Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane

Journal of Membrane Science 280 (2006) 202–209

Modified poly(phenylene oxide) membranes for the separationof carbon dioxide from methane�

S. Sridhar a, B. Smitha b, M. Ramakrishna b, Tejraj M. Aminabhavi a,∗a Membrane Separations Division, Center of Excellence in Polymer Science, Karnatak University, Dharwad 580003, India

b Membrane Separations Group, Chemical Engineering Division, Indian Institute of Chemical Technology, Hyderabad 500007, India

Received 22 September 2005; received in revised form 9 January 2006; accepted 9 January 2006Available online 2 March 2006

Abstract

Two types of poly(phenylene oxide) (PPO) membranes were prepared: one by chemical modification through sulfonation using chlorosulfonicacid and another by physical incorporation with a heteropolyacid (HPA), viz., phosphotungstic acid. These membranes were tested for the separationof CO2/CH4 mixtures. Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction techniques were used to confirm the modified structureof PPO as well as to understand its interactions with gaseous molecules. Scanning electron microscopy (SEM) was used to investigate the membranemmsmCrt©

K

1

omdisobAbsat

0d

orphology. Thermal stability of the modified polymers was assessed by differential scanning calorimetry (DSC), while the tensile strength waseasured to evaluate their mechanical stability. Both chemical and physical modifications did not adversely affect the thermally and mechanical

tabilities. Experiments with pure CO2 and CH4 gases showed that CO2 selectivity (27.2) for SPPO increased by a factor of 2.2, while the PPO–HPAembrane exhibited 1.7 times increase in selectivity with a reasonable permeability of 28.2 Barrer. An increase in flux was observed for the binaryO2/CH4 mixture permeation with an increasing feed concentration (5–40 mol%) of CO2. An enhancement in feed pressure from 5 to 40 kg/cm2

esulted in reduced CO2 permeability and selectivity due to the competitive sorption of methane. Both the modified PPO membranes were foundo be promising for enrichment of methane despite exhibiting lower permeability values than the pristine PPO membrane.

2006 Elsevier B.V. All rights reserved.

eywords: Gas separation; Heteropolyacids; Sulfonated poly(phenylene oxide); Mixed matrix membranes; CO2/CH4 separation

. Introduction

Separation and purification of gases by selective permeationf one or more components of a gaseous mixture through a poly-eric membrane has attracted considerable interest over the past

ecades [1–4]. One of the challenging gas separation problemsn process engineering is that of CO2/CH4 system due to itsignificance in natural gas purification. However, identificationf new polymeric membrane materials for this separation haseen an important objective of membrane researchers [5–10].part from mechanical strength, chemical resistance and dura-ility, the two most important criteria governing the membraneelection for CO2/CH4 separation or any other gaseous mixture,re the productivity and separation efficiency [2]. Generally, arade-off exists between gas permeability and separation factor

� This paper is CEPS Communication #99.∗ Corresponding author. Tel.: +91 836 2215372; fax: +91 836 2771275.

E-mail address: [email protected] (T.M. Aminabhavi).

for the common engineering polymers [11]. Modifications inmorphology or chemical structure of the polymer bring aboutimprovements in either flux or selectivity [12–14].

Development of mixed matrix materials such as polymermembranes filled with nano-sized particles has been underfocus in recent times [15–19]. Duval et al. [15] observed thatzeolites such as silicalite-1, 13X and KY tend to improveCO2/CH4 separation properties of the poorly selective rub-bery polymers. Martin et al. [17] used the interfacial poly-merization technique to deposit thin films of polypyrrole,poly(N-methylpyrrole) and polyaniline onto the surfaces ofmicroporous support membranes. The CO2/CH4 selectivities of16.2 and 31.9 were obtained for undoped and doped poly(N-methylpyrrole) membranes, respectively, but their poor mechan-ical strength restricted any further technological advance-ment. This has prompted researchers to develop mixed matrixmembranes using the conductive fillers that are capable ofenhancing gas separation properties without compromisingon mechanical strength. Examples of such fillers are zir-conium phosphate (Zr(HPO4)2·nH2O), phosphotungstic acid

376-7388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
oi:10.1016/j.memsci.2006.01.019

S. Sridhar et al. / Journal of Membrane Science 280 (2006) 202–209 203

(HPA) (H3PW12O40·nH2O) and silicotungstic acid (SiWA)(H4SiW12O40·nH2O).

