus, the packing density ofthe polymer chains decreased during the membraneformation process. Additionally, the characterizationof molecular packing for the above polyamides weremeasured by using a X-ray diffractometer. All aro-matic polyamides are essentially crystalline expectthat based on 5-t-butyl-iso-phthalic acid (H-3 andF-3), as shown in Table 4. This could be explained bythe fact that the 5-t-butyl-iso-phthalic acid has a bulkypendant group which results in significant steric hin-drance. From the view point of molecular structure, the

Table 4Characterization of molecular packing for the aromatic polyamidemembranes

Membranes Specific volume(cm3 g−1)

d-Spacinga

(Å)Crystallinity

H-1 0.552 4.0 Semi-crystallineH-2 0.58 4.0 Semi-crystallineH-3 0.622 – AmorphousF-1 0.613 5.2 Semi-crystallineF-2 0.613 5.2 Semi-crystallineF-3 0.645 – Amorphous

a X-ray diffraction; λ = 1.54 Å.

Table 5Results on the separation of benzene/cyclohexane by pervaporation

Membranea Benzene infeed (wt.%)

Feed solutiontemperature (◦C)

Permeation flux, Ql(kgmm m−2 h−1)

Separationfactor, αBz/CHx

References

H-3 50 50 16.3 4.9 This studyF-3 50 50 29.4 4 This studyLDPE 50 25 10.8 1.6 [11]PVDF 53 56 1.5 5.4 [12]PAS 50 50 2.6 22.5 [17]PEMA–EGDM 10 40 8.7 6.7 [15]N6–PEMA 50 50 0.15 12 [9]PU–TEOS 50 50 0.65 19 [10]

a LDPE: low density polyethylene; PVDF: poly(vinylidine fluoride); PAS: poly(acrylnitrile-co-styrene); PEMA–EGDM: poly(ethylmethacrylate)–ethylene glycol dimethacrylate; N6–PEMA: nylon 6–poly(ethyl methacrylate); PU–TEOS: polyurethane–tetraethyl-ortho-silicate.

polymer with a larger substituted group gives a higherbarrier to chain rotation than that with a smaller one.The higher barrier to rotation in the polyamide mem-branes may also inhibit local segmental motion. Theabove discussion can explain why the permeation rateof F-3 membranes is higher than that of the other mem-branes. These results agree well with the results fromthe pervaporation experiment, as shown in Table 3.Consequently, the optimum pervaporation results giv-ing a separation factor toward benzene, a permeationrate, and a pervaporation separation index (PSI) valuethrough the fluorine-containing polyamide membrane(F-3) for a 50 wt.% feed benzene concentration were4.0, 1470 and 5903 g m−2 h−1, respectively. In addi-tion, it is interesting to compare the pervaporation per-formances of the aromatic polyamide membranes (H-3and F-3) for benzene/cyclohexane separation with thatof the other polymers reported in the literature. All per-meation rate values were converted to kgmm m−2 h−1.The membrane performances for pervaporationseparation of benzene/cyclohexane mixtures, being ata relatively high level, is listed in Table 5.

3.2. Effect of feed concentration on thepervaporation performance of F-3 membrane

The effect of the feed benzene concentration on thepervaporation performances of the F-3 membranesare shown in Fig. 1. The permeation rate increases asthe feed concentration of benzene increases, but theseparation factor increases with the feed benzeneconcentration up to 50%, then decreases. These resultsmight be due to the fact that the benzene molecule


Fig. 1. Effect of the benzene concentration on the total permeationrate and separation factor for F-3 membrane at 50◦C.

has p-electrons, it may have a stronger interactionwith the amide group of the F-3 membranes, result-ing in the permeation rate increases with the benzeneconcentration in the feed solution. The degree ofswelling increases as the feed benzene concentra-tion increases for the F-3 membranes, as shown inFig. 2. These results correspond well with the resultsmentioned above, as indicated in Fig. 1. Moreover,the molecular size of the permeants has stronglyinfluenced the separation of benzene/cyclohexanemixtures in pervaporation. Table 6 shows the mo-lar volume and the difference between the solubilityparameter of the F-3 membrane and the permeates.According to the solubility parameter, the hydrogenbonding component of benzene is higher than thatof the cyclohexane. Thus, the hydrogen bonding in-teraction between the benzene molecules and theamide group of the F-3 membranes become strong,

Table 6Molar volume and the difference between the solubility (δ) of aromatic polyamide membrane (F-3) and permeates

Permeates Molar volume (cm3 mol−1) Solubility, δa ((cal cm−3)1/2) δbF-3 − δpermeates((cal cm−3)1/2)

δD δP δH δ

Benzene 89.4 9.1 0 1 9.2 0.3Cyclohexane 108.7 8.3 0 0.1 8.4 1.3

a δ = Hansen solubility parameter,δD = dispersive forces contribution component,δP = polar component,δH = hydrogen bondingcomponent,δ2 = δ2

D + δ2P + δ2

H.b By the small method,δF-3 calculated was 9.5.

