Structure–properties relationship for the gas transport properties of new fluoro-containing aromatic polymers

Journal of Membrane Science 385–386 (2011) 277–284

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Structure–properties relationship for the gas transport properties of new fluoro-containing aromatic polymers ˜ b,∗ , M. Teresa Guzmán-Gutiérrez a , M. Humberto Rios-Dominguez b , F. Alberto Ruiz-Trevino a a c,∗∗ d Mikhail G. Zolotukhin , Jorge Balmaseda , Detlev Fritsch , Evgen Prokhorov a

Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, CU, Coyoacán, 04510 México D.F., Mexico Departamento de Ingeniería y de Ingeniería y Ciencias Químicas, Universidad Iberoamericana, Prol. Paseo de la Reforma No. 880, 01219 México D.F., Mexico c Institute of Materials Science, Helmholtz-Zentrum Geesthacht, Centre for Materials and Coastal Research GmbH, Geesthacht, Germany d CINVESTAV del IPN, Unidad Querétaro, Libramiento Norponiente 2000, Fracc. Real de Juriquilla, C.P. 76230, Querétaro, Mexico b

a r t i c l e

i n f o

Article history: Received 24 August 2011 Received in revised form 7 October 2011 Accepted 9 October 2011 Available online 13 October 2011 Keywords: 3F aromatic polymers Gas transport properties Structure–properties relationship

a b s t r a c t Super acid catalysis offers the syntheses of a rich variety of similar but different polymer structures otherwise not easily attainable. By this synthetic method, a set of 22 polymers was prepared and their gas permeability coefficients for pure gases, with their associated ideal selectivity, were measured. An analysis of their selectivity and permeability combination protocol revealed that their membranes have combinations, specifically for the O2 /N2 , CO2 /CH4 and CO2 /N2 separations, that are in the same order of magnitude as those reported for the polysulfone and some polyimide families. For CO2 /N2 separations, polymers CF3 PhSO3 H-T and CF3 Ph-T offer membranes with CO2 permeability coefficients of 83 and 220 Barrer, with corresponding selectivity of 31 and 24. In addition, by the systematic incorporation of Me, Ph, flouro atoms or CF3 groups into their chemical structure it is possible to understand the structure/properties relationship of gas transport for this series of polymers. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Membrane separation processes for gas mixtures can be a cost effective alternative of separation in competition to other, well established processes [1]. By its modular process design and the choice of membrane type this separation process is applicable either to small or even very large process streams. Besides the entire process design the basic permselectivity of the applied membrane is crucial for the overall effectiveness of the separation process. Improved membrane materials may lead to increase the efficiency level of membrane separation processes. As shown by assembling and interpreting all available data on gas permeation polymer materials [2,3] within roughly two decades promising new materials appeared. As a result, the so-called upper bound was shifted to higher permeability as well as selectivity. So as to develop target-oriented improved materials for gas separation the understanding of the structure properties relationship [4] is expected to be a straight forward guideline to new materials. Moreover, most

suited polymer materials for membrane applications have to have ‘good’ mechanical properties that may be simply summarized in the ability of forming flexible, free-standing films. The recently developed method of super acid catalyzed polyhydroxyalkylation [5–9] offers a rich variety of fluorine-containing structures otherwise not obtainable. Those polymers exhibit high molecular weights at low or medium polydispersity accompanied by very well suited mechanical properties as mandatory for good membrane materials. The incorporation of fluorine atoms or groups that contain fluorine atoms increases polymer solubility, glass transition temperatures, thermal stability, and chemical resistance, while moisture absorption, dielectric constant and color are decreased [10]. In this paper we selected similar polymers containing fluorine in their structures to report on their permselectivity and discuss their structure–properties relationship also in relation to known polymers. 2. Experimental 2.1. Materials

∗ Corresponding author. Tel.: +52 55 5950 4389; fax: +52 55 5950 4225. ∗∗ Corresponding author. ˜ E-mail addresses: [email protected] (F.A. Ruiz-Trevino), [email protected] (D. Fritsch). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.10.009

Solvents: N-methylpyrrolidone (NMP), N,N-dimethylacetamide (DMAc), tetrahydrofuran (THF), chloroform, methanol (MeOH).