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) is a wellknown polymer with a high glass transition temperature(Tg > 200 ◦C) and good mechanical strength. PPO is known toexhibit highest permeability to gases among many other aro-matic polymers, but its low selectivity is due to the absence ofpolar groups on the polymer backbone [20]. Hence, PPO hasbeen modified by different methods to improve its permselec-tive property [21–23]. Previous researchers have demonstratedthat selectivity of PPO could be enhanced by introducing polargroups through sulfonation, which would induce stronger inter-actions within the polymer matrix as well as between the mem-brane and polar feed gases such as CO2 [24–27]. The recentpublication by Hamad and Matsuura [27] reported a two-foldincrease in selectivity from 16.7 to 33.2 with a reasonablyhigh permeability of 58.8 for the sulfonated brominated PPOmembrane of 19.7% degree of sulfonation (DS). However, themajority of literature on modified PPO membrane is based onpermeability studies with pure gases, but a comprehensive studywith binary mixtures is lacking. The present study attempts tofulfill this requirement.

In this research, heteropolyacid (HPA), a known proton con-ducting agent, was evaluated as an additive to enhance the gaspermeability characteristics of high-performance PPO polymerfor the separation of industrially important CO /CH gaseousmtTDtgf

2

2

aCM

cHti2w

2

Pprn(a

Fig. 1. Chemical structures of (a) PPO and (b) SPPO membranes.

dust-free glass plate with a uniform thickness, and subsequentlydried in ambient condition (56% relative humidity) to obtain theHPA–PPO blend membrane.

2.1.2. Sulfonated polymer synthesis and membranepreparation

Sulfonation was carried out in chloroform at ambient con-ditions using chlorosulfonic acid as a sulfonating agent [28].About 10 g of the neutralized PPO was then added to 100 mLof neutralized chloroform taken in a three-necked round bottomreaction flask, and the mixture was stirred for about 30 min atambient temperature to form a 10 wt.% solution. A 5% (v/v)solution of chlorosulfonic acid prepared in 100 mL of chloro-form was transferred to a cone-shaped dropping funnel and partof it was gradually added to the polymer solution over a periodof 20 min, and the solution was stirred vigorously at ambienttemperature. The precipitated polymer SPPO was washed withdistilled water repeatedly and dried in air for 24 h at ambi-ent temperature followed by vacuum drying for about 48 h.Chemical structures of PPO and SPPO polymers are given inFig. 1.

2.2. Membrane characterization

2.2.1. FTIR studiesFTIR spectra of the membranes were scanned between 4000

at

2

ssgiip

2 4ixtures. Results obtained for PPO–HPA were compared with

hose obtained from the chemically modified PPO of 20% DS.he PPO-based membranes were characterized by FTIR, XRD,SC and SEM as well as free volume fraction (FVF) determina-

ion. Furthermore, the effect of feed composition and pressureradient on the performance of PPO membrane and its modifiedorms were investigated in detail.

. Experimental

.1. Materials and methods

Chloroform, methanol, sulfuric acid and phosphotungsticcid, H3PW12O40·6H2O (HPA), were purchased from Lobahemie, Mumbai, India. PPO polymer of Mn = 32000 and

¯ w = 244, 000 with a density of 1.06 g/cm3 at 25 ◦C was pur-hased from Aldrich Chemical Co. (Milwaukee, WI, USA).igh molecular weight grade was chosen keeping in view

he mechanical stability required during high-pressure exper-ments. Its Tg was 211 ◦C and melting temperature, Tm, was68 ◦C. Experimentally determined values of density and Tgere 1.014 g/cm3 and 215 ◦C, respectively.

.1.1. Preparation of PPO–HPA blendIt was found that HPA was highly soluble in methanol, while

PO was soluble in chloroform. HPA was blended with theolymer using a mixed solvent system of methanol and chlo-oform, since both the solvents are highly miscible. A homoge-eous solution of HPA (1.22 wt.%), PPO (6.9 wt.%), methanol4.41 wt.%) and chloroform (87.47 wt.%) was prepared at thembient temperature. This solution was spread over a clean

nd 400 cm−1 using a Perkin-Elmer-283B FTIR Spectropho-ometer.

