je060208l

Pervaporation Properties of Polyimide Membranes for Separation of Ethanol+ Water Mixtures

Yexin Xu,?Cuixian Chen,*,?Pengxia Zhang,?Benhui Sun,?and Jiding Li?

Department of Chemical Engineering,Tsinghua University,Beijing100084,People’s Republic of China,and School of Material Science and Engineering,Beijing University of Chemical Technology,Beijing100029,People’s Republic of China Twelve kinds of polyimide membranes have been prepared using three dianhydrides(including2,2-bis[4-(3,4-

dicarboxyphenoxy)phenyl]propane dianhydride(BPADA),3,3′4,4′-benzophenonetetracarboxylic dianhydride

(BTDA),and3,3′4,4′-biphenyltetracarboxylic dianhydride(ODPA))and four diamines(including benzidine(BZD),

bis(4-aminophenyl)phenyl phosphate(BAPP),4,4′-diaminodiphenylmethane(MDA),and4,4′-diaminodiphenyl

ether(ODA))via a two-step method.The polyimides were characterized by FT-IR,DSC,and wide-angle X-ray

diffraction(WAXD).The permeation experiments of water+ethanol mixtures through12polyimide membranes

were carried out at333K.The temperature dependence of pervaporation performances of ODA-based polyimide

membranes is also investigated.The flux of ethanol+water mixtures through the polyimide membranes with the

same dianhydrides increases following the order of BZD

with increase in temperature and the relationships between the flux and temperature can be described by the

Arrhenius equation.According to the Arrhenius equation,the active energies of water+ethanol mixtures in

BTDA+ODA,BPADA+ODA,and ODPA+ODA membranes are(19.3,26.0,and30.6)kJ?mol-1,respectively.

However,the relationship between the separation factor and the temperature is not so clear.In addition,the

natural logarithm of flux J(ln J)increases linearly with the mean interchain distance.

Introduction

Pervaporation(PV)is one of the membrane-separation process involving the partial vaporization of a liquid mixture through a dense membrane whose downstream side is usually kept under vacuum.1In general,separation by PV can be performed using membranes based on the solution-diffusion mechanism of transport.The mass transport across the permselective mem-branes involves three successive steps,that is,(i)sorption of the penetrant from the feed to the membrane,(ii)diffusion of the penetrant in the membrane,and(iii)desorption of the penetrant from the membrane on the downstream side of the membrane.2The steady-state mass transport regime depends on several parameters such as upstream pressure,downstream pressure,temperature,film thickness,properties of solvent molecule,and membrane materials.Consequently,the mass transport through a dense polymer membrane is a complex process.

Polyimides are a class of thermally stable polymers that are often prepared from dianhydride and diamine monomers.They are usually prepared by the so-called two-step method in which a dianhydride and a diamine are allowed to undergo condensa-tion polymerization to form a polyamic acid precursor.Subse-quently,the precursor is converted thermally or chemically to the final polyimide.They have a number of outstanding properties such as high thermal resistance,dimension stability, excellent electrical,and mechanical properties3,4and have been widely used in applications such as aircraft parts,electronic packaging,adhesives,and membranes,especially the vapor separation membranes.5,6In the development of polyimides as vapor separation membranes,particularly for dehumidification of gases,air,and organic vapors,polyimide membranes with high water vapor permeability have attracted great interest.In contrast,there have been a few reports on the use of aromatic polyimide membranes in pervaporation.7

In the present work,12kinds of polyimide membranes have been prepared using three dianhydrides(including2,2-bis[4-(3,4-dicarboxyphenoxy)-phenyl]propane dianhydride(BPADA), 3,3′4,4′-benzophenonetetracarboxylic dianhydride(BTDA),and 3,3′4,4′-biphenyltetracarboxylic dianhydride(ODPA))and four diamines(including benzidine(BZD),bis(4-aminophenyl)phenyl phosphate(BAPP),4,4′-diaminodiphenylmethane(MDA),and 4,4′-diaminodiphenyl ether(ODA))via a two-step method.The polyimides were characterized by FT-IR,DSC,and wide-angle X-ray diffraction(WAXD).We also carry out the PV experi-ments of ethanol+water mixtures through12kinds of dense polyimide membranes at333K to study the influence of polymer structure on the PV performances of membranes.In addition,the temperature dependence of PV performances of some polyimide membranes is investigated.

