Gas, Liquid and eric Separations Using

8/8/2019 Gas, Liquid and eric Separations Using

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Gas, liquid and enantiomeric separations using polyanilineRichard B. Kaner

Department of Chemistry and Biochemistry, Exotic Materials Institute, University of California, Los Angeles, CA 90095-1569, USA

Abstract

Conjugated polymers are excellent candidates for membrane separations since their porosity on the molecular level can be controlledthrough chemical doping. Polyaniline is especially attractive since its simple acid/base doping/dedoping chemistry enables a controllablelevel of doping to be readily achieved using dopants of different size, shape or even chirality. Adding dopants to polyaniline leads to adecrease in permeability to gases, while removing these dopants leads to enhanced permeability. By partially redoping polyaniline, amembrane with one of the highest selectivities for the permeation of oxygen over nitrogen has been created. Liquids can be selectivelypermeated in a process known as pervaporation. As-cast polyaniline shows little selectivity for water over the organic acids formic, aceticand propionic. This is due to the hydrophobic nature of the emeraldine base form of polyaniline leading to a solubility selectivity that favorsorganic acids over water, while diffusivity selectivity favors the smaller water molecules over the larger organic molecules. Highselectivities for water over organic acids can be achieved by using fully hydrochloric acid-doped polyaniline membranes which becomehydrophilic and thus favor water over the organic acids due to both solubility and diffusivity. Chiral polyaniline lms can be created bydoping with a strong chiral acid such as S ( )-camphorsulfonic acid during lm formation. Removal of the dopants with base creates anew form of chiral polyaniline that can distinguish between the amino acids L-phenylalanine and its enantiomer D-phenylalanine.# 2001 Elsevier Science B.V. All rights reserved.

Keywords: Chiral recognition; Gas separation; Pervaporation; Polyaniline

1. Introduction

Our research focuses on the use of conjugated polymersfor gas [1,2] and liquid [3,4] separations and for enan-tiomeric recognition [5]. The lessons learned in AlanMacDiarmid's [6,7] laboratory making polyacetylene/ lithium batteries during the early days of conducting poly-mer research have been put to good use in my laboratoryat UCLA in the synthesis of two-dimensionally orderedthermoelectric materials via doping with lithium [8,9], inthe development of new routes to layered materials [1012]and in the synthesis of alkali metal-doped, superconductingfullerenes [13,14].

The use of polymers for separations depends on the poresizes of the resulting membranes as shown in Fig. 1. If amembrane has a pore size in the 10100 mm regime, it can beused for conventional ltration such as in removing starch(106 A) from solution. If the pore size is in the 0.0810 mmrange, then the membrane can carry out microltration suchas in removing bacteria (e.g. staphylococcus 10 4 A) orviruses (e.g. inuenza 10 3 A) from solutions. If the mem-brane has a pore size between 30 and 1000 A, then it can beused for ultraltration such as in separating hemoglobin

(140 A) from blood. If a membrane is fullydense, then it willhave a pore size of less than 30 A and can, therefore, be usedfor reverse osmosis, pervaporation or gas separation.

Pervaporation, a process which separates liquids bydifferential permeation through a membrane, is especiallyattractive for applications such as the separation of closeboiling liquids or azeotropes which cannotbe separatedusingstandard distillation processes [15]. Gas separation is attrac-tive for converting air into its constituents nitrogen andoxygen, two of the most useful commodity chemicals.Nitrogen-enriched air is especially important in protectingperishables such as apples, enabling long-term transportand storage without the need for refrigeration.Gas separationmodules can be added as needed and are especially attractivefor remote uses of the gas produced such as by farmers wherethe cost of shipping nitrogen-enriched air is prohibitive.

The inspiration for using conjugated polymers for separa-tions stems from the ability to control morphology of apolymer on the molecular level via the doping process eitherduring or after its synthesis. Controlling the structure andporosity of a polymer after its synthesis is very difcult toaccomplish with conventional polymers since they cannotreadily be doped. Several investigations have focused onusing the conjugated polymers polypyrrole, polythiopheneand polyaniline as ion gate and electron transfer membranes

Synthetic Metals 125 (2002) 6571

E-mail address : [email protected] (R.B. Kaner).