Fig. 2. Effect of the benzene concentration in the feed solution onthe degree of swelling of F-3 membrane at 50◦C.

resulting in the benzene-permselectivity increase withincreasing the feed benzene concentration from 10to 50 wt.%. Moreover, when the benzene concentra-tion is higher, the amorphous regions of membraneare more swollen. Hence, the polymer chain in theswellon region becomes more flexible and the energyrequired for diffusive transport also decreases, result-ing in the larger size component can easily permeatethrough the membrane. Thus, the separation factordecreased with the feed benzene concentration higherthan 50 wt.%. Fig. 3 shows that the permeation rateincreases with increasing the temperature and thebenzene concentration of the feed solution. At higherfeed solution temperature, causing an increase in F-3chain mobility and swelling of the membrane matrix,a higher permeation rate is observed. Moreover, ac-cording to the free-volume theory, the thermal motion


Fig. 3. Effect of the benzene concentration on the total permeationrate for F-3 membrane with varying temperature: (d) 30; (s) 40;(j) 50; (h) 60◦C.

of the polymer chains in the amorphous regions ran-domly produces the free volume. As the feed solutiontemperature increases, the frequency and amplitudeof the chain jumping increase and the resulting freevolume becomes larger; therefore, the permeationof the permeating molecules through the membranebecomes easier, resulting in an increase in the totalpermeation rate.

3.3. Effect of benzene concentration in the feed onthe solubility and permeability of the F-3 membrane

In order to investigate the effects of solubility anddiffusivity on the membrane permselectivity, sorptionexperiments for the F-3 membranes were made. Fig. 4shows the effect of the benzene concentration in thefeed on the benzene concentration in the permeateand in the F-3 membrane. It can be seen from Fig.4 that the permeate and sorption composition curveslie under the diagonal line, indicating that benzenemolecules are selectively dissolved into the membraneand are predominantly permeated through the mem-brane. Additionally, the benzene concentration in themembrane is lower than that in the permeate for ben-zene concentration in the range of 10–50 wt.%. Thesephenomena might be due to the fact that the molarvolume of benzene is smaller than that of the cyclo-

Fig. 4. Effect of benzene concentration in the feed on the ben-zene concentration in the permeate (d) and composition of ben-zene/cyclohexane sorbed into the F-3 membrane (j) for ben-zene/cyclohexane mixture at 50◦C.

hexane (Table 6). Once the benzene molecules are in-corporated into the F-3 membrane, they can easily dif-fuse through the F-3 membrane. However, as the feedbenzene concentration higher than 70 wt.% an oppo-site trend was obtained. These results might be at-tributed to the excessive swelling. That is, excessiveswelling (Fig. 2) due to the selective component (ben-zene) causes a non-selective component (cyclohexane)to permeate through the F-3 membrane. Hence, thecyclohexane permeated through the F-3 membrane inspite of its low affinity toward the membrane, result-ing in the benzene concentration in the membrane ishigher than that in the permeate. Furthermore, accord-ing to the solution-diffusion model, the permeability(P) of a permeant through a membrane is a product ofthe solubility (S) and the diffusivity (D). The effects ofthe benzene concentration in the feed on the sorptionselectivity and the diffusion selectivity are discussed inthe following section. The sorption experiments wereperformed to determine the separation factor of sorp-tion, αS, for the F-3 membranes. Thus, the diffusionselectivity,αD, can be calculated from the followingequation

αD = α

αS


Fig. 5. Effect of benzene concentration on separation factor (α),solubility selectivity (αS), and diffusivity selectivity (αD) of F-3membrane:α, (d); αS, (h); αD, (4), operating temperature at50◦C.

The effect of the benzene concentration in the feedon the separation factor (α), sorption selectivity(αS), and the diffusion selectivity (αD) are shown inFig. 5. It shows that the sorption selectivity increaseswith an increasing the benzene concentration in thefeed, which can be accounted for by the high affin-ity between the benzene and the amide groups. Thediffusion selectivity, shown in Fig. 5, first increasesbut then decreases with an increasing the benzeneconcentration in the feed. The diffusion selectivity isstrongly related to the membrane structure and themolar volume of permeant: a denser membrane struc-ture results in a higher diffusion selectivity. However,the degree of swelling of membranes becomes higheras the feed benzene concentration further increases(Fig. 2). The more swollen membrane structure resultsthe diffusion selectivity decreases shown in Fig. 5.Obviously, the diffusion selectivity dominates the be-havior of pervaporation because the separation factorof pervaporation follow the same trend as the diffusionselectivity but not the trend of the sorption selectivity.In addition, the reason why the sorption selectiv-ity had a minimum value at 50 wt.% feed benzeneconcentration is due to the azeotropic composition(52/48 wt.%) of benzene/cyclohexane solution. It islikely that for a mixture of benzene and cyclohexane,the interactions between these molecules are much

stronger at the azeotropic composition. Therefore,when the benzene molecules adsorbed into the mem-brane, much cyclohexane will also adsorbed into themembrane in the vicinity of azeotropic composition.

4. Conclusions

In the present study, a serious of aromatic polyamidemembranes were used for the pervaporation of ben-zene/cyclohexane mixtures. The membranes werebenzene-permselectivity for all the feed composi-tions. The permeation rate and the PSI value offluorine-containing polyamide membranes based onthe BAPPH were higher than that of the aromaticpolyamide membranes based on the BAPPP. In addi-tion, the interaction between the benzene moleculesand the aromatic polyamide membrane is higher thanthat of the cyclohexane. Thep-electrons in benzeneand the polar group in aromatic polyamide play a ma-jor role in improving permeation rate and selectivitytoward benzene. The optimum pervaporation resultswere obtained by the fluorine-containing polyamidemembrane (F-3) for a 50 wt.% feed benzene concen-tration, giving a separation factor toward benzene of4.0, a permeation rate of 1470 g m−2 h−1, and a 5903PSI value.

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pervaporation of benzene/cyclohexane mixtures through aromatic polyamide membranes

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