Experimental Section

Materials.All monomers were purchased from Acros Organ-ics Inc.and Fluka Chemical Corp.BTDA,BPADA,ODPA, ODA,MDA,BAPP,and BZD were purified before polymer-ization.The BPADA,BTDA,and BAPP monomers were purified in a vacuum oven at different temperatures.The ODPA, BZD,MDA,and ODA monomers were purified by recrystal-ization.N,N-Dimethylformamide(DMF)was obtained from Beijing Yili Chemicals Co.Ltd.,purified on distillation under reduced pressure over calcium hydride,and stored over mo-lecular sieves(4?).Two penetrants(i.e.,water and ethanol) were also obtained from the Beijing Yili Chemicals Co.Ltd.,

*Corresponding author.Tel.:+86-10-62773234.Fax:+86-10-62770304. E-mail:cxchen@https://www.wendangku.net/doc/7a3007869.html,.

?Tsinghua University.

?Beijing University of Chemical Technology.1841

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10.1021/je060208l CCC:$30.25?2006American Chemical Society

Published on Web08/04/2006

and they were of reagent grade.All the monomers of polyimides are shown in Figure 1.The structure of ODPA +ODA as an example polyimide is also shown there.

Preparation of Polyimide Membrane.The polyimide mem-brane was prepared by the so-called two-step method (Figure 1).The polyamic acid (PAA)precursors of polyimides were prepared by solution condensation polymerization at ambient temperature and at a composition of 12%solids by mass DMF.The aromatic dianhydride monomer was added to an equimolar amount of aromatic diamine dissolved in the solvent all at once.The obtained PAA solution was cast onto a soda-lime glass plate to form a casting membrane.Then the casting film was placed into a vacuum dryer for 24h to remove some residual solvent.Thermal conversion of the PAA membranes to the corresponding polyimides was performed by heating in a vacuum oven for 2h each at (453,523,and 573)K.After cooling,the membranes were removed from the glass plates.Finally,the homogeneous polyimide membranes with uniform thickness were obtained.In this study,the thicknesses of polyimide membranes were all (14to 16)μm.

Characterization.The physical property parameters of poly-imide membranes are presented in Table 1.The density of polyimide membrane at 298K was measured by flotation in a mixed solution.The solution was prepared by ethanol (F )0.79g ?cm -3)and carbon tetrachloride (F )1.599g ?cm -3).The glass transition temperature (T g )of polyimide was determined by differential scanning calorimetry (DSC)with a Seiko EX-STAR6000instrument at a heating rate of 10K ?min -1,from

(298to 673)K.WAXD patterns were measured by a Bruker D8wide-angle X-ray diffractometer using Cu K R radiation wavelength,and the WAXD pattern of the BTDA +ODA polyimide is displayed in Figure 2.The top of broad peaks on each X-ray pattern for noncrystalline polymers is attributed to intersegmental interference and is thus a representative of average intersegmental distance called as the mean interchain distance (d -spacing).The d -spacing can be calculated from Bragg’s equation.8Free volumes are calculated by Bondi’s model of group contribution,and the parameters and the details have been described elsewhere.

9

Figure 1.Structures of polyimide monomers.

Table 1.Physical Property Parameters of Polyimide Membranes

F

T g M a d -spacing

V m b polyimide g ?cm -3K g ?mol -1?cm 3?mol -1BTDA +BZD 1.386558470.39 4.1237.10BTDA +ODA 1.373544486.39 4.3242.60BTDA +MDA 1.337531484.39 4.9247.33BTDA +BAPP 1.308522690.61 5.2446.30ODPA +BZD 1.374558458.37 4.9230.90ODPA +ODA 1.348538474.37 5.0236.00ODPA +MDA 1.306534472.43 5.1241.13ODPA +BAPP 1.307531678.60 5.3440.10BPADA +BZD 1.339525662.65 5.7403.90BPADA +ODA 1.325506678.66 6.1409.40BPADA +MDA 1.297502676.64 6.4414.13BPADA +BAPP

1.279

488

870.82

6.9

532.20

a

M is the molar weight of the repeat unit.b V m is the molar van der Waals volume of the repeat unit calculated from the group-contribution method.9

1842Journal of Chemical and Engineering Data,Vol.51,No.5,2006

The polyimides were analyzed by Fourier transform infared (FT-IR)spectra and were recorded on a Nicolet IR560spec-trometer with polymer membrane.Spectra in the optical range of (400to 4000)cm -1were obtained by averaging 32scans at a resolution of 4cm -1.The IR spectra of BTDA-based polyimide membranes as examples are shown in Figure 3.Polyimide membranes showed representative imide carbonyl peaks,but no distinct amide groups.This indicated that the polyimides had been fully imidized.