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2.2. Polymer synthesis

2.4. Gas permeation measurement

A simple synthetic method was developed recently [8] and it is in short outlined below. At room temperature conditions inactive fluoroketones or fluoroaldehydes reacts with non activated aromatic hydrocarbons that are activated for polycondensation by the Brønsted suparacid trifluoromethanesulfonic acid (CF3 SO3 H, TFSA). TFSA may be used as pure solvent or diluted by chlorinated solvents, e.g., dichloromethane. As an example, in a typical synthesis to produce CF3 Me-B a 10 mL three-neck flask equipped with a mechanical stirrer was charged with trifluoroacetone (0.504 g, 4.49 mmol), biphenyl (0.6936 g, 4.49 mmol) and dichloromethane (3.3 mL). The solution was cooled to 5 ◦ C, and TFSA (3.3 mL) was added in one portion to the solution and the reaction mixture was stirred for 30 min. Thereafter, the temperature was raised to 20 ◦ C over 1 h and reaction was continued at this temperature for 25 h. The resulting dark-brown, gel-like mass was then shredded and poured slowly into methanol. The precipitated, white solid was filtered, extracted with refluxing methanol and dried in air overnight and at 100 ◦ C under vacuum. The resulting pure white fibrous polymer CF3 Me-B (0.971 g, 87% yield) had an inherent viscosity of 2.57 dL/g when a 0.2 wt% polymer solution in 1-methyl2-pyrrolidone was measured at 25 ◦ C in an Ubbelohode viscometer. All other polymers, with film forming properties well-suited for polymer membranes, were synthesized under the same conditions and reported number-average molecular weights, Mn , and poly dispersive index, PDI, as determined by gel permeation chromatography (GPC-MALLS) of polymer solutions in THF, in the range of 20,000 < Mn (g/mol) < 610,000; 1.3 < PDI > 3.7.

The solvent-free polymer films were tested for pure gas permeation in two constant-volume permeation cells following well-standardized procedures in two different laboratories. For the membranes cast from the high boiling solvents, the permeability coefficients were determined at 1 bar upstream pressures and 30 ◦ C using an effective area of 13.2 cm2 . For the polymer membranes cast from the low boiling CHCl3 , the permeability coefficients were determined at 2 bar and 35 ◦ C using an effective area of 1.3 cm2 . In every permeation cell, the permeate pressure increase with time was recorded at the corresponding temperature and the permeability coefficients for each gas, P(i), were calculated from the slope of the permeate pressure versus time data in the steady-state region (usually between 4 and 8 times of the observed experimental time lag). Apparent diffusion coefficients, D(i), were estimated from the time-lag  by D(i) = l2 /6 (l being the film thickness) and are given in 10−8 cm2 /s. Apparent solubility coefficients, S(i), were calculated from S(i) = P(i)/D(i) and are given in 10−3 cm3 /cm3 cm Hg. Ultrahigh purity gases H2 , He, O2 , N2 , CH4 and CO2 were measured in that order.

2.3. Membrane preparation and fractional free volume characterization Polymer films for permeation measurements were cast following two well-known casting procedures. In the first procedure, homogeneous, dense or pore-free polymer films, 30–90 ␮m thickness, were cast from 5 wt% polymer solutions in NMP, DMAc or THF. After solvent evaporation and drying, the residual high boiling temperature solvents were removed from all membranes by solvent exchange immersing the membranes into MeOH for 24 h. TGA measurements were used to check for residual solvent. All MeOH treated membranes were vacuum dried for 16 h at 120 ◦ C in an oil-free high vacuum (about 10−4 mbar) drier. Without solvent exchange the residual solvent is difficult to remove below Tg and it will alter the membrane properties as well as potentially the casting procedure and choice of solvent [11]. In the second procedure, polymer dense films, with thickness around 30 ␮m, were cast from 5 wt.% polymer-CHCl3 solutions and then vacuum dried for several days (50 ◦ C/day temperature ramp) from 30 to 180 ◦ C were they remain for at least 2 days to produce solvent-free membranes. The membrane densities were determined using dry polymer films in a density gradient column containing aqueous chloride solutions at 30 ◦ C. As an indirect measurement of the packing/swelling of the polymer chains that constitute the membranes, the density data were used to calculate the fractional free volume (FFV) applying the following relation:

FFV =

V − 1.3Vw V

(1)

where V is the polymer-specific volume, at 30 C, and Vw is the specific Van der Waals volume of the polymer repeating unit calculated using the group contribution method suggested by Bondi [12].

