Journal of Membrane Science 447 (2013) 345–354

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Journal of Membrane Science

0376-73http://d

n CorrE-m

journal homepage: www.elsevier.com/locate/memsci

Permeation and sorption of organic solvents and separationof their mixtures through an amorphous perfluoropolymermembrane in pervaporation

John Tang a, Kamalesh K. Sirkar a,n, Sudipto Majumdar b

a Otto York Department of Chemical, Biological and Pharmaceutical Engineering, New Jersey Institute of Technology, University Heights, Newark,NJ 07102-1982, USAb Compact Membrane Systems Inc., Wilmington, DE 19804, USA

a r t i c l e i n f o

Article history:Received 6 February 2013Received in revised form24 May 2013Accepted 23 June 2013Available online 15 July 2013

Keywords:PDD–TFE copolymer membranePervaporationOrganic solvent mixturesLongest molecular dimensionSolvent solubilities

88/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.memsci.2013.06.036

esponding author. Tel.: +1 973 596 8447; fax:ail address: sirkar@njit.edu (K.K. Sirkar).

a b s t r a c t

The permeation and sorption of a few common organic solvents and separation of their mixtures througha polymeric membrane of perfluoro-2,2-dimethyl-1,1,3-dioxole copolymerized with tetrafluoroethylene(PDD–TFE) was investigated by vacuum-based pervaporation. Pure component permeations of commonpharmaceutical solvents such as, toluene, methanol, ethyl acetate and tetrahydrofuran (THF), werecarried out using a 25 μm thick dense polymeric membrane supported by a porous polytetrafluoroethy-lene (PTFE) sheet. Overall permeability coefficients for these solvents along with those of aprotic solventsdimethylformamide (DMF) and dimethylacetamide (DMAc) determined earlier (Tang and Sirkar, 2012[1]), plotted against their longest molecular diameters were correlated to yield a relationship betweenthe molecular solvent size and their permeation through the membrane. The sorption of each puresolvent and a few of their mixtures were determined. Separation of a few organic–organic mixtures ofthese solvents was also implemented. These systems include: toluene–methanol, toluene–ethyl acetateand toluene–tetrahydrofuran. Modest separation factors were achieved using the PDD–TFE membraneunlike the very large values achieved earlier between water and aprotic solvents such as DMF, DMAc andDMSO (Tang and Sirkar, 2012 [1]). The maximum separation factor value for all systems wasapproximately 7.8 for a 75 wt% toluene and 25 wt% methanol feed at 30 1C. The separation factorobtained using a 50 wt% toluene and 50 wt% THF feed at 50 1C was close to 1. Maximum flux valuesthrough a 25 μm thick PDD–TFE membrane were approximately 5 g/(m2 h) for toluene–ethyl acetatesystems and 2.2 g/(m2 h) for toluene–methanol and toluene–THF systems. The membrane was typicallyselective for the organic solvent with the smaller molecular dimensions. The solubility of the individualcomponents in a mixture provided unique insights into such behavior. The observed separation behaviorprovides a window into the complex phenomena of organic solvent transport through the PDD–TFEmembrane.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Pervaporation as a means of separation has been used toseparate a variety of organic–organic and organic–aqueous mix-tures. An additional benefit of pervaporation is its ability to handleazeotropic systems that are difficult to separate via traditionalmeans, namely distillation. An excellent commercialized exampleis alcohol–water azeotrope breakup to dehydrate ethanol. We haverecently demonstrated an extraordinary separation capabilityin the dehydration of aprotic solvents such as DMF, DMAc, andDMSO using a novel amorphous perfluoropolymer membrane [1].

ll rights reserved.

+1 973 642 4854.

One wonders about the utility of such membranes in the pharma-ceutical industry where mixtures of a variety of organic solvents arecreated as by-products of drug synthesis. In addition, a desirableseparation process would be selective for solvents with smallermolecular dimensions. This would allow the process to be appliedto a number of solvent mixture systems while maintaining consistentperformance. An incomplete list of solvents used in pharmaceuticalindustry includes toluene, methanol, ethyl acetate, tetrahydrofuran(THF), N,N-dimethylsulfoxide (DMSO), N,N-dimethylformamide(DMF) and N,N-dimethylacetamide (DMAc). The membrane selectedfor this application must be able to withstand these harsh solvents aswell as provide significant flux and selectivity.

Vacuum driven pervaporation is a process in which the feedcontacts the membrane as a liquid while the permeate exists asvapor. The driving force is created by pulling a vacuum across the

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354346

membrane. Separation is ultimately a function of solvent–mem-brane interaction, membrane properties and solvent volatilities.Solvent transport can be described as a sequence of three steps:(1) selective sorption of the solvents into the membrane, (2)diffusion of the solvent molecules through the membrane, and(3) desorption of solvent molecules from the membrane. Typically,steps 1 and 2 are the limiting factors in solvent transport. Step 3 isoften considered to be very rapid when compared to the sorptionand diffusion steps. Diffusion of species through the membrane isusually the rate-limiting step and is a complex phenomenon. Astwo or more species are simultaneously present in the membrane,there is competition for free volume-based transport corridors.Thus, diffusion of these species can be seen as a coupled process.Transport properties may vary considerably when comparing purecomponent diffusion with diffusion of a mixture.

Transport and sorption of various gaseous species in a glassyamorphous membrane is influenced by two types of sorptions,Henry's Law sorption and Langmuir sorption. Competitive sorptionin Langmuir sites in the case of gaseous mixtures influencestrongly the transport and separation behavior in the case of gasmixtures. One would expect that such phenomena will alsoinfluence the sorption and diffusion behavior of organic liquidmixtures through glassy amorphous membranes.

