Membrane Gas Separation

PEPA 
6FDA 
Imidization 
NH
2
N
H
2
+
+
O
O
O
FDA
O
O
O
O
O
O
N
CF
3
CF
3
N
O
O
N
N
n
O
O
CF
3
F
3
C
O
O
O
O
(CH
3
CO)
2
, (C
2
H
5
)
3
N
Scheme 1.6
+
+
C
H
3
CH
3
C
H
3
CH
3
NH
2
N
H
2
O
O
CF
3
F
3
C
O
O
O
O
N
H
2
Imidization 
(CH
3
CO)
2
, (C
2
H
5
)
3
N
PAS 
6FDA 
TeMPD 
CF
3
F
3
C
N
N
O
O
O
O
C
H
3
C
H
3
CH
3
CH
3
n
CF
3
F
3
C
N
N
O
O
O
O
Scheme 1.7

---

12
Membrane Gas Separation
O
O
CF
3
F
3
C
O
O
O
O
O
C
O
N
H
O
O
CH
3
O
O
N
N
CF
3
F
3
C
O
O
O
O
n
CH
3
C
H
3
C
H
3
CH
3
N
N
CF
3
F
3
C
O
O
O
O
O
C
O
N
H
O
O
C
H
3
O
O
++
N
N
CF
3
F
3
C
O
O
O
O
n
CH
3
C
H
3
C
H
3
CH
3
O
H
N
N
CF
3
F
3
C
O
O
O
O
OH
 
A
P
A
 
D
P
M
e
T
 
A
D
F
6
Imidization 
Δ
,p
-xylene 
C
H
3
CH
3
C
H
3
CH
3
NH
2
N
H
2
N
H
2
OH
CH
3
O
O
O
O
NCO
Scheme 1.8 

---

Synthesis and Gas Permeability of Hyperbranched and Cross-linked Polyimide Membranes
13
The reaction in vinyl or acryl - terminated polyimides was preceded by UV irradiation 
under the presence of a photo initiator. Formation of free 
- 
standing membranes was 
signifi cantly enhanced by UV irradiation.
1.4
Gas Permeation Properties 
In this section, we shall consider gas permeation properties of fi lms based on cross - linked 
and hyperbranched polyimides. We shall focus on CO 
2
capture from the fl ue gases of 
power plants (CO 
2
/N 
2
), CO 
2
separation from natural gas (CO 
2
/CH 
4
), hydrogen purifi cation 
(H 
2
/N 
2
) and oxygen and/or nitrogen enrichment of air (O 
2
/N 
2
) [28] . We shall consider 
permeability coeffi cients P
i
and ideal separation factors for gas pairs A and B (  
α
  (A/B)) 
defi ned as the ratio P (A)/ P (B). The trade - off relationship has been recognized between 
gas permeability and selectivity in polymeric membranes: highly permeable polymers 
have low selectivity and vice versa. The general trade - off relationship between gas perme-
ability and selectivity has been summarized by Robeson [79,80] , who suggested an idea 
of the upper bound. Traditionally, an interest of researchers was directed to materials 
whose data points are located at the Robeson diagrams close to or above the upper bound 
lines. 
1.4.1
Amorphous Cross - linked Polyimides (Type II ) 
The gas permeability and selectivity of cross - linked membranes prepared by monoesteri-
fi cation and transesterifi cation reactions between the carboxylic acid in polyimides and 
diol agents are summarized in Table 1.1 [34 – 38, 41, 42] . These gas permeation properties 
are strongly affected by the structures of the diol agent, polyimide composition and the 
annealing temperature after membrane formation. For example, the range of variation of 
P (CO 
2
) is 10 – 145 Barrer while the selectivity  
α
  (CO 
2
/CH 
4
) is in the range 29 – 87 at 35 ° C. 
The 6FDA - TMPD/DABA (3:2) cross - linked polyimides with 1,3 - propanediol have better 
gas separation performance among the other cross - linked polyimides using monoesteri-
fi cation and transesterifi cation reactions: P (CO 
2
) of cross - linked 6FDA - TMPD/DABA 
(3:2) is 77 Barrer and  
α
  (CO 
2
/CH 
4
) is 40 at 35 ° C and 4.4 atm [42] .
The decarboxylation - induced cross - linked 6FDA - TMPD:DABA (2:1) polyimide 
reveals enhanced resistance to plasticization by high pressure of CO 
2
[43] . This can be a 
result of high annealing temperature which leads to decarboxylation of the pendant acid 
groups. In this process phenyl radicals are formed that are capable of attacking other 
portions of the polyimide macromolecules. The CO 
2
permeability of the cross - linked 
membrane prepared by rapid quenching from above the glass transition temperature is 
260 Barrer at 35 ° C and 6.8 atm. 
The gas permeability and selectivity of cross - linked membranes prepared by the imide 
ring - opening reaction between the imide ring in polyimides and primary diamine agents 
are summarized in Table 1.2 [45 – 56] . As the reaction time increases, the gas permeability 
gradually decreases, whereas the selectivity increases. However, excess cross - linking 
leads to a reduction of both permeability and selectivity. The observed transport param-
eters could be strongly affected by the structures of diamine agent. For example, the 
P (CO 
2
) is in the range 1.9 – 568 Barrer and the selectivity,  
α
  (CO 
2
/CH 
4
) is in the range 