.2.2. XRD studiesA Siemens D 5000 powder X-ray diffractometer was used to

tudy the solid state morphology of PPO and its modified ver-ions in powdered form. X-rays of 1.5406 A wavelength wereenerated by a Cu K� source. The angle of diffraction was var-ed from 0◦ to 65◦ to identify changes in crystal structure andntermolecular distances between intersegmental chains of theolymer matrix.

204 S. Sridhar et al. / Journal of Membrane Science 280 (2006) 202–209

2.2.3. Thermal analysisThermal stability of the polymer films was examined using a

Seiko 220TG/DTA analyzer from 25 to 700 ◦C at a heating rateof 10 ◦C/min with inert nitrogen gas flushed at 200 mL/min.

2.2.4. Mechanical strength analysisTensile strength and %elongation at break of the poly-

mer membranes were measured by using Universal TestingMachine (AGS-10kNG, Shimadzu, Japan). The films of thick-ness ∼0.2 mm, gauge length 50 mm and width of 10 mm werestretched at a crosshead speed of 20 mm/min.

2.2.5. Determination of ion exchange capacity and degreeof substitution

Ion exchange capacity (IEC) indicates the number of milli-equivalents of ions in 1 g of the dry polymer. The degree of sub-stitution (DS) indicates the average number of sulfonic groupspresent in the sulfonated polymer. To determine the degree ofsubstitution by acid groups, sulfonated membranes and unmodi-fied specimens of similar weight were soaked in 50 mL of 0.01Nsodium hydroxide solution for 12 h at ambient temperature.Then, 10 mL of the solution was titrated with 0.01N sulfuricacid [29]. The sample was regenerated with 1 M hydrochloricacid, washed free of acid with water and dried to a constantweight. The IEC was calculated as:

I

wtNmfaub

D

2

F

wvq

V

M

Cc

measured with an accuracy of ±0.001 by floatation method atambient temperature using the mixtures of ethylene glycol andwater [21].

2.3. Gas separation experiments

A permeability cell of stainless steel-316 was designed andfabricated indigenously. The effective membrane area in the cellwas about 10 cm2. Feed and permeate lines in the manifold weremade of 0.635 × 10−2 m (1/4 in.) SS piping connected togetherby means of compression fittings. The vacuum line consistedof a network of high vacuum rubber-glass valve connectionscapable of giving a pressure of as low as 0.05 Torr. Detaileddescription and process flow diagram of the set-up was reportedearlier [30]. The continuous flow method was used to carry outthe permeability studies.

2.3.1. Detailed experimental procedurePure gas permeabilities of CH4 and CO2 were determined

through the membranes by maintaining a constant pressure dif-ferential of 30 kg/cm2 across the membrane. For the binarymixtures of CO2 and CH4, both composition and pressure wereindividually varied keeping the other parameters constant. Allexperiments were performed at 30 ◦C. Feed and permeate lineswere evacuated by means of a vacuum pump (Hind High VacuumCstigsCbwEtotFipocu

K

wppf(p

α

EC = B − P × 0.01 × 5

m(1)

here IEC is the ion exchange capacity (mmol/g (meq./g)), Bhe sulfuric acid used to neutralize the blend sample soaked inaOH (mL), P the sulfuric acid used to neutralize the sulfonatedembranes soaked in NaOH (mL), 0.01 the normality of sul-

uric acid, 5 a numerical factor corresponding to the ratio ofmount of NaOH taken to dissolve the polymer to the amountsed for titration and m is the sample weight (g). The relationshipetween DS and IEC is:

S = 120IEC

1000 + 120IEC − 200IEC. (2)

.2.6. Determination of free volume fractionFVF was calculated according to the following equation [27]:

VF = 1 − Vwρ

M(3)

here ρ is the membrane density, Vw the van der Waals molarolume of the polymer and M is its molecular weight. Theseuantities are calculated using the equations:

w = 244000

120

[71.1

(1 − DS

100

)+ 96

(DS

100

)](4)