Previous studies using pervaporation with organic solventstypically involved a single membrane that is selective for aparticular solvent. Interactions between the solvent and membraneusually dictate the selectivity and performance of the process.Separating organic–organic mixtures requires a material that istailored for the removal a specific solvent. Due to this, a wide arrayof membranes can be formulated for the same application. Forexample, separation of the azeotrope between methanol andtoluene has been explored extensively with a myriad of materials[2–6]. Zhou et al. [2] discovered a high performing membrane usingpolypyrrole doped with hexafluorophosphate. This was applied toseparating binary mixtures of methanol with toluene and methanolwith isopropyl alcohol. The highest selectivity value of 590 formethanol over toluene was found when separating a mixture of95 wt% toluene and 5 wt% methanol. Methanol flux at this concen-tration was reported as 0.230 kg/(m2 h). Another study by Patil et al.[3] employed a mixed matrix membrane containing silicalite zeoliteand chitosan to separate alcohol–toluene mixtures. The materialwas preferential for toluene and achieved a selectivity of 264 andflux values in the range of 0.019–0.027 kg/(m2 h) for the methanol–toluene systems. A similar performance was found with mixtures oftoluene and ethanol.

This study focuses on using a perfluoropolymer membranehaving a very high fractional free volume as well as excellentmechanical, chemical and thermal stability to separate particularorganic–organic mixtures frequently encountered in pharmaceu-tical processing. These physical attributes arise due to the materialbeing a copolymer of poly-2,2-dimethyl-1,1,3-dioxole and tetra-fluoroethylene (PDD–TFE). This membrane has already been intro-duced commercially to separate water vapor and air from gasolinevapor mixtures [7]. Also, it has been able to separate water fromlubricant fluids and alcohols as well [8,9]. Tang and Sirkar [1] haveobserved recently that this membrane is extraordinarily selectivefor water over aprotic organic solvents e.g. N,N-dimethylsulfoxide,N,N-dimethylacetamide, N,N-dimethylformamide with water–organic solvent separation factors in the range of 1000–12,000.The difference in the dimensions and vapor pressures of theseaprotic organic solvents and water are considerable. It is useful toexplore how this membrane will perform when the organiccompounds to be separated are closer to each other in suchproperties than water and the aprotic organic solvents mentioned.

This work proposes to study the permeation of individualorganic solvent species through the membrane and the effective

separation performance of this membrane in organic–organicmixtures and test its sieving properties. The fractional free volumeof this membrane material is considerably higher than that oftraditional amorphous glassy polymers. Furthermore, the dimen-sions of the average free volume regions in the copolymer are inthe range of 5.9–6.3 Å [10]. These combined characteristics givethis membrane potential to achieve high flux and selectivity valuesfor smaller organic molecules over larger, more bulky solvents.

There are a few published studies in regard to permeation oforganic vapors and pervaporation of mixtures involving chlori-nated solvents through a related membrane, Teflon AF 2400[11–14]. This study will explore the influence of various para-meters such as feed composition, and temperature in the perva-poration of selected organic solvents through a PDD–TFEmembrane of the CMS-3 variety. First, pure component permea-tion tests were conducted with ethyl acetate, toluene, tetrahydro-furan and methanol along with pure component solubilitymeasurements. These performances were placed in the contextof those measured earlier with the aprotic solvents studied earlier[1]. Then, mixtures using those common solvents were selected totest the separation performance characteristics of the membrane.Solubility measurements of pure components of all solvents and afew mixtures were also carried out. Pervaporation separation andsorption of the following mixtures were studied: toluene–metha-nol, toluene–ethyl acetate and toluene–tetrahydrofuran.

2. Materials and methods

The solvents used include acetone (GC Resolv), methanol(Histological Grade), ethyl acetate (99.5% A.C.S), toluene (499.5%A.C.S.), tetrahydrofuran (anhydrous, 499.9% inhibitor-free),1-butanol (99.9%), N,N-dimethylformamide (≥99.8% ACS reagent),and N,N-dimethylacetamide (anhydrous, 99.8%) (Sigma-Aldrich,St. Louis, MO). Table 1 lists the solvents [not the aprotic solvents]and a few of their properties.

The membranes used were supplied by Compact MembraneSystems (Wilmington, DE). They were composed of an amorphousperfluorinated PDD–TFE copolymer, CMS-3, of varying thicknesses,100 mm and 25 mm. CMS-3 membrane has a high fractional freevolume, approximately 33–36% [10], when compared to otherpolymeric membranes. Fig. 1 shows the formula of the basic unitalong with those for related materials, Hyflon AD and Cytop; forCMS-3, the values of m and n are 0.66 and 0.34 respectively. Thereis another variety, CMS-7, whose m and n are 0.86 and 0.14. Wehave not studied it due to its more open structure.

The experimental setup utilizes a water bath, a positivedisplacement feed pump, flat membrane cell, condenser, and avacuum pump. The water bath was used to heat the feed to thedesired temperature. At the same time, the feed pump recirculatedthe feed solution from the feed reservoir to the membrane cell andback again. The pressure on the feed side was maintained atatmospheric conditions while the vacuum pump on the permeateside created a low pressure of approximately 7–9 Torr. In order tocollect the permeate sample, the condenser was placed in a Dewarflask containing liquid nitrogen.

Initial investigations determined that the sample volumes ofthe permeate were very small especially with membranes having100 mm thickness. Therefore all experiments reported here used25 mm thick membranes placed on a porous polytetrafluoroethy-lene (PTFE) mesh support. Even then the sample volume collectedwas extremely low. To ensure no loss in the sample, a diluentwhich was a relatively non-volatile solvent was passed throughthe condenser at the time of collection. The diluent was selected tobe completely soluble to both species in the feed solution as wellas having a relatively low vapor pressure at standard temperature

Table 1Various properties and structural dimensions of a few organic solvents.