0379-6779/01/$ see front matter # 2001 Elsevier Science B.V. All rights reserved.PII: S 0 3 7 9 - 6 7 7 9 ( 0 1 ) 0 0 5 1 2 - 4


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[1622]. Polyaniline appears to be the most useful of theconjugated polymers for tailoring membrane properties since

it possesses a simple acid/base doping/dedoping chemistryand is stable in both air and water. Our work has demon-strated that controlling the doping level from fully doped topartially redoped to completely dedoped is important fordifferent separation and recognition processes [15]. Here, itwill be demonstrated that fully doped polyaniline canbe usedas a pervaporation membrane to separate acids such as aceticacid from water, partially redoped polyaniline will be shownto have one of the highest selectivities found so far forpermeating oxygen over nitrogen, and completely dedopedchiral polyaniline will be shown to possess enantiomericrecognition properties by being able to distinguish betweenthe amino acids L-phenylaniline and D-phenylalanine.

2. Experimental

Polyaniline was synthesized in its emeraldine oxidationstate by the oxidative polymerization of aniline in 1.0 molarhydrochloric acid using a standard procedure developed byMacDiarmid et al. [23]. Films for gas and liquid perme-ability experiments were formed from a dispersion of 57%of the emeraldine base form of polyaniline in N -methyl-2-pyrrolidinone (NMP) formed by grinding the powder witha mortar and pestle and then adding in the NMP. This

dispersion was poured into a glass Petri dish and cured at1108C until dry (13 h). After cooling, the lms wereremoved from the Petri dish by soaking in water resultingin fully dense free-standing, purple-bronze membranes.Doping was carried out in appropriate acids such as1.0 M aqueous HCl for 610 h which turned the lms to

a deep blue color. Dedoped lms were made by soaking in0.1 M aqueous ammonia for 410 h resulting in gold-bronze-colored lms. The 40100 mm thick lms werecut using a die into 2.5 cm diameter membranes for usein pervaporation and gas permeability experiments. A com-plete description of the apparatus used for gas and liquidseparations can be found in Ref. [24]. For chiral recognitionexperiments, emeraldine base powder and either S ( )- or R()-camphorsulfonic acid was ground together in a mortarand pestle in a 2:1 mass (5:1 molar) ratio, dispersed in NMP,ltered through cotton, spread on glass plates and cured at608C for 2 h to create thin lms. Circular dichroism spectrawere recorded on a Jasco J715 Spectropolarimeter using afast Fourier transform to lter out high frequency noise.

3. Results and discussion

3.1. Pervaporation

Fully dense membranes have pore sizes of less than 30 A and depend on molecular interactions between the permeantand the polymer to affect separation. For liquidliquidseparations, known as pervaporation, the processes involvedare illustrated in Fig. 2. A feed solution comprised of two or

more liquids is brought in contact with a dense membrane.Molecules in the feed adsorb onto the polymer surface,diffuse through it and desorb on the downstream side. Theoverall process can be described in terms of permeability ( P ),diffusivity ( D) and solubility ( S ) as given in Eq. (1), assumingthat the penetrant does not plasticize the polymer:

P DS (1)

Fig. 1. Separation processes depend on the pore size of the membranesused, ranging from conventional filtration (10100 mm), microfiltration(0.0810 mm) and ultrafiltration (301000 A) down to pervaporation, gasseparation and reverse osmosis (330 A). Fully dense membranes such aspolyaniline can be used for the latter three processes.

Fig. 2. A schematic diagram illustrating the processes involved inpermeation through a membrane. Molecules in the feed solution adsorbonto the membrane, diffuse through it and desorb on the downstream side.The more permeable molecules (small gray circles) become the favoredcomponent in the permeant vapor.

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In a typical experiment, such as that shown in Fig. 3, therewill be a time lag ( t ) before molecules have permeatedthrough the membrane and can be measured as they desorbfrom the downstream side. Once molecules begin desorbing,their rate rapidly reaches a steady-state. The permeability(P in g mm/(m 2 h)) can be determined from the slope of thesteady-state flux as given in Eq. (2):

Py pl

A

(2)

where y p is the slope, i.e. the flowrate in g/h, l the membranethickness in mm and A the area of the membrane in m 2 . Thesteady-state slope can be extrapolated back to the x-interceptto yield the time lag ( t ). Using the solution to Fick's law of diffusion for a flat sheet membrane, as given in Eq. (3), thediffusion coefficient ( D in cm2 /s) can be

Dl2

6t(3)

calculated [15]. Solubility can then be obtained from Eq. (1)or determined in separate experiments.

