Membrane Gas Separation Edited by Yuri Yampolskii and Benny Freeman
© 2010 John Wiley & Sons, Ltd
6
Glassy Perfl uorolymer – Zeolite
Hybrid Membranes for Gas
Separations
Giovanni Golemme
a,b
, Johannes Carolus Jansen
b
, Daniela Muoio
a
, Andrea Bruno
a
,
Raffaella Manes
a
, Maria Giovanna Buonomenna
a
, Jungkyu Choi
c
and Michael Tsapatsis
c
a
Universit à della Calabria, Dipartimento di Ingegneria Chimica e dei Materiali, and INSTM
Consortium, Rende, Italy
b
Istituto per la Tecnologia delle Membrane, ITM - CNR, Rende, Italy
c
University of Minnesota, Department of Chemical Engineering and Materials Science,
Minneapolis, USA
6.1
Introduction
Commercial polymeric membranes are cheap, but very often not suffi ciently permselec-
tive. For any given gas pair, polymers typically show high selectivities with modest
permeabilities, or high permeabilities coupled with reduced selectivities, so that in a
selectivity vs. permeability log – log plot all polymers fall below a so - called upper bound
line [1] . On the other hand, inorganic membranes are often very permselective,
easy to
clean, thermally and chemically resistant, but are usually expensive, brittle, diffi cult
to be prepared in a reproducible way, and are typically characterized by a low surface -
to - volume ratio in modules which, in turn, affects industrial applications (increased dead
volume, need of larger compressors) and therefore translates into higher investment and
running costs.
Mixed - matrix membranes (MMMs), also referred to as hybrid membranes, contain a
separating layer made of a continuous phase (usually a polymer)
embedding a second,
114
Membrane Gas Separation
dispersed phase, the chemical nature of which is different. They represent a viable
opportunity for enhancing the separation capabilities of polymeric membranes for gas
separation (GS). The combination of two materials with different gas diffusivity and
solubility, in fact, allows a combination of high permselectivity of the fi ller (e.g. carbon
molecular sieves, zeolites, inorganic particles) with an ease of production of polymeric
membranes [2] .
In general, the preparation of defect - free MMMs is not an easy task, especially dealing
with rigid fi llers in glassy polymers. In fact, the poor adhesion
between the two phases
can produce voids at the interface and preferential paths for the diffusing species, which
spoil the selectivity of the composite membrane [3] . This problem is particularly severe
for Hyfl ons AD and Tefl ons AF, high free volume, amorphous and glassy perfl uoropoly-
mers with interesting GS properties for hydrocarbon mixtures, for the sweetening of
natural gas and for the N
2
/CH
4
separation [4] : the well - known
diffi culty of sticking
anything on the surface of perfl uorinated polymers, such as polytetrafl uoroethylene, can
be rationalized by considering their very low solubility parameters [5] . In fact, in the
literature the only composite membranes based on amorphous glassy perfl uoropolymers
are made of dispersions of non - porous nano - sized amorphous SiO
2
in Tefl on AF 2400
[6] and AF 1600 [7] . SiO
2
nanoparticles in Tefl ons AF interfere with the packing ability
of the polymer and increase the amount of free volume. The increase in the permeability
of large, condensable molecules exceeds the one of small molecules, thereby decreasing
the intrinsic size selectivity of the polymer:
as a consequence, a selectivity reversal for
the n - butane/CH
4
pair takes place above a 18 wt % SiO
2
content in Tefl on AF 2400 [6] .
The dispersion of suitable porous fi llers inside such polymers offers a new degree of
freedom for improvement of their permselectivity, by enhancing the solubility selectivity
and/or the mobility selectivity or, in the limiting case, the sieving capabilities of the
zeolites. The main goal of this work is to demonstrate that porous fi llers can be effectively
dispersed in amorphous glassy perfl uoropolymers. Silicalite
-
1,
the siliceous form of
ZSM - 5 (structure topology MFI) [8] , has a three - dimensional channel structure with
straight channels of 5.4 – 5.6 Å along the
b axis and zigzag channels of 5.6 Å in the
ac
plane. The zigzag channels connect subsequent layers of straight channels. There were
several reasons for choosing it as the porous fi ller: fi rst, because this zeolite is hydrophobic
and, unlike zeolites containing aluminium, its pores do not absorb signifi cant amounts of
moisture; secondly, because it can be easily prepared in a wide range of sizes and shapes;
and fi nally, because the sorption and mobility data of several gases and vapours in it are
already available in the literature.
Several mixed matrix membranes made of silicalite - 1 crystals
of different size and
shape embedded in Tefl on AF 1600, Tefl on AF 2400 and Hyfl on AD 60X have been
prepared. The gas permeation properties of the membranes have been tested with pure
gases, and some experiments with n - C
4
H
10
/CH
4
mixtures have also been carried out on
silicalite - 1/Tefl on AF 2400 membranes.
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