Structure Name Boiling pt. (1C) Density (g/cm3) Smallest diametera (Å) Largest diametera (Å) Areaa (Å2) Volumea (Å3)Mol. wt. (g/mol)

Methanol 65 0.7918 1.757 2.844 61.23 40.7532.04

Ethyl acetate 77.1 0.897 3.108 6.004 125.77 99.88

88.11

Tetrahydrofuran 66 0.8892 3.306 4.191 105.06 86.33

72.11

Toluene 110.8 0.8669 4.254 5.842 133.77 116.46

92.14

a As estimated using WaveFunction Spartan program.

Fig. 1. The formulas for the basic unit of Hyflon AD, Cytop and PDD–TFE.

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354 347

and pressure. For mixture components such as methanol, ethylacetate, toluene, THF etc. the diluent was n-butanol.

The effective area of the membrane in the flat membrane cellwas 11.4 cm2. Each experimental run was conducted for at least7 h. A gas chromatograph (Varian CP-3800, Walnut Creek, CA)equipped with a DB 5 ms column (Agilent, Wilmington, DE) wasused to analyze the diluted samples (sample diluted with thediluent) at a temperature of 100 1C. The concentration in thepermeate sample could be determined by applying calibrationcurves for various known concentrations of the species in thediluent, n-butanol. By measuring the exact volume of the perme-ate sample, the experimental mass flux, Ji, of each species could becalculated:

Ji ¼ρiCiVp

Amtð1Þ

where ρi is the density of species i (g/cm3), Ci is the concentrationof species i in the permeate by volume (mL/mL), Vp is the totalvolume of the permeate sample in milliliter, Am is the effectivemembrane area in m2 and t is the collection period in hours. Theflux for each component in the mixture is expressed in termsof g/(m2 h).

Species flux in pervaporation is often described as a function ofa species permeability coefficient using the following equation:

Ji ¼Qi

δmðγif xif Psat

i �yipPpÞ ð2Þ

where Ji is mass flux of species i in g/(m2 h), Qi is permeabilitycoefficient of species i expressed in terms of (g m)/(m2 h mm Hg),δm is membrane thickness in meters, γif is its activity coefficient inthe feed solution contacting the membrane, xif is mole fraction ofspecies i in the feed, Pisat is the saturated vapor pressure of speciesi at the given temperature in mm Hg, yip is the mole fraction ofspecies i in the permeate and Pp is the permeate side total pressurein mm Hg. Most of these parameters are known either experi-mentally or through literature values, with the exception of theactivity coefficient which is unity for pure components. Formixtures, activity coefficients illustrated in Table 2 for threedifferent systems were determined via non-random two liquidmodel (NRTL) in conjunction with solubility parameters obtainedfrom the computer software package, ASPEN. It should be notedthat the above equation assumes no coupling occurs whenseparating mixtures. Overall permeability coefficient, Qi, isunknown and is to be determined from the experimentallydetermined flux, Ji. It has been reported as mentioned in termsof (g m)/(m2 h mm Hg) to maintain consistency with the units offlux. When divided by Mi, the molecular mass of species i in gmper gm mole, it has units of (gmol m)/(m2 h mm Hg); this unit hasbeen used in one figure only.

The separation performance of a pervaporation process for twospecies, i and j, can be described through its separation factor, αij. Theseparation factor compares the concentrations of the components in

Table 2Solvent activity coefficients at varying compositions and temperatures.

Methanol–toluene (A–B) Ethyl acetate–toluene (A–B) THF–toluene (A–B)

30 50 60 30 50 60 30 50 60

A–B γA γB γA γB γA γB γA γB γA γB γA γB γA γB γA γB γA γB

25–75% 1.66 1.75 1.62 1.70 1.61 1.68 1.47 1.00 1.35 1.01 1.29 1.01 0.70 0.94 0.79 0.96 0.83 0.9750–50% 1.20 2.88 1.19 2.76 1.18 2.71 1.21 1.06 1.16 1.07 1.13 1.07 0.83 0.85 0.89 0.89 0.91 0.9175–25% 1.07 4.15 1.06 3.93 1.06 3.83 1.05 1.26 1.04 1.23 1.03 1.21 0.93 0.72 0.96 0.80 0.97 0.83

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354348

the permeate to those in the feed. A higher value signifies greaterseparation between the components and higher overall performanceof the process. It can be calculated by

αij ¼wt:f ractionip=wt:f ractionif

wt:f ractionjp=wt:f ractionjfð3Þ

where i and j indicate different species in the mixtures, p refers to thepermeate side and f refers to the feed side. Separation factor can bere-written as a function of total weight (w) and total moles (n) ofeach species in the feed and permeate given by Eq. (4):

αij=wt�f raction ¼wip=wif

wjp=wjf¼ nip=nif

njp=njfð4Þ

If we divide each of nip and njp by (nip+njp) and divide each of nif andnjf by (nif+njf), the left hand side still remains the same separationfactor of Eq. (4). However the right hand side can now be written asthe appropriate ratios of mole fractions; this quantity is the definitionof separation factor based on mole fractions commonly used so that

αij=wt�f raction ¼yipxjfyjpxif

¼ αij=mole�f raction ð5Þ

Here as mentioned next to Eq. (2), x and y with appropriatesubscripts represent mole fractions in the feed and permeaterespectively.

The solubility of a solvent in the membrane was measuredusing the following procedure. For pure components, dry weightof a membrane sample was determined through the use of amicrobalance (Cahn C-31, Cerritos, CA). This sample was soaked inthe desired solvent(s) at room temperature for 8 h or overnight.Excess solvent on the surface was carefully removed using aKimwipe. The sample was then weighed again on the microba-lance. Weight gained between the sorbed weight and the dryweight reflected the amount of solvent absorbed by the membranesample. Results to be reported are average of two samples forthese measurements as well as those reported in the nextparagraph.