Permeation Experiments.The permeation apparatus of liquid feeds through flat-sheet membranes is shown schematically in Figure 4.In the membrane cell,the membrane sample was supported on a porous sintered stainless plate,and its active surface area is 21.35cm 2.Vacuum was produced by a vacuum pump (model 2X-15from Beijing Equipment Factory)in the permeation compartment of the membrane cell.In each per-

meation test,the feed solution was circulated between the membrane cell and a large feed vessel of about 3.5L,which kept the feed at a constant temperature.The steady state of the permeation experiment was reached after 3h.Then the permeate sample was drawn out by the vacuum pump and collected by a cold trap cooled with liquid nitrogen once an hour and then analyzed by the gas chromatogram (GC-14CPTF,SHIMADZU Co.,Japan)to obtain the mass fraction of each penetrant component.The PV performances were evaluated by the separation factor (R )and the permeation flux (J ).The separation factor is defined by

where Y 0and Y w represent the weight fractions of organic solvent and water in the permeate,and X 0and X w are those in the feed,respectively.The permeation flux (J )was calculated using the expression:

where Q is the total mass of permeate collected through the effective area of membrane (A )during time T after the steady state has been reached.

In this study,the liquid feed is the ethanol +water mixtures,and the mass fraction of ethanol is 88.9%.The permeation experiments of all membranes at 333K are carried out to investigate the influence of polyimide structure on the PV performance of membranes.Moreover,the permeation experi-ments of ODA-based polyimide membranes are carried out at (323,333,338,and 343)K,respectively,to study the temper-ature dependence of pervaporation performance of membranes.The permeation experiment of a polyimide membrane at each temperature is repeated three times,and pressure in the permeation compartment is 450Pa.

Results and Discussion

PV is a complex membrane-based separation process affected by many factors such as the physicochemical properties of the mixtures to be separated,the chemical nature of the macromol-ecules that comprise the membrane,and the physical structure of the membrane.Table 1presents the physical properties of 12polyimide membranes.It has been shown that the free volume values of polyimide with the same dianhydride increase in the following order:BZD

The values of steady permeation flux and separation factor of water +ethanol mixtures in 12kinds of polyimide mem-branes at 333K are listed in Table 2.It can be seen that,with the same diahydrides,the flux of solvents increases following the order of BZD

for

Figure 2.WAXD pattern of BTDA +ODA polyimide (RI denotes the relative

intensity).

Figure 3.IR spectra of BTDA-based polyimide membranes (T denotes the transmittance):1,BTDA +MDA;2,BTDA +BZD;3,BTDA +BAPP;4,BTDA +

ODA.

Figure 4.Flowsheet of steady permeation experiments.1,vacuum pump;2,permeate collector;3,cold trap;4,polyimide membrane;5,feed pump;6,feed;7,thermostat (V1-3,valves).

R )

Y w ?X 0X w ?Y 0

(1)

J )

Q A ×T

(2)

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BPADA-based polyimides,and those of ethanol+water mixtures in BTDA-and ODPA-based polyimide membranes increase following the order:MDA