---

14
Membrane Gas Separation
Table 1.1  Gas permeability coeffi cients and selectivity (at 35 ° C) of cross - linked polyimides (Type II) using monoesterifi cation and 
transesterifi cation reaction of free carboxylic acid 
Polyimide
Cross - linkable
Feed pressure 
(atm)
P (O 
2
)
P (CO 
2
)
 
α
  (O 
2
/
N 
2
)
 
α
  (CO 
2
/
N 
2
)
 
α
  (CO 
2
/
CH 
4
)
Reference
6FDA - DABA
linear (Type I)
2
1.01
3.4
8.0
26.9
63.0
[34]
Thermal cross - linked
3.74
2.69
10.40
6.7
26.0
87.0
[34]
6FDA - mPD
linear (Type I)
3.74
2.60
11.03
6.5
27.6
58.0
[34]
6FDA - mPD/
DABA (9:1)
linear (Type I)
3.74
1.71
6.53
6.9
26.1
65.3
[34]
Ethylene Glycol
3.74
1.81
9.50
6.8
35.2
63.3
[34]
6FDA - 6FpDA/
DABA (1:2)
linear (Type I)
10
–
29
–
–
45
[35]
Ion compound (Al(AcAc) 
3
)
10
–
19
–
–
–
[36]
6FDA - 6FpDA/
DABA (2:1)
linear (Type I)
10
–
29
–
–
45
[37]
Ion compound (Al(AcAc) 
3
)
10
–
25
–
–
46
[36,37]
Ethylene glycol
10
6.0
35
–
–
42
[37]
Butylene glycol (140 ° C)
2
42.8
6.2
44.2
47
[38]
6FDA - TMPD/
DABA (2:1)
Thermal cross - linked (130 ° C)
10
–
133
–
–
29
[41]
Thermal cross - linked (220 ° C)
10
–
115
–
–
27
[35,41]
Thermal cross - linked (295 ° C)
10
–
110
–
–
30
[41]
Ethylene Glycol (140 ° C)
10
–
90
–
–
30
[35]
1,4 - cyclohexanedimethanol (140 ° C)
10
–
21
–
–
30
[35]
1,4 - cyclohexanedimethanol (220 ° C)
10
–
22
–
–
30
[41]
1,4 - cyclohexanedimethanol (295 ° C)
10
–
79
–
–
29
[41]
Butylene glycol (140 ° C)
10
–
44
–
–
34
[35,41]
Butylene glycol (140 ° C)
2
13.7
51
4.8
17.9
37
[38]
Butylene glycol (220 ° C)
10
–
46
–
–
34
[35,41]
Butylene glycol (295 ° C)
10
–
138
–
–
30
[35,41]
6FDA - TMPD/
DABA (3:2)
1,3 - propanediol (220 ° C)
4.4
–
57.5
–
–
37.1
[42]
1,3 - propanediol (295 ° C)
4.4
–
77.3
–
–
39.9
[42]
1 Barrer = 10 
−
10
cm 
3
(STP) cm cm 
−
2
s 
−
1
cmHg 
−
1
.