= 244000

120

[120

(1 − DS

100

)+ 198.9

(DS

100

)]. (5)

onstants in the above equations were determined by groupontribution method [27]. The density of the membranes was

o., Model ED-18, Mumbai, India). Feed gas was introducedlowly into the upper chamber by means of a mass flow con-roller (MFC), keeping the outlet valve closed until the dial gaugendicated the desired pressure. High purity (99.9%) grade nitro-en was used as a carrier to sweep the permeate to SS-316 gasample containers (capacity 100 mL) for subsequent analysis.arrier gas was introduced after 3 h of equilibration of the mem-rane with the feed gas to ensure steady state. Permeate sampleas collected for about 4–6 h at a low pressure of 0.5 kg/cm2.ven though back diffusion of N2 carrier gas was expected

o occur, it was assumed to be negligible since the pressuref sample collection in permeate side was very low comparedo the feed side pressures, which were as high as 30 kg/cm2.eed gas was analyzed before and after the experiment and

ts composition remained the same, which means that N2 backermeation into the feed chamber was minimum. Compositionf the feed and permeate streams were determined by a gashromatograph. The permeability coefficient, K was calculatedsing:

= Q

tA(P1 − P2)l (6)

here Q is the permeation volume of the gas [cm3 (STP)], t theermeation time (s), A the effective membrane area (cm2) for gasermeation, l the membrane thickness (cm) and P1 and P2 are theeed side and permeate side partial pressures (1.333224 × 103 PacmHg)), respectively. Selectivity was determined as the ratio ofermeability coefficients of the two gases:

= KCO2

KCH4

. (7)


2.3.2. Analytical procedureFeed and permeate compositions were determined with a

Nucon gas chromatograph (GC) equipped with a thermal con-ductivity detector (TCD) using a Haysep ‘Q’ packed columnof 2 m length and 0.3175 × 10−2 m (1/8 in.) i.d. The oven tem-perature was maintained at 50 ◦C, while injector and detectortemperatures were set at 150 ◦C each. Hydrogen at 0.9 kg/cm2

pressure was used as the carrier gas for analysis. The samplesize was 1 mL throughout. GC was calibrated for response fac-tors of the detector for CO2 and CH4 gases, which had differentthermal conductivity coefficients.

3. Results and discussion

3.1. Effect of blending ratio

HPA was incorporated in varying amounts (0–30 wt.%) intoPPO matrix. It was noticed that an increase in HPA content(>15 wt.%) in the blend renders the membrane brittle as evi-denced by the membrane stability test. Stability of the membranewas assessed by bending test. The membrane is considered tobe stable if its mechanical strength is restored after bending,i.e., it does not break upon bending. Hence, the polymer matrixcontaining 15 wt.% HPA was considered in the present studies.

3.2. Effect of sulfonation on membrane performance

wTpw

3

a

3

F9eiopbabcrpif

ps

Fig. 2. (A) FTIR spectra of (a) PPO, (b) PPO–HPA and (c) SPPO membranes.(B) FTIR spectra of pure HPA.

but a new peak at 520 cm−1 representing para-substitution hasoccurred. This is indicative of the attachment of sulfonic acidgroups at the para position of the benzene ring. The spectraof SPPO shows the SO3 symmetric stretching vibrations in theregion 1000–1060 cm−1. The peak observed at 1360 cm−1 isdue to asymmetric stretching of S O bond. Symmetric vibra-tion of this bond produced a characteristic split band around1150–1185 cm−1.

3.3.2. XRD studiesFig. 3(a–c) shows the wide-angle X-ray diffractograms of

PPO, PPO–HPA and SPPO polymers. XRD pattern of PPO(a) shows sharp peaks at 2θ = 10◦ and 20◦, indicating the crys-talline nature of the polymer. The XRD patterns of PPO–HPA (b)exhibits few sharp peaks at 2θ = 10◦ and broad peaks at 2θ = 20◦,indicating that HPA added to PPO did not exist in a crystal form,but it could be in an amorphous state. Thus, HPA may be presentas agglomerates in PPO in some regions as well as finely dis-

PPO was sulfonated from 0 to 60%. Sulfonated membranesere tested for their stability under a pressure of 30 kg/cm2.he sulfonated membranes with DS > 20% failed to withstandressures due to a loss in mechanical strength. Hence, SPPOith DS = 20% was considered in all our experiments.

