zeolites and CMS. MMM/silica tends to improve the CO 2 /
CH 4 selectivity by restricting polymer chain mobility and
hindered larger molecules to permeate (Ahn et al. 2010).
CNT possesses unique characteristics such as superior
mechanical and thermal stability with high electrical properties and is thus widely regarded as versatile materials. As
gas adsorption separation media, CNT tends to adsorb polar
molecules such as H 2 S (dipole moment of 0.97 Debye),
while discriminating non-polar molecules, despite CNT
being non-polar materials (Nour et al. 2013; Vinoba et al.
2017). CNT alone as gas separation medium is not considered to be attractive due to its non-polar nature;
non-selective sorption; and large pores do not provide
molecular sieving. Nevertheless, the incorporation of CNT
into polymer matrix has been reported to be beneficial to gas
separation properties. The limitations of CNT alone as gas
separation media are compensated by its relatively low
density materials. Compared to other class of fillers, a large
number of CNT materials can be incorporated into a polymer
matrix at similar mass loading with other fillers. As a result,
even low loading of CNT embedded in the membrane can
provide a significant contribution toward overall membrane
properties and hence separation performance (Moghadassi
and Rajabi 2014).
Application of metal–organic frameworks (MOFs) for gas
separation is not well developed comparing other applications. Most of the studies on MOF focused as a catalyst, gas
storage, and gas adsorbent, while as a gas separation media
is discouraging despite their highly permeable properties due
to poor gas pair selectivity. Nevertheless, MOF has higher
BET surface area, no dead volume, high uptake capacity,
and low desorption energy compared to zeolites and CMS.
Most importantly, the presence of organic linker within its
structure has provided good interaction between polymer
matrices which could minimize the interfacial defects. The
development of MMM is a surely interesting approach with
robustness, moderate cost, and high thermal, chemical, and
mechanical stability. Selection of fillers remains as the heart
of MMM development to ensure good compatibility with the
polymer matrix, consequently, boost the membrane performance exceeding the Robeson upper bound.
3.3.2 Challenges in MMM
Generally, the incorporation of inorganic particles into the
polymer matrix would surely improve the membrane properties due to the superiority of the dispersed phase itself. In
actual practice, MMMs development is often encountered
with the deterioration of membrane performance (Chung
et al. 2007). The incompatibility between polymer-inorganic
is regarded as the main factor that leads to the defective
interface. The defective MMM can be identified through its
gas separation performance relative to its neat polymer
membrane. Unselective voids would be formed due to
incompatibility between polymer and filler. Besides the
incompatibility between polymer and filler, the formation of
the voids is due to interfacial cracking at high loading,
elongation stress during spinning hollow fiber, and repulsive
force between polymer and filler (Xing and Ho 2009).
Consequently, the presence of voids interface would lead to
poor gas pair selectivity. Formation of voids can be reduced
by using the treatment on filler before dispersed into the
polymer solution. To address the issue, Zornoza et al.
(2011a, b) proposed filler treatment on mesoporous silica
spheres by using calcinations and chemical extraction.
Filler surface modification is also helpful to enhance its
affinity toward polymer. This approach acts as adhesive agent
at filler-polymer interface hence minimizes the formation of
unselective voids. The common surface modification methods were octadecylamine (Hashemifard et al. 2011), silane
coupling agent (Nik et al. 2011), diethanolamine (Clarizia
et al. 2008), and amine modification (Nordin, et al. 2014). It
was also reported that the incorporation of unmodified zeolite
has diminished CO 2 /CH 4 selectivity up to 80% due to the
formation of unselective voids. Whereas, the incorporated
modified zeolites have boosted CO 2 /CH 4 selectivity, 50%
better than the neat membrane. With the absent of unselective
voids after surface modification, the larger penetrant, CH 4 ,
traveled in longer permeation path while CO 2 can access
through the filler and give an improvement in CO 2 /CH 4
selectivity. Particle distribution is one of the important factors
to be considered in fabricating MMM. Poor particle distribution would provoke particles to agglomerate with each
other thus deteriorating the membrane performance. Poor
Table 5 Comparison of
polymeric, inorganic, and MMM
properties (Ismail et al. 2009)
Properties
Polymeric
Inorganic
MMM
Cost
Economical
High
Moderate
Chemical and thermal stability
Moderate
High
High
Mechanical Strength
Good
Poor
Excellent
Compatibility to solvent
Limited
Wide range
Limited
Separation performance
Moderate
High
Moderate
Handling
Robust
Brittle
Robust
66
I. U. Khan et al.
CH 4 selectivity by restricting polymer chain mobility and
hindered larger molecules to permeate (Ahn et al. 2010).