Pervaporation experiments in the laboratory entail place-ment of a membrane on a porous metal support, evacuationof the pervaporation apparatus, addition of a solution on theupstream side of the membrane and utilization of liquidnitrogen on the downstream side to drive the solutionthrough the membrane [24]. In commercial separations, tolower costs, evacuation and cooling are replaced by passinggas across the downstream side of the membrane whichsweeps away the permeant as it emerges from the mem-brane. The composition of the permeant is traditionallydetermined by either gas chromatography combined withmass spectroscopy (GCMS), density or refractometry [15].We have found that 1 H NMR is another useful and highly

accurate method for determining compositions of anyliquids containing hydrogen [24]. Another useful techniqueis to employ a graduated collection vessel so that thecollected frozen material can be thawed periodically andits volume determined. This is how the ux versus time plotfor methanol diffusion through a fully dedoped polyaniline

membrane was obtained. From Fig. 3, it can be seen that thetime lag, t , for methanol to permeate through dedopedpolyaniline is 10 h leading to a value for the diffusioncoefcient of 1:6 1010 cm2=s. This is comparable tothe diffusion coefcient for methanol through a conven-tional polymer such as poly(methylmethacrylate) [25]. Theslope gives a y p value of 0.0045(3) g/h yielding a perme-ability value of 0.85(1) g mm/(m 2 h).

Pervaporation experiments on as-cast (or undoped) poly-aniline membranes indicate that this form of the conjugatedpolymer has little selectivity for water over organic acids.This can be seen in the upper diagram of Fig. 4 where amixture of water, formic acid, acetic acid and propionic acidwere used as the feed in a pervaporation experiment througha polyaniline membrane. After the permeation experimentwas completed, the components of the permeant in thecollection vessel were determined by integrating the areasunder the peaks in the 1 H NMR spectrum. The water contentincreased a little from 27.7% in the feed to 46% in thepermeant. This occurred due to a slight decrease in theamount of formic and acetic acids and a larger decreasein the amount of propionic acid. By separating the compo-nents of the permeability into solubility and diffusivity,a clearer mechanistic picture emerges explaining theseresults. Basically, undoped polyaniline is hydrophobic lead-

ing to a solubility selectivity that favors the organic com-ponent of the solution over water. Conversely, diffusivityfavors water over the organics due to size. Water has a radiusof about 1.4 Awhile formic, acetic and proprionic acids haveradii of 3.5, 4.2 and 4.8 A, respectively [26,27]. The perme-ability (and diffusivity) values for water, acetic acid andpropionic acid are 0.11 g mm/(m 2 h) 5:0 1010 cm2=s ,0.3 g mm/(m 2 h) 1:2 1010 cm2=s and 0.02 g mm/ (m2 h) 1:2 1010 cm2=s , respectively. No values areavailable for formic acid since it swells undoped polyanilinemaking it porous.

Pervaporation experiments with fully hydrochloric acid-doped polyaniline indicate that this form of the conjugatedpolymer has excellent selectivity for water over organicacids. This can be seen in the lower diagram of Fig. 4 wherethe same mixture of water, formic acid, acetic acid andpropionic acid as studied with the as-cast membrane wasused in a pervaporation experiment through a fully dopedpolyaniline membrane. Here the water content in the per-meant increased from 27.7 to 94.5% with a concomitantdecrease in the formic acid concentration from 21.8 to 4.3%,the acetic acid concentration from 26.6 to 1.1% and thepropionic acid concentration from 24.0 to


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water over the organic acids since the fully hydrochloric

acid-doped membrane is now hydrophilic. Diffusivity alsogreatly favors the smaller water molecules over the largerorganics. Hence, the favorable combination of high diffu-sivity selectivity, due primarily to size and solubility selec-tivity, due to increased hydrophilicity, leads to a very highpermeability selectivity for water especially over the largerorganic acids.

Unfortunately, hydrochloric acid-doped polyaniline mem-branes have one major drawback in separations of aqueoussolutions, namely the dopants slowly leach out of the mem-branes and into the feed solutions during prolonged contactwith water. This leads to a decline in the rate of permeationover time. One way to eliminate this problem is to usepolymeric dopants that cannot leach out of the polymer evenif left in water for long periods of time. Blends of polyanilinewith either polyamic acid or polyacrylic acid have proven tobe useful in this regard [2830]. However, there is a trade-off between the permeation rate and selectivity [24].

3.2. Gas separation

In order for polymers to be effective at selectively per-meating different gases they must be made into fully dense,nonporous membranes. This prevents many conjugatedpolymers from being considered for gas separations. For

example, polyacetylene synthesized from a Ziegler-Natta

catalyst typically produces lms of about one-third fulldensity [31]. Although the bright gold-colored polyacety-lene lms appear reectiveand dense they are not and are, infact, highly permeable to all gases tested. Polyaniline, on theother hand, can be made into fully dense, free-standing lmswhen cast from dispersions in NMP. Nitrogen gas adsorptionexperiments of polyaniline membranes indicate an averagepore size of much less than 20 A, the lower limit of the BET(BrunauerEmmettTeller) characterization method [1].