will result in high selectivity,and loose packing of segment will be good for increasing the flux.The mobility and flexibility of BZD-based polyimide chains are weak because of the symmetric biphenyl group,so the polymer chains are difficult to rotate.BZD-based polyimides exhibit the highest T g as shown in Table1.All of these factors hinder the diffusion of small molecules through the polymer membranes and lead to the decrease of the flux.Consequently,the BTDA+BZD poly-imide membrane with strong rigid chains shows the highest selectivity and the lowest flux as shown in Table2. Furthermore,the ODA-based polyimide membranes was chosen to carry out the permeation experiments of ethanol+ water mixtures at different temperatures in order to investigate the temperature dependence of PV performances.As shown in Table3,the flux through all the membranes increases with increase in the feed temperature,while the correlation between separation factor and temperature is not so clear.The mobility of polymer chain segment increases with increase in temperature, and it will increase the flexibility of polymer chains as well as the free volume of polymer.All of these will increase the diffusion of small molecules through the polymer membrane. In general,the relationship between the permeation flux and temperature can be described by an Arrhenius type relationship: where J is the permeation flux,J0is a constant,T is the temperature of liquid feed,R denotes the universal gas constant, and E a is the activation energy.Clearly,there is a linear relationship between ln J and1/T as shown in Figure5,and E a can be obtained from the slope of the straight line.Table4lists the values of activation energy.It can be seen that the activation energy of solvents in ODPA-based polyimides is larger than that of solvents in BTDA-and BPADA-based polyimides. Consequently,the flux of solvents in ODPA-based polyimide membranes increases rapidly while that of solvents in BTDA-and BPADA-based polyimide membranes increases more slowly with an increase in temperature,as shown in Table3and Figure 5.

In addition,the gas permeability of polymer membranes in gas separation process is influenced not only by the temperature but also by other properties such as the glass transition temperature,d-spacing,and the free volume of the polymers. Similarly,we investigated the relationship between the PV performance and d-spacing of polyimide membranes.In a glassy polymer,the d-spacing calculated from the data of WAXD offers an index of the distance between neighboring segments.In some polymers,gas permeability was discussed with the d-spacing. O’Brien et al.10and Charati et al.11discussed a correlation of d-spacing and diffusivities for CO2and CH4and suggested that d-spacing might correlate with diffusion coefficient and perme-ability of the penetrants in membranes.Values of d-spacing measured in this study are summarized in Table1,and the correlation between ln J and d-spacing is shown in Figure6. Clearly,ln J increases linearly with d-spacing.The result maybe implies that the enough distance between neighboring segments is the prerequisite for transport behavior of small molecules in polymer membranes.The d-spacing expresses the mean distance

Table2.Values of J and r of Water+Ethanol Mixtures in Polyimide Membranes at333K

J

membrane R g?m-2?h-1 BTDA+BZD234110.8

BTDA+ODA70318.9

BTDA+MDA12821.0

BTDA+BAPP27730.5

ODPA+BZD159415.2

ODPA+ODA29920.4

ODPA+MDA13327.5

ODPA+BAPP17538.1

BPADA+BZD127218.4

BPADA+ODA58626.6

BPADA+MDA16632.6

BPADA+BAPP9241.6 Table3.Temperature Dependence of Pervaporation Performances of Polyimide Membranes

T J membrane K R g?m-2?h-1 BTDA+ODA323162415.7

33370318.9

33852221.6

34360823.8 ODPA+ODA323101115.1

33329920.4

33828525.8

34329029.0 BPADA+ODA32338020.8

33358326.6

33833832.5

34311836.1

J)J

0exp(-E a RT)

(3)

Figure5.Temperature dependence of the flux J for water+ethanol

mixtures in polyimide membranes:9,BTDA-based polyimide;9,BPADA-

based polyimide;2,ODPA-based

polyimide.

Figure6.Relation of flux for water+ethanol and d-spacing of polyimide

membranes at333K:9,BTDA-based polyimide;2,BPADA-based

polyimide;O,ODPA-based polyimide.

Table4.Values of Activation Energy

membrane BTDA+ODA ODPA+ODA BPADA+ODA

E a/kJ?mol-119.330.626.0

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of the neighboring segments and can influence the PV perfor-mance of polymer membranes.

Conclusions

Twelve polyimide membranes have been prepared using three dianhydrides(including BPADA,BTDA,and ODPA)and four diamines(including BZD,BAPP,MDA,and ODA)via a two-step method,and the polyimides were characterized by FT-IR, DSC,and WAXD.The steady permeation experiments of water +ethanol mixtures through12polyimide membranes were

carried out at333K.The temperature dependence of PV performances of ODA-based polyimide membranes is also investigated.

According to the experimental results,the flux of ethanol+ water mixtures through the polyimide membranes with the same dianhydrides increases following the order of BZD

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Received for review May11,2006.Accepted June30,2006.The authors greatly appreciate the financial support of the Major State Basic Research Program of China(973Program)(2003CB615701),the National Natural Science Foundation of China(20576059),the SINOPEC Foundation(X505002),and the CNPC Innovation Foundation.

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