For determining the solubility of a solvent present in a solventmixture, a membrane sample was soaked in a prepared solutionfor 8 h or overnight. Again, excess liquid was removed from theouter surfaces using a Kimwipe. To analyze the extent of compe-titive sorption, headspace analysis using a gas chromatograph(Varian CP-3800, Walnut Creek, CA) equipped with an autosam-pler (CTC Analytics CombiPAL, Zwingen, Switzerland) wasemployed to determine the concentration of each solvent thatremained within the membrane sample; the column employedhas been identified earlier. In order to properly strip the sorbedsolvents, the sample was heated to 100 1C in the incubation unit ofthe autosampler prior to injection into the gas chromatograph.Headspace calibration utilized a similar methodology, using asmall drop of solvent mixture, no more than 0.2 mL, as a sample.After heating, the resulting vapor was then injected into the gaschromatograph for analysis. Various concentrations of the solventmixtures were used to generate an applicable calibration curve.

Estimations of selective sorption of solvents into the membranesample were also made using Flory–Huggins relationships. Thishas been done for example by Magalad et al. [15] for a poly (vinylalcohol)–poly (vinyl pyrrolidone) used in the dehydration ofethanol. The overall equation for sorption selectivity, αs, is asfollows:

lnαs ¼ lnΦ1

Φ2

� ��ln

v1v2

� �¼ V1

V2�1

� �ln

Φ2

v2

� �

�χ12ðΦ2�Φ1Þ�χ12ðv1�v2Þ�ΦP χ1P�V1

V2χ2P

� �ð6Þ

Here, Φi represents the volume fraction of component i in theswollen polymer membrane, vi is the volume fraction of compo-nent i in the bulk feed, Vi is the corresponding molar volume ofeach component i, and χij is the interaction parameter betweencomponents i and j. The subscripts 1, 2 and P refer to the solventsin the binary mixture and the polymer (PDD–TFE). Values fromexperimental data have been used for Φi for various solventsystems. Φp is able to be calculated using

ΦP ¼ 1þ ρpρs

Ma

Mb

� �� ρp

ρs

� �� ��1

ð7Þ

where ρp and ρs are the densities of the polymer and solvent andMa and Mb refer to membrane mass after and before swelling,respectively. Due to the significant inability of the PDD–TFE toswell after being submerged in organic solvent, the ratio of Ma toMb has been assumed to be equal, making Φp unity. The interactionparameter, χij, between either solvents was determined by:

χ12 ¼x1lnðx1=v1Þ þ x2lnðx2=v2Þ þ ðΔGE=RTÞ

x1v2ð8Þ

where xi represents the mole fraction of solvent i, ΔGE/RT (J mol�1)is the excess Gibbs energy of mixing, R is the universal gasconstant (J mol�1 K�1) and T is the absolute temperature inKelvin. ΔGE was calculated using:

ΔGE ¼ RTðx1lnγ1 þ x2lnγ2Þ ð9ÞActivity coefficients for each solvent system were determinedusing the NRTL equation via ASPEN. Finally, the interactionparameter, χip, was calculated from

χiP ¼ViðδP�δiÞ2

RTð10Þ

where δ represents the solubility parameter for either the polymeror solvent. The solubility parameter of Teflon was used in lieu ofthat for PDD–TFE due to lack of data.

3. Results and discussion

3.1. Pure component permeation

Previous studies made by Smuleac et al. [9] imply that theseparation mechanism by this membrane is governed by size

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354 349

exclusion. Various molecular dimensions of different solventspecies were obtained for each organic solvent through the useof the commercial package Spartan (Wavefunction Inc., Irvine, CA).This can be seen in Table 1. The molecular dimensions of theaprotic solvents are available in [1].

Due to the wide spread of sizes and shapes of the solvents on amolecular level, an initial investigation was carried out to determinethe effect of molecular radius, area and volume on permeationthrough the CMS-3 membrane. As a function of temperature, mostsolvents exhibit an increase in flux that is somewhat linearlydependent on temperature as seen in Fig. 2. The exception is ethylacetate, whose flux decreases from 1.76 g/(m2 h) to approximately1.10 g/(m2 h) as temperature increases from 30 to 70 1C. This may bedue to the polarity of the solvent having a relatively larger molecularsize in a highly hydrophobic environment provided by PDD–TFE.Sorption (especially Langmuir sorption) and diffusion of larger, morepolar solvents can be hindered through this highly hydrophobicmaterial as we will increasingly encounter in this study.

When the permeability coefficient is plotted against temperature, anegative temperature dependence can be observed for all puresolvents studied as seen in Fig. 3. These results are similar to thatobtained by Pinnau and Toy [16] for gas permeation using a relatedamorphous copolymer membrane, Teflon AF 2400. Their studyfocused on determining the permeability coefficients for hydrogen,nitrogen, helium, oxygen, carbon dioxide, ethane, propane and chlor-odifluoromethane. We will deliberate more on this near the end.

Pure Solvent Flux vs. Temperature

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70 80

Temperature ( °C)

Flux

(g/m

2 -h)

Toluene

THF

EthylAcetateMethanol

Fig. 2. Increasing flux observed with temperature for pure organic solventsbetween 30 and 70 1C.

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+000 10 20 30 40 50 60 70 80

Perm

eabi

lity

Coe

ffici

ent (

g-m

/m2 -h

-mm

Hg)

Temperature (°C)

Pure Component Permeability Coefficient vs. Temperature

TolueneTHFEthyl AcetateMethanol

Fig. 3. Decreasing permeability coefficient with temperature for pure solventcomponents through a CMS-3 membrane.

To compare the effect of molecular size on the permeabilitycoefficient (g m/m2 h mm Hg), data obtained at 60 1C have beenplotted against the longest molecular diameter of each solvent.The DMF and DMAc values were obtained from Tang and Sirkar[1]. Fig. 4a shows that as the longest molecular diameter decreasesfrom approximately 6 Å, overall permeability coefficient increases.However this does not appear to hold true for methanol andtetrahydrofuran. If the permeability coefficients are reported interms of gram moles instead of gram (Fig. 4b), the THF value stilldeviates quite a bit.