3. Results and discussion 3.1. Polymer synthesis In Fig. 1 are summarized the polymer structures and their shortcuts according to the monomers. This overview on selected, similar structures demonstrates the easily obtainable variety of different structures well suited to analyze structure–properties relationship for gas transport. The ketones or aldehydes 2,2,2-trifluoroacetone (CF3 Me), 1,1,1-trifluoropropane2-one (CF3 Et), trifluoroacetophenone (CF3 Ph), fluorinated trifluoroacetophenones (CF3 PhF, CF3 PhF5 ), sulfonated or aminated trifluoroacetophenones (CF3 PhSO3 H, CF3 PhN(C)2) and perfluorinated benzaldehyde (HPhF5 ) were reacted with the aromats biphenyl (B) or terphenyl (T) or aromats with ether bond (E), diether-keto bonds (D) and diether-diketo bonds (K) as outlined in the experimental part. All polymers were of high molecular weight and formed transparent and flexible films cast from suited solvents (see Section 2). 3.2. Permselectivity and structure–properties relationship To discuss structure–properties relationship of gas transport in polymers some conditions have to be considered: (i) a set of comparable structures have to be available; (ii) the data shall be obtained by membrane films of a certain thickness to overcome fast aging effects of thin films [13–15]; (iii) film preparation shall follow a standardized procedure to overcome preparation, solvent and temperature effects that may influence or alter the permselectivity data [11,16–18]. In Table 1 are summarized the gas permeability coefficients of He, H2 , O2 and CO2 , and the corresponding selectivity for the gas pairs He/CH4 , H2 /CH4 , O2 /N2 and CO2 /CH4 for 22 ketone-aromatics polymer membranes synthesized via super acid catalysis and using non-reactive monomers. The permeability and selectivity of at least 5 of the membranes shown in this table have been already reported elsewhere [8]. In general, it is observed that polymers synthesized with the more flexible linkages, E, D or K, lead to the formation of membranes with lower permeability coefficients for all gases, but however more selective, than their counterparts synthesized with the more rigid aromatic monomers based on B, T or BT. Anyone of these membranes show a very competitive selectivity and permeability combination of properties, for some separations, as compared to the selectivity and

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279

X C Ar n Y X C Y CF3 C

Aromatic hydrocarbons

CF3Ph

B

T CF3 C

CF3PhSO3H BT

SO3H O

CF3 C

CF3PhF

O O O

F

O

CF3 C F

E

O

D

O

K

O

CF3PhF5 F

F

F F CF3 C

H C

CF3PhN(CH3)2 F

F

F H3C

N

F F

CH3

CF3 C CH3

HPhF5

CF3Me

CF3 C H2C CH3

CF3Et

Fig. 1. Polymer structures and acronyms.

permeability combination reported for regular polysulfone, PSF [19] and/or fluoro-containing polyimides, PIs [20]. Polymers 17, 18 and 19 (Table 1) show O2 /N2 separation factors from 5.1 to 6.1 with permeability coefficients going from 0.8 to 3.5 Barrer, a combination of properties quite similar to the regular and commercial PSF. With these 22 polymers based on the well-known CF3 group, a new structure–property relationship can be assessed for a family of polymer membranes that can be synthesized with non-reactive monomers via super acid catalysis as it will be shown latter. Before any structure–property relationship is assessed and since some of the polymer structures described in Table 1 cannot be dissolved by a single solvent, it is important to learn what will be the effect of solvents and thermal drying history in the

packing of polymer chains during the membrane formation. Table 2 presents the permeability coefficients for H2 , O2 and CO2 , and their corresponding selectivity for the gas pairs H2 /CH4 , O2 /N2 and CO2 /CH4 for membranes formed with CHCl3 and NMP, and then subjected to different treatments. The specific volume and fractional free volume (FFV) have been determined as an indirect measurement of the packing/swelling effects. The membranes based on CF3 Me-BT and CF3 Ph-BT were cast from CHCl3 , two of them subjected to at least two different thermal protocols, and a third membrane was immersed in methanol to evaluate the combined effect of MeOH and thermal protocol on the packing/swelling of their polymer chains. The last membranes for each the CF3 MeBT and CF3 Ph-T reports on the effect of type of solvent used in the

