interaction between polymer matrices and the filler is the
main factor for particles to agglomerate. The agglomeration
caused the filler to be inaccessible by penetrant and cause
severe CO 2 /CH 4 reduction, thus hindering the potential of
MMM. The addition of excess particle is also increased the
chance of particles to agglomerate. Car et al. (2006) demonstrated that the CO 2 /CH 4 selectivity began to decrease when
Cu 3 (BTC) 2 loading exceeds 30 wt% in the PDMS membrane. At higher filler loading, the filler began to agglomerate
and disrupt the polymer chain severely. Several methods
have been proposed to avoid particle to agglomerate. Ge et al.
(2011) applied the pretreatment of CNT with acid and phase
transition method which has improved the particle distribution in the polymer matrix. Using H 2 SO 4 /HNO 3 and fast
phase inversion, the absent of particle agglomeration at
1–5 wt% of CNT was observed. However, a further increase
in CNT loading up to 10 wt%, particle agglomerations took
place as a cluster of nanotubes formed due to intermolecular
force in the polymer matrix.
Pore blockage can be categorized into the partial blockage and total blockage. Partial blockage allows molecules
smaller than blocking pores to pass through while total
blockage acts as an impermeable filler. Pore blockage can be
caused by the sorbent, solvent, contaminant, a component in
the polymer chain before, during or after membrane fabrication. The pore blockage phenomenon often resulted from
excessive filler loading that creates inaccessible pores after
agglomeration occurs. Moreover, low compatibility between
filler and polymer also lead to pores blockage. By functionalization or alteration of polymer or filler with introducing mutually interactive functional group will enhance its
compatibility and minimize pore blockage (Bhuwania et al.
2014). Another important phenomenon was the plasticization of the membrane during the gas separation process. CO 2
has been shown to plasticize a wide range of glassy polymers at elevated feed pressure (Scholes et al. 2010). Plasticization of glassy polymers occurred due to a reduction in
the interaction between polymer and filler which decline the
gas separation performance of the resulted membranes. The
CO 2 increases the mobility of polymer chain segments,
thereby increasing the diffusion coefficients of all penetrants
in the membrane. Generally, cross-linking or modification
methods were widely applied for improving the plasticization resistance (Adewole et al. 2013). Cross-linking process
leads to a reduction in chain mobility by improving adhesion
between polymer and inorganic filler. Subsequently, the gas
separation performance and long-term stability could be
achieved by overcoming CO 2 -induced plasticization.
3.3.3 Promising Nanoparticles for Mixed Matrix
Membranes
Metal–organic frameworks (MOFs) have recently emerged
as important materials for the catalytic application, gas
storage, adsorption, and gas separation. MOFs are crystalline
compounds consisting of metal ions and secondary building
units (SBUs) or organic ligands (Fig. 5). Interesting characteristics of MOF’s such as high micropore volume, large
pore sizes, high phase crystallinity, and high metal content
offering valuable active sites are the key features of this new
and emerging class of porous materials (Schlichte et al.
2004). Large surface area and pore volume of MOF give
advantages over other porous materials like activated carbon
and zeolite. The high surface area gives more contact with
the targeted species thus increasing the effectiveness of the
particles. Previous researchers had synthesized MOF with
high BET surface areas such as MOF-5 (3000 m
2 /g) (Perez
and Balkus 2009), Co-MOF-74 (1314 m
2 /g) (Cho et al.
2012), Mg-MOF-74 (1332 m
2 /g) (Bao et al. 2011), higher
than commercial zeolite Y (900 m
2 /g), and zeolite beta
(710 m
2 /g). The role of MOF’s in enhancing MMM performance can be summarized as follows; (1) enhancement of
CO 2 diffusivity coefficient through polymer chain disruption
exceeding CH 4 (Song et al. 2012a, b); (2) interaction
between quadrupole moment of CO 2 with the weak electrostatic field of MOF (Basu et al. 2011); and (3) molecular
sieving induced by MOF pores (Ordoñez and Balkus 2010).
One of the attractive properties of MOFs is their adsorption
capability. The high surface area of the materials with open
metal sites act as a large platform for adsorption of specific
gases (Bao et al. 2011). The HKUST-1 was able to adsorb
12.5 mmol CO 2 /g at 15 bar and still did not reach its saturation point due to a large surface area and pore volume
(Liang et al. 2009). Other notable MOF’s with high CO 2
adsorption capacities were Cu-MOF with 10.9 mmol/g
(Lincke et al. 2011) and Mg-MOF-74 with 15 mmol/g
(Choi et al. 2012). In additions, due to the structural flexibility
of MOF’s, their adsorption capacity has increased even at
high pressure while retaining its structure (Li et al. 2011b). It
was reported that the adsorption capacity of ZIF-69 increases
up to 40% when changing to larger pores at high pressure (Li
et al. 2011a) and able to retain 82.6 L CO 2 for every liter of
ZIF-69 (Banerjee et al. 2008). MOFs for gas separation were
also reported in recent years. Since MOF’s are highly porous
materials with high surface area, gases are able to permeate
across the membrane with high permeation rates.