In the case of these more polar solvents, methanol and THF, ithas been demonstrated that these molecules potentially formclusters in highly hydrophobic polymers. Methanol clusteringhas been reported while studying time lag in related membranessuch as Teflon AF and Hyflon AD. Diffusion coefficients for thesedimers and trimers significantly decrease methanol diffusion byorders of magnitude [17]. Therefore, the solubility and diffusion ofpossible dimers or even trimers through the membrane wouldstrongly affect the permeability of a much smaller molecule,methanol. Jansen et al. [17] showed that the permeation wasessentially describable by a methanol dimer.

If we use 5.69 Å as the longest molecular diameter representinga methanol dimer, then the permeability coefficient (g m/m2 h mm Hg) of methanol falls on the curve for all other solventmolecules instead of being an outlier (Fig. 4a). We observe asimilar behavior for the methanol dimer when g-mole is usedinstead of gram in Qi (Fig. 4b). This may also be valid fortetrahydrofuran which has been said to form dimers in both theliquid and gas state [18]. In Fig. 4a, therefore we have also added a

Dimethylformamide

Dimethylacetamide

Methanol (dimer)

Toluene

Ethyl Acetate

THFTHF (dimer)

Methanol

Permeability Coefficient × 107 vs. Longest Molecular Diameter

Permeability Coefficient × 107 vs. Longest Molecular Diameter

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5 6 7 8 9

Longest Molecular Diameter (Ǻ)

0 1 2 3 4 5 6 7 8 9

Longest Molecular Diameter (Ǻ)

Perm

eabi

lity

Coe

ffici

ent

× 10

7 (g

-m/m

2 -h-m

m H

g)

0

0.005

0.01

0.015

0.02

0.025

0.03

Perm

eabi

lity

Coe

ffici

ent

× 10

7

(gm

ol-m

/m2 -h

-mm

Hg)

THF (dimer)

Ethyl Acetate

Toluene

Methanol (dimer)

Dimethylacetamide

Dimethylformamide

Methanol

THF

Fig. 4. (a) Permeability coefficients of solvents through CMS-3 membrane as afunction of a single dimension, longest molecular diameter and (b) Permeabilitycoefficients for pure solvents calculated on a per gram mole basis as a function ofthe longest molecular diameter.

Table 3Solubility of pure components by weight at roomtemperaturea.

Solvent % Gain from dry weight

Ethyl acetate 0.50Tetrahydrofuran 0.55Toluene 0.56Methanol 0.29DMF 0.20DMAc 0.60DMSO 0.55

a Variation of data was less than 7%.

ln Q vs. 103/T

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

02.85 2.90 2.95 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35

103/T (1/Kelvin)

ln Q

Toluene

THF

EthylAcetate

Methanol

Fig. 5. A plot of the natural log of the permeability coefficient to determine theactivation energy of permeation of pure solvents e.g. toluene, THF, ethyl acetate andmethanol through CMS-3.

Table 4Solubility of mixtures by compositiona.

Mixture (A–B) Sorption of A (%) Sorption of B (%)

Methanol–toluene 0.5 99.5Ethyl acetate–toluene 45 55Tetrahydrofuran–toluene 42.7 57.3

a Variability was found to be less than 12%.

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354350

point corresponding to a THF dimer. We observe that such abehavior is in line with the overall behavior. In addition in the caseof methanol, the highly polar molecule has a much lower sorption,as we will soon see, than the other solvents in the membrane(Table 3); that will reduce its permeation rate considerably fromwhat otherwise would have been a very high value for methanol.

There is an Arrhenius type relationship between the perme-ability coefficient and the activation energy of permeation. It isgiven by the following equation:

Qi ¼ Qioexpð�Ep=RTÞ ð11ÞWhen plotted, the slope is used to determine the required

activation energy for each organic solvent. This is illustrated inFig. 5. The values of activation energy for ethyl acetate, toluene,tetrahydrofuran and methanol are as follows: �54.0 kJ/mol,�34.8 kJ/mol, �73.4 kJ/mol, and �37.6 kJ/mol respectively.

3.2. Sorption of pure components and mixtures

As sorption is the initial step in the pervaporation process, it isuseful to deliberate more on the information regarding competi-tive solubility gathered here to help understand the separationfactor of one solvent over another. Preliminary measurementsusing pure components (Table 3) yielded limited results in termsof differentiating one solvent from another except in the case ofmethanol. Solubility for most solvents was limited in this highlyinert, hydrophobic polymer; the weight gain by percent for thepure solvent did not exceed over 0.56%. More polar solvents suchas methanol demonstrated significantly lower solubility in the

CMS-3 membrane than non-polar solvents. It has also been foundthat more polar solvents often had lower solubility in membranesof similar formulations [19]. This can apply to methanol, ethylacetate and tetrahydrofuran when compared to that of toluene.Furthermore, methanol is a much smaller molecule. But the dimerwill have dimensions around 5.69 Å. The range of average freevolume dimensions in this membrane is between 5.9 and 6.3 Å.Therefore, a dimer would have some difficulty in getting sorbed inthe Langmuir sites. We will point out later the potentiallydominant role of the Langmuir sites in such a membrane. As aresult methanol solubility is very low in this membrane.

A more useful approach would be to compare competitivesorption when both solvents are present. These results aresummarized in Table 4. Headspace analysis via gas chromatogra-phy allows for the determination of the concentration of eachsolvent sorbed into the membrane, not the total amount sorbed. Inthis fashion, the distribution of solvents present in the membranecan be found. Using mixtures comprised of 50 wt%–50 wt% organicsolvents, it was discovered that toluene is typically more solublethan THF, methanol and ethyl acetate. In mixtures of THF andtoluene, toluene made up approximately 57.3% of the total liquidsorbed. A similar amount was found between ethyl acetate andtoluene, toluene comprising 55% of the liquid sorbed. Preliminaryresults from pure component sorption tests were reflected incompetitive sorption for methanol. Only 0.29% by weight wassorbed in pure component solubility measurements. Here inmixture studies, methanol consisted of only 0.5% of the totalamount of liquid sorbed in the presence of toluene; toluenevirtually occupied all free volume regions in this membrane inpreference to methanol. This will affect the separation factor formethanol over toluene which otherwise should have been muchlarger as we will see soon.