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Table 1 Permeability–selectivity combination of properties, measured at 1–2 bar upstream pressure and 30–35 ◦ C, for the fluoro-containing aromatic polymer membranes synthesized via super acid catalysis. Entry

Ketone or aldehydes

Aromatic

Solvent

Permeability coefficientsa , P(i)

Ideal selectivity, P(i)/P(j)

He

H2

O2

CO2

H2 /CH4

O2 /N2

CO2 /CH4

CO2 /N2

1 2

CF3 Me

B BT

CHCl3 CHCl3

99 68

107 90

21 18.8

136 98

9 16

4.2 3.9

11 18

27 20

3 4

CF3 Et

T BT

CHCl3 CHCl3

50 45

60 48

10.5 7.7

54 38

18 22

4.0 4.1

16 18

21 20

5 6 7

CF3 Ph

Bb Tb BTb

THF/MeOH NMP/MeOH THF/MeOH

84 82 86

114 126 125

25 32 30

166 220 207

11 10 10

3.8 3.6 3.8

17 17 17

25 24 26

8 9 10

CF3 PhF

B T BT

CHCl3 CHCl3 CHCl3

82 74 76

100 104 100

21.9 23.1 20.7

111 125 104

16 15 17

4.0 3.9 4.1

18 18 18

20 21 20

11 12

CF3 PhF5

T BT

CHCl3 CHCl3

97 126

119 145

29 36.4

127 167

16 15

3.8 3.4

17 17

17 15

13

CF3 PhSO3 H

T

DMAc/MeOH

63

78

12

83

30

4.5

32

31

14 15

CF3 Ph5FN(CH2 )2

T BT

CHCl3 NMP/MeOH

62 58

95 83

19.6 15.5

113 108

17 17

4.1 4.0

21 22

23 28

16 17

CF3 Me

Eb D

THF/MeOH THF/MeOH

27 12

20 10

2.1 0.8

8.7 3.4

51 91

4.7 5.6

22 31

19 24

18 19

CF3 Ph

E Db

NMP/MeOH THF/MeOH

29 12

28 11

3.5 0.9

17 4.2

43 –

5.1 6.1

26 –

25 28

20 21 22

HPhF5

T D K

THF/MeOH THF/MeOH THF/MeOH

41 16 13

49 14 11

9.5 1.4 1.0

53 5.8 4.2

18 54 69

4.0 4.3 4.5

20 22 26

22 18 19

a b

P(i) in Barrer, 1 Barrer = 10−10 cm3 gas (STP) cm/cm2 s cm Hg. As reported in Ref. [8].

casting procedure. For the membranes cast from CHCl3 and without the MeOH treatment, the gas permeability coefficients are dependent on the thermal history, as it will be expected, but however, the corresponding selectivity for the gas pairs shown is not altered appreciably. The permeability coefficients for the membranes cast from CHCl3 , and that were first convection dried for 1 day at 80 ◦ C and then vacuum dried at 120 ◦ C for 16 h, show a more open structure, as can be seen from their FFV, than the membranes that were vacuum dried from 30 ◦ C up to 180 ◦ C. The more open structure left by this thermal treatment increases the permeability without losing selectivity. The combined effect of thermal and MeOH immersion, a typical treatment used to get rid of residual high boiling solvents, on the permeability and selectivity combination of membranes cast from CHCl3 still pushes the permeability coefficients to higher values but the corresponding selectivity practically remains constant, i.e. the CF3 Me-BT or CF3 Ph-BT membranes cast from CHCl3 /MeOH are more permeable in 20–44% for H2 , 26–53% for O2 and 28–56% for CO2 than the directly vacuum-dried membranes cast from CHCl3 . The increase in permeability is associated to an increase in FFV caused by the swelling/de-swelling process imposed to the original packing of the polymer chains. By comparing the permeability coefficients measured for the CF3 Ph-BT membranes cast from CHCl3 /MeOH or NMP/MeOH, it is possible to learn that the type of solvent, CHCl3 or NMP does not play an important role on the packing of the polymer membranes studied in this work, as long as they are treated with the same thermal history. Thus, when analyzing the structure/property relationship it is important to mind the thermal process, and, if applied, the MeOH immersion since any change in these two variables will have a significant influence on permeability, diffusivity and solubility. Table 3 shows the polymer structure, gas permeability coefficients, ideal selectivity, specific volume and FFV calculated for five polymer membranes, cast from CHCl3 and vacuum dried up to 180 ◦ C. The main interest here is to show the effect that different