.3. Membrane characterization

Membranes were characterized by FTIR, XRD, DSC, SEMnd tensile testing techniques.

.3.1. FTIR studiesFTIR spectra of PPO, PPO–HPA and SPPO are shown in

ig. 2A. Characteristic bands of HPA are observed at 792, 887,81 and 1080 cm−1, respectively, in Fig. 2B. Test conditionsmployed, i.e., 140 ◦C after vacuum drying, should generate annfrared spectrum of pure HPA that represents its stable sec-ndary structure, which contains six protonated water moleculeser polyanion. Specific interactions in the PPO–HPA polymerlend represent one of the most important factors influencingggregation, physico-chemical and mechanical properties of thelend membrane. Fig. 2A(b) pertaining to PPO–HPA shows theharacteristic band of HPA at 1080 cm−1, implying that HPAetains its Keggin-type structure even after blending with theolymer. Other three bands coincide with the bands of PPO itself,ndicating no chemical interaction between HPA and PPO; thisurther suggests that interactions are mostly physical in nature.

From the FTIR spectra of SPPO shown in Fig. 2A(c), a sharpeak observed at 780 cm−1 corresponding to mono-substitutioneen in Fig. 2A(a) for the unmodified PPO has disappeared,


Fig. 3. XRD diffractograms of (a) PPO, (b) PPO–HPA and (c) SPPO.

persed particles in other regions of the polymer matrix. Thisis attributed to the blending pattern of PPO with HPA, whichrequires the usage of two miscible solvents. The diffractogramof SPPO (c) shows two broad peaks around 10◦ and 20◦ of 2θ,indicating the amorphous nature of the sulfonated polymer.

3.3.3. DSC resultsDSC tracings of PPO and SPPO are shown in Fig. 4. The Tg

value decreased from 215 ◦C for PPO (a) to 140 ◦C for SPPO (b).Sulfonic groups attached to aromatic ring of the PPO backboneare not thermally stable and they could decompose at 175 ◦C oreven at lower temperatures [26]. Higher the degree of substitu-tion, greater would be the free volume of the sulfonated product,thus enabling a change in the state of the polymer from a morecrystalline state to a more amorphous state, thereby resulting ina reduction of Tg. The reduction in Tg may be attributed to theplasticization by sorbed water at ambient humidity occurringdue to strong interaction between polar groups of the chains andwater vapor present in the air. However, the Tg value is still high

Fig. 4. DSC tracings of (a) PPO and (b) SPPO membranes.

enough for employing SPPO as a membrane for the removal ofCO2 from natural gas mixtures, which are generally available at45–50 ◦C at off-shore wells.

3.3.4. SEM studiesFig. 5 shows SEM micrographs of PPO (a) and PPO–HPA

(b) containing 15 wt.% of HPA. A distinctive difference can beseen between the surface morphologies. The HPA-free poly-mer appears dense and homogeneous. The surface of PPO–HPAblend shows clusters in few regions, indicating that HPA is notonly partially dispersed as agglomerates, but also is concentratedmore on the surface of the polymer matrix. Thus, HPA exists asagglomerates in some parts and is distributed uniformly in otherparts of the polymer surface [31]. When HPA is blended alongwith PPO, the highly acidic groups of HPA increase intermolec-ular interactions through the polar ionic sites, which is calledan ionomer effect. However, due to high volatility of the sol-vent, ion sites in the ionomers aggregate into clusters and willremain as agglomerates in some parts of the polymer matrix.Undoubtedly, there might be some different and complex inter-

O an
Fig. 5. SEM pictures of (a) PP d (b) PPO–HPA membranes.