It is useful to deliberate here on the calculation methods for thesolubility of liquid solutes in polymers. Flory–Huggins theory iscommonly applied to calculate the solubility of liquids in amor-phous polymers that are in rubbery or gel-like state. There are anumber of other successful approaches including those based onvarious equations of state; see the recent article by Sarti and DeAngelis [20] for additional details. It is also well known that suchapproaches are not very useful for amorphous glassy polymerswhose nonequilibrium state militiates against the use of modelsbased on equilibrium thermodynamics. Consequently we see theextensive use of empirical dual sorption models for correlation ofsolubility of gases and vapors in glassy polymers.

Sarti and De Angelis [20] have recently explored the applicationof non-equilibrium thermodynamics of glassy polymers (NET-GP)using non-equilibrium equation of state models. The approach hassignificant requirements in terms of data and information needed.They have successfully calculated the sorption of water andethanol in polycarbonate and water in polysulfone. There needsto be a few more efforts at verifying the predictive capability ofthis approach for glassy polymers and other liquid solutes beforeits adoption for calculating solubility coefficients for volatile liquidsolvents in PDD–TFE copolymer.

Also, the copolymer, PDD–TFE, we have studied is radicallydifferent from conventional glassy polymers such as polycarbonate

Flux vs. Temperature for Ethyl Acetate-Toluene Systems

0

1

2

3

4

5

6

7

0 10 20 30 40 50 60 70

Temperature (°C)

Flux

(g/m

2 -h)

Toluene (25 Tol/75 EA)

Toluene (50 Tol/50 EA)

Toluene (75 Tol/25 EA)

Ethyl Acetate (25 Tol/75 EA)

Ethyl Acetate (50 Tol/50 EA)

Ethyl Acetate (75 Tol/25 EA)

Fig. 6. Flux behavior of toluene and ethyl acetate at varying compositions withrespect to temperature [CMS-3 membrane].

Separation Factor vs. Composition of Ethyl Acetate-Toluene Systems

0

1

2

3

4

5

6

7

8

0 0.25 0.5 0.75 1

Weight Fraction of Toluene in Feed

Sepa

ratio

n Fa

ctor

(Tol

uene

/ Et

hyl A

ceta

te)

30°C50°C60°C

Fig. 7. Separation factor of the CMS-3 membrane as a function of feed compositionfor systems of ethyl acetate and toluene at varying temperatures.

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354 351

and polysulfone. The fractional free volumes (FFVs) of the latterpolymers are very low compared to the extraordinarily high FFVvalue of the PDD–TFE copolymer studied here. Further the freevolume dimensions are unusually high in PDD–TFE. In addition thepolymer is almost completely inert with extremely low solubilityfor all solvents studied here or otherwise except perfluorosolvents.Therefore the free volume regions having large dimensions play avery significant role in sorption long with any preferential groupinteractions in a highly hydrophobic environment.

From Eqs. (6)–(10), the overall sorption selectivity of PDD–TFEwas determined for 50–50 wt% mixtures of methanol–toluene,ethyl acetate–toluene, and tetrahydrofuran–toluene. In one case,this estimated value was approximate to experimental values. Thesorption selectivity for ethyl acetate–toluene was calculated to be1.2. Experimentally, it was also found to be approximately 1.2 aswell. However, the calculated value for αs for methanol–tolueneand THF–toluene were determined respectively to be 664 and0.95. These values are significantly different from their experi-mentally observed values of 199 and 1.3, respectively. This dis-crepancy is due to the fact that Flory–Huggins relationships do notaccount for the high fractional free volumes, the large size of thefree volume regions in the polymer and may not accurately modelglassy polymers such as PDD–TFE.

3.3. Permeation and separation of mixtures

By varying both temperature and composition of each mixture,flux and selectivity may change. As different solvent moleculespermeate through the membrane, there is considerable competi-tion for sorption sites. In this way, rates of sorption and diffusioncannot be reliably predicted from pure component permeationresults. As a real process, diffusion is a coupled process duringpervaporation. Permeation of a faster molecule may speed up thepermeation of a slower one and vice versa.

In the toluene–ethyl acetate system, Fig. 6 shows that the fluxbehavior with temperature and composition appears to be gov-erned by the dominant component in the mixture. At a 50–50 wt%mixture, toluene flux is reduced from 5.0 to 2.4 g/(m2 h) as thetemperature was increased from 30 to 60 1C; the flux of ethylacetate behaves similarly. However, when toluene makes up 75 wt% with ethyl acetate completing the balance, toluene flux illus-trates the more expected positive temperature dependence,increasing from 3.3 to 5.0 g/(m2 h).

It should be noted that at any given composition of toluene andethyl acetate, the flux of either species is higher than their purecomponent permeation. This is likely to be due to coupling. Weknow from pure component permeation behavior (Fig. 2) thattoluene flux increases with temperature. For the 75 toluene–25ethyl acetate mixture, as we go from 50 to 60 1C, we see asignificant increase in ethyl acetate flux as well (much larger thanthat for pure ethyl acetate) presumably due to coupling with thetoluene transport. On the other hand, since the pure componentflux of ethyl acetate drops significantly from 30 to 50 1C, weobserve that for two other mixtures (25% and 50% ethyl acetate),toluene flux is strongly influenced by the behavior of ethyl acetate.

