Membrane Gas Separation

Kinetic diameter / nm 
Kinetic diameter / nm 
Kinetic diameter / nm 
Perm. / 10
-15
 Nm
3
 m m
-2
 Pa
-1
 sec
-1
S / 10
-5
 Nm
3
 m m
-3
 Pa
-1
D / 10
-10
 m
2
 sec
-1
He
H
2
CO
2
O
2
CH
4
N
2
CH
3
OH
(a)
0,1
1
10
100
P/
P(
C
H
4
) (-)
He
H
2
CO
2
O
2
N
2
CH
3
OH
(b)
0,01
0,1
1
10
100
CO
2
O
2
N
2
CH
4
(c)
0,1
1
10
100
1000
e
/k / K
N
2
O
2
CH
4
CO
2
(d)
Figure 6.4 Pure gas transport data at 25 ° C of membranes AF1600 ( 
䊊
 ), AF16_350_30 ( 
䊐
 ), 
AF16_80_15 ( 
䉱
 ), AF16_80_30 ( 
䊏
 ), AF16_80_40 ( 
•
 ), silicalite - 1 ( 
䉫
 ) as derived from 
literature data (see text), and predictions of the Maxwell model for a AF16/MFI 30% 
membrane ( * ); (a) Pure gas steady state permeability vs kinetic diameter of the permeating 
molecules; (b) gas/methane separation factor; (c) gas diffusion coeffi cients from time - lag 
experiments vs kinetic diameter; (d) gas solubility vs the  
ε
/k Lennard - Jones parameter

---

Glassy Perfl uorolymer–Zeolite Hybrid Membranes for Gas Separations
121
1
10
100
0,1
1
10
100
1000
P(CO
2
) / 10
-15
 Nm
3
 m m
-2
 Pa
-1
 s
-1
P(CO
2
)/P(CH
4
), (-)
Cytop
Hyflon AD60 (MP)
AD60_1500_42
Hyflon AD60
Teflon AF 2400
2008 Upper Bound
(a)
0,1
1
10
0,001
0,01
0,1
1
10
100
P(N
2
) / 10
-15
 Nm
3
 m m
-2
 Pa
-1
 s
-1
P(N
2
)/P(CH
4
), (-)
PFAVE
Cytop
Hyflon AD60
Hyflon AD60 (MP)
AD60_1500_42
Teflon AF 2400
2008 Upper Bound
(b)
Figure 6.5 Permeability vs. selectivity plots at 25 ° C of perfl uorinated polymers [4,13,14]
and hybrid membranes for the CO 
2
 /CH 
4
pair (a) and for the N 
2
 /CH 
4
separation (b)
AF1600: a fact that can be rationalized in terms of a loosened polymer packing at the 
interface with the inorganic phase. When the size of the crystal passes from 350 to 80 nm, 
the external surface of the inorganic phase increases of a factor of 19, and the volume of 
the poorly packed polymer increases proportionally. It is possible that such an increase 
of the volume of the more permeable ‘ shells ’ around the individual crystals might cause 
them to coalesce and to give rise to percolation paths of higher permeability and reduced 
selectivity. In the literature, in similar systems formed by nano - sized fumed silica dis-
persed in amorphous and glassy high free volume polymers, it was assumed that the fi llers 
disrupt the chain packing at the interface [6,7] . The sustained diffusivity of the gases 
examined in the membranes containing 80 nm MFI crystals (Figure 6.4 c) and the increase 
in the gas solubility they exhibit with the increase of the content of the fi ller (Figure 6.4 d), 
are also consistent with this picture, by considering that a larger free volume can host 
more molecules. 
It must be pointed out that the information on gas diffusivity and solubility reported in 
Figure 6.4 are qualitative, since they are derived from a simplifi ed treatment of the tran-
sient permeation behaviour; equilibrium gas sorption experiments would be required to 
obtain detailed information on the different role of polymer and zeolites in these mixed 
matrix membranes. 
The data regarding Tefl on AF1600 mixed matrix membranes therefore seem to indicate 
that, when the zeolite particles are small, the gas fl ow runs mainly around them, whereas 
with larger particle sizes the overall resistance increases, and the gas molecules have a 
higher probability to cross the membrane surface and pass through the zeolites. For this 
reasons some membranes have been prepared with silicalite - 1 crystals of about 1.5
μ
m 
and with dC5 MFI. 
The properties of Hyfl on AD 60X are clearly improved by the presence of silicalite - 1. 
As can be seen in Figure 6.5 a the CO 
2
/CH 
4
separation factor gets closer to the Robeson ’ s 
upper bound. Although other polymers have higher separation factors, the resistance to 
plasticization of medium free - volume perfl uorinated polymers [4,9] makes this polymer/
fi ller combination of some interest.
The best results of the AD60_1500_42 membrane is in the diffi cult N 
2
/CH 
4
separation 
(Figure 6.5 b), beyond the Robeson ’ s upper bound. 

---

122
Membrane Gas Separation
A similar effect can be seen in Figure 6.6 a for Tefl on AF 2400: when compared to 
hybrid membranes containing fumed silica, the AF24_1500_40 membrane shows better 
butane permeability and butane/CH 
4
separation factors in pure gas permeation experi-
ments. Mixed gas permeation experiments at 25 
° 
C with equimolar butane 
– 
methane 
mixtures demonstrate that the increase of the butane partial pressure from 50 to 106 kPa 
can revert the methane selective membrane to butane selective. In these experiments the 
n - butane permeability grows monotonically from 2.4 to 3.2
×
10 
−
15
m 
3
m m 
−
2
Pa 
−
1
s 
−
1
. The 
experiments with the membrane AF24_dC5_40, containing large and thin silicalite - 1 
crystals, the results of which are not shown here, always yield better butane/CH 
4
selectiv-
ity and same permeability than with the AF24_1500_40 membrane, and demonstrate the 
importance of the shape of the fi ller on the performance of mixed matrix membranes [24] .