Table 1Single gas permeability and selectivity of PPO membrane and its modified forms(feed pressure = 30 kg/cm2)

Membrane Permeability (Barrera) Selectivity P (CO2/CH4)

CO2 CH4

PPO 43.7 3.6 12.1PPO–HPA 28.2 1.36 20.6SPPO 18.4 0.67 27.2

a 1 Barrer = 10−10 cm3 (STP) cm/cm2 s cmHg.

actions between blending components depending upon the kindof polymer material used. However, the fundamental reasons forthis non-uniform dispersion have not been fully elucidated yet[32]. The SEM micrograph of PPO–HPA blends corroborateswell with the XRD results.

3.3.5. Mechanical strengthTensile strength studies were conducted to assess the mechan-

ical stability of all the polymers before and after modification.From UTM studies, it was noticed that the unmodified PPOmembrane exhibited a tensile strength of 176.8 N/mm2 with anelongation at break of 13.45%. The physically modified PPO,i.e., HPA–PPO exhibited a tensile strength of 173.3 N/mm2 and%elongation of 12.78, whereas the chemically modified form,i.e., sulfonated PPO caused a reduction in tensile strength to146.6 N/mm2 and %elongation at break to 5.67. Therefore, boththe modified forms of PPO showed relatively lower mechanicalstability than the unmodified polymer membrane.

3.4. Gas permeation experiments

3.4.1. Single-gas permeation behavior of CO2 and CH4

From the values of pure gas permeability and ideal selectivitydetermined at 30 kg/cm2 (Table 1), it is evident that of the threemembranes employed, the unmodified PPO membrane showedtioP(smtbwTaobdismh0o

Table 2Free volume fractions of PPO and sulfonated PPO membranes

Polymer ρ (g/cm3) Vw (cm3/mol) M (g/mol) FVF

PPO 1.014 144570 244000 0.399SPPO 1.152 155672 276086 0.350

again, hydrogen bonding interactions between –SO3H groups ofthe adjacent chains are responsible for increased compactness,which explains the reduction in gas permeability. Therefore, inthis study we have restricted the degree of sulfonation to anoptimum level of 20% [24].

3.4.2. Effect of feed concentrationPPO and its modified membranes were tested for the separa-

tion of CO2/CH4 binary mixtures at 30 ◦C by varying the feedCO2 concentration between 5 and 40 mol% at the constant pres-sure of 30 kg/cm2. Flux and selectivity data observed for thebinary gas mixtures were expectedly lower than those obtainedwith the pure gases owing to the reduced partial pressure of CO2gas. From Fig. 6, it is observed that an increase in CO2 con-centration from 5 to 40 mol% caused a corresponding increasein flux of the unmodified PPO membrane from 10.36 × 10−5

to 84.6 × 10−5 cm3/cm2 s. Sulfonated and HPA incorporatedmembranes have shown similar trends with the SPPO mem-branes by exhibiting an increase in flux from 4.3 × 10−5 to39.5 × 10−5 cm3/cm2 s, while the PPO–HPA membrane exhib-ited an enhancement from 6.5 × 10−5 to 60.0 × 10−5 cm3/cm2 s.The rise in flux may be attributed to the increasing partial pres-sure gradient of the preferentially permeating CO2 gas, whichis the driving force for gas transport across the membrane [1].

The corresponding enhancement in selectivities is displayedin Fig. 7. The electronegative oxygen atoms in CO are expectedttpgtt

F

he highest permeability of 43.7 Barrer with the lowest selectiv-ty of 12.1. On the other hand, SPPO gave the highest selectivityf 27.2, which is 2.2 times higher than that of the unmodifiedPO. The PPO–HPA membrane showed a lower permeability28.2 Barrer) as compared to the unmodified PPO, but a higherelectivity of 20.6. Improvement in the performance of both theodified membranes in terms of selectivity may be attributed

o the availability of large number of polar sites in the mem-ranes for the sorption of CO2 gas [26]. The CO2 gas interactsith the polar HPA filler particles through hydrogen bonding.he other component CH4 has a low affinity for HPA particlesnd prefers to move into the bulk polymer rather than adsorbn the filler surface. However, the filler acts as an obstacle foroth CO2 and CH4 gases by determining a more tortuous pathuring the permeation process [19]. Even though HPA load-ng was only 15 wt.% of the polymer matrix, SEM studies havehown that most of the particles remain on the surface of theembrane itself, which reduces the permeability. On the other

and, in case of SPPO, a reduction in free volume fraction from.39 to 0.35 is expected to occur due to an increased stiffeningf the polymer backbone upon sulfonation (see Table 2). Once

2o hydrogen-bond with the hydrogen atoms present in phospho-ungstic acid of HPA–PPO blend and the –SO3H group of SPPOolymer. However, increasing feed concentration brings aboutreater sorption of the gas in the polymer membrane due tohe availability of more number of CO2 molecules for interac-ion with the membrane. Furthermore, the permeation of CH4

ig. 6. Variation of CO2 flux with feed concentration (feed pressure: 30 kg/cm2).