---

Synthesis and Gas Permeability of Hyperbranched and Cross-linked Polyimide Membranes
15
Table 1.2  Gas permeability coeffi cients and selectivity (at 35 ° C) of cross - linked polyimides (Type II) using imide ring - opening reaction by 
primary amine agents 
Polyimide
Cross - linkable
Feed pressure 
(atm)
P (O 
2
)
P (CO 
2
)
 
α
  (O 
2
/N 
2
)
 
α
  (CO 
2
/N 
2
)
 
α
  (CO 
2
/CH 
4
)
Reference
Matrimid 5218
linear (Type I)
10
1.7
6.5
6.6
25.2
34
[45]
p - xylenediamine (1 day)
10
1.9
7.4
6.5
25.3
36
[45]
p - xylenediamine (7 day)
10
1.5
5.1
7.0
23.8
33
[45]
p - xylenediamine (14 day)
10
1.4
4.7
7.0
23.5
34
[45]
p - xylenediamine (32 day)
10
0.9
1.9
6.9
14.6
28
[45]
linear (Type I)
2
–
5.39
–
–
36
[46]
Poly(ethylene oxide) 2000 (1:0.2)
2
–
7.51
–
–
22
[46]
Poly(ethylene oxide) 2000 (1:0.5)
2
–
59.16
–
–
18
[46]
Poly(ethylene oxide) 2000 (1:1)
2
–
115.18
–
–
17
[46]
Poly(ethylene oxide) 600 (1:1)
2
–
1.47
–
–
23
[47]
6FDA - TeMPD
linear (Type I)
10
125
456
3.7
13.5
–
[48]
p - xylenediamine (5 min)
10
45.2
136
4.1
12.3
–
[48]
p - xylenediamine (10 min)
10
28.9
91.8
4.4
14.0
–
[48]
p - xylenediamine (15 min)
10
26.5
70.0
4.4
11.6
–
[48]
p - xylenediamine (30 min)
10
13.7
30.3
4.8
10.6
–
[48]
p - xylenediamine (60 min)
10
2.34
2.14
5.9
5.4
–
[48]
6FDA - TeMPD
linear (Type I)
10
186
612
3.36
11.0
13.6
[49]
1,3 - cyclohexanebis(methylamine)
CHMA (30 min)
10
20.2
55.2
4.67
12.7
19.3
[49]
CHMA (100 ° C)
10
14.6
37.2
4.85
12.4
19.9
[49]
CHMA (150 ° C)
10
17.1
54.5
4.57
14.6
22.7
[49]
CHMA (200 ° C)
10
17.4
59.8
4.70
16.2
26.2
[49]
(continued overleaf)

---

16
Membrane Gas Separation
Polyimide
Cross - linkable
Feed pressure 
(atm)
P (O 
2
)
P (CO 
2
)
 
α
  (O 
2
/N 
2
)
 
α
  (CO 
2
/N 
2
)
 
