Membrane Gas Separation

6.2
Materials and Methods 
The random co - polymers of tetrafl uoroethylene (TFE) used in this work are amorphous 
and glassy. They are Tefl on AF 1600 and 2400 (second monomer perfl uoro - 2,2 - dimethyl -
1,3 - dioxole, DuPont) and Hyfl on AD 60X (second monomer perfl uoro - 4 - methoxy - 

---

Glassy Perfl uorolymer–Zeolite Hybrid Membranes for Gas Separations
115
1,3 - dioxole, courtesy of Dr. V. Arcella, Solvay Solexis). Their properties are listed in 
Table 6.1 .
Silicalite - 1 crystals of different size and habits have been prepared according to litera-
ture procedures [10,11] . In a typical synthesis tetrapropylammonium hydroxide (TPAOH) 
or bis - 1,5 - (tripropylammonium)pentane diiodide (dC5), sodium or potassium hydroxide, 
and tetraethylorthosilicate (TEOS) were stirred overnight to hydrolyze. The solution was 
fi ltered (qualitative paper) into polypropylene bottles (Nalgene) or Tefl on - lined stainless 
steel autoclaves and heated statically or under rotation for the due time at the given tem-
perature. After quenching in cold water, crystals were recovered by repeated centrifuga-
tion and washing with water until pH was 8, and dried at 90 ° C overnight. The larger 
crystals were then calcined in air (2 ° C/min) at 625 ° C for 10 h; the sub - micron crystals 
instead were embedded in a polyacrylamide gel that was dried at 110 ° C, heated under 
N 
2
for 5 h at 550 ° C (heating and cooling 1 ° C/min) and fi nally calcined in air for 5 h at 
550 ° C (heating and cooling 1 ° C/min) [12] . 
The MFI crystals were made fl uorophilic by means of a surface chemical modifi cation, 
the details of which will be given elsewhere, and then dispersed in highly fl uorinated 
solvents (Galden HT 110 from Solvay Solexis, FC - 72 from 3M, and 1 - methoxyperfl uor-
obutane from Alfa Aesar) by sonication for 1 h. The homogeneous polymer solutions were 
added to the MFI suspensions, and the mixtures were sonicated further for 1 h prior to 
pouring them into casting rings on glass or Tefl on at room temperature. The rings were 
covered with a lid or a glass plate in order to reduce the evaporation rate of the solvent. 
In two or three days the 17 – 151
μ
m thick membranes became dry, and detached spontane-
ously from the surface in a few hours after fi lling the rings with water. The residual solvent 
was removed by heating (1 – 4 ° C/min) in vacuum up to 200 ° C (MMMs with Tefl on AF 
2400) or a few degrees beyond the T
g
of the polymer [13] . The thickness of the membranes 
was determined with a Mahr micrometer as the average value of 8 – 20 measurements in 
different points. 
The SEM observations were carried on different instruments (Jeol 6500 FEG, Cambridge 
Stereoscan 360, and FEI Quanta 200) after sputtering a conductive layer of C, Pt or Au 
on the samples. The samples for the TEM observation (Zeiss EM900) were included in 
an Epon Araldite matrix and cured at 60 ° C for 3 days; the cross - sections were cut in 
80 nm thick slices with a Leica Ultracut UCT ultramicrotome. 
Table 6.1  Copolymers used in this work and their relevant physical – chemical properties 
Name
Repeat units
T
g
/ ° C
d /g cm 
−
3
FFV (%)
Reference
Tefl on 
®
AF 2400
O
O
F
F
CF
2
CF
2
F
3
C
CF
3
87
13
240
1.74
33
4
Tefl on 
®
AF 1600
O
O
F
F
CF
2
CF
2
F
3
C
CF
3
65
35
160
1.84
31
4
Hyfl on
®
AD 60X
O
CF
2
O
O
F
CF
2
CF
2
60
40
F
3
C
121
1.93
23
9

---

116
Membrane Gas Separation
The pure gas (99.99% or higher purity) transport properties were tested with an instru-
ment (GKSS, Geesthacht, Germany) with constant permeate volume described elsewhere 
[14] . The short response time of the instrument allows one to record transient permeation 
behaviours of less than 1 second. The pure gas permeability is the amount of gas per-
meating in the unit time, multiplied by the thickness of the membrane and normalized for 
the membrane surface and the pressure gradient. The recorded pressure vs. time plots 
were used to derive diffusion coeffi cients from the initial transient permeation, and 
permeability at the steady state. The time - lag  
θ
  is the intercept of the linear part of the 
pressure vs. time curve with the axis of time. In a homogeneous membrane in which the 
solubility of a gas obeys Henry ’ s law, its diffusion coeffi cient D can be calculated from 
the ratio:
D
l
=
2
6
θ
(6.1)
where l represents the thickness of the membrane. On the GKSS instrument the value 
of D is obtained with an average uncertainty of less than
±
20%, and permeability within 
±
10%. When adsorptive fi ller are present, a modifi cation of the simple Equation (6.1) is 
required [15] . In this work, however, the simplifi ed Equation (6.1) is used throughout in 
order to draw qualitative conclusions. Before each measurements the system was evacu-
ated. Feed pressure was 1 bar, the permeate pressure never reached 1 mbar. The separation 
factor is calculated as the ratio of the single gas permeability coeffi cients; the gas solubil-
ity coeffi cients ( S ) is found from the permeability ( P ) and the diffusion coeffi cients ( D ) 
through the well known relation P = DS valid for membranes with a solution - diffusion 
transport mechanism [16] . 
Binary CH 
4
/n - butane permeation properties were determined using a continuous fl ow 
system [17] . Typically, a He sweep fl ow rate of 100 cm 
3
/min and an i - butane internal 
standard fl ow rate of 11 cm 
3
/min were used, and the equimolar mixed gas feed was 
100 cm 
3
/min. At 25 ° C the backfl ow of helium was negligible. The pressure on either side 
of the membrane was increased up to about two bars by using a back pressure regulator. 
Films were loaded into a Wicke – Kallenbach cell, the feed gas was introduced, and the 
permeating species across the membranes swept by the He stream were analyzed by GC 
(HP 6890). Three measurements were taken at 1 or 3 
h intervals. The temperatures 
used was 25 ° C; between each pressure increase/decrease, the membrane was allowed to 
equilibrate, typically for 3 – 22 h, before the next set of measurements was taken. The 
selectivity P
i
/ P
j
was calculated according to the following expression:
P P
C C
C C
i
j
i
p
j
f
i
f
j
p
=
where the superscript of the concentration C of the generic component refers to the feed 
or to the permeate streams.