Membrane Gas Separation

a
a
b
f
g
h
i
j 
10 
m
m
c
d
e
1 
m
m
0.1 
m
m
Figure 6.3 (a) SEM pictures of top view and (b) bottom view of membrane AF16_80_40; 
(c), (d), (e) TEM pictures of the cross sections of membrane AF16_80_40 at different 
magnifi cations; SEM pictures of top view (f) and cross section (g) of membrane 
AF16_350_30; SEM pictures of bottom view (h), top view (i) and cross section (j) of 
membrane AD60_1500_42; SEM pictures of bottom view (k), top view (l) and cross 
section (m) of membrane AF24_1500_40; SEM pictures of bottom view (n), top view (o) 
and cross section (p) of membrane AF24_dC5_40

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Glassy Perfl uorolymer–Zeolite Hybrid Membranes for Gas Separations
119
k
k
l
m
n
o
p
Figure 6.3 (continued)
total. The membranes are defect free and still fl exible up to about 40 wt% of zeolite. The 
top and the bottom surfaces of membrane AF16_80_40 (Figures 6.3 a and b) are smooth, 
although the dispersion of the small 80 nm MFI crystals in the polymer matrix is very 
poor even at a 15 wt% loading. Figures 6.3 c – e show that the 80 nm crystals form mainly 
aggregates containing voids. The TEM picture at the largest magnifi cation in Figure 6.3 e, 
however, indicates the presence of polymer (highlighted by the arrows) around each single 
crystal in the aggregates, and a good wetting of zeolite surface by Tefl on AF 1600. 
Zeolites of 350 nm or larger instead are well dispersed in Tefl on AF 1600 (Figures 6.3 f 
and 6.3 g), Hyfl on AD 60X (Figures 6.3 h – j), Tefl on AF 2400 (Figures 6.3 k – p).
Before considering the permeability of the membranes, it is useful to evaluate the 
transport of gas in the silicalite - 1 phase. If we assume a solution - diffusion transport 
mechanism inside a MFI crystal, then an estimate of the permeability can be obtained by 
multiplying gas solubility and diffusion coeffi cient at the specifi c temperature and 
pressures of our permeation experiments. The diffusion coeffi cients of CO 
2
and CH 
4
in 
silicalite - 1, at 25 ° C and infi nite dilution, reported in Figure 6.4 c, were derived from the 
data of Nijhuis et al. [18] , because the pulse - response technique used in that work yields 
diffusivity values which are in good agreement with the self - diffusion coeffi cients deter-
mined by pulse - fi eld - gradient NMR under equilibrium conditions [19] . In other words, 
the diffusion coeffi cients reported refer to the transport inside the crystal and do not 
consider the presence of barriers for the transport of matter at the crystal surface. The 
solubility of N 
2
, CH 
4
and CO 
2
in silicalite - 1 (Figure 6.4 d) was calculated from literature 
data [20,21] by considering an average value between 0 and 10 
5
Pa at 25 ° C. It can be 
seen from Figures 6.4 c and d that both the solubility and the diffusion coeffi cients of N 
2
, 

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120
Membrane Gas Separation
CH 
4
and CO 
2
in silicalite - 1 are higher than in Tefl on AF1600, and therefore, if no barrier 
to transport exists in the MFI/Tefl on AF1600 membranes, their permeability should be 
higher than that of the polymer. The permeability to CO 
2
and CH 
4
of a Tefl on AF1600/
MFI 30% membrane, as calculated by means of the Maxwell model (random distributions 
of non interacting solid spheres in a continuous matrix) [22,23] , is shown in Figure 6.4 a: 
none of the two membranes containing 30 wt% MFI crystals is adequately described by 
this simplifi ed model.
The pure gas permeabilities of a Tefl on AF 1600 membrane and of four AF1600/MFI 
membranes (Figure 6.4 a) indicate a good correlation with the kinetic diameter of the 
penetrant molecules. However, the membrane AF24_350_30, containing larger crystals, 
is less permeable and more selective than the others (Figure 6.4 b). Therefore, we can 
conclude that a potential energy barrier to transport exists at or near the surface of 
the zeolite crystals. The other data in Figure 6.4 exclude that this barrier might be due 
to polymer densifi cation at the interface. In fact, the permeability of the membranes 
containing 80 nm crystals is always larger, and the selectivity smaller, than in Tefl on 
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