The CMS-3 membrane seems to be selective for toluene overethyl acetate, despite the much higher boiling point of toluene andthe differences in their molecular dimensions. Fig. 7 illustratesseparation factor remaining relatively constant at 6 over allcompositions of this binary system and temperatures. This mightbe in part due to ethyl acetate's long chain-like structure versustoluene's more globular shape (Table 1). The longest diameter ofethyl acetate is approximately 6 Å whereas for toluene, it isslightly smaller at 5.8 Å. The average free volume dimensions ofthese membrane materials have been reported to be approxi-mately 5.9–6.3 Å [10]. Though the molecular dimensions of these

two solvents are slightly different, the differences in hydrophobicinteractions with the membrane material may allow one solvent tooccupy preferentially the free volume regions and therefore passthrough significantly easier than the other. Competitive sorptionresults of this mixture show that the more hydrophobic andglobular toluene has a significantly higher solubility than ethylacetate (Table 4).

Fig. 8 illustrates the behaviors of the permeability coefficientsof both toluene and ethyl acetate with temperature. The decreaseswith temperature for both species for various mixtures reflect thebehavior we have observed earlier with the pure componentpermeability coefficients (Fig. 3).

Results from separating methanol–toluene mixtures reflectpure component permeation somewhat more so than in thetoluene–ethyl acetate system. Fig. 9 shows that the separationfactor for methanol over toluene increases from 2.8 to 7.7 astoluene concentration in the feed increases. This is likely to be dueto the increase in the activity coefficient of methanol in mixtureswith higher toluene concentration (Table 2). The overall perfor-mance of the membrane can be attributed to a sieving behaviorwith caveats. Methanol is a significantly smaller molecule com-pared to toluene in all dimensions, radius, area and volume.Methanol has a much higher vapor pressure than toluene. Thus,

Permeability Coefficient vs. Temperature for Toluene-Ethyl Acetate Systems

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+000 10 20 30 40 50 60 70

Temperature (°C)

Perm

eabi

lity

Coe

ffici

ent (

g*m

/m2 *h

*mm

Hg)

Toluene (25 Tol/75 EA)

Toluene (50 Tol/50 EA)

Toluene (75 Tol/25 EA)

Ethyl Acetate (25 Tol/ 75 EA)

Ethyl Acetate (50 Tol/50 EA)

Ethyl Acetate (75 Tol/25 EA)

Fig. 8. Decreasing permeability coefficient with temperature for ethyl acetate–toluene systems through a CMS-3 membrane.

Separation Factor vs. Compostition of Methanol-Toluene Systems

0

1

2

3

4

5

6

7

8

9

0 0.25 0.5 0.75 1

Weight Fraction of Toluene in Feed

Sepa

ratio

n Fa

ctor

(Met

hano

l / T

olue

ne) 30°C

50°C60°C

Fig. 9. Separation factor of the CMS-3 membrane as a function of feed compositionfor systems of methanol and toluene at varying temperatures.

Flux vs. Temperature for Methanol-Toluene Systems

0

0.5

1

1.5

2

2.5

3

0 10 20 30 40 50 60 70

Temperature (°C)

Flux

(g/m

2 -h)

MeOH (25 Tol/75 MeOH)

MeOH (50 Tol/ 50 MeOH)

MeOH (75 Tol/ 25 MeOH)

Toluene (25 Tol/75 MeOH)

Toluene (50 Tol/50 MeOH)

Fig. 10. Flux behavior of toluene and methanol at varying compositions withrespect to temperature.

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354352

it should pass through the membrane much more easily thantoluene. We also speculate that the flux and the selectivity ofmethanol would have been much higher if there were no self-association of methanol molecules; further, methanol being highlypolar and potentially present as a dimer undergoes very poorsorption in the free volume holes in the membrane compared totoluene (Table 4). The flux values of individual species in toluene–methanol mixtures are shown in Fig. 10. Methanol permeatesthrough the membrane at a much higher rate at all temperaturesand compositions, ranging from 0.9 to 2.3 g/(m2 h). Toluene fluxvalues were between 0.08 and 0.63 g/(m2 h). Overall permeabilitycoefficients for this system are shown in Fig. 11.

Similar observations are made when separating mixtures oftoluene and tetrahydrofuran (Figs. 12–14). Both solvents havemolecular dimensions that are similar in shape and value. How-ever, tetrahydrofuran is smaller by a couple of angstroms in somecases. At lower temperatures, 30 1C, the membrane achievesseparation factors of only 1.2 to 1.5 for tetrahydrofuran overtoluene over all composition ranges. Note, however that THF hasa much lower boiling point and a much higher vapor pressure. Onewould therefore expect high selectivity for THF over toluene.However, it has a significantly lower sorption than toluene in a

Permeability Coefficient vs. Temperature for Methanol-Toluene Systems

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+000 10 20 30 40 50 60 70

Temperature (°C)

Perm

eabi

lity

Coe

ffici

ent (

g-m

/m2 -h

-mm

Hg)

Toluene (25 Tol/75 MeOH)

Toluene (50 Tol/50 MeOH)

Toluene (75 Tol/25 MeOH)

MeOH (25 Tol/75 MeOH)

MeOH (50 Tol/50 MeOH)

MeOH (75 Tol/25 MeOH)

Fig. 11. Decreasing permeability coefficient with temperature for methanol–toluene systems through a CMS-3 membrane.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.25 0.5 0.75 1

Sepa

ratio

n Fa

ctor

(TH

F / T

olue

ne)

Weight Fraction of Toluene in Feed

Separation Factor vs. Composition for THF-Toluene Systems

30°C50°C60°C

Fig. 12. Separation factor of the CMS-3 membrane as a function of feed composi-tion for systems of tetrahydrofuran and toluene at varying temperatures.

Flux vs. Temperature for THF-Toluene Systems

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60 70

Temperature (°C)

Flux

(g/m

2 -h)

THF (25 Tol/75 THF)

THF (50 Tol/50 THF)

THF (75 Tol/25 THF)

Toluene (25 Tol/75 THF)

Toluene (50 Tol/50 THF)

Toluene (75 Tol/25 THF)

Fig. 13. Flux behavior of toluene and THF at varying compositions with respect totemperature.