Although MOF possesses a unique characteristic, it
should be noted that MOF is the relatively new class of
materials. Thus, several insights such as influences of different gases and vapors and the cost of the materials are
necessary before it can be used for various industrial applications (Tagliabue and Farrusseng 2009). Besides, the
selective sorption of the MOF is relatively low compared to
other classes of materials (Yeo et al. 2014). Although the
versatility of its organic ligands allowed vast modification
approaches to compensate its limitations, an intensive study
regarding this matter is necessary. Table 6 summarizes the
High Performance Membrane for Natural Gas Sweetening Plants
67
main factor for particles to agglomerate. The agglomeration
caused the filler to be inaccessible by penetrant and cause
severe CO 2 /CH 4 reduction, thus hindering the potential of
MMM. The addition of excess particle is also increased the
chance of particles to agglomerate. Car et al. (2006) demonstrated that the CO 2 /CH 4 selectivity began to decrease when
Cu 3 (BTC) 2 loading exceeds 30 wt% in the PDMS membrane. At higher filler loading, the filler began to agglomerate
and disrupt the polymer chain severely. Several methods
have been proposed to avoid particle to agglomerate. Ge et al.
(2011) applied the pretreatment of CNT with acid and phase
transition method which has improved the particle distribution in the polymer matrix. Using H 2 SO 4 /HNO 3 and fast
phase inversion, the absent of particle agglomeration at
1–5 wt% of CNT was observed. However, a further increase
in CNT loading up to 10 wt%, particle agglomerations took
place as a cluster of nanotubes formed due to intermolecular
force in the polymer matrix.
Pore blockage can be categorized into the partial blockage and total blockage. Partial blockage allows molecules
smaller than blocking pores to pass through while total
blockage acts as an impermeable filler. Pore blockage can be
caused by the sorbent, solvent, contaminant, a component in
the polymer chain before, during or after membrane fabrication. The pore blockage phenomenon often resulted from
excessive filler loading that creates inaccessible pores after
agglomeration occurs. Moreover, low compatibility between
filler and polymer also lead to pores blockage. By functionalization or alteration of polymer or filler with introducing mutually interactive functional group will enhance its
compatibility and minimize pore blockage (Bhuwania et al.
2014). Another important phenomenon was the plasticization of the membrane during the gas separation process. CO 2
has been shown to plasticize a wide range of glassy polymers at elevated feed pressure (Scholes et al. 2010). Plasticization of glassy polymers occurred due to a reduction in
the interaction between polymer and filler which decline the
gas separation performance of the resulted membranes. The
CO 2 increases the mobility of polymer chain segments,
thereby increasing the diffusion coefficients of all penetrants
in the membrane. Generally, cross-linking or modification
methods were widely applied for improving the plasticization resistance (Adewole et al. 2013). Cross-linking process
leads to a reduction in chain mobility by improving adhesion
between polymer and inorganic filler. Subsequently, the gas
separation performance and long-term stability could be
achieved by overcoming CO 2 -induced plasticization.
3.3.3 Promising Nanoparticles for Mixed Matrix
Membranes
Metal–organic frameworks (MOFs) have recently emerged
as important materials for the catalytic application, gas
storage, adsorption, and gas separation. MOFs are crystalline
compounds consisting of metal ions and secondary building
units (SBUs) or organic ligands (Fig. 5). Interesting characteristics of MOF’s such as high micropore volume, large
pore sizes, high phase crystallinity, and high metal content
offering valuable active sites are the key features of this new
and emerging class of porous materials (Schlichte et al.
2004). Large surface area and pore volume of MOF give
advantages over other porous materials like activated carbon
and zeolite. The high surface area gives more contact with
the targeted species thus increasing the effectiveness of the
particles. Previous researchers had synthesized MOF with
high BET surface areas such as MOF-5 (3000 m
2 /g) (Perez
and Balkus 2009), Co-MOF-74 (1314 m
2 /g) (Cho et al.
2012), Mg-MOF-74 (1332 m
2 /g) (Bao et al. 2011), higher
than commercial zeolite Y (900 m
2 /g), and zeolite beta
(710 m
2 /g). The role of MOF’s in enhancing MMM performance can be summarized as follows; (1) enhancement of
CO 2 diffusivity coefficient through polymer chain disruption
exceeding CH 4 (Song et al. 2012a, b); (2) interaction
between quadrupole moment of CO 2 with the weak electrostatic field of MOF (Basu et al. 2011); and (3) molecular
sieving induced by MOF pores (Ordoñez and Balkus 2010).
One of the attractive properties of MOFs is their adsorption
capability. The high surface area of the materials with open
metal sites act as a large platform for adsorption of specific
gases (Bao et al. 2011). The HKUST-1 was able to adsorb
12.5 mmol CO 2 /g at 15 bar and still did not reach its saturation point due to a large surface area and pore volume
(Liang et al. 2009). Other notable MOF’s with high CO 2
adsorption capacities were Cu-MOF with 10.9 mmol/g
(Lincke et al. 2011) and Mg-MOF-74 with 15 mmol/g
(Choi et al. 2012). In additions, due to the structural flexibility
of MOF’s, their adsorption capacity has increased even at
high pressure while retaining its structure (Li et al. 2011b). It
was reported that the adsorption capacity of ZIF-69 increases
up to 40% when changing to larger pores at high pressure (Li
et al. 2011a) and able to retain 82.6 L CO 2 for every liter of
ZIF-69 (Banerjee et al. 2008). MOFs for gas separation were
also reported in recent years. Since MOF’s are highly porous
materials with high surface area, gases are able to permeate
across the membrane with high permeation rates.
Although MOF possesses a unique characteristic, it
should be noted that MOF is the relatively new class of
materials. Thus, several insights such as influences of different gases and vapors and the cost of the materials are
necessary before it can be used for various industrial applications (Tagliabue and Farrusseng 2009). Besides, the
selective sorption of the MOF is relatively low compared to
other classes of materials (Yeo et al. 2014). Although the
versatility of its organic ligands allowed vast modification
approaches to compensate its limitations, an intensive study
regarding this matter is necessary. Table 6 summarizes the
High Performance Membrane for Natural Gas Sweetening Plants
67