CNT possesses unique characteristics such as superior
mechanical and thermal stability with high electrical properties and is thus widely regarded as versatile materials. As
gas adsorption separation media, CNT tends to adsorb polar
molecules such as H 2 S (dipole moment of 0.97 Debye),
while discriminating non-polar molecules, despite CNT
being non-polar materials (Nour et al. 2013; Vinoba et al.
2017). CNT alone as gas separation medium is not considered to be attractive due to its non-polar nature;
non-selective sorption; and large pores do not provide
molecular sieving. Nevertheless, the incorporation of CNT
into polymer matrix has been reported to be beneficial to gas
separation properties. The limitations of CNT alone as gas
separation media are compensated by its relatively low
density materials. Compared to other class of fillers, a large
number of CNT materials can be incorporated into a polymer
matrix at similar mass loading with other fillers. As a result,
even low loading of CNT embedded in the membrane can
provide a significant contribution toward overall membrane
properties and hence separation performance (Moghadassi
and Rajabi 2014).
Application of metal–organic frameworks (MOFs) for gas
separation is not well developed comparing other applications. Most of the studies on MOF focused as a catalyst, gas
storage, and gas adsorbent, while as a gas separation media
is discouraging despite their highly permeable properties due
to poor gas pair selectivity. Nevertheless, MOF has higher
BET surface area, no dead volume, high uptake capacity,
and low desorption energy compared to zeolites and CMS.
Most importantly, the presence of organic linker within its
structure has provided good interaction between polymer
matrices which could minimize the interfacial defects. The
development of MMM is a surely interesting approach with
robustness, moderate cost, and high thermal, chemical, and
mechanical stability. Selection of fillers remains as the heart
of MMM development to ensure good compatibility with the
polymer matrix, consequently, boost the membrane performance exceeding the Robeson upper bound.
3.3.2 Challenges in MMM
Generally, the incorporation of inorganic particles into the
polymer matrix would surely improve the membrane properties due to the superiority of the dispersed phase itself. In
actual practice, MMMs development is often encountered
with the deterioration of membrane performance (Chung
et al. 2007). The incompatibility between polymer-inorganic
is regarded as the main factor that leads to the defective
interface. The defective MMM can be identified through its
gas separation performance relative to its neat polymer
membrane. Unselective voids would be formed due to
incompatibility between polymer and filler. Besides the
incompatibility between polymer and filler, the formation of
the voids is due to interfacial cracking at high loading,
elongation stress during spinning hollow fiber, and repulsive
force between polymer and filler (Xing and Ho 2009).
Consequently, the presence of voids interface would lead to
poor gas pair selectivity. Formation of voids can be reduced
by using the treatment on filler before dispersed into the
polymer solution. To address the issue, Zornoza et al.
(2011a, b) proposed filler treatment on mesoporous silica
spheres by using calcinations and chemical extraction.
Filler surface modification is also helpful to enhance its
affinity toward polymer. This approach acts as adhesive agent
at filler-polymer interface hence minimizes the formation of
unselective voids. The common surface modification methods were octadecylamine (Hashemifard et al. 2011), silane
coupling agent (Nik et al. 2011), diethanolamine (Clarizia
et al. 2008), and amine modification (Nordin, et al. 2014). It
was also reported that the incorporation of unmodified zeolite
has diminished CO 2 /CH 4 selectivity up to 80% due to the
formation of unselective voids. Whereas, the incorporated
modified zeolites have boosted CO 2 /CH 4 selectivity, 50%
better than the neat membrane. With the absent of unselective
voids after surface modification, the larger penetrant, CH 4 ,
traveled in longer permeation path while CO 2 can access
through the filler and give an improvement in CO 2 /CH 4
selectivity. Particle distribution is one of the important factors
to be considered in fabricating MMM. Poor particle distribution would provoke particles to agglomerate with each
other thus deteriorating the membrane performance. Poor
Table 5 Comparison of
polymeric, inorganic, and MMM
properties (Ismail et al. 2009)
Properties
Polymeric
Inorganic
MMM
Cost
Economical
High
Moderate
Chemical and thermal stability
Moderate
High
High
Mechanical Strength
Good
Poor
Excellent
Compatibility to solvent
Limited
Wide range
Limited
Separation performance
Moderate
High
Moderate
Handling
Robust
Brittle
Robust
66
I. U. Khan et al.