Permeability Coefficient vs. Temperature for THF-Toluene Systems

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+000 10 20 30 40 50 60 70

Temperature (°C)

Perm

eabi

lity

Coe

ffici

ent (

g-m

/m2-

h-m

m H

g)

THF (25 Tol/75 THF)

THF (50 Tol/50 THF)

THF (75 Tol/25 THF)

Toluene (25 Tol/75 THF)

Toluene (50 Tol/50THF)

Toluene (75 Tol/25 THF)

Fig. 14. Decreasing permeability coefficient with temperature for THF–toluenesystems through a CMS-3 membrane.

Table 5Sorption parameters of Teflon AF 2400 for various gases and contributions madefrom Henry's Law and Langmuir sorption developed from data in [21].

Penetrant kd [cm3 (STP)/(cm3 atm)]

C′h [cm3

(STP)/(cm3)]b [1/atm]

Henry's Lawcontribution

Langmuircontribution

O2 0.21 44 0.015 0.244 0.756N2 0.11 38 0.015 0.164 0.836CO2 1.60 26 0.070 0.485 0.515CH4 0.35 25 0.036 0.287 0.713C2H6 1.50 16 0.220 0.342 0.658C3H8 4.20 13 0.830 0.416 0.584CF4 0.45 29 0.082 0.170 0.830C2F6 1.60 18 0.590 0.193 0.807C3F8 6.40 19 2.200 0.329 0.671

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354 353

mixture presumably due to its polar nature (Table 4). In addition,the more polar THF undergoes association (Fig. 4) which willreduce its sorption, diffusion and therefore permeation.

Permeability coefficients for all solvents, pure or in mixture,decrease as a function of temperature (Figs. 3, 8, 11 and 14). This isan unexpected result as the overall permeability coefficients forsolvents through polymers typically increase with increasingtemperature. However, the overall permeability coefficient is aproduct of two parameters, the diffusion coefficient and thesolubility coefficient. Diffusion coefficient increases significantlywith temperature, while the solubility coefficient behaves in theopposite manner. For this reason, the solubility coefficient maygive further information about solvent permeation through theamorphous glassy perfluoropolymer membranes of the type beingstudied.

From a previous study of permeation of various gases through aTeflon AF 2400 membrane by Merkel et al. [21], several sorptionparameters were determined for a variety of gases for both Henry'sLaw and Langmuir sorption (based on the empirical dual sorptionmodel). An initial analysis of the data (see Table 5) had shown thatLangmuir sorption contributes much more (around 60–75%)towards total sorption than the sorption attributed to Henry's

Law [22]. It should also be noted that the Langmuir sorptionparameter also decreases strongly with increasing temperature.We speculate that Langmuir sorption influences overall sorption ofthe solvents onto the CMS-3 membrane. Correspondingly thepermeability coefficients will decrease with temperature due tothe very strong influence of Langmuir sorption.

4. Concluding remarks

The CMS-3 type of PDD–TFE perfluoropolymer membraneexhibits separation capabilities that has some similarities to thata molecular sieve. However, the average free volume dimensionsof the membrane are approximately close to the molecular sizes ofthe common organic solvents studied. In the most extreme cases,such as separating mixtures of tetrahydrofuran and toluene, verylittle, if any, separation occurred. The highest separation factorsachieved was approximately 8 when separating a mixture of 75 wt% of toluene and 25 wt% of methanol. The membrane was selectivefor methanol, the solvent with the smaller molecular dimensions,despite being much more polar. Sorption of solvents through themembrane is likely to depend much more upon Langmuir sorptionrather than Henry's Law. The overall solubility of various solventsin the membrane was found to be very little, approximately 0.56%at most reflecting the inert nature of this membrane. The perme-ability coefficients of the solvents appear to be correlated with thelongest molecular dimension of the solvents. There is greaterpotential for this membrane to be used in separating mixtures inwhich solvents have a greater disparity in physical sizes, especiallyin applications such as dehydration [1].

Acknowledgments

We acknowledge funding for this research from MAST (Mem-brane Science, Engineering & Technology) Center, National ScienceFoundation, USA and Compact Membrane Systems.

Appendix A

The molecular dimensions of the organic solvents, toluene,methanol, ethyl acetate and THF, were estimated using the soft-ware package Spartan Student Edition for Windows (Waveufnc-tion Inc., Irvine, CA). For each solute, its molecular geometry wasdetermined using the Hartree–Fock approximation along with a 3-21G Gaussian basis set. The calculated distances vary dependingon the selected atom pairs. The largest molecular diameter isdefined as the maximum distance found between opposite atom

J. Tang et al. / Journal of Membrane Science 447 (2013) 345–354354

pairs. The smallest diameter is the minimum distance betweenopposite atom pairs.

Nomenclature

Am Effective membrane area (m2)C Concentration by volume, mL/mLEp Activation energy of permeation (kJ/mol)Ji Flux of component i, (g/m2 h)n Number of molesP Pressure (mm Hg)Q Overall permeability coefficient (g m/m2 h mm Hg)R Universal gas constant (J/mol K)T Temperature (Kelvin)t Time (hours)Vp Total volume of permeate (mL)V Molar volume of a species (cm3/mol)v Volume fraction in bulk feedw Weightx, y Mole fraction in the feed and permeate,

respectivelyαij Separation factor of species i over jαs Sorption selectivity of species i over jδm Membrane thickness (m)δ Solubility parameter (J/cm3)1/2

γ Activity coefficient of a speciesρ Density (g/cm3)Φ Volume fraction in swollen polymerχ Flory–Huggins interaction parameter

Subscripts

i Pertaining to species if Pertaining to the feedj Pertaining to species jP Pertaining to the polymeric membranep Pertaining to the permeate

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