Fig. 7. Variation of membrane selectivity with feed concentration (feed pressure:30 kg/cm2).

molecules would be impeded due to increasing polarization ofCO2 molecules near the membrane surface. SPPO gave a rea-sonable selectivity of 5.2 at 5 mol% of CO2 which increasedto 13.5 at 40% feed concentration. The selectivity values forPPO–HPA and unmodified PPO were in the range 3.7–9.5 and2.1–5.8, respectively.

3.4.3. Effect of feed pressureFig. 8 displays the relationship between CO2 permeability

and feed pressure for the three membranes tested at a constanttemperature of 30 ◦C and feed composition of 5% CO2 and 95%CH4. It was noticed that unmodified PPO membrane exhibiteda reduction in CO2 permeability from 4.7 to 2.5 Barrer withincreasing feed pressure from 5 to 40 kg/cm2. PPO–HPA andSPPO membranes have shown similar trends of reduction in per-meability from 3.2 to 1.6 and 2.1 to 1.2 Barrer, respectively. Incase of semi-crystalline polymer membranes sorption is knownto occur in the amorphous phase, which consists of microvoids.

F5

Fig. 9. Effect of varying feed pressure on membrane selectivity (feed composi-tion: 5% CO2 + 95% CH4).

However, with rising pressure, sorption of the secondary com-ponents, such as CH4, becomes more and more competitive inthese voids, which results in the exclusion of CO2 molecules[3]. Thus, membrane selectivity dropped gradually from 3.3 to2.0 for PPO, 5.5 to 3.6 for PPO–HPA and 7.3 to 5.1 for SPPOmembrane as revealed in Fig. 9.

4. Conclusions

In this research, PPO was successfully modified physically byincorporating HPA filler and chemically by sulfonation. Keepingthe mechanical strength in view, the concentration of HPA anddegree of sulfonation were restricted to 15 and 20 wt.%, respec-tively. Incorporation of an inorganic filler into the PPO matrixas well as modification by sulfonation rendered the polymeramorphous. These effects were confirmed by XRD. All the mem-branes exhibited good thermal and mechanical stability duringthe experimental conditions. Studies with single gas as well asbinary mixtures showed that SPPO membranes gave the highestselectivity, while unmodified PPO membrane was the most per-meable. It is further demonstrated that incorporation of inorganicmaterial such as HPA could dramatically increase the CO2 selec-tivity of the membrane due to interactions between polar sites ofthe membrane and the gas molecules. Increasing the CO2 feedconcentration had a positive effect on flux and selectivity duetscbttfis

A

C
ig. 8. Effect of varying feed pressure on CO2 permeability (feed composition:% CO2 + 95% CH4).
o increasing partial pressures, whereas a rise in feed pressurehowed a negative impact on membrane performance due to theompetitive sorption of CH4 molecules. Modified PPO mem-ranes of this study have shown a good potential to separatehe CO2/CH4 mixtures from natural gas/landfill gas purifica-ion sites. The preparation of highly selective SPPO as a thinlm blend membrane would be ideal for commercial applicationince the drawback of low permeability would be overcome.

cknowledgements

Professor T.M. Aminabhavi is thankful to University Grantsommission (UGC), New Delhi, for a major support to establish


Center of Excellence in Polymer Science (CEPS) at KarnatakUniversity. We also thank Mr. B. Sridhar of CESCA at IICTfor his help in membrane characterization and Mr. Saibabu ofDesign Cell for figure tracings. The research is performed underthe MoU between CEPS and IICT.

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Modified poly(phenylene oxide) membranes for the separation of carbon dioxide from methane