When as-cast polyaniline membranes are exposed todifferent gases, the measured permeabilities are found tobe dependent on the size of the penetrant gas [1]. Thevalues range from 20 Barrers for the relatively small gashelium (2.6 A) to 0.03 Barrers for the much larger gasmethane (3.8 A). Note that 1 Barrer 1010 cm3 STP cm=cm2 s cmHg , where cm 3 (STP) is the volume of gas at

standard temperature and pressure, cm is the thickness of themembrane, cm 2 is the exposed membrane area on the feedside and cmHg is the pressure of the feed gas. The kineticdiameters of the gases are known from sorption experimentsin xeolites [32]. Selectively permeating oxygen over nitro-gen is especially challenging since nitrogen (3.64 A ) is only0.18 A larger than oxygen (3.46 A).

As-cast polyaniline permeates oxygen over nitrogen by aselectivity factor a of up to 9 a P O2 =P N2 , as can be

Fig. 4. Pervaporation of a mixture of organic acids and water through a dedoped polyaniline membrane leads to little selectivity (top). When the samemixture is permeated through a fully HCl-doped polyaniline membrane, a high selectivity for water over the organic acids is observed (bottom).



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seen from the rst set of bars in Fig. 5. This oxygen/nitrogenselectivity is as good as any conventional polymer such aspolyimides and better than polymers typically used incommercial gas separations such as polysulfones [33]. Fullydoping polyaniline with hydrochloric acid reduces the free-volume in the polymer and leads to a decrease in gaspermeabilities (Fig. 5, second set of bars). This also slightlylowers the oxygen/nitrogen selectivity. Dedoping the poly-aniline with aqueous ammonia results in dramaticallyincreased permeabilities due to an increase in free-volume(Fig. 5, third set of bars). This results in higher permeabil-

ities for both oxygen and nitrogen, but only restores theselectivity of the original polyaniline membrane. Partiallyredoping the polyaniline using a 0.0175 M hydrochloric acidsolution results in lower overall permeabilities for bothoxygen and nitrogen, but the decrease in oxygen perme-ability is less (Fig. 5, fourth set of bars), resulting in veryhigh selectivity values for oxygen over nitrogen of about 29.

These high oxygen/nitrogen selectivity values wereunprecedented when rst reported [1]. Several independentreports conrmed that partially redoped polyaniline doesindeed have the highest oxygen/nitrogen selectivity known[34,35]. However, there was originally some concern regard-ing how readily enhanced permeability of dedoped polyani-line could be achieved. The inability to obtain enhancedpermeabilities in dedoped lms is most likely caused bycuring the polyaniline lms at too high a temperature and/orfor too long. Excessive heating of the lms will result in agreat deal of cross-linking [36], leading to membraneswhose free-volume cannot be increased with doping anddedoping. Permeability to oxygen for fully doped polyani-line should increase from about 0.1 Barrers to about0.25 Barrers after dedoping. A recent report by the chemicalengineering group of Lee et al. [37] indicates that thepermeability of polyaniline membranes is highly dependenton the doping level, that doping/dedoping does enhance

oxygen permeability and by redoping membranes for 2 h in0.02 M hydrochloric acid, an oxygen/nitrogen selectivityvalue of 28 can be obtained.

3.3. Chiral recognition

One of the most important challenges facing the pharma-ceutical industry today is to nd a simple and effectivemethod for separating a racemic mixture into individualenantiomers. Over two-third of the drugs being developedtoday are chiral and often only one enantiomer is useful

while the other may be inactive or have deleterious sideeffects [38]. Most methods currently used for enantiomericseparations including crystallization of diastereomeric salts,chromatography using a chiral stationary phase and mole-cular imprinting require expensive chiral starting reagents[39,40]. Here, we examine the possibility of creating aninexpensive chiral recognition polymer by using polyanilineand doping acids that can be recycled.