α
  (CO 
2
/CH 
4
)
Reference
6FDA - TeMPD
linear (Type I)
10
190
640
3.5
11.8
14
[50]
Ethylenediamine (1 min)
10
120
490
4.0
16.3
23
[50]
Ethylenediamine (2 min)
10
30
120
4.3
17.2
32
[50]
Ethylenediamine (5 min)
10
12
25
6.0
12.5
28
[50]
6FDA - TeMPD
linear (Type I)
3.5
186
612
3.4
11.0
13.6
[52]
ethylenediamine (1 min 10 wt%)
3.5
108
435
3.8
15.3
21.1
[52]
1,3 - propanediamine (1 min 
10 wt%)
3.5
26.4
81.1
4.9
15.2
27.8
[52]
1,4 - butanediamine (1 min 
10 wt%)
3.5
55.5
218
4.2
16.5
24.7
[52]
6FDA - TeMPD
linear (Type I)
10
148.3
517
3.46
12.1
15.7
[53]
PAMAM (20 min)
10
137.6
568
3.93
16.2
22.8
[53]
PAMAM (60 min)
10
115.6
451
4.30
16.8
25.1
[53]
PAMAM (24h)
10
44.1
157
5.38
19.1
35.6
[53]
PAMAM (7days)
10
7.33
21.6
6.42
18.9
46.8
[53]
6FDA - TeMPD
linear (Type I)
3.5
186
612
3.4
11.0
13.6
[54]
G1 - DAB - AM (5 min)
3.5
130
435
4.6
15.3
23.0
[54]
G1 - DAB - AM (20 min)
3.5
74
220
5.1
15.2
24.4
[54]
G1 - DAB - AM (30 min)
3.5
70
177
5.7
14.5
24.5
[54]
G1 - DAB - AM (60 min)
3.5
50
98
6.9
13.6
21.7
[54]
6FDA - TeMPD
G0 - PAMAM (1 day)
10
–
160
–
–
36
[56]
G0 - PAMAM (1 day, 120 ° C)
10
–
70
–
–
37
[56]
G0 - PAMAM (1 day, 250 ° C)
10
–
84
–
–
30
[56]
1 Barrer = 10 
−
10
cm 
3
(STP) cm cm 
−
2
s 
−
1
cmHg 
−
1
.
Table 1.2 (continued)

---

Synthesis and Gas Permeability of Hyperbranched and Cross-linked Polyimide Membranes
17
14 – 47 at 35 ° C. The 6FDA - TeMPD cross - linked by PAMAM dendrimer has higher gas 
permeability among the other cross 
- 
linked polyimides using the imide ring 
- 
opening 
reaction [53] : P (CO 
2
) = 568 Barrer and  
α
  (CO 
2
/CH 
4
) = 22.8 at 35 ° C and 10 atm. PAMAM 
dendrimers have good gas separation performance especially for CO 
2
/N 
2
because their 
amine groups have excellent affi nity to CO 
2
[81] . The reactions using dendrimers were 
studied by several authors [53 – 56] . The cross - linking modifi cation by PAMAM and 
amine - terminated diaminobutane (DAB - AM) results in a decrease in permeabilities for 
most gases. In the case of the polyimide cross - linked with DAB - AM dendrimer, the 
maximum selectivity increases by about 400, 300 and 265% for the gas pairs of He/N 
2
, 
H 
2
/N 
2
and H 
2
/CO 
2
, respectively [54] . The gas permeability decreases in the order of 
G1
>
G2
>
G3, which is consistent with the increasing order of the degree of gel 
contents.
The gas permeability and selectivity of cross - linked membranes obtained by UV irra-
diation of polyimides are summarized in Table 1.3 [63 – 69] . The gas permeability decreases 
with the increase in UV irradiation time and is in the range 0.2 – 21 Barrer for O 
2
and 
0.6 – 99 Barrer for CO 
2
at 35 ° C. The selectivities are 4.3 – 13.8 for  
α
  (O 
2
/N 
2
) and 21.2 – 111.4 
for  
α
  (CO 
2
/CH 
4
).
The gas permeation parameters have also been reported for polymers obtained via ether 
reaction of epoxy and diamines [57,58] , for polymer blends with acetylene - terminated 
oligomer [73,74] and internal acetylene polyimide [72,75] . The gas permeability of cross -
linked internal acetylene - containing polymer, 6FDA - TeMPD/ p - intA (4:1) declines from 
612 to 186 Barrer, while the selectivity,  
α
  (CO 
2
/CH 
4
) increases from 14 to 25 at 35 ° C and 
10 atm [75] . Moreover, this cross - linked 6FDA - TeMPD/ p - intA (4:1) membrane is still 
stable under CO 
2
pressure of about 47 atm.