groups such as Me, Et, Ph, PhF and PhF5 have on the mentioned properties. The specific volume and FFV for these polymers fall in the range of the recently published fluorinated poly(ether imide) membranes [21]. Membranes with Et groups have lower permeability and higher selectivity at least for the gas pairs H2 /CH4 and O2 /N2 , than those based on Me groups and apparently this is a result of a “better” overall packing of the polymer chains, as indicated by the lower FFV. The permeability and selectivity combination of properties measured for the membranes based on Ph and Me moieties are practically identical, but interestingly, as the amount of fluorine atoms are incorporated into the Ph moiety, to produce the CF3 PhF and CF3 PhF5 membranes, the gas permeability coefficients increase with some gains or losses in the selectivity of the pair of gases shown there. It is important to observe that the increase in the permeability coefficients, caused by the incorporation of fluorine, is accompanied by a reduction in the FFV. The FFV decreases as the content of fluorine atoms increases and this observation may be supported by the fact that the specific volume is also decreasing with the content of fluorine atoms. Table 4 reports the H2 , O2 and CO2 diffusivity and solubility coefficients calculated from time-lag measurements, as well as the corresponding diffusivity and solubility selectivity factors for the gas pairs H2 /CH4 , O2 /N2 and CO2 /CH4 . The replacement of Me by Et groups decrease the diffusivity coefficients, as well as the solubility coefficients, and also the FFV as it would be expected. However, this is not the case for the relatively highly fluorinated polymer membranes based on CF3 Ph, CF3 PhF, and CF3 PhF5 since the incorporation of fluorine atoms into the Ph moiety increases the diffusivity coefficients, as well as the solubility coefficients for O2 and CO2 , even though the FFV values have decreased. It is not the goal of this work to speculate about the relationship of diffusivity with FFV in these membranes fully loaded with fluorine atoms, since it is well-known that there are other forces or factors that may affect the packing in the solid state.

CF3 C CH3

CF3

CF3 C m

CH3

CF3

C

C p

m

p

n

n

. Membrane type

Solvent and thermal treatment

Permeability coefficienta , P(i)

Ideal selectivity, P(i)/P(j)

V, cm3 /g

FFV

H2

O2

CO2

H2 /CH4

O2 /N2

CO2 /CH4

CF3 Me-BT

CHCl3 b CHCl3 c CHCl3 /MeOHd CHCl3 /MeOHe

90 111 107 108

19 25 24 25

98 136 125 150

17 14 15 19

3.9 3.7 4.0 4.3

18 17 17 27

0.8216 0.8244 0.8243 –

0.190 0.193 0.193 –

CF3 Ph-BT

CHCl3 b CHCl3 c CHCl3 /MeOHd NMP/MeOHf

87 118 125 125

19 26 29 30

104 147 162 207

15 14 13 10

3.9 3.8 3.8 4.0

18 17 16 17

0.8258 0.8295 0.8293 –

0.192 0.196 0.196 –

a b c d e f

P(i) in Barrer. 1 Barrer = 1 × 10−10 cm3 gas (STP) cm/cm2 s cm Hg. Cast from CHCl3 and then vacuum dried from 30 ◦ C up to 180 ◦ C where they stay for 2 days. Temperature ramp 50 ◦ C/day. Cast from CHCl3 , dried 1 day at 80 ◦ C, then dried at 50 ◦ C for 4 h and then vacuum dried at 120 ◦ C for 16 h. Cast from CHCl3 , dried 1 day at 80 ◦ C, immersed in methanol for 24 h, and then dried at 50 ◦ C for 4 h and then vacuum dried at 120 ◦ C for 16 h. Cast from CHCl3 , immersed in methanol for 24 h and convection dried for 4 h at 50 ◦ C and then vacuum dried at 120 ◦ C for 16 h. Cast from NMP, immersed in methanol for 24 h and convection dried for 4 h at 50 ◦ C and then at 120 ◦ C for 16 h (as reported in Ref. [8]).