Thin lms of chiral polyaniline were synthesized bymodifying a procedure rst described by Wallace and cow-orkers [41] and Havinga et al. [42]. Polyaniline powder inthe emeraldine oxidation state is dispersed in NMP witheither S ( ) or R()-camphorsulfonic acid, ltered, castonto glass plates and cured at 60 8C for 2 h. Circular dichro-ism (CD) spectra of the resulting lms conrm that the twopolyaniline lms are chiral and essentially mirror imagesof each other. The S ( )-camphorsulfonic acid-doped poly-aniline has a large dopant-induced peak in the negativedirection at about 450 nm, while the R()-camphorsulfonicacid-doped polyaniline has a large peak in the positivedirection at about 450 nm. This is due to the polyaniline-camphorsulfonic acid complex and not due to the camphor-sulfonic acid itself which only absorbs below 300 nm. Notethat the original polyaniline powder used was in a hydratedstate, if dehydrated polyaniline is used, the spectra will

Fig. 5. The permeability and selectivity of a polyaniline membrane toward oxygen (gray bars) and nitrogen (black bars) is dependent on the doping level inthe polymer. Doping with 1 M HCl reduces the permeability of both gases, dedoping with base increases the permeability and partial redoping using0.0175 M HCl leads to an enhanced selectivity ( a ) for oxygen over nitrogen.

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appear inverted [43]. After treating the doped polymer withaqueous ammonia, a dedoped chiral form of polyaniline isproduced. It is this dedoped chiral polyaniline that is used inchiral recognition studies.

The CD spectrum for chiral-dedoped polyaniline is shownas the dashed line in Fig. 6. This is the spectrum resultingfrom the removal of S ( )-camphorsulfonic acid. Uponexposure to L-phenylalanine for 3 weeks, a new peak appears just below 450 nm (solid line in Fig. 6). This peak iscomparable in intensity to the S ( )-camphorsulfonic aciddopant-induced peak. However, since phenylalanine occurs

in its zwitterionic form, it is incapable of doping the weaklybasic emeraldine base form of polyaniline. Therefore, theinteraction of phenylaniline with chiral-dedoped polyanilineis truly remarkable. A control sample in which a comparablededoped chiral polyaniline lm was exposed to D-phenyla-lanine for 3 weeks indicated that no change in the spectrum,from that given by the dotted line in Fig. 6, occurred. Hence,chiral-dedoped polyaniline recognizes L-phenylalanine, butnot D-phenylalanine. Work on separations using columnchromatography is in progress [44].

4. Conclusions

Polyaniline can be used as a membrane for the selectivepermeation of liquids and gases. Pervaporation experimentsindicate that as-cast polyaniline membranes have littleability to separate water from organic acids. This is dueto their hydrophobic nature leading to a solubility selectivityfavoring organic acids over water, while their diffusivityselectivity favors the smallerwater molecules over the largeracids. On the other hand, fully hydrochloric acid-dopedpolyaniline has a remarkable selectivity for water overorganic acids. This is due to the hydrophilicity of dopedpolyaniline which now favors water over the organics in

terms of solubility selectivity combined with the diffusivityselectivity due to the smaller size of water in comparisonwith the organic acids. Leaching of hydrochloric aciddopants into the aqueous feed solutions is a challenge thatcan be met by using polymeric dopants such as polyacrylicor polyamic acids; however, the permeation rate will be

decreased by these large dopants. Gas permeation experi-ments indicate that the permeabilities of polyaniline mem-branes are dependent on their doping level. Adding dopantsto polyaniline decreases its free-volume and therefore low-ers its permeability to all gases. Dedoping polyaniline withbase leads to enhanced permeabilities relative to as-castmembranes for all gases due to increases in free-volume.Partially redoping polyaniline can lead to some of thehighest selectivity values known for permeating oxygenover nitrogen. Chiral-dedoped polyaniline interacts andrecognizes the chiral amino acid L-phenyalanine, but doesnot recognize its enantiomer D-phenylalanine.

Acknowledgements

The author thanks Prof. Alan MacDiarmid for providingan exciting place and eld to carry out his graduate work andProf. Alan Heeger for collaborations on polyacetylene dur-ing his Ph.D. studies. The work presented here has beengenerously supported over the past decade by the Ofce of Naval Research, a Campus University-Los AlamosResearch(CULAR) grant and fellowships from the Packard, SloanandDreyfus Foundations. Thestudents whodid this researchinclude Dr. Benjamin Mattes, Dr. Mark Anderson, Dr.

Jeanine Conklin, Prof. Shu-Chuan Huang, Prof. Tim Su,Dr. Ian Ball, Dr. Hailan Guo and Dr. Veronica Egan. Helpfulcollaborations have been carried out with Prof. HowardReiss on gas separations and Prof. Charles Knobler on chiralrecognition.

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