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Table 2 The effect of thermal history and MeOH treatment on the gas permeability and selectivity, at 35 ◦ C and 2 bar, and also on the specific volume and fractional free volume determined at 30 ◦ C.

281

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Table 3 Effect of different types of substituent on the selectivity–permeability combination of properties for some copolymers studied in this work. Permeability coefficienta , P(i) Ideal selectivity, P(i)/P(j)

Membrane type

He CF3

H2

O2

CO2

He/N2

H2 /CH4

O2 /N2

CO2 /CH4

V (cm3 /g)

FFV

CF3

C

C

CH3

p

CH3

m

n CF 3

68

90

19

98

14

17

3.9

18

0.8216

0.190

45

48

8

38

24

22

4.1

18

0.8272

0.180

65

87

19

104

14

15

3.9

18

0.8258

0.192

76

100

21

104

15

17

4.1

18

0.7981

0.189

126

145

36

167

12

15

3.4

17

0.7065

0.176

CF 3

C

C m

CH2

p

CH2

CH3

CH3

CF3

n

CF3

C

C p

m

n CF 3

CF 3

C

C p

m

n F

F CF 3

CF 3

C F

m

F F

F

p

F F

F

F a

C

F

n

F

P(i) in Barrer. 1 Barrer = 10−10 cm3 gas (STP) cm/cm2 s cm Hg.

3.3. Selectivity–permeability combination of properties Figs. 2–5 show the selectivity and permeability combination of properties measured for the membranes based on the 22 polymers

synthesized with non-reactive monomers via super acid catalysis. The updated Robeson’s upper bound experimental observation for the O2 /N2 , CO2 /CH4 , H2 /CH4 and CO2 /N2 separations have been included as a reference line. The new polymers described in this

10 2008 Updated Upper Bound 19 17 18

P (O2) / P (N2)

13

16

22

21 20

1

6, 11 7 12

1 0.1

1

10

100

1000

P (O2), Barrers Fig. 2. Selectivity–permeability relationship, for O2 –N2 separation, for the ketonearomatic polymers synthesized via super acid catalysis. Solid line represents the “updated upper bound” observed by Robeson [3], solid triangles represents some polyimides [20] and open triangles PIM 1 and 7 [3].

Fig. 3. Selectivity–permeability relationship, for CO2 –CH4 separation, shown by the ketone-aromatic polymers synthesized via super acid catalysis. Solid line represents the “updated upper bound” proposed by Robeson [3]. Open triangles represents PIM 1 and 7 [3].

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283

Table 4 Gas diffusivity and solubility coefficients, determined at 35 ◦ C and 2 bar upstream pressure, in the ketone-aromatics polymers synthesized in this work, and their contribution to the diffusivity and solubility separation factors. Membrane type

CF3

Diffusivitya , D(i)

Diffusivity separation factor

Solubilityb , S(i)

Solubility separation factor

H2

O2

CO2

H2 /CH4

O2 /N2

CO2 /CH4

H2

O2

CO2

H2 /CH4

O2 /N2

CO2 /CH4

592

13

7

455

2.9

5.2

0.12

1.09

11.0

0.036

1.31

3.33

6.1 3

854

3.1

6.2

0.09

0.96

9.5

0.030

1.37

3.21

CF3

C

C m

CH3

p

CH3

n CF 3

CF 3

C

C m

CH2

p

CH2

CH3

CH3

CF3

n

427

CF3

C

C p

m

n CF 3

268

12

7

223

3.1

5.5

0.25

1.20

12.0

0.065

1.33

3.13

476

11

6

433

2.9

5.3

0.16

1.39

13.7

0.039

1.36

3.36

528

20

9

278

2.7

4.8

0.21

1.41

13.8

0.054

1.26

3.58

CF 3

C

C p

m

n F

F CF 3

CF 3

C F

F F

F F a b

m

C p

F

F

F

F

n

F

Diffusivity coefficient, D(i) × 10−8 cm2 /s. Solubility coefficient, S(i) cm3 gas (STP)/cm3 atm.

work fall below the updated upper bound limits set for these 4 pair of gases. Nevertheless, there are important features that deserve to be considered since there are several challenges in the formation of new membrane materials.

For O2 /N2 separation, polymers 1, 6, 11, 13 and 18 represent an attractive selectivity–permeability combination, which has been marked with a discontinuous straight line that runs parallel the updated upper bound. Their combination falls close to the 100

1,000

2008 Updated Upper Bound

100

P (CO2) / P (N2)

P (H2) / P (CH4)

2008 Updated Upper Bound

17 21

16 18

13

20

12

10

13 15

1 5

7 6

6,7

10

1 1

10

100

1000

P (H2), Barrers Fig. 4. Selectivity–permeability relationship, for H2 –CH4 separation, shown by the ketone-aromatic polymers synthesized via super acid catalysis. Solid line represents the “updated upper bound” proposed by Robeson [3].

1

10

100

1000

P (CO2), Barrers Fig. 5. Selectivity–permeability relationship, for CO2 –N2 separation, shown by the ketone-aromatics polymers synthesized via super acid catalysis. Solid line represents the “updated upper bound” proposed by Robeson [3].

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combination reported for some fluoro-containing polyimides, PIs [20]. There are at least two important jumps that deserve to be noted: (1) in membranes synthesized with the E or D monomers, compare polymers 16–18 and 17–19, the replacement of Me by Ph lead to more permeable and more selective membranes; and (2) in membranes synthesized with the PhF5 group, compare membrane 20–11, the replacement of a hydrogen atom by the CF3 group lead to more permeable membranes and with practically no changes in the O2 /N2 selectivity. It would be interesting to learn if better membranes, in terms of their O2 /N2 selectivity and O2 permeability combination of properties, can be tailored by the relationship defined by the mentioned polymers (1, 6, 11, 13, and 18). For CO2 /CH4 separation, it is observed that polymer 13 (CF3 PhSO3 H-T) results with the best selectivity and permeability combination of properties and this may be due to an increased solubility originated by the interaction of CO2 with the sulfonic acid group. In general, there are membranes with high permeability coefficients for CO2 , higher than 200 Barrer, and with a still high selectivity for CO2 /CH4 separations, higher than 15, such as CF3 Ph-T and CF3 Ph-BT. As in the case for the O2 /N2 separation, the substitution of the Me groups by the Ph groups, compare polymers 16–18, lead to a more permeable and more selective membrane. Unfortunately, it was not possible to measure the permeability coefficient for CH4 in the case of polymer 19, so a direct comparison between polymers 17 and 19 is not possible at the present time. Similarly, the replacement of the hydrogen atom by the CF3 group, compare polymers 20–11, lead to a more permeable membrane with a very small lost in selectivity. For H2 /CH4 separation, the “best” selectivity–permeability trade-off is set by polymer membranes 12 and 13. The replacement of Me by Ph, compare polymers 16–18, leads to an increase in permeability but at the expense of a reduction in the H2 /CH4 selectivity, which is slightly different to the gas pairs O2 /N2 and CO2 /CH4 . However, the replacement of a hydrogen atom by the CF3 group still increases the permeability without losing selectivity. For CO2 /N2 separation, some membranes may have “attractive” combinations since their permeability coefficients for CO2 are above 100 Barrer and their selectivity towards N2 is higher than 20. In fact, polymers 1, 5, 6, 7, 13, and 15 define the “best” performance trade-off with respect to the performance shown by the rest of the polymers synthesized in this work. 4. Conclusions It has been shown that super acid catalyzed synthesis, at practically room temperature conditions, of ketones or aldehydes with no activated hydrocarbons lead to the formation of relatively high number-average molecular weight polymers which form excellent films for gas permeation. Through the examination of the gas transport properties determined for 22 polymer membranes, a new structure–property relationship for these fluoro-containing aromatic polymers was studied. In general, it was found that membranes containing the more flexible moieties E, D or K are less permeable, but however more selective, than membranes containing the more rigid aromatic moieties B, T or BT. Through small and systematic structural changes introduced in the monomers that make up the polymer repeating unit of these membranes, it was shown that the incorporation of fluorine atoms into the Ph moiety of the CF3 Ph-BT copolymers increases the gas permeability coefficients at the expense of small or practically no changes in the selectivity of the pair of gases studied even though the FFV decrease. In membranes with E linkages it was found that the replacement of Me by Ph groups in the CF3 -monomers increases both the permeability and selectivity for the gas pairs O2 /N2 , CO2 /CH4 and CO2 /N2 ,

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